PROAPOPTOTIC PEPTIDES, DEPENDENCE POLYPEPTIDES AND METHODS OF USE

Abstract: The present invention provides substantially pure proapoptotic dependence peptides. The peptides consist substantially of the sequence of an active dependence domain selected from the group of dependence polypeptides consisting of p75<NTR>, androgen receptor, DCC, huntingtin polypeptide, Machado-Joseph disease gene product, SCA1, SCA2, SCA6 and atrophin-1 polypeptide. Substantially pure proapoptotic dependence peptides include SATLDALLAALRRI (SEQ ID NO:3), Q14 (SEQ ID NO:7), SATLDALLAALGGI (SEQ ID NO:4), SATLDALLAALRGI (SEQ ID NO:5), SATLQALLAALRRI (SEQ ID NO:6), tat-GG-SATLDALLAALRRI (SEQ ID NO:37) and tat-GG-Q14 (SEQ ID NO:36). The invention also provides a method of increasing cell survival. The method consists of inhibiting the function of an active proapoptotic dependence domain. A method of increasing cell survival consisting of preventing or reducing the rate of formation of an active proapoptotic dependence domain is also provided. The invention further provides a method of identifying compounds which prevent or inhibit apoptosis. The method consists essentially of administering a test compound to a cell undergoing dependence domain mediated apoptosis, and determining whether the compound increases cell survival. A method of reducing the severity of a proapoptotic dependence domain mediated pathological condition is also provided. The method consists of inhibiting the function of an active dependence domain. Additionally provided is a method of reducing the severity of a pathological condition mediated by unregulated cell growth. The method consists of cytoplasmically administering a proapoptotic dependence peptide.

Description: PROAPOPTOTIC PEPTIDES. DEPENDENCE POLYPEPTIDES AND METHODS OF USE This invention was made with government support under grant number CA69381 awarded by the National Institutes of Health. The United States Government has certain rights in this invention. BACKGROUND OF THE INVENTION This invention relates to negative signal transduction and cell death signaling and, more specifically to the particular amino acid sequences and structures which directly mediate cell death through negative signaling. Apoptosis is a normal physiological process of cell death that plays a critical role in the regulation of tissue homeostasis by ensuring that the rate of new cell accumulation produced by cell division is offset by a commensurate rate of cell loss due to death. It has now become clear that disturbances in apoptosis, also referred to as physiological cell death or programmed cell death, that prevent or delay normal cell turnover can be just as important to the pathogenesis of diseases as are known abnormalities in the regulation of proliferation and the cell cycle. Like cell division, which is controlled through complex interactions between cell cycle regulatory proteins, apoptosis is similarly regulated under normal circumstances by the interaction of gene products that either induce or inhibit cell death. The stimuli which regulate the function of these apoptotic gene products include both extracellular and intracellular signals. Either the presence or the removal of a particular stimulus can be sufficient to evoke a positive or negative apoptotic signal. For example, physiological stimuli that prevent or inhibit apoptosis include, for example, growth factors, extracellular matrix, CD40 ligand, viral gene products, zinc, estrogen and androgens . In contrast, stimuli which promote apoptosis include growth factors such as tumor necrosis factor (TNF) , Fas, and transforming growth factor β (TGFβ) , growth factor withdrawal, loss of extracellular matrix attachment, intracellular calcium and glucocorticoids, for example. Other stimuli, including those of environmental and pathogenetic origins, also exist which can either induce or inhibit programmed cell death. Although apoptosis is mediated by diverse signals and complex interactions of cellular gene products, the results of these interactions is thought to feed into a cell death pathway that is evolutionarily conserved between humans, other mammals and invertebrates . Several gene products which modulate the apoptotic process have now been identified. These gene products include cell survival polypeptides such as Bcl-2, cell death polypeptides such as Bax, and cysteine aspartate proteases (caspases) . The interaction and regulation of these gene products with cell surface or cytoplasmic receptors which transduce cell survival or death signals from outside the cell is as yet fairly uncharacterized. Additionally, it is unclear as to how many other genes exist which participate in apoptosis or what role they may play in the programmed cell death pathway. Finally, it also is unclear what the physiological control mechanisms are which regulate programmed cell death or how the cell death pathways interact with other physiological processes within the organism. Thus, there exists a need for the elucidation of cell death pathways and the identification of novel molecular components which mediate apoptosis. Such molecular components can be used for the treatment or diagnosis of cell death mediated diseases. The present invention satisfies this need and provides related advantages as well. SUMMARY OF THE INVENTION The present invention provides substantially pure proapoptotic dependence peptides. The peptides consist substantially of the sequence of an active dependence domain selected from the group of dependence polypeptides consisting of p75 NTR , androgen receptor, DCC, huntingtin polypeptide, Machado-Joseph disease gene product, SCAl, SCA2, SCA6 and atrophin-1 polypeptide. Substantially pure proapoptotic dependence peptides include SATLDALLAALRRI (SEQ ID NO:3), Q14 (SEQ ID NO:7), SATLDALLAALGGI (SEQ ID NO:4), SATLDALLAALRGI (SEQ ID NO: 5), SATLQALLAALRRI (SEQ ID NO: 6), tat-GG-SATLDALLAALRRI (SEQ ID NO: 37) and tat-GG-Q14 (SEQ ID NO: 36) . The invention also provide a method of increasing cell survival. The method consists of inhibiting the function of an active proapoptotic dependence domain. A method of increasing cell survival consisting of preventing or reducing the rate of formation of an active proapoptotic dependence domain is also provided. The invention further provides a method of identifying compounds which prevent or inhibit apoptosis. The method consists essentially of administering a test compound to a cell undergoing dependence domain mediated apoptosis, and determining whether the compound increases cell survival. A method of reducing the severity of a proapoptotic dependence domain mediated pathological condition is also provided. The method consists of inhibiting the function of an active dependence domain. Additionally provided is a method of reducing the severity of a pathological condition mediated by unregulated cell growth. The method consists of cytoplasmically administering a proapoptotic dependence peptide. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the ability of p75 TR , p75 NTR variants and p75 NTR /TNFR I chimeras to stimulate apoptosis. Figure 2 shows the ability of a proapoptotic dependence peptide and related peptides to stimulate apoptosis . Figure 3 shows that the stimulation of apoptosis by proapoptotic dependence peptides is accompanied by mitochondrial swelling (A) , cytochrome c release (B) , and caspase-3 cleavage (C) . DETAILED DESCRIPTION OF THE INVENTION This invention is directed to proapoptotic peptides, which are capable of inducing cell death, and methods of using proapoptotic peptides. The proapoptotic peptides, also termed proapoptotic dependence peptides, are generally derived from negative signaling polypeptides or other molecules participating in cell death. Negative signaling polypeptides induce cell death when these polypeptides fail to interact with their respective ligands or are otherwise activated by some form of structural alteration. The proapoptotic dependence peptides of the invention are advantageous in that they can directly mediate cellular apoptosis. Thus, the peptides are useful for the treatment of various pathological conditions characterized by unregulated cell growth or survival such as cancer, autoimmune and fibrotic disorders. Moreover, proapoptotic dependence peptides derived from negative signaling polypeptides are advantageous in that they can be used for the identification of compounds which inhibit cell death mediated by negative signaling polypeptides. In one embodiment, the invention is directed to a proapoptotic dependence peptide derived from or modeled after the dependence polypeptide p75 NTR (SEQ ID NO:2). The neurotrophin receptor, or p75 NTR , is a negative signaling polypeptide that mediates apoptosis, neuronal atrophy and decreased neurite outgrowth in the absence of bound neurotrophin. The presence of the neurotrophin receptor p75 TR therefore creates a state of dependence on neurotrophin for the survival of neuronal cells. It is a region of the cytoplasmic domain of p75 NTR , the proapoptotic dependence domain, that directly induces apoptosis in the absence of neurotrophin. The region within the cytoplasmic domain which confers this dependent state and exhibits proapoptotic activity is a region of about fourteen amino acid residues having the sequence SATLDALLAALRRI (SEQ ID NO:3). In another embodiment, the invention is directed to proapoptotic dependence peptides derived from or modeled after other dependence polypeptides such as the androgen receptor (SEQ ID N0:11), the Machado-Joseph disease polypeptide (SEQ ID N0:13), the huntingtin polypeptide (SEQ ID N0:15), and the SCA1 (SEQ ID N0:17), SCA2 (SEQ ID N0:19), SCA6 (SEQ ID N0:21) and atrophin-1 (DRPLA; SEQ ID NO: 23) polypeptides. These dependence polypeptides contain a polyglutamine sequence of variable length that when synthesized as a peptide exhibits proapoptotic activity that directly induces programmed cell death when introduced or expressed intracellularly . The region of the dependence polypeptide that confers this dependent state and exhibits proapoptotic activity is a polyglutamine region of about fourteen amino acids having the sequence QQQQQQQQQQQQQQ (SEQ ID NO: 7) . The invention is also directed to proapoptotic dependence peptides in which the polyglutamine sequence region is between about 6 to 100 amino acid residues, sometimes about 200 amino acid residues, generally about 14 to 40 amino acids. As used herein, the term "proapoptotic" refers to a peptide that is capable in itself of inducing apoptosis or programmed cell death when expressed or introduced intracellularly. The induction of apoptosis by proapoptotic peptides does not depend upon normal physiological stimuli such as the absence of growth or survival factors, or the presence of cell death stimuli. Although proapoptotic dependence peptides function in the absence of physiological stimuli, these peptides can additionally increase the rate or extent of apoptosis when expressed or introduced into a cell which has been induced to undergo apoptosis by such physiological stimuli. Proapoptotic dependence peptides can also induce apoptosis at different rates, and at different points of the cell cycle, depending on the nature of the 7 peptide and the cells in which the dependence peptide is expressed. As used herein, the term "dependence domain" when used in reference to a dependence polypeptide is intended to mean the portion or domain of a dependence polypeptide which can be induced to stimulate apoptosis. Dependence domains can exist in a range of apoptotically active states or be in an inactive state in the dependence polypeptide. To stimulate apoptosis, a dependence domain is induced to the apoptotically active state and, once induced, the dependence domain can directly stimulate apoptosis. A dependence domain can be induced to an apoptotically active state by a conformational change of a dependence polypeptide or a structural change mediated by altered or induced processing of the dependence polypeptide. A dependence domain therefore requires the induction of a conformational or structural change within the larger dependence polypeptide to enable its interaction with a component of the cellular apoptotic machinery to stimulate apoptosis. Conformational or structural changes can occur, for example, by the removal of a growth or survival factor from a dependence polypeptide which functions as a receptor for the growth or survival factor. In this situation removal of the growth factor ligand activates the dependence domain. Alternatively, addition of a ligand to a dependence polypeptide can induce a conformational or structural change which activates the dependence domain. Likewise, a dependence polypeptide other than a cell surface receptor, for example an intracellular protein, can undergo a conformational or structural change induced by binding to a ligand or dissociation from a ligand. A conformational or structural change also can be induced by processing of the dependence polypeptide. For example, proteolytic cleavage of the dependence polypeptide in vivo can liberate an apoptotically active dependence domain that is accessible to the cellular apoptotic machinery. Alternatively, cleavage of an apoptotically active dependence polypeptide can inactivate the proapoptotic activity of the dependence domain . A dependence domain also can be activated by association with another molecule, such as an effector molecule that induces a conformational or structural change upon a dependence domain. For example, a ligand other than a receptor agonist can bind to the dependence polypeptide and induce a conformational or structural change that activates the proapoptotic activity of the dependence domain. A conformational or .structural change also can be induced by an effector molecule that, for example, phosphorylates the dependence polypeptide. Specific examples of dependence domains include, for example, regions within the cytoplasmic domain of receptors which negatively signal cell death such as p75 NTR (neurotrophin receptor; SEQ ID NO:2), DCC (deleted in colonic carcinoma; SEQ ID NO: 25) and CD40 (SEQ ID NO: 27) . A dependence domain of p75 NTR contains, for example, the sequence SATLDALLAALRRI (SEQ ID NO: 3) . Other examples of dependence domains include the polyglutamine regions of the androgen receptor (SEQ ID NO:ll), the Machado-Joseph polypeptide (SEQ ID NO:13), the huntingtin polypeptide (SEQ ID NO: 15), the atrophin-1 polypeptide (SEQ ID NO:23), and the SCA1 (SEQ ID NO:17), SCA2 (SEQ ID NO: 19) and SCA6 (SEQ ID NO:21) polypeptides. Dependence domains are known to exist in other dependence polypeptides, and can be identified by those skilled in the art