Netrin 1 mediates spinal cord oligodendrocyte precursor dispersal

In spinal cord, oligodendrocyte precursors that give rise to myelin-forming cells originate in a restricted domain of the ventral ventricular zone. During development, these cells migrate widely throughout the spinal cord. Netrin 1 is expressed at the ventral ventricular zone during oligodendrocyte precursors emigration, and, in vitro, netrin 1 acts as chemorepellent and antagonizes platelet-derived growth factor (PDGF) chemoattraction. Oligodendrocyte precursors express the netrin receptors DCC and UNC5 and function-blocking anti-DCC antibody inhibits chemorepulsion of ventral spinal cord explants and netrin-secreting cells. In spinal cord slice preparations, addition of function-blocking anti-DCC antibody or netrin 1 dramatically inhibits oligodendrocyte precursor migration from the ventral ventricular zone. These data indicate the initial dispersal of oligodendrocyte precursors from their localized origin is guided by a chemorepellent response to netrin 1.     

Notch1 control of oligodendrocyte differentiation in the spinal cord

We have selectively inhibited Notch1 signaling in oligodendrocyte precursors (OPCs) using the Cre/loxP system in transgenic mice to investigate the role of Notch1 in oligodendrocyte (OL) development and differentiation. Early development of OPCs appeared normal in the spinal cord. However, at embryonic day 17.5, premature OL differentiation was observed and ectopic immature OLs were present in the gray matter. At birth, OL apoptosis was strongly increased in Notch1 mutant animals. Premature OL differentiation was also observed in the cerebrum, indicating that Notch1 is required for the correct spatial and temporal regulation of OL differentiation in various regions of the central nervous system. These findings establish a widespread function of Notch1 in the late steps of mammalian OPC development in vivo.     

Intermediate filament proteins in the developing chick spinal cord

The distribution of different intermediate filament (IF) proteins in the embryonic chick spinal cord was examined at several stages of development using immunohistochemical techniques, analytic gel electrophoresis, and electron microscopy. We have found that: (1) the fibroblast-type IF protein (vimentin) is present in virtually all of the replicating neuroepithelial cells of the early neural tube, as well as in radial glia, astrocytes, and Schwann cells in later stages of development; (2) the fibroblast-type IF protein is not detectable in definitive neurons; (3) the neurofilament proteins are first detectable in postmitotic neuroblasts at about the time of initial axon formation and they are restricted to neurons; (4) the astrocyte-type IF protein (glial fibrillary acidic protein) is in definitive astrocytes, but not in radial glia; (5) the prekeratin proteins are restricted to cells of the leptomeninges; and (6) the muscle-type IF protein (desmin) is restricted to vascular tissue in and around the developing spinal cord. These findings suggest that the fibroblast-type IF protein is the only IF protein in the early neuroepithelial cells and that the progeny of these cells will follow one of three different patterns of IF protein expression: (1) continued expression of only the fibroblast-type IF protein (radial glia); (2) expression of both the fibroblast-type IF protein and the astrocyte-type IF protein (astrocytes); or (3) expression of only the neurofilament proteins (neurons).     

Prostacyclin promotes oligodendrocyte precursor recruitment and remyelination after spinal cord demyelination

Adult oligodendrocyte precursor cells (OPCs) are located adjacent to demyelinated lesion and contribute to myelin repair. The crucial step in remyelination is the migration of OPCs to the demyelinated area; however, the mechanism of OPC migration remains to be fully elucidated. Here we show that prostacyclin (prostaglandin I₂, PGI₂) promotes OPC migration, thereby promoting remyelination and functional recovery in mice after demyelination induced by injecting lysophosphatidylcholine (LPC) into the spinal cord. Prostacyclin analogs enhanced OPC migration via a protein kinase A (PKA)-dependent mechanism, and prostacyclin synthase expression was increased in the spinal cord after LPC injection. Notably, pharmacological inhibition of prostacyclin receptor (IP receptor) impaired remyelination and motor recovery, whereas the administration of a prostacyclin analog promoted remyelination and motor recovery after LPC injection. Our results suggest that prostacyclin could be a key molecule for facilitating the migration of OPCs that are essential for repairing demyelinated areas, and it may be useful in treating disorders characterized by demyelination.     

