METHODS OF TREATING DISORDERS OF THE EYE

Abstract: The present invention relates to methods for the prophylaxis or treatment of retinal cells by the administration of a therapeutically effective amount of a neuregulin polypeptide.

Description: METHODS OF TREATING DISORDERS OF THE EYE GOVERNMENT SUPPORT This invention was made with the support of a federal grant from the U.S. Government (Grant No. 5ROINS28308-06). The Government has certain rights in the invention. FIELD OF THE INVENTION This invention relates to methods of affecting retinal cell function. BACKGROUND OF THE INVENTION The invention relates to prophylactic or affirmative treatment of diseases and disorders of retina and associated tissues of the eye by administering polypeptides found in vertebrate species, which polypeptides are growth, differentiation and survival factors for several cell types. Normal function of retinal cells including survival, proliferation, differentiation, and maintenance is dependent upon the controlled expression of a variety of peptide growth factors. Some of these factors can be produced by neuronal cells and by other cells of the retina, which provide a signal to regulate retinal cell function. Anatomy and Function of the Retina The retina is that component of the visual system which senses light and transmits impulses via the optic nerve to the visual cortex where the signals are deciphered and interpreted as images. The retina is comprised of a series of layers and cell types as illustrated in Figure 1. The basic function of the retina is to transduce the visual image into a pattern of electrical potential changes that can be processed by the visual centers in the brain. The changes in electrical potentials in the retinal cells are then relayed to the brain. The structure of the retina reflects these functions (Figure 1 ). The cells of the retina are arrayed in three layers: ( 1 ) the outer nuclear layer, which contains the photoreceptor cells; (2) the inner nuclear layer, which contains the cell nuclei of most of the retinal intemeurons and glia; and (3) the ganglion cell layer, which contains the cell bodies of the cells that relay the visual information to the brain via the optic nerve. In addition to these nuclear layers, there are three other distinct layers in the retina. The outermost layer is composed of the outer segments of the photoreceptor cells; this is where the actual process of light-to-electrical signal transduction take place. The outer plexiform layer lies between the outer and inner nuclear layers. It is made up of synapses between the terminals of the photoreceptors and the dendrites of the retinal intemeurons of the inner nuclear layer. The inner plexiform layer lies between the inner nuclear layer and the ganglion cell layer. This layer is where the intemeurons of the inner nuclear layer synapse with the retinal ganglion cell dendrites. The retina is composed of five classes of neurons, and two classes of supporting cells (Principles of Neural Science, 3rd ed., Ed. by E.R. Kandel, J. H. Schwartz, and T. M. Jessell. Elsevier, New York, NY 1991 ). Of the neuronal types, the receptor cells are the cells that transduce light into electrical signals. Receptor cells are of two subtypes: cones - which mediate form and color perception in daylight, and rods - which mediate form perception in dim light. Ganglion cells of the retina project axons into the brain via the optic nerve and are the output cells of the retina. The remaining neuronal types are intemeurons that modulate retinal output: bipolar cells connect receptor cells to ganglion cells; horizontal cells mediate lateral interactions between receptors and bipolar cells; and amacrine cells mediate lateral interactions between bipolar cells and ganglion cells. The supporting cell types are the glial cells of the retina, Miiller cells, and the pigment epithelium cells. The latter cell type plays an important role in the maintenance of receptor cells. The basic flow of information through the retina is as follows (Refer to Figure 1): (1) light passes through the cells of the retina and is absorbed by the outer segments of the photoreceptor cells; (2) the photons are transduced into potential changes in the photoreceptor cells; (3) this change in potential is relayed to one type of retinal intemeuron in the inner nuclear layer, the bipolar cell, via synapses in the outer plexiform layer; (4) the bipolar cells relay the electrical potential changes to the ganglion cells through their synapses in the inner plexiform layer; and (5) the ganglion cells convert the potential changes into action potentials that are sent along the optic nerve to the brain. This process results in a pattern of action potentials in the optic nerves that reflects the pattern of light and dark in the visual world. Some initial processing of the visual information takes place in the retina before it is relayed to the other visual areas in the brain. Proper development and maintenance of the retina is necessary for sustaining normal vision. Degeneration of components of the retina can lead to partial or total blindness. Peptide Growth Factors The development and physiology of multicellular organisms requires multiple modes of intercellular communication. Such communication may be systemic, as in the case of hormones delivered via the bloodstream, or can be highly localized. In the latter case two modes are commonly recognized: synaptic signaling from neurons, and paracrine signaling from adjacent or nearby cells (Molecular Biology of the Cell. Alberts et al., 2nd ed. Garland Publishing, New York, NY 1989). A function of such signaling is to coordinate cell survival, proliferation, differentiation, and/or metabolic activity. The molecules that serve as transmitted signals vary in their chemical composition; one group of molecules are proteins, the peptide growth factors. Peptide growth factors act upon cells by binding to cell surface receptors. These receptors are coupled to intracellular signal transduction pathways that give rise to the above described activities when activated by growth factor binding. The genesis and differentiation of the varied retinal cell types and the generation of distinct layers in the retina from progenitor cells of the optic cup are the result of developmental events that are mediated by intercellular communication involving peptide growth factors. Peptide Growth Factors in the retina The roles of growth factors in the development and maintenance of the retina have been studied in cell culture, by molecular analysis of the expressed growth factors and their receptors, and in animal models of disease or injury. As an example of in vitro studies, explants and partially-dissociated chick retinal pigmented epithelium (RPE) can trans-differentiate into neural retina in the presence of bFGF (Coulombre and Coulombre, Dev. Biol. 12:79, 1965). Proliferation of dissociated RPE cells is stimulated by αFGF, BFGF, EGF, PDGF, IGF, and insulin; and it is inhibited by TGFB (Stemfeld et al., Curr. Eye Res. £: 1029, 1989; Leschey et al., Invest. Ophthalmol. Vis. Sci. 3 .: 839. 1990; Song and Lui, J. Cell Physiol. 143: 196, 1990). Cultured RPE cells are induced by cytokines to release nitric oxide, which is cytotoxic— and the induction can be blocked by FGF (Goureau et al., Biochem. Biophys Res. Comm. 13&854, 1992; op. cit., ___: 120, 1994). Further, retinal explants from the rd mouse are rescued from cell death by combined treatment with NGF and bFGF (Caffe et al., Cwrr. Eye Res. 12:719, 1993). The presence of growth factor receptors in retinal cells has been demonstrated by a variety of molecular analytical techniques, including immunostaining, in situ hybridization and tissue binding using radio-labeled ligands. Cells in the RPE express FGF receptors (Malecaze et al., J. Cell Physiol. __4: 1 105, 1993). Ganglion cells and Miiller cells express receptors for BDNF, CNTF, FGF, trkA and trkB (Jelsma et al., J. Neurobiol. 24:1207, 1993; Takahashi et al., Neurosci. Lett. 111: 174, 1993; Carmignoto et al., Exp. Neurol. 1 11:302 191; reviewed in Steinberg, Curr. Opin. Neurobiol. 4:515, 1994). Mϋller cells also express PDGF receptors (Mudhar et al.. Development ___{■ 539, 1993). Receptors for IGF are detected on photoreceptor cells (Waldbillig et al., Exp. Eye Res. 47:587 1988; Ocrant et al.. Exp. Eye Res. 5.2:581, 1991), and depending on the species and developmental stage that are analyzed receptors for bFDF have been localized on several cell types, including retinal ganglion cells (Sternfeld et al., Curr. Eye Res. 8: 1029, 1992; Schweigerer et al., Biochem Biophys. Res. Comm. 142:934, 1987). Studies on retinal ganglion cell survival in vivo in animal models of optic nerve axotomy and retinal ischemia have demonstrated effects due to FGF (Sievers et al., Neurosci. Lett. 7&157, 1987), NGF (Carmignoto et al., J. Neurosci. 2: 1263, 1989), CNTF (Mey and Thanos, Brain Res. 602:304, 1993), BDNF (Mansour-Robaey et al., PNAS USA 21: 1632, 1994; Mey and Thanos, Brain Res. 602:304, 1993), NT4/5 (Cohen et al., J. Neurobiol. 21:953, 1994) and bFGF (Ferguson et al., J. Neurosci. 1Q:2176, 1990). Some undesirable retinal complications, including macrophage proliferation, inflammation, disorganization of retinal structure and angiogenesis are associated with treatment of the retina with several of the above factors. Neuregulins A recently described family of growth factors, the neuregulins (reviewed by Mudge, CMΓΓ. Biol. 2:361 , 1993; Peles and Yarden, Bioessays J!:815, 1993), are synthesized by neurons (Marchionni et al. Nature 362:313. 1993) and by mesenchymal cells from several parenchymal organs (Meyer and Birchmeier, PNAS 91:1064, 1994). The neuregulins and related factors that bind pl85 ero ^^ have been purified, cloned and expressed (Benveniste et al. PNAS, £2:3930, 1985; Kimura et al., Nature ___:251 , 1990; Davis and Stroobant, J. Cell Biol. H : 1353, 1990; Wen et al., Cell __.:559, 1992; Yarden and Ullrich, Ann. Rev. Biochem. 57:443, 1988; Dobashi et al., Proc. Natl. Acad. Sci. 8 :8582, 1991-. Lupu et al., Proc. Natl. Acad. Sci. 8_9_:2287, 1992; Wen et al., Mol. Cell. Biol. 14: 1909, 1994). Recombinant neuregulins have been shown to be mitogenic for peripheral glia (Marchionni et al.. Nature 362:313. 1993) and have been shown to influence the formation of the neuromuscular junction (Falls et al., Cell 72:801, 1993; Jo et al., Nature _T_: 158, 1995; Chu et al., Cell _4: 329, 1995). The neuregulin gene consists of at least thirteen exons. The neuregulin transcripts are alternatively spliced and these encode many distinct peptide growth factors, which are referred to as the neuregulins (Marchionni et al., Nature 362:313. 1993). DNA sequence comparisons revealed that neu differentiation factor (NDF) (Wen et al., Cell 62:559, 1992) and heregulins (Holmes et al., Science 216: 1205, 1992), which were purified as ligands of the pl85 er °B- (also known as neu or HER2) receptor tyrosine kinase, also are splicing variants of the neuregulin gene. The acetylcholine receptor inducing activity (ARIA) also is a product of the neuregulin gene (Falls et al., Cell 72:801, 1993). Common structural features of the neuregulins are the presence of a single immunoglobulin-like (Ig) fold and a single epidermal growth factor-like (EGF) domain. The sites of neuregulin gene expression have been characterized by use of nucleic acid probes to analyze RNA samples by a variety of methods, such as Northern blotting, RNase protection, or in situ hybridization. Transcripts have been detected in the nervous system and in a variety of other tissues (Holmes et al., Science 256:1205. 1992 Wen et al., Cell 62:559, 1992; Meyer and Birchmeier, PNAS 21: 1064, 1994). Sites of gene expression have been localized in the brain and spinal chord and in other tissues. (Orr-Urteger et al., PNAS 20 867, 1993; Falls et al., Cell 72:801, 1993; Marchionni et al.. Nature 262:313, 1993; Meyer and Birchmeier, PNAS 21: 1064, 1994 ; Chen et al., J. Comp. Neurol. ___; 389, 1994; Corfas et al., Neuron 14: 103, 1995). Specifically in the retinal neurepithelium, expression of neuregulin transcripts has been detected at embryonic day 18 in rat (Meyer and Birchmeier, PNAS 21: 1064, 1994). Although a large number of neuregulins may be produced by alternative splicing, they can be broadly sorted into the putative membrane-bound and the soluble isoforms. The former contains a putative trans-membrane domain and may be presented at the cell surface. Membrane-anchored peptide growth factors may mediate cell-cell interactions through cell- adhesion or juxtacrine mechanisms (reviewed by Massague and Pandiella, Ann. Rev. Biochem. 