CD36 is a ditopic glycoprotein with the N-terminal domain implicated in intracellular transport

Expression profiling and antibacterial analysis of cd36 in mandarin fish, Siniperca chuatsi

Cd36 is classified as a class B scavenger receptor and has also been identified as a pattern recognition receptor. In this study, we investigated the genomic structure and molecular characteristics of cd36 in mandarin fish (Siniperca chuatsi), examined its tissue distribution, and evaluated its antibacterial activity. Genomic structure analysis showed that Sccd36 consists of 12 exons and 11 introns. Sequencing analysis confirmed that the open reading frame of Sccd36 contains 1410 bp, encoding 469 amino acids. Sccd36 is deeply conserved with other vertebrates in terms of genomic structure, gene loci and molecular evolution, and the feature of two transmembrane was observed in ScCd36 through structural prediction. Sccd36 was constitutively expressed in all tissues tested, with the strongest expression in the intestine, followed by the heart and the kidney. Dramatic changes of Sccd36 mRNA were detected in mucosal tissues, including the intestine, gill and skin, when stimulated by the microbial ligands lipopolysaccharide and lipoteichoic acid. In addition, ScCd36 was identified as having strong binding ability to microbial ligands and antibacterial activity against the gram-negative bacteria Aeromonas hydrophila and the gram-positive bacteria Streptococcus lactis. Furthermore, we verified that the genetic ablation of cd36 impaired the resistance of fish to bacterial challenge by using zebrafish cd36 knockout line. In conclusion, our findings suggest that ScCd36 plays a crucial role in the innate immune response of mandarin fish against bacterial infections. This also sets the stage for further exploration into the antibacterial function of Cd36 in lower vertebrate species.

Endothelial Cell CD36 Reduces Atherosclerosis and Controls Systemic Metabolism

High-fat Western diets contribute to tissue dysregulation of fatty acid and glucose intake, resulting in obesity and insulin resistance and their sequelae, including atherosclerosis. New therapies are desperately needed to interrupt this epidemic. The significant idea driving this research is that the understudied regulation of fatty acid entry into tissues at the endothelial cell (EC) interface can provide novel therapeutic targets that will greatly modify health outcomes and advance health-related knowledge. Dysfunctional endothelium, defined as activated, pro-inflammatory, and pro-thrombotic, is critical in atherosclerosis initiation, in modulating thrombotic events that could result in myocardial infarction and stroke, and is a hallmark of insulin resistance. Dyslipidemia from high-fat diets overwhelmingly contributes to the development of dysfunctional endothelium. CD36 acts as a receptor for pathological ligands generated by high-fat diets and in fatty acid uptake, and therefore, it may additionally contribute to EC dysfunction. We created EC CD36 knockout (CD36°) mice using cre-lox technology and a cre-promoter that does not eliminate CD36 in hematopoietic cells (Tie2e cre). These mice were studied on different diets, and crossed to the low density lipoprotein receptor (LDLR) knockout for atherosclerosis assessment. Our data show that EC CD36° and EC CD36°/LDLR° mice have metabolic changes suggestive of an uncompensated role for EC CD36 in fatty acid uptake. The mice lacking expression of EC CD36 had increased glucose clearance compared with controls when fed with multiple diets. EC CD36° male mice showed increased carbohydrate utilization and decreased energy expenditure by indirect calorimetry. Female EC CD36°/LDLR° mice have reduced atherosclerosis. Taken together, these data support a significant role for EC CD36 in systemic metabolism and reveal sex-specific impact on atherosclerosis and energy substrate use.

Molecular characterization and expression of sensory neuron membrane proteins in the parasitoid Microplitis mediator (Hymenoptera: Braconidae)

Sensory neuron membrane proteins (SNMPs), homologs of the human fatty acid transport protein CD36 family, are observed to play a significant role in chemoreception, especially in detecting sex pheromone in Drosophila and some lepidopteran species. In the current study, two full-length SNMP transcripts, MmedSNMP1 and MmedSNMP2, were identified in the parasitoid Microplitis mediator (Hymenoptera: Braconidae). Quantitative real-time polymerase chain reaction analysis showed that the expression of MmedSNMP1 was significantly higher in antennae than in other tissues of both sexes. In addition, the MmedSNMP1 transcript was increased dramatically in newly emerged adults and there were no significant differences between adults with or without mating and parasitic experiences. However, compared with MmedSNMP1, the expression of MmedSNMP2 was widely found in various tissues, significantly increased at half-pigmented pupae stage and remained at a relatively constant level during the following developmental stages. It was found that MmedSNMP1 contained eight exons and seven introns, which was highly conserved compared with other insect species. In situ hybridization assay demonstrated that MmedSNMP1 transcript was distributed widely in antennal flagella. Among selected chemosensory genes (odorant binding protein, odorant receptor, and ionotropic receptor genes), MmedSNMP1 only partially overlapped with MmedORco in olfactory sensory neurons of antennae. Subsequent immunolocalization results further indicated that MmedSNMP1 was mainly expressed in sensilla placodea of antennae and possibly involved in perceiving plant volatiles and sex pheromones. These findings lay a foundation for further investigating the roles of SNMPs in the chemosensation of parasitoids.

