Myocardin is a serum response factor (SRF) cofactor expressed in cardiac and smooth muscle (SM) cell lineages.1
Although myocardin lacks intrinsic DNA-binding ability, it forms a stable ternary complex with SRF to potently activate muscle-specific genes through the consensus sequence CC(A/T)6
GG, known as a CArG box.1–3
In addition to activating cardiac gene expression and the cardiogenesis program, myocardin is also a potent transactivator for smooth muscle cell (SMC) differentiation and SM-related gene expression.1,4–13
Despite the significant role of myocardin in controlling muscle gene expression, upstream signaling pathways that regulate myocardin activity remain unknown.
Bone morphogenetic proteins (BMPs) are growth and differentiation factors of the transforming growth factor (TGF)-β
Signaling by this superfamily is mediated by Smad proteins. There are three classes of Smads: receptor-regulated Smads (R-Smads), co-Smad (Smad4), and inhibitory Smads. To date, 3 R-Smads (Smad1, -5, and -8) participate in BMP signaling.15
Once activated, R-Smads form a heteromeric complex with Smad4 that translocates into the nucleus to regulate expression of BMP-responsive genes.15
Smad proteins bind DNA relatively weakly, but are strongly recruited to specific target genes by interacting with other transcription factors.15
BMPs and downstream BMP signaling effectors are essential for cardiovascular development.16–21
However, it is clear BMP signaling alone is insufficient to activate the cardiac gene program because BMP-signaling pathway components are expressed in a wide range of tissues and cell types outside of cardiac muscle. Indeed, the ability of BMP signaling to commit specific mesodermal cells to a cardiac fate requires the interpretation of BMP signaling in a cell type–specific mechanism. How BMP signaling interacts with cardiac transcriptional networks is largely unknown. In light of the role of BMP signaling and myocardin in cardiovascular development, we investigated whether BMP signaling might regulate myocardin-mediated cardiac gene expression.
In this report, we show that Smad1 synergistically activates myocardin-dependent cardiac gene expression. Interestingly, the CArG box is necessary and sufficient for such synergy, whereas no obvious Smad-binding elements (SBEs) are involved. Consistent with their functional interaction, myocardin and Smad1 proteins physically interact. Myocardin transactivity was repressed by Smad7, an inhibitory Smad, and enhanced by constitutively activated activin receptor-like kinase (ALK)-3, a type I BMP receptor. Furthermore, myocardin protein expression was dramatically induced by BMP-2 treatment in cardiomyocytes. These findings suggest a role for BMP signaling in regulating myocardin expression and activity to control cardiac gene expression.