Acid extrusion from the intraerythrocytic malaria parasite is not via a Na(+)/H(+) exchanger

Cation ATPase (ATP4) Orthologue Replacement in the Malaria Parasite Plasmodium knowlesi Reveals Species-Specific Responses to ATP4-Targeting Drugs

Several unrelated classes of antimalarial compounds developed against Plasmodium falciparum target a parasite-specific P-type ATP-dependent Na+ pump, PfATP4. We have previously shown that other malaria parasite species infecting humans are less susceptible to these compounds. Here, we generated a series of transgenic Plasmodium knowlesi orthologue replacement (OR) lines in which the endogenous pkatp4 locus was replaced by a recodonized P. knowlesi atp4 (pkatp4) coding region or the orthologous coding region from P. falciparum, Plasmodium malariae, Plasmodium ovale subsp. curtisi, or Plasmodium vivax. Each OR transgenic line displayed a similar growth pattern to the parental P. knowlesi line. We found significant orthologue-specific differences in parasite susceptibility to three chemically unrelated ATP4 inhibitors, but not to comparator drugs, among the P. knowlesi OR lines. The PfATP4OR transgenic line of P. knowlesi was significantly more susceptible than our control PkATP4OR line to three ATP4 inhibitors: cipargamin, PA21A092, and SJ733. The PvATP4OR and PmATP4OR lines were similarly susceptible to the control PkATP4OR line, but the PocATP4OR line was significantly less susceptible to all ATP4 inhibitors than the PkATP4OR line. Cipargamin-induced inhibition of Na+ efflux was also significantly greater with the P. falciparum orthologue of ATP4. This confirms that species-specific susceptibility differences previously observed in ex vivo studies of human isolates are partly or wholly enshrined in the primary amino acid sequences of the respective ATP4 orthologues and highlights the need to monitor efficacy of investigational malaria drugs against multiple species. P. knowlesi is now established as an important in vitro model for studying drug susceptibility in non-falciparum malaria parasites. IMPORTANCE Effective drugs are vital to minimize the illness and death caused by malaria. Development of new drugs becomes ever more urgent as drug resistance emerges. Among promising compounds now being developed to treat malaria are several unrelated molecules that each inhibit the same protein in the malaria parasite-ATP4. Here, we exploited the genetic tractability of P. knowlesi to replace its own ATP4 genes with orthologues from five human-infective species to understand the drug susceptibility differences among these parasites. We previously estimated the susceptibility to ATP4-targeting drugs of each species using clinical samples from malaria patients. These estimates closely matched those of the corresponding "hybrid" P. knowlesi parasites carrying introduced ATP4 genes. Thus, species-specific ATP4 inhibitor efficacy is directly determined by the sequence of the gene. Our novel approach to understanding cross-species susceptibility/resistance can strongly support the effort to develop antimalarials that effectively target all human malaria parasite species.

Characterization of the ATP4 ion pump in Toxoplasma gondii

The Plasmodium falciparum ATPase PfATP4 is the target of a diverse range of antimalarial compounds, including the clinical drug candidate cipargamin. PfATP4 was originally annotated as a Ca²⁺ transporter, but recent evidence suggests that it is a Na⁺ efflux pump, extruding Na⁺ in exchange for H⁺ Here we demonstrate that ATP4 proteins belong to a clade of P-type ATPases that are restricted to apicomplexans and their closest relatives. We employed a variety of genetic and physiological approaches to investigate the ATP4 protein of the apicomplexan Toxoplasma gondii, TgATP4. We show that TgATP4 is a plasma membrane protein. Knockdown of TgATP4 had no effect on resting pH or Ca²⁺ but rendered parasites unable to regulate their cytosolic Na⁺ concentration ([Na⁺]_cyt). PfATP4 inhibitors caused an increase in [Na⁺]_cyt and a cytosolic alkalinization in WT but not TgATP4 knockdown parasites. Parasites in which TgATP4 was knocked down or disrupted exhibited a growth defect, attributable to reduced viability of extracellular parasites. Parasites in which TgATP4 had been disrupted showed reduced virulence in mice. These results provide evidence for ATP4 proteins playing a key conserved role in Na⁺ regulation in apicomplexan parasites.

