During cardiac development, valve progenitors first arise in the atrioventricular canal as a result of a TGF?-mediated epithelial-mesenchymal cell transformation (EMT). Prior studies showed that this EMT can be divided into two stages, an activation stage and an invasion stage. Stage 1 is mediated, in part, by transforming growth factor beta 2 (TGF?2) and stage 2 is mediated, in part, by TGF?3. Previous work showed that EMT can be disrupted by the inhibition of 5 different TGF? receptors, TGF? receptor II, TGF? receptor III, Endoglin, ALK2 and ALK5. It is our hypothesis that there are two more discrete signaling complexes mediating TGF?-regulated EMT. To continue the analysis of TGF?-mediated regulation of normal heart development, the first aim will examine the correlation between specific TGF? isoforms and receptors in mediating EMT regulation. Microarray experiments will be performed to assess the effect loss of specific TGF? isoforms on altered gene expression in tissues prior to EMT, during EMT and in mesenchymal cells after EMT. Selected markers among the TGF?-regulated genes will then be used to assess whether gene expression is altered as each of the specific receptors is disrupted. Correlations between these data will point to specific receptor and ligand signaling complexes that mediate EMT. These putative complexes will be confirmed at the protein level. Data from other labs has implicated a variety of signal transduction mechanisms and intracellular regulators in cardiac EMT including ErbB2, NF-1, VEGF and NFATc1. In the second aim, experiments will be undertaken to determine whether disruption of TGF? signaling alters any of the other candidate mechanisms or whether loss of these molecules alters TGF? ligand or receptor expression. It was observed in the neural crest system that forced expression of Sox 8, 9 or 10 in the neural tube would cause an EMT and produce ectopic neural crest cells. We conjecture that this is analogous to the cell invasion step in the heart and that it requires some TGF? activity. In the third aim, experiments will be undertaken to determine whether exogenous expression of Sox genes will force EMT and whether Sox gene expression is regulated by TGF? signal transduction or regulates TGF?-mediated responses. Together these experiments will shed new light on the mechanisms of EMT regulation and aid in an understanding of congenital heart disease.
