Enhanced Ca2+ uptake by mouse erythrocytes in malarial (Plasmodium berghei) infection

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.

Calcium-Responsive Liposomes via a Synthetic Lipid Switch

Liposomal drug delivery would benefit from enhanced control over content release. Here, we report a novel avenue for triggering release driven by chemical composition using liposomes sensitized to calcium-a target chosen due to its key roles in biology and disease. To demonstrate this principle, we synthesized calcium-responsive lipid switch 1, designed to undergo conformational changes upon calcium binding. The conformational change perturbs membrane integrity, thereby promoting cargo release. This was shown through fluorescence-based release assays via dose-dependent response depending on the percentage of 1 in liposomes, with minimal background leakage in controls. DLS experiments indicated dramatic changes in particle size upon treatment of liposomes containing 1 with calcium. In a comparison of ten naturally occurring metal cations, calcium provided the greatest release. Finally, STEM images showed significant changes in liposome morphology upon treatment of liposomes containing 1 with calcium. These results showcase lipid switches driven by molecular recognition principles as an exciting avenue for controlling membrane properties.

Chelation of Ca²⁺ ions by a peptide from the repeat region of the Plasmodium falciparum circumsporozoite protein

BACKGROUND

Elegant efforts towards the determination of the structural tendencies of peptides derived from the Plasmodium falciparum circumsporozoite protein allowed the proposal of a left-handed helical conformation for this protein. The use of circular dichroism and Fourier-transformed infrared spectroscopy applied to various peptides derived from this protein, indicated that they bind Ca²⁺ ions in helical environments. The essential role of calcium in cell function and biological mechanisms is well known. It influences the development of several stages of the P. falciparum parasite. However, there is very little knowledge regarding calcium coordination to circumsporozoite proteins. In the present investigation the chelation of Ca²⁺ by the (NANPNVDP)₃NANP peptide, which contains the first seven 4-amino-acid blocks of the repeat region of the P. falciparum circumsporozoite protein, is tested with the use of circular dichroism and nuclear magnetic resonance spectroscopies. Spectroscopy-based solution conformations of the Ca-bound peptide are also determined.

METHODS

NMR spectroscopy and circular dichroism were used to test Ca²⁺ coordination by the peptide (NANPNVDP)3NANP. Solution conformations for the Ca-bound peptide were determined through molecular dynamics calculations.

RESULTS

The NMR spectra collected for (NANPNVDP)₃NANP indicate that the signals generated by some of the amino acids located at its C-terminal end are shifted from their original positions upon Ca²⁺ addition. The solution conformations determined for the Ca-bound peptide indicate that the metal ion can be either six- or seven-coordinate.

CONCLUSIONS

The investigation described herein strongly supports the coordination of Ca²⁺ ions to some of the amino acids located at the C-terminus of the peptide (NANPNVDP)₃NANP. The solution conformations determined for the Ca-bound congener of this peptide display many structural features associated to Ca-binding proteins.

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.

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.

Antimalarial drugs disrupt ion homeostasis in malarial parasites

Plasmodium chabaudi malaria parasite organelles are major elements for ion homeostasis and cellular signaling and also target for antimalarial drugs. By using confocal imaging of intraerythrocytic parasites we demonstrated that the dye acridine orange (AO) is accumulated into P. chabaudi subcellular compartments. The AO could be released from the parasite organelles by collapsing the pH gradient with the K+/H+ ionophore nigericin (20 microM), or by inhibiting the H+-pump with bafilomycin (4 microM). Similarly, in isolated parasites loaded with calcium indicator Fluo 3-AM, bafilomycin caused calcium mobilization of the acidic calcium pool that could also be release with nigericin. Interestingly after complete release of the acidic compartments, addition of thapsigargin at 10 microM was still effective in releasing parasite intracellular calcium stores in parasites at trophozoite stage. The addition of antimalarial drugs chloroquine and artemisinin resulted in AO release from acidic compartments and also affected maintenance of calcium in ER store by using different drug concentrations.

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.

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.

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.

