Global architecture of the F-actin cytoskeleton regulates cell shape-dependent endothelial mechanotransduction

A human pluripotent stem cell-based somitogenesis model using microfluidics

Emerging human pluripotent stem cell (hPSC)-based embryo models are useful for studying human embryogenesis. Particularly, there are hPSC-based somitogenesis models using free-floating culture that recapitulate somite formation. Somitogenesis in vivo involves intricately orchestrated biochemical and biomechanical events. However, none of the current somitogenesis models controls biochemical gradients or biomechanical signals in the culture, limiting their applicability to untangle complex biochemical-biomechanical interactions that drive somitogenesis. Herein, we develop a human somitogenesis model by confining hPSC-derived presomitic mesoderm (PSM) tissues in microfabricated trenches. Exogenous microfluidic morphogen gradients imposed on the PSM tissues cause axial patterning and trigger spontaneous rostral-to-caudal somite formation. A mechanical theory is developed to explain the size dependency between somites and the PSM. The microfluidic somitogenesis model is further exploited to reveal regulatory roles of cellular and tissue biomechanics in somite formation. This study presents a useful microengineered, hPSC-based model for understanding the biochemical and biomechanical events that guide somite formation.

Impact of uniaxial cyclic stretching on matrix-associated endothelial cell responses

Uniaxial cyclic stretching plays a pivotal role in the fields of tissue engineering and regenerative medicine, influencing cell behaviors and functionality based on physical properties, including matrix morphology and mechanical stimuli. This study delves into the response of endothelial cells to uniaxial cyclic strain within the geometric constraints of micro-nano fibers. Various structural scaffold forms of poly(l-lactide-co-caprolactone) (PLCL), such as flat membranes, randomly oriented fiber membranes, and aligned fiber membranes, were fabricated through solvent casting and electrospinning methods. Our investigation focuses on the morphological variation of endothelial cells under diverse geometric constraints and the mechanical-dependent release of nitric oxide (NO) on oriented fibrous membranes. Our results indicate that while uniaxial cyclic stretching promotes endothelial cell spreading, the anisotropy of the matrix morphology remains the primary driving factor for cell alignment. Additionally, uniaxial cyclic stretching significantly enhances NO release, with a notably stronger effect correlated to the increasing strain amplitude. Importantly, this study reveals that uniaxial cyclic stretching enhances the mRNA expression of key proteins, including talin, vinculin, rac, and nitric oxide synthase (eNOS).

Physical biology of cell-substrate interactions under cyclic stretch

Mechanosensitive focal adhesion (FA) complexes mediate dynamic interactions between cells and substrates and regulate cellular function. Integrins in FA complexes link substrate ligands to stress fibers (SFs) and aid load transfer and traction generation. We developed a one-dimensional, multi-scale, stochastic finite element model of a fibroblast on a substrate that includes calcium signaling, SF remodeling, and FA dynamics. We linked stochastic dynamics, describing the formation and clustering of integrins to substrate ligands via motor-clutches, to a continuum level SF contractility model at various locations along the cell length. We quantified changes in cellular responses with substrate stiffness, ligand density, and cyclic stretch. Results show that tractions and integrin recruitments varied along the cell length; tractions were maximum at lamellar regions and reduced to zero at the cell center. Optimal substrate stiffness, based on maximum tractions exerted by the cell, shifted toward stiffer substrates at high ligand densities. Mean tractions varied biphasically with substrate stiffness and peaked at the optimal substrate stiffness. Cytosolic calcium increased monotonically with substrate stiffness and accumulated near lamellipodial regions. Cyclic stretch increased the cytosolic calcium, integrin concentrations, and tractions at lamellipodial and intermediate regions on compliant substrates. The optimal substrate stiffness under stretch shifted toward compliant substrates for a given ligand density. Stretch also caused cell deadhesions beyond a critical substrate stiffness. FA's destabilized on stiff substrates under cyclic stretch. An increase in substrate stiffness and cyclic stretch resulted in higher fibroblast contractility. These results show that chemomechanical coupling is essential in mechanosensing responses underlying cell-substrate interactions.

