Simulation of intrathrombus fluid and solute transport using in vivo clot structures with single platelet resolution

Concizumab improves clot formation in hemophilia A under flow

BACKGROUND

Inhibition of tissue factor pathway inhibitor (TFPI) is an emerging therapeutic strategy for treatment of hemophilia. Concizumab is a monoclonal antibody that binds TFPI and blocks its inhibition of factor (F)Xa thereby extending the initiation of coagulation and compensating for lack of FVIII or FIX.

OBJECTIVES

The objective of this in vitro study was to evaluate how concizumab affects clot formation in hemophilia A under flow.

METHODS

Blood was collected from normal controls or people with hemophilia A. An anti-FVIII antibody was added to normal controls to simulate hemophilia A with inhibitory antibodies to FVIII. Whole blood and recombinant activated FVII (rFVIIa, 25 nM) or concizumab (200, 1000, and 4000 ng/mL) were perfused at 100 s-1 over a surface micropatterned with tissue factor (TF) and collagen-related peptide. Platelet and fibrin(ogen) accumulation were measured by confocal microscopy. Static thrombin generation in plasma was measured in response to rFVIIa and concizumab.

RESULTS

Concizumab (1000 and 4000 ng/mL) and rFVIIa both rescued (93%-101%) total platelet accumulation, but only partially rescued (53%-63%) fibrin(ogen) incorporation to normal control levels in simulated hemophilia A. Results using congenital hemophilia A blood confirmed effects of rFVIIa and concizumab. While these 2 agents had similar effect on clot formation under flow, concizumab enhanced thrombin generation in plasma under static conditions to a greater extent than rFVIIa.

CONCLUSION

TFPI inhibition by concizumab enhanced activation and aggregation of platelets and fibrin clot formation in hemophilia A to levels comparable with that of rFVIIa.

Clots reveal anomalous elastic behavior of fiber networks

The adaptive mechanical properties of soft and fibrous biological materials are relevant to their functionality. The emergence of the macroscopic response of these materials to external stress and intrinsic cell traction from local deformations of their structural components is not well understood. Here, we investigate the nonlinear elastic behavior of blood clots by combining microscopy, rheology, and an elastic network model that incorporates the stretching, bending, and buckling of constituent fibrin fibers. By inhibiting fibrin cross-linking in blood clots, we observe an anomalous softening regime in the macroscopic shear response as well as a reduction in platelet-induced clot contractility. Our model explains these observations from two independent macroscopic measurements in a unified manner, through a single mechanical parameter, the bending stiffness of individual fibers. Supported by experimental evidence, our mechanics-based model provides a framework for predicting and comprehending the nonlinear elastic behavior of blood clots and other active biopolymer networks in general.

Effect of mechanical thrombectomy with and without intravenous thrombolysis on the functional outcome of patients with different degrees of thrombus perviousness

PURPOSE

This study aimed to investigate the long-term functional outcome of patients with different degrees of thrombus perviousness (TP) undergoing mechanical thrombectomy alone and those undergoing combined intravenous thrombolysis (IVT) plus mechanical thrombectomy.

METHODS

We conducted a retrospective analysis of consecutive patients with acute ischemic stroke due to large vessel occlusion who underwent mechanical thrombectomy alone or bridging therapy between January 2016 and October 2020. TP was quantified by thrombus attenuation increase (TAI) on admission computed tomography angiography compared with non-contrast computed tomography. After dichotomization of TAI as higher or lower perviousness, Fisher exact tests were performed to estimate the associations of different therapies with favorable functional outcomes [Modified Ranking Scale score at 90 days (90-day mRS) of 0 to 2].

RESULTS

A total of 73 patients were included in our study. 35 (47.9%) thrombi were classified as higher-perviousness clots with TAI of ≥ 24 HU, and the other 38 thrombi were lower-perviousness clots. A favorable outcome with a 90-day mRS of 0 to 2 was observed in 32 patients. In patients with thrombi of lower perviousness, favorable outcome was more common in the bridging therapy group than in the thrombectomy-alone group (p = 0.013), whereas in patients with thrombi of higher perviousness, the long-term neurological outcome did not significantly differ between two therapy groups (p = 0.094).

CONCLUSION

Patients with thrombi of lower perviousness were recommended to undergo intravenous alteplase followed by endovascular thrombectomy, and those with thrombi of higher perviousness could undergo thrombectomy alone.

Image-based flow simulation of platelet aggregates under different shear rates

Hemodynamics is crucial for the activation and aggregation of platelets in response to flow-induced shear. In this paper, a novel image-based computational model simulating blood flow through and around platelet aggregates is presented. The microstructure of aggregates was captured by two different modalities of microscopy images of in vitro whole blood perfusion experiments in microfluidic chambers coated with collagen. One set of images captured the geometry of the aggregate outline, while the other employed platelet labelling to infer the internal density. The platelet aggregates were modelled as a porous medium, the permeability of which was calculated with the Kozeny-Carman equation. The computational model was subsequently applied to study hemodynamics inside and around the platelet aggregates. The blood flow velocity, shear stress and kinetic force exerted on the aggregates were investigated and compared under 800 s-1, 1600 s-1 and 4000 s-1 wall shear rates. The advection-diffusion balance of agonist transport inside the platelet aggregates was also evaluated by local Péclet number. The findings show that the transport of agonists is not only affected by the shear rate but also significantly influenced by the microstructure of the aggregates. Moreover, large kinetic forces were found at the transition zone from shell to core of the aggregates, which could contribute to identifying the boundary between the shell and the core. The shear rate and the rate of elongation flow were investigated as well. The results imply that the emerging shapes of aggregates are highly correlated to the shear rate and the rate of elongation. The framework provides a way to incorporate the internal microstructure of the aggregates into the computational model and yields a better understanding of the hemodynamics and physiology of platelet aggregates, hence laying the foundation for predicting aggregation and deformation under different flow conditions.

Occult blood flow patterns distal to an occluded artery in acute ischemic stroke

Residual blood flow distal to an arterial occlusion in patients with acute ischemic stroke (AIS) is associated with favorable patient outcome. Both collateral flow and thrombus permeability may contribute to such residual flow. We propose a method for discriminating between these two mechanisms, based on determining the direction of flow in multiple branches distal to the occluding thrombus using dynamic Computed Tomography Angiography (dynamic CTA). We analyzed dynamic CTA data of 30 AIS patients and present patient-specific cases that identify typical blood flow patterns and velocities. We distinguished patterns with anterograde (N = 10), retrograde (N = 9), and both flow directions (N = 11), with a large variability in velocities for each flow pattern. The observed flow patterns reflect the interplay between permeability and collaterals. The presented method characterizes distal flow and provides a tool to study patient-specific distal tissue perfusion.

Endovascular Treatment Effect Diminishes With Increasing Thrombus Perviousness: Pooled Data From 7 Trials on Acute Ischemic Stroke

Supplemental Digital Content is available in the text.