62: 15, 1993). Alternatively, the putative membrane-bound isoforms may be cleaved from the cell surface and function as soluble proteins (Wen et al., Cell 62:559, 1992; Falls et al., Cc// 7_2:801 , 1993). The soluble neuregulin isoforms contain sequence corresponding to the extracellular domains of the putative membrane-bound isoforms. but terminate before the transmembrane domain. These neuregulin isoforms may be secreted, and hence could affect cells at a distance; or they may be present in the cytoplasm, but could be released upon cellular injury. In the latter case, neuregulins may function as injury factors, as has been postulated for the ciliary neurotrophic factor (Stockli et al., Nature 342:920. 1989). Any one of these modes of action of the neuregulins may occur in the retina. Cellular targets of peptide growth factors are those which bear receptors for the factor(s) and those that are shown to respond in a bioassay either in vitro or in vivo. Based on studies demonstrating phosphorylation on tyrosine residues or cross-linking experiments, neuregulins are candidate ligands for the receptor tyrosine kinases pl85 erDB 2 (or HER-2 in human), pl85 erbB 3 (HER-3 in human), pl 85 erbB4 (or HER-4 in human) or related members of the EGFR gene family. Collectively, these receptors can be referred to as erbB receptors. Though the precise ligand-receptor relationship of each neuregulin protein with each member of the EGFR family is yet to be clarified, several lines of evidence suggest that binding of ligands is mediated by either erbB3 and erbB4 , but signaling occurs through either erbB2. erbB4 and heterodimers of the various subunits (e.g., Carraway and Cantley, Cell 1__5, 1994). These receptors are known to be present on Schwann cells and muscle cells (Jo et al., Nature 373: 158, 1995), and other neuregulin targets, such as cell lines derived from various tumor tissues, such as breast and gastric epithelia. Sites of expression of the HER -4 gene have been localized by in situ hybridization to several regions of the brain, including: hippocampus, dentate gyrus, neo cortex, medial habenula, reticular nucleus of the thalamus, and the amygdala (Lai and Lemke, Neuron 6:691, 1991). The distribution of the HER-4 receptor has not been studied by methods that allow detection of the protein or the activated receptor tyrosine kinase in vivo or in cultures of primary cells. The expression pattern of erbB2, erbB3 and erbB4 in the retina has not been described. Neuregulins have been shown to have a variety of biological activities depending on the cell type being studied. Several neuregulins, including native bovine GGFI, II and III and recombinant human GGF2 (rhGGF2) are mitogenic for Schwann cells (Marchionni et al., Nature 162:313, 1993), as is heregulin Bl (Levi et al, J Neurosci. 15:1329, 1995). On human muscle culture, rhGGF2 has a potent trophic effect on myotubes (Sklar et al.. U.S. Pat. Applic. # 08/059. 022). The differentiation response to rhGGF2 also includes induction of acetylcholine receptors in cultured myotubes (Jo et al., Nature 373: 158, 1995). This activity is associated with other forms of neuregulin, including ARIA (Falls et al.. Cell 72:801. 1993) and heregulin B l (Chu et al.. Neuron 14:329, 1995), as well as with rhGGF2. Further, ARIA has been shown to induce synthesis of voltage-gated sodium channels in chick skeletal muscle (Corfas and Fischbach, J. Neurosci. 12:21 18, 1993). Glial growth factor (GGF), and more specifically rhGGF2, can restrict neural crest stem cells to differentiate into glial cells in vitro (Shah et al., Cell 77:349, 1994). Activities of neuregulin on retinal cells have not been described. In summary, there are examples of neuregulin proteins affecting proliferation, survival and differentiation of target cells. Pharmaceutical need for treating disorders of the eye A variety of retinal diseases and related disorders are known that produce impaired vision and in some cases progress to total blindness. These disorders of the eye include, but are not necessarily limited to: various retinopathies, such as hypertensive retinopathy, diabetic retinopathy and occlusive retinopathy; also injuries and disorders resulting in retinal degeneration, such as retinal tearing and detachment and inherited diseases, such as retinitis pigmentosa; also age-related macular degeneration: diseases of the optic nerve; glaucoma and retinal ischemia. Diabetic Retinopathy is the leading cause of blindness in patients 25-74 years. It is responsible for 12,000-24,000 new cases of blindness per year in the United States. Of the 6 million diabetics in the US 50% show detectable retinopathy after 7 years of diabetes. Age- related macular degeneration (ARMD) is estimated to be present in over 9% of the population 52 years and older and in 33% of the population 75 years and older. Glaucoma is associated with chronically high intraocular pressure and approximately 2 million people in the US are currently being treated. In the US approximately 100.000 people are blinded each year by glaucoma. There is precedent for the use of growth factors that have been shown to be active on retinal cultures in the treatment of retinal degenerative diseases. FGF supports the survival of photoreceptor cells in culture and has been injected into the extracellular space surrounding the rods and cones or into the vitreous body to rescue the photoreceptors in rats which have degeneration as a result of light damage or because of an inherited disease (LaVail et al, PNAS __.: 1 1249, 1992, Faktorovich et al J. NeuroSci ___: 3554, 1992). Similarly TGFB2 has been used for the treatment of Macular holes in humans. The TGFB used was derived from bovine sources and was administered by directly infusing the factor into the area of the macular hole (Glaser et al., Opthalmol. 22: 1 162, 1992). Currently, there are limited options for therapy for the promotion of retinal cell function, including survival, proliferation, differentiation, growth and changes in gene activity and metabolic activity. Such a therapy would be useful for treatment of a variety of eye disorders resulting in loss of sight. SUMMARY OF THE INVENTION In general, the present invention provides methods for promoting the function of retinal cells using neuregulins. A novel aspect of the invention involves the use of neuregulins as growth factors to promote survival of retinal cells. Treating of the retinal cells to provide these effects may be achieved by contacting retinal cells with a polypeptide described herein. The treatments may be provided to slow or halt net cell loss or to increase the amount or quality of retinal tissue present in the vertebrate. Neuregulins are a family of protein factors heretofore described as glial growth factors, acetylcholine receptor inducing activity (ARIA), heregulins, neu differentiation factor, which are encoded by one gene. A variety of messenger RNA splicing variants (and their resultant proteins) are derived from this gene and many of these products show binding to and activation of erbB2 (neu) and closely related receptors erbB3 and erbB4. The invention provides methods for using all of the known products of the neuregulin gene, as well as, other not yet discovered splicing variants of the neuregulin gene. Thus, the above factors, regulatory compounds that induce synthesis of these factors, and small molecules which mimic the effect of these factors by binding to the receptors on retinal tissues or by stimulating through other means the second messenger systems activated by the ligand- receptor complex are all extremely useful as prophylactic and affirmative therapies for retinal tissue diseases and related disorders of the eye. The survival of retinal cells as used herein refers to the prevention of loss of retinal cells by necrosis or apoptosis or the prevention of other mechanisms of retinal loss. Survival as used herein indicates a decrease in the rate of cell death of at least 10%, more preferably by at least 50%, and most preferably by at least 100% relative to an untreated control. The rate of survival may be measured by counting cells stainable with a dye specific for dead cells (such as propidium iodide) in culture. Methods for treatment of diseases or disorders using the polypeptides or other compounds described herein are also part of the invention. Examples of retinal tissue disorders that may be treated include eye diseases and disorders resulting from sensorineural pathologies, such as loss of sight, which may also be treated using the methods of the invention. These disorders of the eye include, but are not necessarily limited to: various retinopathies, such as hypertensive retinopathy, diabetic retinopathy and occlusive retinopathy; also injuries and disorders resulting in retinal degeneration, such as retinal tearing and detachment and inherited diseases, such as retinitis pigmentosa; also age-related macular degeneration; diseases of the optic nerve; glaucoma and retinal ischemia. The methods of the invention make use of the fact that the various neuregulin proteins are encoded by the same gene. A variety of messenger RNA splicing variants (and their resultant proteins) are derived from this gene and many of these products show binding to pl85 er °B2 (or related receptors erbB3 and erbB4) and activation of the same. Products of this gene are used to show retinal cell survival activity (see Example 2, below). This invention provides a use for all of the known products of the neuregulin gene (described herein and in the references listed above), which have the stated activities as promoting retinal cell function. Most preferably, recombinant human GGF2 (rhGGF2) is used in these methods. The invention also relates to the use of other, not yet naturally isolated, splicing variants of the neuregulin gene. Figure 12 shows the known patterns of splicing. These patterns are derived from polymerase chain reaction experiments (on reverse transcribed RNA), analysis of cDNA clones (as presented within) and from analysis of published sequences encoding neuregulins (Peles et al.. Cell 62:205, 1992;Wen et al., Cell 62:559, 1992; Wen et al., Mol. Cell Biol. 14: 1909, 1994) These patterns, as well as additional patterns disclosed herein, represent probable splicing variants which exist. The splicing variants are fully described in Goodearl et al., USSN 08/036,555, filed March 24, 1993, incorporated herein by reference. Advantages of the present invention include the development of new therapeutic approaches to injury or diseases of the eye, more specifically degenerative diseases of the retina, based on the promotion of retinal cell function through the use of neuregulins. Loss of retinal cells is a common feature of degenerative eye diseases, and there are no available treatments, including growth factors , that prevent the death of retinal ganglion cells. The factor can be formulated for intraocular injection and administered to patients that suffer from degenerative disorders , which lead to loss of sight. Thus, this approach to therapy can halt or slow the progressive loss of sight, which ensues in various eye diseases. BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows the series of layers and cell types which form the retina: (1) corresponds to the ganglion cell layer; (2) corresponds to the inner plexiform layer; (3) corresponds to the inner nuclear layer; (4) corresponds to the outer plexiform layer; (5) corresponds to the outer nuclear layer. Figure 2 is immunostaining showing that neuregulin protein is expressed in the retinal ganglion cell layer during embryonic retinal development. Arrows point to labeling in the developing ganglion cell layer. Figure 3 is in situ hybridization showing that neuregulin RNA is expressed in cells of the retinal ganglion cell layer during embryonic development. Arrows point to the labeling in the ganglion cell layer, showing that the distribution is similar to the neuregulin immunoreactivity shown in Figure 2. Figure 4 is immunostaining showing that neuregulin protein is present in the inner and outer plexiform layers of the adult retina. Figure 5 is immunostaining showing that TUJl immunoreactivity is expressed in the newborn rat retina and shows that retinal ganglion cells are the primary cell class that expresses this antigen at this stage of development. The retinal ganglion cell layer is marked with large arrows, while the labeled amacrine cells are marked with the small arrows and the labeled horizontal cells are marked with the arrowheads. Figure 6 is an immunostained culture of rat retinal neurons, which were grown for two days in the presence of rhGGF2 (neuregulin) on collagen gels showing extended long processes labeled with the TUJl antibody. Figure 7 is an immunostained culture of rat retinal cells showing that neuregulin (rhGGF2) causes a significant increase in TUJl immunoreactive in embryonic day 18 rat retinal cells