Impact of transcription factor Sox8 on oligodendrocyte specification in the mouse embryonic spinal cord

The myelin-forming oligodendrocytes of the mouse embryonic spinal cord express the three group E Sox proteins Sox8, Sox9, and Sox10. They require Sox9 for their specification from neuroepithelial cells of the ventricular zone and Sox10 for their terminal differentiation and myelination. Here, we show that during oligodendrocyte development, Sox8 is expressed after Sox9, but before Sox10. Loss of Sox8 did not impair oligodendrocyte specification by itself, but enhanced the Sox9-dependent defect. Oligodendrocyte progenitors were still generated in the Sox9-deficient spinal cord, albeit at 20-fold lower rates than in the wildtype. Combined loss of Sox8 and Sox9, in contrast, led to a near complete loss of oligodendrocytes. Other cell types such as ventricular zone cells and radial glia remained unaffected in their numbers as well as their rates of proliferation and apoptosis. Oligodendrocyte development thus relies on the differential contribution of all three group E Sox proteins at various phases.     

Spatial and temporal regulation of ventral spinal cord precursor specification by Hedgehog signaling

Graded Hedgehog (Hh) signaling patterns the spinal cord dorsoventral axis by inducing and positioning distinct precursor domains, each of which gives rise to a different type of neuron. These domains also generate glial cells, but the full range of cell types that any one precursor population produces and the mechanisms that diversify cell fate are unknown. By fate mapping and clonal analysis in zebrafish, we show that individual ventral precursor cells that express olig2 can form motoneurons, interneurons and oligodendrocytes. However, olig2+ precursors are not developmentally equivalent, but instead produce subsets of progeny cells in a spatially and temporally biased manner. Using genetic and pharmacological manipulations, we provide evidence that these biases emerge from Hh acting over time to set, maintain, subdivide and enlarge the olig2+ precursor domain and subsequently specify oligodendrocyte development. Our studies show that spatial and temporal differences in Hh signaling within a common population of neural precursors can contribute to cell fate diversification.     

MicroRNA-26b inhibits oligodendrocyte precursor cell differentiation by targeting adrenomedullin in spinal cord injury

Oligodendrocyte precursor cells (OPCs) serve as a reservoir of newborn oligodendrocytes (OLs) in pathological and homeostatic conditions. After spinal cord injury (SCI), OPCs are activated to generate myelinating OLs, contributing to remyelination and functional recovery; however, the underlying molecular mechanisms remain unclear. Here, microRNA-26b (miR-26b) expression in the spinal cord tissues of SCI rats was examined by real-time polymerase chain reaction analysis. The influences of miR-26b on locomotor recovery following SCI were assessed utilizing Basso, Beattie, and Bresnahan (BBB) scores. The effects of miR-26b on OPC differentiation were explored using immunofluorescence and western blot analyses in vitro and in vivo. The potential targets that are modulated by miR-26b were identified by bioinformatics, luciferase reporter assays, and western blot analyses. The effects of adrenomedullin (ADM) on OPC differentiation were explored in vitro using immunofluorescence and western blot analyses. We demonstrated that miR-26b was significantly downregulated after SCI. BBB scores showed that miR-26b exacerbated the locomotor function deficits induced by SCI. In vitro, miR-26b inhibited the differentiation of primary rat OPCs. In vivo, miR-26b suppressed OPC differentiation in SCI rats. Bioinformatics analyses and experimental detection revealed that miR-26b directly targeted ADM in OPCs. In addition, knockdown of ADM suppressed the differentiation of primary rat OPCs. Our study provides evidence that ADM may mediate miR-26b-inhibited OPC differentiation in SCI.     

Dorsal spinal cord inhibits oligodendrocyte development

Oligodendrocytes are the myelinating cells of the mammalian central nervous system. In the mouse spinal cord, oligodendrocytes are generated from strictly restricted regions of the ventral ventricular zone. To investigate how they originate from these specific regions, we used an explant culture system of the E12 mouse cervical spinal cord and hindbrain. In this culture system O4(+) cells were first detected along the ventral midline of the explant and were subsequently expanded to the dorsal region similar to in vivo. When we cultured the ventral and dorsal spinal cords separately, a robust increase in the number of O4(+) cells was observed in the ventral fragment. The number of both progenitor cells and mature cells also increased in the ventral fragment. This phenomenon suggests the presence of inhibitory factor for oligodendrocyte development from dorsal spinal cord. BMP4, a strong candidate for this factor that is secreted from the dorsal spinal cord, did not affect oligodendrocyte development. Previous studies demonstrated that signals from the notochord and ventral spinal cord, such as sonic hedgehog and neuregulin, promote the ventral region-specific development of oligodendrocytes. Our present study demonstrates that the dorsal spinal cord negatively regulates oligodendrocyte development.     

