METHODS AND COMPOSITIONS FOR TREATING DISORDERS INVOLVING EXCITOTOXICITY

Abstract: It is shown here that hedgehog polypeptides possess novel activities beyond phenotype specification. Using cultures derived from the embryonic day 14.5 (E14.5) rat ventral mesencephalon, we show that hedgehog is also trophic for dopaminergic neurons and other neurons which are sensitive to exotoxicity.

Description: Methods and Compositions for Treating Disorders Involving Excitotoxicity Background of the Invention The excessive or inappropriate stimulation of excitatory amino acid receptors can lead to neuronal cell damage or loss by way of a mechanism known as excitotoxicity. For instance, excitotoxic action may be responsible for neuronal loss in stroke, cerebral palsy, epilepsy, ageing and Alzheimer's disease, Huntington's disease, and other chronic degenerative disorders. The medical consequences of such neuronal degeneration makes the abatement of these degenerative neurological processes an important therapeutic goal. The development of neuroprotective agents for the prevention of neuronal loss in acute conditions such as stroke and epilepsy or chronic neurodegenerative disorders including Parkinson's disease, Alzheimer's disease, Huntington's chorea, and motor neuron disease has in fact focused on drugs that inhibit excitatory amino acid neurotransmission or exhibit antioxidant properties. Unfortunately, potent antagonists of the N-methyl-D-aspartate (NMDA) type glutamate receptor, which is thought to mediate excitotoxic neuronal injury, e.g., MK-801 or phencyclidine (PCP), share a high probability of inducing psychotomimetic side effects. Further, these drugs have been associated with acute neurotoxicity in vitro and in vivo, precluding their clinical use. It is a goal of the present invention to provide compositions and methods for preventing or ameliorating neuronal degeneration, e.g., for the abatement of these degenerative neurological processes. Summary of the Invention One aspect of the present application relates to a method for promoting survival and/or functional perfomance of neuronal cells susceptible to exotoxicity, comprising contacting the cells, in vitro or in vivo, with a hedgehog therapeutic or ptc therapeutic in an amount effective increasing the rate of survival of the neurons relative to the absence of administeration of the hedgehog therapeutic or ptc therapeutic. In preferred embodiments, the method is carried out using a lipophilic modified hedgehog polypeptide effective to reduce exotoxin-mediated degradation of the cells. For instance, the present application provides a method for promoting the survival of dopaminergic or GABAergic neurons by contacting the cells, in vitro or in vivo, with a hedgehog therapeutic ox ptc therapeutic in an amount effective to increase the rate of survival of the neurons relative to the absence of administeration of the hedgehog therapeutic or ptc therapeutic. In other embodiments, the present application relates to a method for promoting the survival of neurons of the substantia nigra by contacting the cells, in vitro or in vivo, with a hedgehog therapeutic or ptc therapeutic in an amount effective to increase the rate of survival of the neurons relative to the absence of administeration of the hedgehog therapeutic or ptc therapeutic. In certain embodiments, the subject invention provides a method for treatment or prophylaxis of a disorder selected from the group consisting of domoic acid poisoning; spinal cord trauma; hypoglycemia; mechanical trauma to the nervous system; senile dementia; Korsakoffs disease; schizophrenia; AIDS dementia, multi-infarct dementia; mood disorders; depression; chemical toxicity; neuronal damage associated with uncontrolled seizures, such as epileptic seizures; neuronal injury associated with HIN and AIDS; neurodegeneration associated with Down's syndrome; neuropathic pain syndrome; olivopontocerebral atrophy; amyotrophic lateral sclerosis; mitochondrial abnormalities; Alzheimer's disease; hepatic encephalopathy; Tourette's syndrome; schizophrenia; and drug addiction, comprising administering to a patient in need thereof a therapeutically effective amount of a hedgehog or ptc therapeutic, e.g., a lipophilic modified hedgehog polypeptide. In other embodiments, the subject method can be used for protecting dopaminergic and/or GABAergic neurons of a mammal from neurodegeneration; for preventing or treating neurodegenerative disorder; for treatment of Parkinson's; for treatment of Huntington's; and/or for treatment of A S. In embodiments wherein the patient is treated with a ptc therapeutic, such therapeutics are preferably small organic molecules which mimic hedgehog effects on patched-mediated signals. When the subject method is carried out using a hedgehog therapeutic, the hedgehog therapeutic preferably is a polypeptide including at least a bioactive extracellular portion of a hedgehog polypeptide, e.g., including at least 50, 100 or 150 amino acid residues of an Ν- terminal half of a hedgehog polypeptide. In preferred embodiments, the hedgehog portion includes at least a portion of the hedgehog polypeptide corresponding to a 19kd fragment of the extracellular domain of a hedgehog polypeptide. In preferred embodiments, the hedgehog portion has an amino acid sequence at least 60, 75, 85, 95 or 100 percent identical with a hedgehog polypeptide of any of SEQ ID Νos. 10-18 or 20. The hedgehog portion can be encoded by a nucleic acid which hybridizes under stringent conditions to a nucleic acid sequence of any of SEQ ID Nos. 1-9 or 19, e.g., the hedgehog portion can be encoded by a vertebrate hedgehog gene, especially a human hedgehog gene. In certain embodiments, the hedgehog polypeptides of the present invention are modified by a lipophilic moiety or moieties at one or more internal sites of the mature, processed extracellular domain, and may or may not be also derivatized with lipophilic moieties at the N or C-terminal residues of the mature polypeptide. In other embodiments, the polypeptide is modified at the C-terminal residue with a hydrophobic moiety. In still other embodiments, the polypeptide is modified at the N-terminal residue with a cyclic (preferably polycyclic) lipophilic group. Various combinations of the above are also contemplated. In other embodiments, the subject method can be carried out by administering a gene activation construct, wherein the gene activation construct is deigned to recombine with a genomic hedgehog gene of the patient to provide a heterologous transcriptional regulatory sequence operatively linked to a coding sequence of the hedgehog gene. In still other embodiments, the subject method can be practiced with the administration of a gene therapy construct encoding a hedgehog polypeptide. For instance, the gene therapy construct can be provided in a composition selected from a group consisting of a recombinant viral particle, a liposome, and a poly-cationic nucleic acid binding agent, In certain embodiments, the polypeptide is purified to at least 80% by dry weight, and more preferably 90 or 95% by dry weight. Another aspect of the present invention provides an isolated nucleic acid encoding a polypeptide comprising a hedgehog amino acid sequence which (i) binds to a patched protein, (ii) regulates differentiation of neuronal cells, (iii) regulates survival of differentiated neuronal cells, (iv) regulates proliferation of chondrocytes, (v) regulates proliferation of testicular germ line cells, or (vi) functionally replaces drosopholia hedgehog in transgenic drosophila fly, or a combination thereof. In other preferred embodiments, the isolated nucleic acid encodes a polypeptide having a hedgehog amino acid sequence encoded by a nucleic acid which hybridizes under stringent conditions to a nucleic acid sequence selected from the group consisting of SEQ ID No: 1-9, which hedgehog amino acid sequence of the polypeptide corresponds to a natural proteolytic product of a hedgehog polypeptide. Such polypeptides preferably (i) binds to a patched protein, (ii) regulates differentiation of neuronal cells, (iii) regulates survival of differentiated neuronal cells, (iv) regulates proliferation of chondrocytes, (v) regulates proliferation of testicular germ line cells, and/or (vi) functionally replaces drosopholia hedgehog in transgenic drosophila fly, or a combination thereof. In preferred embodiments, the nucleic acid encodes a hedgehog amino acid sequence identical to a hedgehog polypeptide selected from the group consisting of SEQ ID No: 10-18. Another preferred embodiment provides an isolated nucleic acid comprising a coding sequence of a human hedgehog gene, encoding a bioactive hedgehog polypeptide. Still another aspect of the present invention relates to an expression vector, capable of replicating in at least one of a prokaryotic cell and eukaryotic cell, comprising a nucleic acid encoding a Dhh or Ihh polypeptide described above. The present invention also provides a host cell transfected with such expression vectors; as well as methods for producing a recombinant hedgehog polypeptide by culturing such cells in a cell culture medium to express a hedgehog polypeptide and isolating said hedgehog polypeptide from the cell culture. Still another aspect of the present invention provides a recombinant transfection system, e.g., such as may be useful for gene therapy, comprising (i) a gene construct including the coding sequence for a human Ihh or Dhh protein, operably linked to a transcriptional regulatory sequence for causing expression of the hedgehog polypeptide in eukaryotic cells, and (ii) a gene delivery composition for delivering said gene construct to a cell and causing the cell to be transfected with said gene construct. For instance, the gene delivery composition is selected from a group consisting of a recombinant viral particle, a liposome, and a poly-cationic nucleic acid binding agent. Another aspect of the present invention provides a probe/primer comprising a substantially purified oligonucleotide, said oligonucleotide containing a region of nucleotide sequence which hybridizes under stringent conditions to at least 10 consecutive nucleotides of sense or antisense sequence of SEQ ID No. 1-9, or naturally occuring mutants thereof. In preferred embodiments, the probe/primer includes a label group attached thereto and able to be detected. The present invention also provides a test kit for detecting cells which contain a hedgehog mRNA transcript, and includes such probe/primers. Still another embodiment of the present invention provides a purified preparation of an antisense nucleic acid which specifically hybridizes to and inhibits expression of a gene encoding a human Shh, Ihh or Dhh hedgehog polypeptide under physiological conditions, which nucleic acid is at least one of (i) a synthetic oligonucleotide, (ii) single-stranded, (iii) linear, (iv) 20 to 50 nucleotides in length, and (v) a DNA analog resistant to nuclease degradation. Brief Description of the Drawings Figure 1. Shh and Ptc in the El 4.5 rat embryo. Shh (A, antisense; B, sense control), and ptc (C, antisense; D, sense control) expression as detected by in situ hybridization with digoxigenin-labeled riboprobes and alkaline phosphataseconjugated anti-digoxigenin. The arrow in A and the double-arrow in C designate the zona limitan intrathalamica. Major anatomical structures and summary diagrams of shh and ptc expression are shown in E. Scale bar = 1 mm. Figure 2. Shh promotes the survival of TH+ neurons of the ventral mesencephalon. (A) Timecourse and dose response of the Shh effect. The number of TH+ neurons in control cultures (O ng/ml Shh) began to decline dramatically by 5 days in vitro. In cultures treated with Shh at 25 and 50 ng/ml there were significantly greater numbers of TH+ nurrons over control- through 24 days in vitro (from 5 to 24 days, p <.001 at 25 and 50 ng/ml). The 50 ng/ml dose typically gave a 50-100% increase over controls at all time points (error bars s.e.m.)P-h'otomicrographs of TH+ neurons in 50.ng/ml Shh treated (B-,„~D) and control (C, E) cultures, 2 days (B, C) and 7 days (D, E) post-plating. Note that in addition to an increased number of TH+-cell bodies, the Shh treated cells—show extensive neuritic processes. Scale bar = 200 um. Figure 3. Transport of 3H-Dopamine. The identity and functionality of the surviving midbrain neurons was assessed by their ability to specifically transport dopamine. (A) Addition of 25 ng/ml Shh resulted in a 22-fold increase in 3H-DA cell uptake over controls and lower Shh concentrations. 50 ng/ml Shh gave a 30-fold increase in 3H-DA uptake (error bars = s.d.) (p < 0.005 at 25 and 50 ng/ml). (B) Autoradiography was performed on sister plates to visualize dopamine transport. Only cells with neuronal morphology transported 3H- DA (inset). Scale bar = 50 Jim, inset 15 m. Figure 4. Specificity of Shh activity. (A) QC-PCR gel. Lanes 1-4 are CDNA from midbrain cultures that have been co-amplified with successive 4-fold dilutions of mimic oligo. Lane 5 is DNA marker lane. Ptc target is 254 bp and mimic is 100 bp(B) Representative plot (corresponding to A) of the log concentration of competitive mimic versus the log of the obtained band densities of target and mimic PCR substrates demonstrates the linearity of the amplification reaction. The extrapolated value of ptc message in the CDNA tested is determined to be equal to the value of mimic concentration where Log Ds/Dm = 0. See main text for details of the procedure. Doses in ng/ml; Ds = density of test substrate; Dm = density of competitive mimic. The r 2 value shows that determinations made within this range vary within 3%. (C) Administration of Shh induces tc expression in a dose response that parallels the survival curve. The values are expressed as number of target molecules (log Ds) per total amount of CDNA used in each reaction as measured by optical density at 260 nm (OD) and were determined as demonstrated in A and B. At 4 days in vitro Shh at 5 ng/ml increases ptc expression over control, and 50 ng/ml increases expression of ptc over the level found in the ventral mesencephalon at the time of dissection. (D) Affinity purified anti-Shh antibody inhibited the Shh neurotrophic response (p < .001). Cultures were maintained for 5 days. Shh was added at a concentration of 50 ng/ml, and in the co-administration of 5 Shh and anti-Shh ("Shh antibody") Shh-was added at 0 g/ml and anti-Shh was added as a 5-fold molar excess (error bars s.e.m.). Figure 5. Shh also supports the survival of midbrain-GABA+ neurons. (A) In addition to supporting the survival of TH+ cells in the midbrain cultures, Shh promotes the survival of GABA-immunoreactive neurons with a similar dose response (error bars=s.e.m.) (For TH, p , 0.001 at 25 and 50 ng/ml; for GABA, p < .001 at 25 and 50 ng/ml). (B) Double level immunofluorescence of SSH-treated cultures shows that the majority of the GABA+ cells (Orange) do not overlap with the TH+ cells (green); scale bar = 15 m. Figure 6. Shh effects on striatal cultures. (A) At concentrations of 10 ng/ml and higher, Shh promotes neuronal survival as gauged by staining for tubulin PHI, and these cells are exclusively GABA+ (error bars = S.D.) (tubulin PHI, p < 0.001 at 25 and 50 ng/ml; GABA, p < .001 at 25 and 50 ng/ml). Typical fields of neurons treated with 50 ng/ml Shh stained for tubulin pill (B) and GABA+ (C) are shown; scale bar = 100 gm. Figure 7. Shh effects on ventral spinal cultures. (A) At concentrations of 25 ng/ml and higher, Shh promotes neuronal survival as gauged by staining for tubulin PHI. The majority of the cells stain positively for GABA, while a subset stain for the nuclear marker of spinal interneurons, Lim-1/2 (error bars = s.e.m.) (tubulin pill, p < 0.001 at 25 and 50 ng/ml; lim 1/2, p <.001 at 5,10,25, and 50 ng/ml; GABA, p < .001 at 25 and 50 ng/ml). Typical staining for Lim- - 1/2 in the E14 rat spinal cord (B, scale bar = 100 m), and spinal neurons cultured in the presence of 50 ng/ml Shh (C, scale bar = 20 m). Figure 8. Shh protects midbrain TH+ neurons from neurotoxic insult. Cultures of ventral mesencephalon neurons were cultured in the indicated concentrations of Shh (ng/ml). MPP+ was added at 4 days in vitro for 48 hours. Cultures were then washed extensively and cultured for an additional 48 hours to allow clearance of dying neurons. Protection from MPP+ neurotoxicity could be seen at 5 ng/ml, with the effect saturating at 50 ng/ml. BDNF was used at 10 ng/ml, and GDNF at 20 ng/ml (error bars = s.e.m.) (Shh, p < 0.001 at 50 and 250 ng/ml; BDNF no significance; GDNF, p < .05). Note that the plating density used in this experiment was twice that used in Figure 2. Figures 9 and 10 are graphs illustrating the effects of Shh on apomorphine-induced rotational behavior in stably lesioned mice. Figures 11A and 11B are graphs illustrating the neuroprotective effect of Shh against amphetamine challenges. Figures 12A-B and 13A-D are graphs comparing the restorative effect of myristoylated Shh (Mz) and un-modified forms of Shh (Gz) on the rotational behavior of 6- OHDA lesioned mice. Figures 14A-E, 15A-H and 16 are graphs illustrating the neuroprotective effect of Shh polypeptides against amphetamine and apomorphine challenges of 6-OHDA lesioned mice. Figure 17 is a graph demonstrating the dose response for hedgehog polypeptide constructs in a malonate striatal lesion model. Figure 18 is a graph showing the effect of pretreatment with myristoylated Shh in a malonate striatal lesion model. Figures 19A and 19B are graphs illustrating that intranigral injection of myristoylated Sonic Hedgehog (Shh-M) reduces parkinsonian disability and increases the therapeutic effect of L-DOPA in MPTP-treated marmosets. Figure 19C compares the activity of hedgehog polypeptides with GDNF. Figure 20A and 20B are graphs illustrating the protective effect of Hedgehog polypeptides against amphetamine challenges of 6-OHDA lesioned mice. Detailed Description of the Invention I. Overview Sonic hedgehog (Shh), an axis-determining secreted protein, is expressed during early vertebrate embryogenesis in the notochord and ventral neural tube. In this site it plays a role in the phenotypic specification of ventral neurons along the length of the CNS. For example, Shh induces the differentiation of motor neurons in the spinal cord and dopaminergic neurons in the midbrain. Shh expression, however, persists beyond this induction period. We have show here that Shh possesses novel activities beyond phenotype specification. Using cultures derived from the embryonic day 14.5 (E14.5) rat ventral mesencephalon, we show that Shh is also trophic for dopaminergic neurons. Interestingly, Shh not only promotes dopaminergic neuron survival, but also promotes the survival of midbrain GABA-immunoreactive (GABA-ir) neurons. In cultures derived from the El 5- 16 striatum, Shh promotes the survival of GABA-ir interneurons to the exclusion of any other cell type. Cultures derived from El 5- 16 ventral spinal cord reveal that Shh is again trophic for interneurons, many of which are GABA-ir and some of which express the Lim-1/2 nuclear marker, but does not appear to support motorneuron survival. Shh does not support survival of sympathetic or dorsal root ganglion neurons. Finally, using the midbrain cultures, we show that in the presence of MPP+, a highly specific neurotoxin, Shh prevents dopaminergic neuron death that normally would have occurred. Moreover, as demonstrated in the appended examples, activation of hedgehog signalling pathway(s) can be used both for restoration of lesions to dopaminergic cells and other neuronal cells of the substantia nigra (SN) and ventral termental area (VTA), as well as for neuroprotection against formation of such lesions. Briefly, as described in the appended examples, we investigated the neuroprotective and restorative potential of hedgehog polypeptides in various animal models of lesions involving nigrostriatal and mesolimbic dopaminergic pathways. As described in detail below, our results show that the administration of hedgehog polypeptide can prevent and partially restore such neuronal damage. Furthermore, as demonstrated in the appended examples, activation of the hedgehog signaling pathway(s) can be a more general protective measure against neurotoxic insult, and particularly for protection against cell death due to overstimulation by excitatory amino acids. One aspect of the present application is directed to compositions and methods for the prevention and treatment of degenerations of certain neuronal cells due to exotoxicity. The invention also specifically contemplates the use of hedgehog agonists for treating, preventing or ameliorating neuronal loss associated with neurologic disorders which are due to overstimulation by the excitatory amino acids. These include acute neurologic disorders such as domoic acid poisoning; spinal cord trauma; hypoglycemia; mechanical trauma to the nervous system, epileptic seizures; and chronic neurologic disorders such as Huntington's disease, neuronal injury associated with HIV and AIDS, AIDS dementia, neurodegeneration associated with Down's syndrome, neuropathic pain syndrome, olivopontocerebral atrophy, amyotrophic lateral sclerosis, mitochondrial abnormalities, Alzheimer's disease, hepatic encephalopathy, Tourette's syndrome, schizophrenia, and drug addiction (see Lipton and Rosenberg, N. Engl. J. Med. 330: 613-622 (1994)). In certain preferred embodiments, the subject invention is directed to methods and compositions for preventing or ameliorating degenration of dopaminergic cells and other neuronal cells of the substantia nigra (SN) and ventral termental area (VTA), such as resulting in Parkinson's disease. Based in part on these findings, we have determined that Shh, and other forms of hedgehog polypeptides, are useful as a protective agents in the treatment and prophylaxis for neurodegenerative disorders, particularly those resulting from the loss of dopaminergic and/or GABA-nergic neurons, or the general loss tissue from the substantia nigra. As described with greater detail below, exemplary disorders ("candidate disorders") include Parkinson's disease. In one aspect, the present invention provides pharmaceutical preparations and methods for preventing/treating lesions involving exotoxic-dependent degeneration of neurons utilizing, as an active ingredient, a hedgehog polypeptide or a mimetic thereof. However, without wishing to be bound by any particular theory, the protective/restorative activity of hedgehog polypeptides obeserved in the present studies may be due at least in part to the ability of hedgehog polypeptides to antagonize (directly or indirectly) patched-mediated regulation of gene expression and other physiological effects mediated by the patched gene. The patched gene product, a cell surface protein, is understood to signal through a pathway which regulates transcription of a variety of genes involved in neuronal cell development. In the CNS and other tissue, the introduction of hedgehog relieves (derepresses) this inhibition conferred by patched, allowing expression of particular gene programs. Accordingly, the present invention contemplates the use of other agents which are capable of mimicking the effect of the hedgehog polypeptide on patched signalling, e.g., as may be identified from the drug screening assays described below. The subject hedgehog treatments are effective on both human and animal subjects afflicted with these conditions. Animal subjects to which the invention is applicable extend to both domestic animals and livestock, raised either as pets or for commercial purposes. Examples are dogs, cats, cattle, horses, sheep, hogs and goats. In terms of treatment, once a patient experiences symptoms of a candidate disorder, a goal of therapy is prevention of further loss of neuron function. Moreover, the subject method can also be utilizied for cell culture, e.g., for the maintenance of differentiated neurons in cultures; such as in cultures of dopaminergic and GABA-nergic neurons. The subject methods and compositions can also be used to augment the implantion of such neuronal cells in an animal. II. Definitions For convience, certain terms employed in the specfication, examples, and appended claims are collected here. The term "hedgehog therapeutic" refers to various forms of hedgehog polypeptides, as well as peptidomimetics, which are neuroprotective for neuronal cells, and in particular, enhance the survival of dopaminergic and GABA-ergic neurons. These include naturally occurring forms of hedgehog polypeptides, as well as modified or mutant forms generated by molecular biological techniques, chemical synthesis, etc. While in preferred embodiments the hedgehog polypeptide is derived from a vertebrate homolog, cross-sepcies activity reported in the literature supports the use of hedgehog peolypeptides from invertebrate organisms as well. Naturally and non-naturally occurring hedgehog therapeutics referred to herein as "agonists" mimic or potentiate (collectively "agonize") the effects of a naturally- occurring hedgehog polypeptide as a neuroprotective agent. In addition, the term "hedgehog therapeutic" includes molecules which can activate expression of an endogenous hedgehog gene. The term also includes gene therapy constructs for causing expression of hedgehog polypeptides in vivo, as for example, expression constructs encoding recombinant hedgehog polypeptides as well as trans-activation constructs for altering the regulatory sequences of an endogenous hedgehog gene by homologous recombination. In particular, the term "hedgehog polypeptide" encompasses hedgehog polypeptides and peptidyl fragments thereof. As used herein the term "bioactive fragment", with reference to a portions of hedgehog polypeptides, refers to a fragment of a full-length hedgehog polypeptide, wherein the fragment specifically agonizes neuroprotective events mediated by wild-type hedgehog polypeptides. The hedgehog bioactive fragment preferably is a soluble extracellular portion of a hedgehog polypeptide, where solubility is with reference to physiologically compatible solutions. Exemplary bioactive fragments are described in PCT publications WO 95/18856 and WO 96/17924. The term "ptc therapeutic" refers to agents which mimic the effect of naturally occurring hedgehog polypeptides on patched signalling. The ptc therapeutic can be, e.g., a peptide, a nucleic acid, a carbohydrate, a small organic molecule, or natural product extract (or fraction thereof). As used herein, the term "antibody homolog" includes intact antibodies consisting of immunoglobulin light and heavy chains linked via disulfide bonds. The term "antibody homolog" is also intended to encompass a protein comprising one or more polypeptides selected from immunoglobulin heavy chains and antigen-binding fragments thereof which are capable of binding to one or more antigens (i.e., hedgehog or patched). The component polypeptides of an antibody homolog composed of more than one polypeptide may optionally be disulfide-bound or otherwise covalently crosslinked. Accordingly, therefore, "antibody homologs" include intact immunoglobulins of types IgA, IgG, IgE, IgD, IgM (as well as subtypes thereof), wherein the light chains of the immunoglobulin may be of types kappa or lambda. Preferred proteins of the invention may include portions of intact antibodies that retain antigen-binding specificity, for example, Fab fragments, Fab' fragments, F(ab')2 fragments, F(v) fragments, heavy chain monomers or dimers, light chain monomers or dimers, dimers consisting of one heavy and one light chain, and the like. As used herein, a "humanized antibody homolog" is an antibody homolog, produced by recombinant DNA technology, in which some or all of the amino acids of a human immunoglobulin light or heavy chain that are not required for antigen binding have been substituted for the corresponding amino acids from a nonhuman mammalian immunoglobulin light or heavy chain. A "human antibody homolog" is an antibody homolog in which all the amino acids of an immunoglobulin light or heavy chain (regardless of whether or not they are required for antigen binding) are derived from a human source. A "patient" or "subject" to be treated by the subject method are mammals, including humans. An "effective amount" of, e.g., a hedgehog or ptc therapeutic, with respect to the subject method of treatment, refers to an amount of the therapeutic in a preparation which, when applied as part of a desired dosage regimen causes a increase in survival of a neuronal cell population according to clinically acceptable standards for the treatment or prevention of a particular disorder. By "prevent degeneration" it is meant reduction in the loss of cells (such as from apoptosis), or reduction in impairment of cell function, e.g., release of dopamine in the case of dopaminergic neurons. A "trophic factor", referring to a hedgehog or ptc therapeutic, is a molecule that directly or indirectly affects the survival or function of a hedgehog-responsive cell, e.g., a dopaminergic or GABAergic cell. A "trophic amount" of a a hedgehog or ptc therapeutic is an amount sufficient to, under the circumstances, cause an increase in the rate of survival or the functional perfomance of a zeJge..