IDENTIFICATION OF AN AUTOCRINE SIGNALING PATHWAY THAT AMPLIFIES INDUCTION OF ENDOCARDIAL CUSHION TISSUE IN THE AVIAN HEART

Endocardial cushion tissue is formed by an epithelial-mesenchymal tran sformation of endocardial cells, a process which results from an induc tive interaction between the myocardium and endocardium within the atr ioventricular (AV) and outflow tract (OT) regions of the heart. We rep ort here that a protein previously found to be required for myocardial ly induced transformation of endocardial cells in vitro, ES/130, is hi ghly expressed within the AV and OT regions not only by myocardial cel ls, but also by the endocardium and its mesenchymal progeny. Given the se findings and others, we have tested the hypothesis that endocardial cushion tissue secretes factors which autoregulate its transformation to mesenchyme. Endocardial cushion tissue was cultured and its condit ioned growth medium was harvested and applied to nontransformed endoca rdial cells maintained in the absence of the inductive myocardium. Thi s treatment resulted in endocardial cell invasion into three-dimension al collagen gels plus increased expression of proteins associated with endocardial cell transformation in vivo. Whereas endocardial cushion tissue was found to express ES/130 protein in vivo and in vitro, minim al detection of ES/130 in its conditioned growth medium was observed i n immunoblots. Attempts to inhibit the mesenchyme-promoting activity o f the conditioned medium with ES/130 antisense were unsuccessful. Howe ver, strong intracellular ES/130 expression was detected in endocardia l cells, and this expression correlated with the ability of endocardia l cells to transform. For example, the minority of endocardial culture s that failed to transform in response to conditioned medium treatment also failed to undergo increased expression of ES/130. These observat ions are interpreted to suggest that (i) endocardial cushion tissue se cretes factors that promote its transformation to mesenchyme, and (ii) while endocardial cushion tissue appears to signal through secretion of factors other than or in addition to ES/130, intracellular ES/130 e xpression nevertheless may be a target endocardial cell response requi red for endocardial cell transformation.