A novel pathway for Ca++ entry into Plasmodium falciparum-infected blood cells

CDPKs: The critical decoders of calcium signal at various stages of malaria parasite development

Calcium ions are used as important signals during various physiological processes. In malaria parasites, Plasmodium spp., calcium dependent protein kinases (CDPKs) have acquired the unique ability to sense and transduce calcium signals at various critical steps during the lifecycle, either through phosphorylation of downstream substrates or mediating formation of high molecular weight protein complexes. Calcium signaling cascades establish important crosstalk events with signaling pathways mediated by other secondary messengers such as cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). CDPKs play critical roles at various important physiological steps during parasite development in vertebrates and mosquitoes. They are also important for transmission of the parasite between the two hosts. Combined with the fact that CDPKs are not present in humans, they continue to be pursued as important targets for development of anti-malarial drugs.

Calcium in the Backstage of Malaria Parasite Biology

The calcium ion (Ca2+) is a ubiquitous second messenger involved in key biological processes in prokaryotes and eukaryotes. In Plasmodium species, Ca2+ signaling plays a central role in the parasite life cycle. It has been associated with parasite development, fertilization, locomotion, and host cell infection. Despite the lack of a canonical inositol-1,4,5-triphosphate receptor gene in the Plasmodium genome, pharmacological evidence indicates that inositol-1,4,5-triphosphate triggers Ca2+ mobilization from the endoplasmic reticulum. Other structures such as acidocalcisomes, food vacuole and mitochondria are proposed to act as supplementary intracellular Ca2+ reservoirs. Several Ca2+-binding proteins (CaBPs) trigger downstream signaling. Other proteins with no EF-hand motifs, but apparently involved with CaBPs, are depicted as playing an important role in the erythrocyte invasion and egress. It is also proposed that a cross-talk among kinases, which are not members of the family of Ca2+-dependent protein kinases, such as protein kinases G, A and B, play additional roles mediated indirectly by Ca2+ regulation. This statement may be extended for proteins directly related to invasion or egress, such as SUB1, ERC, IMC1I, IMC1g, GAP45 and EBA175. In this review, we update our understanding of aspects of Ca2+-mediated signaling correlated to the developmental stages of the malaria parasite life cycle.

Promises and Pitfalls of Parasite Patch-clamp

Protozoan parasites acquire essential ions, nutrients, and other solutes from their insect and vertebrate hosts by transmembrane uptake. For intracellular stages, these solutes must cross additional membranous barriers. At each step, ion channels and transporters mediate not only this uptake but also the removal of waste products. These transport proteins are best isolated and studied with patch-clamp, but these methods remain accessible to only a few parasitologists due to specialized instrumentation and the required training in both theory and practice. Here, we provide an overview of patch-clamp, describing the advantages and limitations of the technology and highlighting issues that may lead to incorrect conclusions. We aim to help non-experts understand and critically assess patch-clamp data in basic research studies.

A prospective mechanism and source of cholesterol uptake by Plasmodium falciparum-infected erythrocytes co-cultured with HepG2 cells

Plasmodium falciparum (P. falciparum) parasites still cause lethal infections worldwide, especially in Africa (https://www.who.int/publications/i/item/world-malaria-report-2019). During P. falciparum blood-stage infections in humans, low-density lipoprotein, high-density lipoprotein and cholesterol levels in the blood become low. Because P. falciparum lacks a de novo cholesterol synthesis pathway, it must import cholesterol from the surrounding environment. However, the origin of the cholesterol and how it is taken up by the parasite across the multiple membranes that surround it is not fully understood. To answer this, we used a cholesterol synthesis inhibiter (simvastatin), a cholesterol transport inhibitor (ezetimibe), and an activating ligand of the peroxisome proliferator-activated receptor α, called ciprofibrate, to investigate the effects of these agents on the intraerythrocytic growth of P. falciparum, both with and without HepG2 cells as the lipoprotein feeders. P. falciparum growth was inhibited in the presence of ezetimibe, but ezetimibe was not very effective at inhibiting P. falciparum growth when used in the co-culture system, unlike simvastatin, which strongly promoted parasite growth in this system. Ezetimibe is known to inhibit cholesterol absorption by blocking the activity of Niemann-Pick C1 like 1 (NPC1L1) protein, and simvastatin is known to enhance NPC1L1 expression in the human body's small intestine. Collectively, our results support the possibility that cholesterol import by P. falciparum involves hepatocytes, and cholesterol uptake into the parasite occurs via NPC1L1 protein or an NPC1L1 homolog during the erythrocytic stages of the P. falciparum lifecycle.

