Functional reconstitution of purified chloroquine resistance membrane transporter expressed in yeast

Epitope-specific IgG pools identify PfCRT monomer and homodimer polypeptides that are differentially phosphorylated at Ser411 in Plasmodium falciparum

The Plasmodium falciparum chloroquine resistance transporter (PfCRT) is a phospho-protein with three identified phosphorylation sites (Ser³³, Ser⁴¹¹ and Thr⁴¹⁶) at its cytosolic N- and C-termini. In this study, we report on the characterization of PfCRT anti-serum and show the presence of three epitope-specific immunoglobulin (IgG) pools (i.e., IgG-P1, P2, and P3), each recognizing a different epitope in PfCRT cytoplasmic C-terminal. IgG-P2 bound the heptapeptide sequence (⁴⁰⁸NEDSEGE⁴¹⁴), including Ser⁴¹¹. The effect of Ser⁴¹¹ phosphorylation on the binding specificity of IgG-P2 was confirmed using heptapeptides and full-length PfCRT with substitutions of Ser⁴¹¹ with aspartic acid (phospho-serine mimic) and alanine residues. Moreover, using purified IgG-P2, we show the presence of PfCRT homodimer that has un-phosphorylated Ser⁴¹¹ and migrates with an apparent molecular mass of 90 kDa on SDS-PAGE. In addition, parasite lysates showed PfCRT to be more phosphorylated at Ser⁴¹¹ in CQ-sensitive (3D7) than CQ-resistant (Dd2-H) strains of P. falciparum. Taken together, the findings of this study suggest a role for Ser⁴¹¹ phosphorylation in PfCRT structure-function.

Multiple genetic loci define Ca++ utilization by bloodstream malaria parasites

Background

Bloodstream malaria parasites require Ca⁺⁺ for their development, but the sites and mechanisms of Ca⁺⁺ utilization are not well understood. We hypothesized that there may be differences in Ca⁺⁺ uptake or utilization by genetically distinct lines of P. falciparum. These differences, if identified, may provide insights into molecular mechanisms.

Results

Dose response studies with the Ca⁺⁺ chelator EGTA (ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid) revealed stable differences in Ca⁺⁺ requirement for six geographically divergent parasite lines used in previous genetic crosses, with the largest difference seen between the parents of the HB3 x Dd2 cross. Genetic mapping of Ca⁺⁺ requirement yielded complex inheritance in 34 progeny clones with a single significant locus on chromosome 7 and possible contributions from other loci. Although encoded by a gene in the significant locus and a proposed Ca⁺⁺ target, PfCRT (P. falciparum chloroquine resistance transporter), the primary determinant of clinical resistance to the antimalarial drug chloroquine, does not appear to contribute to this quantitative trait. Stage-specific application of extracellular EGTA also excluded determinants associated with merozoite egress and erythrocyte reinvasion.

Conclusions

We have identified differences in Ca⁺⁺ utilization amongst P. falciparum lines. These differences are under genetic regulation, segregating as a complex trait in genetic cross progeny. Ca⁺⁺ uptake and utilization throughout the bloodstream asexual cycle of malaria parasites represents an unexplored target for therapeutic intervention.

Engineering and purification of a thermostable, high-yield, variant of PfCRT, the Plasmodium falciparum chloroquine resistance transporter

Historically chloroquine was used to treat the most deadly form of malaria, caused by the parasite Plasmodium falciparum. The selective pressure of chloroquine therapy led to the rapid emergence of chloroquine resistant parasites. Resistance has been attributed to the Plasmodium falciparum Chloroquine Resistance Transporter (PfCRT), an integral membrane protein of unknown structure. A PfCRT structure would provide new insights into how the protein confers chloroquine resistance and thereby also yield novel opportunities for developing anti-malarial therapies. Although PfCRT is an attractive target for characterisation and structure determination, very little work has been published on its expression and purification. Here we present a medium throughput protocol, employing Sf9 insect cells, for testing the expression, stability and purification yield of rationally designed PfCRT mutant constructs and constructs of a PfCRT orthologue from Neospora caninum (NcCRT). We have identified a conserved cysteine residue in PfCRT that results in elevated protein stability when mutated. Combining this mutation with the insertion of T4-lysozyme into a specific surface loop further augments PfCRT protein yield and thermostability. Screening also identified an NcCRT construct with an elevated purification yield. Furthermore it was possible to purify both PfCRT and NcCRT constructs at milligram-scales, with high purities and with size exclusion chromatography profiles that were consistent with monodispersed, homogeneous protein.

