Binding of low density lipoprotein to platelet apolipoprotein E receptor 2' results in phosphorylation of p38MAPK

ApoE Receptor-2 R952Q Variant in Macrophages Elevates Soluble LRP1 to Potentiate Hyperlipidemia and Accelerate Atherosclerosis in Mice

BACKGROUND

apoER2 (apolipoprotein E receptor-2) is a transmembrane receptor in the low-density lipoprotein receptor (LDLR) family with unique tissue expression. A single-nucleotide polymorphism that encodes the R952Q sequence variant has been associated with elevated plasma cholesterol levels and increased myocardial infarction risk in humans. The objective of this study was to delineate the mechanism underlying the association between the apoER2 variant with arginine-to-glutamine substitution at residue 952 (R952Q) and increased atherosclerosis risk.

METHODS

An apoER2 R952Q mouse model was generated and intercrossed with LDLR knockout mice, followed by feeding a Western-type high-fat high-cholesterol diet for 16 weeks. Atherosclerosis was investigated by immunohistology. Plasma lipids and lipid distributions among the various lipoprotein classes were analyzed by colorimetric assay. Tissue-specific effects of the R952Q sequence variant on atherosclerosis were analyzed by bone marrow transplant studies. sLRP1 (soluble low-density lipoprotein receptor-related protein 1) was measured in plasma and conditioned media from bone marrow-derived macrophages by ELISA and GST-RAP (glutathione S-transferase-receptor-associated protein) pull-down, respectively. P38 MAPK (mitogen-activated protein kinase) phosphorylation in VLDL (very-low-density lipoprotein)-treated macrophages was determined by Western blot analysis.

RESULTS

Consistent with observations in humans with this sequence variant, the apoER2 R952Q mutation exacerbated diet-induced hypercholesterolemia, via impediment of plasma triglyceride-rich lipoprotein clearance, to accelerate atherosclerosis in Western diet-fed LDLR knockout mice. Reciprocal bone marrow transplant experiments revealed that the apoER2 R952Q mutation in bone marrow-derived cells instead of non-bone marrow-derived cells was responsible for the increase in hypercholesterolemia and atherosclerosis. Additional data showed that the apoER2 R952Q mutation in macrophages promotes VLDL-induced LRP1 (low-density lipoprotein receptor-related protein 1) shedding in a p38 MAPK-dependent manner.

CONCLUSIONS

The apoER2 R952Q mouse model recapitulates characteristics observed in human disease. The underlying mechanism is that the apoER2 R952Q mutation in macrophages exacerbates VLDL-stimulated sLRP1 production in a p38 MAPK-dependent manner, resulting in its competition with cell surface LRP1 to impede triglyceride-rich lipoprotein clearance, thereby resulting in increased hypercholesterolemia and accelerated atherosclerosis.

The Missing Protein: Is T-Cadherin a Previously Unknown GPI-Anchored Receptor on Platelets?

The membrane of platelets contains at least one uncharacterized glycosylphosphatidylinositol (GPI)-anchored protein according to the literature. Moreover, there is not enough knowledge on the receptor of low-density lipoproteins (LDL) mediating rapid Ca²⁺ signaling in platelets. Coincidentally, expression of a GPI-anchored protein T-cadherin increases LDL-induced Ca²⁺ signaling in nucleated cells. Here we showed evidence that supports the hypothesis about the presence of T-cadherin on platelets. The presence of T-cadherin on the surface of platelets and megakaryocytes was proven using antibodies whose specificity was tested on several negative and positive control cells by flow cytometry and confocal microscopy. Using phosphatidylinositol-specific phospholipase C, the presence of glycosylphosphatidylinositol anchor in the platelet T-cadherin form as well as in other known forms was confirmed. We showed by immunoblotting that the significant part of T-cadherin was detected in specific membrane domains (detergent Triton X-114 resistant) and the molecular weight of this newly identified protein was greater than that of T-cadherin from nucleated cells. Nevertheless, polymerase chain reaction data confirmed only the presence of isoform-1 of T-cadherin in platelets and megakaryocytes, which was also present in nucleated cells. We observed the redistribution of this newly identified protein after the activation of platelets, but only further work may explain its functional importance. Thus, our data described T-cadherin with some post-translational modifications as a new GPI-anchored protein on human platelets.

Lipoprotein receptor signalling in atherosclerosis

The founding member of the lipoprotein receptor family, low-density lipoprotein receptor (LDLR) plays a major role in the atherogenesis through the receptor-mediated endocytosis of LDL particles and regulation of cholesterol homeostasis. Since the discovery of the LDLR, many other structurally and functionally related receptors have been identified, which include low-density lipoprotein receptor-related protein (LRP)1, LRP5, LRP6, very low-density lipoprotein receptor, and apolipoprotein E receptor 2. The scavenger receptor family members, on the other hand, constitute a family of pattern recognition proteins that are structurally diverse and recognize a wide array of ligands, including oxidized LDL. Among these are cluster of differentiation 36, scavenger receptor class B type I and lectin-like oxidized low-density lipoprotein receptor-1. In addition to the initially assigned role as a mediator of the uptake of macromolecules into the cell, a large number of studies in cultured cells and in in vivo animal models have revealed that these lipoprotein receptors participate in signal transduction to modulate cellular functions. This review highlights the signalling pathways by which these receptors influence the process of atherosclerosis development, focusing on their roles in the vascular cells, such as macrophages, endothelial cells, smooth muscle cells, and platelets. Human genetics of the receptors is also discussed to further provide the relevance to cardiovascular disease risks in humans. Further knowledge of the vascular biology of the lipoprotein receptors and their ligands will potentially enhance our ability to harness the mechanism to develop novel prophylactic and therapeutic strategies against cardiovascular diseases.

Platelet lipidome: Dismantling the "Trojan horse" in the bloodstream

The platelet-lipid chapter in the story of atherothrombosis is an old one, recapitulated and revised in many contexts. For decades several stimulating facets have been added to it, both unraveling and increasing the perplexity of platelet-lipid interplay and its pathophysiological consequences. The recent paradigm shift in our perspective has evolved with lipidomic analysis of the intraplatelet compartment and platelet releasate. These investigations have disclosed that platelets are in constant interaction with circulatory lipids, often reflected in their lipid repertoire. In addition, they offer a shielded intracellular space for oxidative lipid metabolism generating "toxic" metabolites that escape degradation by plasma lipases and antioxidant defense, circulate undetected by conventional plasma lipid profile, and deposited at atherosclerotic lesions or thrombus. Lipidomics divulges this silent invader in platelet vehicles, thereby providing potential biomarkers of pathologic manifestations and therapeutic targets to be exploited, which is surmised in this review.

