Identification of an ordered compact structure within the recombinant bovine fibrinogen alphaC-domain fragment by NMR

The Story of the Fibrin(ogen) αC-Domains: Evolution of Our View on Their Structure and Interactions

Although much has been established concerning the overall structure and function of fibrinogen, much less has been known about its two αC regions, each consisting of an αC-connector and an αC-domain, but new information has been accumulating. This review summarizes the state of our current knowledge of the structure and interactions of fibrinogen's αC regions. A series of studies with isolated αC regions and their fragments demonstrated that the αC-domain forms compact ordered structures consisting of N- and C-terminal subdomains including β sheets and suggested that the αC-connector has a poly(L-proline) type II structure. Functionally, the αC-domains interact intramolecularly with each other and with the central region of the molecule, first demonstrated by electron microscopy and then quantified by optical trap force spectroscopy. Upon conversion of fibrinogen into fibrin, the αC-domains switch from intra- to intermolecular interactions to form ordered αC polymers. The formation of αC polymers occurs mainly through the homophilic interaction between the N-terminal subdomains; interaction between the C-terminal subdomains and the αC-connectors also contributes to this process. Considerable evidence supports the idea that the αC-regions accelerate fibrin polymerization and affect the final structure of fibrin clots. The interactions between αC-regions are important for the mechanical properties of clots, increasing their stiffness and extensibility. Conversion of fibrinogen into fibrin results in exposure of multiple binding sites in its αC regions, providing interaction of fibrin with different proteins and cell types during hemostasis and wound healing. This heretofore mysterious part of the fibrinogen molecule is finally giving up its secrets.

Development of Transient Recombinant Expression and Affinity Chromatography Systems for Human Fibrinogen

Fibrin forms the structural scaffold of blood clots and has great potential for biomaterial applications. Creating recombinant expression systems of fibrinogen, fibrin’s soluble precursor, would advance the ability to construct mutational libraries that would enable structure–function studies of fibrinogen and expand the utility of fibrin as a biomaterial. Despite these needs, recombinant fibrinogen expression systems, thus far, have relied on the time-consuming creation of stable cell lines. Here we present tests of a transient fibrinogen expression system that can rapidly generate yields of 8–12 mg/L using suspension HEK Expi293^TM cells. We report results from two different plasmid systems encoding the fibrinogen cDNAs and two different transfection reagents. In addition, we describe a novel, affinity-based approach to purifying fibrinogen from complex media such as human plasma. We show that using a high-affinity peptide which mimics fibrin’s knob ‘A’ sequence enables the purification of 50–75% of fibrinogen present in plasma. Having robust expression and purification systems of fibrinogen will enable future studies of basic fibrin(ogen) biology, while paving the way for the ubiquitous use of fibrin as a biomaterial.

Extension of the Human Fibrinogen Database with Detailed Clinical Information—The αC-Connector Segment

Fibrinogen, an abundant plasma glycoprotein, is involved in the final stage of blood coagulation. Decreased fibrinogen levels, which may be caused by mutations, are manifested mainly in bleeding and thrombotic disorders. Clinically relevant mutations of fibrinogen are listed in the Human Fibrinogen Database. For the αC-connector (amino acids Aα240–410, nascent chain numbering), we have extended this database, with detailed descriptions of the clinical manifestations among members of reported families. This includes the specification of bleeding and thrombotic events and results of coagulation assays. Where available, the impact of a mutation on clotting and fibrinolysis is reported. The collected data show that the Human Fibrinogen Database reports considerably fewer missense and synonymous mutations than the general COSMIC and dbSNP databases. Homozygous nonsense or frameshift mutations in the αC-connector are responsible for most clinically relevant symptoms, while heterozygous mutations are often asymptomatic. Symptomatic subjects suffer from bleeding and, less frequently, from thrombotic events. Miscarriages within the first trimester and prolonged wound healing were reported in a few subjects. All mutations inducing thrombotic phenotypes are located at the identical positions within the consensus sequence of the tandem repeats.

Fibrinogen αC-subregions critically contribute blood clot fibre growth, mechanical stability, and resistance to fibrinolysis

Fibrinogen is essential for blood coagulation. The C-terminus of the fibrinogen α-chain (αC-region) is composed of an αC-domain and αC-connector. Two recombinant fibrinogen variants (α390 and α220) were produced to investigate the role of subregions in modulating clot stability and resistance to lysis. The α390 variant, truncated before the αC-domain, produced clots with a denser structure and thinner fibres. In contrast, the α220 variant, truncated at the start of the αC-connector, produced clots that were porous with short, stunted fibres and visible fibre ends. These clots were mechanically weak and susceptible to lysis. Our data demonstrate differential effects for the αC-subregions in fibrin polymerisation, clot mechanical strength, and fibrinolytic susceptibility. Furthermore, we demonstrate that the αC-subregions are key for promoting longitudinal fibre growth. Together, these findings highlight critical functions of the αC-subregions in relation to clot structure and stability, with future implications for development of novel therapeutics for thrombosis.

Structure and function of fibrinogen BβN-domains

Two BβN-domains of fibrinogen are formed by the N-terminal portions of its two Bβ chains including amino acid residues Bβ1-65. Although their folding status is not well understood and the recombinant disulfide-linked (Bβ1-66)2 fragment corresponding to a pair of these domains was found to be unfolded, some data suggest that these domains may be folded in the parent molecule. In contrast, their major functional properties are well established. Removal of fibrinopeptides B (amino acid residues Bβ1-14) from these domains upon fibrinogen to fibrin conversion results in the exposure of multiple binding sites in fibrin βN-domains (residues β15-65). These sites provide interactions of the βN-domains with different proteins and cells and their participation in various physiological and pathological processes including fibrin assembly, fibrin-dependent angiogenesis, and fibrin-dependent leukocyte transmigration and thereby inflammation. The major goal of the present review is to summarize current view on the structure and function of these domains in fibrinogen and fibrin and their role in the above-mentioned processes.

Fibrinogen αC-regions are not directly involved in fibrin polymerization as evidenced by a "Double-Detroit" recombinant fibrinogen mutant and knobs-mimic peptides

BACKGROUND

Fibrin polymerization, following fibrinopeptides A and B (FpA, FpB) cleavage, relies on newly exposed α- and β-chains N-termini (GPR, GHR; A-, B-knobs, respectively) engaging preexistent a and b pockets in other fibrin(ogen) molecules' γ- and (B)β-chains C-terminal regions. A role for mostly disordered (A)α-chains C-terminal regions "bridging" between fibrin molecules/fibrils has been proposed.

