Functional consequences of stably expressing a mutant calsequestrin (CASQ2D307H) in the CASQ2 null background

Abstract

The role of calsequestrin (CASQ2) in cardiac sarcoplasmic reticulum (SR) calcium (Ca2+) 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 CASQ2D307Hmutant protein in the CASQ2 null mouse. We recently reported that the mutant CASQ2D307Hprotein can be stably expressed in CASQ2 null hearts, and it targets appropriately to the junctional SR (Kalyanasundaram A, Bal NC, Franzini-Armstrong C, Knollmann BC, Periasamy M. J Biol Chem 285: 3076–3083, 2010). In this study, we found that introduction of CASQ2D307Hprotein 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 Ca2+load. We also found that the Ca2+transient decays slower in the CASQ2 null and CASQ2D307Hcells. CASQ2D307Hmyocytes, when rhythmically paced and challenged with isoproterenol, exhibit spontaneous Ca2+waves similar to CASQ2 null myocytes; however, the stability of Ca2+cycling was increased in the CASQ2D307Hmyocytes. In the presence of isoproterenol, Ca2+-transient amplitude in CASQ2D307Hmyocytes was significantly decreased, possibly indicating an inherent defect in Ca2+buffering capacity and release from the mutant CASQ2 at high Ca2+concentrations. We also observed polymorphic ventricular tachycardia in the CASQ2D307Hmice, although lesser than in the CASQ2 null mice. These data suggest that CASQ2D307Hpoint mutation may affect Ca2+buffering capacity and Ca2+release. We propose that poor interaction between CASQ2D307Hand triadin 1 could affect ryanodine receptor 2 stability, thereby increasing susceptibility to delayed afterdepolarizations and triggered arrhythmic activity.