Parasite-induced permeation of nucleosides in Plasmodium falciparum malaria

Down the membrane hole: Ion channels in protozoan parasites

Parasitic diseases caused by protozoans are highly prevalent around the world, disproportionally affecting developing countries, where coinfection with other microorganisms is common. Control and treatment of parasitic infections are constrained by the lack of specific and effective drugs, plus the rapid emergence of resistance. Ion channels are main drug targets for numerous diseases, but their potential against protozoan parasites is still untapped. Ion channels are membrane proteins expressed in all types of cells, allowing for the flow of ions between compartments, and regulating cellular functions such as membrane potential, excitability, volume, signaling, and death. Channels and transporters reside at the interface between parasites and their hosts, controlling nutrient uptake, viability, replication, and infectivity. To understand how ion channels control protozoan parasites fate and to evaluate their suitability for therapeutics, we must deepen our knowledge of their structure, function, and modulation. However, methodological approaches commonly used in mammalian cells have proven difficult to apply in protozoans. This review focuses on ion channels described in protozoan parasites of clinical relevance, mainly apicomplexans and trypanosomatids, highlighting proteins for which molecular and functional evidence has been correlated with their physiological functions.

Characterisation of complexes formed by parasite proteins exported into the host cell compartment of Plasmodium falciparum infected red blood cells

During its intraerythrocytic life cycle, the human malaria parasite Plasmodium falciparum supplements its nutritional requirements by scavenging substrates from the plasma through the new permeability pathways (NPPs) installed in the red blood cell (RBC) membrane. Parasite proteins of the RhopH complex: CLAG3, RhopH2, RhopH3, have been implicated in NPP activity. Here, we studied 13 exported proteins previously hypothesised to interact with RhopH2, to study their potential contribution to the function of NPPs. NPP activity assays revealed that the 13 proteins do not appear to be individually important for NPP function, as conditional knockdown of these proteins had no effect on sorbitol uptake. Intriguingly, reciprocal immunoprecipitation assays showed that five of the 13 proteins interact with all members of the RhopH complex, with PF3D7_1401200 showing the strongest association. Mass spectrometry‐based proteomics further identified new protein complexes; a cytoskeletal complex and a Maurer's clefts/J‐dot complex, which overall helps clarify protein–protein interactions within the infected RBC (iRBC) and is suggestive of the potential trafficking route of the RhopH complex itself to the RBC membrane.

Development of dried blood spot quality control materials for adenosine deaminase severe combined immunodeficiency and LC-MS/MS method for their characterization

Adenosine deaminase severe combined immunodeficiency (ADA-SCID) is an autosomal recessive disorder in which a lack of ADA enzyme prevents the maturation of T- and B-cells; early intervention is crucial for restoring immune function in affected neonates. ADA is responsible for purine metabolism and-in its absence-adenosine, deoxyadenosine, and S-adenosylhomocysteine build up and can be detected in the blood. Preparing dried blood spot (DBS) quality control (QC) materials for these analytes is challenging because enrichments are quickly metabolized by the endogenous ADA in normal donor blood. Adding an inhibitor, erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA), has been previously reported to minimize enzyme activity, although this adds additional cost and complexity. We describe an alternative method using unnatural L-enantiomer nucleosides (L-adenosine and 2'-deoxy-L-adenosine) which eliminates the need for enzyme inhibition. We also present a novel method for characterization of the materials using liquid chromatography mass spectrometry to quantify the analytes of interest.

Plasmodium Purine Metabolism and Its Inhibition by Nucleoside and Nucleotide Analogues

Malaria still affects around 200 million people and is responsible for more than 400,000 deaths per year, mostly children in subequatorial areas. This disease is caused by parasites of the Plasmodium genus. Only a few WHO-recommended treatments are available to prevent or cure plasmodial infections, but genetic mutations in the causal parasites have led to onset of resistance against all commercial antimalarial drugs. New drugs and targets are being investigated to cope with this emerging problem, including enzymes belonging to the main metabolic pathways, while nucleoside and nucleotide analogues are also a promising class of potential drugs. This review highlights the main metabolic pathways targeted for the development of potential antiplasmodial therapies based on nucleos(t)ide analogues, as well as the different series of purine-containing nucleoside and nucleotide derivatives designed to inhibit Plasmodium falciparum purine metabolism.

Targeting Channels and Transporters in Protozoan Parasite Infections

Infectious diseases caused by pathogenic protozoa are among the most significant causes of death in humans. Therapeutic options are scarce and massively challenged by the emergence of resistant parasite strains. Many of the current anti-parasite drugs target soluble enzymes, generate unspecific oxidative stress, or act by an unresolved mechanism within the parasite. In recent years, collections of drug-like compounds derived from large-scale phenotypic screenings, such as the malaria or pathogen box, have been made available to researchers free of charge boosting the identification of novel promising targets. Remarkably, several of the compound hits have been found to inhibit membrane proteins at the periphery of the parasites, i.e., channels and transporters for ions and metabolites. In this review, we will focus on the progress made on targeting channels and transporters at different levels and the potential for use against infections with apicomplexan parasites mainly Plasmodium spp. (malaria) and Toxoplasma gondii (toxoplasmosis), with kinetoplastids Trypanosoma brucei (sleeping sickness), Trypanosoma cruzi (Chagas disease), and Leishmania ssp. (leishmaniasis), and the amoeba Entamoeba histolytica (amoebiasis).

Plasmodium falciparum invasion and intraerythrocytic development are impaired by 2', 3'-dialdehyde adenosine

Purine nucleotide synthesis in protozoa takes place exclusively via the purine salvage pathway and S-adenosyl-l-homocysteine hydrolase (SAHH) is an important enzyme in the Plasmodium salvage pathway which is not present in erythrocytes. Here, we describe the antimalarial effect of 2'3'-dialdehyde adenosine or oxidized adenosine (oADO), inhibitor of SAHH, on in vitro infection of human erythrocytes by P. falciparum. Treatment of infected erythrocytes with oADO inhibits parasite development and reinvasion of new cells. Erythrocytes pre-treated with oADO have a reduced susceptibility to invasion. Our results suggest that oADO interferes with one or more parasitic enzymes of the purine salvage pathway.