The "pushmi-pullyu" of resistance to chloroquine in malaria

Malarial infection continues to impart devastating health problems in the developing world. Treatment of malaria has involved chemotherapy since 168 BC, with the most prevalent and successful forms using plant alkaloids. Perhaps the greatest treatment success against malaria was by chloroquine, a synthetic derivative of the quinines found in the Cinchona tree bark. Chloroquine is able to kill parasites by interfering with haem metabolism in the parasite's digestive vacuole. The widespread use of chloroquine predictably resulted in the development of drug-resistant malaria and the most highly implicated resistance mediators are the transporter proteins P-glycoprotein (P-gp) homologue 1 (P-gh1) and Plasmodium falciparum chloroquine-resistance transporter (PfCRT), which reside on the parasite's digestive vacuole. The presence of PfCRT and P-gh1 on the vacuole membrane is analogous to the two-headed fictional creature known as the "Pushmi-Pullyu". P-gh1 (Pushmi) increases influx of chloroquine into the vacuole, while PfCRT (Pullmi) causes efflux of chloroquine from the vacuole. This review describes how drug-resistant malarial parasites co-ordinate chloroquine distribution through adaptive mutations to promote their survival in the presence of this cytotoxic drug.

Protective immunity in gibel carp, Carassius gibelio of the truncated proteins of cyprinid herpesvirus 2 expressed in Pichia pastoris

Cyprinid herpesvirus 2 (CyHV-2) infection is a newly emerged infectious disease of farmed gibel carp (Carassius gibelio) in China and causes huge economic losses to the aquaculture industry. In this study, the three membrane proteins encoded by genes ORF25, ORF25C, and ORF25D of CyHV-2 were truncated and expressed in yeast, Pichia pastoris. Screening of the recombinant yeasts was done by detecting the truncated proteins using Western blot. Through immunogold labeling, it was shown that proteins binding the colloidal gold were presented on the surface of cells. In the experiment of inhibition of virus binding by the recombinant truncated proteins, the TCID50 of the tORF25 group (10(4.1)/ml) was lower than that of tORF25C (10(4.6)/ml) or tORF25D groups (10(5)/ml). These results suggested that the proteins may be involved in attachment of the virus to the cell surface. Healthy gibel carp were immunized with 20 μg of tORF25, tORF25C, and tORF25D proteins, and the control group received PBS. Interleukin 11 (IL-11) expression in the spleens of the immunized fish peaked at day 4 and the complement component C3 (C3) genes were significantly up-regulated at day 7 post-immunization. Specific antibodies were measured in the three immunized groups and the titer detected in the tORF25 group reached 327, that was significantly higher than the tORF25C (247) or tORF25D (228) groups. When the immunized fish were challenged with live CyHV-2 by intraperitoneal injection the relative percent survival (RPS) of the tORF25, tORF25C, and tORF25D immunized groups was 75%, 63%, and 54%, respectively. The feasibility of the P. pastoris yeast expression system for the production of the recombinant truncated proteins and their apparent bioactivity suggests that tORF25, tORF25C, and tORF25D are potential candidate vaccines against Cyprinid herpesvirus 2 infection in gibel carp.

Characterization of the commercially-available fluorescent chloroquine-BODIPY conjugate, LynxTag-CQGREEN, as a marker for chloroquine resistance and uptake in a 96-well plate assay

Chloroquine was a cheap, extremely effective drug against Plasmodium falciparum until resistance arose. One approach to reversing resistance is the inhibition of chloroquine efflux from its site of action, the parasite digestive vacuole. Chloroquine accumulation studies have traditionally relied on radiolabelled chloroquine, which poses several challenges. There is a need for development of a safe and biologically relevant substitute. We report here a commercially-available green fluorescent chloroquine-BODIPY conjugate, LynxTag-CQ_GREEN, as a proxy for chloroquine accumulation. This compound localized to the digestive vacuole of the parasite as observed under confocal microscopy, and inhibited growth of chloroquine-sensitive strain 3D7 more extensively than in the resistant strains 7G8 and K1. Microplate reader measurements indicated suppression of LynxTag-CQ_GREEN efflux after pretreatment of parasites with known reversal agents. Microsomes carrying either sensitive- or resistant-type PfCRT were assayed for uptake; resistant-type PfCRT exhibited increased accumulation of LynxTag-CQ_GREEN, which was suppressed by pretreatment with known chemosensitizers. Eight laboratory strains and twelve clinical isolates were sequenced for PfCRT and Pgh1 haplotypes previously reported to contribute to drug resistance, and pfmdr1 copy number and chloroquine IC₅₀s were determined. These data were compared with LynxTag-CQ_GREEN uptake/fluorescence by multiple linear regression to identify genetic correlates of uptake. Uptake of the compound correlated with the logIC₅₀ of chloroquine and, more weakly, a mutation in Pgh1, F1226Y.

