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

Soluble glycoprotein VI predicts abdominal aortic aneurysm growth rate and is a novel therapeutic target

ABSTRACT

A common feature in patients with abdominal aortic aneurysms (AAAs) is the formation of a nonocclusive intraluminal thrombus (ILT) in regions of aortic dilation. Platelets are known to maintain hemostasis and propagate thrombosis through several redundant activation mechanisms, yet the role of platelet activation in the pathogenesis of AAA-associated ILT is still poorly understood. Thus, we sought to investigate how platelet activation affects the pathogenesis of AAA. Using RNA sequencing, we identified that the platelet-associated transcripts are significantly enriched in the ILT compared with the adjacent aneurysm wall and healthy control aortas. We found that the platelet-specific receptor glycoprotein VI (GPVI) is among the top enriched genes in AAA ILT and is increased on the platelet surface of patients with AAAs. Examination of a specific indicator of platelet activity, soluble GPVI (sGPVI), in 2 independent cohorts of patients with AAAs is highly predictive of an AAA diagnosis and associates more strongly with aneurysm growth rate than D-dimer in humans. Finally, intervention with the anti-GPVI antibody (JAQ1) in mice with established aneurysms blunted the progression of AAA in 2 independent mouse models. In conclusion, we show that the levels of sGPVI in humans can predict a diagnosis of AAA and AAA growth rate, which may be critical in the identification of high-risk patients. We also identify GPVI as a novel platelet-specific AAA therapeutic target, with minimal risk of adverse bleeding complications, for which none currently exists.

The Use of Clot Strength as a Predictor of Thrombosis in Peripheral Artery Disease

BACKGROUND

Graft/stent thrombosis is the leading cause of amputation in patients over 60, and while dual antiplatelet therapy is the standard of care, there is a significant variability in platelet response and limited guidance on measuring effectiveness. Thromboelastography with platelet mapping (TEG-PM) can objectively detail an individual's coagulation profile, namely the strength of the clot and its response to antiplatelet medication. Although TEG-PM has been used for predicting postoperative bleeding and assessing platelet dysfunction in traumatic brain injury, its application in thrombosis diseases such as peripheral artery disease remains unexplored. The aim of this observational study was to determine if objective measures of clot strength could predict a high clinical risk of thrombosis.

METHODS

Patients >60 years with peripheral artery disease undergoing revascularization were prospectively evaluated from 2021 to 2023. They were clinically followed for 1 year to detect any thrombotic events. TEG-PM was used to objectively evaluate coagulation profiles in patients at 1, 3, 6, and 9 months. These follow-up periods were chosen based on studies showing that 1-3 month intervals in the first year after lower extremity revascularization optimize therapy and risk control. The TEG-PM data preceding a thrombotic/stenotic event in patients with thrombosis was compared to the last known well TEG-PM event in those without a thrombotic/stenotic event. We stratified the groups based on the occurrence of thrombosis/stenotic events. Descriptive statistics were applied to characterize each group and a chi-square test was conducted to assess the variance between both groups. An unpaired t-test was run to identify differences in platelet function. Receiver operating characteristic analysis was performed to determine the optimal TEG-PM cutoff for predicting a higher risk of thrombosis.

RESULTS

One hundred and fifty-eight patients were analyzed, from whom 28 (17.7%) experienced a thrombotic event. The thrombosis cohort exhibited significantly greater MAADP, MAFibrin, and MAThrombin [50.2 vs. 40.0, P < 0.05], [18.19 vs. 14.64, P < 0.05], and [63.8 vs. 58.5, P < 0.05], respectively, indicative of greater clot strength. By receiver operating characteristic analysis, the optimal predictor cut-off for MAADP, indicating a higher risk of thrombosis, was >42 mm [P < 0.05] with 82% sensitivity and 50% specificity.

CONCLUSIONS

An increase in clot strength was found to be predictive of thrombosis/stenosis within 30 days. Using a MAADP cut-off greater than 42 mm might serve as an alternative approach to tailor the use of antiplatelet medication, potentially reducing the risk of thrombosis.

Thrombin cleavage of the hepatitis E virus polyprotein at multiple conserved locations is required for genome replication

The genomes of positive-sense RNA viruses encode polyproteins that are essential for mediating viral replication. These viral polyproteins must undergo proteolysis (also termed polyprotein processing) to generate functional protein units. This proteolysis can be performed by virally-encoded proteases as well as host cellular proteases, and is generally believed to be a key step in regulating viral replication. Hepatitis E virus (HEV) is a leading cause of acute viral hepatitis. The positive-sense RNA genome is translated to generate a polyprotein, termed pORF1, which is necessary and sufficient for viral genome replication. However, the mechanism of polyprotein processing in HEV remains to be determined. In this study, we aimed to understand processing of this polyprotein and its role in viral replication using a combination of in vitro translation experiments and HEV sub-genomic replicons. Our data suggest no evidence for a virally-encoded protease or auto-proteolytic activity, as in vitro translation predominantly generates unprocessed viral polyprotein precursors. However, seven cleavage sites within the polyprotein (suggested by bioinformatic analysis) are susceptible to the host cellular protease, thrombin. Using two sub-genomic replicon systems, we demonstrate that mutagenesis of these sites prevents replication, as does pharmacological inhibition of serine proteases including thrombin. Overall, our data supports a model where HEV uses host proteases to support replication and could have evolved to be independent of a virally-encoded protease for polyprotein processing.

Thrombosis in the pathogenesis of abdominal aortic aneurysm

Background

Abdominal aortic aneurysms (AAAs) are a relatively common vascular pathology of the elderly with high morbidity potential. Irreversible degeneration of the aortic wall leads to lethal rupture if left untreated. Nearly all AAAs contain intraluminal thrombus (ILT) to a varying degree, yet the mechanisms explaining how thrombosis is disturbed in AAA are relatively unknown. This review examined the thrombotic complications associated with AAA, the impact of thrombosis on AAA surgical outcomes and AAA pathogenesis, and the use of antithrombotic therapy in the management of this disease.

Methods

A literature search of the PubMed database was conducted using relevant keywords related to thrombosis and AAAs.

Results

Thrombotic complications are relatively infrequent in AAA yet carry significant morbidity risks. The ILT can impact endovascular aneurysm repair by limiting anatomic suitability and influence the risk of endoleaks. Many of the pathologic mechanisms involved in AAA development, including hemodynamics, inflammation, oxidative stress, and aortic wall remodeling, contain pathways that interact with thrombosis. Conversely, the ILT can also be a source of biochemical stress and exacerbate these aneurysmal processes. In animal AAA models, antithrombotic therapies have shown favorable results in preventing and stabilizing AAA. Antiplatelet agents may be beneficial for reducing risks of major adverse cardiovascular events in AAA patients; however, neither antiplatelet nor anticoagulation is currently used solely for the management of AAA.