APOB gene polymorphisms may affect the risk of minor or minimal bleeding complications in patients on warfarin maintaining therapeutic INR

The purpose of this study was to investigate influence of gene polymorphisms of APOB and APOE on risk of bleeding complications at therapeutic INR, during warfarin treatment in Korean patients with mechanical cardiac valves. The study included 142 patients from the EwhA-Severance Treatment Group (EAST) of Warfarin. A total of 12 SNPs was investigated. Five SNPs of APOB (c.13013G>A, c.1853C>T, c.1594C>T, c.293C>T, and c.7545C>T) and five SNPs of APOE (g.4798T>G, g.6406G>A, g.10413T>C, c.388T>C, and c.526C>T) were selected. In addition to selected SNPs, VKORC1 g.6399C>T, and CYP2C9 c.1075A>C, which were known to have significant effects on warfarin stable doses, were also included in the study. Two SNPs of APOB (c.293C>T and c.1853C>T) were associated with bleeding complications. T allele carriers of c.293C>T had 8.6 times (95% CI 2.9-25.5, p < 0.001) increased risk of bleeding, and attributable risk was 88.3%. C allele carriers of c.1853C>T had 6.4 times (95% CI 2.3-17.9, p < 0.001) increased risk of bleeding after adjusting for covariates (attributable risk of 84.3%). AUROC values of models that included c.1853C>T and c.293C>T were 0.771 and 0.802, respectively. Among demographic characteristics, age was the only significant factor. This study revealed that APOB was associated with bleeding complications in patients with warfarin treatment after mechanical cardiac valves.

Large triglyceride-rich lipoproteins from fasting patients with type 2 diabetes activate platelets

AIMS

Type 2 diabetes (T2D) patients present with risk factors for atherothrombosis such as fasting hypertriglyceridaemia and platelet hyperactivity. Our study objective was to determine the effect of large triglyceride-rich lipoproteins (TGRL) from fasting T2D patients on platelet aggregation and, if any, to identify the signaling pathway involved.

METHODS

Large TGRL were isolated from the plasma of 25 T2D patients by ultracentrifugation (density < 1.000 g/mL). Platelets were isolated from healthy blood donors (HBD) and suspended in buffer, then preincubated in the presence or absence of TGRL and stimulated with either collagen or thrombin. Platelet aggregation and the arachidonic acid (AA) signaling pathway were studied.

RESULTS

Fasting T2D large TGRL were mostly of hepatic origin (apoB100/apoB48 ratio: 42 ± 7) and rich in triglycerides (TG/total apoB ratio: 4.2 ± 0.5), and able to potentiate agonist-stimulated platelet aggregation (collagen: +68%, P < 0.05; thrombin: +771%, P < 0.05). It should also be mentioned that TGRL from the plasma of HBD (n = 7) had no effect on platelet aggregation. In addition, T2D large TGRL increased thromboxane B₂ (TxB₂) concentration in platelets stimulated with either collagen (+34%, P < 0.05) or thrombin (+37%, P < 0.05) compared with platelets stimulated with either of these agonists without TGRL. Phosphorylation of p38 MAPK and cytosolic phospholipase A₂ (cPLA₂) was enhanced after incubation of platelets with T2D TGRL and thrombin (+87% and +32%, respectively, P < 0.05) compared with platelets incubated with thrombin only.

CONCLUSION

Large TGRL from fasting T2D patients may play a role in the development of atherothrombosis by increasing platelet aggregation and activating the platelet AA signaling pathway.

Platelet Activation and Platelet-Leukocyte Aggregates in Type I Diabetes Mellitus

Hyperglycemia alone may not explain the increased risk of cardiovascular diseases (CVDs) in patients with type 1 diabetes (T1D) compared with type 2. This study emphases on the evaluation of some platelet activity markers in patients with T1D, with relevance to some metabolic disorders as hyperlipidemia and hyperglycemia. This study was performed on 35 patients with T1D and 20 healthy controls. All participants were subjected to full history taking, clinical examination and assay of glycated hemoglobin (HbA_1c), and lipid profile. The expression of CD62P and CD36 on platelets and the frequency of platelet–monocyte, and platelet–neutrophil aggregates were assessed by flow cytometry. Patients showed significantly higher expression of CD62P and CD36 than the control group. Platelets aggregates with monocytes were also higher among patients than the control group. Levels of CD36⁺ platelets, CD62P⁺ platelets, and platelet–monocyte aggregates revealed significant correlations with the levels of HbA_1c, total cholesterol, low-density lipoprotein, and triglycerides. Hyperlipidemia and hyperglycemia accompanying T1D have a stimulatory effect on platelet activation which probably makes those patients vulnerable to CVD than nondiabetics.

Further Investigations of the Effects of Anti-β2GP1 Antibodies on Collagen-Induced Platelet Aggregation

Anti-beta-2-glycoprotein 1 (anti-β₂GP1) antibodies are associated with increased thrombotic risk in patients with autoimmune disease. There is conflicting evidence on the effects of anti-β₂GP1 antibodies on platelets, with both enhanced and inhibited aggregation previously reported. However, previous studies did not include isotype antibodies to ensure the observed effects were due to anti-β₂GP1 antibodies. The aims of this study were to (1) investigate the effects of anti-β₂GP1 antibodies on collagen-induced platelet aggregation in parallel with negative control (buffer normal saline) and isotype control antibodies and (2) determine the lupus anticoagulant (LA) activity of anti-β₂GP1 antibodies used. Three animal-derived anti-human-β₂GP1 antibodies (1.25, 2.5, and 5 μg/mL) incubated with healthy platelet-rich plasma were activated by collagen (2.5 μg/mL). Each anti-β₂GP1 antibody demonstrated the inhibition of aggregation compared to negative control, but not to isotype control. No anti-β₂GP1 antibody demonstrated LA activity, suggesting they were probably nonpathological. This study highlights both negative and isotype control markers are important to validate the effects of anti-β₂GP1 antibodies. Assays to measure anti-domain I-β₂GP1 antibodies are recommended to be used in conjunction with functional measures to further investigate the effects of anti-β₂GP1 antibodies.

