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

Glial grafting for demyelinating disease

Remyelination of demyelinated central nervous system (CNS) axons is considered as a potential treatment for multiple sclerosis, and it has been achieved in experimental models of demyelination by transplantation of pro-myelinating cells. However, the experiments undertaken have not addressed the need for tissue-type matching in order to achieve graft-mediated remyelination since they were performed in conditions in which the chance for graft rejection was minimized. This article focuses on the factors determining survival of allogeneic oligodendrocyte lineage cells and their contribution to the remyelination of demyelinating CNS lesions. The immune status of the CNS as well as the suitability of different models of demyelination for graft rejection studies are discussed, and ways of enhancing allogeneic oligodendrocyte-mediated remyelination are presented. Finally, the effects of glial graft rejection on host remyelination are described, highlighting the potential benefits of the acute CNS inflammatory response for myelin repair.

Progenitor cell-based myelination as a model for cell-based therapy of the central nervous system

Diseases of the brain and spinal cord are 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. Foremost among these are the disorders of myelin, in which oligodendrocytes are the specific and often sole victims of the underlying disease process. These include not only the vascular, traumatic, and inflammatory demyelinations of adulthood, but also the congenital and childhood dysmyelinating syndromes of the pediatric leukodystrophies. These congenital disorders of myelin formation and maintenance may present especially compelling targets for cell-based neurological therapy.

Stem cells for the treatment of myelin loss

Treatment of myelin loss is particularly suited to therapeutic strategies based on cell replacement. Demyelination represents a defined and functionally debilitating deficit, and remyelination can be accomplished by supplying regions of demyelination with myelinogenic cell populations. Clinical interest in stem cells as a source of myelinogenic cells arises from their ability to provide an apparently unlimited cell supply for transplantation, and from recent demonstrations that they can be directed to myelinogenic phenotypes with high purity. Here, I present the emerging perspective that stem-cell-mediated remyelination of the adult CNS is a viable therapeutic strategy, and discuss the challenges to remyelination posed by the environment of acute and chronic injuries.

Stem and progenitor cell-based therapy of the human central nervous system

Multipotent neural stem cells, capable of giving rise to both neurons and glia, line the cerebral ventricles of all adult animals, including humans. In addition, distinct populations of nominally glial progenitor cells, which also have the capacity to generate several cell types, are dispersed throughout the subcortical white matter and cortex. A number of approaches have evolved for using neural progenitor cells in cell therapy. Four strategies are especially attractive for clinical translation: first, transplantation of oligodendrocyte progenitor cells as a means of treating the disorders of myelin; second, transplantation of phenotypically restricted neuronal progenitor cells to treat diseases of discrete loss of a single neuronal phenotype, such as Parkinson disease; third, implantation of mixed progenitor pools to treat diseases characterized by the loss of several discrete phenotypes, such as spinal cord injury; and fourth, mobilization of endogenous neural progenitor cells to restore neurons lost as a result of neurodegenerative diseases, in particular Huntington disease. Together, these may present the most compelling strategies and near-term disease targets for cell-based neurological therapy.

Distinctive properties of human adult brain-derived myelin progenitor cells

We used expression of the ganglioside A2B5 to isolate putative myelin progenitor cells from adult human central nervous system parenchyma and compared their phenotypic (expression of myelin lineage molecules) and functional (survival, proliferation) properties with mature oligodendrocytes (OLGs) derived from the same adult material and with A2B5(+) cells isolated from human fetal brain. A2B5(+) cells represented 3 to 5% of the total cell suspension derived from adult specimens. Results of protein (immunostaining) and RNA (polymerase chain reaction) analyses indicated that the adult A2B5(+) cells were more committed to the OLG lineage than their fetal counterparts while continuing to retain properties of progenitor cells compared to the postmitotic mature OLGs. Although the adult A2B5(+) cells retained the capacity to divide, albeit at a reduced rate compared to fetal A2B5(+) cells, they showed reduced survival and process outgrowth compared not only to fetal cells but also to mature OLGs. Our results confirm the presence of progenitor cells committed to the OLG lineage in the adult human central nervous system but raise the issues regarding the intrinsic capacity of these cells to contribute to the process of remyelination that may be necessary during demyelinating diseases.