Conclusions

Thrombosis and ILT may have detrimental effects on AAA growth, rupture risk, and patient outcomes, yet there is limited understanding of the pathologic thrombotic mechanisms in aneurysmal disease at the molecular level. Preventing ILT using platelet and coagulation inhibitors may be a reasonable theoretical target for aneurysm progression and stability; however, the practical benefits of current antithrombotic therapies in AAA are unclear. Further research is needed to demonstrate the extent to which thrombosis impacts AAA pathogenesis and to develop novel pharmacologic strategies for the medical management of this disease.

Fibrin clot properties in cardiovascular disease: from basic mechanisms to clinical practice

Fibrinogen conversion into insoluble fibrin and the formation of a stable clot is the final step of the coagulation cascade. Fibrin clot porosity and its susceptibility to plasmin-mediated lysis are the key fibrin measures, describing the properties of clots prepared ex vivo from citrated plasma. Cardiovascular disease (CVD), referring to coronary heart disease, heart failure, stroke, and hypertension, has been shown to be associated with the formation of dense fibrin networks that are relatively resistant to lysis. Denser fibrin mesh characterized acute patients at the onset of myocardial infarction or ischaemic stroke, while hypofibrinolysis has been identified as a persistent fibrin feature in patients following thrombotic events or in those with stable coronary artery disease. Traditional cardiovascular risk factors, such as smoking, diabetes mellitus, hyperlipidaemia, obesity, and hypertension, have also been linked with unfavourably altered fibrin clot properties, while some lifestyle modifications and pharmacological treatment, in particular statins and anticoagulants, may improve fibrin structure and function. Prospective studies have suggested that prothrombotic fibrin clot phenotype can predict cardiovascular events in short- and long-term follow-ups. Mutations and splice variants of the fibrinogen molecule that have been proved to be associated with thrombophilia or increased cardiovascular risk, along with fibrinogen post-translational modifications, prothrombotic state, inflammation, platelet activation, and neutrophil extracellular traps formation, contribute also to prothrombotic fibrin clot phenotype. Moreover, about 500 clot-bound proteins have been identified within plasma fibrin clots, including fibronectin, α2-antiplasmin, factor XIII, complement component C3, and histidine-rich glycoprotein. This review summarizes the current knowledge on the mechanisms underlying unfavourable fibrin clot properties and their implications in CVD and its thrombo-embolic manifestations.

Congruent identification of imbalanced fibrinolysis by 2 distinct clot lysis time assays

Background

A plasma-based clot lysis time (CLT) assay is an established research test to assess plasma fibrinolytic potential, with application in hyperfibrinolytic or hypofibrinolytic conditions. Interprotocol variations make comparisons between laboratories challenging. The aim of this study was to compare the results of 2 different CLT assays performed by 2 distinct research laboratories by using their own protocol.

Methods

We evaluated fibrinolysis in the plasma of 60 patients undergoing hepatobiliary surgery and in plasma from a healthy donor that was spiked with commonly used anticoagulant drugs (enoxaparin, dabigatran, and rivaroxaban) in 2 distinct laboratories (Aarhus and Groningen) by using 2 different assays that differ, among others, in tissue plasminogen activator (tPA) concentration.

Results

Overall conclusions on fibrinolytic potential in patients undergoing hepatobiliary surgery were similar between the 2 CLT assays, with hyperfibrinolytic and hypofibrinolytic profiles identified at the same time points during and after surgery. Severe hypofibrinolysis was less commonly reported in the Aarhus assay (36/319 samples; 11%) than in the Groningen assay (55/319 samples; 17%). No clot formation was observed in 31 of 319 samples in the Aarhus assay vs 0 of 319 samples in the Groningen assay. Clotting times increased much more profoundly on the addition of all 3 anticoagulants in the Aarhus assay.

Conclusions

Despite the differences in laboratory, protocol, reagents, operator, data processing, and analysis, overall conclusions on fibrinolytic capacity are similar between the 2 laboratories. With a higher concentration of tPA in the Aarhus assay, the test becomes less sensitive for the detection of hypofibrinolysis and is more sensitive to the addition of anticoagulants.

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

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

Fibrin Clot Properties in Atherosclerotic Vascular Disease: From Pathophysiology to Clinical Outcomes

Fibrin is a major component of thrombi formed on the surface of atherosclerotic plaques. Fibrin accumulation as a consequence of local blood coagulation activation takes place inside atherosclerotic lesions and contributes to their growth. The imbalance between thrombin-mediated fibrin formation and fibrin degradation might enhance atherosclerosis in relation to inflammatory states reflected by increased fibrinogen concentrations, the key determinant of fibrin characteristics. There are large interindividual differences in fibrin clot structure and function measured in plasma-based assays and in purified fibrinogen-based systems. Several observational studies have demonstrated that subjects who tend to generate denser fibrin networks displaying impaired clot lysis are at an increased risk of developing advanced atherosclerosis and arterial thromboembolic events. Moreover, the majority of cardiovascular risk factors are also associated with unfavorably altered fibrin clot properties, with their improvement following effective therapy, in particular with aspirin, statins, and anticoagulant agents. The prothrombotic fibrin clot phenotype has been reported to have a predictive value in terms of myocardial infarction, ischemic stroke, and acute limb ischemia. This review article summarizes available data on the association of fibrin clot characteristics with atherosclerotic vascular disease and its potential practical implications.

