The Plasmodium falciparum–CD36 Interaction Is Modified by a Single Amino Acid Substitution in CD36

Plasmodium falciparum-CD36 Structure-Function Relationships Defined by Ortholog Scanning Mutagenesis

BACKGROUND

The interaction of Plasmodium falciparum-infected erythrocytes (IEs) with the host receptor CD36 is among the most studied host-parasite interfaces. CD36 is a scavenger receptor that binds numerous ligands including the cysteine-rich interdomain region (CIDR)α domains of the erythrocyte membrane protein 1 family (PfEMP1) expressed on the surface of IEs. CD36 is conserved across species, but orthologs display differential binding of IEs.

METHODS

In this study, we exploited these differences, combined with the recent crystal structure and 3-dimensional modeling of CD36, to investigate malaria-CD36 structure-function relationships and further define IE-CD36 binding interactions.

RESULTS

We show that a charged surface in the membrane-distal region of CD36 is necessary for IE binding. Moreover, IE interaction with this binding surface is influenced by additional CD36 domains, both proximal to and at a distance from this site.

CONCLUSIONS

Our data indicate that subtle sequence and spatial differences in these domains modify receptor conformation and regulate the ability of CD36 to selectively interact with its diverse ligands.

Comparative Studies of Vertebrate Platelet Glycoprotein 4 (CD36)

Platelet glycoprotein 4 (CD36) (or fatty acyl translocase [FAT], or scavenger receptor class B, member 3 [SCARB3]) is an essential cell surface and skeletal muscle outer mitochondrial membrane glycoprotein involved in multiple functions in the body. CD36 serves as a ligand receptor of thrombospondin, long chain fatty acids, oxidized low density lipoproteins (LDLs) and malaria-infected erythrocytes. CD36 also influences various diseases, including angiogenesis, thrombosis, atherosclerosis, malaria, diabetes, steatosis, dementia and obesity. Genetic deficiency of this protein results in significant changes in fatty acid and oxidized lipid uptake. Comparative CD36 amino acid sequences and structures and CD36 gene locations were examined using data from several vertebrate genome projects. Vertebrate CD36 sequences shared 53-100% identity as compared with 29-32% sequence identities with other CD36-like superfamily members, SCARB1 and SCARB2. At least eight vertebrate CD36 N-glycosylation sites were conserved which are required for membrane integration. Sequence alignments, key amino acid residues and predicted secondary structures were also studied. Three CD36 domains were identified including cytoplasmic, transmembrane and exoplasmic sequences. Conserved sequences included N- and C-terminal transmembrane glycines; and exoplasmic cysteine disulphide residues; TSP-1 and PE binding sites, Thr92 and His242, respectively; 17 conserved proline and 14 glycine residues, which may participate in forming CD36 'short loops'; and basic amino acid residues, and may contribute to fatty acid and thrombospondin binding. Vertebrate CD36 genes usually contained 12 coding exons. The human CD36 gene contained transcription factor binding sites (including PPARG and PPARA) contributing to a high gene expression level (6.6 times average). Phylogenetic analyses examined the relationships and potential evolutionary origins of the vertebrate CD36 gene with vertebrate SCARB1 and SCARB2 genes. These suggested that CD36 originated in an ancestral genome and was subsequently duplicated to form three vertebrate CD36 gene family members, SCARB1, SCARB2 and CD36.

Role of human CD36 in bacterial recognition, phagocytosis, and pathogen-induced JNK-mediated signaling

