Endothelial PAI-1 (Plasminogen Activator Inhibitor-1) Blocks the Intrinsic Pathway of Coagulation, Inducing the Clearance and Degradation of FXIa (Activated Factor XI)

Objective- Activation of coagulation FXI (factor XI) by FXIIa (activated factor XII) is a prothrombotic process. The endothelium is known to play an antithrombotic role by limiting thrombin generation and platelet activation. It is unknown whether the antithrombotic role of the endothelium includes sequestration of FXIa (activated factor XI) activity. This study aims to determine the role of endothelial cells (ECs) in the regulation of the intrinsic pathway of coagulation. Approach and Results- Using a chromogenic assay, we observed that human umbilical veins ECs selectively blocked FXIa yet supported kallikrein and FXIIa activity. Western blotting and mass spectrometry analyses revealed that FXIa formed a complex with endothelial PAI-1 (plasminogen activator inhibitor-1). Blocking endothelial PAI-1 increased the cleavage of a chromogenic substrate by FXIa and the capacity of FXIa to promote fibrin formation in plasma. Western blot and immunofluorescence analyses showed that FXIa-PAI-1 complexes were either released into the media or trafficked to the early and late endosomes and lysosomes of ECs. When baboons were challenged with Staphylococcus aureus to induce a prothrombotic phenotype, an increase in circulating FXIa-PAI-1 complex levels was detected by ELISA within 2 to 8 hours postchallenge. Conclusions- PAI-1 forms a complex with FXIa on ECs, blocking its activity and inducing the clearance and degradation of FXIa. Circulating FXIa-PAI-1 complexes were detected in a baboon model of S. aureus sepsis. Although ECs support kallikrein and FXIIa activity, inhibition of FXIa by ECs may promote the clearance of intravascular FXIa. Visual Overview- An online visual overview is available for this article.

Targeting ELTD1, an angiogenesis marker for glioblastoma (GBM), also affects VEGFR2: molecular-targeted MRI assessment

Glioblastomas (GBM) are deadly brain tumors that currently do not have long-term patient treatments available. GBM overexpress the angiogenesis factor VEGF and its receptor VEGFR2. ETLD1 (epidermal growth factor, latrophilin and seven transmembrane domain-containing protein 1), a G-protein coupled receptor (GPCR) protein, we discovered as a biomarker for high-grade gliomas, is also a novel regulator of angiogenesis. Since it was established that VEGF regulates ELTD1, we wanted to establish if VEGFR2 is also associated with ELTD1, by targeted antibody inhibition. G55 glioma-bearing mice were treated with either anti-ELTD1 or anti-VEGFR2 antibodies. With the use of MRI molecular imaging probes, we were able to detect in vivo levels of either ELTD1 (anti-ELTD1 probe) or VEGFR2 (anti-VEGFR2 probe). Protein expressions were obtained for ELTD1, VEGF or VEGFR2 via immunohistochemistry (IHC). VEGFR2 levels were significantly decreased (P < 0.05) with anti-ELTD1 antibody treatment, and ELTD1 levels were significantly decreased (P < 0.05) with anti-VEGFR2 antibody treatment, both compared to untreated tumors. IHC from mouse tumor tissues established that VEGFR2 and ELTD1 were co-localized. The mouse anti-ELTD1 antibody treatment study indicated that anti-VEGFR2 antibody treatment does not significantly increase survival, decrease tumor volumes, or alter tumor perfusion (measured as relative cerebral blood flow or rCBF). Conversely, anti-ELTD1 antibody treatment was able to significantly increase animal survival (P < 0.01), decrease tumor volumes (P < 0.05), and reduce change in rCBF (P < 0.001), when compared to untreated or IgG-treated tumor bearing mice. Anti-ELTD1 antibody therapy could be beneficial in targeting ELTD1, as well as simultaneously affecting VEGFR2, as a possible GBM treatment.

Excessive Plasmin Compromises Hepatic Sinusoidal Vascular Integrity After Acetaminophen Overdose

The serine protease plasmin degrades extracellular matrix (ECM) components both directly and indirectly through activation of matrix metalloproteinases. Excessive plasmin activity and subsequent ECM degradation cause hepatic sinusoidal fragility and hemorrhage in developing embryos. We report here that excessive plasmin activity in a murine acetaminophen (APAP) overdose model likewise compromises hepatic sinusoidal vascular integrity in adult animals. We found that hepatic plasmin activity is up-regulated significantly at 6 hours after APAP overdose. This plasmin up-regulation precedes both degradation of the ECM component fibronectin around liver vasculature and bleeding from centrilobular sinusoids. Importantly, administration of the pharmacological plasmin inhibitor tranexamic acid or genetic reduction of plasminogen, the circulating zymogen of plasmin, ameliorates APAP-induced hepatic fibronectin degradation and sinusoidal bleeding. Conclusion: These studies demonstrate that reduction of plasmin stabilizes hepatic sinusoidal vascular integrity after APAP overdose. (Hepatology 2018; 00:1-13).

