Serum Response Factor, an Enriched Cardiac Mesoderm Obligatory Factor, Is a Downstream Gene Target for Tbx Genes

Abstract

We tested the idea that T-box factors direct serum response factor (SRF) gene activity early in development. Analysis of SRF-LacZ "knock-in" mice showed highly restricted expression in early embryonic cardiac and skeletal muscle mesoderm and neuroectoderm. Examination of the SRF gene for regulatory regions by linking the promoter and 5-flanking sequences, up to 5.5 kb, failed to target LacZ transgene activity to the heart and the tail pre-somitic mesenchyme. However, linkage of a minimal SRF promoter with the SRF 3-untranslated region (UTR), inundated with multimeric T-box binding sites (TBEs), restored robust reporter gene activity to embryonic heart and tail. Finer dissection of the 3-UTR to a small cluster of TBEs also stimulated transgene activity in the cardiac forming region and the tail, however, when the TBEs contained within these DNA sequences were mutated, preventing Tbx binding, transgene activity was lost. Tbx2, Tbx5, and the cardiac-enriched MYST family histone acetyltransferase TIP60, were observed to be mutual interactive cofactors through the TIP60 zinc finger and the T-box of the Tbx factors. In SRF-null ES cells, TIP60, Tbx2, and Tbx5 were sufficient to stimulate co-transfected SRF reporter activity, however this activity required the presence of the SRF 3-UTR. SRF gene transactivation was blocked by two distinct TIP60 mutants, in which either the histone acetyltransferase domain was inactivated or the Zn finger-protein binding domain was excised. Our study supports the idea that SRF embryonic cardiac gene expression is dependent upon the SRF 3-UTR enhancer, Tbx2, Tbx5, and TIP60 histone acetyltransferase activity.During early development, a subset of pluripotent mesodermal cells becomes increasingly committed to the cardiac muscle lineage, through the combinatorial interactions of transcription factors, which result in the expression of cardiac-specific genes. Although this is a continuous developmental process, one transcription factor, serum response factor (SRF), 1 may play a leading role in the commitment of pre-cardiac cells. SRF is a 67-kDa DNA-binding protein that was first cloned from a HeLa cDNA library and was generally presumed to be a ubiquitous transcription factor (1). SRF binds DNA as a dimer and symmetrically contacts various serum response elements with a consensus sequence CC(A/T) 6 GG. SRF is member of the "MADS" box transcription factor family (2, 3), and, despite their similarities, MADS box proteins have evolved to perform disparate important biological functions. Functions of MADS transcription factors include specification of mating type in yeast, homeotic activities in plants, pharyngeal muscle specification in Caenorhabditis elegans, pulmonary development in Drosophila, and elaboration of mesoderm structures in vertebrates (reviewed in Ref. 4). Recent homologous recombinant knock-out of the murine SRF gene locus supports the observation that SRF is absolutely required for the appearance of mesoderm during mouse gastrulation (5).Analysis of SRF null mice not only reve...

Serum response factor micromanaging cardiogenesis

“…B The second diagram shows cardiogenic specific regulation of the SRF gene. Tbx2, Tbx5, and TIP60, were shown to strongly activate SRF activity (2). Also shown is the competitive inhibition of SRF -cardiogenic cofactor interactions by ETS factors.…”

“…SRF is expressed in a highly restrictive pattern throughout mouse development (2). As development proceeds, SRF transcripts becomes restricted to the cardiac crescent and greatly increases in the heart tube and presomitic mesenchyme in the tail and somites (Fig.3).…”

Role of serum response factor in the pathogenesis of disease

“…50 Expression of SRF is already evident in vascular SMCs at Be10.5 of development, 34 a time coinciding with the expression of several SMC contractile genes [51][52][53][54] and the establishment of a functional circulatory system. 55 The only reported knockout of SRF in vascular SMCs used the Sm22a promoter, 32 which is active in embryonic SMCs, the developing heart, and somites.…”

“…SRF expression is first seen in the mouse cardiac crescent at embryonic day 7.75 (e7.75) 34 where cardiac progenitor cells migrate to the midline of the embryo to form a linear heart tube at Be8.25 followed by cardiac looping and septation. 35 As the embryo turns on its axis, the heart has already formed chambers and is contracting with abundant expression of SRF in the proliferating compact zone and finger-like projections called trabeculations that extend into the interior of the heart (Figure 2a).…”

MicroRNAs Telltale Effects on Signaling Networks in Cardiomyopathy

“…As miRNAs are transcribed like other genes, they are regulated by transcription factors and expression of transcription factors determines the miRNA expression. SRF is a cardiac enriched transcription factor that regulates sarcomere organization in the heart and SRF expression follows a restrictive pattern during development (Olson and Schneider, 2003;Niu et al, 2007;Barron et al, 2005). SRF expression is very important as multiple SRF binding sites have been identified in promoters of genes regulating contractility, cell movement, and growth signaling (Sun et al, 2006;Zhang et al, 2005).…”