Photoaffinity labeling of chloroquine-binding proteins in Plasmodium falciparum

Disruption of Plasmodium falciparum kinetochore proteins destabilises the nexus between the centrosome equivalent and the mitotic apparatus

Plasmodium falciparum is the causative agent of malaria and remains a pathogen of global importance. Asexual blood stage replication, via a process called schizogony, is an important target for the development of new antimalarials. Here we use ultrastructure-expansion microscopy to probe the organisation of the chromosome-capturing kinetochores in relation to the mitotic spindle, the centriolar plaque, the centromeres and the apical organelles during schizont development. Conditional disruption of the kinetochore components, PfNDC80 and PfNuf2, is associated with aberrant mitotic spindle organisation, disruption of the centromere marker, CENH3 and impaired karyokinesis. Surprisingly, kinetochore disruption also leads to disengagement of the centrosome equivalent from the nuclear envelope. Severing the connection between the nucleus and the apical complex leads to the formation of merozoites lacking nuclei. Here, we show that correct assembly of the kinetochore/spindle complex plays a previously unrecognised role in positioning the nascent apical complex in developing P. falciparum merozoites.

Synthesis and Photolysis Properties of a New Chloroquine Photoaffinity Probe

A new chloroquine-derived photoaffinity probe has been prepared by a convergent synthesis from derivative of 4,7-dichloroquinoline and N1,N1-diethyl-N4-methylpentane. The features of this probe are a unique 3-azido photolabel, the pyridine ring of the quinoline, and the presence of a secondary amine at the 4-position of the quinoline. These features, particularly the 4-amino methylation, prevent triazole formation through combination of the 3-azide and the 4-amine. This undergoes facile cleavage with exposure to a medium-pressure mercury lamp with a 254 nm excitation wavelength. Trapping of the nitrene byproduct is accomplished with its reaction with N-phenylmaleimide as its cycloazidation product. The structure of a ring-opened DBU amine has been structurally characterized.

Anti-malarial activity of the alkaloid, heptaphylline, and the furanocoumarin, imperatorin, from Clausena anisata against human Plasmodium falciparum malaria parasites: ex vivo trophozoitocidal, schizonticidal and gametocytocidal approach

BACKGROUND

The erythrocytic stage of the life cycle of the malaria parasite, Plasmodium falciparum, consists of trophozoite, schizont and gametocyte stages in humans. Various anti-malarial agents target different stages of the parasite to produce treatment outcomes. This study reports on the stage-specific anti-malarial activity of heptaphylline and imperatorin against human P. falciparum in addition to their cytotoxicity and selectivity indices (SI).

METHODS

The compounds were isolated from Clausena anisata using column chromatography and their structures elucidated using NMR spectroscopy. The anti-malarial activity was determined by measuring the trophozoitocidal, schizonticidal and gametocytocidal activities of the compounds using the SYBR green assay. Cytotoxicity was evaluated using the tetrazolium-based colorimetric assay.

RESULTS

Heptaphylline and imperatorin produced trophozoitocidal, schizonticidal and gametocytocidal activities with IC50s of 1.57 (0.2317)-26.92 (0.3144) µM with those of artesunate (the standard drug) being 0.00024 (0.0036)-0.0070 (0.0013) µM. In the cytotoxicity assay, the compounds produced CC50S greater than 350 µM and SI of 13.76-235.90. Also, the trophozoitocidal and schizonticidal activities of the compounds were more pronounced than their gametocytocidal activity. Imperatorin was 42.04% more trophozoitocidal than hepthaphyline. However, hepthaphyline has more schizonticidal and gametocytocidal properties than imperatorin.

CONCLUSION

Heptaphylline and imperatorin are promising anti-malarial agents, since they possess potent anti-malarial activity with weak cytotoxicity on RBCs. However, imperatorin is a better anti-malarial prophylactic agent whereas heptaphylline is a better malaria treatment agent.

