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Genome‐scale metabolic reconstructions can serve as important tools for hypothesis generation and high‐throughput data integration. Here, we present a metabolic network reconstruction and flux‐balance analysis (FBA) of Plasmodium falciparum , the primary agent of malaria. The compartmentalized metabolic network accounts for 1001 reactions and 616 metabolites. Enzyme–gene associations were established for 366 genes and 75% of all enzymatic reactions. Compared with other microbes, the P. falciparum metabolic network contains a relatively high number of essential genes, suggesting little redundancy of the parasite metabolism. The model was able to reproduce phenotypes of experimental gene knockout and drug inhibition assays with up to 90% accuracy. Moreover, using constraints based on gene‐expression data, the model was able to predict the direction of concentration changes for external metabolites with 70% accuracy. Using FBA of the reconstructed network, we identified 40 enzymatic drug targets (i.e. in silico essential genes), with no or very low sequence identity to human proteins. To demonstrate that the model can be used to make clinically relevant predictions, we experimentally tested one of the identified drug targets, nicotinate mononucleotide adenylyltransferase, using a recently discovered small‐molecule inhibitor. Synopsis Malaria remains one of the most severe public health challenges worldwide (WHO, 2008 ). Although several available drugs have been successful in controlling malaria in the past, most of them are rapidly losing efficacy due to acquired drug resistance in the most lethal causative agent, Plasmodium falciparum (Mackinnon and Marsh, 2010 ). This creates an urgent need for new drugs and treatments, as well as better understanding of the parasite physiology. With this in mind, we built a genome‐scale flux‐balance model of the P. falciparum metabolism. Given the complex life cycle of Plasmodium , the flux‐balanced model is of direct relevance to the ongoing search to identify new therapeutic drug targets. The model can be used to explore diverse metabolic states of the parasite and identify essential metabolic genes in the context of known alternative pathways (Oberhardt et al , 2009 ). The reconstructed model, which is based on Plasmodium ‐specific databases, genomic annotations, and literature reports, includes 366 genes, 1001 reactions, 616 metabolic species, and 4 cellular compartments. We applied flux‐balance analysis (FBA) (Orth et al , 2010 ) to identify the genes and reactions that are required to produce a set of necessary biomass components. Interestingly, compared with the yeast metabolic network (Duarte et al , 2004 ), a model eukaryote with a similar genome size, the Plasmodium network has a significantly higher proportion of essential genes; we confirmed this result using a comparative analysis of known gene knockouts in the two microbes. This low level of genetic robustness, which is likely due to the parasitic lifestyle, suggests that many metabolic genes of the parasite can be used as effective drug targets. Indeed, based on the in silico analysis we identified 40 essential P. falciparum genes with no or very little sequence identity to their human homologs. We used a recently described small‐molecule inhibitor (compound 1_03 ; Sorci et al , 2009 ) to experimentally verify one of the enzymes identified as essential: nicotinate mononucleotide adenylyltransferase (NMNAT; Figure 2A ). This enzyme, and the corresponding NAD synthesis and recycling pathway, have been recently used for anti‐microbial development (Magni et al , 2009 ). However, to the best of our knowledge, they have not been used against P. falciparum . The compound 1_03 was able to completely block host cell escape and reinvasion by arresting parasites in the trophozoite growth stage (Figure 2B ). These results demonstrate that the inhibitory compound may be a good starting lead for new anti‐malarials. Importantly, the metabolic model of the parasite can be also used to integrate various genomic data, such as gene expression (Oberhardt et al , 2009 ). To illustrate these possibilities, we applied gene‐expression data as constraints for the flux‐balance model (Colijn et al , 2009 ) in order to predict changes in metabolic exchange fluxes. We found that the model was able to correctly predict the changes in external metabolite concentrations (Olszewski et al , 2009 ) with about 70% accuracy (Figure 3 ). The availability of a human metabolic network reconstruction (Duarte et al , 2007 ) would allow, in the future, to analyze the combined parasite–host network, which would deepen understanding of the P. falciparum metabolic vulnerabilities. Future improvements of the presented P. falciparum metabolic model, for example incorporation of missing activities and yet undiscovered pathways, will lead to a better understanding of parasite physiology. Ultimately, the improved understanding should significantly accelerate the identification and development of desperately needed new drugs against this devastating disease. In the paper we present a metabolic reconstruction and flux‐balance analysis (FBA) of Plasmodium falciparum , the primary agent of malaria. The compartmentalized metabolic network of the parasite accounts for 1001 reactions and 616 metabolites. Enzyme–gene associations were established for 366 genes and 75% of all enzymatic reactions. The model was able to reproduce phenotypes of experimental gene knockout and drug inhibition assays with up to 90% accuracy. The model also can be used to efficiently integrate mRNA‐expression data to improve the accuracy of metabolic predictions. Using FBA of the reconstructed metabolic network, we identified 40 enzymatic drug targets (i.e. in silico essential genes) with no or very low sequence identity to human proteins. We experimentally tested one of the identified drug targets, nicotinate mononucleotide adenylyltransferase, using a recently discovered small‐molecule inhibitor.

