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

Spatial Gene Expression of Human Coronary Arteries Revealed the Molecular Features of Diffuse Intimal Thickening in Explanted Hearts

Diffuse intimal thickening (DIT) is a pre-clinical stage of atherosclerosis characterized by thickened intima. The molecular basis of its susceptibility to atherogenesis is unknown, and mechanistic investigations cannot be performed in commonly used mouse models, in which DIT does not exist. Vascular smooth muscle cells (SMCs) are the predominant cell type that occupies the intima and media of DIT. The molecular differences between these two layers may reveal the earliest phenotypic changes in SMCs to promote atherosclerosis. We benchmarked the RNA quality of human coronary arteries from autopsies (n = 7) and explanted hearts (n = 7) and performed Visium spatial gene expression on tissue sections with DIT. Although autopsy samples met the RNA quality standard for Visium (DV200 ≥ 30%), only arteries from explanted hearts exhibited reliable sequencing performance. Genes enriched in TGF-β-mediated remodeling of the extracellular matrix were overrepresented in the intima. SMCs enriched in the intima are dedifferentiated, but unlike those in the atherosclerotic lesions, they are not pro-inflammatory. Our findings indicate that autopsy samples are not ideal to distinguish subtle differences among cell phenotypes. SMCs in thickened intima may lead to lipid retention but not necessarily the onset of atherosclerosis.

An intimal-lumen model in a microfluidic device: potential platform for atherosclerosis-related studies

Atherosclerosis is a chronic inflammatory vascular disorder driven by factors such as endothelial dysfunction, hypertension, hyperlipidemia, and arterial calcification, and is considered a leading global cause of death. Existing atherosclerosis models have limitations due to the absence of an appropriate hemodynamic microenvironment in vitro and interspecies differences in vivo. Here, we develop a simple but robust microfluidic intimal-lumen model of early atherosclerosis using interconnected dual channels for studying monocyte transmigration and foam cell formation at an arterial shear rate. To the best of our knowledge, this is the first study that creates a physiologically relevant microenvironment under an arterial shear rate to modulate lipid-laden foam cells on a microfluidic platform. As a proof of concept, we use murine endothelial cells to develop a vascular lumen in one channel and collagen-embedded murine smooth muscle cells to mimic the subendothelial intimal layer in another channel. The model successfully triggers endothelial dysfunction upon TNF-α stimulation, initiating monocyte adhesion to the endothelial monolayer under the arterial shear rate. Unlike existing in vitro models, native low-density lipoprotein (LDL) is added in the culture media instead of ox-LDL to stimulate subendothelial lipid accumulation, thereby mimicking more accurate physiology. The subendothelial transmigration of adherent monocytes and subsequent foam cell formation is also achieved under flow conditions in the model. The model also investigates the inhibitory effect of aspirin in monocyte adhesion and transmigration. The model exhibits a significant dose-dependent reduction in monocyte adhesion and transmigration upon aspirin treatment, making it an excellent tool for drug testing.

Harnessing the power of bioprinting for the development of next-generation models of thrombosis

Thrombosis, a leading cause of cardiovascular morbidity and mortality, involves the formation of blood clots within blood vessels. Current animal models and in vitro systems have limitations in recapitulating the complex human vasculature and hemodynamic conditions, limiting the research in understanding the mechanisms of thrombosis. Bioprinting has emerged as a promising approach to construct biomimetic vascular models that closely mimic the structural and mechanical properties of native blood vessels. This review discusses the key considerations for designing bioprinted vascular conduits for thrombosis studies, including the incorporation of key structural, biochemical and mechanical features, the selection of appropriate biomaterials and cell sources, and the challenges and future directions in the field. The advancements in bioprinting techniques, such as multi-material bioprinting and microfluidic integration, have enabled the development of physiologically relevant models of thrombosis. The future of bioprinted models of thrombosis lies in the integration of patient-specific data, real-time monitoring technologies, and advanced microfluidic platforms, paving the way for personalized medicine and targeted interventions. As the field of bioprinting continues to evolve, these advanced vascular models are expected to play an increasingly important role in unraveling the complexities of thrombosis and improving patient outcomes. The continued advancements in bioprinting technologies and the collaboration between researchers from various disciplines hold great promise for revolutionizing the field of thrombosis research.

Multi-parametric thrombus profiling microfluidics detects intensified biomechanical thrombogenesis associated with hypertension and aging

Arterial thrombosis is a leading cause of death and disability worldwide with no effective bioassay for clinical prediction. As a symbolic feature of arterial thrombosis, severe stenosis in the blood vessel creates a high-shear, high-gradient flow environment that facilitates platelet aggregation towards vessel occlusion. Here, we present a thrombus profiling assay that monitors the multi-dimensional attributes of thrombi forming in such biomechanical conditions. Using this assay, we demonstrate that different receptor-ligand interactions contribute distinctively to the composition and activation status of the thrombus. Our investigation into hypertensive and older individuals reveals intensified biomechanical thrombogenesis and multi-dimensional thrombus profile abnormalities, endorsing the diagnostic potential of the assay. Furthermore, we identify the hyperactivity of GPIbα-integrin αIIbβ3 mechanosensing axis as a molecular mechanism that contributes to hypertension-associated arterial thrombosis. By studying drug-disease interactions and inter-individual variability, our work reveals a need for personalized anti-thrombotic drug selection that accommodates each patient's pathological profile.

