Effects of disturbed flow on endothelial cells

Role of Flow-Sensitive Endothelial Genes in Atherosclerosis and Antiatherogenic Therapeutics Development

Atherosclerosis is a chronic inflammatory disease that is the underlying cause of cardiovascular disease which initiates from endothelial dysfunction from genetic and environmental risk factors, including biomechanical forces: blood flow. Endothelial cells (ECs) lining the inner arterial wall regions exposed to disturbed flow are prone to atherosclerosis development, whereas the straight regions exposed to stable flow are spared from the disease. These flow patterns induce genome- and epigenome-wide changes in gene expression in ECs. Through the sweeping changes in gene expression, disturbed flow reprograms ECs from athero-protected cell types under the stable flow condition to pro-atherogenic cell conditions. The pro-atherogenic changes induced by disturbed flow, in combination with additional risk factors such as hypercholesterolemia, lead to the progression of atherosclerosis. The flow-sensitive genes and proteins are critical in understanding the mechanisms and serve as novel targets for antiatherogenic therapeutics.

An imprint-based approach to replicate nano- to microscale roughness on gelatin hydrogel scaffolds: surface characterization and effect on endothelialization

Biologization of biomaterials with endothelial cells (ECs) is an important step in vascular tissue engineering, aiming at improving hemocompatibility and diminishing the thrombo-inflammatory response of implants. Since subcellular topography in the scale of nano to micrometers can influence cellular adhesion, proliferation, and differentiation, we here investigate the effect of surface roughness on the endothelialization of gelatin hydrogel scaffolds. Considering the micron and sub-micron features of the different native tissues underlying the endothelium in the body, we carried out a biomimetic approach to replicate the surface roughness of tissues and analyzed how this impacted the adhesion and proliferation of human umbilical endothelial cells (HUVECs). Using an imprinting technique, nano and micro-roughness ranging from Sa= 402 nm to Sa= 8 μm were replicated on the surface of gelatin hydrogels. Fluorescent imaging of HUVECs on consecutive days after seeding revealed that microscale topographies negatively affect cell spreading and proliferation. By contrast, nanoscale roughnesses of Sa= 402 and Sa= 538 nm promoted endothelialization as evidenced by the formation of confluent cell monolayers with prominent VE-cadherin surface expression. Collectively, we present an affordable and flexible imprinting method to replicate surface characteristics of tissues on hydrogels and demonstrate how nanoscale roughness positively supports their endothelialization.

Disturbed fluid flow reinforces endothelial tractions and intercellular stresses

Disturbed fluid flow is well understood to have significant ramifications on endothelial function, but the impact disturbed flow has on endothelial biomechanics is not well understood. In this study, we measured tractions, intercellular stresses, and cell velocity of endothelial cells exposed to disturbed flow using a custom-fabricated flow chamber. Our flow chamber exposed cells to disturbed fluid flow within the following spatial zones: zone 1 (inlet; length 0.676-2.027 cm): 0.0037 ± 0.0001 Pa; zone 2 (middle; length 2.027-3.716 cm): 0.0059 ± 0.0005 Pa; and zone 3 (outlet; length 3.716-5.405 cm): 0.0051 ± 0.0025 Pa. Tractions and intercellular stresses were observed to be highest in the middle of the chamber (zone 2) and lowest at the chamber outlet (zone 3), while cell velocity was highest near the chamber inlet (zone 1), and lowest near the middle of the chamber (zone 2). Our findings suggest endothelial biomechanical response to disturbed fluid flow to be dependent on not only shear stress magnitude, but the spatial shear stress gradient as well. We believe our results will be useful to a host of fields including endothelial cell biology, the cardiovascular field, and cellular biomechanics in general.

Transient low shear-stress preconditioning influences long-term endothelial traction and alignment under high shear flow

Endothelial cells (ECs) within the vascular system encounter fluid shear stress (FSS). High, laminar FSS promotes vasodilation and anti-inflammatory responses, whereas low or disturbed FSS induces dysfunction and inflammation. However, the adaptation of endothelial cells (ECs) to dynamically changing FSS patterns remains underexplored. Here, by combining traction force microscopy with a custom flow chamber, we examined human umbilical vein endothelial cells adapting their traction during transitions from short-term low shear to long-term high shear stress. We discovered that the initial low FSS elevates the traction by only half of the amount in response to direct high FSS even after flow changes to high FSS. However, in the long term under high FSS, the flow started with low FSS triggers a substantial second rise in traction for over 10 h. In contrast, the flow started directly with high FSS results in a quick traction surge followed by a huge reduction below the baseline traction in <30 min. Importantly, we find that the orientation of traction vectors is steered by initial shear exposure. Using Granger causality analysis, we show that the traction that aligns in the flow direction under direct high FSS functionally causes cell alignment toward the flow direction. However, EC traction that orients perpendicular to the flow that starts with temporary low FSS functionally causes cell orientation perpendicular to the flow. Taken together, our findings elucidate the significant influence of initial short-term low FSS on lasting changes in endothelial traction that induces EC alignment.NEW & NOTEWORTHY In our study, we uncover that preconditioning with low shear stress yields enduring impacts on endothelial cell traction and orientation, persisting even after transitioning to high-shear conditions. Using Granger causality analysis, we demonstrate a functional link between the direction of cell traction and subsequent cellular alignment across varying shear environments.

A mechanical modeling framework to study endothelial permeability

The inner lining of blood vessels, the endothelium, is made up of endothelial cells. Vascular endothelial (VE)-cadherin protein forms a bond with VE-cadherin from neighboring cells to determine the size of gaps between the cells and thereby regulate the size of particles that can cross the endothelium. Chemical cues such as thrombin, along with mechanical properties of the cell and extracellular matrix are known to affect the permeability of endothelial cells. Abnormal permeability is found in patients suffering from diseases including cardiovascular diseases, cancer, and COVID-19. Even though some of the regulatory mechanisms affecting endothelial permeability are well studied, details of how several mechanical and chemical stimuli acting simultaneously affect endothelial permeability are not yet understood. In this article, we present a continuum-level mechanical modeling framework to study the highly dynamic nature of the VE-cadherin bonds. Taking inspiration from the catch-slip behavior that VE-cadherin complexes are known to exhibit, we model the VE-cadherin homophilic bond as cohesive contact with damage following a traction-separation law. We explicitly model the actin cytoskeleton and substrate to study their role in permeability. Our studies show that mechanochemical coupling is necessary to simulate the influence of the mechanical properties of the substrate on permeability. Simulations show that shear between cells is responsible for the variation in permeability between bicellular and tricellular junctions, explaining the phenotypic differences observed in experiments. An increase in the magnitude of traction force due to disturbed flow that endothelial cells experience results in increased permeability, and it is found that the effect is higher on stiffer extracellular matrix. Finally, we show that the cylindrical monolayer exhibits higher permeability than the planar monolayer under unconstrained cases. Thus, we present a contact mechanics-based mechanochemical model to investigate the variation in the permeability of endothelial monolayer due to multiple loads acting simultaneously.

The influence of blood velocity and vessel geometric parameters on wall shear stress

Vascular geometry was proposed to be one risk factor of atherosclerosis (AS). When developing this hypothesis, the discussion of geometry-wall shear stress (WSS) has often been included. However, further exploration on how various geometric parameters were affecting WSS was needed. The purpose of this study was to investigate the influence degree of vessel geometric parameters and blood velocity on WSS. A computational fluid dynamics (CFD) analyses of the vertebral and basilar arteries (VA and BA, respectively) was used. Twenty patients with no plaques or vessel wall thickening at the VA and BA were included. CFD analyses using both specific vessel models and flow conditions measured by ultrasound Doppler were performed. Subsequently, CFD results were post-processed with multiple linear regression to investigate numerical correlations between geometrical and flow parameters and WSS. The results of the multiple linear regression analysis further demonstrated that the BA proximal velocity was the most influential factor positively influencing BA WSS. The lower the WSS was, the stronger the influence brought by BA average diameter would be. The regression demonstrated that the contributions brought by average diameter and proximal velocity in lower WSS regions were lower than that in higher WSS regions. Tortuosity was only positively correlated with 97.5th WSS percentile, and vessel length and curvature showed no correlation with WSS. This study quantified the influence degree of BA morphology and flow velocity on WSS, which may have practical implications for predicting hemodynamic risks.

