Multi-scale computational modeling of calmodulin mutations associated with CPVT - Project Summary Approximately 1 in 5 people living today will die at some point in the future due to a cardiac arrhythmia. However, despite decades of research, the underlying mechanisms remain poorly understood. Genetic studies have provided important new insights into the underlying mechanisms by identifying mutations in calcium (Ca) cycling proteins that cause arrhythmias. One of the best studied is Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT), which is an inherited disorder that can cause sudden death during exercise or stress in patients with structurally normal hearts. However, it is not understood how a point mutation in a Ca signaling protein can ultimately lead to a cardiac arrhythmia, which involves the breakdown of electrical excitations at the organ scale. To address this challenge, the applicants propose to develop a multi-scale computational framework to link CPVT mutations to dangerous electrical excitations in cardiac tissue. Their approach is to perform molecular dynamics (MD) simulations of the binding between Calmodulin (CaM) and the Ryanodine Receptor type 2 (RyR2), in wild type (WT) and mutants, and then to link this data to higher scale computational models of the cell and tissue electrophysiology. This computational framework will link the following important scales relevant to arrhythmias: 1. Molecular scale interactions at the CaM-RyR2 binding interface. 2. Ca release within a cardiac cell due to Ca sparks and waves. 3. Ca triggered electrical excitations in cardiac tissue. The specific aims of this project will be devoted to developing a computational toolkit that is tailored to each relevant scale, and which can be systematically linked to scales above and below. In aim 1, they will use high resolution crystal structures to construct MD simulations of the CaM-RyR2 binding interface. Based on these simulations they will compute the binding affinity between CaM and RyR2 in the presence of an array of point mutations on CaM. Also, they will apply MD to determine the key binding interactions that are disrupted by specific CaM mutations. In aim 2, they will construct a biophysically based model of the RyR2 channel that accounts for the reaction kinetics between CaM and RyR2. This model will utilize the binding affinity computations in aim 1 to determine the difference between WT and mutants. Once this model is constructed, they will then evaluate how a specific CaM mutation regulates Ca leak from the SR via spontaneous Ca sparks and induces dangerous action potential (AP) perturbations due to Ca waves. In aim 3, they will develop a phenomenological model of Ca and voltage dynamics of a cardiac Purkinje cell that can be used to simulate millions of cells in the Purkinje fiber system. Using this model, they will explore how Ca waves are synchronized in the Purkinje fiber network leading to wave break and reentry. This project is innovative because computational methods developed in this proposal will be the first to address all the important scales in the problem and is significant because it will thus yield important new insights into the underlying mechanism for CPVT.
