CD36 associates with CD9 and integrins on human blood platelets

Targeting Cysteine Oxidation in Thrombotic Disorders

Oxidative stress increases the risk for clinically significant thrombotic events, yet the mechanisms by which oxidants become prothrombotic are unclear. In this review, we provide an overview of cysteine reactivity and oxidation. We then highlight recent findings on cysteine oxidation events in oxidative stress-related thrombosis. Special emphasis is on the signaling pathway induced by a platelet membrane protein, CD36, in dyslipidemia, and by protein disulfide isomerase (PDI), a member of the thiol oxidoreductase family of proteins. Antioxidative and chemical biology approaches to target cysteine are discussed. Lastly, the knowledge gaps in the field are highlighted as they relate to understanding how oxidative cysteine modification might be targeted to limit thrombosis.

A CD36 transmembrane domain peptide interrupts CD36 interactions with membrane partners on macrophages and inhibits atherogenic functions

CD36 is a transmembrane glycoprotein receptor for oxidized low density lipoprotein (LDL) and other endogenous danger signals and promotes athero-thrombotic processes. CD36 has been shown to associate physically with other transmembrane proteins, including integrins, tetraspanins, and toll-like receptors, which modulate CD36-mediated cell signaling. The CD36 N-terminal transmembrane domain (nTMD) contains a GXXXG sequence motif that mediates protein-protein interactions in many membrane proteins. We thus hypothesized that the nTMD is involved in CD36 interactions with other membrane proteins. CD36 interactions with partner cell surface proteins on murine peritoneal macrophages were detected with an immunofluorescence-based proximity ligation cross linking assay (PLA) and confirmed by immunoprecipitation/immunoblot. Prior to performing these assays, cells were incubated with a synthetic 29 amino acid peptide containing the 22 amino acid of CD36 nTMD or a control peptide in which the glycine residues in GXXXG motif were replaced by valines. In functional experiments, macrophages were preincubated with peptides and then treated with oxLDL to assess LDL uptake, foam cell formation, ROS formation and cell migration. CD36 nTMD peptide treated cells compared to untreated or control peptide treated cells showed decreased CD36 surface associations with tetraspanin CD9 and ameliorated pathologically important CD36 mediated responses to oxLDL, including uptake of DiI-labeled oxLDL, foam cell formation, ROS generation, and inhibition of migration.

Counter Regulation of Tumor Angiogenesis by Vascular Endothelial Growth Factor and Thrombospondin-1

Considerable progress has been made in our understanding of the process of angiogenesis in the context of normal and tumor tissue over the last fifty years. Angiogenesis, like most physiological processes, is carefully controlled by dynamic and opposing effects of positive factors, such as vascular endothelial growth factor (VEGF), and negative factors, such as thrombospondin-1. In most cases, the progression of a small mass of cancerous cells to a life-threatening tumor depends upon the initiation of angiogenesis and involves the dysregulation of the angiogenic balance. Whereas our newfound appreciation for the role of angiogenesis in cancer has opened up new avenues for treatment, the success of these treatments, which have focused almost exclusively on antagonizing the VEGF pathway, has been limited to date. It is anticipated that this situation will improve as more therapeutics that target other pathways are developed, more strategies for combination therapies are advanced, more detailed stratification of patient populations occurs, and a better understanding of resistance to anti-angiogenic therapy is gained.

Oxidised Low-Density Lipoprotein-Induced Platelet Hyperactivity—Receptors and Signalling Mechanisms

Dyslipidaemia leads to proatherogenic oxidative lipid stress that promotes vascular inflammation and thrombosis, the pathologies that underpin myocardial infarction, stroke, and deep vein thrombosis. These prothrombotic states are driven, at least in part, by platelet hyperactivity, and they are concurrent with the appearancxe of oxidatively modified low-density lipoproteins (LDL) in the circulation. Modified LDL are heterogenous in nature but, in a general sense, constitute a prototype circulating transporter for a plethora of oxidised lipid epitopes that act as danger-associated molecular patterns. It is well-established that oxidatively modified LDL promote platelet activation and arterial thrombosis through a number of constitutively expressed scavenger receptors, which transduce atherogenic lipid stress to a complex array of proactivatory signalling pathways in the platelets. Stimulation of these signalling events underlie the ability of modified LDL to induce platelet activation and blunt platelet inhibitory pathways, as well as promote platelet-mediated coagulation. Accumulating evidence from patients at risk of arterial thrombosis and experimental animal models of disease suggest that oxidised LDL represents a tangible link between the dyslipidaemic environment and increased platelet activation. The aim of this review is to summarise recent advances in our understanding of the pro-thrombotic signalling events induced in platelets by modified LDL ligation, describe the contribution of individual platelet scavenger receptors, and highlight potential future challenges of targeting these pathways.

Endothelial Cell Behavior Is Determined by Receptor Clustering Induced by Thrombospondin-1

The thrombospondins (TSPs) are a family of multimeric extracellular matrix proteins that dynamically regulate cellular behavior and response to stimuli. In so doing, the TSPs directly and indirectly affect biological processes such as embryonic development, wound healing, immune response, angiogenesis, and cancer progression. Many of the direct effects of Thrombospondin 1 (TSP-1) result from the engagement of a wide range of cell surface receptors including syndecans, low density lipoprotein receptor-related protein 1 (LRP1), CD36, integrins, and CD47. Different or even opposing outcomes of TSP-1 actions in certain pathologic contexts may occur, depending on the structural/functional domain involved. To expedite response to external stimuli, these receptors, along with vascular endothelial growth factor receptor 2 (VEGFR2) and Src family kinases, are present in specific membrane microdomains, such as lipid rafts or tetraspanin-enriched microdomains. The molecular organization of these membrane microdomains and their constituents is modulated by TSP-1. In this review, we will describe how the presence of TSP-1 at the plasma membrane affects endothelial cell signal transduction and angiogenesis.

Unraveling the Molecular Signature of Extracellular Vesicles From Endometrial-Derived Mesenchymal Stem Cells: Potential Modulatory Effects and Therapeutic Applications

Endometrial-derived Mesenchymal Stem Cells (endMSCs) are involved in the regeneration and remodeling of human endometrium, being considered one of the most promising candidates for stem cell-based therapies. Their therapeutic effects have been found to be mediated by extracellular vesicles (EV-endMSCs) with pro-angiogenic, anti-apoptotic, and immunomodulatory effects. Based on that, the main goal of this study was to characterize the proteome and microRNAome of these EV-endMSCs by proteomics and transcriptomics approaches. Additionally, we hypothesized that inflammatory priming of endMSCs may contribute to modify the therapeutic potential of these vesicles. High-throughput proteomics revealed that 617 proteins were functionally annotated as Extracellular exosome (GO:0070062), corresponding to the 70% of the EV-endMSC proteome. Bioinformatics analyses allowed us to identify that these proteins were involved in adaptive/innate immune response, complement activation, antigen processing/presentation, negative regulation of apoptosis, and different signaling pathways, among others. Of note, multiplexed quantitative proteomics and Systems Biology analyses showed that IFNγ priming significantly modulated the protein profile of these vesicles. As expected, proteins involved in antigen processing and presentation were significantly increased. Interestingly, immunomodulatory proteins, such as CSF1, ERAP1, or PYCARD were modified. Regarding miRNAs expression profile in EV-endMSCs, Next-Generation Sequencing (NGS) showed that the preferred site of microRNAome targeting was the nucleus (n = 371 microTargets), significantly affecting signal transduction (GO:0007165), cell proliferation (GO:0008283), and apoptotic processes (GO:0006915), among others. Interestingly, NGS analyses highlighted that several miRNAs, such as hsa-miR-150-5p or hsa-miR-196b-5p, were differentially expressed in IFNγ-primed EV-endMSCs. These miRNAs have a functional involvement in glucocorticoid receptor signaling, IL-6/8/12 signaling, and in the role of macrophages. In summary, these results allowed us to understand the complexity of the molecular networks in EV-endMSCs and their potential effects on target cells. To our knowledge, this is the first comprehensive study based on proteomic and genomic approaches to unravel the therapeutic potential of these extracellular vesicles, that may be used as immunomodulatory effectors in the treatment of inflammatory conditions.

