Domain 5 of Beta 2 glycoprotein I: Friend or foe in health? Context matters.

Beta 2 glycoprotein I (ß2GPI) is the major autoantigen in the antiphospholipid syndrome, an autoimmune disorder characterized by thrombotic and obstetric complications. The autoantibodies that target beta 2 glycoprotein I are pathogenic and contribute to disease pathogenesis. The ß2GPI molecule is composed of 5 domains that are numbered 1 through to 5. Autoantibodies bind mainly to domain 1 whereas the majority of the biological functions of the ß2GPI molecule in diverse processes such as apoptotic cell clearance, complement regulation, lipopolysaccharide clearance and anticoagulation have been localised to domain 5 and its unique biochemistry, reviewed in this article. The role of purified domain 5 peptide as a potential therapeutic agent in APS and ischemia reperfusion injury is discussed.

Βeta-2-glycoprotein I exerts antithrombotic function through its domain V in mice.

Little is known about the physiological role of beta-2-glycoprotein I (ß2GPI) despite it being the major auto-antigen in the antiphospholipid syndrome. A systematic study of the role of ß2GPI in thrombus formation in vivo has not been performed to date. Herein, we report that ß2GPI deficient (-/-) mice have enhanced thrombus formation compared to wild type (WT) mice in a laser-induced arteriole and venule model of thrombosis. Furthermore, neutrophil accumulation and elastase activity was enhanced in thrombi of ß2GPI -/- compared with WT mice. The antithrombotic function of ß2GPI is dependent on its fifth domain (domain V); intravenous administration of the ß2GPI domain deletion mutant lacking domain V (human recombinant domain I-IV) had no effect on platelet and fibrin thrombus size in ß2GPI -/- or WT mice. On the contrary, intravenous administration of human recombinant domain V significantly inhibited platelet and fibrin thrombus size in both ß2GPI -/- mice and WT mice. These findings reveal a major role for ß2GPI as a natural anticoagulant and implicate domain V of ß2GPI as a potential antithrombotic therapy.

Free Thiol ß2-GPI (ß-2-Glycoprotein-I) Provides a Link Between Inflammation and Oxidative Stress in Atherosclerotic Coronary Artery Disease.

OBJECTIVE: Atherosclerotic coronary artery disease is well recognised as an inflammatory disorder that is also influenced by oxidative stress. ß2-GPI (ß-2-glycoprotein-I) is a circulating plasma protein that undergoes post-translational modification and exists in free thiol as well as oxidized forms. The aim of this study was to assess the association between these 2 post-translational redox forms of ß2-GPI and atherosclerotic coronary artery disease. Approach and Results: Stable patients presenting for elective coronary angiography or CT coronary angiography were prospectively recruited. A separate group of patients after reperfused ST-segment-elevation myocardial infarction formed an acute coronary syndrome subgroup. All patients had collection of fasting serum and plasma for quantification of total and free thiol ß2-GPI. Coronary artery disease extent was quantified by the Syntax and Gensini scores. A total of 552 patients with stable disease and 44 with acute coronary syndrome were recruited. While total ß2-GPI was not associated with stable coronary artery disease, a higher free thiol ß2-GPI was associated with its presence and extent. This finding remained significant after correcting for confounding variables, and free thiol ß2-GPI was a better predictor of stable coronary artery disease than hs-CRP (high-sensitivity C-reactive protein). Paradoxically, there were lower levels of free thiol ß2-GPI after ST-segment-elevation myocardial infarction. CONCLUSIONS: Free thiol ß2-GPI is a predictor of coronary artery disease presence and extent in stable patients. Free thiol ß2-GPI was a better predictor than high-sensitivity C-reactive protein.

Posttranslational forms of beta 2-glycoprotein I in the pathogenesis of the antiphospholipid syndrome.

