The ultrastructure of fibrinogen Caracas II molecules, fibers, and clots

High fibrinogen γ' levels in patient plasma increase clot formation at arterial and venous shear

Fibrinogen γ' accounts for 3% to 40% of plasma fibrinogen. Earlier studies indicated that fibrinogen γ' forms altered fibrin clots under static conditions, whereas clinically, altered plasma γ' levels are associated with arterial and venous thrombosis. However, the effects of static vs flow conditions on the role of γ' throughout the pathophysiological range is unknown. This study explores the effects of γ' levels on clot formation and structure in static and flow conditions. Coagulation of plasma samples with low (n = 41; 3%), normal (n = 45; 10%), or high (n = 33; 30%) γ' levels were compared with that of purified fibrinogen mixtures with increasing ratios of γ' (3%, 10%, 30%). Clots were analyzed by confocal microscopy, permeation, turbidity, and lysis techniques. In a novel 2-step flow-perfusion model, fibrinogen-deficient plasma repleted with increasing ratios of γ' (3%, 10%, 30%) or plasmas with low (n = 5, 3%) or high (n = 5, 30%) γ' were flowed over preformed platelet aggregates at arterial (500 s-1) and venous (150 s-1) shear rates. Increasing γ' percentages within the pathophysiological range (3%-30%) did not result in any change in clot-formation rates; however, it led to significantly higher clot density, thinner fibers, and slower lysis in static conditions. Under flow at arterial shear, high γ' (30%) led to faster (+44.1%-75.3%) and increased (+104%-123%) fibrin deposition, with clots exhibiting a larger volume (+253%-655%) and height (+130%-146%). These trends were magnified at venous shear. Overall, our findings demonstrate the significant impact of pathophysiological fibrinogen γ' levels on clot structure and provide new flow-dependent mechanisms to explain how γ' increases thrombosis risk.

Fibers Generated by Plasma Des-AA Fibrin Monomers and Protofibril/Fibrinogen Clusters Bind Platelets: Clinical and Nonclinical Implications

Objective  Soluble fibrin (SF) is a substantial component of plasma fibrinogen (fg), but its composition, functions, and clinical relevance remain unclear. The study aimed to evaluate the molecular composition and procoagulant function(s) of SF. Materials and Methods  Cryoprecipitable, SF-rich (FR) and cryosoluble, SF-depleted (FD) fg isolates were prepared and adsorbed on one hydrophilic and two hydrophobic surfaces and scanned by atomic force microscopy (AFM). Standard procedures were used for fibrin polymerization, crosslinking by factor XIII, electrophoresis, and platelet adhesion. Results  Relative to FD fg, thrombin-induced polymerization of FR fg was accelerated and that induced by reptilase was markedly delayed, attributable to its decreased (fibrinopeptide A) FpA. FR fg adsorption to each surface yielded polymeric clusters and co-cryoprecipitable solitary monomers. Cluster components were crosslinked by factor XIII and comprised ≤21% of FR fg. In contrast to FD fg, FR fg adsorption on hydrophobic surfaces resulted in fiber generation enabled by both clusters and solitary monomers. This began with numerous short protofibrils, which following prolonged adsorption increased in number and length and culminated in surface-linked three-dimensional fiber networks that bound platelets. Conclusion  The abundance of adsorbed protofibrils resulted from (1) protofibril/fg clusters whose fg was dissociated during adsorption, and (2) adsorbed des-AA monomers that attracted solution counterparts initiating protofibril assembly and elongation by their continued incorporation. The substantial presence of both components in transfused plasma and cryoprecipitate augments hemostasis by accelerating thrombin-induced fibrin polymerization and by tightly anchoring the resulting clot to the underlying wound or to other abnormal vascular surfaces.

Patients with paroxysmal nocturnal hemoglobinuria demonstrate a prothrombotic clotting phenotype which is improved by complement inhibition with eculizumab

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare hematological disorder, characterized by complement-mediated intravascular hemolysis and thrombosis. The increased incidence of PNH-driven thrombosis is still poorly understood, but unlike other thrombotic disorders, is thought to largely occur through complement-mediated mechanisms. Treatment with a C5 inhibitor, eculizumab, has been shown to significantly reduce the number of thromboembolic events in these patients. Based on previously described links between changes in fibrin clot structure and thrombosis in other disorders, our aim was to investigate clot structure as a possible mechanism of thrombosis in patients with PNH and the anti-thrombotic effects of eculizumab treatment on clot structure. Clot structure, fibrinogen levels and thrombin generation were examined in plasma samples from 82 patients from the National PNH Service in Leeds, UK. Untreated PNH patients were found to have increased levels of fibrinogen and thrombin generation, with subsequent prothrombotic changes in clot structure. No link was found between increasing disease severity and fibrinogen levels, thrombin generation, clot formation or structure. However, eculizumab treated patients showed decreased fibrinogen levels, thrombin generation and clot density, with increasing time spent on treatment augmenting these antithrombotic effects. These data suggest that PNH patients have a prothrombotic clot phenotype due to increased fibrinogen levels and thrombin generation, and that the antithrombotic effects of eculizumab are, in-part, due to reductions in fibrinogen and thrombin generation with downstream effects on clot structure.

Structure, Mechanical, and Lytic Stability of Fibrin and Plasma Coagulum Generated by Staphylocoagulase From Staphylococcus aureus

Staphylococcus aureus causes localized infections or invasive diseases (abscesses or endocarditis). One of its virulence factors is staphylocoagulase (SCG), which binds prothrombin to form a complex with thrombin-like proteolytic activity and leads to uncontrolled fibrin generation at sites of bacterial inoculation. The aim of this study was to characterize the formation, structure, mechanical properties and lysis of SCG-generated clots. Recombinant SCG was expressed in Escherichia coli, purified and the amidolytic activity of its complexes with human prothrombin (SCG-PT) and thrombin (SCG-T) was determined using human thrombin as a reference. Fibrin clots were prepared from purified fibrinogen and human plasma using thrombin, SCG-PT or SCG-T as a coagulase. The kinetics of clot formation and lysis by tissue-type plasminogen activator (tPA) were monitored with turbidimetric assays. Fibrin ultrastructure was examined with scanning electron microscopy and small-angle X-ray scattering (SAXS). Fibrin clot porosity was characterized with fluid permeation assays, whereas the viscoelastic properties and mechanical stability were evaluated with oscillation rheometry. Compared to thrombin, the amidolytic and clotting activity of SCG-PT was 1.6- to 2.5-fold lower on a molar basis. SCG-T had equivalent amidolytic, but reduced clotting activity both on pure fibrinogen (1.6-fold), and in plasma (1.3-fold). The SCG-PT and SCG-T generated fibrin with thicker fibers (10-60% increase in median diameter) than thrombin due to increased number of fibrin protofibrils per fiber cross-section. According to the fluid permeability of the clots SCG-PT and SCG-T promoted the formation of more porous structures. The shear stress resistance in the pure fibrin and plasma clots generated by SCG-PT was significantly lower than in the thrombin clots (243.8 ± 22.0 Pa shear stress was sufficient for disassembly of SCG-PT fibrin vs. 937.3 ± 65.6 Pa in thrombin clots). The tPA-mediated lysis of both pure fibrin and plasma clots produced by SCG-PT or SCG-T was accelerated compared to thrombin, resulting in up to a 2.1-fold increase in tPA potency. Our results indicate that SCG generates a thrombus scaffold with a structure characterized by impaired mechanical stability and increased lytic susceptibility. This proneness to clot disintegration could have implications in the septic embolism from endocardial bacterial vegetation.

Fibrinogen αC domain: Its importance in physiopathology

ABSTRACT

Fibrinogen, involved in coagulation, is a soluble protein composed of two sets of disulfide-bridged Aα, Bβ, and γ-chains. In this review, we present the clinical implications of the αC domain of the molecule in Alzheimer's disease, hereditary renal amyloidosis and a number of thrombotic and hemorrhagic disorders. In Alzheimer's disease, amyloid beta peptide (Aβ) is increased and binds to the αC domain of normal fibrinogen, triggering increased fibrin(ogen) deposition in patients' brain parenchyma. In hereditary renal amyloidosis, fibrinogen is abnormal, with mutations located in the fibrinogen αC domain. The mutant αC domain derived from fibrinogen degradation folds incorrectly so that, in time, aggregates form, leading to amyloid deposits in the kidneys. In these patients, no thrombotic tendency has been observed. Abnormal fibrinogens with either a point mutation in the αC domain or a frameshift mutation resulting in absence of a part of the αC domain are often associated with either thrombotic events or bleeding. Mutation of an amino acid into cysteine (as in fibrinogens Dusart and Caracas V) or a frameshift mutation yielding an unpaired cysteine in the αC domain is often responsible for thrombotic events. Covalent binding of albumin to the unpaired cysteine via a disulphide bridge leads to decreased accessibility to the fibrinolytic enzymes, hence formation of poorly degradable fibrin clots, which explains the high incidence of thrombosis. In contrast, anomalies due to a frameshift mutation in the αC connector of the molecule, provoking deletion of a great part of the αC domain, are associated with bleeding.

