Regulation of ryanodine receptors by luminal calcium

Muscle contraction is initiated by Ca2+ release from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) upon membrane depolarization. In addition to this depolarization-triggered SR Ca2+ release, spontaneous Ca2+ release via RyRs occurs under the conditions of SR Ca2+ overload, a process we referred to as store-overload-induced Ca2+ release (SOICR). Although evidence has shown the importance of SOICR in cardiac arrhythmia, the molecular mechanism underlying SOICR is unclear. We have recently demonstrated that RyR2 mutations associated with cardiac arrhythmia and sudden death enhance SOICR by increasing the sensitivity of the channel to luminal Ca2+ activation. The overall objective of this thesis is to investigate the molecular basis of SOICR and luminal Ca2+ activation, and the impact of some arrhythmogenic agents on SOICR.; It is known that skeletal and cardiac muscle differ markedly in SOICR activities. We have demonstrated that the intrinsic properties of RyR1 and RyR2 channels are in part responsible for this difference. Moreover, studies using chimeras of RyR1 and RyR2 have shown that the C-terminal region of the RyR is a critical determinant of both SOICR and luminal Ca2+ activation. To further define the role of the C-terminal domain in SOICR and luminal Ca2+ sensing, site-directed mutagenesis was performed on unique residues in or near the channel pore region. These studies reveal that the predicted channel gating domain may be involved in the luminal Ca 2+ activation of RyRs.; Given its link to cardiac arrhythmia caused by natural mutations, whether SOICR is associated with drug-induced arrhythmia is also investigated. The results of this study show that the arrhythmogenic methylxanthines enhance the propensity of cells expressing RyR2 for SOICR by preferentially sensitizing the RyR2 channel to activation by luminal Ca2+, which mimics the effects of mutations causing catecholaminergic polymorphic ventricular tachycardia (CPVT) on RyR2.; Overall, these studies provide new insights into the molecular basis of SOICR and the regulation of RyRs by luminal Ca2+, and have important implication for the treatment of cardiac and other muscle diseases.