Synergistic Malaria Parasite Killing by Two Types of Plasmodial Surface Anion Channel Inhibitors

Malaria parasites increase their host erythrocyte's permeability to a broad range of ions and organic solutes. The plasmodial surface anion channel (PSAC) mediates this uptake and is an established drug target. Development of therapies targeting this channel is limited by several problems including interactions between known inhibitors and permeating solutes that lead to incomplete channel block. Here, we designed and executed a high-throughput screen to identify a novel class of PSAC inhibitors that overcome this solute-inhibitor interaction. These new inhibitors differ from existing blockers and have distinct effects on channel-mediated transport, supporting a model of two separate routes for solute permeation though PSAC. Combinations of inhibitors specific for the two routes had strong synergistic action against in vitro parasite propagation, whereas combinations acting on a single route produced only additive effects. The magnitude of synergism depended on external nutrient concentrations, consistent with an essential role of the channel in parasite nutrient acquisition. The identified inhibitors will enable a better understanding of the channel's structure-function and may be starting points for novel combination therapies that produce synergistic parasite killing.

Transfection with thymidine kinase permits bromodeoxyuridine labelling of DNA replication in the human malaria parasite Plasmodium falciparum

BACKGROUND

Plasmodium falciparum, the causative agent of severe human malaria, is an early-diverging protozoan whose lifecycle has many unusual features, including its modes of replication. Research on the Plasmodium cell cycle, which occurs primarily via schizogony instead of canonical binary fission, has been hampered by a lack of tools and markers that can be transferred from cell cycle studies in model organisms. A common tool used to study DNA replication and the cell cycle in human cells is the labelling of newly-replicated DNA with the modified nucleotide bromodeoxyuridine (BrdU), followed by immunofluorescent detection. Plasmodium parasites, however, do not incorporate BrdU because they rely only on de novo synthesis of pyrimidines and do not salvage thymidine analogues like BrdU for conversion into nucleotides.

METHODS

Analysis of biochemical pathways in Plasmodium indicated that the absence of the enzyme thymidine kinase (TK) may be the only impediment to BrdU incorporation in this organism. A TK gene from Herpes simplex was, therefore, introduced into the Plasmodium falciparum 3D7 strain and the effect on BrdU labelling was assessed by enzyme-linked immunosorbent assay and immunofluorescence microscopy.

RESULTS

Introduction of a TK gene produces parasites that can indeed incorporate BrdU. This forms a sensitive indicator of DNA replication, which can be detected by both quantitative and qualitative assays on either a population level or a single-cell level. Plasmodium falciparum, when expressing TK, becomes unusually sensitive to BrdU toxicity.

CONCLUSIONS

BrdU labelling represents a significant new tool for investigating DNA replication and the cell cycle in Plasmodium.

High guanidinium permeability reveals dehydration-dependent ion selectivity in the plasmodial surface anion channel

Malaria parasites grow within vertebrate erythrocytes and increase host cell permeability to access nutrients from plasma. This increase is mediated by the plasmodial surface anion channel (PSAC), an unusual ion channel linked to the conserved clag gene family. Although PSAC recognizes and transports a broad range of uncharged and charged solutes, it must efficiently exclude the small Na⁺ ion to maintain infected cell osmotic stability. Here, we examine possible mechanisms for this remarkable solute selectivity. We identify guanidinium as an organic cation with high permeability into human erythrocytes infected with Plasmodium falciparum, but negligible uptake by uninfected cells. Transport characteristics and pharmacology indicate that this uptake is specifically mediated by PSAC. The rank order of organic and inorganic cation permeabilities suggests cation dehydration as the rate-limiting step in transport through the channel. The high guanidinium permeability of infected cells also allows rapid and stringent synchronization of parasite cultures, as required for molecular and cellular studies of this pathogen. These studies provide important insights into how nutrients and ions are transported via PSAC, an established target for antimalarial drug development.

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.

Why do malaria parasites increase host erythrocyte permeability?

Malaria parasites increase erythrocyte permeability to diverse solutes including anions, some cations, and organic solutes, as characterized with several independent methods. Over the past decade, patch-clamp studies have determined that the permeability results from one or more ion channels on the infected erythrocyte host membrane. However, the biological role(s) served by these channels, if any, remain controversial. Recent studies implicate the plasmodial surface anion channel (PSAC) and a role in parasite nutrient acquisition. A debated alternative role in remodeling host ion composition for the benefit of the parasite appears to be nonessential. Because both channel activity and the associated clag3 genes are strictly conserved in malaria parasites, channel-mediated permeability is an attractive target for development of new therapies.

Advances in nanomedicines for malaria treatment

Malaria is an infectious disease that mainly affects children and pregnant women from tropical countries. The mortality rate of people infected with malaria per year is enormous and became a public health concern. The main factor that has contributed to the success of malaria proliferation is the increased number of drug resistant parasites. To counteract this trend, research has been done in nanotechnology and nanomedicine, for the development of new biocompatible systems capable of incorporating drugs, lowering the resistance progress, contributing for diagnosis, control and treatment of malaria by target delivery. In this review, we discussed the main problems associated with the spread of malaria and the most recent developments in nanomedicine for anti-malarial drug delivery.

Ion and nutrient uptake by malaria parasite-infected erythrocytes

Erythrocytes infected with malaria parasites have increased permeability to diverse organic and inorganic solutes. While these permeability changes have been known for decades, the molecular basis of transport was unknown and intensively debated. CLAG3, a parasite protein previously thought to function in cytoadherence, has recently been implicated in formation of the plasmodial surface anion channel (PSAC), an unusual small conductance ion channel that mediates uptake of most solutes. Consistent with transport studies, the clag genes are conserved in all plasmodia but are absent from other genera. The encoded protein is integral to the host membrane, as also predicted by electrophysiology. An important question is whether functional channels are formed by CLAG3 alone or through interactions with other proteins. In either case, gene identification should advance our understanding of parasite biology and may lead to new therapeutics.