T. Vo A, Tristan A, Carreño L, Quijada M, Nguyen VT, Perry G, Larionov OV, Lleonart R, Fernández PL. Identification of Inhibitors of CD36-Amyloid Beta Binding as Potential Agents for Alzheimer's Disease

Neuroinflammation is one of the hallmarks of Alzheimer's disease pathology. Amyloid β has a central role in microglia activation and the subsequent secretion of inflammatory mediators that are associated with neuronal toxicity. The recognition of amyloid β by microglia depends on the expression of several receptors implicated in the clearance of amyloid and in cell activation. CD36 receptor expressed on microglia interacts with fibrils of amyloid inducing the release of proinflammatory cytokines and amyloid internalization. The interruption of the interaction CD36-amyloid β compromises the activation of microglia cells. We have developed and validated a new colorimetric assay to identify potential inhibitors of the binding of amyloid β to CD36. We have found seven molecules, structural analogues of the Trichodermamide family of natural products that interfere with the interaction CD36-amyloid β. By combining molecular docking and dynamics simulations, we suggested the second fatty acids binding site within the large luminal hydrophobic tunnel, present in the extracellular domain of CD36, as the binding pocket of these compounds. Free energy calculations predicted the nonpolar component as the driving force for the binding of these inhibitors. These molecules also inhibited the production of TNF-α, IL-6, and IL-1β by peritoneal macrophages stimulated with fibrils of amyloid β. This work serves as a platform for the identification of new potential anti-inflammatory agents for the treatment of Alzheimer's disease.

Developmental expression and immune role of the class B scavenger receptor cd36 in zebrafish

CD36 is a transmembrane glycoprotein belonging to the scavenger receptor class B family which plays crucial roles in innate immunity. Although CD36 is widely documented in mammals, the study of its functions in fish is still limited. Here we report the identification of a zebrafish cd36 homologue. Zebrafish cd36 has a higher gene expression in the tissues of intestine and liver but very low in kidney and swim bladder. We find cd36 mRNA is maternally expressed and is mainly restricted to the intestine, branchial arches and regions around the lips after the segmentation stage during embryogenesis. Functionally, the recombinant Cd36 corresponding to the large extracellular loop is capable of binding both the Gram-negative and Gram-positive bacteria. These results indicate that zebrafish Cd36 is a microbial-binding molecule. The study expands our knowledge of the function of scavenger receptor molecules in fish innate immune process.

Circulating CD36+ microparticles are not altered by docosahexaenoic or eicosapentaenoic acid supplementation

BACKGROUND AND AIMS

Circulating microparticles (MP) are the source of a plasma derived form of the scavenger receptor CD36, termed soluble (s)CD36, the levels of which correlate with markers of atherosclerosis and risk of cardiovascular disease. Long chain n-3 polyunsaturated fatty acids have cardioprotective effects that we have previously reported to be gender specific. The aim of this study was to determine if dietary docosahexaenoic acid (DHA) and/or eicosapentaenoic acid (EPA) supplementation affect circulating CD36 + MP levels, and if this occurs differentially in healthy men and women.

METHODS AND RESULTS

Participants (43M, 51F) aged 39.6 ± 1.7 years received 4 weeks of daily supplementation with DHA rich (200 mg EPA; 1000 mg DHA), EPA rich (1000 mg EPA; 200 mg DHA), or placebo (sunola) oil in a double-blinded, randomised, placebo controlled trial. Plasma CD36 + MP were enumerated by flow cytometry and differences between genders and treatments were evaluated by Student's or paired t-test and one way ANOVA. Males and females had similar levels of CD36 + MP at baseline (mean = 1018 ± 325 vs 980 ± 318; p = 0.577) and these were not significantly changed after DHA (M, p = 0.571; F, p = 0.444) or EPA (M, p = 0.361; F, p = 0.901) supplementation. Likewise, the overall percent change in these levels were not different between supplemented cohorts compared to placebo when all participants were combined (% change in CD36 + MP: DHA = 5.7 ± 37.5, EPA = -3.4 ± 35.4, placebo = -11.5 ± 32.9; p = 0.158) or stratified by gender (M, DHA = -2.6 ± 30.6, EPA = -15.1 ± 20.1, placebo = -21.4 ± 28.7, p = 0.187; F, DHA = 11.7 ± 41.5, EPA = 6.8 ± 42.9, placebo = -2.8 ± 34.7, p = 0.552).

CONCLUSION

The cardioprotective effects of DHA and EPA do not act through a CD36 + MP mechanism.

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.

