HDP-a novel heme detoxification protein from the malaria parasite

Role Transformation of HSPA8 to Heme-peroxidase After Binding Hemin to Catalyze Heme Polymerization

Hemin, a byproduct of hemoglobin degradation, inflicts oxidative insult to cells. Following its accumulation, several proteins are recruited for heme detoxification with heme oxygenase playing the key role. Chaperones play a protective role primarily by preventing protein degradation and unfolding. They also are known to have miscellaneous secondary roles during similar situations. To discover a secondary role of chaperones during heme stress we studied the role of the chaperone HSPA8 in the detoxification of hemin. In-silico studies indicated that HSPA8 has a well-defined biophoric environment to bind hemin. Through optical difference spectroscopy, we found that HSPA8 binds hemin through its N-terminal domain with a Kd value of 5.9 ± 0.04 µM and transforms into a hemoprotein. The hemoprotein was tested for exhibiting peroxidase activity using guaiacol as substrate. The complex formed reacts with H2O2 and exhibits classical peroxidase activity with an ability to oxidize aromatic and halide substrates. HSPA8 is dose-dependently catalyzing heme polymerization through its N-terminal domain. The IR results reveal that the polymer formed exhibits structural similarities to β-hematin suggesting its covalent nature. The polymerization mechanism was tested through optical spectroscopy, spin-trap, and activity inhibition experiments. The results suggest that the polymerization occurs through a peroxidase-H2O2 system involving a one-electron transfer mechanism, and the formation of free radical and radical-radical interaction. It highlights a possible role of the HSPA8-hemin complex in exhibiting cytoprotective function during pathological conditions like malaria, sickle cell disease, etc.

A computational strategy for systematic virtual screening of plasmodium falciparum heme detoxification protein inhibitors from the Drugbank database

Antimalarial drug resistance poses one of the greatest threats to malaria treatment, resulting in increased morbidity and mortality. Heme Detoxification Protein (HDP) is among the essential hemoglobinases of P. falciparum (Pf), a vital molecular target for the treatment of malaria. In this study, we utilized the virtual screening workflow tool of the Schrodinger suite to find the best hits for the PfHDP from the DrugBank library. A total of 14,942 compounds were identified against the PfHDP. The top compounds with the highest docking scores and least energy scores were subjected to molecular simulations for 500 nanosecond to check the stability of the protein-drug complexes. The top three DrugBank compounds were found to be stable over 500 ns, namely DB09298 (silibinin), DB07426 (1-Hydroxy-2-(1,1':3',1''-Terphenyl-3-Yloxy) Ethane-1,1-Diyl] Bis (Phosphonic Acid), and DB07410 [(2-(3-Dibenzofuran-4-yl-Phenyl)-1-Hydroxy-1-Phosphono-Ethyl]-Phosphonic Acid). Overall analysis suggests that the top three compounds, DB09298, DB07426, and DB07410, have good stability for 500 ns. Their scaffolds can be used to design and develop new analogs of the target HDP protein. Silibinin, the anti-cancer drug, was found to be highly stable for the entire simulation period as compared to the other compounds. DB07426 shows its therapeutic effect on bones, especially in the treatment of osteoporosis, and DB07410 has anti-tumor, antibacterial, anti-oxidative, and anti-viral activities. All three compounds can be considered for repurposing as antimalarial drugs to evaluate the binding capacity or inhibition potential of these compounds. Further in-vivo and in-vitro analysis against the PfHDP protein should be conducted.Communicated by Ramaswamy H. Sarma.

In Vitro Investigation Unveiling New Insights into the Antimalarial Mechanism of Chloroquine: Role in Perturbing Nucleation Events during Heme to β-Hematin Transformation

Malaria parasites generate toxic heme during hemoglobin digestion, which is neutralized by crystallizing into inert hemozoin (β-hematin). Chloroquine blocks this detoxification process, resulting in heme-mediated toxicity in malaria parasites. However, the exact mechanism of chloroquine's action remains unknown. This study investigates the impact of chloroquine on the transformation of heme into β-hematin. The results show that chloroquine does not completely halt the transformation process but rather slows it down. Additionally, chloroquine complexation with free heme does not affect substrate availability or inhibit β-hematin formation. Scanning electron microscopy (SEM) and X-ray powder diffraction (XRD) studies indicate that the size of β-hematin crystal particles and crystallites increases in the presence of chloroquine, suggesting that chloroquine does not impede crystal growth. These findings suggest that chloroquine delays hemozoin production by perturbing the nucleation events of crystals and/or the stability of crystal nuclei. Thus, contrary to prevailing beliefs, this study provides a new perspective on the working mechanism of chloroquine.

Significance of Plasmodium berghei Amino Acid Transporter 1 in Food Vacuole Functionality and Its Association with Cerebral Pathogenesis

The food vacuole plays a central role in the blood stage of parasite development by digesting host hemoglobin acquired from red blood cells and detoxifying the host heme released during hemoglobin digestion into hemozoin. Blood-stage parasites undergo periodic schizont bursts, releasing food vacuoles containing hemozoin. Clinical studies in malaria-infected patients and in vivo animal studies have shown the association of hemozoin with disease pathogenesis and abnormal host immune responses in malaria. Here, we perform a detailed in vivo characterization of putative Plasmodium berghei amino acid transporter 1 localized in the food vacuole to understand its significance in the malaria parasite. We show that the targeted deletion of amino acid transporter 1 in Plasmodium berghei leads to a swollen food vacuole phenotype with the accumulation of host hemoglobin-derived peptides. Plasmodium berghei amino acid transporter 1-knockout parasites produce less hemozoin, and the hemozoin crystals display a thin morphology compared with wild-type parasites. The knockout parasites show reduced sensitivity to chloroquine and amodiaquine by showing recrudescence. More importantly, mice infected with the knockout parasites are protected from cerebral malaria and display reduced neuronal inflammation and cerebral complications. Genetic complementation of the knockout parasites restores the food vacuole morphology with hemozoin levels similar to that of wild-type parasites, causing cerebral malaria in the infected mice. The knockout parasites also show a significant delay in male gametocyte exflagellation. Our findings highlight the significance of amino acid transporter 1 in food vacuole functionality and its association with malaria pathogenesis and gametocyte development. IMPORTANCE Food vacuoles of the malaria parasite are involved in the degradation of red blood cell hemoglobin. The amino acids derived from hemoglobin degradation support parasite growth, and the heme released is detoxified into hemozoin. Antimalarials such as quinolines target hemozoin formation in the food vacuole. Food vacuole transporters transport hemoglobin-derived amino acids and peptides from the food vacuole to the parasite cytosol. Such transporters are also associated with drug resistance. Here, we show that the deletion of amino acid transporter 1 in Plasmodium berghei leads to swollen food vacuoles with the accumulation of hemoglobin-derived peptides. The transporter-deleted parasites generate less hemozoin with thin crystal morphology and show reduced sensitivity to quinolines. Mice infected with transporter-deleted parasites are protected from cerebral malaria. There is also a delay in male gametocyte exflagellation, affecting transmission. Our findings uncover the functional significance of amino acid transporter 1 in the life cycle of the malaria parasite.

Limited genetic diversity and expression profile of Plasmodium falciparum haem detoxification protein: a possible diagnostic target

BACKGROUND

Haem detoxification protein (HDP) is a significant protein in the erythrocytic stage of the Plasmodium lifecycle. HDP could be of paramount interest as a diagnostic biomarker for accurate diagnosis of malaria. We thus explored HDP genetic variation, expression levels of HDP and immune response.

