Illuminating Plasmodium falciparum-infected red blood cells

The malaria parasite undergoes a remarkable series of morphological transformations, which underpin its life in both human and mosquito hosts. The advent of molecular transfection technology coupled with the ability to introduce fluorescent reporter proteins that faithfully track and expose the activities of parasite proteins has revolutionized our view of parasite cell biology. The greatest insights have been realized in the erythrocyte stages of Plasmodium falciparum. P. falciparum invades and remodels the human erythrocyte: it feeds on haemoglobin, grows and divides, and subverts the physiology of its hapless host. Fluorescent proteins have been employed to track and dissect each of these processes and have revealed details and exposed new paradigms.     

Deformability limits of Plasmodium falciparum-infected red blood cells

Splenic filtration of infected red blood cells (RBCs) may contribute to innate immunity and variable outcomes of malaria infections. We show that filterability of individual RBCs is well predicted by the minimum cylindrical diameter (MCD) which is calculated from a RBC's surface area and volume. The MCD describes the smallest diameter tube or smallest pore that a cell may fit through without increasing its surface area. A microfluidic device was developed to measure the MCD from thousands of individual infected RBCs (IRBCs) and uninfected RBCs (URBCs). Average MCD changes during the blood-stage cycle of Plasmodium falciparum were tracked for the cytoadherent strain ITG and the knobless strain Dd2. The MCD values for IRBCs and URBCs raise several new intriguing insights into how the spleen may remove IRBCs: some early-stage ring-IRBCs, and not just late-stage schizont-IRBCs, may be highly susceptible to filtration. In addition, knobby parasites may limit surface area expansions and thus confer high MCDs on IRBCs. Finally, URBCs, in culture with IRBCs, show higher surface area loss which makes them more susceptible to filtration than naive URBCs. These findings raise important basic questions about the variable pathology of malaria infections and metabolic process that affect volume and surface area of IRBCs.     

Protein trafficking in Plasmodium falciparum-infected red blood cells

Plasmodium falciparum inhabits a niche within the most highly terminally differentiated cell in the human body--the mature red blood cell. Life inside this normally quiescent cell offers the parasite protection from the host's immune system, but provides little in the way of cellular infrastructure. To survive and replicate in the red blood cell, the parasite exports proteins that interact with and dramatically modify the properties of the host red blood cell. As part of this process, the parasite appears to establish a system within the red blood cell cytosol that allows the correct trafficking of parasite proteins to their final cellular destinations. In this review, we examine recent developments in our understanding of the pathways and components involved in the delivery of important parasite-encoded proteins to their final destination in the host red blood cell. These complex processes are not only fundamental to the survival of malaria parasites in vivo, but are also major determinants of the unique pathogenicity of this parasite.     

The Plasmodium falciparum-infected red blood cell

Plasmodium falciparum, the most virulent of the human malaria parasites, causes up to one million deaths per year. The parasite spends part of its lifecycle inside the red blood cells (RBCs) of its host. As it grows it ingests the RBC cytoplasm, digesting it in an acidic vacuole. Free haem released during haemoglobin digestion is detoxified by conversion to inert crystals of haemozoin. Malaria pathology is evident during the blood stage of the infection and is exacerbated by adhesion of infected RBCs to blood vessel walls, which prevents splenic clearance of the infected cells. Cytoadherence is mediated by surface-exposed virulence proteins that bind to endothelial cell receptors. These 'adhesins' are exported to the RBC surface via an exomembrane system that is established outside the parasite in the host cell cytoplasm. Antimalarial drugs that interfere with haem detoxification, or target other parasite-specific processes, have been effective in the treatment of malaria, but their use has been dogged by the development of resistance. Similarly, efforts to develop an effective blood vaccine are hindered by the variability of surface-exposed antigens.     

Lipid transfer protein-catalyzed exchange of cholesteryl ester between high density lipoproteins and apoB-containing lipoproteins

Low density lipoproteins (LDL), lipoprotein (a)(Lp(a)), and lipoprotein(a) after removal of the a-protein (Lp(a-)) were compared with respect to their ability to accept cholesteryl ester from high density lipoproteins (HDL). The incubations were performed at constant concentrations of HDL and various concentrations of either LDL, Lp(a), or Lp(a-). Lp(a) exchanged cholesteryl ester with HDL, but at a rate that was only 48.5 +/- 3.8% of the exchange rate found in the presence of autologous LDL. Cleavage of the apo(a) from Lp(a) resulted in Lp(a-), an LDL-like particle, with characteristics of cholesteryl ester exchange very similar to LDL.     