Low Shear in Short-Term Impacts Endothelial Cell Traction and Alignment in Long-Term

Within the vascular system, endothelial cells (ECs) are exposed to fluid shear stress (FSS), a mechanical force exerted by blood flow that is critical for regulating cellular tension and maintaining vascular homeostasis. The way ECs react to FSS varies significantly; while high, laminar FSS supports vasodilation and suppresses inflammation, low or disturbed FSS can lead to endothelial dysfunction and increase the risk of cardiovascular diseases. Yet, the adaptation of ECs to dynamically varying FSS remains poorly understood. This study focuses on the dynamic responses of ECs to brief periods of low FSS, examining its impact on endothelial traction-a measure of cellular tension that plays a crucial role in how endothelial cells respond to mechanical stimuli. By integrating traction force microscopy (TFM) with a custom-built flow chamber, we analyzed how human umbilical vein endothelial cells (HUVECs) adjust their traction in response to shifts from low to high shear stress. We discovered that initial exposure to low FSS prompts a marked increase in traction force, which continues to rise over 10 hours before slowly decreasing. In contrast, immediate exposure to high FSS causes a quick spike in traction followed by a swift reduction, revealing distinct patterns of traction behavior under different shear conditions. Importantly, the direction of traction forces and the resulting cellular alignment under these conditions indicate that the initial shear experience dictates long-term endothelial behavior. Our findings shed light on the critical influence of short-lived low-shear stress experiences in shaping endothelial function, indicating that early exposure to low FSS results in enduring changes in endothelial contractility and alignment, with significant consequences for vascular health and the development of cardiovascular diseases.

Mechanical factors influence β-catenin localization and barrier properties

Mechanical forces are of major importance in regulating vascular homeostasis by influencing endothelial cell behavior and functions. Adherens junctions are critical sites for mechanotransduction in endothelial cells. β-catenin, a component of adherens junctions and the canonical Wnt signaling pathway, plays a role in mechanoactivation. Evidence suggests that β-catenin is involved in flow sensing and responds to tensional forces, impacting junction dynamics. The mechanoregulation of β-catenin signaling is context-dependent, influenced by the type and duration of mechanical loads. In endothelial cells, β-catenin's nuclear translocation and signaling are influenced by shear stress and strain, affecting endothelial permeability. The study investigates how shear stress, strain, and surface topography impact adherens junction dynamics, regulate β-catenin localization, and influence endothelial barrier properties. Insight box Mechanical loads are potent regulators of endothelial functions through not completely elucidated mechanisms. Surface topography, wall shear stress and cyclic wall deformation contribute overlapping mechanical stimuli to which endothelial monolayer respond to adapt and maintain barrier functions. The use of custom developed flow chamber and bioreactor allows quantifying the response of mature human endothelial to well-defined wall shear stress and gradients of strain. Here, the mechanoregulation of β-catenin by substrate topography, wall shear stress, and cyclic stretch is analyzed and linked to the monolayer control of endothelial permeability.

NanoIEA: A Nanopatterned Interdigitated Electrode Array-Based Impedance Assay for Real-Time Measurement of Aligned Endothelial Cell Barrier Functions

A nanopatterned interdigitated electrode array (nanoIEA)-based impedance assay is developed for quantitative real-time measurement of aligned endothelial cell (EC) barrier functions in vitro. A bioinspired poly(3,4-dihydroxy-L-phenylalanine) (poly (l-DOPA)) coating is applied to improve the human brain EC adhesion onto the Nafion nanopatterned surfaces. It is found that a poly (l-DOPA)-coated Nafion grooved nanopattern makes the human brain ECs orient along the nanopattern direction. Aligned human brain ECs on Nafion nanopatterns exhibit increased expression of genes encoding tight and adherens junction proteins. Aligned human brain ECs also have enhanced impedance and resistance versus unaligned ones. Treatment with a glycogen synthase kinase-3 inhibitor (GSK3i) further increases impedance and resistance, suggesting synergistic effects occur on the cell-cell tightness of in vitro human brain ECs via a combination of anisotropic matrix nanotopography and GSK3i treatment. It is found that this enhanced cell-cell tightness of the combined approach is accompanied by increased expression of claudin protein. These data demonstrate that the proposed nanoIEA assay integrated with poly (l-DOPA)-coated Nafion nanopatterns and interdigitated electrode arrays can make not only biomimetic aligned ECs, but also enable real-time measurement of the enhanced barrier functions of aligned ECs via tighter cell-cell junctions.

cSTAR analysis identifies endothelial cell cycle as a key regulator of flow-dependent artery remodeling

Fluid shear stress (FSS) from blood flow is sensed by vascular endothelial cells (ECs) to determine vessel stability, remodeling and susceptibility to atherosclerosis and other inflammatory diseases but the regulatory networks that govern these behaviors are only partially understood. We used cSTAR, a powerful new computational method, to define EC transcriptomic states under low shear stress (LSS) that triggers vessel inward remodeling, physiological shear stress (PSS) that stabilizes vessels, high shear stress (HSS) that triggers outward remodeling, and oscillatory shear stress (OSS) that confers disease susceptibility, all in comparison to cells under static conditions (STAT). We combined these results with the LINCS database where EC transcriptomic responses to drug treatments to define a preliminary regulatory network in which the cyclin-dependent kinases CDK1/2 play a central role in promoting vessel stability. Experimental analysis showed that PSS induced a strong late G1 cell cycle arrest in which CDK2 was activated. EC deletion of CDK2 in mice resulted in inward artery remodeling and both pulmonary and systemic hypertension. These results validate use of cSTAR to determine EC state and in vivo vessel behavior, reveal unexpected features of EC phenotype under different FSS conditions, and identify CDK2 as a key element within the EC regulatory network that governs artery remodeling.

New Dawn for Atherosclerosis: Vascular Endothelial Cell Senescence and Death

Endothelial cells (ECs) form the inner linings of blood vessels, and are directly exposed to endogenous hazard signals and metabolites in the circulatory system. The senescence and death of ECs are not only adverse outcomes, but also causal contributors to endothelial dysfunction, an early risk marker of atherosclerosis. The pathophysiological process of EC senescence involves both structural and functional changes and has been linked to various factors, including oxidative stress, dysregulated cell cycle, hyperuricemia, vascular inflammation, and aberrant metabolite sensing and signaling. Multiple forms of EC death have been documented in atherosclerosis, including autophagic cell death, apoptosis, pyroptosis, NETosis, necroptosis, and ferroptosis. Despite this, the molecular mechanisms underlying EC senescence or death in atherogenesis are not fully understood. To provide a comprehensive update on the subject, this review examines the historic and latest findings on the molecular mechanisms and functional alterations associated with EC senescence and death in different stages of atherosclerosis.

Nuclear mechanosensing of the aortic endothelium in health and disease

The endothelium, the monolayer of endothelial cells that line blood vessels, is exposed to a number of mechanical forces, including frictional shear flow, pulsatile stretching and changes in stiffness influenced by extracellular matrix composition. These forces are sensed by mechanosensors that facilitate their transduction to drive appropriate adaptation of the endothelium to maintain vascular homeostasis. In the aorta, the unique architecture of the vessel gives rise to changes in the fluid dynamics, which, in turn, shape cellular morphology, nuclear architecture, chromatin dynamics and gene regulation. In this Review, we discuss recent work focusing on how differential mechanical forces exerted on endothelial cells are sensed and transduced to influence their form and function in giving rise to spatial variation to the endothelium of the aorta. We will also discuss recent developments in understanding how nuclear mechanosensing is implicated in diseases of the aorta.