Diverse antimalarials from whole-cell phenotypic screens disrupt malaria parasite ion and volume homeostasis

Four hundred structurally diverse drug-like compounds comprising the Medicines for Malaria Venture's 'Pathogen Box' were screened for their effect on a range of physiological parameters in asexual blood-stage malaria (Plasmodium falciparum) parasites. Eleven of these compounds were found to perturb parasite Na+, pH and volume in a manner consistent with inhibition of the putative Na+ efflux P-type ATPase PfATP4. All eleven compounds fell within the subset of 125 compounds included in the Pathogen Box on the basis of their having been identified as potent inhibitors of the growth of asexual blood-stage P. falciparum parasites. All eleven compounds inhibited the Na+-dependent ATPase activity of parasite membranes and showed reduced efficacy against parasites carrying mutations in PfATP4. This study increases the number of chemically diverse structures known to show a 'PfATP4-associated' phenotype, and adds to emerging evidence that a high proportion (7-9%) of the structurally diverse antimalarial compounds identified in whole cell phenotypic screens share the same mechanism of action, exerting their antimalarial effect via an interaction with PfATP4.

Cell Swelling Induced by the Antimalarial KAE609 (Cipargamin) and Other PfATP4-Associated Antimalarials

For an increasing number of antimalarial agents identified in high-throughput phenotypic screens, there is evidence that they target PfATP4, a putative Na⁺ efflux transporter on the plasma membrane of the human malaria parasite Plasmodium falciparum For several such "PfATP4-associated" compounds, it has been noted that their addition to parasitized erythrocytes results in cell swelling. Here we show that six structurally diverse PfATP4-associated compounds, including the clinical candidate KAE609 (cipargamin), induce swelling of both isolated blood-stage parasites and intact parasitized erythrocytes. The swelling of isolated parasites is dependent on the presence of Na⁺ in the external environment and may be attributed to the osmotic consequences of Na⁺ uptake. The swelling of the parasitized erythrocyte results in an increase in its osmotic fragility. Countering cell swelling by increasing the osmolarity of the extracellular medium reduces the antiplasmodial efficacy of PfATP4-associated compounds, consistent with cell swelling playing a role in the antimalarial activity of this class of compounds.

Unexpected selections of Plasmodium falciparum polymorphisms in previously treatment-naïve areas after monthly presumptive administration of three different anti-malarial drugs in Liberia 1976-78

Background

To assess the effect on malaria prevalence, village specific monthly administrations of pyrimethamine, chlorproguanil, chloroquine or placebo were given to children in four previously treatment-naïve Liberian villages, 1976–78. Plasmodium falciparum in vivo resistance developed to pyrimethamine only. Selection of molecular markers of P. falciparum resistance after 2 years of treatment are reported.

Methods

Blood samples were collected from 191 study children in a survey in 1978. Polymorphisms in pfcrt, pfmdr1, pfdhfr, pfdhps, pfmrp1 and pfnhe1 genes were determined using PCR-based methods.

Results

Pfcrt 72–76 CVIET was found in one chloroquine village sample, all remaining samples had pfcrt CVMNK. Pfmdr1 N86 prevalence was 100%. A pfmdr1 T1069_ACT→ACG synonymous polymorphism was found in 30% of chloroquine village samples and 3% of other samples (P = 0.008). Variations in pfnhe1 block I were found in all except the chloroquine treated village (P < 0.001). Resistance associated pfdhfr 108N prevalence was 2% in the pyrimethamine village compared to 45–65% elsewhere, including the placebo village (P = 0.001).