The antiphospholipid syndrome (APS) is an autoimmune disease characterised by a procoagulant state that predisposes to recurrent thrombosis and miscarriages. Two major discoveries have advanced our understanding of the underlying complex pathogenesis of the APS. The first was the discovery that beta-2 glycoprotein-1 (ß2GPI) is the major auto antigen in APS. The second was the discovery in more recent years that ß2GPI contains allosteric disulphide bonds susceptible to posttranslational modification that may be involved in the development of autoantibodies in APS. The main allosteric disulphide bond in the fifth domain of ß2GPI can exist in two redox states: free thiol or oxidised. It is the conformational transformation of ß2GPI from its free thiol form to its more immunogenic oxidised form that exposes neo-epitopes on the first and fifth domains. The purpose of this review is to highlight the recent findings on the posttranslational forms of ß2GPI in the pathogenesis of APS. We suggest that novel assays quantitating the different redox forms of ß2GPI in plasma or serum may be used to supplement existing clinical and laboratory assays to more accurately stratify risk of thrombosis or miscarriage in APS patients.

Factor XI is a substrate for oxidoreductases: enhanced activation of reduced FXI and its role in antiphospholipid syndrome thrombosis.

Factor XI (FXI), a disulfide-linked covalent homodimer, circulates in plasma, and upon activation initiates the intrinsic/consolidation phase of coagulation. We present evidence that disulfide bonds in FXI are reduced to free thiols by oxidoreductases thioredoxin-1 (TRX-1) and protein disulfide isomerase (PDI). We identified that Cys362-Cys482 and Cys118-Cys147 disulfide bonds are reduced by TRX-1. The activation of TRX-1-treated FXI by thrombin, FXIIa or FXIa was significantly increased compared to non-reduced FXI, indicating that the reduced factor is more efficiently activated than the oxidized protein. Using a novel ELISA system, we compared the amount of reduced FXI in antiphospholipid syndrome (APS) thrombosis patients with levels in healthy controls, and found that APS patients have higher levels of reduced FXI. This may have implication for understanding the contribution of FXI to APS thrombosis, and the predisposition to thrombosis in patients with elevated plasma levels of reduced FXI.

Naturally occurring free thiols within beta 2-glycoprotein I in vivo: nitrosylation, redox modification by endothelial cells, and regulation of oxidative stress-induced cell injury.

ß2-Glycoprotein I (ß2GPI) is an evolutionary conserved, abundant circulating protein. Although its function remains uncertain, accumulated evidence points toward interactions with endothelial cells and components of the coagulation system, suggesting a regulatory role in vascular biology. Our group has shown that thioredoxin 1 (TRX-1) generates free thiols in ß2GPI, a process that may have a regulatory role in platelet adhesion. This report extends these studies and shows for the first time evidence of ß2GPI with free thiols in vivo in both multiple human and murine serum samples. To explore how the vascular surface may modulate the redox status of ß2GPI, unstimulated human endothelial cells and EAhy926 cells are shown to be capable of amplifying the effect of free thiol generation within ß2GPI. Multiple oxidoreductase enzymes, such as endoplasmic reticulum protein 46 (ERp 46) and TRX-1 reductase, in addition to protein disulfide isomerase are secreted on the surface of endothelial cells. Furthermore, one or more of these generated free thiols within ß2GPI are also shown to be nitrosylated. Finally, the functional significance of these findings is explored, by showing that free thiol-containing ß2GPI has a powerful effect in protecting endothelial cells and EAhy926 cells from oxidative stress-induced cell death.

Novel assays of thrombogenic pathogenicity in the antiphospholipid syndrome based on the detection of molecular oxidative modification of the major autoantigen ß2-glycoprotein I.