Uroplakins play conserved roles in egg fertilization and acquired additional urothelial functions during mammalian divergence

Uroplakin (UP) tetraspanins and their associated proteins are major mammalian urothelial differentiation products that form unique two-dimensional crystals of 16-nm particles (“urothelial plaques”) covering the apical urothelial surface. Although uroplakins are highly expressed only in mammalian urothelium and are often referred to as being urothelium specific, they are also expressed in several mouse nonurothelial cell types in stomach, kidney, prostate, epididymis, testis/sperms, and ovary/oocytes. In oocytes, uroplakins colocalize with CD9 on cell-surface and multivesicular body-derived exosomes, and the cytoplasmic tail of UPIIIa undergoes a conserved fertilization-dependent, Fyn-mediated tyrosine phosphorylation that also occurs in Xenopus laevis eggs. Uroplakin knockout and antibody blocking reduce mouse eggs’ fertilization rate in in vitro fertilization assays, and UPII/IIIa double-knockout mice have a smaller litter size. Phylogenetic analyses showed that uroplakin sequences underwent significant mammal-specific changes. These results suggest that, by mediating signal transduction and modulating membrane stability that do not require two-dimensional-crystal formation, uroplakins can perform conserved and more ancestral fertilization functions in mouse and frog eggs. Uroplakins acquired the ability to form two-dimensional-crystalline plaques during mammalian divergence, enabling them to perform additional functions, including umbrella cell enlargement and the formation of permeability and mechanical barriers, to protect/modify the apical surface of the modern-day mammalian urothelium.

Tetraspanin CD9: A Key Regulator of Cell Adhesion in the Immune System

The tetraspanin CD9 is expressed by all the major subsets of leukocytes (B cells, CD4⁺ T cells, CD8⁺ T cells, natural killer cells, granulocytes, monocytes and macrophages, and immature and mature dendritic cells) and also at a high level by endothelial cells. As a typical member of the tetraspanin superfamily, a prominent feature of CD9 is its propensity to engage in a multitude of interactions with other tetraspanins as well as with different transmembrane and intracellular proteins within the context of defined membranal domains termed tetraspanin-enriched microdomains (TEMs). Through these associations, CD9 influences many cellular activities in the different subtypes of leukocytes and in endothelial cells, including intracellular signaling, proliferation, activation, survival, migration, invasion, adhesion, and diapedesis. Several excellent reviews have already covered the topic of how tetraspanins, including CD9, regulate these cellular processes in the different cells of the immune system. In this mini-review, however, we will focus particularly on describing and discussing the regulatory effects exerted by CD9 on different adhesion molecules that play pivotal roles in the physiology of leukocytes and endothelial cells, with a particular emphasis in the regulation of adhesion molecules of the integrin and immunoglobulin superfamilies.

Microarray Integrated Analysis of a Gene Network for the CD36 Myocardial Phenotype

CD36 is a multifunctional membrane-type receptor glycoprotein that reacts with oxidized low-density lipoprotein and long-chain fatty acid (LCFA). However, much remains to be understood about the molecular mechanism of the cardio-myopathy observed in CD36-KO mice. In this study, we identify different genes pathways involved in response to CD36 cardio-myopathy phenotype by identifying the differences among biological processes, molecular pathways and networks of interactions that emerge from knocking CD3 and using different bioinformatics tools such as STRING, GeneMANIA and Cytoscape. We were able list all the CD36-regulated genes, their related function and their specific networks. Data analysis showed that CD36-regulated genes differentially expressed are involved in biological processes such as FA metabolism, angiogenesis/apoptosis and cell structure. These results provide the first look at mechanisms involved in CD36 deficiency and development of cardio-myopathy and the opportunity to identify new therapeutic targets.

Anxiety leads to up-regulation of CD36 on the monocytes of chronic hepatitis B-infected patients

Introduction It has been hypothesized that mental disorders including depression and anxiety can affect immune responses. The study was done to evaluate the relation between depression and anxiety and expression levels of CD36, CD68, and CD9 on peripheral blood monocytes of chronic hepatitis B (CHB) patients. Methods Sixty CHB patients were selected with various ranges of depression and anxiety. Depression and anxiety were evaluated using a standard questionnaire by an expert psychiatrist according to BECK's Depression Inventory II and Hamilton Anxiety Rating Scale, respectively. The levels of CD36, CD68, and CD9 on the peripheral blood monocytes have been performed using flow cytometry technique. Results The results demonstrated that levels of CD36 were significantly increased on the peripheral blood monocytes of CHB patients when compared with CHB patients with no anxiety. Expression levels of CD68 and CD9 were not significantly altered on the CHB patients with various ranges of anxiety. Expression levels of CD36, CD68, and CD9 were also not significantly altered on the CHB patients with various ranges of depression. Discussion It seems that anxiety induces inflammation in the CHB patients by induction of alteration in several molecules including up-regulation of CD36. CD36 plays important roles in the induction of tissue damage; hence, it may be hypothesized that anxiety may participate in the induction of some hepatitis B complications.

Transcriptomic signatures in whole blood of patients who acquire a chronic inflammatory response syndrome (CIRS) following an exposure to the marine toxin ciguatoxin

Background

Ciguatoxins (CTXs) are polyether marine neurotoxins found in multiple reef-fish species and are potent activators of voltage-gated sodium channels. It is estimated that up to 500,000 people annually experience acute ciguatera poisoning from consuming toxic fish and a small percentage of these victims will develop a chronic, multisymptom, multisystem illness, which can last years, termed a Chronic Inflammatory Response Syndrome (CIRS). Symptoms of ciguatera CIRS include fatigue, cognitive deficits, neurologic deficits, pain and sensitivity to light. There are few treatment options for ciguatera CIRS since little is known about its pathophysiology.

Methods

This study characterizes the transcriptional profile in whole blood of 11 patients with ciguatera-induced CIRS and 11 normal controls run in duplicate using Agilent one color whole genome microarrays. Differential expression was determined by using a combination of moderated t-test p-value and fold change (FC). Significant genes were subjected to gene ontology, principal component analysis and SVM classification. Seven significant genes found by microarray were validated by PCR.

Results

Using a low stringency (p < 0.05 and FC > 1.4) and a high stringency (p < 0.01 and FC > 1.5) filter, the resulting gene sets of 185 and 55, respectively, showed clear separation of cases and controls by PCA as well as 100% classification accuracy by SVM, indicating that the gene profiles can separate patients from controls. PCR results of 7 genes showed a 95% correlation to microarray data. Several genes identified by microarray are important in wound healing (CD9, CD36, vWF and Factor XIII), adaptive immunity (HLA-DQB1, DQB2, IL18R1 and IL5RA) and innate immunity (GZMK, TOLLIP, SIGIRR and VIPR2), overlapping several areas shown to be disrupted in a mouse model of acute exposure to ciguatoxin. Another area of interest was differential expression of long, non-coding sequences, or lncRNA.

