Hemostasis and thrombosis beyond biochemistry: roles of geometry, flow and diffusion

Kinetics and regulation of coagulation factor X activation by intrinsic tenase on phospholipid membranes

BACKGROUND

Factor X activation by the phospholipid-bound intrinsic tenase complex is a critical membrane-dependent reaction of blood coagulation. Its regulation mechanisms are unclear, and a number of questions regarding diffusional limitation, pathways of assembly and substrate delivery remain open.

METHODS

We develop and analyze here a detailed mechanism-driven computer model of intrinsic tenase on phospholipid surfaces. Three-dimensional reaction-diffusion-advection and stochastic simulations were used where appropriate.

RESULTS

Dynamics of the system was predominantly non-stationary under physiological conditions. In order to describe experimental data, we had to assume both membrane-dependent and solution-dependent delivery of the substrate. The former pathway dominated at low cofactor concentration, while the latter became important at low phospholipid concentration. Factor VIIIa-factor X complex formation was the major pathway of the complex assembly, and the model predicted high affinity for their lipid-dependent interaction. Although the model predicted formation of the diffusion-limited layer of substrate for some conditions, the effects of this limitation on the fXa production were small. Flow accelerated fXa production in a flow reactor model by bringing in fIXa and fVIIIa rather than fX.

CONCLUSIONS

This analysis suggests a concept of intrinsic tenase that is non-stationary, employs several pathways of substrate delivery depending on the conditions, and is not particularly limited by diffusion of the substrate.

Chemical Adjustment of Fibrinolysis

Fibrinolysis is the process of the fibrin-platelet clot dissolution initiated after bleeding has been stopped. It is regulated by a cascade of proteolytic enzymes with plasmin at its core. In pathological cases, the balance of normal clot formation and dissolution is replaced by a too rapid lysis, leading to bleeding, or an insufficient one, leading to an increased thrombotic risk. The only approved therapy for emergency thrombus lysis in ischemic stroke is recombinant tissue plasminogen activator, though streptokinase or urokinase-type plasminogen activators could be used for other conditions. Low molecular weight compounds are of great interest for long-term correction of fibrinolysis dysfunctions. Their areas of application might go beyond the hematology field because the regulation of fibrinolysis could be important in many conditions, such as fibrosis. They enhance or weaken fibrinolysis without significant effects on other components of hemostasis. Here we will describe and discuss the main classes of these substances and their mechanisms of action. We will also explore avenues of research for the development of new drugs, with a focus on the use of computational models in this field.

Platelets provide robustness of spatial blood coagulation to the variation of initial conditions

Activated platelets provide phospholipid surface and secrete coagulation factors, enhancing blood clotting. We investigated the role of platelets in the regulation of blood coagulation spatial dynamics. We activated blood clotting with tissue factor-bearing (TF) surface in platelet-rich plasma (PRP) or platelet-free plasma (PFP). When blood coagulation was initiated by high TF density, clot growth rate (V) in PRP (2 × 105/μL platelets) was only 15 % greater than in PFP. Spatial distribution of thrombin in PRP had a peak-like shape in the area of the fibrin clot edge, while in PFP thrombin was distributed in the shape of descending plateau. Platelet inhibition with prostaglandin E1 or cytochalasin D made spatial thrombin distribution look like in the case of PFP. Inhibition of blood coagulation by natural endogenous inhibitor heparin was diminished in PRP, while the effect of the exogenous or artificial inhibitors (rivaroxaban, nitrophorin, hirudin) remained undisturbed in the presence of platelets. Ten times decrease of the TF surface density greatly depressed blood coagulation in PFP. In PRP only clotting initiation phase was, while the propagation phase remained intact. Coagulation factor deficiency greatly reduced amount of thrombin and decreased V in PFP rather than in PPR. Thus, platelets were redundant for clotting in normal plasma under physiological conditions but provided robustness of the coagulation system to the changes in initial conditions.

Delineating septo-optic dysplasia

BACKGROUND

Septo-optic dysplasia (SOD), once a variable triad of septum pellucidum defects (SPDs), optic nerve hypoplasia (ONH), and hypopituitarism, has had multiple findings added, with uncertain causes, definitions, and limits.

