Medical Biology - Theses
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ItemAspartic proteases and their potential for transmission blocking strategies( 2019)Sexual stage development in Plasmodium spp. is essential for transmission through the mosquito and to the human host. It represents objects to study a broad range of biological processes, including stage conversion and parasite/host co-adaptation. After the bloodmeal, male and female gametes emerge from intracellular gametocytes and zygote formation follows fertilization. Ookinetes develop from the zygote and traverse through the midgut epithelial cell layer to the basal lamina side of outer wall and develop into oocysts, the only parasite developmental stage that grows extracellularly and this growth and development creates thousands of sporozoites. Once fully developed and egressed, these sporozoites are released into the mosquito hemocoel and they migrate to the salivary gland ready to infect next mammalian host and continue their life cycle. This sexual stage also represents a major bottleneck during the life cycle of Plasmodium as, in mosquito midgut, parasites have to persevere for up to 24 hours outside host cell, exposed themselves to various risk factors such as components of human immune system included within bloodmeal, natural midgut microbial flora in mosquito midgut, and mosquito innate immune system. This exposure can lead up to an approximate 300-fold decrease in parasite survivability during the transmission to mosquito. Due to this unique feature, sexual stage is prime target for transmission blocking intervention strategies aimed to inhibit spread of the disease by the mosquito. Protease enzymes are essential during many steps of malaria parasite development in the blood and transmission stages and an important group of these enzymes are the plasmepsins, of which there are 10 in Plasmodium acting at various points through the life cycle. So far, only 4 plasmepsins are identified to be involved in critical processes and required for transmission. Firstly, plasmepsin VI is highly expressed during sexual stages and was previously shown to be involved in sporozoite development in P. berghei. Secondly, plasmepsin VIII is expressed in mature sporozoite and responsible for sporozoite motility in P. berghei. Finally, PMIX and X are found to be essential in both blood and mosquito stages, making them stand out as promising drug targets. In this study, we attempted to determine the biological functions of plasmepsin VI, IX, and X during transmission of malaria parasites. We found that plasmepsin VI is required for transmission of P. falciparum and might plays an important role in sporozoite egress process instead of sporozoite development as observed in P. berghei. We also found that our dual inhibitor that target both plasmepsin IX and X is able to block the transmission of P. falciparum to mosquito while another antimalaria compound that target only plasmepsin X is enough to block transmission of P. berghei from mouse to mosquito suggesting that both plasmepsin IX and X are essential for transmission. Taken together, our data has identified 3 plasmepsins that play important roles in sexual stage of malaria parasites and more works are needed in order to determine the mechanism of action of these 3 proteases.
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ItemMolecular mechanism of cell traversal by Plasmodium falciparum( 2016)Malaria is an infectious mosquito-borne disease caused by apicomplexan parasites of the genus Plasmodium. Each year, malaria affects over 200 million people, causing considerable morbidity and mortality. A central feature of the virulence of malaria parasites is the ability of the liver-infective form of the parasite, known as sporozoites, to migrate from the mosquito bite site in the skin through host tissues to the target organ, the liver. The ability of sporozoites to traverse through different host cell types is crucial for the establishment and development of parasites within the mammalian host. Over the past decade, our understanding of traversal has become clearer through important studies using rodent models of malaria, such as P. berghei and P. yoelii. However, it remains unclear how these findings apply to malaria parasite species that infect humans, such as P. falciparum and P. vivax. Furthermore, proteins involved in the process, as well as a step-wise molecular model of it, remain unknown. In order to address these questions, the work presented in this thesis utilises molecular genetics and cellular biology to investigate the role of proteins in the traversal mechanism. Overall, this study has identified a novel role for two well-known proteins, Apical Membrane Antigen 1 (AMA1) and Merozoite Apical Erythrocyte Binding Ligand (MAEBL), in the traversal process. Furthermore, this study has validated the role Sporozoite Protein Essential for Cell Traversal (SPECT) and Perforin-Like Protein 1 (PLP1) in P. falciparum sporozoites, which are two proteins that previously have been identified as playing a crucial role in traversal using rodent models of malaria. Using mice engrafted with human hepatocytes, this study also demonstrated the importance of traversal for P. falciparum sporozoites to establish infection of human hepatocytes in vivo. Together, these findings provide the first molecular understanding of cell traversal by P. falciparum and give valuable insights into the complexity of traversal and allowed the formation of a basic molecular model for this process.