Induction of palate epithelial mesenchymal transition by transforming growth factor β3 signaling

Transforming growth factor (TGF beta)3 is essential for palate development, particularly during the late phase of palatogenesis when the disintegration of the palatal medial edge seam (MES) occurs resulting in mesenchymal confluence. The MES is composed of medial-edge epithelium (MEE) of opposite palatal shelves; its complete disintegration is essential for mediating correct craniofacial morphogenesis. This phenomenon is initiated by TGF beta 3 upon adherence of opposing palatal shelves, and subsequently epithelialmesenchymal transition (EMT) instigates the loss of E-Cadherin, causing the MES to break into small epithelial islands forming confluent palatal mesenchyme; however, apoptosis and cell migration or in combination of all are other established mechanisms of seam disintegration. To investigate the molecular mechanisms that cause this E-Cadherin loss, we isolated and cultured murine embryonic primary MES cells from adhered palates and employed several biological approaches to explore the mechanism by which TGF beta 3 facilitates palatal seam disintegration. Here, we demonstrate that TGF beta 3 signals by activating both Smad-dependent and Smad-independent pathways. However, activation of the two most common EMT related transcription factors, Snail and SIP, was facilitated by Smad-independent pathways, contrary to the commonly accepted Smad-dependent pathway. Finally, we provide the first evidence that TGF beta 3-activated Snail and SIP1, combined with Smad4, bind to the E-Cadherin promoter to repress its transcription in response to TGF beta 3 signaling. These results suggest that TGF beta 3 uses multiple pathways to activate Snail and SIP1 and these transcription factors repress the cellcell adhesion protein, E-Cadherin, to induce palatal epithelial seam EMT. Manipulation and intervention of the pathways stimulated by TGF beta 3 during palate development may have a significant therapeutic potential.