Induction of Endocardial Cushion Tissue in the Avian Heart is Regulated, in Part, by TGFβ-3-Mediated Autocrine Signaling

Abstract

Valvuloseptal morphogenesis of the primitive heart tube into a four-chambered organ requires the formation of endocardial cushion tissue. The latter is the outcome of an inductive interaction in which endocardial (endothelial) cells are induced to transform into mesenchyme by paracrine signals secreted by the adjacent myocardium. In this study, we propose that transforming endothelial/mesenchymal cells themselves secrete a factor—TGFβ-3—that functions in an autocrine mode to promote/sustain mesenchyme formation and possibly in a paracrine manner to amplify the original (myocardial) inductive event. Cushion mesenchyme-conditioned medium, previously demonstrated to be an endogenous source of autocrine, migration-promoting factors, was found in the present study to contain TGFβ-3, as detected by immunoblot analysis. Immunoneutralization of TGFβ-3 in preparations of cushion mesenchyme-conditioned medium resulted in a failure of treated target endocardial cells to migrate as mesenchyme, whereas inclusion of a control antibody did not inhibit the migration-promoting activity of the conditioned medium. Similar to treatment with the conditioned medium, direct addition of TGFβ-3 to target endocardial cells also elicited invasive migration but only in cultures which had been activatedin vivoby inductive interaction with the myocardium prior to treatment. Selective inhibition of TGFβ-3-mediated autocrine signaling in continuous cocultures of endocardium plus myocardium resulted in endocardial cells which did not migrate, even though they had expressed early markers associated with endocardial cell activation (e.g., α-smooth muscle actin, ES/130, and TGFβ-3). Collectively, these results suggest that (i) two signaling pathways, myocardial and endocardial, are required to start and complete epithelial–mesenchymal transformation in cushion-forming regions of the heart and (ii) the endocardial pathway signals through iteration of TGFβ-3 and is not functionally redundant to the myocardial pathway.