Pharmacologic targeting of coagulation factors XII and XI by monoclonal antibodies reduces thrombosis in nitinol stents under flow

BACKGROUND

Cardiovascular implantable devices, such as vascular stents, are critical for the treatment of cardiovascular diseases. However, their success is dependent on robust and often long-term antithrombotic therapies. Yet, the current standard-of-care therapies often pose significant bleeding risks to patients. Coagulation factor (F)XI and FXII have emerged as potentially safe and efficacious targets to safely reduce pathologic thrombin generation in medical devices.

OBJECTIVES

To study the efficacy of monoclonal antibody-targeting FXII and FXI of the contact pathway in preventing vascular device-related thrombosis.

METHODS

The effects of inhibition of FXII and FXI using function-blocking monoclonal antibodies were examined in a nonhuman primate model of nitinol stent-related thrombosis under arterial and venous flow conditions.

RESULTS

We found that function-blocking antibodies of FXII and FXI reduced markers of stent-induced thrombosis in vitro and ex vivo. However, FXI inhibition resulted in more effective mitigation of thrombosis markers under varied flow conditions.

CONCLUSION

This work provides further support for the translation of contact pathway of coagulation inhibitors for their adjunctive clinical use with cardiovascular devices.

Protein disulfide isomerase cleaves allosteric disulfides in histidine-rich glycoprotein to regulate thrombosis

The essence of difference between hemostasis and thrombosis is that the clotting reaction is a highly fine-tuned process. Vascular protein disulfide isomerase (PDI) represents a critical mechanism regulating the functions of hemostatic proteins. Herein we show that histidine-rich glycoprotein (HRG) is a substrate of PDI. Reduction of HRG by PDI enhances the procoagulant and anticoagulant activities of HRG by neutralization of endothelial heparan sulfate (HS) and inhibition of factor XII (FXIIa) activity, respectively. Murine HRG deficiency (Hrg-/-) leads to delayed onset but enhanced formation of thrombus compared to WT. However, in the combined FXII deficiency (F12-/-) and HRG deficiency (by siRNA or Hrg-/-), there is further thrombosis reduction compared to F12-/- alone, confirming HRG's procoagulant activity independent of FXIIa. Mutation of target disulfides of PDI leads to a gain-of-function mutant of HRG that promotes its activities during coagulation. Thus, PDI-HRG pathway fine-tunes thrombosis by promoting its rapid initiation via neutralization of HS and preventing excessive propagation via inhibition of FXIIa.

Increased factor XI but not factor XII is associated with enhanced inflammation and impaired fibrin clot properties in patients with eosinophilic granulomatosis with polyangiitis

OBJECTIVES

In eosinophilic granulomatosis with polyangiitis (EGPA) a prothrombotic state, including formation of denser fibrin networks with reduced lysability has been observed. Little is known about the intrinsic pathway in EGPA. We investigated whether coagulation factors (F)XI and FXII are associated with eosinophil-driven prothrombotic state.

METHODS

In 34 consecutive EGPA patients with remission we assessed FXI and FXII levels along with plasma fibrin clot permeability (Ks), fibrin clot morphology using scanning electron microscopy, and efficiency of fibrinolysis, expressed as lysis time (t50%) and maximum rate of increase in D-dimer levels (D-Drate).

RESULTS

Increased FXI level (>130%, the upper reference limit) was found in 8 (23.5%) patients. Compared to patients with FXI levels ≤130%, those with increased FXI had higher eosinophil count (+365%) and reduced percentage of neutrophils (-20.4%), along with reduced Ks (-20.5%). In patients with FXI>130% clots were composed of thinner fibrin fibers (-17.5%). FXI was not associated with C-reactive protein and fibrinogen levels or anti-neutrophil cytoplasmic antibodies titers. There were no correlations between FXI and FXII levels as well as between FXII and eosinophil count (all p>0.05).

CONCLUSIONS

To our knowledge, this study is the first to show association between FXI and a prothrombotic state in EGPA. Given clinical trials on FXI inhibition as an antithrombotic option, our findings suggest that this therapeutic approach could be useful in diseases with hypereosinophilia.

Tackling the Root Cause of Surface-Induced Coagulation: Inhibition of FXII Activation to Mitigate Coagulation Propagation and Prevent Clotting

Factor XII (FXII) is a zymogen present in blood that tends to adsorb onto the surfaces of blood-contacting medical devices. Once adsorbed, it becomes activated, initiating a cascade of enzymatic reactions that lead to surface-induced coagulation. This process is characterized by multiple redundancies, making it extremely challenging to prevent clot formation and preserve the properties of the surface. In this study, a novel modulatory coating system based on C1-esterase inhibitor (C1INH) functionalized polymer brushes, which effectively regulates the activation of FXII is proposed. Using surface plasmon resonance it is demonstrated that this coating system effectively repels blood plasma proteins, including FXII, while exhibiting high activity against activated FXII and plasma kallikrein under physiological conditions. This unique property enables the modulation of FXII activation without interfering with the overall hemostasis process. Furthermore, through dynamic Chandler loop studies, it is shown that this coating significantly improves the hemocompatibility of polymeric surfaces commonly used in medical devices. By addressing the root cause of contact activation, the synergistic interplay between the antifouling polymer brushes and the modulatory C1INH is expected to lay the foundation to enhance the hemocompatibility of medical device surfaces.

Bombyx batryticatus extract activates coagulation factor Ⅻ to promote angiogenesis in rats with cerebral ischemia/reperfusion injury

ETHNOPHARMACOLOGICAL RELEVANCE

Bombyx batryticatus is traditionally used to treat patients with stroke, but its mechanism remains unclear.

AIM OF THE STUDY

To explore the interventional effect of Bombyx batryticatus extract as an activator of FⅫ on angiogenesis of rats with cerebral ischemia/reperfusion injury.

MATERIALS AND METHODS

Firstly, the mechanism of Bombyx batryticatus interfering with IS was predicted by systematic pharmacology method, and then it was further verified by animal experiments. The effects of Bombyx batryticatus extract on plasma coagulation were detected, and the activation of coagulation factor Ⅻ (FⅫ) and its downstream substrate kallikrein kinase (KK) was detected in vitro. The brain morphology and expressions of FXII, KK, vascular endothelial growth factors (VEGF), CD31, Brdu/von Willebrand Factor (vWF) were detected. The morphological changes, cell proliferation and VEGF expression of brain microvascular endothelial cells were detected by oxygen glucose deprivation model. The pharmacodynamic substances of Bombyx batryticatus extract were identified by Liquid Chromatography - Mass Spectrometry (LC-MS).

RESULTS

The results of systematic pharmacology found that the treatment of IS by Bombyx batryticatus may be related to blood coagulation and other processes. In vitro, Bombyx batryticatus extract prolonged the activated partial thromboplastin time (APTT), prothrombin time (PT), thrombin time (TT) (P < 0.05), activated FⅫ and promoted the production of downstream substrate KK, with dose-dependent (P < 0.05). Bombyx batryticatus extract improved the neuronal damage of rats, activated FXII and increased the production of KK and the expressions of VEGF, CD31, Brdu/vWF (P < 0.05). Bombyx batryticatus extract also increased the proliferation of brain microvascular endothelial cells and expression of VEGF in rats (P < 0.05). A total of 809 metabolites in Bombyx batryticatus extract were identified by LC-MS.

CONCLUSION

Bombyx batryticatus extract may ameliorate the injury of nerve function in rats with cerebral ischemia/reperfusion injury.

[New perspective of anticoagulation in intensive care unit: basic and clinical advances in coagulation factor XII and XI inhibitors]

Anticoagulation therapy stands as a key treatment for thrombotic diseases. The consequential bleeding risk tied to existing anticoagulation methods significantly impacts patient prognosis. In the intensive care unit (ICU), patients often necessitate organ support, leading to the inevitable placement of artificial devices in blood vessels, thereby requiring anticoagulation treatment to avert clot formation that might impede organ support. Nevertheless, these patients commonly encounter a heightened risk of bleeding. Hemophilia B, identified in 1953, manifests as a deficiency in coagulation factor XI (FXI), which focused people's perspective on the endogenous coagulation pathway, that is, the contact pathway. Upon interaction between the surface of artificial devices and FXII, FXII activates, subsequently triggering FXI and initiating the "coagulation cascade" within the contact pathway. Inhibitors targeting the contact pathway encompass two primary categories: FXII inhibitors and FXI inhibitors, capable of impeding this process. This article reviews the role of FXII and FXI in activating the contact pathway, seeking to illuminate their contributions to thrombus formation. By listing the relatively mature drugs and their indications, clinicians are familiar with this new anticoagulant.

