Bone morphogenetic protein-2 can mediate myocardial regulation of atrioventricular cushion mesenchymal cell formation in mice.

Transformation of endocardial endothelial cells into invasive mesenchyme is a critical antecedent of cardiac cushion tissue formation. The message for bone morphogenetic protein (BMP)-2 is known to be expressed in myocardial cells in a manner consistent with the segmental pattern of cushion formation [Development 109(1990) 833]. In the present work, we localized BMP-2 protein in atrioventricular (AV) myocardium in mice at embryonic day (ED) 8.5 (12 somite stage) before the onset of AV mesenchymal cell formation at ED 9.5. BMP-2 protein expression was absent from ventricular myocardium throughout the stages examined. After cellularization of the AV cushion at ED 10.5, myocardial BMP-2 protein expression was diminished in AV myocardium, whereas cushion mesenchymal cells started expressing BMP protein. Expression of BMP-2 in cushion mesenchyme persisted during later stages of development, ED 13.5-16, during valuvulogenesis. Intense expression of BMP-2 persisted in the valve tissue in adult mice. Based on the expression pattern, we performed a series of experiments to test the hypothesis that BMP-2 mediates myocardial regulation of cardiac cushion tissue formation in mice. When BMP-2 protein was added to the 16-18 somite stage (ED 9.25) AV endocardial endothelium in culture, cushion mesenchymal cells were formed in the absence of AV myocardium, which invaded into collagen gels and expressed the mesenchymal marker, smooth muscle (SM) alpha-actin; whereas the endothelial marker, PECAM-1, was lost from the invaded cells. In contrast, when noggin, a specific antagonist to BMPs, was applied together with BMP-2 to the culture medium, AV endothelial cells remained as an epithelial monolayer with little expression of SM alpha-actin, and expression of PECAM-1 was retained in the endocardial cells. When noggin was added to AV endothelial cells cocultured with associated myocardium, it blocked endothelial transformation to mesenchyme. AV endothelium treated with BMP-2 expressed elevated levels of TGFbeta-2 in the absence of myocardium, as observed in the endothelium cocultured with myocardium. BMP-2-supported elevation of TGFbeta-2 expression in endocardial cells was abolished by noggin treatment. These data indicated that BMP signaling is required in and BMP-2 is sufficient for myocardial segmental regulation of AV endocardial cushion mesenchymal cell formation in mice.