Factor seven activating protease (FSAP): does it activate factor VII?

Factor VII Activating Protease (FSAP) and Its Importance in Hemostasis—Part I: FSAP Structure, Synthesis and Activity Regulation: A Narrative Review

Factor VII activating protease (FSAP) was first isolated from human plasma less than 30 years ago. Since then, many research groups have described the biological properties of this protease and its role in hemostasis and other processes in humans and other animals. With the progress of knowledge about the structure of FSAP, several of its relationships with other proteins or chemical compounds that may modulate its activity have been explained. These mutual axes are described in the present narrative review. The first part of our series of manuscripts on FSAP describes the structure of this protein and the processes leading to the enhancement and inhibition of its activities. The following parts, II and III, concern the role of FSAP in hemostasis and in the pathophysiology of human diseases, with particular emphasis on cardiovascular diseases.

Prostasin and hepatocyte growth factor B in factor VIIa generation: Serine protease knockdowns in zebrafish

BACKGROUND

Blood clotting in humans is initiated by the binding of tissue factor to activated coagulation factor VII (FVIIa) in the plasma. Previous studies have reported that hepsin and factor VII (FVII)-activating protease are responsible for generating FVIIa.

OBJECTIVES

We aimed to identify other proteases that may activate FVII using zebrafish as a model.

METHODS

We screened 179 genes encoding serine protease domains using the piggyback knockdown method to identify genes involved in the activation of zebrafish Fvii. A prolonged kinetic prothrombin time (kPT) assay was used to detect gene knockdown effects.

RESULTS

In the primary screen, 21 genes showed prolonged kPT. In the secondary screen, 14 of 21 genes showed positive results. In the tertiary screen, all 14 genes showed prolonged kPT. These 14 genes were knocked down again to estimate relative levels of zebrafish Fviia. Six genes, including known genes, such as f10 and novel prostasin and hepatocyte growth factor B (hgfb), showed lower Fviia levels. Fvii levels were affected only by the knockdown of f7 and not by the knockdown of the other five genes.

CONCLUSIONS

Prostasin and hgfb are involved in generating Fviia. We hypothesize that prostasin exerts serine protease activity directly or indirectly to activate Fvii. As Hgfb has a mutated serine protease domain, it may not cleave Fvii but may bind to Fvii to induce autoactivation. The approach developed here may be extended to design other large-scale knockdown screens.

Characterization of the enzymatic activity of the serine protease domain of Factor VII activating protease (FSAP)

Factor VII (FVII) activating protease (FSAP) is a circulating serine protease. Human genetic studies, based on the Marburg I (MI) (Gly221Glu, chymotrypsin numbering system) polymorphism, implicate FSAP in the pathogenesis of many diseases. Here, we describe the molecular and functional changes caused by the Gly221Glu substitution in the 220 loop using recombinant proteins expressed in E. coli. The serine protease domain (SPD) of wild type (WT) FSAP displayed auto-catalytic activation whereas the MI isoform displayed very low autocatalytic activation and low proteolytic activity against the chromogenic substrate S-2288, Factor VII, tissue factor pathway inhibitor as well as pro-urokinase. Introduction of a thermolysin cleavage site in the activation position (Arg15Gln) led to cleavage of both WT- and MI-SPD and the resulting WT-SPD, but not the MI-SPD, was active. Mutating the Gly221 position to Asp, Gln and Leu led to a loss of activity whereas the Ala substitution was partially active. These results suggest a disturbance of the active site, or non-accessibility of the substrate to the active site in MI-SPD. With respect to regulation with metal ions, calcium, more than sodium, increased the enzymatic activity of WT-SPD. Thus, we describe a novel method for the production of recombinant FSAP-SPD to understand the role of the MI-single nucleotide polymorphism (SNP) in the regulation of its activity.

