Modes and consequences of thrombin's interaction with fibrin

Investigation of thrombin concentration at the time of clot formation in simultaneous thrombin and fibrin generation assays

Thrombin generation (TG) and fibrin clot formation represent the central process of blood coagulation. Up to 95% of thrombin is considered to be generated after the clot is formed. However, this was not investigated in depth. In this study, we conducted a quantitative analysis of the Thrombin at Clot Time (TCT) parameter in 5758 simultaneously recorded TG and clot formation assays using frozen plasma samples from commercial sources under various conditions of activation. These samples were supplemented with clotting factor concentrates, procoagulant lipid vesicles and a fluorogenic substrate and triggered with tissue factor (TF). We found that TCT is often close to a 10% of thrombin peak height (TPH) yet it can be larger or smaller depending on whether the sample has low or high TPH value. In general, the samples with high TPH are associated with elevated TCT. TCT appeared more sensitive to some procoagulant phenotypes than other commonly used parameters such as clotting time, TPH or Thrombin Production Rate (TPR). In a minority of cases, TCT were not predicted from TG parameters. For example, elevated TCT (above 15% of TPH) was associated with either very low or very high TPR values. We conclude that clotting and TG assays may provide complementary information about the plasma sample, and that the TCT parameter may serve as an additional marker for the procoagulant potential in plasma sample.

Fibrinogen γ' promotes host survival during Staphylococcus aureus septicemia in mice

BACKGROUND

Staphylococcus aureus is a common gram-positive bacterium that is the causative agent for several human diseases, including sepsis. A key virulence mechanism is pathogen binding to host fibrinogen through the C-terminal region of the γ-chain. Previous work demonstrated that FggΔ5 mice expressing mutant fibrinogen γΔ5 lacking a S. aureus binding motif had significantly improved survival following S. aureus septicemia. Fibrinogen γ' is a human splice variant that represents about 10% to 15% of the total fibrinogen in plasma and circulates as a fibrinogen γ'-γ heterodimer (phFibγ'-γ). The fibrinogen γ'-chain is also expected to lack S. aureus binding function.

OBJECTIVE

Determine if human fibrinogen γ'-γ confers host protection during S. aureus septicemia.

METHODS

Analyses of survival and the host response following S. aureus septicemia challenge in FggΔ5 mice and mice reconstituted with purified phFibγ'-γ or phFibγ-γ.

RESULTS

Reconstitution of fibrinogen-deficient or wildtype mice with purified phFibγ'-γ prior to infection provided a significant prolongation in host survival relative to mice reconstituted with purified phFibγ-γ, which was superior to that observed with heterozygous FggΔ5 mice. Improved survival could not be accounted for by quantitative differences in fibrinogen-dependent adhesion or clumping, but phFibγ'-γ-containing mixtures generated notably smaller bacterial aggregates. Importantly, administration of phFibγ'-γ after infection also provided a therapeutic benefit by prolonging host survival relative to administration of phFibγ-γ.

CONCLUSION

These findings provide the proof-of-concept that changing the ratio of naturally occurring fibrinogen variants in blood could offer significant therapeutic potential against bacterial infection and potentially other diseases.

Mathematical modeling to understand the role of bivalent thrombin-fibrin binding during polymerization

Thrombin is an enzyme produced during blood coagulation that is crucial to the formation of a stable clot. Thrombin cleaves soluble fibrinogen into fibrin, which polymerizes and forms an insoluble, stabilizing gel around the growing clot. A small fraction of circulating fibrinogen is the variant γA/γ′, which has been associated with high-affinity thrombin binding and implicated as a risk factor for myocardial infarctions, deep vein thrombosis, and coronary artery disease. Thrombin is also known to be strongly sequestered by polymerized fibrin for extended periods of time in a way that is partially regulated by γA/γ′. However, the role of γA/γ′-thrombin interactions during fibrin polymerization is not fully understood. Here, we present a mathematical model of fibrin polymerization that considered the interactions between thrombin, fibrinogen, and fibrin, including those with γA/γ′. In our model, bivalent thrombin-fibrin binding greatly increased thrombin residency times and allowed for thrombin-trapping during fibrin polymerization. Results from the model showed that early in fibrin polymerization, γ′ binding to thrombin served to localize the thrombin to the fibrin(ogen), which effectively enhanced the enzymatic conversion of fibrinogen to fibrin. When all the fibrin was fully generated, however, the fibrin-thrombin binding persisted but the effect of fibrin on thrombin switched quickly to serve as a sink, essentially removing all free thrombin from the system. This dual role for γ′-thrombin binding during polymerization led to a paradoxical decrease in trapped thrombin as the amount of γ′ was increased. The model highlighted biochemical and biophysical roles for fibrin-thrombin interactions during polymerization and agreed well with experimental observations.

