PROJECT SUMMARY Long before the onset of permanent AF, AF risk factors promote fibrotic remodeling with ensuing progressive atriomyopathy. After the onset of permanent AF, electrical remodeling and transcriptional remodeling have occurred. Mechanisms underlying how these three remodeling processes relate to one another and to the evolution of AF from susceptibility in the clinical phase to permanence in the clinical phase are unknown. If the critical gap of knowledge regarding the cellular determinants of preclinical electrical remodeling is not elucidated, golden opportunities for intervention with risk-modifying and preventive therapies provided by the long preclinical phase of AF will remain missed. The long-term goal is to discover a novel preventive strategy for permanent AF. The overall objective in this application is to determine whether myocyte electrical remodeling is the final common pathway in preclinical atriomyopathy that increases the vulnerability of atrial myocytes to arrhythmia triggers and consequently of the fibrotic atrial tissue to AF. The central hypothesis is that whereas fibrosis initiates the preclinical phase of AF susceptibility, subsequent electrical remodeling is critical to transition from the preclinical to clinical phase of AF permanence. This hypothesis is formulated based on compelling preliminary data from the applicant's six novel robust and clinically relevant rat and rabbit models of AF susceptibility and permanence. An integrative multidisciplinary approach that incorporates technical innovations in electrophysiology, genomics, and computational biology will be used to test a novel, transformative mechanistic concept. Two aims are proposed to define how myofibroblast- and myocyte-centric mechanisms work independently and in synergy with stress to contribute to AF development and maintenance. Specifically, each aim is designed to determine whether preclinical electrical remodeling of atrial myocytes is caused by myocyte-myofibroblast electrical communication (Aim 1) and/or (2) remodeling of the gene expression of critical myocyte ionic currents (Aim 2). Both mechanisms will be investigated in the absence and presence with stress synergy. The proposal is significant because it will identify potential targets for early risk-modifying intervention in the preclinical phase. The study is conceptually innovative because we introduce six novel robust animal models of AF susceptibility and permanence and advance the novel mechanistic concept of atrial AP ventricularization as a driver for the transformation of AF from susceptibility to permanence. The study is also technically innovative because it will generate the first RiboTag rat model, the first single cell transcriptomic profiling using 10x Genomics for cardiomyocytes, and the first dynamic clamp application in atrial tissue.