Glial cell transplantation and remyelination of the central nervous system

Remyelination of Demyelinated CNS Axons by Transplanted Human Schwann Cells: The Deleterious Effect of Contaminating Fibroblasts

Areas of demyelination can be remyelinated by transplanting myelin-forming cells. Schwann cells are the naturally remyelinating cells of the peripheral nervous system and have a number of features that may make them attractive for cell implantation therapies in multiple sclerosis, in which spontaneous but limited Schwann cell remyelination has been well documented. Schwann cells can be expanded in vitro, potentially affording the opportunity of autologous transplantation; and they might also be spared the demyelinating process in multiple sclerosis. Although rat, cat, and monkey Schwann cells have been transplanted into rodent demyelinating lesions, the behavior of transplanted human Schwann cells has not been evaluated. In this study we examined the consequences of injecting human Schwann cells into areas of acute demyelination in the spinal cords of adult rats. We found that transplants containing significant fibroblast contamination resulted in deposition of large amounts of collagen and extensive axonal degeneration. However, Schwann cell preparations that had been purified by positive immunoselection using antibodies to human low-affinity nerve growth factor receptor containing less than 10% fibroblasts were associated with remyelination. This result indicates that fibroblast contamination of human Schwann cells represents a greater problem than would have been appreciated from previous studies.

Myelin recovery in multiple sclerosis: the challenge of remyelination

Multiple sclerosis (MS) is the most common demyelinating and an autoimmune disease of the central nervous system characterized by immune-mediated myelin and axonal damage, and chronic axonal loss attributable to the absence of myelin sheaths. T cell subsets (Th1, Th2, Th17, CD8⁺, NKT, CD4⁺CD25⁺ T regulatory cells) and B cells are involved in this disorder, thus new MS therapies seek damage prevention by resetting multiple components of the immune system. The currently approved therapies are immunoregulatory and reduce the number and rate of lesion formation but are only partially effective. This review summarizes current understanding of the processes at issue: myelination, demyelination and remyelination—with emphasis upon myelin composition/architecture and oligodendrocyte maturation and differentiation. The translational options target oligodendrocyte protection and myelin repair in animal models and assess their relevance in human. Remyelination may be enhanced by signals that promote myelin formation and repair. The crucial question of why remyelination fails is approached is several ways by examining the role in remyelination of available MS medications and avenues being actively pursued to promote remyelination including: (i) cytokine-based immune-intervention (targeting calpain inhibition), (ii) antigen-based immunomodulation (targeting glycolipid-reactive iNKT cells and sphingoid mediated inflammation) and (iii) recombinant monoclonal antibodies-induced remyelination.

The road to remyelination in demyelinating diseases: current status and prospects for clinical treatment

Within CNS disorders, demyelinating diseases are among the most devastating and cost intensive due to long-term disabilities affecting relatively young patients. Multiple sclerosis, a chronic inflammatory demyelinating disease in which the persistent inhibitory microenvironment of the resident oligodendrocyte precursor cells abrogates regeneration of myelin sheaths, is the most prominent disease in the spectrum of demyelinating diseases. The essential goal is to stimulate creation of new myelin sheaths on the demyelinated axons, leading to restoration of saltatory conduction and resolving functional deficits. The past few decades witnessed significant efforts to understand the cellular interactions at the lesion site with studies suggesting efficient remyelination as a prerequisite for functional repair. Despite its proven efficacy in experimental models, immunosuppression has not had profound clinical consequences in multiple sclerosis, which argued for a paradigm shift in the design of therapeutics aiming to achieve remyelination. For example, targeting oligodendrocytes themselves may drive remyelination in the CNS. This group and others have demonstrated that natural autoreactive antibodies directed at oligodendrocyte progenitors participate in remyelination. Accordingly, the authors developed a recombinant autoreactive natural human IgM antibody with therapeutic potential for remyelination.

Cell Therapy From Bench to Bedside Translation in CNS Neurorestoratology Era

Recent advances in cell biology, neural injury and repair, and the progress towards development of neurorestorative interventions are the basis for increased optimism. Based on the complexity of the processes of demyelination and remyelination, degeneration and regeneration, damage and repair, functional loss and recovery, it would be expected that effective therapeutic approaches will require a combination of strategies encompassing neuroplasticity, immunomodulation, neuroprotection, neurorepair, neuroreplacement, and neuromodulation. Cell-based restorative treatment has become a new trend, and increasing data worldwide have strongly proven that it has a pivotal therapeutic value in CNS disease. Moreover, functional neurorestoration has been achieved to a certain extent in the CNS clinically. Up to now, the cells successfully used in preclinical experiments and/or clinical trial/treatment include fetal/embryonic brain and spinal cord tissue, stem cells (embryonic stem cells, neural stem/progenitor cells, hematopoietic stem cells, adipose-derived adult stem/precursor cells, skin-derived precursor, induced pluripotent stem cells), glial cells (Schwann cells, oligodendrocyte, olfactory ensheathing cells, astrocytes, microglia, tanycytes), neuronal cells (various phenotypic neurons and Purkinje cells), mesenchymal stromal cells originating from bone marrow, umbilical cord, and umbilical cord blood, epithelial cells derived from the layer of retina and amnion, menstrual blood-derived stem cells, Sertoli cells, and active macrophages, etc. Proof-of-concept indicates that we have now entered a new era in neurorestoratology.

Molecular and immunogenic features of myelin lipids: incitants or modulators of multiple sclerosis?

Myelin lipids have long been thought to play intriguing roles in the pathogenesis of multiple sclerosis (MS). This review summarizes current understanding of the molecular basis of MS with emphasis on the: (i.) physico-chemical properties, organization and accessibility of the lipids and their distribution within the myelin multilayer; (ii.) characterization of myelin lipid structures, and structure-function relationships relevant to MS mechanisms, and; (iii.) immunogenic and other features of lipids in MS including molecular mimicry, lipid enzyme genetic knockouts, glycolipid-reactive NKT cells, and monoclonal antibody-induced remyelination. New findings associate anti-lipid antibodies with pathophysiological biomarkers and suggest clinical utility. The structure of CD1d-lipid complexed with the lipophilic invariant T cell receptor (iTCR) may be crucial to understanding MS pathogenesis, and design of lipid antigen-specific therapeutics. Novel immuno-modulatory tools for treatment of autoimmune diseases including MS in which there is both constraint of inflammation and stimulation of remyelination are now emerging.

