Thrombotic dysfibrinogenemia. Fibrinogen "Caracas V" relation between very tight fibrin network and defective clot degradability

Novel heterozygous dysfibrinogenemia, Sumida (AαC472S), showed markedly impaired lateral aggregation of protofibrils and mildly lower functional fibrinogen levels

INTRODUCTION

We encountered a 6-year-old girl with systemic lupus erythematosus. Although no bleeding or thrombotic tendency was detected, routine coagulation screening tests revealed slightly lower plasma fibrinogen levels, as determined by functional and antigenic measurements (functional/antigenic ratio=0.857), suggesting hypodysfibrinogenemia.

MATERIALS AND METHODS

DNA sequence and functional analyses were performed on purified plasma fibrinogen, and recombinant variant fibrinogen was synthesized in Chinese hamster ovary cells based on the results obtained.

RESULTS

DNA sequencing revealed a heterozygous AαC472S substitution (mature protein residue number) in the αC-domain. AαC472S fibrinogen indicated the presence of additional disulfide-bonded molecules, and markedly impaired lateral aggregation of protofibrils in spite of slightly lower functional plasma fibrinogen levels. Scanning electron microscopic observations showed a thin fiber fibrin clot, and t-PA and plasminogen-mediated clot lysis was similar to that of a normal control. Recombinant variant fibrinogen-producing cells demonstrated that destruction of the Aα442C-472C disulfide bond did not prevent the synthesis or secretion of fibrinogen, whereas the variant Aα chain of the secreted protein was degraded faster than that of the normal control.

CONCLUSION

Our results suggest that AαC472S fibrinogen may cause dysfibrinogenemia, but not hypofibrinogenemia. The destruction and steric hindrance of the αC-domain of variant fibrinogen led to the impaired lateral aggregation of protofibrils and t-PA and plasminogen-mediated fibrinolysis, as well as several previously reported variants located in the αC-domain, and demonstrated the presence of disulfide-bonded molecules.

The interplay between tissue plasminogen activator domains and fibrin structures in the regulation of fibrinolysis: kinetic and microscopic studies

Regulation of tissue-type plasminogen activator (tPA) depends on fibrin binding and fibrin structure. tPA structure/function relationships were investigated in fibrin formed by high or low thrombin concentrations to produce a fine mesh and small pores, or thick fibers and coarse structure, respectively. Kinetics studies were performed to investigate plasminogen activation and fibrinolysis in the 2 types of fibrin, using wild-type tPA (F-G-K1-K2-P, F and K2 binding), K1K1-tPA (F-G-K1-K1-P, F binding), and delF-tPA (G-K1-K2-P, K2 binding). There was a trend of enzyme potency of tPA > K1K1-tPA > delF-tPA, highlighting the importance of the finger domain in regulating activity, but the differences were less apparent in fine fibrin. Fine fibrin was a better surface for plasminogen activation but more resistant to lysis. Scanning electron and confocal microscopy using orange fluorescent fibrin with green fluorescent protein-labeled tPA variants showed that tPA was strongly associated with agglomerates in coarse but not in fine fibrin. In later lytic stages, delF-tPA-green fluorescent protein diffused more rapidly through fibrin in contrast to full-length tPA, highlighting the importance of finger domain-agglomerate interactions. Thus, the regulation of fibrinolysis depends on the starting nature of fibrin fibers and complex dynamic interaction between tPA and fibrin structures that vary over time.

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.

Impaired fibrin gel permeability by high homocysteine levels

Mechanisms involved in the relationship between hyperhomocysteinemia and thrombosis are still unclear. In previous reports we have shown that high homocysteine concentrations led to more compact and branched fibrin networks than controls. These clots showed an impaired lysis associated to their architecture. The aim of this study was to evaluate the effects of homocysteine on permeation of clots obtained from plasma and purified systems. Fibrin gels were prepared with normal plasma incubated with homocysteine and, in the purified systems, with fibrinogen and factor XIII treated with the amino acid. Permeability constants (K(s)) were determined through flow measurements. Linear regression curve between K(s) values and homocysteine levels in the plasmatic assays showed a negative correlation coefficient, r = -0.997 (p = 0.003). K(s) of fibrin gels obtained from purified systems with fibrinogen incubated with homocysteine was (7.07 ± 0.27) × 10(-9) cm(2), control was (11.40 ± 0.37) × 10(-9) cm(2) (n = 3; p < 0.01). K(s) of fibrin gels obtained with factor XIII treated with homocysteine was (1.47 ± 0.17) × 10(-9) cm (2), and control was (3.31 ± 0.31) × 10(-9) cm(2) (n = 3; p<0.01). Plasma incubated with high homocysteine concentrations produced fibrin clots significantly less permeable than controls in a dose dependent manner, and the results showed that fibrinogen and factor XIII were involved in that detrimental effect. These findings might explain the impaired fibrinolysis related to increased homocysteine levels and contribute to understanding the association between the amino acid and thrombosis.

