Modeling and Parameter Subset Selection for Fibrin Polymerization Kinetics with Applications to Wound Healing

During the hemostatic phase of wound healing, vascular injury leads to endothelial cell damage, initiation of a coagulation cascade involving platelets, and formation of a fibrin-rich clot. As this cascade culminates, activation of the protease thrombin occurs and soluble fibrinogen is converted into an insoluble polymerized fibrin network. Fibrin polymerization is critical for bleeding cessation and subsequent stages of wound healing. We develop a cooperative enzyme kinetics model for in vitro fibrin matrix polymerization capturing dynamic interactions among fibrinogen, thrombin, fibrin, and intermediate complexes. A tailored parameter subset selection technique is also developed to evaluate parameter identifiability for a representative data curve for fibrin accumulation in a short-duration in vitro polymerization experiment. Our approach is based on systematic analysis of eigenvalues and eigenvectors of the classical information matrix for simulations of accumulating fibrin matrix via optimization based on a least squares objective function. Results demonstrate robustness of our approach in that a significant reduction in objective function cost is achieved relative to a more ad hoc curve-fitting procedure. Capabilities of this approach to integrate non-overlapping subsets of the data to enhance the evaluation of parameter identifiability are also demonstrated. Unidentifiable reaction rate parameters are screened to determine whether individual reactions can be eliminated from the overall system while preserving the low objective cost. These findings demonstrate the high degree of information within a single fibrin accumulation curve, and a tailored model and parameter subset selection approach for improving optimization and reducing model complexity in the context of polymerization experiments.

Congenital fibrinogen disorder with a compound heterozygote possessing two novel FGB mutations, one qualitative and the other quantitative

INTRODUCTION

Congenital fibrinogen disorders result from genetic mutations in FGA, FGB, or FGG resulting in quantitative fibrinogen deficiencies (afibrinogenemia or hypofibrinogenemia) or qualitative fibrinogen deficiencies (dysfibrinogenemia). Hypodysfibrinogenemia sharing features with hypo- and dysfibrinogenemia is rare. We performed genetic and functional analyses of a 31-year-old woman with suspected hypodysfibrinogenemia.

MATERIALS AND METHODS

Functional and antigenic fibrinogen values of patient were 1.05 and 1.24 g/L, respectively. DNA sequence and western blotting analyses for plasma fibrinogen were performed. A minigene incorporating the mutational region was transfected into a Chinese hamster ovary cell line (CHO), and reverse transcription products were analyzed. Assembly and secretion were examined using the recombinant variant fibrinogen. We purified the patient's plasma fibrinogen and analyzed thrombin-catalyzed fibrin polymerization (TCFP).

RESULTS AND CONCLUSIONS

DNA sequencing revealed compound heterozygous nucleotide mutations with FGB 35 bp c.1245-17_1262 or -16_1263 del and FGB c.510T>A (resulting in Bβp.N170K substitution) on different alleles. We did not detect shortened Bβ-chain peptides in the plasma using western blotting analysis. A minigene incorporating the deletion DNA showed two aberrant mRNA products. The secretion of Bβp.N170K-fibrinogen-CHO was almost same as normal Bβ-fibrinogen-CHO. TCFP of plasma Bβp.N170K fibrinogen was slightly lower than that of normal plasma fibrinogen. Aberrant splicing products derived from the 35 bp deletion caused hypofibrinogenemia due to nonsense-mediated mRNA decay and suggested the presence of only Bβp.N170K fibrinogen in patient's plasma. Bβp.N170K caused dysfibrinogenemia due to a delay in lateral aggregation. These findings demonstrated that these mutations respectively affected the fibrinogen quality and quantity, resulting in hypodysfibrinogenemia.

