Illuminating Plasmodium falciparum-infected red blood cells

Human plasma plasminogen internalization route in Plasmodium falciparum-infected erythrocytes

Background

The intra-erythrocytic development of the malaria parasite Plasmodium falciparum depends on the uptake of a number of essential nutrients from the host cell and blood plasma. It is widely recognized that the parasite imports low molecular weight solutes from the plasma and the consumption of these nutrients by P. falciparum has been extensively analysed. However, although it was already shown that the parasite also imports functional proteins from the vertebrate host, the internalization route through the different infected erythrocyte membranes has not yet been elucidated. In order to further understand the uptake mechanism, the study examined the trafficking of human plasminogen from the extracellular medium into P. falciparum-infected red blood cells.

Methods

Plasmodium falciparum clone 3D7 was cultured in standard HEPES-buffered RPMI 1640 medium supplemented with 0.5% AlbuMAX. Exogenous human plasminogen was added to the P. falciparum culture and the uptake of this protein by the parasites was analysed by electron microscopy and Western blotting. Immunoprecipitation and mass spectrometry were performed to investigate possible protein interactions that may assist plasminogen import into infected erythrocytes. The effect of pharmacological inhibitors of different cellular physiological processes in plasminogen uptake was also tested.

Results

It was observed that plasminogen was selectively internalized by P. falciparum-infected erythrocytes, with localization in plasma membrane erythrocyte and parasite’s cytosol. The protein was not detected in parasitic food vacuole and haemoglobin-containing vesicles. Furthermore, in erythrocyte cytoplasm, plasminogen was associated with the parasite-derived membranous structures tubovesicular network (TVN) and Maurer’s clefts. Several proteins were identified in immunoprecipitation assay and may be involved in the delivery of plasminogen across the P. falciparum multiple compartments.

Conclusion

The findings here reported reveal new features regarding the acquisition of plasma proteins of the host by P. falciparum-infected erythrocytes, a mechanism that involves the exomembrane system, which is distinct from the haemoglobin uptake, clarifying a route that may be potentially targeted for inhibition studies.

Live and Let Dye: Visualizing the Cellular Compartments of the Malaria Parasite Plasmodium falciparum

Malaria remains one of the deadliest diseases worldwide and it is caused by the protozoan parasite Plasmodium spp. Parasite visualization is an important tool for the correct detection of malarial cases but also to understand its biology. Advances in visualization techniques promote new insights into the complex life cycle and biology of Plasmodium parasites. Live cell imaging by fluorescence microscopy or flow cytometry are the foundation of the visualization technique for malaria research. In this review, we present an overview of possibilities in live cell imaging of the malaria parasite. We discuss some of the state-of-the-art techniques to visualize organelles and processes of the parasite and discuss limitation and advantages of each technique. © 2019 International Society for Advancement of Cytometry.

Development of a conditional localization approach to control apicoplast protein trafficking in malaria parasites

Secretory proteins are of particular importance to apicomplexan parasites and comprise over 15% of the genomes of the human pathogens that cause diseases like malaria, toxoplasmosis and babesiosis as well as other diseases of agricultural significance. Here, we developed an approach that allows us to control the trafficking destination of secretory proteins in the human malaria parasite Plasmodium falciparum. Based on the unique structural requirements of apicoplast transit peptides, we designed three conditional localization domains (CLD1, 2 and 3) that can be used to control protein trafficking via the addition of a cell permeant ligand. Studies comparing the trafficking dynamics of each CLD show that CLD2 has the most optimal trafficking efficiency. To validate this system, we tested whether CLD2 could conditionally localize a biotin ligase called holocarboxylase synthetase 1 (HCS1) without interfering with the function of the enzyme. In a parasite line expressing CLD2-HCS1, we were able to control protein biotinylation in the apicoplast in a ligand-dependent manner, demonstrating the full functionality of the CLD tool. We have developed and validated a novel molecular tool that may be used in future studies to help elucidate the function of secretory proteins in malaria parasites.

