LOW OXYGEN CULTURING OF CENTRAL NERVOUS SYSTEM PROGENITOR CELLS

Abstract: The present invention relates to the growth of cells in culture under conditions that promote cell survival, proliferation, and/or cellular differentiation. The present inventors have found that proliferation was promoted and apoptosis reduced when cells were grown in lowered oxygen as compared to environmental oxygen conditions traditionally employed in cell culture techniques. Further, the inventors found that differentiation of precursor cells to specific fates also was enhanced in lowered oxygen where a much greater number and fraction of dopaminergic neurons were obtained when mesencephalic precursors were expanded and differentiated in lowered oxygen conditions. Thus at more physiological oxygen levels the proliferation and differentiation of CNS precursors is enhanced, and lowered oxygen is a useful adjunct for ex vivo generation of specific neuron types. Methods and compositions exploiting these findings are described.

Description: LOW OXYGEN CULTURING OF CENTRAL NERVOUS SYSTEM PROGENITOR CELLS RELATED APPLICATIONS The present application is a continuation-in-part of U.S. Application number 09/195,569 filed November 18, 1998. The entire text of the above referenced application is incorporated herein by reference without prejudice or disclaimer. FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT The U.S. Government may have rights in the present invention pursuant to the terms of grant numbers A 40780-8 and AR42671-05 awarded by the National Institutes of Health and DARPA/AFOSR grant number F49620-98-1-0487. FIELD OF THE INVENTION The present invention relates to the growth of cells in culture. More particularly, the present invention provides methods and compositions for increasing cell survival, cell proliferation and/or cell differentiation along specific pathways by growing the cells in low ambient oxygen conditions. BACKGROUND OF THE INVENTION In a time of critical shortages of donor organs, efforts to bring cellular transplantation into the clinical arena are urgently needed (Neelakanta & Csete, 1996). Indeed, cellular and tissue transplantation is now well recognized as a desirable technique for the therapeutic intervention of a variety of disorders including cystic fibrosis (lungs), kidney failure, degenerative heart diseases and neurodegenerative disease. However, although this may be a desirable and much needed intervention, a major impediment to this type of therapeutic intervention is the lack of an available supply of viable, differentiated cells. Generally differentiated cells cannot be readily expanded in culture. Thus, methods of increasing the number and/or availability of differentiated, viable cells are needed. The central nervous system (CNS) (brain and spinal cord) has poor regenerative capacity which is exemplified in a number of neurodegenerative disorders, such as Parkinson's Disease. Although such diseases can be somewhat controlled using pharmacological intervention (L-dopa in the case of Parkinson's Disease), the neuropathological damage and the debilitating progression is not reversed. Cell transplantation offers a potential alternative for reversing neuropathological damage as opposed to merely treating the consequences of such damage. Cultured CNS stem cells can self-renew, and after mitogen withdrawal, have an intrinsic capacity to generate oligodendrocytes, astrocytes, and neurons in predictable proportions (Johe et al, 1996). Manipulation of this intrinsic differentiation capacity in culture has been used to define a complex array of factors that maintain, amplify, or diminish a particular differentiated phenotype. Most such studies emphasize a primary role for transcription factors in defining CNS lineage identity, as well as growth and trophic factors acting locally and over long distances (Johe et al, 1996, Panchinsion et al, 1998). Dopaminergic neurons and their progenitors from these cultures are of special interest as potential sources of replacement cellular therapies for Parkinson's Disease patients (reviewed in Olanow et al, 1996). Ideally, ex vivo culture conditions should reproduce the in vivo cellular environment with perfect fidelity. This ideal is especially pertinent when explants are used to study development, because conditions may be defined for cell fate choice and differentiation. For CNS stem cell cultures, in particular, maximizing survival, proliferation, and cell fate choice leading to dopaminergic neurons is essential for future cellular transplant therapies. Thus, understanding and control of the differentiation of such cells is crucial for providing a viable, useful product that can be used in transplantation or for studying the behavior of CNS cells, in vitro, in response to various conditions. In embryogenesis, each tissue and organ develops by an exquisitely organized progression in which relatively unspecialized or "undifferentiated" progenitor or stem cells give rise to progeny that ultimately assume distinctive, differentiated identities and functions. Mature tissues and organs are composed of many types of differentiated cells, with each cell type expressing a particular subset of genes that in turn specifies that cell's distinctive structure, specialized function, and capacity to interact with and respond to environmental signals and nutrients. These molecular, structural and functional capacities and properties comprise the cell phenotype. Similarly, coupled cell proliferation and/or differentiation occurs, in the presence of changing local O 2 supply, when an injured or degenerating adult tissue undergoes repair and regeneration. The level of oxygen is especially pertinent in many regeneration paradigms in which normal blood supply is reduced or even transiently stopped by trauma or embolic events (myocardial infarction, stroke and the like). Therefore, in clinical settings, gases are appreciated as a primary variable in organ survival, with oxygen as the critical gas parameter. Virtually all modern cell culture is conducted at 37°C in a gas atmosphere of 5% CO 2 and 95% air. These conditions match core human body temperature and approximate quite well physiologic C0 2 concentrations. For example, mean brain tissue CO 2 is 60 mm Hg or about 7% (Hoffman et al, 1998). However, in striking contract, oxygen in standard tissue culture does not reflect physiologic tissue levels and is, in fact, distinctly hyperoxic. At sea level, (unhumidified) room air contains 21% O 2 which translates into an oxygen partial pressure of 160 mm Hg [0.21(760 mm Hg)]. However, the body mean tissue oxygen levels are much lower than this level. Alveolar air contains 14% oxygen, arterial oxygen concentration is 12%, venous oxygen levels are 5.3%, and mean tissue intracellular oxygen concentration is only 3% (Guyton, and Hall, 1996). Furthermore, direct microelectrode measurements of tissue O 2 reveal that parts of the brain normally experience O 2 levels considerably lower than total body mean tissue oxygen levels, reflecting the high oxygen utilization in brain. These studies also highlight considerable regional variation in average brain oxygen levels (Table 1) that have been attributed to local differences in capillary density. Mean brain tissue oxygen concentration in adult rates is 1.5% (Silver and Erecinska, 1988), and mean fetal sheep brain oxygen tension has also been estimated at 1.6% (Koos and Power, 1987). Table 1 Regional rat brain tissue partial pressures of oxygen measured by microelectrode Brain area %O 2 Cortex (gray) 2.5-5.3 Cortex (white) 0.8-2.1 Hypothalamus 1.4-2.1 Hippocampus 2.6-3.9 Pons, fornix OJ-0.4 Adapted from Silver, L, Erecinska, M. Oxygen and ion concentrations in normoxic and hypoxic brain cells. In Oxygen Transport to Tissue XX, 7-15, edited by Hudetz and Bruley, Plenum Press, New York (1988). Thus, from the discussion above it is clear that under standard culture conditions, the ambient oxygen levels are distinctly hyperoxic, and not at all within physiologic ranges. These conditions of cell growth are have been historically inadequate for generating cells and tissues for transplantation into the brain or other area of the body or for providing an accurate in vitro model of what is occurring in vivo. Thus, there remains a need for methods to produce differentiated cells which can be used for therapeutic and research purposes. The present invention is directed to providing such methods. BRIEF SUMMARY OF THE INVENTION The present invention is directed to growing cells in low ambient oxygen conditions in order to mimic the physiological oxygen conditions with greater fidelity. The growth of these cells in such conditions provides certain surprising and unexpected results. These results are exploited and described in further detail herein. More particularly, the present invention describes methods that may independently be useful in increasing cell survival, cell proliferation and/or cell differentiation along specific pathways. In specific embodiments, the present invention describes a method of increasing cell differentiation comprising culturing undifferentiated central nervous system (CNS) cells in low ambient oxygen conditions, wherein the low ambient oxygen conditions promotes the cellular differentiation of the neuronal cells. The definitions of low ambient oxygen conditions are described in depth elsewhere in the specification. However, it is contemplated that in specific embodiments the low ambient oxygen conditions comprise an ambient oxygen condition of between about 0.25% to about 18% oxygen. In other embodiments, the ambient oxygen conditions comprise an ambient oxygen condition of between about 0.5% to about 15% oxygen. In still other embodiments, the low ambient oxygen conditions comprise an ambient oxygen condition of between about 1 % to about 10% oxygen. In further embodiments, the low ambient oxygen conditions comprise an ambient oxygen condition of between about 1.5% to about 6% oxygen. Of course, these are exemplary ranges of ambient oxygen conditions to be used in culture and it should be understood that those of skill in the art will be able to employ oxygen conditions falling in any of these ranges generally or an oxygen conditions between any of these ranges that mimics physiological oxygen conditions for CNS cells. Thus, one of skill in the art could set the oxygen culture conditions at 0.5%>, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 11.5%, 18%, 18.5%), or any other oxygen condition between any of these figures. The cells employed in the method described may be any cells that are routinely used for CNS studies. As such, the cells may be primary tissue culture cells or derived from a cell line. The cells may be fetal cells or adult cells. In specific embodiments, it is contemplated that the cells may be selected from the group consisting of central nervous system stem cells, spinal cord-derived progenitor cells, glial cells, astrocytes, neuronal stem cells, central nervous system neural crest-derived cells, neuronal precursor cells, neuronal cells, hepatocytes, and bone marrow derived cells. In preferred embodiments, it is contemplated that the cells may be mecencephalic progenitor cells, lateral ganglionic precursor cells, cortical precursor cells, astrocytes or neuroblasts. The method may comprise determining the amount, level or degree of differentiation. Those of skill in the art are familiar with technologies employed to determine cellular differentiation. The differentiation may determined by monitoring a differentiation specific phenotype in the cells. For example, the differentiation specific phenotype determined may be by monitoring message level, protein level, subcelmlar localization, functional assays or morphological changes. There are various techniques that may be employed for determining message level including but not limited to PCR™, in situ hybridization, RNAse protection assay, or single cell PCR™. In specific embodiments, the present invention may monitor the message level for nestin, tyrosine hydroxylase, GAPDH; BDNF; GDNF; FGFR3; Enl; FGF8; SHH; Ptx3; Nurrl; VEGF; EPO; HIFlα or VHL. Of course these are exemplary differentiation markers for CNS cells or markers of cellular responses to oxygen and it is contemplated that those of skill in the art will be able to substitute additional similar markers for the markers specifically described herein without undue experimentation. Other embodiments monitor protein level by, for example, using antibody staining, HPLC, western blotting or immunoprecipitation. In more particular embodiments, the protein level monitored is the level of nestin, tyrosine hydroxylase, dopamine β-hydroxylase or dopamine transporter. The functional assay typically will be one that monitors a particular function of the selected CNS cells. A particularly useful functional assay may be one which monitors the rate of dopamine production. A preferred feature of the present invention is that the low oxygen conditions produce a cell population that is enriched in dopaminergic neurons as compared to a similar cell population that is grown in 20%> oxygen incubator conditions. Another preferred embodiment is that the low oxygen conditions produce a cell population that is enriched in serotoninergic neurons as compared to a similar cell population that is grown in 20% oxygen incubator conditions. In still additional embodiments, the low oxygen conditions produce a cell population that is depleted in GABAnergic neurons as compared to a similar cell population that is grown in 20% oxygen incubator conditions. Further, certain methods of the present invention will provide low oxygen conditions to produce a cell population that is depleted in glutaminergic neurons as compared to a similar cell population that is grown