Molecular identification and differential expression of sensory neuron membrane proteins in the antennae of the black cutworm moth Agrotis ipsilon

The insect sensory neuron membrane proteins (SNMPs) SNMP1 and SNMP2 are transmembrane domain-containing proteins and are homologs of the vertebrate CD36 transmembrane proteins. It has been suggested that SNMPs play a significant role in insect chemoreception. Previous studies have demonstrated that SNMP1 is expressed in the pheromone-sensitive olfactory receptor neurons (ORNs), whereas SNMP2 is expressed in the supporting cells. In this study, we identified two full-length SNMP transcripts, AipsSNMP1 and AipsSNMP2, in the black cutworm moth Agrotis ipsilon (Hufnagel). The qRT-PCR results indicated that the AipsSNMP1 and AipsSNMP2 transcripts were expressed significantly higher in the antennae than in other tissues of both sexes. The expression of AipsSNMP1 and AipsSNMP2 in the antennae from different development stages of both sexes was investigated and was shown to begin to express in the pupae stage from 3days before emergence and then increased dramatically at the day of the emergence, and the high expression levels were maintained during the following 4days after the emergence in both sexes. The mating status had no effect on the expression levels of the AipsSNMP1 and AipsSNMP2 transcripts. Consistent with previous in situ hybridization studies in other Lepidoptera insects, our immunolocalization results at protein level demonstrated that both AipsSNMP1 and AipsSNMP2 were expressed in pheromone-sensitive sensilla trichodea but with a completely different expression profile. AipsSNMP1 is more uniformed and highly expressed along the membrane of the ORN dendrites, whereas AipsSNMP2 is widely distributed at the bottom of the sensilla trichodea and highly localized in the sensillum lymph. Our studies provide further detailed evidence for the involvement and general functional role of insect SNMPs in the detection of sex pheromones and general odorant molecules.

The origin of circulating CD36 in type 2 diabetes

Objective:

Elevated plasma levels of the fatty acid transporter, CD36, have been shown to constitute a novel biomarker for type 2 diabetes mellitus (T2DM). We recently reported such circulating CD36 to be entirely associated with cellular microparticles (MPs) and aim here to determine the absolute levels and cellular origin(s) of these CD36+MPs in persons with T2DM.

Design:

An ex vivo case-control study was conducted using plasma samples from 33 obese individuals with T2DM (body mass index (BMI)=39.9±6.4 kg m⁻²; age=57±9 years; 18 male:15 female) and age- and gender-matched lean and obese non-T2DM controls (BMI=23.6±1.8 kg m⁻² and 33.5±5.9 kg m⁻², respectively). Flow cytometry was used to analyse surface expression of CD36 together with tissue-specific markers: CD41, CD235a, CD14, CD105 and phosphatidyl serine on plasma MPs. An enzyme-linked immunosorbent assay was used to quantify absolute CD36 protein concentrations.

Results:

CD36+MP levels were significantly higher in obese people with T2DM (P<0.00001) and were primarily derived from erythrocytes (CD235a+=35.8±14.6%); although this did not correlate with haemoglobin A_1c. By contrast, the main source of CD36+MPs in non-T2DM individuals was endothelial cells (CD105+=40.9±8.3% and 33.9±8.3% for lean and obese controls, respectively). Across the entire cohort, plasma CD36 protein concentration varied from undetectable to 22.9 μg ml⁻¹ and was positively correlated with CD36+MPs measured by flow cytometry (P=0.0006) but only weakly associated with the distribution of controls and T2DM (P=0.021). Multivariate analysis confirmed that plasma CD36+MP levels were a much better biomarker for diabetes than CD36 protein concentration (P=0.009 vs P=0.398, respectively).

Conclusions:

Both the levels and cellular profile of CD36+MPs differ in T2DM compared with controls, suggesting that these specific vesicles could represent distinct biological vectors contributing to the pathology of the disease.

Identification, evolution and expression of a CD36 homolog in the basal chordate amphioxus Branchiostoma japonicum

CD36, as one member of scavenger receptor class B (SRB) family, is a transmembrane glycoprotein and has been associated with diverse normal physiological processes and pathological conditions. However, little is known about it in amphioxus, a model organism for insights into the origin and evolution of vertebrates. In this paper, CD36 homologs in amphioxus were identified. Evolutionary analysis suggested that amphioxus BfCD36F-a/b, which were more similar to vertebrate CD36, might represent the primitive form before the splitting of CD36, SRB1 and SRB2 genes during evolution. Then the BjCD36F-a cDNA was cloned from Branchiostoma japonicum using RACE technology. Real-time PCR and in situ hybridization revealed the expression of BjCD36F-a in all the tissues detected with the highest expression in the hepatic caecum. The BjCD36F-a expression was obviously up-regulated after feeding and down-regulated during fasting, indicating a role of BjCD36F-a in feeding regulation. Besides, the up-regulation expression of BjCD36F-a transcripts was also found after either Lipoteichoic acid (LTA) treatment in the BjCD36F-a-transfected FG cells or Escherichia coli (E. coli) challenge in vivo, implying an immune-related function for BjCD36F-a. Collectively, we identify and characterize a conserved gene that is important in the fundamental process of immune and nutritional regulation. These are the first such data in amphioxus, laying a foundation for further study of their physiological functions.