Simulated Point Mutations in the Aα-Chain of Human Fibrinogen Support a Role of the αC Domain in the Stabilization of Fibrin Gel

The hydrophobicity pattern distribution in the Aalpha-, Bbeta- and gamma-chains of human fibrinogen has been studied by a nonlinear method, recurrence quantification analysis, in the wild type and in a number of naturally occurring or simulated mutants. The aim was to find a structural basis for distinguishing between silent and pathological mutants. We were successful in the case of mutations on the Aalpha-chain, thanks to the peculiar features of this chain as compared to the other two. Relevant findings concerning the point mutants of the Aalpha-chain are the following: (a) the recurrence quantification analysis-based classification of such mutants is in good agreement with the clinical classification, and (b) the location of the mutated residue on the sequence plays a more relevant role than its hydrophobic features. Artificial point mutants in the terminal zone (600-866 residues) of the extended isoform of the Aalpha-chain cluster together with the natural hemorrhagic mutants of the first (1-207) residues.

Laboratory testing for fibrinogen abnormalities

Fibrinogen is essential for the formation of a fibrin clot. Acquired and congenital disorders of fibrinogen may result in decreased concentration or altered function of fibrinogen, often leading to an increased risk of bleeding. Routine coagulation testing and specialized laboratory investigations can guide diagnosis in patients suspected of having a fibrinogen abnormality. This article summarizes the types of laboratory assays that are used to assess fibrinogen disorders, and key abnormalities found in different types of fibrinogen disorders.

Common variation in the C-terminal region of the fibrinogen beta-chain: effects on fibrin structure, fibrinolysis and clot rigidity

Fibrinogen BbetaArg448Lys is a common polymorphism, positioned within the carboxyl terminus of the Bbeta-chain of the molecule. Studies suggest that it is associated with severity of coronary artery disease and development of stroke. The effects of the amino acid substitution on clot structure remains controversial, and the aim of this study was to investigate the effect(s) of this polymorphism on fibrin clot structure using recombinant techniques. Permeation, turbidity, and scanning electron microscopy showed that recombinant Lys448 fibrin had a significantly more compact structure, with thin fibers and small pores, compared with Arg448. Clot stiffness, measured by means of a novel method using magnetic tweezers, was significantly higher for the Lys448 compared with the Arg448 variant. Clots made from recombinant protein variants had similar lysis rates outside the plasma environment, but when added to fibrinogen-depleted plasma, the fibrinolysis rates for Lys448 were significantly slower compared with Arg448. This study demonstrates for the first time that clots made from recombinant BbetaLys448 fibrinogen are characterized by thin fibers and small pores, show increased stiffness, and appear more resistant to fibrinolysis. Fibrinogen BbetaArg448Lys is a primary example of common genetic variation with a significant phenotypic effect at the molecular level.

Stimulation of tPA-dependent provisional extracellular fibrin matrix degradation by human recombinant soluble melanotransferrin

Tissue-type plasminogen activator (tPA) and its substrate plasminogen (Plg) are key components in the fibrinolytic system. We have recently demonstrated, that truncated human recombinant soluble melanotransferrin (sMTf) could stimulate the activation of Plg by urokinase plasminogen activator and inhibit angiogenesis. Since various angiogenesis inhibitors were shown to stimulate tPA-mediated plasminogen activation, we examined the effects of sMTf on tPA-dependent fibrinolysis. This study demonstrated that sMTf enhanced tPA-activation of Plg by 6-fold. sMTf also increased the release of [125I]-fibrin fragments by tPA-activated plasmin. Moreover, we observed that the interaction of sMTf with Plg provoked a change in the fibrin clot structure by cleaving the fibrin alpha and beta chains. Overall, the present study shows that sMTf modulates tPA-dependent fibrinolysis by modifying the clot structure. These results also suggest that sMTf properties could involve enhanced dissolution of the provisional extracellular fibrin matrix.