Macrophage cell morphology-imprinted substrates can modulate mesenchymal stem cell behaviors and macrophage M1/M2 polarization for wound healing applications

Mesenchymal stem cells and macrophages (MQ) are two very important cells involved in the normal wound healing process. It is well understood that topological cues and mechanical factors can lead to different responses in stem cells and MQ by influencing their shape, cytoskeleton proliferation, migration, and differentiation, which play an essential role in the success or failure of biomaterial implantation and more importantly wound healing. On the other hand, the polarization of MQ from proinflammatory (M1) to prohealing (M2) phenotypes has a critical role in the acceleration of wound healing. In this study, the morphology of different MQ subtypes (M0, M1, and M2) was imprinted on a silicon surface (polydimethylsiloxane [PDMS]) to prepare a nano-topography cell-imprinted substrate with the ability to induce anti-inflammatory effects on the mouse adipose-derived stem cells (ADSCs) and RAW264.7 monocyte cell line (MO). The gene expression profiles and flow cytometry of MQ revealed that the cell shape microstructure promoted the MQ phenotypes according to the specific shape of each pattern. The ELISA results were in agreement with the gene expression profiles. The ADSCs on the patterned PDMS exhibited remarkably different shapes from no-patterned PDMS. The MOs grown on M2 morphological patterns showed a significant increase in expression and section of anti-inflammatory cytokine compared with M0 and M1 patterns. The ADSCs homing in niches heavily deformed the cytoskeletal, which is probably why the gene expression and phenotype unexpectedly changed. In conclusion, wound dressings with M2 cell morphology-induced surfaces are suggested as excellent anti-inflammatory and antiscarring dressings.

A human pluripotent stem cell-based somitogenesis model using microfluidics

Emerging human pluripotent stem cell (hPSC)-based embryo models are useful for studying human embryogenesis. Particularly, there are hPSC-based somitogenesis models using free-floating culture that recapitulate somite formation. Somitogenesis in vivo involves intricately orchestrated bio-chemical and -mechanical events. However, none of the current somitogenesis models controls biochemical gradients or biomechanical signals in the culture, limiting their applicability to untangle complex biochemical-biomechanical interactions that drive somitogenesis. Here we report a new human somitogenesis model by confining hPSC-derived presomitic mesoderm (PSM) tissues in microfabricated trenches. Exogenous microfluidic morphogen gradients imposed on PSM cause axial patterning and trigger spontaneous rostral-to-caudal somite formation. A mechanical theory is developed to explain the size dependency between somites and PSM. The microfluidic somitogenesis model is further exploited to reveal regulatory roles of cellular and tissue biomechanics in somite formation. This study presents a useful microengineered, hPSC-based model for understanding the bio-chemical and -mechanical events that guide somite formation.

Zyxin and actin structure confer anisotropic YAP mechanotransduction

Tissues and the embedded cells experience anisotropic deformations due to their functions and anatomical locations. The resident cells, such as tenocytes and muscle cells, are often restricted by their extracellular matrix and organize parallel to their major loading direction, yet most studies on cellular responses to strains use isotropic substrates without predetermined organizations. To understand how confined cells sense and respond to anisotropic loading, we combine cell patterning and uniaxial stretch to have precise geometric control. Dynamic stretch parallel to the long axis of the cell activates YAP nuclear translocation, but not when stretched in the perpendicular direction. Looking at the initial cytoskeleton response, parallel stretch leads to actin breakage and repair within the first minute, mediated by zyxin, the focal adhesion protein. In addition, this zyxin-mediated repair response is controlled by focal adhesion kinase (FAK) and leads to YAP signaling. As these factors are intimately involved in a wide range of mechanical regulation, our findings point to new roles of zyxin and YAP in anisotropic mechanotransduction, and may provide additional perspectives in cellular adaptive responses and tissue homeostasis. STATEMENT OF SIGNIFICANCE: Structure and deformation of tissues control gene expression, migration, and proliferation of the resident cells. In an effort to understand the underlying mechanisms, we find that the transcription cofactor YAP respond to mechanical stretch in a direction-dependent manner. We demonstrate that parallel stretch induces actin cytoskeleton damage, focal adhesion kinase (FAK) activation, and zyxin relocation, which are involved in the anisotropic YAP signaling. Our findings provide additional perspectives in the interactions of tissue structure and cell mechanotransduction.

