Nucleosome-releasing factor: a new role for factor VII-activating protease (FSAP)

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.

NETosis and Nucleosome Biomarkers in Septic Shock and Critical COVID-19 Patients: An Observational Study

Background: Neutrophil extracellular traps’ (NETs’) formation is a mechanism of defense that neutrophils deploy as an alternative to phagocytosis, to constrain the spread of microorganisms. Aim: The aim was to evaluate biomarkers of NETs’ formation in a patient cohort admitted to intensive care unit (ICU) due to infection. Methods: Forty-six septic shock patients, 22 critical COVID-19 patients and 48 matched control subjects were recruited. Intact nucleosomes containing histone 3.1 (Nu.H3.1), or citrullinated histone H3R8 (Nu.Cit-H3R8), free citrullinated histone (Cit-H3), neutrophil elastase (NE) and myeloperoxidase (MPO) were measured. Results: Significant differences in Nu.H3.1 and NE levels were observed between septic shock and critical COVID-19 subjects as well as with controls (p-values < 0.05). The normalization of nucleosome levels according to the neutrophil count improved the discrimination between septic shock and critical COVID-19 patients. The ratio of Nu.Cit-H3R8 to Nu.H3.1 allowed the determination of nucleosome citrullination degree, presumably by PAD4. Conclusions: H3.1 and Cit-H3R8 nucleosomes appear to be interesting markers of global cell death and neutrophil activation when combined. Nu.H3.1 permits the evaluation of disease severity and differs between septic shock and critical COVID-19 patients, reflecting two distinct potential pathological processes in these conditions.

Extracellular Nucleosomes Accelerate Microglial Inflammation via C-Type Lectin Receptor 2D and Toll-Like Receptor 9 in mPFC of Mice With Chronic Stress

Damage-associated molecular patterns (DAMPs) are the primary promoter of progressive neuroinflammation and are associated with chronic stress-related emotional disorders. The present study investigated the role and mechanism of extracellular nucleosomes and histones, the newly defined DAMPs, in mice with chronic stress. C57BL/6 mice were exposed to chronic unpredictable mild stress (CUMS) and corticosterone drinking, respectively, for 4 weeks. Negative emotional behaviors were comprehensively investigated. Microglial morphology, oxidative stress, and inflammation, as well as C-type lectin receptor 2D (Clec2d) and Toll-like receptor 9 (TLR9) expression in medial prefrontal cortex (mPFC) were assessed with flow cytometer and cell sorting. Specifically, microglial pro-inflammatory activation and inflammation were further investigated with stereotactic injection of recombinant nucleosomes and histones in mPFC and further evaluated with AAV-Clec2d knocking-down, DNase I, and activated protein C (APC) pretreatment. Moreover, the rescue effect by AAV-Clec2d knocking-down was observed in mice with chronic stress. Mice with chronic stress were presented as obviously depressive- and anxiety-like behaviors and accompanied with significant microglial oxidative stress and inflammation, indicating by reactive oxygen species (ROS) production, primed nuclear factor-κB (NF-κB) signaling pathway, activated NACHT, LRR, and PYD domain–containing protein 3 (NLRP3) inflammasome, and upregulated Clec2d and TLR9 in mPFC, together with histones dictation in cerebrospinal fluid and extracellular trap formation. Stereotactic injection of nucleosomes was contributed to promote microglial inflammation rather than histones in mPFC, indicating that the pro-inflammatory role was derived from extracellular histones-bound DNA but not freely histones. AAV-Clec2d knocking-down, DNase I, and APC were all effective to inhibit nucleosome-induced microglial oxidative stress and inflammation. Moreover, AAV-Clec2d knocking-down in mice with chronic stress exhibited reduced microglial inflammation and improved negative emotional behaviors. Our findings reveal a novel mechanism of DAMP-associated inflammation that extracellular nucleosomes accelerate microglial inflammation via Clec2d and TLR9, and then contribute to chronic stress-induced emotional disorders.

Factor VII activating protease (FSAP) is not essential in the pathophysiology of angioedema in patients with C1 inhibitor deficiency

BACKGROUND

Excessive bradykinin (BK) generation from high molecular weight kininogen (HK) by plasma kallikrein (PK) due to lack of protease inhibition is central to the pathophysiology of hereditary angioedema (HAE). Inadequate protease inhibition may contribute to HAE through a number of plasma proteases including factor VII activating protease (FSAP) that can also cleave HK.

OBJECTIVE

To investigate the interaction between FSAP and C1 inhibitor (C1Inh) and evaluate the potential role of FSAP in HAE with C1Inh deficiency.

MATERIALS AND METHODS

Plasma samples from 20 persons with HAE types 1 or 2 in remission were studied and compared to healthy controls. We measured and compared antigenic FSAP levels, spontaneous FSAP activity, FSAP generation potential, activation of plasma pre-kallikrein (PPK) by FSAP, and the formation of FSAP-C1Inh and FSAP-alpha2-antiplasmin (FSAP-α2AP) complexes. Furthermore, we measured HK cleavage and PK activation after activation of endogenous pro-FSAP and after addition of exogenous FSAP.

RESULTS

In plasma from HAE patients, there is increased basal FSAP activity compared to healthy volunteers. HAE plasma exhibits decreased formation of FSAP-C1Inh complexes and increased formation of FSAP-α2AP complexes in histone-activated plasma. Although exogenous FSAP can cleave HK in plasma, this was not seen when endogenous plasma pro-FSAP was activated with histones in either group. PK was also not activated by FSAP in plasma.

