PROJECT SUMMARY Cardiovascular disease is the leading cause of death and pharmacologic treatment strategies have improved cardiovascular outcomes and survival. However, the prognoses of affected individuals remain poor; hence, the need to elucidate underlying mechanisms and develop better therapies for cardiovascular diseases. Our proposed studies focus on circRNA as an unexplored component of the cardiovascular system to yield new understanding and tools for treatment of myocardial infarction. The overarching objective of this current revised application is to reveal how circular RNAs (circRNAs), a recently identified class of RNAs that are generated from many protein-coding genes, impact mouse models of myocardial infarction (MI). Preliminary data generated for the current proposal shows that modulation of a specific circRNA from the Fibronectin Type III Domain Containing 3B (FNDC3b) gene (circFNDC3b) in ischemic myocardium can modulate cardiac repair after myocardial infarction in mice. In particular, we found: 1) decreased circFNDC3b expression in post-MI mouse hearts and in left ventricular tissues of ischemic cardiomyopathy patients, 2) and in endothelial cells and cardiomyocytes in post-MI mouse hearts; 3) AAV9 mediated circFNDC3b overexpression enhances endothelial cell angiogenesis and inhibits hypoxia-induced cardiomyoblast apoptosis in vitro, 4) and improves left ventricular function, remodeling and neovascularization post-MI in mice; 5) circFNDC3b bind RNA binding protein fused in sarcoma (FUS-1) in endothelial cells and cardiomyoblasts and regulate FUS- 1 levels. Our central hypothesis is that rescue of circFNDC3b promotes positive cardiac remodeling after MI by enhancing neovascularization and improves cardiomyocyte cell survival through FUS-1 inhibition, thus improving cardiac function. This research will create a multi-modal platform for future work, accelerating progress in this field as it relates to therapeutic targets and underlying mechanisms. The hypotheses will be tested under the following 3 specific aims: Specific Aim 1: To determine the physiological role of circFNDC3b on myocardial injury repair post-MI in mice. Specific Aim 2: To elucidate the molecular mechanism by which circFNDC3b regulates cardiomyocyte cell survival and enhances endothelial cell function. Specific Aim 3: To develop novel therapeutic strategies to manipulate other circRNAs to promote myocardial injury repair post-MI in mice. Successful completion of the proposed research will potentially identify circRNAs as potentially novel therapeutic targets for ischemic myocardial repair.