Origin and diversification of the plasminogen activation system among chordates

Background

The plasminogen (PLG) activation system is composed by a series of serine proteases, inhibitors and several binding proteins, which together control the temporal and spatial generation of the active serine protease plasmin. As this proteolytic system plays a central role in human physiology and pathophysiology it has been extensively studied in mammals. The serine proteases of this system are believed to originate from an ancestral gene by gene duplications followed by domain gains and deletions. However, the identification of ancestral forms in primitive chordates supporting these theories remains elusive. In addition, evolutionary studies of the non-proteolytic members of this system are scarce.

Results

Our phylogenetic analyses place lamprey PLG at the root of the vertebrate PLG-group, while lamprey PLG-related growth factors represent the ancestral forms of the jawed-vertebrate orthologues. Furthermore, we find that the earliest putative orthologue of the PLG activator group is the hyaluronan binding protein 2 (HABP2) gene found in lampreys. The prime plasminogen activators (tissue- and urokinase-type plasminogen activator, tPA and uPA) first occur in cartilaginous fish and phylogenetic analyses confirm that all orthologues identified compose monophyletic groups to their mammalian counterparts. Cartilaginous fishes exhibit the most ancient vitronectin of all vertebrates, while plasminogen activator inhibitor 1 (PAI-1) appears for the first time in cartilaginous fishes and is conserved in the rest of jawed vertebrate clades. PAI-2 appears for the first time in the common ancestor of reptiles and mammals, and represents the latest appearing plasminogen activator inhibitor. Finally, we noted that the urokinase-type plasminogen activator receptor (uPAR)—and three-LU domain containing genes in general—occurred later in evolution and was first detectable after coelacanths.

Conclusions

This study identifies several primitive orthologues of the mammalian plasminogen activation system. These ancestral forms provide clues to the origin and diversification of this enzyme system. Further, the discovery of several members—hitherto unknown in mammals—provide new perspectives on the evolution of this important enzyme system.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1353-z) contains supplementary material, which is available to authorized users.

Hereditary angioedema: Linking complement regulation to the coagulation system

Congenital deficiency of C1 inhibitor, the main inhibitor of the classic complement system pathway, leads to paroxysmal angioedema (hereditary angioedema) that can be debilitating or life-threatening for affected patients. In the past few years many new insights on the pathogenesis of angioedema formation in the presence of low levels of C1 inhibitor has been accumulated. There is a central role for bradykinin that is released upon activation of the kallikrein-kinin system that is insufficiently controlled by adequate levels of C1 inhibitor. As C1 inhibitor also possesses a central regulatory role of other plasma systems, including the contact activation system of coagulation and the plasminogen-plasmin system that governs endogenous fibrinolysis, it is interesting to observe the effects of C1 inhibitor deficiency on activation of these systems and relevance for hemostasis in vivo and thrombo-embolic disease. Interestingly, and despite significant activation of these pathways, C1 inhibitor deficiency is not at all associated with a hemorrhagic tendency or prothrombotic state. New therapeutic options for treatment of C1 inhibitor efficiency have become available in recent years, including various forms of C1 inhibitor concentrate. Restoration of C1 inhibitor levels in patients with hereditary angioedema has not resulted in thrombotic complications or any other relevant disorder associated with the hemostatic system.

Genome-wide analysis of genetic determinants of circulating factor VII-activating protease (FSAP) activity

Essentials Knowledge of genetic regulators of plasma factor VII activating protease (FSAP) levels is limited. We performed a genome-wide analysis of variants influencing FSAP activity in Scandinavian cohorts. We replicated an association for Marburg-1 and identified an association for a HABP2 stop variant. We identified a novel locus near ADCY2 as a potential additional regulator of FSAP activity.