METHODS

Phylogenetic analysis was carried out using Pfhdp orthologues sequences of various Plasmodium species. Blood samples were collected from patients in central India. Pfhdp gene was amplified, and sequenced by sanger DNA sequencing. B-cell epitopes were identified in PfHDP using Bepipred Linear Epitope Prediction 2.0, and median-joining network was constructed using global PfHDP sequences. Pfhdp expression levels during erythrocytic stage were assessed using real-time qPCR at 4-h intervals. An IgG immune response against synthetic PfHDP peptides was analysed using ELISA.

RESULTS

Phylogenetic analysis revealed the conserved nature of Pfhdp gene. Diversity analysis revealed one non-synonymous mutation (F91L) among all isolates. Neutrality tests indicated negative selection for Pfhdp gene. HDP was expressed throughout the erythrocytic cycle, and comparatively, high expression was observed in the late trophozoite and schizont stages. High IgG response against both peptides was observed, and no polymorphism was seen in any of the seven predicted B-cell epitopes.

CONCLUSIONS

Findings of the present study indicate the possibility of HDP being exploited as a diagnostic biomarker for Plasmodium falciparum malaria after proteomic validation studies.

Diffraction contrast in cryo-scanning transmission electron tomography reveals the boundary of hemozoin crystals in situ

Malaria is a potentially fatal infectious disease caused by the obligate intracellular parasite Plasmodium falciparum. The parasite infects human red blood cells (RBC) and derives nutrition by catabolism of hemoglobin. As amino acids are assimilated from the protein component, the toxic heme is released. Molecular heme is detoxified by rapid sequestration to physiologically insoluble hemozoin crystals within the parasite's digestive vacuole (DV). Common antimalarial drugs interfere with this crystallization process, leaving the parasites vulnerable to the by-product of their own metabolism. A fundamental debate with important implications on drug mechanism regards the chemical environment of crystallization in situ, whether aqueous or lipid. This issue had been addressed previously by cryogenic soft X-ray tomography. We employ cryo-scanning transmission electron tomography (CSTET) to probe parasite cells throughout the life cycle in a fully hydrated, vitrified state at higher resolution. During the acquisition of CSTET data, Bragg diffraction from the hemozoin provides a uniquely clear view of the crystal boundary at nanometer resolution. No intermediate medium, such as a lipid coating or shroud, could be detected surrounding the crystals. The present study describes a unique application of CSTET in the study of malaria. The findings can be extended to evaluate new drug candidates affecting hemozoin crystal growth.

An assay procedure to investigate the transformation of toxic heme into inert hemozoin via plasmodial heme detoxification protein

Most antimalarial therapeutics, including chloroquine and artemisinin, induce free heme-mediated toxicity in Plasmodium. This cytotoxic heme is produced as a by-product during the large-scale digestion of host hemoglobin. Conversion of this host-derived heme into inert crystalline hemozoin is the only defense mechanism in Plasmodium against heme-induced cytotoxicity. Heme detoxification protein (HDP), a highly conserved plasmodial protein, is reported to be the most efficient biological mediator for heme to hemozoin transformation. Despite its significance, HDP has never been extensively studied for heme transformation into hemozoin. Therefore, we wish to develop a method to study the HDP-mediated transformation of heme into hemozoin. We have adopted, modified, and optimized the pyridine hemochrome assay to study HDP catalysis and use substrate and time kinetics to study the HDP-mediated transformation of heme into hemozoin. In contrast to the previously reported assay for HDP, we found that the new assay is more precise, accurate, and handy, making it more suitable for kinetic studies. HDP-mediated transformation of heme into hemozoin is not a single-step process, and involves a transient intermediate, most likely a cyclic heme dimer. The kinetics and the manner of HDP-mediated hemozoin production are dependent on the substrate concentration, and a small fraction of substrate remains untransformed to hemozoin irrespective of reaction time. Combining HDP as a catalyst and the pyridine hemochrome assay will facilitate the efficient screening of future antimalarials.

Innate immunity to malaria: The good, the bad and the unknown

Malaria is the cause of 600.000 deaths annually. However, these deaths represent only a tiny fraction of total malaria cases. Repeated natural infections with the causative agent, Plasmodium sp. parasites, induce protection from severe disease but not sterile immunity. Thus, immunity to Plasmodium is incomplete. Conversely, immunization with attenuated sporozoite stage parasites can induce sterile immunity albeit after multiple vaccinations. These different outcomes are likely to be influenced strongly by the innate immune response to different stages of the parasite lifecycle. Even small numbers of sporozoites can induce a robust proinflammatory type I interferon response, which is believed to be driven by the sensing of parasite RNA. Moreover, induction of innate like gamma-delta cells contributes to the development of adaptive immune responses. Conversely, while blood stage parasites can induce a strong proinflammatory response, regulatory mechanisms are also triggered. In agreement with this, intact parasites are relatively weakly sensed by innate immune cells, but isolated parasite molecules, notably DNA and RNA can induce strong responses. Thus, the innate response to Plasmodium parasite likely represents a trade-off between strong pro-inflammatory responses that may potentiate immunity and regulatory processes that protect the host from cytokine storms that can induce life threatening illness.

Babesia, Theileria, Plasmodium and Hemoglobin

The Propagation of Plasmodium spp. and Babesia/Theileria spp. vertebrate blood stages relies on the mediated acquisition of nutrients available within the host’s red blood cell (RBC). The cellular processes of uptake, trafficking and metabolic processing of host RBC proteins are thus crucial for the intraerythrocytic development of these parasites. In contrast to malarial Plasmodia, the molecular mechanisms of uptake and processing of the major RBC cytoplasmic protein hemoglobin remain widely unexplored in intraerythrocytic Babesia/Theileria species. In the paper, we thus provide an updated comparison of the intraerythrocytic stage feeding mechanisms of these two distantly related groups of parasitic Apicomplexa. As the associated metabolic pathways including proteolytic degradation and networks facilitating heme homeostasis represent attractive targets for diverse antimalarials, and alterations in these pathways underpin several mechanisms of malaria drug resistance, our ambition is to highlight some fundamental differences resulting in different implications for parasite management with the potential for novel interventions against Babesia/Theileria infections.

Malaria parasite heme biosynthesis promotes and griseofulvin protects against cerebral malaria in mice

Heme-biosynthetic pathway of malaria parasite is dispensable for asexual stages, but essential for mosquito and liver stages. Despite having backup mechanisms to acquire hemoglobin-heme, pathway intermediates and/or enzymes from the host, asexual parasites express heme pathway enzymes and synthesize heme. Here we show heme synthesized in asexual stages promotes cerebral pathogenesis by enhancing hemozoin formation. Hemozoin is a parasite molecule associated with inflammation, aberrant host-immune responses, disease severity and cerebral pathogenesis. The heme pathway knockout parasites synthesize less hemozoin, and mice infected with knockout parasites are protected from cerebral malaria and death due to anemia is delayed. Biosynthetic heme regulates food vacuole integrity and the food vacuoles from knockout parasites are compromised in pH, lipid unsaturation and proteins, essential for hemozoin formation. Targeting parasite heme synthesis by griseofulvin-a FDA-approved antifungal drug, prevents cerebral malaria in mice and provides an adjunct therapeutic option for cerebral and severe malaria.

