Fetal and adult human oligodendrocyte progenitor cell isolates myelinate the congenitally dysmyelinated brain

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.

[Stem cells use in the treatment of neurologic disorders--has the future already arrived?]

<zakljucak> Buducnost celijske terapije u lecenju neuroloskih bolesti je pocela. Ostaju vazna pitanja na koja odgovor mogu dati samo dobro osmisljene medjunarodne studije, a ticu se vrste celija, nacina njihovog umnozavanja i aplikacije, kao i optimalnog vremenskog okvira za ovakvu vrstu lecenja.

Myelin repair strategies: a cellular view

PURPOSE OF REVIEW

The development of successful myelin repair strategies depends on the detailed knowledge of the cellular and molecular processes underlying demyelination and remyelination in the central nervous system of animal models and in patients with multiple sclerosis (MS). Based on the complexity of the demyelination and remyelination processes, it should be expected that effective therapeutic approaches will require a combination of strategies for immunomodulation, neuroprotection, and myelin replacement. This brief review highlights recent cellular and molecular findings and indicates that future therapeutic strategies to enhance remyelination may also require combinatorial treatment to accomplish.

RECENT FINDINGS

The relapsing-remitting course of some forms of multiple sclerosis has typically fueled hope for effective repair of multiple sclerosis lesions, if demyelinating activity could be attenuated. Recent findings support the potential of endogenous neural stem cells and progenitor cells to generate remyelinating oligodendrocytes. Importantly, interactions with viable axons and supportive astrocytic responses are required for endogenous immature cells to fulfill their potential remyelinating capacity.

SUMMARY

The research described here will help in identifying the major obstacles to effective remyelination and potential therapeutic targets to guide development of comprehensive approaches for testing in animal models and eventual treatment of patients with multiple sclerosis.

Neural stem cell transplantation in mouse brain

Neural stem cells (NSCs) are the most primordial, least committed cells of the nervous system, and transplantation of these multipotent cells holds the promise of regenerative therapy for many central nervous system (CNS) diseases. This unit describes methods for NSC transplantation into neonatal mouse pups, embryonic mouse brain, and adult mouse brain. A description of options for detection of labeled donor cells in engrafted mouse brain is provided along with an example protocol for detecting lacZ-expressing cells in situ. Also included is a protocol for preparing NSCs for transplantation.

From fish to man: understanding endogenous remyelination in central nervous system demyelinating diseases

In the central nervous system (CNS) of man, evolutionary pressure has preserved some capability for remyelination while axonal regeneration is very limited. In contrast, two efficient programmes of regeneration exist in the adult fish CNS, neurite regrowth and remyelination. The rapidity of CNS remyelination is critical since it not only restores fast conduction of nerve impulses but also maintains axon integrity. If myelin repair fails, axons degenerate, leading to increased disability. In the human CNS demyelinating disease multiple sclerosis (MS), remyelination often takes place in the midst of inflammation. Here, we discuss recent studies that address the innate repair capabilities of the axon-glia unit from fish to man. We propose that expansion of this research field will help find ways to maintain or enhance spontaneous remyelination in man.

Lineage selection of functional and cryopreservable human embryonic stem cell-derived neurons

A major prerequisite for the biomedical application of human embryonic stem cells (hESC) is the derivation of defined and homogeneous somatic cell types. Here we present a human doublecortin (DCX) promoter-based lineage-selection strategy for the generation of purified hESC-derived immature neurons. After transfection of hESC-derived neural precursors with a DCX-enhanced green fluorescent protein construct, fluorescence-activated cell sorting enables the enrichment of immature human neurons at purities of up to 95%. Selected neurons undergo functional maturation and are able to establish synaptic connections. Considering that the applicability of purified hESC-derived neurons would largely benefit from an efficient cryopreservation technique, we set out to devise defined freezing conditions involving caspase inhibition, which yield post-thaw recovery rates of up to 83%. Combined with our lineage-selection procedure this cryopreservation technique enables the generation of human neurons in a ready-to-use format for a large variety of biomedical applications.

Use of human neural tissue for the generation of progenitors

Accumulating evidence suggests that a better understanding of normal human brain stem cells and tumor stem cells (TSCs) will have profound implications for treating central nervous system disease during the next decade. Neurosurgeons routinely resect excess surgical tissue containing either normal brain stem cells or TSCs. These cells are immediately available for expansion and use in basic biological assays, animal implantation, and comparative analysis studies. Although normal stem cells have much slower kinetics of expansion than TSCs, they are easily expandable and can be frozen for future use in stem cell banks. This nearly limitless resource holds promise for understanding the basic biology of normal brain stem cells and TSCs, which will likely direct the next major shift in therapeutics for brain tumors, brain and spinal cord injury, and neurodegenerative disease. This report reviews the progress that has been made in harvesting and expanding both normal and tumor-derived stem cells and emphasizes the integral role neurosurgeons will play in moving the neural stem cell field forward.

