SCN5A Knockout Cell Line (HEK293)

Fibroblast growth factor homologous factors (FHFs) bind to the cytoplasmic C-terminus of voltage-gated sodium channels (VGSCs) and modulate channel function. Variants in FHFs or VGSCs perturbing that bimolecular interaction are associated with arrhythmias. Like some channel auxiliary subunits, FHFs exert additional cellular regulatory roles, but whether these alternative roles affect VGSC regulation is unknown. Using a separation-of-function strategy, we show that a structurally guided, binding-incompetent, mutant fibroblast growth factor 13 (FGF13; the major FHF in mouse heart), confers complete regulation of VGSC steady-state inactivation (SSI), the canonical effect of FHFs. In cardiomyocytes isolated from Fgf13-KO mice, expression of the mutant FGF13 completely restores WT regulation of SSI. FGF13 regulation of SSI derives from effects on local accessible membrane cholesterol, which is unexpectedly polarized and concentrated in cardiomyocytes at the intercalated disc (ID), where most VGSCs localize. Fgf13-KO eliminates the polarized cholesterol distribution and causes loss of VGSCs from the ID. Moreover, we show that the previously described FGF13-dependent stabilization of VGSC currents at elevated temperatures depends on the cholesterol mechanism. These results provide new insights into how FHFs affect VGSCs and alter the canonical model by which channel auxiliary subunits exert influence.

AIMS:SCN5A encodes cardiac sodium channel Nav1.5 that maintains normal electrophysiological functions of hearts. Loss-of-function variants of Nav1.5 reduce sodium current densities (INa) and cause arrhythmias such as cardiac conduction block or Brugada syndrome. The regulatory mechanisms of Nav1.5 functions are not fully understood. The aim of this study was to identify novel proteins that interact with Nav1.5 and characterize their regulatory mechanisms on Nav1.5 and arrhythmias. METHODS AND RESULTS:GST pull-down coupled with mass spectrometry, co-immunoprecipitation, and mutational analysis were used to identify de-ubiquitinating enzyme USP10 as a novel Nav1.5-interacting protein, and showed that USP10 reduces Nav1.5 protein expression and INa densities in vitro. AAV9-mediated cardiac overexpression of USP10 in mice reduced Nav1.5 protein expression, INa and ICa-L densities, shortened APD, and caused delayed ventricular activation, spontaneous atrioventricular conduction block, sinus pause, and ventricular tachycardia induced with electrical pacing. Cardiac knockdown of USP10 in Scn5a+/- mice restored Nav1.5, INa, and ICa-L to levels comparable to wild-type mice, and alleviated the conduction delay and premature ventricular contractions. Mechanistically, USP10 increased Nav1.5 protein degradation through chaperone-mediated autophagy (CMA) as the effect was blocked by lysosome inhibitor CQ and inhibition of CMA using siRNA targeting LAMP2A or HSC70, but not by proteasomal inhibitor MG132. Mutational analysis identified the key CMA degradation motif of Nav1.5 as EKRFQ431-435. USP10 decreased Nav1.5 ubiquitination and increased binding of Nav1.5 to HSC70. Mutational analysis identified K430 of Nav1.5 as the USP10 de-ubiquitination site, and K430R mutation blocked regulation of Nav1.5 by USP10. CONCLUSION:We identified a novel CMA-mediated pathway regulating degradation of Nav1.5 by coupling with USP10-mediated de-ubiquitination at K430 of Nav1.5, which resulted in reduced INa densities and cardiac conduction defects. Knockdown of USP10 alleviated arrhythmias in Scn5a+/- mice, providing a novel therapeutic strategy for treating arrhythmias with reduced INa.