Sensing adhesion forces between erythrocytes and γ' fibrinogen, modulating fibrin clot architecture and function

Plasma fibrinogen includes an alternatively spliced γ-chain variant (γ'), which mainly exists as a heterodimer (γAγ') and has been associated with thrombosis. We tested γAγ' fibrinogen-red blood cells (RBCs) interaction using atomic force microscopy-based force spectroscopy, magnetic tweezers, fibrin clot permeability, scanning electron microscopy and laser scanning confocal microscopy. Data reveal higher work necessary for RBC-RBC detachment in the presence of γAγ' rather than γAγA fibrinogen. γAγ' fibrinogen-RBCs interaction is followed by changes in fibrin network structure, which forms an heterogeneous clot structure with areas of denser and highly branched fibrin fibers. The presence of RBCs also increased the stiffness of γAγ' fibrin clots, which are less permeable and more resistant to lysis than γAγA clots. The modifications on clots promoted by RBCs-γAγ' fibrinogen interaction could alter the risk of thrombotic disorders.

Promyelocytic extracellular chromatin exacerbates coagulation and fibrinolysis in acute promyelocytic leukemia

Despite routine treatment of unselected acute promyelocytic leukemia (APL) with all-trans-retinoic acid (ATRA), early death because of hemorrhage remains unacceptably common, and the mechanism underlying this complication remains elusive. We have recently demonstrated that APL cells undergo a novel cell death program, termed ETosis, which involves release of extracellular chromatin. However, the role of promyelocytic extracellular chromatin in APL-associated coagulation remains unclear. Our objectives were to identify the novel role of ATRA-promoted extracellular chromatin in inducing a hypercoagulable and hyperfibrinolytic state in APL and to evaluate its interaction with fibrin and endothelial cells (ECs). Results from a series of coagulation assays have shown that promyelocytic extracellular chromatin increases thrombin and plasmin generation, causes a shortening of plasma clotting time of APL cells, and increases fibrin formation. DNase I but not anti-tissue factor antibody could inhibit these effects. Immunofluorescence staining showed that promyelocytic extracellular chromatin and phosphatidylserine on APL cells provide platforms for fibrin deposition and render clots more resistant to fibrinolysis. Additionally, coincubation assays revealed that promyelocytic extracellular chromatin is cytotoxic to ECs, converting them to a procoagulant phenotype. This cytotoxity was blocked by DNase I by 20% or activated protein C by 31%. Our current results thus delineate the pathogenic role of promyelocytic extracellular chromatin in APL coagulopathy. Furthermore, the remaining coagulation disturbance in high-risk APL patients after ATRA administration may be treatable by intrinsic pathway inhibition via accelerating extracellular chromatin degradation.

Post-translational oxidative modification of fibrinogen is associated with coagulopathy after traumatic injury

Victims of trauma often develop impaired blood clot formation (coagulopathy) that contributes to bleeding and mortality. Fibrin polymerization is one critical component of clot formation that can be impacted by post-translational oxidative modifications of fibrinogen after exposure to oxidants. In vitro evidence suggests that Aα-C domain methionine sulfoxide formation, in particular, can induce conformational changes that prevent lateral aggregation of fibrin protofibrils during polymerization. We used mass spectrometry of plasma from trauma patients to find that fibrinogen Aα-C domain methionine sulfoxide content was selectively-increased in patients with coagulopathy vs. those without coagulopathy. This evidence supports a novel linkage between oxidative stress, coagulopathy, and bleeding after injury.

Procoagulant changes in fibrin clot structure in patients with cirrhosis are associated with oxidative modifications of fibrinogen

Essentials Patients with cirrhosis have hemostatic changes, which may contribute to a risk of thrombosis. This in vitro study compares clot formation and structure between patients and healthy subjects. Clot formation is delayed in patients; ultimately, however, clot permeability is decreased. The thrombogenic structure of fibrin clots may contribute to the thrombotic risk in cirrhosis.

ABSTRACT

Background and Objectives Patients with cirrhosis can be at risk of thrombotic complications due to an imbalance between hemostatic components. However, little is known on how the disease affects clot generation or how alterations in the structure of fibrin clots may affect the hemostatic function of these patients. Methods We investigated the formation and structure of clots generated with plasma and purified fibrinogen of 42 patients with cirrhosis. Clots generated with plasma and fibrinogen of 29 healthy volunteers were studied for comparison. Clot formation and structure were assessed by turbidity, permeation studies, confocal laser and scanning electron microscopy (SEM). The extent of fibrinogen oxidation was assessed by measuring the carbonyl content of purified fibrinogen samples. Results Tissue factor and thrombin-induced clotting of plasma was delayed in patients. The clotting rate was also decreased, but change in turbidity, fibrin density and fiber thickness were largely comparable to healthy volunteers. Conversely, clot permeability was significantly decreased in patients. When clots were generated with purified fibrinogen, differences in clot formation and structure similar to those in plasma were found. The carbonyl content was increased in patient fibrinogen and correlated with disease severity and clot permeability. Conclusions Delayed clot formation in cirrhosis ultimately results in decreased clot permeability. Similar alterations in clots generated with purified fibrinogen suggest that modifications of the molecule are (partly) responsible. Taken together, these findings are indicative of hypercoagulable features of clots of patients with cirrhosis, which may explain the increased risk of thrombosis associated with this condition.

Ex vivo addition of fibrinogen concentrate improves the fibrin network structure in plasma samples taken during liver transplantation

BACKGROUND

Optimal hemostatic management during orthotopic liver transplantation (OLT) remains a challenge. The cause of bleeding during OLT is multifactorial, and may include hemostatic imbalance. Fibrinogen concentrates are increasingly being used to control perioperative bleeding during OLT. However, administration is based on arbitrary thresholds of fibrinogen levels. Importantly, studies on fibrin clot structure during OLT are lacking.

OBJECTIVE

We determined the hemostatic efficacy of fibrinogen concentrate in correcting the fibrin structure.

METHODS

Plasma samples taken at various times during OLT from 15 patients and 15 healthy controls were spiked with 1 g L(-1) fibrinogen concentrate or saline. Turbidity, fibrin fiber density and permeability of the fibrin clots were assessed.

RESULTS

Clotting rate and turbidity were significantly decreased at the start of surgery, and decreased even further during surgery. Addition of fibrinogen significantly increased the clotting rate and turbidity at all time points, but did not normalize it. Fibrin density was significantly reduced after reperfusion as compared with the density at the start of surgery and in healthy controls. Fibrin density improved significantly after addition of fibrinogen in samples taken at the start of surgery and after reperfusion. The severely impaired polymerization and decreased density after reperfusion were accompanied by significantly increased permeability of the clot as compared with the start of surgery and in controls, which was completely restored after addition of fibrinogen.

CONCLUSIONS

Ex vivo addition of fibrinogen concentrate during OLT substantially improves the structural properties of the fibrin clot, which, particularly after reperfusion, shows hypocoagulable features. These data support the use of fibrinogen concentrate to control bleeding complications during OLT.

Human plasma protein N-glycosylation

Glycosylation is the most abundant and complex protein modification, and can have a profound structural and functional effect on the conjugate. The oligosaccharide fraction is recognized to be involved in multiple biological processes, and to affect proteins physical properties, and has consequentially been labeled a critical quality attribute of biopharmaceuticals. Additionally, due to recent advances in analytical methods and analysis software, glycosylation is targeted in the search for disease biomarkers for early diagnosis and patient stratification. Biofluids such as saliva, serum or plasma are of great use in this regard, as they are easily accessible and can provide relevant glycosylation information. Thus, as the assessment of protein glycosylation is becoming a major element in clinical and biopharmaceutical research, this review aims to convey the current state of knowledge on the N-glycosylation of the major plasma glycoproteins alpha-1-acid glycoprotein, alpha-1-antitrypsin, alpha-1B-glycoprotein, alpha-2-HS-glycoprotein, alpha-2-macroglobulin, antithrombin-III, apolipoprotein B-100, apolipoprotein D, apolipoprotein F, beta-2-glycoprotein 1, ceruloplasmin, fibrinogen, immunoglobulin (Ig) A, IgG, IgM, haptoglobin, hemopexin, histidine-rich glycoprotein, kininogen-1, serotransferrin, vitronectin, and zinc-alpha-2-glycoprotein. In addition, the less abundant immunoglobulins D and E are included because of their major relevance in immunology and biopharmaceutical research. Where available, the glycosylation is described in a site-specific manner. In the discussion, we put the glycosylation of individual proteins into perspective and speculate how the individual proteins may contribute to a total plasma N-glycosylation profile determined at the released glycan level.