Fibrin clot properties to assess the bleeding phenotype in unrelated patients with hypodysfibrinogenemia due to novel fibrinogen mutations

Congenital hypodysfibrinogenemia is a rare fibrinogen disorder, defined by decreased levels of a dysfunctional fibrinogen. We present the functional and structural characterization of two new fibrinogen variants. A duplication of 32 bases in FGA exon 5, p.Ser382GlyfsTer50 was identified in a patient (P1) with history of hemoptysis and traumatic cerebral bleeding. A missense mutation in FGG exon 8, p.Ala353Ser was identified in two siblings (P2 and P3) with tendency to bruising and menorrhagia. Fibrin polymerization was studied in plasma and in purified fibrinogen by turbidimetry. Fibrin structure was studied by a permeability assay, laser scanning confocal microscopy (LSCM) and scanning electron microscopy (SEM). In both plasma and purified fibrinogen samples, all patients had an abnormal polymerization characterized by a decreased maximal absorption compared to controls. The permeation constant (Ks) was markedly increased in all patients: 31 ± 9 × 10^-9 cm² in P1, and 20 ± 0.1 × 10^-9 cm² in P2 and P3, compared to 6 ± 2 × 10^-9 cm² in the control (p < 0.05). The presence of very large pores that accounts for the increased Ks was confirmed by LSCM and SEM patients' clots images. By SEM, the patients' fibrin fibers diameters were thicker: 90 ± 25 nm in P1, 162 ± 64 nm in P2 and 132 ± 46 nm in P3 compared to 74 ± 25 nm in control (p < 0.0001). In conclusion, both new causative fibrinogen mutations altered clot structure by forming thick fibers, diminishing fiber branching, and increasing pore filling space. These structural changes to clots explain the patients' bleeding phenotypes.

Fibrinogen Modification and Fibrin Formation in Patients with an End-Stage Renal Disease Subjected to Peritoneal Dialysis

End-stage renal disease (ESRD) is a condition accompanied by increased inflammation, oxidative stress, risk of cardiovascular complications, and coagulopathies. The structure of fibrinogen and characteristics of fibrin from plasma samples of ESRD patients on peritoneal dialysis (PD) was investigated. Fibrinogen from ESRD patients had a higher degree of carbonylation than fibrinogen from healthy individuals. The Aα chain was the most susceptible to oxidation, followed by the Bβ chain, whereas the γ-chain was the most resistant to oxidation. Spectrofluorimetric analysis suggested a higher extent of modification of amino acid side chains in fibrinogen from ESRD patients. The tertiary structure of fibrinogen was more affected than its secondary structure. The kinetics (time and rate) of fibrinogen coagulation did not differ between the tested groups. Fibrin prepared from the isolated fibrinogen had a similar structure in both groups. Our results confirm that oxidation and structural alterations of fibrinogen occur in ESRD patients on PD, although these modifications produce no direct effect on the fibrin formation. Taking into account that some patients suffer from bleeding, whereas others develop thrombotic complications, further research on this subject is required to identify other components and processes that contribute to the outcome.

Recent progress on nanoparticles for targeted aneurysm treatment and imaging

An abdominal aortic aneurysm (AAA) is a localized dilatation of the aorta that plagues millions. Its rupture incurs high mortality rates (~80-90%), pressing an urgent need for therapeutic methods to prevent this deadly outcome. Judiciously designed nanoparticles (NPs) have displayed a unique potential to fulfill this need. Aneurysms feature excessive inflammation and extracellular matrix (ECM) degradation. As such, typically inflammatory cells and exposed ECM proteins have been targeted with NPs for therapeutic, diagnostic, or theranostic purposes in experimental models. NPs have been used not only for encapsulation and delivery of drugs and biomolecules in preclinical tests, but also for enhanced imaging to monitor aneurysm progression in patients. Moreover, they can be readily modified with various molecules to improve lesion targeting, detectability, biocompatibility, and circulation time. This review updates on the progress, limitations, and prospects of NP applications in the context of AAA.

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

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

Fibrin clot properties independently predict adverse clinical outcome following acute coronary syndrome: a PLATO substudy

Aims

To determine whether fibrin clot properties are associated with clinical outcomes following acute coronary syndrome (ACS).

Methods and results

Plasma samples were collected at hospital discharge from 4354 ACS patients randomized to clopidogrel or ticagrelor in the PLATelet inhibition and patient Outcomes (PLATO) trial. A validated turbidimetric assay was employed to study plasma clot lysis time and maximum turbidity (a measure of clot density). One-year rates of cardiovascular (CV) death, spontaneous myocardial infarction (MI) and PLATO-defined major bleeding events were assessed after sample collection. Hazard ratios (HRs) were estimated using Cox proportional hazards models. After adjusting for CV risk factors, each 50% increase in lysis time was associated with CV death/spontaneous MI [HR 1.17, 95% confidence interval (CI) 1.05-1.31; P < 0.01] and CV death alone (HR 1.36, 95% CI 1.17-1.59; P < 0.001). Similarly, each 50% increase in maximum turbidity was associated with increased risk of CV death (HR 1.24, 95% CI 1.03-1.50; P = 0.024). After adjustment for other prognostic biomarkers (leukocyte count, high-sensitivity C-reactive protein, high-sensitivity troponin T, cystatin C, N-terminal pro B-type natriuretic peptide, and growth differentiation factor-15), the association with CV death remained significant for lysis time (HR 1.2, 95% CI 1.01-1.42; P = 0.042) but not for maximum turbidity. These associations were consistent regardless of randomized antiplatelet treatment (all interaction P > 0.05). Neither lysis time nor maximum turbidity was associated with major bleeding events.

Conclusion

Fibrin clots that are resistant to lysis independently predict adverse outcome in ACS patients. Novel therapies targeting fibrin clot properties might be a new avenue for improving prognosis in patients with ACS.

Prognostic value of D-dimer and markers of coagulation for stratification of abdominal aortic aneurysm growth

Abdominal aortic aneurysm (AAA) is associated with high morbidity and mortality and is an established cause of unbalanced hemostasis. A number of hemostatic biomarkers have been associated with AAA; however, the utility of hemostatic biomarkers in AAA diagnosis and prognosis is unclear. The aim of the present study was to characterize the potential prognostic value of D-dimer and markers of altered hemostasis in a large cohort of patients with AAAs characterized by either fast or slow aneurysm growth (frequency matched for baseline diameter) and subaneurysmal dilations. We measured plasma concentrations of thrombin-antithrombin (TAT) complex, platelet factor 4 (PF4), and D-dimer in 352 patients with either fast-growing AAAs (>2 mm/y), slow-growing AAAs (<2 mm/y), subaneurysmal aortic dilations, or nonaneurysmal aortas. Plasma D-dimer and TAT were significantly elevated in both AAA and subaneurysmal dilation patients compared with controls. Individuals with D-dimer levels ≥500 ng/mL had 3.09 times the odds of subaneurysms, 6.23 times the odds of slow-growing AAAs, and 7.19 times the odds of fast-growing AAAs than individuals with D-dimer level <500 ng/mL. However, no differences in D-dimer concentration were noted between fast- and slow-growing aneurysms. Plasma D-dimer and TAT were strong independent predictors of AAA growth rate with multivariate analysis revealing a 500-ng/mL increase in D-dimer or 1-µg/mL increase in TAT led to additional 0.21-mm and 0.24-mm changes in aortic diameter per year, respectively. Rising levels of plasma TAT, in addition to D-dimer, may predict disease progression and aneurysm growth in patients with AAA or subaneurysmal dilation.

