Time-dependent degradation and tissue factor addition mask the ability of platelet polyphosphates in activating factor XII–mediated coagulation

Polyphosphate as an antithrombotic target and hemostatic agent

Polyphosphate (PolyP) is a polymer comprised of linear phosphate units connected by phosphate anhydride bonds. PolyP exists in a diverse range of eukaryotes and prokaryotes with varied chain lengths ranging from six to thousands of phosphate units. Upon activation, human platelets and neutrophils release short-chain PolyP, along with other components, to initiate the coagulation pathway. Long-chain PolyP derived from cellular or bacterial organelles exhibits higher proinflammatory and procoagulant effects compared to short-chain PolyP. Notably, PolyP has been identified as a low-hemorrhagic antithrombotic target since neutralizing plasma PolyP suppresses the thrombotic process without impairing the hemostatic functions. As an inorganic polymer without uniform steric configuration, PolyP is typically targeted by cationic polymers or recombinant polyphosphatases rather than conventional antibodies, small-molecule compounds, or peptides. Additionally, because of its procoagulant property, PolyP has been incorporated in wound-dressing materials to facilitate blood hemostasis. This review summarizes current studies on PolyP as a low-hemorrhagic antithrombotic target and the development of hemostatic materials based on PolyP.

Mechanism, Functions, and Diagnostic Relevance of FXII Activation by Foreign Surfaces

Factor XII (FXII) is a serine protease zymogen produced by hepatocytes and secreted into plasma. The highly glycosylated coagulation protein consists of six domains and a proline-rich region that regulate activation and function. Activation of FXII results from a conformational change induced by binding ("contact") with negatively charged surfaces. The activated serine protease FXIIa drives both the proinflammatory kallikrein-kinin pathway and the procoagulant intrinsic coagulation cascade, respectively. Deficiency in FXII is associated with a prolonged activated partial thromboplastin time (aPTT) but not with an increased bleeding tendency. However, genetic or pharmacological deficiency impairs both arterial and venous thrombosis in experimental models. This review summarizes current knowledge of FXII structure, mechanisms of FXII contact activation, and the importance of FXII for diagnostic coagulation testing and thrombosis.

Antithrombotic Effect of shRNA Target F12 Mediated by Adeno-Associated Virus

Coagulation factor XII (FXII) plays a crucial role in thrombosis. Moreover, deficiencies in FXII are not associated with excessive bleeding, and its depletion exhibits satisfactory protective effect on thrombus formation. Several strategies targeting FXII have been applied to inhibit thrombosis formation. In this study, C57BL/6 mice were injected with adeno-associated virus (AAV) to identify the role of short hairpin RNA (shRNA) in thrombosis. Differences in liver FXII, coagulation function, and thrombus formation were detected. The potential side effects of FXII were then evaluated through analysis of tail bleeding, biochemical indices, and pathological sections. Results showed that shRNAs, especially shRNA2, carried by AAV, effectively reduced the expression of FXII. Furthermore, only shRNA2 demonstrated an anti-thrombosis effect on multiple models without hemorrhage and side effects. Hence the novel approach of AAV-based shRNA is specific and safe for inhibiting FXII and thrombosis.

Coagulation factor XII in thrombosis and inflammation

Combinations of proinflammatory and procoagulant reactions are the unifying principle for a variety of disorders affecting the cardiovascular system. The factor XII-driven contact system starts coagulation and inflammatory mechanisms via the intrinsic pathway of coagulation and the bradykinin-producing kallikrein-kinin system, respectively. The biochemistry of the contact system in vitro is well understood; however, its in vivo functions are just beginning to emerge. Challenging the concept of the coagulation balance, targeting factor XII or its activator polyphosphate, provides protection from thromboembolic diseases without interfering with hemostasis. This suggests that the polyphosphate/factor XII axis contributes to thrombus formation while being dispensable for hemostatic processes. In contrast to deficiency in factor XII providing safe thromboprotection, excessive FXII activity is associated with the life-threatening inflammatory disorder hereditary angioedema. The current review summarizes recent findings of the polyphosphate/factor XII-driven contact system at the intersection of procoagulant and proinflammatory disease states. Elucidating the contact system offers the exciting opportunity to develop strategies for safe interference with both thrombotic and inflammatory disorders.