Chemical activation of a high-affinity glutamate transporter in human erythrocytes and its implications for malaria-parasite–induced glutamate uptake

Human erythrocytes have a low basal permeability to L-glutamate and are not known to have a functional glutamate transporter. Here, treatment of human erythrocytes with arsenite was shown to induce the uptake of L-glutamate and D-aspartate, but not that of D-glutamate or L-alanine. The majority of the arsenite-induced L-glutamate influx was via a high-affinity, Na+-dependent system showing characteristics of members of the “excitatory amino acid transporter” (EAAT) family. Western blots and immunofluorescence assays revealed the presence of a member of this family, EAAT3, on the erythrocyte membrane. Erythrocytes infected with the malaria parasite Plasmodium falciparum take up glutamate from the extracellular environment. Although the majority of uptake is via a low-affinity Na+-independent pathway there is, in addition, a high-affinity uptake component, raising the possibility that the parasite activates the host cell glutamate transporter.

Voltage-dependent inactivation of the plasmodial surface anion channel via a cleavable cytoplasmic component

Erythrocytes infected with malaria parasites have increased permeability to ions and various nutrient solutes, mediated by a parasite ion channel known as the plasmodial surface anion channel (PSAC). The parasite clag3 gene family encodes PSAC activity, but there may also be additional unidentified components of this channel. Consistent with a lack of clag3 homology to genes of other ion channels, PSAC has a number of unusual functional properties. Here, we report that PSAC exhibits an unusual form of voltage-dependent inactivation. Inactivation was readily detected in the whole-cell patch-clamp configuration after steps to negative membrane potentials. The fraction of current that inactivates, its kinetics, and the rate of recovery were all voltage-dependent, though with a modest effective valence (0.7±0.1 elementary charges). These properties were not affected by solution composition or charge carrier, suggesting inactivation intrinsic to the channel protein. Intriguingly, inactivation was absent in cell-attached recordings and took several minutes to appear after obtaining the whole-cell configuration, suggesting interactions with soluble cytosolic components. Inactivation could also be largely abolished by application of intracellular, but not extracellular protease. The findings implicate inactivation via a charged cytoplasmic channel domain. This domain may be tethered to one or more soluble intracellular components under physiological conditions.

Malaria parasite clag3 genes determine channel-mediated nutrient uptake by infected red blood cells

Development of malaria parasites within vertebrate erythrocytes requires nutrient uptake at the host cell membrane. The plasmodial surface anion channel (PSAC) mediates this transport and is an antimalarial target, but its molecular basis is unknown. We report a parasite gene family responsible for PSAC activity. We used high-throughput screening for nutrient uptake inhibitors to identify a compound highly specific for channels from the Dd2 line of the human pathogen P. falciparum. Inheritance of this compound's affinity in a Dd2 × HB3 genetic cross maps to a single parasite locus on chromosome 3. DNA transfection and in vitro selections indicate that PSAC-inhibitor interactions are encoded by two clag3 genes previously assumed to function in cytoadherence. These genes are conserved in plasmodia, exhibit expression switching, and encode an integral protein on the host membrane, as predicted by functional studies. This protein increases host cell permeability to diverse solutes.

PfNT2, a permease of the equilibrative nucleoside transporter family in the endoplasmic reticulum of Plasmodium falciparum

The survival and proliferation of the obligate intracellular malaria parasite Plasmodium falciparum require salvage of essential purines from the host. Genetic studies have previously shown that the parasite plasma membrane purine permease, PfNT1, plays an essential function in the transport of all naturally occurring purine nucleosides and nucleobases across the parasite plasma membrane. Here, we describe an intracellular permease, PfNT2. PfNT2 is, like PfNT1, a member of the equilibrative nucleoside transporter family. Confocal and immunoelectron microscopic analyses of transgenic parasites harboring green fluorescent protein- or hemagglutinin-tagged PfNT2 demonstrated endoplasmic reticulum localization. This localization was confirmed by colocalization with the endoplasmic reticulum marker PfBiP. Using yeast as a surrogate system, we show that targeting PfNT2 to the plasma membrane of fui1Delta cells lacking the plasma membrane nucleoside transporter Fui1 confers sensitivity to the toxic nucleoside analog 5-fluorouridine. This study provides the first evidence of an intracellular purine permease in apicomplexan parasites and suggests a novel biological function for the parasite endoplasmic reticulum during malaria infection.

Uptake of purines in Plasmodium falciparum-infected human erythrocytes is mostly mediated by the human equilibrative nucleoside transporter and the human facilitative nucleobase transporter

BACKGROUND

Plasmodium parasites are unable to synthesize purines de novo and have to salvage them from the host. Due to this limitation in the parasite, purine transporters have been an area of focus in the search for anti-malarial drugs. Although the uptake of purines through the human equilibrative nucleoside transporter (hENT1), the human facilitative nucleobase transporter (hFNT1) and the parasite-induced new permeation pathway (NPP) has been studied, no information appears to exist on the relative contribution of these three transporters to the uptake of adenosine and hypoxanthine. Using the appropriate transporter inhibitors, the role of each of these salvage pathways to the overall purine transport in intraerythrocytic Plasmodium falciparum was systematically investigated.

METHODS

The transport of adenosine, hypoxanthine and adenine into uninfected and P. falciparum-infected human erythrocytes was investigated in the presence or absence of classical inhibitors of the hFNT1, hENT1 and NPP. The effective inhibition of the various transporters by the classical inhibitors was verified using appropriate known substrates. The ability of high concentration of unlabelled substrates to saturate these transporters was also studied.

RESULTS

Transport of exogenous purine into infected or uninfected erythrocytes occurred primarily through saturable transporters rather than through the NPP. Hypoxanthine and adenine appeared to enter erythrocytes mainly through the hFNT1 nucleobase transporter whereas adenosine entered predominantly through the hENT1 nucleoside transporter. The rate of purine uptake was approximately doubled in infected cells compared to uninfected erythrocytes. In addition, it was found that the rate of adenosine uptake was considerably higher than the rate of hypoxanthine uptake in infected human red blood cells (RBC). It was also demonstrated that furosemide inhibited the transport of purine bases through hFNT1.

CONCLUSION

Collectively, the data obtained in this study clearly show that the endogenous host erythrocyte transporters hENT1 and hFNT1, rather than the NPP, are the major route of entry of purine into parasitized RBC. Inhibitors of hENT1 and hFNT1, as well as the NPP, should be considered in the development of anti-malarials targeted to purine transport.

