Molecular characterization and localization of Plasmodium falciparum choline kinase

Identification and Characterization of Entamoeba histolytica Choline Kinase

PURPOSE

Entamoeba histolytica is one of the death-causing parasites in the world. Study on its lipid composition revealed that it is predominated by phosphatidylcholine and phosphatidylethanolamine. Further study revealed that its phosphorylated metabolites might be produced by the Kennedy pathway. Here, we would like to report on the characterizations of enzymes from this pathway that would provide information for the design of novel inhibitors against these enzymes in future.

METHODOLOGY

E. histolytica HM-1:IMSS genomic DNA was isolated and two putative choline/ethanolamine kinase genes (EhCK1 and EhCK2) were cloned and expressed from Escherichia coli BL21 strain. Enzymatic characterizations were further carried out on the purified enzymes.

RESULTS

EhCK1 and EhCK2 were identified from E. histolytica genome. The deduced amino acid sequences were more identical to its homologues in human (35-48%) than other organisms. The proteins were clustered as ethanolamine kinase in the constructed phylogeny tree. Sequence analysis showed that they possessed all the conserved motifs in choline kinase family: ATP-binding loop, Brenner's phosphotransferase motif, and choline kinase motif. Here, the open reading frames were cloned, expressed, and purified to apparent homogeneity. EhCK1 showed activity with choline but not ethanolamine. The biochemical characterization showed that it had a Vmax of 1.9 ± 0.1 µmol/min/mg. Its Km for choline and ATP was 203 ± 26 µM and 3.1 ± 0.4 mM, respectively. In contrast, EhCK2 enzymatic activity was only detected when Mn2+ was used as the co-factor instead of Mg2+ like other choline/ethanolamine kinases. Highly sensitive and specific antibody against EhCK1 was developed and used to confirm the endogenous EhCK1 expression using immunoblotting.

CONCLUSIONS

With the understanding of EhC/EK importance in phospholipid metabolism and their unique characteristic, EhC/EK could be a potential target for future anti-amoebiasis study.

Tackling the emerging Artemisinin-resistant malaria parasite by modulation of defensive oxido-reductive mechanism via nitrofurantoin repurposing

Oxidative stress-mediated cell death has remained the prime parasiticidal mechanism of front line antimalarial, artemisinin (ART). The emergence of resistant Plasmodium parasites characterized by oxidative stress management due to impaired activation of ART and enhanced reactive oxygen species (ROS) detoxification has decreased its clinical efficacy. This gap can be filled by development of alternative chemotherapeutic agents to combat resistance defense mechanism. Interestingly, repositioning of clinically approved drugs presents an emerging approach for expediting antimalarial drug development and circumventing resistance. Herein, we evaluated the antimalarial potential of nitrofurantoin (NTF), a clinically used antibacterial drug, against intra-erythrocytic stages of ART-sensitive (Pf3D7) and resistant (PfKelch13R539T) strains of P. falciparum, alone and in combination with ART. NTF exhibited growth inhibitory effect at submicro-molar concentration by arresting parasite growth at trophozoite stage. It also inhibited the survival of resistant parasites as revealed by ring survival assay. Concomitantly, in vitro combination assay revealed synergistic association of NTF with ART. NTF was found to enhance the reactive oxygen and nitrogen species, and induced mitochondrial membrane depolarization in parasite. Furthermore, we found that exposure of parasites to NTF disrupted redox balance by impeding Glutathione Reductase activity, which manifests in enhanced oxidative stress, inducing parasite death. In vivo administration of NTF, alone and in combination with ART, in P. berghei ANKA-infected mice blocked parasite multiplication and enhanced mean survival time. Overall, our results indicate NTF as a promising repurposable drug with therapeutic potential against ART-sensitive as well as resistant parasites.

Choline Kinase: An Unexpected Journey for a Precision Medicine Strategy in Human Diseases

Choline kinase (ChoK) is a cytosolic enzyme that catalyzes the phosphorylation of choline to form phosphorylcholine (PCho) in the presence of ATP and magnesium. ChoK is required for the synthesis of key membrane phospholipids and is involved in malignant transformation in a large variety of human tumours. Active compounds against ChoK have been identified and proposed as antitumor agents. The ChoK inhibitory and antiproliferative activities of symmetrical bispyridinium and bisquinolinium compounds have been defined using quantitative structure-activity relationships (QSARs) and structural parameters. The design strategy followed in the development of the most active molecules is presented. The selective anticancer activity of these structures is also described. One promising anticancer compound has even entered clinical trials. Recently, ChoKα inhibitors have also been proposed as a novel therapeutic approach against parasites, rheumatoid arthritis, inflammatory processes, and pathogenic bacteria. The evidence for ChoKα as a novel drug target for approaches in precision medicine is discussed.

