JNK2, a Newly-Identified SERCA2 Enhancer, Augments an Arrhythmic [Ca 2+ ] SR Leak-Load Relationship

Abstract

Rationale: We recently discovered pivotal contributions of stress kinase JNK2 (c-Jun N-terminal kinase isoform 2) in increased risk of atrial fibrillation through enhanced diastolic sarcoplasmic reticulum (SR) calcium (Ca 2+ ) leak via RyR2 (ryanodine receptor isoform 2). However, the role of JNK2 in the function of the SERCA2 (SR Ca 2+ -ATPase), essential in maintaining SR Ca 2+ content cycling during each heartbeat, is completely unknown. Objective: To test the hypothesis that JNK2 increases SERCA2 activity SR Ca 2+ content and exacerbates an arrhythmic SR Ca 2+ content leak-load relationship. Methods and Results: We used confocal Ca 2+ imaging in myocytes and HEK-RyR2 (ryanodine receptor isoform 2-expressing human embryonic kidney 293 cells) cells, biochemistry, dual Ca 2+ /voltage optical mapping in intact hearts from alcohol-exposed or aged mice (where JNK2 is activated). We found that JNK2, but not JNK1 (c-Jun N-terminal kinase isoform 1), increased SERCA2 uptake and consequently elevated SR Ca 2+ content load. JNK2 also associates with and phosphorylates SERCA2 proteins. JNK2 causally enhances SERCA2-ATPase activity via increased maximal rate, without altering Ca 2+ affinity. Unlike the CaMKII (Ca 2+ /calmodulin-dependent kinase II)-dependent JNK2 action in SR Ca 2+ leak, JNK2-driven SERCA2 function was CaMKII independent (not prevented by CaMKII inhibition). With CaMKII blocked, the JNK2-driven SR Ca 2+ loading alone did not significantly raise leak. However, with JNK2-CaMKII–driven SR Ca 2+ leak present, the JNK2-enhanced SR Ca 2+ uptake limited leak-induced reduction in SR Ca 2+ , normalizing Ca 2+ transient amplitude, but at a higher arrhythmogenic SR Ca 2+ leak. JNK2-specific inhibition completely normalized SR Ca 2+ handling, attenuated arrhythmic Ca 2+ activities, and alleviated atrial fibrillation susceptibility in aged and alcohol-exposed myocytes and intact hearts. Conclusions: We have identified a novel JNK2-induced activation of SERCA2. The dual action of JNK2 in CaMKII-dependent arrhythmic SR Ca 2+ leak and a CaMKII-independent uptake exacerbates atrial arrhythmogenicity, while helping to maintain normal levels of Ca 2+ transients and heart function. JNK2 modulation may be a novel therapeutic target for atrial fibrillation prevention and treatment.