Thrombomodulin: a multifunctional receptor modulating the endothelial quiescence

Thrombomodulin (TM) is a type 1 receptor best known for its function as an anticoagulant cofactor for thrombin activation of protein C on the surface of vascular endothelial cells. In addition to its anticoagulant cofactor function, TM also regulates fibrinolysis, complement, and inflammatory pathways. TM is a multidomain receptor protein with a lectin-like domain at its N-terminus that has been shown to exhibit direct anti-inflammatory functions. This domain is followed by 6 epidermal growth factor-like domains that support the interaction of TM with thrombin. The interaction inhibits the procoagulant function of thrombin and enables the protease to regulate the anticoagulant and fibrinolytic pathways by activating protein C and thrombin-activatable fibrinolysis inhibitor. TM has a Thr/Ser-rich region immediately above the membrane surface that harbors chondroitin sulfate glycosaminoglycans, and this region is followed by a single-spanning transmembrane and a C-terminal cytoplasmic domain. The structure and physiological function of the extracellular domains of TM have been extensively studied, and numerous excellent review articles have been published. However, the physiological function of the cytoplasmic domain of TM has remained poorly understood. Recent data from our laboratory suggest that intracellular signaling by the cytoplasmic domain of TM plays key roles in maintaining quiescence by modulating phosphatase and tensin homolog signaling in endothelial cells. This article briefly reviews the structure and function of extracellular domains of TM and focuses on the mechanism and possible physiological importance of the cytoplasmic domain of TM in modulating phosphatase and tensin homolog signaling in endothelial cells.

Thrombomodulin Switches Signaling and Protease-Activated Receptor 1 Cleavage Specificity of Thrombin

BACKGROUND

Cleavage of the extracellular domain of PAR1 (protease-activated receptor 1) by thrombin at Arg41 and by APC (activated protein C) at Arg46 initiates paradoxical cytopathic and cytoprotective signaling in endothelial cells. In the latter case, the ligand-dependent coreceptor signaling by EPCR (endothelial protein C receptor) is required for the protective PAR1 signaling by APC. Here, we investigated the role of thrombomodulin in determining the specificity of PAR1 signaling by thrombin.

METHODS

We prepared a PAR1 knockout (PAR1-/-) EA.hy926 endothelial cell line by CRISPR/Cas9 and transduced PAR1-/- cells with lentivirus vectors expressing PAR1 mutants in which either Arg41 or Arg46 was replaced with an Ala. Furthermore, human embryonic kidney 293 cells were transfected with wild-type or mutant PAR1 cleavage reporter constructs carrying N-terminal Nluc (NanoLuc luciferase) and C-terminal enhanced yellow fluorescent protein tags.

RESULTS

Characterization of transfected cells in signaling and receptor cleavage assays revealed that, upon interaction with thrombomodulin, thrombin cleaves Arg46 to elicit cytoprotective effects by a β-arrestin-2 biased signaling mechanism. Analysis of functional data and cleavage rates indicated that thrombin-thrombomodulin cleaves Arg46>10-fold faster than APC. Upon interaction with thrombin, the cytoplasmic domain of thrombomodulin recruited both β-arrestin-1 and -2 to the plasma membrane. Thus, the thrombin cleavage of Arg41 was also cytoprotective in thrombomodulin-expressing cells by β-arrestin-1-biased signaling. APC in the absence of EPCR cleaved Arg41 to initiate disruptive signaling responses like thrombin.

CONCLUSIONS

These results suggest that coreceptor signaling by thrombomodulin and EPCR determines the PAR1 cleavage and signaling specificity of thrombin and APC, respectively.

Thrombomodulin Regulates PTEN/AKT Signaling Axis in Endothelial Cells

BACKGROUND

We recently demonstrated that deletion of thrombomodulin gene from endothelial cells results in upregulation of proinflammatory phenotype. In this study, we investigated the molecular basis for the altered phenotype in thrombomodulin-deficient (TM-/-) cells.

METHODS

Different constructs containing deletions or mutations in the cytoplasmic domain of thrombomodulin were prepared and introduced to TM-/- cells. The phenotype of cells expressing different derivatives of thrombomodulin and tissue samples of thrombomodulin-knockout mice were analyzed for expression of distinct regulatory genes in established signaling assays.

RESULTS

The phosphatase and tensin homolog were phosphorylated and its recruitment to the plasma membrane was impaired in TM-/- cells, leading to hyperactivation of AKT (protein kinase B) and phosphorylation-dependent nuclear exclusion of the transcription factor, forkhead box O1. The proliferative/migratory properties of TM-/- cells were enhanced, and cells exhibited hypersensitivity to stimulation by angiopoietin 1 and vascular endothelial growth factor. Reexpression of wild-type thrombomodulin in TM-/- cells normalized the cellular phenotype; however, thrombomodulin lacking its cytoplasmic domain failed to restore the normal phenotype in TM-/- cells. Increased basal permeability and loss of VE-cadherin were restored to normal levels by reexpression of wild-type thrombomodulin but not by a thrombomodulin construct lacking its cytoplasmic domain. A thrombomodulin cytoplasmic domain deletion mutant containing 3-membrane-proximal Arg-Lys-Lys residues restored the barrier-permeability function of TM-/- cells. Enhanced phosphatase and tensin homolog phosphorylation and activation of AKT and mTORC1 (mammalian target of rapamycin complex 1) were also observed in the liver of thrombomodulin-KO mice.

CONCLUSIONS

These results suggest that the cytoplasmic domain of thrombomodulin interacts with the actin cytoskeleton and plays a crucial role in regulation of phosphatase and tensin homolog/AKT signaling in endothelial cells.

Activated protein C inhibits mesothelial-to-mesenchymal transition in experimental peritoneal fibrosis

BACKGROUND

In addition to its anticoagulant function in downregulating thrombin generation, activated protein C (APC) evokes pleiotropic cytoprotective signaling activities when it binds to endothelial protein C receptor (EPCR) to activate protease-activated receptor 1 (PAR1) in endothelial cells.

OBJECTIVES

To investigate the protective effect of APC in a chlorhexidine gluconate (CG)-induced peritoneal fibrosis model.

METHODS

Peritoneal fibrosis was induced in wild-type as well as EPCR- and PAR1-deficient mice via daily injection of CG (0.2 mL of 0.1% CG in 15% ethanol and 85% saline) for 21 days with or without concomitant injection of recombinant human APC derivatives (50 μg/kg of bodyweight). The expression of proinflammatory cytokines and profibrotic markers as well as collagen deposition were analyzed using established methods.

RESULTS

CG significantly upregulated the expression of transforming growth factor-β1 in peritoneal tissues, which culminated in the deposition of excessive extracellular matrix proteins, thickening of the peritoneal membrane, and mesothelial-to-mesenchymal transition in damaged tissues. APC potently inhibited CG-induced peritoneal fibrosis and downregulated the expression of proinflammatory cytokines, collagen deposition, Smad3 phosphorylation, and markers of mesothelial-to-mesenchymal transition (α-smooth muscle actin, vimentin, and N-cadherin). APC also inhibited transforming growth factor-β1-mediated upregulation of α-smooth muscle actin, Smad3, and fibronectin in human primary mesothelial cells. Employing signaling-selective and anticoagulant-selective variants of APC and mutant mice deficient for either EPCR or PAR1, we demonstrated that the EPCR-dependent signaling function of APC through PAR1 activation was primarily responsible for its antifibrotic activity in the CG-induced peritoneal fibrosis model.

CONCLUSION

APC and signaling-selective variants of APC may have therapeutic potential for preventing or treating pathologies associated with peritoneal fibrosis.

Illustrated State-of-the-Art Capsules of the ISTH 2022 Congress

The ISTH London 2022 Congress is the first held (mostly) face-to-face again since the COVID-19 pandemic took the world by surprise in 2020. For 2 years we met virtually, but this year's in-person format will allow the ever-so-important and quintessential creativity and networking to flow again. What a pleasure and joy to be able to see everyone! Importantly, all conference proceedings are also streamed (and available recorded) online for those unable to travel on this occasion. This ensures no one misses out. The 2022 scientific program highlights new developments in hemophilia and its treatment, acquired and other inherited bleeding disorders, thromboinflammation, platelets and coagulation, clot structure and composition, fibrinolysis, vascular biology, venous thromboembolism, women's health, arterial thrombosis, pediatrics, COVID-related thrombosis, vaccine-induced thrombocytopenia with thrombosis, and omics and diagnostics. These areas are elegantly reviewed in this Illustrated Review article. The Illustrated Review is a highlight of the ISTH Congress. The format lends itself very well to explaining the science, and the collection of beautiful graphical summaries of recent developments in the field are stunning and self-explanatory. This clever and effective way to communicate research is revolutionary and different from traditional formats. We hope you enjoy this article and will be inspired by its content to generate new research ideas.

Activated Protein C Strengthens Cardiac Tolerance to Ischemic Insults in Aging

BACKGROUND

APC (activated protein C) is a plasma serine protease with anticoagulant and anti-inflammatory activities. EPCR (Endothelial protein C receptor) is associated with APC's activity and mediates its downstream signaling events. APC exerts cardioprotective effects during ischemia and reperfusion (I/R). This study aims to characterize the role of the APC-EPCR axis in ischemic insults in aging.

METHODS

Young (3-4 months) and aged (24-26 months) wild-type C57BL/6J mice, as well as EPCR point mutation (EPCR^R84A/R84A) knockin C57BL/6J mice incapable of interaction with APC and its wild type of littermate C57BL/6J mice, were subjected to I/R. Wild-type APC, signaling-selective APC-2Cys, or anticoagulant-selective APC-E170A were administrated before reperfusion.

