Establishing the Transient Mass Balance of Thrombosis: From Tissue Factor to Thrombin to Fibrin Under Venous Flow

In situ polymer gelation in confined flow controls intermittent dynamics

Polymer flows through pores, nozzles and other small channels govern engineered and naturally occurring dynamics in many processes, from 3D printing to oil recovery in the earth's subsurface to a wide variety of biological flows. The crosslinking of polymers can change their material properties dramatically, and it is advantageous to know a priori whether or not crosslinking polymers will lead to clogged channels or cessation of flow. In this study, we investigate the flow of a common biopolymer, alginate, while it undergoes crosslinking by the addition of a crosslinker, calcium, driven through a microfluidic channel at constant flow rate. We map the boundaries defining complete clogging and flow as a function of flow rate, polymer concentration, and crosslinker concentration. Interestingly, the boundaries of the dynamic behavior qualitatively match the thermodynamic jamming phase diagram of attractive colloidal particles. That is, polymer clogging occurs in a region analogous to colloids in a jammed state, while the polymer flows in regions corresponding to colloids in a liquid phase. However, between the dynamic regimes of complete clogging and unrestricted flow, we observe a remarkable phenomenon in which the crosslinked polymer intermittently clogs the channel. This pattern of deposition and removal of a crosslinked gel is simultaneously highly reproducible, long-lasting, and controllable by system parameters. Higher concentrations of polymer and cross-linker result in more frequent ablation, while gels formed at lower component concentrations ablate less frequently. Upon ablation, the eluted gel maintains its shape, resulting in micro-rods several hundred microns long. Our results suggest both rich dynamics of intermittent flows in crosslinking polymers and the ability to control them.

Fluorescent peptide for detecting factor XIIIa activity and fibrin in whole blood clots forming under flow

Background

During clotting, thrombin generates fibrin monomers and activates plasma-derived transglutaminase factor (F) XIIIa; collagen and thrombin-activated platelets offer thrombin-independent cellular FXIIIa (cFXIIIa) for clotting. Detecting fibrin on collagen and tissue factor surfaces in whole blood clotting typically uses complex reagents like fluorescent fibrinogen or antifibrin antibody.

Objectives

We want to test whether the peptide using the α2- antiplasmin crosslinking mechanism by FXIIIa is a useful tool in both monitoring FXIIIa activity, and visualize and monitor fibrin formation, deposition, and extent of crosslinking within fibrin structures in whole blood clots formed under flow.

Methods

We tested a fluorescent peptide derived from α2-antiplasmin sequence (Ac-GNQEQVSPLTLLKWC-fluorescein) to monitor the location of transglutaminase activity and fibrin during whole blood clotting under microfluidic flow (wall shear rate, 100 s-1).

Results

The peptide rapidly colocated with accumulating fibrin due to transglutaminase activity, confirmed by Phe-Pro-Arg-chloromethylketone inhibiting fibrin and peptide labeling. The FXIIIa inhibitor T101 had no effect on fibrin generation but ablated the labeling of fibrin by the peptide. Similarly, Gly-Pro-Arg-Pro abated fibrin formation and thus strongly attenuated the peptide signal. At arterial wall shear rate (1000 s-1), less fibrin was formed, and consequently, less peptide labeling of fibrin was detected compared with venous conditions. The addition of tissue plasminogen activator caused a reduction of both fibrin and peptide signals. Also, the peptide strongly colocalized with fibrin (but not platelets) in clots from laser-injured mouse cremaster arterioles. For clotting under flow, FXIIIa activity was most likely plasma-derived since a RhoA inhibitor did not block α2-antiplasmin fragment cross-linking to fibrin.

Conclusion

Under flow, the majority of FXIIIa-dependent fibrin labeling with peptide during clotting was distal of thrombin activity. The synthetic peptide provided a strong and sustained labeling of fibrin as it formed under flow.

