Measurement of whole blood thrombus formation using parallel-plate flow chambers - a practical guide

Thrombotic microenvironment responsive crosslinking cyclodextrin metal-organic framework nanocarriers for precise targeting and thrombolysis

Global public health is seriously threatened by thrombotic disorders because of their high rates of mortality and disability. Most thrombolytic agents, especially protein-based pharmaceuticals, have a short half-life in circulation, reducing their effectiveness in thrombolysis. The creation of an intelligent drug delivery system that delivers medication precisely and releases it under regulated conditions at nearby thrombus sites is essential for effective thrombolysis. In this article, we present a unique medication delivery system (MCRUA) that selectively targets platelets and releases drugs by stimulation from the thrombus' microenvironment. The thrombolytic enzyme urokinase-type plasminogen-activator (uPA) and the anti-inflammatory medication Aspirin (acetylsalicylic acid, ASA) are both loaded onto pH-sensitive CaCO3/cyclodextrin crosslinking metal-organic frameworks (MC) that make up the MCRUA system. c(RGD) is functionalized on the surface of MC, which is functionalized by RGD to an esterification reaction. Additionally, the thrombus site's acidic microenvironment causes MCRUA to disintegrate to release uPA for thrombolysis and aiding in vessel recanalization. Moreover, cyclodextrin-encapsulated ASA enables the treatment of the inflammatory environment within the thrombus, enhancing the antiplatelet aggregation effects and promoting cooperative thrombolysis therapy. When used for thrombotic disorders, our drug delivery system (MCRUA) promotes thrombolysis, suppresses rethrombosis, and enhances biosafety with fewer hemorrhagic side effects.

Dissecting thrombus-directed chemotaxis and random movement in neutrophil near-thrombus motion in flow chambers

BACKGROUND

Thromboinflammation is caused by mutual activation of platelets and neutrophils. The site of thromboinflammation is determined by chemoattracting agents release by endothelium, immune cells, and platelets. Impaired neutrophil chemotaxis contributes to the pathogenesis of Shwachman-Diamond syndrome (SDS). In this hereditary disorder, neutrophils are known to have aberrant chemoattractant-induced F-actin properties. Here, we aim to determine whether neutrophil chemotaxis could be analyzed using our previously developed ex vivo assay of the neutrophils crawling among the growing thrombi.

METHODS

Adult and pediatric healthy donors, alongside with pediatric patients with SDS, were recruited for the study. Thrombus formation and granulocyte movement in hirudinated whole blood were visualized by fluorescent microscopy in fibrillar collagen-coated parallel-plate flow chambers. Alternatively, fibrinogen, fibronectin, vWF, or single tumor cells immobilized on coverslips were used. A computational model of chemokine distribution in flow chamber with a virtual neutrophil moving in it was used to analyze the observed data.

RESULTS

The movement of healthy donor neutrophils predominantly occurred in the direction and vicinity of thrombi grown on collagen or around tumor cells. For SDS patients or on coatings other than collagen, the movement was characterized by randomness and significantly reduced velocities. Increase in wall shear rates to 300-500 1/s led to an increase in the proportion of rolling neutrophils. A stochastic algorithm simulating leucocyte chemotaxis movement in the calculated chemoattractant field could reproduce the experimental trajectories of moving neutrophils for 72% of cells.

CONCLUSIONS

In samples from healthy donors, but not SDS patients, neutrophils move in the direction of large, chemoattractant-releasing platelet thrombi growing on collagen.

Optimal thrombin injection method for the treatment of femoral artery pseudoaneurysm

BACKGROUND

Iatrogenic femoral artery pseudoaneurysm (IFP) incidence is increasing with increase in diagnostic and therapeutic angiography, and so, the less invasive percutaneous thrombin injection (PTI) is the most widely used treatment. Moreover, studies that minimize PTI complications and highlight therapeutic effects are lacking.

OBJECTIVES

This study performed in vitro thrombosis modeling of pseudoaneurysms and analyzed thrombosis within and thromboembolism outside the sac during thrombin injection.

METHODS

We evaluated PTI in terms of thrombin injection location (at the junction of the IFP sac and neck, the center, and the dome, located farthest from the neck of the sac), thrombin injection time (5 and 8 seconds), and blood flow rate (ranging from 210 mL/min to 300 mL/min). Porcine blood was used as the working fluid in this study.

RESULTS

Thrombin injection at the junction of the IFP sac and the pseudoaneurysm neck led to less thrombosis within the sac but substantial thrombi consistently outside the sac, whereas thrombin injected at the sac center mostly led to complete thrombosis within the sac, preventing further blood flow into the sac and reducing likelihood of thrombi outside the sac. A longer thrombin injection time enhanced the therapeutic effect and decreased the possibility of thromboembolism. Thromboembolism occurred more frequently at flow rates of >240 mL/min.

CONCLUSION

The thrombin injection site in a pseudoaneurysm significantly influences thrombogenesis within and thromboembolism outside the sac. Thus, slow and deliberate injection of thrombin into the center of the sac could potentially reduce complications and enhance treatment efficacy.

Advanced in vitro hemocompatibility assessment of biomaterials using a new flow incubation system

Physiologically relevant in vitro hemocompatibility assessment of biomaterials remains challenging. We present a new setup that enables standardized whole blood incubation of biomedical materials under flow. A blood volume of 2 mL is recirculated over test surfaces in a custom-made parallel plate incubation system to determine the activation of hemostasis and inflammation. Controlled physiological shear rates between 125 s-1 and 1250 s-1 and minimized contact to air are combined with a natural-like pumping process. A unique feature of this setup allows tracing adhesion of blood cells to test surfaces microscopically in situ. Validation testing was performed in comparison to previously applied whole blood incubation methodologies. Experiments with the newly developed setup showed that even small obstacles to blood flow activate blood (independent of materials-induced blood activation levels); that adhesion of blood cells to biomaterials equilibrates within 5 to 10 min; that high shear rates (1250 compared to 375 s-1) induce platelet activation; and that hemolysis, platelet factor 4 (PF4) release and platelet loss - but not thrombin formation - depend on shear rate (within the range investigated, 125 to 1250 s-1).

Animal and Cellular Models in Thrombosis and Hemostasis

STANDARDIZED IN VITRO AND IN VIVO MODEL SYSTEMS TO SIMPLIFY COMPLEXITY-THAT'S HOW WE LEARN: The discovery of new target molecules and translational progress in the development and refinement of antithrombotic therapies as well as the improved treatment of bleeding disorders strongly relies on standardized ex vivo and in vivo models that closely resemble the respective human pathologies. The standardization of these models requires sound training in specialized hemostasis and thrombosis research laboratories as well as a consistent daily routine. In this theme issue of Hämostaseologie-Progress in Haemostasis, four review articles cover key models that have proven instrumental to gain mechanistic insights on thrombogenesis and hemostatic processes. In recent decades, these models have moved our field forward and enabled translation across scales, from cell-based research to isolated flow chamber systems, to mouse thrombosis models reflecting the pathologic situations as observed in patients, to large animal models.

Microfluidic Organ-Chips and Stem Cell Models in the Fight Against COVID-19

SARS-CoV-2, the virus underlying COVID-19, has now been recognized to cause multiorgan disease with a systemic effect on the host. To effectively combat SARS-CoV-2 and the subsequent development of COVID-19, it is critical to detect, monitor, and model viral pathogenesis. In this review, we discuss recent advancements in microfluidics, organ-on-a-chip, and human stem cell-derived models to study SARS-CoV-2 infection in the physiological organ microenvironment, together with their limitations. Microfluidic-based detection methods have greatly enhanced the rapidity, accessibility, and sensitivity of viral detection from patient samples. Engineered organ-on-a-chip models that recapitulate in vivo physiology have been developed for many organ systems to study viral pathology. Human stem cell-derived models have been utilized not only to model viral tropism and pathogenesis in a physiologically relevant context but also to screen for effective therapeutic compounds. The combination of all these platforms, along with future advancements, may aid to identify potential targets and develop novel strategies to counteract COVID-19 pathogenesis.