Molecular imprinting as a simple way for the long-term maintenance of the stemness and proliferation potential of adipose-derived stem cells: an in vitro study

Culturing adipose-derived stem cells (ADSCs) on the biomimetic ADSC-imprinted substrate is a simple way for long-term maintenance of their stemness and proliferation potential.

Microfluidic models of the human circulatory system: versatile platforms for exploring mechanobiology and disease modeling

The human circulatory system is a marvelous fluidic system, which is very sensitive to biophysical and biochemical cues. The current animal and cell culture models do not recapitulate the functional properties of the human circulatory system, limiting our ability to fully understand the complex biological processes underlying the dysfunction of this multifaceted system. In this review, we discuss the unique ability of microfluidic systems to recapitulate the biophysical, biochemical, and functional properties of the human circulatory system. We also describe the remarkable capacity of microfluidic technologies for exploring the complex mechanobiology of the cardiovascular system, mechanistic studying of cardiovascular diseases, and screening cardiovascular drugs with the additional benefit of reducing the need for animal models. We also discuss opportunities for further advancement in this exciting field.

High stretch induces endothelial dysfunction accompanied by oxidative stress and actin remodeling in human saphenous vein endothelial cells

The rate of the remodeling of the arterialized saphenous vein conduit limits the outcomes of coronary artery bypass graft surgery (CABG), which may be influenced by endothelial dysfunction. We tested the hypothesis that high stretch (HS) induces human saphenous vein endothelial cell (hSVEC) dysfunction and examined candidate underlying mechanisms. Our results showed that in vitro HS reduces NO bioavailability, increases inflammatory adhesion molecule expression (E-selectin and VCAM1) and THP-1 cell adhesion. HS decreases F-actin in hSVECs, but not in human arterial endothelial cells, and is accompanied by G-actin and cofilin’s nuclear shuttling and increased reactive oxidative species (ROS). Pre-treatment with the broad-acting antioxidant N-acetylcysteine (NAC) supported this observation and diminished stretch-induced actin remodeling and inflammatory adhesive molecule expression. Altogether, we provide evidence that increased oxidative stress and actin cytoskeleton remodeling play a role in HS-induced saphenous vein endothelial cell dysfunction, which may contribute to predisposing saphenous vein graft to failure.

Modulation of Human Mesenchymal Stem Cells by Electrical Stimulation Using an Enzymatic Biofuel Cell

Enzymatic biofuel cells (EBFCs) have excellent potential as components in bioelectronic devices, especially as active biointerfaces to regulate stem cell behavior for regenerative medicine applications. However, it remains unclear to what extent EBFC-generated electrical stimulation can regulate the functional behavior of human adipose-derived mesenchymal stem cells (hAD-MSCs) at the morphological and gene expression levels. Herein, we investigated the effect of EBFC-generated electrical stimulation on hAD-MSC cell morphology and gene expression using next-generation RNA sequencing. We tested three different electrical currents, 127 ± 9, 248 ± 15, and 598 ± 75 nA/cm2, in mesenchymal stem cells. We performed transcriptome profiling to analyze the impact of EBFC-derived electrical current on gene expression using next generation sequencing (NGS). We also observed changes in cytoskeleton arrangement and analyzed gene expression that depends on the electrical stimulation. The electrical stimulation of EBFC changes cell morphology through cytoskeleton re-arrangement. In particular, the results of whole transcriptome NGS showed that specific gene clusters were up- or down-regulated depending on the magnitude of applied electrical current of EBFC. In conclusion, this study demonstrates that EBFC-generated electrical stimulation can influence the morphological and gene expression properties of stem cells; such capabilities can be useful for regenerative medicine applications such as bioelectronic devices.

Integrating Biophysics in Toxicology

Integration of biophysical stimulation in test systems is established in diverse branches of biomedical sciences including toxicology. This is largely motivated by the need to create novel experimental setups capable of reproducing more closely in vivo physiological conditions. Indeed, we face the need to increase predictive power and experimental output, albeit reducing the use of animals in toxicity testing. In vivo, mechanical stimulation is essential for cellular homeostasis. In vitro, diverse strategies can be used to model this crucial component. The compliance of the extracellular matrix can be tuned by modifying the stiffness or through the deformation of substrates hosting the cells via static or dynamic strain. Moreover, cells can be cultivated under shear stress deriving from the movement of the extracellular fluids. In turn, introduction of physical cues in the cell culture environment modulates differentiation, functional properties, and metabolic competence, thus influencing cellular capability to cope with toxic insults. This review summarizes the state of the art of integration of biophysical stimuli in model systems for toxicity testing, discusses future challenges, and provides perspectives for the further advancement of in vitro cytotoxicity studies.