Conclusions

Chloroquine treatment possibly resulted in the development of pfcrt 72–76 CVIET. Selection of pfmdr1 T1069_ACG and a pfnhe1 block 1 genotypes indicates that chloroquine treatment exerted a selective pressure on P. falciparum. Pyrimethamine resistance associated pfdhfr 108N was present prior to the introduction of any drug. Decreased pfdhfr 108N frequency concurrent with development of pyrimethamine resistance suggests a non-pfdhfr polymorphisms mediated resistance mechanism.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1747-6) contains supplementary material, which is available to authorized users.

Ion Regulation in the Malaria Parasite

Some hours after invading the erythrocytes of its human host, the malaria parasite Plasmodium falciparum induces an increase in the permeability of the erythrocyte membrane to monovalent ions. The resulting net influx of Na(+) and net efflux of K(+), down their respective concentration gradients, converts the erythrocyte cytosol from an initially high-K(+), low-Na(+) solution to a high-Na(+), low-K(+) solution. The intraerythrocytic parasite itself exerts tight control over its internal Na(+), K(+), Cl(-), and Ca(2+) concentrations and its intracellular pH through the combined actions of a range of membrane transport proteins. The molecular mechanisms underpinning ion regulation in the parasite are receiving increasing attention, not least because PfATP4, a P-type ATPase postulated to be involved in Na(+) regulation, has emerged as a potential antimalarial drug target, susceptible to inhibition by a wide range of chemically unrelated compounds.

Na+ Influx Induced by New Antimalarials Causes Rapid Alterations in the Cholesterol Content and Morphology of Plasmodium falciparum

Among the several new antimalarials discovered over the past decade are at least three clinical candidate drugs, each with a distinct chemical structure, that disrupt Na+ homeostasis resulting in a rapid increase in intracellular Na+ concentration ([Na+]i) within the erythrocytic stages of Plasmodium falciparum. At present, events triggered by Na+ influx that result in parasite demise are not well-understood. Here we report effects of two such drugs, a pyrazoleamide and a spiroindolone, on intraerythrocytic P. falciparum. Within minutes following the exposure to these drugs, the trophozoite stage parasite, which normally contains little cholesterol, was made permeant by cholesterol-dependent detergents, suggesting it acquired a substantial amount of the lipid. Consistently, the merozoite surface protein 1 and 2 (MSP1 and MSP2), glycosylphosphotidylinositol (GPI)-anchored proteins normally uniformly distributed in the parasite plasma membrane, coalesced into clusters. These alterations were not observed following drug treatment of P. falciparum parasites adapted to grow in a low [Na+] growth medium. Both cholesterol acquisition and MSP1 coalescence were reversible upon the removal of the drugs, implicating an active process of cholesterol exclusion from trophozoites that we hypothesize is inhibited by high [Na+]i. Electron microscopy of drug-treated trophozoites revealed substantial morphological changes normally seen at the later schizont stage including the appearance of partial inner membrane complexes, dense organelles that resemble "rhoptries" and apparent nuclear division. Together these results suggest that [Na+]i disruptor drugs by altering levels of cholesterol in the parasite, dysregulate trophozoite to schizont development and cause parasite demise.

Membrane transport in the malaria parasite and its host erythrocyte

As it grows and replicates within the erythrocytes of its host the malaria parasite takes up nutrients from the extracellular medium, exports metabolites and maintains a tight control over its internal ionic composition. These functions are achieved via membrane transport proteins, integral membrane proteins that mediate the passage of solutes across the various membranes that separate the biochemical machinery of the parasite from the extracellular environment. Proteins of this type play a key role in antimalarial drug resistance, as well as being candidate drug targets in their own right. This review provides an overview of recent work on the membrane transport biology of the malaria parasite-infected erythrocyte, encompassing both the parasite-induced changes in the membrane transport properties of the host erythrocyte and the cell physiology of the intracellular parasite itself.