Influence of short-term refrigeration on collagen-dependent signalling mechanisms in stored platelets

Platelet concentrates (PC) are used to treat patients with thrombocytopenia and hemorrhage, but there is still the demand to find the optimal strategy for temperature-dependent storage of PC. Recently, we could show that cold storage for 1 h (short-term refrigeration) is sufficient to induce enhanced platelet responsiveness. The aim of this study was to investigate effects of cold storage on collagen-dependent activating signalling pathways in platelets from apheresis-derived PC (APC). APC on day 1 or day 2 of storage, were either continuously kept at room temperature (RT, 22 °C), or for comparison, additionally kept at cold temperature (CT, 4 °C) for 1 h. CD62P expression was determined by flow cytometry. Western Blot technique was used to analyze collagen-induced phosphorylation of p38, ERK1/2 or Akt/PKB and its inhibition by prostaglandin E1 (PGE1) or nitric monoxide donor. Adhesion of platelets on collagen-coated surfaces and intracellular phosphorylation of vasodilator-stimulated phosphoprotein (VASP) was visualized by immune fluorescence microscopy. CD62P expression was increased after short-term refrigeration. CT exposition for 1 h induced an elevation of basal ERK1/2 phosphorylation and an alleviation of PGE1- or DEA/NO-suppressed ERK1/2 phosphorylation in APC on day 1 and 2 of storage. Similar, but more moderate effects were observable for p38 phosphorylation. Akt/PKB phosphorylation was increased only in APC on day 2. Refrigeration for 1 h promoted platelet adhesion and reduced basal VASP phosphorylation in adherent platelets. The attenuation of inhibitory signalling in short-term refrigerated stored platelets is associated with enhanced reactivity of activating signalling pathways, especially ERK1/2. Functionally, these processes correlate with increased adhesion of refrigerated platelets on collagen-coated surfaces. The results help to further optimize temperature-dependent strategies for platelet storage.

Multi-parametric thrombus profiling microfluidics detects intensified biomechanical thrombogenesis associated with hypertension and aging

Arterial thrombosis, which represents a critical complication of cardiovascular diseases, is a leading cause of death and disability worldwide with no effective bioassay for clinical prediction. As a symbolic feature of arterial thrombosis, severe stenosis in the blood vessel creates a high-shear, high-gradient flow environment that effectively facilitates platelet aggregation towards vessel occlusion even with platelet amplification loops inhibited. However, no approach is currently available to comprehensively characterize the size, composition and platelet activation status of thrombi forming under this biorheological condition. Here, we present a thrombus profiling assay that monitors the multi-dimensional attributes of thrombi forming in conditions mimicking the physiological scenario of arterial thrombosis. Using this platform, we demonstrate that different receptor-ligand interactions contribute distinctively to the composition and activation status of the thrombus. Our investigation into hypertensive and older individuals reveals intensified biomechanical thrombogenesis and multi-dimensional thrombus profile abnormalities, demonstrating a direct contribution of mechanobiology to arterial thrombosis and endorsing the diagnostic potential of the assay. Furthermore, we identify the hyperactivity of GPIbα-integrin αIIbβ3 mechanosensing axis as a molecular mechanism that contributes to hypertension-associated arterial thrombosis. By studying the interactions between anti-thrombotic inhibitors and hypertension, and the inter-individual variability in personal thrombus profiles, our work reveals a critical need for personalized anti-thrombotic drug selection that accommodates each patient's pathological profile.

Extracellular vesicles selective capture by peptide-functionalized hollow fiber membranes

Recently, membrane devices and processes have been applied for the separation and concentration of subcellular components such as extracellular vesicles (EVs), which play a diagnostic and therapeutic role in many pathological conditions. However, the separation and isolation of specific EV populations from other components found in biological fluids is still challenging. Here, we developed a peptide-functionalized hollow fiber (HF) membrane module to achieve the separation and enrichment of highly pure EVs derived from the culture media of human cardiac progenitor cells. The strategy is based on the functionalization of PSf HF membrane module with BPt, a peptide sequence able to bind nanovesicles characterized by highly curved membranes. HF membranes were modified by a nanometric coating with a copoly azide polymer to limit non-specific interactions and to enable the conjugation with peptide ligand by click chemistry reaction. The BPt-functionalized module was integrated into a TFF process to facilitate the design, rationalization, and optimization of EV isolation. This integration combined size-based transport of species with specific membrane sensing ligands. The TFF integrated BPt-functionalized membrane module demonstrated the ability to selectively capture EVs with diameter < 200 nm into the lumen of fibers while effectively removing contaminants such as albumin. The captured and released EVs contain the common markers including CD63, CD81, CD9 and syntenin-1. Moreover, they maintained a round shape morphology and structural integrity highlighting that this approach enables EVs concentration and purification with low shear stress. Additionally, it achieved the removal of contaminants such as albumin with high reliability and reproducibility, reaching a removal of 93%.

