Fibrinogen heterogeneity: inherited and noninherited

Dysfibrinogenemia and hypofibrinogenemia - Spectrum of pathogenic variants in Slovak patients

INTRODUCTION

Congenital hypofibrinogenemia (CH) and congenital dysfibrinogenemia (CD) are rare coagulation disorders caused by quantitative or qualitative defects in the fibrinogen gene. The aim of this study was to characterize the genetic background and the clinical manifestations of congenital fibrinogen disorders in the patients from Slovakia registered at the National Haemophilia Centre.

MATERIALS AND METHODS

Results of genetic analysis of the fibrinogen genes FGA, FGB and FGG using polymerase chain reaction followed by direct sequencing were evaluated in 36 patients.

RESULTS

Molecular-genetic analysis revealed six novel variants - FGA c.923_968dup p.(Gly324Lysfs*44) and FGG c.1105C>T p.(His369Tyr) were identified in CD patients. In CH patients, in the FGG gene c.8G>A p.(Trp3*), c.823G>T p.(Glu275*) and c.323C>A p.(Ala108Asp) variants were detected. In the FGB gene c.1427C>T p.(Ser476Leu) was identified.

CONCLUSION

This study is a positive contribution towards expanding knowledge about genetic variants in patients with congenital fibrinogen disorders.

Interplay of fibrinogen αEC globular domains and factor XIIIa cross-linking dictates the extensibility and strain stiffening of fibrin networks

BACKGROUND

Fibrinogen is a plasma protein forming the fibrin scaffold of blood clots. Its mechanical properties therefore affect the risk of bleeding as well as thrombosis. There has been much recent interest in the biophysical mechanisms controlling fibrin mechanics; however, the role of molecular heterogeneity of the circulating fibrinogen in determining clot mechanical function remains poorly characterized.

OBJECTIVES

By comparing 2 fibrinogen variants where the only difference is the Aα-chain length, with one variant having a globular domain at its C-terminus, this study aimed to reveal how the molecular structure impacts the structure and mechanics of fibrin networks.

METHODS

We characterized the mechanical response to large shear for networks formed from 2 recombinant fibrinogen variants: the most prevalent variant in circulation with a molecular weight of 340 kDa (recombinant human fibrinogen [rFib] 340) and a minor variant with a molecular weight of 420 kDa (rFib420).

RESULTS

We show that the elastic properties of the 2 variants are identical when fibrin is cross-linked with factor XIIIa but differ strongly in its absence. Uncross-linked rFib420 networks are softer and up to 3-fold more extensible than rFib340 networks. Electron microscopy imaging showed that the 2 variants formed networks with a comparable structure, except at 4 mg/mL, where rFib420 formed denser networks.

CONCLUSION

We propose that the αEC domains of rFib420 increase the extensibility of uncross-linked fibrin networks by promoting protofibril sliding, which is blocked by FXIIIa cross-linking. Our findings can help explain the functional role of different circulating fibrinogen variants in blood clot mechanics and tissue repair.

Validation of a fibrinogen γ' enzyme-linked immunosorbent assay using a new monoclonal antibody: effects of bariatric surgery

Background

Fibrinogen γ' is a naturally occurring 20-amino-acid splice variant of the fibrinogen γ chain. Animal studies link variations in fibrinogen to obesity, but it is unknown how fibrinogen γ' is associated with obesity in humans.

Objectives

To develop and validate an enzyme-linked immunosorbent assay (ELISA) for fibrinogen γ' quantification in human plasma and analyze fibrinogen γ' before and after bariatric surgery.

Methods

We generated C-terminal fibrinogen γ' specific mouse monoclonal antibodies and developed a γ' ELISA. Validation included measures of accuracy, sensitivity, and precision. Fibrinogen γ' and total fibrinogen were measured in 60 individuals before and 6 months after bariatric surgery and in 19 normal-weight controls and 120 blood donors.

Results

Highly specific fibrinogen γ' monoclonal antibodies were produced and successfully used in the ELISA. Recovery was 88%, and limits of detection and quantification were 0.003 mg/mL and 0.014 mg/mL, respectively. Coefficients of variation were 3% for repeatability and 7% for within-laboratory variation. The fibrinogen γ' reference interval was 0.25 to 0.80 mg/mL. Fibrinogen γ' concentrations were reduced after bariatric surgery and were higher in individuals with obesity than in those with normal weight. The fibrinogen γ'/total fibrinogen ratio was unchanged after surgery but was higher than the ratio in normal-weight individuals.

Conclusion

We developed a precise and sensitive ELISA for fibrinogen γ'. Levels of fibrinogen γ', but not the fibrinogen γ'/fibrinogen ratio, were reduced 6 months after bariatric surgery. Absolute and relative levels of fibrinogen γ' were increased in individuals with obesity compared to normal-weight individuals.

