Altered Fibrin Architecture Is Associated With Hypofibrinolysis and Premature Coronary Atherothrombosis

Altered fibrin clot structure/function in patients with antiphospholipid syndrome: association with thrombotic manifestation

We tested the hypothesis that plasma fibrin clot structure/function is unfavourably altered in patients with antiphospholipid syndrome (APS). Ex vivo plasma clot permeability, turbidity and susceptibility to lysis were determined in 126 consecutive patients with APS enrolled five months or more since thrombotic event vs 105 controls. Patients with both primary and secondary APS were characterised by 11% lower clot permeability (p<0.001), 4.8% shorter lag phase (p<0.001), 10% longer clot lysis time (p<0.001), and 4.7% higher maximum level of D-dimer released from clots (p=0.02) as compared to the controls. Scanning electron microscopy images confirmed denser fibrin networks composed of thinner fibres in APS. Clots from patients with "triple-antibody positivity" were formed after shorter lag phase (p=0.019) and were lysed at a slower rate (p=0.004) than in the remainder. Clots from APS patients who experienced stroke and/or myocardial infarction were 8% less permeable (p=0.01) and susceptible to lysis (10.4% longer clot lysis time [p=0.006] and 4.5% slower release of D-dimer from clots [p=0.01]) compared with those following venous thromboembolism alone. Multivariate analysis adjusted for potential confounders showed that in APS patients, lupus anticoagulant and "triple-positivity" were the independent predictors of clot permeability, while "triple-positivity" predicted lysis time. We conclude that APS is associated with prothrombotic plasma fibrin clot phenotype, with more pronounced abnormalities in arterial thrombosis. Molecular background for this novel prothrombotic mechanism in APS remains to be established.

Thromboelastographic phenotypes of fibrinogen and its variants: clinical and non-clinical implications

INTRODUCTION

Thromboelastography (TEG), a widely used clinical point of care coagulation test, is poorly understood. To investigate its fibrin determinants we used normal and variant fibrinogen isolates.

MATERIALS AND METHODS

We focused mainly on the TEG maximum signal amplitude (MA), a shear modulus and clot stiffness indicator. Isolates included normal des-αC, cord, and abnormal congenital variants with amino acid substitutions or deletions that impaired fibrin polymerization. Heterophenotypic congenital isolates were from cryoprecipitate-depleted plasma owing to their more diminished clot MA than their cryoprecipitate counterparts. By colorimetric assay, the amount of fibrinogen adsorbed by untreated TEG cups was 83.5±12.4 pM/cm(2), n=18. Thrombin-induced clots were obtained at pH6.4 or 7.4, the latter containing 8mM CaCl2, and 14% afibrinogenemic plasma with and without gel-sieved platelets.

RESULTS AND CONCLUSIONS

Measured by the water droplet contact angle, >90% reduction of surface hydrophobicity by exposure of TEG cup and pin to ozone plasma decreased MA by 74%. Increasing normal fibrinogen or thrombin concentrations progressively increased MA. Platelets increased MA further ~2 fold, except for ≥10 fold for des-αC clots. Examined in the absence of platelets, MA of heterophenotypic fibrin variants averaged 21%, n=15. The results imply that essential MA determinants include hydrophobic fibrinogen/fibrin adsorption and each polymerization contact site, with substantial enhancement by platelets. Also, cryoprecipitate-harvested soluble fibrinogen/fibrin complexes contained mostly normal molecules, while cryoprecipitate-depleted plasma contained mostly variant molecules. Moreover, significantly decreased MA by fibrinogen anomalies and/or low level thrombin generation can potentially impact clinical interpretation of MA.

Distinct associations of complement C3a and its precursor C3 with atherosclerosis and cardiovascular disease. The CODAM study

Complement C3 is a novel risk factor for cardiovascular disease (CVD), but the underlying mechanism is currently unknown. We determined the associations of the anaphylatoxin C3a, the activation product of C3, and of C3 itself with estimates of atherosclerosis and CVD. We studied associations of C3a and C3 with carotid intima-media thickness (cIMT), ankle-arm blood pressure index (AAIx) and CVD in cross-sectional analyses among 545 participants of the Cohort on Diabetes and Atherosclerosis Maastricht (CODAM) study (62% men, 59.4 ± 6.9 years) and examined effect modification by smoking. We conducted linear and logistic regression analyses with adjustments for age, sex, glucose metabolism status, lipids, adiposity, renal function, blood pressure, pack-years smoked, physical activity, use of medication and investigated mediation by inflammation. C3a was independently associated with cIMT (β=0.032 mm, [95% confidence interval: 0.004; 0.060]) and AAIx (β=-0.022, [-0.043; -0.001]), but C3 was not. Effect modification by smoking was only observed for CVD (P(smoking*C3a)=0.008, P(smoking*C3)=0.018), therefore these associations were stratified for smoking behaviour. Both C3a (odds ratio [OR] =2.96, [1.15; 7.62]) and C3 (OR =1.98, [1.21; 3.22]) were independently associated with CVD in heavy smokers. The association of C3 with CVD was independent of C3a. Low-grade inflammation did partially explain the association of C3a with AAIx, but not the other observed associations. This suggests that C3a and C3 have distinct roles in pathways leading to CVD. C3a may promote atherosclerosis and additionally advance CVD in heavy smokers. Conversely, C3 may be associated with CVD in heavy smokers via pathways other than atherosclerosis.

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.

Lipoprotein (a), LPA Ile4399Met, and fibrin clot properties

INTRODUCTION

Elevated lipoprotein(a) (Lp(a)) levels were reported to be associated with dense fibrin clots. The apo(a) component of Lp(a) is encoded by LPA, and the Met allele of the LPA Ile4399Met polymorphism is associated with elevated Lp(a) levels and cardiovascular disease risk. We investigated whether Ile4399Met was associated with fibrin clot properties.

MATERIALS AND METHODS

We determined plasma Lp(a) levels, fibrin clot permeability and lysis time for 64 LPA 4399Met carriers and 128 noncarriers matched for age, sex, ethnicity, and enrollment site.

RESULTS

Elevated Lp(a) levels were associated with reduced clot permeability and prolonged lysis time (P<0.0001). Carriers of 4399Met had higher Lp(a) levels compared with noncarriers (P=0.0003). However, this association differed by ethnicity (P=0.003 for interaction between genotype and ethnicity): compared with noncarriers, 4399Met carriers had 2.89 fold higher Lp(a) levels among Caucasians while no difference was observed among non-Caucasians (primarily East Asians and Hispanics). Among all subjects, no association was observed between Ile4399Met and clot properties, but this relationship also differed by ethnicity: among non-Caucasians, 4399Met carriers had increased clot permeability and shorter lysis time; whereas among Caucasians, the trend was for decreased permeability and longer lysis time (P<0.01 for interactions between genotype and ethnicity).

CONCLUSIONS

We confirmed that elevated Lp(a) levels are associated with dense fibrin clots, and found that the association of LPA 4399Met carriers and clot permeability as well as lysis time differ by ethnicity.

