Factor XII: form determines function

Factor XII contact activation can be prevented by targeting 2 unique patches in its epidermal growth factor-like 1 domain with a nanobody

BACKGROUND

Factor (F)XII triggers contact activation by binding to foreign surfaces, with the epidermal growth factor-like 1 (EGF-1) domain being the primary binding site. Blocking FXII surface-binding might hold therapeutic value to prevent medical device-induced thrombosis.

OBJECTIVES

To unravel and prevent EGF-1-mediated FXII surface-binding with a variable domain of heavy chain-only antibody (VHH).

METHODS

FXII variants with glutamine substitutions of 2 positively charged amino acid patches within the EGF-1 domain were created. Their role in FXII contact activation was assessed using kaolin pull-down experiments, amidolytic activity assays, and clotting assays. FXII EGF-1 domain-specific VHHs were raised to inhibit EGF-1-mediated FXII contact activation while preserving quiescence.

RESULTS

Two unique, positively charged patches in the EGF-1 domain were identified (upstream, 73K74K76K78H81K82H; downstream, 87K113K). Neutralizing the charge of both patches led to a 99% reduction in FXII kaolin binding, subsequent decrease in autoactivation of 94%, and prolongation of clot formation in activated partial thromboplastin time assays from 36 (±2) to 223 (±13) seconds. Three FXII EGF-1-specific VHHs were developed that are capable of inhibiting kaolin binding and subsequent contact system activation in plasma. The most effective VHH "F2" binds the positively charged patches and thereby dose-dependently extends activated partial thromboplastin time clotting times from 29 (±2) to 43 (±3) seconds without disrupting FXII quiescence.

CONCLUSION

The 2 unique, positively charged patches in FXII EGF-1 cooperatively mediate FXII surface-binding, making both patches crucial for contact activation. Targeting these with FXII EGF-1-specific VHHs can exclusively decrease FXII surface-binding and subsequent contact activation, while preserving zymogen quiescence. These patches thus have potential as druggable targets in preventing medical device-induced thrombosis.

Steps Toward Recapitulating Endothelium: A Perspective on the Next Generation of Hemocompatible Coatings

Endothelium, the lining in this blood vessel, orchestrates three main critical functions such as protecting blood components, modulating of hemostasis by secreting various inhibitors, and directing clot digestion (fibrinolysis) by activating tissue plasminogen activator. No other surface can perform these tasks; thus, the contact of blood and blood-contacting medical devices inevitably leads to the activation of coagulation, often causing device failure, and thromboembolic complications. This perspective, first, discusses the biological mechanisms of activation of coagulation and highlights the efforts of advanced coatings to recapitulate one characteristic of endothelium, hereafter single functions of endothelium and noting necessity of the synergistic integration of its three main functions. Subsequently, it is emphasized that to overcome the challenges of blood compatibility an endothelium-mimicking system is needed, proposing a synergy of bottom-up synthetic biology, particularly synthetic cells, with passive- and bioactive surface coatings. Such integration holds promise for developing advanced biomaterials capable of recapitulating endothelial functions, thereby enhancing the hemocompatibility and performance of blood-contacting medical devices.

Mast cell degranulation and bradykinin-induced angioedema - searching for the missing link

Initiation of the bradykinin generation cascade is responsible for the occurrence of attacks in some types of angioedema without wheals. Hereditary angioedema due to C1 inhibitor deficiency (HAE-C1-INH) is one such clinical entity. In this paper, we explore the existing evidence that mast cells (MCs) degranulation may contribute to the activation of the kallikrein-kinin system cascade, followed by bradykinin formation and angioedema. We present the multidirectional effects of MC-derived heparin and other polyanions on the major components of the kinin-kallikrein system, particularly on the factor XII activation. Although, bradykinin- and histamine-mediated symptoms are distinct clinical phenomena, they share some common features, such as some similar triggers and a predilection to occur at sites where mast cells reside, namely the skin and mucous membranes. In addition, recent observations indicate a high incidence of hypersensitivity reactions associated with MC degranulation in the HAE-C1-INH patient population. However, not all of these can be explained by IgE-dependent mechanisms. Mast cell-related G protein-coupled receptor-X2 (MRGPRX2), which has recently attracted scientific interest, may be involved in the activation of MCs through a different pathway. Therefore, we reviewed MRGPRX2 ligands that HAE-C1-INH patients may be exposed to in their daily lives and that may affect MCs degranulation. We also discussed the known inter- and intra-individual variability in the course of HAE-C1-INH in relation to factors responsible for possible variability in the strength of the response to MRGPRX2 receptor stimulation. The above issues raise several questions for future research. It is not known to what extent a prophylactic or therapeutic intervention targeting the pathways of one mechanism (mast cell degranulation) may affect the other (bradykinin production), or whether the number of mast cells at a specific body site and their reactivity to triggers such as pressure, allergens or MRGPRX2 agonists may influence the occurrence of HAE-C1-INH attacks at that site.

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.

Increased contact activated endogenous thrombin potential in pregnant women with preeclampsia

Preeclampsia is a worldwide contributor to maternal and fetal morbidity and mortality. Women with preeclampsia are in a hyper-coagulable state with increased risk of thromboembolic disease later in life compared with normal pregnant women. The contact system (CAS) in plasma can mediate thrombin generation and is an important contributor to thrombus growth, but the activation of CAS during pregnancy complicated by preeclampsia is not yet elucidated, and CAS may play a role in the pathophysiology of preeclampsia. Therefore, the aim of the study is to address thrombin generation, and in particular, the capacity of the CAS-mediated pathway in patients with preeclampsia compared with pregnant controls. One hundred and seventeen women with preeclampsia and matched controls were included. The project was registered at www.clinicaltrials.gov as NCT04825145. CAS and tissue factor induced thrombin generation, proteins C and S, antithrombin, and histidine-rich glycoprotein (HRG) were assessed. Women with preeclampsia had significantly increased CAS and tissue factor-induced endogenous thrombin potential (ETP), and HRG compared with controls, P  = 0.022, P  = 0.024, and P  = 0.02, respectively. The concentrations of protein C and antithrombin were significantly reduced in the preeclampsia group, P  = 0.024 and P  < 0.0001, respectively. No significant difference in the concentration of protein S was detected, P  = 0.06. This study demonstrates a significant increased CAS-induced ETP and an overall decrease of important regulators of coagulation in women with preeclampsia compared with controls. These aspects can contribute to the hyper-coagulable state characterizing preeclampsia.

