New permeability pathways induced by the malarial parasite in the membrane of its host erythrocyte: potential routes for targeting of drugs into infected cells

Fluorescent Nitrogen-Doped Carbon Dots for Label Live Elder Blood-Stage Plasmodium falciparum through New Permeability Pathways

To verify the size and emergence time of new permeability pathways (NPPs) in malaria parasites, the permeability of the Plasmodium falciparum-infected erythrocytes was tested with different particle sizes of nanomaterials by flow cytometry assay. The results confirmed the permeability of the host cell membrane increases with parasite maturation for the stage-development evolution of NPPs, and especially found that a particle size of about 50 nm had higher efficiency. As a kind of the novel nanomaterials, nitrogen-doped carbon dots (NCDs) showed no toxicity, specificity binding ability to the malaria parasites, and could label live elder blood-stage P. falciparum through NPPs, indicating the potential application in cell imaging. NPPs and some nanomaterials such as NCDs deserve more attention and exploration for the elimination and prevention of malaria.

Improved efficacy of doxycycline in liposomes against Plasmodium falciparum in culture and Plasmodium berghei infection in mice

The rate at which Plasmodium falciparum is developing resistance to clinically used antimalarial drugs is alarming. Therefore, there is a compelling need to develop an efficient drug delivery system to improve the efficacy of existing antimalarial agents and circumvent drug resistance. Here, we report the antibacterial drug doxycycline (DOXY) in liposomal formulations exhibits enhanced antiplasmodial activity against blood stage forms of P. falciparum (3D7) in culture and established Plasmodium berghei NK-65 infection in murine model. Parasite killing on blood stage forms in culture was determined by a radiolabeled [³H] hypoxanthine incorporation assay and infected erythrocytes stained with Giemsa were counted using microscopy in vivo. The 50% inhibitory concentration (IC₅₀) of DOXY-stearylamine liposome (IC₅₀ 0.36 μM) and DOXY-SPC:Chol-liposome (IC₅₀ 0.85 μM) exhibited marked growth inhibition of parasites compared with free DOXY (IC₅₀ 14 μM), with minimal toxicity to normal erythrocytes. Administration of polyethylene glycol distearoyl phosphatidylethanolamine-methoxy-polyethylene glycol2000 (DSPE-mPEG-2000) coated liposomes loaded with DOXY at 2.5 mg/kg per day resulted in efficacious killing of blood parasites with improved survival in mice relative to the free drug in both chloroquine sensitive and resistant strains of P. berghei infection. This is the first report to demonstrate that DOXY in liposomal system has immense chemotherapeutic potential against plasmodial infections at lower dosages.

Three-dimensional analysis of morphological changes in the malaria parasite infected red blood cell by serial block-face scanning electron microscopy

The human malaria parasite, Plasmodium falciparum, exhibits morphological changes during the blood stage cycle in vertebrate hosts. Here, we used serial block-face scanning electron microscopy (SBF-SEM) to visualize the entire structures of P. falciparum-infected red blood cells (iRBCs) and to examine their morphological and volumetric changes at different stages. During developmental stages, the parasite forms Maurer's clefts and vesicles in the iRBC cytoplasm and knobs on the iRBC surface, and extensively remodels the iRBC structure for proliferation of the parasite. In our observations, the Maurer's clefts and vesicles in the P. falciparum-iRBCs, resembling the so-called tubovesicular network (TVN), were not connected to each other, and continuous membrane networks were not observed between the parasitophorous vacuole membrane (PVM) and the iRBC cytoplasmic membrane. In the volumetric analysis, the iRBC volume initially increased and then decreased to the end of the blood stage cycle. This suggests that it is necessary to absorb a substantial amount of nutrients from outside the iRBC during the initial stage, but to release waste materials from inside the iRBC at the multinucleate stage. Transportation of the materials may be through the iRBC membrane, rather than a special structure formed by the parasite, because there is no direct connection between the iRBC membrane and the parasite. These results provide new insights as to how the malaria parasite grows in the iRBC and remodels iRBC structure during developmental stages; these observation can serve as a baseline for further experiments on the effects of therapeutic agents on malaria.

