The Role of the Ankyrin-Binding Protein NrCAM in Node of Ranvier Formation

Molecular events involved in Na⁺ channel clustering at the node of Ranvier have been investigated during early development. NrCAM, an ankyrin_G-binding protein, precedes Na⁺ channels at cluster sites adjacent to the tips of Schwann cell processes. Both Na ⁺ channel and ankyrin_G sequestration at developing nodes are delayed in NrCAM null mutants. The action of NrCAM is manifest locally at individual nodes, rather than affecting overall neuronal expression, and is linked to glial interactions. During remyelination, Na⁺ channel clusters at new nodes are initially labile, and anchoring to the cytoskeleton appears to grow progressively with time. The distance between Na⁺ channel clusters across remyelinating Schwann cells (nascent internodes) increases markedly from 83 to 274 μm during node formation, arguing against schemes in which the loci of nodes are fixed in advance by the axon. A hypothesis for node formation in which axonal Na⁺ channels move by lateral diffusion from regions of Schwann cell contact, with clustering dependent on linkage to the cytoskeleton by ankyrin_G, is proposed and tested in a computational model. To match experimental measurements, this latter reaction needs fast kinetics, and the early arrival of NrCAM is postulated to contribute to this requirement.