Repurposing the mitotic machinery to drive cellular elongation and chromatin reorganisation in Plasmodium falciparum gametocytes

The sexual stage gametocytes of the malaria parasite, Plasmodium falciparum, adopt a falciform (crescent) shape driven by the assembly of a network of microtubules anchored to a cisternal inner membrane complex (IMC). Using 3D electron microscopy, we show that a non-mitotic microtubule organizing center (MTOC), embedded in the parasite's nuclear membrane, orients the endoplasmic reticulum and the nascent IMC and seeds cytoplasmic microtubules. A bundle of microtubules extends into the nuclear lumen, elongating the nuclear envelope and capturing the chromatin. Classical mitotic machinery components, including centriolar plaque proteins, Pfcentrin-1 and -4, microtubule-associated protein, End-binding protein-1, kinetochore protein, PfNDC80 and centromere-associated protein, PfCENH3, are involved in the nuclear microtubule assembly/disassembly process. Depolymerisation of the microtubules using trifluralin prevents elongation and disrupts the chromatin, centromere and kinetochore organisation. We show that the unusual non-mitotic hemispindle plays a central role in chromatin organisation, IMC positioning and subpellicular microtubule formation in gametocytes.

Identification of antimalarial targets of chloroquine by a combined deconvolution strategy of ABPP and MS-CETSA

BACKGROUND

Malaria is a devastating infectious disease that disproportionally threatens hundreds of millions of people in developing countries. In the history of anti-malaria campaign, chloroquine (CQ) has played an indispensable role, however, its mechanism of action (MoA) is not fully understood.

METHODS

We used the principle of photo-affinity labeling and click chemistry-based functionalization in the design of a CQ probe and developed a combined deconvolution strategy of activity-based protein profiling (ABPP) and mass spectrometry-coupled cellular thermal shift assay (MS-CETSA) that identified the protein targets of CQ in an unbiased manner in this study. The interactions between CQ and these identified potential protein hits were confirmed by biophysical and enzymatic assays.

RESULTS

We developed a novel clickable, photo-affinity chloroquine analog probe (CQP) which retains the antimalarial activity in the nanomole range, and identified a total of 40 proteins that specifically interacted and photo-crosslinked with CQP which was inhibited in the presence of excess CQ. Using MS-CETSA, we identified 83 candidate interacting proteins out of a total of 3375 measured parasite proteins. At the same time, we identified 8 proteins as the most potential hits which were commonly identified by both methods.

CONCLUSIONS

We found that CQ could disrupt glycolysis and energy metabolism of malarial parasites through direct binding with some of the key enzymes, a new mechanism that is different from its well-known inhibitory effect of hemozoin formation. This is the first report of identifying CQ antimalarial targets by a parallel usage of labeled (ABPP) and label-free (MS-CETSA) methods.

Chemical Biology Tools To Investigate Malaria Parasites

Parasitic diseases like malaria tropica have been shaping human evolution and history since the beginning of mankind. After infection, the response of the human host ranges from asymptomatic to severe and may culminate in death. Therefore, proper examination of the parasite's biology is pivotal to deciphering unique molecular, biochemical and cell biological processes, which in turn ensure the identification of treatment strategies, such as potent drug targets and vaccine candidates. However, implementing molecular biology methods for genetic manipulation proves to be difficult for many parasite model organisms. The development of fast and straightforward applicable alternatives, for instance small-molecule probes from the field of chemical biology, is essential. In this review, we will recapitulate the highlights of previous molecular and chemical biology approaches that have already created insight and understanding of the malaria parasite Plasmodium falciparum. We discuss current developments from the field of chemical biology and explore how their application could advance research into this parasite in the future. We anticipate that the described approaches will help to close knowledge gaps in the biology of P. falciparum and we hope that researchers will be inspired to use these methods to gain knowledge - with the aim of ending this devastating disease.

Chemical Proteomics and Super-resolution Imaging Reveal That Chloroquine Interacts with Plasmodium falciparum Multidrug Resistance-Associated Protein and Lipids

