Analysis of adhesive domains from the A4VAR Plasmodium falciparum erythrocyte membrane protein-1 identifies a CD36 binding domain

Malian adults maintain serologic responses to virulent PfEMP1s amid seasonal patterns of fluctuation

Plasmodium falciparum erythrocyte membrane protein-1s (PfEMP1s), diverse malaria proteins expressed on the infected erythrocyte surface, play an important role in pathogenesis, mediating adhesion to host vascular endothelium. Antibodies to particular non-CD36-binding PfEMP1s are associated with protection against severe disease. We hypothesized that given lifelong P. falciparum exposure, Malian adults would have broad PfEMP1 serorecognition and high seroreactivity levels during follow-up, particularly to non-CD36-binding PfEMP1s such as those that attach to endothelial protein C receptor (EPCR) and intercellular adhesion molecule-1 (ICAM-1). Using a protein microarray, we determined serologic responses to 166 reference PfEMP1 fragments during a dry and subsequent malaria transmission season in Malian adults. Malian adult sera had PfEMP1 serologic responses throughout the year, with decreased reactivity to a small subset of PfEMP1 fragments during the dry season and increases in reactivity to a different subset of PfEMP1 fragments during the subsequent peak malaria transmission season, especially for intracellular PfEMP1 domains. For some individuals, PfEMP1 serologic responses increased after the dry season, suggesting antigenic switching during asymptomatic infection. Adults were more likely to experience variable serorecognition of CD36-binding PfEMP1s than non-CD36-binding PfEMP1s that bind EPCR or ICAM-1, which remained serorecognized throughout the year. Sustained seroreactivity to non-CD36-binding PfEMP1s throughout adulthood amid seasonal fluctuation patterns may reflect underlying protective severe malaria immunity and merits further investigation.

Human erythrocyte band 3 is a host receptor for Plasmodium falciparum glutamic acid-rich protein

Malaria remains a major global threat to human health and economic development. Microvascular lesions caused by Plasmodium falciparum-infected human erythrocytes/red blood cells are hallmarks of severe pathogenesis contributing to high mortality, particularly in children from sub-Saharan Africa. In this study, we used a phage display complementary DNA library screening strategy to identify P falciparum glutamic acid-rich protein (PfGARP) as a secreted ligand that recognizes an ectodomain of human erythrocyte anion-exchanger, band 3/AE1, as a host receptor. Domain mapping of PfGARP revealed distinct nonoverlapping repeats encoding the immune response epitopes and core erythrocyte-binding activity. Synthetic peptides derived from the erythrocyte-binding repeats of PfGARP induced erythrocyte aggregation reminiscent of the rosetting phenomenon. Using peptides derived from the immunogenic repeats, a quantitative immunoassay was developed to detect a selective immune response against PfGARP in human plasma samples obtained from patients in rural Mali, suggesting the feasibility of PfGARP as a potential biomarker of disease progression. Collectively, our results suggest that PfGARP may play a functional role in enhancing the adhesive properties of human erythrocytes by engaging band 3 as a host receptor. We propose that immunological and pharmacological inhibition of PfGARP may unveil new therapeutic options for mitigating lesions in cerebral and pregnancy-associated malaria.

Lysosomal integral membrane protein-2 as a phospholipid receptor revealed by biophysical and cellular studies

Lysosomal integral membrane protein-2 (LIMP-2/SCARB2) contributes to endosomal and lysosomal function. LIMP-2 deficiency is associated with neurological abnormalities and kidney failure and, as an acid glucocerebrosidase receptor, impacts Gaucher and Parkinson's diseases. Here we report a crystal structure of a LIMP-2 luminal domain dimer with bound cholesterol and phosphatidylcholine. Binding of these lipids alters LIMP-2 from functioning as a glucocerebrosidase-binding monomer toward a dimeric state that preferentially binds anionic phosphatidylserine over neutral phosphatidylcholine. In cellular uptake experiments, LIMP-2 facilitates transport of phospholipids into murine fibroblasts, with a strong substrate preference for phosphatidylserine. Taken together, these biophysical and cellular studies define the structural basis and functional importance of a form of LIMP-2 for lipid trafficking. We propose a model whereby switching between monomeric and dimeric forms allows LIMP-2 to engage distinct binding partners, a mechanism that may be shared by SR-BI and CD36, scavenger receptor proteins highly homologous to LIMP-2.

In vitro inhibition and reversal of Plasmodium falciparum cytoadherence to endothelium by monoclonal antibodies to ICAM-1 and CD36

Background

Sequestration of parasitized red blood cells from the peripheral circulation during an infection with Plasmodium falciparum is caused by an interaction between the parasite protein PfEMP1 and receptors on the surface of host endothelial cells, known as cytoadherence. Several lines of evidence point to a link between the pathology of severe malaria and cytoadherence, therefore blocking adhesion receptors involved in this process could be a good target to inhibit pRBC sequestration and prevent disease. In a malaria endemic setting this is likely to be used as an adjunct therapy by reversing existing cytoadherence. Two well-characterized parasite lines plus three recently derived patient isolates were tested for their cytoadherence to purified receptors (CD36 and ICAM-1) as well as endothelial cells. Monoclonal antibodies against human CD36 and ICAM-1 were used to inhibit and reverse infected erythrocyte binding in static and flow-based adhesion assays.

Results

Anti-ICAM-1 and CD36 monoclonal antibodies were able to inhibit and reverse P. falciparum binding of lab and recently adapted patient isolates in vitro. However, reversal of binding was incomplete and varied in its efficiency between parasite isolates.

Conclusions

The results show that, as a proof of concept, disturbing existing ligand–receptor interactions is possible and could have potential therapeutic value for severe malaria. The variation seen in the degree of reversing existing binding with different parasite isolates and the incomplete nature of reversal, despite the use of high affinity inhibitors, suggest that anti-adhesion approaches as adjunct therapies for severe malaria may not be effective, and the focus may need to be on inhibitory approaches such as vaccines.

Cytoadhesion to gC1qR through Plasmodium falciparum Erythrocyte Membrane Protein 1 in Severe Malaria

Cytoadhesion of Plasmodium falciparum infected erythrocytes to gC1qR has been associated with severe malaria, but the parasite ligand involved is currently unknown. To assess if binding to gC1qR is mediated through the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family, we analyzed by static binding assays and qPCR the cytoadhesion and var gene transcriptional profile of 86 P. falciparum isolates from Mozambican children with severe and uncomplicated malaria, as well as of a P. falciparum 3D7 line selected for binding to gC1qR (Pf3D7^gC1qR). Transcript levels of DC8 correlated positively with cytoadhesion to gC1qR (rho = 0.287, P = 0.007), were higher in isolates from children with severe anemia than with uncomplicated malaria, as well as in isolates from Europeans presenting a first episode of malaria (n = 21) than Mozambican adults (n = 25), and were associated with an increased IgG recognition of infected erythrocytes by flow cytometry. Pf3D7^gC1qR overexpressed the DC8 type PFD0020c (5.3-fold transcript levels relative to Seryl-tRNA-synthetase gene) compared to the unselected line (0.001-fold). DBLβ12 from PFD0020c bound to gC1qR in ELISA-based binding assays and polyclonal antibodies against this domain were able to inhibit binding to gC1qR of Pf3D7^gC1qR and four Mozambican P. falciparum isolates by 50%. Our results show that DC8-type PfEMP1s mediate binding to gC1qR through conserved surface epitopes in DBLβ12 domain which can be inhibited by strain-transcending functional antibodies. This study supports a key role for gC1qR in malaria-associated endovascular pathogenesis and suggests the feasibility of designing interventions against severe malaria targeting this specific interaction.

Plasmodium falciparum sequesters in vital organs. This phenomenon mediated by cytoadhesion of infected-erythrocytes to host receptors in the microvasculature, contributes to the development of severe malaria. Although cytoadhesion to Endothelial Protein-C Receptor has a central role in severe malaria, other host receptors are also likely to be involved. Our results generated by the analysis of P. falciparum isolates from Mozambican patients and laboratory parasite lines indicate that a specific domain (DBLβ12) from DC8-type PfEMP1s can bind to the human receptor gC1qR, previously associated with severe malaria. Our findings revealed that antibodies against PfEMP1 could provide strain-transcending inhibition of gC1qR-binding. Overall, these results support a key role for the adhesion to gC1qR in malaria-associated endovascular pathogenesis and the feasibility of new interventions targeting this specific interaction.