Scavenger receptor CD36 mediates Staphylococcus aureus phagocytosis and initiates TLR2/6 signaling. We analyzed the role of CD36 in the uptake and TLR-independent signaling of various bacterium, including Escherichia coli, Klebsiella pneumoniae, Salmonella typhimurium, S. aureus, and Enterococcus faecalis. Expression of human CD36 in HeLa cells increased the uptake of both gram-positive and gram-negative bacteria compared with the control mock-transfected cells. Bacterial adhesion was associated with pathogen phagocytosis. Upon CD36 transfection, HEK293 cells, which demonstrate no TLR2/4 expression, acquired LPS responsiveness as assessed by IL-8 production. The cells demonstrated a marked 5- to 15-fold increase in cytokine release upon exposure to gram-negative bacteria, while the increase was much smaller (1.5- to 3-fold) with gram-positive bacteria and lipoteichoic acid. CD36 down-regulation utilizing CD36 small interfering RNA reduced cytokine release by 40-50% in human fibroblasts induced by both gram-negative and gram-positive bacteria as well as LPS. Of all MAPK signaling cascade inhibitors tested, only the inhibitor of JNK, a stress-activated protein kinase, potently blocked E. coli/LPS-stimulated cytokine production. NF-kappaB inhibitors were ineffective, indicating direct TLR-independent signaling. JNK activation was confirmed by Western blot analyses of phosphorylated JKN1/2 products. Synthetic amphipathic peptides with an alpha-helical motif were shown to be efficient inhibitors of E. coli- and LPS-induced IL-8 secretion as well as JNK1/2 activation/phosphorylation in CD36-overexpressing cells. These results indicate that CD36 functions as a phagocytic receptor for a variety of bacteria and mediates signaling induced by gram-negative bacteria and LPS via a JNK-mediated signaling pathway in a TLR2/4-independent manner.

CD44, a signal receptor for the inhibition of the cytoadhesion of CD36-binding Plasmodium falciparum-infected erythrocytes by CSA-binding infected erythrocytes

The cytoadhesion of Plasmodium falciparum-infected erythrocytes (IEs) in organ microvessels is a key event in the pathogenesis of cerebral malaria and pulmonary edema. Identification of the molecules involved in the interaction between IEs and endothelial cells has been a major goal of research into severe forms of malaria. In contrast, the consequences of cytoadhesion for endothelial cells have been largely ignored. By combining phenotypic selection, cytoadhesion assays and flow cytometry, we demonstrated that the cytoadhesion of CSA-binding IEs inhibited the cytoadhesion of CD36-binding IEs. We identified CD44 as a signal receptor for CSA-binding IEs cytoadhesion, and demonstrated that the signal was transduced to CD36 through a pathway involving the Src-kinase family and MEK. CD36-mediated cytoadhesion was modulated independently of changes in CD36 expression. These results provide the first evidence that some IEs can downregulate the cytoadhesion of IEs of another phenotype, by modifying endothelial cells via a signaling pathway relating CD44 to CD36. Mimicking this phenomenon may constitute an interesting therapeutic strategy for inhibiting the adhesion of CD36-binding IEs -- the most abundant phenotype among field isolates -- and promoting their degradation in the spleen.

Cellular adhesive phenomena in apicomplexan parasites of red blood cells

The apicomplexan parasites Babesia and Plasmodium are related, yet phylogenetically distinct haemoprotozoa that infect red blood cells and cause severe diseases of major human and veterinary importance. A variety of cellular and molecular interactions are pivotal in many aspects of the pathogenicity of these two parasites. Comparison of the cellular and molecular mechanisms that culminate in accumulation of parasitised red blood cells in the microvasculature of cattle infected with Babesia bovis (babesiosis) and humans infected with Plasmodium falciparum (falciparum malaria) is particularly instructive given the striking similarities in the pathophysiology of these two important medical and veterinary diseases. While such adhesive phenomena have been studied extensively in malaria, they have received relatively little attention in babesiosis. In this review, we summarise the findings of more than 25 years of research into cellular adhesive phenomena in malaria and speculate on how this body of work can now be applied to Babesia parasites. Such information is fundamental if we are to learn more about the biology of Babesia parasites, the cellular and molecular mechanisms by which they cause infection and disease and how to develop novel therapeutic strategies or vaccines for both Babesia and malaria infections.

