Formation of the Human Fibrinogen Subclass Fib420: Disulfide Bonds and Glycosylation in Its Unique (EChain) Domains

Interplay of fibrinogen αEC globular domains and factor XIIIa cross-linking dictates the extensibility and strain stiffening of fibrin networks

BACKGROUND

Fibrinogen is a plasma protein forming the fibrin scaffold of blood clots. Its mechanical properties therefore affect the risk of bleeding as well as thrombosis. There has been much recent interest in the biophysical mechanisms controlling fibrin mechanics; however, the role of molecular heterogeneity of the circulating fibrinogen in determining clot mechanical function remains poorly characterized.

OBJECTIVES

By comparing 2 fibrinogen variants where the only difference is the Aα-chain length, with one variant having a globular domain at its C-terminus, this study aimed to reveal how the molecular structure impacts the structure and mechanics of fibrin networks.

METHODS

We characterized the mechanical response to large shear for networks formed from 2 recombinant fibrinogen variants: the most prevalent variant in circulation with a molecular weight of 340 kDa (recombinant human fibrinogen [rFib] 340) and a minor variant with a molecular weight of 420 kDa (rFib420).

RESULTS

We show that the elastic properties of the 2 variants are identical when fibrin is cross-linked with factor XIIIa but differ strongly in its absence. Uncross-linked rFib420 networks are softer and up to 3-fold more extensible than rFib340 networks. Electron microscopy imaging showed that the 2 variants formed networks with a comparable structure, except at 4 mg/mL, where rFib420 formed denser networks.

CONCLUSION

We propose that the αEC domains of rFib420 increase the extensibility of uncross-linked fibrin networks by promoting protofibril sliding, which is blocked by FXIIIa cross-linking. Our findings can help explain the functional role of different circulating fibrinogen variants in blood clot mechanics and tissue repair.

Engineered Molecular Therapeutics Targeting Fibrin and the Coagulation System: a Biophysical Perspective

The coagulation cascade represents a sophisticated and highly choreographed series of molecular events taking place in the blood with important clinical implications. One key player in coagulation is fibrinogen, a highly abundant soluble blood protein that is processed by thrombin proteases at wound sites, triggering self-assembly of an insoluble protein hydrogel known as a fibrin clot. By forming the key protein component of blood clots, fibrin acts as a structural biomaterial with biophysical properties well suited to its role inhibiting fluid flow and maintaining hemostasis. Based on its clinical importance, fibrin is being investigated as a potentially valuable molecular target in the development of coagulation therapies. In this topical review, we summarize our current understanding of the coagulation cascade from a molecular, structural and biophysical perspective. We highlight single-molecule studies on proteins involved in blood coagulation and report on the current state of the art in directed evolution and molecular engineering of fibrin-targeted proteins and polymers for modulating coagulation. This biophysical overview will help acclimatize newcomers to the field and catalyze interdisciplinary work in biomolecular engineering toward the development of new therapies targeting fibrin and the coagulation system.

Fibrinogen alpha chain O-glycopeptides as possible markers of urinary tract infection

Urinary tract infection (UTI) is the most common bacterial infection leading to substantial morbidity and considerable health care expenditures across all ages. Here we present an exploratory UPLC-MS study of human urine in the context of febrile, complicated urinary tract infection aimed to reveal and identify possible markers of a host response on infection. A UPLC-MS based workflow, taking advantage of Ultra High Resolution (UHR) Qq-ToF-MS, and multivariate data handling were applied to a carefully selected group of 39 subjects with culture-confirmed febrile Escherichia coli UTI. Using a combination of unsupervised and supervised multivariate modeling we have pinpointed a number of peptides specific for UTI. An unequivocal structural identification of these peptides, as O-glycosylated fragments of the human fibrinogen alpha 1 chain, required MS2 and MS3 experiments on two different MS platforms: ESI-UHR-Qq-ToF and ESI-ion trap, a blast search and, finally, confirmation was achieved by matching experimental tandem mass spectra with those of custom synthesized candidate-peptides. In conclusion, exploiting non-targeted UPLC-MS based approach for the investigation of UTI related changes in urine, we have identified and structurally characterized unique O-glycopeptides, which are, to our knowledge, the first demonstration of O-glycosylation of human fibrinogen alpha 1-chain.

