Substrate stiffness and VE-cadherin mechano-transduction coordinate to regulate endothelial monolayer integrity

Stretch-induced damage in endothelial monolayers

Endothelial cells are constantly exposed to mechanical stimuli, of which mechanical stretch has shown various beneficial or deleterious effects depending on whether loads are within physiological or pathological levels, respectively. Vascular properties change with age, and on a cell-scale, senescence elicits changes in endothelial cell mechanical properties that together can impair its response to stretch. Here, high-rate uniaxial stretch experiments were performed to quantify and compare the stretch-induced damage of monolayers consisting of young, senescent, and aged endothelial populations. The aged and senescent phenotypes were more fragile to stretch-induced damage. Prominent damage was detected by immunofluorescence and scanning electron microscopy as intercellular and intracellular void formation. Damage increased proportionally to the applied level of deformation and, for the aged and senescent phenotype, induced significant detachment of cells at lower levels of stretch compared to the young counterpart. Based on the phenotypic difference in cell-substrate adhesion of senescent cells indicating more mature focal adhesions, a discrete network model of endothelial cells being stretched was developed. The model showed that the more affine deformation of senescent cells increased their intracellular energy, thus enhancing the tendency for cellular damage and impending detachment. Next to quantifying for the first-time critical levels of endothelial stretch, the present results indicate that young cells are more resilient to deformation and that the fragility of senescent cells may be associated with their stronger adhesion to the substrate.

Personalized Vascularized Models of Breast Cancer Desmoplasia Reveal Biomechanical Determinants of Drug Delivery to the Tumor

Desmoplasia in breast cancer leads to heterogeneity in physical properties of the tissue, resulting in disparities in drug delivery and treatment efficacy among patients, thus contributing to high disease mortality. Personalized in vitro breast cancer models hold great promise for high-throughput testing of therapeutic strategies to normalize the aberrant microenvironment in a patient-specific manner. Here, tumoroids assembled from breast cancer cell lines (MCF7, SKBR3, and MDA-MB-468) and patient-derived breast tumor cells (TCs) cultured in microphysiological systems including perfusable microvasculature reproduce key aspects of stromal and vascular dysfunction causing impaired drug delivery. Models containing SKBR3 and MDA-MB-468 tumoroids show higher stromal hyaluronic acid (HA) deposition, vascular permeability, interstitial fluid pressure (IFP), and degradation of vascular HA relative to models containing MCF7 tumoroids or models without tumoroids. Interleukin 8 (IL8) secretion is found responsible for vascular dysfunction and loss of vascular HA. Interventions targeting IL8 or stromal HA normalize vascular permeability, perfusion, and IFP, and ultimately enhance drug delivery and TC death in response to perfusion with trastuzumab and cetuximab. Similar responses are observed in patient-derived models. These microphysiological systems can thus be personalized by using patient-derived cells and can be applied to discover new molecular therapies for the normalization of the tumor microenvironment.

Fabrication of chitin-fibrin hydrogels to construct the 3D artificial extracellular matrix scaffold for vascular regeneration and cardiac tissue engineering

As the cornerstone of tissue engineering and regeneration medicine research, developing a cost-effective and bionic extracellular matrix (ECM) that can precisely modulate cellular behavior and form functional tissue remains challenging. An artificial ECM combining polysaccharides and fibrillar proteins to mimic the structure and composition of natural ECM provides a promising solution for cardiac tissue regeneration. In this study, we developed a bionic hydrogel scaffold by combining a quaternized β-chitin derivative (QC) and fibrin-matrigel (FM) in different ratios to mimic a natural ECM. We evaluated the stiffness of those composite hydrogels with different mixing ratios and their effects on the growth of human umbilical vein endothelial cells (HUVECs). The optimal hydrogels, QCFM1 hydrogels were further applied to load HUVECs into nude mice for in vivo angiogenesis. Besides, we encapsulated human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) into QCFM hydrogels and employed 3D bioprinting to achieve batch fabrication of human-engineered heart tissue (hEHT). Finally, the myocardial structure and electrophysiological function of hEHT were evaluated by immunofluorescence and optical mapping. Designed artificial ECM has a tunable modulus (220-1380 Pa), which determines the different cellular behavior of HUVECs when encapsulated in these. QCFM1 composite hydrogels with optimal stiffness (800 Pa) and porous architecture were finally identified, which could adapt for in vitro cell spreading and in vivo angiogenesis of HUVECs. Moreover, QCFM1 hydrogels were applied in 3D bioprinting successfully to achieve batch fabrication of both ring-shaped and patch-shaped hEHT. These QCFM1 hydrogels-based hEHTs possess organized sarcomeres and advanced function characteristics comparable to reported hEHTs. The chitin-derived hydrogels are first used for cardiac tissue engineering and achieve the batch fabrication of functionalized artificial myocardium. Specifically, these novel QCFM1 hydrogels provided a reliable and economical choice serving as ideal ECM for application in tissue engineering and regeneration medicine.

Facile Determination of the Poisson's Ratio and Young's Modulus of Polyacrylamide Gels and Polydimethylsiloxane

Polyacrylamide hydrogels (PAH gel) and polydimethylsiloxane (PDMS, an elastomer) are two soft materials often used in cell mechanics and mechanobiology, in manufacturing lab-on-a-chip applications, among others. This is partly due to the ability to tune their elasticity with ease in addition to various chemical modifications. For affine polymeric networks, two (of three) elastic constants, Young's modulus (E), the shear modulus (G), and Poisson's ratio (ν), describe the purely elastic response to external forces. However, the literature addressing the experimental determination of ν for PAH (sometimes called PAA gels in the literature) and the PDMS elastomer is surprisingly limited when compared to the literature that reports values of the elastic moduli, E and G. Here, we present a facile method to obtain the Poisson's ratio and Young's modulus for PAH gel and PDMS elastomer based on static tensile tests. The value of ν obtained from the deformation of the sample is compared to the value determined by comparing E and G via a second independent method that utilizes small amplitude shear rheology. We show that the Poisson's ratio may vary significantly from the value for incompressible materials (ν = 0.5), often assumed in the literature even for soft compressible hydrogels. Surprisingly, we find a high degree of agreement between elastic constants obtained by shear rheology and macroscopic static tension test data for polyacrylamide hydrogels but not for elastomeric PDMS.

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 microenvironment stiffness on endothelial cell fate: Implication for occurrence and progression of atherosclerosis

Atherosclerosis, the primary cause of cardiovascular diseases (CVDs), is characterized by phenotypic changes in fibrous proliferation, chronic inflammation and lipid accumulation mediated by vascular endothelial cells (ECs) and vascular smooth muscle cells (SMCs) which are correlated with the stiffening and ectopic remodeling of local extracellular matrix (ECM). The native residents, ECs and SMCs, are not only affected by various chemical factors including inflammatory mediators and chemokines, but also by a range of physical stimuli, such as shear stress and ECM stiffness, presented in the microenvironmental niche. Especially, ECs, as a semi-selective barrier, can sense mechanical forces, respond quickly to changes in mechanical loading and provide context-specific adaptive responses to restore homeostasis. However, blood arteries undergo stiffening and lose their elasticity with age. Reports have shown that the ECM stiffening could influence EC fate by changing the cell adhesion, spreading, proliferation, cell to cell contact, migration and even communication with SMCs. The cell behaviour changes mediated by ECM stiffening are dependent on the activation of a signaling cascade of mechanoperception and mechanotransduction. Although the substantial evidence directly indicates the importance of ECM stiffening on the native ECs, the understanding about this complex interplay is still largely limited. In this review, we systematically summarize the roles of ECM stiffening on the behaviours of endothelial cells and elucidate the underlying details in biological mechanism, aiming to provide the process of how ECs integrate ECM mechanics and the highlights for bioaffinity of tissue-specific engineered scaffolds.