Whole-exome sequencing reveals a novel frameshift mutation in a consanguineous family with a hereditary coagulation factor XII deficiency

BACKGROUND AND AIMS

We aimed to elucidate a hereditary mutation of coagulation factor XII (FXII) in a consanguineous Chinese family.

METHODS

Mutations were investigated using Sanger and whole-exome sequencing. FXII (FXII:C) activity and FXII antigen (FXII:Ag) were assessed using clotting assays and ELISA, respectively. Gene variants were annotated and the likelihood that amino acid mutations would affect protein function was predicted using bioinformatics.

RESULTS

Activated partial thromboplastin time was prolonged to >170 s (reference range, 22.3-32.5 s), and FXII:C and FXII:Ag were decreased to 0.3% and 1%, respectively, (normal range for both, 72%-150%) in the proband. Sequencing revealed a homozygous frameshift mutation c.150delC (p.Phe51Serfs*44) site in the F12 gene exon 3. This mutation results in premature termination of the encoded protein translation and the protein is truncated. Bioinformatic findings indicated a novel pathogenic frameshift mutation.

CONCLUSION

The c.150delC frameshift mutation p.Phe51Serfs*44 in the F12 gene likely explains the low FXII level and the molecular pathogenesis of an inherited FXII deficiency in a consanguineous family.

Factor XII Structure-Function Relationships

Factor XII (FXII), the zymogen of the protease FXIIa, contributes to pathologic processes such as bradykinin-dependent angioedema and thrombosis through its capacity to convert the homologs prekallikrein and factor XI to the proteases plasma kallikrein and factor XIa. FXII activation and FXIIa activity are enhanced when the protein binds to a surface. Here, we review recent work on the structure and enzymology of FXII with an emphasis on how they relate to pathology. FXII is a homolog of pro-hepatocyte growth factor activator (pro-HGFA). We prepared a panel of FXII molecules in which individual domains were replaced with corresponding pro-HGFA domains and tested them in FXII activation and activity assays. When in fluid phase (not surface bound), FXII and prekallikrein undergo reciprocal activation. The FXII heavy chain restricts reciprocal activation, setting limits on the rate of this process. Pro-HGFA replacements for the FXII fibronectin type 2 or kringle domains markedly accelerate reciprocal activation, indicating disruption of the normal regulatory function of the heavy chain. Surface binding also enhances FXII activation and activity. This effect is lost if the FXII first epidermal growth factor (EGF1) domain is replaced with pro-HGFA EGF1. These results suggest that FXII circulates in blood in a "closed" form that is resistant to activation. Intramolecular interactions involving the fibronectin type 2 and kringle domains maintain the closed form. FXII binding to a surface through the EGF1 domain disrupts these interactions, resulting in an open conformation that facilitates FXII activation. These observations have implications for understanding FXII contributions to diseases such as hereditary angioedema and surface-triggered thrombosis, and for developing treatments for thrombo-inflammatory disorders.

A site on factor XII required for productive interactions with polyphosphate

BACKGROUND

During plasma contact activation, factor XII (FXII) binds to surfaces through its heavy chain and undergoes conversion to the protease FXIIa. FXIIa activates prekallikrein and factor XI (FXI). Recently, we showed that the FXII first epidermal growth factor-1 (EGF1) domain is required for normal activity when polyphosphate is used as a surface.

OBJECTIVES

The aim of this study was to identify amino acids in the FXII EGF1 domain required for polyphosphate-dependent FXII functions.

METHODS

FXII with alanine substitutions for basic residues in the EGF1 domain were expressed in HEK293 fibroblasts. Wild-type FXII (FXII-WT) and FXII containing the EGF1 domain from the related protein Pro-HGFA (FXII-EGF1) were positive and negative controls. Proteins were tested for their capacity to be activated, and to activate prekallikrein and FXI, with or without polyphosphate, and to replace FXII-WT in plasma clotting assays and a mouse thrombosis model.

RESULTS

FXII and all FXII variants were activated similarly by kallikrein in the absence of polyphosphate. However, FXII with alanine replacing Lys73, Lys74, and Lys76 (FXII-Ala73,74,76) or Lys76, His78, and Lys81 (FXII-Ala76,78,81) were activated poorly in the presence of polyphosphate. Both have <5% of normal FXII activity in silica-triggered plasma clotting assays and have reduced binding affinity for polyphosphate. Activated FXIIa-Ala73,74,76 displayed profound defects in surface-dependent FXI activation in purified and plasma systems. FXIIa-Ala73,74,76 reconstituted FXII-deficient mice poorly in an arterial thrombosis model.

CONCLUSION

FXII Lys73, Lys74, Lys76, and Lys81 form a binding site for polyanionic substances such as polyphosphate that is required for surface-dependent FXII function.

Mechanisms involved in hereditary angioedema with normal C1-inhibitor activity

Patients with the inherited disorder hereditary angioedema (HAE) suffer from episodes of soft tissue swelling due to excessive bradykinin production. In most cases, dysregulation of the plasma kallikrein-kinin system due to deficiency of plasma C1 inhibitor is the underlying cause. However, at least 10% of HAE patients have normal plasma C1 inhibitor activity levels, indicating their syndrome is the result of other causes. Two mutations in plasma protease zymogens that appear causative for HAE with normal C1 inhibitor activity have been identified in multiple families. Both appear to alter protease activity in a gain-of-function manner. Lysine or arginine substitutions for threonine 309 in factor XII introduces a new protease cleavage site that results in formation of a truncated factor XII protein (Δ-factor XII) that accelerates kallikrein-kinin system activity. A glutamic acid substitution for lysine 311 in the fibrinolytic protein plasminogen creates a consensus binding site for lysine/arginine side chains. The plasmin form of the variant plasminogen cleaves plasma kininogens to release bradykinin directly, bypassing the kallikrein-kinin system. Here we review work on the mechanisms of action of the FXII-Lys/Arg309 and Plasminogen-Glu311 variants, and discuss the clinical implications of these mechanisms.

