Anopheles/Plasmodium interactions at the ookinete-to-oocyst developmental transition

Abstract

The ookinete to oocyst developmental transition of the Plasmodium parasite represents a major population bottleneck in the malaria life cycle. This suggests that it could be a target for intervention strategies, such as transmission blocking vaccines, provided essential parasite target molecules can be identified. A recent microarray analysis has identified a large number of transcripts differentially expressed during the parasite’s developmental transitions. Genes differentially regulated during the ookinete-to-oocyst transition may determine the development of the parasite within the mosquito host, as well as, participating directly in parasite/mosquito interactions. Yet, the function of the majority of such molecules is largely unknown. This PhD thesis aims to identify and functionally characterise genes putatively involved in ookinete development and/or the interactions between the parasite and the mosquito host in the model system Plasmodium berghei. Thirty three proteins likely to be implicated in the parasite’s interaction with the mosquito immune system and local epithelial response were identified based on their expression pattern and predicted structural features. Generation of knock-out mutants through targeted gene disruption by homologous recombination was the first step towards functional characterization of these candidates.Successful mutants were assessed for their ability to complete their sexual sporogonic development, as well as, their impact on mosquito immunity following infection of Anopheline mosquitoes of various immune backgrounds. Interestingly, two of the successful mutants were hampered in their ability to undergo normal differentiation during ookinete development while the third one’s ability to invade the mosquito midgut epithelium was impaired. The inability to invade implies a potential interaction of this gene product with mosquito midgut ligands. Eventually malaria transmission through Anopheline mosquitoes was affected in all three mutants. Moreover, challenging of a mosquito protein LRIM1, a major parasite antagonist, also revealed potential involvement of the three mutants in mosquito/parasite immune response pathways. Genetic crosses with parasite lines deficient in the production of either male or female fertile gametes demonstrated in the case of two mutants that, this defect in ookinete development is sex dependent, thus underlining the critical importance of maternal and/or paternal control during the first few hours of parasite development in the mosquito.