It is well established that chloroquine, a quinoline antimalarial, inhibits hemozoin formation in the malaria parasite. Counterintuitively, this archetypal antimalarial is also used in the treatment of diseases in which hemozoin biocrystallization does not play a role. Hence, we decided to investigate whether chloroquine possesses binding targets other than Fe(III) protoporphyrin IX in blood stage Plasmodium falciparum parasites and whether these are related to sites of accumulation within the parasite other than the digestive vacuole. A 7-nitrobenz-2-oxa-1,3-diazole (NBD)-labeled fluorescent derivative of chloroquine, especially sensitive to regions outside the digestive vacuole and retaining the antiplasmodial pharmacophore, was synthesized to investigate subcellular localization in the parasite. Super-resolution microscopy revealed association with membranes including the parasite plasma membrane, the endoplasmic reticulum, and possibly also the mitochondrion. A drug-labeled affinity matrix was then prepared to capture protein binding targets of chloroquine. SDS-PAGE revealed a single prominent band between 200 and 250 kDa from the membrane-associated fraction. Subsequent proteomic analysis revealed that this band corresponded to P. falciparum multidrug resistance-associated protein (PfMRP1). Intrigued by this finding, we demonstrated pull-down of PfMRP1 by matrices labeled with Cinchona alkaloids quinine and quinidine. While PfMRP1 has been implicated in resistance to quinolines and other antimalarials, this is the first time that these drugs have been found to bind directly to this protein. Based on previous reports, PfMRP1, the only prominent protein found to bind to quinolines in this work, is likely to modulate the activity of these antimalarials in P. falciparum rather than act as a drug target.

Comparison of the Exposure Time Dependence of the Activities of Synthetic Ozonide Antimalarials and Dihydroartemisinin against K13 Wild-Type and Mutant Plasmodium falciparum Strains

Fully synthetic endoperoxide antimalarials, namely, OZ277 (RBx11160; also known as arterolane) and OZ439 (artefenomel), have been approved for marketing or are currently in clinical development. We undertook an analysis of the kinetics of the in vitro responses of Plasmodium falciparum to the new ozonide antimalarials. For these studies we used a K13 mutant (artemisinin resistant) isolate from a region in Cambodia and a genetically matched (artemisinin sensitive) K13 revertant. We used a pulsed-exposure assay format to interrogate the time dependence of the response. Because the ozonides have physicochemical properties different from those of the artemisinins, assay optimization was required to ensure that the drugs were completely removed following the pulsed exposure. Like that of artemisinins, ozonide activity requires active hemoglobin degradation. Short pulses of the ozonides were less effective than short pulses of dihydroartemisinin; however, when early-ring-stage parasites were exposed to drugs for periods relevant to their in vivo exposure, the ozonide antimalarials were markedly more effective.

Red blood cells polarize green laser light revealing hemoglobin's enhanced non-fundamental Raman modes

In general, the first overtone modes produce weak bands that appear at approximately twice the wavenumber value of the fundamental transitions in vibrational spectra. Here, we report the existence of a series of enhanced non-fundamental bands in resonance Raman (RR) spectra recorded for hemoglobin (Hb) inside the highly concentrated heme environment of the red blood cell (RBC) by exciting with a 514.5 nm laser line. Such bands are most intense when detecting parallel-polarized light. The enhancement is explained through excitonic theory invoking a type C scattering mechanism and bands have been assigned to overtone and combination bands based on symmetry arguments and polarization measurements. By using malaria diagnosis as an example, we demonstrate that combining the non-fundamental and fundamental regions of the RR spectrum improves the sensitivity and diagnostic capability of the technique. The discovery will have considerable implications for the ongoing development of Raman spectroscopy for blood disease diagnoses and monitoring heme perturbation in response to environmental stimuli.

Detection and quantification of early-stage malaria parasites in laboratory infected erythrocytes by attenuated total reflectance infrared spectroscopy and multivariate analysis

New diagnostic modalities for malaria must have high sensitivity and be affordable to the developing world. We report on a method to rapidly detect and quantify different stages of malaria parasites, including ring and gametocyte forms, using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FT-IR) and partial least-squares regression (PLS). The absolute detection limit was found to be 0.00001% parasitemia (<1 parasite/μL of blood; p < 0.008) for cultured early ring stage parasites in a suspension of normal erythrocytes. Future development of universal and robust calibration models can significantly improve malaria diagnoses, leading to earlier detection and treatment of this devastating disease.