RESULTS

The results demonstrated that cardiac I/R reduces APC activity, and the APC activity was impaired in the aged versus young hearts possibly attributable to the declined EPCR level with aging. Serum EPCR measurement showed that I/R triggered the shedding of membrane EPCR into circulation, while administration of APC attenuated the I/R-induced EPCR shedding in both young and aged hearts. Subsequent echocardiography showed that APC and APC-2Cys but not APC-E170A ameliorated cardiac dysfunction during I/R in both young and aged mice. Importantly, APC elevated the resistance of the aged heart to ischemic insults through stabilizing EPCR. However, all these cardioprotective effects of APC were blunted in the EPCR^R84A/R84A mice versus its wild-type littermates. The ex vivo working heart and metabolomics results demonstrated that AMPK (AMP-activated protein kinase) mediates acute adaptive response while AKT (protein kinase B) is involved in chronic metabolic programming in the hearts with APC treatment.

CONCLUSIONS

I/R stress causes shedding of the membrane EPCR in the heart, and administration of APC prevents I/R-induced cardiac EPCR shedding that is critical for limiting cardiac damage in aging.

Extracellular Histones Bind Vascular Glycosaminoglycans and Inhibit the Anti-Inflammatory Function of Antithrombin

BACKGROUND/AIMS

Binding of histones to molecular pattern recognition receptors on endothelial cells and leukocytes provokes proinflammatory responses and promotes activation of coagulation. Histones also bind therapeutic heparins, thereby neutralizing their anticoagulant functions. The aim of this study was to test the hypothesis that histones can interact with the antithrombin (AT)-binding vascular glycosaminoglycans (GAGs) to induce inflammation and inhibit the anti-inflammatory function of AT.

METHODS

We evaluated the heparin-binding function of histones by an AT-dependent protease-inhibition assay. Furthermore, we treated endothelial cells with histones in the absence and presence of AT and monitored cellular phenotypes employing established signaling assays.

RESULTS

Histones neutralized AT-dependent anticoagulant function of heparin in both purified protease-inhibition and plasma-based assays. Histones also disrupted endothelial cell barrier-permeability function by a GAG-dependent mechanism as evidenced by the GAG-antagonist, surfen, abrogating their disruptive effects. Further studies revealed histones and AT compete for overlapping binding-sites on GAGs, thus increasing concentrations of one protein abrogated effects of the other. Histones elicited proapoptotic effects by inducing nuclear localization of PKC-δ in endothelial cells and barrier-disruptive effects by destabilizing VE-cadherin, which were inhibited by AT, but not by a D-helix mutant of AT incapable of interacting with GAGs. Finally, histones induced release of Weibel-Palade body contents, VWF and angiopoietin-2, and promoted expression of cell adhesion molecules on endothelial cells, which were all downregulated by AT but not by D-helix mutant of AT.

CONCLUSION

We conclude that histones and AT compete for overlapping binding sites on vascular GAGs to modulate coagulation and inflammation.

L-SIGN is a receptor on liver sinusoidal endothelial cells for SARS-CoV-2 virus

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), remains a pandemic. Severe disease is associated with dysfunction of multiple organs, but some infected cells do not express ACE2, the canonical entry receptor for SARS-CoV-2. Here, we report that the C-type lectin receptor L-SIGN interacted in a Ca²⁺-dependent manner with high-mannose–type N-glycans on the SARS-CoV-2 spike protein. We found that L-SIGN was highly expressed on human liver sinusoidal endothelial cells (LSECs) and lymph node lymphatic endothelial cells but not on blood endothelial cells. Using high-resolution confocal microscopy imaging, we detected SARS-CoV-2 viral proteins within the LSECs from liver autopsy samples from patients with COVID-19. We found that both pseudo-typed virus enveloped with SARS-CoV-2 spike protein and authentic SARS-CoV-2 virus infected L-SIGN–expressing cells relative to control cells. Moreover, blocking L-SIGN function reduced CoV-2–type infection. These results indicate that L-SIGN is a receptor for SARS-CoV-2 infection. LSECs are major sources of the clotting factors vWF and factor VIII (FVIII). LSECs from liver autopsy samples from patients with COVID-19 expressed substantially higher levels of vWF and FVIII than LSECs from uninfected liver samples. Our data demonstrate that L-SIGN is an endothelial cell receptor for SARS-CoV-2 that may contribute to COVID-19–associated coagulopathy.

Antithrombin Resistance Rescues Clotting Defect of Homozygous Prothrombin-Y510N Dysprothrombinemia

A patient with hematuria in our clinic was diagnosed with urolithiasis. Analysis of the patient's plasma clotting time indicated that both activated partial thromboplastin time (52.6 seconds) and prothrombin time (19.4 seconds) are prolonged and prothrombin activity is reduced to 12.4% of normal, though the patient exhibited no abnormal bleeding phenotype and a prothrombin antigen level of 87.9%. Genetic analysis revealed the patient is homozygous for prothrombin Y510N mutation. We expressed and characterized the prothrombin-Y510N variant in appropriate coagulation assays and found that the specificity constant for activation of the mutant zymogen by factor Xa is impaired approximately fivefold. Thrombin generation assay using patient's plasma and prothrombin-deficient plasma supplemented with either wild-type or prothrombin-Y510N revealed that both peak height and time to peak for the prothrombin mutant are decreased; however, the endogenous thrombin generation potential is increased. Further analysis indicated that the thrombin mutant exhibits resistance to antithrombin and is inhibited by the serpin with approximately 12-fold slower rate constant. Protein C activation by thrombin-Y510N was also decreased by approximately 10-fold; however, thrombomodulin overcame the catalytic defect. The Na⁺-concentration-dependence of the amidolytic activities revealed that the dissociation constant for the interaction of Na⁺ with the mutant has been elevated approximately 20-fold. These results suggest that Y510 (Y184a in chymotrypsin numbering) belongs to network of residues involved in binding Na⁺. A normal protein C activation by thrombin-Y510N suggests that thrombomodulin modulates the conformation of the Na⁺-binding loop of thrombin. The clotting defect of thrombin-Y510N appears to be compensated by its markedly lower reactivity with antithrombin, explaining patient's normal hemostatic phenotype.

Role of Gly197 in the structure and function of protein C

We previously demonstrated that heterozygous Gly197 to Arg mutation in PROC is associated with venous thrombosis due to the mutation abrogating both zymogenic and enzymatic activities of protein C and activated protein C (APC). In this study, we investigated the role of Gly197 on the structure and function of protein C by replacing it with Ala, Lys and Glu in separate constructs. Characterization of protein C mutants indicated their activation by thrombin is improved ~5-20-fold with the order of PC-G197K > PC-G197E > PC-G197A > PC-WT. Interestingly, the cofactor function of thrombomodulin (TM) in promoting the activation of zymogens by thrombin followed the reverse order of PC-WT > PC-G197A > PC-G197E > PC-G197K. The thrombin-generation inhibitory profiles of zymogens in a tissue factor-mediated thrombin generation assay using protein C-deficient plasma with or without supplementation with TM followed the same order of zymogen activation in the purified system. Evaluation of anticoagulant activities of APC derivatives by prothrombinase and aPTT assays revealed a normal activity for APC-G197A but dramatically impaired activity for the other two mutants. In the endothelial cell permeability assay, APC-G197A exhibited normal antiinflammatory activity, but the other two mutants were nearly inactive. These results suggest that Gly197 plays a key role in TM cofactor-dependent protein C activation by thrombin. It facilitates the recognition of protein C by thrombin in the presence of TM but impedes it in the absence of the cofactor. In APC, a small residue at this position is required for the proper folding/reactivity of the active-site pocket of the protease, a hypothesis supported by structural modeling.

Protective Role of Activated Protein C against Viral Mimetic Poly(I:C)-Induced Inflammation

Activated protein C (APC) is an anticoagulant plasma serine protease which exhibits potent cytoprotective and anti-inflammatory activities. Here, we studied protective effects of APC on the proinflammatory function of polyinosinic:polycytidylic acid [poly(I:C)], a synthetic analog of viral double-stranded RNA, in cellular and animal models. Poly(I:C) induced histone H3 extranuclear translocation via interaction with toll-like receptor 3 in two established endothelial cell lines. Furthermore, poly(I:C) induced histone H3 extranuclear translocation in J774A.1 macrophages and human neutrophils and formation of macrophage and neutrophil extracellular traps (ETs). Mechanistically, poly(I:C) was found to upregulate expression of peptidylarginine deiminase 4 and enhance its interaction with histone H3, thereby leading to increased histone citrullination and neutrophil ET formation. Poly(I:C) elicited proinflammatory signaling responses by inducing nuclear factor kappa B activation and disrupting endothelial cell permeability. In vivo, poly(I:C) enhanced cell surface expression of Mac-1 on neutrophils in mice and facilitated their infiltration to lung tissues. Poly(I:C) also downregulated thrombomodulin expression in mouse tissues and reduced its circulating soluble level in plasma. We demonstrate in this study that APC and a signaling-selective mutant of APC effectively inhibit proinflammatory signaling effects of poly(I:C) in both cellular and animal models. We further demonstrate that unlike the requirement for endothelial protein C receptor on endothelial cells, the integrin Mac-1 is involved in the protease-activated receptor 1-dependent APC inhibition of macrophage ET formation in J774A.1 cells. Taken together, these results support a key role for APC signaling in inhibiting the viral mimetic-induced proinflammatory signaling responses and histone translocation-associated formation of ETs by innate immune cells.

