Progenitor cells derived from the adult human subcortical white matter disperse and differentiate as oligodendrocytes within demyelinated lesions of the rat brain

Progenitor-derived oligodendrocyte culture system from human fetal brain

Differentiation of human neural progenitors into neuronal and glial cell types offers a model to study and compare molecular regulation of neural cell lineage development. In vitro expansion of neural progenitors from fetal CNS tissue has been well characterized. Despite the identification and isolation of glial progenitors from adult human sub-cortical white matter and development of various culture conditions to direct differentiation of fetal neural progenitors into myelin producing oligodendrocytes, acquiring sufficient human oligodendrocytes for in vitro experimentation remains difficult. Differentiation of galactocerebroside(+) (GalC) and O4(+) oligodendrocyte precursor or progenitor cells (OPC) from neural precursor cells has been reported using second trimester fetal brain. However, these cells do not proliferate in the absence of support cells including astrocytes and neurons, and are lost quickly over time in culture. The need remains for a culture system to produce cells of the oligodendrocyte lineage suitable for in vitro experimentation. Culture of primary human oligodendrocytes could, for example, be a useful model to study the pathogenesis of neurotropic infectious agents like the human polyomavirus, JCV, that in vivo infects those cells. These cultured cells could also provide models of other demyelinating diseases of the central nervous system (CNS). Primary, human fetal brain-derived, multipotential neural progenitor cells proliferate in vitro while maintaining the capacity to differentiate into neurons (progenitor-derived neurons, PDN) and astrocytes (progenitor-derived astrocytes, PDA) This study shows that neural progenitors can be induced to differentiate through many of the stages of oligodendrocytic lineage development (progenitor-derived oligodendrocytes, PDO). We culture neural progenitor cells in DMEM-F12 serum-free media supplemented with basic fibroblast growth factor (bFGF), platelet derived growth factor (PDGF-AA), Sonic hedgehog (Shh), neurotrophic factor 3 (NT-3), N-2 and triiodothyronine (T3). The cultured cells are passaged at 2.5e6 cells per 75cm flasks approximately every seven days. Using these conditions, the majority of the cells in culture maintain a morphology characterized by few processes and express markers of pre-oligodendrocyte cells, such as A2B5 and O-4. When we remove the four growth factors (GF) (bFGF, PDGF-AA, Shh, NT-3) and add conditioned media from PDN, the cells start to acquire more processes and express markers specific of oligodendrocyte differentiation, such as GalC and myelin basic protein (MBP). We performed phenotypic characterization using multicolor flow cytometry to identify unique markers of oligodendrocyte.

Experimental and therapeutic opportunities for stem cells in multiple sclerosis

Multiple Sclerosis (MS) is an inflammatory demyelinating neurodegenerative disorder of the brain and spinal cord that causes significant disability in young adults. Although the precise aetiopathogenesis of MS remains unresolved, its pathological hallmarks include inflammation, demyelination, axonal injury (acute and chronic), astrogliosis and variable remyelination. Despite major recent advances in therapeutics for the early stage of the disease there are currently no disease modifying treatments for the progressive stage of disease, whose pathological substrate is axonal degeneration. This represents the great and unmet clinical need in MS. Against this background, human stem cells offer promise both to improve understanding of disease mechanism(s) through in-vitro modeling as well as potentially direct use to supplement and promote remyelination, an endogenous reparative process where entire myelin sheaths are restored to demyelinated axons. Conceptually, stem cells can act directly to myelinate axons or indirectly through different mechanisms to promote endogenous repair; importantly these two mechanisms of action are not mutually exclusive. We propose that discovery of novel methods to invoke or enhance remyelination in MS may be the most effective therapeutic strategy to limit axonal damage and instigate restoration of structure and function in this debilitating condition. Human stem cell derived neurons and glia, including patient specific cells derived through reprogramming, provide an unprecedented experimental system to model MS “in a dish” as well as enable high-throughput drug discovery. Finally, we speculate upon the potential role for stem cell based therapies in MS.

Gene expression in the spinal cord in female lewis rats with experimental autoimmune encephalomyelitis induced with myelin basic protein

Background

Experimental autoimmune encephalomyelitis (EAE), the best available model of multiple sclerosis, can be induced in different animal strains using immunization with central nervous system antigens. EAE is associated with inflammation and demyelination of the nervous system. Micro-array can be used to investigate gene expression and biological pathways that are altered during disease. There are few studies of the changes in gene expression in EAE, and these have mostly been done in a chronic mouse EAE model. EAE induced in the Lewis with myelin basic protein (MBP-EAE) is well characterised, making it an ideal candidate for the analysis of gene expression in this disease model.

Methodology/Principal Findings

MBP-EAE was induced in female Lewis rats by inoculation with MBP and adjuvants. Total RNA was extracted from the spinal cords and used for micro-array analysis using AffimetrixGeneChip Rat Exon 1.0 ST Arrays. Gene expression in the spinal cords was compared between healthy female rats and female rats with MBP-EAE. Gene expression in the spinal cord of rats with MBP-EAE differed from that in the spinal cord of normal rats, and there was regulation of pathways involved with immune function and nervous system function. For selected genes the change in expression was confirmed with real-time PCR.

Conclusions/Significance

EAE leads to modulation of gene expression in the spinal cord. We have identified the genes that are most significantly regulated in MBP-EAE in the Lewis rat and produced a profile of gene expression in the spinal cord at the peak of disease.

Glial progenitor cell-based treatment and modeling of neurological disease

The diseases of myelin are among the most prevalent and disabling conditions in neurology. These diseases include both the vascular and inflammatory demyelinating disorders of adulthood, as well as the childhood leukodystrophies and cerebral palsy. These fundamentally glial disorders may be amenable to treatment by glial progenitor cells (GPCs), which give rise to astroglia and myelin-producing oligodendrocytes. Given the development of new methods for generating and isolating human GPCs, the myelin disorders may now be compelling targets for cell-based therapy. In addition, the efficient engraftment and expansion of human GPCs in murine hosts has led to the development of human glial chimeric mouse brains, which provides new opportunities for studying the species-specific roles of human glia in cognition, as well as in disease pathogenesis.

Histone deacetylase activity is required for human oligodendrocyte progenitor differentiation

The molecular mechanisms controlling human oligodendrocyte development are poorly characterized. Microarray analysis of human oligodendrocyte progenitor cells (OPCs) and immature oligodendrocytes revealed that specific-class I histone deacetylase (HDAC) target genes were actively repressed during oligodendrocyte commitment. Although epigenetic regulation of oligodendrocyte differentiation has been established in rodent development, the role of HDACs in human OPCs remains undefined. We used HDAC inhibitors (HDACi) trichostatin A (TSA) and sodium butyrate to determine the importance of HDAC activity in human primary OPC differentiation. Treatment with either drug resulted in significant dose-dependent inhibition of O4(+) oligodendrocyte cell differentiation, reduction of oligodendrocyte morphological maturation, and downregulation of myelin basic protein mRNA. High dose TSA treatment was also associated with reduction in OPC proliferation. HDACi treatment prevented downregulation of SOX2, ID4, and TCF7L2 mRNAs but did not regulate HES5, suggesting that targets of HDAC repression may differ between species. These results predict that HDACi treatment would impair proliferation and differentiation by parenchymal oligodendrocyte progenitors, and thereby degrade their potential for endogenous repair in human demyelinating disease. © 2012 Wiley Periodicals, Inc.

