Degradation of type IV collagen by matrix metalloproteinases is an important step in the epithelial-mesenchymal transformation of the endocardial cushions

Morphogenesis of some tissues and organs in the developing embryo requires the transformation of epithelial cells into mesenchyme followed by cell motility and invasion of surrounding connective tissues. Details of the mechanisms involved in this important process are beginning to be elucidated. The epithelial-mesenchymal transformation (EMT) process involves many steps, one of which is the upregulation and activation of specific extracellular proteinases including members of the matrix metalloproteinase (MMP) family. Here we analyze the role of MMPs in the initiation of the mesenchymal cell phenotype in the developing heart, and find that they are necessary for the invasion of mesenchymal cells into the extracellular matrix of the endocardial cushion tissues. An important requirement in the formation of this mesenchyme is the turnover of type IV collagen along the basal surface of endocardial cells. In vitro experiments suggest that type IV collagen does not provide a suitable migratory substrate for endocardial cushion cells unless MMP-2 and MT-MMP are active. Relevant MMPs were found to be upregulated by factors known to be involved in the induction of the EMT such as TGFbeta3. These results provide evidence of an important role for MMPs during a specific stage of the epithelial mesenchymal transformation in the embryonic heart, and suggest that specific cell-matrix interactions which facilitate cell migration only occur when the composition of the surrounding extracellular matrix is proteolytically altered.