PDZ Domain–Binding Motif Regulates Cardiomyocyte Compartment-Specific Na V 1.5 Channel Expression and Function

Abstract

Background— Sodium channel Na V 1.5 underlies cardiac excitability and conduction. The last 3 residues of Na V 1.5 (Ser-Ile-Val) constitute a PDZ domain–binding motif that interacts with PDZ proteins such as syntrophins and SAP97 at different locations within the cardiomyocyte, thus defining distinct pools of Na V 1.5 multiprotein complexes. Here, we explored the in vivo and clinical impact of this motif through characterization of mutant mice and genetic screening of patients. Methods and Results— To investigate in vivo the regulatory role of this motif, we generated knock-in mice lacking the SIV domain (ΔSIV). ΔSIV mice displayed reduced Na V 1.5 expression and sodium current ( I Na ), specifically at the lateral myocyte membrane, whereas Na V 1.5 expression and I Na at the intercalated disks were unaffected. Optical mapping of ΔSIV hearts revealed that ventricular conduction velocity was preferentially decreased in the transversal direction to myocardial fiber orientation, leading to increased anisotropy of ventricular conduction. Internalization of wild-type and ΔSIV channels was unchanged in HEK293 cells. However, the proteasome inhibitor MG132 rescued ΔSIV I Na , suggesting that the SIV motif is important for regulation of Na V 1.5 degradation. A missense mutation within the SIV motif (p.V2016M) was identified in a patient with Brugada syndrome. The mutation decreased Na V 1.5 cell surface expression and I Na when expressed in HEK293 cells. Conclusions— Our results demonstrate the in vivo significance of the PDZ domain–binding motif in the correct expression of Na V 1.5 at the lateral cardiomyocyte membrane and underline the functional role of lateral Na V 1.5 in ventricular conduction. Furthermore, we reveal a clinical relevance of the SIV motif in cardiac disease.