Modeling HIV-associated neurocognitive disorders in mice: new approaches in the changing face of HIV neuropathogenesis

It is well established that infection with the human immunodeficiency virus (HIV) leads to immune suppression. Less well known is the fact that long-term, progressive HIV disease is associated with the development of cognitive deficits. Since the introduction of combined antiretroviral therapy (cART), the clinical presentation of HIV infection has evolved into a chronic illness with very low levels of viral replication and chronic immune activation, with compliant affected individuals surviving for decades with a high quality of life. Despite these advances, many HIV-infected individuals develop some degree of neurodegeneration and cognitive impairment. The underlying pathophysiological mechanisms are not well understood, and there are no effective treatments. Thus, there is an unmet need for animal models that enable the study of HIV-associated neurocognitive disorders (HAND) and the testing of new therapeutic approaches to combat them. Here, we review the pros and cons of existing mouse models of HIV infection for addressing these aims and propose a detailed strategy for developing a new mouse model of HIV infection.

The myelin mutants as models to study myelin repair in the leukodystrophies

The leukodystrophies are rare and serious genetic disorders of the central nervous system that primarily affect children who frequently die early in life or have significantly delayed motor and mental milestones that result in long-term disability. Although with some of these disorders, early intervention with bone marrow or cord blood transplantation has been proven useful, it has not yet been determined that such therapies promote myelin repair of the central nervous system. Research on experimental therapies aimed at myelin repair is aided by the ability to test cell replacement strategies in genetic models in which the mutations and neuropathology match the human disorder. Thus, models exist of Pelizaeus-Merzbacher disease and the lysosomal storage disorder, Krabbe disease, which reflect the clinical and pathological course of the human disorders. Collectively, animals with mutations in myelin genes are called the myelin mutants, and they include rodent models such as the shiverer mouse that have been extensively used to study myelination by exogenous cell transplantation. These studies have encompassed many permutations of the age of the recipient, type of transplanted cell, site of engraftment, and so forth, and they offer hope that the scaling up of myelin produced by transplanted cells will have clinical significance in treating patients. Here we review these models and discuss their relative importance and use in such translational approaches. We discuss how grafts are identified and functional outcomes are measured. Finally, we briefly discuss the cells that have been successfully transplanted, which may be used in future clinical trials.

Cell therapy for multiple sclerosis

The spontaneous recovery observed in the early stages of multiple sclerosis (MS) is substituted with a later progressive course and failure of endogenous processes of repair and remyelination. Although this is the basic rationale for cell therapy, it is not clear yet to what degree the MS brain is amenable for repair and whether cell therapy has an advantage in comparison to other strategies to enhance endogenous remyelination. Central to the promise of stem cell therapy is the therapeutic plasticity, by which neural precursors can replace damaged oligodendrocytes and myelin, and also effectively attenuate the autoimmune process in a local, nonsystemic manner to protect brain cells from further injury, as well as facilitate the intrinsic capacity of the brain for recovery. These fundamental immunomodulatory and neurotrophic properties are shared by stem cells of different sources. By using different routes of delivery, cells may target both affected white matter tracts and the perivascular niche where the trafficking of immune cells occur. It is unclear yet whether the therapeutic properties of transplanted cells are maintained with the duration of time. The application of neural stem cell therapy (derived from fetal brain or from human embryonic stem cells) will be realized once their purification, mass generation, and safety are guaranteed. However, previous clinical experience with bone marrow stromal (mesenchymal) stem cells and the relative easy expansion of autologous cells have opened the way to their experimental application in MS. An initial clinical trial has established the probable safety of their intravenous and intrathecal delivery. Short-term follow-up observed immunomodulatory effects and clinical benefit justifying further clinical trials.

Cellular therapy and induced neuronal replacement for Huntington's disease

Huntington's disease (HD) is an inherited, relentlessly progressive neurodegenerative disease with an invariably fatal outcome. HD is inherited in an autosomal dominant fashion, and is characterized pathologically by the loss of cortical and striatal neurons, and clinically by involuntary choreiform movements accompanied by progressive cognitive impairment and emotional lability. The disorder is caused by an expanded cystosine adenine guanine (CAG) tri-nucleotide repeat encoding polyglutamine (polyQ) in the first exon of the Huntingtin gene. There is a correlation between the number of CAG repeats and disease onset, such that in patients with CAG repeat lengths of 36 to 60, disease symptoms typically manifest after 35 years of age, whereas CAG repeat lengths >60 yield the more severe juvenile form of the disease. Even though mutant huntingtin is expressed throughout the brain, it is characterized by the selective degeneration of medium spiny neurons of the caudate and putamen, which heralds more widespread neuronal degeneration with disease progression. The mechanisms of cell dysfunction and death in HD have been the subjects of a number of studies, which have led to therapeutic strategies largely based on the amelioration of mutant huntingtin-related metabolic impairment and cellular toxicity. Each of these approaches has aimed to delay or stop the preferential degeneration of medium spiny neurons early in the disease course. Yet, in later stages of the disease, after cell death has become prominent, cell replacement therapy (whether by direct cell transplantation or by the mobilization of endogenous progenitors) may comprise a stronger potential avenue for therapy. In this review, we will consider recent progress in the transplantation of fetal striatal cells to the HD brain, as well as emerging alternative sources for human striatal progenitor cells. We will then consider the potential application of gene therapy toward the induction of striatal neurogenesis and neuronal recruitment, with an eye toward its potential therapeutic use in HD.

Precursor cell biology and the development of astrocyte transplantation therapies: lessons from spinal cord injury

This review summarizes current progress on development of astrocyte transplantation therapies for repair of the damaged central nervous system. Replacement of neurons in the injured or diseased central nervous system is currently one of the most popular therapeutic goals, but if neuronal replacement is attempted in the absence of appropriate supporting cells (astrocytes and oligodendrocytes), then the chances of restoring neurological functional are greatly reduced. Although the past 20 years have offered great progress on oligodendrocyte replacement therapies, astrocyte transplantation therapies have been both less explored and comparatively less successful. We have now developed successful astrocyte transplantation therapies by pre-differentiating glial restricted precursor (GRP) cells into a specific population of GRP cell-derived astrocytes (GDAs) by exposing the GRP cells to bone morphogenetic protein-4 (BMP) prior to transplantation. When transplanted into transected rat spinal cord, rat and human GDAs(BMP) promote extensive axonal regeneration, rescue neuronal cell survival, realign tissue structure, and restore behavior to pre-injury levels on a grid-walk analysis of volitional foot placement. Such benefits are not provided by GRP cells themselves, demonstrating that the lesion environment does not direct differentiation in a manner optimally beneficial for the restoration of function. Such benefits also are not provided by transplantation of a different population of astrocytes generated from GRP cells exposed to ciliary neurotrophic factor (GDAs(CNTF)), thus providing the first transplantation-based evidence of functional heterogeneity in astrocyte populations. Moreover, lessons learned from the study of rat cells are strongly predictive of outcomes using human cells. Thus, these studies provide successful strategies for the use of astrocyte transplantation therapies for restoration of function following spinal cord injury.

CD140a identifies a population of highly myelinogenic, migration-competent and efficiently engrafting human oligodendrocyte progenitor cells

Experimental models of myelin disorders can be treated by the transplantation of oligodendrocyte progenitor cells (OPCs) into the affected brain or spinal cord. OPCs express gangliosides recognized by MAb A2B5, but this marker also identifies lineage-restricted astrocytes and immature neurons. To establish a more efficient means of isolating myelinogenic OPCs, we asked if FACS could be used to sort PDGFα receptor+ cells from fetal human forebrain, based on expression of the PDGFRα epitope CD140a. CD140a+ isolates were maintained as mitotic bipotential progenitors that could be instructed to either oligodendrocyte or astrocyte fate. Transplanted CD140a+ cells were highly migratory, and rapidly and robustly myelinated the hypomyelinated shiverer mouse brain, more efficiently than did A2B5-sorted cells. Microarray analysis of CD140a+ cells revealed their differential expression of CD9, as well as of PTN-PTPRZ1, wnt, notch and BMP pathway components, indicating the dynamic interaction of self-renewal and fate-restricting pathways in these cells.