Hydrogels to Recapture Extracellular Matrix Cues That Regulate Vascularization

The ECM (extracellular matrix) is a 3-dimensional network that supports cellular responses and maintains structural tissue integrity in healthy and pathological conditions. The interactions between ECM and cells trigger signaling cascades that lead to phenotypic changes and structural and compositional turnover of the ECM, which in turn regulates vascular cell behavior. Hydrogel biomaterials are a powerful platform for basic and translational studies and clinical applications due to their high swelling capacity and exceptional versatility in compositions and properties. This review highlights recent developments and uses of engineered natural hydrogel platforms that mimic the ECM and present defined biochemical and mechanical cues for vascularization. Specifically, we focus on modulating vascular cell stimulation and cell-ECM/cell-cell interactions in the microvasculature that are the established biomimetic microenvironment.

Impaired angiogenesis in ageing: the central role of the extracellular matrix

Each step in angiogenesis is regulated by the extracellular matrix (ECM). Accumulating evidence indicates that ageing-related changes in the ECM driven by cellular senescence lead to a reduction in neovascularisation, reduced microvascular density, and an increased risk of tissue ischaemic injury. These changes can lead to health events that have major negative impacts on quality of life and place a significant financial burden on the healthcare system. Elucidating interactions between the ECM and cells during angiogenesis in the context of ageing is neceary to clarify the mechanisms underlying reduced angiogenesis in older adults. In this review, we summarize ageing-related changes in the composition, structure, and function of the ECM and their relevance for angiogenesis. Then, we explore in detail the mechanisms of interaction between the aged ECM and cells during impaired angiogenesis in the older population for the first time, discussing diseases caused by restricted angiogenesis. We also outline several novel pro-angiogenic therapeutic strategies targeting the ECM that can provide new insights into the choice of appropriate treatments for a variety of age-related diseases. Based on the knowledge gathered from recent reports and journal articles, we provide a better understanding of the mechanisms underlying impaired angiogenesis with age and contribute to the development of effective treatments that will enhance quality of life.

The role of blood flow in vessel remodeling and its regulatory mechanism during developmental angiogenesis

Vessel remodeling is essential for a functional and mature vascular network. According to the difference in endothelial cell (EC) behavior, we classified vessel remodeling into vessel pruning, vessel regression and vessel fusion. Vessel remodeling has been proven in various organs and species, such as the brain vasculature, subintestinal veins (SIVs), and caudal vein (CV) in zebrafish and yolk sac vessels, retina, and hyaloid vessels in mice. ECs and periendothelial cells (such as pericytes and astrocytes) contribute to vessel remodeling. EC junction remodeling and actin cytoskeleton dynamic rearrangement are indispensable for vessel pruning. More importantly, blood flow has a vital role in vessel remodeling. In recent studies, several mechanosensors, such as integrins, platelet endothelial cell adhesion molecule-1 (PECAM-1)/vascular endothelial cell (VE-cadherin)/vascular endothelial growth factor receptor 2 (VEGFR2) complex, and notch1, have been shown to contribute to mechanotransduction and vessel remodeling. In this review, we highlight the current knowledge of vessel remodeling in mouse and zebrafish models. We further underline the contribution of cellular behavior and periendothelial cells to vessel remodeling. Finally, we discuss the mechanosensory complex in ECs and the molecular mechanisms responsible for vessel remodeling.

Development of a three-dimensional in vitro blood-brain barrier using the chitosan-alginate polyelectrolyte complex as the extracellular matrix

Polyelectrolyte complexes (PECs) consist of a spontaneous assembly of oppositely charged polysaccharides. PECs can be used to obtain a hydrogel tissue scaffold in tissue culture. In this study, it is aimed to use PEC as a blood-brain barrier (BBB) model scaffold. By mixing polycationic chitosan and polyanionic alginate solutions at a certain ratio it was obtained a 3D hydrogel scaffold and mimicked in vivo environment of the tissue. The PEC hydrogel scaffold’s chemical, physical, and mechanical characterizations were performed with FTIR, DSC, DMA, and Micro-CT analyses. In order to develop an in vitro BBB model, the human neuroblastoma cell line (SH-SY5Y) and mouse astrocyte cell line (C8-D1A) were mixed into a hydrogel, which is the abluminal side of the BBB. Human microvascular endothelial cells (HBEC-5i) were seeded on the hydrogel, and it was aimed to mimic the luminal side of the BBB. The characterization of the BBB model was determined by measuring the TEER, observation of the cell morphology with SEM, performing the permeability of Lucifer Yellow, and observation of tight junction proteins with immunofluorescence staining. As a result, HBEC-5i cells expressed tight junction proteins (ZO-1 and Claudin-5), showed TEER of 340 ± 22 Ω.cm2, and the Lucifer Yellow permeability of 7.4 × 10−7 ± 2.7 × 10−7 cm/s, which was suitable for use as an in vitro BBB model. Using a hydrogel PEC composed of chitosan and alginate as an extracellular matrix increased the direct interaction of endothelial cells, astrocytes, and neurons with each other and thus obtained a much less permeable model compared to other standard transwell models.

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Mechanical stress driven by rigidity sensing governs epithelial stability

Epithelia act as a barrier against environmental stress and abrasion and in vivo they are continuously exposed to environments of various mechanical properties. The impact of this environment on epithelial integrity remains elusive. By culturing epithelial cells on 2D hydrogels, we observe a loss of epithelial monolayer integrity through spontaneous hole formation when grown on soft substrates. Substrate stiffness triggers an unanticipated mechanical switch of epithelial monolayers from tensile on soft to compressive on stiff substrates. Through active nematic modelling, we find that spontaneous half-integer defect formation underpinning large isotropic stress fluctuations initiate hole opening events. Our data show that monolayer rupture due to high tensile stress is promoted by the weakening of cell-cell junctions that could be induced by cell division events or local cellular stretching. Our results show that substrate stiffness provides feedback on monolayer mechanical state and that topological defects can trigger stochastic mechanical failure, with potential application towards a mechanistic understanding of compromised epithelial integrity during immune response and morphogenesis.

Single-Cell Transcriptomic Census of Endothelial Changes Induced by Matrix Stiffness and the Association with Atherosclerosis

Vascular endothelial cell (EC) plasticity plays a critical role in the progression of atherosclerosis by giving rise to mesenchymal phenotypes in the plaque lesion. Despite the evidence for arterial stiffening as a major contributor to atherosclerosis, the complex interplay among atherogenic stimuli in vivo has hindered attempts to determine the effects of extracellular matrix (ECM) stiffness on endothelial-mesenchymal transition (EndMT). To study the regulatory effects of ECM stiffness on EndMT, an in vitro model is developed in which human coronary artery ECs are cultured on physiological or pathological stiffness substrates. Leveraging single-cell RNA sequencing, cell clusters with mesenchymal transcriptional features are identified to be more prevalent on pathological substrates than physiological substrates. Trajectory inference analyses reveal a novel mesenchymal-to-endothelial reverse transition, which is blocked by pathological stiffness substrates, in addition to the expected EndMT trajectory. ECs pushed to a mesenchymal character by pathological stiffness substrates are enriched in transcriptional signatures of atherosclerotic ECs from human and murine plaques. This study characterizes at single-cell resolution the transcriptional programs that underpin EC plasticity in both physiological or pathological milieus, and thus serves as a valuable resource for more precisely defining EndMT and the transcriptional programs contributing to atherosclerosis.

