Tissue factor activation: is disulfide bond switching a regulatory mechanism?

Alterations to Sphingomyelin Metabolism Affect Hemostasis and Thrombosis

BACKGROUND

Our recent studies suggest that sphingomyelin levels in the plasma membrane influence TF (tissue factor) procoagulant activity. The current study was performed to investigate how alterations to sphingomyelin metabolic pathway would affect TF procoagulant activity and thereby affect hemostatic and thrombotic processes.

METHODS

Macrophages and endothelial cells were transfected with specific siRNAs or infected with adenoviral vectors to alter sphingomyelin levels in the membrane. TF activity was measured in factor X activation assay. Saphenous vein incision-induced bleeding and the inferior vena cava ligation-induced flow restriction mouse models were used to evaluate hemostasis and thrombosis, respectively.

RESULTS

Overexpression of SMS (sphingomyelin synthase) 1 or SMS2 in human monocyte-derived macrophages suppresses ATP-stimulated TF procoagulant activity, whereas silencing SMS1 or SMS2 increases the basal cell surface TF activity to the same level as of ATP-decrypted TF activity. Consistent with the concept that sphingomyelin metabolism influences TF procoagulant activity, silencing of acid sphingomyelinase or neutral sphingomyelinase 2 or 3 attenuates ATP-induced enhanced TF procoagulant activity in macrophages and endothelial cells. Niemann-Pick disease fibroblasts with a higher concentration of sphingomyelin exhibited lower TF activity compared with wild-type fibroblasts. In vivo studies revealed that LPS+ATP-induced TF activity and thrombin generation were attenuated in ASMase^-/- mice, while their levels were increased in SMS2^-/- mice. Further studies revealed that acid sphingomyelinase deficiency leads to impaired hemostasis, whereas SMS2 deficiency increases thrombotic risk.

CONCLUSIONS

Overall, our data indicate that alterations in sphingomyelin metabolism would influence TF procoagulant activity and affect hemostatic and thrombotic processes.

Tissue factor in cancer-associated thromboembolism: possible mechanisms and clinical applications

Venous and arterial thromboses, called as cancer-associated thromboembolism (CAT), are common complications in cancer patients that are associated with high mortality. The cell-surface glycoprotein tissue factor (TF) initiates the extrinsic blood coagulation cascade. TF is overexpressed in cancer cells and is a component of extracellular vesicles (EVs). Shedding of TF⁺EVs from cancer cells followed by association with coagulation factor VII (fVII) can trigger the blood coagulation cascade, followed by cancer-associated venous thromboembolism in some cancer types. Secretion of TF is controlled by multiple mechanisms of TF⁺EV biogenesis. The procoagulant function of TF is regulated via its conformational change. Thus, multiple steps participate in the elevation of plasma procoagulant activity. Whether cancer cell-derived TF is maximally active in the blood is unclear. Numerous mechanisms other than TF⁺EVs have been proposed as possible causes of CAT. In this review, we focused on a wide variety of regulatory and shedding mechanisms for TF, including the effect of SARS-CoV-2, to provide a broad overview for its role in CAT. Furthermore, we present the current technical issues in studying the relationship between CAT and TF.

ERO1-PDI Redox Signaling in Health and Disease

Significance: Protein disulfide isomerase (PDI) and endoplasmic reticulum oxidoreductase 1 (ERO1) are crucial for oxidative protein folding in the endoplasmic reticulum (ER). These enzymes are frequently overexpressed and secreted, and they contribute to the pathology of neurodegenerative, cardiovascular, and metabolic diseases. Recent Advances: Tissue-specific knockout mouse models and pharmacologic inhibitors have been developed to advance our understanding of the cell-specific functions of PDI and ERO1. In addition to their roles in protecting cells from the unfolded protein response and oxidative stress, recent studies have revealed that PDI and ERO1 also function outside of the cells. Critical Issues: Despite the well-known contributions of PDI and ERO1 to specific disease pathology, the detailed molecular and cellular mechanisms underlying these activities remain to be elucidated. Further, although PDI and ERO1 inhibitors have been identified, the results from previous studies require careful evaluation, as many of these agents are not selective and may have significant cytotoxicity. Future Directions: The functions of PDI and ERO1 in the ER have been extensively studied. Additional studies will be required to define their functions outside the ER.

Murine tissue factor disulfide mutation causes a bleeding phenotype with sex specific organ pathology and lethality

Tissue factor is highly expressed in sub-endothelial tissue. The extracellular allosteric disulfide bond Cys186-Cys209 of human tissue factor shows high evolutionary conservation and in vitro evidence suggests that it significantly contributes to tissue factor procoagulant activity. To investigate the role of this allosteric disulfide bond in vivo, we generated a C213G mutant tissue factor mouse by replacing Cys213 of the corresponding disulfide Cys190-Cys213 in murine tissue factor. A bleeding phenotype was prominent in homozygous C213G tissue factor mice. Pre-natal lethality of 1/3rd of homozygous offspring was observed between E9.5 and E14.5 associated with placental hemorrhages. After birth, homozygous mice suffered from bleedings in different organs and reduced survival. Homozygous C213G tissue factor male mice showed higher incidence of lung bleedings and lower survival rates than females. In both sexes, C213G mutation evoked a reduced protein expression (about 10-fold) and severely reduced pro-coagulant activity (about 1000-fold). Protein glycosylation was impaired and cell membrane exposure decreased in macrophages in vivo. Single housing of homozygous C213G tissue factor males reduced the occurrence of severe bleeding and significantly improved survival, suggesting that inter-male aggressiveness might significantly account for the sex differences. These experiments show that the tissue factor allosteric disulfide bond is of crucial importance for normal in vivo expression, post-translational processing and activity of murine tissue factor. Although C213G tissue factor mice do not display the severe embryonic lethality of tissue factor knock-out mice, their postnatal bleeding phenotype emphasizes the importance of fully functional tissue factor for hemostasis.

Beyond thrombosis: the impact of tissue factor signaling in cancer

Tissue factor (TF) is the primary initiator of the coagulation cascade, though its effects extend well beyond hemostasis. When TF binds to Factor VII, the resulting TF:FVIIa complex can proteolytically cleave transmembrane G protein-coupled protease-activated receptors (PARs). In addition to activating PARs, TF:FVIIa complex can also activate receptor tyrosine kinases (RTKs) and integrins. These signaling pathways are utilized by tumors to increase cell proliferation, angiogenesis, metastasis, and cancer stem-like cell maintenance. Herein, we review in detail the regulation of TF expression, mechanisms of TF signaling, their pathological consequences, and how it is being targeted in experimental cancer therapeutics.

Protein disulfide isomerase in cardiovascular disease

Protein disulfide isomerase (PDI) participates in the pathogenesis of numerous diseases. Increasing evidence indicates that intravascular cell-derived PDI plays an important role in the initiation and progression of cardiovascular diseases, including thrombosis and vascular inflammation. Recent studies with PDI conditional knockout mice have advanced our understanding of the function of cell-specific PDI in disease processes. Furthermore, the identification and development of novel small-molecule PDI inhibitors has led into a new era of PDI research that transitioned from the bench to bedside. In this review, we will discuss recent findings on the regulatory role of PDI in cardiovascular disease.