OBJECTIVES

Fibrinogen Detroit is a clinically observed mutation (AαR19 → S) with nonengaging GPS A-knobs. By analogy, a similar Bβ-chain mutation, BβR17 → S, should produce nonengaging GHS B-knobs. A homozygous "Double-Detroit" mutant (AαR19 → S, BβR17 → S; DD-FG) was developed: with A-a and B-b engagements endogenously blocked, other interactions would become apparent.

METHODS

DD-FG, wild-type recombinant (WT-FG), and human plasma (hp-FG) fibrinogen self-association was studied by turbidimetry coupled with fibrinopeptides release high-performance liquid chromatography (HPLC)/mass spectrometry analyses, and by light-scattering following size-exclusion chromatography (SE-HPLC).

RESULTS

In contrast to WT-FG and hp-FG, DD-FG produced no turbidity increase, irrespective of thrombin concentration. The SE-HPLC profile of concentrated DD-FG was unaffected by thrombin treatment, and light-scattering, at lower concentration, showed no intensity and hydrodynamic radius changes. Compared with hp-FG, both WT-FG and DD-FG showed no FpA cleavage difference, while ~50% FpB was not recovered. Correspondingly, SDS-PAGE/Western-blots revealed partial Bβ-chain N-terminal and Aα-chain C-terminal degradation. Nevertheless, ~70% DD-FG molecules bearing (A)αC-regions potentially able to associate were available. Higher-concentration, nearly intact hp-FG with 500-fold molar excess GPRP-NH2 /GHRP-NH2 knobs-mimics experiments confirmed these no-association findings.

CONCLUSIONS

(A)αC-regions interactions appear too weak to assist native fibrin polymerization, at least without knobs engagement. Their role in all stages should be carefully reconsidered.

Adsorption of Fibrinogen on Silica Surfaces-The Effect of Attached Nanoparticles

When a biomaterial is inserted into the body, proteins rapidly adsorb onto its surface, creating a conditioning protein film that functions as a link between the implant and adhering cells. Depending on the nano-roughness of the surface, proteins will adsorb in different amounts, with different conformations and orientations, possibly affecting the subsequent attachment of cells to the surface. Thus, modifications of the surface nanotopography of an implant may prevent biomaterial-associated infections. Fibrinogen is of particular importance since it contains adhesion epitopes that are recognized by both eukaryotic and prokaryotic cells, and can therefore influence the adhesion of bacteria. The aim of this study was to model adsorption of fibrinogen to smooth or nanostructured silica surfaces in an attempt to further understand how surface nanotopography may affect the orientation of the adsorbed fibrinogen molecule. We used a coarse-grained model, where the main body of fibrinogen (visible in the crystal structure) was modeled as rigid and the flexible α C-chains (not visible in the crystal structure) were modeled as completely disordered. We found that the elongated fibrinogen molecule preferably adsorbs in such a way that it protrudes further into solution on a nanostructured surface compared to a flat one. This implicates that the orientation on the flat surface increases its bio-availability.

Oxidation-induced destabilization of the fibrinogen αC-domain dimer investigated by molecular dynamics simulations

Upon activation, fibrinogen is converted to insoluble fibrin, which assembles into long strings called protofibrils. These aggregate laterally to form a fibrin matrix that stabilizes a blood clot. Lateral aggregation of protofibrils is mediated by the αC domain, a partially structured fragment located in a disordered region of fibrinogen. Polymerization of αC domains links multiple fibrin molecules with each other enabling the formation of thick fibrin fibers and a fibrin matrix that is stable but can also be digested by enzymes. However, oxidizing agents produced during the inflammatory response have been shown to cause thinner fibrin fibers resulting in denser clots, which are harder to proteolyze and pose the risk of deep vein thrombosis and lung embolism. Oxidation of Met⁴⁷⁶ located within the αC domain is thought to hinder its ability to polymerize disrupting the lateral aggregation of protofibrils and leading to the observed thinner fibers. How αC domains assemble into polymers is still unclear and yet this knowledge would shed light on the mechanism through which oxidation weakens the lateral aggregation of protofibrils. This study used temperature replica exchange molecular dynamics simulations to investigate the αC-domain dimer and how this is affected by oxidation of Met⁴⁷⁶ . Analysis of the trajectories revealed that multiple stable binding modes were sampled between two αC domains while oxidation decreased the likelihood of dimer formation. Furthermore, the side chain of Met⁴⁷⁶ was observed to act as a docking spot for the binding and this function was impaired by its conversion to methionine sulfoxide.

Optimization of quantitative proteomic analysis of clots generated from plasma of patients with venous thromboembolism

Background

It is well known that fibrin network binds a large variety of proteins, including inhibitors and activators of fibrinolysis, which may affect clot properties, such as stability and susceptibility to fibrinolysis. Specific plasma clot composition differs between individuals and may change in disease states. However, the plasma clot proteome has not yet been in-depth analyzed, mainly due to technical difficulty related to the presence of a highly abundant protein—fibrinogen and fibrin that forms a plasma clot.

Methods

The aim of our study was to optimize quantitative proteomic analysis of fibrin clots prepared ex vivo from citrated plasma of the peripheral blood drawn from patients with prior venous thromboembolism (VTE). We used a multiple enzyme digestion filter aided sample preparation, a multienzyme digestion (MED) FASP method combined with LC–MS/MS analysis performed on a Proxeon Easy-nLC System coupled to the Q Exactive HF mass spectrometer. We also evaluated the impact of peptide fractionation with pipet-tip strong anion exchange (SAX) method on the obtained results.

Results

Our proteomic approach revealed 476 proteins repeatedly identified in the plasma fibrin clots from patients with VTE including extracellular vesicle-derived proteins, lipoproteins, fibrinolysis inhibitors, and proteins involved in immune responses. The MED FASP method using three different enzymes: LysC, trypsin and chymotrypsin increased the number of identified peptides and proteins and their sequence coverage as compared to a single step digestion. Peptide fractionation with a pipet-tip strong anion exchange (SAX) protocol increased the depth of proteomic analyses, but also extended the time needed for sample analysis with LC–MS/MS.

Conclusions

The MED FASP method combined with a label-free quantification is an excellent proteomic approach for the analysis of fibrin clots prepared ex vivo from citrated plasma of patients with prior VTE.

Electronic supplementary material

The online version of this article (10.1186/s12014-017-9173-x) contains supplementary material, which is available to authorized users.