Immunofluorescence staining protocol for STED nanoscopy of Plasmodium-infected red blood cells

Immunofluorescence staining is the key technique for visualizing organization of endogenous cellular structures in single cells. Labeling and imaging of blood stage Plasmodium falciparum has always been challenging since it is a small intracellular parasite. A widely-used standard for parasite immunofluorescence is fixation in suspension with addition of minute amounts of glutaraldehyde to the paraformaldehyde-based solution. While this maintains red blood cell integrity, it has been postulated that antigenicity of the parasite proteins was, if at all, only slightly reduced. Here we show the deleterious effect that even these small quantities of glutaraldehyde can have on immunofluorescence staining quality and present an alternative cell seeding protocol that allows fixation with only paraformaldehyde. The highly improved signal intensity and staining efficiency enabled us to carry out RescueSTED nanoscopy on microtubules and nuclear pores and describe their organization in greater detail throughout the blood stage cycle.

Machine learning-based in-line holographic sensing of unstained malaria-infected red blood cells

Accurate and immediate diagnosis of malaria is important for medication of the infectious disease. Conventional methods for diagnosing malaria are time consuming and rely on the skill of experts. Therefore, an automatic and simple diagnostic modality is essential for healthcare in developing countries that lack the expertise of trained microscopists. In the present study, a new automatic sensing method using digital in-line holographic microscopy (DIHM) combined with machine learning algorithms was proposed to sensitively detect unstained malaria-infected red blood cells (iRBCs). To identify the RBC characteristics, 13 descriptors were extracted from segmented holograms of individual RBCs. Among the 13 descriptors, 10 features were highly statistically different between healthy RBCs (hRBCs) and iRBCs. Six machine learning algorithms were applied to effectively combine the dominant features and to greatly improve the diagnostic capacity of the present method. Among the classification models trained by the 6 tested algorithms, the model trained by the support vector machine (SVM) showed the best accuracy in separating hRBCs and iRBCs for training (n = 280, 96.78%) and testing sets (n = 120, 97.50%). This DIHM-based artificial intelligence methodology is simple and does not require blood staining. Thus, it will be beneficial and valuable in the diagnosis of malaria.

Tomographic phase microscopy: principles and applications in bioimaging [Invited]

Tomographic phase microscopy (TPM) is an emerging optical microscopic technique for bioimaging. TPM uses digital holographic measurements of complex scattered fields to reconstruct three-dimensional refractive index (RI) maps of cells with diffraction-limited resolution by solving inverse scattering problems. In this paper, we review the developments of TPM from the fundamental physics to its applications in bioimaging. We first provide a comprehensive description of the tomographic reconstruction physical models used in TPM. The RI map reconstruction algorithms and various regularization methods are discussed. Selected TPM applications for cellular imaging, particularly in hematology, are reviewed. Finally, we examine the limitations of current TPM systems, propose future solutions, and envision promising directions in biomedical research.

Erythrocyte lysis and Xenopus laevis oocyte rupture by recombinant Plasmodium falciparum hemolysin III

Malaria kills more than 1 million people per year worldwide, with severe malaria anemia accounting for the majority of the deaths. Malaria anemia is multifactorial in etiology, including infected erythrocyte destruction and decrease in erythrocyte production, as well as destruction or clearance of noninfected erythrocytes. We identified a panspecies Plasmodium hemolysin type III related to bacterial hemolysins. The identification of a hemolysin III homologue in Plasmodium suggests a potential role in host erythrocyte lysis. Here, we report the first characterization of Plasmodium falciparum hemolysin III, showing that the soluble recombinant P. falciparum hemolysin III is a pore-forming protein capable of lysing human erythrocytes in a dose-, time-, and temperature-dependent fashion. The recombinant P. falciparum hemolysin III-induced hemolysis was partially inhibited by glibenclamide, a known channel antagonist. Studies with polyethylene glycol molecules of different molecular weights indicated a pore size of approximately 3.2 nm. Heterologous expression of recombinant P. falciparum hemolysin III in Xenopus oocytes demonstrated early hypotonic lysis similar to that of the pore-forming aquaporin control. Live fluorescence microscopy localized transfected recombinant green fluorescent protein (GFP)-tagged P. falciparum hemolysin III to the essential digestive vacuole of the P. falciparum parasite. These transfected trophozoites also possessed a swollen digestive vacuole phenotype. Native Plasmodium hemolysin III in the digestive vacuole may contribute to lysis of the parasitophorous vacuole membrane derived from the host erythrocyte. After merozoite egress from infected erythrocytes, remnant P. falciparum hemolysin III released from digestive vacuoles could potentially contribute to lysis of uninfected erythrocytes to contribute to severe life-threatening anemia.

