Influence of γ′ Fibrinogen Splice Variant on Fibrin Physical Properties and Fibrinolysis Rate

Extreme γ' fibrinogen levels in COVID-19 patients

COVID-19 disease progression can be accompanied by a "cytokine storm" that leads to secondary sequelae such as acute respiratory distress syndrome. Several inflammatory cytokines have been associated with COVID-19 disease progression, but have high daily intra-individual variability. In contrast, we have shown that the inflammatory biomarker γ' fibrinogen (GPF) has a 6-fold lower coefficient of variability compared to other inflammatory markers such as hs-CRP. The aims of the study were to measure GPF in serial blood samples from COVID-19 patients at a tertiary care medical center in order to investigate its association with clinical measures of disease progression. COVID-19 patients were retrospectively enrolled between 3/16/2020 and 8/1/2020. GPF was measured using a commercial ELISA. We found that COVID-19 patients can develop extraordinarily high levels of GPF. Our results showed that ten out of the eighteen patients with COVID-19 had the highest levels of GPF ever recorded. The previous highest GPF level of 80.3 mg/dL was found in a study of 10,601 participants in the ARIC study. GPF levels were significantly associated with the need for ECMO and mortality. These findings have potential implications regarding prophylactic anticoagulation of COVID-19 patients.

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(ogen) as a Therapeutic Target: Opportunities and Challenges

Fibrinogen is one of the key molecular players in haemostasis. Thrombin-mediated release of fibrinopeptides from fibrinogen converts this soluble protein into a network of fibrin fibres that form a building block for blood clots. Thrombin-activated factor XIII further crosslinks the fibrin fibres and incorporates antifibrinolytic proteins into the network, thus stabilising the clot. The conversion of fibrinogen to fibrin also exposes binding sites for fibrinolytic proteins to limit clot formation and avoid unwanted extension of the fibrin fibres. Altered clot structure and/or incorporation of antifibrinolytic proteins into fibrin networks disturbs the delicate equilibrium between clot formation and lysis, resulting in either unstable clots (predisposing to bleeding events) or persistent clots that are resistant to lysis (increasing risk of thrombosis). In this review, we discuss the factors responsible for alterations in fibrin(ogen) that can modulate clot stability, in turn predisposing to abnormal haemostasis. We also explore the mechanistic pathways that may allow the use of fibrinogen as a potential therapeutic target to treat vascular thrombosis or bleeding disorders. Better understanding of fibrinogen function will help to devise future effective and safe therapies to modulate thrombosis and bleeding risk, while maintaining the fine balance between clot formation and lysis.

Association of fibrinogen and plasmin inhibitor, but not coagulation factor XIII gene polymorphisms with coronary artery disease

BACKGROUND

In the final phase of clot formation, fibrinogen constitutes frame, whereas factor XIII (FXIII) active form is responsible for the covalent cross-linking of fibrin fibres and plasmin inhibitor (PI), thus contributing to clot stability. It could be expected that any change of coagulation factors' structure affects the clot formation and modulates the atherothrombotic risk. The aim was to determine the frequency of four single nucleotide polymorphisms: (i) A > G in codon 312 of the fibrinogen α-chain gene (rs6050, Thr312AlaFGA), (ii) C > T at position 10034 of the 3 - untranslated region in the fibrinogen γ-chain gene (rs2066865, 10034C > T FGG), (iii) C > T in codon 564 of the FXIII-A subunit gene (rs5982, Pro564LeuFXIII-A), and (iv) C > T in codon 6 of the plasmin inhibitor gene (rs2070863, Arg6TrpPI) in Croatian patients and their association with coronary artery disease (CAD).

METHODS

We performed the unrelated case-control association study on the consecutive sample of patients 18 years old, who had undergone coronary angiography for investigation of chest pain and suspected CAD. The cases were patients with confirmed CAD (N=201), and the controls were the subjects with no CAD (N=119). Samples were genotyped using PCR-RFLP analysis.

RESULTS

Observed frequencies of the rare alleles of Thr312Ala FGA, 10034C > T FGG, Leu564Pro FXIII-A and Arg6Trp PI polymorphisms were 21%, 17%, 14%, 20%, respectively. Patients with 10034C > T FGG CC genotype had 3.5 times (95% CI 1.02-12.03) higher adjusted odds for CAD than patients with 10034C > T FGG TT genotype. Patients with Arg6Trp PI CC genotype had 3.86 times (95% CI 1.23-12.12) higher odds for CAD than patients with Arg6Trp PI TT genotype. It seems that those genotype-related higher odds are also male-gender related. No difference was observed regarding any other investigated polymorphism.

CONCLUSIONS

Our finding suggests that 10034C > T FGG and Arg6Trp PI are associated with CAD.