Levels of Fibrinogen Variants Are Altered in Severe COVID-19

Background  Fibrinogen variants as a result of alternative messenger RNA splicing or protein degradation can affect fibrin(ogen) functions. The levels of these variants might be altered during coronavirus disease 2019 (COVID-19), potentially affecting disease severity or the thrombosis risk. Aim  To investigate the levels of fibrinogen variants in plasma of patients with COVID-19. Methods  In this case-control study, we measured levels of functional fibrinogen using the Clauss assay. Enzyme-linked immunosorbent assays were used to measure antigen levels of total, intact (nondegraded Aα chain), extended Aα chain (α _E ), and γ' fibrinogen in healthy controls, patients with pneumococcal infection in the intensive care unit (ICU), ward patients with COVID-19, and ICU patients with COVID-19 (with and without thrombosis, two time points). Results  Healthy controls and ward patients with COVID-19 ( n  = 10) showed similar fibrinogen (variant) levels. ICU patients with COVID-19 who later did ( n  = 19) or did not develop thrombosis ( n  = 18) and ICU patients with pneumococcal infection ( n  = 6) had higher absolute levels of functional, total, intact, and α _E fibrinogen than healthy controls ( n  = 7). The relative α _E fibrinogen levels were higher in ICU patients with COVID-19 than in healthy controls, while relative γ' fibrinogen levels were lower. After diagnosis of thrombosis, only the functional fibrinogen levels were higher in ICU patients with COVID-19 and thrombosis than in those without, while no differences were observed in the other fibrinogen variants. Conclusion  Our results show that severe COVID-19 is associated with increased levels of α _E fibrinogen and decreased relative levels of γ' fibrinogen, which may be a cause or consequence of severe disease, but this is not associated with the development of thrombosis.

Impaired microvascular circulation in distant organs following renal ischemia

Mortality in acute kidney injury (AKI) patients remains very high, although very important advances in understanding the pathophysiology and in diagnosis and supportive care have been made. Most commonly, adverse outcomes are related to extra-renal organ dysfunction and failure. We and others have documented inflammation in remote organs as well as microvascular dysfunction in the kidney after renal ischemia. We hypothesized that abnormal microvascular flow in AKI extends to distant organs. To test this hypothesis, we employed intravital multiphoton fluorescence imaging in a well-characterized rat model of renal ischemia/reperfusion. Marked abnormalities in microvascular flow were seen in every organ evaluated, with decreases up to 46% observed 48 hours postischemia (as compared to sham surgery, p = 0.002). Decreased microvascular plasma flow was found in areas of erythrocyte aggregation and leukocyte adherence to endothelia. Intravital microscopy allowed the characterization of the erythrocyte formations as rouleaux that flowed as one-dimensional aggregates. Observed microvascular abnormalities were associated with significantly elevated fibrinogen levels. Plasma flow within capillaries as well as microthrombi, but not adherent leukocytes, were significantly improved by treatment with the platelet aggregation inhibitor dipyridamole. These microvascular defects may, in part, explain known distant organ dysfunction associated with renal ischemia. The results of these studies are relevant to human acute kidney injury.

Thrombosis-associated hypofibrinogenemia: novel abnormal fibrinogen variant FGG c.8G>A with oxidative posttranslational modifications

Here, we present the first case of fibrinogen variant FGG c.8G>A. We investigated the behaviour of this mutated fibrinogen in blood coagulation using fibrin polymerization, fibrinolysis, fibrinopeptides release measurement, mass spectrometry (MS), and scanning electron microscopy (SEM). The case was identified by routine coagulation testing of a 34-year-old man diagnosed with thrombosis. Initial genetic analysis revealed a heterozygous mutation in exon 1 of the FGG gene encoding gamma chain signal peptide. Fibrin polymerization by thrombin and reptilase showed the normal formation of the fibrin clot. However, maximal absorbance within polymerization was lower and fibrinolysis had a longer degradation phase than healthy control. SEM revealed a significant difference in clot structure of the patient, and interestingly, MS detected several posttranslational oxidations of fibrinogen. The data suggest that the mutation FGG c.8G>A with the combination of the effect of posttranslational modifications causes a novel case of hypofibrinogenemia associated with thrombosis.

Mutations Accounting for Congenital Fibrinogen Disorders: An Update

Fibrinogen is a complex protein that plays a key role in the blood clotting process. It is a hexamer composed of two copies of three distinct chains: Aα, Bβ, and γ encoded by three genes, FGA, FGB, and FGG, clustered on the long arm of chromosome 4. Congenital fibrinogen disorders (CFDs) are divided into qualitative deficiencies (dysfibrinogenemia, hypodysfibrinogenemia) in which the mutant fibrinogen molecule is present in the circulation and quantitative deficiencies (afibrinogenemia, hypofibrinogenemia) with no mutant molecule present in the bloodstream. Phenotypic manifestations are variable, patients may be asymptomatic, or suffer from bleeding or thrombosis. Causative mutations can occur in any of the three fibrinogen genes and can affect one or both alleles. Given the large number of studies reporting on novel causative mutations for CFDs since the review on the same topic published in 2016, we performed an extensive search of the literature and list here 120 additional mutations described in both quantitative and qualitative disorders. The visualization of causative single nucleotide variations placed on the coding sequences of FGA, FGB, and FGG reveals important structure function insight for several domains of the fibrinogen molecule.

Fibrin(ogen) as a Therapeutic Target: Opportunities and Challenges

Fibrinogen is one of the key molecular players in haemostasis. Thrombin-mediated release of fibrinopeptides from fibrinogen converts this soluble protein into a network of fibrin fibres that form a building block for blood clots. Thrombin-activated factor XIII further crosslinks the fibrin fibres and incorporates antifibrinolytic proteins into the network, thus stabilising the clot. The conversion of fibrinogen to fibrin also exposes binding sites for fibrinolytic proteins to limit clot formation and avoid unwanted extension of the fibrin fibres. Altered clot structure and/or incorporation of antifibrinolytic proteins into fibrin networks disturbs the delicate equilibrium between clot formation and lysis, resulting in either unstable clots (predisposing to bleeding events) or persistent clots that are resistant to lysis (increasing risk of thrombosis). In this review, we discuss the factors responsible for alterations in fibrin(ogen) that can modulate clot stability, in turn predisposing to abnormal haemostasis. We also explore the mechanistic pathways that may allow the use of fibrinogen as a potential therapeutic target to treat vascular thrombosis or bleeding disorders. Better understanding of fibrinogen function will help to devise future effective and safe therapies to modulate thrombosis and bleeding risk, while maintaining the fine balance between clot formation and lysis.

