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

The simultaneous occurrence of both hypercoagulability and hypofibrinolysis in blood and serum during systemic inflammation, and the roles of iron and fibrin(ogen)

Although the two phenomena are usually studied separately, we summarise a considerable body of literature to the effect that a great many diseases involve (or are accompanied by) both an increased tendency for blood to clot (hypercoagulability) and the resistance of the clots so formed (hypofibrinolysis) to the typical, 'healthy' or physiological lysis. We concentrate here on the terminal stages of fibrin formation from fibrinogen, as catalysed by thrombin. Hypercoagulability goes hand in hand with inflammation, and is strongly influenced by the fibrinogen concentration (and vice versa); this can be mediated via interleukin-6. Poorly liganded iron is a significant feature of inflammatory diseases, and hypofibrinolysis may change as a result of changes in the structure and morphology of the clot, which may be mimicked in vitro, and may be caused in vivo, by the presence of unliganded iron interacting with fibrin(ogen) during clot formation. Many of these phenomena are probably caused by electrostatic changes in the iron-fibrinogen system, though hydroxyl radical (OH˙) formation can also contribute under both acute and (more especially) chronic conditions. Many substances are known to affect the nature of fibrin polymerised from fibrinogen, such that this might be seen as a kind of bellwether for human or plasma health. Overall, our analysis demonstrates the commonalities underpinning a variety of pathologies as seen in both hypercoagulability and hypofibrinolysis, and offers opportunities for both diagnostics and therapies.

Protein N-linked homocysteine is associated with recurrence of venous thromboembolism

BACKGROUND

Recently, protein N-linked homocysteine (Hcy) has been measured in healthy subjects and patients with marked hyperhomocysteinemia. Since elevated total Hcy (tHcy) levels are associated with increased risk of venous thromboembolism (VTE), we aimed to investigate protein N-linked Hcy levels in patients with VTE.

METHODS

We studied 200 consecutive patients with VTE (89 men, 111 women, aged from 17 to 83 years), including 57 subjects with a subsequent episode of VTE (recurrent VTE) during 24 months of follow-up. Protein N-linked Hcy was assayed using high-performance liquid chromatography with an on-column derivatization with o-phthaldialdehyde and fluorescence detection.

RESULTS

The median protein N-linked Hcy was 1.404 μM (interquartile range [IQR] 0.859-2.066), while the median tHcy (IQR) was 9.1 μM (6.8-11.2). In the whole group protein N-linked Hcy correlated only with C-reactive protein (CRP; r = 0.44, p < 0.0001). In patients with recurrent VTE protein N-linked Hcy correlated with C-reactive protein (r = 0.43, p < 0.0001), tHcy (r = 0.42, p = 0.001) and age (r = 0.32, p = 0.014), but not with thrombophilia, unprovoked VTE or the current anticoagulation. Hyperhomocysteinemia, defined as tHcy ≥ 15 μM (n = 14.7%), was not associated with higher protein N-linked Hcy. Patients with recurrent VTE had higher levels of protein N-linked Hcy compared to those who experienced a single episode of VTE (1.553 μM, 1.157-2.445 vs. 1.27 μM, 0.826-1.884; p = 0.002). Multiple regression adjusted for potential confounders showed that the only independent predictor of protein N-linked Hcy in the upper quartile was CRP > 3mg/L (odds ratio 3.04, 95% confidence interval 2.12-4.36, p < 0.0001).

CONCLUSION

Elevated protein N-linked Hcy concentrations, indicating enhanced protein homocysteinylation in vivo, characterize patients with recurrent VTE and this phenomenon is associated with enhanced inflammatory state.

A novel bioreactor for the generation of highly aligned 3D skeletal muscle-like constructs through orientation of fibrin via application of static strain

The generation of functional biomimetic skeletal muscle constructs is still one of the fundamental challenges in skeletal muscle tissue engineering. With the notion that structure strongly dictates functional capabilities, a myriad of cell types, scaffold materials and stimulation strategies have been combined. To further optimize muscle engineered constructs, we have developed a novel bioreactor system (MagneTissue) for rapid engineering of skeletal muscle-like constructs with the aim to resemble native muscle in terms of structure, gene expression profile and maturity. Myoblasts embedded in fibrin, a natural hydrogel that serves as extracellular matrix, are subjected to mechanical stimulation via magnetic force transmission. We identify static mechanical strain as a trigger for cellular alignment concomitant with the orientation of the scaffold into highly organized fibrin fibrils. This ultimately yields myotubes with a more mature phenotype in terms of sarcomeric patterning, diameter and length. On the molecular level, a faster progression of the myogenic gene expression program is evident as myogenic determination markers MyoD and Myogenin as well as the Ca(2+) dependent contractile structural marker TnnT1 are significantly upregulated when strain is applied. The major advantage of the MagneTissue bioreactor system is that the generated tension is not exclusively relying on the strain generated by the cells themselves in response to scaffold anchoring but its ability to subject the constructs to individually adjustable strain protocols. In future work, this will allow applying mechanical stimulation with different strain regimes in the maturation process of tissue engineered constructs and elucidating the role of mechanotransduction in myogenesis.

STATEMENT OF SIGNIFICANCE

Mechanical stimulation of tissue engineered skeletal muscle constructs is a promising approach to increase tissue functionality. We have developed a novel bioreactor-based 3D culture system, giving the user the possibility to apply different strain regimes like static, cyclic or ramp strain to myogenic precursor cells embedded in a fibrin scaffold. Application of static mechanical strain leads to alignment of fibrin fibrils along the axis of strain and concomitantly to highly aligned myotube formation. Additionally, the pattern of myogenic gene expression follows the temporal progression observed in vivo with a more thorough induction of the myogenic program when static strain is applied. Ultimately, the strain protocol used in this study results in a higher degree of muscle maturity demonstrated by enhanced sarcomeric patterning and increased myotube diameter and length. The introduced bioreactor system enables new possibilities in muscle tissue engineering as longer cultivation periods and different strain applications will yield tissue engineered muscle-like constructs with improved characteristics in regard to functionality and biomimicry.