Bazedoxifene, a Postmenopausal Drug, Acts as an Antimalarial and Inhibits Hemozoin Formation

Despite a remarkable improvement in health care and continued drug discovery efforts, malaria control efforts are continuously challenged by the emergence of drug-resistant parasite strains. Given a long and risky development path of new drugs, repurposing existing drugs for the treatment of malaria is an attractive and shorter path. Tamoxifen, a selective estrogen receptor modulator (SERM) for the treatment and prevention of estrogen receptor-positive breast cancer, possesses antibacterial, antifungal, and antiparasitic activities. Hence, we assessed tamoxifen, raloxifene, and bazedoxifene, which represent the first-, second-, and third-generation SERMs, respectively, for antimalarial activity. Raloxifene and bazedoxifene inhibited the erythrocytic development of Plasmodium falciparum with submicromolar 50% inhibitory concentration (IC₅₀) values. Among the three, bazedoxifene was the most potent and also decreased P. berghei infection in female mice but not in male mice. However, bazedoxifene similarly inhibited P. falciparum growth in erythrocytes of male and female origin, which highlights the importance of sex-specific host physiology in drug efficacy. Bazedoxifene was most potent on early ring-stage parasites, and about 35% of the treated parasites did not contain hemozoin in the food vacuole. Bazedoxifene-treated parasites had almost 34% less hemozoin content than the control parasites. However, both control and bazedoxifene-treated parasites had similar hemoglobin levels, suggesting that bazedoxifene inhibits hemozoin formation and that toxicity due to accumulation of free heme could be a mechanism of its antimalarial activity. Because bazedoxifene is in clinical use and bazedoxifene-chloroquine combination shows an additive antiparasitic effect, bazedoxifene could be an adjunctive partner of currently used antimalarial regimens.

IMPORTANCE The emergence and spread of drug-resistant strains of the human malaria parasite Plasmodium falciparum has necessitated new drugs. Selective estrogen receptor modulators are in clinical use for the prevention and treatment of breast cancer and postmenopausal osteoporosis. We demonstrate that bazedoxifene, a third-generation selective estrogen receptor modulator, has potent inhibitory activity against both susceptible and drug-resistant strains of Plasmodium falciparum. It also blocked the development of Plasmodium berghei in mice. The inhibitory effect was strongest on the ring stage and resulted in the inhibition of hemozoin formation, which could be the major mechanism of bazedoxifene action. Hemozoin is a nontoxic polymer of heme, which is a by-product of hemoglobin degradation by the malaria parasite during its development within the erythrocyte. Because bazedoxifene is already in clinical use for the treatment of postmenopausal osteoporosis, our findings support repurposing of bazedoxifene as an antimalarial.

Adsorption to the surface of hemozoin crystals: Structure-based design and synthesis of new amino-phenoxazine β-hematin inhibitors

In silico adsorption of eight antimalarials that inhibit β-hematin (synthetic hemozoin) formation identified a primary binding site on the (001) face, which accommodates inhibitors via formation of predominantly π-π interactions. A good correlation (r2 = 0.64, P = 0.017) between adsorption energies and the logarithm of β-hematin inhibitory activity was found for this face. Of 53 monocyclic, bicyclic and tricyclic scaffolds, the latter yielded the most favorable adsorption energies. Five new amino-phenoxazine compounds were pursued as β-hematin inhibitors based on adsorption behaviour. The 2-substituted phenoxazines show good to moderate β-hematin inhibitory activity (< 100 μM) and Plasmodium falciparum blood stage activity against the 3D7 strain. N1,N1-diethyl-N4-(10H-phenoxazin-2-yl)pentane-1,4-diamine (P2a) is the most promising hit with IC50 values of 4.7 ± 0.6 and 0.64 ± 0.05 μM, respectively. Adsorption energies are predictive of β-hematin inhibitory activity, and thus the in silico approach is a beneficial tool for structure-based development of new non-quinoline inhibitors.

Hemozoin: a Complex Molecule with Complex Activities

Purpose of Review

Malaria is a disease caused by parasites that reside in host red blood cells and use hemoglobin as a nutrient source. Heme released by hemoglobin catabolism is modified by the parasite to produce hemozoin (HZ), which has toxic effects on the host. Experimentation aiming to elucidate how HZ contributes to malaria pathogenesis has utilized different preparations of this molecule, complicating interpretation and comparison of findings. We examine natural synthesis and isolation of HZ and highlight studies that have used multiple preparations, including synthetic forms, in a comparative fashion.

Recent Findings

Recent work utilizing sophisticated imaging and detection techniques reveals important molecular characteristics of HZ synthesis and biochemistry. Other recent studies further refine understanding of contributions of HZ to malaria pathogenesis yet highlight the continuing need to characterize HZ preparations and contextualize experimental conditions in the in vivo infection milieu.

Summary

This review highlights the necessity of collectively determining what is physiologically relevant HZ. Characterization of isolated natural HZ and use of multiple preparations in each study are recommended with application of in vivo studies whenever possible. Adoption of such practices is expected to improve reproducibility of results and elucidate the myriad of ways that HZ participates in malaria pathogenesis.

Reactive Oxygen Species as the Brainbox in Malaria Treatment

Several measures are in place to combat the worldwide spread of malaria, especially in regions of high endemicity. In part, most common antimalarials, such as quinolines and artemisinin and its derivatives, deploy an ROS-mediated approach to kill malaria parasites. Although some antimalarials may share similar targets and mechanisms of action, varying levels of reactive oxygen species (ROS) generation may account for their varying pharmacological activities. Regardless of the numerous approaches employed currently and in development to treat malaria, concerningly, there has been increasing development of resistance by Plasmodium falciparum, which can be connected to the ability of the parasites to manage the oxidative stress from ROS produced under steady or treatment states. ROS generation has remained the mainstay in enforcing the antiparasitic activity of most conventional antimalarials. However, a combination of conventional drugs with ROS-generating ability and newer drugs that exploit vital metabolic pathways, such antioxidant machinery, could be the way forward in effective malaria control.

Non-ribosomal insights into ribosomal P2 protein in Plasmodium falciparum-infected erythrocytes

The enormous complexity of the eukaryotic ribosome has been a real challenge in unlocking the mechanistic aspects of its amazing molecular function during mRNA translation and many non‐canonical activities of ribosomal proteins in eukaryotic cells. While exploring the uncanny nature of ribosomal P proteins in malaria parasites Plasmodium falciparum, the 60S stalk ribosomal P2 protein has been shown to get exported to the infected erythrocyte (IE) surface as an SDS‐resistant oligomer during the early to the mid‐trophozoite stage. Inhibiting IE surface P2 either by monoclonal antibody or through genetic knockdown resulted in nuclear division arrest of the parasite. This strange and serendipitous finding has led us to explore more about un‐canonical cell biology and the structural involvement of P2 protein in Plasmodium in the search for a novel biochemical role during parasite propagation in the human host.