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

Background

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

Results

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

Conclusions

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

ATPe Dynamics in Protozoan Parasites. Adapt or Perish

In most animals, transient increases of extracellular ATP (ATPe) are used for physiological signaling or as a danger signal in pathological conditions. ATPe dynamics are controlled by ATP release from viable cells and cell lysis, ATPe degradation and interconversion by ecto-nucleotidases, and interaction of ATPe and byproducts with cell surface purinergic receptors and purine salvage mechanisms. Infection by protozoan parasites may alter at least one of the mechanisms controlling ATPe concentration. Protozoan parasites display their own set of proteins directly altering ATPe dynamics, or control the activity of host proteins. Parasite dependent activation of ATPe conduits of the host may promote infection and systemic responses that are beneficial or detrimental to the parasite. For instance, activation of organic solute permeability at the host membrane can support the elevated metabolism of the parasite. On the other hand ecto-nucleotidases of protozoan parasites, by promoting ATPe degradation and purine/pyrimidine salvage, may be involved in parasite growth, infectivity, and virulence. In this review, we will describe the complex dynamics of ATPe regulation in the context of protozoan parasite⁻host interactions. Particular focus will be given to features of parasite membrane proteins strongly controlling ATPe dynamics. This includes evolutionary, genetic and cellular mechanisms, as well as structural-functional relationships.

Increased Ca++ uptake by erythrocytes infected with malaria parasites: Evidence for exported proteins and novel inhibitors

Malaria parasites export many proteins into their host erythrocytes and increase membrane permeability to diverse solutes. Although most solutes use a broad-selectivity channel known as the plasmodial surface anion channel, increased Ca⁺⁺ uptake is mediated by a distinct, poorly characterised mechanism that appears to be essential for the intracellular parasite. Here, we examined infected cell Ca⁺⁺ uptake with a kinetic fluorescence assay and the virulent human pathogen, Plasmodium falciparum. Cell surface labelling with N-hydroxysulfosuccinimide esters revealed differing effects on transport into infected and uninfected cells, indicating that Ca⁺⁺ uptake at the infected cell surface is mediated by new or altered proteins at the host membrane. Conditional knockdown of PTEX, a translocon for export of parasite proteins into the host cell, significantly reduced infected cell Ca⁺⁺ permeability, suggesting involvement of parasite-encoded proteins trafficked to the host membrane. A high-throughput chemical screen identified the first Ca⁺⁺ transport inhibitors active against Plasmodium-infected cells. These novel chemical scaffolds inhibit both uptake and parasite growth; improved in vitro potency at reduced free [Ca⁺⁺ ] is consistent with parasite killing specifically via action on one or more Ca⁺⁺ transporters. These inhibitors should provide mechanistic insights into malaria parasite Ca⁺⁺ transport and may be starting points for new antimalarial drugs.

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.

