Lineage tracing using Wnt2b-2A-CreERT2 knock-in mice reveals the contributions of Wnt2b-expressing cells to novel subpopulations of mesothelial/epicardial cell lineages during mouse development.

Mesothelial and epicardial cells give rise to various types of mesenchymal cells via epithelial (mesothelial)-to-mesenchymal transition during development. However, the genes controlling the differentiation and diversification of mesothelial/epicardial cells remain unclear. Here, we examined Wnt2b expression in the embryonic mesothelium and epicardium and performed lineage tracing of Wnt2b-expressing cells by using novel Wnt2b-2A-CreERT2 knock-in and LacZ-reporter mice. Wnt2b was expressed in mesothelial cells covering visceral organs, but the expression was restricted in their subpopulations. Wnt2b-expressing cells labeled at embryonic day (E) 10.5 were distributed to the mesothelium and mesenchyme in the lungs, abdominal wall, stomach, and spleen in Wnt2b;R26R mice at E13.0. Wnt2b was initially expressed in the proepicardial organ (PEO) at E9.5 and then in the epicardium after E10.0. Wnt2b-expressing PEO cells labeled at E9.5 differentiated into a small fraction of cardiac fibroblasts and preferentially localized at the left side of the postnatal heart. LacZ epicardium-derived cells labeled at E10.5 differentiated into a small fraction of fibroblasts and smooth muscle cells in the postnatal heart. Taken together, our results reveal novel subpopulations of PEO and mesothelial/epicardial cells that are distinguishable by Wnt2b expression and elucidate the unique contribution of Wnt2b-expressing PEO and epicardial cells to the postnatal heart.