Antithrombotic therapy in abdominal aortic aneurysm: beneficial or detrimental?

Abdominal aortic aneurysm (AAA) is a degenerative vascular pathology resulting in significant morbidity and mortality in older adults due to rupture and sudden death. Despite 150 000 new cases and nearly 15 000 deaths annually, the only approved treatment of AAA is surgical or endovascular intervention when the risk for aortic rupture is increased. The goal of the scientific community is to develop novel pharmaceutical treatment strategies to reduce the need for surgical intervention. Because most clinically relevant AAAs contain a complex structure of fibrin, inflammatory cells, platelets, and red blood cells in the aneurysmal sac known as an intraluminal thrombus (ILT), antithrombotic therapies have emerged as potential pharmaceutical agents for the treatment of AAA progression. However, the efficacy of these treatments has not been shown, and the effects of shrinking the ILT may be as detrimental as they are beneficial. This review discusses the prospect of anticoagulant and antiplatelet (termed collectively as antithrombotic) therapies in AAA. Herein, we discuss the role of the coagulation cascade and platelet activation in human and animal models of AAA, the composition of ILT in AAA, a possible role of the ILT in aneurysm stabilization, and the implications of antithrombotic drugs in AAA treatment.

Assessment and determinants of whole blood and plasma fibrinolysis in patients with mild bleeding symptoms

Enhanced clot lysis is associated with bleeding, but assessment of lysis capacity remains difficult. The plasma turbidity lysis and whole blood tissue Plasminogen Activator-Rotational Thromboelastometry (tPA-ROTEM) assays estimate fibrinolysis under more physiological conditions than clinically used assays. We hypothesized that these assays could find signs of enhanced lysis capacity in patients who report bleeding symptoms, but are not diagnosed with bleeding disorders. We also aimed to gain insight in determinants of the results of these lysis assays. Data from 240 patients with and 95 patients without self-reported bleeding symptoms were obtained, who were included in a study that primarily aimed to assess prevalence of haemostatic abnormalities in preoperative patients. ROTEM and turbidity assays were performed with rtPA. Blood counts, fibrinolysis and coagulation factor activities were determined. Data were analysed using multivariable linear regression models. Remarkably, patients reporting bleeding symptoms showed signs of significantly impaired lysis capacity in the tPA-ROTEM, but not in the turbidity lysis assay. In these patients, the tPA-ROTEM results depended on FII, FXII, plasminogen, α2-antiplasmin, PAI-1 and TAFI levels. The turbidity lysis results were significantly influenced by fibrinogen, α2-antiplasmin, PAI-1 and TAFI. In conclusion, the tPA-ROTEM and the turbidity lysis assay could not detect enhanced fibrinolytic capacity in patients with bleeding symptoms. This suggests that these symptoms are not caused by enhanced fibrinolytic activity. As both assays were sensitive to important determinants of fibrinolysis they may be able to detect a fibrinolytic imbalance, but this needs to be validated in patients with known hypo- or hyperfibrinolytic disorders.

Impact of γ'γ' fibrinogen interaction with red blood cells on fibrin clots

AIM

γ' fibrinogen has been associated with thrombosis. Here the interactions between γ'γ' or γAγA fibrinogen and red blood cells (RBCs), and their role on fibrin clot properties were studied.

MATERIALS & METHODS

Atomic Force microscopy (AFM)-based force spectroscopy, rheological, electron and confocal microscopy, and computational approaches were conducted for both fibrinogen variants.

RESULTS & CONCLUSION

AFM shows that the recombinant human (rh)γ'γ' fibrinogen increases the binding force and the frequency of the binding to RBCs compared with rhγAγA, promoting cell aggregation. Structural changes in rhγ'γ' fibrin clots, displaying a nonuniform fibrin network were shown by microscopy approaches. The presence of RBCs decreases the fibrinolysis rate and increases viscosity of rhγ'γ' fibrin clots. The full length of the γ' chain structure, revealed by computational analysis, occupies a much wider surface and is more flexible, allowing an increase of the binding between γ' fibers, and eventually with RBCs.

Structural changes of fibrinogen as a consequence of cirrhosis

Cirrhosis is a disease which may develop as a consequence of various conditions. In advanced liver disease, blood coagulation can be seriously affected. Portal hypertension, vascular abnormalities and/or a dysbalance in coagulation factors may result in bleeding disorders or in the development of thrombosis. Fibrinogen is the main protein involved in clot formation and wound healing. The aim of this work was to analyse the glycosylation pattern of the isolated fibrinogen molecules by lectin-based protein microarray, together with the carbonylation pattern of the individual fibrinogen chains, possible changes in the molecular secondary and tertiary structure and reactivity with the insulin-like growth factor-binding protein 1 (IGFBP-1) in patients with cirrhosis. The results pointed to an increase in several carbohydrate moieties: tri/tetra-antennary structures, Gal β-1,4 GlcNAc, terminal α-2,3 Sia and α-1,3 Man, and a decrease in core α-1,6 Fuc and bi-antennary galactosylated N-glycans with bisecting GlcNAc. Fibrinogen Aα chain was the most susceptible to carbonylation, followed by the Bβ chain. Cirrhosis induced additional protein carbonylation, mostly on the α chain. Spectrofluorimetry and CD spectrometry detected reduction in the α-helix content, protein unfolding and/or appearance of modified amino acid residues in cirrhosis. The amount of complexes which fibrinogen forms with IGFBP-1, another factor involved in wound healing was significantly greater in patients with cirrhosis than in healthy individuals. A more detailed knowledge of individual molecules in coagulation process may contribute to deeper understanding of coagulopathies and the results of this study offer additional information on the possible mechanisms involved in impaired coagulation due to cirrhosis.

