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

Clot contraction: compression of erythrocytes into tightly packed polyhedra and redistribution of platelets and fibrin

Contraction of blood clots is necessary for hemostasis and wound healing and to restore flow past obstructive thrombi, but little is known about the structure of contracted clots or the role of erythrocytes in contraction. We found that contracted blood clots develop a remarkable structure, with a meshwork of fibrin and platelet aggregates on the exterior of the clot and a close-packed, tessellated array of compressed polyhedral erythrocytes within. The same results were obtained after initiation of clotting with various activators and also with clots from reconstituted human blood and mouse blood. Such close-packed arrays of polyhedral erythrocytes, or polyhedrocytes, were also observed in human arterial thrombi taken from patients. The mechanical nature of this shape change was confirmed by polyhedrocyte formation from the forces of centrifugation of blood without clotting. Platelets (with their cytoskeletal motility proteins) and fibrin(ogen) (as the substrate bridging platelets for contraction) are required to generate the forces necessary to segregate platelets/fibrin from erythrocytes and to compress erythrocytes into a tightly packed array. These results demonstrate how contracted clots form an impermeable barrier important for hemostasis and wound healing and help explain how fibrinolysis is greatly retarded as clots contract.

High ferritin levels have major effects on the morphology of erythrocytes in Alzheimer's disease

Introduction: Unliganded iron both contributes to the pathology of Alzheimer's disease (AD) and also changes the morphology of erythrocytes (RBCs). We tested the hypothesis that these two facts might be linked, i.e., that the RBCs of AD individuals have a variant morphology, that might have diagnostic or prognostic value.

Methods: We included a literature survey of AD and its relationships to the vascular system, followed by a laboratory study. Four different microscopy techniques were used and results statistically compared to analyze trends between high and normal serum ferritin (SF) AD individuals.

Results: Light and scanning electron microscopies showed little difference between the morphologies of RBCs taken from healthy individuals and from normal SF AD individuals. By contrast, there were substantial changes in the morphology of RBCs taken from high SF AD individuals. These differences were also observed using confocal microscopy and as a significantly greater membrane stiffness (measured using force-distance curves).

Conclusion: We argue that high ferritin levels may contribute to an accelerated pathology in AD. Our findings reinforce the importance of (unliganded) iron in AD, and suggest the possibility both of an early diagnosis and some means of treating or slowing down the progress of this disease.

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.

Fibronectin alters the rate of formation and structure of the fibrin matrix

Plasma fibronectin is a vital component of the fibrin clot; however its role on clot structure is not clearly understood. The goal of this study was to examine the influence of fibronectin on the kinetics of formation, structural characteristics and composition of reconstituted fibrin clots or fibrin matrices. Fibrin matrices were formed by adding thrombin to 1, 2 or 4 mg/ml fibrinogen supplemented with 0-0.4 mg/ml fibronectin. The rate of fibrin matrix formation was then monitored by measuring light absorbance properties at different time points. Confocal microscopy of fluorescein conjugated fibrinogen was used to visualize the structural characteristics of fibrin matrices. The amount of fibronectin in fibrin matrices was determined through electrophoresis and immunoblotting of solubilized matrices. Fibronectin concentration positively correlated with the initial rate of fibrin matrix formation and with steady state light absorbance values of fibrin matrices. An increase in fibronectin concentration resulted in thinner and denser fibers in the fibrin matrices. Electrophoresis and immunoblotting showed that fibronectin was covalently and non-covalently bound to fibrin matrices and in the form of high molecular weight multimers. The formation of fibronectin multimers was attributed to cross-linking of fibronectin by trace amounts Factor XIIIa. These findings are novel because they link results from light absorbance studies to microcopy analyses and demonstrate an influence of fibronectin on fibrin matrix structural characteristics. This data is important in developing therapies that destabilize fibrin clots.

