This R01 application will investigate a novel signaling pathway, the Hippo pathway, in mammalian heart regeneration. The long-term goal is to develop new treatments for patients with heart failure by generating new cardiomyocytes in the adult heart. The objectives of this application are to gain insight into biomechanical properties of cardiac tissue that are regulated by Hippo signaling. The central hypothesis is that Hippo signaling is a negative regulatory pathway that prevents cardiomyocyte regeneration in the adult mammalian heart by promoting a more fetal-like elasticity profile of cardiac muscle. The specific aims are to define the Hippo regulated biomechanical properties and cardiac aging, to determine dynamic changes in biomechanical properties after myocardial infarction, and to determine the functional characteristics of regenerated cardiomyocytes in Hippo mutant hearts. The project is both conceptually and technically innovative. The concepts to be tested are new ideas in cardiomyocyte biology and new and cutting edge imaging technologies are used to address hypotheses. The significance is high because there are no treatments for heart failure due to cardiomyocyte loss. Devising ways to generate new cardiomyocytes is highly significant.