Stage-specific effects of bone morphogenetic proteins on the oligodendrocyte lineage

Oligodendrocyte maturation is regulated by multiple secreted factors present in the brain during critical stages of development. Whereas most of these factors promote oligodendrocyte proliferation and survival, members of the bone morphogenetic protein family (BMPs) recently have been shown to inhibit oligodendrocyte differentiation in vitro. Oligodendrocyte precursors treated with BMPs differentiate to the astrocyte lineage. Given that cells at various stages of the oligodendrocyte lineage have distinct responses to growth factors, we hypothesized that the response to BMP would be stage-specific. Using highly purified, stage-specific cultures, we found that BMP has distinct effects on cultured oligodendrocyte preprogenitors, precursors, and mature oligodendrocytes. Oligodendrocyte preprogenitors (PSA-NCAM+, A2B5-) treated with BMP2 or BMP4 developed a novel astrocyte phenotype characterized by a morphological change and expression of glial fibrillary acidic protein (GFAP) but little glutamine synthetase expression and no labeling with A2B5 antibody. In contrast, treating oligodendrocyte precursors with BMPs resulted in the accumulation of cells with the traditional type 2 astrocyte phenotype (GFAP+, A2B5+). However, many of the cells with an astrocytic morphology did not express GFAP or glutamine synthetase unless thyroid hormone was present in the medium. The addition of fibroblast growth factor along with BMP to either oligodendrocyte preprogenitor or the oligodendrocyte precursor cells inhibited the switch to the astrocyte lineage, whereas platelet-derived growth factor addition had no effect. Treatment of mature oligodendrocytes with BMP elicited no change in morphology or expression of GFAP. These data suggest that as cells progress through the oligodendrocyte lineage, they show developmentally restricted responses to the BMPs.     

Symposium 8: Regulation of Oligodendrocyte Development. Guidance of oligodendrocyte precursor migration in the developing spinal cord

Oligodendrocyte precursors arise in restricted regions of the developing neuroepithelium due to local signals that include sonic hedgehog. In the spinal cord the founder cells of the oligodendrocyte lineage develop in a specific domain of the ventral ventricular zone. These cells or their progeny subsequently migrate long distances to populate the entire spinal cord and myelinate axons in the peripheral presumptive white matter. The majority of migration in the oligodendrocyte lineage is accomplished by immature precursors, which then stop, proliferate and differentiate in the appropriate location. Several distinct mechanisms appear to regulate this migration. The initial dispersal of cells from the ventral ventricular zone is guided by chemorepellent cues including netrin‐1 present in the ventral ventricular domain. Migratory precursors are arrested in particular locations within the developing spinal cord as the result of the localized expression of the chemokine, CXCL1 by astrocytes. This chemokine, signalling through the CXCR2 receptor combines with PDGF to inhibit cell migration and enhance cell proliferation thereby facilitating the local expansion of the oligodendrocyte lineage and myelination of all relevant axons.     

Bone morphogenetic proteins in vertebrate hematopoietic development

During embryonic development, the hematopoietic system is the first to generate terminally differentiated, functional cell types. The urgent necessity for the early formation of blood and blood vessels during embryogenesis means that the induction, expansion, and maturation of these systems must be rapidly and precisely controlled. Bone morphogenic proteins (BMPs) have been implicated in hematopoietic development in the vertebrate embryo and stimulate the proliferation and/or differentiation of human cord blood hematopoietic stem cells (HSC) and embryonic stem cells in vitro. Here we review the mechanisms of action and potential roles of these soluble signaling molecules in vertebrate hematopoiesis.     

Bone morphogenetic proteins in the early development of zebrafish

Bone morphogenetic proteins (BMPs) are known to be widely involved in various biological processes. Many of the members of the BMP family, as well as related factors, receptors and molecules in the BMP signaling pathway, have been isolated, but their precise functions are still unclear. In addition to the 'classical' model organism Xenopus, zebrafish, Danio rerio, is now considered to be a suitable model organism to study the roles of the BMP signaling pathway during embryogenesis. Mutagenesis screens have identified a number of mutants in the pathway. Although they do not cover the entire members of the BMP signaling cascade that are currently known, they serve as a powerful tool to broaden our understanding of BMP functions, in combination with other experimental techniques.     

Bone morphogenetic proteins and their receptors in the eye

The human genome encodes at least 42 different members of the transforming growth factor-beta superfamily of growth factors. Bone morphogenetic proteins (BMPs) are the largest subfamily of proteins within the transforming growth factor-beta superfamily and are involved in numerous cellular functions including development, morphogenesis, cell proliferation, apoptosis, and extracellular matrix synthesis. This article first reviews BMPs and BMP receptors, BMP signaling pathways, and mechanisms controlling BMP signaling. Second, we review BMP and BMP receptor expression during embryonic ocular development/ differentiation and in adult ocular tissues. Lastly, future research directions with respect to BMP, BMP receptors, and ocular tissues are suggested.