Heme Detoxification in the Malaria Parasite: A Target for Antimalarial Drug Development

Over the last century, malaria deaths have decreased by more than 85%. Nonetheless, there were 405 000 deaths in 2018, mostly resulting from Plasmodium falciparum infection. In the 21st century, much of the advance has arisen from the deployment of insecticide-treated bed nets and artemisinin combination therapy. However, over the past few decades parasites with a delayed artemisinin clearance phenotype have appeared in Southeast Asia, threatening further gains. The effort to find new drugs is thus urgent. A prominent process in blood stage malaria parasites, which we contend remains a viable drug target, is hemozoin formation. This crystalline material consisting of heme can be readily seen when parasites are viewed microscopically. The process of its formation in the parasite, however, is still not fully understood.In early work, we recognized hemozoin formation as a biomineralization process. We have subsequently investigated the kinetics of synthetic hemozoin (β-hematin) crystallization catalyzed at lipid-aqueous interfaces under biomimetic conditions. This led us to the use of neutral detergent-based high-throughput screening (HTS) for inhibitors of β-hematin formation. A good hit rate against malaria parasites was obtained. Simultaneously, we developed a pyridine-based assay which proved successful in measuring the concentrations of hematin not converted to β-hematin.The pyridine assay was adapted to determine the effects of chloroquine and other clinical antimalarials on hemozoin formation in the cell. This permitted the determination of the dose-dependent amounts of exchangeable heme and hemozoin in P. falciparum for the first time. These studies have shown that hemozoin inhibitors cause a dose-dependent increase in exchangeable heme, correlated with decreased parasite survival. Electron spectroscopic imaging (ESI) showed a relocation of heme iron into the parasite cytoplasm, while electron microscopy provided evidence of the disruption of hemozoin crystals. This cellular assay was subsequently extended to top-ranked hits from a wide range of scaffolds found by HTS. Intriguingly, the amounts of exchangeable heme at the parasite growth IC₅₀ values of these scaffolds showed substantial variation. The amount of exchangeable heme was found to be correlated with the amount of inhibitor accumulated in the parasitized red blood cell. This suggests that heme-inhibitor complexes, rather than free heme, lead to parasite death. This was supported by ESI using a Br-containing compound which showed the colocalization of Fe and Br as well as by confocal Raman microscopy which confirmed the presence of a complex in the parasite. Current evidence indicates that inhibitors block hemozoin formation by surface adsorption. Indeed, we have successfully introduced molecular docking with hemozoin to find new inhibitors. It follows that the resulting increase in free heme leads to the formation of the parasiticidal heme-inhibitor complex. We have reported crystal structures of heme-drug complexes for several aryl methanol antimalarials in nonaqueous media. These form coordination complexes but most other inhibitors interact noncovalently, and the determination of their structures remains a major challenge.It is our view that key future developments will include improved assays to measure cellular heme levels, better in silico approaches for predicting β-hematin inhibition, and a concerted effort to determine the structure and properties of heme-inhibitor complexes.

A Plasmodium falciparum protein tyrosine phosphatase inhibitor identified from the ChEMBL-NTD database blocks parasite growth

Post‐translational modifications, especially reversible phosphorylation, are among the most common mechanisms that regulate protein function and biological processes in Plasmodium species. Of the Plasmodium phosphatases, phosphatase of regenerating liver (PfPRL) is secreted and is an essential phosphatase. Here, we expressed PfPRL in a heterologous expression system, and then purified and characterized its phosphatase activity. We found that Novartis_003209, a previously identified inhibitor, inhibited the PfPRL phosphatase activity of recombinant PfPRL and blocked in vitro parasite growth in a dose‐dependent manner. Further, in silico docking analysis of Novartis_003209 with all four P. falciparum tyrosine phosphatases (PTP) demonstrated that Novartis_003209 is a Plasmodium PTP inhibitor. Overall, our results identify a scaffold as a potential starting point to design a PTP‐specific inhibitor.

Combating multi-drug resistant malaria parasite by inhibiting falcipain-2 and heme-polymerization: Artemisinin-peptidyl vinyl phosphonate hybrid molecules as new antimalarials

Artemisinin-based combination therapies (ACTs) have been able to reduce the clinical and pathological malaria cases in endemic areas around the globe. However, recent reports have shown a progressive decline in malaria parasite clearance in South-east Asia after ACT treatment, thus envisaging a need for new artemisinin (ART) derivatives and combinations. To address the emergence of drug resistance to current antimalarials, here we report the synthesis of artemisinin-peptidyl vinyl phosphonate hybrid molecules that show superior efficacy than artemisinin alone against chloroquine-resistant as well as multidrug-resistant Plasmodium falciparum strains with EC₅₀ in pico-molar ranges. Further, the compounds effectively inhibited the survival of ring-stage parasite for laboratory-adapted artemisinin-resistant parasite lines as compared to artemisinin. These hybrid molecules showed complete parasite clearance in vivo using P. berghei mouse malaria model in comparison to artemisinin alone. Studies on the mode of action of hybrid molecules suggested that these artemisinin-peptidyl vinyl phosphonate hybrid molecules possessed dual activities: inhibited falcipain-2 (FP-2) activity, a P. falciparum cysteine protease involved in hemoglobin degradation, and also blocked the hemozoin formation in the food-vacuole, a step earlier shown to be blocked by artemisinin. Since these hybrid molecules blocked multiple steps of a pathway and showed synergistic efficacies, we believe that these lead compounds can be developed as effective antimalarials to prevent the spread of resistance to current antimalarials.

Malaria Pigment Crystals: The Achilles' Heel of the Malaria Parasite

The biogenic formation of hemozoin crystals, a crucial process in heme detoxification by the malaria parasite, is reviewed as an antimalarial drug target. We first focus on the in‐vivo formation of hemozoin. A model is presented, based on native‐contrast 3D imaging obtained by X‐ray and electron microscopy, that hemozoin nucleates at the inner membrane leaflet of the parasitic digestive vacuole, and grows in the adjacent aqueous medium. Having observed quantities of hemoglobin and hemozoin in the digestive vacuole, we present a model that heme liberation from hemoglobin and hemozoin formation is an assembly‐line process. The crystallization is preceded by reaction between heme monomers yielding hematin dimers involving fewer types of isomers than in synthetic hemozoin; this is indicative of protein‐induced dimerization. Models of antimalarial drugs binding onto hemozoin surfaces are reviewed. This is followed by a description of bromoquine, a chloroquine drug analogue, capping a significant fraction of hemozoin surfaces within the digestive vacuole and accumulation of the drug, presumably a bromoquine–hematin complex, at the vacuole's membrane.

Disruption of Plasmodium falciparum histidine-rich protein 2 may affect haem metabolism in the blood stage

Background

Haem is a key metabolic factor in the life cycle of the malaria parasite. In the blood stage, the parasite acquires host haemoglobin to generate amino acids for protein synthesis and the by-product haem for metabolic use. The malaria parasite can also synthesize haem de novo on its own. Plasmodium falciparum-specific histidine-rich protein 2 (PfHRP2) has a haem-binding site to mediate the formation of haemozoin, a biocrystallized form of haem aggregates. Notably, the gene regulates the mechanism of haemoglobin-derived haem metabolism and the de novo haem biosynthetic pathway in the Pfhrp2-disrupted parasite line during the intraerythrocytic stages.

Methods

The CRISPR/Cas9 system was used to disrupt the gene locus of Pfhrp2. DNA was extracted from the transgenic parasite, and PCR, Southern blotting and Western blotting were used to confirm the establishment of transgenic parasites. RNA-sequencing and comparative transcriptome analysis were performed to identify differences in gene expression between 3D7 and Pfhrp2^--3D7 parasites.