Molecular probes for imaging myelinated white matter in CNS

Abnormalities and changes in myelination in the brain are seen in many neurodegenerative disorders such as multiple sclerosis (MS). Direct detection and quantification of myelin content in vivo is desired to facilitate diagnosis and therapeutic treatments of myelin-related diseases. The imaging studies require use of myelin-imaging agents that readily enter the brain and selectively bind to myelinated regions. For this purpose, we have systematically evaluated a series of stilbene derivatives as myelin imaging agents. Spectrophotometry-based and radioligand-based binding assays showed that these stilbene derivatives exhibited relatively high myelin-binding affinities. In vitro myelin staining exhibited that the compounds selectively stained intact myelinated regions in wild type mouse brain. In situ tissue staining demonstrated that the compounds readily entered the mouse brain and selectively labeled myelinated white matter regions. These studies suggested that these stilbene derivatives can be used as myelin-imaging probes to monitor myelin pathology in vivo.

Neural precursor cells inhibit multiple inflammatory signals

Intravenous neural precursor cell (NPCs) injection attenuates experimental autoimmune encephalomyelitis by reducing autoreactive T cell encephalitogenicity in lymph nodes in vivo. Here we examined NPC-lymphocyte interactions in vitro. NPCs inhibited the induction of T cell activation marker IL-2-Receptor alpha, ICOS, PD-1 and CTLA-4 and inhibited T cell proliferation. NPCs inhibited T cell activation and proliferation in response to Concavalin-A and to anti-CD3/anti-CD28, which are T cell receptor (TCR)-mediated stimuli, but not in response to phorbol myristate acetate/ionomycin, a TCR-independent stimulus. The suppressive effect was not mediated via downregulation of CD3epsilon or induction of apoptosis. We next examined NPCs effects on inflammatory-cytokine signaling. NPCs impaired IL-2-mediated phosphorylation of JAK3 in lymphocytes, and inhibited IL-6 mediated proliferation of B9 murine hybridoma cells. In conclusion, NPCs ameliorate TCR-mediated T cell activation and inhibit inflammatory cytokines' signaling in immune cells. These findings may underlie the broad anti-inflammatory effects of NPCs in vivo.

Remyelination-promoting human IgMs: developing a therapeutic reagent for demyelinating disease

Promoting remyelination following injury to the central nervous system (CNS) promises to be an effective neuroprotective strategy to limit the loss of surviving axons and prevent disability. Studies confirm that multiple sclerosis (MS) and spinal cord injury lesions contain myelinating cells and their progenitors. Recruiting these endogenous cells to remyelinate may be of therapeutic value. This review addresses the use of antibodies reactive to CNS antigens to promote remyelination. Antibody-induced remyelination in a virus-mediated model of chronic spinal cord injury was initially observed in response to treatment with CNS reactive antisera. Monoclonal mouse and human IgMs, which bind to the surface of oligodendrocytes and myelin, were later identified that were functionally equivalent to antisera. A recombinant form of a human remyelination-promoting IgM (rHIgM22) targets areas of CNS injury and promotes maximal remyelination within 5 weeks after a single low dose (25 microg/kg). The IgM isoform of this reparative antibody is required for in vivo function. We hypothesize that the IgM clusters membrane domains and associated signaling molecules on the surface of target cells. Current therapies for MS are designed to modulate inflammation. In contrast, remyelination promoting IgMs are the first potential therapeutic molecules designed to induce tissue repair by acting within the CNS at sites of damage on the cells responsible for myelin synthesis.

Effectors of demyelination and remyelination in the CNS: implications for multiple sclerosis

Most of the research on multiple sclerosis (MS) has focused on the early events that trigger demyelination and subsequent remyelination. Less attention has been given to the factors that directly mediate the demyelination that is the hallmark of the disease. Effector cells or molecules are those factors directly responsible for mediating the damage in the disease. Similarly, there are effector molecules that are critical for remyelination in the central nervous system (CNS). By understanding those effector molecules in demyelination and remyelination that directly influence the pathologic process, we should be able to generate specific therapies with the greatest potential for benefiting MS patients. This review focuses on effector cells and molecules that are critical for demyelination and remyelination in MS but also in experimental models of the disease including experimental autoimmune encephalomyelitis (EAE), virus-induced models of demyelination (Theiler's virus, murine hepatitis virus), and toxic models of demyelination (lysolecithin, ethidium bromide, and cuprizone). These are models in which the effector molecules for demyelination and remyelination have been most precisely evaluated.

Role of Endogenous Neural Stem Cells in Neurological Disease and Brain Repair

These examples show that stem-cell-based therapy of neuro-psychiatric disorders will not follow a single scheme, but rather include widely different approaches. This is in accordance with the notion that the impact of stem cell biology on neurology will be fundamental, providing a shift in perspective, rather than introducing just one novel therapeutic tool. Stem cell biology, much like genomics and proteomics, offers a "view from within" with an emphasis on a theoretical or real potential and thereby the inherent openness, which is central to the concept of stem cells. Thus, stem cell biology influences many other, more traditional therapeutic approaches, rather than introducing one distinct novel form of therapy. Substantial advances have been made i n neural stemcell research during the years. With the identification of stem and progenitor cells in the adult brain and the complex interaction of different stem cell compartments in the CNS--both, under physiological and pathological conditions--new questions arise: What is the lineage relationship between t he different progenitor cells in the CNS and how much lineage plasticity exists? What are the signals controlling proliferation and differentiation of neural stem cells and can these be utilized to allow repair of the CNS? Insights in these questions will help to better understand the role of stem cells during development and aging and the possible relation of impaired or disrupted stem cell function and their impact on both the development and treatment of neurological disease. A number o f studies have indicated a limited neuronal and glial regeneration certain pathological conditions. These fundamental observations have already changed our view on understanding neurological disease and the brain's capacity for endogenous repair. The following years will have to show how we can influence andmodulate endogenous repair nisms by increasing the cellular plasticity in the young and aged CNS.