Plasmodium sporozoites are deposited in the skin by Anopheles mosquitoes. They then find their way to the liver, where they specifically invade hepatocytes in which they develop to yield merozoites infective to red blood cells. Relatively little is known of the molecular interactions during these initial obligatory phases of the infection. Recent data suggested that many of the inoculated sporozoites invade hepatocytes an hour or more after the infective bite. We hypothesised that this pre-invasive period in the mammalian host prepares sporozoites for successful hepatocyte infection. Therefore, the genes whose expression becomes modified prior to hepatocyte invasion would be those likely to code for proteins implicated in the subsequent events of invasion and development. We have used P. falciparum sporozoites and their natural host cells, primary human hepatocytes, in in vitro co-culture system as a model for the pre-invasive period. We first established that under co-culture conditions, sporozoites maintain infectivity for an hour or more, in contrast to a drastic loss in infectivity when hepatocytes were not included. Thus, a differential transcriptome of salivary gland sporozoites versus sporozoites co-cultured with hepatocytes was established using a pan-genomic P. falciparum microarray. The expression of 532 genes was found to have been up-regulated following co-culture. A fifth of these genes had no orthologues in the genomes of Plasmodium species used in rodent models of malaria. Quantitative RT-PCR analysis of a selection of 21 genes confirmed the reliability of the microarray data. Time-course analysis further indicated two patterns of up-regulation following sporozoite co-culture, one transient and the other sustained, suggesting roles in hepatocyte invasion and liver stage development, respectively. This was supported by functional studies of four hitherto uncharacterized proteins of which two were shown to be sporozoite surface proteins involved in hepatocyte invasion, while the other two were predominantly expressed during hepatic parasite development. The genome-wide up-regulation of expression observed supports the hypothesis that the shift from the mosquito to the mammalian host contributes to activate quiescent salivary gland sporozoites into a state of readiness for the hepatic stages. Functional studies on four of the up-regulated genes validated our approach as one means to determine the repertoire of proteins implicated during the early events of the Plasmodium infection, and in this case that of P. falciparum, the species responsible for the severest forms of malaria.

There is an urgent need for new drugs to treat malaria, with broad therapeutic potential and novel modes of action, to widen the scope of treatment and to overcome emerging drug resistance. Here we describe the discovery of DDD107498, a compound with a potent and novel spectrum of antimalarial activity against multiple life-cycle stages of the Plasmodium parasite, with good pharmacokinetic properties and an acceptable safety profile. DDD107498 demonstrates potential to address a variety of clinical needs, including single-dose treatment, transmission blocking and chemoprotection. DDD107498 was developed from a screening programme against blood-stage malaria parasites; its molecular target has been identified as translation elongation factor 2 (eEF2), which is responsible for the GTP-dependent translocation of the ribosome along messenger RNA, and is essential for protein synthesis. This discovery of eEF2 as a viable antimalarial drug target opens up new possibilities for drug discovery. The description of a compound (DDD107498) with antimalarial activity against multiple life-cycle stages of Plasmodium falciparum and good pharmacokinetic and safety properties, with potential for single-dose treatment, chemoprotection and prevention of transmission. A new antimalarial agent With artemisinin resistance spreading, there is an urgent need to develop new therapeutics to target Plasmodium falciparum , the causative agent of malaria. Here Ian Gilbert and colleagues report the discovery of a compound (DDD107498) with antimalarial activity against multiple life-cycle stages of the parasite and good pharmacokinetic and safety properties. It is non-mutagenic and has potential for both single-dose treatment and once-weekly chemoprotection. DDD107498 acts through inhibition of cytosolic protein synthesis, with translation elongation factor eEF2 as its target.