Na(+) regulation in the malaria parasite Plasmodium falciparum involves the cation ATPase PfATP4 and is a target of the spiroindolone antimalarials

The malaria parasite Plasmodium falciparum establishes in the host erythrocyte plasma membrane new permeability pathways that mediate nutrient uptake into the infected cell. These pathways simultaneously allow Na(+) influx, causing [Na(+)] in the infected erythrocyte cytosol to increase to high levels. The intraerythrocytic parasite itself maintains a low cytosolic [Na(+)] via unknown mechanisms. Here we present evidence that the intraerythrocytic parasite actively extrudes Na(+) against an inward gradient via PfATP4, a parasite plasma membrane protein with sequence similarities to Na(+)-ATPases of lower eukaryotes. Mutations in PfATP4 confer resistance to a potent class of antimalarials, the spiroindolones. Consistent with this, the spiroindolones cause a profound disruption in parasite Na(+) homeostasis, which is attenuated in parasites bearing resistance-conferring mutations in PfATP4. The mutant parasites also show some impairment of Na(+) regulation. Taken together, our results are consistent with PfATP4 being a Na(+) efflux ATPase and a target of the spiroindolones.

Loss of pH control in Plasmodium falciparum parasites subjected to oxidative stress

The intraerythrocytic malaria parasite is susceptible to oxidative stress and this may play a role in the mechanism of action of some antimalarial agents. Here we show that exposure of the intraerythrocytic malaria parasite to the oxidising agent hydrogen peroxide results in a fall in the intracellular ATP level and inhibition of the parasite's V-type H⁺-ATPase, causing a loss of pH control in both the parasite cytosol and the internal digestive vacuole. In contrast to the V-type H⁺-ATPase, the parasite's digestive vacuole H⁺-pyrophosphatase is insensitive to hydrogen peroxide-induced oxidative stress. This work provides insights into the effects of oxidative stress on the intraerythrocytic parasite, as well as providing an alternative possible explanation for a previous report that light-induced oxidative stress causes selective lysis of the parasite's digestive vacuole.

Malaria parasites tolerate a broad range of ionic environments and do not require host cation remodelling

Malaria parasites grow within erythrocytes, but are also free in host plasma between cycles of asexual replication. As a result, the parasite is exposed to fluctuating levels of Na(+) and K(+) , ions assumed to serve important roles for the human pathogen, Plasmodium falciparum. We examined these assumptions and the parasite's ionic requirements by establishing continuous culture in novel sucrose-based media. With sucrose as the primary osmoticant and K(+) and Cl(-) as the main extracellular ions, we obtained parasite growth and propagation at rates indistinguishable from those in physiological media. These conditions abolish long-known increases in intracellular Na(+) via parasite-induced channels, excluding a requirement for erythrocyte cation remodelling. We also dissected Na(+) , K(+) and Cl(-) requirements and found that unexpectedly low concentrations of each ion meet the parasite's demands. Surprisingly, growth was not adversely affected by up to 148 mM K(+) , suggesting that low extracellular K(+) is not an essential trigger for erythrocyte invasion. At the same time, merozoite egress and invasion required a threshold ionic strength, suggesting critical electrostatic interactions between macromolecules at these stages. These findings provide insights into transmembrane signalling in malaria and reveal fundamental differences between host and parasite ionic requirements.

Glutathione export from human erythrocytes and Plasmodium falciparum malaria parasites

Glutathione export from uninfected human erythrocytes was compared with that from cells infected with the malaria parasite Plasmodium falciparum using two separate methods that distinguish between oxidized (GSSG) and reduced (GSH) glutathione. One involved enzymatic recycling with or without thiol-masking; the other involved rapid derivatization followed by HPLC. Glutathione efflux from uninfected erythrocytes under physiological conditions occurred predominantly as GSH. On exposure of the cells to oxidative challenge, efflux of GSSG exceeded that of GSH. Efflux of both species was blocked by MK571, an inhibitor of mammalian multidrug-resistance proteins. Glutathione efflux from parasitized erythrocytes was substantially greater than that from uninfected erythrocytes. Under physiological conditions, the exported species was GSH, whereas under energy-depleted conditions, GSSG efflux occurred. Glutathione export from parasitized cells was inhibited partially by MK571 and more so by furosemide, an inhibitor of the 'new permeability pathways' induced by the parasite in the host erythrocyte membrane. Efflux from isolated parasites occurred as GSH. On exposure to oxidative challenge, this GSH efflux decreased, but no GSSG export was detected. These results are consistent with the view that the parasite supplies its host erythrocyte with GSH, much of which is exported from the infected cell via parasite-induced pathways.