4-Chlorothymol Exerts Antiplasmodial Activity Impeding Redox Defense System in Plasmodium falciparum

Malaria remains one of the major health concerns due to the resistance of Plasmodium species toward the existing drugs warranting an urgent need for new antimalarials. Thymol derivatives were known to exhibit enhanced antimicrobial activities; however, no reports were found against Plasmodium spp. In the present study, the antiplasmodial activity of thymol derivatives was evaluated against chloroquine-sensitive (NF-54) and -resistant (K1) strains of Plasmodium falciparum. Among the thymol derivatives tested, 4-chlorothymol showed potential activity against sensitive and resistant strains of P. falciparum. 4-Chlorothymol was found to increase the reactive oxygen species and reactive nitrogen species level. Furthermore, 4-chlorothymol could perturb the redox balance by modulating the enzyme activity of GST and GR. 4-Chlorothymol also showed synergy with chloroquine against chloroquine-resistant P. falciparum. 4-Chlorothymol was found to significantly suppress the parasitemia and increase the mean survival time in in vivo assays. Interestingly, in in vivo assay, 4-chlorothymol in combination with chloroquine showed higher chemosuppression as well as enhanced mean survival time at a much lower concentration as compared to individual doses of chloroquine and 4-chlorothymol. These observations clearly indicate the potential use of 4-chlorothymol as an antimalarial agent, which may also be effective in combination with the existing antiplasmodial drugs against chloroquine-resistant P. falciparum infection. In vitro cytotoxicity/hemolytic assay evidently suggests that 4-chlorothymol is safe for further exploration of its therapeutic properties.

4-chloro eugenol interacts synergistically with artesunate against drug resistant P. falciparum inducing oxidative stress

4-chloro eugenol (4CE), a semisynthetic analog of phytomolecule eugenol exhibited potent antiplasmodial activity with IC₅₀ in the range of 1.5-5 μM against sensitive as well as drug resistant strain of P. falciparum. This analog also showed synergy with a clinically used antimalarial drug artesunate and was able to curtail the IC₅₀ of artesunate up to 4-5 folds. Although, 4CE did not show any effect on heme polymerization, the most common drug target in the malaria parasite, it could increase the level of reactive oxygen species (ROS) and reactive nitrogen species (RNS) alone as well as in combination with artesunate. Further, 4CE induced oxidative stress was observed to affect the macromolecules in terms of DNA damage, protein carbonylation and lipid peroxidation. At the physiological level, cellular organelles like mitochondria and endoplasmic reticulum were observed to be get affected by 4CE in terms of membrane depolarization and calcium ion leakage respectively. These observations could be validated by expression analysis of oxidative stress responsive genes and proteins. Further, in in vivo assay, 4CE showed significant chemo-suppression of parasitemia as well as an increase in mean survival time in the murine malaria model. Interestingly, in combination with artesunate, 4CE showed higher chemo-suppression as well as enhanced mean survival time at a much lower concentrations of both the partners as compared to an individual dose of artesunate and 4CE. A combination of 4CE and artesunate was also observed to attenuate cerebral malaria pathogenesis.

ChoK-ing the Pathogenic Bacteria: Potential of Human Choline Kinase Inhibitors as Antimicrobial Agents

Novel antimicrobial agents are crucial to combat antibiotic resistance in pathogenic bacteria. Choline kinase (ChoK) in bacteria catalyzes the synthesis of phosphorylcholine, which is subsequently incorporated into the cell wall or outer membrane. In certain species of bacteria, phosphorylcholine is also used to synthesize membrane phosphatidylcholine. Numerous human ChoK inhibitors (ChoKIs) have been synthesized and tested for anticancer properties. Inhibition of S. pneumoniae ChoK by human ChoKIs showed a promising effect by distorting the cell wall and retarded the growth of this pathogen. Comparison of amino acid sequences at the catalytic sites of putative choline kinases from pathogenic bacteria and human enzymes revealed striking sequence conservation that supports the potential application of currently available ChoKIs for inhibiting bacterial enzymes. We also propose the combined use of ChoKIs and nanoparticles for targeted delivery to the pathogen while shielding the human host from any possible side effects of the inhibitors. More research should focus on the verification of putative bacterial ChoK activities and the characterization of ChoKIs with active enzymes. In conclusion, the presence of ChoK in a wide range of pathogenic bacteria and the distinct function of this enzyme has made it an attractive drug target. This review highlighted the possibility of "choking" bacterial ChoKs by using human ChoKIs.

Hydrazonophenol, a Food Vacuole-Targeted and Ferriprotoporphyrin IX-Interacting Chemotype Prevents Drug-Resistant Malaria

The rapid emergence of resistance against frontline antimalarial drugs essentially warrants the identification of new-generation antimalarials. Here, we describe the synthesis of ( E)-2-isopropyl-5-methyl-4-((2-(pyridin-4-yl)hydrazono)methyl)phenol (18), which binds ferriprotoporphyrin-IX (Fe^III-PPIX) ( K_d = 33 nM) and offers antimalarial activity against chloroquine-resistant and sensitive strains of Plasmodium falciparum in vitro. Structure-function analysis reveals that compound 18 binds Fe^III-PPIX through the -C═N-NH- moiety and 2-pyridyl substitution at the hydrazine counterpart plays a critical role in antimalarial efficacy. Live cell confocal imaging using a fluorophore-tagged compound confirms its accumulation inside the acidic food vacuole (FV) of P. falciparum. Furthermore, this compound concentration-dependently elevates the pH in FV, implicating a plausible interference with Fe^III-PPIX crystallization (hemozoin formation) by a dual function: increasing the pH and binding free Fe^III-PPIX. Different off-target bioassays reduce the possibility of the promiscuous nature of compound 18. Compound 18 also exhibits potent in vivo antimalarial activity against chloroquine-resistant P. yoelii and P. berghei ANKA (causing cerebral malaria) in mice with negligible toxicity.