DNA, histones and neutrophil extracellular traps exert anti-fibrinolytic effects in a plasma environment

In response to various inflammatory stimuli, neutrophils secrete neutrophil extracellular traps (NETs), web-like meshworks of DNA, histones and granular components forming supplementary scaffolds in venous and arterial thrombi. Isolated DNA and histones are known to promote thrombus formation and render fibrin clots more resistant to mechanical forces and tissue-type plasminogen activator (tPA)-induced enzymatic digestion. The present study extends our earlier observations to a physiologically more relevant environment including plasma clots and NET-forming neutrophils. A range of techniques was employed including imaging (scanning electron microscopy (SEM), confocal laser microscopy, and photoscanning of macroscopic lysis fronts), clot permeability measurements, turbidimetric lysis and enzyme inactivation assays. Addition of DNA and histones increased the median fibre diameter of plasma clots formed with 16 nM thrombin from 108 to 121 and 119 nm, respectively, and decreased their permeability constant from 6.4 to 3.1 and 3.7×10(-9) cm(2). Histones effectively protected thrombin from antithrombin-induced inactivation, while DNA inhibited plasminogen activation on the surface of plasma clots and their plasmin-induced resolution by 20 and 40 %, respectively. DNA and histones, as well as NETs secreted by phorbol-myristate-acetate-activated neutrophils, slowed down the tPA-driven lysis of plasma clots and the latter effect could be reversed by the addition of DNase (streptodornase). SEM images taken after complete digestion of fibrin in NET-containing plasma clots evidenced retained NET scaffold that was absent in DNase-treated clots. Our results show that DNA and histones alter the fibrin architecture in plasma clots, while NETs contribute to a decreased lytic susceptibility that can be overcome by DNase.

The effect of blood coagulation factor XIII on fibrin clot structure and fibrinolysis

BACKGROUND

Factor XIII is a 320 kDa tetramer, comprising two enzymatic A-subunits and two carrier B-subunits (FXIII A₂ B₂). Activated FXIII (FXIIIa) catalyses the formation of ε-(γ-glutamyl)lysyl covalent bonds between γ-γ, γ-α and α-α chains of adjacent fibrin molecules and also cross-links the major plasmin inhibitor, α2-antiplasmin, to fibrin.

OBJECTIVES

We investigated the role of FXIII cross-linking of fibrin directly in clot morphology and its functional effect on clot formation and lysis, in the absence of α2-antiplasmin.

RESULTS AND CONCLUSIONS

Our data show that the presence of FXIII during clot formation results in fibrin clots that have a significant 2.1-fold reduction in pore size, as determined by the Darcy constant, Ks, and formed thinner fibers (74.7 ± 1.5 nm) and higher density of fibers compared with those without FXIII (86.0 ± 1.7 nm, P < 0.001), as determined by scanning electron microscopy. Additionally, fibrinolysis showed a significant increase in the time to lysis for clots formed in the presence of FXIII in both static and flow systems. These data demonstrate that independent of α2-antiplasmin, FXIII activity plays a role in increasing the stability of the fibrin clot by altering its structure and increasing the resistance to fibrinolysis.

Clot Architecture Is Altered in Abdominal Aortic Aneurysms and Correlates With Aneurysm Size

Objective—

Abdominal aortic aneurysm (AAA) is characterized by widening of the aorta. Once the aneurysm exceeds 5.5 cm, there is a 10% risk of death due to rupture. AAA is also associated with mortality due to other cardiovascular disease. Our aim was to investigate clot structure in AAA and its relationship to aneurysm size.

Methods and Results—

Plasma was obtained from 49 controls, 40 patients with small AAA, and 42 patients with large AAA. Clot formation was studied by turbidity, fibrin pore structure by permeation, and time to half lysis by turbidity with tissue plasminogen activator. Plasma clot pore size showed a stepwise reduction from controls to small to large AAA. Lag phase for plasma clot formation and time to half lysis were prolonged, with smaller AAA samples showing intermediate response. Clot structure was normal in clots made with fibrinogen purified from patients compared with controls, suggesting a role for other plasma factors. Endogenous thrombin potential and turbidity using tissue factor indicated that the effects were independent of changes in thrombin generation.

Conclusion—

Patients with AAA form denser, smaller pored plasma clots that are more resistant to fibrinolysis, and these characteristics correlate with aneurysm size. Clot structure may play a role in AAA development and concomitant cardiovascular disease.

Biomedical Nanoscience: Electrospinning Basic Concepts, Applications, and Classroom Demonstration

Electrospinning is an old polymer processing technique that has recently been rediscovered. It allows for the easy creation of nano- to micro-fibers that can be collected to form a non-woven structure, which can then be used to fabricate novel structures for various applications including tissue engineering scaffolds, clothing, drug delivery vehicles, and filtration media. Current research in our laboratories is focused on the processing of synthetic and biological polymers to create materials with tailored properties and functions for tissue engineering scaffolds and various other medical applications. This technology is revolutionizing the biomaterials and nanotechnology fields and has prompted us to incorporate its history, basic concepts, and applications into diverse courses such as Biomaterials, Tissue Engineering, Polymers in Medicine, and Senior Design in Chemical and Biomedical Engineering. This Innovation of the Curriculum is timely and crucial for multiple reasons. There is a need for a systematic approach to course structure that ties historical concepts to new materials and processes and, ultimately, to practical applications. Combining this lecture organization with active learning in the forms of open discussions and hands-on experiments/demonstrations will enhance learning outcomes (including retention and critical thinking) at all levels of education. At the undergraduate and graduate levels in the courses mentioned, discussions of electrospinning can create a classroom atmosphere of creative thinking, and an actual demonstration of nanomaterial fabrication can serve as a visual aid to the students. More importantly, this curriculum innovation can be used at the high school level to demonstrate nanotechnology and its applications to medicine, which will aid in sparking the interest of future generations of tissue engineers, biomaterial scientists, nanotechnologists, and scientists and engineers in general.

Thrombin flux and wall shear rate regulate fibrin fiber deposition state during polymerization under flow

Thrombin is released as a soluble enzyme from the surface of platelets and tissue-factor-bearing cells to trigger fibrin polymerization during thrombosis under flow conditions. Although isotropic fibrin polymerization under static conditions involves protofibril extension and lateral aggregation leading to a gel, factors regulating fiber growth are poorly quantified under hemodynamic flow due to the difficulty of setting thrombin fluxes. A membrane microfluidic device allowed combined control of both thrombin wall flux (10(-13) to 10(-11) nmol/mum(2) s) and the wall shear rate (10-100 s(-1)) of a flowing fibrinogen solution. At a thrombin flux of 10(-12) nmol/mum(2) s, both fibrin deposition and fiber thickness decreased as the wall shear rate increased from 10 to 100 s(-1). Direct measurement and transport-reaction simulations at 12 different thrombin flux-wall shear rate conditions demonstrated that two dimensionless numbers, the Peclet number (Pe) and the Damkohler number (Da), defined a state diagram to predict fibrin morphology. For Da < 10, we only observed thin films at all Pe. For 10 < Da < 900, we observed either mat fibers or gels, depending on the Pe. For Da > 900 and Pe < 100, we observed three-dimensional gels. These results indicate that increases in wall shear rate quench first lateral aggregation and then protofibril extension.

[Molecular mechanisms of the polymerization of fibrin and the formation of its three-dimensional network]

The results of biochemical, immunochemical, and X-ray studies of the structures of fibrinogen and fibrin molecules were analyzed. The mechanisms of the successive formation of the fibrin three-dimensional network were described: the polymerization of monomeric molecules with the formation of bifilar protofibrils, the lateral association of protofibrils, and the embranchment of the forming fibrils. Data on the electron and confocal microscopy of the polymeric fibrin were considered. The role of the known polymerization centers of fibrin which participated in the formation of protofibrils and their lateral association was discussed. Data on the existence of the previously unknown polymerization centers were given. In particular, the experimental results demonstrated that one of such centers which participated in the formation of protofibrils was located in the Bbeta12-46 fragment, and did not require the cleavage of fibrinopeptide B for its functioning. The results of the computer modeling of the spatial structure of the fibrin(ogen) molecule and the intermolecular interactions in the course of the fibrin polymerization were presented. The location of the alphaC domains in the fibrin(ogen) molecule and their role in the polymerization process were discussed. Information on the structure of the calcium-binding sites of fibrin(ogen) and the functional role of Ca2+ in fibrin polymerization was published. The structure of factor XIII(a) and the mechanisms of fibrin stabilization by this factor were briefly described.