using the methods described herein. The size of the dependence domain can vary as they are contained within the parent dependence polypeptide. Such size differences are to be included within the meaning of the term so long as the dependence domain retains the ability to be induced to an apoptotically active state. As used herein, the term "active" or "apoptotically active" when used to describe the state of a dependence domain is intended to mean that the domain exhibits a conformation or structure which can directly induce or stimulate apoptosis. It is the occurrence of a conformational or structural change within a dependence polypeptide which yields an active dependence domain capable of stimulating apoptosis. For example, when used in reference to a dependence polypeptide which is a receptor for a cell survival or growth factor, such as p75 NTR , DCC or the estrogen receptor, the dependence domain of the receptor is active when the factor is removed from the receptor. In the particular example of p75 NTR , removal of a dependence domain from a larger inhibitory context, for example, from an inactive dependence polypeptide, similarly yields an active dependence domain that is capable of directly stimulating apoptosis. Additional examples of active dependence domains are regions of the cytoplasmic domains of unliganded receptors such as p75 NTR , DCC and CD40, an N-terminal apopain cleavage fragment of the huntingtin polypeptide (SEQ ID NOS:28-31), a polyglutamine region containing between about 10 to 25 glutamine residues (Q10; SEQ ID NO: 8 and Q25; SEQ ID NO: 9, for example) that 10 is a cleavage product of unliganded androgen receptor, and the polyglutamine regions from the Machado-Joseph, SCA1, SCA2, SCA6 and atrophin-1 polypeptides. Other examples of active dependence domains exist as well and are known or can be identified by those skilled in the art . As used herein, the term "dependence peptide" when used in reference to a proapoptotic peptide is intended to mean a peptide having substantially the same amino acid sequence, or functional equivalent or fragment thereof, as a dependence domain. A proapoptotic dependence peptide can directly stimulate apoptosis when expressed or introduced into a cell. A proapoptotic dependence peptide is therefore a constitutively active dependence domain, or functional fragment thereof, whose proapoptotic activity is independent of .a conformational or structural change. Dependence peptides can be as large or larger than the entire dependence domain or as small as 10 amino acids or less. Where the natural dependence polypeptide is known to be processed by a protease such as a caspase, the dependence peptide can be less than the naturally occurring processed polypeptide. A specific example of a proapoptotic dependence peptide is that derived from a dependence domain of p75 NTR having the sequence SATLDALLAALRRI (SEQ ID NO: 3) . Another example is the polyglutamine peptide Q14 (SEQ ID NO: 7) derived from a dependence domain of the androgen receptor, the Machado-Joseph polypeptide, the huntingtin polypeptide and the SCA1, SCA2 and atrophin-1 polypeptides. Additional examples include modified forms of a p75 NTR derived dependence peptide which have the sequences SATLDALLAALGGI (SEQ ID NO:4), SATLDALLAALRGI (SEQ ID N0:5) and SATLQALLAALRRI (SEQ ID NO:6). Thus, proapoptotic dependence peptides of the invention are 11 substantially pure proapoptotic peptides that are derived from or include dependence domains. It is intended that various lengths of polyglutamine-containing proapoptotic dependence peptides derived from or modeled after dependence polypeptides are within the scope of the invention. As used herein, the term "functional equivalent" is intended to mean a peptide that has proapoptotic activity and is modeled after or derived from a dependence peptide. Peptides modeled after or derived from dependence peptides refers to an amino acid sequence or chemical structure that is deduced or produced from the amino acid or encoding nucleotide sequence of the dependence peptide. Functionally equivalent dependence peptides can be identified as those that stimulate apoptosis when introduced or expressed in cells. Specific examples of such functionally equivalent dependence peptides are described further below in Example III. A functionally equivalent dependence peptide can have a relatively high or low apoptotic activity and can be essentially any sequence modeled after or derived from a dependence peptide so long as it induces apoptosis in one or more cell types. Functionally equivalent dependence peptides include those substituted at the level of the primary sequence, for example amino acid substitutions that include natural and nonnatural amino acids, such as penicillamine, and their derivatives or analogs, or those modified at the level of secondary structure, for example changes in cyclization mediated by disulfide bond formation. A functionally equivalent dependence peptide can be artificial, for example it can be engineered or be a chimera, or naturally occurring, for example it can be 12 obtained from a dependence domain or fragment thereof, or be a peptidomimetic . Furthermore, a functional equivalent can be phosphorylated or otherwise modified by the addition of lipid and carbohydrate chains. Such substitutions and modifications of the proapoptotic dependence peptide are to be included within the meaning of the term so long as the peptide stimulates apoptosis in one or more cell types. A "contingency peptide" as used herein, is intended to refer to a particular type of dependence peptide which corresponds substantially to the sequence of a natural in vivo proteolytic cleavage product or otherwise processed peptide or polypeptide that exhibits proapoptotic activity. Specific examples of contingency peptides include, for example, an amino-terminal apopain cleavage fragment of the huntingtin polypeptide (SEQ ID NOS:28-31) and the amino-terminal cleavage product of an unliganded androgen receptor (SEQ ID NO:32). It is noted that alternative cleavages can form different contingency peptides derived from the same dependence polypeptide. As the term proapoptotic dependence peptide is used in reference to the compositions of the invention, the definition of this term is intended to exclude those isolated naturally occurring peptides that are known to possess inherent proapoptotic activity in the native peptide. Specific examples of known isolated naturally occurring proapoptotic peptides are the wasp venom peptide toxin mastoparan and the β-amyloid peptide. The definition however explicitly does not exclude the use of any of such compositions in the methods of the invention. 13 As used herein, terms which reference specific dependence polypeptides, unless stated to the contrary, are intended to maintain the meaning of these terms as they are commonly referred to in the art. Moreover, the nucleotide and amino acid sequences of each of these polypeptides are similarly intended to be substantially that which is known in the art. For example, the nucleotide and predicted amino acid sequence of the following dependence polypeptides can be found published in, for example, P75 NTR (SEQ ID NO:l and SEQ ID N0:2; Johnson et al. Cell 47:545-554 (1986)), DCC (SEQ ID NO:24 and SEQ ID NO:25; Hedrick et al. Genes Dev. 8:1174-1183 (1994)), androgen receptor (SEQ ID NO:10 and SEQ ID NO:ll; Chang et al . Proc. Natl Acad. Sci USA 85:7211-7215 (1988)), estrogen receptor (SEQ ID NO:34 and SEQ ID NO:35; Greene et al . Science 231:1150-1154 (1986)), huntingtin (SEQ ID NO: 14 and SEQ ID NO: 15; Trottier et al. Nat. Genet. 10:104-110 (1995)); Ambrose et al . Somat. Cell. Mol. Genet. 20:27-38 (1994)), CD40 (SEQ ID NO:26 and SEQ ID NO:27; Stamenkovic et al . EMBO J. 8:1403-1410 (1989)), SCA1 (SEQ ID NO:16 and SEQ ID NO:17; Banfi et al. Nat. Genet. 7:513-519 (1994)), SCA2 (SEQ ID NO:18 and SEQ ID NO:19; Sanpei et al . Nat. Genet. 14:277-291 (1996)), SCA6 (SEQ ID NO:20 and SEQ ID NO:21; Zhuchenko et al. Nat. Genet. 15:62-69 (1997)), atrophin-1 (SEQ ID NO: 22 and SEQ ID NO: 23; Onodera et al. Am. J. Hum. Genet. 57:1050-1060 (1995)) and Machado-Joseph disease (SEQ ID NO: 12 and SEQ ID NO: 13; Kawaguchi et al . Nat. Genet. 8:221-228 (1994)). The sequences of the dependence polypeptides listed above are of human origin, however, it is noted that the sequences of the dependence polypeptides from other species are known and are intended to be included within the meaning of the term as used herein. Likewise, other dependence polypeptides are known or can be identified by those skilled in the art 14 and are intended to be included within the meaning of the term as used herein. As used herein, the term "peptide" when used in reference to the proapoptotic molecules of the invention is intended to mean any string of two or more amino acids covalently joined through a peptide bond. The proapoptotic peptides of the invention are generally less than about 250 residues, preferably the .proapoptotic peptides are less than about 100 amino acids, and more preferably the proapoptotic peptides are between about 5 and 50 amino acids in length. Specific dependence peptides exemplified herein have sizes of 14 amino acid residues. The peptides can be obtained by biochemical, recombinant or synthetic means known to those skilled in the art. The term similarly includes natural and nonnatural amino acids as well as functionally alternative forms such as derivatives, analogs and mimetics thereof so long as the peptide or alternate form maintains its activity to directly stimulate apoptosis. The synthesis, testing and function of such amino acid derivatives, analogs and mimetics is well known to those skilled in the art. As used herein, the term "heterologous functional domain" is intended to mean a non-proapoptotic domain that imparts a second function onto the proapoptotic peptides of the invention. For example, a heterologous functional domain can impart targeting capabilities or facilitate cell entry, enhance apoptosis, or modulate the proapoptotic activity of the dependence peptide. Heterologous functional domains can consist of peptide and polypeptide domains as well as other domains consisting of small organic and inorganic molecules, nucleic acids, carbohydrates, lipids and combinations 15 thereof. Heterologous functional domains also can include chemical moieties such as a drug. Specific examples of heterologous functional domains include ligands to cell surface proteins or domains that otherwise facilitate cell entry which therefore function to target the proapoptotic peptides to specific cells and tissues. The HIV tat protein is such a heterologous functional domain which facilitates cellular entry. Heterologous functional domains also include, for example, cytotoxic and cytostatic chemical moieties that enhance apoptosis, or those that regulate activity, for example, modular derepressible motifs such as the glucocorticoid receptor hormone binding domain. Additional examples of heterologous functional domains are known to those skilled in the art and are intended to be included within the meaning of the term so long as they impart a second function onto the proapoptotic peptides of the invention. As used herein, the term "ligand" is intended to mean a molecule or molecules that selectively interacts with another molecule. A ligand can consist of virtually any chemical structure and have any biological function so long as its interaction with another molecule is selective. Examples include, but are not limited to, a hormone receptor interacting with its hormone ligand, an enzyme interacting with a substrate, any protein-protein interaction such as an antibody interacting with an antigen, or a protein-lipid or protein-DNA interaction. The invention provides a substantially pure proapoptotic dependence peptide. The peptide consists essentially of the sequence of an active dependence domain selected from the group of dependence polypeptides 16 consisting of p75 NTR , androgen receptor, huntingtin polypeptide, Machado-Joseph polypeptide, SCAl, SCA2, SCA6 and atrophin-1 (DRPLA) polypeptide. Also provided are substantially pure proapoptotic dependence peptides consisting substantially of the amino acid sequence SATLDALLAALRRI (SEQ ID NO: 3), SATLDALLAALGGI (SEQ ID N0:4), SATLDALLAALRGI (SEQ ID N0:5) and SATLQALLAALRRI (SEQ ID N0:6), or functional equivalents thereof. A proapoptotic dependence peptide comprising a polyglutamine region or functional equivalent thereof is also provided. The cell surface neurotrophin receptor p75 NTR (SEQ ID NO: 2) is a negative cell signaling polypeptide that can be induced to stimulate apoptosis. For example, in the presence of bound neurotrophin or other ligand agonist, p75 NTR is apoptotically inactive whereas in the absence of neurotrophin, unliganded p75 NTR stimulates cellular apoptosis. Apoptosis is therefore mediated by a conformational or structural modulation of P75 NTR induced by ligand release. The conformational or structural modulation of p75 NTR can be inhibited by dimerization or multimerization with a different protein indicating that a monomeric form of p75 NTR is the active form which can stimulate apoptosis. A region of the cytoplasmic domain of p75 NTR that can mediate proapoptotic activity is included in an about fourteen amino acid region having substantially the sequence SATLDALLAALRRI (SEQ ID NO: 3) . When expressed or introduced into a cell, a peptide consisting essentially of the sequence SATLDALLAALRRI or functional equivalent thereof directly stimulates apoptosis. Thus, a region of p75 NTR which contains this sequence is a dependence domain and a peptide containing the sequence SATLDALLAALRRI is a 17 proapoptotic dependence peptide. This proapoptotic sequence is conserved across species and the identical sequence is found to be expressed in the human and rat p75 NTR cytoplasmic domains. The proapoptotic peptide SATLDALLAALRRI further exhibits an α-helical secondary structure . The cell surface DCC gene product (SEQ ID N0:25) also is a negative cell signaling polypeptide that can be induced to stimulate apoptosis. For example, in the presence of netrin or other ligand agonist, DCC is apoptotically inactive. The removal of netrin induces a conformational or structural change of the DCC receptor which results in a concomitant stimulation of apoptosis. A region of the amino-terminus of DCC (SEQ ID NO:33), which in intact cells is intracellular, can mediate proapoptotic activity of this dependence polypeptide. The intracellular androgen receptor, or AR (SEQ ID NO:ll), is another dependence polypeptide that can stimulate apoptosis. Apoptosis can be stimulated by the AR in response to a cell death signal. The apoptotic signal results in the induction of a structural or conformational change in the androgen receptor which stimulates the cell death pathway. One structural or conformational change that occurs in the AR is a proteolytic cleavage which liberates a contingency peptide of about 154 amino acids (SEQ ID NO:32). It is this contingency peptide that is capable of stimulating apoptosis . In the above specific example, the contingency peptide released by caspase-3 mediated cleavage contains a dependence domain consisting of a polyglutamine containing sequence. A peptide containing this domain is 18 capable of directly stimulating apoptosis. The size of the polyglutamine domain ranges from about 11 to 66 amino acids and a peptide of about 14 polyglutamine amino acids when synthesized and introduced into cells (Q14; SEQ ID NO: 7) also can induce apoptosis. This Q14 peptide or other polyglutamine-containing peptides modeled after the AR dependence domain exhibits proapoptotic activity and is therefore a proapoptotic dependence peptide. Similarly, the cytoplasmic huntingtin polypeptide (SEQ ID NO: 15) is another dependence polypeptide that can be induced to stimulate apoptosis. Apoptosis can be stimulated by the huntingtin polypeptide in response to a cell death signal. As with the AR, the apoptotic signal induces a conformational or structural change in the huntingtin polypeptide which activates the cell death pathway. A particular type of structural or conformational change that occurs is a proteolytic cleavage which liberates a contingency peptide and thereby stimulates apoptosis. Apopain-mediated cleavage is one protease which can release an about 80 kDa contingency peptide which corresponds to an amino terminal peptide fragment of the huntingtin dependence polypeptide. The cleavage can occur at any of a cluster of four DXXD (SEQ ID NO: 68) apopain cleavage-recognition motifs that are present in the huntingtin polypeptide. These motifs include DSVD, DEED, DLND and DGTD (SEQ ID NOS: 69-72, respectively) and can be found at residues 510-513, 527-530, 549-552 and 586-589, respectively. (Goldberg et al . Nat. Genet. 