Identification and characterization of two sensory neuron membrane proteins from Cnaphalocrocis medinalis (Lepidoptera: Pyralidae)

Sensory neuron membrane proteins (SNMPs), which are located on the dendritic membrane of olfactory neurons, were considered as important components involved in pheromone reception in insects. In Drosophila melanogaster, mutants without SNMP are unable to evoke neuronal activities in the presence of pheromone cis-vaccenyl acetate (cVA). So deeply understanding the SNMPs functions may help to develop pheromone-mediated insect pest management tactics. The present study reports the identification and characterization of CmedSNMP1 and CmedSNMP2, two candidate SNMPs in the rice leaffolder, Cnaphalocrocis medinalis, one of the serious rice insect pests in Asia. The comparison of amino acid sequences shows that CmedSNMP1 and CmedSNMP2 are very similar to the previously reported SNMPs isolated from moths such as Ostrinia nubilalis and O. furnacalis, respectively, but the two CmedSNMPs share low identity with each other. The distribution patterns of two CmedSNMPs in different tissues of adult moths were examined using RT-PCR and quantitative real-time PCR. Although the two genes are expressed not only in antennae but also in nonolfactory tissues such as wings, legs, and body; the relative transcription level shows both CmedSNMP1 and CmedSNMP2 are highly enriched in antennae. The dN/dS ratios of the two CmedSNMPs indicate that the two genes are all subject to purifying selection and evolved to be functional genes. This work presents for the first time a study on the SNMPs of C. medinalis, which may help in providing guidance to future functional research of moth SNMPs.

CD36 level and trafficking are determinants of lipolysis in adipocytes

CD36 has been linked to the etiology of insulin resistance and inflammation. We explored its function in regulating adipose tissue lipolysis, which influences fat accumulation by liver and muscle and overall metabolism. Knockdown of CD36 in differentiated 3T3-L1 adipocytes decreased lipolysis in response to 10 μM of the β-adrenergic agonist isoproterenol (by 42%), 10 μM of the adenyl cyclase activator forskolin (by 32%), and 500 μM of the phosphodiesterase (PDE) inhibitor isobutylmethylxanthine (by 33%). All three treatments in the knockdown adipocytes were associated with significant decreases of cAMP levels and of the hormone-sensitive lipase (HSL) and perilipin phosphorylation. An important role for PDE was supported by the lack of inhibition of the lipolysis induced by the poorly hydrolyzable dibutyryl cAMP analog. An additional contributory mechanism was diminished activation of the Src-ERK1/2 pathway. Regulation of lipolysis and lipolytic signaling by CD36 was reproduced with adipose tissue from CD36(-/-) mice. The importance of surface CD36 in this regulation was suggested by the finding that the plasma membrane-impermeable CD36 inhibitor sulfo-N-succinimidyl oleate (20 μM) decreased lipolysis. Interestingly, isoproterenol induced CD36 internalization, and this process was blocked by HSL inhibition, suggesting feedback regulation of adipocyte lipolysis via CD36 trafficking.

Oxidized low density lipoprotein, stem cells, and atherosclerosis

Oxidized low density lipoprotein (ox-LDL), a risk factor of atherosclerosis, facilitates the formation and vulnerability of atherosclerotic plaque, thus contributing to several clinical complications. Stem cells participate in vascular repair after damage and atherosclerosis is a process of inflammation accompanied with vascular injury. Researchers have proposed that stem cells participate in the formation of atherosclerotic plaque. Also, because ox-LDL is capable of inducing toxic effects on stem cells, it is reasonable to postulate that ox-LDL promotes the progress of atherosclerosis via acting on stem cells. In the present article, we review the relationship between ox-LDL, stem cells, and atherosclerosis and a portion of the associated mechanisms.

Microglial scavenger receptors and their roles in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is increasing in prevalence with the aging population. Deposition of amyloid-β (Aβ) in the brain of AD patients is a hallmark of the disease and is associated with increased microglial numbers and activation state. The interaction of microglia with Aβ appears to play a dichotomous role in AD pathogenesis. On one hand, microglia can phagocytose and clear Aβ, but binding of microglia to Aβ also increases their ability to produce inflammatory cytokines, chemokines, and neurotoxic reactive oxygen species (ROS). Scavenger receptors, a group of evolutionally conserved proteins expressed on the surface of microglia act as receptors for Aβ. Of particular interest are SCARA-1 (scavenger receptor A-1), CD36, and RAGE (receptor for advanced glycation end products). SCARA-1 appears to be involved in the clearance of Aβ, while CD36 and RAGE are involved in activation of microglia by Aβ. In this review, we discuss the roles of various scavenger receptors in the interaction of microglia with Aβ and propose that these receptors play complementary, nonredundant functions in the development of AD pathology. We also discuss potential therapeutic applications for these receptors in AD.

Apical CD36 immunolocalization in human and porcine taste buds from circumvallate and foliate papillae

CD36 is the receptor for long chain fatty acids (LCFA), and is expressed in lingual taste cells from rodents. In these animals, CD36 has been proposed to play an important role in oral detection of LCFA, and subsequently, determines their dietary fat preference. Humans also seem to detect LCFA in the oral cavity, however, information on the molecular mechanism of this human orosensory LCFA recognition is currently lacking. The aim of our study was to investigate whether CD36 is also expressed in lingual human and porcine taste buds cells. Using fluorescence immunohistochemistry, apical CD36 expression was revealed in human and porcine taste bud cells from circumvallate and foliate papillae. These data suggest CD36 as the putative orosensory receptor for dietary LCFA in human, and, therefore, may be involved in our preference for fatty foods.

