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.     

FGF-dependent generation of oligodendrocytes by a hedgehog-independent pathway

During development, spinal cord oligodendrocyte precursors (OPCs) originate from the ventral, but not dorsal, neuroepithelium. Sonic hedgehog (SHH) has crucial effects on oligodendrocyte production in the ventral region of the spinal cord; however, less is known regarding SHH signalling and oligodendrocyte generation from neural stem cells (NSCs). We show that NSCs isolated from the dorsal spinal cord can generate oligodendrocytes following FGF2 treatment, a MAP kinase dependent phenomenon that is associated with induction of the obligate oligogenic gene Olig2. Cyclopamine, a potent inhibitor of hedgehog signalling, did not block the formation of oligodendrocytes from FGF2-treated neurosphere cultures. Furthermore, neurospheres generated from SHH null mice also produced oligodendrocytes, even in the presence of cyclopamine. These findings are compatible with the idea of a hedgehog independent pathway for oligodendrocyte generation from neural stem cells.     

Distinct sites of origin of oligodendrocytes and somatic motoneurons in the chick spinal cord: oligodendrocytes arise from Nkx2.2-expressing progenitors by a Shh-dependent mechanism

In the vertebrate spinal cord, oligodendrocytes arise from the ventral part of the neuroepithelium, a region also known to generate somatic motoneurons. The emergence of oligodendrocytes, like that of motoneurons, depends on an inductive signal mediated by Sonic hedgehog. We have defined the precise timing of oligodendrocyte progenitor specification in the cervico-brachial spinal cord of the chick embryo. We show that ventral neuroepithelial explants, isolated at various development stages, are unable to generate oligodendrocytes in culture until E5 but become able to do so in an autonomous way from E5.5. This indicates that the induction of oligodendrocyte precursors is a late event that occurs between E5 and E5.5, precisely at the time when the ventral neuroepithelium stops producing somatic motoneurons. Analysis of the spatial restriction of oligodendrocyte progenitors, evidenced by their expression of O4 or PDGFR(&agr;), indicate that they always lie within the most ventral Nkx2.2-expressing domain of the neuroepithelium, and not in the adjacent domain characterized by Pax6 expression from which somatic motoneurons emerge. We then confirm that Shh is necessary between E5 and E5.5 to specify oligodendrocyte precursors but is no longer required beyond this stage to maintain ongoing oligodendrocyte production. Furthermore, Shh is sufficient to induce oligodendrocyte formation from ventral neuroepithelial explants dissected at E5. Newly induced oligodendrocytes expressed Nkx2.2 but not Pax6, correlating with the in vivo observation. Altogether, our results show that, in the chick spinal cord, oligodendrocytes originate from Nkx2.2-expressing progenitors.     

Sonic hedgehog signaling is required during the appearance of spinal cord oligodendrocyte precursors

Spinal cord oligodendrocyte precursors arise in the ventral ventricular zone as a result of local signals. Ectopic oligodendrocyte precursors can be induced by sonic hedgehog (Shh) in explants of chick dorsal spinal cord over an extended developmental period. The role of Shh during normal oligodendrocyte development is, however, unclear. Here we demonstrate that Shh is localized to the ventral spinal cord immediately prior to, and during the appearance of oligodendrocyte precursors. Continued expression of Shh is required for the appearance of spinal cord oligodendrocyte precursors as neutralization of Shh signaling both in vivo and in vitro during a defined developmental period blocked their emergence. The inhibition of oligodendrocyte precursor emergence in the absence of Shh signaling was not the result of inhibiting precursor cell proliferation, and the neutralization of Shh signaling after the emergence of oligodendrocyte precursors had no effect on the appearance of additional cells or their subsequent differentiation. Similar concentrations of Shh induce motor neurons and oligodendrocytes in dorsal spinal cord explants. However, in explants from early embryos the motor neuron lineage is preferentially expanded while in explants from older embryos the oligodendrocyte lineage is preferentially expanded.     

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.     

Delta-Notch signaling regulates oligodendrocyte specification

Oligodendrocytes, the myelinating cell type of the central nervous system, arise from a ventral population of precursors that also produces motoneurons. Although the mechanisms that specify motoneuron development are well described, the mechanisms that generate oligodendrocytes from the same precursor population are largely unknown. By analysing mutant zebrafish embryos, we found that Delta-Notch signaling is required for spinal cord oligodendrocyte specification. Using a transgenic, conditional expression system, we also learned that constitutive Notch activity could promote formation of excess oligodendrocyte progenitor cells (OPCs). However, excess OPCs are induced only in ventral spinal cord at the time that OPCs normally develop. Our data provide evidence that Notch signaling maintains subsets of ventral spinal cord precursors during neuronal birth and, acting with other temporally and spatially restricted factors, specifies them for oligodendrocyte fate.     