Immunobiology of Plasmodium in liver and brain

Malaria remains one of the most serious health problems globally, but our understanding of the biology of the parasite and the pathogenesis of severe disease is still limited. Multiple cellular effector mechanisms that mediate parasite elimination from the liver have been described, but how effector cells use classical granule-mediated cytotoxicity to attack infected hepatocytes and how cytokines and chemokines spread via the unique fluid pathways of the liver to reach the parasites over considerable distances remains unknown. Similarly, a wealth of information on cerebral malaria (CM), one of the most severe manifestations of the disease, was gained from post-mortem analyses of human brain and murine disease models, but the cellular processes that ultimately cause disease are not fully understood. Here, we discuss how imaging of the local dynamics of parasite infection and host response as well as consideration of anatomical and physiological features of liver and brain can provide a better understanding of the initial asymptomatic hepatic phase of the infection and the cascade of events leading to CM. Given the increasing drug resistance of both parasite and vector and the unavailability of a protective vaccine, the urgency to reduce the tremendous morbidity and mortality associated with severe malaria is obvious.

Cytometry in malaria--a practical replacement for microscopy?

Malaria, caused by protozoan Plasmodium parasites, kills ~800,000 people each year. Exact figures are uncertain because presumptive diagnoses are often made without identifying parasites in patients' blood either by microscopy, using Giemsa's century-old stain, or by simpler tests that are ultimately dependent on microscopy for quality control. Microscopy itself relies on trained observers' ability to detect subtle morphological features of parasitized red blood cells, only a few of which may be present on a slide. Quantitative and objective flow cytometric measurements of cellular constituents such as DNA, RNA, and the malaria pigment hemozoin are now useful in research in malaria biology and pharmacology, and can provide more reliable identification of parasite species and developmental stages and better detection of low-density parasitemia than could microscopy. The same measurements can now be implemented in much smaller, simpler, cheaper imaging cytometers, potentially providing a more accurate and precise diagnostic modality.

Characterization of a glycerophosphodiesterase with an unusual tripartite distribution and an important role in the asexual blood stages of Plasmodium falciparum

Catabolism of glycerophospholipids during the rapid growth of the asexual intraerythrocytic malaria parasite may contribute to membrane recycling and the acquisition of lipid biosynthetic precursors from the host. To better understand the scope of lipid catabolism in Plasmodium falciparum, we have characterized a malarial homolog of bacterial glycerophosphodiesterases. These enzymes catalyze the hydrolysis of glycerophosphodiesterases that are generated by phospholipase-catalyzed removal of the two acyl groups from glycerophospholipids. The P. falciparum glycerophosphodiesterase (PfGDPD) exhibits an unusual tripartite distribution during the asexual blood stage with pools of enzyme in the parasitophorous vacuole, food vacuole and cytosol. Efforts to disrupt the chromosomal PfGDPD coding sequence were unsuccessful, which implies that the enzyme is important for efficient parasite growth. Tagging of the endogenous pool of PfGDPD with a conditional aggregation domain partially perturbed the distribution of the enzyme in the parasitophorous vacuole but had no discernable effect on growth in culture. Kinetic characterization of the hydrolysis of glycerophosphocholine by recombinant PfGDPD, an Mg(2+)-dependent enzyme, yielded steady-state parameters that were comparable to those of a homologous bacterial glycerophosphodiesterase. Together, these results suggest a physiological role for PfGDPD in glycerophospholipid catabolism in multiple subcellular compartments. Possibilities for what this role might be are discussed.

Determination of protein subcellular localization in apicomplexan parasites

Parasites from the phylum Apicomplexa include causative agents of serious diseases including malaria (Plasmodium spp.) and toxoplasmosis (Toxoplasma gondii). Apicomplexan parasites infect thousands of types of animal cells and send their proteins to an array of compartments within their own cell, as well as exporting proteins into and beyond their host cell. Ascertaining destinations to which individual proteins are delivered allows researchers to better understand parasite biology and to identify potential targets for therapeutic interventions. Our toolkit for establishing subcellular locations of apicomplexan proteins is becoming more extensive and specialized, and here we review developments in this technology.