Morphometric characterization of fibrinogen's αC regions and their role in fibrin self-assembly and molecular organization

The flexible C-terminal parts of fibrinogen's Aα chains named the αC regions have been shown to play a role in fibrin self-assembly, although many aspects of their structure and functions remain unknown. To examine the involvement of the αC regions in the early stages of fibrin formation, we used high-resolution atomic force microscopy to image fibrinogen and oligomeric fibrin. Plasma-purified full-length human fibrinogen or des-αC fibrinogen lacking most of the αC regions, untreated or treated with thrombin, was imaged. Up to 80% of the potentially existing αC regions were visualized and quantified; they were highly heterogeneous in their length and configurations. Conversion of fibrinogen to fibrin was accompanied by an increase in the incidence and length of the αC regions as well as transitions from more compact conformations, such as a globule on a string, to extended and more flexible offshoots. Concurrent dynamic turbidimetry, confocal microscopy, and scanning electron microscopy revealed that trimming of the αC regions slowed down fibrin formation, which correlated with longer protofibrils, thinner fibers, and a denser network. No structural distinctions, except for the incidence of the αC regions, were revealed in the laterally aggregated protofibrils made of the full-length or des-αC fibrinogens, suggesting a pure kinetic effect of the αC regions on the fibrin architecture. This work provides a structural molecular basis for the promoting role of the αC regions in the early stages of fibrin self-assembly and reveals this stage of fibrin formation as a potential therapeutic target to modulate the structure and mechanical properties of blood clots.

Leptospira Immunoglobulin-Like Protein B (LigB) Binds to Both the C-Terminal 23 Amino Acids of Fibrinogen αC Domain and Factor XIII: Insight into the Mechanism of LigB-Mediated Blockage of Fibrinogen α Chain Cross-Linking

The coagulation system provides a primitive but effective defense against hemorrhage. Soluble fibrinogen (Fg) monomers, composed of α, β and γ chains, are recruited to provide structural support for the formation of a hemostatic plug. Fg binds to platelets and is processed into a cross-linked fibrin polymer by the enzymatic clotting factors, thrombin and Factor XIII (FXIII). The newly formed fibrin-platelet clot can act as barrier to protect against pathogens from entering the bloodstream. Further, injuries caused by bacterial infections can be confined to the initial wound site. Many pathogenic bacteria have Fg-binding adhesins that can circumvent the coagulation pathway and allow the bacteria to sidestep containment. Fg expression is upregulated during lung infection providing an attachment surface for bacteria with the ability to produce Fg-binding adhesins. Fg binding by leptospira might play a crucial factor in Leptospira-associated pulmonary hemorrhage, the main factor contributing to lethality in severe cases of leptospirosis. The 12^th domain of Leptospira immunoglobulin-like protein B (LigB12), a leptospiral adhesin, interacts with the C-terminus of FgαC (FgαCC). In this study, the binding site for LigB12 was mapped to the final 23 amino acids at the C-terminal end of FgαCC (FgαCC8). The association of FgαCC8 with LigB12 (ELISA, K_D = 0.76 μM; SPR, K_D = 0.96 μM) was reduced by mutations of both charged residues (R608, R611 and H614 from FgαCC8; D1061 from LigB12) and hydrophobic residues (I613 from FgαCC8; F1054 and A1065 from LigB12). Additionally, LigB12 bound strongly to FXIII and also inhibited fibrin formation, suggesting that LigB can disrupt coagulation by suppressing FXIII activity. Here, the detailed binding mechanism of a leptospiral adhesin to a host hemostatic factor is characterized for the first time and should provide better insight into the pathogenesis of leptospirosis.

Leptospirosis, caused by pathogenic Leptospira spp., has been increasingly recognized as an emerging zoonosis worldwide. In human cases, clinical presentation can vary from a mild flu-like syndrome to severe multi-organ failure including hepatitis, nephritis and occasionally meningitis. Particularly, pulmonary hemorrhage has become one of the major factors leading to fatality. The host coagulation system normally can be activated to confine damage caused by bacteria. However, this spirochete has developed several virulence proteins to manipulate hemostatic factors including fibrinogen (Fg). Previously, we had observed that Leptospira immunoglobulin-like protein B (LigB) can bind to Fg and inhibit fibrin clot formation. In this study, the LigB binding site on fibrinogen was fine-mapped. The key amino acids contributing to this strong pathogen-host interaction were also identified. In addition, LigB bound to factor XIII and further interfered with the cross-linking of Fg. For the first time, a potential mechanism of leptospiral adhesin binding to fibrinogen was revealed, which should provide a better understanding of the pathogenesis of leptospirosis.

Structural properties of fracture haematoma: current status and future clinical implications

Blood clots (haematomas) that form immediately following a bone fracture have been shown to be vital for the subsequent healing process. During the clotting process, a number of factors can influence the fibrin clot structure, such as fibrin polymerization, growth factor binding, cellular infiltration (including platelet retraction), protein concentrations and cytokines. The modulation of the fibrin clot structure within the fracture site has important clinical implications and could result in the development of multifunctional scaffolds that mimic the natural structure of a haematoma. Artificial haematoma structures such as these can be created from the patient's own blood and can therefore act as an ideal bone defect filling material for potential clinical application to accelerate bone regeneration. Copyright © 2016 John Wiley & Sons, Ltd.

Molecular Dynamics Simulations of the Initial Adsorption Stages of Fibrinogen on Mica and Graphite Surfaces

Fibrinogen, a blood glycoprotein of vertebrates, plays an essential role in blood clotting by polymerizing into fibrin when activated. Upon adsorption on material surfaces, it also contributes to determine their biocompatibility and has been implicated in the onset of thrombosis and inflammation at medical implants. Here we present the first fully atomistic simulations of the initial stages of the adsorption process of fibrinogen on mica and graphite surfaces. The simulations reveal a weak adsorption on mica that allows frequent desorption and reorientation events. This adsorption is driven by electrostatic interactions between the protein and the silicate surface as well as the counterion layer. Preferred adsorption orientations for the globular regions of the protein are identified. The adsorption on graphite is found to be stronger with fewer reorientation and desorption events and shows the onset of denaturation of the protein.

The Internal Dynamics of Fibrinogen and Its Implications for Coagulation and Adsorption

Fibrinogen is a serum multi-chain protein which, when activated, aggregates to form fibrin, one of the main components of a blood clot. Fibrinolysis controls blood clot dissolution through the action of the enzyme plasmin, which cleaves fibrin at specific locations. Although the main biochemical factors involved in fibrin formation and lysis have been identified, a clear mechanistic picture of how these processes take place is not available yet. This picture would be instrumental, for example, for the design of improved thrombolytic or anti-haemorrhagic strategies, as well as, materials with improved biocompatibility. Here, we present extensive molecular dynamics simulations of fibrinogen which reveal large bending motions centered at a hinge point in the coiled-coil regions of the molecule. This feature, likely conserved across vertebrates according to our analysis, suggests an explanation for the mechanism of exposure to lysis of the plasmin cleavage sites on fibrinogen coiled-coil region. It also explains the conformational variability of fibrinogen observed during its adsorption on inorganic surfaces and it is supposed to play a major role in the determination of the hydrodynamic properties of fibrinogen. In addition the simulations suggest how the dynamics of the D region of fibrinogen may contribute to the allosteric regulation of the blood coagulation cascade through a dynamic coupling between the a- and b-holes, important for fibrin polymerization, and the integrin binding site P1.