TG haplotype in the LRP8 is associated with myocardial infarction in south Indian population

Myocardial infarction (MI) is a complex multifactorial cardiovascular disease. India experiences a much greater burden of MI, also suggesting an experimental increase of this burden in the future. The absolute reasons for MI are context dependent and differ with different geographical settings. Several reports indicate that SNPs that are associated with certain diseases in other populations may not be associated with Indian population. It is, therefore, important to validate the association of SNPs. Low density lipoprotein receptor related protein 8 (LRP8) gene plays central role in human lipoprotein metabolism as it facilitates the clearance of bad cholesterol LDL, VLDL from plasma and is reported to be associated with MI in the western population. However, this gene has not been studied in the South Indian population. We aim to test the role of the LRP8 gene variants correlating with the lipid profile in MI patients in South Indian population. We sequenced regions of SNPs rs10788952, rs7546246, rs2297660 and rs5174 of LRP8 in 100 MI patients and 100 age-matched controls. Our result revealed a total of 4 variations. None of the SNPs were significantly associated with MI (p>0.973). Interestingly, haplotype based association analysis showed TG and CG of rs10788952 and rs7546246 significantly associated with MI (p<0.01 and p<0.00005) and in particular, haplotype TG was positively correlated with the risk of MI, as this increased the LDL and total cholesterol level in MI patients in south Indians. Our results suggest that haplotype TG is a risk factor for MI in South Indian population.

Discovery and refinement of genetic loci associated with cardiometabolic risk using dense imputation maps

Large-scale whole-genome sequence data sets offer novel opportunities to identify genetic variation underlying human traits. Here we apply genotype imputation based on whole-genome sequence data from the UK10K and 1000 Genomes Project into 35,981 study participants of European ancestry, followed by association analysis with 20 quantitative cardiometabolic and hematological traits. We describe 17 new associations, including 6 rare (minor allele frequency (MAF) < 1%) or low-frequency (1% < MAF < 5%) variants with platelet count (PLT), red blood cell indices (MCH and MCV) and HDL cholesterol. Applying fine-mapping analysis to 233 known and new loci associated with the 20 traits, we resolve the associations of 59 loci to credible sets of 20 or fewer variants and describe trait enrichments within regions of predicted regulatory function. These findings improve understanding of the allelic architecture of risk factors for cardiometabolic and hematological diseases and provide additional functional insights with the identification of potentially novel biological targets.

β2GP1, Anti-β2GP1 Antibodies and Platelets: Key Players in the Antiphospholipid Syndrome

Anti-beta 2 glycoprotein 1 (anti-β2GP1) antibodies are commonly found in patients with autoimmune diseases such as the antiphospholipid syndrome (APS) and systemic lupus erythematosus (SLE). Their presence is highly associated with increased risk of vascular thrombosis and/or recurrent pregnancy-related complications. Although they are a subtype of anti-phospholipid (APL) antibody, anti-β2GP1 antibodies form complexes with β2GP1 before binding to different receptors associated with anionic phospholipids on structures such as platelets and endothelial cells. β2GP1 consists of five short consensus repeat termed "sushi" domains. It has three interchangeable conformations with a cryptic epitope at domain 1 within the molecule. Anti-β2GP1 antibodies against this cryptic epitope are referred to as 'type A' antibodies, and have been suggested to be more strongly associated with both vascular and obstetric complications. In contrast, 'type B' antibodies, directed against other domains of β2GP1, are more likely to be benign antibodies found in asymptomatic patients and healthy individuals. Although the interactions between anti-β2GP1 antibodies, β2GP1, and platelets have been investigated, the actual targeted metabolic pathway(s) and/or receptor(s) involved remain to be clearly elucidated. This review will discuss the current understanding of the interaction between anti-β2GP1 antibodies and β2GP1, with platelet receptors and associated signalling pathways.

Non-HDL cholesterol is an independent risk factor for aspirin resistance in obese patients with type 2 diabetes

OBJECTIVE

We evaluated the prevalence of aspirin resistance and predictive factors for aspirin resistance in Korean type 2 diabetes patients.

APPROACH AND RESULTS

A total of 1045 type 2 diabetes patients from 11 hospitals who were taking aspirin (100 mg/day for ≥2 weeks) and no other antiplatelet agents were studied to evaluate aspirin resistance. Aspirin resistance was measured in aspirin reaction units using VerifyNow(®). Aspirin resistance was defined as ≥550 aspirin reaction units. Aspirin resistance was detected in 102 of the 1045 subjects (prevalence 9.8%). Aspirin resistance was associated with total cholesterol (P = 0.013), LDL-cholesterol (P = 0.028), and non-HDL cholesterol (P = 0.008) concentrations in univariate analysis. In multivariate logistic regression analysis, only non-HDL cholesterol was associated with aspirin resistance in obese (BMI >25 kg/m(2)) type 2 diabetes patients (adjusted odds ratio 3.55, 95% CI: 1.25-10.05, P = 0.017).

CONCLUSIONS

The prevalence of aspirin resistance in Korean type 2 diabetes patients is 9.8%. Non-HDL cholesterol is an independent risk factor for aspirin resistance, especially in obese type 2 diabetes patients.

Platelets in atherosclerosis

Beyond obvious functions in haemostasis and thrombosis, platelets are considered to be essential in proinflammatory surroundings such as atherosclerosis, allergy, rheumatoid arthritis and even cancer. In atherosclerosis, platelets facilitate the recruitment of inflammatory cells towards the lesion sites and release a plethora of inflammatory mediators, thereby enriching and boosting the inflammatory milieu. Platelets do so by interacting with endothelial cells, circulating leukocytes (monocytes, neutrophils, dendritic cells, T-cells) and progenitor cells. This cross-talk enforces leukocyte activation, adhesion and transmigration. Furthermore, platelets are known to function in innate host defense through the release of antimicrobial peptides and the expression of pattern recognition receptors. In severe sepsis, platelets are able to trigger the formation of neutrophil extracellular traps (NETs), which bind and clear pathogens. The present antiplatelet therapies that target key pathways of platelet activation and aggregation therefore hold the potential to modulate platelet-derived immune functions by reducing cellular interactions of platelets with other immune components and by reducing the secretion of inflammatory proteins into the milieu. The objective of this review is to update and discuss the current perceptions of the platelet immune constituents and their prospect as therapeutic targets in an atherosclerotic setting.