Advances in nanomedicines for malaria treatment

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

Fosmidomycin uptake into Plasmodium and Babesia-infected erythrocytes is facilitated by parasite-induced new permeability pathways

Background

Highly charged compounds typically suffer from low membrane permeability and thus are generally regarded as sub-optimal drug candidates. Nonetheless, the highly charged drug fosmidomycin and its more active methyl-derivative FR900098 have proven parasiticidal activity against erythrocytic stages of the malaria parasite Plasmodium falciparum. Both compounds target the isoprenoid biosynthesis pathway present in bacteria and plastid-bearing organisms, like apicomplexan parasites. Surprisingly, the compounds are inactive against a range of apicomplexans replicating in nucleated cells, including Toxoplasma gondii.

Methodology/Principal Findings

Since non-infected erythrocytes are impermeable for FR90098, we hypothesized that these drugs are taken up only by erythrocytes infected with Plasmodium. We provide evidence that radiolabeled FR900098 accumulates in theses cells as a consequence of parasite-induced new properties of the host cell, which coincide with an increased permeability of the erythrocyte membrane. Babesia divergens, a related parasite that also infects human erythrocytes and is also known to induce an increase in membrane permeability, displays a similar susceptibility and uptake behavior with regard to the drug. In contrast, Toxoplasma gondii-infected cells do apparently not take up the compounds, and the drugs are inactive against the liver stages of Plasmodium berghei, a mouse malaria parasite.

Conclusions/Significance

Our findings provide an explanation for the observed differences in activity of fosmidomycin and FR900098 against different Apicomplexa. These results have important implications for future screens aimed at finding new and safe molecular entities active against P. falciparum and related parasites. Our data provide further evidence that parasite-induced new permeability pathways may be exploited as routes for drug delivery.

X-ray microanalysis investigation of the changes in Na, K, and hemoglobin concentration in plasmodium falciparum-infected red blood cells

Plasmodium falciparum is responsible for severe malaria. During the ∼48 h duration of its asexual reproduction cycle in human red blood cells, the parasite causes profound alterations in the homeostasis of the host red cell, with reversal of the normal Na and K gradients across the host cell membrane, and a drastic fall in hemoglobin content. A question critical to our understanding of how the host cell retains its integrity for the duration of the cycle had been previously addressed by modeling the homeostasis of infected cells. The model predicted a critical contribution of excess hemoglobin consumption to cell integrity (the colloidosmotic hypothesis). Here we tested this prediction with the use of electron-probe x-ray microanalysis to measure the stage-related changes in Na, K, and Fe contents in single infected red cells and in uninfected controls. The results document a decrease in Fe signal with increased Na/K ratio. Interpreted in terms of concentrations, the results point to a sustained fall in host cell hemoglobin concentration with parasite maturation, supporting a colloidosmotic role of excess hemoglobin digestion. The results also provide, for the first time to our knowledge, comprehensive maps of the elemental distributions of Na, K, and Fe in falciparum-infected red blood cells.

The homeostasis of Plasmodium falciparum-infected red blood cells

The asexual reproduction cycle of Plasmodium falciparum, the parasite responsible for severe malaria, occurs within red blood cells. A merozoite invades a red cell in the circulation, develops and multiplies, and after about 48 hours ruptures the host cell, releasing 15–32 merozoites ready to invade new red blood cells. During this cycle, the parasite increases the host cell permeability so much that when similar permeabilization was simulated on uninfected red cells, lysis occurred before ∼48 h. So how could infected cells, with a growing parasite inside, prevent lysis before the parasite has completed its developmental cycle? A mathematical model of the homeostasis of infected red cells suggested that it is the wasteful consumption of host cell hemoglobin that prevents early lysis by the progressive reduction in the colloid-osmotic pressure within the host (the colloid-osmotic hypothesis). However, two critical model predictions, that infected cells would swell to near prelytic sphericity and that the hemoglobin concentration would become progressively reduced, remained controversial. In this paper, we are able for the first time to correlate model predictions with recent experimental data in the literature and explore the fine details of the homeostasis of infected red blood cells during five model-defined periods of parasite development. The conclusions suggest that infected red cells do reach proximity to lytic rupture regardless of their actual volume, thus requiring a progressive reduction in their hemoglobin concentration to prevent premature lysis.