Factor XII Deficiency in Mexico: High Prevalence in the General Population and Patients with Venous Thromboembolic Disease

INTRODUCTION

Thrombosis is one of the leading causes of morbidity and mortality worldwide. Venous thromboembolic disease (VTD) is considered a new epidemic. FXII deficiency is supposed to be a cause of thrombosis. To search for unknown causes of thrombosis in our population, our aim was to determine if FXII deficiency can be considered a risk factor for VTD.

METHODS

Young adult Mexican patients with at least one VTD episode and healthy controls were included in this prospective, observational, controlled study. Liver and renal function tests, blood cytometry, and blood coagulation assays were performed. Plasma FXII activity and its concentration were evaluated.

RESULTS

Over a two-year period, 250 patients and 250 controls were included. FXII activity was significantly lower in the control group compared to patients with VTD (p = 0.005). However, percentage of patients and controls with FXII deficiency was 8.8 and 9.2%, respectively (p = 1.000). No significant association was found between FXII deficiency and VTD (p = 1.0). FXII plasma concentration was lower in controls vs. patients with VTD: 4.05 vs. 6.19 ng/mL (p <0.001). Percentage of patients with low FXII plasma concentration was 1.6% and 6.0% in patients and controls, respectively (p = 0.010).

CONCLUSIONS

FXII deficiency is a frequent finding in patients with VTD and controls in Mexico. Some patients with FXII deficiency had normal APTT result, an effect not described above. FXII plasma concentration was lower in patients with low activity.

Titanium is a potent inducer of contact activation: implications for intravascular devices

BACKGROUND

Titanium (Ti) and its alloys are widely used in manufacturing medical devices because of their strength and resistance to corrosion. Although Ti compounds are considered compatible with blood, they appear to support plasma contact activation and may be thrombogenic.

OBJECTIVES

The objective of this study was to compare Ti and titanium nitride (TiN) with known activators of contact activation (kaolin and silica) in plasma-clotting assays and to assess binding and activation of factor XII, (FXII), factor XI (FXI), prekallikrein, and high-molecular-weight kininogen (HK) with Ti/TiN.

METHODS

Ti-based nanospheres and foils were compared with kaolin, silica, and aluminum in plasma-clotting assays. Binding and activation of FXII, prekallikrein, HK, and FXI to surfaces was assessed with western blots and chromogenic assays.

RESULTS

Using equivalent surface amounts, Ti and TiN were comparable with kaolin and superior to silica, for inducing coagulation and FXII autoactivation. Similar to many inducers of contact activation, Ti and TiN are negatively charged; however, their effects on FXII are not neutralized by the polycation polybrene. Antibodies to FXII, prekallikrein, or FXI or coating Ti with poly-L-arginine blocked Ti-induced coagulation. An antibody to FXII reduced FXII and PK binding to Ti, kallikrein generation, and HK cleavage.

CONCLUSION

Titanium compounds induce contact activation with a potency comparable with that of kaolin. Binding of FXII with Ti shares some features with FXII binding to soluble polyanions but may have unique features. Inhibitors targeting FXII or FXI may be useful in mitigating Ti-induced contact activation in patients with titanium-based implants that are exposed to blood.

Factor XII contributes to thrombotic complications and vaso-occlusion in sickle cell disease

A hypercoagulable state, chronic inflammation, and increased risk of venous thrombosis and stroke are prominent features in patients with sickle cell disease (SCD). Coagulation factor XII (FXII) triggers activation of the contact system that is known to be involved in both thrombosis and inflammation, but not in physiological hemostasis. Therefore, we investigated whether FXII contributes to the prothrombotic and inflammatory complications associated with SCD. We found that when compared with healthy controls, patients with SCD exhibit increased circulating biomarkers of FXII activation that are associated with increased activation of the contact pathway. We also found that FXII, but not tissue factor, contributes to enhanced thrombin generation and systemic inflammation observed in sickle cell mice challenged with tumor necrosis factor α. In addition, FXII inhibition significantly reduced experimental venous thrombosis, congestion, and microvascular stasis in a mouse model of SCD. Moreover, inhibition of FXII attenuated brain damage and reduced neutrophil adhesion to the brain vasculature of sickle cell mice after ischemia/reperfusion induced by transient middle cerebral artery occlusion. Finally, we found higher FXII, urokinase plasminogen activator receptor, and αMβ2 integrin expression in neutrophils of patients with SCD compared with healthy controls. Our data indicate that targeting FXII effectively reduces experimental thromboinflammation and vascular complications in a mouse model of SCD, suggesting that FXII inhibition may provide a safe approach for interference with inflammation, thrombotic complications, and vaso-occlusion in patients with SCD.

Coming soon to a pharmacy near you? FXI and FXII inhibitors to prevent or treat thromboembolism

Anticoagulants have been in use for nearly a century for the treatment and prevention of venous and arterial thromboembolic disorders. The most dreaded complication of anticoagulant treatment is the occurrence of bleeding, which may be serious and even life-threatening. All available anticoagulants, which target either multiple coagulation factors or individual components of the tissue factor (TF) factor VIIa or the common pathways, have the potential to affect hemostasis and thus to increase bleeding risk in treated patients. While direct oral anticoagulants introduced an improvement in care for eligible patients in terms of safety, efficacy, and convenience of treatment, there remain unmet clinical needs for patients requiring anticoagulant drugs. Anticoagulant therapy is sometimes avoided for fear of hemorrhagic complications, and other patients are undertreated due to comorbidities and the perception of increased bleeding risk. Evidence suggests that the contact pathway of coagulation has a limited role in initiating physiologic in vivo coagulation and that it contributes to thrombosis more than it does to hemostasis. Because inhibition of the contact pathway is less likely to promote bleeding, it is an attractive target for the development of anticoagulants with improved safety. Preclinical and early clinical data indicate that novel agents that selectively target factor XI or factor XII can reduce venous and arterial thrombosis without an increase in bleeding complications.

DDB1- and CUL4-associated factor 12-like protein 1 (Dcaf12l1) is not essential for male fertility in mice

Male infertility is a common condition affecting at least 7% of men worldwide and is often genetic in origin. Using whole exome sequencing, we recently discovered three hemizygous, likely damaging variants in DDB1- and CUL4-associated factor 12-like protein 1 (DCAF12L1) in men with azoospermia. DCAF12L1 is located on the X-chromosome and as identified by single cell sequencing studies, its expression is enriched in human testes and specifically in Sertoli cells and spermatogonia. However, very little is known about the role of DCAF12L1 in spermatogenesis, thus we generated a knockout mouse model to further explore the role of DCAF12L1 in male fertility. Knockout mice were generated using CRISPR/Cas9 technology to remove the entire coding region of Dcaf12l1 and were assessed for fertility over a broad range of ages (2-8 months of age). Despite outstanding genetic evidence in men, loss of DCAF12L1 had no discernible impact on male fertility in mice, as highlighted by breeding trials, histological assessment of the testis and epididymis, daily sperm production and evaluation of sperm motility using computer assisted methods. This disparity is likely due to the parallel evolution, and subsequent divergence, of DCAF12 family members in mice and men or the presence of compounding environmental factors in men.

Recent advances in factor XII structure and function

PURPOSE OF REVIEW

Factor XII (FXII), the precursor of the protease FXIIa, contributes to pathologic processes including angioedema and thrombosis. Here, we review recent work on structure-function relationships for FXII based on studies using recombinant FXII variants.

RECENT FINDINGS

FXII is a homolog of pro-hepatocyte growth factor activator (Pro-HGFA). We prepared FXII in which domains are replaced by corresponding parts of Pro-HGA, and tested them in FXII activation and activity assays. In solution, FXII and prekallikrein undergo reciprocal activation to FXIIa and kallikrein. The rate of this process is restricted by the FXII fibronectin type-2 and kringle domains. Pro-HGA replacements for these domains accelerate FXII and prekallikrein activation. When FXII and prekallikrein bind to negatively charged surfaces, reciprocal activation is enhanced. The FXII EGF1 domain is required for surface binding.

SUMMARY

We propose a model in which FXII is normally maintained in a closed conformation resistant to activation by intramolecular interactions involving the fibronectin type-2 and kringle domains. These interactions are disrupted when FXII binds to a surface through EGF1, enhancing FXII activation and prekallikrein activation by FXIIa. These observations have important implications for understanding the contributions of FXII to disease, and for developing therapies to treat thrombo-inflammatory disorders.

Identification of the histidine-rich glycoprotein domains responsible for contact pathway inhibition

BACKGROUND

Previously, we showed that histidine-rich glycoprotein (HRG) binds factor (F) XIIa with high affinity, inhibits FXII autoactivation and FXIIa-mediated activation of FXI, and attenuates ferric chloride-induced arterial thrombosis in mice. Therefore, HRG downregulates the contact pathway in vitro and in vivo.