Enhanced lysis and accelerated establishment of viscoelastic properties of fibrin clots are associated with pulmonary embolism

The factors that contribute to pulmonary embolism (PE), a potentially fatal complication of deep vein thrombosis (DVT), remain poorly understood. Whereas fibrin clot structure and functional properties have been implicated in the pathology of venous thromboembolism and the risk for cardiovascular complications, their significance in PE remains uncertain. Therefore, we systematically compared and quantified clot formation and lysis time, plasminogen levels, viscoelastic properties, activated factor XIII cross-linking, and fibrin clot structure in isolated DVT and PE subjects. Clots made from plasma of PE subjects showed faster clot lysis times with no differences in lag time, rate of clot formation, or maximum absorbance of turbidity compared with DVT. Differences in lysis times were not due to alterations in plasminogen levels. Compared with DVT, clots derived from PE subjects showed accelerated establishment of viscoelastic properties, documented by a decrease in lag time and an increase in the rate of viscoelastic property formation. The rate and extent of fibrin cross-linking by activated factor XIII were similar between clots from DVT and PE subjects. Electron microscopy revealed that plasma fibrin clots from PE subjects exhibited lower fiber density compared with those from DVT subjects. These data suggest that clot structure and functional properties differ between DVT and PE subjects and provide insights into mechanisms that may regulate embolization.

Prolonged duration of type 2 diabetes is associated with increased thrombin generation, prothrombotic fibrin clot phenotype and impaired fibrinolysis

It has been shown that type 2 diabetes (DM) is associated with enhanced thrombin generation and formation of denser fibrin clots of reduced lysability. We sought to investigate the impact of diabetes duration versus glycaemia control on fibrin clot phenotype and its determinants in type 2 diabetic patients. In 156 consecutive Caucasian patients with type 2 diabetes we investigated ex vivo thrombin generation, fibrinolytic proteins, along with plasma fibrin clot permeation (Ks), compaction, turbidity, and efficiency of tissue plasminogen activator (t-PA)-mediated fibrinolysis. Patients with longer diabetes duration (>5 years, median; n=68) had higher peak thrombin generation (+16.3%, p<0.001), plasminogen activator inhibitor-1 (PAI-1) antigen (+14.8%, p=0.001), t-PA antigen (+13.9%, p=0.002) compared with those with duration ≤5 years (n=88). No such differences were observed between patients with inadequate glycaemic control, defined as glycated haemoglobin (HbA1C) >6.5% (48 mmol/mol) (n=77), versus those with HbA1C≤6.5% (n=79). Fibrinogen, thrombin-activatable fibrinolysis inhibitor antigen, plasminogen and soluble thrombomodulin were unaffected by disease duration or glycaemia control. Lower clot permeability, longer clot lysis, and higher maximum D-dimer levels released from clots (all p<0.05 after adjustment for fibrinogen, age, body mass index, insulin, acetylsalicylic acid treatment, and HbA1c or diabetes duration) were also observed in patients with diabetes duration >5 years and those with HbA1C>6.5%. We conclude that prolonged duration of type 2 diabetes is associated with increased thrombin formation, hypofibrinolysis, and prothrombotic fibrin clot phenotype. The impact of disease duration on coagulation is different and stronger than that observed during inadequate glycaemia control.

Functional impact of oxidative posttranslational modifications on fibrinogen and fibrin clots

Fibrinogen is a circulating multifunctional plasma protein vital for hemostasis. Activation of the coagulation cascade converts soluble fibrinogen to insoluble polymerized fibrin, which, along with platelets, forms the hemostatic clot. However, inappropriate formation of fibrin clots may result in arterial and venous thrombotic disorders that may progress to life-threatening adverse events. Often thrombotic disorders are associated with inflammation and the production of oxidants. Fibrinogen represents a potential target for oxidants, and several oxidative posttranslational modifications that influence fibrinogen structure and function have been associated with disease pathogenesis. Here, we review various oxidative modifications of fibrinogen and the consequences of these modifications on protein structure and the ability to form fibrin and how the resulting alterations affect fibrin architecture and viscoelastic and biochemical properties that may contribute to disease.

Reduced plasma fibrin clot permeability and susceptibility to lysis in patients with inflammatory bowel disease: a novel prothrombotic mechanism

BACKGROUND

Inflammatory bowel disease (IBD) is associated with an increased risk of thromboembolism. Its mechanism is still unclear. Altered fibrin clot properties have been reported in patients with thromboembolism and those with chronic inflammatory states. We investigated whether fibrin characteristics are abnormal in IBD.

METHODS

Ex vivo plasma fibrin clot permeability (Ks), compaction, turbidity, and efficiency of fibrinolysis were assessed in 85 consecutive patients with IBD, including 47 with ulcerative colitis (UC) and 38 with Crohn's disease (CD), all with no history of thromboembolism. Forty-eight patients matched for age and sex served as controls.

RESULTS

Compared with controls, patients with UC and CD had 29.5% and 35.7% lower Ks associated with 13.8% and 23.1% lower compaction, respectively (all P < 0.001). Patients with UC and CD had higher maximum clot absorbance (+8.9%, P = 0.008, and +15.2%, P < 0.0001, respectively), higher maximum D-dimer released from clots (D-D(max), +27.0%, P = 0.01, and +28.7%, P < 0.0001, respectively), and prolonged clot lysis time (+19.0%, P < 0.0001, and +25.5%, P < 0.0001, respectively). Lag phase was similar in both group of patients. D-D(max) was the only parameter that differed between patients in the UC and CD groups, being higher in CD (P = 0.04). The multiple linear regression model showed that in patients with UC, but not with CD, Ks, compaction, lysis time, and D-D(max) were all independently associated with disease activity. In patients with CD, Ks and lysis time were independently predicted by fibrinogen and C-reactive protein.

CONCLUSIONS

Both UC and CD are characterized by formation of dense fibrin networks relatively resistant to lysis. Prothrombotic clot phenotype might represent a novel mechanism increasing thrombotic risk in IBD.

Correlation between classical rheometry and supersonic shear wave imaging in blood clots

The assessment of coagulating blood elasticity has gained importance as a result of several studies that have correlated it to cardiovascular pathologic conditions. In this study we use supersonic shear wave imaging (SSI) to measure viscoelastic properties of blood clots. At the same time, classical rheometry experiments were carried out on the same blood samples taken within the first few seconds of coagulation. Using SSI, phase velocities of the shear wave indicated increasing dispersion with time. In all cases, the frequency bandwidth of propagating shear waves changed from 20-50 Hz at the first few min of coagulation to around 300 Hz toward the end of experiments. Using the values of G' and G″ from the rheometry studies, the theoretical shear wave velocities were calculated and correlated with SSI measurements. Results of the two techniques were in very good agreement, confirming that SSI provides accurate measurements of viscoelastic properties as corroborated by conventional rheometric measurements.