Molecular mining of GGAA tagged transcripts and their expression in water buffalo Bubalus bubalis

Repeat sequences are involved in regulation of gene expression both at the transcriptional and translational level. In the mammalian genomes, tri- and tetranucleotide repeats like ATA, AATA, GGAA and GAAA have been associated with diseases. In silico analysis of (GGAA)5 distribution across the species showed maximum number of this repeat in the mouse transcriptome compared to that in other species. Following this, we conducted minisatellite associated sequence amplification (MASA) to explore the buffalo's transcriptome using cDNA from different tissues and an oligo based on (GGAA)5 repeats. MASA uncovered twenty six mRNA transcripts showing homology to known genes in the database. qPCR studies showed the highest expression of twelve transcripts in the spleen. A transcript, pLRC107 with its partial sequence of 203 nucleotides showed sequence variation at several positions in spleen as compared to other four tissues examined. Transcript pLRC100 was found to represent the partial coding sequence of Bos taurus HECT {(homologous to E6-associated protein (UBE3A) carboxyl-terminus domain) and RCC1 (CHC1)-like domain (RLD) 1}, mRNA. We ascertained full length coding sequence of HECT gene and localized the same on buffalo chromosome 10 employing FISH. This gene was found to be conserved across the species. Another gene LRP8 uncovered in the process showed copy number variation between buffalo males (4-9) and females (34-54). The MASA approach enabled us to identify several genes in Bubalus bubalis without screening an entire cDNA library. The highest expression of 12 mRNA transcripts in spleen suggests their likely involvement with immuno transaction. A comprehensive knowledge of the repeat tagged transcriptomes is envisaged to help in understanding their significance in genome organization and evolution forming rich basis of functional and comparative genomics.

Contribution of the P2Y12 receptor-mediated pathway to platelet hyperreactivity in hypercholesterolemia

BACKGROUND

In hypercholesterolemia, platelets demonstrate increased reactivity and promote the development of cardiovascular disease.

OBJECTIVE

This study was carried out to investigate the contribution of the ADP receptor P2Y12-mediated pathway to platelet hyperreactivity due to hypercholesterolemia.

METHODS

Low-density lipoprotein receptor-deficient mice and C57Bl/6 wild-type mice were fed on normal chow and high-fat (Western or Paigen) diets for 8 weeks to generate differently elevated cholesterol levels. P2Y12 receptor-induced functional responses via G(i) signaling were studied ex vivo when washed murine platelets were activated by 2MeSADP and PAR4 agonist AYPGKF in the presence and absence of indomethacin. Platelet aggregation and secretion, α(IIb)β(3) receptor activation and the phosphorylation of extracellular signal-regulated protein kinase (ERK) and Akt were analyzed.

RESULTS

Plasma cholesterol levels ranged from 69 ± 10 to 1011 ± 185 mg dL(-1) depending on diet in mice with different genotypes. Agonist-dependent aggregation, dense and α-granule secretion and JON/A binding were gradually and significantly (P < 0.05) augmented at low agonist concentration in correlation with the increasing plasma cholesterol levels, even if elevated thromboxane generation was blocked. These functional responses were induced via increased levels of G(i) -mediated ERK and Akt phosphorylation in hypercholesterolemic mice vs. normocholesterolemic animals. In addition, blocking of the P2Y12 receptor by AR-C69931MX (Cangrelor) resulted in strongly reduced platelet aggregation in mice with elevated cholesterol levels compared with normocholesterolemic controls.

CONCLUSIONS

These data revealed that the P2Y12 receptor pathway was substantially involved in platelet hyperreactivity associated with mild and severe hypercholesterolemia.

Apolipoprotein E induces antiinflammatory phenotype in macrophages

OBJECTIVE

Apolipoprotein E (apoE) exerts potent antiinflammatory effects. Here, we investigated the effect of apoE on the functional phenotype of macrophages.

METHODS AND RESULTS

Human apoE receptors very-low-density lipoprotein receptor (VLDL-R) and apoE receptor-2 (apoER2) were stably expressed in RAW264.7 mouse macrophages. In these cells, apoE downregulated markers of the proinflammatory M1 phenotype (inducible nitric oxide synthase, interleukin [IL]-12, macrophage inflammatory protein-1α) but upregulated markers of the antiinflammatory M2 phenotype (arginase I, SOCS3, IL-1 receptor antagonist [IL-1RA]). In addition, M1 macrophage responses (migration, generation of reactive oxygen species, antibody-dependent cell cytotoxicity, phagocytosis), as well as poly(I:C)- or interferon-γ-induced production of proinflammatory cytokines; cyclooxygenase-2 expression; and activation of nuclear factor-κB, IκB, and STAT1, were suppressed in VLDL-R- or apoER2-expressing cells. Conversely, the suppression of the M2 phenotype and the enhanced response to poly(I:C) were observed in apoE-producing bone marrow macrophages derived from VLDL-R-deficient mice but not wild-type or low-density lipoprotein receptor-deficient mice. The modulatory effects of apoE on macrophage polarization were inhibited in apoE receptor-expressing RAW264.7 cells exposed to SB220025, a p38 mitogen-activated protein kinase inhibitor, and PP1, a tyrosine kinase inhibitor. Accordingly, apoE induced tyrosine kinase-dependent activation of p38 mitogen-activated protein kinase in VLDL-R- or apoER2-expressing macrophages. Under in vivo conditions, apoE-/- mice transplanted with apoE-producing wild-type bone marrow showed increased plasma IL-1RA levels, and peritoneal macrophages of transplanted animals were shifted to the M2 phenotype (increased IL-1RA production and CD206 expression).