13:442-449 (1996)). The 80 kDa contingency peptide derived from the huntingtin polypeptide includes a polyglutamine containing dependence domain. The number of polyglutamine residues within this domain can vary and 19 generally ranges from 7 to 28 amino acids in length but can exceed 36 amino acids in length. A peptide modeled after or derived from the polyglutamine-containing dependence domain of the huntingtin polypeptide exhibits substantially the same proapoptotic activity as the active dependence domain. Additionally,, a peptide having a polyglutamine sequence of any of the sizes exhibited by the huntingtin polypeptide also exhibits substantially the same proapoptotic activity as the active dependence domain. Therefore, a peptide containing a polyglutamine region of huntingtin is one proapoptotic dependence peptide provided by the invention. The intracellular Machado-Joseph polypeptide (SEQ ID NO: 13) is another dependence polypeptide that can be induced into an active proapoptotic state through a conformational or structural change within a dependence domain. As with the AR and the huntingtin polypeptide, the dependence domain within the polypeptide is a polyglutamine-containing region. This region is the carboxy-terminal region of the Machado-Joseph protein and contains from about 13 to 36 or up to about 68 to 79 glutamine amino acids. Peptides containing this polyglutamine region sequence function as proapoptotic dependence peptides. Moreover, peptides consisting of polyglutamine residues within any of these ranges exhibit proapoptotic activity. Therefore, a peptide modeled after or derived from the dependence domain or the polyglutamine containing region of this domain is another proapoptotic dependence peptide provided by the invention. Other dependence polypeptides which contain dependence domains that can be induced into an active state also are known to exist. These other polypeptides 20 include, for example, the polypeptides encoded by the SCAl, SCA2, SCA6, atrophin-1 and CD40 genes. In particular, the SCAl, SCA2, SCA6 and atrophin-1 polypeptides include at least a polyglutamine-containing dependence domain similar to that previously described. A peptide modeled after or derived from the polyglutamine-containing dependence domain from any of these gene products induces apoptosis and is therefore a proapoptotic dependence peptide. A peptide containing a polyglutamine sequence within any of these polypeptides will similarly induce apoptosis and is therefore a proapoptotic dependence peptide. Thus, the invention provides proapoptotic dependence peptides selected from the group of dependence polypeptides SCAl, SCA2, SCA6 and atrophin-1. The invention further provides proapoptotic dependence peptides consisting of a polyglutamine sequence. The polyglutamine sequence can be a variety of lengths so long as the peptide maintains its activity to induce apoptosis. The lengths of such polyglutamine containing dependence peptides can be from about 6 to 100 amino acid residues, sometimes up to about 250 amino acids. Preferably the length is about 10 to 100 amino acids, more preferably about 14 to 40 amino acids. Therefore, the invention provides dependence peptides of less than or equal to 40 amino acid residues. Specific examples of dependence peptides that are derived from or modeled after dependence peptides are SATLDALLAALRRI (SEQ ID NO : 3 ) , SATLDALLAALGGI (SEQ ID NO: 4), SATLDALLAALRGI (SEQ ID NO: 5) and SATLQALLAALRRI (SEQ ID NO: 6). These peptides were identified by generating variants of the p75 NTR dependence peptide 21 SATLDALLAALRRI and then testing for those which exhibit apoptotic activity. Proapoptotic dependence peptides can be derived from or modeled after dependence domains . Dependence domains can exhibit a low- or non-apoptotic activity or alternatively, exhibit a moderate or high activity depending on the amino acid sequence of the domain and its conformational or structural state. In contrast, the activity of proapoptotic dependence peptides is independent of changes in conformation or structure and are therefore in a constitutively active state. Factors that contribute to conformational and structural changes resulting in a dependence domain having more or less apoptotic activity can include, for example, the degree of ligand association. Specifically, in the case of a negative signaling molecule, a high affinity ligand can associate with a dependence polypeptide for a longer period of time than a low affinity ligand. This association can result in a dependence domain that is in an apoptotically active state for a comparatively longer period of time which prolongs the accessibility of the active dependence domain to the apoptotic machinery thereby enhancing apoptosis. In a cell, the apoptotic activity of the dependence domain and therefore the induction of apoptosis also can be affected by the degree of ligand association with a dependence polypeptide that is intracellular. A dependence polypeptide also can exhibit different apoptotically active conformations and therefore different apoptotic activities by binding to a different ligand. For example, ligands with a similar 22 affinity can bind to different sites on a dependence polypeptide and induce a conformational change that is specific for that site. The site of ligand binding on a dependence polypeptide therefore determines a level of apoptotic activity of a dependence domain. Multiple ligand-binding sites of a dependence polypeptide can result in a dependence domain that is capable of having a broad range of apoptotic activity. Alternatively, a single binding site on a dependence polypeptide can bind to different ligands having different structures. The structure of a ligand also can control a conformation of a dependence polypeptide thereby determining the apoptotic activity of a dependence domain. Thus, the structure of a cell death or survival signal, such as a ligand, received by a dependence polypeptide can modulate its conformational state and therefore the proapoptotic activity of the dependence domain. In contrast, a contingency peptide of defined length produced by a structural change will likely contain a dependence domain that exhibits only a few variations in conformation that affect its apoptotic activity. Another way in which the activity of a dependence domain can vary or be modulated is through the reversal of the conformational change associated with dependence polypeptide activation. Such a reversal can occur by, for example, the removal of ligand or addition of an antagonist. However, the ability to prevent or reverse the apoptotic activity of the dependence domain and therefore apoptosis after formation of an active dependence domain will be affected by the type of change required for dependence domain activation as described below. 23 In a cell, the level of apoptotic activity exhibited by a dependence domain is determined by, in part, the amount of a proapoptotic dependence domain that accumulates. The amount of active dependence domain that is needed for the stimulation of apoptosis in cells can be as few as a single proapoptotic dependence domain molecule or significantly more, for example, 10,000 molecules or greater. The amount needed to stimulate apoptosis can be highly variable among cell types and is largely determined by the apoptotic machinery within a particular cell and the interaction or regulation of the proapoptotic dependence domain with that apoptotic machinery. Dependence polypeptides can be identified by a variety of methods known to those skilled in the art. Briefly, all that is required is to test for the induction of apoptosis following a conformational or structural change in a polypeptide that is mediated by a stimulus. Alternatively, those skilled in the art know or can determine if a particular stimulus induces programmed cell death and such stimuli can then be tested for the induction of a conformational or structural change in the polypeptide. Selection of the particular stimulus and corresponding polypeptide can be made by those skilled in the art based on current knowledge and accepted interpretations of experimental results known in the art. Proapoptotic polypeptides that undergo a structural or conformational change are potential candidates for the dependence polypeptides of the invention. Dependence polypeptides are identified as those polypeptides which yield proapoptotic peptides. 24 Selection of a polypeptide or stimulus to assess can be made by, for example, choosing molecules which are involved in programmed cell death or play a role in cell proliferation, differentiation, survival or growth. For example, receptors for cell regulatory factors can be tested for a change in conformation or structure of a domain and a concomitant induction of apoptosis in the presence or absence of ligand. Similarly, cytoplasmic or nuclear proteins can also be tested for a change in conformation or structure of a domain with a concomitant induction of apoptosis in the presence or absence of a stimulus. A specific example of such a cytoplasmic protein is where the stimulus is a growth factor. Other potential cellular dependence polypeptides include, for example, steroid hormone receptors, signal transduction molecules such as JAK, JNK and STAT, SH2 and SH3 containing proteins and a variety of transcription factors. Such molecules can all be tested in the presence or absence of a ligand or stimulus to determine the induction of a conformational or structural change which mediates apoptosis. A variety of methods exist for determining conformational or structural changes and the concomitant induction of apoptosis. For example, a selected molecule can be introduced or expressed in a cellular background which enables the determination of the functional properties of the polypeptide, ligand or stimulus. Using cell regulatory factor receptors as a specific example, such polypeptides can be expressed in apoptotically competent cells which normally do not express the receptors or in which the endogenous receptor can be selectively inhibited. 25 Cells that express or that are made to express, a candidate cell regulatory factor can then be tested for apoptosis in the presence or absence of the particular cell regulatory factor. Induction of apoptosis mediated through a change in conformation or structure of the receptor identifies that polypeptide as a potential candidate for a dependence polypeptide. ' Synthesis and testing for apoptotic activity of peptide fragments corresponding to different portions of the dependence polypeptide will confirm or refute that the potential candidate is a dependence polypeptide. Alternatively, dependence polypeptides can be identified by first selecting ligands or polypeptides that are known or predicted to play a role in cell growth, proliferation, differentiation or survival. Such ligands or polypeptides can be tested for their ability to induce a conformational or structural change in a cognate binding partner which can then mediate apoptosis. The identification of a cognate binding partner can be performed using methods well known to those skilled in the art. Such methods include, for example, affinity and immunoaffinity selection using ligands, antibodies and anti-idiotype antibodies, for example. Chromatography, affinity precipitation such as immunoaffinity precipitation, solid phase blotting procedures and panning methods are applicable for the identification of ligand or polypeptide binding partners. Numerous formats of such methods are known to those skilled in the art and can be used or modified according to the need and the particular type of binding partner to be identified. Additionally, biochemical purification methods and cloning procedures such as expression cloning with the ligand or polypeptide labeled so as to allow 26 detection of binding interactions. Alternatively, the binding partner can be determined by selection of cells from an expression library for survival or death in the presence or absence of the ligand or polypeptide. Dependence polypeptides also can be identified by hybridization techniques using nucleic acid probes that encode a polyglutamine containing sequence or other sequences such as SATLDALLAALRRI (SEQ ID N0:3), SATLDALLAALGGI (SEQ ID NO : 4 ) , SATLDALLAALRGI (SEQ ID NO: 5) or SATLQALLAALRRI (SEQ ID NO: 6) to screen a nucleic acid library. Probes derived from or modeled after nucleotide or amino acid sequences from other dependence domains or proapoptotic peptides can similarly be used to screen libraries for the identification of dependence polypeptides. Additionally, such nucleotide sequences can be used to search for similar or related sequences in EST and other databases. Dependence polypeptides also can be identified by having regions of amino acid sequence homology to known dependence domains. For example, polypeptides having a polyglutamine region equal to or greater than an about 6 amino acid residue sequence can be selected and tested for dependence polypeptide function. Similarly, polypeptides identified as having a region of homology to the SATLDALLAALRRI (SEQ ID NO: 3) dependence domain or modified forms of a dependence domain, SATLDALLAALGGI (SEQ ID NO:4), SATLDALLAALRGI (SEQ ID NO:5) or SATLQALLAALRRI (SEQ ID NO: 6) can be dependence polypeptides. These and other methods are well known to those skilled in the art and can be used to identify dependence polypeptides. 