Efforts to untangle the functions of a large family of enzymes could lead researchers to new therapies for diverse cardiovascular diseases. Members of the protein disulfide isomerase (PDI) family chemically modify other proteins in ways that can alter both their structure and biological activity. Jaehyung Cho of the University of Illinois at Chicago, USA and coworkers have reviewed numerous studies linking PDI with cardiovascular diseases, including thrombosis, heart attack, vascular inflammation, and stroke. The authors also report progress in developing small-molecule PDI inhibitors that could yield the treatment for these conditions.

Thrombosis in diabetes: a shear flow effect?

Cardiovascular events are the major cause of morbidity and mortality in Type 2 diabetes (T2D). This condition is associated with heightened platelet reactivity, contributing to increased atherothrombotic risk. Indeed, individuals with diabetes respond inadequately to standard antiplatelet therapy. Furthermore, they often experience recurrent events as well as side effects that include excess bleeding. This highlights the need for identification of novel regulators of diabetes-associated thrombosis to target for therapeutic intervention. It is well established that platelet aggregation, a process essential for thrombus formation, is tightly regulated by shear stress; however, the mechanisms underlying shear activation of platelets, particularly in the setting of diabetes, are still poorly understood. This review will address the limitations of current diagnostic systems to assess the importance of shear stress in the regulation of thrombus formation in T2D, and the inability to recapitulate the pro-thrombotic phenotype seen clinically in the setting of T2D. Moreover, we will discuss recent findings utilizing new technologies to define the importance of shear stress in thrombus formation and their potential application to the setting of diabetes. Finally, we will discuss the potential of targeting shear-dependent mechanisms of thrombus formation as a novel therapeutic approach in the setting of T2D.

Sphingomyelin encrypts tissue factor: ATP-induced activation of A-SMase leads to tissue factor decryption and microvesicle shedding

A majority of tissue factor (TF) on cell surfaces exists in an encrypted state with minimal to no procoagulant activity. At present, it is unclear whether limited availability of phosphatidylserine (PS) and/or a specific membrane lipid in the outer leaflet of the plasma membrane contributes to TF encryption. Sphingomyelin (SM) is a major phospholipid in the outer leaflet, and SM metabolism is shown to be altered in many disease settings that cause thrombotic disorders. The present study is carried out to investigate the effect of SM metabolism on TF activity and TF+ microvesicles (MVs) release. In vitro studies using TF reconstituted into liposomes containing varying molar ratios of SM showed that a high molar ratio of SM in the proteoliposomes inhibits TF coagulant activity. Treatment of macrophages with sphingomyelinase (SMase) that hydrolyzes SM in the outer leaflet results in increased TF activity at the cell surface and TF+ MVs release without increasing PS externalization. Adenosine triphosphate (ATP) stimulation of macrophages that activates TF and induces MV shedding also leads to translocation of acid-sphingomyelinase (A-SMase) to the plasma membrane. ATP stimulation increases the hydrolysis of SM in the outer leaflet. Inhibition of A-SMase expression or activity not only attenuates ATP-induced SM hydrolysis, but also inhibits ATP-induced TF decryption and TF+ MVs release. Overall, our novel findings show that SM plays a role in maintaining TF in an encrypted state in resting cells and hydrolysis of SM following cell injury removes the inhibitory effect of SM on TF activity, thus leading to TF decryption.

The Role of Putative Phosphatidylserine-Interactive Residues of Tissue Factor on Its Coagulant Activity at the Cell Surface

Exposure of phosphatidylserine (PS) on the outer leaflet of the cell membrane is thought to play a critical role in tissue factor (TF) decryption. Recent molecular dynamics simulation studies suggested that the TF ectodomain may directly interact with PS. To investigate the potential role of TF direct interaction with the cell surface phospholipids on basal TF activity and the enhanced TF activity following the decryption, one or all of the putative PS-interactive residues in the TF ectodomain were mutated and tested for their coagulant activity in cell systems. Out of the 9 selected TF mutants, five of them -TFS160A, TFS161A, TFS162A, TFK165A, and TFD180A- exhibited a similar TF coagulant activity to that of the wild-type TF. The specific activity of three mutants, TFK159A, TFS163A, and TFK166A, was reduced substantially. Mutation of the glycine residue at the position 164 markedly abrogated the TF coagulant activity, resulting in ~90% inhibition. Mutation of all nine lipid binding residues together did not further decrease the activity of TF compared to TFG164A. A similar fold increase in TF activity was observed in wild-type TF and all TF mutants following the treatment of THP-1 cells with either calcium ionomycin or HgCl2, two agents that are commonly used to decrypt TF. Overall, our data show that a few select TF residues that are implicated in interacting with PS contribute to the TF coagulant activity at the cell surface. However, our data also indicate that TF regions outside of the putative lipid binding region may also contribute to PS-dependent decryption of TF.

The C-terminal CGHC motif of protein disulfide isomerase supports thrombosis

Protein disulfide isomerase (PDI) has two distinct CGHC redox-active sites; however, the contribution of these sites during different physiologic reactions, including thrombosis, is unknown. Here, we evaluated the role of PDI and redox-active sites of PDI in thrombosis by generating mice with blood cells and vessel wall cells lacking PDI (Mx1-Cre Pdifl/fl mice) and transgenic mice harboring PDI that lacks a functional C-terminal CGHC motif [PDI(ss-oo) mice]. Both mouse models showed decreased fibrin deposition and platelet accumulation in laser-induced cremaster arteriole injury, and PDI(ss-oo) mice had attenuated platelet accumulation in FeCl3-induced mesenteric arterial injury. These defects were rescued by infusion of recombinant PDI containing only a functional C-terminal CGHC motif [PDI(oo-ss)]. PDI infusion restored fibrin formation, but not platelet accumulation, in eptifibatide-treated wild-type mice, suggesting a direct role of PDI in coagulation. In vitro aggregation of platelets from PDI(ss-oo) mice and PDI-null platelets was reduced; however, this defect was rescued by recombinant PDI(oo-ss). In human platelets, recombinant PDI(ss-oo) inhibited aggregation, while recombinant PDI(oo-ss) potentiated aggregation. Platelet secretion assays demonstrated that the C-terminal CGHC motif of PDI is important for P-selectin expression and ATP secretion through a non-αIIbβ3 substrate. In summary, our results indicate that the C-terminal CGHC motif of PDI is important for platelet function and coagulation.