Endothelial protein C receptor is shown to be the receptor for Plasmodium falciparum erythrocyte membrane protein 1 variants associated with severe malaria. Drug target in childhood malaria Severe childhood malaria, still causing about a million deaths every year, is triggered by the binding of red blood cells infected with the parasite Plasmodium falciparum to the walls of the host's blood vessels. P. falciparum erythrocyte membrane protein 1 (PfEMP1) containing domain cassettes 8 and 13 is known to be associated with severe malaria, and here Thomas Lavstsen and colleagues identify the receptor for PfEMP1 as endothelial protein C receptor (EPCR), a protein involved in regulating blood coagulation and the inflammatory response. This work could help to explain why some episodes of malaria are life-threatening and involve severe inflammation and suggests a target for future antimalarials. Sequestration of Plasmodium falciparum -infected erythrocytes in host blood vessels is a key triggering event in the pathogenesis of severe childhood malaria, which is responsible for about one million deaths every year 1 . Sequestration is mediated by specific interactions between members of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family and receptors on the endothelial lining 2 . Severe childhood malaria is associated with expression of specific PfEMP1 subtypes containing domain cassettes (DCs) 8 and 13 (ref. 3 ), but the endothelial receptor for parasites expressing these proteins was unknown 4 , 5 . Here we identify endothelial protein C receptor (EPCR), which mediates the cytoprotective effects of activated protein C 6 , as the endothelial receptor for DC8 and DC13 PfEMP1. We show that EPCR binding is mediated through the amino-terminal cysteine-rich interdomain region (CIDRα1) of DC8 and group A PfEMP1 subfamilies, and that CIDRα1 interferes with protein C binding to EPCR. This PfEMP1 adhesive property links P. falciparum cytoadhesion to a host receptor involved in anticoagulation and endothelial cytoprotective pathways, and has implications for understanding malaria pathology and the development of new malaria interventions.

During blood stage Plasmodium falciparum infection, merozoites invade uninfected erythrocytes via a complex, multistep process involving a series of distinct receptor-ligand binding events. Understanding each element in this process increases the potential to block the parasite's life cycle via drugs or vaccines. To investigate specific receptor-ligand interactions, they were systematically blocked using a combination of genetic deletion, enzymatic receptor cleavage and inhibition of binding via antibodies, peptides and small molecules, and the resulting temporal changes in invasion and morphological effects on erythrocytes were filmed using live cell imaging. Analysis of the videos have shown receptor-ligand interactions occur in the following sequence with the following cellular morphologies; 1) an early heparin-blockable interaction which weakly deforms the erythrocyte, 2) EBA and PfRh ligands which strongly deform the erythrocyte, a process dependant on the merozoite's actin-myosin motor, 3) a PfRh5-basigin binding step which results in a pore or opening between parasite and host through which it appears small molecules and possibly invasion components can flow and 4) an AMA1-RON2 interaction that mediates tight junction formation, which acts as an anchor point for internalization. In addition to enhancing general knowledge of apicomplexan biology, this work provides a rational basis to combine sequentially acting merozoite vaccine candidates in a single multi-receptor-blocking vaccine.

The putative Plasmodium translocon of exported proteins (PTEX) is essential for transport of malarial effector proteins across a parasite-encasing vacuolar membrane into host erythrocytes, but the mechanism of this process remains unknown. Here we show that PTEX is a bona fide translocon by determining structures of the PTEX core complex at near-atomic resolution using cryo-electron microscopy. We isolated the endogenous PTEX core complex containing EXP2, PTEX150 and HSP101 from Plasmodium falciparum in the ‘engaged’ and ‘resetting’ states of endogenous cargo translocation using epitope tags inserted using the CRISPR–Cas9 system. In the structures, EXP2 and PTEX150 interdigitate to form a static, funnel-shaped pseudo-seven-fold-symmetric protein-conducting channel spanning the vacuolar membrane. The spiral-shaped AAA+ HSP101 hexamer is tethered above this funnel, and undergoes pronounced compaction that allows three of six tyrosine-bearing pore loops lining the HSP101 channel to dissociate from the cargo, resetting the translocon for the next threading cycle. Our work reveals the mechanism of P. falciparum effector export, and will inform structure-based design of drugs targeting this unique translocon. Cryo-electron microscopy analysis of the purified Plasmodium translocon of exported proteins (PTEX) reveals two distinct resolved states, suggesting a mechanism by which Plasmodium falciparum exports malarial effector proteins into erythrocytes.

In the malaria parasite Plasmodium falciparum , the switch from asexual multiplication to sexual differentiation into gametocytes is essential for transmission to mosquitos. The transcription factor PfAP2-G is a key determinant of sexual commitment that orchestrates this crucial cell fate decision. Here we identify the direct targets of PfAP2-G and demonstrate that it dynamically binds hundreds of sites across the genome. We find that PfAP2-G is a transcriptional activator of early gametocyte genes, and identify differences in PfAP2-G occupancy between gametocytes derived via next-cycle and same-cycle conversion. Our data implicate PfAP2-G not only as a transcriptional activator of gametocyte genes, but also as a potential regulator of genes important for red blood cell invasion. We also find that regulation by PfAP2-G requires interaction with a second transcription factor, PfAP2-I. These results clarify the functional role of PfAP2-G during sexual commitment and early gametocytogenesis. The transcription factor PfAP2-G is a key determinant of sexual commitment in Plasmodium falciparum . Here, Josling et al. define the transcriptional regulatory network of PfAP2-G by identifying its DNA binding sites genome-wide, which vary depending on the route of sexual conversion and rely on interactions with the PfAP2-I transcription factor.