Peptidoglycan induces disseminated intravascular coagulation in baboons through activation of both coagulation pathways

Anthrax infections exhibit progressive coagulopathies that may contribute to the sepsis pathophysiology observed in fulminant disease. The hemostatic imbalance is recapitulated in primate models of late-stage disease but is uncommon in toxemic models, suggesting contribution of other bacterial pathogen-associated molecular patterns (PAMPs). Peptidoglycan (PGN) is a bacterial PAMP that engages cellular components at the cross talk between innate immunity and hemostasis. We hypothesized that PGN is critical for anthrax-induced coagulopathies and investigated the activation of blood coagulation in response to a sterile PGN infusion in primates. The PGN challenge, like the vegetative bacteria, induced a sepsis-like pathophysiology characterized by systemic inflammation, disseminated intravascular coagulation (DIC), organ dysfunction, and impaired survival. Importantly, the hemostatic impairment occurred early and in parallel with the inflammatory response, suggesting direct engagement of coagulation pathways. PGN infusion in baboons promoted early activation of contact factors evidenced by elevated protease-serpin complexes. Despite binding to contact factors, PGN did not directly activate either factor XII (FXII) or prekallikrein. PGN supported contact coagulation by enhancing enzymatic function of active FXII (FXIIa) and depressing its inhibition by antithrombin. In parallel, PGN induced de novo monocyte tissue factor expression in vitro and in vivo, promoting extrinsic clotting reactions at later stages. Activation of platelets further amplified the procoagulant state during PGN challenge, leading to DIC and subsequent ischemic damage of peripheral tissues. These data indicate that PGN may be a major cause for the pathophysiologic progression of Bacillus anthracis sepsis and is the primary PAMP behind the pathogen-induced coagulopathy in late-stage anthrax.

Factor XII Activation Promotes Platelet Consumption in the Presence of Bacterial-Type Long-Chain Polyphosphate In Vitro and In Vivo

Objective- Terminal complications of bacterial sepsis include development of disseminated intravascular consumptive coagulopathy. Bacterial constituents, including long-chain polyphosphates (polyP), have been shown to activate the contact pathway of coagulation in plasma. Recent work shows that activation of the contact pathway in flowing whole blood promotes thrombin generation and platelet activation and consumption distal to thrombus formation ex vivo and in vivo. Here, we sought to determine whether presence of long-chain polyP or bacteria in the bloodstream promotes platelet activation and consumption in a coagulation factor (F)XII-dependent manner. Approach and Results- Long-chain polyP promoted platelet P-selectin expression, microaggregate formation, and platelet consumption in flowing whole blood in a contact activation pathway-dependent manner. Moreover, long-chain polyP promoted local fibrin formation on collagen under shear flow in a FXI-dependent manner. Distal to the site of thrombus formation, platelet consumption was dramatically enhanced in the presence of long-chain polyP in the blood flow in a FXI- and FXII-dependent manner. In a murine model, long-chain polyP promoted platelet deposition and fibrin generation in lungs in a FXII-dependent manner. In a nonhuman primate model of bacterial sepsis, pre-treatment of animals with an antibody blocking FXI activation by FXIIa reduced lethal dose100 Staphylococcus aureus-induced platelet and fibrinogen consumption. Conclusions- This study demonstrates that bacterial-type long-chain polyP promotes platelet activation in a FXII-dependent manner in flowing blood, which may contribute to sepsis-associated thrombotic processes, consumptive coagulopathy, and thrombocytopenia.

Thrombin Receptor Activating Peptide (TRAP) Stimulates Mitogenesis, c-fos and PDGF-A Gene Expression in Human Vascular Smooth Muscle Cells

The synthetic peptide SFLLRNPNDKYEPF, identical in sequence to the new amino-terminus of the thrombin receptor generated following cleavage by thrombin, acts as a thrombin receptor agonist/ activating peptide (TRAP). In this study, Northern blot analysis showed that cultured human vascular smooth muscle cells (HVSMC) express a thrombin receptor transcript. TRAP, in contrast to thrombin was shown to be a weak mitogen for HVSMC. A combination of TRAP and enzymatically-inactivated thrombin (PPACK-thrombin) which provides receptor occupancy, did not potentiate TRAP-induced mitogenesis, indicating that TRAP and PPACK-thrombin do not reproduce the mitogenic effect of enzymatically-active thrombin. Both thrombin and TRAP, induced the expression of c-fos and the PDGF-A gene in a pertussis toxin (PTX)-insensitive manner. Examination of thrombin and TRAP-treated cells by immunofluorescence staining followed by computer assisted image analysis revealed that thrombin and to a lesser extent TRAP induced PDGF-AA protein expression. Antibodies to PDGF-AA partially inhibited thrombin but not TRAP-induced mitogenesis in HVSMC. This study indicates that in addition to the common signalling pathways utilised by thrombin and TRAP, enzymatically-active thrombin activates other signalling pathways and hence TRAP does not mimic fully the biological effect of thrombin on HVSMC.

