Biochemistry and Pharmacology - Theses
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ItemAnalysis of architectural rearrangements in Plasmodium falciparum gametocytes( 2013)Transmission of the most virulent human malaria parasite, Plasmodium falciparum, is dependent on the parasite’s ability to produce viable gametocytes. Over two weeks the parasite prepares itself for transmission into mosquitoes by undergoing a series of cellular rearrangements. Once the metamorphosis is completed, mature gametocytes release from their sequestration sites, enter the circulation and become accessible to feeding mosquitoes. Whilst the formation of mature gametocytes represents a bottleneck in the parasite’s lifecycle, and thus an attractive target for transmission blocking strategies, very little of the basic biology of this lifecycle stage has been described. As the intraerythrocytic asexual stage parasite develops, it modifies its host red blood cell (RBC) by forming an exomembrane network of parasite-derived protein sorting organelles that facilitate the delivery of proteins to the RBC cytoplasm and membrane. Despite early ultrastructural descriptions of the gametocyte exomembrane network, no molecular characterisation of this system has been performed. In this thesis, modern high-resolution microscopy and immuno labelling techniques were used to re-evaluate the fine structure and molecular identity of several key components of the exomembrane system in the gametocyte. Early ultrastructural studies identified a sub-pellicular membrane complex in gametocytes. This structure consists of a flattened cisternal membrane beneath the parasite plasma membrane, which is supported by a network of microtubules. We have further described the molecular composition and origin of the sub-pellicular membrane complex, by identifying the presence of glideosome-associated proteins in gametocytes. We show that the gametocyte pellicle is analogous to the inner membrane complex (IMC), an organelle with structural and motor functions that is conserved across the Apicomplexan phylum. Thus we have proposed that the sub-pellicular membrane complex be renamed the gametocyte IMC (gIMC). We have also shown that the coordinated assembly of the microtubule network alongside the gIMC is responsible for driving shape change in the P. falciparum gametocyte. Interestingly, changes in cellular deformability correspond with gametocyte shape shifting. An increase in deformability coincides with the time the banana-shaped stage V gametocytes reappear in circulation. It has therefore been postulated that modifying the deformability of the host cell enables the gametocyte to circulate in the blood stream without being detected and removed by the mechanical filtering mechanisms in the host's spleen. Further investigation of how these changes occur show that the loss of cellular deformability is not dependent of the tubulin cytoskeleton of the parasite. Instead, it is associated with an altered molecular organization of the host cell membrane, as indicated by a loss of lateral mobility of the major RBC membrane protein, Band 3. Our studies have shown that several parasite proteins, which modulate rigidity of the RBC membrane in asexual stage parasites, are not exported in gametocytes. It appears that changes in gametocyte deformability are due to the rearrangement of cytoskeletal components, including host cell actin. The work presented in this thesis demonstrates that changes in gametocyte morphology and cellular deformability are crucial for the development of the P. falciparum gametocyte and discusses their likely contribution to the survival and transmission of the parasite.