DNMT1 mediates the disturbed flow-induced endothelial to mesenchymal transition through disrupting β-alanine and carnosine homeostasis

Background: Increasing evidence suggests that hemodynamic disturbed flow induces endothelial dysfunction via a complex biological process so-called endothelial to mesenchymal transition (EndoMT). Recently, DNA methyltransferases (DNMTs) was reported as a key molecular mediator to promote EndoMT. Our understanding of how DNMTs, particularly the maintenance DNMTs, DNMT1, coordinate EndoMT is still lacking. Methods: A parallel-plate flow apparatus and perfusion devices were used to apply fluid with endothelial protective pulsatile shear (PS, to mimic the laminar flow) or harmful oscillatory shear (OS, to mimic the disturbed flow) to cultured endothelial cells (ECs). Endothelial lineage tracing mice and conditional EC Dnmt1 knockout mice were subjected to a surgery of carotid partial ligation to generate the flow-accelerated atherogenesis models. Western blotting, quantitative RT-PCR, immunofluorescent staining, methylation-specific PCR, chromatin immunoprecipitation, endothelial functional assays, and assessments for neointimal formation and atherosclerosis were performed. Results: Inhibition of DNMTs with 5-aza-2'-deoxycytidine (5-Aza) suppressed the disturbed flow/OS-induced EndoMT, both in cultured cells and the endothelial lineage tracing mice. 5-Aza also ameliorated the downregulation of aldehyde dehydrogenases (ALDHs) and β-alanine biosynthesis caused by disturbed flow/OS. Knockdown of the ALDH family proteins, ALDH2, ALDH3A1, and ALDH6A1, showed an EndoMT-induction effect as OS. Supplementation of cells with the functional metabolites of β-alanine, carnosine and acetyl-CoA (acetate), reversed EndoMT, likely via inhibiting the phosphorylation of Smad2/3. Endothelial-specific knockout of Dnmt1 protected the vasculature from disturbed flow-induced remodeling and atherosclerosis. Conclusions: Endothelial DNMT1 acts as one of the key epigenetic factors to mediate the hemodynamically regulated EndoMT at least through repressing the expression of ALDH2, ALDH3A1, and ALDH6A1. Supplementation with carnosine and acetate may have a great potential in the prevention and treatment of atherosclerosis.

Endothelial mechanobiology in atherosclerosis

Cardiovascular disease (CVD) is a serious health challenge, causing more deaths worldwide than cancer. The vascular endothelium, which forms the inner lining of blood vessels, plays a central role in maintaining vascular integrity and homeostasis and is in direct contact with the blood flow. Research over the past century has shown that mechanical perturbations of the vascular wall contribute to the formation and progression of atherosclerosis. While the straight part of the artery is exposed to sustained laminar flow and physiological high shear stress, flow near branch points or in curved vessels can exhibit 'disturbed' flow. Clinical studies as well as carefully controlled in vitro analyses have confirmed that these regions of disturbed flow, which can include low shear stress, recirculation, oscillation, or lateral flow, are preferential sites of atherosclerotic lesion formation. Because of their critical role in blood flow homeostasis, vascular endothelial cells (ECs) have mechanosensory mechanisms that allow them to react rapidly to changes in mechanical forces, and to execute context-specific adaptive responses to modulate EC functions. This review summarizes the current understanding of endothelial mechanobiology, which can guide the identification of new therapeutic targets to slow or reverse the progression of atherosclerosis.

Possible molecular mechanisms underlying the development of atherosclerosis in cancer survivors

Cancer survivors undergone treatment face an increased risk of developing atherosclerotic cardiovascular disease (CVD), yet the underlying mechanisms remain elusive. Recent studies have revealed that chemotherapy can drive senescent cancer cells to acquire a proliferative phenotype known as senescence-associated stemness (SAS). These SAS cells exhibit enhanced growth and resistance to cancer treatment, thereby contributing to disease progression. Endothelial cell (EC) senescence has been implicated in atherosclerosis and cancer, including among cancer survivors. Treatment modalities for cancer can induce EC senescence, leading to the development of SAS phenotype and subsequent atherosclerosis in cancer survivors. Consequently, targeting senescent ECs displaying the SAS phenotype hold promise as a therapeutic approach for managing atherosclerotic CVD in this population. This review aims to provide a mechanistic understanding of SAS induction in ECs and its contribution to atherosclerosis among cancer survivors. We delve into the mechanisms underlying EC senescence in response to disturbed flow and ionizing radiation, which play pivotal role in atherosclerosis and cancer. Key pathways, including p90RSK/TERF2IP, TGFβR1/SMAD, and BH4 signaling are explored as potential targets for cancer treatment. By comprehending the similarities and distinctions between different types of senescence and the associated pathways, we can pave the way for targeted interventions aim at enhancing the cardiovascular health of this vulnerable population. The insights gained from this review may facilitate the development of novel therapeutic strategies for managing atherosclerotic CVD in cancer survivors.

Treatment strategy of different enhanced external counterpulsation frequencies for coronary heart disease and cerebral ischemic stroke: A hemodynamic numerical simulation study

BACKGROUND AND OBJECTIVES

Currently, enhanced external counterpulsation (EECP) devices mainly produce one counterpulsation per cardiac cycle. However, the effect of other frequencies of EECP on the hemodynamics of coronary and cerebral arteries is still unclear. It should be investigated whether one counterpulsation per cardiac cycle leads to the optimal therapeutic effect in patients with different clinical indications. Therefore, we measured the effects of different frequencies of EECP on the hemodynamics of coronary and cerebral arteries to determine the optimal counterpulsation frequency for the treatment of coronary heart disease and cerebral ischemic stroke.

METHODS

We established 0D/3D geometric multi-scale hemodynamics model of coronary and cerebral arteries in two healthy individuals, and performed clinical trials of EECP to verify the accuracy of the multi-scale hemodynamics model. The pressure amplitude (35 kPa) and pressurization duration (0.6 s) were fixed. The global and local hemodynamics of coronary and cerebral arteries were studied by changing counterpulsation frequency. Three frequency modes, including one counterpulsation in one, two and three cardiac cycles, were applied. Global hemodynamic indicators included diastolic / systolic blood pressure (D/S), mean arterial pressure (MAP), coronary artery flow (CAF), and cerebral blood flow (CBF), whereas local hemodynamic effects included area-time-averaged wall shear stress (ATAWSS) and oscillatory shear index (OSI). The optimal counterpulsation frequency was verified by analyzing the hemodynamic effects of different frequency modes of counterpulsation cycles and full cycles.

RESULTS

In the full cycle, CAF, CBF and ATAWSS of coronary and cerebral arteries were the highest when one counterpulsation per cardiac cycle was applied. However, in the counterpulsation cycle, the global and local hemodynamic indicators of coronary and cerebral artery reached the highest when one counterpulsation in one cardiac cycle or two cardiac cycles was applied.

CONCLUSIONS

For clinical application, the results of global hemodynamic indicators in the full cycle have more clinical practical significance. Combined with the comprehensive analysis of local hemodynamic indicators, it can be concluded that for coronary heart disease and cerebral ischemic stroke, applying one counterpulsation per cardiac cycle may provide the optimal benefit.

Understanding development of jugular bulb stenosis in vein of galen malformations: identifying metrics of complex flow dynamics in the cerebral venous vasculature of infants

Introduction: Computational fluid dynamics (CFD) assess biological systems based on specific boundary conditions. We propose modeling more advanced hemodynamic metrics, such as core line length (CL) and critical points which characterize complexity of flow in the context of cerebral vasculature, and specifically cerebral veins during the physiologically evolving early neonatal state of vein of Galen malformations (VOGM). CFD has not been applied to the study of arteriovenous shunting in Vein of Galen Malformations but could help illustrate the pathophysiology of this malformation. Methods: Three neonatal patients with VOGM at Boston Children's Hospital met inclusion criteria for this study. Structural MRI data was segmented to generate a mesh of the VOGM and venous outflow. Boundary condition flow velocity was derived from PC-MR sequences with arterial and venous dual velocity encoding. The mesh and boundary conditions were applied to model the cerebral venous flow. We computed flow variables including mean wall shear stress (WSSmean), mean OSI, CL, and the mean number of critical points (nCrPointsmean) for each patient specific model. A critical point is defined as the location where the shear stress vector field is zero (stationary point) and can be used to describe complexity of flow. Results: The division of flow into the left and right venous outflow was comparable between PC-MR and CFD modeling. A high complexity recirculating flow pattern observed on PC-MR was also identified on CFD modeling. Regions of similar WSSmean and OSImean (<1.3 fold) in the left and right venous outflow channels of a single patient have several-fold magnitude difference in higher order hemodynamic metrics (> 3.3 fold CL, > 1.7 fold nCrPointsmean). Specifically, the side which developed JBS in each model had greater nCrPointsmean compared to the jugular bulb with no stenosis (VOGM1: 4.49 vs. 2.53, VOGM2: 1.94 vs. 0, VOGM3: 1 vs. 0). Biologically, these regions had subsequently divergent development, with increased complexity of flow associating with venous stenosis. Discussion: Advanced metrics of flow complexity identified in computational models may reflect observed flow phenomena not fully characterized by primary or secondary hemodynamic parameters. These advanced metrics may indicate physiological states that impact development of jugular bulb stenosis in VOGM.

