Nathan E. Hudson

The coagulation cascade generates a three-dimensional fibrin gel that seals wounds and prevents the loss of blood. Extensive research has defined the structural and mechanical characteristics of fibrin gels and the various biochemical, physiological, and pathological factors that alter those properties. However, there has been no direct visualization of fibrin polymerization with the spatial and temporal resolution to characterize the fibrin branching process. Thus, a mechanistic explanation for the origins of fibrin structures remains unresolved, with several hypothesized models for how fibrin branching occurs. Using SimView light-sheet microscopy, we captured videos of fibrin fibers forming junctions and gelling. Results: We observed four distinct pathways for fibrin fibers to interact and bind, all predicated on the observation that tumbling and diffusing fibers collide and stick together. Branching occurred due to the binding of mature fibers, not through the splitting of fibers or through the lateral association of small fibrin oligomers (protofibrils), as had been previously hypothesized. Diffusion simulations of rod-like fibers confirmed the branching mechanisms, the relative frequency of the various binding pathways, and the distribution of fiber angles after binding. Scanning Electron Microscopy (SEM) imaging also corroborated the relative frequency of various types of fibrin junctions. Moreover, SEM enabled the visualization of junctions at different timepoints, demonstrating that additional fibrin strands adhere to the initial binding site, strengthening the interaction. These results establish a mechanistic framework for fibrin branching and gel formation and define an innovative experimental approach for future interrogations of fibrin polymerization.

Background: In the final steps of the coagulation cascade, thrombin converts fibrinogen to fibrin, which polymerizes into a fibrin network that stabilizes blood clots. The concentration of these two proteins can affect network structure. The fibrin network is dissolved by plasmin, by cleaving fibrin peptide chains. In internal fibrinolysis, tissue plasminogen activator (tPA) activates endogenous plasminogen to plasmin, with activation efficiency much enhanced when tPA and plasminogen are bound to fibrin. The effect of varying fibrinogen and thrombin concentrations and the effect of the resulting varying network structure on fibrinolysis are convoluted, with contradictory trends reported in the literature. Objectives: Investigate how fibrinogen and thrombin concentration quantitatively affect fibrin network structure, and how fibrinogen, thrombin concentrations and the resulting, altered network structure affect fibrinolysis, at a given, rate-limiting tPA concentration. Methods: Fibrin clots were generated from diluted, platelet-poor plasma and purified PEAK 1 fibrinogen, at varying fibrinogen and exogenous thrombin concentrations. The effects of varying fibrinogen and thrombin concentrations on network structure were quantified using non-linear power-law equations derived from confocal images. Internal clot lysis time (CLT) was determined using turbidity-based assays as a function of fibrinogen and thrombin concentrations and network structure at fixed, rate-limiting tPA, and therefore fixed plasmin concentration. Results and Conclusions: Network structural features (fiber density, pore size) and CLT fit well to empirical equations of the form k([Fgn]^α× [Thr]^β). Generally, increasing fibrinogen resulted in higher fiber density and thicker fibers, whereas increasing thrombin resulted in higher fiber density and thinner fibers. Internal CLT showed strong dependence on fibrinogen, and negligible dependence on thrombin. This implies that fibrinogen is the main determinant of internal CLT in these systems, while thrombin and, thus, thrombin-dependent formation kinetics and network structure, per se, have a near-negligible effect.

