Mechanisms of epicardium-directed coronary vessel patterning

The coronary blood vasculature provides the heart with oxygen and nutrients, and removes metabolic waste. Organization of this contiguous network requires the maturation of vascular endothelial cells (EC) into arterial and venous fates based upon their location in the heart. While many of the guidance factors that control vascular patterning have been defined, it is not clear how spatial information controls cell behavior and identity. The epicardium is a single layer of mesothelial cells on the surface of the heart that harbors an important population of cardiovascular progenitors. We previously reported that epicardial epithelial-to-mesenchymal transition (EMT) is required for coronary EC maturation. New preliminary data reveals profound EC patterning and specification defects upon disruption of the epicardium, culminating with the inappropriate localization of angiogenic ECs in the sub-epicardium. To define the cellular and molecular mechanisms linking epicardial EMT to EC patterning and maturation we performed single cell (sc) RNA-sequencing of epicardium-derived cells and ECs isolated from the embryonic mouse heart at key developmental timepoints. This study defined epicardium-derived “shepherding” and “guidepost” cells that express unique angiogenic chemokine signatures. We provide in vitro and in vivo evidence that suggest a common mechanism controls EMT and the expression of genes that encode important guidance cues. We also find that EC localization and arteriovenous fate specification may be controlled by a common molecular mechanism. Based on previously published and preliminary data, we hypothesize that EMT controls the expression and localization of epicardium-derived chemokines that coordinate coronary EC patterning and AV fate specification in the fetal heart. The current study will interrogate this novel paradigm of epicardium-directed coronary EC patterning (localization and branching) and maturation (arteriovenous specification). Here, we will use genetically modified mice, time-lapse live embryo multi-photon imaging, scRNA-seq and spatial transcriptomics, and cell and molecular biology approaches to: 1) Define a common mechanism regulating EMT and the expression of genes that encode EC guidance cues; and 2) Interrogate the mechanisms coordinating epicardium-directed EC patterning and AV fate specification. We expect these studies will provide important insights into the mechanisms that control vascular patterning. This study may also advance our understanding of the developmental origins of coronary artery disease, and lead to therapeutic strategies that stimulate revascularization and repair of ischemic heart tissue.