Vascular Network Formation on Macroporous Polydioxanone Scaffolds

In this study, microvascular network structures for tissue engineering were generated on newly developed macroporous polydioxanone (PDO) scaffolds. PDO represents a polymer biodegradable within months and offers optimal material properties such as elasticity and nontoxic degradation products. PDO scaffolds prepared by porogen leaching and cryo-dried to achieve pore sizes of 326 ± 149.67 μm remained stable with equivalent values for Young's modulus after 4 weeks. Scaffolds were coated with fibrin for optimal cell adherence. To exclude interindividual differences, autologous fibrin was prepared out of human plasma-derived fibrinogen and proved a comparable quality to nonautologous commercially available fibrinogen. Fibrin-coated scaffolds were seeded with recombinant human umbilical vein endothelial cells expressing GFP (GFP-HUVECs) in coculture with adipose tissue-derived mesenchymal stem cells (AD-hMSCs) to form vascular networks. The growth factor content in culture media was optimized according its effect on network formation, quantified and assessed by AngioTool^®. A ratio of 2:3 GFP-HUVECs/AD-hMSCs in medium enriched with 20 ng/mL vascular endothelial growth factor, basic fibroblast growth factor, and hydrocortisone was found to be optimal. Network structures appeared after 2 days of cultivation and stabilized until day 7. The resulting networks were lumenized that could be verified by dextran staining. This new approach might be suitable for microvascular tissue patches as a useful template to be used in diverse vascularized tissue constructs.

Biomarker association with cardiovascular disease and mortality - The role of fibrinogen. A report from the NHANES study

BACKGROUND

While fibrinogen is a known cardiovascular disease (CVD) risk marker, its quantitative input to mortality risk is a topic of debate.

METHODS

We investigated the contribution of fibrinogen, among that of other biomarkers, to prevalent CVD and incident CVD mortality in 4487 participants of the US National Health and Nutrition Examination Survey (NHANES). Participants were observed for a median period of 14 years, resulting in more than 58,000 person-years.

RESULTS

At baseline 551 participants had CVD and during follow up, 1339 all-cause deaths occurred, 321 (24%) of which were due to CVD. Hierarchical cluster analysis and principal component analysis (PCA) were performed to derive clusters of association between biomarkers. Next, structural equation modelling was performed to investigate the association of these clusters with baseline CVD and all-cause and CVD mortality during follow-up. Mediation analysis was used to determine which biomarkers played a mediatory role between prevalent CVD and future mortality. Fibrinogen clustered with C-reactive protein only and was associated with CVD at baseline (p < 0.0001) and with all-cause (p < 0.001) and CVD (p < 0.001) mortality at follow-up. Only fibrinogen (4.7%), followed by gamma-glutamyl transferase (GGT) (3.5%) and uric acid (2.3%) were identified as possible mediators between CVD status and all-cause mortality, with fibrinogen (3.2%) and GGT (3.1%) the only mediators between CVD status and CVD mortality.

CONCLUSION

This data shows that fibrinogen is not only cross-sectionally associated with CVD, but also contributes to all-cause and CVD mortality at follow-up. It furthermore appears to mediate the association between prevalent CVD and both all-cause and CVD mortality.

Fibrin(ogen) in human disease: both friend and foe

Fibrinogen is an abundant protein synthesized in the liver, present in human blood plasma at concentrations ranging from 1.5-4 g/L in healthy individuals with a normal half-life of 3-5 days. With fibrin, produced by thrombin-mediated cleavage, fibrinogen plays important roles in many physiological processes. Indeed, the formation of a stable blood clot, containing polymerized and cross-linked fibrin, is crucial to prevent blood loss and drive wound healing upon vascular injury. A balance between clotting, notably the conversion of fibrinogen to fibrin, and fibrinolysis, the proteolytic degradation of the fibrin mesh, is essential. Disruption of this equilibrium can cause disease in distinct manners. While some pathological conditions are the consequence of altered levels of fibrinogen, others are related to structural properties of the molecule. The source of fibrinogen expression and the localization of fibrin(ogen) protein also have clinical implications. Low levels of fibrinogen expression have been detected in extra-hepatic tissues, including carcinomas, potentially contributing to disease. Fibrin(ogen) deposits at aberrant sites including the central nervous system or kidney, can also be pathological. In this review, we discuss disorders in which fibrinogen and fibrin are implicated, highlighting mechanisms that may contribute to disease.

Effects of Post-Translational Modifications of Fibrinogen on Clot Formation, Clot Structure, and Fibrinolysis: A Systematic Review

Supplemental Digital Content is available in the text.

Post-translational modifications of fibrinogen influence the occurrence and progression of thrombotic diseases. In this systematic review, we assessed the current literature on post-translational modifications of fibrinogen and their effects on fibrin formation and clot characteristics.