Thrombospondin-1: multiple paths to inflammation

Inflammation is a defensive process against tissue injury. Once this self-protective strategy is initiated, an effective resolution of the process is crucial to avoid major and unnecessary tissue damage. If the underlying event inducing inflammation is not addressed and homeostasis is not restored, this process can become chronic and lead to angiogenesis and carcinogenesis. Thrombospondin-1 (TSP-1) is a matricellular protein involved in angiogenesis, cancer, and inflammation. The effects of TSP-1 have been studied in many preclinical tumor models, and mimetic peptides are being tested in cancer clinical trials. However, the molecular mechanisms explaining its role in inflammatory processes are not well understood. This paper will discuss the role of TSP-1 in inflammation and its interaction with key receptors that may explain its functions in that process. Recent literature will be reviewed showing novel mechanisms by which this multifaceted protein could modulate the inflammatory process and impact its resolution.

Palmitoylation of CD36/FAT regulates the rate of its post-transcriptional processing in the endoplasmic reticulum

CD36/FAT is a transmembrane glycoprotein that functions in the cellular uptake of long-chain fatty acids and also as a scavenger receptor. As such it plays an important role in lipid homeostasis and, pathophysiologically, in the progression of type 2 diabetes and atherosclerosis. CD36 expression is tightly regulated at the levels of both transcription and translation. Here we show that its expression and location are also regulated post-translationally, by palmitoylation. Although palmitoylation of CD36 was not required for receptor maturation and cell surface expression, inhibition of palmitoylation either pharmacologically with cerulenin or by mutation of the relevant cysteines delayed processing at the ER and trafficking through the secretory pathway. The absence of palmitoylation also reduced the half life of the CD36 protein. Additionally, the CD36 palmitoylation mutant did not incorporate efficiently into lipid rafts, a site known to be required for its function of fatty acid uptake, and this reduced the efficiency of uptake of oxidized low density lipoprotein. These findings provide an added level of sophistication where translocation of CD36 to the plasma membrane may be physiologically regulated by palmitoylation.

Discovery of antagonists for human scavenger receptor CD36 via an ELISA-like high-throughput screening assay

CD36, a member of the class B scavenger receptor, is a high-affinity receptor for oxidatively modified low-density lipoprotein (oxLDL). Extensive evidence points to a significant role of CD36 in atherosclerosis and suggests that CD36 could be a potential target for treatment of atherosclerosis. Here, the extracellular domain of human CD36 (Gly(30)-Asn(439)) was expressed in Escherichia coli as His(6)-tagged soluble CD36 (sCD36), which could bind oxLDL specifically and effectively inhibit the uptake of oxLDL by murine macrophage RAW 264.7 cells. An enzyme-linked immunosorbent assay (ELISA)-like high-throughput screening (HTS) assay was developed for the discovery of CD36 antagonists, based on the competition of sCD36 binding to immobilized oxLDL and detection with a monoclonal antibody against His-tag. This assay was suitable for HTS in a 96-well format and was robust and reliable according to the evaluation parameter Z' value of 0.82. The developed HTS assay was applied to both pure chemical compounds and microbial secondary metabolite crude extracts to identify CD36 antagonists. Three active compounds-sodium danshensu (DSS), rosmarinic acid (RA), and salvianolic acid B (SAB)-were shown to be antagonistic to sCD36-oxLDL binding and further validated by their inhibition of oxLDL uptake in RAW 264.7 cells. These results suggest that the ELISA-like assay represents a promising screening for identifying bioactive molecules targeting atherosclerosis at the level of CD36-ligand binding.

A CD36-related transmembrane protein is coordinated with an intracellular lipid-binding protein in selective carotenoid transport for cocoon coloration

The transport pathway of specific dietary carotenoids from the midgut lumen to the silk gland in the silkworm, Bombyx mori, is a model system for selective carotenoid transport because several genetic mutants with defects in parts of this pathway have been identified that manifest altered cocoon pigmentation. In the wild-type silkworm, which has both genes, Yellow blood (Y) and Yellow cocoon (C), lutein is transferred selectively from the hemolymph lipoprotein to the silk gland cells where it is accumulated into the cocoon. The Y gene encodes an intracellular carotenoid-binding protein (CBP) containing a lipid-binding domain known as the steroidogenic acute regulatory protein-related lipid transfer domain. Positional cloning and transgenic rescue experiments revealed that the C gene encodes Cameo2, a transmembrane protein gene belonging to the CD36 family genes, some of which, such as the mammalian SR-BI and the fruit fly ninaD, are reported as lipoprotein receptors or implicated in carotenoid transport for visual system. In C mutant larvae, Cameo2 expression was strongly repressed in the silk gland in a specific manner, resulting in colorless silk glands and white cocoons. The developmental profile of Cameo2 expression, CBP expression, and lutein pigmentation in the silk gland of the yellow cocoon strain were correlated. We hypothesize that selective delivery of lutein to specific tissue requires the combination of two components: 1) CBP as a carotenoid transporter in cytosol and 2) Cameo2 as a transmembrane receptor on the surface of the cells.