27 Conformational or structural changes can also be determined by a variety of methods known to those skilled in the art. For example, if there is a structural change such as the cleavage of a domain fragment from the intact polypeptide, such a cleavage can be assessed by assaying for the change in size of the intact polypeptide. Alternatively, such a cleavage can be assessed by assaying for the appearance of the cleaved fragment. Immunoaffinity and electrophoretic methods known to those skilled in the art are amenable for such determinations. Other well known methods also exist and can similarly be used to assess a change in structure of a candidate dependence polypeptide. Conformational changes can similarly be determined using a variety of methods known to those skilled in the art. For example, changes in conformation can be assessed by, for example, determining the binding of conformation-specific antibodies or other binding probes, construction and testing of methods known or predicted to influence conformational changes or stability of a polypeptide or by biophysical methods known in the art. Such biophysical methods include, for example, nuclear magnetic resonance, (NMR) and x-ray crystallography. In addition, the importance of a conformational change can be determined by altering its conformational state, for example, by examining the effect that multimerization with one or more additional proteins has on its apoptotic activity, as compared to the monomeric state. Testing of the dependence domain in a candidate dependence polypeptide can be performed by, for example, recombinantly modifying the suspected dependence domain in the candidate polypeptide and testing whether the 28 modified polypeptide maintains its ability to undergo a conformational or structural change with concomitant stimulation of apoptosis. Loss of dependence domain mediated apoptosis localizes the dependence domain to the modified sequences. Such modifications can be made by, for example, deletions, insertions or mutation of selected regions of sequences within the candidate polypeptide. Alternatively, testing of the dependence domain in a candidate dependence polypeptide can be performed by, for example, synthesizing the domain and determining if it directly induces apoptosis. Such peptides can be made by a variety of methods known to those skilled in the art. For example, peptides can be obtained from commercial vendors or be synthesized on an automated apparatus. Such chemical synthesis enables the introduction of nonnatural and derivatized amino acids as well as structural modifications thereof. Recombinant expression of a dependence domain encoding nucleic acid also can be used to produce large quantities of protein. Mammalian, yeast, bacterial and insect cell systems are examples of expression systems well known in the art which can be used to recombinantly produce proapoptotic dependence domain peptides. Such synthesized or recombinantly produced dependence domain peptides can then be introduced into cells to determine their ability to directly induce apoptosis. Alternatively, a nucleic acid which encodes the dependence domain portion of the candidate dependence polypeptide can be expressed in cells to determine if it directly induces apoptosis. Various expression systems are well known to those skilled in the art and can be used for constitutive or conditional expression of the 29 encoded dependence domain polypeptide. Such methods and modes of expression are described in, for example, Sambrook et al . Molecular Cloning: A Laboratory Manual, 2nd Ed, Vols 1 to 3, Cold Spring Harbor Laboratory Press, New York (1989) . Dependence domain peptides that directly induce apoptosis can be further analyzed to determine which portions, or the portion of the domain which is sufficient to induce cell death. All of such peptides can be considered to be proapoptotic dependence peptides. The analysis can be performed by, for example, producing successively smaller fragments of the domain to identify those regions, or an individual sequence which still exhibits apoptotic activity. Additionally, site-directed mutagenesis can be used to further define the portion of the domain or the amino acids that are required for the proapoptotic activity of the dependence peptides. In addition, randomly generated mutations of a nucleic acid encoding a proapoptotic dependence peptide combined with cell transfections and sequencing analysis of the peptides that have proapoptotic activity can collectively be used to formulate a consensus motif of a proapoptotic dependence peptide . The apoptotic activity of the dependence domains can be determined by a variety of methods known in the art. Such methods include, for example, induction of mitochondrial swelling, cytochrome c release and caspase-3 cleavage (Ellerby et al. J. Neurosci. 17:6165-6178 (1997)). Other methods known in the art exist and can similarly be used for determining the apoptotic activity of dependence polypeptides, domains or peptides . 30 The proapoptotic dependence peptides can be introduced into cells by methods well known to those skilled in the art. As described previously, a nucleic acid encoding a dependence peptide can be contained within a suitable expression vector, for example, a retroviral vector, and introduced into cells. The viral vector can have a natural or engineered cell tropism which can be used to facilitate cell entry or provide targeting. The use of such a tropic vector can enhance the transfection efficiency of cells. Proapoptotic dependence peptides themselves also can be introduced into cells by nonspecific endocytosis, or through the use of heterologous targeting domain. For example, in a particular embodiment described below, an HIV tat protein, when linked to a dependence peptide, facilitates cellular entry. Lipid carriers also can be used to introduce the nucleic acids encoding proapoptotic dependence peptides, or the peptide itself, directly into cells. Other methods of expressing or introducing proapoptotic dependence peptides into cells are known and can be used by those skilled in the art. The invention provides a proapoptotic dependence peptide that contains a heterologous functional domain. The invention also provides a heterologous functional domain consisting of a targeting domain or a domain which facilitates cellular entry. The invention additionally provides a heterologous functional domain consisting of a tat peptide. The invention also provides substantially pure proapoptotic dependence peptides having a sequence consisting of SATLDALLAALRRI (SEQ ID NO:3), tat-GG-SATLDALLAALRRI (SEQ ID NO:37), Q14 (SEQ ID NO:7) and tat-GG-Q14 (SEQ ID NO:36). Also provided are substantially pure proapoptotic dependence peptides having a sequence consisting of 31 SATLDALLAALGGI (SEQ ID NO:4), tat-GG-SATLDALLAALGGI (SEQ ID NO:38), SATLDALLAALRGI (SEQ ID NO:5), tat-GG-SATLDALLAALRGI (SEQ ID NO:39), SATLQALLAALRRI (SEQ ID NO: 6) and tat-GG-SATLQALLAALRRI (SEQ ID NO: 40) or functional equivalents thereof. The proapoptotic dependence peptides can be combined with one or more heterologous functional domains to impart distinct or complimentary functions onto the proapoptotic peptides of the invention. The distinct or complimentary function of the heterologous functional domain can provide targeting functions and additional apoptotic activity onto the proapoptotic peptides of the invention. Additionally, a heterologous functional domain can also function as a regulator of the apoptotic activity of the peptide, for example. A heterologous functional domain can consist of a domain that facilitates entry of a proapoptotic dependence peptide. One example of such a heterologous functional domain that facilitates entry into a cell is the HIV tat protein. This protein or functional equivalents thereof, when coupled to a proapoptotic dependence peptide increases the apoptotic activity of the peptide 30-fold compared to the peptide alone. Additional heterologous domains that provide a cell targeting function or facilitate cellular entry also are known to those skilled in the art. Such domains include, for example, ligands to extracellular proteins or receptors, ligands to other cell surface receptors, antibodies, a natural or engineered viral protein with a desired cell tropism, toxin subunits which facilitate toxin entry and functional fragments thereof. 32 A heterologous functional domain also can augment the cell death activity of the proapoptotic dependence peptide by linking one or more additional cell death or inhibitory activities onto the proapoptotic dependence peptide. Such cell death or inhibitory activities include, for example, domains which exhibit apoptotic, cytotoxic or cytostatic activity. Domains which exhibit apoptotic activity include, for example, ligands or agonists to receptors which induce programmed cell death. Fas ligands or anti-Fas antibodies are two specific examples of such apoptotic domains. A domain which activates caspase protease activity is another example of a heterologous functional domain which exhibits apoptotic activity. Domains which exhibit cytotoxic or cytostatic activity include, for example, toxins and chemotherapeutic agents such as doxorubicin, methotrexate, vincristine and cyclophosphamide can be conjugated to a dependence peptide. Other agents exist as well and are known to those skilled in the art and can be linked to proapoptotic peptides to augment their cell death function. Additionally, agents which enhance apoptosis through cell cycle regulation can be used as a heterologous functional domain. For example, genes that are required for cell proliferation or cell cycle progression can be inhibited by a heterologous domain that is an antisense nucleic acid of that gene. Cell cycle progression also can be inhibited by a negative regulator of the cell cycle, for example, a suppressor gene such as Rb or p53 or active fragment thereof. Such an inhibitor of cell cycle progression can enhance apoptosis in cells. 33 Alternatively, in other cell types, the apoptotic machinery can be, for example, more prevalent or more receptive to initiation by an active dependence domain in actively growing cells than cells in stationary phase. In these cells, stimulation of apoptosis by the dependence peptide can be enhanced by a heterologous domain that stimulates proliferation. A heterologous functional domain also can be a regulatable moiety that modulates the activity of a proapoptotic dependence peptide. When linked to a proapoptotic dependence peptide, a modular domain can impart ligand dependent activation or repression of its proapoptotic activity. For example, many different ligand-dependent transcription factors having inducible ligand-binding domains are known in the art. A heterologous functional domain also can provide a variety of other useful functions known to those skilled in the art. For example, it can be a lipid-based agent to facilitate cell entry, or an agent that increases or decreases the stability of the proapoptotic dependence peptide either intra- or extra-cellularly . A heterologous functional domain also can provide an imaging and/or visualization function which is mediated by an isotopic, colorimetric or fluorometric agent. Such an imaging function is useful for screening an expression library for interacting proteins, or for detecting or localizing apoptosis in vivo. A proapoptotic dependence peptide of the invention also can contain more than one heterologous functional domain. For example, a molecule containing a proapoptotic dependence domain attached to two or more 34 identical domains or moieties or attached to two or more different domains or moieties. An example of such a molecule containing two or more different domains is a dependence peptide attached to a cell targeting domain and a chemotherapeutic moiety. The exact chemical nature and structural organization of such a heterologous domain/dependence peptide construct will be known by those skilled in the art and can be determined based on the particular application. A heterologous functional domain can consist of a variety of different types of moieties ranging from small molecules to large macromolecules. Such moieties can be, for example, nucleic acid, polypeptide or peptide, carbohydrate, lipid, or small molecule compounds. Both natural and non-naturally occurring compounds and derivatives are similarly included. The invention further provides a method of increasing cell survival. The method consists of inhibiting the function of an active dependence domain. Dependence domain mediated pathological conditions which are characterized by abnormal or enhanced cellular apoptosis can be treated by inhibiting the function of an active dependence domain. Inhibition can be achieved by, for example, inhibiting the apoptotic stimulus which induces the change. Alternatively, inhibiting the structural or conformational change associated with the formation of an active dependence domain or inhibiting the activity of the active dependence domain or contingency peptide can inhibit the function of an active dependence domain. Depending on the apoptotic stimulus, a variety of different methods known in the art can be used to inhibit the stimulus and, 35 therefore, the induction of an active dependence domain. For example, if the apoptotic stimulus is removal of a cell growth or survival factor, addition of such a factor can be used to inhibit apoptosis. Alternatively, if the apoptotic stimulus is production of a cell death signal, removal of the signal can be used to inhibit apoptosis. Methods of inhibiting a conformational or structural change in dependence polypeptides are similarly well known in the art and will depend on the type of change sought to be inhibited. Such methods include direct inhibition of active