METHOD

Literature review.

RESULTS

SOD is a complex vascular sequence with confounders.

CONCLUSIONS

Proximal anterior cerebral artery trunk disruptions cause overlapping primary effects, giving ONH alone most often, and isolated SPD less. ONH disruptions can spread to pituitary, SPD disruptions to the cerebral cortex, causing schizencephaly and related anomalies. Pituitary defects are rare without ONH, and cortical findings are rare without SPD. Extensions are unidirectional, so isolated pituitary or cortical defects are separate from SOD. Micro- an- ophthalmia, a suggested ONH variant, is not part of SOD. Disruption by-products can affect development, causing cognitive and endocrine issues, and structural anomalies such as corpus callosum thinning, ventriculomegaly, and hippocampal and olfactory findings. Limbic extensions may also contribute to the same structural defects as by-products. Midline CNS developmental anomalies can act as disruptive foci, most likely through vascular variants, but have separate pathogenesis. Relative frequencies of specific pituitary hormone defects change as SOD rates increase. Increasing relative rates of midline CNS developmental defects and cortical anomalies are consistent with rising levels of exogenous exposures sensitizing to midline predispositions.

Assessment of Fibrinogen Macromolecules Interaction with Red Blood Cells Membrane by Means of Laser Aggregometry, Flow Cytometry, and Optical Tweezers Combined with Microfluidics

An elevated concentration of fibrinogen in blood is a significant risk factor during many pathological diseases, as it leads to an increase in red blood cells (RBC) aggregation, resulting in hemorheological disorders. Despite the biomedical importance, the mechanisms of fibrinogen-induced RBC aggregation are still debatable. One of the discussed models is the non-specific adsorption of fibrinogen macromolecules onto the RBC membrane, leading to the cells bridging in aggregates. However, recent works point to the specific character of the interaction between fibrinogen and the RBC membrane. Fibrinogen is the major physiological ligand of glycoproteins receptors IIbIIIa (GPIIbIIIa or αIIββ3 or CD41/CD61). Inhibitors of GPIIbIIIa are widely used in clinics for the treatment of various cardiovascular diseases as antiplatelets agents preventing the platelets’ aggregation. However, the effects of GPIIbIIIa inhibition on RBC aggregation are not sufficiently well studied. The objective of the present work was the complex multimodal in vitro study of the interaction between fibrinogen and the RBC membrane, revealing the role of GPIIbIIIa in the specificity of binding of fibrinogen by the RBC membrane and its involvement in the cells’ aggregation process. We demonstrate that GPIIbIIIa inhibition leads to a significant decrease in the adsorption of fibrinogen macromolecules onto the membrane, resulting in the reduction of RBC aggregation. We show that the mechanisms underlying these effects are governed by a decrease in the bridging components of RBC aggregation forces.

Determination of fibrin clot growth and spatial thrombin propagation in the presence of different types of phospholipid surfaces

In this work, we present a new method-Thrombodynamics-4D-for the assessment of both plasma and platelet contributions to clotting. Thrombodynamics-4D potentially allows for the determination of plasma or platelet disorders and the effects of various drugs on plasma clotting or on platelet procoagulant function. In this assay, clot formation in platelet-rich plasma or platelet-free plasma supplemented with phospholipids is activated with tissue factor immobilized on a surface. Spatial fibrin clot growth and thrombin concentration dynamics are registered by measuring light scattering of the fibrin clot and fluorescence of the product formed by cleavage of the synthetic fluorogenic substrate by thrombin, respectively. Here, we describe the preanalytical requirements, measurement methodology and calculation principles of assay parameters. Preanalytical and analytical variability and reference ranges of the assay are given. Additionally, we show some clinical examples, which determine the effect of anticoagulants, measure clotting dysfunction in patients with platelet or coagulation disorders and evaluate the effect of surgery.

Tissue Factor-Independent Coagulation Correlates with Clinical Phenotype in Factor XI Deficiency and Replacement Therapy

BACKGROUND

 In factor XI (FXI) deficiency, bleeding cannot be predicted by routine analyses. Since FXI is involved in tissue factor (TF)-independent propagation loop of coagulation, we hypothesized that investigating the spatiotemporal separated phases of coagulation (TF-dependent and -independent) could improve diagnostics.