Type 2 diabetes as a modifier of fibrin clot properties in patients with coronary artery disease

Altered fibrin clot structure has been reported both in patients with coronary artery disease (CAD) and those with type 2 diabetes mellitus (DM2). The aim of the present study was to evaluate plasma fibrin clot permeability and susceptibility to lysis in patients with DM2 and CAD. We studied 132 consecutive CAD patients, including 67 subjects with DM2, scheduled for elective coronary artery bypass grafting surgery. Ex vivo plasma fibrin clot permeability (K(s)) and lysis time (t(50%)) induced by 1 μg/mL recombinant tissue plasminogen activator (tPA), along with plasma levels of plasminogen activator inhibitor-1 (PAI-1), thrombin activatable fibrinolysis inhibitor (TAFI), tPA, von Willebrand factor (vWF), P-selectin, soluble CD40 ligand (sCD40L), were measured. Diabetic and non-diabetic patients did not differ in regard to demographics and remaining cardiovascular risk factors. Concomitant DM2 was associated with higher glucose (+24.3%, p < 0.001), fibrinogen (+9.0%, p = 0.037), PAI-1 (+58.7%, p < 0.001), tPA (+24.0%, p < 0.001) and P-selectin (+12.2%, p < 0.001). Compared with the non-diabetic group, the CAD patients with DM2 had lower K(s) (-6.1%, p = 0.02) and prolonged t(50%) (+5.1%, p = 0.04). Multiple regression analysis of the whole study group showed that vWF, PAI-1, fibrinogen and DM2 were the independent predictors of t(50%) (R(2) = 0.58, p < 0.001), while only vWF was an independent predictor of K(s) (R(2) = 0.22, p < 0.001). This study indicates that DM2 is potent enough to unfavorably affect plasma fibrin clot characteristics despite abnormal clot phenotype typically observed in CAD. Of note, platelet and endothelial markers appear to contribute to fibrin clot properties in CAD concomitant with DM2.

Fibrin clot structure and platelet aggregation in patients with aspirin treatment failure

BACKGROUND

Aspirin is a cornerstone in prevention of cardiovascular events and modulates both platelet aggregation and fibrin clot formation. Some patients experience cardiovascular events whilst on aspirin, often termed aspirin treatment failure (ATF). This study evaluated both platelet aggregation and fibrin clot structure in patients with ATF.

METHODS

We included 177 stable coronary artery disease patients on aspirin monotherapy. Among these, 116 (66%) had ATF defined as myocardial infarction (MI) whilst on aspirin. Platelet aggregation was assessed by Multiplate® aggregometry and VerifyNow®, whereas turbidimetric assays and scanning electron microscopy were employed to study fibrin clot characteristics.

RESULTS

Enhanced platelet aggregation was observed in patients with ATF compared with non-MI patients following stimulation with arachidonic acid 1.0 mM (median 161 (IQR 95; 222) vs. 97 (60; 1776) AU*min, p = 0.005) and collagen 1.0 µg/mL (293 (198; 427) vs. 220 (165; 370) AU*min, p = 0.03). Similarly, clot maximum absorbance, a measure of fibrin network density, was increased in patients with ATF (0.48 (0.41; 0.52) vs. 0.42 (0.38; 0.50), p = 0.02), and this was associated with thinner fibres (mean ± SD: 119.7±27.5 vs. 127.8±31.1 nm, p = 0.003) and prolonged lysis time (552 (498; 756) vs. 519 (468; 633) seconds; p = 0.02). Patients with ATF also had increased levels of C-reactive protein (CRP) (1.34 (0.48; 2.94) and 0.88 (0.32; 1.77) mg/L, p = 0.01) compared with the non-MI group. Clot maximum absorbance correlated with platelet aggregation (r = 0.31-0.35, p-values<0.001) and CRP levels (r = 0.60, p<0.001).

CONCLUSIONS

Patients with aspirin treatment failure showed increased platelet aggregation and altered clot structure with impaired fibrinolysis compared with stable CAD patients without previous MI. These findings suggest that an increased risk of aspirin treatment failure may be identified by measuring both platelet function and fibrin clot structure.

Obesity-related increased γ' fibrinogen concentration in children and its reduction by a physical activity-based lifestyle intervention: a randomized controlled study

OBJECTIVE

To determine if elevated plasma γ'-fibrinogen, typically involved in the formation of fibrinolysis-resistant clots, confers an increased risk for cardiovascular disease (CVD) and thrombosis in children as it does in adults. Although obesity-related hyperfibrinogenemia is frequently reported in children, the role of γ' fibrinogen and its response to physical activity-based lifestyle are less clear in this population.

STUDY DESIGN

In a randomized controlled 3-month physical activity-based lifestyle intervention, γ' fibrinogen concentration was measured in 21 children (aged 14-18 years; Tanner stage > IV), including 15 in the obese group and 6 in the normal weight group, with body mass index percentiles for age and sex of >95 and <85, respectively.

RESULTS

The relationships between γ' fibrinogen and other risk factors for CVD, such as markers of insulin resistance and subclinical inflammation, along with body composition (as measured by dual-energy X-ray absortiometry), were assessed before and after the intervention. γ' fibrinogen concentration was higher in the obese group compared with the normal weight group (P < .05) and was correlated with other risk factors for CVD (adjusted R(2) = 0.9; P < .05), and insulin emerged as the major predictor of γ' fibrinogen. The intervention reduced γ'-fibrinogen concentration (P < .05).

CONCLUSION

Our data reveal: (1) elevated γ' fibrinogen concentrations in obese insulin-resistant children compared with normal lean controls; (2) a relationship between γ' fibrinogen and other CVD risk factors; and (3) physical activity-induced reduction in γ' fibrinogen in obese children.

Progressive improvement in wound healing with increased therapy in haemophilia B mice

Previous work has shown that normalized haemostasis only at the time of an injury is not sufficient to promote optimal wound healing in haemophilia B (HB) mice. However, the duration of treatment required for optimal healing has not been established. The goal of these studies was to determine the effect of different durations of replacement or bypassing therapy [factor IX(FIX) or factor VIIa (FVIIa)] on wound healing parameters in a mouse model of HB. A dermal wound was placed on the back of HB mice. Animals were either untreated or pretreated and then subsequently treated for 3 days, 5 days, or 7 days with FIX or FVIIa. Wound area, time to wound healing, haematoma formation and iron deposition were measured. All treated animals showed shortened time to healing relative to untreated animals. Haematoma formation was prevented by treatment and bleeding into the wounds, measured by iron scores, was reduced by treatment. In addition, there was a progressive improvement in healing with 7 days of treatment more effective than 5 days which was more effective than 3 days. Replacement therapy with FIX had slightly shorter healing times than bypassing therapy with FVIIa. HB mice treated with FIX had slightly smaller wound area than untreated animals; by contrast, FVIIa-treated animals had much smaller wound areas that were close to the wound areas seen in wild-type animals. The data suggest that sustained therapy is required for normal wound healing.

Fibrin(ogen) and thrombotic disease

Fibrinogen is an abundant plasma protein that, when converted to fibrin by thrombin, provides the main building blocks for the clot. Dys-, a-, and hypo-fibrinogenemias have been variably linked to a normal phenotype, bleeding or even thrombosis. Meanwhile, increased fibrinogen concentrations in the blood have been associated with risk for thrombosis. More recently, studies have focussed on abnormal fibrin structure as a cause for thrombosis. Fibrin clots that have high fiber density and increased resistance to fibrinolysis have been consistently associated with risk for thrombosis. Fibrin structure measurements can (i) provide an overall assessment of hemostatic capacity of a sample, (ii) include effects of thrombin generation and fibrinogen concentrations, (iii) include effects of fibrinogen mutations, polymorphisms, and modifications, and (iv) give an indication of clot mechanical strength and resistance to fibrinolysis. A fibrinogen splice variation of the γ-chain (γ') is discussed as a model for changes in fibrin structure in relation to thrombosis. Results from prospective studies on fibrin structure are awaited. Studies of fibrin formation under flow, interactions of fibrin with blood cells, the mechanical properties of the fibrin clot, and nanoscale/molecular characterization of fibrin formation are likely to expose new causal mechanisms for the role of fibrin in thrombotic disease. Future studies into the causality and mechanisms may lead to new opportunities using fibrin structure in the diagnosis or treatment of thrombosis.

