Cloning of a new cation ATPase from Plasmodium falciparum: conservation of critical amino acids involved in calcium binding in mammalian organellar Ca(2+)-ATPases

The genetic Ca2+ sensor GCaMP3 reveals multiple Ca2+ stores differentially coupled to Ca2+ entry in the human malaria parasite Plasmodium falciparum

Cytosolic Ca²⁺ regulates multiple steps in the host-cell invasion, growth, proliferation, and egress of blood-stage Plasmodium falciparum, yet our understanding of Ca²⁺ signaling in this endemic malaria parasite is incomplete. By using a newly generated transgenic line of P. falciparum (PfGCaMP3) that expresses constitutively the genetically encoded Ca²⁺ indicator GCaMP3, we have investigated the dynamics of Ca²⁺ release and influx elicited by inhibitors of the sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase pumps, cyclopiazonic acid (CPA), and thapsigargin (Thg). Here we show that in isolated trophozoite phase parasites: (i) both CPA and Thg release Ca²⁺ from intracellular stores in P. falciparum parasites; (ii) Thg is able to induce Ca²⁺ release from an intracellular compartment insensitive to CPA; (iii) only Thg is able to activate Ca²⁺ influx from extracellular media, through a mechanism resembling store-operated Ca²⁺ entry, typical of mammalian cells; and (iv) the Thg-sensitive Ca²⁺ pool is unaffected by collapsing the mitochondria membrane potential with the uncoupler carbonyl cyanide m-chlorophenyl hydrazone or the release of acidic Ca²⁺ stores with nigericin. These data suggest the presence of two Ca²⁺ pools in P. falciparum with differential sensitivity to the sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase pump inhibitors, and only the release of the Thg-sensitive Ca²⁺ store induces Ca²⁺ influx. Activation of the store-operated Ca²⁺ entry-like Ca²⁺ influx may be relevant for controlling processes such as parasite invasion, egress, and development mediated by kinases, phosphatases, and proteases that rely on Ca²⁺ levels for their activation.

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.

The malaria parasite cation ATPase PfATP4 and its role in the mechanism of action of a new arsenal of antimalarial drugs

The intraerythrocytic malaria parasite, Plasmodium falciparum, maintains a low cytosolic Na(+) concentration and the plasma membrane P-type cation translocating ATPase 'PfATP4' has been implicated as playing a key role in this process. PfATP4 has been the subject of significant attention in recent years as mutations in this protein confer resistance to a growing number of new antimalarial compounds, including the spiroindolones, the pyrazoles, the dihydroisoquinolones, and a number of the antimalarial agents in the Medicines for Malaria Venture's 'Malaria Box'. On exposure of parasites to these compounds there is a rapid disruption of cytosolic Na(+). Whether, and if so how, such chemically distinct compounds interact with PfATP4, and how such interactions lead to parasite death, is not yet clear. The fact that multiple different chemical classes have converged upon PfATP4 highlights its significance as a potential target for new generation antimalarial agents. A spiroindolone (KAE609, now known as cipargamin) has progressed through Phase I and IIa clinical trials with favourable results. In this review we consider the physiological role of PfATP4, summarise the current repertoire of antimalarial compounds for which PfATP4 is implicated in their mechanism of action, and provide an outlook on translation from target identification in the laboratory to patient treatment in the field.

Using genetic methods to define the targets of compounds with antimalarial activity

Although phenotypic cellular screening has been used to drive antimalarial drug discovery in recent years, in some cases target-based drug discovery remains more attractive. This is especially true when appropriate high-throughput cellular assays are lacking, as is the case for drug discovery efforts that aim to provide a replacement for primaquine (4-N-(6-methoxyquinolin-8-yl)pentane-1,4-diamine), the only drug that can block Plasmodium transmission to Anopheles mosquitoes and eliminate liver-stage hypnozoites. At present, however, there are no known chemically validated parasite protein targets that are important in all Plasmodium parasite developmental stages and that can be used in traditional biochemical compound screens. We propose that a plethora of novel, chemically validated, cross-stage antimalarial targets still remain to be discovered from the ~5,500 proteins encoded by the Plasmodium genomes. Here we discuss how in vitro evolution of drug-resistant strains of Plasmodium falciparum and subsequent whole-genome analysis can be used to find the targets of some of the many compounds discovered in whole-cell phenotypic screens.