CONCLUSIONS

ApoE signaling via VLDL-R or apoER2 promotes macrophage conversion from the proinflammatory M1 to the antiinflammatory M2 phenotype. This effect may represent a novel antiinflammatory activity of apoE.

Scavenger receptors: Implications in atherothrombotic disorders

Scavenger receptors are modified lipoprotein binding receptors, expressed on the surface of a variety of cells including endothelial, macrophages and platelets. The most extensively studied class B scavenger receptors comprise of CD36 and SR-BI and have been found to bind to native and modified LDL. Interaction of modified LDL to CD36 accelerates foam cell formation, the key step in atherosclerotic plaque deposition. Recently scavenger receptors have also been implicated in thrombosis. Platelet CD36 serves as a sensor of oxidative stress and modulator of platelet reactivity under hyperlipidemic conditions thus, inducing prothrombotic signals. In contrast, targeting platelet SR-BI corresponds to reduce platelet hyperreactivity in hyperlipidemia suggesting that targeting these receptors could be a promising strategy for the treatment of atherothrombotic disorders.

Apolipoprotein E receptor 2 is involved in the thrombotic complications in a murine model of the antiphospholipid syndrome

Antiphospholipid (aPL)/anti-β(2) glycoprotein I (anti-β(2)GPI) antibodies stimulates tissue factor (TF) expression within vasculature and in blood cells, thereby leading to increased thrombosis. Several cellular receptors have been proposed to mediate these effects, but no convincing evidence for the involvement of a specific one has been provided. We investigated the role of Apolipoprotein E receptor 2 (ApoER2') on the pathogenic effects of a patient-derived polyclonal aPL IgG preparation (IgG-APS), a murine anti-β(2)GPI monoclonal antibody (E7) and of a constructed dimeric β(2)GPI I (dimer), which in vitro mimics β(2)GPI-antibody immune complexes, using an animal model of thrombosis, and ApoER2-deficient (-/-) mice. In wild type mice, IgG-APS, E7 and the dimer increased thrombus formation, carotid artery TF activity as well as peritoneal macrophage TF activity/expression. Those pathogenic effects were significantly reduced in ApoER2 (-/-) mice. In addition, those effects induced by the IgG-APS, by E7 and by the dimer were inhibited by treatment of wild-type mice with soluble binding domain 1 of ApoER2 (sBD1). Altogether these data show that ApoER2 is involved in pathogenesis of antiphospholipids antibodies.

High-density lipoproteins, platelets and the pathogenesis of atherosclerosis

1. Prospective and interventional studies demonstrate an inverse relationship between plasma high-density lipoprotein (HDL)-cholesterol and the incidence of coronary artery disease. Although the atheroprotective effects of HDL are usually attributed to the reverse cholesterol transport, in which HDL shuttles cholesterol from cells in the arterial wall to the liver, other mechanisms are also under investigation. 2. Platelets are involved in both the initiation and progression of atherosclerotic lesions. In addition, the formation of thrombi over ruptured atherosclerotic plaques results in the narrowing or complete occlusion of coronary arteries. Current experimental evidence suggests that HDL may exert antiplatelet effects and thereby counteract the development of atherothrombotic vascular disease. 3. In vitro studies show that HDL inhibits agonist-stimulated platelet aggregation, fibrinogen binding, granule secretion and liberation of thromboxane A(2). Inhibitory effects of HDL are mediated, in part, by scavenger receptor type B1 and/or the apolipoprotein E receptor apoER2/LRP8 and are linked to the induction of intracellular signalling cascades encompassing stimulation of protein kinase C, cytoplasmatic alkalization and generation of nitric oxide. 4. Populational studies demonstrate that there is an inverse association between plasma HDL levels and recurrent venous thromboembolism. In addition, HDL-cholesterol has been identified as an independent predictor of acute platelet thrombus formation. The administration of reconstituted HDL particles in humans attenuates ex vivo platelet activation. 5. The present review summarizes recent advances in understanding HDL-platelet interactions and discusses the potential use of HDL-like particles in the therapy of thrombosis.

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.

Identification of coagulation factor XI as a ligand for platelet apolipoprotein E receptor 2 (ApoER2)

OBJECTIVE

Factor XI (FXI) promotes hemostasis and thrombosis through enhancement of thrombin generation and has been shown to play a critical role in the formation of occlusive thrombi in arterial injury models. The aim of this study was to investigate the mechanisms governing interactions between FXI and platelets.

METHODS AND RESULTS

Platelet adhesion to immobilized FXI was abrogated in the presence of the low-density lipoprotein (LDL) receptor antagonist, receptor-associated protein (RAP), soluble recombinant apolipoprotein E receptor 2 (ApoER2), or the LDL-binding domain 1 or 2 of ApoER2. FXI supported wild-type murine platelet binding; in contrast, ApoER2-deficient murine platelets did not adhere to FXI. In the presence of shear, platelet aggregates formed on FXI or activated FXI (FXIa) surfaces, whereas the presence of RAP, binding domain 1 of ApoER2, or an anti-GPIb alpha mAb blocked platelet adhesion to FXI or FXIa under shear. Soluble FXI bound to immobilized ApoER2' with an affinity of 61 nmol/L.

CONCLUSIONS

This study has identified apolipoprotein E receptor 2 (ApoER2, LRP8), a member of the LDL receptor family, as a platelet receptor for FXI. The interaction of FXI with other cell types that express ApoER2 remains to be explored.

Plasminogen kringle 5 induces apoptosis of brain microvessel endothelial cells: sensitization by radiation and requirement for GRP78 and LRP1

Recombinant plasminogen kringle 5 (rK5) has been shown to induce apoptosis of dermal microvessel endothelial cells (MvEC) in a manner that requires glucose-regulated protein 78 (GRP78). As we are interested in antiangiogenic therapy for glioblastoma tumors, and the effectiveness of antiangiogenic therapy can be enhanced when combined with radiation, we investigated the proapoptotic effects of rK5 combined with radiation on brain MvEC. We found that rK5 treatment of brain MvEC induced apoptosis in a dose- and time-dependent manner and that prior irradiation significantly sensitized (500-fold) the cells to rK5-induced apoptosis. The rK5-induced apoptosis of both unirradiated and irradiated MvEC required expression of GRP78 and the low-density lipoprotein receptor-related protein 1 (LRP1), a scavenger receptor, based on down-regulation studies with small interfering RNA, and blocking studies with either a GRP78 antibody or a competitive inhibitor of ligand binding to LRP1. Furthermore, p38 mitogen-activated protein kinase was found to be a necessary downstream effector for rK5-induced apoptosis. These data suggest that irradiation sensitizes brain MvEC to the rK5-induced apoptosis and that this signal requires LRP1 internalization of GRP78 and the activation of p38 mitogen-activated protein kinase. Our findings suggest that prior irradiation would have a dose-sparing effect on rK5 antiangiogenic therapy for brain tumors and further suggest that the effects of rK5 would be tumor specific, as the expression of GRP78 protein is up-regulated on the brain MvEC in glioblastoma tumor biopsies compared with the normal brain.