Fibrinogen, a protein found in the blood of vertebrates, when activated, aggregates and forms fibrin fibers, the basis of a blood clot. Clots are broken down by the enzyme plasmin, which cuts fibrin fibers at specific places, thus helping the regulation of clot persistence. A mechanistic understanding of fibrin degradation by plasmin is still missing. An important determinant of this process might be the flexibility of fibrinogen. The flexible nature of fibrinogen is reported, for example, by the great variety of conformations observed when fibrinogen adsorbs on material surfaces. However, limits in the spatial resolution of these experiments preclude the identification of the atomistic mechanism behind this flexibility. Here, we perform computer simulations that help identifying with atomistic detail large bending motions occurring at a specific hinge on the molecule. We show how these bending motions can explain the variable conformations observed in experiments and how they help exposing sites where plasmin can cut fibrinogen. Furthermore, our simulations let us identify cooperative effects involving several distant parts of fibrinogen that may play a role in the assembly of fibrin fibers. Both the bending and the cooperative effects, thus, represent potential mechanisms for the regulation of blood clotting.

Visualization of fibrinogen αC regions and their arrangement during fibrin network formation by high-resolution AFM

BACKGROUND

Fibrinogen has been intensively studied with transmission electron microscopy and x-ray diffraction. But until now, a complete 3D structure of the molecule has not yet been available because the two highly flexible αC regions could not be resolved in fibrinogen crystals. This study was aimed at determining whether the αC regions can be visualized by high-resolution atomic force microscopy.

METHODS

Atomic force microscopy with super high resolution was used to image single molecules of fibrinogen and fibrin associates. The key approach was to use a graphite surface modified with the monolayer of amphiphilic carbohydrate-glycine molecules and unique supersharp cantilevers with 1 nm tip diameter.

RESULTS

Fibrinogen αC regions were visualized along with the complete domain structure of the protein. In almost all molecules at pH 7.4 the D domain regions had one or two protrusions of average height 0.4 ± 0.1 nm and length 21 ± 6 nm. The complex, formed between thrombin and fibrinogen, was also visualized. Images of growing fibrin fibers with clearly visible αC regions have been obtained.

CONCLUSIONS

Fibrin αC regions were visible in protofibrils and large fibers; αC regions intertwined near a branchpoint and looked like a zipper. These results support the idea that αC regions are involved in the thickening of fibrin fibers. In addition, new details were revealed about the behavior of individual fibrin molecules during formation of the fibrin network. Under the diluted condition, the positioning of the αC regions could suggest their involvement in long-range interactions between fibrin but not fibrinogen molecules.

Structural insights into fibrinogen dynamics using amide hydrogen/deuterium exchange mass spectrometry

We determined the amide hydrogen/deuterium exchange profile of native human fibrinogen under physiologic conditions. After optimization of the quench and proteolysis conditions, more than 1,200 peptides were identified by mass spectrometry, spanning more than 90% of the constituent Aα, Bβ, and γ chain amino acid sequences. The compact central and distal globular regions of fibrinogen were well protected from deuterium exchange, with the exception of the unfolded amino-terminal segments of the Aα and Bβ chains extending from the central region, and the short γ chain "tail" extending from each distal globular region. The triple-helical coiled-coil regions, which bridge the central region to each distal region, were also well protected with the exception of a moderately fast-exchanging area in the middle of each coiled-coil adjacent to the γ chain carbohydrate attachment site. These dynamic regions appear to provide flexibility to the fibrinogen molecule. The γ chain "out loop" contained within each coiled-coil also exchanged rapidly. The αC domain (Aα 392-610) exchanged rapidly, with the exception of a short segment sandwiched between a conserved disulfide linkage in the N-terminal αC subdomain. This latter finding is consistent with a mostly disordered structure for the αC domain in native fibrinogen. Analysis of the dysfibrinogen Bβ 235 Pro/Leu, which exhibits abnormal fibrin structure, revealed enhanced deuterium exchange surrounding the Pro/Leu substitution site as well as in the vicinity of the high affinity calcium binding site and the A knob polymerization pocket within the γC domain. The implication of these changes with respect to fibrin structure is discussed.

Hydrodynamic characterization of recombinant human fibrinogen species

INTRODUCTION

Fibrinogen is a key component of the blood coagulation system and plays important, diverse roles in several relevant pathologies such as thrombosis, hemorrhage, and cancer. It is a large glycoprotein whose three-dimensional molecular structure is not fully known. Furthermore, circulating fibrinogen is highly heterogeneous, mainly due to proteolytic degradation and alternative mRNA processing. Recombinant production of human fibrinogen allows investigating the impact on the three-dimensional structure of specific changes in the primary structure.

METHODS

We performed analytical ultracentrifugation analyses of a full-length recombinant human fibrinogen, its counterpart purified from human plasma, and a recombinant human fibrinogen with both Aα chains truncated at amino acid 251, thus missing their last 359 amino acid residues.

RESULTS

We have accurately determined the translational diffusion and sedimentation coefficients (Dt(20,w)(0), s(20,w)(0)) of all three species. This was confirmed by derived molecular weights within 1% for the full length species, and 5% for the truncated species, as assessed by comparison with SDS-PAGE/Western blot analyses and primary structure data. No significant differences in the values of Dt(20,w)(0) and s(20,w)(0) were found between the recombinant and purified full length human fibrinogens, while slightly lower and higher values, respectively, resulted for the recombinant truncated human fibrinogen compared to a previously characterized purified human fibrinogen fragment X obtained by plasmin digestion.

CONCLUSIONS

Full-length recombinant fibrinogen is less polydisperse but hydrodynamically indistinguishable from its counterpart purified from human plasma. Recombinant Aα251-truncated human fibrinogen instead behaves differently from fragment X, suggesting a role for the Bβ residues 1-52 in inter-molecular interactions. Overall, these new hydrodynamic data will constitute a reliable benchmark against which models of fibrinogen species could be compared.

