500 Naturwissenschaften und Mathematik 570 Biowissenschaften; Biologie
Plasmodium sporozoites are the infectious forms of the malaria parasite transmitted by mosquitoes. Upon transmission to the mammalian host, sporozoites embark on a complex journey to the liver where they infect hepatocytes to generate pathogenic merozoites which infect red blood cells. It is largely unknown how sporozoites achieve this journey. The aim of this thesis was to expand our knowledge of sporozoite surface and secreted proteins, since they are likely involved in host-parasite interactions in this initial phase of malaria infection. The Plasmodium 6-Cys protein P38 is a member of a small, conserved family of parasite surface and secreted proteins. Several 6-Cys proteins have been implicated in cell-cell interactions and represent targets for malaria vaccine development. Here, it is shown by epitope-tagging and immunofluorescence microscopy that P38 localizes to secretory organelles in salivary gland sporozoites in the rodent malaria parasite P. yoelii. Targeted deletion of P38 does not reveal a function during the entire parasite life cycle. Cell surface biotinylation and mass spectrometry were used to identify novel putative sporozoite surface proteins in P. yoelii and in the human parasite P. falciparum. Four proteins identified in both Plasmodium species were selected for functional analysis in P. yoelii. Three of them are encoded by genes that were previously found to be upregulated in infectious sporozoites (UIS): a putative sugar transporter and two proteins that lack any functional annotation. Several UIS proteins are essential for liver infection. However, using a reverse genetics approach, it is shown that none of the here analyzed UIS proteins is required for establishing infection in mice. The fourth protein is a type I transmembrane protein encoded by the sporozoite-specific gene S23. Epitope-tagging and specific antibodies to S23 in conjunction with immunoelectron microscopy confirm surface localization of S23 in salivary gland sporozoites. Interestingly, the ectodomain of S23 appeared to be inaccessible to antibodies in non-permeabilized sporozoites. Antibody-induced shedding of the major surface protein circumsporozoite protein exposed the S23 ectodomain to antibodies in some sporozoites. Targeted deletion of S23 adversely affected sporozoite locomotion in vitro, which surprisingly did not impact sporozoite infectivity.
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