Anticoagulant and signaling functions of antithrombin

Antithrombin (AT) is a major plasma glycoprotein of the serpin superfamily that regulates the proteolytic activity of the procoagulant proteases of both intrinsic and extrinsic pathways. Two important structural features that participate in the regulatory function of AT include a mobile reactive center loop that binds to active site of coagulation proteases, trapping them in the form of inactive covalent complexes, and a basic D-helix that binds to therapeutic heparins and heparan sulfate proteoglycans (HSPGs) on vascular endothelial cells. The binding of D-helix of AT by therapeutic heparins promotes the reactivity of the serpin with coagulation proteases by several orders of magnitude by both a conformational activation of the serpin and a template (bridging) mechanism. In addition to its essential anticoagulant function, AT elicits a potent anti-inflammatory signaling response when it binds to distinct vascular endothelial cell HSPGs, thereby inducing prostacyclin synthesis. Syndecans-4 has been found as a specific membrane-bound HSPG receptor on endothelial cells that relays the signaling effect of AT to the relevant second messenger molecules in the signal transduction pathways inside the cell. However, following cleavage by coagulation proteases and/or by spontaneous conversion to a latent form, AT loses both its anti-inflammatory activity and high-affinity interaction with heparin and HSPGs. Interestingly, these low-affinity heparin conformers of AT elicit potent proapoptotic and antiangiogenic activities by also binding to specific HSPGs by unknown mechanisms. This review article will summarize current knowledge about mechanisms through which different conformers of AT exert their serine protease inhibitory and intracellular signaling functions in these biological pathways.

Podocyte Integrin-β 3 and Activated Protein C Coordinately Restrict RhoA Signaling and Ameliorate Diabetic Nephropathy

BACKGROUND

Diabetic nephropathy (dNP), now the leading cause of ESKD, lacks efficient therapies. Coagulation protease-dependent signaling modulates dNP, in part via the G protein-coupled, protease-activated receptors (PARs). Specifically, the cytoprotective protease-activated protein C (aPC) protects from dNP, but the mechanisms are not clear.

METHODS

A combination of in vitro approaches and mouse models evaluated the role of aPC-integrin interaction and related signaling in dNP.

RESULTS

The zymogen protein C and aPC bind to podocyte integrin-β ₃, a subunit of integrin-α _v β ₃. Deficiency of this integrin impairs thrombin-mediated generation of aPC on podocytes. The interaction of aPC with integrin-α _v β ₃ induces transient binding of integrin-β ₃ with G _α13 and controls PAR-dependent RhoA signaling in podocytes. Binding of aPC to integrin-β ₃ via its RGD sequence is required for the temporal restriction of RhoA signaling in podocytes. In podocytes lacking integrin-β ₃, aPC induces sustained RhoA activation, mimicking the effect of thrombin. In vivo, overexpression of wild-type aPC suppresses pathologic renal RhoA activation and protects against dNP. Disrupting the aPC-integrin-β ₃ interaction by specifically deleting podocyte integrin-β ₃ or by abolishing aPC's integrin-binding RGD sequence enhances RhoA signaling in mice with high aPC levels and abolishes aPC's nephroprotective effect. Pharmacologic inhibition of PAR1, the pivotal thrombin receptor, restricts RhoA activation and nephroprotects RGE-aPC^high and wild-type mice.Conclusions aPC-integrin-α _v β ₃ acts as a rheostat, controlling PAR1-dependent RhoA activation in podocytes in diabetic nephropathy. These results identify integrin-α _v β ₃ as an essential coreceptor for aPC that is required for nephroprotective aPC-PAR signaling in dNP.

Plasmodium falciparum histidine rich protein HRPII inhibits the anti-inflammatory function of antithrombin

BACKGROUND

It has been reported that histidine-rich protein II (HRPII), secreted by the malaria parasite, Plasmodium falciparum (Pf), inhibits the heparin-dependent anticoagulant activity of antithrombin (AT) in vitro and in plasma-based assay systems.

OBJECTIVE

The objective of this study was to test the hypothesis that HRPII may also interact with the AT-binding vascular glycosaminoglycans (GAGs), thereby inhibiting the anti-inflammatory signaling function of the serpin.

METHODS

We expressed HRPII in bacteria, purified it to homogeneity and studied its effect on endothelial cell signaling in the absence and presence of AT employing established signaling assays.

RESULTS

We demonstrate that a low concentration of HRPII potently disrupts the barrier permeability function of endothelial cells. Moreover, HRPII competitively inhibits the protective effect of AT by a concentration-dependent manner. Similarly, AT inhibits the pro-inflammatory activity of HRPII by a concentration-dependent manner. The siRNA knockdown of 3-O-sulfotransferase 1 (3-OST-1), the enzyme responsible for the essential 3-O-sulfation of the AT-binding GAGs, downregulates the pro-inflammatory function of HRPII in endothelial cells, supporting the hypothesis that HRPII competitively inhibits the interaction of AT with 3-OS containing vascular GAGs. Histidine-rich protein II elicits its barrier-disruptive effect by the Src-dependent phosphorylation of vascular endothelial (VE)-cadherin and AT counteracts this effect. We further demonstrate that inorganic polyphosphates bind HRPII with a high affinity to amplify the pro-inflammatory signaling function of HRPII in both cellular and in vivo permeability models.

CONCLUSION

We postulate that Pf-derived HRPII and polyphosphate can contribute to the pathogenesis of malaria infection by downregulating the AT-dependent anti-inflammatory and anticoagulant pathways.

Thr90Ser Mutation in Antithrombin is Associated with Recurrent Thrombosis in a Heterozygous Carrier

Antithrombin (AT) is a serine protease inhibitor that regulates the activity of coagulation proteases of both intrinsic and extrinsic pathways. We identified an AT-deficient patient with a heterozygous Thr90Ser (T90S) mutation who experiences recurrent venous thrombosis. To understand the molecular basis of the clotting defect, we expressed AT-T90S in mammalian cells, purified it to homogeneity, and characterized its properties in established kinetics, binding, and coagulation assays. The possible effect of mutation on the AT structure was also evaluated by molecular modeling. Results demonstrate the inhibitory activity of AT-T90S toward thrombin and factor Xa has been impaired three- to fivefold in both the absence and presence of heparin. The affinity of heparin for AT-T90S has been decreased by four- to fivefold. Kinetic analysis revealed the stoichiometry of AT-T90S inhibition of both thrombin and factor Xa has been elevated by three- to fourfold in both the absence and presence of heparin, suggesting that the reactivity of coagulation proteases with AT-T90S has been elevated in the substrate pathway. The anticoagulant activity of AT-T90S has been significantly impaired as analyzed in the AT-deficient plasma supplemented with AT-T90S. The anti-inflammatory effect of AT-T90S was also decreased. Structural analysis predicts the shorter side-chain of Ser in AT-T90S has a destabilizing effect on the structure of AT and/or the AT-protease complex, possibly increasing the size of an internal cavity and altering a hydrogen-bonding network that modulates conformations of the allosterically linked heparin-binding site and reactive center loop of the serpin. This mutational effect increases the reactivity of AT-T90S with coagulation proteases in the substrate pathway.

PKC (Protein Kinase C)-δ Modulates AT (Antithrombin) Signaling in Vascular Endothelial Cells

OBJECTIVE

Native and latent conformers of AT (antithrombin) induce anti-inflammatory and proapoptotic signaling activities, respectively, in vascular endothelial cells by unknown mechanisms. Synd-4 (syndecan-4) has been identified as a receptor that is involved in transmitting signaling activities of AT in endothelial cells. Approach and Results: In this study, we used flow cytometry, signaling assays, immunoblotting and confocal immunofluorescence microscopy to investigate the mechanism of the paradoxical signaling activities of high-affinity heparin (native) and low-affinity heparin (latent) conformers of AT in endothelial cells. We discovered that native AT binds to glycosaminoglycans on vascular endothelial cells via its heparin-binding D-helix to induce anti-inflammatory signaling responses by recruiting PKC (protein kinase C)-δ to the plasma membrane and promoting phosphorylation of the Synd-4 cytoplasmic domain at Ser179. By contrast, the binding of latent AT to endothelial cells to a site(s), which is not competed by the native AT, induces a proapoptotic effect by localizing PKC-δ to the perinuclear/nuclear compartment in endothelial cells. Overexpression of a dominant-negative form of PKC-δ resulted in inhibition of anti-inflammatory and proapoptotic signaling activities of both native and latent AT.

CONCLUSIONS

These results indicate that the native and latent conformers of AT may exert their distinct intracellular signaling effects through differentially modulating the subcellular localization of PKC-δ in endothelial cells.

Gly197Arg mutation in protein C causes recurrent thrombosis in a heterozygous carrier

BACKGROUND

Activated protein C (APC) downregulates thrombin generation by inactivating procoagulant cofactors Va and VIIIa by limited proteolysis. We identified two protein C-deficient patients both of whom carry a heterozygous Gly197 to Arg (G197R) mutation in PROC and experience venous thrombosis.

OBJECTIVE

The objective of this study was to determine the molecular basis of the clotting defect in patients carrying the G197R mutation.

METHODS

We expressed protein C-G197R in mammalian cells and characterized its properties in established coagulation and anti-inflammatory assay systems.

RESULTS

The activation of protein C-G197R by thrombin was improved ~10-fold; however, its activation by thrombin was not promoted by thrombomodulin (TM). In a tissue factor-mediated thrombin generation assay, the addition of soluble TM to protein C-deficient plasma, supplemented with protein C-G197R, did not have a significant inhibitory effect on thrombin generation parameters. APC-G197R did not exhibit a significant anticoagulant activity in either purified or plasma-based assay systems. APC-G197R was essentially inactive because it showed no activity in an aPTT assay. Anti-inflammatory activity of APC-G197R was also dramatically impaired as determined by an endothelial cell permeability assay. Structural modeling predicted that the side-chain of Arg cannot be accommodated at this site of APC without a major distortion of the local structure that appears to propagate and adversely affect the reactivity/folding of the catalytic pocket.

CONCLUSION

The G197R mutation in patients appears to be functionally equivalent to a heterozygous protein C knockout with half of the protein having no significant activity and thus causing thrombosis.

Analysis of protein missense alterations by combining sequence- and structure-based methods

BACKGROUND

Different types of in silico approaches can be used to predict the phenotypic consequence of missense variants. Such algorithms are often categorized as sequence based or structure based, when they necessitate 3D structural information. In addition, many other in silico tools, not dedicated to the analysis of variants, can be used to gain additional insights about the possible mechanisms at play.