The assembly of nonadhesive fibrinogen matrices depends on the αC regions of the fibrinogen molecule

Adsorption of fibrinogen on fibrin clots and other surfaces strongly reduces integrin-mediated adhesion of platelets and leukocytes with implications for the surface-mediated control of thrombus growth and blood compatibility of biomaterials. The underlying mechanism of this process is surface-induced aggregation of fibrinogen, resulting in the assembly of a nanoscale multilayered matrix. The matrix is extensible, which makes it incapable of transducing strong mechanical forces via cellular integrins, resulting in insufficient intracellular signaling and weak cell adhesion. To determine the mechanism of the multilayer formation, the physical and adhesive properties of fibrinogen matrices prepared from human plasma fibrinogen (hFg), recombinant normal (rFg), and fibrinogen with the truncated αC regions (FgAα251) were compared. Using atomic force microscopy and force spectroscopy, we show that whereas hFg and rFg generated the matrices with a thickness of ∼8 nm consisting of 7-8 molecular layers, the deposition of FgAα251 was terminated at two layers, indicating that the αC regions are essential for the multilayer formation. The extensibility of the matrix prepared from FgAα251 was 2-fold lower than that formed from hFg and rFg. In agreement with previous findings that cell adhesion inversely correlates with the extensibility of the fibrinogen matrix, the less extensible FgAα251 matrix and matrices generated from human fibrinogen variants lacking the αC regions supported sustained adhesion of leukocytes and platelets. The persistent adhesiveness of matrices formed from fibrinogen derivatives without the αC regions may have implications for conditions in which elevated levels of these molecules are found, including vascular pathologies, diabetes, thrombolytic therapy, and dysfibrinogenemia.

A modular fibrinogen model that captures the stress-strain behavior of fibrin fibers

We tested what to our knowledge is a new computational model for fibrin fiber mechanical behavior. The model is composed of three distinct elements: the folded fibrinogen core as seen in the crystal structure, the unstructured α-C connector, and the partially folded α-C domain. Previous studies have highlighted the importance of all three regions and how they may contribute to fibrin fiber stress-strain behavior. Yet no molecular model has been computationally tested that takes into account the individual contributions of all these regions. Constant velocity, steered molecular dynamics studies at 0.025 Å/ps were conducted on the folded fibrinogen core and the α-C domain to determine their force-displacement behavior. A wormlike chain model with a persistence length of 0.8 nm (Kuhn length = 1.6 nm) was used to model the mechanical behavior of the unfolded α-C connector. The three components were combined to calculate the total stress-strain response, which was then compared to experimental data. The results show that the three-component model successfully captures the experimentally determined stress-strain behavior of fibrin fibers. The model evinces the key contribution of the α-C domains to fibrin fiber stress-strain behavior. However, conversion of the α-helical coiled coils to β-strands, and partial unfolding of the protein, may also contribute.

On the mechanism of αC polymer formation in fibrin

Our previous studies revealed that the fibrinogen αC-domains undergo conformational changes and adopt a physiologically active conformation upon their self-association into αC polymers in fibrin. In the present study, we analyzed the mechanism of αC polymer formation and tested our hypothesis that self-association of the αC-domains occurs through the interaction between their N-terminal subdomains and may include β-hairpin swapping. Our binding experiments performed by size-exclusion chromatography and optical trap-based force spectroscopy revealed that the αC-domains self-associate exclusively through their N-terminal subdomains, while their C-terminal subdomains were found to interact with the αC-connectors that tether the αC-domains to the bulk of the molecule. This interaction should reinforce the structure of αC polymers and provide the proper orientation of their reactive residues for efficient cross-linking by factor XIIIa. Molecular modeling of self-association of the N-terminal subdomains confirmed that the hypothesized β-hairpin swapping does not impose any steric hindrance. To "freeze" the conformation of the N-terminal subdomain and prevent the hypothesized β-hairpin swapping, we introduced by site-directed mutagenesis an extra disulfide bond between two β-hairpins of the bovine Aα406-483 fragment corresponding to this subdomain. The experiments performed by circular dichroism revealed that Aα406-483 mutant containing Lys429Cys/Thr463Cys mutations preserved its β-sheet structure. However, in contrast to wild-type Aα406-483, this mutant had lower tendency for oligomerization, and its structure was not stabilized upon oligomerization, in agreement with the above hypothesis. On the basis of the results obtained and our previous findings, we propose a model of fibrin αC polymer structure and molecular mechanism of assembly.

Factor XIIa regulates the structure of the fibrin clot independently of thrombin generation through direct interaction with fibrin

Recent data indicate an important contribution of coagulation factor (F)XII to in vivo thrombus formation. Because fibrin structure plays a key role in clot stability and thrombosis, we hypothesized that FXII(a) interacts with fibrin(ogen) and thereby regulates clot structure and function. In plasma and purified system, we observed a dose-dependent increase in fibrin fiber density and decrease in turbidity, reflecting a denser structure, and a nonlinear increase in clot stiffness with FXIIa. In plasma, this increase was partly independent of thrombin generation, as shown in clots made in prothrombin-deficient plasma initiated with snake venom enzyme and in clots made from plasma deficient in FXII and prothrombin. Purified FXII and α-FXIIa, but not β-FXIIa, bound to purified fibrinogen and fibrin with nanomolar affinity. Immunostaining of human carotid artery thrombi showed that FXII colocalized with areas of dense fibrin deposition, providing evidence for the in vivo modulation of fibrin structure by FXIIa. These data demonstrate that FXIIa modulates fibrin clot structure independently of thrombin generation through direct binding of the N-terminus of FXIIa to fibrin(ogen). Modification of fibrin structure by FXIIa represents a novel physiologic role for the contact pathway that may contribute to the pathophysiology of thrombosis.

Structure, stability, and interaction of fibrin αC-domain polymers

Our previous studies revealed that in fibrinogen the αC-domains are not reactive with their ligands, suggesting that their binding sites are cryptic and become exposed upon its conversion to fibrin, in which these domains form αC polymers. On the basis of this finding, we hypothesized that polymerization of the αC-domains in fibrin results in the exposure of their binding sites and that these domains adopt the physiologically active conformation only in αC-domain polymers. To test this hypothesis, we prepared a recombinant αC region (residues Aα221-610) including the αC-domain (Aα392-610), demonstrated that it forms soluble oligomers in a concentration-dependent and reversible manner, and stabilized such oligomers by covalently cross-linking them with factor XIIIa. Cross-linked Aα221-610 oligomers were stable in solution and appeared as ordered linear, branching filaments when analyzed by electron microscopy. Spectral studies revealed that the αC-domains in such oligomers were folded into compact structures of high thermal stability with a significant amount of β-sheets. These findings indicate that cross-linked Aα221-610 oligomers are highly ordered and mimic the structure of fibrin αC polymers. The oligomers also exhibited functional properties of polymeric fibrin because, in contrast to the monomeric αC-domain, they bound tPA and plasminogen and stimulated activation of the latter by the former. Altogether, the results obtained with cross-linked Aα221-610 oligomers clarify the structure of the αC-domains in fibrin αC polymers and confirm our hypothesis that their binding sites are exposed upon polymerization. Such oligomers represent a stable, soluble model of fibrin αC polymers that can be used for further structure-function studies of fibrin αC-domains.

