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Glial pathology is a causal contributor to the striatal neuronal dysfunction of Huntington's disease (HD). We investigate mutant HTT-associated changes in gene expression by mouse and human striatal astrocytes, as well as in mouse microglia, to identify commonalities in glial pathobiology across species and models. Mouse striatal astrocytes are fluorescence-activated cell sorted (FACS) from R6/2 and zQ175 mice, which respectively express exon1-only or full-length mHTT, and human astrocytes are generated either from human embryonic stem cells (hESCs) expressing full-length mHTT or from fetal striatal astrocytes transduced with exon1-only mHTT. Comparison of differential gene expression across these conditions, all with respect to normal HTT controls, reveals cell-type-specific changes in transcription common to both species, yet with differences that distinguish glia expressing truncated mHTT versus full-length mHTT. These data indicate that the differential gene expression of glia expressing truncated mHTT may differ from that of cells expressing full-length mHTT, while identifying a conserved set of dysregulated pathways in HD glia.

Progressive multifocal leukoencephalopathy (PML) is a demyelinating infection of the immunosuppressed brain, mediated by the gliotropic polyomavirus JCV. JCV replicates in human glial progenitor cells and astrocytes, which undergo viral T antigen-triggered mitosis, enabling viral replication. We asked if JCV spread might therefore be accelerated by glial proliferation. We found that dividing human astrocytes supported JCV propagation more than mitotically quiescent cells. Both bulk and single cell RNA-seq revealed that these JCV-infected glia exhibited cell cycle-linked perturbation of DNA damage response and transcriptional regulatory pathways. In vivo, JCV infection of humanized glial chimeras was greatly accentuated by cuprizone-induced demyelination and its associated mobilization of GPCs. Infection triggered the death of uninfected as well as infected glia in vivo, reflecting significant bystander death. These data suggest that demyelination-associated glial proliferation accelerates the spread of JCV, and argue for the aggressive prevention of new demyelinating events in patients at risk for PML.

Summary: Neonatally transplanted human glial progenitor cells (hGPCs) can myelinate the brains of myelin-deficient shiverer mice, rescuing their phenotype and survival. Yet, it has been unclear whether implanted hGPCs are similarly able to remyelinate the diffusely demyelinated adult CNS. We, therefore, ask if hGPCs could remyelinate both congenitally hypomyelinated adult shiverers and normal adult mice after cuprizone demyelination. In adult shiverers, hGPCs broadly disperse and differentiate as myelinating oligodendrocytes after subcortical injection, improving both host callosal conduction and ambulation. Implanted hGPCs similarly remyelinate denuded axons after cuprizone demyelination, whether delivered before or after demyelination. RNA sequencing (RNA-seq) of hGPCs back from cuprizone-demyelinated brains reveals their transcriptional activation of oligodendrocyte differentiation programs, while distinguishing them from hGPCs not previously exposed to demyelination. These data indicate the ability of transplanted hGPCs to disperse throughout the adult CNS, to broadly myelinate regions of dysmyelination, and also to be recruited as myelinogenic oligodendrocytes later in life, upon demyelination-associated demand.

Human glial progenitor cells (hGPCs) can completely myelinate the brains of congenitally hypomyelinated shiverer mice, rescuing the phenotype and extending or normalizing the lifespan of these mice. We asked if implanted hGPCs might be similarly able to broadly disperse and remyelinate the diffusely and/or multicentrically-demyelinatedadultCNS. In particular, we asked if fetal hGPCs could effectively remyelinate both congenitally hypomyelinated adult axons,andaxons acutely demyelinated in adulthood, using adultshiverermice and cuprizone-demyelinated mice, respectively. We found that hGPCs broadly infiltrate the adult CNS after callosal injection, and robustly myelinate congenitally-unmyelinated axons in adultshiverer. Moreover, implanted hGPCs similarly remyelinated denuded axons after cuprizone demyelination, whether they were delivered prior toorafter initial cuprizone demyelination. Extraction and FACS of hGPCs from cuprizone-demyelinated brains in which they had been resident, followed by RNA-seq of the isolated human hGPCs, revealed their activation of transcriptional programs indicating their initiation of oligodendrocyte differentiation and myelination. These data indicate the ability of transplanted hGPCs to disperse throughout the adult CNS, to myelinate dysmyelinated regions encountered during their parenchymal colonization, and to also be recruited as myelinating oligodendrocytes at later points in life, upon demyelination-associated demand.