OBJECTIVE: Beta-2-glycoprotein I (ß2 GPI) constitutes the major autoantigen in the antiphospholipid syndrome (APS), a common acquired cause of arterial and venous thrombosis. We recently described the novel observation that ß2 GPI may exist in healthy individuals in a free thiol (biochemically reduced) form. The present study was undertaken to quantify the levels of total, reduced, and posttranslationally modified oxidized ß2 GPI in APS patients compared to various control groups. METHODS: In a retrospective multicenter analysis, the proportion of ß2 GPI with free thiols in serum from healthy volunteers was quantified. Assays for measurement of reduced as well as total circulating ß2 GPI were developed and tested in the following groups: APS (with thrombosis) (n=139), autoimmune disease with or without persistent antiphospholipid antibodies (aPL) but without APS (n=188), vascular thrombosis without APS or aPL (n=38), and healthy volunteers (n=91). RESULTS: Total ß2 GPI was significantly elevated in patients with APS (median 216.2 µg/ml [interquartile range 173.3-263.8]) as compared to healthy subjects (median 178.4 µg/ml [interquartile range 149.4-227.5] [P<0.0002]) or control patients with autoimmune disease or vascular thrombosis (both P<0.0001). The proportion of total ß2 GPI in an oxidized form (i.e., lacking free thiols) was significantly greater in the APS group than in each of the 3 control groups (all P<0.0001). CONCLUSION: This large retrospective multicenter study shows that posttranslational modification of ß2 GPI via thiol-exchange reactions is a highly specific phenomenon in the setting of APS thrombosis. Quantification of posttranslational modifications of ß2 GPI in conjunction with standard laboratory tests for APS may offer the potential to more accurately predict the risk of occurrence of a thrombotic event in the setting of APS.

How I treat the antiphospholipid syndrome.

This article discusses how we approach medical decision making in the treatment of the various facets of the antiphospholipid syndrome (APS), including secondary prophylaxis in the setting of venous and arterial thrombosis, as well as treatment for the prevention of recurrent miscarriages and fetal death. The role of primary thromboprophylaxis is also discussed in depth. Great emphasis is given to incorporating the most up-to-date and relevant evidence base both from the APS literature, and from large, recent, randomized controlled trials (RCTs) of primary and secondary thrombotic prophylaxis in the general population setting (ie, the population that has not been specifically investigated for APS).

How we diagnose the antiphospholipid syndrome.

The antiphospholipid syndrome (APS) is an acquired thrombophilia, characterized by the occurrence of venous and arterial events. This article examines the laboratory and key clinical aspects of APS. Particular focus is given to anti-beta 2-glycoprotein I (beta(2)GPI) antibodies in view of their recent inclusion in the APS classification criteria. The clinical utility of using the beta(2)GPI enzyme-linked immunosorbent assay, in conjunction with the established lupus anticoagulant assays and cardiolipin enzyme-linked immunosorbent assay, for diagnosing and risk stratifying patients suspected of having APS is discussed. The relative importance of the various assays in diagnosing obstetric APS (early and late gestation miscarriages) is explored. The implications of recent epidemiologic findings for possibly understanding the underlying pathophysiologic mechanisms of obstetric APS are highlighted. Insights into which patients with obstetric APS may be at most risk of thrombotic complications are presented.

New insights into the biology and pathobiology of beta2-glycoprotein I.

ß2-glycoprotein I (ß2GPI) is the major autoantigen in the antiphospholipid syndrome. The central importance of understanding ß2GPI physiology from the perspective of the rheumatologist is that it forms the foundation for understanding the pathophysiology underlying autoantibody generation, and the diverse mechanisms by which anti-ß2GPI antibodies in complex with ß2GPI may predispose an individual to the antiphospholipid syndrome clinical phenotype. This review examines some of the latest novel findings in this area.

Oxidative post-translational modification of ßeta 2-glycoprotein I in the pathophysiology of the anti-phospholipid syndrome.

The anti-phospholipid syndrome (APS) is a prothrombotic autoimmune disorder characterized by either thrombosis or pregnancy complications in the setting of persistent anti-phospholipid antibodies (aPL). ßeta 2-glycoprotein I (ß2-GPI) is the major autoantigen in APS that binds anionic phospholipids as well as specific receptors on platelets and endothelial cells resulting in activation of prothrombotic pathways. ß2-GPI consists of 5 Domains that exist in a circular or linear form, with the latter occurring after binding to anionic phospholipids. ß2-GPI also undergoes dynamic posttranslational modification between oxidized and free thiol forms. The relationship between posttranslational modification and structural conformation is yet to be definitively clarified. Compared with controls, patients with the APS have higher levels of total ß2-GPI and lower levels of free thiol ß2-GPI. This raises the possibility of using quantification of ß2-GPI posttranslational modification as a redox biomarker in the management and diagnosis of the APS.