Background and Purpose:

Thrombus perviousness estimates residual flow along a thrombus in acute ischemic stroke, based on radiological images, and may influence the benefit of endovascular treatment for acute ischemic stroke. We aimed to investigate potential endovascular treatment (EVT) effect modification by thrombus perviousness.

Methods:

We included 443 patients with thin-slice imaging available, out of 1766 patients from the pooled HERMES (Highly Effective Reperfusion Evaluated in Multiple Endovascular Stroke trials) data set of 7 randomized trials on EVT in the early window (most within 8 hours). Control arm patients (n=233) received intravenous alteplase if eligible (212/233; 91%). Intervention arm patients (n=210) received additional EVT (prior alteplase in 178/210; 85%). Perviousness was quantified by thrombus attenuation increase on admission computed tomography angiography compared with noncontrast computed tomography. Multivariable regression analyses were performed including multiplicative interaction terms between thrombus attenuation increase and treatment allocation. In case of significant interaction, subgroup analyses by treatment arm were performed. Our primary outcome was 90-day functional outcome (modified Rankin Scale score), resulting in an adjusted common odds ratio for a one-step shift towards improved outcome. Secondary outcomes were mortality, successful reperfusion (extended Thrombolysis in Cerebral Infarction score, 2B–3), and follow-up infarct volume (in mL).

Results:

Increased perviousness was associated with improved functional outcome. After adding a multiplicative term of thrombus attenuation increase and treatment allocation, model fit improved significantly (P=0.03), indicating interaction between perviousness and EVT benefit. Control arm patients showed significantly better outcomes with increased perviousness (adjusted common odds ratio, 1.2 [95% CI, 1.1–1.3]). In the EVT arm, no significant association was found (adjusted common odds ratio, 1.0 [95% CI, 0.9–1.1]), and perviousness was not significantly associated with successful reperfusion. Follow-up infarct volume (12% [95% CI, 7.0–17] per 5 Hounsfield units) and chance of mortality (adjusted odds ratio, 0.83 [95% CI, 0.70–0.97]) decreased with higher thrombus attenuation increase in the overall population, without significant treatment interaction.

Conclusions:

Our study suggests that the benefit of best medical care including alteplase, compared with additional EVT, increases in patients with more pervious thrombi.

Increased Perviousness on CT for Acute Ischemic Stroke is Associated with Fibrin/Platelet-Rich Clots

BACKGROUND AND PURPOSE

Clot perviousness in acute ischemic stroke is a potential CT imaging biomarker for mechanical thrombectomy efficacy. We investigated the association among perviousness, clot cellular composition, and first-pass effect.

MATERIALS AND METHODS

In 40 mechanical thrombectomy-treated cases of acute ischemic stroke, we calculated perviousness as the difference in clot density on CT angiography and noncontrast CT. We assessed the proportion of fibrin/platelet aggregates, red blood cells, and white blood cells on clot histopathology. We tested for linear correlation between histologic components and perviousness, differences in components between "high" and "low" pervious clots defined by median perviousness, and differences in perviousness/composition between cases that did and did not achieve a first-pass effect.

RESULTS

Perviousness significantly positively and negatively correlated with the percentage of fibrin/platelet aggregates (P = .001) and the percentage of red blood cells (P = .001), respectively. Higher pervious clots had significantly greater fibrin/platelet aggregate content (P = .042). Cases that achieved a first-pass effect (n = 14) had lower perviousness, though not significantly (P = .055). The percentage of red blood cells was significantly higher (P = .028) and the percentage of fibrin/platelet aggregates was significantly lower (P = .016) in cases with a first-pass effect. There was no association between clot density on NCCT and clot composition or first-pass effect. Receiver operating characteristic analysis indicated that clot composition was the best predictor of first-pass effect (area under receiver operating characteristic curve: percentage of fibrin/platelet aggregates  = 0.731, percentage of red blood cells  = 0.706, perviousness = 0.668).

CONCLUSIONS

Clot perviousness on CT is associated with a higher percentage of fibrin/platelet aggregate content. Histologic data and, to a lesser degree, perviousness may have value in predicting first-pass outcome. Imaging metrics that more strongly reflect clot biology than perviousness may be needed to predict a first-pass effect with high accuracy.

Poroelasticity of (bio)polymer networks during compression: theory and experiment

Soft living tissues like cartilage can be considered as biphasic materials comprising a fibrous complex biopolymer network and a viscous background liquid. Here, we show by a combination of experiment and theoretical analysis that both the hydraulic permeability and the elastic properties of (bio)polymer networks can be determined with simple ramp compression experiments in a commercial rheometer. In our approximate closed-form solution of the poroelastic equations of motion, we find the normal force response during compression as a combination of network stress and fluid pressure. Choosing fibrin as a biopolymer model system with controllable pore size, measurements of the full time-dependent normal force during compression are found to be in excellent agreement with the theoretical calculations. The inferred elastic response of large-pore (μm) fibrin networks depends on the strain rate, suggesting a strong interplay between network elasticity and fluid flow. Phenomenologically extending the calculated normal force into the regime of nonlinear elasticity, we find strain-stiffening of small-pore (sub-μm) fibrin networks to occur at an onset average tangential stress at the gel-plate interface that depends on the polymer concentration in a power-law fashion. The inferred permeability of small-pore fibrin networks scales approximately inverse squared with the fibrin concentration, implying with a microscopic cubic lattice model that the number of protofibrils per fibrin fiber cross-section decreases with protein concentration. Our theoretical model provides a new method to obtain the hydraulic permeability and the elastic properties of biopolymer networks and hydrogels with simple compression experiments, and paves the way to study the relation between fluid flow and elasticity in biopolymer networks during dynamical compression.

Is the endothelial cell responsible for the thrombus core and shell architecture?

Ischemia leading to heart attacks and strokes is the major cause of deaths in the world. This report explores the possibility that intracellular material from ruptured endothelial cells is partially responsible for the heterogeneous core-and-shell blood clot architecture, typically observed using intravital microscopy. As evidence, we present a fluid dynamic argument that platelet agonists emanating from the injury cannot activate platelets in the thrombus core, given that they would have to travel against flow of blood escaping into the extravascular. Furthermore, we demonstrate visual evidence that the core material appears to be continuous and originating from the damaged endothelium. Finally, we present a mechanism, illustrating the steps of platelet recruitment into the thrombus and sealing of the injury. If correct, the model presented herein will be beneficial to the understanding and treatment of heart attacks, strokes and hemophilia.