Homing of placenta-derived mesenchymal stem cells after perinatal intracerebral transplantation in a rat model

OBJECTIVE

The aim of this study is to assess early homing of placenta-derived stem cells after perinatal intracerebral transplantation in rats.

STUDY DESIGN

Neonatal Wistar rats (2-4 days old) were anesthetized, and 250,000 human placenta-derived mesenchymal stem cells (MSC) injected into the lateral ventricle or the paraventricular white matter using a stereotactic frame. Donor MSC were detected by immunohistochemistry using an antihuman HLA-ABC antibody.

RESULTS

In all, 84% of the animals survived the transplantation. Donor cells were detected in the brain ventricle 1-2 hours posttransplantation. After 4 hours, donor cells migrated throughout the ventricular system. At 1-4 weeks after transplantation, some cells had migrated into the periventricular white matter.

CONCLUSION

Human placenta-derived MSC were successfully transplanted into the lateral ventricles of neonatal rats. Donor cells survived, homed, and migrated in the recipient brains. Proliferation and differentiation analysis and functional tests will assess the therapeutic effects of stem cell transplantation.

Human neural progenitors from different foetal forebrain regions remyelinate the adult mouse spinal cord

Improving oligodendroglial differentiation from human foetal neural progenitor cells remains a primordial issue to accomplish successful cell-based therapies in myelin diseases. Here, we combined in situ, in vitro and in vivo approaches to assess the oligodendrogenic potential of different human foetal forebrain regions during the first trimester of gestation. We show for the first time that the initial wave of oligodendrocyte progenitor emergence in the ventral telencephalon onsets as early as 7.5 weeks into gestation. Interestingly, in vitro, isolation of ganglionic eminences yielded oligodendrocyte progenitor-enriched cultures, as compared with cortex and thalamus. Most importantly, single injection of human neural progenitors into rodent models of focal gliotoxic demyelination revealed the great capacity of these cells to survive, extensively migrate and successfully remyelinate the spinal cord, irrespective of their origin. Thus, our study brings novel insights into the biology of early human foetal neural progenitor cells and offers new support for the development of cellular therapeutics for myelin disorders.

Predifferentiated brain-derived adult human progenitor cells migrate toward ischemia after transplantation to the adult rat brain

BACKGROUND

The adult human brain contains neural stem/progenitor cells (AHNPCs) that can survive transplantation into the adult rat brain, migrate toward a lesion, and display limited neuronal differentiation in vivo.

OBJECTIVE

To investigate the effect of manipulating AHNPCs before grafting by predifferentiation, ie, initiating neuronal differentiation before transplantation, and to determine whether this cell priming would affect their ability to migrate in vivo.

METHODS

AHNPCs were prepared from temporal lobe resections for epilepsy. Seven days after global brain ischemia, predifferentiated AHNPCs (exposed to basic fibroblast growth factor, heparin, and laminin) were transplanted to the left hippocampus. Four and 10 weeks after transplantation, brain sections were analyzed by immunohistochemistry.

RESULTS

Transplanted primed cells expressed committed neuronal markers at a much earlier stage compared with nonprimed AHNPCs and were found colabeled with human markers within the damaged CA1 region 4 weeks after grafting. Furthermore, predifferentiated AHNPCs migrated preferentially into an ischemic lesion, similar to their undifferentiated counterparts. The chemoattractant effect from the expression of stromal cell-derived factor-1α (SDF-1α) in ischemic CA1 on AHNPCs expressing CXC chemokine receptor 4 (CXCR4) may explain this preference in migration in vivo.

CONCLUSION

The plasticity of neural progenitors derived from the adult human brain may be greater than previously assumed in that manipulation before grafting may influence the phenotypes seen in vivo. The SDF-1α-CXCR4 axis is involved in the targeted migration toward an ischemic lesion in the adult rat brain, similar to previous reports on endogenous progenitors in rats and grafted fetal human neural progenitors.

Cells of the oligodendroglial lineage, myelination, and remyelination

Myelin is critical in maintaining electrical impulse conduction in the central nervous system. The oligodendrocyte is the cell type responsible for myelin production within this compartment. The mutual supply of trophic support between oligodendrocytes and the underlying axons may indicate why demyelinated axons undergo degeneration more readily; the latter contributes to the neural decline in multiple sclerosis (MS). Myelin repair, termed remyelination, occurs in acute inflammatory lesions in MS and is associated with functional recovery and clinical remittances. Animal models have demonstrated that remyelination is mediated by oligodendrocyte progenitor cells (OPCs) which have responded to chemotactic cues, migrated into the lesion, proliferated, differentiated into mature oligodendrocytes, and ensheathed demyelinated axons. The limited remyelination observed in more chronic MS lesions may reflect intrinsic properties of neural cells or extrinsic deterrents. Therapeutic strategies currently under development include transplantation of exogenous OPCs and promotion of remyelination by endogenous OPCs. All currently approved MS therapies are aimed at dampening the immune response and are not directly targeting neural processes.

Deciphering the oligodendrogenic program of neural progenitors: cell intrinsic and extrinsic regulators

In the developing and adult CNS, neural stem/progenitor cells (NSPCs) and oligodendroglial progenitor cells (OPCs) follow an oligodendrogenic process with the aim of myelinating axons. This process is to a high degree regulated by an oligodendrogenic program (OPr) composed of intrinsic and extrinsic factors that modulate the different steps required for NSPCs to differentiate into myelinating oligodendrocytes. Even though NSPCs and OPCs are present in the diseased CNS and have the capacity to generate oligodendrocytes, sparse remyelination of axons constitutes a major constraint in therapies toward multiple sclerosis (MS) and spinal cord injury (SCI). Lack of pro-oligodendrogenic factors and presence of anti-oligodendrogenic activities are thought to be the main reasons for this limitation. Thus, molecular and cellular strategies aiming at remyelination and at targeting such pro- and anti-oligodendrogenic mechanisms are currently under investigation. The present review summarizes the current knowledge on the OPr; it implements our own findings on mesenchymal stem cell-derived pro-oligodendroglial factors and on the role of p57/kip2 in oligodendroglial differentiation. Moreover, it describes molecular and cellular approaches for the development of future therapies toward remyelination.

Stem cell biology in traumatic brain injury: effects of injury and strategies for repair

Approximately 350,000 individuals in the US are affected annually by severe and moderate traumatic brain injuries (TBI) that may result in long-term disability. This rate of injury has produced approximately 3.3 million disabled survivors in the US alone. There is currently no specific treatment available for TBI other than supportive care, but aggressive prehospital resuscitation, rapid triage, and intensive care have reduced mortality rates. With the recent demonstration that neurogenesis occurs in all mammals (including man) throughout adult life, albeit at a low rate, the concept of replacing neurons lost after TBI is now becoming a reality. Experimental rodent models have shown that neurogenesis is accelerated after TBI, especially in juveniles. Two approaches have been followed in these rodent models to test possible therapeutic approaches that could enhance neuronal replacement in humans after TBI. The first has been to define and quantify the phenomenon of de novo hippocampal and cortical neurogenesis after TBI and find ways to enhance this (for example by exogenous trophic factor administration). A second approach has been the transplantation of different types of neural progenitor cells after TBI. In this review the authors discuss some of the processes that follow after acute TBI including the changes in the brain microenvironment and the role of trophic factor dynamics with regard to the effects on endogenous neurogenesis and gliagenesis. The authors also discuss strategies to clinically harness the factors influencing these processes and repair strategies using exogenous neural progenitor cell transplantation. Each strategy is discussed with an emphasis on highlighting the progress and limiting factors relevant to the development of clinical trials of cellular replacement therapy for severe TBI in humans.