OBJECTIVES

 This article investigates the correlation of parameters describing TF-dependent and -independent coagulation with the clinical phenotype of FXI deficiency and their ability to assess hemostasis after FXI replacement.

METHODS

 We analyzed: (1) plasma from healthy controls (n = 53); (2) normal plasma (n = 4) spiked with increasing concentrations of a specific FXI inhibitor (C7P); (3) plasma from FXI-deficient patients (n = 24) with different clinical phenotypes (13 bleeders, 8 non-bleeders, 3 prothrombotics); (4) FXI-deficient plasma spiked with FXI concentrate (n = 6); and (5) plasma from FXI-deficient patients after FXI replacement (n = 7). Thrombin generation was measured with the reference method calibrated automated thrombogram and with Thrombodynamics (TD), a novel global assay differentiating TF-dependent and -independent coagulation.

RESULTS

 C7P dose-dependently decreased FXI activity, prolonged activated partial thromboplastin time, and hampered TF-independent coagulation. In FXI-deficient bleeders, TD parameters describing TF-independent propagation of coagulation and fibrin clot formation were reduced compared with controls and FXI-deficient nonbleeders and increased in FXI-deficient patients with prothrombotic phenotype. Receiver operating characteristic analysis indicated that TF-independent parameters were useful for discriminating FXI-deficient bleeders from non-bleeders. In FXI-deficient plasma spiked with FXI concentrate and in patients receiving FXI replacement, TD parameters were shifted toward hypercoagulation already at plasma FXI levels around 20%.

CONCLUSION

 TF-independent coagulation parameters assessed by TD have the potential to identify the clinical phenotype in FXI-deficient patients and to monitor FXI replacement therapy.

In Silico Hemostasis Modeling and Prediction

Computational physiology, i.e., reproduction of physiological (and, by extension, pathophysiological) processes in silico, could be considered one of the major goals in computational biology. One might use computers to simulate molecular interactions, enzyme kinetics, gene expression, or whole networks of biochemical reactions, but it is (patho)physiological meaning that is usually the meaningful goal of the research even when a single enzyme is its subject. Although exponential rise in the use of computational and mathematical models in the field of hemostasis and thrombosis began in the 1980s (first for blood coagulation, then for platelet adhesion, and finally for platelet signal transduction), the majority of their successful applications are still focused on simulating the elements of the hemostatic system rather than the total (patho)physiological response in situ. Here we discuss the state of the art, the state of the progress toward the efficient "virtual thrombus formation," and what one can already get from the existing models.

Redistribution of TPA Fluxes in the Presence of PAI-1 Regulates Spatial Thrombolysis

The fibrin clot is gelatinous matter formed upon injury to stop blood loss and is later destroyed by fibrinolysis, an enzymatic cascade with feedback. Pharmacological fibrinolysis stimulation is also used to destroy pathological, life-threatening clots and thrombi (thrombolysis). The regulation of the nonlinear spatially nonuniform fibrinolytic process in thrombolysis is not currently well understood. We developed a reaction-diffusion-advection model of thrombolysis by tissue plasminogen activator (TPA) in an occluded vessel with a pressure gradient. Sensitivity-analysis-based model reduction was used to reveal the critical processes controlling different steps of thrombolysis. The propagation of thrombolysis in the system without flow was predominantly controlled by TPA diffusion, whereas transport of other active components was rendered nonessential either by their high fibrin-binding parameters and short lifetimes or their initial uniform distribution. The concentration of the main TPA inhibitor plasminogen activator inhibitor 1 (PAI-1) controlled both the extent of lysis propagation and the shape of fibrin spatial distribution during lysis. Interestingly, PAI-1 remained important even when its concentration was an order of magnitude below that of TPA because of its role at the edge of the diffusing TPA front. The system was robust to reaction rate constant perturbations. Using these data, a reduced model of thrombolysis was proposed. In the presence of flow, convection of TPA was the critical controlling process; although the role of PAI-1 concentration was much less in the presence of flow, its influence became greater in the presence of collateral bypassing vessels, which sufficiently reduced TPA flux through the thrombus. Flow bypass through the collateral vessel caused a decrease in TPA flux in the clotted vessel, which increased the PAI-1/TPA ratio, thus making PAI-1-induced inhibition relevant for the regulation of spatial lysis up to its arrest.