Longer Length of Delayed-Contrast Filling of Clot on 4-Dimensional Computed Tomographic Angiography Predicts Cardiogenic Embolism

Background and Purpose- We hypothesized the length of delayed-contrast filling sign (DCFS) of intraarterial clot, indicating contrast medium penetration into the thrombus, was associated with stroke etiology. Methods- We retrospectively included patients with large vessel occlusion in anterior circulation who underwent computed tomographic perfusion within 24 hours poststroke onset. We defined DCFS as contrast medium diffusion through the thrombi after the arterial peak phase on 4-dimensional computed tomographic angiography derived from computed tomographic perfusion. We measured the length of DCFS and investigated its value for predicting the stroke etiology. Results- Three hundred twenty-one patients were analyzed, and their stroke etiologies included cardiogenic embolism (CE, n=167), large artery atherosclerosis (n=64), other etiology group (n=4), and undetermined etiology (n=86). CE patients had longer length of DCFS than non-CE patients (2.3 versus 0.5 mm; P<0.001). The optimal cutoff value of DCFS length for predicting CE was 1.5 mm. The sensitivity, specificity, positive predictive value, and negative predictive value of a length of DCFS >1.5 mm for predicting CE were 83.2%, 70.8%, 75.5%, and 79.6%. Conclusions- Longer length of DCFS was associated with CE in patients with large vessel occlusion in anterior circulation, which may provide stroke etiology information.

Clot permeability and histopathology: is a clot's perviousness on CT imaging correlated with its histologic composition?

BACKGROUND

Clot perviousness in large vessel occlusion has been shown to be associated with improved recanalization outcomes with mechanical thrombectomy and intravenous thrombolysis.

OBJECTIVE

To evaluate the association between clot perviousness based on thrombus attenuation increase (TAI) on CT, and histologic composition of clots in acute ischemic stroke (AIS).

METHODS

A retrospective review was completed of patients with AIS secondary to large vessel occlusion, non-contrast CT (NCCT) and CT angiography (CTA) images, and histologic analysis of the retrieved clot. TAI was measured by subtracting clot attenuation on NCCT from the attenuation on CTA. Up to 3 regions of interest (ROIs) were evaluated on each clot; the average attenuation was used for analysis if multiple ROIs were assessed. Pervious clots were defined as TAI ≥10 Hounsfield units (HUs); impervious clots had TAI <10 HU. Histopathologic analyses of clots were assessed for relative compositions of red blood cells (RBCs), white blood cells (WBCs), fibrin, and platelets/other.

RESULTS

57 patients were included. Pervious clots were more likely to be RBC rich (p=0.04); impervious clots were more likely to be fibrin and WBC rich (p=0.01 for both). Pervious clots also had greater RBC density than impervious clots (49.8% and 33.0%, respectively; p=0.006); fibrin density of pervious clots was lower than that of impervious clots (17.8% and 23.2%, respectively; p=0.02).

CONCLUSION

Clot perviousness, assessed on NCCT and CTA imaging, is associated with higher RBC density and lower fibrin density, offering a possible explanation for the higher rates of successful thrombectomy and favorable clinical outcome seen in such patients.

In vivo measurement of blood clot mechanics from computational fluid dynamics based on intravital microscopy images

Ischemia which leads to heart attacks and strokes is one of the major causes of death in the world. Whether an occlusion occurs or not depends on the ability of a growing thrombus to resist flow forces exerted on its structure. This manuscript provides the first known in vivo measurement of how much stress a clot can withstand, before yielding to the surrounding blood flow. Namely, Lattice-Boltzmann Method flow simulations are performed based on 3D clot geometries, which are estimated from intravital microscopy images of laser-induced injuries in cremaster microvasculature of live mice. In addition to reporting the blood clot yield stresses, we also show that the thrombus "core" does not experience significant deformation, while its "shell" does. This indicates that the shell is more prone to embolization. Therefore, drugs should be designed to target the shell selectively, while leaving the core intact to minimize excessive bleeding. Finally, we laid down a foundation for a nondimensionalization procedure which unraveled a relationship between clot mechanics and biology. Hence, the proposed framework could ultimately lead to a unified theory of thrombogenesis, capable of explaining all clotting events. Thus, the findings presented herein will be beneficial to the understanding and treatment of heart attacks, strokes and hemophilia.

Interrelationships between structure and function during the hemostatic response to injury

Extensive studies have detailed the molecular regulation of individual components of the hemostatic system, including platelets, coagulation factors, and regulatory proteins. Questions remain, however, about how these elements are integrated at the systems level within a rapidly changing physical environment. To answer some of these questions, we developed a puncture injury model in mouse jugular veins that combines high-resolution, multimodal imaging with functional readouts in vivo. The results reveal striking spatial regulation of platelet activation and fibrin formation that could not be inferred from studies performed ex vivo. As in the microcirculation, where previous studies have been performed, gradients of platelet activation are readily apparent, as is an asymmetrical distribution of fibrin deposition and thrombin activity. Both are oriented from the outer to the inner surface of the damaged vessel wall, with a greater extent of platelet activation and fibrin accumulation on the outside than the inside. Further, we show that the importance of P2Y₁₂ signaling in establishing a competent hemostatic plug is related to the size of the injury, thus limiting its contribution to hemostasis to specific physiologic contexts. Taken together, these studies offer insights into the organization of hemostatic plugs, provide a detailed understanding of the adverse bleeding associated with a widely prescribed class of antiplatelet agents, and highlight differences between hemostasis and thrombosis that may suggest alternative therapeutic approaches.

Regulation of Platelet Activation and Coagulation and Its Role in Vascular Injury and Arterial Thrombosis

Hemostasis requires tightly regulated interaction of the coagulation system, platelets, blood cells, and vessel wall components at a site of vascular injury. Dysregulation of this response may result in excessive bleeding if the response is impaired, and pathologic thrombosis with vessel occlusion and tissue ischemia if the response is robust. Studies have elucidated the major molecular signaling pathways responsible for platelet activation and aggregation. Antithrombotic agents targeting these pathways are in clinical use. This review summarizes research examining mechanisms by which these multiple platelet signaling pathways are integrated at a site of vascular injury to produce an optimal hemostatic response.

Numerical accuracy comparison of two boundary conditions commonly used to approximate shear stress distributions in tissue engineering scaffolds cultured under flow perfusion

INTRODUCTION

Flow-induced shear stresses have been found to be a stimulatory factor in pre-osteoblastic cells seeded in 3D porous scaffolds and cultured under continuous flow perfusion. However, due to the complex internal structure of the scaffolds, whole scaffold calculations of the local shear forces are computationally intensive. Instead, representative volume elements (RVEs), which are obtained by extracting smaller portions of the scaffold, are commonly used in literature without a numerical accuracy standard.

OBJECTIVE

Hence, the goal of this study is to examine how closely the whole scaffold simulations are approximated by the two types of boundary conditions used to enable the RVEs: "wall boundary condition" (WBC) and "periodic boundary condition" (PBC).

METHOD

To that end, lattice Boltzmann method fluid dynamics simulations were used to model the surface shear stresses in 3D scaffold reconstructions, obtained from high-resolution microcomputed tomography images.