A Mendelian randomization of γ' and total fibrinogen levels in relation to venous thromboembolism and ischemic stroke

Fibrinogen is a key component of the coagulation cascade, and variation in its circulating levels may contribute to thrombotic diseases, such as venous thromboembolism (VTE) and ischemic stroke. Gamma prime (γ') fibrinogen is an isoform of fibrinogen that has anticoagulant properties. We applied 2-sample Mendelian randomization (MR) to estimate the causal effect of total circulating fibrinogen and its isoform, γ' fibrinogen, on risk of VTE and ischemic stroke subtypes using summary statistics from genome-wide association studies. Genetic instruments for γ' fibrinogen and total fibrinogen were selected, and the inverse-variance weighted MR approach was used to estimate causal effects in the main analysis, complemented by sensitivity analyses that are more robust to the inclusion of pleiotropic variants, including MR-Egger, weighted median MR, and weighted mode MR. The main inverse-variance weighted MR estimates based on a combination of 16 genetic instruments for γ' fibrinogen and 75 genetic instruments for total fibrinogen indicated a protective effect of higher γ' fibrinogen and higher total fibrinogen on VTE risk. There was also a protective effect of higher γ' fibrinogen levels on cardioembolic and large artery stroke risk. Effect estimates were consistent across sensitivity analyses. Our results provide evidence to support effects of genetically determined γ' fibrinogen on VTE and ischemic stroke risk. Further research is needed to explore mechanisms underlying these effects and their clinical applications.

γ' fibrinogen levels are associated with blood clot strength in traumatic brain injury patients

BACKGROUND

γ' fibrinogen is an alternatively-spliced fibrinogen variant that displays different coagulation parameters in vitro than the major form of fibrinogen. Purified γ' fibrinogen has slower clotting kinetics than unfractionated fibrinogen, but forms clots that are stronger and resistant to fibrinolysis. However, these properties have only been investigated in human populations in a limited number of studies. We therefore performed a retrospective analysis to test the hypothesis that γ' fibrinogen levels influence coagulation in vivo.

METHODS

In the present study, we utilized blood samples that were collected from traumatic brain injury patients to probe the relationship between γ' fibrinogen levels and traditional coagulation parameters.

RESULTS

The results show that the levels of γ' fibrinogen were inversely associated with clotting kinetics, indicated by a shortened INR. In addition, the levels of γ' fibrinogen were associated with stronger clots by thrombelastography. However, these changes were not associated with significant changes in hemorrhage progression.

CONCLUSIONS

These findings verify that γ' fibrinogen properties observed in purified systems result in similar properties in a clinical setting, and may affect coagulation.

Fibrin γ/γ' influences the secretion of fibrinolytic components and clot structure

Background

In healthy subjects fibrinogen γ/γ‘ circulates at 8–15% of the total plasma fibrinogen concentration. Elevated levels of this variant have been associated with arterial thrombosis, and its diminution with venous thrombosis. The aims of the present work were to analyze the structure of the fibrin network formed on the top of human dermal microvascular endothelial cells (HMEC-1) at different fibrinogen γ/γ‘ concentrations, as well as its influence on the secretion of fibrinolytic components.

The kinetics of fibrin polymerization on top of HMEC-1 cells with 3, 10, and 30% fibrinogen γ/γ‘ was followed at 350 nm. The secretion of urokinase-type plasminogen activator (uPA) and plasminogen activator inhibitor type 1 (PAI 1) by HMEC-1 were measured in the supernatant and cell lysates, after incubation with 1 nM thrombin, fibrin with 3, and 30% fibrinogen γ/γ‘, using commercial kits. The influence of fibrinogen γ/γ‘ on fibrin structure on the surface of the HMEC-1 was followed with laser scanning confocal microscopy (LSCM).

Results

The kinetics of fibrin formation on HMEC-1 with 3 and 10% fibrinogen γ/γ‘ were similar. However, with 30% fibrinogen γ/γ‘ both the slope and final turbity were approximately 50% less. The LSCM images showed the dramatic effects of increasing fibrinogen γ/γ‘ from 3 to 30%. The uPA and PAI 1 concentrations in culture supernatants HMEC-1 cells treated with thrombin or 30% γ/γ‘ fibrin were two-fold increased as compared to basal culture supernatants and 3% γ/γ‘ fibrin-treated HMEC-1. In all stimulatory conditions the intracellular concentration of uPA was higher than in supernatants. In contrast, the intracellular PAI 1 concentration was decreased as compared to that measured in the supernatant, including the basal condition.

Conclusion

A concentration of 30% fibrin γ/γ‘ alter drastically fibrin structure on the cell surface and affects the secretion of uPA and PAI 1 through its capacity to bind thrombin.

Improving fibrinolysis in venous thromboembolism: impact of fibrin structure

Introduction. Fibrinolysis is of key importance in maintaining vessel patency. Impaired fibrinolysis associated with more compact fibrin structure has been shown in patients with venous thromboembolism (VTE), including deep-vein thrombosis and pulmonary embolism (PE). Currently, recombinant or modified plasminogen activators are the only commonly available thrombolytic agents. However, they are fraught with side effects and suboptimal effectiveness. Areas covered. Based on the available literature, the current evidence linking fibrinolysis with VTE and potential therapeutic targets among fibrinolysis proteins are presented. Expert opinion. Prolonged clot lysis time has been reported as a new predictor of first-time and recurrent VTE, including PE. Anticoagulant therapy, including non-vitamin K antagonist oral anticoagulants, has a favorable impact on fibrinolysis in VTE patients. Several VTE risk factors are also related to lower efficiency of fibrinolysis and their treatment improve fibrinolysis, in part by alterations to fibrin properties. There is an increasing number of studies aiming at developing novel profibrinolytic therapeutic agents for treatment of VTE patients, mostly targeting the antifibrinolytic proteins, i.e. antiplasmin, plasminogen activator inhibitor-1 and thrombin-activatable fibrinolysis inhibitor.