Clot Accumulation in 3D Microfluidic Bifurcating Microvasculature Network

The microvasculature, which makes up the majority of the cardiovascular system, plays a crucial role in the process of thrombosis, with the pathological formation of blood clots inside blood vessels. Since blood microflow conditions significantly influence platelet activation and thrombosis, accurately mimicking the structure of bifurcating microvascular networks and emulating local physiological blood flow conditions are valuable for understanding blood clot formation. In this work, we present an in vitro model for blood clotting in microvessels, focusing on 3D bifurcations that align with Murray's law, which guides vascular networks by maintaining a constant wall shear rate throughout. Using these models, we demonstrate that microvascular bifurcations act as sites facilitating thrombus formation compared to straight models. Additionally, by culturing endothelial cells on the luminal surfaces of the models, we show the potential of using our in vitro platforms to recapitulate the initial clotting in diseases involving endothelial dysfunction, such as Thrombotic Thrombocytopenic Purpura.

Blood shear stress during the cardiac cycle and endothelial cell orientation and polarity in the carotid artery of male and female mice

Introduction: Blood flow produces fluid shear stress (SS), a frictional force parallel to the blood flow, on the endothelial cell (EC) layer of the lumen of the vessels. ECs themselves are sensitive to this frictional force in terms of directionality and intensity. The aim of this study was to determine the physiological shear stress value during the cardiac cycle and EC polarity and orientation from blood flow in healthy male and female mouse carotid artery. Methods: Experimentation is done on anesthetized male and female 8-week-old C5BL/6J mice. In vivo measurements of maximum blood velocity and vessel diameter in diastole and systole were performed on the right common carotid artery by Doppler ultrasound imaging. Blood viscosity (total and plasmatic) and hematocrit were determined on blood samples. For SS calculation, we developed a new method assuming heterogenous blood flow, i.e., a red cell central plug flow surrounded by a peripheral plasma sheath flow, and computing SS from vessel diameter and hemodynamical measurements (maximal blood velocity, hematocrit and plasmatic viscosity). Results: Results were compared with the classical method assuming a homogenous blood flow with constant apparent total blood viscosity. EC polarity and orientation were determined ex vivo on the carotid endothelium by confocal imaging after labeling of the EC nucleus and Golgi apparatus. Diastolic and systolic SS were 6 ± 2.5 Pa and 30 ± 6.5 Pa, respectively. Total blood and plasmatic viscosity was 4 ± 0.5 cP and 1.27 cP, respectively. ECs were polarized and significantly oriented against blood flow. No sex difference was identified.

A double-edged sword: The complex interplay between engineered nanoparticles and platelets

Nanoparticles (NP) play a crucial role in nanomedicine, serving as carriers for localized therapeutics to allow for precise drug delivery to specific disease sites and conditions. When injected systemically, NP can directly interact with various blood cell types, most critically with circulating platelets. Hence, the potential activation/inhibition of platelets following NP exposure must be evaluated a priori due to possible debilitating outcomes. In recent years, various studies have helped resolve the physicochemical parameters that influence platelet-NP interactions, and either emphasize nanoparticles' therapeutic role such as to augment hemostasis or to inhibit thrombus formation, or conversely map their potential undesired side effects upon injection. In the present review, we discuss some of the main effects of several key NP types including polymeric, ceramic, silica, dendrimers and metallic NPs on platelets, with a focus on the physicochemical parameters that can dictate these effects and modulate the therapeutic potential of the NP. Despite the scientific and clinical significance of understanding Platelet-NP interactions, there is a significant knowledge gap in the field and a critical need for further investigation. Moreover, improved guidelines and research methodologies need to be developed and implemented. Our outlook includes the use of biomimetic in vitro models to investigate these complex interactions under both healthy physiological and disease conditions.

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.

Fundamental considerations for designing endothelialized in vitro models of thrombosis

Endothelialized in vitro models for cardiovascular disease have contributed greatly to our current understanding of the complex molecular mechanisms underlying thrombosis. To further elucidate these mechanisms, it is important to consider which fundamental aspects to incorporate into an in vitro model. In this review, we will focus on the design of in vitro endothelialized models of thrombosis. Expanding our understanding of the relation and interplay between the different pathways involved will rely in part on complex models that incorporate endothelial cells, blood, the extracellular matrix, and flow. Importantly, the use of tissue-specific endothelial cells will help in understanding the heterogeneity in thrombotic responses between different vascular beds. The dynamic and complex responses of endothelial cells to different shear rates underlines the importance of incorporating appropriate shear in in vitro models. Alterations in vascular extracellular matrix composition, availability of bioactive molecules, and gradients in concentration and composition of these molecules can all regulate the function of both endothelial cells and perivascular cells. Factors modulating these elements in in vitro models should therefore be considered carefully depending on the research question at hand. As the complexity of in vitro models increases, so can the variability. A bottom-up approach to designing such models will remain an important tool for researchers studying thrombosis. As new techniques are continuously being developed and new pathways are brought to light, research question-dependent considerations will have to be made regarding what aspects of thrombosis to include in in vitro models.