A Mathematical Model of Bivalent Binding Suggests Physical Trapping of Thrombin within Fibrin Fibers

Thrombin is an enzyme that plays many important roles in the blood clotting process; it activates platelets, cleaves coagulation proteins within feedback loops, and cleaves fibrinogen into fibrin, which polymerizes into fibers to form a stabilizing gel matrix in and around growing clots. Thrombin also binds to the formed fibrin matrix, but this interaction is not well understood. Thrombin-fibrin binding is often described as two independent, single-step binding events, one high-affinity and one low-affinity. However, kinetic schemes describing these single-step binding events do not explain experimentally-observed residency times of fibrin-bound thrombin. In this work, we study a bivalent, sequential-step binding scheme as an alternative to the high-affinity event and, in addition to the low-affinity one. We developed mathematical models for the single- and sequential-step schemes consisting of reaction-diffusion equations to compare to each other and to experimental data. We then used Bayesian inference, in the form of Markov chain Monte Carlo, to learn model parameter distributions from previously published experimental data. For the model to best fit the data, we made an additional assumption that thrombin was irreversibly sequestered; we hypothesized that this could be due to thrombin becoming physically trapped within fibrin fibers as they formed. We further estimated that ∼30% of thrombin in the experiments to which we compare our model output became physically trapped. The notion of physically trapped thrombin may provide new insights into conflicting observations regarding the speed of fibrinolysis. Finally, we show that our new model can be used to further probe scenarios dealing with thrombin allostery.

Dynamics of Thrombin Generation and Flux from Clots during Whole Human Blood Flow over Collagen/Tissue Factor Surfaces

Coagulation kinetics are well established for purified blood proteases or human plasma clotting isotropically. However, less is known about thrombin generation kinetics and transport within blood clots formed under hemodynamic flow. Using microfluidic perfusion (wall shear rate, 200 s^-1) of corn trypsin inhibitor-treated whole blood over a 250-μm long patch of type I fibrillar collagen/lipidated tissue factor (TF; ∼1 TF molecule/μm²), we measured thrombin released from clots using thrombin-antithrombin immunoassay. The majority (>85%) of generated thrombin was captured by intrathrombus fibrin as thrombin-antithrombin was largely undetectable in the effluent unless Gly-Pro-Arg-Pro (GPRP) was added to block fibrin polymerization. With GPRP present, the flux of thrombin increased to ∼0.5 × 10^-12 nmol/μm²-s over the first 500 s of perfusion and then further increased by ∼2-3-fold over the next 300 s. The increased thrombin flux after 500 s was blocked by anti-FXIa antibody (O1A6), consistent with thrombin-feedback activation of FXI. Over the first 500 s, ∼92,000 molecules of thrombin were generated per surface TF molecule for the 250-μm-long coating. A single layer of platelets (obtained with α_IIbβ₃ antagonism preventing continued platelet deposition) was largely sufficient for thrombin production. Also, the overall thrombin-generating potential of a 1000-μm-long coating became less efficient on a per μm² basis, likely due to distal boundary layer depletion of platelets. Overall, thrombin is robustly generated within clots by the extrinsic pathway followed by late-stage FXIa contributions, with fibrin localizing thrombin via its antithrombin-I activity as a potentially self-limiting hemostatic mechanism.

Batroxobin binds fibrin with higher affinity and promotes clot expansion to a greater extent than thrombin