Clinical Consequences and Molecular Bases of Low Fibrinogen Levels

The study of inherited fibrinogen disorders, characterized by extensive allelic heterogeneity, allows the association of defined mutations with specific defects providing significant insight into the location of functionally important sites in fibrinogen and fibrin. Since the identification of the first causative mutation for congenital afibrinogenemia, studies have elucidated the underlying molecular pathophysiology of numerous causative mutations leading to fibrinogen deficiency, developed cell-based and animal models to study human fibrinogen disorders, and further explored the clinical consequences of absent, low, or dysfunctional fibrinogen. Since qualitative disorders are addressed by another review in this special issue, this review will focus on quantitative disorders and will discuss their diagnosis, clinical features, molecular bases, and introduce new models to study the phenotypic consequences of fibrinogen deficiency.

Recombinant fibrinogen reveals the differential roles of α- and γ-chain cross-linking and molecular heterogeneity in fibrin clot strain-stiffening

Essentials Fibrinogen circulates in human plasma as a complex mixture of heterogeneous molecular variants. We measured strain-stiffening of recombinantly produced fibrinogen upon clotting. Factor XIII and molecular heterogeneity alter clot elasticity at the protofibril and fiber level. This highlights the hitherto unknown role of molecular composition in fibrin clot mechanics.

SUMMARY

Background Fibrin plays a crucial role in haemostasis and wound healing by forming strain-stiffening fibrous networks that reinforce blood clots. The molecular origin of fibrin's strain-stiffening behavior remains poorly understood, primarily because plasma fibrinogen is a complex mixture of heterogeneous molecular variants and is often contaminated by plasma factors that affect clot properties. Objectives and methods To facilitate mechanistic dissection of fibrin nonlinear elasticity, we produced a homogeneous recombinant fibrinogen corresponding to the main variant in human plasma, termed rFib610. We characterized the structure of rFib610 clots using turbidimetry, microscopy and X-ray scattering. We used rheology to measure the strain-stiffening behavior of the clots and determined the fiber properties by modeling the clots as semi-flexible polymer networks. Results We show that addition of FXIII to rFib610 clots causes a dose-dependent stiffness increase at small deformations and renders the strain-stiffening response reversible. We find that γ-chain cross-linking contributes to clot elasticity by changing the force-extension behavior of the protofibrils, whereas α-chain cross-linking stiffens the fibers, as a consequence of tighter coupling between the constituent protofibrils. Interestingly, rFib610 protofibrils have a 25% larger bending rigidity than plasma-purified fibrin protofibrils and a delayed strain-stiffening, indicating that molecular heterogeneity influences clot mechanics at the protofibril scale. Conclusions Fibrinogen molecular heterogeneity and FXIII affect the mechanical function of fibrin clots by altering the nonlinear viscoelastic properties at the protofibril and fiber scale. This work provides a starting point to investigate the role of molecular heterogeneity of plasma fibrinogen in fibrin clot mechanics and haemostasis.

Fibrin Formation, Structure and Properties

Fibrinogen and fibrin are essential for hemostasis and are major factors in thrombosis, wound healing, and several other biological functions and pathological conditions. The X-ray crystallographic structure of major parts of fibrin(ogen), together with computational reconstructions of missing portions and numerous biochemical and biophysical studies, have provided a wealth of data to interpret molecular mechanisms of fibrin formation, its organization, and properties. On cleavage of fibrinopeptides by thrombin, fibrinogen is converted to fibrin monomers, which interact via knobs exposed by fibrinopeptide removal in the central region, with holes always exposed at the ends of the molecules. The resulting half-staggered, double-stranded oligomers lengthen into protofibrils, which aggregate laterally to make fibers, which then branch to yield a three-dimensional network. Much is now known about the structural origins of clot mechanical properties, including changes in fiber orientation, stretching and buckling, and forced unfolding of molecular domains. Studies of congenital fibrinogen variants and post-translational modifications have increased our understanding of the structure and functions of fibrin(ogen). The fibrinolytic system, with the zymogen plasminogen binding to fibrin together with tissue-type plasminogen activator to promote activation to the active proteolytic enzyme, plasmin, results in digestion of fibrin at specific lysine residues. In spite of a great increase in our knowledge of all these interconnected processes, much about the molecular mechanisms of the biological functions of fibrin(ogen) remains unknown, including some basic aspects of clotting, fibrinolysis, and molecular origins of fibrin mechanical properties. Even less is known concerning more complex (patho)physiological implications of fibrinogen and fibrin.

The Relationship Between Circulating Fibrinogen and Lipoprotein (a) Levels in Patients With Primary Dyslipidemia

The correlation between 2 predictors of vascular events, plasma fibrinogen and serum lipoprotein (a), was evaluated in patients referred to a specialist clinic because of primary hyperlipidemia. A significant correlation existed between fibrinogen and lipoprotein (a) in nonsmokers but not in smokers. Plasma fibrinogen concentration correlated positively and significantly with serum lipoprotein (a) levels in men nonsmokers without cardiovascular disease and in women nonsmokers with cardiovascular disease. Nonsmoker women without cardiovascular disease had significantly higher plasma fibrinogen (3.63 g/L versus 3.07 g/L, P < .0001) than the corresponding men. Nonsmoker women with and without cardiovascular disease had significantly higher lipoprotein (a) levels than the corresponding groups of men (0.36 versus 0.18 g/L; P = .0015 and 0.40 versus 0.26 g/L; P = .008), respectively. The relationship between fibrinogen and lipoprotein (a) levels alters markedly depending on the population selected. This relationship is influenced by gender, the presence of cardiovascular disease and smoking status.