Tenocyte-imprinted substrate: a topography-based inducer for tenogenic differentiation in adipose tissue-derived mesenchymal stem cells

Tendon tissue engineering based on stem cell differentiation has attracted a great deal of attention in recent years. Previous studies have examined the effect of cell-imprinted polydimethylsiloxane (PDMS) substrate on induction differentiation in stem cells. In this study, we used tenocyte morphology as a positive mold to create a tenocyte-imprinted substrate on PDMS. The morphology and topography of this tenocyte replica on PDMS was evaluated with scanning electron microscopy (SEM) and atomic force microscopy. The tenogenic differentiation induction capacity of the tenocyte replica in adipose tissue-derived mesenchymal stem cells (ADSCs) was then investigated and compared with other groups, including tissue replica (which was produced similarly to the tenocyte replica and was evaluated by SEM), decellularized tendon, and bone morphogenic protein (BMP)-12, as other potential inducers. This comparison gives us an estimate of the ability of tenocyte-imprinted PDMS (called cell replica in the present study) to induce differentiation compared to other inducers. For this reason, ADSCs were divided into five groups, including control, cell replica, tissue replica, decellularized tendon and BMP-12. ADSCs were seeded on each group separately and investigated by the real-time reverse transcription polymerase chain reaction (RT-PCR) technique after seven and 14 days. Our results showed that in spite of the higher effect of the growth factor on tenogenic differentiation, the cell replica can also induce tenocyte marker expression (scleraxis and tenomodulin) in ADSCs. Moreover, the tenogenic differentiation induction capacity of the cell replica was greater than tissue replica. Immunocytochemistry analysis revealed that ADSCs seeding on the cell replica for 14 days led to scleraxis and tenomodulin expression at the protein level. In addition, immunohistochemistry indicated that contrary to the promising results in vitro, there was little difference between ADSCs cultured on tenocyte-imprinted PDMS and untreated ADSCs. The results of such studies could lead to the production of inexpensive cell culture plates or biomaterials that can induce differentiation in stem cells without growth factors or other supplements.

Embryonic stem cells become mechanoresponsive upon exit from ground state of pluripotency

Stem cell fate decisions are driven by a broad array of signals, both chemical and mechanical. Although much progress has been made in our understanding of the impact of chemical signals on cell fate choice, much less is known about the role and influence of mechanical signalling, particularly in embryonic stem (ES) cells. Many studies use substrates with different stiffness to study mechanical signalling, but changing substrate stiffness can induce secondary effects which are difficult to disentangle from the direct effects of forces/mechanical signals. To probe the direct impact of mechanical stress on cells, we developed an adaptable cell substrate stretcher to exert specific, reproducible forces on cells. Using this device to test the response of ES cells to tensile strain, we found that cells experienced a transient influx of calcium followed by an upregulation of the so-called immediate and early genes. On longer time scales, however, ES cells in ground state conditions were largely insensitive to mechanical stress. Nonetheless, as ES cells exited the ground state, their susceptibility to mechanical signals increased, resulting in broad transcriptional changes. Our findings suggest that exit from ground state of pluripotency is unaffected by mechanical signals, but that these signals could become important during the next stage of lineage specification. A better understanding of this process could improve our understanding of cell fate choice in early development and improve protocols for differentiation guided by mechanical cues.

Rapid and Robust Coating Method to Render Polydimethylsiloxane Surfaces Cell-Adhesive

