Origin of oligodendrocytes within the human spinal cord

Regulators of Oligodendrocyte Differentiation

Myelination has evolved as a mechanism to ensure fast and efficient propagation of nerve impulses along axons. Within the central nervous system (CNS), myelination is carried out by highly specialized glial cells, oligodendrocytes. The formation of myelin is a prolonged aspect of CNS development that occurs well into adulthood in humans, continuing throughout life in response to injury or as a component of neuroplasticity. The timing of myelination is tightly tied to the generation of oligodendrocytes through the differentiation of their committed progenitors, oligodendrocyte precursor cells (OPCs), which reside throughout the developing and adult CNS. In this article, we summarize our current understanding of some of the signals and pathways that regulate the differentiation of OPCs, and thus the myelination of CNS axons.

Development of myelinating glia: An overview

Myelin is essential to nervous system function, playing roles in saltatory conduction and trophic support. Oligodendrocytes (OLs) and Schwann cells (SCs) form myelin in the central and peripheral nervous systems respectively and follow different developmental paths. OLs are neural stem-cell derived and follow an intrinsic developmental program resulting in a largely irreversible differentiation state. During embryonic development, OL precursor cells (OPCs) are produced in distinct waves originating from different locations in the central nervous system, with a subset developing into myelinating OLs. OPCs remain evenly distributed throughout life, providing a population of responsive, multifunctional cells with the capacity to remyelinate after injury. SCs derive from the neural crest, are highly dependent on extrinsic signals, and have plastic differentiation states. SC precursors (SCPs) are produced in early embryonic nerve structures and differentiate into multipotent immature SCs (iSCs), which initiate radial sorting and differentiate into myelinating and non-myelinating SCs. Differentiated SCs retain the capacity to radically change phenotypes in response to external signals, including becoming repair SCs, which drive peripheral regeneration. While several transcription factors and myelin components are common between OLs and SCs, their differentiation mechanisms are highly distinct, owing to their unique lineages and their respective environments. In addition, both OLs and SCs respond to neuronal activity and regulate nervous system output in reciprocal manners, possibly through different pathways. Here, we outline their basic developmental programs, mechanisms regulating their differentiation, and recent advances in the field.

The orphan G protein-coupled receptor GPR149 is a negative regulator of myelination and remyelination

Myelin sheath, formed by oligodendrocytes (OLs) in the central nervous system (CNS) and Schwann cells in periphery, plays a critical role in supporting neuronal functions. OLs, differentiated from oligodendrocyte precursor cells (OPCs), are important for myelination during development and myelin repair in CNS demyelinating disease. To identify mechanisms of myelin development and remyelination after myelin damage is of great clinical interest. Here we show that the orphan G protein-coupled receptor GPR149, enriched in OPCs, negatively regulate OPC to OL differentiation, myelination, as well as remyelination. The expression of GPR149 is downregulated during OPCs differentiation into OLs. GPR149 deficiency does not affect the number of OPCs, but promotes OPC to OL differentiation which results in earlier development of myelin. In cuprizone-induced demyelination model, GPR149 deficiency significantly enhances myelin regeneration. Further study indicates that GPR149 may regulate OL differentiation and myelin formation via MAPK/ERK pathway. Our study suggests that deleting or blocking GPR149 might be an intriguing way to promote myelin repair in demyelinating diseases.

Sulfatide in health and disease. The evaluation of sulfatide in cerebrospinal fluid as a possible biomarker for neurodegeneration

Sulfatide (3-O-sulfogalactosylceramide, SM4) is a glycosphingolipid, highly multifunctional and particularly enriched in the myelin sheath of neurons. The role of sulfatide has been implicated in various biological fields such as the nervous system, immune system, host-pathogen recognition and infection, beta cell function and haemostasis/thrombosis. Thus, alterations in sulfatide metabolism and production are associated with several human diseases such as neurological and immunological disorders and cancers. The unique lipid-rich composition of myelin reflects the importance of lipids in this specific membrane structure. Sulfatide has been shown to be involved in the regulation of oligodendrocyte differentiation and in the maintenance of the myelin sheath by influencing membrane dynamics involving sorting and lateral assembly of myelin proteins as well as ion channels. Sulfatide is furthermore essential for proper formation of the axo-glial junctions at the paranode together with axonal glycosphingolipids. Alterations in sulfatide metabolism are suggested to contribute to myelin deterioration as well as synaptic dysfunction, neurological decline and inflammation observed in different conditions associated with myelin pathology (mouse models and human disorders). Body fluid biomarkers are of importance for clinical diagnostics as well as for patient stratification in clinical trials and treatment monitoring. Cerebrospinal fluid (CSF) is commonly used as an indirect measure of brain metabolism and analysis of CSF sulfatide might provide information regarding whether the lipid disruption observed in neurodegenerative disorders is reflected in this body fluid. In this review, we evaluate the diagnostic utility of CSF sulfatide as a biomarker for neurodegenerative disorders associated with dysmyelination/demyelination by summarising the current literature on this topic. We can conclude that neither CSF sulfatide levels nor individual sulfatide species consistently reflect the lipid disruption observed in many of the demyelinating disorders. One exception is the lysosomal storage disorder metachromatic leukodystrophy, possibly due to the genetically determined accumulation of non-metabolised sulfatide. We also discuss possible explanations as to why myelin pathology in brain tissue is poorly reflected by the CSF sulfatide concentration. The previous suggestion that CSF sulfatide is a marker of myelin damage has thereby been challenged by more recent studies using more sophisticated laboratory techniques for sulfatide analysis as well as improved sample selection criteria due to increased knowledge on disease pathology.

Fibroblast growth factor 10 is a negative regulator of postnatal neurogenesis in the mouse hypothalamus

New neurons are generated in the postnatal rodent hypothalamus, with a subset of tanycytes in the third ventricular (3V) wall serving as neural stem/progenitor cells. However, the precise stem cell niche organization, the intermediate steps and the endogenous regulators of postnatal hypothalamic neurogenesis remain elusive. Quantitative lineage-tracing in vivo revealed that conditional deletion of fibroblast growth factor 10 (Fgf10) from Fgf10-expressing β-tanycytes at postnatal days (P)4-5 results in the generation of significantly more parenchymal cells by P28, composed mostly of ventromedial and dorsomedial neurons and some glial cells, which persist into adulthood. A closer scrutiny in vivo and ex vivo revealed that the 3V wall is not static and is amenable to cell movements. Furthermore, normally β-tanycytes give rise to parenchymal cells via an intermediate population of α-tanycytes with transient amplifying cell characteristics. Loss of Fgf10 temporarily attenuates the amplification of β-tanycytes but also appears to delay the exit of their α-tanycyte descendants from the germinal 3V wall. Our findings suggest that transience of cells through the α-tanycyte domain is a key feature, and Fgf10 is a negative regulator of postnatal hypothalamic neurogenesis.

