Structural Basis for Antiarrhythmic Drug Action

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Voltage-gated sodium (Nav1.5) channels initiate action potentials and voltage-gated calcium (Cav1.2) channels initiate excitation/contraction coupling in cardiac myocytes. They are molecular targets for mutations that cause arrhythmias and for antiarrhythmic drugs (AADs) used in control and prevention of life-threatening arrhythmias. We have determined the structures of the bacterial Nav channel NavAb and the model calcium channel CavAb by X-ray crystallography, and we have determined the structure of the primary cardiac Nav channel Nav1.5 at high resolution by cryogenic electron microscopy (cryo-EM). This work gave new insight into the structure of the ion selectivity filter and mechanism of Na and Ca selectivity, the structure of the voltage sensors and mechanisms of voltage-dependent gating, the activation and inactivation gates and their functional interaction, the receptor sites for AADs, and the mechanism of access of AADs to their receptor site through the open activation gate and fenestrations in the sides of the pore. Mutations in Nav1.5 that cause inherited cardiac arrhythmias, including Dilated Cardiomyopathy and Long QT Syndrome, map onto the pore module, voltage sensor, activation gate, and fast inactivation gate of Nav and Cav channels, opening the way to probing the pathophysiology of these mutations at the structural level. Here we will investigate the structural basis for Nav and Cav channel function, the complex pore-blocking mechanisms of AADs, and the mechanisms underlying the pathophysiological effects of arrhythmia mutations. Aim 1. We will determine the structure of Nav1.5 channels in resting, open, and inactivated states and analyze the molecular mechanisms for ion selectivity and conductance in the open state of Nav1.5. Aim 2. We will prepare complexes of Nav1.5 in the closed, open, and inactivated states with AADs bound, and we will resolve their structures at high resolution. We will probe structural differences in the drug- receptor complexes formed by Class IA, IB, and IC AADs in order to understand the structural basis for the differences in drug action that lead to their different clinical uses. We will examine the role of the fenestrations in resting-state block by Class IA, IB, and IC AADs. Aim 3. We will insert mutations that cause Dilated Cardiomyopathy and Long QT Syndrome Type-3 into NavAb and Nav1.5, characterize their pathophysiological effects on Na currents and gating pore currents, and resolve their structures at high resolution by X-ray crystallography and/or cryo-EM. Aim 4. We will insert mutations that cause Timothy Syndrome into CavAb and Cav1.2, determine their pathophysiological effects on Ca and Ba currents, and resolve their structures at high resolution by X-ray crystallography and cryo-EM. Overall, these studies open the exciting possibility of understanding cardiac Nav and Cav channels in atomic detail in native and pathogenic conformations and learning how to manipulate the structures of AADs to make them more specific, more effective, and safer.