Polyphosphates form antigenic complexes with platelet factor 4 (PF4) and enhance PF4-binding to bacteria

Short chain polyphosphates (polyP) are pro-coagulant and pro-inflammatory platelet released inorganic polymers. The platelet chemokine platelet factor 4 (PF4) binds to lipid A on bacteria, inducing an antibody mediated host defense mechanism, which can be misdirected against PF4/heparin complexes leading to the adverse drug reaction heparin-induced thrombocytopenia (HIT). Here, we demonstrate that PF4 complex formation with soluble short chain polyP contributes to host defense mechanisms. Circular dichroism spectroscopy and isothermal titration calorimetry revealed that PF4 changed its structure upon binding to polyP in a similar way as seen in PF4/heparin complexes. Consequently, PF4/polyP complexes exposed neoepitopes to which human anti-PF4/heparin antibodies bound. PolyP enhanced binding of PF4 to Escherichia coli, hereby facilitating bacterial opsonisation and, in the presence of human anti-PF4/polyanion antibodies, phagocytosis. Our study indicates a role of polyP in enhancing PF4-mediated defense mechanisms of innate immunity.

In vivo activation and functions of the protease factor XII

Combinations of proinflammatory and procoagulant reactions are the unifying principle for a variety of disorders affecting the cardiovascular system. Factor XII (FXII, Hageman factor) is a plasma protease that initiates the contact system. The biochemistry of the contact system in vitro is well understood; however, its in vivo functions are just beginning to emerge. The current review concentrates on activators and functions of the FXII-driven contact system in vivo. Elucidating its physiologic activities offers the exciting opportunity to develop strategies for the safe interference with both thrombotic and inflammatory diseases.

Factor XII-Driven Inflammatory Reactions with Implications for Anaphylaxis

Anaphylaxis is a life-threatening allergic reaction. It is triggered by the release of pro-inflammatory cytokines and mediators from mast cells and basophils in response to immunologic or non-immunologic mechanisms. Mediators that are released upon mast cell activation include the highly sulfated polysaccharide and inorganic polymer heparin and polyphosphate (polyP), respectively. Heparin and polyP supply a negative surface for factor XII (FXII) activation, a serine protease that drives contact system-mediated coagulation and inflammation. Activation of the FXII substrate plasma kallikrein leads to further activation of zymogen FXII and triggers the pro-inflammatory kallikrein-kinin system that results in the release of the mediator bradykinin (BK). The severity of anaphylaxis is correlated with the intensity of contact system activation, the magnitude of mast cell activation, and BK formation. The main inhibitor of the complement system, C1 esterase inhibitor, potently interferes with FXII activity, indicating a meaningful cross-link between complement and kallikrein-kinin systems. Deficiency in a functional C1 esterase inhibitor leads to a severe swelling disorder called hereditary angioedema (HAE). The significance of FXII in these disorders highlights the importance of studying how these processes are integrated and can be therapeutically targeted. In this review, we focus on how FXII integrates with inflammation and the complement system to cause anaphylaxis and HAE as well as highlight current diagnosis and treatments of BK-related diseases.

Factor XII as a Therapeutic Target in Thromboembolic and Inflammatory Diseases

Coagulation factor XII (FXII, Hageman factor) is a plasma protease that in its active form (FXIIa) initiates the procoagulant and proinflammatory contact system. This name arises from FXII’s unique mechanism of activation that is induced by binding (contact) to negatively charged surfaces. Various substances have the capacity to trigger FXII contact-activation in vivo including mast cell–derived heparin, misfolded protein aggregates, collagen, nucleic acids, and polyphosphate. FXII deficiency is not associated with bleeding, and for decades, the factor was considered to be dispensable for coagulation in vivo. However, despite the fact that humans and animals with deficiency in FXII have a normal hemostatic capacity, animal models revealed a critical role of FXIIa-driven coagulation in thromboembolic diseases. In addition to its role in thrombosis, FXIIa contributes to inflammation through the activation of the inflammatory bradykinin-producing kallikrein-kinin system. Pharmacological inhibition of FXII/FXIIa interferes with thrombosis and inflammation in animal models. Thus, targeting the FXIIa-driven contact system seems to be a promising and safe therapeutic anticoagulation treatment strategy, with additional anti-inflammatory effects. Here, we discuss novel functions of FXIIa in cardiovascular thrombotic and inflammatory disorders.