Alteration of clot architecture using bone substitute biomaterials (beta-tricalcium phosphate) significantly delays the early bone healing process

When a bone substitute biomaterial is implanted into the body, the material's surface comes into contact with circulating blood, which results in the formation of a peri-implant hematoma or blood clot. Although hematoma formation is vital for the early bone healing process, knowledge concerning the biomaterial-induced structural properties of blood clots is limited. Here, we report that implantation of beta-tricalcium phosphate (β-TCP) in a bone defect healing model in rats resulted in significantly delayed early bone healing compared to empty controls (natural healing). In vitro studies showed that β-TCP had a profound effect on the overall structure of hematomas, as was observed by fibrin turbidity, scanning electron microscopy (SEM), compaction assays, and fibrinolysis. Under the influence of β-TCP, clot formation had a significantly shortened lag time and there was enhanced lateral fibrin aggregation during the clot polymerization, which resulted in clots composed of thinner fibers. Furthermore, fibrin clots that formed around β-TCP exhibited reduced compaction and increased resistance to fibrinolysis. Together, these results provide a plausible mechanism for how implanted bone-substitute materials may impact the structural properties of the hematoma, thereby altering the early bone healing processes, such as cell infiltration, growth factor release and angiogenesis.

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.

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

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

High-molecular-weight kininogen and the intrinsic coagulation pathway in patients with de novo acute myocardial infarction

After an acute ischaemic event serum proteins may change reflecting the ischaemic damage. Proteomic studies could provide new insights into potential biomarkers in the evolution of ischaemic syndromes. In this study we have investigated the coordinated changes in coagulation-related proteins in the evolution after an acute myocardial infarction (AMI). Serum proteome (2D-electrophoresis and MALDI-TOF/ TOF) of AMI-patients within the first 6 hours after event onset (admission-time) and 3 days after were compared to controls. Systems biology and bioinformatic analysis were performed to identify the differentially expressed canonical pathways. In silico analysis of differential proteins revealed changes in the intrinsic coagulation pathway in the early phase post-AMI. The two identified high-molecular weight kininogen (HMWK) clusters were inversely correlated in AMI patients at admission, being the intensity of the low-molecular-weight form inversely related to myocardial necrosis (p<0.05). Factor XI (FXI) levels were decreased in AMI patients at admission and normalised 3 days after (p<0.05). There was an early increase in fibrinogen gamma and D-dimer at admission, followed by a decrease in fibrinogen turnover 3 days after (p<0.05). The influence of elapsed time of ischaemia on fibrinogen distribution changes was validated in coronary thrombi retrieved by thromboaspiration. In conclusion, our results demonstrate an active exchange between HMWK forms and a decrease in FXI indicative of intrinsic pathway activation, together with an increase in fibrinogen gamma turnover and D-dimer formation in the early phase post-AMI. Moreover, coronary thrombi showed a dynamic evolution in fibrinogen composition depending on the duration of ischaemia influencing serum fibrinogen-related products content.

Fibrinogen-Related Proteins in Tissue Repair: How a Unique Domain with a Common Structure Controls Diverse Aspects of Wound Healing

Significance: Fibrinogen-related proteins (FRePs) comprise an intriguing collection of extracellular molecules, each containing a conserved fibrinogen-like globe (FBG). This group includes the eponymous fibrinogen as well as the tenascin, angiopoietin, and ficolin families. Many of these proteins are upregulated during tissue repair and exhibit diverse roles during wound healing. Recent Advances: An increasing body of evidence highlights the specific expression of a number of FRePs following tissue injury and infection. Upon induction, each FReP uses its FBG domain to mediate quite distinct effects that contribute to different stages of tissue repair, such as driving coagulation, pathogen detection, inflammation, angiogenesis, and tissue remodeling. Critical Issues: Despite a high degree of homology among FRePs, each contains unique sequences that enable their diversification of function. Comparative analysis of the structure and function of FRePs and precise mapping of regions that interact with a variety of ligands has started to reveal the underlying molecular mechanisms by which these proteins play very different roles using their common domain. Future Directions: Fibrinogen has long been used in the clinic as a synthetic matrix serving as a scaffold or a delivery system to aid tissue repair. Novel therapeutic strategies are now emerging that harness the use of other FRePs to improve wound healing outcomes. As we learn more about the underlying mechanisms by which each FReP contributes to the repair response, specific blockade, or indeed potentiation, of their function offers real potential to enable regulation of distinct processes during pathological wound healing.

Reduced plasminogen binding and delayed activation render γ'-fibrin more resistant to lysis than γA-fibrin

