Hypoxia differentially affects coronary vessel formation during heart development.

AIMS: The coronary vessel system is a dense and diverse network of arteries, veins and capillaries formed by endothelial cells from a variety of sources. While hypoxia is a known stimulus for angiogenic sprouting generally, the exact mechanisms by which hypoxia, and resultant increased VEGFA, influences vessel growth in the heart are not clearly delineated. METHODS AND RESULTS: We used a genetic model to mimic hypoxia through ectopic stabilisation of myocardial HIFα. This enabled us to study the consequences of hypoxia without vascular depletion. Changes in coronary ECs in these hearts relative to littermate controls were assessed by single cell RNA-sequencing, and by examining the activity of enhancer:reporter transgenes active in different coronary vessel beds downstream of distinct vascular regulatory pathways. Analysis of hypoxia-mimic hearts found increased angiogenic gene expression alongside expanded activity of the VEGFA-MEF2-driven angiogenic regulatory pathway in a pattern that indicated increased endocardial-derived angiogenic sprouting. Conversely, regulatory pathways specifically active in the sinus venosus (SV)-derived plexus showed little variance, and sprouting from the SV was not expanded. Although hypoxia and increased VEGFA levels have been previously linked to increased arterial differentiation, we saw little change in initial arterial EC differentiation in the experimental hypoxia-mimic hearts. However, mature coronary arterial formation was delayed. CONCLUSIONS: These observations further emphasize a direct and specific link between the hypoxia pathway and endocardial coronary vessel sprouting and suggest a role of hypoxia/VEGFA in guiding coronary arterial coalescence.