Static and dynamic imaging of erythrocyte invasion and early intra-erythrocytic development in Plasmodium falciparum

Cellular imaging has reemerged in recent years as a powerful approach to provide researchers with a direct measure of essential molecular events in a cell's life, ranging in scale from broad morphological observations of whole cells to intricate single molecule imaging. When combined with quantitative image analysis, the available imaging techniques can act as a critical means to confirm hypotheses, drive the formation of new theories or provide accurate determination of protein localization at subcellular and nanometer scales. Here, we describe two methodological approaches for imaging the transient step of malaria parasite invasion of the human erythrocyte. When applied to image the most virulent human malaria parasite, Plasmodium falciparum, the first approach, using live time-lapse wide-field microscopy, allows the capture of transient events during invasion and postinvasion intra-erythrocytic development, while the second, using immunofluorescence assay (IFA) of fixed samples, allows high-definition exploration of parasite architecture on multiple platforms.

Mitosis in the human malaria parasite Plasmodium falciparum

Malaria is caused by intraerythrocytic protozoan parasites belonging to Plasmodium spp. (phylum Apicomplexa) that produce significant morbidity and mortality, mostly in developing countries. Plasmodium parasites have a complex life cycle that includes multiple stages in anopheline mosquito vectors and vertebrate hosts. During the life cycle, the parasites undergo several cycles of extreme population growth within a brief span, and this is critical for their continued transmission and a contributing factor for their pathogenesis in the host. As with other eukaryotes, successful mitosis is an essential requirement for Plasmodium reproduction; however, some aspects of Plasmodium mitosis are quite distinct and not fully understood. In this review, we will discuss the current understanding of the architecture and key events of mitosis in Plasmodium falciparum and related parasites and compare them with the traditional mitotic events described for other eukaryotes.

Methodology and application of flow cytometry for investigation of human malaria parasites

Historically, examinations of the inhibition of malaria parasite growth/invasion, whether using drugs or antibodies, have relied on the use of microscopy or radioactive hypoxanthine uptake. These are considered gold standards for measuring the effectiveness of antimalarial treatments, however, these methods have well known shortcomings. With the advent of flow cytometry coupled with the use of fluorescent DNA stains allowed for increased speed, reproducibility, and qualitative estimates of the effectiveness of antibodies and drugs to limit malaria parasite growth which addresses the challenges of traditional techniques. Because materials and machines available to research facilities are so varied, different methods have been developed to investigate malaria parasites by flow cytometry. This review is intended to serve as a reference guide for advanced users and importantly, as a primer for new users, to support expanded use and improvements to malaria flow cytometry, particularly in endemic countries.

Cryo transmission X-ray imaging of the malaria parasite, P. falciparum

Cryo transmission X-ray microscopy in the "water window" of photon energies has recently been introduced as a method that exploits the natural contrast of biological samples. We have used cryo tomographic X-ray imaging of the intra-erythrocytic malaria parasite, Plasmodium falciparum, to undertake a survey of the cellular features of this important human pathogen. We examined whole hydrated cells at different stages of growth and defined some of the structures with different X-ray density, including the parasite nucleus, cytoplasm, digestive vacuole and the hemoglobin degradation product, hemozoin. As the parasite develops from an early cup-shaped morphology to a more rounded shape, puncta of hemozoin are formed; these coalesce in the mature trophozoite into a central compartment. In some trophozoite stage parasites we observed invaginations of the parasite surface and, using a selective permeabilization process, showed that these remain connected to the RBC cytoplasm. Some of these invaginations have large openings consistent with phagocytic structures and we observed independent endocytic vesicles in the parasite cytoplasm which appear to play a role in hemoglobin uptake. In schizont stage parasites staggered mitosis was observed and X-ray-dense lipid-rich structures were evident at their apical ends of the developing daughter cells. Treatment of parasites with the antimalarial drug artemisinin appears to affect parasite development and their ability to produce the hemoglobin breakdown product, hemozoin.