Fibrin (Fn) clots formed from γ'-fibrinogen (γ'-Fg), a variant with an elongated γ-chain, are resistant to lysis when compared with clots formed from the predominant γA-Fg, a finding previously attributed to differences in clot structure due to delayed thrombin-mediated fibrinopeptide (FP) B release or impaired cross-linking by factor XIIIa. We investigated whether slower lysis of γ'-Fn reflects delayed plasminogen (Pg) binding and/or activation by tissue plasminogen activator (tPA), reduced plasmin-mediated proteolysis of γ'-Fn, and/or altered cross-linking. Clots formed from γ'-Fg lysed more slowly than those formed from γA-Fg when lysis was initiated with tPA/Pg when FPA and FPB were both released, but not when lysis was initiated with plasmin, or when only FPA was released. Pg bound to γ'-Fn with an association rate constant 22% lower than that to γA-Fn, and the lag time for initiation of Pg activation by tPA was longer with γ'-Fn than with γA-Fn. Once initiated, however, Pg activation kinetics were similar. Factor XIIIa had similar effects on clots formed from both Fg isoforms. Therefore, slower lysis of γ'-Fn clots reflects delayed FPB release, which results in delayed binding and activation of Pg. When clots were formed from Fg mixtures containing more than 20% γ'-Fg, the upper limit of the normal level, the delay in lysis was magnified. These data suggest that circulating levels of γ'-Fg modulate the susceptibility of clots to lysis by slowing Pg activation by tPA and provide another example of the intimate connections between coagulation and fibrinolysis.

Specific effects of fibrinogen and the γA and γ'-chain fibrinogen variants on angiogenesis and wound healing

In a newly formed wound, the natural fibrin network provides the first temporary matrix for tissue repair. Topical application of fibrin to a new wound may improve wound healing. A matrix of the common natural γ' fibrin variant may further improve wound healing because it is expected to have a different architecture and this will influence angiogenesis, because it possesses increased thrombin and factor XIII binding and decreased platelet binding, when compared with the common γA fibrin matrix. Our objective was to determine the effect of fibrinogen and its γA and γ' variants on angiogenesis and wound healing. We used in vitro angiogenesis models and an in vivo rat full-thickness excisional wound healing model. When comparing γA and γ' fibrin in vitro, more tube-like structures were formed on day 7 in γA fibrin than in γ' fibrin (13.83±6.12 AU vs. 6.1±1.46 AU). Wounds treated with fibrin demonstrated improved healing in vivo with more perfusion (47%±3% vs. 26%±4%, p<0.01 in placebo) and higher CD34 density score (2.0±0.4 vs. 2.8±0.1, p<0.01) on day 21 with fibrin matrices when compared with placebo-treated wounds. Increased perfusion was observed in γA fibrin-treated wounds on day 21 (53%±10% vs. 41%±7% for γ' fibrin). The other parameters showed slightly improved (not significant) wound healing with γA fibrin compared with γ' fibrin matrices. In conclusion, the use of fibrin and fibrin variant matrices offers an interesting methodology to stimulate the wound healing process.

The fibrinogen γA/γ' isoform does not promote acute arterial thrombosis in mice

BACKGROUND

Elevated plasma fibrinogen is associated with arterial thrombosis in humans and promotes thrombosis in mice by increasing fibrin formation and thrombus fibrin content. Fibrinogen is composed of six polypeptide chains: (Aα, Bβ, and γ)2. Alternative splicing of the γ chain leads to a dominant form (γA/γA) and a minor species (γA/γ'). Epidemiological studies have detected elevated γA/γ' fibrinogen in patients with arterial thrombosis, suggesting that this isoform promotes thrombosis. However, in vitro data show that γA/γ' is anticoagulant due to its ability to sequester thrombin and suggest its expression is upregulated in response to inflammatory processes.

OBJECTIVE

To determine whether γA/γ' fibrinogen is prothrombotic in vivo.

METHODS

We separated γA/γA and γA/γ' fibrinogen from human plasma-purified fibrinogen and determined the effects on in vitro plasma clot formation and on in vivo thrombus formation and circulating thrombin-antithrombin complexes in mice.

RESULTS AND CONCLUSIONS

Both γA/γA and γA/γ' fibrinogen were cleaved by murine and human thrombin and were incorporated into murine and human clots. When γA/γA or γA/γ' was spiked into plasma, γA/γA increased the fibrin formation rate to a greater extent than γA/γ'. In mice, compared to controls, γA/γA infusion shortened the time to carotid artery occlusion, whereas γA/γ' infusion did not. Additionally, γA/γ' infusion led to lower levels of plasma thrombin-antithrombin complexes following arterial injury, whereas γA/γA infusion did not. These data suggest that γA/γ' binds thrombin in vivo and decreases prothrombotic activity. Together, these findings indicate that elevated levels of γA/γA fibrinogen promote arterial thrombosis in vivo, whereas γA/γ' does not.

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

OBJECTIVE

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

STUDY DESIGN

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

RESULTS

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

CONCLUSION

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

Fibrin(ogen) and thrombotic disease

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

Evidence that fibrinogen γ′ regulates plasma clot structure and lysis and relationship to cardiovascular risk factors in black Africans

This paper describes the effect of fibrinogen γ′ on clot structure in plasma (previously shown in purified systems). This paper also describes the respective roles of total fibrinogen, fibrinogen γ′ concentration, and ratio on clot structure and lysis rates.

Differential regulation of fibrinogen γ chain splice isoforms by interleukin-6

INTRODUCTION

Fibrinogen is a major structural protein in blood clots, and is also a well-known acute phase reactant. The γ chain gene of fibrinogen has two alternative splice variants, γA and γ' chains. γ' fibrinogen constitutes about 7% of total fibrinogen. Total fibrinogen levels and γ' fibrinogen levels have been associated with cardiovascular disease, but the mechanisms regulating the production of the two isoforms are unknown. Several inflammatory cytokines are known to influence the production of total fibrinogen, but the role of cytokines in the production of γ' fibrinogen has not been examined. However, epidemiologic studies have shown an association between γ' fibrinogen levels and inflammatory markers in humans.

