Fluid shear stress threshold regulates angiogenic sprouting

Perfusion and permeability as diagnostic biomarkers of cavernous angioma with symptomatic hemorrhage

Cavernous angiomas with symptomatic hemorrhage (CASH) have a high risk of rebleeding, and hence an accurate diagnosis is needed. With blood flow and vascular leak as established mechanisms, we analyzed perfusion and permeability derivations of dynamic contrast-enhanced quantitative perfusion (DCEQP) MRI in 745 lesions of 205 consecutive patients. Thirteen respective derivations of lesional perfusion and permeability were compared between lesions that bled within a year prior to imaging (N = 86), versus non-CASH (N = 659) using machine learning and univariate analyses. Based on logistic regression and minimizing the Bayesian information criterion (BIC), the best diagnostic biomarker of CASH within the prior year included brainstem lesion location, sporadic genotype, perfusion skewness, and high-perfusion cluster area (BIC = 414.9, sensitivity = 74%, specificity = 87%). Adding a diagnostic plasma protein biomarker enhanced sensitivity to 100% and specificity to 85%. A slightly modified derivation achieved similar accuracy (BIC = 321.6, sensitivity = 80%, specificity = 82%) in the cohort where CASH occurred 3-12 months prior to imaging after signs of hemorrhage would have disappeared on conventional MRI sequences. Adding the same plasma biomarker enhanced sensitivity to 100% and specificity to 87%. Lesional blood flow on DCEQP may distinguish CASH after hemorrhagic signs on conventional MRI have disappeared and are enhanced in combination with a plasma biomarker.

Mechanomaterials: A Rational Deployment of Forces and Geometries in Programming Functional Materials

The knowledge of mechanics of materials has been extensively implemented in developing functional materials, giving rise to recent advances in soft actuators, flexible electronics, mechanical metamaterials, tunable mechanochromics, regenerative mechanomedicine, etc. While conventional mechanics of materials offers passive access to mechanical properties of materials in existing forms, a paradigm shift is emerging toward proactive programming of materials' functionality by leveraging the force-geometry-property relationships. Here, such a rising field is coined as "mechanomaterials". To profile the concept, the design principles in this field at four scales is first outlined, namely the atomic scale, the molecular scale, the manipulation of nanoscale materials, and the microscale design of structural materials. A variety of techniques have been recruited to deliver the multiscale programming of functional mechanomaterials, such as strain engineering, capillary assembly, topological interlocking, kirigami, origami, to name a few. Engineering optical and biological functionalities have also been achieved by implementing the fundamentals of mechanochemistry and mechanobiology. Nonetheless, the field of mechanomaterials is still in its infancy, with many open challenges and opportunities that need to be addressed. The authors hope this review can serve as a modest spur to attract more researchers to further advance this field.

Luminal flow actuation generates coupled shear and strain in a microvessel-on-chip

In the microvasculature, blood flow-derived forces are key regulators of vascular structure and function. Consequently, the development of hydrogel-based microvessel-on-chip systems that strive to mimic thein vivocellular organization and mechanical environment has received great attention in recent years. However, despite intensive efforts, current microvessel-on-chip systems suffer from several limitations, most notably failure to produce physiologically relevant wall strain levels. In this study, a novel microvessel-on-chip based on the templating technique and using luminal flow actuation to generate physiologically relevant levels of wall shear stress and circumferential stretch is presented. Normal forces induced by the luminal pressure compress the surrounding soft collagen hydrogel, dilate the channel, and create large circumferential strain. The fluid pressure gradient in the system drives flow forward and generates realistic pulsatile wall shear stresses. Rigorous characterization of the system reveals the crucial role played by the poroelastic behavior of the hydrogel in determining the magnitudes of the wall shear stress and strain. The experimental measurements are combined with an analytical model of flow in both the lumen and the porous hydrogel to provide an exceptionally versatile user manual for an application-based choice of parameters in microvessels-on-chip. This unique strategy of flow actuation adds a dimension to the capabilities of microvessel-on-chip systems and provides a more general framework for improving hydrogel-basedin vitroengineered platforms.

Angiogenic Sprouting Dynamics Mediated by Endothelial-Fibroblast Interactions in Microfluidic Systems

Angiogenesis, the development of new blood vessels from existing vasculature, is a key process in normal development and pathophysiology. In vitro models are necessary for investigating mechanisms of angiogenesis and developing antiangiogenic therapies. Microfluidic cell culture models of angiogenesis are favored for their ability to recapitulate 3D tissue structures and control spatiotemporal aspects of the microenvironments. To capture the angiogenesis process, microfluidic models often include endothelial cells and a fibroblast component. However, the influence of fibroblast organization on resulting angiogenic behavior remains unclear. Here a comparative study of angiogenic sprouting on a microfluidic chip induced by fibroblasts in 2D monolayer, 3D dispersed, and 3D spheroid culture formats, is conducted. Vessel morphology and sprout distribution for each configuration are measured, and these observations are correlated with measurements of secreted factors and numerical simulations of diffusion gradients. The results demonstrate that angiogenic sprouting varies in response to fibroblast organization with correlating variations in secretory profile and secreted factor gradients across the microfluidic device. This study is anticipated to shed light on how sprouting dynamics are mediated by fibroblast configuration such that the microfluidic cell culture design process includes the selection of a fibroblast component where the effects are known and leveraged.

Pirfenidone Reduces Epithelial-Mesenchymal Transition and Spheroid Formation in Breast Carcinoma through Targeting Cancer-Associated Fibroblasts (CAFs)

The aim of this study was to assess the effects of pirfenidone (PFD) on promoting epithelial-mesenchymal-transition (EMT) and stemness features in breast carcinoma cells through targeting cancer-associated-fibroblasts (CAFs). Using The Cancer Genome Atlas (TCGA) database, we analyzed the association between stromal index, EMT, and stemness-related genes across 1084 breast cancer patients, identifying positive correlation between YAP1, EMT, and stemness genes in samples with a high-stromal index. We monitored carcinoma cell invasion and spheroid formation co-cultured with CAFs in a 3D microfluidic device, followed by exposing carcinoma cells, spheroids, and CAFs with PFD. We depicted a positive association between the high-stromal index and the expression of EMT and stemness genes. High YAP1 expression in samples correlated with more advanced EMT status and stromal index. Additionally, we found that CAFs promoted spheroid formation and induced the expression of YAP1, VIM, and CD44 in spheroids. Treatment with PFD reduced carcinoma cell migration and decreased the expression of these genes at the protein level. The cytokine profiling showed significant depletion of various EMT- and stemness-regulated cytokines, particularly IL8, CCL17, and TNF-beta. These data highlight the potential application of PFD on inhibiting EMT and stemness in carcinoma cells through the targeting of critical cytokines.

