Ducts, channels and transporters in Plasmodium-infected erythrocytes

Bio-inspired artemether-loaded human serum albumin nanoparticles for effective control of malaria-infected erythrocytes

AIM

The intra-erythrocytic development of the malarial parasite is dependent on active uptake of nutrients, including human serum albumin (HSA), into parasitized red blood cells (pRBCs). We have designed HSA-based nanoparticles as a potential drug-delivery option for antimalarials.

METHODS

Artemether-loaded nanoparticles (AANs) were designed and antimalarial activity evaluated in vitro/in vivo using Plasmodium falciparum/Plasmodium berghei species, respectively.

RESULTS

Selective internalization of AAN into Plasmodium-infected RBCs in preference to healthy erythrocytes was observed using confocal imaging. In vitro studies showed 50% dose reduction for AAN as compared with drug-only controls to achieve IC₅₀ levels of inhibition. The nanoparticles exhibited twofold higher peak drug concentrations in RBCs with antimalarial activity at 50% of therapeutic doses in P. bergei infected mice.

CONCLUSION

Novel HSA-based nanoparticles offer safe and effective approach for selective targeting of antimalarial drugs.

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

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

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.

A homologue of N-ethylmaleimide-sensitive factor in the malaria parasite Plasmodium falciparum is exported and localized in vesicular structures in the cytoplasm of infected erythrocytes in the brefeldin A-sensitive pathway

N-Ethylmaleimide-sensitive factor (NSF) and its homologues play a central role in vesicular trafficking in eukaryotic cells. We have identified a NSF homologue in Plasmodium falciparum (PfNSF). The reported PfNSF gene sequence (GenBank accession number CAB10575) indicated that PfNSF comprises 783 amino acids with a calculated molecular weight of 89,133. The overall identities of its gene and amino acid sequences with those of rat NSF are 50.9 and 48.8%, respectively. Reverse transcription-polymerase chain reaction analysis and Northern blotting with total P. falciparum RNA indicated expression of the PfNSF gene. Polyclonal antibodies against a conserved region of NSF specifically recognized an 89-kDa polypeptide in the parasite cells. After homogenization of the parasite cells, approximately 90% of an 89-kDa polypeptide is associated with particulate fraction, suggesting membrane-bound nature of PfNSF. PfNSF was present within both the parasite cells and the vesicular structure outside of the parasite cells. The export of PfNSF outside of the parasite cells appears to occur at the early trophozoite stage and to terminate at the merozoite stage. The export of PfNSF is inhibited by brefeldin A, with 9 microM causing 50% inhibition. Immunoelectromicroscopy indicated that intracellular PfNSF was associated with organelles such as food vacuoles and that extracellular PfNSF was associated with vesicular structures in the erythrocyte cytoplasm. These results indicate that PfNSF expressed in the malaria parasite is exported to the extracellular space and then localized in intraerythrocytic vesicles in a brefeldin A-sensitive manner. It is suggested that a vesicular transport mechanism is involved in protein export targeted to erythrocyte membranes during intraerythrocytic development of the malaria parasite.

The malaria-infected red blood cell: structural and functional changes

The asexual stage of malaria parasites of the genus Plasmodium invade red blood cells of various species including humans. After parasite invasion, red blood cells progressively acquire a new set of properties and are converted into more typical, although still simpler, eukaryotic cells by the appearance of new structures in the red blood cell cytoplasm, and new proteins at the red blood cell membrane skeleton. The red blood cell undergoes striking morphological alterations and its rheological properties are considerably altered, manifesting as red blood cells with increased membrane rigidity, reduced deformability and increased adhesiveness for a number of other cells including the vascular endothelium. Elucidation of the structural changes in the red blood cell induced by parasite invasion and maturation and an understanding of the accompanying functional alterations have the ability to considerably extend our knowledge of structure-function relationships in the normal red blood cell. Furthermore, interference with these interactions may lead to previously unsuspected means of reducing parasite virulence and may lead to the development of novel antimalarial therapeutics.

Import of host delta-aminolevulinate dehydratase into the malarial parasite: identification of a new drug target

The parasite Plasmodium berghei imports the enzyme delta-aminolevulinate dehydratase (ALAD), and perhaps the subsequent enzymes of the pathway from the host red blood cell to sustain heme synthesis. Here we have studied the mechanism of this import. A 65-kDa protein on the P. berghei membrane specifically bound to mouse red blood cell ALAD, and a 93-amino-acid fragment (ALAD-DeltaNC) of the host erythrocyte ALAD was able to compete with the full-length enzyme for binding to the P. berghei membrane. ALAD-DeltaNC was taken up by the infected red blood cell when added to a culture of P. falciparum and this led to a substantial decrease in ALAD protein and enzyme activity and, subsequently, heme synthesis in the parasite, resulting in its death.

