To investigate whether dantrolene (DAN), cardiac ryanodine receptor (RyR2) stabilizer, improves impaired diastolic function in an early pressure-overloaded hypertrophied heart, pressure-overload hypertrophy was induced by transverse aortic constriction (TAC) in mice. Wild-type (WT) mice were divided into four groups: sham-operated mice (Sham), sham-operated mice treated with DAN (DAN+Sham), TAC mice (TAC), and TAC mice treated with DAN (DAN+TAC). The mice were then followed up for 2 weeks. Left ventricular (LV) hypertrophy was induced in TAC, but not DAN+TAC mice, 2 weeks after TAC. There were no differences in LV fractional shortening among the four groups. Catheter tip micromanometer showed that the time constant of LV pressure decay, an index of diastolic function, was significantly prolonged in TAC but not in DAN+TAC mice. Diastolic function was significantly impaired in TAC, but not in DAN+TAC mice as determined by cell shortening and Ca^{2+} transients. An increase in diastolic Ca^{2+} leakage and a decrease in calmodulin (CaM) binding affinity to RyR2 were observed in TAC mice, while diastolic Ca^{2+} leakage improved in DAN+TAC mice. Thus, DAN prevented the progression of hypertrophy and improved the impairment of LV relaxation by inhibiting diastolic Ca^{2+} leakage through RyR2 and the dissociation of CaM from RyR2.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is caused by a single point mutation in the cardiac type 2 ryanodine receptor (RyR2). Using knock-in mouse (KI) model (R2474S/+), we previously reported that a single point mutation within the RyR2 sensitized the channel to agonists, primarily mediated by defective inter-domain interaction within the RyR2 and subsequent dissociation of calmodulin (CaM) from the RyR2. Here, we examined whether CPVT can be genetically rescued by enhancing the binding affinity of CaM to the RyR2. We first determined whether there was a possible amino-acid substitution within the CaM-binding domain in the RyR2 (3584-3603) that can enhance its binding affinity to CaM, and found that V3599K substitution showed the highest binding affinity of CaM to CaM-binding domain. Hence, we generated a heterozygous KI mouse model (V3599K/+) with a single amino acid substitution in the CaM-binding domain of the RyR2, and crossbred it with the heterozygous CPVT –associated R2474S/+ KI mouse to obtain a double heterozygous R2474S/V3599K KI mouse model. The CPVT phenotypes, bidirectional or polymorphic ventricular tachycardia, were inhibited in the R2474S/V3599K mice. Thus, enhancement of the CaM binding affinity of the RyR2 is essential to prevent CPVT-associated arrhythmogenesis.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited disease characterized by stress- or exercise- induced ventricular tachycardia, frequently leading to sudden cardiac death. A considerable body of evidence accumulated over recent years suggests that mutation-linked cardiac ryanodine receptor (RyR2) defects cause Ca^2+ leak from sarcoplasmic reticulum, which triggers delayed afterdepolarization and leads to CPVT. However, the underlying mechanism, by which a single mutation in such a large molecule causes drastic effects on the channel function, remains elusive. Here we report that introduction of a human CPVT mutation S2246L into the mouse RyR2 induces aberrant activation of channel gating by forming abnormally tight domain-domain interaction between the S2246L mutable domain and the K201-binding domain. This produces more global conformational change in the RyR2: namely, an aberrant domain unzipping between the N-terminal (a.a. 1–600) domain and the central (a.a. 2000–2500) domain owing to the allosteric conformational coupling mechanism. Pharmacological correction of the defective inter-domain interactions can stop the aberrant Ca^2+ release and lethal arrhythmia. These results provide a new pathogenic mechanism of CPVT and a novel therapeutic strategy against CPVT