Altered structure and function of fibrinogen after cleavage by Factor VII Activating Protease (FSAP)

Factor VII Activating Protease (FSAP) is a plasma protease affecting both coagulation and fibrinolysis. Although a role in hemostasis is still unclear, the identification of additional physiologic substrates will help to elucidate its role in this context. FSAP has been reported to cleave fibrinogen, but the functional consequences of this are not known. We have therefore undertaken this study to determine the implications of this cleavage for fibrin-clot formation and its lysis. Treatment of human fibrinogen with FSAP released an N-terminal peptide from the Bβ chain (Bβ1-53) and subsequently the fibrinopeptide B; within the Aα chain a partial truncation of the αC-region by multiple cleavages was seen. The truncated fibrinogen showed a delayed thrombin-catalyzed polymerization and formed fibrin clots of reduced turbidity, indicative of thinner fibrin fibers. Confocal laser scanning and scanning electron microscopy of these clots revealed a less coarse fibrin network with thinner fibers and a smaller pore size. A lower pore size was also seen in permeability studies. Unexpectedly, FSAP-treated fibrinogen or plasma exhibited a significantly faster tPA-driven lysis, which correlated exclusively with cleavage of fibrinogen and not with activation of plasminogen activators. Similar observations were also made in plasma after activation of endogenous zymogen FSAP, but not in plasma of carrier of the rare Marburg I single nucleotide polymorphism. In conclusion, altering fibrin clot properties by fibrinogenolysis is a novel function of FSAP in the vasculature, which facilitates clot lysis and may in vivo contribute to reduced fibrin deposition during thrombosis.

Congenital hypodysfibrinogenemia associated with a novel deletion of three residues (γAla289_Asp291del) in fibrinogen

Hypodysfibrinogenemia is the least frequently reported congenital fibrinogen disorder, characterized by both quantity and quality defects of fibrinogen. In this study, we investigated the molecular basis of hypodysfibrinogenemia in a Chinese family. Functional fibrinogen was measured by Clauss method, and the antigenic fibrinogen was measured by immunoturbidimetry assay. All the exons and exon-intron boundaries of fibrinogen genes (FGA, FGB and FGG) were analysed by direct DNA sequencing. To further evaluate its molecular and functional characterizations, fibrinogen was purified from the plasma of propositus, then SDS-PAGE, fibrin polymerization, clot lysis, and electron microscopy scanning were all performed. The propositus showed a slight decrease of immunologic fibrinogen (1.52 g/L) but dramatically reduced functional fibrinogen (0.3 g/L). DNA sequencing revealed a novel heterozygous CCTTTGATG deletion in the exon 8 of FGG, leading to the deletion of Ala289, Phe290, and Asp291 in fibrinogen γ-chain. The polymerization of the fibrinogen from the propositus was markedly impaired, with prolonged lag period and decreased final turbidity. The fibrinogen clottability showed a reduced fraction of participating clot formation. While the clot lysis showed normal. Scanning electron microscopy revealed that the fibers of the propositus were thicker than normal, with larger pores and curlier meshworks. We conclude that γAla289_Asp291del is responsible for the hypodysfibrinogenemia in this case.

Favorable 2'-substitution in the loop region of a thrombin-binding DNA aptamer

Simple 2'-OMe-chemical modification in the loop region of the 15mer G-rich DNA sequence GGTTGGTGTGGTTGG is reported. The G-quadruplex structure of this thrombin-binding aptamer (TBA), is stabilized by single modifications (T → 2'-OMe-U), depending on the position of the modification. The structural stability also renders significantly increased inhibition of thrombin-induced fibrin polymerization, a process closely associated with blood-clotting.

