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

Induction of Olig2 precursors by FGF involves BMP signalling blockade at the Smad level

During normal development oligodendrocyte precursors (OPCs) are generated in the ventral spinal cord in response to Sonic hedgehog (Shh) signalling. There is also a second, late wave of oligodendrogenesis in the dorsal spinal cord independent of Shh activity. Two signalling pathways, controlled by bone morphogenetic protein and fibroblast growth factor (FGF), are active players in dorsal spinal cord specification. In particular, BMP signalling from the roof plate has a crucial role in setting up dorsal neural identity and its inhibition is sufficient to generate OPCs both in vitro and in vivo. In contrast, FGF signalling can induce OPC production from dorsal spinal cord cultures in vitro. In this study, we examined the cross-talk between mitogen-activated protein kinase (MAPK) and BMP signalling in embryonic dorsal spinal cord cultures at the SMAD1/5/8 (SMAD1) transcription factor level, the main effectors of BMP activity. We have previously shown that FGF2 treatment of neural precursor cells (NPCs) derived from rat E14 dorsal spinal cord is sufficient to generate OPCs in vitro. Utilising the same system, we now show that FGF prevents BMP-induced nuclear localisation of SMAD1-phosphorylated at the C-terminus (C-term-pSMAD1). This nuclear exclusion of C-term-pSMAD1 is dependent on MAPK activity and correlates with OLIG2 upregulation, the obligate transcription factor for oligodendrogenesis. Furthermore, inhibition of the MAPK pathway abolishes OLIG2 expression. We also show that SMAD4, which acts as a common partner for receptor-regulated Smads including SMAD1, associates with a Smad binding site in the Olig2 promoter and dissociates from it upon differentiation. Taken together, these results suggest that FGF can promote OPC generation from embryonic NPCs by counteracting BMP signalling at the Smad1 transcription factor level and that Smad-containing transcriptional complexes may be involved in direct regulation of the Olig2 promoter.

BMP signaling through BMPRIA in astrocytes is essential for proper cerebral angiogenesis and formation of the blood-brain-barrier

Bone morphogenetic protein (BMP) signaling is involved in differentiation of neural precursor cells into astrocytes, but its contribution to angiogenesis is not well characterized. This study examines the role of BMP signaling through BMP type IA receptor (BMPRIA) in early neural development using a conditional knockout mouse model, in which Bmpr1a is selectively disrupted in telencephalic neural stem cells. The conditional mutant mice show a significant increase in the number of cerebral blood vessels and the level of vascular endothelial growth factor (VEGF) is significantly upregulated in the mutant astrocytes. The mutant mice also show leakage of immunoglobulin around cerebral microvessels in neonatal mice, suggesting a defect in formation of the blood-brain-barrier. In addition, astrocytic endfeet fail to encircle cortical blood vessels in the mutant mice. These results suggest that BMPRIA signaling in astrocytes regulates the expression of VEGF for proper cerebrovascular angiogenesis and has a role on in the formation of the blood-brain-barrier.

Glial precursor cell transplantation therapy for neurotrauma and multiple sclerosis

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

Specification of CNS glia from neural stem cells in the embryonic neuroepithelium

All the neurons and glial cells of the central nervous system are generated from the neuroepithelial cells in the walls of the embryonic neural tube, the 'embryonic neural stem cells'. The stem cells seem to be equivalent to the so-called 'radial glial cells', which for many years had been regarded as a specialized type of glial cell. These radial cells generate different classes of neurons in a position-dependent manner. They then switch to producing glial cells (oligodendrocytes and astrocytes). It is not known what drives the neuron-glial switch, although downregulation of pro-neural basic helix-loop-helix transcription factors is one important step. This drives the stem cells from a neurogenic towards a gliogenic mode. The stem cells then choose between developing as oligodendrocytes or astrocytes, of which there might be intrinsically different subclasses. This review focuses on the different extracellular signals and intracellular responses that influence glial generation and the choice between oligodendrocyte and astrocyte fates.

Bone morphogenetic proteins 4, 6, and 7 are up-regulated in mouse spinal cord during experimental autoimmune encephalomyelitis

