Uninfected red cells from malaria-infected blood: alteration of fatty acid composition involving a serum protein: an in vivo and in vitro study

A patatin-like phospholipase functions during gametocyte induction in the malaria parasite Plasmodium falciparum

Patatin-like phospholipases (PNPLAs) are highly conserved enzymes of prokaryotic and eukaryotic organisms with major roles in lipid homeostasis. The genome of the malaria parasite Plasmodium falciparum encodes four putative PNPLAs with predicted functions during phospholipid degradation. We here investigated the role of one of the plasmodial PNPLAs, a putative PLA₂ termed PNPLA1, during blood stage replication and gametocyte development. PNPLA1 is present in the asexual and sexual blood stages and here localizes to the cytoplasm. PNPLA1-deficiency due to gene disruption or conditional gene-knockdown had no effect on intraerythrocytic growth, gametocyte development and gametogenesis. However, parasites lacking PNPLA1 were impaired in gametocyte induction, while PNPLA1 overexpression promotes gametocyte formation. The loss of PNPLA1 further leads to transcriptional down-regulation of genes related to gametocytogenesis, including the gene encoding the sexual commitment regulator AP2-G. Additionally, lipidomics of PNPLA1-deficient asexual blood stage parasites revealed overall increased levels of major phospholipids, including phosphatidylcholine (PC), which is a substrate of PLA₂ . PC synthesis is known to be pivotal for erythrocytic replication, while the reduced availability of PC precursors drives the parasite into gametocytogenesis; we thus hypothesize that the higher PC levels due to PNPLA1-deficiency prevent the blood stage parasites from entering the sexual pathway.

Metabolic Crosstalk Between Host and Parasitic Pathogens

A complex network that embraces parasite-host intrinsic factors and the microenvironment regulated the interaction between a parasite and its host. Nutritional pressures exerted by both elements of this duet thus dictate this host-parasite niche. To survive and proliferate inside a host and a harsh nutritional environment, the parasites modulate different nutrient sensing pathways to subvert host metabolic pathways. Such mechanism is able to change the flux of distinct nutrients/metabolites diverting them to be used by the parasites. Apart from this nutritional strategy, the scavenging of nutrients, particularly host fatty acids, constitutes a critical mechanism to fulfil parasite nutritional requirements, ultimately defining the host metabolic landscape. The host metabolic alterations that result from host-parasite metabolic coupling can certainly be considered important targets to improve diagnosis and also for the development of future therapies. Metabolism is in fact considered a key element within this complex interaction, its modulation being crucial to dictate the final infection outcome.

Phospholipases during membrane dynamics in malaria parasites

Plasmodium parasites, the causative agents of malaria, display a well-regulated lipid metabolism required to ensure their survival in the human host as well as in the mosquito vector. The fine-tuning of lipid metabolic pathways is particularly important for the parasites during the rapid erythrocytic infection cycles, and thus enzymes involved in lipid metabolic processes represent prime targets for malaria chemotherapeutics. While plasmodial enzymes involved in lipid synthesis and acquisition have been studied in the past, to date not much is known about the roles of phospholipases for proliferation and transmission of the malaria parasite. These phospholipid-hydrolyzing esterases are crucial for membrane dynamics during host cell infection and egress by the parasite as well as for replication and cell signaling, and thus they are considered important virulence factors. In this review, we provide a comprehensive bioinformatic analysis of plasmodial phospholipases identified to date. We further summarize previous findings on the lipid metabolism of Plasmodium, highlight the roles of phospholipases during parasite life-cycle progression, and discuss the plasmodial phospholipases as potential targets for malaria therapy.

Transfer of 4-hydroxynonenal from parasitized to non-parasitized erythrocytes in rosettes. Proposed role in severe malaria anemia