Effects of dabigatran on the cellular and protein phase of coagulation in patients with coronary artery disease on dual antiplatelet therapy with aspirin and clopidogrel

There is growing interest in understanding the effects of adding an oral anticoagulant in patients on dual antiplatelet therapy (DAPT). Vitamin K antagonists (VKAs) and clopidogrel represent the most broadly utilised oral anticoagulant and P2Y12 receptor inhibitor, respectively. However, VKAs can interfere with clopidogrel metabolism via the cytochrome P450 (CYP) system which in turn may result in an increase in platelet reactivity. Dabigatran is a direct acting (anti-II) oral anticoagulant which does not interfere with CYP and has favourable safety and efficacy profiles compared with VKAs. The pharmacodynamic (PD) effects on platelet reactivity and clot kinetic of adjunctive dabigatran therapy in patients on DAPT are poorly explored. In this prospective, randomised, double-blind, placebo-controlled PD study, patients (n=30) on maintenance DAPT with aspirin and clopidogrel were randomised to either dabigatran 150 mg bid or placebo for seven days. PD testing was performed before and after treatment using four different assays exploring multiple pathways of platelet aggregation and fibrin clot kinetics: light transmittance aggregometry (LTA), multiple electrode aggregometry (MEA), kaolin-activated thromboelastography (TEG) and turbidimetric assays. There were no differences in multiple measures of platelet reactivity investigating purinergic and non-purinergic signaling pathways assessed by LTA, MEA and TEG platelet mapping. Dabigatran significantly increased parameters related to thrombin activity and thrombus generation, and delayed fibrin clot formation, without affecting clot structure or fibrinolysis. In conclusion, in patients on DAPT with aspirin and clopidogrel, adjunctive dabigatran therapy is not associated with modulation of profiles of platelet reactivity as determined by several assays assessing multiple platelet signalling pathways. However, dabigatran significantly interferes with parameters related to thrombin activity and delays fibrin clot formation.

Data-driven Modeling of Hemodynamics and its Role on Thrombus Size and Shape in Aortic Dissections

Aortic dissection is a pathology that manifests due to microstructural defects in the aortic wall. Blood enters the damaged wall through an intimal tear, thereby creating a so-called false lumen and exposing the blood to thrombogenic intramural constituents such as collagen. The natural history of this acute vascular injury thus depends, in part, on thrombus formation, maturation, and possible healing within the false lumen. A key question is: Why do some false lumens thrombose completely while others thrombose partially or little at all? An ability to predict the location and extent of thrombus in subjects with dissection could contribute significantly to clinical decision-making, including interventional design. We develop, for the first time, a data-driven particle-continuum model for thrombus formation in a murine model of aortic dissection. In the proposed model, we simulate a final-value problem in lieu of the original initial-value problem with significantly fewer particles that may grow in size upon activation, thus representing the local concentration of blood-borne species. Numerical results confirm that geometry and local hemodynamics play significant roles in the acute progression of thrombus. Despite geometrical differences between murine and human dissections, mouse models can provide considerable insight and have gained popularity owing to their reproducibility. Our results for three classes of geometrically different false lumens show that thrombus forms and extends to a greater extent in regions with lower bulk shear rates. Dense thrombi are less likely to form in high-shear zones and in the presence of strong vortices. The present data-driven study suggests that the proposed model is robust and can be employed to assess thrombus formation in human aortic dissections.

Potential biomechanical roles of risk factors in the evolution of thrombus-laden abdominal aortic aneurysms

Abdominal aortic aneurysms (AAAs) typically harbour an intraluminal thrombus (ILT), yet most prior computational models neglect biochemomechanical effects of thrombus on lesion evolution. We recently proposed a growth and remodelling model of thrombus-laden AAAs that introduced a number of new constitutive relations and associated model parameters. Because values of several of these parameters have yet to be elucidated by clinical data and could vary significantly from patient to patient, the aim of this study was to investigate the possible extent to which these parameters influence AAA evolution. Given that some of these parameters model potential effects of factors that influence the risk of rupture, this study also provides insight into possible roles of common risk factors on the natural history of AAAs. Despite geometrical limitations of a cylindrical domain, findings support current thought that smoking, hypertension, and female sex likely increase the risk of rupture. Although thrombus thickness is not a reliable risk factor for rupture, the model suggests that the presence of ILT may have a destabilizing effect on AAA evolution, consistent with histological findings from human samples. Finally, simulations support two hypotheses that should be tested on patient-specific geometries in the future. First, ILT is a potential source of the staccato enlargement observed in many AAAs. Second, ILT can influence rupture risk, positively or negatively, via competing biomechanical (eg, stress shielding) and biochemical (ie, proteolytic) effects. Although further computational and experimental studies are needed, the present findings highlight the importance of considering ILT when predicting aneurysmal enlargement and rupture risk.

Fibrin clot properties and their modulation in thrombotic disorders

Accumulating evidence indicates that accelerated formation of fibrin clots composed of compact, highly-branched networks with thin fibres which are relatively resistant to plasmin-mediated lysis can be commonly observed in patients with venous or arterial thrombosis. This review discusses characteristics of fibrin clot structure and function in patients with various thromboembolic manifestations, in particular myocardial infarction, ischaemic stroke and venous thromboembolism, based on the publications till December 2013. Moreover, factors will be presented that in vivo unfavourably determine altered fibrin clot properties in thrombotic disorders and modalities that can improve clot phenotype.

Structural and functional changes of fibrinogen due to aging

Different factors affect coagulation process. Since fibrinogen is the main coagulation factor, the influence of aging on fibrinogen structure and function was investigated in this study. Fibrinogen was isolated from plasma obtained from healthy persons in the age range 21-83 and examined. Lectin microarray analysis demonstrated increased glycosylation of fibrinogen due to aging, with predominant increase in high-mannose or hybrid type N-glycans, as well as tri-/tetraantennary complex N-glycans with greater content of galactose and N-acetylglucosamine residues. Spectrofluorimetric analysis indicated that fibrinogen molecules have more densely packed structure, but there are no additional advanced glycation end products with increasing age. According to the results of functional analysis, fibrinogen molecules isolated from older persons exhibited reduced clotting time, with significant positive correlation with age, but there were no differences in clotting speed, maximal optical density of fibrin clot, diameter of fibrin fibres, fibrin porosity or reactivity with the insulin-like growth factor binding protein 1. Glycosylation changes of fibrinogen in healthy aging most likely affect its structure and function, namely clotting time. Structural and functional studies of proteins in relation to healthy aging contribute to deeper understanding of mechanisms responsible for longevity.

Thrombin-activatable fibrinolysis inhibitor in human abdominal aortic aneurysm disease

Essentials Patients with abdominal aortic aneurysms (AAA) develop dense clots that are resistant to lysis. This study explores the role of thrombin-activatable fibrinolysis inhibitor (TAFI) in human AAA. There is evidence of chronically increased TAFI activation in patients with AAA. TAFI may represent a pharmacological target for cardiovascular risk reduction in AAA.