CD36 signaling inhibits the translation of heat shock protein 70 induced by oxidized low density lipoprotein through activation of peroxisome proliferators-activated receptor gamma

Oxidized LDL (OxLDL), a causal factor in atherosclerosis, induces the expression of heat shock proteins (Hsp) in a variety of cells. In this study, we investigated the role of CD36, an OxLDL receptor, and peroxisome proliferator-activated receptor gamma (PPARgamma) in OxLDL-induced Hsp70 expression. Overexpression of dominant-negative forms of CD36 or knockdown of CD36 by siRNA transfection increased OxLDL-induced Hsp70 protein expression in human monocytic U937 cells, suggesting that CD36 signaling inhibits Hsp70 expression. Similar results were obtained by the inhibition of PPARgamma activity or knockdown of PPARgamma expression. In contrast, overexpression of CD36, which is induced by treatment of MCF-7 cells with troglitazone, decreased Hsp70 protein expression induced by OxLDL. Interestingly, activation of PPARgamma through a synthetic ligand, ciglitazone or troglitazone, decreased the expression levels of Hsp70 protein in OxLDL-treated U937 cells. However, major changes in Hsp70 mRNA levels were not observed. Cycloheximide studies demonstrate that troglitazone attenuates Hsp70 translation but not Hsp70 protein stability. PPARgamma siRNA transfection reversed the inhibitory effects of troglitazone on Hsp70 translation. These results suggest that CD36 signaling may inhibit stress- induced gene expression by suppressing translation via activation of PPARgamma in monocytes. These findings reveal a new molecular basis for the anti-inflammatory effects of PPARgamma.

The Human Scavenger Receptor CD36: glycosylation status and its role in trafficking and function

Human CD36 is a class B scavenger receptor expressed in a variety of cell types such as macrophage and adipocytes. This plasma membrane glycoprotein has a wide range of ligands including oxidized low density lipoprotein and long chain fatty acids which involves the receptor in diseases such as atherosclerosis and insulin resistance. CD36 is heavily modified post-translationally by N-linked glycosylation, and 10 putative glycosylation sites situated in the large extracellular loop of the protein have been identified; however, their utilization and role in the folding and function of the protein have not been characterized. Using mass spectrometry on purified and peptide N-glycosidase F-deglycosylated CD36 and also by comparing the electrophoretic mobility of different glycosylation site mutants, we have determined that 9 of the 10 sites can be modified by glycosylation. Flow cytometric analysis of the different glycosylation mutants expressed in mammalian cells established that glycosylation is necessary for trafficking to the plasma membrane. Minimally glycosylated mutants that supported trafficking were identified and indicated the importance of carboxyl-terminal sites Asn-247, Asn-321, and Asn-417. However, unlike SRBI, no individual site was found to be essential for proper trafficking of CD36. Surprisingly, these minimally glycosylated mutants appear to be predominantly core-glycosylated, indicating that mature glycosylation is not necessary for surface expression in mammalian cells. The data also show that neither the nature nor the pattern of glycosylation is relevant to binding of modified low density lipoprotein.

Cd36, a class B scavenger receptor, functions as a monomer to bind acetylated and oxidized low-density lipoproteins

Cd36 is a small-molecular-weight integral membrane protein expressed in a diverse, but select, range of cell types. It has an equally diverse range of ligands and physiological functions, which has implicated Cd36 in a number of diseases including insulin resistance, diabetes, and, most notably, atherosclerosis. The protein is reported to reside in detergent-resistant microdomains within the plasma membrane and to form homo- and hetero-intermolecular interactions. These data suggest that this class B scavenger receptor may gain functionality for ligand binding, and/or ligand internalization, by formation of protein complexes at the cell surface. Here, we have overexpressed Cd36 in insect cells, purified the recombinant protein to homogeneity, and analyzed its stability and solubility in a variety of nonionic and zwitterionic detergents. Octylglucoside conferred the greatest degree of stability, and by analytical ultracentrifugation we show that the protein is monomeric. A solid-phase ligand-binding assay demonstrated that the purified monomeric protein retains high affinity for acetylated and oxidized low-density lipoproteins. Therefore, no accessory proteins are required for interaction with ligand, and binding is a property of the monomeric fold of the protein. Thus, the highly purified and functional Cd36 should be suitable for crystallization in octylglucoside, and the in vitro ligand-binding assay represents a promising screen for identification of bioactive molecules targeting atherogenesis at the level of ligand binding.

Molecular basis of human CD36 gene mutations

CD36 is a transmembrane glycoprotein of the class B scavenger receptor family. The CD36 gene is located on chromosome 7 q11.2 and is encoded by 15 exons. Defective CD36 is a likely candidate gene for impaired fatty acid metabolism, glucose intolerance, atherosclerosis, arterial hypertension, diabetes, cardiomyopathy, Alzheimer disease, and modification of the clinical course of malaria. Contradictory data concerning the effects of antiatherosclerotic drugs on CD36 expression indicate that further investigation of the role of CD36 in the development of atherosclerosis may be important for the prevention and treatment of this disease. This review summarizes current knowledge of CD36 gene structure, splicing, and mutations and the molecular, metabolic, and clinical consequences of these phenomena.

