JPH-2 interacts with Cai-handling proteins and ion channels in dyads: Contribution to premature ventricular contraction–induced cardiomyopathy

Abstract

Background In a canine model of premature ventricular contraction-induced cardiomyopathy (PVC-CM), Cav1.2 is downregulated and misplaced from t-tubules. Junctophilin-2 (JPH-2) is also downregulated. Objective To understand the role of JPH-2 in PVC-CM, and to probe changes in other proteins involved in dyad structure and function. Methods We quantify t-tubule contents (di-8-ANEPPS fluorescence in live myocytes), examine myocyte ultra-structures (electron microscopy), probe JPH-2 interacting proteins (co-immunoprecipitation), quantify dyad and non-dyad protein levels (immunoblotting), and examine subcellular distributions of dyad proteins (immunofluorescence/confocal microscopy). We also test direct JPH-2 modulation of channel function (vs indirect modulation through dyad formation) using heterologous expression. Results PVC myocytes have reduced t-tubule contents but otherwise normal ultra-structures. Among nineteen proteins examined, only JPH-2, bridging-integrator-1 (BIN-1) and Cav1.2 are highly downregulated in PVC hearts. However, statistical analysis indicates a general reduction of dyad protein levels when JPH-2 is downregulated. Furthermore, several dyad proteins, including Na/Ca exchanger, are missing or shifted from dyads to peripheral surface in PVC myocytes. JPH-2 directly or indirectly interacts with Cai-handling proteins, Cav1.2 and KCNQ1, although not BIN-1 or other scaffolding proteins tested. Expression in mammalian cells, that do not have dyads, confirms direct JPH-2 modulation of ICaL (Cav1.2/Cavβ2) and IKs (KCNQ1/KCNE1). Conclusion JPH-2 is more than a ‘dyad glue’: it can modulate Cai-handling and ion channel function in the dyad region. Downregulation of JPH-2, BIN-1 and Cav1.2 plays a deterministic role in PVC-CM. Dissecting the hierarchical relationship among the three is necessary for the design of therapeutic interventions to prevent the progression of PVC-CM.

Regulation of Cardiomyocyte T-Tubular Structure: Opportunities for Therapy

“…Similarly, a mutation of JPH2 (E169K) which reduces its binding to RyR was observed to increase RyR opening (leak) [90], while JPH2 overexpression was associated with inhibition of Ca 2+ sparks [88]. Other proposed regulatory roles of JPH2 in the dyad include modulation of LTCCs, as reported previously in skeletal muscle [91] and recently in heart [92]. …”

Junctophilin-2 at the intersection of arrhythmia and pathologic cardiac remodeling

“…This model was used to demonstrate a loss of JPH2 expression associated with impaired excitation-contraction coupling and decreased systolic function in PVC-induced cardiomyopathy. 8 Jiang et al 2 expand on this work by highlighting the role of JPH2 as a central player in the mechanism of PVC-induced cardiomyopathy, in which several dyadic proteins including Cav1.2 and JPH2 are downregulated. The authors delve further into the downstream consequences of JPH2 loss by performing a co-immunoprecipitation screening of JPH2 with various dyadic proteins including Ca handling, scaffolding, and ion channel proteins.…”

“…1 This so-called PVC-induced cardiomyopathy represents a mechanism by which dyssynchro-nous electrical activation of the heart progressed to pathologic remodeling. In this issue of Heart Rhythm, Jiang et al 2 use a canine model to explore the link between PVCs and the development of heart failure. This study provides some interesting novel mechanistic insights into the mechanisms underlying Ca handling in the heart.…”

“…3 JPH2 dysregulation has been implicated in both hypertrophic and dilated cardiomyopathy in humans and in rodent models of disease. In this article, Jiang et al 2 identify an association between canine PVC-induced cardiomyopathy and loss of JPH2 expression with derangement in T-tubule ultrastruc-ture. This work builds on initial observations that loss of JPH2 in mouse hearts leads to T-tubule disruption and subsequent cardiomyopathy owing to inefficient Ca signaling.…”

“…Jiang et al 2 highlight the unique regulatory role that JPH2 has in cardiac ion channels. They demonstrated that JPH2 positively regulates Cav1.2 when overexpressed in HEK293 cells.…”