Membrane and host cytoskeleton reorganization during malaria parasite egress from erythrocytes

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Malaria is a major global killer, most deadly for children in the developing world. Of the five species that infect humans, Plasmodium falciparum is the most lethal. Although there are currently effective anti malarial drugs, the appearance and spread of resistant strains of P falciparum pose an increasing threat. The parasite has a complex life cycle with several different stages in its mosquito and human hosts, but the clinical symptoms in humans arise from waves of parasite release during the asexual blood stages, in which parasites invade red blood cells and multiply within an internal membrane compartment called a vacuole. The growing parasites hijack their host blood cells, consume their haemoglobin, and redirect the cell activity for the benefit of the parasite. Of particular importance, the parasite exports some of its own proteins to build new structures on the surface of the blood cell. A uniquely lethal aspect of P. falciparum is that it creates surface protrusions (called knobs) on the blood cell that make it adhere to the lining of blood vasculature. This prevents the infected cells from being captured and destroyed by the spleen, but can also block brain blood capillaries, the main cause of death in malaria infection. Once they have matured, 16-24 daughter parasites break through their surrounding vacuole membrane as well as the red blood cell membrane (a process collectively called egress) to enter the bloodstream, where they immediately invade a fresh round of blood cells. The processes of invasion, adhesion and egress are regulated through a cascade of enzyme reactions, as well as expression and transport of structural components. Some of these components are unique to malaria, making them potential targets for future drug development. Currently, most of them are poorly characterised.In this project, we focus on the steps by which the mature parasites break through the two bounding membranes to undergo egress. A highly regulated series of steps leads to the explosive release of parasites from the blood cell. To examine these membrane breakage events in detail, we use electron microscopy to image at nanoscale resolution the three-dimensional structures of the infected cells during the late stages of parasite development. This imaging is combined with use of parasite mutants and drug-like molecules and enzyme inhibitors to trap the parasites at different steps of egress. This approach has already led us to discover a new, initial step in egress that had not been previously detected. We now know that the process begins with the parasites causing the membrane surrounding their vacuole to become leaky. Subsequently, this membrane is completely disrupted, allowing the parasites to move freely inside the blood cell. Shortly after that, the blood cell membrane itself becomes leaky and then finally the cell membrane and its underlying cytoskeleton rupture to allow the parasites to escape and invade new host blood cells. With recent advances in gene editing technology, it has now become possible to conditionally modify gene expression in P. falciparum, and we will use this powerful technology to probe the molecular nature, functions and subcellular localisations of key malaria components involved in the sequence of steps during egress. The results of these studies could form the basis for future development of novel therapeutics.