Differential Expression and Regulation by 20-hydroxyecdysone of Mosquito Ultraspiracle Isoforms

Ultraspiracle (USP), the insect homologue of the vertebrate retinoid X receptor, is an obligatory dimerization partner for the ecdysteroid receptor (EcR). Two USP isoforms, USP-A and USP-B, with distinct N-termini, occur in the mosquito Aedes aegypti. In the fat body and ovary, USP-A mRNA is highly expressed during the pre- and late vitellogenic stages, corresponding to a period of low ecdysteroid titer, while USP-B mRNA exhibits its highest levels during the vitellogenic period, correlating with a high ecdysteroid titer. Remarkably, 20-hydroxyecdysone (20E) has opposite effects on USP isoform transcripts in in vitro fat body culture. This steroid hormone upregulates USP-B transcription and its presence is required to sustain a high level of USP-B expression. In contrast, 20E inhibits activation of USP-A transcription. Although EcR.USP-A recognizes the same ecdysteroid-responsive elements, EcR.USP-B binds them with an affinity twofold higher than that of EcR.USP-A. Likewise, EcR.USP-B transactivates a reporter gene in CV-1 cells twofold more strongly than EcR.USP-A. These results suggest that USP-B functions as a major heterodimerization partner for EcR during the vitellogenic response to 20E in the mosquito.