TGF-beta 3-induced palatogenesis requires matrix metalloproteinases

Cleft lip and palate syndromes are among the most common congenital malform ations in humans. Mammalian palatogenesis is a complex process involving hi ghly regulated interactions between epithelial and mesenchymal cells of the palate to permit correct positioning of the palatal shelves, the remodelin g of the extracellular matrix (ECM), and subsequent fusion of the palatal s helves. Here we show that several matrix metalloproteinases (MMPs), includi ng a cell membrane-associated MMP (MT1-MMP) and tissue inhibitor of metallo proteinase-2 (TIMP-2) were highly expressed by the medial edge epithelium ( MEE). MMP-13 was expressed both in MEE and in adjacent mesenchyme, whereas gelatinase A (MMP-2) was expressed by mesenchymal cells neighboring the MEE . Transforming growth factor (TGF)-beta3-deficient mice, which suffer from clefting of the secondary palate, showed complete absence of TIMP-2 in the midline and expressed significantly lower levels of MMP-13 and slightly red uced levels of MMP-2. In concordance with these findings, MMP-13 expression was strongly induced by TGF-beta3 in palatal fibroblasts. Finally, palatal shelves from prefusion. wild-type mouse embryos cultured in the presence o f a synthetic inhibitor of MMPs or excess of TIMP-2 failed to fuse and MEE cells did not transdifferentiate, phenocopying the defect of the TGF-beta3- deficient mice. Our observations indicate for the first time that the prote olytic degradation of the ECM by MMPs is a necessary step for palatal fusio n.