Increase of a calcium independent transglutaminase activity in the erythrocyte during the infection with Plasmodium falciparum

We have studied the activity of a calcium dependent transglutaminase (EC 2.3.2.13) during the growth of the parasite Plasmodium falciparum inside the infected human erythrocyte. There is only one detectable transglutaminase in the two-cell-system, and its origin is erythrocytic. No activity was detected in preparations of the parasite devoid of erythrocyte cytoplasm. The Michaelis Menten constants (Km) of the enzyme for the substrates N'N' dimethylcaseine and putrescine were undistinguishable whether the cell extracts used in their determination were obtained from normal or from infected red cells. The total activity of transglutaminase in stringently synchronized cultures, measured at 0.5 mM Ca2+, decreased with the maturation of the parasite. However, a fraction which became irreversibly activated and independent of calcium concentration was detected. The proportion of this fraction grew with maturation; it represented only 20% of the activity in 20 hr-old-trophozoites while in 48-hr-schizonts it was more than 85% of the total activity. The activation of this fraction of transglutaminase did not depend on an increase in the erythrocyte cytoplasmic calcium, since most of the calcium was shown to be located in the parasite.

Acidic calcium pools in intraerythrocytic malaria parasites

Calcium uptake by permeabilized P. chabaudi malaria parasites was measured at the trophozoite stage to assess calcium accumulation by the parasite organelles. As determined with 45Ca2+, the total calcium in the parasite was found to be 11 pmoles/10(7) cells. When the K+/H+ uncoupling agent, nigericin was present, this level fell to 6.5 pmoles/10(7) cells. A similar regulatory mechanism operates in P. falciparum, since addition of nigericin to intact parasites in calcium free-medium resulted in a transient elevation of free calcium in the parasite cytosol, as judged by fluorescent imaging of single cells loaded with the calcium indicator fluo-3,AM. 7-Chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-Cl) and monensin, inhibitors of H+ ATPases and K+/H+ ionophore respectively, induced calcium elevation in fluo-3, AM-labeled intact P. chabaudi parasites. We conclude that malaria parasites utilize acidic intracellular compartments to regulate their cytosolic free calcium concentration.

Inositol 1,4,5-trisphosphate induced Ca2+ release from chloroquine-sensitive and -insensitive intracellular stores in the intraerythrocytic stage of the malaria parasite P. chabaudi

Isolated P. chabaudi parasites were permeabilized with digitonin and the function of intracellular Ca2+ stores was studied using the Ca2+ indicators arsenazo III or Fluo 3-acid in the medium. Addition of the second messenger InsP3 (5 microM) to permeabilized parasites leads to Ca2+ release into the medium, with the mean extent of release being 40 nmol Ca2+/10(8) cells. This Ca2+ release was completely abolished in the presence of heparin, an InsP3 receptor antagonist. The amount of Ca2+ released was approximately 50% reduced when InsP3 was added subsequent to the discharge of the endoplasmic reticulum (ER) Ca2+ pool with the SERCA (sarcoplasmic ER Ca2+ ATPase) inhibitors thapsigargin and tBHQ (2,5-di(ter-butyl)-1,4 benzohydroquinone). The thapsigargin- and tBHQ-sensitive pool account for 20 nmol of Ca2+/10(8) cells. If InsP3 was added after the discharge of the residual Ca2+ by addition of either the K+/H+ uncoupler nigericin or the antimalarial drug chloroquine, no further Ca2+ release was observed. This is the first report of InsP3-induced Ca2+ release in a parasite protozoa. In addition our finding that chloroquine depletes an InsP3-sensitive Ca2+ compartment, raises the possibility that the InsP3-dependent Ca2+ release from this store might be important for the regulation of growth and differentiation of the parasite.

The role of Ca2+ in the process of cell invasion by intracellular parasites

In order to replicate, many parasites must invade host cells. Changes in the intracellular Ca(2+) concentration ([Ca(2+)](i)) of different parasites and tissue culture cells during their interaction have been studied. An increase in cytosolic Ca(2+) in Trypanosoma cruzi trypomastigotes occurs after association of the parasites with host cells. Ca(2+) mobilization in the host cells also takes place upon contact with T. cruzi trypomastigotes, Leishmania donovani amastigotes or Plasmodium falciparum merozoites. When Ca(2+) transients are prevented by intracellular Ca(2+) chelators, a decrease in parasite association to host cells is observed. This reveals the importance of [Ca(2+)](i) in the process of parasite-host cell interaction, as discussed here by Roberto Docampo and Silvia Moreno.