Investigating thrombin-loaded fibrin in whole blood clot microfluidic assay via fluorogenic peptide

During clotting under flow, thrombin rapidly generates fibrin, whereas fibrin potently sequesters thrombin. This co-regulation was studied using microfluidic whole blood clotting on collagen/tissue factor, followed by buffer wash, and a start/stop cycling flow assay using the thrombin fluorogenic substrate, Boc-Val-Pro-Arg-AMC. After 3 min of clotting (100 s-1) and 5 min of buffer wash, non-elutable thrombin activity was easily detected during cycles of flow cessation. Non-elutable thrombin was similarly detected in plasma clots or arterial whole blood clots (1000 s-1). This thrombin activity was ablated by Phe-Pro-Arg-chloromethylketone (PPACK), apixaban, or Gly-Pro-Arg-Pro to inhibit fibrin. Reaction-diffusion simulations predicted 108 nM thrombin within the clot. Heparin addition to the start/stop assay had little effect on fibrin-bound thrombin, whereas addition of heparin-antithrombin (AT) required over 6 min to inhibit the thrombin, indicating a substantial diffusion limitation. In contrast, heparin-AT rapidly inhibited thrombin within microfluidic plasma clots, indicating marked differences in fibrin structure and functionality between plasma clots and whole blood clots. Addition of GPVI-Fab to blood before venous or arterial clotting (200 or 1000 s-1) markedly reduced fibrin-bound thrombin, whereas GPVI-Fab addition after 90 s of clotting had no effect. Perfusion of AF647-fibrinogen over washed fluorescein isothiocyanate (FITC)-fibrin clots resulted in an intense red layer around, but not within, the original FITC-fibrin. Similarly, introduction of plasma/AF647-fibrinogen generated substantial red fibrin masses that did not penetrate the original green clots, demonstrating that fibrin cannot be re-clotted with fibrinogen. Overall, thrombin within fibrin is non-elutable, easily accessed by peptides, slowly accessed by average-sized proteins (heparin/AT), and not accessible to fresh fibrinogen.

Activation of Most Toll-Like Receptors in Whole Human Blood Attenuates Platelet Deposition on Collagen under Flow

Platelets have toll-like receptors (TLRs); however, their function in thrombosis or hemostasis under flow conditions is not fully known. Thrombin-inhibited anticoagulated whole blood was treated with various TLR agonists and then perfused over fibrillar collagen using microfluidic assay at venous wall shear rate (100 s^-1). Platelet deposition was imaged with fluorescent anti-CD61. For perfusion of whole blood without TLR agonist addition, platelets rapidly accumulated on collagen and eventually occluded the microchannels. Interestingly, most of the tested TLR agonists (Pam₃CKS₄, MALP-2, polyinosinic-polycytidylic acid HMW, imiquimod, and CpG oligodeoxynucleotides) strongly reduced platelet deposition on collagen, while only the TLR4 agonist endotoxin lipopolysaccharide (LPS) enhanced deposition. Following 90 sec of deposition under flow of untreated blood, the addition of various TLR-7 agonists (imiquimod, vesatolimod, and GSK2245035) all caused immediate blockade of further platelet deposition. Since TLR signaling can activate nuclear factor-kappaB (NF-κB), the IKK-inhibitor (IKK inhibitor VII) and NF-κB inhibitor (Bay 11-7082) were tested. The IKK/NF-κB inhibitors strongly inhibited platelet deposition under flow. Furthermore, addition of Pam3CSK4 (TLR1/2 ligand), MALP-2 (TLR2/6 ligand), and Imquimod (TLR7 ligand) reduced phosphotidylserine (PS) exposure. Activation of TLR1/2, TLR2/6, TLR3, TLR7, and TLR9 in whole blood reduced platelet deposition under flow on collagen; however, LPS (major Gram negative bacterial pathogenic component) activation of LTR4 was clearly prothrombotic.

Anti-GPVI Fab reveals distinct roles for GPVI signaling in the first platelet layer and subsequent layers during microfluidic clotting on collagen with or without tissue factor

The collagen receptor glycoprotein VI (GPVI) drives strong platelet activation, however its role at later stages of clotting remains less clear. Controlled timing of addition of anti-human GPVI Fab (clone E12) with microfluidic venous whole blood flow over collagen (± lipidated tissue factor, TF) produced distinct effects on platelets, fibrin, P-selectin exposure, and phosphatidylserine (PS) exposure. On collagen alone, Fab present initially potently reduced platelet deposition on collagen, while Fab added 90 s after initial platelet deposition, stopped subsequent platelet accumulation (despite the absence of fibrin). With thrombin generation via TF, Fab added at either t = 0 or 90 s had no effect on platelet deposition. However, Fab added initially, but not at 90-s, blocked fibrin formation. Gly-Pro-Arg-Pro ablated fibrin formation without effect on platelet accumulation (regardless of Fab added at t = 0 or 90 s), indicating thrombin signaling can suffice over GPVI signaling. Still, Fab moderately reduced P-selectin exposure with thrombin present and fibrin absent. On collagen/TF, Fab present initially ablated PS exposure, but had no effect when added 30 to 90-s later. The thrombin generated via PS exposure had an important role in driving platelet deposition in the presence of Fab, since inhibition of PS via annexin V binding in the presence of Fab significantly inhibited platelet deposition. We conclude GPVI signaling in the first platelet layer on collagen dictates thrombin and fibrin production, but the role of GPVI at subsequent times after formation of the first monolayer is obscured by thrombin-induced signaling.