Abstract 037: Coagulation Factor XII Promotes Platelet Consumption in the Presence of Microbial Polyphosphate Under Shear Flow

Background: Terminal complications of bacterial sepsis include development of disseminated intravascular consumptive coagulopathy. Bacterial constituents, including long-chain polyphosphates (polyP), have been shown to activate the contact pathway of coagulation in plasma. Recent work shows that activation of the contact pathway in flowing whole blood can promote thrombin generation and platelet activation and consumption distal to thrombus formation ex vivo and in vivo .

Aim: Determine whether presence of long-chain polyP in the bloodstream promotes platelet activation and consumption in a coagulation factor (F)XII-dependent manner.

Methods/Results: The addition of long-chain polyP to human whole blood promoted platelet P-selectin expression, microaggregate formation and platelet consumption in the bloodstream under shear in a FXII-dependent manner. Moreover, long-chain polyP accelerated thrombus formation on immobilized collagen surfaces under shear flow in a thrombin generation-dependent manner. Distal to the site of thrombus formation, platelet consumption was dramatically enhanced in the presence of long-chain polyP in the bloodstream. Inhibiting contact activation of coagulation using established and novel agents reduced fibrin formation on collagen as well as platelet consumption in the bloodstream distal to the site of thrombus formation. In vivo , FXII deficiency was protective against long-chain polyP occlusive lung thrombus formation in mice. Lastly, in a non-human primate model of sepsis, pretreatment of animals with an antibody blocking FXI activation by FXIIa (14E11) diminished LD 100 S. aureus -induced platelet and fibrinogen consumption.

Conclusions: This study demonstrates that bacterial-type long-chain polyP promotes FXII-mediated thrombin generation and platelet activation in the flowing blood and could contribute to sepsis-associated thrombotic processes, consumptive coagulopathy and thrombocytopenia.

Abstract 091: Coagulation Factor XII Promotes Platelet Consumption in the Presence of Microbial Polyphosphate Under Shear Flow

Background: Terminal complications of bacterial sepsis include development of disseminated intravascular consumptive coagulopathy. Bacterial constituents, including long-chain polyphosphates (polyP), have been shown to activate the contact pathway of coagulation in plasma. Recent work shows that activation of the contact pathway in flowing whole blood can promote thrombin generation and platelet activation and consumption distal to thrombus formation ex vivo and in vivo .

Aim: Determine whether presence of long-chain polyP in the bloodstream promotes platelet activation and consumption in a coagulation factor (F)XII-dependent manner.

Methods/Results: The addition of long-chain polyP to human whole blood promoted platelet P-selectin expression, microaggregate formation and platelet consumption in the bloodstream under shear in a FXII-dependent manner. Moreover, long-chain polyP accelerated thrombus formation on immobilized collagen surfaces under shear flow in a thrombin generation-dependent manner. Distal to the site of thrombus formation, platelet consumption was dramatically enhanced in the presence of long-chain polyP in the bloodstream. Inhibiting contact activation of coagulation using established and novel agents reduced fibrin formation on collagen as well as platelet consumption in the bloodstream distal to the site of thrombus formation. In vivo , FXII deficiency was protective against long-chain polyP occlusive lung thrombus formation in mice. Lastly, in a non-human primate model of sepsis, pretreatment of animals with an antibody blocking FXI activation by FXIIa (14E11) diminished LD 100 S. aureus -induced platelet and fibrinogen consumption.

Conclusions: This study demonstrates that bacterial-type long-chain polyP promotes FXII-mediated thrombin generation and platelet activation in the flowing blood and could contribute to sepsis-associated thrombotic processes, consumptive coagulopathy and thrombocytopenia.