An automated method for fibrin clot permeability assessment

The fibrin clot permeability coefficient (Ks) is a useful measure of porosity of the fibrin network, which is determined by a number of genetic and environmental factors. Currently available methods to evaluate Ks are time-consuming, require constant supervision and provide only one parameter. We present an automated method in which drops are weighed individually, buffer is dosed by the pump and well defined clot washing is controlled by the software. The presence of a straight association between drop mass and their dripping time allows to shorten the measurement time twice. In 40 healthy individuals, Ks, the number of drops required to reach the plateau (DTP), the time to achieve the plateau (TTP) and the DTP/TTP ratio (DTR) were calculated. There was a positive association between Ks (r = 0.69, P < 0.0001) evaluated by using the manual [median of 4.17 (3.60-5.18) ·10⁻⁹ cm²) and the automated method [median of 4.35 (3.74-5.38) ·10⁻⁹ cm²]. The correlation was stronger (r = 0.85, P < 0.001) in clots with DTP of 7 or less (n = 12). DTP was associated with total homocysteine (tHcy) (r = 0.35, P < 0.05) and activated partial thromboplastin time (APTT) (r = -0.34, P < 0.05), TTP with Ks (r = -0.55, P < 0.01 for the manual method and r = -0.44, P < 0.01 for the automated method) and DTP (r = 0.75, P < 0.0001), and DTR with Ks (r = 0.70, P < 0.0001 for the manual method and r = 0.76, P < 0.0001 for the automated method), fibrinogen (r = -0.58, P < 0.0001) and C-reactive protein (CRP) (r = -0.47, P < 0.01). The automated method might be a suitable tool for research and clinical use and may offer more additional parameters describing fibrin clot structure.

Multimodal Molecular Imaging Reveals High Target Uptake and Specificity of 111In- and 68Ga-Labeled Fibrin-Binding Probes for Thrombus Detection in Rats

We recently showed the high target specificity and favorable imaging properties of 64Cu and Al18F PET probes for noninvasive imaging of thrombosis. Here, our aim was to evaluate new derivatives labeled with either with 68Ga, 111In, or 99mTc as thrombus imaging agents for PET and SPECT. In this study, the feasibility and potential of these probes for thrombus imaging was assessed in detail in 2 animal models of arterial thrombosis. The specificity of the probes was further evaluated using a triple-isotope approach with multimodal SPECT/PET/CT imaging.

METHODS

Radiotracers were synthesized using a known fibrin-binding peptide conjugated to 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid monoamide (DOTA-MA), or a diethylenetriamine ligand (DETA-propanoic acid [PA]), followed by labeling with 68Ga (FBP14, 68Ga-NODAGA), 111In (FBP15, 111In-DOTA-MA), or 99mTc (FBP16, 99mTc(CO)3-DETA-PA), respectively. PET or SPECT imaging, biodistribution, pharmacokinetics, and metabolic stability were evaluated in rat models of mural and occlusive carotid artery thrombosis. In vivo target specificity was evaluated by comparing the distribution of the SPECT and PET probes with preformed 125I-labeled thrombi and with a nonbinding control probe using SPECT/PET/CT imaging.

RESULTS

All 3 radiotracers showed affinity similar to soluble fibrin fragment DD(E) (inhibition constant=0.53-0.83 μM). After the kidneys, the highest uptake of 68Ga-FBP14 and 111In-FBP15 was in the thrombus (1.0±0.2 percentage injected dose per gram), with low off-target accumulation. Both radiotracers underwent fast systemic elimination (half-life, 8-15 min) through the kidneys, which led to highly conspicuous thrombi on PET and SPECT images. 99mTc-FBP16 displayed low target uptake and distribution consistent with aggregation or degradation. Triple-isotope imaging experiments showed that both 68Ga-FBP14 and 111In-FBP15, but not the nonbinding derivative 64Cu-D-Cys-FBP8, detected the location of the 125I-labeled thrombus, confirming high target specificity.

CONCLUSION

68Ga-FBP14 and 111In-FBP15 have high fibrin affinity and thrombus specificity and represent useful PET and SPECT probes for thrombus detection.

Altered Fibrin Clot Properties in Patients With Cerebral Venous Sinus Thrombosis: Association With the Risk of Recurrence

BACKGROUND AND PURPOSE

Venous thromboembolism and ischemic stroke are associated with unfavorable fibrin clot structure and function. We hypothesized that denser fibrin networks displaying impaired lysability characterize patients with cerebral venous sinus thrombosis (CVST).

METHODS

We assessed plasma fibrin clot properties in 50 patients (aged 38.9±9.8 years, 36 women) after the first CVST unrelated to trauma or malignancy after anticoagulation withdrawal and 50 well-matched controls. Recurrences were recorded during follow-up (18-46; median, 36 months).

RESULTS

Clot permeability was lower in patients with CVST than in controls (Ks, 6.43±0.97 versus 7.3±1.2 10(-9) cm(2); P<0.001) and was associated with prolonged clot lysis time (103.0±16.8 versus 92.4±16.2 minutes; P<0.001), lower maximum rate of D-dimer release from clots (0.068 [0.064-0.071] versus 0.072 [0.067-0.078] mg/L per minute; P<0.001) and higher maximum D-dimer levels in the lysis assay (4.39±0.56 versus 4.19±0.46 mg/L, respectively; P=0.03). Patients with CVST had a slightly shorter lag phase (P=0.02) and higher maximum absorbance of fibrin gels on turbidimetry (P<0.001) compared with controls. Deficiencies in natural anticoagulants or antiphospholipid syndrome, and factor V Leiden occurred more often in the patients (P<0.05). CVST recurred in 6 patients (12%) and was associated with 21% higher baseline fibrinogen (P=0.007), 20% lower Ks (P=0.04) and 17% greater D-Dmax (P=0.01). Multiple logistic regression showed that only elevated D-Dmax (>4.83 mg/L) predicted CVST recurrence (odds ratio, 5.1; 95% confidence interval, 1.63-16.19) after adjustment for fibrinogen.

CONCLUSIONS

CVST is associated with the formation of more compact plasma fibrin clots and resistance to fibrinolysis, which may predispose to the recurrence.

A portable blood plasma clot micro-elastometry device based on resonant acoustic spectroscopy

Abnormal blood clot stiffness is an important indicator of coagulation disorders arising from a variety of cardiovascular diseases and drug treatments. Here, we present a portable instrument for elastometry of microliter volume blood samples based upon the principle of resonant acoustic spectroscopy, where a sample of well-defined dimensions exhibits a fundamental longitudinal resonance mode proportional to the square root of the Young's modulus. In contrast to commercial thromboelastography, the resonant acoustic method offers improved repeatability and accuracy due to the high signal-to-noise ratio of the resonant vibration. We review the measurement principles and the design of a magnetically actuated microbead force transducer applying between 23 pN and 6.7 nN, providing a wide dynamic range of elastic moduli (3 Pa-27 kPa) appropriate for measurement of clot elastic modulus (CEM). An automated and portable device, the CEMport, is introduced and implemented using a 2 nm resolution displacement sensor with demonstrated accuracy and precision of 3% and 2%, respectively, of CEM in biogels. Importantly, the small strains (<0.13%) and low strain rates (<1/s) employed by the CEMport maintain a linear stress-to-strain relationship which provides a perturbative measurement of the Young's modulus. Measurements of blood plasma CEM versus heparin concentration show that CEMport is sensitive to heparin levels below 0.050 U/ml, which suggests future applications in sensing heparin levels of post-surgical cardiopulmonary bypass patients. The portability, high accuracy, and high precision of this device enable new clinical and animal studies for associating CEM with blood coagulation disorders, potentially leading to improved diagnostics and therapeutic monitoring.