Summary: Generation of new hypothalamic neurons after birth is a multistep process involving cell division and cell movements that are controlled by Fgf10.

iPSCs from people with MS can differentiate into oligodendrocytes in a homeostatic but not an inflammatory milieu

Multiple sclerosis (MS) is an inflammatory and demyelinating disease of the central nervous system (CNS) that results in variable severities of neurodegeneration. The understanding of MS has been limited by the inaccessibility of the affected cells and the lengthy timeframe of disease development. However, recent advances in stem cell technology have facilitated the bypassing of some of these challenges. Towards gaining a greater understanding of the innate potential of stem cells from people with varying degrees of disability, we generated induced pluripotent stem cells (iPSCs) from peripheral blood mononuclear cells derived from stable and progressive MS patients, and then further differentiated them into oligodendrocyte (OL) lineage cells. We analyzed differentiation under both homeostatic and inflammatory conditions via sustained exposure to low-dose interferon gamma (IFNγ), a prominent cytokine in MS. We found that all iPSC lines differentiated into mature myelinating OLs, but chronic exposure to IFNγ dramatically inhibited differentiation in both MS groups, particularly if exposure was initiated during the pre-progenitor stage. Low-dose IFNγ was not toxic but led to an early upregulation of interferon response genes in OPCs followed by an apparent redirection in lineage commitment from OL to a neuron-like phenotype in a significant portion of the treated cells. Our results reveal that a chronic low-grade inflammatory environment may have profound effects on the efficacy of regenerative therapies.

Matrix metalloproteinases shape the oligodendrocyte (niche) during development and upon demyelination

The oligodendrocyte lineage cell is crucial to proper brain function. During central nervous system development, oligodendrocyte progenitor cells (OPCs) migrate and proliferate to populate the entire brain and spinal cord, and subsequently differentiate into mature oligodendrocytes that wrap neuronal axons in an insulating myelin layer. When damage occurs to the myelin sheath, OPCs are activated and recruited to the demyelinated site, where they differentiate into oligodendrocytes that remyelinate the denuded axons. The process of OPC attraction and differentiation is influenced by a multitude of factors from the cell's niche. Matrix metalloproteinases (MMPs) are powerful and versatile enzymes that do not only degrade extracellular matrix proteins, but also cleave cell surface receptors, growth factors, signaling molecules, proteases and other precursor proteins, leading to their activation or degradation. MMPs are markedly upregulated during brain development and upon demyelinating injury, where their broad functions influence the behavior of neural progenitor cells (NPCs), OPCs and oligodendrocytes. In this review, we focus on the role of MMPs in (re)myelination. We will start out in the developing brain with describing the effects of MMPs on NPCs, OPCs and eventually oligodendrocytes. Then, we will outline their functions in oligodendrocyte process extension and developmental myelination. Finally, we will review their potential role in demyelination, describe their significance in remyelination and discuss the evidence for a role of MMPs in remyelination failure, focusing on multiple sclerosis. In conclusion, MMPs shape the oligodendrocyte (niche) both during development and upon demyelination, and thus are important players in directing the fate and behavior of oligodendrocyte lineage cells throughout their life cycle.

Oligodendrocytes in Development, Myelin Generation and Beyond

Oligodendrocytes are the myelinating cells of the central nervous system (CNS) that are generated from oligodendrocyte progenitor cells (OPC). OPC are distributed throughout the CNS and represent a pool of migratory and proliferative adult progenitor cells that can differentiate into oligodendrocytes. The central function of oligodendrocytes is to generate myelin, which is an extended membrane from the cell that wraps tightly around axons. Due to this energy consuming process and the associated high metabolic turnover oligodendrocytes are vulnerable to cytotoxic and excitotoxic factors. Oligodendrocyte pathology is therefore evident in a range of disorders including multiple sclerosis, schizophrenia and Alzheimer’s disease. Deceased oligodendrocytes can be replenished from the adult OPC pool and lost myelin can be regenerated during remyelination, which can prevent axonal degeneration and can restore function. Cell population studies have recently identified novel immunomodulatory functions of oligodendrocytes, the implications of which, e.g., for diseases with primary oligodendrocyte pathology, are not yet clear. Here, we review the journey of oligodendrocytes from the embryonic stage to their role in homeostasis and their fate in disease. We will also discuss the most common models used to study oligodendrocytes and describe newly discovered functions of oligodendrocytes.

How Do Cells of the Oligodendrocyte Lineage Affect Neuronal Circuits to Influence Motor Function, Memory and Mood?

Oligodendrocyte progenitor cells (OPCs) are immature cells in the central nervous system (CNS) that can rapidly respond to changes within their environment by modulating their proliferation, motility and differentiation. OPCs differentiate into myelinating oligodendrocytes throughout life, and both cell types have been implicated in maintaining and modulating neuronal function to affect motor performance, cognition and emotional state. However, questions remain about the mechanisms employed by OPCs and oligodendrocytes to regulate circuit function, including whether OPCs can only influence circuits through their generation of new oligodendrocytes, or can play other regulatory roles within the CNS. In this review, we detail the molecular and cellular mechanisms that allow OPCs, newborn oligodendrocytes and pre-existing oligodendrocytes to regulate circuit function and ultimately influence behavioral outcomes.

Developmental and Repairing Production of Myelin: The Role of Hedgehog Signaling

Since the discovery of its role as a morphogen directing ventral patterning of the spinal cord, the secreted protein Sonic Hedgehog (Shh) has been implicated in a wide array of events contributing to the development, maintenance and repair of the central nervous system (CNS). One of these events is the generation of oligodendrocytes, the glial cells of the CNS responsible for axon myelination. In embryo, the first oligodendroglial cells arise from the ventral ventricular zone in the developing brain and spinal cord where Shh induces the basic helix-loop-helix transcription factors Olig1 and Olig2 both necessary and sufficient for oligodendrocyte production. Later on, Shh signaling participates in the production of oligodendroglial cells in the dorsal ventricular-subventricular zone in the postnatal forebrain. Finally, the modulation of Hedgehog signaling activity promotes the repair of demyelinated lesions. This mini-review article focuses on the Shh-dependent molecular mechanisms involved in the spatial and temporal control of oligodendrocyte lineage appearance. The apparent intricacy of the roles of two essential components of Shh signaling, Smoothened and Gli1, in the postnatal production of myelin and its regeneration following a demyelinating event is also highlighted. A deeper understanding of the implication of each of the components that regulate oligodendrogenesis and myelination should beneficially influence the therapeutic strategies in the field of myelin diseases.

