The heart is one of the first organs to form during vertebrate development and mutations in the genes that govern heart development are associated with congenital heart disease in humans. In this article, Huang et al. used scanning electron microscopy to track heart development in myocardin-deficient mice. This image depicts a cross section of heart harvested from a wild type mouse embryo after 9.5 days of development. They also captured video of the ventricular cardiac contractions of wild type and myocardin-deficient embryos.
Myocardin is a muscle lineage–restricted transcriptional coactivator that has been shown to transduce extracellular signals to the nucleus required for SMC differentiation. We now report the discovery of a myocardin/BMP10 (where BMP10 indicates bone morphogenetic protein 10) signaling pathway required for cardiac growth, chamber maturation, and embryonic survival. Myocardin-null (Myocd) embryos and embryos harboring a cardiomyocyte-restricted mutation in the Myocd gene exhibited myocardial hypoplasia, defective atrial and ventricular chamber maturation, heart failure, and embryonic lethality. Cardiac hypoplasia was caused by decreased cardiomyocyte proliferation accompanied by a dramatic increase in programmed cell death. Defective chamber maturation and the block in cardiomyocyte proliferation were caused in part by a block in BMP10 signaling. Myocardin transactivated the Bmp10 gene via binding of a serum response factor–myocardin protein complex to a nonconsensus CArG element in the Bmp10 promoter. Expression of p57kip2, a BMP10-regulated cyclin-dependent kinase inhibitor, was induced in Myocd–/– hearts, while BMP10-activated cardiogenic transcription factors, including NKX2.5 and MEF2c, were repressed. Remarkably, when embryonic Myocd–/– hearts were cultured ex vivo in BMP10-conditioned medium, the defects in cardiomyocyte proliferation and p57kip2 expression were rescued. Taken together, these data identify a heretofore undescribed myocardin/BMP10 signaling pathway that regulates cardiomyocyte proliferation and apoptosis in the embryonic heart.
Jianhe Huang, John Elicker, Nina Bowens, Xi Liu, Lan Cheng, Thomas P. Cappola, Xiaohong Zhu, Michael S. Parmacek