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.

Neurodegeneration and cell replacement

The past decade has witnessed ground-breaking advances in human stem cell biology with scientists validating adult neurogenesis and establishing methods to isolate and propagate stem cell populations suitable for transplantation. These advances have forged promising strategies against human neurodegenerative diseases. For example, growth factor administration could stimulate intrinsic repair from endogenous neural stem cells, and cultured stem cells engineered into biopumps could be transplanted to deliver neuroprotective or restorative agents. Stem cells could also be transplanted to generate new neural elements that augment and potentially replace degenerating central nervous system (CNS) circuitry. Early efforts in neural tissue transplantation have shown that these strategies can improve functional outcome, but the ultimate success of clinical stem cell-based strategies will depend on detailed understanding of stem cell biology in the degenerating brain and detailed evaluation of their functional efficacy and safety in preclinical animal models.

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.

Human embryonic stem cells for brain repair?

Cell therapy has been perceived as the main or ultimate goal of human embryonic stem (ES) cell research. Where are we now and how are we going to get there? There has been rapid success in devising in vitro protocols for differentiating human ES cells to neuroepithelial cells. Progress has also been made to guide these neural precursors further to more specialized neural cells such as spinal motor neurons and dopamine-producing neurons. However, some of the in vitro produced neuronal types such as dopamine neurons do not possess all the phenotypes of their in vivo counterparts, which may contribute to the limited success of these cells in repairing injured or diseased brain and spinal cord in animal models. Hence, efficient generation of neural subtypes with correct phenotypes remains a challenge, although major hurdles still lie ahead in applying the human ES cell-derived neural cells clinically. We propose that careful studies on neural differentiation from human ES cells may provide more immediate answers to clinically relevant problems, such as drug discovery, mechanisms of disease and stimulation of endogenous stem cells.

Stem cells: an overview

Stem cells are specialized cells that possess a capacity to undergo self-renewal while at the same time having the ability to give rise to at least one or more differentiated or mature cell type. They therefore represent a fundamental cornerstone during the life of all vertebrates, playing central roles in the production of new and replacement cells for tissues during development and homeostasis, including repair following disease or injury. This unit is a review of stem cells, their roles in development, and their potentials as therapeutic agents.

Central nervous system therapy for lysosomal storage disorders

✓ Most lysosomal storage disorders are characterized by progressive central nervous system impairment, with or without systemic involvement. Affected individuals have an array of symptoms related to brain dysfunction, the most devastating of which is neurodegeneration following a period of normal development. The blood–brain barrier has represented a significant impediment to developing therapeutic approaches to treat brain disease, but novel approaches—including enzyme replacement, small-molecule, gene, and cell-based therapies—have given children afflicted by these conditions and those who care for them hope for the future.

A potential role for bone morphogenetic protein signalling in glial cell fate determination following adult central nervous system injury in vivo

Bone morphogenetic proteins (BMPs) and their endogenous inhibitors, including noggin, chordin and follistatin, have roles in pattern formation and fate specification of neuronal and glial cells during nervous system development. We have examined their influence on glial reactions in the injured central nervous system (CNS). We show that penetrating injuries to the brain and spinal cord resulted in the upregulation of BMP-2/4, BMP-7, and noggin, with the latter being expressed almost exclusively by reactive astrocytes at the injury site, and we show that astrocytes in vitro produce noggin. As BMPs have been shown to drive cultured NG2-positive oligodendrocyte precursors (OPCs) towards a multipotential phenotype (type II astrocytes), we investigated the effects of inhibiting noggin with a function-blocking antibody (noggin-FbAb). In vitro, BMP-driven conversion of OPCs to type 2 astrocytes was inhibited by noggin, an effect that was reversed by noggin-FbAb. Noggin-FbAb also increased the number of type 2 astrocytes generated from cultured OPCs exposed to an astrocyte feeder layer, consistent with astrocytes producing both BMPs and noggin. In knife cut injuries in vivo, noggin-FbAb treatment resulted in an increase in the number of NG2-positive cells and small GFAP-positive cells in the injury site, and the appearance of glial cells with the morphological and antigenic characteristics of type 2 astrocytes (as generated in vitro), with coexpression of both GFAP and NG2. This potential conversion of inhibitory OPCs to type 2 astrocyte-like cells in vivo suggests that endogenous BMPs, unmasked by noggin antagonism, might be exploited to manipulate cell fate following CNS trauma.

