Endocardium-to-coronary artery differentiation during heart development and regeneration involves sequential roles of Bmp2 and Cxcl12/Cxcr4.

Regenerating coronary blood vessels has the potential to ameliorate ischemic heart disease, yet there is currently no method of stimulating clinically effective cardiac angiogenesisis. Endocardial cells, a particularly plastic cell type during development, line the heart lumen and are natural coronary vessel progenitors. Their intrinsic angiogenic potential is lost in adults, but studying the endocardial-to-coronary developmental pathway could identify methods of re-instating this process in diseased hearts. Here, we use a combination of mouse genetics and scRNAseq of lineage-traced endothelial cells to identify novel regulators of endocardial angiogenesis and precisely assess the role of Cxcl12/Cxcr4 signaling. Time-specific lineage tracing demonstrated that endocardial cells differentiated earlier than previously thought, largely at mid-gestation. A new mouse line reporting the activity of Cxcr4 revealed that, despite widespread Cxcl12 and Cxcr4 expression, only a small subset of these coronary endothelial cells activated the receptor, which were mostly in arteries. In accordance with these two findings, Cxcr4 deletion in the endocardial lineage only affected artery formation and only when deleted before mid-gestation. Integrating scRNAseq data of coronary endothelial cells from the endocardial lineage at both mid- and late-gestation identified a transitioning population that was specific to the earlier timepoint that specifically expressed Bmp2. Recombinant Bmp2 stimulated endocardial angiogenesis in an in vitro explant assay and in neonatal mouse hearts upon myocardial infarction. Our data shed light on how understanding the molecular mechanisms underlying endocardial-to-coronary transitions can identify new potential therapeutic targets that could promote revascularization of the injured heart.