Pelizaeus-Merzbacher disease (PMD) is an X-linked disorder caused by mutation in the proteolipid protein-1 (PLP1) gene, which encodes the proteolipid protein of myelinating oligodendroglia. PMD exhibits phenotypic variability that reflects its considerable genotypic heterogeneity, but all forms of the disease result in central hypomyelination, associated in most cases with early neurological dysfunction, progressive deterioration, and ultimately death. PMD may present as a connatal, classic and transitional forms, or as the less severe spastic paraplegia type 2 and PLP-null phenotypes. These disorders are most often associated with duplications of the PLP1 gene, but can also be caused by coding and noncoding point mutations as well as full or partial deletion of the gene. A number of genetically-distinct but phenotypically-similar disorders of hypomyelination exist which, like PMD, lack any effective therapy. Yet as relatively pure CNS hypomyelinating disorders, with limited involvement of the PNS and relatively little attendant neuronal pathology, PMD and similar hypomyelinating disorders are attractive therapeutic targets for neural stem cell and glial progenitor cell transplantation, efforts at which are now underway in a number of research centers.

As new methods for producing and isolating human glial progenitor cells (hGPCs) have been developed, the disorders of myelin have become especially compelling targets for cell-based therapy. Yet as animal modeling of glial progenitor cell-based therapies has progressed, it has become clear that transplanted hGPCs not only engraft and expand within murine hosts, but dynamically outcompete the resident progenitors so as to ultimately dominate the host brain. The engrafted human progenitor cells proceed to generate parenchymal astrocytes, and when faced with a hypomyelinated environment, oligodendrocytes as well. As a result, the recipient brains may become inexorably humanized with regards to their resident glial populations, yielding human glial chimeric mouse brains. These brains provide us a fundamentally new tool by which to assess the species-specific attributes of glia in modulating human cognition and information processing. In addition, the cellular humanization of these brains permits their use in studying glial infectious and inflammatory disorders unique to humans, and the effects of those disorders on the glial contributions to cognition. Perhaps most intriguingly, by pairing our ability to construct human glial chimeras with the production of patient-specific hGPCs derived from pluripotential stem cells, we may now establish mice in which a substantial proportion of resident glia are both human and disease-derived. These mice in particular may provide us new opportunities for studying the human-specific contributions of glia to human psychopathology, as well as to higher cognition. As such, the assessment of human glial chimeric mice may provide us new insight into the species-specific contributions of glia to human cognitive evolution, as well as to the pathogenesis of human neurological and neuropsychiatric disease.

Summary In this study, we investigated whether intrinsic glial dysfunction contributes to the pathogenesis of schizophrenia (SCZ). Our approach was to establish humanized glial chimeric mice using glial progenitor cells (GPCs) produced from induced pluripotent stem cells derived from patients with childhood-onset SCZ. After neonatal implantation into myelin-deficient shiverer mice, SCZ GPCs showed premature migration into the cortex, leading to reduced white matter expansion and hypomyelination relative to controls. The SCZ glial chimeras also showed delayed astrocytic differentiation and abnormal astrocytic morphologies. When established in myelin wild-type hosts, SCZ glial mice showed reduced prepulse inhibition and abnormal behavior, including excessive anxiety, antisocial traits, and disturbed sleep. RNA-seq of cultured SCZ human glial progenitor cells (hGPCs) revealed disrupted glial differentiation-associated and synaptic gene expression, indicating that glial pathology was cell autonomous. Our data therefore suggest a causal role for impaired glial maturation in the development of schizophrenia and provide a humanized model for its in vivo assessment.

