Abstract
BACKGROUND: In a canine model of premature ventricular contraction-induced cardiomyopathy (PVC-CM), Cav1.2 is downregulated and misplaced from transverse tubules (T tubules). Junctophilin-2 (JPH-2) is also downregulated. OBJECTIVES: The objectives of this study were 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 ultrastructures (electron microscopy), probe JPH-2-interacting proteins (co-immunoprecipitation), quantify dyad and nondyad 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 ultrastructures. Among 19 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 in dyad protein levels when JPH-2 is downregulated. Furthermore, several dyad proteins, including Na/Ca exchanger, are missing or shifted from dyads to the 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 the L-type Ca channel current (Cav1.2/voltage-gated Ca channel β subunit 2) and slow delayed rectifier current (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.