3D In Vitro Blood-Brain-Barrier Model for Investigating Barrier Insults

Blood-brain-barrier (BBB) disruption has been associated with a variety of central-nervous-system diseases. In vitro BBB models enable to investigate how the barrier reacts to external injury events, commonly referred to as insults. Here, a human-cell-based BBB platform with integrated, transparent electrodes to monitor barrier tightness in real time at high resolution is presented. The BBB model includes human cerebral endothelial cells and primary pericytes and astrocytes in a 3D arrangement within a pump-free, open-microfluidic platform. With this platform, this study demonstrates that oxygen-glucose deprivation (OGD), which mimics the characteristics of an ischemic insult, induces a rapid remodeling of the cellular actin structures and subsequent morphological changes in the endothelial cells. High-resolution live imaging shows the formation of large actin stress-fiber bundles in the endothelial layer during OGD application, which ultimately leads to cell shrinkage and barrier breakage. Simultaneous electrical measurements evidence a rapid decrease of the barrier electrical resistance before the appearance of stress fibers, which indicates that the barrier function is compromised already before the appearance of drastic morphological changes. The results demonstrate that the BBB platform recapitulates the main barrier functions in vitro and can be used to investigate rapid reorganization of the BBB upon application of external stimuli.

Participation of Krüppel-like Factors in Atherogenesis

Atherosclerosis is an important problem in modern medicine, the keys to understanding many aspects of which are still not available to clinicians. Atherosclerosis develops as a result of a complex chain of events in which many cells of the vascular wall and peripheral blood flow are involved. Endothelial cells, which line the vascular wall in a monolayer, play an important role in vascular biology. A growing body of evidence strengthens the understanding of the multifaceted functions of endothelial cells, which not only organize the barrier between blood flow and tissues but also act as regulators of hemodynamics and play an important role in regulating the function of other cells in the vascular wall. Krüppel-like factors (KLFs) perform several biological functions in various cells of the vascular wall. The large family of KLFs in humans includes 18 members, among which KLF2 and KLF4 are at the crossroads between endothelial cell mechanobiology and immunometabolism, which play important roles in both the normal vascular wall and atherosclerosis.

Impact of spatial and temporal stability of flow vortices on vascular endothelial cells

PURPOSE

Intracranial aneurysms (IAs) are pathological dilations of cerebrovascular vessels due to degeneration of the mechanical strength of the arterial wall, precluded by altered cellular functionality. The presence of swirling hemodynamic flow (vortices) is known to alter vascular endothelial cell (EC) morphology and protein expression indicative of IAs. Unfortunately, less is known if vortices with varied spatial and temporal stability lead to differing levels of EC change. The aim of this work is to investigate vortices of varying spatial and temporal stability impact on ECs.

METHODS

Vortex and EC interplay was investigated by a novel combination of parallel plate flow chamber (PPFC) design and computational analysis. ECs were exposed to laminar (7.5 dynes/[Formula: see text] wall shear stress) or low (<1 dynes/[Formula: see text]) stress vortical flow using PPFCs. Immunofluorescent imaging analyzed EC morphology, while ELISA tests quantified VE-cadherin (cell-cell adhesion), VCAM-1 (macrophage-EC adhesion), and cleaved caspase-3 (apoptotic signal) expression. PPFC flow was simulated, and vortex stability was calculated via the temporally averaged degree of (volume) overlap (TA-DVO) of vortices within a given area.

RESULTS

EC morphological changes were independent of vortex stability. Increased stability promoted VE-cadherin degradation (correlation coefficient r = [Formula: see text]0.84) and 5-fold increased cleaved caspase-3 post 24 h in stable (TA-DVO 0.736 ± 0.05) vs unstable (TA-DVO 0.606 [Formula: see text]0.2) vortices. ECs in stable vortices displayed a 4.5-fold VCAM-1 increase than unstable counterparts after 12 h.

CONCLUSION

This work demonstrates highly stable disturbed flow imparts increased inflammatory signaling, degraded cell-cell adhesion, and increased cellular apoptosis than unstable vortices. Such knowledge offers novel insight toward understanding IA development and rupture.

Disturbed Flow-Facilitated Margination and Targeting of Nanodisks Protect against Atherosclerosis

Disturbed blood flow induces endothelial pro-inflammatory responses that promote atherogenesis. Nanoparticle-based therapeutics aimed at treating endothelial inflammation in vasculature where disturbed flow occurs may provide a promising avenue to prevent atherosclerosis. By using a vertical-step flow apparatus and a microfluidic chip of vascular stenosis, herein, it is found that the disk-shaped versus the spherical nanoparticles exhibit preferential margination (localization and adhesion) to the regions with the pro-atherogenic disturbed flow. By employing a mouse model of carotid partial ligation, superior targeting and higher accumulation of the disk-shaped particles are also demonstrated within disturbed flow areas than that of the spherical particles. In hyperlipidemia mice, administration of disk-shaped particles loaded with hypomethylating agent decitabine (DAC) displays greater anti-inflammatory and anti-atherosclerotic effects compared with that of the spherical counterparts and exhibits reduced toxicity than "naked" DAC. The findings suggest that shaping nanoparticles to disk is an effective strategy for promoting their delivery to atheroprone endothelia.

A microfluidic impedance platform for real-time, in vitro characterization of endothelial cells undergoing fluid shear stress

We introduce a microfluidic impedance platform to electrically monitor in real-time, endothelium monolayers undergoing fluid shear stress. Our platform incorporates sensing electrodes (SEs) that measure cell behavior and cell-free control electrodes that measure cell culture media resistance simultaneously but independently from SEs. We evaluated three different cellular subpopulations sizes through 50, 100, and 200 μm diameter SEs. We tested their utility in measuring the response of human umbilical vein endothelial cells (HUVECs) at static, constant (17.6 dyne per cm²), and stepped (23.7-35-58.1 dyne per cm²) shear stress conditions. For 14 hours, we collected the impedance spectra (100 Hz-1 MHz) of sheared cells. Using equivalent circuit models, we extracted monolayer permeability (R_TER), cell membrane capacitance, and cell culture media resistance. Platform evaluation concluded that: (1) 50 μm SEs (∼2 cells) suffered interfacial capacitance and reduced cell measurement sensitivity, (2) 100 μm SEs (∼6 cells) was limited to measuring cell behavior only and cannot measure cell culture media resistance, and (3) 200 μm SEs (∼20 cells) detected cell behavior with accurate prediction of cell culture media resistance. Platform-based shear stress studies indicated a shear magnitude dependent increase in R_TER at the onset of acute flow. Consecutive stepped shear conditions did not alter R_TER in the same magnitude after shear has been applied. Finally, endpoint staining of VE-cadherin on the actual SEs and endpoint R_TER measurements were greater for 23.7-35-58.1 dyne per cm² than 17.6 dyne per cm² shear conditions.

Small Molecule BRD4 Inhibitors Apabetalone and JQ1 Rescues Endothelial Cells Dysfunction, Protects Monolayer Integrity and Reduces Midkine Expression

NF-κB signaling is a key regulator of inflammation and atherosclerosis. NF-κB cooperates with bromodomain-containing protein 4 (BRD4), a transcriptional and epigenetic regulator, in endothelial inflammation. This study aimed to investigate whether BRD4 inhibition would prevent the proinflammatory response towards TNF-α in endothelial cells. We used TNF-α treatment of human umbilical cord-derived vascular endothelial cells to create an in vitro inflammatory model system. Two small molecule inhibitors of BRD4—namely, RVX208 (Apabetalone), which is in clinical trials for the treatment of atherosclerosis, and JQ1—were used to analyze the effect of BRD4 inhibition on endothelial inflammation and barrier integrity. BRD4 inhibition reduced the expression of proinflammatory markers such as SELE, VCAM-I, and IL6 in endothelial cells and prevented TNF-α-induced endothelial tight junction hyperpermeability. Endothelial inflammation was associated with increased expression of the heparin-binding growth factor midkine. BRD4 inhibition reduced midkine expression and normalized endothelial permeability upon TNF-α treatment. In conclusion, we identified that TNF-α increased midkine expression and compromised tight junction integrity in endothelial cells, which was preventable by pharmacological BRD4 inhibition.