Rapid Serum-Free Isolation of Oligodendrocyte Progenitor Cells from Adult Rat Spinal Cord

Oligodendrocyte progenitor cells (OPCs) play a pivotal role in both health and disease within the central nervous system, with oligodendrocytes, arising from resident OPCs, being the main myelinating cell type. Disruption in OPC numbers can lead to various deleterious health defects. Numerous studies have described techniques for isolating OPCs to obtain a better understanding of this cell type and to open doors for potential treatments of injury and disease. However, the techniques used in the majority of these studies involve several steps and are time consuming, with current culture protocols using serum and embryonic or postnatal cortical tissue as a source of isolation. We present a primary culture method for the direct isolation of functional adult rat OPCs, identified by neuron-glial antigen 2 (NG2) and platelet derived growth factor receptor alpha (PDGFrα) expression, which can be obtained from the adult spinal cord. Our method uses a simple serum-free cocktail of 3 growth factors - FGF₂, PDGF_AA, and IGF-I, to expand adult rat OPCs in vitro to 96% purity. Cultured cells can be expanded for at least 10 passages with very little manipulation and without losing their phenotypic progenitor cell properties, as shown by immunocytochemistry and RT-PCR. Cultured adult rat OPCs also maintain their ability to differentiate into GalC positive cells when incubated with factors known to stimulate their differentiation. This new isolation method provides a new source of easily accessible adult stem cells and a powerful tool for their expansion in vitro for studies aimed at central nervous system repair.

Multiple roles of Sonic Hedgehog in the developing human cortex are suggested by its widespread distribution

Sonic Hedgehog (Shh) plays an instrumental role in brain development, fine-tuning processes such as cell proliferation, patterning, and fate specification. Although, mutations in the SHH pathway in humans are associated with various neurodevelopmental disorders, ranging from holoprosencephaly to schizophrenia, its expression pattern in the developing human brain is not well established. We now determined the previously not reported wide expression of SHH in the human fetal cerebral cortex during most of the gestation period (10–40 gestational weeks). This spatiotemporal distribution puts Shh in a position to influence the fundamental processes involved in corticogenesis. SHH expression increased during development, shifting from progenitor cells in the proliferative zones to neurons, both glutamatergic and GABAergic, and astrocytes in upper cortical compartments. Importantly, the expression of its downstream effectors and complementary receptors revealed evolutionary differences in SHH-pathway gene expression between humans and rodents.

Hedgehog: A Key Signaling in the Development of the Oligodendrocyte Lineage

The Hedgehog morphogen aroused an enormous interest since it was characterized as an essential signal for ventral patterning of the spinal cord two decades ago. The pathway is notably implicated in the initial appearance of the progenitors of oligodendrocytes (OPCs), the glial cells of the central nervous system which after maturation are responsible for axon myelination. In accordance with the requirement for Hedgehog signaling in ventral patterning, the earliest identifiable cells in the oligodendrocyte lineage are derived from the ventral ventricular zone of the developing spinal cord and brain. Here, we present the current knowledge about the involvement of Hedgehog signaling in the strict spatial and temporal regulation which characterizes the initiation and progression of the oligodendrocyte lineage. We notably describe the ability of the Hedgehog signaling to tightly orchestrate the appearance of specific combinations of genes in concert with other pathways. We document the molecular mechanisms controlling Hedgehog temporal activity during OPC specification. The contribution of the pathway to aspects of OPC development different from their specification is also highlighted especially in the optic nerve. Finally, we report the data demonstrating that Hedgehog signaling-dependency is not a universal situation for oligodendrocyte generation as evidenced in the dorsal spinal cord in contrast to the dorsal forebrain.

Myelin and oligodendrocyte development in the canine spinal cord

We studied the developmental pattern of oligodendrocyte differentiation and myelin formation in the fetal canine spinal cord from E40 to P0. The pattern of development matches what has been described in the spinal cord of humans, rodents, and many other species. Oligodendrocytes were first found at E40, close to the central canal, with their spread in a tangential manner to the ventral and then lateral columns. Myelin development followed the same pattern but was not seen until E46. A clear subpial zone lacking glial cells and myelin was seen in the lateral column in early development, suggesting that there may also be a radial component of migration of oligodendrocyte progenitor cells (OPCs) from a ventral site. This spatial-temporal developmental pattern seen in wild type matches a delay in myelination of the superficial tracts of the spinal cord seen in a canine myelin mutant, suggesting that the mutation prevents the distribution and differentiation of OPCs at an early, but narrow, window of time during fetal development.

The Role of Sonic Hedgehog in the Specification of Human Cortical Progenitors In Vitro

Impaired sonic hedgehog (Shh) signaling is involved in the pathology of cortical formation found in neuropsychiatric disorders. However, its role in the specification of human cortical progenitors is not known. Here, we report that Shh is expressed in the human developing cortex at mid-gestation by radial glia cells (RGCs) and cortical neurons. We used RGC cultures, established from the dorsal (cortical) telencephalon of human brain at mid-gestation to study the effect of Shh signaling. Cortical RGCs in vitro maintained their regional characteristics, expressed components of Shh signaling, and differentiated into Nkx2.1, Lhx6, and calretinin-positive (CalR(+)) cells, potential cortical interneuron progenitors. Treatment with exogenous Shh increased the pool of Nkx2.1(+) progenitors, decreased Lhx6 expression, and suppressed the generation of CalR(+) cells. The blockade of endogenous Shh signaling increased the number of CalR(+) cells, but did not affect Nkx2.1 expression, implying the existence of parallel Shh-independent pathways for cortical Nkx2.1 regulation. These results support the idea that, during human brain development, Shh plays an important role in the specification of cortical progenitors. Since direct functional studies in humans are limited, the in vitro system that we established here could be of great interest for modeling the development of human cortical progenitors.

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

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

Response of human oligodendrocyte progenitors to growth factors and axon signals

We examined the effects of growth factors and axonal signals on the differentiation of human fetal and adult oligodendrocyte progenitor cells (OPCs) and determined whether these effects translated into enhanced axonal ensheathment. Only small numbers of fetal OPCs grown in defined medium expressed the oligodendroglial lineage markers Olig2 and O4. The combination of platelet-derived growth factor-AA and basic fibroblast growth factor enhanced proliferation of Olig2-positive and O4-positive cells; a combination of brain-derived neurotrophic factor and insulin-like growth factor 1 promoted O4-positive cell differentiation, galactocerebroside expression, and morphological complexity. Coculturing with rodent dorsal root ganglion neurons in defined medium alone enhanced OPC differentiation and myelin basic protein expression. The addition of brain-derived neurotrophic factor/insulin-like growth factor 1 further enhanced differentiation, axonal attachment and ensheathment, and clustering of the contactin-associated protein Caspr and Na+ channels. By contrast, most adult OPCs were O4 positive and Olig2 positive in defined medium; both brain-derived neurotrophic factor/insulin-like growth factor 1 and platelet-derived growth factor-AA/basic fibroblast growth factor promoted their myelin basic protein expression and membrane sheet formation; coculture with dorsal root ganglion neurons further increased myelin basic protein expression. Growth factors also enhanced attachment of adult OPCs to axons, but their capacity to ensheath axons was lower than that of fetal OPCs. These results demonstrate that fetal and adult OPCs show measurable responses to selected growth factors and axon signals that correlate with their capacity for axon ensheathment. The distinct properties of fetal and adult OPCs may be related to differences in their chronological age and initial differentiation states.

