Project Summary The ability to produce new myocytes within the heart would revolutionize our ability to treat cardiovascular disease, and new strategies for generating new cardiac muscle cells (cardiomyocytes) after injury, such as myocardial infarction (MI), are needed. Adult mammalian cardiomyocytes (CM) are generally considered to be post-mitotic, hence previous nongenetic strategies to stimulate resident CM proliferation have to date been insufficient to overcome CM loss with injury. However, recent reports demonstrate that adult differentiated CMs in humans and mice have the ability to proliferate, albeit at low frequency, suggesting that these rare events could be harnessed at a larger scale if the underlying mechanisms were fully understood. Developmental studies show that the transcription factor Tbx20 promotes CM proliferation in the fetal and neonatal heart. Tbx20 overexpression (Tbx20OE) in adult CMs induces fetal-like characteristics, including proliferation, smaller size, mononucleation, and fetal contractile protein expression. We hypothesize that induction of adult CM proliferation via a Tbx20-dependent pathway requires both partial dedifferentiation, mediated by increased BMP signaling, and repression of cell cycle inhibitors, including Btg2. The Aims are: 1) Determine if transient or exogenous Tbx20 expression is sufficient to promote adult CM proliferation and repair in vivo. 2) Determine if BMP signaling promotes CM dedifferentiation necessary for induction of adult CM proliferation in vivo. 3) Determine if loss of cell cycle inhibitors Btg1/2 promotes CM proliferation and repair after injury. The ability to induce resident adult mammalian CMs to proliferate by physiologic reversion to a fetal-like state would be an important advance in efforts to generate new myocardium after cardiac injury.