Shaping bio-inspired nanotechnologies to target thrombosis for dual optical-magnetic resonance imaging

Arterial and venous thrombosis are among the most common causes of death and hospitalization worldwide. Nanotechnology approaches hold great promise for molecular imaging and diagnosis as well as tissue-targeted delivery of therapeutics. In this study, we developed and investigated bioengineered nanoprobes for identifying thrombus formation; the design parameters of nanoparticle shape and surface chemistry, i.e. incorporation of fibrin-binding peptides CREKA and GPRPP, were investigated. Two nanoparticle platforms based on plant viruses were studied - icosahedral cowpea mosaic virus (CPMV) and elongated rod-shaped tobacco mosaic virus (TMV). These particles were loaded to carry contrast agents for dual-modality magnetic resonance (MR) and optical imaging, and both modalities demonstrated specificity of fibrin binding in vitro with the presence of targeting peptides. Preclinical studies in a carotid artery photochemical injury model of thrombosis confirmed thrombus homing of the nanoprobes, with the elongated TMV rods exhibiting significantly greater attachment to thrombi than icosahedral (sphere-like) CPMV. While in vitro studies confirmed fibrin-specificity conferred by the peptide ligands, in vivo studies indicated the nanoparticle shape had the greatest contribution toward thrombus targeting, with no significant contribution from either targeting ligand. These results demonstrate that nanoparticle shape plays a critical role in particle deposition at the site of vascular injury. Shaping nanotechnologies opens the door for the development of novel targeted diagnostic and therapeutic strategies (i.e., theranostics) for arterial and venous thrombosis.

TACTIC: Trans-Agency Consortium for Trauma-Induced Coagulopathy

Trauma-induced coagulopathy (TIC) includes heterogeneous coagulopathic syndromes with different underlying causes, and treatment is challenged by limited diagnostic tests to discriminate between these entities in the acute setting. We provide an overview of progress in understanding the mechanisms of TIC and the context for several of the hypotheses that will be tested in 'TACTIC'. Although connected to ongoing clinical trials in trauma, TACTIC itself has no intent to conduct clinical trials. We do anticipate that 'early translation' of promising results will occur. Functions anticipated at this early translational level include: (i) basic science groundwork for future therapeutic candidates; (ii) development of acute coagulopathy scoring systems; (iii) coagulation factor composition-based computational analysis; (iv) characterization of novel analytes including tissue factor, polyphosphates, histones, meizothrombin and α-thrombin-antithrombin complexes, factor XIa, platelet and endothelial markers of activation, signatures of protein C activation and fibrinolysis markers; and (v) assessment of viscoelastic tests and new point-of-care methods.

Denser plasma clot formation and impaired fibrinolysis in paroxysmal and persistent atrial fibrillation while on sinus rhythm: association with thrombin generation, endothelial injury and platelet activation

INTRODUCTION

Formation of compact and poorly lysable fibrin clots have been demonstrated in patients following ischemic stroke. Recently, it has been shown that denser fibrin networks and impaired fibrinolysis occurs in subjects with permanent atrial fibrillation (AF). Fibrin clot phenotype in other types of AF remains to be established. We evaluated fibrin clot properties in paroxysmal (PAF) and persistent AF (PsAF).

MATERIAL AND METHODS

We studied 88 non-anticoagulated patients with AF on sinus rhythm and free of stroke (41 with PAF, 47 with PsAF) versus 50 controls. Ex-vivo plasma fibrin clot permeability (Ks) and clot lysis time (CLT) were evaluated along with von Willebrand factor (vWF), peak thrombin generation (TG), platelet factor 4 (PF4) and fibrinolytic proteins.

RESULTS

Compared with control subjects, clots obtained from plasma of patients with PAF and PsAF had similarly lower Ks (-7.7%, P=0.01; -8.6%, P=0.005, respectively) and prolonged CLT (+10.8%, P=0.006; +7.8% P=0.04, respectively). No associations of Ks and CLT with CHA2DS2-VASc and HAS-BLED score were observed. Patients with AF had higher TG, vWF, PF4 and plasminogen activator inhibitor-1 (PAI-1) antigen compared with controls. Multiple linear regression adjusted for age, gender, body mass index and fibrinogen showed that TG (β=-0.41), vWF (β=-0.29) and PF4 (β=-0.28) are the independent predictors of Ks (R(2)=0.78), while CLT was independently predicted by TG (β=0.37), PAI-1 antigen (β=0.29) and vWF (β=0.26) in the AF group (R(2)=0.39).

CONCLUSIONS

Patients with PAF and PsAF while on sinus rhythm display unfavorably altered fibrin clot properties, which might contribute to thromboembolic complications.

Time-Dependent Thrombus Resolution After Tissue-Type Plasminogen Activator in Patients With Stroke and Mice

BACKGROUND AND PURPOSE

We investigated the relationship between the degree of thrombus resolution and the time from stroke onset or thrombus formation to intravenous tissue-type plasminogen activator (tPA) treatment.

METHODS

In patients with stroke, we measured thrombus volume on thin-section noncontrast brain computed tomographic scans taken at baseline and 1 hour after tPA administration. We determined the association between the time from symptom onset to tPA treatment and the degree of thrombus resolution. In a C57/BL6 mouse model of FeCl3-induced carotid artery thrombosis, we investigated the effect of tPA administered at different time intervals after thrombus formation, using Doppler-based blood flow measurement.

RESULTS

Of 249 patients enrolled, 171 showed thrombus on baseline computed tomography. Thrombus was resolved by ≥50% in 43 patients (25.1%, good volume reduction) and by <50% in 94 patients (55.0%, moderate volume reduction) 1 hour after tPA treatment. In 34 patients (19.9%, nonvolume reduction; either no change or thrombus volume increased), overall thrombus volume increased. The probability of thrombus resolution decreased as the time interval from symptom onset to treatment increased. On multivariate analysis, good volume reduction was independently related with shorter time intervals from symptom onset to tPA treatment (odds ratio, 0.986 per minute saved; 95% confidence interval, 0.974-0.999). In the mouse model, as the interval between thrombus formation and tPA treatment increased, the initiation of recanalization was delayed (P=0.006) and the frequency of final recanalization decreased (P for trends=0.006).

CONCLUSIONS

Early administration of tPA after stroke onset is associated with better thrombus resolution.

Low fasting glucose is associated with enhanced thrombin generation and unfavorable fibrin clot properties in type 2 diabetic patients with high cardiovascular risk

Objective

To investigate the effect of low blood glucose on thrombin generation and fibrin clot properties in type 2 diabetes (T2DM).