Retrovirally mediated telomerase immortalization of neural progenitor cells

Traditional methods of generating immortalized lines of both somatic cells and their progenitors have relied on the use of oncogenes. However, the resulting lines are typically anaplastic in vitro and tumorigenic in vivo, and hence of limited utility. The overexpression of telomerase, as mediated by the induced overexpression of human telomerase reverse transcriptase (hTERT), has permitted the generation of stable, non-oncogenic lines of a variety of cell types. This strategy for immortalization has found special utility in the central nervous system, as few stable lines are available for the study of either human neural progenitor cells, or of neurons or glia of restricted phenotype. We describe the use of retroviral hTERT overexpression for generating lines of immortalized human neural progenitor cells, whose neuronal progeny are phenotypically restricted, post-mitotic and functionally competent. Although we focus here on telomerase immortalization of spinal neural progenitors, this is a broadly applicable protocol for using hTERT to immortalize human fetal neural progenitors of any pre-selected phenotype and for characterizing the cell lines thereby generated.

Replacing cells in multiple sclerosis

Cell transplantation is emerging as a major potential therapeutic approach in the treatment of otherwise untreatable neurodegenerative diseases. In multiple sclerosis (MS), a major direction of current research is to devise strategies that will remyelinate axons and protect them against subsequent ongoing degeneration. Ongoing loss of axons will lead to chronic disability. Oligodendrocytes and their progenitors are lost during multiple relapses in the course of MS and either needs to be replaced from an exogenous source or the remaining progenitors stimulated to differentiate and remyelinate. The successful isolation and purification of human oligodendrocytes from neural or embryonic stem cells offer hope that a source of sufficient cells for translational application might be achievable in the future. Focal repair of strategic lesions followed by more disseminated delivery of exogenous cells will be the short and long-term goals.

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.

Neural stem cells and the future treatment of neurological diseases: raising the standard

Neural stem and progenitor cells offer great potential for treatment of neurological disorders.AQ[16]Both neurologic and neurological were used in text; the latter was more common. Note changes have been made to match in title and text body. The current strategies of isolation, expansion, and characterization of these cells require in vitro manipulations that can change their intrinsic properties, specifically with the acquisition of chromosomal abnormalities. We have analyzed the rationale of using neural stem cells in neurological disorders, the caveats of current isolation and in vitro culture protocols of neural precursors. Addressing these challenges is crucial for translation of neural stem cell therapy to the clinic.

Cellular approaches for stimulating CNS remyelination

Myelination is critical for the normal functioning of the vertebrate nervous system. In the CNS, myelin is produced by oligodendrocytes, and the loss of oligodendrocytes and myelin results in severe functional impairment. Although spontaneous remyelination occurs in chronic demyelinating diseases such as multiple sclerosis, the repair process eventually fails, often resulting in long-term disability. Two distinct general approaches can be considered to promote myelin repair. In one the target is stimulation of the endogenous myelin repair process through delivery of growth factors, and in the second the target is augmentation of the repair process through the delivery of exogenous cells with myelination potential. In both cases, effective treatment of diseases such as multiple sclerosis requires modulation of the immune system, since demyelination is associated with specific immunological activation. Recent studies have shown that some populations of stem cells, including mesenchymal stem cells, have the capacity of promoting endogenous myelin repair and modulating the immune response, prompting an assessment of their use as therapy in demyelinating diseases such as MS. Other types of demyelinating disorders, such as the leukodystrophies, may require multiple repair strategies including both replacement of dysfunctional cells and delivery or supplementation of growth factors, immune modulators or metabolic enzymes. Here we discuss the use of stem cells for the treatment of demyelinating diseases. While the current number of stem cell-based clinical trials for demyelinating diseases is limited, this is likely to increase significantly in the next few years, and a clear understanding of the applicability, limitations and underlying mechanisms mediating stem cell repair is critical.

Prospects for neural stem cell-based therapies for neurological diseases

Neural stem and progenitor cells have great potential for the treatment of neurological disorders. However, many obstacles remain to translate this field to the patient's bedside, including rationales for using neural stem cells in individual neurological disorders; the challenges of neural stem cell biology; and the caveats of current strategies of isolation and culturing neural precursors. Addressing these challenges is critical for the translation of neural stem cell biology to the clinic. Recent work using neural stem cells has yielded novel biologic concepts such as the importance of the reciprocal interaction between neural stem cells and the neurodegenerative environment. The prospect of using transplants of neural stem cells and progenitors to treat neurological diseases requires a better understanding of the molecular mechanisms of both neural stem cell behavior in experimental models and the intrinsic repair capacity of the injured brain.

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.

