Atrial fibrillation (AF) is the most common rhythm disturbance in the US and other developed countries. AF significantly affects the lives of the afflicted, causing symptoms that range from palpitations to fatigue, weakness, activity intolerance, stroke, congestive heart failure and death. The impact on public health is substantial, with more than 450,000 hospital admissions per year and $26 billion in healthcare costs. Adding to the problems caused by AF is the lack of safe and effective therapies for this rhythm disorder. Pharmacotherapy for AF has a long history of poor efficacy and potentially lethal side effects. Ablation strategies have made inroads in paroxysmal AF, but they continue to be long, difficult procedures with less than optimal success rates and too frequent adverse events. Ablation does not cure AF. We propose development of gene therapy as a new strategy to eliminate AF. Like many other effective therapies, gene therapy must focus on disease mechanism as a starting point for development. In the case of AF, electrical and structural remodeling are critical elements of the disease mechanism that we aim to reverse. We have previously shown the ability to eliminate the action potential shortening and conduction velocity slowing elements of electrical remodeling with gene transfer of a dominant negative potassium channel mutation and connexins. We recently found partial reversal of structural remodeling with inhibition of the calcium/calmodulin-dependent protein kinase II. In this proposal, we hypothesize that calcium and mitogen activated protein kinase signaling cause AF-related structural remodeling. We explore this hypothesis in a clinically relevant porcine model of atrial fibrillation and heart failure by using molecular methods to correlate signaling pathway activation to structural remodeling and specific drug or genetic blockers of the relevant signaling pathways to more completely connect pathway activation to structural remodeling. To address our hypothesis, we propose 3 aims: (1) to define and prevent AF-related structural remodeling caused by calcineurin overactivity; (2) to evaluate ERK1/2 signaling in AF; (3) to evaluate the effects of multiple signaling pathway blockade on AF-related structural remodeling. Successful completion of our aims will not only identify critical mechanisms driving AF-related structural remodeling, but it will also complete a substantial component of the preclinical testing necessary to translate these investigational agents into clinical therapies.