Results

Pfhrp2^- transgenic parasites were successfully established by the CRISPR/Cas9 system. A total of 964, 1261, 3138, 1064, 2512 and 1778 differentially expressed genes (DEGs) were identified in the six comparison groups, respectively, with 373, 520, 1499, 353, 1253 and 742 of these DEGs upregulated and 591, 741, 1639, 711, 1259 and 1036 of them downregulated, respectively. Five DEGs related to haem metabolism and synthesis were identified in the comparison groups at six time points (0, 8, 16, 24, 32, and 40 h after merozoite invasion). The genes encoding delta-aminolevulinic acid synthetase and ferrochelatase, both related to haem biosynthesis, were found to be significantly upregulated in the comparison groups, and those encoding haem oxygenase, stromal-processing peptidase and porphobilinogen deaminase were found to be significantly downregulated. No GO terms were significantly enriched in haem-related processes (Q value = 1).

Conclusion

Our data revealed changes in the transcriptome expression profile of the Pfhrp2^--3D7 parasite during the intraerythrocytic stages. The findings provide insight at the gene transcript level that will facilitate further research on and development of anti-malaria drugs.

Type I Interferons and Malaria: A Double-Edge Sword Against a Complex Parasitic Disease

Type I interferons (IFN-Is) are important cytokines playing critical roles in various infections, autoimmune diseases, and cancer. Studies have also shown that IFN-Is exhibit 'conflicting' roles in malaria parasite infections. Malaria parasites have a complex life cycle with multiple developing stages in two hosts. Both the liver and blood stages of malaria parasites in a vertebrate host stimulate IFN-I responses. IFN-Is have been shown to inhibit liver and blood stage development, to suppress T cell activation and adaptive immune response, and to promote production of proinflammatory cytokines and chemokines in animal models. Different parasite species or strains trigger distinct IFN-I responses. For example, a Plasmodium yoelii strain can stimulate a strong IFN-I response during early infection, whereas its isogenetic strain does not. Host genetic background also greatly influences IFN-I production during malaria infections. Consequently, the effects of IFN-Is on parasitemia and disease symptoms are highly variable depending on the combination of parasite and host species or strains. Toll-like receptor (TLR) 7, TLR9, melanoma differentiation-associated protein 5 (MDA5), and cyclic GMP-AMP synthase (cGAS) coupled with stimulator of interferon genes (STING) are the major receptors for recognizing parasite nucleic acids (RNA/DNA) to trigger IFN-I responses. IFN-I levels in vivo are tightly regulated, and various novel molecules have been identified to regulate IFN-I responses during malaria infections. Here we review the major findings and progress in ligand recognition, signaling pathways, functions, and regulation of IFN-I responses during malaria infections.

A lipocalin mediates unidirectional heme biomineralization in malaria parasites

During blood-stage development, malaria parasites are challenged with the detoxification of enormous amounts of heme released during the proteolytic catabolism of erythrocytic hemoglobin. They tackle this problem by sequestering heme into bioinert crystals known as hemozoin. The mechanisms underlying this biomineralization process remain enigmatic. Here, we demonstrate that both rodent and human malaria parasite species secrete and internalize a lipocalin-like protein, PV5, to control heme crystallization. Transcriptional deregulation of PV5 in the rodent parasite Plasmodium berghei results in inordinate elongation of hemozoin crystals, while conditional PV5 inactivation in the human malaria agent Plasmodium falciparum causes excessive multidirectional crystal branching. Although hemoglobin processing remains unaffected, PV5-deficient parasites generate less hemozoin. Electron diffraction analysis indicates that despite the distinct changes in crystal morphology, neither the crystalline order nor unit cell of hemozoin are affected by impaired PV5 function. Deregulation of PV5 expression renders P. berghei hypersensitive to the antimalarial drugs artesunate, chloroquine, and atovaquone, resulting in accelerated parasite clearance following drug treatment in vivo. Together, our findings demonstrate the Plasmodium-tailored role of a lipocalin family member in hemozoin formation and underscore the heme biomineralization pathway as an attractive target for therapeutic exploitation.

Synthesis of Novel Ciprofloxacin-Based Hybrid Molecules toward Potent Antimalarial Activity

Antimalarial drug resistance is a serious obstacle in the persistent quest to eradicate malaria. There is a need for potent chemical agents that are able to act on drug-resistant Plasmodium falciparum populations at reasonable concentrations without any related toxicity to the host. By rational drug design, we envisaged to address this issue by generating a novel hybrid drug possessing two pharmacophores that can act on two unique and independent targets within the cell. We synthesized a new class of ciprofloxacin-based hybrid molecules, which have been integrated with acridine, quinolone, sulphonamide, and cinnamoyl pharmacophores (1-4). We realized a potent chloroquinolone-ciprofloxacin-based antimalarial hybrid (2, CQ-CFX) whose mechanism of action is unlike that of its parent molecules indicating a unique biological target. CQ-CFX is not only potent against CQ-resistant and susceptible strains of Plasmodium falciparum at low nanomolar concentrations (IC₅₀ values are 63.17 ± 1.2 nM and 25.52 ± 4.45 nM, respectively) but is also not toxic to mammalian and bacterial systems up to 20 μM and 1 μM, respectively.

Directing traffic: Chaperone-mediated protein transport in malaria parasites

The ability of eukaryotic parasites from the phylum Apicomplexa to cause devastating diseases is predicated upon their ability to maintain faithful and precise protein trafficking mechanisms. Their parasitic life cycle depends on the trafficking of effector proteins to the infected host cell, transport of proteins to several critical organelles required for survival, as well as transport of parasite and host proteins to the digestive organelles to generate the building blocks for parasite growth. Several recent studies have shed light on the molecular mechanisms parasites utilise to transform the infected host cells, transport proteins to essential metabolic organelles and for biogenesis of organelles required for continuation of their life cycle. Here, we review key pathways of protein transport originating and branching from the endoplasmic reticulum, focusing on the essential roles of chaperones in these processes. Further, we highlight key gaps in our knowledge that prevents us from building a holistic view of protein trafficking in these deadly human pathogens.

The parasitophorous vacuole of the blood-stage malaria parasite

The pathology of malaria is caused by infection of red blood cells with unicellular Plasmodium parasites. During blood-stage development, the parasite replicates within a membrane-bound parasitophorous vacuole. A central nexus for host-parasite interactions, this unique parasite shelter functions in nutrient acquisition, subcompartmentalization and the export of virulence factors, making its functional molecules attractive targets for the development of novel intervention strategies to combat the devastating impact of malaria. In this Review, we explore the origin, development, molecular composition and functions of the parasitophorous vacuole of Plasmodium blood stages. We also discuss the relevance of the malaria parasite's intravacuolar lifestyle for successful erythrocyte infection and provide perspectives for future research directions in parasitophorous vacuole biology.

