Differential expression and regulation by 20-hydroxyecdysone of mosquito ecdysteroid receptor isoforms A and B

Cloning of the AaEcR-A isoform, along with the previously cloned AaEcR-B isoform, has permitted us to evaluate the expression of AaEcR isoforms during mosquito vitellogenesis. Mosquito EcR isoform transcripts exhibited dramatically different patterns of expression after a blood meal-triggered activation of vitellogenesis in the fat body. The AaEcR-B transcript level rose sharply by 4-h post blood meal (PBM), coinciding with the small ecdysteroid peak, and then declined reaching its lowest level at 16-24-h PBM. In contrast, the AaEcR-A transcript peaked at 16-20-h PBM, coinciding with the large ecdysteroid peak. AaEcR-B and AaEcR-A transcripts exhibited a striking difference in sensitivity to 20-hydroxyecdysone (20E), being maximally activated at 10(-8) and 10(-6) M, respectively. Both ecdysteroid receptor (EcR) isoform mRNAs were transcribed in a cycloheximide-independent manner, suggesting that they are direct targets of 20E. However, AaEcR-A transcription requires continuous presence of 20E, while AaEcR-B mRNA level rose for 4 It and then declined under the same conditions. These results indicate the mosquito EcR isoforms play distinct physiological functions during vitellogenesis in the mosquito fat body. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.