Downregulation of platelet responsiveness upon contact with LDL by the protein-tyrosine phosphatases SHP-1 and SHP-2

OBJECTIVE

The sensitivity of platelets to aggregating agents increases when low-density lipoprotein (LDL) binds to apolipoprotein E receptor 2' (apoER2'), triggering activation of p38MAPK and formation of thromboxane A2. LDL signaling is terminated by PECAM-1 through recruitment and activation of the Ser/Thr protein phosphatase PP2A, but platelets remain unresponsive to LDL when PECAM-1 activation disappears. We report a second mechanism that halts LDL signaling and in addition lowers platelet responsiveness to aggregating agents.

METHODS AND RESULTS

After a first stimulation with LDL, platelets remain unresponsive to LDL for 60 minutes, despite normal apoER2' activation by a second dose of LDL. A possible cause is persistent activation of the tyrosine phosphatases SHP-1 and SHP-2, which may not only block a second activation of p38MAPK, PECAM-1, and PP2A by LDL but also seem to reduce aggregation by TRAP, collagen, and ADP.

CONCLUSION

These findings reveal that p38MAPK phosphorylation and platelet activation by LDL are suppressed by two mechanisms: (1) short activation of PECAM-1/PP2A, and (2) prolonged activation of SHP-1 and SHP-2. Activation of SHP-1 and SHP-2 is accompanied by reduced responsiveness to aggregating agents, which--if present in vivo--would make LDL an aggregation inhibitor during prolonged contact with platelets.

Platelets and the antiphospholipid syndrome

The antiphospholipid syndrome is a non-inflammatory autoimmune disease characterised by the presence of antiphospholipid antibodies in the plasma of patients with venous or arterial thrombosis or recurrent complications of pregnancy. The strong relation between the presence of antibodies against anionic phospholipids and thrombo-embolic complications is well established, but how the presence of antiphospholipid antibodies results in the observed clinical manifestations remains a mystery. Experimental observations suggest that an altered regulation of platelet function can cause the thrombotic complications observed in the antiphospholipid syndrome. In this review, we will discuss the evidence that the platelet is an important player in the pathogenesis of the antiphospholipid syndrome.

Deficiency of LRP8 in mice is associated with altered platelet function and prolonged time for in vivo thrombosis

INTRODUCTION

Our group has previously reported genetic studies associating polymorphisms in the low density lipoprotein receptor related protein 8 (LRP8) gene with myocardial infarction. The aim of this study was to define the role of platelet surface LRP8 in thrombosis.

MATERIALS AND METHODS

Flow cytometry, aggregometry, intravital microscopy and tail bleeding assays were used to examine platelet function and hemostasis in LRP8-deficient mice and littermate controls.

RESULTS

We demonstrated that activation of platelets from both LRP8(+/-) and LRP8(-/-) mice was reduced in vitro in response to either ADP or thrombin. In vivo, LRP8-hemizygous and LRP8(-/-) mice demonstrated 200% and 68% increased time for carotid occlusion in response to FeCl(3) injury, respectively. Moreover, lipidated apoE3, a ligand for LRP8, inhibited platelet activation in a dose-dependent fashion. This inhibition was markedly attenuated in LRP8(-/-) but not LRP8(+/-) mice and did not result from membrane cholesterol efflux or a nitric oxide dependent pathway. Tail bleeding times were unaffected in both genotypes.

CONCLUSIONS

Our results suggest that LRP8 is capable of altering thrombosis without affecting normal hemostasis through mechanisms both dependent on and independent of apoE. This suggests a means whereby clot formation could be affected in humans with LRP8 gene variants.

Platelet activation by dimeric beta2-glycoprotein I requires signaling via both glycoprotein Ibalpha and apolipoprotein E receptor 2'

BACKGROUND

Dimerization of beta(2)-glycoprotein I (beta(2)-GPI) by autoantibodies is thought to trigger the clinical manifestations observed in the antiphospholipid syndrome. Arterial thrombosis, a frequently occurring clinical manifestation of the antiphospholipid syndrome, is a process in which platelets play a crucial role. Previous work has shown that binding of dimeric beta(2)-GPI to the platelet receptors apolipoprotein E receptor 2' (ApoER2') and glycoprotein Ibalpha (GPIbalpha) mediates increased platelet activation in an in vitro thrombosis model.

OBJECTIVE

The individual roles of ApoER2' and GPIbalpha in mediating platelet activation by dimeric beta(2)-GPI has hitherto been unclear. In this study, we have determined the roles of either receptor in platelet activation by dimeric beta(2)-GPI.

METHODS

Platelet activation by dimeric beta(2)-GPI was studied under conditions of flow. Intracellular signaling induced by dimeric beta(2)-GPI was subsequently analyzed by means of sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) and western blot analysis.

RESULTS

The increase in platelet deposition onto a fibronectin surface under conditions of flow by dimeric beta(2)-GPI was completely abolished by inhibition of the interaction of dimeric beta(2)-GPI with either GPIbalpha or ApoER2'. Upon platelet stimulation with dimeric beta(2)-GPI, GPIbalpha translocated to the cytoskeleton via the scaffold protein 14-3-3zeta. Concomitantly, ApoER2' dissociated from the adapter protein Disabled1, presumably through phosphorylation of the cytoplasmic tail. Inhibition of one process could not inhibit the other.

CONCLUSION

We show that dimeric beta(2)-GPI signals via two distinct pathways in platelets, both of which are required for platelet activation. Abrogation of either signal results in loss of activation.