Importance of oligodendrocyte protection, BBB breakdown and inflammation for remyelination

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the CNS. A better understanding of why remyelination fails in MS is necessary to improve remyelination strategies. Remyelination is mediated by oligodendrocyte precursor cells (OPCs), which are widely distributed throughout the adult CNS. However, it is still unclear whether OPCs detectable in MS lesions survive the inflammatory response but are unable to myelinate or whether OPC and oligodendrocyte death is primarily responsible for remyelination failure and detectable OPCs enter demyelinated areas from adjacent tissue as the lesion evolves. Remyelination strategies should, therefore, focus on stimulation of differentiation or prevention of apoptosis, as well as establishment of a supportive environment for OPC-mediated remyelination, which may be especially important in chronically demyelinated lesions.

Transplantation of ciliary neurotrophic factor-expressing adult oligodendrocyte precursor cells promotes remyelination and functional recovery after spinal cord injury

Demyelination contributes to the dysfunction after traumatic spinal cord injury (SCI). We explored whether the combination of neurotrophic factors and transplantation of adult rat spinal cord oligodendrocyte precursor cells (OPCs) could enhance remyelination and functional recovery after SCI. Ciliary neurotrophic factor (CNTF) was the most effective neurotrophic factor to promote oligodendrocyte (OL) differentiation and survival of OPCs in vitro. OPCs were infected with retroviruses expressing enhanced green fluorescent protein (EGFP) or CNTF and transplanted into the contused adult thoracic spinal cord 9 d after injury. Seven weeks after transplantation, the grafted OPCs survived and integrated into the injured spinal cord. The survival of grafted CNTF-OPCs increased fourfold compared with EGFP-OPCs. The grafted OPCs differentiated into adenomatus polyposis coli (APC(+)) OLs, and CNTF significantly increased the percentage of APC(+) OLs from grafted OPCs. Immunofluorescent and immunoelectron microscopic analyses showed that the grafted OPCs formed central myelin sheaths around the axons in the injured spinal cord. The number of OL-remyelinated axons in ventrolateral funiculus (VLF) or lateral funiculus (LF) at the injured epicenter was significantly increased in animals that received CNTF-OPC grafts compared with all other groups. Importantly, 75% of rats receiving CNTF-OPC grafts recovered transcranial magnetic motor-evoked potential and magnetic interenlargement reflex responses, indicating that conduction through the demyelinated axons in VLF or LF, respectively, was partially restored. More importantly, recovery of hindlimb locomotor function was significantly enhanced in animals receiving grafts of CNTF-OPCs. Thus, combined treatment with OPC grafts expressing CNTF can enhance remyelination and facilitate functional recovery after traumatic SCI.

Stem cells in genetic myelin disorders

The genetic myelin disorders are a range of diseases that manifest with severe neurological problems, often from infancy. It has been postulated for some time that stem cells might be an effective treatment for these disorders, primarily as agents to restore dysfunctional or lost myelin. Stem cells, however, may offer a wider range of therapeutic potential, for instance as vehicles to replace abnormal enzymes or genes, or to provide trophic support for residual CNS tissue. This article will review several of the more common genetic myelin disorders and currently available therapies, including bone marrow transplantation for adrenoleukodystrophy. Specific stem cell subtypes and their relevance to potential therapeutic use will be discussed and stem cell transplantation in animal model studies will also be reviewed.

Robust generation of oligodendrocyte progenitors from human neural stem cells and engraftment in experimental demyelination models in mice

BACKGROUND

Cell-based therapy holds great promises for demyelinating diseases. Human-derived fetal and adult oligodendrocyte progenitors (OPC) gave encouraging results in experimental models of dysmyelination but their limited proliferation in vitro and their potential immunogenicity might restrict their use in clinical applications. Virtually unlimited numbers of oligodendroglial cells could be generated from long-term self-renewing human (h)-derived neural stem cells (hNSC). However, robust oligodendrocyte production from hNSC has not been reported so far, indicating the need for improved understanding of the molecular and environmental signals controlling hNSC progression through the oligodendroglial lineage. The aim of this work was to obtain enriched and renewable cultures of hNSC-derived oligodendroglial cells by means of epigenetic manipulation.

METHODOLOGY/PRINCIPAL FINDINGS

We report here the generation of large numbers of hNSC-derived oligodendroglial cells by concurrent/sequential in vitro exposure to combinations of growth factors (FGF2, PDGF-AA), neurotrophins (NT3) and hormones (T3). In particular, the combination FGF2+NT3+PDGF-AA resulted in the maintenance and enrichment of an oligodendroglial cell population displaying immature phenotype (i.e., proliferation capacity and expression of PDGFRalpha, Olig1 and Sox10), limited self-renewal and increased migratory activity in vitro. These cells generate large numbers of oligodendroglial progeny at the early stages of maturation, both in vitro and after transplantation in models of CNS demyelination.

CONCLUSIONS/SIGNIFICANCE

We describe a reliable method to generate large numbers of oligodendrocytes from a renewable source of somatic, non-immortalized NSC from the human foetal brain. We also provide insights on the mechanisms underlying the pro-oligodendrogenic effect of the treatments in vitro and discuss potential issues responsible for the limited myelinating capacity shown by hNSC-derived oligodendrocytes in vivo.

Glial progenitor cell transplantation and the generation of chimeric animal models with human brain cells: implications for novel therapeutics

BACKGROUND

The potential of exogenous stem cell or progenitor cell transplantation as a novel therapeutic strategy to address unmet medical needs is a vast and important area of investigation. A recent US patent has been issued to Goldman from the University of Rochester based on pioneering studies with human fetal and adult-derived glial progenitor cells (GPCs), covering the generation of chimeric mouse/human animals.

OBJECTIVE/METHOD

In this patent and associated manuscript, extensive chimerism due to grafting of human GPCs is associated with remyelination and functional rescue of mice congenitally deficient in oligodendrocyte survival and myelination, due to a deletion in the myelin basic protein gene (the shiverer mouse). This review highlights the implications of generating human/mouse chimeric animals for the study of human brain physiology, preclinical studies and the clinical application of progenitor cells towards the development of novel therapeutics for the treatment of demyelinating disorders.

CONCLUSION

The use of GPCs offers promise for remyelination disorders, and the ability of these cells to repopulate the entire rodent nervous system should allow for the investigation of the physiological properties of human glial derivatives in an in vivo context, enhancing the understanding of mechanisms with a primary effect through the modulation of human glial cell biology.