SUMMARY

Background Factor VII-activating protease (FSAP) has roles in both coagulation and fibrinolysis. Recent data indicate its involvement in several other processes, such as vascular remodeling and inflammation. Plasma FSAP activity is highly variable among healthy individuals and, apart from the low-frequency missense variant Marburg-I (rs7080536) in the FSAP-encoding gene HABP2, determinants of this variation are unclear. Objectives To identify novel genetic variants within and outside of the HABP2 locus that influence circulating FSAP activity. Patients/Methods We performed an exploratory genome-wide association study (GWAS) on plasma FSAP activity amongst 3230 Swedish subjects. Directly genotyped rare variants were also analyzed with gene-based tests. Using GWAS, we confirmed the strong association between the Marburg-I variant and FSAP activity. HABP2 was also significant in the gene-based analysis, and remained significant after exclusion of Marburg-I carriers. This was attributable to a rare HABP2 stop variant (rs41292628). Carriers of this stop variant showed a similar reduction in FSAP activity as Marburg-I carriers, and this finding was replicated. A secondary genome-wide significant locus was identified at a 5p15 locus (rs35510613), and this finding requires future replication. This common variant is located upstream of ADCY2, which encodes a protein catalyzing the formation of cAMP. Results and Conclusions This study verified the Marburg-I variant to be a strong regulator of FSAP activity, and identified an HABP2 stop variant with a similar impact on FSAP activity. A novel locus near ADCY2 was identified as a potential additional regulator of FSAP activity.

DNA and factor VII-activating protease protect against the cytotoxicity of histones

Circulating histones have been implicated as major mediators of inflammatory disease because of their strong cytotoxic effects. Histones form the protein core of nucleosomes; however, it is unclear whether histones and nucleosomes are equally cytotoxic. Several plasma proteins that neutralize histones are present in plasma. Importantly, factor VII-activating protease (FSAP) is activated upon contact with histones and subsequently proteolyzes histones. We aimed to determine the effect of FSAP on the cytotoxicity of both histones and nucleosomes. Indeed, FSAP protected against histone-induced cytotoxicity of cultured cells in vitro. Upon incubation of serum with histones, endogenous FSAP was activated and degraded histones, which also prevented cytotoxicity. Notably, histones as part of nucleosome complexes were not cytotoxic, whereas DNA digestion restored cytotoxicity. Histones in nucleosomes were inefficiently cleaved by FSAP, which resulted in limited cleavage of histone H3 and removal of the N-terminal tail. The specific isolation of either circulating nucleosomes or free histones from sera of Escherichia coli challenged baboons or patients with meningococcal sepsis revealed that histone H3 was present in the form of nucleosomes, whereas free histone H3 was not detected. All samples showed signs of FSAP activation. Markedly, we observed that all histone H3 in nucleosomes from the patients with sepsis, and most histone H3 from the baboons, was N-terminally truncated, giving rise to a similarly sized protein fragment as through cleavage by FSAP. Taken together, our results suggest that DNA and FSAP jointly limit histone cytotoxicity and that free histone H3 does not circulate in appreciable concentrations in sepsis.

A Positively Charged Surface Triggers Coagulation Activation Through Factor VII Activating Protease (FSAP)

Contact between biomedical materials and blood often initiates undesirable pro-coagulant and pro-inflammatory processes. On negatively charged materials, blood coagulation is known to be triggered through autoactivation of Factor XII, while activation on cationic surfaces follows a distinct and so far enigmatic mechanism. Because Factor VII activating protease (FSAP) is known to be activated on positively and on negatively charged macromolecules in plasma, we have investigated its interaction with charged biomaterials and its consequences for coagulation. Several activation processes in blood and plasma were characterized after contact with material surfaces with varied charge. FSAP was found to be exclusively activated by the positively charged surfaces polyethylenimine (PEI) and poly-l-lysine (PLL), not by the negatively charged glass or self-assembled monolayer with carboxyl group termination (SAM-COOH), as well as uncharged (Teflon AF) surfaces. Whole blood incubation on PEI showed that this activation was concomitant with coagulation as determined by thrombin and fibrin formation, which was high for glass (F1+2, 138 nM) and PEI (F1+2, 44 nM) but low for Teflon AF (F1+2, 3.3 nM) and SAM COOH (F1+2, 5.8 nM). Contact phase inhibitor diminished coagulation to background levels for all surfaces except PEI (F1+2: ^PEI 43 to 25 nM; glass, 58 to 1.5 nM) indicating that coagulation activation is not dependent on FXII activation on the PEI surface. A decisive role of endogenous FSAP for coagulation however was confirmed with the use of FSAP inhibitory antibodies which showed no influence on Teflon AF, glass and SAM COOH but diminished F1+2 on PEI to less than 50%. We propose that FSAP activation could be a novel mechanism of surface-driven coagulation. An inhibition of this protease might improve hemocompatibility of cationic surfaces and therefore facilitate the application of polycationic surfaces in blood.