Spatial and temporal mapping of the PfEMP1 export pathway in Plasmodium falciparum

The human malaria parasite, Plasmodium falciparum, modifies the red blood cells (RBCs) that it infects by exporting proteins to the host cell. One key virulence protein, P. falciparum Erythrocyte Membrane Protein-1 (PfEMP1), is trafficked to the surface of the infected RBC, where it mediates adhesion to the vascular endothelium. We have investigated the organization and development of the exomembrane system that is used for PfEMP1 trafficking. Maurer's cleft cisternae are formed early after invasion and proteins are delivered to these (initially mobile) structures in a temporally staggered and spatially segregated manner. Membrane-Associated Histidine-Rich Protein-2 (MAHRP2)-containing tether-like structures are generated as early as 4 h post invasion and become attached to Maurer's clefts. The tether/Maurer's cleft complex docks onto the RBC membrane at ~20 h post invasion via a process that is not affected by cytochalasin D treatment. We have examined the trafficking of a GFP chimera of PfEMP1 expressed in transfected parasites. PfEMP1B-GFP accumulates near the parasite surface, within membranous structures exhibiting a defined ultrastructure, before being transferred to pre-formed mobile Maurer's clefts. Endogenous PfEMP1 and PfEMP1B-GFP are associated with Electron-Dense Vesicles that may be responsible for trafficking PfEMP1 from the Maurer's clefts to the RBC membrane.

Origin, composition, organization and function of the inner membrane complex of Plasmodium falciparum gametocytes

The most virulent of the human malaria parasites, Plasmodium falciparum, undergoes a remarkable morphological transformation as it prepares itself for sexual reproduction and transmission via mosquitoes. Indeed P. falciparum is named for the unique falciform or crescent shape of the mature sexual stages. Once the metamorphosis is completed the mature gametocyte releases from sequestration sites and enters the circulation making it accessible to feeding mosquitoes. Early ultrastructural studies showed that gametocyte elongation is driven by the assembly of a system of flattened cisternal membrane compartments underneath the parasite plasma membrane and a supporting network of microtubules. Here we describe the molecular composition and origin of the sub-pellicular membrane complex, and show that it is analogous to the inner membrane complex, an organelle with structural and motor functions that is well conserved across the apicomplexa. We identify novel cross-linking elements that may help stabilize the inner membrane complex during gametocyte development. We show that changes in gametocyte morphology are associated with an increase in cellular deformability and postulate that this enables the gametocytes to circulate in the blood stream without being detected and removed by the mechanical filtering mechanisms in the host's spleen.

Genetic ablation of a Maurer's cleft protein prevents assembly of the Plasmodium falciparum virulence complex

The malaria parasite Plasmodium falciparum assembles knob structures underneath the erythrocyte membrane that help present the major virulence protein, P. falciparum erythrocyte membrane protein-1 (PfEMP1). Membranous structures called Maurer's clefts are established in the erythrocyte cytoplasm and function as sorting compartments for proteins en route to the RBC membrane, including the knob-associated histidine-rich protein (KAHRP), and PfEMP1. We have generated mutants in which the Maurer's cleft protein, the ring exported protein-1 (REX1) is truncated or deleted. Removal of the C-terminal domain of REX1 compromises Maurer's cleft architecture and PfEMP1-mediated cytoadherance but permits some trafficking of PfEMP1 to the erythrocyte surface. Deletion of the coiled-coil region of REX1 ablates PfEMP1 surface display, trapping PfEMP1 at the Maurer's clefts. Complementation of mutants with REX1 partly restores PfEMP1-mediated binding to the endothelial cell ligand, CD36. Deletion of the coiled-coil region or complete deletion of REX1 is tightly associated with the loss of a subtelomeric region of chromosome 2, encoding KAHRP and other proteins. A KAHRP-green fluorescent protein (GFP) fusion expressed in the REX1-deletion parasites shows defective trafficking. Thus, loss of functional REX1 directly or indirectly ablates the assembly of the P. falciparum virulence complex at the surface of host erythrocytes.

NAD(+)-dependent DNA ligase: a novel target waiting for the right inhibitor

DNA ligases (EC.6.5.1.1) are key enzymes that catalyze the formation of phosphodiester bonds at single stranded or double stranded breaks between adjacent 5' phosphoryl and 3' hydroxyl groups of DNA. These enzymes are important for survival because they are involved in major cellular processes like DNA replication/repair and recombination. DNA ligases can be classified into two groups on the basis of their cofactor specificities. NAD(+)-dependent DNA ligases are present in bacteria, some entomopox viruses and mimi virus while ATP-dependent DNA ligases are ubiquitous. The former have recently been drawing a lot of attention as novel targets for antibiotics to overcome current drug resistance issues. Currently a diverse range of inhibitors have been identified. There are several issues to be addressed in the quest for optimized inhibitors of the enzyme. In the first part of the review we summarize current structural work on these enzymes. Subsequently we describe the currently available classes of inhibitors. We also address modalities to improve the specificity and potencies of new inhibitors identified using protein structure based rational approaches. In conclusion, NAD(+)-dependent ligases show great promise and represent a novel drug target whose time has come.