Peristaltic pumps adapted for laminar flow experiments enhance in vitro modeling of vascular cell behavior

Endothelial cells (ECs) are the primary cellular constituent of blood vessels that are in direct contact with hemodynamic forces over their lifetime. Throughout the body, vessels experience different blood flow patterns and rates that alter vascular architecture and cellular behavior. Because of the complexities of studying blood flow in an intact organism, particularly during development, the field has increasingly relied on in vitro modeling of blood flow as a powerful technique for studying hemodynamic-dependent signaling mechanisms in ECs. While commercial flow systems that recirculate fluids exist, many commercially available pumps are peristaltic and best model pulsatile flow conditions. However, there are many important situations in which ECs experience laminar flow conditions in vivo, such as along long straight stretches of the vasculature. To understand EC function under these contexts, it is important to be able to reproducibly model laminar flow conditions in vitro. Here, we outline a method to reliably adapt commercially available peristaltic pumps to study laminar flow conditions. Our proof-of-concept study focuses on 2D models but could be further adapted to 3D environments to better model in vivo scenarios, such as organ development. Our studies make significant inroads into solving technical challenges associated with flow modeling and allow us to conduct functional studies toward understanding the mechanistic role of shear forces on vascular architecture, cellular behavior, and remodeling in diverse physiological contexts.

Lymphatic Tissue and Organ Engineering for In Vitro Modeling and In Vivo Regeneration

The lymphatic system has an important role in maintaining fluid homeostasis and transporting immune cells and biomolecules, such as dietary fat, metabolic products, and antigens in different organs and tissues. Therefore, impaired lymphatic vessel function and/or lymphatic vessel deficiency can lead to numerous human diseases. The discovery of lymphatic endothelial markers and prolymphangiogenic growth factors, along with a growing number of in vitro and in vivo models and technologies has expedited research in lymphatic tissue and organ engineering, advancing therapeutic strategies. In this article, we describe lymphatic tissue and organ engineering in two- and three-dimensional culture systems and recently developed microfluidics and organ-on-a-chip systems in vitro. Next, we discuss advances in lymphatic tissue and organ engineering in vivo, focusing on biomaterial and scaffold engineering and their applications for lymphatic vessels and lymphoid organ regeneration. Last, we provide expert perspective and prospects in the field of lymphatic tissue engineering.

In Vitro Flow Chamber Design for the Study of Endothelial Cell (Patho)Physiology

In the native vasculature, flowing blood produces a frictional force on vessel walls that affects endothelial cell function and phenotype. In the arterial system, the vasculature's local geometry directly influences variations in flow profiles and shear stress magnitudes. Straight arterial sections with pulsatile shear stress have been shown to promote an athero-protective endothelial phenotype. Conversely, areas with more complex geometry, such as arterial bifurcations and branch points with disturbed flow patterns and lower, oscillatory shear stress, typically lead to endothelial dysfunction and the pathogenesis of cardiovascular diseases. Many studies have investigated the regulation of endothelial responses to various shear stress environments. Importantly, the accurate in vitro simulation of in vivo hemodynamics is critical to the deeper understanding of mechanotransduction through the proper design and use of flow chamber devices. In this review, we describe several flow chamber apparatuses and their fluid mechanics design parameters, including parallel-plate flow chambers, cone-and-plate devices, and microfluidic devices. In addition, chamber-specific design criteria and relevant equations are defined in detail for the accurate simulation of shear stress environments to study endothelial cell responses.

Model-based evaluation of local hemodynamic effects of enhanced external counterpulsation

BACKGROUND AND OBJECTIVES

The treatment benefits of enhanced external counterpulsation (EECP) heavily depends on hemodynamics. Global hemodynamics of EECP can cause blood flow redistribution in the circulatory system whereas local hemodynamic effects act on vascular endothelial cells (VECs). Local hemodynamic effects of EECP on VECs are important in the treatment of atherosclerosis, but currently cannot be not evaluated. Herein we aim to establish evaluation models of local hemodynamic effects based on the global hemodynamic indicators.

METHODS

We established 0D/3D geometric multi-scale hemodynamic models of the coronary and cerebral artery of two healthy individuals to calculate the global hemodynamic indicators and the local hemodynamic effects. Clinical EECP trials were performed to verify the accuracy of the multi-scale hemodynamic model. The global hemodynamic indicators included diastolic blood pressure/systolic blood pressure (Q = D/S), mean arterial pressure (MAP), internal carotid artery flow (ICAF) and cerebral blood flow (CBF), whereas local hemodynamic effects focused on time-averaged wall shear stress (TAWSS). The correlation between these indicators was analyzed via Pearson correlation coefficient. Significantly related indicators were selected for curve-fitting to establish evaluation models of the coronary and cerebral artery. Moreover, clinical data of a coronary heart disease patient and a cerebral ischemic stroke patient were collected to verify the effectiveness of the application of the established evaluation models to real patients.

RESULTS

For coronary artery, TAWSS was correlated to Q = D/S and ICAF (P < 0.05), whereas for cerebral artery, TAWSS was correlated to MAP and CBF (P < 0.05). The mean square error (MSE) between the evaluated values using evaluation model and the calculated values using 0D/3D model of TAWSS of the coronary and cerebral artery were 5.4% and 1.0%, respectively. The MSE of evaluation model applied to real patients was greater than that applied to healthy individuals, but within an acceptable range.

CONCLUSIONS

The presented error demonstrated validity and accuracy of the evaluation models in clinical patients. Based on the evaluation models, global hemodynamic indicators could be used to evaluate the local hemodynamic effects under the current counterpulsation mode. With TAWSS range of 4-7 Pa as the target range, EECP strategies can further be optimized.

Fluid Shear Stress Regulates the Landscape of microRNAs in Endothelial Cell-Derived Small Extracellular Vesicles and Modulates the Function of Endothelial Cells

Blood fluid shear stress (FSS) modulates endothelial function and vascular pathophysiology. The small extracellular vesicles (sEVs) such as exosomes are potent mediators of intercellular communication, and their contents reflect cellular stress. Here, we explored the miRNA profiles in endothelial cells (EC)-derived sEVs (EC-sEVs) under atheroprotective laminar shear stress (LSS) and atheroprone low-oscillatory shear stress (OSS) and conducted a network analysis to identify the main biological processes modulated by sEVs’ miRNAs. The EC-sEVs were collected from culture media of human umbilical vein endothelial cells exposed to atheroprotective LSS (20 dyne/cm²) and atheroprone OSS (±5 dyne/cm²). We explored the miRNA profiles in FSS-induced EC-sEVs (LSS-sEVs and OSS-sEVs) and conducted a network analysis to identify the main biological processes modulated by sEVs’ miRNAs. In vivo studies were performed in a mouse model of partial carotid ligation. The sEVs’ miRNAs-targeted genes were enriched for endothelial activation such as angiogenesis, cell migration, and vascular inflammation. OSS-sEVs promoted tube formation, cell migration, monocyte adhesion, and apoptosis, and upregulated the expression of proteins that stimulate these biological processes. FSS-induced EC-sEVs had the same effects on endothelial mechanotransduction signaling as direct stimulation by FSS. In vivo studies showed that LSS-sEVs reduced the expression of pro-inflammatory genes, whereas OSS-sEVs had the opposite effect. Understanding the landscape of EC-exosomal miRNAs regulated by differential FSS patterns, this research establishes their biological functions on a system level and provides a platform for modulating the overall phenotypic effects of sEVs.

In vitro blood brain barrier models: An overview

Understanding the composition and function of the blood brain barrier (BBB) enables the development of novel, innovative techniques for administering central nervous system (CNS) medications and technologies for improving the existing models. Scientific and methodological interest in the pathology of the BBB resulted in the formation of numerous in vitro BBB models. Once successfully studied and modelled, it would be a valuable tool for elucidating the mechanism of action of the CNS disorders prior to their manifestation and the pathogenic factors. Understanding the rationale behind the selection of the models as well as their working may enable the development of state-of-the-art drugs for treating and managing neurological diseases. Hence, to have realistic simulation of the BBB and test its drug permeability the microfluidics-based BBB-on-Chip model has been developed. To summarise, we aim to evaluate the advanced, newly developed and frequently used in vitro BBB models, thereby providing a brief overview of the components essential for in vitro BBB formation, the methods of chip fabrication and cell culturing, its applications and the recent advances in this technological field. This will be critical for developing CNS treatments with improved BBB penetrability and pharmacokinetic properties.