Biomechanics of haemostasis and thrombosis in health and disease: from the macro- to molecular scale

Although the processes of haemostasis and thrombosis have been studied extensively in the past several decades, much of the effort has been spent characterizing the biological and biochemical aspects of clotting. More recently, researchers have discovered that the function and physiology of blood cells and plasma proteins relevant in haematologic processes are mechanically, as well as biologically, regulated. This is not entirely surprising considering the extremely dynamic fluidic environment that these blood components exist in. Other cells in the body such as fibroblasts and endothelial cells have been found to biologically respond to their physical and mechanical environments, affecting aspects of cellular physiology as diverse as cytoskeletal architecture to gene expression to alterations of vital signalling pathways. In the circulation, blood cells and plasma proteins are constantly exposed to forces while they, in turn, also exert forces to regulate clot formation. These mechanical factors lead to biochemical and biomechanical changes on the macro- to molecular scale. Likewise, biochemical and biomechanical alterations in the microenvironment can ultimately impact the mechanical regulation of clot formation. The ways in which these factors all balance each other can be the difference between haemostasis and thrombosis. Here, we review how the biomechanics of blood cells intimately interact with the cellular and molecular biology to regulate haemostasis and thrombosis in the context of health and disease from the macro- to molecular scale. We will also show how these biomechanical forces in the context of haemostasis and thrombosis have been replicated or measured in vitro.

The isolation of fibrinogen monomer dramatically influences fibrin polymerization

Fibrin polymerization begins with the thrombin-catalyzed cleavage of fibrinopeptides from fibrinogen and proceeds through several assembly steps to form an insoluble fibrin clot. Using dynamic light scattering (DLS), we found that purified fibrinogens are polydisperse, containing small amounts of fibrinogen complexes. In order to characterize the impact of these complexes, we used gel filtration chromatography to isolate monomers from three fibrinogens: plasma, recombinant, and recombinant variant Aα251. SDS-PAGE analysis showed that the polypeptides in the monomers were indistinguishable from those in the initial fibrinogen. DLS showed the fibrinogen monomers were monodisperse. We used turbidity to follow polymerization and found the polymerization of fibrinogen monomers was markedly different from the polymerization of the initial fibrinogen; the final optical density (OD) was significantly higher for monomers. Moreover, the polymerization curve for fibrinogen monomers was independent of the polymerization curves of the fibrinogen samples without gel filtration. For example, monomers isolated from two recombinant fibrinogen preparations polymerized similarly even though the final OD increased 2-fold for one preparation and 3-fold for the other. Scanning electron microscopy of the fibrin clots verified the turbidity data; monomer clots had thicker fibers. We conclude that fibrinogen complexes alter the kinetics of polymerization and impair the assembly of monomers into protofibrils and fibers.

Weight Reduction Is Associated With Increased Plasma Fibrin Clot Lysis

Obesity is associated with an increased risk of vascular thrombotic events. We sought to investigate how obesity and weight loss affect plasma fibrin clot properties. A total of 29 obese patients were studied before and after 3-month low-fat diet. Plasma fibrin clot parameters, including fibrin clot permeation coefficient ( Ks), the lag phase of the turbidity curve, clot lysis time ( t50%), maximum rate of increase in D-dimer levels, and maximum D-dimer concentrations, were determined. Low-fat diet resulted in the reduction of body weight ( P < .0001), body mass index ( P < .0001), fat mass ( P < .0001), total cholesterol ( P < .0001), low-density lipoprotein cholesterol ( P = .0005), triglycerides ( P = .008), and plasminogen activator inhibitor 1 ( P = .02), but not in fibrinogen or C-reactive protein. The only change in fibrin clot variables was shorter t50% ( P = .02). Baseline t50%, but not posttreatment, correlated with waist circumference ( r = .44, p = .02). This study demonstrates that weight loss in obese people can increase the efficiency of fibrin clot lysis.

Whole blood clots are more resistant to lysis than plasma clots--greater efficacy of rivaroxaban

INTRODUCTION

Defective thrombolysis, a thrombotic risk factor, can be attributed to the formation of a compact clot poorly accessible to fibrinolytic enzymes. Venous thrombi, rich in red blood cells (RBCs), and arterial thrombi containing various amounts of RBCS, plasma and whole blood (WB) clot permeability and degradability were compared. The effect of rivaroxaban, a potent direct factor Xa inhibitor, was also evaluated.

MATERIALS AND METHODS

Fibrin permeability was determined by flow measurement through the clot. Clot degradability was evaluated by the amount of D-dimer generated by clot perfusion with plasminogen and tissue plasminogen activator. Fibrin clot structure was assessed by confocal microscopy.

RESULTS

WB clot permeability (KS) and degradability were 6.7- and 38-fold lower, respectively, compared with plasma clots. This is attributed to 1) occlusion of fibrin pores by RBCs and 2) a consistent increase in thrombin generation due to platelets and RBCs inducing formation of a tighter clot. Rivaroxaban added to plasma or WB before clotting, in reducing thrombin generation, led to the formation of a looser clot that is more degradable by fibrinolytic enzymes. Permeability and degradability of whole blood clots formed in the presence of rivaroxaban were very similar to those of plasma clots.

CONCLUSION

The resistance to fibrinolysis of WB clots was reduced considerably when clots were formed with rivaroxaban. These results may have implications for the development of antithrombotic agents.

Mechanisms of fibrin polymerization and clinical implications

Research on all stages of fibrin polymerization, using a variety of approaches including naturally occurring and recombinant variants of fibrinogen, x-ray crystallography, electron and light microscopy, and other biophysical approaches, has revealed aspects of the molecular mechanisms involved. The ordered sequence of fibrinopeptide release is essential for the knob-hole interactions that initiate oligomer formation and the subsequent formation of 2-stranded protofibrils. Calcium ions bound both strongly and weakly to fibrin(ogen) have been localized, and some aspects of their roles are beginning to be discovered. Much less is known about the mechanisms of the lateral aggregation of protofibrils and the subsequent branching to yield a 3-dimensional network, although the αC region and B:b knob-hole binding seem to enhance lateral aggregation. Much information now exists about variations in clot structure and properties because of genetic and acquired molecular variants, environmental factors, effects of various intravascular and extravascular cells, hydrodynamic flow, and some functional consequences. The mechanical and chemical stability of clots and thrombi are affected by both the structure of the fibrin network and cross-linking by plasma transglutaminase. There are important clinical consequences to all of these new findings that are relevant for the pathogenesis of diseases, prophylaxis, diagnosis, and treatment.

Impaired thrombolysis: a novel cardiovascular risk factor in end-stage renal disease

AIMS

End-stage renal disease (ESRD) patients have an excess cardiovascular risk, above that predicted by traditional risk factor models. Prothrombotic status may contribute to this increased risk. Global thrombotic status assessment, including measurement of occlusion time (OT) and thrombolytic status, may identify vulnerable patients. Our aim was to assess overall thrombotic status in ESRD and relate this to cardiovascular risk.