Abstract 276: Excessive Plasmin Activity Compromises Hepatic Sinusoidal Vascular Integrity After Acetaminophen Overdose

The serine protease plasmin degrades extracellular matrix (ECM) components both through direct cleavage and indirectly through activation of matrix metalloproteinases. We previously reported that excessive plasmin activity and subsequent ECM degradation causes hepatic sinusoidal fragility and lethal hemorrhage in developing embryos. We now report that excessive plasmin activity in a murine acetaminophen (APAP) overdose model likewise compromises postnatal hepatic sinusoidal vascular integrity. We found that hepatic plasmin activity is upregulated significantly at 6 hr after APAP overdose. This plasmin upregulation precedes both degradation of the ECM component fibronectin around liver vasculature and bleeding from centrilobular sinusoids. Importantly, administration of the pharmacological plasmin inhibitor tranexamic acid or genetic reduction of plasminogen, the circulating zymogen of plasmin, ameliorates APAP-induced hepatic fibronectin degradation and sinusoidal bleeding. In conclusion, these studies demonstrate that reduction of plasmin stabilizes hepatic sinusoidal vascular integrity after APAP overdose.

Inflammation, innate immunity and blood coagulation

Inflammation drives arterial, venous and microvascular thrombosis. Chronic inflammation contributes to arterial thrombotic complications, whereas acute inflammation drives venous thrombosis and microvascular thrombosis. Mechanistically, inflammation modulates thrombotic responses by upregulating procoagulants, downregulating anticoagulants and suppressing fibrinolysis. The inflammatory response can also result in cell apoptosis or necrosis. Products released from the dead cells, particularly histones, propagate further inflammation, tissue death and organ failure.

Inhibition of histone mediated cytotoxicity appears to be a new mechanism for protecting against this deadly cascade.

CD9 Gene Deficiency Does not Affect Smooth Muscle Cell Migration and Neointima Formation after Vascular Injury in Mice

The hypothesis that CD9, a member of the tetraspanin family, plays a role in smooth muscle cell (SMC) migration was tested with the use of a vascular injury model in wild-type (CD9+/+) and CD9-deficient (CD9−/−) mice. Neointima formation 3 weeks after electric injury of the femoral artery was not significantly different in CD9+/+ and CD9−/− mice (area of 0.019 ± 0.0034 mm2 versus 0.013 ± 0.0036 mm2; mean ± SEM, n = 6). The medial areas were also comparable, resulting in intima/media ratio’s of 1.3 ± 0.15 and 0.90 ± 0.22, respectively. Nuclear cell counts in cross-sectional areas of the injured region were comparable in media (33 ± 5 versus 27 ± 2) and neointima (135 ± 16 versus 97 ± 17) of CD9+/+ and CD9−/− arteries. Immunocytochemical analysis revealed expression of CD9 in the endothelium, by SMC in the media and by some fibroblasts in the adventitia of non-injured femoral arteries. Three weeks after injury, there appeared to be a gradient of increased CD9 expression from the adventitia to the neointima, in which SMC are abundantly present. Immunogold labeling and electron microscopy with non-injured femoral arteries of CD9+/+ mice confirmed the presence of CD9 at the surface of adventitial fibroblasts and in SMC or pericytes, as well as in the endothelium.

Thus, in this model CD9 is highly expressed by migrating SMC, but deficiency of CD9 does not affect SMC migration or neointima formation after perivascular injury.

In vivo-generated thrombin and plasmin do not activate the complement system in baboons

Sepsis concurrently activates both coagulation and complement systems. Although complement activation by bacteria is well documented, work in mice and in vitro suggests that coagulation proteases can directly cleave complement proteins. We aimed to determine whether generation of coagulation proteases in vivo can activate the complement cascade in 2 highly coagulopathic models. We compared temporal changes in activation biomarkers of coagulation (thrombin-antithrombin [TAT]), fibrinolysis (plasmin-antiplasmin [PAP]), and complement (C3b, C5a, C5b-9) in baboons infused with factor Xa (FXa) and phospholipids (FXa/phosphatidylcholine-phosphatidylserine [PCPS]) vs LD100 Escherichia coli We found that, albeit with different timing, both FXa/PCPS and E coli infusion led to robust thrombin and plasmin generation. Conversely, only E coli challenge activated the complement system, reaching a maximum at 2 hours postchallenge during the peaks of lipopolysaccharide and bacteremia but not of TAT and PAP. Despite inducing a strong burst of thrombin and plasmin, FXa/PCPS infusion did not produce measurable levels of complement activation in vivo. Similarly, ex vivo incubation of baboon serum with thrombin, plasmin, or FXa did not show noticeable complement cleavage unless supraphysiologic amounts of enzymes were used. Our results suggest that in vivo-generated thrombin and plasmin do not directly activate the complement in nonhuman primates.