RESULTS

It was found that despite the RVEs being sufficiently larger than 6 times the scaffold pore size (which is the only accuracy guideline found in literature), the stresses were still significantly under-predicted by both types of boundary conditions: between 20% and 80% average error, depending on the scaffold's porosity. Moreover, it was found that the error grew with higher porosity. This is likely due to the small pores dominating the flow field, and thereby negating the effects of the unrealistic boundary conditions, when the scaffold porosity is small. Finally, it was found that the PBC was always more accurate and computationally efficient than the WBC. Therefore, it is the recommended type of RVE.

Thrombus Permeability on Dynamic CTA Predicts Good Outcome after Reperfusion Therapy

BACKGROUND AND PURPOSE

Thrombus permeability assessed on conventional CTA is associated with neurologic outcome in patients with acute ischemic stroke. We aimed to investigate whether dynamic CTA can improve the accuracy of thrombus permeability assessment and its predictive value for outcome.

MATERIALS AND METHODS

We reviewed consecutive patients with acute ischemic stroke who had occlusion of the M1 segment of the middle artery cerebral artery and underwent pretreatment perfusion CT. Thrombus permeability, determined by thrombus attenuation increase (TAI), was assessed on 26-phase dynamic CTA derived from perfusion CT. TAI_max was defined as the maximum TAI among phases; TAI_peak, as TAI of peak arterial phase; TAI_con, as TAI on phase 13. Good outcome was defined as a 3-month mRS score of ≤2.

RESULTS

One hundred four patients were enrolled in the final analysis. The median TAI_max, TAI_peak, and TAI_con were 30.1 HU (interquartile range, 13.0-50.2 HU), 9.5 HU (interquartile range, -1.6-28.7 HU), and 6.6 HU (interquartile range, -5.1-24.4 HU), respectively. Multivariable regression analyses showed that TAI_max (OR = 1.027; 95% CI, 1.007-1.048; P = .008), TAI_peak (OR = 1.029; 95% CI, 1.005-1.054; P = .020), and TAI_con (OR = 1.026; 95% CI, 1.002-1.051; P = .037) were independently associated with good outcome. The areas under the ROC curve of TAI_max, TAI_peak, and TAI_con in predicting good outcome were 0.734, 0.701, and 0.658, respectively.

CONCLUSIONS

Thrombus permeability assessed on dynamic CTA could be a better predictor of outcome after reperfusion therapy than that assessed on conventional single-phase CTA.

Model predictions of deformation, embolization and permeability of partially obstructive blood clots under variable shear flow

Thromboembolism, one of the leading causes of morbidity and mortality worldwide, is characterized by formation of obstructive intravascular clots (thrombi) and their mechanical breakage (embolization). A novel two-dimensional multi-phase computational model is introduced that describes active interactions between the main components of the clot, including platelets and fibrin, to study the impact of various physiologically relevant blood shear flow conditions on deformation and embolization of a partially obstructive clot with variable permeability. Simulations provide new insights into mechanisms underlying clot stability and embolization that cannot be studied experimentally at this time. In particular, model simulations, calibrated using experimental intravital imaging of an established arteriolar clot, show that flow-induced changes in size, shape and internal structure of the clot are largely determined by two shear-dependent mechanisms: reversible attachment of platelets to the exterior of the clot and removal of large clot pieces. Model simulations predict that blood clots with higher permeability are more prone to embolization with enhanced disintegration under increasing shear rate. In contrast, less permeable clots are more resistant to rupture due to shear rate-dependent clot stiffening originating from enhanced platelet adhesion and aggregation. These results can be used in future to predict risk of thromboembolism based on the data about composition, permeability and deformability of a clot under specific local haemodynamic conditions.

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

Supplemental Digital Content is available in the text.

Objective—

We investigated the coregulation of thrombin and fibrin as blood flows over a procoagulant surface.

Approach and Results—

Using microfluidic perfusion of factor XIIa-inhibited human whole blood (200 s⁻¹ wall shear rate) over a 250-μm long patch of collagen/TF (tissue factor; ≈1 molecule per μm²) and immunoassays of the effluent for F1.2 (prothrombin fragment 1.2), TAT (thrombin–antithrombin complex), and D-dimer (post–end point plasmin digest), we sought to establish the transient mass balance for clotting under venous flow. F1.2 (but almost no free thrombin detected via TAT assay) continually eluted from clots when fibrin was allowed to form. Low-dose fluorescein-Phe-Pro-Arg-chloromethylketone stained fibrin-bound thrombin—a staining ablated by anti–γ′-fibrinogen or the fibrin inhibitor glypro-arg-pro but highly resistant to 7-minute buffer rinse, demonstrating tight binding of thrombin to γ′-fibrin. With fibrin polymerizing for 500 seconds, 92 000 thrombin molecules and 203 000 clot-associated fibrin monomer equivalents were generated per TF molecule (or per μm²). Fibrin reached 15 mg/mL in the pore space (porosity ≈0.5) of a 15-μm-thick thrombus core by 500 seconds and 30 mg/mL by 800 seconds. For a known rate of ≈60 FPA (fibrinopeptide-A) per thrombin per second, each thrombin molecule generated only 3 fibrin monomer equivalents during 500 seconds, indicating an intraclot thrombin half-life of ≈70 ms, much shorter than its diffusional escape time (≈10 seconds). By 800 seconds, gly-pro-arg-pro allowed 4-fold more F1.2 generation, consistent with gly-pro-arg-pro ablating fibrin’s antithrombin-I activity and facilitating thrombin-mediated FXIa activation.

Conclusions—

Under flow, fibrinogen continually penetrates the clot, and γ′-fibrin regulates thrombin.

Platelet packing density is an independent regulator of the hemostatic response to injury

Essentials Platelet packing density in a hemostatic plug limits molecular movement to diffusion. A diffusion-dependent steep thrombin gradient forms radiating outwards from the injury site. Clot retraction affects the steepness of the gradient by increasing platelet packing density. Together, these effects promote hemostatic plug core formation and inhibit unnecessary growth.

SUMMARY

Background Hemostasis studies performed in vivo have shown that hemostatic plugs formed after penetrating injuries are characterized by a core of highly activated, densely packed platelets near the injury site, covered by a shell of less activated and loosely packed platelets. Thrombin production occurs near the injury site, further activating platelets and starting the process of platelet mass retraction. Tightening of interplatelet gaps may then prevent the escape and exchange of solutes. Objectives To reconstruct the hemostatic plug macro- and micro-architecture and examine how platelet mass contraction regulates solute transport and solute concentration in the gaps between platelets. Methods Our approach consisted of three parts. First, platelet aggregates formed in vitro under flow were analyzed using scanning electron microscopy to extract data on porosity and gap size distribution. Second, a three-dimensional (3-D) model was constructed with features matching the platelet aggregates formed in vitro. Finally, the 3-D model was integrated with volume and morphology measurements of hemostatic plugs formed in vivo to determine how solutes move within the platelet plug microenvironment. Results The results show that the hemostatic mass is characterized by extremely narrow gaps, porosity values even smaller than previously estimated and stagnant plasma velocity. Importantly, the concentration of a chemical species released within the platelet mass increases as the gaps between platelets shrink. Conclusions Platelet mass retraction provides a physical mechanism to establish steep chemical concentration gradients that determine the extent of platelet activation and account for the core-and-shell architecture observed in vivo.