The molecular basis of platelet biogenesis, activation, aggregation and implications in neurological disorders

Platelets are anucleated blood constituents, vital for hemostasis and involved in the pathophysiology of several cardiovascular, neurovascular diseases as well as inflammatory processes and metastasis. Over the past few years, the molecular processes that regulate the function of platelets in hemostasis and thrombosis have emerged revealing platelets to be perhaps more complex than may have been expected. The most understood part of platelets is to respond to a blood vessel injury by altering shape, secreting granule contents, and aggregating. These responses, while advantageous for hemostasis, can become detrimental when they root ischemia or infarction. Only a few transcription and signaling factors involved in platelet biogenesis have been identified till date. Platelets encompass an astonishingly complete array of organelles and storage granules including mitochondria, lysosomes, alpha granules, dense granules, a dense tubular system (analogous to the endoplasmic reticulum of nucleated cells); a highly invaginated plasma membrane system known as the open canalicular system (OCS) and large fields of glycogen. Platelets as a model cells to study neurological disorders have been recommended by several researchers since several counterparts exist between platelets and the brain, which make them interesting for studying the neurobiology of various neurological disorders. This review has been compiled with an aim to integrate the latest research on platelet biogenesis, activation and aggregation focusing on the molecular pathways that power and regulate these processes. The dysregulation of important molecular players affecting fluctuating platelet biology and thereby resulting in neurovascular diseases has also been discussed.

Impact of Tissue Factor Localization on Blood Clot Structure and Resistance under Venous Shear

The structure and growth of a blood clot depend on the localization of tissue factor (TF), which can trigger clotting during the hemostatic process or promote thrombosis when exposed to blood under pathological conditions. We sought to understand how the growth, structure, and mechanical properties of clots under flow are shaped by the simultaneously varying TF surface density and its exposure area. We used an eight-channel microfluidic device equipped with a 20- or 100-μm-long collagen surface patterned with lipidated TF of surface densities ∼0.1 and ∼2 molecules/μm². Human whole blood was perfused at venous shear, and clot growth was continually measured. Using our recently developed computational model of clot formation, we performed simulations to gain insights into the clot’s structure and its resistance to blood flow. An increase in TF exposure area resulted not only in accelerated bulk platelet, thrombin, and fibrin accumulation, but also in increased height of the platelet mass and increased clot resistance to flow. Moreover, increasing the TF surface density or exposure area enhanced platelet deposition by approximately twofold, and thrombin and fibrin generation by greater than threefold, thereby increasing both clot size and its viscous resistance. Finally, TF effects on blood flow occlusion were more pronounced for the longer thrombogenic surface than for the shorter one. Our results suggest that TF surface density and its exposure area can independently enhance both the clot’s occlusivity and its resistance to blood flow. These findings provide, to our knowledge, new insights into how TF affects thrombus growth in time and space under flow.

Review of quantitative systems pharmacological modeling in thrombosis

Hemostasis and thrombosis are often thought as two sides of the same clotting mechanism whereas hemostasis is a natural protective mechanism to prevent bleeding and thrombosis is a blood clot abnormally formulated inside a blood vessel, blocking the normal blood flow. The evidence to date suggests that at least arterial thrombosis results from the same critical pathways of hemostasis. Analysis of these complex processes and pathways using quantitative systems pharmacological model-based approach can facilitate the delineation of the causal pathways that lead to the emergence of thrombosis. In this paper, we provide an overview of the main molecular and physiological mechanisms associated with hemostasis and thrombosis, and review the models and quantitative system pharmacological modeling approaches that are relevant in characterizing the interplay among the multiple factors and pathways of thrombosis. An emphasis is given to computational models for drug development. Future trends are discussed.