Dual roles of different redox forms of complement factor H in protecting against age related macular degeneration.

Complement Factor H (CFH) is an important inhibitor of the alternate complement pathway in Bruch's membrane (BM), located between the choriocapillaris and the retinal pigment epithelium. Furthermore dysfunction of its activity as occurs with certain polymorphisms is associated with an increased risk of age related macular degeneration (AMD). The retina is a site of high generation of reactive oxygen species (ROS) and dysfunction of redox homeostasis in this milieu also contributes to AMD pathogenesis. In this study we wanted to explore if CFH exists in distinct redox forms and whether these species have unique protective biological functions. CFH can be reduced by the naturally occurring thioredoxin - 1 in CFH domains 1-4, 17-20. We found a duality of function between the oxidised and reduced forms of CFH. The oxidised form was more efficient in binding to C3b and lipid peroxidation by-products that are known to accumulate in the retinae and activate the alternate complement pathway. Oxidised CFH enhances Factor I mediated cleavage of C3 and C3b whereas the reduced form loses this activity. In the setting of oxidative stress (hydrogen peroxide)-mediated death of human retinal pigment epithelial cells as can occur in AMD, the free thiol form of CFH offers a protective function compared to the oxidised form. We found for the first time using a novel ELISA system we have developed for free thiol CFH, that both redox forms of CFH are found in the human plasma. Furthermore there is a distinct ratio of these redox forms in plasma depending if an individual has early or late AMD, with individuals with early AMD having higher levels of the free thiol form compared to late AMD.

ß2GPI exerts an anti-obesity effect in female mice by inhibiting lipogenesis and promoting lipolysis.

In humans, males compared to females have increased visceral adipose tissue which contributes to their increased risk of early death. Mice display analogous sexual diamorphism whereby females are protected from weight gain when fed a high fat diet compared to males. A role has recently been reported for ß2-glycoprotein I, an abundant plasma protein, in healthy leanness in humans. In this study we investigated the role of ß2-glycoprotein I in fat metabolism in male and female mice fed a normal chow or high fat diet. We have made a number of novel insights into factors contributing to sexual diamorphism in obesity. Female wild type mice are protected from obesity when fed a high fat diet due to down regulation of lipogenesis in the visceral adipose tissues. This down regulation is due to ß2-glycoprotein I as female mice deficient in this protein have increased levels of lipogenesis enzymes in their visceral adipose tissues with an accompanying increase in weight compared to female wild type controls. Understanding female specific regulators of obesity may lead to sex specific anti-obesity therapies to address this major health problem.

Βeta 2-glycoprotein I protects mice against gram-negative septicaemia in a sexually dimorphic manner.

The immune responses of males and females to bacterial infections display differences. The mechanisms that underlie this sexual dimorphism are multifactorial. Lipopolysaccharide (LPS) contributes to the pathogenesis of endotoxaemia. We have previously demonstrated that the plasma protein beta-2 glycoprotein-1 (ß2GPI) reduces LPS-induced inflammation in male mice. In the present study using a more robust infection model of septicaemia the role of ß2GPI is examined in both male and female wild type (WT) and ß2GPI deficient (ß2GPI-/-) mice challenged with Escherichia coli (E. coli) intravenously. ß2GPI deficiency led to an increase of E. coli colony forming units (CFU) in the circulation of both male and female mice. In male ß2GPI-/- mice this was associated with a worse clinical severity score. This difference was not observed between female ß2GPI-/- and female WT mice. Male WT mice had decreased levels of total and increased levels of free thiol ß2GPI following administration of LPS or E. coli. This pattern of sexual dimorphic response was also observed in our cohort of humans with sepsis. These findings support a role for ß2GPI in modulating the sex-specific susceptibility to gram-negative septicaemia.

Nitration of tyrosines in complement factor H domains alters its immunological activity and mediates a pathogenic role in age related macular degeneration.