The calcium signaling toolkit of the Apicomplexan parasites Toxoplasma gondii and Plasmodium spp

Apicomplexan parasites have complex life cycles, frequently split between different hosts and reliant on rapid responses as the parasites react to changing environmental conditions. Calcium ion (Ca(2+)) signaling is consequently essential for the cellular and developmental changes that support Apicomplexan parasitism. Apicomplexan genomes reveal a rich repertoire of genes involved in calcium signaling, although many of the genes responsible for observed physiological changes remain unknown. There is evidence, for example, for the presence of a nifedipine-sensitive calcium entry mechanism in Toxoplasma, but the molecular components involved in Ca(2+) entry in both Toxoplasma and Plasmodium, have not been identified. The major calcium stores are the endoplasmic reticulum (ER), the acidocalcisomes, and the plant-like vacuole in Toxoplasma, or the food vacuole in Plasmodium spp. Pharmacological evidence suggests that Ca(2+) release from intracellular stores may be mediated by inositol 1,4,5-trisphosphate (IP3) or cyclic ADP ribose (cADPR) although there is no molecular evidence for the presence of receptors for these second messengers in the parasites. Several Ca(2+)-ATPases are present in Apicomplexans and a putative mitochondrial Ca(2+)/H(+) exchanger has been identified. Apicomplexan genomes contain numerous genes encoding Ca(2+)-binding proteins, with the notable expansion of calcium-dependent protein kinases (CDPKs), whose study has revealed roles in gliding motility, microneme secretion, host cell invasion and egress, and parasite differentiation. Microneme secretion has also been shown to depend on the C2 domain containing protein DOC2 in both Plasmodium spp. and Toxoplasma, providing further evidence for the complex transduction of Ca(2+) signals in these organisms. The characterization of these pathways could lead to the discovery of novel drug targets and to a better understanding of the role of Ca(2+) in these parasites.

A kinetic fluorescence assay reveals unusual features of Ca⁺⁺ uptake in Plasmodium falciparum-infected erythrocytes

Background

To facilitate development within erythrocytes, malaria parasites increase their host cell uptake of diverse solutes including Ca⁺⁺. The mechanism and molecular basis of increased Ca⁺⁺ permeability remains less well studied than that of other solutes.

Methods

Based on an appropriate Ca⁺⁺ affinity and its greater brightness than related fluorophores, Fluo-8 was selected and used to develop a robust fluorescence-based assay for Ca⁺⁺ uptake by human erythrocytes infected with Plasmodium falciparum.

Results

Both uninfected and infected cells exhibited a large Ca⁺⁺-dependent fluorescence signal after loading with the Fluo-8 dye. Probenecid, an inhibitor of erythrocyte organic anion transporters, abolished the fluorescence signal in uninfected cells; in infected cells, this agent increased fluorescence via mechanisms that depend on parasite genotype. Kinetic fluorescence measurements in 384-well microplates revealed that the infected cell Ca⁺⁺ uptake is not mediated by the plasmodial surface anion channel (PSAC), a parasite nutrient channel at the host membrane; it also appears to be distinct from mammalian Ca⁺⁺ channels. Imaging studies confirmed a low intracellular Ca⁺⁺ in uninfected cells and higher levels in both the host and parasite compartments of infected cells. Parasite growth inhibition studies revealed a conserved requirement for extracellular Ca⁺⁺.

Conclusions

Nondestructive loading of Fluo-8 into human erythrocytes permits measurement of Ca⁺⁺ uptake kinetics. The greater Ca⁺⁺ permeability of cells infected with malaria parasites is apparent when probenecid is used to inhibit Fluo-8 efflux at the host membrane. This permeability is mediated by a distinct pathway and may be essential for intracellular parasite development. The miniaturized assay presented here should help clarify the precise transport mechanism and may identify inhibitors suitable 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.

Decreased redox-sensitive erythrocyte cation channel activity in aquaporin 9-deficient mice

