ABCH2 transporter in the first line of defense protects malaria vectors from pyrethroids

Abstract

Contact insecticides are primarily used for the control of Anopheles malaria vectors. These chemicals penetrate mosquito legs and other appendages, the first barrier to reaching their neuronal targets. An ATP-Binding Cassette transporter from the H family (ABCH2) is highly expressed in Anopheles coluzzii legs, and further induced upon insecticide exposure. RNAi-mediated silencing of the ABCH2 caused a significant increase in deltamethrin mortality compared to control mosquitoes, coincident with a corresponding increase in 14C-deltamethrin penetration. RT-qPCR analysis and immunolocalization revealed that ABCH2 is mainly localized in the legs and head appendages, and more specifically, the apical part of appendage epidermis, underneath the cuticle. To unravel the molecular mechanism underlying the role of ABCH2 in modulating pyrethroid toxicity, two hypotheses were investigated: An indirect role, based on the orthology with other insect ABCH transporters involved with lipid transport and deposition of CHC lipids in Anopheles legs which may increase cuticle thickness, slowing down the penetration rate of deltamethrin; or the direct pumping of deltamethrin out of the organism. Evaluation of the leg cuticular hydrocarbon (CHC) content showed that this was not affected by ABCH2 silencing, indicating this transporter in is not associated with the transport of leg CHCs. Homology-based modeling suggested that the ABCH2 half-transporter adopts a physiological homodimeric state, in line with its ability to hydrolyze ATP in vitro when expressed on its own in insect cells. Docking analysis revealed a deltamethrin pocket on the homodimeric transporter. Furthermore, deltamethrin-induced ATP hydrolysis in ABCH2-expressing cell membranes, further supports that deltamethrin is indeed a ABCH2 substrate. Overall, our findings pinpoint ABCH2 as a key regulator of deltamethrin toxicity.