Summary: Astrocytic differentiation is developmentally impaired in patients with childhood-onset schizophrenia (SCZ). To determine why, we used genetic gain- and loss-of-function studies to establish the contributions of differentially expressed transcriptional regulators to the defective differentiation of glial progenitor cells (GPCs) produced from SCZ patient-derived induced pluripotent cells (iPSCs). Negative regulators of the bone morphogenetic protein (BMP) pathway were upregulated in SCZ GPCs, including BAMBI, FST, and GREM1, whose overexpression retained SCZ GPCs at the progenitor stage. SMAD4 knockdown (KD) suppressed the production of these BMP inhibitors by SCZ GPCs and rescued normal astrocytic differentiation. In addition, the BMP-regulated transcriptional repressor REST was upregulated in SCZ GPCs, and its KD similarly restored normal glial differentiation. REST KD also rescued potassium-transport-associated gene expression and K+ uptake, which were otherwise deficient in SCZ glia. These data suggest that the glial differentiation defect in childhood-onset SCZ, and its attendant disruption in K+ homeostasis, may be rescued by targeting BMP/SMAD4- and REST-dependent transcription. : Astrocytic differentiation is impaired in childhood-onset schizophrenia (SCZ). Liu et al. report that SMAD4-dependent BMP signaling and REST are upregulated in hiPSC-derived SCZ glia and that SMAD4 and REST knockdown rescue both astroglial differentiation and K+ transport. SCZ astrocytic maturation may thus be rescued by targeting SMAD4- and REST-dependent transcription. Keywords: schizophrenia, stem cells, iPSC, astrocytes, glial progenitor cells, epigenetics, REST, potassium channel, BMP inhibitors, BAMBI

The causal contribution of glial pathology to Huntington disease (HD) has not been heavily explored. To define the contribution of glia to HD, we established human HD glial chimeras by neonatally engrafting immunodeficient mice with mutant huntingtin (mHTT)-expressing human glial progenitor cells (hGPCs), derived from either human embryonic stem cells or mHTT-transduced fetal hGPCs. Here we show that mHTT glia can impart disease phenotype to normal mice, since mice engrafted intrastriatally with mHTT hGPCs exhibit worse motor performance than controls, and striatal neurons in mHTT glial chimeras are hyperexcitable. Conversely, normal glia can ameliorate disease phenotype in transgenic HD mice, as striatal transplantation of normal glia rescues aspects of electrophysiological and behavioural phenotype, restores interstitial potassium homeostasis, slows disease progression and extends survival in R6/2 HD mice. These observations suggest a causal role for glia in HD, and further suggest a cell-based strategy for disease amelioration in this disorder.

Experimental animals with myelin disorders can be treated by transplanting oligodendrocyte progenitor cells (OPCs) into the affected brain or spinal cord. OPCs have been isolated by their expression of 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 sorted fetal human forebrain cells for CD140a, an epitope of platelet derived growth factor receptor (PDGFR)α, which is differentially expressed by OPCs. CD140a(+) cells were isolated as mitotic bipotential progenitors that initially expressed neither mature neuronal nor astrocytic phenotypic markers, yet could be instructed to either oligodendrocyte or astrocyte fate in vitro. Transplanted CD140a(+) cells were highly migratory and robustly myelinated the hypomyelinated shiverer mouse brain more rapidly and efficiently than did A2B5(+)cells. Microarray analysis of CD140a(+) cells revealed overexpression of the oligodendroglial marker CD9, suggesting that CD9(+)/CD140a(+) cells may constitute an even more highly enriched population of myelinogenic progenitor cells.

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.

Neural precursor cells are dispersed widely throughout the adult brain. A population of multipotential progenitor cells, capable of giving rise to both neurons and glia, lines the cerebral ventricles of all adult animals, including humans. In addition, glial progenitor cells, which also have the capacity to generate multiple cell types, are dispersed throughout the subcortical white matter and cortex. Each of these categories of progenitor cells exists in, and has been isolated from, the adult human brain. We have begun to ask how these cells might be used therapeutically. We report here examples of two potential strategies for their use: (1) transplantation of phenotypically restricted progenitors, in which purified isolates of defined human precursor cells are implanted into disease sites, and (2) induction in vivo, in which resident progenitor cells are stimulated by delivered growth factors. We find that each of these strategies has potential therapeutic efficacy, and discuss the most likely and feasible clinical targets for their clinical application.

Neonatally transplanted human glial progenitor cells (hGPCs) densely engraft and myelinate the hypomyelinatedshiverermouse. We found that, in hGPC-xenografted mice, the human donor cells continue to expand throughout the forebrain, systematically replacing the host murine glia. The differentiation of the donor cells is influenced by the host environment, such that more donor cells differentiated as oligodendrocytes in the hypomyelinatedshivererbrain than in myelin wild-types, in which hGPCs were more likely to remain as progenitors. Yet in each recipient, both the number and relative proportion of mouse GPCs fell as a function of time, concomitant with the mitotic expansion and spread of donor hGPCs. By a year after neonatal xenograft, the forebrain GPC populations of implanted mice were largely, and often entirely, of human origin. Thus, neonatally implanted hGPCs outcompeted and ultimately replaced the host population of mouse GPCs, ultimately generating mice with a humanized glial progenitor population. These human glial chimeric mice should permit us to define the specific contributions of glia to a broad variety of neurological disorders, using human cellsin vivo.

Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease triggered by infection with the human gliotropic JC virus (JCV). Due to the human-selective nature of the virus, there are no animal models available to investigate JCV pathogenesis. To address this issue, we developed mice with humanized white matter by engrafting human glial progenitor cells (GPCs) into neonatal immunodeficient and myelin-deficient mice. Intracerebral delivery of JCV resulted in infection and subsequent demyelination of these chimeric mice. Human GPCs and astrocytes were infected more readily than oligodendrocytes, and viral replication was noted primarily in human astrocytes and GPCs rather than oligodendrocytes, which instead expressed early viral T antigens and exhibited apoptotic death. Engraftment of human GPCs in normally myelinated and immunodeficient mice resulted in humanized white matter that was chimeric for human astrocytes and GPCs. JCV effectively propagated in these mice, which indicates that astroglial infection is sufficient for JCV spread. Sequencing revealed progressive mutation of the JCV capsid protein VP1 after infection, suggesting that PML may evolve with active infection. These results indicate that the principal CNS targets for JCV infection are astrocytes and GPCs and that infection is associated with progressive mutation, while demyelination is a secondary occurrence, following T antigen-triggered oligodendroglial apoptosis. More broadly, this study provides a model by which to further assess the biology and treatment of human-specific gliotropic viruses.

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.

Vanishing white matter (VWM) disease is a genetic leukoencephalopathy linked to mutations in the eukaryotic translation initiation factor 2B. It is a disease of infants, children, and adults who experience a slowly progressive neurologic deterioration with episodes of rapid clinical worsening triggered by stress and eventually leading to death. Characteristic neuropathologic findings include cystic degeneration of the white matter with scarce reactive gliosis, dysmorphic astrocytes, and paucity of myelin despite an increase in oligodendrocytic density. To assess whether a defective maturation of macroglia may be responsible for the feeble gliosis and lack of myelin, weinvestigated the maturation status of astrocytes and oligodendrocytes in the brains of 8 VWM patients, 4 patients with other white matter disorders and 6 age-matched controls with a combination of immunocytochemistry, histochemistry, scratch-wound assays, Western blot, and quantitative polymerase chain reaction. We observed increased proliferation and a defect in the maturation of VWM astrocytes. They show an anomalous composition of their intermediate filament network with predominance of the δ-isoform of the glial fibrillary acidic protein and an increase in the heat shock protein αB-crystallin, supporting the possibility that a deficiency in astrocyte function may contribute to the loss of white matter in VWM. We also demonstrated a significant increase in numbers of premyelinating oligodendrocyte progenitors in VWM, which may explain the coexistence of oligodendrocytosis and myelin paucity in the patients' white matter. © 2010 by the American Association of Neuropathologists, Inc.

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.

The diversity of neocortical cytoarchitecture could arise from genetic prespecification of cell types and numbers in the ventricular zone, by interaction of cells with their immediate environment, efferent targets, or afferent inputs. Here, we examine the role of the thalamus as efferent target or afferent source in the early control of cell number and type in the developing neocortex. Electrolytic lesions of the thalamus were made in hamsters at birth prior to the thalamic innervation of layer IV. By postnatal day 7, when migration to the cortex is complete, there were no differences in cell number between the cortical plate contralateral and ipsilateral to the thalamic lesion, showing that the absence of thalamic input does not influence the last phases of neocortical cell generation or migration. However, the incidence of pyknotic cells was elevated in the upper half of the cortical plate at this time. By adulthood, the number of cells per unit column of cortex was reduced, due to the apparent absence of small, nonpyramidal cells of layer IV, as determined from Nissl-stained material. Therefore, some of the cytoarchitectonic variability of the neocortex could arise epigenetically by the interaction of neocortical cells with their afferent connections.

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.

Summary 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 × 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.

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.

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.