Photoaffinity labeling of the Plasmodium falciparum chloroquine resistance transporter with a novel perfluorophenylazido chloroquine

Several models describing how amino acid substitutions in the Plasmodium falciparum chloroquine resistance transporter (PfCRT) confer resistance to chloroquine (CQ) and other antimalarial drugs have been proposed. Further progress requires molecular analysis of interactions between purified reconstituted PfCRT protein and these drugs. We have thus designed and synthesized several perfluorophenyl azido (pfpa) CQ analogues for PfCRT photolabeling studies. One particularly useful probe (AzBCQ) places the pfpa group at the terminal aliphatic N of CQ via a flexible four-carbon ester linker and includes a convenient biotin tag. This probe photolabels PfCRT in situ with high specificity. Using reconstituted proteoliposomes harboring partially purified recombinant PfCRT, we analyze AzBCQ photolabeling versus competition with CQ and other drugs to probe the nature of the CQ binding site. We also inspect how pH, the chemoreversal agent verapamil (VPL), and various amino acid mutations in PfCRT that cause CQ resistance (CQR) affect the efficiency of AzBCQ photolabeling. Upon gel isolation of AzBCQ-labeled PfCRT followed by trypsin digestion and mass spectrometry analysis, we are able to define a single AzBCQ covalent attachment site lying within the digestive vacuolar-disposed loop between putative helices 9 and 10 of PfCRT. Taken together, the data provide important new insight into PfCRT function and, along with previous results, allow us to propose a model for a single CQ binding site in the PfCRT protein.

ATPase activity of purified plasma membranes and digestive vacuoles from Plasmodium falciparum

The ATPase activity of the human malaria parasite, Plasmodium falciparum was investigated using two experimental systems, (i) digestive vacuoles, and (ii) purified plasma membranes isolated from a chloroquine-sensitive and a chloroquine-resistant strain. No correlation between the level of ATPase activity and chloroquine sensitivity could be detected. In both systems, the ATPase activity of the chloroquine-resistant and -sensitive strain was decreased in the presence of the P-glycoprotein inhibitor vanadate. Susceptibility to inhibition by vanadate together with the lack of effect of ouabain implies a P-type ATPase activity in the plasma membrane. Furthermore, the inhibition of Fac8 ATPase activity by oligomycin both in the digestive vacuoles and the plasma membranes would be consistent with higher levels of Pgh1 in Fac8. Our data are consistent with the presence of a V-type H+-ATPase in the parasite food vacuole. Bafilomycin A1 and N-ethylmaleimide decreased the vacuolar ATPase activity in both chloroquine-resistant and -sensitive strains. Interestingly, a 30% decrease was observed between the ATPase activity of plasma membranes isolated from Fac8 and D10 in the presence of bafilomycin A1, suggesting the presence of a V-type ATPase in D10 plasma membrane that is underexpressed or altered in the plasma membrane of the chloroquine-resistant Fac8. The chemosensitisers tested had no effect on the ATPase activity of chloroquine-resistant P. falciparum in both systems suggesting that their activity is not mediated through an ATP-dependent mechanism. No effect was observed on the vacuolar ATPase activity in the presence of the antimalarials tested indicating that an ATP-dependent transport has not been activated.

Kinetics of inhibition of glutathione-mediated degradation of ferriprotoporphyrin IX by antimalarial drugs

We have shown previously that chloroquine and amodiaquine inhibit the glutathione-dependent degradation of ferriprotoporphyrin IX (FP). We have also demonstrated that treatment of human erythrocytes infected with Plasmodium falciparum with chloroquine or amodiaquine results in a dose- and time-dependent accumulation of FP in the membrane fraction of these cells in correlation with parasite killing. High levels of membrane FP are known to perturb the barrier properties of cellular membranes, and could thereby irreversibly disturb the ion homeostasis of the parasite and cause parasite death. We here report on the effect of various 4-aminoquinolines, as well as pyronaridine, halofantrine and some bis-quinolines, on glutathione-mediated destruction of FP in aqueous solution, when FP was bound non-specifically to a protein, and when it was dissolved in human erythrocyte ghost membranes. We showed that all drugs were capable of inhibiting FP degradation in solution. The inhibitory efficacy of some drugs declined when FP was bound non-specifically to protein. Quinine and mefloquine were unable to inhibit the degradation of membrane-associated FP, in line with their inability to increase membrane-associated FP levels in malaria-infected cells following drug treatment. The discrepancy between chloroquine and amodiaquine on the one hand, and quinine and mefloquine on the other, is discussed in terms of the particular location of drugs and FP in the phospholipid membrane, and may suggest differences in the mechanistic details of the antimalarial action of these drugs.