Complex inheritance of the plasmodial surface anion channel in a Plasmodium falciparum genetic cross

Human erythrocytes infected with the malaria parasite Plasmodium falciparum have increased permeabilities to many solutes. The plasmodial surface anion channel (PSAC) may mediate these changes. Despite good understanding of the biochemical and biophysical properties, the genetic basis of PSAC activity remains unknown. Functional polymorphisms in laboratory isolates and two mutants generated by in vitro selection implicate a parasite-encoded channel, although parasite-induced modifications of endogenous channels have not been formally excluded. Here, we identified stable differences in furosemide efficacy against PSAC activity induced by HB3 and 3D7A parasites. This difference was apparent in both single PSAC patch-clamp recordings and in sorbitol-mediated osmotic lysis measurements, confirming that Cl(-) and sorbitol are transported by a single-channel type. Examination of 19 progeny from a genetic cross between HB3 and 3D7A revealed complex inheritance with some cloned progeny exhibiting furosemide affinities outside the range of parental values. Isolates generated by selfing of the 3D7A clone also exhibited altered furosemide affinities, implicating changes in one or more alleles during meiosis or passage through a primate host. PSAC may be encoded by multiple parasite genes (e.g. a multi-gene family or multiple genes that encode distinct channel subunits) or a single polymorphic gene under strong selective pressure.

Molecules targeting the purine salvage pathway in Apicomplexan parasites

The need of intracellular parasites to retrieve nutrients and fulfill their energy requirements is achieved by manipulating the host's metabolism. With the spread of AIDS, research on purine metabolism has gained in importance with the aim to develop drugs against opportunistic infections. Many studies over the past ten years have yielded contradictory results, but this review tries to clarify these findings by exposing the latest data concerning purine transport and the specific activities of the major enzymes of the purine salvage pathway of Toxoplasma gondii, Plasmodium falciparum and Cryptosporidium parvum.

Open and closed states of the plasmodial surface anion channel

BACKGROUND

The malaria parasite, Plasmodium falciparum, causes most of its clinical sequelae through its intracellular growth and multiplication in human red blood cells (RBCs). During this intracellular cycle, the parasite markedly alters the membrane transport properties of the host RBC through the induction of an unusual ion channel known as the plasmodial surface anion channel (PSAC). In addition to its unusual selectivity and pharmacology, PSAC exhibits atypical voltage-dependent gating; single-channel recordings reveal fast-flickering behavior interspersed with periods of inactivity. Detailed study of its gating properties have been complicated by the technical difficulty of obtaining single PSAC recordings, its small conductance, and various sources of error important for fast-flickering channels.

METHODS

Here, we developed an automated algorithm to analyze large amounts of single-channel data with particular emphasis on these sources of error. This algorithm was evaluated with high-quality single-channel and multichannel recordings obtained in the presence and absence of furosemide, a well-known PSAC antagonist.

RESULTS AND CONCLUSIONS

Our analysis reveals that the adequate description of PSAC gating requires only one exponentially decaying open state, but at least three closed channel states. This model was further supported by single-channel recordings in the presence of furosemide, which inhibits PSAC with moderate affinity through an allosteric mechanism.

A blasticidin S-resistant Plasmodium falciparum mutant with a defective plasmodial surface anion channel

Erythrocytes infected with malaria parasites exhibit marked increases in permeability to organic and inorganic solutes. The plasmodial surface anion channel (PSAC), an unusual voltage-dependent ion channel induced on the host membrane after infection, may play a central role in these permeability changes. Here, we identified a functional PSAC mutant through in vitro selection with blasticidin S. Resistance to blasticidin S was generated during culture and correlated with significant reductions in permeability to multiple solutes, consistent with uptake via a common pathway. Single channel recordings revealed marked changes in PSAC gating with the addition of a subconductance state not present in wild-type channels. The channel's selectivity profile and pharmacology also were significantly altered. Eventual loss of the mutant phenotype upon removal of selective pressure and slower growth of mutant parasites suggest that PSAC serves an important role in intracellular parasite survival. These findings provide solid evidence for the uptake of diverse solutes via PSAC and implicate one or more parasite genes in expression of this channel.

Targeting purine and pyrimidine metabolism in human apicomplexan parasites

Synthesis de novo, acquisition by salvage and interconversion of purines and pyrimidines represent the fundamental requirements for their eventual assembly into nucleic acids as nucleotides and the deployment of their derivatives in other biochemical pathways. A small number of drugs targeted to nucleotide metabolism, by virtue of their effect on folate biosynthesis and recycling, have been successfully used against apicomplexan parasites such as Plasmodium and Toxoplasma for many years, although resistance is now a major problem in the prevention and treatment of malaria. Many targets not involving folate metabolism have also been explored at the experimental level. However, the unravelling of the genome sequences of these eukaryotic unicellular organisms, together with increasingly sophisticated molecular analyses, opens up possibilities of introducing new drugs that could interfere with these processes. This review examines the status of established drugs of this type and the potential for further exploiting the vulnerability of apicomplexan human pathogens to inhibition of this key area of metabolism.

Babesia and plasmodia increase host erythrocyte permeability through distinct mechanisms

Human erythrocytes infected with Plasmodium falciparum have markedly increased permeability to diverse solutes, many of which may be mediated by an unusual small conductance ion channel, the plasmodial surface anion channel (PSAC). Because these increases may be essential for parasite survival in the bloodstream, an important question is whether other intraerythrocytic parasites induce similar ion channels. Here, we examined this question using human erythrocytes infected with Babesia divergens, a distantly related apicomplexan parasite that can cause severe disease in immunocompromised humans. Osmotic lysis experiments after enrichment of infected erythrocytes with a new method revealed that these parasites also increase host permeability to various organic solutes. These permeability changes differed significantly from those induced by P. falciparum in transport rates, selectivity profiles and temperature dependence. Cell-attached and whole-cell patch-clamp experiments confirmed and extended these differences because neither PSAC-like channels nor significant increases in whole-cell anion conductance were seen after B. divergens infection. While both babesia and plasmodia increase host erythrocyte permeability to a diverse collection of organic solutes, they utilize fundamentally different mechanisms.

