The research proposed in this application is designed to elucidate the structure-function relationships of a form of exercise-induced sudden death known as catecholaminergic polymorphic ventricular tachycardia (CPVT), caused by mutations in the Type-2 ryanodine receptor (RyR2)/calcium release channel. RyR2 channels are required for the release of calcium (Ca2+) from intracellular stores, a process that triggers cellular functions including excitation-contraction (EC) coupling in the cardiac muscle. RyR2, along with RyR1 and RyR3, are the largest known ion channels, comprised of the four identical ~565 kDa channel-forming protomers, as well as regulatory subunits, enzymes, and their respective targeting/anchoring proteins in a macromolecular complex that exceeds three million daltons. It is known that RyR2 mutations cause arrhythmias including exercise-induced sudden death, or CPVT, and stress- induced post-translational modifications of RyR2 contribute to both CPVT and heart failure progression. The applicants have recently obtained near-atomic-level resolution cryo-electron microscopy (cryo-EM) reconstructions of Type-1 RyR (RyR1) from highly purified rabbit skeletal muscle in both the closed and the open states, defining the transmembrane pore in unprecedented detail and placing all cytosolic domains as tertiary folds, including a Ca2+ domain. Using modeling software, the structure of RyR2 has been modeled based on homology with RyR1. We propose to study the localization, structural effects, and function of at least 11 representative pathogenic CPVT mutations, in order to develop a system for understanding how pathogenic genetic variants in different regions of the channel cause clinical disease. These studies will be conducted by solving cryo-EM structures of mutant RyR2 channels and by functionally testing these mutations using a novel, high bandwidth, high-throughput lipid bilayer technology developed by our team. This technology is capable of identifying channel opening events with nanosecond resolution (compared to current single channel current resolution of millisecond resolution). We will then develop a database of all known genetic variants CPVT-associated and systematically engineer these mutations into recombinant RyR2 in order to study these channels using our novel high-throughput lipid bilayer measurement system. The data from this project will be useful for understanding the underlying mechanisms of CPVT. It will provide an approach that can be used to develop therapies for CPVT. It will advance our understanding of novel technologies for studying other diseases caused by RyR2 dysfunction and for studying other ion channels.
Patients with CPVT have mutations in the cardiac ryanodine receptor/calcium release channels that cause the channel to become ?leaky,? which, during exercise, results in pathologic cellular calcium overload, delayed-after-depolarizations, and lethal arrhythmias. Given the large size of RyR2 it is unclear how mutations in different parts of the channel have similar effects on channel function, and the rarity of this disease makes it difficult to assign pathogenicity to newly identified variants that are associated with lethal arrhythmias. By solving the cryo-EM structure of two RyR2-CPVT mutants in multiple states, testing these mutations using a novel high-bandwidth, high-throughput lipid bilayer system, and developing a comprehensive database of RyR2 variants associated with CPVT, we will advance the understanding of how CPVT mutations cause clinical disease and provide a framework for assigning pathogenicity to variants of unknown significance.