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.

ADP and thromboxane inhibitors both reduce global contraction of clot length, while thromboxane inhibition attenuates internal aggregate contraction

Platelet contractility drives clot contraction to enhance clot density and stability. Clot contraction is typically studied under static conditions, with fewer studies of wall-adherent platelet clots formed under flow. We tested the effect of inhibitors of ADP and/or thromboxane A2 (TXA2) signaling on clot contraction. Using an eight-channel microfluidic device, we perfused PPACK-treated whole blood (WB) ± acetylsalicylic acid (ASA), 2-methylthioAMP (2-MeSAMP), and/or MRS-2179 over collagen (100/s) for 7.5 min, then stopped flow to observe contraction for 7.5 minutes. Two automated imaging methods scored fluorescent platelet percent contraction over the no-flow observation period: (1) “global” measurement of clot length and (2) “local” changes in surface area coverage of the numerous platelet aggregates within the clot. Total platelet fluorescence intensity (FI) decreased with concomitant decrease in global aggregate contraction when ASA, 2-MeSAMP, and/or MRS-2179 were present. Total platelet FI and global aggregate contraction were highly correlated ( R²  = 0.87). In contrast, local aggregate contraction was more pronounced than global aggregate contraction across all inhibition conditions. However, ASA significantly reduced local aggregate contraction relative to conditions without TXA2 inhibition. P-selectin display was significantly reduced by ADP and TXA2 inhibition, but there was limited detection of global or local aggregate contraction in P-selectin-positive platelets across all conditions, as expected for densely packed “core” platelets. Our results demonstrate that global aggregate contraction is inhibited by ASA, 2-MeSAMP, and MRS-2179, while ASA more potently inhibited local aggregate contraction. These results help resolve how different platelet antagonists affect global and local clot structure and function.

Sensitivity analysis of a reduced model of thrombosis under flow: Roles of Factor IX, Factor XI, and γ'-Fibrin

A highly reduced extrinsic pathway coagulation model (8 ODEs) under flow considered a thin 15-micron platelet layer where transport limitations were largely negligible (except for fibrinogen) and where cofactors (FVIIa, FV, FVIII) were not rate-limiting. By including thrombin feedback activation of FXI and the antithrombin-I activities of fibrin, the model accurately simulated measured fibrin formation and thrombin fluxes. Using this reduced model, we conducted 10,000 Monte Carlo (MC) simulations for ±50% variation of 5 plasma zymogens and 2 fibrin binding sites for thrombin. A sensitivity analysis of zymogen concentrations indicated that FIX activity most influenced thrombin generation, a result expected from hemophilia A and B. Averaging all MC simulations confirmed both the mean and standard deviation of measured fibrin generation on 1 tissue factor (TF) molecule per μm2. Across all simulations, free thrombin in the layer ranged from 20 to 300 nM (mean: 50 nM). The top 2% of simulations that produced maximal fibrin were dominated by conditions with low antithrombin-I activity (decreased weak and strong sites) and high FIX concentration. In contrast, the bottom 2% of simulations that produced minimal fibrin were dominated by low FIX and FX. The percent reduction of fibrin by an ideal FXIa inhibitor (FXI = 0) ranged from 71% fibrin reduction in the top 2% of MC simulations to only 34% fibrin reduction in the bottom 2% of MC simulations. Thus, the antithrombotic potency of FXIa inhibitors may vary depending on normal ranges of zymogen concentrations. This reduced model allowed efficient multivariable sensitivity analysis.

