Biochemical and Functional Analysis of Members of the Myocardin Family During Cardiovascular Development

The various stages of muscle development are characterized by distinct patterns of gene expression precisely controlled by combinatorial interaction between a large number of muscle-specific and ubiquitous transcription factors. Myocardin is a cardiac and smooth muscle-specific transcriptional coactivator of serum response factor (SRF) that forms a ternary complex with SRF on DNA and provides its strong transcriptional activation domain (TAD) to SRF. SRF has been shown to stimulate expression of smooth and cardiac muscle genes in association with GATA transcription factors, which play important roles in cardiac and smooth muscle development. I show that GATA transcription factors can either stimulate or suppress the transcriptional activity of myocardin, depending on the target gene. Modulation of myocardin activity by GATA4 is mediated by the physical interaction of myocardin with the DNA binding domain of GATA4 but does not require binding of GATA4 to DNA. The ability of GATA transcription factors to modulate myocardin activity provides a potential mechanism for fine tuning the expression of serum response factor target genes in a gene-specific manner. Two Myocardin Related Transcription Factors, referred to as MRTF-A and B, are expressed in numerous embryonic and adult tissues, implying their potential to modulate SRF target genes in a wide range of tissues. To determine the functions of MRTF-B in vivo, I generated MRTF-B mutant mice by targeted inactivation of the MRTF-B gene. I show that mice homozygous for an MRTF-B loss-of-function mutation die during mid-gestation from a spectrum of cardiovascular defects. These abnormalities are accompanied by a failure in differentiation of smooth muscle cells within the branchial arch arteries, which are derived from the neural crest. The phenotype of MRTF-B mutant mice is distinct from that of mice lacking myocardin and MRTF-A, revealing unique roles for these SRF coactivators in the development of different subsets of smooth muscle cells in vivo.