in 20%> oxygen incubator conditions. In preferred embodiments, the method may further comprise growing the cells in the presence of a neuronal growth stimulant, mitogen, cytokine, neuroprotective factor or an anti-apoptotic agent. The inventors have found that there was a significant increase in EPO expression as a result of lowered oxygen versus 20% O 2. In particular embodiments, the differentiated phenotype is retained after transfer of the cells from the low ambient oxygen conditions to 20% oxygen culture conditions. In specific embodiments, it is contemplated that the cells may be grown in low ambient oxygen conditions for multiple generations prior to transfer to 20% oxygen culture conditions. In other embodiments, the cells may be continuously maintained in low ambient oxygen conditions. Another aspect of the present invention provides a method of inhibiting apoptosis of a CNS cell in culture comprising growing the cell in low ambient oxygen conditions. Yet another embodiment provides a method of increasing the expansion of a CNS cell in culture comprising growing the cell in low ambient oxygen, wherein the cells exhibit increased expansion in the low ambient oxygen as compared to growing the cell in 20% oxygen incubator conditions. In an additional embodiment, the present invention further contemplates a method of increasing cell proliferation in culture comprising growing CNS cells in low ambient oxygen, wherein the growth in low ambient oxygen increases cell proliferation compared to growing the cells in 20%> oxygen incubator conditions. Also provided is a method of preparing a cell for use against a neurodegenerative disorder comprising obtaining a population of CNS cells and growing the cells in low ambient oxygen conditions wherein the low ambient oxygen conditions increases the expression of a gene involved in the neurodegenerative disease. In specific embodiments, the neurodegenerative disease is Parkinson's Disease and the gene is tyrosine hydroxylase (TH). The method further may comprise contacting the cell(s) with a first polynucleotide encoding a dopamine biosynthetic protein under conditions suitable for the expression of the protein wherein the polynucleotide is under the transcriptional control of a promoter active in the cells. In addition, the method further may comprise contacting the cell with a first polynucleotide encoding a dopamine releasing protein under conditions suitable for the expression of the protein wherein the polynucleotide is under the transcriptional control of a promoter active in the cells. Also contemplated is a method further comprising contacting the cell with a second polynucleotide encoding a dopamine releasing protein under conditions suitable for the expression of the protein wherein the polynucleotide is under the transcriptional control of a promoter active in the cells. Other embodiments involve contacting the cell with a second polynucleotide encoding a dopamine biosynthetic protein under conditions suitable for the expression of the protein wherein the polynucleotide is under the transcriptional control of a promoter active in the cells. In more particular embodiments, the dopamine biosynthesis protein may be TH; L-amino acid decarboxylase (AADC), erythropoietin or any other protein directly or indirectly involved in dopamine synthesis. The dopamine releasing protein is a vesicular monoamine transporter (VMAT), which may be VMAT1 or VMAT2. In specific embodiments, the first and second polynucleotides are under control of different promoters. The promoter may be any promoter known to those of skill in the art that will be operative in the cells being used. For example, it is contemplated that the promoter may be CMV IE, SV40 IE, β-actin, TH promoter, AADC promoter, and nestin promoter. It is contemplated that the first and second polynucleotides each may be covalently linked to a polyadenylation signal. Also encompassed by the present invention is a cell produced according to the method comprising obtaining a starting CNS cell and growing the cell in low ambient oxygen conditions wherein the conditions produce a differentiated neuronal cell. In specific embodiments, the starting cell is a nestin-positive cell. More particularly, the low ambient conditions produce a nestin-negative differentiated cell more rapidly and in greater numbers than traditional cell culture conditions. In specific embodiments, the low ambient conditions produce a TH positive cell. In other embodiments, the cell further comprises an expression vector comprising a polynucleotide encoding an exogenous gene wherein the polynucleotide is operatively linked to a promoter. Another aspect of the present invention provides a method of treating Parkinson's disease in a subject comprised of obtaining cells suitable for transplanting in the subject; growing the cells in low ambient oxygen conditions; and implanting the cells grown in the low ambient oxygen conditions into the subject; wherein the implanted cells have an increased capacity to produce dopamine in the subject as compared to similar cells grown in 20%o oxygen incubator conditions. In specific embodiments, the cells are from the subject and have been transduced with a polynucleotide that expresses a protein that increases dopamine production and are treated or expanded in lowered oxygen conditions. In other preferred embodiments, the cells are CNS cells from a source other than the subject. In preferred embodiments, the cells are transduced with a polynucleotide that expresses a protein that increases dopamine production and are treated or expanded in lowered oxygen conditions. Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein. FIG. 1. Effect of lowered oxygen on precursor yield in vitro at varying plating densities. Striatal cultures were expanded with bFGF in lowered or ambient oxygen, and total cell numbers assessed after 5 days of proliferation when over 95% of cells are nestin+ precursors. Significantly increased cell numbers were detected at all densities in lowered O 2 compared to ambient oxygen. FIG. 2. Lowered oxygen culturing leads to increased proliferation of CNS precursors. FIG. 2A. Mesencephalic precursors were pulsed with 10 μM BrdU for 60 minutes immediately before fixation, then stained for BrdU uptake. More BrdU+ cells were seen in lowered oxygen cultures during both proliferation and differentiation. Scale bar = 20 μm. FIG. 2B. Mesencephalic precursors in lowered O 2 yielded an increased percentage of BrdU+ cells and a greater absolute number of BrdU+ cells than cultures maintained at 20%o O 2 . Data are given as mean +/- SEM, n=40. Differences between lowered and 20%o O 2 were statistically significant at all time points and for all parameters (n=8, p < 0.05) except percentage of BrdU+ cells at day 4 of expansion (n=8, p = 0J0). FIG. 3. CNS precursors cultured in lowered (vs. 20%) O 2 have reduced rates of apoptosis. FIG. 3A. Apoptosis was assayed by TUNEL labeling of mesencephalic precursors cultured in parallel at either lowered or 20% O 2 . Representative figures of the expansion phase (2 and 6 days of culture) and the differentiation phase (4 days after bFGF withdrawal) are shown. Scale bar = 20 μm. FIG. 3B. Precursors grown at lowered O 2 showed a significant decrease in the percentage of apoptotic cells (n=8, p < 0.05) compared to traditional cultures. FIG. 4. Basic differentiation patterns of CNS stems in lowered and 20% O 2 cultures. FIG. 4A. Striatal cultures in lowered or 20% O 2 were assessed for the relative percentages of precursor-derived neurons (by TUJl stain), astrocytes (GFAP) and oligodendrocytes (Gal-C) after 5 days of bFGF proliferation followed by four days of cell differentiation (for quantification see text). FIG. 4B. Passaged mesencephalic precursors were proliferated for 6 days and differentiated for 5 days in lowered or 20% O 2 and analyzed for 04, a marker of oligodendrocyte precursors. O4+ cells could be detected only in lowered oxygen cultures. FIG. 4C. Nestin+ clones were derived from single passaged mesencephalic precursor cells after 20 days of bFGF proliferation (left panel). Clones in lowered oxygen differentiated into TUJ1+ neurons upon bFGF withdrawal (right panel). FIG. 4D. Lowered O 2 promotes clone formation efficiency. The yield of clones derived from single precursors was 3- fold higher in lowered O 2 compared to 20% O 2 cultures (left panel). The majority of clones derived from precursors in O 2 oxygen cultures contained 50 - 500 cells whereas clone size in 20 % O 2 cultures was generally 5 - 50 cells (right panel). Scale bar = 20 μm in all panels. FIG. 5. Lowered O 2 culturing improves the yield of functional precursor- derived dopaminergic neurons. FIG. 5A and FIG. 5B. Precursors from E12 mesencephalon were proliferated with bFGF for 5 days followed by 5 days of differentiation, then stained for the neuronal marker TUJl and for TH. A large increase in total number (and percentage) of TH+ neurons was detected (p < 0.001) in lowered O 2 compared to 20% O 2 cultures. Scale bar = 20 μm. FIG. 5C. Quantification of TH protein level by Western blot analysis revealed significantly more TH in samples from lowered (vs. 20%) O 2 cultures. Each lane was loaded with 2.5m μg total protein. FIG. 5D. rp-HPLC with electrochemical detection was used to quantify dopamine levels in conditioned medium (24 hrs), in HBSS after 15 minutes of conditioning (basal release), and in HBSS+56 mM KC1 after 15 minutes (evoked release). Significantly more dopamine was detected in cultures maintained at lowered O 2 compared to those grown at 20% O 2 under all these conditions (conditioned medium p < 0.01; basal and evoked release p < 0.05). Inset shows typical chromatogram for dopamine detection in lowered and 20% O 2 cultures. FIG. 6. Neuronal subtype differentiation from mesencephalic precursors in lowered vs. 20% O 2 . Double immunocytochemical labeling revealed that lowered O 2 culturing markedly increased the representation of dopaminergic and serotonergic neuronal (Tujl+) subtypes, but decreased the representation of GABA+ and Glutamate+ neurons. Colony depicted in GABA stain at 20% O 2 is an unusual example of very high GABA expression under these conditions. TH and GABA were not co-expressed as seen in some developing neurons in vivo. Floor plate cells (FP4+) were more numerous in lowered O 2 cultures as was the percentage of neurons expressing the midbrain transcription factor Enl . Precursor markers nestin and PSA- NCAM were both reduced in lowered O 2 cultures after differentiation compared to 20% O 2 conditions (lower right panels). Scale bars = 20 μm. FIG. 7. Differential gene expression in mesencephalic precursors at lowered and 20% O 2 assessed by RT-PCR. FIG. 7A. Expression of genes involved in the physiological response to changes in oxygen levels. The expression of HIFlα, NHL, EPO and VEGF was assessed after 2 or 6 days of expansion and after differentiation (day 4 of differentiation = day 10 of culture) in lowered and 20% O 2 . Data are normalized to GAPDH expression. A significant increase in EPO expression was detected in lowered oxygen versus 20% O 2 mostly during cell differentiation, whereas NEGF was upregulated during both expansion and differentiation. Surprisingly, no major oxygen-dependent regulation of HIFlα or VHL was observed. FIG. 7B. Candidate genes involved in midbrain development were also tested for O 2 -dependent differential expression. Increased expression of TH and Ptx-3 during cell differentiation confirmed the larger number of functional dopaminergic neurons in lowered oxygen cultures (compare FIGs. 5). Significant lowered O 2 -mediated changes in expression levels of FGF8 and Enl were also detected. FIG. 8. EPO mimics the lowered oxygen effect on dopaminergic differentiation. Saturating concentrations of EPO or EPO neutralizing antibody were added to E12 mesencephalic precursor cultures during both proliferation and differentiation phase (5 days each) in lowered or 20% O 2 . EPO supplementation significantly increased TH+ cell numbers in 20% O 2 cultures (n^ό, p < 0.05). EPO neutralizing antibody decreased TH+ cell numbers in both lowered oxygen (n=6, p < 0.01) and 20% O 2 cultures (n=6, p < 0.05). Scale bar = 20μm. DETAILED DESCRIPTION OF THE INVENTION In order for cell transplantation therapies to become widely and universally used there is a need for availability of appropriately differentiated, viable cells. Preferably, these cells need to be resilient enough that they can be cryopreserved without loss of phenotypic integrity. The high incubator O 2 levels in which the cells are grown at ambient air O 2 levels (referred to herein as traditional O2 conditions; or 20% O 2 culture conditions) do not facilitate the production of such cells. These cells often do not survive, proliferate or differentiate in sufficient numbers to be useful. As such expansion of these cells in traditional culture yields a cell that is at best inadequate for use in in vitro model assay studies let alone for use in transplantation. The present invention is directed towards providing methods and compositions for producing cells that are differentiated, viable, amenable to cryopreservation and provide an accurate indication of how such cells behave biochemically in an in vivo setting. As such, these methods will provide cells that can be used in vitro to perform characterization studies or in vivo as replacement therapies for cells that have been damaged by disease, injury resulting from trauma, ischemia, or a drug-induced injury. Further, it should be noted that the method leads to increased survival of undifferentiated precursors that could also be used for transplantation, which when placed in the appropriate environmental conditions will differentiate down the appropriate pathway. The present invention