Associations Between Collateral Status and Thrombus Characteristics and Their Impact in Anterior Circulation Stroke

BACKGROUND AND PURPOSE

Thrombus characteristics and collateral score are associated with functional outcome in patients with acute ischemic stroke. It has been suggested that they affect each other. The aim of this study is to evaluate the association between clot burden score, thrombus perviousness, and collateral score and to determine whether collateral score influences the association of thrombus characteristics with functional outcome.

METHODS

Patients with baseline thin-slice noncontrast computed tomography and computed tomographic angiography images from the MR CLEAN trial (Multicenter Randomized Clinical Trial of Endovascular Treatment of Acute Ischemic Stroke in the Netherlands) were included (n=195). Collateral score and clot burden scores were determined on baseline computed tomographic angiography. Thrombus attenuation increase was determined by comparing thrombus density on noncontrast computed tomography and computed tomographic angiography using a semiautomated method. The association of collateral score with clot burden score and thrombus attenuation increase was evaluated with linear regression. Mediation and effect modification analyses were used to assess the influence of collateral score on the association of clot burden score and thrombus attenuation increase with functional outcome.

RESULTS

A higher clot burden score (B=0.063; 95% confidence interval, 0.008-0.118) and a higher thrombus attenuation increase (B=0.014; 95% confidence interval, 0.003-0.026) were associated with higher collateral score. Collateral score mediated the association of clot burden score with functional outcome. The association between thrombus attenuation increase and functional outcome was modified by the collateral score, and this association was stronger in patients with moderate and good collaterals.

CONCLUSIONS

Patients with lower thrombus burden and higher thrombus perviousness scores had higher collateral score. The positive effect of thrombus perviousness on clinical outcome was only present in patients with moderate and high collateral scores.

CLINICAL TRIAL REGISTRATION

URL: http://www.trialregister.nl. Unique identifier: NTR1804 and URL: http://www.controlled-trials.com Unique identifier: ISRCTN10888758.

Coordination of platelet agonist signaling during the hemostatic response in vivo

The local microenvironment within an evolving hemostatic plug shapes the distribution of soluble platelet agonists, resulting in a gradient of platelet activation. We previously showed that thrombin activity at a site of vascular injury is spatially restricted, resulting in robust activation of a subpopulation of platelets in the hemostatic plug core. In contrast, adenosine 5'-diphosphate (ADP)/P2Y₁₂ signaling contributes to the accumulation of partially activated, loosely packed platelets in a shell overlying the core. The contribution of the additional platelet agonists thromboxane A₂ (TxA₂) and epinephrine to this hierarchical organization was not previously shown. Using a combination of genetic and pharmacologic approaches coupled with real-time intravital imaging, we show that TxA₂ signaling is critical and nonredundant with ADP/P2Y₁₂ for platelet accumulation in the shell region but not required for full platelet activation in the hemostatic plug core, where thrombin activity is highest. In contrast, epinephrine signaling is dispensable even in the presence of a P2Y₁₂ antagonist. Finally, dual P2Y₁₂ and thrombin inhibition does not substantially inhibit hemostatic plug core formation any more than thrombin inhibition alone, providing further evidence that thrombin is the primary driver of platelet activation in this region. Taken together, these studies show for the first time how thrombin, P2Y₁₂, and TxA₂ signaling are coordinated during development of a hierarchical organization of hemostatic plugs in vivo and provide novel insights into the impact of dual antiplatelet therapy on hemostasis and thrombosis.

Enhanced Fibrinolysis with Magnetically Powered Colloidal Microwheels

Thrombi that occlude blood vessels can be resolved with fibrinolytic agents that degrade fibrin, the polymer that forms between and around platelets to provide mechanical stability. Fibrinolysis rates however are often constrained by transport-limited delivery to and penetration of fibrinolytics into the thrombus. Here, these limitations are overcome with colloidal microwheel (µwheel) assemblies functionalized with the fibrinolytic tissue-type plasminogen activator (tPA) that assemble, rotate, translate, and eventually disassemble via applied magnetic fields. These microwheels lead to rapid fibrinolysis by delivering a high local concentration of tPA to induce surface lysis and, by taking advantage of corkscrew motion, mechanically penetrating into fibrin gels and platelet-rich thrombi to initiate bulk degradation. Fibrinolysis of plasma-derived fibrin gels by tPA-microwheels is fivefold faster than with 1 µg mL^-1 tPA. µWheels following corkscrew trajectories can also penetrate through 100 µm sized platelet-rich thrombi formed in a microfluidic model of hemostasis in ≈5 min. This unique combination of surface and bulk dissolution mechanisms with mechanical action yields a targeted fibrinolysis strategy that could be significantly faster than approaches relying on diffusion alone, making it well-suited for occlusions in small or penetrating vessels not accessible to catheter-based removal.

Systems Analysis of Thrombus Formation

The systems analysis of thrombosis seeks to quantitatively predict blood function in a given vascular wall and hemodynamic context. Relevant to both venous and arterial thrombosis, a Blood Systems Biology approach should provide metrics for rate and molecular mechanisms of clot growth, thrombotic risk, pharmacological response, and utility of new therapeutic targets. As a rapidly created multicellular aggregate with a polymerized fibrin matrix, blood clots result from hundreds of unique reactions within and around platelets propagating in space and time under hemodynamic conditions. Coronary artery thrombosis is dominated by atherosclerotic plaque rupture, complex pulsatile flows through stenotic regions producing high wall shear stresses, and plaque-derived tissue factor driving thrombin production. In contrast, venous thrombosis is dominated by stasis or depressed flows, endothelial inflammation, white blood cell-derived tissue factor, and ample red blood cell incorporation. By imaging vessels, patient-specific assessment using computational fluid dynamics provides an estimate of local hemodynamics and fractional flow reserve. High-dimensional ex vivo phenotyping of platelet and coagulation can now power multiscale computer simulations at the subcellular to cellular to whole vessel scale of heart attacks or strokes. In addition, an integrated systems biology approach can rank safety and efficacy metrics of various pharmacological interventions or clinical trial designs.

