Rapid conduction in myelinated axons depends on the generation of specialized subcellular domains to which different sets of ion channels are localized. Here, we describe the identification of Caspr2, a mammalian homolog of Drosophila Neurexin IV (Nrx-IV), and show that this neurexin-like protein and the closely related molecule Caspr/Paranodin demarcate distinct subdomains in myelinated axons. While contactin-associated protein (Caspr) is present at the paranodal junctions, Caspr2 is precisely colocalized with Shaker-like K+ channels in the juxtaparanodal region. We further show that Caspr2 specifically associates with Kv1.1, Kv1.2, and their Kvbeta2 subunit. This association involves the C-terminal sequence of Caspr2, which contains a putative PDZ binding site. These results suggest a role for Caspr family members in the local differentiation of the axon into distinct functional subdomains.

Molecular events involved in Na+ channel clustering at the node of Ranvier have been investigated during early development. NrCAM, an ankyrinG-binding protein, precedes Na+ channels at cluster sites adjacent to the tips of Schwann cell processes. Both Na+ channel and ankyrinG 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 microm 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 ankyrinG, 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.

Action potential conduction velocity increases dramatically during early development as axons become myelinated. Integral to this process is the clustering of voltage-gated Na + (Nav) channels at regularly spaced gaps in the myelin sheath called nodes of Ranvier. We show here that some aspects of peripheral node of Ranvier formation are distinct from node formation in the CNS. For example, at CNS nodes, Nav1.2 channels are detected first, but are then replaced by Nav1.6. Similarly, during remyelination in the CNS, Nav1.2 channels are detected at newly forming nodes. By contrast, the earliest Nav-channel clusters detected during developmental myelination in the PNS have Nav1.6. Further, during PNS remyelination, Nav1.6 is detected at new nodes. Finally, we show that accumulation of the cell adhesion molecule neurofascin always precedes Nav channel clustering in the PNS. In most cases axonal neurofascin (NF-186) accumulates first, but occasionally paranodal neurofascin is detected first. We suggest there is heterogeneity in the events leading to Nav channel clustering, indicating that multiple mechanisms might contribute to node of Ranvier formation in the PNS.