particularly contemplates the use of culture conditions using subatmospheric/physiological oxygen to culture or enrich a population of neuronal cells with cells that are expanded and/or differentiated to express a particular neuronal phenotype. The increase in cell differentiation may be such that the process of a cell being converted from a primitive undifferentiated state to one in which a particular cellular phenotype (dopaminergic phenotype; GABAergic phenotype; serotoninergic phenotype or the like) is expressed. Specifically, it appears that growth in low O 2 conditions results in an enrichment in dopaminergic and serotinergic neuronal populations, whereas GABAergic and glutaminergic neurons are relatively decreased. These enriched populations may be subject to further enrichment through such methods such as cell sorting. Alternatively, it may be that the increase in differentiation produced by this method is such that the relative percentage of cells that go on to differentiate (as opposed to remaining in an undifferentiated state) is increased in low oxygen. However, incubation of a pluripotent cell line under low O 2 incubation conditions in vitro, will allow the manipulation or skewing of the direction of differentiation of the cell population. Thus, the oxygen is used to control the number and percentage of one type of cell in the population increased or decreased because the differentiation pathway changes under influence of the gas. Thus, enrichment of the CNS cells by physiologic or low levels of oxygen may be the result of one or more mechanisms that include (1) increase in the absolute number of CNS cells, (2) enrichment by selective survival of CNS cells, (3) enrichment of CNS by their selective proliferation or (4) enrichment of specific differentiation pathways. Any increase in the number of CNS cells is significant in that more cells are then available to regenerate a greater volume of new tissue. An enrichment, even without increase in number, is important in applications where limitations on total cell number are pertinent or when the effects of the non-CNS progenitor cell contaminants are negative for the desired outcome or for defining the material adequately. Any enhancement of survival of the CNS cell, even without increase in cell number or any enrichment of cell types is valuable in settings where culture is required (i.e., to handle tissue before administration of cell therapy, or to permit any other procedure during which the cells must survive such as transfection of genes, drug treatment, or enrichment by cell sorting or other additional procedures). A particular embodiment of the present invention demonstrates that growth of CNS cells, or indeed any pluripotent stem cell in subatmospheric culture conditions reduces the level of apoptotic and non-apoptotic cell death. It is likely that the increased survival of the cells may be due to both an inhibition of apoptosis and non- apoptotic death. Apoptosis or programmed cell death is a well known phenomenon and can be measured by techniques well know to those of skill in the art. A particular and novel aspect of the methods of the present invention is that such methods all employ low ambient culture growth conditions. By the term "low ambient oxygen conditions", the present invention refers to any culturing conditions below atmospheric oxygen. Thus in particular embodiments, low ambient O2 conditions are defined as between about 0.5% and about 18%>. Ideally, the culture oxygen conditions are kept as close as possible to the normal physiological oxygen conditions in which a particular cell would be found in in vivo the better. Clearly, this will mean that those conditions employed for cells will depend on the regional origin of a particular cell. For example, cells from an alveolar origin may prefer growth at about 14% O 2 ; cells from an arterial source will prefer an oxygen concentration of about 12%; whereas those from certain regions of the brain may prefer oxygen conditions as low as about 1.5%. It should be noted that the low ambient oxygen conditions are not to be considered the same as "hypoxic" conditions. The low ambient oxygen conditions are intended to mimic physiological conditions. As defined herein "hypoxic conditions" are those in which the oxygen level is less than 0.1% O 2 (Gross et al, 1999). The low ambient oxygen conditions thus will be used to promote differentiation of CNS cells, inhibit apoptosis of cells in culture, increase expansion of cells, and otherwise make such cells amenable for use in transplantation. Such methods and compositions are outlined in further detail below. Definitions The present section provides definitions of the terms used in the present invention in order to facilitate a better understanding of the invention. A "stem cell" is a relatively undifferentiated cell that can be induced to proliferate and that can produce progeny that subsequently differentiate into one or more mature cell types, while also retaining one or more cells with parental developmental potential. In many biological instances, stem cells are also "multipotent" because they can produce progeny of more than one distinct cell type, but this is not required for "stem-ness." Self-renewal is the other classical part of the stem cell definition, and it is essential as used in this document. In theory, self- renewal can occur by either of two major mechanisms. Stem cells may divide asymmetrically, with one daughter retaining the stem state and the other daughter expressing some distinct other specific function and phenotype. Alternatively, some of the stem cells in a population can divide symmetrically into two stems, thus maintaining some stem cells in the population as a whole, while other cells in the population give rise to differentiated progeny only. Formally, it is possible that cells that begin as stem cells might proceed toward a differentiated phenotype, but then "reverse" and re-express the stem cell phenotype. "Progenitor cells" have a cellular phenotype that is more primitive (i.e., is at an earlier step along a developmental pathway or progression than is a fully differentiated cell). Often, progenitor cells also have significant or very high proliferative potential. Progenitor cells may give rise to multiple distinct differentiated cell types or to a single differentiated cell type, depending on the developmental pathway and on the environment in which the cells develop and differentiate. Like stem cells, it is possible that cells that begin as progenitor cells might proceed toward a differentiated phenotype, but then "reverse" and re-express the progenitor cell phenotype. "Differentiation" refers to the developmental process whereby cells assume a specialized phenotype, i.e., acquire one or more characteristics or functions distinct from other cell types. In most uses, the differentiated phenotype refers to a cell phenotype that is at the mature endpoint in some developmental pathway. In many but not all tissues, the process of differentiation is coupled with exit from the cell cycle — in these cases, the cells lose or greatly restrict their capacity to proliferate when they differentiate. "Subatmospheric" conditions mean any oxygen concentration below about 20%, preferably below about 15%, more preferably below about 10%, at sea level. The term subatmoshpheric may be used herein interchangeably with "low oxygen conditions" defined above. "Atmospheric O 2 conditions" are those conditions found in the air, i.e., 20- 21% O 2 . As used herein this term is used interchangeably with the term "traditional" O2 conditions as traditional tissue culture incubators are kept at atmospheric O 2 conditions. "Physiologic" oxygen levels are the range of oxygen levels normally found in healthy tissues and organs. These levels vary depending on tissue type (Table 1). However, it is of note that this rate is below 15% in all tissues and below 8% in most tissues. Thus the physiological oxygen levels can range from about 15% to about 1.5% depending upon the region of the body being measured. "Hypoxia" occurs when the normal physiologic levels of oxygen are not supplied to a cell or tissue. "Normoxia" refers to normal physiologic levels of oxygen for the particular cell type, cell state or tissue in question. "Anoxia" is the absence of oxygen. "Hypoxic conditions" are those leading to cellular hypoxia. These conditions depend on cell type, and on the specific architecture or position of a cell within a tissue or organ, as well as the metabolic status of the cell. A critical point is that in most cell biology research of the past 25 years, ambient atmospheric oxygen levels of 20-21% are routinely called and experimentally taken to be "normoxic," but this assumption is physiologically erroneous. In this historic context, much cell culture literature refers to any condition with oxygen lower than ambient atmospheric as "hypoxic," but this usage is also physiologically incorrect. "Acidosis" means that the pH is below normal physiologic levels. "Enriching" of cells means that the yield (fraction) of cells of one type is increased over the fraction of cells in the starting culture or preparation. "Proliferation" refers to an increase in the number of cells in a population (growth) by means of cell division. Cell proliferation is generally understood to result from the coordinated activation of multiple signal transduction pathways in response to the environment, including growth factors and other mitogens. Cell proliferation may also be promoted by release from the actions of intra- or extracellular signals and mechanisms that block or negatively affect cell proliferation. "Regeneration" means re-growth of a cell population, organ or tissue after disease or trauma. Other terms used throughout the specification will have the meaning commonly assigned by those of skill in the art unless otherwise stated. Central Nervous System Cells As mentioned earlier, a particular advantage of the present invention is that it can be used to generate viable cells or tissue that can be used to ameliorate neurodegenerative disorders. Such cells or tissue upon transplantation can be referred to as a graft. The cells for transplantation can include but are not limited human or animal neurons for stroke, brain and spinal cord injury, Alzheimer's Disease, Huntington's Disease and other neurodegenerative disorders; septal and GABAergic cells for epilepsy; ventral mesencephalic or other CNS dopaminergic cells for treatment of Parkinson's Disease; and trophic factor secreting cells for neurological disorders, or even certain psychiatric disorders. The cells to be used as grafts can be from primary tissue or even from certain cell lines. Further it should be understood that any of the cell types mentioned herein throughout may be adult cells or from a fetal origin. For treatment of neurological disorders, the present invention will produce differentiated neural stem cells that proliferate and differentiate. Undifferentiated neural progenitor cells differentiate into neuroblasts and glioblasts which give rise to neurons and glial cells. During development, cells that are derived from the neural tube give rise to neurons and glia of the CNS. Certain factors present during development, such as nerve growth factor (NGF), promote the growth of neural cells. Methods of isolating and culturing neural stem cells and progenitor cells are well known to those of skill in the art (Hazel and Muller, 1997; U.S. Patent No. 5,750,376). Suitable neural cells may be obtained from suitable solid tissues these include any organ or tissue from adult, post-natal, fetal or embryonic mammalian tissue. Any mammal can be used in this invention, including mice, cattle, sheep, goat, pigs, dogs, rats, rabbits, and primates (including human). Specific examples of suitable solid tissues include neurons or central nervous system supporting cells derived from brain tissue, germ cells or embryonic stem cells. Stem cells and progenitor cells isolated from any other solid organ (liver, pancreas, spleen, kidney, thyroid, etc.) or those from marrow, spleen or blood are also amenable candidates for culturing under physiologic or hypoxic conditions. Hazel and Muller describe methods of isolating, culturing, and differentiating rat brain neuroepithelial stem cells from both fetal and adult rat brains. Briefly, neural precursors are removed from desired regions of the fetal rat central nervous system by dissection, dissociated to a single-cell suspension, and plated on tissue culture dishes in medium containing the mitogen basic fibroblast growth factor (bFGF). Initially, many of the differentiated neurons die. Proliferating cells are then harvested in a buffered solution. The passaged cells are relatively homogenous for multipotent precursors. To induce differentiation to neurons and glia, the media containing bFGF is removed and replaced with media lacking bFGF. Subatmospheric culturing conditions can be used in such a protocol from the start of stem cell isolation, in order to enrich the stem cell pool and enhance differentiation into a greater number of cells. Subatmospheric/physiologic culture conditions can also be used after initial plating and division, to up-regulate certain gene products in the more differentiated brain cells. Subatmospheric/physiologic culture conditions can also be used throughout the process to enhance the function of the entire population for transplantation. Detection of neural stem cell derivatives can be determined by antibody staining. For example, central nervous system multipotential stems are marked by high level expression of the intermediate filament, nestin (Hazel & Muller, 1997). The differentiated neurons are marked by the antibody TUJl (O'Rourke et al, 1997), oligodendrocytes by GalC (Bosio et al, 1996), and astrocytes by GFAP antibodies (Rutka et a/., 1997). The methods of the present invention may be used to produce neural cells containing a heterologous gene. Methods of producing cells of neural origin comprising a heterologous gene and uses of such cells are described in U.S. Patent No. 5,750,376 (incorporated herein by reference). Culture Conditions Suitable medium and conditions for generating primary cultures are well known in the art and vary depending on cell type. For example, skeletal muscle, bone, neurons, skin, liver, and embryonic stem cells are all grown in media differing in their specific contents. Furthermore, media for one cell type may differ significantly from lab to lab and institution to institution. As a general principle, when the goal of culturing is to keep cells dividing, serum is added to the medium in relatively large quantities (10-20% by volume). Specific purified growth factors or cocktails of multiple growth factors can also be added or sometimes used in lieu of serum. As a general principle, when the goal of culturing is to reinforce differentiation, serum with its mitogens is generally limited (serum about 1-2% by volume). Specific factors or hormones that promote differentiation and/or promote cell cycle arrest can also be used. Physiologic oxygen and subatmospheric oxygen conditions can be used at any time during the growth and differentiation of cells in culture, as a critical adjunct to selection of specific cell phenotypes, growth and proliferation of specific cell types, or differentiation of specific cell types. In general, physiologic or low oxygen-level culturing is accompanied by methods that limit acidosis of the cultures, such as addition of strong buffer to medium (such as Hepes), and frequent medium changes and changes in CO 2 concentration. Cells can be exposed to the low oxygen conditions using a variety of means. Specialized laboratory facilities may have completely enclosed environments in which the oxygen levels are controlled throughout a dedicated, isolated room. In such specialized areas, low oxygen levels can be maintained throughout the isolation, growth and differentiation of cells without interruption. Very few laboratories have such specialized areas. Physiologic or low oxygen culturing conditions also can be maintained by using commercially-available chambers which are flushed with a predetermined gas mixture (e.g., as available from Billups-Rothenberg, San Diego CA). As an adjunct, medium can be flushed with the same gas mixture prior to cell feeding. In general, it is not possible to maintain physiologic or low oxygen conditions during cell feeding and passaging using these smaller enclosed units, and so, the time for these manipulations should be minimized as much as possible. Any sealed unit can be used for physiologic oxygen or low oxygen level culturing provided that adequate humidification, temperature, and carbon dioxide are provided. In addition to oxygen, the other gases for culture typically are about 5% carbon dioxide and the remainder is nitrogen, but optionally may contain varying amounts of nitric oxide (starting as low as 3 ppm), carbon monoxide and other gases, both inert and biologically active. Carbon dioxide concentrations typically range around 5% as noted above, but may vary between 2-10%. Both nitric oxide and carbon monoxide are typically administered in very small amounts (i.e. in the ppm range), determined empirically or from the literature. The optimal physiologic or low oxygen level conditions for any given cell type or any particular desired outcome will vary. A skilled artisan could determine suitable subatmospheric conditions by generating an oxygen dose response curve, in which carbon dioxide is kept constant, and oxygen levels are varied (with nitrogen as the remaining gas). For example, to determine the optimal ambient oxygen culturing conditions for expansion of a CNS cell, one would establish cultures from an organ system. The initial culture is mixed, consisting of some differentiated cells, cells of other developmental lineages or pathways, as well as CNS cells. After exposure to the various oxygen levels (e.g. 1%, 2%, 5%, 10% and 15%), the number and function of CNS cells is assessed by methods appropriate to the system. In some cases, a constellation of molecular markers is available to rapidly identify the cell population. But in other cases, a single marker coupled with proliferation assays is appropriate, while in other cases proliferation assays alone are appropriate. In some cases all or some of the above assays are coupled with bioassays to follow the differentiation potential of the presumed stem cells. Overall, the precise assays used to determine stem cell and/or progenitor response to oxygen levels are dependent on the nature of the system examined as well as available markers and techniques specific to that system. The timing of physiologic or low oxygen conditions is also part of the oxygen dose response curve. Some cells may be more or less sensitive to oxygen during isolation or immediately after isolation while some cells may respond only after some time in culture. The timing of physiologic or low oxygen conditions absolutely and in relation to other manipulations of the cultures is part of assessing the optimal oxygen culturing conditions. Furthermore, the mitogenic effects of other gases may be synergistic with physiologic or low oxygen conditions. Different gene regulatory networks may be induced by low/physiologic oxygen culturing during different phases of culture. During expansion of the cells, low oxygen may induce gene expression distinct from that induced by low oxygen during differentiation. The cells are typically exposed to low oxygen level conditions for a time sufficient to enrich the population of progenitor/stem cells compared to other cell types. Typically this is for 1 or more hours, preferably 3 or more hours, more preferably 6 or more hours, and most preferably 12 or more hours, and may be continuous. The temperature during the culture is typically reflective of core body temperature, or about 37°C, but may vary between about 32°C and about 40°C. Other important embodiments may simply achieve an increase in cell absolute number or promote the survival of cells. Following an initial exposure to low or physiologic oxygen culturing conditions, cells can be maintained in these conditions or returned to normal laboratory oxygen conditions, depending on the desired outcome. It is understood that the initial medium for isolating the CNS cells, the medium for proliferation of these cells, and the medium for differentiation of these cells can be the same or different. All can be used in conjunction with low or physiologic oxygen level culturing. The medium can be supplemented with a variety of growth factors, cytokines, serum, etc. Examples of suitable growth factors are basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), transforming growth factors (TGFα and TGFβ), platelet derived growth factors (PDGFs), hepatocyte growth factor (HGF), insulin-like growth factor (IGF), insulin, erythropoietin (EPO), and colony stimulating factor (CSF). Examples of suitable hormone medium additives are estrogen, progesterone, testosterone or glucocorticoids such as dexamethasone. Examples of cytokine medium additives are interferons, interleukins, or tumor necrosis factor-α (TNFα). One skilled in the art will test additives and culture components at varied oxygen levels, as the oxygen level may alter cell response to, active lifetime of additives or other features affecting their bioactivity. In addition, the surface on which the cells are grown can be plated with a variety of substrates that contribute to survival, growth and/or differentiation of the cells. These substrates include but are not limited to laminin, poly-L-lysine, poly-D-lysine, polyornithine and fibronectin. Additional Factors for Promotion of Growth and Differentiation As described herein, the present invention provides methods of increasing the survival, differentiation and phenotypic integrity of CNS cells. This method generally involves growing these cells in vitro within physiological oxygen parameters. There is now a wealth of literature pointing to other factors that may increase the survival of such cells. It is contemplated that the use of some of these factors in combination with the growth conditions of the present invention will be useful. Much of this interest has focussed on finding trophic factors. These factors are able to increase the survival of dopaminergic cells prepared for transplantation; maintain the in situ survival post-transplantation of embryonic neurons transplanted into the striatum; as well as increase graft volume, and thereby re-innervate a larger part of the caudate and putamen which has been shown to have effect both in vitro and in vivo. Trophic factors such as NGF, bFGF, EGF, IGF I and II, TGFβl-3, PDGF, brain derived growth factor (BDNF), ganglion derived growth (GDNF), neurotrophin (NT)-3, NT-4, and ciliary neuronal trophic factor (CNTF), (Engele and Bohn, 1996; Mayer et al, 1993a and 1993b; Knusel et al, 1990, 1991; Poulsen et al, 1994; Nikkhah et al, 1993; Othberg et al, 1995; Hyman et al, 1991) have been investigated and shown to have pronounced effects in vitro. MPTP and 6-OHDA lesions in primates are models of certain neurodegenerative disorders. It has been shown that the effects of such lesions can be reversed in primates and rats (Gash et al, 1996) by the addition of NGF or bFGF to the cell suspension prior to grafting. The same factors also have been shown to increase graft survival, if added to the cell suspension prior to grafting (Chen et al, 1996; Dunnett and Bjorkland., 1994). Additional studies showed an increased graft survival rate in transplanted neurons derived from a neural progenitor (CINP) cell line, that were retrovirally transduced with NGF (Martinez-Serrano et al, 1995) and astrocytes transduced with BDNF (Yoshimoto et al, 1995). GDNF has been shown to increase graft survival, extend fiber outgrowth and alleviate behavioral effects after 6- hydroxydopamine lesions in the striatum of rats (Sauer et al, 1994; Bowenkamp et al, 1995; Rosenblad et β/., 1996; Olson, 1996). Thus these and other factors that may prolong the survival of the CNS cells either in vitro or in vivo are contemplated for use in the growth and maintenance conditions described in the present invention. Induction of differentiation with EPO at atmospheric O 2 In another embodiment, EPO can be used to mimic the lowered oxygen effect on dopaminergic differentiation. That is, EPO at atmospheric oxygen levels is used to promote differentiation of stem cells, CNS progenitor cells, or progeny thereof to produce dopaminergic cells. Mesencephalic precursor cells are particularly preferred. The amount of EPO used in this embodiment is an amount sufficient to produce greater than at least about 10%, preferably 20%>, more preferably 50% of dopaminergic cells. Preferably, saturating concentrations of EPO are added the culture either during proliferation, during differentiation or during both phases. The cultures are contacted with EPO for greater than at least about 1, preferably 2, more preferably 5 days. For example, a culture of CNS progenitor cells could be cultured in an atmosphere containing 5% CO2, 20% O2 (balance N 2 ) for 5 days at saturating concentrations of EPO during the proliferative phase, and subsequently cultured for 3 days at 10% EPO during the differentiation phase. The atmosphere preferably contains 20% 0„ although lower O 2 concentrations can be used. Transplantation Methods Laboratory and clinical studies have shown the transplantation of cells into the CNS is a potentially significant alternative therapeutic modality for neurodegenerative disorders such as Parkinson's disease (Wictorin et al, 1990; Lindvall et al, 1990; Sanberg et al, 1994; Bjorklund and Stenevi, 1985; Freeman et al, 1994). In some cases, transplanted neural tissue can survive and form connections with the CNS of the recipient, i.e. the host (Wictorin et al, 1990). When successfully accepted by the host, the transplanted cells and/or tissue have been shown to ameliorate the behavioral deficits associated with the disorder (Sanberg et al, 1994). The obligatory step for the success of this kind of treatment is to have enough viable cells available for the transplant. The physiologic/subatmospheric culturing conditions described herein can be used to differentiate specific populations of CNS cells useful for transplantation, and to expand the number of available CNS cells derived from a variety of culture systems. In addition to cell cultures described above, fetal neural tissue is another important source for neural transplantation (Lindvall et al, 1990; Bjorklund, 1992; Isacson et al, 1986; Sanberg et al, 1994). Other viable graft sources include adrenal cells and various cell types that secrete neural growth factors and trophic factors. The field of neural tissue transplantation as a productive treatment protocol for neurodegenerative disorders has received much attention resulting in its progression to clinical trials. To date the major problem with this field has been the lack of ability to obtain enough viable cells. The present invention provides a method of maintaining such tissue in a state that will prevent them from losing their ability to serve as an appropriate graft for neurodegenerative diseases. Methods of grafting cells are now well known to those of skill in art (U.S Patent 5762926; U.S Patent 5650148; U.S Patent 5,082,670) . Neural transplantation or grafting involves transplantation of cells into the central nervous system or into the ventricular cavities or subdurally onto the surface of a host brain. Conditions