Engineered matrix microenvironments reveal the heterogeneity of liver sinusoidal endothelial cell phenotypic responses

Fibrosis is one of the hallmarks of chronic liver disease and is associated with aberrant wound healing. Changes in the composition of the liver microenvironment during fibrosis result in a complex crosstalk of extracellular cues that promote altered behaviors in the cell types that comprise the liver sinusoid, particularly liver sinusoidal endothelial cells (LSECs). Recently, it has been observed that LSECs may sustain injury before other fibrogenesis-associated cells of the sinusoid, implicating LSECs as key actors in the fibrotic cascade. A high-throughput cellular microarray platform was used to deconstruct the collective influences of defined combinations of extracellular matrix (ECM) proteins, substrate stiffness, and soluble factors on primary human LSEC phenotype in vitro. We observed remarkable heterogeneity in LSEC phenotype as a function of stiffness, ECM, and soluble factor context. LYVE-1 and CD-31 expressions were highest on 1 kPa substrates, and the VE-cadherin junction localization was highest on 25 kPa substrates. Also, LSECs formed distinct spatial patterns of LYVE-1 expression, with LYVE-1+ cells observed in the center of multicellular domains, and pattern size regulated by microenvironmental context. ECM composition also influenced a substantial dynamic range of expression levels for all markers, and the collagen type IV was observed to promote elevated expressions of LYVE-1, VE-cadherin, and CD-31. These studies highlight key microenvironmental regulators of LSEC phenotype and reveal unique spatial patterning of the sinusoidal marker LYVE-1. Furthermore, these data provide insight into understanding more precisely how LSECs respond to fibrotic microenvironments, which will aid drug development and identification of targets to treat liver fibrosis.

Extracellular matrix regulates force transduction at VE-cadherin junctions

Increased tension on VE-cadherin (VE-cad) complexes activates adaptive cell stiffening and local cytoskeletal reinforcement--two key signatures of intercellular mechanotransduction. Here we demonstrate that tugging on VE-cad receptors initiates a cascade that results in downstream integrin activation. The formation of new integrin adhesions potentiates vinculin and actin recruitment to mechanically reinforce stressed cadherin adhesions. This cascade differs from documented antagonistic effects of integrins on intercellular junctions. We identify focal adhesion kinase, Abl kinase, and RhoA GTPase as key components of the positive feedback loop. Results further show that a consequence of integrin involvement is the sensitization of intercellular force transduction to the extracellular matrix (ECM) not by regulating junctional tension but by altering signal cascades that reinforce cell-cell adhesions. On type 1 collagen or fibronectin substrates, integrin subtypes α2β1 and α5β1, respectively, differentially control actin remodeling at VE-cad adhesions. Specifically, ECM-dependent differences in VE-cad force transduction mirror differences in the rigidity sensing mechanisms of α2β1 and α5β1 integrins. The findings verify the role of integrins in VE-cad force transduction and uncover a previously unappreciated mechanism by which the ECM impacts the mechanical reinforcement of interendothelial junctions.

Directing Multicellular Organization by Varying the Aspect Ratio of Soft Hydrogel Microwells

Multicellular organization with precise spatial definition is essential to various biological processes, including morphogenesis, development, and healing in vascular and other tissues. Gradients and patterns of chemoattractants are well-described guides of multicellular organization, but the influences of 3D geometry of soft hydrogels are less well defined. Here, the discovery of a new mode of endothelial cell self-organization guided by combinatorial effects of stiffness and geometry, independent of protein or chemical patterning, is described. Endothelial cells in 2 kPa microwells are found to be ≈30 times more likely to migrate to the edge to organize in ring-like patterns than in stiff 35 kPa microwells. This organization is independent of curvature and significantly more pronounced in 2 kPa microwells with aspect ratio (perimeter/depth) < 25. Physical factors of cells and substrates that drive this behavior are systematically investigated and a mathematical model that explains the organization by balancing the dynamic interaction between tangential cytoskeletal tension, cell-cell, and cell-substrate adhesion is presented. These findings demonstrate the importance of combinatorial effects of geometry and stiffness in complex cellular organization that can be leveraged to facilitate the engineering of bionics and integrated model organoid systems with customized nutrient vascular networks.

Microindentation of Fluid-Filled Cellular Domes Reveals the Contribution of RhoA-ROCK Signaling to Multicellular Mechanics

Cellular mechanics encompass both mechanical properties that resist forces applied by the external environment and internally generated forces applied at the location of cell-cell and cell-matrix junctions. Here, the authors demonstrate that microindentation of cellular domes formed by cell monolayers that locally lift off the substrate provides insight into both aspects of cellular mechanics in multicellular structures. Using a modified Hertz contact equation, the force-displacement curves generated by a micro-tensiometer are used to measure an effective dome stiffness. The results indicate the domes are consistent with the Laplace-Young relationship for elastic membranes, regardless of biochemical modulation of the RhoA-ROCK signaling axis. In contrast, activating RhoA, and inhibiting ROCK both alter the relaxation dynamics of the domes deformed by the micro-tensiometer, revealing an approach to interrogate the role of RhoA-ROCK signaling in multicellular mechanics. A finite element model incorporating a Mooney-Rivlin hyperelastic constitutive equation to describe monolayer mechanics predicts effective stiffness values that are consistent with the micro-tensiometer measurements, verifying previous measurements of the response of cell monolayers to tension. Overall, these studies establish microindentation of fluid-filled domes as an avenue to investigate the contribution of cell-generated forces to the mechanics of multicellular structures.

Organ-Specific Endothelial Cell Differentiation and Impact of Microenvironmental Cues on Endothelial Heterogeneity

Endothelial cells throughout the body are heterogeneous, and this is tightly linked to the specific functions of organs and tissues. Heterogeneity is already determined from development onwards and ranges from arterial/venous specification to microvascular fate determination in organ-specific differentiation. Acknowledging the different phenotypes of endothelial cells and the implications of this diversity is key for the development of more specialized tissue engineering and vascular repair approaches. However, although novel technologies in transcriptomics and proteomics are facilitating the unraveling of vascular bed-specific endothelial cell signatures, still much research is based on the use of insufficiently specialized endothelial cells. Endothelial cells are not only heterogeneous, but their specialized phenotypes are also dynamic and adapt to changes in their microenvironment. During the last decades, strong collaborations between molecular biology, mechanobiology, and computational disciplines have led to a better understanding of how endothelial cells are modulated by their mechanical and biochemical contexts. Yet, because of the use of insufficiently specialized endothelial cells, there is still a huge lack of knowledge in how tissue-specific biomechanical factors determine organ-specific phenotypes. With this review, we want to put the focus on how organ-specific endothelial cell signatures are determined from development onwards and conditioned by their microenvironments during adulthood. We discuss the latest research performed on endothelial cells, pointing out the important implications of mimicking tissue-specific biomechanical cues in culture.

Mechanisms of Mechanical Force Induced Pulmonary Vascular Endothelial Hyperpermeability

Mechanical ventilation is a supportive therapy for patients with acute respiratory distress syndrome (ARDS). However, it also inevitably produces or aggravates the original lung injury with pathophysiological changes of pulmonary edema caused by increased permeability of alveolar capillaries which composed of microvascular endothelium, alveolar epithelium, and basement membrane. Vascular endothelium forms a semi-selective barrier to regulate body fluid balance. Mechanical ventilation in critically ill patients produces a mechanical force on lung vascular endothelium when the endothelial barrier was destructed. This review aims to provide a comprehensive overview of molecular and signaling mechanisms underlying the endothelial barrier permeability in ventilator-induced lung jury (VILI).

The Mechanobiology of Endothelial-to-Mesenchymal Transition in Cardiovascular Disease

Endothelial cells (ECs) lining the cardiovascular system are subjected to a highly dynamic microenvironment resulting from pulsatile pressure and circulating blood flow. Endothelial cells are remarkably sensitive to these forces, which are transduced to activate signaling pathways to maintain endothelial homeostasis and respond to changes in the environment. Aberrations in these biomechanical stresses, however, can trigger changes in endothelial cell phenotype and function. One process involved in this cellular plasticity is endothelial-to-mesenchymal transition (EndMT). As a result of EndMT, ECs lose cell-cell adhesion, alter their cytoskeletal organization, and gain increased migratory and invasive capabilities. EndMT has long been known to occur during cardiovascular development, but there is now a growing body of evidence also implicating it in many cardiovascular diseases (CVD), often associated with alterations in the cellular mechanical environment. In this review, we highlight the emerging role of shear stress, cyclic strain, matrix stiffness, and composition associated with EndMT in CVD. We first provide an overview of EndMT and context for how ECs sense, transduce, and respond to certain mechanical stimuli. We then describe the biomechanical features of EndMT and the role of mechanically driven EndMT in CVD. Finally, we indicate areas of open investigation to further elucidate the complexity of EndMT in the cardiovascular system. Understanding the mechanistic underpinnings of the mechanobiology of EndMT in CVD can provide insight into new opportunities for identification of novel diagnostic markers and therapeutic interventions.