Methods

In 165 patients with T2DM and high cardiovascular risk, we measured ex vivo plasma fibrin clot permeation [K_s], turbidity and efficiency of fibrinolysis including clot lysis time [t_50%], together with thrombin generation and platelet activation markers in relation to fasting blood glucose.

Results

As compared to patients in medium (4.5-6.0 mmol/l, n = 52) and higher (>6.0 mmol/l, n = 75) glucose group, subjects with low glycemia (<4.5 mmol/l, n = 38) had lower K_s by 11% (p < 0.001) and 8% (p = 0.01), respectively, prolonged t_50% by 10% (p < 0.001) and 7% (p = 0.016), respectively, and higher peak thrombin generation by 21% and 16%, respectively (p < 0.001 for both). There were no significant differences in K_s and t_50% between patients in medium and higher glucose group. In the whole group, a J-shape relationship was observed between glycemia and the following factors: peak thrombin generation, K_s and t_50%. Only in patients with HbA1c < 6.0% (42 mmol/mol) (n = 26) fasting glucose positively correlated with K_s (r = 0.53, P = 0.006) and inversely with t_50% (r = −0.46, P = 0.02).

By multiple regression analysis, after adjustment for age, fibrinogen, HbA1c, insulin treatment and T2DM duration, fasting glycemia was the independent predictor of K_s (F = 6.6, df = 2, P = 0.002), t_50% (F = 8.0, df = 2, P < 0.001) and peak thrombin generation (F = 13.5, df = 2, P < 0.0001).

Conclusions

In T2DM patients fasting glycemia <4.5 mmol/l is associated with enhanced thrombin formation and formation of denser fibrin clots displaying lower lysability, especially when strict glycemia control was achieved (HbA1c<6.0%).

Development and validation of a new assay for assessing clot integrity

INTRODUCTION

Research and routine laboratory assessment of clot integrity can be time consuming, expensive, and cannot be batched as it is generally performed in real time. To address these issues, we developed and validated a micro-titre based assay to quantify thrombogenesis and fibrinolysis, the purpose being to assess patients at risk of cardiovascular events by virtue of hypercoagulability. In further validation, thrombogenesis results were compared to similar indices from the thrombelastograph (TEG).

METHODS

Our assay determines three indices of thrombogenesis (lag time to the start of thrombus formation (LT), rate of clot formation (RCF), and maximum clot density (MCD)) and two of fibrinolysis (rate of clot dissolution (RCD) and time for 50% of the clot to lyse (T50)). Plasma was tested fresh and again after being frozen at -70°C. Some samples were tested immediately, others after being left at room temperature for up to 24h.

RESULTS

The intra-assay coefficients of variation (CVs) of the three thrombogenesis measures (LT, RCF, MCD) and two fibrinolysis measures (RCD, T50) varied between 2.7 and 12.0% in fresh plasma and between 1.3% and 10.8% in frozen plasma respectively. Similarly, the inter-assay coefficients of variation of the thrombogenesis and fibrinolysis measures were 4.9-10.8% in fresh plasma and 2.2-6.5% in frozen plasma respectively. TEG assays intra- and inter assay CVs were around 25%. There were no significant differences in all plate assay indices up to 6h at room temperature. Certain plate assay thrombogenesis data were comparable to TEG indices after analysis by Pearson's correlation. The reagent processing cost per sample is £15 for TEG and £2 for the plate assays.

CONCLUSION

Our micro-titre based assay assessing plasma thrombogenesis and fibrinolysis has good intra- and inter-assay CVs, can assess plasma up to 6h after venepuncture, is more efficient (in terms of throughput) and is more economical than that of the TEG.

Fibrin gel as an injectable biodegradable scaffold and cell carrier for tissue engineering

Due to the increasing needs for organ transplantation and a universal shortage of donated tissues, tissue engineering emerges as a useful approach to engineer functional tissues. Although different synthetic materials have been used to fabricate tissue engineering scaffolds, they have many limitations such as the biocompatibility concerns, the inability to support cell attachment, and undesirable degradation rate. Fibrin gel, a biopolymeric material, provides numerous advantages over synthetic materials in functioning as a tissue engineering scaffold and a cell carrier. Fibrin gel exhibits excellent biocompatibility, promotes cell attachment, and can degrade in a controllable manner. Additionally, fibrin gel mimics the natural blood-clotting process and self-assembles into a polymer network. The ability for fibrin to cure in situ has been exploited to develop injectable scaffolds for the repair of damaged cardiac and cartilage tissues. Additionally, fibrin gel has been utilized as a cell carrier to protect cells from the forces during the application and cell delivery processes while enhancing the cell viability and tissue regeneration. Here, we review the recent advancement in developing fibrin-based biomaterials for the development of injectable tissue engineering scaffold and cell carriers.

Focal embolic cerebral ischemia in the rat

Animal models of focal cerebral ischemia are well accepted for investigating the pathogenesis and potential treatment strategies for human stroke. Occlusion of the middle cerebral artery (MCA) with an endovascular filament is a widely used model to induce focal cerebral ischemia. However, this model is not amenable to thrombolytic therapies. As thrombolysis with recombinant tissue plasminogen activator (rtPA) is a standard of care within 4.5 h of human stroke onset, suitable animal models that mimic cellular and molecular mechanisms of thrombosis and thrombolysis of stroke are required. By occluding the MCA with a fibrin-rich allogeneic clot, we previously developed an embolic model of MCA occlusion in the rat, which recapitulates the key components of thrombotic development and of thrombolytic therapy of rtPA observed from human ischemic stroke. Here we describe in detail the surgical procedures of our model, including preparing emboli from rat donors. These procedures can be typically completed within ∼30 min, and they are highly adaptable to other strains of rats, as well as mice, in both sexes. Thus, this model provides a powerful tool for translational stroke research.

Multimodal assessment of non-specific hemostatic agents for apixaban reversal

BACKGROUND

Non-specific hemostatic agents, namely activated prothrombin complex concentrate (aPCC), PCC and recombinant activated factor (F) VII (rFVIIa), can be used, off-label, to reverse the effects of FXa inhibitors in the rare cases of severe hemorrhages, as no approved specific antidote is available. We have evaluated the ability of aPCC, PCC and rFVIIa to reverse apixaban.

METHODS

Healthy volunteer whole blood was spiked with therapeutic or supra-therapeutic apixaban concentrations and two doses of aPCC, PCC or rFVIIa. Tests performed included a turbidimetry assay for fibrin polymerization kinetics analysis, scanning electron microscopy for fibrin network structure observation, thrombin generation assay (TGA), thromboelastometry, prothrombin time and activated partial thromboplastin time.

RESULTS

aPCC generated a dense clot constituting thin and branched fibers similar to those of a control without apixaban, increased fibrin polymerization velocity and improved quantitative (endogenous thrombin potential and peak height) as well as latency (clotting and lag times) parameters. Adding PCC also improved the fibrin and increased quantitative parameters, but fibrin polymerization kinetics and latency parameters were not corrected. Finally, rFVIIa improved latency parameters but failed to restore the fibrin network structure, fibrin polymerization velocity and quantitative parameters.