Polydimethylsiloxane (PDMS) is a synthetic material with excellent properties for biomedical applications because of its easy fabrication method, high flexibility, permeability to oxygen, transparency, and potential to produce high-resolution structures in the case of lithography. However, PDMS needs to be modified to support homogeneous cell attachments and spreading. Even though many physical and chemical methods, like plasma treatment or extracellular matrix coatings, have been developed over the last decades to increase cell-surface interactions, these methods are still very time-consuming, often not efficient enough, complex, and can require several treatment steps. To overcome these issues, we present a novel, robust, and fast one-step PDMS coating method using engineered anchor peptides fused to the cell-adhesive peptide sequence (glycine-arginine-glycine-aspartate-serine, GRGDS). The anchor peptide attaches to the PDMS surface predominantly by hydrophobic interactions by simply dipping PDMS in a solution containing the anchor peptide, presenting the GRGDS sequence on the surface available for cell adhesion. The binding performance and kinetics of the anchor peptide to PDMS are characterized, and the coatings are optimized for efficient cell attachment of fibroblasts and endothelial cells. Additionally, the applicability is proven using PDMS-based directional nanotopographic gradients, showing a lower threshold of 5 μm wrinkles for fibroblast alignment.

Energy-Mediated Machinery Drives Cellular Mechanical Allostasis

Allostasis is a fundamental biological process through which living organisms achieve stability via physiological or behavioral changes to protect against internal and external stresses, and ultimately better adapt to the local environment. However, an full understanding of cellular-level allostasis is far from developed. By employing an integrated micromechanical tool capable of applying controlled mechanical stress on an individual cell and simultaneously reporting dynamic information of subcellular mechanics, individual cell allostasis is observed to occur through a biphasic process; cellular mechanics tends to restore to a stable state through a mechanoadaptative process with excitative biophysical activity followed by a decaying adaptive phase. Based on these observations, it is found that cellular allostasis occurs through a complex balance of subcellular energy and cellular mechanics; upon a transient and local physical stimulation, cells trigger an allostatic state that maximizes energy and overcomes a mechanical "energy barrier" followed by a relaxation state that reaches its mechanobiological stabilization and energy minimization. Discoveries of energy-driven cellular machinery and conserved mechanotransductive pathways underscore the critical role of force-sensitive cytoskeleton equilibrium in cellular allostasis. This highlight the biophysical origin of cellular mechanical allostasis, providing subcellular methods to understand the etiology and progression of certain diseases or aging.

Innovative Tools for Mechanobiology: Unraveling Outside-In and Inside-Out Mechanotransduction

Cells and tissues can sense and react to the modifications of the physico-chemical properties of the extracellular environment (ECM) through integrin-based adhesion sites and adapt their physiological response in a process called mechanotransduction. Due to their critical localization at the cell-ECM interface, transmembrane integrins are mediators of bidirectional signaling, playing a key role in “outside-in” and “inside-out” signal transduction. After presenting the basic conceptual fundamentals related to cell mechanobiology, we review the current state-of-the-art technologies that facilitate the understanding of mechanotransduction signaling pathways. Finally, we highlight innovative technological developments that can help to advance our understanding of the mechanisms underlying nuclear mechanotransduction.

Probing Single-Cell Mechanical Allostasis Using Ultrasound Tweezers

Introduction

In response to external stress, cells alter their morphology, metabolic activity, and functions to mechanically adapt to the dynamic, local environment through cell allostasis. To explore mechanotransduction in cellular allostasis, we applied an integrated micromechanical system that combines an 'ultrasound tweezers'-based mechanical stressor and a Förster resonance energy transfer (FRET)-based molecular force biosensor, termed "actinin-sstFRET," to monitor in situ single-cell allostasis in response to transient stimulation in real time.

Methods

The ultrasound tweezers utilize 1 Hz, 10-s transient ultrasound pulses to acoustically excite a lipid-encapsulated microbubble, which is bound to the cell membrane, and apply a pico- to nano-Newton range of forces to cells through an RGD-integrin linkage. The actinin-sstFRET molecular sensor, which engages the actin stress fibers in live cells, is used to map real-time actomyosin force dynamics over time. Then, the mechanosensitive behaviors were examined by profiling the dynamics in Ca²⁺ influx, actomyosin cytoskeleton (CSK) activity, and GTPase RhoA signaling to define a single-cell mechanical allostasis.

Results

By subjecting a 1 Hz, 10-s physical stress, single vascular smooth muscle cells (VSMCs) were observed to remodeled themselves in a biphasic mechanical allostatic manner within 30 min that caused them to adjust their contractility and actomyosin activities. The cellular machinery that underscores the vital role of CSK equilibrium in cellular mechanical allostasis, includes Ca²⁺ influx, remodeling of actomyosin CSK and contraction, and GTPase RhoA signaling. Mechanical allostasis was observed to be compromised in VSMCs from patients with type II diabetes mellitus (T2DM), which could potentiate an allostatic maladaptation.