Although spontaneous remyelination occurs in multiple sclerosis (MS), the extent of myelin repair is often inadequate to restore normal function. Oligodendrocyte precursors remaining in nonremyelinating MS plaques may be restricted by an inhibitory signal. Bone morphogenetic proteins (BMPs) have been implicated as repressors of oligodendrocyte development and inducers of astrogliogenesis. We hypothesized that BMPs are up-regulated in MS lesions and play a role in demyelination and astrogliosis. We examined expression of BMPs in an animal model of MS, chronic experimental autoimmune encephalomyelitis (EAE) induced by the myelin oligodendrocyte glycoprotein (MOG) peptide in C57BL/6 mice. By 14 days postimmunization, compared to those of control mice, the lumbar spinal cords of MOG-peptide EAE mice demonstrated prominent astrogliosis, infiltration of inflammatory cells, and disrupted expression of myelin proteins. Quantitative RT-PCR showed that expression of BMP4, BMP6, and BMP7 mRNA increased 2- to 4-fold in the lumbar spinal cords of animals with symptomatic EAE versus in vehicle-treated and untreated controls on days 14, 21, and 42 postimmunization. BMP2 mRNA expression was not altered. BMP4 mRNA was much more abundant in the spinal cords of all animals than was mRNA encoding BMP2, BMP6, and BMP7. Immunoblot analysis confirmed the increased expression of BMP4 in the EAE animals. Immunohistochemistry revealed increased BMP4 immunoreactivity in areas of inflammation in MOG-peptide EAE animals. BMP4 labeling was mostly limited to macrophages but was sometimes associated with astrocytes and oligodendrocytes. These results indicate that members of the BMP family are differentially expressed in adult spinal cord and are up-regulated during EAE. (c) 2007 Wiley-Liss, Inc.

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

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

Transcriptional control of oligodendrogenesis

Oligodendrocytes (OGs) assemble the myelin sheath around axons in the central nervous system. Specification of cells into the OG lineage is largely the result of interplay between bone morphogenetic protein, sonic hedgehog and Notch signaling pathways, which regulate expression of transcription factors (TFs) dictating spatial and temporal aspects of oligodendrogenesis. Many of these TFs and others then direct OG development through to a mature myelinating OG. Here we describe signaling pathways and TFs that are inductive, inhibitory, and/or permissive to OG specification and maturation. We develop a basic transcriptional network and identify similarities and differences between regulation of oligodendrogenesis in the spinal cord and brain.

Evidence that nucleocytoplasmic Olig2 translocation mediates brain-injury-induced differentiation of glial precursors to astrocytes

The mechanisms by which neural and glial progenitor cells in the adult brain respond to tissue injury are unknown. We studied the responses of these cells to stab wound injury in rats and in two transgenic mouse models in which Y/GFP is driven either by Sox2 (a neural stem cell marker) or by Talpha-1 (which marks newly born neurons). The response of neural progenitors was low in all nonneurogenic regions, and no neurogenesis occurred at the injury site. Glial progenitors expressing Olig2 and NG2 showed the greatest response. The appearance of these progenitors preceded the appearance of reactive astrocytes. Surprisingly, we found evidence of the translocation of the transcription factor Olig2 into cytoplasm in the first week after injury, a mechanism that is known to mediate the differentiation of astrocytes during brain development. Translocation of Olig2, down-regulation of NG2, and increased glial fibrillary acidic protein expression were recapitulated in vitro after exposure of glial progenitors to serum components or bone morphogentic protein by up-regulation of Notch-1. The glial differentiation and Olig2 translocation could be blocked by inhibition of Notch-1 with the gamma-secretase inhibitor DAPT. Together, these data indicate that the prompt maturation of numerous Olig2(+) glial progenitors to astrocytes underlies the repair process after a traumatic injury. In contrast, neural stem cells and neuronal progenitor cells appear to play only a minor role in the injured adult CNS.

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

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

Nkx2.2 Expression in Differentiation of Oligodendrocyte Precursor Cells and Inhibitory Factors for Differentiation of Oligodendrocytes after Traumatic Spinal Cord Injury

Because successful remyelination does not occur following traumatic spinal cord injury, patients suffer from long tract dysfunction. However, demyelination is followed by remyelination in early multiple sclerosis. Oligodendrocyte precursor cells constitute a large cell population in the adult mammalian central nervous system. We demonstrated the proliferation, migration, and differentiation of oligodendrocyte precursor cells in chemically induced demyelination, a model for multiple sclerosis, and reported that Nkx2.2 expression may regulate oligodendrocyte precursor cell differentiation, making it a key factor in the differentiation. To investigate what factors disturb remyelination in spinal cord injury, we examined the oligodendrocyte precursor cell proliferation and differentiation, and the expression of Nkx2.2 using contusive injury in rats as a model for traumatic spinal cord injury. This study showed that oligodendrocyte precursor cells proliferated after contusive injury but did not subsequently differentiate. The number of Nkx2.2-positive oligodendrocyte precursor cells did not significantly change in the tissue surrounding the lesion. Within the demyelinating lesion, the peak of Nkx2.2-positive oligodendrocyte precursor cell was delayed, and its level was lower than in the chemical models. No clearly recognizable oligodendrocytes were found in the demyelinating lesion throughout the observation period. To assess whether environmental changes differ between these two models, mRNA expressions of various cytokines were evaluated and compared. IL-1beta and IL-6 mRNA significantly increased in the contusion-induced injury model, 6 h after the injury. These results suggest that environmental factors such as cytokines may affect Nkx2.2 expression or oligodendrocyte precursor cell differentiation in the contusion-induced spinal cord injury model.