A Flow Cytometry-Based Assay for Procoagulant Platelet Polyphosphate

BACKGROUND

Platelet polyphosphate is an inorganic procoagulant polymer of orthophosphate units that is stored in dense granules and is released upon platelet activation. Here, we describe an assay to measure polyphosphate on the surface of procoagulant human platelets.

METHODS AND RESULTS

Recombinant Escherichia coli-expressed exopolyphosphatase deletion mutant PPX_Δ12 labeled with fluorescent Alexa488 dye was used as a polyphosphate probe in flow cytometry. PPX_Δ12-Alexa488-signal dose-dependently increased with long-chain polyphosphate binding to platelets. In contrast, short-chain polyphosphate that is found in the supernatant of activated platelets, did not bind to the platelet surface. Both exopolyphosphatase treatment and polyphosphate pre-incubation abolished PPX_Δ12-Alexa488 binding to polyphosphate on platelets. Stimulation of platelets with thrombin receptor agonist Trap6, and P2Y₁₂ receptor activator ADP increased polyphosphate accumulation on platelet surfaces and PPX_Δ12-Alexa488 signal in a dose-dependent manner.

CONCLUSION

This study indicates that long-chain polyphosphate binds to platelet plasma membranes and presents a promising diagnostic assay to measure this interaction on human platelets in platelet-rich plasma. Future investigations will aim to determine if polyphosphate can be used as a novel biomarker of thrombosis. © 2016 International Clinical Cytometry Society.

Contact system revisited: an interface between inflammation, coagulation, and innate immunity

The contact system is a plasma protease cascade initiated by factor XII (FXII) that activates the proinflammatory kallikrein-kinin system and the procoagulant intrinsic coagulation pathway. Anionic surfaces induce FXII zymogen activation to form proteolytically active FXIIa. Bacterial surfaces also have the ability to activate contact system proteins, indicating an important role for host defense using the cooperation of the inflammatory and coagulation pathways. Recent research has shown that inorganic polyphosphate found in platelets activates FXII in vivo and can induce coagulation in pathological thrombus formation. Experimental studies have shown that interference with FXII provides thromboprotection without a therapy-associated increase in bleeding, renewing interest in the FXIIa-driven intrinsic pathway of coagulation as a therapeutic target. This review summarizes how the contact system acts as the cross-road of inflammation, coagulation, and innate immunity.

Factor XII: a novel target for safe prevention of thrombosis and inflammation

Plasma protein factor XII (FXII) activates the procoagulant and proinflammatory contact system that drives both the kallikrein-kinin system and the intrinsic pathway of coagulation. When zymogen FXII comes into contact with negatively charged surfaces, it auto-activates to the serine proteaseactivated FXII (FXIIa). Recently, various in vivo activators of FXII have been identified including heparin, misfolded protein aggregates, polyphosphate and nucleic acids. Murine models have established a central role of FXII in arterial and venous thrombosis. Despite its central function in thrombosis, deficiency in FXII does not impair haemostasis in animals and humans. In a preclinical cardiopulmonary bypass system in large animals, the FXIIa-blocking antibody 3F7 prevented thrombosis; however, in contrast to traditional anticoagulants, bleeding was not increased. In addition to its function in thrombosis, FXIIa initiates formation of the inflammatory mediator bradykinin. This mediator increases vascular leak, causes vasodilation, and induces chemotaxis with implications for septic, anaphylactic and allergic disease states. Therefore, targeting FXIIa appears to be a promising strategy for thromboprotection without associated bleeding risks but with anti-inflammatory properties.