The homeobox gene Hb9 is expressed selectively by motor neurons (MNs) in the developing CNS. Previous studies have identified a 9-kb 5′ fragment of the mouse Hb9 gene that is sufficient to direct gene expression to spinal MNs in vivo. Here, we sought to identify more discrete MN-specifying elements, using homology searches between genomic sequences of evolutionarily distant species. Based on homology screening of the mouse and human Hb9 promoters, we identified a 3.6-kb Hb9 enhancer that proved sufficient to drive MN-specific lacZ expression. We then compared mouse, human, and pufferfish (Fugu rubripes) genomic sequences, and identified a conserved 438-bp sequence, consisting of noncontiguous 313-bp and 125-bp fragments, residing within the 3.6-kb Hb9 enhancer. The zebrafish (Danio rerio) Hb9 genomic region was then found to have two identical copies of the 125-bp sequence, but no counterpart for the 313-bp sequence. Transgenic analysis showed that the 125-bp alone was both necessary and sufficient to direct spinal MN-specific lacZ expression, whereas the 313-bp sequence had no such enhancer activity. Moreover, the 125-bp Hb9 enhancer was found to harbor two Hox/Pbx consensus-binding sequences, mutations of which completely disrupted thoracolumbar Hb9 expression. These data suggest that Hox/Pbx plays a critical role in the segmental specification of spinal MNs. Together, these results indicate that the molecular pathways regulating Hb9 expression are evolutionarily conserved, and that MN-specific gene expression may be directed and achieved using a small 125-bp 5′ enhancer.

Publisher Summary Oligodendrocyte progenitor cells (OPCs) seem to be extraordinarily widespread in the adult mammalian brain. The principal class of OPCs in adult rodents is a bipotential astrocyte-oligodendrocyte progenitor cell designated the O-2A progenitor, by virtue of its generation in vitro of oligodendrocytes and type 2 astrocytes, the latter comprising the traditionally recognized fibrous astrocytes of the white matter. OPCs may be specifically targeted and isolated from the white matter. This chapter outlines basic strategies for isolating OPCs from the adult white matter, using either fluorescence-activated cell sorting (FACS) or a higher-yield, less-specific alternative immunomagnetic isolation (MACS). Adult-human white-matter progenitor cells (WMPCs) give rise to multipotential neurospheres. Nonetheless, the very existence of multipotential progenitors scattered throughout the white-matter parenchyma forces one to reconsider understanding of both the nature and incidence of neural stem cells in the adult brain, and challenges the conception of the supposed rarity of adult neural progenitor and stem cells. In doing so, they point to an abundant and widespread source of cells that may be used both as a target for pharmacological induction and as a cell type appropriate for therapeutic engraftment to the diseased adult brain.

Lineage-restricted progenitors of the central nervous system (CNS) are not readily expandable because their mitotic competence is limited. Here we used retroviral overexpression of human telomerase reverse transcriptase (hTERT) to immortalize progenitors from human fetal spinal cord. The hTERT-immortalized cells divided in basic fibroblast growth factor (bFGF) expressed high telomerase activity, and gave rise to phenotypically restricted subpopulations of either glia or neurons. The latter included a prototypic line, hSC11V-TERT, that gave rise only to neurons. These included both chx10(+) interneurons and Islet1(+)/Hb9(+)/ChAT(+) motor neurons; the latter were recognized by green fluorescent protein (GFP) driven by the Hb9 enhancer. The neurons were postmitotic and achieved electrophysiologic competence. Upon xenograft to both fetal rat brain and injured adult spinal cord, they matured as neurons and survived for 6 months, with no evident tumorigenesis. The cells have survived168 doublings in vitro, with karyotypic normalcy and without replicative senescence. hTERT overexpression thus permits the generation of progenitor lines able to give rise to phenotypically restricted neurons.

A distinct population of white matter progenitor cells (WMPCs), competent but not committed to generate oligodendrocytes, remains ubiquitous in the adult human subcortical white matter. These cells are present in both sexes and into senescence and may constitute as much as 4% of the cells of adult human capsular white matter. Transduction of adult human white matter dissociates with plasmids bearing early oligodendrocytic promoters driving fluorescent reporters permits the separation of these cells at high yield and purity, as does separation based on their expression of A2B5 immunoreactivity. Isolates of these cells survive xenograft to lysolecithin-demyelinated brain and migrate rapidly to infiltrate these lesions, without extending into normal white matter. Within several weeks, implanted progenitors mature as oligodendrocytes, and develop myelin-associated antigens. Lentiviral tagging with green fluorescent protein confirmed that A2B5-sorted progenitors develop myelin basic protein expression within regions of demyelination and that they fail to migrate when implanted into normal brain. Adult human white matter progenitor cells can thus disperse widely through regions of experimental demyelination and are able to differentiate as myelinating oligodendrocytes. This being the case, they may constitute appropriate vectors for cell-based remyelination strategies.