Hyperfibrinogenemia predicts long-term risk of death after ischemic stroke

In stroke patients higher levels of plasma fibrinogen are associated with increased risk of unfavourable functional outcome and short-term mortality. The aim of our study was to determine the relationship between plasma fibrinogen level and long-term risk of death in ischemic stroke patients. Seven hundred thirty six patients (median age 71; 47.1 % men) admitted to the stroke unit within 24 h after stroke were included. Plasma fibrinogen level was measured on day 1 of hospitalisation. Hyperfibrinogenemia was defined as plasma fibrinogen concentration >3.5 g/L. The maximal follow-up period was 84 months. Hyperfibrinogenemia was found in 25.0 % of patients. On multivariate logistic regression analysis, after adjustment for age, stroke severity, atrial fibrillation, smoking, white blood cell count, fever, in-hospital pneumonia and hyperglycemia, hyperfibrinogenemia was associated with increased case fatality (HR 1.71, 95 % CI 1.29–2.26, P < 0.01). Hyperfibrinogenemia predicts the long-term risk of death in ischemic stroke patients.

A central role for intermolecular dityrosine cross-linking of fibrinogen in high molecular weight advanced oxidation protein product (AOPP) formation

BACKGROUND

Advanced oxidation protein products (AOPPs) are dityrosine cross-linked and carbonyl-containing protein products formed by the reaction of plasma proteins with chlorinated oxidants, such as hypochlorous acid (HOCl). Most studies consider human serum albumin (HSA) as the main protein responsible for AOPP formation, although the molecular composition of AOPPs has not yet been elucidated. Here, we investigated the relative contribution of HSA and fibrinogen to generation of AOPPs.

METHODS

AOPP formation was explored by SDS-PAGE, under both reducing and non-reducing conditions, as well as by analytical gel filtration HPLC coupled to fluorescence detection to determine dityrosine and pentosidine formation.

RESULTS

Following exposure to different concentrations of HOCl, HSA resulted to be carbonylated but did not form dityrosine cross-linked high molecular weight aggregates. Differently, incubation of fibrinogen or HSA/fibrinogen mixtures with HOCl at concentrations higher than 150 μM induced the formation of pentosidine and high molecular weight (HMW)-AOPPs (>200 k Da), resulting from intermolecular dityrosine cross-linking. Dityrosine fluorescence increased in parallel with increasing HMW-AOPP formation and increasing fibrinogen concentration in HSA/fibrinogen mixtures exposed to HOCl. This conclusion is corroborated by experiments where dityrosine fluorescence was measured in HOCl-treated human plasma samples containing physiological or supra-physiological fibrinogen concentrations or selectively depleted of fibrinogen, which highlighted that fibrinogen is responsible for the highest fluorescence from dityrosine.

CONCLUSIONS

A central role for intermolecular dityrosine cross-linking of fibrinogen in HMW-AOPP formation is shown.

GENERAL SIGNIFICANCE

These results highlight that oxidized fibrinogen, instead of HSA, is the key protein for intermolecular dityrosine formation in human plasma.

Factor XIII stiffens fibrin clots by causing fiber compaction

BACKGROUND

Factor XIII-induced cross-linking has long been associated with the ability of fibrin blood clots to resist mechanical deformation, but how FXIII can directly modulate clot stiffness is unknown.

OBJECTIVES AND METHODS

We hypothesized that FXIII affects the self-assembly of fibrin fibers by altering the lateral association between protofibrils. To test this hypothesis, we studied the cross-linking kinetics and the structural evolution of the fibers and clots during the formation of plasma-derived and recombinant fibrins by using light scattering, and the response of the clots to mechanical stresses by using rheology.

RESULTS

We show that the lateral aggregation of fibrin protofibrils initially results in the formation of floppy fibril bundles, which then compact to form tight and more rigid fibers. The first stage is reflected in a fast (10 min) increase in clot stiffness, whereas the compaction phase is characterized by a slow (hours) development of clot stiffness. Inhibition of FXIII completely abrogates the slow compaction. FXIII strongly increases the linear elastic modulus of the clots, but does not affect the non-linear response at large deformations.

CONCLUSIONS

We propose a multiscale structural model whereby FXIII-mediated cross-linking tightens the coupling between the protofibrils within a fibrin fiber, thus making the fiber stiffer and less porous. At small strains, fiber stiffening enhances clot stiffness, because the clot response is governed by the entropic elasticity of the fibers, but once the clot is sufficiently stressed, the modulus is independent of protofibril coupling, because clot stiffness is governed by individual protofibril stretching.

Identification and characterization of α1 -antitrypsin in fibrin clots

BACKGROUND AND OBJECTIVES

Preliminary studies indicated that α1 -antitrypsin (A1AT) is the most abundant protein that is non-covalently bound to fibrin clots prepared from plasma. The aim of this study was to identify and characterize fibrin(ogen)-bound A1AT.

