Project Summary Adult hearts lack the capacity to regenerate. Since mature (adult) cardiomyocytes (CMs; cardiac muscle cells) are post-mitotic, the fibrotic scar formed by cardiac fibroblasts (CFs) after a myocardial infarction is irreversible. This scar often contributes to left ventricular dysfunction and heart failure, the leading causes of death in US. Recent studies have found that the mammalian neonatal heart has a remarkable capacity to resolve fibrosis and regenerate after an injury. This unique phenomenon may be mediated, in part, by CFs through intrinsic intracellular factors or via extrinsic signals in the neonatal cardiac extracellular matrix. Evaluation of published RNA-sequencing data has revealed an enrichment of Amphiregulin (i.e. Areg) in the heart during the neonatal regenerative window. Areg is a pro-regenerative cytokine that has been also shown to stimulate CF proliferation and fibrosis in the adult heart. However, the cellular responses to Areg may be different in the highly regenerative neonatal heart. In this study, we will evaluate the contributions of neonatal CFs to cardiac repair (Aim 1) through selective cell depletion and RNA sequencing while simultaneously defining their response to pro-regenerative cytokines such as Areg (Aim 2) through in vivo and in vitro studies. Establishing the unique role of CFs during neonatal cardiac regeneration will lead to more targeted approaches to combat the irreversible cardiac fibrosis present after adult cardiac injury. This proposal also details a training plan of clinical activities to complement the proposed research plan of the applicant. The longitudinal clinical electives in cardiovascular and regenerative medicine will provide a better understanding of the clinical context of the research. Under the guidance of the sponsor, thesis committee, and advisory committee, the applicant will develop her knowledge in experimental design, scientific communications, and techniques in cardiovascular research. This proposal contributes to current knowledge in cardiac fibrosis while training the applicant to be a clinician-scientist.