Global analysis of Plasmodium falciparum Na(+)/H(+) exchanger (pfnhe-1) allele polymorphism and its usefulness as a marker of in vitro resistance to quinine

The aim of this study was to provide a comprehensive analysis of the worldwide genetic polymorphism of ms4760 alleles of the pfnhe-1 gene and to discuss their usefulness as molecular marker of quinine resistance (QNR). A new numbering of ms4760 allele, classification grouping ms4760 alleles according to the number of DNNND and DDNHNDNHNND repeat motifs in blocks II and V was also proposed. A total of 1508 ms4760 sequences from isolates, culture-adapted parasites or reference strains from various geographical regions were retrieved from GenBank (last update on 15th June 2012) or from publications and were used for genetic analyses. The association of different alleles of pfnhe-1 with resistance to quinoline antimalarial drugs showed marked geographic disparities. The validity and reliability of candidate polymorphisms in pfnhe-1 gene as molecular markers of QNR appeared restricted to endemic areas from South Asia or possibly East African countries and needs to be confirmed.

Polyamine uptake by the intraerythrocytic malaria parasite, Plasmodium falciparum

Polyamines and the enzymes involved in their biosynthesis are present at high levels in rapidly proliferating cells, including cancer cells and protozoan parasites. Inhibition of polyamine biosynthesis in asexual blood-stage malaria parasites causes cytostatic arrest of parasite development under in vitro conditions, but does not cure infections in vivo. This may be due to replenishment of the parasite's intracellular polyamine pool via salvage of exogenous polyamines from the host. However, the mechanism(s) of polyamine uptake by the intraerythrocytic parasite are not well understood. In this study, the uptake of the polyamines, putrescine and spermidine, into Plasmodium falciparum parasites functionally isolated from their host erythrocyte was investigated using radioisotope flux techniques. Both putrescine and spermidine were taken up into isolated parasites via a temperature-dependent process that showed cross-competition between different polyamines. There was also some inhibition of polyamine uptake by basic amino acids. Inhibition of polyamine biosynthesis led to an increase in the total amount of putrescine and spermidine taken up from the extracellular medium. The uptake of putrescine and spermidine by isolated parasites was independent of extracellular Na(+) but increased with increasing external pH. Uptake also showed a marked dependence on the parasite's membrane potential, decreasing with membrane depolarization and increasing with membrane hyperpolarization. The data are consistent with polyamines being taken up into the parasite via an electrogenic uptake process, energised by the parasite's inwardly negative membrane potential.

Know your enemy: understanding the role of PfCRT in drug resistance could lead to new antimalarial tactics

The prevention and treatment of malaria is heavily dependent on antimalarial drugs. However, beginning with the emergence of chloroquine (CQ)-resistant Plasmodium falciparum parasites 50 years ago, efforts to control the disease have been thwarted by failed or failing drugs. Mutations in the parasite's 'chloroquine resistance transporter' (PfCRT) are the primary cause of CQ resistance. Furthermore, changes in PfCRT (and in several other transport proteins) are associated with decreases or increases in the parasite's susceptibility to a number of other antimalarial drugs. Here, we review recent advances in our understanding of CQ resistance and discuss these in the broader context of the parasite's susceptibilities to other quinolines and related drugs. We suggest that PfCRT can be viewed both as a 'multidrug-resistance carrier' and as a drug target, and that the quinoline-resistance mechanism is a potential 'Achilles' heel' of the parasite. We examine a number of the antimalarial strategies currently undergoing development that are designed to exploit the resistance mechanism, including relatively simple measures, such as alternative CQ dosages, as well as new drugs that either circumvent the resistance mechanism or target it directly.