Platelet accumulation in an endothelium-coated elastic vein valve model of deep vein thrombosis is mediated by GPIbα-VWF interaction

Deep vein thrombosis is a life-threatening disease that takes millions of people's lives worldwide. Given both technical and ethical issues of using animals in research, it is necessary to develop an appropriate in vitro model that would recapitulate the conditions of venous thrombus development. We present here a novel microfluidics vein-on-a-chip with moving valve leaflets to mimic the hydrodynamics in a vein, and Human Umbilical Vein Endothelial Cell (HUVEC) monolayer. A pulsatile flow pattern, typical for veins, was used in the experiments. Unstimulated human platelets, reconstituted with the whole blood, accumulated at the luminal side of the leaflet tips proportionally to the leaflet flexibility. Platelet activation by thrombin induced robust platelet accrual at the leaflet tips. Inhibition of glycoprotein (GP) IIb-IIIa did not decrease but, paradoxically, slightly increased platelet accumulation. In contrast, blockade of the interaction between platelet GPIbα and A1 domain of von Willebrand factor completely abolished platelet deposition. Stimulation of the endothelium with histamine, a known secretagogue of Weibel-Palade bodies, promoted platelet accrual at the basal side of the leaflets, where human thrombi are usually observed. Thus, platelet deposition depends on the leaflet flexibility, and accumulation of activated platelets at the valve leaflets is mediated by GPIbα-VWF interaction.

Dynamic investigation of zein-based degradable and hemocompatible coatings for drug-eluting stents: a microfluidic approach

Biodegradable stent coatings have shown great potential in terms of delivering drugs to a damaged vessel wall, and their release profiles are key elements governing the overall performance of drug-eluting stents (DESs). However, release and degradation kinetics are usually not tested under simulated physiological conditions or in dynamic environments, both essential aspects in the design of novel DESs. To bridge this gap, fused silica-based microfluidic systems, with either round or square channel cross-sections, were designed to mimic the microenvironment of a stented vessel. In particular, we fabricated and characterized microfluidic chips based on customizable channels, which were spray-coated with a naturally-derived, rutin-loaded zein solution, to perform a comprehensive study under flow conditions. Dynamic assays after 6 hours showed how the degradation of the zein matrix was affected by the cross-sectional conformation (∼69% vs. ∼61%, square and round channel, respectively) and the simulated blood fluid components (∼55%, round channel with a more viscous solution). The released amount of rutin was ∼81% vs. ∼77% and ∼78% vs. ∼74% from the square and round channels, using the less and more viscous blood-simulated fluids, respectively. Fitting the drug release data to Korsmeyer-Peppas and first-order mathematical models provided further insight into the mechanism of rutin release and coating behavior under flowing conditions. More importantly, whole blood tests with our newly developed microfluidic platforms confirmed the hemocompatibility of our zein-based coating. In detail, in-flow and static studies on the blood cell behavior showed a significant reduction of platelet adhesion (∼73%) and activation (∼93%) compared to the stainless-steel substrate, confirming the benefits of using such naturally-derived coatings to avoid clogging. Overall, our microfluidic designs can provide a key practical tool for assessing polymer degradation and drug release from degradable matrices under flowing conditions, thus aiding future studies on the development of hemocompatible, controlled-release coatings for DESs.

Thrombosis Models: An Overview of Common In Vivo and In Vitro Models of Thrombosis

Occlusions in the blood vessels caused by blood clots, referred to as thrombosis, and the subsequent outcomes are leading causes of morbidity and mortality worldwide. In vitro and in vivo models of thrombosis have advanced our understanding of the complex pathways involved in its development and allowed the evaluation of different therapeutic approaches for its management. This review summarizes different commonly used approaches to induce thrombosis in vivo and in vitro, without detailing the protocols for each technique or the mechanism of thrombus development. For ease of flow, a schematic illustration of the models mentioned in the review is shown below. Considering the number of available approaches, we emphasize the importance of standardizing thrombosis models in research per study aim and application, as different pathophysiological mechanisms are involved in each model, and they exert varying responses to the same carried tests. For the time being, the selection of the appropriate model depends on several factors, including the available settings and research facilities, the aim of the research and its application, and the researchers' experience and ability to perform surgical interventions if needed.

“Going with the flow” in modeling fibrinolysis

The formation of thrombi is shaped by intravascular shear stress, influencing both fibrin architecture and the cellular composition which has downstream implications in terms of stability against mechanical and fibrinolytic forces. There have been many advancements in the development of models that incorporate flow rates akin to those found in vivo. Both thrombus formation and breakdown are simultaneous processes, the balance of which dictates the size, persistence and resolution of thrombi. Therefore, there is a requirement to have models which mimic the physiological shear experienced within the vasculature which in turn influences the fibrinolytic degradation of the thrombus. Here, we discuss various assays for fibrinolysis and importantly the development of novel models that incorporate physiological shear rates. These models are essential tools to untangle the molecular and cellular processes which govern fibrinolysis and can recreate the conditions within normal and diseased vessels to determine how these processes become perturbed in a pathophysiological setting. They also have utility to assess novel drug targets and antithrombotic drugs that influence thrombus stability.

Roles of Focal Adhesion Kinase PTK2 and Integrin αIIbβ3 Signaling in Collagen- and GPVI-Dependent Thrombus Formation under Shear

Glycoprotein (GP)VI and integrin αIIbβ3 are key signaling receptors in collagen-dependent platelet aggregation and in arterial thrombus formation under shear. The multiple downstream signaling pathways are still poorly understood. Here, we focused on disclosing the integrin-dependent roles of focal adhesion kinase (protein tyrosine kinase 2, PTK2), the shear-dependent collagen receptor GPR56 (ADGRG1 gene), and calcium and integrin-binding protein 1 (CIB1). We designed and synthetized peptides that interfered with integrin αIIb binding (pCIB and pCIBm) or mimicked the activation of GPR56 (pGRP). The results show that the combination of pGRP with PTK2 inhibition or of pGRP with pCIB > pCIBm in additive ways suppressed collagen- and GPVI-dependent platelet activation, thrombus buildup, and contraction. Microscopic thrombus formation was assessed by eight parameters (with script descriptions enclosed). The suppressive rather than activating effects of pGRP were confined to blood flow at a high shear rate. Blockage of PTK2 or interference of CIB1 no more than slightly affected thrombus formation at a low shear rate. Peptides did not influence GPVI-induced aggregation and Ca2+ signaling in the absence of shear. Together, these data reveal a shear-dependent signaling axis of PTK2, integrin αIIbβ3, and CIB1 in collagen- and GPVI-dependent thrombus formation, which is modulated by GPR56 and exclusively at high shear. This work thereby supports the role of PTK2 in integrin αIIbβ3 activation and signaling.

Structure-Based Cyclic Glycoprotein Ibα-Derived Peptides Interfering with von Willebrand Factor-Binding, Affecting Platelet Aggregation under Shear

The plasmatic von Willebrand factor (VWF) circulates in a compact form unable to bind platelets. Upon shear stress, the VWF A1 domain is exposed, allowing VWF-binding to platelet glycoprotein Ib-V-IX (GPIbα chain). For a better understanding of the role of this interaction in cardiovascular disease, molecules are needed to specifically interfere with the opened VWF A1 domain interaction with GPIbα. Therefore, we in silico designed and chemically synthetized stable cyclic peptides interfering with the platelet-binding of the VWF A1 domain per se or complexed with botrocetin. Selected peptides (26–34 amino acids) with the lowest-binding free energy were: the monocyclic mono- vOn Willebrand factoR-GPIbα InTerference (ORbIT) peptide and bicyclic bi-ORbIT peptide. Interference of the peptides in the binding of VWF to GPIb-V-IX interaction was retained by flow cytometry in comparison with the blocking of anti-VWF A1 domain antibody CLB-RAg35. In collagen and VWF-dependent whole-blood thrombus formation at a high shear rate, CLB-RAg35 suppressed stable platelet adhesion as well as the formation of multilayered thrombi. Both peptides phenotypically mimicked these changes, although they were less potent than CLB-RAg35. The second-round generation of an improved peptide, namely opt-mono-ORbIT (28 amino acids), showed an increased inhibitory activity under flow. Accordingly, our structure-based design of peptides resulted in physiologically effective peptide-based inhibitors, even for convoluted complexes such as GPIbα-VWF A1.