PfCRT-mediated drug transport in malarial parasites

A wide range of drug transport studies using intact infected red blood cells, isolated malarial parasites, heterologous expression systems, and purified protein, combined with elegant genetic experiments, have suggested that chloroquine transport by the Plasmodium falciparum chloroquine resistance transporter (PfCRT) is a key aspect of the molecular mechanism of quinoline antimalarial drug resistance. However, many questions remain. This short review summarizes data that have led to drug channel versus drug pump hypotheses for PfCRT and suggests ways in which recent contrasting interpretations might be reconciled.

Purified E255L mutant SERCA1a and purified PfATP6 are sensitive to SERCA-type inhibitors but insensitive to artemisinins

The antimalarial drugs artemisinins have been described as inhibiting Ca(2+)-ATPase activity of PfATP6 (Plasmodium falciparum ATP6) after expression in Xenopus oocytes. Mutation of an amino acid residue in mammalian SERCA1 (Glu(255)) to the equivalent one predicted in PfATP6 (Leu) was reported to induce sensitivity to artemisinin in the oocyte system. However, in the present experiments, we found that artemisinin did not inhibit mammalian SERCA1a E255L either when expressed in COS cells or after purification of the mutant expressed in Saccharomyces cerevisiae. Moreover, we found that PfATP6 after expression and purification from S. cerevisiae was insensitive to artemisinin and significantly less sensitive to thapsigargin and 2,5-di(tert-butyl)-1,4-benzohydroquinone than rabbit SERCA1 but retained higher sensitivity to cyclopiazonic acid, another type of SERCA1 inhibitor. Although mammalian SERCA and purified PfATP6 appear to have different pharmacological profiles, their insensitivity to artemisinins suggests that the mechanism of action of this class of drugs on the calcium metabolism in the intact cell is complex and cannot be ascribed to direct inhibition of PfATP6. Furthermore, the successful purification of PfATP6 affords the opportunity to develop new antimalarials by screening for inhibitors against PfATP6.

Transporters as mediators of drug resistance in Plasmodium falciparum

Drug resistance represents a major obstacle in the radical control of malaria. Drug resistance can arise in many different ways, but recent developments highlight the importance of mutations in transporter molecules as being major contributors to drug resistance in the human malaria parasite Plasmodium falciparum. While approximately 2.5% of the P. falciparum genome encodes membrane transporters, this review concentrates on three transporters, namely the chloroquine resistance transporter PfCRT, the multi-drug resistance transporter 1 PfMDR1, and the multi-drug resistance-associated protein PfMRP, which have been strongly associated with resistance to the major antimalarial drugs. The studies that identified these entities as contributors to resistance, and the possible molecular mechanisms that can bring about this phenotype, are discussed. A deep understanding of the underpinning mechanisms, and of the structural specificities of the players themselves, is a necessary basis for the development of the new drugs that will be needed for the future armamentarium against malaria.

Chloroquine transport in Plasmodium falciparum. 2. Analysis of PfCRT-mediated drug transport using proteoliposomes and a fluorescent chloroquine probe

