Functional and phenotypic characterization of the Stearoyl CoA desaturase gene of Anopheles coluzzii

Abstract

Malaria is an infectious disease caused by Plasmodium parasites that are transmitted by the bite of female Anopheles mosquitoes. Successful acquisition and transmission of malaria parasites requires a female mosquito obtaining a blood meal from human hosts. The blood meal, which is rich in protein, is required for egg development. Most of the ingested protein is converted to lipid and stored in the fat body where vitellogenesis takes place. In this process, saturated fatty acids are converted to unsaturated fatty acids by the stearoyl-CoA desaturase (SCD1). Unsaturated fatty acids are also essential for maintaining cell membrane fluidity and other housekeeping functions. The main aim of this thesis was to functionally and phenotypically characterize the function of SCD1 during blood meal metabolism in the African mosquito vector Anopheles coluzzii. RNA interference (RNAi) silencing of the SCD1 gene and administration of a small molecule inhibitor of SCD1 had a significant impact on the survival of female mosquitoes following a blood meal. SCD1 knockdown (KD) caused a 100% mortality within 48 h after a human blood meal, while addition of the SCD1 small molecule inhibitor sterculic acid (SA) in the blood meal caused a 50% mortality within 72 h of blood meal. Microscopic analysis showed that SCD1 KD mosquitoes failed to develop eggs in response to the blood meal, while their thorax was filled with blood at 24 h post blood meal. These findings were highly consistent with electron microscopy data that showed increased plasma membrane rigidity and depletion of lipid droplets in the midgut epithelial cells. Transcriptional profiling using A. coluzzii oligonucleotide DNA microarrays showed that genes involved in protein, lipid and carbohydrate metabolism, as well as a large number of immunity genes were the most affected in blood-fed SCD1 KD versus control mosquitoes. Metabolomics profiling highlighted the biochemical framework by which the SCD1 KD phenotype is manifested after a blood meal, revealing increased amounts of saturated fatty acids and TCA cycle (and other interlinked pathway) intermediates in SCD1 KD and SA-treated mosquitoes. The data reported in this thesis reveal that silencing of SCD1 in female A. coluzzii mosquitoes leads to a metabolic syndrome primarily associated with the increase of saturated fatty acids and TCA cycle intermediates, which affects important biological functions leading to premature mosquito death. The accumulation of saturated fatty acids is also the likely cause of a potent immune response observed in the absence of infection, which resembles an auto-inflammatory reaction. These data provide important leads for the development of novel interventions aiming to block transmission of mosquito-borne diseases.