Sensitivity and Robustness of Spatially Dependent Thrombin Generation and Fibrin Clot Propagation

Blood coagulation is a delicately regulated space- and time-dependent process that leads to the formation of fibrin clots preventing blood loss upon vascular injury. The sensitivity of the coagulation network was previously investigated without accounting for transport processes. To investigate its sensitivity to coagulation factor deficiencies in a spatial reaction-diffusion system, we combined an in vitro experimental design with a computational systems biology model. Clot formation in platelet-free plasma supplemented with phospholipids was activated with identical amounts of tissue factor (TF) either homogeneously distributed (concentration 5 pM, homogeneous model) or immobilized on the surface (surface density 100 pmole/m², spatially heterogeneous model). Fibrin clot growth and thrombin concentration dynamic in space were observed using video microscopy in plasma of healthy donors or patients with deficiencies in factors (F) II, FV, FVII, FVIII, FIX, FX, or FXI. In the spatially heterogeneous model, near-activator thrombin generation was decreased in FV-, FVII-, and FX-deficient plasma. In the homogeneous model, clotting was not registered in these samples. The simulation and experiment data showed that the coagulation threshold depended on the TF concentration. Our data indicate that the velocity of spatial clot propagation correlates linearly with the concentration of thrombin at the clot wave front but not with the overall thrombin wave amplitude. Spatial clot growth in normal plasma at early stages was neither reaction nor diffusion limited but became diffusion limited later. In contrast, clot growth was always diffusion limited in FV-, FVII-, and FX-deficient plasma and reaction limited in FVIII-, FIX-, and FXI-deficient plasma. We conclude that robustness of the spatially heterogeneous coagulation system was achieved because of the combination of 1) a local high TF surface density that overcomes activation thresholds, 2) diffusion control being shared between different active factors, and 3) an early saturated stimulus-response dependence of fibrin clot formation by thrombin.

Carboxypeptidase U (CPU, carboxypeptidase B2, activated thrombin-activatable fibrinolysis inhibitor) inhibition stimulates the fibrinolytic rate in different in vitro models

Essentials AZD9684 is a potent inhibitor of carboxypeptidase U (CPU, TAFIa, CPB2). The effect of AZD9684 on fibrinolysis was investigated in four in vitro systems. The CPU system also attenuates fibrinolysis in more advanced hemostatic systems. The size of the observed effect on fibrinolysis is dependent on the exact experimental conditions.

SUMMARY

Background Carboxypeptidase U (CPU, carboxypeptidase B2, activated thrombin-activatable fibrinolysis inhibitor) is a basic carboxypeptidase that attenuates fibrinolysis. This characteristic has raised interest in the scientific community and pharmaceutical industry for the development of inhibitors as profibrinolytic agents. Objectives Little is known about the contribution of CPU to clot resistance in more advanced hemostatic models, which include blood cells and shear stress. The aim of this study was to evaluate the effects of the CPU system in in vitro systems for fibrinolysis with different grades of complexity. Methods The contribution of the CPU system was evaluated in the following systems: (i) plasma clot lysis; (ii) rotational thromboelastometry (ROTEM) in whole blood; (iii) front lysis with confocal microscopy in platelet-free and platelet-rich plasma; and (iv) a microfluidic system with whole blood under arterial shear stress. Experiments were carried out in the presence or absence of AZD9684, a specific CPU inhibitor. Results During plasma clot lysis, addition of AZD9684 resulted in 33% faster lysis. In ROTEM, the lysis onset time was decreased by 38%. For both clot lysis and ROTEM, an AZD9684 dose-dependent response was observed. CPU inhibition in front lysis experiments resulted in 47% and 50% faster lysis for platelet-free plasma and platelet-rich plasma, respectively. Finally, a tendency for faster lysis was observed only in the microfluidic system when AZD9684 was added. Conclusions Overall, these experiments provide novel evidence that the CPU system can also modulate fibrinolysis in more advanced hemostatic systems. The extent of the effects appears to be dependent upon the exact experimental conditions.

Coated platelets introduce significant delay in onset of peak thrombin production: Theoretical predictions

Platelets play a crucial role in the initiation, progress, termination as well as regulation of blood coagulation. Recent studies have confirmed that not all but only a small percentage of thrombin-activated platelets ("coated" platelets) exhibit procoagulant properties (namely the expression of phosphatidylserine binding sites) required for the acceleration and progress of coagulation. A mechanistic model is developed for in vitro coagulation whose key features are distinct equations for coated platelets, thrombin dose-dependence for coated platelets, and competitive binding of coagulation factors to platelet membrane. Model predictions show significant delay in the onset of peak Va production, and peak thrombin production when dose-dependence is incorporated instead of a fixed theoretical maximum percentage of coated platelets. Further, peak thrombin concentration is significantly overestimated when either fractional presence of coated platelets is ignored (by 299.4%) or when dose-dependence on thrombin is ignored (by 24.7%).