Oxygen tension controls the expansion of human CNS precursors and the generation of astrocytes and oligodendrocytes

Human neural precursor proliferation and potency is limited by senescence and loss of oligodendrocyte potential. We found that in vitro expansion of human postnatal brain CD133(+) nestin(+) precursors is enhanced at 5% oxygen, while raising oxygen tension to 20% depletes precursors and promotes astrocyte differentiation even in the presence of mitogens. Higher cell densities yielded more astrocytes regardless of oxygen tension. This was reversed by noggin at 5%, but not 20%, oxygen due to a novel repressive effect of low oxygen on bone morphogenetic protein (BMP) signaling. When induced to differentiate by mitogen withdrawal, 5% oxygen-expanded precursors generated 17-fold more oligodendrocytes than cells expanded in 20% oxygen. When precursors were expanded at 5% oxygen and then differentiated at 20% oxygen, oligodendrocyte maturation was further enhanced 2.5-fold. These results indicate that dynamic control of oxygen tension regulates different steps in fate and maturation and may be crucial for treating neurodegenerative diseases.

[Promoting myelin repair in disorders such as multiple sclerosis and some types of leukodystrophy: current studies]

Several ways of promoting myelin repair in myelin disorders such as multiple sclerosis and certain types of leukodystrophies are currently being investigated. Numerous studies suggest that it is possible to repair the central nervous system (CNS) by cell transplantation or by enhancing endogenous remyelination. Investigations in animal models indicate that cell therapy results in robust anatomical and functional recovery of acute myelin lesions. These models are also used to explore and validate the role of candidate molecules to stimulate endogenous remyelination by activating the myelin competent population or providing neuroprotection. However, in view of the heterogeneity of the lesion environment in MS, it seems more likely that cell therapy alone will not be able to contribute efficiently to the repair of the lesion. Further developments should indicate whether combining multiple approaches will be more powerful to achieve global myelin repair in the CNS than applying these strategies alone.

Oxygen tension regulates survival and fate of mouse central nervous system precursors at multiple levels

Despite evidence that oxygen regulates neural precursor fate, the effects of changing oxygen tensions on distinct stages in precursor differentiation are poorly understood. We found that 5% oxygen permitted clonal and long-term expansion of mouse fetal cortical precursors. In contrast, 20% oxygen caused a rapid decrease in hypoxia-inducible factor 1alpha and nucleophosmin, followed by the induction of p53 and apoptosis of cells. This led to a decrease in overall cell number and particularly a loss of astrocytes and oligodendrocytes. Clonal analysis revealed that apoptosis in 20% oxygen was due to a complete loss of CD133(lo)CD24(lo) multipotent precursors, a substantial loss of CD133(hi)CD24(lo) multipotent precursors, and a failure of remaining CD133(hi)CD24(lo) cells to generate glia. In contrast, committed neuronal progenitors were not significantly affected. Switching clones from 5% to 20% oxygen only after mitogen withdrawal led to a decrease in total clone numbers but an even greater decrease in oligodendrocyte-containing clones. During this late exposure to 20% oxygen, bipotent glial (A2B5+) and early (platelet-derived growth factor receptor alpha) oligodendrocyte progenitors appeared and disappeared more quickly, relative to 5% oxygen, and late stage O4+ oligodendrocyte progenitors never appeared. These results indicate that multipotent cells and oligodendrocyte progenitors are more susceptible to apoptosis at 20% oxygen than committed neuronal progenitors. This has important implications for optimizing ex vivo production methods for cell replacement therapies.

Revisiting the astrocyte–oligodendrocyte relationship in the adult CNS

The lineages of both astrocytes and oligodendrocytes have been popular areas of research in the last decade. The source of these cells in the mature CNS is relevant to the study of the cellular response to CNS injury. A significant amount of evidence exists to suggest that resident precursor cells proliferate and differentiate into mature glial cells that facilitate tissue repair and recovery. Additionally, the re-entry of mature astrocytes into the cell cycle can also contribute to the pool of new astrocytes that are observed following CNS injury. In order to better understand the glial response to injury in the adult CNS we must revisit the astrocyte-oligodendrocyte relationship. Specifically, we argue that there is a common glial precursor cell from which astrocytes and oligodendrocytes differentiate and that the microenvironment surrounding the injury determines the fate of the stimulated precursor cell. Ideally, better understanding the origin of new glial cells in the injured CNS will facilitate the development of therapeutics targeted to alter the glial response in a beneficial way.