Conclusions

Disruptions of innate and adaptive immune mechanisms were recorded at both the genomic and proteomic level. A disruption in the HLA-T cell receptor axis could indicate HLA haplotype sensitivity for this chronic syndrome, as noted in many autoimmune conditions. Taken together, these indicators of illness provide additional insights into pathophysiology and potential therapies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12920-015-0089-x) contains supplementary material, which is available to authorized users.

Cancer stem cell-specific scavenger receptor CD36 drives glioblastoma progression

Glioblastoma (GBM) contains a self-renewing, tumorigenic cancer stem cell (CSC) population which contributes to tumor propagation and therapeutic resistance. While the tumor microenvironment is essential to CSC self-renewal, the mechanisms by which CSCs sense and respond to microenvironmental conditions are poorly understood. Scavenger receptors are a broad class of membrane receptors well characterized on immune cells and instrumental in sensing apoptotic cellular debris and modified lipids. Here, we provide evidence that CSCs selectively use the scavenger receptor CD36 to promote their maintenance using patient-derived CSCs and in vivo xenograft models. CD36 expression was observed in GBM cells in addition to previously described cell types including endothelial cells, macrophages, and microglia. CD36 was enriched in CSCs and was able to functionally distinguish self-renewing cells. CD36 was coexpressed with integrin alpha 6 and CD133, previously described CSC markers, and CD36 reduction resulted in concomitant loss of integrin alpha 6 expression, self-renewal, and tumor initiation capacity. We confirmed oxidized phospholipids, ligands of CD36, were present in GBM and found that the proliferation of CSCs, but not non-CSCs, increased with exposure to oxidized low-density lipoprotein. CD36 was an informative biomarker of malignancy and negatively correlated to patient prognosis. These results provide a paradigm for CSCs to thrive by the selective enhanced expression of scavenger receptors, providing survival, and metabolic advantages.

Regulation of AMPK activation by CD36 links fatty acid uptake to β-oxidation

Increases in muscle energy needs activate AMPK and induce sarcolemmal recruitment of the fatty acid (FA) translocase CD36. The resulting rises in FA uptake and FA oxidation are tightly correlated, suggesting coordinated regulation. We explored the possibility that membrane CD36 signaling might influence AMPK activation. We show, using several cell types, including myocytes, that CD36 expression suppresses AMPK, keeping it quiescent, while it mediates AMPK activation by FA. These dual effects reflect the presence of CD36 in a protein complex with the AMPK kinase LKB1 (liver kinase B1) and the src kinase Fyn. This complex promotes Fyn phosphorylation of LKB1 and its nuclear sequestration, hindering LKB1 activation of AMPK. FA interaction with CD36 dissociates Fyn from the protein complex, allowing LKB1 to remain cytosolic and activate AMPK. Consistent with this, CD36(-/-) mice have constitutively active muscle and heart AMPK and enhanced FA oxidation of endogenous triglyceride stores. The molecular mechanism described, whereby CD36 suppresses AMPK, with FA binding to CD36 releasing this suppression, couples AMPK activation to FA availability and would be important for the maintenance of cellular FA homeostasis. Its dysfunction might contribute to the reported association of CD36 variants with metabolic complications of obesity in humans.

CD36, a scavenger receptor implicated in atherosclerosis

CD36 is a membrane glycoprotein that is present on various types of cells, including monocytes, macrophages, microvascular endothelial cells, adipocytes and platelets. Macrophage CD36 participates in atherosclerotic arterial lesion formation through its interaction with oxidized low-density lipoprotein (oxLDL), which triggers signaling cascades for inflammatory responses. CD36 functions in oxLDL uptake and foam cell formation, which is the initial critical stage of atherosclerosis. In addition, oxLDL via CD36 inhibits macrophage migration, which may be a macrophage-trapping mechanism in atherosclerotic lesions. The role of CD36 was examined in in vitro studies and in vivo experiments, which investigated various functions of CD36 in atherosclerosis and revealed that CD36 deficiency reduces atherosclerotic lesion formation. Platelet CD36 also promotes atherosclerotic inflammatory processes and is involved in thrombus formation after atherosclerotic plaque rupture. Because CD36 is an essential component of atherosclerosis, defining the function of CD36 and its corresponding signaling pathway may lead to a new treatment strategy for atherosclerosis.

Transient lesion in the splenium of the corpus callosum in acute uncomplicated falciparum malaria

Patients with acute uncomplicated Plasmodium falciparum malaria have no evident neurologic disorder, vital organ dysfunction, or other severe manifestations of infection. Nonetheless, parasitized erythrocytes cytoadhere to the endothelium throughout their microvasculature, especially within the brain. We aimed to determine if 3 Tesla magnetic resonance imaging studies could detect evidence of cerebral abnormalities in these patients. Within 24 hours of admission, initial magnetic resonance imaging examinations found a lesion with restricted water diffusion in the mid-portion of the splenium of the corpus callosum of 4 (40%) of 10 male patients. The four patients who had a splenial lesion initially had evidence of more severe hemolysis and thrombocytopenia than the six patients who had no apparent abnormality. Repeat studies four weeks later found no residua of the lesions and resolution of the hematologic differences. These observations provide evidence for acute cerebral injury in the absence of severe or cerebral malaria.

Members of the thrombospondin gene family bind stromal interaction molecule 1 and regulate calcium channel activity

The thrombospondins (TSPs) are a family of matricellular proteins that regulate cellular phenotype through interactions with a myriad of other proteins and proteoglycans. We have identified a novel interaction of the members of the TSP gene family with stromal interaction molecule 1 (STIM1). This association is robust since it is preserved in Triton X-100, can be detected with multiple anti-TSP-1 and anti-STIM1 antibodies, and is detected in a wide range of cell types. We have also found that STIM1 co-immunoprecipitates with TSP-4 and cartilage oligomeric matrix protein (COMP), and that a recombinant version of the N-terminal domain of STIM1 binds to the signature domain of TSP-1 and COMP. The association of the TSPs with STIM1 is observed in both the presence and absence of calcium indicating that the calcium-dependent conformation of the signature domain of TSPs is not required for binding. Thus, this interaction could occur in the ER under conditions of normal or low calcium concentration. Furthermore, we observed that the expression of COMP in HEK 293 cells decreases STIM1-mediated calcium release activated calcium (CRAC) channel currents and increases arachidonic acid calcium (ARC) channel currents. These data indicate that the TSPs regulate STIM1 function and participate in the reciprocal regulation of two channels that mediate calcium entry into the cell.