Links between allergy and cardiovascular or hemostatic system

In addition to a well-known immunologic background of atherosclerosis and influences of inflammation on arterial and venous thrombosis, there is growing evidence for the presence of links between allergy and vascular or thrombotic disorders. In this interpretative review, five pretty well-documented areas of such overlap are described and discussed, including: (1) links between atherosclerosis and immunoglobulin E or atopy, (2) mutual effects of blood lipids and allergy, (3) influence of atopy and related disorders on venous thromboembolism, (4) the role of platelets in allergic diseases, and (5) the functions of protein C system in atopic disorders.

Cardioembolic stroke: Protective effect of a severe internal carotid artery stenosis in a patient with cardiac embolism

Cardioembolic stroke is generally caused by intracranial artery occlusion. Clots may be identified in the intracranial vessels by means of conventional neuroimaging in the acute phase. High-resolution ultrasonography may show some features suggestive of cardiac emboli when occluding extracranial carotid arteries. We describe a patient with cardioembolic ischemic stroke in the right hemisphere in whom a left internal carotid artery stenosis paradoxically protected the ipsilateral hemisphere from distal intracranial embolism. The patient also presented multiple acute ischemic embolic lesions in the right middle cerebral artery territory and in the right occipital lobe, which was fed by the posterior cerebral artery, anomally originating from the right carotid siphon. Interestingly, the left internal carotid artery--which showed a severe preexisting stenosis--was occluded by the cardiac clot, whereas the right internal carotid artery only presented a moderate stenosis that had probably allowed the clots to pass. Therefore, the severe left internal carotid artery stenosis may have blocked the cardiac embolus, preventing it from reaching the ipsilateral hemisphere.

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.

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.

Alterations of fibrin network structure mediated by dermatan sulfate

Dermatan sulfate (DS) is well-known for its anticoagulant activity through binding to heparin cofactor II (HCII) to enhance thrombin inhibition. It has also been reported that DS has a profibrinolytic effect. We have evaluated the effects of DS solutions (4-20 μg/mL) on the formation (by kinetic studies), structure (by electron microscopy and compaction assays) and lysis (with urokinase-type plasminogen activator) of plasma fibrin networks. The results showed that DS significantly prolonged the lag phase and decreased the fibrin formation rate and the optical density of the final networks versus control, in a concentration dependent way. DS-associated networks presented a minor network percentage compared with control, composed of lower number of fibers per field, which resulted significantly thinner and longer. Moreover, DS rendered gels more sensible to rupture by centrifugal force and more susceptible to lysis. When fibrin formation kinetic assays were performed with purified fibrinogen instead of plasma, in the absence of HCII, the optical density of final DS-associated networks was statistically lower than control. Therefore, a direct effect of DS on the thickness of fibers was observed. Since in all in vitro assays low DS concentrations were used, it could be postulated that the fibrin features described above are plausible to be found in in vivo thrombi and therefore, DS would contribute to the formation of less thrombogenic clots.

Effect of tobacco smoking and smoking cessation on plasma lipoproteins and associated major cardiovascular risk factors: a narrative review

Cigarette smoking, active or passive, kills about 6 million people each year worldwide. Cardiovascular disease (CVD) is responsible for 40% of all smoking-related deaths, lung cancer accounts for 20% of all smoking-related deaths, and chronic obstructive pulmonary disease is related to another 20% of deaths. In this narrative review we consider the relationship between cigarette smoking and CVD. We discuss disease states and/or CVD risk factors related to smoking, such as dyslipidaemia, vascular inflammation, endothelial dysfunction, arterial stiffness, insulin resistance, type 2 diabetes mellitus (T2DM), chronic kidney disease (CKD), and non-alcoholic fatty liver disease (NAFLD) as well as their complex interrelations. Smoking cessation can correct abnormalities related to smoking; however, success rates are relatively low. In cases of inability to quit, measures to minimize the adverse effects of smoking specifically related to CVD should be taken. Smokers should receive best practice treatment, according to guidelines, as for non-smokers.

Effect of multispecies probiotic supplements on metabolic profiles, hs-CRP, and oxidative stress in patients with type 2 diabetes

BACKGROUND

We are aware of no study that has indicated the effects of daily consumption of multispecies probiotic supplements on metabolic profiles, high-sensitivity C-reactive protein (hs-CRP), and oxidative stress in diabetic patients.