Assessment of flow within developing chicken vasculature and biofabricated vascularized tissues using multimodal imaging techniques

Fluid flow shear stresses are strong regulators for directing the organization of vascular networks. Knowledge of structural and flow dynamics information within complex vasculature is essential for tuning the vascular organization within engineered tissues, by manipulating flows. However, reported investigations of vascular organization and their associated flow dynamics within complex vasculature over time are limited, due to limitations in the available physiological pre-clinical models, and the optical inaccessibility and aseptic nature of these models. Here, we developed laser speckle contrast imaging (LSCI) and side-stream dark field microscopy (SDF) systems to map the vascular organization, spatio-temporal blood flow fluctuations as well as erythrocytes movements within individual blood vessels of developing chick embryo, cultured within an artificial eggshell system. By combining imaging data and computational simulations, we estimated fluid flow shear stresses within multiscale vasculature of varying complexity. Furthermore, we demonstrated the LSCI compatibility with bioengineered perfusable muscle tissue constructs, fabricated via molding techniques. The presented application of LSCI and SDF on perfusable tissues enables us to study the flow perfusion effects in a non-invasive fashion. The gained knowledge can help to use fluid perfusion in order to tune and control multiscale vascular organization within engineered tissues.

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.

Endothelial barrier function is co-regulated at vessel bifurcations by fluid forces and sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid mediator of endothelial barrier function. Prior studies have implicated mechanical stimulation due to intravascular laminar shear stress in co-regulating S1P signaling in endothelial cells (ECs). Yet, vascular networks in vivo consist of vessel bifurcations, and this geometry generates hemodynamic forces at the bifurcation point distinct from laminar shear stress. However, the role of these forces at vessel bifurcations in regulating S1P-dependent endothelial barrier function is not known. In this study, we implemented a microfluidic platform that recapitulates the flow dynamics of vessel bifurcations with in situ quantification of the permeability of microvessel analogues. Co-application of S1P with impinging bifurcated fluid flow, which is characterized by approximately zero shear stress and 38 dyn•cm-2 stagnation pressure at the vessel bifurcation point, promotes vessel stabilization. Similarly, co-treatment of S1P with 3 dyn•cm-2 laminar shear stress is also protective of endothelial barrier function. Moreover, it is shown that vessel stabilization due to bifurcated fluid flow and laminar shear stress is dependent on S1P receptor 1 or 2 signaling. Collectively, these findings demonstrate the endothelium-protective function of fluid forces at vessel bifurcations and their involvement in coordinating S1P-dependent regulation of vessel permeability.

From remodeling to quiescence: The transformation of the vascular network

The vascular system is essential for embryogenesis, healing, and homeostasis. Dysfunction or deregulated blood vessel function contributes to multiple diseases, including diabetic retinopathy, cancer, hypertension, or vascular malformations. A balance between the formation of new blood vessels, vascular remodeling, and vessel quiescence is fundamental for tissue growth and function. Whilst the major mechanisms contributing to the formation of new blood vessels have been well explored in recent years, vascular remodeling and quiescence remain poorly understood. In this review, we highlight the cellular and molecular mechanisms responsible for vessel remodeling and quiescence during angiogenesis. We further underline how impaired remodeling and/or destabilization of vessel networks can contribute to vascular pathologies. Finally, we speculate how addressing the molecular mechanisms of vascular remodeling and stabilization could help to treat vascular-related disorders.

Therapeutic angiogenesis in coronary artery disease: a review of mechanisms and current approaches

INTRODUCTION

Despite tremendous advances, the shortcomings of current therapies for coronary disease are evidenced by the fact that it remains the leading cause of death in many parts of the world. There is hence a drive to develop novel therapies to tackle this disease. Therapeutic approaches to coronary angiogenesis have long been an area of interest in lieu of its incredible, albeit unrealized potential.

AREAS COVERED

This paper offers an overview of mechanisms of native angiogenesis and a description of angiogenic growth factors. It progresses to outline the advances in gene and stem cell therapy and provides a brief description of other investigational approaches to promote angiogenesis. Finally, the hurdles and limitations unique to this particular area of study are discussed.

EXPERT OPINION

An effective, sustained, and safe therapeutic option for angiogenesis truly could be the paradigm shift for cardiovascular medicine. Unfortunately, clinically meaningful therapeutic options remain elusive because promising animal studies have not been replicated in human trials. The sheer complexity of this process means that numerous major hurdles remain before therapeutic angiogenesis truly makes its way from the bench to the bedside.

Microfluidic Printing of Tunable Hollow Microfibers for Vascular Tissue Engineering

Bioprinting of vascular tissues holds great potential in tissue engineering and regenerative medicine. However, challenges remain in fabricating biocompatible and versatile scaffolds for the rapid engineering of vascular tissues and vascularized organs. Here, we report novel bioink-enabled microfluidic printing of tunable hollow microfibers for the rapid formation of blood vessels. By compositing biomaterials including sodium alginate, gelatin methacrylate (GelMA), and glycidyl-methacrylate silk fibroin (SilkMA), we prepared a novel composite bioink with excellent printability and biocompatibility. This composite bioink can be printed into hollow microfibers with tunable dimensions using a microfluidic co-axial printing. After seeding human umbilical vein endothelial cells (HUVEC) into the hollow chambers via a microfluidic prefusion device, these cells can adhere to, grow, proliferate, and then cover the internal surface of the printed hollow scaffolds to form vessel-like tissue structures within three days. By combining the unique composite bioink, microfluidic printing of vascular scaffolds, and microfluidic cell seeding and culturing, our strategy can fabricate vascular-like tissue structures with high viability and tunable dimension within three days. The presented method may engineer in vitro vasculatures for the broad applications in basic research and translational medicine including in vitro disease models, tissue microcirculation, and tissue transplantation.

Treatment of Limb Ischemia with Conducted Effects of Catheter-Based Endovascular Ultrasound

Ultrasound (US) is known to stimulate endogenous shear-dependent pathways, and can lower microvascular resistance through mediators that are conducted downstream from US exposure. We hypothesized that endovascular US, already in use for thrombolysis in humans, can improve tissue perfusion in the setting of acute limb ischemia through downstream-conducted effects. Models of severe peripheral arterial disease were developed in mice and in rhesus macaques. An endovascular US catheter (2.3 MHz, 0.5-1.1 MPa) was used to expose the limb adductor in mice for 10 min or the femoral artery distal to stenosis in macaques for 15 min. Quantitative contrast-enhanced ultrasound perfusion imaging was performed to assess flow augmentation in the adductor muscle of mice and the calf muscle of macaques. Microvascular blood flow in the ischemic limb relative to the contralateral control limb was reduced to 22 ± 8% in mice and 36 ± 20% in macaques. US produced immediate 2.3- and 3-fold increases (p < 0.05) in the murine and macaque ischemic limbs, respectively. In macaques, perfusion in the ischemic limb was increased to a normal level. We conclude that non-cavitating US produced by endovascular catheters that are used to enhance thrombolysis in humans can reduce vascular resistance and increase limb perfusion in the setting of acute ischemia.