Targeting the Contact Pathway of Coagulation for the Prevention and Management of Medical Device-Associated Thrombosis

Hemorrhage remains a major complication of anticoagulants, with bleeding leading to serious and even life-threatening outcomes in rare settings. Currently available anticoagulants target either multiple coagulation factors or specifically coagulation factor (F) Xa or thrombin; however, inhibiting these pathways universally impairs hemostasis. Bleeding complications are especially salient in the medically complex population who benefit from medical devices. Extracorporeal devices-such as extracorporeal membrane oxygenation, hemodialysis, and cardiac bypass-require anticoagulation for optimal use. Nonetheless, bleeding complications are common, and with certain devices, highly morbid. Likewise, pharmacologic prophylaxis to prevent thrombosis is not commonly used with many medical devices like central venous catheters due to high rates of bleeding. The contact pathway members FXI, FXII, and prekallikrein serve as a nexus, connecting biomaterial surface-mediated thrombin generation and inflammation, and may represent safe, druggable targets to improve medical device hemocompatibility and thrombogenicity. Recent in vivo and clinical data suggest that selectively targeting the contact pathway of coagulation through the inhibition of FXI and FXII can reduce the incidence of medical device-associated thrombotic events, and potentially systemic inflammation, without impairing hemostasis. In the following review, we will outline the current in vivo and clinical data encompassing the mechanism of action of drugs targeting the contact pathway. This new class of inhibitors has the potential to herald a new era of effective and low-risk anticoagulation for the management of patients requiring the use of medical devices.

Factor XII Explored with AlphaFold - Opportunities for Selective Drug Development

Medical device associated thrombosis is an important clinical problem. This type of thrombosis can result from Factor XII (FXII) binding to non-natural surface materials and subsequent activation of the contact pathway. This drives the development of new therapeutic strategies to block this pathway and information on the structural properties of FXII should catalyse this quest. Presently, there is no publicly available crystal structure of full-length FXII. However, the AlphaFold Protein Structure Database provides a model structure. We here explore this model in combination with previous structure-function studies to identify opportunities for selective pharmacological blockade of the contribution of FXII in medical device associated thrombosis. Previous studies demonstrated that FXII activation is dependent on molecular cleavage after R353. We subsequently proposed that protein conformation protects this cleavage site to ensure zymogen quiescence and prevent inappropriate FXII activation. The AlphaFold model shows that a small loop containing R353 indeed is buried in the globular molecule. This is the result of intra-molecular interactions between the (N-terminal) Fibronectin type II domain, (central) kringle and (C-terminal) protease domain, in a structure that resembles a three-point harness. Furthermore, this interaction pushes the intermediate domains, as well as the flexible proline-rich region (PRR), outward while encapsulating R353 in the molecule. The outward directed positively charged patches are likely to be involved in binding to anionic surfaces. The binding of FXII to surfaces (and several monoclonal antibodies) acccelerates its activation by inducing conformational changes. For prevention of medical device associated thrombosis, it is therefore important to target the surface binding sites of FXII without causing structural changes.

HDX-MS study on garadacimab binding to activated FXII reveals potential binding interfaces through differential solvent exposure

Hageman factor (FXII) is an essential component in the intrinsic coagulation cascade and a therapeutic target for the prophylactic treatment of hereditary angioedema (HAE). CSL312 (garadacimab) is a novel high-affinity human antibody capable of blocking activated FXII activity that is currently undergoing Phase 3 clinical trials in HAE. Structural studies using hydrogen/deuterium exchange coupled to mass spectrometry revealed evidence of interaction between the antibody and regions surrounding the S1 specificity pocket of FXII, including the 99-loop, 140-loop, 180-loop, and neighboring regions. We propose complementarity-determining regions (CDRs) in heavy-chain CDR2 and CDR3 as potential paratopes on garadacimab, and the 99-loop, 140-loop, 180-loop, and 220-loop as binding sites on the beta chain of activated FXII (β-FXIIa).

Triazol-1-yl Benzamides Promote Anticoagulant Activity via Inhibition of Factor XIIa

BACKGROUND

Human factor XIIa (FXIIa) is a plasma serine protease that plays a significant role in several physiological and pathological processes. Animal models have revealed an important contribution of FXIIa to thromboembolic diseases. Remarkably, animals and patients with FXII deficiency appear to have normal hemostasis. Thus, FXIIa inhibition may serve as a promising therapeutic strategy to attain safer and more effective anticoagulation. Very few small molecule inhibitors of FXIIa have been reported. We synthesized and investigated a focused library of triazol-1-yl benzamide derivatives for FXIIa inhibition.

METHODS

We chemically synthesized, characterized, and investigated a focused library of triazol- 1-yl benzamide derivatives for FXIIa inhibition. Using a standardized chromogenic substrate hydrolysis assay, the derivatives were evaluated for inhibiting human FXIIa. Their selectivity over other clotting factors was also evaluated using the corresponding substrate hydrolysis assays. The best inhibitor affinity to FXIIa was also determined using fluorescence spectroscopy. Effects on the clotting times (prothrombin time (PT) and activated partial thromboplastin time (APTT)) of human plasma were also studied.

RESULTS

We identified a specific derivative (1) as the most potent inhibitor in this series. The inhibitor exhibited nanomolar binding affinity to FXIIa. It also exhibited significant selectivity against several serine proteases. It also selectively doubled the activated partial thromboplastin time of human plasma.

CONCLUSION

Overall, this work puts forward inhibitor 1 as a potent and selective inhibitor of FXIIa for further development as an anticoagulant.

Depressed Kallikrein Generation in Women With Preeclampsia: A Matched Cross-Sectional Study

Objective

The pathophysiology of preeclampsia is not fully understood. Disturbances in the contact system are associated with preeclampsia. Few studies have investigated the association between preeclampsia and alterations in the contact system in plasma. This study aims to elucidate whether this basic biological system is affected in preeclampsia using new methods focusing on the dynamic interactions and total capacity of the contact system in blood.

Design

Cross-sectional study matching women with preeclampsia and controls without preeclampsia regarding age, pregestational body mass index, and gestational age at onset of the disease.

Setting

Two Danish University hospitals.

Sample

A cohort of 117 women with preeclampsia and 117 controls.

Methods

The turnover and capacity of the contact system were determined with new methods. Paired t-test, Wilcoxon signed-pairs signed rank test, Mann-Whitney or Chi²-test were applied, as appropriate.

Main Outcome Measurements

Kallikrein generation (peak kallikrein concentration and endogenous kallikrein potential), coagulation factor XII, prekallikrein, H-kininogen, cleaved H-kininogen, and complement C1 esterase inhibitor.

Results

The endogenous kallikrein potential, peak kallikrein concentration, prekallikrein and cleaved H-kininogen were significantly lower in women with preeclampsia compared to the controls, p ≤ 0.005, whereas the concentration of coagulation factor XII, H-kininogen and complement C1 esterase inhibitor was not significantly different, p > 0.05.

Conclusion

This study demonstrates significant reduction in kallikrein generating capacity, prekallikrein and cleaved H-kininogen indicating that the contact system is affected in preeclampsia suggesting a link to the pathophysiology of the disease.