OBJECTIVE

This study was designed to determine the effects of multispecies probiotic supplements on metabolic profiles, hs-CRP, and oxidative stress in diabetic patients.

METHODS

This randomized double-blind placebo-controlled clinical trial was performed on 54 diabetic patients aged 35-70 years. Subjects were randomly assigned to take either a multispecies probiotic supplement (n = 27) or placebo (n = 27) for 8 weeks. The multispecies probiotic supplement consisted of 7 viable and freeze-dried strains: Lactobacillus acidophilus (2 × 10(9) CFU), L. casei (7 × 10(9) CFU), L. rhamnosus (1.5 × 10(9) CFU), L. bulgaricus (2 × 10(8) CFU), Bifidobacterium breve (2 × 10(10) CFU), B. longum (7 × 10(9) CFU), Streptococcus thermophilus (1.5 × 10(9) CFU), and 100 mg fructo-oligosaccharide. Fasting blood samples were taken at baseline and after intervention to measure metabolic profiles, hs-CRP, and biomarkers of oxidative stress including plasma total antioxidant capacity and total glutathione (GSH).

RESULTS

Between-group comparisons of fasting plasma glucose (FPG) revealed that consumption of probiotic supplements prevented a rise in FPG (+28.8 ± 8.5 for placebo vs. +1.6 ± 6 mg/dl for probiotic group, p = 0.01). Although a significant within-group increase in serum insulin and low-density lipoprotein cholesterol levels was found in both the probiotic group and the placebo group, the changes were similar between the two groups. We observed a significant increase in HOMA-IR (homeostasis model of assessment-insulin resistance) in both the probiotic group (p = 0.02) and the placebo group (p = 0.001); however, the increase in the placebo group was significantly higher than that in the probiotic group (+2.38 vs. +0.78, p = 0.03). Mean changes in serum hs-CRP were significantly different between the two groups (-777.57 for the probiotic group vs. +878.72 ng/ml for the placebo group, p = 0.02). Probiotic supplementation led to a significant increase in plasma GSH levels compared to placebo (240.63 vs. -33.46 µmol/l, p = 0.03).

CONCLUSION

In conclusion, multispecies probiotic supplementation, compared with placebo, for 8 weeks in diabetic patients prevented a rise in FPG and resulted in a decrease in serum hs-CRP and an increase in plasma total GSH.

Mechanisms of Coronary Thrombosis in Cigarette Smoke Exposure

Acute rupture or erosion of a coronary atheromatous plaque and subsequent coronary artery thrombosis cause the majority of sudden cardiac deaths and myocardial infarctions. Cigarette smoking is a major risk factor for acute coronary thrombosis. Indeed, a majority of sudden cardiac deaths attributable to acute thrombosis are in cigarette smokers. Both active and passive cigarette smoke exposure seem to increase the risk of coronary thrombosis and myocardial infarctions. Cigarette smoke exposure seems to alter the hemostatic process via multiple mechanisms, which include alteration of the function of endothelial cells, platelets, fibrinogen, and coagulation factors. This creates an imbalance of antithrombotic/prothrombotic factors and profibrinolytic/antifibrinolytic factors that support the initiation and propagation of thrombosis.

Architecture of fibrin network inside thrombotic material obtained from the right atrium and pulmonary arteries: flow and location matter

Pulmonary embolectomy is a treatment option in selected patients with high-risk pulmonary embolism (PE). Efficiency of thrombus degradation in PE largely depends on the architecture of its fibrin network, however little is known about its determinants. We present the case of a 56-year-old woman with high-risk PE and proximal deep-vein thrombosis, whose thrombotic material removed during embolectomy from the right atrium and pulmonary (lobar and segmental) arteries has been studied using scanning electron microscopy (SEM). SEM images showed that distally located thrombi are richer in densely-packed fibrin fibers and contain more white cells and less erythrocytes than the proximal ones and the atrial thrombus. Fibrin fibers alignment along the flow vector was observed in the thrombi removed from high-velocity flow pulmonary arteries, and not in the atrial thrombus. The content of denser fibrin network and platelet aggregates was increased in segmental thromboemboli. Our findings describe the relation between thrombus architecture and location, and might help to elucidate thrombus resistance to anticoagulant therapy in some PE patients.