Bone morphogenetic protein signaling in stem cells − one signal, many consequences

Bone morphogenetic protein (BMP) signals play key roles throughout embryology, from the earliest patterning events, via tissue specification, through organ development and again in germ cell differentiation. While both input and the transducer molecules are rather well studied, the final outcome of a BMP signal is basically unpredictable and differs enormously between previously studied cell types. As already suggested by their name, BMPs exhibit most of their (known) functions on stem cells and precursor cells, usually driving them into various types of differentiation or death. In this minireview, some prime examples of BMP effects on several very different stem-cell types are discussed.

BMP signaling mutant mice exhibit glial cell maturation defects

Bone morphogenetic proteins have been implicated in the development of oligodendrocytes and astrocytes, however, a role for endogenous BMP signaling in glial development has not been demonstrated in a genetic model. Using mice in which signaling via type I BMP receptors Bmpr1a and Bmpr1b have been inactivated in the neural tube, we demonstrate that BMP signaling contributes to the maturation of glial cells in vivo. At P0, mutant mice exhibited a 25-40% decrease in GFAP+ or S100beta+ astrocytes in the cervical spinal cord. The number of oligodendrocyte precursors and the timing of their emergence was unchanged in the mutant mice compared to the normals, however myelin protein expression and mature oligodendrocyte numbers were significantly reduced. These data indicate that BMP signaling promotes the generation of astrocytes and mature, myelinating oligodendrocytes in vivo but does not affect oligodendrocyte precursor development, thus suggesting tight regulation of BMP signaling to ensure proper gliogenesis.

Bone morphogenetic protein-mediated modulation of lineage diversification during neural differentiation of embryonic stem cells

Embryonic stem cells (ES cells) can give rise to a broad spectrum of neural cell types. The biomedical application of ES cells will require detailed knowledge on the role of individual factors modulating fate specification during in vitro differentiation. Bone morphogenetic proteins (BMPs) are known to exert a multitude of diverse differentiation effects during embryonic development. Here, we show that exposure to BMP2 at distinct stages of neural ES cell differentiation can be used to promote specific cell lineages. During early ES cell differentiation, BMP2-mediated inhibition of neuroectodermal differentiation is associated with an increase in mesoderm and smooth muscle differentiation. In fibroblast growth factor 2-expanded ES cell-derived neural precursors, BMP2 supports the generation of neural crest phenotypes, and, within the neuronal lineage, promotes distinct subtypes of peripheral neurons, including cholinergic and autonomic phenotypes. BMP2 also exerts a density-dependent promotion of astrocyte differentiation at the expense of oligodendrocyte formation. Experiments involving inhibition of the serine threonine kinase FRAP support the notion that these effects are mediated via the JAK/STAT pathway. The preservation of diverse developmental BMP2 effects in differentiating ES cell cultures provides interesting prospects for the enrichment of distinct neural phenotypes in vitro.

Bone morphogenetic protein signalling and vertebrate nervous system development

Transforming growth factor-beta (TGFbeta) signalling, particularly signalling from the bone morphogenetic protein (BMP) members of this protein family, is crucial for the development of both the central and peripheral nervous systems in vertebrates. Experimental embryology and genetics performed in a range of organisms are providing insights into how BMPs establish the neural tissue and control the types and numbers of neurons formed. These studies also highlight the interactions between different developmental signals that are necessary to form a functional nervous system. The challenges ahead will be to uncover functions of TGFbeta signalling in later stages of CNS development, as well as to determine possible associations with neurological diseases.

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

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

NG2 and Olig2 expression provides evidence for phenotypic deregulation of cultured central nervous system and peripheral nervous system neural precursor cells

Neural stem cells cultured with fibroblast growth factor 2 (FGF2)/epidermal growth factor (EGF) generate clonal expansions called neurospheres (NS), which are widely used for therapy in animal models. However, their cellular composition is still poorly defined. Here, we report that NS derived from several embryonic and adult central nervous system (CNS) regions are composed mainly of remarkable cells coexpressing radial glia markers (BLBP, RC2, GLAST), oligodendrogenic/neurogenic factors (Mash1, Olig2, Nkx2.2), and markers that in vivo are typical of the oligodendrocyte lineage (NG2, A2B5, PDGFR-alpha). On NS differentiation, the latter remain mostly expressed in neurons, together with Olig2 and Mash1. Using cytometry, we show that in growing NS the small population of multipotential self-renewing NS-forming cells are A2B5(+) and NG2(+). Additionally, we demonstrate that these NS-forming cells in the embryonic spinal cord were initially NG2(-) and rapidly acquired NG2 in vitro. NG2 and Olig2 were found to be rapidly induced by cell culture conditions in spinal cord neural precursor cells. Olig2 expression was also induced in astrocytes and embryonic peripheral nervous system (PNS) cells in culture after EGF/FGF treatment. These data provide new evidence for profound phenotypic modifications in CNS and PNS neural precursor cells induced by culture conditions.