Single-Cell RNA-Seq Reveals Coronary Heterogeneity and Identifies CD133+TRPV4high Endothelial Subpopulation in Regulating Flow-Induced Vascular Tone in Mice

BACKGROUND

Single-cell RNA-Seq analysis can determine the heterogeneity of cells between different tissues at a single-cell level. Coronary artery endothelial cells (ECs) are important to coronary blood flow. However, little is known about the heterogeneity of coronary artery ECs, and cellular identity responses to flow. Identifying endothelial subpopulations will contribute to the precise localization of vascular endothelial subpopulations, thus enabling the precision of vascular injury treatment.

METHODS

Here, we performed a single-cell RNA sequencing of 31 962 cells and functional assays of 3 branches of the coronary arteries (right coronary artery/circumflex left coronary artery/anterior descending left coronary artery) in wild-type mice.

RESULTS

We found a compendium of 7 distinct cell types in mouse coronary arteries, mainly ECs, granulocytes, cardiac myocytes, smooth muscle cells, lymphocytes, myeloid cells, and fibroblast cells, and showed spatial heterogeneity between arterial branches. Furthermore, we revealed a subpopulation of coronary artery ECs, CD133+TRPV4high ECs. TRPV4 (transient receptor potential vanilloid 4) in CD133+TRPV4high ECs is important for regulating vasodilation and coronary blood flow.

CONCLUSIONS

Our study elucidates the nature and range of coronary arterial cell diversity and highlights the importance of coronary CD133+TRPV4high ECs in regulating coronary vascular tone.

Hemocompatibility and biophysical interface of left ventricular assist devices and total artificial hearts

ABSTRACT

Over the past 2 decades, there has been a significant increase in the utilization of long-term mechanical circulatory support (MCS) for the treatment of cardiac failure. Left ventricular assist devices (LVADs) and total artificial hearts (TAHs) have been developed in parallel to serve as bridge-to-transplant and destination therapy solutions. Despite the distinct hemodynamic characteristics introduced by LVADs and TAHs, a comparative evaluation of these devices regarding potential complications in supported patients, has not been undertaken. Such a study could provide valuable insights into the complications associated with these devices. Although MCS has shown substantial clinical benefits, significant complications related to hemocompatibility persist, including thrombosis, recurrent bleeding, and cerebrovascular accidents. This review focuses on the current understanding of hemostasis, specifically thrombotic and bleeding complications, and explores the influence of different shear stress regimens in long-term MCS. Furthermore, the role of endothelial cells in protecting against hemocompatibility-related complications of MCS is discussed. We also compared the diverse mechanisms contributing to the occurrence of hemocompatibility-related complications in currently used LVADs and TAHs. By applying the existing knowledge, we present, for the first time, a comprehensive comparison between long-term MCS options.

Clots reveal anomalous elastic behavior of fiber networks

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

Nanoscaled Titanium Oxide Layer Provokes Quick Osseointegration on 3D-Printed Dental Implants: A Domino Effect Induced by Hydrophilic Surface

Three-dimensional printing is a revolutionary strategy to fabricate dental implants. Especially, 3D-printed dental implants modified with nanoscaled titanium oxide layer (H-SLM) have impressively shown quick osseointegration, but the accurate mechanism remains elusive. Herein, we unmask a domino effect that the hydrophilic surface of the H-SLM facilitates blood wetting, enhances the blood shear rate, promotes blood clotting, and changes clot features for quick osseointegration. Combining computational fluid dynamic simulation and biological verification, we find a blood shear rate during blood wetting of the hydrophilic H-SLM 1.2-fold higher than that of the raw 3D-printed implant, which activates blood clot formation. Blood clots formed on the hydrophilic H-SLM demonstrate anti-inflammatory and pro-osteogenesis effects, leading to a 1.5-fold higher bone-to-implant contact and a 1.8-fold higher mechanical anchorage at the early stage of osseointegration. This mechanism deepens current knowledge between osseointegration speed and implant surface characteristics, which is instructive in surface nanoscaled modification of multiple 3D-printed intrabony implants.

An in vivo investigation on the effects of stent implantation on hematological and hemorheological parameters

BACKGROUND

Even though cardiovascular stenting is widely used for the treatment of coronary artery disease, information on how it can affect the hematological and hemorheological profile is scarce in the literature. Most of the work on this issue is based on theoretical or computational fluid dynamics models, lacking in-depth in vitro and in vivo experimental verification.