after two days of culture. Figure 8 is an immunostained culture of rat retinal cells showing that the neuregulin (rhGGF2)- induced cell survival is age dependent: neuregulin (rhGGF2) does not cause a significant increase in TUJl immunoreactive embryonic day 15 rat retinal cells after two days of culture. Figure 9 is a bar graph of the results of three separate experiments with embryonic day 18 rat retina] cells. Figure 10 is a bar graph of the experimental results showing the effects of GGF on retinal cell survival. Figure 11A is a listing of the coding strand DNA sequence and deduced amino aid sequence of the cDNA obtained from the splicing pattern of GGFBPP1 shown in Figure 12. Potential glycosylation sites are underlined (along with polyadenylation signal AATAAA); Figure 11B is a listing of the coding strand DNA sequence and deduced amino acid sequence of the cDNA obtained from splicing pattern of GGF2BPP2. Potential glycosylation sites are underlined (along with polyadenylation signal AATAAA); Figure 11C is a listing of the coding strand DNA sequence and deduced amino acid sequence of the cDNA obtained from splicing pattern of GGF2BPP3. Potential glycosylation sites are underlined (along with polyadenylation signal AATAAA). Figure 12 shows products of the neuregulin gene. Figure 13 is a listing of the DNA sequences and predicted peptide sequences of the coding segments of GGF. Line 1 is a listing of the predicted amino acid sequences of bovine GGF, line 2 is a listing of the nucleotide sequences of bovine GGF, line 3 is a listing of the nucleotide sequences of human GGF (heregulin) (nucleotide base matches are indicated with a vertical line) and line 4 is a listing of the predicted amino acid sequences of human GGF/heregulin where it differs from the predicted bovine sequence. Coding segments E, A' and K represent only the bovine sequences. Coding segment D' represents only the human (heregulin) sequence. Figure 14 is the predicted GGF2 amino acid sequence and nucleotide sequence of BPP5. The upper line is the nucleotide sequence and the lower line is the predicted amino acid sequence. Figure 15 is the predicted amino acid sequence and nucleotide sequence of GGF2BPP2. The upper line is the nucleotide sequence and the lower line is the predicted amino acid sequence. Figure 16 is the predicted amino acid sequence and nucleotide sequence of GGF2BPP4. The upper line is the nucleotide sequence and the lower line is the predicted amino acid sequence. Figure 17 is a list of splicing variants derived from the sequences shown in Figure 13. Figure 18 is the predicted amino acid sequence, bottom, and nucleic sequence, top, of EGFL1. Figure 19 is the predicted amino acid sequence, bottom, and nucleic sequence, top, of EGFL2. Figure 20 is the predicted amino acid sequence, bottom, and nucleic sequence, top, of EGFL3. Figure 21 is the predicted amino acid sequence, bottom, and nucleic sequence, top, of EGFL4. Figure 22 is the predicted amino acid sequence, bottom, and nucleic sequence, top, of EGFL5. Figure 23 is the predicted amino acid sequence, bottom, and nucleic sequence, top, of EGFL6. Figure 24 is the predicted amino acid sequence (middle) and nucleic sequence (top) of GGF2HBS5. The bottom (intermittent) sequence represents peptide sequences derived from GGF-II preparations. Figure 25 is the sequences of GGFHBS5, GGFHB 1 and GGFBPP5 polypeptides. Figure 26 is the amino acid sequence of cDNA encoding mature hGGF2. Figure 27 depicts a stretch of the putative bovine GGF-II gene sequence from the recombinant bovine genomic phage GGF2BG1. The figure is the coding strand of the DNA sequence and the deduced amino acid sequence in the third reading frame. DETAILED DESCRIPTION OF THE INVENTION It is intended that all references cited shall be incorporated herein by reference. General The invention pertains to methods of promoting function of retinal cells. The function is affected by the administration of a neuregulin to a vertebrate where the neuregulin interacts with a retinal cell to promote one or more aspects of retinal cell function, including proliferation, differentiation, growth, survival, changes in the pattem of gene expression and secretion, and metabolic change of the retinal cell. Definition of key terms The term administration as used herein refers to the act of delivering a substance, including but not limited to the following routes: parenteral, intravenous, subcutaneous, intramuscular, intraorbital, ophthalmic, intravitreal. subretinal. intraperitoneal. topical, intranasal, aerosol or oral. The term affecting as used herein refers to the induction of a quantitative change in the response of a target cell, as a result of an interaction with a neuregulin. The term amacrine cell as used herein refers to local intemeurons in the inner plexiform layer of the retina that mediate interactions between bipolar and ganglion cells. The term bipolar cell as used herein refers to the intemeurons of the retina that connect the photoreceptor cells with the retinal ganglion cells. The term differentiation as used herein refers to a morphological and/or chemical change that results in the generation of a different cell type or state of specialization. The differentiation of cells as used herein refers to the induction of a cellular developmental program which specifies one or more components of a cell type. The therapeutic usefulness of differentiation can be seen, in increases in quantity of any component of a cell type in diseased tissue by at least 10% or more, more preferably by 50% or more, and most preferably by more than 100% relative to the equivalent tissue in a similarly treated control animal. The term disorder as used herein refers to a disturbance of function and/or structure of a living organism, resulting from an extemal source, a genetic predisposition, a physical or chemical trauma, or a combination of the above, including but not limited to any mammalian disease. The term erbB receptor as used herein refers to erbB2. erbB3 and erbB4 (also HER- 2. HER-3 and HER-4 of human) existing as monomeric. homodimeric and heterodimeric (e.g.. erbB2/erbB3) cell surface receptor tyrosine kinases that bind and/or are activated by one or more neuregulins. The term function as used herein refers to any activity or response of a cell. These include but are not limited to proliferation, differentiation, growth, survival, changes in the pattem of gene expression and secretion, and metabolic changes. The term horizontal cell used herein refers to local intemeurons in the outer plexiform layer of the retina that mediate interactions between bipolar and receptors cells. The term mammal as used herein describes a member of the Class Mammalia (Subphylum Vertebrata). The term mitosis as used herein refers to the division of a cell where each daughter nucleus receives identical complements of the numbers of chromosomes characteristic of the somatic cells of the species. Mitosis as used herein refers to any cell division which results in the production of new cells in the patient. More specifically, a useful therapeutic is defined in vitro as an increase in mitotic index relative to untreated cells of 50%, more preferably 100%, and most preferably 300%), when the cells are exposed to labeling agent for a time equivalent to two doubling times. The mitotic index is the fraction of cells in the culture which have labeled nuclei when grown in the presence of a tracer which only incorporates during S phase (i.e., BrdU) and the doubling time is defined as the average time required for the number of cells in the culture to increase by a factor of two. The term neuregulin as used herein refers to the glial growth factors, the heregulins. neu differentiation factor, acetylcholine receptor inducing activity, and erbB2, 3 and 4 binding proteins. A more complete definition of neuregulins can be found in the specification herein and in the following materials: U.S. Patent No. 5,237,056; U.S. Patent Application SN 08/249,322: WO 92/20798; EPO 0 505 148 Al ; Marchionni, et al.. Nature 162:313, 1993; Benveniste, et al., PNAS £2:3930, 1985; Ki ura, et al.. Nature _4_:251, 1990; Davis and Stroobant, J. Cell. Biol. 110: 1353, 1990; Wen, et al., Cell 62:559, 1992; Yarden and Ullrich, Ann. Rev. Biochem. 57:443, 1988; Holmes, et al., Science 256:1205. 1992; Dobashi, et al., Proc. Natl. Acad. Sci. £8:8582, 1991 ; Lupu, et al., Proc. Natl. Acad. Sci. £2:2287. 1992; Peles and Yarden, BioEssays 11:815, 1993, Mudge, Current Biology 1:361. 1993, all hereby incorporated by reference. The term neurological disorder as described herein refers to a disorder of the nervous system. The term photoreceptor cell as used herein refers to two retinal cell types, rods and cones, that are the cells that transduce light into an electrical signal. The term retinal cell as used herein refers to any of the cell types that comprise the retina, such as retinal ganglion cells, amacrine cells, horizontal cells, bipolar cells, and photoreceptor cells including rods and cones, Mϋller glial cells and retinal pigmented epithelium. The term retinal ganglion cell as used herein refers to neurons of the retina that project axons via the optic nerve to the lateral geniculate nucleus and the superior colliculus. The term survival as used herein refers to any process where a cell avoids death. The term survival as used herein also refers to the prevention of cell loss as evidenced by necrosis or apoptosis or the prevention of other mechanisms of cell loss. Survival as used herein indicates a decrease in the rate of cell death by at least 10%, more preferably by at least 50%, and most preferably by at least 100% relative to an untreated control. The rate of survival may be measured by counting cells stainable with a dye specific for dead cells (such as propidium iodide) in culture. The term therapeutically effective amount as used herein refers to that amount which will produce a desirable result upon administration and which will vary depending upon a number of issues, including the dosage to be administered, and the route of administration. The term treating as used herein may refer to a procedure (e.g. medical procedure) designed to exert a beneficial effect on a disorder. Treating as used herein means any administration of a substance described herein for the purpose of increasing retinal cell function. Most preferably, the treating is for the purpose of reducing or diminishing the symptoms or progression of a disease or disorder of retinal cells. Treating as used herein also means the administration of a substance to increase or alter the cells in healthy individuals. The treating may be brought about by the contacting of the cells which are sensitive or responsive to the neuregulins described herein with an effective amount of the neuregulin. The term TUJl as used herein refers to an antibody that recognizes a neural-specific form of β-tubulin, which is expressed in the longitudinal cells, amacrine cells and ganglion cells of the retina. The term vertebrate as used herein refers to an animal that is a member of the Subphylum Vertebrata (Phylum Chordata). Neuregulins A novel aspect of the present invention relates to the ability of neuregulins to affect retinal cell function. Neuregulins are the products of a gene which produce a number of variably-sized, differentially-spliced RNA transcripts that give rise to a series of proteins. These proteins are of different lengths and contain some common peptide sequences and some unique peptide sequences. The conclusion that these factors are encoded by a single gene is supported by the differentially-spliced RNA sequences which are recoverable from bovine posterior pituitary, human spinal chord and human breast cancer cells (MDA-MB- 231 ). Further support for this conclusion derives from the size range of proteins which act as ligands for the erbB receptors (see below). Further evidence to support the fact a single gene encodes the various neuregulins derives from nucleotide sequence comparisons. Holmes et al., (Science 256: 1205. 1992) demonstrate the purification of a 45-kilodalton human protein (Heregulin-a) which specifically interacts with the receptor protein pl85erbB2. p e les et al., (Cell 62-559, 1992) describe a complementary DNA isolated from rat cells encoding a protein call "neu differentiation factor" (NDF). The translation product of the NDF cDNA has pl 85erbB2 binding activity. Several other groups have reported the purification of proteins of various molecular weights with erbB receptor binding activity. These groups include the following: Lupu et al., Proc. Natl. Acad. Sci. USA £2:2287, 1992; Yarden and Peles, Biochemistry 10:3543. 1991 : Lupu et al.. Science 242: 1552. 1990; Dobashi et al.. Biochem. Biophys. Res. Comm. 172: 1536, 1991: and Huang et al., J. Biol. Chem. 211: 1 1508. 