Batroxobin is a thrombin-like serine protease from the venom of Bothrops atrox moojeni that clots fibrinogen. In contrast to thrombin, which releases fibrinopeptide A and B from the NH2-terminal domains of the Aα- and Bβ-chains of fibrinogen, respectively, batroxobin only releases fibrinopeptide A. Because the mechanism responsible for these differences is unknown, we compared the interactions of batroxobin and thrombin with the predominant γA/γA isoform of fibrin(ogen) and the γA/γ' variant with an extended γ-chain. Thrombin binds to the γ'-chain and forms a higher affinity interaction with γA/γ'-fibrin(ogen) than γA/γA-fibrin(ogen). In contrast, batroxobin binds both fibrin(ogen) isoforms with similar high affinity (Kd values of about 0.5 μM) even though it does not interact with the γ'-chain. The batroxobin-binding sites on fibrin(ogen) only partially overlap with those of thrombin because thrombin attenuates, but does not abrogate, the interaction of γA/γA-fibrinogen with batroxobin. Furthermore, although both thrombin and batroxobin bind to the central E-region of fibrinogen with a Kd value of 2-5 μM, the α(17-51) and Bβ(1-42) regions bind thrombin but not batroxobin. Once bound to fibrin, the capacity of batroxobin to promote fibrin accretion is 18-fold greater than that of thrombin, a finding that may explain the microvascular thrombosis that complicates envenomation by B. atrox moojeni. Therefore, batroxobin binds fibrin(ogen) in a manner distinct from thrombin, which may contribute to its higher affinity interaction, selective fibrinopeptide A release, and prothrombotic properties.

High thrombin concentrations in fibrin sealants induce apoptosis in human keratinocytes

Over the last century many studies have been performed to assess the impact of fibrin sealant (FS) components on cells. Because of the noncovalent bonding of thrombin to fibrin during fibrin clot formation, we wanted to further evaluate the impact of fibrin bound thrombin on cell viability. Initially, we quantified the activity of thrombin in three different, commercially available FS. This information was used to prepare fibrin clots covering a range of thrombin concentrations from 4 to 820 IU mL(-1), but which were identical with respect to all other constituents. Although these fibrin clots did not differ in their three-dimensional structure, clots prepared with highly concentrated thrombin (820 IU mL(-1)) failed to support adhesion and spreading of primary human keratinocytes (NHEK). The number of attached cells was also significantly reduced on high thrombin activity clots. We hypothesized that these observations are not only the consequence of decreased proliferation but of apoptotic mechanisms, since the expression of cleaved caspase 3 and 7 was strongly enhanced on fibrin clots with high thrombin activity. This was accompanied by an induction of expression of Trail-R2 which is a receptor known to mediate apoptosis signals. Blocking of thrombin activity by hirudin led to an improvement of cell morphology and to an increase in number of attached cells. In addition, the induction of caspase 3 and 7 was also reduced. Thus, here we report for the first time that fibrin bound thrombin does not only decrease proliferation (as already published by others), it also does induce NHEK apoptosis when present at high concentrations.

Fibrinopeptides A and B release in the process of surface fibrin formation

Fibrinogen adsorption on a surface results in the modification of its functional characteristics. Our previous studies revealed that fibrinogen adsorbs onto surfaces essentially in 2 different orientations depending on its concentration in the solution: "side-on" at low concentrations and "end-on" at high concentrations. In the present study, we analyzed the thrombin-mediated release of fibrinopeptides A and B (FpA and FpB) from fibrinogen adsorbed in these orientations, as well as from surface-bound fibrinogen-fibrin complexes prepared by converting fibrinogen adsorbed in either orientation into fibrin and subsequently adding fibrinogen. The release of fibrinopeptides from surface-adsorbed fibrinogen and from surface-bound fibrinogen-fibrin complexes differed significantly compared with that from fibrinogen in solution. The release of FpB occurred without the delay (lag phase) characteristic of its release from fibrinogen in solution. The amount of FpB released from end-on adsorbed fibrinogen and from adsorbed fibrinogen-fibrin complexes was much higher than that of FpA. FpB is known as a potent chemoattractant, so its preferential release suggests a physiological purpose in the attraction of cells to the site of injury. The N-terminal portions of fibrin β chains including residues Bβ15-42, which are exposed after cleavage of FpB, have been implicated in many processes, including angiogenesis and inflammation.

Cell Compatibility of Fibrin Sealants: In Vitro Study with Cells Involved in Soft Tissue Repair