Workgroup report: incorporating in vitro alternative methods for developmental neurotoxicity into international hazard and risk assessment strategies

This is the report of the first workshop on Incorporating In Vitro Alternative Methods for Developmental Neurotoxicity (DNT) Testing into International Hazard and Risk Assessment Strategies, held in Ispra, Italy, on 19-21 April 2005. The workshop was hosted by the European Centre for the Validation of Alternative Methods (ECVAM) and jointly organized by ECVAM, the European Chemical Industry Council, and the Johns Hopkins University Center for Alternatives to Animal Testing. The primary aim of the workshop was to identify and catalog potential methods that could be used to assess how data from in vitro alternative methods could help to predict and identify DNT hazards. Working groups focused on two different aspects: a) details on the science available in the field of DNT, including discussions on the models available to capture the critical DNT mechanisms and processes, and b) policy and strategy aspects to assess the integration of alternative methods in a regulatory framework. This report summarizes these discussions and details the recommendations and priorities for future work.

Whole Genome Analysis of Human Neural Stem Cells Derived from Embryonic Stem Cells and Stem and Progenitor Cells Isolated from Fetal Tissue

Multipotent neural stem cells (NSC) have been derived from human embryonic stem cells (hESC) as well as isolated from fetal tissues. However, there have been few exclusive markers of NSC identified to date, and the differences between NSC from various sources are poorly understood. Although cells isolated from these two sources share many important characteristics, it is not clear how closely they are related in terms of gene expression. Here, we compare the gene expression profiles of 11 lines of NSC derived from hESC (ES_NSC), four lines of NSC isolated from fetus (F_NSC), and two lines of restricted progenitors in order to characterize these cell populations and identify differences between NSC derived from these two sources. We showed that ES_NSC were clustered together with high transcriptional similarities but were distinguished from F_NSC, oligodendrocyte precursor cells, and astrocyte precursor cells. There were 17 genes expressed in both ES_NSC and F_NSC whose expression was not identified in restricted neural progenitors. Furthermore, the major differences between ES_NSC and F_NSC were mostly observed in genes related to the key neural differentiation pathways. Here, we show that comparison of global gene expression profiles of ES_NSC, F_NSC, and restricted neural progenitor cells makes it possible to identify some of the common characteristics of NSC and differences between similar stem cell populations derived from hESCs or isolated from fetal tissue. Disclosure of potential conflicts of interest is found at the end of this article.

Myelination of congenitally dysmyelinated spinal cord axons by adult neural precursor cells results in formation of nodes of Ranvier and improved axonal conduction

Emerging evidence suggests that cell-based remyelination strategies may be a feasible therapeutic approach for CNS diseases characterized by myelin deficiency as a result of trauma, congenital anomalies, or diseases. Although experimental demyelination models targeted at the transient elimination of oligodendrocytes have suggested that transplantation-based remyelination can partially restore axonal molecular structure and function, it is not clear whether such therapeutic approaches can be used to achieve functional remyelination in models associated with long-term, irreversible myelin deficiency. In this study, we transplanted adult neural precursor cells (aNPCs) from the brain of adult transgenic mice into the spinal cords of adult Shiverer (shi/shi) mice, which lack compact CNS myelin. Six weeks after transplantation, the transplanted aNPCs expressed oligodendrocyte markers, including MBP, migrated extensively along the white matter tracts of the spinal cord, and formed compact myelin. Conventional and three-dimensional confocal and electron microscopy revealed axonal ensheathment, establishment of paranodal junctional complexes leading to de novo formation of nodes of Ranvier, and partial reconstruction of the juxtaparanodal and paranodal molecular regions of axons based on Kv1.2 and Caspr (contactin-associated protein) expression by the transplanted aNPCs. Electrophysiological recordings revealed improved axonal conduction along the transplanted segments of spinal cords. We conclude that myelination of congenitally dysmyelinated adult CNS axons by grafted aNPCs results in the formation of compact myelin, reconstruction of nodes of Ranvier, and enhanced axonal conduction. These data suggest the therapeutic potential of aNPCs to promote functionally significant myelination in CNS disorders characterized by longstanding myelin deficiency.

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.

Activation of the subventricular zone in multiple sclerosis: evidence for early glial progenitors

In multiple sclerosis (MS), oligodendrocyte and myelin destruction lead to demyelination with subsequent axonal loss. Experimental demyelination in rodents has highlighted the activation of the subventricular zone (SVZ) and the involvement of progenitor cells expressing the polysialylated form of neural cell adhesion molecule (PSA-NCAM) in the repair process. In this article, we studied the distribution of early PSA-NCAM(+) progenitors in the SVZ and MS lesions in human postmortem brains. Compared with controls, MS SVZ showed a 2- to 3-fold increase in cell density and proliferation, which correlated with enhanced numbers of PSA-NCAM(+) and glial fibrillary acidic protein-positive (GFAP(+)) cells. PSA-NCAM(+) progenitors mainly were Sox9(+), and a few expressed Sox10 and Olig2, markers of oligodendroglial specification. PSA-NCAM(+) progenitors expressing Sox10 and Olig2 also were detected in demyelinated MS lesions. In active and chronic active lesions, the number of PSA-NCAM(+) progenitors was 8-fold higher compared with chronic silent lesions, shadow plaques, and normal-appearing white matter. In active and chronic active lesions, PSA-NCAM(+) progenitors were more frequent in periventricular lesions (30-50%) than in lesions remote from the ventricular wall. These data indicate that, as in rodents, activation of gliogenesis in the SVZ occurs in MS and suggest the mobilization of SVZ-derived early glial progenitors to periventricular lesions, where they could give rise to oligodendrocyte precursors. These early glial progenitors could be a potential target for therapeutic strategies designed to promote myelin repair in MS.