The protein-binding and drug release properties of macromolecular conjugates containing daptomycin and dextran

Prototype daptomycin-dextran macromolecular conjugates were prepared in an attempt to modify the biodistribution and protein-binding properties of daptomycin. Synthesis of daptomycin macromolecular conjugates involved dextran activation, daptomycin-dextran coupling, and purification. The reaction mixtures were separated on a Sephadex G-100 column using 10% acetronitrile in water as a mobile phase. UV and fluorescence characteristics of high molecular weight fractions demonstrated imine product formation while the lower molecular weight fractions contained free daptomycin, imine, and anilide products. Daptomycin macromolecular conjugates were characterized by drug loading, drug release, and binding affinity for fibrinogen using HPLC analysis and surface plasmon resonance. Drug loading was calculated to be 160mg of daptomycin per gram of macromolecule. Approximately 9% of the conjugated daptomycin was released from the macromolecular conjugates in aqueous media in the pH range of 1-7.4. The conjugates possessed higher affinity for fibrinogen than that of daptomycin.

Fibrinogen Guarenas, an abnormal fibrinogen with an Aalpha-chain truncation due to a nonsense mutation at Aalpha 467 Glu (GAA)-->stop (TAA)

Fibrinogen Guarenas is a dysfibrinogenemia with a nonsense mutation at G4731T that causes an Aalpha-chain truncation at Ser 466. This abnormal fibrinogen is associated with a bleeding diathesis, severe in the proposita and mild in one brother, even though the fibrinogen levels in plasma are normal. All other family members are asymptomatic. Fibrinogens from the proposita and one family member, the mother of the proposita, both heterozygous for the mutation, were studied. Turbidity curves of fibrin polymerization showed that the lateral association of protofibrils was impaired and the maximum rate of polymerization was slightly diminished. The binding of albumin to fibrinogen was increased compared to control due to the presence of a free sulfhydryl group because of the missing disulphide bridge between Aalpha-Cys 442-472 in the mutated molecules. The abnormal fibrinogen formed much less alpha-polymer, and gamma-dimer formation was delayed compared to the control. Plasminogen activation by t-PA in the presence of fibrin was decreased. When Guarenas clots were perfused with fibrinolytic enzymes, clot degradation was retarded. Clot structure studied by confocal 3D microscopy showed that the fibrin network was dense, made up of thin and highly branched fibers, which accounted for the decreased flow rates by buffer permeation and increased rigidity of the fibrin clots, measured using a torsion pendulum. It seems that the increased clot rigidity, decreased porosity, hypofibrinolysis and t-PA induced fibrinolysis, by itself are not necessarily associated with thrombotic disorders in dysfibrinogenemia.

Modifications of flow measurement to determine fibrin gel permeability and the preliminary use in research and clinical materials

Our earlier investigations employing a flow measurement have yielded intriguing findings as to what governs the fibrin network porosity. To make the method suitable for use by more groups with various laboratory conditions, sample materials or study purposes, we simplified the essential equipment and thereby minimized the sample volume to 250 microl in comparison with the need for 3000 microl in the previous method. To assess whether the fibrin gel permeability depends on changes in thrombin generation potential and/or fibrinogen clotting property, different concentrations of thrombin with or without frozen-thawed platelets, serving as phospholipids, were used. The platelets and 0.05 IU/ml thrombin were added to plasma samples from patients with previous myocardial infarction. The fibrin gel permeability, expressed as Darcy constant (Ks), was decreased compared with that in controls, supporting findings about high risk of thromboembolism in this disease due to increases of thrombin activity and fibrinogen function. When 0.4 IU/ml thrombin was used in samples provided by 10 healthy individuals treated with acetysalicylic acid, Ks levels were increased during versus before therapy. Since almost no thrombin generation was found in the samples with the higher dose of exogenous thrombin, we considered that modifications in fibrinogen clotting property by acetysalicylic acid rendered the fibrin network more permeable. In summary, as the reproducibility remains satisfactory (coefficient of variation < 10%) despite aforementioned modifications in the equipment and reagents, any interested laboratory ought to be able to repeat the method. Assays of fibrin permeability in such a simple way may help to determine the fibrin clot stability in pathological/pharmacological studies, and probably serve as a tool to estimate thromboembolism risk in clinical materials, such as patients with cardiovascular diseases.