E-selectin negatively regulates polymorphonuclear neutrophil transmigration through altered endothelial junction integrity

Transendothelial migration (TEM) of neutrophils under blood flow is critical in the inflammatory cascade. However, the role of endothelial plasticity in this process is not fully understood. Therefore, we used an in vitro model to test the dynamics of human polymorphonuclear neutrophil (PMN) TEM across lipopolysaccharide-treated human umbilical vein endothelial cell (HUVEC) monolayers. Interestingly, shRNA-E-selectin knockdown in HUVECs destabilized endothelial junctional integrity by reducing actin branching and increasing stress fiber at cell-cell junctions. This process is accomplished by downregulating the activation of cortactin and Arp2/3, which in turn alters the adhesive function of VE-cadherin, enhancing PMN transmigration. Meanwhile, redundant P-selectins possess overlapping functions in E-selectin-mediated neutrophil adhesion, and transmigration. These results demonstrate, to our knowledge, for the first time, that E-selectins negatively regulate neutrophil transmigration through alterations in endothelial plasticity. Furthermore, it improves our understanding of the mechanisms underlying actin remodeling, and junctional integrity, in endothelial cells mediating leukocyte TEM.

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.

Molecular tension in syndecan-1 is regulated by extracellular mechanical cues and fluidic shear stress

The endothelium plays a central role in regulating vascular homeostasis and is key in determining the response to materials implanted in the vascular system. Endothelial cells are uniquely sensitive to biophysical cues from applied forces and their local cellular microenvironment. The glycocalyx is a layer of proteoglycans, glycoproteins and glycosaminoglycans that lines the luminal surface of the vascular endothelium, interacting directly with the components of the blood and the forces of blood flow. In this work, we examined the changes in mechanical tension of syndecan-1, a cell surface proteoglycan that is an integral part of the glycocalyx, in response to substrate stiffness and fluidic shear stress. Our studies demonstrate that syndecan-1 has higher mechanical tension in regions of cell adhesion, on and in response to nanotopographical cues. In addition, we found that substrate stiffness also regulated the mechanical tension of syndecan-1 and altered its binding to actin, myosin iiB and signaling intermediates including Src, PKA and FAK. Application of fluidic shear stress created a gradient in tension in syndecan-1 and led to enhanced association with actin, Src, myosin IIb and other cytoskeleton related molecules. Overall, our studies support that syndecan-1 is responsive to the mechanical environment of the cells and alters its association with actin and signaling intermediates in response to mechanical stimuli.

A junctional PACSIN2/EHD4/MICAL-L1 complex coordinates VE-cadherin trafficking for endothelial migration and angiogenesis

Angiogenic sprouting relies on collective migration and coordinated rearrangements of endothelial leader and follower cells. VE-cadherin-based adherens junctions have emerged as key cell-cell contacts that transmit forces between cells and trigger signals during collective cell migration in angiogenesis. However, the underlying molecular mechanisms that govern these processes and their functional importance for vascular development still remain unknown. We previously showed that the F-BAR protein PACSIN2 is recruited to tensile asymmetric adherens junctions between leader and follower cells. Here we report that PACSIN2 mediates the formation of endothelial sprouts during angiogenesis by coordinating collective migration. We show that PACSIN2 recruits the trafficking regulators EHD4 and MICAL-L1 to the rear end of asymmetric adherens junctions to form a recycling endosome-like tubular structure. The junctional PACSIN2/EHD4/MICAL-L1 complex controls local VE-cadherin trafficking and thereby coordinates polarized endothelial migration and angiogenesis. Our findings reveal a molecular event at force-dependent asymmetric adherens junctions that occurs during the tug-of-war between endothelial leader and follower cells, and allows for junction-based guidance during collective migration in angiogenesis.

Communication between endothelial leader and follower cells during collective cell migration is crucial for vascular development. Here, the authors show that PACSIN2 guides collective cell migration and angiogenesis by recruiting a protein trafficking complex to asymmetric cell-cell junctions, controlling local junction plasticity.

Development of Polymeric Nanoparticles for Blood-Brain Barrier Transfer-Strategies and Challenges

Neurological disorders such as Alzheimer's disease, stroke, and brain cancers are difficult to treat with current drugs as their delivery efficacy to the brain is severely hampered by the presence of the blood-brain barrier (BBB). Drug delivery systems have been extensively explored in recent decades aiming to circumvent this barrier. In particular, polymeric nanoparticles have shown enormous potentials owing to their unique properties, such as high tunability, ease of synthesis, and control over drug release profile. However, careful analysis of their performance in effective drug transport across the BBB should be performed using clinically relevant testing models. In this review, polymeric nanoparticle systems for drug delivery to the central nervous system are discussed with an emphasis on the effects of particle size, shape, and surface modifications on BBB penetration. Moreover, the authors critically analyze the current in vitro and in vivo models used to evaluate BBB penetration efficacy, including the latest developments in the BBB-on-a-chip models. Finally, the challenges and future perspectives for the development of polymeric nanoparticles to combat neurological disorders are discussed.

Elucidating the Biomechanics of Leukocyte Transendothelial Migration by Quantitative Imaging

Leukocyte transendothelial migration is crucial for innate immunity and inflammation. Upon tissue damage or infection, leukocytes exit blood vessels by adhering to and probing vascular endothelial cells (VECs), breaching endothelial cell-cell junctions, and transmigrating across the endothelium. Transendothelial migration is a critical rate-limiting step in this process. Thus, leukocytes must quickly identify the most efficient route through VEC monolayers to facilitate a prompt innate immune response. Biomechanics play a decisive role in transendothelial migration, which involves intimate physical contact and force transmission between the leukocytes and the VECs. While quantifying these forces is still challenging, recent advances in imaging, microfabrication, and computation now make it possible to study how cellular forces regulate VEC monolayer integrity, enable efficient pathfinding, and drive leukocyte transmigration. Here we review these recent advances, paying particular attention to leukocyte adhesion to the VEC monolayer, leukocyte probing of endothelial barrier gaps, and transmigration itself. To offer a practical perspective, we will discuss the current views on how biomechanics govern these processes and the force microscopy technologies that have enabled their quantitative analysis, thus contributing to an improved understanding of leukocyte migration in inflammatory diseases.

Three-dimensional endothelial cell incorporation within bioactive nanofibrous scaffolds through concurrent emulsion electrospinning and coaxial cell electrospraying