Mutations in the PfCRT protein cause chloroquine resistance (CQR), and earlier studies from our laboratory using plasma membrane inside-out vesicles (ISOV) prepared from yeast expressing recombinant PfCRT [Zhang, H., et al. (2004) Biochemistry 43, 8290-8296] suggested that the putative transporter mediates downhill facilitated diffusion of charged chloroquine (CQ). However, more recent experiments with a fluorescent CQ probe (NBD-CQ) presented in the accompanying paper (DOI 10.1021/bi901034r ) indicated that the CQR phenotype in live parasites is associated with a reduced rate of ATP-dependent CQ uptake into the digestive vacuole (DV). An altered rate constant for uptake has multiple interpretations. To further investigate this phenomenon, PfCRT proteins found in chloroquine-sensitive (CQS) and CQR strains of Plasmodium falciparum were purified from yeast engineered to express "yeast optimized" pfcrt genes, reconstituted into proteoliposomes (PL), and efflux of NBD-CQ was measured from these PL. A membrane-impermeant quencher was used to distinguish intra-PL NBD-CQ from extra-PL NBD-CQ vs time as well as resolve initial rates and rate constants for efflux. Efflux was investigated at a range of NBD-CQ concentrations, in the presence vs absence of pH gradients (DeltapH) and transmembrane potentials (DeltaPsi). Explicit turnover numbers for apparent PfCRT-mediated transport were then calculated under these conditions. Our data are consistent with a model wherein PfCRT catalyzes electrochemically downhill diffusion of NBD-CQ out of the DV, in response to DeltaPsi or DeltapH, at a rate that can partially compete with the ATP-dependent uptake of NBD-CQ by CQS parasites described in the previous paper. These data allow us to propose a refined model for altered CQ accumulation in CQR malarial parasites.

Membrane transport proteins of the malaria parasite

The malaria parasite-infected erythrocyte is a multi-compartment structure, incorporating numerous different membrane systems. The movement of nutrients, metabolites and inorganic ions into and out of the intraerythrocytic parasite, as well as between subcellular compartments within the parasite, is mediated by transporters and channels - integral membrane proteins that facilitate the movement of solutes across the membrane bilayer. Proteins of this type also play a key role in antimalarial drug resistance. Genes encoding transporters and channels account for at least 2.5% of the parasite genome. However, ascribing functions and physiological roles to these proteins, and defining their roles in drug resistance, is not straightforward. For any given membrane transport protein, a full understanding of its role(s) in the parasitized erythrocyte requires a knowledge of its subcellular localization and substrate specificity, as well as some knowledge of the effects on the parasite of modifying the sequence and/or level of expression of the gene involved. Here we consider recent work in this area, describe a number of newly identified transport proteins, and summarize the likely subcellular localization and putative substrate specificity of all of the candidate membrane transport proteins identified to date.

Expression of SARS-coronavirus spike glycoprotein in Pichia pastoris

To establish a rapid and economical method for the expression of viral proteins in high yield and purity by Pichia pastoris, the S protein of the SARS-CoV was selected in this study. Six S glycoprotein fragments were expressed in Escherichiacoli BL21 and yeast KM71H strains. After purification by affinity chromatography, the protein identities were confirmed by western blot analysis, N-terminal sequencing and mass spectrometry. The proteins expressed in E. coli were low in solubility and bound by GroEL. They still formed soluble aggregates even when the GroEL was removed by urea. The proteins expressed in P. pastoris were relatively soluble. The maximal yield of the RBD reached 46 mg/l with purity greater than 95%. Pull-down assay revealed that ACE2 was specifically captured from cell lysate, indicating that the RBD was biologically active. The glycosylated and deglycosylated RBD was then subjected to SEC and results showed that deglycosylated RBD formed soluble aggregates again. Taken together, pure and biological active RBD of the S protein could be expressed in P. pastoris, and the P. pastoris expression platform will be a good alternative for the expression of viral proteins, in particular, the highly glycosylated surface proteins that mediate the tissue tropism and viral entry.

Heterologous expression of plasmodial proteins for structural studies and functional annotation

Malaria remains the world's most devastating tropical infectious disease with as many as 40% of the world population living in risk areas. The widespread resistance of Plasmodium parasites to the cost-effective chloroquine and antifolates has forced the introduction of more costly drug combinations, such as Coartem^®. In the absence of a vaccine in the foreseeable future, one strategy to address the growing malaria problem is to identify and characterize new and durable antimalarial drug targets, the majority of which are parasite proteins. Biochemical and structure-activity analysis of these proteins is ultimately essential in the characterization of such targets but requires large amounts of functional protein. Even though heterologous protein production has now become a relatively routine endeavour for most proteins of diverse origins, the functional expression of soluble plasmodial proteins is highly problematic and slows the progress of antimalarial drug target discovery. Here the status quo of heterologous production of plasmodial proteins is presented, constraints are highlighted and alternative strategies and hosts for functional expression and annotation of plasmodial proteins are reviewed.