Fate determination of adult human glial progenitor cells

Glial progenitor cells (GPCs) comprise the most abundant population of progenitor cells in the adult human brain. They are responsible for central nervous system (CNS) remyelination, and likely contribute to the astrogliotic response to brain injury and degeneration as well. Adult human GPCs are biased to differentiate as oligodendrocytes and elaborate new myelin, and yet they retain multilineage plasticity, and can give rise to neurons as well as astrocytes and oligodendrocytes once removed from the adult parenchymal environment. GPCs retain strong mechanisms for cell-autonomous self-renewal, and yet both their phenotype and fate may be dictated by their microenvironment. Using the transcriptional profiles of acutely isolated GPCs, we have begun to understand the operative ligand-receptor interactions involved in these processes, and have identified several key signaling pathways by which adult human GPCs may be reliably instructed to either oligodendrocytic or astrocytic fate. In addition, we have noted significant differences between the expressed genes and dominant signaling pathways of fetal and adult human GPCs, as well as between rodent and human GPCs. The latter data in particular call into question therapeutic strategies predicated solely upon data obtained using rodents, while perhaps highlighting the extent to which evolution has been attended by the phylogenetic modification of glial phenotype and function.

Isolation of human multipotent neural progenitors from adult filum terminale

Stem cells have been isolated from several CNS regions, including the spinal cord. However, the terminal end of the spinal cord, filum terminale, has been referred to as a fibrovascular tag without neurogenic potential and of no clinical significance. Recently, we were fortunate to acquire some samples of this tissue. We show for the first time that progenitor cells exhibiting the hallmarks of stem cells can be isolated from adult human filum terminale (FTNPs). More specifically, FTNPs self-renew and proliferate to form neurospheres, and exhibit tripotent differentiation into neurons, astrocytes, and oligodendrocytes. Equally important, FTNPs develop the electrophysiological profile of neurons and glia. Whole-cell patch-clamp recordings show beta-III-tubulin(+) neurons exhibiting overshooting action potentials, displaying both the fast inactivating TTX-sensitive sodium current as well as 4-AP and TEA sensitive potassium currents. To assess potency in vivo, FTNPs were transplanted into the posterior periventricular region of control or ischemic rat brains. Despite a vigorous immune response against the xenograft, FTNPs survived and were found not only in the graft area but had also migrated to the lesioned CA1 region. Notwithstanding the immune response, FTNPs differentiated into astrocytes, but no neuronal differentiation was observed in the transplant milieu tested. However, neuronal differentiation in vivo cannot be ruled out and assessment of the conditions necessary to promote neurogenesis in vivo requires more research. Significantly, no tumor formation or aberrant cell morphology was seen in or adjacent to the graft area. Thus, filum terminale provides a novel source of adult human neural progenitor cells that develop into functional neurons with possible clinical applications.

The role of oxygen in regulating neural stem cells in development and disease

Oxygen (O2) is a substrate for energy production in the cell and is a rapid regulator of cellular metabolism. Recent studies have also implicated O2 and its signal transduction pathways in controlling cell proliferation, fate, and morphogenesis during the development of many tissues, including the nervous system. O2 tensions in the intact brain are much lower than in room air, and there is evidence that dynamic control of O2 availability may be a component of the in vivo neural stem cell (NSC) niche. At lower O2 tensions, hypoxia-inducible factor 1alpha (HIF1alpha) facilitates signal transduction pathways that promote self-renewal (e.g., Notch) and inhibits pathways that promote NSC differentiation or apoptosis (e.g., bone morphogenetic proteins). Increasing O2 tension degrades HIF1alpha, thus promoting differentiation or apoptosis of NSCs and progenitors. These dynamic changes in O2 tension can be mimicked to optimize ex vivo production methods for cell replacement therapies. Conversely, disrupted O2 availability may play a critical role in disease states such as stroke or brain tumor progression. Hypoxia during stroke activates precursor proliferation in vivo, while glioblastoma stem cells proliferate maximally in a more hypoxic environment than normal stem cells, which may make them resistant to certain anti-neoplastic therapies. These findings suggest that O2 response is central to the normal architecture and dynamics of NSC regulation and in the etiology and treatment of brain diseases.

Interruption of beta-catenin signaling reduces neurogenesis in Alzheimer's disease

The neuronal loss associated with Alzheimer's disease (AD) affects areas of the brain that are vital to cognition. Although recent studies have shown that new neurons can be generated from progenitor cells in the neocortices of healthy adults, the neurogenic potential of the stem/progenitor cells of AD patients is not known. To answer this question, we compared the properties of glial progenitor cells (GPCs) from the cortices of healthy control (HC) and AD subjects. The GPCs from AD brain samples displayed reduced renewal capability and reduced neurogenesis compared with GPCs from HC brains. To investigate the mechanisms underlying this difference, we compared beta-catenin signaling proteins in GPCs from AD versus HC subjects and studied the effect of amyloid beta peptide (Abeta, a hallmark of AD pathology) on GPCs. Interestingly, GPCs from AD patients exhibited elevated levels of glycogen synthase kinase 3beta (GSK-3beta, an enzyme known to phosphorylate beta-catenin), accompanied by an increase in phosphorylated beta-catenin and a decrease in nonphosphorylated beta-catenin compared with HC counterparts. Furthermore. we found that Abeta treatment impaired the ability of GPCs from HC subjects to generate new neurons and caused changes in beta-catenin signaling proteins similar to those observed in GPCs from AD patients. Similar results were observed in GPCs isolated from AD transgenic mice. These results suggest that Abeta-induced interruption of beta-catenin signaling may contribute to the impairment of neurogenesis in AD progenitor cells.

Neonatal chimerization with human glial progenitor cells can both remyelinate and rescue the otherwise lethally hypomyelinated shiverer mouse

Congenitally hypomyelinated shiverer mice fail to generate compact myelin and die by 18-21 weeks of age. Using multifocal anterior and posterior fossa delivery of sorted fetal human glial progenitor cells into neonatal shiverer x rag2(-/-) mice, we achieved whole neuraxis myelination of the engrafted hosts, which in a significant fraction of cases rescued this otherwise lethal phenotype. The transplanted mice exhibited greatly prolonged survival with progressive resolution of their neurological deficits. Substantial myelination in multiple regions was accompanied by the acquisition of normal nodes of Ranvier and transcallosal conduction velocities, ultrastructurally normal and complete myelination of most axons, and a restoration of a substantially normal neurological phenotype. Notably, the resultant mice were cerebral chimeras, with murine gray matter but a predominantly human white matter glial composition. These data demonstrate that the neonatal transplantation of human glial progenitor cells can effectively treat disorders of congenital and perinatal hypomyelination.

Statin therapy inhibits remyelination in the central nervous system

Remyelination of lesions in the central nervous system contributes to neural repair following clinical relapses in multiple sclerosis. Remyelination is initiated by recruitment and differentiation of oligodendrocyte progenitor cells (OPCs) into myelinating oligodendrocytes. Simvastatin, a blood-brain barrier-permeable statin in multiple sclerosis clinical trials, has been shown to impact the in vitro processes that have been implicated in remyelination. Animals were fed a cuprizone-supplemented diet for 6 weeks to induce localized demyelination in the corpus callosum; subsequent return to normal diet for 3 weeks stimulated remyelination. Simvastatin was injected intraperitoneally during the period of coincident demyelination and OPC maturation (weeks 4 to 6), throughout the entire period of OPC responses (weeks 4 to 9), or during the remyelination-only phase (weeks 7 to 9). Simvastatin treatment (weeks 4 to 6) caused a decrease in myelin load and both Olig2(strong) and Nkx2.2(strong) OPC numbers. Simvastatin treatment (weeks 4 to 9 and 7 to 9) caused a decrease in myelin load, which was correlated with a reduction in Nkx2.2(strong) OPCs and an increase in Olig2(strong) cells, suggesting that OPCs were maintained in an immature state (Olig2(strong)/Nkx2.2(weak)). NogoA+ oligodendrocyte numbers were decreased during all simvastatin treatment regimens. Our findings suggest that simvastatin inhibits central nervous system remyelination by blocking progenitor differentiation, indicating the need to monitor effects of systemic immunotherapies that can access the central nervous system on brain tissue-repair processes.