The value of flow cytometry in the measurement of platelet activation and aggregation in human immunodeficiency virus infection

Human immunodeficiency deficiency virus (HIV) infection is associated with chronic inflammation and an increased risk of thrombotic events. Activated platelets (PLTs) play an important role in both thrombosis and inflammation, and HIV has been shown to induce PLT activation by both direct and indirect mechanisms. P-selectin (CD62P) is a well-described marker of PLT activation, and PLT glycoprotein (GP) IV (CD36) has been identified as a marker of PLT aggregation. Data on PLT function in the context of HIV infection remain inconclusive. Laboratory techniques, such as flow cytometry, enable the assessment of PLTs in their physiological state and environment, with minimal artifactual in vitro activation and aggregation. In this study, we describe a novel flow cytometry PLT assay, which enabled the measurement of PLT function in HIV infection. Forty-one antiretroviral-naïve HIV-positive individuals and 41 HIV-negative controls were recruited from a clinic in the Western Cape. Platelet function was evaluated by assessing the response of platelets to adenosine diphosphate (ADP) at two concentrations (0.04 mM, 0.2 mM). The percentage expression and mean fluorescence intensity (MFI) of CD62P and CD36 was used to evaluate platelet function. These were then correlated with platelet (PLT) count; CD4 count; % CD38/8; viral load and D-dimers. The % CD62P levels were higher in HIV-positive patients (HIV % CD62P 11.33[5.96-29.36] vs. control 2.48[1.56-6.04]; p < 0.0001). In addition, the HIV group showed higher CD62P MFI levels (HIV CD62P MFI 3.25 ± 7.23 vs. control 2.35 ± 1.31, p = 0.0292). Baseline levels of %CD36 expression were significantly higher in HIV-positive patients (%CD36 12.41[6.31-21.83] vs. control 6.04[1.34-13.15]; p = 0.0091). However, the baseline CD36MFI showed no significant difference between the two groups (HIV CD36 MFI 3.09 ± 0.64 vs. control 2.44 ± 0.11, p = 0.4591). The HIV group showed higher levels of % CD36 expression post stimulation with 0.04 mM ADP 43.32 ± 27.41 vs. control 27.47 ± 12.95; p < 0.0214) and no significant difference at 0.2 mM ADP (HIV % CD36 39.06 ± 17.91 vs. control 44.61 ± 18.76; p = 0.3277). Furthermore, the HIV group showed a single phase response to ADP as compared to the control group, which showed a normal biphasic response. We concluded that PLT flow cytometry is valuable in the assessment of levels of PLT activation, and further, that the addition of an endogenous agonist, such as ADP, enabled the measurement of PLT function in HIV infection. We were able to show that, although PLTs are significantly activated in HIV compared to uninfected controls, they retain their functional capacity.

The effect of albumin on podocytes: the role of the fatty acid moiety and the potential role of CD36 scavenger receptor

Evidence is emerging that podocytes are able to endocytose proteins such as albumin using kinetics consistent with a receptor-mediated process. To date the role of the fatty acid moiety on albumin uptake kinetics has not been delineated and the receptor responsible for uptake is yet to be identified. Albumin uptake studies were carried out on cultured human podocytes exposed to FITC-labelled human serum albumin either carrying fatty acids (HSA+FA) or depleted of them (HSA-FA). Receptor-mediated endocytosis of FITC-HSA+FA over 60 min was 5 times greater than that of FITC-HSA-FA. 24h exposure of podocytes to albumin up-regulated nephrin expression and induced the activation of caspase-3. These effects were more pronounced in response to HSA-FA. Individually, anti-CD36 antibodies had no effect upon endocytosis of FITC-HSA. However, a cocktail of 2 antibodies reduced uptake by nearly 50%. Albumin endocytosis was enhanced in the presence of the CD36 specific inhibitor sulfo-N-succinimidyl oleate (SSO) while knock-down of CD36 using CD36siRNA had no effect on uptake. These data suggest that receptor-mediated endocytosis of albumin by podocytes is regulated by the fatty acid moiety, although, some of the detrimental effects are induced independently of it. CD36 does not play a direct role in the uptake of albumin.

Evidence of promiscuous endothelial binding by Plasmodium falciparum-infected erythrocytes

The adhesion of infected red blood cells (iRBCs) to human endothelium is considered a key event in the pathogenesis of cerebral malaria and other life‐threatening complications caused by the most prevalent malaria parasite Plasmodium falciparum. In the past 30 years, 14 endothelial receptors for iRBCs have been identified. Exposing 10 additional surface proteins of endothelial cells to a mixture of P. falciparum isolates from three Ghanaian malaria patients, we identified seven new iRBC receptors, all expressed in brain vessels. This finding strongly suggests that endothelial binding of P. falciparum iRBCs is promiscuous and may use a combination of endothelial surface moieties.

CD36-specific antibodies block release of HIV-1 from infected primary macrophages and its transmission to T cells

HIV-1-infected macrophages likely represent viral reservoirs, as they accumulate newly formed virions in internal virus-containing compartments (VCCs). However, the nature and biogenesis of VCCs remain poorly defined. We show that upon HIV-1 infection of primary human macrophages, Gag is recruited to preexisting compartments containing the scavenger receptor CD36, which then become VCCs. Silencing of CD36 in HIV-1-infected macrophages decreases the amount of virions released. Strikingly, soluble anti-CD36 antibodies, but not the natural ligands of CD36, inhibit release of virions from HIV-1-infected macrophages and the transmission of virus to CD4(+) T cells. The effect of the antibodies is potent, rapid, and induces the retention of virions within VCCs. Ectopic expression of CD36 in HeLa cells renders them susceptible to the inhibitory effect of the anti-CD36 mAb upon HIV-1 infection. We show that the anti-CD36 mAb inhibits HIV-1 release by clustering newly formed virions at their site of budding, and that signaling via CD36 is not required. Thus, HIV-1 reservoirs in macrophages may be tackled therapeutically using anti-CD36 antibodies to prevent viral dissemination.

CD36 recruits α₅β₁ integrin to promote cytoadherence of P. falciparum-infected erythrocytes

The adhesion of Plasmodium falciparum-infected erythrocytes (IRBC) to receptors on different host cells plays a divergent yet critical role in determining the progression and outcome of the infection. Based on our ex vivo studies with clinical parasite isolates from adult Thai patients, we have previously proposed a paradigm for IRBC cytoadherence under physiological shear stress that consists of a recruitment cascade mediated largely by P-selectin, ICAM-1 and CD36 on primary human dermal microvascular endothelium (HDMEC). In addition, we detected post-adhesion signaling events involving Src family kinases and the adaptor protein p130CAS in endothelial cells that lead to CD36 clustering and cytoskeletal rearrangement which enhance the magnitude of the adhesive strength, allowing adherent IRBC to withstand shear stress of up to 20 dynes/cm². In this study, we addressed whether CD36 supports IRBC adhesion as part of an assembly of membrane receptors. Using a combination of flow chamber assay, atomic force and confocal microscopy, we showed for the first time by loss- and gain-of function assays that in the resting state, the integrin α₅β₁ does not support adhesive interactions between IRBC and HDMEC. Upon IRBC adhesion to CD36, the integrin is recruited either passively as part of a molecular complex with CD36, or actively to the site of IRBC attachment through phosphorylation of Src family kinases, a process that is Ca²⁺-dependent. Clustering of β₁ integrin is associated with an increase in IRBC recruitment as well as in adhesive strength after attachment (∼40% in both cases). The adhesion of IRBC to a multimolecular complex on the surface of endothelial cells could be of critical importance in enabling adherent IRBC to withstand the high shear stress in the microcirculations. Targeting integrins may provide a novel approach to decrease IRBC cytoadherence to microvascular endothelium.