Differential association of Plasmodium falciparum Na+/H+ exchanger polymorphism and quinine responses in field- and culture-adapted isolates of Plasmodium falciparum

Plasmodium falciparum isolates with decreased susceptibility to quinine are increasingly being found in malaria patients. Mechanisms involved in this resistance are not yet understood. Several studies claim that alongside mutations in the Pfcrt and Pfmdr1 genes, the Pfnhe-1 Na(+)/H(+) exchanger polymorphism plays a role in decreasing susceptibility. However, conflicting results on the link between the Pfnhe-1 gene and quinine resistance arise from field- and culture-adapted isolates. We tested the association between Pfnhe-1, Pfcrt, and Pfmdr1 polymorphisms in field- and culture-adapted isolates from various countries with their in vitro susceptibility to quinine. Field isolates presented a higher diversity of the Pfnhe-1 microsatellite sequence than culture-adapted isolates. In culture-adapted isolates but not in field isolates, mutations in the Pfcrt and Pfmdr1 genes, as well as a higher number of DNNND repeats in the Pfnhe-1 gene, were associated with a higher 50% inhibitory concentration (IC(50)) of quinine. Furthermore, most of the culture-adapted isolates with more than one DNNND repeat in the Pfnhe-1 gene also harbored mutated Pfcrt and Pfmdr1 genes with an apparent cumulative effect on quinine susceptibility. This study supports the involvement of the Pfnhe-1 gene in the modulation of the in vitro quinine response when associated with mutated Pfcrt and Pfmdr1 genes. Culture adaptation could be responsible for selection of specific haplotypes of these three genes. Methods used for drug testing might thus influence the association between Pfnhe-1 polymorphism and quinine susceptibility. However, we do not exclude the possibility that in particular settings, Pfnhe-1 polymorphism can be used as a molecular marker for surveillance of quinine resistance.

Polymorphism of Plasmodium falciparum Na(+)/H(+) exchanger is indicative of a low in vitro quinine susceptibility in isolates from Viet Nam

Background

The Plasmodium falciparum NA+/H+ exchanger (pfnhe1, gene PF13_0019) has recently been proposed to influence quinine (QN) susceptibility. However, its contribution to QN resistance seems to vary geographically depending on the genetic background of the parasites. Here, the role of this gene was investigated in in vitro QN susceptibility of isolates from Viet Nam.

Method

Ninety-eight isolates were obtained from three different regions of the Binh Phuoc and Dak Nong bordering Cambodia provinces during 2006-2008. Among these, 79 were identified as monoclonal infection and were genotyped at the microsatellite pfnhe1 ms4760 locus and in vitro QN sensitivity data were obtained for 51 isolates. Parasite growth was assessed in the field using the HRP2 immunodetection assay.

Results

Significant associations were found between polymorphisms at pfnhe1 microsatellite ms4760 and susceptibility to QN. Isolates with two or more DNNND exhibited much lower susceptibility to QN than those harbouring zero or one DNNND repeats (median IC₅₀ of 682 nM versus median IC₅₀ of 300 nM; p = 0.0146) while isolates with one NHNDNHNNDDD repeat presented significantly reduced QN susceptibility than those who had two (median IC₅₀ of 704 nM versus median IC₅₀ of 375 nM; p < 0.01). These QNR associated genotype features were mainly due to the over representation of profile 7 among isolates (76.5%). The majority of parasites had pfcrt76T and wild-type pfmdr1 (> 95%) thus preventing analysis of associations with these mutations. Interestingly, area with the highest median QN IC₅₀ showed also the highest percentage of isolates carrying the pfnhe1 haplotype 7.

Conclusions

The haplotype 7 which is the typical Asian profile is likely well-adapted to high drug pressure in this area and may constitute a good genetic marker to evaluate the dissemination of QNR in this part of the world.

