Myelin is a multilayered membrane that ensheathes axons in vertebrate nervous systems. In the peripheral nervous system, specialized glia called Schwann cells wrap themselves around axonal segments to form the myelin sheath. Myelin is essential for trophic support of neurons and provides insulation to efficiently conduct nerve impulses. Disruption of the peripheral myelin sheath can result in devastating neuropathies such as Charcot-Marie-Tooth (CMT) disease. There are currently no treatments to prevent de-myelination or stimulate re-myelination, and development of these therapies is hindered by an incomplete understanding of the genetic and molecular factors that drive Schwann cells to form the myelin sheath. Schwann cell myelination requires Gpr126, an orphaned adhesion family G protein-coupled receptor (aGPCR). Loss of Gpr126 function in vertebrate models results in arrested development of Schwann cells at the promyelinating state and reduced or absent peripheral myelin. Although Gpr126 is essential for peripheral myelination, very little is known about the mechanism by which it functions. This study will utilize the zebrafish model system to define the role of Gpr126 in myelination. In the first aim, I will use structure-function analysis o define the molecular domains of Gpr126 that regulate key steps in peripheral myelination. I predict that the large N-terminal domain of Gpr126 has a distinct function in sorting each Schwann cell around a single large-caliber axon. In contrast, I hypothesize the transmembrane C-terminus of Gpr126 is sufficient to direct the Schwann cell to wrap the axon following sorting. I will generate specific zebrafish mutants and perform in vivo analyses to test these hypotheses. In my second aim, I will perform an innovative forward genetic screen for mutations that enhance or suppress myelin defects in gpr126 hypomorphic mutants. This enhancer/suppressor screen will identify regulators of the Gpr126 myelination program as well as novel factors that function in parallel to Gpr126 for myelination. By defining the molecular and genetic factors that promote myelination, these studies will provide the basis for identifying causative mutations for genetic neuropathies like CMT. This work also will also deepen our understanding of Schwann cell biology and can lay the foundation for developing therapies to treat demyelination and promote re-myelination of peripheral nerves.
Loss of myelin is a primary cause of neuropathies such as Charcot-Marie-Tooth disease, which causes loss of sensation and pain and is the most common inherited neurological disorder. Normal myelin development requires the adhesion family G protein-coupled receptor Gpr126, which represents an excellent pharmaceutical target for therapeutics that treat myelinopathies. This study will define the critical molecular domains of Gpr126 that promote myelination and will identify crucial genetic factors that interact with Gpr126 to form the myelin sheath.
|Liebscher, Ines; Schön, Julia; Petersen, Sarah C et al. (2014) A tethered agonist within the ectodomain activates the adhesion G protein-coupled receptors GPR126 and GPR133. Cell Rep 9:2018-26|