Mechanistic insights into the interaction of cardiac sodium channel Na_v1.5 with MOG1 and a new molecular mechanism for Brugada syndrome

Background

Mutations in cardiac sodium channel Na_v1.5 cause Brugada syndrome (BrS). MOG1 is a chaperone that binds to Na_v1.5, facilitates Na_v1.5 trafficking to the cell surface, and enhances the amplitude of sodium current I_Na.

Objective

The purpose of this study was to identify structural elements involved in MOG1-Na_v1.5 interaction and their relevance to the pathogenesis of BrS.

Methods

Systematic analyses of large deletions, microdeletions, and point mutations, and glutathione S-transferases pull-down, co-immunoprecipitation, cell surface protein quantification, and patch-clamping of I_Na were performed.

Results

Large deletion analysis defined the MOG1-Na_v1.5 interaction domain to amino acids S₄₇₆-H₅₈₅ of Na_v1.5 Loop I connecting transmembrane domains I and II. Microdeletion and point mutation analyses further defined the domain to F₅₃₀T₅₃₁F₅₃₂R₅₃₃R₅₃₄R₅₃₅. Mutations F530A, F532A, R533A, and R534A, but not T531A and R535A, significantly reduced MOG1-Na_v1.5 interaction and eliminated MOG1-enhanced I_Na. Mutagenesis analysis identified D24, E36, D44, E53, and E101A of MOG1 as critical residues for interaction with Na_v1.5 Loop I. We then characterized 3 mutations at the MOG1-Na_v1.5 interaction domain: p.F530V, p.F532C, and p.R535Q reported from patients with long QT syndrome and BrS. We found that p.F532C reduced MOG1-Na_v1.5 interaction and eliminated MOG1 function on I_Na; p.R535Q is also a loss-of-function mutation that reduces I_Na amplitude in a MOG1-independent manner, whereas p.F530V is benign as it does not have an apparent effect on MOG1 and I_Na.

Conclusion

Our findings define the MOG1-Na_v1.5 interaction domain to a 5-amino-acid motif of F₅₃₀T₅₃₁F₅₃₂R₅₃₃R₅₃₄ in Loop I. Mutation p.F532C associated with BrS abolishes Na_v1.5 interaction with MOG1 and reduces MOG1-enhanced I_Na density, thereby uncovering a novel molecular mechanism for the pathogenesis of BrS.

Introduction

Gain-of-function mutations in the SCN5A gene encoding cardiac sodium channel Na_v1.5 cause type 3 long QT syndrome (LQTS),¹ and loss-of-function mutations cause Brugada syndrome (BrS), cardiac conduction disease, sick sinus syndrome, and atrial fibrillation (AF).^2,3 Na_v1.5 is a large transmembrane protein, which consists of an intracellular N-terminus, 4 homologous transmembrane domains (DI to DIV) that are connected to each other by intracellular Loop I, Loop II, and Loop III, and the intracellular C-terminus.² Full function of the cardiac sodium channel requires the assembly of a macromolecular complex with more than 20 other proteins, including 4 β-subunits, ankyrin, calmodulin, Nedd4-2, FGF13, SAP97, αB-Crystallin, and MOG1.⁴

We previously used yeast 2-hybrid screen to identify MOG1 as a chaperone that interacts with Na_v1.5 and increases I_Na density by promoting endoplasmic reticulum exit of Na_v1.5 and increasing Na_v1.5 trafficking to plasma membranes.5, 6, 7 Knockdown of mog1 significantly decreased the heart rate and induced abnormal cardiac morphogenesis in zebrafish embryos.⁸ Knockout of mog1 led to cardiac hypertrophy and heart failure, prolonged QRS and QTc intervals on electrocardiogram (ECG), and reduced the heart rate in adult zebrafish.⁹ Kattygnarath et al¹⁰ found a p.E83D mutation of MOG1 in a patient affected with BrS.

In this study, we investigated the mechanism by which Na_v1.5 interacts with MOG1. Moreover, we characterized 1 BrS mutation (p.F532C)¹¹ and 2 LQTS mutations (p.F530V and p.R535Q)¹² located in and immediately after the Na_v1.5-MOG1 domain, and identified a novel molecular mechanism for the pathogenesis of BrS.