Directing human neural stem/precursor cells into oligodendrocytes by overexpression of Olig2 transcription factor

Multipotential neural stem/precursor cells of the central nervous system were extensively studied for their properties of generating myelinating oligodendrocytes both in vitro and in vivo upon engraftment in animal models of myelin disorders, such as leucodystrophy and multiple sclerosis. These studies provided proof-of-principle that efficient myelination can be achieved by cell transplantation. However, one major drawback of cell-based therapy of myelin diseases is the difficulty in generating oligodendrocytes efficiently from human fetal neural stem/precursor cells (hNPC). Here we explored whether overexpression of the basic helix-loop-helix (bHLH) transcription factor Olig2 in fetal hNPC could enhance the generation of oligodendrocytes both in vitro and in vivo. We report that transduction of hNPC with Olig2-encoding lentiviral vectors enhances their commitment toward an oligodendroglial fate. Moreover, Olig2-transduced hNPC, grafted into the dysmyelinated shiverer mouse brain, survived up to 9 weeks, migrated extensively, and differentiated into MBP(+) myelinating oligodendrocytes. In contrast, control hNPC remained at a less mature stage and generated very few myelinating oligodendrocytes. Our study indicates that bHLH transcription factors, such as Olig2, are interesting targets for directing hNPC into myelinating oligodendrocytes.

Novel neuronal proteolipid protein isoforms encoded by the human myelin proteolipid protein 1 gene

The human myelin proteolipid protein 1 gene (hPLP1), which encodes the major structural myelin proteins of the central nervous system (CNS), is classically described as expressed in the oligodendrocytes, the CNS myelinating cells. We identified two new exons in the intron 1 of the hPLP1 gene that lead to the expression of additional mRNA and protein isoforms mainly expressed in neurons instead of oligodendrocytes. Those novel neuronal PLP isoforms are detected as soon as human fetal development and their concomitant expression is specific of the human species. As classical PLP proteins, the novel protein isoforms seem to be addressed to the plasma membrane. These results suggest for the first time that PLP may have functions in humans not only in oligodendrocytes but also in neurons and could be implicated in axono-glial communication. Moreover, this neuronal expression of the hPLP1 gene might explain the neuronal dysfunctions in patients carrying hPLP1 gene mutations.

Targeted overexpression of a golli-myelin basic protein isoform to oligodendrocytes results in aberrant oligodendrocyte maturation and myelination

Recently, several in vitro studies have shown that the golli–myelin basic proteins regulate Ca²⁺ homoeostasis in OPCs (oligodendrocyte precursor cells) and immature OLs (oligodendrocytes), and that a number of the functions of these cells are affected by cellular levels of the golli proteins. To determine the influence of golli in vivo on OL development and myelination, a transgenic mouse was generated in which the golli isoform J37 was overexpressed specifically within OLs and OPCs. The mouse, called JOE (J37-overexpressing), is severely hypomyelinated between birth and postnatal day 50. During this time, it exhibits severe intention tremors that gradually abate at later ages. After postnatal day 50, ultrastructural studies and Northern and Western blot analyses indicate that myelin accumulates in the brain, but never reaches normal levels. Several factors appear to underlie the extensive hypomyelination. In vitro and in vivo experiments indicate that golli overexpression causes a significant delay in OL maturation, with accumulation of significantly greater numbers of pre-myelinating OLs that fail to myelinate axons during the normal myelinating period. Immunohistochemical studies with cell death and myelin markers indicate that JOE OLs undergo a heightened and extended period of cell death and are unable to effectively myelinate until 2 months after birth. The results indicate that increased levels of golli in OPC/OLs delays myelination, causing significant cell death of OLs particularly in white matter tracts. The results provide in vivo evidence for a significant role of the golli proteins in the regulation of maturation of OLs and normal myelination.

Human fetal radial glia cells generate oligodendrocytes in vitro

Limited knowledge about human oligodendrogenesis prompted us to explore the lineage relationship between cortical radial glia (RG) cells and oligodendrocytes (OLs) in the human fetal forebrain. RG cells were isolated from cortical ventricular/subventricular zone and their progeny was followed in vitro. One portion of RG cells differentiated into cells of OL lineage identified by cell-type specific antibodies, including platelet-derived growth factor receptor-alpha (PDGFRalpha), NG2, O4, myelin basic protein, and myelin oligodendrocyte glycoprotein. Moreover, using Cre Lox fate mapping (brain lipid binding protein-Cre/Floxed-yellow fluorescent protein) we established a direct link between RG cells and OL progenitors. In vitro generation of RG-derived O4(+) OL progenitors was enhanced by addition of sonic hedgehog (SHH) and reduced by the SHH inhibitor, cyclopamine, suggesting the role of SHH signaling in this process. In summary, our in vitro experiments revealed that a portion of cortical RG cells isolated from human forebrain at the second trimester of gestation generates OL progenitors and this suggests a role of SHH in this process.

In brain, Axl recruits Grb2 and the p85 regulatory subunit of PI3 kinase; in vitro mutagenesis defines the requisite binding sites for downstream Akt activation

Axl is a receptor tyrosine kinase implicated in cell survival following growth factor withdrawal and other stressors. The binding of Axl's ligand, growth arrest-specific protein 6 (Gas6), results in Axl autophosphorylation, recruitment of signaling molecules, and activation of downstream survival pathways. Pull-down assays and immunoprecipitations using wildtype and mutant Axl transfected cells determined that Axl directly binds growth factor receptor-bound protein 2 (Grb2) at pYVN and the p85 subunit of phosphatidylinositol-3 kinase (PI3 kinase) at two pYXXM sites (pY779 and pY821). Also, p85 can indirectly bind to Axl via an interaction between p85's second proline-rich region and the N-terminal SH3 domain of Grb2. Further, Grb2 and p85 can compete for binding at the pY821VNM site. Gas6-stimulation of Axl-transfected COS7 cells recruited activated PI3 kinase and phosphorylated Akt. An interaction between Axl, p85 and Grb2 was confirmed in brain homogenates, enriched populations of O4+ oligodendrocytes, and O4- flow-through prepared from day 10 mouse brain, indicating that cells with active Gas6/Axl signal through Grb2 and the PI3 kinase/Akt pathways.

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.

The molecular orchestra of the migration of oligodendrocyte precursors during development

During development of the central nervous system (CNS), postmitotic cells (including neurons and myelin-generating cells, the oligodendrocytes) migrate from the germinal areas of the neural tube where they originate to their final destination sites. The migration of neurons during development has been extensively studied and has been the topic of detailed reviews. The migration of oligodendrocyte precursor cells (OPCs) is also an extremely complex and precise event, with a widespread migration of OPCs across many regions to colonize the entire CNS. Different mechanisms have been shown to direct the migration of OPCs, among them contact-mediated mechanisms (adhesion molecules) and long-range cues (chemotropic molecules). This review provides a detailed overview and discussion of the cellular and molecular basis of OPCs migration during development. Because it has been shown that neuronal and oligodendroglial lineages share some of these mechanisms, we briefly summarize similarities and differences between these two types of neural cells. We also summarize the changes in the normal migration of OPCs during development that would be relevant for different neurological diseases (including demyelinating diseases, such as multiple sclerosis, and glial cancers). We also examine the relevance of these migratory properties of the oligondendrocytic cell lineage for the repair of neural damage.