Structure-Function Relationship of the Interaction between Tissue Factor and Factor VIIa

Interactions between tissue factor and factor VIIa are the primary initiators of coagulation in hemostasis and certain thrombotic diseases. Tissue factor, an integral membrane protein expressed extensively outside of the vasculature, is the regulatory protein cofactor for coagulation factor VIIa. Factor VIIa, a trypsin-like serine protease homologous with other blood coagulation proteases, is weakly active when free in solution and must bind its membrane-bound cofactor for physiologically relevant activity. Tissue factor allosterically activates factor VIIa by several mechanisms such as active site positioning, spatial stabilization, and direct interactions with the substrate. Protein-membrane interactions between tissue factor, factor VIIa, and substrates all play critical roles in modulating the activity of this enzyme complex. Additionally, divalent cations such as Ca(2+) and Mg(2+) are critical for correct protein folding, as well as protein-membrane and protein-protein interactions. The contributions of these factors toward tissue factor-factor VIIa activity are discussed in this review.

The Aryl Hydrocarbon Receptor is a Critical Regulator of Tissue Factor Stability and an Antithrombotic Target in Uremia

Patients with CKD suffer high rates of thrombosis, particularly after endovascular interventions, yet few options are available to improve management and reduce thrombotic risk. We recently demonstrated that indoxyl sulfate (IS) is a potent CKD-specific prothrombotic metabolite that induces tissue factor (TF) in vascular smooth muscle cells (vSMCs), although the precise mechanism and treatment implications remain unclear. Because IS is an agonist of the aryl hydrocarbon receptor (AHR), we first examined the relationship between IS levels and AHR-inducing activity in sera of patients with ESRD. IS levels correlated significantly with both vSMC AHR activity and TF activity. Mechanistically, we demonstrated that IS activates the AHR pathway in primary human aortic vSMCs, and further, that AHR interacts directly with and stabilizes functional TF. Antagonists directly targeting AHR enhanced TF ubiquitination and degradation and suppressed thrombosis in a postinterventional model of CKD and endovascular injury. Furthermore, AHR antagonists inhibited TF in a manner dependent on circulating IS levels. In conclusion, we demonstrated that IS regulates TF stability through AHR signaling and uncovered AHR as an antithrombotic target and AHR antagonists as a novel class of antithrombotics. Together, IS and AHR have potential as uremia-specific biomarkers and targets that may be leveraged as a promising theranostic platform to better manage the elevated thrombosis rates in patients with CKD.

Therapeutic implications of protein disulfide isomerase inhibition in thrombotic disease

The study of thrombus formation has increasingly applied in vivo tools such as genetically modified mice and intravital microscopy to the evaluation of molecular and cellular mechanisms of thrombosis. Among several unexpected findings of this approach was the discovery that protein disulfide isomerase serves an essential role in thrombus formation at sites of vascular injury. The observation that the commonly ingested quercetin flavonoid, quercetin-3-rutinoside, inhibits protein disulfide isomerase and blocks thrombus formation in preclinical studies has set the stage for clinical trials using protein disulfide isomerase antagonists as antithrombotics. Although the mechanisms by which protein disulfide isomerase facilitates platelet activation and fibrin formation have yet to be elucidated, protein disulfide isomerase antagonists are currently being developed as antithrombotics. This review will consider what is known about the role of protein disulfide isomerase in platelet accumulation and fibrin generation with a focus on pharmacological strategies for blocking protein disulfide isomerase activity in the context of thrombus formation. Potential indications and clinical trial design for testing the efficacy of protein disulfide isomerase inhibition to reduce the incidence of thrombosis will be considered.

Breast cancer phenotypes regulated by tissue factor-factor VII pathway: Possible therapeutic targets

Breast cancer is a leading cause of cancer death in women, worldwide. Fortunately, breast cancer is relatively chemosensitive, with recent advances leading to the development of effective therapeutic strategies, significantly increasing disease cure rate. However, disease recurrence and treatment of cases lacking therapeutic molecular targets, such as epidermal growth factor receptor 2 and hormone receptors, referred to as triple-negative breast cancers, still pose major hurdles in the treatment of breast cancer. Thus, novel therapeutic approaches to treat aggressive breast cancers are essential. Blood coagulation factor VII (fVII) is produced in the liver and secreted into the blood stream. Tissue factor (TF), the cellular receptor for fVII, is an integral membrane protein that plays key roles in the extrinsic coagulation cascade. TF is overexpressed in breast cancer tissues. The TF-fVII complex may be formed in the absence of injury, because fVII potentially exists in the tissue fluid within cancer tissues. The active form of this complex (TF-fVIIa) may stimulate the expression of numerous malignant phenotypes in breast cancer cells. Thus, the TF-fVII pathway is a potentially attractive target for breast cancer treatment. To date, a number of studies investigating the mechanisms by which TF-fVII signaling contributes to breast cancer progression, have been conducted. In this review, we summarize the mechanisms controlling TF and fVII synthesis and regulation in breast cancer cells. Our current understanding of the TF-fVII pathway as a mediator of breast cancer progression will be also described. Finally, we will discuss how this knowledge can be applied to the design of future therapeutic strategies.

Differential roles of tissue factor and phosphatidylserine in activation of coagulation

It has been suggested that the main physiological trigger of coagulation, tissue factor, possesses limited procoagulant activity and occurs in an inactive or so-called encrypted state. For the conversion of encrypted into decrypted tissue factor with sufficient procoagulant activity, four distinct models have been proposed: 1; dimer formation, 2; lipid rafts, 3; disulfide bonds, and 4; phosphatidylserine exposure. Pro and cons can be given for each of these mechanisms of tissue factor encryption/decryption, however, it seems most likely that two or more mechanisms act together in activating the procoagulant activity. The exposure of phosphatidylserine in the outer layer of cell membranes supports coagulation through enhanced formation of the tenase (factors IXa, VIIIa and X) and prothrombinase (factors Xa, Va and prothrombin) complexes. The proposed role for phosphatidylserine in decryption of tissue factor could contribute to the correct orientation of the tissue factor - factor VII complex. Overall, the contribution of both tissue factor and phosphatidylserine to coagulation seems distinct with tissue factor being the physiological activator and phosphatidylserine the driving force of propagation of coagulation.

Synergies of phosphatidylserine and protein disulfide isomerase in tissue factor activation

Tissue factor (TF), the cellular receptor and cofactor for factor VII/VIIa, initiates haemostasis and thrombosis. Initial tissue distribution studies suggested that TF was sequestered from the circulation and only present at perivascular sites. However, there is now clear evidence that TF also exists as a blood-borne form with critical contributions not only to arterial thrombosis following plaque rupture and to venous thrombosis following endothelial perturbation, but also to various other clotting abnormalities associated with trauma, infection, or cancer. Because thrombin generation, fibrin deposition, and platelet aggregation in the contexts of haemostasis, thrombosis, and pathogen defence frequently occur without TF de novo synthesis, considerable efforts are still directed to understanding the molecular events underlying the conversion of predominantly non-coagulant or cryptic TF on the surface of haematopoietic cells to a highly procoagulant molecule following cellular injury or stimulation. This article will review some of the still controversial mechanisms implicated in cellular TF activation or decryption with particular focus on the coordinated effects of outer leaflet phosphatidylserine exposure and thiol-disulfide exchange pathways involving protein disulfide isomerase (PDI). In this regard, our recent findings of ATP-triggered stimulation of the purinergic P2X7 receptor on myeloid and smooth muscle cells resulting in potent TF activation and shedding of procoagulant microparticles as well as of rapid monocyte TF decryption following antithymocyte globulin-dependent membrane complement fixation have delineated specific PDI-dependent pathways of cellular TF activation and thus illustrated additional and novel links in the coupling of inflammation and coagulation.