Impairing flow-mediated endothelial remodeling reduces extravasation of tumor cells

Tumor progression and metastatic dissemination are driven by cell-intrinsic and biomechanical cues that favor the growth of life-threatening secondary tumors. We recently identified pro-metastatic vascular regions with blood flow profiles that are permissive for the arrest of circulating tumor cells. We have further established that such flow profiles also control endothelial remodeling, which favors extravasation of arrested CTCs. Yet, how shear forces control endothelial remodeling is unknown. In the present work, we aimed at dissecting the cellular and molecular mechanisms driving blood flow-dependent endothelial remodeling. Transcriptomic analysis of endothelial cells revealed that blood flow enhanced VEGFR signaling, among others. Using a combination of in vitro microfluidics and intravital imaging in zebrafish embryos, we now demonstrate that the early flow-driven endothelial response can be prevented upon specific inhibition of VEGFR tyrosine kinase and subsequent signaling. Inhibitory targeting of VEGFRs reduced endothelial remodeling and subsequent metastatic extravasation. These results confirm the importance of VEGFR-dependent endothelial remodeling as a driving force of CTC extravasation and metastatic dissemination. Furthermore, the present work suggests that therapies targeting endothelial remodeling might be a relevant clinical strategy in order to impede metastatic progression.

Integration of substrate- and flow-derived stresses in endothelial cell mechanobiology

Endothelial cells (ECs) lining all blood vessels are subjected to large mechanical stresses that regulate their structure and function in health and disease. Here, we review EC responses to substrate-derived biophysical cues, namely topography, curvature, and stiffness, as well as to flow-derived stresses, notably shear stress, pressure, and tensile stresses. Because these mechanical cues in vivo are coupled and are exerted simultaneously on ECs, we also review the effects of multiple cues and describe burgeoning in vitro approaches for elucidating how ECs integrate and interpret various mechanical stimuli. We conclude by highlighting key open questions and upcoming challenges in the field of EC mechanobiology.

Endothelial cell mechanics and blood flow forces in vascular morphogenesis

The vertebrate cardiovascular system is made up by a hierarchically structured network of highly specialised blood vessels. This network emerges during early embryogenesis and evolves in size and complexity concomitant with embryonic growth and organ formation. Underlying this plasticity are actin-driven endothelial cell behaviours, which allow endothelial cells to change their shape and move within the vascular network. In this review, we discuss the cellular and molecular mechanisms involved in vascular network formation and how these intrinsic mechanisms are influenced by haemodynamic forces provided by pressurized blood flow. While most of this review focusses on in vivo evidence from zebrafish embryos, we also mention complementary findings obtained in other experimental systems.

Cold atmospheric plasma improves cutaneous microcirculation in standardized acute wounds: Results of a controlled, prospective cohort study

BACKGROUND

Given the high prevalence of wounds and their challenging treatment, the research of therapies to improve wound healing is of great clinical interest. In addition, the general consequences of developing chronic wounds constitute a large health economic aspect, which underscores the interest in the development of efficient treatment strategies. Direct cold atmospheric plasma (di_CAP) has been shown to have beneficial effects on microcirculation of human tissue (Kisch et al., 2016a). It also affects microbial settlements, which may have supportive effects on wound healing processes (Balzer et al., 2015). To treat these adequately, in our view, the positive effects on wound healing should be objectified by application on standardized wounds. However, wound healing is a complex process, depending on nutrient and oxygen supply by cutaneous blood circulation. In spite of microcirculation has been shown to improve in healthy skin by CAP, a quantification of the effect in a standardized wound model has never been evaluated (Kisch et al., 2016a). Based on this, we hypothesize that CAP also influences the microcirculation in standardized acute wounds in a prospective cohort study.

METHODS

Microcirculatory data of 20 healthy subjects (14 males, 6 females; mean age 40.85 ± 15.84 years; BMI 26.83 ± 7.27 kg/m2) were recorded continuously at a standardized acute wound after skin transplantation (donor site) at the thigh. Under standardized conditions, microcirculatory measurements were performed using a combined laser Doppler and photospectrometry system. After baseline measurement, CAP was applied by a dielectric barrier discharge (DBD) plasma device for 90 s to the acute wound area. Immediately after the application, cutaneous microcirculation was assessed for 30 min (min) at the same site.

RESULTS

After CAP application, tissue oxygen saturation immediately increased by 5% (92,66 ± 4,76% vs. Baseline 88,21 ± 6,52%, p < 0,01) in the first 60 s and remained significantly elevated for 4 min. Capillary blood flow increased by 19.3% within the first minute of CAP therapy (220.14 ± 65.91 AU vs. Baseline 184.52 ± 56.77 AU, p < 0.001). The statistically highly significant increase in blood flow continued over the entire measurement time. A maximum value was shown in the blood flow in the 15th minute (232.15 ± 58.90 AU, p < 0.001) according to CAP application. With regard to the output measurement, it represents a percentage increase of 25.8%. The measurement of post-capillary venous filling pressure at a tissue depth of 6-8 mm was 59.39 ± AU 12.94 at baseline measurement. After application, there were no significant changes.

CONCLUSION

CAP increases cutaneous tissue oxygen saturation and capillary blood flow at the standardized acute wound healing model. These results support recently published data on wound healing after CAP treatment. However, further studies are needed to determine if this treatment can improve the reduced microcirculation in chronic wounds. Moreover, repetitive application protocols have to be compared with a single session treatment approach.

Autophagy modulates FSS-induced epithelial-mesenchymal transition in hepatocellular carcinoma cells

Hepatocellular carcinoma is a highly fatal disease and threatens human health seriously. Fluid shear stress (FSS), which is caused by the leakage of plasma from abnormally permeable tumor blood vessels and insufficient lymphatic drainage, has been identified as contributing pathologically to cancer metastasis. Autophagy and epithelial-mesenchymal transition (EMT) are both reported to be involved in cancer cell migration and invasion, but little has been revealed about the interaction between autophagy and EMT under a tumor mechanical microenvironment. Here, we identified that exposure to 1.4 dyne/cm² FSS could promote the formation of autophagosomes and significantly increase the expressions of autophagy-related markers of beclin1 and ATG7, and the ratio of LC3Ⅱ/Ⅰ in both of HepG2 and QGY-7703 cells. The FSS loading also elevated the levels of mesenchymal markers N-cadherin, Vimentin, Twist, Snail, and β-catenin, while the epithelial markers E-cadherin showed a decrease. Once the autophagy was blocked by 3-methyladenine (3-MA) or knocking ATG5 down, the occurrence of FSS-induced EMT was inhibited dramatically according to the expression and translocation of E-cadherin, N-cadherin, and β-catenin. Given the effect of EMT on cell migration, we observed that inhibition of autophagy could impede FSS-induced cell migration. Collectively, this study demonstrated that autophagy played a crucial role in FSS-induced EMT and cell migration in hepatocellular carcinoma.

Perfusion and endothelialization of engineered tissues with patterned vascular networks

As engineered tissues progress toward therapeutically relevant length scales and cell densities, it is critical to deliver oxygen and nutrients throughout the tissue volume via perfusion through vascular networks. Furthermore, seeding of endothelial cells within these networks can recapitulate the barrier function and vascular physiology of native blood vessels. In this protocol, we describe how to fabricate and assemble customizable open-source tissue perfusion chambers and catheterize tissue constructs inside them. Human endothelial cells are seeded along the lumenal surfaces of the tissue constructs, which are subsequently connected to fluid pumping equipment. The protocol is agnostic with respect to biofabrication methodology as well as cell and material composition, and thus can enable a wide variety of experimental designs. It takes ~14 h over the course of 3 d to prepare perfusion chambers and begin a perfusion experiment. We envision that this protocol will facilitate the adoption and standardization of perfusion tissue culture methods across the fields of biomaterials and tissue engineering.