Role of glycine 221 in catalytic activity of hyaluronan-binding protein 2

HABP2 (hyaluronan-binding protein 2) is a Ca²⁺-dependent serine protease with putative roles in blood coagulation and fibrinolysis. A G221E substitution, known as the Marburg I polymorphism, reportedly affects HABP2 function and has been associated with increased risk for cardiovascular disease. However, the importance of Gly-221 for HABP2 activity is unclear. Here, we used G221E, G221A, and G221S mutants to assess the role of Gly-221 in HABP2 catalysis. The G221E variant failed to activate the single-chain urokinase-type plasminogen activator, and the G221A and G221S variants displayed moderately reduced single-chain urokinase-type plasminogen activator activation. Activity toward the peptide substrate S-2288 was markedly decreased in all HABP2 variants, with G221E being the most defective and G221A being the least defective. In the absence of Ca²⁺, S-2288 cleavage by wild-type HABP2 was Na⁺-dependent, with K_m decreasing from 3.0 to 0.6 mm upon titration from 0 to 0.3 m Na⁺ In the presence of 5 mm Ca²⁺, K_m was further reduced to 0.05 mm, but without an appreciable contribution of Na⁺ At physiological concentrations of Na⁺ and Ca²⁺, the three HABP2 variants, and particularly G221E, displayed a major K_m increase for S-2288. Chemical footprinting revealed that Ile-16 is significantly less protected from chemical modification in G221E than in wild-type HABP2, suggesting impaired insertion of the N terminus into the G221E protease domain, with a concomitant impact on catalytic activity. Homology modeling suggested that the Glu-221 side chain could sterically hinder insertion of the N terminus into the HABP2 protease domain, helping to explain the detrimental effects of Glu-221 substitution on HABP2 activity.

The various assays for measuring activity states of factor VIIa in plasma and therapeutic products: Diagnostic value and analytical usefulness in various pathophysiological states

The key coagulation factor FVII, and its activated form FVIIa, present a major interest for their role at the initiation phase of blood coagulation, and because they can activate all blood coagulation cascade, through the extrinsic, but also the intrinsic pathway. Blood activation initiated through FVII is first presented, as it is understood nowadays. Measurement of FVII and FVIIa were of main interest for epidemiological studies, but FVIIa contribution to assay results was only deduced. The introduction of specific FVIIa assays, functional or immunoassays, allowed measuring directly FVIIa without any interference of non-activated FVII, or other coagulation factors or their activated forms. The various methods available, and their characteristics are presented, with a special focus on two assays developed by our group for FVIIa (a clotting one and a chromogenic one). The FVIIa clotting assay shows evident superiority for measuring its activity in plasma, in pathophysiological conditions. The normal range is <2.5ng/ml, which represents less than 0.5% of the FVII protein. FVIIa is elevated in some pathological states. The chromogenic assay is of interest for assigning the potency of FVIIa concentrates, as it has a higher dynamic range. Both assays are fully automatable on laboratory instruments, and standardized in a satisfactory manner thanks to the use of the FVIIa concentrate WHO International Standard (NIBSC). The various applications and usefulness of FVIIa laboratory assays are discussed, for the measurement of therapeutic products, or for following recoveries in treated patients, including hemophiliacs with inhibitors, patients with severe bleeding risk (liver diseases, surgery, trauma, …), and lastly for measurement of its activity in therapeutic products.