Conclusions

By integrating tools that simultaneously permit localized mechanical perturbation and map actomyosin forces, we revealed distinct cellular mechanical allostasis profiles in our micromechanical system. Our findings of cell mechanical allostasis and maladaptation provide the potential for mechanophenotyping cells to reveal their pathogenic contexts and their biophysical mediators that underlie multi-etiological diseases such as diabetes, hypertension, or aging.

CYLD deficiency exacerbates lipopolysaccharide (LPS)-induced pyroptosis in astrocytes of mice with sepsis

Sepsis is a clinical syndrome occurring in patients following infection or injury, and is a leading cause of mortality in the world. However, the pathogenesis that contributes to sepsis and its associated brain injury is still unclear. Cylindromatosis (CYLD) is integrally involved in various physiological processes, such as immune responses, inflammation and cell cycle control. In the study, in vitro and in vivo models with sepsis by lipopolysaccharide (LPS) were performed to investigate the role of CYLD in inflammasome and its related astrocyte (AST) pyroptosis. The in vitro results suggested that LPS treatment significantly resulted in inflammasome activation and pyroptosis in AST. CYLD expression was progressively degraded upon LPS incubation in AST. Importantly, CYLD knockout (KO) could further promote pyroptosis and accelerate histone release in LPS-stimulated AST. In addition, the in vivo experiments suggested that LPS treatment in mice enhanced Caspase-1 immunoreactivity and resulted in high expression of phosphorylated nuclear factor-κB p65 (NF-κB p65), interleukin-1β (IL-1β) and IL-18 in hippocampus tissue samples, which were markedly exacerbated by CYLD knockout. Moreover, mice with CYLD deficiency exhibited higher mortality rate induced by LPS treatment. In conclusion, these findings suggested that CYLD decrease might accelerate astrocyte activity by promoting pyroptosis to promote sepsis-associated brain injury, and CYLD might be a potential therapeutic target for the management of this disease.

Cytological analysis and structural quantification of FtsZ1-2 and FtsZ2-1 network characteristics in Physcomitrella patens

Although the concept of the cytoskeleton as a cell-shape-determining scaffold is well established, it remains enigmatic how eukaryotic organelles adopt and maintain a specific morphology. The Filamentous Temperature Sensitive Z (FtsZ) protein family, an ancient tubulin, generates complex polymer networks, with striking similarity to the cytoskeleton, in the chloroplasts of the moss Physcomitrella patens. Certain members of this protein family are essential for structural integrity and shaping of chloroplasts, while others are not, illustrating the functional diversity within the FtsZ protein family. Here, we apply a combination of confocal laser scanning microscopy and a self-developed semi-automatic computational image analysis method for the quantitative characterisation and comparison of network morphologies and connectivity features for two selected, functionally dissimilar FtsZ isoforms, FtsZ1-2 and FtsZ2-1. We show that FtsZ1-2 and FtsZ2-1 networks are significantly different for 8 out of 25 structural descriptors. Therefore, our results demonstrate that different FtsZ isoforms are capable of generating polymer networks with distinctive morphological and connectivity features which might be linked to the functional differences between the two isoforms. To our knowledge, this is the first study to employ computational algorithms in the quantitative comparison of different classes of protein networks in living cells.

Techniques to stimulate and interrogate cell-cell adhesion mechanics

Cell-cell adhesions maintain the mechanical integrity of multicellular tissues and have recently been found to act as mechanotransducers, translating mechanical cues into biochemical signals. Mechanotransduction studies have primarily focused on focal adhesions, sites of cell-substrate attachment. These studies leverage technical advances in devices and systems interfacing with living cells through cell-extracellular matrix adhesions. As reports of aberrant signal transduction originating from mutations in cell-cell adhesion molecules are being increasingly associated with disease states, growing attention is being paid to this intercellular signaling hub. Along with this renewed focus, new requirements arise for the interrogation and stimulation of cell-cell adhesive junctions. This review covers established experimental techniques for stimulation and interrogation of cell-cell adhesion from cell pairs to monolayers.

