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

Soluble Receptors Affecting Stroke Outcomes: Potential Biomarkers and Therapeutic Tools

Soluble receptors are widely understood to be freestanding moieties formed via cleavage from their membrane-bound counterparts. They have unique structures, are found among various receptor families, and have intriguing mechanisms of generation and release. Soluble receptors’ ability to exhibit pleiotropic action by receptor modulation or by exhibiting a dual role in cytoprotection and neuroinflammation is concentration dependent and has continually mystified researchers. Here, we have compiled findings from preclinical and clinical studies to provide insights into the role of soluble/decoy receptors, focusing on the soluble cluster of differentiation 36, the soluble cluster of differentiation 163, and soluble lipoprotein-related protein 1 (sCD36, sCD163, and sLRP1, respectively) and the functions they could likely serve in the management of stroke, as they would notably regulate the bioavailability of the hemoglobin and heme after red blood cell lysis. The key roles that these soluble receptors play in inflammation, oxidative stress, and the related pharmacotherapeutic potential in improving stroke outcomes are described. The precise pleiotropic physiological functions of soluble receptors remain unclear, and further scientific investigation/validation is required to establish their respective role in diagnosis and therapy.

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.

The putative diabetic plasma marker, soluble CD36, is non-cleaved, non-soluble and entirely associated with microparticles

BACKGROUND

CD36 is a widely expressed cell surface receptor that binds lipoproteins, and its function has been implicated in many complications of the metabolic syndrome. A cell-free form of CD36, soluble CD36 (sCD36), has been reported in human plasma, found to be elevated in obesity and diabetes, and claimed as a marker of insulin resistance.

OBJECTIVE

To determine the nature of sCD36; in particular, whether sCD36 is truly soluble or, as hypothesized, is found as a component of circulating microparticles (MPs).

METHODS

Lipoproteins were fractionated by density gradient centrifugation, and plasma MPs were isolated by ultracentrifugation, size exclusion, and immunoprecipitation with CD36 detected by immunoblotting. MPs from plasma and activated platelets were analyzed by multicolor flow cytometry, with a DyLight-488 anti-CD36 conjugate in combination with antibodies against different cellular markers.

RESULTS

Cell-free plasma CD36 was not observed associated with lipoproteins and was not a proteolytic fragment; rather, it was associated with the plasma MP fraction, suggesting that sCD36 in the plasma of normal subjects is a product of circulating MPs. Cytometric and immunoblotting analyses of plasma from normal donors showed that these MPs were derived mainly from platelets. Analysis of in vitro activated platelets also showed that CD36 to be secreted in the form of MPs.

CONCLUSIONS

sCD36 is not a proteolytic product, but rather is associated with a specific subset of circulating MPs that can readily be analysed. This finding will enable more specific investigations into the cellular source of the increased levels of plasma CD36 found in subjects with diabetes.

Ectophosphorylation of CD36 regulates cytoadherence of Plasmodium falciparum to microvascular endothelium under flow conditions

The adhesion of Plasmodium falciparum-infected erythrocytes (IRBCs) to human dermal microvascular endothelial cells (HDMECs) under flow conditions is regulated by a Src family kinase- and alkaline phosphatase (AP)-dependent mechanism. In this study, we showed that the target of the phosphatase activity is the ectodomain of CD36 at threonine-92 (Thr92). Mouse fibroblasts (NIH 3T3 cells) transfected with wild-type CD36 or a mutant protein in which Thr92 was substituted by Ala supported the rolling and adhesion of IRBCs. However, while the Src family kinase inhibitors PP1 and PP2 and the specific AP inhibitor levamisole significantly reduced IRBC adhesion to wild-type CD36 transfectants as with HDMECs, the inhibitors had no effect on IRBC adhesion to the mutant cells. Using a phosphospecific antibody directed at a 12-amino-acid peptide spanning Thr92, we demonstrated directly that CD36 was constitutively phosphorylated and could be dephosphorylated by exogenous AP. Endothelial CD36 was likewise constitutively phosphorylated. The phosphospecific antibody inhibited IRBC adhesion to HDMECs that could be reversed by preincubating the antibody with the phosphorylated but not the nonphosphorylated peptide. Pretreatment of HDMECs with AP abrogated the effect of PP1 on IRBC adhesion. Collectively, these results are consistent with a critical role for CD36 dephosphorylation through Src family kinase activation in regulating IRBC adhesion to vascular endothelium.

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.

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

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

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

The CD36 receptor sequence predicts two hydrophobic domains located at the N- and C-termini of the protein, but there are conflicting reports as to whether the N-terminal uncleaved leader sequence functions as a transmembrane domain. To investigate the topology of CD36, we generated a panel of mutants lacking either one or both hydrophobic regions and analyzed their folding and transport in COS-7 cells. The N- and the C-terminal hydrophobic regions were both sufficient to anchor CD36 in the membrane, and a FLAG epitope inserted at the N-terminus was located intracellularly. These results indicate that CD36 adopts a ditopic configuration. Accordingly, neither N- nor C-terminal truncation mutants were secreted. Analysis with conformation-specific monoclonal antibodies showed that the N-terminal transmembrane domain truncated molecule was slowly transported through the exocytic pathway and largely accumulated intracellularly. Thus, dual membrane insertion dictates the correct topogenesis and seems to be necessary for efficient folding and intracellular transport.