Proliferation and migration of glial precursor cells in the developing rat spinal cord

The mechanisms that control the production and differentiation of glial cells during development are difficult to unravel because of displacement of precursor cells from their sites of origin to their permanent location. The two main neuroglial cells in the rat spinal cord are oligodendrocytes and astrocytes. Considerable evidence supports the view that oligodendrocytes in the spinal cord are derived from a region of the ventral ventricular zone (VZ). Some astrocytes, at least, may arise from radial glia. In this study a 5-Bromo-2′-deoxyuridine (BrdU) incorporation assay was used to identify proliferating cells and examine the location of proliferating glial precursor cells in the embryonic spinal cord at different times post BrdU incorporation. In this way the migration of proliferating cells into spinal cord white matter could be followed. At E14, most of the proliferating cells in the periventricular region were located dorsally and these cells were probably proliferating neuronal precursors. At E16 and E18, the majority of the proliferating cells in the periventricular region were located ventrally. In the white matter the number of proliferating cells increased as the animals increased in age and much of this proliferation occurred locally. BrdU labelling showed that glial precursor cells migrate from their ventral and dorsal VZ birth sites to peripheral regions of the cord. Furthermore although the majority of proliferating cells in the spinal cord at E16 and E18 were located in the ventral periventricular region, some proliferating cells remained in the dorsal VZ region of the cord.     

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

Ectopic myelinating oligodendrocytes in the dorsal spinal cord as a consequence of altered semaphorin 6D signaling inhibit synapse formation

Different types of sensory neurons in the dorsal root ganglia project axons to the spinal cord to convey peripheral information to the central nervous system. Whereas most proprioceptive axons enter the spinal cord medially, cutaneous axons typically do so laterally. Because heavily myelinated proprioceptive axons project to the ventral spinal cord, proprioceptive axons and their associated oligodendrocytes avoid the superficial dorsal horn. However, it remains unclear whether their exclusion from the superficial dorsal horn is an important aspect of neural circuitry. Here we show that a mouse null mutation of Sema6d results in ectopic placement of the shafts of proprioceptive axons and their associated oligodendrocytes in the superficial dorsal horn, disrupting its synaptic organization. Anatomical and electrophysiological analyses show that proper axon positioning does not seem to be required for sensory afferent connectivity with motor neurons. Furthermore, ablation of oligodendrocytes from Sema6d mutants reveals that ectopic oligodendrocytes, but not proprioceptive axons, inhibit synapse formation in Sema6d mutants. Our findings provide new insights into the relationship between oligodendrocytes and synapse formation in vivo, which might be an important element in controlling the development of neural wiring in the central nervous system.     

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.     

Local sonic hedgehog signaling regulates oligodendrocyte precursor appearance in multiple ventricular zone domains in the chick metencephalon

In the chick metencephalon, oligodendrocyte precursors arise in distinct domains of the ventricular zone. During development, the earliest oligodendrocyte precursors appear in the metencephalic ventral ventricular zone adjacent to the midline, consistent with their location in the spinal cord. In contrast to spinal cord, however, distinct domains in the lateral and dorsal metencephalic ventricular zone subsequently generate oligodendrocyte precursors. All oligodendrogenic domains of the metencephalon appear in close apposition to regions that transiently express sonic hedgehog (Shh). Inhibition studies demonstrate a functional requirement for Shh signaling in the early appearance of metencephalic oligodendrocyte precursors, while in vitro studies suggest a dose-dependent increase in the number of oligodendrocyte precursors in response to Shh. In purified cultures of oligodendrocyte precursors, Shh promotes cell survival and proliferation, suggesting that Shh can act directly on these cells. These data suggest that Shh may be responsible for the localized appearance of oligodendrocyte precursors throughout the CNS, irrespective of the dorso-ventral neural axis.     

Vitronectin promotes oligodendrocyte differentiation during neurogenesis of human embryonic stem cells

We demonstrate enhanced differentiation of oligodendrocytes during neurogenesis of human embryonic stem cells (hESCs) using an extracellular matrix protein, vitronectin (VN). We show that VN is expressed in the ventral part of the developing human spinal cord. Combined treatment of retinoic acid, sonic hedgehog, and noggin in the presence of VN allows hESCs to differentiate into O4-positive oligodendrocytes. Particularly, VN profoundly promotes the derivation of oligodendrocyte progenitors that proliferate and differentiate into oligodendrocytes in response to mitogenic and survival factors. These results support the beneficial effect of VN on oligodendrocytic differentiation of hESCs.