Control of Schwann Cell differentiation.

Myelin is essential for the rapid axonal conduction required by vertebrate nervous systems; in the peripheral nervous system, it is formed by Schwann cells. In addition to its clinical significance, Schwann cell maturation provides a model for investigating cell-cell interactions in development, and the transcriptional regulatory events that produce terminally differentiated cell phenotypes. Elucidation of the mechanisms by which peripheral myelination is controlled may reveal how terminal differentiation is regulated in other cell types.

The POU domain transcription factor Oct-6 (Tst-1, SCIP) encoded by the Pou3f1 gene, is required for timely peripheral myelination (Bermingham et al., Genes and Development 10: 1751-1762, 1996: PubMed Abstract). Most Oct-6 homozygous pups die soon after birth, but a few live long enough to reveal that peripheral myelination is delayed. In collaboration with Dr. Steve Scherer of the University of Pennsylvania, we observed that Oct-6 mutant Schwann cells ensheathe axons, but are delayed in forming multiple wraps of myelin. Oct-6 was thought previously to function as a repressor of myelin gene expression, but Oct-6 mutant Schwann cells normally express transcripts that encode several major myelin structural proteins. We used representational difference analysis (RDA) to identify Oct-6 downstream effector genes that are either direct or indirect Oct-6 targets. To isolate differentially expressed genes from the small amounts of RNA that one could obtain from the sciatic nerves of newborn mutant mice before they died, we performed RDA in concert with a carrier RNA. We tested this technique using RNA from Schwann cells cultured with or without the adenylyl cyclase activator forskolin, which mimics some aspects of Schwann cell differentiation, and we identified several forskolin activated or repressed genes that may function in Schwann cell differentiation (Bermingham et al., J. Neuroscience Research 63: 516-524 (2001) PubMed (Abstract). From Oct-6 (+/+) and (-/-) sciatic nerves we isolated six putative Oct-6 target genes that are dowregulated in Oct-6 mutant mice, and therefore are activated, directly or indirectly, by Oct-6. However, no Oct-6-repressed genes were found, suggesting that Oct-6 functions as an activator, not a repressor, in Schwann cell differentiation. A paper describing these genes has been published (Bermingham et al, J Neuroscience 22, 10217-10231 (2002). One of the novel genes, Slc36a2, encodes a protein that we have named tramdorin, for transmembrane domain rich protein. It defines a new family of amino acid transport proteins (Bermingham and Pennington, Mammalian Genome 14, 114-125 (2004). Another Oct-6 downstream effector gene, gliomedin, encodes a type II transmembrane protein with extracellular collagen and olfactomedin domains; it was identified independently by Ori Peles of the Weizmann Institute. Gliomedin is required for formation of the node of Ranvier (Eshed et al, Neuron (2005).

The spontaneous claw paw (clp) mutation causes a delay in peripheral myelination whereas CNS myelination is normal. In collaboration with Dies Meijer of Erasmus University, Rotterdam and Jeffrey Milbrandt of Washington University, St. Louis, we positionally cloned the claw paw mutation, and found that it affects splicing of the Lgi4 gene. This gene encodes a secreted leucine rich repeat protein of unknown function. In clp/clp mice, Lgi4 exon 4 is skipped, resulting in a protein in which two leucine rich repeats are fused into a single, hybrid repeat. This mutant protein is not secreted. We have shown that siRNA specific for Lgi4 prevents myelination in cocultures, and that exogenous Lgi4 permits clp/clp cocultures to myelinate. Thus Schwann cells secrete a protein that is required for their differentiation, and we are attempting to determine the mechanism by which it acts.


John Bermingham, Updated 11/10/05