Neural stem cells as a potential source of oligodendrocytes for myelin repair

Neural stem cells (NSCs) are considered to have widespread therapeutic possibilities on account of their ability to provide large numbers of cells whilst retaining multi-potentiality. Application to human demyelinating diseases requires improved understanding of the signalling requirements underlying the generation of oligodendrocytes from NSCs. During development, spinal cord oligodendrocyte precursors (OPCs) originate from the ventral, but not dorsal neuroepithelium due to the regulatory effects of the morphogen Sonic hedgehog (Shh). The developing human spinal cord shows comparable ventral-dorsal gradient of oligodendrocyte differentiation potential to the embryonic rodent spinal cord. In contrast expanded human neural precursors derived from both isolated ventral or dorsal cultures show a reduced capacity to generate oligodendrocytes, whereas comparable rodent cultures demonstrate a marked increase in oligodendrocyte formation by a hedgehog independent pathway. Inter-species difference in the capacity of neural precursors to generate oligodendrocytes emphasises the need for greater study of human derived stem cell populations.

Nogo and Nogo-66 receptor in human and chick: implications for development and regeneration

Antibodies to the myelin protein Nogo increase axonal regrowth after central nervous system injury. We have investigated whether Nogo expression contributes to loss of regenerative potential during development by using chick embryos, which regenerate their spinal cord until embryonic day (E) 13, when myelination begins. We show that Nogo-A and the Nogo receptor (NgR) are developmentally regulated both in chick and human embryos, are first detected at developmental stages when the chick spinal cord regenerates, and are not down-regulated after injury at permissive stages for regeneration. Therefore, expression of Nogo-A and NgR in pre-E13 chick spinal cords is not sufficient to inhibit regeneration. Nogo-A expression in the chick early embryo is primarily observed in axons, whereas NgR is mainly located on neuronal cell bodies, both in spinal cord and eye, and in striated muscle including the heart. With the onset of myelination, there is down-regulation of Nogo-A expression in neurons. Therefore, loss of regenerative potential might be linked to changes in its cellular localization. The possibility that only Nogo expressed in mature oligodendrocytes can exercise inhibitory effects would reconcile the lack of inhibition we observe in developing chick spinal cords before the onset of myelination with evidence from other laboratories on the inhibitory effects of Nogo in mature central nervous system. The distinctive and complementary patterns of Nogo-A and NgR expression and their conservation throughout evolution support the view that Nogo signaling represents a key pathway in nervous system and striated muscle development. Its putative role in target innervation and establishment of neural circuitry is discussed.

Sequence of oligodendrocyte development in the human fetal telencephalon

Oligodendrocytes (OL), cells that myelinate axons in the CNS, differentiate from early to late oligodendrocyte progenitor cells (OPC) to become mature OL. Unlike the case in the rodent brain, myelin formation starts prenatally in the human brain, but the sequence of OL development and the onset of myelination are not well understood. We studied the human fetal forebrain at midgestation (17-23 gestational weeks, g.w.) using OL lineage-specific antibodies and mRNA probes. Early OPC were present in a gradient from the subventricular zone to the cortical plate. Their close apposition to radial glia fibers suggests a possible role of these fibers in OPC migration. Late OPC reached peak density in the subplate layer, whereas multipolar cells with the morphology of mature OL were restricted to the emerging white matter. At 20 g.w., myelinated axons were observed in the diencephalon, but not in the telencephalon, consistent with caudal-to-rostral progression of myelination. Interestingly, in organotypic slice cultures of the same gestational ages, the subventricular zone contained a considerably greater number of the mature OL cells, suggesting the presence of inhibitory signals in vivo. Overall, in addition to considerable similarities with rodents, important differences in temporal and spatial distribution and regulatory signals for OL differentiation exist in the human brain.

Interactions between ID and OLIG proteins mediate the inhibitory effects of BMP4 on oligodendroglial differentiation

Bone morphogenetic protein (BMP) signaling inhibits the generation of oligodendroglia and enhances generation of astrocytes by neural progenitor cells both in vitro and in vivo. This study examined the mechanisms underlying the effects of BMP signaling on glial lineage commitment. Treatment of cultured neural progenitor cells with BMP4 induced expression of all four members of the inhibitor of differentiation (ID) family of helix-loop-helix transcriptional inhibitors and blocked oligodendrocyte (OL) lineage commitment. Overexpression of Id4 or Id2 but not Id1 or Id3 in cultured progenitor cells reproduced both the inhibitory effects of BMP4 treatment on OL lineage commitment and the stimulatory effects on astrogliogenesis. Conversely, decreasing the levels of Id4 mRNA by RNA interference enhanced OL differentiation and inhibited the effects of BMP4 on glial lineage commitment. This suggests that induction of Id4 expression mediates effects of BMP signaling. Bacterial two-hybrid and co-immunoprecipitation studies demonstrated that ID4,and to a lesser extent ID2, complexed with the basic-helix-loop-helix transcription (bHLH) factors OLIG1 and OLIG2, which are required for the generation of OLs. By contrast, ID1 and ID3 did not complex with the OLIG proteins. In addition, the OLIG and ID proteins both interacted with the E2A proteins E12 and E47. Further, exposure of cultured progenitor cells to BMP4 changed the intracellular localization of OLIG1 and OLIG2 from a predominantly nuclear to a predominantly cytoplasmic localization. These observations suggest that the induction of ID4 and ID2, and their sequestration of both OLIG proteins and E2A proteins mediate the inhibitory effects of BMP signaling on OL lineage commitment and contribute to the generation of astrocytes.

Effect of a traditional Chinese herbal medicine, Ren-Shen-Yang-Rong-Tang (Japanese name: Ninjin-Youei-To), on oligodendrocyte precursor cells from aged-rat brain

Understanding of oligodendrocyte precursor cells and their role in the generation of oligodendrocytes in developing and adult rodents has been considered, particularly much less is known about aged-rodent oligodendrocyte precursor cells and their cell lineage. In this present study, we have developed oligodendrocyte cultures from the 30-month-old rat brain and examined whether oligodendrocyte precursor cells can proliferate in vitro. Adult oligodendrocyte precursor cells (O1(-), O4(+)) and oligodendrocytes (O1(+), O4(+)) are present in the cultures of the 30-month-old rat brain. They are also capable of proliferating and differentiating in the cultures. These capabilities increased four- to fivefold, when the aged rats are treated with Ninjin-Youei-To for 3 months in comparison with those of control aged rats. These results suggest that Ninjin-Youei-To has a potential mitotic effect on oligodendrocyte precursor cells in aged-rat brains and may be expected to have a therapeutic effect on brain aging.

