PROJECT SUMMARY Normal coronary artery formation is essential for heart growth and function. Malformed coronary arteries are a clinically significant birth defect that can cause life-threatening cardiac complications, including ventricular noncompaction, myocardial ischemia, and sudden cardiac death. Yet, developmental mechanisms that drive proper coronary artery formation are incompletely understood, which has hindered our ability to develop the heart-specific interventions for this devastating disease. The long-term goal of this project is therefore to reveal the molecular and cellular mechanisms underlying coronary artery development so that we may identify key regulatory factors for developing new targeted therapies to combat this congenital condition. We have addressed this goal during previous finding period. Our studies have shown that embryonic coronary arteries in the inner compact myocardium are formed by ventricular endocardial cells through angiogenesis regulated by the VEGF- NOTCH signaling. Furthermore, our studies have revealed that these embryonic coronary arteries undergo angiogenic expansion perinatally to add the neovessels to the growing compact myocardium. However, in contrast to the vascularization of the compact myocardium, we know little about vascularization of trabecular myocardium which remains largely avascular until birth. We have recently identified a subpopulation of coronary progenitor cells among ventricular endocardial cells which are committed to the coronary arteries in the trabecular myocardium. We named these cells as the second wave coronary progenitors (SCPs) to separate them from the first wave coronary progenitors (FCPs) for the coronary vessels at the compact myocardium. SCPs acquire angiogenic potential earlier in embryonic development through a previously unknown endocardial to mesenchymal transformation (EMT) long before they undergo angiogenesis later during perinatal periods to vascularize the trabecular myocardium. In this renewal application, we propose to characterize this new angiogenic-EMT paradigm (angioEMT) by SCPs. Our overarching hypothesis is that vascularization of trabecular myocardium by SCPs is regulated by a “two-hit” mechanism involving sequential angioEMT and hypoxia signaling. We plan to test this hypothesis in three Specific Aims. Aim 1 will characterize SCPs by distinguishing them from FCPs using a lineage-based single cell RNA-sequencing (scRNA-seq) analysis and a modified functional angioEMT assay. Aim 2 will define the angioEMT signaling in the early fate decision by SCPs using genetic loss-of-function approaches investigating the TGFb signaling. Aim 3 will decipher the angiogenic signaling in the later angiogenic activation of SCPs focusing on VEGFA-VEGFR3 and DLL4-NOTCH1 signaling. Vascularization of trabecular myocardium as well as trabecular compaction in the individual nulls will be examined by histology, immunostaining, and RNAscope in situ hybridization. The changes in ...