Insights into the phosphatidylcholine and phosphatidylethanolamine biosynthetic pathways in Leishmania parasites and characterization of a choline kinase from Leishmania infantum

The protozoan parasite Leishmania infantum is a causative agent of the disease visceral leishmaniasis, which can be fatal if not properly treated. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) biosynthesis pathways are attractive targets for new antileishmanial compounds since these Leishmania cell membrane phospholipids are important for parasite morphology and physiology. In this work we observed Leishmania synthesize PC and PE from extracellular choline and ethanolamine, respectively, suggesting the presence of CDP-choline and CDP-ethanolamine pathways. In addition, Leishmania converted PE to PC, indicating the parasite possesses phosphatidylethanolamine N-methyltransferase (PEMT) activity. The first step in the biosynthesis of PC or PE requires the phosphorylation of choline or ethanolamine by a kinase. We cloned the gene encoding a putative choline/ethanolamine kinase from Leishmania infantum and expressed and purified the encoded recombinant protein. The enzyme possesses choline kinase activity with a V_max of 3.52μmol/min/mg and an apparent K_m value of 0.089mM with respect to choline. The enzyme can also phosphorylate ethanolamine in vitro, but the apparent K_m for ethanolamine is 850-fold greater than for choline. In an effort to probe requirements for small molecule inhibition of Leishmania choline kinase, the recombinant enzyme was evaluated for the ability to be inhibited by novel quaternary ammonium salts. The most effective inhibitor was N-iodomethyl-N,N,-dimethyl-N-(6,6-diphenyl hex-5-en-1-yle) ammonium iodide, denoted compound C6. In the presence of 4mM compound C6, the V_max/K_m decreased to approximately 1% of the wild-type catalytic efficiency. In addition, in Leishmania cells treated with compound C6 choline transport was inhibited.

Antimalarial Activity of Small-Molecule Benzothiazole Hydrazones

We synthesized a new series of conjugated hydrazones that were found to be active against malaria parasite in vitro, as well as in vivo in a murine model. These hydrazones concentration-dependently chelated free iron and offered antimalarial activity. Upon screening of the synthesized hydrazones, compound 5f was found to be the most active iron chelator, as well as antiplasmodial. Compound 5f also interacted with free heme (KD [equilibrium dissociation constant] = 1.17 ± 0.8 μM), an iron-containing tetrapyrrole released after hemoglobin digestion by the parasite, and inhibited heme polymerization by parasite lysate. Structure-activity relationship studies indicated that a nitrogen- and sulfur-substituted five-membered aromatic ring present within the benzothiazole hydrazones might be responsible for their antimalarial activity. The dose-dependent antimalarial and heme polymerization inhibitory activities of the lead compound 5f were further validated by following [(3)H]hypoxanthine incorporation and hemozoin formation in parasite, respectively. It is worth mentioning that compound 5f exhibited antiplasmodial activity in vitro against a chloroquine/pyrimethamine-resistant strain of Plasmodium falciparum (K1). We also evaluated in vivo antimalarial activity of compound 5f in a murine model where a lethal multiple-drug-resistant strain of Plasmodium yoelii was used to infect Swiss albino mice. Compound 5f significantly suppressed the growth of parasite, and the infected mice experienced longer life spans upon treatment with this compound. During in vitro and in vivo toxicity assays, compound 5f showed minimal alteration in biochemical and hematological parameters compared to control. In conclusion, we identified a new class of hydrazone with therapeutic potential against malaria.

Kinetic modelling of phospholipid synthesis in Plasmodium knowlesi unravels crucial steps and relative importance of multiple pathways

BACKGROUND

Plasmodium is the causal parasite of malaria, infectious disease responsible for the death of up to one million people each year. Glycerophospholipid and consequently membrane biosynthesis are essential for the survival of the parasite and are targeted by a new class of antimalarial drugs developed in our lab. In order to understand the highly redundant phospholipid synthethic pathways and eventual mechanism of resistance to various drugs, an organism specific kinetic model of these metabolic pathways need to be developed in Plasmodium species.

RESULTS

Fluxomic data were used to build a quantitative kinetic model of glycerophospholipid pathways in Plasmodium knowlesi. In vitro incorporation dynamics of phospholipids unravels multiple synthetic pathways. A detailed metabolic network with values of the kinetic parameters (maximum rates and Michaelis constants) has been built. In order to obtain a global search in the parameter space, we have designed a hybrid, discrete and continuous, optimization method. Discrete parameters were used to sample the cone of admissible fluxes, whereas the continuous Michaelis and maximum rates constants were obtained by local minimization of an objective function.The model was used to predict the distribution of fluxes within the network of various metabolic precursors.The quantitative analysis was used to understand eventual links between different pathways. The major source of phosphatidylcholine (PC) is the CDP-choline Kennedy pathway.In silico knock-out experiments showed comparable importance of phosphoethanolamine-N-methyltransferase (PMT) and phosphatidylethanolamine-N-methyltransferase (PEMT) for PC synthesis.The flux values indicate that, major part of serine derived phosphatidylethanolamine (PE) is formed via serine decarboxylation, whereas major part of phosphatidylserine (PS) is formed by base-exchange reactions.Sensitivity analysis of CDP-choline pathway shows that the carrier-mediated choline entry into the parasite and the phosphocholine cytidylyltransferase reaction have the largest sensitivity coefficients in this pathway, but does not distinguish a reaction as an unique rate-limiting step.