dependence domain formation by, for example, binding a ligand or other specifically reactive molecule to the dependence domain so as to prevent activation or revert it to an inactive conformation. Multimerization of p75 NTR inhibits the change in conformation associated with apoptotic activation and can therefore similarly be employed as a direct method of inhibition. An indirect method for inhibition can be, for example, binding a ligand or specifically reactive molecule to an adjacent domain which allosterically inhibits the change in conformation. For the inhibition of a structural change such as a cleavage event which produces a contingency peptide, agents which bind to or near the cleavage site that mask its recognition motif can be used to prevent cleavage and formation of the apoptotic fragment. Alternatively, inhibitors of the protease which cleaves the dependence polypeptide can also be used to inhibit the structural change . Finally, pathological conditions mediated by dependence polypeptides activated by a conformational or structural change induced by proteolytic cleavage can be 36 treated by inhibiting an association between a contingency peptide and the cellular apoptotic machinery. Such methods are described in greater detail below and, as with those described above, are similarly well known to those skilled in the art. The invention further provides a method of increasing cell survival by inhibiting the function of an active dependence domain by selectively binding a ligand to a dependence polypeptide containing the active dependence domain. The activity of a dependence domain in dependence polypeptides can be inhibited by selectively binding a ligand to the dependence polypeptide so as to prevent negative signaling and apoptosis . Ligand binding can inhibit dependence domain function either indirectly or directly. For example, a ligand can bind to the dependence polypeptide and revert the dependence domain to an apoptotically inactive conformation. Alternatively, a ligand can bind, for example, to an active dependence domain and directly inhibit its interaction with a component of the apoptotic machinery. Similarly, in the case of a dependence polypeptide activated by a structural change, direct inhibition by ligand binding at or near the active dependence domain can prevent its interaction with a component of the cellular apoptotic machinery. For dependence polypeptides that are activated to their proapoptotic state by ligand binding, antagonists also can be used to inhibit the function of a dependence domain. An antagonist can be in excess of a ligand or exhibit a higher affinity than the ligand in order to displace it from a dependence polypeptide and 37 inhibit a conformational or structural change associated with dependence domain activation. Ligands that directly or indirectly inhibit the function of an active dependence domain can be identified and used by those skilled in the art. Such ligands can essentially be any compound or macromolecule . Combinatorial libraries of such molecules can be used to identify suitable ligands having a desired property. Once identified, those skilled in the art can determine by titration, for example, the amount to be used to inhibit the function of an active dependence domain to increase cell survival. It should be recognized that ligands, such as agonists, antagonists or those that directly inhibit interaction with the apoptotic machinery can have a high or low binding affinity. Those skilled in the art can select a ligand based on the characteristics desired and the particular application. The invention further provides a method of inhibiting the function of a dependence domain by inhibiting the association of an active dependence domain with an interacting molecule. Inhibitors of an association between an active dependence domain and the apoptotic machinery can include, for example, molecules that selectively bind to an active dependence domain as well as those that otherwise bind and inhibit the association. Such molecules that otherwise inhibit an association can do so by, for example, steric hinderence when bound adjacent to an active dependence domain. For example, a peptide domain or mimetic of an interacting component of the apoptotic machinery, can bind to a dependence domain and inhibit its association with the component of the apoptotic machinery to enhance cell survival. Such a mimetic can be derived from or modeled after an interacting component of the apoptotic machinery. Alternatively, an inhibitor of an association can selectively bind to a component of the apoptotic machinery, for example, a peptide domain or mimetic of an active dependence domain. Such a dependence domain mimetic would mimic binding to a component of the apoptotic machinery, but would not mimic induction of apoptosis. The binding of such a non-apoptotic dependence domain mimetic to a component of the apoptotic machinery can prevent an association between an active dependence domain and a component of apoptotic machinery. It is noted that inhibition of an association between an active dependence domain and a component of the apoptotic machinery does not require that the binding molecules described above be a peptide domain or mimetic. Rather, any molecule that can bind selectively to an active or inactive dependence domain or a component of the apoptotic machinery can inhibit the association of an active dependence domain with an interacting molecule. A method of identifying selectively-binding molecules that inhibit an association is further described below. In a similar fashion, a repressor molecule also can directly or indirectly inhibit an association between an active dependence domain and a component of the apoptotic machinery. For example, the ligand-bound neurotrophin receptor p75 NTR is apoptotically inactive and forms a homodimer that represses the activity of a dependence domain. In contrast, in the absence of neurotrophin, p75 NTR is monomeric and stimulates apoptosis. Thus, a repressor molecule that directly or 39 indirectly promotes p75 NTR homodimer or multimer formation can inhibit an association with the apoptotic machinery. Formation of homodimers or multimers also can be induced by, for example, phosphorylation or other post-translational modifications known to those skilled in the art. The invention provides a method of increasing cell survival by preventing or reducing the rate of formation of an active proapoptotic dependence domain. The invention provides a method of identifying compounds which prevent or inhibit apoptosis. The method consists of administering a test compound to a cell undergoing proapoptotic dependence domain mediated apoptosis and determining whether the compound increases cell survival. Further provided is a method wherein apoptosis is induced by unliganded p75 NTR . Identifying compounds useful for treating pathologies mediated by inappropriate or unregulated proapoptotic dependence domain mediated apoptosis, can be performed using cells that express a dependence polypeptide. The cells are administered a test compound under conditions which allow the induction of apoptosis. An increase in cell survival can be determined by assaying for the ability of the cells to remain viable, proliferate or by measuring other apoptotic determinants known in the art. Viability can be measured by, for example, trypan blue exclusion, whereas proliferation can be determined by, for example, tritium incorporation. In one embodiment, cells that express the P75 NTR neurotrophin receptor can be used to identify compounds that prevent or inhibit apoptosis. The cells can be 40 administered a test compound in the presence and absence of neurotrophin, and cells that survive or proliferate in the absence of neurotrophin can be counted and compared to control cells that were administered neurotrophin. A test compound that increases cell survival in the absence of neurotrophin can be further tested, for example, for the relative efficacy and the concentrations needed to inhibit apoptosis using titration experiments. The test compound also can be administered before, during, or after withdrawal of neurotrophin from the cells to determine the time of optimal efficacy. Such procedures are well known in the art and given the teachings provided herein, can be used to identify and optimize compounds which inhibit proapoptotic dependence domain mediated apoptosis. Additional cell-based assay systems using other dependence polypeptides and functional equivalents or fragments thereof can similarly identify compounds that increase cell survival by preventing or inhibiting proapoptotic dependence domain mediated apoptosis. For example, cells expressing a proapoptotic dependence peptide under the control of a regulatable promoter, such as an MMTV promoter, can be administered a test compound before, during, or after exposure of the cells to glucocorticoid hormone to determine if the test compound can increase cell survival in the presence of the stimulus which induces active dependence domain formation. Regulatable expression of a dependence peptide in cells is advantageous in that different dependence peptides can be expressed and test compounds administered. Test compounds found to increase cell survival can be tested against a variety of different dependence peptides to determine their range of efficacy. Compounds which display an ability to increase the 41 survival of cells expressing different dependence polypeptides or proapoptotic dependence peptides can be a broad spectrum inhibitor of apoptosis arid be useful in the therapeutic methods of the invention. Compounds that can be tested for their ability to increase cell survival can be small organic molecules, nucleic acids, carbohydrates, proteins or peptides, and mimetics or fragments thereof or combinations thereof. Large scale screening of combinatorial libraries of biologically active substances are known in the art and can be administered as test compounds. The test compounds can be added to the culture media and directly interact with cell surface dependence polypeptides or, if hydrophobic, can directly enter cells. Alternatively, in the event that the dependence polypeptide or functional equivalent is intracellular, a test compound can be conjugated to a targeting moiety, for example, the HIV tat protein, to facilitate cell entry. Incorporation of the test compound into liposomes is another method which can be used to facilitate cell entry. Those skilled in the art can readily determine the appropriate delivery method of a test compound depending on the particular system used. Apoptosis participates in the maintenance of tissue homeostasis in a number of physiological processes such as embryonic development, hematopoietic cell regulation and normal cell turnover. Recent advances indicate that dysfunction, or loss of regulated apoptosis, can lead to a variety of pathological disease states. For example, the loss of apoptosis in cells can lead to the pathological accumulation of self-reactive lymphocytes, virally infected cells, hyperproliferative cells such as neoplastic or tumor cells and cells that 42 contribute to fibrotic conditions. Inappropriate activation of apoptosis also can contribute to a variety of pathological disease states including, for example, acquired immunodeficiency syndrome (AIDS) , neurodegenerative diseases and ischemic injury. Treatments which are specifically designed to modulate the apoptotic pathways in these and other pathological conditions can alter the progression of -many of these diseases . The invention provides a method of reducing the severity of a proapoptotic dependence domain mediated pathological condition. The method consists of inhibiting the function of an active dependence domain. Further provided is a method of inhibiting the association of an active proapoptotic dependence domain with an interacting molecule. The invention also provides a method of reducing the severity of a dependence domain mediated pathological condition by inhibiting or reducing the rate of formation of an active proapoptotic dependence domain. Dependence domain mediated pathological conditions that are characterized by cells that exhibit aberrant increases in cell death can be treated by inhibiting the function of an active dependence domain. Dependence domain function can be inhibited by inhibiting the cell death stimulus which induces the conformational or structural change of a dependence polypeptide, as previously described. In addition, ligand agonists, antagonists and other inhibitory binding molecules can inhibit the conformation or structural change of a dependence polypeptide thereby reducing the severity of a dependence domain mediated pathological condition. Such ligands can revert a dependence polypeptide to an 43 apoptotically inactive state or directly or indirectly inhibit the function of the dependence domain by preventing its interaction with a component of the apoptotic machinery. The inhibition of apoptosis using these agents can reduce the severity of the dependence domain mediated pathology. Methods that inhibit or reduce dependence domain formation by inhibiting a conformational or structural change to increase cell survival have been described previously. Such methods also can be used to reduce the severity of a dependence domain mediated pathological condition. The severity of pathologies mediated by negative signaling dependence polypeptides can be reduced by administering a therapeutic ligand, such as an agonist, antagonist, protease inhibitor, or other binding inhibitor, as previously described, to inhibit or reduce the rate of formation of an active dependence domain. An individual exhibiting the pathology or an afflicted tissue can be administered such a ligand in a pharmaceutically acceptable carrier. Therapeutic ligands can enter the tissue by passive diffusion, or alternatively, by a delivery vehicle. A lipid-based vessicle is one example of a delivery vehicle that can be used to facilitate entry of a peptide molecule. Additionally, a targeting domain can be associated with the therapeutic ligand or a lipid vessicle carrier which contains the therapeutic ligand. Alternatively, a nucleic acid can encode a peptide or polypeptide therapeutic ligand which can be introduced and expressed into the appropriate cells or tissues by methods known in the art. Such compositions can be administered by intravenous 44 injection into the bloodstream or directly injected into the afflicted region. Dependence polypeptides containing polyglutamine sequence dependence domains have been identified as mediators of pathologies associated with abnormal induction of apoptosis. For example, a direct correlation exists between polyglutamine sequence expansion of a dependence polypeptide and clinical onset of a disease. In particular, expansion of a huntingtin polypeptide polyglutamine sequence beyond 36 amino acids is associated with Huntingtin' s disease (Macdonald et al . Cell 72:971-983 (1993)). Similarly, expansion of a polyglutamine sequence in AR from a normal range of about 11 to 33 to about 38 to 66 residues is associated with the manifestation of Spinal and Bulbar muscular atrophy (LaSpada et al . Nature 352:77-79(1991)). Furthermore, expansion of a polyglutamine dependence domain of atrophin-1, Machado-Joseph, SCAl, SCA2 and SCA6 is associated with a manifestation of the respective dentatorubropallidoluysian atrophy, Machado-Joseph disease, spinocerebellar ataxia type 1, spinocerebellar ataxia type 2 and spinocerebellar ataxia type 6 pathologies (Koide et al . Nat. Genet. 