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Exploring the Roles of Phosphoinositides in the Biology of the Malaria Parasite Plasmodium Falciparum

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Exploring the Roles of Phosphoinositides in the Biology of the Malaria Parasite Plasmodium Falciparum Exploring the roles of phosphoinositides in the biology of the malaria parasite Plasmodium falciparum Thèse Zeinab Ebrahimzadeh Doctorat en microbiologie-immunologie Philosophiæ doctor (Ph. D.) Québec, Canada © Zeinab Ebrahimzadeh, 2019 Exploring the roles of phosphoinositides in the biology of the malaria parasite Plasmodium falciparum Thèse Zeinab Ebrahimzadeh M. Sc. Dave Richard, directeur de recherche Résumé Plasmodium falciparum est un parasite appartenant au phylum Apicomplexa et est à l’origine de la forme la plus sévère de la malaria. Dans les zones endémiques d'Afrique subsaharienne, la plupart des victimes sont des enfants de moins de cinq ans. L’entrée de P. falciparum dans sa cellule cible, le globule rouge, repose sur la sécrétion de protéines par des organites spécialisés : les micronèmes, les rhoptries et les granules denses. Les mécanismes de biogenèse de ces organites et la coordination de la libération de leur contenu lors de l'invasion sont cependant pour la plupart inconnus. Il a été toutefois été démontré que les protéines destinées à ces organites apicaux se concentrent dans des microdomaines de l’appareil de Golgi, dont la composition en lipides et en protéines détermine leur destination finale. À ce jour, les mécanismes de sélection et de transport des protéines apicales vers les organites d'invasion ainsi que leurs mécanismes de sécrétion durant l’invasion sont pour la plupart inconnus. Nous avons donc posé l’hypothèse que les phosphoinositides (PI) et leurs protéines effectrices sont impliqués dans ces processus chez P. falciparum. Les PI sont sept lipides phosphorylés retrouvés de façon minoritaire dans les différentes membranes cellulaires. Chaque membrane subcellulaire contient une espèce caractéristique de PI qui peut être reconnue et liée spécifiquement par des protéines effectrices. Une large gamme de processus biologiques sont régulés par les PI, tels le trafic vésiculaire, les canaux ioniques, les pompes d’efflux et les transporteurs, ainsi que certains processus endocytiques et exocytaires. Des études antérieures ont été en mesure de détecter seulement cinq des sept espèces de PI chez P. falciparum. Dans le cadre d’un premier projet, nous avons étudié la distribution de six PI, à savoir PI3P, PI4P, PI5P, PI (4,5)P2, PI(3,4)P2 et PI(3,4,5)P3, chez P. falciparum. Pour ce faire, nous avons exprimé chez le parasite des rapporteurs spécifiques correspondant à des domaines humains de liaison aux PI, fusionnés à une protéine fluorescente. Cette méthode nous a permis de confirmer des rapports antérieurs sur la localisation du PI3P dans la membrane de la vacuole alimentaire, dans de petites vésicules près ou sur la membrane plasmique du parasite ainsi qu’à l’apicoplaste. De plus, nous avons révélé pour la première fois la iii présence de PI5P chez P. falciparum et montré qu’il se localisait à la membrane plasmique, au noyau et potentiellement dans le réticulum endoplasmique de transition. Nous avons aussi montré que le PI4P est localisé dans la membrane plasmique ainsi que dans l’appareil de Golgi et que le PI(4,5)P2 est présent dans la membrane plasmique tout au long du cycle érythrocytaire. Cette carte de la distribution subcellulaire des PI constitue un excellent outil pour mieux déchiffrer les rôles de ces lipides chez le parasite P. falciparum. Dans le cadre d’un second projet, nous avons caractérisé une protéine possédant un domaine conservé chez les Apicomplexa, le domain d’homologie de la Pleckstrine, la protéine PfPH2. En utilisant la stratégie de Knock-sideways pour inactiver conditionnellement la protéine d’intérêt, nous avons montré que PfPH2 est impliquée dans l’attachement initial du mérozoite à la surface du globule rouge. Cet effet est directement lié à un défaut de sécrétion d'une population spécifique de micronèmes en l’absence de la protéine PfPH2. Enfin, nous avons mis en évidence que le domaine PH de PfPH2, lorsque exprimé sous forme de protéine recombinante, se lie aux PI avec une grande spécificité. Pris ensemble, nos résultats démontrent le rôle essentiel des PI dans le processus d’invasion et proposent un modèle mécanistique pour l'exocytose des micronèmes. iv Abstract Plasmodium falciparum belongs to the phylum of Apicomplexa and causes the most severe form of malaria. In endemic areas of sub-Saharan Africa, most of the victims are among children under the age of five. P. falciparum relies on proteins released from sophisticated invasion organelles called micronemes, rhoptries and dense granules to enter human erythrocytes. The mechanism of biogenesis of invasion organelles and the coordinated release of their contents during invasion