1992. We have found that proteins that bind pl85 e rbB2 and related receptors (i.e., pl85ert>B3 and p l 85erbB4) affect retinal cell survival (Example 2). Further, the presence of immunologically-detectable neuregulin protein (Example 1 ) in retinal ganglion cells in vivo indicates that neuregulin has a role in retinal cell survival in vivo. These neuregulins may be identified using the protocols described herein and in Holmes et al., Science ___ι 1205, 1992; Peles et al.. Cell 62:205, 1992; Wen et al.. Cell 62:559. 1992; Lupu et al., Proc. Natl. Acad. Sci. USA £2:2287. 1992; Yarden and Peles. Biochemistry 10:3543, 1991 ; Lupu et al.. Science 249: 1552. 1990; Dobashi et al.. Biochem. Biophys. Res. Comm. 172: 1536. 1991 Huang et al., J. Biol. Chem. 212: 1 1508- 1 1512. 1992: Marchionni et al., Nature 362:313. 1993; and in U.S. Patent Application Serial No. 07/931.041, filed August 17, 1992, all of which are incorporated herein by reference. Specifically, the invention provides for use of polypeptides of a specified formula, and DNA sequences encoding those polypeptides. The polypeptides have the formula WYBAZCX wherein WYBAZCX is composed of the amino acid sequences shown in Figure 13; wherein W comprises the polypeptide segment F, or is absent: wherein Y comprises the polypeptide segment E. or is absent; wherein Z comprises the polypeptide segment G or is absent: and wherein X comprises the polypeptide segments C D HKL. C/D H. C D HL, CIO D, C/D' HL, C/D' HKL, C/D' H, C/D' D. C D C D' HKL. C D C D' H, C/D C/D' HL. C D C/D' D, C/D D' H, C D D' HL. C/D D" HKL, C/D' D' H, C D" D' HL, C/D' D' HKL, C D C D' D' H, C/D C/D' D' HL, or C/D C/D' D' HKL: provided that, either a) at least one of F, Y. B. A. Z. C, or X is of bovine origin; or b) Y comprises the polypeptide segment E; or c) X comprises the polypeptide segments C/D HKL, C D D, C D' HKL, C/D C/D' HKL. C/D C D' D, C/D D' H, C/D D' HL, C/D D' HKL, C/D' D' H, C/D' D' HKL, C/D C/D' D' H, C/D C/D' D' HL. C/D C D' D' HKL, C DΗ, C D C/D'H, or C/D C/D' HL. In addition, the invention includes the use of the DNA sequence comprising coding segments -5 FBA 3 as well as the with corresponding polypeptide segments having the amino acid sequences shown in Figure 13; the DNA sequence comprising the coding segments S ' FBA' 3' as well as the corresponding polypeptide segments having the amino acid sequences shown in Figure 13; the DNA sequence comprising the coding segments ^ FEBA 3' as well as the corresponding polypeptide segments having the amino acid sequences shown in Figure 13; the DNA sequence comprising the coding segments ^ FEBA' 3 as well as the corresponding polypeptide segments having the amino acid sequences shown in Figure 13; the DNA sequence comprising the polypeptide coding segments of the GGF2HBS5 cDNA clone (ATCC Deposit No. 75298, deposited September 2, 1992), also known as GGF-D. The invention further includes the use of peptides of the formula FBA, FEBA, FBA' FEBA' and DNA sequences encoding these peptides wherein the polypeptide segments correspond to amino acid sequences shown in Figure 13. The polypeptide purified GGF-II polypeptide is also included as part of the invention. Also included in this invention is the mature GGF peptide and the DNA encoding said peptide, exclusive of the N-terminal signal sequence, which is also useful for treatment of conditions involving abnormalities in retinal cell function. Furthermore, the invention includes a method of retinal cell function by the application to a vertebrate of a - 30 kD polypeptide factor isolated from the MDA - MB 231 human breast cell line: or - 35 D polypeptide factor isolated from the rat I-EJ transformed fibroblast cell line to the glial cell: or -75 kD polypeptide factor isolated from the SKBR-3 human breast cell line; or -44 kD polypeptide factor isolated from the rat I-EJ transformed fibroblast cell line, or -25 kD polypeptide factor isolated from activated mouse peritoneal macrophages: or -45 kD polypeptide factor isolated from the MDA - MB 231 human breast cell; or -7 to 14 kD polypeptide factor isolated from the ATL-2 human T-cell line to the glial cell; or -25 kD polypeptide factor isolated from the bovine kidney cell; or -42 kD polypeptide factor (ARIA) isolated from brains. The invention further includes a method for the use of the EGFL1, EGFL2, EGFL3, EGFL4, EGFL5, and EGFL6 polypeptides, Figures 18 to 23 and respectively, for the methods of affecting retinal cell function in vivo and in vitro. Also included in the invention is the administration of the GGF-II polypeptide whose sequence is shown in Figure 24 for affecting retinal cell function. Thus, the invention further embraces a polypeptide factor capable of affecting retinal cell function and including an amino acid sequence encoded by: (a) a DNA sequence shown in Figure 1 1 ; (b) a DNA sequence shown in Figure 27; (c) the DNA sequence represented by nucleotides 281-557 of the sequences shown in Figure 1 1 ; or (d) a DNA sequence hybridizable to any one of the DNA sequences according to (a), (b) or (c). The invention further includes sequences which have greater than 60%. preferably 80%. sequence identity of homology to the sequences indicated above. While the present invention is not limited to a particular set of hybridization conditions, the following protocol gives general guidance which may, if desired, be followed DNA probes may be labeled to high specific activity (approximately lθ8 to 10^ 32p dpm/μg) by nick-translation or by PCR reactions according to Schowalter and Sommer (Anal Biochem. 177:90. 1989) and purified by desalting on G-150 Sephadex columns Probes may be denatured (10 minutes in boiling water followed by immersion into ice water), then added to hybridization solutions of 80% buffer B (2g polyvinylpyrolidine, 2g Fιcoll-400, 2g bovine serum albumin. 50ml 1 M Tris HCL (pH 7.5), 58g NaCl. Ig sodium pyrophosphate, lOg sodium dodecyl sulfate, 950 ml H2O) containing 10% dextran sulfate at lθ6 dpm 3-P per ml and incubated overnight (approximately 16 hours) at 60° C. The filters may then be washed at 60° C first in buffer B for 15 minutes followed by three 20-mιnute washes in 2X SSC, 0.1% SDS then one for 20 minutes in lXSSC, 0 1% SDS In other respects, the invention provides: (a) a basic polypeptide factor which has, if obtained from bovine pituitary mateπal, an observed molecular weight, whether in reducing conditions or not, of from about 30 kD to about 36 kD on SDS-polyacrylamide gel electrophoresis using the following molecular weight standards Lysozyme (hen egg white) 14,400 Soybean trypsin inhibitor 21.500 Carbonic anhydrase (bovine) 31 ,000 Ovalbumin (hen egg white) 45,000 Bovine serum albumin 66,200 Phosphorylase B (rabbit muscle) 97,400, which factor has glial cell mitogenic activity including stimulating the division of rat Schwann cells in the presence of fetal calf plasma, and when isolated using reversed-phase HPLC retains at least 50% of said activity after 10 weeks incubation in 0 1 % tπfluoroacetic acid at 4° C. and (b) a basic polypeptide factor which has, if obtained from bovine pituitary material, an observed molecular weight, under non-reducing conditions, or from about 55 kD to about 63 kD on SDS-polyacrylamide gel electrophoresis using the following molecular weight standards Lysozyme (hen egg white) 14,400 Soybean trypsin inhibitor 21 ,500 Carbonic anhydrase (bovine) 31 ,000 Ovalbumin (hen egg white) 45.000 Bovine serum albumin 66,200 Phosphorylase B (rabbit muscle) 97,400; which factor the human equivalent of which is encoded by DNA clone GGF2HBS5 described herein and is capable of affecting retinal cell function. For convenience of description only, the lower molecular weight and higher molecular weight factors of this invention are referred to hereafter as * 'GGF-I" and "GGF- II". respectively. The "GGF2" designation is used for all clones isolated with peptide sequence data derived from GGF-II protein (i.e., GGF2HBS5, GGF2BPP3). It will be appreciated that the molecular weight range limits quoted are not exact, but are subject to slight variations depending upon the source of the particular polypeptide factor. A variation of, say, about 10% would not, for example, be impossible for material from another source. Another important aspect of the invention is a DNA sequence encoding a polypeptide capable of affecting retinal cell function and comprising: (a) a DNA sequence shown Figure 1 1 ; (b) a DNA sequence shown in Figure 27; (c) the DNA sequence represented by nucleotides 281 -557 of the sequence shown in Figure 11 ; or (d) a DNA sequence hybridizable to any one of the DNA sequences according to (a), (b) or (c). Thus other important aspects of the invention are: (a) A series of human and bovine polypeptide factors capable of affecting retinal cell function. These peptide sequences are shown in Figures 13, 14. 15 and 16 respectively. (b) A series of polypeptide factors capable of affecting retinal cell function and purified and characterized according to the procedures outlined by Lupu et al.. Science 242: 1552, 1990; Lupu et al., Proc. Natl. Acad. Sci USA _9: 2287, 1992; Holmes et al., Science 216-1205, 1992; Peles et al., Cell 62:205, 1992; Yarden and Peles, Biochemistry 10:3543, 1991; Dobashi et al., Proc. Natl. Acad. Sci. __: 8582, 1991 ; Davis et al., Biochem. Biophys. Res. Commun. 1 79 : 1536. 1991 ; Beaumont et al., Patent Application PCT/US91/03443 (1990): Greene et al., Patent Application PCT/US91/02331 (1990); Usdin and Fischbach, J. Cell. Biol. 101:493, 1986; Falls et al., Cold Spring Harbor Symp. Quant. Biol. 51397. 1990; Harris et al., Proc. Natl. Acad. Sci. USA 8£:7664. 1991: and Falls et al., Cell 22:801. 1993. (c) A polypeptide factor (GGFBPP5) is capable of affecting retinal cell function. The amino acid sequence is shown in Figure 14. and is encoded by the bovine DNA sequence shown in Figure 14. The novel human peptide sequences described above and presented Figures 13, 14. 15. and 16. respectively, represent a series of splicing variants which can be isolated as full length complementary DNAs (cDNAs) from natural sources (cDNA libraries prepared from the appropriate tissues) or can be assembled as DNA constructs with individual exons (e.g., derived as separate exons) by someone skilled in the art. Other compounds, in particular peptides, which bind specifically to erbB receptors can also be used according to the invention as effectors of retinal cell function. A candidate compound can be routinely screened for erbB receptor binding, and, if it binds, can then be screened for affecting retinal cell function, more specifically, retinal cell survival, using the methods described herein. The invention includes any modifications or equivalents of the above polypeptide factors which do not exhibit a significantly reduced activity. For example, modifications in which amino acid content or sequence is altered without substantially adversely affecting activity are included. By way of illustration, in EP-A 109748 mutations of native proteins are disclosed in which the possibility of unwanted disulfide bonding is avoided by replacing any cysteine in the native sequence which is not necessary for biological activity with a neutral amino acid. The statements of effect and use contained herein are therefore to be construed accordingly, with such uses and effects employing modified or equivalent factors being part of the invention. The new sequences of the invention open up the benefits of recombinant technology. The invention thus also includes the following aspects: (a) DNA constructs comprising DNA sequences as defined above in operable reading frame position within vectors (positioned relative to control sequences so as to permit expression of the sequences) in chosen host cells after transformation thereof by the constructs (preferably the control sequence includes regulatable promoters, e.g. Tip). It will be appreciated that the selection of a promoter and regulatory sequences (if any) are matters of choice for those of skill in the art: (b) host cells modified by incorporating constructs as defined in (a) immediately above so that said DNA sequences may be expressed in said host cells - the choice of host is not critical, and chosen cells may be prokaryotic or eukaryotic and may be genetically modified to incorporate said constructs by methods known in the art; and. (c) a process for the preparation of factors as defined above comprising cultivating the modified host cells under conditions permitting expression of the DNA sequences. These conditions can be readily determined, for any particular embodiment, by those of skill in the art of recombinant DNA technology. Glial cell mitogens prepared by this means are included in the present invention. None of the factors described in the art has the combination of characteristics possessed by the present new polypeptide factors. The invention also includes a neuregulin as defined above, by extracting vertebrate brain material to obtain protein, subjecting the resulting extract to chromatographic purification by hydroxyapatite HPLC and then subjecting these fractions to SDS- polyacrylamide gel electrophoresis. The fraction which as an observed molecular weight of about 30 kD to 36 kD and/or the fraction which has an observed molecular weight of about 55 kD to 63 kD is collected. In either case, the fraction is subjected to SDS-polyacrylamide gel electrophoresis using the following molecular weight standards: Lysozyme (hen egg white) 14,400 Soybean trypsin inhibitor 21,500 Carbonic anhydrase (bovine) 31 ,000 Ovalbumin (hen egg white) 45,000 Bovine serum albumin 66,200 Phosphorylase B (rabbit muscle) 97,400 In the case of the smaller molecular weight fraction, the SDS-polyacrylamide gel is run in non-reducing conditions in reducing conditions or, and in the case of the larger molecular weight fraction the gel is run under non-reducing conditions. The fractions are then tested for activity stimulating the division of rat Schwann cells against a background of fetal calf plasma. Preferably, the above process starts by isolating a relevant fraction obtained by carboxymethyl cellulose chromatography, e.g. from bovine pituitary material. It is also preferred that hydroxyapatite HPLC, cation exchange chromatography, gel filtration, and/or reversed-phase HPLC be employed prior to the SDS-Polyacrylamide gel electrophoresis. At each stage in the process, activity may be determined using Schwann cell incorporation of radioactive iododeoxyuridine as a measure in an assay generally as described by Brockes in Meth. Enz. 142:217. 