6:9-13(1994)); Kawaguchi et al . Nat. Genet. 8:221-228 (1994); Orr et al . Nat. Genet. 4:221-226 (1993); Sanpei et al . Nat. Genet. 14:277-284 (1996); Zhuchenko et al . Nat. Genet. 15:62-69 (1997) ) . Diseases characterized by abnormal levels of cellular dependence domain mediated apoptosis can be treated by using the previously described methods that inhibit dependence domain activation thereby altering the course of the disease. Such methods include, for example, inhibiting the apoptotic stimulus that induces a 45 conformational or structural change of a dependence polypeptide. Therapeutic ligands, antagonists and other inhibitory binding molecules can inhibit or prevent an association between an active dependence domain and a component of the apoptotic machinery or inhibit proteolytic cleavage and contingent peptide formation thereby alleviating the pathology. Such therapeutic ligands and binding inhibitors can be administered to a subject at the site of the pathology. Alternatively, a nucleic acid encoding an inhibitory peptide in a suitable expression vector, or an antisense nucleic acid derived from or modeled after a proapoptotic dependence domain can be contained in a lipid-based vessicle or a viral vector and can be administered to a subject to alleviate the pathology. Introduction of such therapeutic ligands, inhibitors and antisense molecules into a sufficient number of diseased cells can inhibit or decrease the rate of dependence-domain mediated apoptosis of these cells which can therefore alter the course of the pathology. Thus, the invention also provides a method of reducing the severity of a dependence domain-mediated pathological condition of Huntingtin' s disease, Alzheimer's disease, Kennedy's disease, Spinocerebellar atrophy, dentatorubropallidoluysian atrophy, Machado-Joseph disease, stroke and head trauma. The invention provides a method of reducing the severity of a pathological condition mediated by unregulated cell proliferation or cell survival consisting of cytoplasmically administering a proapoptotic dependence peptide. Further provided is a method of reducing the severity of a pathological condition consisting of neoplastic, malignant, autoimmune 46 or fibrotic conditions by cytoplasmically administering a proapoptotic dependence peptide. A proapoptotic dependence peptide can be administered into the afflicted region or regions characterized by unregulated cell growth or survival to reduce the severity of the pathological condition. Proapoptotic dependence peptides can include, for example, Q14 (SEQ ID N0:7), SATLDALLAALRRI (SEQ ID N0:3), SATLDALLAALRGI (SEQ ID NO: 5) or SATLQALLAALRRI (SEQ ID NO:6), or a functional equivalent or fragment thereof. If desired, a dependence peptide that exhibits relatively less apoptotic activity as compared to SATLDALLAALRRI, such as SATLDALLAALGGI (SEQ ID NO:4), can be administered into the afflicted region. The peptides can be introduced into the cell by, for example, a heterologous targeting domain or using a lipid based carrier. A formulation containing a proapoptotic dependence peptide that provides stability or resistance to serum proteases additionally can be used as well as other formulations known in the art. For the treatment of a neoplastic or fibrotic condition, the proapoptotic dependence peptide can be administered by direct injection into a solid tumor mass or into a region of fibrosis. Additional modes of administration are known and can be determined by those skilled in the art depending on the pathological condition to be treated. The invention further provides a method of reducing the severity of a pathological condition mediated by unregulated cell proliferation or cell survival by cytoplasmically administering a nucleic acid encoding a proapoptotic dependence peptide. 47 A nucleic acid encoding a proapoptotic dependence peptide or functional equivalent or fragment thereof can be delivered into an appropriate tissue to alleviate the severity of a pathological condition characterized by unregulated cell growth or survival. Expression of the nucleic acid can be provided by a constitutively active or regulatable promoter. For example, a tissue specific promoter can be used to restrict expression of a proapoptotic dependence peptide to those cells and tissues that characterize the pathology. A regulatable promoter can be used to control the induction of apoptosis or to restrict apoptosis to cells exposed to an inducer. Such vectors, promoters and expression constructs for nucleic acids are known to those skilled in the art. Viral vectors containing a natural or engineered envelope protein also can be used to target a nucleic acid encoding a proapoptotic dependence peptide to neoplastic, malignant or autoimmune tissues of cells expressing an appropriate cell surface protein. Thus, disorders characterized by cells that abnormally proliferate can be selectively targeted for apoptosis . It is understood that modifications which do not substantially affect the activity of the various embodiments of this invention are also included within the definition of the invention provided herein. Accordingly, the following examples are intended to illustrate but not limit the present invention. 48 EXAMPLE I Restoration of Neurotrophin Dependence and Negative Apoptotic Signaling in Prostate Carcinoma Cells This Example shows that the restoration of p75 NTR expression in prostate carcinoma cells confers neurotrophin dependence and negative apoptotic signaling. Prostrate carcinoma is characterized by a gradual decline in the level of p75 NTR expression from the development of benign prostatic hypertrophy to progression into metastatic carcinoma. Human PC3 prostate carcinoma cells do not express p75 NTR , nor are they neurotrophin dependent. To determine if p75 NTR expression confers a state of neurotrophin dependence in PC3 cells, p75 NTR was expressed in the PC3 cells and the viability of the transfected PC3 cells was determined in the presence and absence of neurotrophins . Briefly, PC3 prostate carcinoma cells were grown in DMEM/F12 (50/50) supplemented with 5% fetal bovine serum (FBS) and seeded at a density of 50% on 10 cm tissue culture dishes. For transfections, 10 μg of the pBabepuro-p75 NTR expression vector or ' insert-less pBabepuro plasmid DNA (Morgenstern and Land Nucl . Acids Res. 18:1068 (1990)) was added to 50 μl of the lipofection reagent DOTAP (Boehringer Mannheim Biochemicals, Indianapolis, IN) in a polystyrene tube, mixed, and the volume was adjusted to 500 μl with HBS (20 mM Hepes, 150 mM NaCl) . After 30 minutes, the DNA/lipofection solution was added directly to the PC3 cells. PC3 cell transfectants were selected by growing the cells in 5 μg/ml of puromycin. The cells also were incubated in the presence or absence of a 2 mM mixture of the following neurotrophins: nerve growth factor, 4 9 brain-derived neurotrophic factor, or neurotrophic factor 3. After puromycin selection and propagation of the transformed cells over the course of 15 to 18 days, the number of surviving cells were counted. The results indicate that in the absence of exogenous neurotrophins, the viability of the p75 NTR transfected PC3 cells was approximately 50 to 80% less than control cells transfected with the insert-less pBabepuro plasmid. In addition, the p75 NTR transfected PC3 cells incubated in 2 mM of neurotrophin exhibited a significant improvement in colony number. These results show that a state of neurotrophin dependence was created by expressing p75 NTR in PC3 cells. EXAMPLE II Identification of a Dependence Domain in p!5 mR This Example shows that the stimulation of apoptosis by p75 NTR can be mediated by a domain near the carboxy-terminus and that mutating a region similar to the Fas/Apo-1 and TNFR I death domains in p75 N R does not affect the apoptotic activity of p75 NTR . This Example also shows that multimerization of p75 NTR can inhibit proapoptotic activity. Expression constructs containing wild type p75 NTR , p75 NTR variants and p75 NTR /TNFR II chimeras were constructed and are shown in Figure 1. The P75 TR variants consisted of single point mutations, double point mutations, carboxy-terminal deletions and internal deletions. The p75 NTR /TNFR II chimeras consisted of the p75 NTR amino-terminal half fused to TNFR II carboxy-terminal half, ECρ75, and the TNFR II 50 amino-terminal half fused to the p75 NTR carboxy-terminal half, ECp70. Each construct was expressed in NRA5 mutant PC12 neural cells, which do not normally express p75 NTR , to determine the region of p75 NTR that confers neurotrophin dependence. The results are shown in Figure 1. Briefly, cloning of the wild type p75 NTR and the variant p75 NTR cDNAs into the pBabepuro mammalian expression vector was performed as described (Rabizadeh et al. Science 261:345-348 (1993)). p75 NTR variants containing single point mutations at positions 348, 359 and 370, in which glutamic acid was replaced with alanine (E348A) , tryptophan was replaced with glycine (W359G) and leucine was replaced with lysine (L370K) , were generated using the Altered Sites II in vi tro Mutagenesis System (Promega, Madison, WI ) with a single stranded template of p75 NTR cDNA. The primers used were 5 ' -CCTTTACCCACGCGGCCTGCCCAGT-3 ' (E348A; .SEQ ID NO:57), 5 ' -CTGCTGGCCAGCGGGGGTGCCCAG-3 ' (W359G; SEQ ID NO:58), and 5 ' -ACGCTTGATGCCAAATTAGCCGCCCTGCGA-3 ' (L370K; SEQ ID NO:59) . The p75 NTR carboxy-terminal deletion variants of 19 amino acids, p75ΔC19, and 33 amino acids, p75ΔC33, were generated by PCR amplification with the Pfu polymerase enzyme (Stratagene, La Jolla, CA) . The 5' PCR primer contains the unique Bam HI site located at 700 bp of the rat p75 cDNA and is 5 ' -ATGGATCCCAAGGTCTACGCC-3 ' (SEQ ID NO: 60). Both 3' PCR primers contained Sal I sites which introduce a stop codon following isoleucine 377 or asparagine 363, and are 5 ' -CGCTGGTCGACTAGATGCGTCGCAG-3 ' (SEQ ID 'NO: 61) for p75ΔC19 and 5 ' -CGCTGGTCGACTAGTCCTGGGCACC-3 ' (SEQ ID 51 NO: 62) for p75ΔC33. The pBabepuro-p75ΔC19 and pBabepuro-p75ΔC33 expression vectors were constructed by replacing the Bam Hi-Sal I fragment in pBabepuro-p75 with the corresponding PCR products. A third p75 NTR carboxy-terminal deletion variant of 38 amino acids, p75ΔC38, was produced by a partial Pvu II digestion of the p75 NTR cDNA in a pUC18 cloning plasmid. The construct was then digested with Xba I and the restriction sites were filled in with the Klenow fragment of DNA Polymerase I to generate blunt ends. The resulting 1.3 kb DNA fragment was agarose gel fractionated, purified and religated to create the pUC18-p75ΔC38 plasmid. The p75ΔC38 cDNA was then excised from this plasmid and cloned into the pBabepuro expression vector as described above. The p75 NTR variant Ml contained two point mutations in which both arginines at positions 375 and 376 were replaced with glycine. The p75 NTR variant M2 contained two point mutations in which both leucines at positions 370 and 371 were replaced with lysine and proline, respectively. The Ml and M2 variant p75 NTR cDNAs were generated from a pUC18-p75 plasmid by first removing a Bam Hl-Xba I fragment from the plasmid and then replacing it with two fragments generated by PCR amplification using Pfu. The first PCR product spanned from the Bam HI site within the p75 NTR open reading frame to a new Hind III site which contained the desired mutation. The second PCR product spanned from the same new Hind III site to the Xba I site in the pUCl8 plasmid. The PCR products were digested and ligated into the Bam HI and Xba I digested pUC18-p75 plasmid to generate a cDNA encoding the Ml or M2 variant p75 NTR . The oligonucleotides used to amplify the first PCR product were 5 ' -ATCCCTGGTCGATGGATCCCAA-3 ' (SEQ ID NO: 63), which 52 contained the Bam HI site, and 5 ' -TCTCTGGATCCCTCCCAGGGCG-3 ' (SEQ ID NO: 64) which contained the Hind III site and the Ml mutation, or 5 ' -CTGGATCCGTCGCAGGGCGGCTGGTTTGG-3 ' (SEQ ID NO: 65), which contained the Hind III site and the M2 mutation. For the second PCR product, the oligonucleotides were 5'-CTGCGACGGATCCAGAGAGCTG-3 * (SEQ ID NO:66), which contained the Hind III site and 5'-GCTCTAGAACATCAGTCGTCGGA-3' (SEQ ID NO:67), which contained the Xba I site. The p75 NTR internal deletion variant lacking a Fas/Apo-1 like region spanning amino acids 328 to 348 is denoted p75Δ328-48 and was constructed using a strategy similar to that described above. Briefly, PCR amplification was used to generate two fragments that flanked the desired deletion which contained either one of the restriction sites Bam HI or Xba I. After Bam HI or Xba I digestion, the two flanking sequence fragments were religated into a Bam HI and Xba I digested pUC18-p75 plasmid. The p75 TR internal deletion variant cDNA was excised from this plasmid and cloned into the pBabepuro expression vector as described above. The chimeric p75 NTR /TNFR II expression constructs were obtained from E. Shooter (constructed as described by Rovelli et al. Proc. Natl. Acad. Sci. USA 90:8717-8721 (1993)) and then subcloned into the pBabepuro expression vector. For the chimeric constructs, the gray regions indicate p75 NTR and the white regions indicate TNFR II and are shown in Figure 1. The nucleotide sequence of all constructs was confirmed by DNA sequencing. The expression of p75 NTR protein was detected by flow cytometry using monoclonal antibody 192, 53 and im unoblotting using anti-p75 antiserum (Promega, Madison, WI) . The FKBPl2-tagging vector MF1E/MF3E, which included an amino-terminal myristylation site for membrane insertion (Spencer et al . Science 262:1019-1024 (1993)), contains one and three repeats of the FK-binding protein (FKBP) sequence. The FKBP12 vector served as a PCR template and was amplified using primers flanked by Nhe I (5' primer) or Nde I (3' primer) sites to produce DNA fragments consisting of one or three FK-binding domains (FKBP) . The resulting PCR products contained either one or three FKBP sequence repeats and were subcloned into pcDNA3.1. A DNA fragment encoding an intracytoplasmic form of p75 NTR was removed from the pUC18-p75 plasmid by digestion with Nde I and Bam HI, and the DNA fragment was ligated to the carboxy-terminus of the FKBP sequences within the pcDNA3.1-FKBP construct. The resulting two expression vectors encoded FKBP/p75 NTR chimeras comprising one or three FKBP repeats at the amino-terminus fused to an intracytoplasmic form of p75 NTR at the carboxy-terminus . PC12 NRA5 cells were grown and maintained as described previously (Rabizadeh et al . Science 261:345-348 (1993)). For transfection, the cells were exposed to the cationic lipid DOTAP (Boehringer Mannheim Biochemicals, Indianapolis, IN) containing the particular p75 NTR expression vector using the manufacturer's protocol. To obtain stable transfectants, the cells were selected in 5 μg/ml puromycin, and pools of puromycin resistant cell transfectants were compared in the analysis (Zhong et al . Proc. Natl. Acad. Sci. USA 90:4533-4537 (1993)). The expression of p75 NTR protein in the transfected cells was detected by flow cytometry 54 using the monoclonal antibody 192 (Baldwin et al . J. Immunol . 