RhoA signaling via serum response factor plays an obligatory role in myogenic differentiation

Serum response factor (SRF) plays a central role during myogenesis, being required for the expression of striated alpha-actin genes. As shown here, the small GTPase RhoA-dependent activation of SRF results in the expression of muscle-specific genes, thereby promoting myogenic differentiation in myoblast cell lines. Co-expression of activated V14-RhoA and SRF results in an approximately 10-fold activation of the skeletal alpha-actin promoter in replicating myoblasts, while SRFpm1, a dominant negative SRF mutant, blocks RhoA dependent skeletal alpha-actin promoter activity. Serum withdrawal further potentiates RhoA- and SRF-mediated activation of alpha-actin promoter to about 30-fold in differentiated myotubes. In addition, the proximal SRE1 in the skeletal alpha-actin promoter is sufficient to mediate RhoA signaling via SRF. Furthermore, SRFpm1 and to a lesser extent dominant negative N19-RhoA inhibit myoblast fusion, postreplicative myogenic differentiation, and expression of direct SRF targets such as skeletal alpha-actin and indirect targets such as myogenin and alpha-myosin heavy chain. Moreover, RhoA also stimulates the autoregulatable murine SRF gene promoter in myoblasts, and the expression level of SRF is reduced in myoblasts overexpressing N19-RhoA. Our study supports the concept that RhoA signaling via SRF serves as an obligatory muscle differentiation regulatory pathway.     

Analysis of elastin gene expression in the developing chick aorta using cloned elastin cDNA

A segment of chick elastin cDNA cloned in the plasmid pBR322 was sequenced by the method of Maxam and Gilbert ((1977) Proc. Natl. Acad. Sci. U. S. A. 74, 560-564) and the 192-base pair insert was found to be derived from the nontranslated region of the 3' end of the mRNA. The nick-translated cDNA was used to identify and to estimate the relative amounts of elastin mRNA in the developing chick embryo aorta by blot hybridization. A single species of 3.5 kilobases hybridized to the cDNA probe and this species increased greatly between day 7 and day 14 of embryonic development. This increase was paralleled by an increase in translatable aortic elastin mRNA and by the in vivo rate of elastin synthesis, demonstrating that the change in synthesis during development is largely controlled by the elastin mRNA content of the aorta.