CONCLUSION

In this study, we established that FSAP activity is increased and the pattern of FSAP-inhibitor complexes is altered in HAE patients. However, we did not find evidence suggesting that FSAP contributes directly to HAE attacks.

The role of extracellular DNA (exDNA) in cellular processes

Nowadays, extracellular DNA or circulating cell-free DNA is considered to be a molecule with clinical applications (diagnosis, prognosis, monitoring of treatment responses, or patient follow-up) in diverse pathologies, especially in cancer. Nevertheless, because of its molecular characteristics, it can have many other functions. This review focuses on the participation of extracellular DNA (exDNA) in fundamental processes such as cell signaling, coagulation, immunity, evolution through horizontal transfer of genetic information, and adaptive response to inflammatory processes. A deeper understanding of its role in each of these processes will allow development of better tools to monitor and control pathologies, as well as helping to generate new therapeutic options, beyond the applicability of DNA in liquid biopsy.

Extracellular histones promote fibrinolysis by single-chain urokinase-type plasminogen activator in a factor seven activating protease-dependent way

INTRODUCTION

Extracellular histones inhibit tissue plasminogen activator (t-PA)-mediated fibrinolysis by modifying fibrin structure and rheological properties. However, other plasminogen activators involved in intravascular and extravascular fibrinolysis have not been considered yet.

OBJECTIVES

We investigated the effect of histones on fibrinolysis driven by different plasminogen activators.

METHODS

Clot lysis induced by t-PA, urokinase (u-PA) and its single chain precursor (scu-PA) was evaluated by turbidimetry. Conversion of scu-PA to u-PA and activation of factor seven activating protease (FSAP) were assessed by fluorogenic and chromogenic assays, respectively.

RESULTS

Histones delayed t-PA- and u-PA-mediated fibrinolysis but strongly accelerated scu-PA-driven clot lysis through the enhancement of scu-PA to u-PA conversion. This effect required a plasma factor identified as FSAP by the following findings: 1) histones enhanced neither scu-PA activation nor scu-PA-mediated clot lysis under purified conditions; 2) in plasma, the enhancement of fibrinolytic activity by histones was abolished by a neutralizing anti-FSAP antibody; and 3) histones promoted the activation of plasma FSAP. The effect of the natural mixture of histones on scu-PA-driven fibrinolysis was differentially recapitulated by the individual recombinant histones, H4 displaying the strongest activity. When complexed to DNA, histones still accelerated scu-PA-mediated fibrinolysis but with a lesser efficiency due to a reduced FSAP activation. Finally, preincubation of histones with heparin or activated protein C, two known inhibitors of histones, further amplified histone-mediated boost of scu-PA-driven fibrinolysis.

CONCLUSIONS

Enhancement of FSAP-mediated scu-PA activity by histones may play yet unforeseen roles in intravascular fibrinolysis and contribute to extravascular proteolysis and tissue damage.

Neutrophils mitigate the systemic host response during endotoxemia in mice

Recent studies have suggested that neutrophils can exert anti-inflammatory effects. To determine the role of neutrophils in the acute response to lipopolysaccharide (LPS), a component of the Gram-negative bacterial cell wall, we challenged neutrophil-depleted and control mice with LPS and analyzed the plasma concentrations of biomarkers indicative of the cytokine and chemokine network, activation of coagulation and the vascular endothelium, and cellular injury. We here show that neutrophils serve an anti-inflammatory role upon LPS administration, as reflected by sustained elevations of multiple cytokines and chemokines, and enhanced release of nucleosomes in mice depleted of neutrophils, compared with control mice.

Altered structure and function of fibrinogen after cleavage by Factor VII Activating Protease (FSAP)

Factor VII Activating Protease (FSAP) is a plasma protease affecting both coagulation and fibrinolysis. Although a role in hemostasis is still unclear, the identification of additional physiologic substrates will help to elucidate its role in this context. FSAP has been reported to cleave fibrinogen, but the functional consequences of this are not known. We have therefore undertaken this study to determine the implications of this cleavage for fibrin-clot formation and its lysis. Treatment of human fibrinogen with FSAP released an N-terminal peptide from the Bβ chain (Bβ1-53) and subsequently the fibrinopeptide B; within the Aα chain a partial truncation of the αC-region by multiple cleavages was seen. The truncated fibrinogen showed a delayed thrombin-catalyzed polymerization and formed fibrin clots of reduced turbidity, indicative of thinner fibrin fibers. Confocal laser scanning and scanning electron microscopy of these clots revealed a less coarse fibrin network with thinner fibers and a smaller pore size. A lower pore size was also seen in permeability studies. Unexpectedly, FSAP-treated fibrinogen or plasma exhibited a significantly faster tPA-driven lysis, which correlated exclusively with cleavage of fibrinogen and not with activation of plasminogen activators. Similar observations were also made in plasma after activation of endogenous zymogen FSAP, but not in plasma of carrier of the rare Marburg I single nucleotide polymorphism. In conclusion, altering fibrin clot properties by fibrinogenolysis is a novel function of FSAP in the vasculature, which facilitates clot lysis and may in vivo contribute to reduced fibrin deposition during thrombosis.