Survival of the malaria pathogen Plasmodium falciparum in host erythrocytes requires the opening of new permeability pathways (NPPs) in the host cell membrane, accomplishing entry of nutrients, exit of metabolic waste products such as lactate and movement of inorganic ions such as Cl⁻, Na⁺ and Ca²⁺. The molecular identity of NPPs has remained largely elusive but presumably involves several channels, which partially can be activated by oxidative stress in uninfected erythrocytes. One NPP candidate is aquaporin 9 (AQP9), a glycerol-permeable water channel expressed in erythrocytes. Gene-targeted mice lacking functional AQP9 (aqp⁻/⁻) survive infection with the malaria pathogen Plasmodium berghei better than their wild-type littermates (aqp9⁺/⁺). In the present study whole-cell patch-clamp recordings were performed to explore whether ion channel activity is different in erythrocytes from aqp⁻/⁻ and aqp9⁺/⁺ mice. As a result, the cation conductance (K⁺ > Na⁺ > Ca²⁺ ≫ NMDG⁺) was significantly lower in erythrocytes from aqp⁻/⁻ than in erythrocytes from aqp9⁺/⁺ mice. Oxidative stress by exposure for 15-30 min to 1 mM H₂O₂ or 1 mM tert-butyl-hydroperoxide enhanced the cation conductance and increased cytosolic Ca²⁺ concentration, effects significantly less pronounced in erythrocytes from aqp⁻/⁻ than in erythrocytes from aqp9⁺/⁺ mice. In conclusion, lack of AQP9 decreases the cation conductance of erythrocytes, an effect that possibly participates in the altered susceptibility of AQP9-deficient mice to infection with P. berghei.

Calcium storage and function in apicomplexan parasites

Calcium is relevant for several vital functions in apicomplexan parasites, including host cell invasion, parasite motility and differentiation. The ER (endoplasmic reticulum) and calcium-rich acidocalcisomes have been identified as major calcium stores. Other potential calcium-storage organelles include the Golgi, the mitochondrion, the apicoplast and the recently described plant-like vacuole in Toxoplasma gondii. Compared with most eukaryotic systems, apicomplexan parasites contain a reduced number of calcium-related genes, a vast majority of which remain uncharacterized. Several Ca²⁺-ATPases have been described in apicomplexans, several of which are annotated in the different genomes. There is experimental evidence for an IP3 (inositol 1,4,5-trisphosphate)-dependent calcium response in Plasmodium spp. and T. gondii, although no IP3 or ryanodine receptors have been identified. Genes encoding potential calcium channels are present in T. gondi, but not in Plasmodium spp. and Cryptosporidium spp. Effector calcium-binding proteins including calmodulins and CDPK (calcium-dependent protein kinase) genes mainly found in plants have also been described. The characterized CDPKs were found to play important roles in protein secretion, host cell invasion and parasite differentiation. Taken together, the available information on calcium storage and function in apicomplexans, although fragmented, suggest the existence of unique calcium-mediated pathways in these parasites. An in-depth functional characterization of the apicomplexan calcium-related genes could lead to the identification of novel therapeutic targets, and will improve our understanding of the role of calcium in parasite development and virulence.

Generation of second messengers in Plasmodium

Signalling in malaria parasites is a field of growing interest as its components may prove to be valuable drug targets, especially when one considers the burden of a disease that is responsible for up to 500 million infections annually. The scope of this review is to discuss external stimuli in the parasite life cycle and the upstream machinery responsible for translating them into intracellular responses, focussing particularly on the calcium signalling pathway.

PfCHA is a mitochondrial divalent cation/H+ antiporter in Plasmodium falciparum

The human malaria parasite Plasmodium falciparum is capable of adapting to vastly different extracellular Ca(2+) environments while maintaining tight control of its intracellular Ca(2+) concentration. The mechanisms underpinning Ca(2+) homeostasis in this important pathogen are only partly understood. Here we have functionally expressed the putative Ca(2+)/H(+) antiporter PfCHA in Xenopus laevis oocytes. Our data suggest that PfCHA mediates H(+)-coupled Ca(2+) and Mn(2+) exchange. The apparent dissociation constant K(M) for Ca(2+) of 2.2 +/- 0.7 mM and the maximal velocity V(max) of 0.6 +/- 0.1 nmol per oocyte per hour are consistent with PfCHA being a low-affinity high-capacity Ca(2+) carrier. In the parasite, PfCHA was found to localize to the mitochondrion. Physiological studies conducted with live parasitized erythrocytes, and using Fluo-4 and Rhod-2 to monitor cytoplasmic and mitochondrial Ca(2+) dynamics, suggest that the mitochondrion serves as a dynamic Ca(2+) store and that PfCHA functions as a Ca(2+) efflux system expelling excess Ca(2+) from the mitochondrion. PfCHA lacks appreciable homologies to the human mitochondrial Ca(2+)/H(+) exchanger and might represent an evolutionary divergent class of mitochondrial cation antiporter, which, in turn, might provide novel opportunities for intervention.