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.

High-density cholesterol and apolipoprotein AI as modifiers of plasma fibrin clot properties in apparently healthy individuals

Low high-density lipoprotein cholesterol (HDL-C) increases cardiovascular risk, whereas its high levels protect against atherosclerosis via multiple beneficial effects. Dense and poorly lysable fibrin clot formation is observed in cardiovascular disease. We sought to investigate whether HDL-C and its major component apolipoprotein A (Apo A)-I affect fibrin clot properties. In 136 apparently healthy individuals (99 men, 37 women, aged 49-69 years) we determined plasma fibrin clot permeability (Ks coefficient) and lysis time (t50%) together with Apo A-I and lipoprotein (a) [Lp(a)] levels. The median HDL-C level was 1.33  mmol/l (range from 0.77 to 2.19  mmol/l). HDL-C was positively associated with Apo A-I (r = 0.62, P < 0.00001). HDL-C and Apo A-I were positively correlated with Ks (r = 0.52, P < 0.00001 and r = 0.44, P < 0.00001, respectively) and inversely with t50% (r = -0.44, P < 0.00001 and r = -0.35, P = 0.00003, respectively). No such associations were seen for other lipid variables. Ks and t50% were associated with Lp(a) (r = -0.42, P < 0.00001 and r = 0.42, P < 0.00001, respectively) and fibrinogen (r = -0.31, P = 0.00024 and r = 0.39, P < 0.00001, respectively). Individuals with HDL-C at least 1.4 mmol/l (n = 54) had 19% higher Ks (P = 0.00016) and 17% shorter t50% (P = 0.0012) than the remainder. After adjustment for age, fibrinogen, and Lp(a), HDL-C was the independent predictor of Ks (β = 0.7, P < 0.00001) and t50% (β = -0.62, P < 0.00001). This study shows that elevated HDL-C levels are associated with improved fibrin clot permeability and lysis, indicating a novel antithrombotic mechanism underlying the postulated beneficial effects of therapy targeted at HDL-C.

Venous thromboembolism in patients with prior stroke

Patients with prior stroke are susceptible to venous thromboembolism (VTE). We studied patients with stroke in the Worcester VTE study of 2488 consecutive patients hospitalized with VTE. In all, 288 (11.6%) had a clinical history of stroke and 2200 (88.4%) did not. Patients with stroke were more likely to die inhospital (9.2% vs 4%) and within 30 days of VTE diagnosis (16.7% vs 6.9%) compared with patients without stroke (all P < .001). Recent immobilization (adjusted odds ratio [OR] 2.15; 95% confidence interval [CI] 1.15-4.09) and inferior vena cava (IVC) filter insertion (adjusted OR 2.1; 95% CI 1.15-3.83) were associated with a doubling of inhospital death. Recent immobilization (adjusted OR 1.84; 95% CI 1.19-2.83) and IVC filter insertion (adjusted OR 1.94; 95% CI 1.2-3.14) were associated with an increased risk of death within 30 days of VTE. In conclusion, patients with VTE and prior stroke were more than twice as likely to die while hospitalized and within 30 days of VTE diagnosis.

Counting 1 fibrin molecule at a time

In this issue of Blood, Hategan et al report on the development of a novel method to study single molecule kinetics of fibrin polymerization.(1)

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.

Impaired fibrinolysis is associated with the severity of aortic stenosis in humans

BACKGROUND

A role of fibrinolysis in the pathogenesis of aortic valve stenosis (AS) is unknown, although fibrinolytic proteins have been detected in aortic stenotic valves.

OBJECTIVE

To investigate whether impaired fibrinolysis could be associated with AS.