METHODS AND RESULTS

Thrombotic and thrombolytic status of ESRD patients (n = 216) on haemodialysis was assessed using the Global Thrombosis Test. This novel, near-patient test measures the time required to form (OT) and time required to lyse (lysis time, LT) an occlusive platelet thrombus. Patients were followed-up for 276 ± 166 days for major adverse cardiovascular events (MACE, composite of cardiovascular death, non-fatal MI, or stroke). Peripheral arterial or arterio-venous fistula thrombosis was a secondary endpoint. Occlusion time was reduced (491 ± 177 vs. 378 ± 96 s, P < 0.001) and endogenous thrombolysis was impaired (LT median 1820 vs.1053 s, P < 0.001) in ESRD compared with normal subjects. LT ≥ 3000 s occurred in 42% of ESRD patients, and none of the controls. Impaired endogenous thrombolysis (LT ≥ 3000 s) was strongly associated MACE (HR = 4.25, 95% CI = 1.58-11.46, P = 0.004), non-fatal MI and stroke (HR = 14.28, 95% CI = 1.86-109.90, P = 0.01), and peripheral thrombosis (HR = 9.08, 95% CI = 2.08-39.75, P = 0.003). No association was found between OT and MACE.

CONCLUSION

Impaired endogenous thrombolysis is a novel risk factor in ESRD, strongly associated with cardiovascular events.

Anti-inflammatory therapy in type 1 diabetes

Type 1 diabetes (T1D) is a multi-factorial, organ-specific autoimmune disease in genetically susceptible individuals, which is characterized by a selective and progressive loss of insulin-producing β-cells. Cells mediating innate as well as adaptive immunity infiltrate pancreatic islets, thereby generating an aberrant inflammatory process called insulitis that can be mirrored by a pathologic autoantibody production and autoreactive T-cells. In tight cooperation with infiltrating innate immune cells, which secrete high levels of pro-inflammatory cytokines like IL-1β, TNFα, and INFγ effector T-cells trigger the fatal destruction process of β-cells. There is ongoing discussion on the contribution of inflammation in T1D pathogenesis, ranging from a bystander reaction of autoimmunity to a dysregulation of immune responses that initiate inflammatory processes and thereby actively promoting β-cell death. Here, we review recent advances in anti-inflammatory interventions in T1D animal models and preclinical studies and discuss their mode of action as well as their capacity to interfere with T1D development.

S-nitrosoglutathione acts as a small molecule modulator of human fibrin clot architecture

BACKGROUND

Altered fibrin clot architecture is increasingly associated with cardiovascular diseases; yet, little is known about how fibrin networks are affected by small molecules that alter fibrinogen structure. Based on previous evidence that S-nitrosoglutathione (GSNO) alters fibrinogen secondary structure and fibrin polymerization kinetics, we hypothesized that GSNO would alter fibrin microstructure.

METHODOLOGY/PRINCIPAL FINDINGS

Accordingly, we treated human platelet-poor plasma with GSNO (0.01-3.75 mM) and imaged thrombin induced fibrin networks using multiphoton microscopy. Using custom designed computer software, we analyzed fibrin microstructure for changes in structural features including fiber density, diameter, branch point density, crossing fibers and void area. We report for the first time that GSNO dose-dependently decreased fibrin density until complete network inhibition was achieved. At low dose GSNO, fiber diameter increased 25%, maintaining clot void volume at approximately 70%. However, at high dose GSNO, abnormal irregularly shaped fibrin clusters with high fluorescence intensity cores were detected and clot void volume increased dramatically. Notwithstanding fibrin clusters, the clot remained stable, as fiber branching was insensitive to GSNO and there was no evidence of fiber motion within the network. Moreover, at the highest GSNO dose tested, we observed for the first time, that GSNO induced formation of fibrin agglomerates.

CONCLUSIONS/SIGNIFICANCE

Taken together, low dose GSNO modulated fibrin microstructure generating coarse fibrin networks with thicker fibers; however, higher doses of GSNO induced abnormal fibrin structures and fibrin agglomerates. Since GSNO maintained clot void volume, while altering fiber diameter it suggests that GSNO may modulate the remodeling or inhibition of fibrin networks over an optimal concentration range.

High frequency ultrasound imaging of whole blood gelation and retraction during in vitro coagulation

Blood coagulation is a series of biochemical reactions resulting in the mechanical transformation of liquid blood into a gel. As a consequence, ultrasound, being mechanical waves, can provide specific details on the dynamics of coagulation. In fact, previous high-frequency ultrasound monitoring studies have shown drastic changes in ultrasound velocity and attenuation during whole blood coagulation and a model discussing the observed mechanical transformations was proposed. In this paper, a technique of visualization of the clotting mechanism is introduced, which complements and revises the previous hypotheses. This method is based on the monitoring of scatterers (red blood cells) movement through a time correlation of 20 MHZ rf signals. It allows the computing of both a displacement map revealing local details and disparities and a parameter quantifying the global structural behavior. Qualitative results for two typical samples show that the technique provides new insights on the gelation dynamics. A quantitative analysis computed from 12 healthy subjects found that the changes in the structural parameters are significantly correlated to the changes in velocity and attenuation, both dependent on the mechanical transformations in the sample. The previous model is therefore revised and a new way to measure gel and retraction times is proposed.

Effects of MASP-1 of the complement system on activation of coagulation factors and plasma clot formation

BACKGROUND

Numerous interactions between the coagulation and complement systems have been shown. Recently, links between coagulation and mannan-binding lectin-associated serine protease-1 (MASP-1) of the complement lectin pathway have been proposed. Our aim was to investigate MASP-1 activation of factor XIII (FXIII), fibrinogen, prothrombin, and thrombin-activatable fibrinolysis inhibitor (TAFI) in plasma-based systems, and to analyse effects of MASP-1 on plasma clot formation, structure and lysis.

METHODOLOGY/PRINCIPAL FINDINGS

We used a FXIII incorporation assay and specific assays to measure the activation products prothrombin fragment F1+2, fibrinopeptide A (FPA), and activated TAFI (TAFIa). Clot formation and lysis were assessed by turbidimetric assay. Clot structure was studied by scanning electron microscopy. MASP-1 activated FXIII and, contrary to thrombin, induced FXIII activity faster in the Val34 than the Leu34 variant. MASP-1-dependent generation of F1+2, FPA and TAFIa showed a dose-dependent response in normal citrated plasma (NCP), albeit MASP-1 was much less efficient than FXa or thrombin. MASP-1 activation of prothrombin and TAFI cleavage were confirmed in purified systems. No FPA generation was observed in prothrombin-depleted plasma. MASP-1 induced clot formation in NCP, affected clot structure, and prolonged clot lysis.

CONCLUSIONS/SIGNIFICANCE

We show that MASP-1 interacts with plasma clot formation on different levels and influences fibrin structure. Although MASP-1-induced fibrin formation is thrombin-dependent, MASP-1 directly activates prothrombin, FXIII and TAFI. We suggest that MASP-1, in concerted action with other complement and coagulation proteins, may play a role in fibrin clot formation.

