Morphological response of human endothelial cells subjected to cyclic strain in vitro

Cell orientation under stretch: A review of experimental findings and mathematical modelling

The key role of electro-chemical signals in cellular processes had been known for many years, but more recently the interplay with mechanics has been put in evidence and attracted substantial research interests. Indeed, the sensitivity of cells to mechanical stimuli coming from the microenvironment turns out to be relevant in many biological and physiological circumstances. In particular, experimental evidence demonstrated that cells on elastic planar substrates undergoing periodic stretches, mimicking native cyclic strains in the tissue where they reside, actively reorient their cytoskeletal stress fibres. At the end of the realignment process, the cell axis forms a certain angle with the main stretching direction. Due to the importance of a deeper understanding of mechanotransduction, such a phenomenon was studied both from the experimental and the mathematical modelling point of view. The aim of this review is to collect and discuss both the experimental results on cell reorientation and the fundamental features of the mathematical models that have been proposed in the literature.

Fluidic Force Microscopy and Atomic Force Microscopy Unveil New Insights into the Interactions of Preosteoblasts with 3D-Printed Submicron Patterns

Physical patterns represent potential surface cues for promoting osteogenic differentiation of stem cells and improving osseointegration of orthopedic implants. Understanding the early cell-surface interactions and their effects on late cellular functions is essential for a rational design of such topographies, yet still elusive. In this work, fluidic force microscopy (FluidFM) and atomic force microscopy (AFM) combined with optical and electron microscopy are used to quantitatively investigate the interaction of preosteoblasts with 3D-printed patterns after 4 and 24 h of culture. The patterns consist of pillars with the same diameter (200 nm) and interspace (700 nm) but distinct heights (500 and 1000 nm) and osteogenic properties. FluidFM reveals a higher cell adhesion strength after 24 h of culture on the taller pillars (32 ± 7 kPa versus 21.5 ± 12.5 kPa). This is associated with attachment of cells partly on the sidewalls of these pillars, thus requiring larger normal forces for detachment. Furthermore, the higher resistance to shear forces observed for these cells indicates an enhanced anchorage and can be related to the persistence and stability of lamellipodia. The study explains the differential cell adhesion behavior induced by different pillar heights, enabling advancements in the rational design of osteogenic patterns.

Significance of mechanical loading in bone fracture healing, bone regeneration, and vascularization

In 1892, J.L. Wolff proposed that bone could respond to mechanical and biophysical stimuli as a dynamic organ. This theory presents a unique opportunity for investigations on bone and its potential to aid in tissue repair. Routine activities such as exercise or machinery application can exert mechanical loads on bone. Previous research has demonstrated that mechanical loading can affect the differentiation and development of mesenchymal tissue. However, the extent to which mechanical stimulation can help repair or generate bone tissue and the related mechanisms remain unclear. Four key cell types in bone tissue, including osteoblasts, osteoclasts, bone lining cells, and osteocytes, play critical roles in responding to mechanical stimuli, while other cell lineages such as myocytes, platelets, fibroblasts, endothelial cells, and chondrocytes also exhibit mechanosensitivity. Mechanical loading can regulate the biological functions of bone tissue through the mechanosensor of bone cells intraosseously, making it a potential target for fracture healing and bone regeneration. This review aims to clarify these issues and explain bone remodeling, structure dynamics, and mechano-transduction processes in response to mechanical loading. Loading of different magnitudes, frequencies, and types, such as dynamic versus static loads, are analyzed to determine the effects of mechanical stimulation on bone tissue structure and cellular function. Finally, the importance of vascularization in nutrient supply for bone healing and regeneration was further discussed.

A comprehensive review of computational and image analysis techniques for quantitative evaluation of striated muscle tissue architecture

Unbiased evaluation of morphology is crucial to understanding development, mechanics, and pathology of striated muscle tissues. Indeed, the ability of striated muscles to contract and the strength of their contraction is dependent on their tissue-, cellular-, and cytoskeletal-level organization. Accordingly, the study of striated muscles often requires imaging and assessing aspects of their architecture at multiple different spatial scales. While an expert may be able to qualitatively appraise tissues, it is imperative to have robust, repeatable tools to quantify striated myocyte morphology and behavior that can be used to compare across different labs and experiments. There has been a recent effort to define the criteria used by experts to evaluate striated myocyte architecture. In this review, we will describe metrics that have been developed to summarize distinct aspects of striated muscle architecture in multiple different tissues, imaged with various modalities. Additionally, we will provide an overview of metrics and image processing software that needs to be developed. Importantly to any lab working on striated muscle platforms, characterization of striated myocyte morphology using the image processing pipelines discussed in this review can be used to quantitatively evaluate striated muscle tissues and contribute to a robust understanding of the development and mechanics of striated muscles.

Cell reorientation on a cyclically strained substrate

Cyclic strain avoidance, the phenomenon of cell and cytoskeleton alignment perpendicular to the direction of cyclic strain of the underlying 2D substrate, is an important characteristic of the adherent cell organization. This alignment has typically been attributed to the stress-fiber reorganization although observations clearly show that stress-fiber reorganization under cyclic loading is closely coupled to cell morphology and reorientation of the cells. Here, we develop a statistical mechanics framework that couples the cytoskeletal stress-fiber organization with cell morphology under imposed cyclic straining and make quantitative comparisons with observations. The framework accurately predicts that cyclic strain avoidance stems primarily from cell reorientation away from the cyclic straining rather than cytoskeletal reorganization within the cell. The reorientation of the cell is a consequence of the cell lowering its free energy by largely avoiding the imposed cyclic straining. Furthermore, we investigate the kinetics of the cyclic strain avoidance mechanism and demonstrate that it emerges primarily due to the rigid body rotation of the cell rather than via a trajectory involving cell straining. Our results provide clear physical insights into the coupled dynamics of cell morphology and stress-fibers, which ultimately leads to cellular organization in cyclically strained tissues.

Mechanical stimulation induces vasa vasorum capillary alignment in a fibrin-based tunica adventitia

Generation of bioartificial blood vessels with a physiological three-layered wall architecture is a long pursued goal in vascular tissue engineering. While considerable advances have been made to resemble the physiological tunica intima and media morphology and function in bioartificial vessels, only very few studies have targeted the generation of a tunica adventitia including its characteristic vascular network known as the vasa vasorum, which are essential for graft nutrition and integration. In healthy native blood vessels, capillary vasa vasorum are aligned longitudinally to the vessel axis. Thus, inducing longitudinal alignment of capillary tubes to generate a physiological tunica adventitia morphology and function may be advantageous in bioengineered vessels as well. In this study, we investigated the effect of two biomechanical stimulation parameters, longitudinal tension and physiological cyclic stretch, on tube alignment in capillary networks formed by self-assembly of human umbilical vein endothelial cells in tunica adventitia-equivalents of fibrin-based bioartificial blood vessels. Moreover, the effect of changes of the biomechanical environment on network remodeling after initial tube formation was analyzed. Both, longitudinal tension and cyclic stretch by pulsatile perfusion induced physiological capillary tube alignment parallel to the longitudinal vessel axis. This effect was even more pronounced when both biomechanical factors were applied simultaneously, which resulted in alignment of 57.2% ± 5.2% within 5° of the main vessel axis. Opposed to that, random tube orientation was observed in vessels incubated statically. Scanning electron microscopy showed that longitudinal tension also resulted in longitudinal alignment of fibrin fibrils, which may function as a guidance structure for directed capillary tube formation. Moreover, existing microvascular networks showed distinct remodeling in response to addition or withdrawal of mechanical stimulation with corresponding increase or decrease of the degree of alignment. With longitudinal tension and cyclic stretch, we identified two mechanical stimuli that facilitate the generation of a pre-vascularized tunica adventitia-equivalent with physiological tube alignment in bioartificial vascular grafts.