Netrin-1 is a chemorepellent for oligodendrocyte precursor cells in the embryonic spinal cord

Netrin-1, secreted by floor plate cells, orients axon extension in relation to the ventral midline of the embryonic spinal cord. Oligodendrocyte precursor (OP) cells are born close to the ventral midline and migrate away from the floor plate. Here we show that OP cells, identified by expression of the platelet-derived growth factor alpha receptor, express the netrin receptors dcc and unc5h1 but do not express netrin-1. Using a microchemotaxis assay, we demonstrate that migrating OPs are repelled by a gradient of netrin-1 in vitro. Furthermore, application of netrin-1 to OPs in vitro triggers retraction of OP processes. In the absence of netrin-1 or Deleted in Colorectal Cancer (DCC) function in vivo, fewer OP cells migrate from the ventral to the dorsal embryonic spinal cord, consistent with netrin-1 acting as a repellent. In addition to their role regulating cell movement, DCC and UNC-5 homologs have been suggested to function as proapoptotic dependence receptors, triggering cell death in the absence of netrin-1. In contrast, we report no evidence of increased OP cell death in vivo or in vitro in the absence of either netrin-1 or DCC. These findings indicate that netrin-1 is a repellent cue for migrating OPs in the embryonic spinal cord.

Identification of molecular subtypes of glioblastoma by gene expression profiling

Epidermal growth factor receptor (EGFR) overexpression occurs in nearly 50% of cases of glioblastoma (GBM), but its clinical and biological implications are not well understood. We have used Affymetrix high-density oligonucleotide arrays to demonstrate that EGFR-overexpressing GBMs (EGFR+) have a distinct global gene transcriptional profile. We show that the expression of 90 genes can distinguish EGFR+ from EGFR nonexpressing (EGFR-) GBMs, including a number of genes known to act as growth/survival factors for GBMs. We have also uncovered two additional novel molecular subtypes of GBMs, one of which is characterized by coordinate upregulation of contiguous genes on chromosome 12q13-15 and expression of both astrocytic and oligodendroglial genes. These results define distinct molecular subtypes of GBMs that may be important in disease stratification, and in the discovery and assessment of GBM treatment strategies.

Remyelinating strategies for the treatment of multiple sclerosis

Demyelination is the pathological hallmark of multiple sclerosis (MS) lesions. The concept of remyelination has gained acceptance in recent years, but naturally occurring remyelination is incomplete. To improve repair processes, a number of strategies have been explored experimentally and clinical trials are being carried out. In principle, remyelination can be achieved by either promoting endogenous repair mechanisms or by providing an exogenous source of myelinating cells via transplantation. Both approaches have been successful in animal models of demyelination. Besides, many studies have elucidated principal mechanisms of oligodendrocyte biology and remyelination in the central nervous system (CNS). This progress in knowledge also allowed for more specific interventions. First clinical trials to enhance endogenous remyelination have been performed, unfortunately with disappointingly negative results. This illustrates that experimental data cannot be easily transferred to human disease, and more detailed knowledge on the regulatory mechanisms of remyelination in MS is required. Recently, the first MS patient received a transplant of autologous Schwann cells. Many other cell types are being studied experimentally, including stem cells. Despite the ethical problems associated with an embryonic cell source, new developments in stem cell biology indicate that adult stem cells or bone marrow-derived cells may substitute for embryonic cells in the future. In this review, we describe the current views on oligodendrocyte biology, myelination and remyelination, and focus on recent developments leading to reconstructing, remyelinating strategies in MS.

Bone morphogenetic proteins negatively control oligodendrocyte precursor specification in the chick spinal cord

In the vertebrate spinal cord, oligodendrocytes originate from a restricted region of the ventral neuroepithelium. This ventral localisation of oligodendrocyte precursors (OLPs) depends on the inductive influence of sonic hedgehog (Shh) secreted by ventral midline cells. We have investigated whether the ventral restriction of OLP specification might also depend on inhibiting signals mediated by bone morphogenetic proteins (BMPs). BMPs invariably and markedly inhibited oligodendrocyte development in ventral neural tissue both in vitro and in vivo. Conversely, in vivo ablation of the dorsal most part of the chick spinal cord or inactivation of BMP signalling using grafts of noggin-producing cells promoted the appearance of neuroepithelial OLPs dorsal to their normal domain of emergence, showing that endogenous BMPs contribute to the inhibition of oligodendrocyte development in the spinal cord. BMPs were able to oppose the Shh-mediated induction of OLPs in spinal cord neuroepithelial explants dissected before oligodendrocyte induction,suggesting that BMPs may repress OLP specification by interfering with Shh signalling in vivo. Strikingly, among the transcription factors involved in OLP specification, BMP treatment strongly inhibited the expression of Olig2 but not of Nkx2.2, suggesting that BMP-mediated inhibition of oligodendrogenesis is controlled through the repression of the former transcription factor. Altogether, our data show that oligodendrogenesis is not only regulated by ventral inductive signals such as Shh, but also by dorsal inhibiting signals including BMP factors. They suggest that the dorsoventral position of OLPs depends on a tightly regulated balance between Shh and BMP activities.

ATP-binding cassette transporter ABCA2 (ABC2) expression in the developing spinal cord and PNS during myelination

We examined developmental characteristics of the ATP-binding cassette transporter ABCA2 (or ABC2) -expressing cells in rat spinal cord and peripheral nerves. In adult spinal cord, ABCA2 immunoreactivity was detected in lysosome-like organelles of mature oligodendrocyte cell bodies, and a single specific band was detected by Western blot analysis. In postnatal developing spinal cord, ABCA2 immunolabeling was first detected in a small number of cells restricted to the ventral marginal area and the dorsal funiculus at birth (P0). ABCA2-positive cells were co-immunolabeled by O4, a marker for late progenitor and immature oligodendrocytes. At the same time, myelin basic protein was apparent in the same restricted regions. The number of ABCA2 and O4 co-immunolabeled cells increased quickly in both dorsal and ventral regions from P2 and reached a peak at P8. After transient expression from P0 to P8, O4 labeling in white matter tracts decreased and disappeared. In contrast, ABCA2-positive oligodendrocytes persisted in gray and white matter throughout the spinal cord into adulthood. These data suggest a role for the ABCA2 transporter in maturation of oligodendrocyte lineage cells and the onset of myelination in the central nervous system. In addition, ABCA2 immunoreactivity was detected in the ciliated region of the ependyma in the central canal from early postnatal development. ABCA2 immunoreactivity was also detected in the Schwann cell lineage in developing spinal nerves and in adult trigeminal and sciatic nerves. ABCA2 was also expressed in numerous undetermined cells distributed in para-nerve connective tissues and nerve sheaths throughout early postnatal development. These data indicate multiple levels of involvement for ABCA2 in nervous system development especially with strong evidence for a role in myelination.