An LRP8 variant is associated with familial and premature coronary artery disease and myocardial infarction

Our previous genomewide linkage scan of 428 nuclear families (GeneQuest) identified a significant genetic susceptibility locus for premature myocardial infarction (MI) on chromosome 1p34-36. We analyzed candidate genes in the locus with a population-based association study involving probands with premature coronary artery disease (CAD) and/or MI from the GeneQuest families (381 cases) and 560 controls without stenosis detectable by coronary angiography. A nonconservative substitution, R952Q, in LRP8 was significantly associated with susceptibility to premature CAD and/or MI by use of both population-based and family-based designs. Three additional white populations were used for follow-up replication studies: another independent cohort of CAD- and/or MI-affected families (GeneQuest II: 441 individuals from 22 pedigrees), an Italian cohort with familial MI (248 cases) and 308 Italian controls, and a separate Cleveland GeneBank cohort with sporadic MI (1,231 cases) and 560 controls. The association was significantly replicated in two independent populations with a family history of CAD and/or MI, the GeneQuest II family-based replication cohort and the Italian cohort, but not in the population with sporadic disease. The R952Q variant of LRP8 increased activation of p38 mitogen-activated protein kinase by oxidized low-density lipoprotein. This extensive study, involving multiple independent populations, provides the first evidence that genetic variants in LRP8 may contribute to the development of premature and familial CAD and MI.

Platelet activation by oxidized low density lipoprotein is mediated by CD36 and scavenger receptor-A

OBJECTIVE

The interaction of platelets with low density lipoprotein (LDL) contributes to the development of cardiovascular disease. Platelets are activated by native LDL (nLDL) through apoE Receptor 2' (apoER2')-mediated signaling to p38(MAPK) and by oxidized LDL (oxLDL) through lysophosphatidic acid (LPA) signaling to Rho A and Ca2+. Here we report a new mechanism for platelet activation by oxLDL.

METHODS AND RESULTS

Oxidation of nLDL increases p38(MAPK) activation through a mechanism that is (1) independent of LPA, and (2) unlike nLDL-signaling not desensitized by prolonged platelet-LDL contact or inhibited by receptor-associated protein or chondroitinase ABC. Antibodies against scavenger receptors CD36 and SR-A alone fail to block p38(MAPK) activation by oxLDL but combined blockade inhibits p38(MAPK) by >40% and platelet adhesion to fibrinogen under flow by >60%. Mouse platelets deficient in either CD36 or SR-A show normal p38(MAPK) activation by oxLDL but combined deficiency of CD36 and SR-A disrupts oxLDL-induced activation of p38(MAPK) by >70%.

CONCLUSION

These findings reveal a novel platelet-activating pathway stimulated by oxLDL that is initiated by the combined action of CD36 and SR-A.

Platelet activation by low density lipoprotein and high density lipoprotein

Cardiovascular disease is the main cause of death and disability in the Western society. Lipoproteins are important in the development of cardiovascular disease since they change the properties of different cells involved in atherosclerosis and thrombosis. The interaction of platelets with lipoproteins has been under intense investigation. Particularly the initiation of platelet signaling pathways by low density lipoprotein (LDL) has been studied thoroughly, since platelets of hypercholesterolemic patients, whose plasma contains elevated LDL levels due to absent or defective LDL receptors, show hyperaggregability in vitro and enhanced activity in vivo. These observations suggest that LDL enhances platelet responsiveness. Several signaling pathways induced by LDL have been revealed in vitro, such as signaling via p38 mitogen-activated protein kinase and p125 focal adhesion kinase. High density lipoprotein (HDL) consists of two subtypes, HDL(2) and HDL(3), which have opposing effects on platelet activation. This review provides a summary of the activation of signaling pathways after platelet-LDL and platelet-HDL interaction, with special emphasis on their role in the development of thrombosis and atherosclerosis.

Current concepts on the pathogenesis of the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is an important cause of acquired thrombophilia. It is characterized by the core clinical manifestations of thrombosis, either venous or arterial, and in women it can also be associated with recurrent fetal loss. The detection of persistently elevated levels of antiphospholipid antibodies (aPL Abs) is a requisite laboratory feature for the diagnosis to be made. The dominant antigenic targets in APS are beta 2-glycoprotein I (β2-GPI) and prothrombin. There is an accumulating body of experimental evidence that suggests that specific subgroups of aPL Abs may directly contribute to disease pathogenesis. This review critically examines the experimental evidence underlying the various propositions made to explain how these antibodies may predispose to disease in humans. Furthermore, it also examines the evidence relating to the immunologic mechanisms that may contribute to the breakage of peripheral tolerance in this disorder. Delineating the strengths and limitations of the experimental evidence accumulated thus far will hopefully stimulate further experimentation toward achieving the ultimate goal of precisely defining the dominant pathogenic mechanisms operational in APS. This may pave the way for the development of improved therapies.

Hepatitis C virus interacts with human platelet glycoprotein VI

Hepatitis C virus (HCV) interacts with human platelets in vivo as a potential transport of infectious virions to the target liver. The binding of native viral particles with the platelet membrane glycoprotein VI (GPVI) was analysed. A consistent interaction between HCV from plasma or after purification by two different methods and the recombinant extracellular immunoglobulin (Ig)-like domains of human GPVI (hD1D2) was observed with two independent experimental approaches: pull-down and ELISA assays. Between 2 and 7 % of HCV particles were specifically bound to hD1D2. The binding was inhibited by an anti-hD1D2 in a dose-dependent manner. Human D1D2 interaction with HCV was significantly higher than the murine D1D2, supporting the specificity of the interaction and to the single human domains (D1 and D2), suggesting that both Ig-like domains of the molecule are required for efficient binding. GPVI may be a platelet surface ligand for HCV playing a role in viral transport and persistence.

Lipoprotein-associated proteins involved in platelet signaling

Platelets and lipoproteins are both key elements in the development of atherosclerosis and thrombosis. Based on their density, five classes of lipoproteins have been identified which all influence platelets via distinct mechanisms. The activation of platelets starts with binding of apolipoproteins to different platelet receptors and is followed by the activation of signaling pathways resulting in activation or inhibition of platelet functions like aggregation or secretion. In addition to apolipoproteins, lipoproteins are also associated to a large amount of proteins, enzymes and lipids that also can induce platelet activation or inhibition. This review provides a summary of the activation of signaling pathways after platelet-lipoprotein interactions initiated by lipoprotein-associated proteins and lipids.