The structural basis for CD36 binding by the malaria parasite

CD36 is a scavenger receptor involved in fatty acid metabolism, innate immunity and angiogenesis. It interacts with lipoprotein particles and facilitates uptake of long chain fatty acids. It is also the most common target of the PfEMP1 proteins of the malaria parasite, Plasmodium falciparum, tethering parasite-infected erythrocytes to endothelial receptors. This prevents their destruction by splenic clearance and allows increased parasitaemia. Here we describe the structure of CD36 in complex with long chain fatty acids and a CD36-binding PfEMP1 protein domain. A conserved hydrophobic pocket allows the hugely diverse PfEMP1 protein family to bind to a conserved phenylalanine residue at the membrane distal tip of CD36. This phenylalanine is also required for CD36 to interact with lipoprotein particles. By targeting a site on CD36 that is required for its physiological function, PfEMP1 proteins maintain the ability to tether to the endothelium and avoid splenic clearance.

Targeting of the CD36 scavenger receptor by the malaria parasite effector PfEMP1 prevents splenic clearance of infected erythrocytes. Here, the authors propose that diverse PfEMP1 achieve this by binding to a conserved phenylalanine residue in CD36 that is also required for lipoprotein binding.

The role of PfEMP1 as targets of naturally acquired immunity to childhood malaria: prospects for a vaccine

The Plasmodium falciparum erythrocyte membrane protein 1 antigens that are inserted onto the surface of P. falciparum infected erythrocytes play a key role both in the pathology of severe malaria and as targets of naturally acquired immunity. They might be considered unlikely vaccine targets because they are extremely diverse. However, several lines of evidence suggest that underneath this molecular diversity there are a restricted set of epitopes which may act as effective targets for a vaccine against severe malaria. Here we review some of the recent developments in this area of research, focusing on work that has assessed the potential of these molecules as possible vaccine targets.

Diverse functional outcomes of Plasmodium falciparum ligation of EPCR: potential implications for malarial pathogenesis

Plasmodium falciparum-infected erythrocytes (IRBC) expressing the domain cassettes (DC) 8 and 13 of the cytoadherent ligand P. falciparum erythrocyte membrane protein 1 adhere to the endothelial protein C receptor (EPCR). By interfering with EPCR anti-coagulant and pro-endothelial barrier functions, IRBC adhesion could promote coagulation and vascular permeability that contribute to the pathogenesis of cerebral malaria. In this study, we examined the adhesion of DC8- and DC13-expressing parasite lines to endothelial cells from different microvasculature, and the consequences of EPCR engagement on endothelial cell function. We found that IRBC from IT4var19 (DC8) and IT4var07 (DC13) parasite lines adhered to human brain, lung and dermal endothelial cells under shear stress. However, the relative contribution of EPCR to parasite cytoadherence on different types of endothelial cell varied. We also observed divergent functional outcomes for DC8 cysteine-rich interdomain region (CIDR)α1.1 and DC13 CIDRα1.4 domains. IT4var07 CIDRα1.4 inhibited generation of activated protein C (APC) on lung and dermal endothelial cells and blocked the APC-EPCR binding interaction on brain endothelial cells. IT4var19 CIDRα1.1 inhibited thrombin-induced endothelial barrier dysfunction in lung endothelial cells, whereas IT4var07 CIDRα1.4 inhibited the protective effect of APC on thrombin-induced permeability. Overall, these findings reveal a much greater complexity of how CIDRα1-expressing parasites may modulate malaria pathogenesis through EPCR adhesion.

Phylogeography of var gene repertoires reveals fine-scale geospatial clustering of Plasmodium falciparum populations in a highly endemic area

Plasmodium falciparum malaria is a major global health problem that is being targeted for progressive elimination. Knowledge of local disease transmission patterns in endemic countries is critical to these elimination efforts. To investigate fine-scale patterns of malaria transmission, we have compared repertoires of rapidly evolving var genes in a highly endemic area. A total of 3680 high-quality DBLα-sequences were obtained from 68 P. falciparum isolates from ten villages spread over two distinct catchment areas on the north coast of Papua New Guinea (PNG). Modelling of the extent of var gene diversity in the two parasite populations predicts more than twice as many var gene alleles circulating within each catchment (Mugil = 906; Wosera = 1094) than previously recognized in PNG (Amele = 369). In addition, there were limited levels of var gene sharing between populations, consistent with local parasite population structure. Phylogeographic analyses demonstrate that while neutrally evolving microsatellite markers identified population structure only at the catchment level, var gene repertoires reveal further fine-scale geospatial clustering of parasite isolates. The clustering of parasite isolates by village in Mugil, but not in Wosera was consistent with the physical and cultural isolation of the human populations in the two catchments. The study highlights the microheterogeneity of P. falciparum transmission in highly endemic areas and demonstrates the potential of var genes as markers of local patterns of parasite population structure.

Cytoadherence phenotype of Plasmodium falciparum-infected erythrocytes is associated with specific pfemp-1 expression in parasites from children with cerebral malaria

Background

Cytoadherence of Plasmodium falciparum-infected erythrocytes (IEs) in deep microvasculature endothelia plays a major role in the pathogenesis of cerebral malaria (CM). This biological process is thought to be mediated by P. falciparum erythrocyte membrane protein-1 (PfEMP-1) and human receptors such as CD36 and ICAM-1. The relationship between the expression of PfEMP-1 variants and cytoadherence phenotype in the pathology of malaria is not well established.

Methods

Cytoadherence phenotypes of IEs to CD36, ICAM-1, CSPG and the transcription patterns of A, B, var2csa, var3, var gene groups and domain cassettes DC8 and DC13 were assessed in parasites from children with CM and uncomplicated malaria (UM) to determine if cytoadherence is related to a specific transcription profile of pfemp-1 variants.

Results

Parasites from CM patients bind significantly more to CD36 than those from UM patients, but no difference was observed in their binding ability to ICAM-1 and CSPG. CM isolates highly transcribed groups A, B, var2csa, var3, DC8 and DC13 compared to UM parasites. The high transcription levels of var genes belonging to group B positively correlated with increased binding level to CD36.

Conclusion

CM isolates bind significantly more to CD36 than to ICAM-1, which was correlated with high transcription level of group B var genes, supporting their implication in malaria pathogenesis.

The role of PfEMP1 adhesion domain classification in Plasmodium falciparum pathogenesis research

The Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family has a key role in parasite survival, transmission, and virulence. PfEMP1 are exported to the erythrocyte membrane and mediate binding of infected erythrocytes to the endothelial lining of blood vessels. This process aids parasite survival by avoiding spleen-dependent killing mechanisms, but it is associated with adhesion-based disease complications. Switching between PfEMP1 proteins enables parasites to evade host immunity and modifies parasite tropism for different microvascular beds. The PfEMP1 protein family is one of the most diverse adhesion modules in nature. This review covers PfEMP1 adhesion domain classification and the significant role it is playing in deciphering and deconvoluting P. falciparum cytoadhesion and disease.

Surface antigens of Plasmodium falciparum-infected erythrocytes as immune targets and malaria vaccine candidates

Understanding the targets and mechanisms of human immunity to malaria caused by Plasmodium falciparum is crucial for advancing effective vaccines and developing tools for measuring immunity and exposure in populations. Acquired immunity to malaria predominantly targets the blood stage of infection when merozoites of Plasmodium spp. infect erythrocytes and replicate within them. During the intra-erythrocytic development of P. falciparum, numerous parasite-derived antigens are expressed on the surface of infected erythrocytes (IEs). These antigens enable P. falciparum-IEs to adhere in the vasculature and accumulate in multiple organs, which is a key process in the pathogenesis of disease. IE surface antigens, often referred to as variant surface antigens, are important targets of acquired protective immunity and include PfEMP1, RIFIN, STEVOR and SURFIN. These antigens are highly polymorphic and encoded by multigene families, which generate substantial antigenic diversity to mediate immune evasion. The most important immune target appears to be PfEMP1, which is a major ligand for vascular adhesion and sequestration of IEs. Studies are beginning to identify specific variants of PfEMP1 linked to disease pathogenesis that may be suitable for vaccine development, but overcoming antigenic diversity in PfEMP1 remains a major challenge. Much less is known about other surface antigens, or antigens on the surface of gametocyte-IEs, the effector mechanisms that mediate immunity, and how immunity is acquired and maintained over time; these are important topics for future research.