OBJECTIVE

To identify the domains on HRG responsible for contact pathway inhibition.

METHODS

Recombinant HRG domain constructs (N-terminal [N1, N2, and N1N2], proline-rich regions, histidine-rich region [HRR], and C-terminal) were expressed and purified. The affinities of plasma-derived HRG, HRG domain constructs, and synthetic HRR peptides for FXII, FXIIa, β-FXIIa, and polyphosphate (polyP) were determined using surface plasmon resonance, and their effects on polyP-induced FXII autoactivation, FXIIa-mediated activation of FXI and prekallikrein, the activated partial thromboplastin time (APTT), and thrombin generation were examined.

RESULTS

HRG and HRG domain constructs bind FXIIa, but not FXII or β-FXII. HRR, N1, and N1N2 bind FXIIa with affinities comparable with that of HRG, whereas the remaining domains bind with lower affinity. Synthetic HRR peptides bind FXIIa and polyP with high affinity. HRG and HRR significantly inhibit FXII autoactivation and prolong the APTT. Like HRG, synthetic HRR peptides inhibit FXII autoactivation, attenuate FXIIa-mediated activation of prekallikrein and FXI, prolong the APTT, and attenuate thrombin generation.

CONCLUSION

The interaction of HRG with FXIIa and polyP is predominantly mediated by the HRR domain. Like intact HRG, HRR downregulates the contact pathway and contributes to HRG-mediated down regulation of coagulation.

Plasmin-mediated cleavage of high-molecular-weight kininogen contributes to acetaminophen-induced acute liver failure

Acetaminophen (APAP)-induced liver injury is associated with activation of coagulation and fibrinolysis. In mice, both tissue factor-dependent thrombin generation and plasmin activity have been shown to promote liver injury after APAP overdose. However, the contribution of the contact and intrinsic coagulation pathways has not been investigated in this model. Mice deficient in individual factors of the contact (factor XII [FXII] and prekallikrein) or intrinsic coagulation (FXI) pathway were administered a hepatotoxic dose of 400 mg/kg of APAP. Neither FXII, FXI, nor prekallikrein deficiency mitigated coagulation activation or hepatocellular injury. Interestingly, despite the lack of significant changes to APAP-induced coagulation activation, markers of liver injury and inflammation were significantly reduced in APAP-challenged high-molecular-weight kininogen-deficient (HK-/-) mice. Protective effects of HK deficiency were not reproduced by inhibition of bradykinin-mediated signaling, whereas reconstitution of circulating levels of HK in HK-/- mice restored hepatotoxicity. Fibrinolysis activation was observed in mice after APAP administration. Western blotting, enzyme-linked immunosorbent assay, and mass spectrometry analysis showed that plasmin efficiently cleaves HK into multiple fragments in buffer or plasma. Importantly, plasminogen deficiency attenuated APAP-induced liver injury and prevented HK cleavage in the injured liver. Finally, enhanced plasmin generation and HK cleavage, in the absence of contact pathway activation, were observed in plasma of patients with acute liver failure due to APAP overdose. In summary, extrinsic but not intrinsic pathway activation drives the thromboinflammatory pathology associated with APAP-induced liver injury in mice. Furthermore, plasmin-mediated cleavage of HK contributes to hepatotoxicity in APAP-challenged mice independently of thrombin generation or bradykinin signaling.

Factor XII plays a pathogenic role in organ failure and death in baboons challenged with Staphylococcus aureus

Activation of coagulation factor (F) XI promotes multiorgan failure in rodent models of sepsis and in a baboon model of lethal systemic inflammation induced by infusion of heat-inactivated Staphylococcus aureus. Here we used the anticoagulant FXII-neutralizing antibody 5C12 to verify the mechanistic role of FXII in this baboon model. Compared with untreated control animals, repeated 5C12 administration before and at 8 and 24 hours after bacterial challenge prevented the dramatic increase in circulating complexes of contact system enzymes FXIIa, FXIa, and kallikrein with antithrombin or C1 inhibitor, and prevented cleavage and consumption of high-molecular-weight kininogen. Activation of several coagulation factors and fibrinolytic enzymes was also prevented. D-dimer levels exhibited a profound increase in the untreated animals but not in the treated animals. The antibody also blocked the increase in plasma biomarkers of inflammation and cell damage, including tumor necrosis factor, interleukin (IL)-1β, IL-6, IL-8, IL-10, granulocyte-macrophage colony-stimulating factor, nucleosomes, and myeloperoxidase. Based on clinical presentation and circulating biomarkers, inhibition of FXII prevented fever, terminal hypotension, respiratory distress, and multiorgan failure. All animals receiving 5C12 had milder and transient clinical symptoms and were asymptomatic at day 7, whereas untreated control animals suffered irreversible multiorgan failure and had to be euthanized within 2 days after the bacterial challenge. This study confirms and extends our previous finding that at least 2 enzymes of the contact activation complex, FXIa and FXIIa, play critical roles in the development of an acute and terminal inflammatory response in baboons challenged with heat-inactivated S aureus.

FXII regulates the formation of deep vein thrombosis via the PI3K/AKT signaling pathway in mice

Deep vein thrombosis (DVT) is a common peripheral vascular disease, which may result in pulmonary embolism and is accompanied by endothelial injury. However, the pathogenesis of DVT remains unclear. Coagulation factor XII (FXII), as an important coagulation factor, has been reported to be closely associated with thrombosis. However, the association between FXII protein and DVT formation is not yet fully understood. The present study examined the effects of FXII protein on DVT formation and aimed to reveal the underlying mechanism. In the present study, histological characterization of the femoral vein tissue was examined by hematoxylin and eosin staining. The damage to the femoral vein tissue was examined by TUNEL assay. Superoxide dismutase (SOD) and malondialdehyde (MDA) concentrations were examined using ELISA. Tumor necrosis factor (TNF)‑α, interleukin (IL)‑6, IL‑8 and phosphoinositide 3‑kinase (PI3K)/AKT signaling were determined by ELISA, immunohistochemical staining and western blot analysis. The results demonstrated that thrombosis, FXII protein, cell apoptosis and the SOD concentrations were decreased, while the MDA concentrations were increased in mice with DVT compared with the control or sham groups. TNF‑α, IL‑6, IL‑8 and PI3K/AKT signaling was also upregulated in the mice with DVT. Furthermore, the knockdown of FXII significantly upregulated the SOD concentrations and downregulated thrombosis and cell apoptosis, as well as the MDA concentrations in mice with DVT. The knockdown of FXII also significantly downregulated the protein expression of TNF‑α, IL‑6 and IL‑8, and the activation of PI3K/AKT signaling. Additionally, LY294002 pre‑treatment markedly downregulated thrombosis and cell apoptosis and the MDA content, whereas it upregulated the SOD concentrations in mice with DVT. LY294002 pre‑treatment also significantly downregulated the TNF‑α, IL‑6 and IL‑8 protein levels. Taken together, the present study demonstrates that FXII protein promotes DVT via the activation of PI3K/AKT signaling by inducing an inflammatory response. Targeting FXII protein may thus prove to be a potential approach for the treatment of DVT.

Proteolytic activity of contact factor zymogens

Contact activation is triggered when blood is exposed to compounds or "surfaces" that promote conversion of the plasma zymogens factor XII (FXII) and prekallikrein to the active proteases FXIIa and kallikrein. FXIIa promotes blood coagulation by converting zymogen factor XI (FXI) to the protease FXIa. Contact activation appears to represent an enhancement of the propensity for FXII and prekallikrein to reciprocally activate each other by surface-independent limited proteolysis. The nature of the activities that perpetuate this process, and that trigger contact activation, are debated. FXII and prekallikrein, like most members of the chymotrypsin/trypsin protease family, are synthesized as single polypeptides that are presumed to be in an inactive state. Internal cleavage leads to conformational changes in the protease domain that convert the enzyme active site from a closed conformation to an open conformation accessible to substrates. We observed that FXII expresses a low level of activity as a single-chain zymogen that catalyzes prekallikrein activation in solution, as well as surface-dependent activation of prekallikrein, FXI, and FXII (autoactivation). Prekallikrein also expresses activity that promotes cleavage of kininogen to release bradykinin, and surface-dependent FXII activation. Modeling suggests that a glutamine residue at position 156 in the FXII and prekallikrein protease domains stabilizes an open active site conformation by forming hydrogen bonds with Asp194. The activity inherent in FXII and prekallikrein suggests a mechanism for sustaining reciprocal activation of the proteins and for initiating contact activation, and supports the premise that zymogens of some trypsin-like enzymes are active proteases.