1987. but modified by substituting 10% FCP for 10% FCS. Compounds can be assayed for their usefulness in vitro using the methods provided in the description and examples below. Following the in vitro demonstration of the effect of neuregulins on retinal cell function, the in vivo therapeutic benefit of the effect can be accomplished by the administration of neuregulins, neuregulin producing cells or DNA encoding neuregulins to a vertebrate requiring therapy. In Vitro Assays of Neuregulin Effects on Retinal Cells Several in vitro assays are used to determine which neuregulin protein(s) promote retinal cell function and which retinal cell types are affected by contacting neuregulin protein. Described below are methods for detecting the ability of a neuregulin to promote function of a retinal cell. In vitro assays for determining neuregulin effects on retinal cell function depend on establishing retinal cultures. A general reference on cell and tissue culture is Cell and Tissue Culture: Laboratory Procedures (Ed. by A. Doyle, J. B. Griffiths, and D. G. Newell. John Wiley and Sons, New York, NY, 1994). General references on the culture of neural cells and tissues are Methods in Neurosciences, Vol. 2 (Ed. by P. M. Conn. Academic Press, Sand Diego, CA, 1990) and Culturing Nerve Cells (Ed. by G. Banker and K. Goslin, MIT Press, Cambridge, MA, 1991). General references of immunocytochemistry are Antibodies: A Laboratory Manual (E. Harlow and D. Lane, Cold Spring Harbor Laboratory. Cold Spring Harbor, NY, 1988), and Immunocytochemistry II (Ed. by A. C. Cuello. John Wiley and Sons. New York, NY, 1993). The retinal cells from a vertebrate used in this invention may be cultured in a variety of media. Commercially available media such as Ham ' s FlO(Sigma), Minimal Essential Medium ([MEM], Sigma), RPMI-1640 (Sigma), and Dulbecco's Modified Eagle ' s Medium ([DMEM], Sigma) are suitable for culturing retinal cells. In addition, any of the media described in Ham and Wallace. Meth. Enz. 5_£:44, 1979; Bames and Sato, Anal. Biochem. 102:255. 1980; U.S. Pat. Nos. 4,767,704; 4,657,866; 4,927,762; or 4.560,655: WO 90/03430; WO 87/00195 and U.S. Pat. Re. 30,985, may be used as culture media for retinal cells. Any of these media may be supplemented as necessary with hormones and/or other growth factors (such as insulin, transferrin, or epidermal growth factor), salts (such as sodium chloride, calcium, magnesium, and phosphate), buffers (such as HEPES), nucleosides (such as adenosine and thymidine), antibiotics (such as Gentamycin™ drug), trace elements (defined as inorganic compounds usually present at final concentrations in the micromolar range), and glucose or an equivalent energy source. Any other necessary supplements may also be included at appropriate concentrations that would be known to those skilled in the art. The culture conditions, such as temperature, pH. and the like, will be apparent to the ordinarily skilled artisan. The use of retinal cell cultures to demonstrate that neuregulin promotes retinal cell function is in accordance with methods described in general terms above and further described in Pittack et al., Devel. 1 13:577. 1991. The retina is dissected from either embryonic or adult vertebrate animals and placed into Ca + 2/Mg + 2-free Hepes-buffered sterile saline (HBSS) for 15 min., followed by treatment with 0.25% trypsin for an additional 15 min. The trypsin is inactivated by the addition of 1 % fetal bovine serum. The cells are subsequently resuspended in fresh medium and gently triturated to yield a single - cell suspension. Cells are plated into wells of 24- well plates and cultured at 37°C The types of retinal cells present in the culture can be identified through the use of immunocytochemical markers. Specific molecular markers can be stained immunocytochemically for the identification of cell types in the retina: for photoreceptors — e.g., rhodopsin, and red and green cone opsins; for amacrine cells ~ e.g., cellular retinoic acid binding protein; for bipolar cells — e.g., a specific form of protein kinase C and its substrate protein PCP2; for retinal ganglion cells— Thy 1 and B3-tubulin and; for horizontal cells— B3-tubulin. After maintaining the cultures for varying periods of time, preferably greater than 1 day and less than 7 days, a variety of assays can be utilized to assess various aspects of cellular phenotype such as, but not limited to, cell survival, proliferation, differentiation, morphology, and production of enzymes and secreted products. In Vitro Method I The survival function is assayed by methods that identify and count either viable cells or dead retinal cells following culture at low density (e.g., for retinal ganglion cells 10.000 cells/cm^) over a period from one to six days in the presence of varying amounts of neuregulin added to the culture medium. Included in these methods are specific stains for dead cells, such as propidium iodide, which enters the nucleus of dead cells and is detected by fluorescence microscopy. Alternatively, the counting of retinal cells adhering to the culture substratum over a six day period also can be used as an indicator of cell survival. In Vitro Method II An alternative procedure to monitor retinal cell death utilizes labeling of nicked DNA strands, which are characteristic of cells undergoing apoptotic cell death, with digoxygenin- 1 1 -dUTP using terminal deoxynucleotidyl transferase (TUNEL) according to the protocol described in Gavrieli et al., J. Cell Biol. ϊ__: 493-501, 1992. The labeled DNA strands are detected using standard kits available from commercial vendors (e.g., Genius kit from Boehringer Mannheim). Further, a cell death detection ELISA system, which is based on the DNA fragmentation that occurs in dying cells (Boehringer Mannheim catalog no. 1585 045) can be utilized to quantify cell death in accordance with the instructions provided by the commercial vendor. In Vitro Method III The release into the culture medium of the cytosolic enzyme lactate dehydrogenase (LDH) also can be used to quantify the extent of retinal cell death in vitro (Kirk et al, J. Pharmacol. Exper. Therapeut. 271 :1080. 1994). LDH levels are measured by an automated kinetic colorimetric assay in which oxidation of lactate to pyruvate is coupled to reduction of the tetrazolium dye. INT. Briefly, 80 ul samples of the culture medium are mixed with an equal volume of the substrate solution containing (in mg/1) INT, 334; phenazine methosulfate, 86; nicotinamide adenine dinucleotide, 862; L-(+)-lactate. 4900 (lithium salt); and 0.1% Triton X- 100 in 0.2 M Tris buffer, pH 8.2. In the assay, LDH activity is directly proportional to the rate of appearance of the resulting INT formazan (absorbance max. at 492 nm). The product is monitored quantitatively in a microplate reader (UVmax, Molecular Devices. Menlo Park. CA) as the change in absorbance at 490 nm over a 2 min. interval. In Vitro Method IV The proliferative function of neuregulins on retinal cells can be assayed by incorporation of l-^I-Urd, 3^.^ or BrdU into replicating DNA strands of dividing cells, or by cell counting. The assays developed to measure the mitogenic activity of neuregulins on Schwann cells by incorporation of DNA synthesis precursors (Brockes et al.. Brain Res. 161:105, 1979; Davis and Stroobant, J. Cell Biol. ___: 1353, 1990) can be adapted to retinal cells by one of normal skill in the art of cell culture. In Vitro Method V The differentiation function of neuregulin on retinal cells can be assayed by employing analytical methods, such as immunostaining or in situ hybridization, which can detect and quantify marker proteins associated with the various cell types of the retina. Retinal ganglion cells are recognized by staining with the specific tubulin antibody TUJl, as shown in Example 2 (Figure 4). The glial cells of the retina, Mϋller glia, are recognized by staining with antibodies that recognize glial fibrillary acidic protein (GFAP). For example, neurogenesis of retinal cells in culture can be achieved by dissociating embryonic retinal progenitor cells of the rat (from El 5 through El 8), then contacting the cells with the neuregulin and quantifying the distribution of various cell types identified by immunostaining using the markers described herein. In addition to this assay, which is based on determining activity in retinal neurogenesis, the differentiation function of neuregulin can be assayed in mature cultures (e.g., differentiated in culture for approximately two weeks). As such, changes in the level of specific proteins expressed in particular retinal cell types can be quantified. In Vitro Method VI Several peptide growth factors and their receptors have been identified in the retina, as described in the prior act. Methods utilized to detect those molecules and activities can be employed to demonstrate a differentiation function of neuregulin on retinal cells. Neuregulins can be shown to induce the synthesis of growth factors and/or their receptors expressed in the retina. The analysis can be by in situ hybridization or other methods of quantitative RNA analysis, such as, but not limited to, reverse transcription-PCR, RNAse protection and Northern blotting. Alternatively, induced expression of growth factors or their receptors can be assayed by immunocytochemical staining or cell biological assays designed to measure growth factor activity. The in vitro assays described above to identify neuregulins that have biological activity on retinal cells can be applied to dissociated cells, semi-dissociated cells, explants of whole retina and parts thereof, such as preparations of retinal pigmented epithelium and other layers of the retina. The cultures can be established and maintained using methods described above. In some cases, minor modifications or substitutions to the procedures described herein, which do not alter the reduction to practice of the invention, can be provided by one of ordinary skill in the art. In Vivo Assays of Neuregulin Effects on Retinal Cells Neuregulin activity on retinal cells also can be shown through in vivo assays. Some in vivo assays represent animal models of retinal degeneration and other diseases and disorders of the eye. For example, photoreceptor cells are lost in inherited retinal degeneration and in age-related macular degeneration. Retinal ganglion cells die in glaucoma and in optic nerve injuries, such as retinal ischemia or axotomy. In Vivo Method I The rescue of photoreceptor cells can be demonstrated in Royal College of Surgeons (RCS) rats, which have an inherited retinal degeneration (Faktorovich et al., Nature 347:83. 