Specific inhibition of the plasmodial surface anion channel by dantrolene

The plasmodial surface anion channel (PSAC), induced on human erythrocytes by the malaria parasite Plasmodium falciparum, is an important target for antimalarial drug development because it may contribute to parasite nutrient acquisition. However, known antagonists of this channel are quite nonspecific, inhibiting many other channels and carriers. This lack of specificity not only complicates drug development but also raises doubts about the exact role of PSAC in the well-known parasite-induced permeability changes. We recently identified a family of new PSAC antagonists structurally related to dantrolene, an antagonist of muscle Ca++ release channels. Here, we explored the mechanism of dantrolene's actions on parasite-induced permeability changes. We found that dantrolene inhibits the increased permeabilities of sorbitol, two amino acids, an organic cation, and hypoxanthine, suggesting a common pathway shared by these diverse solutes. It also produced parallel reductions in PSAC single-channel and whole-cell Cl- currents. In contrast to its effect on parasite-induced permeabilities, dantrolene had no measurable effect on five other classes of anion channels, allaying concerns of poor specificity inherent to other known antagonists. Our studies indicate that dantrolene binds PSAC at an extracellular site distinct from the pore, where it inhibits the conformational changes required for channel gating. Its affinity for this site depends on ionic strength, implicating electrostatic interactions in dantrolene binding. In addition to the potential therapeutic applications of its derivatives, dantrolene's specificity and its defined mechanism of action on PSAC make it a useful tool for transport studies of infected erythrocytes.

Transport of nucleosides across the Plasmodium falciparum parasite plasma membrane has characteristics of PfENT1

Like all parasitic protozoa, the human malaria parasite Plasmodium falciparum lacks the enzymes required for de novo synthesis of purines and it is therefore reliant upon the salvage of these compounds from the external environment. P. falciparum equilibrative nucleoside transporter 1 (PfENT1) is a nucleoside transporter that has been localized to the plasma membrane of the intraerythrocytic form of the parasite. In this study we have characterized the transport of purine and pyrimidine nucleosides across the plasma membrane of 'isolated' trophozoite-stage P. falciparum parasites and compared the transport characteristics of the parasite with those of PfENT1 expressed in Xenopus oocytes. The transport of nucleosides into the parasite: (i) was, in the case of adenosine, inosine and thymidine, very fast, equilibrating within a few seconds; (ii) was of low affinity [K(m) (adenosine) = 1.45 +/- 0.25 mM; K(m) (thymidine) = 1.11 +/- 0.09 mM]; and (iii) showed 'cross-competition' for adenosine, inosine and thymidine, but not cytidine. The kinetic characteristics of nucleoside transport in intact parasites matched very closely those of PfENT1 expressed in Xenopus oocytes [K(m) (adenosine) = 1.86 +/- 0.28 mM; K(m) (thymidine) = 1.33 +/- 0.17 mM]. Furthermore, PfENT1 transported adenosine, inosine and thymidine, with a cross-competition profile the same as that seen for isolated parasites. The data are consistent with PfENT1 serving as a major route for the uptake of nucleosides across the parasite plasma membrane.

The plasma membrane permease PfNT1 is essential for purine salvage in the human malaria parasite Plasmodium falciparum

The human malaria parasite Plasmodium falciparum relies on the acquisition of host purines for its survival within human erythrocytes. Purine salvage by the parasite requires specialized transporters at the parasite plasma membrane (PPM), but the exact mechanism of purine entry into the infected erythrocyte, and the primary purine source used by the parasite, remain unknown. Here, we report that transgenic parasites lacking the PPM transporter PfNT1 (P. falciparum nucleoside transporter 1) are auxotrophic for hypoxanthine, inosine, and adenosine under physiological conditions and are viable only if these normally essential nutrients are provided at excess concentrations. Transport measurements across the PPM revealed a severe reduction in hypoxanthine uptake in the knockout, whereas adenosine and inosine transport were only partially affected. These data provide compelling evidence for a sequential pathway for exogenous purine conversion into hypoxanthine using host enzymes followed by PfNT1-mediated transport into the parasite. The phenotype of the conditionally lethal mutant establishes PfNT1 as a critical component of purine salvage in P. falciparum and validates PfNT1 as a potential therapeutic target.

Purine and pyrimidine transport in pathogenic protozoa: From biology to therapy

Purine salvage is an essential function for all obligate parasitic protozoa studied to date and most are also capable of efficient uptake of preformed pyrimidines. Much progress has been made in the identification and characterisation of protozoan purine and pyrimidine transporters. While the genes encoding protozoan or metazoan pyrimidine transporters have yet to be identified, numerous purine transporters have now been cloned. All protozoan purine transporter-encoding genes characterised to date have been of the Equilibrative Nucleoside Transporter family conserved in a great variety of eukaryote organisms. However, these protozoan transporters have been shown to be sufficiently different from mammalian transporters to mediate selective uptake of therapeutic agents. Recent studies are increasingly addressing the structure and substrate recognition mechanisms of these vital transport proteins.

A Malaria Parasite-encoded Vacuolar H+-ATPase Is Targeted to the Host Erythrocyte

The asexual development of malaria parasites inside the erythrocyte is accompanied by changes in the composition, structure, and function of the host cell membrane and cytoplasm. The parasite exports a membrane network into the host cytoplasm and several proteins that are inserted into the erythrocyte membrane, although none of these proteins has been shown to have enzymatic activity. We report here that a functional malaria parasite-encoded vacuolar (V)-H(+)-ATPase is exported to the erythrocyte and localized in membranous structures and in the plasma membrane of the infected erythrocyte. This localization was determined by separation of parasite and erythrocyte membranes and determination of enzyme marker activities and by immunofluorescence microscopy assays using antibodies against the B subunit of the malarial V-H(+)-ATPase and erythrocyte (spectrins) and parasite (merozoite surface protein 1) markers. Our results suggest that this pump has a role in the maintenance of the intracellular pH (pH(i)) of the infected erythrocyte. Our results also indicate that although the pH(i) maintained by the V-H(+)-ATPase is important for maximum uptake of small metabolites at equilibrium, it does not appear to affect transport across the erythrocyte membrane and is, therefore, not involved in the previously described phenomenon of increased permeability of infected erythrocytes that is sensitive to chloride channel inhibitors (new permeation pathway). This constitutes the first report of the presence of a functional enzyme of parasite origin in the plasma membrane of its host.