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.

Dynamic Tuning of Viscoelastic Hydrogels with Carbonyl Iron Microparticles Reveals the Rapid Response of Cells to Three-Dimensional Substrate Mechanics

Current methods to dynamically tune three-dimensional hydrogel mechanics require specific chemistries and substrates that make modest, slow, and often irreversible changes in their mechanical properties, exclude the use of protein-based scaffolds, or alter the hydrogel microstructure and pore size. Here, we rapidly and reversibly alter the mechanical properties of hydrogels consisting of extracellular matrix proteins and proteoglycans by adding carbonyl iron microparticles (MPs) and applying external magnetic fields. This approach drastically alters hydrogel mechanics: rheology reveals that application of a 4000 Oe magnetic field to a 5 mg/mL collagen hydrogel containing 10 wt % MPs increases the storage modulus from approximately 1.5 to 30 kPa. Cell morphology experiments show that cells embedded within these hydrogels rapidly sense the magnetically induced changes in ECM stiffness. Ca²⁺ transients are altered within seconds of stiffening or subsequent softening, and slower but still dynamic changes occur in YAP nuclear translocation in response to time-dependent application of a magnetic field. The near instantaneous change in hydrogel mechanics provides new insight into the effect of changing extracellular stiffness on both acute and chronic changes in diverse cell types embedded in protein-based scaffolds. Due to its flexibility, this method is broadly applicable to future studies interrogating cell mechanotransduction in three-dimensional substrates.

The biomechanical basis of biased epithelial tube elongation in lung and kidney development

During lung development, epithelial branches expand preferentially in a longitudinal direction. This bias in outgrowth has been linked to a bias in cell shape and in the cell division plane. How this bias arises is unknown. Here, we show that biased epithelial outgrowth occurs independent of the surrounding mesenchyme, of preferential turnover of the extracellular matrix at the bud tips and of FGF signalling. There is also no evidence for actin-rich filopodia at the bud tips. Rather, we find epithelial tubes to be collapsed during early lung and kidney development, and we observe fluid flow in the narrow tubes. By simulating the measured fluid flow inside segmented narrow epithelial tubes, we show that the shear stress levels on the apical surface are sufficient to explain the reported bias in cell shape and outgrowth. We use a cell-based vertex model to confirm that apical shear forces, unlike constricting forces, can give rise to both the observed bias in cell shapes and tube elongation. We conclude that shear stress may be a more general driver of biased tube elongation beyond its established role in angiogenesis. This article has an associated 'The people behind the papers' interview.

Effect of Weight Loss Surgery on Biomarkers of Angiogenesis in Obese Patients

BACKGROUND

The present study aims to clarify the effects of weight loss on biomarkers associated with angiogenesis in patients who underwent laparoscopic sleeve gastrectomy (SG) or adjustable gastric banding (LAGB) in the 12-month follow-up study.

MATERIALS AND METHODS

We studied 24 obese patients who underwent laparoscopic weight loss surgery, 13 of whom underwent SG and 11 of whom underwent LAGB. We evaluated the circulating level of angiogenesis biomarkers preoperatively and 12 months after surgery.

RESULTS

Before surgery, the following angiogenic circulating factors were significantly higher than those of healthy subjects: angiopoietin 2 (ANG-2) (p < .05), granulocyte colony-stimulating factor (G-CSF) (p < .05), hepatocyte growth factor (HGF) (p < .01), platelet endothelial cell adhesion molecule (PECAM-1) (p < .01), and vascular endothelial growth factor (VEGF) (p < .05). The following angiogenesis biomarkers decreased significantly after weight loss compared with their baseline values: ANG-2 (p < .05), follistatin (p < .05), HGF (p < .01), PECAM-1 (p < .01), and VEGF (p < .05). There were no significant differences in the circulating levels of angiogenesis biomarkers between individuals who underwent SG and those who underwent LAGB; however, HGF, PECAM-1, and VEGF tended to be lower after SG. %BMI correlated negatively with HGF, PECAM-1, and VEGF. A similar significant negative correlation was found for %WL and %EWL. WHR correlated with PDGF-B and VEGF.

CONCLUSIONS

We concluded that weight loss surgery induces the changes of circulating levels of angiogenesis biomarkers in obese patients. The changes in angiogenesis status in obese patients who lost weight after bariatric surgery depended on the amount of weight loss.

Microvascularized tumor organoids-on-chips: advancing preclinical drug screening with pathophysiological relevance

Recent developments of organoids engineering and organ-on-a-chip microfluidic technologies have enabled the recapitulation of the major functions and architectures of microscale human tissue, including tumor pathophysiology. Nevertheless, there remain challenges in recapitulating the complexity and heterogeneity of tumor microenvironment. The integration of these engineering technologies suggests a potential strategy to overcome the limitations in reconstituting the perfusable microvascular system of large-scale tumors conserving their key functional features. Here, we review the recent progress of in vitro tumor-on-a-chip microfluidic technologies, focusing on the reconstruction of microvascularized organoid models to suggest a better platform for personalized cancer medicine.

Stem cell-based vascularization of microphysiological systems

Microphysiological systems (MPSs) (i.e., tissue or organ chips) exploit microfluidics and 3D cell culture to mimic tissue and organ-level physiology. The advent of human induced pluripotent stem cell (hiPSC) technology has accelerated the use of MPSs to study human disease in a range of organ systems. However, in the reduction of system complexity, the intricacies of vasculature are an often-overlooked aspect of MPS design. The growing library of pluripotent stem cell-derived endothelial cell and perivascular cell protocols have great potential to improve the physiological relevance of vasculature within MPS, specifically for in vitro disease modeling. Three strategic categories of vascular MPS are outlined: self-assembled, interface focused, and 3D biofabricated. This review discusses key features and development of the native vasculature, linking that to how hiPSC-derived vascular cells have been generated, the state of the art in vascular MPSs, and opportunities arising from interdisciplinary thinking.

Engineering Vascularized Organoid-on-a-Chip Models

Recreating human organ-level function in vitro is a rapidly evolving field that integrates tissue engineering, stem cell biology, and microfluidic technology to produce 3D organoids. A critical component of all organs is the vasculature. Herein, we discuss general strategies to create vascularized organoids, including common source materials, and survey previous work using vascularized organoids to recreate specific organ functions and simulate tumor progression. Vascularization is not only an essential component of individual organ function but also responsible for coupling the fate of all organs and their functions. While some success in coupling two or more organs together on a single platform has been demonstrated, we argue that the future of vascularized organoid technology lies in creating organoid systems complete with tissue-specific microvasculature and in coupling multiple organs through a dynamic vascular network to create systems that can respond to changing physiological conditions.