In situ Analysis of Tissue Factor-Dependent Thrombin Generation in Human Atherosclerotic Vessels

Tissue factor (TF) is expressed in human atherosclerotic plaques where it may contribute to the thrombogenicity of the lesions and their progression toward unstable syndromes and acute myocardial infarction. In this study we tested the hypothesis that thrombin generation takes place in the lesion. Localisation of TF, factor VII (FVII), factor X/Xa (FX/Xa), thrombin, thrombin receptor PAR-1 and FXa receptor EPR-1 was done by immunostaining, ligand binding, or immunogold electron microscopy. Quantitation of TF antigen was done using a modified ELISA on fixed tissue sections. The amount of antigen was correlated with the pattern and intensity of immunostaining as detected on consecutive sections using confocal microscopy. TFdependent generation of FXa on cryosections was used to assess the functional activity of TF. Active thrombin was detected using hirudin as a specific probe, followed by anti-hirudin IgG. Our light microscopy and immunogold electron microscopy results showed that the factors involved in TF-dependent coagulation are localised in atherosclerotic plaques in close proximity and colocalise with active thrombin and fibrin deposits. We have detected 3 to 7-fold increase of TF antigen and TF-dependent FXa generation in atherosclerotic vessels as compared with controls. Hirudin binding proved that active thrombin is present within the lesions. In conclusion, our data show that active coagulation factors are generated within atherosclerotic lesions and co-localise with their cellular receptors. These findings may suggest possible roles of the TF-dependent coagulation pathway in the intramural fibrin deposition and the progression of the atherosclerotic lesions.

Bemiparin and Fluid Flow Modulate the Expression, Activity and Release of Tissue Factor Pathway Inhibitor in Human Endothelial Cells In Vitro

We investigated the localisation, gene expression, and activity of tissue factor pathway inhibitor (TFPI) in endothelial cells (EC) grown in static conditions or under shear stress, in the presence of unfractionated heparin (UFH) and two low-molecular-weight heparins (LMWHs), dalteparin and bemiparin (a second generation of LMWHs). All three preparations induced increased release, cellular redistribution, and enhanced activity of TFPI on the cell surface in static EC. In EC grown under shear stress (0.27, 4.1 and 19 dyne/cm2) and incubated with each heparin for 24 h, the release of TFPI was significantly correlated with the level of flow for bemiparin and dalteparin, but not for UFH. For all three levels of flow tested, bemiparin induced the highest secretion and increase of both cellular TFPI and cell surface activity of the inhibitor. The expression of TFPI mRNA, determined by Northern blotting, was specifically modulated by heparins. All three preparations increased the expression of TFPI by 60 to 120% in EC under minimal flow, but only bemiparin enhanced TFPI mRNA in EC under the arterial flow. Immunogold electron microscopy revealed that EC exhibited strong cellular labelling for TFPI when grown under arterial flow in the presence of bemiparin. We conclude that in EC subjected to shear stress in vitro bemiparin is more efficient than UFH or dalteparin in modulating the expression, release and activity of TFPI. We therefore suggest that bemiparin may be superior over the conventional heparins in maintaining the anticoagulant properties of the endothelium.

Release of Soluble Urokinase Receptor from Vascular Cells*

Urokinase-type plasminogen activator (uPA) and its cell surface-receptor (uPAR) regulate cellular functions linked to adhesion and migration and contribute to pericellular proteolysis in tissue remodelling processes. Soluble uPAR (suPAR) is present in the circulation, peritoneal and ascitic fluid and in the cystic fluid from ovarian cancer. We have investigated the origin and the vascular distribution of the soluble receptor, which accounts for 10-20% of the total receptor in vascular endothelial and smooth muscle cells. Phase separation analysis of the cell conditioned media with Triton X-114 indicated that suPAR associates with the aqueous phase, indicative of the absence of the glycolipid anchor. There was a polarized release of suPAR from cultured endothelial cells towards the basolateral direction, whereas the membrane-bound receptor was found preferentially on the apical surface. Both, uPAR and suPAR became upregulated 2-4 fold after activation of protein kinase C with phorbol ester, which required de-novo protein biosynthesis. Interleukin-1β (IL-1β), basic fibroblast growth factor (bFGF) or vascular endothelial growth factor increased suPAR release from endothelial cells, whereas platelet derived growth factor-BB, bFGF or IL-1β stimulated suPAR release from vascular smooth muscle cells. Immune electron microscopy indicated that in atherosclerotic vessels (s)uPAR was observed on cell membranes as well as in the extracellular matrix. These findings indicate that (s)uPAR from vascular cells is upregulated by proangiogenic as well as proatherogenic growth factors and cytokines, is preferentially released towards the basolateral side of endothelial cells and accumulates in the vessel wall.