Nitrosative stress has been implicated in the pathogenesis of age related macular degeneration (AMD). Tyrosine nitration is a unique type of post translational modification that occurs in the setting of inflammation and nitrosative stress. To date, the significance and functional implications of tyrosine nitration of complement factor H (CFH), a key complement regulator in the eye has not been explored, and is examined in this study in the context of AMD pathogenesis.Sections of eyes from deceased individuals with AMD (n = 5) demonstrated the presence of immunoreactive nitrotyrosine CFH. We purified nitrated CFH from retinae from 2 AMD patients. Mass spectrometry of CFH isolated from AMD eyes revealed nitrated residues in domains critical for binding to heparan sulphate glycosaminoglycans (GAGs), lipid peroxidation by-products and complement (C) 3b.Functional studies revealed that nitrated CFH did not bind to lipid peroxidation products, nor to the GAG of perlecan nor to C3b. There was loss of cofactor activity for Factor I mediated cleavage of C3b with nitrated CFH compared to non-nitrated CFH. CFH inhibits, but nitrated CFH significantly potentiates, the secretion of the pro-inflammatory and angiogenic cytokine IL-8 from monocytes that have been stimulated with lipid peroxidation by-products. AMD patients (n = 30) and controls (n = 30) were used to measure plasma nitrated CFH using a novel ELISA. AMD patients had significantly elevated nitrated CFH levels compared to controls (p = 0.0117). These findings strongly suggest that nitrated CFH contributes to AMD progression, and is a target for therapeutic intervention.

The Fifth Domain of Beta 2 Glycoprotein I Protects from Natural IgM Mediated Cardiac Ischaemia Reperfusion Injury.

Reperfusion after a period of ischemia results in reperfusion injury (IRI) which involves activation of the inflammatory cascade. In cardiac IRI, IgM natural antibodies (NAb) play a prominent role through binding to altered neoepitopes expressed on damaged cells. Beta 2 Glycoprotein I (ß2GPI) is a plasma protein that binds to neoepitopes on damaged cells including anionic phospholipids through its highly conserved Domain V. Domain I of ß2GPI binds circulating IgM NAbs and may provide a link between the innate immune system, IgM NAb binding and cardiac IRI. This study was undertaken to investigate the role of Β2GPI and its Domain V in cardiac IRI using wild-type (WT), Rag-1 -/- and ß2GPI deficient mice. Compared with control, treatment with Domain V prior to cardiac IRI prevented binding of endogenous ß2GPI to post-ischemic myocardium and resulted in smaller myocardial infarction size in both WT and ß2GPI deficient mice. Domain V treatment in WT mice also resulted in less neutrophil infiltration, less apoptosis and improved ejection fraction at 24 h. Rag-1 -/- antibody deficient mice reconstituted with IgM NAbs confirmed that Domain V prevented IgM NAb induced cardiac IRI. Domain V remained equally effective when delivered at the time of reperfusion which has therapeutic clinical relevance.Based upon this study Domain V may function as a universal inhibitor of IgM NAb binding in the setting of cardiac IRI, which offers promise as a new therapeutic strategy in the treatment of cardiac IRI.

Gram Negative Bacterial Inflammation Ameliorated by the Plasma Protein Beta 2-Glycoprotein I.

Lipopolysaccharide (LPS) is a major component of the outer wall of gram negative bacteria. In high doses LPS contributes to the inflammation in gram negative sepsis, and in low doses contributes to the low grade inflammation characteristic of the metabolic syndrome. We wanted to assess the role of beta2-glycoprotein I (ß2GPI) a highly conserved plasma protein and its different biochemical forms in a mouse model of LPS systemic inflammation. Normal and ß2GPI deficient mice were administered LPS through their veins and assessed for a range of inflammation markers in their blood and liver. Different biochemical forms of ß2GPI were measured in normal mice given either saline or LPS. We show that ß2GPI has a significant role in inhibiting LPS induced inflammation. In this study we provide some evidence that ß2GPI serves a protective role in a mouse model of LPS inflammation. This resolves the controversy of previous studies which used LPS and ß2GPI in test tube based models of LPS induced activation of white cells. We also highlight the potential relevance of a newly discovered biochemical form of ß2GPI in LPS mediated inflammation and we speculate that this form has a protective role against LPS induced pathology.