267:8352-8359 (1992)). Cell death was quantitated by propidium iodide as previously described (Rabizadeh et al . Science 261:345-348 (1993) and Kane et al. J. Neurosci. Res. 40:269-275 (1995)). The results shown in Figure 1 indicate the percentage of cell death stimulated by particular p75 TR constructs after normalization to that stimulated by wild type p75 NTR . Each p75 NTR construct was analyzed in 3 to 7 separate transfections and the statistical significance was assessed by the two-tailed t-test with bars indicating standard error; p < 0.05 is indicated by *, and p < 0.01 by **. The asterisks over the constructs indicate mutation sites and the t symbol indicates mutants that induced cell death at least as effectively as P 75 NTR . The results indicate that wild type p75 NTR , p75WT, stimulates apoptosis and has an EC 50 of about 10-50 μm. In contrast, a p75 NTR /TNFR II chimeric protein having an amino-terminal p75 NTR portion fused to a carboxy-terminal TNFR II portion, ECp75, failed to stimulate apoptosis in NRA 5 cells whereas a TNFR II/p75 NTR chimeric protein having an amino-terminal TNFR II portion fused to a carboxy-terminal p75 NTR portion, ECp70, stimulated apoptosis in NRA 5 cells. These findings indicate that a proapoptotic dependence domain is located in a carboxy-terminal region of ρ75 NTR . Therefore, additional mutations within the carboxy-terminal region of p75 NTR were analyzed. 55 The effect of amino acid deletions at or near the carboxy-terminus of p75 NTR on the apoptotic activity was determined. Deletion of the carboxy-terminal 19 amino acids of p75 NTR , p75ΔC19, did not diminish the ability of this p75 TR variant to stimulate apoptosis; in fact, a slight increase in apoptosis was observed. However, extending the carboxy-terminal deletion an additional 14 residues for a total of 33 amino acids, p75ΔC33, abolished the ability of this p75 NTR variant to induce apoptosis in the absence of neurotrophin. The 14 amino acid internal near the carboxy-terminus sequence of p75 NTR that confers neurotrophin dependence lies just to the carboxyl side of a sequence region that exhibits sequence similarity to the Fas/Apo-1 and TNFR I death domains. This Fas/Apo-1 and TNFR I like region was tested for its ability to confer neurotrophin dependence in p75 NTR by deletion analysis and site directed mutagenesis. An internal deletion of 21 amino acids that removed the Fas/Apo-1 and TNFR I like sequence region, p75Δ328-48, did not inhibit the ability of this p75 N R variant to induce apoptosis. Similarly, point mutations of the native TNFR I protein which abolish TNFR I's ability to stimulate cellular apoptosis, when introduced into the Fas/Apo-1 and TNFR I like region of p75 NTR , had little or no effect on neurotrophin dependence. Specifically, point mutations in which the tryptophan at position 359 was replaced with glycine, p75W359G, or the glutamic acid "at position 369 was replaced with alanine, p75E348A, had little or no effect on the ability of these p75 NTR variants to stimulate apoptosis. Thus, a Fas/Apo-1 and TNFR like death domain located immediately to the aminyl side of 56 the 14 amino acid sequence region of p75 NTR is not required for the stimulation of apoptosis. To further confirm the importance of the 14 amino acid domain, p75 NTR variants containing single or double point mutations in the domain were analyzed for their ability to stimulate apoptosis. Specifically, replacing leucine with lysine at position 370 (L370K) of p75 NTR abolished proapoptotic activity. Similarly, replacing the two arginines with glycine at positions 375 and 376 in p75 NTR , p75Ml, or replacing the two leucines at positions 370 and 371 with lysine and proline in p75 NTR , respectively, p75M2, decreased the apoptotic activity. Specifically, the p75 NTR variants p75Ml and p75M2 exhibited a 75% and 60% decrease in the stimulation of apoptosis, respectively, in comparison to wild type p75 NTR . These results demonstrate the importance of particular amino acids within the 14 amino acid proapoptotic dependence domain of p75 NTR for the stimulation of apoptosis and further demonstrate that this domain confers neurotrophin dependence. The stimulation of cellular apoptosis by Fas and TNFR I is induced by ligand binding which triggers multimerization of Fas and TNFR I. The assembly of such a death-inducing signaling complex contributes to cellular apoptosis by activating caspase-8. The effect that dimerization or multimerization has on the ability of p75 NTR to stimulate apoptosis was analyzed. FKBP/p75 NTR protein chimeras containing one or three copies of an FKBP fused to an intracytoplasmic form of p75 TR were expressed in cells. Cross-linking studies indicated that FKBP expressed in cells could be induced to form dimers or multimers by exposing the cells to the FK1012 agent. 57 Therefore, a single copy FKBP/p75 NTR protein chimera expressed in cells could be induced to form a dimer in the presence of the FK1012 dimerizing agent. Expression of a triple copy FKBP/p75 NTR protein chimera in cells could be induced to form a multimer in the presence of FK1012. Briefly, 293T cells were grown and maintained in DMEM supplemented with 10% FBS at 37 °C and plated at a density of 5 x 10 5 cells into each well of a 6-well plate. The cells were transiently transfected with 5 μg of plasmid DNA containing either a single copy or triple copy of the FKBP cDNA fused to intracytoplasmic p75 NTR in the presence or absence of 2 μM FK1012 using the calcium phosphate method (Sambrook et al . Molecular Cloning: A Laboratory Manual Chapter 16 (1989)). After an 18 hour incubation, the cells were washed with DMEM and placed on DMEM supplemented with 3% FBS and 2 μM FK1012 as before. After an additional 18 hour incubation, transfected cells were placed on DMEM supplemented with 1.5% FBS, 2 μM FK1012 as before, and 35 μM tamoxifen to induce apoptosis . These studies indicated that expression of a monomeric intracytoplasmic form of p75 NTR in cells stimulates apoptosis. In contrast, apoptosis was blocked when cells containing the single copy or triple copy FKBP/p75 NTR protein chimera were exposed to FK1012. These results demonstrate that dimerization or multimerization of p75 NTR with a different protein can inhibit apoptosis and that a monomeric form of p75 NTR can stimulate apoptosis. 58 EXAMPLE III Induction of Cell Death with Proapoptotic Peptides This Example shows the induction of cell death by the p75 NTR dependence domain proapoptotic peptide SATLDALLAALRRI (SEQ ID NO: 3) and by the polyglutamine proapoptotic peptide Q14 (SEQ ID NO: 7) . A region of a dependence polypeptide that mediates apoptosis in cells was analyzed for its ability to stimulate apoptosis in cells. Various cell types were treated with peptide fragments modeled after a p75 NTR dependence domain SATLDALLAALRRI (blue; SEQ ID NO: 3, tat-blue; SEQ ID NO: 37) and the polyglutamine-containing dependence domains tat-GG-Q14 (SEQ ID NO: 36) . The effect of replacing leucine with lysine at position 7 (purple, SATLDAKLAALRRI; SEQ ID NO: 1; tat-purple, tat-GG-SATLDAKLAALRRI; SEQ ID NO:42), removing the carboxy-terminal "RRI" sequence (gray, SATLDALLAAL; SEQ ID NO: 43; tat-gray, tat-GG-SATLDALLAAL; SEQ ID NO: 44) or amino-terminal "SATLD" sequence (green; ALLAALRRI; SEQ ID NO: 45) on the proapoptotic activity of a dependence peptide was examined. Negative control peptides, for example, the helicity controls (turquoise, KDRNLRRITRMVLV; SEQ ID NO: 46; tat-turquoise, tat-GG-KDRNLRRITRMVLV; SEQ ID NO: 47 and red, LDENFKRCFREFCI; SEQ ID NO:48), scrambled sequence (tat-yellow, tat-GG-DLSLARLATARLAI; SEQ ID NO:50), and positive control peptides, for example, the mastoparan peptide (MP, INLKALAALAKKIL; SEQ ID NO: 51) also were examined. The 12 amino acid HIV tat protein fragment (GRKKRRQRRRPP; SEQ ID NO:52; hereinafter termed "tat"), which facilitates cellular entry, also was included on the amino terminus of some of the peptides tested. This HIV tat sequence did not affect the function of the 59 peptide to which it was linked, as shown below. For convenience, the hyphen in the above amino acid sequences is a nomenclature intended to set apart the proapoptotic dependence peptides and variants thereof or control peptides from other amino acid residues contained in the peptide. Briefly, NTera 2 human neuronal cells, R2 neural cells, CSM14.1 neural cells, LNCaP cells, SH-SY5Y human neuroblastoma cells and PC12 NRA5 cells were grown in DMEM/F12 (50/50) supplemented with 5% fetal bovine serum and seeded onto 96-well plates. The peptides were synthesized and HPLC purified (Coast Scientific, San Diego, CA) . The purified peptides were dissolved in tissue culture grade water and diluted to 50 μM and 100 μM in serum free medium and directly added to the cells in 96-well plates. The cells were incubated at 37°C for 18 hours and 20 μM propidium iodide was added. Cell viability was determined using a fluorimeter as previously described (Kane et al . J. Neurosci. Res. 40:269-275 (1995)). The presence of the dependence peptides lacking the tat sequence in cells was confirmed by confocal microscopy. The results of these studies shown in Table 1 reveal that cells treated with a SATLDALLAALRRI (blue; SEQ ID NO: 3) dependence peptide underwent apoptosis as did cells treated with the positive mastoparan peptide control (MP) . Similarly, an all D-enantiomer of the dependence peptide stimulated apoptosis. In contrast, cells treated with either helicity control peptide (turquoise or red) did not undergo apoptosis. The leucine to lysine point mutation at position 7 (purple) , the carboxy-terminal "RRI" (gray) and the amino-terminal "SATLD" (green) sequences were critical to the apoptotic 60 function of SATLDALLAALRRI; these forms of the dependence peptide were incapable of stimulating apoptosis. The proapoptotic dependence peptides containing the HIV tat sequence also stimulated apoptosis in cells. These studies indicated that tat-GG-SATLDALLAALRRI exhibited a 30-fold increase in apoptosis compared to the SATLDALLAALRRI dependence peptide lacking the tat sequence. Similar results were obtained for tat-GG-Q14 in comparison to Q14. Specifically, the viability of cells treated with 50 μM tat-GG-SATLDALLAALRRI was 1.5% for COS-7, 4.2% for PC3, 0% for LNCaP, 1.3% for NTera 2, 0% for R2, and 0% for NRA 5 cells (100 μM peptide) . However, cells exposed to the tat sequence alone did not undergo apoptosis. Peptides which did not exhibit apoptotic activity without the amino-terminal tat sequence similarly did not exhibit apoptotic activity with the linked tat sequence. Specifically, cell viability after exposure to tat-purple was 97.8% for COS-7, 92.8% for PC3 and 69.3% for NTera 2 cells. For tat-gray, cell viability was 97.1% for COS-7, 90.5% for PC3, 59.1% for LNCaP and 76.7% for NTera 2 cells. For tat-turquoise, cell viability was 87.9% for PC3, 46.7% for LNCaP, 67.6% for NTera 2, 92.6% for R2 and 95.7% for NRA 5 cells (100 μM peptide) . Similarly, for tat-yellow, PC3 cell viability was 97%. These findings indicate that the tat sequence itself could neither confer apoptotic activity upon a peptide lacking apoptotic activity or inhibit the inherent apoptotic activity of a proapoptotic dependence peptide. 61 Table 1 : Induction of Cell Death by Proapoptotic Peptides Peptide Effect on designation Sequence apoptosis Blue SATL DALL AAL RRI Apoptotic Purple SATL DAKL AAL RRI None Turquoise KDRN LRRI TRM VLV None Red LDEN FKRC FRE FCI None MP INLK ALAA LAK KIL Apoptotic Gray SATL DALL AAL None Green ALL AAL RRI None tat-blue tat-GG-SATL DALL AAL RRI Apoptotic tat-purple tat-GG-SATL DAKL AAL RRI None tat-gray tat-GG-SATL DALL AAL None tat-turquoise tat-GG-KDRN LRRI TRM VLV None tat-yellow tat-GG-DLSL ARLA TAR LAI None tat-GG-Q14 tat-GG-QQQQ QQQQ QQQ QQQ Apoptotic tat GRKK RRQR RRP P None The results in Table 1 show the identification of the dependence domains of several dependence polypeptides. In addition, Table 1 shows the effect of carboxy-terminal deletions, amino-terminal deletions and introducing a point mutation on the apoptotic activity of a dependence peptide modeled after a p75 NTR dependence domain. The results also show that dependence peptides modeled after dependence domains stimulate apoptosis when introduced into every cell type examined. The stimulation of apoptosis in such diverse cell types indicates that the dependence peptides of the invention can be used to treat many different pathological conditions characterized by different cell types. 62 To further analyze the effect of particular point mutations on apoptosis, additional studies employing dependence peptides and mutated variants linked to tat were performed in SH-SY5Y cells. The results shown in Figure 2 are of studies in which quadruplicate samples were averaged, and the studies were repeated 2 to 10 times for each peptide. Each column represents the percentage cell death and the bars indicate the standard error. The amount of peptide added to the cells is indicated above each column. These studies demonstrated that the presence or absence of apoptotic activity observed for particular peptides in SH-SY5Y cells is the same as that observed in the other cell lines described above indicating that apoptotic activity is independent of cell line. Specifically, tat-blue (tat-GG-SATLDALLAALRRI) exhibited apoptotic activity whereas tat-turqoise (tat-GG-KDRNLRRITRMVLV) , tat-gray (tat-GG-SATLDALLAAL) , tat-yellow (tat-GG-DLSLARLATARLAI) and tat-purple (tat-GG-SATLDAKLAALRRI) did not. These studies also demonstrate that particular amino acid residues are critical to the apoptotic activity of the dependence peptide SATLDALLAALRRI. For example, replacing two arginine residues at positions 12 and 13 with glutamic acid residues (tat-GG-SATLDALLAALF ± EI; SEQ ID NO: 53) abolished the ability of the peptide to induce apoptosis. Similarly, replacing the arginine residues with glycine residues (tat-GG-SATLDALLAALG_GI; SEQ ID NO: 38) or glutamine residues (tat-GG-SATLDALLAALQ_QI; SEQ ID NO:54) at positions 12 and 13 decreased the ability of the peptides to stimulate SH-SY5Y cell death by 70% and 80%, respectively. 