A verapamil-sensitive chloroquine-associated H+ leak from the digestive vacuole in chloroquine-resistant malaria parasites

Chloroquine resistance in the malaria parasite Plasmodium falciparum has made malaria increasingly difficult to control. Chloroquine-resistant parasites accumulate less chloroquine than their chloroquine-sensitive counterparts; however, the mechanism underlying this remains unclear. The primary site of accumulation and antimalarial action of chloroquine is the internal acidic digestive vacuole of the parasite, the acidity of which is maintained by inwardly-directed H+ pumps, working against the (outward) leak of H+. In this study we have investigated the leak of H+ from the digestive vacuole of the parasite by monitoring the alkalinisation of the vacuole following inhibition of the H+-pumping V-type ATPase by concanamycin A. The rates of alkalinisation observed in three chloroquine-resistant strains were two- to fourfold higher than those measured in three chloroquine-sensitive strains. On addition of chloroquine there was a dramatic increase in the rate of alkalinisation in the chloroquine-resistant strains, whereas chloroquine caused the rate of alkalinisation to decrease in the chloroquine-sensitive strains. The chloroquine-associated increase in the rate of alkalinisation seen in chloroquine-resistant parasites was inhibited by the chloroquine-resistance reversal agent verapamil. The data are consistent with the hypothesis that in chloroquine-resistant parasites chloroquine effluxes from the digestive vacuole, in association with H+, via a verapamil-sensitive pathway.

Dissecting the components of quinine accumulation in Plasmodium falciparum

Although quinine, the active ingredient of chinchona bark, has been used in the treatment of malaria for several centuries, there is little information regarding the interactions of this drug with the human malaria parasite Plasmodium falciparum. To better understand quinine's mode of action and the mechanism underpinning reduced responsiveness, we have investigated the factors that contribute to quinine accumulation by parasites that differ in their susceptibility to quinine. Interestingly, passive distribution, in accordance with the intracellular pH gradients, and intracellular binding could account for only a small fraction of the high amount of quinine accumulated by the parasites investigated. The results of trans-stimulation kinetics suggest that high accumulation of quinine is brought about by a carrier-mediated import system. This import system seems to be weakened in parasites with reduced quinine susceptibility. Other data show that polymorphisms within PfCRT are causatively linked with an increased verapamil-sensitive quinine efflux that, depending on the genetic background, resulted in reduced quinine accumulation. The polymorphisms within PfMDR1 investigated did not affect quinine accumulation. Our data are consistent with the model that several factors, including acidotropic trapping, binding to intracellular sites and carrier-mediated import and export transport systems, contribute to steady-state intracellular quinine accumulation.

Is PfCRT a channel or a carrier? Two competing models explaining chloroquine resistance in Plasmodium falciparum

Chloroquine (CQ), an antimalarial drug with a long history, now frequently fails in the field owing to the rapid spread of resistant Plasmodium falciparum strains. CQ resistance is linked to a K76T mutation in PfCRT, a membrane-located food vacuolar protein and member of the drug-metabolite transporter superfamily, but there is as yet no agreed mechanism of how mutated PfCRT brings about CQ resistance. Current models suggest that mutated PfCRT acts either as a channel or a transporter of CQ, enabling CQ to leave the digestive food vacuole of the parasite, in which the CQ accumulates. Here, we review the pros and cons of the carrier and transporter models in light of recent developments in the field.

Differences in trans-stimulated chloroquine efflux kinetics are linked to PfCRT in Plasmodium falciparum

The mechanism underpinning chloroquine drug resistance in the human malarial parasite Plasmodium falciparum has remained controversial. Currently discussed models include a carrier or a channel for chloroquine, the former actively expelling the drug, the latter facilitating its passive diffusion, out of the parasite's food vacuole, where chloroquine accumulates and inhibits haem detoxification. Here we have challenged both models using an established trans-stimulation efflux protocol. While carriers may demonstrate trans-stimulation, channels do not. Our data reveal that extracellular chloroquine stimulates chloroquine efflux in the presence and absence of metabolic energy in both chloroquine-sensitive and -resistant parasites, resulting in a hyperbolic increase in the apparent initial efflux rates as the concentration of external chloroquine increases. In the absence of metabolic energy, the apparent initial efflux rates were comparable in both parasites. Significant differences were only observed in the presence of metabolic energy, where consistently higher apparent initial efflux rates were found in chloroquine-resistant parasites. As trans-stimulation is characteristic of a carrier, and not a channel, we interpret our data in favour of a carrier for chloroquine being present in both chloroquine-sensitive and -resistant parasites, however, with different transport modalities.