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

The formation of fibrin clots that are relatively resistant to lysis represents the final step in blood coagulation. We discuss the genetic and environmental regulators of fibrin structure in relation to thrombotic disease. In addition, we discuss the implications of fibrin structure for treatment of thrombosis. Fibrin clots composed of compact, highly branched networks with thin fibers are resistant to lysis. Altered fibrin structure has consistently been reported in patients with several diseases complicated by thromboembolic events, including patients with acute or prior myocardial infarction, ischemic stroke, and venous thromboembolism. Relatives of patients with myocardial infarction or venous thromboembolism display similar fibrin abnormalities. Low-dose aspirin, statins, lowering of homocysteine, better diabetes control, smoking cessation, and suppression of inflammatory response increase clot permeability and susceptibility to lysis. Growing evidence indicates that abnormal fibrin properties represent a novel risk factor for arterial and venous thrombotic events, particularly of unknown etiology in young and middle-aged patients.

Comparative NMR analysis of an 80-residue G protein-coupled receptor fragment in two membrane mimetic environments

Fragments of integral membrane proteins have been used to study the physical chemical properties of regions of transporters and receptors. Ste2p(G31-T110) is an 80-residue polypeptide which contains a portion of the N-terminal domain, transmembrane domain 1 (TM1), intracellular loop 1, TM2 and part of extracellular loop 1 of the α-factor receptor (Ste2p) from Saccharomyces cerevisiae. The structure of this peptide was previously determined to form a helical hairpin in lyso-palmitoylphosphatidyl-glycerol micelles (LPPG) [1]. Herein, we perform a systematic comparison of the structure of this protein fragment in micelles and trifluoroethanol (TFE):water in order to understand whether spectra recorded in organic:aqueous medium can facilitate the structure determination in a micellar environment. Using uniformly labeled peptide and peptide selectively protonated on Ile, Val and Leu methyl groups in a perdeuterated background and a broad set of 3D NMR experiments we assigned 89% of the observable atoms. NOEs and chemical shift analysis were used to define the helical regions of the fragment. Together with constraints from paramagnetic spin labeling, NOEs were used to calculate a transiently folded helical hairpin structure for this peptide in TFE:water. Correlation of chemical shifts was insufficient to transfer assignments from TFE:water to LPPG spectra in the absence of further information.

The molecular origins of the mechanical properties of fibrin

When normal blood circulation is compromised by damage to vessel walls, clots are formed at the site of injury. These clots prevent bleeding and support wound healing. To sustain such physiological functions, clots are remarkably extensible and elastic. Fibrin fibers provide the supporting framework of blood clots, and the properties of these fibers underlie the mechanical properties of clots. Recent studies, which examined individual fibrin fibers or cylindrical fibrin clots, have shown that the mechanical properties of fibrin depend on the mechanical properties of the individual fibrin monomers. Within the fibrin monomer, three structures could contribute to these properties: the coiled-coil connectors the folded globular nodules and the relatively unstructured αC regions. Experimental data suggest that each of these structures contributes. Here we review the recent work with a focus on the molecular origins of the remarkable biomechanical properties of fibrin clots.

Role of fibrin structure in thrombosis and vascular disease

Fibrin clot formation is a key event in the development of thrombotic disease and is the final step in a multifactor coagulation cascade. Fibrinogen is a large glycoprotein that forms the basis of a fibrin clot. Each fibrinogen molecule is comprised of two sets of Aα, Bβ, and γ polypeptide chains that form a protein containing two distal D regions connected to a central E region by a coiled-coil segment. Fibrin is produced upon cleavage of the fibrinopeptides by thrombin, which can then form double-stranded half staggered oligomers that lengthen into protofibrils. The protofibrils then aggregate and branch, yielding a three-dimensional clot network. Factor XIII, a transglutaminase, cross-links the fibrin stabilizing the clot protecting it from mechanical stress and proteolytic attack. The mechanical properties of the fibrin clot are essential for its function as it must prevent bleeding but still allow the penetration of cells. This viscoelastic property is generated at the level of each individual fiber up to the complete clot. Fibrinolysis is the mechanism of clot removal, and involves a cascade of interacting zymogens and enzymes that act in concert with clot formation to maintain blood flow. Clots vary significantly in structure between individuals due to both genetic and environmental factors and this has an effect on clot stability and susceptibility to lysis. There is increasing evidence that clot structure is a determinant for the development of disease and this review will discuss the determinants for clot structure and the association with thrombosis and vascular disease.

Fibrinogen triggers astrocyte scar formation by promoting the availability of active TGF-beta after vascular damage

Scar formation in the nervous system begins within hours after traumatic injury and is characterized primarily by reactive astrocytes depositing proteoglycans that inhibit regeneration. A fundamental question in CNS repair has been the identity of the initial molecular mediator that triggers glial scar formation. Here we show that the blood protein fibrinogen, which leaks into the CNS immediately after blood-brain barrier (BBB) disruption or vascular damage, serves as an early signal for the induction of glial scar formation via the TGF-beta/Smad signaling pathway. Our studies revealed that fibrinogen is a carrier of latent TGF-beta and induces phosphorylation of Smad2 in astrocytes that leads to inhibition of neurite outgrowth. Consistent with these findings, genetic or pharmacologic depletion of fibrinogen in mice reduces active TGF-beta, Smad2 phosphorylation, glial cell activation, and neurocan deposition after cortical injury. Furthermore, stereotactic injection of fibrinogen into the mouse cortex is sufficient to induce astrogliosis. Inhibition of the TGF-beta receptor pathway abolishes the fibrinogen-induced effects on glial scar formation in vivo and in vitro. These results identify fibrinogen as a primary astrocyte activation signal, provide evidence that deposition of inhibitory proteoglycans is induced by a blood protein that leaks in the CNS after vasculature rupture, and point to TGF-beta as a molecular link between vascular permeability and scar formation.

[Molecular mechanisms of the polymerization of fibrin and the formation of its three-dimensional network]

The results of biochemical, immunochemical, and X-ray studies of the structures of fibrinogen and fibrin molecules were analyzed. The mechanisms of the successive formation of the fibrin three-dimensional network were described: the polymerization of monomeric molecules with the formation of bifilar protofibrils, the lateral association of protofibrils, and the embranchment of the forming fibrils. Data on the electron and confocal microscopy of the polymeric fibrin were considered. The role of the known polymerization centers of fibrin which participated in the formation of protofibrils and their lateral association was discussed. Data on the existence of the previously unknown polymerization centers were given. In particular, the experimental results demonstrated that one of such centers which participated in the formation of protofibrils was located in the Bbeta12-46 fragment, and did not require the cleavage of fibrinopeptide B for its functioning. The results of the computer modeling of the spatial structure of the fibrin(ogen) molecule and the intermolecular interactions in the course of the fibrin polymerization were presented. The location of the alphaC domains in the fibrin(ogen) molecule and their role in the polymerization process were discussed. Information on the structure of the calcium-binding sites of fibrin(ogen) and the functional role of Ca2+ in fibrin polymerization was published. The structure of factor XIII(a) and the mechanisms of fibrin stabilization by this factor were briefly described.