Severe anaemia is a life-threatening complication of falciparum malaria associated with loss of predominantly non-parasitized red blood cells (npRBCs). This poorly elucidated process might be influenced by (i) rosettes, i.e. npRBCs cytoadherent to haemozoin-containing parasitized RBCs (pRBCs) and (ii) generation in pRBCs of 4-hydroxynonenal (4-HNE) through haemozoin-catalysed lipid peroxidation. We explored whether close proximity in rosettes may facilitate 4-HNE transfer to npRBCs, which is likely to enhance their phagocytosis and contribute to malaria anaemia. Fluorescence microscopy and flow cytometry data indicated 4-HNE transfer to npRBCs in rosettes. Rosettes were formed by 64·8 ± 1·8% varO-expressing pRBCs, and 8·7 ± 1·1% npRBCs were positive for 4-HNE-protein-conjugates, while low-rosetting parasites generated only 2·4 ± 1·1% 4-HNE-conjugate-positive npRBCs. 4-HNE transfer decreased after blocking rosetting by monoclonal antibodies. A positive linear relationship between rosette frequency and 4-HNE-conjugates in npRBCs was found in 40 malaria patients, a first indication for a role of rosetting in npRBCs modifications in vivo. Children with severe malaria anaemia had significantly higher percentages of 4-HNE-conjugate-positive npRBCs compared to children with uncomplicated malaria. In conclusion, 4-HNE transfer from pRBCs to npRBCs in rosettes is suggested to play a role in the phagocytic removal of large numbers of npRBCs, the hallmark of severe malaria anaemia.

Crystallization of synthetic haemozoin (beta-haematin) nucleated at the surface of lipid particles

The mechanism of formation of haemozoin, a detoxification by-product of several blood-feeding organisms including malaria parasites, has been a subject of debate; however, recent studies suggest that neutral lipids may serve as a catalyst. In this study, a model system consisting of an emulsion of neutral lipid particles was employed to investigate the formation of beta-haematin, the synthetic counterpart of haemozoin, at the lipid-water interface. A solution of monoglyceride, either monostearoylglycerol (MSG) or monopalmitoylglycerol (MPG), dissolved in acetone and methanol was introduced to an aqueous surface. Fluorescence, confocal and transmission electron microscopic (TEM) imaging and dynamic light scattering analysis of samples obtained from beneath the surface confirmed the presence of homogeneous lipid particles existing in two major populations: one in the low micrometre size range and the other in the hundred nanometre range. The introduction of haem (Fe(iii)PPIX) to this lipid particle system under biomimetic conditions (37 degrees C, pH 4.8) produced beta-haematin with apparent first-order kinetics and an average half life of 0.5 min. TEM of monoglycerides (MSG or MPG) extruded through a 200 nm filter with haem produced beta-haematin crystals aligned and parallel to the lipid-water interface. These TEM data, together with a model system replacing the lipid with an aqueous organic solvent interface using either methyl laurate or docosane demonstrated that the OH and C[double bond, length as m-dash]O groups are apparently necessary for efficient nucleation. This suggests that beta-haematin crystallizes via epitaxial nucleation at the lipid-water interface through interaction of Fe(iii)PPIX with the polar head group. Once nucleated, the crystal grows parallel to the interface until growth is terminated by the curvature of the lipid particle. The hydrophobic nature of the mature crystal favours an interior transport resulting in crystals aligned parallel to the lipid-water interface and each other, strikingly similar to that seen in malaria parasites.

Serum lipoproteins promote efficient presentation of the malaria virulence protein PfEMP1 at the erythrocyte surface

The virulence of the malaria parasite Plasmodium falciparum is related to its ability to express a family of adhesive proteins known as P. falciparum erythrocyte membrane protein 1 (PfEMP1) at the infected red blood cell surface. The mechanism for the transport and delivery of these adhesins to the erythrocyte membrane is only poorly understood. In this work, we have used specific immune reagents in a flow cytometric assay to monitor the effects of serum components on the surface presentation of PfEMP1. We show that efficient presentation of the A4 and VAR2CSA variants of PfEMP1 is dependent on the presence of serum in the bathing medium during parasite maturation. Lipid-loaded albumin supports parasite growth but allows much less efficient presentation of PfEMP1 at the red blood cell surface. Analysis of the serum components reveals that lipoproteins, especially those of the low-density lipoprotein fraction, promote PfEMP1 presentation. Cytoadhesion of infected erythrocytes to the host cell receptors CD36 and ICAM-1 is also decreased in infected erythrocytes cultured in the absence of serum. The defect appears to be in the transfer of PfEMP1 from parasite-derived structures known as the Maurer's clefts to the erythrocyte membrane or in surface conformation rather than a down-regulation or switching of particular PfEMP1 variants.