Fibrinogen Yecheon: congenital dysfibrinogenemia with gamma methionine-310 to threonine substitution

This case study reports a rare fibrinogen variant, gamma Met310Thr mutation, for the first time in Korea. The case shows a point mutation from T to C in the 1,007th nucleotide of the FGG gene. This report describes a variant fibrinogen, hereinafter called "fibrinogen Yecheon", using the name after the town where the patient was living at the time of diagnosis. Fibrinogen Yecheon has a de novo heterozygous point mutation of FGG resulting in gamma Met310Thr and subsequent extra N-glycosylation at gamma Asn308. Extra N-glycosylated fibrinogen is considered a main inhibitor of normal fibrinogen activity.

Effects of Aspirin on Clot Structure and Fibrinolysis Using a Novel In Vitro Cellular System

Objectives— The purpose of this study was to investigate the direct effects of aspirin on fibrin structure/function.

Methods and Results— Chinese Hamster Ovary cell lines stably transfected with fibrinogen were grown in the absence (0) and presence of increasing concentrations of aspirin. Fibrinogen was purified from the media using affinity chromatography, and clots were made from recombinant protein. Mean final turbidity [OD(±SEM)] was 0.083(±0.03), 0.093(±0.002), 0.101(±0.005), and 0.125(±0.003) in clots made from 0, 1, 10, and 100 mg/L aspirin-treated fibrinogen, respectively ( P <0.05). Permeability coefficient (Ks cm 2 ×10 −8 ) was 1.68(±0.29) and 4.13(±0.33) comparing fibrinogen produced from cells grown with 0 mg/L and 100 mg/L aspirin respectively ( P <0.05). Scanning electron microscopy confirmed a looser clot structure and increased fiber thickness of clots made from aspirin-treated fibrinogen, whereas rheometer studies showed a significant 30% reduction in clot rigidity. Fibrinolysis was quicker in clots made from aspirin-treated fibrinogen. Ex vivo studies in 3 normal volunteers given 150 mg aspirin daily for 1 week demonstrated similar changes in clot structure/function.

Conclusion— Aspirin directly altered clot structure resulting in the formation of clots with thicker fibers and bigger pores, which are easier to lyse. This study clearly demonstrates an alternative mode of action for aspirin, which should be considered in studies evaluating the biochemical efficacy of this agent.

The presence of gamma' chain impairs fibrin polymerization

INTRODUCTION

A fraction of fibrinogen molecules contain an alternatively spliced variant chain called gamma'. Plasma levels of this variant have been associated with both myocardial infarction and venous thrombosis. Because clot structure has been associated with cardiovascular risk, we examined the effect of gamma' chain on clot structure.

MATERIALS AND METHODS

We expressed three fibrinogen variants in Chinese hamster ovary (CHO) cells: gamma/gamma homodimer, gamma/gamma' heterodimer, and gamma'/gamma' homodimer. We observed thrombin-catalyzed fibrinopeptide release by HPLC, fibrin polymerization by turbidity, and clot structure by scanning electron microscopy. We characterized post-translational modifications by mass spectrometry.

RESULTS

Fibrinopeptide A was released at the same rate for all three fibrinogens, while fibrinopeptide B was released faster from the gamma'/gamma' homodimer. The rise in turbidity was slower and final absorbance was lower during polymerization of gamma'-containing fibrinogens than for gamma/gamma fibrinogen. Micrographs showed that gamma'/gamma' fibrin clots are composed of very thin fibers, while the diameter of gamma/gamma' fibers is similar to gamma/gamma fibers. Further, the fiber networks formed from gamma'-containing samples were non-uniform. Mass spectrometry showed heterogeneous addition of N-glycans and tyrosine sulfation in the gamma' chain.

CONCLUSIONS

The presence of gamma' chains slows lateral aggregation and alters fibrin structure. We suggest these changes are likely due to charge-charge repulsion, such that polymerization of the gamma'/gamma' homodimer is more impaired than the heterodimer since these repulsions are partially offset by incorporation of gamma chains in the gamma/gamma' heterodimer.

Glycaemic control improves fibrin network characteristics in type 2 diabetes - a purified fibrinogen model

Diabetic subjects have been shown to have altered fibrin network structures. One proposed mechanism for this is non-enzymatic glycation of fibrinogen due to high blood glucose. We investigated whether glycaemic control would result in altered fibrin network structures due to decreased fibrinogen glycation. Twenty uncontrolled type 2 diabetic subjects were treated with insulin in order to achieve glycaemic control. Twenty age- and body mass index (BMI)-matched non-diabetic subjects were included as a reference group. Purified fibrinogen, isolated from plasma samples was used for analysis. There was a significant decrease in fibrinogen glycation (6.81 to 5.02 mol glucose/mol fibrinogen) with a corresponding decrease in rate of lateral aggregation (5.86 to 4.62) and increased permeability (2.45 to 2.85 x 10(-8) cm(2)) and lysis rate (3.08 to 3.27 microm/min) in the diabetic subjects after glycaemic control. These variables correlated with markers of glycaemic control. Fibrin clots of non-diabetic subjects had a significantly higher ratio of inelastic to elastic deformation than the diabetic subjects (0.10 vs. 0.09). Although there was no difference in median fiber diameter between diabetic and non-diabetic subjects, there was a small increase in the proportion of thicker fibers in the diabetic samples after glycaemic control. Results from SDS-PAGE indicated no detectable difference in factor XIIIa-crosslinking of fibrin clots between uncontrolled and controlled diabetic samples. Diabetic subjects may have altered fibrin network formation kinetics which contributes to decreased pore size and lysis rate of fibrin clots. Achievement of glycaemic control and decreased fibrinogen glycation level improves permeability and lysis rates in a purified fibrinogen model.

Polyphosphate enhances fibrin clot structure

Polyphosphate, a linear polymer of inorganic phosphate, is present in platelet dense granules and is secreted on platelet activation. We recently reported that polyphosphate is a potent hemostatic regulator, serving to activate the contact pathway of blood clotting and accelerate factor V activation. Because polyphosphate did not alter thrombin clotting times, it appeared to exert all its procoagulant actions upstream of thrombin. We now report that polyphosphate enhances fibrin clot structure in a calcium-dependent manner. Fibrin clots formed in the presence of polyphosphate had up to 3-fold higher turbidity, had higher mass-length ratios, and exhibited thicker fibers in scanning electron micrographs. The ability of polyphosphate to enhance fibrin clot turbidity was independent of factor XIIIa activity. When plasmin or a combination of plasminogen and tissue plasminogen activators were included in clotting reactions, fibrin clots formed in the presence of polyphosphate exhibited prolonged clot lysis times. Release of polyphosphate from activated platelets or infectious microorganisms may play an important role in modulating fibrin clot structure and increasing its resistance to fibrinolysis. Polyphosphate may also be useful in enhancing the structure of surgical fibrin sealants.

Homophenotypic Aalpha R16H fibrinogen (Kingsport): uniquely altered polymerization associated with slower fibrinopeptide A than fibrinopeptide B release

We detail for the first time the uniquely altered fibrin polymerization of homophenotypic Aalpha R16H dysfibrinogen. By polymerase chain reaction amplification and DNA sequencing, our new proposita's genotype consisted of a G>A transition encoding for Aalpha R16H, and an 11 kb Aalpha gene deletion. High-performance liquid chromatography disclosed fibrinopeptide A release approximately six times slower than its fibrinopeptide B. Turbidimetric analyses revealed unimpaired fibrin repolymerization, and abnormal thrombin-induced polymerization (1-7 mumol/l fibrinogen, > 96% coagulable), consisting of a prolonged lag time, slow rate, and abnormal clot turbidity maxima, all varying with thrombin concentration. For example, at 0.2-3 U/ml, the resulting turbidity maxima ranged from lower to higher than normal control values. By scanning electron microscopy, clots formed by 0.3 and 3 thrombin U/ml displayed mean fibril diameters 42 and 254% of the respective control values (n = 400). Virtually no such differences from control values were demonstrable, however, when clots formed in the presence of high ionic strength (micro = 0.30) or of monoclonal antibeta(15-42)IgG. The latter also prolonged the thrombin clotting time approximately three-fold. Additionally, thrombin-induced clots displayed decreased elastic moduli, with G' values of clots induced by 0.3, 0.7 and 3 thrombin U/ml corresponding to 11, 34, and 45% of control values. The results are consistent with increased des-BB fibrin monomer generation preceding and during polymerization. This limited the inherent gelation delay, decreased the clot stiffness, and enabled a progressively coarser, rather than finer, network induced by increasing thrombin concentrations. We hypothesize that during normal polymerization these constitutive des-BB fibrin monomer properties attenuate their des-AA fibrin counterparts.