Brush-Like Interface on Surface-Attached Hydrogels Repels Proteins and Bacteria

Interfacing artificial materials with biological tissues remains a challenge. The direct contact of their surface with the biological milieu results in multiscale interactions, in which biomacromolecules adsorb and act as transducers mediating the interactions with cells and tissues. So far, only antifouling polymer brushes have been able to conceal the surface of synthetic materials. However, their complex synthesis has precluded their translation to applications. Here, we show that ultra-thin surface-attached hydrogel coatings of N-(2-hydroxypropyl) methacrylamide (HPMA) and carboxybetaine methacrylamide (CBMAA) provided the same level of protection as brushes. In spite of being readily applicable, these coatings prevented the fouling from whole blood plasma and provided a barrier to the adhesion of Gram positive and negative bacteria. The analysis of the components of the surface free energy and nanoindentation experiments revealed that the excellent antifouling properties stem from the strong surface hydrophilicity and the presence of a brush-like structure at the water interface. Moreover, these coatings could be functionalized to achieve antimicrobial activity while remaining stealth and non-cytotoxic to eukaryotic cells. Such level of performance was previously only achieved with brushes. Thus, we anticipate that this readily applicable strategy is a promising route to enhance the biocompatibility of real biomedical devices. This article is protected by copyright. All rights reserved.

A Model for Surface-Dependent Factor XII Activation: The Roles of Factor XII Heavy Chain Domains

The FXII EGF1 domain promotes surface binding, FXII activation on surfaces, and FXIIa activation of prekallikrein on surfaces.

The FXII FN2 and KNG domains are part of a mechanism that restricts FXII activation in the absence of a surface.

Factor XII (FXII) is the zymogen of a plasma protease (FXIIa) that contributes to bradykinin generation by converting prekallikrein to the protease plasma kallikrein (PKa). FXII conversion to FXIIa by autocatalysis or PKa-mediated cleavage is enhanced when the protein binds to negatively charged surfaces such as polymeric orthophosphate. FXII is composed of noncatalytic (heavy chain) and catalytic (light chain) regions. The heavy chain promotes FXII surface-binding and surface-dependent activation but restricts activation when FXII is not surface bound. From the N terminus, the heavy chain contains fibronectin type 2 (FN2), epidermal growth factor-1 (EGF1), fibronectin type 1 (FN1), EGF2, and kringle (KNG) domains and a proline-rich region. It shares this organization with its homolog, pro–hepatocyte growth factor activator (Pro-HGFA). To study the importance of heavy chain domains in FXII function, we prepared FXII with replacements of each domain with corresponding Pro-HGFA domains and tested them in activation and activity assays. EGF1 is required for surface-dependent FXII autoactivation and surface-dependent prekallikrein activation by FXIIa. KNG and FN2 are important for limiting FXII activation in the absence of a surface by a process that may require interactions between a lysine/arginine binding site on KNG and basic residues elsewhere on FXII. This interaction is disrupted by the lysine analog ε-aminocaproic acid. A model is proposed in which an ε-aminocaproic acid–sensitive interaction between the KNG and FN2 domains maintains FXII in a conformation that restricts activation. Upon binding to a surface through EGF1, the KNG/FN2-dependent mechanism is inactivated, exposing the FXII activation cleavage site.

Assessment of the State of Platelet Haemostasis and Adhesive - Aggregation Properties of Platelets as a Factor of Increasing the Tendency to Thrombosis in Chronic Inflammation

In recent decades, considerable progress has been made in understanding the functional mechanisms of platelets and the correction of platelet haemostasis. Platelets are considered the most important participants in both the normal and pathological thrombotic processes characteristic of a variety of diseases and conditions. Alterations in various limbs of haemostasis are found in many somatic diseases (atherosclerosis, coronary heart disease, stroke), surgical procedures, oncological and immunological diseases. Inflammation underlies most diseases and remains an urgent problem in medicine. In the leukocyte infiltration of the inflammatory focus, the mechanism of its self-preservation is of great importance. The activation of haematopoiesis during inflammation is triggered by factors released by stimulated leukocytes of the focus and peripheral blood. Therefore, the problem of the state of the haemostasis system should be the focus of constant attention of clinicians, and with the help of laboratory monitoring of the state of the haemostasis system, it is possible to carry out drug correction of the haemocoagulation potential.

Factor XII protects neurons from apoptosis by epidermal and hepatocyte growth factor receptor-dependent mechanisms

BACKGROUND

Factor XII (FXII) is a serine protease that participates in the intrinsic coagulation pathway. Several studies have shown that plasma FXII exerts a deleterious role in cerebral ischemia and traumatic brain injury by promoting thrombo-inflammation. Nevertheless, the impact of FXII on neuronal cell fate remains unknown.

OBJECTIVES

We investigated the role of FXII and FXIIa in neuronal injury and apoptotic cell death.

METHODS

We tested the neuroprotective roles of FXII and FXIIa in an experimental model of neuronal injury induced by stereotaxic intracerebral injection of N-methyl-D-aspartic acid (NMDA) in vivo and in a model of apoptotic death of murine primary neuronal cultures through serum deprivation in vitro.

RESULTS

Here, we found that exogenous FXII and FXIIa reduce brain lesions induced by NMDA injection in vivo. Furthermore, FXII protects cultured neurons from apoptosis through a growth factor--like effect. This mechanism was triggered by direct interaction with epidermal growth factor (EGF) receptor and subsequent activation of this receptor. Interestingly, the "proteolytically" active and two-chain form of FXII, FXIIa, exerts its protective effects by an alternative signaling pathway. FXIIa activates the pro-form of hepatocyte growth factor (HGF) into HGF, which in turn activated the HGF receptor (HGFR) pathway.

CONCLUSION

This study describes two novel mechanisms of action of FXII and identifies neurons as target cells for the protective effects of single and two-chain forms of FXII. Therefore, inhibition of FXII in neurological disorders may have deleterious effects by preventing its beneficial effects on neuronal survival.

Unraveling the Mechanism and Kinetics of Binding of an LCI-eGFP-Polymer for Antifouling Coatings

The ability of proteins to adsorb irreversibly onto surfaces opens new possibilities to functionalize biological interfaces. Herein, the mechanism and kinetics of adsorption of protein-polymer macromolecules with the ability to equip surfaces with antifouling properties are investigated. These macromolecules consist of the liquid chromatography peak I peptide from which antifouling polymer brushes are grafted using single electron transfer-living radical polymerization. Surface plasmon resonance spectroscopy reveals an adsorption mechanism that follows a Langmuir-type of binding with a strong binding affinity to gold. X-ray reflectivity supports this by proving that the binding occurs exclusively by the peptide. However, the lateral organization at the surface is directed by the cylindrical eGFP. The antifouling functionality of the unimolecular coatings is confirmed by contact with blood plasma. All coatings reduce the fouling from blood plasma by 8894% with only minor effect of the degree of polymerization for the studied range (DP between 101 and 932). The excellent antifouling properties, combined with the ease of polymerization and the straightforward coating procedure make this a very promising antifouling concept for a multiplicity of applications.