Thrombosis and Hemodynamics: external and intrathrombus gradients

Distinct from dilute, isotropic, and homogeneous reaction systems typically used in laboratory kinetic assays, blood is concentrated, two-phase, flowing, and highly anisotropic when clotting on a surface. This review focuses on spatial gradients that are generated and can dictate thrombus structure and function. Novel experimental and computational tools have recently emerged to explore reaction-transport coupling during clotting. Multiscale simulations help bridge tissue length scales (the coronary arteries) to millimeter scales of a growing clot to the microscopic scale of single-cell signaling and adhesion. Microfluidic devices help create and control pathological velocity profiles, albeit at a low Reynolds number. Since rate processes and force loading are often coupled, this review highlights prevailing convective-diffusive transport physics that modulate cellular and molecular processes during thrombus formation.

Effects of Heparin and Bivalirudin on Thrombin-Induced Platelet Activation: Differential Modulation of PAR Signaling Drives Divergent Prothrombotic Responses

Heparin and bivalirudin are widely used as anticoagulants in the setting of acute thrombosis. In this study, we investigated how these drugs affect the ability of thrombin to generate a prothrombotic platelet response via activation of the protease-activated receptors (PARs) 1 and 4. We examined the effects of heparin/antithrombin and bivalirudin on PAR1- and PAR4-mediated intracellular calcium mobilization, aggregation, α-granule release, and procoagulant membrane exposure in platelets exposed to thrombin concentrations likely to be encountered in the thrombus microenvironment during thrombosis. At physiological antithrombin levels, heparin treatment resulted in complete and sustained inhibition of thrombin-induced PAR4-mediated platelet activation, but transient PAR1 signaling was sufficient to elicit significant α-granule release and platelet aggregation. In contrast, bivalirudin treatment resulted in rapid and profound inhibition of signaling from both PAR receptors, followed by a delayed phase of PAR4-mediated platelet activation, resulting in a robust prothrombotic response. Combination treatment with bivalirudin and subtherapeutic concentrations of heparin completely inhibited the residual platelet activation observed with single drug treatment at all time-points. Our results show that heparin and bivalirudin have different and complementary inhibitory effects on the activation of PAR1 and PAR4 by thrombin.

Thermal shift assay to probe melting of thrombin, fibrinogen, fibrin monomer, and fibrin: Gly-Pro-Arg-Pro induces a fibrin monomer-like state in fibrinogen

BACKGROUND

Thrombin activates fibrinogen and binds the fibrin E-domain (K_d ~ 2.8 μM) and the splice variant γ'-domain (K_d ~ 0.1 μM). We investigated if the loading of D-Phe-Pro-Arg-chloromethylketone inhibited thrombin (PPACK-thrombin) onto fibrin could enhance fibrin stability.

METHODS

A 384-well plate thermal shift assay (TSA) with SYPRO-orange provided melting temperatures (T_m) of thrombin, PPACK-thrombin, fibrinogen, fibrin monomer, and fibrin.

RESULTS

Large increases in T_m indicated that calcium led to protein stabilization (0 vs. 2 mM Ca²⁺) for fibrinogen (54.0 vs. 62.3 °C) and fibrin (62.3 vs. 72.2 °C). Additionally, active site inhibition with PPACK dramatically increased the T_m of thrombin (58.3 vs. 78.3 °C). Treatment of fibrinogen with fibrin polymerization inhibitor GPRP increased fibrinogen stability by ΔT_m = 9.3 °C, similar to the ΔT_m when fibrinogen was converted to fibrin monomer (ΔT_m = 8.8 °C) or to fibrin (ΔT_m = 10.4 °C). Addition of PPACK-thrombin at high 5:1 M ratio to fibrin(ogen) had little effect on fibrin(ogen) T_m values, indicating that thrombin binding does not detectably stabilize fibrin via a putative bivalent E-domain to γ'-domain interaction.

CONCLUSIONS

TSA was a sensitive assay of protein stability and detected: (1) the effects of calcium-stabilization, (2) thrombin active site labeling, (3) fibrinogen conversion to fibrin, and (4) GPRP induced changes in fibrinogen stability being essentially equivalent to that of fibrin monomer or polymerized fibrin.

SIGNIFICANCE

The low volume, high throughput assay has potential for use in understanding interactions with rare or mutant fibrin(ogen) variants.