Murine malaria parasite sequestration: CD36 is the major receptor, but cerebral pathology is unlinked to sequestration

Sequestration of malaria-parasite-infected erythrocytes in the microvasculature of organs is thought to be a significant cause of pathology. Cerebral malaria (CM) is a major complication of Plasmodium falciparum infections, and PfEMP1-mediated sequestration of infected red blood cells has been considered to be the major feature leading to CM-related pathology. We report a system for the real-time in vivo imaging of sequestration using transgenic luciferase-expressing parasites of the rodent malaria parasite Plasmodium berghei. These studies revealed that: (i) as expected, lung tissue is a major site, but, unexpectedly, adipose tissue contributes significantly to sequestration, and (ii) the class II scavenger-receptor CD36 to which PfEMP1 can bind is also the major receptor for P. berghei sequestration, indicating a role for alternative parasite ligands, because orthologues of PfEMP1 are absent from rodent malaria parasites, and, importantly, (iii) cerebral complications still develop in the absence of CD36-mediated sequestration, dissociating parasite sequestration from CM-associated pathology. Real-time in vivo imaging of parasitic processes may be used to evaluate the molecular basis of pathology and develop strategies to prevent pathology.

The role of scavenger receptors in pathogen recognition and innate immunity

Scavenger receptors represent a large family of structurally unrelated distinct gene products, expressed by myeloid and selected endothelial cells and able to recognise modified low-density lipoproteins. They also bind and internalise a variety of microbial pathogens, as well as modified or endogenous molecules derived from the host, and contribute to a range of physiological or pathological processes.

CD36-mediated nonopsonic phagocytosis of erythrocytes infected with stage I and IIA gametocytes of Plasmodium falciparum

Gametocytes, the sexual stages of malaria parasites (Plasmodium spp.) that are transmissible to mosquitoes, have been the focus of much recent research as potential targets for novel drug and vaccine therapies. However, little is known about the host clearance of gametocyte-infected erythrocytes (GEs). Using a number of experimental strategies, we found that the scavenger receptor CD36 mediates the uptake of nonopsonized erythrocytes infected with stage I and IIA gametocytes of Plasmodium falciparum by monocytes and culture-derived macrophages (Mphis). Light microscopy and immunofluorescence assays revealed that stage I and IIA gametocytes were readily internalized by monocytes and Mphis. Pretreating monocytes and Mphis with a monoclonal antibody that blocked CD36 resulted in a significant reduction in phagocytosis, as did treating GEs with low concentrations of trypsin to remove P. falciparum erythrocyte membrane protein 1 (PfEMP-1), a parasite ligand for CD36. Pretreating monocytes and Mphis with peroxisome proliferator-activated receptor gamma-retinoid X receptor agonists, which specifically upregulate CD36, resulted in a significant increase in the phagocytosis of GEs. Murine CD36 on mouse Mphis also mediated the phagocytosis of P. falciparum stage I and IIA gametocytes, as determined by receptor blockade with anti-murine CD36 monoclonal antibodies and the lack of uptake by CD36-null Mphis. These results indicate that phagocytosis of stage I and IIA gametocytes by monocytes and Mphis appears to be mediated to a large extent by the interaction of PfEMP-1 and CD36, suggesting that CD36 may play a role in innate clearance of these early sexual stages.

Identification of CD226 ligand on colo205 cell surface

AIM: To confirm the existence of CD226 ligand and its distribution, which is a novel molecule that was cloned in 1996.

METHODS: The mRNA was extracted from TPA activated Jurkat cells and used as a template for reverse-transcription. After PCR amplification, the fragment including CD226 extracellular region and the splice donor sequence “ACTTACCTGT” was obtained and cloned into fusion expression vector pIG. The recombinant vector pCD226/Ig was transfected in COS-7 cells by DEAE-Dextran method, the secreting fusion protein was identified by Sandwich ELISA, and was purified by anti-CD226 affinity chromatography. This fusion protein was used as a probe in the investigation of CD226 ligand by immunohistochemistry. Existence of CD226 ligand was further identified by adhesion experiment.