CONCLUSION

aPCC was more effective than PCC or rFVIIa in reversing in vitro the effects of apixaban. aPCC rapidly triggered the development of an apparently normal fibrin network and corrected latency and quantitative parameters, whereas PCC or rFVIIa had only a partial effect.

Shotgun analysis of plasma fibrin clot-bound proteins in patients with acute myocardial infarction

INTRODUCTION

The presence and amount of the proteins within a plasma clot may influence clot properties, like susceptibility to fibrinolysis, however, the clot proteome has not yet been extensively described. The aim of the study was to investigate the protein composition of clots of four patients with acute myocardial infarction (AMI) in two time points: in the acute ischemic phase and two months later during the standard therapy.

MATERIALS AND METHODS

Shotgun proteomic method (2DLC-MS/MS) was used to investigate time-dependent protein composition changes of clots prepared ex vivo from citrated plasma of the peripheral blood of patients with AMI.

RESULTS

Proteomic analysis revealed a total number of 62 proteins identified in all 8 samples grouping into several distinct functional clusters (e.g. cholesterol transporter activity, immunoglobulin binding and peptidase regulatory activity). The protein signatures of clots differed significantly depending on time after ACS, showing 30% greater variability in protein composition of the clots prepared in the plasma two months after the onset of AMI. Several proteins potentially involved in clot formation and resolution showed an interesting pattern of changes over time.

CONCLUSION

We provided the first qualitative analysis of proteomes of fibrin clots generated ex vivo in plasma taken from patients with AMI showing differences between clots generated in the acute ischemic phase and those prepared two months later. It might be hypothesized that differences involving proteins of potential influence on within-clot fibrinolysis and clot stability may partially explain time-dependent changes in the clots structure and firmness in patients with AMI.

Increased Oxidation as an Additional Mechanism Underlying Reduced Clot Permeability and Impaired Fibrinolysis in Type 2 Diabetes

AIMS

We sought to investigate whether enhanced oxidation contributes to unfavorable fibrin clot properties in patients with diabetes.

METHODS

We assessed plasma fibrin clot permeation (K s , a measure of the pore size in fibrin networks) and clot lysis time induced by recombinant tissue plasminogen activator (CLT) in 163 consecutive type 2 diabetic patients (92 men and 71 women) aged 65 ± 8.8 years with a mean glycated hemoglobin (HbA1c) of 6.8%. We also measured oxidative stress markers, including nitrotyrosine, the soluble form of receptor for advanced glycation end products (sRAGE), 8-iso-prostaglandin F2α (8-iso-PGF2α ), oxidized low-density lipoprotein (oxLDL), and advanced glycation end products (AGE).

RESULTS

There were inverse correlations between K s and nitrotyrosine, sRAGE, 8-iso-PGF2α , and oxLDL. CLT showed a positive correlation with oxLDL and nitrotyrosine but not with other oxidation markers. All these associations remained significant for K s after adjustment for fibrinogen, disease duration, and HbA1c (all P < 0.05), while oxLDL was the only independent predictor of CLT.

CONCLUSIONS

Our study shows that enhanced oxidative stress adversely affects plasma fibrin clot properties in type 2 diabetic patients, regardless of disease duration and glycemia control.

Fibrinolysis and the control of blood coagulation

Fibrin plays an essential role in hemostasis as both the primary product of the coagulation cascade and the ultimate substrate for fibrinolysis. Fibrinolysis efficiency is greatly influenced by clot structure, fibrinogen isoforms and polymorphisms, the rate of thrombin generation, the reactivity of thrombus-associated cells such as platelets, and the overall biochemical environment. Regulation of the fibrinolytic system, like that of the coagulation cascade, is accomplished by a wide array of cofactors, receptors, and inhibitors. Fibrinolytic activity can be generated either on the surface of a fibrin-containing thrombus, or on cells that express profibrinolytic receptors. In a widening spectrum of clinical disorders, acquired and congenital defects in fibrinolysis contribute to disease morbidity, and new assays of global fibrinolysis now have potential predictive value in multiple clinical settings. Here, we summarize the basic elements of the fibrinolytic system, points of interaction with the coagulation pathway, and some recent clinical advances.

Diagnostic morphology: biophysical indicators for iron-driven inflammatory diseases

Most non-communicable diseases involve inflammatory changes in one or more vascular systems, and there is considerable evidence that unliganded iron plays major roles in this. Most studies concentrate on biochemical changes, but there are important biophysical correlates. Here we summarize recent microscopy-based observations to the effect that iron can have major effects on erythrocyte morphology, on erythrocyte deformability and on both fibrinogen polymerization and the consequent structure of the fibrin clots formed, each of which contributes significantly and negatively to such diseases. We highlight in particular type 2 diabetes mellitus, ischemic thrombotic stroke, systemic lupus erythematosus, hereditary hemochromatosis and Alzheimer's disease, while recognizing that many other diseases have co-morbidities (and similar causes). Inflammatory biomarkers such as ferritin and fibrinogen are themselves inflammatory, creating a positive feedback that exacerbates disease progression. The biophysical correlates we describe may provide novel, inexpensive and useful biomarkers of the therapeutic benefits of successful treatments.

Impaired thrombin generation in Reelin-deficient mice: a potential role of plasma Reelin in hemostasis

BACKGROUND

Reelin is a large extracellular glycoprotein that is present in the peripheral blood. That Reelin interacts with the coagulation components and elicits a functional role in hemostasis has not yet been elucidated.

OBJECTIVES

The hemostatic activity of Reelin is investigated and defined in this study.

METHODS

The interplay of Reelin with coagulation components was elucidated by far-Western and liposome/platelet binding assays. In vivo and ex vivo hemostasis-related analyses of Reelin-deficient mice and plasma were also performed.

RESULTS

Reelin interacted with the liposomes containing phosphatidylserine (PS) or phosphatidylcholine. Instead of interacting with known Reelin receptors (ApoE receptor 2, very low density lipoprotein receptor and integrin β1), Reelin interacted with PS of the activated platelets. The interaction between Reelin and the coagulation factors of thrombin and FXa was also demonstrated with the Kd of 11.7 and 21.2 nm, respectively. Reelin-deficient mice displayed a prolonged bleeding time and an increase in rebleeding rate. Despite the fact that Reelin deficiency had no significant effect on the clotting time of prothrombin and activated partial thromboplastin time, the fibrin clot formation was abnormal and the fibrin clot structure was relatively loosened with reduced clot strength. Abnormal fibrinogen expression did not account for the hemostatic defects associated with Reelin deficiency. Instead, thrombin generation was impaired concomitant with an altered prothrombin cleavage pattern.