Factor XII and kininogen asymmetric assembly with gC1qR/C1QBP/P32 is governed by allostery

The contact system is composed of factor XII (FXII), prekallikrein (PK), and cofactor high-molecular-weight kininogen (HK). The globular C1q receptor (gC1qR) has been shown to interact with FXII and HK. We reveal the FXII fibronectin type II domain (FnII) binds gC1qR in a Zn2+-dependent fashion and determined the complex crystal structure. FXIIFnII binds the gC1qR trimer in an asymmetric fashion, with residues Arg36 and Arg65 forming contacts with 2 distinct negatively charged pockets. gC1qR residues Asp185 and His187 coordinate a Zn2+ adjacent to the FXII-binding site, and a comparison with the ligand-free gC1qR crystal structure reveals the anionic G1-loop becomes ordered upon FXIIFnII binding. Additional conformational changes in the region of the Zn2+-binding site reveal an allosteric basis for Zn2+ modulation of FXII binding. Mutagenesis coupled with surface plasmon resonance demonstrate the gC1qR Zn2+ site contributes to FXII binding, and plasma-based assays reveal gC1qR stimulates coagulation in a FXII-dependent manner. Analysis of the binding of HK domain 5 (HKD5) to gC1qR shows only 1 high-affinity binding site per trimer. Mutagenesis studies identify a critical G3-loop located at the center of the gC1qR trimer, suggesting steric occlusion as the mechanism for HKD5 asymmetric binding. Gel filtration experiments reveal that gC1qR clusters FXII and HK into a higher-order 500-kDa ternary complex. These results support the conclusion that extracellular gC1qR can act as a chaperone to cluster contact factors, which may be a prelude for initiating the cascades that drive bradykinin generation and the intrinsic pathway of coagulation.

Annexin A4 inhibits sulfatide-induced activation of coagulation factor XII

BACKGROUND

Factor XII (FXII) is a plasma serine protease that initiates the intrinsic pathway of blood coagulation upon contact with anionic substances, such as the sulfated glycolipid sulfatide. Annexins (ANXs) have been implicated in the regulation of the blood coagulation reaction by binding to anionic surfaces composed of phospholipids and sulfated glycoconjugates, but their physiological importance is only partially understood.

OBJECTIVE

To test the hypothesis that ANXs are involved in suppressing the intrinsic pathway initiated by sulfatide, we examined the effect of eight recombinant ANX proteins on the intrinsic coagulation reaction and their sulfatide binding activities.

METHODS

Recombinant ANXs were prepared in Escherichia coli expression systems and their anticoagulant effects on the intrinsic pathway initiated by sulfatide were examined using plasma clotting assay and chromogenic assay. ANXA4 active sites were identified by alanine scanning and fold deletion in the core domain.

RESULTS AND CONCLUSIONS

We found that ANXA3, ANXA4, and ANXA5 strongly inhibited sulfatide-induced plasma coagulation. Wild-type and mutated ANXA4 were used to clarify the molecular mechanism involved in inhibition. ANXA4 inhibited sulfatide-induced auto-activation of FXII to FXIIa and the conversion of its natural substrate FXI to FXIa but showed no effect on the protease activity of FXIIa or FXIa. Alanine scanning showed that substitution of the Ca2+ -binding amino acid residue in the fourth fold of the core domain of ANXA4 reduced anticoagulant activity, and deletion of the entire fourth fold of the core domain resulted in complete loss of anticoagulant activity.

Protease activity in single-chain prekallikrein

Prekallikrein (PK) is the precursor of the trypsin-like plasma protease kallikrein (PKa), which cleaves kininogens to release bradykinin and converts the protease precursor factor XII (FXII) to the enzyme FXIIa. PK and FXII undergo reciprocal conversion to their active forms (PKa and FXIIa) by a process that is accelerated by a variety of biological and artificial surfaces. The surface-mediated process is referred to as contact activation. Previously, we showed that FXII expresses a low level of proteolytic activity (independently of FXIIa) that may initiate reciprocal activation with PK. The current study was undertaken to determine whether PK expresses similar activity. Recombinant PK that cannot be converted to PKa was prepared by replacing Arg371 with alanine at the activation cleavage site (PK-R371A, or single-chain PK). Despite being constrained to the single-chain precursor form, PK-R371A cleaves high-molecular-weight kininogen (HK) to release bradykinin with a catalytic efficiency ∼1500-fold lower than that of kallikrein cleavage of HK. In the presence of a surface, PK-R371A converts FXII to FXIIa with a specific activity ∼4 orders of magnitude lower than for PKa cleavage of FXII. These results support the notion that activity intrinsic to PK and FXII can initiate reciprocal activation of FXII and PK in solution or on a surface. The findings are consistent with the hypothesis that the putative zymogens of many trypsin-like proteases are actually active proteases, explaining their capacity to undergo processes such as autoactivation and to initiate enzyme cascades.

Phenotype of ribonuclease 1 deficiency in mice

Biological roles for extracellular RNA (eRNA) have become apparent. For example, eRNA can induce contact activation in blood via activation of the plasma proteases factor XII (FXII) and factor XI (FXI). We sought to reveal the biological role of the secretory enzyme ribonuclease 1 (RNase 1) in an organismal context by generating and analyzing RNase 1 knockout (Rnase1^-/-) mice. We found that these mice are viable, healthy, and fertile, though larger than Rnase1^+/+ mice. Rnase1^-/- plasma contains more RNA than does the plasma of Rnase1^+/+ mice. Moreover, the plasma of Rnase1^-/- mice clots more rapidly than does wild-type plasma. This phenotype appeared to be due to increased levels of the active form of FXII (FXIIa) in the plasma of Rnase1^-/- mice compared to Rnase1^+/+ mice, and is consistent with the known effects of eRNA on FXII activation. The apparent activity of FXI in the plasma of Rnase1^-/- mice was 1000-fold higher when measured in an assay triggered by a low concentration of tissue factor than in assays based on recalcification, consistent with eRNA enhancing FXI activation by thrombin. These findings suggest that one of the physiological functions of RNase 1 is to degrade eRNA in blood plasma. Loss of this function facilitates FXII and FXI activation, which could have effects on inflammation and blood coagulation. We anticipate that Rnase1^-/- mice will be a useful tool for evaluating other hypotheses about the functions of RNase 1 and of eRNA in vivo.

Precise Structure and Anticoagulant Activity of Fucosylated Glycosaminoglycan from Apostichopus japonicus: Analysis of Its Depolymerized Fragments

Apostichopus japonicus is one of the most economically important species in sea cucumber aquaculture in China. Fucosylated glycosaminoglycan from A. japonicus (AjFG) has shown multiple pharmacological activities. However, results from studies on the structure of AjFG are still controversial. In this study, the deaminative depolymerization method that is glycosidic bond-selective was used to prepare the depolymerized products from AjFG (dAjFG), and then a series of purified oligosaccharide fragments such as tri-, hexa-, nona-, and dodecasaccharides were obtained from dAjFG by gel permeation chromatography. The 1D/2D NMR and ESI-MS spectrometry analyses showed that these oligosaccharides had the structural formula of l-FucS-α1,3-d-GlcA-β1,3-{d-GalNAc_4S6S-β1,4-[l-FucS-α1,3-]d-GlcA-β1,3-}_n-d-anTal-diol_4S6S (n = 0, 1, 2, 3; FucS represents Fuc_2S4S, Fuc_3S4S, or Fuc_4S). Thus, the unambiguous structure of native AjFG can be rationally deduced: it had the backbone of {-4-d-GlcA-β1,3-d-GalNAc_4S6S-β1-}_n, which is similar to chondroitin sulfate E, and each d-GlcA residue in the backbone was branched with a l-FucS monosaccharide at O-3. Bioactivity assays confirmed that dAjFG and nonasaccharides and dodecasaccharides from AjFG had potent anticoagulant activity by intrinsic FXase inhibition while avoiding side effects such as FXII activation and platelet aggregation.

