Fast opening and closing of voltage-gated sodium channels are crucial for proper propagation of the action potential through excitable tissues. Unlike potassium channels, sodium channel α-subunits are believed to form functional monomers. Yet, an increasing body of literature shows inconsistency with the traditional idea of a single α-subunit functioning as a monomer. Here we demonstrate that sodium channel α-subunits not only physically interact with each other but they actually assemble, function and gate as a dimer. We identify the region involved in the dimerization and demonstrate that 14-3-3 protein mediates the coupled gating. Importantly we show conservation of this mechanism among mammalian sodium channels. Our study not only shifts conventional paradigms in regard to sodium channel assembly, structure, and function but importantly this discovery of the mechanism involved in channel dimerization and biophysical coupling could open the door to new approaches and targets to treat and/or prevent sodium channelopathies.

Computational modeling indicates that cardiac conduction may involve ephaptic coupling – intercellular communication involving electrochemical signaling across narrow extracellular clefts between cardiomyocytes. We hypothesized that β1(SCN1B) –mediated adhesion scaffolds trans-activating NaV1.5 (SCN5A) channels within narrow (<30 nm) perinexal clefts adjacent to gap junctions (GJs), facilitating ephaptic coupling. Super-resolution imaging indicated preferential β1 localization at the perinexus, where it co-locates with NaV1.5. Smart patch clamp (SPC) indicated greater sodium current density (INa) at perinexi, relative to non-junctional sites. A novel, rationally designed peptide, βadp1, potently and selectively inhibited β1-mediated adhesion, in electric cell-substrate impedance sensing studies. βadp1 significantly widened perinexi in guinea pig ventricles, and selectively reduced perinexal INa, but not whole cell INa, in myocyte monolayers. In optical mapping studies, βadp1 precipitated arrhythmogenic conduction slowing. In summary, β1-mediated adhesion at the perinexus facilitates action potential propagation between cardiomyocytes, and may represent a novel target for anti-arrhythmic therapies.

Abstract-Calmodulin (CaM) is a calcium-sensing protein that binds to Na ϩ channels, with unknown functional consequences. Wild-type CaM produced a hyperpolarizing shift in the steady-state availability of expressed skeletal muscle (1) but not cardiac (hH1) Na ϩ channels. Mutant CaM 1234 did not alter the voltage dependence or the kinetics of gating of either 1 or hH1. Mutation of the highly conserved IQ motif in the carboxyl terminus of both isoforms (IQ/AA) slowed the kinetics of current decay and abolished the effect of wild-type CaM on 1, but did not alter hH1 currents. The IQ/AA mutation eliminated CaM binding to the carboxyl terminus of both 1 and hH1 channels. Inhibition of Ca 2ϩ /CaM kinase (CaM-K) slowed the current decay, the rate of entry into inactivation, and shifted the voltage dependence of hH1 in the depolarizing direction independent of CaM overexpression with no effect on 1 Na ϩ channels. CaM signaling modulates Na ϩ currents in an isoform-specific manner, via direct interaction with skeletal muscle Na ϩ channels and through CaM-K in the case of the cardiac isoform.