METHODS

Here we applied different computational approaches to a set of 20 known missense variants present on different proteins (CYP, complement factor B, antithrombin and blood coagulation factor VIII). The tools that were used include fast computational approaches and web servers such as PolyPhen-2, PopMusic, DUET, MaestroWeb, SAAFEC, Missense3D, VarSite, FlexPred, PredyFlexy, Clustal Omega, meta-PPISP, FTMap, ClusPro, pyDock, PPM, RING, Cytoscape, and ChannelsDB.

RESULTS

We observe some conflicting results among the methods but, most of the time, the combination of several engines helped to clarify the potential impacts of the amino acid substitutions.

CONCLUSION

Combining different computational approaches including some that were not developed to investigate missense variants help to predict the possible impact of the amino acid substitutions. Yet, when the modified residues are involved in a salt-bridge, the tools tend to fail, even when the analysis is performed in 3D. Thus, interactive structural analysis with molecular graphics packages such as Chimera or PyMol or others are still needed to clarify automatic prediction.

Ile73Asn mutation in protein C introduces a new N-linked glycosylation site on the first EGF-domain of protein C and causes thrombosis

Activated protein C exerts its anticoagulant activity by protein S-dependent inactivation of factors Va and VIIIa by limited proteolysis. We identified a venous thrombosis patient who has plasma protein C antigen level of 63% and activity levels of 44% and 23%, as monitored by chromogenic and clotting assays. Genetic analysis revealed the proband carries compound heterozygous mutations (c.344T>A, p.I73N and c.1181G>A, p.R352Q) in PROC. We individually expressed protein C mutations and discovered that thrombin-thrombomodulin activates both variants normally and the resulting activated protein C mutants exhibit normal amidolytic and proteolytic activities. However, while protein S-dependent catalytic activity of activated protein C-R352Q toward factor Va was normal, it was significantly impaired for activated protein C-I73N. These results suggest that the Ile to Asn substitution impairs interaction of activated protein C-I73N with protein S. This conclusion was supported by a normal anticoagulant activity for activated protein C-I73N in protein S-deficient but not in normal plasma. Further analysis revealed Ile to Asn substitution introduces a new glycosylation site on first EGF-like domain of protein C, thereby adversely affecting interaction of activated protein C with protein S. Activated protein C-R352Q only exhibited reduced activity in sub-physiological concentrations of Na⁺ and Ca²⁺, suggesting that this residue contributes to metal ion-binding affinity of the protease, with no apparent adverse effect on its function in the presence of physiological levels of metal ions. These results provide insight into the mechanism by which I73N/R352Q mutations in activated protein C cause thrombosis in proband carrying this compound heterozygous mutation.

Inorganic Polyphosphate Amplifies High Mobility Group Box 1-Mediated Von Willebrand Factor Release and Platelet String Formation on Endothelial Cells

Objective- Inorganic polyphosphate (polyP) is known to modulate coagulation, inflammation, and metabolic pathways. It also amplifies inflammatory responses of HMGB1 (high mobility group box 1) in endothelial cells. The objective of this study was to evaluate the effect of polyP on von Willebrand factor (VWF) release from endothelial cells with or without HMGB1. Approach and Results- EA.hy926 endothelial cells were treated with different concentrations of polyP₇₀ alone or in combination with different concentrations of HMGB1. VWF release was measured by an ELISA assay in the absence or presence of pharmacological inhibitors of the receptor for advanced glycation end products, P2Y₁, and Ca²⁺. A flow chamber assay was used to monitor polyP₇₀-mediated platelet recruitment and VWF-platelet string formation. PolyP₇₀ and HMGB1 induced VWF release from endothelial cells by a concentration-dependent manner. PolyP₇₀ amplified HMGB1-mediated VWF release from endothelial cells. This was also true if boiled platelet releasate was used as the source of polyP. Gene silencing or pharmacological inhibitors of receptor for advanced glycation end products, P2Y₁, and Ca²⁺ significantly inhibited VWF release. PolyP₇₀ and HMGB1 synergistically promoted VWF-platelet string formation in the flow chamber assay, which was inhibited by the anti-GPIbα (glycoprotein Ib alpha) antibody. VWF release by polyP₇₀-HMGB1 complex required phosphorylation of Src and phospholipase C because inhibitors of Src, phospholipase C, and Ca²⁺ signaling significantly decreased VWF secretion. The polyP₇₀-HMGB1 complex also increased angiopoietin-2 release, indicating that Weibel-Palade body exocytosis is involved in the VWF release. Conclusions- PolyP₇₀ can promote thrombotic and inflammatory pathways by inducing VWF release and platelet string formation on endothelial cells.

Activated protein C inhibits lipopolysaccharide-mediated acetylation and secretion of high-mobility group box 1 in endothelial cells

Essentials APC elicits cytoprotective responses in endothelial cells via EPCR-dependent cleavage of PAR1. APC inhibits LPS-mediated translocation and extracellular secretion of HMGB1 in endothelial cells. Signaling activity of APC inhibits LPS-mediated acetylation of HMGB1 by epigenetic mechanisms. APC inhibits LPS-mediated HMGB1 expression in CD31-positive endothelial cells in cremaster muscle. SUMMARY: Background Activated protein C (APC) inhibits high-mobility group box 1 (HMGB1) signaling and its lipopolysaccharide (LPS)-mediated release by endothelial protein C receptor (EPCR)-dependent activation of protease-activated receptor 1 (PAR1) in endothelial cells. Post-translational acetylation is known to modulate the subcellular localization of HMGB1, and its hyperacetylated form is translocated to the cytoplasm of innate immune cells before being secreted into the extracellular space. Objective To determine whether APC inhibits LPS-mediated HMGB1 secretion from endothelial cells by modulating its acetylation status. Methods The subcellular localization of HMGB1 in LPS-treated endothelial cells was monitored in the absence and presence of APC by western blot analysis of fractionated cell lysates and confocal immunofluorescence microscopy. Results Both western blot and immunofluorescence data indicated that APC effectively inhibits LPS-mediated translocation of HMGB1 from the nucleus to the cytoplasm by EPCR-dependent and PAR1-dependent mechanisms. When EPCR was ligated by the Gla-domain of protein C/APC, thrombin also inhibited LPS-mediated HMGB1 translocation. Further studies revealed that APC inhibits the translocation of HMGB1 from the nucleus to the cytoplasm by inhibiting LPS-mediated hyperacetylation of HMGB1 by (de)acetylating enzymes. Furthermore, the translocated HMGB1 was found to be associated with lysosome-associated membrane protein 1 in LPS-treated endothelial cells. The in vivo relevance of these findings was investigated in the mouse cremaster muscle, and this demonstrated that both wild-type APC and a signaling-selective mutant of APC inhibit LPS-mediated HMGB1 expression and translocation in CD31-positive endothelial cells. Conclusion These results suggest that APC inhibits LPS-mediated cytoplasmic translocation and secretion of HMGB1 in endothelial cells by epigenetic mechanisms.

Expression and functional characterization of two natural heparin-binding site variants of antithrombin

Essentials Heparin-binding site (HBS) variants of antithrombin (AT) are associated with thrombosis risk. HSB variants have, in general, normal progressive inhibitory activity but reduced heparin affinity. Thrombosis in HSB carriers has been primarily attributed to the loss of heparin cofactor activity. Results here demonstrate that HSB variants of AT also lack anti-inflammatory signaling functions.

SUMMARY

Background Several heparin-binding site (HBS) variants of antithrombin (AT) have been identified that predispose carriers to a higher incidence of thrombosis. Thrombosis in carriers of HBS variants has been primarily attributed to a loss in their heparin-dependent anticoagulant function. Objective The objective of this study was to determine whether HSB mutations affect the anti-inflammatory functions of variants. Methods Two HBS variants of AT (AT-I7N and AT-L99F), which are known to be associated with a higher incidence of thrombosis, were expressed in mammalian cells and purified to homogeneity. These variants were characterized by kinetic assays followed by analysis of their activities in established cellular and/or in vivo inflammatory models. The possible effects of mutations on AT structure were also evaluated by molecular modeling. Results The results indicated that, whereas progressive inhibitory activities of variants were minimally affected, their heparin affinity and inhibitory activity in the presence of heparin were markedly decreased. Unlike wild-type AT, neither AT variant was capable of inhibiting activation of nuclear factor-κB or downregulation of expression of cell adhesion molecules in response to lipopolysaccharide (LPS). Similarly, neither variant elicited barrier protective activity in response to LPS. Structural analysis suggested that the L99F substitution locally destabilizes AT structure. Conclusions It is concluded that the L99F mutation of AT is associated with destabilization of the serpin structure, and that the loss of anti-inflammatory signaling function of the HBS variants may also contribute to enhanced thrombosis in carriers of HBS mutations.

Activated protein C protects against pressure overload-induced hypertrophy through AMPK signaling

We found that the anticoagulant plasma protease, activated protein C (APC), stimulates the energy sensor kinase, AMPK, in the stressed heart by activating protease-activated receptor 1 (PAR1) on cardiomyocytes. Wild-type (WT) and AMPK-kinase dead (KD) transgenic mice were subjected to transverse aortic constriction (TAC) surgery. The results demonstrated that while no phenotypic differences can be observed between WT and AMPK-KD mice under normal physiological conditions, AMPK-KD mice exhibit significantly larger hearts after 4 weeks of TAC surgery. Analysis by echocardiography suggested that the impairment in the cardiac function of AMPK-KD hearts is significantly greater than that of WT hearts. Immunohistochemical staining revealed increased macrophage infiltration and ROS generation in AMPK-KD hearts after 4 weeks of TAC surgery. Immunoblotting results demonstrated that the redox markers, pShc⁶⁶, 4-hydroxynonenal and ERK, were all up-regulated at a higher extent in AMPK-KD hearts after 4 weeks of TAC surgery. Administration of APC-WT and the signaling selective APC-2Cys mutant, but not the anticoagulant selective APC-E170A mutant, significantly attenuated pressure overload-induced hypertrophy and fibrosis. Macrophage infiltration and pShc⁶⁶ activation caused by pressure overload were also inhibited by APC and APC-2Cys but not by APC-E170A. Therefore, the cardiac AMPK protects against pressure overload-induced hypertrophy and the signaling selective APC-2Cys may have therapeutic potential for treating hypertension-related hypertrophy without increasing the risk of bleeding.