Improving Hemocompatibility: How Can Smart Surfaces Direct Blood To Fight against Thrombi

Nature utilizes endothelium as a blood interface that perfectly controls hemostasis, preventing the uncontrolled formation of thrombi. The management of positive and negative feedback that finely tunes thrombosis and fibrinolysis is essential for human life, especially for patients who undergo extracorporeal circulation (ECC) after a severe respiratory or cardiac failure. The exposure of blood to a surface different from healthy endothelium inevitably initiates coagulation, drastically increasing the mortality rate by thromboembolic complications. In the present study, an ultrathin antifouling fibrinolytic coating capable of disintegrating thrombi in a self-regulated manner is reported. The coating system is composed of a polymer brush layer that can prevent any unspecific interaction with blood. The brushes are functionalized with a tissue plasminogen activator (tPA) to establish localized fibrinolysis that solely and exclusively is active when it is required. This interactive switching between the dormant and active state is realized through an amplification mechanism that increases (positive feedback) or restores (negative feedback) the activity of tPA depending on whether a thrombus is detected and captured or not. Thus, only a low surface density of tPA is necessary to lyse real thrombi. Our work demonstrates the first report of a coating that self-regulates its fibrinolytic activity depending on the conditions of blood.

Coagulation, inflammation, and CD46 transgene expression in neonatal porcine islet xenotransplantation

BACKGROUND

Thrombosis is a known consequence of intraportal islet transplantation, particularly for xenogeneic islets. To define the origins of thrombosis after islet xenotransplantation and relate it to early inflammation, we examined porcine islets transplanted into non-human primates using a dual-transplant model to directly compare islet characteristics.

METHODS

α1,3-Galactosyltransferase gene-knockout (GTKO) islets with and without expression of the human complement regulatory transgene CD46 (hCD46) were studied. Biologically inert polyethylene microspheres were used to examine the generic pro-thrombotic effects of particle embolization. Immunohistochemistry was performed 1 and 24 hours after transplantation.

RESULTS

Xeno-islet transplantation activated both extrinsic and intrinsic coagulation pathways. The intrinsic pathway was also initiated by microsphere embolization, while extrinsic pathway tissue factor (TF) and platelet aggregation were more specific to engrafted islets. hCD46 expression significantly reduced TF, platelet, fibrin, and factor XIIIa accumulation in and around islets but did not alter intrinsic factor activation. Layers of TF⁺ cells emerged around islets within 24 hours, particularly co-localized with vimentin, and identified as CD3+ and CD68+ cells inflammatory cells.

CONCLUSIONS

These findings detail the origins of thrombosis following islet xenotransplantation, relate it to early immune activation, and suggest a role for transgenic hCD46 expression in its mitigation. Layers of TF-positive inflammatory cells and fibroblasts around islets at 24 hours may have important roles in the progressive events of thrombosis, inflammatory cell recruitment, rejection, and the ultimate outcome of transplanted grafts. These suggest that the strategies targeting these elements could yield more progress toward successful xenogeneic islet engraftment and survival.

The Role of the Plasminogen Activation System in Angioedema: Novel Insights on the Pathogenesis

The main physiological functions of plasmin, the active form of its proenzyme plasminogen, are blood clot fibrinolysis and restoration of normal blood flow. The plasminogen activation (PA) system includes urokinase-type plasminogen activator (uPA), tissue-type PA (tPA), and two types of plasminogen activator inhibitors (PAI-1 and PAI-2). In addition to the regulation of fibrinolysis, the PA system plays an important role in other biological processes, which include degradation of extracellular matrix such as embryogenesis, cell migration, tissue remodeling, wound healing, angiogenesis, inflammation, and immune response. Recently, the link between PA system and angioedema has been a subject of scientific debate. Angioedema is defined as localized and self-limiting edema of subcutaneous and submucosal tissues, mediated by bradykinin and mast cell mediators. Different forms of angioedema are linked to uncontrolled activation of coagulation and fibrinolysis systems. Moreover, plasmin itself can induce a potentiation of bradykinin production with consequent swelling episodes. The number of studies investigating the PA system involvement in angioedema has grown in recent years, highlighting its relevance in etiopathogenesis. In this review, we present the components and diverse functions of the PA system in physiology and its importance in angioedema pathogenesis.

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.

The evolution of factor XI and the kallikrein-kinin system

Factor XI (FXI) is the zymogen of a plasma protease (FXIa) that contributes to hemostasis by activating factor IX (FIX). In the original cascade model of coagulation, FXI is converted to FXIa by factor XIIa (FXIIa), a component, along with prekallikrein and high-molecular-weight kininogen (HK), of the plasma kallikrein-kinin system (KKS). More recent coagulation models emphasize thrombin as a FXI activator, bypassing the need for FXIIa and the KKS. We took an evolutionary approach to better understand the relationship of FXI to the KKS and thrombin generation. BLAST searches were conducted for FXI, FXII, prekallikrein, and HK using genomes for multiple vertebrate species. The analysis shows the KKS appeared in lobe-finned fish, the ancestors of all land vertebrates. FXI arose later from a duplication of the prekallikrein gene early in mammalian evolution. Features of FXI that facilitate efficient FIX activation are present in all living mammals, including primitive egg-laying monotremes, and may represent enhancement of FIX-activating activity inherent in prekallikrein. FXI activation by thrombin is a more recent acquisition, appearing in placental mammals. These findings suggest FXI activation by FXIIa may be more important to hemostasis in primitive mammals than in placental mammals. FXI activation by thrombin places FXI partially under control of the vitamin K-dependent coagulation mechanism, reducing the importance of the KKS in blood coagulation. This would explain why humans with FXI deficiency have a bleeding abnormality, whereas those lacking components of the KKS do not.

The FXII c.-4T>C Polymorphism as a Disease Modifier in Patients With Hereditary Angioedema Due to the FXII p.Thr328Lys Variant

Background

Hereditary angioedema due to the Thr328Lys variant in the coagulation factor XII (HAE-FXII) affects mainly women in whom the symptomatology is dependent on high estrogen levels. Clinical variability and incomplete penetrance are challenging features that hinder the diagnosis and management of HAE-FXII. The c.-4T>C Kozak polymorphism is the only common variation accounting for FXII plasma levels and was previously shown to modify the course of HAE due to C1-Inhibitor deficiency.