CONCLUSION

We provide a fully parametrized kinetic model for the multiple phospholipid synthetic pathways in P. knowlesi. This model has been used to clarify the relative importance of the various reactions in these metabolic pathways. Future work extensions of this modelling strategy will serve to elucidate the regulatory mechanisms governing the development of Plasmodium during its blood stages, as well as the mechanisms of action of drugs on membrane biosynthetic pathways and eventual mechanisms of resistance.

Antiplasmodial activity and mechanism of action of RSM-932A, a promising synergistic inhibitor of Plasmodium falciparum choline kinase

We have investigated the mechanism of action of inhibition of the choline kinase of P. falciparum (p.f.-ChoK) by two inhibitors of the human ChoKα, MN58b and RSM-932A, which have previously been shown to be potent antitumoral agents. The efficacy of these inhibitors against p.f.-ChoK is investigated using enzymatic and in vitro assays. While MN58b may enter the choline/phosphocholine binding site, RSM-932A appears to have an altogether novel mechanism of inhibition and is synergistic with respect to both choline and ATP. A model of inhibition for RSM-932A in which this inhibitor traps p.f.-ChoK in a phosphorylated intermediate state blocking phosphate transfer to choline is presented. Importantly, MN58b and RSM-932A have in vitro inhibitory activity in the low nanomolar range and are equally effective against chloroquine-sensitive and chloroquine-resistant strains. RSM-932A and MN58b significantly reduced parasitemia and induced the accumulation of trophozoites and schizonts, blocking intraerythrocytic development and interfering with parasite egress or invasion, suggesting a delay of the parasite maturation stage. The present data provide two new potent structures for the development of antimalarial compounds and validate p.f.-ChoK as an accessible drug target against the parasite.

Malarial kinases: novel targets for in silico approaches to drug discovery

Malaria, the disease caused by infection with protozoan parasites from the genus Plasmodium, claims the lives of nearly 1 million people annually. Developing nations, particularly in the African Region, bear the brunt of this malaria burden. Alarmingly, the most dangerous etiologic agent of malaria, Plasmodium falciparum, is becoming increasingly resistant to current first-line antimalarials. In light of the widespread devastation caused by malaria, the emergence of drug-resistant P. falciparum strains, and the projected decrease in funding for malaria eradication that may occur over the next decade, the identification of promising new targets for antimalarial drug design is imperative. P. falciparum kinases have been proposed as ideal drug targets for antimalarial drug design because they mediate critical cellular processes within the parasite and are, in many cases, structurally and mechanistically divergent when compared with kinases from humans. Identifying a molecule capable of inhibiting the activity of a target enzyme is generally an arduous and expensive process that can be greatly aided by utilizing in silico drug design techniques. Such methods have been extensively applied to human kinases, but as yet have not been fully exploited for the exploration and characterization of antimalarial kinase targets. This review focuses on in silico methods that have been used for the evaluation of potential antimalarials and the Plasmodium kinases that could be explored using these techniques.

Novel antimalarial drug targets: hope for new antimalarial drugs

Malaria is a major global threat, that results in more than 2 million deaths each year. The treatment of malaria is becoming extremely difficult due to the emergence of drug-resistant parasites, the absence of an effective vaccine, and the spread of insecticide-resistant vectors. Thus, malarial therapy needs new chemotherapeutic approaches leading to the search for new drug targets. Here, we discuss different approaches to identifying novel antimalarial drug targets. We have also given due attention to the existing validated targets with a view to develop novel, rationally designed lead molecules. Some of the important parasite proteins are claimed to be the targets; however, further in vitro or in vivo structure-function studies of such proteins are crucial to validate these proteins as suitable targets. The interactome analysis among apicoplast, mitochondrion and genomic DNA will also be useful in identifying vital pathways or proteins regulating critical pathways for parasite growth and survival, and could be attractive targets. Molecules responsible for parasite invasion to host erythrocytes and ion channels of infected erythrocytes, essential for intra-erythrocyte survival and stage progression of parasites are also becoming attractive targets. This review will discuss and highlight the current understanding regarding the potential antimalarial drug targets, which could be utilized to develop novel antimalarials.

Aryl aryl methyl thio arenes prevent multidrug-resistant malaria in mouse by promoting oxidative stress in parasites

We have synthesized a new series of aryl aryl methyl thio arenes (AAMTAs) and evaluated antimalarial activity in vitro and in vivo against drug-resistant malaria. These compounds interact with free heme, inhibit hemozoin formation, and prevent Plasmodium falciparum growth in vitro in a concentration-dependent manner. These compounds concentration dependently promote oxidative stress in Plasmodium falciparum as evident from the generation of intraparasitic oxidants, protein carbonyls, and lipid peroxidation products. Furthermore, AAMTAs deplete intraparasite GSH levels, which is essential for antioxidant defense and survival during intraerythrocytic stages. These compounds displayed potent antimalarial activity not only in vitro but also in vivo against multidrug-resistant Plasmodium yoelii dose dependently in a mouse model. The mixtures of enantiomers of AAMTAs containing 3-pyridyl rings were found to be more efficient in providing antimalarial activity. Efforts have been made to synthesize achiral AAMTAs 17-23 and among them, compound 18 showed significant antimalarial activity in vivo.