Axon behaviour at Schwann cell - astrocyte boundaries: manipulation of axon signalling pathways and the neural adhesion molecule L1 can enable axons to cross

Axon regeneration in vivo is blocked at boundaries between Schwann cells and astrocytes, such as occur at the dorsal root entry zone and around peripheral nerve or Schwann cell grafts. We have created a tissue culture model of these boundaries in Schwann cell - astrocyte monolayer co-cultures. Axon behaviour resembles that in vivo, with axons showing a strong preference for Schwann cells over astrocytes. At boundaries between the two cell types, axons growing on astrocytes cross readily onto Schwann cells, but only 15% of axons growing on Schwann cells are able to cross onto astrocytes. Treatment with chondroitinase or chlorate to reduce inhibition by proteoglycans did not change this behaviour. The neural adhesion molecule L1 is present on Schwann cells and not astrocytes, and manipulation of L1 by application of an antibody, L1-Fc in solution, or adenoviral transduction of L1 into astrocytes increased the proportion of axons able to cross onto astrocytes to 40-50%. Elevating cAMP levels increased crossing from Schwann cells onto astrocytes in live and fixed cultures, and had a co-operative effect with NT-3 but not with NGF. Inactivation of Rho with a cell-permeant form of C3 exoenzyme also increased crossing from Schwann cells to astrocytes. Our experiments indicate that the preference of axons for Schwann cells is largely mediated by the presence of L1 on Schwann cells but not astrocytes, and that manipulation of growth cone signalling pathways can allow axons to disregard boundaries between the two cell types.

A human antibody that promotes remyelination enters the CNS and decreases lesion load as detected by T2‐weighted spinal cord MRI in a virus‐induced murine model of MS

The human monoclonal antibody rHIgM22 enhances remyelination following spinal cord demyelination in a virus-induced murine model of multiple sclerosis. Using three-dimensional T2-weighted in vivo spinal cord magnetic resonance imaging (MRI), we have therefore assessed the extent of spinal cord demyelination, before and after 5 weeks of treatment with rHIgM22, to determine whether antibody enhanced remyelination can be detected by MRI. A significant decrease was seen in T2 high signal lesion volume following antibody treatment. Histologic examination of the spinal cord tissue reveals that this decrease in lesion volume correlates with antibody promoted remyelination. To show that rHIgM22 enters the spinal cord and colocalizes with demyelinating lesions, we used ultrasmall superparamagnetic iron oxide particle (USPIO)-labeled antibodies. This may be considered as additional evidence to the hypothesis that rHIgM22 promotes remyelination by local effects in the lesions, likely by binding to CNS cells. The reduction in high signal T2-weighted lesion volume may be an important outcome measure in future clinical trials in humans.

Remyelination-promoting antibodies activate distinct Ca2+ influx pathways in astrocytes and oligodendrocytes: relationship to the mechanism of myelin repair

Our laboratory has identified mouse and human monoclonal antibodies that promote myelin repair in multiple models of demyelinating disease. We have proposed that these antibodies promote remyelination by directly activating central nervous system glia. Intracellular calcium concentration was monitored using a Fura2 ratiometric assay. Repair-promoting antibodies induced distinct Ca2+ signals in both astrocytes and oligodendrocytes. Astrocyte Ca2+ signaling is mediated by a phospholipase C-dependent pathway while oligodendrocyte Ca2+ signaling is mediated via AMPA-sensitive glutamate receptors. An antibody's ability to induce Ca2+ signals is statistically correlated with promotion of myelin repair. These findings support the hypothesis that remyelination-promoting antibodies are acting directly at the surface of glial cells to induce calcium-dependent physiologic reparative function.

Heterogeneity of pathogenesis in multiple sclerosis: implications for promotion of remyelination

Enhancing myelin repair remains an important therapeutic goal in primary demyelinating diseases of the central nervous system (CNS) such as multiple sclerosis (MS). The emerging heterogeneity of pathology within MS lesions, and differential oligodendrocyte survival in particular, suggests that therapeutic strategies may need to be tailored to an individual patient's requirements. A number of therapeutic strategies have been proposed to enhance myelin repair in the CNS: cell transplantation, growth factor therapy, and antibody therapy, but each proposed therapy has different implications with respect to pathogenetic mechanisms of demyelination. Of these, antibody therapy is the most amenable to immediate application in patients-but a combination of therapeutic approaches may be required in practice.

In vivo magnetic resonance tracking of magnetically labeled cells after transplantation

During the last few years, the therapeutic use of stem and progenitor cells as a substitute for malfunctioning endogenous cell populations has received considerable attention. Unlike their current use in animal models, the introduction of therapeutic cells in patients will require techniques that can monitor their tissue biodistribution noninvasively. Among the different imaging modalities, magnetic resonance (MR) imaging offers both near-cellular (i.e., 25- to 50-mu) resolution and whole-body imaging capability. In order to be visualized, cells must be labeled with an intracellular tracer molecule that can be detected by MR imaging. Methods have now been developed that make it possible to incorporate sufficient amounts of superparamagnetic iron oxide into cells, enabling their detection in vivo using MR imaging. This is illustrated for (neural stem cell-derived) magnetically labeled oligodendroglial progenitors, transplanted in the central nervous system of dysmyelinated rats. Cells can be followed in vivo for at least 6 weeks after transplantation, with a good histopathologic correlation including the formation of myelin. Now that MR tracking of magnetically labeled cells appears feasible, it is anticipated that this technique may ultimately become an important tool for monitoring the efficacy of clinical (stem) cell transplantation protocols.

Embryonic-derived glial-restricted precursor cells (GRP cells) can differentiate into astrocytes and oligodendrocytes in vivo

We have isolated and characterized a unique glial-restricted precursor cell (GRP) from the embryonic spinal cord. Clonal analysis demonstrated that these cells are able to generate oligodendrocytes and two distinct type of astrocytes (type 1 and type 2) when exposed to appropriate signals in vitro. We now show that many aspects of these cells are retained in vivo. GRP cells are restricted to the glial lineage in vivo as they seem to be unable to generate neuronal phenotypes in an in vivo neurogenic environment. GRP cells survive and migrate in the neonatal and adult brain. Transplanted GRP cells differentiate into myelin-forming oligodendrocytes in a myelin-deficient background and also generate immature oligodendrocytes in the normal neonatal brain. In addition, GRP cells also consistently generated glial fibrillary protein-expressing cells in the neonatal and adult brain, a property not consistently expressed by other glial precursor cells like the O-2A/OPC cells. We suggest that the lineage restriction of GRP cells and their ability to generate both oligodendrocytes and astrocytes in vivo together with their embryonic character that allows for extensive in vitro expansion of the population makes the cell useful for clinical application.

