Functional consequences of stably expressing a mutant calsequestrin (CASQ2 ^D307H) in the CASQ2 null background

The role of calsequestrin (CASQ2) in cardiac sarcoplasmic reticulum (SR) calcium (Ca ²⁺) transport has gained significant attention since point mutations in CASQ2 were reported to cause ventricular arrhythmia. In the present study, we have critically evaluated the functional consequences of expressing the CASQ2 ^D307H mutant protein in the CASQ2 null mouse. We recently reported that the mutant CASQ2 ^D307H protein can be stably expressed in CASQ2 null hearts, and it targets appropriately to the junctional SR (Kalyanasundaram A, Bal NC, Franzini-Arm-strong C, Knollmann BC, Periasamy M. J Biol Chem 285: 3076-3083, 2010). In this study, we found that introduction of CASQ2 ^D307H protein in the CASQ2 null background partially restored triadin 1 levels, which were decreased in the CASQ2 null mice. Despite twofold expression (relative to wild-type CASQ2), the mutant protein failed to increase SR Ca ²⁺ load. We also found that the Ca ²⁺ transient decays slower in the CASQ2 null and CASQ2 ^D307H cells. CASQ2 ^D307H myocytes, when rhythmically paced and challenged with isoproterenol, exhibit spontaneous Ca ²⁺ waves similar to CASQ2 null myocytes; however, the stability of Ca ²⁺ cycling was increased in the CASQ2 ^D307H myocytes. In the presence of isoproterenol, Ca ²⁺-transient amplitude in CASQ2 ^D307H myocytes was significantly decreased, possibly indicating an inherent defect in Ca ²⁺ buffering capacity and release from the mutant CASQ2 at high Ca ²⁺ concentrations. We also observed polymorphic ventricular tachycardia in the CASQ2 ^D307H mice, although lesser than in the CASQ2 null mice. These data suggest that CASQ2 ^D307H point mutation may affect Ca ²⁺ buffering capacity and Ca ²⁺ release. We propose that poor interaction between CASQ2 ^D307H and triadin 1 could affect ryanodine receptor 2 stability, thereby increasing susceptibility to delayed after-depolarizations and triggered arrhythmic activity.