Molecular characterization and tissue localization of glutathione S-transferase from adult Ancylostoma ceylanicum

Glutathione S-transferases (GSTs) are a detoxifying enzyme family that is essential for parasite blood-feeding and survival, and represent potential targets for hookworm vaccine development. Multiple GST-encoding complementary DNAs (cDNAs) have been cloned from Ancylostoma caninum and Necator americanus, but there are no reports about the cloning of this enzyme from Ancylostoma ceylanicum, the animal-derived zoonotic hookworm. To study the molecular nature and tissue localization of GST of A. ceylanicum (Ace-GST), we designed primers based on the GST gene sequence of A. ceylanicum in GenBank, amplified the Ace-GST cDNA by reverse transcription polymerase chain reaction, and analysed its homology and genetic evolution relationship. The amplified product was cloned into the pET-32a vector and transformed into Escherichia coli BL21 (DE3) for expression. To prepare anti-GST polyclonal antibodies, the recombinant protein was purified and used to immunize Kunming mice. The level of immunoglobulin G (IgG) antibody in the serum of immunized mice was detected by indirect enzyme-linked immunosorbent assay, and the Ace-GST localization in adult worm was determined using the immunofluorescence method. The results showed that the full-length cDNA encoding Ace-GST was 468 bp, which had the highest homology with Ac-GST-1 (60.1%) and clustered into one branch (v-class) with Ac-GST-1 and Na-GST-1 in a phylogenetic tree. Mice immunized with recombinant Ace-GST showed specific IgG antibody response. Immunolocalization revealed that natural Ace-GST is mainly located in the epidermis, muscle and intestine of the adult. These results may lay a foundation for further studies on the biological function of Ace-GST.

Iron in parasitic protists – from uptake to storage and where we can interfere

A comprehensive review of iron metabolism in parasitic protists and its potential use as a drug target.

Antimalarial Quinoline Drugs Inhibit β-Hematin and Increase Free Hemin Catalyzing Peroxidative Reactions and Inhibition of Cysteine Proteases

Malaria caused by Plasmodium affects millions people worldwide. Plasmodium consumes hemoglobin during its intraerythrocytic stage leaving toxic heme. Parasite detoxifies free heme through formation of hemozoin (β-hematin) pigment. Proteolysis of hemoglobin and formation of hemozoin are two main targets for antimalarial drugs. Quinoline antimarial drugs and analogs (β-carbolines or nitroindazoles) were studied as inhibitors of β-hematin formation. The most potent inhibitors were quinacrine, chloroquine, and amodiaquine followed by quinidine, mefloquine and quinine whereas 8-hydroxyquinoline and β-carbolines had no effect. Compounds that inhibited β-hematin increased free hemin that promoted peroxidative reactions as determined with TMB and ABTS substrates. Hemin-catalyzed peroxidative reactions were potentiated in presence of proteins (i.e. globin or BSA) while antioxidants and peroxidase inhibitors decreased peroxidation. Free hemin increased by chloroquine action promoted oxidative reactions resulting in inhibition of proteolysis by three cysteine proteases: papain, ficin and cathepsin B. Glutathione reversed inhibition of proteolysis. These results show that active quinolines inhibit hemozoin and increase free hemin which in presence of H₂O₂ that abounds in parasite digestive vacuole catalyzes peroxidative reactions and inhibition of cysteine proteases. This work suggests a link between the action of quinoline drugs with biochemical processes of peroxidation and inhibition of proteolysis.

Toxic but tasty - temporal dynamics and network architecture of heme-responsive two-component signaling in Corynebacterium glutamicum

Heme is an essential cofactor and alternative iron source for almost all bacterial species but may cause severe toxicity upon elevated levels and consequently, regulatory mechanisms coordinating heme homeostasis represent an important fitness trait. A remarkable scenario is found in several corynebacterial species, e.g. Corynebacterium glutamicum and Corynebacterium diphtheriae, which dedicate two paralogous, heme-responsive two-component systems, HrrSA and ChrSA, to cope with the Janus nature of heme. Here, we combined experimental reporter profiling with a quantitative mathematical model to understand how this particular regulatory network architecture shapes the dynamic response to heme. Our data revealed an instantaneous activation of the detoxification response (hrtBA) upon stimulus perception and we found that kinase activity of both kinases contribute to this fast onset. Furthermore, instant deactivation of the P_hrtBA promoter is achieved by a strong ChrS phosphatase activity upon stimulus decline. While the activation of detoxification response is uncoupled from further factors, heme utilization is additionally governed by the global iron regulator DtxR integrating information on iron availability into the regulatory network. Altogether, our data provide comprehensive insights how TCS cross-regulation and network hierarchy shape the temporal dynamics of detoxification (hrtBA) and utilization (hmuO) as part of a global homeostatic response to heme.

Parasite Recognition and Signaling Mechanisms in Innate Immune Responses to Malaria

Malaria caused by the Plasmodium family of parasites, especially P.falciparum and P. vivax, is a major health problem in many countries in the tropical and subtropical regions of the world. The disease presents a wide array of systemic clinical conditions and several life-threatening organ pathologies, including the dreaded cerebral malaria. Like many other infectious diseases, malaria is an inflammatory response-driven disease, and positive outcomes to infection depend on finely tuned regulation of immune responses that efficiently clear parasites and allow protective immunity to develop. Immune responses initiated by the innate immune system in response to parasites play key roles both in protective immunity development and pathogenesis. Initial pro-inflammatory responses are essential for clearing infection by promoting appropriate cell-mediated and humoral immunity. However, elevated and prolonged pro-inflammatory responses owing to inappropriate cellular programming contribute to disease conditions. A comprehensive knowledge of the molecular and cellular mechanisms that initiate immune responses and how these responses contribute to protective immunity development or pathogenesis is important for developing effective therapeutics and/or a vaccine. Historically, in efforts to develop a vaccine, immunity to malaria was extensively studied in the context of identifying protective humoral responses, targeting proteins involved in parasite invasion or clearance. The innate immune response was thought to be non-specific. However, during the past two decades, there has been a significant progress in understanding the molecular and cellular mechanisms of host-parasite interactions and the associated signaling in immune responses to malaria. Malaria infection occurs at two stages, initially in the liver through the bite of a mosquito, carrying sporozoites, and subsequently, in the blood through the invasion of red blood cells by merozoites released from the infected hepatocytes. Soon after infection, both the liver and blood stage parasites are sensed by various receptors of the host innate immune system resulting in the activation of signaling pathways and production of cytokines and chemokines. These immune responses play crucial roles in clearing parasites and regulating adaptive immunity. Here, we summarize the knowledge on molecular mechanisms that underlie the innate immune responses to malaria infection.

Current scenario and future strategies to fight artemisinin resistance

Despite several setbacks in the fight against malaria such as insecticide and drug resistance as well as low efficacy of available vaccines, considerable success in reducing malaria burden has been achieved in the past decade. Artemisinins (ARTs and their combination therapies, ACTs), the current frontline drugs against uncomplicated malaria, rapidly kill plasmodial parasites and are non-toxic at short exposures. Though the exact mode of action remains unclear, the endoperoxide bridge, indispensable for ART activity, is thought to react with heme released from hemoglobin hydrolysis and generate free radicals that alkylate multiple protein targets, thereby disrupting proteostasis pathways. However, rapid development of ART resistance in recent years with no potential alternatives on the horizon threaten the elimination efforts. The Greater Mekong Subregion in South-East Asia continues to churn out mutants resistant to multiple ACTs and detected in increasingly expanding geographies. Extensive research on ART-resistant strains have identified a potential candidate Kelch13, crucial for mediating ART resistance. Parasites with mutations in the propeller domains of Plasmodium falciparum Kelch13 protein were shown to have enhanced phosphatidylinositol 3-kinase levels that were concomitant with delayed parasite clearance. Current research focused on understanding the mechanism of Kelch13-mediated ART resistance could provide better insights into Plasmodium resistome. This review covers the current proposed mechanisms of ART activity, resistance strategies adopted by the parasite in response to ACTs and possible future approaches to mitigate the spread of resistance from South-East Asia.