Transplantation of neural precursor cells into the dysmyelinated CNS of mutant mice deficient in the myelin-associated glycoprotein and Fyn tyrosine kinase

We have studied in long-term experiments the fate of intraventricularly transplanted neural precursor cells in a dysmyelinated mouse brain. Precursor cells were isolated from striata or spinal cords of transgenic mouse embryos ubiquitously expressing enhanced green fluorescent protein (EGFP). Cells were expanded in vitro in the presence of mitogens for up to 14 weeks, and injected into the lateral ventricle of young postnatal mouse mutants deficient in the myelin-associated glycoprotein (MAG) and the nonreceptor-type tyrosine kinase Fyn. The CNS of these mutants is severely hypomyelinated and most myelin sheaths display ultrastructural abnormalities. Despite this phenotype, MAG/Fyn-deficient mice have a normal longevity. Analysis of mutant brains 1 to 6 months after transplantation revealed widespread distribution of EGFP-positive cells in the recipient tissue. Grafted cells preferentially populated white matter tracts and differentiated into a variety of morphologically distinct cell types. A significant fraction of donor cells was identified as oligodendrocytes. Electron microscopic analysis revealed the presence of numerous donor-derived, ultrastructurally intact, myelin sheaths around host axons. EGFP-positive oligodendrocytes and myelin survived for up to 6 months after transplantation, the latest time point investigated. Remarkably, the number of donor-derived oligodendrocytes increased significantly with increasing time intervals after transplantation, resulting in widespread myelination of 6-month-old host brains. These long-term experiments thus demonstrate that extensive myelination of a dysmyelinated brain can be achieved after a single injection of neural precursor cells.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Depletion of endogenous oligodendrocyte progenitors rather than increased availability of survival factors is a likely explanation for enhanced survival of transplanted oligodendrocyte progenitors in X-irradiated compared to normal CNS

Oligodendrocyte progenitors (OPs) survive and migrate following transplantation into adult rat central nervous system (CNS) exposed to high levels of X-irradiation but fail to do so if they are transplanted into normal adult rat CNS. In the context of developing OP transplantation as a potential therapy for repairing demyelinating diseases it is clearly of some importance to understand what changes have occurred in X-irradiated CNS that permit OP survival. This study addressed two alternative hypotheses. Firstly, X-irradiation causes an increase in the availability of OP survival factors, allowing the CNS to support a greater number of progenitors. Secondly, X-irradiation depletes the endogenous OP population thereby providing vacant niches that can be occupied by transplanted OPs. In situ hybridization was used to examine whether X-irradiation causes an increase in mRNA expression of five known OP survival factors, CNTF, IGF-I, PDGF-A, NT-3 and GGF-2. The levels of expression of these factors at 4 and 10 days following exposure of the adult rat spinal cord to X-irradiation remain the same as the expression levels in normal tissue. Using intravenous injection of horseradish peroxidase, no evidence was found of X-irradiation-induced change in blood-brain barrier permeability that might have exposed X-irradiated tissue to serum-derived survival factors. However, in support of the second hypothesis, a profound X-irradiation-induced decrease in the number of OPs was noted. These data suggest that the increased survival of transplanted OPs in X-irradiated CNS is not a result of the increases in the availability of the OP survival factors examined in this study but rather the depletion of endogenous OPs creating 'space' for transplanted OPs to integrate into the host tissue.

Cell delivery to the central nervous system

A dysfunctional central nervous system (CNS) resulting from neurological disorders and diseases impacts all of humanity. The outcome presents a staggering health care issue with a tremendous potential for developing interventive therapies. The delivery of therapeutic molecules to the CNS has been hampered by the presence of the blood-brain barrier (BBB). To circumvent this barrier, putative therapeutic molecules have been delivered to the CNS by such methods as pumps/osmotic pumps, osmotic opening of the BBB, sustained polymer release systems and cell delivery via site-specific transplantation of cells. This review presents an overview of some of the CNS delivery technologies with special emphasis on transplantation of cells with and without the use of polymer encapsulation technology.

Multiple sclerosis and chronic autoimmune encephalomyelitis: a comparative quantitative study of axonal injury in active, inactive, and remyelinated lesions

Recent magnetic resonance (MR) studies of multiple sclerosis lesions indicate that axonal injury is a major correlate of permanent clinical deficit. In the present study we systematically quantified acute axonal injury, defined by immunoreactivity for beta-amyloid-precursor-protein in dystrophic neurites, in the central nervous system of 22 multiple sclerosis patients and 18 rats with myelin-oligodendrocyte glycoprotein (MOG)-induced chronic autoimmune encephalomyelitis (EAE). The highest incidence of acute axonal injury was found during active demyelination, which was associated with axonal damage in periplaque and in the normal appearing white matter of actively demyelinating cases. In addition, low but significant axonal injury was also observed in inactive demyelinated plaques. In contrast, no significant axonal damage was found in remyelinated shadow plaques. The patterns of axonal pathology in chronic active EAE were qualitatively and quantitatively similar to those found in multiple sclerosis. Our studies confirm previous observations of axonal destruction in multiple sclerosis lesions during active demyelination, but also indicate that ongoing axonal damage in inactive lesions may significantly contribute to the clinical progression of the disease. The results further emphasize that MOG-induced EAE may serve as a suitable model for testing axon-protective therapies in inflammatory demyelinating conditions.

Gliogenesis in the central nervous system

Multipotential neuroepithelial stem cells are thought to give rise to all the differentiated cells of the central nervous system (CNS). The developmental potential of these multipotent stem cells becomes more restricted as they differentiate into progressively more committed cells and ultimately into mature neurons and glia. In studying gliogenesis, the optic nerve and spinal cord have become invaluable models and the progressive stages of differentiation are being clarified. Multiple classes of glial precursors termed glial restricted precursors (GRP), oligospheres, oligodendrocyte-type2 astrocyte (O-2A) and astrocyte precursor cells (APC) have been identified. Similar classes of precursor cells can be isolated from human neural stem cell cultures and from embryonic stem (ES) cell cultures providing a non-fetal source of such cells. In this review, we discuss gliogenesis, glial stem cells, putative relationships of these cells to each other, factors implicated in gliogenesis, and therapeutic applications of glial precursors.