In vivo imaging of infection immunology--4I's!

As predicted by the red queen hypothesis, microbial pathogens are probably the major reason for the evolution of the immune system (Paterson et al., Nature 464:275-278, 2010). In general, at the population level, i.e., for most of us, most of the time, the immune response to infection is highly effective. However, there remain significant challenges with particularly intransigent organisms or those that are crossing species barriers. Thus, in some cases, efforts to develop new and effective vaccines and drugs have met with limited success. To paraphrase Rudyard Kipling, "I keep six honest serving men--they taught me all I know; their names are what, and why, and when and how and where and who". Addressing these key tenets will be key to understanding the interaction between infection and the immune system. This is particularly important, as the early events during induction of an immune response influence the acquisition of effector function and development of memory responses. Our understanding of the interactions of pathogens with the host immune system has largely been derived through in vitro or static in vivo study. This is a significant issue, as the component parts of the immune system do not work in isolation, and their interactions occur in distinct and specialized micro- and macro anatomical locations that can only be assessed in the physiological context, dynamically in vivo. To this end, the increasing availability of genetically manipulable pathogens and high resolution, real-time in vivo imaging over the preceding 5 years has greatly enhanced our ability to understand and evaluate factors involved in host-pathogen interactions in vivo. This article will review the current status of this area, highlight why progress has been faster with some pathogens and tissues (e.g., protozoa and accessible site such as skin), and speculate on what recent developments in biology and imaging will tell us about pathogen-specific immune responses in the future. This will be done by following the chronological development of the infection process from invasion, to recognition, and dissemination.

Synthetic peptides from conserved regions of the Plasmodium falciparum early transcribed membrane and ring exported proteins bind specifically to red blood cell proteins

Severe malaria pathology is directly associated with cytoadherence of infected red blood cells (iRBCs) to healthy RBCs and/or endothelial cells occurring during the intraerythrocytic development of Plasmodium falciparum. We synthesized, as 20-mer long peptides, the members of the ring exported (REX) protein family encoded in chromosome 9, as well as the early transcribed membrane proteins (E-TRAMP) 10.2 and 4, to identify specific RBC binding regions in these proteins. Twelve binding peptides were identified (designated as HABPs): three were identified in REX1, two in REX2, one in REX3, two in REX4 and four in E-TRAMP 10.2. The majority of these HABPs was conserved among different P. falciparum strains, according to sequence analysis. No HABPs were found in E-TRAMP 4. Bindings of HABPs were saturable and sensitive to the enzymatic treatment of RBCs and HABPs had different structural features, according to circular dichroism studies. Our results suggest that the REX and E-TRAMP families participate in relevant interactions with RBC membrane proteins, which highlight these proteins as potential targets for the development of fully effective immunoprophylactic methods.

Oleic acid biosynthesis in Plasmodium falciparum: characterization of the stearoyl-CoA desaturase and investigation as a potential therapeutic target

Background

Plasmodium falciparum parasitization of erythrocytes causes a substantial increase in the levels of intracellular fatty acids, notably oleic acid. How parasites acquire this monounsaturated fatty acid has remained enigmatic. Here, we report on the biochemical and enzymatic characterization of stearoyl-CoA desaturase (SCD) in P. falciparum.

Methodology/Principal Findings

Metabolic labeling experiments allowed us to demonstrate the production of oleic acid from stearic acid both in lysates of parasites incubated with [¹⁴C]-stearoyl-CoA and in parasite-infected erythrocytes labeled with [¹⁴C]-stearic acid. Optimal SCD activity was detected in schizonts, the stage of maximal membrane synthesis. This activity correlated with a late trophozoite stage-specific induction of PFE0555w transcripts. PFE0555w harbors a typical SCD signature. Similar to mammalian SCDs, this protein was found to be associated with the endoplasmic reticulum, as determined with PFE0555w-GFP tagged transgenic P. falciparum. Importantly, these parasites exhibited increased rates of stearic to oleic acid conversion, providing additional evidence that PFE0555w encodes the plasmodial SCD (PfSCD). These findings prompted us to assess the activity of sterculic acid analogues, known to be specific Δ9-desaturase inhibitors. Methyl sterculate inhibited the synthesis of oleic acid both with parasite lysates and infected erythrocytes, most likely by targeting PfSCD. This compound exhibited significant, rapid and irreversible antimalarial activity against asexual blood stages. This parasiticidal effect was antagonized by oleic acid.