Olig2 Overexpression Induces the In Vitro Differentiation of Neural Stem Cells into Mature Oligodendrocytes

Differentiation induction of neural stem cells (NSCs) into oligodendrocytes during embryogenesis is the result of a complex interaction between local induction factors and intracellular transcription factors. At the early stage of differentiation, in particular, the helix-loop-helix transcription factors Olig1 and Olig2 have been shown to be essential for oligodendrocyte lineage determination. In view of the possible application of NSCs as a source for remyelinating cell transplants in demyelinating diseases (e.g., multiple sclerosis), in vitro procedures need to be developed to drive the oligodendrocyte differentiation process. Mere culture in medium supplemented with major embryonic oligodendrogenic induction factors, such as Sonic hedgehog, results in oligodendrocyte differentiation of only about 10% of NSCs. We previously showed that induction of Olig1 expression by gene transfection could indeed initiate the first stage of oligodendrocyte differentiation in NSCs, but appeared to be unable to generate fully mature, functional oligodendrocytes. In this study, we transfected NSCs isolated from the embryonic mouse brain with the Olig2 gene and found that the introduced overexpression of Olig2 could induce the development of fully mature oligodendrocytes expressing the transcription factor Nkx2.2 and all major myelin-specific proteins. Moreover, Olig2-transfected NSCs, in contrast to nontransfected NSCs, developed into actively remyelinating oligodendrocytes after transplantation into the corpus callo-sum of long-term cuprizonefed mice, an animal model for demyelination. Our results show that transfection of genes encoding for oligodendrogenic transcription factors can be an efficient way to induce the differentiation of NSCs into functional oligodendrocytes.

Prospects of cell therapy for disorders of myelin

Recent advances in stem cell biology have raised expectations that both diseases of, and injuries to, the central nervous system may be ameliorated by cell transplantation. In particular, cell therapy has been studied for inducing efficient remyelination in disorders of myelin, including both the largely pediatric disorders of myelin formation and maintenance and the acquired demyelinations of both children and adults. Potential cell-based treatments of two major groups of disorders include both delivery of myelinogenic replacements and mobilization of residual oligodendrocyte progenitor cells as a means of stimulating endogenous repair; the choice of modality is then predicated upon the disease target. In this review we consider the potential application of cell-based therapeutic strategies to disorders of myelin, highlighting the promises as well as the problems and potential perils of this treatment approach.

Stem cell-based strategies for treating pediatric disorders of myelin

The pediatric leukodystrophies comprise a category of disease manifested by neonatal or childhood deficiencies in myelin production or maintenance; these may be due to hereditary defects in one or more genes critical to the initiation of myelination, as in Pelizaeus-Merzbacher Disease, or to enzymatic deficiencies with aberrant substrate accumulation-related dysfunction, as in the lysosomal storage disorders. Despite differences in both phenotype and natural history, these disorders are all essentially manifested by a profound deterioration in neurological function with age. A congenital deficit in forebrain myelination is also noted in children with the periventricular leukomalacia of cerebral palsy, another major source of neurological morbidity. In light of the wide range of disorders to which congenital hypomyelination and/or postnatal demyelination may contribute, and the relative homogeneity of central oligodendrocytes and their progenitors, the pediatric leukodystrophies may be especially attractive targets for cell-based therapeutic strategies. As a result, glial progenitor cells (GPCs), which can give rise to new myelinogenic oligodendrocytes, have become of great interest as potential therapeutic vectors for the restoration of myelin to the hypomyelinated or dysmyelinated childhood CNS. In addition, by distributing themselves throughout the deficient host neuraxis after perinatal allograft, and giving rise to astrocytes as well as oligodendrocytes, glial progenitors appear to be of potential great utility in rectifying enzymatic deficiencies. In this review, we focus on current efforts to develop the use of isolated human GPCs as transplantable agents both for mediating enzymatic restoration to the enzyme-deficient brain and for therapeutic myelination in the disorders of congenital hypomyelination.

Central nervous system-directed effects of FTY720 (fingolimod)

FTY720, also known as fingolimod, is an orally administered sphingosine-1-phosphate (S1P) analogue that is under investigation as a therapy for both relapsing-remitting (RR) and progressive forms of multiple sclerosis (MS). The demonstrated beneficial effect of FTY720 on disease activity in RR-MS patients and in the animal model experimental autoimmune encephalomyelitis (EAE) is largely attributed to effects on the systemic immune system. In addition, unlike other current systemic immuno-modulators used in MS, the lipophilic nature of FTY720 allows it to cross the blood-brain barrier (BBB). Since S1P receptors are expressed on all cell types, FTY720 has the potential to exert effects directly on the BBB and on resident cells of the CNS. The latter include cells implicated in regulating immune reactivity within the CNS (astrocytes, microglia), those that are targeted by the disease process (oligodendrocytes, neurons), and those involved in repair (oligodendrocyte progenitor cells). In vitro studies document the dose-dependent effects of FTY720 on neural cell survival, differentiation, and cytoskeletal dynamics. Animal model studies, specifically EAE, indicate an overall neuroprotective effect of FTY720 mediated at least in part by its actions within the CNS. Ongoing studies will need to define the direct and indirect (via immune-modulation) effects of FTY720 on the CNS across the broad clinical spectrum of MS.

Glial precursor cell transplantation therapy for neurotrauma and multiple sclerosis

Traumatic injury to the brain or spinal cord and multiple sclerosis (MS) share a common pathophysiology with regard to axonal demyelination. Despite advances in central nervous system (CNS) repair in experimental animal models, adequate functional recovery has yet to be achieved in patients in response to any of the current strategies. Functional recovery is dependent, in large part, upon remyelination of spared or regenerating axons. The mammalian CNS maintains an endogenous reservoir of glial precursor cells (GPCs), capable of generating new oligodendrocytes and astrocytes. These GPCs are upregulated following traumatic or demyelinating lesions, followed by their differentiation into oligodendrocytes. However, this innate response does not adequately promote remyelination. As a result, researchers have been focusing their efforts on harvesting, culturing, characterizing, and transplanting GPCs into injured regions of the adult mammalian CNS in a variety of animal models of CNS trauma or demyelinating disease. The technical and logistic considerations for transplanting GPCs are extensive and crucial for optimizing and maintaining cell survival before and after transplantation, promoting myelination, and tracking the fate of transplanted cells. This is especially true in trials of GPC transplantation in combination with other strategies such as neutralization of inhibitors to axonal regeneration or remyelination. Overall, such studies improve our understanding and approach to developing clinically relevant therapies for axonal remyelination following traumatic brain injury (TBI) or spinal cord injury (SCI) and demyelinating diseases such as MS.

Effect of chronic antipsychotic exposure on astrocyte and oligodendrocyte numbers in macaque monkeys

BACKGROUND

Both in vivo and postmortem studies suggest that oligodendrocyte and myelination alterations are present in individuals with schizophrenia. However, it is unclear whether prolonged treatment with antipsychotic medications contributes to these disturbances. We recently reported that chronic exposure of macaque monkeys to haloperidol or olanzapine was associated with a 10%-18% lower glial cell number in the parietal grey matter. Consequently, in this study we sought to determine whether the lower glial cell number was due to fewer oligodendrocytes as opposed to lower numbers of astrocytes.