A Mechanistic Motor-Clutch Model That Explains Cell Shape Dynamics to Cyclic Stretch

Many cells in the body experience cyclic mechanical loading, which can impact cellular processes and morphology. In vitro studies often report that cells reorient in response to cyclic stretch of their substrate. To explore cellular mechanisms involved in this reorientation, a computational model was developed by adapting previous computational models of the actin–myosin–integrin motor-clutch system developed by others. The computational model predicts that under most conditions, actin bundles align perpendicular to the direction of applied cyclic stretch, but under specific conditions, such as low substrate stiffness, actin bundles align parallel to the direction of stretch. The model also predicts that stretch frequency impacts the rate of reorientation and that proper myosin function is critical in the reorientation response. These computational predictions are consistent with reports from the literature and new experimental results presented here. The model suggests that the impact of different stretching conditions (stretch type, amplitude, frequency, substrate stiffness, etc.) on the direction of cell alignment can largely be understood by considering their impact on cell–substrate detachment events, specifically whether detachments preferentially occur during stretching or relaxing of the substrate.

Cyclic stretching-induced epithelial cell reorientation is driven by microtubule-modulated transverse extension during the relaxation phase

Many types of adherent cells are known to reorient upon uniaxial cyclic stretching perpendicularly to the direction of stretching to facilitate such important events as wound healing, angiogenesis, and morphogenesis. While this phenomenon has been documented for decades, the underlying mechanism remains poorly understood. Using an on-stage stretching device that allowed programmable stretching with synchronized imaging, we found that the reorientation of NRK epithelial cells took place primarily during the relaxation phase when cells underwent rapid global retraction followed by extension transverse to the direction of stretching. Inhibition of myosin II caused cells to orient along the direction of stretching, whereas disassembly of microtubules enhanced transverse reorientation. Our results indicate distinct roles of stretching and relaxation in cell reorientation and implicate a role of myosin II-dependent contraction via a microtubule-modulated mechanism. The importance of relaxation phase also explains the difference between the responses to cyclic and static stretching.

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.

Using Wet Electrospun PCL/Gelatin/CNT Yarns to Fabricate Textile-Based Scaffolds for Vascular Tissue Engineering

Incorporating conductive materials in scaffolds has shown advantages in regulating adhesion, mitigation, and proliferation of electroactive cells for tissue engineering applications. Among various conductive materials, carbon nanotubes (CNTs) have shown great promises in tissue engineering because of their good mechanical properties. However, the broad application of CNTs in tissue engineering is limited by current methods to incorporate CNTs in polymers that require miscible solvents to dissolve CNTs and polymers or CNT surface modification. These methods either limit polymer selections or adversely affect the properties of polymer/CNT composites. Here, we report a novel method to fabricate polymer/CNT composite yarns by electrospinning polycaprolactone/gelatin into a bath of CNT dispersion and extracting electrospun fibers out of the bath. The concentration of CNTs in the bath affects the thermal and mechanical properties and the yarns' degradation behavior. In vitro biological test results show that within a limited range of CNT concentrations in the bath, the yarns exhibit good biocompatibility and the ability to guide cell elongation and alignment. We also report the design and fabrication of a vascular scaffold by knitting the yarns into a textile fabric and combining the textile fabric with gelatin. The scaffold has similar mechanical properties to native vessels and supports cell proliferation. This work demonstrates that the wet electrospun polymer/CNT yarns are good candidates for constructing vascular scaffolds and provides a novel method to incorporate CNTs or other functional materials into biopolymers for tissue engineering applications.

Actin polymerization downstream of integrins: signaling pathways and mechanotransduction

A cell constantly adapts to its environment. Cell decisions to survive, to proliferate or to migrate are dictated not only by soluble growth factors, but also through the direct interaction of the cell with the surrounding extracellular matrix (ECM). Integrins and their connections to the actin cytoskeleton are crucial for monitoring cell attachment and the physical properties of the substratum. Cell adhesion dynamics are modulated in complex ways by the polymerization of branched and linear actin arrays, which in turn reinforce ECM-cytoskeleton connection. This review describes the major actin regulators, Ena/VASP proteins, formins and Arp2/3 complexes, in the context of signaling pathways downstream of integrins. We focus on the specific signaling pathways that transduce the rigidity of the substrate and which control durotaxis, i.e. directed migration of cells towards increased ECM rigidity. By doing so, we highlight several recent findings on mechanotransduction and put them into a broad integrative perspective that is the result of decades of intense research on the actin cytoskeleton and its regulation.

Characterizing the cellular architecture of dynamically remodeling vascular tissue using 3-D image analysis and virtual reconstruction

Epithelial tubules form critical structures in lung, kidney, and vascular tissues. However, the processes that control their morphogenesis and physiological expansion and contraction are not well understood. Here we examine the dynamic remodeling of epithelial tubes in vivo using a novel model system: the extracorporeal vasculature of Botryllus schlosseri, in which the disruption of the basement membrane triggers rapid, massive vascular retraction without loss of barrier function. We developed and implemented 3-D image analysis and virtual reconstruction tools to characterize the cellular morphology of the vascular wall in unmanipulated vessels and during retraction. In both control and regressed conditions, cells within the vascular wall were planar polarized, with an integrin- and curvature-dependent axial elongation of cells and a robust circumferential alignment of actin bundles. Surprisingly, we found no measurable differences in morphology between normal and retracting vessels under extracellular matrix (ECM) disruption. However, inhibition of integrin signaling through focal adhesion kinase inhibition caused disruption of cellular actin organization. Our results demonstrate that epithelial tubes can maintain tissue organization even during extreme remodeling events, but that the robust response to mechanical signals—such as the response to loss of vascular tension after ECM disruption—requires functional force sensing machinery via integrin signaling.

Biofabrication Strategies and Engineered In Vitro Systems for Vascular Mechanobiology

The vascular system is integral for maintaining organ-specific functions and homeostasis. Dysregulation in vascular architecture and function can lead to various chronic or acute disorders. Investigation of the role of the vascular system in health and disease has been accelerated through the development of tissue-engineered constructs and microphysiological on-chip platforms. These in vitro systems permit studies of biochemical regulation of vascular networks and parenchymal tissue and provide mechanistic insights into the biophysical and hemodynamic forces acting in organ-specific niches. Detailed understanding of these forces and the mechanotransductory pathways involved is necessary to develop preventative and therapeutic strategies targeting the vascular system. This review describes vascular structure and function, the role of hemodynamic forces in maintaining vascular homeostasis, and measurement approaches for cell and tissue level mechanical properties influencing vascular phenomena. State-of-the-art techniques for fabricating in vitro microvascular systems, with varying degrees of biological and engineering complexity, are summarized. Finally, the role of vascular mechanobiology in organ-specific niches and pathophysiological states, and efforts to recapitulate these events using in vitro microphysiological systems, are explored. It is hoped that this review will help readers appreciate the important, but understudied, role of vascular-parenchymal mechanotransduction in health and disease toward developing mechanotherapeutics for treatment strategies.

