Molecular basis of ryanodine receptor regulation by luminal ca2+ and its roles in diseases
Creators: name: Chen, Wenqian
Publisher: name: University of Calgary
Year: 2012

description: Bibliography: p. 220-245
description: Some pages are in colour.
description: Includes copy of animal protocol approval. Original copy with original Partial Copyright Licence.
description: It has been known since the 1970s that intracellular Ca2+ store overload triggers spontaneous local Ca2+ release and propagating Ca2+ waves in the heart. Spontaneous Ca2+ release, also known as store-overload-induced Ca2+ release (SOICR), can trigger life-threatening ventricular tachyarrhythrnias (VTs) in the heart. It has been well elucidated that reduced threshold for SOICR is a common feature of cardiac ryanodine receptor (RyR2) mutations associated with catecholaminergic polymorphic ventricular tachycardia (CPVT). However, the molecular mechanism underlying SOI CR has remained a mystery for decades. The overall objective of this study is to investigate the molecular mechanism by which intra-luminal Ca2+ activates RyR2 and triggers spontaneous Ca2+ release, and to determine the role of spontaneous Ca2+ release in congenital diseases caused by mutations in skeletal ryanodine receptor (RyRl) and RyR2. A multi-disciplinary approach was applied to investigate the molecular mechanism underlying congenital skeletal muscle diseases associated with RyRl mutations and to identify the molecular determinant of SOICR. Mutations in RyRl, which share a similar distribution with mutations in RyR2 in the linear sequence, have been found to be linked to malignant hyperthermia (MH) and central core disease (CCD). The disease-causing mutations in RyRl were systemically characterized. It was found that the naturally occurring mutations in RyRl cause MH by increasing the channel's sensitivity to luminal Ca2+ activation, resulting in a decreased threshold for spontaneous Ca2+ release. Dantrolene, the unique drug for treating the MH episode, acts by suppressing spontaneous Ca2+ release via RyRl mutants. Since altered threshold for spontaneous Ca2+ release, which is dependent on the luminal Ca2+ sensitivity of the RyR channel, is a common feature of the disease-causing RyRl and RyR2 mutations, the molecular determinant of the lurninal Ca2 sensing of RyR2 was investigated. A luminal Ca2+ sensor in the helix-bundle crossing region of the RyR2 channel pore was identified. It was found that residue E4872 is the major determinant for luminal Ca2+ sensing of the RyR2 channel. Removing the negative charge from this residue abolished the luminal, but not cytosolic, Ca2+ activation of the channel. Introducing metal-binding histidines at this site converts RyR2 into a Ni2+ gated channel. Relocating the negative charge to the adjacent residue G4871 restored channel's sensitivity to luminal Ca2+ activation. To further investigate the physiological and pathological significance of the luminal Ca2+ sensor, a knock-in mouse model harboring a luminal Ca2+-sensing mutation, E4872Q, was generated. It was found that the hearts from E4872Q+/- mice are resistant to SOICR and are completely protected against SOICR-evoked VTs. Thus, the RyR2 gate contains a store Ca2+ sensor that governs luminal Ca2+ regulation ofRyR2, SOICR, and SOICR-evoked VTs. This novel store-sensing gate is conserved in all RyRs and inositol 1,4,5-trisphosphate receptors, implying that this may be a common mode of Ca2+ release channel regulation. Overall, this study advances our general understanding of the essential role of RyR2 in the initiation of SOI CR, and provides insight into the physiological and pathological significance of SOICR in the heart.