The factor XIIa blocking antibody 3F7: a safe anticoagulant with anti-inflammatory activities

The plasma protein factor XII (FXII) is the initiating protease of the procoagulant and proinflammatory contact system. FXII activates both the bradykinin (BK) producing kallikrein-kinin system and the intrinsic pathway of coagulation. Contact with negatively charged surfaces induces auto-activation of zymogen FXII that results in activated FXII (FXIIa). Various in vivo activators of FXII have been identified including heparin, misfolded protein aggregates, nucleic acids and polyphosphate. Murine models have established a central role of FXII in arterial and venous thromboembolic diseases. Despite the central function of FXII in pathologic thrombosis, its deficiency does not impair hemostasis in animals or humans. The selective role of FXIIa in thrombosis, but not hemostasis, offers an exciting novel strategy for safe anticoagulation based on interference with FXIIa. We have generated the recombinant fully human FXIIa-blocking antibody 3F7, which abolished FXIIa enzymatic activity and prevented thrombosis in a cardiopulmonary bypass system in large animals, in the absence of increased therapy-associated bleeding. Furthermore, 3F7 also interfered with BK-driven edema in the severe swelling disorder hereditary angioedema (HAE) type III. Taken together, targeting FXIIa with 3F7 appears to be a promising approach to treat edema disorders and thrombosis.

The polyphosphate-factor XII pathway drives coagulation in prostate cancer-associated thrombosis

Cancer is a leading cause of thrombosis. We identify a new procoagulant mechanism that contributes to thromboembolism in prostate cancer and allows for safe anticoagulation therapy development. Prostate cancer-mediated procoagulant activity was reduced in plasma in the absence of factor XII or its substrate of the intrinsic coagulation pathway factor XI. Prostate cancer cells and secreted prostasomes expose long chain polyphosphate on their surface that colocalized with active factor XII and initiated coagulation in a factor XII-dependent manner. Polyphosphate content correlated with the procoagulant activity of prostasomes. Inherited deficiency in factor XI or XII or high-molecular-weight kininogen, but not plasma kallikrein, protected mice from prostasome-induced lethal pulmonary embolism. Targeting polyphosphate or factor XII conferred resistance to prostate cancer-driven thrombosis in mice, without increasing bleeding. Inhibition of factor XII with recombinant 3F7 antibody reduced the increased prostasome-mediated procoagulant activity in patient plasma. The data illustrate a critical role for polyphosphate/factor XII-triggered coagulation in prostate cancer-associated thrombosis with implications for anticoagulation without therapy-associated bleeding in malignancies.

FXIa and platelet polyphosphate as therapeutic targets during human blood clotting on collagen/tissue factor surfaces under flow

Factor XIIa (FXIIa) and factor XIa (FXIa) contribute to thrombosis in animal models, whereas platelet-derived polyphosphate (polyP) may potentiate contact or thrombin-feedback pathways. The significance of these mediators in human blood under thrombotic flow conditions on tissue factor (TF) -bearing surfaces remains inadequately resolved. Human blood (corn trypsin inhibitor treated [4 μg/mL]) was tested by microfluidic assay for clotting on collagen/TF at TF surface concentration ([TF]wall) from ∼0.1 to 2 molecules per μm(2). Anti-FXI antibodies (14E11 and O1A6) or polyP-binding protein (PPXbd) were used to block FXIIa-dependent FXI activation, FXIa-dependent factor IX (FIX) activation, or platelet-derived polyP, respectively. Fibrin formation was sensitive to 14E11 at 0 to 0.1 molecules per µm(2) and sensitive to O1A6 at 0 to 0.2 molecules per µm(2). However, neither antibody reduced fibrin generation at ∼2 molecules per µm(2) when the extrinsic pathway became dominant. Interestingly, PPXbd reduced fibrin generation at low [TF]wall (0.1 molecules per µm(2)) but not at zero or high [TF]wall, suggesting a role for polyP distinct from FXIIa activation and requiring low extrinsic pathway participation. Regardless of [TF]wall, PPXbd enhanced fibrin sensitivity to tissue plasminogen activator and promoted clot retraction during fibrinolysis concomitant with an observed PPXbd-mediated reduction of fibrin fiber diameter. This is the first detection of endogenous polyP function in human blood under thrombotic flow conditions. When triggered by low [TF]wall, thrombosis may be druggable by contact pathway inhibition, although thrombolytic susceptibility may benefit from polyP antagonism regardless of [TF]wall.