Of the several species of malaria parasites that infect humans, disease caused by Plasmodium falciparum is responsible for most of the deaths. The unique pathological finding of this infection is the intense adhesion of infected red blood cells (IRBC) in the microcirculation, resulting in obstruction to blood flow and organ dysfunction. The focus of our research is to identify the molecules on host endothelial cells that support the adhesion as potential therapeutic targets. In this report, we showed for the first time a functional association between CD36, a well studied adhesion molecule for parasite adhesion, and α₅β₁, a member of the integrin family of adhesion molecules that are important for adhesion of leukocytes to blood vessels and cell adhesion to the extracellular matrix. We found that by itself, α₅β₁ integrin does not support IRBC adhesion. When IRBC adhere to CD36, the integrin is recruited to the site of adhesion through activation of the Src family kinase signaling pathway. As a result, both the number of adherent IRBC and the strength with which they adhere is greatly increased. These results demonstrate that IRBC adhesion is a dynamic and complex process. We need to identify each of the functional components to design anti-adhesive therapy.

Thrombospondin-1 modulates VEGF signaling via CD36 by recruiting SHP-1 to VEGFR2 complex in microvascular endothelial cells

Thrombospondin-1 (TSP-1) inhibits growth factor signaling at the receptor level in microvascular endothelial cells (MVEC), and CD36 has been suggested to be involved in this inhibition, but the mechanisms are not known. We hypothesized that CD36-TSP-1 interaction recruits Src homology 2 domain-containing protein tyrosine phosphatase (SHP)-1 to the vascular endothelial growth factor receptor 2 (VEGFR2) signaling complex and attenuates vascular endothelial growth factor (VEGF) signaling. Western blots of anti-CD36 and anti-VEGFR2 immunoprecipitates from VEGF-treated MVEC showed that exposure of the cells to a recombinant protein containing the CD36 binding domain of thrombospondin-1 (known as the TSR domain) induced association of SHP-1 with the VEGFR2/CD36 signaling complex and thereby suppressed VEGFR2 phosphorylation. SHP-1 phosphatase activity was increased in immunoprecipitated VEGFR2 complexes from TSR-treated cells. Silencing CD36 expression in MVEC by small interfering RNA (siRNA) or genetic deletion of cd36 in mice showed that TSR-induced SHP-1/VEGFR2 complex formation required CD36 in vitro and in vivo. Silencing SHP-1 expression in MVEC by siRNA abrogated TSR-mediated inhibition of VEGFR2 phosphorylation as well as TSR-mediated inhibition of VEGF-induced endothelial cell migration and tube formation. These studies reveal a SHP-1-mediated antiangiogenic pathway induced by CD36-TSP-1 interaction that inhibits VEGFR2 signaling and they provide a novel target to modulate angiogenesis therapeutically.

The involvement of CD36 in monocyte activation by antiphospholipid antibodies

Background

CD36, known as a scavenger receptor, is a transmembrane glycoprotein expressed on monocytes, platelets and endothelial cells, recognizes multiple ligands, including phosphatidylserine, and regulates atherogenesis and thrombosis. The objective of this study is to investigate the possible involvement of CD36 in the pathophysiology of thrombosis in patients with antiphospholipid syndrome (APS).

Methods

First, rs3765187, a missense mutation linked to CD36 deficiency, was investigated by TaqMan polymerase chain reaction (PCR) genotyping method in 819 Japanese, including 132 patients with APS, 265 with systemic lupus erythematosus (SLE) in the absence of APS, and 422 healthy subjects. Then, the involvement of CD36 in antiphospholipid antibody (aPL)-induced tissue factor (TF) expression was examined using CD36-null mice or anti-CD36. Purified IgG from patients with APS and a monoclonal phosphatidylserine-dependent antiprothrombin antibody were used in these experiments. TF expression was tested by real-time PCR and flow cytometry.

Results

Minor allele carrier of rs3765187 was less frequent in patients with APS (3.8% p = 0.032), but not in patients with SLE in the absence of APS (7.9% p = 0.32), compared with healthy subjects (10.2%). The aPL-induced TF expression was significantly suppressed on peritoneal macrophages from CD36-null mice compared to wild type and significantly inhibited by anti-CD36 on human monocytes.

Conclusions

The gene mutation linked to CD36 deficiency was less frequent in patients with APS. The deficient or suppressed CD36 function significantly reduced aPL-induced TF expression in vitro. Taken together, in a susceptible background CD36 scavenger receptor function may be involved in the thrombotic pathophysiology in patients with APS.

Multimolecular signaling complexes enable Syk-mediated signaling of CD36 internalization

CD36 is a versatile receptor known to play a central role in the development of atherosclerosis, the pathogenesis of malaria, and the removal of apoptotic cells. Remarkably, the short cytosolically exposed regions of CD36 lack identifiable motifs, which has hampered elucidation of its mode of signaling. Using a combination of phosphoprotein isolation, mass spectrometry, superresolution imaging, and gene silencing, we have determined that the receptor induces ligand internalization through a heteromeric complex consisting of CD36, β1 and/or β2 integrins, and the tetraspanins CD9 and/or CD81. This receptor complex serves to link CD36 to the adaptor FcRγ, which bears an immunoreceptor tyrosine activation motif. By coupling to FcRγ, CD36 is able to engage Src-family kinases and Syk, which in turn drives the internalization of CD36 and its bound ligands.

Thermodynamic characterization of 3-[(3-cholamidopropyl)-dimethylammonium]-1-propanesulfonate (CHAPS) micellization using isothermal titration calorimetry: temperature, salt, and pH dependence

A systematic investigation of the micellization process of a biocompatible zwitterionic surfactant 3-[(3-cholamidopropyl)-dimethylammonium]-1-propanesulfonate (CHAPS) has been carried out by isothermal titration calorimetry (ITC) at temperatures between 278.15 K and 328.15 K in water, aqueous NaCl (0.1, 0.5, and 1 M), and buffer solutions (pH = 3.0, 6.8, and 7.8). The effect of different cations and anions on the micellization of CHAPS surfactant has been also examined in LiCl, CsCl, NaBr, and NaI solutions at 308.15 K. It turned out that the critical micelle concentration, cmc, is only slightly shifted toward lower values in salt solutions, whereas in buffer media it remains similar to its value in water. From the results obtained, it could be assumed that CHAPS behaves as a weakly charged cationic surfactant in salt solutions and as a nonionic surfactant in water and buffer medium. Conventional surfactants alike, CHAPS micellization is endothermic at low and exothermic at high temperatures, but the estimated enthalpy of micellization, ΔHM0, is considerably lower in comparison with that obtained for ionic surfactants in water and NaCl solutions. The standard Gibbs free energy, ΔGM0, and entropy, ΔSM0, of micellization were estimated by fitting the model equation based on the mass action model to the experimental data. The aggregation numbers of CHAPS surfactant around cmc, obtained by the fitting procedure also, are considerably low (nagg ≈ 5 ± 1). Furthermore, some predictions about the hydration of the micelle interior based on the correlation between heat capacity change, Δcp,M0, and changes in solvent-accessible surface upon micelle formation were made. CHAPS molecules are believed to stay in contact with water upon aggregation, which is somehow similar to the micellization process of short alkyl chain cationic surfactants.

