CD36 signals to the actin cytoskeleton and regulates microglial migration via a p130Cas complex

The pattern recognition receptor CD36 initiates a signaling cascade that promotes microglial activation and recruitment to beta-amyloid deposits in the brain. In the present study we identify the focal adhesion-associated proteins p130Cas, Pyk2, and paxillin as novel members of the tyrosine kinase signaling pathway downstream of CD36 and show that assembly of this complex is essential for microglial migration. In primary microglia and macrophages exposed to beta-amyloid, the scaffolding protein p130Cas is rapidly tyrosine-phosphorylated and co-localizes with CD36 to membrane ruffles contemporaneous with F-actin polymerization. These beta-amyloid-stimulated events are not detected in CD36 null cells and are dependent on CD36 activation of Src family tyrosine kinases. Fyn, a Src kinase known to interact with CD36, co-precipitates with p130Cas and is an essential upstream intermediate in the signaling pathways leading to phosphorylation of the p130Cas substrate domain. Furthermore, the p130Cas-interacting kinase Pyk2 and the cytoskeletal adapter protein paxillin also demonstrate CD36-dependent phosphorylation, identifying these focal adhesion molecules as additional members of this beta-amyloid signaling cascade. Disruption of this p130Cas complex by small interfering RNA silencing inhibits p44/42 mitogen-activated protein kinase phosphorylation and microglial migration, illustrating the importance of this pathway in microglial activation and recruitment. Together, these data are the first to identify the signaling cascade that directly links CD36 to the actin cytoskeleton and, thus, implicates it in diverse processes such as cellular migration, adhesion, and phagocytosis.

Response to Staphylococcus aureus requires CD36-mediated phagocytosis triggered by the COOH-terminal cytoplasmic domain

Phagocyte recognition and clearance of bacteria play essential roles in the host response to infection. In an on-going forward genetic screen, we identify the Drosophila melanogaster scavenger receptor Croquemort as a receptor for Staphylococcus aureus, implicating for the first time the CD36 family as phagocytic receptors for bacteria. In transfection assays, the mammalian Croquemort paralogue CD36 confers binding and internalization of Gram-positive and, to a lesser extent, Gram-negative bacteria. By mutational analysis, we show that internalization of S. aureus and its component lipoteichoic acid requires the COOH-terminal cytoplasmic portion of CD36, specifically Y463 and C464, which activates Toll-like receptor (TLR) 2/6 signaling. Macrophages lacking CD36 demonstrate reduced internalization of S. aureus and its component lipoteichoic acid, accompanied by a marked defect in tumor necrosis factor-α and IL-12 production. As a result, Cd36^−/− mice fail to efficiently clear S. aureus in vivo resulting in profound bacteraemia. Thus, response to S. aureus requires CD36-mediated phagocytosis triggered by the COOH-terminal cytoplasmic domain, which initiates TLR2/6 signaling.

Loss of receptor-mediated lipid uptake via scavenger receptor A or CD36 pathways does not ameliorate atherosclerosis in hyperlipidemic mice

Macrophage internalization of modified lipoproteins is thought to play a critical role in the initiation of atherogenesis. Two scavenger receptors, scavenger receptor A (SR-A) and CD36, have been centrally implicated in this lipid uptake process. Previous studies showed that these receptors mediated the majority of cholesterol ester accumulation in macrophages exposed to oxidized LDL and that mice with deletions of either receptor exhibited marked reductions in atherosclerosis. This work has contributed to an atherosclerosis paradigm: scavenger receptor-mediated oxidized lipoprotein uptake is required for foam cell formation and atherogenesis. In this study, Apoe-/- mice lacking SR-A or CD36, backcrossed into the C57BL/6 strain for 7 generations, were fed an atherogenic diet for 8 weeks. Hyperlipidemic Cd36-/-Apoe-/- and Msr1-/-Apoe-/- mice showed significant reductions in peritoneal macrophage lipid accumulation in vivo; however, in contrast with previous reports, this was associated with increased aortic sinus lesion areas. Characterization of aortic sinus lesions by electron microscopy and immunohistochemistry showed abundant macrophage foam cells, indicating that lipid uptake by intimal macrophages occurs in the absence of CD36 or SR-A. These data show that alternative lipid uptake mechanisms may contribute to macrophage cholesterol ester accumulation in vivo and suggest that the roles of SR-A and CD36 as proatherosclerotic mediators of modified LDL uptake in vivo need to be reassessed.