Strategies for achieving and monitoring myelin repair

A number of factors more or less unique to multiple sclerosis have suggested that this disease may be particularly amenable to cell-based reparative therapies. The relatively focussed damage to oligodendrocytes and myelin at least in early disease implies that only a single population of cells need be replaced-and that the daunting problem of re-establishing connectivity does not apply. The presence of significant though partial spontaneous myelin repair in multiple sclerosis proves there to be no insurmountable barrier to remyelination intrinsic to the CNS: the therapeutic challenge becomes that of supplementing this spontaneous process, rather than creating repair de novo. Finally, the large body of available knowledge concerning the biology of oligodendrocytes, and the success of experimental myelin repair, have allowed cautious optimism that future prospects for such therapies are not unrealistic. Nonetheless, particular and significant problems are not hard to list: the occurrence of innumerable lesions scattered throughout the CNS, axon loss, astrocytosis, and a continuing inflammatory process, to name but a few. Here we review the progress and the areas where difficulties have yet to be resolved in efforts to develop remyelinating therapies for multiple sclerosis.

Demyelinating diseases and potential repair strategies

Demyelination is associated with a number of neurological disorders including multiple sclerosis (MS), spinal cord injury and nerve compression. MS lesions often show axon loss and therefore reparative therapeutic goals include remyelination and neuroprotection of vulnerable axons. Experimental cellular transplantation has proven successful in a number of demyelination and injury models to remyelinate and improve functional outcome. Here we discuss the remyelination and neuroprotective potential of several myelin-forming cells types and their behavior in different demyelination and injury models. Better understanding of these models and current cell-based strategies for remyelination and neuroprotection offer exciting opportunities to develop strategies for clinical studies.

Human embryonic stem cell-derived neural precursors develop into neurons and integrate into the host brain

Whether and how in-vitro-produced human neural precursors mature and integrate into the brain are crucial to the utility of human embryonic stem (hES) cells in treating neurological disorders. After transplantation into the ventricles of neonatal immune-deficient mice, hES-cell-derived neural precursors stopped expressing the cell division marker Ki67, except in neurogenic areas, and differentiated into neurons and then glia in a temporal course intrinsic to that of human cells regardless of location. The human cells located in the gray matter became neurons in the olfactory bulb and striatum, whereas those in the white matter produced exclusively glia. Importantly, the grafted human cells formed synapses. Thus, the in-vitro-produced human neural precursors follow their intrinsic temporal program to produce neurons and glia and, in response to environmental signals, generate cells appropriate to their target regions and integrate into the brain.

Evolving Therapies For Multiple Sclerosis

The introduction of immunomodulatory and immunosuppressive agents for treatment of multiple sclerosis (MS) has forever altered the natural course of this incurable and disabling neurodegenerative disorder. Despite early diagnosis of relapsing-remitting MS and early initiation of therapy, patients still experience breakthrough relapses and progression of their underlying MS pathology. The imperfect effectiveness, side effects, and toxicity of these agents, emphasize the necessity for development of more effective medications with less adverse events. This chapter presents readers with the most current information on the nature, mechanism(s) of action, and side effects of the most promising experimental agents currently under clinical trials. Some of the agents now at different stages of clinical trial have emerged as both safe and promising. The understanding of MS etiology will lead to the development of increasingly specific, safer, and effective treatments for MS by neuroscientists and neurologists.

Human oligodendrocytes derived from embryonic stem cells: Effect of noggin on phenotypic differentiation in vitro and on myelination in vivo

In attempts to produce mature oligodendrocytes from human embryonic stem (huES) cells, we searched conditions inducing transcription factors Olig1/2, as well as Nkx2.2 and Sox10, which are needed for maturation. This was obtained by retinoic acid treatment followed by noggin, an antagonist of bone morphogenetic proteins (BMPs). We found that retinoic acid induces BMPs in huES cells. Addition of noggin at a specific step was essential to form numerous mature oligodendrocytes with ramified branches and producing myelin basic protein (MBP). We describe a procedure converting huES cells into enriched populations of oligodendrocyte precursors that can be expanded and passaged repeatedly and subsequently differentiated into mature cells. Transplantation of such precursors showed that pretreatment by noggin markedly stimulates their capacity to myelinate in the brain of MBP-deficient shiverer mice in organotypic cultures and in living animals. Arrays of numerous long MBP+ fibers were generated over extended areas in the brain, with evidence of cell migration after transplantation and with formation of compact myelin sheaths.

