Essential functions of mosquito ecdysone importers in development and reproduction

Abstract

The primary insect steroid hormone ecdysone requires a membrane transporter to enter its target cells. Although an organic anion-transporting polypeptide (OATP) named Ecdysone Importer (EcI) serves this role in the fruit fly Drosophila melanogaster and most likely in other arthropod species, this highly conserved transporter is apparently missing in mosquitoes. Here we report three additional OATPs that facilitate cellular incorporation of ecdysone in Drosophila and the yellow fever mosquito Aedes aegypti. These additional ecdysone importers (EcI-2, 3, and 4) are dispensable for development and reproduction in Drosophila, consistent with the predominant role of EcI. In contrast, in Aedes, EcI-2 is indispensable for ecdysone-mediated development, whereas EcI-4 is critical for vitellogenesis induced by ecdysone in adult females. Altogether, our results indicate unique and essential functions of these additional ecdysone importers in mosquito development and reproduction, making them attractive molecular targets for species- and stage-specific control of ecdysone signaling in mosquitoes.

Functional characterization of putative ecdysone transporters in lepidopteran pests

“…Similar results were observed when disruption of the organic anion transporter EcI-2 of Aedes aegypti significantly reduced egg survival [26]. Interestingly, DmOatp74D null mutants did not exhibit any significant embryonic lethality, which contradicts with the increased embryonic lethality induced by knocking out SfOatp74D (Fig 4B ) and EcI-2 of Aedes aegypti [24,26]. This raises the question whether there is a different mechanism for cellular uptake of ecdysone in the Drosophila embryo or the phenotype is masked because of maternal deposition in the eggs of Oatp74D mRNA or protein.…”

“…Interestingly, apart from identifying Oatp74D orthologs in S. frugiperda and H. armigera, it was found that mosquitoes do not have Oatp74D orthologs, although they belong to arthropods (Fig 1). In agreement with this result, a very recent study reports the existence of additional ecdysone importers EcI-2, EcI-3 and EcI-4 in Aedes aegypti, with EcI-2 being necessary for development [26]. The fact that orthologs of the latter ecdysone transporters exist in D. melanogaster, but have no dominant role in development, exemplifies why further functional characterization of Oatp74D was needed to assess its essentiality in the two lepidopteran pests of our interest.…”

“…Therefore, the reduced egg hatching rate caused by SfOatp74D disruption (Fig 4B) could be explained by its essential role in ecdysone transport. Similar results were observed when disruption of the organic anion transporter EcI-2 of Aedes aegypti significantly reduced egg survival [26]. Interestingly, DmOatp74D null mutants did not exhibit any significant embryonic lethality, which contradicts with the increased embryonic lethality induced by knocking out SfOatp74D (Fig 4B ) and EcI-2 of Aedes aegypti [24,26].…”

“…Furthermore, OATP74D was found to regulate the ecdysone signaling pathway and to be necessary for ecdysone-dependent gene expression in cultured cells [24]. It is noteworthy to mention that although three additional OATPs mediating ecdysone import have been identified in Drosophila, they were dispensable compared to OATP74D (26). Ecdysone was suggested as a substrate for different OATPs, and this has been demonstrated also in mosquitoes which lack an OATP74D ortholog [26].…”

“…It is noteworthy to mention that although three additional OATPs mediating ecdysone import have been identified in Drosophila, they were dispensable compared to OATP74D (26). Ecdysone was suggested as a substrate for different OATPs, and this has been demonstrated also in mosquitoes which lack an OATP74D ortholog [26]. Considering that human OATPs have been functionally associated with hormone transport [27,28], it could be suggested that the mechanism of cellular uptake of hormones via OATPs is conserved.…”