MATERIALS AND METHODS

The expression of γ' fibrinogen and total fibrinogen by HepG2 liver cells was quantitated after treatment with interleukin-1β, transforming growth factor-β, tumor necrosis factor-α, and interleukin-6.

RESULTS

Interleukin-1β, transforming growth factor-β, and tumor necrosis factor-α, known down-regulators of total fibrinogen synthesis, also downregulate γ' fibrinogen synthesis in HepG2 cells. However, interleukin-6 differentially up-regulates the production of total and γ' fibrinogen, leading to a 3.6-fold increase in γA mRNA, but an 8.3-fold increase in γ' mRNA.

CONCLUSIONS

These findings indicate that γ' fibrinogen is disproportionately up-regulated by inflammatory responses induced by interleukin-6.

Evidence that fibrinogen γ' directly interferes with protofibril growth: implications for fibrin structure and clot stiffness

BACKGROUND

Fibrinogen contains an alternatively spliced γ-chain (γ'), which mainly exists as a heterodimer with the common γA-chain (γA/γ'). Fibrinogen γ' has been reported to inhibit thrombin and modulate fibrin structure, but the underlying mechanisms are unknown.

OBJECTIVE

We aimed to investigate the molecular mechanism underpinning the influence of γ' on fibrin polymerization, structure and viscoelasticity.

METHODS

γA/γA and γA/γ' fibrinogens were separated using anion exchange chromatography. Cross-linking was controlled with purified FXIIIa and a synthetic inhibitor. Fibrin polymerization was analyzed by turbidity and gel-point time was measured using a coagulometer. We used atomic force microscopy (AFM) to image protofibril formation while final clot structure was assessed by confocal and scanning electron microscopy. Clot viscoelasticity was measured using a magnetic microrheometer.

RESULTS

γA/γ' fibrin formed shorter oligomers by AFM than γA/γA, which in addition gelled earlier. γA/γ' clots displayed a non-homogenous arrangement of thin fibers compared with the uniform arrangements of thick fibers for γA/γA clots. These differences in clot structure were not due to thrombin inhibition as demonstrated in clots made with reptilase. Non-cross-linked γA/γA fibrin was approximately 2.7 × stiffer than γA/γ'. Cross-linking by FXIIIa increased the stiffness of both fibrin variants; however, the difference in stiffness increased to approximately 4.6 × (γA/γA vs. γA/γ').

CONCLUSIONS

Fibrinogen γ' is associated with the formation of mechanically weaker, non-uniform clots composed of thin fibers. This is caused by direct disruption of protofibril formation by γ'.

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

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

Assessment of genetic determinants of the association of γ' fibrinogen in relation to cardiovascular disease

OBJECTIVE

γ' fibrinogen is a newly emerging biomarker that is associated with cardiovascular disease (CVD). However, the genetic determinants of γ' fibrinogen levels are unknown. We therefore conducted a genome-wide association study on 3042 participants from the Framingham Heart Study Offspring Cohort.

METHODS AND RESULTS

A genome-wide association study with 2.5 million single-nucleotide polymorphisms (SNPs) was carried out for γ' fibrinogen levels from the cycle 7 examination. Fifty-four SNPs in or near the fibrinogen gene locus demonstrated genome-wide significance (P<5.0×10(-8)) for association with γ' fibrinogen levels. The top-signal SNP was rs7681423 (P=9.97×10(-110)) in the fibrinogen gene locus near FGG, which encodes the γ chain. Conditional on the top SNP, the only other SNP that remained genome-wide significant was rs1049636. Associations between SNPs, γ' fibrinogen levels, and prevalent CVD events were examined using multiple logistic regression. γ' fibrinogen levels were associated with prevalent CVD (P=0.02), although the top 2 SNPs associated with γ' fibrinogen levels were not associated with CVD. These findings contrast those for total fibrinogen levels, which are associated with different genetic loci, particularly FGB, which encodes the Bβ chain.

CONCLUSIONS

γ' fibrinogen is associated with prevalent CVD and with SNPs exclusively in and near the fibrinogen gene locus.

Association between γ' fibrinogen levels and inflammation

The γ' fibrinogen isoform produces clots that are stiffer and more resistant to breakdown than the more common fibrinogen isoform, γA. Increased levels of γ' fibrinogen are associated with several forms of cardiovascular disease. The purpose of this cross-sectional study was to investigate the relationship between γ' fibrinogen, an emerging risk factor for cardiovascular disease, and inflammatory markers in subjects with a chronic inflammatory state. The 284 subjects for this study came from the Periodontitis And Vascular Events (PAVE) study, and γ' fibrinogen and total fibrinogen in plasma were measured by ELISA. Information on patient demographics and health status, as well as levels of C-reactive protein (CRP), an inflammatory marker, have previously been collected for this study. The mean (SE) γ' fibrinogen level in the subjects was 0.622 (0.017) mg/ml. Levels of γ' fibrinogen were correlated with CRP (p = 0.006), with a one unit increase in CRP associated with a 1.9% increase in γ' fibrinogen, after adjustment for potential confounders. Total fibrinogen was not correlated with γ' fibrinogen in these subjects. The number of dental sites with evidence of tissue inflammation was also significantly associated with γ' fibrinogen levels. These results provide an important step in the evolution of γ' fibrinogen not only as a general risk factor for cardiovascular disease, but as a potentially useful biomarker for assessing a patient's inflammatory state and associated cardiovascular disease risk.