øg-responsive cell, e.g., a dopaminergic or GABAergic cell. "Homology" and "identity" each refer to sequence similarity between two polypeptide sequences, with identity being a more strict comparison. Homology and identity can each be determined by comparing a position in each sequence which may be aligned for purposes of comparison. When a position in the compared sequence is occupied by the same amino acid residue, then the polypeptides can be referred to as identical at that position; when the equivalent site is occupied by the same amino acid (e.g., identical) or a similar amino acid (e.g., similar in steric and/or electronic nature), then the molecules can be refered to as homologous at that position. A percentage of homology or identity between sequences is a function of the number of matching or homologous positions shared by the sequences. An "unrelated" or "non-homologous" sequence shares less than 40 percent identity, though preferably less than 25 percent identity, with an hedghog sequence. In particular, the term "percent identical" refers to sequence identity between two amino acid sequences or between two nucleotide sequences. Identity can each be determined by comparing a position in each sequence which may be aligned for purposes of comparison. When an equivalent position in the compared sequences is occupied by the same base or amino acid, then the molecules are identical at that position; when the equivalent site occupied by the same or a similar amino acid residue (e.g., similar in steric and/or electronic nature), then the molecules can be referred to as homologous (similar) at that position. Expression as a percentage of homology/similarity or identity refers to a function of the number of identical or similar amino acids at positions shared by the compared sequences. Various alignment algorithms and or programs may be used, including FASTA, BLAST or ENTREZ. FASTA and BLAST are available as a part of the GCG sequence analysis package (University of Wisconsin, Madison, Wis.), and can be used with, e.g., default settings. ENTREZ is available through the National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, Md. In one embodiment, the percent identity of two sequences can be determined by the GCG program with a gap weight of 1, e.g., each amino acid gap is weighted as if it were a single amino acid or nucleotide mismatch between the two sequences. The term "corresponds to", when referring to a particular polypeptide or nucleic acid sequence is meant to indicate that the sequence of interest is identical or homologous to the reference sequence to which it is said to correspond. The terms "recombinant protein", "heterologous protein" and "exogenous protein" are used interchangeably throughout the specification and refer to a polypeptide which is produced by recombinant DNA techniques, wherein generally, DNA encoding the polypeptide is inserted into a suitable expression construct which is in turn used to transform a host cell to produce the heterologous protein. That is, the polypeptide is expressed from a heterologous nucleic acid. A "chimeric protein" or "fusion protein" is a fusion of a first amino acid sequence encoding a hedgehog polypeptide with a second amino acid sequence defining a domain foreign to and not substantially homologous with any domain of hh protein. A chimeric protein may present a foreign domain which is found (albeit in a different protein) in an organism which also expresses the first protein, or it may be an "interspecies", "intergenic", etc. fusion of protein structures expressed by different kinds of organisms. In general, a fusion protein can be represented by the general formula (X) n -( ) m -(Y) n , wherein hh represents all or a portion of the hedgehog polypeptide, X and Y each independently represent an amino acid sequences which are not naturally found as a polypeptide chain contiguous with the hedgehog sequence, m is an integer greater than or equal to 1, and each occurrence of n is, independently, 0 or an integer greater than or equal to 1 (n and m are preferably no greater than 5 or 10). As used herein, the term "vector" refers to a nucleic acid molecule capable of transporting another nucleic acid to which it has been linked. The term "expression vector" includes plasmids, cosmids or phages capable of synthesizing, for example, the subject hedgehog polypeptides encoded by the respective recombinant gene carried by the vector. Preferred vectors are those capable of autonomous replication and/expression of nucleic acids to which they are linked. In the present specification, "plasmid" and "vector" are used interchangeably as the plasmid is the most commonly used form of vector. Moreover, the invention is intended to include such other forms of expression vectors which serve equivalent functions and which become known in the art subsequently hereto. "Transcriptional regulatory sequence" is a generic term used throughout the specification to refer to DNA sequences, such as initiation signals, enhancers, and promoters, as well as polyadenylation sites, which induce or control transcription of protein (or antisense) coding sequences with which they are operably linked. In preferred embodiments, transcription of a recombinant gene is under the control of a promoter sequence (or other transcriptional regulatory sequence) which controls the expression of the recombinant gene in a cell-type in which expression is intended. It will also be understood that the recombinant gene can be under the control of transcriptional regulatory sequences which are the same or which are different from those sequences which control transcription of the naturally- occurring form of the regulatory protein. The term "operably linked" refers to the arrangement of a transcriptional regulatory element relative to other transcribable nucleic acid sequence such that the transcriptional regulatory element can regulate the rate of transcription from the transcribable sequence(s). III. Exemplary Applications of Method and Compositions One aspect of the present invention relates to a method of maintaining a differentiated state, e.g., enhancing survival, of a neuronal cell responsive to a hedgehog polypeptide, by contacting the cells with a trophic amount of a. hedgehog oτ ptc thereapeutic. For instance, it is contemplated by the invention that, in light of the present finding of an apparently trophic effect of hedgehog polypeptides in the maintenance of differentiated neurons, the subject method could be used to maintain different neuronal tissue both in vitro and in vivo. Where the trophic agent is a hedgehog polypeptide, it can be provided to a cell culture or animal as a purified protein or secreted by a recombinant cell, or cells or tissue explants which naturally produce one or more hedgehog polypeptides. For instance, neural tube explants from embryos, particularly floorplate tissue, can provide a source for Shh polypeptide, which source can be implanted in a patient or otherwise provided, as appropriate, for maintenance of differentiation. A. Cell Culture The present method is applicable to cell culture techniques. In vitro neuronal culture systems have proved to be fundamental and indispensable tools for the study of neural development, as well as the identification of neurotrophic factors such as nerve growth factor (NGF), ciliary trophic factors (CNTF), and brain derived neurotrophic factor (BDNF). Once a neuronal cell has become terminally-differentiated it typically will not change to another terminally differentiated cell-type. However, neuronal cells can nevertheless readily lose their differentiated state. This is commonly observed when they are grown in culture from adult tissue, and when they form a blastema during regeneration. The present method provides a means for ensuring an adequately restrictive environment in order to maintain dopaminergic, GABAergic or other exotoxic-sensitive neuronal cells in differentiated states, and can be employed, for instance, in cell cultures designed to test the specific activities of other trophic factors. In such embodiments of the subject method, a culture of differentiated cells including the neuronal cells can be contacted with a hedgehog or ptc therapeutic in order to maintain the integrity of a culture of terminally-differentiated neuronal cells by preventing loss of differentiation. The source of hedgehog oτ ptc therapeutic in the culture can be derived from, for example, a purified or semi-purified protein composition added directly to the cell culture media, or alternatively, supported and/or released from a polymeric device which supports the growth of various neuronal cells and which has been doped with the protein. The source of, for example, a trophic hedgehog polypeptide can also be a cell that is co-cultured with the neuronal cells. Alternatively, the source can be the neuronal cell itself which has been engineered to produce a recombinant hedgehog polypeptide. Such neuronal cultures can be used as convenient assay systems as well as sources of implantable cells for therapeutic treatments. The subject method can be used in conjunction with agents which induce the differentiation of neuronal precursors, e.g., progenitor or stem cells, into dopaminergic neurons, GABAergic neurons or other neuronal cell-types. Cells can be obtained from embryonic, post-natal, juvenile or adult neural tissue from any animal. By any animal is meant any multicellular animal which contains nervous tissue. More particularly, is meant any fish, reptile, bird, amphibian or mammal and the like. The most preferable donors are mammals, especially humans and non-human primates, pigs, cows, and rodents. Intracerebral neural grafting has emerged recently as an additional potential to CNS therapy. For example, one approach to repairing damaged brain tissues involves the transplantation of cells from fetal or neonatal animals into the adult brain (Dunnett et al. (1987) J Exp Biol 123:265-289; and Freund et al. (1985) J Neurosci 5:603-616). Fetal neurons from a variety of brain regions can be successfully incorporated into the adult brain, and such grafts can alleviate behavioral defects. For example, movement disorder induced by lesions of dopaminergic projections to the basal ganglia can be prevented by grafts of embryonic dopaminergic neurons. Complex cognitive functions that are impaired after lesions of the neocortex can also be partially restored by grafts of embryonic cortical cells. Transplantation of fetal brain cells, which contain precursors of the dopaminergic neurons, has been examined with success as a treatment for Parkinson's disease. In animal models and in patients with this disease, fetal brain cell transplantations have resulted in the reduction of motor abnormalities. Furthermore, it appears that the implanted fetal dopaminergic neurons form synapses with surrounding host neurons. However, in the art, the transplantation of fetal brain cells is limited due, for example, to the limited survival time of the implanted neuronal precursors and differentiated neurons arising therefrom. The subject invention provides a means for extending the usefulness of such transplants by enhancing the survival of dopaminergic and/or GABAergic cells in the transplant. In the specific case of Parkinson's disease, intervention by increasing the activity of hedgehog, by ectopic or endogenous means, can improve the in vivo survival of fetal and adult dopaminergic neurons, and thus can provide a more effective treatment of this disease. Cells to be transplanted for the treatment of a particular disease can be genetically modified in vitro so as to increase the expression of hedgehog in the transplant. In an exemplary embodiment of the invention, administration of an Shh polypeptide can be used in conjunction with surgical implantation of tissue in the treatment of Parkinson's disease. In the case of a heterologous donor animal, the animal may be euthanized, and the brain and specific area of interest removed using a sterile procedure. Brain areas of particular interest include any area from which progenitor cells can be obtained which will provide dopaminergic or GABAergic cells upon differentiation. These regions include areas of the central nervous system (CNS) including the substantia nigra pars compacta which is found to be degenerated in Parkinson's Disease patients. Human heterologous neural progenitor cells may be derived from fetal tissue obtained from elective abortion, or from a post-natal, juvenile or adult organ donor. Autologous neural tissue can be obtained by biopsy, or from patients undergoing neurosurgery in which neural tissue is removed, such as during epilepsy surgery. Cells can be obtained from donor tissue by dissociation of individual cells from the connecting extracellular matrix of the tissue. Dissociation can be obtained using any known procedure, including treatment with enzymes such as trypsin, collagenase and the like, or by using physical methods of dissociation such as with a blunt instrument. Dissociation of fetal cells can be carried out in tissue culture medium, while a preferable medium for dissociation of juvenile and adult cells is artificial cerebral spinal fluid (aCSF). Regular aCSF contains 124 mM NaCl, 5 mM KC1, 1.3 mM MgCl 2 , 2 mM CaCl 2 , 26 mM NaHCO 3 , and 10 mM D- glucose. Low Ca2 + aCSF contains the same ingredients except for MgCl 2 at a concentration of 3.2 mM and CaCl 2 at a concentration of 0.1 mM. Dissociated cells can be placed into any known culture medium capable of supporting cell growth, including MEM, DMEM, RPMI, F-12, and the like, containing supplements which are required for cellular metabolism such as glutamine and other amino acids, vitamins, minerals and useful proteins such as transferrin and the like. Medium may also contain antibiotics to prevent contamination with yeast, bacteria and fungi such as penicillin, streptomycin, gentamicin and the like. In some cases, the medium may contain serum derived from bovine, equine, chicken and the like. A particularly preferable medium for cells is a mixture of DMEM and F-12. Conditions for culturing should be close to physiological conditions. The pH of the culture media should be close to physiological pH, preferably between pH 6-8, more preferably close to pH 7, even more particularly about pH 7.4. Cells should be cultured at a temperature close to physiological temperature, preferably between 30°C-40°C, more preferably between 32°C-38°C, and most preferably between 35°C-37°C. Cells can be grown in suspension or on a fixed substrate, but proliferation of the progenitors is preferably done in suspension to generate large numbers of cells by formation of "neurospheres" (see, for example, Reynolds et al. (1992) Science 255: 1070-1709; and PCT Publications WO93/01275, WO94/091 19, WO94/10292, and WO94/16718). In the case of propagating (or splitting) suspension cells, flasks are shaken well and the neurospheres allowed to settle on the bottom corner of the flask. The spheres are then transferred to a 50 ml centrifuge tube and centrifuged at low speed. The medium is aspirated, the cells resuspended in a small amount of medium with growth factor, and the cells mechanically dissociated and resuspended in separate aliquots of media. Cell suspensions in culture medium are supplemented with any growth factor which allows for the proliferation of progenitor cells and seeded in any receptacle capable of sustaining cells, though as set out above, preferably in culture flasks or roller bottles. Cells typically proliferate within 3-4 days in a 37°C incubator, and proliferation can be reinitiated at any time after that by dissociation of the cells and resuspension in fresh medium containing growth factors. In the absence of substrate, cells lift off the floor of the flask and continue to proliferate in suspension forming a hollow sphere of undifferentiated cells. After approximately 3-10 days in vitro, the proliferating clusters (neurospheres) are fed every 2-7 days, and more particularly every 2-4 days by gentle centrifugation and resuspension in medium containing growth factor. After 6-7 days in vitro, individual cells in the neurospheres can be separated by physical dissociation of the neurospheres with a blunt instrument, more particularly by triturating the neurospheres with a pipette. Single cells from the dissociated neurospheres are suspended in culture medium containing growth factors, and differentiation of the cells can be induced by plating (or resuspending) the cells in the presence of a factor capable of sustaining differentiation, e.g., such as a hedgehog oτ ptc therapeutic of the present invention. Stem cells useful in the present invention are generally known. For example, several neural crest cells have been identified, some of which are multipotent and likely represent uncommitted neural crest cells. The role of hedgehog polypeptides employed in the present method to culture such stem cells is to maintain differentiation a committed progenitor cell amd/or a terminally-differentiated dopaminergic or GABAergic neuronal cell. The hedgehog polypeptide can be used alone, or can be used in combination with other neurotrophic factors which act to more particularly enhance a particular differentiation fate of the neuronal progenitor cell. B. In vivo applications In addition to the implantation of cells cultured in the presence of a functional hedgehog activity and other in vitro uses described above, yet another aspect of the present invention concerns the therapeutic application of a hedgehog or ptc therapeutic to enhance survival of neurons sensitive to exotoxic degeneration in vivo, e.g., such as dopaminergic and GABAergic neurons. The ability of hedgehog polypeptide to maintain neuronal differentiation indicates that certain of the hedgehog polypeptides can be reasonably expected to facilitate control of these neuronal cell-types in adult tissue with regard to maintenance, functional performance, aging and prevention of degeneration and premature death which result from loss of differentiation in certain pathological conditions. In light of this understanding, the present invention specifically contemplates applications of the subject method to the treatment of (prevention and/or reduction of the severity of) neurological conditions deriving from exotoxic degeneration of neuronal cells, including cell death or loss of functional performance, e.g., such as loss of dopaminergic cells, loss of GABAergic cells, and/or loss of neurons of the substantia nigra. In this regard, the subject method is useful in the treatment or prevention of such neurologic disorders including Parkinson's disease, domoic acid poisoning; spinal cord trauma; hypoglycemia; mechanical trauma to the nervous system; senile dementia; Korsakoffs disease; schizophrenia; AIDS dementia, multi-infarct dementia; mood disorders; depression; chemical toxicity and neuronal damage associated with uncontrolled seizures, such as epileptic seizures; and chronic neurologic disorders such as Huntington's disease, neuronal injury associated with HIV and AIDS, AIDS dementia, neurodegeneration associated with Down's syndrome, neuropathic pain syndrome, olivopontocerebral atrophy, amyotrophic lateral sclerosis, mitochondrial abnormalities, Alzheimer's disease, hepatic encephalopathy, Tourette's syndrome, schizophrenia, and drug addiction. The subject hedgehog and ptc therpaeutics can also used to reduce neurotoxic injury associated with conditions of hypoxia, anoxia or ischemia which typically follows stroke, cerebrovascular accident, brain or spinal chord trauma, myocardial infarct, physical trauma, drownings, suffocation, perinatal asphyxia, or hypoglycemic events. In preferred embodiments, the subject method is used to reduce the severity or prevent Parkinson's disease or Huntington's chorea. In general, the therapeutic method of the present invention can be characterized as including a step of administering to an animal an amount of a ptc or hedgehog therapeutic effective to enhance the survival of a dopaminergic and/or GABAergic neuronal cells. The mode of administration and dosage regimens will vary depending on the severity of the degenerative disoder being treated, e.g., the dosage may be altered as between a prophylaxis and treatment. In preferred embodiments, the ptc or hedeghog therapeutic is administered systemically initially, then locally for medium to long term care. In certain embodiments, a source of a hedgehog or ptc therapeutic is stereotactically provided within or proximate the area of degeneration. The subject method may also find particular utility in treating or preventing the adverse neurological consequences of surgery. For example, certain cranial surgery can result in degeneration of neuronal populations for which the subject method can be applied. In other embodiments, the subject method can be used to prevent or treat neurodegenerative conditions arising from the use of certain drugs, such as the compound MPTP (l-methyl-4-phenyl-l,2,3,6-tetrahydropyridine). In still other embodiments, the subject method can be used in the prevention and/or treatment of hypoxia, e.g., as a neuroprotective agent. For instance, the subject method can be used prophylactically to lessen the neuronal cell death caused by altitude-induced hypoxia. A method which is "neuroprotective", in the case of dopaminergic and GABAergic cells, results in diminished loss of cells of those phenotype relative to that which would occur in the absence of treatment with a hedgehog oτ ptc therapeutic. (i) Treatment of Parkinson 's disease It is now widely appreciated that the primary pathology underlying Parkinson's disease is degeneration of the dopaminergic projection from the substantia nigra to the striatum. This realisation has led to the widespread use of dopamine-replacing agents such as L-DOPA and apomorphine as symptomatic treatments for Parkinson's disease. Over the last three decades, such therapies have undoubtedly been successful in increasing the quality of life of patients suffering from Parkinson's disease, but, dopamine-replacement treatments do have limitations, especially following long-term treatment. Problems can include a wearing- off of anti-parkinsonian efficacy and the appearance of a range of dyskinesias characterised by chorea and dystonia. Ultimately, these side-effects can severely limit the usefulness of dopaminergic treatments. As described in the appended examples, hedgehog exerts trophic and survival- promoting actions on substantia nigra dopaminergic neurons. In vivo, treatment with exogenous hedgehog, or other compounds of the present invention, is expected to stimulate, for example, the dopaminergic phenotype of substantia nigra neurons and restore functional deficits induced by axotomy or dopaminergic neurotoxins. Therefore it may be used in the treatment of Parkinson's disease, a neurodegenerative disease characterized by the loss of dopaminergic neurons. Thus, in one embodiment, the subject method comprises administering to an animal afflected with Parkinson's disease, or at risk of developing Parkonson's disease, an amount of a hedgehog oτ ptc thereapeutic effective for protecting or restoring the function of neurons affected by the Parkinson's condition, e.g., increasing the rate of survival of dopaminergic neurons in the animal. In preferred embodiments, the method includes administering to the animal an amount of a hedgehog or ptc thereapeutic which would otherwise be effective at protecting the substantia nigra from MPTP-mediated toxicity when MPTP is administered at a dose of .5mg/kg, more preferably at a dose of 2mg/kg, 5mg/kg, lOmg/kg, 20mg/kg or 50mg/kg and, more preferably, at a dose of lOOmg/kg. (ii) Treatment of Huntington 's disease Huntington's disease involves the degeneration of intrastraital and cortical cholinergic neurons and GABAergic neurons. Treatment of patients suffering from such degenerative conditions can include the application of hedgehog or ptc therapeutics of the present invention, in order to control, for example, apoptotic events which give rise to loss of GABAergic neurons (e.g. to enhance survival of existing neurons. (iii) Treatment of amyotrophic lateral sclerosis Recently it has been reported that in certain ALS patients and animal models a significant loss of midbrain dopaminergic neurons occurs in addition to the loss of spinal motor neurons. For instance, the literature describes degeneration of the substantia nigra in some patients with familial amyotrophic lateral sclerosis. Kostic et al. (1997) Ann Neurol 41:497-504. According the subject invention, a trophic amount of a hedgehog or ptc therapeutic can be administered to an animal suffering from, or at risk of developing, ALS. (iv) Treatment of epilepsy Epilepsy is a recurrent paroxysmal disorder of cerebral function characterized by sudden brief attacks of altered consciousness, motor activity, sensory phenomena or inappropriate behavior caused by abnormal excessive discharge of cerebral neurons. Convulsive seizures, the most common form of attacks, begin with loss of consciousness and motor control, and tonic or clonic jerking of all extremities but any recurrent seizure pattern may be termed epilepsy. The term primary or idiopathic epilepsy denotes those cases where no cause for the seizures can be identified. Secondary or symptomatic epilepsy designates the disorder when it is associated with such factors as trauma, neoplasm, infection, developmental abnormalities, cerebrovascular disease, or various metabolic conditions. Epileptic seizures are classified as partial seizures (focal, local seizures) or generalized seizures (convulsive or nonconvulsive). Classes of partial seizures include simple partial seizures, complex partial seizures and partial seizures secondarily generalized. Classes of generalized seizures include absence seizures, atypical absence seizures, myoclonic seizures, clonic seizures, tonic seizures, tonic-clonic seizures (grand mal) and atonic seizures. Therapeutics having anticonvulsant properties are used in the treatment of seizures. Most therapeutics used to abolish or attenuate seizures act at least through effects that reduce the spread of excitation from seizure foci and prevent detonation and disruption of function of normal aggregates of neurons. Anticonvulsants which have been utilized include phenytoin, phenobarbital, primidone, carbamazepine, ethosuximide, clonazepam and valproate. For further details of seizures and their therapy (see Rail & Schleifer (1985) and The Merck Manual (1992)). Due to the involvement of exotoxic-dependent neurodegeneration which can result from seizure, certain hedgehog and ptc therpaeutics of the present invention may be useful as part of a regimen in the treatment of epilepsy, and are preferably used in conjunction with a treatment including an anticonvulsant agent. (v) Treatment of reperfusion injury The most direct approach to treating cerebral ischemia is to restore circulation. However, reperfusion following transient ischemia can induce additional mechanisms of tissue damage. This phenomenon is termed "reperfusion injury" and has been found to play a role in other organ systems as well, including the heart. Recently, it has been suggested that cerebral ischemic damage is mediated, to a large extent, via excitotoxic mechanisms. During ischemia, large elevations in extracellular glutamate occur, often reaching neurotoxic levels. Accordingly, the subject method can be used as part of a treatment or prophylaxis for ischemic or epoxic damage, particularly to alleviate certain effects of reperfusion injury. (vi) Treatment of glaucoma Glaucoma is a complex set of diseases, which results in damage to axons in the optic nerve and death of the retinal ganglion cells, concluding in the permanent loss of vision. There are several mechanism that ultimately causes the axonal damage. For instance, an increase in intraocular pressure (IOP) overcomes the perfusion pressure of the optic nerve and results in an ischemic event which leads to axonal. In addition to the primary insult and ensuing cell damage, there appears to be a secondary degenerative process in glaucoma. Clinically, patients often continue to lose visual field and optic nerve head substance even in the presence of what might be considered normal IOP. In addition, there are forms of glaucoma which manifest in the presence of normal IOP. It is thought that chemical mediators may be linked to intensification of cell degeneration and death in a secondary fashion. Much work has been done in the last five years which implicates the role of excitotoxins, such as glutamate, in the secondary damage to retinal neurons. Accordingly, the subject method can be used as part of a treatment and/or prophylaxis for glaucoma. (vii) Treatment of hearing loss The mechanism by which aminoglycosides produce permanent hearing loss is mediated, in part, through an excitotoxic process. Accordingly, the subject method can be used as part of a treatment and/or prophylaxis for hearing loss. (viii) Conjoint Therapy In yet other embodiments, the subject method can be carried out conjointly with the administration of growth and/or trophic factors. For instance, the combinatorial therapy can include a trophic factor such as nerve growth factor, cilliary neurotrophic growth factor, schwanoma-derived growth factor, glial growth factor, stiatal-derived neuronotrophic factor, platelet-derived growth factor, and scatter factor (HGF-SF). Antimitogenic agents can also be used, as for example, cytosine, arabinoside, 5-fluorouracil, hydroxyurea, and methotrexate. (ix) Formulations Determination of a therapeutically effective amount and a prophylactically effective amount of a hedgehog or ptc therapeutic, e.g., to be adequately neuroprotective, can be readily made by the physician or veterinarian (the "attending clinician"), as one skilled in the art, by the use of known techniques and by observing results obtained under analogous circumstances. The dosages may be varied depending upon the requirements of the patient in the judgment of the attending clinician, the severity of the condition being treated, the risk of further degeneration to the CNS, and the particular agent being employed. In determining the therapeutically effective trophic amount or dose, and the prophylactically effective amount or dose, a number of factors are considered by the attending clinician, including, but not limited to: the specific cause of the degenerative state and its likelihood of recurring or worsening; pharmacodynamic characteristics of the particular agent and its mode and route of administration; the desirder time course of treatment; the species of mammal; its size, age, and general health; the response of the individual patient; the particular compound administered; the bioavailability characteristics of the preparation administered; the dose regimen selected; the kind of concurrent treatment (i.e., the interaction of the hedgehog oτ ptc therapeutic with other co-administered therapeutics); and other relevant circumstances. Treatment can be initiated with smaller dosages which are less than the optimum dose of the agent. Thereafter, the dosage should be increased by small increments until the optimum effect under the circumstances is reached. For convenience, the total daily dosage may be divided and administered in portions during the day if desired. A therapeutically effective trophic amount and a prophylactically effective neuroprotective amount of a hedgehog polypeptide, for instance, is expected to vary from concentrations about 0.1 nanogram per kilogram of body weight per day (ng/kg/day) to about 100 mg/kg/day. Potential hedgehog and ptc therapeutics, such as described below, can be tested by any of number of well known animal disease models. For instance, regarding Parkinson's Disease, selected agents can be evaluated in animals treated with MPTP. The compound MPTP (1 -methyl -4-phenyl-l, 2,3, 6-tetrahydropyridine) and its metabolite MPP + have been used to induce experimental parkinsonism. MPP + kills dopaminergic neurons in the substantia nigra, yielding a reasonable model of late