Cyclicity of Mosquito Vitellogenic Ecdysteroid-Mediated Signaling Is Modulated by Alternative Dimerization of the RXR Homologue Ultraspiracle

In anautogenous mosquitoes, egg maturation requires a blood meal. As a consequence, mosquitoes are vectors of numerous devastating human diseases. Blood feeding triggers a 20-hydroxyecdysone (20E) hormonal cascade, which activates yolk protein precursor (YPP) genes in the female fat body, an insect metabolic tissue. An important adaptation for anautogeny is the previtellogenic arrest preventing activation of YPP genes. Equally essential is termination of their expression, so that another arrest is achieved after a batch of eggs is laid. Here, we report that mosquito Seven-up (AaSvp), a chicken ovalbumin upstream promoter-transcription factor homologue, is involved in regulating the cyclicity of vitellogenic ecdysteroid-mediated signaling through heterodimerization with a retinoid X receptor homologue Ultraspiracle (USP), the obligatory functional ecdysteroid receptor (EcR) partner. AaSvp inhibits 20E-dependent activation of the vitellogenin (Vg) gene in transfection assays. Two-hybrid and GST pull-down analyses demonstrate that in vitro AaSvp interacts with both AaUSP and AaEcR. However, the coimmunoprecipitation using fat body nuclear extracts reveals that at 33-36 h postblood meal, when the 20E titer sharply declines and YPP gene expression ceases, AaSvp replaces AaEcR in USP heterodimers. The chromatin immunoprecipitation assay indicates that protein-protein interaction rather than binding competition for the Vg ecdysteroid response element accounts for the inhibition of Vg expression by AaSvp.