Sox10 is required for Schwann cell identity and progression beyond the immature Schwann cell stage

Mutations in the transcription factor SOX10 cause neurocristopathies, including Waardenburg-Hirschsprung syndrome and peripheral neuropathies in humans. This is partly attributed to a requirement for Sox10 in early neural crest for survival, maintenance of pluripotency, and specification to several cell lineages, including peripheral glia. As a consequence, peripheral glia are absent in Sox10-deficient mice. Intriguingly, Sox10 continues to be expressed in these cells after specification. To analyze glial functions after specification, we specifically deleted Sox10 in immature Schwann cells by conditional mutagenesis. Mutant mice died from peripheral neuropathy before the seventh postnatal week. Nerve alterations included a thinned perineurial sheath, increased lipid and collagen deposition, and a dramatically altered cellular composition. Nerve conduction was also grossly aberrant, and neither myelinating nor nonmyelinating Schwann cells formed. Instead, axons of different sizes remained unsorted in large bundles. Schwann cells failed to develop beyond the immature stage and were unable to maintain identity. Thus, our study identifies a novel cause for peripheral neuropathies in patients with SOX10 mutations.

Sox9 and Sox10 influence survival and migration of oligodendrocyte precursors in the spinal cord by regulating PDGF receptor alpha expression

Specification of the myelin-forming oligodendrocytes of the central nervous system requires the Sox9 transcription factor, whereas terminal differentiation depends on the closely related Sox10. Between specification and terminal differentiation, Sox9 and Sox10 are co-expressed in oligodendrocyte precursors and are believed to exert additional functions. To identify such functions, we have deleted Sox9 specifically in already specified oligodendrocyte precursors of the spinal cord. In the absence of Sox9, oligodendrocyte precursors developed normally and started terminal differentiation on schedule. However, when Sox10 was additionally deleted, oligodendrocyte precursors exhibited an altered migration pattern and were present in reduced numbers because of increased apoptosis rates. Remaining precursors continued to express many characteristic oligodendroglial markers. Aberrant expression of astrocytic and neuronal markers was not observed. Strikingly, we failed to detect PDGF receptor alpha expression in the mutant oligodendrocyte precursors, arguing that PDGF receptor alpha is under transcriptional control of Sox9 and Sox10. Altered PDGF receptor alpha expression is furthermore sufficient to explain the observed phenotype, as PDGF is both an important survival factor and migratory cue for oligodendrocyte precursors. We thus conclude that Sox9 and Sox10 are required in a functionally redundant manner in oligodendrocyte precursors for PDGF-dependent survival and migration.

The high-mobility-group domain of Sox proteins interacts with DNA-binding domains of many transcription factors

Sox proteins are widely believed to team up with other transcription factors as partner proteins to perform their many essential functions during development. In this study, yeast two-hybrid screens identified transcription factors as a major group of interacting proteins for Sox8 and Sox10. Interacting transcription factors were very similar for these two group E Sox proteins and included proteins with different types of DNA-binding domains, such as homeodomain proteins, zinc finger proteins, basic helix–loop–helix and leucine zipper proteins. In all cases analyzed, the interaction involved the DNA-binding domain of the transcription factor which directly contacted the C-terminal part of the high-mobility-group (HMG) domain. In particular, the C-terminal tail region behind helix 3 of the HMG domain was shown by mutagenesis to be essential for interaction and transcription factor recruitment. The HMG domain thus not only possesses DNA-binding and DNA-bending but also protein-interacting ability which may be equally important for the architectural function of Sox proteins on their target gene promoters.