Lipid transfer between human serum high density lipoproteins and egg yolk lipoproteins in incubation mixtures

Ultracentrifugally isolated human serum high density lipoproteins of d 1.063-1.21 (HDL) were incubated with egg yolk lipoproteins of d < 1.006 for up to 24 hr at various concentrations. Transfer of HDL cholesterol esters to egg yolk lipoproteins occurred simultaneously with transfer of glycerides from egg yolk lipoproteins to HDL. These observations show that exchange of lipids can take place between lipoproteins in the absence of other serum proteins and enzymes. The mole ratios of HDL cholesterol esters to glycerides approached an integral value of 1 : 1 during the course of the incubation. These results suggest that lipid components form complexes within the HDL structure.     

Lipid trafficking between high density lipoproteins and Babesia divergens-infected human erythrocytes.

A two-fold increase in the amount of phospholipids was observed in Babesia divergens infected human red blood cells. In vitro incubation with [32P]-phosphorus and [3H]-glycerol demonstrated that B divergens has the ability to synthesize the phospholipid backbone. On the other hand, the low incorporation of [14C]acetate indicated the absence of a de novo fatty acid synthesis and suggested the necessity of an exogenous lipid source for the parasite. Several intra-erythrocytic growth cycles of B divergens could be achieved in vitro, using a serum-free medium supplemented only with fractions of human high density lipoproteins (HDL). At an HDL concentration of 0.5 mg/ml (protein concentration) and with a 1% starting parasitaemia, parasite growth was similar to that observed under standard culture conditions with 10% human serum, at least for the first 24 h, a time equivalent to three parasite erythrocytic life-cycles. Lipid transfer from HDL to the intra-erythrocytic parasites was demonstrated by uptake and exchange of fluorescent NBD-phosphatidylcholine (NBD-PC) loaded HDL at different temperatures. Kinetic experiments with [3H]-oleyl-PC-loaded HDL demonstrated a unidirectional transfer of lipids from radiolabelled HDL to the parasite; partial conversion of PC to phosphatidylethanolamine (PE) was also observed. In the semi-defined medium, the HDL fraction appeared to be the major source of lipids for the growth of B divergens in human erythrocytes.     

Calcium transport and compartment analysis of free and exchangeable calcium in Plasmodium falciparum-infected red blood cells.

Calcium (Ca2+) is indispensable for normal development of the various stages of the asexual erythrocytic cycle of malaria parasites. However, the mechanisms involved in Ca2+ uptake, compartmentalization and cellular regulation are poorly understood. To clarify some of these issues, we have measured total, exchangeable, and free Ca2+ in normal red cells (RBCs) and Plasmodium falciparum (FCR-3)-infected cells (IRBCs) as a function of parasite development. All three forms of Ca2+ were found to be substantially higher in IRBCs than in RBCs, and to increase with parasite maturation up to the trophozoite stage and decline thereafter. Exchangeable and free [Ca2+] in host cell and parasite compartments were determined by selectively lysing IRBCs with Sendai virus, and estimating these parameters in the lysate (host cytosol) and the pellet (parasite cytosol). Levels of both exchangeable and free [Ca2+] were found to be higher in host cytosol than in parasite cytosol. The Ca2+ gradient across the parasite membrane can be maintained by the pH gradient across this membrane by means of a Ca2+/H+ antiporter. Host cytosol free [Ca2+] reached levels known to produce structural, physiological and biochemical changes in RBCs, and could account for similar features normally seen in malaria-infected red cells. Uptake of Ca2+ into IRBCs was nonsaturable and substantially faster than the saturable Ca2+ uptake into RBCs. The rate of Ca2+ uptake across the parasite membrane was even faster suggesting that the rate-limiting step in uptake into intact IRBCs is the translocation of Ca2+ across the host cell membrane.     

