Nuclear receptors in mosquito vitellogenesis

Vitellogenesis in insects involves the coordinated activity of the fat body , which produces large amounts of yolk protein precursors (YP), and oocytes , which specifically accumulate these proteins. The expression of YP genes is achieved through strict sex-, tissue-, and hormone specific control in t he female fat body.. In mosquitoes, expression of YP genes is controlled by 20-hydroxyecdysone (20E). To elucidate the role of 20E in mosquito vitello genesis, we cloned cDNAs encoding the Aedes aegypti ecdysteroid receptor (A aEcR) and two isoforms of its heterodimeric partner, the Ultraspiracle homo logue (AaUSP). The two AaUSP Isoforms differ in their A/B domains and have distinct expression patterns. The ecdysone regulation of YP genes likely in volves products of early genes. We cloned the gene of the mosquito homologu e to the Drosophila early gene E75 (AaE75) belonging to the nuclear recepto r superfamily. Kinetics of AaE75 expression correlate with the expression o f YP genes, suggesting that AaE75 may have a regulatory role in YP gene exp ression. A second nuclear receptor superfamily member, the NGFI-B homologue AaHR38 is implicated in repression of the ecdysone-signaling pathway in th e fat body of the previtellogenic female mosquito at the state-of-arrest. F inally, three isoforms of the hepatocyte nuclear factor I (HNF-J) homologue AaHNF-4 are differentially expressed in the mosquito fat body during vitel logenesis, suggesting their involvement in regulating vitellogenic events i n this tissue.