Scaling up single-cell mechanics to multicellular tissues - the role of the intermediate filament-desmosome network

Cells and tissues sense, respond to and translate mechanical forces into biochemical signals through mechanotransduction, which governs individual cell responses that drive gene expression, metabolic pathways and cell motility, and determines how cells work together in tissues. Mechanotransduction often depends on cytoskeletal networks and their attachment sites that physically couple cells to each other and to the extracellular matrix. One way that cells associate with each other is through Ca2+-dependent adhesion molecules called cadherins, which mediate cell-cell interactions through adherens junctions, thereby anchoring and organizing the cortical actin cytoskeleton. This actin-based network confers dynamic properties to cell sheets and developing organisms. However, these contractile networks do not work alone but in concert with other cytoarchitectural elements, including a diverse network of intermediate filaments. This Review takes a close look at the intermediate filament network and its associated intercellular junctions, desmosomes. We provide evidence that this system not only ensures tissue integrity, but also cooperates with other networks to create more complex tissues with emerging properties in sensing and responding to increasingly stressful environments. We will also draw attention to how defects in intermediate filament and desmosome networks result in both chronic and acquired diseases.

The nature and intensity of mechanical stimulation drive different dynamics of MRTF-A nuclear redistribution after actin remodeling in myoblasts

Serum response factor and its cofactor myocardin-related transcription factor (MRTF) are key elements of muscle-mass adaptation to workload. The transcription of target genes is activated when MRTF is present in the nucleus. The localization of MRTF is controlled by its binding to G-actin. Thus, the pathway can be mechanically activated through the mechanosensitivity of the actin cytoskeleton. The pathway has been widely investigated from a biochemical point of view, but its mechanical activation and the timescales involved are poorly understood. Here, we applied local and global mechanical cues to myoblasts through two custom-built set-ups, magnetic tweezers and stretchable substrates. Both induced nuclear accumulation of MRTF-A. However, the dynamics of the response varied with the nature and level of mechanical stimulation and correlated with the polymerization of different actin sub-structures. Local repeated force induced local actin polymerization and nuclear accumulation of MRTF-A by 30 minutes, whereas a global static strain induced both rapid (minutes) transient nuclear accumulation, associated with the polymerization of an actin cap above the nucleus, and long-term accumulation, with a global increase in polymerized actin. Conversely, high strain induced actin depolymerization at intermediate times, associated with cytoplasmic MRTF accumulation.

Unbiased pattern analysis reveals highly diverse responses of cytoskeletal systems to cyclic straining

In mammalian cells, actin, microtubules, and various types of cytoplasmic intermediate filaments respond to external stretching. Here, we investigated the underlying processes in endothelial cells plated on soft substrates from silicone elastomer. After cyclic stretch (0.13 Hz, 14% strain amplitude) for periods ranging from 5 min to 8 h, cells were fixed and double-stained for microtubules and either actin or vimentin. Cell images were analyzed by a two-step routine. In the first step, micrographs were segmented for potential fibrous structures. In the second step, the resulting binary masks were auto- or cross-correlated. Autocorrelation of segmented images provided a sensitive and objective measure of orientational and translational order of the different cytoskeletal systems. Aligning of correlograms from individual cells removed the influence of only partial alignment between cells and enabled determination of intrinsic cytoskeletal order. We found that cyclic stretching affected the actin cytoskeleton most, microtubules less, and vimentin mostly only via reorientation of the whole cell. Pharmacological disruption of microtubules had barely any influence on actin ordering. The similarity, i.e., cross-correlation, between vimentin and microtubules was much higher than the one between actin and microtubules. Moreover, prolonged cyclic stretching slightly decoupled the cytoskeletal systems as it reduced the cross-correlations in both cases. Finally, actin and microtubules were more correlated at peripheral regions of cells whereas vimentin and microtubules correlated more in central regions.

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.

Modeling the mechanosensitivity of fast-crawling cells on cyclically stretched substrates

The mechanosensitivity of cells, which determines how they are able to respond to mechanical signals, is crucial for the functioning of biological systems. Experimentally, this is investigated by studying the reorientation of cells on cyclically stretched substrates. The reorientation depends on the type of cell and on the stretching protocol, but the mechanisms responsible for the response are still not completely understood. Here, we introduce a computational model for fast crawling cells on cyclically stretched substrates that accounts for the sub-cellular elements responsible for cell shape and motility. This includes the dynamics of the cell membrane, the actin cytoskeleton, and the focal adhesions with the stretching substrate. These processes evolve over characteristic time scales that can vary by orders of magnitude and naturally give rise to the frequency dependent reorientation observed experimentally. Depending on which processes are being probed by the stretching and on the type of coupling with the substrate, our simulations predict either no reorientation, a bi-stability in the parallel and perpendicular directions, or a complete reorientation in either the parallel or perpendicular direction. In particular, we show that an asymmetry in the adhesion dynamics during the loading and unloading phases of the stretching, whether it comes from the response of the cell itself or from the precise stretching protocol, can be used to selectively align the cells. Our results provide further evidence for the importance of focal adhesion dynamics in determining the mechanosensitive response of cells, as well as a way to interpret recent experiments.

Multifactorial bottom-up bioengineering approaches for the development of living tissue substitutes

Bottom-up bioengineering utilizes the inherent capacity of cells to build highly sophisticated structures with high levels of biomimicry. Despite the significant advancements in the field, monodomain approaches require prolonged culture time to develop an implantable device, usually associated with cell phenotypic drift in culture. Herein, we assessed the simultaneous effect of macromolecular crowding (MMC) and mechanical loading in enhancing extracellular matrix (ECM) deposition while maintaining tenocyte (TC) phenotype and differentiating bone marrow stem cells (BMSCs) or transdifferentiating neonatal and adult dermal fibroblasts toward tenogenic lineage. At d 7, all cell types presented cytoskeleton alignment perpendicular to the applied load independently of the use of MMC. MMC enhanced ECM deposition in all cell types. Gene expression analysis indicated that MMC and mechanical loading maintained TC phenotype, whereas tenogenic differentiation of BMSCs or transdifferentiation of dermal fibroblasts was not achieved. Our data suggest that multifactorial bottom-up bioengineering approaches significantly accelerate the development of biomimetic tissue equivalents.-Gaspar, D., Ryan, C. N. M., Zeugolis, D. I. Multifactorial bottom-up bioengineering approaches for the development of living tissue substitutes.