Nanofibrous scaffolds hold great promise in tissue engineering owing to their extracellular matrix (ECM)-mimicking architectures. Electrospinning, with its ease for producing nanofibrous scaffolds, has therefore been widely employed for various tissue engineering applications. However, electrospun nanofibrous scaffolds have faced the inherent challenge of three-dimensional (3D) cell distribution due to the small sizes of interconnected pores in these scaffolds when conventional approach of scaffold fabrication with subsequent cell seeding is adopted, which severely limits their applications in repairing/regenerating human body tissues with thick and vascularized structures. In this study, we demonstrate a method to directly place living endothelial cells within bioactive nanofibrous scaffolds in 3D through concurrent emulsion electrospinning and coaxial cell electrospraying. Using this concurrent manufacturing method, endothelial cells are encapsulated in hydrogel microspheres and deposited along with vascular endothelial growth factor (VEGF)-containing nanofibers in the scaffold fabrication process, resulting in nanofibrous scaffolds with 3D embedded cell-encapsulated microspheres. After selective disruption of the hydrogel microspheres, the encapsulated endothelial cells are released, yielding bioactive nanofibrous scaffolds with tissue-like 3D cell-incorporated nanofibrous structures. It is shown that cell viability is well preserved (>98%) during the concurrent manufacturing process and that a deep cell distribution (~100 μm) through the scaffold thickness has been achieved. With combined structural and biochemical cues via the 3D cell-incorporated architectures, endothelial cells can freely stretch, display enhanced intercellular connections, and maintain the phenotype in the bioactive nanofibrous scaffolds. Our investigations offer a promising platform technology for creating bioactive nanofibrous scaffolds with 3D cell incorporation and for overcoming inherent problems of electrospun nanofibrous scaffolds, which should open new avenues for biomanufacturing tissue-mimicking constructs with vascularized structures and complex anatomy. STATEMENT OF SIGNIFICANCE: Electrospun nanofibrous scaffolds face challenges in three-dimensional (3D) cell incorporation and vascularization. Enhancing cell penetration via enlarged interconnected pores is a common strategy to address that. However, there are conflicts between cell penetration and structural integrity for scaffolds formed using such strategy, as deep cell penetration, if possible, can only achieve in highly loose architectures. In this investigation, we demonstrate a concurrent emulsion electrospinning and coaxial cell electrospraying technique, realizing 3D endothelial cell incorporation in electrospun nanofibrous scaffolds independent of cell penetration. Our technology appropriately addresses the conflict between deep 3D cell incorporation and structural integrity. In the scaffolds, the 3D incorporated endothelial cells show well-preserved viability, phenotype and functions, implying improved vascularization potential.

You Talking to Me? Cadherin and Integrin Crosstalk in Biomaterial Design

While much work has been done in the design of biomaterials to control integrin-mediated adhesion, less emphasis has been put on functionalization of materials with cadherin ligands. Yet, cell-cell contacts in combination with cell-matrix interactions are key in driving embryonic development, collective cell migration, epithelial to mesenchymal transition, and cancer metastatic processes, among others. This review focuses on the incorporation of both cadherin and integrin ligands in biomaterial design, to promote what is called the "adhesive crosstalk." First, the structure and function of cadherins and their role in eliciting mechanotransductive processes, by themselves or in combination with integrin mechanosensing, are introduced. Then, biomaterials that mimic cell-cell interactions, and recent applications to get insights in fundamental biology and tissue engineering, are critically discussed.

Biomechanical Regulation of Hematopoietic Stem Cells in the Developing Embryo

PURPOSE OF REVIEW

The contribution of biomechanical forces to hematopoietic stem cell (HSC) development in the embryo is a relatively nascent area of research. Herein, we address the biomechanics of the endothelial-to-hematopoietic transition (EHT), impact of force on organelles, and signaling triggered by extrinsic forces within the aorta-gonad-mesonephros (AGM), the primary site of HSC emergence.

RECENT FINDINGS

Hemogenic endothelial cells undergo carefully orchestrated morphological adaptations during EHT. Moreover, expansion of the stem cell pool during embryogenesis requires HSC extravasation into the circulatory system and transit to the fetal liver, which is regulated by forces generated by blood flow. Findings from other cell types also suggest that forces external to the cell are sensed by the nucleus and mitochondria. Interactions between these organelles and the actin cytoskeleton dictate processes such as cell polarization, extrusion, division, survival, and differentiation.

SUMMARY

Despite challenges of measuring and modeling biophysical cues in the embryonic HSC niche, the past decade has revealed critical roles for mechanotransduction in governing HSC fate decisions. Lessons learned from the study of the embryonic hematopoietic niche promise to provide critical insights that could be leveraged for improvement in HSC generation and expansion ex vivo.

A hybrid vascular graft harnessing the superior mechanical properties of synthetic fibers and the biological performance of collagen filaments

Tissue-engineered small caliber vascular grafts have attracted much research attention as a viable alternative to traditional vascular grafts with their biocompatibility and potential to achieve complete healing. However, the major challenge is to fabricate a scaffold with both satisfactory mechanical properties and fast endothelialization. In this study, a hybrid tubular vascular tissue engineered scaffold has been circular-knitted using novel electrochemically aligned collagen (ELAC) filaments plied together with traditional poly(lactic acid) (PLA) yarn. The collagen component was able to promote the recruitment and proliferation of endothelial cells by increasing the initial cell adhesion 10-fold and the eventual cell population 3.2 times higher than the PLA scaffold alone. At the same time, the PLA yarn was able to provide sufficient mechanical strength and structural stability, as well as facilitate scaffold fabrication on high speed textile production equipment. The tubular hybrid scaffold exhibited excellent bursting strength (1.89 ± 0.43 MPa) and suture retention strength (10.86 ± 0.49 N), and had comparable compliance (3.98 ± 1.94%/100 mmHg) to that of the coronary artery (3.8 ± 0.3%/100 mmHg) under normotensive pressure. With its excellent mechanical and biological performance, this prototype hybrid scaffold is a promising candidate for the construction of a clinically successful and easily translatable tissue-engineered small caliber vascular graft.

Adaptation of endothelial cells to shear stress under atheroprone conditions by modulating internalization of vascular endothelial cadherin and vinculin

Background

Endothelial cells play a pivotal role in cardiovascular physiology and pathology by providing a barrier to the bloodstream. In the current study, we investigated the phenotype and barrier function of endothelial cells in response to shear stress under pro-atherogenic conditions.

Methods

Endothelial cells were exposed to laminar shear stress (LSS) in a parallel-plate flow chamber containing oxidized low-density lipoprotein (oxLDL) in the perfusion solution, or remained static. We quantified the response of endothelial monolayers to LSS and oxLDL in terms of cell viability, barrier integrity, vascular endothelial cadherin (VE-cadherin) availability, focal adhesion (FA) remodeling, and monocyte-endothelial interactions.

Results

Our results showed that oxLDL stimulation and static conditions synergized to enhance endothelial barrier disruption. Under the same oxLDL challenge, the application of 25 dynes/cm² LSS on the endothelial monolayer decreased the passage of fluorescein isothiocyanate (FITC)-dextran by 37.79%, increased transendothelial electrical resistance (TEER) by 24.97% compared with static cells (P<0.05), which was accompanied by reduced intercellular gap formation, relatively solid cell-substrate adhesion. Compared with static cells, endothelial cells exposed to both laminar flow and oxLDL had less small FAs, less monocyte adhesion and transmigration, and alleviated overexpression of VCAM-1 and MCP-1. Meanwhile, the oxLDL-induced internalization of VE-cadherin and vinculin were also attenuated by laminar flow, and this change was more pronounced at LSS of 25 dynes/cm² than 5 dynes/cm².

Conclusions

Static conditions favor, whereas physiologically higher levels of LSS ameliorate endothelial barrier disruption under pro-atherogenic stress, which is related to the improved availability of VE-cadherin and vinculin on the cell surface.

A discrete interface in matrix stiffness creates an oscillatory pattern of endothelial monolayer disruption

Intimal stiffening upregulates endothelial cell contractility, disrupting barrier integrity; however, intimal stiffening is non-uniform. The impact of local changes in intimal stiffness on proximal and distal cell-cell interactions is unknown. To investigate the range at which matrix stiffness heterogeneities impact neighboring endothelial cells within a monolayer, we built a micropillar system with adjacent regions of stiff and compliant matrix. The stiffness interface results in an oscillatory pattern of neutrophil transendothelial migration, symmetrical about the interface and well-fit by a sinusoid function. 'Peaks' of the sinusoid were found to have increased cellular contractility and decreased barrier function relative to 'troughs' of the sinusoid. Pharmacological modulation of contractility was observed to break symmetry, altering the amplitude and wavelength of the sinusoid, indicating that contractility may regulate this effect. This work illuminates a novel biophysical phenomenon of the role of stiffness-mediated cell-matrix interactions on cell-cell interactions at a distance. Additionally, it provides insight into the range at which intimal matrix stiffness heterogeneities will impact endothelial barrier function and potentially contribute to atherogenesis.