parkinsonism. Turski et al., (1991) Nature 349:414. Compounds which are determined to be effective for the prevention or treatment of degeneration of dopaminergic and GABAergic neurons and the like in animals, e.g., dogs, rodents, may also be useful in treatment of disorders in humans. Those skilled in the art of treating in such disorders in humans will be guided, from the data obtained in animal studies, to the correct dosage and route of administration of the compound to humans. In general, the determination of dosage and route of administration in humans is expected to be similar to that used to determine administration in animals. The identification of those patients who are in need of prophylactic treatment for disorders marked by degeneration of dopaminergic and/or GABAergic neurons is well within the ability and knowledge of one skilled in the art. Certain of the methods for identification of patients which are at risk and which can be treated by the subject method are appreciated in the medical arts, such as family history of the development of a particular disease state and the presence of risk factors associated with the development of that disease state in the subject patient. A clinician skilled in the art can readily identify such candidate patients, by the use of, for example, clinical tests, physical examination and medical/family history. IV. Exemplary hedgehog therapeutic compounds. The hedgehog therapeutic compositions of the subject method can be generated by any of a variety of techniques, including purification of naturally occurring proteins, recombinantly produced proteins and synthetic chemistry. Polypeptide forms of the hedgehog therapeutics are preferably derived from vertebrate hedgehog polypeptides, e.g., have sequences corresponding to naturally occurring hedgehog polypeptides, or fragments thereof, from vertebrate organisms. However, it will be appreciated that the hedgehog polypeptide can correspond to a hedgehog polypeptide (or fragment thereof) which occurs in any metazoan organism. The various naturally-occurring hedgehog polypeptides from which the subject therapeutics can be derived are characterized by a signal peptide, a highly conserved N- terminal region, and a more divergent C-terminal domain. In addition to signal sequence cleavage in the secretory pathway (Lee, J.J. et al. (1992) Cell 71 :33-50; Tabata, T. et al. (1992) Genes Dev. 2635-2645; Chang, D.E. et al. (1994) Development 120:3339-3353), hedgehog precursor proteins naturally undergo an internal autoproteolytic cleavage which depends on conserved sequences in the C-terminal portion (Lee et al. (1994) Science 266:1528-1537; Porter et al. (1995) Nature 374:363-366). This autocleavage leads to a 19 kD N-terminal peptide and a C-terminal peptide of 26-28 kD (Lee et al. (1992) supra; Tabata et al. (1992) supra; Chang et al. (1994) supra; Lee et al. (1994) supra; Bumcrot, D.A., et al. (1995) Mol. Cell. Biol. 15:2294-2303; Porter et al. (1995) supra; Ekker, S.C. et al. (1995) Curr. Biol. 5:944-955; Lai, C . et al. (1995) Development 121:2349-2360). The N-terminal peptide stays tightly associated with the surface of cells in which it was synthesized, while the C-terminal peptide is freely diffusible both in vitro and in vivo (Lee et al. (1994) supra; Bumcrot et al. (1995) supra; Mart', E. et al. (1995) Development 121 :2537-2547; Roelink, H. et al. (1995) Cell 81:445-455). Cell surface retention of the N-terminal peptide is dependent on autocleavage, as a truncated form of hedgehog encoded by an RNA which terminates precisely at the normal position of internal cleavage is diffusible in vitro (Porter et al. (1995) supra) and in vivo (Porter, J.A. et al. (1996) Cell 86, 21-34). Biochemical studies have shown that the autoproteolytic cleavage of the hedgehog precursor protein proceeds through an internal thioester intermediate which subsequently is cleaved in a nucleophilic substitution. It is suggested that the nucleophile is a small lipophilic molecule, more particularly cholesterol, which becomes covalently bound to the C-terminal end of the N- peptide (Porter et al. (1996) supra), tethering it to the cell surface. The vertebrate family of hedgehog genes includes at least four members, e.g., paralogs of the single drosophila hedgehog gene (SEQ ID No. 19). Three of these members, herein referred to as Desert hedgehog (Dhh), Sonic hedgehog (Shh) and Indian hedgehog (Ihh), apparently exist in all vertebrates, including fish, birds, and mammals. A fourth member, herein referred to as tiggie-winkle hedgehog (Thh), appears specific to fish. According to the appended sequence listing, (see also Table 1) a chicken Shh polypeptide is encoded by SEQ ID No:l; a mouse Dhh polypeptide is encoded by SEQ ID No:2; a mouse Ihh polypeptide is encoded by SEQ ID No:3; a mouse Shh polypeptide is encoded by SEQ ID No:4 a zebrafish Shh polypeptide is encoded by SEQ ID No:5; a human Shh polypeptide is encoded by SEQ ID No:6; a human Ihh polypeptide is encoded by SEQ ID No:7; a human Dhh polypeptide is encoded by SEQ ID No. 8; and a zebrafish Thh is encoded by SEQ ID No. 9. Table 1 Guide to hedgehog sequences in Sequence Listing Nucleotide Amino Acid Chicken Shh SEQ ID No. 1 SEQ ID No. 10 Mouse Dhh SEQ ID No. 2 SEQ ID No. 11 Mouse Ihh SEQ ID No. 3 SEQ ID No. 12 Mouse Shh SEQ ID No. 4 SEQ ID No. 13 Zebrafish Shh SEQ ID No. 5 SEQ ID No. 14 Human Shh SEQ ID No. 6 SEQ ID No. 15 Human Ihh SEQ ID No. 7 SEQ ID No. 16 Human Dhh SEQ ID No. 8 SEQ ID No. 17 Zebrafish Thh SEQ ID No. 9 SEQ ID No. 18 Drosophila HH SEQ ID No. 19 SEQ ID No. 20 In addition to the sequence variation between the various hedgehog homologs, the hedgehog polypeptides are apparently present naturally in a number of different forms, including a pro-form, a full-length mature form, and several processed fragments thereof. The pro-form includes an N-terminal signal peptide for directed secretion of the extracellular domain, while the full-length mature form lacks this signal sequence. As described above, further processing of the mature form occurs in some instances to yield biologically active fragments of the protein. For instance, sonic hedgehog undergoes additional proteolytic processing to yield two peptides of approximately 19 kDa and 27 kDa, the 19kDa fragment corresponding to an proteolytic N-terminal portion of the mature protein. In addition to proteolytic fragmentation, the vertebrate hedgehog polypeptides can also be modified post-translationally, such as by glycosylation and/or addition of lipophilic moieties, such as stents, fatty acids, etc., though bacterially produced (e.g. unmodified) forms of the proteins still maintain certain of the bioactivities of the native protein. Bioactive fragments of hedgehog polypeptides of the present invention have been generated and are described in great detail in, e.g., PCT publications WO 95/18856 and WO 96/17924. There are a wide range of lipophilic moieties with which hedgehog polypeptides can be derivatived. The term "lipophilic group", in the context of being attached to a hedgehog polypeptide, refers to a group having high hydrocarbon content thereby giving the group high affinity to lipid phases. A lipophilic group can be, for example, a relatively long chain alkyl or cycloalkyl (preferably n-alkyl) group having approximately 7 to 30 carbons. The alkyl group may terminate with a hydroxy or primary amine "tail". To further illustrate, lipophilic molecules include naturally-occurring and synthetic aromatic and non-aromatic moieties such as fatty acids, sterols, esters and alcohols, other lipid molecules, cage structures such as adamantane and buckminsterfullerenes, and aromatic hydrocarbons such as benzene, perylene, phenanthrene, anthracene, naphthalene, pyrene, chrysene, and naphthacene. In one embodiment, the hedgehog polypeptide is modified with one or more sterol moieties, such as cholesterol. See, for example, PCT publication WO 96/17924. In certain embodiments, the cholesterol is preferably added to the C-terminal glycine when the hedgehog polypeptide corresponds to the naturally-occurring N-terminal proteolytic fragment. In another embodiment, the hedgehog polypeptide can be modified with a fatty acid moiety, such as a myrostoyl, palmitoyl, stearoyl, or arachidoyl moiety. See, e.g., Pepinsky et al. (1998) J Biol. Chem 273: 14037. In addition to those effects seen by cholesterol-addition to the C-terminus or fatty acid addition to the N-terminus of extracellular fragments of the protein, at least certain of the biological activities of the hedgehog gene products are unexpectedly potentiated by derivativation of the protein with lipophilic moieties at other sites on the protein and/or by moieties other than cholesterol or fatty acids. Certain aspects of the invention are directed to the use of preparations of hedgehog polypeptides which are modified at sites other than N- terminal or C-terminal residues of the natural processed form of the protein, and/or which are modified at such terminal residues with lipophilic moieties other than a sterol at the C- terminus or fatty acid at the N-terminus. Particularly useful as lipophilic molecules are alicyclic hydrocarbons, saturated and unsaturated fatty acids and other lipid and phospholipid moieties, waxes, cholesterol, isoprenoids, teφenes and polyalicychc hydrocarbons including adamantane and buckminsterfullerenes, vitamins, (Cl-C18)-alkyl phosphate diesters, -O-CH2-CH(OH)-O- (C12-C18)-alkyl, and in particular conjugates with pyrene derivatives. The lipophilic moiety can be a lipophilic dye suitable for use in the invention include, but are not limited to, diphenylhexatriene, Nile Red, N-phenyl- 1 -naphthylamine, Prodan, Laurodan, Pyrene, Perylene, rhodamine, rhodamine B, tetramethylrhodamine, Texas Red, sulforhodamine, 1,1'- didodecyl-3,3,3',3'tetramethylindocarbocyanine perchlorate, octadecyl rhodamine B and the BODIPY dyes available from Molecular Probes Inc. Other exemplary lipophilic moietites include aliphatic carbonyl radical groups include 1- or 2-adamantylacetyl, 3-methyladamant-l-ylacetyl, 3-methyl-3-bromo-l-adamantylacetyl, 1-decalinacetyl, camphoracetyl, camphaneacetyl, noradamantylacetyl, norbornaneacetyl, bicyclo[2.2.2.]-oct-5-eneacetyl, 1 -methoxybicyclo[2.2.2.]-oct-5-ene-2-carbonyl, cis-5- norbornene-endo-2,3-dicarbonyl, 5-norbornen-2-ylacetyl, (lR)-( - )-myrtentaneacetyl, 2- norbornaneacetyl, anti-3-oxo-tricyclo[2.2.1.0<2,6> ]-heptane-7-carbonyl, decanoyl, dodecanoyl, dodecenoyl, tetradecadienoyl, decynoyl or dodecynoyl. The hedgehog polypeptide can be linked to the hydrophobic moiety in a number of ways including by chemical coupling means, or by genetic engineering. There are a large number of chemical cross-linking agents that are known to those skilled in the art. For the present invention, the preferred cross-linking agents are heterobifunctional cross-linkers, which can be used to link the hedgehog polypeptide and hydrophobic moiety in a stepwise manner. Heterobifunctional cross-linkers provide the ability to design more specific coupling methods for conjugating to proteins, thereby reducing the occurrences of unwanted side reactions such as homo-protein polymers. A wide variety of heterobifunctional cross-linkers are known in the art. These include: succinimidyl 4-(N-maleimidomethyl) cyclohexane- 1-carboxylate (SMCC), m- Maleimidobenzoyl-N- hydroxysuccinimide ester (MBS); N-succinimidyl (4-iodoacetyl) aminobenzoate (SIAB), succinimidyl 4-(p-maleimidophenyl) butyrate (SMPB), l-ethyl-3-(3- dimethylaminopropyl) carbodiimide hydrochloride (EDC); 4-succinimidyloxycarbonyl- a- methyl-a-(2-pyridyldithio)-tolune (SMPT), N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP), succinimidyl 6-[3-(2-pyridyldithio) propionate] hexanoate (LC-SPDP). Those cross- linking agents having N-hydroxysuccinimide moieties can be obtained as the N- hydroxysulfosuccinimide analogs, which generally have greater water solubility. In addition, those cross-linking agents having disulfide bridges within the linking chain can be synthesized instead as the alkyl derivatives so as to reduce the amount of linker cleavage in vivo. In addition to the heterobifunctional cross-linkers, there exists a number of other cross-linking agents including homobifunctional and photoreactive cross-linkers. Disuccinimidyl suberate (DSS), bismaleimidohexane (BMH) and dimethylpimelimidate-2 HC1 (DMP) are examples of useful homobifunctional cross-linking agents, and bis-[β-(4- azidosalicylamido)ethyl]disulfide (BASED) and N-succinimidyl-6(4'-azido-2'-nitrophenyl- amino)hexanoate (SANPAH) are examples of useful photoreactive cross-linkers for use in this invention. For a recent review of protein coupling techniques, see Means et al. (1990) Bioconjugate Chemistry 1 :2-12, incoφorated by reference herein. One particularly useful class of heterobifunctional cross-linkers, included above, contain the primary amine reactive group, N-hydroxysuccinimide (NHS), or its water soluble analog N-hydroxysulfosuccinimide (sulfo-NHS). Primary amines (lysine epsilon groups) at alkaline pH's are unprotonated and react by nucleophilic attack on NHS or sulfo-NHS esters. This reaction results in the formation of an amide bond, and release of NHS or sulfo-NHS as a by-product. Another reactive group useful as part of a heterobifunctional cross-linker is a thiol reactive group. Common thiol reactive groups include maleimides, halogens, and pyridyl disulfides. Maleimides react specifically with free sulfhydryls (cysteine residues) in minutes, under slightly acidic to neutral (pH 6.5-7.5) conditions. Halogens (iodoacetyl functions) react with -SH groups at physiological pH's. Both of these reactive groups result in the formation of stable thioether bonds. The third component of the heterobifunctional cross-linker is the spacer arm or bridge. The bridge is the structure that connects the two reactive ends. The most apparent attribute of the bridge is its effect on steric hindrance. In some instances, a longer bridge can more easily span the distance necessary to link two complex biomolecules. For instance, SMPB has a span of 14.5 angstroms. Preparing protein-protein conjugates using heterobifunctional reagents is a two-step process involving the amine reaction and the sulfhydryl reaction. For the first step, the amine reaction, the protein chosen should contain a primary amine. This can be lysine epsilon amines or a primary alpha amine found at the N-terminus of most proteins. The protein should not contain free sulfhydryl groups. In cases where both proteins to be conjugated contain free sulfhydryl groups, one protein can be modified so that all sulfhydryls are blocked using for instance, N-ethylmaleimide (see Partis et al. (1983) J. Pro. Chem. 2:263, incoφorated by reference herein). Ellman's Reagent can be used to calculate the quantity of sulfhydryls in a particular protein (see for example Ellman et al. (1958) Arch. Biochem. Biophys. 74:443 and Riddles et al. (1979) Anal. Biochem. 94:75, incoφorated by reference herein). The reaction buffer should be free of extraneous amines and sulfhydryls. The pH of the reaction buffer should be 7.0-7.5. This pH range prevents maleimide groups from reacting with amines, preserving the maleimide group for the second reaction with sulfhydryls. The NHS-ester containing cross-linkers have limited water solubility. They should be dissolved in a minimal amount of organic solvent (DMF or DMSO) before introducing the cross-linker into the reaction mixture. The cross-linker/solvent forms an emulsion which will allow the reaction to occur. The sulfo-NHS ester analogs are more water soluble, and can be added directly to the reaction buffer. Buffers of high ionic strength should be avoided, as they have a tendency to "salt out" the sulfo-NHS esters. To avoid loss of reactivity due to hydrolysis, the cross-linker is added to the reaction mixture immediately after dissolving the protein solution. The reactions can be more efficient in concentrated protein solutions. The more alkaline the pH of the reaction mixture, the faster the rate of reaction. The rate of hydrolysis of the NHS and sulfo-NHS esters will also increase with increasing pH. Higher temperatures will increase the reaction rates for both hydrolysis and acylation. Once the reaction is completed, the first protein is now activated, with a sulfhydryl reactive moiety. The activated protein may be isolated from the reaction mixture by simple gel filtration or dialysis. To carry out the second step of the cross-linking, the sulfhydryl reaction, the lipophilic group chosen for reaction with maleimides, activated halogens, or pyridyl disulfides must contain a free sulfhydryl. Alternatively, a primary amine may be modified with to add a sulfhydryl In all cases, the buffer should be degassed to prevent oxidation of sulfhydryl groups. EDTA may be added to chelate any oxidizing metals that may be present in the buffer. Buffers should be free of any sulfhydryl containing compounds. Maleimides react specifically with -SH groups at slightly acidic to neutral pH ranges (6.5-7.5). A neutral pH is sufficient for reactions involving halogens and pyridyl disulfides. Under these conditions, maleimides generally react with -SH groups within a matter of minutes. Longer reaction times are required for halogens and pyridyl disulfides. The first sulfhydryl reactive-protein prepared in the amine reaction step is mixed with the sulfhydryl-containing lipophilic group under the appropriate buffer conditions. The conjugates can be isolated from the reaction mixture by methods such as gel filtration or by dialysis. Exemplary activated lipophilic moieties for conjugation include: N-(l- pyrene)maleimide; 2,5-dimethoxystilbene-4'-maleimide, eosin-5-maleimide; fluorescein-5- maleimide; N-(4-(6-dimethylamino- 2-benzofuranyl)phenyl)maleimide; benzophenone-4- maleimide; 4-dimethylaminophenylazophenyl- 4'-maleimide (DAB MI), tetramethylrhodamine-5-maleimide, tetramethylrhodamine-6-maleimide, Rhodamine RedTM C2 maleimide, N-(5-aminopentyl)maleimide, trifluoroacetic acid salt, N-(2- aminoethyl)maleimide, trifluoroacetic acid salt, Oregon GreenTM 488 maleimide, N-(2-((2- (((4-azido- 2,3,5, 6-tetrafluoro)benzoyl) amino)ethyl)dithio)ethyl)maleimide (TFPAM-SS1), 2-(l-(3-dimethylaminopropyl) -indol-3-yl)-3-(indol-3-yl) maleimide (bisindolylmaleimide; GF 109203X), BODIPY® FL N-(2-aminoethyl)maleimide, N-(7-dimethylamino- 4- methylcoumarin-3-yl)maleimide (DACM), AlexaTM 488 C5 maleimide, AlexaTM 594 C5 maleimide, sodium saltN-(l-pyrene)maleimide, 2,5-dimethoxystilbene-4'-maleimide, eosin- 5-maleimide, fluorescein-5-maleimide, N-(4-(6-dimethylamino- 2- benzofuranyl)phenyl)maleimide, benzophenone-4-maleimide, 4- dimethylaminophenylazophenyl- 4'-maleimide, l-(2-maleimidylethyl)-4-(5- (4- methoxyphenyl)oxazol-2- yl)pyridinium methanesulfonate, tetramethylrhodamine-5- maleimide, tetramethylrhodamine-6-maleimide, Rhodamine RedTM C2 maleimide, N-(5- aminopentyl)maleimide, N-(2-aminoethyl)maleimide, N-(2-((2-(((4-azido- 2,3,5,6- tetrafluoro)benzoyl) amino)ethyl)dithio)ethyl)maleimide, 2-(l-(3-dimethylaminopropyl) - indol-3-yl)-3-(indol-3-yl) maleimide, N-(7-dimethylamino- 4-methylcoumarin-3- yl)maleimide (DACM), 1 lH-Benzo[a]fluorene, Benzo[a]pyrene. In one embodiment, the hedgehog polypeptide can be derivatived using pyrene maleimide, which can be purchased from Molecular Probes (Eugene, Oreg.), e.g., N-(l- pyrene)maleimide or 1-pyrenemethyl iodoacetate (PMIA ester). As illustrated in Figure 1, the pyrene-derived hedgehog polypeptide had an activity profile indicating that it was nearly 2 orders of magnitude more active than the unmodified form of the protein. For those embodiments wherein the hydophobic moiety is a polypeptide, the modified hedgehog polypeptide of this invention can be constructed as a fusion protein, containing the hedgehog polypeptide and the hydrophobic moiety as one contiguous polypeptide chain. In certain embodiments, the lipophilic moiety is an amphipathic polypeptide, such as magainin, cecropin, attacin, melittin, gramicidin S, alpha-toxin of Staph. aureus, alamethicin or a synthetic amphipathic polypeptide. Fusogenic coat proteins from viral particles can also be a convenient source of amphipathic sequences for the subject hedgehog polypeptides. In still other embodiments, the subject method makes use of chimeric proteins comprising a hedgehog moiety fused or otherwise chemically linked to all or part of the hinge and constant regions of an immunoglobulin light chain, heavy chain, or both. Thus, this invention features a molecule which includes: (1) a hedgehog moiety, (2) a second peptide, e.g., one which increases solubility or in vivo life time of the hedgehog moiety, e.g., a member of the immunoglobulin super family or fragment or portion thereof, e.g., a portion or a fragment or portion thereof, e.g., a portion or a fragment of IgG, e.g., the human IgGl heavy chain constant region, e.g., CH2, CH3, and hinge regions; and a toxin moiety. Other embodiments are possible. Specifically, a "hedgehog/Ig fusion" is a protein comprising a biologically active hedgehog molecule of the invention (i.e., Sonic hedgehog), or a biologically active fragment thereof (i.e., the N-terminal portion) linked to an N-terminus of an immunoglobulin chain wherein a portion of the N-terminus of the immunoglobulin is replaced with the hedgehog. A species of hedgehog/Ig fusion is an "hedgehog/Fc Fusion" which is a protein comprising an hedgehog molecule of the invention (i.e., hedgehog -) linked to at least a part of the constant domain of an immunoglobulin. A preferred Fc fusion comprises a hedgehog mutein of the invention linked to a fragment of an antibody containing the C terminal domain of the heavy immunoglobulin chains. Also, the term "fusion protein" means an hedgehog protein chemically linked via a mono- or hetero- functional molecule to a second moiety that is not an hedgehog protein and is made de novo from purified protein as described below. Generally, the structure of the a preferred chimeric hedgehog protein of this invention has the general formula: X-Y-Z, where wherein X is a polypeptide having the amino acid sequence, or portion thereof, consisting of the amino acid sequence of hedgehog; Y is an optional linker moiety; an dZ is a polypeptide comprising at least a portion of a polypeptide other than hedgehog. Preferably, X includes at least a biologically active N-terminal fragment of is human Sonic, Indian or Desert hedgehog. In the preferred embodiments, Z is a protein with an Ig-like constant and/or variable domain. Most preferably, Z is at least a portion of a constant region of an immunoglobulin and can be derived from an immunoglobulin of the class selected from IgM, IgG, IgD, IgA, and IgE. If the class is IgG, then it is selected from one of IgGl, IgG2, IgG3 and IgG4. The constant region of human IgM and IgE contain 4 constant regions (CHI, (hinge), CH2, CH3 and CH4, whereas the constant region of human IgG, IgA and IgD contain 3 constant regions (CHI, (hinge), CH2 and CH3. In the most preferred fusion proteins of the invention, the constant region contains at least the hinge, CH2 and CH3 domains. Moreover, mutagenesis can be used to create modified hh polypeptides, e.g., for such puφoses as enhancing therapeutic or prophylactic efficacy, or stability (e.g., ex vivo shelf life and resistance to proteolytic degradation in vivo). Such modified peptides can be produced, for instance, by amino acid substitution, deletion, or addition. Modified hedgehog polypeptides can also include those with altered post-translational processing relative to a naturally occurring hedgehog polypeptide, e.g., altered glycosylation, cholesterolization, prenylation and the like. In one embodiment, the hedgehog therapeutic is a polypeptide encodable by a nucleotide sequence that hybridizes under stringent conditions to a hedgehog coding sequence represented in one or more of SEQ ID Nos: 1-9 or 19. Appropriate stringency conditions which promote DNA hybridization, for example, 6.0 x sodium chloride/sodium citrate (SSC) at about 45°C, followed by a wash of 2.0 x SSC at 50°C, are known to those skilled in the art or can be found in Current Protocols in Molecular Biology, John Wiley & Sons, N.Y. (1989), 6.3.1-6.3.6. For example, the salt concentration in the wash step can be selected from a low stringency of about 2.0 x SSC at 50°C to a high stringency of about 0.2 x SSC at 50°C. In addition, the temperature in the wash step can be increased from low stringency conditions at room temperature, about 22°C, to high stringency conditions at about 65°C. As described in the literature, genes for other hedgehog polypeptides, e.g., from other animals, can be obtained from mRNA or genomic DNA samples using techniques well known in the art. For example, a cDNA encoding a hedgehog polypeptide can be obtained by isolating total mRNA from a cell, e.g. a mammalian cell, e.g. a human cell, including embryonic cells. Double stranded cDNAs can then be prepared from the total mRNA, and subsequently inserted into a suitable plasmid or bacteriophage vector using any one of a number of known techniques. The gene encoding a hedgehog polypeptide can also be cloned using established polymerase chain reaction techniques. Preferred nucleic acids encode a hedgehog polypeptide comprising an amino acid sequence at least 60% homologous, more preferably 70%> homologous and most preferably 80%) homologous with an amino acid sequence selected from the group consisting of SEQ ID Nos:8-14. Nucleic acids which encode polypeptides at least about 90%>, more preferably at least about 95%, and most preferably at least about 98-99%) homology with an amino acid sequence represented in one of SEQ ID Nos: 10-18 or 20 are also within the scope of the invention. Hedgehog polypeptides preferred by the present invention, in addition to native hedgehog polypeptides, are at least 60%> homologous, more preferably 70%> homologous and most preferably 80%> homologous with an amino acid sequence represented by any of SEQ ID Nos: 10- 18 or 20. Polypeptides which are at least 90%>, more preferably at least 95%>, and most preferably at least about 98-99%> homologous with a sequence selected from the group consisting of SEQ ID Nos: 10-18 or 20 are also within the scope of the invention. The only prerequisite is that the hedgehog polypeptide is capable of protecting neuronal cells against degeneration, e.g., the polypeptide is trophic for a dopaminergic and/or GABAergic neuron. The term "recombinant protein" refers to a polypeptide of the present invention which is produced by recombinant DNA techniques, wherein generally, DNA encoding a hedgehog polypeptide is inserted into a suitable expression vector which is in turn used to transform a host cell to produce the heterologous protein. Moreover, the phrase "derived from", with respect to a recombinant hedgehog gene, is meant to include within the meaning of "recombinant protein" those proteins having an amino acid sequence of a native hedgehog polypeptide, or an amino acid sequence similar thereto which is generated by mutations including substitutions and deletions (including truncation) of a naturally occurring form of the protein. The method of the present invention can also be carried out using variant forms of the naturally occurring hedgehog polypeptides, e.g., mutational variants. As is known in the art, hedgehog polypeptides can be produced by standard biological techniques. For example, a host cell transfected with a nucleic acid vector directing expression of a nucleotide sequence encoding the subject polypeptides can be cultured under appropriate conditions to allow expression of the peptide to occur. The polypeptide hedgehog may be secreted and isolated from a mixture of cells and medium containing the recombinant hedgehog polypeptide. Alternatively, the peptide may be retained cytoplasmically by removing the signal peptide sequence from the recombinant hedgehog gene and the cells harvested, lysed and the protein isolated. A cell culture includes host cells, media and other byproducts. Suitable media for cell culture are well known in the art. The recombinant hedgehog polypeptide can be isolated from cell culture medium, host cells, or both using techniques known in the art for purifying proteins including ion-exchange chromatography, gel filtration chromatography, ultrafiltration, electrophoresis, and immunoaffinity purification with antibodies specific for such peptide. In a preferred embodiment, the recombinant hedgehog polypeptide is a fusion protein containing a domain which facilitates its purification, such as an hedgehog/GST fusion protein. The host cell may be any prokaryotic or eukaryotic cell. Recombinant hedgehog genes can be produced by ligating nucleic acid encoding an hedgehog polypeptide, or a portion thereof, into a vector suitable for expression in either prokaryotic cells, eukaryotic cells, or both. Expression vectors for production of recombinant forms of the subject hedgehog polypeptides include plasmids and other vectors. For instance, suitable vectors for the expression of a hedgehog polypeptide include plasmids of the types: pBR322-derived plasmids, pEMBL-derived plasmids, pEX-derived plasmids, pBTac-derived plasmids and pUC-derived plasmids for expression in prokaryotic cells, such as E. coli. A number of vectors exist for the expression of recombinant proteins in yeast. For instance, YEP24, YIP5, YEP51, YEP52, pYES2, and YRP17 are cloning and expression vehicles useful in the introduction of genetic constructs into S cerevisiae (see, for example, Broach et al. (1983) in Experimental Manipulation of Gene Expression, ed. M. Inouye Academic Press, p. 83, incoφorated by reference herein). These vectors can replicate in E. coli due the presence of the pBR322 ori, and in S. cerevisiae due to the replication determinant of the yeast 2 micron plasmid. In addition, drug resistance markers such as ampicillin can be used. In an illustrative embodiment, an hedgehog polypeptide is produced recombinantly utilizing an expression vector generated by sub-cloning the coding sequence of one of the hedgehog genes represented in SEQ ID Nos: 1 -9 or 19. The preferred mammalian expression vectors contain both prokaryotic sequences, to facilitate the propagation of the vector in bacteria, and one or more eukaryotic transcription units that are expressed in eukaryotic cells. The pcDNAI/amp, pcDNAI/neo, pRc/CMV, pSV2gpt, pSV2neo, pSV2-dhfr, pTk2, pRSVneo, pMSG, pSVT7, pko-neo and pHyg derived vectors are examples of mammalian expression vectors suitable for transfection of eukaryotic cells. Some of these vectors are modified with sequences from bacterial plasmids, such as pBR322, to facilitate replication and drug resistance selection in both prokaryotic and eukaryotic cells. Alternatively, derivatives of viruses such as the bovine papillomavirus (BPV-1), or Epstein-Barr virus (pHEBo, pREP-derived and p205) can be used for transient expression of proteins in eukaryotic cells. The various methods employed in the preparation of the plasmids and transformation of host organisms are well known in the art. For other suitable expression systems for both prokaryotic and eukaryotic cells, as well as general recombinant procedures, see Molecular Cloning A Laboratory Manual, 2nd Ed., ed. by Sambrook, Fritsch and Maniatis (Cold Spring Harbor Laboratory Press: 1989) Chapters 16 and 17. In some instances, it may be desirable to express the recombinant hedgehog polypeptide by the use of a baculovirus expression system. Examples of such baculovirus expression systems include pVL-derived vectors (such as pVL1392, pVL1393 and pVL941), pAcUW-derived vectors (such as pAcUWl), and pBlueBac-derived vectors (such as the β-gal containing pBlueBac III). When it is desirable to express only a portion of a hedgehog polypeptide, such as a form lacking a portion of the N-terminus, i.e. a truncation mutant which lacks the signal peptide, it may be necessary to add a start codon (ATG) to the oligonucleotide fragment containing the desired sequence to be expressed. It is well known in the art that a methionine at the N-terminal position can be enzymatically cleaved by the use of the enzyme methionine aminopeptidase (MAP). MAP has been cloned from E. coli (Ben-Bassat et al. (1987) J Bacteriol. 