Acetylation and glycation of fibrinogen in vitro occur at specific lysine residues in a concentration dependent manner: a mass spectrometric and isotope labeling study

Aspirin may exert part of its antithrombotic effects through platelet-independent mechanisms. Diabetes is a condition in which the beneficial effects of aspirin are less prominent or absent - a phenomenon called "aspirin resistance". We investigated whether acetylation and glycation occur at specific sites in fibrinogen and if competition between glucose and aspirin in binding to fibrinogen occurs. Our hypothesis was that such competition might be one explanation to "aspirin resistance" in diabetes. After incubation of fibrinogen in vitro with aspirin (0.8 mM, 24 h) or glucose (100 mM, 5-10 days), we found 12 modified sites with mass spectrometric techniques. Acetylations in the α-chain: αK191, αK208, αK224, αK429, αK457, αK539, αK562, in the β-chain: βK233, and in the γ-chain: γK170 and γK273. Glycations were found at βK133 and γK75, alternatively γK85. Notably, the lysine 539 is a site involved in FXIII-mediated cross-linking of fibrin. With isotope labeling in vitro, using [(14)C-acetyl]salicylic acid and [(14)C]glucose, a labeling of 0.013-0.084 and 0.12-0.5 mol of acetylated and glycated adduct/mol fibrinogen, respectively, was found for clinically (12.9-100 μM aspirin) and physiologically (2-8 mM glucose) relevant plasma concentrations. No competition between acetylation and glycation could be demonstrated. Thus, fibrinogen is acetylated at several lysine residues, some of which are involved in the cross-linking of fibrinogen. This may mechanistically explain why aspirin facilitates fibrin degradation. We find no support for the idea that glycation of fibrin(ogen) interferes with acetylation of fibrinogen.

Monomeric C-reactive protein alters fibrin clot properties on endothelial cells

Elevated plasma levels of C-reactive protein (CRP) are independently associated with increased risk of atherothrombosis. Several lines of evidence suggest that CRP has prothrombogenic effects on injured vessel wall(s) by enhancing tissue factor (TF) expression. Abnormal fibrin formation is correlated with increased thrombotic risk. However, the impact of localized, cell surface-driven in situ tissue factor generation by CRP on clot dynamics and fibrin architecture has not previously been evaluated. We examined the impact of native CRP and modified or monomeric CRP (mCRP) on the fibrin formation and structure in Human Umbilical Vein Endothelial Cells (HUVECs). Fibrin formation and structure were examined using laser scanning confocal microscopy. Incubation with mCRP on the cell surface had faster fibrin polymerization by the analysis of turbidimetry. Confocal microscopy of fibrin clots showed a significantly increased density in the treatment of mCRP compared with native CRP and control in the proximal versus distal relationship to the cell surface. The increased expression and activity of TF on the cell surface was observed by addition of mCRP. Blockage of tissue factor and lipid rafts significantly reduced the density of fibrin network produced by mCRP-stimulated endothelial cells. mCRP changes clot dynamics and alters fibrin architecture by enhancing TF on the endothelial cell surface. These results support the concept that elevated CRP levels may induce fibrinolytic resistance and endothelial dysfunction by altering fibrin clot structure.

A novel mechanism for hypofibrinolysis in diabetes: the role of complement C3

AIMS/HYPOTHESIS

Impaired fibrin clot lysis is a key abnormality in diabetes and complement C3 is one protein identified in blood clots. This work investigates the mechanistic pathways linking C3 and hypofibrinolysis in diabetes using ex vivo/in vitro studies.

METHODS

Fibrinolysis and C3 plasma levels were determined in type 1 diabetic patients and healthy controls, and the effects of glycaemia investigated. C3 incorporation into fibrin clots and modulation of fibrinolysis were analysed by ELISA, immunoblotting, turbidimetric assays and electron and confocal microscopy.

RESULTS

Clot lysis time was longer in diabetic children than in controls (599 ± 18 and 516 ± 12 s respectively; p < 0.01), C3 levels were higher in diabetic children (0.55 ± 0.02 and 0.43 ± 0.02 g/l respectively; p < 0.01) and both were affected by improving glycaemia. An interaction between C3 and fibrin was confirmed by the presence of lower protein levels in sera compared with corresponding plasma and C3 detection in plasma clots by immunoblot. In a purified system, C3 was associated with thinner fibrin fibres and more prolongation of lysis time of clots made from fibrinogen from diabetic participants compared with controls (244 ± 64 and 92 ± 23 s respectively; p < 0.05). Confocal microscopy showed higher C3 incorporation into diabetic clots compared with controls, and fully formed clot lysis was prolonged by 764 ± 76 and 428 ± 105 s respectively (p < 0.05). Differences in lysis, comparing diabetes and controls, were not related to altered plasmin generation or C3-fibrinogen binding assessed by plasmon resonance.

CONCLUSIONS/INTERPRETATION

C3 incorporation into clots from diabetic fibrinogen is enhanced and adversely affects fibrinolysis. This may be one novel mechanism for compromised clot lysis in diabetes, potentially offering a new therapeutic target.

Validation study toward measuring the mechanical properties of blood clots using resonant acoustic spectroscopy with optical vibrometry

Clot elastic modulus (CEM) has recently been shown to correlate with various hemostatic and thrombotic disorders and may be an important diagnostic parameter in cardiovascular diseases. Current methods of CEM measurement lack repeatability and require large sample volume. We present a novel method named resonant acoustic spectroscopy with optical vibrometry (RASOV) that has the potential to assess CEM with higher accuracy and speed, and lower sample volume. To validate RASOV, we measured the acoustic spectrum of agarose gel with varied concentrations in open-faced rectangular wells. Results showed a linear relationship between the natural resonant frequency and agarose content within a concentration range of 4 to 12 mg/mL. Furthermore, we observed that the resonant frequencies decrease with increasing transducer mass. As a highly accurate, resonance-based method, RASOV has great potential for biomechanical properties measurement, especially for human blood.

High-dose aspirin is required to influence plasma fibrin network structure in patients with type 1 diabetes

OBJECTIVE

Patients with type 1 diabetes form a less permeable fibrin network, which could contribute to their increased risk of cardiovascular disease (CVD). Low-dose aspirin treatment is the standard in the management of CVD; however, the effect seems reduced in patients with diabetes. We investigated the effects of low- and high-dose aspirin treatment on fibrin network formation in patients with type 1 diabetes (primary aim) and the possible interaction between the treatment effects of aspirin on fibrin network permeability and glycemic control in these patients (secondary aim).

RESEARCH DESIGN AND METHODS

Forty-eight patients (24 subjects with good [HbA(1c) <7.4%] and 24 subjects with poor [HbA(1c) >8.4%] glycemic control) were randomly assigned to treatment with 75 or 320 mg/day aspirin during 4 weeks in a crossover fashion. A 4-week washout period separated the treatment periods. The plasma fibrin network was assessed by determination of the permeability coefficient (K(s)).

RESULTS

Treatment with 75 mg aspirin did not influence fibrin network permeability (K(s)). However, K(s) increased significantly during treatment with 320 mg aspirin (P = 0.004), and a significant treatment effect was seen compared with treatment with 75 mg aspirin (P = 0.009). The increase in K(s) during high-dose aspirin treatment was significant in patients with poor glycemic control (P = 0.02), whereas K(s) only tended to increase in patients with good glycemic control (P = 0.06).

CONCLUSIONS

A high dose of aspirin is required to influence fibrin network permeability in patients with type 1 diabetes. The observed lack of effect with low-dose aspirin may contribute to aspirin treatment failure in diabetes.