The remyelinating potential and in vitro differentiation of MOG-expressing oligodendrocyte precursors isolated from the adult rat CNS

There is a long-standing controversy as to whether oligodendrocytes may be capable of cell division and thus contribute to remyelination. We recently published evidence that a subpopulation of myelin oligodendrocyte glycoprotein (MOG)-expressing cells in the adult rat spinal cord co-expressed molecules previously considered to be restricted to oligodendrocyte progenitors [G. Li et al. (2002) Brain Pathol., 12, 463-471]. To further investigate the properties of MOG-expressing cells, anti-MOG-immunosorted cells were grown in culture and transplanted into acute demyelinating lesions. The immunosorting protocol yielded a cell preparation in which over 98% of the viable cells showed anti-MOG- and O1-immunoreactivity; 12-15% of the anti-MOG-immunosorted cells co-expressed platelet-derived growth factor alpha receptor (PDGFRalpha) or the A2B5-epitope. When cultured in serum-free medium containing EGF and FGF-2, 15-18% of the anti-MOG-immunosorted cells lost anti-MOG- and O1-immunoreactivity and underwent cell division. On removal of these growth factors, cells differentiated into oligodendrocytes, or astrocytes and Schwann cells when the differentiation medium contained BMPs. Transplantation of anti-MOG-immunosorted cells into areas of acute demyelination immediately after isolation resulted in the generation of remyelinating oligodendrocytes and Schwann cells. Our studies indicate that the adult rat CNS contains a significant number of oligodendrocyte precursors that express MOG and galactocerebroside, molecules previously considered restricted to mature oligodendrocytes. This may explain why myelin-bearing oligodendrocytes were considered capable of generating remyelinating cells. Our study also provides evidence that the adult oligodendrocyte progenitor can be considered as a source of the Schwann cells that remyelinate demyelinated CNS axons following concurrent destruction of oligodendrocytes and astrocytes.

Neural progenitor cell transplants promote long-term functional recovery after traumatic brain injury

Studies demonstrating the versatility of neural progenitor cells (NPCs) have recently rekindled interest in neurotransplantation methods aimed at treating traumatic brain injury (TBI). However, few studies have evaluated the safety and functional efficacy of transplanted NPCs beyond a few months. The purpose of this study was to assess the long-term survival, migration, differentiation and functional significance of NPCs transplanted into a mouse model of TBI out to 1 year post-transplant. NPCs were derived from E14.5 mouse brains containing a transgene-expressing green fluorescent protein (GFP) and cultured as neurospheres in FGF2-containing medium. Neurospheres were injected into the ipsilateral striatum of adult C57BL/6 mice 1 week following unilateral cortical impact injury. Behavioral testing revealed significant improvements in motor abilities in NPC-treated mice as early as 1 week, and the recovery was sustained out to 1 year post-transplant. In addition, mice receiving NPC transplants showed significant improvement in spatial learning abilities at 3 months and 1 year, whereas an intermediate treatment effect on this behavioral parameter was detected at 1 month. At 14 months post-transplant, GFP(+) NPCs were observed throughout the injured hippocampus and adjacent cortical regions of transplanted brains. Immunohistochemical analysis revealed that the majority of transplanted cells co-labeled for NG2, an oligodendrocyte progenitor cell marker, but not for neuronal, astrocytic or microglial markers. In conclusion, transplanted NPCs survive in the host brain up to 14 months, migrate to the site of injury, enhance motor and cognitive recovery, and may play a role in trophic support following TBI.

Metabotropic glutamate receptors are expressed in adult human glial progenitor cells

Glial precursor cells (GPCs) are present in the adult human central nervous system (CNS) and they can be isolated and maintained in culture for in vitro studies. This study analysed expression of mGluR3 and mGluR5 metabotropic glutamate receptor (mGluR) mRNAs in GPCs. A2B5 surface antigen positive GPCs were isolated using immunomagnetic selection from dissociated temporal lobe subcortical white matter cells. The separated GPCs were maintained in cultures and characterised by immunoreactivity for the differentiation markers A2B5 and human platelet-derived growth factor-alpha receptor (PDGFalphaR). Reverse transcription followed by multiplex PCR analysis showed that the GPCs expressed both mGluR3 and mGluR5a mRNAs. Double immunostaining for glial progenitor markers and mGluR5 proteins demonstrated that all A2B5 and PDGFalphaR-positive cells were also positive for mGluR5. The results indicate that GPCs present in the adult human CNS express mGluR3 and mGluR5a. These neurotransmitter receptors may be involved in the proliferation and differentiation of glial cells.

Remyelination in multiple sclerosis: a new role for neurotrophins?

Multiple sclerosis (MS) is a common neurological disease, which affects young adults. Its course is unpredictable and runs over decades. It is considered as an autoimmune disease, and is neuropathologically characterized by demyelination, variable loss of oligodendroglial cells, and axonal degeneration. Demyelination provides a permitting condition for axonal degeneration, which seems to be causative of permanent neurological deficits. Hence, the current treatment, which works preferentially immunmodulatory, should be complemented by therapeutics, which improves remyelination not only for restoring conduction velocity but also for preventing an irreversible axonal damage. One strategy to achieve this aim would be to promote remyelination by stimulating oligodendroglial cells remaining in MS lesions. While central nervous system neurons were already known to respond to neurotrophins (NT), interactions with glial cells became apparent more recently. In vitro and in vivo studies have shown that NT influence proliferation, differentiation, survival, and regeneration of mature oligodendrocytes and oligodendroglial precursors in favor of a myelin repair. Two in vivo models provided direct evidence that NT can improve remyelination. In addition, their neuroprotective and anti-inflammatory role would support a repair. Hence, a wealth of data point to NT as promising therapeutical candidates.

