Factor XIII stiffens fibrin clots by causing fiber compaction

Abstract

To cite this article: Kurniawan NA, Grimbergen J, Koopman J, Koenderink GH. Factor XIII stiffens fibrin clots by causing fiber compaction. 2014; 12: 1687-96. Summary. Background: Factor XIII-induced cross-linking has long been associated with the ability of fibrin blood clots to resist mechanical deformation, but how FXIII can directly modulate clot stiffness is unknown. Objectives and Methods: We hypothesized that FXIII affects the selfassembly of fibrin fibers by altering the lateral association between protofibrils. To test this hypothesis, we studied the cross-linking kinetics and the structural evolution of the fibers and clots during the formation of plasmaderived and recombinant fibrins by using light scattering, and the response of the clots to mechanical stresses by using rheology. Results: We show that the lateral aggregation of fibrin protofibrils initially results in the formation of floppy fibril bundles, which then compact to form tight and more rigid fibers. The first stage is reflected in a fast (10 min) increase in clot stiffness, whereas the compaction phase is characterized by a slow (hours) development of clot stiffness. Inhibition of FXIII completely abrogates the slow compaction. FXIII strongly increases the linear elastic modulus of the clots, but does not affect the nonlinear response at large deformations. Conclusions: We propose a multiscale structural model whereby FXIIImediated cross-linking tightens the coupling between the protofibrils within a fibrin fiber, thus making the fiber stiffer and less porous. At small strains, fiber stiffening enhances clot stiffness, because the clot response is governed by the entropic elasticity of the fibers, but once the clot is sufficiently stressed, the modulus is independent of protofibril coupling, because clot stiffness is governed by individual protofibril stretching. J Thromb Haemost

Fibrin structural and diffusional analysis suggests that fibers are permeable to solute transport

“…for fiber radii and the number of protofibrils are in line with those from literature [17,18,23,27] and the same trends were observed for the influence of thrombin on these two parameters ( Fig. 5a and b) [17].…”

“…Note that in this study 20LH and 20HH hydrogels were prepared with the lowest concentration of CaCl2 compared to other conditions, only 4 M (Table 1), which is in line with literature findings where an absence of Ca ++ causes lower mass to length ratios [21] and consequently a lower number of protofibrils. Moreover, factor XIII is known to induce fiber compaction, and thus increase the number of protofibrils [23], which suggests that the observed differences in the number of protofibrils between low and high factor XIII concentrations are probably attributed to differences in the concentration of CaCl2 and not factor XIII. Furthermore, the (weak) influence from factor XIII in compositions of 5 and 10 mg/mL fibrinogen, where the differences in CaCl2 are smaller, also suggests that the observed differences in number of protofibrils, fiber internal density and fiber volume fraction at 20 mg/mL fibrinogen stem from the 5-fold difference in CaCl2.…”

“…Given the spectral range of the measurement ( 1 -2) the method works under the assumption that fibrin hydrogels consist of a network of randomly oriented, thin (rf << 1) and long (L >> 2) cylindrical fibers [19]. Then the mass to length ratio ( ) and the average radius (rf) of the fibers can be estimated as [23]: (1) where (2) (3) t= OD l ln (10), is the wavelength, OD is the optical density, l is the path length in cm, ns is the refractive index of the solvent, dn/dc is the specific refractive index increment for fibrin (= 0.17594 cm 3 /g) [18], c is the fibrinogen concentration in g/mL and NA is the Avogadro number. In the wavelength range of 500-800 nm the dependence of t 5 to 2 is typically linear and the slope of that line gives , while the intercept is dependent on both and rf.…”

“…The complexity of fibrin hydrogels is not only limited to their structure, but it is also evident in their mechanical behavior, which also depends on the fibrin composition [23] and therefore on the structural details of the hydrogel [34,35]. Dilute fibrin networks (0.5-8 mg/mL) exhibit a complex nonlinear elastic response with a transition from entropic elasticity at small strains to enthalpic elasticity at large strains [34].…”

“…Finally, changes in factor XIIIa concentration have been shown to affect the packing of protofibrils within the fibers, with higher concentrations of factor XIIIa leading to fibers with a smaller distance between the constituent protofibrils [23].…”