OBJECTIVE

This work investigates, in an in vivo setting, the effects of stenting and the implantation time-course on hematological and hemorheological parameters that could potentially compromise the device's functionality and longevity.

METHODS

Custom-made self-expanding nitinol stents were implanted in the common carotid artery of male CD1 mice. Whole blood samples were collected from control (non-stented) and stented animals at 5 and 10 weeks post-implantation. Hematological measurements and blood viscosity, red blood cell aggregation, and deformability were performed using standard techniques.

RESULTS

Implant-induced changes were observed in some of the hematological and hemorheological indices. Blood viscosity seems to have been negatively affected by an increased hematocrit and reduced RBC deformability, at 10 weeks post-implantation, despite a slight decrease in RBC aggregation.

CONCLUSIONS

Although the alterations observed may be the result of the peri-implant inflammatory response, the physiological consequences due to hemorheological changes need to be further investigated.

Time- and distance-resolved robotic imaging of fluid flow in vertical microfluidic strips: a new technique for quantitative, multiparameter measurement of global haemostasis

Measuring the complex processes of blood coagulation, haemostasis and thrombosis that are central to cardiovascular health and disease typically requires a choice between high-resolution low-throughput laboratory assays, or simpler less quantitative tests. We propose combining mass-produced microfluidic devices with open-source robotic instrumentation to enable rapid development of affordable and portable, yet high-throughput and performance haematological testing. A time- and distance-resolved fluid flow analysis by Raspberry Pi imaging integrated with controlled sample addition and illumination, enabled simultaneous tracking of capillary rise in 120 individual capillaries (∼160, 200 or 270 μm internal diameter), in 12 parallel disposable devices. We found time-resolved tracking of capillary rise in each individual microcapillary provides quantitative information about fluid properties and most importantly enables quantitation of dynamic changes in these properties following stimulation. Fluid properties were derived from flow kinetics using a pressure balance model validated with glycerol-water mixtures and blood components. Time-resolved imaging revealed fluid properties that were harder to determine from a single endpoint image or equilibrium analysis alone. Surprisingly, instantaneous superficial fluid velocity during capillary rise was found to be largely independent of capillary diameter at initial time points. We tested if blood function could be measured dynamically by stimulating blood with thrombin to trigger activation of global haemostasis. Thrombin stimulation slowed vertical fluid velocity consistent with a dynamic increase in viscosity. The dynamics were concentration-dependent, with highest doses reducing flow velocity faster (within 10 s) than lower doses (10-30 s). This open-source imaging instrumentation expands the capability of affordable microfluidic devices for haematological testing, towards high-throughput multi-parameter blood analysis needed to understand and improve cardiovascular health.

Intravital Imaging of Thrombosis Models in Mice

Intravital microscopy is a powerful tool to study thrombosis in real time. The kinetics of thrombus formation and progression in vivo is studied after inflicting damage to the endothelium through mechanical, chemical, or laser injury. Mouse models of atherosclerosis are also used to induce thrombus formation. Vessels of different sizes and from different vascular beds such as carotid artery or vena cava, mesenteric or cremaster arterioles, can be targeted. Using fluorescent dyes, antibodies, or reporter mouse strains allows to visualize key cells and factors mediating the thrombotic processes. Here, we review the latest literature on using intravital microscopy to study thrombosis as well as thromboinflammation following transient middle cerebral artery occlusion, infection-induced immunothrombosis, and liver ischemia reperfusion.

An agent-based model of cardiac allograft vasculopathy: toward a better understanding of chronic rejection dynamics

Cardiac allograft vasculopathy (CAV) is a coronary artery disease affecting 50% of heart transplant (HTx) recipients, and it is the major cause of graft loss. CAV is driven by the interplay of immunological and non-immunological factors, setting off a cascade of events promoting endothelial damage and vascular dysfunction. The etiology and evolution of tissue pathology are largely unknown, making disease management challenging. So far, in vivo models, mostly mouse-based, have been widely used to study CAV, but they are resource-consuming, pose many ethical issues, and allow limited investigation of time points and important biomechanical measurements. Recently, agent-based models (ABMs) proved to be valid computational tools for deciphering mechanobiological mechanisms driving vascular adaptation processes at the cell/tissue level, augmenting cost-effective in vivo lab-based experiments, at the same time guaranteeing richness in observation time points and low consumption of resources. We hypothesize that integrating ABMs with lab-based experiments can aid in vivo research by overcoming those limitations. Accordingly, this work proposes a bidimensional ABM of CAV in a mouse coronary artery cross-section, simulating the arterial wall response to two distinct stimuli: inflammation and hemodynamic disturbances, the latter considered in terms of low wall shear stress (WSS). These stimuli trigger i) inflammatory cell activation and ii) exacerbated vascular cell activities. Moreover, an extensive analysis was performed to investigate the ABM sensitivity to the driving parameters and inputs and gain insights into the ABM working mechanisms. The ABM was able to effectively replicate a 4-week CAV initiation and progression, characterized by lumen area decrease due to progressive intimal thickening in regions exposed to high inflammation and low WSS. Moreover, the parameter and input sensitivity analysis highlighted that the inflammatory-related events rather than the WSS predominantly drive CAV, corroborating the inflammatory nature of the vasculopathy. The proof-of-concept model proposed herein demonstrated its potential in deepening the pathology knowledge and supporting the in vivo analysis of CAV.