The parasite Plasmodium falciparum is responsible for severe malaria in humans. The 48 hour asexual reproduction cycle of the parasite within red blood cells is responsible for the symptoms in this disease. Within this period, the parasite causes massive changes in the host red cell, increasing some metabolic activities hundredfold, making it leaky to many nutrients and waste products, and consuming most of the cell's hemoglobin, far more than it needs for its own metabolism. The challenge that we faced was to explain how the infected cell maintained its integrity throughout such a violent cycle. Seeking clues, we developed a mathematical model of an infected cell in which we encoded our current knowledge and understanding of the complex processes that control cell homeostasis. We present here for the first time a detailed description of the model and a critical analysis of its predictions in relation to the available experimental evidence. The results support the view that host-cell integrity is maintained by the progressive reduction in the hemoglobin concentration within the host cell, resulting in a reduced rate and extent of swelling.

Malaria parasite transporters as a drug-delivery strategy

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

Plasmodium falciparum-induced channels

To survive within a red blood cell, the malaria parasite alters dramatically the permeability of the host's plasma membrane (allowing the uptake of essential nutrients and the removal of potentially hazardous metabolites). The pathway(s) responsible for the increased permeability have been proposed as putative chemotherapeutic targets and/or selective routes for antimalarial agents that target the internal parasite. This review covers our current understanding of this parasite-induced phenomenon in Plasmodium falciparum-infected human red blood cells. In particular, recent electrophysiological studies, using the patch-clamp technique, are reviewed.

Plasmodium falciparum-activated Chloride Channels Are Defective in Erythrocytes from Cystic Fibrosis Patients

An inwardly rectifying anion channel in malaria-infected red blood cells has been proposed to function as the "new permeation pathway" for parasite nutrient acquisition. As the channel shares several properties with the cystic fibrosis transmembrane conductance regulator (CFTR), we tested their interrelationship by whole-cell current measurements in Plasmodium falciparum-infected and uninfected red blood cells from control and cystic fibrosis (CF) patients. A CFTR-like linear chloride conductance as well as a malaria parasite-induced and a shrinkage-activated endogenous inwardly rectifying chloride conductance with properties identical to the malaria-induced channel were all found to be defective in CF erythrocytes. Surprisingly, the absence of the inwardly rectifying chloride conductance in CF erythrocytes had no gross effect on in vitro parasite growth or new permeation pathway activity, supporting an argument against a close association between the Plasmodium-activated chloride channel and the new permeation pathway. The functional expression of CFTR in red blood cells opens new perspectives to exploit the erythrocyte as a readily available cell type in electrophysiological, diagnostic, and therapeutic studies of CF.

Amino acid transport into cultured McCoy cells infected with Chlamydia trachomatis

Amino acid transport into McCoy cells infected with strains representative of the two major biovars of Chlamydia trachomatis has been studied to determine if uptake is increased during infection. Preliminary work suggested that the transport systems L, A/ASC (for neutral amino acid transport), N (for transport of Asn, Gln, and His) and y+ (for cationic amino acids) were present in McCoy cells. With lymphogranuloma venereum biovar strain 434, little difference in the influx of representative amino acids Trp, His, and Lys or the analogue 2-aminoisobutyric acid (AIB) was observed during infection. With trachoma biovar strain DK20, a small increase in the initial entry rate and equilibrium concentration of each amino acid was found. McCoy cells appear to have great capacity for concentrating amino acids, which might obviate the need for transport induction by chlamydiae under conditions favoring the growth of infectious organisms.