*Part of this work was supported by grants (Pr 327/1-4) from the Deutsche Forschungsgemeinschaft (Bonn, Germany) and the Novartis-Foundation (Nürnberg, Germany). This work is part of the MD/PhD-thesis of T.C. at the Institute for Biochemistry, Department of Medicine, Justus-Liebig-Universität, Giessen, Germany.

Abbreviations: bFGF: basic fibroblast growth factor, GPI: glycosyl-phosphatidylinositol, FCS: fetal calf serum, HUVEC: human umbilical vein endothelial cells, HVSMC: human vascular smooth muscle cells, IL-1β: interleukin-1β, mAb: monoclonal antibody, PBS: phosphate-buffered saline, PDGF-BB: platelet derived growth factor-BB, piPLC: phosphatidylinositol-specific phospholipase C, piPLD: phosphatidylinositol-specific phospholipase D, PMA: phorbol myristate acetate, scuPA: single chain uPA, suPAR: soluble urokinase receptor, uPA: urokinase- type plasminogen activator, uPAR: urokinase receptor, VN: vitronectin

Adipose Tissue Expression of Gelatinases in Mouse Models of Obesity

Following the observation by Brown et al. (Am J Physiol 1997; 272: C937-49) that primary rat adipocytes in culture secrete gelatinase A (MMP-2), we have evaluated gelatinase expression in adipose tissue with the use of mouse models of obesity. Wild-type mice were kept on a standard fat diet (SFD) or on a high fat diet (42% fat, HFD) and genetically obese db/db mice were kept on SFD; gonadal and subcutaneous fat pads were removed and analysed ex vivo. These studies revealed that: 1) the HFD induced adipocyte hypertrophy; 2) after 32 weeks, significantly higher levels of 70 kDa (p <0.05) and 65 kDa proMMP-2 (p < 0.01) were observed in extracts of gonadal fat pads of mice on HFD; 3) the contribution of active MMP-2 to the total level was comparable in SFD and HFD groups (20 to 30%); and 4) gelatinase B (MMP-9) was not consistently detected. These findings were confirmed by gelatin zymography and by mRNA determination using competitive RT-PCR. The presence of MMP-2 in the adipose tissue was confirmed immunologically and its localization in adipocytes revealed by immunogold electron microscopy. The potential functional role of MMP-2 in adipose tissue remains to be determined.

Acute Release of Tissue Factor Pathway Inhibitor after In Vivo Thrombin Generation in Baboons

Tissue factor pathway inhibitor (TFPI), the major downregulator of the procoagulant activity of tissue factor (TF), is synthesised by endothelial cells (EC) and acutely released in vitro after thrombin stimulation. Expression of TF on EC and subsequent thrombin generation occurs in vivo during sepsis or malignancy, inducing disseminated intravascular coagulation (DIC). The present study investigates the changes in plasma TFPI in relation to markers of in vivo thrombin generation induced by injection of factor Xa (FXa)/phospholipids in baboons at dosages leading to partial (48%) or complete fibrinogen depletion. The plasma concentrations of thrombin-antithrombin III (TAT) and fibrinopeptide A (FpA), as markers of in vivo generation of thrombin, were strongly enhanced after injection of FXa/phospholipids. TFPI levels, whether measured as antigen or activity, increased significantly in both treatment groups within few minutes, and were dependent on the dose of FXa/phospholipids. Significant positive correlations between plasma levels of TFPI and of TAT or FpA were observed. Altogether, our results indicate that experimentally induced in vivo generation of thrombin causes the acute release of TFPI, high-lighting a possible novel function of thrombin in downregulation of the coagulation process, potentially relevant for the outcome of DIC.

Temporal and Topographic Matrix Metalloproteinase Expression after Vascular Injury in Mice

Temporal and topographic expression of matrix metalloproteinases (MMPs) after perivascular electric injury was studied in wild-type (WT) and urokinase-deficient (u-PA-/-) mice. Neointima formation after injury of the femoral artery was significantly reduced in u-PA-/- mice as compared to WT mice (area of 0.002 ± 0.0007 mm2 versus 0.008 ± 0.002 mm2 at 3 weeks after injury; p <0.001), associated with impaired cellular migration (nuclear cell counts of 44 ± 5 versus 82 ± 9 in cross-sectional areas; p <0.001).