Therapeutic strategies in multiple sclerosis. II. Long-term repair

Spontaneous myelin repair in multiple sclerosis (MS) provides a striking example of the brain's inherent capacity for sustained and stable regenerative tissue repair--but also clearly emphasizes the limitations of this capacity; remyelination ultimately fails widely in many patients, and disability and handicap accumulate. The observation of endogenous partial myelin repair has raised the possibility that therapeutic interventions designed to supplement or promote remyelination might have a useful and significant impact both in the short term, in restoring conduction, and in the long term, in safeguarding axons. Therapeutic remyelination interventions must involve manipulations to either the molecular or the cellular environment within lesions; both depend crucially on a detailed understanding of the biology of the repair process and of those glia implicated in spontaneous repair, or capable of contributing to exogenous repair. Here we explore the biology of myelin repair in MS, examining the glia responsible for successful remyelination, oligodendrocytes and Schwann cells, their 'target' cells, neurons and the roles of astrocytes. Options for therapeutic remyelinating strategies are reviewed, including glial cell transplantation and treatment with growth factors or other soluble molecules. Clinical aspects of remyelination therapies are considered--which patients, which lesions, which stage of the disease, and how to monitor an intervention--and the remaining obstacles and hazards to these approaches are discussed.

Polysialylated neural cell adhesion molecule-positive CNS precursors generate both oligodendrocytes and Schwann cells to remyelinate the CNS after transplantation

Transplantation offers a means of identifying the differentiation and myelination potential of early neural precursors, features relevant to myelin regeneration in demyelinating diseases. In the postnatal rat brain, precursor cells expressing the polysialylated (PSA) form of the neural cell adhesion molecule NCAM have been shown to generate mostly oligodendrocytes and astrocytes in vitro (Ben-Hur et al., 1998). Immunoselected PSA-NCAM+ newborn rat CNS precursors were expanded as clusters with FGF2 and grafted into a focal demyelinating lesion in adult rat spinal cord. We show that these neural precursors can completely remyelinate such CNS lesions. While PSA-NCAM+ precursor clusters contain rare P75+ putative neural crest precursors, they do not generate Schwann cells in vitro even in the presence of glial growth factor. Yet they generate oligodendrocytes, astrocytes, and Schwann cells in vivo when confronted with demyelinated axons in a glia-free area. We confirmed the transplant origin of these Schwann cells using Y chromosome in situ hybridization and immunostaining for the peripheral myelin protein P0 of tissue from female rats that had been grafted with male cell clusters. The number and distribution of Schwann cells within remyelinated tissue, and the absence of P0 mRNAs in donor cells, indicated that Schwann cells were generated by expansion and differentiation of transplanted PSA-NCAM+ neural precursors and were not derived from contaminating Schwann cells. Thus, transplantation into demyelinated CNS tissue reveals an unexpected differentiation potential of a neural precursor, resulting in remyelination of CNS axons by PNS and CNS myelin-forming cells.

Expression of a homologue of rat NG2 on human microglia

The expression of NG2 chondroitin sulfate has been widely associated with oligodendrocyte precursors in rodents. We used a monoclonal antibody (9.2.27) against the human homologue of the rat NG2 to determine whether expression of this molecule was associated with a specific glial cell population present in dissociated cell preparations derived from adult and fetal human brain tissue. Our data, derived using FACS and immunocytochemical analyses of immediately ex vivo or cultured glial cells, indicate that the large majority of NG2 expressing cells belonged to the microglial lineage (CD68, CD11c) rather than to the oligodendrocyte lineage (O4, A2B5, GalC). In situ immunohistochemistry performed on non-fixed normal spinal cord tissue confirmed the observation that NG2 is expressed by mononuclear phagocytes of the CNS. In contrast, peripheral blood-derived monocytes were NG2(-). Cells from fetal brain tissue showed only small numbers of NG2(+) cells, which was consistent with the number of microglial cells in this preparation. In absence of additional markers, we cannot exclude that this anti-NG2 mAb might also recognize human oligodendrocyte progenitor cells.

Multipotent and restricted precursors in the central nervous system

Acquisition of cell type-specific properties in the nervous system is likely a process of sequential restriction in developmental potential. At least two classes of pluripotent stem cells, neuroepithelial (NEP) stem cells and EGF-dependent neurosphere stem cells, have been identified in distinct spatial and temporal domains. Pluripotent stem cells likely generate central nervous system (CNS) and peripheral nervous system (PNS) derivatives via the generation of intermediate lineage-restricted precursors that differ from each other and from multipotent stem cells. Neuronal precursors termed neuronal-restricted precursors (NRPs), multiple classes of glial precursors termed glial-restricted precursors (GRPs), oligodendrocyte-type 2 astrocytes (O2As), astrocyte precursor cells (APCs), and PNS precursors termed neural crest stem cells (NCSCs) have been identified. Multipotent stem cells and restricted precursor cells can be isolated from embryonic stem (ES) cell cultures providing a non-fetal source of such cells. Analysis in multiple species illustrates similarities between rat, mouse, and human cell differentiation raising the possibility that similar factors and markers may be used to isolate precursor cells from human tissue or ES cells. Anat Rec (New Anat): 257:137-143, 1999.

Establishment and properties of neural stem cell clones: plasticity in vitro and in vivo

The study of the basic physiology of the neural precursors generated during brain development is driven by two inextricably linked goals. First, such knowledge is instrumental to our understanding of how the high degree of cellular complexity of the mature central nervous system (CNS) is generated, and how to dissect the steps of proliferation, fate commitment, and differentiation that lead early pluripotent neural progenitors to give rise to mature CNS cells. Second, it is hoped that the isolation, propagation, and manipulation of brain precursors and, particularly, of multipotent neural stem cells (NSCs), will lead to therapeutic applications in neurological disorders. The debate is still open concerning the most appropriate definition of a stem cell and on how it is best identified, characterized, and manipulated. By adopting an operational definition of NSCs, we review some of the basic findings in this area and elaborate on their potential therapeutic applications. Further, we discuss recent evidence from our two groups that describe, based on that rigorous definition, the isolation and propagation of clones of NSCs from the human fetal brain and illustrate how they have begun to show promise for neural cell replacement and molecular support therapy in models of degenerative CNS diseases. The extensive propagation and engraftment potential of human CNS stem cells may, in the not-too-distant-future, be directed towards genuine clinical therapeutic ends, and may open novel and multifaceted strategies for redressing a variety of heretofore untreatable CNS dysfunctions.