Common variation in the C-terminal region of the fibrinogen beta-chain: effects on fibrin structure, fibrinolysis and clot rigidity

Fibrinogen BbetaArg448Lys is a common polymorphism, positioned within the carboxyl terminus of the Bbeta-chain of the molecule. Studies suggest that it is associated with severity of coronary artery disease and development of stroke. The effects of the amino acid substitution on clot structure remains controversial, and the aim of this study was to investigate the effect(s) of this polymorphism on fibrin clot structure using recombinant techniques. Permeation, turbidity, and scanning electron microscopy showed that recombinant Lys448 fibrin had a significantly more compact structure, with thin fibers and small pores, compared with Arg448. Clot stiffness, measured by means of a novel method using magnetic tweezers, was significantly higher for the Lys448 compared with the Arg448 variant. Clots made from recombinant protein variants had similar lysis rates outside the plasma environment, but when added to fibrinogen-depleted plasma, the fibrinolysis rates for Lys448 were significantly slower compared with Arg448. This study demonstrates for the first time that clots made from recombinant BbetaLys448 fibrinogen are characterized by thin fibers and small pores, show increased stiffness, and appear more resistant to fibrinolysis. Fibrinogen BbetaArg448Lys is a primary example of common genetic variation with a significant phenotypic effect at the molecular level.

Incorporation of fibrin molecules containing fibrinopeptide A alters clot ultrastructure and decreases permeability

Previous studies have shown that a heterozygous mutation in the fibrinogen Aalpha chain gene, which results in an Aalpha R16C substitution, causes fibrinolytic resistance in the fibrin clot. This mutation prevents thrombin cleavage of fibrinopeptide A from mutant Aalpha R16C chains, but not from wild-type Aalpha chains. However, the mechanism underlying the fibrinolytic resistance is unclear. Therefore, this study investigated the biophysical properties of the mutant fibrin that contribute to fibrinolytic resistance. Fibrin clots made from the mutant fibrinogen incorporated molecules containing fibrinopeptide A into the polymerised clot, which resulted in a 'spiky' clot ultrastructure with barbed fibrin strands. The clots were less stiff than normal fibrin and were cross-linked slower by activated FXIII, but had an increased average fiber diameter, were more dense, had smaller pores and were less permeable. Protein sequencing showed that unclottable fibrinogen remaining in the supernatant consisted entirely of homodimeric Aalpha R16C fibrinogen, whereas both cleaved wild-type alpha chains and uncleaved Aalpha R16C chains were in the fibrin clot. Therefore, fibrinolytic resistance of the mutant clots is probably a result of altered clot ultrastructure caused by the incorporation of fibrin molecules containing fibrinopeptide A, resulting in larger diameter fibers and decreased permeability to fibrinolytic enzymes.

Direct evidence for specific interactions of the fibrinogen alphaC-domains with the central E region and with each other

The carboxyl-terminal regions of the fibrinogen Aalpha chains (alphaC regions) form compact alphaC-domains tethered to the bulk of the molecule with flexible alphaC-connectors. It was hypothesized that in fibrinogen two alphaC-domains interact intramolecularly with each other and with the central E region preferentially through its N-termini of Bbeta chains and that removal of fibrinopeptides A and B upon fibrin assembly results in dissociation of the alphaC regions and their switch to intermolecular interactions. To test this hypothesis, we studied the interactions of the recombinant alphaC region (Aalpha221-610 fragment) and its subfragments, alphaC-connector (Aalpha221-391) and alphaC-domain (Aalpha392-610), between each other and with the recombinant (Bbeta1-66)2 and (beta15-66)2 fragments and NDSK corresponding to the fibrin(ogen) central E region, using laser tweezers-based force spectroscopy. The alphaC-domain, but not the alphaC-connector, bound to NDSK, which contains fibrinopeptides A and B, and less frequently to desA-NDSK and (Bbeta1-66)2 containing only fibrinopeptides B; it was poorly reactive with desAB-NDSK and (beta15-66)2 both lacking fibrinopeptide B. The interactions of the alphaC-domains with each other and with the alphaC-connector were also observed, although they were weaker and heterogeneous in strength. These results provide the first direct evidence for the interaction between the alphaC-domains and the central E region through fibrinopeptide B, in agreement with the hypothesis given above, and indicate that fibrinopeptide A is also involved. They also confirm the hypothesized homomeric interactions between the alphaC-domains and display their interaction with the alphaC-connectors, which may contribute to covalent cross-linking of alpha polymers in fibrin.

Functional analysis of fibrin {gamma}-chain cross-linking by activated factor XIII: determination of a cross-linking pattern that maximizes clot stiffness

Activated coagulation factor XIII (FXIIIa) cross-links the gamma-chains of fibrin early in clot formation. Cross-linking of the alpha-chains occurs more slowly, leading to high molecular weight multimer formations that can also contain gamma-chains. To study the contribution of FXIIIa-induced gamma-chain cross-linking on fibrin structure and function, we created 2 recombinant fibrinogens (gammaQ398N/Q399N/K406R and gammaK406R) that modify the gamma-chain cross-linking process. In gammaK406R, gamma-dimer cross-links were absent, but FXIIIa produced a cross-linking pattern similar to that observed in tissue transglutaminase cross-linked fibrin(ogen) with mainly alpha-gamma cross-links. In Q398N/Q399N/K406R, cross-links with any gamma-chain involvement were completely absent, and only alpha-chain cross-linking occurred. Upon cross-linking, recombinant normal fibrin yielded a 3.5-fold increase in stiffness, compared with a 2.5-fold increase by alpha-chain cross-linking alone (gammaQ398N/Q399N/K406R). gammaK406R fibrin showed a 1.5-fold increase in stiffness after cross-linking. No major differences in clot morphology, polymerization, and lysis rates were observed, although fiber diameter was slightly lower in cross-linked normal fibrin relative to the variants. Our results show that gamma-chain cross-linking contributes significantly to clot stiffness, in particular through gamma-dimer formation; alpha-gamma hybrid cross-links had the smallest impact on clot stiffness.

Platelet factor 4 (CXCL4) seals blood clots by altering the structure of fibrin

Platelet factor-4 (PF4/CXCL4) is an orphan chemokine released in large quantities in the vicinity of growing blood clots. Coagulation of plasma supplemented with a matching amount of PF4 results in a translucent jelly-like clot. Saturating amounts of PF4 reduce the porosity of the fibrin network 4.4-fold and decrease the values of the elastic and loss moduli by 31- and 59-fold, respectively. PF4 alters neither the cleavage of fibrinogen by thrombin nor the cross-linking of protofibrils by activated factor XIII but binds to fibrin and dramatically transforms the structure of the ensuing network. Scanning electron microscopy showed that PF4 gives rise to a previously unreported pattern of polymerization where fibrin assembles to form a sealed network. The subunits constituting PF4 form a tetrahedron having at its corners a RPRH motif that mimics (in reverse orientation) the Gly-His-Arg-Pro-amide peptides that co-crystallize with fibrin. Molecular modeling showed that PF4 could be docked to fibrin with remarkable complementarities and absence of steric clashes, allowing the assembly of irregular polymers. Consistent with this hypothesis, as little as 50 microm the QVRPRHIT peptide derived from PF4 affects the polymerization of fibrin.

A classification of the fibrin network structures formed from the hereditary dysfibrinogens

OBJECTIVE

The main objective was to study the relationships of the molecular defects in 38 dysfibrinogens with their fibrin networks.