for successful transplantation include: 1) viability of the implant; 2) retention of the graft at the site of transplantation; and 3) minimum amount of pathological reaction at the site of transplantation. Methods for transplanting various nerve tissues, for example embryonic brain tissue, into host brains have been described in Neural Grafting in the Mammalian CNS, Bjorklund and Stenevi, eds., (1985) Das, Ch. 3 pp. 23-30; Freed, Ch. 4, pp. 31- 40; Stenevi et al, Ch. 5, pp. 41-50; Brundin et al, Ch. 6, pp. 51-60; David et al, Ch. 7, pp. 61-70; Seiger, Ch. 8, pp. 71-77 (1985), incorporated by reference herein. These procedures include intraparenchymal transplantation, i.e. within the host brain (as compared to outside the brain or extraparenchymal transplantation) achieved by injection or deposition of tissue within the host brain so as to be opposed to the brain parenchyma at the time of transplantation (Das, supra). The two main procedures for intraparenchymal transplantation are: 1) injecting the donor cells within the host brain parenchyma or 2) preparing a cavity by surgical means to expose the host brain parenchyma and then depositing the graft into the cavity (Das, supra). Both methods provide parenchymal apposition between the graft and host brain tissue at the time of grafting, and both facilitate anatomical integration between the graft and host brain tissue. This is of importance if it is required that the graft become an integral part of the host brain and to survive for the life of the host. Alternatively, the graft may be placed in a ventricle, e.g. a cerebral ventricle or subdurally, i.e. on the surface of the host brain where it is separated from the host brain parenchyma by the intervening pia mater or arachnoid and pia mater. Grafting to the ventricle may be accomplished by injection of the donor cells or by growing the cells in a substrate such as 3% collagen to form a plug of solid tissue which may then be implanted into the ventricle to prevent dislocation of the graft. For subdural grafting, the cells may be injected around the surface of the brain after making a slit in the dura. Injections into selected regions of the host brain may be made by drilling a hole and piercing the dura to permit the needle of a microsyringe to be inserted. The microsyringe is preferably mounted in a stereotaxic frame and three dimensional stereotaxic coordinates are selected for placing the needle into the desired location of the brain or spinal cord. The donor cells may also be introduced into the putamen, nucleus basalis, hippocampus cortex, striatum or caudate regions of the brain, as well as the spinal cord. For grafting, the cell suspension is drawn up into the syringe and administered to anesthetized graft recipients. Multiple injections may be made using this procedure. The age of the donor tissue, i.e., the developmental stage, may affect the success of cell survival after grafting. The cellular suspension procedure thus permits grafting of donor cells to any predetermined site in the brain or spinal cord, is relatively non-traumatic, allows multiple grafting simultaneously in several different sites or the same site using the same cell suspension, and permits mixtures of cells from different anatomical regions. Multiple grafts may consist of a mixture of cell types, and/or a mixture of transgenes inserted into the cells. Preferably from approximately 10 to approximately 10 cells are introduced per graft. For transplantation into cavities, which may be preferred for spinal cord grafting, tissue is removed from regions close to the external surface of the CNS to form a transplantation cavity, for example by removing bone overlying the brain and stopping bleeding with a material such a gelfoam (Stenevi et al, Brain Res. 114:1-20 (1976)). Suction may be used to create the cavity. The graft is then placed in the cavity. More than one transplant may be placed in the same cavity using injection of cells or solid tissue implants. Grafting of donor cells into a traumatized brain will require different procedures, for example, the site of injury must be cleaned and bleeding stopped before attempting to graft. In addition, the donor cells should possess sufficient growth potential to fill any lesion or cavity in the host brain to prevent isolation of the graft in the pathological environment of the traumatized brain. Measurement of Phenotype of Cells In specific embodiments, it may be necessary to monitor the phenotype of the cell that has been grown in the subatmospheric oxygen conditions so as to determine whether differentiation or other modification of the cell has occurred. Various methods may be used to achieve this, including monitoring message level, protein level, subcellular localization, functional assays or morphological changes. The methods for monitoring message level include PCR™ (U.S. Patent 5,364,790; U.S. Patent 4,800,159; U.S. Patent 4,683,195), In situ hybridization (U.S. Patent 4,888,278; U.S. Patent 4,886,741; U.S. Patent 5,506,098; U.S. Patent 5,225,326; U.S. Patent 5,521,061; U.S. Patent 5,538,869; U.S. Patent 5,665,540), RNAse protection assay, and single cell PCR™. The methods for monitoring protein level may use antibody staining, HPLC, western blotting or immunoprecipitation. These techniques are all well known to those of skill in the art. The ability to detect genes that are differentially expressed in two cell types or populations combined with advances of rapid gene detection and sequencing technologies may be used to compare gene expression in cells cultured under varying oxygen concentrations. Methods of differential display have been used to elucidate the genes responsible for a difference in phenotypes between two relatively similar cell types or during sequential changes of a cell from one state to another. For example, using the differential display technique, Kocher et al (1995) selected for genes that were up- regulated in renal cell carcinoma compared with normal renal parenchyma. Through this method, Kocher et al (1995) were able to isolate a gene (DD96) that was rarely expressed in normal epithelial cell populations, expressed diffusely in malignant epithelial cells of the wide majority of carcinomas, and markedly overexpressed in carcinomas originating from the colon, breast, and lung. A similar technique can be used to compare gene expression in cells incubated under traditional versus low oxygen level conditions. Genes up-regulated in one population over the other then may be used as a probe to screen for expression of that gene in other cell populations or the same cell population under different culturing conditions (i.e., in the presence of compounds or environmental stimuli that may affect the expression of the gene). Kang et al (1998) have developed a reciprocal subtraction differential RNA display (RSDD) method that permits the rapid and efficient identification and cloning of both abundant and rare differentially expressed genes. The technology was used to analyze gene expression alterations resulting during cancer progression as adenovirus- transformed rodent cells developed an aggressive transformed state (Kang et al, 1998). The approach resulted in the identification and cloning of known and unknown sequences that displayed expression as a function of progression and suppressed expression as a function of progression (Kang et al, 1998). The RSDD technique may be used to compare gene expression between cells during maintenance, proliferation and/or differentiation of the cells from progenitor or stem cells to fully differentiated cells in room air versus subatmospheric conditions. The methods of differential display may be used in conjunction with rapid DNA sequencing and detection methods, allowing for the ability to screen for or sequence a large number of genes in a relatively short amount of time. U.S. Patent No. 5,800,992 provides methods for detecting the differential expression of a plurality of genes between two cell types using complimentary polynucleotides in an array. Such technology is commonly referred to as "DNA chip" technology because the polynucleotides are deposited on a substrate that resemble computer microprocessor chips. Also described are methods of sequencing genes using DNA chips. Additionally, similar techniques are described in U.S. Patent No. 5,834,181 which utilizes similar technology to detect minor alterations in genes such as single nucleotide substitution, allowing detection of mutations in genes that lead to a change in the phenotype of a cell. Single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) technique, also will be useful in monitoring the phenotype of the cells grown in the present invention. Such a technique is described by, for example, Cornelison and Wold (1997). The single cell RT-PCR technique of Cornelison and Wold allows determination of expression of a number of genes at one time and may be used to identify skeletal muscle satellite cells and determine their activation state when incubated in the low/physiological oxygen conditions of the present invention. Another detection method commonly used is an RNAse protection assay in which a radiolabeled RNA probe is mixed with a test RNA population, such as total cellular RNA from an individual, under conditions where complementary segments of the RNA probe and the test RNA will hybridize. RNAse is then added to the mixture to destroy unprotected (unhybridized), single-stranded probe and test RNA. When all single-stranded RNA has been destroyed, only short fragments of protected RNA remains that can be analyzed electrophoretically to diagnose the particular RNA composition of the test RNA. The protected double-stranded RNA fragments are denatured before analysis, to make available the detectable, labeled single stranded RNA probe fragment. Disease Models Once a particular set of cells have been generated it will of course be necessary to test that these cells would apply to a disease model. Animal models of Parkinson's Disease and other neurodegenerative diseases are now well known to those of skill in the art. For example, a rat model of Parkinson's disease can be created by giving a unilateral injection of saline-ascorbate 6-hydroxy-dopamine (6-OHDA) into the medial forebrain bundle. This produces a lesion that ultimately mimics Parkinsonian behavior. Completeness of the lesion produced can by monitoring either apomorphine or amphetamine induced rotational behavior (Ungerstedt and Arbuthnott, 1970). Animals turning at a rate of more than 7 turns per minute (Schmidt et al, 1982) can be inferred to have the appropriate lesion (at least 7 contralateral rotations/min following apomorphine administration and at least 7 ipsilateral rotations/min towards the side of the lesion following amphetamine administration). Using such a model a baseline rotation behavior can be established. After that, the cells grown in the present invention can then be transplanted into the rat model as described herein above. Any decrease in the rotational behavior would be indicative of the cellular transplant having an appropriate therapeutic value. Gene Replacement/ Augmentation Applications Optionally, the CNS cells obtained using the method of the present invention can be manipulated to express desired gene products. Gene therapy can be used to either modify a cell to replace a gene product, to facilitate regeneration of tissue, to treat disease, or to improve survival of the cells following implantation into a patient (i.e. prevent rejection). In this embodiment, the CNS cells are transfected prior to expansion and differentiation. Techniques for transfecting cells are known in the art. A skilled artisan could envision a multitude of genes which would convey beneficial properties to the transfected cell or, more indirectly, to the recipient patient/animal. The added gene may ultimately remain in the recipient cell and all its progeny, or may only remain transiently, depending on the embodiment. For example, genes encoding tyrosine hydroxylase, or a monoamine transporter such as VMAT 1 or VMAT 2 could be transfected into certain CNS cells to provide an appropriate therapeutic cell suitable for grafting into a subject with Parkinson's disease. Other genes that could be used include GABA-decarboxylase, enkephalin, dopa decarboxylase (AADC), ciliary neuronal trophic factor (CNTF), brain derived neurotrophic factor (BDNF), neurotrophin (NT)-3, NT-4, and basic fibroblast growth factor (bFGF). In some situations, it may be desirable to transfect the cell with more than one gene. Of course the above therapeutic genes are only exemplary and those of skill in the art will understand that any neurodegenerative disorder that results from an aberration in gene expression and/or function can be treated by gene replacement and/or augmentation. Such disorders and their related genes are well known to those of skill in the art. In some instances, it is desirable to have the gene product secreted. In such cases, the gene product preferably contains a secretory signal sequence that facilitates secretion of the protein. The viral vectors used herein may be adenoviral (U.S. Patent No. 5,824,544; U.S. Patent No. 5,707,618; U.S. Patent No. 5,693,509; U.S. Patent No. 5,670,488; U.S. Patent No. 5,585,362; each incorporated herein by reference), retroviral (U.S. Patent No. 5,888,502; U.S. Patent No. 5,830,725; U.S. Patent No. 5,770,414; U.S. Patent No. 5,686,278; U.S. Patent No. 4,861,719 each incorporated herein by reference), an adeno-associated viral (U.S. Patent No. 5,474,935; U.S. Patent No. 5,139,941; U.S. Patent No. 5,622,856; U.S. Patent No. 5,658,776; U.S. Patent No. 5,773,289; U.S. Patent No. 5,789,390; U.S. Patent No. 5,834,441; U.S. Patent No. 5,863,541; U.S. Patent No. 5,851,521; U.S. Patent No. 5,252,479; each