Microphysiological systems in early stage drug development: Perspectives on current applications and future impact

Microphysiological systems (MPS) are making advances to provide more standardized and predictive physiologically relevant responses to test articles in living tissues and organ systems. The excitement surrounding the potential of MPS to better predict human responses to medicines and improving clinical translation is overshadowed by their relatively slow adoption by the pharmaceutical industry and regulators. Collaboration between multiorganizational consortia and regulators is necessary to build an understanding of the strengths and limitations of MPS models and closing the current gaps. Here, we review some of the advances in MPS research, focusing on liver, intestine, vascular system, kidney and lung and present examples highlighting the context of use for these systems. For MPS to gain a foothold in drug development, they must have added value over existing approaches. Ideally, the application of MPS will augment in vivo studies and reduce the use of animals via tiered screening with less reliance on exploratory toxicology studies to screen compounds. Because MPS support multiple cell types (e.g. primary or stem-cell derived cells) and organ systems, identifying when MPS are more appropriate than simple 2D in vitro models for understanding physiological responses to test articles is necessary. Once identified, MPS models require qualification for that specific context of use and must be reproducible to allow future validation. Ultimately, the challenges of balancing complexity with reproducibility will inform the promise of advancing the MPS field and are critical for realization of the goal to reduce, refine and replace (3Rs) the use of animals in nonclinical research.

Induction of scaffold angiogenesis by recipient vasculature precision micropuncture

The success of engineered tissues continues to be limited by time to vascularization and perfusion. Here, we studied the effects of precision injury to a recipient macrovasculature in promoting neovessel formation in an adjacently placed scaffold. Segmental 60 μm diameter micropunctures (MP) were created in the recipient rat femoral artery and vein followed by coverage with a simple collagen scaffold. Scaffolds were harvested at 24, 48, 72, and 96 h post-implantation for detailed analysis. Those placed on top of an MP segment showed an earlier and more robust cellular infiltration, including both endothelial cells (CD31) and macrophages (F4/80), compared to internal non-micropunctured control limbs (p < 0.05). At the 96-hour timepoint, MP scaffolds demonstrated an increase in physiologic perfusion (p < 0.003) and a 2.5-fold increase in capillary network formation (p < 0.001). These were attributed to an overall upsurge in small vessel quantity. Furthermore, MP positioned scaffolds demonstrated significant increases in many modulators of angiogenesis, including VEGFR2 and Tie-2 despite a decrease in HIF-1α at all timepoints. This study highlights a novel microsurgical approach that can be used to rapidly vascularize or inosculate contiguously placed scaffolds and grafts. Thereby, offering an easily translatable route towards the creation of thicker and more clinically relevant engineered tissues.

Microfluidic Biomaterials

Since their initial description in 2005, biomaterials that are patterned to contain microfluidic networks ("microfluidic biomaterials") have emerged as promising scaffolds for a variety of tissue engineering and related applications. This class of materials is characterized by the ability to be readily perfused. Transport and exchange of solutes within microfluidic biomaterials is governed by convection within channels and diffusion between channels and the biomaterial bulk. Numerous strategies have been developed for creating microfluidic biomaterials, including micromolding, photopatterning, and 3D printing. In turn, these materials have been used in many applications that benefit from the ability to perfuse a scaffold, including the engineering of blood and lymphatic microvessels, epithelial tubes, and cell-laden tissues. This article reviews the current state of the field and suggests new areas of exploration for this unique class of materials.

Flow shear stress controls the initiation of neovascularization via heparan sulfate proteoglycans within a biomimetic microfluidic model

Endothelial cells (ECs) in vivo are subjected to three forms of shear stress induced by luminal blood flow, transendothelial flow and interstitial flow simultaneously. It is controversial that shear stress, especially the component induced by luminal flow, was thought to inhibit the initialization of angiogenesis and trigger arteriogenesis. Here, we combined microfabrication techniques and delicate numerical simulations to reconstruct the initial physiological microenvironment of neovascularization in vitro, where ECs experience high luminal shear stress, physiological transendothelial flow and various vascular endothelial growth factor (VEGF) distributions simultaneously. With the biomimetic microfluidic model, cell alignment and endothelial sprouting assays were carried out. We found that luminal shear stress inhibits endothelial sprouting and tubule formation in a dose-dependent manner. Although a high concentration of VEGF increases EC sprouting, neither a positive nor a negative VEGF gradient additionally affects the degree of sprouting, and luminal shear stress significantly attenuates neovascularization even in the presence of VEGF. Heparinase was used to selectively degrade the heparan sulfate proteoglycan (HSPG) coating on ECs and messenger RNA profiles in ECs were analyzed. It turned out that HSPGs could act as a mechanosensor to sense the change of fluid shear stress, modulate multiple EC gene expressions, and hence affect neovascularization. In summary, distraction from the stabilized state, such as decreased luminal shear stress, increased VEGF and the destructed mechanotransduction of HSPGs would induce the initiation of neovascularization. Our study highlights the key role of the magnitude and forms of shear stress in neovascularization.

Environmental toxicology wars: Organ-on-a-chip for assessing the toxicity of environmental pollutants

Environmental pollution is a widespread problem, which has seriously threatened human health and led to an increase of human diseases. Therefore, it is critical to evaluate environmental pollutants quickly and efficiently. Because of obvious inter-species differences between animals and humans, and lack of physiologically-relevant microenvironment, animal models and in vitro two-dimensional (2D) models can not accurately describe toxicological effects and predicting actual in vivo responses. To make up the limitations of conventional environmental toxicology screening, organ-on-a-chip (OOC) systems are increasingly developing. OOC systems can provide a well-organized architecture with comparable to the complex microenvironment in vivo and generate realistic responses to environmental pollutants. The feasibility, adjustability and reliability of OCC systems make it possible to offer new opportunities for environmental pollutants screening, which can study their metabolism, collective response, and fate in vivo. Further progress can address the challenges to make OCC systems better investigate and evaluate environmental pollutants with high predictive power.

Strategies for re-vascularization and promotion of angiogenesis in trauma and disease

The maintenance of a healthy vascular system is essential to ensure the proper function of all organs of the human body. While macrovessels have the main role of blood transportation from the heart to all tissues, microvessels, in particular capillaries, are responsible for maintaining tissues' functionality by providing oxygen, nutrients and waste exchanges. Occlusion of blood vessels due to atherosclerotic plaque accumulation remains the leading cause of mortality across the world. Autologous vein and artery grafts bypassing are the current gold standard surgical procedures to substitute primarily obstructed vascular structures. Ischemic scenarios that condition blood supply in downstream tissues may arise from blockage phenomena, as well as from other disease or events leading to trauma. The (i) great demand for new vascular substitutes, arising from both the limited availability of healthy autologous vessels, as well as the shortcomings associated with small-diameter synthetic vascular grafts, and (ii) the challenging induction of the formation of adequate and stable microvasculature are current driving forces for the growing interest in the development of bioinspired strategies to ensure the proper function of vasculature in all its dimensional scales. Here, a critical review of well-established technologies and recent biotechnological advances to substitute or regenerate the vascular system is provided.

