Pro-Epicardial Cells are Heterogeneous with Specified Smooth Muscle-Like and Pacemaker Progenitor Cells

The heterogeneity and specification of pro-epicardial cells (pro-EpiCs) to fibroblast and smooth muscle cell (SMC) are unknown. We applied single-cell RNA sequencing (scRNA-seq) paired with RNAScope, bioinformatics, and lineage tracing in an unbiased manner to identify the previously uncharacterized molecular heterogeneity of the pro-EpiCs isolated from pro-epicardium (PE). We found that pro-EpiCs labeled by Tbx18 Cre/+ are heterogeneous, with three clusters displaying differential expression profiles and distinct spatial locations. Cluster 1 are mesothelial cells, and Cluster 2 express SMC markers. Surprisingly, Cluster 3 express Isl1 and markers of pacemaker progenitor cells (PMPC) but not marker of atrial cardiomyocytes. Our studies conclude that pro-EpiCs are heterogeneous and SMC-like cells are specified in the PE, while fibroblasts are not specified in PE but epicardium. We identified a region in PE that contains PMPCs, which translocate through the inflow tract to the sinoatrial node. The expression profile of Cluster 3 cells unifies previous studies regarding the origins and markers of PMPCs.