API-INVASION · The role of host cell actin cytoskeleton in invasion by Apicomplexa parasites

Members of the Apicomplexa phylum are obligatory intracellular parasites many of which being important human pathogens. Plasmodium sp., the etiological agent of malaria is the most devastating but Toxoplasma gondii can be severe in both immune-compromised individuals where it causes encephalitis or pneumonia and infect fetuses. These parasites invade host cells by transforming into specialized stages called zoites and most of our current knowledge on the few second-lasting entry process comes from studies using Toxoplasma tachyzoites and Plasmodium sporozoites. Upon contact between the two cells, the close apposition between membranes forms a unique type of cell-cell junction which accompanies parasite active propelling into the host cell. The current dogma stipulates that the zoite driving force critically depends on its actomyosin motor while the host cell remains essentially passive. However, new evidence from Tardieux’ laboratory recently contradicts this view. Live video and confocal microscopy indicate that de novo actin polymerization is important for entry of P. berghei sporozoites and T. gondii tachyzoites to promote anchoring of the zoite-cell junction on which the parasite pulls to penetrate the host cell. In this proposal, using both Plasmodium and Toxoplasma zoites, we aim at dissecting the molecular mechanisms that control actin assembly/disassembly in the host cell during entry. To this end, we plan to identify (i) the up-stream host cell signalling factors leading to the host response (ii) the parasite proteins released into the host cell which potentially contribute to this response (iii) assess the function of these parasite factors during invasion. To address these points, live confocal imaging of invasion will be combined with in vitro biochemical actin assays and proteomic analysis. Our results will address a brand new issue on how parasites trigger signalling at the host cell membrane during invasion.