Thrombin inhibits nuclear factor κB and RhoA pathways in cytokine-stimulated vascular endothelial cells when EPCR is occupied by protein C

The occupancy of endothelial protein C receptor (EPCR) by protein C switches the protease activated receptor 1 (PAR-1)-dependent signalling specificity of thrombin from a permeability enhancing to a barrier protective response in vascular endothelial cells. In this study, the modulatory effects of thrombin and thrombin receptor agonist peptides (TRAP) on tumour necrosis factor (TNF)-α-stimulated HUVECs in the absence and presence of the catalytically inactive protein C-S195A were evaluated by monitoring the expression of cell surface adhesion molecules (VCAM-1, ICAM-1 and E-selectin), adhesion of freshly isolated neutrophils to cytokine-stimulated endothelial cells, regulation of the Rho family of small GTPases and the activation of nuclear factor-κB (NF-κB) pathway. The analysis of results indicate that both thrombin and TRAP initiate proinflammatory responses in endothelial cells, thus neither PAR-1 agonist in-fluenced the proinflammatory effects of TNF-α in the absence of the protein C mutant. Interestingly, however, the occupancy of EPCR by the protein C mutant switched the PAR-1-dependent signaling specificity of thrombin, thus leading to thrombin inhibition of the expression of all three adhesion molecules as well as the binding of neutrophils to TNF-α-activated endothelial cells. Furthermore, similar to activated protein C, both thrombin and TRAP activated Rac1 and inhibited the activation of RhoA and NF-κB pathways in response to TNF-α in cells pre-treated with protein C-S195A. Based on these results we conclude that when EPCR is ligated by protein C, the cleavage of PAR-1 by thrombin initiates antiinflammatory responses, thus leading to activation of Rac1 and inhibition of RhoA and NF-κB signalling cascades in vascular endothelial cells.

Activated Protein C Attenuates Severe Inflammation by Targeting VLA-3high Neutrophil Subpopulation in Mice

The host injury involved in multiorgan system failure during severe inflammation is mediated, in part, by massive infiltration and sequestration of hyperactive neutrophils in the visceral organ. A recombinant form of human activated protein C (rhAPC) has shown cytoprotective and anti-inflammatory functions in some clinical and animal studies, but the direct mechanism is not fully understood. Recently, we reported that, during endotoxemia and severe polymicrobial peritonitis, integrin VLA-3 (CD49c/CD29) is specifically upregulated on hyperinflammatory neutrophils and that targeting the VLA-3high neutrophil subpopulation improved survival in mice. In this article, we report that rhAPC binds to human neutrophils via integrin VLA-3 (CD49c/CD29) with a higher affinity compared with other Arg-Gly-Asp binding integrins. Similarly, there is preferential binding of activated protein C (PC) to Gr1highCD11bhighVLA-3high cells isolated from the bone marrow of septic mice. Furthermore, specific binding of rhAPC to human neutrophils via VLA-3 was inhibited by an antagonistic peptide (LXY2). In addition, genetically modified mutant activated PC, with a high affinity for VLA-3, shows significantly improved binding to neutrophils compared with wild-type activated PC and significantly reduced neutrophil infiltration into the lungs of septic mice. These data indicate that variants of activated PC have a stronger affinity for integrin VLA-3, which reveals novel therapeutic possibilities.

Characterization of Protein Z-Dependent Protease Inhibitor/Antithrombin Chimeras Provides Insight into the Serpin Specificity of Coagulation Proteases

Protein Z (PZ)-dependent protease inhibitor (ZPI) and antithrombin (AT) are two physiological serpin inhibitors involved in the regulation of proteolytic activities of the blood coagulation cascade. ZPI has restricted protease specificity capable of inhibiting factors Xa (FXa) and XIa (FXIa) but exhibiting no reactivity with other coagulation proteases. Unlike ZPI, AT is a general inhibitor of all coagulation proteases and the only physiological inhibitor of factor IXa (FIXa). To understand the molecular determinants of protease specificity of the two serpins, we engineered two ZPI mutants in which the P12-P3' residues of the reactive center loop of ZPI were replaced with either P12-P3' or P12-P7' residues of AT (ZPI-AT^P12-P3' and ZPI-AT^P12-P7'). The reactivity of chimeras with FXa was improved ∼4-25-fold in the absence of PZ. Both chimeras inhibited FIXa with rate constants that were ∼2 orders of magnitude higher than the rate of the AT inhibition of the protease. PZ improved the reactivity of chimeras with FIXa by another 2 orders of magnitude, rendering the chimeras potent inhibitors of FIXa so that the PZ-mediated inhibitory activity of the ZPI-AT chimeras toward FIXa was ∼20-fold higher than that of the fondaparinux-catalyzed inhibition of FIXa by AT. Further studies revealed that the substitution of P1-Tyr of ZPI with an Arg is sufficient to convert the serpin to an effective inhibitor of FIXa. The potential therapeutic utility of the serpin chimeras as specific inhibitors of FIXa was diminished by findings that the chimeras function as effective substrates for other coagulation proteases.

Gly74Ser mutation in protein C causes thrombosis due to a defect in protein S-dependent anticoagulant function

Protein C is a vitamin K-dependent serine protease zymogen in plasma which upon activation by thrombin in complex with thrombomodulin (TM) down-regulates the clotting cascade by a feedback loop inhibition mechanism. Activated protein C (APC) exerts its anticoagulant function through protein S-dependent degradation of factors Va and VIIIa. We recently identified a venous thrombosis patient whose plasma level of protein C antigen is normal, but its anticoagulant activity is only 34 % of the normal level. Genetic analysis revealed that the proband and her younger brother carry a novel heterozygous mutation c.346G>A, p.Gly74Ser (G74S) in PROC. Thrombin generation assay indicated that the TM-dependent anticoagulant activity of the proband's plasma has been significantly impaired. We expressed protein C-G74S in mammalian cells and characterised its properties in established coagulation assays. We demonstrate that the protein C variant can be normally activated by the thrombin-TM complex and the resulting APC mutant also exhibits normal amidolytic and proteolytic activities toward both FVa and FVIIIa. However, it was discovered the protein S-dependent catalytic activity of APC variant toward both procoagulant cofactors has been significantly impaired. Protein S concentration-dependence of FVa degradation revealed that the capacity of APC variant to interact with the cofactor has been markedly impaired. The same results were obtained for inactivation of FVa-Leiden suggesting that the protein S-dependent activity of APC variant toward cleavage of Arg-306 site has been adversely affected. These results provide insight into the mechanism through which G74S substitution in APC causes thrombosis in the proband carrying this mutation.

Paradoxical bleeding and thrombotic episodes of dysprothrombinaemia due to a homozygous Arg382His mutation

We have characterised the pathogenic basis of dysprothrombinaemia in a patient exhibiting paradoxical bleeding and thrombotic defects during pregnancy and postpartum. Genetic analysis revealed that the proband is homozygous for the prothrombin Arg382His mutation, possessing only ~1 % clotting activity. The proband experienced severe bleeding episodes during her pregnancy, which required treatment with prothrombin complex concentrates, and then pulmonary embolism and deep-vein thrombosis at 28 days postpartum, which required treatment with LMWH and fresh frozen plasma. Analysis of haemostatic parameters revealed that the subject had elevated FDP and DD and decreased fibrinogen levels, indicating the presence of hyperfibrinolysis. Thrombin generation and clotting assays with the proband's plasma in the presence of soluble thrombomodulin and tissue-type plasminogen activator indicated a defect in activation of both protein C and thrombin activatable fibrinolysis inhibitor (TAFI). Unlike normal plasma, no TAFI activation could be detected in the patient's plasma. The expression and characterisation of recombinant prothrombin Arg382His indicated that zymogen activation by prothrombinase was markedly impaired and the activation of protein C and TAFI by thrombin-Arg382His was impaired 600-fold and 2500-fold, respectively. The recombinant thrombin mutant exhibited impaired catalytic activity toward both fibrinogen and PAR1 as determined by clotting and signalling assays. However, the mutant activated factor XI normally in both the absence and presence of polyphosphates. Arg382 is a key residue on (pro)exosite-1 of prothrombin and kinetic analysis of substrate activation suggested that the poor zymogenic activity of the mutant is due to its inability to bind factor Va in the prothrombinase complex.

Inorganic polyphosphate promotes cyclin D1 synthesis through activation of mTOR/Wnt/β-catenin signaling in endothelial cells

Essentials Polyphosphate (polyP) activates mTOR but its role in Wnt/β-catenin signaling is not known. PolyP-mediated cyclin D1 expression (β-catenin target gene) was monitored in endothelial cells. PolyP and boiled platelet-releasates induced the expression of cyclin D1 by similar mechanisms. PolyP establishes crosstalk between mTOR and Wnt/β-catenin signaling in endothelial cells.