Malaria parasite mutants with altered erythrocyte permeability: a new drug resistance mechanism and important molecular tool

Erythrocytes infected with plasmodia, including those that cause human malaria, have increased permeability to a diverse collection of organic and inorganic solutes. While these increases have been known for decades, their mechanistic basis was unclear until electrophysiological studies revealed flux through one or more ion channels on the infected erythrocyte membrane. Current debates have centered on the number of distinct ion channels, which channels mediate the transport of each solute and whether the channels represent parasite-encoded proteins or human channels activated after infection. This article reviews the identification of the plasmodial surface anion channel and other proposed channels with an emphasis on two distinct channel mutants generated through in vitro selection. These mutants implicate parasite genetic elements in the parasite-induced permeability, reveal an important new antimalarial drug resistance mechanism and provide tools for molecular studies. We also critically examine the technical issues relevant to the detection of ion channels by electrophysiological methods; these technical considerations have general applicability for interpreting studies of various ion channels proposed for the infected erythrocyte membrane.

Anion channels in Plasmodium-falciparum-infected erythrocytes and protein kinase A

By replicating within red blood cells, malaria parasites are largely hidden from immune recognition; however, in the cells, nutrients are limiting and hazardous metabolic end products can rapidly accumulate. Therefore, to survive within erythrocytes, parasites alter the permeability of the host plasma membrane, either by upregulating existing transporters or by creating new permeation pathways. Recent electrophysiological studies of Plasmodium-infected erythrocytes have demonstrated that membrane permeability is mediated by transmembrane transport through ion channels in the infected erythrocyte. This article discusses the evidence and controversies concerning the nature of these channels and surveys the potential role of phosphorylation in activating anion channels that could be important in developing novel strategies for future malarial chemotherapies.

Artemisinin inhibits cation currents in malaria-infected human erythrocytes

BACKGROUND

Previous patch-clamp studies have demonstrated inwardly and outwardly rectifying anion currents, ClC-2 Cl- currents, and nonselective Ca(++)-permeable cation currents in Plasmodium falciparum-infected human erythrocytes.

METHODS

The current work studied the effect of the potent antimalarial drug artemisinin on the P falciparum infection-induced whole cell currents in human erythrocyte.

RESULTS

Artemisinin had no significant effect on the outwardly rectifying anion currents but inhibited the cation-selective currents with an apparent half-maximal inhibitory concentration of < or =10 micromol/L.

CONCLUSION

Because artemisinin reportedly inhibits the asexual parasite amplification with much higher potency, the antimalarial action of the drug cannot be attributed to the artemisinin effect on the cation currents. However, artemisinin may be used as a pharmacologic tool to dissect different current fractions in P falciparum-infected erythrocytes.

Quantitative Calcium Measurements in Subcellular Compartments of Plasmodium falciparum-infected Erythrocytes

The acidic food vacuole exerts several important functions during intraerythrocytic development of the human malarial parasite Plasmodium falciparum. Hemoglobin taken up from the host erythrocyte is degraded in the food vacuole, and the heme liberated during this process is crystallized to inert hemozoin. Several anti-malarial drugs target food vacuolar pathways, such as hemoglobin degradation and heme crystallization. Resistance and sensitization to some antimalarials is associated with mutations in food vacuolar membrane proteins. Other studies suggest a role of the food vacuole in ion homeostasis, and release of Ca2+ from the food vacuole may mediate adopted physiological responses. To investigate whether the food vacuole is an intracellular Ca2+ store, which in turn may affect other physiological functions in which this organelle partakes, we have investigated total and exchangeable Ca2+ within the parasite's food vacuole using x-ray microanalysis and quantitative confocal live cell Ca2+ imaging. Apparent free Ca2+ concentrations of approximately 90, approximately 350, and approximately 400 nM were found in the host erythrocyte cytosol, the parasite cytoplasm, and the food vacuole, respectively. In our efforts to determine free intracellular Ca2+ concentrations, we evaluated several Ca2+-sensitive fluorochromes in a live cell confocal setting. We found that the ratiometric Ca2+ indicator Fura-Red provides reliable determinations, whereas measurements using the frequently used Fluo-4 are compromised due to problems arising from phototoxicity, photobleaching, and the strong pH dependence of the dye. Our data suggest that the food vacuole contains only moderate amounts of Ca2+, disfavoring a role as a major intracellular Ca2+ store.