Sequence variation and structural conservation allows development of novel function and immune evasion in parasite surface protein families

Trypanosoma and Plasmodium species are unicellular, eukaryotic pathogens that have evolved the capacity to survive and proliferate within a human host, causing sleeping sickness and malaria, respectively. They have very different survival strategies. African trypanosomes divide in blood and extracellular spaces, whereas Plasmodium species invade and proliferate within host cells. Interaction with host macromolecules is central to establishment and maintenance of an infection by both parasites. Proteins that mediate these interactions are under selection pressure to bind host ligands without compromising immune avoidance strategies. In both parasites, the expansion of genes encoding a small number of protein folds has established large protein families. This has permitted both diversification to form novel ligand binding sites and variation in sequence that contributes to avoidance of immune recognition. In this review we consider two such parasite surface protein families, one from each species. In each case, known structures demonstrate how extensive sequence variation around a conserved molecular architecture provides an adaptable protein scaffold that the parasites can mobilise to mediate interactions with their hosts.

Inferring developmental stage composition from gene expression in human malaria

In the current era of malaria eradication, reducing transmission is critical. Assessment of transmissibility requires tools that can accurately identify the various developmental stages of the malaria parasite, particularly those required for transmission (sexual stages). Here, we present a method for estimating relative amounts of Plasmodium falciparum asexual and sexual stages from gene expression measurements. These are modeled using constrained linear regression to characterize stage-specific expression profiles within mixed-stage populations. The resulting profiles were analyzed functionally by gene set enrichment analysis (GSEA), confirming differentially active pathways such as increased mitochondrial activity and lipid metabolism during sexual development. We validated model predictions both from microarrays and from quantitative RT-PCR (qRT-PCR) measurements, based on the expression of a small set of key transcriptional markers. This sufficient marker set was identified by backward selection from the whole genome as available from expression arrays, targeting one sentinel marker per stage. The model as learned can be applied to any new microarray or qRT-PCR transcriptional measurement. We illustrate its use in vitro in inferring changes in stage distribution following stress and drug treatment and in vivo in identifying immature and mature sexual stage carriers within patient cohorts. We believe this approach will be a valuable resource for staging lab and field samples alike and will have wide applicability in epidemiological studies of malaria transmission.

Clonal variants of Plasmodium falciparum exhibit a narrow range of rolling velocities to host receptor CD36 under dynamic flow conditions

Cytoadhesion of Plasmodium falciparum parasitized red blood cells (pRBCs) has been implicated in the virulence of malaria infection. Cytoadhesive interactions are mediated by the protein family of Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1). The PfEMP1 family is under strong antibody and binding selection, resulting in extensive sequence and size variation of the extracellular domains. Here, we investigated cytoadhesion of pRBCs to CD36, a common receptor of P. falciparum field isolates, under dynamic flow conditions. Isogeneic parasites, predominantly expressing single PfEMP1 variants, were evaluated for binding to recombinant CD36 under dynamic flow conditions using microfluidic devices. We tested if PfEMP1 size (number of extracellular domains) or sequence variation affected the pRBC-CD36 interaction. Our analysis showed that clonal parasite variants varied ∼5-fold in CD36 rolling velocity despite extensive PfEMP1 sequence polymorphism. In addition, adherent pRBCs exhibited a characteristic hysteresis in rolling velocity at microvascular flow rates, which was accompanied by changes in pRBC shape and may represent important adaptations that favor stable binding.

Glutathione transport: a new role for PfCRT in chloroquine resistance

AIMS

Chloroquine (CQ) kills Plasmodium falciparum by binding heme, preventing its detoxification to hemozoin in the digestive vacuole (DV) of the parasite. CQ resistance (CQR) is associated with mutations in the DV membrane protein P. falciparum chloroquine resistance transporter (PfCRT), mediating the leakage of CQ from the DV. However, additional factors are thought to contribute to the resistance phenotype. This study tested the hypothesis that there is a link between glutathione (GSH) and CQR.

RESULTS

Using isogenic parasite lines carrying wild-type or mutant pfcrt, we reveal lower levels of GSH in the mutant lines and enhanced sensitivity to the GSH synthesis inhibitor l-buthionine sulfoximine, without any alteration in cytosolic de novo GSH synthesis. Incubation with N-acetylcysteine resulted in increased GSH levels in all parasites, but only reduced susceptibility to CQ in PfCRT mutant-expressing lines. In support of a heme destruction mechanism involving GSH in CQR parasites, we also found lower hemozoin levels and reduced CQ binding in the CQR PfCRT-mutant lines. We further demonstrate via expression in Xenopus laevis oocytes that the mutant alleles of Pfcrt in CQR parasites selectively transport GSH.

INNOVATION

We propose a mechanism whereby mutant pfcrt allows enhanced transport of GSH into the parasite's DV. The elevated levels of GSH in the DV reduce the level of free heme available for CQ binding, which mediates the lower susceptibility to CQ in the PfCRT mutant parasites.

CONCLUSION

PfCRT has a dual role in CQR, facilitating both efflux of harmful CQ from the DV and influx of beneficial GSH into the DV.

CD36 and Fyn kinase mediate malaria-induced lung endothelial barrier dysfunction in mice infected with Plasmodium berghei

Severe malaria can trigger acute lung injury characterized by pulmonary edema resulting from increased endothelial permeability. However, the mechanism through which lung fluid conductance is altered during malaria remains unclear. To define the role that the scavenger receptor CD36 may play in mediating this response, C57BL/6J (WT) and CD36−/− mice were infected with P. berghei ANKA and monitored for changes in pulmonary endothelial barrier function employing an isolated perfused lung system. WT lungs demonstrated a >10-fold increase in two measures of paracellular fluid conductance and a decrease in the albumin reflection coefficient (σ_alb) compared to control lungs indicating a loss of barrier function. In contrast, malaria-infected CD36−/− mice had near normal fluid conductance but a similar reduction in σ_alb. In WT mice, lung sequestered iRBCs demonstrated production of reactive oxygen species (ROS). To determine whether knockout of CD36 could protect against ROS-induced endothelial barrier dysfunction, mouse lung microvascular endothelial monolayers (MLMVEC) from WT and CD36−/− mice were exposed to H₂O₂. Unlike WT monolayers, which showed dose-dependent decreases in transendothelial electrical resistance (TER) from H₂O₂ indicating loss of barrier function, CD36−/− MLMVEC demonstrated dose-dependent increases in TER. The differences between responses in WT and CD36−/− endothelial cells correlated with important differences in the intracellular compartmentalization of the CD36-associated Fyn kinase. Malaria infection increased total lung Fyn levels in CD36−/− lungs compared to WT, but this increase was due to elevated production of the inactive form of Fyn further suggesting a dysregulation of Fyn-mediated signaling. The importance of Fyn in CD36-dependent endothelial signaling was confirmed using in vitro Fyn knockdown as well as Fyn−/− mice, which were also protected from H₂O₂- and malaria-induced lung endothelial leak, respectively. Our results demonstrate that CD36 and Fyn kinase are critical mediators of the increased lung endothelial fluid conductance caused by malaria infection.

Cytoadherence of Plasmodium berghei-infected red blood cells to murine brain and lung microvascular endothelial cells in vitro

Sequestration of infected red blood cells (iRBC) within the cerebral and pulmonary microvasculature is a hallmark of human cerebral malaria (hCM). The interaction between iRBC and the endothelium in hCM has been studied extensively and is linked to the severity of malaria. Experimental CM (eCM) caused by Plasmodium berghei ANKA reproduces most features of hCM, although the sequestration of RBC infected by P. berghei ANKA (PbA-iRBC) has not been completely delineated. The role of PbA-iRBC sequestration in the severity of eCM is not well characterized. Using static and flow cytoadherence assays, we provide the first direct in vitro evidence for the binding of PbA-iRBC to murine brain and lung microvascular endothelial cells (MVEC). We found that basal PbA-iRBC cytoadherence to MVECs was significantly higher than that of normal red blood cells (NRBC) and of RBC infected with P. berghei K173 (PbK173-iRBC), a strain that causes noncerebral malaria (NCM). MVEC prestimulation with tumor necrosis factor (TNF) failed to promote any further significant increase in mixed-stage iRBC adherence. Interestingly, enrichment of the blood for mature parasites significantly increased PbA-iRBC binding to the MVECs prestimulated with TNF, while blockade of VCAM-1 reduced this adhesion. Our study provides evidence for the firm, flow-resistant binding to endothelial cells of iRBC from strain ANKA-infected mice, which develop CM, and for less binding of iRBC from strain K173-infected mice, which develop NCM. An understanding of P. berghei cytoadherence may help elucidate the importance of sequestration in the development of CM and aid the development of antibinding therapies to help reduce the burden of this syndrome.