Natural killer (NK) cell-mediated cytolysis of Plasmodium falciparum-infected human red blood cells in vitro

The ability of human NK cells to inhibit the growth in vitro of the asexual blood stages of Plasmodium falciparum was tested. Purified NK cells from donors with no prior exposure to malaria significantly inhibited parasite growth after 48 hours of co-culture in the presence of human immune serum. This inhibition was completely abrogated by pre-treatment of the NK cells with an anti-CD95 (anti-Fas) monoclonal antibody and human Fas-Fc soluble protein. The level of growth inhibition was also substantially reduced by pre-treatment with an anti-CD56 antibody. These two antibodies caused reductions, to varying levels, of the amounts of NK cell-derived granzyme B (GrB) and pro-inflammatory cytokines, but only the anti-CD95 antibody affected the production of soluble Fas ligand (sFasL). Direct destruction of parasite-infected red cells by NK cells, in the absence of serum, was also observed in a standard 51Cr cytotoxicity test, during which N-carboxybenzoxy-L-lysine thiobenzil ester (BLT esterase) activity, which catalyzes serine protease granule release, was detected. The results obtained are indicative of a novel mechanism of NK cell-mediated cytotoxic activity against Plasmodium falciparum-infected red cells, which is mediated in part by both Fas and by GrB.     

Selective permeabilization of the host cell membrane of Plasmodium falciparum-infected red blood cells with streptolysin O and equinatoxin II

Plasmodium falciparum develops within the mature RBCs (red blood cells) of its human host in a PV (parasitophorous vacuole) that separates the host cell cytoplasm from the parasite surface. The pore-forming toxin, SLO (streptolysin O), binds to cholesterol-containing membranes and can be used to selectively permeabilize the host cell membrane while leaving the PV membrane intact. We found that in mixtures of infected and uninfected RBCs, SLO preferentially lyses uninfected RBCs rather than infected RBCs, presumably because of differences in cholesterol content of the limiting membrane. This provides a means of generating pure preparations of viable ring stage infected RBCs. As an alternative permeabilizing agent we have characterized EqtII (equinatoxin II), a eukaryotic pore-forming toxin that binds preferentially to sphingomyelin-containing membranes. EqtII lyses the limiting membrane of infected and uninfected RBCs with similar efficiency but does not disrupt the PV membrane. It generates pores of up to 100 nm, which allow entry of antibodies for immunofluorescence and immunogold labelling. The present study provides novel tools for the analysis of this important human pathogen and highlights differences between Plasmodium-infected and uninfected RBCs.     

Cholesterol transport between red blood cells and lipoproteins contributes to cholesterol metabolism in blood

Lipoproteins play a key role in transport of cholesterol to and from tissues. Recent studies have also demonstrated that red blood cells (RBCs), which carry large quantities of free cholesterol in their membrane, play an important role in reverse cholesterol transport. However, the exact role of RBCs in systemic cholesterol metabolism is poorly understood. RBCs were incubated with autologous plasma or isolated lipoproteins resulting in a significant net amount of cholesterol moved from RBCs to HDL, while cholesterol from LDL moved in the opposite direction. Furthermore, the bi-directional cholesterol transport between RBCs and plasma lipoproteins was saturable and temperature-, energy-, and time-dependent, consistent with an active process. We did not find LDLR, ABCG1, or scavenger receptor class B type 1 in RBCs but found a substantial amount of ABCA1 mRNA and protein. However, specific cholesterol efflux from RBCs to isolated apoA-I was negligible, and ABCA1 silencing with siRNA or inhibition with vanadate and Probucol did not inhibit the efflux to apoA-I, HDL, or plasma. Cholesterol efflux from and cholesterol uptake by RBCs from Abca1^+/+ and Abca1^−/− mice were similar, arguing against the role of ABCA1 in cholesterol flux between RBCs and lipoproteins. Bioinformatics analysis identified ABCA7, ABCG5, lipoprotein lipase, and mitochondrial translocator protein as possible candidates that may mediate the cholesterol flux. Together, these results suggest that RBCs actively participate in cholesterol transport in the blood, but the role of cholesterol transporters in RBCs remains uncertain.     

Protein traffic in Plasmodium infected-red blood cells

To survive within erythrocytes, Plasmodium parasites have to put into place different membrane and sub-cellular compartments in order to import different nutrients and to export proteins/antigens. Infected cells pose not only a major world health risk by killing two million people per year, but also a very interesting cell biology problem, as within the erythrocyte the parasite resides inside a vacuole called the parasitophorous vacuole and as a consequence, it is separated from the blood stream by three membrane barriers, its own plasma membrane, the parasitophorous vacuole membrane and the erythrocyte plasma membrane. In spite of these three barriers the parasite is capable of secreting antigens and importing nutrients, and to do this, it has developed a complex vesicular system that extends into the red blood cell cytoplasm to the plasma membrane. Understanding how the parasite controls this extensive vesicular traffic has driven research into Plasmodium Rabs, whose potential role is discussed.     

