Abstract 13430: ‘Dyads’ Function as Ion Channel Hubs in Cardiac Myocytes and Junctophilin-2 Plays a Critical Role in Stabilizing Channel Expression in Dyads
Background: In cardiomyocytes t-tubules and junctional sarcoplasmic reticulum (jSR) come close to form ‘dyads’, where Ca influx through L-type Ca channels effectively activates Ca release through RyR2. Many other channels also cluster to t-tubules. How are they stabilized here is not clear. Junctophilin-2 (JPH-2) provides a scaffold stabilizing dyads in cardiomyocytes. Whether JPH-2 is also involved in stabilizing channels in t-tubules is not clear. We reported that in a canine model of premature ventricular contraction (PVC)-induced cardiomyopathy JPH-2 protein was conspicuously reduced. This allows us to study how changes in JPH-2 may impact on protein level and distribution of ion channels in dyads.
Methods: Immunoblotting, immunofluorescence/confocal microscopy, [Ca]i monitoring and patch clamp.
Results: (1) JPH-2 reduction is associated with decrease in Cav1.2, RyR2, KCNQ1, and ERG1, but increase in KCNE1. There is no correlation between JPH-2 and KCNE2, Kv4.3, KChIP2, Kir2.1 or Kir2.2. Thus, decrease in JPH-2 selectively impacts on L-type Ca, RyR2, IKs and IKr channels, but not Ito or IK1 channels. (2) There is no correlation between ion channels and other dyad scaffolding proteins: BIN1, caveolin-3, ankyrin-B, and telethonin. (3) PVC myocytes have more and larger LAMP1+ (lysosomal marker) vesicles than CON myocytes, and most LAMP1+ vesicles are also positive for JPH-2, suggesting that PVC-associated stress activates autophagy which enhances JPH-2 degradation. (4) Loss of JPH-2 impairs E-C coupling, due to downregulation of RyR2 and Cav1.2, and a misalignment between the two. (5) Loss of JPH-2 reduces t-tubule density, causing a redistribution of KCNQ1 from dyads to lateral surface membrane. (6) Loss of JPH-2 paradoxically increases IKs, that helps action potential repolarization when IK1 and Ito are both down-regulated, i.e. IKs as a ‘repolarization reserve’.
Conclusion: JPH-2 is critical in stabilizing ion channels in dyads. Stress-induced autophagy targets JPH-2, leading to dyad remodeling, downregulation and/or redistribution of key ion channel subunits. The functional consequences can be detrimental (E-C coupling impairment) but also protective (increase in IKs to protect against excessive action potential prolongation).
Author Disclosures: M. Jiang: None. Y. Wang: None. J.M. Eltit: None. M. Zhang: None. A. Tan: None. K. Kaszala: None. J.F. Huizar: None. R. Balijepalli: None. G. Tseng: None.
This research has received full or partial funding support from the American Heart Association.
- © 2014 by American Heart Association, Inc.