Pairing your SOX: The Role of HMG-Domain Transcription Factors in Peripheral Nerve Myelination

Abstract

Compact myelin is an innovative acquisition in jawed vertebrates that is formed by the wrapping of glial cell membranes around neuronal axons in the central and peripheral nervous systems. The major functions of compact myelin include providing trophic support to axons and increasing the propagation of nerve impulses, which has facilitated the escape and predatory behaviors of vertebrates. Schwann cells are neural crest derived glia that form compact myelin in the peripheral nervous system (PNS). The SRY-related HMG box 10 (SOX10) transcription factor is essential for all stages of Schwann cell development. Specifically, SOX10 activates the expression of key myelin genes in the PNS and has therefore been reported as a pro-myelination transcription factor. Previously identified SOX10 target genes have been shown to be critical for Schwann cell function. Thus, the identification of additional genes regulated by SOX10 will improve our understanding of myelination in the PNS. We developed a stringent, computational method for genome-wide identification of SOX10 response elements. Experimental validation of a set of predicted SOX10 response elements revealed SOX5, SOX6, and NFIB as novel SOX10 target genes. To further explore the utility of our computational data we compared our predictions to published SOX10 ChIP-seq data from rat sciatic nerve and our own DNase-seq data generated from cultured Schwann cells. This analysis—along with subsequent functional studies—revealed SOX10 response elements that map to HES1, MYCN, ID2, and ID4. Remarkably, SOX5, SOX6, HES1, MYCN, ID2, and ID4 all encode proteins that inhibit myelination. Thus, our computationally anchored strategy revealed a putative novel function for SOX10 in Schwann cells, which suggests a model where SOX10 activates the expression of genes that inhibit myelination during non-myelinating stages of Schwann cell development. We then deeply characterized the SOX10 response element at SOX6, which revealed that this element resides at a previously unreported alternative promoter that directs the expression of a specific mRNA isoform. SOX6 was previously reported to inhibit glial cell differentiation in the central nervous system; however, the role of SOX6 in Schwann cells has not yet been characterized. To explore the role of SOX6 in Schwann cells, we set out to identify SOX6 target loci via overexpression of SOX6 in culture Schwann cells followed by RNA-seq analysis. Gene ontology analysis of up-regulated genes revealed a putative role for SOX6 in Schwann cell proliferation and regeneration following nerve injury, which should be explored using further computational and functional studies. Importantly, the computational and functional datasets we present here will be valuable for the study of transcriptional regulation, SOX protein function, and glial cell biology.