PE transformation is stimulated by AV cushion cells that stimulate fibrous ECM production in developing valves

The contribution of epicardially‐derived and their progenitors, proepicardial (PE) cells, to the development of the atrioventricular (AV) valves have been recently described in mouse linage tracing experiments. Results presented here indicate that PE cell epithelial to mesenchymal transformation (EMT) is dependent on the presence of AV cushions. Once in the AV cushions, PE cells appear to regulate the expression and deposition of fibrous extracellular matrix (ECM) proteins. Using a tubular, 3D culturing system, we show that AV cushions induce PE cells to undergo an EMT and migrate into AV cushions where they stimulate the expression and deposition of ECM proteins including tenascin C, periostin and collagen I, which are critical for proper valve function. It was tested if members of the TGFβ family of proteins were sufficient to stimulate PE EMT and ECM expression. Only the combination of TGFβ 1, 2 and 3, was found to regulate these processes. When PE cells were cultured on tube scaffolds containing TGFβ 1, 2 and 3 together, PE cell EMT increased compared to cultures with only a single TGFβ isoform. qRT‐PCR and confocal analysis found increased expression and localization of ECM molecules in these transformed cells. These results indicate all three isoforms of TGFβ are required for epicardial EMT, which appear to be necessary the critical expression and deposition of fibrous ECM proteins during valve development.