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J. Cell Biol.,
Volume 140, Number 3, February 9, 1998 675-684
-Spectrin Is Colocalized with Both Voltage-gated Sodium
Channels and AnkyrinG at the Adult Rat Neuromuscular Junction
School of Neurosciences, The Medical School, University of Newcastle upon Tyne NE2 4HH, United Kingdom
Voltage-gated sodium channels (VGSCs) are
concentrated in the depths of the postsynaptic folds at
mammalian neuromuscular junctions (NMJs) where
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, 
-spectrin and ankyrin,
which may help to maintain the high local density of
VGSCs. Here we show in skeletal muscle, using immunofluorescence, that -spectrin is precisely colocalized
with both VGSCs and ankyrinG, the nodal isoform of ankyrin. In en face views of rat NMJs, acetylcholine receptors (AChRs), and utrophin immunolabeling are organized in distinctive linear arrays corresponding to the
crests of the postsynaptic folds. In contrast, -spectrin,
VGSCs, and ankyrinG have a punctate distribution that
extends laterally beyond the AChRs, consistent with a
localization in the depths of the folds. Double antibody labeling shows that -spectrin is precisely colocalized
with both VGSCs and ankyrinG at the NMJ. Furthermore, quantification of immunofluorescence in labeled
transverse sections reveals that -spectrin is also concentrated in perijunctional regions, in parallel with an
increase in labeling of VGSCs and ankyrinG, but not of
dystrophin. These observations suggest that interactions with -spectrin and ankyrinG 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.
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