The neurobiology of multiple sclerosis: genes, inflammation, and neurodegeneration

The autoimmune model of multiple sclerosis (MS) pathogenesis provided for many years a useful but incomplete conceptual framework for understanding the complex array of factors that lead to the loss of immune homeostasis, myelin and axonal injury, and progressive neurological symptoms. The availability of novel tools in molecular neurogenetics and increasingly sophisticated neuroimaging technologies, together with the revitalization of MS neuropathology, has created a new paradigm for the multidisciplinary study of this disease. This is reflected by the growing resolution of the MS genomic map, discovery of delicate inflammatory networks that are perturbed in MS, identification of mediators of demyelination, and recognition that cumulative axonal loss and neuronal injury are the histological correlates of neurological disability. Together, these advances have set the stage for the development of therapeutic approaches designed to target the demyelinating and neurodegenerative components of the disease and promote repair.

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.

Stem cells for the treatment of neurological disorders

Many common neurological disorders, such as Parkinson's disease, stroke and multiple sclerosis, are caused by a loss of neurons and glial cells. In recent years, neurons and glia have been generated successfully from stem cells in culture, fueling efforts to develop stem-cell-based transplantation therapies for human patients. More recently, efforts have been extended to stimulating the formation and preventing the death of neurons and glial cells produced by endogenous stem cells within the adult central nervous system. The next step is to translate these exciting advances from the laboratory into clinically useful therapies.

Skin-derived precursors generate myelinating Schwann cells for the injured and dysmyelinated nervous system

Although neural stem cells hold considerable promise for treatment of the injured or degenerating nervous system, their current human sources are embryonic stem cells and fetally derived neural tissue. Here, we asked whether rodent and human skin-derived precursors (SKPs), neural crest-related precursors found in neonatal dermis, represent a source of functional, myelinating Schwann cells. Specifically, cultured SKPs responded to neural crest cues such as neuregulins to generate Schwann cells, and these Schwann cells proliferated and induced myelin proteins when in contact with sensory neuron axons in culture. Similar results were obtained in vivo; 6 weeks after transplantation of naive SKPs or SKP-derived Schwann cells into the injured peripheral nerve of wild-type or shiverer mutant mice (which are genetically deficient in myelin basic protein), the majority of SKP-derived cells had associated with and myelinated axons. Naive rodent or human SKPs also generated Schwann cells that myelinated CNS axons when transplanted into the dysmyelinated brain of neonatal shiverer mice. Thus, neonatal SKPs generate functional neural progeny in response to appropriate neural crest cues and, in so doing, provide a highly accessible source of myelinating cells for treatment of nervous system injury, congenital leukodystrophies, and dysmyelinating disorders.

Future of multiple sclerosis therapy: combining antigen-specific immunotherapy with strategies to promote myelin repair

Persistent CNS inflammation and the failure of myelin repair during multiple sclerosis (MS) trigger a progressive deterioration in neurophysiological function and permanent clinical debilitation. Current treatment consists of immunosuppressive therapies targeted against the immune response, which have only been moderately successful in ameliorating disease relapses and have little or no benefit in slowing disease progression or enhancing remyelination. Recent breakthroughs have revealed new targets and more selective techniques for inhibiting autoreactive T-cell responses and promoting lesion repair in animal models of MS. In light of these new findings and the limitations of current treatments, the authors hypothesize that the future of MS therapy will progress towards the development of a combinatorial therapeutic strategy that consists of specific tolerance of autoreactive T cells, myelin repair and axonal protection.