Gamma' fibrinogen: evaluation of a new assay for study of associations with cardiovascular disease

BACKGROUND

Studies of disease associations with gamma' fibrinogen, a newly emerging risk factor for cardiovascular disease, have been hampered by the lack of a standardized and well-characterized assay.

METHODS

We developed an immunometric technique to measure gamma' fibrinogen concentrations in plasma and studied the clinical utility of this test in samples from healthy individuals enrolled in the Framingham Offspring Study and in a separate case/control study of coronary artery disease (CAD). Monoclonal antibody 2.G2.H9, specific for the unique carboxyl terminal peptide of the fibrinogen gamma' chain, was used as capture antibody. Sheep antihuman fibrinogen/horseradish peroxidase conjugate was used for detection, with 3,3',5,5'-tetramethylbenzidine as substrate. We evaluated the linearity, imprecision, analytical specificity, and lower limit of quantification of the assay. We determined the reference interval for gamma' fibrinogen in healthy individuals from the Framingham Offspring Study (n = 2879) and quantified associations between gamma' fibrinogen and cardiovascular disease risk factors. The sensitivity and specificity of gamma' fibrinogen in evaluating CAD patients (n = 133) was determined with ROC curve analysis.

RESULTS

The gamma' fibrinogen ELISA had within-run CVs of 13.4% at 0.127 g/L and 4.8% at 0.416 g/L. The limit of quantification at an imprecision of 20% was 0.10 g/L. The reference interval for healthy individuals was 0.088-0.551 g/L. ROC curve analysis of results from patients with CAD yielded an area under the curve of 0.76, with a diagnostic accuracy of 0.78 at a decision threshold of 0.30 g/L.

CONCLUSIONS

gamma' Fibrinogen shows excellent utility for cardiovascular risk analysis.

The pleiotropic role of the fibrinogen γ′ chain in hemostasis

A fraction of fibrinogen contains a differently spliced γ chain called γ′, which presents itself mainly as heterodimer with the common γA chain as γA/γ′ fibrinogen. The γ′ chain differs from the γA chain in its C-terminus and has important functional implications for fibrinogen. The presence of the γ′ chain modulates thrombin and FXIII activity, influences clot architecture, and eliminates a platelet-binding site. Associations of γA/γ′ fibrinogen levels with arterial and venous thrombosis have been reported, indicating that the functional effects of γA/γ′ fibrinogen may contribute to the pathology of thrombosis. This review summarizes the key biologic aspects of this interesting variant of fibrinogen and discusses inconsistencies in current reports.

The presence of gamma' chain impairs fibrin polymerization

INTRODUCTION

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Polyphosphate enhances fibrin clot structure

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

Fibrinogen gamma' chain carboxy terminal peptide selectively inhibits the intrinsic coagulation pathway

The minor gammaA/gamma' isoform of fibrinogen contains a high affinity binding site for thrombin exosite II that is lacking in the major fibrinogen isoform, gammaA/gammaA fibrinogen. The biological consequences of gamma' chain binding to thrombin were therefore investigated. Coagulation assays, thrombin activity assays, and a primate thrombosis model were used to characterize the biological effects of the gamma' 410-427 peptide. The gamma' peptide had little effect on thrombin cleavage of the small peptidyl substrate tosyl-glycyl-prolyl-arginine-4-nitranilide acetate. However, in vitro assays demonstrated that the gamma' peptide inhibited thrombin cleavage of larger proteinaceous substrates, including fibrinogen and factor VIII. The gamma' peptide inhibited the activated partial thromboplastin time in plasma and showed greater inhibition of activated partial thromboplastin time assays than prothrombin time assays, consistent with the inhibition of factor VIII cleavage. Studies in a baboon thrombosis model showed that the gamma' 410-427 peptide inhibited fibrin-rich thrombus formation (typical of venous thrombi) and, to a lesser extent, platelet-rich thrombus formation (typical of arterial thrombi). These results indicate that binding of thrombin exosite II by the gamma' peptide has selective effects on the intrinsic pathway.

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

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

Elevated plasma fibrinogen gamma' concentration is associated with myocardial infarction: effects of variation in fibrinogen genes and environmental factors

BACKGROUND

Fibrinogen gamma', a fibrinogen gamma-chain variant generated via alternative mRNA processing, has been associated with susceptibility to thrombotic disease.

OBJECTIVE

The present case-control study searched for potential determinants of the plasma fibrinogen gamma' concentration and examined the relationship between this variant and risk of myocardial infarction (MI).

PATIENTS AND METHODS

The Stockholm Coronary Artery Risk Factor study, comprising 387 postinfarction patients and 387 healthy individuals, was employed. The fibrinogen gamma (FGG) 9340T > C [rs1049636], fibrinogen alpha (FGA) 2224G > A [rs2070011] and fibrinogen beta (FGB) 1038G > A [rs1800791] polymorphisms were determined. The plasma fibrinogen gamma' concentration was measured by enzyme-linked immunosorbent assay. The multifactor dimensionality reduction method was used for interaction analyses on risk of MI.