Chloroquine - some open questions on its antimalarial mode of action and resistance

During the digestion of its host cell hemoglobin, large amounts of toxic ferriprotoporphyrin IX (FPIX) are generated in the intraerythrocytic malaria parasite. FPIX is detoxified either by being polymerized into hemozoin inside the food vacuole, or through its degradation by glutathione in the cytosol. Chloroquine is able to complex with FPIX, thus inhibiting both processes and thereby generating receptors for its own uptake. These leads to the accumulation of FPIX in the membrane fraction of infected cells that results in membrane permeabilization and disruption of cation homeostasis and concluded in parasite death. Several unresolved questions, such as the site of FPIX:chloroquine complex formation, the role of pH gradient in drug accumulation and resistance, the role of Pgh-1 in resistance, the mode of action of reversers and the involvement of proteins and their mutants in resistance, are discussed. Copyright 1999 Harcourt Publishers Ltd.

A comparison and analysis of several ways to promote haematin (haem) polymerisation and an assessment of its initiation in vitro

We compared several methods for producing haematin polymerisation at physiological temperatures (i.e., 37 degrees) and found that a trophozoite lysate-mediated reaction was inappropriate for measuring compound inhibition of haematin polymerisation. Using this method, we obtained significantly higher IC50 values (concentration inhibiting haematin polymerisation by 50%) for certain compounds than when other methods were used, including a food vacuole lysate-mediated reaction. This difference was probably due to the binding of these compounds to cytosolic parasite proteins, as proteinase K treatment of the trophozoite lysate reversed this effect. The initiation of haematin polymerisation was also investigated using several assays. It was found that haematin polymerisation occurred spontaneously, in the absence of preformed haemozoin, over a period of several days, but that the process was more rapid when an acetonitrile extract of malarial trophozoites was added. This extract contained no detectable protein, and its activity could be replicated using an extract from uninfected erythrocytes and by using lipids. We therefore postulate that no protein or parasite-specific material is absolutely required for the initiation of haematin polymerisation. The formation of beta-haematin de novo using the acetonitrile extract is more pH-dependent than the generation of newly synthesised beta-haematin from preformed haemozoin and cannot proceed much above pH = 6. We postulate that the initiation of haematin polymerisation is more sensitive to the equilibrium of haematin between its monomeric and mu-oxo dimer form and requires a higher concentration of monomer than for the elongation phase of polymerisation.

Synthetic peptides corresponding to a repetitive sequence of malarial histidine rich protein bind haem and inhibit haemozoin formation in vitro

Synthetic peptides containing a repetitive hexapeptide sequence (Ala-His-His-Ala-Ala-Asp) of malarial histidine-rich protein II were evaluated for binding with haem in vitro. The pattern of haem binding suggested that each repeat unit of this sequence provides one binding site for haem. Chloroquine inhibited the haem-peptide complex formation with preferential formation of a haem chloroquine complex. In vitro studies on haem polymerisation showed that none of the peptides could initiate haemozoin formation. However, they could inhibit haemozoin formation promoted by a malarial parasite extract, possibly by competitively binding free haem. These results indicate this hexapeptide sequence represents the haem binding site of the malarial histidine-rich protein and possibly the site of nucleation for haem polymerisation.

Active efflux by multidrug transporters as one of the strategies to evade chemotherapy and novel practical implications of yeast pleiotropic drug resistance

Mankind is faced by the increasing emergence of resistant pathogens, including cancer cells. An overview of the different strategies adopted by a variety of cells to evade chemotherapy is presented, with a focus on the mechanisms of multidrug transport. In particular, we analyze the yeast network for pleiotropic drug resistance and assess the potentiality of this system for further understanding of the mechanism of broad specificity and for development of novel practical applications.