Molecular basis of human CD36 gene mutations

CD36 is a transmembrane glycoprotein of the class B scavenger receptor family. The CD36 gene is located on chromosome 7 q11.2 and is encoded by 15 exons. Defective CD36 is a likely candidate gene for impaired fatty acid metabolism, glucose intolerance, atherosclerosis, arterial hypertension, diabetes, cardiomyopathy, Alzheimer disease, and modification of the clinical course of malaria. Contradictory data concerning the effects of antiatherosclerotic drugs on CD36 expression indicate that further investigation of the role of CD36 in the development of atherosclerosis may be important for the prevention and treatment of this disease. This review summarizes current knowledge of CD36 gene structure, splicing, and mutations and the molecular, metabolic, and clinical consequences of these phenomena.

CD36: implications in cardiovascular disease

CD36 is a broadly expressed membrane glycoprotein that acts as a facilitator of fatty acid uptake, a signaling molecule, and a receptor for a wide range of ligands, including apoptotic cells, modified forms of low density lipoprotein, thrombospondins, fibrillar beta-amyloid, components of Gram positive bacterial walls and malaria infected erythrocytes. CD36 expression on macrophages, dendritic and endothelial cells, and in tissues including muscle, heart, and fat, suggest diverse roles, and indeed, this is truly a multi-functional receptor involved in both homeostatic and pathological conditions. Despite an impressive increase in our knowledge of CD36 functions, in depth understanding of the mechanistic aspects of this protein remains elusive. This review focuses on CD36 in cardiovascular disease-what we know, and what we have yet to learn.

Novel Immunoblotting Monoclonal Antibodies Against Human and Rat CD36/Fat Used to Identify an Isoform of CD36 in Rat Muscle

CD36, a surface membrane glycoprotein, functions as a class B scavenger receptor that binds to several distinct ligands. Included among these is oxidized low-density lipoprotein (Ox-LDL), a major trigger of atherosclerotic lesions, and the levels of CD36 activity and Ox-LDL uptake may have an impact on coronary artery disease. In addition, recent studies in rodents have shown that CD36, also known as FAT, controls the levels of free fatty acids and triglycerides in plasma, and is an important regulator of the metabolic pathways involved in insulin resistance. Despite the importance of measuring CD36 expression in different tissues there is a paucity of good immunoblotting antibodies, particularly for rodent tissue. Here, using GST-fusion proteins incorporating the cysteine cluster encoded by exons VIII, IX, and X of the CD36 gene as immunogen, we have generated a panel of monoclonal antibodies that are excellent blotting reagents for human and rat CD36. With these reagents we were able to visualize an additional, faster migrating CD36 band in rat muscle, likely representing a minor splice variant of CD36 (CD36var.1) hitherto seen only in the human HEL cell line.

Identification of CD36 molecular features required for its in vitro angiostatic activity

Thrombospondin-1 (TSP-1), a natural inhibitor of angiogenesis, acts directly on endothelial cells (EC) via CD36 to inhibit their migration and morphogenesis induced by basic fibroblast growth factor. Here we show that CD36 triggered by TSP-1 inhibits in vitro angiogenesis stimulated by vascular endothelial growth factor-A (VEGF-A). To demonstrate that the TSP-1 inhibitory signal was mediated by CD36, we transduced CD36 in CD36-deficient endothelial cells. Both TSP-1 and the agonist anti-CD36 mAb SMO, which mimics TSP-1 activity, reduced the VEGF-A165-induced migration and sprouting of CD36-ECs. To address the mechanisms by which CD36 may exert its angiostatic function, we investigated the functional components of the C-terminal cytoplasmic tail by site-directed mutagenesis. Our results indicate that C464, R467, and K469 of CD36 are required for the inhibitory activity of TSP-1. In contrast, point mutation of C466 did not alter TSP-1 ability to inhibit EC migration and sprouting. Moreover, we show that activation of CD36 by TSP-1 down-modulates the VEGF receptor-2 (VEGFR-2) and p38 mitogen-associated protein kinase phosphorylation induced by VEGF-A165, and this effect was specifically abolished by point mutation at C464. These results identify specific amino acids of the C-terminal cytoplasmic tail of CD36 crucial for the in vitro angiostatic activity of TSP-1 and extend our knowledge of regulation of VEGFR-2-mediated biological activities on ECs.