Structure, stability, and interaction of the fibrin(ogen) alphaC-domains

Our recent study established the NMR structure of the recombinant bAalpha406-483 fragment corresponding to the NH(2)-terminal half of the bovine fibrinogen alphaC-domain and revealed that at increasing concentrations this fragment forms oligomers (self-associates). The major goals of the study presented here were to determine the structure and self-association of the full-length human fibrinogen alphaC-domains. To accomplish these goals, we prepared a recombinant human fragment, hAalpha425-503, homologous to bovine bAalpha406-483, and demonstrated using NMR, CD, and size-exclusion chromatography that its overall fold and ability to form oligomers are similar to those of bAalpha406-483. We also prepared recombinant hAalpha392-610 and bAalpha374-568 fragments corresponding to the full-length human and bovine alphaC-domains, respectively, and tested their structure, stability, and ability to self-associate. Size-exclusion chromatography revealed that both fragments form reversible oligomers in a concentration-dependent manner. Their oligomerization was confirmed in sedimentation equilibrium experiments, which also established the self-association affinities of these fragments and revealed that the addition of each monomer to assembling alphaC-oligomers substantially increases the stabilizing free energy. In agreement, unfolding experiments monitored by CD established that self-association of both fragments results in a significant increase in their thermal stability. Analysis of CD spectra of both fragments revealed that alphaC self-association results in an increase in the level of regular structure, implying that the COOH-terminal half of the alphaC-domain adopts an ordered conformation in alphaC-oligomers and that this domain contains two independently folded subdomains. Altogether, these data further clarify the structure of the human and bovine alphaC-domains and the molecular mechanism of their self-association into alphaC-polymers in fibrin.

Hydrodynamic and mass spectrometry analysis of nearly-intact human fibrinogen, chicken fibrinogen, and of a substantially monodisperse human fibrinogen fragment X

The shape and solution properties of fibrinogen are affected by the location of the C-terminal portion of the Aalpha chains, which is presently still controversial. We have measured the hydrodynamic properties of a human fibrinogen fraction with these appendages mostly intact, of chicken fibrinogen, where they lack 11 characteristic 13-amino acids repeats, and of human fragment X, a plasmin early degradation product in which they have been removed. The human fibrinogen/fragment X samples were extensively characterized by SDS-PAGE/Western blotting and mass spectrometry, allowing their composition to be precisely determined. The solution properties of all samples were then investigated by analytical ultracentrifugation and size-exclusion HPLC coupled with multi-angle light scattering and differential pressure viscometry detectors. The measured parameters suggest that the extra repeats have little influence on the overall fibrinogen conformation, while a significant change is brought about by the removal of the C-terminal portion of the Aalpha chains beyond residue Aalpha200.

Crystal structure of human fibrinogen

A crystal structure of human fibrinogen has been determined at approximately 3.3 A resolution. The protein was purified from human blood plasma, first by a cold ethanol precipitation procedure and then by stepwise chromatography on DEAE-cellulose. A product was obtained that was homogeneous on SDS-polyacrylamide gels. Nonetheless, when individual crystals used for X-ray diffraction were examined by SDS gel electrophoresis after data collection, two species of alpha chain were present, indicating that some proteolysis had occurred during the course of operations. Amino-terminal sequencing on post-X-ray crystals showed mostly intact native alpha- and gamma-chain sequences (the native beta chain is blocked). The overall structure differs from that of a native fibrinogen from chicken blood and those reported for a partially proteolyzed bovine fibrinogen in the nature of twist in the coiled-coil regions, likely due to weak forces imparted by unique crystal packing. As such, the structure adds to the inventory of possible conformations that may occur in solution. Other features include a novel interface with an antiparallel arrangement of beta chains and a unique tangential association of coiled coils from neighboring molecules. The carbohydrate groups attached to beta chains are unusually prominent, the full sweep of 11 sugar residues being positioned. As was the case for native chicken fibrinogen, no resolvable electron density could be associated with alphaC domains.

Identification of the Staphylococcus aureus MSCRAMM clumping factor B (ClfB) binding site in the alphaC-domain of human fibrinogen

Clumping factor B (ClfB) of Staphylococcus aureus binds to cytokeratin 10 and to fibrinogen. In this study the binding site in human fibrinogen was localized to a short region within the C terminus of the Aalpha-chain. ClfB only bound to the Aalpha-chain of fibrinogen in a ligand-affinity blot and in solid-phase assays with purified recombinant fibrinogen chains. A variant of fibrinogen with wild-type Bbeta- and gamma-chains but with a deletion that lacked the C-terminal residues from 252-610 of the Aalpha-chain did not support adherence of S. aureus Newman expressing ClfB. A series of truncated mutants of the recombinant Aalpha-chain were tested for their ability to support adherence of S. aureus Newman ClfB(+), which allowed the binding site to be localized to a short segment of the unfolded flexible repeated sequence within the C terminus of the Aalpha-chain. This was confirmed by two amino acid substititions within repeat 5 of the recombinant Aalpha-chain which did not support adherence of Newman ClfB(+). Lactococcus lactis expressing ClfB mutants with amino acid substitutions (N256 and Q235) located in the putative ligand-binding trench between domains N2 and N3 of the A-domain were defective in adherence to immobilized fibrinogen and cytokeratin 10, suggesting that both ligands bind to the same or overlapping regions.

A comparison of the mechanical and structural properties of fibrin fibers with other protein fibers

In the past few years a great deal of progress has been made in studying the mechanical and structural properties of biological protein fibers. Here, we compare and review the stiffness (Young's modulus, E) and breaking strain (also called rupture strain or extensibility, epsilon(max)) of numerous biological protein fibers in light of the recently reported mechanical properties of fibrin fibers. Emphasis is also placed on the structural features and molecular mechanisms that endow biological protein fibers with their respective mechanical properties. Generally, stiff biological protein fibers have a Young's modulus on the order of a few Gigapascal and are not very extensible (epsilon(max) < 20%). They also display a very regular arrangement of their monomeric units. Soft biological protein fibers have a Young's modulus on the order of a few Megapascal and are very extensible (epsilon(max) > 100%). These soft, extensible fibers employ a variety of molecular mechanisms, such as extending amorphous regions or unfolding protein domains, to accommodate large strains. We conclude our review by proposing a novel model of how fibrin fibers might achieve their extremely large extensibility, despite the regular arrangement of the monomeric fibrin units within a fiber. We propose that fibrin fibers accommodate large strains by two major mechanisms: (1) an alpha-helix to beta-strand conversion of the coiled coils; (2) a partial unfolding of the globular C-terminal domain of the gamma-chain.