Prospects and Pitfalls of Pregnancy-Associated Malaria Vaccination Based on the Natural Immune Response to Plasmodium falciparum VAR2CSA-Expressing Parasites

Pregnancy-associated malaria, a manifestation of severe malaria, is the cause of up to 200,000 infant deaths a year, through the effects of placental insufficiency leading to growth restriction and preterm delivery. Development of a vaccine is one strategy for control. Plasmodium falciparum-infected red blood cells accumulate in the placenta through specific binding of pregnancy-associated parasite variants that express the VAR2CSA antigen to chondroitin sulphate A on the surface of syncytiotrophoblast cells. Parasite accumulation, accompanied by an inflammatory infiltrate, disrupts the cytokine balance of pregnancy with the potential to cause placental damage and compromise foetal growth. Multigravid women develop immunity towards VAR2CSA-expressing parasites in a gravidity-dependent manner which prevents unfavourable pregnancy outcomes. Although current vaccine design, targeting VAR2CSA antigens, has succeeded in inducing antibodies artificially, this candidate may not provide protection during the first trimester and may only protect those women living in areas endemic for malaria. It is concluded that while insufficient information about placental-parasite interactions is presently available to produce an effective vaccine, incremental progress is being made towards achieving this goal.

Clag9 is not essential for PfEMP1 surface expression in non-cytoadherent Plasmodium falciparum parasites with a chromosome 9 deletion

BACKGROUND

The expression of the clonally variant virulence factor PfEMP1 mediates the sequestration of Plasmodium falciparum infected erythrocytes in the host vasculature and contributes to chronic infection. Non-cytoadherent parasites with a chromosome 9 deletion lack clag9, a gene linked to cytoadhesion in previous studies. Here we present new clag9 data that challenge this view and show that surface the non-cytoadherence phenotype is linked to the expression of a non-functional PfEMP1.

METHODOLOGY/PRINCIPAL FINDINGS

Loss of adhesion in P. falciparum D10, a parasite line with a large chromosome 9 deletion, was investigated. Surface iodination analysis of non-cytoadherent D10 parasites and COS-7 surface expression of the CD36-binding PfEMP1 CIDR1α domain were performed and showed that these parasites express an unusual trypsin-resistant, non-functional PfEMP1 at the erythrocyte surface. However, the CIDR1α domain of this var gene expressed in COS-7 cells showed strong binding to CD36. Atomic Force Microscopy showed a slightly modified D10 knob morphology compared to adherent parasites. Trafficking of PfEMP1 and KAHRP remained functional in D10. We link the non-cytoadherence phenotype to a chromosome 9 breakage and healing event resulting in the loss of 25 subtelomeric genes including clag9. In contrast to previous studies, knockout of the clag9 gene from 3D7 did not interfere with parasite adhesion to CD36.

CONCLUSIONS/SIGNIFICANCE

Our data show the surface expression of non-functional PfEMP1 in D10 strongly indicating that genes other than clag9 deleted from chromosome 9 are involved in this virulence process possibly via post-translational modifications.

Plasmodium falciparum glutamate dehydrogenase a is dispensable and not a drug target during erythrocytic development

Background

Plasmodium falciparum contains three genes encoding potential glutamate dehydrogenases. The protein encoded by gdha has previously been biochemically and structurally characterized. It was suggested that it is important for the supply of reducing equivalents during intra-erythrocytic development of Plasmodium and, therefore, a suitable drug target.

Methods

The gene encoding the NADP(H)-dependent GDHa has been disrupted by reverse genetics in P. falciparum and the effect on the antioxidant and metabolic capacities of the resulting mutant parasites was investigated.

Results

No growth defect under low and elevated oxygen tension, no up- or down-regulation of a number of antioxidant and NADP(H)-generating proteins or mRNAs and no increased levels of GSH were detected in the D10^Δgdha parasite lines. Further, the fate of the carbon skeleton of [¹³C] labelled glutamine was assessed by metabolomic studies, revealing no differences in the labelling of α-ketoglutarate and other TCA pathway intermediates between wild type and mutant parasites.

Conclusions

First, the data support the conclusion that D10^Δgdha parasites are not experiencing enhanced oxidative stress and that GDHa function may not be the provision of NADP(H) for reductive reactions. Second, the results imply that the cytosolic, NADP(H)-dependent GDHa protein is not involved in the oxidative deamination of glutamate but that the protein may play a role in ammonia assimilation as has been described for other NADP(H)-dependent GDH from plants and fungi. The lack of an obvious phenotype in the absence of GDHa may point to a regulatory role of the protein providing glutamate (as nitrogen storage molecule) in situations where the parasites experience a limiting supply of carbon sources and, therefore, under in vitro conditions the enzyme is unlikely to be of significant importance. The data imply that the protein is not a suitable target for future drug development against intra-erythrocytic parasite development.

Malaria: modification of the red blood cell and consequences in the human host

Residence in the human erythrocyte is essential for the lifecycle of all Plasmodium that infect man. It is also the phase of the life cycle that causes disease. Although the red blood cell (RBC) is a highly specialized cell for its function of carrying oxygen to and carbon dioxide away from tissues, it is devoid of organelles and lacks any cellular machinery to synthesize new protein. Therefore in order to be able to survive and multiply within the RBC membrane the parasite needs to make many modifications to the infected RBC (iRBC). Plasmodium falciparum (P. falciparum) also expresses parasite-derived proteins on the surface of the iRBC that enable the parasite to cytoadhere to endothelial and other intravascular cells. These RBC modifications are at the root of malaria pathogenesis and, in this ancient disease of man, have formed the epicentre of a genetic 'battle' between parasite and host. This review discusses some of the critical modifications of the RBC by the parasite and some of the consequences of these adaptations on disease in the human host, with an emphasis on advances in understanding of the pathogenesis of severe and cerebral malaria (CM) from recent research.

The chondroitin sulfate A-binding site of the VAR2CSA protein involves multiple N-terminal domains

Malaria during pregnancy is a major health problem for African women. The disease is caused by Plasmodium falciparum malaria parasites, which accumulate in the placenta by adhering to chondroitin sulfate A (CSA). The interaction between infected erythrocytes and the placental receptor is mediated by a parasite expressed protein named VAR2CSA. A vaccine protecting pregnant women against placental malaria should induce antibodies inhibiting the interaction between VAR2CSA and CSA. Much effort has been put into defining the part of the 350 kDa VAR2CSA protein that is responsible for binding. It has been shown that full-length recombinant VAR2CSA binds specifically to CSA with high affinity, however to date no sub-fragment of VAR2CSA has been shown to interact with CSA with similar affinity or specificity. In this study, we used a biosensor technology to examine the binding properties of a panel of truncated VAR2CSA proteins. The experiments indicate that the core of the CSA-binding site is situated in three domains, DBL2X-CIDR_PAM and a flanking domain, located in the N-terminal part of VAR2CSA. Furthermore, recombinant VAR2CSA subfragments containing this region elicit antibodies with high parasite adhesion blocking activity in animal immunization experiments.