Food vacuole-associated lipid bodies and heterogeneous lipid environments in the malaria parasite, Plasmodium falciparum

The malaria parasite Plasmodium falciparum induces a sixfold increase in the phospholipid content of infected erythrocytes during its intraerythrocytic growth. We have characterized the lipid environments in parasitized erythrocyte using the hydrophobic probe, Nile Red. Spectral imaging with a confocal microscope revealed heterogeneous lipid environments in parasite-infected erythrocytes. An insight into the nature of these environments was gained by comparing these spectra with those of triacylglycerol/phospholipid emulsions and phospholipid membranes. Using this approach, we identified a population of intensely stained particles of a few hundred nanometers in size that are closely associated with the digestive vacuole of the parasite and appear to be composed of neutral lipids. Electron microscopy and isolation of food vacuoles confirmed the size of these particles and their intimate association respectively. Lipid analysis suggests that these neutral lipid bodies are composed of di- and triacylgycerols and may represent storage organelles for lipid intermediates that are generated during digestion of phospholipids in the food vacuole. Mono-, di- and triacylglycerol suspensions promote beta-haematin formation, suggesting that these neutral lipid bodies, or their precursors, may also be involved in haem detoxification. We also characterized other compartments of the infected erythrocyte that were stained less intensely with the Nile Red probe. Both the erythrocyte membrane and the parasite membrane network exhibit red shifts compared with the neutral lipid bodies that are consistent with cholesterol-rich and cholesterol-poor membranes respectively. Ratiometric imaging revealed more subtle variations in the lipid environments within the parasite membrane network.

Alterations of uninfected red blood cells in malaria

Red blood cell (RBC) negative charges and resistance to linoleic acid (LNA)-induced lysis were studied in Plasmodium yoelii-infected mice and in malaria (P. falciparum)-affected individuals. RBCs from mice infected with P. yoelii showed a progressive decrease in the net surface negative charges at 24 h after infection, reaching a minimal value on day 3, followed by a second phase that was characterised by a recovery to normal levels on day 6. Resistance to linoleic acid follows similar kinetics. These alterations preceded the appearance of parasites in the peripheral blood. A similar increase in LNA-induced lysis was observed in RBCs from malaria-affected individuals. These early membrane alterations of uninfected RBCs could be responsible for spreading of infection and RBC lysis during infection.

Plasmodium knowlesi induces alterations in phosphatidylcholine and phosphatidylethanolamine molecular species composition of parasitized monkey erythrocytes

Using high performance liquid chromatography and gas-liquid chromatography, we have characterized the phosphatidylcholine and phosphatidylethanolamine molecular species composition of trophozoite and schizont forms of Plasmodium knowlesi parasitized erythrocytes. Similarly, we determined these parameters in the erythrocyte membranes of trophozoite parasitized cells, unparasitized erythrocytes from infected monkeys before and after a chloroquine treatment and erythrocytes from monkeys that had never been infected. Plasma phosphatidylcholine molecular species composition was also studied. P. knowlesi parasitized erythrocytes presented higher amounts of 16:0/18:2-phosphatidylcholine than the various control cells, which appeared to be compensated for by a decrease in 18:0/20:4-, 16:0/20:3-, 16:0/18:1-, 18:0/18:2-, 18:0/20:3-, 16:0/16:0- and 16:0/18:0-phosphatidylcholines. In the case of phosphatidylethanolamine, the alterations were quantitatively of greater importance and consisted of an increase in, again, 16:0/18:2-phosphatidylethanolamine and a decrease in several species containing 20:4, namely 16:0/20:4-, 18:0/20:4- and 18:1/20:4-phosphatidylethanolamine; also the levels of alkoxy-phosphatidylethanolamines were markedly decreased. P. knowlesi development within monkey erythrocytes therefore appears to be associated with changes in phosphatidylcholine and phosphatidylethanolamine molecular species in the whole parasitized cell. These alterations are also exhibited by the host cell membrane, which provides the first experimental evidence that the parasite is able to manipulate the erythrocyte membrane lipid species composition. The consequences of these alterations on membrane physiology are discussed, as well as the implications that these data may have on the trafficking of phosphatidylcholine and phosphatidylethanolamine in the erythrocytes of P. knowlesi infected monkeys.