Synthesis and bio-evaluation of human macrophage migration inhibitory factor inhibitor to develop anti-inflammatory agent

Macrophage migration inhibitory factor (MIF), a pro-inflammatory cytokine, is involved in the development of an array of inflammatory disorders including rheumatoid arthritis, inflammatory bowel disease, psoriasis, multiple sclerosis and sepsis. The synthesis of MIF-inhibitor is a rationale approach to develop novel anti-inflammatory agent to treat multitude of inflammatory diseases. In this work, we have synthesized and evaluated MIF-inhibitory activity of a series of small molecules containing isoxazoline skeleton. Mode of binding of this inhibitor to human MIF (huMIF) was determined by docking studies. The synthesized molecules inhibit tautomerase activity of huMIF. The anti-inflammatory activity of the most active inhibitor, 4-((3-(4-hydroxy-3-methoxyphenyl)-4, 5-dihydroisoxazol-5-yl) methoxy) benzaldehyde (4b) was evaluated against huMIF-induced inflammation in a cellular model (RAW 264.7 cell). Compound 4b significantly inhibits huMIF-mediated NF-κB translocation to the nucleus, up-regulation of inducible nitric oxide synthase and nitric oxide production in RAW 264.7 cell which are the markers for inflammation. The compound 4b is not cytotoxic as evident from cell viability assay. Hence, the compound 4b has potential to be a novel anti-inflammatory agent.

Identification and molecular characterization of an Alba-family protein from human malaria parasite Plasmodium falciparum

We have investigated the DNA-binding nature as well as the function of a putative Alba (Acetylation lowers binding affinity) family protein (PfAlba3) from Plasmodium falciparum. PfAlba3 possesses DNA-binding property like Alba family proteins. PfAlba3 binds to DNA sequence non-specifically at the minor groove and acetylation lowers its DNA-binding affinity. The protein is ubiquitously expressed in all the erythrocytic stages of P. falciparum and it exists predominantly in the acetylated form. PfAlba3 inhibits transcription in vitro by binding to DNA. Plasmodium falciparum Sir2 (PfSir2A), a nuclear localized deacetylase interacts with PfAlba3 and deacetylates the lysine residue of N-terminal peptide of PfAlba3 specific for DNA binding. PfAlba3 is localized with PfSir2A in the periphery of the nucleus. Fluorescence in situ hybridization studies revealed the presence of PfAlba3 in the telomeric and subtelomeric regions. ChIP and ChIP ReChIP analyses further confirmed that PfAlba3 binds to the telomeric and subtelomeric regions as well as to var gene promoter.

Cysteine-3 and cysteine-4 are essential for the thioredoxin-like oxidoreductase and antioxidant activities of Plasmodium falciparum macrophage migration inhibitory factor

Plasmodium falciparum macrophage migration inhibitory factor (PfMIF) exhibits thioredoxin (Trx)-like oxidoreductase activity but the active site for this activity and its function have not been evaluated. A bioinformatics search revealed that the conserved CXXC motif, which is responsible for Trx-like oxidoreductase activity, is absent from PfMIF. In contrast, the adjacent N-terminal Cys-3 and Cys-4 are conserved in MIF across species of malarial parasites. Mutation of either vicinal Cys-3 or Cys-4 of PfMIF abolished the Trx-like activity, whereas the mutation of the remaining Cys-59 or Cys-103 did not affect it. PfMIF has an antioxidant function. It prevents reactive oxygen species-mediated lipid peroxidation and oxidative damage of DNA as evident from DNA nicking assay. Interestingly, chemical modification of the vicinal cysteines by phenylarsine oxide (PAO), a specific vicinal thiol modifier, significantly prevented this antioxidant activity. Modification of Cys-3 and Cys-4 was confirmed by MALDI-TOF mass spectroscopy of peptide fragments obtained after cyanogen bromide digestion of PAO-modified PfMIF. Furthermore, mutation of either Cys-3 or Cys-4 of PfMIF resulted in the loss of both Trx-like oxidoreductase and antioxidant activities of PfMIF. Altogether, our results suggest that the vicinal Cys-3 and Cys-4 play a critical role in the Trx-like oxidoreductase activity and antioxidant property of PfMIF.