Isolation and serum-free culture of primary Schwann cells from human fetal peripheral nerve

We have developed a method for isolating Schwann cells (SC) from human fetal peripheral nerve and maintaining these SC in vitro under serum-free conditions. This method yields essentially pure SC which have a bipolar, spindle-shaped morphology; align in fascicles; and express typical glial cell markers. Human fetal SC can be maintained for months under serum-free conditions with the neuregulin NDF beta. These human fetal SC can mimic axonal contact in vivo by retaining the functional capacity to strongly associate with neurites of cultured human fetal dorsal root ganglia. These isolation, culture, and coculture techniques provide a method for investigating SC-neuron interactions as well as development and function of human fetal SC.

Isolation and cloning of multipotential stem cells from the embryonic human CNS and establishment of transplantable human neural stem cell lines by epigenetic stimulation

Stem cells that can give rise to neurons, astroglia, and oligodendroglia have been found in the developing and adult central nervous system (CNS) of rodents. Yet, their existence within the human brain has not been documented, and the isolation and characterization of multipotent embryonic human neural stem cells have proven difficult to accomplish. We show that the developing human CNS embodies multipotent precursors that differ from their murine counterpart in that they require simultaneous, synergistic stimulation by both epidermal and fibroblast growth factor-2 to exhibit critical stem cell characteristics. Clonal analysis demonstrates that human C NS stem cells are multipotent and differentiate spontaneously into neurons, astrocytes, and oligodendrocytes when growth factors are removed. Subcloning and population analysis show their extensive self-renewal capacity and functional stability, their ability to maintain a steady growth profile, their multipotency, and a constant potential for neuronal differentiation for more than 2 years. The neurons generated by human stem cells over this period of time are electrophysiologically active. These cells are also cryopreservable. Finally, we demonstrate that the neuronal and glial progeny of long-term cultured human CNS stem cells can effectively survive transplantation into the lesioned striatum of adult rats. Tumor formation is not observed, even in immunodeficient hosts. Hence, as a consequence of their inherent biology, human CNS stem cells can establish stable, transplantable cell lines by epigenetic stimulation. These lines represent a renewable source of neurons and glia and may significantly facilitate research on human neurogenesis and the development of clinical neural transplantation.

Identification of A2B5-positive putative oligodendrocyte progenitor cells and A2B5-positive astrocytes in adult human white matter

Spontaneous remyelination of previously demyelinated axons is found in a substantial minority of acute and chronic lesions in multiple sclerosis. In the rodent, central remyelination restores saltatory conduction and helps restore limb function, and it seems likely that endogenous myelin repair contributes to neurological recovery in multiple sclerosis. However, the identity of the remyelinating cell remains enigmatic. Fully differentiated oligodendrocytes have very limited capacity for recapitulating their developmental activities and re-engaging myelination pathways. Proliferative oligodendrocyte progenitors--often known as O-2A cells because of their ability to differentiate in vitro into either oligodendrocytes or ("type 2") astrocytes--are, in contrast, extremely efficient at myelin repair either spontaneously, or after transplantation into the de- or dysmyelinated CNS. Oligodendrocyte progenitors are present in both developing and adult rodent CNS. We have previously demonstrated that proliferative oligodendrocyte progenitors are present in cultures prepared from the adult human CNS. Here, using fresh tissue print preparations, we report that cells with processes and the A2B5-positive immunophenotype of proliferative oligodendrocyte progenitors are present in situ in adult human white matter. This technique also reveals the occurrence of A2B5-positive astrocytes, a cell also not previously identified in the normal adult human CNS. In the light of the rodent data showing the importance of oligodendrocyte progenitors in myelin repair, our findings suggesting the presence of progenitors in the adult human brain may have significant implications for spontaneous remyelination in multiple sclerosis and other demyelinating conditions.

Demyelination and remyelination of the caudal cerebellar peduncle of adult rats following stereotaxic injections of lysolecithin, ethidium bromide, and complement/anti-galactocerebroside: a comparative study

Experimentally induced demyelination due to the direct injection of gliotoxic agents has provided powerful models for studying the biology of remyelination. For the most part, these models have involved injection into white matter tracts of the spinal cord. However, the spinal cord has a number of limitations, such as the size of lesions that it is possible to make and its unsuitability for long-term direct cannulation for the delivery of putative remyelination-enhancing agents. In this study, we describe the natural history of three new models of demyelination/remyelination based on the stereotaxic injection of three gliotoxins: lysolecithin, ethidium bromide, and a combination of anti-galactocerebroside antibody and complement (GalC-ab/comp) into the caudal cerebellar peduncle of adult rats. All three agents produced large areas of demyelination with minimal axonal damage, which undergo extensive remyelination. Ethidium bromide- and GalC-ab/comp-induced lesions remyelinated more slowly than those induced by lysolecithin. The contribution to the remyelination of the lesion by Schwann cells reflects the degree of astrocyte damage incurred within the demyelinated area and is greatest for ethidium bromide-induced demyelination. These new models not only provide further insights into the mechanisms of CNS remyelination but also provide a valuable new resource for addressing a series of key issues relevant to current efforts to promote CNS remyelination either by the enhancement of intrinsic processes or by the transplantation of myelinogenic cells.