Ex vivo observation of granulocyte activity during thrombus formation

Background

The process of thrombus formation is thought to involve interactions between platelets and leukocytes. Leukocyte incorporation into growing thrombi has been well established in vivo, and a number of properties of platelet-leukocyte interactions critical for thrombus formation have been characterized in vitro in thromboinflammatory settings and have clinical relevance. Leukocyte activity can be impaired in distinct hereditary and acquired disorders of immunological nature, among which is Wiskott-Aldrich Syndrome (WAS). However, a more quantitative characterization of leukocyte behavior in thromboinflammatory conditions has been hampered by lack of approaches for its study ex vivo. Here, we aimed to develop an ex vivo model of thromboinflammation, and compared granulocyte behavior of WAS patients and healthy donors.

Results

Thrombus formation in anticoagulated whole blood from healthy volunteers and patients was visualized by fluorescent microscopy in parallel-plate flow chambers with fibrillar collagen type I coverslips. Moving granulocytes were observed in hirudinated or sodium citrate-recalcified blood under low wall shear rate conditions (100 s⁻¹). These cells crawled around thrombi in a step-wise manner with an average velocity of 90–120 nm/s. Pre-incubation of blood with granulocyte priming agents lead to a significant decrease in mean-velocity of the cells and increase in the number of adherent cells. The leukocytes from patients with WAS demonstrated a 1.5-fold lower mean velocity, in line with their impaired actin polymerization. It is noteworthy that in an experimental setting where patients’ platelets were replaced with healthy donor’s platelets the granulocytes’ crawling velocity did not change, thus proving that WASP (WAS protein) deficiency causes disruption of granulocytes’ behavior. Thereby, the observed features of granulocytes crawling are consistent with the neutrophil chemotaxis phenomenon. As most of the crawling granulocytes carried procoagulant platelets teared from thrombi, we propose that the role of granulocytes in thrombus formation is that of platelet scavengers.

Conclusions

We have developed an ex vivo experimental model applicable for observation of granulocyte activity in thrombus formation. Using the proposed setting, we observed a reduction of motility of granulocytes of patients with WAS. We suggest that our ex vivo approach should be useful both for basic and for clinical research.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01238-x.

Multiparameter platelet function analysis of bleeding patients with a prolonged platelet function analyser closure time

Patients referred for evaluation of bleeding symptoms occasionally have a prolonged platelet function analyser (PFA) closure time, without evidence for von Willebrand disease or impaired platelet aggregation. The aim of this study was to establish a shear‐dependent platelet function defect in these patients. Patients were included based on high bleeding score and prior PFA prolongation. Common tests of von Willebrand factor (VWF) and platelet function and exome sequencing were performed. Microfluidic analysis of shear‐dependent collagen‐induced whole‐blood thrombus formation was performed. In 14 PFA‐only patients, compared to healthy volunteers, microfluidic tests showed significantly lower platelet adhesion and thrombus formation parameters. This was accompanied by lower integrin activation, phosphatidylserine exposure and P‐selectin expression. Principal components analysis indicated VWF as primary explaining variable of PFA prolongation, whereas conventional platelet aggregation primarily explained the reduced thrombus parameters under shear. In five patients with severe microfluidic abnormalities, conventional platelet aggregation was in the lowest range of normal. No causal variants in Mendelian genes known to cause bleeding or platelet disorders were identified. Multiparameter assessment of whole‐blood thrombus formation under shear indicates single or combined effects of low–normal VWF and low–normal platelet aggregation in these patients, suggesting a shear‐dependent platelet function defect, not detected by static conventional haemostatic tests.

Temporal Roles of Platelet and Coagulation Pathways in Collagen- and Tissue Factor-Induced Thrombus Formation

In hemostasis and thrombosis, the complex process of thrombus formation involves different molecular pathways of platelet and coagulation activation. These pathways are considered as operating together at the same time, but this has not been investigated. The objective of our study was to elucidate the time-dependency of key pathways of thrombus and clot formation, initiated by collagen and tissue factor surfaces, where coagulation is triggered via the extrinsic route. Therefore, we adapted a microfluidics whole-blood assay with the Maastricht flow chamber to acutely block molecular pathways by pharmacological intervention at desired time points. Application of the technique revealed crucial roles of glycoprotein VI (GPVI)-induced platelet signaling via Syk kinase as well as factor VIIa-induced thrombin generation, which were confined to the first minutes of thrombus buildup. A novel anti-GPVI Fab EMF-1 was used for this purpose. In addition, platelet activation with the protease-activating receptors 1/4 (PAR1/4) and integrin αIIbβ3 appeared to be prolongedly active and extended to later stages of thrombus and clot formation. This work thereby revealed a more persistent contribution of thrombin receptor-induced platelet activation than of collagen receptor-induced platelet activation to the thrombotic process.

Emerging Microfluidic Approaches for Platelet Mechanobiology and Interplay With Circulatory Systems

Understanding how platelets can sense and respond to hemodynamic forces in disturbed blood flow and complexed vasculature is crucial to the development of more effective and safer antithrombotic therapeutics. By incorporating diverse structural and functional designs, microfluidic technologies have emerged to mimic microvascular anatomies and hemodynamic microenvironments, which open the floodgates for fascinating platelet mechanobiology investigations. The latest endothelialized microfluidics can even recapitulate the crosstalk between platelets and the circulatory system, including the vessel walls and plasma proteins such as von Willebrand factor. Hereby, we highlight these exciting microfluidic applications to platelet mechanobiology and platelet–circulatory system interplay as implicated in thrombosis. Last but not least, we discuss the need for microfluidic standardization and summarize the commercially available microfluidic platforms for researchers to obtain reproducible and consistent results in the field.

An "occlusive thrombosis-on-a-chip" microfluidic device for investigating the effect of anti-thrombotic drugs

Cardiovascular disease remains one of the world's leading causes of death. Myocardial infarction (heart attack) is triggered by occlusion of coronary arteries by platelet-rich thrombi (clots). The development of new anti-platelet drugs to prevent myocardial infarction continues to be an active area of research and is dependent on accurately modelling the process of clot formation. Occlusive thrombi can be generated in vivo in a range of species, but these models are limited by variability and lack of relevance to human disease. Although in vitro models using human blood can overcome species-specific differences and improve translatability, many models do not generate occlusive thrombi. In those models that do achieve occlusion, time to occlusion is difficult to measure in an unbiased and objective manner. In this study we developed a simple and robust approach to determine occlusion time of a novel in vitro microfluidic assay. This highlighted the potential for occlusion to occur in thrombosis microfluidic devices through off-site coagulation, obscuring the effect of anti-platelet drugs. We therefore designed a novel occlusive thrombosis-on-a-chip microfluidic device that reliably generates occlusive thrombi at arterial shear rates by quenching downstream coagulation. We further validated our device and methods by using the approved anti-platelet drug, eptifibatide, recording a significant difference in the "time to occlude" in treated devices compared to control conditions. These results demonstrate that this device can be used to monitor the effect of antithrombotic drugs on time to occlude, and, for the first time, delivers this essential data in an unbiased and objective manner.