A Periplasmic Fluorescent Reporter Protein and its Application in High-throughput Membrane Protein Topology Analysis

We have developed a periplasmic fluorescent reporter protein suitable for high-throughput membrane protein topology analysis in Escherichia coli. The reporter protein consists of a single chain (scFv) antibody fragment that binds to a fluorescent hapten conjugate with high affinity. Fusion of the scFv to membrane protein sites that are normally exposed in the periplasmic space tethers the scFv onto the inner membrane. Following permealization of the outer membrane to allow diffusion of the fluorescent hapten into the periplasm, binding to the anchored scFv renders the cells fluorescent. We show that cell fluorescence is an accurate and sensitive reporter of the location of residues within periplasmic loops. For topological analysis, a set of nested deletions in the membrane protein gene is employed to construct two libraries of gene fusions, one to the scFvand one to the cytoplasmic reporter green fluorescent protein (GFP). Fluorescent clones are isolated by flow cytometry and the sequence of the fusion junctions is determined to identify amino acid residues within periplasmic and cytoplasmic loops, respectively. We applied this methodology to the topology analysis of E. coli TatC protein for which previous studies had led to conflicting results. The ease of screening libraries of fusions by flow cytometry enabled the rapid identification of almost 90 highly fluorescent scFv and GFP fusions, which, in turn, allowed the fine mapping of TatC membrane topology.

ERp44, a novel endoplasmic reticulum folding assistant of the thioredoxin family

In human cells, Ero1-Lalpha and -Lbeta (hEROs) regulate oxidative protein folding by selectively oxidizing protein disulfide isomerase. Specific protein--protein interactions are probably crucial for regulating the formation, isomerization and reduction of disulfide bonds in the endoplasmic reticulum (ER). To identify molecules involved in ER redox control, we searched for proteins interacting with Ero1-Lalpha. Here, we characterize a novel ER resident protein (ERp44), which contains a thioredoxin domain with a CRFS motif and is induced during ER stress. ERp44 forms mixed disulfides with both hEROs and cargo folding intermediates. Whilst the interaction with transport-competent Ig-K chains is transient, ERp44 binds more stably with J chains, which are retained in the ER and eventually degraded by proteasomes. ERp44 does not bind a short-lived ribophorin mutant lacking cysteines. Its overexpression alters the equilibrium of the different Ero1-Lalpha redox isoforms, suggesting that ERp44 may be involved in the control of oxidative protein folding.

Antennal SNMPs (sensory neuron membrane proteins) of Lepidoptera define a unique family of invertebrate CD36-like proteins

SNMP1-Apol is an antennal-specific protein of the wild silk moth Antheraea polyphemus; the protein is abundantly expressed and localized to the receptor membranes of sex-pheromone specific olfactory sensory neurons (OSNs). SNMP1-Apol is thought to function in odor detection based on its olfactory-specific expression, localization within OSNs, developmental time of expression, and apparent homology to the CD36 family of membrane-bound receptor proteins. In the current study, SNMP1-Apol homologues were identified from the moths Bombyx mori, Heliothis virescens, and Manduca sexta. These species posses antennal mRNAs encoding proteins with amino acid sequence identities ranging from 75-80%; these proteins are collectively designated SNMP1. A second M. sexta SNMP homologue, previously identified and partially sequenced [Robertson et al.: Insect Mol Biol 8:501-518, 1999] was fully sequenced and characterized. The encoded protein shares only 26-27% sequence identity with the SNMP1 proteins, and is thus designated SNMP2-Msex. The SNMP sequences were used to identify 14 and four possible homologues in Drosophila melanogaster and Caenorhabditis elegans genome databases, respectively; thus, greatly expanding CD36 family membership among the invertebrate lineages. Despite their sequence difference, SNMP1-Msex and SNMP2-Msex expression is localized to OSNs and occurs simultaneously with the onset of olfactory function. These findings suggest that SNMPs play a central role in odor detection in insects, and that the CD36 gene family is widely represented among animal phyla. The SNMPs are the only identified neuronal members of the CD36 family, and as such expand the activities of this gene family into roles influencing brain function and behavioral action.

Analysis of chimeric receptors shows that multiple distinct functional activities of scavenger receptor, class B, type I (SR-BI), are localized to the extracellular receptor domain

Scavenger receptor BI (SR-BI) mediates the selective uptake of high-density lipoprotein (HDL) cholesteryl ester (CE), a process by which HDL CE is taken into the cell without degradation of the HDL particle. In addition, SR-BI stimulates the bi-directional flux of free cholesterol (FC) between cells and lipoproteins, an activity that may be responsible for net cholesterol efflux from peripheral cells as well as the rapid hepatic clearance of FC from plasma HDL. SR-BI also increases cellular cholesterol mass and alters cholesterol distribution in plasma membrane domains as judged by the enhanced sensitivity of membrane cholesterol to extracellular cholesterol oxidase. In contrast, CD36, a closely related class B scavenger receptor, has none of these activities despite binding HDL with high affinity. In the present study, analyses of chimeric SR-BI/CD36 receptors and domain-deleted SR-BI have been used to test the various domains of SR-BI for functional activities related to HDL CE selective uptake, bi-directional FC flux, and the alteration of membrane cholesterol mass and distribution. The results show that each of these activities localizes to the extracellular domain of SR-BI. The N-terminal cytoplasmic tail and transmembrane domains appear to play no role in these activities other than targeting the receptor to the plasma membrane. The C-terminal tail of SR-BI is dispensable for activity as well for targeting to the plasma membrane. Thus, multiple distinct functional activities are localized to the SR-BI extracellular domain.