Characterization of the enhanced transport of L- and D-lactate into human red blood cells infected with Plasmodium falciparum suggests the presence of a novel saturable lactate proton cotransporter

Human erythrocytes parasitized with the malarial protozoan Plasmodium falciparum showed rates of L-lactate, D-lactate, and pyruvate uptake many fold greater than control cells. Thus it was necessary to work at 0 degrees C to resolve true initial rates of transport. Studies on the dependence of the rate of transport on substrate concentration implied the presence in parasitized cells of both a saturable mechanism blocked by alpha-cyano-4-hydroxycinnamate (CHC) and a nonsaturable mechanism insensitive to CHC. The former was dominant at physiological substrate concentrations with Km values for pyruvate and D-lactate of 2.3 and 5.2 mM, respectively, with no stereoselectivity for L- over D-lactate. CHC was significantly less effective as an inhibitor of lactate transport in parasitized erythrocytes than in uninfected cells, whereas p-chloromercuribenzenesulfonate, a potent inhibitor in control cells, gave little or no inhibition of lactate transport into parasitized erythrocytes. Inhibition of transport into infected cells was also observed with phloretin, furosemide, niflumic acid, stilbenedisulfonate derivatives, and 5-nitro-2-(3-phenylpropylamino)benzoic acid at concentrations similar to those that inhibit the lactate carrier of control erythrocytes. These compounds were more effective inhibitors of the rapid transport of chloride into infected cells than of lactate transport, whereas CHC was more effective against lactate transport. This implies that different pathways are involved in the parasite-induced transport pathways for lactate and chloride. The transport of L-lactate into infected erythrocytes was also inhibited by D-lactate, pyruvate, 2-oxobutyrate, and 2-hydroxybutyrate. The intracellular accumulation of L-lactate at equilibrium was dependent on the transmembrane pH gradient, suggesting a protogenic transport mechanism. Our data are consistent with lactate and pyruvate having direct access to the malarial parasite, perhaps via the proposed parasitophorous duct or some close contact between the host cell and parasite plasma membranes, with transport across the latter by both a proton-linked carrier (CHC-sensitive, saturable, and the major route) and free diffusion of the undissociated acid (CHC-insensitive, unsaturable, and a minor route).

Parasite-regulated membrane transport processes and metabolic control in malaria-infected erythrocytes

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Stage-dependent effects of analogs of gramicidin A on the growth of Plasmodium falciparum in vitro

Tryptophan-N-formylated gramicidin A, a nonhemolytic derivative of the toxic peptide antibiotic gramicidin A, has previously been shown to induce potassium leakage from Plasmodium falciparum-infected erythrocytes in vitro and to inhibit the growth of the parasite. In the present study the antimalarial activities of two other nonhemolytic derivatives of gramicidin A, viz., acylated gramicidin A and desformylated gramicidin A, were tested and compared with those of gramicidin A and tryptophan-N-formylated gramicidin A. The 50% growth-inhibitory concentrations (IC50 values) of the four compounds varied from 0.3 to 18.3 nM, and complete growth inhibition was detected within one parasitic growth cycle. Using highly synchronized cultures of P. falciparum, it was furthermore shown that the gramicidin analogs are inhibitory to all developmental stages of the parasite, although their efficiency in accomplishing growth inhibition was found, as expected, to be clearly stage-dependent and to increase with the age of the parasite.

Enhanced choline and Rb+ transport in human erythrocytes infected with the malaria parasite Plasmodium falciparum

Human erythrocytes infected in vitro with the malaria parasite Plasmodium falciparum showed a markedly increased rate of choline influx compared with normal cells. Choline transport into uninfected cells (cultured in parallel with infected cells) obeyed Michaelis-Menten kinetics (Km approximately 11 microM). In malaria-parasite-infected cells there was an additional choline-transport component which failed to saturate at extracellular concentrations of up to 500 microM. This component was less sensitive than the endogenous transporter to inhibition by the Cinchona bark alkaloids quinine, quinidine, cinchonine and cinchonidine, but showed a much greater sensitivity than the native system to inhibition by piperine. The sensitivity of the induced choline transport to these reagents was similar to that of the malaria-induced (ouabain- and bumetanide-resistant) Rb(+)-transport pathway; however, the relative magnitudes of the piperine-sensitive choline and Rb+ fluxes in malaria-parasite-infected cells varied between cultures. This suggests either that the enhanced transport of the two cations was via functionally distinct (albeit pharmacologically similar) pathways, or that the transport was mediated by a pathway with variable substrate selectivity.

