Dynamic imaging of fibrin network formation correlated with other measures of polymerization

Beneficial effect of cigarette smoking cessation on fibrin clot properties

To examine the associations between cigarette smoking and preferable clot properties. Plasma fibrin clots from 21 randomly selected current smokers (n = 7), former smokers (n = 7) and non-smokers (n = 7) were analyzed, using scanning electron microscopy (SEM). With the use of the turbidimetric clotting and lysis assay in plasma, the maximum absorbance (MaxAbs(C), MaxAbs(L)) was measured and lysis time (Lys(50%)) was calculated. Smoking cessation significantly influenced fibrin fiber branching and density. Median fiber diameter was not changed. Lys(50%) was the highest in current smokers and was reduced in former smokers to the non-smoker level (2120 ± 385 versus 1771 ± 122 and 1724 ± 272 s; P = 0.04). Smoking cessation improves fibrin clot architecture which results in the lesser resistance to lysis.

Kinetic characterization of inhibition of human thrombin with DNA aptamers by turbidimetric assay

A sensitive turbidimetric method for detecting fibrin association was used to study the kinetics of fibrinogen hydrolysis with thrombin. The data were complemented by high-performance liquid chromatography (HPLC) measurements of the peptide products, fibrinopeptides released during hydrolysis. Atomic force microscopy (AFM) data showed that the fibril diameter is the main geometric parameter influencing the turbidity. The turbidimetric assay was validated using thrombin with the standard activity. To study thrombin inhibitors, a kinetic model that allows estimating the inhibition constants and the type of inhibition was proposed. The kinetic model was used to study the inhibitory activity of the two DNA aptamers 15-TBA (thrombin-binding aptamer) and 31-TBA, which bind to thrombin exosites. For the first time, 31-TBA was shown to possess the competitive inhibition type, whereas the shortened aptamer 15-TBA has the noncompetitive inhibition type.

Nanostructure of the fibrin clot

The nanostructure of the fibrin fibers in fibrin clots is investigated by using spectrometry and small angle x-ray scattering measurements. First, an autocoherent analysis of the visible light spectra transmitted through formed clots is demonstrated to provide robust measurements of both the radius and density of the fibrin fibers. This method is validated via comparison with existing small-angle and dynamic light-scattering data. The complementary use of small angle x-ray scattering spectra and light spectrometry unambiguously shows the disjointed nature of the fibrin fibers. Indeed, under quasiphysiological conditions, the fibers are approximately one-half as dense as their crystalline fiber counterparts. Further, although the fibers are locally crystalline, they appear to possess a lateral fractal structure.

Visualization and identification of the structures formed during early stages of fibrin polymerization

We determined the sequence of events and identified and quantitatively characterized the mobility of moving structures present during the early stages of fibrin-clot formation from the beginning of polymerization to the gel point. Three complementary techniques were used in parallel: spinning-disk confocal microscopy, transmission electron microscopy, and turbidity measurements. At the beginning of polymerization the major structures were monomers, whereas at the middle of the lag period there were monomers, oligomers, protofibrils (defined as structures that consisted of more than 8 monomers), and fibers. At the end of the lag period, there were primarily monomers and fibers, giving way to mainly fibers at the gel point. Diffusion rates were calculated from 2 different results, one based on sizes and another on the velocity of the observed structures, with similar results in the range of 3.8-0.1 μm²/s. At the gel point, the diffusion coefficients corresponded to very large, slow-moving structures and individual protofibrils. The smallest moving structures visible by confocal microscopy during fibrin polymerization were identified as protofibrils with a length of approximately 0.5 μm. The sequence of early events of clotting and the structures present are important for understanding hemostasis and thrombosis.

"Ta panta rhei"

In this issue of Blood, Evans et al report that fractal analysis of the mechanical properties of whole-blood clots defines a unique property of the incipient clot that can be used as a functional biomarker of hemostasis.

