Abstract 2870: The Human Histidine-Rich Ca Binding Protein Ser96Ala Variant Enhances Ryanodine Receptor Activity and Induces Arrhythmias
The histidine-rich calcium binding protein (HRC) is located in the luminal side of sarcoplasmic reticulum (SR). Previous biochemical studies have shown that HRC can bind to triadin (a component of ryanodine receptors complex) and SERCA2a, which suggested that it may play a role in regulating SR Ca2+release and uptake. We identified an HRC Ser96Ala polymorphism, which appears to correlate with ventricular arrhythmias and sudden death in dilated cardiomyopathy (DCM) patients. In the present study, we accessed the mechanism underlying human arrhythmias, by adeno-viral infection of adult rat ventricular cardiomyocytes with wild-type (HRCWT) or mutant HRC (HRCS96A). Total HRC protein was consistently increased by ~50% in both HRCWT and HRCS96A infected myocytes, without any effects on the levels of other major SR Ca2+cycling proteins (SERCA2a, phospholamban, calsequestrin, junctin and triadin). Expression of HRCS96A decreased Ca2+transient amplitude by 30%, while it prolonged transient decay time by 29%, compared with HRCWT. In addition, the frequency of spontaneous Ca2+sparks was increased by 2-fold, which reflected enhanced RyR activity under resting conditions. Caffeine-induced SR Ca2+release was also reduced by 16% in HRCS96A cells, which may be related to leaky RyRs and depressed SERCA2a function. Furthermore, stress conditions induced by 1μM isoproterenol and 2Hz field stimulation, resulted in elevated frequency of Ca2+waves in HRCS96A infected cells, compared with HRCWT. Thus, the HRC Ser96Ala variant increased the rate of Ca2+leak from the SR and enhanced propensity of arrhythmogenic Ca2+waves under stress conditions, suggesting a link between this genetic variant and life-threatening ventricular arrhythmias in human carriers.