Effect of diamide on nucleoside and glucose transport in Plasmodium falciparum and Babesia bovis infected erythrocytes

Normal human erythrocytes, preincubated with the oxidizing agent diamide, did not demonstrate any increased permeability, but showed a significant decrease in their ability to transport the nucleoside adenosine. Diamide appeared to have little effect on glucose permeation in uninfected and Plasmodium falciparum infected cells. The inhibition of adenosine transport in human erythrocytes by diamide pretreatment appeared to be unrelated to the inhibition by the established nucleoside transport inhibitor, nitrobenzylthioinosine (NBMPR). An ID50 for diamide of 0.3 mM was determined for 1 microM adenosine transport in human erythrocytes after preincubation for 45 min at 37 degrees C. However, preincubation of diamide (20 mM, 60 min at 37 degrees C) with Babesia bovis-infected bovine erythrocytes resulted in complete inhibition of the capacity of the parasitised cell to transport adenosine and partial inhibition of glucose permeation. By contrast, diamide was shown to have little or no effect on the new or induced nucleoside permeation site in P. falciparum (trophozoite) infected erythrocytes nor on the glucose transporter in these cells. The results further indicate the differences between the normal human erythrocyte nucleoside and glucose transporters and those new or altered transporters in the membrane of P. falciparum or B. bovis-infected red blood cells.

Characteristics of 86Rb+ transport in human erythrocytes infected with Plasmodium falciparum

Human red cells infected in vitro with Plasmodium falciparum showed a significant increase in the rate of both ouabain-sensitive and ouabain-insensitive 86Rb+ influx. The increase in ouabain-insensitive 86Rb+ influx was due, in part, to increased transport via a bumetanide-sensitive system and, in part to transport via a pathway that was absent (or at least inactive) in uninfected cells. The parasite-induced pathway was inhibited by piperine and had a dose response very similar to that of the Gardos channel of uninfected cells but was less sensitive than the Gardos channel to inhibition by quinine.

Water and urea transport in human erythrocytes infected with the malaria parasite Plasmodium falciparum

The permeability properties of the human red cell membrane to various solutes are altered by malarial infection. In the present work we show that the permeability of the red cell membrane to water is also affected by the intraerythrocytic growth of the malaria parasite Plasmodium falciparum, whereas urea permeability appears unchanged. The data from infected cells show decreases in membrane surface area, cell volume, the osmotically active water fraction (Weff), and osmotic water permeability (Pf) as measured by stopped-flow spectroscopy. On the other hand, the data suggest an increase in diffusive water permeability (Pd) in infected cells with no change in urea permeability when measured by the continuous flow method. The decreased Pf/Pd ratio of infected cell membranes and its implications in the geometry of the red cell membrane water channel or pore are discussed.

Association of Plasmodium berghei proteins with the host erythrocyte membrane: binding to inside-out vesicles

Two acidic phosphoproteins of Plasmodium berghei origin, of 65 and 46 kDa, are associated with the plasma membrane of the host mouse erythrocyte. The 65-kDa protein partitions between a soluble and particulate phase upon host cell lysis, whereas the 46-kDa protein is localized exclusively in the particulate fraction. Both proteins bind to inside-out vesicles derived from erythrocyte ghosts and the conditions of the reassociation reaction indicate that the binding is specific and that the proteins interact only with the cytoplasmic face of the erythrocyte membrane. The 65-kDa protein appears to exist in two membrane-associated states; one loosely bound, which readily dissociates from the membrane, and a more tightly associated state, which does not dissociate under non-denaturing conditions. The 46-kDa protein is tightly bound to the host erythrocyte membrane and does not dissociate. Cross-linking studies suggest that both of these parasite proteins interact with the submembrane cytoskeleton of the erythrocyte, and that the 65-kDa protein also appears to interact simultaneously with the lipid bilayer and erythrocyte membrane proteins. However, direct interaction between the malarial proteins and distinct erythrocyte membrane proteins could not be demonstrated. In summary, these findings indicate that the acidic phosphoproteins of the malarial parasite interact with the cytoplasmic face of the erythrocyte membrane both in vivo and in vitro.