Development of O4+/O1- immunopanned pro-oligodendroglia in vitro

In this study, O4+/O1- pro-oligodendroglia isolated by immunopanning from cerebral hemispheres of P3-P5 rats were evaluated during their maturation in culture. Immunopanning yielded 3-4 x 10(5) cells/cerebrum, with 98% O4+ and 6% O1+. There was heterogeneity in the morphologies of immunopanned cells ranging from simple bipolar cells to more complex multipolar cells. As a first step in determining potential differentiative responses of mature oligodendroglia, we examined glial fibrillary acidic protein (GFAP) expression in response to fetal bovine serum (FBS) by cultures established from O4+/O1- immunopanned cells grown for 1, 14, or 21 days, exposed to 20% FBS for 6-7 days and fixed and immunostained on days 7, 21 or 28 in culture (DIC). When immunopanned cells were exposed to FBS following 1 day in serum-free medium, 88% expressed GFAP and when immunopanned cells were cultured for 14 days prior to FBS exposure, 78% expressed GFAP. By contrast, when cells were cultured for 21 days prior to FBS exposure (when a majority of the cells expressed O1 and myelin basic protein (MBP)), only 19% of the cells expressed GFAP (p < 0.001). Cells that were O4+/GFAP- even in the presence of FBS often exhibited a mature oligodendroglial morphology. Among immunopanned cells that responded to FBS by expression of GFAP, both process-bearing (similar to type 2 astroglia) and flattened, polygonal (similar to type 1 astroglia) GFAP+ cells were observed. These results confirm the utility of immunopanning for the isolation of pro-oligodendroglia and demonstrate that oligodendroglia that develop in vitro from O4+/O1- immunopanned cells become resistant to GFAP induction by FBS.

Generation and regulation of developing immortalized neural cell lines

The genetic and environmental signals that regulate progressive lineage elaboration in the mammalian brain are poorly understood. In addition, characterization of the developmental profiles of early central nervous system (CNS) stem/ progenitor cells and analysis of the mechanisms involved in their clonal expansion, lineage restriction, and cellular maturation have been fragmentary and elusive. These seminal neurodevelopmental issues have been examined using a series of clonally derived neural stem/progenitor cell lines established by retroviral transduction of embryonic (E16.5-E17.5) murine hippocampal and cerebellar cells using temperature-sensitive alleles (A58/U19) of the simian virus (SV) 40 large tumor (T) antigen. Under conditions permissive for T-antigen expression (33 degrees C), single neural stem cells exhibited self-renewal, clonal expansion, and both symmetric and asymmetric modes of cell division. By contrast, at the nonpermissive temperature for T-antigen expression (39 degrees C), specific sets of cytokines potentiated the progressive elaboration of neuronal, oligodendroglial, and astroglial lineage species. These observations demonstrate that a spectrum of genetic and epigenetic signals and distinct cellular processes are involved in orchestrating the evolution of individual neural lineages from regional CNS stem/progenitor species. Further, the availability of conditionally immortalized neural cell lines that can be transplanted back into the mammalian brain may represent an important experimental resource for the detailed characterization of cellular and molecular mechanisms involved in the developmental sculpting, plasticity, and regeneration of the mammalian CNS.

Transplanting myelin-forming cells into the central nervous system: principles and practice

Although transplantation of myelin-forming cells into the central nervous system (CNS) has recently attracted much attention as a potential therapy for repairing persistent demyelination found in the demyelinating diseases such as multiple sclerosis and the leukodystrophies, it is worth remembering that the technique was originally conceived of as an experimental technique for manipulating in vivo environments to study interactions between different cell types in either repair or development. It is in this capacity that the technique is still predominantly used. Nevertheless, information, both technical and biological, that the continued use of the technique yields also often provides material for assessing the feasibility of glial cell transplantation as a therapeutic procedure. In this article, we describe some of the guiding principles of transplantation of myelinogenic cells into the mammalian CNS, focusing initially on the recipient environment and then considering the donor material. The division of the discussion into recipient and donor is one of convenience since in reality the interactions between the two cannot be considered in isolation.

Remyelination: cellular and gene therapy

Dysfunctional myelination or oligodendroglial abnormalities play a prominent role in a vast array of pediatric neurological diseases of genetic, inflammatory, immunological, traumatic, ischemic, developmental, metabolic, and infectious causes. Recent advances in glial cell biology have suggested that effective remyelination strategies may, indeed, be feasible. Evidence for myelin repair is accumulating in various experimental models of dysmyelinating and demyelinating disease. Attempts at remyelination have either been directed towards creating myelin de novo from exogenous sources of myelin-elaborating cells or promoting an intrinsic spontaneous remyelinating process. Ultimately, some disorders of myelin may require multiple repair strategies, not only the replacement of dysfunctional cells (oligodendroglia) but also the delivery or supplementation of gene products (i.e., growth factors, immune modulators, metabolic enzymes). Although primary oligodendrocytes or oligodendroglial precursors may be effective for glial cell replacement in certain discrete regions and circumstances and although various genetic vectors may be effective for the delivery of therapeutic molecules, multipotent neural stem cells may be most ideally suited for both gene transfer and cell replacement on transplantation into multiple regions of the central nervous system under a wide range of pathological conditions. We propose that, by virtue of their inherent biological properties, neural stem cells possess the multifaceted therapeutic capabilities that many diseases characterized by myelin dysfunction in the pediatric population may demand.

Purification and characterization of adult oligodendrocyte precursor cells from the rat optic nerve

Oligodendrocyte precursor cells (OPCs) persist in substantial numbers in the adult brain in a quiescent state suggesting that they may provide a source of new oligodendrocytes after injury. To determine whether adult OPCs have the capacity to divide rapidly, we have developed a method to highly purify OPCs from adult optic nerve and have directly compared their properties with their perinatal counterparts. When cultured in platelet-derived growth factor (PDGF), an astrocyte-derived mitogen, perinatal OPCs divided approximately once per day, whereas adult OPCs divided only once every 3 or 4 d. The proliferation rate of adult OPCs was not increased by addition of fibroblast growth factor (FGF) or of the neuregulin glial growth factor 2 (GGF2), two mitogens that are normally produced by retinal ganglion cells. cAMP elevation has been shown previously to be essential for Schwann cells to survive and divide in response to GGF2 and other mitogens. Similarly we found that when cAMP levels were elevated, GGF2 alone was sufficient to induce perinatal OPCs to divide slowly, approximately once every 4 d, but adult OPCs still did not divide. When PDGF was combined with GGF2 and cAMP elevation, however, the adult OPCs began to divide rapidly. These findings indicate that adult OPCs are intrinsically different than perinatal OPCs. They are not senescent cells, however, because they retain the capacity to divide rapidly. Thus, after demyelinating injuries, enhanced axonal release of GGF2 or a related neuregulin might collaborate with astrocyte-derived PDGF to induce rapid division of adult OPCs.

Do olfactory glia have advantages over Schwann cells for CNS repair?