METHODS AND RESULTS

We studied 74 patients with AS (43 male, 31 female, aged 62.7 ± 10.7 years) without documented atherosclerotic valvular disease scheduled for isolated valve replacement and 68 controls. The plasma fibrin clot lysis time (CLT) in the presence of tissue factor (TF) and tissue plasminogen activator (tPA), along with plasma plasminogen activator inhibitor-1 (PAI-1) were determined. Valvular expression of fibrin and PAI-1 together with macrophages and mast cells (MC) was evaluated by immunostaining. Patients with AS compared with controls were characterized by a prolonged CLT (median, 110 [54-153] vs. 92.5 [58-115] min, P = 0.0007) and increased plasma PAI-1 (78.6 [35.5-149] vs. 38.5 [18-61] ng mL(-1) , P < 0.0001). CLT was correlated with maximal (r = 0.43, P = 0.0002) and mean (r = 0.38, P = 0.001) transvalvular pressure gradients, and aortic valve area (r = -0.59, P < 0.0001). In AS patients, the CLT was positively correlated with the valve leaflet thickness (r = 0.67, P = 0.003), the degree of valve calcification (r = 0.65, P < 0.00001), valvular fibrin (r = 0.54, P = 0.007) and PAI-1 expression (r = 0.48, P = 0.007). Double-immunostaining revealed colocalization of valvular PAI-1 with MC (87 ± 17% cells) and macrophages (48 ± 11% cells) within stenotic valves.

CONCLUSIONS

Hypofibrinolysis might be a marker of severe AS and be implicated in AS progression.

Acidosis, magnesium and acetylsalicylic acid: effects on thrombin

Thrombin, an enzyme from the hydrolase family, is the main component of the blood coagulation system. In ischemic stroke it acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin forming blood clots in the brain. It has been found to phosphoresce at room temperature in the millisecond and microsecond ranges. The phosphorescence of thrombin was studied under physiological conditions, in acidosis (decrease of pH from 8.0 to 5.0) and on the addition of salts (magnesium sulfate and sodium chloride) and of acetylsalicylic acid, and its connection with thrombin function is discussed. Acidosis significantly increased the internal dynamics of thrombin. We propose that lactate-acidosis plays a protective role in stroke, preventing the formation of clots. The addition of NaCl and MgSO(4) in different concentrations increased the internal dynamics of thrombin. Also, the addition of MgSO(4) decreased thrombin-induced platelet aggregation. However, magnesium sulfate and acetylsalicylic acid in the therapeutic concentrations used for treatment of ischemic stroke had no effect on thrombin internal dynamics. The data obtained will help to elucidate the conformational stability of thrombin under conditions modulating lactate-acidosis and in the presence of magnesium sulfate.

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.

Common threads in cardiac fibrosis, infarct scar formation, and wound healing

Wound healing, cardiac fibrosis, and infarct scar development, while possessing distinct features, share a number of key functional similarities, including extracellular matrix synthesis and remodeling by fibroblasts and myofibroblasts. Understanding the underlying mechanisms that are common to these processes may suggest novel therapeutic approaches for pathologic situations such as fibrosis, or defective wound healing such as hypertrophic scarring or keloid formation. This manuscript will briefly review the major steps of wound healing, and will contrast this process with how cardiac infarct scar formation or interstitial fibrosis occurs. The feasibility of targeting common pro-fibrotic growth factor signaling pathways will be discussed. Finally, the potential exploitation of novel regulators of wound healing and fibrosis (ski and scleraxis), will be examined.

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.

New insights into the mechanisms of venous thrombosis

Venous thrombosis is a leading cause of morbidity and mortality in industrialized countries, especially in the elderly. Many risk factors have been identified for venous thrombosis that alter blood flow, activate the endothelium, and increase blood coagulation. However, the precise mechanisms that trigger clotting in large veins have not been fully elucidated. The most common site for initiation of the thrombus appears to be the valve pocket sinus, due to its tendency to become hypoxic. Activation of endothelial cells by hypoxia or possibly inflammatory stimuli would lead to surface expression of adhesion receptors that facilitate the binding of circulating leukocytes and microvesicles. Subsequent activation of the leukocytes induces expression of the potent procoagulant protein tissue factor that triggers thrombosis. Understanding the mechanisms of venous thrombosis may lead to the development of new treatments.

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.

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.