Fibrin Formation, Structure and Properties

Fibrinogen and fibrin are essential for hemostasis and are major factors in thrombosis, wound healing, and several other biological functions and pathological conditions. The X-ray crystallographic structure of major parts of fibrin(ogen), together with computational reconstructions of missing portions and numerous biochemical and biophysical studies, have provided a wealth of data to interpret molecular mechanisms of fibrin formation, its organization, and properties. On cleavage of fibrinopeptides by thrombin, fibrinogen is converted to fibrin monomers, which interact via knobs exposed by fibrinopeptide removal in the central region, with holes always exposed at the ends of the molecules. The resulting half-staggered, double-stranded oligomers lengthen into protofibrils, which aggregate laterally to make fibers, which then branch to yield a three-dimensional network. Much is now known about the structural origins of clot mechanical properties, including changes in fiber orientation, stretching and buckling, and forced unfolding of molecular domains. Studies of congenital fibrinogen variants and post-translational modifications have increased our understanding of the structure and functions of fibrin(ogen). The fibrinolytic system, with the zymogen plasminogen binding to fibrin together with tissue-type plasminogen activator to promote activation to the active proteolytic enzyme, plasmin, results in digestion of fibrin at specific lysine residues. In spite of a great increase in our knowledge of all these interconnected processes, much about the molecular mechanisms of the biological functions of fibrin(ogen) remains unknown, including some basic aspects of clotting, fibrinolysis, and molecular origins of fibrin mechanical properties. Even less is known concerning more complex (patho)physiological implications of fibrinogen and fibrin.

Factor XIII deficiency enhances thrombin generation due to impaired fibrin polymerization - An effect corrected by Factor XIII replacement

INTRODUCTION

Factor XIII (FXIII) cross-links fibrin, completing blood coagulation. Congenital FXIII deficiency is managed with plasma-derived FXIII (pdFXIII) or recombinant FXIII (rFXIII) concentrates.

AIM

As the mechanisms protecting patients with low FXIII levels (<5IU/dL) from spontaneous bleeds remain unknown we assessed the interplay between thrombin generation (TG), fibrin formation and clot kinetics before and after FXIII administration in three patients with FXIII deficiency.

METHODS

Patients received initially rFXIII (35IU/kg, A-subunit) following with pdFXIII at 1250IU or 2500IU (12-30IU/kg) monthly. TG (CAT), thromboelastometry (ROTEM), prothrombin fragments F1+2, fibrinogen and FXIII activity (FXIII:C) were measured at baseline and one-hour recovery.

RESULTS

FXIII was at the target level of 20±6IU/dL at the 4-week trough. rFXIII corrected FXIII to 98±15 and high-dose pdFXIII to a level of 90±6, whereas low-dose/half dose pdFXIII reached 45±4IU/dL. Although fibrinogen (Clauss Method) was normal, coagulation in FIBTEM was impaired, which FXIII administration tended to correct. CAT implied 1.6- to 1.9-fold enhanced TG, which FXIII administration normalized. Inhibition of fibrin polymerization by Gly-Pro-Arg-Pro peptide mimicked FXIII deficiency in CAT by enhancing TG both in control and FXIII recovery plasma. Antithrombin, α2-macroblobulin-thrombin complex and prothrombin were normal, whereas F1+2 were elevated compatible with in vivo TG.

DISCUSSION

FXIII deficiency impairs fibrinogen function and fibrin formation simultaneously enhancing TG on the poorly polymerizing fibrin strands, when fibrin's antithrombin I -like function is absent. Our study suggests an inverse link between low FXIII levels and enhanced TG modifying structure-function relationship of fibrin to support hemostasis.

Fibrin(ogen)olytic and platelet modulating activity of a novel protease from the Echis multisquamatis snake venom

The variety of enzymes including serine proteases that possess fibrin(ogen)olytic and platelet modulating activity have been discovered in different snake venoms. In our work the fibrin(ogen)olytic and platelet modulating activity of a new protease from Echis multisquamatis snake venom was studied. It was shown that purified enzyme cleaved the ВβR42-A43 bond of fibrinogen during first contact with the substrate following much slower hydrolysis of C-terminus of fibrinogen Aα-chain. Protease hydrolysed fibrin clot too, but at much slower rate and cleaved both C-terminus of Aα-chain and ВβR42-A43 bond of Bβ-chain simultaneously. Preincubation of fibrinogen with protease dramatically elongated thrombin clotting time and the clot formed from a mixture of native fibrinogen and fibrinogen desВβ(1-42)2 digested by plasmin much faster than a native fibrin clot. The protease did not activate platelets nor cause changes in their shape and granularity, but it reduced platelets aggregation induced by ADP.