The antimalarial drug, chloroquine, interacts with lactate dehydrogenase from Plasmodium falciparum

We have previously shown that a radioiodinated photoreactive analogue of chloroquine, [125I]N-(4-(4-diethylamino-1-methylbutylamino)quinolin-6-yl) -4-azido-2-hydroxybenzamide ([125I]ASA-Q), specifically labels two proteins in Plasmodium falciparum with apparent molecular weights (Mr) of 42 and 33 kDa (Foley M, Deady LW, Ng K, Cowman AF, Tilley L. J Biol Chem 1994:269:6955-6961). We now report the identification of the 33 kDa protein. The 33 kDa protein was purified from Plasmodium falciparum using photoaffinity labeling with [125I]ASA-Q to monitor the enrichment process. N-terminal sequence analysis of the purified protein revealed exact identity of the first 35 amino acids with P. falciparum lactate dehydrogenase (PfLDH). The plasmodial enzyme was cloned and expressed in E. coli and the recombinant protein used to produce a rabbit antiserum. Immunoprecipitation using affinity-purified anti-PfLDH antibodies confirmed the identity of the 33 kDa CQ-binding protein. The enzyme activity of purified PfLDH was not significantly affected by chloroquine indicating that PfLDH is not a direct target of CQ. PfLDH was, however, shown to be exquisitely sensitive to inhibition by free heme and chloroquine protected against this inhibitory effect.

Quinoline antimalarials: mechanisms of action and resistance

The quinoline-containing antimalarial drugs, chloroquine, quinine and mefloquine, are a vital part of our chemotherapeutic armoury against malaria. These drugs are thought to act by interfering with the digestion of haemoglobin in the blood stages of the malaria life cycle. Chloroquine is a dibasic drug which diffuses down the pH gradient to accumulate about a 1000-fold in the acidic vacuole of the parasite. The high intravacuolar concentration of chloroquine is proposed to inhibit the polymerisation of haem. As a result, the haem which is released during haemoglobin breakdown builds up to poisonous levels, thereby killing the parasite with its own toxic waste. The more lipophilic quinolinemethanol drugs, mefloquine and quinine, are not concentrated so extensively in the food vacuole and probably have alternative sites of action. The technique of photoaffinity labelling has been used to identify a series of proteins which interact specifically with mefloquine. These studies have led us to speculate that the quinolinemethanols bind to high density lipoproteins in the serum and are delivered to the erythrocytes where they interact with an erythrocyte membrane protein, known as stomatin, and are then transferred to the intracellular parasite via a pathway used for the uptake of exogenous phospholipid. The final target(s) of quinine and mefloquine action are not yet fully characterised, but may include parasite proteins with apparent molecular weights of 22 kDa and 36 kDa. As resistance to the quinoline antimalarials rises inexorably, there is an urgent need to understand the molecular basis for decreased drug sensitivity. A parasite-encoded homologue of P-glycoprotein has been implicated in the development of drug resistance, possibly by controlling the level of accumulation of the quinoline-containing drugs. As our molecular understanding of these processes increases, it should be possible to design novel antimalarial strategies which circumvent the problem of drug resistance.

Photoaffinity labeling of mefloquine-binding proteins in human serum, uninfected erythrocytes and Plasmodium falciparum-infected erythrocytes

A photoreactive quinolinemethanol analog, N-[4-[1-hydroxy-2-(dibutylamino)ethyl]quinolin-8yl]-4- azido-2-salicylamide (ASA-MQ) has been synthesized which closely mimics the action of mefloquine. ASA-MQ possesses potent antimalarial activity against a mefloquine-sensitive strain of Plasmodium falciparum and shows decreased activity against a mefloquine-resistant parasite strain. Radioiodinated ASA-MQ has been used in photoaffinity labeling studies to identify mefloquine-interacting proteins in serum, uninfected erythrocytes and Plasmodium falciparum-infected erythrocytes. We have shown that mefloquine interacts specifically with apo-A1, the major protein of serum high density lipoproteins. In addition, mefloquine was shown to interact specifically with the erythrocyte membrane protein, band 7.2b (stomatin). A further two high affinity mefloquine-binding proteins with apparent molecular masses of 22 and 36 kDa were identified in three different strains of Plasmodium falciparum. We suggest that these two mefloquine-binding parasite proteins may be involved in the uptake of mefloquine or may represent macromolecular targets of mefloquine action in malaria parasites.

