Expression and function of bone morphogenetic proteins in the development of the embryonic endocardial cushions

Bone morphogenetic proteins (BMPs) are considered to be significant factors in the morphogenesis of the endocardial cushions of the developing embryonic heart. Previous studies have suggested that they are involved in the epithelial-mesenchymal transformation and migration of the cells forming the cushions, or in triggering an apoptotic cascade in a sub-population of cushion cells. We investigated the expression and function of BMP2 and BMP4 proteins in the developing heart of the chick and mouse embryos. In the chick, by immunocytochemistry, we find expression of BMP2 protein in the endocardial cushions of the outflow tract (OT) and atrio-ventricular (AV) regions at embryonic days (ED) 5-6, as well as in adjacent myocardial layers. Immunoblotting indicated that such expression persisted through ED 4-7, but peaked at ED4-5 in the OT and 5-6 in the AV cushions. This temporal sequence correlated with the peaks of apoptotic cell death found previously in the OT and AV cushions of the chick embryo. At equivalent stages in mouse, no such expression of BMP2 was found in the cushions, although expression was detected in adjacent myocardial layers. In the case of BMP4, in both chick and mouse, expression was found only in the myocardia and not in the cushions. Furthermore, BMP-specific receptors were found in the cushions, but not the myocardia, in both the AV and OT regions of the chick embryo. These results provide circumstantial evidence to support the contention that BMPs, originating from the myocardium, could be significant in the induction of apoptosis in chick embryo cushion cells, and confirms that there is species-specific variation in the expression pattern of BMP proteins, as had been predicted from previous studies of mRNA expression. Culture media conditioned by the growth of tissues from various regions of the developing heart were tested for their ability to induce apoptosis in cushion cells in culture. It was found that medium derived from the myocardia induced significant levels of cell death in the cushion cells, and that BMP4 could be detected in such media; however, retroviral over-expression of constitutively active (CA) and dominant-negative (DN) isoforms of BMP-specific receptors 1A and 1B (BMPR-1A and BMPR-1B) in cultured cells of the AV cushions did not alter levels of apoptosis or cell proliferation. Similar over-expression in cultured endocardial cells resulted in a significant change in cell shape, from endothelial to fibroblastic, with BMPR-1A CA and BMPR-1B DN, suggesting an influence of these receptors on cell transformation and/or cell migration. Taken together, these results provide support for the contention that BMP2 and BMP4 are important factors in the phenotypic transformational events involved in the morphogenesis of the chick embryo endocardial cushions, and could be involved in the induction of apoptosis in the cushion cells.