Corrigendum: Fetal and adult human oligodendrocyte progenitor cells effectively myelinate dysmyelinated brain

The effects of potential excess innervation on cell survival in the superior colliculus and related structures during the period of normally occurring cell death was examined. A unilateral, partial lesion of the superficial layers of the superior colliculus on the day of birth, which results in a compression of the retinotectal map into the remaining area, was the manipulation used to produce the potential excess innervation. Cell density was reduced in the tectal fragment early in development, consistent with hyperinnervation, but had returned to normal by the end of the period of normally occurring cell death. The overall incidence of cell degeneration in the remaining partial colliculus was not different from the undamaged contralateral colliculus or from normal, though there was evidence of a transitory depression and later elevation of cell loss. Cell loss in the retina contralateral to the lesion was increased in the late part of the period of normal cell loss and there were fewer cells in the retinal ganglion cell layer at maturity. The amount of the cell loss in the retina was small compared to the amount of target removal. These results suggest that the survival of neurons with branching axons does not sensitively reflect target availability.

The effect of unilateral deletion of the visual cortex on early cell death and eventual cell number in various structures of the visual system was examined. At minimum, this manipulation potentially provides excess retinal afference to the superior colliculi, partially denervates the superior colliculi, reduces normal retinal terminal area and opens up potential target space for the retina and superior colliculus in those areas where they share terminal space with the visual cortex. All layers of the superior colliculus, bilaterally, showed an initial decrease in the rate of cell death relative to normal followed by an increase in cell death rates. No change in the number or distribution of cells in the retinal ganglion cell layer resulted despite a substantial loss of retinal terminal area, and a substantial alteration of the pattern of retinal central termination. These results are interpreted as evidence for two stages in normally occurring cell death, a first in which axons compete to colonize any available terminal space, and a second in which axon-to-target specificity must be matched. These results also provide evidence that the amount of target required for neuron survival is clearly variable.

To determine if cell death participates in the regulation of cell number between interconnecting populations of the neocortex, we sectioned the corpus callosum of neonatal hamsters, thus depriving callosally projecting cells of their normal targets and callosally-recipient cells of their normal afference. The numbers of neurons per unit column in two areas of the cortex which have heavy callosal projections (the 17-18a border and area 6) and one area that is relatively acallosal (area 3) were compared in animals with early corpus callosum sections and controls. No differences were found, either for a 'unit cortical column,' or for the callosally-projecting layers (II-III and V). Mean soma sizes in layers II-III and V of all three areas were likewise unchanged. In area 6 and part of area 3, however, the distribution of soma sizes in callosally projecting and recipient laminae was significantly altered. The change in size distribution without change in mean soma area suggests that the cortex responds to the elimination of the callosal pathway in more than one way. Since no role for cell death in removal of diffuse connectivity or in target regulation of neuron number has yet been found, a new hypothesis for the function of cell death in local cytoarchitectural differentiation of the cortex is proposed.

In hamster, following removal of one eye on the day of birth the amount of normally occurring degeneration in the retinal ganglion cell layer of the remaining eye is reduced, particularly in the temporal retina. To examine the changes in the number and distribution of cells indicated by the alterations in early cell degeneration, adult retinas from hamsters who had one eye removed at birth were compared to those of normal adults. Normal adult retinal ganglion cell layers were found to contain an average of 157,223 cells (a population which includes retinal ganglion cells, "displaced" amacrine cells, and glia). At adulthood, the remaining retinas of early enucleates had an average of 169,863 cells in the ganglion cell layer, an increase of 8%. If only cells having Nissl substance and a soma diameter in excess of 10 micron (a group likely to consist entirely of retinal ganglion cells) are considered, the increase observed was 19%. Cells having Nissl substance and soma diameters between 5 and 10 micron, a group which includes both retinal ganglion cells and displaced amacrines, show a 13% change. Cells less than 10 micron with no Nissl substance visible, which include displaced amacrines and glial cells showed no net change (75,883 versus 75,409). The increase in cell number was found across the entire retina, but was largest in the temporal retina. These results show that alteration in early neuronal survival is a component of early plastic changes in the central nervous system, and that early cell degeneration rates are good predictors of later cell number and distribution.