Characteristics of fibrin formation and clot stability in individuals with congenital type IIb protein C deficiency

Many studies have shown that unregulated or excessive thrombin formation is potentially a cause of thrombosis; however, studies examining processes that contribute to fibrin stabilization in individuals predisposed to thrombosis are limited. In this study, we investigate a family with familial thrombosis via type IIb protein C (PC) deficiency. Using contact pathway inhibited whole blood, thrombin generation, fibrin clot formation and factor (f)XIII activation were monitored over time in control (n=5) and PC deficient (n=4, 34 - 69% PC by activity) subjects. The dynamics of thrombin generation varied significantly with the time required to reach the maximal rate of thrombin-antithrombin formation being much shorter in PC deficiency (5.8 ± 0.4 minutes) than in controls (9.7 ± 0.4 minutes; p < 0.001). PC deficient clots were significantly heavier than control clots (p < 0.001) and this difference could not be contributed exclusively to differences in fibrinogen levels between groups. FXIII was consumed faster in PC deficient subjects (23.7 ± 2.0 nM/minute) than in controls (5.1 ± 1.5 nM/minute; p < 0.0001) suggesting increased fXIII activation and incorporation of fXIIIa substrates into the clot. In plasma, the clot lysis time was increased in PC deficiency by both TAFIa dependent and independent mechanisms. Since PC deficient clots are both denser and show a greater degree of resistance to fibrinolysis, these clots would likely resist fibrinolysis and potentiate fibrin deposition observed in thrombosis.

α-α Cross-links increase fibrin fiber elasticity and stiffness

Fibrin fibers, which are ~100 nm in diameter, are the major structural component of a blood clot. The mechanical properties of single fibrin fibers determine the behavior of a blood clot and, thus, have a critical influence on heart attacks, strokes, and embolisms. Cross-linking is thought to fortify blood clots; though, the role of α-α cross-links in fibrin fiber assembly and their effect on the mechanical properties of single fibrin fibers are poorly understood. To address this knowledge gap, we used a combined fluorescence and atomic force microscope technique to determine the stiffness (modulus), extensibility, and elasticity of individual, uncross-linked, exclusively α-α cross-linked (γQ398N/Q399N/K406R fibrinogen variant), and completely cross-linked fibrin fibers. Exclusive α-α cross-linking results in 2.5× stiffer and 1.5× more elastic fibers, whereas full cross-linking results in 3.75× stiffer, 1.2× more elastic, but 1.2× less extensible fibers, as compared to uncross-linked fibers. On the basis of these results and data from the literature, we propose a model in which the α-C region plays a significant role in inter- and intralinking of fibrin molecules and protofibrils, endowing fibrin fibers with increased stiffness and elasticity.

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.

Beneficial effect of cigarette smoking cessation on fibrin clot properties

To examine the associations between cigarette smoking and preferable clot properties. Plasma fibrin clots from 21 randomly selected current smokers (n = 7), former smokers (n = 7) and non-smokers (n = 7) were analyzed, using scanning electron microscopy (SEM). With the use of the turbidimetric clotting and lysis assay in plasma, the maximum absorbance (MaxAbs(C), MaxAbs(L)) was measured and lysis time (Lys(50%)) was calculated. Smoking cessation significantly influenced fibrin fiber branching and density. Median fiber diameter was not changed. Lys(50%) was the highest in current smokers and was reduced in former smokers to the non-smoker level (2120 ± 385 versus 1771 ± 122 and 1724 ± 272 s; P = 0.04). Smoking cessation improves fibrin clot architecture which results in the lesser resistance to lysis.

Factor XIIa regulates the structure of the fibrin clot independently of thrombin generation through direct interaction with fibrin

Recent data indicate an important contribution of coagulation factor (F)XII to in vivo thrombus formation. Because fibrin structure plays a key role in clot stability and thrombosis, we hypothesized that FXII(a) interacts with fibrin(ogen) and thereby regulates clot structure and function. In plasma and purified system, we observed a dose-dependent increase in fibrin fiber density and decrease in turbidity, reflecting a denser structure, and a nonlinear increase in clot stiffness with FXIIa. In plasma, this increase was partly independent of thrombin generation, as shown in clots made in prothrombin-deficient plasma initiated with snake venom enzyme and in clots made from plasma deficient in FXII and prothrombin. Purified FXII and α-FXIIa, but not β-FXIIa, bound to purified fibrinogen and fibrin with nanomolar affinity. Immunostaining of human carotid artery thrombi showed that FXII colocalized with areas of dense fibrin deposition, providing evidence for the in vivo modulation of fibrin structure by FXIIa. These data demonstrate that FXIIa modulates fibrin clot structure independently of thrombin generation through direct binding of the N-terminus of FXIIa to fibrin(ogen). Modification of fibrin structure by FXIIa represents a novel physiologic role for the contact pathway that may contribute to the pathophysiology of thrombosis.

Platelet response to aspirin 50 and 100 mg in patients with coronary heart disease over a five-year period

Aspirin has been shown to decrease cardiovascular (CV) events by ∼25%. Despite aspirin therapy 10% to 20% of patients with arterial vascular disease develop atherothrombotic events. A meta-analysis of antiplatelet therapy showed a progressive decrease in clinical efficacy of aspirin after 2 years. Whether this is due to a decreased sensitivity to aspirin during long-term therapy remains unclear. A prospective randomized clinical trial with serial monitoring over 5 years was conducted in 100 patients with documented coronary heart disease. We investigated whether long-term treatment with aspirin 50 and 100 mg affects platelet response similarly. Occurrence of CV events was documented. Platelet sensitivity to aspirin, prostacyclin, and adenosine diphosphate-, collagen-, and epinephrine-induced platelet aggregation were evaluated over time. In addition, β-thromboglobulin and inflammatory markers were measured. Four patients were lost to follow-up and 10 patients died. Eleven patients developed nonfatal CV events. In the 2 groups platelet response to aspirin and the referenced variables remained unchanged over 5 years. In patients who developed CV events, the last monitoring interval revealed no difference in platelet response to aspirin. However, patients with nonfatal and fatal CV events showed increased inflammatory markers versus patients without CV events independent of aspirin 50 or 100 mg intake. In conclusion, our study revealed no difference in antiplatelet response to aspirin 50 versus 100 mg or CV events over 5 years in patients with coronary heart disease.

Fibrin clot structure and function: a role in the pathophysiology of arterial and venous thromboembolic diseases

The formation of fibrin clots that are relatively resistant to lysis represents the final step in blood coagulation. We discuss the genetic and environmental regulators of fibrin structure in relation to thrombotic disease. In addition, we discuss the implications of fibrin structure for treatment of thrombosis. Fibrin clots composed of compact, highly branched networks with thin fibers are resistant to lysis. Altered fibrin structure has consistently been reported in patients with several diseases complicated by thromboembolic events, including patients with acute or prior myocardial infarction, ischemic stroke, and venous thromboembolism. Relatives of patients with myocardial infarction or venous thromboembolism display similar fibrin abnormalities. Low-dose aspirin, statins, lowering of homocysteine, better diabetes control, smoking cessation, and suppression of inflammatory response increase clot permeability and susceptibility to lysis. Growing evidence indicates that abnormal fibrin properties represent a novel risk factor for arterial and venous thrombotic events, particularly of unknown etiology in young and middle-aged patients.