METHODS AND RESULTS

Plasma clots were prepared and extensively washed with saline. Clot-bound A1AT could only be extracted using denaturing agents such as urea, thiourea or SDS, pointing to an apparently strong association. Purified fibrinogen, but still containing A1AT as a contaminant, was gel filtered, which showed that the A1AT was bound to fibrinogen. A specific ELISA detected the presence of A1AT-fibrinogen complexes in both purified fibrinogen and pooled normal plasma. Finally, fibrin(ogen)-Sepharose chromatography indicated that A1AT purified from plasma contained a small fraction of fibrin(ogen)-binding A1AT. To study the inhibitory activity of fibrin(ogen)-bound A1AT, both fibrinogen containing A1AT and washed plasma clots were incubated with increasing amounts of elastase. SDS-PAGE and Western blotting showed under both conditions the generation of the A1AT-elastase complex as well as cleaved A1AT. The inhibitory activity of fibrin(ogen)-bound A1AT was also demonstrated by measuring elastase-induced lysis of fibrin clots.

CONCLUSION

Fibrin clots contain strongly bound A1AT, which is functionally active as a serine protease inhibitor (serpin). This A1AT might play a role in the local regulation of proteases involved in coagulation or fibrinolysis and represent a novel link between the inflammatory and hemostatic systems.

The assembly of nonadhesive fibrinogen matrices depends on the αC regions of the fibrinogen molecule

Adsorption of fibrinogen on fibrin clots and other surfaces strongly reduces integrin-mediated adhesion of platelets and leukocytes with implications for the surface-mediated control of thrombus growth and blood compatibility of biomaterials. The underlying mechanism of this process is surface-induced aggregation of fibrinogen, resulting in the assembly of a nanoscale multilayered matrix. The matrix is extensible, which makes it incapable of transducing strong mechanical forces via cellular integrins, resulting in insufficient intracellular signaling and weak cell adhesion. To determine the mechanism of the multilayer formation, the physical and adhesive properties of fibrinogen matrices prepared from human plasma fibrinogen (hFg), recombinant normal (rFg), and fibrinogen with the truncated αC regions (FgAα251) were compared. Using atomic force microscopy and force spectroscopy, we show that whereas hFg and rFg generated the matrices with a thickness of ∼8 nm consisting of 7-8 molecular layers, the deposition of FgAα251 was terminated at two layers, indicating that the αC regions are essential for the multilayer formation. The extensibility of the matrix prepared from FgAα251 was 2-fold lower than that formed from hFg and rFg. In agreement with previous findings that cell adhesion inversely correlates with the extensibility of the fibrinogen matrix, the less extensible FgAα251 matrix and matrices generated from human fibrinogen variants lacking the αC regions supported sustained adhesion of leukocytes and platelets. The persistent adhesiveness of matrices formed from fibrinogen derivatives without the αC regions may have implications for conditions in which elevated levels of these molecules are found, including vascular pathologies, diabetes, thrombolytic therapy, and dysfibrinogenemia.

A novel missense mutation in the FGB g. 3354 T>A (p. Y41N), fibrinogen Caracas VIII

A novel dysfibrinogenaemia with a replacement of Tyr by Asn at Bβ41 has been discovered (fibrinogen Caracas VIII). An asymptomatic 39-year-old male was diagnosed as having dysfibrinogenaemia due to a mildly prolonged thrombin time (+ 5.8 seconds); his fibrinogen concentration was in the low normal range, both by Clauss and gravimetric determination, 1.9 g/l and 2.1 g/l, respectively. The plasma polymerization process was slightly impaired, characterised by a mildly prolonged lag time and a slightly increased final turbidity. Permeation through the patients' clots was dramatically increased, with the Darcy constant around four times greater than that of the control (22 ± 2 x 10(-9) cm² compared to 6 ± 0.5 x 10(-9) cm² in controls). The plasma fibrin structure of the patient, by scanning electron microscopy, featured a mesh composed of thick fibres (148 ± 50 nm vs. 120 ± 31 nm in controls, p<0.05) and larger pores than those of the control fibrin clot. The viscoelastic properties of the clot from the patient were also altered, as the storage modulus (G', 310 ± 30) was much lower than in the control (831 ± 111) (p ≤0.005). The interaction of the fibrin clot with a monolayer of human microvascular endothelial cells, by confocal laser microscopy, revealed that the patients' fibrin network had less interaction with the cells. These results demonstrate the significance of the amino terminal end of the β chain of fibrin in the polymerisation process and its consequences on the clot organisation on the surface of endothelial cells.

Molecular weight fibrinogen variants alter gene expression and functional characteristics of human endothelial cells

BACKGROUND

Fibrin is a temporary matrix that not only seals a wound, but also provides a temporary matrix structure for invading cells during wound healing. Two naturally occurring fibrinogen variants, high molecular weight (HMW) and low molecular weight (LMW) fibrinogen, display different properties in supporting angiogenesis in vivo and in vitro.

OBJECTIVES

This study was aimed at investigating the functional characteristics and molecular mechanisms of human microvascular endothelial cells (HMVECs) cultured on HMW and LMW fibrin matrices.

METHODS AND RESULTS

HMVECs on HMW fibrin matrices showed increased proliferation and tube formation as compared with their counterparts on unfractionated and LMW fibrin. Degradation of HMW fibrin was markedly enhanced by the presence of HMVECs, that of LMW fibrin was enhanced only slightly. However, the expression levels of fibrinolysis-regulating proteins and integrins were similar. Subsequent microarray analysis revealed that the expression of 377 genes differed significantly between HMVECs cultured on HMW fibrin and those cultured on LMW fibrin. Among these genes, UNC5B, DLL4 and the DLL4-Notch downstream targets Hey1, Hey2 and Hes1 showed increased expression in HMVECs on LMW fibrin. However, pharmacologic and genetic (DLL4 small interfering RNA) inhibition of DLL4-Notch signaling blunted rather than enhanced proliferation and tube formation by HMVECs on both fibrin variants.

