Tbx18-positive cells differentiated from murine ES cells serve as proepicardial progenitors to give rise to vascular smooth muscle cells and fibroblasts.

Proepicardium (PE) cells generate cardiac fibroblasts, smooth muscle cells (SMCs) and endothelial cells that form coronary arteries. T-box18 (Tbx18) is a well-known marker of PE cells and epicardium. We examined whether Tbx18-positive cells differentiated from murine embryonic stem (ES) cells serve as PE progenitors to give rise to vascular SMCs and fibroblasts. To collect Tbx18-positive cells, we established Tbx18-EGFP knock-in mouse ES cells using the CRISPR/Cas9 system. We harvested the Tbx18-EGFP-positive cells on day 8, 10 and 14 after the initiation of differentiation; Tbx18 mRNA was enriched on day 8 to 14 and Snai2 mRNA was enriched on day 8 and 10, indicating successful collection of Tbx18-positive cells. Tbx18-EGFP-positive cells expressed the PE marker WT1 on day 8 and 10. They also expressed the SMC marker Acta2 and fibroblast markers Thy1 and Fsp1 on day 8 to 14, but did not express the endothelial cell marker PECAM or the cardiac cell marker CD166 or Myh7. In conclusion, Tbx18-positive cells represent a part of PE cells in the initial phase of differentiation and subsequently include SMCs as well as fibroblasts. These results indicate that Tbx18-positive cells serve as a PE progenitor to supply a variety of cells that contribute to the formation of coronary arteries.