Nuclear Receptors in Mosquito Vitellogenesis

Abstract

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 the female fat body. In mosquitoes, expression of YP genes is controlled by 20-hydroxyecdysone (20E). To elucidate the role of 20E in mosquito vitellogenesis, we cloned cDNAs encoding the Aedes aegypti ecdysteroid receptor (AaEcR) and two isoforms of its heterodimeric partner, the Ultraspiracle homologue (AaUSP). The two AaUSP isoforms differ in their A/B domains and have distinct expression patterns. The ecdysone regulation of YP genes likely involves products of early genes. We cloned the gene of the mosquito homologue to the Drosophila early gene E75 (AaE75) belonging to the nuclear receptor superfamily. Kinetics of AaE75 expression correlate with the expression of YP genes, suggesting that AaE75 may have a regulatory role in YP gene expression. A second nuclear receptor superfamily member, the NGFI-B homologue AaHR38 is implicated in repression of the ecdysone-signaling pathway in the fat body of the previtellogenic female mosquito at the state-of-arrest. Finally, three isoforms of the hepatocyte nuclear factor 4 (HNF-4) homologue AaHNF-4 are differentially expressed in the mosquito fat body during vitellogenesis, suggesting their involvement in regulating vitellogenic events in this tissue.