Co-expression of PDGF alpha receptor and NG2 by oligodendrocyte precursors in human CNS and multiple sclerosis lesions

Following inflammatory demyelination in multiple sclerosis (MS), partial remyelination occurs. Studies in rodents have indicated that oligodendrocyte precursor cells (OPCs) are responsible for this remyelination. Rodent OPCs are identified in situ with antibodies against platelet-derived growth factor alpha receptor (PDGFalphaR) and NG2 chondroitin sulfate proteoglycan. In human CNS tissue, studies of NG2 and PDGFalphaR expression are limited and controversy exists as to whether these molecules are specific OPC markers. This study has investigated whether PDGFalphaR and NG2 are co-expressed on OPCs in human CNS, and whether OPCs are associated with remyelination in MS. MS brain tissue was examined for PDGFalphaR and NG2 immunoreactivity and for expression of NG2 mRNA by in situ hybridisation. Putative OPCs, expressing both NG2 and PDGFalphaR, were present within normal-appearing white matter and within areas of active demyelination in MS, but not in chronic silent lesions. They were also seen in association with remyelination in MS tissue and with developmental myelination in human spinal cord. NG2+ cells that did not express PDGFalphaR were also detected. Given their lack of reactivity with microglial or astrocyte markers, these NG2+/PDGFalphaR- cells probably represented more mature OPCs that had lost PDGFalphaR expression. The distribution of OPCs observed in this study strongly suggests these cells are potential sources of remyelinating oligodendrocytes in active lesions in MS.

Intrinsic resistance of neural stem cells to toxic metabolites may make them well suited for cell non-autonomous disorders: evidence from a mouse model of Krabbe leukodystrophy

While transplanted neural stem cells (NSCs) have been shown to hold promise for cell replacement in models of a number of neurological disorders, these examples have typically been under conditions where the host cells become dysfunctional due to a cell autonomous etiology, i.e. a 'sick' cell within a relatively supportive environment. It has long been held that cell replacement in a toxic milieu would not likely be possible; donor cells would succumb in much the same way as endogenous cells had. Many metabolic diseases are characterized by this situation, suggesting that they would be poor targets for cell replacement therapies. On the other hand, models of such diseases could prove ideal for testing the capacity for cell replacement under such challenging conditions. In the twitcher (twi ) mouse -- as in patients with Krabbe or globoid cell leukodystrophy (GLD), for which it serves as an authentic model -- loss of galactocerebrosidase (GalC) activity results in the accumulation of psychosine, a toxic glycolipid. Twi mice, like children with GLD, exhibit inexorable neurological deterioration presumably as a result of dysfunctional and ultimately degenerated oligodendrocytes with loss of myelin. It is believed that GLD pathophysiology is related to a psychosine-filled environment that kills not only host oligodendrocytes but theoretically any new cells placed into that milieu. Through the implantation of NSCs into the brains of both neonatal and juvenile/young adult twi mice, we have determined that widespread oligodendrocyte replacement and remyelination is feasible. NSCs appear to be intrinsically resistant to psychosine -- more so in their undifferentiated state than when directed ex vivo to become oligodendrocytes. This resistance can be enhanced by engineering the NSCs to over-express GalC. Some twi mice grafted with such engineered NSCs had thicker white tracts and lived 2-3 times longer than expected. While their brains had detectable levels of GalC, it was probably more significant that their psychosine levels were lower than in twi mice that died at a younger age. This concept of resistance based on differentiation state extended to human NSCs which could similarly survive within the twi brain. Taken together, these results suggest a number of points regarding cellular therapies against degenerative diseases with a prominent cell non-autonomous component: Cell replacement is possible if cells resistant to the toxic environment are employed. Furthermore, an important aspect of successful treatment will likely be not only cell replacement but also cross-correction of host cells to provide them with enzyme activity and hence resistance. While oligodendrocyte replacement alone was not a sufficient treatment for GLD (even when extensive), the replacement of both cells and molecules -- e.g. with NSCs that could both become oligodendrocytes and 'pumps' for GalC -- emerges as a promising basis for a multidisciplinary strategy. Most neurological disease are complex in this way and will likely require multifaceted approaches, perhaps with NSCs serving as the 'glue'.

Emerging therapies in multiple sclerosis

Multiple sclerosis (MS) is the most common neurological cause of disability in young people. The disease-modifying treatments, IFN-beta and glatiramer acetate, have been widely available over the last decade and have shown a beneficial effect on relapse rate and magnetic resonance imaging parameters of disease activity; however, their effect on disease progression and disability is modest. Therefore, the search for alternative treatment strategies continues. As understanding of the heterogeneous pathophysiology of MS has increased, emphasis has shifted to more selective therapy that targets components of the inflammatory cascade and the promotion of remyelination and neuroprotection. These agents target the blood-brain barrier, systemic immune dysfunction, local inflammation and neurodegeneration. Combination therapies are being investigated for patients who fail first-line treatments. Many new drugs are being developed and tested that address these issues with the aim of finding a more effective and convenient therapy. These include humanized monoclonal antibodies such as daclizumab (IL-2 antagonist), oral immunomodulators such as sirolimus and statins and neuroprotective agents such as NMDA antagonists and Na+-channel blockers. Many of the treatments discussed in this review are still at early stages of development, but provide exciting potential treatment options; others have proved disappointing in larger extended-phase studies.

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.