Spiroindolones, a potent compound class for the treatment of malaria

Recent reports of increased tolerance to artemisinin derivatives--the most recently adopted class of antimalarials--have prompted a need for new treatments. The spirotetrahydro-beta-carbolines, or spiroindolones, are potent drugs that kill the blood stages of Plasmodium falciparum and Plasmodium vivax clinical isolates at low nanomolar concentration. Spiroindolones rapidly inhibit protein synthesis in P. falciparum, an effect that is ablated in parasites bearing nonsynonymous mutations in the gene encoding the P-type cation-transporter ATPase4 (PfATP4). The optimized spiroindolone NITD609 shows pharmacokinetic properties compatible with once-daily oral dosing and has single-dose efficacy in a rodent malaria model.

Plasmodium in the postgenomic era: new insights into the molecular cell biology of malaria parasites

In this review, we bring together some of the approaches toward understanding the cellular and molecular biology of Plasmodium species and their interaction with their host red blood cells. Considerable impetus has come from the development of new methods of molecular genetics and bioinformatics, and it is important to evaluate the wealth of these novel data in the context of basic cell biology. We describe how these approaches are gaining valuable insights into the parasite-host cell interaction, including (1) the multistep process of red blood cell invasion by the merozoite; (2) the mechanisms by which the intracellular parasite feeds on the red blood cell and exports parasite proteins to modify its cytoadherent properties; (3) the modulation of the cell cycle by sensing the environmental tryptophan-related molecules; (4) the mechanism used to survive in a low Ca(2+) concentration inside red blood cells; (5) the activation of signal transduction machinery and the regulation of intracellular calcium; (6) transfection technology; and (7) transcriptional regulation and genome-wide mRNA studies in Plasmodium falciparum.

Comparative genomic and phylogenetic analyses of calcium ATPases and calcium-regulated proteins in the apicomplexa

The phylum Apicomplexa comprises a large group of early branching eukaryotes that includes a number of human and animal parasites. Calcium controls a number of vital processes in apicomplexans including protein secretion, motility, and differentiation. Despite the importance of calcium as a second messenger, very little is known about the systems that control homeostasis or that regulate calcium signaling in parasites. The recent completion of many apicomplexan genomes provides new opportunity to define calcium response pathways in this group of parasites in comparison to model organisms. Whole-genome comparison between the apicomplexans Plasmodium spp., Cryptosporidium spp., and Toxoplasma gondii revealed the presence of several P-Type Ca2+ transporting ATPases including a single endoplasmic reticulum (ER)-type sarcoplasmic-endoplasmic reticulum Ca2+ ATPase, several Golgi-like Ca2+ ATPases, and a single Ca2+/H+ exchanger. Only T. gondii showed evidence of plasma membrane-type Ca2+ ATPases or voltage-gated calcium channels. Despite pharmacological evidence for IP3 and ryanodine-mediated calcium release, animal-type calcium channels were not readily identified in parasites, indicating they are more similar to plants. Downstream of calcium release, a variety of EF-hand-containing proteins regulate calcium responses. Our analyses detected a single conserved calmodulin (CaM) homologue, 3 distinct centrin (CETN)-caltractin-like proteins, one of which is shared with ciliates, and a variety of deep-branching, CaM-CETN-like proteins. Apicomplexans were also found to contain a wide array of calcium-dependent protein kinases (CDPKs), which are commonly found in plants. Toxoplasma gondii contains more than 20 CDPK or CDPK-related kinases, which likely regulate a variety of responses including secretion, motility, and differentiation. Genomic and phylogenetic comparisons revealed that apicomplexans contain a variety of unusual calcium response pathways that are distinct from those seen in vertebrates. Notably, plant-like pathways for calcium release channels and calcium-dependent kinases are found in apicomplexans. The experimental flexibility of T. gondii should allow direct experimental manipulation of these pathways to validate their biological roles. The central importance of calcium in signaling and development, and the novel characteristics of many of these systems, indicates that parasite calcium pathways may be exploited as new therapeutic targets for intervention.