Utility of Platelet Endothelial Cell Adhesion Molecule 1 in the Platelet Activity Assessment in Mouse and Human Blood

In our previous study, we introduced the platelet endothelial cell adhesion molecule 1 (PECAM-1)/thrombus ratio, which is a parameter indicating the proportion of PECAM-1 in laser-induced thrombi in mice. Because PECAM-1 is an antithrombotic molecule, the higher the PECAM-1/thrombus ratio, the less activated the platelets. In this study, we used an extracorporeal model of thrombosis (flow chamber model) to verify its usefulness in the assessment of the PECAM-1/thrombus ratio in animal and human studies. Using the lipopolysaccharide (LPS)-induced inflammation model, we also evaluated whether the PECAM-1/thrombus ratio determined in the flow chamber (without endothelium) differed from that calculated in laser-induced thrombosis (with endothelium). We observed that acetylsalicylic acid (ASA) decreased the area of the thrombus while increasing the PECAM-1/thrombus ratio in healthy mice and humans in a dose-dependent manner. In LPS-treated mice, the PECAM-1/thrombus ratio decreased as the dose of ASA increased in both thrombosis models, but the direction of change in the thrombus area was inconsistent. Our study demonstrates that the PECAM-1/thrombus ratio can more accurately describe the platelet activation status than commonly used parameters such as the thrombus area, and, hence, it can be used in both human and animal studies.

Adrenaline May Contribute to Prothrombotic Condition via Augmentation of Platelet Procoagulant Response, Enhancement of Fibrin Formation, and Attenuation of Fibrinolysis

Background: Adrenaline is believed to play a role in thrombosis and hemostasis. The complex effect of its clinically relevant concentrations on thrombus formation, coagulation and fibrinolysis in human blood has never been specifically studied. Methods: Confocal microscopy was used to study thrombus formation under flow, exposure of phosphatidylserine (PS) in adhered platelets, to evaluate clots density, and to measure kinetics of fibrin formation and external fibrinolysis under flow. Flow cytometry was utilized to assess PS exposure in non-adhered platelets. Kinetics of clot formation and internal fibrinolysis was evaluated by thromboelastometry. Platelet aggregation was measured by optical aggremometry. Kinetics of clot retraction was assessed by using digital camera. Results: We found that adrenaline (1-10 nM) is able to enhance platelet activation evoked by subthreshold collagen (150 ng/ml), resulting in augmentation of platelet aggregation, thrombus formation under arterial flow conditions, platelet PS exposure, and formation of platelet-fibrin clots. The development of platelet procoagulant response evoked by adrenaline + low collagen was associated with the formation of denser platelet-fibrin clots and the decrease in rate of fibrinolysis despite whether lysis was initiated inside (internal fibrinolysis) or outside the clot (external fibrinolysis). The above phenomena were abolished by the α₂-adrenergic receptor antagonist, rauwolscine. Adrenaline-collagen synergism, expressed as PS exposure, was significantly reduced by cyclooxygenase inhibitor (acetylsalicic acid), GPIIb/IIIa receptor blocker (tirofiban), and P2Y₁₂ receptor antagonist (PSB 0739). Conclusion: Clinically relevant concentrations of adrenaline may significantly augment responses of human platelets in the presence of subthreshold concentrations of collagen, which should be considered during therapies involving adrenaline infusion. Routinely used antiplatelet drugs may reduce the prothrombotic state evoked by adrenaline-collagen synergism.

Multiparameter microfluidics assay of thrombus formation reveals increased sensitivity to contraction and antiplatelet agents at physiological temperature

INTRODUCTION

Current developments to assess qualitative and quantitative platelet traits in flowed whole-blood are based on microfluidic devices that mostly operate at room temperature. However, operation at physiological temperature (37 °C) may increase the assay's sensitivity, and facilitates the comparison to other platelet function tests of the diagnostic laboratory.

MATERIALS AND METHODS

We adapted the conventional microspot-based microfluidic device with a simple thermo-coupled pre-heating module. Automated analysis of microscopic images assisted in obtaining five time-dependent parameters of thrombus formation over collagen microspots (shear rate 1000 s^-1). These modifications allowed rapid testing of control and patient blood samples at physiological temperature.

RESULTS AND CONCLUSION

The higher temperature enhanced platelet adhesion and aggregation as well as late thrombus characteristics such as size and contraction, when compared to room temperature. Moreover, assessment at 37 °C indicated a time-dependent impairment of the thrombus parameters in blood from patients taking common antiplatelet medication, i.e. aspirin and/or clopidogrel. This pointed to increased contribution of the autocrine platelet agonists thromboxane A₂ and ADP in the buildup of contracted thrombi under flow. Overall, this study underlined the advantage of multiparameter assessment of microfluidic thrombus formation in detecting an acquired platelet dysfunction, when operating at physiological temperature. This work may bring microfluidics tests closer to the diagnostic laboratory.

Confocal Blood Flow Videomicroscopy of Thrombus Formation over Human Arteries and Local Targeting of P2X7

Atherothrombosis exposes vascular components to blood. Currently, new antithrombotic therapies are emerging. Herein we investigated thrombogenesis of human arteries with/without atherosclerosis, and the interaction of coagulation and vascular components, we and explored the anti-thrombogenic efficacy of blockade of the P2X purinoceptor 7 (P2X7). A confocal blood flow videomicroscopy system was performed on cryosections of internal mammary artery (IMA) or carotid plaque (CPL) determining/localizing platelets and fibrin. Blood from healthy donors elicited thrombi over arterial layers. Confocal microscopy associated thrombus with tissue presence of collagen type I, laminin, fibrin(ogen) and tissue factor (TF). The addition of antibodies blocking TF (aTF) or factor XI (aFXI) to blood significantly reduced fibrin deposition, variable platelet aggregation and aTF + aFXI almost abolished thrombus formation, showing synergy between coagulation pathways. A scarce effect of aTF over sub-endothelial regions, more abundant in tissue TF and bundles of laminin and collagen type I than deep intima, may suggest tissue thrombogenicity as molecular structure-related. Consistently with TF-related vascular function and expression of P2X7, the sections from CPL but not IMA tissue cultures pre-treated with the P2X7 antagonist A740003 demonstrated poor thrombogenesis in flow experiments. These data hint to local targeting studies on P2X7 modulation for atherothrombosis prevention/therapy.

Healthy and diseased in vitro models of vascular systems

Irregular hemodynamics affects the progression of various vascular diseases, such atherosclerosis or aneurysms. Despite the extensive hemodynamics studies on animal models, the inter-species differences between humans and animals hamper the translation of such findings. Recent advances in vascular tissue engineering and the suitability of in vitro models for interim analysis have increased the use of in vitro human vascular tissue models. Although the effect of flow on endothelial cell (EC) pathophysiology and EC-flow interactions have been vastly studied in two-dimensional systems, they cannot be used to understand the effect of other micro- and macro-environmental parameters associated with vessel wall diseases. To generate an ideal in vitro model of the vascular system, essential criteria should be included: 1) the presence of smooth muscle cells or perivascular cells underneath an EC monolayer, 2) an elastic mechanical response of tissue to pulsatile flow pressure, 3) flow conditions that accurately mimic the hemodynamics of diseases, and 4) geometrical features required for pathophysiological flow. In this paper, we review currently available in vitro models that include flow dynamics and discuss studies that have tried to address the criteria mentioned above. Finally, we critically review in vitro fluidic models of atherosclerosis, aneurysm, and thrombosis.