RESULTS: Expression of a secreting fusion protein was identified by sandwich ELISA, indicating that both CD226 extracellular domain and IgGFc domain could be recognized respectively by anti-CD226 and anti-hIgFc mAb. About 130 μg CD226/Ig fusion protein could be obtained from 100 mL COS-7 culture supernatants by anti-CD226 affinity chromatography purification. SDS-PAGE showed that this fusion protein has a molecular mass of 83 ku. It was confirmed by immunohistochemistry that CD226 ligand expressed on the Colo205 cells, but not on Jurkat cell, U937 cell and mixed lymphocyte culture cells. In adhesive assay, resting Jurkat cells did not have significant adhesion to Colo205 cells. In contrast, activated Jurkat cells could bind to colon carcinoma Colo205 cells and this adhesive reaction could be blocked by CD226/Ig fusion protein or anti-CD226 mAb. Immunochemical experiment showed that Colo205 cells could be specifically stained by CD226/Ig, indicating that CD226 ligand exists on the surface of Colo205 cells.

CONCLUSION: Existence of CD226 ligand on the surface of Colo205 cells was identified by immunohistochemistry and adhesion blocking experiment. In addition, the secreting CD226/Ig fusion protein prepared in this study will be a potential tool for further investigation of CD226 ligand.

Cytoadherence of Plasmodium falciparum-infected erythrocytes is mediated by a redox-dependent conformational fraction of CD36

The adherence of Plasmodium falciparum-infected RBC (IRBC) to postcapillary venular endothelium is an important determinant of the pathogenesis of severe malaria complications. Cytoadherence of IRBC to endothelial cells involves specific receptor/ligand interactions. The glycoprotein CD36 expressed on endothelial cells is the major receptor involved in this interaction. Treatment of CD36-expressing cells with reducing agents, such as DTT and N-acetylcysteine, was followed by CD36 conformational change monitorable by the appearance of the Mo91 mAb epitope. Only a fraction of the surface expressed CD36 molecules became Mo91 positive, suggesting the presence of two subpopulations of molecules with different sensitivities to reduction. The Mo91 epitope has been localized on a peptide (residues 260-279) of the C-terminal, cysteine-rich region of CD36. Treatment with reducing agents inhibited the CD36-dependent cytoadherence of IRBC to CD36-expressing cells and dissolved pre-existent CD36-mediated IRBC/CD36-expressing cell aggregates. CD36 reduction did not impair the functionality of CD36, since the reactivity of other anti-CD36 mAbs as well as the binding of oxidized low density lipoprotein, a CD36 ligand, were maintained. The modifications induced by reduction were reversible. After 14 h CD36 was reoxidized, the cells did not express the Mo91 epitope, and cytoadherence to IRBC was restored. The results indicate that IRBCs bind only to a redox-modulated fraction of CD36 molecules expressed on the cell surface. The present data indicate the therapeutic potential of reducing agents, such as the nontoxic drug N-acetylcysteine, to prevent or treat malaria complications due to IRBC cytoadhesion.

Peroxisome proliferator-activated receptor gamma-retinoid X receptor agonists increase CD36-dependent phagocytosis of Plasmodium falciparum-parasitized erythrocytes and decrease malaria-induced TNF-alpha secretion by monocytes/macrophages

Severe and fatal malaria is associated with the failure of host defenses to control parasite replication, excessive secretion of proinflammatory cytokines such as TNF-alpha, and sequestration of parasitized erythrocytes (PEs) in vital organs. The identification of CD36 as a major sequestration receptor has led to the assumption that it contributes to the pathophysiology of severe malaria and has prompted the development of antiadherence therapies to disrupt the CD36-PE interaction. This concept has been challenged by unexpected evidence that individuals deficient in CD36 are more susceptible to severe and cerebral malaria. In this study, we demonstrate that CD36 is the major receptor mediating nonopsonic phagocytosis of PEs by macrophages, a clearance mechanism of potential importance in nonimmune hosts at the greatest risk of severe malaria. CD36-mediated uptake of PEs occurs via a novel pathway that does not involve thrombospondin, the vitronectin receptor, or phosphatidylserine recognition. Furthermore, we show that proliferator-activated receptor gamma-retinoid X receptor agonists induce an increase in CD36-mediated phagocytosis and a decrease in parasite-induced TNF-alpha secretion. Specific up-regulation of monocyte/macrophage CD36 may represent a novel therapeutic strategy to prevent or treat severe malaria.