Platelet Adhesion Mediated by von Willebrand Factor at High Shear Rates Is Associated with Premature Coronary Artery Disease

This study investigated von Willebrand factor (VWF)-mediated platelet adhesion at high shear rates in patients with premature coronary artery disease (CAD). The study included 84 patients with stable premature CAD and 64 patients without CAD. Whole blood samples were perfused through a microfluidic cell over a collagen-coated surface at a shear rate of 1300 s^-1. Measurements were performed before and after the inhibition of VWF-specific platelet GPIb receptors with an anti-GPIb monoclonal antibody (mAb). Platelet adhesion decreased by 77.0% (55.9; 84.7) in patients with premature CAD and by 29.6% (0.0; 59.7) in control patients after the inhibition of VWF-platelet interaction with anti-GPIb mAb (p < 0.001). After adjusting for traditional risk factors, the odds ratio for premature CAD per 1% decrease in GPIb-mediated platelet adhesion was 1.03 (95% CI, 1.02-1.05; p < 0.001). The optimal cut-off level value of GPIb-mediated platelet adhesion was 62.8%, with 70.2% sensitivity and 81.2% specificity for CAD. The plasma levels of VWF or antiplatelet therapy did not affect the GPIb-mediated component of platelet adhesion. Thus, the GPIb-mediated component of platelet adhesion was more pronounced in patients with premature CAD. This may indicate the possible role of excessive VWF-platelet interactions in the development of premature CAD.

The relative importance of platelet integrins in hemostasis, thrombosis and beyond

Integrins are heterodimeric transmembrane receptors composed of α and β chains, with an N-terminal extracellular domain forming a globular head corresponding to the ligand binding site. Integrins regulate various cellular functions including adhesion, migration, proliferation, spreading and apoptosis. On platelets, integrins play a central role in adhesion and aggregation on subendothelial matrix proteins of the vascular wall, thereby ensuring hemostasis. Platelet integrins belong either to the β1 family (α2β1, α5β1 and α6β1) or to the β3 family (αIIbβ3 and αvβ3). On resting platelets, integrins can engage their ligands when the latter are immobilized but not in their soluble form. The effects of various agonists promote an inside-out signal in platelets, increasing the affinity of integrins for their ligands and conveying a modest signal reinforcing platelet activation, called outside-in signaling. This outside-in signal ensures platelet adhesion, shape change, granule secretion and aggregation. In this review, we examine the role of each platelet integrin in hemostatic plug formation, hemostasis and arterial thrombosis and also beyond these classical functions, notably in tumor metastasis and sepsis.

A Bayesian approach to blood rheological uncertainties in aortic hemodynamics

Computational hemodynamics has received increasing attention recently. Patient-specific simulations require questionable model assumptions, for example, for geometry, boundary conditions, and material parameters. Consequently, the credibility of these simulations is much doubted, and rightly so. Yet, the matter may be addressed by a rigorous uncertainty quantification. In this contribution, we investigated the impact of blood rheological models on wall shear stress uncertainties in aortic hemodynamics obtained in numerical simulations. Based on shear-rheometric experiments, we compare the non-Newtonian Carreau model to a simple Newtonian model and a Reynolds number-equivalent Newtonian model. Bayesian Probability Theory treats uncertainties consistently and allows to include elusive assumptions such as the comparability of flow regimes. We overcome the prohibitively high computational cost for the simulation with a surrogate model, and account for the uncertainties of the surrogate model itself, too. We have two main findings: (1) The Newtonian models mostly underestimate the uncertainties as compared to the non-Newtonian model. (2) The wall shear stresses of specific persons cannot be distinguished due to largely overlapping uncertainty bands, implying that a more precise determination of person-specific blood rheological properties is necessary for person-specific simulations. While we refrain from a general recommendation for one rheological model, we have quantified the error of the uncertainty quantification associated with these modeling choices.

Structural hierarchy of mechanical extensibility in human von Willebrand factor multimers

The von Willebrand factor (VWF) is a multimeric glycoprotein composed of 80- to 120-nm-long protomeric units and plays a fundamental role in mediating platelet function at high shear. The exact nature of the shear-induced structural transitions have remained elusive; uncovering them requires the high-resolution quantitative analysis of gradually extended VWF. Here, we stretched human blood-plasma-derived VWF with molecular combing and analyzed the axial structure of the elongated multimers with atomic force microscopy. Protomers extended through structural intermediates that could be grouped into seven distinct topographical classes. Protomer extension thus progresses through the uncoiling of the C_1-6 domain segment, rearrangements among the N-terminal VWF domains, and unfolding and elastic extension of the A₂ domain. The least and most extended protomer conformations were localized at the ends and the middle of the multimer, respectively, revealing an apparent necking phenomenon characteristic of plastic-material behavior. The structural hierarchy uncovered here is likely to provide a spatial control mechanism to the complex functions of VWF.