Wall shear rates in human and mouse arteries: Standardization of hemodynamics for in vitro blood flow assays: Communication from the ISTH SSC subcommittee on biorheology

Hemodynamics play a central role in hemostasis and thrombosis by affecting all aspects linked to platelet functions and coagulation. In vitro flow devices are extensively used in basic research, pharmacological studies, antiplatelet agent screening, and development of diagnostic tools. Because hemodynamic conditions vary tremendously throughout the vascular tree and among different (patho)physiological processes, it is important to use flow conditions based on relevant biorheological reference ranges. Surprisingly, it is particularly difficult to find a concise overview of relevant hemodynamic parameters in various human and mouse vessels. To our knowledge, this is the first time an inventory of flow conditions in healthy, non-diseased, human and mouse vessels has been created. The objective of providing such a repertoire is to aid researchers in the field of hemostasis and thrombosis in choosing rheological conditions relevant in in vitro flow experiments and to promote harmonization of flow-based assays to facilitate comparative evaluations between studies. With reference to the human, we discuss relevant similarities and discrepancies in wall shear rates in the mouse, which are typically one order of magnitude greater in agreement with allometric scaling laws between species. Importantly, we bring the attention of the researchers to the fact that the relevant range of average wall shear rates in human arteries where clinically relevant arterial thrombosis occurs may fall as low as 100 to 200 s-1, thus significantly overlapping with what are considered "venous" shear rates. The same range for the murine arteries used for arterial thrombosis models may significantly exceed 1000 s-1 reaching values considered to be "pathological."

Effects of Astragalus Polysaccharides Nanoparticles on Cerebral Thrombosis in SD Rats

Objective

To investigate the efficacy and improvement of Astragalus polysaccharides (APS) and APS-nano on cerebral thrombosis in rats.

Methods

A total of 72 SD rats were randomly divided into NC group, Model group, APS-Nano group, and APS group. The cerebral thrombosis Model of SD rats was established by injecting compound thrombus inducer into the internal carotid artery. After 14 days of different intervention treatments, the TTC staining of brain tissue were performed, and A/left brain wet weight ratio, left brain/right brain wet weight ratio, blood rheology indexes, and coagulation function indexes of cerebral thrombosis were measured. ELISA was used to measure the contents of thromboxane 2 (TXB2), 6-keto-prostaglandin F1α (6-Keto-PGF1α), tissue factor (TF), neuron-specific enolase (NSE), S-100β, catenin (CAT), superoxide dismutase (SOD), as well as malondialdehyde (MDA). The binding specificity between miR-885-3p and TF was verified by the double-luciferin reporting experiment, and western blot was used to measure the expression level of TF protein.

Results

Compared with the Model group, after treatment with APS-nano or APS, the ratio of left brain/right brain wet weight decreased significantly. Whole blood low shear viscosity (WBLSV), whole blood high shear viscosity (WBHSV), plasma viscosity (PV), and erythrocyte aggregation index (Arbc) was all reduced. In addition, prothrombin time (PT) and activated partial thromboplastin time (APTT) were increased, and fibrinogen (FIB) content was decreased. The expression of TXB2, 6-Keto-PGF1α, and TF showed a downward trend. Similarly, the expression of TF protein was decreased. Furthermore, the contents of NSE and S-100β proteins were all decreased, whereas the contents of CAT and SOD were increased, and the contents of MDA was decreased. At the same dose, compared with APS treatment, APS-nano treatment had a significant inhibitory effect on cerebral thrombosis in rats. Finally, we found that TF is a target gene of miR-885-3p and specifically binds to miR-885-3p.

Conclusion

APS has a significant inhibitory effect on the formation of cerebral thrombosis induced by compound thrombus inducers. Moreover, APS-nano has a more significant inhibitory effect on cerebral thrombosis. Meanwhile, the regulation of miR-885-3p regulating TF expression may be related to the occurrence of cerebral thrombosis.

Core and shell platelets of a thrombus: A new microfluidic assay to study mechanics and biochemistry

BACKGROUND

Hemostatic clots have a P-selectin positive platelet core covered with a shell of P-selectin negative platelets.

OBJECTIVE

To develop a new human blood microfluidic assay to interrogate core/shell mechanics.

METHODS

A 2-stage assay perfused whole blood over collagen/± tissue factor (TF) for 180 seconds at 100 s-1 wall shear rate, followed by buffer perfusion at either 100 s-1 (venous) or 1000 s-1 (arterial). This microfluidic assay used an extended channel height (120 µm), allowing buffer perfusion well before occlusion.