Efflux of a range of antimalarial drugs and 'chloroquine resistance reversers' from the digestive vacuole in malaria parasites with mutant PfCRT

Chloroquine-resistant malaria parasites (Plasmodium falciparum) show an increased leak of H(+) ions from their internal digestive vacuole in the presence of chloroquine. This phenomenon has been attributed to the transport of chloroquine, together with H(+), out of the digestive vacuole (and hence away from its site of action) via a mutant form of the parasite's chloroquine resistance transporter (PfCRT). Here, using transfectant parasite lines, we show that a range of other antimalarial drugs, as well as various 'chloroquine resistance reversers' induce an increased leak of H(+) from the digestive vacuole of parasites expressing mutant PfCRT, consistent with these compounds being substrates for mutant forms, but not the wild-type form, of PfCRT. For some compounds there were significant differences observed between parasites having the African/Asian Dd2 form of PfCRT and those with the South American 7G8 form of PfCRT, consistent with there being differences in the transport properties of the two mutant proteins. The finding that chloroquine resistance reversers are substrates for mutant PfCRT has implications for the mechanism of action of this class of compound.

An acid-loading chloride transport pathway in the intraerythrocytic malaria parasite, Plasmodium falciparum

The intraerythrocytic malaria parasite exerts tight control over its ionic composition. In this study, a combination of fluorescent ion indicators and (36)Cl(-) flux measurements was used to investigate the transport of Cl(-) and the Cl(-)-dependent transport of "H(+)-equivalents" in mature (trophozoite stage) parasites, isolated from their host erythrocytes. Removal of extracellular Cl(-), resulting in an outward [Cl(-)] gradient, gave rise to a cytosolic alkalinization (i.e. a net efflux of H(+)-equivalents). This was reversed on restoration of extracellular Cl(-). The flux of H(+)-equivalents was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and, when measured in ATP-depleted parasites, showed a pronounced dependence on the pH of the parasite cytosol; the flux was low at cytosolic pH values < 7.2 but increased steeply with cytosolic pH at values > 7.2. (36)Cl(-) influx measurements revealed the presence of a Cl(-) uptake mechanism with characteristics similar to those of the Cl(-)-dependent H(+)-equivalent flux. The intracellular concentration of Cl(-) in the parasite was estimated to be approximately 48 mm in situ. The data are consistent with the intraerythrocytic parasite having in its plasma membrane a 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive transporter that, under physiological conditions, imports Cl(-) together with H(+)-equivalents, resulting in an intracellular Cl(-) concentration well above that which would occur if Cl(-) ions were distributed passively in accordance with the parasite's large, inwardly negative membrane potential.

Monitoring of in vitro susceptibilities and molecular markers of resistance of Plasmodium falciparum isolates from Thai-Myanmar border to chloroquine, quinine, mefloquine and artesunate

Malaria is one of the major causes of morbidity and mortality worldwide. The major factor which has aggravated the situation is the emergence of multidrug resistant Plasmodium falciparum malaria. To successfully deal with the problem, thorough understanding of the molecular bases for reduced parasite sensitivity to existing antimalarial drugs is of considerable importance. The objective of this work was to broaden the insight into the molecular mechanisms of resistance of P. falciparum to quinoline-containing antimalarials and artemisinin derivatives. Polymorphisms of the candidate genes pfmdr1 and pfcrt were investigated in relation to the susceptibility (in vitro sensitivity) of P. falciparum isolates to chloroquine (CQ), mefloquine (MQ), quinine (QN) and the artemisinin derivative - artesunate (AS). A total of 26 P. falciparum isolates were successful cultured. In vitro sensitivity results indicate the increase in susceptibility of P. falciparum strains in Thailand to CQ, while the susceptibility to MQ and QN was markedly declined. The pattern of cross-resistance was observed between MQ vs QN vs AS. Only one point mutation in the pfmdr1 gene, i.e., N86Y was observed with low prevalence of 7.7% (2/26). In contrast, the mutations at positions 76T, 220S, 271E, 326S, 356T and 371I in the pfcrt gene were identified in almost all isolates (25 isolates, 96.2%). The association between polymorphisms of the pfmdr1 and susceptibility of the parasite to MQ and QN was observed (increased susceptibilities to MQ and QN in isolates with mutations). Moreover, the correlation between pfmdr1 gene amplification and susceptibility of the parasite to MQ, QN and AS was observed (decreased susceptibilities to MQ, QN and AS in isolates with increased pfmdr1 copy number).