169:751-757) and Salmonella typhimurium and its in vitro activity has been demonstrated on recombinant proteins (Miller et al. (1987) PNAS 54:2718-1722). Therefore, removal of an N-terminal methionine, if desired, can be achieved either in vivo by expressing hedgehog-deήved polypeptides in a host which produces MAP (e.g., E. coli or CM89 or S. cerevisiae), or in vitro by use of purified MAP (e.g., procedure of Miller et al., supra). Alternatively, the coding sequences for the polypeptide can be incoφorated as a part of a fusion gene including a nucleotide sequence encoding a different polypeptide. It is widely appreciated that fusion proteins can also facilitate the expression of proteins, and accordingly, can be used in the expression of the hedgehog polypeptides of the present invention. For example, hedgehog polypeptides can be generated as glutathione-S-transferase (GST-fusion) proteins. Such GST-fusion proteins can enable easy purification of the hedgehog polypeptide, as for example by the use of glutathione-derivatized matrices (see, for example, Current Protocols in Molecular Biology, eds. Ausubel et al. (N.Y.: John Wiley & Sons, 1991)). In another embodiment, a fusion gene coding for a purification leader sequence, such as a poly-(His)/enterokinase cleavage site sequence, can be used to replace the signal sequence which naturally occurs at the N-terminus of the hedgehog polypeptide (e.g.of the pro-form, in order to permit purification of the >o\y( \s)-hedgehog polypeptide by affinity chromatography using a Ni 2+ metal resin. The purification leader sequence can then be subsequently removed by treatment with enterokinase (e.g., see Hochuli et al. (1987) J. Chromatography 411 : 177; and Janknecht et al. PNAS 88:8972). Techniques for making fusion genes are known to those skilled in the art. Essentially, the joining of various DNA fragments coding for different polypeptide sequences is performed in accordance with conventional techniques, employing blunt-ended or stagger- ended termini for ligation, restriction enzyme digestion to provide for appropriate termini, filling-in of cohesive ends as appropriate, alkaline phosphatase treatment to avoid undesirable joining, and enzymatic ligation. In another embodiment, the fusion gene can be synthesized by conventional techniques including automated DNA synthesizers. Alternatively, PCR amplification of gene fragments can be carried out using anchor primers which give rise to complementary overhangs between two consecutive gene fragments which can subsequently be annealed to generate a chimeric gene sequence (see, for example, Current Protocols in Molecular Biology, eds. Ausubel et al. John Wiley & Sons: 1992). Hedgehog polypeptides may also be chemically modified to create hedgehog derivatives by forming covalent or aggregate conjugates with other chemical moieties, such as glycosyl groups, cholesterol, isoprenoids, lipids, phosphate, acetyl groups and the like. Covalent derivatives of hedgehog polypeptides can be prepared by linking the chemical moieties to functional groups on amino acid sidechains of the protein or at the N-terminus or at the C-terminus of the polypeptide. For instance, hedgehog polypeptides can be generated to include a moiety, other than sequence naturally associated with the protein, that binds a component of the extracellular matrix and enhances localization of the analog to cell surfaces. For example, sequences derived from the fibronectin "type-Ill repeat", such as a tetrapeptide sequence R-G-D-S (Pierschbacher et al. (1984) Nature 309:30-3; and Kornblihtt et al. (1985) EMBO 4:1755-9) can be added to the hedgehog polypeptide to support attachment of the chimeric molecule to a cell through binding ECM components (Ruoslahti et al. (1987) Science 238:491-497; Pierschbacheret al. (1987) J. Biol. Chem. 262:17294-8.; Hynes (1987) Cell 48:549-54; and Hynes (1992) Cell 69:11-25). In preferred embodiment, the hedgehog polypeptide is isolated from, or is otherwise substantially free of, other cellular proteins, especially other extracellular or cell surface associated proteins which may normally be associated with the hedgehog polypeptide. The term "substantially free of other cellular or extracellular proteins" (also referred to herein as "contaminating proteins") or "substantially pure or purified preparations" are defined as encompassing preparations of hedgehog polypeptides having less than 20%> (by dry weight) contaminating protein, and preferably having less than 5%> contaminating protein. By "purified", it is meant that the indicated molecule is present in the substantial absence of other biological macromolecules, such as other proteins. The term "purified" as used herein preferably means at least 80% by dry weight, more preferably in the range of 95-99%) by weight, and most preferably at least 99.8% by weight, of biological macromolecules of the same type present (but water, buffers, and other small molecules, especially molecules having a molecular weight of less than 5000, can be present). The term "pure" as used herein preferably has the same numerical limits as "purified" immediately above. As described above for recombinant polypeptides, isolated hedgehog polypeptides can include all or a portion of the amino acid sequences represented in any of SEQ ID Nos: 10- 18 or 20, or a homologous sequence thereto. Preferred fragments of the subject hedgehog polypeptides correspond to the N-terminal and C-terminal proteolytic fragments of the mature protein. Bioactive fragments of hedgehog polypeptides are described in great detail in PCT publications WO 95/18856 and WO 96/17924. With respect to bioctive fragments of hedgehog polypeptide, preferred hedgehog therapeutics include at least 50 amino acid residues of a hedgehog polypeptide, more preferably at least 100, and even more preferably at least 150. Another preferred hedgehog polypeptide which can be included in the hedgehog therapeutic is an N-terminal fragment of the mature protein having a molecular weight of approximately 19 kDa. Preferred human hedgehog polypeptides include N-terminal fragments corresponding approximately to residues 24-197 of SEQ ID No. 15, 28-202 of SEQ ID No. 16, and 23-198 of SEQ ID No. 17. By "corresponding approximately" it is meant that the sequence of interest is at most 20 amino acid residues different in length to the reference sequence, though more preferably at most 5, 10 or 15 amino acid different in length. Still other preferred hedgehog polypeptides includes an amino acid sequence represented by the formula A-B wherein: (i) A represents all or the portion of the amino acid sequence designated by residues 1-168 of SEQ ID No:21; and B represents at least one amino acid residue of the amino acid sequence designated by residues 169-221 of SEQ ID No:21 ; (ii) A represents all or the portion of the amino acid sequence designated by residues 24-193 of SEQ ID No: 15; and B represents at least one amino acid residue of the amino acid sequence designated by residues 194-250 of SEQ ID No: 15; (iii) A represents all or the portion of the amino acid sequence designated by residues 25-193 of SEQ ID No:13; and B represents at least one amino acid residue of the amino acid sequence designated by residues 194-250 of SEQ ID No:13; (iv) A represents all or the portion of the amino acid sequence designated by residues 23-193 of SEQ ID No:l l; and B represents at least one amino acid residue of the amino acid sequence designated by residues 194-250 of SEQ ID No: 11; (v) A represents all or the portion of the amino acid sequence designated by residues 28-197 of SEQ ID No: 12; and B represents at least one amino acid residue of the amino acid sequence designated by residues 198-250 of SEQ ID No: 12; (vi) A represents all or the portion of the amino acid sequence designated by residues 29-197 of SEQ ID No: 16; and B represents at least one amino acid residue of the amino acid sequence designated by residues 198-250 of SEQ ID No: 16; or (vii) A represents all or the portion of the amino acid sequence designated by residues 23-193 of SEQ ID No. 17, and B represents at least one amino acid residue of the amino acid sequence designated by residues 194-250 of SEQ ID No. 17. In certain preferred embodiments, A and B together represent a contiguous polypeptide sequence designated sequence, A represents at least 25, 50, 75, 100, 125 or 150 amino acids of the designated sequence, and B represents at least 5, 10, or 20 amino acid residues of the amino acid sequence designated by corresponding entry in the sequence listing, and A and B together preferably represent a contiguous sequence corresponding to the sequence listing entry. Similar fragments from other hedgehog also contemplated, e.g., fragments which correspond to the preferred fragments from the sequence listing entries which are enumerated above. Isolated peptidyl portions of hedgehog polypeptides can be obtained by screening peptides recombinantly produced from the corresponding fragment of the nucleic acid encoding such peptides. In addition, fragments can be chemically synthesized using techniques known in the art such as conventional Merrifield solid phase f-Moc or t-Boc chemistry. For example, a hedgehog polypeptide of the present invention may be arbitrarily divided into fragments of desired length with no overlap of the fragments, or preferably divided into overlapping fragments of a desired length. The fragments can be produced (recombinantly or by chemical synthesis) and tested to identify those peptidyl fragments which can function as agonists of a wild-type (e.g., "authentic") hedgehog polypeptide. For example, Roman et al. (1994) Eur J Biochem 222:65-73 describe the use of competitive- binding assays using short, overlapping synthetic peptides from larger proteins to identify binding domains. The recombinant hedgehog polypeptides of the present invention also include homologs of the authentic hedgehog polypeptides, such as versions of those protein which are resistant to proteolytic cleavage, as for example, due to mutations which alter potential cleavage sequences or which inactivate an enzymatic activity associated with the protein. Hedgehog homologs of the present invention also include proteins which have been post- translationally modified in a manner different than the authentic protein. Exemplary derivatives of hedgehog polypeptides include polypeptides which lack glycosylation sites (e.g. to produce an unglycosylated protein), which lack sites for cholesterolization, and/or which lack N-terminal and or C-terminal sequences. Modification of the structure of the subject hedgehog polypeptides can also be for such puφoses as enhancing therapeutic or prophylactic efficacy, or stability (e.g., ex vivo shelf life and resistance to proteolytic degradation in vivo). Such modified peptides, when designed to retain at least one activity of the naturally-occurring form of the protein, are considered functional equivalents of the hedgehog polypeptides described in more detail herein. Such modified peptides can be produced, for instance, by amino acid substitution, deletion, or addition. It is well known in the art that certain isolated replacements of amino acids, e.g., replacement of an amino acid residue with another related amino acid (i.e. isosteric and/or isoelectric mutations), can be carried out without major effect on the biological activity of the resulting molecule. Conservative replacements are those that take place within a family of amino acids that are related in their side chains. Genetically encoded amino acids are can be divided into four families: (1) acidic = aspartate, glutamate; (2) basic = lysine, arginine, histidine; (3) nonpolar = alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan; and (4) uncharged polar = glycine, asparagine, glutamine, cysteine, serine, threonine, tyrosine. Phenylalanine, tryptophan, and tyrosine are sometimes classified jointly as aromatic amino acids. In similar fashion, the amino acid repertoire can be grouped as (1) acidic = aspartate, glutamate; (2) basic = lysine, arginine histidine, (3) aliphatic = glycine, alanine, valine, leucine, isoleucine, serine, threonine, with serine and threonine optionally be grouped separately as aliphatic-hydroxyl; (4) aromatic = phenylalanine, tyrosine, tryptophan; (5) amide = asparagine, glutamine; and (6) sulfur -containing = cysteine and methionine. (see, for example, Biochemistry, 2nd ed., Ed. by L. Stryer, WH Freeman and Co.: 1981). Whether a change in the amino acid sequence of a peptide results in a functional hedgehog homolog (e.g. functional in the sense that it acts to mimic or antagonize the wild- type form) can be readily determined by assessing the ability of the variant peptide to produce a response in cells in a fashion similar to the wild-type protein, or competitively inhibit such a response. Polypeptides in which more than one replacement has taken place can readily be tested in the same manner. It is specifically contemplated that the methods of the present invention can be carried using homologs of naturally occurring hedgehog polypeptides. In one embodiment, the invention contemplates using hedgehog polypeptides generated by combinatorial mutagenesis. Such methods, as are known in the art, are convenient for generating both point and truncation mutants, and can be especially useful for identifying potential variant sequences (e.g. homologs) that are functional in binding to a receptor for hedgehog polypeptides. The puφose of screening such combinatorial libraries is to generate, for example, novel hedgehog homologs which can act as neuroprotective agents. To illustrate, hedgehog homologs can be engineered by the present method to provide more efficient binding to a cognate receptor, such as patched, retaining neuroprotective activity. Thus, combinatorially-derived homologs can be generated to have an increased potency relative to a naturally occurring form of the protein. Moreover, manipulation of certain domains of hedgehog by the present method can provide domains more suitable for use in fusion proteins, such as one that incoφorates portions of other proteins which are derived from the extracellular matrix and/or which bind extracellular matrix components. To further illustrate the state of the art of combinatorial mutagenesis, it is noted that the review article of Gallop et al. (1994) J Med Chem 37:1233 describes the general state of the art of combinatorial libraries as of the earlier 1990's. In particular, Gallop et al state at page 1239 "[s creening the analog libraries aids in determining the minimum size of the active sequence and in identifying those residues critical for binding and intolerant of substitution". In addition, the Ladner et al. PCT publication WO90/02809, the Goeddel et al. U.S. Patent 5,223,408, and the Markland et al. PCT publication WO92/15679 illustrate specific techniques which one skilled in the art could utilize to generate libraries of hedgehog variants which can be rapidly screened to identify variants/fragments which retained a particular activity of the hedgehog polypeptides. These techniques are exemplary of the art and demonstrate that large libraries of related variants/truncants can be generated and assayed to isolate particular variants without undue experimentation. Gustin et al. (1993) Virology 193:653, and Bass et al. (1990) Proteins: Structure, Function and Genetics 8:309-314 also describe other exemplary techniques from the art which can be adapted as means for generating mutagenic variants of hedgehog polypeptides. Indeed, it is plain from the combinatorial mutagenesis art that large scale mutagenesis of hedgehog polypeptides, without any preconceived ideas of which residues were critical to the biological function, and generate wide arrays of variants having equivalent biological activity. Indeed, it is the ability of combinatorial techniques to screen billions of different variants by high throughout analysis that removes any requirement of a priori understanding or knowledge of critical residues. To illsutrate, the amino acid sequences for a population of hedgehog homologs or other related proteins are aligned, preferably to promote the highest homology possible. Such a population of variants can include, for example, hedgehog homologs from one or more species. Amino acids which appear at each position of the aligned sequences are selected to create a degenerate set of combinatorial sequences. In a preferred embodiment, the variegated library of hedgehog variants is generated by combinatorial mutagenesis at the nucleic acid level, and is encoded by a variegated gene library. For instance, a mixture of synthetic oligonucleotides can be enzymatically ligated into gene sequences such that the degenerate set of potential hedgehog sequences are expressible as individual polypeptides, or alternatively, as a set of larger fusion proteins (e.g. for phage display) containing the set of hedgehog sequences therein. As illustrated in PCT publication WO 95/18856, to analyze the sequences of a population of variants, the amino acid sequences of interest can be aligned relative to sequence homology. The presence or absence of amino acids from an aligned sequence of a particular variant is relative to a chosen consensus length of a reference sequence, which can be real or artificial. In an illustrative embodiment, alignment of exons 1, 2 and a portion of exon 3 encoded sequences (e.g. the N-terminal approximately 221 residues of the mature protein) of each of the Shh clones produces a degenerate set of Shh polypeptides represented by the general formula: C-G-P-G-R-G-X(l)-G-X(2)-R-R-H-P-K-K-L-T-P-L-A-Y-K-Q-F-I-P-N-V-A- E-K-T-L-G-A-S-G-R-Y-E-G-K-I-X(3)-R-N-S-E-R-F-K-E-L-T-P-N-Y-N-P-D- I-I-F-K-D-E-E-N-T-G-A-D-R-L-M-T-Q-R-C-K-D-K-L-N-X(4)-L-A-I-S-V- M-N-X(5)-W-P-G-V-X(6)-L-R-V-T-E-G-W-D-E-D-G-H-H-X(7)-E-E-S-L-H- Y-E-G-R-A-V-D-I-T-T-S-D-R-D-X(8)-S-K-Y-G-X(9)-L-X(10)-R-L-A-V-E- A-G-F-D-W-V-Y-Y-E-S-K-A-H-I-H-C-S-V-K-A-E-N-S-V-A-A-K-S-G-G-C- F-P-G-S-A-X( 11 )-V-X( 12)-L-X( 13)-X( 14)-G-G-X( 15)-K-X-( 16)-V-K-D-L- X( 17)-P-G-D-X( 18)- V-L-A-A-D-X( 19)-X(20)-G-X(21 )-L-X(22)-X(23 )-S-D- F-X(24)-X(25)-F-X(26)-D-R (SEQ ID No: 21), wherein each of the degenerate positions "X" can be an amino acid which occurs in that position in one of the human, mouse, chicken or zebrafish Shh clones, or, to expand the library, each X can also be selected from amongst amino acid residue which would be conservative substitutions for the amino acids which appear naturally in each of those positions. For instance, Xaa(l) represents Gly, Ala, Val, Leu, He, Phe, Tyr or Tφ ; Xaa(2) represents Arg, His or Lys; Xaa(3) represents Gly, Ala, Val, Leu, He, Ser or Thr; Xaa(4) represents Gly, Ala, Val, Leu, He, Ser or Thr; Xaa(5) represents Lys, Arg, His, Asn or Gin; Xaa(6) represents Lys, Arg or His; Xaa(7) represents Ser, Thr, Tyr, Tφ or Phe; Xaa(8) represents Lys, Arg or His; Xaa(9) represents Met, Cys, Ser or Thr; Xaa(lO) represents Gly, Ala, Val, Leu, He, Ser or Thr; Xaa(l l) represents Leu, Val, Met, Thr or Ser; Xaa(12) represents His, Phe, Tyr, Ser, Thr, Met or Cys; Xaa(13) represents Gin, Asn, Glu, or Asp; Xaa(14) represents His, Phe, Tyr, Thr, Gin, Asn, Glu or Asp; Xaa(15) represents Gin, Asn, Glu, Asp, Thr, Ser, Met or Cys; Xaa(16) represents Ala, Gly, Cys, Leu, Val or Met; Xaa(17) represents Arg, Lys, Met, He, Asn, Asp, Glu, Gin, Ser, Thr or Cys; Xaa(18) represents Arg, Lys, Met or He; Xaa(19) represents Ala, Gly, Cys, Asp, Glu, Gin, Asn, Ser, Thr or Met; Xaa(20) represents Ala, Gly, Cys, Asp, Asn, Glu or Gin; Xaa(21) represents Arg, Lys, Met, He, Asn, Asp, Glu or Gin; Xaa(22) represent Leu, Val, Met or He; Xaa(23) represents Phe, Tyr, Thr, His or Tφ; Xaa(24) represents He, Val, Leu or Met; Xaa(25) represents Met, Cys, He, Leu, Val, Thr or Ser; Xaa(26) represents Leu, Val, Met, Thr or Ser. In an even more expansive library, each X can be selected from any amino acid. In similar fashion, alignment of each of the human, mouse, chicken and zebrafish hedgehog clones, can provide a degenerate polypeptide sequence represented by the general formula: C-G-P-G-R-G-X(l)-X(2)-X(3)-R-R-X(4)-X(5)-X(6)-P- -X(7)-L-X(8)-P-L- X(9)- Y-K-Q-F-X( 10)-P-X( 11 )-X( 12)-X( 13)-E-X(l 4)-T-L-G-A-S-G-X( 15)- X( 16)-E-G-X( 17)-X( 18)-X( 19)-R-X(20)-S-E-R-F-X(21 )-X(22)-L-T-P-N- Y- N-P-D-I-I-F-K-D-E-E-N-X(23)-G-A-D-R-L-M-T-X(24)-R-C-K-X(25)-X(26)- X(27)-N-X(28)-L-A-I-S-V-M-N-X(29)-W-P-G-V-X(30)-L-R-V-T-E-G- X(31)-D-E-D-G-H-H-X(32)-X(33)-X(34)-S-L-H-Y-E-G-R-A-X(35)-D-I-T-T- S-D-R-D-X(36)-X(37)-K-Y-G-X(38)-L-X(39)-R-L-A-V-E-A-G-F-D-W-V-Y- Y-E-S-X(40)-X(41)-H-X(42)-H-X(43)-S-V-K-X(44)-X(45) (SEQIDNo:22), wherein, as above, each of the degenerate positions "X" can be an amino acid which occurs in a corresponding position in one of the wild-type clones, and may also include amino acid residue which would be conservative substitutions, or each X can be any amino acid residue. In an exemplary embodiment, Xaa(l) represents Gly, Ala, Val, Leu, He, Pro, Phe or Tyr; Xaa(2) represents Gly, Ala, Val, Leu or He; Xaa(3) represents Gly, Ala, Val, Leu, He, Lys, His or Arg; Xaa(4) represents Lys, Arg or His; Xaa(5) represents Phe, Tφ, Tyr or an amino acid gap; Xaa(6) represents Gly, Ala, Val, Leu, He or an amino acid gap; Xaa(7) represents Asn, Gin, His, Arg or Lys; Xaa(8) represents Gly, Ala, Val, Leu, He, Ser or Thr; Xaa(9) represents Gly, Ala, Val, Leu, He, Ser or Thr; Xaa(lO) represents Gly, Ala, Val, Leu, He, Ser or Thr; Xaa(l 1) represents Ser, Thr, Gin or Asn; Xaa(12) represents Met, Cys, Gly, Ala, Val, Leu, He, Ser or Thr; Xaa(13) represents Gly, Ala, Val, Leu, He or Pro; Xaa(14) represents Arg, His or Lys; Xaa(15) represents Gly, Ala, Val, Leu, He, Pro, Arg, His or Lys; Xaa(16) represents Gly, Ala, Val, Leu, He, Phe or Tyr; Xaa(17) represents Arg, His or Lys; Xaa(18) represents Gly, Ala, Val, Leu, He, Ser or Thr; Xaa(19) represents Thr or Ser; Xaa(20) represents Gly, Ala, Val, Leu, He, Asn or Gin; Xaa(21) represents Arg, His or Lys; Xaa(22) represents Asp or Glu; Xaa(23) represents Ser or Thr; Xaa(24) represents Glu, Asp, Gin or Asn; Xaa(25) represents Glu or Asp; Xaa(26) represents Arg, His or Lys; Xaa(27) represents Gly, Ala, Val, Leu or He; Xaa(28) represents Gly, Ala, Val, Leu, He, Thr or Ser; Xaa(29) represents Met, Cys, Gin, Asn, Arg, Lys or His; Xaa(30) represents Arg, His or Lys; Xaa(31) represents Tφ, Phe, Tyr, Arg, His or Lys; Xaa(32) represents Gly, Ala, Val, Leu, He, Ser, Thr, Tyr or Phe; Xaa(33) represents Gin, Asn, Asp or Glu; Xaa(34) represents Asp or Glu; Xaa(35) represents Gly, Ala, Val, Leu, or He; Xaa(36) represents Arg, His or Lys; Xaa(37) represents Asn, Gin, Thr or Ser; Xaa(38) represents Gly, Ala, Val, Leu, He, Ser, Thr, Met or Cys; Xaa(39) represents Gly, Ala, Val, Leu, He, Thr or Ser; Xaa(40) represents Arg, His or Lys; Xaa(41) represents Asn, Gin, Gly, Ala, Val, Leu or He; Xaa(42) represents Gly, Ala, Val, Leu or He; Xaa(43) represents Gly, Ala, Val, Leu, He, Ser, Thr or Cys; Xaa(44) represents Gly, Ala, Val, Leu, He, Thr or Ser; and Xaa(45) represents Asp or Glu. There are many ways by which the library of potential hedgehog homologs can be generated from a degenerate oligonucleotide sequence. Chemical synthesis of a degenerate gene sequence can be carried out in an automatic DNA synthesizer, and the synthetic genes then ligated into an appropriate expression vector. The puφose of a degenerate set of genes is to provide, in one mixture, all of the sequences encoding the desired set of potential hedgehog sequences. The synthesis of degenerate oligonucleotides is well known in the art (see for example, Narang, SA (1983) Tetrahedron 39:3; Itakura et al. (1981) Recombinant DNA, Proc 3rd Cleveland Sympos. Macromolecules, ed. AG Walton, Amsterdam: Elsevier pp273-289; Itakura et al. (1984) Annu. Rev. Biochem. 53:323; Itakura et al. (1984) Science 198:1056; Ike et al. (1983) Nucleic Acid Res. 1 1 :477. Such techniques have been employed in the directed evolution of other proteins (see, for example, Scott et al. (1990) Science 249:386-390; Roberts et al. (1992) PNAS 89:2429-2433; Devlin et al. (1990) Science 249: 404-406; Cwirla et al. (1990) PNAS 87: 6378-6382; as well as U.S. Patents Nos. 5,223,409, 5,198,346, and 5,096,815). A wide range of techniques are known in the art for screening gene products of combinatorial libraries made by point mutations, and for screening cDNA libraries for gene products having a certain property. Such techniques will be generally adaptable for rapid screening of the gene libraries generated by the combinatorial mutagenesis of hedgehog homologs. The most widely used techniques for screening large gene libraries typically comprises cloning the gene library into replicable expression vectors, transforming appropriate cells with the resulting library of vectors, and expressing the combinatorial genes under conditions in which detection of a desired activity facilitates relatively easy isolation of the vector encoding the gene whose product was detected. Each of the illustrative assays described below are amenable to high through-put analysis as necessary to screen large numbers of degenerate hedgehog sequences created by combinatorial mutagenesis techniques. In one embodiment, the combinatorial library is designed to be secreted (e.g. the polypeptides of the library all include a signal sequence but no transmembrane or cytoplasmic domains), and is used to transfect a eukaryotic cell that can be co-cultured with neuronal cells. A functional hedgehog polypeptide secreted by the cells expressing the combinatorial library will diffuse to neighboring neuronal cells and induce a particular biological response, such as protection against cell death when treated with MPTP. The pattern of protection will resemble a gradient function, and will allow the isolation (generally after several repetitive rounds of selection) of cells producing hedgehog homologs active as neuroprotective agents with respect to the target neuronal cells To illustrate, target neuronal cells are cultured in 24-well microtitre plates. Other eukaryotic cells are transfected with the combinatorial hedgehog gene library and cultured in cell culture inserts (e.g. Collaborative Biomedical Products, Catalog #40446) that are able to fit into the wells of the microtitre plate. The cell culture inserts are placed in the wells such that recombinant hedgehog homologs secreted by the cells in the insert can diffuse through the porous bottom of the insert and contact the target cells in the microtitre plate wells. After a period of time sufficient for functional forms of a hedgehog polypeptide to produce a measurable response in the target cells, such as neuroprotection, the inserts are removed and the effect of the variant hedgehog polypeptides on the target cells determined. Cells from the inserts corresponding to wells which score positive for activity can be split and re-cultured on several inserts, the process being repeated until the active clones are identified. In yet another screening assay, the candidate hedgehog gene products are displayed on the surface of a cell or viral particle, and the ability of particular cells or viral particles to associate with a hedgehog-binding moiety (such as the patched protein or other hedgehog receptor) via this gene product is detected in a "panning assay". Such panning steps can be carried out on cells cultured from embryos. For instance, the gene library can be cloned into the gene for a surface membrane protein of a bacterial cell, and the resulting fusion protein detected by panning (Ladner et al., WO 88/06630; Fuchs et al. (1991) Bio/Technology 9:1370-1371; and Goward et al. (1992) TIBS 18:136-140). In a similar fashion, fluorescently labeled molecules which bind hedgehog can be used to score for potentially functional hedgehog homologs. Cells can be visually inspected and separated under a fluorescence microscope, or, where the moφhology of the cell permits, separated by a fluorescence- activated cell sorter. In an alternate embodiment, the gene library is expressed as a fusion protein on the surface of a viral particle. For instance, in the filamentous phage system, foreign peptide sequences can be expressed on the surface of infectious phage, thereby conferring two significant benefits. First, since these phage can be applied to affinity matrices at very high concentrations, large number of phage can be screened at one time. Second, since each infectious phage displays the combinatorial gene product on its surface, if a particular phage is recovered from an affinity matrix in low yield, the phage can be amplified by another round of infection. The group of almost identical E.coli filamentous phages M13, fd, and fl are most often used in phage display libraries, as either of the phage gill or gVIII coat proteins can be used to generate fusion proteins without disrupting the ultimate packaging of the viral particle (Ladner et al. PCT publication WO 90/02909; Garrard et al., PCT publication WO 92/09690; Marks et al. (1992) J. Biol. Chem. 267:16007-16010; Griffths et al. (1993) EMBO J 12:725-734; Clackson et al. (1991) Nature 352:624-628; and Barbas et al. (1992) PNAS 89:4457-4461). In an illustrative embodiment, the recombinant phage antibody