Artemisinin partial resistance may facilitate selection of Plasmodium falciparum resistant to combination therapy partner drugs. We evaluated 99  P. falciparum isolates collected in 2021 from northern Uganda, where resistance-associated PfK13 C469Y and A675V mutations have emerged, and eastern Uganda, where these mutations are uncommon. With the ex vivo ring survival assay, isolates with the 469Y mutation (median survival 7.3% for mutant, 2.5% mixed, and 1.4% wild type) and/or mutations in Pfcoronin or falcipain-2a, had significantly greater survival; all isolates with survival >5% had mutations in at least one of these proteins. With ex vivo growth inhibition assays, susceptibility to lumefantrine (median IC 50 14.6 vs. 6.9 nM, p  < 0.0001) and dihydroartemisinin (2.3 vs. 1.5 nM, p  = 0.003) was decreased in northern vs. eastern Uganda; 14/49 northern vs. 0/38 eastern isolates had lumefantrine IC 50  > 20 nM ( p  = 0.0002). Targeted sequencing of 819 isolates from 2015–21 identified multiple polymorphisms associated with altered drug susceptibility, notably PfK13 469Y with decreased susceptibility to lumefantrine ( p  = 6 × 10 −8 ) and PfCRT mutations with chloroquine resistance ( p  = 1 × 10 −20 ). Our results raise concern regarding activity of artemether-lumefantrine, the first-line antimalarial in Uganda. In this work, susceptibilities to two key antimalarials, dihydroartemisinin and lumefantrine, were associated with multiple genetic polymorphisms in Plasmodium falciparum , and were lower in northern Uganda, where resistance-mediating mutations have emerged, compared to eastern Uganda.

An unexpectedly large fraction of genes in metazoans (human, mouse, zebrafish, worm, fruit fly) express high levels of circularized RNAs containing canonical exons. Here we report that circular RNA isoforms are found in diverse species whose most recent common ancestor existed more than one billion years ago: fungi (Schizosaccharomyces pombe and Saccharomyces cerevisiae), a plant (Arabidopsis thaliana), and protists (Plasmodium falciparum and Dictyostelium discoideum). For all species studied to date, including those in this report, only a small fraction of the theoretically possible circular RNA isoforms from a given gene are actually observed. Unlike metazoans, Arabidopsis, D. discoideum, P. falciparum, S. cerevisiae, and S. pombe have very short introns (∼ 100 nucleotides or shorter), yet they still produce circular RNAs. A minority of genes in S. pombe and P. falciparum have documented examples of canonical alternative splicing, making it unlikely that all circular RNAs are by-products of alternative splicing or 'piggyback' on signals used in alternative RNA processing. In S. pombe, the relative abundance of circular to linear transcript isoforms changed in a gene-specific pattern during nitrogen starvation. Circular RNA may be an ancient, conserved feature of eukaryotic gene expression programs.

The Plasmodium falciparum chloroquine resistance transporter (PfCRT) is a key contributor to multidrug resistance and is also essential for the survival of the malaria parasite, yet its natural function remains unresolved. We identify host-derived peptides of 4-11 residues, varying in both charge and composition, as the substrates of PfCRT in vitro and in situ, and show that PfCRT does not mediate the non-specific transport of other metabolites and/or ions. We find that drug-resistance-conferring mutations reduce both the peptide transport capacity and substrate range of PfCRT, explaining the impaired fitness of drug-resistant parasites. Our results indicate that PfCRT transports peptides from the lumen of the parasite’s digestive vacuole to the cytosol, thereby providing a source of amino acids for parasite metabolism and preventing osmotic stress of this organelle. The resolution of PfCRT’s native substrates will aid the development of drugs that target PfCRT and/or restore the efficacy of existing antimalarials. Plasmodium falciparum chloroquine resistance transporter (PfCRT) mediates multidrug resistance, but its natural function remains unclear. Here, Shafik et al. show that PfCRT transports host-derived peptides of 4-11 residues but not other ions or metabolites, and that drug-resistance-conferring PfCRT mutants have reduced peptide transport.