Haemodynamic flow conditions at the initiation of high-shear platelet aggregation: a combined in vitro and cellular in silico study

The influence of the flow environment on platelet aggregation is not fully understood in high-shear thrombosis. The objective of this study is to investigate the role of a high shear rate in initial platelet aggregation. The haemodynamic conditions in a microfluidic device are studied using cell-based blood flow simulations. The results are compared with in vitro platelet aggregation experiments performed with porcine whole blood (WB) and platelet-rich-plasma (PRP). We studied whether the cell-depleted layer in combination with high shear and high platelet flux can account for the distribution of platelet aggregates. High platelet fluxes at the wall were found in silico. In WB, the platelet flux was about twice as high as in PRP. Additionally, initial platelet aggregation and occlusion were observed in vitro in the stenotic region. In PRP, the position of the occlusive thrombus was located more downstream than in WB. Furthermore, the shear rates and stresses in cell-based and continuum simulations were studied. We found that a continuum simulation is a good approximation for PRP. For WB, it cannot predict the correct values near the wall.

Microengineered Human Vein-Chip Recreates Venous Valve Architecture and Its Contribution to Thrombosis

Deep vein thrombosis (DVT) and its consequences are lethal, but current models cannot completely dissect its determinants-endothelium, flow, and blood constituents-together called Virchow's triad. Most models for studying DVT forego assessment of venous valves that serve as the primary sites of DVT formation. Therefore, the knowledge of DVT formed at the venous cusps has remained obscure due to lack of experimental models. Here, organ-on-chip methodology is leveraged to create a Vein-Chip platform integrating fully vascularized venous valves and its hemodynamic, as seen in vivo. These Vein-Chips reveal that vascular endothelium of valve cusps adapts to the locally disturbed microenvironment by expressing a different phenotype from the regions of uniform flow. This spatial adaptation of endothelial function recreated on the in vitro Vein-Chip platform is shown to protect the vein from thrombosis from disturbed flow in valves, but interestingly, cytokine stimulation reverses the effect and switches the valve endothelium to becoming prothrombotic. The platform eventually modulates the three factors of Virchow's triad and provides a systematic approach to investigate the determinants of fibrin and platelet dynamics of DVT. Therefore, this Vein-Chip offers a new preclinical approach to study venous pathophysiology and show effects of antithrombotic drug treatment.

Platelet Imaging

The knowledge gained through imaging platelets has formed the backbone of our understanding of their biology in health and disease. Early investigators relied on conventional light microscopy with limited resolution and were primarily able to identify the presence and basic morphology of platelets. The advent of high resolution technologies, in particular, electron microscopy, accelerated our understanding of the dynamics of platelet ultrastructure dramatically. Further refinements and improvements in our ability to localize and reliably identify platelet structures have included the use of immune-labeling techniques, correlative-fluorescence light and electron microscopy, and super-resolution microscopies. More recently, the expanded development and application of intravital microscopy in animal models has enhanced our knowledge of platelet functions and thrombus formation in vivo, as these experimental systems most closely replicate native biological environments. Emerging improvements in our ability to characterize platelets at the ultrastructural and organelle levels include the use of platelet cryogenic electron tomography with quantitative, unbiased imaging analysis, and the ability to genetically label platelet features with electron dense markers for analysis by electron microscopy.

Vascular protective effect of aspirin and rivaroxaban upon endothelial denudation of the mouse carotid artery

While in recent trials the dual pathway inhibition with aspirin plus rivaroxaban has shown to be efficacious in patients with atherosclerotic cardiovascular disease, little is known about the effects of this combination treatment on thrombus formation and vascular remodelling upon vascular damage. The aim of this study was to examine the effects of aspirin and/or rivaroxaban on injury-induced murine arterial thrombus formation in vivo and in vitro, vessel-wall remodelling, and platelet-leukocyte aggregates. Temporary ligation of the carotid artery of C57BL/6 mice, fed a western type diet, led to endothelial denudation and sub-occlusive thrombus formation. At the site of ligation, the vessel wall stiffened and the intima-media thickened. Aspirin treatment antagonized vascular stiffening and rivaroxaban treatment led to a positive trend towards reduced stiffening. Local intima-media thickening was antagonized by both aspirin or rivaroxaban treatment. Platelet-leukocyte aggregates and the number of platelets per leukocyte were reduced in aspirin and/or rivaroxaban treatment groups. Furthermore, rivaroxaban restricted thrombus growth and height in vitro. In sum, this study shows vascular protective effects of aspirin and rivaroxaban, upon vascular injury of the mouse artery.

Assessment of Shear-Dependent Kinetics of Primary Haemostasis With a Microfluidic Acoustic Biosensor

Primary haemostasis is a complex dynamic process, which involves in-flow interactions between platelets and sub-endothelial matrix at the area of the damaged vessel wall. It results in a first haemostatic plug, which stops bleeding, before coagulation ensues and consolidates it. The diagnosis of primary haemostasis defect would benefit from evaluation of the whole sequence of mechanisms involved in platelet plug formation in flow. This work proposes a new approach that is based on characterization of the shear-dependent kinetics that enables the evaluation of the early stages of primary haemostasis. We used a label-free method with a quartz crystal microbalance (QCM) biosensor to measure the platelet deposits over time onto covalently immobilized type I fibrillar collagen. We defined three metrics: total frequency shift, lag time, and growth rate. The measurement was completed at four predefined shear rates prevailing in small vessels (500, 770, 1000 and 1500 s^-1) during five minutes of perfusion with anticoagulated normal whole blood. The rate of the frequency shift over the first five minutes was strongly influenced by shear rate conditions, presenting a maximum around 770 s^-1, and varying by a factor larger than three in the studied shear rate range. To validate the biosensor signal, the total frequency shift was compared to results obtained by atomic force microscopy (AFM) on final platelet deposits. The results show that shear-dependent kinetic assays are promising as an advanced method for screening of primary haemostasis.

Effects of bacterial lipopolysaccharides on platelet function: inhibition of weak platelet activation

Platelets are anucleate blood cells with reported roles in hemostasis and immune responses, which possess a functional receptor for bacterial lipopolysaccharides (LPSs), the well-known inducers of inflammation. However, LPSs effects on platelets are contradictory. Here we aim to investigate mechanisms of platelet functioning in the presence of LPS and to find the cause of the discrepancy in the previously published data. Cell activity was analyzed by flow cytometry, western blotting, and aggregometry. Thrombus growth was assessed by fluorescent microscopy. LPS' activity was checked by their capability to induce PMN activation. However, LPSs did not substantially affect either thrombus growth in flow chambers, irreversible platelet aggregation, or platelet responses to strong activation. Platelet aggregation in response to 1 μM of ADP was significantly inhibited by LPSs. Flow cytometry analysis revealed that platelet activation responses to weak stimulation were also diminished by LPSs, while VASP phosphorylation was weakly increased. Additionally, LPSs were capable of inhibition of ADP-induced P2-receptor desensitization. Incubation of platelets with a pan-PDE inhibitor IBMX significantly enhanced the LPSs-induced platelet inhibition, implying cAMP/cGMP dependent mechanism. The discrepancy in the previously published data could be explained by LPS-induced weak inhibition of platelet activation and the prevention of platelet desensitization.

Impaired iloprost-induced platelet inhibition and phosphoproteome changes in patients with confirmed pseudohypoparathyroidism type Ia, linked to genetic mutations in GNAS

Patients diagnosed with pseudohypoparathyroidism type Ia (PHP Ia) suffer from hormonal resistance and abnormal postural features, in a condition classified as Albright hereditary osteodystrophy (AHO) syndrome. This syndrome is linked to a maternally inherited mutation in the GNAS complex locus, encoding for the GTPase subunit Gsα. Here, we investigated how platelet phenotype and omics analysis can assist in the often difficult diagnosis. By coupling to the IP receptor, Gsα induces platelet inhibition via adenylyl cyclase and cAMP-dependent protein kinase A (PKA). In platelets from seven patients with suspected AHO, one of the largest cohorts examined, we studied the PKA-induced phenotypic changes. Five patients with a confirmed GNAS mutation, displayed impairments in Gsα-dependent VASP phosphorylation, aggregation, and microfluidic thrombus formation. Analysis of the platelet phosphoproteome revealed 2,516 phosphorylation sites, of which 453 were regulated by Gsα-PKA. Common changes in the patients were: (1) a joint panel of upregulated and downregulated phosphopeptides; (2) overall PKA dependency of the upregulated phosphopeptides; (3) links to key platelet function pathways. In one patient with GNAS mutation, diagnosed as non-AHO, the changes in platelet phosphoproteome were reversed. This combined approach thus revealed multiple phenotypic and molecular biomarkers to assist in the diagnosis of suspected PHP Ia.