The immune response to secondary necrotic cells

When apoptotic cells are not cleared in an efficient and timely manner, they progress to secondary necrosis and lose their membrane integrity. This results in a leakage of immunostimulatory, danger associated molecular patterns (DAMPs), similar to accidental (or primary) necrosis. However, primary necrosis is a sudden event with an inadvertent release of almost unmodified DAMPs. Secondary necrotic cells, in contrast, have gone through various modifications during the process of apoptosis. Recent research revealed that the molecules released from the cytoplasm or exposed on the cell surface differ between primary necrosis, secondary necrosis, and regulated necrosis such as necroptosis. This review gives an overview of these differences and focusses their effects on the immune response. The implications to human physiology and diseases are manifold and will be discussed in the context of cancer, neurodegenerative disorders and autoimmunity.

FSAP, a new player in inflammation?

Factor VII-activating protease (FSAP) is a serine protease in plasma that has a role in coagulation and fibrinolysis. FVII could be activated by purified FSAP in a tissue factor independent manner and pro-urokinase has been demonstrated to be a substrate for purified FSAP in-vitro. However, the physiological role of FSAP in haemostasis remains unclear. More recently FSAP is suggested to be involved in inflammation. It modulates vascular permeability directly and indirectly by the generation of bradykinin. Furthermore, FSAP is activated by dead cells induced by the inflammatory response and subsequently removes nucleosomes from apoptotic cells. FSAP activation can be detected in sepsis patients as well. However, whether FSAP activation upon inflammation is beneficial or detrimental remains an open question.

In this review the structure, activation mechanisms and the possible role of FSAP in inflammation are discussed.

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.

Interaction of factor VII activating protease (FSAP) with neutrophil extracellular traps (NETs)

The circulating zymogen form of Factor VII activating protease (FSAP) can be activated by histones and nucleosomes in vivo. These cell-death-associated nuclear factors are also actively extruded into the extracellular space by neutrophils through a process called neutrophil extracellular trap (NET) formation (NETosis). NETs are thought to be involved in host defense, inflammation as well as thrombosis. We have investigated the bidirectional interactions of FSAP and NETs. Phorbol ester-mediated NET formation was marginally stimulated by FSAP. Plasma-derived FSAP as well as exogenous FSAP bound to NETs. There was co-localization of FSAP and NETs in coronary thrombi from patients with acute myocardial infarction. Contrary to our expectations no activation of pro-FSAP by NETs was evident. However, after disintegration of NETs with DNase, a robust activation of pro-FSAP, due to release of histones from nucleosomes, was detected. The released histones were in turn degraded by FSAP. Histone cytotoxicity towards endothelial cells was neutralized by FSAP more potently than by activated protein C (APC). One more consequence of histone degradation was a decrease in nucleosome release from apoptotic neutrophils. Taken together, NETs bind to FSAP, but do not activate pro-FSAP unless histones are released from NETs by DNAse. This activation of FSAP is likely to be important in diminishing the cytotoxic effect of histones, thus limiting the damaging effect of NETosis.

Complement in removal of the dead - balancing inflammation

Recognition and removal of apoptotic and necrotic cells must be efficient and highly controlled to avoid excessive inflammation and autoimmune responses to self. The complement system, a crucial part of innate immunity, plays an important role in this process. Thus, apoptotic and necrotic cells are recognized by complement initiators such as C1q, mannose binding lectin, ficolins, and properdin. This triggers complement activation and opsonization of cells with fragments of C3b, which enhances phagocytosis and thus ensures silent removal. Importantly, the process is tightly controlled by the binding of complement inhibitors C4b-binding protein and factor H, which attenuates late steps of complement activation and inflammation. Furthermore, factor H becomes actively internalized by apoptotic cells, where it catalyzes the cleavage of intracellular C3 to C3b. The intracellularly derived C3b additionally opsonizes the cell surface further supporting safe and fast clearance and thereby aids to prevent autoimmunity. Internalized factor H also binds nucleosomes and directs monocytes into production of anti-inflammatory cytokines upon phagocytosis of such complexes. Disturbances in the complement-mediated clearance of dying cells result in persistence of autoantigens and development of autoimmune diseases like systemic lupus erythematosus, and may also be involved in development of age-related macula degeneration.

Extracellular nucleic acids in immunity and cardiovascular responses: between alert and disease

Severe inflammatory complications are a potential consequence in patients with predetermined conditions of infections, pulmonary diseases, or cardiovascular disorders. Notably, the amplitude of the inflammatory response towards these complications can dictate the disease progression and outcome. During the recent years, evidence from basic research as well as from clinical studies has identified self-extracellular nucleic acids as important players in the crosstalk between immunity and cardiovascular diseases. These stress- or injury-induced endogenous polymeric macromolecules not only serve as "alarmins" or "Danger-associated molecular patterns" (DAMPs), but their functional repertoire goes far beyond such activities in innate immunity. In fact, (patho-) physiological functions of self-extracellular DNA and RNA are associated and in many cases causally related to arterial and venous thrombosis, atherosclerosis, ischemia-reperfusion injury or tumour progression. Yet, the underlying molecular mechanisms are far from being completely understood. Interestingly enough, however, novel antagonistic approaches in vitro and in vivo, particularly using natural endonucleases or synthetic nucleic acid binding polymers, appear to be promising and safe therapeutic options for future studies. The aim of this review article is to provide an overview of the current state of (patho-) physiological functions of self-extracellular nucleic acids with special emphasis on their role as beneficial / alerting or adverse / damaging factors in connection with immune responses, inflammation, thrombosis, and cardiovascular diseases.

