Myelinated nerve fibers are critical for efficient propagation of impulses via saltatory conduction. Myelin is organized anatomically and physiologically into longitudinal segments (internodes) that are interrupted by the nodes of Ranvier. High concentrations of voltage-gated sodium channels are located in the node, and specific potassium channels are restricted primarily to the paranodal regions. The nodes are critical for saltatory conduction along the nerve, but the molecular signals that determine node formation are poorly understood. Recent studies demonstrated that a subset of cell adhesion molecules (CAMs) in the Ig Superfamily (i.e., Nr-CAM and neurofascin) are specifically localized to the node at early stages of its formation. We have recently demonstrated that a recombinant form of Nr-CAM can perturb node formation using a co-culture system involving dorsal root ganglion (DRG) neurons and Schwann cells. The results strongly indicate a role for Nr-CAM and its ligands in node formation. Therefore, we propose to explore the functions of Nr-CAM and proteins that can interact with it, i.e., TAG-1, contactin, RPTPbeta, neurofascin and ankyrin, in the formation and maintenance of the node of Ranvier. Our first goal is to determine the patterns of expression of Nr-CAM and proteins known to interact with it in neurons and Schwann cells at different stages of nerve differentiation. We will also use mice that we have recently generated that are null for the Nr-CAM protein to search for potential alterations in the structure and development of the node and in nerve conduction. Our second goal is to analyze the role of Nr-CAM and proteins that can interact with it in node formation using a Schwann cell-neuron co-culture system. Recombinant forms of CAMs as Fc fusion proteins and specific antibodies against Nr-CAM and its binding partners will be used to test their ability to perturb node formation in the co-culture system. Node formation will be analyzed by staining with antibodies against ankyrin and sodium channels that normally become clustered at the nodes in culture. Our third goal is to identify specific binding regions in the Nr-CAM receptors involved in node formation and to analyze molecular mechanisms that signal protein clustering at nodes. The proposed studies will clarify the functions of Nr-CAM and its binding partners in node development in the PNS and may also provide new insights into the molecular basis of nerve development in the CNS. In the future, such information may lead to new therapeutic strategies to prevent injury and restore function in myelinated nerves that become damaged and remyelinated in multiple sclerosis.
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|Sakurai, T; Lustig, M; Babiarz, J et al. (2001) Overlapping functions of the cell adhesion molecules Nr-CAM and L1 in cerebellar granule cell development. J Cell Biol 154:1259-73|
|Lustig, M; Erskine, L; Mason, C A et al. (2001) Nr-CAM expression in the developing mouse nervous system: ventral midline structures, specific fiber tracts, and neuropilar regions. J Comp Neurol 434:13-28|