CD36 inhibitors reduce postprandial hypertriglyceridemia and protect against diabetic dyslipidemia and atherosclerosis

CD36 is recognized as a lipid and fatty acid receptor and plays an important role in the metabolic syndrome and associated cardiac events. The pleiotropic activity and the multiple molecular associations of this scavenger receptor with membrane associated molecules in different cells and tissues have however questioned its potential as a therapeutic target. The present study shows that it is possible to identify low molecular weight chemicals that can block the CD36 binding and uptake functions. These inhibitors were able to reduce arterial lipid deposition, fatty acid intestinal transit, plasma concentration of triglycerides and glucose, to improve insulin sensitivity, glucose tolerance and to reduce the plasma concentration of HbAc1 in different and independent rodent models. Correlation between the anti-CD36 activity of these inhibitors and the known pathophysiological activity of this scavenger receptor in the development of atherosclerosis and diabetes were observed at pharmacological doses. Thus, CD36 might represent an attractive therapeutic target.

Multicolor flow cytometry for evaluation of platelet surface antigens and activation markers

INTRODUCTION

Flow cytometry allows the analysis of multiple antigens in a single tube at a single cell level. We present a rapid and sensitive two tube flow cytometric protocol for the detection of multiple platelet antigens and activation markers gated on a pure platelet population.

MATERIALS AND METHODS

The presence of platelet specific antigens was analyzed in citrated whole blood of normal platelets and from patients diagnosed with platelet abnormalities. Quiescent platelets as well as stimulated platelets were analyzed using a gating strategy based on ubiquitously expressed platelet membrane markers. A ubiquitously expressed platelet marker was combined with antibodies against the activated alpha2b-beta3 (PAC-1), Lysosomal Activated Membrane Protein (CD63) and P-selectin (CD62P).

RESULTS

We were able to detect the platelet antigens CD36, CD41, CD42a, CD42b and CD61 in one single tube. Our approach allowed the single tube determination of PAC-1, CD63 and CD62P after activation of platelets by thrombin, collagen, ADP and PAR-1, and determination of platelet abnormalities.

CONCLUSIONS

Our two tube multi-parameter screening protocol is suited for the analysis of platelet antigens expressed on quiescent and activated platelets and allows the detection of aberrancies as found in blood of patients with thrombocytopathy such as Glanzmann Thrombasthenia, storage pool disease with diminished granule content and patients treated with clopidogrel and acetylsalicylic acid.

CD9 tetraspanin interacts with CD36 on the surface of macrophages: a possible regulatory influence on uptake of oxidized low density lipoprotein

CD36 is a type 2 scavenger receptor with multiple functions. CD36 binding to oxidized LDL triggers signaling cascades that are required for macrophage foam cell formation, but the mechanisms by which CD36 signals remain incompletely understood. Mass spectrometry analysis of anti-CD36 immuno-precipitates from macrophages identified the tetraspanin CD9 as a CD36 interacting protein. Western blot showed that CD9 was precipitated from mouse macrophages by anti-CD36 monoclonal antibody and CD36 was likewise precipitated by anti-CD9, confirming the mass spectrometry results. Macrophages from cd36 null mice were used to demonstrate specificity. Membrane associations of the two proteins on intact cells was analyzed by confocal immunofluorescence microscopy and by a novel cross linking assay that detects proteins in close proximity (<40 nm). Functional significance was determined by assessing lipid accumulation, foam cell formation and JNK activation in wt, cd9 null and cd36 null macrophages exposed to oxLDL. OxLDL uptake, lipid accumulation, foam cell formation, and JNK phosphorylation were partially impaired in cd9 null macrophages. The present study demonstrates that CD9 associates with CD36 on the macrophage surface and may participate in macrophage signaling in response to oxidized LDL.

Signaling role of CD36 in platelet activation and thrombus formation on immobilized thrombospondin or oxidized low-density lipoprotein

BACKGROUND AND OBJECTIVE

Platelets abundantly express glycoprotein CD36 with thrombospondin-1 (TSP1) and oxidized low-density lipoprotein (oxLDL) as proposed ligands. How these agents promote platelet activation is still poorly understood.

METHODS AND RESULTS

Both TSP1 and oxLDL caused limited activation of platelets in suspension. However, immobilized TSP1 and oxLDL, but not LDL, strongly supported platelet adhesion and spreading with a major role of CD36. Platelet spreading was accompanied by potent Ca(2+) rises, and resulted in exposure of P-selectin and integrin activation, all in a CD36-dependent manner with additional contributions of α(IIb) β(3) and ADP receptor stimulation. Signaling responses via CD36 involved activation of the protein tyrosine kinase Syk. In whole blood perfusion, co-coating of TSP1 or oxLDL with collagen enhanced thrombus formation at high-shear flow conditions, with increased expression on platelets of activated α(IIb) β(3), P-selectin and phosphatidylserine, again in a CD36-dependent way.

CONCLUSIONS

Immobilized TSP1 and oxLDL activate platelets partly via CD36 through a Syk kinase-dependent Ca(2+) signaling mechanism, which enhances collagen-dependent thrombus formation under flow. These findings provide novel insight into the role of CD36 in hemostasis.

Tetraspanin microdomains: fine-tuning platelet function

Platelets are crucial for preventing excessive blood loss at sites of injury by plugging holes in damaged blood vessels through thrombus formation. Platelet thrombi can, however, cause heart attack or stroke by blocking diseased vessels upon rupture of atherosclerotic plaques. Current anti-platelet therapy is not effective in all patients and carries a risk of bleeding. As such, a major goal in platelet research is to identify new drug targets to specifically inhibit platelets in disease processes. Tetraspanins are potential candidates because of their capacity to regulate other proteins in microdomains, and their defined roles in cell adhesion and signalling. In the last 6 years, analyses of tetraspanin-deficient mice have suggested that tetraspanins are indeed important for fine-tuning platelet responses. The future characterization of novel regulatory mechanisms in tetraspanin microdomains may lead to new drug targets for the prevention and treatment of heart attack and stroke.

Priming of the vascular endothelial growth factor signaling pathway by thrombospondin-1, CD36, and spleen tyrosine kinase

CD36 plays a critical role in the inhibition of angiogenesis through binding to the type 1 repeats of thrombospondin-1 (TSP-1) and activating Fyn tyrosine kinase and MAPK pathways. Here, we reveal a novel association of CD36 with VEGFR-2 and spleen tyrosine kinase (Syk). We also address the correlation between the expression of CD36 and Syk by demonstrating that overexpression of CD36 in HUVECs up-regulates endogenous Syk expression. We also define a new role for TSP-1 and CD36 in the activation of the VEGFR-2 signaling pathway that requires Syk. Our findings also identify a role for Syk as a stimulator of VEGF-A-induced angiogenesis by increasing phosphorylation of Y1175 in VEGFR-2, which is a major tyrosine for promoting VEGF-A-induced endothelial cell migration. Together, these studies introduce a new signaling pathway for TSP-1, CD36, and Syk, and address the role of these proteins in regulating the angiogenic switch.

Regulation of platelet function by class B scavenger receptors in hyperlipidemia

Platelets constitutively express class B scavenger receptors CD36 and SR-BI, 2 closely related pattern recognition receptors best known for their roles in lipoprotein and lipid metabolism. The biological role of scavenger receptors in platelets is poorly understood. However, in vitro and in vivo data suggest that class B scavenger receptors modulate platelet function and contribute significantly to thrombosis by sensing pathological or physiological ligands, inducing prothrombotic signaling, and increasing platelet reactivity. Platelet CD36 recognizes a novel family of endogenous oxidized choline phospholipids that accumulate in plasma of hyperlipidemic mice and in plasma of subjects with low high-density lipoprotein levels. This interaction leads to the activation of specific signaling pathways and promotes platelet activation and thrombosis. Platelet SR-BI, on the other hand, plays a critical role in the induction of platelet hyperreactivity and accelerated thrombosis under conditions associated with increased platelet cholesterol content. Intriguingly, oxidized high-density lipoprotein, an SR-BI ligand, can suppress platelet function. These recent findings demonstrate that platelet class B scavenger receptors play roles in thrombosis in dyslipidemia and may contribute to acute cardiovascular events in vivo in hypercholesterolemia.