Characterization of peroxynitrite-oxidized low density lipoprotein binding to human CD36

Peroxynitrite-mediated oxidation may be an important physiological mechanism for oxidation of low density lipoprotein (LDL), however, the molecular basis for the interaction of peroxynitrite oxidized LDL (OxLDL) with scavenger receptors such as CD36, has not been characterized. In this study, we compared the biochemical characteristics and receptor binding of LDL that was oxidized using: (1) Cu2+, a standard method of oxidizing LDL in vitro; and (2) 3-morpholinosydnonimine (SIN-1), a source of peroxynitrite. Both methods of oxidation caused an increase in electrophoretic migration of LDL, but greater mobility was observed with Cu2+-OxLDL. In addition, greater fragmentation of apolipoprotein B was observed following Cu2+ oxidation than after SIN-1 oxidation. The levels of lipid peroxides and thiobarbituric acid reactive substances were similar after 20 h of oxidation by both methods, although the time-course was distinct. Cu2+ and SIN-1-OxLDL bound specifically to the macrophage scavenger receptor CD36 with high affinity. Binding of the 20 h SIN-1 treated LDL to CD36 was comparable to a 4 h Cu2+ modified LDL. The binding of Cu2+ and SIN-1-OxLDL to CD36 was similar under different biochemical conditions and modifications of the receptor, suggesting that OxLDL particles, generated by either method, bind to the same domain of CD36. The results demonstrate that SIN-1 produced an oxidized LDL particle that binds specifically to CD36 and suggests that peroxynitrite OxLDL may represent a more physiologically relevant model than Cu2+-OxLDL for studying the interactions of OxLDL with cells and lipoprotein receptors in vitro.

Nonopsonic monocyte/macrophage phagocytosis of Plasmodium falciparum–parasitized erythrocytes: a role for CD36 in malarial clearance

Plasmodium falciparum is the most lethal form of malaria and is increasing both in incidence and in its resistance to antimalarial agents. An improved understanding of the mechanisms of malarial clearance may facilitate the development of new therapeutic interventions. We postulated that the scavenger receptor CD36, an important factor in cytoadherence of P falciparum–parasitized erythrocytes (PEs), might also play a role in monocyte- and macrophage-mediated malarial clearance. Exposure of nonopsonized PEs to Fc receptor–blocked monocytes resulted in significant PE phagocytosis, accompanied by intense clustering of CD36 around the PEs. Phagocytosis was blocked 60% to 70% by monocyte pretreatment with monoclonal anti-CD36 antibodies but not by antibodies to αvβ3, thrombospondin, intercellular adhesion molecule-1, or platelet/endothelial cell adhesion molecule-1. Antibody-induced CD36 cross-linking did result in the early increase of surface CD11b expression, but there was no increase in, or priming for, tumor necrosis factor (TNF)-α secretion following either CD36 cross-linking or PE phagocytosis. CD36 clustering does support intracellular signaling: Antibody-induced cross-linking initiated intracellular tyrosine phosphorylation as well as extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) phosphorylation. Both broad-spectrum tyrosine kinase inhibition (genistein) and selective ERK and p38 MAPK inhibition (PD98059 and SB203580, respectively) reduced PE uptake to almost the same extent as CD36 blockade. Thus, CD36-dependent binding and signaling appears to be crucial for the nonopsonic clearance of PEs and does not appear to contribute to the increase in TNF-α that is prognostic of poor outcome in clinical malaria.

CD36 folding revealed by conformational epitope expression is essential for cytoadherence of Plasmodium falciparum-infected red blood cells

CD36 is a membrane glycoprotein and a putative scavenger receptor expressed by several cell types. In capillary endothelial cells, it mediates the adherence of erythrocytes infected with Plasmodium falciparum. The CD36 sequence contains two hydrophobic domains located at the amino-and carboxyl-termini of the protein, but the topology of this protein and the functional significance of these domains are still not clearly defined. We generated soluble CD36-IgG chimeric molecules by fusion of the extracellular domains of CD36 with human immunoglobulin domains. The construct containing the N-terminal hydrophobic domain of CD36 was completely retained intracellularly as membrane-associated molecule, suggesting that the N-terminal hydrophobic domain of the CD36 is a real transmembrane domain and that CD36 has hairpin topology. A small amount of the CD36-IgG chimeric construct lacking both transmembrane domains escaped retention, was correctly processed, and accumulated in the extracellular medium as a soluble molecule. This CD36-IgG construct failed to bind Plasmodium falciparum-infected erythrocytes. Using monoclonal antibodies specific for either conformational or structural epitopes, we demonstrate that failure of this CD36-IgG construct to bind infected erythrocytes was due to incorrect folding of the soluble chimeric molecule.