1990). The histological analysis (Method HI ) consists of vascular perfusion of anesthetized animals, embedding the eye in epoxy resin, then staining 1 micron sections with toluidine blue. In untreated RCS rats at 53 days after birth (P53) the outer nuclear layer, which contains the photoreceptor cells, is reduced in thickness to only a few rows of cells (approximately 20% of the thickness found in normal rats at the same age). A therapeutically effective dose of neuregulin administered by intravitreal administration ( a single injection of 1 microliter) can restore the thickness of the outer nuclear layer, and hence rescue photoreceptor cells. Alternatively, rescue of photoreceptor cells can be demonstrated in the Sprague-Dawley rat models (2-to-3 month old males) of exposure to constant light (115-200 foot-candles) for 1 week (LaVail et al., PNAS USA £2: 1 1249, 1992). Neuregulin can be injected (1 ul) into the subretinal space or into the vitreous humor 48 hours prior to the onset of continuous illumination. Histological analysis (Method HI ) of retinas following a fixed recovery period (usually 10 days) is used to assess the damage to and rescue of photoreceptor cells. Retinal detachment also leads to the death of photoreceptor cells, which provides another animal model (Erickson et al., J. Struct. Biol. 108: 148. 1992) to demonstrate the in vivo survival activity of neuregulin on retinal cells. In Vivo Method II Several mouse genetic models of photoreceptor degeneration (e.g., rd-- mutant of b subunit of cGMP phosphodiesterase; rds-- mutant of peripherin) can be used to show neuregulin survival effects in vivo using the modes of administration described above. The rd and rds animals show retinal degeneration within a few weeks after birth and following intravitreal injection of neuregulin tissues can be analyzed by histological methods described above (e.g.. Method HI ). Further, retinal explants from rd mice cultured in neuregulin- containing medium can be assayed for thickness of the outer nuclear layer using methods described in Caffe et al., Curr. Eye Res. 12:719, 1993. Mouse pups are enucleated 48 hours after birth and treated with proteinase K. After enzyme treatment, the neural retina with the retinal pigmented epithelium (RPE) attached is recovered, placed into a multi-well culture dish and incubated in 1.2 ml culture medium (e.g.. R16) for up to 4 weeks at 37 C with 5 % CO2- Immunocytochemical staining for opsin of fixed (e.g., 4% paraformaldehyde) sections is used to assess the degeneration and rescue of photoreceptor cells. In the rd mouse the outer nuclear layer (photoreceptor cells) degenerate after 2-to-4 weeks in culture. The media can be supplemented with varying doses of neuregulin to achieve an effect on retinal cell function, such as rescue of the outer nuclear layer from degeneration. Survival effects also can be shown using the TUNEL method on sections of retina analyzed in the models described above. In Vivo Method III In response to injury to the retina Muller cells undergo a proliferative gliosis. The mitotic activity of neuregulin on Muller glia can be shown by labeling dividing cells with a DNA synthesis precursor following administration of the factor. Labeled cells can be detected by autoradiography ( 3 H-dT) or by immunostaining (BrdU labeling) and quantified. In Vivo Method IV Neuregulins can be shown to promote retinal ganglion cell survival following optic nerve axotomy or nerve crash using methods described in Sievers et al.. Neurosci. Lett. 26: 157, 1987; Carmignoto et al., J. Neurosci. 2: 1263, 1989; Mey and Thanos, Brain Res. 602:304. 1993. Briefly, 4-to-6 week old mice are anesthetized, the optic nerve exposed and crushed intraorbitally 2-4 mm posterior to the optic disk between fine forceps for 30-60 sec. Alternatively, the nerve is transected surgically. Administration of neuregulin by intravitreal or subretinal injection is done after the animals recover from surgery using a therapeutically effective dose. The survival of retinal ganglion cells is assessed at several time points between 3 days and 6 weeks after injection by histological analysis (Method HI ) or by immunostaining using antibodies that recognize retinal ganglion cells as described herein. In Vivo Method V Ischemia can be produced in the retina of the albino Lewis rat by raising intraocular pressure by intraocular injection of saline (Unoki and LaVail. Invest Ophthalmol Vis. Sci. 11:907, 1994). The thickness of the inner retinal layer is reduced due to loss of retinal ganglion cells when retinas are analyzed histologically (Method HI ) at 7 days post- ischemia. An intravitreal injection of a therapeutically effective amount of neuregulin given two days prior to ischemia can reduce the ischemic damage. In Vivo Method VI Other compounds, in particular peptides, which specifically bind and/or activate erbB receptors also can be used according to the invention as effectors of retinal cell function. A candidate compound can be routinely screened for erbB receptor binding, and if it binds, can then be screened for affecting retinal cell function using the methods described herein. In Vivo Method VII Inadequate amounts of survival-promoting factors can lead to degenerative eye disorders, such as macular degeneration. The present invention demonstrates the survival - promoting activity of neuregulin indicating that these factors may be used to promote retinal cell survival in a vertebrate (preferably a mammal, more preferably a human) by administering to the vertebrate an effective amount of a polypeptide or a related compound. Neuregulin effects on retinal cells may occur, for example, by preventing the extent of naturally-occurring programmed cell death that occurs during the embryonic development of the retina. In a rat model, retinal ischemia can be induced by increasing intraocular pressure via injection of saline into the eye (Buchi et al., Ophthalmologic. 201:138, 1991; Hughes, Exp. Eye Res. __:513, 1991). This model has been used to evaluate the efficacy of bFGF, CNTF and BDNF in decreasing neuronal loss (Unoki and LaVail, Invest. Ophthalmol. Vis. Sci. H:907, 1994). Neuregulins administered by intraocular injection can be shown to decrease neuronal loss associated with retinal ischemia in this animal model. Neuregulin effects on retinal cell survival can be shown in genetic and transgenic mouse models for Retinitis Pigmentosa (rp). Histological analysis (Method HI ) of the retina of rp mice following intravitreal administration of neuregulin can be used to rescue retinal cell degeneration. The demonstration of biological activity of the neuregulins by promoting retinal cell function in any of the animal models described above indicates efficacy in treating disorders of the eye. A variety of retinal diseases and related disorders are known that produce impaired vision and in some cases progress to total blindness. These disorders of the eye include, but are not necessarily limited to: various retinopathies. such as hypertensive retinopathy, diabetic retinopathy and occlusive retinopathy; also injuries and disorders resulting in retinal degeneration, such as retinal tearing and detachment and inherited diseases, such as retinitis pigmentosa; also age-related macular degeneration (ARMD) and related diseases, such as idiopathic central serous chorioretinopathy, central areolar choroidal dystrophy, macular holes, macular coloboma. Stargardt hereditary dystrophy, trauma, diabetic circinate maculopathy, angioid streaks and choroidal neovascularization. presumed ocular histoplasmosis and choroidal neovascularization, angiomatosis retinae, choroidal rupture and choroidal neovascularization, toxoplasmosis and choroidal neovascularization: diseases.of the optic nerve; and glaucoma and retinal ischemia. Thus, administration of neuregulin in a therapeutically effective amount can provide a treatment for disorders of the eye, which otherwise left untreated would result in the loss of sight. The invention includes the use of any modifications or equivalents of the above polypeptide factors which do not exhibit a significantly reduced activity related to affecting retinal cell function For example, modifications in which amino acid content or sequence is altered without substantially adversely affecting activity are included The statements of effect and use contained herein are therefore to be construed accordingly, with such uses and effects employing modified or equivalent factors being part of the invention The invention includes the use of the above named family of proteins (i.e neuregulins) as extracted from natural sources (tissues or cell lines) or as prepared by recombinant means The human peptide sequences described above represent a series of splicing variants which can be isolated as full length complementary DNAs (cDNAs) from natural sources (cDNA libraries prepared from the appropriate tissues) or can be assembled as DNA constructs with individual exons (e.g., derived as separate exons) by someone skilled in the art The invention includes methods for the use of any protein which is substantially homologous to the coding segments in Figure 13, as well as other naturally occurring neuregulin polypeptides for the purpose of promoting retinal cell function Also included are the use of: allelic vanations: natural mutants; induced mutants, proteins encoded by DNA that hybridizes under high or low stringency conditions to a nucleic acid naturally occurring (for definitions of high and low stringency see Current Protocols in Molecular Biology, (1989) John Wiley & Sons. New York, NY. 6.3 1 - 6.3 6. hereby incorporated by reference), and the use of polypeptides or proteins specifically bound by antisera to GGF polypeptides The term also includes the use of chimeπc polypeptides that include the GGF polypeptides comprising sequences from Figure 13 Use of Neuregulins A novel aspect of the invention involves the use of neuregulins as factors to promote retinal cell function. Treatment of the cells to achieve these effects may be achieved by contacting cells with a polypeptide descπbed herein. The methods of the invention make use of the fact that the neuregulin proteins are encoded by the same gene. A variety of messenger RNA splicing variants (and their resultant proteins) are derived from this gene and many of these products show binding to erbB receptors and activation of the same. This invention provides a use for all of the known products of the neuregulin gene (described herein and in the references listed above). Most preferably, recombinant human GGF2 (rhGGF2) is used in these methods. The invention also relates to the use of other, not yet naturally isolated, splicing variants of the neuregulin gene. Figure 12 shows the known pattems of splicing. These pattems are derived from polymerase chain reaction experiments (on reverse transcribed RNA), analysis of cDNA clones (as presented within), and analysis of published sequences encoding neuregulins (Peles et al.. Cell 62:205, 1992 and Wen et al., Cell 62:559, 1992) These pattems, as well as additional pattems disclosed herein, represent probable splicing vaπants which exist. The splicing variants are fully described in Goodearl et al., USSN 08/036,555, filed March 24, 1993, incorporated herein by reference. More specifically, effects on retinal cell function may be achieved by contacting cells with a polypeptide defined by the formula WYBAZCX wherein WYBAZCX is composed of the polypeptide segments shown in Figure 13; wherein W comprises the polypeptide segment F, or is absent, wherein Y comprises the polypeptide segment E, or is absent; wherein Z comprises the polypeptide segment G, or is absent; and wherein X comprises the polypeptide segment C/D HKL, C/D H, C/D HL, C/D D, C/D' HL, C/D' HKL, C/D' H, C/D' D, C/D C/D' HKL, C/D C/D' H, C/D C/D' HL, C/D C/D' D, C/D D' H, C/D D' HL, C/D D' HKL, C/D' D' H, C/D' D' HL, C/D' D' HKL, C/D C/D' D' H, C/D C/D' D' HL, or C/D C/D' D' HKL. Furthermore, the invention includes a method of treating retinal cells by the application to the retinal cell of a -30 kD polypeptide factor isolated from the MDA-MB 231 human breast cell line, or -35 kD polypeptide factor isolated from the rat I-EJ transformed fibroblast cell line to the glial cell, or -75 kD polypeptide factor isolated from SKBR-3 human breast cell line, or -44 kD polypeptide factor isolated from the rat I-EJ transformed fibroblast cell line, or -25 kD polypeptide factor isolated from activated mouse peritoneal macrophages; or -45 kD polypeptide factor isolated from the MDA-MB 231 human breast cell; or -7 to 14 kD polypeptide factor isolated from the ATL-2 human T-cell line to the glial cell: or -25 kD polypeptide factor isolated from the bovine kidney cells; or -42 kD ARIA polypeptide factor isolated from brain: or -46-47 kD polypeptide factor which stimulates 0-2A glial progenitor cells: or -43-45 kD polypeptide factor. GGFIII. U.S. patent application Serial No. 07/931.041. filed August 17. 