Human In Vitro Models for Assessing the Genomic Basis of Chemotherapy-Induced Cardiovascular Toxicity

Chemotherapy-induced cardiovascular toxicity (CICT) is a well-established risk for cancer survivors and causes diseases such as heart failure, arrhythmia, vascular dysfunction, and atherosclerosis. As our knowledge of the precise cardiovascular risks of each chemotherapy agent has improved, it has become clear that genomics is one of the most influential predictors of which patients will experience cardiovascular toxicity. Most recently, GWAS-led, top-down approaches have identified novel genetic variants and their related genes that are statistically related to CICT. Importantly, the advent of human-induced pluripotent stem cell (hiPSC) models provides a system to experimentally test the effect of these genomic findings in vitro, query the underlying mechanisms, and develop novel strategies to mitigate the cardiovascular toxicity liabilities due to these mechanisms. Here we review the cardiovascular toxicities of chemotherapy drugs, discuss how these can be modeled in vitro, and suggest how these models can be used to validate genetic variants that predispose patients to these effects.

Bacterial biofilms in infective endocarditis: an in vitro model to investigate emerging technologies of antimicrobial cardiovascular device coatings

Objective

In spite of the progress in antimicrobial and surgical therapy, infective endocarditis (IE) is still associated with a high morbidity and mortality. IE is characterized by bacterial biofilms of the endocardium, especially of the aortic and mitral valve leading to their destruction. About one quarter of patients with formal surgery indication cannot undergo surgery. This group of patients needs further options of therapy, but due to a lack of models for IE prospects of research are low. Therefore, the purpose of this project was to establish an in vitro model of infective endocarditis to allow growth of bacterial biofilms on porcine aortic valves, serving as baseline for further research.

Methods and results

A pulsatile two-chamber circulation model was constructed that kept native porcine aortic valves under sterile, physiologic hemodynamic and temperature conditions. To create biofilms on porcine aortic valves the system was inoculated with Staphylococcus epidermidis PIA 8400. Aortic roots were incubated in the model for increasing periods of time (24 h and 40 h) and bacterial titration (1.5 × 10⁴ CFU/mL and 1.5 × 10⁵ CFU/mL) with 5 L cardiac output per minute. After incubation, tissue sections were analysed by fluorescence in situ hybridization (FISH) for direct visualization of the biofilms. Pilot tests for biofilm growth showed monospecies colonization consisting of cocci with time- and inocula-dependent increase after 24 h and 40 h (n = 4). In n = 3 experiments for 24 h, with the same inocula, FISH visualized biofilms with ribosome-containing, and thus metabolic active cocci, tissue infiltration and similar colonization pattern as observed by the FISH in human IE heart valves infected by S. epidermidis.

Conclusion

These results demonstrate the establishment of a novel in vitro model for bacterial biofilm growth on porcine aortic roots mimicking IE. The model will allow to identify predilection sites of valves for bacterial adhesion and biofilm growth and it may serve as baseline for further research on IE therapy and prevention, e.g. the development of antimicrobial transcatheter approaches to IE.

Graphic abstract

Targeting of externalized αB-crystallin on irradiated endothelial cells with pro-thrombotic vascular targeting agents: Potential applications for brain arteriovenous malformations

BACKGROUND

Vascular targeting uses molecular markers on the surface of diseased vasculature for ligand-directed drug delivery to induce vessel occlusion or destruction. In the absence of discriminatory markers, such as in brain arteriovenous malformations (AVMs), stereotactic radiosurgery may be used to prime molecular changes on the endothelial surface. This study explored αB-crystallin (CRYAB) as a radiation induced target and pre-tested the specificity and efficacy of a CRYAB-targeting coaguligand for in vitro thrombus induction.

METHODS

A parallel-plate flow system was established to circulate human whole blood over a layer of human brain endothelial cells. A conjugate of anti-CRYAB antibody and thrombin was injected into the circuit to compare binding and thrombus formation on cells with or without prior radiation treatment (0-25 Gy).

RESULTS

Radiation increased CRYAB expression and surface exposure in human brain endothelial cells. In the parallel-plate flow system, the targeted anti-CRYAB-thrombin conjugate increased thrombus formation on the surface of irradiated cells relative to non-irradiated cells and to a non-targeting IgG-thrombin conjugate. Fibrin deposition and accumulation of fibrinogen degradation products increased significantly at radiation doses at or above 15 Gy with conjugate concentrations of 1.25 and 2.5 μg/mL.

CONCLUSIONS

CRYAB exposure can be detected at the surface of human brain endothelial cells in response to irradiation. Pro-thrombotic CRYAB-targeting conjugates can bind under high flow conditions and in the presence of whole blood induce stable thrombus formation with high specificity and efficacy on irradiated surfaces. CRYAB provides a novel radiation marker for potential vascular targeting in irradiated brain AVMs.

ELMO1 deficiency enhances platelet function

Phosphatidylinositol 3-kinase is an important signaling molecule that, once activated, leads to the generation of phosphatidylinositol (3,4,5)-trisphosphate (PIP₃). We performed a proteomic screen to identify PIP₃-interacting proteins in human platelets_. Among these proteins, we found engulfment and cell motility 1 (ELMO1), a scaffold protein with no catalytic activity. ELMO1 is expressed in platelets and interacts with active RhoG. However, the function of ELMO1 in platelets is not known. The focus of this study was to determine the function of ELMO1 in platelets utilizing ELMO1^-/- mice. Platelet aggregation, granule secretion, integrin αIIbβ3 activation, and thromboxane generation were enhanced in ELMO1^-/- platelets in response to glycoprotein VI (GPVI) agonists but unaltered when a protease-activated receptor 4 agonist was used. The kinetics of spreading on immobilized fibrinogen was enhanced in ELMO1^-/- platelets compared with wild-type (WT) littermate controls. This suggests that ELMO1 plays a role downstream of the GPVI and integrin αIIbβ3 pathway. Furthermore, whole blood from ELMO1^-/- mice perfused over collagen exhibited enhanced thrombus formation compared with WT littermate controls. ELMO1^-/- mice showed reduced survival compared with control following pulmonary embolism. ELMO1^-/- mice also exhibited a shorter time to occlusion using the ferric-chloride injury model and reduced bleeding times compared with WT littermate controls. These results indicate that ELMO1 plays an important role in hemostasis and thrombosis in vivo. RhoG activity was enhanced in ELMO1^-/- murine platelets compared with WT littermate controls in response to GPVI agonist. Together, these data suggest that ELMO1 negatively regulates GPVI-mediated thrombus formation via RhoG.

Rolling or Two-Stage Aggregation of Platelets on the Surface of Thin Ceramic Coatings under in Vitro Simulated Blood Flow Conditions

The process of modern cardiovascular device fabrication should always be associated with an investigation of how surface properties modulate its hemocompatibility through plasma protein adsorption as well as blood morphotic element activation and adhesion. In this work, a package of novel assays was used to correlate the physicochemical properties of thin ceramic coatings with hemocompatibility under dynamic conditions. Different variants of carbon-based films were prepared on polymer substrates using the magnetron sputtering method. The microstructural, mechanical, and surface physicochemical tests were performed to characterize the coatings, followed by investigation of whole human blood quality changes under blood flow conditions using the "Impact R" test, tubes' tester, and radial flow chamber assay. The applied methodology allowed us to determine that aggregate formation on hydrophobic and hydrophilic carbon-based coatings may follow one of the two different mechanisms dependent on the type and conformational changes of adsorbed blood plasma proteins.