Evidence for age-related differences in human fibrinogen

Fibrinogen has previously been demonstrated to exist in a 'fetal' form, in cord blood of term infants, with increased sialic acid content compared to adult fibrinogen. The functional implications of these differences are reflected in prolonged thrombin clotting times in newborns as well as differences in polymerization of fibrin from 'fetal' fibrinogen. Despite numerous studies of fibrinogen structure and function, the age at which 'fetal' fibrinogen reverts to the adult form, as well as the physiological significance of this phenomenon remains unknown. This study was designed to determine whether the difference between the 'fetal' and the 'adult' fibrinogen molecule persists in a 'childhood' form throughout progression from infancy to adulthood. The results demonstrate that although the concentration of fibrinogen from day 1 neonates is decreased compared to adult fibrinogen, functional activity of this protein is comparable in both age groups. In addition, despite there being quantitatively less fibrinogen in day 3 and 11-16-year age groups, this protein is functionally more active compared to adult fibrinogen. In addition, the molecular weight of the Aα fibrinogen chain was consistently higher by up to 1500 Da in neonates and children compared to adults, suggesting age-specific differences in posttranslational modification of this chain of the protein.These age-related differences in fibrinogen could provide a protective mechanism against excessive polymerization and proteolysis of this protein, providing a possible explanation of the thromboprotective mechanism that is functioning in neonates and children.

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.

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 non-inherited heterogeneity and its relationship to function in health and disease

In healthy individuals fibrinogen occurs in more than one million non-identical forms because of the many possible combinations of biosynthetically or postbiosynthetically modified or genetically polymorphic sites. The various forms may show considerable differences in their functional properties. Normal variant sites are due to alternative splicing, modification of certain amino acid residues, and proteolysis. Both the A alpha and the gamma chain occur in two splice forms, and it is known that only the shorter gamma chain can interact with platelets, but the longer may bind thrombin and factor XIII. Many types of posttranslationally modified amino acid residues are present in fibrinogen. The A alpha chain is partially phosphorylated at two sites, possibly leading to protection against proteolysis. The B beta chain is N-glycosylated and partially proline hydroxylated, each at one site. The gamma chain is N-glycosylated at one site and the longer splice form doubly tyrosine-sulfated. The glycosylations are believed to protect against polymerization and proteolysis. All three chains are partially oxidized at methionine residues and deamidated at asparagine and glutamine residues. The A alpha and gamma chain are partially carboxy-terminally degraded by proteolysis, the shorter forms causing a decrease in polymerization, crosslinking, and clot stability. Abnormal variants occur in patients with diabetes mellitus, in the form of glycated lysine residues; in patients with certain types of cancer, in the form of crosslinked degradation products; in patients with certain types of autoimmune disease, in the form of complexes with antibodies; in cigarette smokers; and in individuals treated with acetylsalicylic acid, in the form of acetylated lysine residues.

Contribution of the alpha EC domain to the structure and function of fibrinogen-420

In addition to the conventional fibrinogen with its alpha, beta, and gamma subunit chains, there is a subclass of fibrinogen molecules, accounting for one percent of the total in human adults, in which both alpha chains have been replaced by extended alpha chains (alpha E) that sport a globular C-terminal domain (alpha EC) comparable to beta C and gamma C. Using nomenclature based on molecular weight, the subclass of alpha E-containing molecules has been named fibrinogen-420 to differentiate it from the better known fibrinogen, now referred to as fibrinogen-340. Review of the events leading to the discovery of fibrinogen-420 in the early 1990s and its subsequent characterization, culminating in the crystal structure of its unique alpha EC domains, highlights special aspects of its evolutionary history, outstanding features of its structure, and the perplexities of its biology. Various working hypotheses that have driven prior investigation are evaluated and practical insights are offered to spur further research into the role of fibrinogen-420.

Confirmation of mendelian properties of heterodimeric fibrinogen molecules in a heterozygotic dysfibrinogenemia, "fibrinogen Amarillo," using gprphoresis to differentiate semifibrin molecules from fibrinogen and fibrin