METHODS AND RESULTS

Scanning electron microscopic analyses revealed that all the fibrins formed under the same conditions had networks composed of either normal thickness fibers or thin fibers, accompanied by a variety of alterations in the network structure and characteristics. We classified these fibrin networks into five classes, designated normal, less-ordered, porous A, porous B and lace-like networks. The dysfibrinogens with defects in fibrinopeptide A release or the E:D binding sites formed normal or less-ordered networks, while those with defects in the D:D association formed porous A networks composed of many tapered terminating fibers, despite having fibers of normal width, and containing many pores or spaces. The porous B and lace-like networks were composed of highly branched thin fibers because of defects in the lateral association among protofibrils, and the major difference between them was the porosity of the porous B networks. All the porous B networks were easily damaged by mechanical stress, whereas the lace-like networks retained high resistance to such stress, indicating that the network strength was not dependent on the fiber width, but on the porosity that led to fragility of the network.

CONCLUSION

Impairment of the D:D association is the major disturbing factor that leads to the formation of porous fibrin networks. The porosity may be introduced by severe impairment of the D:D association, as well as the lateral association, as has often been observed by extra glycosylation or defects in Ca2+ binding.

Fibrinogen Denver: a dysfibrinogenemia associated with an abnormal Reptilase time and significant bleeding

This paper reports a new dysfibrinogenemia with an unusual pattern of laboratory assays. The patient, a 51-year-old female with a lifelong moderate bleeding history, was initially diagnosed with von Willebrand disease based on routine coagulation assays and the clinical bleeding presentation. During recent testing as part of a preoperative screen and without any current history of treatment, levels of von Willebrand factor (VWF) antigen, VWF activity, and factor VIII activity were all significantly elevated, which was unexpected given her previous diagnosis. Additional testing was performed looking for other heritable causes for her considerable bleeding tendency. Interestingly, the patient had a significantly prolonged Reptilase time, minimally short thrombin time, and an abnormal fibrinogen-crossed immunoelectrophoresis pattern. Clearly, this patient had a fibrinogen abnormality that had been missed when only routine coagulation screening assays were performed. A brief review of the fibrinogen literature revealed no other dysfibrinogenemias reported with a similar pattern of test results, and thus this defect was designated fibrinogen Denver.

A new optimized method for the determination of fibrin clot permeability

The measurement of fibrin clot permeability is important in the investigation of fibrin clot characteristics. A major disadvantage of the standard measurement techniques is that they are time consuming as they require constant adjustments and monitoring. Presented here is a new method that is easy to perform and does not need continuous adjustments. In addition, the supervision required during the measurement is greatly minimized. When performing the measurement in quadruple, the intra-run coefficient of variation is 11.7%, which is comparable with other reported methods. A (5.1-13.1) x 10(-9) cm reference interval of the permeability coefficient (Ks) among 22 healthy volunteers was established.

The alphaC domains of fibrinogen affect the structure of the fibrin clot, its physical properties, and its susceptibility to fibrinolysis

The functions of the alphaC domains of fibrinogen in clotting and fibrinolysis, which have long been enigmatic, were determined using recombinant fibrinogen truncated at Aalpha chain residue 251. Scanning electron microscopy and confocal microscopy revealed that the fibers of alpha251 clots were thinner and denser, with more branch points than fibers of control clots. Consistent with these results, the permeability of alpha251 clots was nearly half that of control clots. Together, these results suggest that in normal clot formation, the alphaC domains enhance lateral aggregation to produce thicker fibers. The viscoelastic properties of alpha251 fibrin clots differed markedly from control clots; alpha251 clots were much less stiff and showed more plastic deformation, indicating that interactions between the alphaC domains in normal clots play a major role in determining the clot's mechanical properties. Comparing factor XIIIa cross-linked alpha251 and control clots showed that gamma chain cross-linking had a significant effect on clot stiffness. Plasmin-catalyzed lysis of alpha251 clots, monitored with both macroscopic and microscopic methods, was faster than lysis of control clots. In conclusion, these studies provide the first definitive evidence that the alphaC domains play an important role in determining the structure and biophysical properties of clots and their susceptibility to fibrinolysis.

Fibrinogen and fibrin

Fibrinogen is a large, complex, fibrous glycoprotein with three pairs of polypeptide chains linked together by 29 disulfide bonds. It is 45 nm in length, with globular domains at each end and in the middle connected by alpha-helical coiled-coil rods. Both strongly and weakly bound calcium ions are important for maintenance of fibrinogen's structure and functions. The fibrinopeptides, which are in the central region, are cleaved by thrombin to convert soluble fibrinogen to insoluble fibrin polymer, via intermolecular interactions of the "knobs" exposed by fibrinopeptide removal with "holes" always exposed at the ends of the molecules. Fibrin monomers polymerize via these specific and tightly controlled binding interactions to make half-staggered oligomers that lengthen into protofibrils. The protofibrils aggregate laterally to make fibers, which then branch to yield a three-dimensional network-the fibrin clot-essential for hemostasis. X-ray crystallographic structures of portions of fibrinogen have provided some details on how these interactions occur. Finally, the transglutaminase, Factor XIIIa, covalently binds specific glutamine residues in one fibrin molecule to lysine residues in another via isopeptide bonds, stabilizing the clot against mechanical, chemical, and proteolytic insults. The gene regulation of fibrinogen synthesis and its assembly into multichain complexes proceed via a series of well-defined steps. Alternate splicing of two of the chains yields common variant molecular isoforms. The mechanical properties of clots, which can be quite variable, are essential to fibrin's functions in hemostasis and wound healing. The fibrinolytic system, with the zymogen plasminogen binding to fibrin together with tissue-type plasminogen activator to promote activation to the active enzyme plasmin, results in digestion of fibrin at specific lysine residues. Fibrin(ogen) also specifically binds a variety of other proteins, including fibronectin, albumin, thrombospondin, von Willebrand factor, fibulin, fibroblast growth factor-2, vascular endothelial growth factor, and interleukin-1. Studies of naturally occurring dysfibrinogenemias and variant molecules have increased our understanding of fibrinogen's functions. Fibrinogen binds to activated alphaIIbbeta3 integrin on the platelet surface, forming bridges responsible for platelet aggregation in hemostasis, and also has important adhesive and inflammatory functions through specific interactions with other cells. Fibrinogen-like domains originated early in evolution, and it is likely that their specific and tightly controlled intermolecular interactions are involved in other aspects of cellular function and developmental biology.

The mechanical properties of fibrin for basic scientists and clinicians

In this review, I set forth some basic information about the mechanical properties of fibrin clots and attempt to identify the big questions remaining. The intent is to make this topic understandable to both basic scientists who are interested in blood clotting and to hematologists or cardiologists, since I believe that this is something everyone working in these fields should know. The viscoelastic properties of fibrin are remarkable and unique among polymers. Moreover, these properties are essential to the physiology of blood clotting and are important for understanding and therefore preventing and treating thrombosis.

The effects of additional carbohydrate in the coiled-coil region of fibrinogen on polymerization and clot structure and properties: characterization of the homozygous and heterozygous forms of fibrinogen Lima (Aalpha Arg141-->Ser with extra glycosylation)

Fibrinogen Lima is an abnormal fibrinogen with an Aalpha Arg141-->Ser substitution resulting in an extra N-glycosylation at Aalpha Asn139, which seems to be responsible for the impairment of fibrin polymerization. We have studied the polymerization and properties of clots made from both plasma and purified fibrinogen of both the homozygous and heterozygous forms. The clot permeation studies with both plasma and purified protein revealed a normal flux through the network for the heterozygous form but very decreased permeation in the homozygous form. Consistent with turbidity results, the clot network of the homozygous form, seen by scanning electron microscopy, was tight and composed of thin fibers, with many branch points, while the appearance of clots from the heterozygous form was similar to that of control clots, but in both cases the fibers were more curved than those of control clots. The rheological properties of clots from the homozygous form were also altered, with rigidity being increased in plasma clots, but decreased in the purified system, a consequence of the balance between numbers of branch points and fiber curvature. From these results it seems that the extra carbohydrate moiety, located in the alpha coiled-coil region close to the betaC domains, impairs the protofibril lateral association process, giving rise to thinner, more curved fibers, with the structural anomalies being most pronounced in the clots from the homozygous plasma. These studies support a model for fibrin polymerization in which the betaC-betaC interactions are involved in lateral aggregation.

A monoclonal antibody to the fibrinogen gamma-chain alters fibrin clot structure and its properties by producing short, thin fibers arranged in bundles

BACKGROUND

We previously reported that hamster monoclonal antibody 7E9, which reacts with the C-terminus of the gamma-chain of mouse fibrinogen, inhibits factor (F)XIIIa-mediated cross-linking, platelet adhesion to fibrinogen, and platelet-mediated clot retraction; in addition, it facilitates thrombolysis.

