Serum response factor orchestrates nascent sarcomerogenesis and silences the biomineralization gene program in the heart

Abstract

Our conditional serum response factor (SRF) knockout, Srf Cko , in the heart-forming region blocked the appearance of rhythmic beating myocytes, one of the earliest cardiac defects caused by the ablation of a cardiac-enriched transcription factor. The appearance of Hand1 and Smyd1, transcription and chromatin remodeling factors; Acta1, Acta2, Myl3, and Myom1, myofibril proteins; and calcium-activated potassium-channel gene activity (KCNMB1), the channel protein, were powerfully attenuated in the Srf CKO mutant hearts. A requisite role for combinatorial cofactor interactions with SRF, as a major determinant for regulating the appearance of organized sarcomeres, was shown by viral rescue of SRF-null ES cells with SRF point mutants that block cofactor interactions. In the absence of SRF genes associated with biomineralization, GATA-6, bone morphogenetic protein 4 (BMP4), and periostin were strongly up-regulated, coinciding with the down regulation of many SRF dependent microRNA, including miR1, which exerted robust silencer activity over the induction of GATA-6 leading to the down regulation of BMP4 and periostin.heart development ͉ microRNA ͉ periostin ͉ cardiogenesis ͉ GATA6

Role of serum response factor in the pathogenesis of disease

“…In SRF mutants, cardiomyocytes show severe disruption in cardiac sarcomerogenesis, which likely contributes to the heart failure phenotype. 32,37 A similar ultrastructure phenotype is manifest in adult mouse hearts where SRF is inducibly inactivated using a tamoxifen-responsive Cre recombinase. 38 A mosaic knockout of SRF using the Desmin promoter driving Cre resulted in mutant cardiomyocytes in close apposition to wild-type cardiomyocytes.…”

“…31,32,36,37 This would support an indirect mechanism involving downregulated expression of SRF-dependent transcription factors that normally activate these genes, or upregulation of repressive miRs. Interestingly, loss of expression of SRF in cardiomyocytes can activate a subset of genes.…”

“…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). The most obvious phenotype in mice with cardiac muscle-specific inactivation of SRF is a thin-walled compact zone and poorly developed trabeculations of the ventricular chambers resulting in a dilated, heart failure-like phenotype with embryo demise occurring at Be11.5 of development 31,32,36,37 (Figure 2b). Wild-type embryonic cardiomyocytes have few bands of sarcomeres that somehow are sufficient to coordinate enough force to propel blood throughout the embryo.…”

“…39 Not surprisingly, biochemical data from knockout mice show dramatic decreases in an array of SRF-target genes encoding for contractile elements and calcium-handling proteins. 31,32,36,37,39 Reduced expression of such genes explains the inability of mutant hearts to organize functional sarcomeres and generate sufficient contractile force. As SRF activates a growing number of actin cytoskeletal genes important in cell migration, 27 the poorly Figure 1 The CArG box.…”

“…Interestingly, loss of expression of SRF in cardiomyocytes can activate a subset of genes. 23,37 Although SRF is not considered to be a direct repressor of gene expression, it may indirectly silence gene activity through its ability to bind to and activate CArG-containing miRs. 25,40,41 In fact, some upregulated genes in SRF-null cardiomyocytes have been shown to increase because of the loss in the expression of SRF-dependent miRs that normally limit such gene expression.…”