Inhibition of the in vitro growth of Plasmodium falciparum by acyclic nucleoside phosphonates

Forty-eight acyclic nucleoside phosphonates (putative prodrugs of acyclic nucleoside triphosphate inhibitors of DNA replication) have been evaluated for in vitro antiplasmodial activity. Only certain purine derivatives with a hydroxyl group attached to the acyclic sugar moiety displayed antiplasmodial activity. The two most active analogs were (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine ((S)-HPMPA, IC50=0.18+/-0.07 microM) and (S)-3-deaza-HPMPA (IC50=0.29+/-0.08 microM). Their cyclic derivatives, containing an ester bond between the phosphonate and the hydroxyl group, were slightly less active. All tested compounds that lacked the hydroxyl group, including potent antiretrovirus analogs such as 9-(2-phosphonylmethoxyethyl)adenine (PMEA) and the (S)-HPMPA derivatives (R)-PMPA and (S)-FPMPA, did not show any activity, even at very high concentrations ( >250 microM). Similarly, pyrimidine analogs of (S)-HPMPA, such as (S)-HPMPT, (S)-HPMPU and the anti-herpesvirus analog (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl) cytosine ((S)-HPMPC), were devoid of any antiplasmodial activity. In addition, 11 acyclic nucleoside (non-phosphorylated) analogs--which in contrast to the acyclic nucleoside phosphonates require the presence of a monophosphorylating enzyme for the first activation step--were tested. None of them inhibited the growth of the parasite. In short three chemical entities seem to be imperative for antiplasmodial activity: a purine base, a hydroxyl group in the acyclic side chain and a phosphonate group terminating this chain.

Growth-stimulating influence of human chorionic gonadotropin (hCG) on Plasmodium falciparum in vitro

In this study, it is reported that human Chorionic Gonadotropin (hCG), being one of the most important hormones of pregnancy, has a growth-stimulating effect on the asexual stages of Plasmodium falciparum in vitro. On the one hand, it is shown that the effect of the hormone is dose-related: The highest growth-rates of Plasmodium falciparum in vitro are achieved, when doses of 8.32 i.u./ml (= 50 i.u. hCG/6 ml) and 16.67 I.U./ml (= 100 i.u. hCG/6 ml) are added to the culture medium. These doses correspond to the physiological peak amounts of hCG between the 9th and 16th week of pregnancy, when parasitaemia also reaches its highest rate. On the other hand, it is shown, that any growth-stimulating effect disappears after inactivation of the hormone by heating at 120 degrees C for 20 minutes. These data support the hypothesis, that hCG does not only possess immunosuppressive properties acting on the response of T-lymphocytes, but also increases the growth of Plasmodium falciparum in vitro. The combination of both effects may explain why malaria still remains one of the most serious complications of pregnancy.

Intracellular trafficking in Plasmodium-infected erythrocytes

The past few years have witnessed considerable progress in molecular and biochemical studies of intracellular trafficking in malaria-infected red cells. Highlights include the identification of solute channels in the vacuolar membrane and the red blood cell membrane, a tubovesicular membrane network that delivers exogenous nutrients and drugs to the parasite, and parasite gene families that mediate adherence to endothelial cells and red cells.

Characterization and cloning of the gene encoding the vacuolar membrane protein EXP-2 from Plasmodium falciparum

As a contribution to the characterization of the parasitophorous vacuolar membrane from Plasmodium falciparum we have begun the identification of vacuolar membrane proteins. Exported protein-2 (EXP-2) is a vacuolar membrane protein exposed into the vacuolar space. To further characterize EXP-2, it was purified, and the 45 N-terminal amino acids were determined by micro-sequencing. Based on this information, partial cDNA and genomic fragments were amplified by PCR and used as probes for the isolation of complete cDNA and genomic DNA clones. The single copy gene is located on chromosome 14, and is transcribed during the ring stage of parasite development. The open reading frame encodes an N-terminal signal sequence which is cleaved from the mature protein. The amino acid composition of EXP-2 is characterized by charged amino acids, with a high abundance of aspartate residues in the C-terminal portion of the protein. In contrast to EXP-1, an integral protein of the vacuolar membrane, EXP-2 lacks a typical hydrophobic transmembrane domain. We suggest that EXP-2 may associate with the vacuolar membrane via an amphipathic helix located in the N-terminal half of the protein.

Selective cytotoxicity of dermaseptin S3 toward intraerythrocytic Plasmodium falciparum and the underlying molecular basis

The antimicrobial activity of various naturally occurring microbicidal peptides was reported to result from their interaction with microbial membrane. In this study, we investigated the cytotoxicity of the hemolytic peptide dermaseptin S4 (DS4) and the nonhemolytic peptide dermaseptin S3 (DS3) toward human erythrocytes infected by the malaria parasite Plasmodium falciparum. Both DS4 and DS3 inhibited the parasite's ability to incorporate [3H]hypoxanthine. However, while DS4 was toxic toward both the parasite and the host erythrocyte, DS3 was toxic only toward the intraerythrocytic parasite. To gain insight into the mechanism of this selective cytotoxicity, we labeled the peptides with fluorescent probes and investigated their organization in solution and in membranes. In Plasmodium-infected cells, rhodamine-labeled peptides interacted directly with the intracellular parasite, in contrast to noninfected cells, where the peptides remained bound to the erythrocyte plasma membrane. Binding experiments to phospholipid membranes revealed that DS3 and DS4 had similar binding characteristics. Membrane permeation studies indicated that the peptides were equally potent in permeating phosphatidylserine/phosphatidylcholine vesicles, whereas DS4 was more permeative with phosphatidylcholine vesicles. In aqueous solutions, DS4 was found to be in a higher aggregation state. Nevertheless, both DS3 and DS4 spontaneously dissociated to monomers upon interaction with vesicles, albeit with different kinetics. In light of these results, we propose a mechanism by which dermaseptins permeate cells and affect intraerythrocytic parasites.