Identification of a role for the PfEMP1 semi-conserved head structure in protein trafficking to the surface of Plasmodium falciparum infected red blood cells

Transport of Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) variants to the red blood cell (RBC) surface enables malarial parasite evasion of host immunity by modifying the antigenic and adhesive properties of infected RBCs. In this study, we applied the Bxb1 integrase system to integrate transgenes encoding truncated PfEMP1-GFP fusions into cytoadherent A4 parasites and characterize their surface transport requirements. Our studies revealed that the semi-conserved head structure of PfEMP1 proteins, in combination with the predicted transmembrane region and cytoplasmic tail, encodes sufficient information for RBC surface display. In contrast, miniPfEMP1 proteins with truncated head structures were exported to the RBC cytoplasm but were not detected at the RBC surface by flow cytometry or immuno-electron microscopy. We demonstrated the absence of a mechanistic barrier to having native and miniPfEMP1 proteins displayed simultaneously at the RBC surface. However, surface-exposed miniPfEMP1 proteins did not convey cytoadherence properties to their host cells, implicating potential steric considerations in host-receptor interactions or the need for multiple domains to mediate cell binding. This study establishes a new system to investigate PfEMP1 transport and demonstrates that the PfEMP1 semi-conserved head structure is under selection for protein transport, in addition to its known roles in adhesion.

Sequestration and tissue accumulation of human malaria parasites: can we learn anything from rodent models of malaria?

The sequestration of Plasmodium falciparum–infected red blood cells (irbcs) in the microvasculature of organs is associated with severe disease; correspondingly, the molecular basis of irbc adherence is an active area of study. In contrast to P. falciparum, much less is known about sequestration in other Plasmodium parasites, including those species that are used as models to study severe malaria. Here, we review the cytoadherence properties of irbcs of the rodent parasite Plasmodium berghei ANKA, where schizonts demonstrate a clear sequestration phenotype. Real-time in vivo imaging of transgenic P. berghei parasites in rodents has revealed a CD36-dependent sequestration in lungs and adipose tissue. In the absence of direct orthologs of the P. falciparum proteins that mediate binding to human CD36, the P. berghei proteins and/or mechanisms of rodent CD36 binding are as yet unknown. In addition to CD36-dependent schizont sequestration, irbcs accumulate during severe disease in different tissues, including the brain. The role of sequestration is discussed in the context of disease as are the general (dis)similarities of P. berghei and P. falciparum sequestration.

Structural insights into chondroitin sulfate binding in pregnancy-associated malaria

Malaria during pregnancy is caused when parasite-infected erythrocytes accumulate within the placenta through interactions between the VAR2CSA protein on the infected erythrocyte surface and placental CSPGs (chondroitin sulfate proteoglycans). This interaction is the major target for therapeutics to treat or prevent pregnancy-associated malaria. Here we review the structural characterization of CSPG-binding DBL (Duffy-binding like) domains from VAR2CSA and summarize the growing evidence that the exquisite ligand specificity of VAR2CSA results from the adoption of higher-order architecture in which these domains fold together to form a ligand-binding pocket.

Surface co-expression of two different PfEMP1 antigens on single plasmodium falciparum-infected erythrocytes facilitates binding to ICAM1 and PECAM1

The Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) antigens play a major role in cytoadhesion of infected erythrocytes (IE), antigenic variation, and immunity to malaria. The current consensus on control of variant surface antigen expression is that only one PfEMP1 encoded by one var gene is expressed per cell at a time. We measured var mRNA transcript levels by real-time Q-PCR, analysed var gene transcripts by single-cell FISH and directly compared these with PfEMP1 antigen surface expression and cytoadhesion in three different antibody-selected P. falciparum 3D7 sub-lines using live confocal microscopy, flow cytometry and in vitro adhesion assays. We found that one selected parasite sub-line simultaneously expressed two different var genes as surface antigens, on single IE. Importantly, and of physiological relevance to adhesion and malaria pathogenesis, this parasite sub-line was found to bind both CD31/PECAM1 and CD54/ICAM1 and to adhere twice as efficiently to human endothelial cells, compared to infected cells having only one PfEMP1 variant on the surface. These new results on PfEMP1 antigen expression indicate that a re-evaluation of the molecular mechanisms involved in P. falciparum adhesion and of the accepted paradigm of absolutely mutually exclusive var gene transcription is required.

Plasmodium falciparum is the most pathogenic human malaria parasite and its virulence has been linked to its capacity to express different adhesion proteins that enable the developing parasitized erythrocyte to bind to capillaries of the host, thereby avoiding removal by the spleen. Each parasite has approximately 60 genes encoding different versions of this adhesion protein, and a switch in surface display of these proteins enables the parasite to evade the immune system. Here we show that different variants of these binding proteins can be found expressed simultaneously on single infected red blood cells mediating binding to different endothelial receptors.

Var gene expression and human Plasmodium pathogenesis

Plasmodium falciparum is responsible for most of the morbidity and mortality associated with malaria and is unique in its ability to sequester in organ postcapillary venules. Specific host-parasite interactions mediate this phenomenon and the P. falciparum erythrocyte membrane protein 1 is the predominant ligand responsible for adhering to host endothelial receptors. This review focuses on the current knowledge regarding this protein family, evidence for its role in various pathogenic mechanisms and on insights that have been gained in this area from field studies.

Molecular aspects of Plasmodium falciparum Infection during pregnancy

Cytoadherence of Plasmodium-falciparum-parasitized red blood cells (PRBCs) to host receptors is the key phenomenon in the pathological process of the malaria disease. Some of these interactions can originate poor outcomes responsible for 1 to 3 million annual deaths mostly occurring among children in sub-Saharan Africa. Pregnancy-associated malaria (PAM) represents an important exception of the disease occurring at adulthood in malaria endemic settings. Consequences of this are shared between the mother (maternal anemia) and the baby (low birth weight and infant mortality). Demonstrating that parasites causing PAM express specific variant surface antigens (VSA(PAM)), including the P. falciparum erythrocyte membrane protein 1 (P f EMP1) variant VAR2CSA, that are targets for protective immunity has strengthened the possibility for the development of PAM-specific vaccine. In this paper, we review the molecular basis of malaria pathogenesis attributable to the erythrocyte stages of the parasites, and findings supporting potential anti-PAM vaccine components evidenced in PAM.

Immunization with VAR2CSA-DBL5 recombinant protein elicits broadly cross-reactive antibodies to placental Plasmodium falciparum-infected erythrocytes

Pregnancy-associated malaria is a severe clinical syndrome associated with the sequestration of Plasmodium falciparum-infected erythrocytes in the placenta. Placental binding is mediated by VAR2CSA, a member of the large and diverse P. falciparum erythrocyte membrane 1 (PfEMP1) protein family. To better understand if conserved regions in VAR2CSA can be targeted by antibodies, we immunized rabbits with VAR2CSA-DBL1 and -DBL5 recombinant proteins produced in Pichia pastoris and developed a panel of seven chondroitin sulfate A (CSA)-binding parasites from diverse geographic origins. Overall, no two parasites in the panel expressed the same VAR2CSA sequence. The DBL1 domains averaged 80% amino acid identity (range, 72 to 89%), and the DBL5 domains averaged 86% amino acid identity (range, 83 to 99%), similar to a broader sampling of VAR2CSA sequences from around the world. Whereas antibodies generated against the VAR2CSA-DBL1 recombinant protein had only limited breadth and reacted with three or four parasites in the panel, immunization with DBL5 recombinant proteins elicited broadly cross-reactive antibodies against all or most parasites in the panel, as well as to fresh clinical isolates from pregnant women. These findings demonstrate that the major PfEMP1 variant expressed by placental isolates exposes strain-transcendent epitopes that can be targeted by vaccination and may have application for pregnancy malaria vaccine development.

Can any lessons be learned from the ambiguous glycan binding of PfEMP1 domains?

Pregnancy-associated malaria (PAM) is caused by Plasmodium falciparum-infected erythrocytes (IEs) accumulating in the placenta and has dire consequences for both mother and child. The multi-domain antigen VAR2CSA confers specific adhesion of IEs to chondroitin sulphate A (CSA) in the placenta, and is the leading PAM vaccine candidate. Recent data from different laboratories show that the binding properties of individual VAR2CSA domains do not reflect the native CSA-specific adhesion of IEs, which questions the relevance of the information obtained from single domain binding assays and co-crystallization experiments. Here, we discuss the implications of these findings for VAR2CSA vaccine development and highlight the need for studying the native structure of this protein.

The role of platelets in the pathogenesis of cerebral malaria

Malaria is a major cause of morbidity and mortality in the developing world and cerebral malaria is responsible for the majority of malaria-associated deaths. There is a strong association between thrombocytopenia and outcome in malaria, suggesting a role for platelets in the pathogenesis of malaria. This thrombocytopenia is likely due to platelet activation possibly through an interaction between PfEMP1 on plasmodium and CD36 on platelets. Platelet activation by plasmodium has two potential consequences. It can lead to the formation of micro-aggregates of infected red blood cells and platelets which can occlude blood vessels and it also leads to binding to and activation of the endothelium.