Animal Models of Thrombosis From Zebrafish to Nonhuman Primates: Use in the Elucidation of New Pathologic Pathways and the Development of Antithrombotic Drugs

Thrombosis is a leading cause of morbidity and mortality worldwide. Animal models are used to understand the pathological pathways involved in thrombosis and to test the efficacy and safety of new antithrombotic drugs. In this review, we will first describe the central role a variety of animal models of thrombosis and hemostasis has played in the development of new antiplatelet and anticoagulant drugs. These include the widely used P2Y12 antagonists and the recently developed orally available anticoagulants that directly target factor Xa or thrombin. Next, we will describe the new players, such as polyphosphate, neutrophil extracellular traps, and microparticles, which have been shown to contribute to thrombosis in mouse models, particularly venous thrombosis models. Other mouse studies have demonstrated roles for the factor XIIa and factor XIa in thrombosis. This has spurred the development of strategies to reduce their levels or activities as a new approach for preventing thrombosis. Finally, we will discuss the emergence of zebrafish as a model to study thrombosis and its potential use in the discovery of novel factors involved in thrombosis and hemostasis. Animal models of thrombosis from zebrafish to nonhuman primates are vital in identifying pathological pathways of thrombosis that can be safely targeted with a minimal effect on hemostasis. Future studies should focus on understanding the different triggers of thrombosis and the best drugs to prevent each type of thrombotic event.

The cellular basis of platelet secretion: Emerging structure/function relationships

Platelet activation has long been known to be accompanied by secretion from at least three types of compartments. These include dense granules, the major source of small molecules; α-granules, the major protein storage organelle; and lysosomes, the site of acid hydrolase storage. Despite ~60 years of research, there are still many unanswered questions about the cell biology of platelet secretion: for example, how are these secretory organelles organized to support cargo release and what are the key routes of cargo release, granule to plasma membrane or granule to canalicular system. Moreover, in recent years, increasing evidence points to the platelet being organized for secretion of the contents from other organelles, namely the dense tubular system (endoplasmic reticulum) and the Golgi apparatus. Conceivably, protein secretion is a widespread property of the platelet and its organelles. In this review, we concentrate on the cell biology of the α-granule and its structure/function relationships. We both review the literature and discuss the wide array of 3-dimensional, high-resolution structural approaches that have emerged in the last few years. These have begun to reveal new and unanticipated outcomes and some of these are discussed. We are hopeful that the next several years will bring rapid advances to this field that will resolve past controversies and be clinically relevant.

Multi-Constituent Simulation of Thrombus Deposition

In this paper, we present a spatio-temporal mathematical model for simulating the formation and growth of a thrombus. Blood is treated as a multi-constituent mixture comprised of a linear fluid phase and a thrombus (solid) phase. The transport and reactions of 10 chemical and biological species are incorporated using a system of coupled convection-reaction-diffusion (CRD) equations to represent three processes in thrombus formation: initiation, propagation and stabilization. Computational fluid dynamic (CFD) simulations using the libraries of OpenFOAM were performed for two illustrative benchmark problems: in vivo thrombus growth in an injured blood vessel and in vitro thrombus deposition in micro-channels (1.5 mm × 1.6 mm × 0.1 mm) with small crevices (125 μm × 75 μm and 125 μm × 137 μm). For both problems, the simulated thrombus deposition agreed very well with experimental observations, both spatially and temporally. Based on the success with these two benchmark problems, which have very different flow conditions and biological environments, we believe that the current model will provide useful insight into the genesis of thrombosis in blood-wetted devices, and provide a tool for the design of less thrombogenic devices.

Platelets and hemostasis: a new perspective on an old subject

Publisher's Note: This article has a companion Counterpoint by Kapur and Semple. Publisher's Note: Join in the discussion of these articles at Blood Advances Community Conversations.

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.

Permeable Thrombi Are Associated With Higher Intravenous Recombinant Tissue-Type Plasminogen Activator Treatment Success in Patients With Acute Ischemic Stroke

BACKGROUND AND PURPOSE

Preclinical studies showed that thrombus permeability improves recombinant tissue-type plasminogen activator (r-tPA) efficacy. We hypothesize that thrombus permeability estimated from radiological imaging is associated with improved recanalization after treatment with intravenously administered r-tPA (r-tPA) and with better functional outcome.

METHODS

We assessed thrombus attenuation increase (TAI) in patients from the Dutch Acute Stroke Study with an occlusion of an intracranial artery on computed tomographic angiography. Patients were included within 9 hours after the stroke onset. After dichotomization of TAI as pervious or impervious, logistic regressions analyses were performed to estimate associations of intravenous r-tPA therapy with complete recanalization and with favorable functional outcome (modified Rankin Scale score of ≤2).

RESULTS

Three hundred eight patients matched the inclusion criteria. The median TAI was 20.1 (interquartile range, 8.5-37.8) Hounsfield unit (HU). We found a significant increase in the odds of complete recanalization with increasing TAI for patients treated with intravenous r-tPA (P=0.030). One hundred thirty-one (42%) thrombi were classified as pervious with TAI of ≥23 HU. In patients with a pervious thrombus, complete recanalization was more frequent after treatment with intravenous r-tPA than after conservative treatment (odds ratio, 6.26; 95% confidence interval, 2.4-16.8; P<0.001). In patients with an impervious thrombus, the effect of intravenous r-tPA was not significant (odds ratio, 1.4; 95% confidence interval, 0.5-4.1; P=0.47). Favorable outcome was more common in patients with a pervious thrombi than without (odds ratio, 2.1; 95% confidence interval, 1.3-3.4; P=0.001).

CONCLUSIONS

Thrombus perviousness, as measured on computed tomography in the acute stage of ischemic stroke, is strongly associated with recanalization after intravenous r-tPA treatment and with favorable functional outcome.

Transport physics and biorheology in the setting of hemostasis and thrombosis

The biophysics of blood flow can dictate the function of molecules and cells in the vasculature with consequent effects on hemostasis, thrombosis, embolism, and fibrinolysis. Flow and transport dynamics are distinct for (i) hemostasis vs. thrombosis and (ii) venous vs. arterial episodes. Intraclot transport changes dramatically the moment hemostasis is achieved or the moment a thrombus becomes fully occlusive. With platelet concentrations that are 50- to 200-fold greater than platelet-rich plasma, clots formed under flow have a different composition and structure compared with blood clotted statically in a tube. The platelet-rich, core/shell architecture is a prominent feature of self-limiting hemostatic clots formed under flow. Importantly, a critical threshold concentration of surface tissue factor is required for fibrin generation under flow. Once initiated by wall-derived tissue factor, thrombin generation and its spatial propagation within a clot can be modulated by γ'-fibrinogen incorporated into fibrin, engageability of activated factor (FIXa)/activated FVIIIa tenase within the clot, platelet-derived polyphosphate, transclot permeation, and reduction of porosity via platelet retraction. Fibrin imparts tremendous strength to a thrombus to resist embolism up to wall shear stresses of 2400 dyne cm(-2) . Extreme flows, as found in severe vessel stenosis or in mechanical assist devices, can cause von Willebrand factor self-association into massive fibers along with shear-induced platelet activation. Pathological von Willebrand factor fibers are A Disintegrin And Metalloprotease with ThromboSpondin-1 domain 13 resistant but are a substrate for fibrin generation due to FXIIa capture. Recently, microfluidic technologies have enhanced the ability to interrogate blood in the context of stenotic flows, acquired von Willebrand disease, hemophilia, traumatic bleeding, and drug action.