Malaria parasite transporters as a drug-delivery strategy

The recent characterization of the choline carrier of the malaria parasite and its role in the selective delivery of novel antimalarial drugs has reignited interest in parasite transporters as a drug-delivery strategy. In this article, we discuss these findings in relation to the wider context of developing a sustainable antimalarial-drug-development portfolio.

Malaria Parasites Are Rapidly Killed by Dantrolene Derivatives Specific for the Plasmodial Surface Anion Channel

Dantrolene was recently identified as a novel inhibitor of the plasmodial surface anion channel (PSAC), an unusual ion channel on Plasmodium falciparum-infected human red blood cells. Because dantrolene is used clinically, has a high therapeutic index, and has desirable chemical synthetic properties, it may be a lead compound for antimalarial development. However, dantrolene derivatives would need to preferentially interact with PSAC over the sarcoplasmic reticulum (SR) Ca2+ release channel to avoid unwanted side effects from antimalarial therapy. Furthermore, dantrolene's modest affinity for PSAC (K(m) of 1.2 microM) requires improvement. In this study, we tested 164 derivatives of dantrolene to examine whether these hurdles can be surmounted. A simple screen for PSAC block defined the minimal scaffold needed and identified compounds with > or =5-fold higher affinity. Single-channel patch-clamp recordings on infected human red blood cells with two derivatives also revealed increased blocking affinity that resulted from slower unbinding from a site on the extracellular face of PSAC. We tested these derivatives in a frog skeletal muscle contractility assay and found that, in contrast to dantrolene, they had little or no effect on SR Ca2+ release. Finally, these blockers kill in vitro parasite cultures at lower concentrations than dantrolene, consistent with an essential role for PSAC. Because, as a class, these derivatives fulfil the requirements for drug leads and can be studied with simple screening technology, more extensive medicinal chemistry is warranted to explore antimalarial development.

Plasmodial surface anion channel-independent phloridzin resistance in Plasmodium falciparum

The plasmodial surface anion channel (PSAC) is an unusual ion channel induced on the human red blood cell membrane after infection with the malaria parasite, Plasmodium falciparum. Because PSAC is permeant to small metabolic precursors essential for parasite growth and is present on red blood cells infected with geographically divergent parasite isolates, it may be an ideal target for future antimalarial development. Here, we used chemically induced mutagenesis and known PSAC antagonists that inhibit in vitro parasite growth to examine whether resistance mutations in PSAC can be readily induced. Stable mutants resistant to phloridzin were generated and selected within 3 weeks after treatment with 1-methyl-3-nitro-1-nitrosoguanidine. These mutants were evaluated with osmotic lysis and electrophysiological transport assays, which indicate that PSAC inhibition by phloridzin is complex with at least two different modes of inhibition. Mutants resistant to the growth inhibitory effects of phloridzin expressed PSAC activity indistinguishable from that on sensitive parasites, indicating selection of resistance via mutations in one or more other parasite targets. Failure to induce mutations in PSAC activity is consistent with a highly constrained channel protein less susceptible to resistance mutations; whether this protein is parasite- or host-encoded remains to be determined.

Patch-clamp analysis of the "new permeability pathways" in malaria-infected erythrocytes

The intraerythrocytic amplification of the malaria parasite Plasmodium falciparum induces new pathways of solute permeability in the host cell's membrane. These pathways play a pivotal role in parasite development by supplying the parasite with nutrients, disposing of the parasite's metabolic waste and organic osmolytes, and adapting the host's electrolyte composition to the parasite's needs. The "new permeability pathways" allow the fast electrogenic diffusion of ions and thus can be analyzed by patch-clamp single-channel or whole-cell recording. By employing these techniques, several ion-channel types with different electrophysiological profiles have been identified in P. falciparum-infected erythrocytes; they have also been identified in noninfected cells. This review discusses a possible contribution of these channels to the new permeability pathways on the one hand and their supposed functions in noninfected erythrocytes on the other.

Plasmodium falciparum likely encodes the principal anion channel on infected human erythrocytes

Invasion by the human malaria parasite, Plasmodium falciparum, is associated with marked yet selective increases in red blood cell (RBC) membrane permeability. We previously identified an unusual voltage-dependent ion channel, the plasmodial surface anion channel (PSAC), which may account for these increases. Since then, controversy has arisen about whether there are additional parasite-induced anion channels on the RBC membrane and whether these channels are parasite-encoded proteins or the result of modifications of an endogenous host protein. Here, we used genetically divergent parasite isolates and quantitative transport measurements to examine these questions. Our studies indicate that PSAC alone can adequately account for the increased permeability of infected RBCs to key solutes. Two distinct parasite isolates, grown in RBCs from a single donor, exhibit channel activity with measurably different voltage-dependent gating, a finding difficult to reconcile with simple activation or modification of a host protein. Instead, this difference in channel gating can be conservatively explained by a small number of polymorphisms in a parasite gene that encodes PSAC. The absence of known eukaryotic ion channel homologues in the completed P falciparum genome suggests a novel channel gene, and substantiates PSAC as a target for antimalarial development.

Antiplasmodial chalcones inhibit sorbitol-induced hemolysis of Plasmodium falciparum-infected erythrocytes

A series of alkoxylated and hydroxylated chalcones previously reported to have antiplasmodial activities in vitro were investigated for their effects on the new permeation pathways induced by the malaria parasite in the host erythrocyte membrane. Of 21 compounds with good antiplasmodial activities (50% inhibitory concentrations [IC(50)s], < or = 20 microM), 8 members were found to inhibit sorbitol-induced lysis of parasitized erythrocytes to a significant extent (< or = 40% of control values) at a concentration (10 microM) that was close to their antiplasmodial IC(50)s. Qualitative structure-activity analysis suggested that activity was governed to a greater extent by a substitution on ring B than on ring A of the chalcone template. Most of the active compounds had methoxy or dimethoxy groups on ring B. Considerable variety was permitted on ring A in terms of the electron-donating or -withdrawing property. Lipophilicity did not appear to be an important determinant for activity. Although they are not exceptionally potent as inhibitors (lowest IC(50), 1.9 microM), the chalcones compare favorably with other more potent inhibitors in terms of their selective toxicities against plasmodia and their neutral character.