FXIII-A Leu34 genetic variant in premature coronary artery disease: a genotype--phenotype case control study

The FXIII-A Leu34 genetic variant increases and accelerates fibrin stabilisation; however, its association with premature coronary artery disease (CAD) and thrombotic events remains controversial. FXIII Val34Leu genotype was determined in 242 young individuals (<45 years old) who survived a myocardial infarction (MI) and 242 healthy controls matched for age and gender. We evaluated its effect on long-term clinical outcome defined as a composite of cardiovascular death, recurrent MI and urgent revascularisation. In addition, fibrin clot stiffness (elastic modulus or EM) and response to rt-PA-mediated fibrinolysis (fibrinolysis rate) were measured ex vivo using the Hemodyne analyser and confocal microscopy as surrogate endpoint. FXIII-A Leu34 genetic variant was not associated with premature CAD (adj. odds ratio 0.83 [0.49-1.4]) nor did it influence clinical outcome in patients, during a median follow-up of 6.3 (± 2.4) years. Patients produced stiffer fibrin clots (median [IQR] EM = 20.3 [14.9-28.1] vs. 12.8 [9.6-17.1] kdynes/cm²; p<0.0001) and displayed reduced response to fibrinolysis with lower fibrinolysis rate (6.7 [3.4-11.0] vs. 9.0 [5.0-16.7] sec-¹ x 10(-4); p<0.0001) than healthy controls. Carriage of factor XIII-A Leu34 led to a stepwise decrease in fibrinolysis rate with a significant gene-dose-effect in patients (7.7 [4.1-12.2] vs. 4.8 [3.0-8.5] vs. 4.3 [2.4-8.1] sec-¹ x 10(-4), for wild-type, heterozygous and homozygous, p for trend = 0.003) and a non-significant trend in controls (p = 0.01). In conclusion, FXIII-A Leu34 is a polymorphism which provides a strong resistance to fibrinolysis with a gene-dose effect, but does not relate to premature CAD or to recurrent coronary events in this study.

Imaging and Elastometry of Blood Clots Using Magnetomotive Optical Coherence Tomography and Labeled Platelets

Improved methods for imaging and assessment of vascular defects are needed for directing treatment of cardiovascular pathologies. In this paper, we employ magnetomotive optical coherence tomography (MMOCT) as a platform both to detect and to measure the elasticity of blood clots. Detection is enabled through the use of rehydrated, lyophilized platelets loaded with superparamagnetic iron oxides (SPIO-RL platelets) that are functional infusion agents that adhere to sites of vascular endothelial damage. Evidence suggests that the sensitivity for detection is improved over threefold by magnetic interactions between SPIOs inside RL platelets. Using the same MMOCT system, we show how elastometry of simulated clots, using resonant acoustic spectroscopy, is correlated with the fibrin content of the clot. Both methods are based upon magnetic actuation and phase-sensitive optical monitoring of nanoscale displacements using MMOCT, underscoring its utility as a broad-based platform to detect and measure the molecular structure and composition of blood clots.

Reduced thrombin formation and altered fibrin clot properties induced by polyunsaturated omega-3 fatty acids on top of dual antiplatelet therapy in patients undergoing percutaneous coronary intervention (OMEGA-PCI clot)

OBJECTIVE

The goal of this study was to investigate whether omega-3 polyunsaturated fatty acids (n-3 PUFA) are able to alter plasma fibrin clot properties and reduce thrombin formation in stable coronary artery disease patients undergoing percutaneous coronary intervention (PCI).

METHODS AND RESULTS

In an investigator-initiated, prospective, double-blind, placebo-controlled, randomized study, patients undergoing PCI who received standard pharmacotherapy were assigned to the treatment with 1 g/day n-3 PUFA (n = 30) or placebo (n = 24) for 1 month. Plasma fibrin clot permeability (K(s)); lysis time (t(50%)); prothrombin fragment 1.2; and peak thrombin generation from automated thrombogram, 8-isoprostaglandin F(2α) (8-iso-PGF(2α), an oxidative stress marker), and C-reactive protein were determined at baseline, 3 to 5 days after randomization, and 30 days after randomization. At baseline, both treatment groups did not differ significantly. A 1-month treatment with n-3 PUFA compared with placebo was associated with 15.3% higher K(s), indicating larger pores in the fibrin network (P = 0.0005); 14.3% shorter t(50%), indicating increased susceptibility to fibrinolysis (P<0.0001); 33.8% lower prothrombin fragment 1.2 (P = 0.0013); 13.4% lower peak thrombin generation (P = 0.04); and 13.1% lower 8-iso-PGF(2α) (P = 0.009). Treatment with n-3 PUFA had no effect on fibrinogen and C-reactive protein. After 1 month of treatment, fibrinogen (r = -0.53, P<0.0001), treatment assignment (r = 0.29, P = 0.006) and 8-iso-PGF(2α) (r = -0.27, P = 0.015) were independently associated with clot permeability (P<0.0001, R(2) = 0.66).

CONCLUSIONS

Adding n-3 PUFA to standard therapy in stable patients undergoing PCI significantly decreases thrombin formation and oxidative stress and favorably alters fibrin clot properties. These findings indicate novel antithrombotic effects induced by n-3 PUFA in humans.

Composition of coronary thrombus in acute myocardial infarction

OBJECTIVES

We sought to analyze the composition of coronary thrombus in vivo in ST-segment elevation myocardial infarction (STEMI) patients.

BACKGROUND

The dynamic process of intracoronary thrombus formation in STEMI patients is poorly understood.

METHODS

Intracoronary thrombi (n = 45) were obtained by thromboaspiration in 288 consecutive STEMI patients presenting for primary percutaneous intervention, and analyzed using high-definition pictures taken with a scanning electron microscope. Plasma biomarkers (TnI, CRPus, IL-6, PAI-1, sCD40 ligand, and TNF-α) and plasma fibrin clot viscoelastic properties were measured simultaneously on peripheral blood.

RESULTS

Thrombi were mainly composed of fibrin (55.9 ± 18%) with platelets (16.8 ± 18%), erythrocytes (11.5 ± 9%), cholesterol crystals (5.2 ± 8.4%), and leukocytes (1.3 ± 2.0%). The median ischemic time was 175 min (interquartile range: 140 to 297). Ischemic time impacted thrombi composition, resulting in a positive correlation with intracoronary thrombus fibrin content, r = 0.38, p = 0.01, and a negative correlation with platelet content, r = -0.34, p = 0.02. Thus, fibrin content increased with ischemic time, ranging from 48.4 ± 21% (<3 h) up to 66.9 ± 9% (>6 h) (p = 0.02), whereas platelet content decreased from 24.9 ± 23% (<3 h) to 9.1 ± 6% (>6 h) (p = 0.07). Soluble CD40 ligand was positively correlated to platelet content in the thrombus (r = 0.40, p = 0.02) and negatively correlated with fibrin content (r = -0.36; p = 0.04). Multivariate analysis indicated that ischemic time was the only predictor of thrombus composition, with a 2-fold increase of fibrin content per ischemic hour (adjusted odds ratio: 2.00 [95% confidence interval: 1.03 to 3.7]; p = 0.01).

CONCLUSIONS

In acute STEMI, platelet and fibrin contents of the occlusive thrombus are highly dependent on ischemia time, which may have a direct impact on the efficacy of drugs or devices used for coronary reperfusion.