Antimalarial drugs inhibit calcium-dependent backward swimming and calcium currents in Paramecium calkinsi

The antimalarial drugs, quinacrine, chloroquine, quinine, primaquine, and mefloquine, share structural similarities with W-7, a compound that inhibits calcium-dependent backward swimming and calcium currents in Paramecium. Therefore, we tested whether antimalarial drugs also inhibit backward swimming and calcium currents in P. calkinsi. When the Paramecium is depolarized in high potassium medium, voltage-dependent calcium channels in the ciliary membrane open causing the cell to swim backward for 30 to 70 s. Application of calcium channel inhibitors, such as W-7, reduce the duration of backward swimming. In 0.05 mM calcium, quinacrine, mefloquine, quinine, chloroquine, primaquine and W-7 all reduced the duration of backward swimming. These effects were seen in sodium-containing and sodium-free high potassium solutions as well as sodium-free depolarizing solutions containing potassium channel blockers. In these low calcium solutions, backward swimming was inhibited by 50% at concentrations ranging from 100 nM to 30 microM. At higher calcium concentrations (1 mM or 15 mM), the effects of the antimalarials and W-7 were reduced. The effects of quinacrine and W-7 were tested directly on calcium currents using the two microelectrode voltage clamp technique. In 15 mM calcium, 100 microM quinacrine and 100 microM W-7 reduced the peak calcium current by 51% and 42%, respectively. Thus, antimalarial drugs reduce calcium currents in Paramecium calkinsi.

Parasite-induced changes to localized erythrocyte membrane deformability in Plasmodium falciparum cultures

The effect of intra-erythrocyte development of the Plasmodium falciparum parasite on local deformability of human erythrocyte membranes was studied by aspiration of cells into 0.56 micron diameter pores in polycarbonate filters and examination, after fixing, with a scanning electron microscope. As the aspiration pressure increased, the erythrocyte membrane was extruded into the filter pores. The pressure dependence of the protrusion length and the minimum pressure required to produce any deformation provided measures of the membrane shear and the bending moduli, respectively. At the trophozoite and, to a greater extent, schizont stage of development, host cell membrane deformability was significantly decreased. There was no appreciable difference between uninfected and ring-infected erythrocytes.

The role of calcium in the invasion of human erythrocytes by Plasmodium falciparum

The role of calcium in the invasion of human erythrocytes by Plasmodium falciparum merozoites has been investigated using a variety of techniques. It has been demonstrated using calcium-depleted medium that invasion is dependent upon the presence of calcium and that neither magnesium, manganese or zinc may substitute for it, suggesting that the effect is calcium specific and not dependent upon a non-specific, charge-based mechanism. Using resealed erythrocyte ghosts and altering the internal and external concentrations of calcium and the chelator EGTA, it has been shown that the role of calcium in invasion, at least as far as the target cell is concerned, is in the extracellular environment. Similarly, loading either the schizont-infected, or target erythrocyte with the membrane permeant calcium chelator Indo-1, at concentrations sufficient to chelate approximately 100 times the concentration of resting cell calcium, produced no change in the parasite invasion rate. Consequently we conclude that calcium plays an extra-cellular role in merozoite invasion of the human erythrocyte.

Trypanosoma cruzi: calcium ion movement during internalization in host HeLa cells

The role of cytosolic Ca2+ and cytoplasmic calcium movement during the parasitization of HeLa cells by T. cruzi were studied. The level of calcium in parasitized cells increased compared to the control cells. Our experiments demonstrate that this cytosolic calcium originates from the release of the intracellular calcium deposits, especially from the mitochondria of the host cell. The parasitization rates decreased after the cells were treated with drugs to increase the cytosolic Ca2+ levels to inhibit the host-cell calmodulin.

Purification of a glycoprotein excreted by Trypanosoma cruzi to increase the permeability of the host-cell membrane

During invasion of the prospective host cell, metacyclic forms of Trypanosoma cruzi render the membrane of HeLa cells permeable to the alpha-sarcin toxin, by excreting a glycoprotein with N-acetyl-D-glucosamine residues. The molecular weight of the glycoprotein is 64,000 dalton and its isoelectric point is 4.8.

Effects of membrane acting-drugs on plasmodium species and sickle cell erythrocytes

The effects of several membrane-acting drugs on malaria and sickle cell anemia was studied. In the initial experiments, propranolol and W-7 were shown to increase red cell density. In vitro, these drugs inhibited the growth of P. falciparum. However, in vivo experiments using the murine malarial parasite, P. vinckei, demonstrated little, if any, anti-parasite activity with the doses of drugs employed. Subsequently, prostaglandin oligomeric derivatives were found to inhibit the growth of P. falciparum in vitro and P. vinckei in vivo. Since prostaglandin oligomers inhibited the formation of dense, dehydrated cells (irreversible sickle cells), they may also have therapeutic efficacy in sickle cell anemia.