Conclusion/Significance

Our study provides evidence that parasite-mediated fatty acid modification is important for blood-stage survival and provides a new strategy to develop a novel antimalarial therapeutic based on the inhibition of PfSCD.

The role of the Maurer's clefts in protein transport in Plasmodium falciparum

Maurer's clefts (MCs) are membranous structures that are formed by Plasmodium falciparum and used by the parasite for protein sorting and protein export. Virulence proteins, as well as other proteins used to remodel the erythrocyte, are exported. Discontinuity between major membrane compartments within the infected erythrocyte cytoplasm suggests multiple traffic routes for exported proteins. The sequences of the conserved Plasmodium export element seem insufficient for export of all parasite proteins. The parasite displays remarkable versatility in the types of proteins exported to the MCs and in the functions of the proteins within the MCs. In this Review, protein export to the MCs and the role of the MCs in the transport of proteins to the erythrocyte membrane are summarized.

High-resolution X-ray imaging of Plasmodium falciparum-infected red blood cells

Methods for imaging cellular architecture and ultimately macromolecular complexes and individual proteins, within a cellular environment, are an important goal for cell and molecular biology. Coherent diffractive imaging (CDI) is a method of lensless imaging that can be applied to any individual finite object. A diffraction pattern from a single biological structure is recorded and an iterative Fourier transform between real space and reciprocal space is used to reconstruct information about the architecture of the sample to high resolution. As a test system for cellular imaging, we have applied CDI to an important human pathogen, the malaria parasite, Plasmodium falciparum. We have employed a novel CDI approach, known as Fresnel CDI, which uses illumination with a curved incident wavefront, to image red blood cells infected with malaria parasites. We have examined the intrinsic X-ray absorption contrast of these cells and compared them with cells contrasted with heavy metal stains or immunogold labeling. We compare CDI images with data obtained from the same cells using scanning electron microscopy, light microscopy, and scanning X-ray fluorescence microscopy. We show that CDI can offer new information both within and at the surface of complex biological specimens at a spatial resolution of better than 40 nm. and we demonstrate an imaging modality that conveniently combines scanning X-ray fluorescence microscopy with CDI. The data provide independent confirmation of the validity of the coherent diffractive image and demonstrate that CDI offers the potential to become an important and reliable new high-resolution imaging modality for cell biology. CDI can detect features at high resolution within unsectioned cells.

Refractive index maps and membrane dynamics of human red blood cells parasitized by Plasmodium falciparum

Parasitization by malaria-inducing Plasmodium falciparum leads to structural, biochemical, and mechanical modifications to the host red blood cells (RBCs). To study these modifications, we investigate two intrinsic indicators: the refractive index and membrane fluctuations in P. falciparum-invaded human RBCs (Pf-RBCs). We report experimental connections between these intrinsic indicators and pathological states. By employing two noninvasive optical techniques, tomographic phase microscopy and diffraction phase microscopy, we extract three-dimensional maps of refractive index and nanoscale cell membrane fluctuations in isolated RBCs. Our systematic experiments cover all intraerythrocytic stages of parasite development under physiological and febrile temperatures. These findings offer potential, and sufficiently general, avenues for identifying, through cell membrane dynamics, pathological states that cause or accompany human diseases.

A conditional export system provides new insights into protein export in Plasmodium falciparum-infected erythrocytes

The human malarial parasite Plasmodium falciparum exports determinants of virulence and pathology to destinations within its host erythrocyte, including the cytoplasm, the plasma membrane and membrane profiles of parasite origin termed Maurer's clefts. While there is some information regarding the signals that allot proteins for export, the trafficking route itself has remained largely obscure, partly due to technical limitations in following protein trafficking with time. To overcome these shortcomings, we have established a conditional protein export system in P. falciparum, based on the previously described conditional aggregation domain (CAD domain) that self-aggregates in the endoplasmic reticulum in a manner that is reversible by the addition of a small molecule. By fusing the CAD domain to the first 80 amino acids of STEVOR and full-length PfSBP1, we were able to control export of a soluble and a transmembrane protein to the erythrocyte cytosol and the Maurer's clefts respectively. The conditional export system allowed us to study the temporal sequence of events of protein export and identify intermediate steps. We further explored the potential of the conditional export system in identifying factors that interact with exported proteins en route. Our data provide evidence for a physical interaction of exported proteins with the molecular chaperone PfBiP during early export steps.

