Characterizing a novel interaction between ecdysone receptor and the AP-1 transcription factor in the regulation of gene expression during Drosophila dorsal closure

Dorsal closure (DC) of the Drosophila embryo is a well-characterized model system for studying morphogenetic events in wound healing and other developmental fusions such as palate fusion and neural tube closure. Prior to DC, a hole occupied with an extraembryonic tissue called amnioserosa (AS) is naturally left at the dorsal side of the embryo. DC begins when the epithelial sheets migrate over a hole and fuse to form a continuous epidermis. A commonly used secretable signal is a member of the transforming growth factor β (TGFβ) family, such as Dpp in Drosophila. During DC, the leading edge cells secrete Dpp into the AS cells to produce the steroid hormone, ecdysone (20E), which then drives AS morphogenesis by triggering gene expression. Here, we provide evidence that ecdysone-mediated gene expression is achieved through a novel interaction between the ecdysone receptor (EcR) and a subunit of the JNK-activated AP-1 transcription factor, Jun. While steroid hormone receptor interactions with AP-1 have been described in vertebrates, to our knowledge they have not been described in invertebrates and our work suggests that these interactions are ancient, predating the split between the vertebrate and invertebrate lineages.