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.

Channel-induced apoptosis of infected host cells-the case of malaria

Infection of erythrocytes by the malaria pathogen Plasmodium falciparum leads to activation of several distinct anion channels and a non-selective, Ca2+-permeable cation channel. All channel types are presumably activated by the oxidative stress generated by the pathogen. Similar or identical channels are activated by oxidation of non-infected erythrocytes. Activation of the non-selective cation channel allows entry of Ca2+ and Na+, both of which are required for intracellular growth of the pathogen. The entry of Ca2+ stimulates an intraerythrocytic scramblase that facilitates bi-directional phospholipid migration across the bilayer, resulting in breakdown of the phosphatidylserine asymmetry of the cell membrane. The exposure of phosphatidylserine at the outer surface of the cell membrane is presumably followed by binding to phosphatidylserine receptors on macrophages and subsequent phagocytosis of the affected erythrocyte. The lysosomal degradation may eventually eliminate the pathogen. The channel may thus play a dual role in pathogen survival. Absence of the channels is not compatible with pathogen growth, enhanced channel activity accelerates erythrocyte "apoptosis" that may represent a host defence mechanism serving to eliminate infected erythrocytes.

Calcium regulation in protozoan parasites

The calcium ion (Ca(2+)) is used as a major signaling molecule in a diverse range of eukaryotic cells including several human parasitic protozoa, such as Trypanosoma cruzi, Trypanosoma brucei, Leishmania spp, Plasmodium spp, Toxoplasma gondii, Cryptosporidium parvum, Entamoeba histolytica, Giardia lamblia and Trichomonas vaginalis. Ca(2+) is critical for invasion of intracellular parasites, and its cytosolic concentration is regulated by the concerted operation of several transporters present in the plasma membrane, endoplasmic reticulum, mitochondria and acidocalcisomes. Recent findings have shed light on the function of these transporters, the roles that they play in cellular metabolism and their potential use for targeting them for new therapies.

Inhibition of Ca2+/calmodulin-dependent protein kinase blocks morphological differentiation of plasmodium gallinaceum zygotes to ookinetes

Once ingested by mosquitoes, malaria parasites undergo complex cellular changes. These include zygote formation, transformation of zygote to ookinete, and differentiation from ookinete to oocyst. Within the oocyst, the parasite multiplies into numerous sporozoites. Modulators of intracellular calcium homeostasis, MAPTAM, and TMB-8 blocked ookinete development as did the calmodulin (CaM) antagonists W-7 and calmidazolium. Ca(2+)/CaM-dependent protein kinase inhibitor KN-93 also blocked zygote elongation, while its ineffective analog KN-92 did not have such effect. In vitro both zygote and ookinete extracts efficiently phosphorylated autocamtide-2, a classic CaM kinase substrate, which could be blocked by calmodulin antagonists W-7 and calmidazolium and CaM kinase inhibitor KN-93. These results demonstrated the presence of calmodulin-dependent CaM kinase activity in the parasite. KN-93-treated parasites, however, expressed the ookinete-specific enzyme chitinase and the ookinete surface antigen Pgs28 normally, suggesting that the morphologically untransformed parasites are biochemically mature ookinetes. In mosquitoes, KN-93-treated parasites did not develop as oocysts, while KN-92-treated parasites produced similar numbers of oocysts as controls. These data suggested that in Plasmodium gallinaceum morphological development of zygote to ookinete, but not its biochemical maturation, relies on Ca(2+)/CaM-dependent protein kinase activity and demonstrated that the morphological differentiation is essential for the further development of the parasite in infected blood-fed mosquitoes.