SUMMARY

Background Inorganic polyphosphate (polyP) elicits intracellular signaling responses in endothelial cells through activation of mTOR complexes 1 and 2. Glycogen synthase kinase 3 (GSK-3) is known to be a negative regulator of mTOR and Wnt/β-catenin signaling pathways. Objective The objective of this study was to investigate the effect of polyP on the expression, degradation and subcellular localization of the Wnt/β-catenin target gene, cyclin D1, in endothelial cells. Methods Regulation of cyclin D1 expression, phosphorylation and subcellular localization by polyP or platelet releasates was monitored in the absence and presence of pharmacological inhibitors and/or siRNA for specific molecules of the upstream mTOR/Wnt/β-catenin signaling network by established methods. Results Both synthetic polyP and boiled-platelet releasates induced the phosphorylation-dependent inactivation of GSK-3, thereby increasing the expression and nuclear localization, but inhibiting the degradation of cyclin D1. Inhibitors of mTORC1 (PI3K, AKT, PLC, PKC), rapamycin and siRNA for raptor (mTORC1-specific component) and β-catenin, all inhibited polyP-mediated regulation of cyclin D1 expression, phosphorylation and subcellular localization in endothelial cells. The signaling effect of polyP was effectively inhibited by the recombinant extracellular domain of the receptor for advanced glycation end products (RAGE) and/or by the RAGE siRNA. Specific pharmacological inhibitors and siRNA knockdown of ERK1/2 and NF-κB pathways indicated that polyP-mediated cyclin D1 expression and nuclear localization are IKKɑ and ERK1/2 dependent, whereas its inhibitory effect on phosphorylation-dependent degradation of cyclin D1 is IKKβ-dependent. Conclusion We conclude that a RAGE-dependent polyP-mediated crosstalk between mTOR and the GSK-3/Wnt/β-catenin signaling network can modulate important physiological processes in endothelial cells.

Activated protein C protects against renal ischaemia/reperfusion injury, independent of its anticoagulant properties

Acute renal failure, a serious condition characterised by a drastic decline in renal function, often follows ischaemia/reperfusion (I/R) episodes. I/R is characterised by necrosis, inflammation and activation of coagulation, in concert causing renal tissue damage. In this context, activated protein C (APC) might be of importance in the pathogenesis of renal I/R. APC is a serine protease which has anticoagulant but also several anti-inflammatory and cytoprotective effects such as protection of endothelial barrier function. It was our objective to study the role of cytoprotective and anticoagulant functions of APC during renal I/R. C57BL/6j mice subjected to renal I/R were treated with intraperitoneally injected exogenous human APC, or two mutant forms of APC (200 µg/kg) which specifically lack anticoagulant or signalling properties. In a different experiment mice received specific monoclonal antibodies (20 mg/kg) that block the cytoprotective and/or anticoagulant properties of endogenous APC. Treatment with APC reduced tubular injury and enhanced renal function without altering the inflammatory response and did reduce renal fibrin deposition. Administration of APC mutant lacking anticoagulant properties reduced renal damage and enhanced renal function. Blocking the anticoagulant and cytoprotective functions of endogenous APC resulted in elevated tubular damage and reduced tubular cell proliferation, however, without influencing renal function or the inflammatory response. Furthermore, blocking both the anticoagulant and cytoprotective effects of APC resulted in dramatic renal interstitial haemorrhage, indicative of impaired vascular integrity. Blocking only the anticoagulant function of APC did not result in interstitial bleeding. In conclusion, the renoprotective effect of APC during I/R is independent of its anticoagulant properties.

Expression and Characterization of Gly-317 Variants of Factor IX Causing Variable Bleeding in Hemophilia B Patients

We recently identified two hemophilia B patients who carried Gly-317 to Arg (FIX-G317R) or Gly-317 to Glu (FIX-G317E) substitutions in their FIX gene. The former mutation caused severe and the latter moderate bleeding in afflicted patients. To understand the molecular basis for the variable clinical manifestation of Gly-317 mutations, we prepared recombinant G317R and G317E derivatives of FIX and compared their kinetic properties to those of recombinant wild-type FIX in appropriate assay systems. Both physiological activators, factor XIa and extrinsic Tenase (factor VIIa-tissue factor), activated both zymogen variants with an ∼1.5-fold elevated K(m); however, extrinsic Tenase activated FIX-G317E with an ∼2-fold improved k(cat). By contrast to zymogen activation, the catalytic activities of both FIXa-G317R and FIXa-G317E enzymes toward the natural substrate, factor X, were dramatically (>4 orders of magnitude) impaired, but their apparent affinity for interaction with factor VIIIa was only slightly (<2-fold) decreased. Further studies revealed that the reactivity of FIXa-G317R and FIXa-G317E with antithrombin has been impaired 10- and 13-fold, respectively, in the absence and 166- and 500-fold, respectively, in the presence of pentasaccharide. As expected, the clotting activities of FIX variants could not be measured by the aPTT assay. These results implicate a critical role for Gly-317 in maintaining normal catalytic function for FIX/FIXa in the clotting cascade. The results further suggest that improved k(cat) of FIX-G317E activation in the extrinsic pathway together with dramatically impaired reactivity of FIXa-G317E with antithrombin may account for the less severe bleeding phenotype of a hemophilia B patient carrying the FIX-G317E mutation.

Inorganic polyphosphate elicits pro-inflammatory responses through activation of the mammalian target of rapamycin complexes 1 and 2 in vascular endothelial cells

BACKGROUND

Inorganic polyphosphate (polyP) elicits pro-inflammatory signaling responses in endothelial cells through interaction with two receptors, RAGE and P2Y1 . It is known that polyP activates mTOR signaling in breast cancer cells.

OBJECTIVES

The objective of this study is to understand the mechanism of the polyP-mediated signaling pathway in endothelial cells and to determine whether polyP exerts its pro-inflammatory effect through activation of mTOR.

METHODS

mTOR activation by polyP or platelet releasates in cellular and animal models was monitored in the absence and presence of pharmacological inhibitors and/or siRNA knockdown of specific signaling molecules.

RESULTS

PolyP effectively induced phosphorylation of mTOR complex 1 (mTORC1) substrate, p70S6K, in endothelial cells by an AKT-dependent but ERK-independent mechanism. The siRNA knockdown of both RAGE and P2Y1 or specific inhibitors of the PI3K/PLC/PKC/Ca(2+) signaling axis inhibited polyP-mediated p70S6K phosphorylation. Moreover, either rapamycin or siRNA knockdown of raptor (mTORC1-specific component) abrogated polyP-mediated phosphorylation of p70S6K. By contrast, the siRNA knockdown of rictor (mTOR complex 2-specific component) but not raptor eliminated the barrier-disruptive effect of polyP. Specific NF-κB inhibitors abrogated polyP-mediated phosphorylation of p70S6K and rapamycin suppressed polyP-induced activation of NF-κB. Finally, specific inhibitors of the mTOR signaling network eliminated polyP-mediated vascular leakage and leukocyte recruitment in animal models.

CONCLUSIONS

PolyP, through interaction with RAGE and P2Y1 , activates both the mTORC1 and mTORC2 signaling network. Both the pro-inflammatory and mTOR signaling functions of polyP are linked.

Antithrombin up-regulates AMP-activated protein kinase signalling during myocardial ischaemia/reperfusion injury

Antithrombin (AT) is a protein of the serpin superfamily involved in regulation of the proteolytic activity of the serine proteases of the coagulation system. AT is known to exhibit anti-inflammatory and cardioprotective properties when it binds to heparan sulfate proteoglycans (HSPGs) on vascular cells. AMP-activated protein kinase (AMPK) plays an important cardioprotective role during myocardial ischaemia and reperfusion (I/R). To determine whether the cardioprotective signaling function of AT is mediated through the AMPK pathway, we evaluated the cardioprotective activities of wild-type AT and its two derivatives, one having high affinity and the other no affinity for heparin, in an acute I/R injury model in C57BL/6J mice in which the left anterior descending coronary artery was occluded. The serpin derivatives were given 5 minutes before reperfusion. The results showed that AT-WT can activate AMPK in both in vivo and ex vivo conditions. Blocking AMPK activity abolished the cardioprotective function of AT against I/R injury. The AT derivative having high affinity for heparin was more effective in activating AMPK and in limiting infraction, but the derivative lacking affinity for heparin was inactive in eliciting AMPK-dependent cardioprotective activity. Activation of AMPK by AT inhibited the inflammatory c-Jun N-terminal protein kinase (JNK) pathway during I/R. Further studies revealed that the AMPK activity induced by AT also modulates cardiac substrate metabolism by increasing glucose oxidation but inhibiting fatty acid oxidation during I/R. These results suggest that AT binds to HSPGs on heart tissues to invoke a cardioprotective function by triggering cardiac AMPK activation, thereby attenuating JNK inflammatory signalling pathways and modulating substrate metabolism during I/R.

Conformational activation of antithrombin by heparin involves an altered exosite interaction with protease

Heparin allosterically activates antithrombin as an inhibitor of factors Xa and IXa by enhancing the initial Michaelis complex interaction of inhibitor with protease through exosites. Here, we investigate the mechanism of this enhancement by analyzing the effects of alanine mutations of six putative antithrombin exosite residues and three complementary protease exosite residues on antithrombin reactivity with these proteases in unactivated and heparin-activated states. Mutations of antithrombin Tyr(253) and His(319) exosite residues produced massive 10-200-fold losses in reactivity with factors Xa and IXa in both unactivated and heparin-activated states, indicating that these residues made critical attractive interactions with protease independent of heparin activation. By contrast, mutations of Asn(233), Arg(235), Glu(237), and Glu(255) exosite residues showed that these residues made both repulsive and attractive interactions with protease that depended on the activation state and whether the critical Tyr(253)/His(319) residues were mutated. Mutation of factor Xa Arg(143), Lys(148), and Arg(150) residues that interact with the exosite in the x-ray structure of the Michaelis complex confirmed the importance of all residues for heparin-activated antithrombin reactivity and Arg(150) for native serpin reactivity. These results demonstrate that the exosite is a key determinant of antithrombin reactivity with factors Xa and IXa in the native as well as the heparin-activated state and support a new model of allosteric activation we recently proposed in which a balance between attractive and repulsive exosite interactions in the native state is shifted to favor the attractive interactions in the activated state through core conformational changes induced by heparin binding.