Basic fibroblast growth factor endows dorsal telencephalic neural progenitors with the ability to differentiate into oligodendrocytes but not gamma-aminobutyric acidergic neurons

Basic fibroblast growth factor (bFGF) is commonly used to enrich and maintain neural stem cells in vitro. Olig2 is an essential transcription factor for oligodendrocyte lineage specification and is expressed predominantly in ventral neuroepithelial cells in the medial and lateral ganglionic eminence (GE), where oligodendrocyte progenitors originate. Here we report significant induction of Olig2 expression in dorsal neuroepithelium-derived cells cultured in the presence of bFGF, in which Olig2-expressing cells were initially negligible. Among Olig2-expressing cells appearing after a 5-day treatment with bFGF, 99.8% coexpressed nestin. There was no significant difference in proliferation or apoptosis in dorsal and ventral neuroepithelial cultures in the presence of bFGF, suggesting that bFGF induces ectopic expression of Olig2 in dorsal "cortical" neuroepithelial cells. Similarly, expression of Mash1, another ventral neuroepithelial cell marker gene, was also induced in cultured dorsal neuroepithelial cells in the presence of bFGF. Conversely, in this culture, expression of dorsal neuroepithelial cell markers, such as Neurogenin1, Neurogenin2, Pax6, and Emx2, was down-regulated. These results suggested a possible ventralizing activity of bFGF. In fact, bFGF-treated dorsal neuroepithelial cells acquired the potential to generate O4-positive oligodendrocytes with efficacy comparable to that observed with GE-derived cells. In marked contrast, bFGF did not enable dorsal neuroepithelial cells to generate gamma-aminobutyric acid (GABA) neurons, which normally develop only from GE in vivo. Thus, bFGF endows dorsal telencephalic neural progenitors with the ability to differentiate into oligodendrocytes but not GABAergic neurons, suggesting the presence of different mechanisms governing specification of dorsoventral cell identities of neuronal and glial cell lineages.

Exogenous and Fibroblast Growth Factor 2/Epidermal Growth Factor-Regulated Endogenous Cytokines Regulate Neural Precursor Cell Growth and Differentiation

Neurospheres (NSs) are clonal cellular aggregates composed of neural stem cells and progenitors. A comprehensive description of their proliferation and differentiation regulation is an essential prerequisite for their use in biotherapies. Cytokines are essential molecules regulating cell precursor fate. Using a gene-array strategy, we conducted a descriptive and functional analysis of endogenous cytokines and receptors expressed by spinal cord-derived NSs during their growth or their differentiation into neuronal and glial cells. NSs were found to express approximately 100 receptor subunits and cytokine/secreted developmental factors. Several angiogenic factors and receptors that could mediate neural precursor cell-endothelial cell relationships were detected. Among them, receptor B for endothelins was highly expressed, and endothelins were found to increase NS growth. In contrast, NSs express receptors for ciliary neurotrophic factor (CNTF), bone morphogenetic protein (BMP), interferon (IFN)-gamma, or tumor necrosis factor (TNF)-alpha, which, when added in the growth phase, led to a dramatic growth reduction followed by a reduction or a loss of oligodendrocyte formation on differentiation. In addition, NSs synthesize fibroblast growth factor 2/epidermal growth factor (FGF2/EGF)-regulated endogenous cytokines that participate in their growth and differentiation. Notably, BMP-7 and CNTF were expressed during expansion, but upon differentiation there was a remarkable switch from BMP-7 to BMP-4 and -6 and a sharp increase of CNTF. Reintroduction of growth factors reverses the BMP expression profile, indicating growth factor-BMP cross-regulations. The role of endogenous CNTF was investigated by deriving NSs from CNTF knockout mice. These NSs have an increased growth rate associated with reduction of apoptosis and generate astrocytes with a reduced glial fibulary acidic protein (GFAP) content. These results demonstrate the combined role of endogenous and exogenous cytokines in neural precursor cell growth and differentiation.

