It is known that sarcoplasmic reticulum (SR) Ca2+ release in cardiac muscle is initiated via cardiac ryanodine receptor (RyR2) through a mechanism called Ca2+-induced Ca2+ release. However, how the SR Ca2+ release is terminated is undetermined. The objective of the current study is to understand the molecular basis and regulation of RyR2-mediated Ca2+ release termination and its role in the pathogenesis of cardiac diseases.
Based on recent 3D structural analyses, the NH2-terminal region of RyR2 interacts with the channel domain via the central domain and undergoes dynamic conformational changes during channel gating. It has also been discovered that the NH2-terminal region consists of three distinct domains. HEK293 cell studies on domain deletions and disease mutations demonstrate that the different domains play different roles in RyR2 function. The NH2-terminal region is a major determinant of Ca2+ release activation and termination.
Enhanced luminal Ca2+ activation of RyR2 has been linked to catecholaminergic polymorphic ventricular tachycardia (CPVT). However, in addition to CPVT, many RyR2 mutations can also cause cardiomyopathies. Knock-in mouse models harboring cardiomyopathy- associated RyR2 mutations have been generated to investigate the causal mechanisms of cardiomyopathies. The exon-3 deletion mouse model exhibited markedly reduced RyR2 expression level and no characteristic phenotype. The RyR2-R420W mouse model showed enhanced susceptibility to CPVT and altered cytosolic Ca2+ transient properties, suggesting that the abnormal cytosolic Ca2+ transient may be a key factor in the pathogenesis of cardiomyopathies.
Calmodulin (CaM) is a regulatory protein that binds and inhibits RyR2 in the presence of cytosolic Ca2+. The inhibitory effect depends on the affinity of CaM for Ca2+ and RyR2. CaM mutations affecting either aspect may result in aberrant regulation of RyR2 activity, hence abnormal Ca2+ release termination. Indeed, arrhythmogenic mutations and most newly discovered CaM variants delayed Ca2+ release termination while others enhanced it, indicating that CaM is a major modulator of RyR2-mediated Ca2+ release termination.
Overall, Ca2+ release termination is an intrinsic property of RyR2 that can be regulated by modulators such as CaM. Altered Ca2+ release termination is critically involved in the pathogenesis of cardiac diseases.