Exploring the Heterogeneity of Nanoparticles in Their Interactions with Plasma Coagulation Factor XII

Tuning the characteristics of nanoparticles (NPs) would be promising in improving their biocompatibilities, regarding biosafety and nanodrug considerations. Due to the high priority of the artificial NPs in contacting the circulatory system, understanding their interactions with plasma zymogens is of great importance. Four kinds of NPs, including 5 nm gold NPs (GNP-5), 5 and 20 nm silver NPs (SNP-5, SNP-20), and 20 nm silica NPs (SiNP-20), were investigated for their interactions with the coagulation factor XII (FXII). GNP-5 adsorbed FXII in a standing-up mode, and exhibited high binding affinity for the heavy chain of the protein without altering its secondary structure or inducing its activation. In contrast to GNP-5, FXII adsorption on the other tested NPs was in a lying-down mode, and their interactions with FXII induced its conformational changes, thus causing the evident zymogen cleavage. The structural alterations and activation of FXII induced by the NPs exhibited in specific surface area dependent manners, which were related with different NP cores and sizes. Additionally, the enzymatic activity of α-FXIIa was also influenced by NP incubation, and the alterations were dependent on the specific characters of the NPs as evidenced by the enzymatic inhibition effect of GNP-5 (noncompetitive) and SNP-5 (competitive), and enhanced enzymatic catalysis abilities of SNP-20 and SiNP-20. The interesting findings on the heterogeneity of NPs in their interactions with plasma FXII not only revealed the underlying mechanism for NP-triggered hematological responses, but also suggested the crucial role of tuning NP parameters in their potential bioapplication, like nanodrug design.

Factor XII truncation accelerates activation in solution

Essentials During contact system activation, factor XII is progressively cleaved by plasma kallikrein. We investigated the role of factor XII truncation in biochemical studies. Factor XII contains naturally occurring truncating cleavage sites for a variety of enzymes. Truncation of factor XII primes it for activation in solution through exposure of R353. SUMMARY: Background The contact activation system and innate immune system are interlinked in inflammatory pathology. Plasma kallikrein (PKa) is held responsible for the stepwise processing of factor XII (FXII). A first cleavage activates FXII (into FXIIa); subsequent cleavages truncate it. This truncation eliminates its surface-binding domains, which negatively regulates surface-dependent coagulation. Objectives To investigate the influence of FXII truncation on its activation and downstream kallikrein-kinin system activation. Methods We study activation of recombinant FXII variants by chromogenic assays, by FXIIa ELISA and western blotting. Results We demonstrate that FXII truncation primes it for activation by PKa in solution. We demonstrate this phenomenon in three settings. (i) Truncation at a naturally occurring PKa-sensitive cleavage site, R334, accelerates FXIIa formation in solution. A site-directed mutant FXII-R334A displays ~50% reduced activity when exposed to PKa. (ii) A pathogenic mutation in FXII that causes hereditary angioedema, introduces an additional plasmin-sensitive cleavage site. Truncation at this site synergistically accelerates FXII activation in solution. (iii) We identify new, naturally occurring cleavage sites in FXII that have so far not been functionally linked to contact system activation. As examples, we show that non-activating truncation of FXII by neutrophil elastase and cathepsin K primes it for activation by PKa in solution. Conclusions FXII truncation, mediated by either pathogenic mutations or naturally occurring cleavage sites, primes FXII for activation in solution. We propose that the surface-binding domains of FXII shield its activating cleavage site, R353. This may help to explain how the contact system contributes to inflammatory pathology.

Anticoagulant and Antithrombotic Properties of Three Structurally Correlated Sea Urchin Sulfated Glycans and Their Low-Molecular-Weight Derivatives

The anticoagulant and antithrombotic properties of three structurally correlated sea urchin-derived 3-linked sulfated α-glycans and their low molecular-weight derivatives were screened comparatively through various in vitro and in vivo methods. These methods include activated partial thromboplastin time, the inhibitory activity of antithrombin over thrombin and factor Xa, venous antithrombosis, the inhibition of platelet aggregation, the activation of factor XII, and bleeding. While the 2-sulfated fucan from Strongylocentrotus franciscanus was observed to be poorly active in most assays, the 4-sulfated fucan from Lytechinus variegatus, the 2-sulfated galactan from Echinometra lucunter and their derivatives showed multiple effects. All marine compounds showed no capacity to activate factor XII and similar low bleeding tendencies regardless of the dose concentrations used to achieve the highest antithrombotic effect observed. The 2-sulfated galactan showed the best combination of results. Our work improves the background about the structure-function relationship of the marine sulfated glycans in anticoagulation and antithrombosis. Besides confirming the negative effect of the 2-sulfated fucose and the positive effect of the 2-sulfated galactose on anticoagulation in vitro, our results also demonstrate the importance of this set of structural requirements on antithrombosis in vivo, and further support the involvement of high-molecular weight and 4-sulfated fucose in both activities.

Factor XII and uPAR upregulate neutrophil functions to influence wound healing

Coagulation factor XII (FXII) deficiency is associated with decreased neutrophil migration, but the mechanisms remain uncharacterized. Here, we examine how FXII contributes to the inflammatory response. In 2 models of sterile inflammation, FXII-deficient mice (F12-/-) had fewer neutrophils recruited than WT mice. We discovered that neutrophils produced a pool of FXII that is functionally distinct from hepatic-derived FXII and contributes to neutrophil trafficking at sites of inflammation. FXII signals in neutrophils through urokinase plasminogen activator receptor-mediated (uPAR-mediated) Akt2 phosphorylation at S474 (pAktS474). Downstream of pAkt2S474, FXII stimulation of neutrophils upregulated surface expression of αMβ2 integrin, increased intracellular calcium, and promoted extracellular DNA release. The sum of these activities contributed to neutrophil cell adhesion, migration, and release of neutrophil extracellular traps in a process called NETosis. Decreased neutrophil signaling in F12-/- mice resulted in less inflammation and faster wound healing. Targeting hepatic F12 with siRNA did not affect neutrophil migration, whereas WT BM transplanted into F12-/- hosts was sufficient to correct the neutrophil migration defect in F12-/- mice and restore wound inflammation. Importantly, these activities were a zymogen FXII function and independent of FXIIa and contact activation, highlighting that FXII has a sophisticated role in vivo that has not been previously appreciated.

The Plasma Kallikrein-Kininogen Pathway Is Critical in the Pathogenesis of Colitis in Mice

The kallikrein-kinin system (KKS) consists of two serine proteases, prekallikrein (pKal) and factor XII (FXII), and a cofactor, high-molecular-weight kininogen (HK). Upon activation of the KKS, HK is cleaved to release bradykinin. Although the KKS is activated in humans and animals with inflammatory bowel disease (IBD), its role in the pathogenesis of IBD has not been characterized. In the present study, we determined the role of the KKS in the pathogenesis of IBD using mice that lack proteins involved in the KKS. In two colitis models, induced by dextran sulfate sodium (DSS) or 2,4,6-trinitrobenzene sulfonic acid (TNBS), mice deficient in HK, pKal, or bradykinin receptors displayed attenuated phenotypes, including body weight loss, disease activity index, colon length shortening, histological scoring, and colonic production of cytokines. Infiltration of neutrophils and inflammatory monocytes in the colonic lamina propria was reduced in HK-deficient mice. Reconstitution of HK-deficient mice through intravenous injection of HK recovered their susceptibility to DSS-induced colitis, increased IL-1β levels in the colon tissue and bradykinin concentrations in plasma. In contrast to the phenotypes of other mice lacking other proteins involved in the KKS, mice lacking FXII had comparable colonic inflammation to that observed in wild-type mice. The concentration of bradykinin was significantly increased in the plasma of wild-type mice after DSS-induced colitis. In vitro analysis revealed that DSS-induced pKal activation, HK cleavage, and bradykinin plasma release were prevented by the absence of pKal or the inhibition of Kal. Unlike DSS, TNBS-induced colitis did not trigger HK cleavage. Collectively, our data strongly suggest that Kal, acting independently of FXII, contributes to experimental colitis by promoting bradykinin release from HK.

Protocols to Assess Coagulation Following In Vitro Infection with Hemorrhagic Fever Viruses

During the course of infection with a hemorrhagic fever virus (HFV), the checks and balances associated with normal coagulation are perturbed resulting in hemorrhage in severe cases and, in some patients, disseminated intravascular coagulopathy (DIC). While many HFVs have animal models that permit the analyses of systemic coagulopathy, animal infection models do not exist for all HFVs and moreover do not always recapitulate the pathology observed in human tissues. Furthermore, molecular analyses of how coagulation is affected are not always straightforward or practical when using ex-vivo animal-derived samples, thus reinforcing the importance of cell culture studies. This chapter highlights procedures utilizing human umbilical vein endothelial cells (HUVECs) as a model system to evaluate components of the intrinsic (prekallikrein (PK), factor XII (FXII), kininogen, and bradykinin (BK)) and extrinsic (Tissue Factor (TF)) systems. Specifically, protocols are included for the generation of a coculture blood vessel model, plating and infection of HUVEC monolayers and assays designed to measure activation of PK and FXII, cleavage of kininogen, and to measure the expression of TF mRNA and protein.