Zymographic and/or microscopic analysis indicated that MMP expression gradually increased to reach a maximum at 1 to 2 weeks after vascular injury. In general, MMP levels were lower in u-PA-/- than in WT mice. In non-injured arteries, MMP-2 (gelatinase A) and MMP-3 (stromelysin-1) were produced mainly by adventitial fibroblasts and/or non-contractile smooth muscle cells (SMC). One week after injury, MMP-2 and MMP-3 levels were enhanced due to an increased number and size of producing cells; 2 to 3 weeks after injury, MMP-2 and MMP-3 were produced also by some contractile SMC, which stained with α-actin antiserum. MMP-9 (gelatinase B), MMP-12 (metalloelastase) and MMP-13 (collagenase-3) were found in macrophages located mainly in the adventitia. Immunogold electron microscopic examination revealed that MMP-2 was located predominantly in association with the cell surface of fibroblasts or SMC, while MMP-9 and MMP-12 were located in well defined storage granules within macrophages. MMP-2, MMP-3 and MMP-13, but not MMP-9 or MMP-12, were also found extracellularly, associated with elastin-containing structures (MMP-2), with the basement membrane and occasionally with collagen fibres (MMP-3), or with proteoglycans, collagen and elastin (MMP-13).

The temporal and topographic expression pattern of MMPs after vascular injury, coinciding with smooth muscle cell migration and neointima formation, thus is compatible with a role in vascular remodeling.

Loss of mucin-type O-glycans impairs the integrity of the glomerular filtration barrier in the mouse kidney

The kidney's filtration activity is essential for removing toxins and waste products from the body. The vascular endothelial cells of the glomerulus are fenestrated, flattened, and surrounded by podocytes, specialized cells that support glomerular endothelial cells. Mucin-type core 1-derived O-glycans (O-glycans) are highly expressed on both glomerular capillary endothelial cells and their supporting podocytes, but their biological role is unclear. Biosynthesis of core 1-derived O-glycans is catalyzed by the glycosyltransferase core 1 β1,3-galactosyltransferase (C1galt1). Here we report that neonatal or adult mice with inducible deletion of C1galt1 (iC1galt1^-/-) exhibit spontaneous proteinuria and rapidly progressing glomerulosclerosis. Ultrastructural analysis of the glomerular filtration barrier components revealed that loss of O-glycans results in altered podocyte foot processes. Further analysis indicated that O-glycan is essential for the normal signaling function of podocalyxin, a podocyte foot process-associated glycoprotein. Our results reveal a new function of O-glycosylation in the integrity of the glomerular filtration barrier.

BRG1 (Brahma-Related Gene 1) Promotes Endothelial Mrtf Transcription to Establish Embryonic Capillary Integrity

OBJECTIVE

The chromatin remodeling enzyme BRG1 (brahma-related gene 1) transcriptionally regulates target genes important for early blood vessel development and primitive hematopoiesis. However, because Brg1 deletion in vascular progenitor cells results in lethal anemia by embryonic day 10.5 (E10.5), roles for BRG1 in embryonic vascular development after midgestation are unknown. In this study, we sought to determine whether endothelial cell BRG1 regulates genes important for vascular development or maintenance later in embryonic development.

APPROACH AND RESULTS

Using mice with temporally inducible deletion of endothelial BRG1 (Brg1^fl/fl;Cdh5(PAC)-Cre^ERT2 ), we found that Brg1 excision between E9.5 and 11.5 results in capillary dilation and lethal hemorrhage by E14.5. This phenotype strongly resembles that seen when the SRF (serum response factor) transcription factor is deleted from embryonic endothelial cells. Although expression of Srf and several of its known endothelial cell target genes are downregulated in BRG1-depleted endothelial cells, we did not detect binding of BRG1 at these gene promoters, indicating that they are not direct BRG1 target genes. Instead, we found that BRG1 binds to the promoters of the SRF cofactors Mrtfa and Mrtfb (myocardin-related transcription factors A and B) in endothelial cells, and these genes are downregulated in Brg1-deficient endothelial cells.

CONCLUSIONS

BRG1 promotes transcription of endothelial Mrtfa and Mrtfb, which elevates expression of SRF and SRF target genes that establish embryonic capillary integrity. These data highlight a new and temporally specific role for BRG1 in embryonic vasculature and provide novel information about epigenetic regulation of Mrtf expression and SRF signaling in developing blood vessels.

Profibrotic Infrapatellar Fat Pad Remodeling Without M1 Macrophage Polarization Precedes Knee Osteoarthritis in Mice With Diet-Induced Obesity

OBJECTIVE

To test the hypothesis that high-fat (HF) diet-induced obesity increases proinflammatory cytokine expression, macrophage infiltration, and M1 polarization in the infrapatellar fat pad (IFP) prior to knee cartilage degeneration.