Platelet Adhesion and Thrombus Formation in Microchannels: The Effect of Assay-Dependent Variables

Microfluidic flow chambers (MFCs) allow the study of platelet adhesion and thrombus formation under flow, which may be influenced by several variables. We developed a new MFC, with which we tested the effects of different variables on the results of platelet deposition and thrombus formation on a collagen-coated surface. Methods: Whole blood was perfused in the MFC over collagen Type I for 4 min at different wall shear rates (WSR) and different concentrations of collagen-coating solutions, keeping blood samples at room temperature or 37 °C before starting the experiments. In addition, we tested the effects of the antiplatelet agent acetylsalicylic acid (ASA) (antagonist of cyclooxygenase-1, 100 µM) and cangrelor (antagonist of P2Y₁₂, 1 µM). Results: Platelet deposition on collagen (I) was not affected by the storage temperature of the blood before perfusion (room temperature vs. 37 °C); (II) was dependent on a shear rate in the range between 300/s and 1700/s; and (III) was influenced by the collagen concentration used to coat the microchannels up to a value of 10 µg/mL. ASA and cangrelor did not cause statistically significant inhibition of platelet accumulation, except for ASA at low collagen concentrations. Conclusions: Platelet deposition on collagen-coated surfaces is a shear-dependent process, not influenced by the collagen concentration beyond a value of 10 µg/mL. However, the inhibitory effect of antiplatelet drugs is better observed using low concentrations of collagen.

Evaluating medical device and material thrombosis under flow: current and emerging technologies

This review highlights the importance of flow in medical device thrombosis and explores current and emerging technologies to evaluate dynamic biomaterial Thrombosis in vitro.

Adenosine Receptor Agonists Exhibit Anti-Platelet Effects and the Potential to Overcome Resistance to P2Y12 Receptor Antagonists

Large inter-individual variation in platelet response to endogenous agonists and pharmacological agents, including resistance to antiplatelet therapy, prompts a search for novel platelet inhibitors and development new antithrombotic strategies. The present in vitro study evaluates the beneficial effects of three adenosine receptor (AR) agonists (regadenoson, LUF 5835 and NECA), different in terms of their selectivity for platelet adenosine receptors, when used alone and in combination with P2Y₁₂ inhibitors, such as cangrelor or prasugrel metabolite. The anti-platelet effects of AR agonists were evaluated in healthy subjects (in the whole group and after stratification of individuals into high- and low-responders to P2Y₁₂ inhibitors), using whole blood techniques, under flow (thrombus formation) and static conditions (study of platelet activation and aggregation). Compared to P2Y₁₂ antagonists, AR agonists were much less or not effective under static conditions, but demonstrated similar antiplatelet activity in flow. In most cases, AR agonists significantly enhanced the anti-platelet effect of P2Y₁₂ antagonists, despite possessing different selectivity profiles and antiplatelet activities. Importantly, their inhibitory effects in combination with P2Y₁₂ antagonists were similar in high- and low-responders to P2Y₁₂ inhibitors. In conclusion, a combination of anti-platelet agents acting via the P1 and P2 purinergic receptors represents a promising alternative to existing antithrombotic therapy.

New approaches for the assessment of platelet activation status in thrombus under flow condition using confocal microscopy

The goal of the study was the assessment of heterogeneous platelet activation status in thrombus. In a ferric(III) chloride (FeCl₃) thrombosis (intravital) model of C57BL/6 J mice, the area of irreversibly activated (phosphatidylserine (PS)-positive) platelets was assessed after 1-s exposure of a vessel to FeCl₃. In a laser-induced thrombosis (intravital) model of GFP mice, the area of the thrombus composed of PS-negative platelets was evaluated. The ratio of the area of PECAM-1 to the area of the thrombus was used as a marker to assess the activity of PS-negative platelets. In the in vitro flow chamber model, the thrombus area (PS-negative and PS-positive platelets) and the platelet activation index (ratio of the area of PS-positive platelets to the area of thrombus) were determined. To assess platelet activation status with these models, acetylsalicylic acid (ASA) and iloprost (Ilo) were used. In the FeCl₃ thrombosis, ASA (10 mg/kg, 100 mg/kg) decreased the area of PS-positive platelets. In the laser thrombosis, ASA (10 mg/kg) decreased the thrombus area, but the decrease in platelet activity was evident even at 3 mg/kg by an increased PECAM-1/thrombus ratio. In the flow chamber, ASA (0.02 mg/ml, 0.2 mg/ml) equally decreased the platelet activation index, whereas only at 0.2 mg/ml, it decreased the thrombus area. Ilo (3.6 ng/ml, 36 ng/ml) decreased the thrombus area but at 36 ng/ml increased the platelet activation index. We showed that intravital models and flow chamber provide a detailed assessment of platelet activation status and the mechanism of drug action.

Engineering a Bi-Conical Microchip as Vascular Stenosis Model

Vascular stenosis is always associated with hemodynamic changes, especially shear stress alterations. Herein, bi-conical shaped microvessels were developed through flexibly and precisely controlled templated methods for hydrogel blood-vessel-like microchip. The blood-vessel-like microvessels demonstrated tunable dimensions, perfusable ability, and good cytocompatibility. The microchips showed blood-vessel-like lumens through fine embeddedness of human umbilical vein endothelial cells (HUVECs) on the interior surface of hydrogel microchannels, which closely reproduced the morphology and functions of human blood vessels. In the gradual narrowing region of bi-conical shape, fluid flow generated wall shear stress, which caused cell morphology variations. Wall shear rates at the gradual narrowing region were simulated by FLUENT software. The results showed that our microchannels qualified for performance as a vascular stenosis-like model in evaluating blood hydrodynamics. In general, our blood-vessel-on-a-chip could offer potential applications in the prevention, diagnosis, and therapy of arterial thrombosis.

Whole Blood Based Multiparameter Assessment of Thrombus Formation in Standard Microfluidic Devices to Proxy In Vivo Haemostasis and Thrombosis

Microfluidic assays are versatile tests which, using only small amounts of blood, enable high throughput analyses of platelet function in several minutes. In combination with fluorescence microscopy, these flow tests allow real-time visualisation of platelet activation with the possibility of examining combinatorial effects of wall shear rate, coagulation and modulation by endothelial cells. In particular, the ability to use blood and blood cells from healthy subjects or patients makes this technology promising, both for research and (pre)clinical diagnostic purposes. In the present review, we describe how microfluidic devices are used to assess the roles of platelets in thrombosis and haemostasis. We place emphasis on technical aspects and on experimental designs that make the concept of "blood-vessel-component-on-a-chip" an attractive, rapidly developing technology for the study of the complex biological processes of blood coagulability in the presence of flow.

Laser speckle decorrelation time-based platelet function testing in microfluidic system

Platelet aggregation and adhesion are critically involved in both normal hemostasis and thrombosis during vascular injury. Before any surgery, it is important to identify the number of platelets and their functionality to reduce the risk of bleeding; therefore, platelet function testing is a requirement. We introduce a novel evaluation method of assessing platelet function with laser speckle contrast imaging. The speckle decorrelation time (SDT) of the blood flowing through a microfluidic channel chip provides a quantitative measure of platelet aggregation. We compared SDTs of whole blood and platelet-poor blood, i.e., whole blood stripped of its buffy coat region, and found a marked reduction in decorrelation time for platelet-poor blood. The measured SDT of platelet-poor blood was 1.04 ± 0.21 ms, while that of whole blood was 2.64 ± 0.83 ms. To further characterize the sensitivity of our speckle decorrelation time-based platelet function testing (SDT-PFT), we added various agonists involved in platelet aggregation, including adenosine diphosphate (ADP), epinephrine (EPI), and arachidonic acid (AA). In this study, the results show that whole blood with ADP resulted in the largest SDT, followed by whole blood with AA, whole blood with EPI, whole blood without agonist, and platelet-poor blood with or without agonist. These findings show that SDT-PFT has the potential for rapid screening of bleeding disorders and monitoring of anti-platelet therapies with only a small volume of blood.