Building Scaffolds for Tubular Tissue Engineering

Hollow organs and tissue systems drive various functions in the body. Many of these hollow or tubular systems, such as vasculature, the intestines, and the trachea, are common targets for tissue engineering, given their relevance to numerous diseases and body functions. As the field of tissue engineering has developed, numerous benchtop models have been produced as platforms for basic science and drug testing. Production of tubular scaffolds for different tissue engineering applications possesses many commonalities, such as the necessity for producing an intact tubular opening and for formation of semi-permeable epithelia or endothelia. As such, the field has converged on a series of manufacturing techniques for producing these structures. In this review, we discuss some of the most common tissue engineered applications within the context of tubular tissues and the methods by which these structures can be produced. We provide an overview of the general structure and anatomy for these tissue systems along with a series of general design criteria for tubular tissue engineering. We categorize methods for manufacturing tubular scaffolds as follows: casting, electrospinning, rolling, 3D printing, and decellularization. We discuss state-of-the-art models within the context of vascular, intestinal, and tracheal tissue engineering. Finally, we conclude with a discussion of the future for these fields.

Recent progress in translational engineered in vitro models of the central nervous system

The complexity of the human brain poses a substantial challenge for the development of models of the CNS. Current animal models lack many essential human characteristics (in addition to raising operational challenges and ethical concerns), and conventional in vitro models, in turn, are limited in their capacity to provide information regarding many functional and systemic responses. Indeed, these challenges may underlie the notoriously low success rates of CNS drug development efforts. During the past 5 years, there has been a leap in the complexity and functionality of in vitro systems of the CNS, which have the potential to overcome many of the limitations of traditional model systems. The availability of human-derived induced pluripotent stem cell technology has further increased the translational potential of these systems. Yet, the adoption of state-of-the-art in vitro platforms within the CNS research community is limited. This may be attributable to the high costs or the immaturity of the systems. Nevertheless, the costs of fabrication have decreased, and there are tremendous ongoing efforts to improve the quality of cell differentiation. Herein, we aim to raise awareness of the capabilities and accessibility of advanced in vitro CNS technologies. We provide an overview of some of the main recent developments (since 2015) in in vitro CNS models. In particular, we focus on engineered in vitro models based on cell culture systems combined with microfluidic platforms (e.g. 'organ-on-a-chip' systems). We delve into the fundamental principles underlying these systems and review several applications of these platforms for the study of the CNS in health and disease. Our discussion further addresses the challenges that hinder the implementation of advanced in vitro platforms in personalized medicine or in large-scale industrial settings, and outlines the existing differentiation protocols and industrial cell sources. We conclude by providing practical guidelines for laboratories that are considering adopting organ-on-a-chip technologies.

Differential Induction of the ADAM17 Regulators iRhom1 and 2 in Endothelial Cells

Background: Endothelial function significantly depends on the proteolytic release of surface expressed signal molecules, their receptors and adhesion molecules via the metalloproteinase ADAM17. The pseudoproteases iRhom1 and 2 independently function as adapter proteins for ADAM17 and are essential for the maturation, trafficking, and activity regulation of ADAM17. Bioinformatic data confirmed that immune cells predominantly express iRhom2 while endothelial cells preferentially express iRhom1.

Objective: Here, we investigate possible reasons for higher iRhom1 expression and potential inflammatory regulation of iRhom2 in endothelial cells and analyze the consequences for ADAM17 maturation and function.

Methods: Primary endothelial cells were cultured in absence and presence of flow with and without inflammatory cytokines (TNFα and INFγ). Regulation of iRhoms was studied by qPCR, involved signaling pathways were studied with transcriptional inhibitors and consequences were analyzed by assessment of ADAM17 maturation, surface expression and cleavage of the ADAM17 substrate junctional adhesion molecule JAM-A.

Results: Endothelial iRhom1 is profoundly upregulated by physiological shear stress. This is accompanied by a homeostatic phenotype driven by the transcription factor KLF2 which is, however, only partially responsible for regulation of iRhom1. By contrast, iRhom2 is most prominently upregulated by inflammatory cytokines. This correlates with an inflammatory phenotype driven by the transcription factors NFκB and AP-1 of which AP-1 is most relevant for iRhom2 regulation. Finally, shear stress exposure and inflammatory stimulation have independent and no synergistic effects on ADAM17 maturation, surface expression and JAM-A shedding.

Conclusion: Conditions of shear stress and inflammation differentially upregulate iRhom1 and 2 in primary endothelial cells which then results in independent regulation of ADAM17.

Transient Support from Fibroblasts is Sufficient to Drive Functional Vascularization in Engineered Tissues

Formation of capillary blood vasculature is a critical requirement for native as well as engineered organs and can be induced in vitro by co-culturing endothelial cells with fibroblasts. However, whether these fibroblasts are required only in the initial morphogenesis of endothelial cells or needed throughout is unknown, and the ability to remove these stromal cells after assembly could be useful for clinical translation. In this study, we introduce a technique termed CAMEO (Controlled Apoptosis in Multicellular Tissues for Engineered Organogenesis), whereby fibroblasts are selectively ablated on demand, and utilize it to probe the dispensability of fibroblasts in vascular morphogenesis. The presence of fibroblasts is shown to be necessary only during the first few days of endothelial cell morphogenesis, after which they can be ablated without significantly affecting the structural and functional features of the developed vasculature. Furthermore, we demonstrate the use of CAMEO to vascularize a construct containing primary human hepatocytes that improved tissue function. In conclusion, this study suggests that transient, initial support from fibroblasts is sufficient to drive vascular morphogenesis in engineered tissues, and this strategy of engineering-via-elimination may provide a new general approach for achieving desired functions and cell compositions in engineered organs.

Engineered Liver Tissue Culture in an In Vitro Tubular Perfusion System

Liver disease and the subsequent loss of liver function is an enormous clinical challenge. A severe shortage of donor liver tissue greatly limits patients' options for a timely transplantation. Tissue engineering approaches offer a promising alternative to organ transplantation by engineering artificial implantable tissues. We have established a platform of cell-laden microbeads as basic building blocks to assemble macroscopic tissues via different mechanisms. This modular fabrication strategy possesses great potential for liver tissue engineering in a bottom-up manner. In this study, we encapsulated human hepatocytes into microbeads presenting a favorable microenvironment consisting of collagen and mesenchymal stem cells, and then we perfused the beads in a three-dimensional printed tubular perfusion bioreactor that promoted oxygen and medium diffusion to the impregnated cells. We noted high cell vitality and retention of parenchymal cell functionality for up to 30 days in this culture system. Our engineering-based approach led to the advancement in tissue size and long-term functionality of an artificial liver tissue in vitro.