Molecular and physiologic basis of quinoline drug resistance in Plasmodium falciparum malaria

30 years before the discovery of the pfcrt gene, altered cellular drug accumulation in drug-resistant malarial parasites had been well documented. Heme released from catabolized hemoglobin was thought to be a key target for quinoline drugs, and additional modifications to quinoline drug structure in order to improve activity against chloroquine-resistant malaria were performed in a few laboratories. However, parasite cell culture methods were still in their infancy, assays for drug susceptibility were not well standardized, and the power of malarial genetics was decades away. The last 10 years have witnessed explosive progress in elucidation of the biochemistry of chloroquine resistance. This review briefly summarizes that progress, and discusses where additional work is needed.

Probing the multifactorial basis of Plasmodium falciparum quinine resistance: evidence for a strain-specific contribution of the sodium-proton exchanger PfNHE

Quinine (QN) continues to be an important treatment option for severe malaria, however resistance to this drug has emerged in field isolates of the etiologic agent Plasmodium falciparum. Quantitative trait loci investigations of QN resistance have mapped three loci of this complex trait. Two coincide with pfcrt and pfmdr1, involved in resistance to chloroquine (CQ) and other quinoline-based antimalarials. A third locus on chromosome 13 contains the sodium-proton exchanger (pfnhe) gene. Previous studies have associated pfnhe polymorphisms with reduced QN sensitivity in culture-adapted field isolates. Here, we provide direct evidence supporting the hypothesis that pfnhe contributes to QN resistance. Using allelic exchange, we reduced pfnhe expression by introducing a truncated 3' untranslated region (UTR) from pfcrt into the endogenous pfnhe 3'UTR. Transfections were performed with 1BB5 and 3BA6 (both CQ- and QN-resistant) as well as GC03 (CQ- and QN-sensitive), all progenies of the HB3xDd2 genetic cross. RNA and protein analyses of the ensuing recombinant clones demonstrated a approximately 50% decrease in pfnhe expression levels. A statistically significant 30% decrease in QN IC(50) values was associated with these decreased expression levels in 1BB5 and 3BA6 but not in GC03. CQ, mefloquine and lumefantrine IC(50) values were unaltered. Cytosolic pH values were similar in all parental lines and recombinant clones. Our observations support a role for pfnhe in QN resistance in a strain-dependent manner, which might be contingent on pre-existing resistance to CQ and/or QN. These data bolster observations that QN resistance is a complex trait requiring the contribution of multiple transporter proteins.

Chloroquine resistance-conferring mutations in pfcrt give rise to a chloroquine-associated H+ leak from the malaria parasite's digestive vacuole

Chloroquine resistance in the malaria parasite Plasmodium falciparum is conferred by mutations in the P. falciparum chloroquine resistance transporter (PfCRT). PfCRT localizes to the membrane of the parasite's internal digestive vacuole, an acidic organelle in which chloroquine accumulates to high concentrations and exerts its toxic effect. Mutations in PfCRT are thought to reduce chloroquine accumulation in this organelle. How they do so is the subject of ongoing debate. Recently we have shown that in the presence of chloroquine there is an increased leak of H+ from the digestive vacuole in chloroquine-resistant but not chloroquine-sensitive parasites. Here, using transfectant parasite strains of a single genetic background and differing only in their pfcrt allele, we show that chloroquine resistance-conferring PfCRT mutations are responsible for this chloroquine-associated H+ leak. This is consistent with the hypothesis that the chloroquine resistance-conferring forms of PfCRT mediate the efflux of chloroquine, in association with H+, from the malaria parasite's digestive vacuole.