CONCLUSIONS

By interacting with platelet phospholipids and the coagulation factors, thrombin and FXa, Reelin plays a selective role in coagulation activation, leading to thrombin generation and formation of a normal fibrin clot.

Serum ferritin is an important inflammatory disease marker, as it is mainly a leakage product from damaged cells

"Serum ferritin" presents a paradox, as the iron storage protein ferritin is not synthesised in serum yet is to be found there. Serum ferritin is also a well known inflammatory marker, but it is unclear whether serum ferritin reflects or causes inflammation, or whether it is involved in an inflammatory cycle. We argue here that serum ferritin arises from damaged cells, and is thus a marker of cellular damage. The protein in serum ferritin is considered benign, but it has lost (i.e. dumped) most of its normal complement of iron which when unliganded is highly toxic. The facts that serum ferritin levels can correlate with both disease and with body iron stores are thus expected on simple chemical kinetic grounds. Serum ferritin levels also correlate with other phenotypic readouts such as erythrocyte morphology. Overall, this systems approach serves to explain a number of apparent paradoxes of serum ferritin, including (i) why it correlates with biomarkers of cell damage, (ii) why it correlates with biomarkers of hydroxyl radical formation (and oxidative stress) and (iii) therefore why it correlates with the presence and/or severity of numerous diseases. This leads to suggestions for how one might exploit the corollaries of the recognition that serum ferritin levels mainly represent a consequence of cell stress and damage.

Spatiotemporal characterization of a fibrin clot using quantitative phase imaging

Studying the dynamics of fibrin clot formation and its morphology is an important problem in biology and has significant impact for several scientific and clinical applications. We present a label-free technique based on quantitative phase imaging to address this problem. Using quantitative phase information, we characterized fibrin polymerization in real-time and present a mathematical model describing the transition from liquid to gel state. By exploiting the inherent optical sectioning capability of our instrument, we measured the three-dimensional structure of the fibrin clot. From this data, we evaluated the fractal nature of the fibrin network and extracted the fractal dimension. Our non-invasive and speckle-free approach analyzes the clotting process without the need for external contrast agents.

Why does aspirin decrease the risk of venous thromboembolism? On old and novel antithrombotic effects of acetyl salicylic acid

It is well established that aspirin, an irreversible inhibitor of platelet cyclooxygenase activity, is effective in secondary prevention of arterial thromboembolic events. The pooled results of the recent randomized, multicenter WARFASA and ASPIRE aspirin trials showed a 32% reduction in the rate of recurrence of venous thromboembolism (VTE) in patients receiving aspirin following VTE. These clinical data support evidence that platelets contribute to the initiation and progression of venous thrombosis and aspirin inhibits thrombin formation and thrombin-mediated coagulant reactions. In addition to the known acetylation of serine 529 residue in platelet cyclooxygenase-1, the postulated mechanisms of aspirin-induced antithrombotic actions also involve the acetylation of other proteins in blood coagulation, including fibrinogen, resulting in more efficient fibrinolysis. This review summarizes current knowledge on the aspirin-induced antithrombotic effects that potentially explain clinical studies showing reduced rates of VTE events in aspirin-treated subjects.

Clot properties and cardiovascular disease

Fibrinogen is cleaved by thrombin to fibrin, which provides the blood clot with its essential structural backbone. As an acute phase protein, the plasma levels of fibrinogen are increased in response to inflammatory conditions. In addition to fibrinogen levels, fibrin clot structure is altered by a number of factors. These include thrombin levels, treatment with common cardiovascular medications, such as aspirin, anticoagulants, statins and fibrates, as well as metabolic disease states such as diabetes mellitus and hyperhomocysteinaemia. In vitro studies of fibrin clot structure can provide information regarding fibre density, clot porosity, the mechanical strength of fibres and fibrinolysis. A change in fibrin clot structure, to a denser clot with smaller pores which is more resistant to lysis, is strongly associated with cardiovascular disease. This pathological change is present in patients with arterial as well as venous diseases, and is also found in a moderate form in relatives of patients with cardiovascular disease. Pharmacological therapies, aimed at both the treatment and prophylaxis of cardiovascular disease, appear to result in positive changes to the fibrin clot structure. As such, therapies aimed at 'normalising' fibrin clot structure may be of benefit in the prevention and treatment of cardiovascular disease.

Eryptosis as a marker of Parkinson's disease

A major trend in recent Parkinson's disease (PD) research is the investigation of biological markers that could help in identifying at-risk individuals or to track disease progression and response to therapies. Central to this is the knowledge that inflammation is a known hallmark of PD and of many other degenerative diseases. In the current work, we focus on inflammatory signalling in PD, using a systems approach that allows us to look at the disease in a more holistic way. We discuss cyclooxygenases, prostaglandins, thromboxanes and also iron in PD. These particular signalling molecules are involved in PD pathophysiology, but are also very important in an aberrant coagulation/hematology system. We present and discuss a hypothesis regarding the possible interaction of these aberrant signalling molecules implicated in PD, and suggest that these molecules may affect the erythrocytes of PD patients. This would be observable as changes in the morphology of the RBCs and of PD patients relative to healthy controls. We then show that the RBCs of PD patients are indeed rather dramatically deranged in their morphology, exhibiting eryptosis (a kind of programmed cell death). This morphological indicator may have useful diagnostic and prognostic significance.

CVD risk factors are related to plasma fibrin clot properties independent of total and or γ' fibrinogen concentration

INTRODUCTION

Cardiovascular disease (CVD) risk factors are associated with total fibrinogen concentration and/or altered clot structure. It is however, unclear whether such associations with clot structure are ascribed to fibrinogen concentration or other independent mechanisms. We aimed to determine whether CVD risk factors associated with increased total and/or γ' fibrinogen concentration, were also associated with altered fibrin clot properties and secondly whether such associations were due to the fibrinogen concentration or through independent associations.

MATERIALS AND METHODS

In a plasma setting CVD risk factors (including total and γ' fibrinogen concentration) were cross-sectionally analysed in 2010 apparently healthy black South African participants. Kinetics of clot formation (lag time, slope and maximum absorbance) as well as clot lysis times were calculated from turbidity curves.

RESULTS

Of the measured CVD risk factors age, metabolic syndrome, C-reactive protein (CRP), high density lipoprotein (HDL)-cholesterol and homocysteine were significantly associated with altered fibrin clot properties after adjustment for total and or γ' fibrinogen concentration. Aging was associated with thicker fibres (p=0.004) while both metabolic syndrome and low HDL-cholesterol levels were associated with lower rates of lateral aggregation (slope), (p=0.0004 and p=0.0009), and the formation of thinner fibres (p=0.007 and p=0.0004). Elevated CRP was associated with increased rates of lateral aggregation (p=0.002) and consequently thicker fibres (p<0.0001). Hyperhomocysteinemia was associated with increased rates of lateral aggregation (p=0.0007) without affecting fibre thickness.