Structural comparison of two CSPG-binding DBL domains from the VAR2CSA protein important in malaria during pregnancy

Severe malaria during pregnancy is associated with accumulation of parasite-infected erythrocytes in the placenta due to interactions between VAR2CSA protein, expressed on the surface of infected-erythrocytes, and placental chondroitin sulfate proteoglycans (CSPG). VAR2CSA contains multiple CSPG-binding domains, including DBL3X and DBL6ɛ. Previous structural studies of DBL3X suggested CSPG to bind to a positively charged patch and sulfate-binding site on the concave surface of the domain. Here we present the structure of the DBL6ɛ domain from VAR2CSA. This domain displays the same overall architecture and secondary structure as that of DBL3X but differs in loop structures, disulfide bond positions and surface charge distribution. In particular, despite binding to CSPG, DBL6ɛ lacks the key features of the CSPG-binding site of DBL3X. Instead DBL6ɛ binds to CSPG through a positively charged surface on the distal side of subdomain 2 that is exposed in intact VAR2CSA on the erythrocyte surface. Finally, unlike intact VAR2CSA, both DBL3X and DBL6ɛ bind to various carbohydrates, with greatest affinity for ligands with high sulfation and negative charge. These studies provide further insight into the structure of DBL domains and suggest a model for the role of individual domains in CSPG binding by VAR2CSA in placental malaria.

A model of the complex between the PfEMP1 malaria protein and the human ICAM-1 receptor

Malaria is caused by protozoan parasites of the genus Plasmodium. Four species of Plasmodium can infect humans: P. falciparum, P. malariae, P. vivax, and P. ovale. P. falciparum is the only able to cytoadhere to the surface of postcapillary endothelial cells. A key role in cytoadherence is played by the interaction between the PfEMP1 P. falciparum protein and the human intracellular adhesion molecule (ICAM-1) although very little is known about the molecular details of this complex. Here we propose a model for this interaction on the basis of a homology model of the functional domain of PfEMP1 and of the ICAM-1 three dimensional structures. Our model is consistent with the results of many experimental observations, provides a rational explanation for the different binding abilities of different strains of P. falciparum and explains the reduced binding affinity of the A4 strain of P. falciparum for the ICAM-1(Kilifi) polymorphism. On the basis of our model, we can also explain why the murine ICAM-1, although sharing 70% sequence similarity with its human homologue, does not bind PfEMP1, and why the binding of fibrinogen and PfEMP1 to ICAM-1 is mutually exclusive. The model of the complex proposed here can serve as a useful tool for the design and interpretation of biochemical and immunological experimental results.

Adhesion of Plasmodium falciparum-infected erythrocytes to human cells: molecular mechanisms and therapeutic implications

Severe malaria has a high mortality rate (15–20%) despite treatment with effective antimalarial drugs. Adjunctive therapies for severe malaria that target the underlying disease process are therefore urgently required. Adhesion of erythrocytes infected with Plasmodium falciparum to human cells has a key role in the pathogenesis of life-threatening malaria and could be targeted with antiadhesion therapy. Parasite adhesion interactions include binding to endothelial cells (cytoadherence), rosetting with uninfected erythrocytes and platelet-mediated clumping of infected erythrocytes. Recent research has started to define the molecular mechanisms of parasite adhesion, and antiadhesion therapies are being explored. However, many fundamental questions regarding the role of parasite adhesion in severe malaria remain unanswered. There is strong evidence that rosetting contributes to severe malaria in sub-Saharan Africa; however, the identity of other parasite adhesion phenotypes that are implicated in disease pathogenesis remains unclear. In addition, the possibility of geographic variation in adhesion phenotypes causing severe malaria, linked to differences in malaria transmission levels and host immunity, has been neglected. Further research is needed to realise the untapped potential of antiadhesion adjunctive therapies, which could revolutionise the treatment of severe malaria and reduce the high mortality rate of the disease.

Ectopic recombination of a malaria var gene during mitosis associated with an altered var switch rate

The Plasmodium falciparum var multigene family encodes P. falciparum erythrocyte membrane protein 1, which is responsible for the pathogenic traits of antigenic variation and adhesion of infected erythrocytes to host receptors during malaria infection. Clonal antigenic variation of P. falciparum erythrocyte membrane protein 1 is controlled by the switching between exclusively transcribed var genes. The tremendous diversity of the var gene repertoire both within and between parasite strains is critical for the parasite's strategy of immune evasion. We show that ectopic recombination between var genes occurs during mitosis, providing P. falciparum with opportunities to diversify its var repertoire, even during the course of a single infection. We show that the regulation of the recombined var gene has been disrupted, resulting in its persistent activation although the regulation of most other var genes is unaffected. The var promoter and intron of the recombined var gene are not responsible for its atypically persistent activity, and we conclude that altered subtelomeric cis sequence is the most likely cause of the persistent activity of the recombined var gene.

Heterologous expression of plasmodial proteins for structural studies and functional annotation

Malaria remains the world's most devastating tropical infectious disease with as many as 40% of the world population living in risk areas. The widespread resistance of Plasmodium parasites to the cost-effective chloroquine and antifolates has forced the introduction of more costly drug combinations, such as Coartem^®. In the absence of a vaccine in the foreseeable future, one strategy to address the growing malaria problem is to identify and characterize new and durable antimalarial drug targets, the majority of which are parasite proteins. Biochemical and structure-activity analysis of these proteins is ultimately essential in the characterization of such targets but requires large amounts of functional protein. Even though heterologous protein production has now become a relatively routine endeavour for most proteins of diverse origins, the functional expression of soluble plasmodial proteins is highly problematic and slows the progress of antimalarial drug target discovery. Here the status quo of heterologous production of plasmodial proteins is presented, constraints are highlighted and alternative strategies and hosts for functional expression and annotation of plasmodial proteins are reviewed.

The cysteine-rich interdomain region from the highly variable plasmodium falciparum erythrocyte membrane protein-1 exhibits a conserved structure

Plasmodium falciparum malaria parasites, living in red blood cells, express proteins of the erythrocyte membrane protein-1 (PfEMP1) family on the red blood cell surface. The binding of PfEMP1 molecules to human cell surface receptors mediates the adherence of infected red blood cells to human tissues. The sequences of the 60 PfEMP1 genes in each parasite genome vary greatly from parasite to parasite, yet the variant PfEMP1 proteins maintain receptor binding. Almost all parasites isolated directly from patients bind the human CD36 receptor. Of the several kinds of highly polymorphic cysteine-rich interdomain region (CIDR) domains classified by sequence, only the CIDR1α domains bind CD36. Here we describe the CD36-binding portion of a CIDR1α domain, MC179, as a bundle of three α-helices that are connected by a loop and three additional helices. The MC179 structure, containing seven conserved cysteines and 10 conserved hydrophobic residues, predicts similar structures for the hundreds of CIDR sequences from the many genome sequences now known. Comparison of MC179 with the CIDR domains in the genome of the P. falciparum 3D7 strain provides insights into CIDR domain structure. The CIDR1α three-helix bundle exhibits less than 20% sequence identity with the three-helix bundles of Duffy-binding like (DBL) domains, but the two kinds of bundles are almost identical. Despite the enormous diversity of PfEMP1 sequences, the CIDR1α and DBL protein structures, taken together, predict that a PfEMP1 molecule is a polymer of three-helix bundles elaborated by a variety of connecting helices and loops. From the structures also comes the insight that DBL1α domains are approximately 100 residues larger and that CIDR1α domains are approximately 100 residues smaller than sequence alignments predict. This new understanding of PfEMP1 structure will allow the use of better-defined PfEMP1 domains for functional studies, for the design of candidate vaccines, and for understanding the molecular basis of cytoadherence.