Coordinate expression of BMP-2, BMP receptors and Noggin in normal mouse spine

The purpose of this study was to determine the localization of bone morphogenetic protein-2 (BMP-2), BMP receptors (BMPRs) and Noggin in mouse spinal tissues. The coordinate expression of these positive and negative regulators of BMP signaling may elucidate regulatory mechanisms for bone induction in the spine. Whole spines from 3-week-old mice were used and the spatial expression profiles of BMP-2, BMPR-1a, -1b, -2 and Noggin were examined using in situ hybridization. BMP-2, BMPR-1b and -2 were observed in bone marrow cells in the vertebrae, chondrocytes, hyaline cartilage cells and fibrous cells in the intervertebral discs and neurons of the spinal cord in the entire spine. BMPR-1a was also observed in these cells, but only in the cervical spine. Noggin was expressed in bone marrow cells in the vertebrae, chondrocytes and hyaline cartilage cells and fibrous cells in the intervertebral discs in the entire spine and in neurons in the spinal cord in the cervical and thoracic regions. Noggin was also expressed in the anterior longitudinal, posterior longitudinal and yellow ligaments in the cervical spine, and in the fibrous cells in the anterior longitudinal and yellow ligaments of the lumbar spine.

Intrinsic and extrinsic inhibition of oligodendrocyte development by rat retina

Cell patterning in the vertebrate CNS reflects the combination of localized cell induction, migration and differentiation. A striking example of patterning is the myelination of visual system. In many species, retinal ganglion cell axons are myelinated in the optic nerve but are unmyelinated in the retina. Here, we confirm that rat and mouse retina lack oligodendrocytes and their precursors and identify multiple mechanisms that might contribute to their absence. Soluble cues from embryonic retina inhibit the induction of oligodendrocytes from neural stem cells and their differentiation from optic nerve precursors. This inhibition is mediated by retinal-derived BMPs. During development BMPs are expressed in the retina and addition of the BMP antagonist Noggin reversed retinal inhibition of oligodendrocyte development. The lack of retinal oligodendrocytes does not simply reflect expression of BMPs, since no oligodendrocytes or their precursors developed when embryonic retinal cells were grown in the presence of Noggin and/or inductive cues such as Shh and IGF-1. Similarly, injection of Noggin into the postnatal rat eye failed to induce oligodendrocyte differentiation. These data combined with the proposed inhibition of OPC migration by molecules selectively expressed at the nerve retina junction suggest that multiple mechanisms combine to suppress retinal myelination during development.

Oligodendrocyte wars

Oligodendrocyte precursors first arise in a restricted ventral part of the embryonic spinal cord and migrate laterally and dorsally from there. Later, secondary sources develop in the dorsal cord. Normally, the ventrally-derived precursors compete with and suppress their dorsal counterparts. There are also ventral and dorsal sources in the forebrain, but here the more dorsal precursors prevail and the ventral-most lineage is eliminated during postnatal life. How do the different populations compete and what is the outcome of the competition? Do different embryonic origins signify different functional subgroups of oligodendrocyte?

Wnt signaling controls the timing of oligodendrocyte development in the spinal cord

During spinal cord development, oligodendrocytes are generated from a restricted region of the ventral ventricular zone and then spread out into the entire spinal cord. These events are controlled by graded inductive and repressive signals derived from a local organizing center. Sonic hedgehog was identified as an essential ventral factor for oligodendrocyte lineage specification, whereas the dorsal cue was less clear. In this study, Wnt proteins were identified as the dorsal factors that directly inhibit oligodendrocyte development. Wnt signaling through a canonical beta-catenin pathway prevents its differentiation from progenitor to an immature state. Addition of rmFz-8/Fc, a Wnt antagonist, increased the number of immature oligodendrocytes in the spinal cord explant culture, demonstrating that endogenous Wnt signaling controls oligodendrocyte development.

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.

Generation of oligodendrocyte precursor cells from mouse dorsal spinal cord independent of Nkx6 regulation and Shh signaling

In the developing spinal cord, early progenitor cells of the oligodendrocyte lineage are induced in the motor neuron progenitor (pMN) domain of the ventral neuroepithelium by the ventral midline signal Sonic hedgehog (Shh). The ventral generation of oligodendrocytes requires Nkx6-regulated expression of the bHLH gene Olig2 in this domain. In the absence of Nkx6 genes or Shh signaling, the initial expression of Olig2 in the pMN domain is completely abolished. In this study, we provide the in vivo evidence for a late phase of Olig gene expression independent of Nkx6 and Shh gene activities and reveal a brief second wave of oligodendrogenesis in the dorsal spinal cord. In addition, we provide genetic evidence that oligodendrogenesis can occur in the absence of hedgehog receptor Smoothened, which is essential for all hedgehog signaling.

Dorsally Derived Oligodendrocytes Come of Age

While oligodendrocyte precursors have previously been localized to the ventral midline, two papers in this issue of Neuron (Cai et al. and Vallstedt et al.) provide compelling evidence for a second dorsal origin of oligodendrocyte precursors in the spinal cord and hindbrain. These cells share expression of the PDGFalpha receptor and Olig2 with their ventral counterparts but differ in the requirement for Nkx6 and hedgehog signaling.

