Overexpression of CaMKIIδc in RyR2R4496C+/− Knock-In Mice Leads to Altered Intracellular Ca2+ Handling and Increased Mortality

ObjectivesWe investigated whether increased Ca2+/calmodulin-dependent kinase II (CaMKII) activity aggravates defective excitation-contraction coupling and proarrhythmic activity in mice expressing R4496C mutated cardiac ryanodine receptors (RyR2).BackgroundRyR2 dysfunction is associated with arrhythmic events in inherited and acquired cardiac disease.MethodsCaMKIIδc transgenic mice were crossbred with RyR2R4496C+/− knock-in mice.ResultsHeart weight-to-body weight ratio in CaMKIIδc/RyR2R4496C 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/RyR2R4496C and CaMKIIδc mice. Sarcoplasmic reticulum Ca2+ content in isolated myocytes was decreased to a similar extent in CaMKIIδc/RyR2R4496C and CaMKIIδc mice. However, relaxation parameters and Ca2+ decay at 1 Hz were prolonged significantly in CaMKIIδc mice versus CaMKIIδc/RyR2R4496C mice. Sarcoplasmic reticulum Ca2+ spark frequency and characteristics indicated increased sarcoplasmic reticulum Ca2+ leak in CaMKIIδc/RyR2R4496C versus CaMKIIδc myocytes (p < 0.05), most likely because of increased RyR2 phosphorylation. Delayed afterdepolarizations were significantly more frequent with increased amplitudes in CaMKIIδc/RyR2R4496C versus CaMKIIδc mice. Increased arrhythmias in vivo (67% vs. 25%; p < 0.05) may explain the increased mortality in CaMKIIδc/RyR2R4496C mice, which died prematurely with only 30% alive (vs. 60% for CaMKIIδc, p < 0.05) after 14 weeks.ConclusionsCaMKIIδc overexpression in RyR2R4496C+/− knock-in mice increases the propensity toward triggered arrhythmias, which may impair survival. CaMKII contributes to further destabilization of a mutated RyR2 receptor.