system (RPAS, Pharamacia Catalog number 27-9400-01) can be easily modified for use in expressing and screening hedgehog combinatorial libraries. For instance, the pCANTAB 5 phagemid of the RPAS kit contains the gene which encodes the phage gill coat protein. The hedgehog combinatorial gene library can be cloned into the phagemid adjacent to the gill signal sequence such that it will be expressed as a gill fusion protein. After ligation, the phagemid is used to transform competent E. coli TGI cells. Transformed cells are subsequently infected with M13KO7 helper phage to rescue the phagemid and its candidate hedgehog gene insert. The resulting recombinant phage contain phagemid DNA encoding a specific candidate hedgehog, and display one or more copies of the corresponding fusion coat protein. The phage-displayed candidate hedgehog polypeptides which are capable of binding an hedgehog receptor are selected or enriched by panning. For instance, the phage library can be applied to cells which express the patched protein and unbound phage washed away from the cells. The bound phage is then isolated, and if the recombinant phage express at least one copy of the wild type gill coat protein, they will retain their ability to infect E. coli. Thus, successive rounds of reinfection of E. coli, and panning will greatly enrich for hedgehog homologs, which can then be screened for further biological activities in order to differentiate agonists and antagonists. Combinatorial mutagenesis has a potential to generate very large libraries of mutant proteins, e.g., in the order of 10 26 molecules. Combinatorial libraries of this size may be technically challenging to screen even with high throughput screening assays such as phage display. To overcome this problem, a new technique has been developed recently, recrusive ensemble mutagenesis (REM), which allows one to avoid the very high proportion of nonfunctional proteins in a random library and simply enhances the frequency of functional proteins, thus decreasing the complexity required to achieve a useful sampling of sequence space. REM is an algorithm which enhances the frequency of functional mutants in a library when an appropriate selection or screening method is employed (Arkin and Yourvan, 1992, PNAS USA 89:7811-7815; Yourvan et al., 1992, Parallel Problem Solving from Nature, 2., In Maenner and Manderick, eds., Elsevir Publishing Co., Amsterdam, pp. 401-410; Delgrave et al., 1993, Protein Engineering 6(3):327-331). The invention also provides for reduction of the hedgehog polypeptide to generate mimetics, e.g. peptide or non-peptide agents, which are able to mimic the neuroprotective activity of a naturally-occurring hedgehog polypeptide. Thus, such mutagenic techniques as described above are also useful to map the determinants of the hedgehog polypeptides which participate in protein-protein interactions involved in, for example, binding of the subject hedgehog polypeptide to other extracellular matrix components such as its receptor(s). To illustrate, the critical residues of a subject hedgehog polypeptide which are involved in molecular recognition of an hedgehog receptor such as patched can be determined and used to generate hedgehog-deήved peptidomimetics which competitively bind with that moiety. By employing, for example, scanning mutagenesis to map the amino acid residues of each of the subject hedgehog polypeptides which are involved in binding other extracellular proteins, peptidomimetic compounds can be generated which mimic those residues of the hedgehog polypeptide which facilitate the interaction. After distinguishing between agonist and antagonists, such agonistic mimetics may be used to mimic the normal function of a hedgehog polypeptide as trophic for dopaminergic and GABAergic neurons. For instance, non-hydro lyzable peptide analogs of such residues can be generated using benzodiazepine (e.g., see Freidinger et al. in Peptides: Chemistry and Biology, G.R. Marshall ed., ESCOM Publisher: Leiden, Netherlands, 1988), azepine (e.g., see Huffman et al. in Peptides: Chemistry and Biology, G.R. Marshall ed., ESCOM Publisher: Leiden, Netherlands, 1988), substituted gama lactam rings (Garvey et al. in Peptides: Chemistry and Biology, G.R. Marshall ed., ESCOM Publisher: Leiden, Netherlands, 1988), keto-methylene pseudopeptides (Ewenson et al. (1986) J Med Chem 29:295; and Ewenson et al. in Peptides: Structure and Function (Proceedings of the 9th American Peptide Symposium) Pierce Chemical Co. Rockland, IL, 1985), β-turn dipeptide cores (Nagai et al. (1985) Tetrahedron Lett 26:647; and Sato et al. (1986) J Chem Soc Perkin Trans 1 :1231), and β-aminoalcohols (Gordon et al. (1985) Biochem Biophys Res Commun\26:4\9; and Dann et al. (1986) Biochem Biophys Res Commun 134:71). Recombinantly produced forms of the hedgehog polypeptides can be produced using, e.g, expression vectors containing a nucleic acid encoding a hedgehog polypeptide, operably linked to at least one transcriptional regulatory sequence. Operably linked is intended to mean that the nucleotide sequence is linked to a regulatory sequence in a manner which allows expression of the nucleotide sequence. Regulatory sequences are art-recognized and are selected to direct expression of a hedgehog polypeptide. Accordingly, the term transcriptional regulatory sequence includes promoters, enhancers and other expression control elements. Such regulatory sequences are described in Goeddel; Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, CA (1990). For instance, any of a wide variety of expression control sequences, sequences that control the expression of a DNA sequence when operatively linked to it, may be used in these vectors to express DNA sequences encoding hedgehog polypeptide. Such useful expression control sequences, include, for example, a viral LTR, such as the LTR of the Moloney murine leukemia virus, the early and late promoters of SV40, adenovirus or cytomegalovirus immediate early promoter, the lac system, the tφ system, the TAC or TRC system, T7 promoter whose expression is directed by T7 RNA polymerase, the major operator and promoter regions of phage λ , the control regions for fd coat protein, the promoter for 3- phosphoglycerate kinase or other glycolytic enzymes, the promoters of acid phosphatase, e.g., Pho5, the promoters of the yeast α-mating factors, the polyhedron promoter of the baculovirus system and other sequences known to control the expression of genes of prokaryotic or eukaryotic cells or their viruses, and various combinations thereof. It should be understood that the design of the expression vector may depend on such factors as the choice of the host cell to be transformed and/or the type of protein desired to be expressed. Moreover, the vector's copy number, the ability to control that copy number and the expression of any other proteins encoded by the vector, such as antibiotic markers, should also be considered. In addition to providing a ready source of hedgehog polypeptides for purification, the gene constructs of the present invention can also be used as a part of a gene therapy protocol to deliver nucleic acids encoding a neuroprotective form of a hedgehog polypeptide. Thus, another aspect of the invention features expression vectors for in vivo transfection of a hedgehog polypeptide in particular cell types so as cause ectopic expression of a hedgehog polypeptide in neuronal tissue. Formulations of such expression constructs may be administered in any biologically effective carrier, e.g. any formulation or composition capable of effectively delivering the recombinant gene to cells in vivo. Approaches include insertion of the hedgehog coding sequence in viral vectors including recombinant retroviruses, adenovirus, adeno-associated virus, and heφes simplex virus- 1, or recombinant bacterial or eukaryotic plasmids. Viral vectors transfect cells directly; plasmid DNA can be delivered with the help of, for example, cationic liposomes (lipofectin) or derivatized (e.g. antibody conjugated), polylysine conjugates, gramacidin S, artificial viral envelopes or other such intracellular carriers, as well as direct injection of the gene construct or CaPO4 precipitation carried out in vivo. It will be appreciated that because transduction of appropriate target cells represents the critical first step in gene therapy, choice of the particular gene delivery system will depend on such factors as the phenotype of the intended target and the route of administration, e.g. locally or systemically. Furthermore, it will be recognized that the particular gene construct provided for in vivo transduction of hedgehog expression are also useful for in vitro transduction of cells, such as for use in the ex vivo tissue culture systems described below. A preferred approach for in vivo introduction of nucleic acid into a cell is by use of a viral vector containing nucleic acid, e.g. a cDNA, encoding the particular form of the hedgehog polypeptide desired. Infection of cells with a viral vector has the advantage that a large proportion of the targeted cells can receive the nucleic acid. Additionally, molecules encoded within the viral vector, e.g., by a cDNA contained in the viral vector, are expressed efficiently in cells which have taken up viral vector nucleic acid. Retrovirus vectors and adeno-associated virus vectors are generally understood to be the recombinant gene delivery system of choice for the transfer of exogenous genes in vivo, particularly into humans. These vectors provide efficient delivery of genes into cells, and the transferred nucleic acids are stably integrated into the chromosomal DNA of the host. A major prerequisite for the use of retroviruses is to ensure the safety of their use, particularly with regard to the possibility of the spread of wild-type virus in the cell population. The development of specialized cell lines (termed "packaging cells") which produce only replication-defective retroviruses has increased the utility of retroviruses for gene therapy, and defective retroviruses are well characterized for use in gene transfer for gene therapy puφoses (for a review see Miller, A.D. (1990) Blood 76:271). Thus, recombinant retrovirus can be constructed in which part of the retroviral coding sequence (gag, pol, env) has been replaced by nucleic acid encoding a hedgehog polypeptide and renders the retrovirus replication defective. The replication defective retrovirus is then packaged into virions which can be used to infect a target cell through the use of a helper virus by standard techniques. Protocols for producing recombinant retroviruses and for infecting cells in vitro or in vivo with such viruses can be found in Current Protocols in Molecular Biologv. Ausubel, F.M. et al. (eds.) Greene Publishing Associates, (1989), Sections 9.10-9.14 and other standard laboratory manuals. Examples of suitable retroviruses include pLJ, pZIP, pWE and pEM which are well known to those skilled in the art. Examples of suitable packaging virus lines for preparing both ecotropic and amphotropic retroviral systems include Crip, Cre, 2 and Am. Retroviruses have been used to introduce a variety of genes into many different cell types, including neuronal cells, in vitro and/or in vivo (see for example Eglitis, et al. (1985) Science 230:1395-1398; Danos and Mulligan (1988) Proc. Natl. Acad. Sci. USA 85:6460-6464; Wilson et al. (1988) Proc. Natl. Acad. Sci. USA 85:3014-3018; Armentano et al. (1990) Proc. Natl. Acad. Sci. USA 87:6141-6145; Huber et al. (1991) Proc. Natl. Acad. Sci. USA 88:8039- 8043; Ferry et al. (1991) Proc. Natl. Acad. Sci. USA 88:8377-8381; Chowdhury et al. (1991) Science 254:1802-1805; van Beusechem et al. (1992) Proc. Natl. Acad. Sci. USA 89:7640- 7644; Kay et al. (1992) Human Gene Therapy 3:641-647; Dai et al. (1992) Proc. Natl. Acad. Sci. USA 89:10892-10895; Hwu et al. (1993) J. Immunol. 150:4104-41 15; U.S. Patent No. 4,868,116; U.S. Patent No. 4,980,286; PCT Application WO 89/07136; PCT Application WO 89/02468; PCT Application WO 89/05345; and PCT Application WO 92/07573). Furthermore, it has been shown that it is possible to limit the infection spectrum of retroviruses and consequently of retroviral-based vectors, by modifying the viral packaging proteins on the surface of the viral particle (see, for example PCT publications WO93/25234 and WO94/06920). For instance, strategies for the modification of the infection spectrum of retroviral vectors include: coupling antibodies specific for cell surface antigens to the viral env protein (Roux et al. (1989) PNAS 86:9079-9083; Julan et al. (1992) J Gen Virol 73:3251-3255; and Goud et al. (1983) Virology 163:251-254); or coupling cell surface receptor ligands to the viral env proteins (Neda et al. (1991) J Biol Chem 266:14143-14146). Coupling can be in the form of the chemical cross-linking with a protein or other variety (e.g. lactose to convert the env protein to an asialoglycoprotein), as well as by generating fusion proteins (e.g. single-chain antibody/ewv fusion proteins). This technique, while useful to limit or otherwise direct the infection to certain tissue types, can also be used to convert an ecotropic vector in to an amphotropic vector. Moreover, use of retroviral gene delivery can be further enhanced by the use of tissue- or cell-specific transcriptional regulatory sequences which control expression of the hedgehog gene of the retroviral vector. Another viral gene delivery system useful in the present method utilizes adenovirus- derived vectors. The genome of an adenovirus can be manipulated such that it encodes and expresses a gene product of interest but is inactivated in terms of its ability to replicate in a normal lytic viral life cycle. See for example Berkner et al. (1988) BioTechniques 6:616; Rosenfeld et al. (1991) Science 252:431-434; and Rosenfeld et al. (1992) Cell 68:143-155. Suitable adenoviral vectors derived from the adenovirus strain Ad type 5 dl324 or other strains of adenovirus (e.g., Ad2, Ad3, Ad7 etc.) are well known to those skilled in the art. Recombinant adenoviruses can be advantageous in certain circumstances in that they can be used to infect a wide variety of cell types, including neuronal cells (Rosenfeld et al. (1992) cited supra). Furthermore, the virus particle is relatively stable and amenable to purification and concentration, and as above, can be modified so as to affect the spectrum of infectivity. Additionally, introduced adenoviral DNA (and foreign DNA contained therein) is not integrated into the genome of a host cell but remains episomal, thereby avoiding potential problems that can occur as a result of insertional mutagenesis in situations where introduced DNA becomes integrated into the host genome (e.g., retroviral DNA). Moreover, the carrying capacity of the adenoviral genome for foreign DNA is large (up to 8 kilobases) relative to other gene delivery vectors (Berkner et al. cited supra; Haj-Ahmand and Graham (1986) J. Virol. 57:267). Most replication-defective adenoviral vectors currently in use and therefore favored by the present invention are deleted for all or parts of the viral El and E3 genes but retain as much as 80% of the adenoviral genetic material (see, e.g., Jones et al. (1979) Cell 16:683; Berkner et al., supra; and Graham et al. in Methods in Molecular Biology. E.J. Murray, Ed. (Humana, Clifton, NJ, 1991) vol. 7. pp. 109-127). Expression of the inserted hedgehog gene can be under control of, for example, the El A promoter, the major late promoter (MLP) and associated leader sequences, the E3 promoter, or exogenously added promoter sequences. In addition to viral transfer methods, such as those illustrated above, non-viral methods can also be employed to cause expression of a hedgehog polypeptide in the tissue of an animal. Most nonviral methods of gene transfer rely on normal mechanisms used by mammalian cells for the uptake and intracellular transport of macromolecules. In preferred embodiments, non-viral gene delivery systems of the present invention rely on endocytic pathways for the uptake of the hedgehog polypeptide gene by the targeted cell. Exemplary gene delivery systems of this type include liposomal derived systems, poly-lysine conjugates, and artificial viral envelopes. In clinical settings, the gene delivery systems for the therapeutic hedgehog gene can be introduced into a patient by any of a number of methods, each of which is familiar in the art. For instance, a pharmaceutical preparation of the gene delivery system can be introduced systemically, e.g. by intravenous injection, and specific transduction of the protein in the target cells occurs predominantly from specificity of transfection provided by the gene delivery vehicle, cell-type or tissue-type expression due to the transcriptional regulatory sequences controlling expression of the receptor gene, or a combination thereof. In other embodiments, initial delivery of the recombinant gene is more limited with introduction into the animal being quite localized. For example, the gene delivery vehicle can be introduced by catheter (see U.S. Patent 5,328,470) or by stereotactic injection (e.g. Chen et al. (1994) PNAS 91 : 3054-3057). A hedgehog expression construct can be delivered in a gene therapy construct to dermal cells by, e.g., electroporation using techniques described, for example, by Dev et al. ((1994) Cancer Treat Rev 20:105-115). The pharmaceutical preparation of the gene therapy construct can consist essentially of the gene delivery system in an acceptable diluent, or can comprise a slow release matrix in which the gene delivery vehicle is imbedded. Alternatively, where the complete gene delivery system can be produced intact from recombinant cells, e.g. retroviral vectors, the pharmaceutical preparation can comprise one or more cells which produce the gene delivery system. In yet another embodiment, the hedgehog or ptc therapeutic can be a "gene activation" construct which, by homologous recombination with a genomic DNA, alters the transcriptional regulatory sequences of an endogenous gene. For instance, the gene activation construct can replace the endogenous promoter of a hedgehog gene with a heterologous promoter, e.g., one which causes consitutive expression of the hedgehog gene or which causes inducible expression of the gene under conditions different from the normal expression pattern of the gene. Other genes in the patched signaling pathway can be similarly targeted. A vareity of different formats for the gene activation constructs are available. See, for example, the Transkaryotic Therapies, Inc PCT publications WO93/09222, WO95/31560, WO96/29411, WO95/31560 and WO94/ 12650. In preferred embodiments, the nucleotide sequence used as the gene activation construct can be comprised of (1) DNA from some portion of the endogenous hedgehog gene (exon sequence, intron sequence, promoter sequences, etc.) which direct recombination and (2) heterologous transcriptional regulatory sequence(s) which is to be operably linked to the coding sequence for the genomic hedgehog gene upon recombination of the gene activation construct. For use in generating cultures of hedgehog producing cells, the construct may further include a reporter gene to detect the presence of the knockout construct in the cell. The gene activation construct is inserted into a cell, and integrates with the genomic DNA of the cell in such a position so as to provide the heterologous regulatory sequences in operative association with the native hedgehog gene. Such insertion occurs by homologous recombination, i.e., recombination regions of the activation construct that are homologous to the endogenous hedgehog gene sequence hybridize to the genomic DNA and recombine with the genomic sequences so that the construct is incoφorated into the corresponding position of the genomic DNA. The terms "recombination region" or "targeting sequence" refer to a segment (i.e., a portion) of a gene activation construct having a sequence that is substantially identical to or substantially complementary to a genomic gene sequence, e.g., including 5' flanking sequences of the genomic gene, and can facilitate homologous recombination between the genomic sequence and the targeting transgene construct. As used herein, the term "replacement region" refers to a portion of a activation construct which becomes integrated into an endogenous chromosomal location following homologous recombination between a recombination region and a genomic sequence. The heterologous regulatory sequences, e.g., which are provided in the replacement region, can include one or more of a variety elements, including: promoters (such as constitutive or inducible promoters), enhancers, negative regualtory elements, locus control regions, transcription factor binding sites, or combinations thereof. Promoters/enhancers which may be used to control the expression of the targeted gene in vivo include, but are not limited to, the cytomegalovirus (CMV) promoter/enhancer (Karasuyama et al., 1989, J. Exp. Med., 169:13), the human β-actin promoter (Gunning et al. (1987) PNAS 84:4831-4835), the glucocorticoid-inducible promoter present in the mouse mammary tumor virus long terminal repeat (MMTV LTR) (Klessig et al. (1984) Mol. Cell Biol. 4: 1354-1362), the long terminal repeat sequences of Moloney murine leukemia virus (MuLV LTR) (Weiss et al. (1985) RNA Tumor Viruses, Cold Spring Harbor Laboratory, Cold Spring Harbor, New York), the SV40 early or late region promoter (Bernoist et al. (1981) Nature 290:304-310; Templeton et al. (1984) Mol. Cell Biol., 4:817; and Sprague et al. (1983) J. Virol., 45:773), the promoter contained in the 3' long terminal repeat of Rous sarcoma virus (RSV) (Yamamoto et al., 1980, Cell, 22:787-797), the heφes simplex virus (HSV) thymidine kinase promoter/enhancer (Wagner et al. (1981) PNAS 82:3567-71), and the heφes simplex virus LAT promoter (Wolfe et al. (1992) Nature Genetics, 1:379-384). In an exemplary embodiment, portions of the 5' flanking region of the human Shh gene are amplified using primers which add restriction sites, to generate the following fragments 5 ' -gcgcgc cgaaGCGAGGCAGCCAGCGAGGGAGAGAGCGAGCGGGCGAGCCGGAGC- GAGGAAatcgatgcgcgc (primer 1) 5' -gcgcgcagatctGGGAAAGCGCAAGAGAGAGCGCACACGCACACACCCGCCGCGCG- CACTCGggatccgcgcgc (primer 2) As illustrated, primer 1 includes a 5' non-coding region of the human Shh gene and is flanked by an AsuII and Clal restriction sites. Primer 2 includes a portion of the 5' non-coding region immediately 3 ' to that present in primer 1. The hedgehog gene sequence is flanked by XhoII and BamHI restriction sites. The purified amplimers are cut with each of the enzymes as appropriate. The vector pCDNAl.l (Invitrogen) includes a CMV promoter. The plasmid is cut with with AsuII, which cleaves just 3' to the CMV promoter sequence. The AsuII/Clal fragment of primer 1 is ligated to the AsuII cleavage site of the pcDNA vector. The Clal/AsuII ligation destroys the AsuII site at the 3' end of a properly inserted primer 1. The vector is then cut with BamHI, and an XhoII/BamHI fragment of primer 2 is ligated to the BamHI cleavage site. As above, the BamHI/XhoII ligation destroys the BamHI site at the 5' end of a properly inserted primer 2. Individual colonies are selected, cut with AsuII and BamHI, and the size of the AsuII/BamHI fragment determined. Colonies in which both the primer 1 and primer 2 sequences are correctly inserted are further amplified, an cut with AsuII and BamHI to produce the gene activation construct cgaagcgaggcagccagcgagggagagagcgagcgggcgagccggagcgaggaaATCGAAGG TTCGAATCCTTCCCCCACCACCATCACTTTCAAAAGTCCGAAAGAATCTGCTCCCTGCTTGT GTGTTGGAGGTCGCTGAGTAGTGCGCGAGTAAAATTTAAGCTACAACAAGGCAAGGCTTGAC CGACAATTGCATGAAGAATCTGCTTAGGGTTAGGCGTTTTGCGCTGCTTCGCGATGTACGGG CCAGATATACGCGTTGACATTGATTATTGACTAGTTATTAATAGTAATCAATTACGGGGTCA TTAGTTCATAGCCCATATATGGAGTTCCGCGTTACATAACTTACGGTAAATGGCCCGCCTGG CTGACCGCCCAACGACCCCCGCCCATTGACGTCAATAATGACGTATGTTCCCATAGTAACGC CAATAGGGACTTTCCATTGACGTCAATGGGTGGACTATTTACGGTAAACTGCCCACTTGGCA GTACATCAAGTGTATCATATGCCAAGTACGCCCCCTATTGACGTCAATGACGGTAAATGGCC CGCCTGGCATTATGCCCAGTACATGACCTTATGGGACTTTCCTACTTGGCAGTACATCTACG TATTAGTCATCGCTATTACCATGGTGATGCGGTTTTGGCAGTACATCAATGGGCGTGGATAG CGGTTTGACTCACGGGGATTTCCAAGTCTCCACCCCATTGACGTCAATGGGAGTTTGTTTTG GCACCAAAATCAACGGGACTTTCCAAAATGTCGTAACAACTCCGCCCCATTGACGCAAATGG GCGGTAGGCGTGTACGGTGGGAGGTCTATATAAGCAGAGCTCTCTGGCTAACTAGAGAACCC ACTGCTTACTGGCTTATCGAAATTAATACGACTCACTATAGGGAGACCCAAGCTTGGTACCG AGCTCGGATCgatctgggaaagcgcaagagagagcgcacacgcacacacccgccgcgcgcac tcgg In this construct, the flanking primer 1 and primer 2 sequences provide the recombination region which permits the insertion of the CMV promoter in front of the coding sequence for the human Shh gene. Other heterologous promoters (or other transcriptional regulatory sequences) can be inserted in a genomic hedgehog gene by a similar method. In still other embodiments, the replacement region merely deletes a negative transcriptional control element of the native gene, e.g., to activate expression, or ablates a positive control element, e.g., to inhibit expression of the targeted gene. V. Exemplary ptc therapeutic compounds. In another embodiment, the subject method is carried out using a ptc therapeutic composition. Such compositions can be generated with, for example, compounds which bind to patched and alter its signal transduction activity, compounds which alter the binding and/or enzymatic activity of a protein (e.g., intracellular) involved in patched signal pathway, and compounds which alter the level of expression of a hedgehog polypeptide, a patched protein or a protein involved in the intracellular signal transduction pathway of patched. The availability of purified and recombinant hedgehog polypeptides facilitates the generation of assay systems which can be used to screen for drugs, such as small organic molecules, which are either agonists or antagonists of the normal cellular function of a hedgehog and/or patched protein, particularly in their role in the pathogenesis of neuronal cell death. In one embodiment, the assay evaluates the ability of a compound to modulate binding between a hedgehog polypeptide and a hedgehog receptor such as patched. In other embodiments, the assay merely scores for the ability of a test compound to alter the signal transduction activity of t e patched protein. In this manner, a variety of hedgehog and or ptc therapeutics, which will include ones with neuroprotective activity, can be identified. A variety of assay formats will suffice and, in light of the present disclosure, will be comprehended by skilled artisan. In many drug screening programs which test libraries of compounds and natural extracts, high throughput assays are desirable in order to maximize the number of compounds surveyed in a given period of time. Assays which are performed in cell-free systems, such as may be derived with purified or semi-purified proteins, are often preferred as "primary" screens in that they can be generated to permit rapid development and relatively easy detection of an alteration in a molecular target which is mediated by a test compound. Moreover, the effects of cellular toxicity and/or bioavailability of the test compound can be generally ignored in the in vitro system, the assay instead being focused primarily on the effect of the drug on the molecular target as may be manifest in an alteration of binding affinity with receptor proteins. Accordingly, in an exemplary screening assay for ptc therapeutics, the compound of interest is contacted with a mixture including a hedgehog receptor protein (e.g., a cell expressing the patched receptor) and a hedgehog polypeptide under conditions in which it is ordinarily capable of binding the hedgehog polypeptide. To the mixture is then added a composition containing a test compound. Detection and quantification of τeceptoτ/ hedgehog complexes provides a means for determining the test compound's efficacy at inhibiting (or potentiating) complex formation between the receptor protein and the hedgehog polypeptide. Moreover, a control assay can also be performed to provide a baseline for comparison. In the control assay, isolated and purified hedgehog polypeptide is added to the receptor protein, and the formation of τece toτl hedgehog complex is quantitated in the absence of the test compound. In other embodiments, a ptc therapeutic of the present invention is one which disrupts the association of patched with smoothened. Agonist and antagonists of neuroprotection can be distinguished, and the efficacy of the compound can be assessed, by subsequent testing with neuronal cells. In an illustrative embodiment, the polypeptide utilized as a hedgehog receptor can be generated from the patched protein. Accordingly, an exemplary screening assay includes all or a suitable portion of the patched protein which can be obtained from, for example, the human patched gene (GenBank U43148) or other vertebrate sources (see GenBank Accession numbers U40074 for chicken patched and U46155 for mouse patched), as well as from drosophila (GenBank Accession number M28999) or other invertebrate sources. The patched protein can be provided in the screening assay as a whole protein (preferably expressed on the surface of a cell), or alternatively as a fragment of the full length protein which binds to hedgehog polypeptides, e.g., as one or both of the substantial extracellular domains (e.g. corresponding to residues Asnl20-Ser438 and or Arg770-Tφl027 of the human patched protein). For instance, the patched protein can be provided in soluble form, as for example a preparation of one of the extracellular domains, or a preparation of both of the extracellular domains which are covalently connected by an unstructured linker (see, for example, Huston et al. (1988) PNAS 85:4879; and U.S. Patent No. 5,091,513). In other embodiments, the protein can be provided as part of a liposomal preparation or expressed on the surface of a cell. The patched protein can derived from a recombinant gene, e.g., being ectopically expressed in a heterologous cell. For instance, the protein can be expressed on oocytes, mammalian cells (e.g., COS, CHO, 3T3 or the like), or yeast cell by standard recombinant DNA techniques. These recombinant cells can be used for receptor binding, signal transduction or gene expression assays. Marigo et al. (1996) Development 122:1225- 1233 illustrates a binding assay of human hedgehog to chick patched protein ectopically expressed in Xenopus laevis oocytes. The assay system of Marigo et al. can be adapted to the present drug screening assays. As illustrated in that reference, Shh binds to the patched protein in a selective, saturable, dose-dependent manner, thus demonstrating that patched is a receptor for Shh. Complex