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.