SUMMARY

Background Intra-luminal thrombosis is a key factor in growth of abdominal aortic aneurysms (AAAs). Patients with AAA form dense clots that are resistant to fibrinolysis. Thrombin-activatable fibrinolysis inhibitor (TAFI) has been shown to influence AAA development in murine models. Objective The aim of this study is to characterize the role of TAFI in human AAA. Methods Plasma levels of TAFI, TAFI activation peptide (TAFI-AP), activated/inactivated TAFI (TAFIa/ai) and plasmin-α2-antiplasmin complex were measured by ELISAs in patients with AAA (n = 202) and controls (n = 188). Results TAFIa/ai and TAFI-AP levels were higher in patients than controls (median [IQR], 20.3 [14.6-32.8] ng mL-1 vs. 14.2 [11.2-19.3] ng mL-1 and 355.0 [232.4-528.1] ng mL-1 vs. 248.6 [197.1-328.1] ng mL-1 ). TAFIa/ai was positively correlated with TAFI-AP (r = 0.164). Intact TAFI levels were not different between patients and controls (13.4 [11.2-16.1] μg mL-1 vs. 12.8 [10.6-15.4] μg mL-1 ). Plasmin-α2-antiplasmin was higher in AAA patients than controls (690.0 [489.1-924.3] ng mL-1 vs. 480.7 [392.6-555.3] ng mL-1 ). Conclusions The increase in TAFIa/ai and TAFI-AP suggests an increased TAFI activation in patients with AAA. Prospective studies are required to further elucidate the role of TAFI and fibrinolysis in AAA pathogenesis.

Inhibition of plasmin-mediated TAFI activation may affect development but not progression of abdominal aortic aneurysms

Objective

Thrombin-activatable fibrinolysis inhibitor (TAFI) reduces the breakdown of fibrin clots through its action as an indirect inhibitor of plasmin. Studies in TAFI-deficient mice have implicated a potential role for TAFI in Abdominal Aortic Aneurysm (AAA) disease. The role of TAFI inhibition on AAA formation in adult ApoE-/- mice is unknown. The aim of this paper was to investigate the effects of TAFI inhibition on AAA development and progression.

Methods

Using the Angiotensin II model of AAA, male ApoE-/- mice were infused with Angiotensin II 750ng/kg/min with or without a monoclonal antibody inhibitor of plasmin-mediated activation of TAFI, MA-TCK26D6, or a competitive small molecule inhibitor of TAFI, UK-396082.

Results

Inhibition of TAFI in the Angiotensin II model resulted in a decrease in the mortality associated with AAA rupture (from 40.0% to 16.6% with MA-TCK26D6 (log-rank Mantel Cox test p = 0.16), and 8.3% with UK-396082 (log-rank Mantel Cox test p = 0.05)). Inhibition of plasmin-mediated TAFI activation reduced the incidence of AAA from 52.4% to 30.0%. However, late treatment with MA-TCK26D6 once AAA were already established had no effect on the progression of AAA in this model.

Conclusions

The formation of intra-mural thrombus is responsible for the dissection and early rupture in the angiotensin II model of AAA, and this process can be prevented through inhibition of TAFI. Late treatment with a TAFI inhibitor does not prevent AAA progression. These data may indicate a role for inhibition of plasmin-mediated TAFI activation in the early stages of AAA development, but not in its progression.

Novel mechanisms that regulate clot structure/function

The structure and function of the blood clot has been associated with altered risk of thrombosis. Dense fibrin structures with small pores increase the risk of thrombosis, and have major functional consequences by increasing the resistance to fibrinolysis and altering the visco-elastic properties of the clot. However, while the structural changes to the overall fibrin network have been extensively characterised, little is known regarding the intrafibrillar structure of fibrin, the way protofibrils are arranged inside the fibrin fibers and the functional consequences of this. This brief paper aims to review recent findings regarding novel mechanisms that regulate fibrin intrafibrillar structure, including the degree of protofibril packing, their functional consequences, and the effects of FXIII activation on clot structure and thrombosis. It is concluded that fibrin intrafibrillar structure represents a major novel mechanism that influences clot structure and stability. Future studies are required to investigate the role of fibrin intrafibrillar structure in the functional characteristics of the blood clot, and in diseases of bleeding and thrombosis.

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

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

ABSTRACT

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

Fibrin clot structure in patients with congenital dysfibrinogenaemia

The clinical phenotype of patients with congenital dysfibrinogenaemia is highly heterogeneous, from absence of symptoms to mild bleeding, or thrombosis. A few mutations are associated with a specific phenotype, but generally the clinical course is not predictable. We investigated whether fibrin clot properties are correlated with the patient's phenotype and/or genotype. Ex vivo plasma fibrin clot characteristics, including turbidity, fibrinolysis, clot permeability and fibrin fibre density assessed by laser scanner confocal microscopy were investigated in 24 genotyped patients with congenital dysfibrinogenaemia compared to normal pool plasma. Compared to normal pool plasma, the patients were characterised by slower fibrin polymerisation (lag time, 345.10 ± 22.98 vs. 166.00s), thinner fibrin fibres (maximum absorbance, 0.15 ± 0.01 vs. 0.31), prolonged clot lysis time (23.72 ± 0.97 vs. 20.32 min) and larger clot pore size (21.5×10(-9) ± 4.48×10(-9) vs. 7.96×10(-9)cm(2)). Laser scanning confocal microscopy images confirmed disorganised fibrin networks in all patients. Patients with tendency to bleed showed an increased permeability compared to asymptomatic patients (p=0.01) and to patients with a thrombotic history (p=0.02) while patients with thrombotic history had a tendency to have a prolonged clot lysis time. Fibrin clot properties were similar among hotspot mutations. Further studies including a larger number of patients are needed to evaluate whether analysis of permeability and clot lysis time may help to distinguish the clinical phenotype in these patients and to assess differences according to the genotype.