Vitexin inhibits APEX1 to counteract the flow-induced endothelial inflammation

Vascular endothelial cells are exposed to shear stresses with disturbed vs. laminar flow patterns, which lead to proinflammatory vs. antiinflammatory phenotypes, respectively. Effective treatment against endothelial inflammation and the consequent atherogenesis requires the identification of new therapeutic molecules and the development of drugs targeting these molecules. Using Connectivity Map, we have identified vitexin, a natural flavonoid, as a compound that evokes the gene-expression changes caused by pulsatile shear, which mimics laminar flow with a clear direction, vs. oscillatory shear (OS), which mimics disturbed flow without a clear direction. Treatment with vitexin suppressed the endothelial inflammation induced by OS or tumor necrosis factor-α. Administration of vitexin to mice subjected to carotid partial ligation blocked the disturbed flow-induced endothelial inflammation and neointimal formation. In hyperlipidemic mice, treatment with vitexin ameliorated atherosclerosis. Using SuperPred, we predicted that apurinic/apyrimidinic endonuclease1 (APEX1) may directly interact with vitexin, and we experimentally verified their physical interactions. OS induced APEX1 nuclear translocation, which was inhibited by vitexin. OS promoted the binding of acetyltransferase p300 to APEX1, leading to its acetylation and nuclear translocation. Functionally, knocking down APEX1 with siRNA reversed the OS-induced proinflammatory phenotype, suggesting that APEX1 promotes inflammation by orchestrating the NF-κB pathway. Animal experiments with the partial ligation model indicated that overexpression of APEX1 negated the action of vitexin against endothelial inflammation, and that endothelial-specific deletion of APEX1 ameliorated atherogenesis. We thus propose targeting APEX1 with vitexin as a potential therapeutic strategy to alleviate atherosclerosis.

Heme Oxygenase-1 at the Nexus of Endothelial Cell Fate Decision Under Oxidative Stress

Endothelial cells (ECs) form the inner lining of blood vessels and are central to sensing chemical perturbations that can lead to oxidative stress. The degree of stress is correlated with divergent phenotypes such as quiescence, cell death, or senescence. Each possible cell fate is relevant for a different aspect of endothelial function, and hence, the regulation of cell fate decisions is critically important in maintaining vascular health. This study examined the oxidative stress response (OSR) in human ECs at the boundary of cell survival and death through longitudinal measurements, including cellular, gene expression, and perturbation measurements. 0.5 mM hydrogen peroxide (HP) produced significant oxidative stress, placed the cell at this junction, and provided a model to study the effectors of cell fate. The use of systematic perturbations and high-throughput measurements provide insights into multiple regimes of the stress response. Using a systems approach, we decipher molecular mechanisms across these regimes. Significantly, our study shows that heme oxygenase-1 (HMOX1) acts as a gatekeeper of cell fate decisions. Specifically, HP treatment of HMOX1 knockdown cells reversed the gene expression of about 51% of 2,892 differentially expressed genes when treated with HP alone, affecting a variety of cellular processes, including anti-oxidant response, inflammation, DNA injury and repair, cell cycle and growth, mitochondrial stress, metabolic stress, and autophagy. Further analysis revealed that these switched genes were highly enriched in three spatial locations viz., cell surface, mitochondria, and nucleus. In particular, it revealed the novel roles of HMOX1 on cell surface receptors EGFR and IGFR, mitochondrial ETCs (MTND3, MTATP6), and epigenetic regulation through chromatin modifiers (KDM6A, RBBP5, and PPM1D) and long non-coding RNA (lncRNAs) in orchestrating the cell fate at the boundary of cell survival and death. These novel aspects suggest that HMOX1 can influence transcriptional and epigenetic modulations to orchestrate OSR affecting cell fate decisions.

The vascular niche in next generation microphysiological systems

In recent years, microphysiological system (MPS, also known as, organ-on-a-chip or tissue chip) platforms have emerged with great promise to improve the predictive capacity of preclinical modeling thereby reducing the high attrition rates when drugs move into trials. While their designs can vary quite significantly, in general MPS are bioengineered in vitro microenvironments that recapitulate key functional units of human organs, and that have broad applications in human physiology, pathophysiology, and clinical pharmacology. A critical next step in the evolution of MPS devices is the widespread incorporation of functional vasculature within tissues. The vasculature itself is a major organ that carries nutrients, immune cells, signaling molecules and therapeutics to all other organs. It also plays critical roles in inducing and maintaining tissue identity through expression of angiocrine factors, and in providing tissue-specific milieus (i.e., the vascular niche) that can support the survival and function of stem cells. Thus, organs are patterned, maintained and supported by the vasculature, which in turn receives signals that drive tissue specific gene expression. In this review, we will discuss published vascularized MPS platforms and present considerations for next-generation devices looking to incorporate this critical constituent. Finally, we will highlight the organ-patterning processes governed by the vasculature, and how the incorporation of a vascular niche within MPS platforms will establish a unique opportunity to study stem cell development.

Collagen Bioinks for Bioprinting: A Systematic Review of Hydrogel Properties, Bioprinting Parameters, Protocols, and Bioprinted Structure Characteristics

Bioprinting is a modern tool suitable for creating cell scaffolds and tissue or organ carriers from polymers that mimic tissue properties and create a natural environment for cell development. A wide range of polymers, both natural and synthetic, are used, including extracellular matrix and collagen-based polymers. Bioprinting technologies, based on syringe deposition or laser technologies, are optimal tools for creating precise constructs precisely from the combination of collagen hydrogel and cells. This review describes the different stages of bioprinting, from the extraction of collagen hydrogels and bioink preparation, over the parameters of the printing itself, to the final testing of the constructs. This study mainly focuses on the use of physically crosslinked high-concentrated collagen hydrogels, which represents the optimal way to create a biocompatible 3D construct with sufficient stiffness. The cell viability in these gels is mainly influenced by the composition of the bioink and the parameters of the bioprinting process itself (temperature, pressure, cell density, etc.). In addition, a detailed table is included that lists the bioprinting parameters and composition of custom bioinks from current studies focusing on printing collagen gels without the addition of other polymers. Last but not least, our work also tries to refute the often-mentioned fact that highly concentrated collagen hydrogel is not suitable for 3D bioprinting and cell growth and development.

Hemodynamics and remodeling of the portal confluence in patients with malignancies of the pancreatic head: a pilot study towards planned and circumferential vein resections

BACKGROUND

We designed a retrospective computational study to evaluate the effects of hemodynamics on portal confluence remodeling in real models of patients with malignancies of the pancreatic head.

METHODS

Patient-specific models were created according to computed tomography data. Fluid dynamics was simulated by using finite-element methods. Computational results were compared to morphological findings.

RESULTS

Five patients underwent total pancreatectomy, one had duodenopancreatectomy. Vein resection was performed en-bloc with the specimen. Histopathological findings showed that in patients without a vein stenosis and a normal hemodynamics, the three-layered wall of the vein was preserved. In patients with a stenosis > 70% of vein diameter and modified hemodynamics, the three-layered structure of the resected vein was replaced by a dense inflammatory infiltrate in absence of tumor infiltration.

CONCLUSIONS

The portal confluence involved by malignancies of the pancreatic head undergoes a remodeling that is not mainly due to a wall infiltration by the tumor but instead to a persistent pathological hemodynamics that disrupts the balance between eutrophic remodeling and degradative process of the vein wall that can lead to the complete upheaval of the three-layered vein wall. This finding can have useful surgical application in planning resection of the vein involved by tumor growth.

Low Levels of MicroRNA-10a in Cardiovascular Endothelium and Blood Serum Are Related to Human Atherosclerotic Disease

Background

MicroRNA-10a (miR-10a) inhibits transcriptional factor GATA6 to repress inflammatory GATA6/VCAM-1 signaling, which is regulated by blood flow to affect endothelial function/dysfunction. This study aimed to identify the expression patterns of miR-10a/GATA6/VCAM-1 in vivo and study their implications in the pathophysiology of human coronary artery disease (CAD), i.e., atherosclerosis.

Methods

Human atherosclerotic coronary arteries and nondiseased arteries were used to detect the expressions of miR-10a/GATA6/VCAM-1 in pathogenic vs. normal conditions. In addition, sera from CAD patients and healthy subjects were collected to detect the level of circulating miR-10a.