Identification of inhibitors for putative malaria drug targets among novel antimalarial compounds

The efficacy of most marketed antimalarial drugs has been compromised by evolution of parasite resistance, underscoring an urgent need to find new drugs with new mechanisms of action. We have taken a high-throughput approach toward identifying novel antimalarial chemical inhibitors of prioritized drug targets for Plasmodium falciparum, excluding targets which are inhibited by currently used drugs. A screen of commercially available libraries identified 5655 low molecular weight compounds that inhibit growth of P. falciparum cultures with EC(50) values below 1.25μM. These compounds were then tested in 384- or 1536-well biochemical assays for activity against nine Plasmodium enzymes: adenylosuccinate synthetase (AdSS), choline kinase (CK), deoxyuridine triphosphate nucleotidohydrolase (dUTPase), glutamate dehydrogenase (GDH), guanylate kinase (GK), N-myristoyltransferase (NMT), orotidine 5'-monophosphate decarboxylase (OMPDC), farnesyl pyrophosphate synthase (FPPS) and S-adenosylhomocysteine hydrolase (SAHH). These enzymes were selected using TDRtargets.org, and are believed to have excellent potential as drug targets based on criteria such as their likely essentiality, druggability, and amenability to high-throughput biochemical screening. Six of these targets were inhibited by one or more of the antimalarial scaffolds and may have potential use in drug development, further target validation studies and exploration of P. falciparum biochemistry and biology.

Glycerophospholipid acquisition in Plasmodium - a puzzling assembly of biosynthetic pathways

Throughout the Plasmodium life cycle, malaria parasites repeatedly undergo rapid cellular growth and prolific divisions, necessitating intense membrane neogenesis and, in particular, the acquisition of high amounts of phospholipids. At the intraerythrocytic stage, glycerophospholipids are the main parasite membrane constituents, which mostly originate from the Plasmodium-encoded enzymatic machinery. Several proteins and entire pathways have been characterized and their features reported, thereby generating a global view of glycerophospholipid synthesis across Plasmodium spp. The malaria parasite displays a panoply of pathways that are seldom found together in a single organism. The major glycerophospholipids are synthesized via ancestral prokaryotic CDP-diacylglycerol-dependent pathways and eukaryotic-type de novo pathways. The parasite exhibits additional reactions that bridge some of these routes and are otherwise restricted to some organisms, such as plants, while base-exchange mechanisms are largely unexplored in Plasmodium. Marked differences between Plasmodium spp. have also been reported in phosphatidylcholine and phosphatidylethanolamine synthesis. Little is currently known about glycerophospholipid acquisition at non-erythrocytic stages, but recent data reveal that intrahepatocytic parasites, oocysts and sporozoites import various host lipids, and that de novo fatty acid synthesis is only crucial at the late liver stage. More studies on the different Plasmodium developmental stages are needed, to further assemble the different pieces of this glycerophospholipid synthesis puzzle, which contains highly promising therapeutic targets.

The Kennedy phospholipid biosynthesis pathways are refractory to genetic disruption in Plasmodium berghei and therefore appear essential in blood stages

Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are the main membrane phospholipids (PLs) of Plasmodium parasites and can be generated by the de novo (Kennedy) CDP-choline and CDP-ethanolamine pathways and by the CDP-diacylglycerol dependent pathway. The Kennedy pathways initiate from exogenous choline and ethanolamine involving choline kinase (CK) and ethanolamine kinase (EK), followed by the choline-phosphate cytidylyltransferase (CCT) and ethanolamine-phosphate cytidylyltransferase (ECT) that catalyse the formation of CDP-choline and CDP-ethanolamine. Finally, in Plasmodium, PC and PE are apparently synthesized by a common choline/ethanolamine-phosphotransferase (CEPT). Here, we have studied the essential nature of the Kennedy pathways in Plasmodium berghei, a rodent malaria parasite. Sequence analysis of the P. berghei CEPT, CCT, ECT and CK enzymes revealed the presence of all catalytic domains and essential residues and motifs necessary for enzymatic activities. Constructs were designed for the generation of gene knockout and GFP-fusions of the cept, cct, ect and ck genes in P. berghei. We found that all four genes were consistently refractory to knockout attempts. At the same time, successful tagging of these proteins with GFP demonstrated that the loci were targetable and indicated that these genes are essential in P. berghei blood stage parasites. GFP-fusions of CCT, ECT and CK were found in the cytosol whereas the GFP-CEPT mainly localised in the endoplasmic reticulum. These results indicate that both CDP-choline and CDP-ethanolamine de novo pathways are essential for asexual P. berghei development and are non-redundant with other possible sources of PC and PE.

Exploring metabolomic approaches to analyse phospholipid biosynthetic pathways in Plasmodium

SUMMARYPlasmodium falciparum, the agent responsible for malaria, is an obligate intracellular protozoan parasite. For proliferation, differentiation and survival, it relies on its own protein-encoding genes, as well as its host cells for nutrient sources. Nutrients and subsequent metabolites are required by the parasites to support their high rate of growth and replication, particularly in the intra-erythrocytic stages of the parasite that are responsible for the clinical symptoms of the disease. Advances in mass spectrometry have improved the analysis of endogenous metabolites and enabled a global approach to identify the parasite's metabolites by the so-called metabolomic analyses. This level of analysis complements the genomic, transcriptomic and proteomic data already available and should allow the identification of novel metabolites, original pathways and networks of regulatory interactions within the parasite, and between the parasite and its hosts. The field of metabolomics is just in its infancy in P. falciparum, hence in this review, we concentrate on the available methodologies and their potential applications for deciphering important biochemical processes of the parasite, such as the astonishingly diverse phospholipid biosynthesis pathways. Elucidating the regulation of the biosynthesis of these crucial metabolites could help design of future anti-malarial drugs.

