Coronary vessel development
and artery differentiation
Distinct molecular pathways of multiple progenitors
Given that multiple sources of coronary vessels exist, we questioned whether the sinus venosus and endocardium grow in response to similar or distinct molecular pathways. Our experiments have identified two ligands required specifically for sinus venosus-derived coronary development, the growth factor VEGF-C and the peptide hormone ELABELA. In contrast, they are not required for endocardium angiogenesis (Chen et al., 2014b; Sharma et al., 2017). Vegfc and Elabela are expressed by the epicardium on the surface of the heart where sinus venosus sprouts emerge, emphasizing the importance of epicardial cells in stimulating sinus venosus sprouting. Sinus venosus sprouting also specifically requires, CCBE1, an extracellular matrix protein that facilitates VEGF-C processing, and the ELABELA receptor, APJ (Sharma et al., 2017). It is relevant to note that the endocardial activators are known for being hypoxia-inducible, while the SV activators are not, and only endocardial sprouts first emerge into tissues labeled with probes that bind hypoxic tissues (Sharma et al., 2017).
Biological advantages of multiple progenitors
Compensation: One hypothesis for why coronaries have multiple origins is that it could provide an alternative progenitor pool if one source is disrupted. Performing endocardial lineage tracing in APJ mutants revealed that endocardial-derived vessels expand when sinus venosus angiogenesis is deficient (Sharma et al., 2017).
Support of rapid tissue growth and ventricular compaction: Another hypothesis for the importance of multiple coronary progenitors is that it allows for faster vascularization that could better support rapid growth of the heart wall. Coincident with the onset of coronary angiogenesis, the ventricular heart wall begins transforming from loosely packed trabeculae to thick, compact muscle. This transformation is called ventricular compaction, and, when defective in humans, causes a cardiomyopathy termed Left Ventricular Non-Compaction. We discovered that endothelial-specific deletion of the chromatin remodeler Ino80 causes ventricular non-compaction in mice due to defective coronary angiogenesis from both the sinus venosus and endocardium. This study (Rhee et al., 2018) revealed that angiogenesis from both coronary progenitor pools is required to promote optimal expansion of the compact heart wall through production of a paracrine factor.