TGF beta-mediated RhoA expression is necessary for epithelial-mesenchymal transition in the embryonic chick heart.

Endothelia in the atrioventricular canal (AVC) of the embryonic heart undergo an epithelial-mesenchymal transition (EMT) and migrate into the underlying extracellular matrix. We explore here whether RhoA mediates this EMT. RhoA was detected in all cells of the chick heart during the stages studied. Expression was elevated when EMT was actively occurring. Explants treated with C3 exoenzyme in collagen gel cultures showed a significant decrease in mesenchymal cell numbers. siRNA was used to inhibit RhoA mRNA, and both activated endothelial and mesenchymal cells decreased significantly with treatment. Loss of RhoA produced a reduction of RhoB, cyclin-b2, and beta-catenin messages showing that these genes are regulated downstream of RhoA. In contrast, runx-2 was not reduced. Inhibition of TGFbeta3 or TGFbeta2 activity caused a large reduction of RhoA message. These data place RhoA in TGFbeta regulated pathways for both endothelial activation and mesenchymal invasion and demonstrate a functional requirement during EMT. Developmental Dynamics 235:1589-1598, 2006. (c) 2006 Wiley-Liss, Inc.