Dependence of nodal sodium channel clustering on paranodal axoglial contact in the developing CNS

Na+ channel clustering at nodes of Ranvier in the developing rat optic nerv e was analyzed to determine mechanisms of localization, including the possi ble requirement for glial contact in vivo. Immunofluorescence labeling for myelin-associated glycoprotein and for the protein Caspr, a component of ax oglial junctions, indicated that oligodendrocytes were present, and paranod al structures formed, as early as postnatal day 7 (P7). However, the first Na+ channel clusters were not seen until P9. Most of these were broad, and all were excluded from paranodal regions of axoglial contact. The number of detected Na+ channel clusters increased rapidly from P12 to P22. During th is same period, conduction velocity increased sharply, and Na+ channel clus ters became much more focal. To test further whether oligodendrocyte contac t directly influences Na+ channel distributions, nodes of Ranvier in the hy pomyelinating mouse Shiverer were examined. This mutant has oligodendrocyte -ensheathed axons but lacks compact myelin and normal axoglial junctions. D uring development Na+ channel clusters in Shiverer mice were reduced in num bers and were in aberrant locations. The subcellular location of Caspr was disrupted, and nerve conduction properties remained immature. These results indicate that in vivo, Na+ channel clustering at nodes depends not only on the presence of oligodendrocytes but also on specific axoglial contact at paranodal junctions. In rats, ankyrin-3/G, a cytoskeletal protein implicate d in Na+ channel clustering, was detected before Na+ channel immunoreactivi ty but extended into paranodes in non-nodal distributions. In Shiverer, ank yrin-3/G labeling was abnormal, suggesting that its localization also depen ds on axoglial contact.