ABC transporters involvement in Anopheles stephensi defense against Azadirachta indica extract

Insecticides are a core component of malaria control programmes, but their massive use led to resistance insurgence in different vector populations that threaten the global malaria control efforts. Alternatives have been searched in botanical compounds used for centuries as traditional remedies. Azadirachta indica, also known as Neem tree, is particularly interesting for its wide action against vector arthropods, parasites and other agents of infection. Neem extracts have shown a strong larvicidal, anti-emergence, repellency, anti-oviposition effects on different mosquito species including Anopheles stephensi, the main malaria vector in Asia. To understand the detoxifying mechanisms of An. stephensi against Neem extracts, we studied the implications of ABC-transporters at different time-points through a) bioassays in combination with the verapamil, an ABC-transporter inhibitor, and b) expression profile of 6 genes of 3 different ABC sub-families (ABC-B, ABC-C, ABC-G) through real-time PCR. Third instar larvae were fed with fish food alone or mixed with A. indica extract. Three pools of larvae were taken at 0.5, 24, 48 and 72 hours for expression analysis. A parallel bioassay with same treatments was performed to assess mortality due to different concentrations of Neem extract. Our study shows an up-regulation of the ABCG4 gene for all time-points and an early activation followed by a down-regulation of ABCB2 and ABCBmember6 genes, when mosquito larvae were fed with food mixed to Neem. After 48h, we observed that mortality is dose-dependent for the insecticide but, surprisingly, verapamil seems to partly preserve larvae from dying when Neem concentration is higher. This can suggest an antagonistic effect of verapamil and Neem. These results demonstrate the involvement of ABC-transporters on the detoxification of the A. indica extract and highlight the need of further investigations to understand the effect of this compound in combination with other toxic compounds against An. stephensi and other mosquitoes in general.