TGFβ2 and TGFβ3 have separate and sequential activities during epithelial-mesenchymal cell transformation in the embryonic heart

Heart valve formation is initiated by an epithelial-mesenchymal cell transformation (EMT) of endothelial cells in the atrioventricular (AV) canal. Mesenchymal cells formed from cardiac EMTs are the initial cellular components of the cardiac cushions and progenitors of valvular and septal fibroblasts. It has been shown that transforming growth factor beta (TGF beta) mediates EMT in the AV canal, and TGF beta 1 and 2 isoforms are expressed in the mouse heart while TGF beta 2 and 3 are expressed in the avian heart. Depletion of TGF beta 3 in avian or TGF beta 2 in mouse leads to developmental defects of heart tissue. These observations raise questions as to whether multiple TGF beta isoforms participate in valve formation. In this study, we examined the localization and function of TGF beta 2 and TGF beta 3 in the chick heart during EMT. TGF beta 2 was present in both endothelium and myocardium before and after EMT. TGF beta 2 antibody inhibited endothelial cell-cell separation. In contrast, TGF beta 3 was present only in the myocardium before EMT and was in the endothelium at the initiation of EMT. TGF beta 3 antibodies inhibited mesenchymal cell formation and migration into the underlying matrix. Both TGF beta 2 and 3 increased fibrillin 2 expression. However, only TGF beta 2 treatment increased cell surface beta-1,4-galactosyltransferase expression. These data suggest that TGF beta 2 and TGF beta 3 are sequentially and separately involved in the process of EMT. TGF beta 2 mediates initial endothelial cell-cell separation while TGF beta 3 is required for the cell morphological change that enables the migration of cells into the underlying ECM. (C) 1999 Academic Press.