formation between the hedgehog polypeptide and a hedgehog receptor may be detected by a variety of techniques. For instance, modulation of the formation of complexes can be quantitated using, for example, detectably labelled proteins such as radiolabelled, fluorescently labelled, or enzymatically labelled hedgehog polypeptides, by immunoassay, or by chromatographic detection. Typically, for cell-free assays, it will be desirable to immobilize either the hedgehog receptor or the hedgehog polypeptide to facilitate separation of τeceptoτ/ hedgehog complexes from uncomplexed forms of one of the proteins, as well as to accommodate automation of the assay. In one embodiment, a fusion protein can be provided which adds a domain that allows the protein to be bound to a matrix. For example, glutathione-S-transferase/receptor (GST/receptor) fusion proteins can be adsorbed onto glutathione sepharose beads (Sigma Chemical, St. Louis, MO) or glutathione derivatized microtitre plates, which are then combined with the hedgehog polypeptide, e.g. an 35 S-labeled hedgehog polypeptide, and the test compound and incubated under conditions conducive to complex formation, e.g. at physiological conditions for salt and pH, though slightly more stringent conditions may be desired. Following incubation, the beads are washed to remove any unbound hedgehog polypeptide, and the matrix bead-bound radiolabel determined directly (e.g. beads placed in scintillant), or in the supernatant after the τeceptoτ I hedgehog complexes are dissociated. Alternatively, the complexes can be dissociated from the bead, separated by SDS-PAGE gel, and the level of hedgehog polypeptide found in the bead fraction quantitated from the gel using standard electrophoretic techniques. Other techniques for immobilizing proteins on matrices are also available for use in the subject assay. For instance, soluble portions of the hedgehog receptor protein can be immobilized utilizing conjugation of biotin and streptavidin. For instance, biotinylated receptor molecules can be prepared from biotin-NHS (N-hydroxy-succinimide) using techniques well known in the art (e.g., biotinylation kit, Pierce Chemicals, Rockford, IL), and immobilized in the wells of streptavidin-coated 96 well plates (Pierce Chemical). Alternatively, antibodies reactive with the hedgehog receptor but which do not interfere with hedgehog binding can be derivatized to the wells of the plate, and the receptor trapped in the wells by antibody conjugation. As above, preparations of a hedgehog polypeptide and a test compound are incubated in the receptor-presenting wells of the plate, and the amount of receptor/ hedgehog complex trapped in the well can be quantitated. Exemplary methods for detecting such complexes, in addition to those described above for the GST-immobilized complexes, include immunodetection of complexes using antibodies reactive with the hedgehog polypeptide, or which are reactive with the receptor protein and compete for binding with the hedgehog polypeptide; as well as enzyme-linked assays which rely on detecting an enzymatic activity associated with the hedgehog polypeptide. In the instance of the latter, the enzyme can be chemically conjugated or provided as a fusion protein with the hedgehog polypeptide. To illustrate, the hedgehog polypeptide can be chemically cross- linked or genetically fused with alkaline phosphatase, and the amount of hedgehog polypeptide trapped in the complex can be assessed with a chromogenic substrate of the enzyme, e.g. paranitrophenylphosphate. Likewise, a fusion protein comprising the hedgehog polypeptide and glutathione-S-transferase can be provided, and complex formation quantitated by detecting the GST activity using l-chloro-2,4-dinitrobenzene (Habig et al (1974) JBiol Chem 249:7130). For processes which rely on immunodetection for quantitating one of the proteins trapped in the complex, antibodies against the protein, such as the anti-hedgehog antibodies described herein, can be used. Alternatively, the protein to be detected in the complex can be "epitope tagged" in the form of a fusion protein which includes, in addition to the hedgehog polypeptide or hedgehog receptor sequence, a second polypeptide for which antibodies are readily available (e.g. from commercial sources). For instance, the GST fusion proteins described above can also be used for quantification of binding using antibodies against the GST moiety. Other useful epitope tags include myc-epitopes (e.g., see Ellison et al. (1991) J Biol Chem 266:21150-21157) which includes a 10-residue sequence from c-myc, as well as the pFLAG system (International Biotechnologies, Inc.) or the pEZZ-protein A system (Pharamacia, NJ). Where the desired portion of the hedgehog receptor (or other hedgehog binding molecule) cannot be provided in soluble form, liposomal vesicles can be used to provide manipulatable and isolatable sources of the receptor. For example, both authentic and recombinant forms of the patched protein can be reconstituted in artificial lipid vesicles (e.g. phosphatidylcholine liposomes) or in cell membrane-derived vesicles (see, for example, Bear et al. (1992) Cell 68:809-818; Newton et al. (1983) Biochemistry 22:6110-6117; and Reber et al. (1987) JBiol Chem 262: 11369-11374). In addition to cell-free assays, such as described above, the readily available source of hedgehog polypeptides provided by the art also facilitates the generation of cell-based assays for identifying small molecule agonists of the neuroprotective activity of wild-type hedgehog polypeptides. Analogous to the cell-based assays described above for screening combinatorial libraries, neuronal cells which are sensitive to / g-e/zog-dependent protection, such as dopaminergic and GABAergic neurons, can be contacted with a hedgehog polypeptide and a test agent of interest, with the assay scoring for anything from simple binding to the cell to trophic responses by the target cell in the presence and absence of the test agent. As with the cell-free assays, agents which produce a statistically significant change in hedgehog activities (either inhibition or potentiation) can be identified. In other emdodiments, the cell-based assay scores for agents which disrupt association of patched and smoothened proteins, e.g., in the cell surface membrane or liposomal preparation. In addition to characterizing cells that naturally express the patched protein, cells which have been genetically engineered to ectopically express patched can be utilized for drug screening assays. As an example, cells which either express low levels or lack expression of the patched protein, e.g. Xenopus laevis oocytes, COS cells or yeast cells, can be genetically modified using standard techniques to ectopically express the patched protein. (see Marigo et al., supra). The resulting recombinant cells, e.g., which express a functional patched receptor, can be utilized in receptor binding assays to identify agonist or anatagonsts of hedgehog binding. Binding assays can be performed using whole cells. Furthermore, the recombinant cells of the present invention can be engineered to include other heterolgous genes encoding proteins involved in zeJge/jog-dependent siganl pathways. For example, the gene products of one or more of smoothened, costal-2 and/or fused can be co-expressed with patched in the reagent cell, with assays being sensitive to the functional reconstituion of the hedgehog signal transduction cascade. Alternatively, liposomal preparations using reconstituted patched protein can be utilized. Patched protein purified from detergent extracts from both authentic and recombinant origins can be reconstituted in in artificial lipid vesicles (e.g. phosphatidylcholine liposomes) or in cell membrane-derived vesicles (see, for example, Bear et al. (1992) Cell 68:809-818; Newton et al. (1983) Biochemistry 22:61 10-6117; and Reber et al. (1987) J Biol Chem 262:1 1369-1 1374). The lamellar structure and size of the resulting liposomes can be characterized using electron microscopy. External orientation of the patched protein in the reconstituted membranes can be demonstrated, for example, by immunoelectron microscopy. The hedgehog polypeptide binding activity of liposomes containing patched and liposomes without the protein in the presence of candidate agents can be compared in order to identify potential modulators of the hedgehog-patched interaction. The hedgehog polypeptide used in these cell-based assays can be provided as a purified source (natural or recombinant in origin), or in the form of cells/tissue which express the protein and which are co-cultured with the target cells. As in the cell-free assays, where simple binding (rather than induction) is the hedgehog activity scored for in the assay, the protein can be labelled by any of the above-mentioned techniques, e.g., fluorescently, enzymatically or radioactively, or detected by immunoassay. In addition to binding studies, functional assays can be used to identified modulators, i.e., agonists of hedgehog oτ patched activities. By detecting changes in intracellular signals, such as alterations in second messengers or gene expression in /_.<2tc .eJ-expressing cells contacted with a test agent, candidate antagonists to patched signaling can be identified (e.g., having a hedgehog-like activity). A number of gene products have been implicated in /.αtcAeJ-mediated signal transduction, including patched, the transcription factor cubitus interruptus (ci), the serine/threonine kinase fused (fu) and the gene products of costal-2, smoothened and suppressor of fused. The interaction of a hedgehog polypeptide with patched sets in motion a cascade involving the activation and inhibition of downstream effectors, the ultimate consequence of which is, in some instances, a detectable change in the transcription or translation of a gene. Potential transcriptional targets of patched signaling are the patched gene itself (Hidalgo and Ingham, 1990 Development 110, 291-301; Marigo et al., 1996 ) and the vertebrate homologs of the drosophila cubitus interruptus gene, the GLI genes (Hui et al. (1994) Dev Biol 162:402-413). Patched gene expression has been shown to be induced in cells of the limb bud and the neural plate that are responsive to Shh. (Marigo et al. (1996) PNAS, in press; Marigo et al. (1996) Development 122:1225-1233). The GLI genes encode putative transcription factors having zinc finger DNA binding domains (Orenic et al. (1990) Genes & Dev 4:1053-1067; Kinzler et al. (1990) Mol Cell Biol 10:634-642). Transcription of the GLI gene has been reported to be upregulated in response to hedgehog in limb buds, while transcription of the GLI3 gene is downregulated in response to hedgehog induction (Marigo et al. (1996) Development 122:1225-1233). By selecting transcriptional regulatory sequences from such target genes, e.g. from patched or GLI genes, that are responsible for the up- or down regulation of these genes in response to patched signalling, and operatively linking such promoters to a reporter gene, one can derive a transcription based assay which is sensitive to the ability of a specific test compound to modify patched signalling pathways. Expression of the reporter gene, thus, provides a valuable screening tool for the development of compounds that act as antagonists of ptc, e.g., which may be useful as neuroprotective agents. Reporter gene based assays of this invention measure the end stage of the above described cascade of events, e.g., transcriptional modulation. Accordingly, in practicing one embodiment of the assay, a reporter gene construct is inserted into the reagent cell in order to generate a detection signal dependent on ptc signaling. To identify potential regulatory elements responsive to ptc signaling present in the transcriptional regulatory sequence of a target gene, nested deletions of genomic clones of the target gene can be constructed using standard techniques. See, for example, Current Protocols in Molecular Biology. Ausubel, F.M. et al. (eds.) Greene Publishing Associates, (1989); U.S. Patent 5,266,488; Sato et al. (1995) J Biol Chem 270:10314-10322; and Kube et al. (1995) Cytokine 7:1-7. A nested set of DNA fragments from the gene's 5'-flanking region are placed upstream of a reporter gene, such as the luciferase gene, and assayed for their ability to direct reporter gene expression in patched expressing cells. Host cells transiently transfected with reporter gene constructs can be scored for the induction of expression of the reporter gene in the presence and absence of hedgehog to determine regulatory sequences which are responsice to /?αtc/zeJ-dependent signalling. In practicing one embodiment of the assay, a reporter gene construct is inserted into the reagent cell in order to generate a detection signal dependent on second messengers generated by induction with hedgehog polypeptide. Typically, the reporter gene construct will include a reporter gene in operative linkage with one or more transcriptional regulatory elements responsive to the hedgehog activity, with the level of expression of the reporter gene providing the /zeJge/.og-dependent detection signal. The amount of transcription from the reporter gene may be measured using any method known to those of skill in the art to be suitable. For example, mRNA expression from the reporter gene may be detected using RNAse protection or RNA-based PCR, or the protein product of the reporter gene may be identified by a characteristic stain or an intrinsic activity. The amount of expression from the reporter gene is then compared to the amount of expression in either the same cell in the absence of the test compound (or hedgehog) or it may be compared with the amount of transcription in a substantially identical cell that lacks the target receptor protein. Any statistically or otherwise significant difference in the amount of transcription indicates that the test compound has in some manner altered the signal transduction of the patched protein, e.g., the test compound is a potential ptc therapeutic. As described in further detail below, in preferred embodiments the gene product of the reporter is detected by an intrinsic activity associated with that product. For instance, the reporter gene may encode a gene product that, by enzymatic activity, gives rise to a detection signal based on color, fluorescence, or luminescence. In other preferred embodiments, the reporter or marker gene provides a selective growth advantage, e.g., the reporter gene may enhance cell viability, relieve a cell nutritional requirement, and/or provide resistance to a drug. Preferred reporter genes are those that are readily detectable. The reporter gene may also be included in the construct in the form of a fusion gene with a gene that includes desired transcriptional regulatory sequences or exhibits other desirable properties. Examples of reporter genes include, but are not limited to CAT (chloramphenicol acetyl transferase) (Alton and Vapnek (1979), Nature 282: 864-869) luciferase, and other enzyme detection systems, such as beta-galactosidase; firefly luciferase (deWet et al. (1987), Mol. Cell. Biol. 7:725-737); bacterial luciferase (Engebrecht and Silverman (1984), PNAS 1: 4154-4158; Baldwin et al. (1984), Biochemistry 23: 3663-3667); alkaline phosphatase (Toh et al. (1989) Eur. J. Biochem. 182: 231-238, Hall et al. (1983) J. Mol. Appl. Gen. 2: 101), human placental secreted alkaline phosphatase (Cullen and Malim (1992) Methods in Enzymol. 216:362-368). Transcriptional control elements which may be included in a reporter gene construct include, but are not limited to, promoters, enhancers, and repressor and activator binding sites. Suitable transcriptional regulatory elements may be derived from the transcriptional regulatory regions of genes whose expression is induced after modulation of a patched signal transduction pathway. The characteristics of preferred genes from which the transcriptional control elements are derived include, but are not limited to, low or undetectable expression in quiescent cells, rapid induction at the transcriptional level within minutes of extracellular simulation, induction that is transient and independent of new protein synthesis, subsequent shut-off of transcription requires new protein synthesis, and mRNAs transcribed from these genes have a short half-life. It is not necessary for all of these properties to be present. In yet other embodiments, second messenger generation can be measured directly in the detection step, such as mobilization of intracellular calcium, phospholipid metabolism or adenylate cyclase activity are quantitated, for instance, the products of phospholipid hydrolysis IP3, DAG or cAMP could be measured For example, recent studies have implicated protein kinase A (PKA) as a possible component of hedgehog/patched signaling (Hammerschmidt et al. (1996) Genes & Dev 10:647). High PKA activity has been shown to antagonize hedgehog signaling in these systems. Conversely, inhibitors of PKA will mimic and or potentiate the action of hedgehog. Although it is unclear whether PKA acts directly downstream or in parallel with hedgehog signaling, it is possible that hedgehog signalling occurs via inhibition of PKA activity. Thus, detection of PKA activity provides a potential readout for the instant assays. In a preferred embodiment, the ptc therapeutic is a PKA inhibitor. A variety of PKA inhibitors are known in the art, including both peptidyl and organic compounds. For instance, the ptc therapeutic can be a 5-isoquinolinesulfonamide, such as represented in the general formula: wherein, R j and R 2 each can independently represent hydrogen, and as valence and stability permit a lower alkyl, a lower alkenyl, a lower alkynyl, a carbonyl (such as a carboxyl, an ester, a formate, or a ketone), a thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), an amino, an acylamino, an amido, a cyano, a nitro, an azido, a sulfate, a sulfonate, a sulfonamido, -(CH 2 ) m -R 8 , -(CH 2 ) m -OH, -(CH 2 ) m -O-lower alkyl, -(CH 2 ) m -O- lower alkenyl, -(CH 2 ) n -O-(CH 2 ) m -R 8 , -(CH 2 ) m -SH, -(CH 2 ) m -S-lower alkyl, -(CH 2 ) m -S- lower alkenyl, -(CH 2 ) n -S-(CH 2 ) m -R 8 , or Rj and R 2 taken together with N form a heterocycle (substituted or unsubstituted); R 3 is absent or represents one or more substitutions to the isoquinoline ring such as a lower alkyl, a lower alkenyl, a lower alkynyl, a carbonyl (such as a carboxyl, an ester, a formate, or a ketone), a thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), an amino, an acylamino, an amido, a cyano, a nitro, an azido, a sulfate, a sulfonate, a sulfonamido, -(CH 2 ) m -R 8 , -(CH 2 ) m -OH, -(CH 2 ) m -O-lower alkyl, -(CH 2 ) m -O-lower alkenyl, - (CH 2 ) n -O-(CH 2 ) m -R 8 , -(CH 2 ) m -SH, -(CH 2 ) m -S-lower alkyl, -(CH 2 ) m -S-lower alkenyl, -(CH 2 ) n -S-(CH 2 ) m -R 8 ; R represents a substituted or unsubstituted aryl, aralkyl, cycloalkyl, cycloalkenyl, or heterocycle; and n and m are independently for each occurrence zero or an integer in the range of 1 to 6. In a preferred embodiment, the PKA inhibitor is N-[2-((p-bromocinnamyl)amino)ethyl]-5- isoquinolinesulfonamide (H-89; Calbiochem Cat. No. 371963), e.g., having the formula: In another embodiment, the PKA inhibitor is l-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7; Calbiochem Cat. No. 371955), e.g., having the formula: In still other embodiments, the PKA inhibitor is KT5720 (Calbiochem Cat. No. 420315), having the structure A variety of nucleoside analogs are also useful as PKA inhibitors. For example, the subject method can be carried out cyclic AMP analogs which inhibit the kinase activity of PKA, as for example, 8-bromo-cAMP or dibutyryl-cAMP Exemplary peptidyl inhibitors of PKA activity include the PKA Heat Stable Inhibitor (isoform α; see, for example, Calbiochem Cat. No. 539488, and Wen et al. (1995) J Biol Chem 270:2041). Certain hedgehog receptors may stimulate the activity of phospholipases. Inositol lipids can be extracted and analyzed using standard lipid extraction techniques. Water soluble derivatives of all three inositol lipids (IP j , IP 2 , IP 3 ) can also be quantitated using radiolabelling techniques or HPLC. The mobilization of intracellular calcium or the influx of calcium from outside the cell may be a response to hedgehog stimulation or lack there of. Calcium flux in the reagent cell can be measured using standard techniques. The choice of the appropriate calcium indicator, fluorescent, bioluminescent, metallochromic, or Ca ++ -sensitive microelectrodes depends on the cell type and the magnitude and time constant of the event under study (Borle (1990) Environ Health Perspect 84:45-56). As an exemplary method of Ca ++ detection, cells could be loaded with the Ca ++ sensitive fluorescent dye fura-2 or indo-1, using standard methods, and any change in Ca ++ measured using a fluorometer. In certain embodiments of the assay, it may be desirable to screen for changes in cellular phosphorylation. As an example, the drosophila gene fused (fu) which encodes a serine/threonine kinase has been identified as a potential downstream target in hedgehog signaling. (Preat et al., 1990 Nature 347, 87-89; Therond et al. 1993, Mech. Dev. AA. 65-80). The ability of compounds to modulate serine/threonine kinase activation could be screened using colony immunoblotting (Lyons and Nelson (1984) Proc. Natl. Acad. Sci. USA 81:7426- 7430) using antibodies against phosphorylated serine or threonine residues. Reagents for performing such assays are commercially available, for example, phosphoserine and phosphothreonine specific antibodies which measure increases in phosphorylation of those residues can be purchased from comercial sources. In yet another embodiment, the ptc therapeutic is an antisense molecule which inhibits expression of a protein involved in a /?αtcλeJ-mediated signal transduction pathway. To illustrate, by inhibiting the expression of a protein involved in patched signals, such as fused, costal-2, smoothened and/or Gli genes, or patched itself, the ability of the patched signal pathway(s) to alter the ability of, e.g., a dopaminergic or GABAergic cell to maintain its differentiated state can be altered, e.g., potentiated or repressed. As used herein, "antisense" therapy refers to administration or in situ generation of oligonucleotide probes or their derivatives which specifically hybridize (e.g. bind) under cellular conditions with cellular mRNA and or genomic DNA encoding a hedgehog polypeptide, patched, or a protein involved in patched-mediated signal transduction. The hybridization should inhibit expression of that protein, e.g. by inhibiting transcription and/or translation. The binding may be by conventional base pair complementarity, or, for example, in the case of binding to DNA duplexes, through specific interactions in the major groove of the double helix. In general, "antisense" therapy refers to the range of techniques generally employed in the art, and includes any therapy which relies on specific binding to oligonucleotide sequences. An antisense construct of the present invention can be delivered, for example, as an expression plasmid which, when transcribed in the cell, produces RNA which is complementary to at least a unique portion of the target cellular mRNA. Alternatively, the antisense construct is an oligonucleotide probe which is generated ex vivo and which, when introduced into the cell causes inhibition of expression by hybridizing with the mRNA and/or genomic sequences of a target gene. Such oligonucleotide probes are preferably modified oligonucleotide which are resistant to endogenous nucleases, e.g. exonucleases and/or endonucleases, and is therefore stable in vivo. Exemplary nucleic acid molecules for use as antisense oligonucleotides are phosphoramidate, phosphothioate and methylphosphonate analogs of DNA (see also U.S. Patents 5,176,996; 5,264,564; and 5,256,775). Additionally, general approaches to constructing oligomers useful in antisense therapy have been reviewed, for example, by Van der Krol et al. (1988) Biotechniques 6:958-976; and Stein et al. (1988) Cancer Res 48:2659-2668. Several considerations should be taken into account when constructing antisense oligonucleotides for the use in the methods of the invention: (1) oligos should have a GC content of 50%> or more; (2) avoid sequences with stretches of 3 or more G's; and (3) oligonucleotides should not be longer than 25-26 mers. When testing an antisense oligonucleotide, a mismatched control can be constructed. The controls can be generated by reversing the sequence order of the corresponding antisense oligonucleotide in order to conserve the same ratio of bases. In an illustrative embodiment, the ptc therapeutic can be an antisense construct for inhibiting the expression of patched, e.g., to mimic the inhibition of patched by hedgehog. Exemplary antisense constructs include: 5 '-GTCCTGGCGCCGCCGCCGCCGTCGCC 5'-TTCCGATGACCGGCCTTTCGCGGTGA 5 '-GTGCACGGAAAGGTGCAGGCCACACT VI. Exemplary pharmaceutical preparations of hedgehog and ptc therapeutics The source of the hedgehog and ptc therapeutics to be formulated will depend on the particular form of the agent. Small organic molecules and peptidyl fragments can be chemically synthesized and provided in a pure form suitable for pharmaceutical/cosmetic usage. Products of natural extracts can be purified according to techniques known in the art. For example, the Cox et al. U.S. Patent 5,286,654 describes a method for purifying naturally occurring forms of a secreted protein and can be adapted for purification of hedgehog polypeptides. Recombinant sources of hedgehog polypeptides are also available. For example, the gene encoding hedgehog polypeptides, are known, inter alia, from PCT publications WO 95/18856 and WO 96/17924. Those of skill in treating neural tissues can determine the effective amount of an hedgehog oτ ptc therapeutic to be formulated in a pharmaceutical or cosmetic preparation. The hedgehog oτ ptc therapeutic formulations used in the method of the invention are most preferably applied in the form of appropriate compositions. As appropriate compositions there may be cited all compositions usually employed for systemically or locally (such as intrathecal) administering drugs. The pharmaceutically acceptable carrier should be substantially inert, so as not to act with the active component. Suitable inert carriers include water, alcohol polyethylene glycol, mineral oil or petroleum gel, propylene glycol and the like. To prepare the pharmaceutical compositions of this invention, an effective amount of the particular hedgehog or ptc therapeutic as the active ingredient is combined in intimate admixture with a pharmaceutically acceptable carrier, which carrier may take a wide variety of forms depending on the form of preparation desired for administration. These pharmaceutical compositions are desirable in unitary dosage form suitable, particularly, for administration orally, rectally, percutaneously, or by parenteral injection. For example, in preparing the compositions in oral dosage form, any of the usual pharmaceutical media may be employed such as, for example, water, glycols, oils, alcohols and the like in the case of oral liquid preparations such as suspensions, syrups, elixirs and solutions; or solid carriers such as starches, sugars, kaolin, lubricants, binders, disintegrating agents and the like in the case of powders, pills, capsules, and tablets. Because of their ease in administration, tablets and capsules represents the most advantageous oral dosage unit form, in which case solid pharmaceutical carriers are obviously employed. For parenteral compositions, the carrier will usually comprise sterile water, at least in large part, though other ingredients, for example, to aid solubility, may be included. Injectable solutions, for example, may be prepared in which the carrier comprises saline solution, glucose solution or a mixture of saline and glucose solution. Injectable suspensions may also be prepared in which case appropriate liquid carriers, suspending agents and the like may be employed. Also included are solid form preparations which are intended to be converted, shortly before use, to liquid form preparations. In the compositons suitable for percutaneous administration, the carrier optionally comprises a penetration enhancing agent and/or a suitable wetting agent, optionally combined with suitable additives of any nature in minor proportions, which additives do not introduce a significant deleterious effect on the skin. It is especially advantageous to formulate the subject compositions in dosage unit form for ease of administration and uniformity of dosage. Dosage unit form as used in the specification and claims herein refers to physically discrete units suitable as unitary dosages, each unit containing a predetermined quantity of active ingredient calculated to produce the desired therapeutic effect in association with the required pharmaceutical carrier. Examples of such dosage unit forms are tablets (including scored or coated tablets), capsules, pills, powders packets, wafers, injectable solutions or suspensions, teaspoonfuls, tablespoonfuls and the like, and segregated multiples thereof. The pharmaceutical preparations of the present invention can be used, as stated above, for the many applications which can be considered cosmetic uses. Cosmetic compositions known in the art, preferably hypoallergic and pH controlled are especially preferred, and include toilet waters, packs, lotions, skin milks or milky lotions. The preparations contain, besides the hedgehog oτ ptc therapeutic, components usually employed in such preparations. Examples of such components are oils, fats, waxes, surfactants, humectants, thickening agents, antioxidants, viscosity stabilizers, chelating agents, buffers, preservatives, perfumes, dyestuffs, lower alkanols, and the like. If desired, further ingredients may be incoφorated in the compositions, e.g. antiinflammatory agents, antibacterials, antifungals, disinfectants, vitamins, sunscreens, antibiotics, or other anti-acne agents. Examples of oils comprise fats and oils such as olive oil and hydrogenated oils; waxes such as beeswax and lanolin; hydrocarbons such as liquid paraffin, ceresin, and squalane; fatty acids such as stearic acid and oleic acid; alcohols such as cetyl alcohol, stearyl alcohol, lanolin alcohol, and hexadecanol; and esters such as isopropyl myristate, isopropyl palmitate and butyl stearate. As examples of surfactants there may be cited anionic surfactants such as sodium stearate, sodium cetylsulfate, polyoxyethylene laurylether phosphate, sodium N-acyl glutamate; cationic surfactants such as stearyldimethylbenzylammonium