Members of the myocyte enhancer binding factor-2 (MEF2) family of MADS (MCM1, agamous, deficiens, and serum response factor) box transcription factors are expressed in the skeletal, cardiac, and smooth muscle lineages of vertebrate and Drosophila embryos. These factors bind an adenine-thymidine-rich DNA sequence associated with muscle-specific genes. The function of MEF2 was determined by generating a loss-of-function of the single mef2 gene in Drosophila (D-mef2). In loss-of-function embryos, somatic, cardiac, and visceral muscle cells did not differentiate, but myoblasts were normally specified and positioned. These results demonstrate that different muscle cell types share a common myogenic differentiation program controlled by MEF2.

The D-mef2 gene encodes a MADS domain transcription factor expressed in differentiated muscles and their precursors in the Drosophila embryo. Embryos deficient for D-MEF2 protein due to a deletion of upstream transcriptional control sequences fail to form muscle, suggesting that the gene is required for muscle cell differentiation. To directly demonstrate a role for D-mef2 in embryonic myogenesis, we isolated gene mutants containing EMS-induced point mutations, characterized the effects of these mutations on D-MEF2 protein stability and nuclear localization, and analyzed the resulting muscle phenotypes. Our results show that in the somatic muscle lineage, D-mef2 is required for both the formation and patterning of body wall muscle. In the absence of somatic myogenesis, there is extensive apoptosis among the myoblast cell population. In contrast, in the cardiac muscle lineage, morphogenesis of the dorsal vessel occurs normally but the three myosin subunit genes are not expressed. Mutant embryos also exhibit an abnormal midgut morphology, which correlates with the absence of alpha PS2 integrin gene expression and muscle-specific enhancer function, suggesting that D-mef2 regulates the inflated locus which encodes this integrin subunit. D-MEF2 is also expressed in adepithelial cells and rare D-mef2 transheterozygous mutant adults fail to fly, consistent with defects observed in the indirect flight muscles. These results demonstrate that the D-mef2 gene has multiple functions in myogenesis and tissue morphogenesis during Drosophila development.

MEF2 is a MADS-box transcription factor required for muscle development in Drosophila. Here, we show that the bHLH transcription factor Twist directly regulates Mef2 expression in adult somatic muscle precursor cells via a 175-bp enhancer located 2245 bp upstream of the transcriptional start site. Within this element, a single evolutionarily conserved E box is essential for enhancer activity. Twist protein can bind to this E box to activate Mef2 transcription, and ectopic expression of twist results in ectopic activation of the wild-type 175-bp enhancer. By use of a temperature-sensitive mutant of twist, we show that activation of Mef2 transcription via this enhancer by Twist is required for normal adult muscle development, and reduction in Twist function results in phenotypes similar to those observed previously in Mef2 mutant adults. The 175-bp enhancer is also active in the embryonic mesoderm, indicating that this enhancer functions at multiple times during development, and its function is dependent on the same conserved E box. In embryos, a reduction in Twist function also strongly reduced Mef2 expression. These findings define a novel transcriptional pathway required for skeletal muscle development and identify Twist as an essential and direct regulator of Mef2 expression in the somatic mesoderm.

The cucumber (Cucumis sativus) fruit is covered with bloom trichomes and warts (composed of spines and tubercules), which have an important impact on the commercial value of the crop. However, little is known about the regulatory mechanism underlying their formation. Here, we reported that the cucumber WD-repeat homolog CsTTG1, which is localized in the nucleus and cytomembrane, plays an important role in the formation of cucumber fruit bloom trichomes and warts. Functional characterization of CsTTG1 revealed that it is mainly expressed in the epidermis of cucumber ovary and that its overexpression in cucumber alters the density of fruit bloom trichomes and spines, thereby promoting the warty fruit trait. Conversely, silencing CsTTG1 expression inhibits the initiation of fruit spines. Molecular and genetic analyses showed that CsTTG1 acts in parallel to Mict/CsGL1, a key trichome formation factor, to regulate the initiation of fruit trichomes, including fruit bloom trichomes and spines, and that the further differentiation of fruit spines and formation of tubercules regulated by CsTTG1 is dependent on Mict. Using yeast two-hybrid assay and bimolecular fluorescence complementation assay, we determined that CsTTG1 directly interacts with Mict. Collectively, our results indicate that CsTTG1 is an important component of the molecular network that regulates fruit bloom trichome and wart formation in cucumber.

No abstract