RyR2 mutations linked to ventricular tachycardia and sudden death reduce the threshold for store-overload-induced Ca 2+ release (SOICR)

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Abstract
The cardiac ryanodine receptor (RyR2) governs the release of Ca 2+ from the sarcoplasmic reticulum, which initiates muscle contraction. Mutations in RyR2 have been linked to ventricular tachycardia (VT) and sudden death, but the precise molecular mechanism is unclear. It is known that when the sarcoplasmic reticulum store Ca 2+ content reaches a critical level, spontaneous Ca 2+ release occurs, a process we refer to as store-overload-induced Ca 2+ release (SOICR). In view of the well documented arrhythmogenic nature of SOICR, we characterized the effects of disease-causing RyR2 mutations on SOICR in human embryonic kidney (HEK)293 cells and found that, at elevated extracellular Ca 2+ levels, HEK293 cells expressing RyR2 displayed SOICR in a manner virtually identical to that observed in cardiac cells. Using this cell model, we demonstrated that the RyR2 mutations linked to VT and sudden death, N4104K, R4496C, and N4895D, markedly increased the occurrence of SOICR. At the molecular level, we showed that these RyR2 mutations increased the sensitivity of single RyR2 channels to activation by luminal Ca 2+ and enhanced the basal level of [ 3 H]ryanodine binding. We conclude that disease-causing RyR2 mutations, by enhancing RyR2 luminal Ca 2+ activation, reduce the threshold for SOICR, which in turn increases the propensity for triggered arrhythmia. Abnormal RyR2 luminal Ca 2+ activation likely contributes to the enhanced SOICR commonly observed in various cardiac conditions, including heart failure, and may represent a unifying mechanism for Ca 2+ overload-associated VT.

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