Importance of Initial Concentration of Factor VIII in a Mechanistic Model of In Vitro Coagulation

This computational study generates a hypothesis for the coagulation protein whose initial concentration greatly influences the course of coagulation. Many clinical malignancies of blood coagulation arise due to abnormal initial concentrations of coagulation factors. Sensitivity analysis of mechanistic models of blood coagulation is a convenient method to assess the effect of such abnormalities. Accordingly, the study presents sensitivity analysis, with respect to initial concentrations, of a recently developed mechanistic model of blood coagulation. Both the model and parameters to which model sensitivity is being analyzed provide newer insights into blood coagulation: the model incorporates distinct equations for plasma-phase and platelet membrane-bound species, and sensitivity to initial concentrations is a new dimension in sensitivity analysis. The results show that model predictions are most uncertain with respect to changes in initial concentration of factor VIII, and this hypothesis is supported by results from other models developed independently.

Modeling thrombosis in silico: Frontiers, challenges, unresolved problems and milestones

Hemostasis is a complex physiological mechanism that functions to maintain vascular integrity under any conditions. Its primary components are blood platelets and a coagulation network that interact to form the hemostatic plug, a combination of cell aggregate and gelatinous fibrin clot that stops bleeding upon vascular injury. Disorders of hemostasis result in bleeding or thrombosis, and are the major immediate cause of mortality and morbidity in the world. Regulation of hemostasis and thrombosis is immensely complex, as it depends on blood cell adhesion and mechanics, hydrodynamics and mass transport of various species, huge signal transduction networks in platelets, as well as spatiotemporal regulation of the blood coagulation network. Mathematical and computational modeling has been increasingly used to gain insight into this complexity over the last 30 years, but the limitations of the existing models remain profound. Here we review state-of-the-art-methods for computational modeling of thrombosis with the specific focus on the analysis of unresolved challenges. They include: a) fundamental issues related to physics of platelet aggregates and fibrin gels; b) computational challenges and limitations for solution of the models that combine cell adhesion, hydrodynamics and chemistry; c) biological mysteries and unknown parameters of processes; d) biophysical complexities of the spatiotemporal networks' regulation. Both relatively classical approaches and innovative computational techniques for their solution are considered; the subjects discussed with relation to thrombosis modeling include coarse-graining, continuum versus particle-based modeling, multiscale models, hybrid models, parameter estimation and others. Fundamental understanding gained from theoretical models are highlighted and a description of future prospects in the field and the nearest possible aims are given.

Modelling and simulation of flow and agglomeration in deep veins valves using discrete multi physics

The hemodynamics in flexible deep veins valves is modelled by means of discrete multi-physics and an agglomeration algorithm is implemented to account for blood accrual in the flow. Computer simulations of a number of valves typologies are carried out. The results show that the rigidity and the length of the valve leaflets play a crucial role on both mechanical stress and stagnation in the flow. Rigid and short membranes may be inefficient in preventing blood reflux, but reduce the volume of stagnant blood potentially lowering the chances of thrombosis. Additionally, we also show that in venous valves, cell agglomeration is driven by stagnation rather than mechanical stress.

Co-ordinated spatial propagation of blood plasma clotting and fibrinolytic fronts

