Clustering of Na+ channels and node of Ranvier formation in remyelinating axons

Abstract

Polyclonal antibodies were raised against a well conserved region of the vertebrate Na+ channel and were affinity purified for use in immunocytochemistry. Focal demyelination of rat sciatic axons was initiated by an intraneural injection of lysolecithin and Na+ channel clustering was followed at several stages of myelin removal and repair. At 1 week post-injection axons contained long, fully demyelinated regions. Na+ channel clusters appeared only at heminodes forming the borders of these zones, and at widely spaced isolated sites that may represent former nodes of Ranvier. Over the next few days proliferating Schwann cells adhered to axons and began to extend processes. Clusters of Na+ channels appeared at the edges of these structures. As the Schwann cells elongated, the clusters seemed to move with them, since they remained at edges and the distance between aggregates increased. Clusters associated with different Schwann cells ultimately approached each other and appeared to fuse. Na+ channels then coalesced further at these sites, forming new nodes of Ranvier in regions that previously were internodal. If Schwann cell proliferation were blocked by mitomycin, no new clusters of Na+ channels appeared within internodes. Under these conditions, heminodal clusters remained visible at 1 week postinjection, but by 2 weeks they were no longer detectable, suggesting that proliferating Schwann cells are required for their maintenance. Clusters at normal nodes of Ranvier remained. It is concluded that Na+ channel aggregation and mobility in demyelinated nerve fibers is controlled by adhering Schwann cells, resulting in the formation of stable new nodes of Ranvier during remyelination.