Cysteine-protease activity elicited by Ca2+ stimulus in Plasmodium

Bloodstage malaria parasites require proteolytic activity for key processes as invasion, hemoglobin degradation and merozoite escape from red blood cells (RBCs). We investigated by confocal microscopy the presence of cysteine-protease activity elicited by calcium stimulus in Plasmodium chabaudi and Plasmodium falciparum in free trophozoites or for the later parasite within RBC using fluorescence resonance energy transfer (FRET) peptides. Peptide probes access, to either free or intraerythrocytic parasites, was also tested by selecting a range of fluorescent peptides (653-3146 Da molecular mass) labeled with Abz or FITC. In the present work we show that Ca2+ stimulus elicited by treatment with either melatonin, thapsigargin, ionomicin or nigericin, promotes an increase of substrate hydrolysis, which was blocked by the specific cysteine-protease inhibitor E-64 and the intracellular Ca2+ chelator, BAPTA. When parasites were treated with cytoplasmic Ca2+ releasing compounds, a cysteine-protease was labeled in the parasite cytoplasm by the fluorescent specific irreversible inhibitor, Ethyl-Eps-Leu-Tyr-Cap-Lys(Abz)-NH2, where Ethyl-Eps is Ethyl-(2S,3S)-oxirane-2,3-dicarboxylate. In summary, we demonstrate that P. chabaudi and P. falciparum have a cytoplasmic dependent cysteine-protease activity elicited by Ca2+.

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

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

Transport processes in Plasmodium falciparum-infected erythrocytes: potential as new drug targets

Plasmodium falciparum infection induces alterations in the transport properties of infected erythrocytes that have recently been defined using electrophysiological techniques. Mechanisms responsible for transport of substrates into intraerythrocytic parasites have also been clarified by studies of three substrate-specific (hexose, nucleoside and aquaglyceroporin) parasite plasma membrane transporters. These have been characterised functionally using the Xenopus laevis oocyte heterologous expression system. The same expression system is currently being used to define the function of parasite 'P' type ATPases responsible for intraparasitic [Ca(2+)] homeostasis. We review studies on these transport processes and examine their potential as novel drug targets.

Transport proteins of Plasmodium falciparum: defining the limits of metabolism

In this review we give an account of transport processes occurring at the membrane interface that separates the asexual stage of Plasmodium falciparum from its host, the infected erythrocyte, and also describe proteins whose activities may be important at this location. We explain the potential clinical value of such studies in the light of the current spread of parasite resistance to conventional antimalarial strategies. We discuss the uptake of substrates critical to the survival of the intracellular malaria parasite, and also the parasite's homeostatic and disposal mechanisms. The use of the Xenopus laevis expression system in the characterisation of a hexose transporter ("PfHT1") and a Ca(2+) ATPase ("PfATP4") of the parasite plasma membrane are described in detail.

Characterization of P-type ATPase 3 in Plasmodium falciparum

We report the nucleotide sequence, derived amino acid sequence and expression profile of P-type ATPase 3 (PfATPase3) from Plasmodium falciparum. An open reading frame of 7362 nucleotides, interrupted by a single intron of 168 nt, encoded a protein product of 2394 amino acids with a predicted MW of 282791 Da. Hydropathy analysis of PfATPase3 revealed six amino-terminal and six carboxyl-terminal membrane spanning regions (M1-12) flanking a large hydrophilic domain with a smaller hydrophilic loop between M4 and M5. Based on a phylogenetic comparison of conserved domains present in P-type ATPases from other organisms, PfATPase3 resembled a Type-V ATPase for which the transport affinity is unknown. The PfATPase3 topology was interrupted by four regions, termed 'inserts', unique to malarial P-type ATPases, which were high in asparagine residues and charged amino acids (inserts I1-I4). Inserts I1 and I3 also contained repeated amino acid motifs. The number and composition of repeated amino acid motifs in insert I3 were variable in seven P. falciparum strains tested. PfATPase3 was 80.2% similar to the non-insert portions of P. yoelii ATPase3, although their inserts differed in length and composition. PfATPase3 mRNA was most abundant relative to beta-tubulin during the latter half of the erythrocytic cycle and was also present in gametocytes. Using affinity-purified antibody to a 14 amino acid PfATPase3 epitope, a 260 kDa protein was detected by Western analysis. Based on immunofluorescence, the PfATPase3 protein was located intracellularly in gametocytes and, to a lesser extent, in late erythrocytic stages.

