Decreased RyR2 refractoriness determines myocardial synchronization of aberrant Ca ²⁺ release in a genetic model of arrhythmia

Abstract

Dysregulated intracellular Ca2+ signaling is implicated in a variety of cardiac arrhythmias, including catecholaminergic polymorphic ventricular tachycardia. Spontaneous diastolic Ca 2+ release (DCR) can induce arrhythmogenic plasma membrane depolarizations, although the mechanism responsible for DCR synchronization among adjacent myocytes required for ectopic activity remains unclear. We investigated the synchronization mechanism(s) of DCR underlying untimely action potentials and diastolic contractions (DCs) in a catecholaminergic polymorphic ventricular tachycardia mouse model with a mutation in cardiac calsequestrin. We used a combination of different approaches including single ryanodine receptor channel recording, optical imaging (Ca 2+ and membrane potential), and contractile force measurements in ventricular myocytes and intact cardiac muscles. We demonstrate that DCR occurs in a temporally and spatially uniform manner in both myocytes and intact myocardial tissue isolated from cardiac calsequestrin mutation mice. Such synchronized DCR events give rise to triggered electrical activity that results in synchronous DCs in the myocardium. Importantly, we establish that synchronization of DCR is a result of a combination of abbreviated ryanodine receptor channel refractoriness and the preceding synchronous stimulated Ca 2+ release/reuptake dynamics. Our study reveals how aberrant DCR events can become synchronized in the intact myocardium, leading to triggered activity and the resultant DCs in the settings of a cardiac rhythm disorder.calcium-induced calcium release | luminal calcium | RyR2 deactivation | sarcoplasmic reticulum

Gene Transfer of Engineered Calmodulin Alleviates Ventricular Arrhythmias in a Calsequestrin‐Associated Mouse Model of Catecholaminergic Polymorphic Ventricular Tachycardia

“…Similar results were obtained with the CPVT CaM N54I (Figure S1). Together with previous results on CASQ2 and RyR2‐mediated CPVT, these data suggests that shortened RyR2 refractoriness may be a common mechanism that promotes arrhythmias caused by both CASQ2 and CaM mutants 7, 8…”

“…We and others have previously demonstrated that the generation of arrhythmogenic Ca release in CPVT involves shortened refractoriness of RyR2 in CPVT models associated with mutations in CASQ2 7, 21. CPVT mutations in CaM and CASQ2 produced additive effects on RyR2 refractoriness and cellular arrhythmogenesis, supporting the notion that shortened refractoriness is a common mechanistic step in CPVT (Figure 2).…”

“…Genetic causes of certain forms of CPVT (eg, mutations in the luminal RyR2 accessory protein CASQ2) involve premature reactivation of RyR2 via abbreviation of the time RyR2 remains refractory 7. Recently, mutations in CaM have also been shown to cause CPVT 31, 32, 33.…”

“…Such Ca dependent binding would be expected to inhibit RyR2 specifically after Ca release lasting late into the diastolic period. Consistent with this notion, mutations in CaM have recently been linked to CPVT, a syndrome associated with a shortened RyR2 refractory period 7, 8, 21. We reasoned that if indeed certain CPVT‐causing CaM mutations act by shortening RyR2 refractoriness, then CaM variants with slowed Ca dissociation could be engineered to prolong RyR2 refractoriness in cardiac disease settings when refractoriness is pathologically shortened.…”

“…Interestingly, previous work from our and other laboratories have demonstrated that impairment of the ability of RyR2 to become refractory, ie, appropriately close after each systolic Ca release event, appears to be a common mechanism for various forms of genetic and acquired arrhythmias, including CPVT and post‐infarction ventricular fibrillation (VF) 7, 8, 9. Specifically, shortened RyR2 refractoriness results in aberrant diastolic SR Ca release that synchronizes across the myocardium to induce triggered activity, thus, precipitating malignant arrhythmias.…”