RESULTS

The FGG 9340T > C and FGA 2224G > A polymorphisms, total plasma concentrations of fibrinogen, insulin and high-density lipoprotein, and gender appeared to be independent determinants of plasma fibrinogen gamma' concentration in patients, and the corresponding determinants in controls included FGG 9340T > C and FGA 2224G > A polymorphisms and plasma fibrinogen concentration. An elevated plasma fibrinogen gamma' concentration proved to be an independent predictor of MI [adjusted odds ratio (OR) (95% CI): 1.24 (1.01, 1.52)]. The plasma fibrinogen gamma' concentration was involved in a high-order interaction with total plasma fibrinogen and the FGG 9340T > C and FGA 2224G > A polymorphisms, associated with a further increased risk of MI [OR (95% CI): 3.22 (2.35, 4.39)].

CONCLUSIONS

Plasma fibrinogen gamma' concentration influences the risk of MI, and this relationship seems to be strengthened by the presence of an elevated total plasma fibrinogen concentration and the FGG 9340T and FGA 2224G alleles.

Decorin Modulates Fibrin Assembly and Structure

Emerging evidence indicates that fibrin clotting is regulated by different external factors. We demonstrated recently that decorin, a regulator of collagen fibrillogenesis and transforming growth factor-beta activity, binds to the D regions of fibrinogen (Dugan, T.A., Yang, V. W.-C., McQuillan, D.J., and Höök, M. (2003) J. Biol. Chem. 278, 13655-13662). We now report that the decorin-fibrinogen interaction alters the assembly, structure, and clearance of fibrin fibers. Relative to fibrinogen, substoichiometric amounts of decorin core protein modulated clotting, whereas an excess of an active decorin peptide was necessary for similar activity. These concentration-dependent effects suggest that decorin bound to the D regions sterically modulates fibrin assembly. Scanning electron microscopy images of fibrin clotted in the presence of increasing concentrations of decorin core protein showed progressively decreasing fiber diameter. The sequestration of Zn(2+) ions from the N-terminal fibrinogen-binding region abrogated decorin incorporation into the fibrin network. Compared with linear thicker fibrin fibers, the curving thin fibers formed with decorin underwent accelerated tissue-type plasminogen activator-dependent fibrinolysis. Collectively, these data demonstrate that decorin can regulate fibrin organization and reveal a novel mechanism by which extracellular matrix components can participate in hemostasis, thrombosis, and wound repair.

Altered Fibrin Architecture Is Associated With Hypofibrinolysis and Premature Coronary Atherothrombosis

OBJECTIVE

Hypofibrinolysis promotes atherosclerosis progression and recurrent ischemic events in premature coronary artery disease. We investigated the role of fibrin physical properties in this particular setting.

METHODS AND RESULTS

Biomarkers of recurrent thrombosis and premature coronary artery disease (CAD) were measured in 33 young post-myocardial infarction patients with angiographic-proven CAD and in 33 healthy volunteers matched for age and sex. Ex vivo plasma fibrin physical properties were assessed by measuring fibrin rigidity and fibrin morphological properties using a torsion pendulum and optical confocal microscopy. The fibrinolysis rate was derived from continuous monitoring of the viscoelastic properties after addition of lytic enzymes. Young CAD patients had a significant increase in plasma concentration of fibrinogen, von Willebrand factor, plasminogen activator inhibitor type 1, and lipoprotein(a) as compared with controls (P<0.05). Fibrin of young CAD patients was stiffer (P=0.002), made of numerous (P=0.002) and shorter fibers (P=0.04), and lysed at a slower rate than that of controls (P=0.03). Fibrin stiffness was an independent predictor for both premature CAD and hypofibrinolysis.

CONCLUSIONS

This first detailed study of clot properties in such a group of patients demonstrated that abnormal plasma fibrin architecture is an important feature of both premature CAD and fibrinolysis rate. The determinants of this particular phenotype warrant further investigation.

Influence of homocysteine on fibrin network lysis

To elucidate some of the links between homocysteine and vascular disease, we have evaluated the effect of the amino acid on the formation (by kinetics studies), structure (by electron microscopy) and lysis of the fibrin network, using tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA). We have studied whether homocysteine could alter the activity of the components involved in fibrinolysis (by amidolytic and thrombolytic methods). The results showed that homocysteine-associated networks were more compact and branched than controls (52 +/- 6 vs 44 +/- 5 fibers/field, P = 0.008), and were formed by shorter and thicker fibers. This clot proved to be more resistant to fibrinolysis with u-PA than control [lysis time 50%: 257 +/- 16 (homocysteine) vs 187 +/- 6 min (control); P < 0.004], but there were no differences with t-PA. Homocysteine did not affect the biological activities of plasmin, or plasminogen activation by t-PA and u-PA. Defective fibrinolysis with u-PA was therefore associated with homocysteine-fibrin structural alterations rather than the homocysteine effect on the biological activities of the fibrinolytic components evaluated. Results suggest that hyperhomocysteinemic patients could produce tight clots, were more resistant to lysis, and generated a procoagulant environment in situ. We believe that our findings may contribute to understanding the mechanisms involved in the homocysteine harmful effect.