METHODS

With fluorescent immunocytochemical techniques, we optimized the visualization of each cell type throughout the entire thickness of tissue sections, while minimizing final tissue shrinkage. As a result, we were able to obtain robust stereological estimates of total oligodendrocyte and astrocyte numbers in the parietal grey matter with the optical fractionator method.

RESULTS

We found a significant 20.5% lower astrocyte number with a non-significant 12.9% lower oligodendrocyte number in the antipsychotic-exposed monkeys. Similar effects were seen in both the haloperidol and olanzapine groups.

CONCLUSIONS

These findings suggest that studies investigating glial cell alterations in schizophrenia must take into account the effect of antipsychotic treatment.

Long-term expansion of adult human brain subventricular zone precursors

OBJECTIVE

Many common neurosurgical procedures, including anterior temporal lobectomy and endoscopic ventricular puncture, allow neurosurgeons to retrieve portions of the germinal subventricular zone (SVZ). Isolation and maintenance of precursor cells from this zone can be used for hypothesis-driven experiments with a goal of improving our understanding of the basic mechanisms of central nervous system injury or disease and the potential of cell-based therapies to treat them. This article details our ability to reliably harvest, isolate, characterize, and maintain normal adult human brain SVZ precursor cells.

METHODS

Normal SVZ specimens were retrieved as part of anterior temporal lobe resections during planned epilepsy surgery. Dissociated SVZ specimens were plated and incubated in epidermal growth factor and basic fibroblast growth factor for more than 1 year to select for and expand normal neural precursor cells.

RESULTS

Self-renewal and immunocytochemical experiments proved the feasibility of long-term expansion of a slowly dividing nestin+, vimentin+, and glial fibrillary acidic protein-positive astrocyte capable of generating new neurons and glia. These mitotically active bipotent human precursors generated a large number of progeny and possessed significant self-renewal capacity, demonstrated by their ability to generate neurospheres. Cryopreservation was reliable with no loss of the precursor phenotype.

CONCLUSION

We have adapted techniques to allow for the isolation and long-term propagation of human adult neural precursors that are capable of generating both neurons and astrocytes in vitro. We have exploited the cell's self-renewal capacity to significantly and consistently expand human neural precursor cells for as long as 20 months. These findings suggest that cells derived from the SVZ during routine surgery may provide a renewable source of human neural precursor cells to study the biological mechanism of central nervous system disease or for application in cell-based human transplantation paradigms.

Myelin repair: the role of stem and precursor cells in multiple sclerosis

Multiple sclerosis is the most common potential cause of neurological disability in young adults. The disease has two distinct clinical phases, each reflecting a dominant role for separate pathological processes: inflammation drives activity during the relapsing-remitting stage and axon degeneration represents the principal substrate of progressive disability. Recent advances in disease-modifying treatments target only the inflammatory process. They are ineffective in the progressive stage, leaving the science of disease progression unsolved. Here, the requirement is for strategies that promote remyelination and prevent axonal loss. Pathological and experimental studies suggest that these processes are tightly linked, and that remyelination or myelin repair will both restore structure and protect axons. This review considers the basic and clinical biology of remyelination and the potential contribution of stem and precursor cells to enhance and supplement spontaneous remyelination.

FTY720 modulates human oligodendrocyte progenitor process extension and survival

OBJECTIVE

FTY720, a sphingosine-1-phosphate (S1P) receptor agonist that crosses the blood-brain barrier, is a potential immuno-therapy for multiple sclerosis. Our objective was to assess the effect of FTY720 on process extension, differentiation, and survival of human oligodendrocyte progenitor cells (OPCs), and link the functional effects with S1P receptor expression and signaling.

METHODS

Functional assays and receptor expression studies were conducted on A2B5+ OPCs derived from the human fetal central nervous system. Cells were treated with physiologically relevant concentrations of the active phosphorylated form of FTY720. S1P receptor/signaling modulators were used to elucidate the basis of the FTY720-induced functional responses.

RESULTS

Short-term (1 day) FTY720 treatment caused initial process retraction that was reversed by uncoupling S1P3 and 5 from their G protein using suramin, and with a Rho-kinase inhibitor H1152. Retraction was associated with RhoA-mediated cytoskeletal signaling and with inhibition of OPC differentiation into more mature phenotypes. Continued FTY720 treatment (2 days) induced process extension and enhanced cell survival associated with increased extracellular signal-regulated kinases 1 and 2 phosphorylation, mimicked with the S1P1-specific agonist SEW2871, but not reversed with suramin. Quantitative real-time polymerase chain reaction showed that FTY720 induced reciprocal and cyclic modulation of S1P1 and S1P5 messenger RNA levels. The observed initial downregulation of S1P5 and subsequently of S1P1 messenger RNA supports functional responses being mediated sequentially by S1P5- and later S1P1-associated signaling.

INTERPRETATION

FTY720 induces time-dependent modulation of S1P receptors on human OPCs with consequent functional responses that are directly relevant for the remyelination process.

Bone morphogenetic protein signaling and olig1/2 interact to regulate the differentiation and maturation of adult oligodendrocyte precursor cells

Promotion of remyelination is an important therapeutic strategy for the treatment of the demyelinating neurological disorders. Adult oligodendrocyte precursor cells (OPCs), which normally reside quiescently in the adult central nervous system (CNS), become activated and proliferative after demyelinating lesions. However, the extent of endogenous remyelination is limited because of the failure of adult OPCs to mature into myelinating oligodendrocytes (OLs) in the demyelinated CNS. Understanding the molecular mechanisms that regulate the differentiation of adult OPCs could lead to new therapeutic strategies to treat these disorders. In this study, we established a stable culture of adult spinal cord OPCs and developed a reliable in vitro protocol to induce their sequential differentiation. Adult OPCs expressed bone morphogenetic protein (BMP) type Ia, Ib, and II receptor subunits, which are required for BMP signal transduction. BMP2 and 4 promoted dose-dependent astrocyte differentiation of adult OPCs with concurrent suppression of OL differentiation. Treatment of OPCs with BMP2 and 4 increased ID4 expression and decreased the expression of olig1 and olig2. Overexpression of olig1 or olig2 blocked the astrocyte differentiation of adult OPCs induced by BMP2 and 4. Furthermore, overexpression of both olig1 and olig2, but not olig1 or olig2 alone, rescued OL differentiation from inhibition by BMP2 and 4. Our results demonstrated that downregulation of olig1 and olig2 is an important mechanism by which BMP2 and 4 inhibit OL differentiation of adult OPCs. These data suggest that blocking BMP signaling combined with olig1/2 overexpression could be a useful therapeutic strategy to enhance endogenous remyelination and facilitate functional recovery in CNS demyelinated disorders. Disclosure of potential conflicts of interest is found at the end of this article.

Disease Targets and Strategies for the Therapeutic Modulation of Endogenous Neural Stem and Progenitor Cells

Neural stem cells, able to self-renew and give rise to both neurons and glia, line the cerebral ventricles of the adult human brain. Humans also harbor lineage-restricted neuronal progenitors in the hippocampus and glial progenitor cells in both the gray and white matter of the forebrain. These various stem and progenitor cell types may provide targets for pharmacotherapy for a variety of disorders of the central nervous system. Each resident progenitor phenotype may be mobilized and induced to differentiate in vivo by the actions of both exogenous growth factors and small molecule modulators of progenitor-selective signaling pathways. This strategy may be particularly efficacious in neurodegenerations such as Huntington's disease, in which lost neurons may be replenished through the directed induction of progenitor cells lining the ventricular wall of the affected striatum. Similarly, the mobilization of glial progenitor cells may permit the introduction of new oligodendrocytes to demyelinated regions of adult white matter. Our rapidly increasing understanding of the molecular control of progenitor cell mobilization and differentiation should provide a wealth of new opportunities for recruiting endogenous progenitors as a means of treating neurological disease.

