Direct and indirect gene repression by the ecdysone cascade during mosquito reproductive cycle

Hematophagous mosquitoes transmit devastating human diseases. Reproduction of these mosquitoes is cyclical, with each egg maturation period supported by a blood meal. Previously, we have shown that in the female mosquito Aedes aegypti, nearly half of all genes are differentially expressed during the postblood meal reproductive period in the fat body, an insect analog of mammalian liver and adipose tissue. This work aims to decipher how transcription networks govern these genes. Bioinformatics tools found 89 putative transcription factor binding sites (TFBSs) on the cis-regulatory regions of more than 1,400 differentially expressed genes. Putative transcription factors that may bind to these TFBSs were identified and used for the construction of temporally coordinated regulatory networks. Further molecular biology analyses have uncovered mechanisms of direct and indirect negative transcriptional regulation by the steroid hormone 20-hydroxyecdysone (20E) through the ecdysone receptor (EcR). Genes within the two groups, early genes and late mid-genes, have distinctly different expression profiles. However, both groups of genes show lower expression at the high titers of 20E and are down-regulated by the 20E/EcR cascade by different molecular mechanisms. Transcriptional repression of early genes is indirect and involves the classic 20E pathway with ecdysone-induced protein E74 functioning as a repressor. Late mid-genes are repressed directly by EcR that recognizes and binds a previously unreported DNA element, different from those utilized in the 20E-mediated gene activation, within the regulatory regions of its target genes and recruits Mi2 that acts as a corepressor, initiating chromatin condensation.