Protein disulfide isomerase in thrombosis and vascular inflammation

Protein disulfide isomerase (PDI) catalyzes disulfide bond oxidation, reduction and isomerization during protein synthesis in the endoplasmic reticulum (ER). In addition to its critical role in the ER, in vitro and in vivo studies with blocking antibodies and conditional knockout mice have demonstrated that cell surface PDI is required for thrombosis, hemostasis and vascular inflammation in a manner dependent on its isomerase activity. This review will focus on our current understanding of the pathophysiologic role of PDI in regulating integrin-mediated platelet and neutrophil functions during vascular disease.

Using annexin V-coated magnetic beads to capture active tissue factor-bearing microparticles from body fluids

Microparticles, found in all body fluids including peripheral blood, are important elements that regulate cellular interactions under both physiological and pathological conditions. They play an important role in blood clot formation and increased cell aggregation. However, little is known about the components of the microparticles and their mechanism of action. A method to quantify and assess the underlying mechanism of action of microparticles in pathologies is therefore desirable. We present a specific method to isolate cell-derived microparticles from malignant effusions using annexin V-coated magnetic microbeads. The microparticles can be detected by flow cytometry. Our results show that the microparticles can be isolated with >80% specificity when bound to annexin V-coated magnetic beads, which was originally developed for the detection of apoptotic cells. We also show that the isolated microparticles were still functionally active and can be used for further analysis. Thus, our method enables isolation as well as structural and functional characterisation of the microparticles which are produced in numerous patho-physiological situations. This should help gain a deeper insight into various disease situations, which in turn should pave the way for the development of novel drugs and specific therapy strategies.

Analysis of tissue factor expression in various cell model systems: cryptic vs. active

BACKGROUND

Tissue factor (TF) encryption plays an important role in regulating TF coagulant activity. Potential differences in experimental cell model systems and strategies hampered our understanding of the TF encryption mechanisms.

OBJECTIVE

To characterize the procoagulant activity status of TF in different cell types, and to determine whether increased TF procoagulant activity following the activation stems from transformation of the cryptic TF to the active form.

METHODS

Simultaneous kinetic analyses of TF-FVIIa activation of FX and FVIIa binding to cell surface TF were performed under identical experimental conditions in fibroblast (WI-38), cancer cell (MDA-231), endothelial cell (HUVEC) and monocytic cell (THP-1) model systems. These data were then utilized to estimate TF coagulant-specific activity and percentages of active and cryptic TF present in these cell types.

RESULTS

MDA-231 and WI-38 cells express 10 to 100 times more TF on their cell surfaces compared with perturbed HUVEC and THP-1 cells. TF-specific activity on cell surfaces of MDA-231, WI-38 and THP-1 cells was very similar. Nearly 80-90% of the TF in MDA-231, WI-38 and THP-1 cells was cryptic. A plasma concentration of FVII would be sufficient to bind both active and cryptic TF on cell surfaces. Increased TF activity following cell activation stems from decryption of cryptic TF rather than increasing the coagulant activity of the active TF.

CONCLUSIONS

Our data demonstrate that TF encryption is not limited to a specific cell type, and unlike previously thought, the majority of the TF expressed in cancer cells is not constitutively procoagulant.

Disulfide reduction abolishes tissue factor cofactor function

BACKGROUND

Tissue factor (TF), an in vivo initiator of blood coagulation, is a transmembrane protein and has two disulfides in the extracellular domain. The integrity of one cysteine pair, Cys186-Cys209, has been hypothesized to be essential for an allosteric "decryption" phenomenon, presumably regulating TF procoagulant function, which has been the subject of a lengthy debate. The conclusions of published studies on this subject are based on indirect evidences obtained by the use of reagents with potentially oxidizing/reducing properties.

METHODS

The status of disulfides in recombinant TF1-263 and natural placental TF in their non-reduced native and reduced forms was determined by mass-spectrometry. Functional assays were performed to assess TF cofactor function.

RESULTS

In native proteins, all four cysteines of the extracellular domain of TF are oxidized. Reduced TF retains factor VIIa binding capacity but completely loses the cofactor function.

CONCLUSION

The reduction of TF disulfides (with or without alkylation) eliminates TF regulation of factor VIIa catalytic function in both membrane dependent FX activation and membrane independent synthetic substrate hydrolysis.

GENERAL SIGNIFICANCE

Results of this study advance our knowledge on TF structure/function relationships.

Rapid activation of monocyte tissue factor by antithymocyte globulin is dependent on complement and protein disulfide isomerase

Lymphocyte depletion with antithymocyte globulin (ATG) can be complicated by systemic coagulation activation. We found that ATG activated tissue factor procoagulant activity (TF PCA) on monocytic cells more potently than other stimuli that decrypt TF, including cell disruption, TF pathway inhibitor inhibition, and calcium ionophore treatment. Induction of TF PCA by ATG was dependent on lipid raft integrity and complement activation. We showed that ATG-mediated TF activation required complement activation until assembly of the C5b-7 membrane insertion complex, but not lytic pore formation by the membrane attack complex C5b-9. Consistently, induction of TF PCA by ATG did not require maximal phosphatidylserine membrane exposure and was not correlated with the magnitude of complement-induced lytic cell injury. Blockade of free thiols, an inhibitory monoclonal antibody to protein disulfide isomerase (PDI), and the small-molecule PDI antagonist quercetin-3-rutinoside prevented ATG-mediated TF activation, and C5 complement activation resulted in oxidation of cell surface PDI. This rapid and potent mechanism of cellular TF activation represents a novel connection between the complement system and cell surface PDI-mediated thiol-disulfide exchange. Delineation of this clinically relevant mechanism of activation of the extrinsic coagulation pathway during immunosuppressive therapy with ATG may have broader implications for vascular thrombosis associated with inflammatory disorders.

New Fundamentals in Hemostasis

Hemostasis encompasses the tightly regulated processes of blood clotting, platelet activation, and vascular repair. After wounding, the hemostatic system engages a plethora of vascular and extravascular receptors that act in concert with blood components to seal off the damage inflicted to the vasculature and the surrounding tissue. The first important component that contributes to hemostasis is the coagulation system, while the second important component starts with platelet activation, which not only contributes to the hemostatic plug, but also accelerates the coagulation system. Eventually, coagulation and platelet activation are switched off by blood-borne inhibitors and proteolytic feedback loops. This review summarizes new concepts of activation of proteases that regulate coagulation and anticoagulation, to give rise to transient thrombin generation and fibrin clot formation. It further speculates on the (patho)physiological roles of intra- and extravascular receptors that operate in response to these proteases. Furthermore, this review provides a new framework for understanding how signaling and adhesive interactions between endothelial cells, leukocytes, and platelets can regulate thrombus formation and modulate the coagulation process. Now that the key molecular players of coagulation and platelet activation have become clear, and their complex interactions with the vessel wall have been mapped out, we can also better speculate on the causes of thrombosis-related angiopathies.

