αII-spectrin is essential for assembly of the nodes of Ranvier in myelinated axons

Saltatory conduction in myelinated axons requires organization of the nodes of Ranvier, where voltage-gated sodium channels are prominently localized [1]. Previous results indicate that alpha II-spectrin, a component of the cortical cytoskeleton [2], is enriched at the paranodes [3, 4], which flank the node of Ranvier, but alpha II-spectrin's function has not been investigated. Starting with a genetic screen in zebrafish, we discovered in all-spectrin (alpha II-spn) a mutation that disrupts nodal sodium-channel clusters in myelinated axons of the PNS and CNS. In alpha II-spn mutants, the nodal sodium-channel clusters are reduced in number and disrupted at early stages. Analysis of chimeric animals indicated that alpha II-spn functions autonomously in neurons. Ultrastructural studies show that myelin forms in the posterior lateral line nerve and in the ventral spinal cord in alpha II-spn mutants and that the node is abnormally long; these findings indicate that alpha II-spn is required for the assembly of a mature node of the correct length. We find that alpha II-spectrin is enriched in nodes and paranodes at early stages and that the nodal expression diminishes as nodes mature. Our results provide functional evidence that all-spectrin in the axonal cytoskeleton is essential for stabilizing nascent sodium-channel clusters and assembling the mature node of Ranvier.