Plasmodium proteins at the parasite-host interface contribute to the liver and blood infection stages

Abstract

Plasmodium parasites extensively remodel their host cells to establish a growth-permissive environment. Research on Plasmodium-host interactions has predominantly focused on the blood-stage of Plasmodium falciparum infection. To gain broader insights into the parasite-host interface across different Plasmodium species and infection stages, this study characterized six candidate exported proteins in Plasmodium berghei, a rodent malaria model. Candidate proteins were selected based on their predicted export signals, conservation across Plasmodium species, and association with exported proteins. Many exported proteins for P. falciparum carry the conserved Plasmodium export element/host-targeting motif (PEXEL/HTS). The two candidate proteins PBANKA_0100200 and PBANKA_1000300 were selected based on the presence of a predicted PEXEL/HTS motif. However, despite their expression during the blood stage of infection, localization studies revealed that PBANKA_0100200 and PBANKA_1000300 are not exported. This finding underscores that the presence of a PEXEL/HTS motif is not always a definitive marker of protein export in P. berghei, suggesting that additional factors or sequence elements influence the export of proteins in this species. PBANKA_0214600 also has a PEXEL/HTS motif, and its export during both liver- and blood-stage infection was shown in this study. PBANKA_0214600 was found to localize to discrete cytoplasmic structures different from the Plasmodium berghei-induced membrane structures (IBIS). While its deletion did not affect parasite growth, it led to an increased number of circulating schizonts, indicating a potential role in sequestration The tryptophan-rich proteins intraerythrocytic Plasmodium-induced structure proteins 2 and 3 (IPIS2 and IPIS3) were immunoprecipitated with IBIS1, a protein known to localize to the IBIS. Functional characterization revealed that deletion of IPIS2 impairs schizont adhesion to CD36, leading to an increased presence of circulating schizonts and a reduced parasite burden in adipose and lung tissue of infected mice. In contrast, deletion of IPIS3 results in an increased number of circulating schizonts but does not significantly affect sequestration in the lung or adipose tissue, suggesting that these proteins are involved in different aspects of the sequestration process. Finally, PbACS9, a putative acyl-CoA synthetase, was chosen as a candidate protein as it was also pulled down with IBIS1 in the same manner as IPIS2 and IPIS3. PbACS9 was shown to be exported to the erythrocyte membrane. PbACS9-deficient parasites exhibited delayed liver-stage development and increased gametocyte numbers in the blood-stage of infection. In order to analyze the function of PbACS9 for the parasite a fatty acid analysis was conducted. In the measured conditions no difference in the uptake or metabolism of fatty acids with less than 18 carbons was measured if compared to WT-parasites. This thesis advances the understanding of Plasmodium-host interactions by identifying and characterizing 6 proteins and revealing their roles in host cell remodeling, immune evasion, and parasite survival across multiple life cycle stages.