Prostaglandin derivatives inhibit the growth of malarial parasites in mice

New prostaglandin oligomeric derivatives, termed MR-256 and MR-356, were found to inhibit the growth of murine malarial parasites, P. chabaudi and P. vinckei, within red blood cells in vivo. When mice were infected with P. chabaudi, both MR-256 and MR-356 suppressed the growth of parasites, but MR-356 had a greater inhibitory effect than MR-256. With P. vinckei, MR-356 also inhibited the growth of parasites, and improved the survival rate. The effect of MR-256 was much less. A possible inhibitory mechanism of action of these drugs is discussed.

Stage-dependent inhibition of Plasmodium falciparum by potent Ca2+ and calmodulin modulators

The effects of Ca2+ channel blockers, verapamil, nicardipine and diltiazem, and of potent calmodulin (CaM) inhibitors, trifluoperazine (TFP), calmidazolium, W-7 and W-5, on Plasmodium falciparum in culture were examined. Among Ca2+ blockers, nicardipine was the most potent with the 50% inhibitory concentration (IC50) of 4.3 microM at 72 h after culture. Parasites were more sensitive to calmidazolium and W-7 with IC50 of 3.4 and 4.5 microM, respectively, than to TFP and W-5. All Ca2+ blockers and CaM inhibitors suppressed parasite development at later stages. Nicardipine, diltiazem, calmidazolium and W-5 also retarded parasite development at earlier stages and/or subsequent growth following pretreatment. Verapamil, nicardipine, TFP and calmidazolium reduced erythrocyte invasion by merozoites. Fluorescence microscopy with the cationic fluorescent dye rhodamine 123 revealed that nicardipine, TFP and calmidazolium depolarized both the plasma membrane and mitochondrial membrane potentials of the parasite. It is therefore considered that although all Ca2+ and CaM antagonists tested here influence parasite development at later stages, they are multifunctional, having effects not directly associated with Ca2+ channels or CaM.

Rapid clearance of Plasmodium yoelii-infected erythrocytes after exposure to the ionophore A23187

1. The effects of Ca2+ and the calcium ionophore A23187 on the intraerythrocytic development of the asexual forms of Plasmodium yoelii were examined. 2. Erythrocyte-free parasites obtained by saponin lysis of infected cells remained viable after exposure to 1 mM Ca2+. 3. A23187 inhibited the growth of P. yoelii and the inhibition was augmented by Ca2+ in cells infected with parasites at young stage of development. 4. A23187-treated infected cells disappeared from the circulation shortly after intravenous injection and this disappearance was profound in infected cells treated with the ionophore in the presence of Ca2+.

The Wellcome Trust lecture. Mechanisms of molecular trafficking in malaria

The asexual stages of Plasmodium living within the erythrocyte result in growth-related changes in the permeability properties of the red cell for substances such as glucose, amino acids, purine nucleosides, sodium, potassium, calcium, zinc, iron and several antimalarial drugs such as chloroquine, amodiaquine and mefloquine. In most cases such changes do not appear to be due to a modification in the affinity or specificity of red cell transporters; indeed, for most substances the membrane-associated transporters are either unaffected or are partially inactivated. In malaria-infected erythrocytes, where a striking increase in influx has been observed, it has been attributed to the insertion of parasite-encoded transporters into the red cell membrane or the formation of aqueous leaks and/or pores. Leak formation, in the vast majority of cases, does not appear to be dependent on the insertion of plasmodial proteins into the red cell membrane. However, since the data presently available are less than satisfactory for discriminating amongst the various possible transport mechanisms future studies will require painstaking efforts and carefully controlled conditions to discriminate amongst the various transport systems which are operational in the malaria-infected red cell and the parasite.