Platelet surface-associated activation and secretion-mediated inhibition of coagulation factor XII

Coagulation factor XII (fXII) is important for arterial thrombosis, but its physiological activation mechanisms are unclear. In this study, we elucidated the role of platelets and platelet-derived material in fXII activation. FXII activation was only observed upon potent platelet stimulation (with thrombin, collagen-related peptide, or calcium ionophore, but not ADP) accompanied by phosphatidylserine exposure and was localised to the platelet surface. Platelets from three patients with grey platelet syndrome did not activate fXII, which suggests that platelet-associated fXII-activating material might be released from α-granules. FXII was preferentially bound by phosphotidylserine-positive platelets and annexin V abrogated platelet-dependent fXII activation; however, artificial phosphotidylserine/phosphatidylcholine microvesicles did not support fXII activation under the conditions herein. Confocal microscopy using DAPI as a poly-phosphate marker did not reveal poly-phosphates associated with an activated platelet surface. Experimental data for fXII activation indicates an auto-inhibition mechanism (k_i/k_a = 180 molecules/platelet). Unlike surface-associated fXII activation, platelet secretion inhibited activated fXII (fXIIa), particularly due to a released C1-inhibitor. Platelet surface-associated fXIIa formation triggered contact pathway-dependent clotting in recalcified plasma. Computer modelling suggests that fXIIa inactivation was greatly decreased in thrombi under high blood flow due to inhibitor washout. Combined, the surface-associated fXII activation and its inhibition in solution herein may be regarded as a flow-sensitive regulator that can shift the balance between surface-associated clotting and plasma-dependent inhibition, which may explain the role of fXII at high shear and why fXII is important for thrombosis but negligible in haemostasis.

New agents for thromboprotection. A role for factor XII and XIIa inhibition

Blood coagulation is essential for hemostasis, however excessive coagulation can lead to thrombosis. Factor XII starts the intrinsic coagulation pathway and contact-induced factor XII activation provides the mechanistic basis for the diagnostic aPTT clotting assay. Despite its function for fibrin formation in test tubes, patients and animals lacking factor XII have a completely normal hemostasis. The lack of a bleeding tendency observed in factor XII deficiency states is in sharp contrast to deficiencies of other components of the coagulation cascade and factor XII has been considered to have no function for coagulation in vivo. Recently, experimental animal models showed that factor XII is activated by an inorganic polymer, polyphosphate, which is released from procoagulant platelets and that polyphosphate-driven factor XII activation has an essential role in pathologic thrombus formation. Cumulatively, the data suggest to target polyphosphate, factor XII, or its activated form factor XIIa for anticoagulation. As the factor XII pathway specifically contributes to thrombosis but not to hemostasis, interference with this pathway provides a unique opportunity for safe anticoagulation that is not associated with excess bleeding. The review summarizes current knowledge on factor XII functions, activators and inhibitors.

Plasma contact system activation drives anaphylaxis in severe mast cell-mediated allergic reactions

BACKGROUND

Anaphylaxis is an acute, potentially lethal, multisystem syndrome resulting from the sudden release of mast cell-derived mediators into the circulation.

OBJECTIVES AND METHODS

We report here that a plasma protease cascade, the factor XII-driven contact system, critically contributes to the pathogenesis of anaphylaxis in both murine models and human subjects.

RESULTS

Deficiency in or pharmacologic inhibition of factor XII, plasma kallikrein, high-molecular-weight kininogen, or the bradykinin B2 receptor, but not the B1 receptor, largely attenuated allergen/IgE-mediated mast cell hyperresponsiveness in mice. Reconstitutions of factor XII null mice with human factor XII restored susceptibility for allergen/IgE-mediated hypotension. Activated mast cells systemically released heparin, which provided a negatively charged surface for factor XII autoactivation. Activated factor XII generates plasma kallikrein, which proteolyzes kininogen, leading to the liberation of bradykinin. We evaluated the contact system in patients with anaphylaxis. In all 10 plasma samples immunoblotting revealed activation of factor XII, plasma kallikrein, and kininogen during the acute phase of anaphylaxis but not at basal conditions or in healthy control subjects. The severity of anaphylaxis was associated with mast cell degranulation, increased plasma heparin levels, the intensity of contact system activation, and bradykinin formation.