CONCLUSIONS

Heterogeneity in naturally occurring fibrinogen strongly influences endothelial cell proliferation and tube formation, and causes alterations in gene expression, including that of DLL4-Notch. The higher fibrinolytic sensitivity of HMW fibrin in the presence of HMVECs contributes to increased tube formation. Although the expression of DLL4-Notch was altered, it did not explain the enhanced tube formation in HMW fibrin. This study provides new perspectives for biological and tissue engineering applications.

Fibrinogen triggers astrocyte scar formation by promoting the availability of active TGF-beta after vascular damage

Scar formation in the nervous system begins within hours after traumatic injury and is characterized primarily by reactive astrocytes depositing proteoglycans that inhibit regeneration. A fundamental question in CNS repair has been the identity of the initial molecular mediator that triggers glial scar formation. Here we show that the blood protein fibrinogen, which leaks into the CNS immediately after blood-brain barrier (BBB) disruption or vascular damage, serves as an early signal for the induction of glial scar formation via the TGF-beta/Smad signaling pathway. Our studies revealed that fibrinogen is a carrier of latent TGF-beta and induces phosphorylation of Smad2 in astrocytes that leads to inhibition of neurite outgrowth. Consistent with these findings, genetic or pharmacologic depletion of fibrinogen in mice reduces active TGF-beta, Smad2 phosphorylation, glial cell activation, and neurocan deposition after cortical injury. Furthermore, stereotactic injection of fibrinogen into the mouse cortex is sufficient to induce astrogliosis. Inhibition of the TGF-beta receptor pathway abolishes the fibrinogen-induced effects on glial scar formation in vivo and in vitro. These results identify fibrinogen as a primary astrocyte activation signal, provide evidence that deposition of inhibitory proteoglycans is induced by a blood protein that leaks in the CNS after vasculature rupture, and point to TGF-beta as a molecular link between vascular permeability and scar formation.

Concentration-dependent effect of fibrinogen on IgG-specific antigen binding and phagocytosis

In this paper, we aim to characterize fibrinogen-IgG interactions, and explore how fibrinogen alters IgG-mediated phagocytosis. Using enzyme-linked binding assays, we found that fibrinogen binding to IgG is optimized for surfaces coated with high levels of IgG. Using a similar method, we have shown that for an antigen unable to specifically bind fibrinogen, fibrinogen enhances binding of antibodies towards that antigen. For binding of IgG antibodies to cells expressing Fc receptors, we found a bimodal binding response, where low levels of fibrinogen enhance binding of antibody to Fc receptors and high levels reduce it. This corresponds to a bimodal effect on phagocytosis of IgG-coated particles, which is inhibited in the presence of excess IgG during coating of the particles with antibodies and fibrinogen. We conclude that fibrinogen can modulate phagocytosis of IgG-coated particles in vitro by changing IgG binding behavior, and that high fibrinogen levels could negatively affect phagocytosis.

Interaction between fibrinogen and IL-6 genetic variants and associations with cardiovascular disease risk in the Cardiovascular Health Study

The inflammatory cytokine interleukin-6 (IL-6) is a main regulator of fibrinogen synthesis, though its interaction with fibrinogen genes (FGA, FGB, FGG) and subsequent impact on cardiovascular disease (CVD) risk is not well-studied. We investigated joint associations of fibrinogen and IL6 tagSNPs with fibrinogen concentrations, carotid intima-media thickness, and myocardial infarction or ischemic stroke in 3900 European-American Cardiovascular Health Study participants. To identify combinations of genetic main effects and interactions associated with outcomes, we used logic regression. We also evaluated whether the relationship between fibrinogen SNPs and fibrinogen level varied by IL-6 level using linear regression models with multiplicative interaction terms. Combinations of fibrinogen and IL6 SNPs were significantly associated with fibrinogen level (p < 0.005), but not with other outcomes. Fibrinogen levels were higher in individuals having FGB1437 (rs1800790) and lacking FGA6534 (rs6050) minor alleles; these SNPs interacted with IL6 rs1800796 to influence fibrinogen level. Marginally significant (p= 0.03) interactions between IL-6 level and FGA and FGG promoter SNPs associated with fibrinogen levels were detected. We identified potential gene-gene interactions influencing fibrinogen levels. Although IL-6 responsive binding sites are present in fibrinogen gene promoter regions, we did not find strong evidence of interaction between fibrinogen SNPs and IL6 SNPs or levels influencing CVD.

Genetic variation in the fibrinogen-alpha and fibrinogen-gamma genes in relation to arterial stiffness: the Rotterdam Study

OBJECTIVE

Arterial stiffness increases with age and predicts cardiovascular disease. Fibrinogen is an acute-phase protein and some studies showed an association with arterial stiffness. We studied genetic variation in the fibrinogen-alpha (FGA) and fibrinogen-gamma (FGG) genes, by means of single nucleotide polymorphisms (FGA: -58 G/A, 1374 G/A, 1526 T/C, 312 Thr/Ala, and FGG: 4288 G/A, 6326 G/A, 7792 T/C) and resultant haplotypes in relation to arterial stiffness.

