Targeted Inactivation of Serum Response Factor in the Developing Heart Results in Myocardial Defects and Embryonic Lethality

Abstract

Serum response factor (SRF) is at the confluence of multiple signaling pathways controlling the transcription of immediate-early response genes and muscle-specific genes. There are active SRF target sequences in more than 50 genes expressed in the three muscle lineages including normal and diseased hearts. However, the role of SRF in heart formation has not been addressed in vivo thus far due to the early requirement of SRF for mesoderm formation. We have generated a conditional mutant of SRF by using Cre-LoxP strategy that will be extremely useful to study the role of SRF in embryonic and postnatal cardiac functions, as well as in other tissues. This report shows that heart-specific deletion of SRF in the embryo by using a new ␤MHC-Cre transgenic mouse line results in lethal cardiac defects between embryonic day 10.5 (E10.5) and E13.5, as evidenced by abnormally thin myocardium, dilated cardiac chambers, poor trabeculation, and a disorganized interventricular septum. At E9.5, we found a marked reduction in the expression of essential regulators of heart development, including Nkx2.5, GATA4, myocardin, and the SRF target gene c-fos prior to overt maldevelopment. We conclude that SRF is crucial for cardiac differentiation and maturation, acting as a global regulator of multiple developmental genes.

Role of serum response factor in the pathogenesis of disease

“…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.…”

“…Three distinct floxed Srf mice have been used to cross with specific Cre mice. Two were designed for deleting exon-1 30 or exon-2, 31 the latter mouse yielding a hypomorphic allele upon Cre-mediated excision. The other floxed mouse was engineered for deletion of the 5 0 promoter and the first exon.…”

The serum response factor (SRF)/megakaryocytic acute leukemia (MAL) network participates in megakaryocyte development

“…Srf flx mice 10 were bred with polyinosine-polycytidine-inducible Mx1-Cre mice, 11 then analyzed 5 weeks post-induction (Supplementary Figure 1). The Mal/Mrft-A À/À mice have been reported.…”