RESULTS

Clot growth on collagen stopped immediately with buffer exchange, revealing ~10% reduction in platelet fluorescence intensity (at 100 s-1) and ~30% (at 1000 s-1) by 1200 seconds. Thrombin generation (on collagen/TF) reduced erosion at either buffer flow rate. P-selectin-positive platelets were stable (no erosion) against 1000 s-1, in contrast to P-selectin negative platelets. Thrombin inhibition (with D-Phe-Pro-Arg-CMK) reduced the number of P-selectin-positive platelets and lowered thrombus stability through the reduction of P-selectin-positive platelets. Interestingly, fibrin inhibition (with H-Gly-Pro-Arg-Pro-OH acetate salt) increased the number of P-selectin-positive platelets but did not lower stability, suggesting that fibrin was only in the core region. Thromboxane inhibition reduced P-selectin-positive platelets and caused a nearly 60% reduction of the clot at arterial buffer flow. P2Y1 antagonism reduced clot size and the number of P-selectin-positive platelets and reduced the stability of P-selectin-negative platelets.

CONCLUSION

The 2-stage assay (extended channel height plus buffer exchange) interrogated platelet stability using human blood. Under all conditions, P-selectin-positive platelets never left the clot.

Intrathrombus Fibrin Attenuates Spatial Sorting of Phosphatidylserine Exposing Platelets during Clotting Under Flow

BACKGROUND

 As thrombosis proceeds, certain platelets in a clot expose phosphatidylserine (PS) on their outer membrane. These PS⁺ platelets subsequently sort to the perimeter of the mass via platelet contraction. It remains unclear how thrombin and fibrin may alter PS⁺ platelet sorting within a clot.

OBJECTIVE

 We investigated the role of fibrin in PS⁺ platelet sorting.

METHODS

 We used an 8-channel microfluidic assay of clotting over collagen (±tissue factor) at 100 s^-1 initial wall shear rate. Temporal PS⁺ platelet sorting was measured using a Pearson's correlation coefficient between the annexin V distribution in a clot at 9 versus 15 minutes. Spatial PS⁺ platelet sorting was measured using an autocorrelation metric of the final annexin V distribution.

RESULTS

 By 6 minutes, PS⁺ platelets were distributed throughout the platelet deposits and became highly spatially sorted by 15 minutes when thrombin and fibrin were blocked with Phe-Pro-Arg-chloromethylketone (PPACK). Fibrin polymerization (no PPACK) attenuated temporal and spatial PS sorting and clot contraction. With Gly-Pro-Arg-Pro (GPRP) added to block fibrin polymerization, PS sorting was prominent as was clot contraction. Exogenously added tissue plasminogen activator drove fibrinolysis that in turn promoted clot contraction and PS sorting, albeit to a lesser degree than the PPACK or GPRP conditions. Clots lacking fibrin displayed 3.6 times greater contraction than clots with fibrin.

CONCLUSION

 PS sorting correlated with clot contraction, as previously reported. However, fibrin inversely correlated with both percent contraction and PS sorting. Fibrin attenuated clot contraction and PS sorting relative to clots without fibrin.

Src family kinases inhibition by dasatinib blocks initial and subsequent platelet deposition on collagen under flow, but lacks efficacy with thrombin generation

Kinase inhibitors can pose bleeding risks as platelet signaling evolves during clotting. Using microfluidics (200 s^-1 wall shear rate) to perfuse Factor XIIa-inhibited or thrombin-inhibited whole blood (WB) over collagen ± tissue factor (TF), we explored the potency of the Src family kinase (SFK) inhibitor dasatinib or the spleen tyrosine kinase (Syk) inhibitor GS-9973 present at clot initiation or added after 90 s (via rapid switch to inhibitor-pretreated WB). When initially present, dasatinib potently inhibited platelet deposition on collagen (no TF). Furthermore, dasatinib immediately inhibited subsequent platelet deposition when introduced 90 s after clot initiation. However, when thrombin was generated, dasatinib was markedly less potent against platelet deposition on collagen/TF (but blocked fibrin deposition) and had no effect when added 90 s after clot initiation. Similarly, dasatinib added at 90 s had no effect on clotting on collagen/TF when fibrin was also blocked with Gly-Pro-Arg-Pro, indicating that strong thrombin-induced signaling (but not fibrin-induced signaling) can bypass the SFK inhibition at later times. The Syk inhibitor GS-9973 was less potent than dasatinib when present initially, but inhibited clot growth when added at 90 s, even in the presence of thrombin (±fibrin). Interestingly, the active form (R-406) of fostamatinib inhibits platelet function in only 2 0f 5 healthy blood samples. SFK-inhibitors may have reduced antithrombotic activity and reduced bleeding risks in settings of high TF and local thrombin generation. For oncology patients, SFK-inhibitors like dasatinib may have reduced antithrombotic activity and reduced bleeding risk in settings of local thrombin generation.