Amodiaquine accumulation in Plasmodium falciparum as a possible explanation for its superior antimalarial activity over chloroquine

Amodiaquine is a 4-aminoquinoline antimalarial whose structure is similar to chloroquine. In contrast to the wealth of information available about chloroquine accumulation and its relationship to activity, little is known about the uptake characteristics of amodiaquine, a drug that is inherently more active against malaria parasites. In this study we have investigated the accumulation of amodiaquine in Plasmodium falciparum in vitro, in order to gain an insight into the mechanisms responsible for its superior activity over chloroquine. The driving force for parasite accumulation of the 4-aminoquinolines is proposed to be a transmembrane proton gradient maintained by a vacuolar ATPase. In the present study, amodiaquine accumulation was greatly reduced, at steady state, in the absence of glucose and at 0 degrees C indicating a clear energy dependence of uptake. Amodiaquine accumulation in Plasmodium falciparum was shown to be 2- to 3-fold greater than chloroquine accumulation. This observation probably accounts for amodiaquine's greater inherent activity but is surprising given that amodiaquine is a weaker base than chloroquine. With this in mind we present evidence for an intraparasitic binding component in the accumulation of the 4-aminoquinolines. Differences in binding affinity of this 'receptor' for amodiaquine and chloroquine may partially explain the greater accumulation and in vitro potency of amodiaquine compared to chloroquine.

Is haemozoin a target for antimalarial drugs?

Malarial parasites utilize a unique pathway to degrade haemoglobin. Most of the haem which is released during haemoglobin degradation is incorporated into haemozoin. The mechanism of haemozoin synthesis and the structure of haemozoin are controversial. Neither chloroquine nor artemisinin appear to affect haemozoin synthesis in vivo. Artemisinin is activated by intraparasitic haem and iron into a free radical which then alkylates specific malarial proteins.

The ATP-binding cassette (ABC) gene family of Plasmodium falciparum

Multidrug resistance (MDR) in mammalian tumour cells is mediated by P-glycoproteins. The apparent similarities between MDR and the chloroquine-resistance phenotype (CQR) in Plasmodium falciparum suggests that homologous proteins may be involved. In mammals, P-glycoproteins are encoded by mdr genes that are a subset of a super-family characterized by ATP-binding cassettes (ABC). Three genes, pfmdr1, pfmdr2 and pfef3-rl, have been identified in P. falciparum that have homology to the ABC transporter gene family. Each protein encoded by these genes has a distinct structure, suggesting functional differences between the three. Justin Rubio and Alan Cowman here discuss the structure and possible function of the ABC proteins from P. falciparum and evidence that the protein encoded by the pfmdr1 gene can influence quinoline-containing antimalarial drug-resistance phenotypes.

Praziquantel: An urgent and exciting challenge

The anthelmintic drug praziquantel has proved useful in the treatment of schistosomiasis. The precise mechanism by which praziquantel kills the parasites has yet to be elucidated. Here, John Kusel and colleagues review the current theories on praziquantel action and suggest future avenues for research, which becomes urgent in the light of some reports of drug resistance.

Dreamtime, devastation and deviation: Australia's contribution to the chemotherapy of human parasitic infections

There are three areas in which Australian scientists have made outstanding contributions to the study of the chemotherapy of human parasitic infections. Naturally occurring products of plants have great potential as antiparasitic agents and although several native species have been shown to have antimalarial and anthelmintic activity, their potential as chemotherapeutic agents has not been fully realised; secondly, the demands of war ensured that the Army Malaria Unit at Cairns carried out meticulous and exceptional studies to evaluate new antimalarial compounds. Not only were they able to prove the effectiveness of atebrin, Proguanil and chloroquine as prophylactics, they also obtained much new information on the pharmacokinetics of antimalarials and about the infection itself. Full recognition of these pioneering studies involving over 1000 volunteers infected with malaria, which can never be repeated, has not been appreciated. The third significant contribution is the molecular studies on the mechanisms of drug resistance Plasmodium falciparum of both the antifolate- and quinoline-containing drugs and the identification and subsequent biochemical and molecular analysis of drug resistance in Giardia intestinalis infections.