A systems approach to hemostasis: 4. How hemostatic thrombi limit the loss of plasma-borne molecules from the microvasculature

Previous studies have shown that hemostatic thrombi formed in response to penetrating injuries have a core of densely packed, fibrin-associated platelets overlaid by a shell of less-activated, loosely packed platelets. Here we asked, first, how the diverse elements of this structure combine to stem the loss of plasma-borne molecules and, second, whether antiplatelet agents and anticoagulants that perturb thrombus structure affect the re-establishment of a tight vascular seal. The studies combined high-resolution intravital microscopy with a photo-activatable fluorescent albumin marker to simultaneously track thrombus formation and protein transport following injuries to mouse cremaster muscle venules. The results show that protein loss persists after red cell loss has ceased. Blocking platelet deposition with an αIIbβ3antagonist delays vessel sealing and increases extravascular protein accumulation, as does either inhibiting adenosine 5'-diphosphate (ADP) P2Y12receptors or reducing integrin-dependent signaling and retraction. In contrast, sealing was unaffected by introducing hirudin to block fibrin accumulation or a Gi2α gain-of-function mutation to expand the thrombus shell. Collectively, these observations describe a novel approach for studying vessel sealing after injury in real time in vivo and show that (1) the core/shell architecture previously observed in arterioles also occurs in venules, (2) plasma leakage persists well beyond red cell escape and mature thrombus formation, (3) the most critical events for limiting plasma extravasation are the stable accumulation of platelets, ADP-dependent signaling, and the emergence of a densely packed core, not the accumulation of fibrin, and (4) drugs that affect platelet accumulation and packing can delay vessel sealing, permitting protein escape to continue.

Spatiotemporal regulation of coagulation and platelet activation during the hemostatic response in vivo

The hemostatic response requires the tightly regulated interaction of the coagulation system, platelets, other blood cells and components of the vessel wall at a site of vascular injury. The dysregulation of this response may result in excessive bleeding if the response is impaired, and pathologic thrombosis with vessel occlusion and tissue ischemia if the response is overly robust. Extensive studies over the past decade have sought to unravel the regulatory mechanisms that coordinate the multiple biochemical and cellular responses in time and space to ensure that an optimal response to vascular damage is achieved. These studies have relied in part on advances in in vivo imaging techniques in animal models, allowing for the direct visualization of various molecular and cellular events in real time during the hemostatic response. This review summarizes knowledge gained with these in vivo imaging and other approaches that provides new insights into the spatiotemporal regulation of coagulation and platelet activation at a site of vascular injury.

Necrotic platelets provide a procoagulant surface during thrombosis

A subpopulation of platelets fulfills a procoagulant role in hemostasis and thrombosis by enabling the thrombin burst required for fibrin formation and clot stability at the site of vascular injury. Excess procoagulant activity is linked with pathological thrombosis. The identity of the procoagulant platelet has been elusive. The cell death marker 4-[N-(S-glutathionylacetyl)amino]phenylarsonous acid (GSAO) rapidly enters a subpopulation of agonist-stimulated platelets via an organic anion-transporting polypeptide and is retained in the cytosol through covalent reaction with protein dithiols. Labeling with GSAO, together with exposure of P-selectin, distinguishes necrotic from apoptotic platelets and correlates with procoagulant potential. GSAO(+) platelets form in occluding murine thrombi after ferric chloride injury and are attenuated with megakaryocyte-directed deletion of the cyclophilin D gene. These platelets form a procoagulant surface, supporting fibrin formation, and reduction in GSAO(+) platelets is associated with reduction in platelet thrombus size and fibrin formation. Analysis of platelets from human subjects receiving aspirin therapy indicates that these procoagulant platelets form despite aspirin therapy, but are attenuated by inhibition of the necrosis pathway. These findings indicate that the major subpopulation of platelets involved in fibrin formation are formed via regulated necrosis involving cyclophilin D, and that they may be targeted independent of platelet activation.

Modulation and pre-amplification of PAR1 signaling by ADP acting via the P2Y12 receptor during platelet subpopulation formation

BACKGROUND

Two major soluble blood platelet activators are thrombin and ADP. Of these two, only thrombin can induce mitochondrial collapse and programmed cell death leading to phosphatidylserine (PS) exposure required for blood clotting reactions acceleration. Thrombin can also greatly potentiate collagen-induced PS exposure. However, ADP acting through the P2Y12 receptor was shown to increase the PS-exposing (PS+) platelets fraction produced by thrombin or thrombin-plus-collagen via an unknown mechanism.

METHODS

We developed a comprehensive multicompartmental computational model of platelet PAR1-and-P2Y12 calcium signal transduction that included cytoplasmic signaling, dense tubular system and mitochondria. To test model predictions, flow cytometry experiments with washed, annexin V-labeled platelets were performed.

RESULTS

Stimulation of thrombin receptor PAR1 in the model induced cytoplasmic calcium oscillations, calcium uptake by mitochondria, opening of the permeability transition pore and collapse of the mitochondrial membrane potential. ADP stimulation of P2Y12 led to cAMP decrease that, in turn, caused changes in phospholipase C phosphorylation by protein kinase A, increase in cytoplasmic calcium level and, consequently, PS+ platelet formation. ADP addition before stimulation of PAR1 produced much greater increase of the PS+ fraction because cAMP concentration had time to go down prior to calcium oscillations; this prediction was also tested and confirmed experimentally.

CONCLUSION

These results suggest a mechanism of ADP-dependent PS exposure regulation and show a likely mode of action that could be important for the PS exposure regulation in thrombi, where ADP is released before thrombin formation.

Modelling of platelet-fibrin clot formation in flow with a DPD-PDE method

The paper is devoted to mathematical modelling of clot growth in blood flow. Great complexity of the hemostatic system dictates the need of usage of the mathematical models to understand its functioning in the normal and especially in pathological situations. In this work we investigate the interaction of blood flow, platelet aggregation and plasma coagulation. We develop a hybrid DPD-PDE model where dissipative particle dynamics (DPD) is used to model plasma flow and platelets, while the regulatory network of plasma coagulation is described by a system of partial differential equations. Modelling results confirm the potency of the scenario of clot growth where at the first stage of clot formation platelets form an aggregate due to weak inter-platelet connections and then due to their activation. This enables the formation of the fibrin net in the centre of the platelet aggregate where the flow velocity is significantly reduced. The fibrin net reinforces the clot and allows its further growth. When the clot becomes sufficiently large, it stops growing due to the narrowed vessel and the increase of flow shear rate at the surface of the clot. Its outer part is detached by the flow revealing the inner part covered by fibrin. This fibrin cap does not allow new platelets to attach at the high shear rate, and the clot stops growing. Dependence of the final clot size on wall shear rate and on other parameters is studied.

Shaping the platelet response to vascular injury

PURPOSE OF REVIEW

Several decades of work by many investigators have elucidated the major signaling pathways responsible for platelet activation. Still to be fully understood is how these pathways are integrated into a single network and how changing conditions within a growing thrombus affect that network. In this review we will consider some of the recent studies that address these issues and describe a model that provides insights into platelet activation as it occurs in vivo.