Comparison of the cellular and biochemical properties of Plasmodium falciparum choline and ethanolamine kinases

The proliferation of the malaria-causing parasite Plasmodium falciparum within the erythrocyte is concomitant with massive phosphatidylcholine and phosphatidylethanolamine biosynthesis. Based on pharmacological and genetic data, de novo biosynthesis pathways of both phospholipids appear to be essential for parasite survival. The present study characterizes PfCK (P. falciparum choline kinase) and PfEK (P. falciparum ethanolamine kinase), which catalyse the first enzymatic steps of these essential metabolic pathways. Recombinant PfCK and PfEK were expressed as His6-tagged fusion proteins from overexpressing Escherichia coli strains, then purified to homogeneity and characterized. Using murine polyclonal antibodies against recombinant kinases, PfCK and PfEK were shown to be localized within the parasite cytoplasm. Protein expression levels increased during erythrocytic development. PfCK and PfEK appeared to be specific to their respective substrates and followed Michaelis-Menten kinetics. The Km value of PfCK for choline was 135.3+/-15.5 microM. PfCK was also able to phosphorylate ethanolamine with a very low affinity. PfEK was found to be an ethanolamine-specific kinase (Km=475.7+/-80.2 microM for ethanolamine). The quaternary ammonium compound hemicholinium-3 and an ethanolamine analogue, 2-amino-1-butanol, selectively inhibited PfCK or PfEK. In contrast, the bis-thiazolium compound T3, which was designed as a choline analogue and is currently in clinical trials for antimalarial treatment, affected PfCK and PfEK activities similarly. Inhibition exerted by T3 was competitive for both PfCK and PfEK and correlated with the impairment of cellular phosphatidylcholine biosynthesis. Comparative analyses of sequences and structures for both kinase types gave insights into their specific inhibition profiles and into the dual capacity of T3 to inhibit both PfCK and PfEK.

Use of thermal melt curves to assess the quality of enzyme preparations

This study sought to determine whether the quality of enzyme preparations can be determined from their melting curves, which may easily be obtained using a fluorescent probe and a standard reverse transcription-polymerase chain reaction (RT-PCR) machine. Thermal melt data on 31 recombinant enzymes from Plasmodium parasites were acquired by incrementally heating them to 90 degrees C and measuring unfolding with a fluorescent dye. Activity assays specific to each enzyme were also performed. Four of the enzymes were denatured to varying degrees with heat and sodium dodecyl sulfate (SDS) prior to the thermal melt and activity assays. In general, melting curve quality was correlated with enzyme activity; enzymes with high-quality curves were found almost uniformly to be active, whereas those with lower quality curves were more varied in their catalytic performance. Inspection of melting curves of bovine xanthine oxidase and Entamoeba histolytica cysteine protease 1 allowed active stocks to be distinguished from inactive stocks, implying that a relationship between melting curve quality and activity persists over a wide range of experimental conditions and species. Our data suggest that melting curves can help to distinguish properly folded proteins from denatured ones and, therefore, may be useful in selecting stocks for further study and in optimizing purification procedures for specific proteins.

Immunomodulation by phosphocholine--biosynthesis, structures and immunological implications of parasitic PC-epitopes

Phosphocholine (PC) as a small haptenic molecule present on antigens of parasites can provoke various effects on immune cells leading to immunomodulation of the host's immune system. This immunomodulation not only allows long-term persistence but also prevents severe pathology due to down-regulation of cellular immune responses. Additionally, PC plays an important role for development and fertility of the parasites. To fully understand the mechanisms of immunomodulation the detailed knowledge of the biosynthesis of the PC-epitopes, their molecular structure and biological function has to be elucidated. The implication of parasite-specific transferases in the biosynthesis of the PC-epitopes and the sensitivity of parasites towards disruption of the choline metabolism offers new perspectives for the development of anti-parasitic drugs and therapies. Furthermore, the immunomodulation provoked by PC-epitopes preventing inflammatory reactions may be useful in the treatment of inflammatory diseases. This review summarizes the current knowledge on the biosynthesis of PC-epitopes, their structures and immunological implications.

Biochemical characterization of the initial steps of the Kennedy pathway in Trypanosoma brucei: the ethanolamine and choline kinases

Ethanolamine and choline are major components of the trypanosome membrane phospholipids, in the form of GPEtn (glycerophosphoethanolamine) and GPCho (glycerophosphocholine). Ethanolamine is also found as an integral component of the GPI (glycosylphosphatidylinositol) anchor that is required for membrane attachment of cell-surface proteins, most notably the variant-surface glycoproteins. The de novo synthesis of GPEtn and GPCho starts with the generation of phosphoethanolamine and phosphocholine by ethanolamine and choline kinases via the Kennedy pathway. Database mining revealed two putative C/EKs (choline/ethanolamine kinases) in the Trypanosoma brucei genome, which were cloned, overexpressed, purified and characterized. TbEK1 (T. brucei ethanolamine kinase 1) was shown to be catalytically active as an ethanolamine-specific kinase, i.e. it had no choline kinase activity. The K_m values for ethanolamine and ATP were found to be 18.4±0.9 and 219±29 μM respectively. TbC/EK2 (T. brucei choline/ethanolamine kinase 2), on the other hand, was found to be able to phosphorylate both ethanolamine and choline, even though choline was the preferred substrate, with a K_m 80 times lower than that of ethanolamine. The K_m values for choline, ethanolamine and ATP were 31.4±2.6 μM, 2.56±0.31 mM and 20.6±1.96 μM respectively. Further substrate specificity analysis revealed that both TbEK1 and TbC/EK2 were able to tolerate various modifications at the amino group, with the exception of a quaternary amine for TbEK1 (choline) and a primary amine for TbC/EK2 (ethanolamine). Both enzymes recognized analogues with substituents on C-2, but substitutions on C-1 and elongations of the carbon chain were not well tolerated.