A method for visualizing surface-exposed and internal PfEMP1 adhesion antigens in Plasmodium falciparum infected erythrocytes

Background

The insertion of parasite antigens into the host erythrocyte membrane and the structure and distribution of Plasmodium falciparum adhesion receptors on that membrane are poorly understood. Laser scanning confocal microscopy (LSCM) and a novel labelling and fixation method have been used to obtain high resolution immuno-fluorescent images of erythrocyte surface PfEMP1 and internal antigens which allow analysis of the accumulation of PfEMP1 on the erythrocyte membrane during asexual development.

Methods

A novel staining technique has been developed which permits distinction between erythrocyte surface PfEMP1 and intracellular PfEMP1, in parasites whose nuclear material is exceptionally well resolved. Primary antibody detection by fluorescence is carried out on the live parasitized erythrocyte. The surface labelled cells are then fixed using paraformaldehyde and permeabilized with a non-ionic detergent to permit access of antibodies to internal parasite antigens. Differentiation between surface and internal antigens is achieved using antibodies labelled with different fluorochromes and confocal microscopy

Results

Surface exposed PfEMP1 is first detectable by antibodies at the trophozoite stage of intracellular parasite development although the improved detection method indicates that there are differences between different laboratory isolates in the kinetics of accumulation of surface-exposed PfEMP1.

Conclusion

A sensitive method for labelling surface and internal PfEMP1 with up to three different fluorochromes has been developed for laser scanning confocal optical microscopy and the analysis of the developmental expression of malaria adhesion antigens.

A 3D view of the host cell compartment in P. falciparum-infected erythrocytes

The most deadly of the human malaria parasites, Plasmodium falciparum, invades the erythrocytes of its host and initiates a remarkable series of morphological rearrangements within the host cell cytoplasm. The mature erythrocyte is effectively a floating sack of haemoglobin with no endogenous protein synthesis or protein trafficking machinery. In order to colonise and remodel its extracellular space, the parasite generates a series of novel structures that are involved in the export of virulence factors to the surface of the host cell. These include extensions of the parasite's vacuolar membrane, known as the tubulovesicular network, and structures referred to as Maurer's clefts. Maurer's clefts are convoluted collections of distorted discs that are tethered to the red blood cell membrane by structures with stalk-like profiles. Recently electron tomography has enabled visualisation--in three dimensions and at unprecedented resolution--the complexity of the membrane systems within the infected RBC cytoplasm.

What can transgenic parasites tell us about the development of Plasmodium-specific immune responses?

Malaria infects 500 million people and kills an estimated 2.7 million annually, representing one of the most significant diseases in the world. However, efforts to develop effective vaccines have met with limited success. One reason is our lack of basic knowledge of how and where the immune system responds to parasite antigens. This is important as the early events during induction of an immune response influence the acquisition of effector function and development of memory responses. Our knowledge of the interactions of Plasmodia with the host immune system has largely been derived through in vitro study. This is a significant issue as the component parts of the immune system do not work in isolation and their interactions occur in distinct and specialized micro- and macro-anatomical locations that can only be assessed in the physiological context, in vivo. In this context, the availability of transgenic malaria parasites over the last 10 years has greatly enhanced our ability to understand and evaluate factors involved in host-parasite interactions in vivo. In this article, we review the current status of this area and speculate on what parasite transgenesis approaches will tell us about the development of Plasmodium-specific immune responses in the future.

Secretion of proteins into host cells by Apicomplexan parasites

The phylum Apicomplexa consists of a diverse group of obligate, intracellular parasites. The distinct evolutionary pressures on these protozoans as they have adapted to their respective niches have resulted in a variety of methods that they use to interact with and modify their hosts. One of these is the secretion and trafficking of parasite proteins into the host cell. We review this process for Theileria, Toxoplasma and Plasmodium. We also present what is known about the mechanisms by which parasite proteins are exported into the host cell, as well as information on their known and putative functions once they have reached their final destination.

