The developmental origin of oligodendrocyte progenitors (OLPs) in the forebrain has been controversial. We now show, by Cre-lox fate mapping in transgenic mice, that the first OLPs originate in the medial ganglionic eminence (MGE) and anterior entopeduncular area (AEP) in the ventral forebrain. From there, they populate the entire embryonic telencephalon including the cerebral cortex before being joined by a second wave of OLPs from the lateral and/or caudal ganglionic eminences (LGE and CGE). Finally, a third wave arises within the postnatal cortex. When any one population is destroyed at source by the targeted expression of diphtheria toxin, the remaining cells take over and the mice survive and behave normally, with a normal complement of oligodendrocytes and myelin. Thus, functionally redundant populations of OLPs compete for space in the developing brain. Notably, the embryonic MGE- and AEP-derived population is eliminated during postnatal life, raising questions about the nature and purpose of the competition.

The molecular mechanisms that determine glial cell fate in the vertebrate nervous system have not been elucidated. Peripheral glial cells differentiate from pluripotent neural crest cells. We show here that the transcription factor Sox10 is a key regulator in differentiation of peripheral glial cells. In mice that carry a spontaneous or a targeted mutation ofSox10, neuronal cells form in dorsal root ganglia, but Schwann cells or satellite cells are not generated. At later developmental stages, this lack of peripheral glial cells results in a severe degeneration of sensory and motor neurons. Moreover, we show that Sox10 controls expression ofErbB3in neural crest cells.ErbB3encodes a Neuregulin receptor, and down-regulation ofErbB3accounts for many changes in development of neural crest cells observed inSox10mutant mice. Sox10 also has functions not mediated by ErbB3, for instance in the melanocyte lineage. Phenotypes observed in heterozygous mice that carry a targetedSox10null allele reproduce those observed in heterozygousSox10^Dommice. Haploinsufficiency ofSox10can thus cause pigmentation and megacolon defects, which are also observed inSox10^Dom/+ mice and in patients with Waardenburg-Hirschsprung disease caused by heterozygousSOX10mutations.

Sox proteins belong to the HMG box superfamily of DNA-binding proteins and are found throughout the animal kingdom. They are involved in the regulation of such diverse developmental processes as germ layer formation, organ development and cell type specifi-cation. Hence, deletion or mutation of Sox proteins often results in developmental defects and congenital disease in humans. Sox proteins perform their function in a complex interplay with other transcription factors in a manner highly dependent on cell type and promoter context. They exhibit a remarkable crosstalk and functional redundancy among each other.

Sox proteins are characterized by possession of a DNA-binding domain with similarity to the high-mobility group domain of the sex determining factor SRY. Here, we report on Sox10, a novel protein with predominant expression in glial cells of the nervous system. During development Sox10 first appeared in the forming neural crest and continued to be expressed as these cells contributed to the forming PNS and finally differentiated into Schwann cells. In the CNS, Sox10 transcripts were originally confined to glial precursors and later detected in oligodendrocytes of the adult brain. Functional studies failed to reveal autonomous transcriptional activity for Sox10. Instead, Sox10 functioned synergistically with the POU domain protein Tst-1/Oct6/SCIP with which it is coexpressed during certain stages of Schwann cell development. Synergy depended on binding to adjacent sites in target promoters, was mediated by the N-terminal regions of both proteins, and could not be observed between Sox10 and several other POU domain proteins. Interestingly, Sox10 also modulated the function of Pax3 and Krox-20, two other transcription factors involved in Schwann cell development. We propose a role for Sox10 in conferring cell specificity to the function of other transcription factors in developing and mature glia.

Sox10 is a high-mobility-group transcriptional regulator in early neural crest. Without Sox10, no glia develop throughout the peripheral nervous system. Here we show that Sox10 is restricted in the central nervous system to myelin-forming oligodendroglia. In Sox10-deficient mice progenitors develop, but terminal differentiation is disrupted. No myelin was generated upon transplantation of Sox10-deficient neural stem cells into wild-type hosts showing the permanent, cell-autonomous nature of the defect. Sox10 directly regulates myelin gene expression in oligodendrocytes, but does not control erbB3 expression as in peripheral glia. Sox10 thus functions in peripheral and central glia at different stages and through different mechanisms.