Gpr126 signaling and Schwann cell-axon interactions during myelination in zebrafish

The scope and complexity of the vertebrate nervous system requires the rapid transmission of neural impulses over long distances. The myelin sheath is an evolutionary adaptation that allows axons to rapidly propagate action potentials. Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS) form myelin by wrapping their cell membranes around axons to form a multilayered membranous sheath that insulates and supports axons. Voltage gated sodium channels cluster at the unmyelinated gaps between myelin segments--the nodes of Ranvier. Depolarization of the axonal membrane at the nodes allows action potentials to propagate in a saltatory manner. Diseases of myelin, including multiple sclerosis in the CNS, and Charcot-Marie-Tooth disease in the PNS, underscore its clinical importance. In this dissertation, I focus on the mechanisms controlling the initiation and maturation of myelin in the PNS, as well as the role of Schwann cells in regulating sodium channel clustering at the nodes of Ranvier. Schwann cells arise from the neural crest in a series of developmental stages and depend upon axonal signals, such as Neuregulin 1 type III (Nrg1-III), for their survival and differentiation into myelinating Schwann cells. Nrg1-III signals control almost every aspect of Schwann cell development, but it has been proposed that other signaling pathways, such as those mediated by cyclic adenosine monophosphate (cAMP), may intersect with the Nrg1 pathway to affect a Schwann cell's response to Nrg1. In Chapter 2, I describe the identification of an orphan adhesion G protein-coupled receptor (GPCR), Gpr126, that was identified in a forward genetic screen in zebrafish. Gpr126 is essential for Schwann cells to initiate myelination, and gpr126 mutant zebrafish are devoid of PNS myelin. I show that Gpr126 is required cell-autonomously in Schwann cells to initiate myelination, and that elevating cAMP by drug treatment rescues myelination in gpr126 mutants in vivo. These results provide strong evidence that Gpr126 is the receptor that activates cAMP signaling to initiate myelination in Schwann cells in vivo. In Chapter 3, I demonstrate that although Gpr126 is essential for the initiation of myelination, it is no longer required for the maturation or maintenance of the myelin sheath. Notably, although Gpr126 signaling is required for the expression of the promyelinating transcription factor Krox20 during the initiation of myelination, Gpr126 signaling is dispensable for the maintenance of Krox20 expression. I further demonstrate that expression of activated protein kinase A (PKA) in Schwann cells is sufficient to rescue myelination in gpr126 mutants in vivo, but that over-expression of Nrg1-III in neurons is not. These results show that Gpr126 has a specific function during the initiation of myelination, and suggest that Gpr126 signaling functions in parallel to Nrg1 signaling. In chapter 4, I describe a novel role for Schwann cells in inhibiting the clustering of axonal sodium channels at the internodes, thereby confining sodium channel clustering to the nodes of Ranvier. Taken together, the findings described in this dissertation identify an orphan adhesion GPCR that regulates the initiation of myelination in Schwann cells, as well as define a novel role for Schwann cells in the regulation of sodium channel clustering.