CONCLUSION

Final clot structure may contribute to increased CVD risk in vivo through associations with other CVD risk factors independent from total or γ' fibrinogen concentration.

Kinetic model facilitates analysis of fibrin generation and its modulation by clotting factors: implications for hemostasis-enhancing therapies

Current mechanistic knowledge of protein interactions driving blood coagulation has come largely from experiments with simple synthetic systems, which only partially represent the molecular composition of human blood plasma. Here, we investigate the ability of the suggested molecular mechanisms to account for fibrin generation and degradation kinetics in diverse, physiologically relevant in vitro systems. We represented the protein interaction network responsible for thrombin generation, fibrin formation, and fibrinolysis as a computational kinetic model and benchmarked it against published and newly generated data reflecting diverse experimental conditions. We then applied the model to investigate the ability of fibrinogen and a recently proposed prothrombin complex concentrate composition, PCC-AT (a combination of the clotting factors II, IX, X, and antithrombin), to restore normal thrombin and fibrin generation in diluted plasma. The kinetic model captured essential features of empirically detected effects of prothrombin, fibrinogen, and thrombin-activatable fibrinolysis inhibitor titrations on fibrin formation and degradation kinetics. Moreover, the model qualitatively predicted the impact of tissue factor and tPA/tenecteplase level variations on the fibrin output. In the majority of considered cases, PCC-AT combined with fibrinogen accurately approximated both normal thrombin and fibrin generation in diluted plasma, which could not be accomplished by fibrinogen or PCC-AT acting alone. We conclude that a common network of protein interactions can account for key kinetic features characterizing fibrin accumulation and degradation in human blood plasma under diverse experimental conditions. Combined PCC-AT/fibrinogen supplementation is a promising strategy to reverse the deleterious effects of dilution-induced coagulopathy associated with traumatic bleeding.

A fibrin-specific monoclonal antibody from a designed phage display library inhibits clot formation and localizes to tumors in vivo

Fibrin formation from fibrinogen is a rare process in the healthy organism but is a pathological feature of thrombotic events, cancer and a wide range of inflammatory conditions. We have designed and constructed an antibody phage display library (containing 13 billion clones) for the selective recognition of the N-terminal peptide of fibrin alpha chain. The key structural feature for selective fibrin binding was a K94E mutation in the VH domain. From this library, an antibody was isolated (termed AP2), which recognizes the five N-terminal amino acids of fibrin with high affinity (Kd=44nM), but does not bind to fibrinogen. The AP2 antibody could be expressed in various formats (scFv, small immune protein and IgG) and inhibited fibrin clot formation in a concentration-dependent manner. Moreover, the AP2 antibody stained the fibrin-rich provisional stroma in solid tumors but did not exhibit any detectable staining toward normal tissues. Using a radioiodinated antibody preparation and quantitative biodistribution studies in tumor-bearing mice, AP2 was shown to selectively localize to fibrin-rich F9 murine teratocarcinomas, but not to SKRC-52 human kidney cancer xenografts. Collectively, the experiments indicate that the AP2 antibody recognizes fibrin in vitro and in vivo. The antibody may facilitate the development of fibrin-specific therapeutic agents.

Platelet Reactivity in Patients With Stroke and Hyperlipidemia, GPIbα Assessment

The aim of this study was to assess platelet reactivity in patients after ischemic stroke and to investigate the influence of hyperlipidemia (HL) on platelet activity markers. A total of 41 patients after ischemic stroke were divided into the following 2 groups: patients with HL and patients with normolipidemia. Expression of CD42b on resting, thrombin-activated blood platelets, and fibrinogen level was assessed. The CD42b-positive platelets were analyzed using the flow cytometer, anti-CD61, and anti-CD42b monoclonal antibodies. The results confirmed increased platelet reactivity to thrombin in all patients after ischemic stroke manifested by significantly lower CD42b expression and percentage of CD42b(+) platelets after activation by thrombin. The influence of HL on the expression of CD42b on resting and thrombin-activated platelets was not found. However, increased level of fibrinogen but no influence of HL on fibrinogen concentration was observed in patients after ischemic stroke. Increased susceptibility to platelet agonists was found in patients after ischemic stroke in the convalescent phase.

Lp(a) is not associated with diabetes but affects fibrinolysis and clot structure ex vivo

Lipoprotein (a) [Lp(a)] is a low density lipoprotein (LDL) with one apolipoprotein (a) molecule bound to the apolipoprotein B-100 of LDL. Lp(a) is an independent risk factor for cardiovascular disease (CVD). However, the relationship of Lp(a) to diabetes and metabolic syndrome, both known for increased CVD risk, is controversial. In a population based study on type two diabetes mellitus (T2DM) development in women, Lp(a) plasma levels showed the well known skewed distribution without any relation to diabetes or impaired glucose tolerance. A modified clot lysis assay on a subset of 274 subjects showed significantly increased clot lysis times in T2DM subjects, despite inhibition of PAI-1 and TAFI. Lp(a) plasma levels significantly increased the maximal peak height of the clot lysis curve, indicating a change in clot structure. In this study Lp(a) is not related to the development of T2DM but may affect clot structure ex vivo without a prolongation of the clot lysis time.

Multiple roles of complement MASP-1 at the interface of innate immune response and coagulation

MASP-1 is a versatile serine protease that cleaves a number of substrates in human blood. In recent years it became evident that besides playing a crucial role in complement activation MASP-1 also triggers other cascade systems and even cells to mount a more powerful innate immune response. In this review we summarize the latest discoveries about the diverse functions of this multi-faceted protease. Recent studies revealed that among MBL-associated serine proteases, MASP-1 is the one responsible for triggering the lectin pathway via its ability to rapidly autoactivate then cleave MASP-2, and possibly MASP-3. The crystal structure of MASP-1 explains its more relaxed substrate specificity compared to the related complement enzymes. Due to the relaxed specificity, MASP-1 interacts with the coagulation cascade and the kinin generating system, and it can also activate endothelial cells eliciting pro-inflammatory signaling.

Prothrombotic alterations in plasma fibrin clot properties in thyroid disorders and their post-treatment modifications

INTRODUCTION

Available data on fibrin clot properties and fibrinolysis in hyperthyroidism and hypothyroidism are inconsistent. Our objective was to assess the impact of effective treatment of hyper- and hypothyroidism on fibrin clot characteristics.

MATERIAL AND METHODS

In a case-control study, ex vivo plasma fibrin clot permeability (Ks) and efficiency of fibrinolysis were assessed in 35 consecutive hyperthyroid and 35 hypothyroid subjects versus 30 controls. All measurements were performed before and after 3months of thyroid function normalizing therapy.