Fibrin sealants can be used to support tissue regeneration or as vehicles for delivery of cells in tissue engineering. Differences in the composition of fibrin sealants, however, could determine the success of such applications. The results presented in this article show clear differences between Fibrin sealant A (FS A) clots and Fibrin sealant B (FS B) clots with respect to their compatibility with primary human cells involved in soft tissue repair. FS A clots, which are characterized by a physiological coarse fibrin structure, promoted attachment, spreading, and proliferation of keratinocytes, fibroblasts, and endothelial cells. In contrast, FS B clots displaying a fine to medium clot structure failed to support spreading of all three cell types. Adhesion of keratinocytes was decreased on FS B clots compared to FS A clots after 3 h incubation, whereas number of attached fibroblasts and endothelial cells was initially comparable between the two fibrin sealants. However, all three cell types proliferated on FS A clots but no sustained proliferation was detected on FS B clots. We further demonstrate that the observed differences between FS A and B clots are partly based upon 1 M sodium chloride extractable constituents, like thrombin, and partly on nonextractable constituents or the fibrin structure. In conclusion, our in vitro results demonstrate that FS A clots serve as a provisional matrix that encourages adhesion and growth of keratinocytes, fibroblasts, and endothelial cells. Therefore, FS A seems to be well suited for applications in tissue engineering.

Tissue factor in coagulation: Which? Where? When?

Tissue factor (TF) is an integral membrane protein, normally separated from the blood by the vascular endothelium, which plays a key role in the initiation of blood coagulation. With a perforating vascular injury, TF becomes exposed to blood and binds plasma factor VIIa. The resulting complex initiates a series of enzymatic reactions leading to clot formation and vascular sealing. In some pathological states, circulating blood cells express TF as a result of exposure to an inflammatory stimulus leading to intravascular clotting, vessel occlusion, and thrombotic pathology. Numerous controversies have arisen related to the influence of structural features of TF, its presentation, and its function. There are contradictory reports about the synthesis and presentation of TF on blood cells and the presence (or absence) of functionally active TF circulating in normal blood either on microparticles or as a soluble protein. In this review we discuss TF structure-function relationships and the role of TF during various phases of the blood coagulation process. We also highlight controversies concerning the expression/presence of TF on various cells and in blood in normal and pathological states.

The nature of the stable blood clot procoagulant activities

The function of tissue factor (Tf)-initiated coagulation is hemorrhage control through the formation and maintenance of an impermeable platelet-fibrin barrier. The catalytic processes involved in the clot maintenance function are not well defined, although the rebleeding problems characteristic of individuals with hemophilias A and B suggest a link between specific defects in the Tf-initiated process and defects in the maintenance function. We have previously demonstrated, using a methodology of "flow replacement" (or resupply) of ongoing Tf-initiated reactions with fresh reactants, that procoagulant complexes are produced during Tf-initiated coagulation, which are capable of reinitiating coagulation without input from extrinsic factor Xase activity (Orfeo, T., Butenas, S., Brummel-Ziedins, K. E., and Mann, K. G. (2005) J. Biol. Chem. 280, 42887-42896). Here we used Tf-initiated reactions in normal and hemophilia blood or in their corresponding proteome mixtures as sources of procoagulant end products and then varied the resupplying material to determine the identity of the catalysts that drive the new cycle of thrombin formation. The central findings are as follows: 1) the prothrombinase complex (fVa-fXa-Ca(2+)-membrane) accumulated during the episode of Tf-initiated coagulation is the primary catalyst responsible for the observed pattern of prothrombin activation after resupply; 2) impairments in intrinsic factor Xase function, i.e. hemophilias A and B, result in an impaired capacity to mount a resupply response; and 3) in normal hemostasis the intrinsic factor Xase function contributes to the durability of the resupply response.

Bivalent binding to gammaA/gamma'-fibrin engages both exosites of thrombin and protects it from inhibition by the antithrombin-heparin complex

Thrombin exosite 1 binds the predominant gamma(A)/gamma(A)-fibrin form with low affinity. A subpopulation of fibrin molecules, gamma(A)/gamma'-fibrin, has an extended COOH terminus gamma'-chain that binds exosite 2 of thrombin. Bivalent binding to gamma(A)/gamma'-fibrin increases the affinity of thrombin 10-fold, as determined by surface plasmon resonance. Because of its higher affinity, thrombin dissociates 7-fold more slowly from gamma(A)/gamma'-fibrin clots than from gamma(A)/gamma(A)-fibrin clots. After 24 h of washing, however, both gamma(A)/gamma'- and gamma(A)/gamma(A)-fibrin clots generate fibrinopeptide A when incubated with fibrinogen, indicating the retention of active thrombin. Previous studies demonstrated that heparin heightens the affinity of thrombin for fibrin by simultaneously binding to fibrin and exosite 2 on thrombin to generate a ternary heparin-thrombin-fibrin complex that protects thrombin from inhibition by antithrombin and heparin cofactor II. In contrast, dermatan sulfate does not promote ternary complex formation because it does not bind to fibrin. Heparin-catalyzed rates of thrombin inhibition by antithrombin were 5-fold slower in gamma(A)/gamma'-fibrin clots than they were in gamma(A)/gamma(A)-fibrin clots. This difference reflects bivalent binding of thrombin to gamma(A)/gamma'-fibrin because (a) it is abolished by addition of a gamma'-chain-directed antibody that blocks exosite 2-mediated binding of thrombin to the gamma'-chain and (b) the dermatan sulfate-catalyzed rate of thrombin inhibition by heparin cofactor II also is lower with gamma(A)/gamma'-fibrin than with gamma(A)/gamma(A)-fibrin clots. Thus, bivalent binding of thrombin to gamma(A)/gamma'-fibrin protects thrombin from inhibition, raising the possibility that gamma(A)/gamma'-fibrin serves as a reservoir of active thrombin that renders thrombi thrombogenic.