Extracellular histones, cell-free DNA, or nucleosomes: differences in immunostimulation

In inflammation, extensive cell death may occur, which results in the release of chromatin components into the extracellular environment. Individually, the purified chromatin components double stranded (ds)DNA and histones have been demonstrated, both in vitro and in vivo, to display various immunostimulatory effects, for example, histones induce cytotoxicity and proinflammatory signaling through toll-like receptor (TLR)2 and 4, while DNA induces signaling through TLR9 and intracellular nucleic acid sensing mechanisms. However, DNA and histones are organized in nucleosomes in the nucleus, and evidence suggests that nucleosomes are released as such in inflammation. The cytotoxicity and proinflammatory signaling induced by nucleosomes have not been studied as extensively as the separate effects brought about by histones and dsDNA, and there appear to be some marked differences. Remarkably, little distinction between the different forms in which histones circulate has been made throughout literature. This is partly due to the limitations of existing techniques to differentiate between histones in their free or DNA-bound form. Here we review the current understanding of immunostimulation induced by extracellular histones, dsDNA and nucleosomes, and discuss the importance of techniques that in their detection differentiate between these different chromatin components.

Propagation of thrombosis by neutrophils and extracellular nucleosome networks

Neutrophils, early mediators of the innate immune defense, are recruited to developing thrombi in different types of thrombosis. They amplify intravascular coagulation by stimulating the tissue factor-dependent extrinsic pathway via inactivation of endogenous anticoagulants, enhancing factor XII activation or decreasing plasmin generation. Neutrophil-dependent prothrombotic mechanisms are supported by the externalization of decondensed nucleosomes and granule proteins that together form neutrophil extracellular traps. These traps, either in intact or fragmented form, are causally involved in various forms of experimental thrombosis as first indicated by their role in the enhancement of both microvascular thrombosis during bacterial infection and carotid artery thrombosis. Neutrophil extracellular traps can be induced by interactions of neutrophils with activated platelets; vice versa, these traps enhance adhesion of platelets via von Willebrand factor. Neutrophil-induced microvascular thrombus formation can restrict the dissemination and survival of blood-borne bacteria and thereby sustain intravascular immunity. Dysregulation of this innate immune pathway may support sepsis-associated coagulopathies. Notably, neutrophils and extracellular nucleosomes, together with platelets, critically promote fibrin formation during flow restriction-induced deep vein thrombosis. Neutrophil extracellular traps/extracellular nucleosomes are increased in thrombi and in the blood of patients with different vaso-occlusive pathologies and could be therapeutically targeted for the prevention of thrombosis. Thus, during infections and in response to blood vessel damage, neutrophils and externalized nucleosomes are major promoters of intravascular blood coagulation and thrombosis.

Factor VII-activating protease deficiency promotes neointima formation by enhancing leukocyte accumulation

Essentials Factor VII-activating protease (FSAP) is a plasma protease involved in vascular processes. Neointima formation was investigated after vascular injury in FSAP-/- mice. The neointimal lesion size and the accumulation of macrophages were increased in FSAP-/- mice. This was due to an increased activity of the chemokine (C-C motif) ligand 2 (CCL2).

SUMMARY

Background Factor VII-activating protease (FSAP) is a multifunctional circulating plasma serine protease involved in thrombosis and vascular remodeling processes. The Marburg I single-nucleotide polymorphism (MI-SNP) in the FSAP-coding gene is characterized by low proteolytic activity, and is associated with increased rates of stroke and carotid stenosis in humans. Objectives To determine whether neointima formation after vascular injury is increased in FSAP-/- mice. Methods and Results The neointimal lesion size and the proliferation of vascular smooth muscle cells (VSMCs) were significantly enhanced in FSAP-/- mice as compared with C57BL/6 control mice after wire-induced injury of the femoral artery. Accumulation of leukocytes and macrophages was increased within the lesions of FSAP-/- mice at day 3 and day 14. Quantitative zymography demonstrated enhanced activity of gelatinases/matrix metalloproteinase (MMP)-2 and MMP-9 within the neointimal lesions of FSAP-/- mice, and immunohistochemistry showed particular costaining of MMP-9 with accumulating leukocytes. Using intravital microscopy, we observed that FSAP deficiency promoted the intravascular adherence and the subsequent transmigration of leukocytes in vivo in response to chemokine ligand 2 (CCL2). CCL2 expression was increased in FSAP-/- monocytes but not in the vessel wall. There was no difference in the expression of platelet-derived growth factor (PDGF-BB). Conclusions FSAP deficiency causes an increase in CCL2 expression and CCL2-mediated infiltration of leukocytes into the injured vessel, thereby promoting SMC proliferation and migration by the activation of leukocyte-derived gelatinases. These results provide a possible explanation for the observed association of the loss-of-function MI-SNP with vascular proliferative diseases.

Extracellular histones reduce survival and angiogenic responses of late outgrowth progenitor and mature endothelial cells

ESSENTIALS: Extracellular histones are highly augmented in sites of neovessel formation, such as regeneration tissues. We studied histone effect on survival and angiogenic activity of mature and progenitor endothelial cells. Extracellular histones trigger apoptosis and pyroptosis and reduce angiogenesis in vivo and in vitro. Histone blockade can be useful as a therapeutic strategy to improve angiogenesis and tissue regeneration.