Fibrinolytic PLGA nanoparticles for slow clot lysis within abdominal aortic aneurysms attenuate proteolytic loss of vascular elastic matrix

Abdominal aortic aneurysms (AAAs) involve chronic overexpression of proteases in the aortic wall that result in disruption of elastic fibers and consequent loss of vessel elasticity. Nearly 75% of AAAs contain flow-obstructing, fibrin-rich intraluminal thrombi (ILT), which act as a) a bioinert shield, protecting the underlying AAA wall from high hemodynamic stresses, and b) a reservoir of inflammatory cells and proteases that cause matrix breakdown. For these reasons, restoring flow through the aorta lumen and facilitating transmural diffusion of therapeutics from circulation to the AAA wall must be achieved by slow thrombolysis of the ILT to render it porous without rapid breakdown. Intravenously dosed tissue plasminogen activator (tPA) has been shown to rapidly lyse ILTs in acute stroke and myocardial infarctions. For future use in opening up AAA segments, in this study, we investigated the ability of tPA released from poly(lactic-co-glycolic acid) nanoparticles (PLGA NPs) to slowly lyse fibrin clots without inducing proteolytic injury and matrix synthesis-inhibitory effects on cultured rat aneurysmal smooth muscle cells (EaRASMCs). Fibrin clot lysis time was greatly extended over that in presence of exogenous tPA. Surface functionalization of NPs with a cationic amphiphile allowed them to bind to anionic fibrin clot, release tPA at a slower rate and to lyse the clot as a front proceeding outwards in unlike the more rapid and homogenous lysis that occurred due to anionic PLGA NPs. Elastic matrix content was decreased in EaRASMC cultures exposed to byproducts of clot lysis with exogenous tPA, but not tPA-NPs, and was likely due to increased proteolytic activity (MMPs, plasmin) in EaRASMC cultures exposed to exogenous tPA-lysed clots. Our results suggest that gradual ILT lysis via slow release of tPA from NPs will be likely beneficial over exogenous tPA delivery in preserving elastic matrix content and attenuating matrilysis in the adjoining AAA wall, in vivo, while rendering the ILT porous to facilitate transmural delivery of endoluminally delivered AAA therapeutics.

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.

Effects of riboflavin and amotosalen photoactivation systems for pathogen inactivation of fresh-frozen plasma on fibrin clot structure

BACKGROUND

Fresh-frozen plasma (FFP) transfusion carries a risk of viral transmission from donor to recipient. Riboflavin (Mirasol) and amotosalen (Intercept) are two pathogen inactivation (PI) methods that may enhance the safety of FFP for transfusion. Our study investigated the effects of Mirasol and Intercept treatment on fibrin formation and clot structure.

STUDY DESIGN AND METHODS

FFP underwent either Mirasol or Intercept treatment, and aliquots were taken before addition of the compound, before illumination (after addition of compound only), and after treatment (addition of compound plus illumination). All samples underwent turbidimetric analysis, lysis analysis, assessment of clot permeation, and analysis by laser scanning confocal microscopy.

RESULTS

After treatment, there was a decrease in optical density of the fibrin network for Mirasol and Intercept, lag time to fibrin formation was prolonged for Mirasol and lysis time for Intercept, clot permeability was significantly decreased, and clot density was increased for both.

CONCLUSIONS

Our study shows that plasma treated with Mirasol and Intercept produces denser clots consisting of thinner fibers and warrants further studies to evaluate the clinical significance of these structural changes in fibrin clot formation.

Platelet Inhibitors Reduce Rupture in a Mouse Model of Established Abdominal Aortic Aneurysm

OBJECTIVE

Rupture of abdominal aortic aneurysms causes a high morbidity and mortality in the elderly population. Platelet-rich thrombi form on the surface of aneurysms and may contribute to disease progression. In this study, we used a pharmacological approach to examine a role of platelets in established aneurysms induced by angiotensin II infusion into hypercholesterolemic mice.

APPROACH AND RESULTS

Administration of the platelet inhibitors aspirin or clopidogrel bisulfate to established abdominal aortic aneurysms dramatically reduced rupture. These platelet inhibitors reduced abdominal aortic platelet and macrophage recruitment resulting in decreased active matrix metalloproteinase-2 and matrix metalloproteinase-9. Platelet inhibitors also resulted in reduced plasma concentrations of platelet factor 4, cytokines, and components of the plasminogen activation system in mice. To determine the validity of these findings in human subjects, a cohort of aneurysm patients were retrospectively analyzed using developed and validated algorithms in the electronic medical record database at Vanderbilt University. Similar to mice, administration of aspirin or P2Y12 inhibitors was associated with reduced death among patients with abdominal aortic aneurysm.

CONCLUSIONS

These results suggest that platelets contribute to abdominal aortic aneurysm progression and rupture.

A Computational Model of Biochemomechanical Effects of Intraluminal Thrombus on the Enlargement of Abdominal Aortic Aneurysms

Abdominal aortic aneurysms (AAAs) typically develop an intraluminal thrombus (ILT), yet most computational models of AAAs have focused on either the mechanics of the wall or the hemodynamics within the lesion, both in the absence of ILT. In the few cases wherein ILT has been modeled directly, as, for example, in static models that focus on the state of stress in the aortic wall and the associated rupture risk, thrombus has been modeled as an inert, homogeneous, load-bearing material. Given the biochemomechanical complexity of an ILT, there is a pressing need to consider its diverse effects on the evolving aneurysmal wall. Herein, we present the first growth and remodeling model that addresses together the biomechanics, mechanobiology, and biochemistry of thrombus-laden AAAs. Whereas it has been shown that aneurysmal enlargement in the absence of ILT depends primarily on the stiffness and turnover of fibrillar collagen, we show that the presence of a thrombus within lesions having otherwise the same initial wall composition and properties can lead to either arrest or rupture depending on the biochemical effects (e.g., release of proteases) and biomechanical properties (e.g., stiffness of fibrin) of the ILT. These computational results suggest that ILT should be accounted for when predicting the potential enlargement or rupture risk of AAAs and highlight specific needs for further experimental and computational research.

Integration of acoustic radiation force and optical imaging for blood plasma clot stiffness measurement

Despite the life-preserving function blood clotting serves in the body, inadequate or excessive blood clot stiffness has been associated with life-threatening diseases such as stroke, hemorrhage, and heart attack. The relationship between blood clot stiffness and vascular diseases underscores the importance of quantifying the magnitude and kinetics of blood's transformation from a fluid to a viscoelastic solid. To measure blood plasma clot stiffness, we have developed a method that uses ultrasound acoustic radiation force (ARF) to induce micron-scaled displacements (1-500 μm) on microbeads suspended in blood plasma. The displacements were detected by optical microscopy and took place within a micro-liter sized clot region formed within a larger volume (2 mL sample) to minimize container surface effects. Modulation of the ultrasound generated acoustic radiation force allowed stiffness measurements to be made in blood plasma from before its gel point to the stage where it was a fully developed viscoelastic solid. A 0.5 wt % agarose hydrogel was 9.8-fold stiffer than the plasma (platelet-rich) clot at 1 h post-kaolin stimulus. The acoustic radiation force microbead method was sensitive to the presence of platelets and strength of coagulation stimulus. Platelet depletion reduced clot stiffness 6.9 fold relative to platelet rich plasma. The sensitivity of acoustic radiation force based stiffness assessment may allow for studying platelet regulation of both incipient and mature clot mechanical properties.