63 The results shown in Figure 2 also reveal that other amino acids were less critical to the apoptotic activity of the dependence peptide SATLDALLAALRRI . For example, replacing the arginine at position 13 with glycine ( tat-GG-SATLDALLAALRG_I; SEQ ID NO: 39) had very little effect on the ability of the peptide to stimulate apoptosis. Similarly, replacing an aspartic acid at position 5 with glutamine (tat-GG-SATLQALLAALRRI; SEQ ID NO: 40) resulted in a peptide that retained most of its apoptotic function; SH-SY5Y cells were 70% killed as compared to tat-GG-SATLDALLAALRRI. The results shown in Figure 2 -demonstrate that particular amino acids are extremely important for apoptotic activity whereas other amino acids appear less critical. Furthermore, the results in Figure 2, in conjunction with the results in Figure 1, indicate that mutating certain amino acids in a dependence peptide can be a means by which one can decrease (see, for example, tat-GG-SATLDALLAAL≤GI and tat-GG-SATLDALLAALQ_0_I ) or increase (see, for example, Figure 1, p75ΔC19) the ability of a dependence peptide to stimulate apoptosis. Such altered forms of dependence peptides can be useful for modulating the degree of apoptosis in cells. EXAMPLE IV Dependence Peptide Mediated Mitochondrial Swelling, Cytochrome c Release and Caspase-3 Cleavage This Example shows that dependence peptides increase mitochondrial swelling, stimulate the release of cytochrome c from mitochondria and activate caspase-3 in a cell free assay system. 64 Many molecules that stimulate cellular apoptosis such as actactyloside, Bax and mastoparan have been shown to stimulate mitochondrial swelling. Consistent with these observations, molecules such as Bcl-2 which inhibit apoptosis inhibit mitochondrial swelling. The effect of a proapoptotic dependence peptide on mitochondrial swelling was determined and the results are shown in Figure 3A. Briefly, mitochondria were prepared as previously described (Ellerby et al . J_ s _ Neurosci. 17:6165-6178 (1997)) except for the following modifications. The rats were sacrificed by C0 2 inhalation without fasting and the mitochondria were isolated in MIB buffer (210 mM mannitol, 70 mM sucrose, .05% BSA, 1 mM EGTA, 5 mM Hepes-NaOH, pH 7.4). The mitochondrial pellet samples resuspended in MCB buffer (300 mM mannitol, 10 mM KH 2 P0 4 , 0.1% BSA, pH 7.2) and applied to a discontinuous sucrose gradient (1.6 M sucrose, 10 mM KH 2 P0 4 , pH 7.5; 1.2 M sucrose, 10 mM KH 2 P0 4 , pH 7.5) were centrifuged at 48,500 g for 1 hour. Centrifugation resulted in the fractionation of mitochondrial layers which were collected, resuspended in 4 volumes of MCB, and centrifuged at 12,000 g for 10 minutes. The mitochondrial pellets were collected, resuspended in MSB, and stored on ice. After the addition of 50 μM of the peptide, mitochondrial swelling was followed spectrophotometrically at 520 nm (Petronilli et al. J. Biol. Chem. 269:16638-16642 (1994)) in CFS (220 mM mannitol, 68 mM sucrose, 2 mM NaCl, 5 mM KH 2 P0 4 , 2 mM MgCl 2 , 5 mM succinate, 10 mM Hepes-NaOH, 2 mM ATP, 50 μg/ml creatine kinase, 10 mM phosphocreatine, 0.75 μg/ml rotenone, pH 7.4). The results shown in Figure 3A indicate that the isolated mitochondria treated with the dependence peptide SATLDALLAALRRI (p75 364 . 377 ) underwent a rapid 65 increase in swelling as indicated by the decreased absorbance at 520 nm. Similarly, mitochondria treated with a 0.5 mM calcium chloride positive control underwent rapid swelling. In contrast, no swelling of mitochondria was observed in incubation buffer alone or after treatment with a scrambled peptide control (yellow, DLSLARLATARLAI; SEQ ID NO: 49). Apoptosis inducing molecules such as actactyloside, Bax and mastoparan also have been shown to stimulate cytochrome c release from mitochondria whereas apoptotic inhibitors such as Bcl-2 inhibit cytochrome c release. The effect of a proapoptotic dependence peptide on cytochrome c release from mitochondria was determined and the results are shown in Figure 3B. Briefly, cytochrome c release studies (1 hour, 37°C) were performed as described (Ellerby et al . J. Neurosci. 17:6165-6178 (1997)). The mitochondria were prepared as described above, washed and resuspended in CFS (50-10 mg/ml) and peptide was added to the mitochondria at a final concentration of 385 μM. Western blot analysis using a cytochrome c specific antibody monitored the amount of cytochrome c released (Ellerby et al. J. Neurosci. 17:6165-6178 (1997)). The results shown in Figure 3B indicate the relative amount of cytochrome c, which was normalized to a negative buffer control. Mitochondria treated with Triton X-100 were used as a positive control. The results demonstrate that cytochrome c release by mitochondria was stimulated by 500 μM of the SATLDALLAALRRI (p75 364 . 377 ; ) and 385 μM of the tat-GG-SATLDALLAALRRI (tat-p75 364 . 377 ) dependence peptides. In contrast, mitochondria exposed to a helicity control (turqoise, SEQ ID NO: 46; helicity determined by Helical 66 Wheel program of GCG) , tat-yellow control peptide (SEQ ID NO: 56) and a peptide that lacks proapoptotic activity due to a point mutation, tat-purple (tat-p75 364 . 377 L370K; SEQ ID NO:42), did not stimulate cytochrome c release from mitochondria. The activation of cellular apoptosis often results in caspase processing which leads to its activation, an event thought to contribute to the apoptotic cascade. For example, the activation of caspase-8 can be triggered by a Fas or TNFR I multimeric death inducing signaling complex. The effect of a proapoptotic dependence peptide on caspase-3 cleavage therefore was determined using a cell free system. The results are shown in Figure 3C. Briefly, neuronal CFS extracts were prepared and cell-free caspase activation studies were performed. For these studies (3 hour, 37°C) , mitochondria were washed and resuspended in CFS (50-100 mg/ml) and the final peptide concentration was 385 μM. Western blot analyses using the caspase-3 specific antibody, CPP32, was performed as described (Ellerby et al . J. Neurosci. 17:6165-6178 (1997)). The results shown in Figure 3C demonstrate that cleavage of caspase-3, indicated by the appearance of a prominent band below the 20 kDa marker, is stimulated by treatment of the CFS extracts with a proapoptotic dependence peptide SATLDALLAALRRI (p75 364 . 377 ) modeled after a p75 NTR dependence domain. In contrast, no cleavage of caspase-3 was observed in extracts treated with a scrambled control peptide DLSLARLATARLAI (SEQ ID NO: 55) . These results demonstrate that the proapoptotic peptides of the invention stimulate mitochondrial swelling, cytochrome c release, and caspase-3 activation. 67 Similarly, an all D-enantiomer of the dependence peptide stimulated mitochondrial swelling, cytochrome c release, and caspase-3 activation indicating that stimulation of apoptosis by dependence peptides is not stereospecific . The observed changes stimulated by proapoptotic dependence peptides may suggest a possible mechanism by which proapoptotic peptides stimulate apoptosis. In addition, such detectable changes provide useful methods to identify dependence polypeptides and their dependence domains. Throughout this application various publications have been referenced within parentheses. The disclosures of these publications in their entireties are hereby incorporated by reference in this application in order to more fully describe the state of the art to which this invention pertains. Although the invention has been described with reference to the disclosed embodiments, those skilled in the art will readily appreciate that the specific experiments detailed are only illustrative of the invention. It should be understood that various modifications can be made without departing from the spirit of the invention. Accordingly, the invention is limited only by the following claims.

Claims: 68 What is claimed is: 1. A substantially pure proapoptotic dependence peptide comprising substantially the sequence of an active dependence domain selected from the group of dependence polypeptides consisting of p75 NTR , androgen receptor, DCC, huntingtin polypeptide, Machado-Joseph disease gene product, SCAl, SCA2, SCA6 and atrophin-1 polypeptide . 2. The proapoptotic dependence peptide of claim 1, wherein the dependence polypeptide is p75 NTR and the proapoptotic dependence peptide further comprises substantially the sequence selected from the group consisting of SATLDALLAALRRI (SEQ ID N0:3), SATLDALLAALGGI (SEQ ID NO:4), SATLDALLAALRGI (SEQ ID NO:5), and SATLQALLAALRRI (SEQ ID NO:6) or functional equivalent thereof. 3. The proapoptotic dependence peptide of claim 1, wherein the dependence polypeptide is the androgen receptor, huntingtin polypeptide, Machado-Joseph disease gene product, SCAl, SCA2, SCA6 or the atrophin-1 polypeptide and the dependence peptide further comprises a polyglutamine region sequence. 4. The proapoptotic dependence peptide of claim 3, wherein said polyglutamine region sequence is between about 6 to 250 amino acid residues, preferably about 10 to 100 amino acids, more preferably about 14 to 40 amino acids. 5. The proapoptotic dependence peptide of claim 1, further comprising less than about 40 amino acids . 69 6. The proapoptotic dependence peptide of claim 1, further comprising a heterologous functional domain. 7. The proapoptotic dependence peptide of claim 6, wherein said heterologous functional domain is a targeting domain or a domain which facilitates cellular entry. 8. The proapoptotic dependence peptide of claim 6, wherein said heterologous functional domain comprises a tat peptide. 9. A substantially pure proapoptotic dependence peptide having a sequence selected from the group consisting of SATLDALLAALRRI (SEQ ID NO:3), SATLDALLAALGGI (SEQ ID NO:4), SATLDALLAALRGI (SEQ ID NO: 5), and SATLQALLAALRRI (SEQ ID NO: 6), tat-GG-SATLDALLAALRRI (SEQ ID NO:37), Q14 (SEQ ID N0:7) and tat-GG-Q14 (SEQ ID NO:36). 10. A method of increasing cell survival, comprising inhibiting the function of an active proapoptotic dependence domain. 11. The method of claim 10, wherein said function is inhibited by selectively binding a ligand to said active proapoptotic dependence domain. 12. The method of claim 10, wherein said function is inhibited by inhibiting the association of an active proapoptotic dependence domain with an interacting molecule . 70 13. A method of increasing cell survival comprising preventing or reducing the rate of formation of an active proapoptotic dependence domain. 14. The method of claim 13, wherein said rate of formation is prevented or reduced by selectively binding a ligand to a dependence polypeptide containing said active proapoptotic dependence domain. 15. The method of claim 13, wherein said rate of formation is prevented or reduced by selectively binding a ligand to said active proapoptotic dependence domain. 16. The method of claim 13, wherein said rate of formation is prevented or reduced by preventing the association of a dependence polypeptide with an interacting molecule. 17. The method of claim 13, wherein said active proapoptotic dependence domain is a contingency peptide. 18. A method of identifying compounds which prevent or inhibit apoptosis comprising administering a test compound to a cell undergoing proapoptotic dependence domain mediated apoptosis and determining whether said compound increases cell survival. 19. The method of claim 18, wherein said proapoptotic dependence domain-mediated apoptosis is induced by unliganded p75 NTR . 71 20. A method of reducing the severity of a proapoptotic dependence domain mediated pathological condition, comprising inhibiting the function of an active dependence domain. 21. The method of claim 20, wherein said function is inhibited by inhibiting the association of an active proapoptotic dependence domain with an interacting molecule . 22. The method of claim 20, wherein said function is inhibited by inhibiting or reducing the rate of formation of an active proapoptotic dependence domain. 23. The method of claim 22, wherein said rate of formation is inhibited or reduced by specifically binding a ligand to a dependence polypeptide containing said active dependence domain. 24. The method of claim 22, wherein said rate of formation is inhibited or reduced by specifically binding a ligand to said active dependence domain. 25. The method of claim 22, wherein said rate of formation is inhibited or reduced by preventing the association of a dependence polypeptide with an interacting molecule. 26. The method of claim 22, wherein said active proapoptotic dependence domain is a contingency peptide . 72 27. The method of claim 20, wherein said pathological condition is selected from the group consisting of Huntington' s disease, Alzheimer's disease, Kennedy's disease, Spinocerebellar ataxias, dentatorubropallidoluysian atrophy, Machado-Joseph disease, stroke and head trauma. 28. A method of reducing the severity of a pathological condition mediated by unregulated cell proliferation or cell survival, comprising cytoplasmically administering a proapoptotic dependence peptide . 29. The method of claim 28, wherein said pathological condition comprises neoplastic, malignant, autoimmune or fibrotic conditions. 30. The method of claim 28, wherein said cytoplasmically administering further comprises expressing a nucleic acid encoding said proapoptotic dependence peptide. 31. The method of claim 28, wherein said cytoplasmically administering further comprises a heterologous domain. 32. The method of claim 28, wherein said cytoplasmically administering further comprises a heterologous targeting domain. 33. The method of claim 32, wherein said heterologous targeting domain mediates cytoplasmic entry.

Inventors: BREDESEN DALE E, RABIZADEH SHAHROOZ, BREDESEN, DALE, E., RABIZADEH, SHAHROOZ
Applicants: BURNHAM INST, THE BURNHAM INSTITUTE
Application year: 1999
Main classification: A61K38
IPC codes: A61K38, A61K48, C07K7, C07K14