Multiple Dorsoventral Origins of Oligodendrocyte Generation in the Spinal Cord and Hindbrain

Studies have indicated that oligodendrocytes in the spinal cord originate from a ventral progenitor domain defined by expression of the oligodendrocyte-determining bHLH proteins Olig1 and Olig2. Here, we provide evidence that progenitors in the dorsal spinal cord and hindbrain also produce oligodendrocytes and that the specification of these cells may result from a dorsal evasion of BMP signaling over time. Moreover, we show that the generation of ventral oligodendrocytes in the spinal cord depends on Nkx6.1 and Nkx6.2 function, while these homeodomain proteins in the anterior hindbrain instead suppress oligodendrocyte specification. The opposing roles for Nkx6 proteins in the spinal cord and hindbrain, in turn, appear to reflect that oligodendrocytes are produced by distinct ventral progenitor domains at these axial levels. Based on these findings, we propose that oligodendrocytes derive from several distinct positional origins and that the activation of Olig1/2 at different positions is controlled by distinct genetic programs.

Msx1 disruption leads to diencephalon defects and hydrocephalus

We have analyzed the expression of the Msx1 gene in the developing mouse brain and examined the brain phenotype in homozygotes. Msx1 is expressed in every cerebral vesicle throughout development, particularly in neuroepithelia, such as those of the fimbria and the medulla. Timing analysis suggests that Msx1(nLacZ) cells delaminate and migrate radially from these epithelia, mainly at embryonic days 14-16, while immunohistochemistry studies reveal that some of the beta-galactosidase migrating cells are oligodendrocytes or astrocytes. Our results suggest that the Msx1 neuroepithelia of fimbria and medulla may be a source of glial precursors. The Msx1 mutants display severe hydrocephalus at birth, while the subcommissural organ, the habenula, and the posterior commissure fail to develop correctly. No label was detected in the mutant subcommissural organ using a specific antibody against Reissner's fiber. Besides, the fasciculus retroflexus deviates close to the subcommissural organ, while the paraventricular thalamic nucleus shows histological disorganization. Our results implicate the Msx1 gene in the differentiation of the subcommissural organ cells and posterior commissure and that Msx1 protein may play a role in the pathfinding and bundling of the fasciculus retroflexus and in the structural arrangement of the paraventricular thalamic nucleus.

IGF-I instructs multipotent adult neural progenitor cells to become oligodendrocytes

Adult multipotent neural progenitor cells can differentiate into neurons, astrocytes, and oligodendrocytes in the mammalian central nervous system, but the molecular mechanisms that control their differentiation are not yet well understood. Insulin-like growth factor I (IGF-I) can promote the differentiation of cells already committed to an oligodendroglial lineage during development. However, it is unclear whether IGF-I affects multipotent neural progenitor cells. Here, we show that IGF-I stimulates the differentiation of multipotent adult rat hippocampus-derived neural progenitor cells into oligodendrocytes. Modeling analysis indicates that the actions of IGF-I are instructive. Oligodendrocyte differentiation by IGF-I appears to be mediated through an inhibition of bone morphogenetic protein signaling. Furthermore, overexpression of IGF-I in the hippocampus leads to an increase in oligodendrocyte markers. These data demonstrate the existence of a single molecule, IGF-I, that can influence the fate choice of multipotent adult neural progenitor cells to an oligodendroglial lineage.

Negative regulatory effect of an oligodendrocytic bHLH factor OLIG2 on the astrocytic differentiation pathway

In the developing vertebrate nervous system, multipotent neural stem cells produce both neurons and glia. OLIG2 is a basic helix-loop-helix transcription factor that plays critical roles in oligodendrocyte and motor neuron development; however, its role in astrocytic development remains elusive. In this study, we analyzed an effect of OLIG2 on cytokine-induced astrocytic differentiation from mouse telencephalic neuroepithelial cells. We show that the presence of OLIG2 protein leads to inhibition of the promoter activation of astrocyte-specific glial fibrillary acidic protein gene. We found that OLIG2 abolishes complex formation between a transcriptional coactivator p300 and a transcription factor, signal transducer and activator of transcription 3 (STAT3), which is activated by astrocytic differentiation-inducing cytokines, such as leukemia inhibitory factor (LIF). The enforced expression of OLIG2 in neuroepithelial cells inhibits the LIF-induced astrocytic differentiation. We also show that the OLIG2 protein in the nuclei of neural precursor cells disappears in accordance with astrocytic differentiation during culture with LIF. Together, these results reveal a novel molecular function of OLIG2 on the astrocyte development. Cell Death and Differentiation (2004) 11, 196-202. doi:10.1038/sj.cdd.4401332 Published online 24 October 2003

Converse control of oligodendrocyte and astrocyte lineage development by Sonic hedgehog in the chick spinal cord

