ABSTRACT In response to persistent neurohormonal/hemodynamic stress or injury, the heart undergoes a pathologic remodeling process typically characterized by cardiomyocyte hypertrophy and fibrosis. Cardiac fibrosis, defined as excess extracellular matrix deposition in the heart, is a major contributor to both systolic and diastolic dysfunction for millions of heart failure patients. Upon injury, resident quiescent fibroblasts undergo a cell state transition to become activated fibroblasts and myofibroblasts that hyper-secrete extracellular matrix proteins and can be contractile. Fibrotic remodeling causes stiffening of the left ventricle, impaired relaxation, reduced compliance, and altered electrical conductance. This drives disease progression and worsens pathology. Despite the well-known contribution of cardiac fibrosis to adverse outcomes associated with heart failure, effective therapies targeting fibrosis in the heart remain elusive. Inhibition of the BET (bromodomain and extra-terminal domain) family of epigenetic reader proteins with chemical small molecules (e.g. JQ1) has proven effective at blocking in vivo cardiac hypertrophy, inflammation, and fibrosis and preserving cardiac function in rodent models of heart failure. Though these inhibitors block all BET proteins in the heart (BRD2, BRD3, BRD4), genetic targeting of individual BET proteins in cell culture experiments support the notion that BRD4 is the main BET protein responsible for activation of pathologic gene expression programs in cardiomyocytes and cardiac fibroblasts. However, recent work has given insight on the cell-type specific role of BRD4 in cardiomyocytes, uncovering an essential role for BRD4 in maintaining mitochondrial homeostasis. Conditional deletion of BRD4 in adult cardiomyocytes resulted in progressive cardiac dysfunction, impaired mitochondrial function, and eventual death in mice. Taken together with previous studies, this suggests that the beneficial effects of small molecule BET inhibitors in mouse models of heart failure are unlikely to be mediated exclusively by inhibition of BRD4 in cardiomyocytes. We hypothesize that genetic in vivo disruption of BRD4 in activated fibroblasts will safely block fibrosis and that multiple BET proteins (i.e. also BRD2 and BRD3) mediate the beneficial effects of small-molecule pan-BET inhibition. In Aim 1, we will determine the in vivo role of BRD4 in activated fibroblasts, and in Aim 2 we will elucidate the previously unexplored role of individual BET proteins in cardiac cell gene expression regulation through ChIP-Seq. This work will be carried out in the laboratory of Dr. Matthew Stratton, an expert in epigenetic regulation of pathologic cardiac remodeling, and under the co-supervision of Dr. Loren Wold, a world-leader in cardiac physiology. Our long-term goal is to develop safe and effective therapeutics to improve clinical outcomes for patients with cardiac fibrosis and HF.