incorporated herein by reference), an adenoviral-adenoassociated viral hybrid (U.S. Patent No. 5,856,152 incorporated herein by reference), a lentiviral vector, a vaccinia viral or a herpesviral (U.S. Patent No. 5,879,934; U.S. Patent No. 5,849,571; U.S. Patent No. 5,830,727; U.S. Patent No. 5,661,033; U.S. Patent No. 5,328,688; each incorporated herein by reference) vector. Delivery of the expression constructs through non- viral vectors also is contemplated. Such delivery may employ microinjection (U.S. Patent No. 5,612,205), electroporation (U.S. Patent No. 5,507,724; U.S. Patent No. 5,869,326; U.S. Patent No. 5,824,547; U.S. Patent No. 5,789,213; U.S. Patent No. 5,749,847; U.S. Patent No. 5,019,034; Tur-Kaspa et al, 1986; Potter et al, 1984), calcium phosphate coprecipitation (Graham and Nan Der Eb, 1973; Chen and Okayama, 1987; Rippe et al, 1990), DEAE dextran introduction (Gopal, 1985), receptor mediated introduction (Wu and Wu, 1987; Wu and Wu, 1988), liposome mediated introduction (U.S. Patent No. 5,631,018; U.S. Patent No. 5,620,689; U.S. Patent No. 5,861,314; U.S. Patent No. 5,855,910; U.S. Patent No. 5,851,818; U.S. Patent No. 5,827,703, U.S. Patent No. 5,785,987; Nicolau and Sene, 1982; Fraley et al, 1979), dendrimer technology (U.S. Patent 5,795,581; U.S. Patent 5,714,166; U.S. Patent 5,661,025), naked DNA injection (Harland and Weintraub, 1985) and particle bombardment (U.S. Patent No. 5,836,905; U.S. Patent No. 5,120,657; Yang et al, 1990). The desired gene is usually operably linked to its own promoter or to a foreign promoter which, in either case, mediates transcription of the gene product. Promoters are chosen based on their ability to drive expression in restricted or in general tissue types, or on the level of expression they promote, or how they respond to added chemicals, drugs or hormones. Particularly contemplated promoters include but are not limited to CMV IE, SV40 IE, β-actin, collagen promoter, TH promoter, AADC promoter and the nestin promoter. Other genetic regulatory sequences that alter expression of a gene may be co- transfected. In some embodiments, the host cell DNA may provide the promoter and/or additional regulatory sequences. Other elements that can enhance expression can also be included such as an enhancer or a system that results in high levels of expression. Methods of targeting genes in mammalian cells are well known to those of skill in the art (U.S. Patent Nos. 5,830,698; 5,789,215; 5,721,367 and 5,612,205). By "targeting genes" it is meant that the entire or a portion of a gene residing in the chromosome of a cell is replaced by a heterologous nucleotide fragment. The fragment may contain primary the targeted gene sequence with specific mutations to the gene or may contain a second gene. The second gene may be operably linked to a promoter or may be dependent for transcription on a promoter contained within the genome of the cell. In a preferred embodiment, the second gene confers resistance to a compound that is toxic to cells lacking the gene. Such genes are typically referred to as antibiotic-resistance genes. Cells containing the gene may then be selected for by culturing the cells in the presence of the toxic compound. Application to Other Cell Types Although the majority of the discussion above is focussed on the growth and culturing of CNS cells, it should be appreciated that techniques of the present invention also will be useful for growth of other types of cells. As such it is contemplated that the techniques provided herein will be useful for growing any cells that are routinely used in transplant therapies. For example, such cells may be islets cells for diabetes; myoblasts for muscular dystrophy; hepatocytes for liver disease; skin grafts for wound healing and/or burns, and bone marrow or stem cells for hematopoietic and genetic disorders. In addition, the disclosure of U.S. Application number 09/195,569 filed November 18, 1998, (specifically incorporated herein by reference) will provide additional examples that may be useful in conjunction with the present invention. Examples The following examples are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the inventor to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. Example 1 Material and Methods Culture of CNS Stem Cells. Animals were housed and treated following NIH guidelines. Cells dissected from rat embryonic lateral ganglionic eminence (El 4) or mesencephalon (El 2) were mechanically dissociated, plated on plastic 24-well plates (Costar) or 12 mm glass cover slips (Carolina Biologicals) precoated with polyornithine/fibronectin, and grown in defined medium with bFGF (Studer et al, 1998; Johe et al, 1996). In general, bFGF was withdrawn from the medium after 4 - 6 days of culture. Clonal assays were carried out in plastic 48-well plates (Costar). In some studies, recombinant human (rh)EPO, rhVEGF 165 or recombinant mouse (rm)FGF8b, or their neutralizing antibodies (all from R&D Systems) were added to cultures at the following concentrations: EPO 0.5 U/ml, EPO neutralizing antibody 10 μg/ml, FGF8 250 ng/ml, FGF8b neutralizing antibody 5 μg/ml, VEGF 50 ng/ml, VEGF neutralizing antibody 0.5 μg/ml. Dose response for EPO was carried out at 0.05 U/ml, 0.5 U/ml, 5 U/ml and 15 U/ml; for anti-EPO at 10 μg/ml and 100 μg/ml. Results of all experiments were confirmed by at least 2 independent culture series. Low oxygen culture. Cultures were placed in humidified portable isolation chambers (Billups-Rothenberg, Del Mar CA), flushed daily with a gas mixture 1% O 2 , 5% CO 2 -r- 94% N 2 . Precise O levels in the surrounding atmosphere depended on the length of chamber flush (90 sec at 15 L/min achieved 6% O 2 , 6 minutes of flush achieved 1.5% O 2 ), which was not standardized until availability of an O 2 -sensitive electrode system (OS2000, Animus Corp., Frazer PA). Thus "lowered O 2 " conditions represent a range of ambient O 2 of 3+2%, which approximates normal brain tissue levels (Table 1). The entire chamber was housed in an incubator to maintain temperature at 37°C. BrdU uptake and TUNEL analysis. Bromodeoxyuridine (10 μM) was added to cultures for exactly 60 minutes, just prior to fixation. Anti-BrdU staining was performed according to the manufacturer's protocol (Amersham Life Sciences). The TUNEL reaction (Boehringer-Mannheim) was also performed according to manufacturer's protocol. TUNEL+ cells were visualized by metal-enhanced DAB reaction (Pierce) after peroxidase conversion of the FITC label. Immunohistochemistry. Cells were fixed in 4% paraformaldehyde + 0.15% picric acid/PBS. Standard immunohistochemical protocols were followed. The following antibodies were used: Stem cell/progenitor characterization: Nestin polyclonal #130 1 :500 (Martha Marvin & Ron McKay), PS A-NCAM, Enl and FP4 (all monoclonal 1 :2, Developmental Studies Hybridoma Bank, provided by Tom Jessel). Stem cell differentiation: β-tubulin type III (Tuj l) monoclonal 1 :500 and polyclonal 1 :500 (both BabCO), O4 monoclonal 1 :5 (Boehringer-Mannheim), galactocerebroside (GalC) monoclonal 1:50 (Boehringer-Mannheim), glial fibrillary acidic protein (GFAP) 1 :100 (ICN Biochemicals). Neuronal subtype differentiation: Tyrosine hydroxylase (TH) polyclonal 1:200-1 :500 (PelFreeze, Rogers AK) or monoclonal 1 :2000 (Sigma), GABA polyclonal 1 :500 (Sigma), glutamate 1 :500 (Sigma), dopamine-β-hydroxylase (DBH) 1 :100 (Protos Biotech Corp). Appropriate fluorescence-tagged (Jackson Immunoresearch) or biotinylated (Vector Laboratories) secondary antibodies followed by metal-enhanced DAB reaction (Pierce) were used for visualization. Cell Counts and Statistical Procedures. Uniform random sampling procedures were used for cell counts and quantified using the fractionator technique (Gundersen, et al, 1988). Statistical comparisons were made by ANOVA with posthoc Dunnett test when more than 2 groups were involved. If data were not normally distributed, a non-parametric test (Mann- Whitney U) was used to compare lowered vs. 20% O 2 results. Data are expressed as mean +SEM. Reverse-phase HPLC determinations of dopamine content. Culture supematants of medium, HBSS, and HBSS+56 mM KC1 were stabilized with orthophosphoric acid and metabisulfite, and stored at -80°C until analysis. Stabilization, aluminum adsorption, equipment, and elution of dopamine have been previously described (Studer et al, 1998; Studer et al, 1996). Results were normalized against dopamine standards at varying flow rates and sensitivities. Western blots. Cell pellets were stored at -80°C. Pellet was lysed in 20 mM Hepes, pH 7.6, 20% glycerol, 10 mM NaCl, 1.5 mM MgCl 2 , 0J mM EDTA, 0.1% Triton-X-100, with protease inhibitors (Complete, Boehringer-Mannheim), homogenized, and incubated on ice for 1 hr. After centrifugation, supernatant protein concentration was assayed by BCA (Pierce). For western, block was 5% milk in TBST, primary TH antibody (Pel-Freeze, Rogers, AK) was used at 1 :500, and secondary was HRP-conjugated goat anti-rabbit (Pierce) at 1 :5000. Signal was detected with SuperSignal (Pierce). RTPCR™. Cultures were washed once in PBS before solubilization in 2 ml (per 6 cm dish) Trizol (Life Technologies) then stored at -80°C. RNA extraction was carried out according to manufacturer's recommendations (Gibco Life Technologies). The Superscript kit (Gibco Life Technologies) was used for reverse transcription of 10 μg RNA per condition. PCR conditions were optimized by varying MgCl concentration and cycle number to determine linear amplification range. Amplification products were identified by size and confirmed by DNA sequencing. TH was kindly provided by Vera Vikodem, NIDDK, 30 eye, 56°C, 300bp. For the other products the primer sequences, cycle numbers, and annealing temperatures were as shown in Table 2. Table 2: primer sequences, cycle numbers, and annealing temperatures for PCR Example 2 Results: Lowered O 2 augments expansion of striatal and mesencephalic precursors. E14 rat striatum, widely used for derivation of CNS precursors, cultured in lowered O 2 yielded an average 2- to 3-fold more cells than 20%> O 2 cultures over a wide range of plating densities in the presence of bFGF. (Basic FGF acts as mitogen for stem cells obtained from many regions of the developing brain. The withdrawal of bFGF initiates differentiation to neurons, astrocytes and oligodendrocytes). (FIG. 1). Identical results were obtained with E12 mesencephalic precursors. For all results, data were verified by at least two independent culture series. Effects on cell proliferation and cell death. To test whether increased cell yield in lowered O 2 is due to increased proliferation, reduced cell death, or both, precursors were pulsed with bromodeoxyuridine (BrdU) for 1 hr immediately before fixation at multiple time points while precursor cells were proliferating or differentiating. Both mesencephalic and striatal precursors showed increased BrdU labeling indices when grown in lowered O 2 as compared to traditional cultures. Lowered O 2 increased the BrdU labeling index in the presence of bFGF and during cell differentiation following mitogen withdrawal from mesencephalic precursors (FIG. 2). BrdU incorporation rates for striatal precursors showed similar patterns {Day 2 of expansion : 18 + 6% in lowered O 2 vs. 11 ± 6% in 20% O 2 (n=24, p < 0.05). Day 6 of expansion : 30 + 8% in lowered O 2 vs. 22 + 5% in 20% O 2 (n=24, p < 0.05). Day 4 of differentiation (10 days in vitro) : 12 + 5% in lowered O 2 vs. 3 + 3% in 20% 0 2 (n=24, p < 0.05)} . In addition to this apparent increase in cell proliferation in lowered O 2 cultures, precursor cells were also less likely to undergo apoptosis than CNS precursors grown in 20% O 2 . Both mesencephalic and striatal precursors revealed significantly reduced percentages of TUNEL-positive cells both during expansion and after bFGF withdrawal. TUNEL data for mesencephalic precursors are summarized in FIG. 3. Thus, both reduced apoptosis and increased cell proliferation contribute to elevated yield of cells at the end of the expansion phase. Cell death is reduced but not entirely eliminated during the differentiation phase by lowering the O 2 levels. Cell lineage and clonal growth. A series of molecular markers were used, together with morphologic assessment in order to characterize how lowered O 2 culturing affects the choice of differentiation pathways and the kinetics of differentiation. Immunoreactivity for the intermediate filament nestin was used to discriminate CNS stem and progenitor cells from more differentiated progeny (Lendahl et al, 1990). Six days after bFGF withdrawal the percentage of nestin- positive cells derived from expanded precursors was grossly reduced in lowered O 2 cultures compared with 20% O 2 cultures, suggesting that differentiation might have been accelerated in lowered O 2 (FIG. 4A and FIG. 6). The sialic acid substituted form of NCAM (PSA-NCAM), a proposed marker for committed neuronal progenitors (Mayer et al, 1997), was conversely reduced in differentiated lowered O 2 cultures (FIG. 6). The idea of accelerated progression to a more differentiated phenotype was supported by the earlier appearance of neuronal and glial markers in lowered O 2 . Neurons were assessed by β-tubulin III (TUJl) staining, astrocytes by glial fibrillary acidic protein (GFAP), oligodendrocyte precursors by O4, and oligodendrocytes by GalC galactocerebroside staining (FIG. 4). Five days after bFGF withdrawal striatal cultures held at low O 2 contained 46%> Tujl -positive cells vs. 34%> in 20% O 2 (n=12, p < 0.05); Six percent were GFAP+ vs. 2% in 20% O 2 (n=12, p < 0.05); and 4% were Gal-C+ vs. 5% in 20% O (p=n.s.). In mesencephalic cultures held at lowered oxygen, 73% were Tujl+ vs. 63%> in 20%> O 2 (n=12, p = 0.06); no GFAP+ cells were detected in either oxygen conditions; 1% were O4+ versus 0%> in 20% O 2 (n=12, p < 0.01). (FIG. 4B). To investigate O 2 effects at clonal densities, mesencephalic precursors were first expanded in bFGF for 6 days in 20% O 2 , replated at a density of 1-5 cells/well, then maintained at either lowered or 20% O 2 . After 20 days, 20 ng/mL bFGF was withdrawn. Clonal cultures with typical multi-lineage differentiation responses were observed in both lowered and 20%> O 2 conditions. FIG. 4C illustrates a typical nestin+ clone (left panel) and clonally derived cells undergoing neuronal differentiation 4 days after mitogen withdrawal (right panel). As expected of stem cells, all three lineages were represented in the clones grown in low oxygen conditions. However, the efficiency of clone formation was 3 times higher in lowered O 2 and the average clone size also increased from <50 cells in 20% O 2 to 50-500 cells in lowered O 2 (FIG. 4D, FIG. 4E). Example 3 Results: Neuronal subtype differentiation The results above establish that lowered oxygen conditions support stem cell proliferation and differentiation to neurons and glia. The inventors' previous work has shown that nestin-positive mesencephalic precursors differentiate into functional dopaminergic neurons (Studer et al, 1998). Next it was determined whether this specific neuronal fate was influenced by lowered O 2 . Mesencephalic precursors in lowered oxygen displayed a striking increase in both the absolute number and fraction of neurons expressing TH (FIG. 5 A, FIG. 5B). In lowered O 2 , large neuronal clusters were seen in which virtually all neurons were TH+. On average, 56%> of neurons (marked by Tujl staining) generated in lowered O 2 were TH+ vs. 18%o in traditional cultures (n=12, p <0.001). Increased TH-immunoreactivity in lowered O cultures correlated with increased TH protein content in Western blots (FIG. 5C). The functional dopaminergic capacity of the TH-positive neurons was further assessed by reverse phase HPLC, which showed significantly increased levels of dopamine in lowered vs. 20% O 2 cultures (FIG. 5D): Conditioned medium (24 hours) showed a 5- fold increase in dopamine (n=5, p < 0.01). Basal release in HBSS revealed a 2- to 3- fold increase (n=5, p < 0.05) and evoked release was 3-fold increased (ι =5, p < 0.05). These results confirm that lowered oxygen favors the differentiation of functional dopaminergic neurons. Mesencephalic precursors give rise to neurons with several distinct neurotransmitter phenotypes in addition to dopaminergic fate (Studer et al, 1998). Interestingly, the percentage of serotonergic neurons was also increased in lowered O 2 , 3.2±1.2% vs. 1.2+0.3% in 20% O 2 (n=12, p < 0.05, FIG. 6). On the other hand GABA+ and Glutamate+ neurons were less likely to be generated in lowered O 2 (FIG. 6: GABA+ cells 6.6+1.8% in lowered O 2 vs. 10.4+1.5% n=12, p < 0.05; Glutamate+ cells 12.8%+3.8% in lowered O 2 cultures vs. 23.6±4.0% in 20% O 2 (n=12, p < 0.01). No double labeling of TH with GABA was detected indicating that TH immunoreactivity corresponded to differentiated dopaminergic neurons and was not a transient developmental phenomenon seen in developing GABAergic neurons (Max et al, 1996). Furthermore, the TH-positive neurons were not fated to a noradrenergic phenotype, since no dopamine-β-hydroxylase staining could be demonstrated. Since lowered O 2 promoted differentiation of dopaminergic and serotonergic neurons, both ventral neuronal phenotypes (Yamada et al, 1991; Hynes et al, 1995; Ye et al, 1998), it was determined whether these changes were associated with an increase in floor plate cells. Immunohistochemistry revealed expanded zones of FP4+ cells in lowered O 2 (FIG. 6). A more striking feature was the increased occurrence of neurons expressing the transcription factor engrailed-1 (Enl) in lowered O 2 . (FIG. 6). Engrailed-1 is critical for normal midbrain development (Joyner, 1996; Danelian and McMahon 1996; Wurst et al, 1994) and has been implicated in control of dopaminergic neuronal fate (Simone et al, 1998). It is important to establish whether the low oxygen condition enhanced dopaminergic differentiation by acting during the proliferation or differentiation phases of the culture system. Mesencephalic precursors were expanded for 5 days in either lowered or 20%> O . Each group was then subdivided for differentiation in either lowered or 20% O 2 . Precursors expanded in lowered O 2 but differentiated in 20%) O yielded 38+6% TH+ neurons, similar to those maintained in lowered O 2 throughout (41+7%>, n=12, p = n.s.) but significantly higher than those maintained in 20% O 2 throughout (17+4%, n=12, p < 0.01). Exposure to lowered O 2 limited to the differentiation phase did not significantly increase the yield of dopaminergic neurons (21+2%, n=12, p=n.s.) compared to cultures maintained in 20% O 2 throughout. From these data, it is shown that the major effect of low O 2 is during the expansion phase. Semi-quantitative RT-PCR was used to assay RNA from cultures at various time points for differential expression of candidate genes involved in dopaminergic neuron development (FIG. 7). A small increase in TH message was detected from lowered O 2 cultures after differentiation, compared to 20%) O 2 . The Ptx3 homeobox gene has also been implicated in dopamine neuron development (Smidt et al, 1997) and was also expressed at increased levels in lowered O 2 suggesting that these conditions promoted the dopaminergic phenotype, not simply upregulation of TH gene expression. Strong evidence links sonic hedgehog (Echelard et al, 1993); and Nurrl (Saucedo-Cardenas et al, 1998) genes to the differentiation of midbrain dopaminergic neurons but no O 2 -dependent changes in expression were detected. However, engrailed-1 was upregulated in lowered O , paralleling the immunohistochemical results (FIG. 6). Fibroblast growth factor 8b (FGF8b) message was dramatically upregulated in lowered O 2 , by the end of the expansion phase. Messages for other regulators of dopaminergic differentiation did not differ significantly between O 2 conditions. Example 4 Discussion: Lowered oxygen cultures favor proliferation and survival of CNS stem cells. Standard conditions for the culture of mammalian cells are 37°C in a gas atmosphere of 5%> CO 2 and 95% air. Thus ambient temperature is adjusted to reflect core mammalian body temperature and CO 2 is adjusted to reflect approximate venous concentrations, while in striking contrast, O 2 levels in culture are not adjusted to reflect physiologic levels. At sea level, unhumidified room air contains 21% O 2 , and a 95% air/5%) CO 2 mixture contains 20% O 2 . Alveolar air contains 14%. O 2 , arterial O 2 concentration is 12%>, venous O 2 levels are 5.3%, and mean tissue intracellular O 2 concentration is 3% (Guyton, and Hall, 1996). Directly relating to this study, mean brain O 2 in the adult rat and in fetal sheep have both been measured at 1.6% (Silver and Erecinska, 1988; Koos and Power, 1987). Physiological tissue O 2 levels in some brain regions are even lower (Table 1). In this work the impact of lowered, more physiologic oxygen levels on CNS stem cell culture was analyzed and showed four major effects: 1) increased proliferation of progenitors; 2) reduced apoptosis; 3) accelerated progression to differentiated states; and 4) elevated absolute number and proportion of TH+- neurons. Lowered O 2 culturing consistently enhanced proliferation of CNS stem cells. A 2- to 4-fold increase in cell number was observed during the proliferation phase when most of the cells are nestin+ precursors. This increase in cell number was also maintained after mitogen withdrawal when proliferation was vastly reduced and differentiation takes place. Although more cells were present in differentiated cultures in lowered O 2 , the proportions of neurons and glia were similar in the two culture conditions. In neural tissue, there is one supporting, though specialized, precedent for mitogenic activity of lowered O 2 in neural crest-derived carotid body chromaffin cells (Nurse and Vollmer, 1997). These dopaminergic glomus cells are functionally specialized O 2 -sensitive chemoreceptors, and so would be expected to be specifically responsive to changes in O 2 levels in the artery. The present results show that lowered oxygen enhances the proliferation and survival of CNS stem cells. Two specific signaling pathways, FGF8 and EPO, were identified as candidates for significant roles in the lowered O 2 response and showed that each can recapitulate part of the lowered O 2 phenotype at 20% O 2 . Lowered O 2 culturing led to relative increases in RNAs encoding erythropoietin and FGF8. In early midbrain development, FGF8 functions as a mitogen (Danelian and McMahon 1996), but significant mitogenic or trophic effects of FGF8 on CNS stem cell cultures have not been reported. Here, the increased cell yield from mesencephalic precursors maintained in 20% O 2 and exposed to 250 ng/ml FGF8 partly recapitulated the proliferation/trophic effects of lowered O 2, with a 30% increase in total number compared to a 200-400% increase in lowered O 2 . In addition to increased proliferation, less apoptosis occurs in CNS stem cells cultured in lowered O 2 . There is a potential toxic role for reactive oxygen intermediates (ROI) produced in room air cultures. However, it cannot simply be assumed that 20% O 2 cultures generate more oxidative stress than lowered O 2 cultures, since free radicals are generated in ischemic conditions (Perez Velazquez et al, 1997). In contrast to the increased cell number seen in lowered O , only minor effects were detected on the final ratio of neurons to astrocytes to oligodendrocytes that were derived from expanded striatal or mesencephalic precursors. This result together with clonal analysis confirms that the nestin+ precursors expanded in lowered oxygen have stem cell properties. Example 5 Discussion: Dopaminergic commitment and differentiation. There is a great deal of evidence that CNS stem cells can give rise to multiple neuron types (Johe et al, 1996; Gritti et al, 1996; Kalyani et al, 1998). For several years the midbrain has been studied as model for neuron subtype specification (Hynes et al, 1995;Ye et al, 1998; Wang et al, 1995; Ericson et al, 1995; (reviewed in Hynes and Rosenthal, 1999). Recently, conditions have been established that allow midbrain precursor cells to proliferate and differentiate to dopaminergic neurons in vitro (Studer et al, 1998). In contrast to primary rat fetal mesencephalic cultures where only 5%> of the neurons are immunoreactive for TH, this number was increased to 24% of neurons in dissociated precursor cultures from El 2 mesencephalon. Here it is shown that 56%. of neurons generated from mesencephalic precursors are TH+, and this finding is associated with increased dopamine production by HPLC. Serotonergic neurons, another ventral neuron type found in this region of the brain, were also generated in increased numbers in lowered O 2 . In contrast the number of GABAergic and Glutamatergic neurons were reduced. The lowered oxygen conditions were most effective in generating dopaminergic neurons during the phase of precursor cell expansion. These results suggest that lowered oxygen conditions enhance the production of ventral fates by a mechanism that acts prior to differentiation. Transcript levels of FGF8 and Enl, accepted mediators of midbrain dopaminergic neuron development (Ye et al, 1998; Simone et al, 1998; Shamim et al, 1999), were upregulated in lowered vs. 20% O 2 cultures. FGF8 has also been implicated in the commitment of serotonergic neurons (Ye et al, 1998). These findings are consistent with a role for FGF8 in the expansion of dopaminergic and serotonergic neuronal subtypes seen in lowered O 2 cultures. However, addition of FGF8 to 20% 0 2 cultures or neutralization of FGF8 in lowered O 2 cultures did not reproduce the 0 2 -dependent neuronal subtype differentiation patterns. The secreted morphogen sonic hedgehog (SHH) has been shown to induce dopaminergic neuron differentiation in explants of the early neural plate (Hynes et al, 1995; Ye et al, 1998; Wang et al, 1995). Purified sonic hedgehog had no effect on expanded mesencephalic precursors under both oxygen conditions. Engrailed-1 mRNA and protein levels were increased in lowered oxygen. Engrailed-1 is thought to act in a pathway with pax2, wnt-1 and FGF8 to regulate the fate of midbrain neurons (Joyner, 1996; Danelian and McMahon, 1996; Wurst et al, 1994; Simone et al, 1998). The FGF8 gene contains a binding site for engrailed (Gemel et al, 1999). In addition it was found that the FGF8 5'-UTR sequence (accession #AF065607) contains a 9 base sequence (CCTCCCTCA) that is also known to control oxygen responsiveness in VEGF and EPO regulatory elements (Scandurro, and Beckman, 1998). The inventors have not yet determined if Enl acts as a direct upstream regulator of FGF8 in lowered O 2 cultures, or whether they act independently. Nonetheless, the prominent expression of Enl in young neurons (FIG. 6) suggests it may be a good candidate for regulating neuronal subtype differentiation. EPO levels are known to be regulated by oxygen in the erythropoietic system. EPO and its receptor are expressed in brain from early development through adulthood (Juul et al, 1999), but no specific role for EPO in CNS development has been described. In the adult CNS, however, EPO has received attention as a neuroprotective agent (Sakanaka et al, 1998), and EPO treatment of PC12 cells has been demonstrated to increase intracellular monoamine levels (Masuda et al, 1993). Here the results show that at 20%> O 2; EPO can mimic part of the lowered O 2 effect. Increases in yield of dopaminergic neurons in 20% O cultures was dose-dependent, but no additional increase in yield was mediated by EPO in lowered oxygen, suggesting