Molecular basis of the clotting defect in a bleeding patient missing the Asp-185 codon in the factor X gene

Factor X (FX) is a vitamin K-dependent plasma zymogen, which following activation to factor Xa (FXa), converts prothrombin to thrombin in the blood clotting cascade. It was recently demonstrated that a natural variant of FX carrying the Asp-185 deletion (FX-D185del, chymotrypsinogen numbering) was associated with mild bleeding in a patient with severe FX deficiency. In this study, we expressed FX-D185del in mammalian cells and characterized its properties in appropriate kinetic assays in purified systems. We discovered that while the FX variant can be normally activated by physiological activators; both amidolytic and proteolytic activities of the mutant are dramatically impaired. Interestingly, factor Va (FVa) significantly improved the proteolytic defect when the mutant protease was assembled into the prothrombinase complex. Thus, in contrast to >50-fold catalytic defect in the absence of FVa, the variant activated prothrombin with only ~2.5-fold decreased catalytic efficiency in the presence of the cofactor. The FXa variant dramatically lost its susceptibility to inhibition by antithrombin and tissue factor pathway inhibitor, thus exhibiting ~2-3 orders of magnitude lower reactivity with the plasma inhibitors. Further studies revealed that Na(+) no longer activates the variant protease, suggesting that the functionally important allosteric linkage between the Na(+)-binding and the P1-binding sites of the protease has been eliminated. These results suggest that the lower catalytic efficiency of FXa-D185del in the bleeding patient may be partially compensated by the loss of its reactivity with plasma inhibitors, possibly explaining the basis for the paradoxical severe FX deficiency with only mild bleeding tendency for this mutation.

Adenosine regulates the proinflammatory signaling function of thrombin in endothelial cells

The plasma level of the regulatory metabolite adenosine increases during the activation of coagulation and inflammation. Here we investigated the effect of adenosine on modulation of thrombin-mediated proinflammatory responses in HUVECs. We found that adenosine inhibits the barrier-disruptive effect of thrombin in HUVECs by a concentration-dependent manner. Analysis of cell surface expression of adenosine receptors revealed that A2A and A2B are expressed at the highest level among the four receptor subtypes (A2B  > A2A  > A1  > A3 ) on HUVECs. The barrier-protective effect of adenosine in response to thrombin was recapitulated by the A2A specific agonist, CGS 21680, and abrogated both by the siRNA knockdown of the A2A receptor and by the A2A -specific antagonists, ZM-241385 and SCH-58261. The thrombin-induced RhoA activation and its membrane translocation were both inhibited by adenosine in a cAMP-dependent manner, providing a molecular mechanism through which adenosine exerts a barrier-protective function. Adenosine also inhibited thrombin-mediated activation of NF-κB and decreased adhesion of monocytic THP-1 cells to stimulated HUVECs via down-regulation of expression of cell surface adhesion molecules, VCAM-1, ICAM-1, and E-selectin. Moreover, adenosine inhibited thrombin-induced elevated expression of proinflammatory cytokines, IL-6 and HMGB-1; and chemokines, MCP-1, CXCL-1, and CXCL-3. Taken together, these results suggest that adenosine may inhibit thrombin-mediated proinflammatory signaling responses, thereby protecting the endothelium from injury during activation of coagulation and inflammation.

Abstract 88: AMP-activated Protein Kinase Signaling Mediates the Cardioprotective effect of Antithrombin against Myocardial Ischemia and Reperfusion Injury

Background: Antithrombin (AT) is an endogenous protein of the serpin superfamily involved in regulation of the proteolytic activity of the serine proteases of the coagulation system. AT is known to exhibit anti-inflammatory and cardioprotective properties when it binds to distinct heparan sulfate proteoglycans (HSPGs) on vascular endothelial cells. The energy sensor AMP-activated protein kinase (AMPK) plays an important cardioprotective role during myocardial ischemia and reperfusion (I/R). The objective of this study was to investigate whether the cardioprotective signaling function of AT against I/R injury is mediated through the AMPK pathway.

Methods and Results: The cardioprotective activities of wild-type (WT) AT and its two recombinant derivatives, one having high affinity and the other no affinity for heparin, were evaluated in an acute I/R (20 min/4 h) injury model in which the left anterior descending coronary artery (LAD) was occluded. The serpin derivatives were given 5 min before reperfusion. The results showed that AT-WT can activate the protective AMPK signaling pathway in both in vivo and ex vivo conditions. Blocking AMPK activity abolished the cardioprotective function of AT against I/R injury. The AT derivative having high affinity for heparin was more effective in activating AMPK, but the derivative lacking any affinity for heparin was inactive in eliciting AMPK-dependent cardioprotective activity. The activation of AMPK by AT inhibited the inflammatory c-Jun N-terminal protein kinase (JNK) pathway during I/R. Further studies revealed that the AMPK activity of AT also modulates cardiac substrate metabolism by increasing glucose oxidation but inhibiting fatty acid oxidation during I/R.

Conclusions: These results suggest that AT binds to vascular HSPGs to invoke a cardioprotective function by triggering cardiac AMPK activation, thereby attenuating JNK inflammatory signaling pathways and modulating substrate metabolism during I/R.

Thrombin inhibits HMGB1-mediated proinflammatory signaling responses when endothelial protein C receptor is occupied by its natural ligand

High mobility group box 1 (HMGB1) is involved in the pathogenesis of vascular diseases. Unlike activated protein C (APC), the activation of PAR-1 by thrombin is known to elicit proinflammatory responses. To determine whether the occupancy of EPCR by the Gla-domain of APC is responsible for the PAR-1-dependent antiinflammatory activity of the protease, we pretreated HUVECs with the PC zymogen and then activated PAR-1 with thrombin. It was found that thrombin downregulates the HMGB1-mediated induction of both TNF-α and IL-6 and inhibits the activation of both p38 MAPK and NF-κB in HUVECs pretreated with PC. Furthermore, thrombin inhibited HMGB1-mediated hyperpermeability and leukocyte adhesion/migration by inhibiting the expression of cell adhesion molecules in HUVECs if EPCR was occupied. Collectively, these results suggest the concept that thrombin can initiate proinflammatory responses in vascular endothelial cells through the activation of PAR-1 may not hold true for normal vessels expressing EPCR under in vivo conditions. [BMB Reports 2013; 46(11): 544-549]

Protease-activated receptor signalling by coagulation proteases in endothelial cells

Endothelial cells express several types of integral membrane protein receptors, which upon interaction and activation by their specific ligands, initiate a signalling network that links extracellular cues in circulation to various biological processes within a plethora of cells in the vascular system. A small family of G-protein coupled receptors, termed protease-activated receptors (PAR1-4), can be specifically activated by coagulation proteases, thereby modulating a diverse array of cellular activities under various pathophysiological conditions. Thrombin and all vitamin K-dependent coagulation proteases, with the exception of factor IXa for which no PAR signalling has been attributed, can selectively activate cell surface PARs on the vasculature. Thrombin can activate PAR1, PAR3 and PAR4, but not PAR2 which can be specifically activated by factors VIIa and Xa. The mechanistic details of the specificity of PAR signalling by coagulation proteases are the subject of extensive investigation by many research groups worldwide. However, analysis of PAR signalling data in the literature has proved to be challenging since a single coagulation protease can elicit different signalling responses through activation of the same PAR receptor in endothelial cells. This article is focused on briefly reviewing the literature with respect to determinants of the specificity of PAR signalling by coagulation proteases with special emphasis on the mechanism of PAR1 signalling by thrombin and activated protein C in endothelial cells.

Characterization of the protein Z-dependent protease inhibitor interactive-sites of protein Z

BACKGROUND

Protein Z (PZ) has been reported to promote the inactivation of factor Xa (FXa) by PZ-dependent protease inhibitor (ZPI) by about three orders of magnitude. Previously, we prepared a chimeric PZ in which its C-terminal pseudo-catalytic domain was grafted on FX light-chain (Gla and EGF-like domains) (PZ/FX-LC). Characterization of PZ/FX-LC revealed that the ZPI interactive-site is primarily located within PZ pseudo-catalytic domain. Nevertheless, the cofactor function and apparent Kd of PZ/FX-LC for interaction with ZPI remained impaired ~6-7-fold, suggesting that PZ contains a ZPI interactive-site outside pseudo-catalytic domain. X-ray structural data indicates that Tyr-240 of ZPI interacts with EGF2-domain of PZ. Structural data further suggests that 3 other ZPI surface loops make salt-bridge interactions with PZ pseudo-catalytic domain. To identify ZPI interactive-sites on PZ, we grafted the N-terminal EGF2 subdomain of PZ onto PZ/FX-LC chimera (PZ-EGF2/FX-LC) and also generated two compensatory charge reversal mutants of PZ pseudo-catalytic domain (Glu-244 and Arg-212) and ZPI surface loops (Lys-239 and Asp-293).

METHODS

PZ chimeras were expressed in mammalian cells and ZPI derivatives were expressed in Escherichia coli.

RESULTS

The PZ EGF2 subdomain fusion restored the defective cofactor function of PZ/FX-LC. The activities of PZ and ZPI mutants were all impaired if assayed individually, but partially restored if the compensatory charge reversal mutants were used in the assay.

CONCLUSIONS

PZ EGF2 subdomain constitutes an interactive-site for ZPI. Data with compensatory charge reversal mutants validates structural data that the identified residues are part of interactive-sites.

GENERAL SIGNIFICANCE

Insight is provided into mechanisms through which specificity of ZPI-PZ-FXa complex formation is determined.