METHODS

We characterized the effect of HF feeding on knee OA pathology, body adiposity, and glucose intolerance in male C57BL/6J mice and identified a diet duration that induces metabolic dysfunction prior to cartilage degeneration. Magnetic resonance imaging and histomorphology were used to quantify changes in the epididymal, subcutaneous, and infrapatellar fat pads and in adipocyte sizes. Finally, we used targeted gene expression and protein arrays, immunohistochemistry, and flow cytometry to quantify differences in fat pad markers of inflammation and immune cell populations.

RESULTS

Twenty weeks of feeding with an HF diet induced marked obesity, glucose intolerance, and early osteoarthritis (OA), including osteophytes and cartilage tidemark duplication. This duration of HF feeding increased the IFP volume. However, it did not increase IFP inflammation, macrophage infiltration, or M1 macrophage polarization as observed in epididymal fat. Furthermore, leptin protein levels were reduced. This protection from obesity-induced inflammation corresponded to increased IFP fibrosis and the absence of adipocyte hypertrophy.

CONCLUSION

The IFP does not recapitulate classic abdominal adipose tissue inflammation during the early stages of knee OA in an HF diet-induced model of obesity. Consequently, these findings do not support the hypothesis that IFP inflammation is an initiating factor of obesity-induced knee OA. Furthermore, the profibrotic and antihypertrophic responses of IFP adipocytes to HF feeding suggest that intraarticular adipocytes are subject to distinct spatiotemporal structural and metabolic regulation among fat pads.

Activated protein C inhibits neutrophil extracellular trap formation in vitro and activation in vivo

Activated protein C (APC) is a multifunctional serine protease with anticoagulant, cytoprotective, and anti-inflammatory activities. In addition to the cytoprotective effects of APC on endothelial cells, podocytes, and neurons, APC cleaves and detoxifies extracellular histones, a major component of neutrophil extracellular traps (NETs). NETs promote pathogen clearance but also can lead to thrombosis; the pathways that negatively regulate NETosis are largely unknown. Thus, we studied whether APC is capable of directly inhibiting NETosis via receptor-mediated cell signaling mechanisms. Here, by quantifying extracellular DNA or myeloperoxidase, we demonstrate that APC binds human leukocytes and prevents activated platelet supernatant or phorbol 12-myristate 13-acetate (PMA) from inducing NETosis. Of note, APC proteolytic activity was required for inhibiting NETosis. Moreover, antibodies against the neutrophil receptors endothelial protein C receptor (EPCR), protease-activated receptor 3 (PAR3), and macrophage-1 antigen (Mac-1) blocked APC inhibition of NETosis. Select mutations in the Gla and protease domains of recombinant APC caused a loss of NETosis. Interestingly, pretreatment of neutrophils with APC prior to induction of NETosis inhibited platelet adhesion to NETs. Lastly, in a nonhuman primate model of Escherichia coli-induced sepsis, pretreatment of animals with APC abrogated release of myeloperoxidase from neutrophils, a marker of neutrophil activation. These findings suggest that the anti-inflammatory function of APC at therapeutic concentrations may include the inhibition of NETosis in an EPCR-, PAR3-, and Mac-1-dependent manner, providing additional mechanistic insight into the diverse functions of neutrophils and APC in disease states including sepsis.

Sphingosine 1-phosphate and its carrier apolipoprotein M in human sepsis and in Escherichia coli sepsis in baboons

Sphingosine 1‐phosphate (S1P) is an important regulator of vascular integrity and immune cell migration, carried in plasma by high‐density lipoprotein (HDL)‐associated apolipoprotein M (apoM) and by albumin. In sepsis, the protein and lipid composition of HDL changes dramatically. The aim of this study was to evaluate changes in S1P and its carrier protein apoM during sepsis. For this purpose, plasma samples from both human sepsis patients and from an experimental Escherichia coli sepsis model in baboons were used. In the human sepsis cohort, previously studied for apoM, plasma demonstrated disease‐severity correlated decreased S1P levels, the profile mimicking that of plasma apoM. In the baboons, a similar disease‐severity dependent decrease in plasma levels of S1P and apoM was observed. In the lethal E. coli baboon sepsis, S1P decreased already within 6–8 hrs, whereas the apoM decrease was seen later at 12–24 hrs. Gel filtration chromatography of plasma from severe human or baboon sepsis on Superose 6 demonstrated an almost complete loss of S1P and apoM in the HDL fractions. S1P plasma concentrations correlated with the platelet count but not with erythrocytes or white blood cells. The liver mRNA levels of apoM and apoA1 decreased strongly upon sepsis induction and after 12 hr both were almost completely lost. In conclusion, during septic challenge, the plasma levels of S1P drop to very low levels. Moreover, the liver synthesis of apoM decreases severely and the plasma levels of apoM are reduced. Possibly, the decrease in S1P contributes to the decreased endothelial barrier function observed in sepsis.