In the developing spinal cord, oligodendrocyte progenitors (OLPs) originate from the ventral neuroepithelium and the specification of this lineage depends on the inductive activity of Sonic hedgehog (Shh) produced by ventral midline cells. On the other hand, it has been shown that OLP identity is acquired by the coexpression of the transcription factors olig2 and nkx2.2. Although initially expressed in adjacent nonoverlapping domains of the ventral neuroepithelium, these transcription factors become coexpressed in the pMN domain at the time of OLP specification through dorsal extension of the Nkx2.2 domain. Here we show that Shh is sufficient to promote the coexpression of Olig2 and Nkx2.2 in neuroepithelial cells. In addition, Shh activity is necessary for this coexpression since blocking Shh signalling totally abolishes Olig2 expression and impedes dorsal extension of Nkx2.2. Although Shh at these stages affects neuroepithelial cell proliferation, the dorsal extension of the Nkx2.2 domain is not due to progenitor proliferation but to repatterning of the ventral neuroepithelium. Finally, Shh not only stimulates OLP specification but also simultaneously restricts the ventral extension of the astrocyte progenitor (AP) domain and reduces astrocyte development. We propose that specification of distinct glial lineages is the result of a choice that depends on Shh signalling.

Oligodendrocyte maturation is inhibited by bone morphogenetic protein

Mature oligodendrocytes myelinate axons in the CNS. The development of the myelin sheath is dependent on the proper maturation of oligodendrocytes from precursors cells, a spatially restricted process that is regulated by inductive and repressive cues. Several members of the bone morphogenetic protein family (BMP2 and 4) have been implicated as repressors of oligodendrocyte development in vitro by shifting oligodendrocyte precursors into the astrocyte lineage. We now report on a second role of BMPs in oligodendrocyte development, regulation of myelin protein expression in immature oligodendrocytes. Purified immature rodent oligodendrocytes treated with BMP4 maintained galactocerebroside (GalC) expression, whereas the expression of three key myelin proteins, proteolipid protein (PLP), myelin basic protein (MBP), and 2'-3'-cyclic nucleotide 3'-phosphodiesterase (CNP), was severely decreased. Paradoxically, BMP-treated oligodendrocytes show increased process extension and complexity, normally a feature of oligodendrocyte maturation. We also investigated whether BMP4 could inhibit myelin protein expression in an E 12.5 mouse explant culture of cervical spinal cord and hindbrain that maintains the in vivo cellular relationships and architecture. Beads soaked in BMP protein implanted into these explants inhibited the expression of myelin proteins, proteolipid protein, and myelin-associated glycoprotein (MAG), in the local area surrounding the bead. Since these explants also contained precursors cells, expression of galactocerebroside and O4, an oligodendrocyte marker, were also decreased by BMP treatment but to a much lesser degree than the myelin markers. Together, these data indicate that BMPs have multiple roles in oligodendrocyte development. At earlier stages, they affect cell lineage decisions and at later stages, they inhibit cell specialization.

Patterning of spinal cord oligodendrocyte development by dorsally derived BMP4

Oligodendrocyte precursors (OPCs) initially arise in the motor neuron domain of the ventral ventricular zone of the developing spinal cord. After dispersal throughout gray and white matter, OPCs differentiate in a characteristic ventral to dorsal sequence. The spatial localization of OPC induction is in part a result of both positive local sonic hedgehog signaling and dorsally derived inhibitory cues. One component of dorsal inhibitory signals seems to be members of the transforming growth factor beta (TGFbeta) superfamily such as the bone morphogenetic proteins (BMPs). We show that during the initial appearance and subsequent maturation of OPCs, BMP4 was expressed specifically in the dorsal midline and its expression was correlated spatially and temporally with phospho-Smad 1+, BMP4-responsive cells. Implantation of sonic hedgehog (Shh)-coated beads adjacent to dorsal spinal cord in Xenopus embryos induced ectopic dorsal OPCs whereas BMP4-coated beads inhibited OPC appearance. More importantly, blocking endogenous dorsal BMP4 with anti-BMP4-coated beads locally induced ectopic OPCs. Similar results were obtained using soluble ligands on slice preparations of rodent spinal cord in vitro. In dissociated cell cultures of embryonic rat spinal cord, Shh and BMP4 had antagonistic effects on OPC development and the sensitivity of oligodendrocyte lineage cells to BMP4 increased with maturation. These data suggest that BMP4 contributes to the pattern of spinal cord oligodendrogenesis by regulating both induction and maturation of spinal cord OPCs.