Uptake of nitrobenzylthioinosine and purine beta-L-nucleosides by intracellular Toxoplasma gondii

Intracellular Toxoplasma gondii grown in human foreskin fibroblast cells transported nitrobenzylthioinosine [NBMPR; 6-[(4-nitrobenzyl)mercapto]-9-beta-D-ribofuranosylpurine], an inhibitor of nucleoside transport in mammalian cells, as well as the nonphysiological beta-L-enantiomers of purine nucleosides, beta-L-adenosine, beta-L-deoxyadenosine, and beta-L-guanosine. The beta-L-pyrimidine nucleosides, beta-L-uridine, beta-L-cytidine, and beta-L-thymidine, were not transported. The uptake of NBMPR and the nonphysiological purine nucleoside beta-L-enantiomers by the intracellular parasites also implies that Toxoplasma-infected cells can transport these nucleosides. In sharp contrast, under the same conditions, uninfected fibroblast cells did not transport NBMPR or any of the unnatural beta-L-nucleosides. beta-D-Adenosine and dipyridamole, another inhibitor of nucleoside transport, inhibited the uptake of NBMPR and beta-L-stereoisomers of the purine nucleosides by intracellular Toxoplasma and Toxoplasma-infected cells. Furthermore, infection with a Toxoplasma mutant deficient in parasite adenosine/purine nucleoside transport reduced or abolished the uptake of beta-D-adenosine, NBMPR, and purine beta-L-nucleosides. Hence, the presence of the Toxoplasma adenosine/purine nucleoside transporters is apparently essential for the uptake of NBMPR and purine beta-L-nucleosides by intracellular Toxoplasma and Toxoplasma-infected cells. These results also demonstrate that, in contrast to the mammalian nucleoside transporters, the Toxoplasma adenosine/purine nucleoside transporter(s) lacks stereospecificity and substrate specificity in the transport of purine nucleosides. In addition, infection with T. gondii confers the properties of the parasite's purine nucleoside transport on the parasitized host cells and enables the infected cells to transport purine nucleosides that were not transported by uninfected cells. These unique characteristics of purine nucleoside transport in T. gondii may aid in the identification of new promising antitoxoplasmic drugs.

Channels and transporters as drug targets in the Plasmodium-infected erythrocyte

Throughout the intraerythrocytic phase of its lifecycle the malaria parasite is separated from the extracellular medium by the plasma membrane of its host erythrocyte and by the parasitophorous vacuole in which the parasite is enclosed. The intracellular parasite itself has, at its surface, a plasma membrane, and has a variety of membrane-bound organelles which carry out a range of biochemical functions. Each of the various membranes of the infected cell have in them proteins that facilitate the movement of molecules and ions from one side of the membrane to the other. These 'channels' and 'transporters' play a central role in the physiology of the parasitised cell. From a clinical viewpoint they are of interest both as potential targets in their own right, and as potential drug targeting routes capable of mediating the entry of cytotoxic drugs into the appropriate compartment of the infected cell. In this review both of these aspects are considered.

Extracellular lysines on the plasmodial surface anion channel involved in Na+ exclusion

The human malaria parasite, Plasmodium falciparum, induces an unusual ion channel, the plasmodial surface anion channel (PSAC), on its host red blood cell (RBC) membrane. PSAC has a broad selectivity with permeability to anions, sugars, amino acids, purines, and certain vitamins, suggesting a role in nutrient acquisition by the intracellular parasite. Permeating solutes cover a range of molecular sizes and may be either neutral or carry a net negative or positive charge. Despite this broad selectivity, PSAC must efficiently exclude Na+ to avoid osmotic lysis of infected RBCs in the bloodstream. Here, we used amine-reactive N-hydroxysulfosuccinimide esters to probe PSAC's unusual selectivity. PSAC permeation rates, measured with both a kinetic osmotic lysis assay and single-channel patch-clamp, irreversibly decrease after treatment with these reagents. Sequential labelings with different esters and the effects of their chain length suggest that PSAC has multiple lysine residues near its extracellular pore mouth and that inhibition occurs via steric hindrance of its pore by the amide-linked side chain. When combined with the effects of pH on permeation, these findings implicate a combination of cation repulsion by pore mouth charges and a weak binding site for permeant solutes in PSAC's broad selectivity yet effective exclusion of Na+.

Effects of purine nucleosides on the in vitro growth of Cryptosporidium parvum

The effect of purine nucleosides on the in vitro growth of Cryptosporidium parvum was studied. Culturing the parasite in THP-1 cells for 72 h in growth medium supplemented with adenosine or inosine improved the parasite yields especially in the first 48 h. Similar results were obtained with parasites cultured in Madin-Darby bovine kidney cells and incubated for 24 h with inosine. The addition of inosine to 72-h cultures enhanced the growth of C. parvum in THP-1 cells, especially the trophic stages, whereas the analogue formycin B was toxic to the parasites and induced a marked decrease in the gamont stages. The monitoring of the added purine nucleosides by high performance liquid chromatography showed that at 37 degrees C in the presence of THP-1 cells, a rapid uptake of inosine occurred with hypoxanthine being the main purine present after 2 h in the medium.

Potential chemotherapeutic targets in the purine metabolism of parasites

Parasites are responsible for a wide variety of infectious diseases in human as well as in domestic and wild animals, causing an enormous health and economical blight. Current containment strategies are not entirely successful and parasitic infections are on the rise. In the absence of availability of antiparasitic vaccines, chemotherapy remains the mainstay for the treatment of most parasitic diseases. However, there is an urgent need for new drugs to prevent or combat some major parasitic infections because of lack of a single effective approach for controlling the parasites (e.g., trypanosomiasis) or because some serious parasitic infections developed resistance to presently available drugs (e.g., malaria). The rational design of a drug is usually based on biochemical and physiological differences between pathogens and host. Some of the most striking differences between parasites and their mammalian host are found in purine metabolism. Purine nucleotides can be synthesized by the de novo and/or the so-called "salvage" pathways. Unlike their mammalian host, most parasites studied lack the pathways for de novo purine biosynthesis and rely on the salvage pathways to meet their purine demands. Moreover, because of the great phylogenic separation between the host and the parasite, there are in some cases sufficient distinctions between corresponding enzymes of the purine salvage from the host and the parasite that can be exploited to design specific inhibitors or "subversive substrates" for the parasitic enzymes. Furthermore, the specificities of purine transport, the first step in purine salvage, diverge significantly between parasites and their mammalian host. This review highlights the unique transporters and enzymes responsible for the salvage of purines in parasites that could constitute excellent potential targets for the design of safe and effective antiparasitic drugs.