ApoER2 is endocytosed by a clathrin-mediated process involving the adaptor protein Dab2 independent of its Rafts' association

The apolipoprotein E receptor 2 (apoER2) is a member of the low-density lipoprotein receptor family which binds ligands such as reelin, apolipoprotein E and apolipoprotein J/clusterin and has been shown to play roles in neuronal migration during development and in male fertility. The function of apoER2 mainly depends on cellular signaling triggered by ligand binding. Although the receptor is internalized, the mechanism and functional significance of its endocytic trafficking remain unclear. Apolipoprotein E receptor 2 partitions into lipid rafts and interacts with caveolin-1, a feature that could modulate its endocytic behavior. Recent evidence also suggested that apoER2 might be endocytosed by a pathway independent of clathrin. Here, we show that despite a raft association, apoER2 internalization depends on its cytoplasmic FxNPXY motif that is similar to canonical motifs for clathrin-mediated endocytosis. This motif mediates receptor binding to the adaptor protein Dab2, which can interact directly with clathrin. Several inhibitory conditions of clathrin-mediated endocytosis, including expression of the dominant negative forms of eps15 and Dab2, decreased apoER2 internalization. In contrast, treatment with the drug nystatin, which blocks the caveolar/raft internalization pathway, has no effect on the receptor's endocytosis. Neither the transmembrane nor the proline-rich insert of the cytoplasmic domain, which has been previously reported to exclude the receptor from the clathrin-mediated pathway, altered apoER2 endocytic activity. These studies indicate that apoER2 internalizes through a clathrin-mediated pathway and that its association with caveolar and noncaveolar rafts does not determine its endocytosis.

The binding site in {beta}2-glycoprotein I for ApoER2' on platelets is located in domain V

The antiphospholipid syndrome is caused by autoantibodies directed against beta(2)-glycoprotein I (beta(2)GPI). Dimerization of beta(2)GPI results in an increased platelet deposition to collagen. We found that apolipoprotein E receptor 2' (apoER2'), a member of the low density lipoprotein receptor family, is involved in activation of platelets by dimeric beta(2)GPI. To identify which domain of dimeric beta(2)GPI interacts with apoER2', we have constructed domain deletion mutants of dimeric beta(2)GPI, lacking domain I (DeltaI), II (DeltaII), or V (DeltaV), and a mutant with a W316S substitution in the phospholipid (PL)-insertion loop of domain V. DeltaI and DeltaII prolonged the clotting time, as did full-length dimeric beta(2)GPI; DeltaV had no effect on the clotting time. Second, DeltaI and DeltaII bound to anionic PL, comparable with full-length dimeric beta(2)GPI. DeltaV and the W316S mutant bound with decreased affinity to anionic PL. Platelet adhesion to collagen increased significantly when full-length dimeric beta(2)GPI, DeltaI, or DeltaII (mean increase 150%) were added to whole blood. No increase was found with plasma beta(2)GPI, DeltaV, or the W316S mutant. Immunoprecipitation indicated that full-length dimeric beta(2)GPI, DeltaI, DeltaII, and the W316S mutant can interact with apoER2' on platelets. DeltaV did not associate with apoER2'. We conclude that domain V is involved in both binding beta(2)GPI to anionic PL and in interaction with apoER2' and subsequent activation of platelets. The binding site in beta(2)GPI for interaction with apoER2' does not overlap with the hydrophobic insertion loop in domain V.

Effect of oxidation on the platelet-activating properties of low-density lipoprotein

OBJECTIVE

Because of the large variation in oxidizing procedures and susceptibility to oxidation of low-density lipoprotein (LDL) and the lack in quantification of LDL oxidation, the role of oxidation in LDL-platelet contact has remained elusive. This study aims to compare platelet activation by native LDL (nLDL) and oxidized LDL (oxLDL).

METHODS AND RESULTS

After isolation, nLDL was dialyzed against FeSO4 to obtain LDL oxidized to well-defined extents varying between 0% and >60%. The oxLDL preparations were characterized with respect to their platelet-activating properties. An increase in LDL oxidation enhances platelet activation via 2 independent pathways, 1 signaling via p38(MAPK) phosphorylation and 1 via Ca2+ mobilization. Between 0% and 15% oxidation, the p38(MAPK) route enhances fibrinogen binding induced by thrombin receptor (PAR-1)-activating peptide (TRAP), and signaling via Ca2+ is absent. At >30% oxidation, p38(MAPK) signaling increases further and is accompanied by Ca2+ mobilization and platelet aggregation in the absence of a second agonist. Despite the increase in p38(MAPK) signaling, synergism with TRAP disappears and oxLDL becomes an inhibitor of fibrinogen binding. Inhibition is accompanied by binding of oxLDL to the scavenger receptor CD36, which is associated with the fibrinogen receptor, alpha(IIb)beta3.

CONCLUSIONS

At >30% oxidation, LDL interferes with ligand binding to integrin alpha(IIb)beta3, thereby attenuating platelet functions.

Survey of the year 2004 commercial optical biosensor literature

The year 2004 represents a milestone for the biosensor research community: in this year, over 1000 articles were published describing experiments performed using commercially available systems. The 1038 papers we found represent an approximately 10% increase over the past year and demonstrate that the implementation of biosensors continues to expand at a healthy pace. We evaluated the data presented in each paper and compiled a 'top 10' list. These 10 articles, which we recommend every biosensor user reads, describe well-performed kinetic, equilibrium and qualitative/screening studies, provide comparisons between binding parameters obtained from different biosensor users, as well as from biosensor- and solution-based interaction analyses, and summarize the cutting-edge applications of the technology. We also re-iterate some of the experimental pitfalls that lead to sub-optimal data and over-interpreted results. We are hopeful that the biosensor community, by applying the hints we outline, will obtain data on a par with that presented in the 10 spotlighted articles. This will ensure that the scientific community at large can be confident in the data we report from optical biosensors.