Calcium and calmodulin antagonists inhibit human malaria parasites (Plasmodium falciparum): implications for drug design

The malaria parasite has an obligate calcium requirement for normal intracellular growth and invasion of host erythrocytes. Calmodulin (CaM) is a vital calcium-dependent protein present in eukaryotes. We found by radioimmunoassay that free parasites contain CaM. Schizont-infected erythrocytes had CaM levels of 23.3 +/- 2.7 ng per 10(6) cells compared to normals (11.2 +/- 1.5 ng per 10(6) cells). CaM levels were proportional to parasite maturity. Immunoelectron microscopy identified CaM diffusely within the cytoplasm of mature parasites and at the apical end of merozoites within the ductule of rhoptries, which may explain the calcium requirement for invasion. Cyclosporin A (CsA) was also found by electron microscopic autoradiography to concentrate in the food vacuole, as do chloroquine and mefloquine, and to distribute within the cytoplasm of mature parasites. The binding of dansylated CsA to schizont-infected erythrocytes was higher than to normal erythrocytes as analyzed by flow cytometry. Kinetic analysis revealed that binding was saturable for normal and infected erythrocytes and possibly free parasites. Competition for binding existed between dansylated CsA and native CsA as well as the CaM inhibitor W-7 and the classic antimalarial chloroquine. The in vitro growth of Plasmodium falciparum was sensitive to CaM antagonists, and in large part inhibition of the parasite was proportional to known anti-CaM potency. Antagonism existed between combinations of these drugs in multi-drug-resistant strains of P. falciparum, suggesting possible competition for the same binding site. In addition, the malaria parasite was also susceptible to calcium antagonists.

The relationship of phosphorylation of membrane proteins with the osmotic fragility and filterability of Plasmodium berghei-infected mouse erythrocytes

Membrane from Plasmodium berghei-infected mouse erythrocytes showed a pattern of protein phosphorylation which was substantially altered from the normal pattern, with an increase in the phosphorylation of the protein with an apparent molecular weight of 43,000 (M 43), which increased from undetectable in uninfected cells to a maximum in the mature trophozoite stage. Phosphorylation levels of this and other minor bands were strongly correlated with osmotic fragility and filterability. The level of M 43 phosphorylation in membranes from cells which remained intact in a hypotonic medium was 3.82 +/- 0.59-times that of lysed cells, compared with the value of 0.76 +/- 0.07 calculated from distribution alone. Results found when intact erythrocytes were phosphorylated by incubation with [32P]Pi prior to partial lysis were similar to those found when membranes from the lysed and unlysed fractions were subsequently phosphorylated with [gamma-32P]ATP. Infected erythrocytes which could pass repeatedly through 3-micron polycarbonate filters had a much higher phosphorylation level for the M 43 region than whole infected cells with similar parasitemia and stage distribution. The phosphorylation change could play a role in the control of osmotic and mechanical properties of the infected erythrocytes during maturation.

Characteristics of membrane protein phosphorylation in Plasmodium berghei-infected mouse erythrocytes

Membrane protein phosphorylation in Plasmodium berghei-infected erythrocytes was studied by incubating intact cells with (32P)orthophosphate and incubating isolated membrane with (gamma-32P)ATP. Phosphorylated proteins were detected by autoradiography after sodium dodecylsulfate (SDS)-polyacrylamide gel electrophoresis or isoelectric focusing followed by gel electrophoresis. New phosphorylated proteins were found in membrane from infected erythrocytes, including a protein with electrophoretic mobility identical to band 5, with Mr 43,000. The molar ratio of phosphate to protein ranged between 0.1 and 0.5. Isoelectric focusing-SDS polyacrylamide gel electrophoresis, peptide mapping, extractability properties, and reduction of susceptibility to DNase I inhibition suggested that this protein is phosphorylated actin. In contrast, spectrin phosphorylation in infected erythrocytes was mostly unchanged.

Plasmodium falciparum: regional differences in lectin and cationized ferritin binding to the surface of the malaria-infected human erythrocyte

The distribution of anionic residues on the surface of erythrocytes infected with Plasmodium falciparum was studied using cationized ferritin (CF) and transmission electron microscopy. CF staining of uninfected erythrocytes or erythrocytes infected with a knobless variant resulted in a dense and uniform distribution of ferritin particles; however, when red cells infected with a knob-inducing variant were exposed to CF, aggregates of ferritin particles were observed in the region of membrane elevation. Lectin binding to the erythrocyte surface was visualized by transmission electron microscopy using ferritin-conjugated lectins and lectin-fetuin-gold. No differences were observed in the lectin-binding patterns of malaria-infected or uninfected erythrocytes using WGA (wheat-germ agglutinin), RCA (ricin), and Limax flavus lectin. In distinct contrast to the uniform distribution of ferritin particles seen with these lectins was the appearance of clusters of ferritin-ConA over the knobby regions. Localized aggregates of ConA were not seen in knob-free areas or on the surface of red cells infected with a knobless variant. No significant differences were found in the agglutination reactions of normal and infected cells with the Cancer antennarius lectin specific for O-acylated sialic acids.