Thrombin flux and wall shear rate regulate fibrin fiber deposition state during polymerization under flow

Thrombin is released as a soluble enzyme from the surface of platelets and tissue-factor-bearing cells to trigger fibrin polymerization during thrombosis under flow conditions. Although isotropic fibrin polymerization under static conditions involves protofibril extension and lateral aggregation leading to a gel, factors regulating fiber growth are poorly quantified under hemodynamic flow due to the difficulty of setting thrombin fluxes. A membrane microfluidic device allowed combined control of both thrombin wall flux (10(-13) to 10(-11) nmol/mum(2) s) and the wall shear rate (10-100 s(-1)) of a flowing fibrinogen solution. At a thrombin flux of 10(-12) nmol/mum(2) s, both fibrin deposition and fiber thickness decreased as the wall shear rate increased from 10 to 100 s(-1). Direct measurement and transport-reaction simulations at 12 different thrombin flux-wall shear rate conditions demonstrated that two dimensionless numbers, the Peclet number (Pe) and the Damkohler number (Da), defined a state diagram to predict fibrin morphology. For Da < 10, we only observed thin films at all Pe. For 10 < Da < 900, we observed either mat fibers or gels, depending on the Pe. For Da > 900 and Pe < 100, we observed three-dimensional gels. These results indicate that increases in wall shear rate quench first lateral aggregation and then protofibril extension.

Toward an understanding of fibrin branching structure

The blood clotting enzyme thrombin converts fibrinogen molecules into fibrin monomers which polymerize to form a fibrous three-dimensional gel. The concentration of thrombin affects the architecture of the resulting gel, in particular, a higher concentration of thrombin produces a gel with more branch points per unit volume and with shorter fiber segments between branch points. We propose a mechanism by which fibrin branching can occur and show that this mechanism can lead to dependence of the gel's structure (at the time of gelation) on the rate at which monomer is supplied. A higher rate of monomer supply leads to a gel with a higher branch concentration and with shorter fiber segments between branch points. The origin of this dependence is explained.

[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.

Polyphosphate modifies the fibrin network and down-regulates fibrinolysis by attenuating binding of tPA and plasminogen to fibrin

Activated platelets secrete a negatively charged polymer, polyphosphate (polyP). Here, we explore the interactions of polyP with fibrin(ogen) and its effect on fibrin structure and fibrinolysis. Electrophoretic mobility and binding assays indicate that polyP interacts with fibrinogen and soluble fibrin. Clots formed in the presence of polyP exhibited reduced turbidity and permeability indicative of a tighter fibrin network, but these changes were not related to cross-linking or fibrinopeptide release. Microscopy showed a change in fibrin distribution in clots formed with polyP; with formation of tight aggregates of fibrin fibers interspaced with large pores in contrast to homogenous fiber distribution in control clots. Lysis by tissue plasminogen activator (tPA) and plasminogen or plasmin was delayed in clots formed with polyP and depended on both the activator and polyP concentration. Adding polyP to the clot after fibrin formation or to repolymerizing soluble fibrin did not affect lysis, indicating changes induced by polyP occur at the level of conversion of fibrinogen to fibrin. Surface plasmon resonance showed that the presence of polyP reduced the binding of both plasminogen and tPA to partially lysed fibrin surfaces. These data show that polyP directly influences fibrin architecture and attenuates fibrinolysis through reduced binding of fibrinolytic proteins.

Effects of cigarette smoke exposure on clot dynamics and fibrin structure: an ex vivo investigation

OBJECTIVE

The purpose of this study was to examine the effect of cigarette smoke exposure (CSE) on clot dynamics and fibrin architecture and to isolate the relative contribution of platelets and fibrinogen to clot dynamics.

METHODS AND RESULTS

From young healthy males smokers (n=34) and nonsmokers (n=34) a baseline blood was drawn, and smokers had another blood draw after smoking 2 regular cigarettes. Using thromboelastography (TEG) the degree of platelet-fibrin interaction was measured. In additional experiments, abciximab (20 microg/mL) was added to the smokers samples (n=27) to reduce the effects of platelet function from the TEG parameters. The maximum clot strength (G) obtained with abciximab measured mainly the contribution of fibrinogen to clot strength (GF). By subtracting GF from G, the contribution of platelets to clot strength (GP) was presumed. A significant difference was found for all TEG parameters between nonsmokers versus postsmoking and pre- versus postsmoking samples. Postsmoking both GF and GP were significantly higher as compared to presmoking. On electron microscopy and turbidity analysis, postsmoking fibrin clots were significantly different compared to presmoking and nonsmoking samples.