are mostly unknown. It has been shown that proteins targeted to the apical organelles accumulate in microdomains of the Golgi apparatus with specific lipid and protein composition that determine the final destination of their cargo. To date, the mechanisms of transport of the cargo molecules to the invasion organelles and their release mechanism are mostly unknown. We proposed that phosphoinositides (PIPs) and their effector proteins could be involved in these processes in P. falciparum. PIPs are seven minor phosphorylated lipids in cellular membranes. Each subcellular membrane contains a characteristic species of PIPs that are specifically bound by PIP- interacting proteins. A wide range of biological processes regulated by PIPs such as vesicular trafficking, ion channels, pumps, and transporters and control both endocytic and exocytic processes. Based on previous reports five out of seven PIP species have been detected in P. falciparum. In my first project, we have studied the distribution of six PIPs namely PI3P, PI4P, PI5P, PI(4,5)P2, PI(3,4)P2 and PI(3,4,5)P3 using expression of specific reporters made up of human PIP-binding domains fused to a fluorescent protein. Here, we have confirmed previous reports on PI3P localization to the food vacuole membrane, small vesicles close/on the parasite plasma membrane and the apicoplast. Also, we have reported for the first time the presence of PI5P in P. falciparum and showed that it localizes to the PM, nucleus and potentially transitional ER. PI4P shows localization to the PM and Golgi and PI(4,5)P2 localizes to the PM all over the erythrocytic cycle. The resulting map of the subcellular distribution of PIPs will now be a great tool to further decipher the roles of these lipids in P. falciparum, In the second project, we have characterized a Pleckstrin Homology domain-containing protein (PfPH2) conserved in all apicomplexan parasites. Using the knock sideways v strategy to conditionally inactivate the protein, we show that PfPH2 is involved in an early step of the invasion process, when the merozoites initially attach to red blood cells. We further demonstrate that this is due to the abrogated secretion of a specific population of micronemes. Finally, we reveal that recombinantly expressed PfPH2 binds PIPs with a broad specificity. Taken together, our results present evidence for the role of PI in invasion and propose a mechanistic model for the exocytosis of micronemes. vi Table of contents Résumé ........................................................................................................................ iii Abstract ........................................................................................................................ v Table of contents ....................................................................................................... vii List of Tables ................................................................................................................ x List of Figures ............................................................................................................ xi Abbreviation list ....................................................................................................... xii Avant-Propos ........................................................................................................... xvi Acknowledgments ................................................................................................. xvi Contributions ....................................................................................................... xviii Introduction ................................................................................................................. 1 Malaria ....................................................................................................................... 1 Etiology and Epidemiology .................................................................................... 1 Disease and Pathology of Plasmodium Infection ................................................... 4 Malaria vector ........................................................................................................ 4 Plasmodium falciparum life cycle ............................................................................. 5 Erythrocytic stage ................................................................................................... 5 Atypical organelles ................................................................................................. 8 Molecular bases of invasion ................................................................................. 10 Malaria Treatment .................................................................................................... 14 Diagnosis, Treatments and Resistance ................................................................. 14 Prevention and
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