Biophysical Investigation of Sodium Channel Interaction with β-Subunit Variants Associated with Arrhythmias

Abstract

Background: Voltage-gated sodium (Na V) channels help regulate electrical activity of the plasma membrane. Mutations in associated subunits can result in pathological outcomes. Here we examined the interaction of Na V channels with cardiac arrhythmia-linked mutations in SCN2B and SCN4B, two genes that encode auxiliary b-subunits. Materials and Methods: To investigate changes in SCN2B R137H and SCN4B I80T function, we combined threedimensional X-ray crystallography with electrophysiological measurements on Na V 1.5, the dominant subtype in the heart. Results: SCN4B I80T alters channel activity, whereas SCN2B R137H does not have an apparent effect. Structurally, the SCN4B I80T perturbation alters hydrophobic packing of the subunit with major structural changes and causes a thermal destabilization of the folding. In contrast, SCN2B R137H leads to structural changes but overall protein stability is unaffected. Conclusion: SCN4B I80T data suggest a functionally important region in the interaction between Na V 1.5 and b4 that, when disrupted, could lead to channel dysfunction. A lack of apparent functional effects of SCN2B R137H on Na V 1.5 suggests an alternative working mechanism, possibly through other Na V channel subtypes present in heart tissue. Indeed, mapping the structural variations of SCN2B R137H onto neuronal Na V channel structures suggests altered interaction patterns.

Late Sodium Current of the Heart: Where Do We Stand and Where Are We Going?

“…Six subtypes of ß subunits (ß1, ß1A, ß1B, ß2-ß4) encoded by four genes have been identified until now, but the presence of ß1A is not proven in mammals [81][82][83][84]. The different ß variants co-assemble with various affinities to different α subunits [85]. Na v 1.5 is shown to combine with all four subtypes, but Na v 1.4 associates only with ß1 [86].…”

“…In the ventricular myocytes of ß1 , knockout mice transient and persistent sodium currents were found to be increased due to increased Na v 1.5 expression [88]. Mutations within ß-subunits are shown to lead to clinical arrhythmias [85]. Recently, Angsutararux et al published interesting data on ß-subunit mutants causing Brugada syndrome, or atrial fibrillation [89].…”

“…The intracellular domain of ß4 subtype is postulated to play a central role in the mechanism of resurgent current in neurons [90]. The presence of resurgent current has not been proved in hearts, but the ß4 subunit has been identified in atrial and ventricular myocytes [85]. Therefore, resurgent current could possibly be present in cardiac muscle too.…”

“…Some of these proteins, such as ß-subunits or ubiquitin, have been the subjects of intensive research for a long time, but others, like small G-proteins and Ankyrin G, have only recently attracted attention. Mutations in some of these accessory proteins have already been linked to various cardiac diseases [85], and we can assume that further harmful mutations are to be uncovered. Recently, there was a boom in this field, resulting in numerous publications.…”