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

Abstract

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.

Matrix GLA Protein Stimulates VEGF Expression through Increased Transforming Growth Factor-β1 Activity in Endothelial Cells

“…BMP-2 is expressed in the embryonic aorta (46), and is known to be involved in interactions between ectodermal and mesodermal cells during development (47,48). Furthermore, it has a role in EC differentiation; BMP-binding endothelial cell precursor-derived regulator, a recently described BMP antagonist in EC, inhibits both BMP signaling and EC differentiation (49).…”

BMP and FGF regulatory pathways in semilunar valve precursor cells

“…BMP and FGF signaling pathways have previously been shown to be important for normal heart development including endocardial cushion formation, cushion elongation, and remodeling (Galvin et al, 2000;Gaussin et al, 2002;Delot et al, 2003;Jiao et al, 2003;Sugi et al, 2003Sugi et al, , 2004Somi et al, 2004;Ma et al, 2005;Lincoln et al, 2006a). A number of BMP ligands have been identified in the developing heart; in particular, BMP4 is expressed in the OFT myocardium adjacent to the endocardial cushions during early stages of valve development (Keyes et al, 2003).…”

Co-ordinating Notch, BMP, and TGF-β signaling during heart valve development

“…Additionally, although explants normally require AVC myocardium for EMT to take place, BMP2 induces EMT in mouse AVC explants without AVC myocardium. Moreover, noggin, a BMP antagonist, blocks EMT [50] and addition of BMP2 to ventricular explants (normally unable to undergo EMT) induces EMT [51]. These together indicate that BMP2 is a critical factor required for EMT and initial formation of the cardiac cushions.…”

“…At E9.5, a heart tube is identifiable in BMP2 −/− embryos but chambers are indistinguishable and thus precludes examination of BMP2 in cardiac cushion formation [49]. Given that BMP2 is highly expressed in the AVC myocardium prior to valve formation [50], Ma et al deleted BMP2 specifically in the myocardium using Nkx2.5-Cre [39] and demonstrated that BMP2 is required for formation of cardiac jelly and for initiation and formation of the cardiac cushions [39]. Additionally, although explants normally require AVC myocardium for EMT to take place, BMP2 induces EMT in mouse AVC explants without AVC myocardium.…”

“…For example levels of TGFβ2 in the AVC are reduced in myocardial-specific ALK3 mutant mice, indicating that myocardial BMP signalling is required to maintain TGFβ2 expression during cushion formation [61]. Moreover, addition of BMP2 to AVC explants increases TGFβ2 [50] and myocardial deletion of Bmp2 leads to loss of TGFβ2 in AV cushions [39].…”