Sonic hedgehog is a potent inducer of rat oligodendrocyte development from cortical precursors in vitro

Sonic Hedgehog (Shh) induces oligodendrocyte development in the ventral neural tube and telencephalon but its role in oligodendrocyte generation in dorsal telencephalon is debated. Transcripts for Shh and its receptor complex were detected in subventricular zone and neocortex from E17 to birth. As Shh is not yet expressed in E15 neocortex, we grew E15 cortical precursors (CP) into neurospheres in the presence of recombinant Octyl-Shh (O-Shh). After sphere adhesion and removal of O-Shh, enhanced neurite outgrowth and cell migration were already observed at 3 h. Three days after O-Shh treatment, oligodendrocyte progenitors (OP) emerged and continued to increase in number for 7 days while the ratio of neuronal cells decreased compared to control. Shh selectively triggered mitosis of OP but not neuronal progenitors and enhanced growth of neonatal OP. Thus Shh in E15-17 embryonic neocortex can signal CP to adopt an oligodendrocyte fate and favors expansion of this lineage.

Identification of Golli and myelin basic proteins in human brain during early development

The myelin basic protein gene (Mbp) encodes for the major myelin structural proteins and it is included in the Golli-Mbp gene complex. Previously, we observed MBP-like proteins in the human central nervous system (CNS) at developmental stages preceding myelination. In an effort to distinguish between Golli (HOG5 and HOG7) and MBP mRNAs and to determine their spatiotemporal distribution, we performed in situ hybridization using two human Golli specific probes: one corresponding to exon 5a absent from all MBP transcripts, and the other corresponding to exon 5c specific for HOG5. HOG7 transcript was observed first, in 5 gestational week-old embryos, whereas both Golli transcripts were detected at 6-7 weeks gestation in the proliferative zones of the entire CNS. Golli proteins immunoreactivity was observed in microglia and early neurons of the developing telencephalon. During midgestation (17-22 weeks gestation), at the onset of myelination, MBP and Golli mRNAs were observed in the telencephalic subventricular zone and occasionally in the future cerebral cortex. Developmental expression of the human Golli-Mbp indicates that the two Golli proteins have different onset of expression, distribution and possibly function. These results support the hypothesis that at least one of them, HOG7, may be involved in the regulation of early neurogenesis, while both may have additional, still undefined function at the onset of myelination.

Sonic hedgehog-dependent emergence of oligodendrocytes in the telencephalon: evidence for a source of oligodendrocytes in the olfactory bulb that is independent of PDGFRα signaling

Most studies on the origin of oligodendrocyte lineage have been performed in the spinal cord. By contrast, molecular mechanisms that regulate the appearance of the oligodendroglial lineage in the brain have not yet attracted much attention. We provide evidence for three distinct sources of oligodendrocytes in the mouse telencephalon. In addition to two subpallial ventricular foci, the anterior entopeduncular area and the medial ganglionic eminence, the rostral telencephalon also gives rise to oligodendrocytes. We show that oligodendrocytes in the olfactory bulb are generated within the rostral pallium from ventricular progenitors characterized by the expression of Plp. We provide evidence that these Plp oligodendrocyte progenitors do not depend on signal transduction mediated by platelet-derived growth factor receptors (PDGFRs), and therefore propose that they belong to a different lineage than the PDGFRα-expressing progenitors. Moreover, induction of oligodendrocytes in the telencephalon is dependent on sonic hedgehog signaling, as in the spinal cord. In all these telencephalic ventricular territories, oligodendrocyte progenitors were detected at about the same developmental stage as in the spinal cord. However, both in vivo and in vitro, the differentiation into O4-positive pre-oligodendrocytes was postponed by 4-5 days in the telencephalon in comparison with the spinal cord. This delay between determination and differentiation appears to be intrinsic to telencephalic oligodendrocytes, as it was not shortened by diffusible or cell-cell contact factors present in the spinal cord.

TNF alpha promotes proliferation of oligodendrocyte progenitors and remyelination

Here we used mice lacking tumor necrosis factor-alpha (TNF alpha) and its associated receptors to study a model of demyelination and remyelination in which these events could be carefully controlled using a toxin, cuprizone. Unexpectedly, the lack of TNF alpha led to a significant delay in remyelination as assessed by histology, immunohistochemistry for myelin proteins and electron microscopy coupled with morphometric analysis. Failure of repair correlated with a reduction in the pool of proliferating oligodendrocyte progenitors (bromodeoxyuridine-labeled NG2(+) cells) followed by a reduction in the number of mature oligodendrocytes. Analysis of mice lacking TNF receptor 1 (TNFR1) or TNFR2 indicated that TNFR2, not TNFR1, is critical to oligodendrocyte regeneration. This unexpected reparative role for TNF alpha in the CNS is important for understanding oligodendrocyte regeneration/proliferation, nerve remyelination and the design of new therapeutics for demyelinating diseases.

Differences between T-cell reactivities to major myelin protein-derived peptides in opticospinal and conventional forms of multiple sclerosis and healthy controls

In Japanese, susceptibility to the conventional form of multiple sclerosis (C-MS) is associated with the HLA-DRB1*1501-DRB5*0101 haplotype while susceptibility to the opticospinal form of MS (OS-MS) is associated with HLA-DPA1*0202-DPB1*0501. To clarify the characteristics of T cells autoreactive to myelin proteins in each MS subtype, we established T-cell lines reactive to such myelin antigens as myelin basic protein (MBP), proteolipid protein (PLP) and myelin oligodendrocyte glycoprotein (MOG) from 5 of 10 OS-MS patients, 6 of 11 C-MS patients and 7 of 13 healthy controls (HCs), and T-cell epitopes and their restriction molecules were determined. We found that (a) intermolecular epitope spreading was found to be significantly more frequent in MS patients than in HCs (P=0.0128), (b) intramolecular epitope spreading also tended to occur more frequently in MS patients than in HCs (P=0.0584), (c) in OS-MS, HLA-DR-restricted and MOG-autoreactive T cells were more frequently established as compared with those reactive to MBP or PLP epitopes and (d) in C-MS, HLA-DQ-restricted and PLP-autoreactive T cells dominated those autoreactive to MBP or MOG epitopes. A DPB1*0501-restricted MBP-reactive T-cell clone from a patient with OS-MS provided evidence that the first HLA class II anchor amino acid of peptide bound to disease-susceptible DP5 molecule was distinct from that for the DR2 molecule. Taken together, these differences in specificities of myelin-autoreactive T cells between C-MS and OS-MS as well as the difference in the anchor motif of the binding peptides between each MS subtype-susceptible HLA class II molecule may contribute to the development of distinct clinical phenotypes.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

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

Developmental pattern of expression of the alpha chemokine stromal cell-derived factor 1 in the rat central nervous system