Tissue factor structure and function

Tissue factor (TF) is an integral membrane protein that is essential to life. It is a component of the factor VIIa-TF complex enzyme and plays a primary role in both normal hemostasis and thrombosis. With a vascular injury, TF becomes exposed to blood and binds plasma factor VIIa, and the resulting complex initiates a series of enzymatic reactions leading to clot formation and vascular sealing. Many cells, both healthy, and tumor cells, produce detectable amounts of TF, especially when they are stimulated by various agents. Despite the relative simplicity and small size of TF, there are numerous contradictory reports about the synthesis and presentation of TF on blood cells and circulation in normal blood either on microparticles or as a soluble protein. Another subject of controversy is related to the structure/function of TF. It has been almost commonly accepted that cell-surface-associated TF has low (if any) activity, that is, is "encrypted" and requires specific conditions/reagents to become active, that is, "decrypted." However there is a lack of agreement related to the mechanism and processes leading to alterations in TF function. In this paper TF structure, presentation, and function, and controversies concerning these features are discussed.

Thioredoxin and thioredoxin reductase control tissue factor activity by thiol redox-dependent mechanism

Abnormally enhanced tissue factor (TF) activity is related to increased thrombosis risk in which oxidative stress plays a critical role. Human cytosolic thioredoxin (hTrx1) and thioredoxin reductase (TrxR), also secreted into circulation, have the power to protect against oxidative stress. However, the relationship between hTrx1/TrxR and TF remains unknown. Here we show reversible association of hTrx1 with TF in human serum and plasma samples. The association is dependent on hTrx1-Cys-73 that bridges TF-Cys-209 via a disulfide bond. hTrx1-Cys-73 is absolutely required for hTrx1 to interfere with FVIIa binding to purified and cell-surface TF, consequently suppressing TF-dependent procoagulant activity and proteinase-activated receptor-2 activation. Moreover, hTrx1/TrxR plays an important role in sensing the alterations of NADPH/NADP(+) states and transducing this redox-sensitive signal into changes in TF activity. With NADPH, hTrx1/TrxR readily facilitates the reduction of TF, causing a decrease in TF activity, whereas with NADP(+), hTrx1/TrxR promotes the oxidation of TF, leading to an increase in TF activity. By comparison, TF is more likely to favor the reduction by hTrx1-TrxR-NADPH. This reversible reduction-oxidation reaction occurs in the TF extracellular domain that contains partially opened Cys-49/-57 and Cys-186/-209 disulfide bonds. The cell-surface TF procoagulant activity is significantly increased after hTrx1-knockdown. The response of cell-surface TF procoagulant activity to H(2)O(2) is efficiently suppressed through elevating cellular TrxR activity via selenium supplementation. Our data provide a novel mechanism for redox regulation of TF activity. By modifying Cys residues or regulating Cys redox states in TF extracellular domain, hTrx1/TrxR function as a safeguard against inappropriate TF activity.

Regulation of tissue factor coagulant activity on cell surfaces

Tissue factor (TF) is a transmembrane glycoprotein and an essential component of the factor VIIa-TF enzymatic complex that triggers activation of the coagulation cascade. Formation of TF-FVIIa complexes on cell surfaces not only trigger the coagulation cascade but also transduce cell signaling via activation of protease-activated receptors. Tissue factor is expressed constitutively on cell surfaces of a variety of extravascular cell types, including fibroblasts and pericytes in and surrounding blood vessel walls and epithelial cells, but is generally absent on cells that come into contact with blood directly. However, TF expression could be induced in some blood cells, such as monocytes and endothelial cells, following an injury or pathological stimuli. Tissue factor is essential for hemostasis, but aberrant expression of TF leads to thrombosis. Therefore, a proper regulation of TF activity is critical for the maintenance of hemostatic balance and health in general. TF-FVIIa coagulant activity at the cell surface is influenced not only by TF protein expression levels but also independently by a variety of mechanisms, including alterations in membrane phospholipid composition and cholesterol content, thiol-dependent modifications of TF allosteric disulfide bonds, and other post-translational modifications of TF. In this article, we critically review the key literature on mechanisms by which TF coagulant activity is regulated at the cell surface in the absence of changes in TF protein levels with specific emphasis on recently published data and provide the authors' perspective on the subject.

Subcellular localization of tissue factor and human coronary artery smooth muscle cell migration

BACKGROUND

Tissue factor (TF) is the most relevant physiological trigger of thrombosis. Additionally TF is a transmembrane receptor with cell signaling functions.

OBJECTIVES

The aim of this study was to investigate TF subcellular localization, function and signaling in human coronary artery smooth muscle cell migration.

METHODS

Coronary arteries and primary cultures of vascular smooth muscle cells (HVSMC) were obtained from human explanted hearts. Wound repair and Boyden chamber assays were used to measure migration in vitro. TF-pro-coagulant activity (TF-PCA) was measured in extracted cellular membranes. Analysis of TF distribution was performed by confocal microscopy. A nucleofector device was used for TF and protease activated receptor 2 (PAR2) silencing. mRNA levels were analyzed by RT-PCR.

RESULTS

In migrating HVSMC TF translocates to the leading edge of the cells showing an intense patch-like staining in the lamellipodia. In the migrating front TF colocalizes with filamin (FLN) in the polarized lipid rafts. TF-PCA was increased in migrating cells. Silencing of the TF gene inhibits RSK-induced FLN-Ser-2152 phosphorylation, down-regulates CDC42, RhoA, and Rac1 protein expression and significantly inhibits cell migration. Silencing PAR2 also inhibits cell migration; however, silencing both TF and PAR2 induces a significantly higher effect on migration. Smooth muscle cells expressing TF have been identified in non-lipid-rich human coronary artery atherosclerotic plaques.

CONCLUSIONS

TF translocates to the cell front in association with cytoskeleton proteins and regulates HVSMC migration by mechanisms dependent and independent of factor (F)VIIa/PAR2. These results extend the functional role of TF to smooth muscle cell trafficking in vessel wall remodeling.

Platelets derived from the bone marrow of diabetic animals show dysregulated endoplasmic reticulum stress proteins that contribute to increased thrombosis

OBJECTIVE

Patients with diabetes mellitus have an increased risk of suffering atherothrombotic syndromes and are prone to clustering cardiovascular risk factors. However, despite their dysregulated glucose metabolism, intensive glycemic control has proven insufficient to reduce thrombotic complications. Therefore, we aimed to elucidate the determinants of thrombosis in a model of type 2 diabetes mellitus with cardiovascular risk factors clustering.