CONCLUSIONS

Acute CSE changes clot dynamics and alters fibrin architecture. Both functional changes in fibrinogen and platelets appear to contribute to heightened thrombogenicity after acute CSE.

In situ formation, manipulation, and imaging of droplet-encapsulated fibrin networks

The protein fibrin plays a principal role in blood clotting and forms robust three dimensional networks. Here, microfluidic devices have been tailored to strategically generate and study these bionetworks by confinement in nanoliter volumes. The required protein components are initially encapsulated in separate droplets, which are subsequently merged by electrocoalescence. Next, distinct droplet microenvironments are created as the merged droplets experience one of two conditions: either they traverse a microfluidic pathway continuously, or they "park" to fully evolve an isotropic network before experiencing controlled deformations. High resolution fluorescence microscopy is used to image the fibrin networks in the microchannels. Aggregation (i.e."clotting") is significantly affected by the complicated flow fields in moving droplets. In stopped-flow conditions, an isotropic droplet-spanning network forms after a suitable ripening time. Subsequent network deformation, induced by the geometric structure of the microfluidic channel, is found to be elastic at low rates of deformation. A shape transition is identified for droplets experiencing rates of deformation higher than an identified threshold value. In this condition, significant densification of protein within the droplet due to hydrodynamic forces is observed. These results demonstrate that flow fields considerably affect fibrin in different circumstances exquisitely controlled using microfluidic tools.

Ultrathin self-assembled fibrin sheets

Fibrin polymerizes into the fibrous network that is the major structural component of blood clots and thrombi. We demonstrate that fibrin from three different species can also spontaneously polymerize into extensive, molecularly thin, 2D sheets. Sheet assembly occurs in physiologic buffers on both hydrophobic and hydrophilic surfaces, but is routinely observed only when polymerized using very low concentrations of fibrinogen and thrombin. Sheets may have been missed in previous studies because they may be very short-lived at higher concentrations of fibrinogen and thrombin, and their thinness makes them very difficult to detect. We were able to distinguish fluorescently labeled fibrin sheets by polymerizing fibrin onto micro-patterned structured surfaces that suspended polymers 10 microm above and parallel to the cover-glass surface. We used a combined fluorescence/atomic force microscope system to determine that sheets were approximately 5 nm thick, flat, elastic and mechanically continuous. Video microscopy of assembling sheets showed that they could polymerize across 25-microm channels at hundreds of microm(2)/sec (approximately 10(13) subunits/s x M), an apparent rate constant many times greater than those of other protein polymers. Structural transitions from sheets to fibers were observed by fluorescence, transmission, and scanning electron microscopy. Sheets appeared to fold and roll up into larger fibers, and also to develop oval holes to form fiber networks that were "pre-attached" to the substrate and other fibers. We propose a model of fiber formation from sheets and compare it with current models of end-wise polymerization from protofibrils. Sheets could be an unanticipated factor in clot formation and adhesion in vivo, and are a unique material in their own right.

Cellular procoagulant activity dictates clot structure and stability as a function of distance from the cell surface

BACKGROUND

Thrombin concentration modulates fibrin structure and fibrin structure modulates clot stability; however, the impact of localized, cell surface-driven in situ thrombin generation on fibrin structure and stability has not previously been evaluated.

METHODS AND RESULTS

Human fibroblasts were incubated with factors Xa, Va, prothrombin and fibrinogen, or plasma. Fibrin formation, structure, and lysis were examined using laser scanning confocal microscopy and transmission electron microscopy. In situ thrombin generation on the cell surface produced clots with a significantly denser fiber network in a 10-microm region proximal versus distal to (40 to 50 microm) the cell surface. This morphology was not altered by addition of integrin-blocking RGDS peptide and was not apparent in clots made by exogenous thrombin addition, suggesting that spatial morphology was dictated predominantly by localized thrombin generation on the fibroblast surface. The fibrin network lysed more rapidly distal versus proximal to the cell surface, suggesting that the structural heterogeneity of the clot affected its fibrinolytic stability.

CONCLUSIONS

In situ thrombin generation on the cell surface modulates the three-dimensional structure and stability of the clot. Thrombus formation in vivo may reflect the ability of the local cell population to support thrombin generation and, therefore, the three-dimensional structure and stability of the fibrin network.