Identification and characterization of the plasma kallikrein-kinin system inhibitor, haemaphysalin, from hard tick, Haemaphysalis longicornis

The plasma kallikrein-kinin system inhibitor, haemaphysalin, from the hard tick, Haemaphysalis longicornis, was identified. It was found that haemaphysalin inhibited activation of the plasma kallikrein-kinin system by interfering with reciprocal activation between factor XII and prekallikrein. It did not, however, inhibit amidolytic activities of factor XIIa and kallikrein. Direct binding assay indicated that factor XII/XIIa and high molecular weight kininogen (HK) are the target molecules of haemaphysalin, and that Zn2+ ions are involved in the interactions of haemaphysalin with these target molecules. This suggests that haemaphysalin interacts with target molecules by recognizing their conformational changes induced by Zn2+ ions. Furthermore, haemaphysalin interacted with the fibronectin type II domain and domain D5, the cell binding domains of factor XII and HK, respectively. This finding suggests that haemaphysalin interferes with the association of factor XII and the prekallikrein-HK complex with a biologic activating surface by binding to these cell-binding domains, leading to inhibition of the reciprocal activation between factor XII and prekallikrein.

Plasma levels of bradykinin are suppressed in factor XII-deficient mice

A genetically-transmissible factor (F) XII-inactivated allele has been produced in mice by targeted replacement of exons 3–8 of the FXII gene with the neomycin resistance gene. Interbreeding of these mice provided offspring homozygous for two inactivated FXII alleles (FXII−/−). Male and female FXII-deficient mice bred normally in all genotypic combinations of the heterozygous and homozygous states, and the offspring survived to adulthood, suggesting that a total FXII deficiency does not affect embryonic development and survival. Neither FXII transcripts nor FXII antigen was found in various tissues of adult FXII−/−mice. No obvious unchallenged coagulopathies were present in FXII−/−adult mice, despite greatly prolonged activated partial thromboplastin times in this mouse cohort. FXII−/−mice were then used to assess the in vivo importance of the plasma FXII/prekallikrein/kininogen pathway in provision of resting plasma bradykinin (BK) levels and in generation of plasma BK stimulated by contact with an artificial surface, using a new and greatly improved plasma BK assay developed during these studies. It was found that approximately 50% of resting BK, and all of the contact-stimulated plasma BK, was provided by this FXII-dependent pathway, without a requirement for FXI. These results provide clear evidence that surface-stimulated BK production, in mice, is dependent on the activation of FXII.

Endothelial cells express a matrix protein which binds activated factor XII in a zinc-independent manner

Recent studies have shown that peptides identified as surface binding regions of high molecular mass kininogen (HK) and factor XII (FXII) inhibit the Zn2+-dependent binding of FXII to confluent layers of human umbilical vein endothelial cells (HUVEC). This indicates that negatively charged FXII binding surfaces, such as sulfatides and dextran sulfate, may interfere with the binding of FXII to confluent layer of HUVEC. Upon investigating this hypothesis it was unexpectedly found that sulfatides enhanced a specific binding of FXII to a matrix protein expressed during growth of the endothelial cells and that this binding was independent of the presence of Zn2+. The function of sulfatides was partly to minimize nonspecific electrostatic binding and partly to induce and enhance autoactivation of FXII generating αFXIIa. Western blot analysis of the extracts of the matrix incubated with FXII and sulfatides showed that the binding was specific for αFXIIa. The dissociation constant for binding αFXIIa was 12. 8 ± 0. 4 nM (n=4). The binding of αFXIIa to ECM was mapped to the heavy chain as no binding was observed of the light chain containing the catalytic domain. HK, which previously has been shown to completely abolish the Zn2+-dependent binding of FXII to confluent layers of HUVEC, did not inhibit the binding of αFXIIa to the matrix but sulfatides enhanced binding of FXII to ECM. This suggests that HK interferes with the binding of FXII to sulfatides and thereby the autoactivation of FXII. Trypsin treatment of the matrix protein completely abolished the binding, and fibronectin but not laminin was found to bea suitable target. The binding of activated FXII to the ECM suggests that FXIIa may bea modulator of cellular adhesion, migration and vascularization.

Alginate microbeads are coagulation compatible, while alginate microcapsules activate coagulation secondary to complement or directly through FXII

Alginate microspheres are presently under evaluation for future cell-based therapy. Their ability to induce harmful host reactions needs to be identified for developing the most suitable devices and efficient prevention strategies. We used a lepirudin based human whole blood model to investigate the coagulation potentials of alginate-based microspheres: alginate microbeads (Ca/Ba Beads), alginate poly-l-lysine microcapsules (APA and AP microcapsules) and sodium alginate-sodium cellulose sulfate-poly(methylene-co-cyanoguanidine) microcapsules (PMCG microcapsules). Coagulation activation measured by prothrombin fragments 1+2 (PTF1.2) was rapidly and markedly induced by the PMCG microcapsules, delayed and lower induced by the APA and AP microcapsules, and not induced by the Ca/Ba Beads. Monocytes tissue factor (TF) expression was similarly activated by the microcapsules, whereas not by the Ca/Ba Beads. PMCG microcapsules-induced PTF1.2 was abolished by FXII inhibition (corn trypsin inhibitor), thus pointing to activation through the contact pathway. PTF1.2 induced by the AP and APA microcapsules was inhibited by anti-TF antibody, pointing to a TF driven coagulation. The TF induced coagulation was inhibited by the complement inhibitors compstatin (C3 inhibition) and eculizumab (C5 inhibition), revealing a complement-coagulation cross-talk. This is the first study on the coagulation potentials of alginate microspheres, and identifies differences in activation potential, pathways and possible intervention points.

STATEMENT OF SIGNIFICANCE

Alginate microcapsules are prospective candidate materials for cell encapsulation therapy. The material surface must be free of host cell adhesion to ensure free diffusion of nutrition and oxygen to the encapsulated cells. Coagulation activation is one gateway to cellular overgrowth through deposition of fibrin. Herein we used a physiologically relevant whole blood model to investigate the coagulation potential of alginate microcapsules and microbeads. The coagulation potentials and the pathways of activation were depending on the surface properties of the materials. Activation of the complement system could also be involved, thus emphasizing a complement-coagulation cross-talk. Our findings points to complement and coagulation inhibition as intervention point for preventing host reactions, and enhance functional cell-encapsulation devices.

Surface dependent contact activation of factor XII and blood plasma coagulation induced by mixed thiol surfaces

Studies of the activation of FXII in both platelet poor plasma and in neat buffer solutions were undertaken for a series of mixed thiol self-assembled monolayers spanning a broad range of water wettability. A wide spectrum of carboxyl/methyl-, hydroxyl/methyl-, and amine/methyl-thiol modified surfaces were prepared, characterized, and then utilized as the procoagulant materials in a series of FXII activation studies. X-ray photoelectron spectroscopy was utilized to verify the sample surface's thiol composition and contact angles measured to determine the sample surface's wettability. These samples were then used in in vitro coagulation assays using a 50% mixture of recalcified plasma in phosphate buffered saline. Alternatively, the samples were placed into purified FXII solutions for 30 min to assess FXII activation in neat buffer solution. Plasma coagulation studies supported a strong role for anionic surfaces in contact activation, in line with the traditional models of coagulation, while the activation results in neat buffer solution demonstrated that FXIIa production is related to surface wettability with minimum levels of enzyme activation observed at midrange wettabilities, and no statistically distinguishable differences in FXII activation seen between highly wettable and highly nonwettable surfaces. Results demonstrated that the composition of the solution and the surface properties of the material all contribute to the observation of contact activation, and the activation of FXII is not specific to anionic surfaces as has been long believed.

Factor XI and factor XII as targets for new anticoagulants

Although the non-vitamin antagonist oral anticoagulants produce less intracranial bleeding than warfarin, serious bleeding still occurs. Therefore, the search for safer anticoagulants continues. Factor XII and factor XI have emerged as promising targets whose inhibition has the potential to prevent thrombosis with little or no disruption of hemostasis. Thus, thrombosis is attenuated in mice deficient in factor XII or factor XI and patients with congenital factor XII deficiency do not bleed and those with factor XI deficiency rarely have spontaneous bleeding. Strategies targeting factor XII and XI include antisense oligonucleotides to decrease their synthesis, inhibitory antibodies or aptamers, and small molecule inhibitors. These strategies attenuate thrombosis in various animal models and factor XI knockdown with an antisense oligonucleotide in patients undergoing knee replacement surgery reduced postoperative venous thromboembolism to a greater extent than enoxaparin without increasing bleeding. Therefore, current efforts are focused on evaluating the efficacy and safety of factor XII and factor XI directed anticoagulant strategies.