METHODS AND RESULTS

Intravital microscopy was used to analyze thrombosis in vivo in Zucker diabetic fatty rats (ZD) and lean normoglycemic controls. Bone marrow (BM) transplants were performed to test the contribution of each compartment (blood or vessel wall) to thrombogenicity. ZD showed significantly increased thrombosis compared with lean normoglycemic controls. BM transplants demonstrated the key contribution of the hematopoietic compartment to increased thrombogenicity. Indeed, lean normoglycemic controls transplanted with ZD-BM showed increased thrombosis with normal glucose levels, whereas ZD transplanted with lean normoglycemic controls-BM showed reduced thrombosis despite presenting hyperglycemia. Significant alterations in megakaryopoiesis and platelet-endoplasmic reticulum stress proteins, protein disulfide isomerase and 78-kDa glucose-regulated protein, were detected in ZD, and increased tissue factor procoagulant activity was detected in plasma and whole blood of ZD.

CONCLUSIONS

Our results indicate that diabetes mellitus with cardiovascular risk factor clustering favors BM production of hyperreactive platelets with altered protein disulfide isomerase and 78-kDa glucose-regulated protein expression that can contribute to increase thrombotic risk independently of blood glucose levels.

Decryption of tissue factor

Tissue factor (TF) is a transmembrane protein which, in complex with factor (F)VIIa, initiates blood coagulation. Numerous studies have determined TF epitopes and individual amino acids which play an important role in the TF/FVIIa complex formation and its activity towards natural substrates. However the subject of cell-surface TF activity remains controversial. It has been almost commonly accepted that TF on the cell surface has low (if any) activity, i.e. is encrypted and requires specific conditions/reagents to become active, i.e. decrypted. One of the leading theories suggests that cell membrane lipid composition plays a crucial role in TF decryption, whereas another assigns the key role to the formation of the Cys(186)-Cys(209) disulfide bond. Despite a number of studies published from several laboratories, the role of this bond in the activity of the TF/FVIIa complex remains elusive and controversial. One of the causes of this controversy could be related to the lack of specificity of the reagents used for the cell treatment leading to possible alterations in other cell surface proteins and cell membrane environment. In conclusion, the influence of the Cys(186)-Cys(209) this bond on cell surface TF function remains unclear.

Tissue factor controversies

Tissue factor plays a primary role in both hemorrhage control and thrombosis depending upon whether its presentation is extravascular or intravascular. The molecular architecture and function of the tissue factor molecule and its role in the activations of factor IX and factor X have been elegantly elucidated but controversies prevail with respect to distinctions between tissue factor sources and tissue factor "activity." This presentation will review data on the architecture and functions of the tissue factor-factor VIIa complex and discuss the elements of the controversies associated with tissue factor presentation in both normal and pathologic milieu.

Role of thiol pathways in TF procoagulant regulation

The generation of procoagulant Tissue Factor (TF) is crucial for thrombosis. TF contains a surface exposed allosteric disulfide bond that stabilizes the carboxyl-terminal domain involved in ligand interactions with coagulation factors VIIa and X. TF procoagulant activation typically occurs following cellular perturbations that also cause the appearance of procoagulant phosphatidylserine in the outer leaflet of cell membranes. However, thiol modifying agents, without suppressing phosphatidylserine exposure, can prevent TF activation, implicating thiol-disulfide exchange reactions in the regulation of TF procoagulant activity of primary cells. Protein disulfide isomerase (PDI), a regulator of extracellular thiol exchange, is associated with cell surface TF and required for TF-dependent thrombosis in vivo. PDI regulates the thiol-dependent biogenesis of procoagulant microparticles that are released from myeloid cells and smooth muscle cells following activation of the purinergic P2X7 receptor. Genetic deletion of P2X7 signaling attenuates FeCl(3)-induced carotid artery thrombosis in mice, indicating that TF prothrombotic activity is regulated by specific cell signaling pathways in vivo.

Tissue factor encryption and decryption: facts and controversies

Tissue factor (TF)-initiated coagulation plays a critical role in both hemostasis and thrombosis. It is generally believed that most of the tissue factor expressed on cell surfaces is maintained in a cryptic, i.e., coagulantly inactive state and an activation step (decryption) is required for the expression of maximum TF procoagulant activity. However, what exactly constitutes cryptic or procoagulant TF, molecular differences between these two forms and mechanisms that are responsible for transformation from one to the other form are not entirely clear and remain highly controversial, thus are a matter of ongoing debate. This brief review discusses pertinent literature on TF encryption/decryption with specific emphasis on the role of membrane phospholipids and reduction/oxidation of the TF Cys186-Cys209 disulfide bond in regulating TF activity at cell surfaces.

Tissue factor: mechanisms of decryption

It is generally believed that only a small fraction of the tissue factor (TF) found on cell surfaces is active whereas the vast majority is cryptic in coagulation. It is unclear how cryptic TF differs from the coagulant active TF or potential mechanisms involved in transformation of cryptic TF to the coagulant active form. Exposure of phosphatidylserine (PS) in response to various chemical or pathophysiological stimuli has been considered as the most potent inducer of TF decryption. In addition to PS, TF self-association and association with specialized membrane domains may also play a role in TF decryption. It has been suggested recently that protein disulfide isomerase regulates TF decryption through its oxidoreductase activity by targeting Cys186-Cys209 disulfide bond in TF extracellular domain or regulating the PS equilibrium at the plasma membrane. However, this hypothesis requires further validation to become an accepted mechanism. In this article, we critically review literature on TF encryption/decryption with specific emphasis on recently published data and provide our perspective on this subject.

Posttranslational modifications of tissue factor

Tissue factor (TF), a membrane protein, is an initiator of blood coagulation in vivo. In this review we discuss how posttranslational modifications affect activity and other properties of TF. Glycosylation of the extracellular domain and the composition of carbohydrates at three glycosylation sites have an influence on TF activity in the extrinsic FXase by increasing the rate of FX proteolysis. No influence of TF glycosylation on the activity of the FVIIa/TF complex towards small synthetic substrates was observed, suggesting that glycosylation has no effect on TF interaction with FVIIa. There are no published data suggesting a direct influence of phosphorylation or palmitoylation in the cytoplasmic domain on TF procoagulant activity. There has been a debate in the recent literature related to the role and formation of the Cys¹⁸⁶-Cys²⁰⁹ disulfide bond. Published opinions from various laboratories range from this bond being essential for the expression of cell TF activity to having no role in it. Overall, it is clear that some modifications of TF have an effect on TF procoagulant activity, signaling functions and trafficking. The influences of other modifications are debatable.

