Developing transgenic models to induce late-life mortality in the malaria vector Anopheles gambiae

Abstract

A key factor that limits the transmission of malaria is the age of its vector, Anopheles gambiae. This is due to the long extrinsic incubation period (EIP) of the parasite which can last a high proportion of the adult mosquito's life. Therefore, only a small percentage of a mosquito population is able to transmit malaria. This bottleneck could be of use in developing novel vector control strategies which are based on artificially shortening the adult life span of mosquitoes through the introduction of suitably tailored transgenic constructs. One strategy was to induce a toxic late-acting amino acid metabolism disease in mosquitoes that kills females after their amino acid rich blood meal. Phenylpyruvate, the toxic accumulation product in human Phenylketonuria disease, was found to cause increased death when fed to mosquitoes. GC-MS analysis showed that similar to PKU patients, phenylpyruvate had been converted to another toxic metabolite phenyllactate. Using RNAi, two enzymes PAH and PPO9 of the phenylalanine pathway were knocked down and their effect on survival, behaviour and melanisation immune response was investigated. While knockdown of one enzyme caused a reduced melanisation in response to parasite infection, reduced activity of the other led to a shortened adult life span but no change in melanisation. This led to the conclusion that both enzymes are regulating different processes and that melanisation is not necessary for mosquito survival and possibly mosquito immunity. Another strategy was to express long stretches of polyglutamines that are responsible for the toxicity of several neurodegenerative diseases, including Huntington's disease. Because uniform CAG repeats containing huntingtin exon 1 were highly instable in vitro and in vivo, different N-terminal huntingtin fragments with alternating CAG/CAA repeats fused to EGFP were inserted by attP/attB integration system in the model system Drosophila melanogaster and in Anopheles gambiae to characterise polyglutamine length dependent aggregation formation in the optic lobe and its effect on survival and behaviour. In both species, the polyglutamine fragments of putative toxic length tended to form aggregates in the photoreceptor cells. This led to a significant reduced adult life span in flies but not in mosquitoes. The latter showed a change in their chromatic vision ability. Alternative promoters were characterised for future HD mosquito models and could help to understand the observed difference between flies and mosquitoes. Both strategies have a potential as promising late-life acting vector control tools that merit further exploration.