CONCLUSIONS

In summary, the data collectively show a role of the contact system in patients with anaphylaxis and support the hypothesis that targeting bradykinin generation and signaling provides a novel and alternative treatment strategy for anaphylactic attacks.

Single-chain factor XII exhibits activity when complexed to polyphosphate

BACKGROUND

The mechanism underpinning factor XII autoactivation was originally characterized with non-physiological surfaces, such as dextran sulfate (DS), ellagic acid, and kaolin. Several 'natural' anionic activating surfaces, such as platelet polyphosphate (polyP), have now been identified.

OBJECTIVE

To analyze the autoactivation of FXII by polyP of a similar length to that found in platelets (polyP70 ).

METHODS AND RESULTS

PolyP70 showed similar efficacy to DS in stimulating autoactivation of FXII, as detected with amidolytic substrate. Western blotting revealed different forms of FXII with the two activating surfaces: two-chain αFXIIa was formed with DS, whereas single-chain FXII (scFXII; 80 kDa) was formed with polyP70 . Dissociation of scFXII from polyP70 abrogated amidolytic activity, suggesting reversible exposure of the active site. Activity of scFXII-polyP70 was enhanced by Zn(2+) and was sensitive to NaCl concentration. A bell-shaped concentration response to polyP70 was evident, as is typical of surface-mediated reactions. Reaction of scFXII-polyP70 with various concentrations of S2302 generated a sigmoidal curve, in contrast to a hyperbolic curve for αFXIIa, from which a Hill coefficient of 3.67 was derived, indicative of positive cooperative binding. scFXII-polyP70 was more sensitive to inhibition by H-d-Pro-Phe-Arg-chloromethylketone and corn trypsin inhibitor than αFXIIa, but inhibition profiles for C1-inhibitor were similar. Active scFXII-polyP70 was also able to cleave its physiological targets FXI and prekallikrein to their active forms.

CONCLUSIONS

Autoactivation of FXII by polyP, of the size found in platelets, proceeds via an active single-chain intermediate. scFXII-polyP70 shows activity towards physiological substrates, and may represent the primary event in initiating contact activation in vivo.

Factor XII: a drug target for safe interference with thrombosis and inflammation

Data from experimental animal models revealed an essential role for factor XII (FXII) in thrombotic occlusive diseases. In contrast to other blood coagulation factors, deficiency in the protease is not associated with abnormal bleeding from injury sites (hemostasis) in patients or in animals. Cumulatively, these findings suggest that FXII could be targeted as a new method of anticoagulation that is devoid of bleeding risks. An FXIIa-neutralizing antibody, 3F7, has been developed that inhibited thrombosis in an extracorporeal membrane oxygenation (ECMO) system as efficiently as heparin. However, in sharp contrast to heparin, 3F7 treatment was not associated with an increase in therapy-associated hemorrhage. In this review, we summarize current knowledge of FXII physiology and pharmacology.

New players in haemostasis and thrombosis

The blood coagulation cascade is essential for haemostasis, but excessive activation can cause thrombosis. Importantly, recent studies have identified factors that contribute to thrombosis but not haemostasis. These include factor XII (FXII), tissue factor-positive microparticles (MPs) and neutrophil extracellular traps (NETs). Studies have shown that FXII plays a role in thrombosis but not haemostasis. FXII is activated in vivo by a variety of negatively-charged polyphosphates, which include extracellular RNA, DNA and inorganic polyphosphate (PolyP) that are released during cell damage and infection. These findings have led to the development of nucleic acid-binding polymers as a new class of anticoagulant drug. Other studies have analysed the role of MPs in experimental thrombosis. MPs are small membrane vesicles released from activated or apoptotic cells. We and others have found that tissue factor-positive MPs enhance thrombosis in mouse models and are elevated in the plasma of pancreatic cancer patients. Finally, NETs have been shown to contribute to experimental venous thrombosis in mouse models and are present in human thrombi. NETs are composed of chromatin fibers that are released from neutrophils undergoing cell death. NETs can capture platelets and increase fibrin deposition. The recent advances in our understanding of the factors contributing to thrombosis in animal models provide new opportunities for the development of safer anticoagulant drugs.