High levels of endothelial and platelet microvesicles in patients with type 1 diabetes irrespective of microvascular complications

INTRODUCTION

Patients with type 1 diabetes have high risk of developing microvascular complications, and microangiopathy contributes to premature cardiovascular disease in this population. The role that microvesicles (MVs) may play in the development of microangiopathy in type 1 diabetes remains unclear.

MATERIALS AND METHODS

Plasma levels of endothelial MVs (EMVs) and platelet MVs (PMVs) in 130 patients with type 1 diabetes without microangiopathy, 106 patients with microangiopathy and 100 matched healthy controls were analyzed using flow cytometry. MV expression of procoagulant phosphatidylserine (PS) and proinflammatory high mobility group box-1 protein (HMGB1) was also assessed.

RESULTS

Patients with type 1 diabetes had markedly elevated levels of EMVs and PS+ EMVs as well as PMVs and PS+ PMVs compared to healthy controls (p < .001 for all). Furthermore, HMGB1+ EMVs and HMGB1+ PMVs were significantly increased in patients (p < .001 for all). After adjusting for potential confounders, there were no clear differences between patients with or without microvascular complications for any of the MV parameters.

CONCLUSION

Type 1 diabetes is a prothrombotic and proinflammatory disease state that, regardless of the presence of clinical microangiopathy, is associated with elevated levels of plasma MVs, in particular those of an endothelial origin. We have for the first time demonstrated that patients with type 1 diabetes have higher levels of HMGB1+ MVs. HMGB1 is an alarmin with potent proinflammatory effects that drive endothelial dysfunction, and it would therefore be of interest to further study the role of HMGB1+ MVs in the development of macrovascular complications in type 1 diabetes.

Biophysical signal transduction in cancer cells: Understanding its role in cancer pathogenesis and treatment

Signaling between cells can promote both the development and progression of cancer. It has been found that chemical and physical signals, together with extracellular factors, can influence cancer progression. In this review, we focus on the physical microenvironment of cancer cells and examine the action of mechanical, electromagnetic, thermal, and acoustic signals on cancer cells, which may provide new directions for cancer research and treatment.

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.

Stiffness of the aligned fibers affects structural and functional integrity of the oriented endothelial cells

Promoting healthy endothelialization of the tissue-engineered vascular grafts is of great importance in preventing the occurrence of undesired post-implantation complications including neointimal hyperplasia, late thrombosis, and neoatherosclerosis. Previous researches have demonstrated the crucial role of scaffold topography or stiffness in modulating the behavior of the monolayer endothelial cells (ECs). However, effects of the stiffness of scaffolds with anisotropic topography on ECs within vivo like oriented morphology has received little attention. In this study, aligned fibrous substrates (AFSs) with tunable stiffness (14.68-2141.72 MPa), similar to the range of stiffness of the healthy and diseased subendothelial matrix, were used to investigate the effects of fiber stiffness on ECs' attachment, orientation, proliferation, function, remodeling and dysfunction. The results demonstrate that stiffness of the AFSs, capable of providing topographical cues, is a crucial endothelium-protective microenvironmental factor by maintaining stable and quiescent endothelium with in vivo like orientation and strong cell-cell junctions. Stiffer AFSs exacerbated the disruption of endothelium integrity, the occurrence of endothelial-to-mesenchymal transition (EndMT), and the inflammation-induced activation in the endothelial monolayer. This study provides new insights into the understanding on how the stiffness of biomimicking anisotropic substrate regulates the structural and functional integrity of the in vivo like endothelial monolayer, and offers essential designing parameters in engineering biomimicking small-diameter vascular grafts for the regeneration of viable blood vessels. STATEMENT OF SIGNIFICANCE: In vascular tissue engineering, promoting endothelialization on scaffold surface has been considered as a paramount strategy to reduce post-implantation complications. Electrospun aligned fibers have been known to provide contact guidance effect in directing endothelial cells' oriented growth, however, whether the formed EC monolayer in 'correct' orientation shape is of 'correct' function hasn't been explored yet. Given the recognized important role of substrate stiffness in endothelial function, AFSs across physiologically relevant range of moduli (14.68-2141.72 MPa) while maintaining consistent surface chemistry and topographical features were employed to investigate the fiber stiffness effects on ECs function in anisotropic morphology. This study will provide more insightful perspectives in the physiologically remodeling progression of vascular endothelium and design of vascular scaffolds.

Materials for blood brain barrier modeling in vitro

Brain homeostasis relies on the selective permeability property of the blood brain barrier (BBB). The BBB is formed by a continuous endothelium that regulates exchange between the blood stream and the brain. This physiological barrier also creates a challenge for the treatment of neurological diseases as it prevents most blood circulating drugs from entering into the brain. In vitro cell models aim to reproduce BBB functionality and predict the passage of active compounds through the barrier. In such systems, brain microvascular endothelial cells (BMECs) are cultured in contact with various biomaterial substrates. However, BMEC interactions with these biomaterials and their impact on BBB functions are poorly described in the literature. Here we review the most common materials used to culture BMECs and discuss their potential impact on BBB integrity in vitro. We investigate the biophysical properties of these biomaterials including stiffness, porosity and material degradability. We highlight a range of synthetic and natural materials and present three categories of cell culture dimensions: cell monolayers covering non-degradable materials (2D), cell monolayers covering degradable materials (2.5D) and vascularized systems developing into degradable materials (3D).

Comparison of the Effect of Two Hyaluronic Acid Preparations on Fibroblast and Endothelial Cell Functions Related to Angiogenesis

Hyaluronic acid (HA) is used in substitutive and aesthetic medicine with various applications. Ultrapure absorbable HA (Bioregen^®) and a mix of reticulated and free low molecular weight HA (Regenyal Idea Bioexpander®) (both provided by Regenyal Laboratories Srl, San Benedetto del Tronto (AP), Italy) represent a reliable hydrating device and skin filler, useful for skin blemishes, lines and wrinkles, and lip widening, respectively. The commercial products are known for their safety, but data on the molecular, cellular, and tissue responses are lacking. We aimed to evaluate the bioavailability and the pro-angiogenic features of the products Bioregen^® and Bioexpander^® in vitro on cultured endothelial cells (ECs) and dermal fibroblasts in vivo when injected into experimental animals. When added to fibroblasts and ECs, Bioexpander^® induced cell migration. The two HA preparations were well tolerated, while a transient proangiogenic behavior of Bioexpander^®, when implanted subcutaneously in mice, was found. The neovascular response was evident in the first week with higher levels of VEGF and FGF-2 before undergoing regression. In conclusion, our data strengthen the safety of HA synthetic preparations both in vitro and in vivo. Even if a proangiogenic response is documented, it is modest and transient, leading to tissue recovery and absence of an inflammatory infiltrate.

Stiffness Sensing by Cells

Physical stimuli are essential for the function of eukaryotic cells, and changes in physical signals are important elements in normal tissue development as well as in disease initiation and progression. The complexity of physical stimuli and the cellular signals they initiate are as complex as those triggered by chemical signals. One of the most important, and the focus of this review, is the effect of substrate mechanical properties on cell structure and function. The past decade has produced a nearly exponentially increasing number of mechanobiological studies to define how substrate stiffness alters cell biology using both purified systems and intact tissues. Here we attempt to identify common features of mechanosensing in different systems while also highlighting the numerous informative exceptions to what in early studies appeared to be simple rules by which cells respond to mechanical stresses.