Studying endothelial cell shedding and orientation using adaptive perfusion-culture in a microfluidic vascular chip

Most tissue-engineered blood vessels are endothelialized by static cultures in vitro. However, it has not been clear whether endothelial cell-shedding and local damage may occur in an endothelial layer formed by static cultures under the effect of blood flow shear postimplantation. In this study, we report a bionic and cost-effective vascular chip platform, and proved that a static culture of endothelialized tissue-engineered blood vessels had the problem of a large number of endothelial cells falling off under the condition imitating the human arterial blood flow, and we addressed this challenge by regulating the flow field in a vascular chip. Electrospun membranes made of highly oriented or randomly distributed poly(ε-caprolactone) fibers were used as the vascular scaffolds, on which endothelial cells proliferated well and eventually formed dense intima layers. We noted that the monolayers gradually adapted to the artery-like microenvironment through the regulation of chip flow field, which also revealed improved cellular orientations. In conclusion, we have proposed a vascular chip with adaptive flow patterns to gradually accommodate the statically cultured vascular endothelia to the shear environment of arterial flow field and enhanced the orientation of the endothelial cells. This strategy may find numerous potential applications such as screening of vascular engineering biomaterials and maturation parameters, studying of vascular biology and pathology, and construction of vessel-on-a-chip models for drug analysis, among others.

Cysteine cathepsins are altered by flow within an engineered in vitro microvascular niche

Throughout the process of vascular growth and remodeling, the extracellular matrix (ECM) concurrently undergoes significant changes due to proteolytic activity—regulated by both endothelial and surrounding stromal cells. The role of matrix metalloproteinases has been well-studied in the context of vascular remodeling, but other proteases, such as cysteine cathepsins, could also facilitate ECM remodeling. To investigate cathepsin-mediated proteolysis in vascular ECM remodeling, and to understand the role of shear flow in this process, in vitro microvessels were cultured in previously designed microfluidic chips and assessed by immunostaining, zymography, and western blotting. Primary human vessels (HUVECs and fibroblasts) were conditioned by continuous fluid flow and/or small molecule inhibitors to probe cathepsin expression and activity. Luminal flow (in contrast to static culture) decreases the activity of cathepsins in microvessel systems, despite a total protein increase, due to a concurrent increase in the endogenous inhibitor cystatin C. Observations also demonstrate that cathepsins mostly co-localize with fibroblasts, and that fibrin (the hydrogel substrate) may stabilize cathepsin activity in the system. Inhibitor studies suggest that control over cathepsin-mediated ECM remodeling could contribute to improved maintenance of in vitro microvascular networks; however, further investigation is required. Understanding the role of cathepsin activity in in vitro microvessels and other engineered tissues will be important for future regenerative medicine applications.

Hydrogels: The Next Generation Body Materials for Microfluidic Chips?

The integration of microfluidics with biomedical research is confronted with considerable limitations due to its body materials. With high content of water, hydrogels own superior biocompatibility and degradability. Can hydrogels become another material choice for the construction of microfluidic chips, particularly biofluidics? The present review aims to systematically establish the concept of hydrogel-based microfluidic chips (HMCs) and address three main concerns: i) why choosing hydrogels? ii) how to fabricate HMCs?, and iii) in which fields to apply HMCs? It is envisioned that hydrogels may be used increasingly as substitute for traditional materials and gradually act as the body material for microfluidic chips. The modifications of conventional process are highlighted to overcome issues arising from the incompatibility between the construction methods and hydrogel materials. Specifically targeting at the "soft and wet" hydrogels, an efficient flowchart of "i) high resolution template printing; ii) damage-free demolding; iii) twice-crosslinking bonding" is proposed. Accordingly, a broader microfluidic chip concept is proposed in terms of form and function. Potential biomedical applications of HMCs are discussed. This review also highlights the challenges arising from the material replacement, as well as the future directions of the proposed concept. Finally, the authors' viewpoints and perspectives for this emerging field are discussed.

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.

From arteries to capillaries: approaches to engineering human vasculature

From micro-scaled capillaries to millimeter-sized arteries and veins, human vasculature spans multiple scales and cell types. The convergence of bioengineering, materials science, and stem cell biology has enabled tissue engineers to recreate the structure and function of different hierarchical levels of the vascular tree. Engineering large-scale vessels has been pursued over the past thirty years to replace or bypass damaged arteries, arterioles, and venules, and their routine application in the clinic may become a reality in the near future. Strategies to engineer meso- and microvasculature have been extensively explored to generate models to study vascular biology, drug transport, and disease progression, as well as for vascularizing engineered tissues for regenerative medicine. However, bioengineering of large-scale tissues and whole organs for transplantation, have failed to result in clinical translation due to the lack of proper integrated vasculature for effective oxygen and nutrient delivery. The development of strategies to generate multi-scale vascular networks and their direct anastomosis to host vasculature would greatly benefit this formidable goal. In this review, we discuss design considerations and technologies for engineering millimeter-, meso-, and micro-scale vessels. We further provide examples of recent state-of-the-art strategies to engineer multi-scale vasculature. Finally, we identify key challenges limiting the translation of vascularized tissues and offer our perspective on future directions for exploration.

Reduction in MicroRNA-4488 Expression Induces NFκB Translocation in Venous Endothelial Cells Under Arterial Flow

PURPOSE

Little is known about the molecular interactions among inflammatory responses that damage venous endothelial cells (vECs) during venous-to-arterial flow transition in vein graft diseases. Because arterial flow triggers excessive autophagy and inflammation in vECs, this study aimed to investigate the mediator of inflammation and methods to prevent vEC damage.

METHODS

Arterial laminar shear stress (ALSS; 12 dynes/cm²) was applied to vECs via in vitro and ex vivo perfusion systems. Inflammation in vECs was measured using inflammatory protein markers, NFκB translocation, cyclooxygenase-2 (COX-2) and COX-2 and NFκB promoter assays. The involvement of microRNA-4488 (miR-4488) was measured and confirmed by altering the specific miR using a miR-4488 mimic or inhibitor. The potential anti-inflammatory drugs and/or nitric oxide (NO) donor L-arginine (L-Arg) to prevent damage to vECs under ALSS was investigated.

RESULTS

ALSS triggered reactive oxygen species production, excessive autophagy, COX-2 protein expression, and NFκB translocation during vEC inflammation. Reduction in miR-4488 expression was detected in inflamed vECs treated with LPS, lipopolysaccharide (LPS) TNFα, and ALSS. Transfection of miR-4488 mimic (50 nM) prior to ALSS application inhibited the accumulation of inflammatory proteins as well as the translocation of NFκB. Combined treatment of vECs with COX-2-specific inhibitor (SC-236) and L-Arg alleviated the ALSS-induced inflammatory responses. Protective effects of the combined treatment on vECs against ALSS-induced damage were abolished by the application of miR-4488 inhibitor.

CONCLUSION

We showed that ALSS triggered the COX-2/NFκB pathway to induce vEC inflammation with a reduction in miR-4488. Combination of SC-236 and L-Arg prevented ALSS-induced vEC damage, thus, shows high potential for preventing vein graft diseases.