Engineering D-helix of antithrombin in alpha-1-proteinase inhibitor confers antiinflammatory properties on the chimeric serpin

Antithrombin (AT) is a heparin-binding serpin in plasma which regulates the proteolytic activity of procoagulant proteases of the clotting cascade. In addition to being an anticoagulant, AT also exhibits antiinflammatory activities when it binds to cell surface heparan sulfate proteoglycans (HSPGs) on the endothelium via its basic residues of D-helix to elicit intracellular signalling responses. By contrast to AT, α1-proteinase inhibitor (α1-PI) is a non-heparin-binding serpin that exhibits very slow reactivity with coagulation proteases and possesses no HSPG-dependent antiinflammatory properties. To determine whether the antiinflammatory signaling specificity of AT can be transferred to α1-PI, we replaced the D-helix of human α1-PI with the corresponding sequence of human AT and expressed the chimeric serpin α1-PI/D-helix) in a bacterial expression system. High molecular weight heparin bound to α1-PI/D-helix and accelerated the inhibition of thrombin by the serpin mutant by a template mechanism reminiscent of the cofactor effect of heparin on inhibition of thrombin by AT. Like AT, α1-PI/D-helix exhibited antiinflammatory properties in both cellular and animal models. Thus, α1-PI/D-helix inhibited the barrier-disruptive effect of proinflammatory cytokines and inhibited the activation of nuclear factor-κB transcription factor in lipopolysaccharide-stimulated endothelial cells by a concentration-dependent manner. Furthermore, the chimeric serpin reduced lipopolysaccharide-mediated lethality, elicited a vascular protective effect and inhibited infiltration of activated leukocytes to the peritoneal cavity of mice in an HMGB1-mediated inflammatory model. These results suggest that grafting the D-helix of AT to α1-PI confers antiinflammatory properties on the serpin and that the chimeric serpin may have therapeutic utility for treating inflammatory disorders.

Abstract 094: Antithrombin Exerts a Cardioprotective Effect against Myocardial Ischemic/reperfusion Injury through Induction of Prostacyclin and Inhibition of NF-kB Signaling

Antithrombin (AT) is a plasma serpin inhibitor that regulates the proteolytic activity of procoagulant proteases of the clotting cascade. In addition to its anticoagulant activity, AT also possesses potent antiinflammatory and antiangiogenic properties. In this study, we investigated the antiinflammatory activity of wild-type AT (AT-WT) and a reactive center loop mutant of AT (AT-RCL) not capable of inhibiting thrombin in a mouse model of ischemia/reperfusion injury in which the left anterior descending coronary artery (LAD) was occluded and released then. The results demonstrate that AT markedly reduces the myocardial infarct size (p<0.05 vs. vehicle) by a mechanism that is independent of its anticoagulant activity. Thus, the AT-RCL mutant, lacking any detectable reactivity with thrombin, attenuated myocardial infarct size to the same extent as the AT-WT in the acute injury model. Further studies revealed that AT binds to vascular heparan sulfate proteoglycans via its heparin-binding domain to exert its protective activity as evidenced by the therapeutic AT-binding pentasaccharide fragment of heparin (fondaparinux) abrogating the cardioprotective activity of AT. We further demonstrate that AT up-regulates the production of prostacyclin in myocardial tissues (p<0.05 vs. vehicle) and inhibits the production of proinflammatory cytokines TNF-α and IL-6 (p<0.05 vs. vehicle) in vivo by attenuating the ischemia/reperfusion-induced JNK and NF-κB signaling pathways. These results suggest that AT, through its antiinflammatory signaling effect, may have therapeutic potential for reducing cardiac ischemia/reperfusion injury and that a normal cardioprotective activity for the AT-RCL variant renders it a superior drug candidate if bleeding becomes a concern in the treatment.

Antithrombin is protective against myocardial ischemia and reperfusion injury

BACKGROUND

Antithrombin (AT) is a plasma serpin inhibitor that regulates the proteolytic activity of procoagulant proteases of the clotting cascade. In addition to its anticoagulant activity, AT also possesses potent anti-inflammatory properties.

OBJECTIVES

The objective of this study was to investigate the anti-inflammatory activity of wild-type AT (AT-WT) and a reactive centre loop mutant of AT (AT-RCL) which is not capable of inhibiting thrombin.

METHODS

The cardioprotective activities of AT-WT and AT-RCL were monitored in a mouse model of ischemia/reperfusion (I/R) injury in which the left anterior descending coronary artery was occluded and then released.

RESULTS

We demonstrate that AT markedly reduces myocardial infarct size by a mechanism that is independent of its anticoagulant activity. Thus, AT-RCL attenuated myocardial infarct size to the same extent as AT-WT in this acute injury model. Further studies revealed that AT binds to vascular heparan sulfate proteoglycans via its heparin-binding domain to exert its protective activity as evidenced by the therapeutic AT-binding pentasaccharide (fondaparinux) abrogating the cardioprotective activity of AT and a heparin-site mutant of AT exhibiting no cardioprotective property. We further demonstrate that AT up-regulates the production of prostacyclin in myocardial tissues and inhibits expression of pro-inflammatory cytokines tumor necrosis factor (TNF)-α and interleukin (IL)-6 in vivo by attenuating ischemia/reperfusion-induced JNK and NF-κB signaling pathways.

CONCLUSIONS

The present results suggest that both AT and the non-anticoagulant AT-RCL, through their anti-inflammatory signaling effects, elicit potent cardioprotective responses. Thus, AT may have therapeutic potential for treating cardiac I/R injury.

The missense Thr211Pro mutation in the factor X activation peptide of a bleeding patient causes molecular defect in the clotting cascade

Factor X (FX) is a vitamin K-dependent coagulation zymogen, which upon activation to factor Xa assembles into the prothrombinase complex to activate prothrombin to thrombin. FX can be activated by either factor VIIa-tissue factor or factor IXa-factor VIIIa in extrinsic and intrinsic pathways, respectively. In this study, we identified a bleeding patient with moderate FX deficiency who exhibits a clotting defect only in the intrinsic pathway. Exome sequencing revealed that the patient carries a novel homozygous missense mutation that results in substitution of Thr211 with Pro in the activation peptide of FX. Thr211 is the site of an O-linked glycosylation in the activation peptide of FX. We postulated that the lack of this post-translational modification specifically impacts the activation of FX by intrinsic Xase, thereby impairing thrombin generation in the subject. To test this hypothesis, we expressed both wild-type FX and FX containing this mutation in mammalian cells and following the purification of the zymogens to homogeneity characterized their properties in both purified and plasma-based assay systems. Analysis of the results suggests that Thr211 to Pro substitution renders the FX mutant a poor substrate for both physiological activators, however, at physiological concentration of the substrate, the clotting defect manifest itself only in the intrinsic pathway, thus explaining the bleeding phenotype for the patient carrying this mutation.

Residues of the 39-loop restrict the plasma inhibitor specificity of factor IXa

The two plasma inhibitors, protein Z-dependent protease inhibitor (ZPI) and tissue factor pathway inhibitor (TFPI), effectively inhibit the activity of activated factor X (FXa); however, neither inhibitor exhibits any reactivity with the homologous protease activated factor IX (FIXa). In this study, we investigated the molecular basis for the lack of reactivity of FIXa with these plasma inhibitors and discovered that unique structural features within residues of the 39-loop are responsible for restricting the inhibitor specificity of FIXa. This loop in FXa is highly acidic and contains three Glu residues at positions 36, 37, and 39. On the other hand, the loop is shorter by one residue in FIXa (residue 37 is missing), and it contains a Lys and an Asp at positions 36 and 39, respectively. We discovered that replacing residues of the 39-loop (residues 31-41) of FIXa with corresponding residues of FXa renders the FIXa chimera susceptible to inactivation by both ZPI and TFPI. Thus, the inactivation rate of the FIXa chimera by ZPI in the presence of protein Z (PZ), negatively charged membrane vesicles, and calcium ions approached the same diffusion-limited rate (>10(7) m(-1) s(-1)) that has been observed for the PZ-dependent inhibition of FXa by ZPI. Interestingly, sequence alignments indicated that, similar to FXa, residue 36 is a Glu in both mouse and bovine FIXa and that both proteases are also susceptible to inhibition by the PZ-ZPI complex. These results suggest that structural features within residues of the 39-loop contribute to the resistance of FIXa to inhibition by plasma inhibitors ZPI and TFPI.

Variable contributions of basic residues forming an APC exosite in the binding and inactivation of factor VIIIa

Basic residues contained in the 39-, 60-, and 70-80-loops of activated protein C (APC) comprise an exosite that contributes to the binding and subsequent proteolytic inactivation of factor (F) VIIIa. Surface plasmon resonance (SPR) showed that WT APC bound to FVIII light chain (LC) and the FVIIIa A1/A3C1C2 dimer with equivalent affinity (Kd = 525 and 546 nM, respectively). These affinity values may reflect binding interactions to the acidic residue-rich a1 and a3 segments adjacent to A1 domain in the A1/A3C1C2 and A3 domain in LC, respectively. Results from SPR, using a panel of APC exosite variants where basic residues were mutated, in binding to immobilized FVIIIa A1/A3C1C2 or LC indicated ~4-10-fold increases in the Kd values relative to WT for several of the variants including Lys39Ala, Lys37-Lys38-Lys39/Pro-Gln-Glu, and Arg67Ala. On the other hand, a number of APC variants including Lys38Ala, Lys62Ala, and Lys78Ala showed little if any change in binding affinity to the FVIII substrates. FXa generation assays and Western blotting, used to monitor rates of FVIIIa inactivation and proteolysis at the primary cleavage site in the cofactor (Arg(336)), respectively, showed marked rate reductions relative to WT for the Lys39Ala, Lys37-Lys38-Lys39/Pro-Gln-Glu, Arg67Ala, and Arg74Ala variants. Furthermore, kinetic analysis monitoring FVIIIa inactivation by APC variants at varying FVIIIa substrate concentration showed ~2.6-4.4-fold increases in Km values relative to WT. These results show a variable contribution of basic residues comprising the APC exosite, with significant contributions from Lys39, Arg67, and Arg74 to forming a FVIIIa-interactive site.