Progesterone utilizes distinct membrane pools of tissue factor to increase coagulation and invasion and these effects are inhibited by TFPI

Tissue factor (TF) serving as the receptor for coagulation factor VII (FVII) initiates the extrinsic coagulation pathway. We previously demonstrated that progesterone increases TF, coagulation and invasion in breast cancer cell lines. Herein, we investigated if tissue factor pathway inhibitor (TFPI) could down-regulate progesterone-increased TF activity in these cells. Classically, TFPI redistributes TF-FVII-FX-TFPI in an inactive quaternary complex to membrane associated lipid raft regions. Herein, we demonstrate that TF increased by progesterone is localized to the heavy membrane fraction, despite progesterone-increased coagulation originating almost exclusively from lipid raft domains, where TF levels are extremely low. The progesterone increase in coagulation is not a rapid effect, but is progesterone receptor (PR) dependent and requires protein synthesis. Although a partial relocalization of TF occurs, TFPI does not require the redistribution to lipid rafts to inhibit coagulation or invasion. Inhibition by TFPI and anti-TF antibodies in lipid raft membrane fractions confirmed the dependence on TF for progesterone-mediated coagulation. Through the use of pathway inhibitors, we further demonstrate that the TF up-regulated by progesterone is not coupled to the progesterone increase in TF-mediated coagulation. However, the progesterone up-regulated TF protein may be involved in progesterone-mediated breast cancer cell invasion, which TFPI also inhibits.

Redox properties of the tissue factor Cys186-Cys209 disulfide bond

TF (tissue factor) is a transmembrane cofactor that initiates blood coagulation in mammals by binding Factor VIIa to activate Factors X and IX. The cofactor can reside in a cryptic configuration on primary cells and de-encryption may involve a redox change in the C-terminal domain Cys(186)-Cys(209) disulfide bond. The redox potential of the bond, the spacing of the reduced cysteine thiols and their oxidation by TF activators was investigated to test the involvement of the dithiol/disulfide in TF activation. A standard redox potential of -278 mV was determined for the Cys(186)-Cys(209) disulfide of recombinant soluble TF. Notably, ablating the N-terminal domain Cys(49)-Cys(57) disulfide markedly increased the redox potential of the Cys(186)-Cys(209) bond, suggesting that the N-terminal bond may be involved in the regulation of redox activity at the C-terminal bond. Using As(III) and dibromobimane as molecular rulers for closely spaced sulfur atoms, the reduced Cys(186) and Cys(209) sulfurs were found to be within 3-6 Å (1 Å=0.1 nm) of each other, which is close enough to reform the disulfide bond. HgCl2 is a very efficient activator of cellular TF and activating concentrations of HgCl2-mediated oxidation of the reduced Cys(186) and Cys(209) thiols of soluble TF. Moreover, PAO (phenylarsonous acid), which cross-links two cysteine thiols that are in close proximity, and MMTS (methyl methanethiolsulfonate), at concentrations where it oxidizes closely spaced cysteine residues to a cystine residue, were efficient activators of cellular TF. These findings further support a role for Cys(186) and Cys(209) in TF activation.

Anticoagulant effects of an antidiabetic drug on monocytes in vitro

INTRODUCTION

Monocyte- and microparticle (MP)-associated tissue factor (TF) is upregulated in diabetes. Lipopolysaccharide (LPS) induces expression of TF and alternatively spliced TF (asTF) and increases MP release from monocytes. Using LPS-stimulated TF-bearing human monocytes, we examined whether glibenclamide, a sulfonylurea used to treat diabetes type 2, might possess anticoagulant properties.

METHODS

We studied the effects of glibenclamide on cell- and supernatant-associated procoagulant activity (Factor Xa-generating assay and clot formation assay), on expression of TF and asTF (flow cytometry, RT-qPCR, western blot) and on cell viability and MP release (flow cytometry).

RESULTS

Glibenclamide dose-dependently decreased procoagulant activity of cells and supernatants. The reduction in cellular procoagulant activity coincided with reduced expression of TF and asTF in cells, whereas cell viability remained almost unchanged. The glibenclamide-induced reduction in procoagulant activity of supernatants appeared to be associated with a decreased number of released MPs.

CONCLUSIONS

Reduction of monocyte- and supernatant-associated procoagulant activity by glibenclamide is associated with decreased expression of TF and asTF and possibly with a reduced MP number. Our data indicate that glibenclamide reduces the prothrombotic state in LPS-stimulated monocytes in vitro. Glibenclamide might therefore also have an anticoagulant effect in vivo, but this needs to be further evaluated.

Glycosylation of tissue factor is not essential for its transport or functions

BACKGROUND

Glycosylation plays an important role in protein function. The importance of glycosylation for tissue factor (TF) function is unclear.

OBJECTIVE

The aim of the present study is to investigate the importance of TF glycosylation in transport to the cell surface and its coagulant and signaling functions.

METHODS

Endothelial cells and peripheral blood mononuclear cells (PBMC) were treated with tunicamycin to inhibit N-linked glycosylation. Site-specific mutagenesis of one or more potential N-linked glycosylation sites in TF was used to generate TF mutants lacking glycans. TF expression at the cell surface was determined in binding assays using (125)I-FVIIa or (125)I-TF mAb and confocal microscopy. TF coagulant activity was measured by factor (F) Xa generation assay, and TF signaling function was assessed by measuring cleavage of protease activated receptor 2 (PAR2) and activation of p44/42 MAPK.

RESULTS

Tunicamycin treatment reduced TF activity at the endothelial cell surface; however, this reduction was found to be the result of decreased TF protein production in tunicamycin-treated cells. Tunicamycin treatment had no significant effect on TF activity or antigen levels in PBMC. No significant differences were observed in TF protein expression and procoagulant activity among cells transfected to express either wild-type TF or TF mutants. A fully non-glycosylated TF is shown to bind FVIIa and interact with FX with the same efficiency as that of wild-type TF. Non-glycosylated TF is also capable of supporting FVIIa cleavage of PAR2 and PAR2-dependent p44/42 MAPK activation.

CONCLUSIONS

Glycosylation is not essential for TF transport and coagulant or signaling functions.

P2X7 receptor signaling contributes to tissue factor-dependent thrombosis in mice

Thrombosis is initiated by tissue factor (TF), a coagulation cofactor/receptor expressed in the vessel wall, on myeloid cells, and on microparticles (MPs) with variable procoagulant activity. However, the molecular pathways that generate prothrombotic TF in vivo are poorly defined. The oxidoreductase protein disulfide isomerase (PDI) is thought to be involved in the activation of TF. Here, we found that in mouse myeloid cells, ATP-triggered signaling through purinergic receptor P2X, ligand-gated ion channel, 7 (P2X7 receptor; encoded by P2rx7) induced activation (decryption) of TF procoagulant activity and promoted release of TF+ MPs from macrophages and SMCs. The generation of prothrombotic MPs required P2X7 receptor-dependent production of ROS leading to increased availability of solvent-accessible extracellular thiols. An antibody to PDI with antithrombotic activity in vivo attenuated the release of procoagulant MPs. In addition, P2rx7-/- mice were protected from TF-dependent FeCl3-induced carotid artery thrombosis. BM chimeras revealed that P2X7 receptor prothrombotic function was present in both hematopoietic and vessel wall compartments. In contrast, an alternative anti-PDI antibody showed activities consistent with cellular activation typically induced by P2X7 receptor signaling. This anti-PDI antibody restored TF-dependent thrombosis in P2rx7-/- mice. These data suggest that PDI regulates a critical P2X7 receptor-dependent signaling pathway that generates prothrombotic TF, defining a link between inflammation and thrombosis with potential implications for antithrombotic therapy.