Oscillatory Strain Promotes Vessel Stabilization and Alignment through Fibroblast YAP-Mediated Mechanosensitivity

Endothelial cells form the interior layer of blood vessels and, as such, are constantly exposed to shear stress and mechanical strain. While the impact of shear stress on angiogenesis is widely studied, the role of mechanical strain is less understood. To this end, endothelial cells and fibroblasts are cocultured under oscillatory strain to create a vessel network. The two cell types show distinctly different sensitivities to the mechanical stimulation. The fibroblasts, sense the stress directly, and respond by increased alignment, proliferation, differentiation, and migration, facilitated by YAP translocation into the nucleus. In contrast, the endothelial cells form aligned vessels by tracking fibroblast alignment. YAP inhibition in constructs under mechanical strain results in vessel destruction whereas less damage is observed in the YAP-inhibited static control. Moreover, the mechanical stimulation enhances vessel development and stabilization. Additionally, vessel orientation is preserved upon implantation into a mouse dorsal window chamber and promotes the invading host vessels to orient in the same manner. This study sheds light on the mechanisms by which mechanical strain affects the development of blood vessels within engineered tissues. This can be further utilized to engineer a more organized and stable vasculature suitable for transplantation of engineered grafts.

Cellular orientation is guided by strain gradients

The strain-induced reorientation response of cyclically stretched cells has been well characterized in uniform strain fields. In the present study, we comprehensively analyse the behaviour of human fibroblasts subjected to a highly non-uniform strain field within a polymethylsiloxane microdevice. Our results indicate that the strain gradient amplitude and direction regulate cell reorientation through a coordinated gradient avoidance response. We provide critical evidence that strain gradient is a key physical cue that can guide cell organization. Specifically, our work suggests that cells are able to pinpoint the location under the cell of multiple physical cues and integrate this information (strain and strain gradient amplitudes and directions), resulting in a coordinated response. To gain insight into the underlying mechanosensing processes, we studied focal adhesion reorganization and the effect of modulating myosin-II contractility. The extracted focal adhesion orientation distributions are similar to those obtained for the cell bodies, and their density is increased by the presence of stretching forces. Moreover, it was found that the myosin-II activity promoter calyculin-A has little effect on the cellular response, while the inhibitor blebbistatin suppresses cell and focal adhesion alignment and reduces focal adhesion density. These results confirm that similar internal structures involved in sensing and responding to strain direction and amplitude are also key players in strain gradient mechanosensing and avoidance.

Active Traction Force Response to Long-Term Cyclic Stretch Is Dependent on Cell Pre-stress

Mechanical stimulation is recognized as a potent modulator of cellular behaviors such as proliferation, differentiation, and extracellular matrix assembly. However, the study of how cell-generated traction force changes in response to stretch is generally limited to short-term stimulation. The goal of this work is to determine how cells actively alter their traction force in response to long-term physiological cyclic stretch as a function of cell pre-stress. We have developed, to our knowledge, a novel method to assess traction force after long-term (24 h) uniaxial or biaxial cyclic stretch under conditions of high cell pre-stress with culture on stiff (7.5 kPa) polyacrylamide gels (with or without transforming growth factor β1 (TGF-β1)) and low pre-stress by treating with blebbistatin or culture on soft gels (0.6 kPa). In response to equibiaxial stretch, valvular interstitial cells on stiff substrates decreased their traction force (from 300 nN to 100 nN) and spread area (from 3000 to 2100 μm(2)). With uniaxial stretch, the cells had similar decreases in traction force and area and reoriented perpendicular to the stretch. TGF-β1-treated valvular interstitial cells had higher pre-stress (1100 nN) and exhibited a larger drop in traction force with uniaxial stretch, but the percentage changes in force and area with stretch were similar to the non-TGF-β1-treated group. Cells with inhibited myosin II motors increased traction force (from 41 nN to 63 nN) and slightly reoriented toward the stretch direction. In contrast, cells cultured on soft gels increased their traction force significantly, from 15 nN to 45 nN, doubled their spread area, elongated from an initially rounded morphology, and reoriented perpendicular to the uniaxial stretch. Contractile-moment measurements provided results consistent with total traction force measurements. The combined results indicate that the change in traction force in response to external cyclic stretch is dependent upon the initial cell pre-stress. This finding is consistent with depolymerization of initially high-tension actin stress fibers, and reinforcement of an initially low-tension actin cytoskeleton.

Biomechanics of cell reorientation in a three-dimensional matrix under compression

Although a number of studies have reported that cells cultured on a stretchable substrate align away from or perpendicular to the stretch direction, how cells sense and respond to compression in a three-dimensional (3D) matrix remains an open question. We analyzed the reorientation of human prostatic normal tissue fibroblasts (NAFs) and cancer-associated fibroblasts (CAFs) in response to 3D compression using a Fast Fourier Transform (FFT) method. Results show that NAFs align to specific angles upon compression while CAFs exhibit a random distribution. In addition, NAFs with enhanced contractile force induced by transforming growth factor β (TGF-β) behave in a similar way as CAFs. Furthermore, a theoretical model based on the minimum energy principle has been developed to provide insights into these observations. The model prediction is in agreement with the observed cell orientation patterns in several different experimental conditions, disclosing the important role of stress fibers and inherent cell contractility in cell reorientation.

Mechanical and Vascular Cues Synergistically Enhance Osteogenesis in Human Mesenchymal Stem Cells

Development and maintenance of a vascular network are critical for bone growth and homeostasis; strategies that promote vascular function are critical for clinical success of tissue-engineered bone constructs. Co-culture of endothelial cells (ECs) with mesenchymal stem cells (MSCs) and exposure to 10% cyclic tensile strain have both been shown to regulate osteogenesis in isolation, but potential synergistic effects have yet to be explored. The objective of this study was to expose an MSC-EC co-culture to 10% cyclic tensile strain to examine the role of this mechanical stimulus on MSC-EC behavior. We hypothesized that paracrine signaling from ECs would stimulate osteogenesis of MSCs, and exposure to 10% cyclic tensile strain would enhance this anabolic signal. Human umbilical vein ECs and human bone marrow-derived MSCs were either monocultured or co-cultured at a 1:1 ratio in a mixed osteo/angiogenic medium, exposed to 10% cyclic tensile strain at 1 Hz for 4 h/day for 2 weeks, and biochemically and histologically analyzed for endothelial and osteogenic markers. While neither 10% cyclic tensile strain nor co-culture alone had a significant effect on osteogenesis, the concurrent application of strain to an MSC-EC co-culture resulted in a significant increase in calcium accretion and mineral deposition, suggesting that co-culture and strain synergistically enhance osteogenesis. Neither co-culture, 10% cyclic tensile strain, nor a combination of these stimuli affected endothelial markers, indicating that the endothelial phenotype remained stable, but unresponsive to the stimuli evaluated in this study. This study is the first to investigate the role of cyclic tensile strain on the complex interplay between ECs and MSCs in co-culture. The results of this study provide key insights into the synergistic effects of 10% cyclic tensile strain and co-culture on osteogenesis. Understanding mechanobiological factors affecting MSC-EC crosstalk will help enhance strategies for creating vascularized tissues in tissue engineering and regenerative medicine.

