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The C-terminal domain of ßIV-spectrin is crucial for KCNQ2 aggregation and excitability at nodes of Ranvier.
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International audience. The spectrin cytoskeleton has an important function in the targeting of proteins to excitable membrane domains. In axons, βIV-spectrin stabilizes voltage-gated sodium (Nav) channel clusters at nodes of Ranvier and axon initial segments, two regions crucial for the generation and conduction of action potentials. Here, I investigated the physiology of the neuromuscular junction and peripheral nerves in quivering-3J mice, which show a frame-shift base insertion in the Spnb4 gene and lack the C-terminus of βIV-spectrin. The quivering-3J mice show prominent spontaneous and evoked hyperactivities at diaphragm neuromuscular junctions. These neuromyotonic and myokymic discharges were more prominent in adult animals when tremors and ataxia were pronounced. Recordings of sciatic and phrenic nerves showed that the hyperactivities originate in myelinated axons distally from nerve terminals. Axon and myelin structure in the PNS were unaffected in quivering-3J mice. Of interest, KCNQ2 subunit aggregates were undetectable at PNS and CNS nodes, whereas Nav and Kv1.1/Kv1.2 channels were properly concentrated at nodal and juxtaparanodal regions, respectively. The protein level of KCNQ2 subunits was normal in mutant animals, suggesting that KCNQ2 subunit absence stems from clustering or trafficking defects in axons. The quivering-3J nodes also presented high densities of ankyrin-G and CK2α, two cytosolic molecules involved with aggregating Nav and KCNQ2/3 channels in axons. Because βIV-spectrin does not interact with KCNQ2/3 subunits, it is suspected that βIV-spectrin regulates the distribution of KCNQ2/3 subunits in axonal subdomains via regulatory partners. Retigabine, an activator of KCNQ2/3 channels, attenuated the repetitive activities in quivering-3J mice, suggesting that depletion of KCNQ2 subunits at nodes initiates neuromyotonic/myokymic discharges. These findings demonstrate that spectrin cytoskeleton finely regulates ion channel distribution and implicates KCNQ2/3 subunits in axonal excitability and in myokymia aetiology.
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