Fibrinolysis is a cascade of proteolytic reactions occurring in blood and soft tissues, which functions to disintegrate fibrin clots when they are no more needed. In order to elucidate its regulation in space and time, fibrinolysis was investigated using an in vitro reaction-diffusion experimental model of blood clot formation and dissolution. Clotting was activated by a surface with immobilized tissue factor in a thin layer of recalcified blood plasma supplemented with tissue plasminogen activator (TPA), urokinase plasminogen activator or streptokinase. Formation and dissolution of fibrin clot was monitored by videomicroscopy. Computer systems biology model of clot formation and lysis was developed for data analysis and experimental planning. Fibrin clot front propagated in space from tissue factor, followed by a front of clot dissolution propagating from the same source. Velocity of lysis front propagation linearly depended on the velocity clotting front propagation (correlation r2 = 0.91). Computer model revealed that fibrin formation was indeed the rate-limiting step in the fibrinolysis front propagation. The phenomenon of two fronts which switched the state of blood plasma from liquid to solid and then back to liquid did not depend on the fibrinolysis activator. Interestingly, TPA at high concentrations began to increase lysis onset time and to decrease lysis propagation velocity, presumably due to plasminogen depletion. Spatially non-uniform lysis occurred simultaneously with clot formation and detached the clot from the procoagulant surface. These patterns of spatial fibrinolysis provide insights into its regulation and might explain clinical phenomena associated with thrombolytic therapy.

Discrete multi-physics: A mesh-free model of blood flow in flexible biological valve including solid aggregate formation

We propose a mesh-free and discrete (particle-based) multi-physics approach for modelling the hydrodynamics in flexible biological valves. In the first part of this study, the method is successfully validated against both traditional modelling techniques and experimental data. In the second part, it is further developed to account for the formation of solid aggregates in the flow and at the membrane surface. Simulations of various types of aggregates highlight the main benefits of discrete multi-physics and indicate the potential of this approach for coupling the hydrodynamics with phenomena such as clotting and calcification in biological valves.

Polymerization Initiated by Particle Contact: A Quiescent State Trigger for Materials Synthesis

Dispersions of particles onto which reactive groups are bound give rise to inhomogeneous concentrations that may afford fundamentally different chemical behavior compared to the same molecular species dissolved in homogeneous solution. An example is bimolecular reactivity of complementary-functionalized particles, whereby interparticle contact is expected to promote fast kinetics localized to the interface, while exhibiting essentially no reactivity elsewhere. Such materials may exhibit unique properties analogous to blood clotting and thereby be useful in self-healing applications. Here, we demonstrate a radical polymerization reaction whose initiation is controlled by the physical proximity of two complementary co-initiators bound to a substrate and/or polymer beads. Polymerization of the surrounding acrylate monomer only occurs when interfaces functionalized with dimethylaniline encounter interfaces bearing benzoyl peroxide. At the interface of the complementary-functionalized beads, polymerization affords a "clot-like" scaffold of beads and polymer. Interestingly, such a scaffold is only attained when the beads are in a quiescent state. These findings open the way to the design of spatially controlled dual initiator systems and novel self-healing strategies and motifs.

Coagulation factors bound to procoagulant platelets concentrate in cap structures to promote clotting

Binding of coagulation factors to phosphatidylserine (PS)-exposing procoagulant-activated platelets followed by formation of the membrane-dependent enzyme complexes is critical for blood coagulation. Procoagulant platelets formed upon strong platelet stimulation, usually with thrombin plus collagen, are large "balloons" with a small (∼1 μm radius) "cap"-like convex region that is enriched with adhesive proteins. Spatial distribution of blood coagulation factors on the surface of procoagulant platelets was investigated using confocal microscopy. All of them, including factors IXa (FIXa), FXa/FX, FVa, FVIII, prothrombin, and PS-sensitive marker Annexin V were distributed nonhomogeneously: they were primarily localized in the "cap," where their mean concentration was by at least an order of magnitude, higher than on the "balloon." Assembly of intrinsic tenase on liposomes with various PS densities while keeping the PS content constant demonstrated that such enrichment can accelerate this reaction by 2 orders of magnitude. The mechanisms of such acceleration were investigated using a 3-dimensional computer simulation model of intrinsic tenase based on these data. Transmission electron microscopy and focal ion beam-scanning electron microscopy with Annexin V immunogold-labeling revealed a complex organization of the "caps." In platelet thrombi formed in whole blood on collagen under arterial shear conditions, ubiquitous "caps" with increased Annexin V, FX, and FXa binding were observed, indicating relevance of this mechanism for surface-attached platelets under physiological flow. These results reveal an essential heterogeneity in the surface distribution of major coagulation factors on the surface of procoagulant platelets and suggest its importance in promoting membrane-dependent coagulation reactions.