Fibrin structure and wound healing

Fibrinogen and fibrin play an important role in blood clotting, fibrinolysis, cellular and matrix interactions, inflammation, wound healing, angiogenesis, and neoplasia. The contribution of fibrin(ogen) to these processes largely depends not only on the characteristics of the fibrin(ogen) itself, but also on interactions between specific-binding sites on fibrin(ogen), pro-enzymes, clotting factors, enzyme inhibitors, and cell receptors. In this review, the molecular and cellular biology of fibrin(ogen) is reviewed in the context of cutaneous wound repair. The outcome of wound healing depends largely on the fibrin structure, such as the thickness of the fibers, the number of branch points, the porosity, and the permeability. The binding of fibrin(ogen) to hemostasis proteins and platelets as well as to several different cells such as endothelial cells, smooth muscle cells, fibroblasts, leukocytes, and keratinocytes is indispensable during the process of wound repair. High-molecular-weight and low-molecular-weight fibrinogen, two naturally occurring variants of fibrin, are important determinants of angiogenesis and differ in their cell growth stimulation, clotting rate, and fibrin polymerization characteristics. Fibrin sealants have been investigated as matrices to promote wound healing. These sealants may also be an ideal delivery vehicle to deliver extra cells for the treatment of chronic wounds.

A new optimized method for the determination of fibrin clot permeability

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

The molecular physiology and pathology of fibrin structure/function

The formation of a fibrin clot is a pivotal event in atherothrombotic vascular disease and there is mounting evidence that the structure of clots is of importance in the development of disease. This review describes the crucial events in the formation and dissolution of a clot, with particular focus on genetic and environmental factors that have been identified as determinants of fibrin structure in vivo, and discusses the substantiation of the relationship between fibrin structure and disease in conjunction with a review of the current literature.

Fibrinogen heterogeneity: inherited and noninherited

PURPOSE OF REVIEW

Many noninherited or inherited variations have been described in fibrinogen and they may affect the different functions of fibrinogen.

RECENT FINDINGS

A number of the acquired variations in fibrinogen affect the properties of the fibrinogen molecule, such as the conversion rate to fibrin and/or the characteristics of the fibrin clot. Also, genetic polymorphisms are known that can affect the function of the fibrinogen molecule. In addition, some other genetic variants are associated with plasma levels of fibrinogen and with the increase of fibrinogen levels during an acute-phase reaction.

SUMMARY

In this review the authors discuss the noninherited and inherited variations of fibrinogen and the clinical implications (e.g. when determining the risk of cardiovascular disease).

Studies on the basis for the properties of fibrin produced from fibrinogen-containing gamma' chains

Human fibrinogen 1 is homodimeric with respect to its gamma chains (gammaA-gammaA'), whereas fibrinogen 2 molecules each contain one gammaA (gammaA1-411V) and one gamma' chain, which differ by containing a unique C-terminal sequence from gamma'408 to 427L that binds thrombin and factor XIII. We investigated the structural and functional features of these fibrins and made several observations. First, thrombin-treated fibrinogen 2 produced finer, more branched clot networks than did fibrin 1. These known differences in network structure were attributable to delayed release of fibrinopeptide (FP) A from fibrinogen 2 by thrombin, which in turn was likely caused by allosteric changes at the thrombin catalytic site induced by thrombin exosite 2 binding to the gamma' chains. Second, cross-linking of fibrin gamma chains was virtually the same for both types of fibrin. Third, the acceleratory effect of fibrin on thrombin-mediated XIII activation was more prominent with fibrin 1 than with fibrin 2, and this was also attributable to allosteric changes at the catalytic site induced by thrombin binding to gamma' chains. Fourth, fibrinolysis of fibrin 2 was delayed compared with fibrin 1. Altogether, differences between the structure and function of fibrins 1 and 2 are attributable to the effects of thrombin binding to gamma' chains.

The mechanical properties of fibrin for basic scientists and clinicians

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

Pathophysiologic roles of the fibrinogen gamma chain

PURPOSE OF REVIEW

Fibrinogen binds through its gamma chains to cell surface receptors, growth factors, and coagulation factors to perform its key roles in fibrin clot formation, platelet aggregation, and wound healing. However, these binding interactions can also contribute to pathophysiologic processes, including inflammation and thrombosis. This review summarizes the latest findings on the role of the fibrinogen gamma chain in these processes, and illustrates the potential for therapeutic intervention.

RECENT FINDINGS

Novel gamma chain epitopes that bind platelet integrin alpha IIbbeta3 and leukocyte integrin alphaMbeta2 have been characterized, leading to the revision of former dogma regarding the processes of platelet aggregation, clot retraction, inflammation, and thrombosis. A series of studies has shown that the gamma chain serves as a depot for fibroblast growth factor-2 (FGF-2), which is likely to play an important role in wound healing. Inhibition of gamma chain function with the monoclonal antibody 7E9 has been shown to interfere with multiple fibrinogen activities, including factor XIIIa crosslinking, platelet adhesion, and platelet-mediated clot retraction. The role of the enigmatic variant fibrinogen gamma chain has also become clearer. Studies have shown that gamma chain binding to thrombin and factor XIII results in clots that are mechanically stiffer and resistant to fibrinolysis, which may explain the association between gammaA/gamma' fibrinogen levels and cardiovascular disease.

SUMMARY

The identification of new interactions with gamma chains has revealed novel targets for the treatment of inflammation and thrombosis. In addition, several exciting studies have shown new functions for the variant gamma chain that may contribute to cardiovascular disease.