Stromal cell-derived factor 1 (SDF-1) is an alpha-chemokine that stimulates migration of haematopoietic progenitor cells and development of the immune system. SDF-1 is also abundantly and selectively expressed in the developing and mature CNS, as we show here. At embryonic day 15, SDF-1 transcripts were detected in the germinal periventricular zone and in the deep layer of the forming cerebral cortex. At birth, granule cells in the cerebellum and glial cells of the olfactory bulb outer layer showed an SDF-1 in situ hybridization signal that decreased progressively within the next 2 weeks. In other regions such as cortex, thalamus and hippocampus, SDF-1 transcripts detected at birth progressively increased in abundance during the postnatal period. SDF-1 protein was identified by immunoblot and/or immunocytochemistry in most brain regions where these transcripts were detected. SDF-1 was selectively localized in some thalamic nuclei and neurons of the fifth cortical layer as well as in pontine and brainstem nuclei which relay the nociceptive response. The presence of SDF-1 transcripts in cerebellar granule cells was correlated with their migration from the external to the inner granular layers with disappearance of the signal when migration was completed. In contrast, SDF1 mRNA signal increased during formation of the hippocampal dentate gyrus and stayed high in this region throughout life. The selective and regulated expression of SDF-1 in these regions suggests a role in precursor migration, neurogenesis and, possibly, synaptogenesis. Thus this alpha chemokine may be as essential to nervous system function as it is to the immune system.

Comparison of neural precursor cell fate in second trimester human brain and spinal cord

Neural transplantation holds promise for the treatment of traumatic brain and spinal cord injury by replacing lost cellular elements as well as repairing neural damage. Fetal human stem cells derived from central nervous system (CNS) tissue are potential transplantable sources for all cell types found in the mature human nervous system including neurons, astrocytes and oligodendroglia. Although nearly all areas of the fetal human neuraxis contain undifferentiated neural precursor cells, the phenotypic fate of the daughter cells might vary from one region to another during a specific developmental period. The purpose of this study was to compare the various cell types derived from neural precursors cultured from second trimester fetal human brain and spinal cord. To this end, brains (n = 8) and spinal cords (n = 8) of 15-24 week fetuses were dissociated and grown in culture medium supplemented with epidermal growth factor (EGF), basic fibroblast growth factor (FGF) and leukemia inhibitory factor (LIF). The proliferating precursor cells from both brain and spinal cord grew as spherical masses that were plated on laminin-coated dishes after seven days in culture. During the next 5-7 days, the cells that emerged from these spheres were fixed and processed for immunocytochemistry. Brain derived spheres gave rise to cells expressing antigens specific for neurons (MAP-2ab and neuron specific-intermediate filaments), astrocytes (GFAP) and oligodendrocytes (A007). In contrast, cells that emerged from spinal cord derived spheres were only immunoreactive for GFAP. These data suggest that neuroepithelial precursor cells from different CNS regions, although similar in their responsiveness to proliferative growth factors, might differ in their ability to generate different cell types in the adult CNS.

Late oligodendrocyte progenitors coincide with the developmental window of vulnerability for human perinatal white matter injury

Hypoxic-ischemic injury to the periventricular cerebral white matter [periventricular leukomalacia (PVL)] results in cerebral palsy and is the leading cause of brain injury in premature infants. The principal feature of PVL is a chronic disturbance of myelination and suggests that oligodendrocyte (OL) lineage progression is disrupted by ischemic injury. We determined the OL lineage stages at risk for injury during the developmental window of vulnerability for PVL (23-32 weeks, postconceptional age). In 26 normal control autopsy human brains, OL lineage progression was defined in parietal white matter, a region of predilection for PVL. Three successive OL stages, the late OL progenitor, the immature OL, and the mature OL, were characterized between 18 and 41 weeks with anti-NG2 proteoglycan, O4, O1, and anti-myelin basic protein (anti-MBP) antibodies. NG2+O4+ late OL progenitors were the predominant stage throughout the latter half of gestation. Between 18 and 27 weeks, O4+O1+ immature OLs were a minor population (9.9 +/- 2.1% of total OLs; n = 9). Between 28 and 41 weeks, an increase in immature OLs to 30.9 +/- 2.1% of total OLs (n = 9) was accompanied by a progressive increase in MBP+ myelin sheaths that were restricted to the periventricular white matter. The developmental window of high risk for PVL thus precedes the onset of myelination and identifies the late OL progenitor as the major potential target. Moreover, the decline in incidence of PVL at approximately 32 weeks coincides with the onset of myelination in the periventricular white matter and suggests that the risk for PVL is related to the presence of late OL progenitors in the periventricular white matter.

Mature oligodendrocyte apoptosis precedes IGF-1 production and oligodendrocyte progenitor accumulation and differentiation during demyelination/remyelination

We have documented changes in the oligodendrocyte population during demyelinating insult to the adult CNS. Feeding of cuprizone to adult mice led to apoptotic death of mature oligodendrocytes followed by profound demyelination of the corpus callosum. A regenerative response was initiated even during active demyelination. Oligodendrocyte progenitors have begun to proliferate and then accumulate within the lesion. Many of these cells may have migrated from the sub-ventricular zone and fornix before their accumulation in the demyelinating corpus callosum. The accumulation of differentiating oligodendrocyte progenitors was followed closely by the reappearance of mature oligodendrocytes and remyelination. Interestingly, an increase in IGF-1 mRNA was detected at Week 3 through Week 7, suggesting potential involvement in remyelination. Other factors, however, such as PDGF, NT3, FGF, jagged, and notch remained unchanged. These results suggest that the mature oligodendroglial population depleted by apoptosis is replaced by a newly formed oligodendroglial population derived from progenitors; these accumulate and seem to differentiate during remyelination.

Tracing human oligodendroglial development in vitro

Human neural precursor cell cultures (neurospheres) were established from fetal brain tissues of 15-20 gestation weeks and propagated for over a year in the presence of epidermal growth factor, basic fibroblast growth factor and leukemia inhibitory factor. Neurospheres were differentiated without the presence of above growth factors to follow the development of oligodendroglia. Oligodendroglial progenitors, identified by their bipolar morphology and expression of platelet-derived growth factor receptor-alpha (PDGFRalpha), emerged from spheres as early as 1 DIV; O4+ cells with bipolar to multipolar processes were observed at 3 DIV whereas O1+ multiprocess-bearing oligodendroglia did not appear until 5-7 DIV. They further differentiated to myelin basic protein-expressing oligodendrocytes after 2-3 weeks in culture. Thus, human oligodendroglial maturation in vitro follows the same pathway as rat cells but takes twice as long as their rodent counterparts. Bromodeoxyuridine incorporation indicated that PDGFRalpha-expressing cells but not O4+ oligodendroglia proliferated. More oligodendroglial progenitors incorporated BrdU and more O4+ cells survived when they were in contact with neurons and astrocytes than when they developed beyond the astrocyte layer. In addition, oligodendroglia on astrocytes had a complex process branching whereas those growing beyond astrocyte layer often formed membrane sheaths. Thus the survival, proliferation and maturation of oligodendroglia are influenced by other cell types.