Bilirubin inhibits Plasmodium falciparum growth through the generation of reactive oxygen species

Free heme is very toxic because it generates highly reactive hydroxyl radicals ((.)OH) to cause oxidative damage. Detoxification of free heme by the heme oxygenase (HO) system is a very common phenomenon by which free heme is catabolized to form bilirubin as an end product. Interestingly, the malaria parasite, Plasmodium falciparum, lacks an HO system, but it forms hemozoin, mainly to detoxify free heme. Here, we report that bilirubin significantly induces oxidative stress in the parasite as evident from the increased formation of lipid peroxide, decrease in glutathione content, and increased formation of H(2)O(2) and (.)OH. Bilirubin can effectively inhibit hemozoin formation also. Furthermore, results indicate that bilirubin inhibits parasite growth and induces caspase-like protease activity, up-regulates the expression of apoptosis-related protein (Gene ID PFI0450c), and reduces the mitochondrial membrane potential. (.)OH scavengers such as mannitol, as well as the spin trap alpha-phenyl-n-tert-butylnitrone, effectively protect the parasite from bilirubin-induced oxidative stress and growth inhibition. These findings suggest that bilirubin, through the development of oxidative stress, induces P. falciparum cell death and that the malaria parasite lacks an HO system probably to protect itself from bilirubin-induced cell death as a second line of defense.

Overexpression, purification and localization of apoptosis related protein from Plasmodium falciparum

A growing body of evidence has ascertained that apoptosis is not only restricted to metazoans but also exists in unicellular parasites. In Plasmodium falciparum, the presence of a putative gene having sequence homology with apoptosis related protein (PfARP) (Gene ID PFI0450c) has raised enormous interest to unravel the function of this unique protein in cell death of malaria parasite. To characterize this protein, the PfARP gene has been amplified from the P. falciparum transcriptome by RT-PCR and the amplified gene has been successfully cloned, over-expressed and purified to homogeneity. The purified PfARP exhibits minimum subunit MW of approximately 24kDa as evident from SDS-PAGE. CD analysis reveals that the alpha and beta content of the recombinant PfARP are 61% and 15%, respectively. Semiquantitative RT-PCR analysis indicates the expression of PfARP at both metabolically less active ring and highly active trophozoite stages of malaria parasite. Immunofluorescence microscopy further supports that PfARP expresses stage specifically with the highest expression at trophozite stage and very little in the schizont stage. PfARP is a cytosolic protein as evident from immunofluorescence microscopy. The role of this protein in P. falciparum cell death and stage progression is not yet known. The identification, purification and characterization would certainly be a step to initiate work on this protein to evaluate its role in P. falciparum growth, multiplication and stage progression.

Inhibition of Plasmodium falciparum choline kinase by hexadecyltrimethylammonium bromide: a possible antimalarial mechanism

Choline kinase is the first enzyme in the Kennedy pathway (CDP-choline pathway) for the biosynthesis of the most essential phospholipid, phosphatidylcholine, in Plasmodium falciparum. In addition, choline kinase also plays a pivotal role in trapping essential polar head group choline inside the malaria parasite. Recently, Plasmodium falciparum choline kinase (PfCK) has been cloned, overexpressed, and purified. However, the function of this enzyme in parasite growth and survival has not been evaluated owing to the lack of a suitable inhibitor. Purified recombinant PfCK enabled us to identify an inhibitor of PfCK, hexadecyltrimethylammonium bromide (HDTAB), which has a very close structural resemblance to hexadecylphosphocholine (miltefosin), the well-known antiproliferative and antileishmanial drug. HDTAB inhibited PfCK in a dose-dependent manner and offered very potent antimalarial activity in vitro against Plasmodium falciparum. Moreover, HDTAB exhibited profound antimalarial activity in vivo against the rodent malaria parasite Plasmodium yoelii (N-67 strain). Interestingly, parasites at the trophozoite and schizont stages were found to be particularly sensitive to HDTAB. The stage-specific antimalarial effect of HDTAB correlated well with the expression pattern of PfCK in P. falciparum, which was observed by reverse transcription-PCR and immunofluorescence microscopy. Furthermore, the antimalarial activity of HDTAB paralleled the decrease in phosphatidylcholine content, which was found to correlate with the decreased phosphocholine generation. These results suggest that inhibition of choline kinase by HDTAB leads to decreased phosphocholine, which in turn causes a decrease in phosphatidylcholine biosynthesis, resulting in death of the parasite.