Critical to the development of the heart is the tissue interaction that takes place between the myocardium and the endothelium of the AV canal. The endothelial cells undergo a transformation into invasive mesenchyme and become the progenitor cells of the valves and septa. These events are similar to other transformations in the embryo including gastrulation, neural crest formation, somite breakdown, and palate formation. In order to dissect the transformation process in the heart, the proposed study will examine mechanisms and molecules common to several of these cell transformations and examine them in the heart. It is hoped that this approach will reveal general themes utilized for cell transformation and identify mechanisms used by cardiac endothelial cells to respond to the complex inductive stimuli. Specifically, using blockade to 2 different signal transduction pathways, the TGFb type II receptor serine/threonine kinase and the G1 protein, the relationship between components of the inducing stimulus, regulatory genes and several phenotypic markers will be identified. Several of the proposed regulatory genes will be targeted for degradation and the relationship of gene expression to subsequent expression of other regulatory genes and phenotypic markers will be studied. The regulatory genes to be examined are the homeobox genes mox-1, msx-1, goosecoid and optomotor type DNA binding protein, brachyury. Markers of phenotypic transformation will include mox-1, collagen I tenascin, galactosyltransferase, and PECAM. Since this transformation is sensitive to the effects of the pertussis toxin, the G1 protein as a mediator of the induction process will be analyzed. The proposed studies will examine which member of this family is involved. The specific aims are 1) to examine and compare the effects of blocked signal transduction in 2 different pathways on markers of gene regulation and phenotypic transformation; 2) determine the order and relevance, if any, between goosecoid, msx-1, mox-1 and brachyury gene expression during the transformation process; 3) identify which Gia isoform found in the embryo mediates the transformation process; and 4) examine other gastrulation specific genes for expression in the heart.