The fibrinogen molecule consists of two sets of Aalpha, Bbeta, and gamma chains assembled into a bilateral disulfide linked (Aalpha, Bbeta, gamma)2 structure. Cleavage of the two A-fibrinopeptides (FPA, Aalpha1-16) from normal Aalpha chains with arginine at position 16 (RFPA) by thrombin or the venom enzyme atroxin transforms fibrinogen into self-aggregating fibrin monomers (alpha, Bbeta, gamma)(2). Mutant Aalpha16R-->H fibrinopeptide (HFPA) cannot be cleaved from fibrinogen by atroxin. Many studies on heterozygous dysfibrinogenemias with this mutation suggested that incorporation of the mutant chains into the molecules was ordered in a manner yielding only (1) homodimeric normal (RFPARFPA) atroxin-coagulable molecules and (2) homodimeric abnormal (H(FPA)HFPA) atroxin-incoagulable molecules in equal quantities. Although heterodimeric molecules (RFPAHFPA) could not be found in studies on the intact protein, Meh et al. demonstrated their existence by showing that CNBr digests of fibrinogens from atroxin-treated Aalpha16R-->H heterozygotic dysfibrinogenemias consistently yielded N-terminal fragments (NDSKs) with partially resolved electrophoretic bands predominantly in between the NDSKs of fibrinogen and alpha-fibrin. An opportunity to confirm and better quantify the heterodimers arose with the recent development of a method (GPRphoresis) for identifying molecules lacking only one FPA, which is applied here in study of a newly presenting case of an Aalpha16R-->H dysfibrinogenemia, "fibrinogen Amarillo." GPRphoresis uses electrophoretic shifts, staged with GPRP-NH(2) to separate the self-aggregating fibrin monomers lacking both FPAs from weakly aggregating "semifibrin" molecules lacking one FPA An antifibrin alpha17-23 antibody is used to measure and differentiate the semifibrin from fibrinogen with FPA fully intact. Applying GPRphoresis to atroxin digests of fibrinogen Amarillo clearly demonstrated RFPARFPA, RFPAHFPA, and HFPAHFPA molecules in nearly perfect Mendelian 1:2:1 proportions. In turn, the high levels of the semifibrin in the terminal atroxin digests provide genetic phenotypic evidence supporting fidelity of the GPRphoresis method.

The αEC domain of human fibrinogen-420 is a stable and early plasmin cleavage product

Human fibrinogen-420, (Eβγ)2, was isolated from plasma and evaluated for its ability to form clots and for its susceptibility to proteolysis. Clotting parameters, including cross-linking of subunit chains, of this subclass and of the more abundant fibrinogen-340 (βγ)2, were found to be similar, suggesting little impact of the unique EC domains of fibrinogen-420 on coagulation. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) analysis of plasmic digestion patterns revealed production from fibrinogen-420 of the conventional fibrinogen degradation products, X, Y, D, and E, to be comparable to that from fibrinogen-340 in all respects except the presence of at least 2 additional cleavage products that were shown by Western blot analysis to contain the EC domain. One was a stable fragment (ECX) comigrating with a 34-kd yeast recombinant EC domain, and the other was an apparent precursor. Their release occurred early, before that of fragments D and E. Two bands of the same mobility and antibody reactivity were found in Western blots of plasma collected from patients with myocardial infarction shortly after the initiation of thrombolytic therapy.

The EC Domains of Human Fibrinogen420Contain Calcium Binding Sites But Lack Polymerization Pockets

The extended  (E) isoform unique to Fibrinogen420 (Fib420) is distinguished from the conventional  chain of Fibrinogen340 by the presence of an additional 236-residue carboxyl terminus globular domain (EC). A recombinant form of EC (rEC), having a predicted mass of 27,653 Daltons, was expressed in yeast (Pichia pastoris) and purified by anion exchange column chromatography. Purified rEC appears to be predominantly intact, as judged by N-terminal sequence analysis, mass spectral analysis of the C-terminal cyanogen bromide (CNBr) fragment, and comparison of recognition by epitope-specific monoclonal antibodies. Carbohydrate determination, coupled with analysis of CNBr digestion fragments, confirms N-linked glycosylation at Asn667, the site at which sugar is attached in E. Analysis of CNBr digestion fragments confirms that two disulfide bridges exist at cysteine pairs E613/644 and E780/793. In the presence of 5 mmol/L EDTA, rEC is highly susceptible to plasmic degradation, but Ca2+ (5 mmol/L) renders rEC resistant. No protective effect from plasmic degradation was conferred to rEC by the peptides GPRPamide or GHRP, nor did rEC bind to a GPR peptide column. These results suggest that the EC domain contains a calcium-binding site, but lacks a polymerization pocket. By analogy with the site elucidated in the γC domain, we predict that the EC calcium binding site involves residues E772-778: DADQWEE.