Characterisation of plasmodium proteins playing critical roles in mosquito midgut infection and malaria transmission

Abstract

Malaria parasites undergo dramatic losses during their gametocyte-to-ookinete-to- oocyst developmental transition. This PhD thesis primarily aimed to use the Signature Tagged Mutagenesis (STM) methodology to simultaneously phenotype pools of individually tagged P. berghei mutants, of genes previously found to be highly upregulated during the gametocyte-to-ookinete-to-oocyst developmental transition stages and possibly involved in mosquito-parasite interactions. Although the first pool of mutants acted as a proof-of-concept experiment, it revealed three novel Plasmodium genes with essential functions during the parasite ookinete-to-oocyst-to- sporozoite transition. Specifically, AQP2 is essential for sporozoite formation in the developing oocyst, N38 is important for the gametocyte-to-ookinete developmental transition and N350 is critical for ookinete motility prior to midgut invasion The mosquito complement-like system is responsible for the greatest parasite population bottleneck observed during the ookinete-to-oocyst developmental transition, which also coincides with the ookinete traversal through the mosquito midgut epithelium. Therefore, this PhD also aimed to shed light on the molecular mechanisms mediated by the mosquito innate immune system to clear the Plasmodium parasites and thus regulate the infection outcome. Several P. berghei genes have been identified to play an essential role in the parasite protection from the mosquito complement responses including c01, PIMMS43, P47 and c57. Knockout of any of these genes leads to ookinete elimination by the mosquito complement-like reactions upon reaching the basal sub-epithelial space, unless silencing key factors of the mosquito complement system. Based on these findings, it was further suggested that (i) either all of these genes have essential functions in parasite immune evasion, or (ii) their loss-of-function bears a fitness cost that exceeds a certain threshold required for parasites to endure the mosquito complement responses. Serial mouse- to-mosquito-to-mouse transmission cycles followed by allele quantification revealed that the mosquito complement system is responsible for instantly removing 99% of the introduced in the population knockout alleles. This study offers new perspectives for further understanding the mosquito-parasite interactions leading to malaria transmission.