BACKGROUND

Extracellular histones are highly augmented in sites of neovessel formation, like regeneration tissues. Their cytotoxic effect has been studied in endothelial cells, although the mechanism involved and their action on endothelial colony-forming cells (ECFCs) remain unknown.

OBJECTIVE

To study the effect of histones on ECFC survival and angiogenic functions and compare it with mature endothelial cells.

METHODS AND RESULTS

Nuclear morphology analysis showed that each human recombinant histone triggered both apoptotic-like and necrotic-like cell deaths in both mature and progenitor endothelial cells. While H1 and H2A exerted a weak toxicity, H2B, H3 and H4 were the most powerful. The percentage of apoptosis correlated with the percentage of ECFCs exhibiting caspase-3 activation and was zeroed by the pan-caspase inhibitor Z-VAD-FMK. Necrotic-like cell death was also suppressed by this compound and the caspase-1 inhibitor Ac-YVAD-CMK, indicating that histones triggered ECFC pyroptosis. All histones, at non-cytotoxic concentrations, reduced migration and H2B, H3 and H4 induced cell cycle arrest and impaired tubulogenesis via p38 activation. Neutrophil-derived histones exerted similar effects. In vivo blood vessel formation in the quail chorioallantoic membrane was also reduced by H2B, H3 and H4. Their cytotoxic and antiangiogenic effects were suppressed by unfractioned and low-molecular-weight heparins and the combination of TLR2 and TLR4 blocking antibodies.

CONCLUSIONS

Histones trigger both apoptosis and pyroptosis of ECFCs and inhibit their angiogenic functions. Their cytotoxic and antiangiogenic effects are similar in mature endothelial cells and disappear after heparin addition or TLR2/TLR4 blockade, suggesting both as therapeutic strategies to improve tissue regeneration.

Increased Nucleosomes and Neutrophil Activation Link to Disease Progression in Patients with Scrub Typhus but Not Murine Typhus in Laos

Cell-mediated immunity is essential in protection against rickettsial illnesses, but the role of neutrophils in these intracellular vasculotropic infections remains unclear. This study analyzed the plasma levels of nucleosomes, FSAP-activation (nucleosome-releasing factor), and neutrophil activation, as evidenced by neutrophil-elastase (ELA) complexes, in sympatric Lao patients with scrub typhus and murine typhus. In acute scrub typhus elevated nucleosome levels correlated with lower GCS scores, raised respiratory rate, jaundice and impaired liver function, whereas neutrophil activation correlated with fibrinolysis and high IL-8 plasma levels, a recently identified predictor of severe disease and mortality. Nucleosome and ELA complex levels were associated with a 4.8-fold and 4-fold increased risk of developing severe scrub typhus, beyond cut off values of 1,040 U/ml for nucleosomes and 275 U/ml for ELA complexes respectively. In murine typhus, nucleosome levels associated with pro-inflammatory cytokines and the duration of illness, while ELA complexes correlated strongly with inflammation markers, jaundice and increased respiratory rates. This study found strong correlations between circulating nucleosomes and neutrophil activation in patients with scrub typhus, but not murine typhus, providing indirect evidence that nucleosomes could originate from neutrophil extracellular trap (NET) degradation. High circulating plasma nucleosomes and ELA complexes represent independent risk factors for developing severe complications in scrub typhus. As nucleosomes and histones exposed on NETs are highly cytotoxic to endothelial cells and are strongly pro-coagulant, neutrophil-derived nucleosomes could contribute to vascular damage, the pro-coagulant state and exacerbation of disease in scrub typhus, thus indicating a detrimental role of neutrophil activation. The data suggest that increased neutrophil activation relates to disease progression and severe complications, and increased plasma levels of nucleosomes and ELA complexes represent independent risk factors for developing severe scrub typhus.

Tropical rickettsial illnesses, especially scrub typhus and murine typhus, are increasingly recognized as a leading cause of treatable undifferentiated febrile illness in Asia, but remain severely neglected and under appreciated diseases in many areas. In this study we investigated the relationship of markers of neutrophil activation and cell death with disease severity in patients with acute scrub typhus and murine typhus in Laos. These easily measurable circulating markers were associated with a 4 to 5-fold increased risk of developing severe clinical disease manifestations in scrub typhus and represent independent predictors of severe disease, and possibly death. We also found strong correlations between circulating markers of cell death and neutrophil activation in patients with scrub typhus, but not murine typhus, providing indirect evidence that neutrophil extracellular traps could contribute to the vascular damage and pro-coagulant state leading to exacerbation of disease in scrub typhus, thus indicating a detrimental role of neutrophil activation. The data suggest that increased neutrophil activation relates to disease progression and severe complications, and increased plasma levels of nucleosomes and ELA complexes represent independent risk factors for developing severe scrub typhus.

Diabetes-independent increase of factor VII-activating protease activation in patients with Gram-negative sepsis (melioidosis)

BACKGROUND

The plasma protease factor VII-activating protease (FSAP) can release nucleosomes from late apoptotic cells. Nucleosomes are markers of cell death, and extracellular cell-free DNA has been suggested to play an important role in inflammation and has been demonstrated to correlate with severity and outcome in sepsis patients.

OBJECTIVE

To investigate FSAP activation in patients suffering from Burkholderia pseudomallei infection (melioidosis), an important cause of Gram-negative sepsis in Southeast Asia. As diabetes mellitus (DM) is the most important risk factor for both melioidosis and sepsis, we were also able to examine the role of DM in FSAP activation in this cohort of patients.