Objectives

To assess the influence of the c.-4T>C polymorphism on disease expression in 39 Spanish HAE-FXII index patients.

Methods

The c.-4T>C polymorphism was sequenced by the standard Sanger method, and HAE severity was calculated according to the score by Cumming et al. (2003) The activation of the contact system was quantified by the kallikrein-like activity of plasma in chromogenic assays upon activation with high-molecular-weight dextran sulfate.

Results

The c.-4CC genotype was overrepresented in the studied cohort: 82% were CC-homozygous (expected frequency = 59%) and 18% were CT-heterozygous (expected frequency = 39%) (p = 0.001). Patients with a c.-4CC genotype exhibited higher kallikrein-like activity (0.9659 ± 0.1136) than those with a c.-4TC genotype (0.7645 ± 0.1235) (p = 0.024) or healthy donors. Moreover, the polymorphism influenced HAE-FXII severity score (c.-4CC = 4.43 ± 2.28 vs c.-4TC = 2.0 ± 1.15; p = 0.006) but not the degree of estrogen dependence or time until remission.

Conclusion

The c.-4T>C polymorphism is overrepresented in a Spanish HAE-FXII cohort and significantly influences the degree of contact system activation and the clinical severity of the disease.

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.

A mutation in the kringle domain of human factor XII that causes autoinflammation, disturbs zymogen quiescence, and accelerates activation

Coagulation factor XII (FXII) drives production of the inflammatory peptide bradykinin. Pathological mutations in the F12 gene, which encodes FXII, provoke acute tissue swelling in hereditary angioedema (HAE). Interestingly, a recently identified F12 mutation, causing a W268R substitution, is not associated with HAE. Instead, FXII-W268R carriers experience cold-inducible urticarial rash, arthralgia, fever, and fatigue. Here, we aimed to investigate the molecular characteristics of the FXII-W268R variant. We expressed wild type FXII (FXII-WT), FXII-W268R, and FXII-T309R (which causes HAE), as well as other FXII variants in HEK293 freestyle cells. Using chromogenic substrate assays, immunoblotting, and ELISA, we analyzed expression media, cell lysates, and purified proteins for FXII activation. Recombinant FXII-W268R forms increased amounts of intracellular cleavage products that are also present in expression medium and display enzymatic activity. The active site-incapacitated variant FXII-W268R/S544A reveals that intracellular fragmentation is largely dependent on autoactivation. Purified FXII-W268R is highly sensitive to activation by plasma kallikrein and plasmin, compared with FXII-WT or FXII-T309R. Furthermore, binding studies indicated that the FXII-W268R variant leads to the exposure of a plasminogen-binding site that is cryptic in FXII-WT. In plasma, recombinant FXII-W268R spontaneously triggers high-molecular-weight kininogen cleavage. Our findings suggest that the W268R substitution influences FXII protein conformation and exposure of the activation loop, which is concealed in FXII-WT. This results in intracellular autoactivation and constitutive low-grade secretion of activated FXII. These findings help to explain the chronically increased contact activation in carriers of the FXII-W268R variant.

Plasminflammation-An Emerging Pathway to Bradykinin Production

Plasminogen activation is essential for fibrinolysis—the breakdown of fibrin polymers in blood clots. Besides this important function, plasminogen activation participates in a wide variety of inflammatory conditions. One of these conditions is hereditary angioedema (HAE), a rare disease with characteristic attacks of aggressive tissue swelling due to unregulated production and activity of the inflammatory mediator bradykinin. Plasmin was already implicated in this disease decades ago, but a series of recent discoveries have made it clear that plasmin actively contributes to this pathology. Collective evidence points toward an axis in which the plasminogen activation system and the contact system (which produces bradykinin) are mechanistically coupled. This is amongst others supported by findings in subtypes of HAE that are caused by gain-of-function mutations in the genes that respectively encode factor XII or plasminogen, as well as clinical experience with the antifibrinolytic agents in HAE. The concept of a link between plasminogen activation and the contact system helps us to explain the inflammatory side effects of fibrinolytic therapy, presenting as angioedema or tissue edema. Furthermore, these observations motivate the development and characterization of therapeutic agents that disconnect plasminogen activation from bradykinin production.

Driving towards Precision Medicine for angioedema without wheals

Evidence accumulated over the last two decades indicates that recurrent angioedema without wheals constitutes a diverse family of disorders with a much higher complexity than was previously regarded. Indicatively, during the last two years, novel variants of three genes other than SERPING1 and F12 have been identified in association with hereditary angioedema. Most interestingly, functional studies of at least one of these variants (the variant c.807G > T of ANGPT1 gene) imply the existence of a new disease endotype in which the altered bradykinin metabolism and function does not play a central role. Therefore, using conventional approaches, it seems that the complexity of this disease cannot be sufficiently elucidated and any attempt to interrelate its many diverse aspects seems unrealistic. Similar to other rare and chronic diseases, a Precision Medicine approach, discovering the right target and giving "the right drug, for the right patient, at the right time, every time" seems the optimal future practice. Herein, we review recent data challenging and dictating the need for a switch of angioedema research into high-throughput approaches and we present the expected advantages for better understanding of the disease and patients management.

Factor XII deficiency is common in domestic cats and associated with two high frequency F12 mutations

Factor XII (FXII) is a coagulation protein that initiates surface-activation of the coagulation cascade in vitro. The protein's in vivo role, however, remains poorly defined. Factor XII deficiency, or Hageman trait, is a rare hereditary disorder that is not associated with bleeding, and wide variations in FXII activity (FXII:C) exist among healthy people. While FXII-deficient knockout mice appear to be resistant to arterial thrombosis, human F12 polymorphisms that influence FXII:C have not been associated with thrombotic risk in population surveys. Factor XII deficiency is a naturally occurring hereditary trait in domestic cats. We undertook phenotypic and genotypic analyses of FXII-deficient cats for comparative studies with the human disease counterpart. A retrospective review of feline submissions to our laboratory revealed that FXII deficiency is common in domestic cats, and also present in many different breeds. The trait has a geographic bias toward the Midwestern United States. Clinical history, coagulation assays, and samples for F12 sequencing were obtained from 26 FXII deficient cats. None of the cats had experienced abnormal bleeding and their residual FXII:C was related to F12 mutation number and mutation-type. We found 2 high frequency F12 mutations: an exon 13 missense mutation (c.1631G > C) and an exon 11 deletion mutation (c.1321delC), and additional sequence variants throughout the gene. Factor XII deficiency in pet cat populations provides an animal model system to help clarify the biologic actions and clinical relevance of FXII protein.