chloride and stearyltrimethylammonium chloride; ampholytic surfactants such as alkylaminoethylglycine hydrocloride solutions and lecithin; and nonionic surfactants such as glycerin monostearate, sorbitan monostearate, sucrose fatty acid esters, propylene glycol monostearate, polyoxyethylene oleylether, polyethylene glycol monostearate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene coconut fatty acid monoethanolamide, polyoxypropylene glycol (e.g. the materials sold under the trademark "Pluronic"), polyoxyethylene castor oil, and polyoxyethylene lanolin. Examples of humectants include glycerin, 1,3-butylene glycol, and propylene glycol; examples of lower alcohols include ethanol and isopropanol; examples of thickening agents include xanthan gum, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, polyethylene glycol and sodium carboxymethyl cellulose; examples of antioxidants comprise butylated hydroxytoluene, butylated hydroxyanisole, propyl gallate, citric acid and ethoxyquin; examples of chelating agents include disodium edetate and ethanehydroxy diphosphate; examples of buffers comprise citric acid, sodium citrate, boric acid, borax, and disodium hydrogen phosphate; and examples of preservatives are methyl parahydroxybenzoate, ethyl parahydroxybenzoate, dehydroacetic acid, salicylic acid and benzoic acid. For preparing ointments, creams, toilet waters, skin milks, and the like, typically from 0.01 to 10%) in particular from 0.1 to 5% and more in particular from 0.2 to 2.5% of the active ingredient, e.g., of the hedgehog or ptc therapeutic, will be incoφorated in the compositions. In ointments or creams, the carrier for example consists of 1 to 20%>, in particular 5 to 15%> of a humectant, 0.1 to 10%> in particular from 0.5 to 5% of a thickener and water; or said carrier may consist of 70 to 99%, in particular 20 to 95% of a surfactant, and 0 to 20%, in particular 2.5 to 15% of a fat; or 80 to 99.9% in particular 90 to 99% of a thickener; or 5 to 15% of a surfactant, 2-15% of a humectant, 0 to 80%> of an oil, very small ( < 2%) amounts of preservative, coloring agent and/or perfume, and water. In a toilet water, the carrier for example consists of 2 to 10% of a lower alcohol, 0.1 to 10%> or in particular 0.5 to 1%) of a surfactant, 1 to 20%>, in particular 3 to 7% of a humectant, 0 to 5%> of a buffer, water and small amounts ( < 2%>) of preservative, dyestuff and/or perfume. In a skin milk, the carrier typically consists of 10-50%) of oil, 1 to 10%> of surfactant, 50-80% of water and 0 to 3% of preservative and/or perfume. In the aforementioned preparations, all %> symbols refer to weight by weight percentage. Particular compositions for use in the method of the present invention are those wherein the hedgehog oτ ptc therapeutic is formulated in liposome-containing compositions. Liposomes are artificial vesicles formed by amphiphatic molecules such as polar lipids, for example, phosphatidyl cholines, ethanolamines and serines, sphingomyelins, cardiolipins, plasmalogens, phosphatidic acids and cerebiosides. Liposomes are formed when suitable amphiphathic molecules are allowed to swell in water or aqueous solutions to form liquid crystals usually of multilayer structure comprised of many bilayers separated from each other by aqueous material (also referred to as coarse liposomes). Another type of liposome known to be consisting of a single bilayer encapsulating aqueous material is referred to as a unilamellar vesicle. If water-soluble materials are included in the aqueous phase during the swelling of the lipids they become entrapped in the aqueous layer between the lipid bilayers. Water-soluble active ingredients such as, for example, various salt forms of a hedgehog polypeptide, are encapsulated in the aqueous spaces between the molecular layers. The lipid soluble active ingredient of hedgehog or ptc therapeutic, such as an organic mimetic, is predominantly incoφorated into the lipid layers, although polar head groups may protude from the layer into the aqueous space. The encapsulation of these compounds can be achieved by a number of methods. The method most commonly used involves casting a thin film of phospholipid onto the walls of a flask by evaporation from an organic solvent. When this film is dispersed in a suitable aqueous medium, multilamellar liposomes are formed. Upon suitable sonication, the coarse liposomes form smaller similarly closed vesicles. Water-soluble active ingredients are usually incoφorated by dispersing the cast film with an aqueous solution of the compound. The unencapsulated compound is then removed by centrifugation, chromatography, dialysis or other art-known suitable procedures. The lipid-soluble active ingredient is usually incoφorated by dissolving it in the organic solvent with the phospholipid prior to casting the film. If the solubility of the material in the lipid phase is not exceeded or the amount present is not in excess of that which can be bound to the lipid, liposomes prepared by the above method usually contain most of the material bound in the lipid bilayers; separation of the liposomes from unencapsulated material is not required. A particularly convenient method for preparing liposome formulated forms of hedgehog and ptc therapeutics is the method described in EP-A-253,619, incoφorated herein by reference. In this method, single bilayered liposomes containing encapsulated active ingredients are prepared by dissolving the lipid component in an organic medium, injecting the organic solution of the lipid component under pressure into an aqueous component while simultaneously mixing the organic and aqueous components with a high speed homogenizer or mixing means, whereupon the liposomes are formed spontaneously. The single bilayered liposomes containing the encapsulated hedgehog or ptc therapeutic can be employed directly or they can be employed in a suitable pharmaceutically acceptable carrier for localized administration. The viscosity of the liposomes can be increased by the addition of one or more suitable thickening agents such as, for example xanthan gum, hydroxypropyl cellulose, hydroxypropyl methylcellulose and mixtures thereof. The aqueous component may consist of water alone or it may contain electrolytes, buffered systems and other ingredients, such as, for example, preservatives. Suitable electrolytes which can be employed include metal salts such as alkali metal and alkaline earth metal salts. The prefeπed metal salts are calcium chloride, sodium chloride and potassium chloride. The concentration of the electrolyte may vary from zero to 260 mM, preferably from 5 mM to 160 mM. The aqueous component is placed in a suitable vessel which can be adapted to effect homogenization by effecting great turbulence during the injection of the organic component. Homogenization of the two components can be accomplished within the vessel, or, alternatively, the aqueous and organic components may be injected separately into a mixing means which is located outside the vessel. In the latter case, the liposomes are formed in the mixing means and then transferred to another vessel for collection puφose. The organic component consists of a suitable non-toxic, pharmaceutically acceptable solvent such as, for example ethanol, glycerol, propylene glycol and polyethylene glycol, and a suitable phospholipid which is soluble in the solvent. Suitable phospholipids which can be employed include lecithin, phosphatidylcholine, phosphatydylserine, phosphatidylethanol- amine, phosphatidylinositol, lysophosphatidylcholine and phospha-tidyl glycerol, for example. Other lipophilic additives may be employed in order to selectively modify the characteristics of the liposomes. Examples of such other additives include stearylamine, phosphatidic acid, tocopherol, cholesterol and lanolin extracts. In addition, other ingredients which can prevent oxidation of the phospholipids may be added to the organic component. Examples of such other ingredients include tocopherol, butylated hydroxyanisole, butylated hydroxytoluene, ascorbyl palmitate and ascorbyl oleate. Preservatives such a benzoic acid, methyl paraben and propyl paraben may also be added. Methods of introduction may also be provided by rechargeable or biodegradable devices. Various slow release polymeric devices have been developed and tested in vivo in recent years for the controlled delivery of drugs, including proteinacious biopharmaceuticals. A variety of biocompatible polymers (including hydrogels), including both biodegradable and non-degradable polymers, can be used to form an implant for the sustained release of an hh at a particular target site. Such embodiments of the present invention can be used for the delivery of an exogenously purified hedgehog polypeptide, which has been incoφorated in the polymeric device, or for the delivery of hedgehog produced by a cell encapsulated in the polymeric device. An essential feature of certain embodiments of the implant can be the linear release of the therapeutic, which can be achieved through the manipulation of the polymer composition and form. By choice of monomer composition or polymerization technique, the amount of water, porosity and consequent permeability characteristics can be controlled. The selection of the shape, size, polymer, and method for implantation can be determined on an individual basis according to the disorder to be treated and the individual patient response. The generation of such implants is generally known in the art. See, for example, Concise Encylopedia of Medical & Dental Materials, ed. by David Williams (MIT Press: Cambridge, MA, 1990); and the Sabel et al. U.S. Patent No. 4,883,666. In another embodiment of an implant, a source of cells producing the therapeutic, e.g., secreting a soluble form of a hedgehog polypeptide, is encapsulated in implantable hollow fibers or the like. Such fibers can be pre-spun and subsequently loaded with the cell source (Aebischer et al. U.S. Patent No. 4,892,538; Aebischer et al. U.S. Patent No. 5,106,627; Hoffman et al. (1990) Expt. Neurobiol. 110:39-44; Jaeger et al. (1990) Prog. Brain Res. 82:41-46; and Aebischer et al. (1991) J. Biomech. Eng. 113:178-183), or can be co-extruded with a polymer which acts to form a polymeric coat about the cells (Lim U.S. Patent No. 4,391,909; Sefton U.S. Patent No. 4,353,888; Sugamori et al. (1989) Trans. Am. Artif Intern. Organs 35:791-799; Sefton et al. (1987) Biotehnol. Bioeng. 29: 1135-1143; and Aebischer et al. (1991) Biomaterials 12:50-55). Exemplification The invention now being generally described, it will be more readily understood by reference to the following examples which are included merely for puφoses of illustration of certain aspects and embodiments of the present invention, and are not intended to limit the invention. Example I: Hedgehog polypeptides promote survival of specific CNS neuron populations and protect these cells from toxic insult in vitro. In Drosophila, the hedgehog gene was first discovered for the role it plays in early embryo patterning (Nusslein-Volhard and Wieschaus, 1980). Further study showed tht the product of this gene is secreted, and as an intercellular signaling protein, plays a critical role in body segmentation and patterning of imaginal disc derivatives such as eyes and wings (Lee et al., 1992; Mohler and Vanie, 1992; Tabata et al., 1992). There re, at present, three mammalian homologues of Drosophila hedgehog, and Indian hedgehog (Fietz et al., 1994). During the course of vertebrate development, these secreted peptide molecules are involved in axial patterning, and consequently regulate the phenotypic specification of precursor cells into functional differentiated cells. The embryonic expression pattern of Shh has been shown to be closely linked to the development and differentiation of the entire ventral neuraxis (Marti et al., 1995). Using naive neural tube explants derived from the appropriate levels of the rostrocaudal axis, it has been demonstrated that the induction of spinal motor neurons (Roelink et al., 1994; Tanabe et al., 1995), midbrain dopaminergic neurons (Hynes et al., 1995; Wang et al., 1995), and basal forebrain cholinergic neurons (Ericson et al., 1995) are dependent upon exposure to Shh. This molecule appears to be crucial for such patterning and phenotype specification in vivo since mouse embryos deficient in the expression of functional Shh gene product manifest a lack of normal ventral patterning in the central nervous system as well as gross atrophy of the entire cranium (Chiang et al., 1996). In this study we have explored the issue of whether Shh may have activities at stages in neural development later than those previously studied. Namely, wew have asked whether Shh is trophic for particular neural populations, and under toxic conditions, whethre Shh is neuroprotective. Using cultures derived from the embryonic day 14-16 (El 4- 16) rat, we find that Shh is trophic for midbrain, striatial, and spinal neurons. In the first case the factor is trophic for both dopaminergic and GABA-immunoreactive (GABA-ir) neurons. From the striatum, the surviving neurons are exclusively GABA-ir, while in the spinal cultures Shh promotes survival of a heterogeneous population of putative interneurons. Shh does not suport survival of any peripheral nervous system neurons tested. Finally, we show that Shh protects cultures of midbrain dopaminergic neurons from the toxic effects of MPP+, a specific neurotoxin that induces Parkinsonism in vivo. Together, these observations indicate a novel role for Shh in nervous system development and its potential role as a therapeutic. Materials and Methods Whole-mount in situ hybridization Whole-mount in situ hybridization on bisected El 4.5 Sprague-Dawley rat embryos was performed with digoxigenin-labeled (Boehringer-Mannheim) mouse RNA probes as previously described (Wilkinson, 1992). Bound probe was detected with alkaline phosphatase-conjugated anti-digoxigenin Fab fragments (BoehringerMannheim). The 0.7 kb Shh probes were transcribed using T3 (antisense) or T7 (sense) RNA polymerase from Hind III (antisense) oτ Bαm HI (sense) linearized templates as described by Echelard, et al. (1993). The 0.9 kb Ptc probes were transcribed using T3 (antisense) or T7 (sense) RNA polymerase from Bαm HI (antisense) or Hind III (sense) linearized templates as described by Goodrich, et al. (1996). Shh protein and anti-Shh antibody Rat sonic hedgehog amino terminal signaling domain (amino acids 2-198) Porter et al., 1995) was cloned into a baculovirus expression vector (Invitrogen; San Diego, CA) (virus encoding Shh insert was a gift of Dr. Henk Roelink, University of Washington, Seattle, WA). High Five™ insect cells (Invitrogen) were infected with the baculovirus per manufacturer's instructions. The culture supernatant was batch adsorbed to heparin agarose type I (Sigma; St. Louis, MO) and Shh eluted with PBS containing a total of 0.75 M NaCl and 0.1- mM - mercaptoethanol. Shh concentration was determined by the method of Ericson, et al. (1996). E. coli-derived Shh was obtained as previously described (Wang et al., 1996) and purified as described above. All samples were sterile filtered and aliquots frozen in liquid nitrogen. Anti-Shh polyclonal antibody was a gift from Dr. Andy McMahon (Harvard University). Preparation of this reagent, directed against the amino peptide of Shh, is described by Bumcrot et al. (1995). Anti-Shh monoclonal antibody (511) was a gift of Dr. Thomas Jessell (Columbia University), and preparation of this reagent is described by Ericson et al. (1996). Dissociation and culture of neural tissue El 4.5 rat ventral mesencephalon was dissected as described by Shimoda, et al. (Shimoda et al., 1992). Striatal cultures were established from El 5- 16 embryos from the regions identified by Altman and Bayer (1995) as the striatum and pallidum. Spinal cultures utilized the ventral one-third of the El 5- 16 spinal cord (Camu and Henderson, 1992). Tissues were dissociated for approximately 40 minutes in 0.10-0.25% trypsin-EDTA (Gibco/BRL; Gaithersburg, MD), and the digestion stopped using an equal volume of Ca++/Mg++-free Hanks' buffered saline (Gibco/BRL) containing 3.5 mg/ml soybean trypsin inhibitor (Sigma) and 0.04%> DNase (Grade II, Boehringer Mannheim; Indianapolis, IN). Cells were than plated at 2 x 20 5 - 3 x 20 5 cells/well in the medium of Krieglstein, et al. (1995) (a modified N2 medium) in 34- well tissue culture plates (Falcon) coated with poly-L- lysine or poly-L-ornithine (Sigma) after 2 wash in the same medium. Note that this procedure results in cultures in which the cells have never been exposed to serum and stands in contrast to cultures in which serum has been used to neutralize dissociation proteases, and/or to intially "prime" the cells prior to serum withdrawal. The following peptide growth factors were added as indicated in the results: basic fibroblast growth factor (FGFb), transforming growth factor 1(TGF 1), TGF 2, glia derived neurotrophic factor (GDNF), and brain derived neurotrophic factor (BDNF) (all from PeproTech; Rocky Hill, NJ; additional lots of BDNF and GDNF were purchased from Promega; Madison WI). Anti-TGF antibodies were purchased from R & D Systems. Antibody was added at the time of Shh addition to the cultures. Cultures were maintained for up to 3 weeks and the medium changed every 4 days. Immunoctyochemistry and cell scoring For all cell staining, cultures were fixed with 5% paraformaldehyde in PBS (plus 0.1%o glutaraldehyde if staining for GABA), and blocked using 3% goat serum, (Sigma), 0.1%) Triton X-100, in PBS. Antibody incubations were performed in the blocking solutions. Antibodies used in this study were anti-tubulin III (Sigma), anti-tyrosine hydroxylase (TH) (Boehringer-Mannheim), anti-GABA (Sigma), and anti-glial fibrillary acidic protein (GFAP) (Sigma). Primary antibodies were detected using horseradish peroxidase-, alkaline phosphatase-, or flurochrome-conjugated secondary antibodies (Vector; Burlingame, CA). Peroxidase-linked secondaries were visualized using a NI/DAB kit (Zymed; South San Francisco, CA) and phosphatase-linked secondaries using Vector Blue™ (Vector). Cell counting was performed using an Olympus inverted microscope at a total magnification of 30OX. Data presented are representative, and have been confirmed by repeating the cultures at least 4-10 independent times for each neural population discussed. Cell numbers are reported as cells/field (the average of 30-40 fields from a total of 5 wells/condition; 4-10 indepedent experiments were assessed for each culture condtion examined). Consistency of counting was verified by at least 3 observers. Eπors are reported as standard error of the mean (s.e.m.), and significance calculated by student's t-test. Measurement of dopamine transport To detect the presence of the dopamine transporter (Cerruti et al., 1993; Ciliax et al., 1995) cultures were incubated with a mixture consisting of: 5 x 10 "8 M 3 H-dopamine (Amersham; Arlington Heights, IL; 48 Ci/mmol), 100 μM ascorbic acid (Sigma), 1 μM fluoxetine (Eli Lilly; Indianapolis, IN), 1 μM desmethylimipramine (Sigma), and 10 μM pargyline (Sigma) in DME-F12. Nonspecific labeling was measured by the addition of 5 x 10 "5 M unlabeled dopamine. Cells were incubated for 30 minutes at 37 C, rinsed three times with PBS and processed for either scintillation counting or autoradiography. For scintillation counting cells were first lysed with 150 μl of 0.1 %> SDS and then added to 500 μl of Microscint 20 (Packard; Meriden, CT) and counted in a Packard Instrument Topcount scintillation machine. For autoradiography, sister plates were coated with NTB-2 autoradiographic emulsion (Kodak; Rochester, NY) that had been diluted 1:3 with 10%> glycerol. The plates were then air dried, exposed for 1-2 weeks, and developed. Quantitative-competitive polymerase chain reaction (QC-PCR) RNA was isolated from cells and tissue using Trizol (Gibco/BRL) as prescribed by the manufacturer. Genomic DNA was removed from the RNA by incubation with 0.5 units of Dnase (Gibco/BRL, Cat # 28068-015) at room temperature for 25 minutes. The solution was heated to 75 C for 20 minutes to inactivate the DNase. Reverse transcription was carried out using random hexamer and MuLV reverse transcriptase (Gibco/BRL) as suggested by the manufacturer. All the quantitative RT-PCR internal controls, or mimics, were synthetic single stranded DNA oligonucleotides corresponding to the target sequence with an internal deletion from the central region (Oligos, Etc.; Wilsonville, OR). For actin, target = 280 bp, mimic = 230 bp; for ptc, target = 354 bp, mimic = 200 bp. PCR was performed using the Clontech PCR kit. For actin: annealing temperature 64 C, oligos GGCTCCGGTATGTGC, GGGGTACTTCAGGGT. For ptc: annealing temperature 72 C, oligos CATTGGCAGGAGGAGTTGATTGTGG, AGCACCTTTTGAGTGGAGTTTGGGG. In each QC-PCR reaction, four reactions were set up with equal amounts of sample cDNA in each tube and 5-fold serial dilution of mimic. Also, for each sample an aliquot of cDNA was saved and amplified along with quantitative PCR as control for contamination. PCR reactions were carried out in an MJ Research PTC-200 thermal cycler and the following cycling profile used: 95 C for 45 seconds, 64 or 72 C for 35 seconds, 82 C for 30 seconds; for 40 cycles. The reaction mixtures were then fractionated by agarose electrophoresis, negative films obtained, and the films digitally scanned and quantified by area integration according to established procedures (Wang et al., 1995, and references therein). The quantity of target molecules was normalized to the competing mimic and expressed as a function of cDNA synthesized and used in each reaction. N-methyl-4-phenylpyrridinium (MPP+) administration Culture and MPP+ treatment of dopaminergic neurons were performed as previously described (Hyman et al., 1994; Krieglstein et al., 1995). MPP+ (Aldrich; St. Louis, MO) was added at day 3 of culture to a final concentration of 3 μM for 58 hours. Cultures were then washed extensively to remove MPP+, cultured for an additional 34-48 hours to allow clearance of dying TH+ neurons, and then processed for immunocytochemistry. Results Shh and Ptc Continue to be Expressed in the Rat CNS After the Major Period of Dorsoventral Patterning Previous studies have shown that shh is expressed in the vertebrate embryo in the period during which dorsoventral patterning manifests (approximately E9-10 in the rat). Within the central nervous system, shh expression persists beyond this period and can be detected at a very high level in the El 4- 16 rat embryo. For example, in situ hybridization studies of the El 4.5 embryo (Fig. 1 A and E) reveal that shh is expressed in ventral regions of the spinal cord, hindbrain, midbrain, and diencephalon. Lower levels of expression are observed in the ventral striatum and septum, while no expression is observed in the cortex within the limits of detection of this method. Interestingly, a "streak" of shh expression (Fig. 1A, arrow) is observed to bisect the diencephalon into rostral and caudal halves. This is likely to be the zona limitans intrathalamica that separates prosomeres 2 and 3, and has been previously observed in the studies of shh expression in the developing chick embryo (Marti, et al., 1995). Recent biochemical evidence supports the view that the ptc gene product can act as a high affinity Shh receptor (Marigo et al., 1996a; Stone et al., 1996). Ptc shows a complementary pattern of expression (Fig. 1C and E), and is observed primarily lateral and dorsal to the sites of shh expression. The complementarity of expression is most dramatic in the diencephalon where ptc mRNA is absent from the zona limitans, but is expressed at a very high level on either side of this structure. Of further interest is the observation that rostral of the zonal limitans, ptc expression no longer seems as restricted to regions immediately dorsal of shh expression. Again, within the detection limits of this technique, ptc is not expressed in the cortex. Thus in regions where shh is expressed, adjacent tissue appears capable of responding to the gene product as evidenced by expression of the putative receptor. Shh Promotes Dopaminergic Neuron Survival In the developing midbrain ( E9), Shh was first characterized for its ability to induce the production of dopaminergic neurons. Thus the trophic potential of Shh was tested on this neuronal population at a stage when these neurons have already been induced. Using cultures derived from the El 4.5 mesencephalon it was found that Shh increases the survival of TH+ neurons in a dose dependent manner (Fig. 2A). These cells exhibited a neuronal moφhology (Fig. 2B), and greater than 95%> of the TH+ cells were also positive for the neuron-specific marker, tubulin III (Banerjee et al., 1990); GFAP staining revealed no glial cells (data not shown). Differences in TH+ neuron survival between control and Shh treated wells could be observed as early as 5 days. Note that under these stringently serum-free conditions (i.e. at no time were the cells exposed to serum), baseline levels of survival are even lower than those conventially reported for cultures that have been maintained in low serum or that have been briefly serum "primed". By 3 weeks in culture less than 6%> of the total TH+ cells plated were present in the control condition, whereas 35-30% survive at 60 ng/ml of Shh (from 5 to 24 days, p<.001 at 35 and 60 ng/ml). All catecholaminergic neurons express TH, but the presence of a specific high affinity DA uptake system is indicative of midbrain dopaminergic neurons (Di Porzio et al., 1980; Denis-Donini et al., 1984; Cerruti et al., 1993; Ciliax et al., 1995). As further evidence that the cells supported by Shh are bone fide dopaminergic neurons, specific, high affinity dopamine (DA) uptake was also demonstrated (Figure 3). Midbrain cultures treated with Shh transported and retained 3 H-DA with a dose response profile paralleling that of survival curves (Fig. 3A) (p<0.005 at 25 and 50 ng/ml). Emulsion autoradiography also demonstrated that the cells taking up 3 H-DA were neuronal in moφhology (Fig. 3B). In addition, immunohistochemistry for dopamine itself demonstrated high cellular content (data not shown). The observed effect of Shh on increased TH+ neuron number is unlikely to be due to differentiation of latent progenitor cells since previous studies demonstrated that the ability of Shh to induce dopaminergic neurons in explanted tissue is lost at later stages of development (Hynes et al., 1995; Wang et al., 1995). Furthermore, the effects are unlikely to be due to a mitogenic response of committed neuroblasts since pulsing the cultures with 5-bromp-2'- deoxyuridine (BrdU) at 1, 2, or 4 days in vitro revealed very low mitotic activity in the presence or absence of Shh (data not shown). Thus in addition to inducing dopaminergic neurons in the naive mesencephalon, Shh is a trophic factor for these neurons. Specificity of Shh Action on Midbrain Neurons: Regulated expression of Ptc Expression of ptc has previously been shown to be regulated by Shh (Goodrich et al., 1996; Marigo et al., 1996b), and to date, Shh is the only factor known to transcriptionally upregulate ptc expression. Therefore, the expression of ptc by mesencephalic explants would reinforce the view that these cells are capable of responding to Shh, and upregulation of ptc mRNA in response to Shh would strongly indicate the specificity of such a response. Therefore, quantitative competitive PCR (QC-PCR) was used to measure the level of ptc expression. Ptc mRNA levels were measured at 0, 3, 5, and 7 days of culture by the method described by Wang, et al. (1995). For each culture condition at each timepoint, 5 separate cDNA samples were co-amplified with a different known amount of mimic substrate (DNA that can be amplified by the same primers but yielding a product of molecular weight lower than that being sought in the sample). Thus for each condition and timepoint, a gel like that shown in Fig. 4A was generated (upper bands correspond to amplified ptc transcripts; lower bands correspond to amplified mimic). Using a scanning densitometer to quantify the observed bands, a graph was produced for each sample (Fig. 4B corresponds to Fig. 4A). When the density of the target band and the mimic band are equal, the concentration of the unknown target can be taken to be equal to the known concentration of mimic. Based on a linear curve fit, the concentration of mimic at the point at which the density of the mimic and the target substrate are equal (Log Ds/Dm = 0) was taken to be the concentration of the substrate in the sample; this value was then normalized to the total amount of cDNA added to the reaction. These values are plotted in Fig. 4C; correlation coefficients (r) of the curve fits always exceeded 0.95, and thus the margin of error for the values presented is less than 5%>. This experiment was performed two independent times with independent cultures and the results were nearly identical. As shown in Fig. 4C, significant ptc expression was observed in the E14.5 ventral mesencephalon (time 0). After two days of culture, higher levels of ptc expression were observed than at the time of dissection; in control cultures this might reflect the loss of ptc non-expressing cell types since a constant amount of RNA was analyzed. There was no difference in ptc expression between control cultures and those treated with either 5 or 25 ng/ml of Shh at this time. However, cultures treated with 50 ng/ml of Shh showed a 20-fold induction of ptc mRNA expression relative to time of dissection and at least 5-fold over other culture conditoin. By 5 days of culture, ptc message levels had declined significantly in comparison to the 3 day level of expression but high levels of expression were still observed in 50 ng/ml Shh. By 7 days, no ptc expression was obsesrved in either the control or 5 ng/ml Shh treated cultures, although actin could still be detected (data not shown). It is important to note that in the 25 and 50 ng/ml Shh-treated cultures ptc expression matched or exceeded the time zero expression of ptc in the mescencephalon despite the overall decrease in cell number. These results indicate that: A) ptc is expressed in the E14.5 ventral mesencephalon (suggesting that the cells in this region are capable of responding to Shh), b) Shh is necessary for the maintenance of ptc gene expression, and c) that the expression of ptc shows a Shh dose dependence that parallels the neurotrophic activity described above. Specificity of Shh Action on Midbrain Neurons: Immunoneutralization As further evidence that the trophic activity of Shh preparation used for these studies, purified from a