Potent Antimalarial Activity of Two Arenes Linked with Triamine Designed To Have Multiple Interactions with Heme

Based on the idea that compounds designed to exhibit high affinity for heme would block hemozoin formation, a critical heme-detoxification process for malarial parasites, we synthesized a series of compounds with two π-conjugated moieties at terminal amino groups of triamine. These compounds exhibited moderate to high antimalarial activities in vitro toward both chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum. In a P. berghei-infected mouse model, 3a and 12a showed potent antimalarial activities compared to artesunate, as well as a prolonged duration of antimalarial effect. We found a good correlation between protective activity against hemin degradation and antimalarial activity. Compounds 8b and 3a strongly inhibited hemozoin formation catalyzed by heme detoxification protein.

Prediction Model for Antimalarial Activities of Hemozoin Inhibitors by Using Physicochemical Properties

The rapid spread of strains of malaria parasites that are resistant to several drugs has threatened global malaria control. Hence, the aim of this study was to predict the antimalarial activity of chemical compounds that possess anti-hemozoin-formation activity as a new means of antimalarial drug discovery. After the initial in vitro anti-hemozoin-formation high-throughput screening (HTS) of 9,600 compounds, a total of 224 hit compounds were identified as hemozoin inhibitors. These 224 compounds were tested for in vitro erythrocytic antimalarial activity at 10 μM by using chloroquine-mefloquine-sensitive Plasmodium falciparum strain 3D7A. Two independent experiments were conducted. The physicochemical properties of the active compounds were extracted from the ChemSpider and SciFinder databases. We analyzed the extracted data by using Bayesian model averaging (BMA). Our findings revealed that lower numbers of S atoms; lower distribution coefficient (log D) values at pH 3, 4, and 5; and higher predicted distribution coefficient (ACD log D) values at pH 7.4 had significant associations with antimalarial activity among compounds that possess anti-hemozoin-formation activity. The BMA model revealed an accuracy of 91.23%. We report new prediction models containing physicochemical properties that shed light on effective chemical groups for synthetic antimalarial compounds and help with in silico screening for novel antimalarial drugs.

Repetitive sequences in malaria parasite proteins

Five species of parasite cause malaria in humans with the most severe disease caused by Plasmodium falciparum. Many of the proteins encoded in the P. falciparum genome are unusually enriched in repetitive low-complexity sequences containing a limited repertoire of amino acids. These repetitive sequences expand and contract dynamically and are among the most rapidly changing sequences in the genome. The simplest repetitive sequences consist of single amino acid repeats such as poly-asparagine tracts that are found in approximately 25% of P. falciparum proteins. More complex repeats of two or more amino acids are also common in diverse parasite protein families. There is no universal explanation for the occurrence of repetitive sequences and it is possible that many confer no function to the encoded protein and no selective advantage or disadvantage to the parasite. However, there are increasing numbers of examples where repetitive sequences are important for parasite protein function. We discuss the diverse roles of low-complexity repetitive sequences throughout the parasite life cycle, from mediating protein-protein interactions to enabling the parasite to evade the host immune system.

The Emerging Role of Pattern Recognition Receptors in the Pathogenesis of Malaria

Despite a global effort to develop an effective vaccine, malaria is still a significant health problem. Much of the pathology of malaria is immune mediated. This suggests that host immune responses have to be finely regulated. The innate immune system initiates and sets the threshold of the acquired immune response and determines the outcome of the disease. Yet, our knowledge of the regulation of innate immune responses during malaria is limited. Theoretically, inadequate activation of the innate immune system could result in unrestrained parasite growth. Conversely, hyperactivation of the innate immune system, is likely to cause excessive production of proinflammatory cytokines and severe pathology. Toll-like receptors (TLRs) have emerged as essential receptors which detect signature molecules and shape the complex host response during malaria infection. This review will highlight the mechanisms by which Plasmodium components are recognized by innate immune receptors with particular emphasis on TLRs. A thorough understanding of the complex roles of TLRs in malaria may allow the delineation of pathological versus protective host responses and enhance the efficacy of anti-malarial treatments and vaccines.

Exploring Heme and Hemoglobin Binding Regions of Plasmodium Heme Detoxification Protein for New Antimalarial Discovery

Hemoglobin degradation/hemozoin formation, essential steps in the Plasmodium life cycle, are targets of existing antimalarials. The pathway still offers vast possibilities to be explored for new antimalarial discoveries. Here, we characterize heme detoxification protein, PfHDP, a major protein involved in hemozoin formation, as a novel drug target. Using in silico and biochemical approaches, we identified two heme binding sites and a hemoglobin binding site in PfHDP. Treatment of Plasmodium falciparum 3D7 parasites with peptide corresponding to the hemoglobin binding domain in PfHDP resulted in food vacuole abnormalities similar to that seen with a cysteine protease inhibitor, E-64 (I-1). Screening of compounds that bound the modeled PfHDP structure in the heme/hemoglobin-binding pockets from Maybridge Screening Collection identified a compound, ML-2, that inhibited parasite growth in a dose-dependent manner, thus paving the way for testing its potential as a new drug candidate. These results provide functional insights into the role of PfHDP in Hz formation and further suggest that PfHDP could be an important drug target to combat malaria.

Modelling the drivers of the spread of Plasmodium falciparum hrp2 gene deletions in sub-Saharan Africa

Rapid diagnostic tests (RDTs) have transformed malaria diagnosis. The most prevalent P. falciparum RDTs detect histidine-rich protein 2 (PfHRP2). However, pfhrp2 gene deletions yielding false-negative RDTs, first reported in South America in 2010, have been confirmed in Africa and Asia. We developed a mathematical model to explore the potential for RDT-led diagnosis to drive selection of pfhrp2-deleted parasites. Low malaria prevalence and high frequencies of people seeking treatment resulted in the greatest selection pressure. Calibrating our model against confirmed pfhrp2-deletions in the Democratic Republic of Congo, we estimate a starting frequency of 6% pfhrp2-deletion prior to RDT introduction. Furthermore, the patterns observed necessitate a degree of selection driven by the introduction of PfHRP2-based RDT-guided treatment. Combining this with parasite prevalence and treatment coverage estimates, we map the model-predicted spread of pfhrp2-deletion, and identify the geographic regions in which surveillance for pfhrp2-deletion should be prioritised.

Unraveling heme detoxification in the malaria parasite by in situ correlative X-ray fluorescence microscopy and soft X-ray tomography

A key drug target for malaria has been the detoxification pathway of the iron-containing molecule heme, which is the toxic byproduct of hemoglobin digestion. The cornerstone of heme detoxification is its sequestration into hemozoin crystals, but how this occurs remains uncertain. We report new results of in vivo rate of heme crystallization in the malaria parasite, based on a new technique to measure element-specific concentrations at defined locations in cell ultrastructure. Specifically, a high resolution correlative combination of cryo soft X-ray tomography has been developed to obtain 3D parasite ultrastructure with cryo X-ray fluorescence microscopy to measure heme concentrations. Our results are consistent with a model for crystallization via the heme detoxification protein. Our measurements also demonstrate the presence of considerable amounts of non-crystalline heme in the digestive vacuole, which we show is most likely contained in hemoglobin. These results suggest a tight coupling between hemoglobin digestion and heme crystallization, highlighting a new link in the crystallization pathway for drug development.