Netrin-1 inhibits leukocyte migration in vitro and in vivo

Cell migration plays important roles in embryonic development and inflammation, and this process is highly regulated to ensure tissue homeostasis. A number of barriers exist to prevent the inappropriate migration of leukocytes into healthy peripheral tissues, including retention of these cells in the inactive state and maintenance of the integrity and charge of the vascular endothelium. However, active signals also are likely to exist that can repulse cells or abolish existing cell migration. One such paradigm exists in the developing nervous system, where neuronal migration is mediated by a balance between chemoattractive and chemorepulsive signals. The ability of the guidance molecule netrin-1 to repulse or abolish attraction of neuronal cells expressing the UNC5b receptor makes it an attractive candidate for the regulation of inflammatory cell migration. Here, we show that netrin-1 is expressed on vascular endothelium, where it is regulated by infection and inflammatory cytokines. The netrin-1 receptor UNC5b is strongly expressed by leukocytes, upon which netrin-1 acts as a potent inhibitor of migration to different chemotactic stimuli both in vivo and in vitro. These data suggest that endothelial expression of netrin-1 may inhibit basal cell migration into tissues and that its down-regulation with the onset of sepsis/inflammation may facilitate leukocyte recruitment.

Oxidation of low-density lipoproteins induces amyloid-like structures that are recognized by macrophages

The macrophage scavenger receptor CD36 plays a key role in the initiation of atherosclerosis through its ability to bind to and internalize oxidized low-density lipoproteins (oxLDL). Prompted by recent findings that the CD36 receptor also recognizes amyloid fibrils formed by beta-amyloid and apolipoprotein C-II, we investigated whether the oxidation of low-density lipoproteins (LDL) generates characteristic amyloid-like structures and whether these structures serve as CD36 ligands. Our studies demonstrate that LDL oxidized by copper ions, 2,2-azobis(2-amidinopropane) dihydrochloride (AAPH), or ozone react with the diagnostic amyloid dyes thioflavin T and Congo Red and bind to serum amyloid P component (SAP), a universal constituent of physiological amyloid deposits. X-ray powder diffraction patterns for native LDL show a diffuse powder diffraction ring with maximum intensity corresponding to an atomic spacing of approximately 4.7 A, consistent with the spacing between beta-strands in a beta-sheet. Ozone treatment of LDL generates an additional diffuse powder diffraction ring with maximum intensity indicating a spacing of approximately 9.8 A. This distance is consistent with the presence of cross-beta-structure, a defining characteristic of amyloid. Evidence that these cross-beta-amyloid structures in oxLDL are recognized by macrophages is provided by the observation that SAP strongly inhibits the association and internalization of (125)I-labeled copper-oxidized LDL by peritoneal macrophages. The ability of SAP to bind to amyloid-like structures in oxLDL and prevent lipid uptake by macrophages highlights the potential importance of these structures and suggests an important preventative role for SAP in foam cell formation and early-stage atherosclerosis.

beta-Amyloid promotes accumulation of lipid peroxides by inhibiting CD36-mediated clearance of oxidized lipoproteins

Background

Recent studies suggest that hypercholesterolemia, an established risk factor for atherosclerosis, is also a risk factor for Alzheimer's disease. The myeloid scavenger receptor CD36 binds oxidized lipoproteins that accumulate with hypercholesterolemia and mediates their clearance from the circulation and peripheral tissues. Recently, we demonstrated that CD36 also binds fibrillar β-amyloid and initiates a signaling cascade that regulates microglial recruitment and activation. As increased lipoprotein oxidation and accumulation of lipid peroxidation products have been reported in Alzheimer's disease, we investigated whether β-amyloid altered oxidized lipoprotein clearance via CD36.

Methods

The availability of mice genetically deficient in class A (SRAI & II) and class B (CD36) scavenger receptors has facilitated studies to discriminate their individual actions. Using primary microglia and macrophages, we assessed the impact of Aβ on: (a) cholesterol ester accumulation by GC-MS and neutral lipid staining, (b) binding, uptake and degradation of ¹²⁵I-labeled oxidized lipoproteins via CD36, SR-A and CD36/SR-A-independent pathways, (c) expression of SR-A and CD36. In addition, using mice with targeted deletions in essential kinases in the CD36-signaling cascade, we investigated whether Aβ-CD36 signaling altered metabolism of oxidized lipoproteins.

Results

In primary microglia and macrophages, Aβ inhibited binding, uptake and degradation of oxidized low density lipoprotein (oxLDL) in a dose-dependent manner. While untreated cells accumulated abundant cholesterol ester in the presence of oxLDL, cells treated with Aβ were devoid of cholesterol ester. Pretreatment of cells with Aβ did not affect subsequent degradation of oxidized lipoproteins, indicating that lysosomal accumulation of Aβ did not disrupt this degradation pathway. Using mice with targeted deletions of the scavenger receptors, we demonstrated that Aβ inhibited oxidized lipoprotein binding and its subsequent degradation via CD36, but not SRA, and this was independent of Aβ-CD36-signaling. Furthermore, Aβ treatment decreased CD36, but not SRA, mRNA and protein, thereby reducing cell surface expression of this oxLDL receptor.

Conclusions

Together, these data demonstrate that in the presence of β-amyloid, CD36-mediated clearance of oxidized lipoproteins is abrogated, which would promote the extracellular accumulation of these pro-inflammatory lipids and perpetuate lipid peroxidation.