A two-compartment model of osmotic lysis in Plasmodium falciparum-infected erythrocytes

We recently identified a voltage-dependent anion channel on the surface of human red blood cells (RBCs) infected with the malaria parasite, Plasmodium falciparum. This channel, the plasmodial erythrocyte surface anion channel (PESAC), likely accounts for the increased permeability of infected RBCs to various small solutes, as assessed quantitatively with radioisotope flux and patch-clamp studies. Whereas this increased permeability has also been studied by following osmotic lysis of infected cells in permeant solutes, these experiments have been limited to qualitative comparisons of lysis rates. To permit more quantitative examination of lysis rates, we have developed a mathematical model for osmotic fragility of infected cells based on diffusional uptake via PESAC and the two-compartment geometry of infected RBCs. This model, combined with a simple light scattering assay designed to track osmotic lysis precisely, produced permeability coefficients that match both previous isotope flux and patch-clamp estimates. Our model and light scattering assay also revealed Michaelian kinetics for inhibition of PESAC by furosemide, suggesting a 1:1 stoichiometry for their interaction.

Localization of the Plasmodium falciparum PfNT1 nucleoside transporter to the parasite plasma membrane

Nutrient transporters play critical roles in parasite metabolism, but the membranes in which they reside have not been clearly defined. The transport of purine nutrients is crucial to the survival of the malaria parasite Plasmodium falciparum, and nucleoside transport activity has been associated with a number of different membrane components within the parasitized erythrocyte. To determine the location of the PfNT1 nucleoside transporter, the first component of the nucleoside permeation pathway to be studied at the molecular level in P. falciparum (Carter, N. S., Ben Mamoun, C., Liu, W., Silva, E. O., Landfear, S. M., Goldberg, D. E., and Ullman, B. (2000) J. Biol. Chem. 275, 10683-10691), polyclonal antisera against the NH2-terminal 36 amino acids of PfNT1 were raised in rabbits. Western blot analysis of parasite lysates revealed that the antibodies were specific for PfNT1 and that the level of PfNT1 protein in the infected erythrocyte is regulated in a stage-specific fashion. The amount of PfNT1 polypeptide increases dramatically during the early trophozoite stage and reaches its maximal level in the late trophozoite and schizont stages. Deconvolution and immunoelectron microscopy using these monospecific antibodies revealed that PfNT1 localizes predominantly, if not exclusively, to the plasma membrane of the parasite and not to the parasitophorous vacuolar or erythrocyte membranes.

Nutrient acquisition by intracellular apicomplexan parasites: staying in for dinner

The intracellular forms of the apicomplexan parasites Plasmodium, Toxoplasma and Eimeria reside within a parasitophorous vacuole. The nutrients required by these intracellular parasites to support their high rate of growth and replication originate from the host cell which, in turn, takes up such compounds from the extracellular milieu. Solutes moving from the external medium to the interior of the parasite, are confronted by a series of three membranes --the host cell membrane, the parasitophorous vacuole membrane and the parasite plasma membrane. Each constitutes a potential permeability barrier which must be either crossed or bypassed. It is the mechanisms by which this occurs that are the subject of this review.

5-(Trifluoromethyl)-beta-l-2'-deoxyuridine, the L-enantiomer of trifluorothymidine: stereospecific synthesis and antiherpetic evaluations

As a part of our ongoing work on beta-L-nucleoside analogues as potential antiviral drugs, we have synthesized 5-(trifluoromethyl)-beta-L-2'-deoxyuridine (L-TFT), the hitherto unknown L-enantiomer of trifluorothymidine (CF(3)dUrd, TFT). We have also studied the effect of L-TFT on human and herpes simplex virus (HSV) type 1 and 2 thymidine kinases, and human thymidine phosphorylase, as well as its anti-HSV-1 and anti-HSV-2 activities in cell cultures. L-TFT has been found: (i) to inhibit HSV-1 TK with activity comparable to TFT, with no effect on human TK, (ii) to be phosphorylated by the viral enzyme with similar efficiency to TFT, (iii) to be resistant, in contrast to TFT, to hydrolysis by human thymidine phosphorylase. Unfortunately, when evaluated in cell cultures, L-TFT did not show any anti-HSV-1 and anti-HSV-2 activities.

Diamidine compounds: selective uptake and targeting in Plasmodium falciparum

Extensive drug resistance in Plasmodium falciparum emphasizes the urgent requirement for novel antimalarial agents. Here we report potent antimalarial activity of a number of diamidine compounds. The lead compound pentamidine is concentrated 500-fold by erythrocytes infected with P. falciparum. Pentamidine accumulation can be blocked by inhibitors of hemoglobin digestion, suggesting that the drug binds to ferriprotoporphyrin IX (FPIX). All of the compounds bound to FPIX in vitro and inhibited the formation of hemozoin. Furthermore, inhibitors of hemoglobin digestion markedly antagonized the antimalarial activity of the diamidines, indicating that binding to FPIX is crucial for the activity of diamidine drugs. Pentamidine was not accumulated into uninfected erythrocytes. Pentamidine transport into infected cells exhibits an initial rapid phase, nonsaturable in the micromolar range and sensitive to inhibition by furosemide and glibenclamide. Changing the counter-ion in the order Cl(-) < Br(-) < NO(2)(-) < I(-) <SCN(-) markedly stimulated pentamidine transport. These data suggest that pentamidine is transported although a pore or ion channel with properties similar to those of the recently characterized 'induced permeability pathway' on the infected red cell membrane. In summary, the diamidines exhibit two levels of selectivity against P. falciparum. The route of entry and molecular target are both specific to malaria-infected cells and are distinct from targets in other protozoa. Drugs that target the hemoglobin degradation pathway of malaria parasites have a proven record of accomplishment. The employment of induced permeability pathways to access this target represents a novel approach to antiparasite chemotherapy and offers an additional level of selectivity.