Intravenous Hemostats: Foundation, Targeting, and Controlled-Release

Uncontrollable blood loss is the greatest cause of mortality in prehospital patients and the main source of disability and death in hospital care. Compared with external hemostats, intravenous hemostats are more appropriate for preventing and treating uncontrolled bleeding in vivo and large bleeding on the body surface. This Review initially establishes intravenous hemostats' response basis, including the coagulation mechanism, fibrinolytic pathway, and protein corona. Second, the study of advancement of intravenous hemostat targeting was expanded from two perspectives, cellular hemostatic agents and synthetic hemostatic agents. Meanwhile, after discussing the progress of controlled-release intravenous hemostats with platelets as the stimuli, this Review offers insight into the possibility of controlled-release intravenous hemostats with microenvironment as the stimuli, combining the studies of controlled-release targeted thrombolysis.

Loss of α4A- and β1-tubulins leads to severe platelet spherocytosis and strongly impairs hemostasis in mice

Native circulating blood platelets present with a discoid flat morphology maintained by a submembranous peripheral ring of microtubules, named marginal band. The functional importance of this particular shape is still debated, but it was initially hypothesized to facilitate platelet interaction with the injured vessel wall and to contribute to hemostasis. The importance of the platelet discoid morphology has since been questioned on the absence of clear bleeding tendency in mice lacking the platelet-specific β1-tubulin isotype, which exhibits platelets with a thinner marginal band and an ovoid shape. Here, we generated a mouse model inactivated for β1-tubulin and α4A-tubulin, an α-tubulin isotype strongly enriched in platelets. These mice present with fully spherical platelets completely devoid of a marginal band. In contrast to the single knockouts, the double deletion resulted in a severe bleeding defect in a tail-clipping assay, which was not corrected by increasing the platelet count to normal values by the thrombopoietin-analog romiplostim. In vivo, thrombus formation was almost abolished in a ferric chloride-injury model, with only a thin layer of loosely packed platelets, and mice were protected against death in a model of thromboembolism. In vitro, platelets adhered less efficiently and formed smaller-sized and loosely assembled aggregates when perfused over von Willebrand factor and collagen matrices. In conclusion, this study shows that blood platelets require 2 unique α- and β-tubulin isotypes to acquire their characteristic discoid morphology. Lack of these 2 isotypes has a deleterious effect on flow-dependent aggregate formation and stability, leading to a severe bleeding disorder.

Autoinhibitory module underlies species difference in shear activation of von Willebrand factor

BACKGROUND

Von Willebrand factor (VWF) is a multimeric plasma protein that bridges the gap between vessel injury and platelet capture at high shear rates. Under high shear or tension, VWF can become activated upon the unfolding of its autoinhibitory module (AIM). AIM unfolding exposes the A1 domain, allowing for binding to platelet glycoprotein (GP)Ibα to initiate primary hemostasis. The characteristics of the AIM and its inhibitory properties within mouse VWF are unknown.

OBJECTIVES

To determine and characterize the autoinhibitory properties of mouse VWF.

METHODS

Recombinant mouse VWF A1 fragments containing or lacking the flanking regions around the A1 domain were generated. We tested the ability of these fragments to bind to human or mouse GPIbα and platelets. We compared the unfolding of mouse AIM-A1 to human AIM-A1 by single-molecule force spectroscopy.

RESULTS

Recombinant mouse AIM-A1 binds with higher affinity to GPIbα than its human counterpart. Recombinant mouse proteins lacking part of the AIM show increased binding to GPIbα. Activated A1 fragments lacking the AIM can effectively agglutinate platelets across the species barrier. Using single-molecule force spectroscopy, we determined that the mouse AIM unfolds under forces similar to the human AIM. Additionally, the human AIM paired with mouse A1 largely recapitulates the behavior of human AIM-A1.

CONCLUSIONS

Our results suggest that the regulation of VWF-GPIbα binding has been specifically tuned to work optimally in different rheological architectures. Differences in the AIM sequence may contribute to the difference in VWF shear response between human and mice.

