Role of host haem biosynthetic enzymes in the Plasmodium falciparum erythrocyte-stage
Date
2019
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Yang, Hao
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Plasmodium falciparum is a parasite that causes severe type of malaria, including cerebral malaria. Its growth and survival in erythrocytes relies on haem, an essential metabolic cofactor participating in various redox reactions of nearly all eukaryotic organisms. Research has therefore focused on interfering with the acquisition of haem by studying three ways parasite can access haem. The first way is through its own de novo haem synthetic pathway (HSP), in which succinyl-CoA and glycine are converted into haem through a series of eight enzymatic steps. The second and third ways are to scavenge either host haem biosynthetic enzymes or haem from haemoglobin degradation, or both, to meet its haem requirements. However, it remains unclear which is the essential way. A recent study has found that host red cells deficient in one of the HSP enzymes, ferrochelatase, gives rise to resistance to malarial infection (in mice) and parasite growth (in human erythropoietic protoporphyric cells), implying that Plasmodium at least requires host FECH enzyme for its growth (Smith, Jerkovic et al. 2015). This project aims to investigate if other host haem biosynthetic enzymes in addition to FECH are required for Plasmodium to infect and propagate in the erythrocyte.
Here we conducted P. falciparum growth assays on human porphyric red blood cells to investigate the parasite requirement for these host haem enzymes. We tested 14 individual AIP patient blood samples; 13 samples showed significant parasite growth inhibitory effect. We also tested the growth of a previously generated P. falciparum HMBS knockout parasite line in normal red blood cells and found the knockout of the PfHmbs had no significant effect on the growth of parasite, which is consistent with a previous study (Sigala, Crowley et al. 2015). However, when cultured in the AIP red cells, the growth of PfHmbs-knockout parasite line was significantly impaired compared to the wild type parental parasite line. This suggests that the lack of PfHmbs may increase the parasite's reliance on the hHMBS. Whole genomic sequencing of the knockout and parental parasite lines was conducted to investigate the hypothesis that the selection and isolation of a viable knockout parasite line occurred through the acquisition of genetic changes that enabled this altered reliance to occur. However, no significant genomic structure changes were consistent with this hypothesis. This work also established an assay to measure human HMBS activity, and used this to screen for inhibitors in a compound library called the Pathogen Box. However none of the approximately 400 compounds inhibited the enzyme. Together, this work identified human HMBS as a novel host enzyme that is required to sustain the parasite growth. Future studies are needed to address the mechanism of enzyme importation by the parasite, and identify small molecule inhibitors of HMBS that may be further developed as novel host-directed antimalarial therapies.
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