METHODS

The present study (n = 3891) was embedded in the Rotterdam Study. Associations of the fibrinogen level, genotypes and haplotypes with aortic stiffness (pulse wave velocity), carotid stiffness (distensibility coefficient) and pulse pressure were investigated in men and women by analyses of variance, linear regression and by haplotype analyses. Analyses were adjusted for age, mean arterial pressure, heart rate, known cardiovascular risk factors and atherosclerosis.

RESULTS

Genotype analyses yielded associations of FGA-58 G/A (P = 0.040, for trend) and FGA-1526 T/C (P = 0.004, for trend) with the fibrinogen levels, but no consistent associations with arterial stiffness, in women. FGA-haplotype 4 was associated with the fibrinogen level (P = 0.02) in women. FGA-haplotype 3 and FGG-haplotype 2 were associated with aortic stiffness (P = 0.05) in women. No associations were found in men.

CONCLUSION

Findings indicate that the fibrinogen level and genetic variation in the FGA and FGG genes may influence arterial stiffness in women.

Simulated Point Mutations in the Aα-Chain of Human Fibrinogen Support a Role of the αC Domain in the Stabilization of Fibrin Gel

The hydrophobicity pattern distribution in the Aalpha-, Bbeta- and gamma-chains of human fibrinogen has been studied by a nonlinear method, recurrence quantification analysis, in the wild type and in a number of naturally occurring or simulated mutants. The aim was to find a structural basis for distinguishing between silent and pathological mutants. We were successful in the case of mutations on the Aalpha-chain, thanks to the peculiar features of this chain as compared to the other two. Relevant findings concerning the point mutants of the Aalpha-chain are the following: (a) the recurrence quantification analysis-based classification of such mutants is in good agreement with the clinical classification, and (b) the location of the mutated residue on the sequence plays a more relevant role than its hydrophobic features. Artificial point mutants in the terminal zone (600-866 residues) of the extended isoform of the Aalpha-chain cluster together with the natural hemorrhagic mutants of the first (1-207) residues.

Reduced clot permeability and susceptibility to lysis in patients with acute coronary syndrome: effects of inflammation and oxidative stress

BACKGROUND

Stable angina is associated with unfavorable fibrin structure/function. It is not known how acute coronary syndromes (ACS) affect fibrin architecture.

OBJECTIVE

We investigated fibrin clot properties and their determinants in ACS patients.

PATIENTS AND METHODS

Clot permeability, turbidity and fibrinolysis were assessed in 40 patients with ACS versus 40 controls with stable angina matched for age, sex, and risk factors.

RESULTS

Patients with ACS had lower clot permeability (p=0.001), faster fibrin polymerization (p=0.008), and prolonged fibrinolysis time (p<0.0001) than controls. C-reactive protein (CRP) and 8-epi-prostaglandin F(2alpha), a marker of oxidative stress, were the only independent predictors of clot permeability (R(2)=-0.74; p<0.0001 and R(2)=-0.65; p<0.0001, respectively) and fibrinolysis time in ACS patients (R(2)=0.60; p<0.0001 and R(2)=0.59; p=0.0002, respectively). In angina patients, fibrinogen and CRP predicted permeability (R(2)=-0.71; p<0.0001 and R(2)=-0.62; p<0.0001), and D-dimer predicted lysis time (R(2)=0.54; p=0.0005). In regression analysis models incorporating all patients, the only independent predictor of all clot variables was being an ACS patient (R(2) 0.51 to 0.85; p<0.001).

CONCLUSIONS

This first study of clot properties in patients during an ACS demonstrated that compared with stable angina patients, their clots are composed of dense networks that are more resistant to lysis and these features are correlated with raised CRP and oxidative stress.

Plasma coagulation factor levels in venous thrombosis

High plasma levels of several coagulation factors have been described to be associated with an increased risk of venous thrombosis. However, the mechanisms underlying these associations, as well as those involved in the regulation of plasma levels of coagulation factors, are mostly unknown. Whether these factors should be included in the workup of patients with venous thrombosis remains to be determined. In this review, we discuss the present knowledge on the effects of plasma levels of coagulation factors on the development of venous thrombosis. Furthermore, we review recent findings and ideas on the mechanisms through which elevated plasma coagulation factor levels may influence thrombosis. Finally, we enter into the matter of the possible determinants of elevated plasma levels of coagulation factors.

Fibrin structure and wound healing

Fibrinogen and fibrin play an important role in blood clotting, fibrinolysis, cellular and matrix interactions, inflammation, wound healing, angiogenesis, and neoplasia. The contribution of fibrin(ogen) to these processes largely depends not only on the characteristics of the fibrin(ogen) itself, but also on interactions between specific-binding sites on fibrin(ogen), pro-enzymes, clotting factors, enzyme inhibitors, and cell receptors. In this review, the molecular and cellular biology of fibrin(ogen) is reviewed in the context of cutaneous wound repair. The outcome of wound healing depends largely on the fibrin structure, such as the thickness of the fibers, the number of branch points, the porosity, and the permeability. The binding of fibrin(ogen) to hemostasis proteins and platelets as well as to several different cells such as endothelial cells, smooth muscle cells, fibroblasts, leukocytes, and keratinocytes is indispensable during the process of wound repair. High-molecular-weight and low-molecular-weight fibrinogen, two naturally occurring variants of fibrin, are important determinants of angiogenesis and differ in their cell growth stimulation, clotting rate, and fibrin polymerization characteristics. Fibrin sealants have been investigated as matrices to promote wound healing. These sealants may also be an ideal delivery vehicle to deliver extra cells for the treatment of chronic wounds.