Project Summary/Abstract Myocardial infarction results from ischemic injury to the coronary arteries, and triggers localized cardiomyocyte (CM) death as well as turnover of damaged cardiac tissue into fibrotic tissue. Adult mammalian CMs fail to sufficiently proliferate post-injury and allow for persistent fibrosis within the heart which elevates patient morbidity. Heart disease remains the leading cause of death in the United States and current therapeutics only attenuate the progression of heart disease: they cannot reverse cardiac damage by inducing CM proliferation. However, it may be possible to stimulate adult CM proliferation as mono- and bi-nucleated CMs were recently shown to proliferate in post-natal, young, mammalian hearts. Therefore, it is imperative to discern the mechanisms driving CM proliferation and if their manipulation can improve pediatric and adult CM proliferation and cardiac regeneration. In this proposal, we will utilize a ventricular amputation model in adult zebrafish to study cardiac regeneration. Adult zebrafish CMs are mono-nucleated, diploid, and display robust proliferation after injury which makes them ideal to identify and study the molecular mechanisms regulating CM proliferation. Using transcriptome profiling (RNA-Seq) after ventricular resection, we identified that Forkhead box M1, Foxm1, expression increased during the early stages of cardiac regeneration. The goals of this proposal are to determine the role of Foxm1 in cardiac regeneration and CM proliferation. We will utilize loss- of-function approaches to study foxm1 mutants after cardiac injury. In addition, we will address the question whether Foxm1 is activated by the Ras/MAPK signaling axis, and investigate downstream foxm1 target genes via transcriptional profiling. These studies will be critical in understanding how CMs are activated to become mitotic after injury. Understanding these intricate mechanisms will provide us with new insights to stimulate cardiomyocyte proliferation and discover genetic targets to enhance cardiac regeneration in mammalian systems.