RECENT FINDINGS

Genetic and pharmacologic studies performed in vivo have demonstrated that platelet activation during hemostasis and thrombosis is heterogeneous. Those studies indicate that distinct platelet activation pathways are not merely redundant, but are coordinated in time and space to achieve an optimal response. This coordination is achieved at least in part by the evolving distribution of platelet agonists and changes in solute transport within a hemostatic plug.

SUMMARY

Studies examining the coordination of platelet signaling in time and space continue to increase our understanding of hemostasis and thrombosis. In addition to helping to decipher platelet biology, the results have implications for the understanding of new and existing antiplatelet agents and their potential risks.

A systems approach to hemostasis: 2. Computational analysis of molecular transport in the thrombus microenvironment

Hemostatic thrombi formed after a penetrating injury have a heterogeneous architecture in which a core of highly activated, densely packed platelets is covered by a shell of less-activated, loosely packed platelets. In the first manuscript in this series, we show that regional differences in intrathrombus protein transport rates emerge early in the hemostatic response and are preserved as the thrombus develops. Here, we use a theoretical approach to investigate this process and its impact on agonist distribution. The results suggest that hindered diffusion, rather than convection, is the dominant mechanism responsible for molecular movement within the thrombus. The analysis also suggests that the thrombus core, as compared with the shell, provides an environment for retaining soluble agonists such as thrombin, affecting the extent of platelet activation by establishing agonist-specific concentration gradients radiating from the site of injury. This analysis accounts for the observed weaker activation and relative instability of platelets in the shell and predicts that a failure to form a tightly packed thrombus core will limit thrombin accumulation, a prediction tested by analysis of data from mice with a defect in clot retraction.

A systems approach to hemostasis: 3. Thrombus consolidation regulates intrathrombus solute transport and local thrombin activity

Hemostatic thrombi formed after a penetrating injury have a distinctive structure in which a core of highly activated, closely packed platelets is covered by a shell of less-activated, loosely packed platelets. We have shown that differences in intrathrombus molecular transport emerge in parallel with regional differences in platelet packing density and predicted that these differences affect thrombus growth and stability. Here we test that prediction in a mouse vascular injury model. The studies use a novel method for measuring thrombus contraction in vivo and a previously characterized mouse line with a defect in integrin αIIbβ3 outside-in signaling that affects clot retraction ex vivo. The results show that the mutant mice have a defect in thrombus consolidation following vascular injury, resulting in an increase in intrathrombus transport rates and, as predicted by computational modeling, a decrease in thrombin activity and platelet activation in the thrombus core. Collectively, these data (1) demonstrate that in addition to the activation state of individual platelets, the physical properties of the accumulated mass of adherent platelets is critical in determining intrathrombus agonist distribution and platelet activation and (2) define a novel role for integrin signaling in the regulation of intrathrombus transport rates and localization of thrombin activity.

A systems approach to hemostasis: 1. The interdependence of thrombus architecture and agonist movements in the gaps between platelets

Hemostatic thrombi develop a characteristic architecture in which a core of highly activated platelets is covered by a shell of less-activated platelets. Here we have used a systems biology approach to examine the interrelationship of this architecture with transport rates and agonist distribution in the gaps between platelets. Studies were performed in mice using probes for platelet accumulation, packing density, and activation plus recently developed transport and thrombin activity probes. The results show that intrathrombus transport within the core is much slower than within the shell. The region of slowest transport coincides with the region of greatest packing density and thrombin activity, and appears prior to full platelet activation. Deleting the contact-dependent signaling molecule, Sema4D, delays platelet activation, but not the emergence of the low transport region. Collectively, these results suggest a timeline in which initial platelet accumulation and the narrowing gaps between platelets create a region of reduced transport that facilitates local thrombin accumulation and greater platelet activation, whereas faster transport rates within the shell help to limit thrombin accumulation and growth of the core. Thus, from a systems perspective, platelet accumulation produces an altered microenvironment that shapes thrombus architecture, which in turn affects agonist distribution and subsequent thrombus growth.

Systems biology of platelet-vessel wall interactions

Platelets are small, anucleated cells that participate in primary hemostasis by forming a hemostatic plug at the site of a blood vessel's breach, preventing blood loss. However, hemostatic events can lead to excessive thrombosis, resulting in life-threatening strokes, emboli, or infarction. Development of multi-scale models coupling processes at several scales and running predictive model simulations on powerful computer clusters can help interdisciplinary groups of researchers to suggest and test new patient-specific treatment strategies.

Fibrin networks regulate protein transport during thrombus development

Thromboembolic disease is a leading cause of morbidity and mortality worldwide. In the last several years there have been a number of studies attempting to identify mechanisms that stop thrombus growth. This paper identifies a novel mechanism related to formation of a fibrin cap. In particular, protein transport through a fibrin network, an important component of a thrombus, was studied by integrating experiments with model simulations. The network permeability and the protein diffusivity were shown to be important factors determining the transport of proteins through the fibrin network. Our previous in vivo studies in mice have shown that stabilized non-occluding thrombi are covered by a fibrin network (‘fibrin cap’). Model simulations, calibrated using experiments in microfluidic devices and accounting for the permeable structure of the fibrin cap, demonstrated that thrombin generated inside the thrombus was washed downstream through the fibrin network, thus limiting exposure of platelets on the thrombus surface to thrombin. Moreover, by restricting the approach of resting platelets in the flowing blood to the thrombus core, the fibrin cap impaired platelets from reaching regions of high thrombin concentration necessary for platelet activation and limited thrombus growth. The formation of a fibrin cap prevents small thrombi that frequently develop in the absence of major injury in the 60000 km of vessels in the body from developing into life threatening events.

To restrict the loss of blood following rupture of blood vessels, the human body rapidly forms a clot consisting mainly of platelets and fibrin. However, to prevent formation of a pathological clot within vessels (thrombus) as a result of vessel damage or dysfunction, the response must be regulated, and clot formation must be limited. Our previous studies demonstrated that as a laser-induced thrombus stabilized in mice, the ratio of fibrin to platelets at the thrombus surface increased significantly. Stabilized non-occluding thrombi were observed to be covered by a fibrin network (‘fibrin cap’). In the present work the role of the fibrin network in protein transport is examined by integrating experiments in microfluidic devices with the hemodynamic thrombus model. The study reveals permeability of the fibrin network and protein diffusivity to be important factors determining the transport of blood proteins inside the thrombus. It is shown that the fibrin network does not dramatically limit the diffusion of thrombin but impairs flowing platelets in blood from reaching regions of high thrombin concentration thus, reducing the probability they are activated and stably integrated into the thrombus. This novel, counter-intuitive mechanism suggests that a fibrin network formed at early stages of thrombus initiation can prevent normally asymptomatic thrombi from developing into pathological clots.