To a large extent the success of axon regeneration and sustained remyelination which distinguishes the PNS from the CNS is attributable to differences in their respective glial environments. For this reason, many have been attracted to the idea that repair of the CNS might be achieved by transplanting Schwann cells into areas of CNS pathology. Schwann cells will not only promote regeneration but will also myelinate axons thereby making them an appropriate cell type to mediate repair of lesions characterised by demyelination as well as axotomy. The recent discovery that olfactory glia are capable of forming myelin sheaths, together with their well-documented ability to support axon regeneration, means that these cells have a range of repair properties similar to that of Schwann cells. It is not clear at present which of these two alternatives, the Schwann cells or the olfactory glial cell, would be of greater benefit for achieving regeneration of axons or remyelination of persistent demyelination following transplantation into the CNS. In this article we review the repair properties of olfactory glia and identify the areas in which their use for repairing the CNS may have advantages over Schwann cells.

Isolation and transplantation of multipotential populations of epidermal growth factor-responsive, neural progenitor cells from the canine brain

Glial cell transplantation into myelin-deficient rodent models has resulted in myelination of axons and restoration of conduction velocity. The shaking (sh) pup canine myelin mutant is a useful model in which to test the ability to repair human myelin diseases, but as in humans, the canine donor supply for allografting is limited. A solution may be provided by self-renewing epidermal growth factor (EGF)-responsive multipotential neural progenitor cell populations ("neurospheres"). Nonadherent spherical clusters, similar in appearance to murine neurospheres, have been obtained from the brain of perinatal wildtype (wt) canine brain and expanded in vitro in the presence of EGF for at least 6 months. Most of the cells in these clusters express a nestin-related protein. Within 1-2 weeks after removal of EGF, cells from the clusters generate neurons, astrocytes, and both oligodendroglial progenitors and oligodendrocytes. Transplantation of lacZ-expressing wt neurospheres into the myelin-deficient (md) rat showed that a proportion of the cells differentiated into oligodendrocytes and produced myelin. In addition, cells from the neurosphere populations survived at least 6 weeks after grafting into a 14-day postnatal sh pup recipient and at least 2 weeks after grafting into an adult sh pup recipient. Thus, neurospheres provide a new source of allogeneic donor cells for transplantation studies in this mutant.

Repair of myelin disease: strategies and progress in animal models

Myelin disorders form an important group of human neurological diseases that are as yet incurable. Recent studies on experimental remyelination have suggested that it might be feasible to repair the CNS, either by transplanting normal myelinating cells or by enhancing endogenous repair. Progress in animal models, particularly in transplanting cells of the oligodendrocyte lineage, has resulted in significant focal remyelination and physiological evidence of restoration of function. These data suggest that focal lesions in multiple sclerosis could be repaired by the transplantation of myelin-forming cells. Future therapies could involve both transplantation and promotion of endogenous repair, and the two approaches could be combined with ex vivo manipulation of the donor tissue.

Signals that initiate myelination in the developing mammalian nervous system

The myelination of axons by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system is essential for the establishment of saltatory conduction. In the absence or destruction of the myelin sheath, as seen in demyelinating diseases, impulse conduction is impeded resulting in severe sensory and motor deficits. Axon myelination is the culmination of a sequence of events that begins with the differentiation of glial cells and proceeds to the transcription and translation of myelin genes, the elaboration of a myelin sheath, and the recognition and ensheathment of axons. This review examines the regulatory mechanisms for each of these steps and compares and contrasts the role of the axon in initiating myelination in the central and peripheral nervous system.

To what extent is oligodendrocyte progenitor migration a limiting factor in the remyelination of multiple sclerosis lesions?

In this article we describe a series of experimental approaches, involving the use of gliotoxin-induced demyelination, X-irradiation and glial cell transplantation, which examine the size of the area around demyelinating lesions from which new remyelinating cells are generated, and the distance over which they are able to migrate. Taken together, these studies suggest that the recruitment of remyelinating cells takes place over a very limited area and that long distance migration of remyelinating cells is not a feature of remyelination. The implications of these findings for spontaneous remyelination of multiple sclerosis plaques, and the development of strategies for enhancing remyelination are discussed.

Inhibition of oligodendrocyte precursor motility by oligodendrocyte processes: implications for transplantation-based approaches to multiple sclerosis

Transplantation of oligodendrocyte precursor cells represents a promising approach to the treatment of the chronic demyelinated lesions of multiple sclerosis. In view of the multi-focal nature of the disease it will be necessary for the transplanted oligodendrocyte precursor cells to migrate through normal white matter between lesions. Work in other systems has shown that differentiated oligodendrocytes within white matter express molecules inhibitory for axon outgrowth. In light of this we have examined the effect of oligodendrocytes on the migration of oligodendrocyte precursors in vitro using time lapse video microscopy. We find that oligodendrocytes induce collapse and loss of motility in oligodendrocyte precursor processes, with this effect being lost as oligodendrocytes undergo programmed cell death. We conclude that the inhibitory factors present on differentiated oligodendrocytes may prevent effective migration between lesion in vivo, and that strategies to overcome this inhibition may be required for successful repair.

Transplanting oligodendrocyte progenitors into the adult CNS

This review covers a number of aspects of the behaviour of oligodendrocyte progenitors following transplantation into the adult CNS. First, an account is given of the ability of transplanted oligodendrocyte progenitors, grown in tissue culture in the presence of PDGF and bFGF, to extensively remyelinate focal areas of persistent demyelination. Secondly, we describe how transplanted clonal cell lines of oligodendrocyte progenitors will differentiate into astrocytes as well oligodendrocytes following transplantation into pathological environments in which both oligodendrocytes and astrocytes are absent, thereby manifesting the bipotentially demonstrable in vitro but not during development. Finally, a series of studies examining the migratory behaviour of transplanted oligodendrocyte progenitors (modelled using the oliodendrocyte progenitor cell line CG4) are described. These show that CG4 cells do not survive (or migrate) when transplanted into the normal adult CNS. However, if they are transplanted into CNS tissue that has previously been exposed to 40 Gy of x-irradiation then transplanted CG4 cells survive, divide and migrate over large distances. Moreover, within an x-irradiated environment, migrating transplanted CG4 cells are able to enter remotely located foci of demyelination and contribute to the remyelination of the demyelinated axons within. These studies demonstrate that although the normal adult CNS does not appear to support survival and migration of the CG4 cell line, it is possible to manipulate the environment in such a way that these nonpermissive properties of the environment can be overcome.