The role of extracellular histones in haematological disorders

Over the past decades, chromosomal alterations have been extensively investigated for their pathophysiological relevance in haematological malignancies. In particular, epigenetic modifications of intra-nuclear histones are now known as key regulators of healthy cell cycles that have also evolved into novel therapeutic targets for certain blood cancers. Thus, for most haematologists, histones are DNA-chained proteins that are buried deep within chromatin. However, the plot has deepened with recent revelations on the function of histones when unchained and released extracellularly upon cell death or from activated neutrophils as part of neutrophil extracellular traps (NETs). Extracellular histones and NETs are increasingly recognized for profound cytotoxicity and pro-coagulant effects. This article highlights the importance of recognizing this new paradigm of extracellular histones as a key player in host defence through its damage-associated molecular patterns, which could translate into novel diagnostic and therapeutic biomarkers in various haematological and critical disorders.

Modeling Disorders of Blood Coagulation in the Zebrafish

Hemostasis, the process of blood clot formation and resolution in response to vascular injury, and thrombosis, the dysregulation of hemostasis leading to pathological clot formation, are widely studied. However, the genetic variability in hemostatic and thrombotic disorders is incompletely understood, suggesting that novel mediators have yet to be uncovered. The zebrafish is developing into a powerful in vivo model to study hemostasis, and its features as a model organism are well suited to (a) develop high-throughput screens to identify novel mediators of hemostasis and thrombosis, (b) validate candidate genes identified in human populations, and (c) characterize the structure/function relationship of gene products. In this review, we discuss conservation of the zebrafish hemostatic system, highlight areas for future study, and outline the utility of this model to study blood coagulation and its dysregulation.

Zebrafish as a model system for the study of hemostasis and thrombosis

PURPOSE OF REVIEW

Although the zebrafish has been established as a research tool over the past two to three decades, in hematology it has primarily been used to investigate areas distinct from coagulation. The advantages of this vertebrate model include high fecundity, rapid and external development, and conservation of virtually all clotting factors in the zebrafish genomic sequence. Here, we summarize the growing application of this technology to the study of hemostasis and thrombosis.

RECENT FINDINGS

Loss of function studies have demonstrated conservation of function for a number of zebrafish coagulation factors. These include positive and negative regulators of coagulation, as well as key components of the thrombus itself, such as von Willebrand factor, fibrinogen, and thrombocytes. Such analyses have also been leveraged to aid in the understanding of human variation and disease, as well as to perform in-vivo structure/function experiments.

SUMMARY

The zebrafish is an organism that lends itself to a number of unique and powerful approaches not possible in mammals. This review demonstrates that there is a high degree of genetic and functional conservation of coagulation, portending future insights into hemostasis and thrombosis through the use of this model.

Complement, thrombotic microangiopathy and disseminated intravascular coagulation

In the blurring boundaries between clinical practice and scientific observations, it is increasingly attractive to propose shared disease mechanisms that could explain clinical experience. With the advent of available therapeutic options for complement inhibition, there is a push for more widespread application in patients, despite a lack of clinically relevant research. Patients with disseminated intravascular coagulation (DIC) and thrombotic microangiopathies (TMA) frequently exhibit complement activation and share the clinical consequences of thrombocytopenia, microangiopathic hemolytic anemia, and microvascular thrombosis. However, they arise from very different molecular etiologies giving rise to cautious questions about inclusive treatment approaches because most clinical observations are associative and not cause-and-effect. Complement inhibition is successful in many cases of atypical hemolytic uremic syndrome, greatly reducing morbidity and mortality of patients by minimizing thrombocytopenia, microangiopathic hemolytic anemia, and microvascular thrombosis. But is this success due to targeting disease etiology or because complement is a sufficiently systemic target or both? These questions are important because complement activation and similar clinical features also are observed in many DIC patients, and there are mounting calls for systemic inhibition of complement mediators despite the enormous differences in the primary diseases complicated by DIC. We are in great need of thoughtful and standardized assessment with respect to both beneficial and potentially harmful consequences of complement activation in these patient populations. In this review, we discuss about what needs to be done in terms of establishing the strategy for complement inhibition in TMA and DIC, based on the current knowledge.