A stage-specific cell-manipulation platform for inducing endothelialization on demand

Endothelialization is of great significance for vascular remodeling, as well as for the success of implanted vascular grafts/stents in cardiovascular disease treatment. However, desirable endothelialization on synthetic biomaterials remains greatly challenging owing to extreme difficulty in offering dynamic guidance on endothelial cell (EC) functions resembling the native extracellular matrix-mediated effects. Here, we demonstrate a bilayer platform with near-infrared-triggered transformable topographies, which can alter the geometries and functions of human ECs by tunable topographical cues in a remote-controlled manner, yet cause no damage to the cell viability. The migration and the adhesion/spreading of human ECs are respectively promoted by the temporary anisotropic and permanent isotropic topographies of the platform in turn, which appropriately meet the requirements of stage-specific EC manipulation for endothelialization. In addition to the potential of promoting the development of a new generation of vascular grafts/stents enabling rapid endothelialization, this stage-specific cell-manipulation platform also holds promise in various biomedical fields, since the needs for stepwise control over different cell functions are common in wound healing and various tissue-regeneration processes.

The Interplay Between Cell-Cell and Cell-Matrix Forces Regulates Cell Migration Dynamics

Cells in vivo encounter and exert forces as they interact with the extracellular matrix (ECM) and neighboring cells during migration. These mechanical forces play crucial roles in regulating cell migratory behaviors. Although a variety of studies have focused on describing single-cell or the collective cell migration behaviors, a fully mechanistic understanding of how the cell-cell (intercellular) and cell-ECM (extracellular) traction forces individually and cooperatively regulate single-cell migration and coordinate multicellular movement in a cellular monolayer is still lacking. Here, we developed an integrated experimental and analytical system to examine both the intercellular and extracellular traction forces acting on individual cells within an endothelial cell colony as well as their roles in guiding cell migratory behaviors (i.e., cell translation and rotation). Combined with force, multipole, and moment analysis, our results revealed that traction force dominates in regulating cell active translation, whereas intercellular force actively modulates cell rotation. Our findings advance the understanding of the intricacies of cell-cell and cell-ECM forces in regulating cellular migratory behaviors that occur during the monolayer development and may yield deeper insights into the single-cell dynamic behaviors during tissue development, embryogenesis, and wound healing.

Endothelial Cell Mechano-Metabolomic Coupling to Disease States in the Lung Microvasculature

Lungs are the most vascular part of humans, accepting the totality of cardiac output in a volume much smaller than the body itself. Due to this cardiac output, the lung microvasculature is subject to mechanical forces including shear stress and cyclic stretch that vary with the cardiac and breathing cycle. Vessels are surrounded by extracellular matrix which dictates the stiffness which endothelial cells also sense and respond to. Shear stress, stiffness, and cyclic stretch are known to influence endothelial cell state. At high shear stress, endothelial cells exhibit cell quiescence marked by low inflammatory markers and high nitric oxide synthesis, whereas at low shear stress, endothelial cells are thought to "activate" into a pro-inflammatory state and have low nitric oxide. Shear stress' profound effect on vascular phenotype is most apparent in the arterial vasculature and in the pathophysiology of vascular inflammation. To conduct the flow of blood from the right heart, the lung microvasculature must be rigid yet compliant. It turns out that excessive substrate rigidity or stiffness is important in the development of pulmonary hypertension and chronic fibrosing lung diseases via excessive cell proliferation or the endothelial-mesenchymal transition. Recently, a new body of literature has evolved that couples mechanical sensing to endothelial phenotypic changes through metabolic signaling in clinically relevant contexts such as pulmonary hypertension, lung injury syndromes, as well as fibrosis, which is the focus of this review. Stretch, like flow, has profound effect on endothelial phenotype; metabolism studies due to stretch are in their infancy.

Balance of mechanical forces drives endothelial gap formation and may facilitate cancer and immune-cell extravasation

The formation of gaps in the endothelium is a crucial process underlying both cancer and immune cell extravasation, contributing to the functioning of the immune system during infection, the unfavorable development of chronic inflammation and tumor metastasis. Here, we present a stochastic-mechanical multiscale model of an endothelial cell monolayer and show that the dynamic nature of the endothelium leads to spontaneous gap formation, even without intervention from the transmigrating cells. These gaps preferentially appear at the vertices between three endothelial cells, as opposed to the border between two cells. We quantify the frequency and lifetime of these gaps, and validate our predictions experimentally. Interestingly, we find experimentally that cancer cells also preferentially extravasate at vertices, even when they first arrest on borders. This suggests that extravasating cells, rather than initially signaling to the endothelium, might exploit the autonomously forming gaps in the endothelium to initiate transmigration.

Three-Dimensional Monolayer Stress Microscopy

Many biological processes involve the collective generation and transmission of mechanical stresses across cell monolayers. In these processes, the monolayer undergoes lateral deformation and bending because of the tangential and normal components of the cell-generated stresses. Monolayer stress microscopy (MSM) methods have been developed to measure the intracellular stress distribution in cell monolayers. However, current methods assume plane monolayer geometry and neglect the contribution of bending to the intracellular stresses. This work introduces a three-dimensional (3D) MSM method that calculates monolayer stress from measurements of the 3D traction stresses exerted by the cells on a flexible substrate. The calculation is carried out by imposing equilibrium of forces and moments in the monolayer, subject to external loads given by the 3D traction stresses. The equilibrium equations are solved numerically, and the algorithm is validated for synthetic loads with known analytical solutions. We present 3D-MSM measurements of monolayer stress in micropatterned islands of endothelial cells of different sizes and shapes. These data indicate that intracellular stresses caused by lateral deformation emerge collectively over long distances; they increase with the distance from the island edge until they reach a constant value that is independent of island size. On the other hand, bending-induced intracellular stresses are more concentrated spatially and remain confined to within one to two cell lengths of bending sites. The magnitude of these bending stresses is highest at the edges of the cell islands, where they can exceed the intracellular stresses caused by lateral deformations. Our data from nonpatterned monolayers suggests that biomechanical perturbations far away from monolayer edges also cause significant localized alterations in bending tension. The localized effect of bending-induced stresses may be important in processes like cellular extravasation, which are accompanied by significant normal deflections of a cell monolayer (i.e., the endothelium) and require localized changes in monolayer permeability.

Endothelial Cell Mechanotransduction in the Dynamic Vascular Environment

The vascular endothelial cells (ECs) that line the inner layer of blood vessels are responsible for maintaining vascular homeostasis under physiological conditions. In the presence of disease or injury, ECs can become dysfunctional and contribute to a progressive decline in vascular health. ECs are constantly exposed to a variety of dynamic mechanical stimuli, including hemodynamic shear stress, pulsatile stretch, and passive signaling cues derived from the extracellular matrix. This review describes the molecular mechanisms by which ECs perceive and interpret these mechanical signals. The translational applications of mechanosensing are then discussed in the context of endothelial-to-mesenchymal transition and engineering of vascular grafts.

Substrate stiffness heterogeneities disrupt endothelial barrier integrity in a micropillar model of heterogeneous vascular stiffening

Intimal stiffening has been linked with increased vascular permeability and leukocyte transmigration, hallmarks of atherosclerosis. However, recent evidence indicates age-related intimal stiffening is not uniform but rather characterized by increased point-to-point heterogeneity in subendothelial matrix stiffness, the impact of which is much less understood. To investigate the impact of spatially heterogeneous matrix rigidity on endothelial monolayer integrity, we develop a micropillar model to introduce closely-spaced, step-changes in substrate rigidity and compare endothelial monolayer phenotype to rigidity-matched, uniformly stiff and compliant substrates. We found equivalent disruption of adherens junctions within monolayers on step-rigidity and uniformly stiff substrates relative to uniformly compliant substrates. Similarly, monolayers cultured on step-rigidity substrates exhibited equivalent percentages of leukocyte transmigration to monolayers on rigidity-matched, uniformly stiff substrates. Adherens junction tension and focal adhesion density, but not size, increased within monolayers on step-rigidity and uniformly stiff substrates compared to more compliant substrates suggesting that elevated tension is disrupting adherens junction integrity. Leukocyte transmigration frequency and time, focal adhesion size, and focal adhesion density did not differ between stiff and compliant sub-regions of step-rigidity substrates. Overall, our results suggest that endothelial monolayers exposed to mechanically heterogeneous substrates adopt the phenotype associated with the stiffer matrix, indicating that spatial heterogeneities in intimal stiffness observed with age could disrupt endothelial barrier integrity and contribute to atherogenesis.