Direct evidence for specific interactions of the fibrinogen alphaC-domains with the central E region and with each other

The carboxyl-terminal regions of the fibrinogen Aalpha chains (alphaC regions) form compact alphaC-domains tethered to the bulk of the molecule with flexible alphaC-connectors. It was hypothesized that in fibrinogen two alphaC-domains interact intramolecularly with each other and with the central E region preferentially through its N-termini of Bbeta chains and that removal of fibrinopeptides A and B upon fibrin assembly results in dissociation of the alphaC regions and their switch to intermolecular interactions. To test this hypothesis, we studied the interactions of the recombinant alphaC region (Aalpha221-610 fragment) and its subfragments, alphaC-connector (Aalpha221-391) and alphaC-domain (Aalpha392-610), between each other and with the recombinant (Bbeta1-66)2 and (beta15-66)2 fragments and NDSK corresponding to the fibrin(ogen) central E region, using laser tweezers-based force spectroscopy. The alphaC-domain, but not the alphaC-connector, bound to NDSK, which contains fibrinopeptides A and B, and less frequently to desA-NDSK and (Bbeta1-66)2 containing only fibrinopeptides B; it was poorly reactive with desAB-NDSK and (beta15-66)2 both lacking fibrinopeptide B. The interactions of the alphaC-domains with each other and with the alphaC-connector were also observed, although they were weaker and heterogeneous in strength. These results provide the first direct evidence for the interaction between the alphaC-domains and the central E region through fibrinopeptide B, in agreement with the hypothesis given above, and indicate that fibrinopeptide A is also involved. They also confirm the hypothesized homomeric interactions between the alphaC-domains and display their interaction with the alphaC-connectors, which may contribute to covalent cross-linking of alpha polymers in fibrin.

NMR solution structure, stability, and interaction of the recombinant bovine fibrinogen alphaC-domain fragment

According to the existing hypothesis, in fibrinogen, the COOH-terminal portions of two Aalpha chains are folded into compact alphaC-domains that interact intramolecularly with each other and with the central region of the molecule; in fibrin, the alphaC-domains switch to an intermolecular interaction resulting in alphaC-polymers. In agreement, our recent NMR study identified within the bovine fibrinogen Aalpha374-538 alphaC-domain fragment an ordered compact structure including a beta-hairpin restricted at the base by a 423-453 disulfide linkage. To establish the complete structure of the alphaC-domain and to further test the hypothesis, we expressed a shorter alphaC-fragment, Aalpha406-483, and performed detailed analysis of its structure, stability, and interactions. NMR experiments on the Aalpha406-483 fragment identified a second loose beta-hairpin formed by residues 459-476, yielding a structure consisting of an intrinsically unstable mixed parallel/antiparallel beta-sheet. Size-exclusion chromatography and sedimentation velocity experiments revealed that the Aalpha406-483 fragment forms soluble oligomers whose fraction increases with an increase in concentration. This was confirmed by sedimentation equilibrium analysis, which also revealed that the addition of each monomer to an assembling alphaC-oligomer substantially increases its stabilizing free energy. In agreement, unfolding experiments monitored by CD established that oligomerization of Aalpha406-483 results in increased thermal stability. Altogether, these experiments establish the complete NMR solution structure of the Aalpha406-483 alphaC-domain fragment, provide direct evidence for the intra- and intermolecular interactions between the alphaC-domains, and confirm that these interactions are thermodynamically driven.

Fibrinogen and fibrin: scaffold proteins in hemostasis

PURPOSE OF REVIEW

Elevated fibrinogen is a cardiovascular risk factor. Recent work provides a rationale for this risk, as abnormal fibrin clot structure, strength and stability correlates with coronary artery disease. This review describes in-vitro experiments whose intent is to define the molecular mechanisms that control clot architecture and function in vivo.

RECENT FINDINGS

Biochemical and structural data continue to define the interactions between monomer units that assemble into a fibrin clot. In particular, 'A: a' interactions dominate the first step in fiber formation, while the analogous 'B: b' interactions have a minor role. Studies show the N-terminus of Bbeta, the C-terminus of Aalpha, and the splice variant gamma' modulate fibrin clot structure. Measurement of the mechanical properties of fibrinogen and fibrin show fibrin fibers are among the strongest in nature. Studies have identified fibrinogen-binding proteins that influence clot structure and function.

SUMMARY

These findings defined mechanisms that control fibrin clot structure, strength and stability. This basic information provides direction for clinical studies to examine clot properties in pathologic thrombosis and pharmaceutical studies to develop therapeutic interventions to prevent or control cardiovascular disease. These studies also establish novel techniques to examine individual bonds, molecules and fibers.

Interaction of the fibronectin COOH-terminal Fib-2 regions with fibrin: further characterization and localization of the Fib-2-binding sites

Incorporation of fibronectin into fibrin clots is important for the formation of a provisional matrix that promotes cell adhesion and migration during wound healing. Previous studies revealed that this incorporation occurs through noncovalent interaction between two NH2-terminal Fib-1 regions of fibronectin (one on each chain) and the alphaC-regions of fibrin, and is further reinforced by factor XIIIa-mediated covalent cross-linking of fibronectin to the fibrin matrix. To clarify the role of another pair of fibrin-binding regions, Fib-2, located at the disulfide-linked COOH-terminal ends of fibronectin, we prepared by limited proteolysis a dimeric 140 kDa (Fib-2)2 fragment containing both Fib-2 regions and tested its interaction with recombinant fragments corresponding to the alphaC regions of fibrin(ogen). In both ELISA and surface plasmon resonance (SPR) experiments 140 kDa (Fib-2)2 bound to the immobilized Aalpha221-610 alphaC-fragment. However, the affinity of binding was substantially lower than that for Fib-1. Ligand blotting and ELISA established that the Fib-2 binding site is located in the connector part of the alphaC region including residues Aalpha221-391. Analysis of the SPR-detected binding of fibronectin to the immobilized Aalpha221-610 alphaC-fragment revealed two types of fibronectin-binding sites, one with high affinity and another one with much lower affinity. Competition experiments revealed about 30% inhibition of the Fib-2 mediated binding by increasing concentrations of Fib-1 fragment suggesting partial overlap of the two sets of binding sites. Based on these results and our previous studies we propose a mechanism of interaction of fibronectin with fibrin in which both Fib-1 and Fib-2 play a role.