Aspirin therapy is associated with less compact fibrin networks and enhanced fibrinolysis in patients with abdominal aortic aneurysm

OBJECTIVE

Thrombotic changes in fibrin networks contribute to increased cardiovascular risk in patients with abdominal aortic aneurysm (AAA). Given that aspirin modulates the fibrin network, we aimed to determine if aspirin therapy is associated with changes in ex-vivo fibrin clot characteristics in AAA patients and also conducted an exploratory analysis of 5-year mortality in these individuals.

METHODS

We recruited 145 male patients, divided into controls (aortic diameter < 3 cm, n = 49), AAA not taking aspirin (AAA-Asp, n = 50) and AAA on 75 mg day(-1) aspirin (AAA+Asp, n = 46), matched for aneurysm size. Characteristics of clots made from plasma and plasma-purified fibrinogen were investigated using turbidimetric analysis, permeation studies, and confocal and electron microscopy. Plasma fibrinogen, D-dimer and inflammatory marker levels were also measured.

RESULTS

Maximum absorbance (MA) of plasma clots from controls was lower than that of AAA patients not on aspirin (AAA-Asp) at 0.30 ± 0.01 and 0.38 ± 0.02 au, respectively (P = 0.002), whereas aspirin-treated subjects had MA similar to controls (0.31 ± 0.02 P = 0.9). Plasma clot lysis time displayed an identical pattern at 482 ± 15, 597 ± 24 and 517 ± 27 s for control, AAA-Asp and AAA+Asp (P = 0.001 and P = 0.8). The lysis time of clots made from purified fibrinogen of AAA-Asp was longer than that of AAA+Asp patients (756 ± 47 and 592 ± 52 s, respectively; P = 0.041). Permeation studies and confocal and electron microscopy showed increased clot density in AAA-Asp compared with the AAA+Asp group. Mortality in AAA-Asp and AAA+Asp was similar, despite increased cardiovascular risk in the latter group, and both exhibited higher mortality than controls.

CONCLUSION

Aspirin improves fibrin clot characteristics in patients with AAA, which may have important clinical implications.

Common FXIII and fibrinogen polymorphisms in abdominal aortic aneurysms

Introduction

Abdominal aortic aneurysms (AAA) are characterized by a progressive dilatation of the abdominal aorta, and are associated with a high risk of rupture once the dilatation exceeds 55 mm in diameter. A large proportion of AAA develops an intraluminal thrombus, which contributes to hypoxia, inflammation and tissue degradation. We have previously shown that patients with AAA produce clots with altered structure which is more resistant to fibrinolysis. The aim of this study was to investigate genetic polymorphisms of FXIII and fibrinogen in AAA to identify how changes to these proteins may play a role in the development of AAA.

Methods

Subjects of Western/European descent, ≥55 years of age (520 AAA patients and 449 controls) were genotyped for five polymorphisms (FXIII-A Val34Leu, FXIII-B His95Arg, FXIII-B Splice Variant (intron K nt29576C-G), Fib-A Thr312Ala and Fib-B Arg448Lys) by RT-PCR. Data were analysed by χ2 test and CubeX.

Results

The FXIII-B Arg95 allele associated with AAA (Relative risk - 1.240, CI 1.093–1.407, P = 0.006). There was no association between FXIII-A Val34Leu, FXIII-B Splice Variant, Fib-A Thr312Ala or Fib-B Arg448Lys and AAA. FXIII-B His95Arg and FXIII-B Splice variant (intron K nt29576C-G) were in negative linkage disequilibrium (D’ = −0.609, p = 0.011).

Discussion

The FXIII-B Arg95 variant is associated with an increased risk of AAA. These data suggest a possible role for FXIII in AAA pathogenesis.

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.

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.

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.

Partial deletion of the αC-domain in the Fibrinogen Perth variant is associated with thrombosis, increased clot strength and delayed fibrinolysis

Genetic fibrinogen (FGN) variants that are associated with bleeding or thrombosis may be informative about fibrin polymerisation, structure and fibrinolysis. We report a four generation family with thrombosis and heritable dysfibrinogenaemia segregating with a c.[1541delC];[=] variation in FGA (FGN-Perth). This deletion predicts a truncated FGN αC-domain with an unpaired terminal Cys at residue 517 of FGN-Aα. In keeping with this, SDS-PAGE of purified FGN-Perth identified a truncated FGN-Aα chain with increased co-purification of albumin, consistent with disulphide bonding to the terminal Cys of the variant FGN-Aα. Clot visco-elastic strength in whole blood containing FGN-Perth was greater than controls and tPA-mediated fibrinolysis was delayed. In FGN-Perth plasma and in purified FGN-Perth, there was markedly reduced final turbidity after thrombin-mediated clot generation. Consistent with this, FGN-Perth formed tighter, thinner fibrin fibres than controls indicating defective lateral aggregation of protofibrils. Clots generated with thrombin in FGN-Perth plasma were resistant to tPA-mediated fibrinolysis. FGN-Perth clot also displayed impaired tPA-mediated plasmin generation but incorporated α2-antiplasmin at a similar rate to control. Impaired fibrinolysis because of defective plasmin generation potentially explains the FGN-Perth clinical phenotype. These findings highlight the importance of the FGN αC-domain in the regulation of clot formation and fibrinolysis.

Biochemomechanics of intraluminal thrombus in abdominal aortic aneurysms

Most computational models of abdominal aortic aneurysms address either the hemodynamics within the lesion or the mechanics of the wall. More recently, however, some models have appropriately begun to account for the evolving mechanics of the wall in response to the changing hemodynamic loads. Collectively, this large body of work has provided tremendous insight into this life-threatening condition and has provided important guidance for current research. Nevertheless, there has yet to be a comprehensive model that addresses the mechanobiology, biochemistry, and biomechanics of thrombus-laden abdominal aortic aneurysms. That is, there is a pressing need to include effects of the hemodynamics on both the development of the nearly ubiquitous intraluminal thrombus and the evolving mechanics of the wall, which depends in part on biochemical effects of the adjacent thrombus. Indeed, there is increasing evidence that intraluminal thrombus in abdominal aortic aneurysms is biologically active and should not be treated as homogeneous inert material. In this review paper, we bring together diverse findings from the literature to encourage next generation models that account for the biochemomechanics of growth and remodeling in patient-specific, thrombus-laden abdominal aortic aneurysms.

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.

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.