The effect of rigid cells on blood viscosity: linking rheology and sickle cell anemia

Sickle cell anemia (SCA) is a disease that affects red blood cells (RBCs). Healthy RBCs are highly deformable objects that under flow can penetrate blood capillaries smaller than their typical size. In SCA there is an impaired deformability of some cells, which are much stiffer and with a different shape than healthy cells, and thereby affect regular blood flow. It is known that blood from patients with SCA has a higher viscosity than normal blood. However, it is unclear how the rigidity of cells is related to the viscosity of blood, in part because SCA patients are often treated with transfusions of variable amounts of normal RBCs and only a fraction of cells will be stiff. Here, we report systematic experimental measurements of the viscosity of a suspension varying the fraction of rigid particles within a suspension of healthy cells. We also perform systematic numerical simulations of a similar mixed suspension of soft RBCs, rigid particles, and their hydrodynamic interactions. Our results show that there is a rheological signature within blood viscosity to clearly identify the fraction of rigidified cells among healthy deformable cells down to a 5% volume fraction of rigidified cells. Although aggregation of RBCs is known to affect blood rheology at low shear rates, and our simulations mimic this effect via an adhesion potential, we show that such adhesion, or aggregation, is unlikely to provide a physical rationalization for the viscosity increase observed in the experiments at moderate shear rates due to rigidified cells. Through numerical simulations, we also highlight that most of the viscosity increase of the suspension is due to the rigidity of the particles rather than their sickled or spherical shape. Our results are relevant to better characterize SCA, provide useful insights relevant to rheological consequences of blood transfusions, and, more generally, extend to the rheology of mixed suspensions having particles with different rigidities, as well as offering possibilities for developments in the field of soft material composites.

Method article: an in vitro blood flow model to advance the study of platelet adhesion utilizing a damaged endothelium

In vitro flow assays utilizing microfluidic devices are often used to study human platelets as an alternative to the costly animal models of hemostasis and thrombosis that may not accurately represent human platelet behavior in vivo. Here, we present a tunable in vitro model to study platelet behavior in human whole blood flow that includes both an inflamed, damaged endothelium and exposed extracellular matrix. We demonstrate that the model is adaptable across various anticoagulants, shear rates, and proteins for endothelial cell culture without the need for a complicated, custom-designed device. Furthermore, we verified the ability of this 'damaged endothelium' model as a screening method for potential anti-platelet or anti-thrombotic compounds using a P2Y₁₂ receptor antagonist (ticagrelor), a pan-selectin inhibitor (Bimosiamose), and a histamine receptor antagonist (Cimetidine). These compounds significantly decreased platelet adhesion to the damaged endothelium, highlighting that this model can successfully screen anti-platelet compounds that target platelets directly or the endothelium indirectly.

Venous puncture wound hemostasis results in a vaulted thrombus structured by locally nucleated platelet aggregates

Primary hemostasis results in a platelet-rich thrombus that has long been assumed to form a solid plug. Unexpectedly, our 3-dimensional (3D) electron microscopy of mouse jugular vein puncture wounds revealed that the resulting thrombi were structured about localized, nucleated platelet aggregates, pedestals and columns, that produced a vaulted thrombus capped by extravascular platelet adherence. Pedestal and column surfaces were lined by procoagulant platelets. Furthermore, early steps in thrombus assembly were sensitive to P2Y₁₂ inhibition and late steps to thrombin inhibition. Based on these results, we propose a Cap and Build, puncture wound paradigm that should have translational implications for bleeding control and hemostasis.

Rhee, Pokrovskaya et al. utilize 3D electron microscopy of mouse jugular vein puncture wounds to reveal thrombi structured around localized, nucleated platelet aggregates that produced a vaulted thrombus capped by extravascular platelet adherence. As a result, the authors propose a “Cap and Build” paradigm of primary hemostasis.

Capillary-size flow of human blood plasma: Revealing hidden elasticity and scale dependence

The dynamical mechanical analysis of blood generally uses models inspired by conventional flows, assuming scale-independent homogeneous flows and without considering fluid-surface boundary interactions. The present experimental study highlights the relevance of using an approach in line with physiological reality providing a strong interaction between the fluid and the boundary interface. New dynamic properties of human blood plasma are found: a finite shear elastic response (solid-like property) is identified in nearly static conditions, which also depends on the scale (being reinforced at small scales). The elastic behavior is confirmed by the induction, without heat transfer, of local hot and cold thermodynamic states evidencing a thermo-mechanical coupling in blood plasma so far known only in elastic materials. This finding opens new routes for medical diagnosis and device fabrication.

Animal blood in translational research: How to adjust animal blood viscosity to the human standard

Animal blood is used in mock circulations or in forensic bloodstain pattern analysis. Blood viscosity is important in these settings as it determines the driving pressure through biomedical devices and the shape of the bloodstain. However, animal blood can never exactly mimic human blood due to erythrocyte properties differing among species. This results in the species-specific shear thinning behavior of blood suspensions, and it is therefore not enough to adjust the hematocrit of an animal blood sample to mimic the behavior of human blood over the entire range of shear rates that are present in the body. In order to optimize experiments that require animal blood, we need models to adapt the blood samples. We here offer mathematical models derived for each species using a multi linear regression approach to describe the influence of shear rate, hematocrit, and temperature on blood viscosity. Results show that pig blood cannot be recommended for experiments at low flow conditions (<200 s-1 ) even though erythrocyte properties are similar in pigs and humans. However, pig blood mimics human blood excellently at high flow condition. Horse blood is unsuitable as experimental model in this regard. For several studied conditions, sheep blood was the closest match to human blood viscosity among the tested species.