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Trafficking and Gating Cooperation Between Deficient Nav1.5-mutant Channels to Rescue INa.

BACKGROUND
Pathogenic variants in SCN5A, the gene encoding the cardiac Na+ channel α-subunit Nav1.5, 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 Nav1.5 mutation has unique consequences on the channel trafficking and gating capabilities. Recently, we established that sodium channel α-subunits Nav1.5, Nav1.1 and Nav1.2 could dimerize, thus, explaining the potency of some Nav1.5 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 Nav1.5 channels can cooperate, or transcomplement each other, to rescue the Na+ current (INa). 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 INa, 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 INa, demonstrating a cooperative trafficking of Nav1.5 α-subunits. Surprisingly, we also showed a cooperation mechanism whereby the R878C gating-deficient channel was able to rescue the slowed inactivation kinetics of the C-terminal truncated R1860X (ΔCter) variant, suggesting coupled gating.


CONCLUSIONS
Altogether, our results add to the evidence that Nav 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.