METHODS

In a prospective observational study, complexes of FSAP with α2 -antiplasmin (AP) were assayed in 44 patients with melioidosis, 34 of whom were classified as diabetic. Eighty-two healthy subjects served as controls (52 with DM and 30 without).

RESULTS

FSAP-AP complex levels were markedly elevated in patients as compared with controls. The FSAP level increased by 16.82 AU mL(-1) in patients with melioidosis after adjustment for the effect of DM in the regression model. As expected, FSAP activation was correlated with nucleosome release (slope = 0.74). No difference in FSAP activation on admission was seen between survivors and non-survivors, but the extent of FSAP activation correlated with stage of the disease; repeated testing during convalescence showed a return towards normal values (day 0 vs. day 28, 4.16 AU mL(-1) , 95% confidence interval [CI] 1.42-12.22).

CONCLUSION

Patients with Gram-negative sepsis caused by B. pseudomallei have abundant FSAP activation, which significantly correlates with stage of disease. The presence of DM, however, does not influence the extent of FSAP activation.

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.

Regulation of monocyte/macrophage function by factor VII activating protease (FSAP)

OBJECTIVE

Factor VII activating protease (FSAP) is a novel regulator of vascular inflammation and hemostasis. However, the molecular mechanism by which circulating FSAP influences inflammatory events and progression of atherosclerosis is not yet entirely understood. Here we have investigated the influence of FSAP on monocyte/macrophage functions.

METHODS

We stimulated human monocyte-derived macrophages with FSAP and analyzed their cellular responses.

RESULTS

FSAP induced IκB-dependent NF-κB activation in a time- and concentration-dependent fashion. FSAP also activated the phosphorylation and proteolytic degradation of the inhibitor protein IκBα. The phosphorylation of the p65 subunit of NF-κB was induced by FSAP, which is known to contribute to the enhancement of DNA-binding activity of NF-κB. Concomitantly, FSAP up-regulated the expression of pro-inflammatory cytokines, matrix metalloproteinases, cell adhesion molecules and tissue factor. In the presence of FSAP there was increased monocytes adhesion and transendothelial migration in a beta2 integrin dependent manner.

CONCLUSIONS

Our findings suggest that FSAP activates the NF-κB pathway and the associated downstream pro-inflammatory factors in monocytic cells. This adds to a spectrum of FSAP effects on the vascular system that may explain its association with cardiovascular diseases.

Nucleosomes and neutrophil activation in sickle cell disease painful crisis

Activated polymorphonuclear neutrophils play an important role in the pathogenesis of vaso-occlusive painful sickle cell crisis. Upon activation, polymorphonuclear neutrophils can form neutrophil extracellular traps. Neutrophil extracellular traps consist of a meshwork of extracellular DNA, nucleosomes, histones and neutrophil proteases. Neutrophil extracellular traps have been demonstrated to be toxic to endothelial and parenchymal cells. This prospective cohort study was conducted to determine neutrophil extracellular trap formation in sickle cell patients during steady state and painful crisis. As a measure of neutrophil extracellular traps, plasma nucleosomes levels were determined and polymorphonuclear neutrophil activation was assessed measuring plasma levels of elastase-α1-antitrypsin complexes in 74 patients in steady state, 70 patients during painful crisis, and 24 race-matched controls using Enzyme Linked Immunosorbent Assay. Nucleosome levels in steady state sickle cell patients were significantly higher than levels in controls. During painful crisis levels of both nucleosomes and elastase-α1-antitrypsin complexes increased significantly. Levels of nucleosomes correlated significantly to elastase-α1-antitrypsin complex levels during painful crisis, (Sr = 0.654, P<0.001). This was seen in both HbSS/HbSβ(0)-thalassemia (Sr=0.55, P<0.001) and HbSC/HbSβ(+-)thalassemia patients (Sr=0.90, P<0.001) during painful crisis. Levels of nucleosomes showed a correlation with length of hospital stay and were highest in patients with acute chest syndrome. These data support the concept that neutrophil extracellular trap formation and neutrophil activation may play a role in the pathogenesis of painful sickle cell crisis and acute chest syndrome.

Extracellular nucleic acids as novel alarm signals in the vascular system. Mediators of defence and disease

Upon vascular injury or tissue damage, the exposed intracellular material such as nucleic acids, histones and other macromolecules may come into contact with vessel wall cells and circulating blood cells and may thus, have an enduring influence on wound healing and body defence processes. This short review summarizes recent work related to extracellular DNA and RNA and their role as prominent alarm signals and inducers of different defence reactions related to innate immunity and thrombus formation. Of particular importance are DNA-histone complexes (nucleosome material) that, having been expelled during stimulation of the neutrophils, not only trap and eliminate bacteria but also promote thrombus formation in the arterial and venous system. Consequently therefore, the administration of DNase exhibits strong antithrombotic functions. Similarly, extracellular RNA provokes activation of the contact phase system of blood coagulation and, by interacting with specific proteins and cytokines, it promotes vascular permeability and oedema formation. The development of RNA-mediated thrombosis, vasogenic oedema or proinflammatory responses are counteracted by the administration of RNase1 in several pathogenetic animal models. Thus, extracellular nucleic acids appear not only to function as host alarm signals that serve to amplify the defence response, but they also provide important links to thrombus formation as part of the innate immune system.

