The interaction between myelinating glial cells and their underlying axons organize the axonal membrane at and around the nodes of Ranvier into distinct domains, each of which contains a unique set of ion channels, cell adhesion molecules and cytoplasmic adaptor proteins. This local differentiation of the axon is essential for the fast and efficient propagation of action potentials and its disruption results in pathophysiological changes often seen in demyelinating diseases. This research proposal sets out to identify the molecular mechanisms by which myelinating Schwann cells (SC) regulate the establishment of the nodes of Ranvier. We have recently identified three novel glial cell adhesion molecules (glial CAMs), Gliomedin, CDO and Zig1 that are localized at the SC microvilli which contact the nodes of Ranvier. Furthermore, we have found that Gliomedin is the long sought-after glial receptor for NrCAM and neurofascin, two cell adhesion molecules that are associated with Na+ channels at the nodal axolemma. We propose to determine whether axon-glial contact mediated by these glial CAMs controls the clustering of Na+ channels along the axonal membrane. First, the distribution of these proteins during development will be examined and correlated with the molecular assembly of the nodes. Secondly, we will determine their role in myelination and the formation of the nodes of Ranvier using an in vitro myelination system, as well as by analyzing mice in which Gliomedin, CDO or Zig1 have been eliminated by gene targeting. Finally, a combined biochemical and molecular approach will be taken to reveal the molecular interactions mediated by these glial CAMs at the nodes. Our studies will yield important insight into the yet elusive mechanisms of axon-glial communication and the coordinated differentiation of axons and myelin-forming cells, which allows myelinated fibers to maximize their conduction velocity. A better understanding of these mechanisms may help to design future therapeutical strategies for demyelinating disorders.
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