Glia as neural progenitor cells

Recent studies have substantially expanded our conception of the roles for glia in function and maintenance of the adult nervous system. Of these reports, several have re-examined the lineage relationships among neural stem cells, their early radial glial derivatives and their mitotically competent neurogenic daughters. These studies have highlighted the role of radial cells in development, and of their glial progeny postnatally, as both progenitors and regulators of neuronal production and phenotype. In the adult mammalian brain, radial cell populations are scant, but their glial derivatives participate in a gliovascular network that organizes not only the structural and functional architecture of the brain but also its generative niches for resident progenitors - glial as well as neuronal. As in other organs, these progenitors can reside as transit-amplifying pools, by which lineage-biased progenitors expand to replenish discrete mature phenotypes. This review will consider the types of transit-amplifying progenitor cells persistent in the adult mammalian CNS, and the extent to which these derive from glial phenotypes. It will also discuss the interactions of progenitor cells with their brethren that could specify their phenotype and fate, while defining the permissive niches for cell genesis in the adult CNS.

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

Both late-gestation and adult human forebrain contain large numbers of oligodendrocyte progenitor cells (OPCs). These cells may be identified by their A2B5(+)PSA-NCAM(-) phenotype (positive for the early oligodendrocyte marker A2B5 and negative for the polysialylated neural cell adhesion molecule). We used dual-color fluorescence-activated cell sorting (FACS) to extract OPCs from 21- to 23-week-old fetal human forebrain, and A2B5 selection to extract these cells from adult white matter. When xenografted to the forebrains of newborn shiverer mice, fetal OPCs dispersed throughout the white matter and developed into oligodendrocytes and astrocytes. By 12 weeks, the host brains showed extensive myelin production, compaction and axonal myelination. Isolates of OPCs derived from adult human white matter also myelinated shiverer mouse brain, but much more rapidly than their fetal counterparts, achieving widespread and dense myelin basic protein (MBP) expression by 4 weeks after grafting. Adult OPCs generated oligodendrocytes more efficiently than fetal OPCs, and ensheathed more host axons per donor cell than fetal cells. Both fetal and adult OPC phenotypes mediated the extensive and robust myelination of congenitally dysmyelinated host brain, although their differences suggested their use for different disease targets.

Identification and isolation of multipotential neural progenitor cells from the subcortical white matter of the adult human brain

The subcortical white matter of the adult human brain harbors a pool of glial progenitor cells. These cells can be isolated by fluorescence-activated cell sorting (FACS) after either transfection with green fluorescent protein (GFP) under the control of the CNP2 promoter, or A2B5-targeted immunotagging. Although these cells give rise largely to oligodendrocytes, in low-density culture we observed that some also generated neurons. We thus asked whether these nominally glial progenitors might include multipotential progenitor cells capable of neurogenesis. We found that adult human white-matter progenitor cells (WMPCs) could be passaged as neurospheres in vitro and that these cells generated functionally competent neurons and glia both in vitro and after xenograft to the fetal rat brain. WMPCs were able to produce neurons after their initial isolation and did not require in vitro expansion or reprogramming to do so. These experiments indicate that an abundant pool of mitotically competent neurogenic progenitor cells resides in the adult human white matter.

Multiple paths towards repair in multiple sclerosis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the CNS, affecting approximately 1/1000 individuals in the Western world. Available treatments limit CNS inflammation and strategies to repair damage in the CNS offer the potential of recovery of both tissue and function. With further fundamental knowledge developing, this area is ripe for 'translation' to clinical application.

Stem cell biology of the central nervous system

Neural stem cells (NSCs) are multipotential progenitor cells that have self-renewal activities. A single NSC is capable of generating various kinds of cells within the central nervous system (CNS), including neurons, astrocytes, and oligodendrocytes. Because of these characteristics, there is increasing interest in NSCs and neural progenitor cells from the aspects of both basic developmental biology and therapeutic applications to the damaged brain. This special issue, dedicated to understanding the nature of the NSCs present in the CNS, presents an introduction to several avenues of research that may lead to feasible strategies for manipulating cells in situ to treat the damaged brain. The topics covered by these studies include the extracellular factors and signal transduction cascades involved in the differentiation and maintenance of NSCs, the population dynamics and locations of NSCs in embryonic and adult brains, prospective identification and isolation of NSCs, the induction of NSCs to adopt particular neuronal phenotypes, and their transplantation into the damaged CNS.