OBJECTIVES

To understand the mechanism(s) by which 7E9 acts, we have now studied the effect of 7E9 IgG, 7E9 F(ab')2, and 7E9 Fab on fibrin clot structure using electron microscopy and measurements of clot physical properties.

RESULTS

By transmission electron microscopy, 7E9 IgG was found to bind primarily to the ends of the fibrinogen molecule. 7E9 IgG and 7E9 F(ab')2, both of which are bivalent, were capable of binding to two fibrinogen molecules simultaneously. Scanning electron microscopy of clots formed in the presence of equimolar concentrations of fibrinogen and 7E9 IgG demonstrated the presence of very short and thin fibers (63% reduction in fiber diameter) arranged in unusual bundles, surrounding large pores. Clots formed in the presence of 7E9 demonstrated a marked increase in permeation (approximately 25-fold increase in perfusion rate at constant pressure), an approximately 50% reduction in dynamic storage modulus (G'; a reflection of decreased clot stiffness), and an approximately 38% increase in loss tangent (tan delta; a reflection of the clot's ability to undergo irreversible deformation). These clots also showed decreased absorbance at 350 nm, reflecting the clot structure produced by 7E9 IgG. The effects of 7E9 IgG were not observed with control hamster IgG, 7E9 F(ab')2, or 7E9 Fab fragments, indicating requirements for both the binding properties and mass of 7E9 IgG.

CONCLUSIONS

These data indicate that 7E9 antibody affects fibrin clot structure in a way that is consistent with the enhanced fibrinolysis we reported previously. Together with our previous observations, we conclude that 7E9 is directed at a strategically important region of fibrinogen with regard to platelet function, FXIIIa-mediated cross-linking, clot retraction, fibrin structure, and fibrinolysis. Thus targeting this region of fibrinogen may have antithrombotic therapeutic potential.

Fibrinogen gamma-chain splice variant gamma' alters fibrin formation and structure

Fibrinogen gammaA/gamma' results from alternative splicing of mRNA. This variant, which constitutes approximately 8% to 15% of plasma fibrinogen, contains FXIII and thrombin binding sites. Our objective was to investigate whether gammaA/gamma' differs in fibrin formation and structure from the more common variant gammaA/gammaA. Both variants were separated and purified by anion-exchange chromatography. Fibrin formation and clot structure of the variants and unfractionated fibrinogen were investigated by turbidity and scanning electron microscopy (SEM). Thrombin cleavage of fibrinopeptides was analyzed by high-performance liquid chromatography (HPLC). Turbidity analysis showed significantly altered polymerization rates and overall fiber thickness in gammaA/gamma' clots compared with gammaA/gammaA and unfractionated fibrinogen. This finding was consistent with a range of thrombin concentrations. HPLC demonstrated reduced rates of fibrinopeptide B (FpB) release from gammaA/gamma' fibrinogen compared with gammaA/gammaA. Delayed FpB release was associated with delayed lateral aggregation of protofibrils and significant differences were found on SEM, with gammaA/gamma' clots consisting of smaller diameter fibers and increased numbers of branch points compared with both gammaA/gammaA and unfractionated fibrinogen. These results demonstrate that the gammaA/gamma' splice variant of fibrinogen directly alters fibrin formation and structure, which may help to explain the increased thrombotic risk associated with this variant.

Functional analysis of the fibrinogen Aalpha Thr312Ala polymorphism: effects on fibrin structure and function

BACKGROUND

The fibrinogen Aalpha Thr312Ala polymorphism occurs within the alphaC domain of fibrinogen, which is important for lateral aggregation and factor XIII-induced cross-linking of fibrin fibers. We have previously shown an association of Ala312 fibrinogen with poststroke mortality in subjects with atrial fibrillation and with pulmonary embolism in subjects with venous thrombosis.

METHODS AND RESULTS

We studied the properties of clots formed from purified Ala312 and Thr312 fibrinogen and found that Ala312 fibrinogen produces stiffer clots, associated with increased alpha chain cross-linking, as demonstrated by SDS-Page. On electron microscopy analysis, we found larger fiber diameters in the Ala312 clots and observed a lower number of fibers per square micrometer. The number of branch points per square micrometer was similar between genotypes.

CONCLUSIONS

Our study shows that Ala312 influences clot structure and properties by increased factor XIII cross-linking and formation of thicker fibrin fibers. These findings may provide a mechanism by which Ala312 fibrinogen could predispose to clot embolization.

Genetic regulation of fibrin structure and function: complex gene-environment interactions may modulate vascular risk

BACKGROUND

Polymorphisms in the fibrinogen and factor XIII genes are associated with atherothrombotic risk, but clinical studies have produced inconsistent results and laboratory studies have not explained these findings. We aimed to investigate interactions between polymorphisms in the factor XIII and fibrinogen genes, fibrinogen concentrations, and other cardiovascular risk factors in relation to fibrin structure and function.

METHODS

We used permeation analysis and electron microscopy to investigate interactions between fibrin structure, factor XIII Val34Leu, fibrinogen Aalpha Thr312Ala, fibrinogen Bbeta Arg448Lys, and fibrinogen concentrations in plasma and purified systems.

FINDINGS

Increased fibrinogen concentrations were associated with decreases in permeability, with tighter clot structures in the presence of factor XIII 34Val alleles compared with those in the presence of 34Leu alleles. Findings were confirmed by scanning electron microscopy of fibrin. Similar changes in permeability were noted for Aalpha fibrinogen 312Ala compared with that for 312Thr.

INTERPRETATION

Our results show interactions between coding polymorphisms in fibrinogen and factor XIII and fibrinogen concentrations that modify fibrin and explain the apparent paradox between epidemiological studies of factor XIII 34Leu and reported in-vitro effects on fibrin structure and function. We suggest a potential complexity of gene-gene and gene-environment interactions in determining cardiovascular risk.

Structure and function of human fibrinogen inferred from dysfibrinogens

Fibrinogen is a 340-kDa plasma protein that is composed of two identical molecular halves, each consisting of three non-identical subunit polypeptides designated as A alpha, B beta- and gamma-chains held together by multiple disulfide bonds. Fibrinogen has a trinodular structure, i.e., one central E domain comprizing the amino-terminal regions of paired individual three polypeptides, and two identical outer D domains. These three nodules are linked by two coiled-coil regions [1,2]. After activation with thrombin, a tripeptide segment consisting of Gly-Pro-Arg is exposed at the amino-terminus of each alpha-chain residing at the center of the E domain and combines with its complementary binding site, called the 'a' site, residing in the carboxyl-terminal region of the gamma-chain in the outer D domain of another molecule. By crystallographic analysis [3], the alpha-amino group of alpha Gly-1 is shown to be juxtaposed between the carboxyl group of gamma Asp-364 and the carboxyamide of Gln-329 in the 'a' site. Half molecule-staggered, double-stranded fibrin protofibrils are thus formed [4,5]. Upon abutment of two adjacent D domains on the same strand, D-D self association takes place involving Arg-275, Tyr-280 and Ser-300 of the gamma-chain on the surface of the abutting two D domains [3]. Thereafter, carboxyl-terminal regions of the fibrin a-chains are thought to be untethered and interact with those of other protofibrils leading to the formation of thick fibrin bundles and interwoven networks after appropriate branching [6-9]. Although many enigmas still remain regarding the mechanisms of these molecular interactions, fibrin assembly proceeds in a highly ordered fashion. In my talk, I would like to discuss these molecular interactions of fibrinogen and fibrin based on the up-date data provided by analyses of normal as well as hereditary dysfibrinogens, particularly in the latter by introducing representative molecules at each step of fibrin clot formation.

The structure and function of the alpha C domains of fibrinogen

The alpha C domains have been localized on fibrinogen and fibrin. Several model systems have been developed to study their functions. Analysis of the amino acid sequence of the alpha C domains suggested that each is made up of a globular and an extended portion. Microcalorimetry confirmed this result and showed that the two alpha C domains interact intramolecularly. Electron microscopy of fibrinogen with a monoclonal antibody to the alpha C domains demonstrated that these regions normally interact with the central portion of the molecule. In the conversion from fibrinogen to fibrin there is a large scale conformational change, such that the alpha C domains dissociate from the central region and are available for intermolecular interaction. Experiments with highly purified and well characterized fragment X monomer, missing either one or both of the alpha C domains, indicate that intermolecular interactions between alpha C domains are important for the enhancement of lateral aggregation during fibrin polymerization. Isolated alpha C fragments polymerized at neutral pH and interacted with the alpha C domains of fibrin monomer to influence clot formation. Several dysfibrinogenemias in which there are amino acid substitutions in, or truncations of, the alpha C domains revealed that these changes can have dramatic effects on polymerization and clot structure. The polymerization of A alpha 251 recombinant fibrinogen, that contains A alpha chains truncated at residue 251, was altered, as were the mechanical properties and the rate of fibrinolysis of the clots. Altogether, these results help to define the role of the alpha C domains in determining the structure and properties of clots.