Reduced model to predict thrombin and fibrin during thrombosis on collagen/tissue factor under venous flow: Roles of γ'-fibrin and factor XIa

During thrombosis, thrombin generates fibrin, however fibrin reversibly binds thrombin with low affinity E-domain sites (K_D = 2.8 μM) and high affinity γ’-fibrin sites (K_D = 0.1 μM). For blood clotting on collagen/tissue factor (1 TF-molecule/μm²) at 200 s^-1 wall shear rate, high μM-levels of intraclot thrombin suggest robust prothrombin penetration into clots. Setting intraclot zymogen concentrations to plasma levels (and neglecting cofactor rate limitations) allowed the linearization of 7 Michaelis-Menton reactions between 6 species to simulate intraclot generation of: Factors FXa (via TF/VIIa or FIXa), FIXa (via TF/FVIIa or FXIa), thrombin, fibrin, and FXIa. This reduced model [7 rates, 2 K_D’s, enzyme half-lives~1 min] predicted the measured clot elution rate of thrombin-antithrombin (TAT) and fragment F1.2 in the presence and absence of the fibrin inhibitor Gly-Pro-Arg-Pro. To predict intraclot fibrin reaching 30 mg/mL by 15 min, the model required fibrinogen penetration into the clot to be strongly diffusion-limited (actual rate/ideal rate = 0.05). The model required free thrombin in the clot (~100 nM) to have an elution half-life of ~2 sec, consistent with measured albumin elution, with most thrombin (>99%) being fibrin-bound. Thrombin-feedback activation of FXIa became prominent and reached 5 pM FXIa at >500 sec in the simulation, consistent with anti-FXIa experiments. In predicting intrathrombus thrombin and fibrin during 15-min microfluidic experiments, the model revealed “cascade amplification” from 30 pM levels of intrinsic tenase to 15 nM prothrombinase to 15 μM thrombin to 90 μM fibrin. Especially useful for multiscale simulation, this reduced model predicts thrombin and fibrin co-regulation during thrombosis under flow.

During blood clotting events, a complex series of reaction are involved. Simulation gives insights to the concentration of different enzymes which are at too low of concentration to be detected. However, the models are often large and difficult to solve for clotting under flow conditions. With a thin film approximation, we were able to simplify clotting under flow with parameters from literature, with only 3 adjusted in order to fit the experimental data. This model gave insights into the dynamics of the species involved, and the roles of γ’-fibrin and thrombin feedback activation. This reduced model may be useful in further multiscale simulations.

Contact Pathway Function During Human Whole Blood Clotting on Procoagulant Surfaces

Microfluidic thrombosis assays allow the control of anticoagulation, hemodynamics, pharmacology, and procoagulant surfaces containing collagen ± tissue factor (TF). With corn trypsin inhibitor (CTI) ranging from low (1–4 μg/mL) to high levels (40–60 μg/mL), the function of Factor XIIa (FXIIa) can be modulated in the presence of low or high surface TF. With high CTI and no collagen/TF in the assay, no thrombin is generated during 15-min microfluidic perfusion. At low CTI (no TF), the generation of FXIa leads to fibrin polymerization at ~300 s after the initiation of flow over collagen, an onset time shortened at zero CTI and prolonged at high CTI. The engagement of FXIa was difficult to observe for clotting on high TF surfaces due to the dominance of the extrinsic pathway. Low TF surfaces allowed observable crosstalk between extrinsic pathway generation of thrombin and thrombin-mediated activation of FXIa, a feedback detected at >5 min and attenuated with polyphosphate inhibitor. From thrombin-antithrombin immunoassay of the effluent of blood flowing over collagen/TF, the majority of thrombin was found captured on intrathrombus fibrin. Additionally, extreme shear rates (>10,000 s⁻¹) can generate massive von Willebrand Factor fibers that capture FXIIa and FXIa to drive fibrin generation, an event that facilitates VWF fiber dissolution under fibrinolytic conditions. Finally, we found that occlusive sterile thrombi subjected to pressure drops >70 mm-Hg/mm-clots have interstitial stresses sufficient to drive NETosis. These microfluidic studies highlight the interaction of contact pathway factors with the extrinsic pathway, platelet polyphosphate, VWF fibers, and potentially shear-induced NETs.