1992, incorporated herein by reference. The invention includes use of any modifications or equivalents of the above polypeptide factors which do not exhibit a significantly reduced activity. For example, modifications in which amino acid content or sequence is altered without substantially adversely affecting activity are included. The statements of effect and use contained herein are therefore to be construed accordingly, with such uses and effects employing modified or equivalent factors being part of the invention. The human peptide sequences described above and presented in Figs. 13, 14, 15. and 16, respectively, represent a series of splicing variants which can be isolated as full-length complementary DNAs (cDNAs) from natural sources (cDNA libraries prepared from the appropriate tissues) or can be assembled as DNA constructs with individual exons (e.g., derived as separate exons) by someone skilled in the art. Another aspect of the invention is the use of a pharmaceutical or veterinary formulation comprising any factor as defined above formulated for pharmaceutical or veterinary use, respectively, optionally together with an acceptable diluent, carrier or excipient and/or in unit dosage form. In using the factors of the invention, conventional pharmaceutical or veterinary practice may be employed to provide suitable formulations or compositions. A medicament is made by administering the polypeptide with a pharmaceutically effective carrier. Neuregulins can be administered intravitreally by insertion of a needle through the sclera, choroid and retina and then injection of factor formulated in an appropriate vehicle for administration. The factor may also be delivered subretinally by a transpleural injection. There is also the option of delivering the factor intraocularly using ethylene-vinyl acetate copolymer implants or by delivery to the comeal surface via eye drops. Thus, the formulations to be used as a part of the invention can be applied to parenteral administration, for example, intravenous, subcutaneous, intramuscular, intraorbital, ophthalmic, intraperitoneal, topical, intranasal, aerosol, transdermal and by other slow release devices (i.e., osmotic pump-driven devices; see also USSN 08/293,465, hereby incorporated by reference). The formulations of this invention may also be administered by the transplantation into the patient of host cells expressing the DNA encoding polypeptides which are effective for the methods of the invention or by the use of surgical implants which release the formulations of the invention. Parenteral formulations may be in the form of liquid solutions or suspensions; for oral administration, formulations maybe in the form of tablets or capsules; and for intranasal formulations, in the form of powders, nasal drops, or aerosols. Methods well-known in the art for making formulations are to be found in, for example, "Remington's Pharmaceutical Sciences." Formulations for parenteral administration may, for example, contain as excipients sterile water or saline, polyalkylene glycols such as polyethylene glycol, oils of vegetable origin, or hydrogenated naphthalenes, biocompatible, biodegradable lactide polymer, or polyoxyethylene-polyoxypropylene copolymers may be used to control the release of the present factors. Other potentially useful parenteral delivery systems for the factors include ethylene-vinyl acetate copolymer particles, osmotic pumps, implantable infusion systems, and liposomes. Formulations for inhalation may contain as excipients, for example, lactose, or may be aqueous solutions containing, for example, polyoxyethylene-9-lauryl ether, glycocholate and deoxycholate, or may be oily solutions for administration in the form of nasal drops, or as a gel to be applied intranasally. Formulations for parenteral administration may also include glycocholate for buccal administration, methoxysalicylate for rectal administration, or citric acid for vaginal administration. The present factors can be used as the sole active agents, or can be used in combination with other active ingredients, e.g., other growth factors which could facilitate neuronal survival in neurological diseases, or peptidase or protease inhibitors. The concentration of the present factors in the formulations of the invention will vary depending upon a number of issues, including the dosage to be administered, and the route of administration. In general terms, the factors of this invention may be provided in an aqueous physiological buffer solution containing about 0.1 to 10% w/v compound for parenteral administration. General dose ranges are from about 1 μg/kg to about lg/kg of body weight per day: a preferred dose range is from about 0.01 mg/kg to 100 mg/kg of body weight per day. The preferred dosage to be administered is likely to depend upon the type and extent of progression of the pathophysiological condition being addressed, the overall health of the patient, the make up of the formulation, and the route of administration. A further general aspect of the invention is the use of a factor of the invention in the manufacture of a medicament, preferably for the treatment of a disease or disorder. The "GGF2" designation is used for all clones which were previously isolated with peptide sequence data derived from GGF-II protein (i.e., GGF2HBS5, GGF2BPP3) and, when present alone (i.e., GGF2 OR rhGGF2), to indicate recombinant human protein encoded by plasmids isolated with peptide sequence data derived from the GGF-II protein (i.e., as produced in insect cells from the plasmid HBS5). Recombinant human GGF from the GGFHBS5 clone is called GGF2, rhGGF2 and GGF2HBS5 polypeptide. Methods for treatment of diseases or disorders using nucleic acid constructs encoding neuregulins or neuregulin producer cells are also part of the invention. Delivery of DNA to a cell or tissue that will take up the DNA, express the DNA and produce neuregulin as shown by Wolff et al., (Science 247: 1465. 1990) and Ascadi et al., (Nature 312:815, 1991) is an aspect of the invention. Genetic modification of cultured cells (or their precursors) such as fibroblasts (as shown by Wolff et al. Proc. Nat'l Acad. Sci. USA £6: 1575, 1988) or such as those derived from the nervous system (as shown by Weiss et al. International Patent Application number PCT/US94/01053; publication number WO 94/16718) to induce the production of neuregulin from the cultured cells is another aspect of this invention. The genetically modified neuregulin producer cells can be transplanted to a position near the retinal cell type and elicit the responses described above. Other Embodiments The invention includes methods for the use of any protein which is substantially homologous to the coding segments in Figure 13 as well as other naturally occurring GGF or neuregulin polypeptides for the purpose of promoting retinal cell function. Also included are the use of: allelic variations: natural mutants; induced mutants; proteins encoded by DNA that hybridizes under high or low stringency conditions to a nucleic acid naturally occurring (for definitions of high and low stringency see Current Protocols in Molecular Biology, John Wiley & Sons, New York, 1989; 6.3.1-6.3.6, hereby incorporated by reference); and the use of polypeptides or proteins specifically bound by antisera to GGF polypeptides. The term also includes the use of chimeric polypeptides that include the GGF polypeptides comprising sequences from Figure 1 1 for the promotion of retinal cell function. As will be seen from Example 2, below, the present factors exhibit survival activity on retinal cells. The general statements of invention above in relation to formulations and/or medicaments and their manufacture should clearly be construed to include appropriate products and uses. A series of experiments follow which provide additional basis for the claims described herein. The following examples relating to the present invention should not be construed as specifically limiting the invention, or such variations of the invention, now known or later developed. The examples illustrate our discovery that recombinant human GGF2 (rhGGF2) confers survival effects on retinal cell culture. These activities indicate efficacy of GGF2 and other neuregulins in inducing wound repair and repair of other retinal tissue damage, and promoting regeneration and prophylactic effects on retinal tissue degeneration. EXAMPLES The following examples are designed to illustrate certain aspects of the present invention. The examples are not intended to be comprehensive of all embodiments of the present invention, and should not be construed as limiting the claims presented herein. Example 1: Neuregulin expression in embryonic and adult retina. Neuregulin is expressed in the retina of the developing embryo and in adult rat. The pattem of expression has been demonstrated by in situ hybridization (Figure 3) and also by immunostaining (Figures 2 and 4). Expression is detected in the retinal ganglion cell layer. The expression occurs at a point in development when the retinal layers are undergoing differentiation and when the retinal ganglion cells are extending their axons and making connections to target of innervation in the brain (lateral geniculate and superior colliculus). The timing and distribution of neuregulin gene products in the retina suggests the neuregulins have a role in the development and/or maintenance of the cells in the retina and their associated tissues. Methods In situ hybridization (see Figure 3). Ten micron frozen section was incubated with a single- stranded digoxigenin-labeled riboprobe (antisense strand) encoding the EGF-like domain through the cytoplasmin domain of the rat cDNA clone GGFRP3 (Marchionni et al.. Nature 162:312. 1993). Immunostaining . Ten micron frozen section of embryonic day 16 rat retina was incubated in CN16 (anti-rhGGF2) antibody at approximately 10 mg/ml for 12 hours, and the antibody binding was revealed using indirect immunohistochemistry with a peroxidase conjugated secondary antibody (see Figure 2). Ten micron frozen section from an adult rat retina was incubated in CN16 as described in Figure 1 , except that a fluorescein conjugated secondary antibody was used to reveal the binding of the primary antibody (see Fig 4). Neuregulin immunoreactivity is present in the synaptic layers of the retina, where the processes of the retinal ganglion cells connect with the retinal intemeurons (inner plexiform layer, large arrows) and in the outer plexiform layer (small arrows, where the processes of the photoreceptors make synapses with the second order retinal neurons, the bipolar cells and the horizontal cells). Ten micron section from a newborn rat retina incubated with TUJl antibody, a mouse monoclonal antibody that recognizes neuron-specific beta -tubulin (from Dr. A. Frankfurter, UVA). The antibody binding was revealed by indirect immunohistochemistry with a fluorescein conjugated secondary antibody (see Figure 5). Example 2: Neuregulin (rhGGF2) promotes survival of retinal cells in vitro. Embryonic and newborn rat retinal cells were cultured for 2 days on collagen gel coated coverslips and then fixed and labeled with an antibody (TUJl ) that identifies primarily retinal ganglion cells at these stages of development (see Figure 6). All the labeled cells with processes on the sample coverslips were counted. Final concentrations of rhGGF2 in the culture wells ranged from 0.01 to 100 ng/ml. No clear dose response was observed, so the data from all rhGGF2 treated wells was combined for the analysis. Three separate experiments with embryonic day 18 cells all showed an increase in the number of TUJl immunoreactive cells with processes after two days in vitro (see Figure 7). Unpaired sample student's T-test showed that the increases in cell survival were statistically significant with p<0.004 and p<0.012. Two experiments with embryonic day 15 cells and one experiment with newborn rat retinal cells did not show any significant differences from control (see Figure 8). From these observations we conclude that rhGGF2 promotes rat retinal cell survival in cell culture in an age-dependent manner. This age-dependence could represent either a changing requirement for this factor of a specific retinal cell population or a change in the relative number of the responsive cells in the population during these developmental stages. When assayed either alone or in combination with EGF, rhGGF2 had no mitogenic activity retinal cells in vitro at any of the ages tested. Methods Embryonic rat retinal cell were dissociated and plated at low density on collagen gels and allowed to survive for two days with 10 ng/ml rhGGF2 (neuregulin) added to the medium. After fixation in 4% paraformaldehyde, the cells were stained with TUJl antibody to reveal the full extent of their processes and all the cells that survived for two days with intact, non- fragmented processes were counted.