One-Stage Factor VIII Assays

Coagulation factor assays using one-stage methodology are widely used to measure factor levels for the purpose of detecting a deficiency, or to monitor replacement therapy. In this chapter, we focus on a method to measure Factor VIII coagulant activity (FVIII:C) by the one-stage assay (FVIII:C-1), with extra information provided to also allow this method to apply to Factor IX (FIX), Factor XI (FXI), and Factor XII (FXII). From the perspective of laboratory testing, these factors are components of the "intrinsic" coagulation pathway and are all measured in test systems based on the correction of the Activated Partial Thromboplastin Time (APTT). Factor activity is assessed by measuring the ability of an unknown sample to correct the prolonged APTT of factor-deficient plasma (deficient in the factor of interest), relative to the effect of a known calibrator. These assays are used for the diagnosis of the many causes of reduced factor levels, including those causing a prolonged APTT. It is important for laboratory staff to understand the impact of method variations, limitations, and result interpretation and these aspects are also discussed.

Factor XII: form determines function

Factor XII is a mysterious plasma protein without a clear physiologic function. It was identified as a clotting factor, but has no clear role in hemostasis. However, FXII also contributes to the production of bradykinin, a short-lived inflammatory peptide. A growing body of mechanistic research from animal models indicates that FXII contributes to thrombotic disease by triggering excessive coagulation. FXII is evolutionarily conserved, suggesting that this molecule does have a physiologic function. This leads to intriguing questions: What does FXII really do? Is it even a real clotting factor at all? Before the groundbreaking discovery of a role for FXII in thrombotic disease, many studies investigated the biochemical properties of FXII and its activators. In this review, we highlight several biochemical studies that reveal much about the natural behavior of FXII. On the basis of these findings, it is possible to draft a conceptual model to explain how FXII reacts to surface materials. We then discuss how this model applies to the activities of FXII in its natural environment. There are two tentative physiologic functions of FXII that can operate exclusively: (i) maintenance of thrombus stability; (ii) local regulation of vascular permeability. Either, or both, of these natural functions may explain the evolutionary development and maintenance of FXII.

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.

Molecular Characterization of a Novel Missense Mutation (Asp538Asn) in a Chinese Patient with Factor XII Deficiency

BACKGROUND

Congenital factor XII (FXH) deficiency is an autosomal recessive disorder whose genetic basis has been described in a relatively small number of cases.

METHODS

Recently, we studied a Chinese family in which the proband had obviously prolonged activated partial thromboplastin time (APTT) associated with low functional and antigen FXII levels, 5% and 6.8%, respectively. To investigate the molecular defects in this FXII-deficient patient, we performed FXII mutation screening and invitro expression studies.

RESULTS

Sequence analysis of the FXII gene revealed a heterozygous G>A transition at nucleotide 8597 in exon 13, causing a novel Asp538Asn mutation in the catalytic domain.

CONCLUSIONS

From the results above, we reasoned that this mutation must confer a cross-reacting material (CRM) negative phenotype. Additional expression studies in COS-7 cells showed that the antigen level of mutant FXII (FXII-Asp538Asn) was lower compared to the wild type in culture media, whereas the corresponding level of FXII antigen in cell lysates was equivalent roughly to that of the wild type. These findings indicated that the Asp538Asn mutation results in intracellular degradation of the mutant FXII and causes FXII deficiency.

The contact activation and kallikrein/kinin systems: pathophysiologic and physiologic activities

The contact activation system (CAS) and kallikrein/kinin system (KKS) are older recognized biochemical pathways that include several proteins that skirt the fringes of the blood coagulation, fibrinolytic, complement and renin-angiotensin fields. These proteins initially were proposed as part of the hemostatic pathways because their deficiencies are associated with prolonged clinical assays. However, the absence of bleeding states with deficiencies of factor XII (FXII), prekallikrein (PK) and high-molecular-weight kininogen indicates that the CAS and KKS do not contribute to hemostasis. Since the discovery of the Hageman factor 60 years ago much has been learned about the biochemistry, cell biology and animal physiology of these proteins. The CAS is a pathophysiologic surface defense mechanism against foreign proteins, organisms and artificial materials. The KKS is an inflammatory response mechanism. Targeting their activation through FXIIa or plasma kallikrein inhibition when blood interacts with the artificial surfaces of modern interventional medicine or in acute attacks of hereditary angioedema restores vascular homeostasis. FXII/FXIIa and products that arise with PK deficiency also offer novel ways to reduce arterial and venous thrombosis without an effect on hemostasis. In summary, there is revived interest in the CAS and KKS due to better understanding of their activities. The new appreciation of these systems will lead to several new therapies for a variety of medical disorders.

Coagulation factor XII genetic variation, ex vivo thrombin generation, and stroke risk in the elderly: results from the Cardiovascular Health Study

BACKGROUND

The relationships of thrombin generation (TG) with cardiovascular disease risk are underevaluated in population-based cohorts.

OBJECTIVES

To evaluate the relationships of TG influenced by the contact and tissue factor coagulation pathways ex vivo with common single-nucleotide polymorphisms (SNPs) and incident cardiovascular disease and stroke.

PATIENTS/METHODS

We measured peak TG (pTG) in baseline plasma samples of Cardiovascular Health Study participants (n = 5411), both with and without inhibitory anti-factor XIa antibody (pTG/FXIa(-) ). We evaluated their associations with ~ 50 000 SNPs by using the IBCv2 genotyping array, and with incident cardiovascular disease and stroke events over a median follow-up of 13.2 years.

RESULTS

The minor allele for an SNP in the FXII gene (F12), rs1801020, was associated with lower pTG in European-Americans (β = - 34.2 ± 3.5 nm; P = 3.3 × 10(-22) ; minor allele frequency [MAF] = 0.23) and African-Americans (β = - 31.1 ± 7.9 nm; P = 9.0 × 10(-5) ; MAF = 0.42). Lower FXIa-independent pTG (pTG/FXIa(-) ) was associated with the F12 rs1801020 minor allele, and higher pTG/FXIa(-) was associated with the ABO SNP rs657152 minor allele (β = 16.3 nm; P = 4.3 × 10(-9) ; MAF = 0.37). The risk factor-adjusted ischemic stroke hazard ratios were 1.09 (95% confidence interval CI 1.01-1.17; P = 0.03) for pTG, 1.06 (95% CI 0.98-1.15; P = 0.17) for pTG/FXIa(-) , and 1.11 (95% CI 1.02-1.21; P = 0.02) for FXIa-dependent pTG (pTG/FXIa(+) ), per one standard deviation increment (n = 834 ischemic strokes). In a multicohort candidate gene analysis, rs1801020 was not associated with incident ischemic stroke (β = - 0.02; standard error = 0.08; P = 0.81).

CONCLUSIONS

These results support the importance of contact activation pathway-dependent TG as a risk factor for ischemic stroke, and indicate the importance of F12 SNPs for TG ex vivo and in vivo.

Negatively charged silver nanoparticles cause retinal vascular permeability by activating plasma contact system and disrupting adherens junction

Silver nanoparticles (AgNPs) have been extensively used as antibacterial component in numerous healthcare, biomedical and consumer products. Therefore, their adverse effects to biological systems have become a major concern. AgNPs have been shown to be absorbed into circulation and redistributed into various organs. It is thus of great importance to understand how these nanoparticles affect vascular permeability and uncover the underlying molecular mechanisms. A negatively charged mecaptoundeonic acid-capped silver nanoparticle (MUA@AgNP) was investigated in this work. Ex vivo experiments in mouse plasma revealed that MUA@AgNPs caused plasma prekallikrein cleavage, while positively charged or neutral AgNPs, as well as Ag ions had no effect. In vitro tests revealed that MUA@AgNPs activated the plasma kallikrein-kinin system (KKS) by triggering Hageman factor autoactivation. By using specific inhibitors aprotinin and HOE 140, we demonstrated that KKS activation caused the release of bradykinin, which activated B2 receptors and induced the shedding of adherens junction protein, VE-cadherin. These biological perturbations eventually resulted in endothelial paracellular permeability in mouse retina after intravitreal injection of MUA@AgNPs. The findings from this work provided key insights for toxicity modulation and biomedical applications of AgNPs.

Factor XI and contact activation as targets for antithrombotic therapy

The most commonly used anticoagulants produce therapeutic antithrombotic effects either by inhibiting thrombin or factor Xa (FXa) or by lowering the plasma levels of the precursors of these key enzymes, prothrombin and FX. These drugs do not distinguish between thrombin generation contributing to thrombosis from thrombin generation required for hemostasis. Thus, anticoagulants increase bleeding risk, and many patients who would benefit from therapy go untreated because of comorbidities that place them at unacceptable risk for hemorrhage. Studies in animals demonstrate that components of the plasma contact activation system contribute to experimentally induced thrombosis, despite playing little or no role in hemostasis. Attention has focused on FXII, the zymogen of a protease (FXIIa) that initiates contact activation when blood is exposed to foreign surfaces, and FXI, the zymogen of the protease FXIa, which links contact activation to the thrombin generation mechanism. In the case of FXI, epidemiologic data indicate this protein contributes to stroke and venous thromboembolism, and perhaps myocardial infarction, in humans. A phase 2 trial showing that reduction of FXI may be more effective than low molecular weight heparin at preventing venous thrombosis during knee replacement surgery provides proof of concept for the premise that an antithrombotic effect can be uncoupled from an anticoagulant effect in humans by targeting components of contact activation. Here, we review data on the role of FXI and FXII in thrombosis and results of preclinical and human trials for therapies targeting these proteins.

