The steroid hormone Ecdysone coordinates larval growth and development through its interaction with the transcriptional repressor Smrter.
Authors: Joanna Wardwell-Ozgo; Colby Schweibenz; Douglas Terry; Ken Moberg
Affiliation: Emory University School of Medicine
Keywords: d. repressors/corepressors; d. repressors/corepressors
Hormones act as systemic signaling cues to coordinate developmental timing, growth, and tissue patterning. In Drosophila, pulses of the steroid hormone ecdysone (Ec) synchronize various physiological and developmental processes required for larval ecdysis, tissue growth, and morphogenesis. Binding of the bioactive form of ecdysone (20-hydroxyecdysone; 20E) to its cognate nuclear hormone receptor, Ecdysone Receptor (EcR), is proposed to switch target genes from a transcriptionally repressed to activated state. Current data indicate that EcR has tissue-specific targets, and that even within a single cell type, developmental fluctuations in 20E levels also coincide with dynamic redistribution of EcR between different loci. Despite a long history of genetic analysis of EcR, mechanisms that direct these temporal/cell-type specific transcriptional responses to 20E are not well defined. To probe this repression-to-activation switch in a single tissue, we created a Gal4/UAS-regulated “EcR sponge” transgene composed of the ligand binding domain (LBD) region of EcR (EcRLBD) fused to RFP. Expression of EcRLBD represses heterologous EcR transcriptional reporters in wing disc cells when 20E titers are high (late L3), and reciprocally activates EcR activity in these same cells during early L3 when 20E levels are low, indicating that EcRLBD interacts with key elements of the repression-to-activation switch. Indeed, loss of the transcriptional repressor Smrter (NCoR) mimics the derepressing effect of EcRLBD in early L3 wing cells, suggesting that Smrter is an EcR-associated repressor in larval disc cells. Consistent with this hypothesis, a version of EcRLBD carrying the A483T mutation, which blocks Smrter binding, (termed ‘EcRDmber’) represses EcR transcriptional reporter activity regardless of developmental age. With regard to endogenous target genes, EcRLBD, but not EcRDmber, relieves repression of multiple EcR targets in wing disc cells and disrupts some, but not all, 20E-EcR-triggered developmental processes in the larval salivary gland and fat body. Significantly, expression of EcRLBD confers a growth advantage to larval wing cells and derepresses a subset of growth genes controlled by the Hippo pathway. In sum, these data reveal distinct molecular roles of 20E-EcR signaling at individual promoters, and suggest that 20E-EcR signaling coordinates proliferation and tissue growth in part through repression of Hippo pathway target genes. Current work centers on proteomic analysis of EcRLBDinteractors in larval cells to identify key activators and repressors that are active in these mechanisms.