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Dissecting cGMP- and calcium-dependent signalling pathways that control gliding of a malaria zoite

Final Report Summary - PLASMODIUM MOTILITY (Dissecting cGMP- and calcium-dependent signalling pathways that control gliding of a malaria zoite)

Malaria caused by Plasmodium spp parasites is a profound human health problem. The development of Plasmodium parasites shows a high level of coordination mediated by intracellular signalling pathways. Ca2+- and cGMP- dependent signalling have recently received much attention in research on malaria parasites, because both second messengers are thought to be responsible for regulating diverse events in the parasite’s life cycle that are critical for pathogenesis and mosquito transmission. During the course of this project, we have provided a solution to the mystery of how these two second messengers jointly control so many key events in Plasmodium development.
Using the motility of P. berghei ookinetes as a signalling paradigm, we first showed that the parasite’s cGMP-dependent protein kinase, PKG, maintains the levels of cytosolic Ca2+ required for gliding motility. Perturbations of PKG signalling had a marked impact on the parasite’s phosphoproteome, with a significant enrichment of in vivo regulated sites in multiple pathways including vesicular trafficking and phosphoinositide metabolism. A global analysis of cellular phospholipids demonstrated that PKG controls phosphoinositide metabolism in gliding ookinetes, by controlling the subcellular localisation or activity of lipid kinases. Phosphorylation of phosphatidylinositol therefore links PKG to cellular Ca2+ levels by providing lipid precursors of inositol 1,4,5-trisphosphate. We found that the same PKG-dependent pathway operates upstream of the Ca2+ signals that mediate activation of P. berghei gametocytes necessary for the mosquito colonisation and the egress of P. falciparum merozoites from infected human erythrocytes. This is an important discovery because it tells us for the first time that a single signalling pathway controls how parasites emerge from erythrocytes, how they recognize the mosquito, and finally how they control an unusual form of gliding motility to invade cells.
PKG thus emerges as unifying factor to control multiple cellular Ca2+ signals essential for malaria parasite development and transmission. This has important consequences for the development of new drugs against malaria parasites: identifying the molecular pathways that operate upstream of cytosolic Ca2+ will reveal apicomplexa-specific but stage-transcending molecular mechanisms, such as those linking PKG to phosphoinositide metabolism. Such pathways are likely to provide targets for inhibitors that block parasite development at multiple stages, such as inhibitors of PKG itself or the lipid kinases.