Mode of perturbation of Asahi fibrin assembly by the extra oligosaccharides

Steric hindrance by the backbone of extra oligosaccharides at gamma-Asn 308 may cause the repulsive force to widen the junction at the D:D interface, and thus, interfere with the longitudinal elongation and lateral association of protofibrils.

Hereditary disorders of fibrinogen

Fibrinogen, a 340-kDa plasma protein, is composed of two identical molecular halves each consisting of three non-identical A alpha-, B beta- and gamma-chain subunits held together by multiple disulfide bonds. Fibrinogen is shown to have a trinodular structure; that is, one central nodule, the E domain, and two identical outer nodules, the D-domains, linked by two coiled-coil regions. After activation with thrombin, a pair of binding sites comprising Gly-Pro-Arg is exposed in the central nodule and combines with its complementary binding site a in the outer nodule of another molecules. By using crystallographic analysis, the alpha-amino group of alpha Gly-1 is shown to be juxtaposed between gamma Asp-364 and gamma Asp-330, and guanidino group of alpha Arg-3 between the carboxyl group of gamma Asp-364 and gamma Gln-329 in the a site. Half molecule-staggered, double-stranded protofibrils are thus formed. Upon abutment of two adjacent D domains on the same strand, D-D self association takes place involving Arg-275, Tyr-280, and Ser-300 of the gamma-chain on the surface of the abutting two D domains. Thereafter, carboxyl-terminal regions of the alpha-chains are untethered and interact with those of other protofibrils leading to the formation of thick fibrin bundles and networks. Although many enigmas still remain concerning the exact mechanisms of these molecular interactions, fibrin assembly proceeds in a highly ordered fashion. In this review, these molecular interactions of fibrinogen and fibrin are discussed on the basis of the data provided by hereditary dysfibrinogens on introducing representative molecules at each step of fibrin clot formation.

End-linked homodimers in fibrinogen Osaka VI with a Bβ-chain extension lead to fragile clot structure

The authors have identified a 12-residue carboxyl-terminal extension of Lys-Ser-Pro-Met-Arg-Arg-Phe-Leu-Leu-Phe-Cys-Met in a dysfibrinogen derived from a woman heterozygotic for this abnormality and associated with severe bleeding. This extension is due to a T-to-A mutation that creates AAG encoding Lys at the stop (TAG) codon, thus translating 36 base pairs in the noncoding region of the Bβ gene. The extra Cys residues appear to be involved in 1 or 2 disulfide bonds between 2 adjacent abnormal fibrinogen molecules, forming a fibrinogen homodimer as indicated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis. Indeed, about half of the fibrinogen molecules exist as end-linked dimers oriented in parallel or with an angle, as observed by transmission electron microscopy. These end-linked dimers may well alter the conformations of D and DD regions on fibrin assembly, leading to increased fiber branching at their sites in the growing protofibrils. By scanning electron microscopy, the Osaka VI fibrin network appears to have a lacelike structure composed of highly branched, thinner fibers than the normal fibrin architecture. Such fibrin networks may be easily damaged to form large pores when fluids are allowed to pass through the gels. The fragility of Osaka VI fibrin clots, further confirmed by permeation and compaction studies, may account for the massive bleeding observed in this patient.

End-linked homodimers in fibrinogen Osaka VI with a Bβ-chain extension lead to fragile clot structure

The authors have identified a 12-residue carboxyl-terminal extension of Lys-Ser-Pro-Met-Arg-Arg-Phe-Leu-Leu-Phe-Cys-Met in a dysfibrinogen derived from a woman heterozygotic for this abnormality and associated with severe bleeding. This extension is due to a T-to-A mutation that creates AAG encoding Lys at the stop (TAG) codon, thus translating 36 base pairs in the noncoding region of the Bβ gene. The extra Cys residues appear to be involved in 1 or 2 disulfide bonds between 2 adjacent abnormal fibrinogen molecules, forming a fibrinogen homodimer as indicated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis. Indeed, about half of the fibrinogen molecules exist as end-linked dimers oriented in parallel or with an angle, as observed by transmission electron microscopy. These end-linked dimers may well alter the conformations of D and DD regions on fibrin assembly, leading to increased fiber branching at their sites in the growing protofibrils. By scanning electron microscopy, the Osaka VI fibrin network appears to have a lacelike structure composed of highly branched, thinner fibers than the normal fibrin architecture. Such fibrin networks may be easily damaged to form large pores when fluids are allowed to pass through the gels. The fragility of Osaka VI fibrin clots, further confirmed by permeation and compaction studies, may account for the massive bleeding observed in this patient.

The factor XIII V34L polymorphism accelerates thrombin activation of factor XIII and affects cross-linked fibrin structure

Factor XIII on activation by thrombin cross-links fibrin. A common polymorphism Val to Leu at position 34 in the FXIII A subunit is under investigation as a risk determinant of thrombosis. Because Val34Leu is close to the thrombin cleavage site, the hypothesis that it would alter the function of FXIII was tested. Analysis of FXIII subunit proteolysis by thrombin using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance liquid chromatography showed that FXIII 34Leu was cleaved by thrombin more rapidly and by lower doses than 34Val. Mass spectrometry of isolated activation peptides confirmed the predicted single methyl group difference and demonstrated that the thrombin cleavage site is unaltered by Val34Leu. Kinetic analysis of activation peptide release demonstrated that the catalytic efficiency (kcat/Km) of thrombin was 0.5 for FXIII 34Leu and 0.2 (μmol/L)−1× sec−1 for 34Val. Presence of fibrin increased the catalytic efficiency to 4.8 and 2.2 (μmol/L)−1 × sec−1, respectively. Although the 34Leu peptide was released at a similar rate as fibrinopeptide A, the 34Val peptide was released more slowly than fibrinopeptide A but more quickly than fibrinopeptide B generation. Cross-linking of γ- and -chains appeared earlier when fibrin was incubated with FXIII 34Leu than with 34Val. Fully activated 34Leu and 34Val FXIII showed similar cross-linking activity. Analysis of fibrin clots prepared using plasma from FXIII 34Leu subjects by turbidity and permeability measurements showed reduced fiber mass/length ratio and porosity compared to 34Val. The structural differences were confirmed by electron microscopy. These results demonstrate that Val34Leu accelerates activation of FXIII by thrombin and consequently affects the structure of the cross-linked fibrin clot.

The factor XIII V34L polymorphism accelerates thrombin activation of factor XIII and affects cross-linked fibrin structure

Factor XIII on activation by thrombin cross-links fibrin. A common polymorphism Val to Leu at position 34 in the FXIII A subunit is under investigation as a risk determinant of thrombosis. Because Val34Leu is close to the thrombin cleavage site, the hypothesis that it would alter the function of FXIII was tested. Analysis of FXIII subunit proteolysis by thrombin using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance liquid chromatography showed that FXIII 34Leu was cleaved by thrombin more rapidly and by lower doses than 34Val. Mass spectrometry of isolated activation peptides confirmed the predicted single methyl group difference and demonstrated that the thrombin cleavage site is unaltered by Val34Leu. Kinetic analysis of activation peptide release demonstrated that the catalytic efficiency (kcat/Km) of thrombin was 0.5 for FXIII 34Leu and 0.2 (μmol/L)−1× sec−1 for 34Val. Presence of fibrin increased the catalytic efficiency to 4.8 and 2.2 (μmol/L)−1 × sec−1, respectively. Although the 34Leu peptide was released at a similar rate as fibrinopeptide A, the 34Val peptide was released more slowly than fibrinopeptide A but more quickly than fibrinopeptide B generation. Cross-linking of γ- and -chains appeared earlier when fibrin was incubated with FXIII 34Leu than with 34Val. Fully activated 34Leu and 34Val FXIII showed similar cross-linking activity. Analysis of fibrin clots prepared using plasma from FXIII 34Leu subjects by turbidity and permeability measurements showed reduced fiber mass/length ratio and porosity compared to 34Val. The structural differences were confirmed by electron microscopy. These results demonstrate that Val34Leu accelerates activation of FXIII by thrombin and consequently affects the structure of the cross-linked fibrin clot.