Stimulatory effect of cobalt ions incorporated into calcium phosphate coatings on neovascularization in an in vivo intramuscular model in goats

Rapid vascularization of bone graft substitutes upon implantation is one of the most important challenges to overcome in order to achieve successful regeneration of large, critical-size bone defects. One strategy for stimulating vascularization during the regeneration process is to create a hypoxic microenvironment by either directly lowering the local oxygen tension, or by applying hypoxia-mimicking factors. Cells compensate for the hypoxic condition by releasing angiogenic factors leading to new blood vessel formation. In the present study, we explored the potential of cobalt ions (Co(2+)), known chemical mimickers of hypoxia, to stimulate vascularization within a bone graft substitute in vivo. To this end, Co(2+) ions were incorporated into calcium phosphate (CaPs) coatings deposited on poly(lactic acid) (PLA) particles with their effect on the formation of new blood vessels studied upon intramuscular implantation in goats. PLA particles and CaP-coated particles without Co(2+) ions served as controls. Pathological scoring of the inflammatory response following a 12-week implantation period showed no significant differences between the four types of materials. Based on histological and immunohistochemical analyses, both blood vessel area and number of blood vessels in CaP-coated PLA particles containing Co(2+) were higher than in the uncoated PLA particles and CaP-coated PLA particles without Co(2+). Analysis of blood vessel size distribution indicated abundant formation of small blood vessels in all the samples, while large blood vessels were predominantly found in PLA particles coated with CaP containing Co(2+) ions. The results of this study support the use of CaPs containing Co(2+) ions to enhance vascularization in vivo.

STATEMENT OF SIGNIFICANCE

In this work, we have investigated the potential of cobalt ions, incorporated into thin calcium phosphate (CaP) coatings that were deposited on particles of poly(lactic acid) (PLA), to induce neovascularization in vivo. Qualitative and quantitative histological and immunohistochemical analyses showed that both the number of blood vessels and the total blood vessel area were higher in CaP-coated PLA particles containing cobalt ions as compared to the uncoated PLA particles and CaP-coated PLA particles without the metallic additive. Furthermore, a wider distribution of blood vessel sizes, varying from very small to large vessels was specifically observed in samples containing cobalt ions. This in vivo study will significantly contribute to the existing knowledge on the use of bioinorganics, which are simple and inexpensive inorganic factors that can be used to control relevant biological process during tissue regeneration, such as vascularization. As such, we are convinced that this manuscript will be of interest to the readers of Acta Biomaterialia.

Genetic variants and cellular stressors associated with exfoliation syndrome modulate promoter activity of a lncRNA within the LOXL1 locus

Exfoliation syndrome (XFS) is a common, age-related, systemic fibrillinopathy. It greatly increases risk of exfoliation glaucoma (XFG), a major worldwide cause of irreversible blindness. Coding variants in the lysyl oxidase-like 1 (LOXL1) gene are strongly associated with XFS in all studied populations, but a functional role for these variants has not been established. To identify additional candidate functional variants, we sequenced the entire LOXL1 genomic locus (∼40 kb) in 50 indigenous, black South African XFS cases and 50 matched controls. The variants with the strongest evidence of association were located in a well-defined 7-kb region bounded by the 3'-end of exon 1 and the adjacent region of intron 1 of LOXL1. We replicated this finding in US Caucasian (91 cases/1031 controls), German (771 cases/1365 controls) and Japanese (1484 cases/1188 controls) populations. The region of peak association lies upstream of LOXL1-AS1, a long non-coding RNA (lncRNA) encoded on the opposite strand of LOXL1. We show that this region contains a promoter and, importantly, that the strongly associated XFS risk alleles in the South African population are functional variants that significantly modulate the activity of this promoter. LOXL1-AS1 expression is also significantly altered in response to oxidative stress in human lens epithelial cells and in response to cyclic mechanical stress in human Schlemm's canal endothelial cells. Taken together, these findings support a functional role for the LOXL1-AS1 lncRNA in cellular stress response and suggest that dysregulation of its expression by genetic risk variants plays a key role in XFS pathogenesis.

Biomechanical strain induces elastin and collagen production in human pluripotent stem cell-derived vascular smooth muscle cells

Blood vessels are subjected to numerous biomechanical forces that work harmoniously but, when unbalanced because of vascular smooth muscle cell (vSMC) dysfunction, can trigger a wide range of ailments such as cerebrovascular, peripheral artery, and coronary artery diseases. Human pluripotent stem cells (hPSCs) serve as useful therapeutic tools that may help provide insight on the effect that such biomechanical stimuli have on vSMC function and differentiation. In this study, we aimed to examine the effect of biomechanical strain on vSMCs derived from hPSCs. The effects of two types of tensile strain on hPSC-vSMC derivatives at different stages of differentiation were examined. The derivatives included smooth muscle-like cells (SMLCs), mature SMLCs, and contractile vSMCs. All vSMC derivatives aligned perpendicularly to the direction of cyclic uniaxial strain. Serum deprivation and short-term uniaxial strain had a synergistic effect in enhancing collagen type I, fibronectin, and elastin gene expression. Furthermore, long-term uniaxial strain deterred collagen type III gene expression, whereas long-term circumferential strain upregulated both collagen type III and elastin gene expression. Finally, long-term uniaxial strain downregulated extracellular matrix (ECM) expression in more mature vSMC derivatives while upregulating elastin in less mature vSMC derivatives. Overall, our findings suggest that in vitro application of both cyclic uniaxial and circumferential tensile strain on hPSC-vSMC derivatives induces cell alignment and affects ECM gene expression. Therefore, mechanical stimulation of hPSC-vSMC derivatives using tensile strain may be important in modulating the phenotype and thus the function of vSMCs in tissue-engineered vessels.

Device-Based In Vitro Techniques for Mechanical Stimulation of Vascular Cells: A Review

The most common cause of death in the developed world is cardiovascular disease. For decades, this has provided a powerful motivation to study the effects of mechanical forces on vascular cells in a controlled setting, since these cells have been implicated in the development of disease. Early efforts in the 1970 s included the first use of a parallel-plate flow system to apply shear stress to endothelial cells (ECs) and the development of uniaxial substrate stretching techniques (Krueger et al., 1971, “An in Vitro Study of Flow Response by Cells,” J. Biomech., 4(1), pp. 31–36 and Meikle et al., 1979, “Rabbit Cranial Sutures in Vitro: A New Experimental Model for Studying the Response of Fibrous Joints to Mechanical Stress,” Calcif. Tissue Int., 28(2), pp. 13–144). Since then, a multitude of in vitro devices have been designed and developed for mechanical stimulation of vascular cells and tissues in an effort to better understand their response to in vivo physiologic mechanical conditions. This article reviews the functional attributes of mechanical bioreactors developed in the 21st century, including their major advantages and disadvantages. Each of these systems has been categorized in terms of their primary loading modality: fluid shear stress (FSS), substrate distention, combined distention and fluid shear, or other applied forces. The goal of this article is to provide researchers with a survey of useful methodologies that can be adapted to studies in this area, and to clarify future possibilities for improved research methods.