Oxidation induces a Cl(-)-dependent cation conductance in human red blood cells

Oxidative stress induces complex alterations of membrane proteins in red blood cells (RBCs) eventually leading to haemolysis. To study changes of membrane ion permeability induced by oxidative stress, whole-cell patch-clamp recordings and haemolysis experiments were performed in control and oxidised human RBCs. Control RBCs exhibited a small cation-selective whole-cell conductance (236 +/- 38 pS; n = 8) which was highly sensitive to the external Cl(-) concentration: replacement of NaCl in the bath by sodium gluconate induced an increase of this cation conductance by about 85 %. Exposing RBCs to t-butylhydroxyperoxide (1 mM for 10 min) induced a twofold increase in this cation conductance which was further stimulated after replacement of extracellular Cl(-) by gluconate, Br(-), I(-) or SCN(-). In addition, lowering the ionic strength of the bath solution by isosmotic substitution of NaCl by sorbitol activated the cation conductance. The Cl(-)-sensitive and oxidation-induced cation conductance was Ca(2+) permeable, exhibited a permselectivity of Cs(+) > K(+) > Na(+) = Li(+) >> NMDG(+), and was partially inhibited by amiloride (1 mM) and almost completely inhibited by GdCl(3) (150 microM), but was insensitive to TEA, BaCl(2), NPPB, flufenamic acid or quinidine. DIDS (100 microM) reversibly inhibited the activation of the cation conductance by removal of external Cl(-). Oxidation induced haemolysis in NaCl-bathed human RBCs. This haemolysis was attenuated by amiloride (1 mM) and inhibited by replacement of bath Na(+) by the impermeant cation NMDG(+). The Na(+)- and Ca(2+)-permeable conductance might be involved in haemolytic diseases induced by elevated oxidative stress, such as glucose-6-phosphate dehydrogenase deficiency.

Plasmodium falciparum activates endogenous Cl(-) channels of human erythrocytes by membrane oxidation

Intraerythrocytic survival of the malaria parasite Plasmodium falciparum requires that host cells supply nutrients and dispose of waste products. This solute transport is accomplished by infection-induced new permeability pathways (NPP) in the erythrocyte membrane. Here, whole-cell patch-clamp and hemolysis experiments were performed to define properties of the NPP. Parasitized but not control erythrocytes constitutively expressed two types of anion conductances, differing in voltage dependence and sensitivity to inhibitors. In addition, infected but not control cells hemolyzed in isosmotic sorbitol solution. Both conductances and hemolysis of infected cells were inhibited by reducing agents. Conversely, oxidation induced identical conductances and hemolysis in non-infected erythrocytes. In conclusion, P.falciparum activates endogenous erythrocyte channels by applying oxidative stress to the host cell membrane.

Kinetics of inhibition of the plasma membrane calcium pump by vanadate in intact human red cells

The lack of specific inhibitors of the plasma membrane Ca2+ pump (PMCA) has made vanadate (VO3-), a non-specific inhibitor, an invaluable tool in the study of PMCA function. However, three important properties of vanadate as an inhibitor of the PMCA in intact cells, namely its speed of action in different experimental conditions, the reversibility of its inhibitory effects at different doses, and its dose-response, had never been characterized, despite extensive use. We report here the speed, reversibility and dose-response of PMCA inhibition by vanadate in intact human red cells. Near maximal inhibitory concentrations (1mM) in the red cell suspension blocked almost instantly the uphill Ca2+ extrusion by the PMCA, regardless of the intracellular Ca2+ concentration, cation composition of the external media, membrane potential or volume-stability of the cell. PMCA inhibition by vanadate, at concentrations of 10mM and 1mM, was not reversed by washing, resuspending, and incubating the cells for up to 2h in vanadate-free media. Vanadate inhibited PMCA-mediated Ca2+ efflux in intact red cells with a K1/2 of approximately 3 microM, a value similar to that described for the Ca2+-ATPase in isolated red cell membranes.