RESULTS

At baseline, hyperthyroid, but not hypothyroid, patients had lower Ks than controls (p<0.0001). Hyperthyroid and hypothyroid groups compared with controls had prolonged clot lysis time (CLT), and lower rate of D-dimer release from clots (D-Drate) (all p<0.05). The regression analysis adjusted for fibrinogen showed that in hyperthyroid patients, pre-treatment thyroid stimulating hormone (TSH) independently predicted Ks, while thrombin activatable fibrinolysis inhibitor (TAFI) antigen predicted CLT. In hypothyroid individuals a similar regression model showed that TSH independently predicts CLT. After 3months of thyroid function normalizing therapy, 32 (91.4%) hyperthyroid and 30 (85.7%) hypothyroid subjects achieved euthyroidism and had improved fibrin clot properties (all p<0.05), with normalization of Ks in hyperthyroid and lysability in hypothyroid patients.

CONCLUSIONS

Both hyper- and mild-to-moderate hypothyroidism are associated with prothrombotic plasma fibrin clot phenotype and restoration of euthyroidism improves clot phenotype. Abnormal fibrin clot phenotype might contribute to thromboembolic risk in thyroid disease.

Altered plasma fibrin clot properties and fibrinolysis in patients with multiple myeloma

BACKGROUND

Multiple myeloma (MM) is associated with increased risk of venous and arterial thromboembolism. Formation of denser and poorly lysable fibrin clots is observed in patients with arterial and venous thromboembolism. We investigated fibrin clot properties and their determinants in MM patients.

MATERIALS AND METHODS

Ex vivo plasma fibrin clot permeability, turbidity and susceptibility to lysis were evaluated in 106 MM patients at the time of diagnosis vs. 100 age- and sex-matched controls. MM patients had lower clot permeability (Ks ), compaction, indicating denser fibrin clots, impaired fibrin polymerization with longer lag phase and lower final turbidity (D-Dmax ), combined with hypofibrinolysis reflected by longer lysis time and slower rate of D-dimer release from fibrin clots (D-Drate ) compared with controls (all P < 0·001).

RESULTS

Patients with IgG MM had lower Ks compared with IgA MM [5·9 (5·1-6·4) vs. 6·3 (5·9-7·2) 10(-9)  cm(2) ; P = 0·007] and longer lysis time compared with light-chain-disease patients [11·4 (10·9-12·3) vs. 10·7 (9·8-11·9) min; P = 0·022]. Of the fibrin variables, only Ks was significantly lower in patients with International Staging System (ISS) grade III than in those with ISS grade I and II [5·9 (4·9-6·6) vs. 6·2 (5·7-6·8) 10(-9)  cm(2) ; P = 0·015]. Multivariate analysis adjusted for age and fibrinogen showed that in MM patients elevated peak thrombin levels determine Ks and D-Dmax , while thrombin-activatable fibrinolysis inhibitor (TAFI) activity predicts Ks , t50% , D-Drate and lag phase.

CONCLUSIONS

Our study demonstrates prothrombotic fibrin clot phenotype in patients with MM, with a significant impact of increased thrombin formation and TAFI activity.

Structural basis for the nonlinear mechanics of fibrin networks under compression

Fibrin is a protein polymer that forms a 3D filamentous network, a major structural component of protective physiological blood clots as well as life threatening pathological thrombi. It plays an important role in wound healing, tissue regeneration and is widely employed in surgery as a sealant and in tissue engineering as a scaffold. The goal of this study was to establish correlations between structural changes and mechanical responses of fibrin networks exposed to compressive loads. Rheological measurements revealed nonlinear changes of fibrin network viscoelastic properties under dynamic compression, resulting in network softening followed by its dramatic hardening. Repeated compression/decompression enhanced fibrin clot stiffening. Combining fibrin network rheology with simultaneous confocal microscopy provided direct evidence of structural modulations underlying nonlinear viscoelasticity of compressed fibrin networks. Fibrin clot softening in response to compression strongly correlated with fiber buckling and bending, while hardening was associated with fibrin network densification. Our results suggest a complex interplay of entropic and enthalpic mechanisms accompanying structural changes and accounting for the nonlinear mechanical response in fibrin networks undergoing compressive deformations. These findings provide new insight into the fibrin clot structural mechanics and can be useful for designing fibrin-based biomaterials with modulated viscoelastic properties.

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.

Effect of clot aging and cholesterol content on ultrasound-assisted thrombolysis

Exposure to 2-MHz transcranial diagnostic ultrasound enhances the thrombolytic activity of intravenously administered tissue plasminogen activator (IV-tPA) in acute ischemic stroke (sonothrombolysis). However, rates of arterial recanalization vary widely, depending upon the clot burden, its location, and stroke subtype. We evaluated the influence of age and cholesterol level of the blood clots on sonothrombolysis in an in vitro model. To "age" the clots, serum was replaced by fresh blood periodically. We increased the cholesterol content of the clots by adding cholesterin to the blood. The clots were lysed by tPA and/or transcranial Doppler ultrasound sonication for 1 h. The extent of thrombolysis induced by various treatment protocols (controls, sonication, tPA, and sonothrombolysis) was evaluated with relative changes in the clot weights and in the clot structure by scanning electron microscopy (SEM) at end of the experiment. Sonothrombolysis induced significantly higher weight reduction in fresh clots (37.3 % in 2-h old clots versus 24.8 % in 10-h ones, p < 0.005) as well as the clots with higher cholesterol levels (41.7 versus 30.6 % in normal cholesterol clots, p < 0.005). SEM demonstrated patterns of clot dissolution among various treatment modalities. Sonothrombolysis induced better clot lysis in fresh thrombi with high cholesterol levels.

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.

Roles of fibrin α- and γ-chain specific cross-linking by FXIIIa in fibrin structure and function

Factor XIII is responsible for the cross-linking of fibrin γ-chains in the early stages of clot formation, whilst α-chain cross-linking occurs at a slower rate. Although γ- and α-chain cross-linking was previously shown to contribute to clot stiffness, the role of cross-linking of both chains in determining clot structure is currently unknown. Therefore, the aim of this study was to determine the role of individual α- and γ-chain cross-linking during clot formation, and its effects on clot structure. We made use of a recombinant fibrinogen (γQ398N/Q399N/K406R), which does not allow for γ-chain cross-linking. In the absence of cross-linking, intact D-D interface was shown to play a potential role in fibre appearance time, clot stiffness and elasticity. Cross-linking of the fibrin α-chain played a role in the thickening of the fibrin fibres over time, and decreased lysis rate in the absence of α2-antiplasmin. We also showed that α-chain cross-linking played a role in the timing of fibre appearance, straightening fibres, increasing clot stiffness and reducing clot deformation. Cross-linking of the γ-chain played a role in fibrin fibre appearance time and fibre density. Our results show that α- and γ-chain cross-linking play independent and specific roles in fibrin clot formation and structure.