Biophysical characterization of fibrinogen Caracas I with an Aα-chain truncation at Aα-466 Ser

Fibrinogen Caracas I is a dysfibrinogenemia with a mild bleeding tendency; a novel nonsense mutation, in the gene coding the Aalpha-chain, identified in this study as G4731T, giving rise to a new stop codon at Aalpha-Glu 467. Fibrinogen from two family members, the mother and sister of the propositus, both heterozygous for the mutation were studied, analyzing clots made from both plasma and purified fibrinogen. Clot structure and properties were characterized by turbidity, permeation, scanning electron microscopy and rheological studies. Permeation through Caracas I plasma clots was decreased, consistent with the decreased final turbidity. As shown by scanning electron microscopy, plasma clots from the patients were composed of very thin fibers, with increased fibrin density and reduced pore size. Viscoelastic measurements revealed that fibrinogen Caracas I plasma clots were much stiffer and less subject to compaction. These results demonstrate a key role of the carboxyl-terminal alpha chains of fibrin in lateral aggregation during polymerization and reinforce the utility of studying plasma clots. It is important to point out that the biophysical studies with fibrinogen purified by two different methods yielded contradictory results, which can be accounted for by selective purification of certain molecular species as seen by two-dimensional electrophoresis.

Elevated prothrombin results in clots with an altered fiber structure: a possible mechanism of the increased thrombotic risk

Individuals with elevated prothrombin levels are at increased risk of venous thrombosis. To understand the mechanism behind this observation, we studied the effect of prothrombin concentration on thrombin generation and fibrin clot structure. The pattern of thrombin generation was directly related to the prothrombin level at all concentrations tested. From 0% to 300% of normal plasma levels of prothrombin, increasing the prothrombin concentration increased the initial rate, peak, and total amount of thrombin generated. Importantly, fibrin clot structure was also affected by the prothrombin concentration. Fibrin clots made from prothrombin concentrations less than 10% of plasma levels were weak and poorly formed. Fibrin clots made at 10% to 100% of plasma levels of prothrombin had similar fiber structures (mass-to-length ratio; mu). However, the fiber mass-to-length ratio decreased with increasing prothrombin levels more than 100% of plasma levels, in a dose-dependent manner. These results suggest that increased levels of prothrombin alter thrombin generation and clot structure. Specifically, elevated prothrombin levels produce clots with reduced fibrin mass-to-length ratios compared with normal clots. We hypothesize that this alteration in fibrin clot structure is an important determinant of the risk of thrombosis.

Analyzing fibrin clot structure using a microplate reader

Fibrin clot structure studies are often performed using optical methods. For example, the clot's fiber structure can be assessed by measuring light scattering as a function of wavelength. From these measurements, one can calculate the mass/length ratio (mu), a relative measure of fibrin thickness. Fiber thickness has important functional correlates in terms of clot stability and resistance to fibrinolysis. Typically, measurements to calculate mass/length ratios are carried out on high-end spectrophotometers. However, limitations of this instrument include the large sample volume required and the inability to read multiple samples at one time. To circumvent these limitations, a plate-reading spectrophotometer is more commonly used to monitor clot formation; increases in absorbance indicate clot formation, while decreases indicate clot lysis. However, it is unclear whether plate-reading spectrophotometers can be used to quantitatively evaluate fibrin fiber structure. In the current study, we compared spectrophotometric analysis of fibrin gels on single-sample and plate-reading spectrophotometers. Results show that a plate-reading spectrophotometer does not give accurate measurements of the fiber mass/length ratio. However, the plate-reading spectrophotometer can provide a qualitative measure of fiber structure for both purified fibrinogen and plasma. We suggest that plate-reading spectrophotometers can provide a convenient, rapid, and inexpensive means of analyzing fibrin clot structure.

A compendium of gene expression in normal human tissues

This study creates a compendium of gene expression in normal human tissues suitable as a reference for defining basic organ systems biology. Using oligonucleotide microarrays, we analyze 59 samples representing 19 distinct tissue types. Of approximately 7,000 genes analyzed, 451 genes are expressed in all tissue types and designated as housekeeping genes. These genes display significant variation in expression levels among tissues and are sufficient for discerning tissue-specific expression signatures, indicative of fundamental differences in biochemical processes. In addition, subsets of tissue-selective genes are identified that define key biological processes characterizing each organ. This compendium highlights similarities and differences among organ systems and different individuals and also provides a publicly available resource (Human Gene Expression Index, the HuGE Index, http://www.hugeindex.org) for future studies of pathophysiology.