Defective glycosylation of coagulation factor XII underlies hereditary angioedema type III

Hereditary angioedema type III (HAEIII) is a rare inherited swelling disorder that is associated with point mutations in the gene encoding the plasma protease factor XII (FXII). Here, we demonstrate that HAEIII-associated mutant FXII, derived either from HAEIII patients or recombinantly produced, is defective in mucin-type Thr309-linked glycosylation. Loss of glycosylation led to increased contact-mediated autoactivation of zymogen FXII, resulting in excessive activation of the bradykinin-forming kallikrein-kinin pathway. In contrast, both FXII-driven coagulation and the ability of C1-esterase inhibitor to bind and inhibit activated FXII were not affected by the mutation. Intravital laser-scanning microscopy revealed that, compared with control animals, both F12-/- mice reconstituted with recombinant mutant forms of FXII and humanized HAEIII mouse models with inducible liver-specific expression of Thr309Lys-mutated FXII exhibited increased contact-driven microvascular leakage. An FXII-neutralizing antibody abolished bradykinin generation in HAEIII patient plasma and blunted edema in HAEIII mice. Together, the results of this study characterize the mechanism of HAEIII and establish FXII inhibition as a potential therapeutic strategy to interfere with excessive vascular leakage in HAEIII and potentially alleviate edema due to other causes.

Coagulation factor XII protease domain crystal structure

BACKGROUND

Coagulation factor XII is a serine protease that is important for kinin generation and blood coagulation, cleaving the substrates plasma kallikrein and FXI.

OBJECTIVE

To investigate FXII zymogen activation and substrate recognition by determining the crystal structure of the FXII protease domain.

METHODS AND RESULTS

A series of recombinant FXII protease constructs were characterized by measurement of cleavage of chromogenic peptide and plasma kallikrein protein substrates. This revealed that the FXII protease construct spanning the light chain has unexpectedly weak proteolytic activity compared to β-FXIIa, which has an additional nine amino acid remnant of the heavy chain present. Consistent with these data, the crystal structure of the light chain protease reveals a zymogen conformation for active site residues Gly193 and Ser195, where the oxyanion hole is absent. The Asp194 side chain salt bridge to Arg73 constitutes an atypical conformation of the 70-loop. In one crystal form, the S1 pocket loops are partially flexible, which is typical of a zymogen. In a second crystal form of the deglycosylated light chain, the S1 pocket loops are ordered, and a short α-helix in the 180-loop of the structure results in an enlarged and distorted S1 pocket with a buried conformation of Asp189, which is critical for P1 Arg substrate recognition. The FXII structures define patches of negative charge surrounding the active site cleft that may be critical for interactions with inhibitors and substrates.

CONCLUSIONS

These data provide the first structural basis for understanding FXII substrate recognition and zymogen activation.

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.

[Modern methods of diagnosis of thrombophylic states and complex treatment of patients with thrombotic complications of severe forms of varicose disease]

Actual issues of surgical treatment of patients, suffering complications of severe forms of varicose disease of the lower extremities (VDLE) are discussed. The causes of unsatisfactory results of treatment in patients, suffering varicothrombophlebitis (VTHPH), the main of which--absence of the only one tactics for operative treatment and anticoagulant therapy, were analyzed. The results of patients examination, suffering thrombotic complications of severe forms of VDLE, while its recurrent course, in conjunction of VTHPH and thrombosis of deep veins of the lower extremities, using diagnostic complex "PLR genetics thrombophilia", are adduced. Differential tactics of treatment in patients, suffering severe forms of VDLE, while various localization of thrombotic process, concerning the presence of thrombophilic states, is proposed.

Lack of secondary microthrombosis after thrombin-induced stroke in mice and non-human primates

BACKGROUND

Secondary microthrombosis is a major pathophysiologic mechanism leading to brain damage following transient mechanical vascular occlusion (TMVO), the most widely used experimental stroke model. Whether secondary microthrombosis also occurs in non-TMVO stroke models represents an important issue for clinical translation of antimicrothrombosis therapeutic strategies.

OBJECTIVES

To assess the occurrence and the pathogenic role of secondary microthrombosis in two thrombin-induced stroke models in mice and non-human primates (Macaca mulatta).

METHODS

Stroke was induced in mice and non-human primates by intra-arterial administration of recombinant thrombin. This method induces the formation of a fibrin-rich thrombus, which is spontaneously dissolved in the following hours by the endogenous fibrinolytic system. Perfusion-weighted imaging and fluorescent-lectin microangiography were performed after recanalization to detect secondary microthrombosis. Moreover, to investigate its pathogenic role, thrombin-induced stroke was induced in bradykinin receptor B1 (B1R) knockout mice, which are protected from the thromboinflammation responsible for secondary microthrombosis in TMVO models.

RESULTS

Reperfusion was stable and complete in all mice and non-human primates tested, revealing no secondary decrease in cerebral blood flow. No evidence of secondary microthrombosis was found in the two models. Accordingly, deficiency in B1R did not protect the mice from brain damage after thrombin-induced stroke.

CONCLUSIONS

Our data demonstrate that secondary microthrombosis does not occur after thrombin-induced stroke. In view of this, the pathophysiologic roles of hematologic players promoting or protecting against secondary microthrombosis (such as factor XII, von Willebrand factor, and T cells) deserve to be re-evaluated in non-TMVO stroke models.

Factor XII activation markers do not reflect FXII dependence of thrombin generation induced by polyvinylchloride

The role of factor XII (FXII) as the main trigger of the coagulation cascade during haemodialysis has been recently challenged. Polyvinylchloride (PVC) is the standard polymer for haemodialysis circuit tubings, but its interaction with FXII has not been extensively characterized. In a modified Chandler tubing loop model using heparinized fresh human whole blood we selectively inhibited coagulation factors VII, X or XII with monospecific antibodies. Contact of whole blood with PVC induced a strong thrombin generation [thrombin-antithrombin complexes (TAT) 64 ± 24 μg/l, before <1 μg/l]. Despite this, levels of FXII coagulation activity, free FXIIa or FXIIa-C1 inhibitor complexes remained unchanged. The anti-FXII antibody abolished thrombin generation (TAT 8 ± 5 μg/l, P < 0.05) and made the free FXIIa undetectable. Inhibition of FVII did not affect coagulation activation (TAT 68 ± 26 μg/l). Our data provide definitive evidence that PVC triggers the coagulation system via FXII. However, all FXII activation markers in plasma failed to detect contact activation.

Factor XII promotes blood coagulation independent of factor XI in the presence of long-chain polyphosphates

BACKGROUND

Inorganic polyphosphates (polyP), which are secreted by activated platelets (short-chain polyP) and accumulate in some bacteria (long-chain polyP), support the contact activation of factor XII (FXII) and accelerate the activation of FXI.

OBJECTIVES

The aim of the present study was to evaluate the role of FXI in polyP-mediated coagulation activation and experimental thrombus formation.

METHODS AND RESULTS

Pretreatment of plasma with antibodies that selectively inhibit FXI activation by activated FXII (FXIIa) or FIX) activation by activated FXI (FXIa) were not able to inhibit the procoagulant effect of long or short-chain polyP in plasma. In contrast, the FXIIa inhibitor, corn trypsin inhibitor, blocked the procoagulant effect of long and short polyP in plasma. In a purified system, long polyP significantly enhanced the rate of FXII and prekallikrein activation and the activation of FXI by thrombin but not by FXIIa. In FXI-deficient plasma, long polyP promoted clotting of plasma in an FIX-dependent manner. In a purified system, the activation of FXII and prekallikrein by long polyP promoted FIX activation and prothombin activation. In an ex vivo model of occlusive thrombus formation, inhibition of FXIIa with corn trypsin inhibitor but not of FXI with a neutralizing antibodies abolished the prothrombotic effect of long polyP.

CONCLUSIONS

We propose that long polyP promotes FXII-mediated blood coagulation bypassing FXI. Accordingly, some polyp-containing pathogens may have evolved strategies to exploit polyP-initiated FXII activation for virulence, and selective inhibition of FXII may improve the host response to pathogens.