Malaria parasites express proteins of the erythrocyte membrane protein-1 family (PfEMP1) on the surfaces of the human red blood cells that they infect. These large proteins vary in sequence extensively, yet bind to host receptors to allow infected cells to adhere to host tissues. PfEMP1 proteins help parasites evade the immune system, as the 60 PfEMP1 genes are expressed one at a time. Sequence comparisons predict that PfEMP1 molecules are modular, made up of Duffy binding-like (DBL) and cysteine-rich interdomain region (CIDR) domains. Many CIDR domains bind to the human receptor CD36. We have analyzed the structure of the CD36-binding portion, known as MC179, of a CIDR domain. The MC179 protein is composed of a bundle of three helices connected by a loop and three additional helices. Based on the structure and sequence similarities, MC179 is a good model for the hundreds of known CIDR sequences. In addition, the MC179 three-helix bundle is remarkably similar to subdomain 3 of the known DBL structures. MC179 provides insight into the relatedness of both kinds of PfEMP1 domains and predicts that the large PfEMP1 molecules are polymers of three-helix bundles and their connecting polypeptides.

The structure of a chondroitin sulfate-binding domain important in placental malaria

Adhesive PfEMP1 proteins are displayed on the surface of malaria-infected red blood cells. They play a critical role in the disease, tethering infected cells away from destruction by the spleen and causing many severe symptoms. A molecular understanding of how these domains maintain their binding properties while evading immune detection will be important in developing therapeutics. In malaria of pregnancy, domains from the var2csa-encoded PfEMP1 protein interact with chondroitin sulfate on the placenta surface. This causes accumulation of infected red blood cells, leading to placental inflammation and block of blood flow to the developing fetus. This is associated with maternal anemia, low birth weight, and premature delivery and can lead to the death of mother and child. Here I present the structure of the chondroitin sulfate-binding DBL3X domain from a var2csa-encoded PfEMP1 protein. The domain adopts a fold similar to malarial invasion proteins, with extensive loop insertions. One loop is flexible in the unliganded structure but observed in the presence of sulfate or disaccharide, where it completes a sulfate-binding site. This loop, and others surrounding this putative carbohydrate-binding site, are flexible and polymorphic, perhaps protecting the binding site from immune detection. This suggests a model for how the domain maintains ligand binding while evading the immune response and will guide future drug and vaccine development.

Structural insight into epitopes in the pregnancy-associated malaria protein VAR2CSA

Pregnancy-associated malaria is caused by Plasmodium falciparum malaria parasites binding specifically to chondroitin sulfate A in the placenta. This sequestration of parasites is a major cause of low birth weight in infants and anemia in the mothers. VAR2CSA, a polymorphic multi-domain protein of the PfEMP1 family, is the main parasite ligand for CSA binding, and identification of protective antibody epitopes is essential for VAR2CSA vaccine development. Attempts to determine the crystallographic structures of VAR2CSA or its domains have not been successful yet. In this study, we propose 3D models for each of the VAR2CSA DBL domains and we show that regions in the fold of VAR2CSA inter-domain 2 and a PfEMP1 CIDR domain seem to be homologous to the EBA-175 and Pkα-DBL fold. This suggests that ID2 could be a functional domain. We also identify regions of VAR2CSA present on the surface of native VAR2CSA by comparing reactivity of plasma containing anti-VAR2CSA antibodies in peptide array experiments before and after incubation with native VAR2CSA. By this method we identify conserved VAR2CSA regions targeted by antibodies that react with the native molecule expressed on infected erythrocytes. By mapping the data onto the DBL models we present evidence suggesting that the S1+S2 DBL sub-domains are generally surface-exposed in most domains, whereas the S3 sub-domains are less exposed in native VAR2CSA. These results comprise an important step towards understanding the structure of VAR2CSA on the surface of CSA-binding infected erythrocytes.

Individuals living in areas with high Plasmodium falciparum transmission acquire immunity to malaria over time and adults have markedly reduced risk of getting severe disease. However, pregnant women constitute an important exception, and they become more susceptible to malaria during pregnancy. This so called pregnancy-associated malaria (PAM) has severe consequences for both mother and child, and a vaccine would save hundreds of thousands of lives each year. PAM is caused by P. falciparum–infected red blood cells that bind to receptors in the placenta. By binding to the placental tissue, the parasites avoid being filtered though the spleen where they would have been killed. The protein mediating this placental binding is a very large multidomain and variant protein named VAR2CSA. Using structural modeling of VAR2CSA and antibody reagents from women who have had PAM, we show that antibodies tend to bind in similar regions, on one side of the individual VAR2CSA domains. In addition, we show that highly conserved parts of this variant protein are accessible for antibodies. This finding correlates with epidemiological data showing that woman acquire immunity towards PAM relatively fast, and the identification of these epitopes is thus a major step towards a protective vaccine.

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.

The impact of IgG antibodies to recombinant Plasmodium falciparum 732var CIDR-1alpha domain in mothers and their newborn babies

Different domains of a novel full-length var gene (termed 732var) isolated from a placenta of a malaria-infected woman were expressed in Escherichia coli as recombinant proteins and analysed biochemically and immunologically. Two of these, the Duffy binding-like (DBL)-3gamma domain and the cysteine-rich interdomain region (CIDR)-1alpha were able to bind chondroitin sulfate A and CD36, respectively. The DBL-3gamma domain was investigated in a previous study and confirmed here to exhibit anti-disease characteristics related to pregnancy-associated malaria. Mothers with high anti-DBL-3gamma antibody levels were protected from placental infection. The novel finding in this study is that babies born to mothers carrying anti-CIDR-1alpha antibodies had a delayed time to the first infection.

Structural modifications to a high-activity binding peptide located within the PfEMP1 NTS domain induce protection against P. falciparum malaria in Aotus monkeys

Binding of P. falciparum-infected erythrocytes to vascular endothelium and to uninfected erythrocytes is mediated by the parasite-derived variant erythrocyte membrane protein PfEMP-1 and various receptors, both on the vascular endothelium and on the erythrocyte surface. Consecutive, non-overlapping peptides spanning the N-terminal segment (NTS) and Duffy-binding-like PfEMP1 sequence alpha-domain (DBLalpha) of this protein were tested in erythrocyte and C32 cell binding assays. Eight peptides specifically bound to C32 cells, and were named high-activity binding peptides (HABPs). No erythrocyte binding HABPs were found in this region. Strikingly, three HABPs [6504 ((1)MVELA KMGPK EAAGG DDIED(20)), 6505 ((21)ESAKH MFDRI GKDVY DKVKE(40)) and 6506 ((41)YRAKE RGKGL QGRLS EAKFEK(60))] are located within the NTS, for which no specific function has yet been described. HABP 6505 is neither immunogenic nor protection-inducing; therefore, based on our previous reports, critical amino acids (shown in bold) in HABP-C32 cell binding were identified and replaced to modify HABP immunogenicity and protectivity. Analogue peptide 12722 (ESAKH KFDRI GKDVY DMVKE) produced high antibody titres and completely protected three out of 12 vaccinated Aotus monkeys and 23410 (KHKFD FIGKI VYDMV KER) also produced high protection-inducing titres and completely protected one out of eight monkeys. (1)H NMR studies showed that all peptides were helical. Binding of these peptides to isolated HLADRbeta1 molecules did not reveal any preference, suggesting that they could bind to molecules not studied here.

Plasmodium falciparum var gene expression is developmentally controlled at the level of RNA polymerase II‐mediated transcription initiation

The Plasmodium falciparum var gene family codes for a major virulence factor in this most lethal of human malaria parasites. A single var protein variant type is expressed on each infected red blood cell, with antigenic variation allowing progeny parasites to escape host immune detection. The control of mutually exclusive var gene expression in the parasite relies on in situ epigenetic changes. Whether control of expression occurs at transcription initiation or post transcription, however, remains to be established. Recent evidence supports existence of a unique var transcription site at the nuclear periphery containing the dominantly expressed var gene, although silent var genes can colocalize to the same region. We demonstrate here that exclusive var gene expression is controlled at the level of transcription initiation during ring stages and that var genes are transcribed by RNA polymerase II. This represents another example where P. falciparum differs from the paradigm for antigenic variation, Trypanosoma brucei.