Low-density lipoprotein (LDL) is a major atherogenic lipoprotein, yet its potential to directly modify the fibrin scaffold of blood clots is incompletely understood. Here, we investigated how LDL alters plasma fibrin network architecture and internal fibrinolysis across defined fibrinogen/thrombin conditions. Pooled normal human plasma was supplemented with LDL and clotted with controlled concentrations of fibrinogen and thrombin. Fibrin architecture was visualized by confocal microscopy and quantified by pore-size analysis; clot formation and lysis were monitored turbidimetrically in the presence of tissue plasminogen activator (tPA). Increasing LDL produced a pronounced reduction in fibrin-network pore size across the tested fibrinogen/thrombin conditions. The LDL dependence of pore diameter was well described by a power-law relationship, D_pore=(6.54 ± 0.11)[LDL]^(-0.12 ± 0.02) , (R² = 0.90), with a significant negative LDL exponent (p = 4 × 10⁻⁵). Increasing LDL also prolonged clot lysis time and altered turbidity kinetics. These findings extend epidemiologic and clinical associations between ApoB-containing lipoproteins and hypofibrinolytic clot phenotypes by demonstrating, in a controlled plasma system, that LDL itself can modify fibrin network architecture and fibrinolytic susceptibility. The results support a structure-function role for LDL within the fibrin biomaterial and motivate direct tests of LDL incorporation, protofibril packing, fibrinolytic-protein binding, and single-fiber mechanics.

Background. In the final steps of the coagulation cascade, thrombin converts fibrinogen to fibrin, which polymerizes into a fibrin network that stabilizes blood clots. The concentration of these two proteins can affect network structure. The fibrin network is dissolved by plasmin, by cleaving fibrin peptide chains. In internal fibrinolysis, tissue plasminogen activator (tPA) activates endogenous plasminogen to plasmin, with activation efficiency much enhanced when tPA and plasminogen are bound to fibrin. The effect of varying fibrinogen and thrombin concentrations and the effect of the resulting varying network structure on fibrinolysis are convoluted, with contradictory trends reported in the literature. Objectives. Investigate how fibrinogen and thrombin concentration quantitatively affect fibrin network structure, and how fibrinogen, thrombin concentrations and the resulting, altered network structure affect fibrinolysis, at a given, rate-limiting tPA concentration. Methods. Fibrin clots were generated from diluted, platelet-poor plasma and purified PEAK 1 fibrinogen, at varying fibrinogen and exogenous thrombin concentrations. The effects of varying fibrinogen and thrombin concentrations on network structure were quantified using non-linear power-law equations derived from confocal images. Internal clot lysis time (CLT) was determined using turbidity-based assays as a function of fibrinogen and thrombin concentrations and network structure at fixed, rate-limiting tPA, and therefore fixed plasmin concentration. Results and Conclusions. Network structural features (fiber density, pore size) and CLT fit well to empirical equations of the form k([Fgn]^α× [Thr]^β). Generally, increasing fibrinogen resulted in higher fiber density and thicker fibers, whereas increasing thrombin resulted in higher fiber density and thinner fibers. Internal CLT showed strong dependence on fibrinogen, and negligible dependence on thrombin. This implies that fibrinogen is the main determinant of internal CLT in these systems, while thrombin and, thus, thrombin-dependent formation kinetics and network structure, per se, have a near-negligible effect.

Altered properties of fibrin clots have been associated with bleeding and thrombotic disorders, including hemophilia or trauma and heart attack or stroke. Clotting factors, such as thrombin and tissue factor, or blood plasma proteins, such as fibrinogen, play critical roles in fibrin network polymerization. The concentrations and combinations of these proteins affect the structure and stability of clots, which can lead to downstream complications. The present work includes clots made from plasma and purified fibrinogen and shows how varying fibrinogen and activation factor concentrations affect the fibrin properties under both conditions. We used a combination of scanning electron microscopy, confocal microscopy, and turbidimetry to analyze clot/fiber structure and polymerization. We quantified the structural and polymerization features and found similar trends with increasing/decreasing fibrinogen and thrombin concentrations for both purified fibrinogen and plasma clots. Using our compiled results, we were able to generate multiple linear regressions that predict structural and polymerization features using various fibrinogen and clotting agent concentrations. This study provides an analysis of structural and polymerization features of clots made with purified fibrinogen or plasma at various fibrinogen and clotting agent concentrations. Our results could be utilized to aid in interpreting results, designing future experiments, or developing relevant mathematical models.