Lipid transfers between reconstituted high density lipoprotein complexes and low density lipoproteins: effects of plasma protein factors

In this study we examined the transfer of lipids between reconstituted high density lipoprotein discs (r-HDL) and human low density lipoproteins (LDL) in the presence and absence of lecithin:cholesterol acyltransferase (LCAT) or of plasma phospholipid transfer protein (PLTP). We found that spontaneous transfer of phospholipids from r-HDL to LDL occurred by an apparent first order reaction with a half-time of 5.8 to 6.9 hr depending on the phospholipid. During the time of incubation of r-HDL with LDL (from 0 to 25 hr), the phospholipid content of r-HDL decreased more than 30%, the free cholesterol content increased 2.5-fold, and low levels of cholesteryl esters appeared in r-HDL. These compositional changes gave rise to small discoidal particles with a limiting diameter of 77 A and two molecules of apoA-I per particle. When LCAT was included in the reaction mixture, the r-HDL lost even more phospholipid, lost some free cholesterol, and gained cholesteryl esters relative to the apolipoprotein content, due to the enzymatic reaction. The products of the LCAT reaction had a diameter of 93 A and three, rather than two, apoA-I molecules per particle. Inclusion of PLTP into the reaction mixture accelerated the transfer of phospholipids (half-time of 1.7 hr) and the formation of the 77 A product. In addition to these compositional and morphological changes, which may be important in the interconversions of native HDL subspecies, the prolonged incubations revealed some slow reactions, such as the esterification of LDL cholesterol by LCAT, a background formation of cholesteryl esters in r-HDL, and an apparent hydrolysis of cholesteryl esters in LDL in the presence of r-HDL.     

Trafficking determinants for PfEMP3 export and assembly under the Plasmodium falciparum-infected red blood cell membrane

During the maturation of intracellular asexual stages of Plasmodium falciparum parasite-encoded proteins are exported into the erythrocyte cytosol. A number of these parasite proteins attach to the host cell cytoskeleton and facilitate transformation of a disk-shaped erythrocyte into a rounded and more rigid infected erythrocyte able to cytoadhere to the vasculature. Knob formation on the surface of infected erythrocytes is critical for this cytoadherence to the host endothelium. P. falciparum proteins have been identified that localize to the parasite-infected erythrocyte membrane: the variant cytoadherence ligand erythrocyte membrane protein 1 (PfEMP1), the knob-associated histidine-rich protein (KAHRP) and the erythrocyte membrane protein 3 (PfEMP3). In this study, we have generated parasites expressing PfEMP3-green fluorescent protein chimeras and identified domains involved in entry to the secretory pathway, export across the parasitophorous vacuolar membrane and attachment to Maurer's clefts and the erythrocyte membrane. Solubility assays, fluorescence photobleaching experiments and immunogold electron microscopy suggest that the exported chimeric proteins are trafficked in a complex rather than in vesicles. This study characterizes elements involved in the tight but transient binding of PfEMP3 to Maurer's clefts and shows that the same elements are necessary for correct assembly under the erythrocyte membrane.     

Itinerary of high density lipoproteins in endothelial cells

High density lipoprotein (HDL) and its main protein component apolipoprotein A-I (ApoA-I) have multiple anti-atherogenic functions. Some of them are exerted within the vessel wall, so that HDL needs to pass the endothelial barrier. To elucidate their itinerary through endothelial cells (ECs), we labelled ApoA-I and HDL either fluorescently or with 1.4 nm nanogold and investigated their cellular localization by using immunofluorescent microscopy (IFM) and electron microscopy (EM). HDL as well as ApoA-I is taken up by ECs into the same route of intracellular trafficking. Time kinetics and pulse chase experiments revealed that HDL is trafficked through different vesicles. HDL partially co-localized with LDL, albumin, and transferrin. HDL did not co-localize with clathrin and caveolin-1. Fluorescent HDL was recovered at small proportions in early endosomes and endosome to trans-golgi network vesicles but not at all in recycling endosomes, in late endosomes or lysosomes. EM identified HDL mainly in large filled vesicles which however upon IFM did not colocalize with markers of multivesicular bodies or autophagosomes. The uptake or cellular distribution of HDL was altered upon pharmacological interference with cytochalasine D, colchicine and dynasore. Blockage of fluid phase uptake with Amiloride or EIPA did not reduce the uptake of HDL. Neither did we observe any co-localization of HDL with dextran as the marker of fluid phase uptake. In conclusion, HDL and ApoA-I are internalized and trafficked by endothelial cells through a non-classical endocytic route.     