that the EPO levels in lowered O 2 were at maximal functional levels for this response. Though EPO supplementation of 20% O 2 cultures significantly improves dopaminergic yield, the full effect of lowered O could not be recapitulated, suggesting that additional factors are involved in promoting dopaminergic differentiation in lowered O 2 . Nonetheless, the finding that EPO affects the differentiation patterns of expanded CNS precursors is novel and identifies EPO as a component of increased dopaminergic neuron yield in lowered oxygen conditions. A recent report highlighted increased dopamine content after differentiated dopaminergic mesencephalic neurons were exposed to hypoxic conditions (O% O 2 gas mixture) (Gross et al, 1999). Another study described a relative increase in TH- expressing neurons in primary neuronal cultures from E14 rats after exposure to 5%> O 2 (Colton et al, 1995). It is also known that hypoxic conditions favor expression of the TH gene (Czyzyk-Krzeska et al, 1994; Paulding, and Czyzyk-Krzeska, 1999). However, this is the first report that lowered oxygen conditions support CNS stem cells during the expansion phase and enhance the production of ventral neuronal subtypes. Compared to 20% O 2 the net expansion of dopaminergic neurons in lowered O 2 was at least 9-fold increased (a three-fold increase in total cell numbers, and a 3- fold increase in the percentage of TH+-neurons). HPLC shows that these neurons produce dopamine. The present results show that oxygen levels much lower than those traditionally used in culture may be useful in mimicking in vivo phenomena. Lowered O culturing has the practical implication of contributing to a more efficient production of dopaminergic neurons for transplant therapy in Parkinson's disease. Finally, effects of lowered, more physiological O 2 on cell cultures are not limited to the CNS, and extend to the PNS and to other non-neuronal tissues. The effect of EPO on dopaminergic differentiation was studied. Saturating concentrations of EPO or EPO neutralizing antibody were added to E12 mesencephalic precursor cultures during both proliferation and differentiation phase (5 days each) in lowered or 20%> 02 using the conditions set forth in example 3. EPO was added to medium at a dose 5 U/ml for 5 days at the time or plating (isolation) or starting at the time of bFGF withdrawal. At the end of 10 days total time in culture, cells were fixed and stained for TH to mark dopaminergic neurons. The number of positive-staining cells were counted. EPO supplementation significantly increased TH+ cell numbers in 20% O2 cultures (n=6, p < 0.05). The experiment was repeatedly with anti-EPO in place of EPO. The anti-EPO concentration was in a saturating concentration. The timing was the same as the EPO experiments, and they were stained and counted at the same 10 day time point. EPO neutralizing antibody decreased TH+ cell numbers in both lowered oxygen (n=6, p < 0.01) and 20% O2 cultures (n=6, p < 0.05). Figure 8 shows the results when EPO was added early. Similar results were obtained with the late addition of EPO. All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain agents which are both chemically and physiologically related may be substituted for the agents described herein while the same or similar results would be achieved. 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Claims: CLAIMS 1. A method of increasing cell differentiation comprising culturing undifferentiated central nervous system (CNS) cells in low ambient oxygen conditions, wherein said low ambient oxygen conditions promotes the cellular differentiation of said neuronal cells. 2. The method of claim 1, wherein said low ambient oxygen conditions comprise an ambient oxygen condition of between about 0J5%> to about 18%> oxygen. 3. The method of claim 1, wherein said low ambient oxygen conditions comprise an ambient oxygen condition of between about 0.5% to about 15% oxygen. 4. The method of claim 1, wherein said low ambient oxygen conditions comprise an ambient oxygen condition of between about 1%> to about 10%> oxygen. 5. The method of claim 1, wherein said low ambient oxygen conditions comprise an ambient oxygen condition of between about 1.5% to about 6% oxygen. 6. The method of claim 1, wherein said low ambient oxygen conditions mimic physiological oxygen conditions for CNS cells. 7. The method of claim 1, wherein said cells are primary tissue culture cells. 8. The method of claim 1, wherein said cells are derived from a cell line. 9. The method of claim 1, wherein said cells are selected from the group consisting of a central nervous system stem cells, spinal cord-derived progenitor cells, glial cells, astrocytes, neuronal stem cells, central nervous system neural crest-derived cells, neuronal precursor cells, neuronal cells, hepatocytes, and bone marrow derived cells. 10. The method of claim 9, wherein said cells are fetal cells. 11. The method of claim 9, wherein said cells are adult cells. 12. The method of claim 1, wherein said cells are selected from the group consisting of mecencephalic progenitor cells, lateral ganglionic precursor cells, cortical precursor cells, astrocytes and neuroblasts. 13. The method of claim 1, wherein said differentiation is determined by monitoring a differentiation specific phenotype in said cells. 14. The method of claim 13, wherein said differentiation specific phenotype determined by monitoring message level, protein level, subcellular localization, functional assays or morphological changes. 15. The method of claim 14, wherein said message level is monitored using PCR™, in situ hybridization, RNAse protection assay, or single cell PCR™. 16. The method of claim 14, wherein said protein level is monitored using antibody staining, HPLC, western blotting or immunoprecipitation. 17. The method of claim 14 wherein said message level monitored is the message for nestin, tyrosine hydroxylase, GAPDH; BDNF; GDNF; FGFR3; Enl ; FGF8; SHH; Ptx3; Nur ; VEGF; EPO; HIFlα or VHL. 18. The method of claim 15 wherein said protein level monitored is the level of nestin, tyrosine hydroxylase, dopamine β-hydroxylase or dopamine transporter. 19. The method of claim 14, wherein said functional assay monitors the rate of dopamine production. 20. The method of claim 1 , wherein said low oxygen conditions produce a cell population that is enriched in dopaminergic neurons as compared to a similar cell population that is grown in 20%> oxygen incubator conditions. 21. The method of claim 1, wherein said low oxygen conditions produce a cell population that is enriched in serotoninergic neurons as compared to a similar cell population that is grown in 20% oxygen incubator conditions. 22. The method of claim 1, wherein said low oxygen conditions produce a cell population that is depleted in GABAnergic neurons as compared to a similar cell population that is grown in 20%o oxygen incubator conditions. 23. The method of claim 1, wherein said low oxygen conditions produce a cell population that is depleted in glutaminergic neurons as compared to a similar cell population that is grown in 20% oxygen incubator conditions. 24. The method of claim 1, further comprising growing said cells in the presence of a neuronal growth stimulant, mitogen, cytokine, neuroprotective factor or an anti-apoptotic agent. 25. The method of claim 1, wherein said differentiated phenotype is retained after transfer of said cells from said low ambient oxygen conditions to 20% oxygen culture conditions. 26. The method of claim 25, wherein said cells are grown in low ambient oxygen conditions for multiple generations prior to transfer to 20%) oxygen culture conditions. 27. The method of claim 1, wherein said cells are continuously maintained in low ambient oxygen conditions. 28. A method of inhibiting apoptosis of a CNS cell in culture comprising growing said cell in low ambient oxygen conditions. 29. The method of claim 28, wherein said low ambient oxygen conditions comprise an ambient oxygen condition of between about 0.25% to about 18%> oxygen. 30. A method of increasing the expansion of a CNS cell in culture comprising growing said cell in low ambient oxygen, wherein said cell exhibit increased expansion in said low ambient oxygen as compared to growing said cell in 20% oxygen incubator conditions. 31. The method of claim 30, wherein said low ambient oxygen conditions comprise an ambient oxygen condition of between about 0.25% to about 18% oxygen. 32. The method of claim 30, wherein said low ambient oxygen conditions mimic physiological oxygen conditions for CNS cells. 33. The method of claim 30, wherein said cell is a primary tissue culture cell. 34. The method of claim 30, wherein said cell is derived from a cell line. 35. The method of claim 30, wherein said cell is a fetal cell. 36. The method of claim 30, wherein said cell is an adult cell. 37. A method of increasing cell proliferation in culture comprising growing CNS cells in low ambient oxygen, wherein said growth in low ambient oxygen increases cell proliferation compared to growing said cells in 20% oxygen incubator conditions. 38. A method of preparing a cell for use against a neurodegenerative disorder comprising a) obtaining a population of CNS cells and b) growing said cells in low ambient oxygen conditions wherein said low ambient oxygen conditions increases the expression of a gene involved in said neurodegenerative disease. 39. The method of claim 38, wherein said neurodegenerative disease is Parkinson's Disease. 40. The method of claim 38 wherein said gene is tyrosine hydroxylase (TH). 41. The method of claim 38, wherein said cell is a primary cell. 42. The method of claim 38, wherein said cell is derived from a cell line. 43. The method of claim 38, further comprising contacting said cell with a first polynucleotide encoding a dopamine biosynthetic protein under conditions suitable for the expression of said protein wherein said polynucleotide is under the transcriptional control of a promoter active in said cells. 44. The method of claim 38, further comprising contacting said cell with a first polynucleotide encoding a dopamine releasing protein under conditions suitable for the expression of said protein wherein said polynucleotide is under the transcriptional control of a promoter active in said cells. 45. The method of claim 43, further comprising contacting said cell with a second polynucleotide encoding a dopamine releasing protein under conditions suitable for the expression of said protein wherein said polynucleotide is under the transcriptional control of a promoter active in said cells. 46. The method of claim 44, further comprising contacting said cell with a second polynucleotide encoding a dopamine biosynthetic protein under conditions suitable for the expression of said protein wherein said polynucleotide is under the transcriptional control of a promoter active in said cells. 47. The method of claim 43, wherein said dopamine biosynthesis protein is selected from the group consisting of TH; L-amino acid decarboxylase (AADC) and erythropoietin. 48. The method of claim 44, wherein said dopamine releasing protein is a vesicular monoamine transporter (VMAT). 49. The method of claim 45, wherein said first and second polynucleotides are under control of different promoters. 50. The method of claim 46, wherein said first and second polynucleotides are under control of different promoters. 51. The method of claim 43, wherein the promoter is selected from the group consisting of CMV IE, SV40 IE, β-actin, TH promoter, AADC promoter, and nestin promoter. 52. The method of claim 45, wherein said first and second polynucleotides each are covalently linked to a polyadenylation signal. 53. The method of claim 46, wherein said first and second polynucleotides each are covalently linked to a polyadenylation signal. 54. A cell produced according to the method comprising obtaining a starting CNS cell and growing said cell in low ambient oxygen conditions wherein said conditions produce a differentiated neuronal cell. 55. The cell of claim 54, wherein said starting cell is a nestin-positive cell. 56. The cell of claim 54, wherein said low ambient conditions produce a nestin- negative cell. 57. The cell of claim 54, wherein said low ambient conditions produce a TH positive cell. 58. The cell of claim 54, wherein said cell further comprises an expression vector comprising a polynucleotide encoding an exogenous gene wherein said polynucleotide is operatively linked to a promoter. 59. A method of treating Parkinson's disease in a subject comprising: a) obtaining cells suitable for transplanting to said subject; b) growing said cells in low ambient oxygen conditions; and c) implanting said cells grown from step (b) into said subject wherein said implanted cells have an increased capacity to produce dopamine in said subject as compared to similar cells grown in 20% oxygen incubator conditions. 60. The method of claim 59, wherein said cells are from said subject and have been transduced with a polynucleotide that expresses a protein that increases dopamine production. 61. The method of claim 59, wherein said cells are CNS cells from a source other than said subject. 62. The method of claim 59, wherein said cells are transduced with a polynucleotide that expresses a protein that increases dopamine production.

Inventors: CESTE MARIE, DOYLE JOHN, MCKAY RON, STUDER LORENZ, WOLD BARBARA J, CESTE, MARIE, DOYLE, JOHN, MCKAY, RON, STUDER, LORENZ, WOLD, BARBARA, J.
Applicants: CALIFORNIA INST OF TECHN, CESTE MARIE, DOYLE JOHN, MCKAY RON, STUDER LORENZ, WOLD BARBARA J, CALIFORNIA INSTITUTE OF TECHNOLOGY, CESTE, MARIE, DOYLE, JOHN, MCKAY, RON, STUDER, LORENZ, WOLD, BARBARA, J.
Application year: 1999
Main classification: C12N5
IPC codes: A61K35, A61K38, A61K48, A61P25, C12N5, C12N15, C12Q1