Physical linkage to drug resistance genes results in conservation of var genes among West Pacific Plasmodium falciparum isolates

The multicopy var gene family encoding the variant surface antigen Plasmodium falciparum erythrocyte membrane protein 1 is highly diverse, with little overlap between different P. falciparum isolates. We report 5 var genes (varS1-varS5) that are shared at relatively high frequency among 63 genetically diverse P. falciparum isolates collected from 5 islands in the West Pacific region. The varS1, varS2, and varS3 genes were localized to the internal region on chromosome 4, approximately 200 kb from pfdhfr-ts, whereas varS4 and varS5 were mapped to an internal region of chromosome 7, within 100 kb of pfcrt. The presence of varS2 and varS3 were significantly correlated with the pyrimethamine-resistant pfdhfr genotype, whereas varS4 was strongly correlated with the chloroquine-resistant pfcrt genotype. Thus, the conservation of these var genes is the result of their physical linkage with drug-resistant genes in combination with the antimalarial drug pressure in the region.

Delivery of the malaria virulence protein PfEMP1 to the erythrocyte surface requires cholesterol-rich domains

The particular virulence of the human malaria parasite Plasmodium falciparum derives from export of parasite-encoded proteins to the surface of the mature erythrocytes in which it resides. The mechanisms and machinery for the export of proteins to the erythrocyte membrane are largely unknown. In other eukaryotic cells, cholesterol-rich membrane microdomains or "rafts" have been shown to play an important role in the export of proteins to the cell surface. Our data suggest that depletion of cholesterol from the erythrocyte membrane with methyl-beta-cyclodextrin significantly inhibits the delivery of the major virulence factor P. falciparum erythrocyte membrane protein 1 (PfEMP1). The trafficking defect appears to lie at the level of transfer of PfEMP1 from parasite-derived membranous structures within the infected erythrocyte cytoplasm, known as the Maurer's clefts, to the erythrocyte membrane. Thus our data suggest that delivery of this key cytoadherence-mediating protein to the host erythrocyte membrane involves insertion of PfEMP1 at cholesterol-rich microdomains. GTP-dependent vesicle budding and fusion events are also involved in many trafficking processes. To determine whether GTP-dependent events are involved in PfEMP1 trafficking, we have incorporated non-membrane-permeating GTP analogs inside resealed erythrocytes. Although these nonhydrolyzable GTP analogs reduced erythrocyte invasion efficiency and partially retarded growth of the intracellular parasite, they appeared to have little direct effect on PfEMP1 trafficking.

Adherence of Plasmodium falciparum infected erythrocytes to CHO-745 cells and inhibition of binding by protein A in the presence of human serum

Adhesion of erythrocytes infected with the malaria parasite Plasmodium falciparum to human host receptors is a process associated with severe malarial pathology. A number of in vitro cell lines are available as models for these adhesive processes, including Chinese hamster ovary (CHO) cells which express the placental adhesion receptor chondroitin-4-sulphate (CSA) on their surface. CHO-745 cells, a glycosaminoglycan-negative mutant CHO cell line lacking CSA and other reported P. falciparum adhesion receptors, are often used for recombinant expression of host receptors and for receptor binding studies. In this study we show that P. falciparum-infected erythrocytes can be easily selected for adhesion to an endogenous receptor on the surface of CHO-745 cells, bringing into question the validity of using these cells as a tool for P. falciparum adhesin expression studies. The adhesive interaction between CHO-745 cells and parasitized erythrocytes described here is not mediated by the known P. falciparum adhesion receptors CSA, CD36, or ICAM-1. However, we found that CHO-745-selected parasitized erythrocytes bind normal human IgM and that adhesion to CHO-745 cells is inhibited by protein A in the presence of serum, but not in its absence, indicating a non-specific inhibitory effect. Thus, protein A, which has been used as an inhibitor for a recently described interaction between infected erythrocytes and the placenta, may not be an appropriate in vitro inhibitor for understanding in vivo adhesive interactions.

Plasmodium falciparum variant surface antigen expression patterns during malaria

The variant surface antigens expressed on Plasmodium falciparum–infected erythrocytes are potentially important targets of immunity to malaria and are encoded, at least in part, by a family of var genes, about 60 of which are present within every parasite genome. Here we use semi-conserved regions within short var gene sequence “tags” to make direct comparisons of var gene expression in 12 clinical parasite isolates from Kenyan children. A total of 1,746 var clones were sequenced from genomic and cDNA and assigned to one of six sequence groups using specific sequence features. The results show the following. (1) The relative numbers of genomic clones falling in each of the sequence groups was similar between parasite isolates and corresponded well with the numbers of genes found in the genome of a single, fully sequenced parasite isolate. In contrast, the relative numbers of cDNA clones falling in each group varied considerably between isolates. (2) Expression of sequences belonging to a relatively conserved group was negatively associated with the repertoire of variant surface antigen antibodies carried by the infected child at the time of disease, whereas expression of sequences belonging to another group was associated with the parasite “rosetting” phenotype, a well established virulence determinant. Our results suggest that information on the state of the host–parasite relationship in vivo can be provided by measurements of the differential expression of different var groups, and need only be defined by short stretches of sequence data.

Hope that it will be possible to develop a malaria vaccine is supported by the fact that individuals who have grown up in malaria endemic regions learn to carry malarial infections without suffering disease. Surprisingly little is still known about how this immunity develops. Much current research focuses on how the host develops immune responses to parasite antigens that are exposed to the host immune system. A major family of such antigens are inserted into the surface of parasite-infected erythrocytes, where they undergo antigenic switching to evade a developing antibody response. These proteins are encoded by a family of approximately 60 var genes, variants of which are present in every parasite genome.

The extreme diversity of the var genes has prevented meaningful comparison of their expression in clinical isolates. However, the authors of this paper show that var genes can be placed in groups that have a similar representation in the genomes of all parasites that the authors collected from Kenyan children. Having demonstrated an underlying similarity at the genomic level, the authors show that the var expression patterns vary markedly between different patients. The expression levels of specific groups of var genes was associated with poorly developed antibody responses in the children and a well-established parasite virulence phenotype. The study provides tools for exploring how host and parasite adapt to one another as immunity develops.

Transcriptome analysis of antigenic variation in Plasmodium falciparum--var silencing is not dependent on antisense RNA

A microarray analysis of Plasmodium falciparum selected to express different var genes suggests that antisense transcripts are not responsible for the transcriptional silencing of non-expressed var genes.

Background

Plasmodium falciparum, the causative agent of the most severe form of malaria, undergoes antigenic variation through successive presentation of a family of antigens on the surface of parasitized erythrocytes. These antigens, known as Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) proteins, are subject to a mutually exclusive expression system, and are encoded by the multigene var family. The mechanism whereby inactive var genes are silenced is poorly understood. To investigate transcriptional features of this mechanism, we conducted a microarray analysis of parasites that were selected to express different var genes by adhesion to chondroitin sulfate A (CSA) or CD36.

Results

In addition to oligonucleotides for all predicted protein-coding genes, oligonucleotide probes specific to each known var gene of the FCR3 background were designed and added to the microarray, as well as tiled sense and antisense probes for a subset of var genes. In parasites selected for adhesion to CSA, one full-length var gene (var2csa) was strongly upregulated, as were sense RNA molecules emanating from the 3' end of a limited subset of other var genes. No global relationship between sense and antisense production of var genes was observed, but notably, some var genes had coincident high levels of both antisense and sense transcript.

Conclusion

Mutually exclusive expression of PfEMP1 proteins results from transcriptional silencing of non-expressed var genes. The distribution of steady-state sense and antisense RNA at var loci are not consistent with a silencing mechanism based on antisense silencing of inactive var genes. Silencing of var loci is also associated with altered regulation of genes distal to var loci.

Host immunity modulates transcriptional changes in a multigene family (yir) of rodent malaria

Variant antigens, encoded by multigene families, and expressed at the surface of erythrocytes infected with the human malaria parasite Plasmodium falciparum and the simian parasite Plasmodium knowlesi, are important in evasion of host immunity. The vir multigene family, encoding a very large number of variant antigens, has been identified in the human parasite Plasmodium vivax and homologues (yir) of this family exist in the rodent parasite Plasmodium yoelii. These genes are part of a superfamily (pir) which are found in Plasmodium species infecting rodents, monkeys and humans (P. yoelii, P. berghei, P. chabaudi, P. knowlesi and P. vivax). Here, we show that YIR proteins are expressed on the surface of erythrocytes infected with late-stage asexual parasites, and that host immunity modulates transcription of yir genes. The surface location and expression pattern of YIR is consistent with a role in antigenic variation. This provides a unique opportunity to study the regulation and expression of the pir superfamily, and its role in both protective immunity and antigenic variation, in an easily accessible animal model system.