The contact system at the crossroads of various key patho- physiological functions: Update on present understanding, laboratory exploration and future perspectives

The contact system initiates the intrinsic pathway of coagulation and is started by Factor XII activation, which then activates prekallicrein to kallicrein and Factor XI to Factor XIa and, in the presence of high molecular weight kininogen, forms a "contact phase activation loop", that amplifies Factor XII activation. FXII deficiency is not associated with bleeding tendencies, but when the blood clots, the thrombus is less dense, thus favoring antithrombotic protection. Activated Factor XII inhibition emerges as an efficient target for preventing thrombo-embolic diseases without inducing a hemorrhagic risk. Activated Factor XII exhibits other activities, in that it can activate complement and provoke inflammation, contributing to innate immunity. It also stimulates fibrinolysis through uPA activation from scu-PA. Among the other components of the contact phase, Factor XI has a more important role in coagulation pathways and can directly activate FX, FVIII and FV, in a FIX independent pathway. Its deficiency is associated with a mild bleeding diathesis ("pseudo-hemophilia" or hemophilia C), with a variable incidence among kindreds. Recently, the occurrence of thrombotic events the same day following infusion of immunoglobulin concentrates has been demonstrated to be caused by the presence of trace amounts of activated Factor XI, pointing out the key role of this factor for thrombogenicity. Prekallicrein can be activated at the endothelial surface in the presence of high molecular weight kininogen, whose cleavage generates bradykinins and contributes to vessel tonicity and inflammation. The contact phase, through its activation loop, is then an important physiological system, which can initiate and regulate various biological functions and is at the crossroads of various biological activities. Many of the body's physiological functions are intimately linked between them, making the global approach of special usefulness for understanding the interactions which can result from any abnormality of one of them. New pharmaceutical drugs targeting a defined activity need to be investigated for all the possible interferences or side effects. In this article we aim to present and summarize the present understanding of contact phase system activation and regulation, its involvement in various physiological functions, and the laboratory tools for its exploration.

A mechanism for hereditary angioedema with normal C1 inhibitor: an inhibitory regulatory role for the factor XII heavy chain

The plasma proteins factor XII (FXII) and prekallikrein (PK) undergo reciprocal activation to the proteases FXIIa and kallikrein by a process that is enhanced by surfaces (contact activation) and regulated by the serpin C1 inhibitor. Kallikrein cleaves high-molecular-weight kininogen (HK), releasing the vasoactive peptide bradykinin. Patients with hereditary angioedema (HAE) experience episodes of soft tissue swelling as a consequence of unregulated kallikrein activity or increased prekallikrein activation. Although most HAE cases are caused by reduced plasma C1-inhibitor activity, HAE has been linked to lysine/arginine substitutions for Thr³⁰⁹ in FXII (FXII-Lys/Arg309). Here, we show that FXII-Lys/Arg309 is susceptible to cleavage after residue 309 by coagulation proteases (thrombin and FXIa), resulting in generation of a truncated form of FXII (δFXII). The catalytic efficiency of δFXII activation by kallikrein is 15-fold greater than for full-length FXII. The enhanced rate of reciprocal activation of PK and δFXII in human plasma and in mice appears to overwhelm the normal inhibitory function of C1 inhibitor, leading to increased HK cleavage. In mice given human FXII-Lys/Arg309, induction of thrombin generation by infusion of tissue factor results in enhanced HK cleavage as a consequence of δFXII formation. The effects of δFXII in vitro and in vivo are reproduced when wild-type FXII is bound by an antibody to the FXII heavy chain (HC; 15H8). The results contribute to our understanding of the predisposition of patients carrying FXII-Lys/Arg309 to angioedema after trauma, and reveal a regulatory function for the FXII HC that normally limits PK activation in plasma.

Virulence Associated Gene 8 of Bordetella pertussis Enhances Contact System Activity by Inhibiting the Regulatory Function of Complement Regulator C1 Inhibitor

Bordetella pertussis is a Gram-negative bacterium and the causative agent of whooping cough. Whooping cough is currently re-emerging worldwide and, therefore, still poses a continuous global health threat. B. pertussis expresses several virulence factors that play a role in evading the human immune response. One of these virulence factors is virulence associated gene 8 (Vag8). Vag8 is a complement evasion molecule that mediates its effects by binding to the complement regulator C1 inhibitor (C1-INH). This regulatory protein is a fluid phase serine protease that controls proenzyme activation and enzyme activity of not only the complement system but also the contact system. Activation of the contact system results in the generation of bradykinin, a pro-inflammatory peptide. Here, the activation of the contact system by B. pertussis was explored. We demonstrate that recombinant as well as endogenous Vag8 enhanced contact system activity by binding C1-INH and attenuating its inhibitory function. Moreover, we show that B. pertussis itself is able to activate the contact system. This activation was dependent on Vag8 production as a Vag8 knockout B. pertussis strain was unable to activate the contact system. These findings show a previously overlooked interaction between the contact system and the respiratory pathogen B. pertussis. Activation of the contact system by B. pertussis may contribute to its pathogenicity and virulence.

Single-chain factor XII: a new form of activated factor XII

PURPOSE OF REVIEW

Exposure of blood to foreign surfaces induces reciprocal conversion of the plasma proteins factor XII (fXII) and plasma prekallikrein (PPK) to the proteases α-fXIIa and α-kallikrein. This process, called contact activation, has a range of effects on host defence mechanisms, including promoting coagulation. The nature of the triggering mechanism for contact activation is debated. One hypothesis predicts that fXII has protease activity, either intrinsically or upon surface-binding, that initiates contact activation. We tested this by assessing the proteolytic activity of a recombinant fXII variant that cannot be converted to α-fXIIa.

RECENT FINDINGS

The proteolytic activity of fXII-T (for 'triple' mutant), a variant with alanine substitutions for arginine at activation cleavage sites (Arg334, Arg344, and Arg353) was tested with known α-fXIIa substrates. FXII-T activates PPK in solution, and the reaction is enhanced by polyphosphate, an inducer of contact activation released from platelets. In the presence of polyphosphate, fXII-T converts fXII to α-fXIIa, and also converts the coagulation protein factor XI to its active form.

SUMMARY

The findings support the hypothesis that contact activation is initiated through activity intrinsic to single-chain fXII, and indicate that preexisting α-fXIIa is not required for induction of contact activation.

