Differences in myelination between spinal cord and corticular tissue transplanted intraocularly in rats

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.

Evidence for a second wave of oligodendrogenesis in the postnatal cerebral cortex of the mouse

The existing view is that cortical oligodendrocytes (OLs) in rodents are born from the cortical subventricular zone (SVZ) after birth, but recent data suggest that many forebrain oligodendrocyte progenitor cells (OPCs) are specified much earlier (between E9.5 and E13.5 in the mouse) in the ventricular zone of the ventral forebrain under the control of sonic hedgehog (Shh) and migrate into the cortex afterward. We examined expression of specific early OL markers (PDGFRalpha, PLP/DM20, Olig2, and NG2) in the developing forebrain to clarify this issue. We propose that OPCs colonize the developing cortex in two temporally distinct waves. The gray matter is at least partially populated by a first wave of OPCs that arises in the medial ganglionic eminence and the entopeduncular area and spreads into the cortex via the developing cortical plate. The cerebral cortex benefits from the second wave of OPCs coming from residential SVZ. In the second wave, there might be two different types of precursor cells: PLP/DM20(+) cells populating only inner layers and PDGFRalpha(+) cells, which might eventually myelinate the outer regions as well.

Sonic hedgehog is required during an early phase of oligodendrocyte development in mammalian brain

Oligodendrocyte precursor development in the embryonic spinal cord is thought to be regulated by the secreted signal, Sonic hedgehog (Shh). Such precursors can be identified by the expression of Olig genes, encoding basic helix-loop-helix factors, in the spinal cord and brain. However, the signaling pathways that govern oligodendrocyte precursor (OLP) development in the rostral central nervous system are poorly understood. Here, we show that Shh is required for oligodendrocyte development in the mouse forebrain and spinal cord, and that Shh proteins are both necessary and sufficient for OLP production in cortical neuroepithelial cultures. Moreover, adenovirus-mediated Olig1 ectopic expression can promote OLP formation independent of Shh activity. Our results demonstrate essential functions for Shh during early phases of oligodendrocyte development in the mammalian central nervous system. They further suggest that a key role of Shh signaling is activation of Olig genes.

Hedgehog-dependent oligodendrocyte lineage specification in the telencephalon

In the caudal neural tube, oligodendrocyte progenitors (OLPs) originate in the ventral neuroepithelium under the influence of Sonic hedgehog (SHH), then migrate throughout the spinal cord and brainstem before differentiating into myelin-forming cells. We present evidence that oligodendrogenesis in the anterior neural tube follows a similar pattern. We show that OLPs in the embryonic mouse forebrain express platelet-derived growth factor alpha-receptors (PDGFRA), as they do in more caudal regions. They first appear within a region of anterior hypothalamic neuroepithelium that co-expresses mRNA encoding SHH, its receptor PTC1 (PTCH) and the transcription factors OLIG1, OLIG2 and SOX10. Pdgfra-positive progenitors later spread through the forebrain into areas where Shh is not expressed, including the cerebral cortex. Cyclopamine inhibited OLP development in cultures of mouse basal forebrain, suggesting that hedgehog (HH) signalling is obligatory for oligodendrogenesis in the ventral telencephalon. Moreover, Pdgfra-positive progenitors did not appear on schedule in the ventral forebrains of Nkx2.1 null mice, which lack the telencephalic domain of Shh expression. However, OLPs did develop in cultures of Nkx2.1−/− basal forebrain and this was blocked by cyclopamine. OLPs also developed in neocortical cultures, even though Shh transcripts could not be detected in the embryonic cortex. Here, too, the appearance of OLPs was suppressed by cyclopamine. In keeping with these findings, we detected mRNA encoding SHH and Indian hedgehog (IHH) in both Nkx2.1−/− basal forebrain cultures and neocortical cultures. Overall, the data are consistent with the idea that OLPs in the telencephalon, possibly even some of those in the cortex, develop under the influence of SHH in the ventral forebrain.

Oligodendrocyte development in the spinal cord and telencephalon: common themes and new perspectives

There are clear parallels between oligodendrocyte development in the spinal cord and forebrain. However, there is new evidence that in both of these regions oligodendrocyte lineage development may be more complex than we earlier thought. This stems from the recent identification of three new transcription factor genes, Olig1, Olig2 and Sox10, that are expressed from the early stages of oligodendrocyte lineage development. In this article, we highlight the common themes underlying specification and early development of oligodendrocytes in the spinal cord and telencephalon. Then, we discuss recent studies of Sox10 and the Olig genes and their implications for oligodendrocyte specification. We conclude that although the mechanisms of oligodendrogenesis appear to be fundamentally similar at different rostro-caudal levels of the neuraxis, there are still many unanswered questions about the details of oligodendrocyte specification.