BETA-SPECTRIN IS COLOCALIZED WITH BOTH VOLTAGE-GATED SODIUM-CHANNELS AND ANKYRIN(G) AT THE ADULT-RAT NEUROMUSCULAR-JUNCTION

Voltage-gated sodium channels (VGSCs) are concentrated in the depths o f the postsynaptic folds at mammalian neuromuscular junctions (NMJs) w here they facilitate action potential generation during neuromuscular transmission. At the nodes of Ranvier and the axon hillocks of central neurons, VGSCs are associated with the cytoskeletal proteins, beta-sp ectrin and ankyrin, which may help to maintain the high local density of VGSCs. Here we show in skeletal muscle, using immunofluorescence, t hat beta-spectrin is precisely colocalized with both VGSCs and ankyrin (G), the nodal isoform of ankyrin, In en face views of rat NMJs, acety lcholine receptors (AChRs), and utrophin immunolabeling are organized in distinctive linear arrays corresponding to the crests of the postsy naptic folds. In contrast, beta-spectrin, VGSCs, and ankyrin(G) have a punctate distribution that extends laterally beyond the AChRs, consis tent with a localization in the depths of the folds. Double antibody l abeling shows that beta-spectrin is precisely colocalized with both VG SCs and ankyrin(G) at the NMJ. Furthermore, quantification of immunofl uorescence in labeled transverse sections reveals that beta-spectrin i s also concentrated in perijunctional regions, in parallel with an inc rease in labeling of VGSCs and ankyrin(G), but not of dystrophin. Thes e observations suggest that interactions with beta-spectrin and ankyri n(G) help to maintain the concentration of VGSCs at the NMJ and that a common mechanism exists throughout the nervous system for clustering VGSCs at a high density.