Signal-Dependent Transcriptional Regulation of Vascular Smooth Muscle Cell Differentiation
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Vascular smooth muscle cells (VSMCs) play a key role in development as they are the major source of extracellular matrix components of vessel walls. During development, VSMCs will both proliferate and differentiate to form components of the vasculature. Differentiated VSMCs (contractile phenotype) line vessel walls to regulate blood flow. The proliferative phenotype of VSMCs (synthetic phenotype) refers to migration and proliferation of these cells to specific sites to contribute to the formation of the vasculature. Interestingly, VSMCs maintain the ability to proliferate post-natally in response to vascular injury. Therefore, the purpose of this body of work was to investigate the signalling pathways that regulate transcriptional control in VSMCs. Calcium sensitivity in VSMCs is regulated by RhoA/ROCK-mediated inhibition of the myosin light chain phosphatase complex, and alterations in smooth muscle gene expression. We found that calcium signalling stimulates ROCK-mediated phosphorylation of the PP1α inhibitor CPI-17 at threonine 38, leading to derepression of MEF2C by PP1α and increased myocardin expression, which lies upstream of smooth muscle-specific structural genes. Furthermore, TGF-β also potently induces VSMC marker genes at the transcriptional and protein levels in 10T1/2 mouse embryonic fibroblast cells. We found that the potent transcriptional regulator and nuclear retention factor, TAZ, is required for TGF-β induction of smooth muscle genes and is required in the maintenance of the differentiated VSMC phenotype. A synergistic interaction between TAZ and SRF in regulating smooth muscle gene activation and differentiation has also been observed, and TAZ expression enhances SRF binding to the smooth muscle α-actin promoter. This work addresses several important aspects of signalling pathways involved in the regulation of the vascular smooth muscle phenotype and provides a further understanding of the role of SRF in vascular development and vascular disease.