Sex-Specific Biology of Plasmodium falciparum

Abstract

Malaria, caused by Plasmodium parasites, is responsible for almost 500,000 deaths annually. Disease transmission requires the parasite to differentiate into male and female gametocytes in human red blood cells prior to sexual reproduction upon ingestion by a mosquito. This thesis addresses fundamental questions about the sex-specific development of Plasmodium gametocytes to guide transmission-blocking antimalarial interventions. Although Plasmodium depends on host nutrients to survive, the lifecycle of the malaria parasite spans radically different host environments, particularly in terms of lipid composition. In culture, asexual P. falciparum proliferation is accompanied by stage-specific lipid modifications such as increasing phosphatidylcholine during membrane biogenesis. In vivo P. berghei lifecycle progression in the mouse and mosquito is accompanied by changes in lipid composition, with gametocytes apparently synthesising lipids de novo in preparation for transmission. Parasite species-specific lipid changes to the host red blood cell reflect differences in the lipid composition of their respective host species, suggesting host-parasite co-evolution of lipid metabolism. Investigation of gametocyte biology in a sex-specific manner required the development of a novel method to sort male and female gametocytes. The technique is applied to validate novel sex-specific gametocyte markers. The role of DNA methylation in sex determination and differentiation in P. falciparum is also investigated as a fundamental biology application of this technique. Lipid metabolism of P. falciparum gametocytes is next investigated in a sex-specific manner. Female gametocytes stockpile neutral lipids and sphingolipids, consistent with female gametocytes being the main contributor of resources for zygote development immediately after fertilisation. Specific lipids appear to be imported from the host and may be essential culture medium supplements for gametocytes. Finally, the biological significance of de novo synthesis of key sex-specific gametocyte lipids is investigated by genetic and chemical inhibition of parasite lipid metabolism. Overall this thesis highlights gametocyte and sex-specific Plasmodium traits that could assist with monitoring and blocking malaria transmission.