INDUCTION OF ENDOCARDIAL CUSHION TISSUE IN THE AVIAN HEART IS REGULATED, IN PART, BY TFG-BETA-3-MEDIATED AUTOCRINE SIGNALING

Valvuloseptal morphogenesis of the primitive heart tube into a four-ch ambered organ requires the formation of endocar--- dial cushion tissue . The latter is the outcome of an inductive interaction in which endoc ardial (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-TGF beta-3-that functions in an autocrine mode to pr omote/sustain mesenchyme formation and possibly in a paracrine manner to amplify the original (myocardial) inductive event. Cushion mesenchy me-conditioned medium, previously demonstrated to be an endogenous sou rce of autocrine, migration-promoting factors, was found in the presen t study to contain TGF beta-3, as detected by immunoblot analysis. Imm unoneutralization of TGF beta-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 antibo dy did not inhibit the migration-promoting activity of the conditioned medium, Similar to treatment with the conditioned medium, direct addi tion of TGF beta-3 to target endocardial cells also elicited invasive migration but only in cultures which had been activated in vivo by ind uctive interaction with the myocardium prior to treatment. Selective i nhibition of TGF beta-3 mediated autocrine signaling in continuous coc ultures of endocardium plus myocardium resulted in endocardial cells w hich did not migrate, even though they had expressed early markers ass ociated with endocardial cell activation (e.g., alpha-smooth muscle ac tin, ES/130, and TGF beta-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 TGF beta-3 and is not functionally redundant to the myocardial pathway. (C) 1997 Academic Press.