Temporal responses of human endothelial and smooth muscle cells exposed to uniaxial cyclic tensile strain

The physiology of vascular cells depends on stimulating mechanical forces caused by pulsatile flow. Thus, mechano-transduction processes and responses of primary human endothelial cells (ECs) and smooth muscle cells (SMCs) have been studied to reveal cell-type specific differences which may contribute to vascular tissue integrity. Here, we investigate the dynamic reorientation response of ECs and SMCs cultured on elastic membranes over a range of stretch frequencies from 0.01 to 1 Hz. ECs and SMCs show different cell shape adaptation responses (reorientation) dependent on the frequency. ECs reveal a specific threshold frequency (0.01 Hz) below which no responses is detectable while the threshold frequency for SMCs could not be determined and is speculated to be above 1 Hz. Interestingly, the reorganization of the actin cytoskeleton and focal adhesions system, as well as changes in the focal adhesion area, can be observed for both cell types and is dependent on the frequency. RhoA and Rac1 activities are increased for ECs but not for SMCs upon application of a uniaxial cyclic tensile strain. Analysis of membrane protrusions revealed that the spatial protrusion activity of ECs and SMCs is independent of the application of a uniaxial cyclic tensile strain of 1 Hz while the total number of protrusions is increased for ECs only. Our study indicates differences in the reorientation response and the reaction times of the two cell types in dependence of the stretching frequency, with matching data for actin cytoskeleton, focal adhesion realignment, RhoA/Rac1 activities, and membrane protrusion activity. These are promising results which may allow cell-type specific activation of vascular cells by frequency-selective mechanical stretching. This specific activation of different vascular cell types might be helpful in improving strategies in regenerative medicine.

Polarized actin structural dynamics in response to cyclic uniaxial stretch

Endothelial cell (EC) alignment to directional flow or stretch supports anti-inflammatory functions, but mechanisms controlling polarized structural adaptation in response to physical cues remain unclear. This study aimed to determine whether factors associated with early actin edge ruffling implicated in cell polarization are prerequisite for stress fiber (SF) reorientation in response to cyclic uniaxial stretch. Time-lapse analysis of EGFP-actin in confluent ECs showed that onset of either cyclic uniaxial or equibiaxial stretch caused a non-directional increase in edge ruffling. Edge activity was concentrated in a direction perpendicular to the stretch axis after 60 min, consistent with the direction of SF alignment. Rho-kinase inhibition caused reorientation of both stretch-induced edge ruffling and SF alignment parallel to the stretch axis. Arp2/3 inhibition attenuated stretch-induced cell elongation and disrupted polarized edge dynamics and microtubule organizing center reorientation, but it had no effect on the extent of SF reorientation. Disrupting localization of p21-activated kinase (PAK) did not prevent stretch-induced SF reorientation, suggesting that this Rac effector is not critical in regulating stretch-induced cytoskeletal remodeling. Overall, these results suggest that directional edge ruffling is not a primary mechanism that guides SF reorientation in response to stretch; the two events are coincident but not causal.

Mechano-transduction: from molecules to tissues

Biological mechano-transduction and force-dependent changes scale from protein conformation (â„« to nm) to cell organization and multi-cell function (mm to cm) to affect cell organization, fate, and homeostasis.

External forces play complex roles in cell organization, fate, and homeostasis. Changes in these forces, or how cells respond to them, can result in abnormal embryonic development and diseases in adults. How cells sense and respond to these mechanical stimuli requires an understanding of the biophysical principles that underlie changes in protein conformation and result in alterations in the organization and function of cells and tissues. Here, we discuss mechano-transduction as it applies to protein conformation, cellular organization, and multi-cell (tissue) function.

The interplay of cyclic stretch and vascular endothelial growth factor in regulating the initial steps for angiogenesis

Angiogenesis is regulated by chemical and mechanical factors in vivo. The regulatory role of mechanical factors and how chemical and mechanical angiogenic regulators work in concert remains to be explored. We investigated the effect of cyclic uniaxial stretch (20%, 1 Hz), with and without the stimulation of vascular endothelial growth factor (VEGF), on sprouting angiogenesis by employing a stretchable three-dimensional cell culture model. When compared to static controls, stretch alone significantly increased the density of endothelial sprouts, and these sprouts aligned perpendicular to the direction of stretch. The Rho-associated kinase (ROCK) inhibitor Y27632 suppressed stretch-induced sprouting angiogenesis and associated sprout alignment. While VEGF is a potent angiogenic stimulus through ROCK-dependent pathways, the combination of VEGF and stretch did not have an additive effect on angiogenesis. In the presence of VEGF stimulation, the ROCK inhibitor suppressed stretch-induced sprout alignment but did not affect stretch-induced sprout density; in contrast, the receptor tyrosine kinase (RTK) inhibitor sunitinib had no effect on stretch-induced alignment but trended toward suppressed stretch-induced sprout density. Our results suggest that the formation of sprouts and their directionality do not have completely identical regulatory pathways, and thus it is possible to separately manipulate the number and pattern of new sprouts.

Differential effects of cyclic stretch on bFGF- and VEGF-induced sprouting angiogenesis

How mechanical factors affect angiogenesis and how they and chemical angiogenic factors work in concert remain not yet well-understood. This study investigated the interactive effects of cyclic uniaxial stretch and two potent proangiogenic molecules [basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF)] on angiogenesis using a stretchable three-dimensional (3-D) cell culture model. Endothelial cells seeded atop a 3-D collagen gel underwent sprouting angiogenesis while being subjected to either 10 or 20% cyclic uniaxial stretch at a frequency of either 1/12 or 1 Hz, in conjunction with an elevated concentration of bFGF or VEGF. Without the presence of additional growth factors, 10 and 20% stretch at 1 Hz induced angiogenesis and the perpendicular alignment of new sprouts, and both inductive effects were abolished by cytochalasin D (an actin polymerization inhibitor). While "10% stretch at 1 Hz," "20% stretch at 1 Hz," bFGF, and VEGF were strong angiogenesis stimulants individually, only the combination of "20% stretch at 1 Hz" and bFGF had an additive effect on inducing new sprouts. Interestingly, the combination of "20% stretch at a lower frequency (1/12 Hz)" and bFGF decreased sprouting angiogenesis, even though the level of perpendicular alignment of new sprouts was the same for both stretch frequencies. Taken together, these results demonstrate that both stretch frequency and magnitude, along with interactions with various growth factors, are essential in mediating formation of endothelial sprouts and vascular patterning. Furthermore, work in this area is warranted to elucidate synergistic or competitive signaling mechanisms.

Mechanosensitive properties in the endothelium and their roles in the regulation of endothelial function

: Vascular endothelial cells (ECs) line the luminal surface of blood vessels, which are exposed constantly to mechanical stimuli, such as fluid shear stress, cyclic strain, and blood pressure. In recent years, more and more evidence indicates that ECs sense these mechanical stimuli and subsequently convert mechanical stimuli into intracellular signals. The properties of ECs that sense the mechanical stimuli are defined as mechanosensors. There are a variety of mechanosensors that have been identified in ECs. These mechanosensors play an important role in regulating the function of the endothelium and vascular function, including blood pressure. This review focuses on the mechanosensors that have been identified in ECs and on the roles that mechanosensors play in the regulation of endothelium function, and in the regulation of vascular function.

