Ectopic expression of MEF2 in the epidermis induces epidermal expression of muscle genes and abnormal muscle development in Drosophila.

Myocyte-specific enhancer-binding factor 2 (MEF2) is a myogenic regulatory factor in vertebrates and Drosophila. Whereas the role of MEF2 in regulating vertebrate myogenesis and muscle genes has been extensively studied, little is known of the role of MEF2 in regulating Drosophila myogenesis. We have shown in a recent analysis of the regulation of the Drosophila Tropomyosin I (TmI) gene in transgenic flies that MEF2 is a positive regulator of TmI expression in the somatic body-wall muscles of embryos, larvae, and adults. To understand further the role of MEF2 in myogenesis and test the role of MEF2 in regulating TmI expression, we have used the yeast GAL4/UAS system to generate embryos in which MEF2 is ectopically expressed in tissues where it is not normally expressed or embryos in which MEF2 is overexpressed in the mesoderm and muscles. We observe that ectopic expression of MEF2 in the epidermis and the ventral midline cells in embryos activates the expression of TmI and other muscle genes in these tissues and that this activation is stage-dependent suggesting a requirement for additional factors. Furthermore, ectopic expression of MEF2 in the epidermis results in a decrease in the expression of signaling molecules in the epidermis and a failure of the embryo to properly form body-wall muscles. These results indicate that MEF2 can function out of context in the epidermis to induce the expression of muscle genes and interfere with a requirement for the epidermis in muscle development. We also find that the level of MEF2 in the mesoderm and/or muscles in embryos is critical to body-wall muscle formation; however, no effect is observed on the development of the visceral muscle or dorsal vessel.