Overexpression of CaMKIIδc in RyR2R4496C+/- knock-in mice leads to altered intracellular Ca2+ handling and increased mortality.

Abstract

We investigated whether increased Ca(2+)/calmodulin-dependent kinase II (CaMKII) activity aggravates defective excitation-contraction coupling and proarrhythmic activity in mice expressing R4496C mutated cardiac ryanodine receptors (RyR2). RyR2 dysfunction is associated with arrhythmic events in inherited and acquired cardiac disease. CaMKIIδc transgenic mice were crossbred with RyR2(R4496C+/-) knock-in mice. Heart weight-to-body weight ratio in CaMKIIδc/RyR2(R4496C) and CaMKIIδc mice was similarly increased approximately 3-fold versus wild-type mice (p < 0.05). Echocardiographic data showed comparable cardiac dilation and impaired contractility in CaMKIIδc/RyR2(R4496C) and CaMKIIδc mice. Sarcoplasmic reticulum Ca(2+) content in isolated myocytes was decreased to a similar extent in CaMKIIδc/RyR2(R4496C) and CaMKIIδc mice. However, relaxation parameters and Ca(2+) decay at 1 Hz were prolonged significantly in CaMKIIδc mice versus CaMKIIδc/RyR2(R4496C) mice. Sarcoplasmic reticulum Ca(2+) spark frequency and characteristics indicated increased sarcoplasmic reticulum Ca(2+) leak in CaMKIIδc/RyR2(R4496C) versus CaMKIIδc myocytes (p < 0.05), most likely because of increased RyR2 phosphorylation. Delayed afterdepolarizations were significantly more frequent with increased amplitudes in CaMKIIδc/RyR2(R4496C) versus CaMKIIδc mice. Increased arrhythmias in vivo (67% vs. 25%; p < 0.05) may explain the increased mortality in CaMKIIδc/RyR2(R4496C) mice, which died prematurely with only 30% alive (vs. 60% for CaMKIIδc, p < 0.05) after 14 weeks. CaMKIIδc overexpression in RyR2(R4496C+/-) knock-in mice increases the propensity toward triggered arrhythmias, which may impair survival. CaMKII contributes to further destabilization of a mutated RyR2 receptor.