Single-cell transcriptomics reveals diversity during heart valve epithelial-to-mesenchymal transitions

Abstract The epithelial-to-mesenchymal transition (EMT) is a process critical for wound healing, fibrosis, and cancer metastasis, but is also essential for atrioventricular valve formation, where distinct EMTs of endocardium (EndMT) and epicardium (EpiMT) generate mesenchyme. To track these processes, we have analyzed over 50,000 murine single-cell transcriptomes from embryonic day (E)7.75 cardiac crescent to E12.5 atrioventricular canals. We detail mesenchymal and endocardial bifurcation during EndMT, identify a unique, Hic1-expressing epicardial population during EpiMT, and reveal epithelial-mesenchymal plasticity (EMP) during both processes. Single-cell and histological analysis of Sox9-deficient valves show the accumulation of cells exhibiting EMP. Lastly, we deconvolve the signaling pathways active during the initiation and progression of EndMT and EpiMT. Overall, these data are the first to reveal mechanisms of emergence of mesenchyme from endocardium or epicardium at single-cell resolution and will serve as an atlas of EMT initiation and progression with broad implications in regenerative medicine and cancer biology.