JNK2, a Newly-Identified SERCA2 Enhancer, Augments an Arrhythmic [Ca <sup>2+</sup> ] <sub>SR</sub> Leak-Load Relationship
Jiajie Yan, Dan J. Bare, Jaime DeSantiago, Weiwei Zhao, Yiming Mei, Zhenhui Chen, Kenneth S. Ginsburg, R. John Solaro, Beata M. Wolska, Donald M. Bers, S.R. Wayne Chen, Xun Ai
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.