The Apicomplexa phylum is defined by the presence of an apical complex of organelles that include two specific secretory granules called micronemes and rhoptries. These organelles sequentially release their contents during invasion: microneme proteins contribute to motility, attachment and invasion, while rhoptry proteins contribute to both invasion and subsequent manipulation of host cell functions. Our research is focused on the formation of a structure (the moving junction) which is essential for host cell invasion in both Toxoplasma and Plasmodium, and depends on the cooperation between microneme and rhoptry proteins. We are also interested by the mechanism of secretion of rhoptries, which remains a major conundrum in the field. Finally, we are also investigating lipid kinases that are important for the homeostasis of the apicoplast (a non-photosynthetic plastid important for parasite metabolism), and for intracellular development.


Host Cell invasion
Apicomplexan parasites are motile and actively invade the cells in which they replicate. They have developped a unique invasion mechanism that is conserved across the phylum. At the core of the invasion machine is an interface of interactions between the host cell and the parasite called Moving Junction (MJ) that anchors the parasite to the host cortical cytoskeleton while the parasite’s actin-myosin motor provides forward motion into the host cell (movie). Described by electron microscopy in the late 70s, the molecular machinery supporting MJ-dependent host cell entry has remained a black box for more than 30 years. In the last 15 years, our lab was at the heart of the discoveries that revealed the molecular connectors between the parasite and host cell plasma membranes at the MJ. These results highlighted the RON2-AMA1 interaction as the main mechanism by which T. gondii tachyzoites and P. falciparum merozoites establish the MJ, opening attractive new strategies for the design and development of inhibitors targeting MJ and also paving the way for vaccination with the AMA1-RON2 complex. We demonstrated how the MJ is assembled at structural level in T. gondii and P. falciparum and defined not only parasite but also host factors hijacked by the parasite. Our current works aim at dissecting the function of these host factors and at defining how the other parasite stages of Toxoplasma (sporozoites, bradyzoites) invade host cells.



Schematic of T. gondii tachyzoite invading a host cell. Details in the inset show how, in the MJ,AMA1 is exposed on the parasite surface and interacts with a short extracellular domain of RON2 incorporated cooperate to recruit adaptor proteins (ALIX, CD2AP, CIN85, TSG101) through multiple specific interaction motifs that physically link the RONs complex to the cortical actin cytoskeleton. This results in the stabilization of the junction between the parasite and the host cortical actin providing a substantial cellular anchor for the AMA1/RON2 bridge at the surface, which is necessary to sustain the parasite invasive force.



Rhoptry secretion
The ability of Apicomplexa to cause disease depends on the coordinated secretion of specialized secretory organelles. The rhoptries are particularly important, because they act as the apicomplexan equivalent of bacterial secretion systems. They inject parasite proteins directly in the cytoplasm of host cells not only for invasion but also to hijack host functions crucial to establish and maintain infection. However, in contrast to bacteria where the secretion machinery has been resolved in details at the atomic scale, how these eukaryotic parasites secrete and inject rhoptry effectors into cells is still an enigma. We aim to dissect the mechanistic steps and the molecular components that assemble the rhoptry secretion machine.


Phosphoinosites function in Toxoplasma
We uncovered original apicoplast-related functions for two lipid kinases in Toxoplasma. Functional characterization of PI3Kinase and the PIKfyve kinase which produce PI(3)P and PI(3,5)P2, respectively. These two phosphoinositides are classically associated with endolysosomal functions in mammalian cells and yeast, but are interestingly crucial for maintaining apicoplast homeostasis in the parasites. We use genetic and cellular approaches to elucidate how PI3P and PI(3,5)P2 exert their function on apicoplast biogenesis.