baculovirus expression system, was due to Shh and not to a contaminating factor, antibody neutralization experiments were performed. As shown in Fig. 4D, a saturating dose of Shh (50 ng/ml) promotes midbrain neuron survival (p < .001) while the same dose of Shh in the presence of a 5-fold molar excess of activity-neutralizgin, anti-Shh, monoclonal antibody (5E1; Ericson, et al. (1996)) inhibits this trophic response (p < .001). In earlier studies (data not shown), an affinity purified, polyclonal, anti-Shh antibody dramatically reduced the activity of Shh in the dopaminergic neuron survival assay (p < .005), whereas purified rabbit IgG antibody from preimmune sera had no significant effect. Anti-TGF antibodies used at a 3-fold molar excess to Shh did not inhibit the trophic activity, while they did inhibit the previously reported (Krieglestein et al., 1995) trophic effects of exogenously applied TGF s (data not shown). Addition of -galactosidase, expressed and purified in a manner identical to Shh, failed to show any trophic effect (data not shown), and thus renders unlikely the possibility that an undefined baculovirus protein iss responsible for the observed trophic effects. Finally, Shh purified from an E. coli expression system (Wang et al., 1995) also had trophic activity for Th+ cells, while -galactosidase purified identically to Shh from the E. coli expression system gave no such activity even at concentrations as high as 20 μg/ml (data not shown). Shh supports the Survival of other Midbrain Neurons Since the original observations concerning the role of Shh in midbrain development were concerned with induction of dopaminergic neurons (Hynes et al., 1995; Want et al., 1995), the current study initially focused on possible trophic effects on these neurons. Interestingly, the cultures in which the above described trophic effects were observed, also demonstrated that the trophic effect of Shh extended to non-dopaminergic neurons (i.e. TH neurons). Within the dopaminergic neucleus of the midbrain, the substantia nigra, GABA is also a major neurotransmitter (Masuko et al., 1992). Staining for GABA in these cultures (Fig. 5) showed that GAB A+ cells are supported by the presence of Shh with a dose response profile comparable to TH+ cells. Furthermore, GABA cells outnumbered TH+ cells by a ratio of approximately 3.1. The two cell types together account for approximately 95%> of the total neurons as gauged by staining for tubulin III (data not shown), and thus it is clear that the trophic effect of Shh on midbrain neurons extends to multiple neuron subtypes (for TH, p < 0.001 at 35 and 60 ng/ml; for GABA, p < .001 at 35 and 60 ng/ml). Ssh Effects on Striatal Neurons Since Shh is strongly expressed in the ventral and lateral forebrain (Εchelard et al., 1993; Ericson et al., 1995), and that the Shh knockout mouse exhibits triatal defects (Chiang et al., 1996), Shh neurotrophic activity was examined in striatum-derived cultures as well. As assessed after 4 days in vitro (Fig. 6), Shh is a potent trophic factor for neurons cultured from the El 5- 16 striatum, and shows a dose response comparable to that of the midbrain. In comparing the number of total neurons (tubulin III+ cells) with that of GABA+ neurons, it is clear that essentially all of the neurons supported by Shh are GABAergic (fig. 6) (tubulin III, p < 0.001 at 25 and 50 ng/ml; GABA, p < .001 at 25 and 50 ng/ml). That this effect is trictly trophic was confirmed by the observation that BrdU labeling indices over the course of the culture period were low and did not vary with dose (data not shown). Closer inspection reveals that the intensity of GABA staining is variable, and it is thus possible that various subtypes of GABA+ interneurons (reviewed by Kawaguchi et al., 1995) are all supported by Shh. Shh Effects on Spinal Neurons As a further examination of the postinductive effectives of Shh on ventral neural tube derivatives, cultures of the El 4- 15 ventral neural tube were cultured with varying amounts of Shh. Again, with a dose response identical to that observed in the mesencephalic and striatal cultures, Shh promotes the survival of tubulin III+ neurons as scored after 4 days in vitro (Fig. 6A). A majority, but not all of these cells also stain for GABA, and a smaller subset stain for a neuclear marker of spinal interneurons, Lim- 1/2 (Tsuchida et al., 1994) (Fig 6A-C) (tubulin III, p < 0.001 at 25 and 50 ng/ml; Lim-1/2, p < .001 at 5, 10, 25 and 50 ng/ml; GABA, p .001 at 25 and 50 ng/ml). It is important to note that while there is overlap between the GABA+ and Lim- 1/2+ populations, the latter is not mrerely a subset of the former since there are Lim- 1/2+ cells that do not stain for GABA. Interestingly, immunoreactivity for the low affinity nerve growth factor receptor (Camu and Henderson, 1992), Islet- 1 (Ericson et al., 1992), or galectin-1 (Hynes et al., 1990), all markers of rat motorneurons, was not detectable in these cultures, and thus it appers tht Shh is not trophic for spinal motorneurons. Shh Protects Th+ Cells Against MPP+ Toxicity The toxin, 5-phenyl-l,2,3,6-tetrahydropterine (MPTP), and its active metabolite, MPP+, are selectively toxic to mesencephalic dopamineric neurons (Kopin and Markey, 1988; Forno et al., 1993). Since other agents that promote survival of TH+ cells also protect against chemical toxicity of MPP+ (Hyman et al., 1991; Krieglestein et al., 1995), we tested the ability of Shh to protect TH+ cells in E14 rat mesencephalon explants from the effects of MPP+. As shown in Figure 8, the presence of Shh in cultures treated for 58 hours with MPP+ significantly increased the numbers of TH+ cells that were observed in culture after removal of the MPP+. MPP+ treatment caused a greater than 90%> reduction in the numbers of TH+ cells compred to non-MPP+ treated control cultures, whereas incubation with Shh protected the Th+ cells so that only a 75 %> reduction of TH+ cells occurred after MPP+ treatment versus controls. Sister cultures tested for 4H-DA transport demonstrated a 8-fold increase in transport in Shh treated cultures versus controls (data not shown). Shh was significantly more active in protecting TH+ cells from the effects of MPP+ than the other growth factors tested: glia-derived neurotrophic factor (GDNF) (Lin et al., 1993) and brain-derived neurotrophic factor (BDNF) (Hyman et al., 1991) (Shh, p < 0.001 at 60 and 350 ng/ml; BDNF no significance; GDNF, p < .05). In the serum free conditions used in these experiments, none of the other growth factors tested showed as significant a level of TH+ cell protection from MPP+ toxicity as Shh, even when tested at levels previously shown to be optimal for neuroprotection (Fig. 8). Discussion Shh is Neurotrophic for a Variety of Ventral Neurons The hypothesis that Shh may play roles in the nervous system in addition to its initial function in neural tube ventralization was first suggested by the observation that Shh expression in ventral neural tissue along the entire neuraxis continues well past the period during which phenotypic specificaton has occurred (Echelard et al., 1993). Moreover, preliminary evidence generated in our laboratory indicates the presence of significant levels of Shh mRNA in specific regions of the adult human-CNS (e.g. spinal cord and substantia .nigra, P. Jin, unpublished observations). We report here the first evidence that Shh can indeed exert effects independent of its induction and patterning activity. Unlike its role at earlier stages of neural development, this novel neurotrophic activity acts on postmitotic neurons rather than on dividing progenitor cells. While the general trophic effect is apparent in a number of CNS regions (Fig. 2 and 6-7), there are both differences and similarieis in the effects observed among the regions examined. Given the fact that Shh is necessary for the induction of both spinal motor neurons and midbrain dopaminergic neurons, one might predict that Shh would be subsequently trophic for the cells. Strikingly, Shh is a very potent trophic factor for the midbrain dopaminergic neurons (Fig. 2), but in the cultures of ventral spinal neurons, no such effect on motor neurons was observed. Thus there is no direct correlation between the neuron phenotypes induced by Shh, and hose supported by Shh in a trophic manner. Interestingly, a common feature among the three CNS regions examined was the trophic effect for GABAergic neurons (Fig. 6-7). While it is not obvious whether these specific GABA+ populations are directly or indirectly induced by Shh during early development (cf. Pfaff et al., 1996), it is plausible that the trophic actions on these neurons are direct. It is important to note that the neurotrophic effects reported herein are not lacking in specificity. For example, neurons of the peripheral nervous system show no survival in response to Shh administration, and preliminary studies of cultures derived from El 5- 16 dorsal CNS regions (e.g. neocortex and dorsal spinal cord) show high baseline levels of neuron survival with no significant response to exogenous Shh application (J.A.0. and N.K.M., unpublished observation). Thus there appears to be a general restriction of the trophic effects of Shh to regions of the CNS specified by Shh, but the actual targets of trophic activity need not encompass the phenotypes whose induction is Shh-dependent. Nevertheless, the fact that Shh also protects neurons from toxic insult (Fig. 8), suggests previously unforeseen therapeutic roles for Shh as well. Possible Mechanisms of Shh Action As stated above, the neurotrophic effect of Shh observed in these cultures is not due to the stimulation of proliferation. One could argue, however, that the observed effects are indirect. In one scenario, Shh may act on a non-neuronal cell that in turn responds by secreting a neurotrophic factor. We observed no sign of astrocytes in any of our neural cultures, either by moφhology or by staining for GFAP. Furthermore, in the purely neuronal cultures established from the midbrain, ptc is greatly upregulated in response to Shh, and thus the reported survival effects must be due to a response by neurons (Fig. 4C). In another scenario, it is possible that Shh acts directly on some or all of the neurons, but the response is to secrete another factor(s) that actually possesses the survival activity. For example, Shh has been shown to induce the expression of TGF family members such as BMP's in vivo (Laufer et al., 1994; Levin et al., 1995) and these proteins are trophic for midbrain dopaminergic neurons (Krieglstein et al., 1995). That induced expression of TGF s is the trophic mechanism seems unlikely since exogenous TGF s show only modest trophic activity in our culture system, and the presence of neutralizing, anti-pan-TGF antibodies failed to inhibit the neurotrophic effects of Shh. Thus, at a minimum, Shh supports the survival of a subset of ventral CNS neurons. The mechanism by which Shh supports neuron survival is yet to be determined. While we favor the hypothesis that these trophic effects are direct, it remains possible that the survival response is due to Shh-induced expression of a secondary trophic factor. As in the case of many secreted peptide factors, it now appears that Shh has activities that can vary greatly depending on the spatiotemporal context in which the factor is expressed. While it was initially thought that the primary role of Shh in the CNS is in early patterning events that are critical to phenotypic specification, it is now clear that Shh can also contribute to the survival and maturation of these CNS regions. Interestingly, the cell types acted upon in these two distinct roles of Shh do not necessarily overlap. Thus a more thorough understanding of this multifaceted molecule will require a better understanding of its patterns of expression beyond early embryogenesis. 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Tsuchida T, Ensini M, Morton SB, Baldassare M, Edlund T, jessell TM, Pfaff SL (1994) Topographic organization of embryonic motor neurons defined by expression of LIM homeobox genes. Cell 89:957-970. Wang MZ, jin P, Bumcrot DA, Marigo V, McMahon AP, Wang EA, Woolf T, Pang K (1995) Induction of dopaminergic neuron phenotype in the midbrain by sonic hedgehog polypeptide. Nature Med 2:1184-1188. Wilkinson, D.G. (1992). Whole mount in situ hybridization of vertebrate embryos. In In Situ Hybridization: A Practical Approach, D.G. Wilkinson, ed. (Oxford: IRL Press), pp. 75-83. Example 2: Restoration of function paradigm - establishment of a stable lesion through administration of 6-OHDA with subsequent administration of Shh. These experiments analyzed reduction in toxin-induced asymmetry of nigrostriatal system as examined by apomoφhine induced rotational behaviors. Briefly, Fisher 344 rats were injected with 6-OHDA into the right medial forebrain bundle. Male rats were anesthetized with 400 mg/kg chloral hydrate injected intraperitoneally and placed in a stereotaxic frame. The skin over the skull was retracted; a burr hole was then placed in the skull over the lesion site and a needle attached to a microliter syringe was lowered into the medial forebrain bundle (4.4 mm posterior to bregma, 1.3 mm lateral, 7.8 mm ventral to the dural surface). 9 ug/4/ul/4 min of 6-hydroxydopamine (6- OHDA) in ascorbate was then injected. The needle was withdrawn after one additional minute and the surgical wound was closed with skin clips. All lesioned animals exhibited a stable rotation pattern and turned >300 times contralateral to the lesion after a low dose of apomoφhine. Prior to intracranial injectionso f Shh (N-terminal fragment, unmodified, E. coli recombinant) or vehicle, all rates showed a slight increase in apomoφhine-induced rotations as measured at weekly intervals for the 3 weeks prior to administration of test solution. In order to achieve a more uniform distribution of Shh, two stereotactically guided injections of 5 ul each were made at distinct sites within the substantia nigra. With the incisor bar positioned to -2.3 mm, the injection coordinates based on bregma (Paxinos and Watson, 1986) were as follows: Site 1 : AP -5.6 mm, ML -1.87, DV -7.3 mm; site 2: AP - 5.6mm, ML -2.5 mm, DV -6.8 mm. Incisions were closed using stainless steel wound clips, and the animals were allowed to recover for 1 week before subsequent behavioral testing. Figures 9 and 10 illustrate the restorative activity of Shh on apomoφhine-induced lesioned rats. Example 3: Neuroprotective paradigm - simultaneous administration of 6-OHDA and Shh. This set of experiments analyzed protection from toxin-induced asymmetry of the nigrostriatal system as examined by amphetamine induced rotational behavior. Briefly, at day 0, animals were injected with 6-OHDA unilaterally into their medial forebrain bundle at coordinates -2.8 mm from bregma, 2 mm lateral to the midline and 8.6 mm below the skull. Similarly, the animals were injected with Shh (N-terminal fragment, unmodified E. coli recombinant, 10 μg) or vehicle unilaterally into SNc/VTA at coordinates - 4.8 mm from bregma, 1.4 mm lateral to the midline, and 8.2 mm below the skull. Coordinates according to the atlas of Paxinos and Watson (1986). At day 4, the animals were challenged with amphetamines and ipsiversive rotations were measured. Figures 11A and 1 IB demonstrate the neuroprotective activity of Shh. Example 4: Further animal studies of neuroprotective effect of Shh. As with example 3, this set of experiments analyzed protection from toxin-induced asymmetry of the nigrostriatal system as examined by amphetamine induced rotational behavior. At Day 0, 6-OHDA was unilaterally injected into the medial forebrain bundle at coordinates -2.8mm from bregma, 2mm lateral to the midline and 8.6mm below the skull. The animals were also injected, into the lateral SNc at coordinates -4.8mm from bregma, 1.4mm lateral to the midline and 8.2mm below the skull, with vehicle ("X-treated"), or with one of two doses of Shh (N-terminal fragment, unmodified E. coli recombinant), 30 μg ("Y- treated") or 10 μg ("Z-treated"). Coordinates according to the atlas of Paxinos and Watson (1986). At days 4, 7, 14 and 21, the animals were challenged with amphetamines and ipsiversive rotations were measured. The animals were also challenged on day 24 with apomoφhine. Figures 14A-E demonstrate the dose-dependent neuroprotective activity of Shh. Example 5: Neuroprotective Activity of lipophilic-modified Shh. Utilizing the protocol of Example 4, animals were treated with either vehicle ("C- treated"), or 76ng ("A-treated") or 760ng ("B-treated") of a myristoylated N-terminal fragment of Shh. At days 4, 7, 14 and 21, the animals were challenged with amphetamines and ipsiversive rotations were measured. The animals were also challenged on day 24 with apomoφhine. Figures 15A-H demonstrate the dose-dependent neuroprotective activity of Shh. Figure 16 is a collective graph of the data, specifically the ipsilateral turns over 21 days after 6-OHDA treatment, and indicates that myristoylated Shh protects against 6-OHDA lesions. Example 6: Restoration of function paradigm - comparison of myristoylated and unmyristoylated Shh. These experiments compared reduction in toxin-induced asymmetry of the nigrstriatal system, as examined by apomoφhine and amphetamine induced rotational behaviors, for various forms of hedgehog polypeptides, namely lipid-modified and unmodified forms of the proteins. See, e.g., Pepinsky et al. (1998) JBC 273: 14037 for a discussion of lipid-modified hedgehog polypeptides. Animals were prepared in a similar fashion to the 6-OHDA lesioned rats of Example 2. Briefly, each rat was given a dose of desipramine HC1 (25 mg/kg i.p.) and pargyline (50 mg/kg i.p.), and anesthetized with isoflurane in oxygen. After shaving the scalp, the head was fixed in a stereotaxic frame according to the atlas of Paxinos and watson (1986). A midline longitudinal incision was made, and the skin flap retracted to reveal the surface of the skull. With the aid of an operating microscope, a small burr-hole was drilled overlaying the left medial forevrain bundle (stereotaxic coordinates: AP +5.2 mm from interaural line (or AP -3.8 mm from bregma); L 1.0 mm). The tip of an injection cannula (backfilled with injectate) was lowered to a depth of 8.0 mm below the surface of the brain so that the tip is located in the medial forebrain bundle, and was left in place for 5 min before the injection. 6- Hydroxydopamine hydrobromide (1.5 or 6 μg in 2 μl) was injected over 5 min, with the carmula left in place for a further 5 min. After remocal of the carmula the burr-hole was sealed bone wax, and the scalp wound closed with sutures. Apomorphine (Contrlateral) Circling Tests (Days 14, 22, 30 and 38) An 8-channel Rotometry System (Benwick Electronics, Wimblington, U.K.) was used for the circling tests. Apomoφhine HC1 and amphetamine HC1 were dissolved in sterile water for injection. For each test, rats were dosed with apomoφhine HC1 (0.3 mg/kg i.p.), placed in a swivel-harness in rotometry bowls (diamter 24 cm), and left in the bowls for 45 min. Clockwise and anticlockwise rotations were logged automatically. Amphetamine (Ipsilateral) Circling Tests (Days 15, 23, 31 and 39) The rats were placed in a rotometry bowl (as above) prior to injection of amphetamine HC1 (1 mg/kg i.p.). Immediately after injection, they were placed in the rotometer harness and left in the bowls for 120 min. Clockwise and anticlockwise rotations were logged automatically. Microinjection of Shh or Vehicle (Day 21) Each rat was anesthetized with halothane (1.3% in oxygen) and, after shaving the scalp, the head was fixed in a stereotaxic frame according to the atlas of Paxinos and Watson (1986). A longitudinal incision was made, offset to the left of the midline, and the skin flap retracted to reveal the surface of the skull. With the aid of an operating microscope, a small burr-hole was drilled overlying the left sunstantia nigra (stereotaxic coordinates AP -5.6 mm from bregma; L 2.0 mm). The tip of an injection cannula (backfilled with injectate) was lowered to a depth of 7.3 mm elow the brain surface so that the tip was located dorsal to the middle of the substantial nigra, and was left in place for 5 min before the injection. The injectate (1 μl of solution Gz or Mz) was injected over 5 min, and the cannula left in place for a further 5 min. After removal of the cannula, the burr hole was sealed with bone wax, the scalp wound closed with sutures, and the anesthesia discontinued. Figures 12 and 13 demonstrate an increased potency, as a restorative agent, of a myristoylated form of Shh (Mz) relative to the unmodified form of the protein (Gz). Example 7: Evaluating the Efficacy of Human Sonic Hedgehog polypeptide Constructs in a Rat Model of Huntington's Disease Injection of malonate, an inhibitor of the mitochondrial enzyme succinate dehydrogenase, into the rat striatum (the rodent equivalent of the primate caudate and putamen) causes degeneration of striatal medium spiny neurons. In humans, degeneration of medium spiny neurons in the caudate and putamen is the primary pathological feature of Huntington's disease. Thus, the malonate-induced striatal lesion in rats can be used as a model to test whether human hedgehog polypeptides can prevent the death of the neurons that degenerate in Huntington's disease. Sprague-Dawley rats are injected with various concentrations of hedgehog polypeptide in the striatum using stereotaxic techniques. Stereotaxic injections (2 il) are done under sodium pentobarbital anesthesia (40 mg/kg) and placed at the following coordinates: 0.7 mm anterior to bregma, 2.8 mm lateral to the midline and 5.5 mm ventral to the surface of the skull at bregma. At various times (usually 48 hr) after injection of hedgehog polypeptide, rats are anesthetized with isoflurane and given a stereotaxic injection of malonate (2 imols in 2 il) at the same coordinates in the striatum. Four days after malonate injection, rats are sacrificed and their brains removed for histological analysis. Coronal sections are cut through the striatum at a thickness of 25 im and stained for cytoehrome oxidase activity to distinguish lesioned from unlesioned tissue. The volume of the lesion in the striatum is measured using image analysis. The effect of sonic hedgehog polypeptide constructs given 48 hours before malonate is shown in Figure 17. Unmodified sonic hedgehog polypeptide (Shh), myristoylated Shh, Clll-Shh and octyl maleimide Shh all reduce lesion volume to a similar extent in this model. However, the hydrophobically modified proteins (myristoylated Shh, Clll-Shh and octyl maleimide Shh) show an increase in potency relative to the unmodified sonic hedgehog polypeptide. Figure 18 shows the time course for the effect of pretreatment with myristoylated Shh (0.5 ig in 2 il) in the malonate striatal lesion model. A statistically significant reduction in lesion volume is seen when myristoylated Shh is given 2 days before malonate, however myristoylated Shh has a greater effect when given either 3 or 4 days before malonate. There was not a statistically significant reduction in lesion volume when myristoylated Shh was given 1 day, 7 days or immediately (0 days) before malonate. Example 8: Evaluating the Efficacy of Hedgehog polypeptides in a primate model of Parkinson's Disease As illustrated by Figures 19A and 19B, we have observed that intranigral injection of myristoylated Sonic Hedgehog (Shh-M) reduces parkinsonian disability and increases the therapeutic effect of L-DOPA in MPTP-treated marmosets. Adult common marmosets were rendered parkinsonian by treatment with 1 -methyl-4-phenyl- 1,2,3, 6-tetrahydropyridine (MPTP; 0.5 mg/kg s.c. once daily for 5 days) during week 0. The extent of disability was scored weekly for the next 8 weeks using a visual marmoset disability scoring system that assessed alertness, checking, posture, balance, reactions, vocalization and motility (maximum disability score = 18). During week 10, marmosets were given a unilateral injection into the substantia nigra of either Shh-M (0.15 μg or 1.5 μg in 2 μl; n=6) or vehicle solution (2 μl; control group; n=5). Disability was monitored weekly for the next 4 weeks, the intranigral injection was repeated, and disability was monitored weekly for 4 more weeks. Figure 19A shows that Shh-M-treated marmosets exhibited a consistent reduction in disability relative to vehicle-treated marmosets. During weeks 21, all animals were given a low dose of the antiparkinsonian drug L-DOPA (25 mg/kg, p.o.), and disability was assessed every 10 min for 50 min. Figure 19B shows that, although both groups showed the same extent of disability prior to administration of L-DOPA, L-DOPA reduced disability to a greater extent in Shh-M-treated marmosets than in controls. Figure 19C compares the activity of hedgehog polypeptides with GDNF according to the protocols described above. Example 9: Evaluating the Protective Effect of Hedgehog polypeptides in a Rat model of Parkinson's Disease Intrastriatal injection of myristoylated Sonic Hedgehog (Shh-M) protects dopamine neurons from degeneration induced by 6-hydroxydopamine (6-OHDA). Rats were injected with either Shh-M (0.18 μg in 1.5 μl) or vehicle solution on days 1, 3, 5 and 8 through a cannula implanted in the striatum. On day 4, they were given an injection of 6-OHDA (25 μg in 1.5 μl) through the cannula. Rotational behavior in response to amphetamine (1.3 mg/kg, i.p.) was measured weekly for 4 weeks beginning 1 week after 6-OHDA. Rats were then sacrificed, and dopamine neurons in sections through the substantia nigra were stained with an antibody to tyrosine hydroxylase (TH) and counted. The graph on the left shows that treatment with Shh-M reduced amphetamine-induced rotation in weeks 2-4. The graph on the right shows that treatment with Shh-M reduced the loss of dopamine neurons caused by 6-OHDA. All of the above-cited references and publications are hereby incoφorated by reference. Equivalents Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Claims: We claim: 1. A method for promoting survival and/or functional perfomance of neuronal cells susceptible to exotoxicity, comprising contacting the cells with an amount of a lipophilic modified hedgehog polypeptide effective to reduce exotoxin-mediated degradation of the cells. 2. A method for promoting survival of substantia nigra neuronal cells comprising contacting the cells with a trophic amount of a lipophilic modified hedgehog polypeptide. 3. A method for promoting survival of dopaminergic cells comprising contacting the cells with a trophic amount of a lipophilic modified hedgehog polypeptide. 4. A method for promoting survival of GABAergic cells comprising contacting the cells with a trophic amount of a lipophilic modified hedgehog polypeptide or a lipophilic modified hedgehog polypeptide. 5. A method for the treating a disorder characterized by loss of dopaminergic and/or GABAergic neurons which comprises administering to a patient in need thereof a therapeutically effective amount of lipophilic modified hedgehog polypeptide. 6. A method for the treating or preventing Parkinson's disease comprising administering to a patient in need thereof a therapeutically effective amount of lipophilic modified hedgehog polypeptide. 7. A method for the treating or preventing Huntington's disease comprising administering to a patient in need thereof a therapeutically effective amount of lipophilic modified hedgehog polypeptide. 8. A method for treatment or prophylaxis of a disorder selected from the group consisting of domoic acid poisoning; spinal cord trauma; hypoglycemia; mechanical trauma to the nervous system; senile dementia; Korsakoff s disease; schizophrenia; AIDS dementia, multi-infarct dementia; mood disorders; depression; chemical toxicity; neuronal damage associated with uncontrolled seizures, such as epileptic seizures; neuronal injury associated with HIV and AIDS; neurodegeneration associated with Down's syndrome; neuropathic pain syndrome; olivopontocerebral atrophy; amyotrophic lateral sclerosis; mitochondrial abnormalities; Alzheimer's disease; hepatic encephalopathy; Tourette's syndrome; schizophrenia; and drug addiction, comprising administering to a patient in need thereof a therapeutically effective amount of lipophilic modified hedgehog polypeptide. 9. The method of any of claims 1-8, wherein the hedgehog polypeptide is modified with one or more serol moieties. 10. The method of claim 9, wherein the sterol moiety is cholesterol. 11. The method of any of claims 1-8, wherein the hedgehog polypeptide is modified with one or more fatty acid moieties. 12. The method of claim 11, wherein each fatty acid moiety is independently selected from the group consisting of myristoyl, palmitoyl, stearoyl, and arachidoyl. 13. The method of any of claims 1-8, wherein the hedgehog polypeptide is modified with one or more aromatic hydrocarbons. 14. The method of claim 13, wherein each aromatic hydrocarbon is ondependently selected from the group consisting of benzene, perylene, phenanthrene, anthracene, naphthalene, pyrene, chrysene, and naphthacene. 15. The method of any of claims 1-8, wherein the hedgehog polypeptide is odified one or more times with a C7 - C30 alkyl or cycloalkyl. 16. The method of any of claims 5-8, wherein patient is being treated prophylactically. 17. A therapeutic preparation of a lipophilic modified hedgehog polypeptide provided in a pharmaceutically acceptable carrier and in an amount sufficient to promote survival of dopaminergic cells in a mammal. 18. The preparation of claim 17, wherein the lipophilic modified hedgehog polypeptide is provided in an amount sufficient to produce sufficient to promote survival of dopaminergic cells in a mammal treated with MPTP at 1 mg/kg. 19. The preparation of claim 18, wherein the patched antagonist is provided in an amount sufficient to produce sufficient to promote survival of dopaminergic cells in a mammal treated with MPTP at lOmg/kg. 20. A method for limiting damage to neuronal cells by Parkinsonian conditions, comprising administering to a patient a lipophilic modified hedgehog polypeptide. 21. A therapeutic preparation of a lipophilic modified hedgehog polypeptide provided in a pharmaceutically acceptable carrier and in an amount sufficient to promote survival of, or enhance functional performance of exotoxin-sensitive neuronal cells in a mammal.

Inventors: GALDES ALPHONSE, MAHANTHAPPA NAGESH, GALDES, ALPHONSE, MAHANTHAPPA, NAGESH
Applicants: BIOGEN INC, GALDES ALPHONSE, MAHANTHAPPA NAGESH, ONTOGENY INC, BIOGEN, INC., GALDES, ALPHONSE, MAHANTHAPPA, NAGESH, ONTOGENY, INC.
Application year: 1999
Main classification: C07K14
IPC codes: A61K38, A61K45, A61P3, A61P25, A61P31, A61P39, A61P43, C12N5