Mechanisms of glial development

Glial cells comprise most of the non-neuronal cells of the brain and peripheral nervous system, and include the myelin-forming oligodendrocytes and Schwann cells, radial glia and astrocytes. Their functions are diverse and include almost every aspect of nervous system function, from the birth and death of cells to the migrations and cell-cell interactions that connect and integrate the working elements of the nervous system. Recent studies have provided exciting insights into the mechanisms that drive the conversion into a glial cell and the developmental signals that guide the behavior of these multifunctional cells. An emerging theme is the so-called glial lineage being more diverse and more plastic than was previously thought. Here, we highlight some recent insights into glial development.

Emerging roles for bone morphogenetic proteins in central nervous system glial biology

Bone morphogenetic proteins, members of the TGFbeta superfamily have been implicated in a variety of roles in the developing and mature nervous system. These divergent functions are a reflection of the closely defined spatial and temporal expression of BMPs in the CNS, and the potential interactions of the BMP signaling pathway with the STAT and MAP kinase pathways. In this review we discuss the roles of BMPs in early patterning of the CNS, determination of neural cell fate, and regulation of oligodendrocyte maturation during CNS development. Additional functions for members of the TGFbeta superfamily in CNS injury responses are emerging suggesting these molecules represent useful targets for manipulating neural responses to CNS insults.

Getting a GR(i)P on oligodendrocyte development

One of the most extensively studied of mammalian cells is the oligodendrocyte, the myelin-forming cell of the central nervous system. The ancestry and development of this cell have been studied with every approach utilized by developmental biologists. Such detailed efforts have the potential of providing paradigms of relevance to those interested in analyzing the ancestry and development of any cell type. One of the striking features of studies on the development of oligodendrocytes is that different analytical approaches have led to strikingly different theoretical views regarding the ancestry of these cells. On one extreme is the hypothesis that the steps leading to the generation of oligodendrocytes begin with the generation of a glial-restricted precursor (GRP) cell from neuroepithelial stem cells. GRP cells are thought to be capable of giving rise to all glial cells (including oligodendrocytes and multiple astrocyte populations), but not to neurons, a process that appears to require progression through further stages of greater lineage restriction. On the other extreme is the hypothesis that oligodendrocytes are derived from a precursor cell that generates only motor neurons and oligodendrocytes, with astrocytes being generated through a separate lineage. In this review, we critically consider the various contributions to understanding the ancestry of oligodendrocytes, with particular attention to the respective merits of the GRP cell vs. the motor neuron-oligodendrocyte precursor (MNOP) cell hypothesis. We draw the conclusion that, at present, the strengths of the GRP cell hypothesis outweigh those of the MNOP hypothesis and other hypotheses suggesting oligodendrocytes are developmentally more related to motor neurons than to astrocytes. Moreover, it is clear from existing data that, following the period of motor neuron generation, the major glial precursor cell in the embryonic spinal cord is the GRP cell, and that multiple previous studies on the earliest stages of oligodendrocyte generation in the developing spinal cord have been focused on a differentiation stage of GRP cells.

Understanding Glial Differentiation in Vertebrate Nervous System Development

Recent progresses in molecular and developmental biology provide us a good idea how differentiation of glial cells in the vertebrate nervous system is regulated. Combinations of positional cues such as secreted proteins and cell-intrinsic mechanisms such as transcription factors are essential for the regulation of astrocyte and oligodendrocyte differentiation from the neural epithelium. In contrast, regulatory mechanisms of glial differentiation from neural crest-derived cells in the peripheral nervous system are less understood. However, recent studies suggest that, at least in part, the peripheral gliogenesis is regulated by mechanisms, such as Notch signaling, that are also important for the gliogenesis in the developing central nervous system.

From cells to circuits: development of the zebrafish spinal cord

The ability of an animal to carry out its normal behavioral repertoire requires generation of an enormous diversity of neurons and glia. The relative simplicity of the spinal cord makes this an especially attractive part of the nervous system for addressing questions about the development of vertebrate neural specification and function. The last decade has witnessed an explosion in our understanding of spinal cord development and the functional interactions among spinal cord neurons and glia. Cellular, genetic, molecular, physiological and behavioral studies in zebrafish have all been important in providing insights into questions that remained unanswered by studies from other vertebrate model organisms. This is the case because many zebrafish spinal neurons can be individually identified and followed over time in living embryos and larvae. In this review, we discuss what is currently known about the cellular, genetic and molecular mechanisms involved in specifying distinct cell types in the zebrafish spinal cord and how these cells establish the functional circuitry that mediates particular behaviors. We start by describing the early signals and morphogenetic movements that form the nervous system, and in particular, the spinal cord. We then provide an overview of the cell types within the spinal cord and describe how they are specified and patterned. We begin ventrally with floor plate and proceed dorsally, through motoneurons and oligodendrocytes, interneurons, astrocytes and radial glia, spinal sensory neurons and neural crest. We next describe axon pathfinding of spinal neurons. Finally, we discuss the roles of particular spinal cord neurons in specific behaviors.