Activated protein C up-regulates procoagulant tissue factor activity on endothelial cells by shedding the TFPI Kunitz 1 domain

Thrombin and activated protein C (APC) signaling can mediate opposite biologic responses in endothelial cells. Given that thrombin induces procoagulant tissue factor (TF), we examined how TF activity is affected by APC. Exogenous or endogenously generated APC led to increased TF-dependent factor Xa activity. Induction required APC's proteolytic activity and binding to endothelial cell protein C receptor but not protease activated receptors. APC did not affect total TF antigen expression or the availability of anionic phospholipids on the apical cell membrane. Western blotting and cell surface immunoassays demonstrated that APC sheds the Kunitz 1 domain from tissue factor pathway inhibitor (TFPI). A TFPI Lys86Ala mutation between the Kunitz 1 and 2 domains eliminated both cleavage and the enhanced TF activity in response to APC in overexpression studies, indicating that APC up-regulates TF activity by endothelial cell protein C receptor-dependent shedding of the Kunitz 1 domain from membrane-associated TFPI. Our results demonstrate an unexpected procoagulant role of the protein C pathway that may have important implications for the regulation of TF- and TFPI-dependent biologic responses and for fine tuning of the hemostatic balance in the vascular system.

Imaging fibrin formation and platelet and endothelial cell activation in vivo

Over the past six decades research employing in vitro assays has identified enzymes, cofactors, cell receptors and associated ligands important to the haemostatic process and its regulation. These studies have greatly advanced our understanding of the molecular and cellular bases of haemostasis and thrombosis. However, in vitro assays cannot simultaneously reproduce the interactions of all of the components of the haemostatic process that occur in vivo nor do they reflect the importance of haemodynamic factors resulting from blood flow. To overcome these limitations investigators have increasingly turned to animal models of haemostasis and thrombosis. In this article we describe some advances in the visualisation of platelet and endothelial cell activation and blood coagulation in vivo and review what we have learned from our intravital microscopy experiments using primarily the laser-induced injury model for thrombosis.

Control of mature protein function by allosteric disulfide bonds

Protein disulfide bonds are the links between the sulfur atoms of two cysteine amino acids. All the known life forms appear to make this bond. Most disulfide bonds perform a structural role by stabilizing the tertiary and quaternary structures. Some perform a functional role and can be characterized as either catalytic or allosteric disulfides. Catalytic disulfides/dithiols transfer electrons between proteins, whereas the allosteric bonds control the function of the protein in which they reside when they undergo redox change. There are currently five clear examples of allosteric disulfide bonds and a number of potential allosteric disulfides at various stages of characterization. The features of these bonds and how they control the activity of the respective proteins are discussed. A common aspect of the allosteric disulfides identified to date is that they all link β-strands or β-loops.

Systems biology of coagulation initiation: kinetics of thrombin generation in resting and activated human blood

Blood function defines bleeding and clotting risks and dictates approaches for clinical intervention. Independent of adding exogenous tissue factor (TF), human blood treated in vitro with corn trypsin inhibitor (CTI, to block Factor XIIa) will generate thrombin after an initiation time (T_i) of 1 to 2 hours (depending on donor), while activation of platelets with the GPVI-activator convulxin reduces T_i to ∼20 minutes. Since current kinetic models fail to generate thrombin in the absence of added TF, we implemented a Platelet-Plasma ODE model accounting for: the Hockin-Mann protease reaction network, thrombin-dependent display of platelet phosphatidylserine, VIIa function on activated platelets, XIIa and XIa generation and function, competitive thrombin substrates (fluorogenic detector and fibrinogen), and thrombin consumption during fibrin polymerization. The kinetic model consisting of 76 ordinary differential equations (76 species, 57 reactions, 105 kinetic parameters) predicted the clotting of resting and convulxin-activated human blood as well as predicted T_i of human blood under 50 different initial conditions that titrated increasing levels of TF, Xa, Va, XIa, IXa, and VIIa. Experiments with combined anti-XI and anti-XII antibodies prevented thrombin production, demonstrating that a leak of XIIa past saturating amounts of CTI (and not “blood-borne TF” alone) was responsible for in vitro initiation without added TF. Clotting was not blocked by antibodies used individually against TF, VII/VIIa, P-selectin, GPIb, protein disulfide isomerase, cathepsin G, nor blocked by the ribosome inhibitor puromycin, the Clk1 kinase inhibitor Tg003, or inhibited VIIa (VIIai). This is the first model to predict the observed behavior of CTI-treated human blood, either resting or stimulated with platelet activators. CTI-treated human blood will clot in vitro due to the combined activity of XIIa and XIa, a process enhanced by platelet activators and which proceeds in the absence of any evidence for kinetically significant blood borne tissue factor.

Clotting of blood involves a series of reactions wherein at each step an inactive zymogen is converted to an active enzyme by the product of the previous step, sometimes in plasma and usually on efficient catalytic surfaces provided by the activating platelet. The protein Tissue Factor (TF) initiates this cascade when blood vessels are disrupted, but how this cascade is triggered in the absence of exogenous TF remains the subject of much debate. First, we validated a high throughput experimental system that allowed the noninvasive quantification of thrombin generation dynamics. Next, we showed that “contact activation,” despite use of the best available inhibitor (CTI) to prevent it, builds up enough autocatalytic strength to trigger coagulation without exogenous TF, particularly upon activated platelets. Further, we build an ODE based model to predict the stability of blood resulting from multiple perturbations with active enzymes at various physiologically realizable concentrations. Unlike existing models, we consider the dynamics of platelet activation on reaction rates due to phosphatiylserine exposure. The “Platelet-Plasma” model lays the groundwork for integration of coagulation reaction kinetics and donor specific descriptions of platelet function.

Endothelium-derived but not platelet-derived protein disulfide isomerase is required for thrombus formation in vivo

Protein disulfide isomerase (PDI) catalyzes the oxidation reduction and isomerization of disulfide bonds. We have previously identified an important role for extracellular PDI during thrombus formation in vivo. Here, we show that endothelial cells are a critical cellular source of secreted PDI, important for fibrin generation and platelet accumulation in vivo. Functional PDI is rapidly secreted from human umbilical vein endothelial cells in culture upon activation with thrombin or after laser-induced stimulation. PDI is localized in different cellular compartments in activated and quiescent endothelial cells, and is redistributed to the plasma membrane after cell activation. In vivo studies using intravital microscopy show that PDI appears rapidly after laser-induced vessel wall injury, before the appearance of the platelet thrombus. If platelet thrombus formation is inhibited by the infusion of eptifibatide into the circulation, PDI is detected after vessel wall injury, and fibrin deposition is normal. Treatment of mice with a function blocking anti-PDI antibody completely inhibits fibrin generation in eptifibatide-treated mice. These results indicate that, although both platelets and endothelial cells secrete PDI after laser-induced injury, PDI from endothelial cells is required for fibrin generation in vivo.