Hemostasis is the cessation of bleeding due to the formation of a blood clot. After the completion of wound healing, the blood clot is typically dissolved through the natural process of fibrinolysis, the enzymatic digestion by plasmin of the fibrin fibers that make up its structural scaffold. In vitro studies of fibrinolysis reveal mechanisms regulating these processes and often employ fluorescent microscopy to observe protein colocalization and fibrin digestion. In this study, we investigate the effects of labeling a fibrin network with 20 nm diameter fluorescent beads (fluorospheres) for the purpose of studying fibrinolysis. We observed fibers and 2-D fibrin networks labeled with fluorospheres during fibrinolysis. We found that the labeling of fibrin with fluorospheres can alter fibrinolytic mechanisms. In previous work, we showed that, during lysis, fibrin fibers are cleaved into two segments at a single location. Herein we demonstrate that fibrinolysis can be altered by the concentration of fluorospheres used to label the fibers, with high concentrations of fluorospheres leading to very minimal cleaving. Furthermore, fibers that are left uncleaved after the addition of plasmin often elongate, losing their inherent tension throughout the imaging process. Elongation was especially prominent among fibers that had bundled together due to other cleavage events and was dependent on the concentration of fluorophores used to label fibers. Of the fibers that do cleave, the site at which they cleave also shows a predictable trend dependent on fluorosphere concentration; low concentrations heavily favor cleavage locations at either end of fibrin fiber and high concentrations show no disparity between the fiber ends and other locations along the fiber. After the initial cleavage event bead concentration also affects further digestion, as higher bead concentrations exhibited a larger population of fibers that did not digest further. The results described in this paper indicate that fluorescent labeling strategies can impact fibrinolysis results.

Turbidimetry is an experimental technique often used to study the structure of filamentous networks. To extract structural properties such as filament diameter from turbidimetric data, simplifications to light scattering theory must be employed. In this work, we evaluate the applicability of three commonly utilized turbidimetric analysis approaches, each using slightly different simplifications. We make a specific application towards analyzing fibrin fibers, which form the structural scaffold of blood clots, but the results are generalizable. Numerical simulations were utilized to assess the applicability of each approach across a range of fiber lengths and diameters. Simulation results indicated that all three turbidimetric approaches commonly underestimate fiber diameter, and that the “Carr-Hermans” approach, utilizing wavelengths in the range of 500–800 nm, provided <10% error for the largest number of diameter/length combinations. These theoretical results were confirmed, under select conditions, via the comparison of fiber diameters extracted from experimental turbidimetric data, with diameters obtained using super-resolution microscopy.

Hemostasis in the arterial circulation is mediated by binding of the A1 domain of the ultralong protein von Willebrand factor (VWF) to GPIbα on platelets to form a platelet plug. A1 is activated by tensile force on VWF concatemers imparted by hydrodynamic drag force. The A1 core is protected from force-induced unfolding by a long-range disulfide that links cysteines near its N- and C-termini. The O-glycosylated linkers between A1 and its neighboring domains, which transmit tensile force to A1, are reported to regulate A1 activation for binding to GPIb, but the mechanism is controversial and incompletely defined. Here, we study how these linkers, and their polypeptide and O-glycan moieties, regulate A1 affinity by measuring affinity, kinetics, thermodynamics, hydrogen deuterium exchange (HDX), and unfolding by temperature and urea. The N-linker lowers A1 affinity 40-fold with a stronger contribution from its O-glycan than polypeptide moiety. The N-linker also decreases HDX in specific regions of A1 and increases thermal stability and the energy gap between its native state and an intermediate state, which is observed in urea-induced unfolding. The C-linker also decreases affinity of A1 for GPIbα, but in contrast to the N-linker, has no significant effect on HDX or A1 stability. Among different models for A1 activation, our data are consistent with the model that the intermediate state has high affinity for GPIbα, which is induced by tensile force physiologically and regulated allosterically by the N-linker.