Expression and functional characterization of a Plasmodium falciparum Ca2+-ATPase (PfATP4) belonging to a subclass unique to apicomplexan organisms

We have obtained a full-length P type ATPase sequence (PfATP4) encoded by Plasmodium falciparum and expressed PfATP4 in Xenopus laevis oocytes to study its function. Comparison of the hitherto incomplete open reading frame with other Ca(2+)-ATPase sequences reveals that PfATP4 differs significantly from previously defined categories. The Ca(2+)-dependent ATPase activity of PfATP4 is stimulated by a much broader range of [Ca(2+)](free) (3.2-320 micrometer) than are an avian SERCA1 pump or rabbit SERCA 1a (maximal activity < 10 micrometer). The activity of PfATP4 is resistant to inhibition by ouabain (200 micrometer) or thapsigargin (0.8 micrometer) but is inhibited by vanadate (1 mM) or cyclopiazonic acid (1 microM). We used a quantitative polymerase chain reaction to assay expression of mRNA encoding PfATP4 relative to that for beta-tubulin in synchronized asexual stages and found variable expression throughout the life cycle with a maximal 5-fold increase in meronts compared with ring stages. This analysis suggests that PfATP4 defines a novel subclass of Ca(2+)-ATPases unique to apicomplexan organisms and therefore offers potential as a drug target.

Membrane transport in the malaria-infected erythrocyte

The malaria parasite is a unicellular eukaryotic organism which, during the course of its complex life cycle, invades the red blood cells of its vertebrate host. As it grows and multiplies within its host blood cell, the parasite modifies the membrane permeability and cytosolic composition of the host cell. The intracellular parasite is enclosed within a so-called parasitophorous vacuolar membrane, tubular extensions of which radiate out into the host cell compartment. Like all eukaryote cells, the parasite has at its surface a plasma membrane, as well as having a variety of internal membrane-bound organelles that perform a range of functions. This review focuses on the transport properties of the different membranes of the malaria-infected erythrocyte, as well as on the role played by the various membrane transport systems in the uptake of solutes from the extracellular medium, the disposal of metabolic wastes, and the origin and maintenance of electrochemical ion gradients. Such systems are of considerable interest from the point of view of antimalarial chemotherapy, both as drug targets in their own right and as routes for targeting cytotoxic agents into the intracellular parasite.

The permeability properties of the parasite cell membrane

The asexual development of the malaria parasite takes place inside the host's erythrocyte, an environment that is different from that of most other eukaryotic organisms. The intense and rapid development of the parasite, as well as the homeostatic regulation of its cellular composition, require an extensive exchange of material between the parasite and its immediate surroundings. Studies on free murine parasite species suggest that a plasma membrane H+ pump is responsible for the maintenance of membrane potential and pH gradient, which are used as driving forces for the uptake of glucose and extrusion of Ca2+ by means of a symporter and an antiporter, respectively. In Plasmodium falciparum, a similar transport of Ca2+ may prevail. Several other transporters have been assigned to the plasma membrane of this parasite, either by direct measurements or by inference: D-glucose, nucleosides, L-amino acids, L-lactate and pantothenic acid. A Na+/H+ antiporter has been demonstrated, and implicated in the regulation of pH, and an ATP/ADP antiporter, whose function remains controversial, has been characterized. The presence of Mg2+ and Na+/K+ pumps and an active extrusion of oxidized glutathione can be inferred from the composition of the parasite cytosol vs. that of the host cell. Several genes coding for cation pumps have been cloned and their functions await characterization.

