PROJECT SUMMARY The ventricular conduction system (VCS) is essential for normal myocardial excitation and contraction. Diseases impacting VCS structure and function are responsible for a substantial burden of cardiovascular disease, including conduction slowing and block, and triggered and reentrant arrhythmias. In previous NIH- funded work, our laboratory established and characterized novel genetic tools to visualize the formation and function of the murine VCS and its constituent Purkinje cells, resulting in the identification of a novel transcriptional pathway critical for VCS development. We also used these tools to create and characterize innovative genetically engineered murine models that firmly established the role of Purkinje cells or juxta- Purkine cells as arrhythmogenic triggers in heritable syndromes including CPVT and LQT3. While our earlier studies significantly advanced the field's understanding of the role of Purkinje cells in mono- and oligogenic forms of arrhythmogenic heart disease, much less is known about the underlying mechanistic basis for pathologic remodeling of the VCS in common acquired cardiomyopathies, such as those that seen in patients with hypertension, diabetes, obesity and other common conditions. These disease states are by far responsible for the greatest burden of cardiovascular morbidity and mortality. We have obtained preliminary data suggesting that paracrine signaling from activated fibroblasts may drive Purkinje cell transcriptional remodeling, as evidenced by repression of the rapid conduction signature and induction of a pro-fibrotic program. Based upon these data, we propose two specific aims designed to provide a deep mechanistic understanding of pathologic remodeling of Purkinje cells. We also will perform exploratory studies to determine if agents that target fibroblast activation can diminish pathologic Purkinje cell remodeling.