Passive modulation of antigenic expression in the surface of normal and malaria-infected erythrocytes

The membranes of Plasmodium falciparum-infected human red blood cells contain antigens of demonstrably cryptic character. We show here, by a cell surface radioimmunoassay using anti-human red cell membrane antisera, that raising the membrane microviscosity of intact cells leads to a marked increase in the cell surface antigen reactivity of normal cells, and even more so in cells infected in vitro with two strains of P. falciparum. A variety of sera from adults and children living in endemic areas and from malaria patients, all of which showed no detectable surface reactivity with either normal or infected red cells, were demonstrably surface-reactive to infected cells whose sterol membrane content has been raised by means conservative of cell integrity. New epitopes become exposed on the surface of infected cells after lipid modification. The present studies indicate that the reduced membrane viscosity reported in malaria-infected cells determines to a considerable extent the expression of cell surface antigens of both host and parasite, and could play a significant role in parasite immune evasion.

Plasmodium falciparum antigens associated with membrane structures in the host erythrocyte cytoplasm

Hybridomas were made from mice immunized with plasma membranes from erythrocytes infected with Plasmodium falciparum. Among the monoclonal antibodies produced, a series reacted with antigens in the host cell cytoplasm. Immunoelectron microscopy, along with indirect fluorescent antibody double labeling experiments, were used to further localize the antigens to membrane structures (presumably Maurer's clefts) in the erythrocyte cytoplasm. The epitopes thus localized are found on three parasite proteins (20 kDa, 29 kDa, and 45 kDa) and one parasite glycoprotein (45 kDa). They are likely to be part of a transport system for the parasite.

Bioflavonoid effects on in vitro cultures of Plasmodium falciparum. Inhibition of permeation pathways induced in the host cell membrane by the intraerythrocytic parasite

A series of polyhydroxyphenol glycosides including bioflavonoid-glycosides structurally-related to phlorizin (phloretin-2-beta-glucoside) have been tested for their capacity to inhibit permeation pathways induced in red cell membranes by intraerythrocytic Plasmodium falciparum parasites. The permeation through these pathways has been assessed on trophozoites by sorbitol-mediated hemolysis based on a novel technique of high sensitivity and time resolution which has been adapted for handling relatively large number of samples in microtitration plates. Changes in the number of phenolic groups and to a lesser extent changes in the relative position of these groups had a substantial effect on the inhibitory efficacy of the phlorizin derivatives. Diglycoside derivatives were completely ineffective while various monoglycoside derivatives had comparable effects. Structure-activity relationship (SAR) studies of 3-monosubstituted phlorizin derivatives indicate that the inhibitory potency varied considerably with the chemical nature of the group substituted in the 3 position. Inhibition correlated best (r = 0.90) with Hammett's constant, underscoring the role of the electron withdrawing capacity of the chemical groups substituted on the hydroxydihydrochalcone moiety. On the other hand, substitution with lipophilic groups had either minimal effects or reduced the inhibitory power of the derivatives. Inhibition of transport correlated with the inhibition of intraerythrocytic parasite growth and provides a basis for new therapeutic approaches of malaria. Based on the SAR studies, a 3-isothiocyano analog of phlorizin was synthesized and shown to block irreversibly the above permeation pathways (20 microM, 10 min reaction at ambient temperature) as well as the intraerythrocytic growth of the parasite. The present study provides proof for the involvement of amino groups in red cell membrane components as controlling elements of the permeation pathways induced by the intraerythrocytic parasite. The putative groups could serve as targets for affinity labeling of the membrane components associated with the permeation function.