Trafficking and Gating Cooperation Between Deficient Na(v)1.5-mutant Channels to Rescue INa

Background: Pathogenic variants in SCN5A, the gene encoding the cardiac Na+ channel alpha-subunit Na(v)1.5, result in life-threatening arrhythmias, e.g., Brugada syndrome, cardiac conduction defects and long QT syndrome. This variety of phenotypes is underlied by the fact that each Na(v)1.5 mutation has unique consequences on the channel trafficking and gating capabilities. Recently, we established that sodium channel alpha-subunits Na(v)1.5, Na(v)1.1 and Na(v)1.2 could dimerize, thus, explaining the potency of some Na(v)1.5 pathogenic variants to exert dominant-negative effect on WT channels, either by trafficking deficiency or coupled gating. Objective: The present study sought to examine whether Na(v)1.5 channels can cooperate, or transcomplement each other, to rescue the Na+ current (I-Na). Such a mechanism could contribute to explain the genotype-phenotype discordance often observed in family members carrying Na+-channel pathogenic variants. Methods: Patch-clamp and immunocytochemistry analysis were used to investigate biophysical properties and cellular localization in HEK293 cells and rat neonatal cardiomyocytes transfected respectively with WT and 3 mutant channels chosen for their particular trafficking and/or gating properties. Results: As previously reported, the mutant channels G1743R and R878C expressed alone in HEK293 cells both abolished I-Na, G1743R through a trafficking deficiency and R878C through a gating deficiency. Here, we showed that coexpression of both G1743R and R878C nonfunctioning channels resulted in a partial rescue of I-Na, demonstrating a cooperative trafficking of Na(v)1.5 alpha-subunits. Surprisingly, we also showed a cooperation mechanism whereby the R878C gatingdeficient channel was able to rescue the slowed inactivation kinetics of the C-terminal truncated R1860X (Delta Cter) variant, suggesting coupled gating. Conclusions: Altogether, our results add to the evidence that Na-v channels are able to interact and regulate each other's trafficking and gating, a feature that likely contributes to explain the genotype-phenotype discordance often observed between members of a kindred carrying a Na+-channel pathogenic variant.