Substrate elasticity regulates adipose-derived stromal cell differentiation towards osteogenesis and adipogenesis through β-catenin transduction

It is generally recognised that mesenchymal stem cells (MSCs) can differentiate into multiple lineages through guidance from the biophysical properties of the substrates. However, the precise biophysical mechanism that enables MSCs to respond to substrate properties remains unclear. In the current study, polydimethylsiloxane (PDMS) substrates with different stiffnesses were fabricated and the way in which the elastic modulus of the substrate regulated differentiation towards osteogenesis and adipogenesis in adipose-derived stromal cells (ASCs) was explored. Initially, a cell morphology change by SEM was observed between the stiff and soft substrates. The cytoskeleton stains including microfilament by F-actin and microtubule by α- and β-tubulin further showed a larger cell spreading area on the stiff substrate. Then the expression of vinculin, in charge for the linkage of adhesion molecules to the actin cytoskeleton, was enhanced on the stiff substrate. This change in focal adhesion plaque further triggered intracellular β-catenin signaling and promoted its nuclear translocation especially on the stiff substrate. The influence of β-catenin signaling on direct differentiation to osteogenic lineages was through direct binding between its downstream protein, Lef-1, and the osteogenic transcriptional factors, Runx2 and Osx, while on differentiation to adipogenic lineages was through modulating the expression of PPARγ. The imbalance of stiffness-induced β-catenin signaling finally induced a stronger osteogenesis and a weaker adipogenesis on the stiff substrate relative to those on the soft substrate. This study indicates the importance of stiffness on ASC differentiation and could help to increase understanding of the mechanism underlying molecular signal transduction from mechanosensing, mechanotransducing to stem cell differentiation.

STATEMENT OF SIGNIFICANCE

Mesenchymal stem cells can differentiate into multiple lineages, such as adipogenesis, myogenesis, neurogenesis, angiogenesis and osteogenesis, through influence of biophysical properties of the extracellular matrix. However, the precise bio-mechanism that triggers stem cell differentiation in response to matrix biophysical properties remains unclear. In the current study, we provide a series of experiments involving the characterization of cell morphology, microfilament, microtubule and adhesion capacity of adipose-derived stromal cells (ASCs) in response to substrate stiffness, and further elucidation of cytoplasmic β-catenin-dependent signal transduction, nuclear translocation and resultant promoter activation of transcriptional factors for osteogenesis and adipogenesis. This study provides an explanation on deeper understanding of bio-mechanism underlying substrate stiffness-triggered β-catenin signal transduction from active mechanosensing, mechanotransducing to stem cell differentiation.

Modeling Tissue Polarity in Context

Polarity is critical for development and tissue-specific function. However, the acquisition and maintenance of tissue polarity is context dependent. Thus, cell and tissue polarity depend on cell adhesion which is regulated by the cytoskeleton and influenced by the biochemical composition of the extracellular microenvironment and modified by biomechanical cues within the tissue. These biomechanical cues include fluid flow induced shear stresses, cell-density and confinement-mediated compression, and cellular actomyosin tension intrinsic to the tissue or induced in response to morphogens or extracellular matrix stiffness. Here, we discuss how extracellular matrix stiffness and fluid flow influence cell-cell and cell-extracellular matrix adhesion and alter cytoskeletal organization to modulate cell and tissue polarity. We describe model systems that when combined with state of the art molecular screens and high-resolution imaging can be used to investigate how force modulates cell and tissue polarity.

Determining mechanical features of modulated epithelial monolayers using subnuclear particle tracking

Force generation within cells, mediated by motor proteins along cytoskeletal networks, maintains the function of multicellular structures during homeostasis and when generating collective forces. Here, we describe the use of chromatin dynamics to detect cellular force propagation [a technique termed SINK (sensors from intranuclear kinetics)] and investigate the force response of cells to disruption of the monolayer and changes in substrate stiffness. We find that chromatin dynamics change in a substrate stiffness-dependent manner within epithelial monolayers. We also investigate point defects within monolayers to map the impact on the strain field of a heterogeneous monolayer. We find that cell monolayers behave as a colloidal assembly rather than as a continuum since the data fit an exponential decay; the lateral characteristic length of recovery from the mechanical defect is ∼50 µm for cells with a 10 µm spacing. At distances greater than this characteristic length, cells behave similarly to those in a fully intact monolayer. This work demonstrates the power of SINK to investigate diseases including cancer and atherosclerosis that result from single cells or heterogeneities in monolayers.This article has an associated First Person interview with the first author of the paper.

Dependence of Tensional Homeostasis on Cell Type and on Cell-Cell Interactions

INTRODUCTION

The ability to maintain a homeostatic level of cell tension is essential for many physiological processes. Our group has recently reported that multicellularity is required for tensional homeostasis in endothelial cells. However, other studies have shown that isolated fibroblasts also maintain constant tension over short time scales without the need of cell-cell contacts. Therefore, in this study, our aim was to determine how different cell types regulate tension as isolated cells or in small clustered groupings and to investigate the role of cell-cell adhesion molecules, such as E-cadherin, in this system.

METHODS

Micropattern traction force microscopy was used to determine how bovine aortic endothelial cells, bovine vascular smooth muscle cells, mouse embryonic fibroblasts, and human gastric adenocarcinoma cells, with or without cell-cell interactions due to E-cadherin, maintain tensional homeostasis over time. Tension temporal fluctuations in single cells and cell clusters were evaluated.

RESULTS

We found that only endothelial cells require clustering for tensional homeostasis. The same was not verified in fibroblasts or vascular smooth muscle cells. Of relevance, in adenocarcinoma cells, we verified that tensional homeostasis was dependent on the competence of the adhesion molecule E-cadherin at both the single cells and multicellular levels.

CONCLUSION

These findings indicate that cell-cell contacts may be critical for tensional homeostasis and, potentially, for barrier function of the endothelium. Furthermore, the cell-cell adhesion molecule E-cadherin is an important regulator of tensional homeostasis, even in the absence of cadherin engagement with neighboring cells, which demonstrates its relevance not only as a structural molecule but also as a signaling moiety.

Substrate stiffness-dependent exacerbation of endothelial permeability and inflammation: mechanisms and potential implications in ALI and PH (2017 Grover Conference Series)

The maintenance of endothelial barrier integrity is absolutely essential to prevent the vascular leak associated with pneumonia, pulmonary edema resulting from inhalation of toxins, acute elevation to high altitude, traumatic and septic lung injury, acute lung injury (ALI), and its life-threatening complication, acute respiratory distress syndrome (ARDS). In addition to the long-known edemagenic and inflammatory agonists, emerging evidences suggest that factors of endothelial cell (EC) mechanical microenvironment such as blood flow, mechanical strain of the vessel, or extracellular matrix stiffness also play an essential role in the control of endothelial permeability and inflammation. Recent studies from our group and others have demonstrated that substrate stiffening causes endothelial barrier disruption and renders EC more susceptible to agonist-induced cytoskeletal rearrangement and inflammation. Further in vivo studies have provided direct evidence that proinflammatory stimuli increase lung microvascular stiffness which in turn exacerbates endothelial permeability and inflammation and perpetuates a vicious circle of lung inflammation. Accumulating evidence suggests a key role for RhoA GTPases signaling in stiffness-dependent mechanotransduction mechanisms defining EC permeability and inflammatory responses. Vascular stiffening is also known to be a key contributor to other cardiovascular diseases such as arterial pulmonary hypertension (PH), although the precise role of stiffness in the development and progression of PH remains to be elucidated. This review summarizes the current understanding of stiffness-dependent regulation of pulmonary EC permeability and inflammation, and discusses potential implication of pulmonary vascular stiffness alterations at macro- and microscale in development and modulation of ALI and PH.