Claims: CLAIMS What is claimed is: 1. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a polypeptide defined by the formula WYBAZCX wherein WYBAZCX is composed of the polypeptide segments shown in Figure 13; wherein W comprises polypeptide segment F, or is absent; wherein Y comprises polypeptide segment E, or is absent; wherein Z comprises polypeptide segment G, or is absent; and wherein X comprises polypeptide segments C/D HKL. C/D HL, C/D D, C/D' HL, C/D' HKL, C/D' H, C/D' D, C/D C/D' HKL, C/D C/D' H, C/D C/D' HL, C/D C/D' D, C/D D' H, C/D D' HL, C/D D' HKL, C/D' D' H, C/D' D' HL, C/D' D' HKL, C/D C/D' D' H, C/D C/D' D' HL, or C/D C/D' D' HKL. 2. A method of claim 1 , wherein X is C/D HKL. 3. A method of claim 1 , wherein X is C/D H. 4. A method of claim 1 , wherein X is C/D HL. 5. A method of claim 1 , wherein X is C/D D. 6. A method of claim 1 , wherein X is C/D' HL. 7. A method of claim 1 , wherein X is C/D' HKL. 8. A method of claim 1. wherein X is C D' H. 9. A method of claim 1 , wherein X is C/D' D. 10. A method of claim 1 , wherein X is C/D C/D' HKL. 1 1. A method of claim 1 , wherein X is C/D C/D' H. 12. A method of claim 1. wherein X is C/D C/D' HL. 13. A method of claim 1, wherein X is C/D C/D' D. 14. A method of claim 1 , wherein X is C/D D' H. 15. A method of claim 1. wherein X is C/D D' HL. 16. A method of claim 1. wherein X is C/D D' HKL. 17. A method of claim 1, wherein X is C/D' D' H. 18. A method of claim 1. wherein X is C D' D' HL. 19. A method of claim 1, wherein X is C/D' D' HKL. 20. A method of claim 1 , wherein X is C/D C/D' D * H. 21. A method of claim 1, wherein X is C/D C/D' D' HL. 22. A method of claim 1 , wherein X is C/D C/D' D' HKL. 23. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a polypeptide comprising FBA polypeptide segments having the amino acid sequences shown in Figure 1 1. 24. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a polypeptide comprising FBA' polypeptide segments having the amino acid sequences shown in Figure 1 1. 25. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a polypeptide comprising FEBA polypeptide segments having the amino acid sequences shown in Figure 11. 26. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a polypeptide comprising FEBA' polypeptide segments having the amino acid sequences shown in Figure 1 1 to said retinal cells. 27. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with GGF2 polypeptide. 28. The method of claim 27, wherein said GGF2 is recombinant human GGF2. 29. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a compound which binds with erbB receptors of said retinal cells. 30. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a polypeptide comprising EGFL1 , having the amino acid sequence shown in Figure 18. 31. A method of treating retinal cells of a mammal, said method comprising contacting said retinal cells with a polypeptide comprising EGFL2, having the amino acid sequence shown in Figure 19. 32. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a polypeptide comprising EGFL3, having the amino acid sequence shown in Figure 20. 33. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a polypeptide comprising EGFL4, having the amino acid sequence shown in Figure 21. 34. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a polypeptide comprising EGFL5, having the amino acid sequence shown in Figure 22. 35. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a polypeptide comprising EGFL6, having the amino acid sequence shown in Figure 23. 36. A method for treating retinal cells of a mammal, said method comprising contacting a 45 kD polypeptide factor isolated from the rat I-EJ ras- transformed fibroblast cell line to said retinal cells. 37. A method for treating retinal cells of a mammal, said method comprising contacting a 75 kD polypeptide factor isolated from SKBR-3 human breast cell line to said retinal cells. 38. A method for treating retinal cells of a mammal, said method comprising contacting a 45 kD polypeptide factor isolated from the MDA-MB231 human breast cell line to said retinal cells. 39. A method for treating retinal cells of a mammal, said method comprising contacting a 7 to 14 kD polypeptide factor isolated from the ATL-2 human T-cell line to said retinal cells. 40. A method for treating retinal cells of a mammal, said method comprising contacting a 25 kD polypeptide factor isolated from activated mouse peritoneal macrophages to said retinal cells. 41. A method for treating retinal cells of a mammal, said method comprising contacting a 25 kD polypeptide factor isolated from bovine kidney to said retinal cells. 42. A method for treating retinal cells of a mammal, said method comprising contacting an ARIA polypeptide to said retinal cells. 43. A method for treating retinal cells of a mammal, said method comprising contacting a 46-47 kD polypeptide factor known to stimulate 0-2A glial progenitor cells to said retinal cells. 44. A method for treating retinal cells of a mammal, said method comprising contacting GGF-III to said retinal cells. 45. A method for treating retinal cells of a mammal, said method comprising administration to said mammal of a DNA sequence encoding a polypeptide of the formula WYBAZCX wherein WYBAZCX is composed of the polypeptide segments shown in Figure 13; wherein W comprises polypeptide segment F, or is absent; wherein Y comprises polypeptide segment E, or is absent; wherein Z comprises polypeptide segment G, or is absent; and wherein X comprises polypeptide segments C/D HKL, C/D H, C/D HL, C/D D, C/D' HL, C/D' HKL, C/D' H, C/D' D, C/D C/D' HKL, C/D C/D' H, C/D C/D' HL, C/D C D' D, C/D D' H, C/D D' HL, C/D D' HKL, C/D' D * H, C/D' D' HL, C/D' D' HKL, C/D C/D' D' H, C/D C/D' D' HL, or C/D C/D' HKL, said DNA in an expressible genetic construction. 46. A method for treating retinal cells of a mammal, said method comprising contacting said retinal cells with a therapeutically effective amount of a neuregulin polypeptide. 47. A method for the prophylaxis or treatment of pathophysiological condition of retinal cells in a mammal in which said condition involves a retinal cell type which is sensitive or responsive to a neuregulin polypeptide, said method comprising administration of a therapeutically effective amount of said neuregulin polypeptide. 48. A method for the treatment of a condition which involves retinal cell damage in a mammal, said method comprising contacting said retinal cell with an effective amount of a neuregulin polypeptide. 49. The method of any one of claims 1 through 28, wherein a result of said treating is decreased atrophy of said retinal cells. 50. The method on any one of the claims 1 through 28, wherein a result of said treating is an increase of said retinal cells present in said mammal. 51. The method on any one of the claims 1 through 28, wherein a result of said treating is an increase in said retinal cells survival in said mammal. 52. A method of any one of the claims 1 through 28. wherein said retinal cells are in a mammal with a retinal cell disease. 53. A method of claim 52. wherein said retinal cell disease is a retinopathy. 54. A method of claim 53, wherein said retinopathy is hypertensive retinopathy. 55. A method of claim 53, wherein said retinopathy is diabetic retinopathy. 56. A method of claim 53, wherein said retinopathy is occlusive retinopathy. 57. A method of claim 52, wherein said retinal cell disease is retinal degeneration. 58. A method of claim 57, wherein said retinal degeneration is caused by injury. 59. A method of claim 57, wherein said retinal degeneration is caused by a genetic disorder. 60. A method of claim 59. wherein said genetic disorder is retinitis pigmentosa. 61. A method of claim 57, wherein said retinal degeneration is age related macular degeneration. 62. A method of claim 52, wherein said retinal disease is caused by elevated intraocular pressure. 63. A method of claim 52, wherein said retinal disease is caused by an optic neuropathy. 64. A method for the prophylaxis or treatment of a pathophysiological condition of a retina in a vertebrate in which said condition involves a retinal cell type which is sensitive or responsive to a neuregulin polypeptide, said method comprising administration to said vertebrate of a therapeutically effective amount of said neuregulin polypeptide. 65. A method of claim 53, wherein said condition involves retinal cell damage. 66. A method of any one of claims 1 through 28. wherein said retinal cell is a retinal ganglion cell. 67. A method of any one of claims 1 through 28, wherein said retinal cell is an amacrine cell. 68. A method of any one of claims 1 through 28, wherein said retinal cell is a horizontal cell. 69. A method of any one of claims 1 through 28, wherein said retinal cell is a bipolar cell. 70. A method of any one of claims 1 through 28, wherein said retinal cell is a photoreceptor cell. 71. A method of any one of claims 1 through 28, wherein said retinal cell is a pigment cell. 72. A method of treating retinal cells of a mammal, said method comprising contacting an N-ARIA polypeptide to said retinal cells.

Inventors: BERMINGHAM-MCDONOGH OLIVIA, GWYNNE DAVID I, MAHANTHAPPA NAGESH K, MARCHIONNI MARK A, MCCABE KATHRYN L, REH THOMAS A, BERMINGHAM-MCDONOGH, OLIVIA, GWYNNE, DAVID, I., MAHANTHAPPA, NAGESH, K., MARCHIONNI, MARK, A., MCCABE, KATHRYN, L., REH, THOMAS, A.
Applicants: BERMINGHAM MCDONOGH OLIVIA, CAMBRIDGE NEUROSCIENCE INC, GWYNNE DAVID I, MAHANTHAPPA NAGESH K, MARCHIONNI MARK A, MCCABE KATHRYN L, REH THOMAS A, BERMINGHAM-MCDONOGH, OLIVIA, CAMBRIDGE NEUROSCIENCE, INC., GWYNNE, DAVID, I., MAHANTHAPPA, NAGESH, K., MARCHIONNI, MARK, A., MCCABE, KATHRYN, L., REH, THOMAS, A.
Application year: 1996
Main classification: C07K14
IPC codes: A61K31, A61K38, A61P27, C07K14, C12N5, C12N15