Proteomic analysis of silica hybrid sol-gel coatings: a potential tool for predicting the biocompatibility of implants in vivo

The interactions of implanted biomaterials with the host organism determine the success or failure of an implantation. Normally, their biocompatibility is assessed using in vitro tests. Unfortunately, in vitro and in vivo results are not always concordant; new, effective methods of biomaterial characterisation are urgently needed to predict the in vivo outcome. As the first layer of proteins adsorbed onto the biomaterial surfaces might condition the host response, mass spectrometry analysis was performed to characterise these proteins. Four distinct hybrid sol-gel biomaterials were tested. The in vitro results were similar for all the materials examined here. However, in vivo, the materials behaved differently. Six of the 171 adsorbed proteins were significantly more abundant on the materials with weak biocompatibility; these proteins are associated with the complement pathway. Thus, protein analysis might be a suitable tool to predict the in vivo outcomes of implantations using newly formulated biomaterials.

FXII promotes proteolytic processing of the LRP1 ectodomain

BACKGROUND

Factor XII (FXII) is a serine protease that is involved in activation of the intrinsic blood coagulation, the kallikrein-kinin system and the complement cascade. Although the binding of FXII to the cell surface has been demonstrated, the consequence of this event for proteolytic processing of membrane-anchored proteins has never been described.

METHODS

The effect of FXII on the proteolytic processing of the low-density lipoprotein receptor-related protein 1 (LRP1) ectodomain was tested in human primary lung fibroblasts (hLF), alveolar macrophages (hAM) and in human precision cut lung slices (hPCLS). The identity of generated LRP1 fragments was confirmed by MALDI-TOF-MS. Activity of FXII and gelatinases was measured by S-2302 hydrolysis and zymography, respectively.

RESULTS

Here, we demonstrate a new function of FXII, namely its ability to process LRP1 extracellular domain. Incubation of hLF, hAM, or hPCLS with FXII resulted in the accumulation of LRP1 ectodomain fragments in conditioned media. This effect was independent of metalloproteases and required FXII proteolytic activity. Binding of FXII to hLF surface induced its conversion to FXIIa and protected FXIIa against inactivation by a broad spectrum of serine protease inhibitors. Preincubation of hLF with collagenase I impaired FXII activation and, in consequence, LRP1 cleavage. FXII-triggered LRP1 processing was associated with the accumulation of gelatinases (MMP-2 and MMP-9) in conditioned media.

CONCLUSIONS

FXII controls LRP1 levels and function at the plasma membrane by modulating processing of its ectodomain.

GENERAL SIGNIFICANCE

FXII-dependent proteolytic processing of LRP1 may exacerbate extracellular proteolysis and thus promote pathological tissue remodeling.

Proteolytic properties of single-chain factor XII: a mechanism for triggering contact activation

When blood is exposed to variety of artificial surfaces and biologic substances, the plasma proteins factor XII (FXII) and prekallikrein undergo reciprocal proteolytic conversion to the proteases αFXIIa and α-kallikrein by a process called contact activation. These enzymes contribute to host-defense responses including coagulation, inflammation, and fibrinolysis. The initiating event in contact activation is debated. To test the hypothesis that single-chain FXII expresses activity that could initiate contact activation, we prepared human FXII variants lacking the Arg353 cleavage site required for conversion to αFXIIa (FXII-R353A), or lacking the 3 known cleavage sites at Arg334, Arg343, and Arg353 (FXII-T, for "triple" mutant), and compared their properties to wild-type αFXIIa. In the absence of a surface, FXII-R353A and FXII-T activate prekallikrein and cleave the tripeptide S-2302, demonstrating proteolytic activity. The activity is several orders of magnitude weaker than that of αFXIIa. Polyphosphate, an inducer of contact activation, enhances PK activation by FXII-T, and facilitates FXII-T activation of FXII and FXI. In plasma, FXII-T and FXII-R353A, but not FXII lacking the active site serine residue (FXII-S544A), shortened the clotting time of FXII-deficient plasma and enhanced thrombin generation in a surface-dependent manner. The effect was not as strong as for wild-type FXII. Our results support a model for induction of contact activation in which activity intrinsic to single-chain FXII initiates αFXIIa and α-kallikrein formation on a surface. αFXIIa, with support from α-kallikrein, subsequently accelerates contact activation and is responsible for the full procoagulant activity of FXII.

Polyphosphate nanoparticles on the platelet surface trigger contact system activation

Polyphosphate is an inorganic polymer that can potentiate several interactions in the blood coagulation system. Blood platelets contain polyphosphate, and the secretion of platelet-derived polyphosphate has been associated with increased thrombus formation and activation of coagulation factor XII. However, the small polymer size of secreted platelet polyphosphate limits its capacity to activate factor XII in vitro. Thus, the mechanism by which platelet polyphosphate contributes to thrombus formation remains unclear. Using live-cell imaging, confocal and electron microscopy, we show that activated platelets retain polyphosphate on their cell surface. The apparent polymer size of membrane-associated polyphosphate largely exceeds that of secreted polyphosphate. Ultracentrifugation fractionation experiments revealed that membrane-associated platelet polyphosphate is condensed into insoluble spherical nanoparticles with divalent metal ions. In contrast to soluble polyphosphate, membrane-associated polyphosphate nanoparticles potently activate factor XII. Our findings identify membrane-associated polyphosphate in a nanoparticle state on the surface of activated platelets. We propose that these polyphosphate nanoparticles mechanistically link the procoagulant activity of platelets with the activation of coagulation factor XII.

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.

Processing of Factor XII during Inflammatory Reactions

The contact system was originally identified as an obsolete part of the coagulation system, but it has been repeatedly implicated in inflammatory states, such as infection, as well as in allergic- and chronic inflammatory disease. Under these conditions, there is surprisingly little evidence that factor XII (FXII) acts as a coagulation factor, and its activity appears to be mainly directed toward activation of the kallikrein–kinin system. The contact system factors interact with pathogens as well as cells of the (innate) immune system on several levels. Among others, these cells may provide negatively charged surfaces that contribute to contact activation as well as release enzymes that feed into this system. Furthermore, cellular receptors have been identified that bind contact factors at sites of inflammation. Based on the accumulated evidence, we propose a model for enzymatic crosstalk between inflammatory cells and the plasma contact system. During these reactions, FXII is enzymatically cleaved by non-contact system enzymes. This generates unactivated FXII fragments that can subsequently be rapidly activated in the fluid phase. The resulting enzyme lacks procoagulant properties, but retains its pro-inflammatory characteristic as a prekallikrein activator.