Cytochemical localisation of calcium ATPase activity during the erythrocytic cell cycle of Plasmodium falciparum

Using a cytochemical technique, we evaluated the levels of Ca(2+)-ATPase activity in the plasmatic and in the parasitophorous vacuole membrane through the different developmental stages of the Plasmodium falciparum parasitised erythrocyte. We found that the activity is detectable and remains unaltered in the plasma membrane throughout the 48 h cell cycle. However, in the parasitophorous membrane, although the activity was very similar to that measured in the plasma membrane of the young stages (younger than 20-h-old parasites), it diminished gradually with maturation and in schizonts it was almost undetectable. These data suggest that the plasma membrane Ca(2+)-ATPase is important in the maintenance of a low erythrocyte cytoplasmic Ca(2+) concentration, and that in addition it could be a way to supply the vital cation to the parasite at the beginning of the infection, when other transport mechanisms have not yet developed.

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 non-selective voltage-activated cation channel in the human red blood cell membrane: reconciliation between two conflicting reports and further characterisation

Using the patch-clamp technique, the non-selective, voltage-activated cation channel in the human red blood cell (RBC) membrane was further characterised. Activity of the cation channel could be demonstrated at a range of salt concentrations with the current-voltage characteristics for monovalent cations going from linear to superlinear functions, depending on the cation concentration in the range of 100-500 mM. The non-selective voltage-activated cation channel was demonstrated to be permeable to the divalent cations Ca2+ and Ba2+, and even Mg2+. The current-voltage relations for the divalent cations were superlinear even at 75 mM salt concentration, but indicated outward rectification in contrast to the I-V curve for monovalent cations. The degree of activation at a given membrane potential depended strongly on the prehistory of the channel. The gating exhibited hysteretic-like behaviour, since the quasi steady-state deactivation and activation curves were displaced by approximately 25 mV. This result fully explains apparent discrepancies between V0.5-values previously obtained by slightly different experimental protocols. The possible physiological/pathophysiological role of the channel is discussed in the context of the demonstrated permeability for divalent cations.

Functional state of the plasma membrane Ca2+ pump in Plasmodium falciparum-infected human red blood cells

The active Ca2+ transport properties of malaria-infected, intact red blood cells are unknown. We report here the first direct measurements of Ca2+ pump activity in human red cells infected with Plasmodium falciparum, at the mature, late trophozoite stage. Ca2+ pump activity was measured by the Co2+-exposure method adapted for use in low-K+ media, optimal for parasitised cells. This required a preliminary study in normal, uninfected red cells of the effects of cell volume, membrane potential and external Na+/K+ concentrations on Ca2+ pump performance. Pump-mediated Ca2+ extrusion in normal red cells was only slightly lower in low-K+ media relative to high-K+ media despite the large differences in membrane potential predicted by the Lew-Bookchin red cell model. The effect was prevented by clotrimazole, an inhibitor of the Ca2+-sensitive K+ (KCa) channel, suggesting that it was due to minor cell dehydration. The Ca2+-saturated Ca2+ extrusion rate through the Ca2+ pump (Vmax) of parasitised red cells was marginally inhibited (2-27 %) relative to that of both uninfected red cells from the malaria-infected culture (cohorts), and uninfected red cells from the same donor kept under identical conditions (co-culture). Thus, Ca2+ pump function is largely conserved in parasitised cells up to the mature, late trophozoite stage. A high proportion of the ionophore-induced Ca2+ load in parasitised red cells is taken up by cytoplasmic Ca2+ buffers within the parasite. Following pump-mediated Ca2+ removal from the host, there remained a large residual Ca2+ pool within the parasite which slowly leaked to the host cell, from which it was pumped out.

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.