Physiological cyclic strain promotes endothelial cell survival via the induction of heme oxygenase-1

Endothelial cells (ECs) are constantly subjected to cyclic strain that arises from periodic change in vessel wall diameter as a result of pulsatile blood flow. Application of physiological levels of cyclic strain inhibits EC apoptosis; however, the underlying mechanism is not known. Since heme oxygenase-1 (HO-1) is a potent inhibitor of apoptosis, the present study investigated whether HO-1 contributes to the antiapoptotic action of cyclic strain. Administration of physiological cyclic strain (6% at 1 Hz) to human aortic ECs stimulated an increase in HO-1 activity, protein, and mRNA expression. The induction of HO-1 was preceded by a rise in reactive oxygen species (ROS) and Nrf2 protein expression. Cyclic strain also stimulated an increase in HO-1 promoter activity that was prevented by mutating the antioxidant responsive element in the promoter or by overexpressing dominant-negative Nrf2. In addition, the strain-mediated induction of HO-1 and activation of Nrf2 was abolished by the antioxidant N-acetyl-l-cysteine. Finally, application of cyclic strain blocked inflammatory cytokine-mediated EC death and apoptosis. However, the protective action of cyclic strain was reversed by the HO inhibitor tin protoporphyrin-IX and was absent in ECs isolated from HO-1-deficient mice. In conclusion, the present study demonstrates that a hemodynamically relevant level of cyclic strain stimulates HO-1 gene expression in ECs via the ROS-Nrf2 signaling pathway to inhibit EC death. The ability of cyclic strain to induce HO-1 expression may provide an important mechanism by which hemodynamic forces promote EC survival and vascular homeostasis.

Effects of extracellular pressure and alcohol on the microglial response to inflammatory stimulation

BACKGROUND

Traumatic brain injury induces a neuroinflammatory response frequently associated with increased intracranial pressure. The aim of this study was to investigate the effects of alcohol and increased extracellular pressure on murine BV-2 microglial proliferation and cytokine responses to lipopolysaccharide (LPS) stimulation.

METHODS

BV-2 cells were cultured under 0 or 30 mm Hg increased extracellular pressure without or with ethanol (100 mmol/L) or LPS (10 ng/mL) for 24 hours. Cell proliferation was assessed using MTS assay and secretion of the proinflammatory cytokines tumor necrosis factor (TNF)-α, interleukin (IL)-6, and monocyte chemotactic protein (MCP)-1 by enzyme-linked immunosorbent assay.

RESULTS

Increased pressure and LPS stimulation each promoted proliferation. Ethanol pretreatment blocked these effects. Basal TNF-α and IL-6 secretion was at the limits of delectability. Basal MCP-1 production was stimulated by pressure, which was blocked by ethanol. Even this low LPS dose stimulated microglial secretion of TNF-α, IL-6, and MCP-1. Pressure inhibited LPS-stimulated production of these proinflammatory cytokines, while ethanol pretreatment blocked LPS-stimulated cytokine production. The combination of pressure and ethanol further reduced TNF-α, IL-6, and MCP-1 secretion by LPS-stimulated microglial cells.

CONCLUSION

Alcohol's anti-inflammatory effects may contribute to the reduced mortality from traumatic brain injury that some have described in acutely intoxicated patients, while pressure down-regulation of inflammatory cytokine release could create a negative feedback that ameliorates inflammation in traumatic brain injury.

Cyclic stretch reveals a mechanical role for intermediate filaments in a desminopathic cell model

Mechanics is now recognized as crucial in cell function. To date, the mechanical properties of cells have been inferred from experiments which investigate the roles of actin and microtubules ignoring the intermediate filaments (IFs) contribution. Here, we analyse myoblasts behaviour in the context of myofibrillar myopathy resulting from p.D399Y desmin mutation which disorganizes the desmin IF network in muscle cells. We compare the response of myoblasts expressing either mutated or wild-type desmin to cyclic stretch. Cells are cultivated on supports submitted to periodic uniaxial stretch of 20% elongation amplitude and 0.3 Hz frequency. We show that during stretching cycles, cells expressing mutated desmin reduce their mean amplitude both for the elongation and spreading area compared to those expressing wild-type desmin. Even more unexpected, the reorientation angles are altered in the presence of p.D399Y desmin. Yet, at rest, the whole set of those parameters are similar for the two cell populations. Thus, we demonstrate that IFs affect the mechanical properties and the dynamics of cell reorientation. Since these processes are known due to actin cytoskeleton, these results suggest the IFs implication in mechanics signal transduction. Further studies may lead to better understanding of their contribution to this process.

Cyclic stretch induces cell reorientation on substrates by destabilizing catch bonds in focal adhesions

A minimal model of cellular mechanosensing system that consists of a single stress fiber adhering on a substrate via two focal adhesions made of catch bonds is adopted to investigate the phenomena of cell reorientation on substrates induced by an applied uniaxial cyclic stretch. The model indicates that the catch bonds in the focal adhesions experience a periodically oscillating internal force with amplitude and frequency controlled by two intrinsic clocks of the stress fiber, one associated with localized activation and the other with homogeneous activation of sarcomere units along the stress fiber. It is shown that this oscillating force due to cyclic stretch tends to destabilize focal adhesions by reducing the lifetime of catch bonds. The resulting slide or relocation of focal adhesions then causes the associated stress fiber to shorten and rotate to configurations nearly perpendicular to the stretching direction. These predicted behaviors from our model are consistent with a wide range of experimental observations.

Microsystems for biomimetic stimulation of cardiac cells

The heart is a complex integrated system that leverages mechanoelectrical signals to synchronize cardiomyocyte contraction and push blood throughout the body. The correct magnitude, timing, and distribution of these signals is critical for proper functioning of the heart; aberrant signals can lead to acute incidents, long-term pathologies, and even death. Due to the heart's limited regenerative capacity and the wide variety of pathologies, heart disease is often studied in vitro. However, it is difficult to accurately replicate the cardiac environment outside of the body. Studying the biophysiology of the heart in vitro typically consists of studying single cells in a tightly controlled static environment or whole tissues in a complex dynamic environment. Micro-electromechanical systems (MEMS) allow us to bridge these two extremes by providing increasing complexity for cell culture without having to use a whole tissue. Here, we carefully describe the electromechanical environment of the heart and discuss MEMS specifically designed to replicate these stimulation modes. Strengths, limitations and future directions of various designs are discussed for a variety of applications.

Vascularized bone tissue engineering: approaches for potential improvement

Significant advances have been made in bone tissue engineering (TE) in the past decade. However, classical bone TE strategies have been hampered mainly due to the lack of vascularization within the engineered bone constructs, resulting in poor implant survival and integration. In an effort toward clinical success of engineered constructs, new TE concepts have arisen to develop bone substitutes that potentially mimic native bone tissue structure and function. Large tissue replacements have failed in the past due to the slow penetration of the host vasculature, leading to necrosis at the central region of the engineered tissues. For this reason, multiple microscale strategies have been developed to induce and incorporate vascular networks within engineered bone constructs before implantation in order to achieve successful integration with the host tissue. Previous attempts to engineer vascularized bone tissue only focused on the effect of a single component among the three main components of TE (scaffold, cells, or signaling cues) and have only achieved limited success. However, with efforts to improve the engineered bone tissue substitutes, bone TE approaches have become more complex by combining multiple strategies simultaneously. The driving force behind combining various TE strategies is to produce bone replacements that more closely recapitulate human physiology. Here, we review and discuss the limitations of current bone TE approaches and possible strategies to improve vascularization in bone tissue substitutes.