Characterization of macromolecular transport pathways in malaria-infected erythrocytes

We have previously provided evidence for a pathway in Plasmodium falciparum-infected erythrocytes, coined the parasitophorous duct pathway, which provides serum (macro)molecules direct access to intraerythrocytic parasites . The present study addresses the purity of the fluorescent macromolecules used to define the duct pathway and provides ultrastructural evidence for its presence. The fluorescent tracers used to characterize transport remain intact during their incubation with infected erythrocytes. Transport of macromolecules in the external medium or host cell cytosol to the intracellular parasites is shown to occur by two distinct pathways. Fluorescent dextrans in the erythrocyte cytosol are ingested by the parasite via a specialized organelle, the cytostome, and are transported to the parasite food vacuole. Transport through this pathway occurs throughout the asexual life cycle. By contrast, fluorescent dextrans in the external medium bypass the erythrocyte cytosol, and are internalized by the parasite by a process resembling fluid-phase endocytosis. Serial sections of mature parasites fixed and stained by various methods for transmission electron microscopy reveal areas of apparent membrane continuity between the erythrocyte membrane and the parasitophorous vacuolar membrane that surrounds the parasite, that could leave the parasites exposed to the external medium. Using carboxylate and amidine-modified fluorescent latex spheres and laser scanning confocal microscopy, macromolecules up to 50-70 nm in diameter are found to have direct access to intraerythrocytic parasites. This size exclusion is consistent with the dimensions of the parasitophorous duct pathway revealed by electron microscopy. This investigation reports for the first time the existence of two, distinct macromolecular transport pathways in malaria-infected erythrocytes.

Defining the minimal domain of the Plasmodium falciparum protein MESA involved in the interaction with the red cell membrane skeletal protein 4.1

During part of its life cycle, the protozoan parasite Plasmodium falciparum lives within the human red blood cell and modifies both the structural and functional properties of the red cell. It does this by synthesizing a number of polypeptides that it transports into the red cell cytoplasm and to the red cell membrane. One of these transported proteins, MESA (mature parasite-infected erythrocyte surface antigen), is anchored to the red cell membrane by noncovalent interaction with erythrocyte protein 4.1. We have utilized a combination of in vitro transcription and translation and a membrane binding assay to identify the protein sequence involved in anchoring MESA to the membrane. Labeled fragments of different regions of the MESA protein were evaluated for their ability to bind to inside-out vesicle membrane preparations of human red cells. Binding was dependent on the presence of red cell membrane proteins and was abolished either by trypsin treatment or by selective depletion of membrane proteins. Binding was specific and could be inhibited by the addition of competing protein, with an IC50 of (6.3 +/- 1.2) x 10(-7) M, indicative of a moderate affinity interaction. Fractionation studies demonstrated that binding fragments interacted most efficiently with membrane protein fractions that had been enriched in protein 4.1. Binding inhibition experiments using synthetic peptides identified the binding domain of MESA for protein 4.1 as a 19-residue sequence near the amino terminus of MESA, a region capable of forming an amphipathic helix.

Membrane modifications in erythrocytes parasitized by Plasmodium falciparum

Plasmodium falciparum malaria parasites invade human red blood cells and immediately begin making significant alterations to the structure of the erythrocyte. These alterations facilitate the movement of nutrients into, and waste products and parasite-derived proteins out of the cell to meet the needs of the growing parasite. A tubovesicular membrane network extending from the parasite vacuole membrane probably has a central role in the transport processes. The parasite also modifies the erythrocyte membrane itself in a way that not only changes its permeability but also places parasite-derived proteins in knob-like protrusions at the cell surface. These proteins enable the parasite to adhere to endothelial cells and thereby avoid clearance from the blood stream by the spleen. Antigenic variation of these proteins allows parasitized erythrocytes to vary their phenotype and produce a sustained and chronic malaria infection. Study of the molecular processes that underlie these parasite-induced modifications of the host red blood cell will lead to improved understanding of malaria pathogenesis and, perhaps, suggest new approaches against the disease.

The varied lifestyles of intracellular pathogens within eukaryotic vacuolar compartments

Many bacterial pathogens and eukaryotic parasites can enter mammalian cells and live intracellularly inside membrane-bound vacuoles. The intravacuolar lifestyle of these pathogens plays a key role in pathogenesis. Understanding the molecular basis of the development of these specialized intracellular compartments is critical to understanding how these organisms cause disease.

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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