Glial progenitor-based repair of demyelinating neurological diseases

Demyelinating diseases of the brain and spinal cord affect more than one-quarter million of Americans, with numbers reaching more than two million across the world. These patients experience not only the vascular, traumatic, and inflammatory demyelinations of adulthood but the congenital and childhood dysmyelinating syndromes of the pediatric leukodystrophies. Several disease-modifying strategies have been developed that slow disease progression, especially in the inflammatory demyelinations and in multiple sclerosis in particular. Yet, currently available disease modifiers typically influence the immune system and are neither intended to nor competent to reverse the structural neurologic damage attending acquired demyelination. Fortunately, however, the disorders of myelin lend themselves well to attempts at structural repair, because central oligodendrocytes are the primary, and often sole, victims of the underlying disease process. Given the relative availability and homogeneity of human oligodendrocyte progenitor cells, the disorders of myelin formation and maintenance may be especially compelling targets for cell-based neurologic therapy.

Cell replacement therapy in neurological disease

Diseases of the brain and spinal cord represent especially daunting challenges for cell-based strategies of repair, given the multiplicity of cell types within the adult central nervous system, and the precision with which they must interact in both space and time. Nonetheless, a number of diseases are especially appropriate for cell-based therapy, in particular those in which single phenotypes are lost, and in which the re-establishment of vectorially specific connections is not entirely requisite for therapeutic benefit. We review here a set of potential therapeutic indications that meet these criteria as potentially benefiting from the transplantation of neural stem and progenitor cells. These include: (i) transplantation of phenotypically restricted neuronal progenitor cells into diseases of a single neuronal phenotype, such as Parkinson's disease; (ii) implantation of mixed progenitor pools into diseases characterized by the loss of a limited number of discrete phenotypes, such as spinal cord injury and the motor neuronopathies; (iii) transplantation of glial and nominally oligodendrocytic progenitor cells as a means of treating disorders of myelin; and (iv) transplantation of neural stem cells as a means of treating lysosomal storage disorders and other diseases of enzymatic deficiency. Among the diseases potentially approachable by these strategies, the myelin disorders, including the paediatric leucodystrophies as well as adult traumatic and inflammatory demyelinations, may present the most compelling targets for cell-based neurological therapy.

Neural subtype specification from embryonic stem cells

One of the keys to using embryonic stem cells (ESCs) in brain research and potential application in neurological diseases is directed differentiation of neuronal and glial subtypes. This may be achieved by application of developmental principles in guiding cell lineage specification from naïve stem cells. Establishment of defined ESC differentiation models that recapitulate in vivo development, especially from human ESCs, will most likely provide a dynamic tool for dissecting molecular mechanisms underlying early embryonic development that is otherwise not readily obtainable. This is also a rational and realistic way of producing enriched populations of functional neurons and glia for pathological analyses as well as possible therapeutic applications.

Immunomagnetic separation of adult human olfactory neural progenitors

Olfactory neuroepithelium (ONe) has lifelong regenerative capacity owing to the presence of mitotically active progenitors. The accessibility of ONe makes it a unique source of progenitors for cell replacement strategies in the CNS. We have established lines of neurosphere forming cells (NSFCs) from adult postmortem ONe and patients undergoing nasal sinus surgery by endoscopic biopsy. These heterogeneous lines are composed primarily of an immature neuronally restricted and a small glial restricted subpopulation. More homogeneous subpopulations of the NSFCs are essential for detailed study of factors influencing their lineage restriction. Immunomagnetic bead separation using an antibody against tyrosine kinase (Trk) receptors (Trk-pan, which recognizes Trk-A, B, C) resulted in viable, enriched positive and negative subpopulations that could be analyzed immunocytochemically. The positive cells remained positive for the first week after which the number of Trk-pan expressing cells decreased. The negative subpopulation began to express Trk-pan immunoreactivity after five days in vitro. Both subpopulations reverted to the heterogeneous composition after two weeks. Furthermore, most NSFCs were positive for Trk-B, a few for Trk-A, while no reactivity was observed for Trk-C. Because NSFCs produce brain derived neurotrophic factor (BDNF) and express Trk B, the specific receptor for BDNF, it is likely that population dynamics are under a paracrine and/or autocrine regulatory mechanism. Lineage restriction analysis demonstrated that the isolated subpopulation had a restriction potential equivalent to the original heterogeneous population. These studies characterize further the NSFCs and support the future potential therapeutic use of ONe-derived progenitors for CNS injury and neurodegenerative disorders.

Complementary patterns of gene expression by human oligodendrocyte progenitors and their environment predict determinants of progenitor maintenance and differentiation

OBJECTIVE

Glial progenitor cells are abundant in adult human white matter. This study was designed to identify signaling pathways regulating their self-renewal and fate.

METHODS

We compared the transcriptional profiles of freshly sorted adult human white matter progenitor cells (WMPCs), purified by A2B5-based immunomagnetic sorting, with those of the white matter from which they derived.

RESULTS

We identified 132 genes differentially expressed by WMPCs; these included principal components of five receptor-defined signaling pathways, represented by platelet derived growth factor receptor alpha (PDGFRA) and type 3 fibroblast growth factor receptor (FGFR3), receptor tyrosine phosphatase-beta/zeta (RTPZ), notch, and syndecan3. WMPCs also differentially expressed the bone morphogenetic protein 4 (BMP4) inhibitors neuralin and BAMBI (BMP and activin membrane-bound inhibitor), suggesting tonic defense against BMP signaling. Differential overexpression of RTPZ was accompanied by that of its modulators pleiotrophin, NrCAM, tenascin, and the chondroitin sulfate proteoglycans, suggesting the importance of RTPZ signaling to WMPCs. When exposed to the RTPZ inhibitor bpV(phen), or lentiviral-shRNAi against RTPZ, WMPCs differentiated as oligodendrocytes. Conversely, when neuralin and BAMBI were antagonized by BMP4, astrocytic differentiation was induced, which was reversible by noggin.

INTERPRETATION

The RTPZ and BMP pathways regulate the self-maintenance of adult human WMPCs, and can be modulated to induce their oligodendrocytic or astrocytic differentiation. As such, they provide targets by which to productively mobilize resident progenitor cells of the adult human brain.

Migrating and myelinating potential of subventricular zone neural progenitor cells in white matter tracts of the adult rodent brain

Adult neural stem cells in the subventricular zone (SVZ) produce neuronal progenitors that migrate along the rostral migratory stream (RMS) and generate olfactory interneurons. Here, we evaluate the migratory potential of SVZ cells outside the RMS and their capacity to generate oligodendrocytes in the adult brain. We show that SVZ cells migrate long distances when grafted into white matter tracts such as the cingulum (Ci) and corpus callosum (CC). Furthermore, 22 days postinjection, most present morphologic and phenotypic characteristics of cells committed to the oligodendrocyte lineage. Cells grafted in shiverer CC and Ci become MBP-positive oligodendrocytes, abundantly myelinating these white matter tracts. Type A progenitors are involved in this myelinating process. Altogether, this study reveals the migrating and myelinating potential of SVZ cells in a new environmental context. Therefore, SVZ cells stand as interesting candidates for the development of novel therapeutic strategies for demyelinating diseases.