Endothelial cell culture model of carotid artery atherosclerosis

Atherosclerotic lesions form non-randomly at locations in bends and bifurcations where the local flow can be classified as 'disturbed flow' and is associated with low shear stress oscillatory or reciprocating flow. Endothelial cells in vivo are constantly exposed to mechanical stimulation due to hemodynamic loading in the form of pulsatile pressure, cyclic stretch and shear stress to maintain phenotype and control function. In conditions like atherosclerosis, the pressure and strain loading remains the same whereas the local fluid flow behavior and shear stress are altered. Common in vitro models of atherosclerosis focus primarily on shear stress without accounting for pressure and strain loading. To overcome this limitation, we used our microfluidic Endothelial Cell Culture Model (ECCM) to achieve accurate replication of pressure, strain and shear stress waveforms associated with both normal flow seen in straight sections of arteries and disturbed flow seen atherosclerosis lesion susceptible regions. We specifically recreated mechanical stresses associated with the proximal internal carotid which is a major risk factor for stroke. Cells cultured using both conditions show distinct differences in alignment and cytoskeletal organization. In summary we recreated pressure, stretch and shear stress loading seen in straight sections and in the proximal internal carotid in a cell culture compatible platform.

Live free or die: stretch-induced apoptosis occurs when adaptive reorientation of annulus fibrosus cells is restricted

High matrix strains in the intervertebral disc occur during physiological motions and are amplified around structural defects in the annulus fibrosus (AF). It remains unknown if large matrix strains in the human AF result in localized cell death. This study investigated strain amplitudes and substrate conditions where AF cells were vulnerable to stretch-induced apoptosis. Human degenerated AF cells were subjected to 1 Hz-cyclic tensile strains for 24h on uniformly collagen coated substrates and on substrates with 40 μm stripes of collagen that restricted cellular reorientation. AF cells were capable of responding to stretch (stress fibers and focal adhesions aligned perpendicular to the direction of stretch), but were vulnerable to stretch-induced apoptosis when cytoskeletal reorientation was restricted, as could occur in degenerated states due to fibrosis and crosslink accumulation and at areas where high strains occur (around structural defects, delaminations, and herniations).

Actin filaments function as a tension sensor by tension-dependent binding of cofilin to the filament

In vitro, actin filament tension correlates with the binding and apparent activity of the filament-severing protein cofilin, suggesting a molecular mechanism by which cells respond to changes in mechanical force.

Intracellular and extracellular mechanical forces affect the structure and dynamics of the actin cytoskeleton. However, the underlying molecular and biophysical mechanisms, including how mechanical forces are sensed, are largely unknown. Actin-depolymerizing factor/cofilin proteins are actin-modulating proteins that are ubiquitously distributed in eukaryotes, and they are the most likely candidate as proteins to drive stress fiber disassembly in response to changes in tension in the fiber. In this study, we propose a novel hypothesis that tension in an actin filament prevents the filament from being severed by cofilin. To test this, we placed single actin filaments under tension using optical tweezers. When a fiber was tensed, it was severed after the application of cofilin with a significantly larger delay in comparison with control filaments suspended in solution. The binding rate of cofilin to an actin bundle decreased when the bundle was tensed. These results suggest that tension in an actin filament reduces the cofilin binding, resulting in a decrease in its effective severing activity.

Cyclic stress at mHz frequencies aligns fibroblasts in direction of zero strain

Recognition of external mechanical signals is vital for mammalian cells. Cyclic stretch, e.g. around blood vessels, is one such signal that induces cell reorientation from parallel to almost perpendicular to the direction of stretch. Here, we present quantitative analyses of both, cell and cytoskeletal reorientation of umbilical cord fibroblasts. Cyclic strain of preset amplitudes was applied at mHz frequencies. Elastomeric chambers were specifically designed and characterized to distinguish between zero strain and minimal stress directions and to allow accurate theoretical modeling. Reorientation was only induced when the applied stretch exceeded a specific amplitude, suggesting a non-linear response. However, on very soft substrates no mechanoresponse occurs even for high strain. For all stretch amplitudes, the angular distributions of reoriented cells are in very good agreement with a theory modeling stretched cells as active force dipoles. Cyclic stretch increases the number of stress fibers and the coupling to adhesions. We show that changes in cell shape follow cytoskeletal reorientation with a significant temporal delay. Our data identify the importance of environmental stiffness for cell reorientation, here in direction of zero strain. These in vitro experiments on cultured cells argue for the necessity of rather stiff environmental conditions to induce cellular reorientation in mammalian tissues.

Force- and Ca2+-dependent internalization of integrins in cultured endothelial cells

The effects of mechanical force applied to the integrin clusters at focal contacts were examined in cultured human umbilical vein endothelial cells. When a fibronectin-coated glass bead was attached to the apical cell surface, focal contacts formed beneath the bead that became linked to focal contacts at the basal cell membrane by actin stress fibers in 5 minutes. Integrin dynamics at the basal focal contacts were monitored in live cells in response to a localized mechanical stimulus generated by displacing the glass bead. Traction force transmitted to the basal focal contacts through the stress fibers was monitored by measuring the deformation of the polyacrylamide gel substratum. The force declined in a few seconds, probably owing to decreases in the elastic modulus of the stress fibers. This transient mechanical stimulus caused the dephosphorylation of paxillin and disassembly of integrin clusters at the basal cell membrane in 20 minutes. The disassembly was mediated mainly by clathrin-dependent endocytosis of integrins. The integrin internalization was inhibited in Ca2+- and K+-free solution, and by phenylarsine oxide, a phosphatase inhibitor. These results suggest that a transient mechanical stimulus applied to focal contacts induces Ca2+-dependent dephosphorylation of some proteins, including paxillin, and facilitates clathrin-dependent endocytosis of integrins.

Microfluidic endothelial cell culture model to replicate disturbed flow conditions seen in atherosclerosis susceptible regions

Atherosclerotic lesions occur non-randomly at vascular niches in bends and bifurcations where fluid flow can be characterized as "disturbed" (low shear stress with both forward and retrograde flow). Endothelial cells (ECs) at these locations experience significantly lower average shear stress without change in the levels of pressure or strain, which affects the local balance in mechanical stresses. Common in vitro models of atherosclerosis focus primarily on shear stress without accounting for pressure and strain loading. To overcome this limitation, we used our microfluidic endothelial cell culture model (ECCM) to achieve accurate replication of pressure, strain, and shear stress waveforms associated with both normal flow seen in straight sections of arteries and disturbed flow seen in the abdominal aorta in the infrarenal segment at the wall distal to the inferior mesenteric artery (IMA), which is associated with high incidence of atherosclerotic lesion formation. Human aortic endothelial cells (HAECs) were cultured within the ECCM under both normal and disturbed flow and evaluated for cell shape, cytoskeletal alignment, endothelial barrier function, and inflammation using immunofluorescence microscopy and flow cytometry. Results clearly demonstrate quantifiable differences between cells cultured under disturbed flow conditions, which are cuboidal with short and randomly oriented actin microfilaments and show intermittent expression of β-Catenin and cells cultured under normal flow. However, in the absence of pro-inflammatory stimulation, the levels of expression of activation markers: intra cellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), platelet endothelial cell adhesion molecule-1 (PECAM-1), and vascular endothelial cell growth factor - receptor 2 (VEGF-R2) known to be involved in the initiation of plaque formation were only slightly higher in HAECs cultured under disturbed flow in comparison to cells cultured under normal flow.