Type 2 scavenger receptor CD36 in platelet activation: the role of hyperlipemia and oxidative stress

Platelet hyper-reactivity and a systemic prothrombotic state are associated with atherosclerosis and other inflammatory conditions. CD36, a member of the Type 2 scavenger receptor family, is a multiligand pattern recognition receptor that recognizes specific oxidized phospholipids, molecules expressed on microbial pathogens, apoptotic cells, and cell-derived microparticles. Recent studies have demonstrated that CD36 binding to oxidized LDL or microparticles activates a specific signaling pathway that induces platelet activation. This pathway is activated in vivo in the setting of hyperlipidemia and oxidant stress. Genetic deletion of CD36 protects mice from pathological thrombosis associated with hyperlipidemia without any apparent effect on normal hemostasis. Targeting CD36 or its signaling pathway could potentially lead to the development of novel antithrombotic therapies for patients with atheroinflammatory disorders.

Uptake of oxidized low density lipoprotein by CD36 occurs by an actin-dependent pathway distinct from macropinocytosis

The class B scavenger receptor CD36 has numerous ligands that include modified forms of low density lipoprotein, fibrillar amyloid, apoptotic cells, and Plasmodium falciparum-infected red blood cells, linking this molecule to atherosclerosis, Alzheimer disease, malaria, and other diseases. We studied the signaling events that follow receptor engagement and lead to CD36 and ligand internalization. We show that oxidized low density lipoprotein or antibody-induced clustering of CD36 triggers macropinocytosis and internalization of the receptor-ligand complex. Remarkably, however, CD36 internalization is independent of macropinocytosis and occurs by a novel endocytic mechanism that depends on actin, but not dynamin. This actin-driven endocytosis requires the activation Src family kinases, JNK, and Rho family GTPases, but, unlike macropinocytosis, it is not affected by inhibitors of phosphatidylinositol 3-kinase or Na/H exchange. Manipulation of this unique mode of internalization may prove helpful in the prevention and management of the wide range of diseases in which CD36 is implicated.

Thrombospondin-1 modulates vascular endothelial growth factor activity at the receptor level

Vascular endothelial growth factor (VEGF) is a well-established stimulator of vascular permeability and angiogenesis, whereas thrombospondin-1 (TSP-1) is a potent angiogenic inhibitor. In this study, we have found that the TSP-1 receptors CD36 and beta1 integrin associate with the VEGF receptor 2 (VEGFR2). The coclustering of receptors that regulate angiogenesis may provide the endothelial cell with a platform for integration of positive and negative signals in the plane of the membrane. Thus, this complex may represent a molecular switch that regulates angiogenesis and determines endothelial cell behavior. In this context, physiological levels of TSP-1 appear to support VEGFR2 function on both the cellular and tissue level, because phosphorylation of VEGFR2 and vascular permeability in response to VEGF are decreased in TSP-1-null mice and isolated endothelial cells. A therapeutic agent based on the antiangiogenic domain of TSP-1, designated 3TSR (for three TSP-1 type 1 repeats), has significant antiangiogenic and antitumor efficacy. Systemic treatment of wild-type mice with 3TSR significantly decreased VEGF-induced permeability. Consistent with this result, VEGF-stimulated phosphorylation of VEGFR2 was also significantly decreased in lung extracts from 3TSR-treated mice. Moreover, 3TSR significantly decreased VEGF-stimulated VEGFR2 phosphorylation in human dermal microvascular endothelial cells in culture. Taken together, the results indicate that TSP-1 and 3TSR modulate the function of VEGFR2.

CD36, a scavenger receptor involved in immunity, metabolism, angiogenesis, and behavior

CD36 is a membrane glycoprotein present on platelets, mononuclear phagocytes, adipocytes, hepatocytes, myocytes, and some epithelia. On microvascular endothelial cells, CD36 is a receptor for thrombospondin-1 and related proteins and functions as a negative regulator of angiogenesis. On phagocytes, through its functions as a scavenger receptor recognizing specific oxidized phospholipids and lipoproteins, CD36 participates in internalization of apoptotic cells, certain bacterial and fungal pathogens, and modified low-density lipoproteins, thus contributing to inflammatory responses and atherothrombotic diseases. CD36 also binds long-chain fatty acids and facilitates their transport into cells, thus participating in muscle lipid utilization, adipose energy storage, and gut fat absorption and possibly contributing to the pathogenesis of metabolic disorders, such as diabetes and obesity. On sensory cells, CD36 is involved in insect pheromone signaling and rodent fatty food preference. The signaling pathways downstream of CD36 involve ligand-dependent recruitment and activation of nonreceptor tyrosine kinases, specific mitogen-activated protein kinases, and the Vav family of guanine nucleotide exchange factors; modulation of focal adhesion constituents; and generation of intracellular reactive oxygen species. CD36 in many cells is localized in specialized cholesterol-rich membrane microdomains and may also interact with other membrane receptors, such as tetraspanins and integrins. Identification of the precise CD36 signaling pathways in specific cells elicited in response to specific ligands may yield novel targets for drug development.

Thrombospondin-1 modulates VEGF-A-mediated Akt signaling and capillary survival in the developing retina

Microvascular development is often perceived to result from a balance of positive and negative factors that impact signaling for proliferation and survival. The survival signaling that results from hypoxia-induced VEGF-A has been well established, but the factors that antagonize this signaling have been poorly studied. As endogenous inhibitors of angiogenesis, thrombospondins (TSPs) are likely candidates to affect survival signaling. Here we report that TSP1 antagonized microvascular survival to retinal hyperoxia, and Akt signaling in both the retina and in cultured endothelial cells. TSP1 expression is correlated with the association of the CD36 receptor with Src versus Fyn. In the presence of TSP1, CD36 is coprecipitated with Fyn as previously shown by others. However, in the absence of TSP1, there is a preferential association with Src. We now demonstrate that these Src family kinases play an important role in modulating microvascular survival in response to TSP1 by crossing tsp1(-/-) mice to the src(-/-) and fyn(-/-) mice and testing the survival of retinal blood vessels in hyperoxia. We find that tsp1(-/-), fyn(-/-), and double-mutant tsp1(-/-)/fyn(-/-) mice have a similar enhancement of capillary survival in oxygen, whereas in a tsp(-/-) background, the loss of only one allele of src restores the balance in survival and apoptosis to that of wild-type mice. Taken together, we hypothesize that TSP1 antagonizes VEGF-driven Akt survival signaling in part through the recruitment of Fyn to membrane domains containing CD36, but when TSP1 is absent, an opposing Src recruitment contributes to VEGF-driven Akt phosphorylation and capillary survival.