Plasmodium chabaudi-infected erythrocytes adhere to CD36 and bind to microvascular endothelial cells in an organ-specific way

Adherence of erythrocytes infected with Plasmodium falciparum to microvascular endothelial cells (sequestration) is considered to play an important role in parasite virulence and pathogenesis. However, the real importance of sequestration for infection and disease has never been fully assessed. The absence of an appropriate in vivo model for sequestration has been a major barrier. We have examined the rodent malaria parasite Plasmodium chabaudi chabaudi AS in mice as a potential model. Erythrocytes infected with this parasite adhere in vitro to purified CD36, a critical endothelium receptor for binding P. falciparum-infected erythrocytes. P. c. chabaudi-infected erythrocytes adhere in vitro to endothelial cells in a gamma interferon-dependent manner, suggesting the involvement of additional adhesion molecules in the binding process, as is also the case with P. falciparum-infected cells. Furthermore, plasma or sera from infected and hyperimmune mice, respectively, have the ability to block binding of infected erythrocytes to endothelial cells. In vivo, erythrocytes containing mature P. c. chabaudi parasites are sequestered from the peripheral circulation. Sequestration is organ specific, occurring primarily in the liver, although intimate contact between infected erythrocytes and endothelial cells is also observed in the spleen and brain. The results are discussed in the context of the use of this model to study (i) the relationship between endothelial cell activation and the level of sequestration and (ii) the primary function of sequestration in malaria infection.

Inhibitory Activity of Human Lactoferrin and Its Peptide on Chondroitin Sulfate A-, CD36-, and Thrombospondin-Mediated Cytoadherence ofPlasmodium falciparum–Infected Erythrocytes

Lactoferrin (LF), a human serum protein, strongly inhibited the adherence of Plasmodium falciparum–infected erythrocytes (PE) to immobilized chondroitin sulfate A (CSA)–conjugated albumin at a concentration of 100 μg/mL and blocked the PE binding to CD36-expressing Chinese hamster ovary (CHO) cells, as well as immobilized CD36 at concentrations of 5 μg/mL and 100 μg/mL, respectively. Biotinylated LF bound to CD36 in a saturable manner, and such binding was inhibited by unlabeled LF and the anti-CD36 monoclonal antibody, 8A6, suggesting specificity of binding. Additionally, LF inhibited PE binding to immobilized thrombospondin (TSP) at a concentration of 100 μg/mL, and specific binding of LF to TSP was confirmed using biotinylated LF. LF inhibited PE binding to C32 amelanotic melanoma cells in a dose-dependent manner. A peptide of LF, Arg-Asn-Met Arg-Lys-Val Arg-Gly-Pro-Pro-Val-Ser-Cys (amino acid residues 25-37 of LF), which has been suggested to contribute to LF binding to various materials, including CSA, inhibited PE binding to immobilized CSA-conjugated albumin, immobilized CD36, CD36-expressing CHO cells, immobilized TSP, and C32 amelanotic melanoma cells, as well as LF itself. These results suggest that LF peptide may provide the basis for developing agents that are able to inhibit CSA-, CD36-, and TSP-mediated cytoadherence of PE.

Role of red blood cells in thrombosis

Most biomedical textbooks teach that coagulation and thrombosis are primarily a function of endothelial cells, platelets, and soluble coagulation factors. Red blood cells, in contrast, are generally regarded as innocent bystanders, passively entrapped in a developing thrombus as they flow through the vasculature. This review summarizes evidence that demonstrates an active role for red cells in normal and pathologic hemostasis. We then evaluate the possible molecular mechanisms whereby a usually inert erythrocyte can actively contribute to the processes of clot formation.