Calcium homeostasis and signaling in the blood-stage malaria parasite

The nature of the mechanisms underlying Ca2+ homeostasis in malaria parasites has puzzled investigators for almost two decades. This review summarizes the current knowledge about Ca2+ homeostasis in Plasmodium spp and highlights some key aspects of this process that are specific to this parasite. Plasmodium spp are exposed, during their intracellular stage, not to the usual millimolar concentrations of Ca2+ found in body fluids, but rather to the very low Ca2+ environment of the host cell cytoplasm. Two crucial questions then arise: (1) how is Ca2+ homeostasis achieved by these protozoa; and (2) do they use Ca2+-based signaling pathways? By critically reviewing the recent literature in the field, Célia Garcia here provides at least some partial answers to these questions.

Identification of an endoplasmic reticulum-resident calcium-binding protein with multiple EF-hand motifs in asexual stages of Plasmodium falciparum

An endoplasmic reticulum-located, calcium-binding protein, with an apparent molecular weight (Mr) of approximately 40,000 (PfERC), has been identified in the asexual stages of the malaria parasite, Plasmodium falciparum. This protein appears to be equivalent to a previously described gametocyte protein, Pfs40, which was reported to be expressed on the gametocyte surface (Rawlings DJ, Kaslow DC. J Biol Chem 1992;267:3976-3982). Sequencing of the 3' end of the gene revealed the omission of a single base in the 3' region of the published sequence. The corrected gene sequence encodes a C-terminal IDEL motif, which indicates residency of the 40 kDa protein within the endoplasmic reticulum. The predicted C-terminal region also appears to contain a sixth EF-hand calcium-binding domain, which suggests that PfERC is related to previously reported ER-localized calcium-binding proteins, namely reticulocalbin and ERC-55 (Ozawa M. J. Biochem. 1995;117:1113-1119; Weis K, Griffiths G, Lamond AI. J. Biol. Chem. 1994;269:19142-19150). The presence of the 40 kDa calcium-binding protein in malaria parasites was confirmed using 45Ca2+-blotting and partial protein sequencing of the corresponding Coomassie blue-stained polypeptide. Confocal immunofluorescence microscopy of asexual stage parasites was used to show that PfERC co-localizes with the known ER-located protein, Pfgrp. Analysis of immunoblots of tightly synchronized parasites showed that expression of PfERC increases with increasing maturity of the parasite. We propose that PfERC is a member of the reticulocalbin family of calcium-binding proteins and may play a role in protein trafficking in the malaria parasite.

Analysis of a cation-transporting ATPase of Plasmodium falciparum

We have cloned and characterised one gene, PfATPase4 which encodes a P-type ATPase containing all the primary sequence motifs characteristic of this class of transmembrane ion transporters, and also a fragment of a second P. falciparum P-type ATPase pseudogene (PfATPase5). Analysis of conserved domains and motifs of specific ATPases reveals that PfATPase4 is most analogous to Ca2+ ATPases of the endoplasmic reticulum. The PfATPase4 gene gives rise to a transcript of 8 kb shortly after erythrocyte invasion. Although this mRNA is not detected in later stages, the protein detected immunologically at 190 kDa persists throughout and is detected in free merozoites. Immunofluorescence microscopy reveals that the PfATPase4 protein is concentrated in discrete compartments at the periphery of the parasite. Detailed sequence and structural analyses of these and the other P-type ATPases of P. falciparum described previously, reveals that they comprise an unusual family in several respects. Firstly, the large number of non-homologous genes so far characterised reflects the complexities of ionic regulation in the diverse environments encountered by the parasite. Secondly, the plasmodial P-type ATPase family may be classified both at primary sequence and structural levels into two distinct groups-those typical of P-type ATPases (including PfATPase4) and those which are much more divergent. A third complexity is illustrated by the fact that one of the other members [1] here termed PfATPase6, has an even greater similarity to the sarcoplasmic reticulum Ca2+ ATPases than does PfATPase4, which raises questions about the possible functional relationship between these two members.