Tetraspanins and vascular functions

Tetraspanins are multiple membrane-spanning proteins that likely function as the organizers of membrane microdomains. Tetraspanins associate with other membrane-bound molecules such as cell-adhesion proteins, growth factor receptors, and Ig superfamily members and regulate key cellular processes such as adhesion, migration, and fusion. Tetraspanins are widely expressed in vascular and haematopoietic cells and are involved in both physiological and pathological processes related to angiogenesis, vascular injury, thrombosis, and haemostasis. A wide body of evidence suggests that tetraspanins directly regulate the development and functions of the vascular system and the pathogenesis of vascular diseases. This article reviews current understanding of the roles of tetraspanins in vascular functions.

CD36 modulates migration of mouse and human macrophages in response to oxidized LDL and may contribute to macrophage trapping in the arterial intima

The trapping of lipid-laden macrophages in the arterial intima is a critical but reversible step in atherogenesis. However, the mechanism by which this occurs is not clearly defined. Here, we tested in mice the hypothesis that CD36, a class B scavenger receptor expressed on macrophages, has a role in this process. Using both in vivo and in vitro migration assays, we found that oxidized LDL (oxLDL), but not native LDL, inhibited migration of WT mouse macrophages but not CD36-deficient cells. We further observed a crucial role for CD36 in modulating the in vitro migratory response of human peripheral blood monocyte-derived macrophages to oxLDL. oxLDL also induced rapid spreading and actin polymerization in CD36-sufficient but not CD36-deficient mouse macrophages in vitro. The underlying mechanism was dependent on oxLDL-mediated CD36 signaling, which resulted in sustained activation of focal adhesion kinase (FAK) and inactivation of Src homology 2-containing phosphotyrosine phosphatase (SHP-2). The latter was due to NADPH oxidase-mediated ROS generation, resulting in oxidative inactivation of critical cysteine residues in the SHP-2-active site. Macrophage migration in the presence of oxLDL was restored by both antioxidants and NADPH oxidase inhibitors, which restored the dynamic activation of FAK. We conclude therefore that CD36 signaling in response to oxLDL alters cytoskeletal dynamics to enhance macrophage spreading, inhibiting migration. This may induce trapping of macrophages in the arterial intima and promote atherosclerosis.

The crystal structure of the heparin-binding reelin-N domain of f-spondin

The extracellular matrix protein F-spondin mediates axon guidance during neuronal development. Its N-terminal domain, termed the reelin-N domain, is conserved in F-spondins, reelins, and other extracellular matrix proteins. In this study, a recombinant human reelin-N domain has been expressed, purified, and shown to bind heparin. The crystal structure of the reelin-N domain resolved to 2.0 A reveals a variant immunoglobulin-like fold and potential heparin-binding sites. Substantial conformational variations even in secondary structure are observed between the two chemically identical reelin-N domains in one crystallographic asymmetric unit. The variations may result from extensive, highly specific interactions across the interface of the two reelin-N domains. The calculated values of buried surface area and the interface's shape complementarity are consistent with the formation of a weak dimer. The homophilic asymmetric dimer can potentially offer advantages in binding to ligands such as glycosaminoglycans, which may, in turn, bridge the two reelin-N domains and stabilize the dimer.

Thrombospondins in cancer

The thrombospondins (TSPs) are a family of five proteins that are involved in the tissue remodeling that is associated with embryonic development, wound healing, synaptogenesis, and neoplasia. These proteins mediate the interaction of normal and neoplastic cells with the extracellular matrix and surrounding tissue. In the tumor microenvironment, TSP-1 has been shown to suppress tumor growth by inhibiting angiogenesis and by activating transforming growth factor beta. TSP-1 inhibits angiogenesis through direct effects on endothelial cell migration and survival, and through effects on vascular endothelial cell growth factor bioavailability. In addition, TSP-1 may affect tumor cell function through interaction with cell surface receptors and regulation of extracellular proteases. Whereas the role of TSP-1 in the tumor microenvironment is the best characterized, the other TSPs may have similar functions. (Part of a Multi-author Review).

Variants in the CD36 gene associate with the metabolic syndrome and high-density lipoprotein cholesterol

A region along chromosome 7q was recently linked to components of the metabolic syndrome (MetS) in several genome-wide linkage studies. Within this region, the CD36 gene, which encodes a membrane receptor for long-chain fatty acids and lipoproteins, is a potentially important candidate. CD36 has been documented to play an important role in fatty acid metabolism in vivo and subsequently may be involved in the etiology of the MetS. The protein also impacts survival to malaria and the influence of natural selection has resulted in high CD36 genetic variability in populations of African descent. We evaluated 36 tag SNPs across CD36 in the HyperGen population sample of 2020 African-Americans for impact on the MetS and its quantitative traits. Five SNPs associated with increased odds for the MetS [P = 0.0027-0.03, odds ratio (OR) = 1.3-1.4]. Coding SNP, rs3211938, previously shown to influence malaria susceptibility, is documented to result in CD36 deficiency in a homozygous subject. This SNP conferred protection against the MetS (P = 0.0012, OR = 0.61, 95%CI: 0.46-0.82), increased high-density lipoprotein cholesterol, HDL-C (P = 0.00018) and decreased triglycerides (P = 0.0059). Fifteen additional SNPs associated with HDL-C (P = 0.0028-0.044). We conclude that CD36 variants may impact MetS pathophysiology and HDL metabolism, both predictors of the risk of heart disease and type 2 diabetes.

Thrombospondin-based antiangiogenic therapy

Thrombospondins (TSPs) are a family of extracellular matrix proteins that regulate tissue genesis and remodeling. TSP-1 plays a pivotal role in the regulation of both physiological and pathological angiogenesis. The inhibitory effects of TSP-1 on angiogenesis have been established in numerous experimental models. Among other TSP members, TSP-2 has equivalent domain structure as TSP-1 and shares most functions of TSP-1. The mechanisms by which TSP-1 and -2 inhibit angiogenesis can be broadly characterized as direct effects on vascular endothelial cells and indirect effects on the various angiogenic regulators. The fact that TSP-1 and -2 are potent endogenous angiogenic inhibitors has prompted studies to explore their therapeutic applications, and detailed understanding of the mechanisms of action of TSP-1 and -2 has facilitated the design of therapeutic strategies to optimize these activities. The therapeutic effects can be achieved by up-regulation of endogenous TSPs, or by the delivery of recombinant proteins or synthetic peptides that contain sequences from the angiogenic domain of TSP-1. In this article, we review the progress in thrombospondin-based antiangiogenic therapy and discuss the perspectives on the significant challenges that remain.

Shed gangliosides provide detergent-independent evidence for Type-3 glycosynapses

Membrane microdomains, or rafts, at the plasma membrane have been invoked to explain many cellular processes. Protein-protein interactions within such microdomains including, for example, the tetraspanin web are reported to provide a scaffold for signal transduction. However, the nature of such protein-protein interactions is not fully elucidated. Hakomori [S.I. Hakomori, The glycosynapse, Proc. Natl. Acad. Sci. USA 99 (2002) 225-232] has advanced the concept that glycosphingolipids, particularly gangliosides, provide the intermediary link between transmembrane receptors and signal transducers and has redefined membrane rafts as Type-1, -2 or -3 glycosynapses. Here, using simple immunofluorescent analysis of the ganglioside complexes naturally released from cellular microprocesses (termed "footprints") we show that the ganglioside can determine the nature of protein-protein associations. Specifically, we demonstrate that CD36 and the tetraspanin CD151, both of which interact with beta1 integrins, associate together only in the presence of the gangliosides GD2/GD3. These results substantiate the glycosynapse hypothesis and suggest that the nature of the tetraspanin web may be determined by gangliosides.