Apolipoprotein-specific populations in high density lipoproteins of human cord blood

High density lipoproteins (HDL) in human cord blood have previously been shown to exhibit particle size profiles distinctly different from those of adult HDL. The adult HDL profile is comprised of separate contributions from two major apolipoprotein-specific populations; one population contains both apolipoproteins AI and AII (HDL(AIwAII], while the other has apolipoprotein AI without AII (HDL(AIw/oAII]. The present studies establish that cord blood HDL are also comprised of HDL(AIwAII) and HDL(AIw/oAII) populations whose particle size profiles closely reflect cholesterol and HDL-cholesterol levels in cord blood. Compared with the adult, cord blood HDL(AIwAII) profiles generally show both a greater subspeciation within HDL2a and HDL3b/3c size intervals as well as relative reduction of material in the HDL3a interval. In the cord blood HDL(AIw/oAII) profile, HDL2b(AIw/oAII) particles also show subspeciation with a major component that is consistently larger than that normally observed in the adult (11.2 vs. 10.3 nm). As in the adult, the HDL3a(AIw/oAII) component is present but, unlike the adult, its relative amount is low; hence, its peak is usually not discernable in the cord blood total HDL profile. Our studies show that the larger-sized HDL2b(AIw/oAII) of cord blood are enriched in phospholipid which probably accounts for their increased size. The protein moiety of the larger-sized HDL2b(AIw/oAII) has a molecular weight equivalent to four apolipoprotein AI molecules per particle similar to the normal-sized adult subpopulation. Phospholipid enrichment of cord blood HDL(AIwAII) subpopulations within the HDL2a size interval was not observed. However, the protein moiety of cord blood HDL2a(AIwAII) is unusual in that it exhibits an apolipoprotein AI:AII molar ratio considerably lower (0.8:1 vs. 1.6:1) than that of adult. We suggest that the unique particle size distribution of cord blood total HDL is due in large part to: (a) a specific enrichment of phospholipid in HDL2b(AIw/oAII) species, producing particles larger than normal adult counterparts and (b) an elevated proportion of apoAII carried by the HDL(AIwAII) particles that may influence subspeciation in the HDL3a/b/c size interval.     

Lipid packing determines protein-membrane interactions: Challenges for apolipoprotein A-I and high density lipoproteins

Protein and protein-lipid interactions, with and within specific areas in the cell membrane, are critical in order to modulate the cell signaling events required to maintain cell functions and viability. Biological bilayers are complex, dynamic platforms, and thus in vivo observations usually need to be preceded by studies on model systems that simplify and discriminate the different factors involved in lipid-protein interactions. Fluorescence microscopy studies using giant unilamellar vesicles (GUVs) as membrane model systems provide a unique methodology to quantify protein binding, interaction, and lipid solubilization in artificial bilayers. The large size of lipid domains obtainable on GUVs, together with fluorescence microscopy techniques, provides the possibility to localize and quantify molecular interactions. Fluorescence Correlation Spectroscopy (FCS) can be performed using the GUV model to extract information on mobility and concentration. Two-photon Laurdan Generalized Polarization (GP) reports on local changes in membrane water content (related to membrane fluidity) due to protein binding or lipid removal from a given lipid domain. In this review, we summarize the experimental microscopy methods used to study the interaction of human apolipoprotein A-I (apoA-I) in lipid-free and lipid-bound conformations with bilayers and natural membranes. Results described here help us to understand cholesterol homeostasis and offer a methodological design suited to different biological systems.     

Cholesterol content of red blood cells and low-density lipoproteins in hypertriglyceridemia

The red blood cells and the low-density lipoproteins in hypertriglyceridemia have a lower ratio of unesterified cholesterol to phospholipid than normal. The low-density lipoproteins are also smaller and more dense in hypertriglyceridemia, and contain only 45% of the normal unesterified cholesterol mass. The phase behavior of the lipids shows that normal red cells and low-density lipoproteins are close to saturation with cholesterol, whereas in hypertriglyceridemia less cholesterol is present. Because newly secreted triacylglycerol-rich lipoproteins are poor in cholesterol, their excess production and transport in hypertriglyceridemia may prevent maintenance of the normal cholesterol content of blood cells and low-density lipoproteins. Partitioning of cholesterol into triacylglycerol-rich lipoproteins is able to account for significant fluxes of unesterified cholesterol in the plasma compartment.