Circulating Nucleosomes and Neutrophil Activation as Risk Factors for Deep Vein Thrombosis

Objective—

The formation of neutrophil extracellular traps and the exposure of nucleosomes on these neutrophil extracellular traps contribute to coagulation activation and the propagation of deep vein thrombosis (DVT) in animal models. However, no data are available on the role of neutrophil extracellular traps or nucleosomes in patients with thrombosis.

Methods and Results—

We conducted a case–control study, in which levels of circulating nucleosomes and neutrophil elastase–α1-antitrypsin complexes were assessed in plasma from 150 patients with objectified symptomatic DVT (cases) and compared with 195 patients with a clinical suspicion of DVT but in whom DVT was excluded (controls). We explored the association between both nucleosomes and elastase–α1-antitrypsin complexes, and the presence of DVT by calculating the odds ratio with corresponding 95% CIs. Elevated levels of both circulating nucleosomes and elastase–α1-antitrypsin complexes were associated with a 3-fold risk of DVT, and the associations remained similar after adjustment for potential confounders (malignancy, smoking, recent immobilization, recent hospitalization). The risk increased with higher nucleosome and elastase–α1-antitrypsin complex levels, suggesting a dose-dependent relationship among circulating nucleosomes, activated neutrophils, and DVT.

Conclusion—

Our study suggests an association among circulating nucleosomes, activated neutrophils, and presence of DVT in humans, which might have implications for treatment and prevention.

Tissue factor pathway inhibitor is an inhibitor of factor VII-activating protease

BACKGROUND

Factor VII-activating protease (FSAP) is a serine protease that circulates in plasma in its inactive single-chain form and can be activated upon contact with dead cells. When activated by apoptotic cells, FSAP leads to the release of nucleosomes. The serpins C1-inhibitor and α(2) -antiplasmin are reported to be the major inhibitors of FSAP. However, regulation of FSAP activity by Kunitz-type inhibitors is not well studied.

OBJECTIVES

To compare the inhibition of FSAP activity and FSAP-induced nucleosome release from apoptotic cells by tissue factor pathway inhibitor (TFPI) with that of C1-inhibitor and α(2) -antiplasmin.

METHODS

Apoptotic cells were incubated with plasma or FSAP in presence of the inhibitor, and nucleosome release was analyzed with flow cytometry. Monoclonal antibodies against TFPI and altered forms of TFPI were used to investigate which domains of TFPI contribute to FSAP inhibition.

RESULTS AND CONCLUSIONS

We show that TFPI abrogates FSAP activity and nucleosome release from apoptotic cells. TFPI is a much more efficient inhibitor than C1-inhibitor or α(2) -antiplasmin. The active site of K2 is required for inhibition of FSAP. A direct binding interaction between FSAP and the C-terminal domain of TFPI is also required for efficient inhibition. Inhibition of FSAP-induced nucleosome release by recombinant TFPI might, in part, explain the anti-inflammatory effects of recombinant TFPI infusion observed in animal and human sepsis.

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

BACKGROUND

Factor seven activating protease (FSAP) was initially reported as an activator of single-chain urokinase-type plasminogen activator (scuPA) and factor VII (FVII). Subsequently, numerous additional substrates have been identified, and multiple other biological effects have been reported. Due to the apparent lack of specificity, the physiological role of FSAP has become increasingly unclear. Rigorous studies have been limited by the difficulty of obtaining intact FSAP from blood or recombinant sources.

OBJECTIVES

Our aim was to produce intact recombinant human FSAP, and to assess its role as a trigger of coagulation and fibrinolysis.

RESULTS

Expression of wild-type FSAP in various mammalian cells invariably resulted in the accumulation of degraded FSAP due to autoactivation and degradation. To overcome this problem, we constructed a variant in which Arg(313) at the natural activation site was replaced by Gln, creating a cleavage site for the bacterial protease thermolysin. HEK293 cells produced FSAP(R313Q) in its intact form. Thermolysin-activated FSAP displayed the same reactivity toward the substrate S-2288 as plasma-derived FSAP, and retained its ability to activate scuPA. Polyphosphate and heparin increased V(max) by 2-3-fold, without affecting K(m) (62 nm) of scuPA activation. Surprisingly, FVII activation by activated FSAP proved negligible, even in the presence of calcium ions, phospholipid vesicles and recombinant soluble tissue factor. On membranes of 100% cardiolipin FVII cleavage did occur, but this resulted in transient activation and rapid degradation.

CONCLUSIONS

While FSAP indeed activates scuPA, FVII appears remarkably resistant to activation. Therefore, reappraisal of the putative role of FSAP in hemostasis seems appropriate.

Extracellular histones increase plasma thrombin generation by impairing thrombomodulin-dependent protein C activation

BACKGROUND

Histones are basic proteins that contribute to cell injury and tissue damage when released into the extracellular space. They have been attributed a prothrombotic activity, because their injection into mice induces diffuse microvascular thrombosis. The protein C-thrombomodulin (TM) system is a fundamental regulator of coagulation, particularly in the microvasculature, and its activity can be differentially influenced by interaction with several cationic proteins.

OBJECTIVE

To evaluate the effect of histones on the protein C-TM system in a plasma thrombin generation assay and in purified systems.

METHODS

The effect of histones on plasma thrombin generation in the presence or absence of TM was analyzed by calibrated automated thrombinography. Protein C activation in purified systems was evaluated by chromogenic substrate cleavage. The binding of TM and protein C to histones was evaluated by solid-phase binding assay.