Effect of substrate and fibrin polymerization inhibitor on determination of plasma thrombin generation in vitro

BACKGROUND

Thrombin generation potential, a critical haemostatic measure, can be determined by continuous detection of total thrombin or direct subsampling. However, differences between methods exist in area under the curve or peak thrombin calculated. Also, impact of anticoagulants on thrombin generation may vary depending on mode of analysis.

OBJECTIVE

We studied the effect of components on thrombin generation in the presence or absence of anticoagulants.

METHODS

The continuous method was conducted with plasma +/- fibrin(ogen) +/- fibrin polymerization inhibitor. Plasma contained slow-reacting TG5134 substrate at 37 degrees C and reaction was started with dilute thromboplastin in CaCl(2)/Tris buffer. Absorbance (405 nm) was recorded over time and free thrombin calculated from total thrombin activity. For the subsampling method, similar plasma mixtures +/- TG5134 were reacted and free thrombin measured directly as the difference in activity against S2238 substrate of timed subsamples taken into EDTA or EDTA + antithrombin + heparin.

RESULTS

Slow-reacting substrate in the continuous method acted as a competitor for thrombin, giving delayed but greater free thrombin than direct subsampling. These differences persisted to varying extents with all anticoagulants tested. In either method, presence of polymerization inhibitor increased the amount of free thrombin. Continuous method detection of alpha(2)macroglobulin complexes was hampered by sensitivity limits leading to inordinate free thrombin calculations. Especially with hirudin, although free thrombin remained at the end of the subsampling method, continuous method calculations assumed no residual free thrombin.

CONCLUSION

In vitro plasma thrombin generation is delayed and increased by slow-acting substrate and fibrin polymerization inhibitor.

Ecotin modulates thrombin activity through exosite-2 interactions

Ecotin is a Escherichia coli-derived protein that has been characterized as a potent inhibitor of serine-proteases. This protein is highly effective against several mammalian enzymes, which includes pancreatic and neutrophil-derived elastases, chymotrypsin, trypsin, factor Xa, and kallikrein. In this work we showed that ecotin binds to human alpha-thrombin via its secondary binding site, and modulates thrombin catalytic activity. Formation of wild type ecotin-alpha-thrombin complex was observed by native PAGE and remarkably, gel filtration chromatography showed an unusual 2:1 ecotin:enzyme stoichiometry. Analysis of the protease inhibitor effects on thrombin biological activities showed that (i) it decreases the inhibition of thrombin by heparin/antithrombin complex (IC50=3.2 microM); (ii) it produces a two-fold increase in the thrombin-induced fibrinogen clotting; and (iii) it inhibits thrombin-induced platelet aggregation (IC50=4.5 microM). Allosteric changes on thrombin structure were then evaluated. Complex formation with ecotin caused a three-fold increase in the rate of thrombin inhibition by BPTI, suggesting a displacement of the enzyme's 60-loop. In addition, ecotin modulated the enzyme's catalytic site, as demonstrated by changes in the fluorescence emission of fluorescein-FPRCK-alpha-thrombin (EC50=3.5 microM). Finally, solid phase competition assays demonstrated that heparin and prothrombin fragment 2 prevents thrombin interaction with ecotin. Altogether, these observations strongly support an ecotin interaction with thrombin anion-binding exosite-2, resulting in modulation of its biological activities. At this point, ecotin might be useful as a new tool for studying thrombin allosteric modulation.