Nucleolar translocalization of GRA10 of Toxoplasma gondii transfectionally expressed in HeLa cells

Toxoplasma gondii GRA10 expressed as a GFP-GRA10 fusion protein in HeLa cells moved to the nucleoli within the nucleus rapidly and entirely. GRA10 was concentrated specifically in the dense fibrillar component of the nucleolus morphologically by the overlap of GFP-GRA10 transfection image with IFA images by monoclonal antibodies against GRA10 (Tg378), B23 (nucleophosmin) and C23 (nucleolin). The nucleolar translocalization of GRA10 was caused by a putative nucleolar localizing sequence (NoLS) of GRA10. Interaction of GRA10 with TATA-binding protein associated factor 1B (TAF1B) in the yeast two-hybrid technique was confirmed by GST pull-down assay and immunoprecipitation assay. GRA10 and TAF1B were also co-localized in the nucleolus after co-transfection. The nucleolar condensation of GRA10 was affected by actinomycin D. Expressed GFP-GRA10 was evenly distributed over the nucleoplasm and the nucleolar locations remained as hollows in the nucleoplasm under a low dose of actinomycin D. Nucleolar localizing and interacting of GRA10 with TAF1B suggested the participation of GRA10 in rRNA synthesis of host cells to favor the parasitism of T. gondii.

Malaria - an overview

Malaria is caused by protozoan parasites of the genus Plasmodium and is a major cause of mortality and morbidity worldwide. These parasites have a complex life cycle in their mosquito vector and vertebrate hosts. The primary factors contributing to the resurgence of malaria are the appearance of drug-resistant strains of the parasite, the spread of insecticide-resistant strains of the mosquito and the lack of licensed malaria vaccines of proven efficacy. This minireview includes a summary of the disease, the life cycle of the parasite, information relating to the genome and proteome of the species lethal to humans, Plasmodium falciparum, together with other recent developments in the field.

The twists and turns of Maurer's cleft trafficking in P. falciparum-infected erythrocytes

The malaria parasite, Plasmodium falciparum, invades the red blood cells (RBCs) of its human host and initiates a series of morphological rearrangements within the host cell cytoplasm. The mature RBC has no endogenous trafficking machinery; therefore, the parasite generates novel structures to mediate protein transport. These include compartments called the Maurer's clefts (MC), which play an important role in the trafficking of parasite proteins to the surface of the host cell. Recent electron tomography studies have revealed MC as convoluted flotillas of flattened discs that are tethered to the RBC membrane, prompting speculation that the MC could, in one respect, represent an extracellular equivalent of the Golgi apparatus. Visualization of both resident and cargo proteins has helped decipher the signals and routes for trafficking of parasite proteins to the MC and beyond.

Electron tomography of the Maurer's cleft organelles of Plasmodium falciparum-infected erythrocytes reveals novel structural features

During intraerythrocytic development, the human malaria parasite, Plasmodium falciparum, establishes membrane-bound compartments, known as Maurer's clefts, outside the confines of its own plasma membrane. The Maurer's compartments are thought to be a crucial component of the machinery for protein sorting and trafficking; however, their ultrastructure is only partly defined. We have used electron tomography to image Maurer's clefts of 3D7 strain parasites. The compartments are revealed as flattened structures with a translucent lumen and a more electron-dense coat. They display a complex and convoluted morphology, and some regions are modified with surface nodules, each with a circular cross-section of approximately 25 nm. Individual 25 nm vesicle-like structures are also seen in the erythrocyte cytoplasm and associated with the red blood cell membrane. The Maurer's clefts are connected to the red blood cell membrane by regions with extended stalk-like profiles. Immunogold labelling with specific antibodies confirms differential labelling of the Maurer's clefts and the parasitophorous vacuole and erythrocyte membranes. Spot fluorescence photobleaching was used to demonstrate the absence of a lipid continuum between the Maurer's clefts and parasite membranes and the host plasma membrane.