The process of revascularization in the neonatal mouse retina following short-term blockade of vascular endothelial growth factor receptors

Misdirected vascular growth frequently occurs in the neovascular diseases in the retina. However, the mechanisms are still not fully understood. In the present study, we created capillary-free zones in the central and peripheral retinas in neonatal mice by pharmacological blockade of vascular endothelial growth factor (VEGF) signaling. Using this model, we investigated the process and mechanisms of revascularization in the central and peripheral avascular areas. After the completion of a 2-day treatment with the VEGF receptor tyrosine kinase inhibitor KRN633 on postnatal day (P) 4 and P5, revascularization started on P8 in the central avascular area where capillaries had been dropped out. The expression levels of VEGF were higher in the peripheral than in the central avascular area. However, the expansion of the vasculature in the peripheral avascular retina remained suppressed until revascularization had been completed in the central avascular area. Additionally, we found disorganized endothelial cell division, misdirected blood vessels with irregular diameters, and abnormal fibronectin networks at the border of the vascular front and the avascular retina. In the central avascular area, a slight amount of fibronectin as non-vascular component re-formed to provide a scaffold for revascularization. Mechanistic analysis revealed that higher levels of VEGF attenuated the migratory response of endothelial cells without decreasing the proliferative activity. These results suggest that the presence of concentration range of VEGF, which enhances both migration and proliferation of the endothelial cells, and the structurally normal fibronectin network contribute to determine the proper direction of angiogenesis.

It Takes Two to Tango: Endothelial TGFβ/BMP Signaling Crosstalk with Mechanobiology

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta (TGFβ) superfamily of cytokines. While some ligand members are potent inducers of angiogenesis, others promote vascular homeostasis. However, the precise understanding of the molecular mechanisms underlying these functions is still a growing research field. In bone, the tissue in which BMPs were first discovered, crosstalk of TGFβ/BMP signaling with mechanobiology is well understood. Likewise, the endothelium represents a tissue that is constantly exposed to multiple mechanical triggers, such as wall shear stress, elicited by blood flow or strain, and tension from the surrounding cells and to the extracellular matrix. To integrate mechanical stimuli, the cytoskeleton plays a pivotal role in the transduction of these forces in endothelial cells. Importantly, mechanical forces integrate on several levels of the TGFβ/BMP pathway, such as receptors and SMADs, but also global cell-architecture and nuclear chromatin re-organization. Here, we summarize the current literature on crosstalk mechanisms between biochemical cues elicited by TGFβ/BMP growth factors and mechanical cues, as shear stress or matrix stiffness that collectively orchestrate endothelial function. We focus on the different subcellular compartments in which the forces are sensed and integrated into the TGFβ/BMP growth factor signaling.

Perfusion promotes endothelialized pore formation in high concentration fibrin gels otherwise unsuitable for tube development

Vascularization of tissue engineered implants is crucial for their survival and integration in the recipient's body. Pre-vascularized, fibrin-based implants offer a solution since low concentration fibrin hydrogels (1 mg/mL) have been shown to promote tube formation of endothelial cells in co-culture with adipogenic stem cells. However, higher fibrinogen concentrations (> 20 mg/mL) enabling the fabrication of stable implants are necessary.We here characterized fibrin gels of 1-30 mg/mL for their rheological properties and whether they support tube formation of endothelial cell-adipogenic stem cell co-cultures for up to 7 days. Moreover, 20 mg/mL gels containing preformed channels and endothelial cell-adipogenic stem cell co-culture were perfused continuously in a customized flow chamber with 3.9 dyn/cm² for 12 days and analyzed for capillary formation.Rheology of fibrin gels showed increasing stability proportional to fibrinogen concentration with 20 mg/mL gels having a storage module of 465 Pa. Complex tube networks stable for 7 days were observed at 1-5 mg/mL gels whereas higher concentrations showed initial sprouting only. However, perfusion of 20 mg/mL fibrin gels resulted in endothelialized pore formation in several layers of the gel with endothelial cell-adipogenic stem cell co-culture.Thus, perfusion supports the formation of capillary-like structures in fibrin gels that are too dense for spontaneous tube formation under static conditions. Future studies are necessary to further increase pore density and to investigate proper nutrition of tissue-specific target cells in the scaffold.

The Importance of Mechanical Forces for in vitro Endothelial Cell Biology

Blood and lymphatic vessels are lined by endothelial cells which constantly interact with their luminal and abluminal extracellular environments. These interactions confer physical forces on the endothelium, such as shear stress, stretch and stiffness, to mediate biological responses. These physical forces are often altered during disease, driving abnormal endothelial cell behavior and pathology. Therefore, it is critical that we understand the mechanisms by which endothelial cells respond to physical forces. Traditionally, endothelial cells in culture are grown in the absence of flow on stiff substrates such as plastic or glass. These cells are not subjected to the physical forces that endothelial cells endure in vivo, thus the results of these experiments often do not mimic those observed in the body. The field of vascular biology now realize that an intricate analysis of endothelial signaling mechanisms requires complex in vitro systems to mimic in vivo conditions. Here, we will review what is known about the mechanical forces that guide endothelial cell behavior and then discuss the advancements in endothelial cell culture models designed to better mimic the in vivo vascular microenvironment. A wider application of these technologies will provide more biologically relevant information from cultured cells which will be reproducible to conditions found in the body.

Blood Flow Forces in Shaping the Vascular System: A Focus on Endothelial Cell Behavior

The endothelium is the cell monolayer that lines the interior of the blood vessels separating the vessel lumen where blood circulates, from the surrounding tissues. During embryonic development, endothelial cells (ECs) must ensure that a tight barrier function is maintained whilst dynamically adapting to the growing vascular tree that is being formed and remodeled. Blood circulation generates mechanical forces, such as shear stress and circumferential stretch that are directly acting on the endothelium. ECs actively respond to flow-derived mechanical cues by becoming polarized, migrating and changing neighbors, undergoing shape changes, proliferating or even leaving the tissue and changing identity. It is now accepted that coordinated changes at the single cell level drive fundamental processes governing vascular network morphogenesis such as angiogenic sprouting, network pruning, lumen formation, regulation of vessel caliber and stability or cell fate transitions. Here we summarize the cell biology and mechanics of ECs in response to flow-derived forces, discuss the latest advances made at the single cell level with particular emphasis on in vivo studies and highlight potential implications for vascular pathologies.

Microfluidics for the study of mechanotransduction

Mechanical forces regulate a diverse set of biological processes at cellular, tissue, and organismal length scales. Investigating the cellular and molecular mechanisms that underlie the conversion of mechanical forces to biological responses is challenged by limitations of traditional animal models and in vitro cell culture, including poor control over applied force and highly artificial cell culture environments. Recent advances in fabrication methods and material processing have enabled the development of microfluidic platforms that provide precise control over the mechanical microenvironment of cultured cells. These devices and systems have proven to be powerful for uncovering and defining mechanisms of mechanotransduction. In this review, we first give an overview of the main mechanotransduction pathways that function at sites of cell adhesion, many of which have been investigated with microfluidics. We then discuss how distinct microfluidic fabrication methods can be harnessed to gain biological insight, with description of both monolithic and replica molding approaches. Finally, we present examples of how microfluidics can be used to apply both solid forces (substrate mechanics, strain, and compression) and fluid forces (luminal, interstitial) to cells. Throughout the review, we emphasize the advantages and disadvantages of different fabrication methods and applications of force in order to provide perspective to investigators looking to apply forces to cells in their own research.