Evidence for a role of hemozoin in metabolism and gametocytogenesis

Hemozoin is generally considered a waste deposit that is formed for the sole purpose of detoxification of free heme that results from the digestion of hemoglobin by Plasmodium parasites. However, several observations of parasite multiplication, both in vertebrate and invertebrate hosts are suggestive of a wider, but overlooked, metabolic role for this product. The presence of clinical peripheral blood samples of P. falciparum with high parasitemia containing only hemozoin-deficient (non-pigmented) asexual forms has been repeatedly confirmed. Such samples stand in contrast with other samples that contain mostly pigmented circulating trophozoites and gametocytes, indicating that pigment accumulation is a prominent feature of gametocytogenesis. The restricted size, i.e. below detection by light microscopy, of hemozoin in asexual merozoites and ringforms of P. falciparum implies its continuous turnover, supporting a role in metabolism. The prominent interaction of hemozoin with several antimalarial drugs, the involvement of proteins in hemozoin formation, and the finding of plasmodial genes coding for a heme-oxygenase-like protein argue for a wider and more active role for hemozoin in the parasite's metabolism. The observed association of hemozoin with crystalloids during ookinete development is consistent with a useful function to it during parasite multiplication in the invertebrate host. Finally, alternative mechanisms, other than hemozoin formation, provide substitute or additional routes for heme detoxification.

Design, Synthesis, and Biological Evaluation of Peptidomimetic N-Substituted Cbz-4-Hyp-Hpa-Amides as Novel Inhibitors of Plasmodium falciparum

A new series of peptidomimetic N-substituted Cbz-4-Hyp-Hpa-amides were designed, synthesized, and evaluated for inhibition of the Plasmodium falciparum. Substituents on the N-atom of the amide group were selected alkyl-, allyl-, aryl-, 2-hydroxyethyl-, 2-cyanoethyl-, cyanomethyl-, 2-hydroxyethyl-, 2,2-diethoxyethyl-, or 2-ethoxy-2-oxoethylamino groups, and about of 40 new compounds were synthesized and evaluated for antiplasmodial activity in vitro. Antimalarial activity has been investigated as for the final peptide mimetics, and their immediate predecessors, carrying TBDMS or TBDPS protecting groups on 4-hydroxyproline residue and 18 derivatives exhibited toxicity against P. falciparum. Of these agents, compound 23e was shown to have potent antimalarial activity with IC₅₀ 528 ng/ml.

Biochemistry of malaria parasite infected red blood cells by X-ray microscopy

Red blood cells infected by the malaria parasite Plasmodium falciparum are correlatively imaged by tomography using soft X-rays as well as by scanning hard nano-X-ray beam to obtain fluorescence maps of various elements such as S and Fe. In this way one can deduce the amount of Fe bound either in hemoglobin or in hemozoin crystals in the digestive vacuole of the malaria parasite as well as determine the hemoglobin concentrations in the cytosols of the red blood cell and of the parasite. Fluorescence map of K shows that in the parasite’s schizont stage the K concentration in the red blood cell cytosol is diminished by a factor of seven relative to a pristine red blood cell but the total amount of K in the infected red blood cell is the same as in the pristine red blood cell.

What Is Known about the Immune Response Induced by Plasmodium vivax Malaria Vaccine Candidates?

Malaria caused by Plasmodium vivax continues being one of the most important infectious diseases around the world; P. vivax is the second most prevalent species and has the greatest geographic distribution. Developing an effective antimalarial vaccine is considered a relevant control strategy in the search for means of preventing the disease. Studying parasite-expressed proteins, which are essential in host cell invasion, has led to identifying the regions recognized by individuals who are naturally exposed to infection. Furthermore, immunogenicity studies have revealed that such regions can trigger a robust immune response that can inhibit sporozoite (hepatic stage) or merozoite (erythrocyte stage) invasion of a host cell and induce protection. This review provides a synthesis of the most important studies to date concerning the antigenicity and immunogenicity of both synthetic peptide and recombinant protein candidates for a vaccine against malaria produced by P. vivax.

High-Throughput Screening and Prediction Model Building for Novel Hemozoin Inhibitors Using Physicochemical Properties

It is essential to continue the search for novel antimalarial drugs due to the current spread of resistance against artemisinin by Plasmodium falciparum parasites. In this study, we developed in silico models to predict hemozoin inhibitors as a potential first-step screening for novel antimalarials. An in vitro colorimetric high-throughput screening assay of hemozoin formation was used to identify hemozoin inhibitors from 9,600 structurally diverse compounds. The physicochemical properties of positive hits and randomly selected compounds were extracted from the ChemSpider database; they were used for developing prediction models to predict hemozoin inhibitors using two different approaches, i.e., traditional multivariate logistic regression and Bayesian model averaging. Our results showed that a total of 224 positive-hit compounds exhibited the ability to inhibit hemozoin formation, with 50% inhibitory concentrations (IC₅₀s) ranging from 3.1 μM to 199.5 μM. The best model according to traditional multivariate logistic regression included the three variables octanol-water partition coefficient, number of hydrogen bond donors, and number of atoms of hydrogen, while the best model according to Bayesian model averaging included the three variables octanol-water partition coefficient, number of hydrogen bond donors, and index of refraction. Both models had a good discriminatory power, with area under the curve values of 0.736 and 0.781 for the traditional multivariate model and Bayesian model averaging, respectively. In conclusion, the prediction models can be a new, useful, and cost-effective approach for the first screen of hemozoin inhibition-based antimalarial drug discovery.

The hookworm Ancylostoma ceylanicum intestinal transcriptome provides a platform for selecting drug and vaccine candidates

Background

The intestine of hookworms contains enzymes and proteins involved in the blood-feeding process of the parasite and is therefore a promising source of possible vaccine antigens. One such antigen, the hemoglobin-digesting intestinal aspartic protease known as Na-APR-1 from the human hookworm Necator americanus, is currently a lead candidate antigen in clinical trials, as is Na-GST-1 a heme-detoxifying glutathione S-transferase.

Methods

In order to discover additional hookworm vaccine antigens, messenger RNA was obtained from the intestine of male hookworms, Ancylostoma ceylanicum, maintained in hamsters. RNA-seq was performed using Illumina high-throughput sequencing technology. The genes expressed in the hookworm intestine were compared with those expressed in the whole worm and those genes overexpressed in the parasite intestine transcriptome were further analyzed.

Results

Among the lead transcripts identified were genes encoding for proteolytic enzymes including an A. ceylanicum APR-1, but the most common proteases were cysteine-, serine-, and metallo-proteases. Also in abundance were specific transporters of key breakdown metabolites, including amino acids, glucose, lipids, ions and water; detoxifying and heme-binding glutathione S-transferases; a family of cysteine-rich/antigen 5/pathogenesis-related 1 proteins (CAP) previously found in high abundance in parasitic nematodes; C-type lectins; and heat shock proteins. These candidates will be ranked for downstream antigen target selection based on key criteria including abundance, uniqueness in the parasite versus the vertebrate host, as well as solubility and yield of expression.

Conclusion

The intestinal transcriptome of A. ceylanicum provides useful information for the identification of proteins involved in the blood-feeding process, representing a first step towards a reverse vaccinology approach to a human hookworm vaccine.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1795-8) contains supplementary material, which is available to authorized users.