Defective thrombus formation in mice lacking endogenous factor VII activating protease (FSAP)

Factor VII (FVII) activating protease (FSAP) is a circulating protease with a putative function in blood coagulation and fibrinolysis. Genetic epidemiological studies have implied a role for FSAP in carotid stenosis, stroke and thrombosis. To date, no in vivo evidence is available to support these claims. We have, for the first time, used FSAP-/- mice to define its role in thrombosis and haemostasis in vivo and to characterise the molecular mechanisms involved. FeCl3-induced arterial thrombosis in carotid and mesenteric artery revealed that the occlusion time was significantly increased in FSAP-/- mice (p< 0.01) and that some FSAP-/- mice did not occlude at all. FSAP-/- mice were protected from lethal pulmonary thromboembolism induced by collagen/ epinephrine infusion (p< 0.01). Although no spontaneous bleeding was evident, in the tail bleeding assay a re-bleeding pattern was observed in FSAP-/- mice. To explain these observations at a mechanistic level we then determined how haemostasis factors and putative FSAP substrates were altered in FSAP-/- mice. Tissue factor pathway inhibitor (TFPI) levels were higher in FSAP-/- mice compared to WT mice whereas FVIIa levels were unchanged. Other coagulation factors as well as markers of platelet activation and function revealed no significant differences between WT and FSAP-/- mice. This phenotype of FSAP-/- mice could be reversed by application of exogenous FSAP. In conclusion, a lack of endogenous FSAP impaired the formation of stable, occlusive thrombi in mice. The underlying in vivo effect of FSAP is more likely to be related to the modulation of TFPI rather than FVIIa.

Role of hepsin in factor VII activation in zebrafish

Factor VII, the initiator of the extrinsic coagulation cascade, circulates in human plasma mainly in its zymogen form, factor VII and in small amounts in its activated form, factor VIIa. However, the mechanism of initial generation of factor VIIa is not known despite intensive research using currently available model systems. Earlier findings suggested serine proteases factor VII activating protease and hepsin play a role in activating factor VII, however, it has remained controversial. In this paper we estimated the levels of factor VIIa and factor VII for the first time in zebrafish adult population and also reevaluated the role of the above two serine proteases in activating factor VII in vivo using zebrafish as a model system. Knockdown of factor VII activating protease and hepsin was performed followed by assaying for their effect on factor VIIa concentration and extrinsic coagulation as measured by the kinetic prothrombin time. Factor VII activating protease knockdown showed no change in kinetic prothrombin time and no effect on factor VIIa levels while hepsin knockdown increased the kinetic prothrombin time and significantly reduced the factor VIIa plasma levels. Our results thus indicate that hepsin plays a physiologically important role in factor VII activation and hemostasis in zebrafish.

Overexpression of factor VII ameliorates bleeding diathesis of factor VIII-deficient mice with inhibitors

INTRODUCTION

Factor VIII (FVIII) treatment for hemophilia A has difficulties in correcting bleeding diathesis in the presence of inhibitors.

MATERIALS AND METHODS

An adeno-associated virus type 8 (AAV8) vector containing the factor VII (FVII) gene or the activated factor VII (FVIIa) gene was used to investigate the therapeutic effect of FVII or FVIIa overexpression in FVIII-deficient mice with inhibitors.

RESULTS

Following repeated human FVIII injection, FVIII-deficient mice developed anti-human FVIII antibodies that cross-reacted with mouse FVIII. High transgene expression of murine FVII or murine FVIIa was achieved using the AAV8 vector and resulted in increased blood FVII activity greater than 800% of normal murine FVII levels in vector-injected FVIII-deficient mice. Thromboelastography analysis showed significant improvements in clotting time, clot formation time, α angle, and mean clot firmness in AAV8 vector-injected FVIII-deficient mice with inhibitors. Overexpression of FVIIa ameliorated the bleeding phenotype of FVIII-deficient mice with inhibitors and significantly increased the survival rate after tail clipping. In addition, overexpression of FVII increased the survival rate of FVIII-deficient mice with inhibitors after tail clipping though it was not as efficient as FVIIa overexpression.

CONCLUSIONS

These data suggest that FVII overexpression is an alternative strategy for the treatment of hemophilia A with inhibitors.

New Fundamentals in Hemostasis

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