Results

The comparison of human atherosclerotic coronary arteries with nondiseased arteries demonstrated that lower levels of endothelial miR-10a are related to human atherogenesis. Moreover, GATA6/VCAM-1 (a downstream target of miR-10a) was highly expressed in the endothelium, accompanied by the reduced levels of miR-10a during the development of human atherosclerosis. In addition, CAD patients had a significantly lower concentration of miR-10a in their serum compared to healthy subjects.

Conclusions

Our findings suggest that low miR-10a and high GATA6/VCAM-1 in the cardiovascular endothelium correlates to the development of human atherosclerotic lesions, suggesting that miR-10a signaling has the potential to be developed as a biomarker for human atherosclerosis.

Aqueous humor dynamics in human eye: A lattice Boltzmann study

This paper presents a lattice Boltzmann model to simulate the aqueous humor (AH) dynamics in the human eye by involving incompressible Navier-Stokes flow, heat convection and diffusion, and Darcy seepage flow. Verifying simulations indicate that the model is stable, convergent and robust. Further investigations were carried out, including the effects of heat convection and buoyancy, AH production rate, permeability of trabecular meshwork, viscosity of AH and anterior chamber angle on intraocular pressure (IOP). The heat convection and diffusion can significantly affect the flow patterns in the healthy eye, and the IOP can be controlled by increasing the anterior chamber angle or decreasing the secretion rate, the drainage resistance and viscosity of AH. However, the IOP is insensitive to the viscosity of AH, which may be one of the causes that the viscosity would not have been considered as a factor for controlling the IOP. It's interesting that all these factors have more significant influences on the IOP in pathologic eye than healthy one. The temperature difference and the eye-orientation have obvious influence on the cornea and iris wall shear stresses. The present model and simulation results are expected to provide an alternative tool and theoretical reference for the study of AH dynamics.

Go with the flow: modeling unique biological flows in engineered in vitro platforms

Interest in recapitulating in vivo phenomena in vitro using organ-on-a-chip technology has grown rapidly and with it, attention to the types of fluid flow experienced in the body has followed suit. These platforms offer distinct advantages over in vivo models with regards to human relevance, cost, and control of inputs (e.g., controlled manipulation of biomechanical cues from fluid perfusion). Given the critical role biophysical forces play in several tissues and organs, it is therefore imperative that engineered in vitro platforms capture the complex, unique flow profiles experienced in the body that are intimately tied with organ function. In this review, we outline the complex and unique flow regimes experienced by three different organ systems: blood vasculature, lymphatic vasculature, and the intestinal system. We highlight current state-of-the-art platforms that strive to replicate physiological flows within engineered tissues while introducing potential limitations in current approaches.

Platelet adhesion potential estimation in a normal and diseased coronary artery model: effects of shear stress magnitude versus shear stress history

Platelets play a salient role in the pathogenesis of coronary diseases; primarily through their adhesion to other platelets, endothelial cells and plasma proteins. It is necessary for platelets to activate in order for them to adhere to these different substrates. One of the key regulatory mechanical factors in platelet activation is shear stress, which has been shown to alter multiple platelet functions through the activation of mechanoreceptors. Our goal was to investigate how different numerical shear stress tracking techniques affect platelet adhesion estimates within physiologically relevant computational models. Previously, we developed a physiological coronary artery computational fluid dynamics model. Shear stress waveforms, obtained from these models, were used to monitor in vitro platelet and endothelial cell adhesion marker expression. In this work, the adhesion marker expression data was regressed to obtain numerical functions for receptor expression predictions. These functions were input into a customized adhesion model utilizing different shear stress tracking techniques. For the normal vascular conditions and minimal pathological disease models, shear stress tracking did not significantly affect the adhesion estimates. However, for the severe pathological model, the two shear stress tracking methods had vastly different estimates. Therefore, shear stress tracking methods must be chosen accurately to predict platelet adhesion potentials for accurate modeling techniques.

Mechanotransduction-on-chip: vessel-chip model of endothelial YAP mechanobiology reveals matrix stiffness impedes shear response

Endothelial mechanobiology is a key consideration in the progression of vascular dysfunction, including atherosclerosis. However mechanistic connections between the clinically associated physical stimuli, vessel stiffness and shear stress, and how they interact to modulate plaque progression remain incompletely characterized. Vessel-chip systems are excellent candidates for modeling vascular mechanobiology as they may be engineered from the ground up, guided by the mechanical parameters present in human arteries and veins, to recapitulate key features of the vasculature. Here, we report extensive validation of a vessel-chip model of endothelial yes-associated protein (YAP) mechanobiology, a protein sensitive to both matrix stiffness and shearing forces and, importantly, implicated in atherosclerotic progression. Our model captures the established endothelial mechanoresponse, with endothelial alignment, elongation, reduction of adhesion molecules, and YAP cytoplasmic retention under high laminar shear. Conversely, we observed disturbed morphology, inflammation, and nuclear partitioning under low, high, and high oscillatory shear. Examining targets of YAP transcriptional co-activation, connective tissue growth factor (CTGF) is strongly downregulated by high laminar shear, whereas it is strongly upregulated by low shear or oscillatory flow. Ankyrin repeat domain 1 (ANKRD1) is only upregulated by high oscillatory shear. Verteporfin inhibition of YAP reduced the expression of CTGF but did not affect ANKRD1. Lastly, substrate stiffness modulated the endothelial shear mechanoresponse. Under high shear, softer substrates showed the lowest nuclear localization of YAP whereas stiffer substrates increased nuclear localization. Low shear strongly increased nuclear localization of YAP across stiffnesses. Together, we have validated a model of endothelial mechanobiology and describe a clinically relevant biological connection between matrix stiffness, shear stress, and endothelial activation via YAP mechanobiology.

Implementation and characterization of a physiologically relevant flow waveform in a 3D microfluidic model of the blood-brain barrier

Previous in vitro studies interrogating the endothelial response to physiologically relevant flow regimes require specialized pumps to deliver time-dependent waveforms that imitate in vivo blood flow. The aim of this study is to create a low-cost and broadly adaptable approach to mimic physiological flow, and then use this system to characterize the effect of flow separation on velocity and shear stress profiles in a three-dimensional (3D) topology. The flow apparatus incorporates a programmable linear actuator that superposes oscillations on a constant mean flow driven by a peristaltic pump to emulate flow in the carotid artery. The flow is perfused through a 3D in vitro model of the blood-brain barrier designed to induce separated flow. Experimental flow patterns measured by microparticle image velocimetry and modeled by computational fluid dynamics reveal periodic changes in the instantaneous shear stress along the channel wall. Moreover, the time-dependent flow causes periodic flow separation zones, resulting in variable reattachment points during the cycle. The effects of these complex flow regimes are assessed by evaluating the integrity of the in vitro blood-brain barrier model. Permeability assays and immunostaining for proteins associated with tight junctions reveal barrier breakdown in the region of disturbed flow. In conclusion, the flow system described here creates complex, physiologically relevant flow profiles that provide deeper insight into the fluid dynamics of separated flow and pave the way for future studies interrogating the cellular response to complex flow regimes.

Single cell morphological metrics and cytoskeletal alignment regulate VCAM-1 protein expression

In the initial stages of atherosclerosis, vascular adhesion molecule-1 (VCAM-1) is a surface protein that mediates leukocyte adhesion to the endothelium's luminal surface. VCAM-1 expression is upregulated on endothelial cells (ECs) under pro-inflammatory conditions and is known to be modulated by fluid shear stress (FSS). High, pulsatile FSS induces endothelial elongation and cytoskeletal alignment and downregulates pro-inflammatory induced VCAM-1 expression, which is associated with an athero-protective EC phenotype. In contrast, athero-prone ECs under low, oscillatory FSS fail to elongate and maintain a cobblestone morphology with random cytoskeletal alignment, while VCAM-1 expression is upregulated. Whether EC shape and cytoskeletal alignment play a role in the regulation of VCAM-1 protein expression independent of FSS has not been previously determined. The goal of this study was to determine the effect of EC morphology, specifically cell elongation and alignment, and cytoskeletal alignment on VCAM-1 protein expression using topographical micropatterning of an endothelial monolayer and single cell image analysis techniques. Elongated ECs with an aligned cytoskeleton significantly downregulated VCAM-1 protein expression in the absence of FSS compared to planar controls. In addition, linear correlations between morphological metrics and protein expression showed that actin alignment had a significantly stronger effect on VCAM-1 expression than cell elongation. Functionally, monocytic U937 cells statically adhered less on micropatterns compared to planar substrates, in a VCAM-1 dependent manner. Therefore, endothelial cellular elongation and alignment as well as cytoskeletal alignment regulate VCAM-1 protein expression and immunogenic functions to produce a less inflammatory phenotype in the absence of hemodynamic effects.