Impact of Deficiency of Intrinsic Coagulation Factors XI and XII on Ex Vivo Thrombus Formation and Clot Lysis

The contributions of coagulation factor XI (FXI) and FXII to human clot formation is not fully known. Patients with deficiency in FXI have a variable mild bleeding risk, whereas FXII deficiency is not associated with bleeding. These phenotypes make FXII and FXI attractive target proteins in anticoagulant therapy. Here, we studied the mechanisms of fibrin clot formation, stability, and fibrinolytic degradation in patients with severe FXI or FXII deficiency. Thrombin generation was triggered in platelet-poor (PPP) and platelet-rich plasma (PRP) with the biological FXII trigger sulfatides. Intrinsic and extrinsic thrombus formation and degradation in whole blood were determined with rotational thromboelastometry (ROTEM). Clot formation under flow was assessed by perfusion of whole blood over collagen microspots with(out) tissue factor (TF). Thrombin generation and clot formation were delayed in FXII- and FXI-deficient patients triggered with sulfatides. In FXI-deficient plasma, this delay was more pronounced in PRP compared to PPP. In whole blood of FXII-deficient patients, clots were smaller but resistance to fibrinolysis was normal. In whole blood of FXI-deficient patients, clot formation was normal but the time to complete fibrinolysis was prolonged. In flow chamber experiments triggered with collagen/TF, platelet coverage was reduced in severe compared with moderate FXI deficiency, and fibrin formation was impaired. We conclude that quantitative defects in FXII and FXI have a substantial impact on contact activation-triggered coagulation. Furthermore, FXI deficiency has a dose-dependent suppressing effect on flow-mediated and platelet/TF-dependent clot formation. These last data highlight the contribution of particularly FXI to hemostasis.

Recent advances in microfluidic platelet function assays: Moving microfluidics into clinical applications

The analysis of platelet aggregation and thrombosis kinetics has significantly advanced with progress in microfluidic technology. However, the results of platelet aggregation tests do not fully reflect the observed clinical outcomes. To address the present unmet clinical needs, the basic but essential biology of platelets should be reconsidered in relation to the characteristics of microfluidic systems employed for platelet tests. To this end, the present article provides an overview of commercially available point of care devices and focuses on recent microfluidic studies, describing their measurement principles. We critically discuss the characteristics of the microfluidics systems used to evaluate the complex processes underlying platelet aggregation, and that are specifically designed to mimic the pathophysiological environment of blood vessels, including hemodynamic factors as well as blood vessel injury. To this end, we summarize unsolved issues related to the application of platelet function tests based on microfluidics. Overall, we confirm that platelet function tests based on microfluidics provide a versatile platform that encompasses a variety of basic research, as well as clinical diagnostic applications.

Idarucizumab, but not procoagulant concentrates, fully restores dabigatran-altered platelet and fibrin components of hemostasis

BACKGROUND

Comparative studies on the restoration of hemostasis with different reversal agents after dabigatran therapy have not been performed. We compared the efficacy and prothrombotic potential of the specific antidote idarucizumab with that of previously recommended non-specific procoagulant concentrates.

STUDY DESIGN AND METHODS

We explored the in vitro effects of dabigatran (184 ng/mL) on fibrin and platelet-aggregate formation onto a damaged vessel under flow conditions (600 s^-1 ). The reversal mechanisms and efficacy of idarucizumab (0.3-3 mg/mL) were compared with that of the non-specific procoagulant concentrates aPCC (25-75 U/Kg), PCC (70 U/Kg), or rFVIIa (120 μg/Kg). Generation of thrombin and prothrombin fragment (F1 + 2), and thromboelastometry parameters of clot formation were measured.

RESULTS

Dabigatran caused pronounced reductions in fibrin (87%) and platelet interactions (36%) with damaged vessels (p < 0.01) and significantly impaired thrombin generation and thromboelastometric parameters (delayed dynamics and reduced firmness). Idarucizumab completely normalized rates of fibrin and platelet coverage to baseline values in flow studies; and reversed the alterations in thrombin generation, F1 + 2 and thromboelastometry parameters produced by dabigatran. In comparison, aPCC and PCC only partially compensated for the dabigatran-induced alterations in fibrin deposition, but were unable to fully restore them to baseline values. Reversal with aPCC or PCC improved the majority of alterations in coagulation-related tests, but tended to overcompensate thrombin generation kinetics and significantly increased F1 + 2 levels.

CONCLUSION

Idarucizumab antagonizes alterations of direct and indirect biomarkers of hemostasis caused by dabigatran. In our studies, idarucizumab was clearly more efficacious than strategies with non-specific procoagulant concentrates and devoid of the excessive procoagulant tendency observed with the latter.

Cardiovascular disease models: A game changing paradigm in drug discovery and screening

Cardiovascular disease is the leading cause of death worldwide. Although investment in drug discovery and development has been sky-rocketing, the number of approved drugs has been declining. Cardiovascular toxicity due to therapeutic drug use claims the highest incidence and severity of adverse drug reactions in late-stage clinical development. Therefore, to address this issue, new, additional, replacement and combinatorial approaches are needed to fill the gap in effective drug discovery and screening. The motivation for developing accurate, predictive models is twofold: first, to study and discover new treatments for cardiac pathologies which are leading in worldwide morbidity and mortality rates; and second, to screen for adverse drug reactions on the heart, a primary risk in drug development. In addition to in vivo animal models, in vitro and in silico models have been recently proposed to mimic the physiological conditions of heart and vasculature. Here, we describe current in vitro, in vivo, and in silico platforms for modelling healthy and pathological cardiac tissues and their advantages and disadvantages for drug screening and discovery applications. We review the pathophysiology and the underlying pathways of different cardiac diseases, as well as the new tools being developed to facilitate their study. We finally suggest a roadmap for employing these non-animal platforms in assessing drug cardiotoxicity and safety.

Advanced in vitro models of vascular biology: Human induced pluripotent stem cells and organ-on-chip technology

The vascular system is one of the first to develop during embryogenesis and is essential for all organs and tissues in our body to develop and function. It has many essential roles including controlling the absorption, distribution and excretion of compounds and therefore determines the pharmacokinetics of drugs and therapeutics. Vascular homeostasis is under tight physiological control which is essential for maintaining tissues in a healthy state. Consequently, disruption of vascular homeostasis plays an integral role in many disease processes, making cells of the vessel wall attractive targets for therapeutic intervention. Experimental models of blood vessels can therefore contribute significantly to drug development and aid in predicting the biological effects of new drug entities. The increasing availability of human induced pluripotent stem cells (hiPSC) derived from healthy individuals and patients have accelerated advances in developing experimental in vitro models of the vasculature: human endothelial cells (ECs), pericytes and vascular smooth muscle cells (VSMCs), can now be generated with high efficiency from hiPSC and used in 'microfluidic chips' (also known as 'organ-on-chip' technology) as a basis for in vitro models of blood vessels. These near physiological scaffolds allow the controlled integration of fluid flow and three-dimensional (3D) co-cultures with perivascular cells to mimic tissue- or organ-level physiology and dysfunction in vitro. Here, we review recent multidisciplinary developments in these advanced experimental models of blood vessels that combine hiPSC with microfluidic organ-on-chip technology. We provide examples of their utility in various research areas and discuss steps necessary for further integration in biomedical applications so that they can be contribute effectively to the evaluation and development of new drugs and other therapeutics as well as personalized (patient-specific) treatments.