RESULTS

Histones dose-dependently increased plasma thrombin generation in the presence of TM, independently of its chondroitin sulfate moiety. This effect was not caused by inhibition of activated protein C activity, but by the impairment of TM-mediated protein C activation. Histones were able to bind to both protein C and TM, but the carboxyglutamic acid domain of protein C was required for their effect. Histones H4 and H3 displayed the highest activity. Importantly, unlike heparin, DNA did not inhibit the potentiating effect of histones on thrombin generation.

CONCLUSIONS

Histones enhance plasma thrombin generation by reducing TM-dependent protein C activation. This mechanism might contribute to microvascular thrombosis induced by histones in vivo at sites of organ failure or severe inflammation.

Extracellular histones promote thrombin generation through platelet-dependent mechanisms: involvement of platelet TLR2 and TLR4

The release of histones from dying cells is associated with microvascular thrombosis and, because histones activate platelets, this could represent a possible pathogenic mechanism. In the present study, we assessed the influence of histones on the procoagulant potential of human platelets in platelet-rich plasma (PRP) and in purified systems. Histones dose-dependently enhanced thrombin generation in PRP in the absence of any trigger, as evaluated by calibrated automated thrombinography regardless of whether the contact phase was inhibited. Activation of coagulation required the presence of fully activatable platelets and was not ascribable to platelet tissue factor, whereas targeting polyphosphate with phosphatase reduced thrombin generation even when factor XII (FXII) was blocked or absent. In the presence of histones, purified polyphosphate was able to induce thrombin generation in plasma independently of FXII. In purified systems, histones induced platelet aggregation; P-selectin, phosphatidylserine, and FV/Va expression; and prothrombinase activity. Blocking platelet TLR2 and TLR4 with mAbs reduced the percentage of activated platelets and lowered the amount of thrombin generated in PRP. These data show that histone-activated platelets possess a procoagulant phenotype that drives plasma thrombin generation and suggest that TLR2 and TLR4 mediate the activation process.

Activation of factor VII-activating protease in human inflammation: a sensor for cell death

INTRODUCTION

Cell death is a central event in the pathogenesis of sepsis and is reflected by circulating nucleosomes. Circulating nucleosomes were suggested to play an important role in inflammation and were demonstrated to correlate with severity and outcome in sepsis patients. We recently showed that plasma can release nucleosomes from late apoptotic cells. Factor VII-activating protease (FSAP) was identified to be the plasma serine protease responsible for nucleosome release. The aim of this study was to investigate FSAP activation in patients suffering from various inflammatory diseases of increasing severity.

METHODS

We developed ELISAs to measure FSAP-C1-inhibitor and FSAP-α2-antiplasmin complexes in plasma. FSAP-inhibitor complexes were measured in the plasma of 20 adult patients undergoing transhiatal esophagectomy, 32 adult patients suffering from severe sepsis and 8 from septic shock and 38 children suffering from meningococcal sepsis.

RESULTS

We demonstrate plasma FSAP to be activated upon contact with apoptotic and necrotic cells by an assay detecting complexes between FSAP and its target serpins α2-antiplasmin and C1-inhibitor, respectively. By means of that assay we demonstrate FSAP activation in post-surgery patients, patients suffering from severe sepsis, septic shock and meningococcal sepsis. Levels of FSAP-inhibitor complexes correlate with nucleosome levels and correlate with severity and mortality in these patients.

CONCLUSIONS

These results suggest FSAP activation to be a sensor for cell death in the circulation and that FSAP activation in sepsis might be involved in nucleosome release, thereby contributing to lethality.

Polyamine-promoted autoactivation of plasma hyaluronan-binding protein

BACKGROUND

Plasma hyaluronan-binding protein (PHBP), a protease implicated in extracellular proteolysis, consists of multiple domains: an N-terminal region (NTR), three epidermal growth factor (EGF)-like domains, a kringle domain, and a protease domain. PHBP circulates as a single-chain proenzyme (pro-PHBP), which is converted to an active, two-chain form through autoproteolysis.

OBJECTIVE

To understand the mechanism of autoactivation. Here, we report that polyamine induces the formation of pro-PHBP autoactivation complex, in which an intermolecular interaction between NTR and the third EGF-like domain (E3) plays a role.

METHODS

Using a series of pro-PHBP mutants that partially lack functional domains, polyamine-induced pro-PHBP autoactivation was investigated in terms of enzyme activity, protein interaction, and inhibition by carminic acid, an anthraquinone compound identified in this study.

RESULTS

Polyamine enhanced intermolecular binding of pro-PHBP, but not of mutant pro-PHBP that partially lacked NTR (DeltaN). Carminic acid inhibited intermolecular pro-PHBP binding and specifically abolished polyamine-induced autoactivation. NTR bound to pro-PHBP and DeltaN, but its binding was minimal to a mutant that lacked E3. The NTR-DeltaN binding was inhibited by a combination of polyamine and carminic acid, but each compound alone was ineffective.

CONCLUSIONS

We infer from the data that (i) polyamine modulates intramolecular NTR-E3 interaction to allow intermolecular binding between NTR and E3 in another pro-PHBP molecule to form an autoactivation complex, and (ii) carminic acid inhibits polyamine-modulated intermolecular NTR-E3 binding. Polyamine concentrations are higher in cells and tissues with inflammation and malignancy. Polyamine leakage from legions through cell death or tissue injury may account for physiologically relevant pro-PHBP activation.