Membrane structure and function of malaria parasites and the infected erythrocyte

According to the World Health Organization the global estimate of malaria is over 200 million infections, the majority of which are caused by the most life-threatening species,Plasmodium falciparum(Report of the Steering Committees of the Scientific Working Groups on Malaria, World Health Organization, June 1983). The causative agent of the disease, the malarial parasite, requires two hosts: a blood-sucking mosquito and a blood-containing vertebrate. Commonly, infection of the vertebrate begins when an infected mosquito bites a suitable vertebrate and injects minute sporozoites into the bloodstream. Within 30 mm the introduced sporozoites leave the bloodstream and enter parenchymal cells of the liver (mammals) or endothelial cells (birds). In these sites the parasite undergoes asexual multiplication (= exo-erythrocytic schizogony) producing daughter progeny called merozoites. The exo-erythrocytic merozoites are released from the tissues into the circulation where they invade red blood cells. Within an erythrocyte the merozoite undergoes asexual multiplication (= erythrocytic schizogony) producing a substantial number of merozoites. The erythrocyte lyses, merozoites are released, and invasion of another erythrocyte may then take place. The synchronous rupture of the red cell and merozoite release is marked by the periodic fever–chill cycles so characteristic of the malarial infection. Some merozoites continue to reinvade other erythrocytes and multiply by asexual means, whereas others enter erythrocytes and differentiate into sexual stages, male or female gametocytes. When a suitable mosquito feeds on an infected vertebrate gametocytes are ingested and the sexual cycle of development is initiated. In the mosquito stomach the gametocytes transform into gametes, fertilization takes place, the resultant worm-like zygote penetrates the cells of the mosquito gut and comes to lie on the outer surface of the stomach. Here each zygote forms a cyst-like body, the oocyst, within which thousands of sporozoites are produced by asexual multiplication. When the swollen oocysts burst, sporozoites are freed and these make their way to the salivary gland. At the next blood feeding the mosquito injects the infective sporozoites and the life-cycle is completed.

Selective stage-specific changes in the permeability to small hydrophilic solutes of human erythrocytes infected with Plasmodium falciparum

The permeability characteristics of Plasmodium falciparum-infected human erythrocytes to various 3H-labelled solutes were measured during the maturation of the parasites in sorbitol-synchronised cultures. Using [14C]inulin as the extracellular marker, estimates were made of the influx kinetics of [3H]amino acids into trichloroacetic acid (TCA)-soluble pools within the erythrocyte and concomitant incorporation into TCA-precipitable material. These measurements provided values of the rates of protein synthesis by the parasite and the initial influx rates for the transport of precursor amino acids into the erythrocyte. For about 12-15 h after parasitisation, the influx of L-[3H]glutamine remained at a low level comparable to that in the uninfected cell (2-9 nmol g-1 cells min-1). As pigment appeared in the trophozoite, the initial rate of influx of L-glutamine increased to a value up to 100-fold higher than in the uninfected erythrocyte. The increase in permeability affected only the parasitised cells in a culture of partially infected erythrocytes, and was selective with respect to substrate since the influx kinetics for both [3H]isoleucine and [3H]arginine were not affected by parasitisation. The permeability changes occurred mainly over a 4-8 h period in the development of the young trophozoite, during which time [3H]glycine influx was enhanced by a factor of 3-10, with a comparable increase in the uptake of myo-[3H]inositol. L-[3H]glutamate, which did not penetrate significantly into uninfected erythrocytes, entered red cells infected with mature trophozoites at a rate which was much less than 1% of the parasite-induced-L-glutamine influx. At the stages when the permeability to L-glutamine was markedly enhanced, parasitised cells remained impermeable to [3H]sucrose. An analysis of the relative 3H activities in glutathione and free amino acid pools indicated that, if L-glutamine permeation did not increase during parasite maturation beyond the ring stage, or was blocked by a potential antimalarial compound, an insufficient supply of L-glutamine would be available for the increased rates of parasite protein synthesis and glutathione turnover within the red cell.