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.

Advanced in vitro Research Models to Study the Role of Endothelial Cells in Solid Organ Transplantation

The endothelium plays a key role in acute and chronic rejection of solid organ transplants. During both processes the endothelium is damaged often with major consequences for organ function. Also, endothelial cells (EC) have antigen-presenting properties and can in this manner initiate and enhance alloreactive immune responses. For decades, knowledge about these roles of EC have been obtained by studying both in vitro and in vivo models. These experimental models poorly imitate the immune response in patients and might explain why the discovery and development of agents that control EC responses is hampered. In recent years, various innovative human 3D in vitro models mimicking in vivo organ structure and function have been developed. These models will extend the knowledge about the diverse roles of EC in allograft rejection and will hopefully lead to discoveries of new targets that are involved in the interactions between the donor organ EC and the recipient's immune system. Moreover, these models can be used to gain a better insight in the mode of action of the currently prescribed immunosuppression and will enhance the development of novel therapeutics aiming to reduce allograft rejection and prolong graft survival.

Effect of transport parameters on atherosclerotic lesion growth: A parameter sensitivity analysis

BACKGROUND

Atherosclerosis is a degenerative disease of the arterial wall. It results in the formation of progressively growing plaque lesions that can harden and narrow their host arteries. Current computational models of the inflammatory process that govern atherosclerosis growth are reliant on a number of parameters that can freely vary and whose precise values are not well known.

METHODS

To identify the significance of variation in such parameters, a parametric sensitivity study had been conducted on the blood density, blood viscosity, plasma viscosity and bulk flow low density lipoprotein (LDL) concentration. Using computational modeling, the significance of variation in these parameters was assessed on the transport of LDL. The simulation was performed via the 2^k factorial experimental design, which was conducted to identify the significance of the select parameters on the intima LDL concentration and endothelial LDL coverage area.

RESULTS

Results identified the blood viscosity and bulk flow LDL concentration are the dominant parameters for the atherosclerotic lesion growth. The coverage of LDL on the arterial wall surface was strongly dependent on the blood viscosity. The significance of these findings was discussed.

CONCLUSION

This statistical study identifies two dominating blood factors, LDL concentration and blood viscosity, and how they influence atherosclerosis which will serves as a guideline for further investigation on the atherosclerosis topic.

Disruption of P2X4 purinoceptor and suppression of the inflammation associated with cerebral aneurysm formation

OBJECTIVE

There are no effective therapeutic drugs for cerebral aneurysms, partly because the pathogenesis remains unresolved. Chronic inflammation of the cerebral arterial wall plays an important role in aneurysm formation, but it is not clear what triggers the inflammation. The authors have observed that vascular endothelial P2X4 purinoceptor is involved in flow-sensitive mechanisms that regulate vascular remodeling. They have thus hypothesized that shear stress-associated hemodynamic stress on the endothelium causes the inflammatory process in the cerebral aneurysm development.

METHODS

To test their hypothesis, the authors examined the role of P2X4 in cerebral aneurysm development by using P2X4-/- mice and rats that were treated with a P2X4 inhibitor, paroxetine, and subjected to aneurysm-inducing surgery. Cerebral aneurysms were induced by unilateral carotid artery ligation and renovascular hypertension.

RESULTS

The frequency of aneurysm induction evaluated by light microscopy was significantly lower in the P2X4-/- mice (p = 0.0488) and in the paroxetine-treated male (p = 0.0253) and female (p = 0.0204) rats compared to control mice and rats, respectively. In addition, application of paroxetine from 2 weeks after surgery led to a significant reduction in aneurysm size in the rats euthanized 3 weeks after aneurysm-inducing surgery (p = 0.0145), indicating that paroxetine suppressed enlargement of formed aneurysms. The mRNA and protein expression levels of known inflammatory contributors to aneurysm formation (monocyte chemoattractant protein-1 [MCP-1], interleukin-1β [IL-1β], tumor necrosis factor-α [TNFα], inducible nitric oxide synthase [iNOS], and cyclooxygenase-2 [COX-2]) were all significantly elevated in the rats that underwent the aneurysm-inducing surgery compared to the nonsurgical group, and the values in the surgical group were all significantly decreased by paroxetine administration according to quantitative polymerase chain reaction techniques and Western blotting. Although immunolabeling densities for COX-2, iNOS, and MCP-1 were not readily observed in the nonsurgical mouse groups, such densities were clearly seen in the arterial wall of P2X4+/+ mice after aneurysm-inducing surgery. In contrast, in the P2X4-/- mice after the surgery, immunolabeling of COX-2 and iNOS was not observed in the arterial wall, whereas that of MCP-1 was readily observed in the adventitia, but not the intima.

CONCLUSIONS

These data suggest that P2X4 is required for the inflammation that contributes to both cerebral aneurysm formation and growth. Enhanced shear stress-associated hemodynamic stress on the vascular endothelium may trigger cerebral aneurysm development. Paroxetine may have potential for the clinical treatment of cerebral aneurysms, given that this agent exhibits efficacy as a clinical antidepressant.

Adaptable pulsatile flow generated from stem cell-derived cardiomyocytes using quantitative imaging-based signal transduction

Endothelial cells (EC) in vivo are continuously exposed to a mechanical microenvironment from blood flow, and fluidic shear stress plays an important role in EC behavior. New approaches to generate physiologically and pathologically relevant pulsatile flows are needed to understand EC behavior under different shear stress regimes. Here, we demonstrate an adaptable pump (Adapt-Pump) platform for generating pulsatile flows from human pluripotent stem cell-derived cardiac spheroids (CS) via quantitative imaging-based signal transduction. Pulsatile flows generated from the Adapt-Pump system can recapitulate unique CS contraction characteristics, accurately model responses to clinically relevant drugs, and simulate CS contraction changes in response to fluidic mechanical stimulation. We discovered that ECs differentiated under a long QT syndrome derived pathological pulsatile flow exhibit abnormal EC monolayer organization. This Adapt-Pump platform provides a powerful tool for modeling the cardiovascular system and improving our understanding of EC behavior under different mechanical microenvironments.

Remodeling of the Microvasculature: May the Blood Flow Be With You

The vasculature ensures optimal delivery of nutrients and oxygen throughout the body, and to achieve this function it must continually adapt to varying tissue demands. Newly formed vascular plexuses during development are immature and require dynamic remodeling to generate well-patterned functional networks. This is achieved by remodeling of the capillaries preserving those which are functional and eliminating other ones. A balanced and dynamically regulated capillary remodeling will therefore ensure optimal distribution of blood and nutrients to the tissues. This is particularly important in pathological contexts in which deficient or excessive vascular remodeling may worsen tissue perfusion and hamper tissue repair. Blood flow is a major determinant of microvascular reshaping since capillaries are pruned when relatively less perfused and they split when exposed to high flow in order to shape the microvascular network for optimal tissue perfusion and oxygenation. The molecular machinery underlying blood flow sensing by endothelial cells is being deciphered, but much less is known about how this translates into endothelial cell responses as alignment, polarization and directed migration to drive capillary remodeling, particularly in vivo. Part of this knowledge is theoretical from computational models since blood flow hemodynamics are not easily recapitulated by in vitro or ex vivo approaches. Moreover, these events are difficult to visualize in vivo due to their infrequency and briefness. Studies had been limited to postnatal mouse retina and vascular beds in zebrafish but new tools as advanced microscopy and image analysis are strengthening our understanding of capillary remodeling. In this review we introduce the concept of remodeling of the microvasculature and its relevance in physiology and pathology. We summarize the current knowledge on the mechanisms contributing to capillary regression and to capillary splitting highlighting the key role of blood flow to orchestrate these processes. Finally, we comment the potential and possibilities that microfluidics offers to this field. Since capillary remodeling mechanisms are often reactivated in prevalent pathologies as cancer and cardiovascular disease, all this knowledge could be eventually used to improve the functionality of capillary networks in diseased tissues and promote their repair.