Empirically Determined Vascular Smooth Muscle Cell Mechano-Adaptation Law

Cardiovascular disease can alter the mechanical environment of the vascular system, leading to mechano-adaptive growth and remodeling. Predictive models of arterial mechano-adaptation could improve patient treatments and outcomes in cardiovascular disease. Vessel-scale mechano-adaptation includes remodeling of both the cells and extracellular matrix. Here, we aimed to experimentally measure and characterize a phenomenological mechano-adaptation law for vascular smooth muscle cells (VSMCs) within an artery. To do this, we developed a highly controlled and reproducible system for applying a chronic step-change in strain to individual VSMCs with in vivo like architecture and tracked the temporal cellular stress evolution. We found that a simple linear growth law was able to capture the dynamic stress evolution of VSMCs in response to this mechanical perturbation. These results provide an initial framework for development of clinically relevant models of vascular remodeling that include VSMC adaptation.

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.

New advances in probing cell-extracellular matrix interactions

The extracellular matrix (ECM) provides structural and biochemical support to cells within tissues. An emerging body of evidence has established that the ECM plays a key role in cell mechanotransduction - the study of coupling between mechanical inputs and cellular phenotype - through either mediating transmission of forces to the cells, or presenting mechanical cues that guide cellular behaviors. Recent progress in cell mechanotransduction research has been facilitated by advances of experimental tools, particularly microtechnologies, engineered biomaterials, and imaging and analytical methods. Microtechnologies have enabled the design and fabrication of controlled physical microenvironments for the study and measurement of cell-ECM interactions. Advances in engineered biomaterials have allowed researchers to develop synthetic ECMs that mimic tissue microenvironments and investigate the impact of altered physicochemical properties on various cellular processes. Finally, advanced imaging and spectroscopy techniques have facilitated the visualization of the complex interaction between cells and ECM in vitro and in living tissues. This review will highlight the application of recent innovations in these areas to probing cell-ECM interactions. We believe cross-disciplinary approaches, combining aspects of the different technologies reviewed here, will inspire innovative ideas to further elucidate the secrets of ECM-mediated cell control.

Self-organized amniogenesis by human pluripotent stem cells in a biomimetic implantation-like niche

Amniogenesis-the development of amnion-is a critical developmental milestone for early human embryogenesis and successful pregnancy. However, human amniogenesis is poorly understood due to limited accessibility to peri-implantation embryos and a lack of in vitro models. Here we report an efficient biomaterial system to generate human amnion-like tissue in vitro through self-organized development of human pluripotent stem cells (hPSCs) in a bioengineered niche mimicking the in vivo implantation environment. We show that biophysical niche factors act as a switch to toggle hPSC self-renewal versus amniogenesis under self-renewal-permissive biochemical conditions. We identify a unique molecular signature of hPSC-derived amnion-like cells and show that endogenously activated BMP-SMAD signalling is required for the amnion-like tissue development by hPSCs. This study unveils the self-organizing and mechanosensitive nature of human amniogenesis and establishes the first hPSC-based model for investigating peri-implantation human amnion development, thereby helping advance human embryology and reproductive medicine.

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.

Cell stretching devices as research tools: engineering and biological considerations

Cells within the human body are subjected to continuous, cyclic mechanical strain caused by various organ functions, movement, and growth. Cells are well known to have the ability to sense and respond to mechanical stimuli. This process is referred to as mechanotransduction. A better understanding of mechanotransduction is of great interest to clinicians and scientists alike to improve clinical diagnosis and understanding of medical pathology. However, the complexity involved in in vivo biological systems creates a need for better in vitro technologies, which can closely mimic the cells' microenvironment using induced mechanical strain. This technology gap motivates the development of cell stretching devices for better understanding of the cell response to mechanical stimuli. This review focuses on the engineering and biological considerations for the development of such cell stretching devices. The paper discusses different types of stretching concepts, major design consideration and biological aspects of cell stretching and provides a perspective for future development in this research area.

Three Dimensional and Homogenous Single Cell Cyclic Stretch within a Magnetic Micropillar Array (mMPA) for a Cell Proliferation Study

The physical properties of the extracellular matrix (ECM) are a key aspect of the cell microenvironment. A biological system is a highly dynamic organization. In our study, we designed and prepared a large area of magnetic PDMS elastomer micropillar array (mMPA) with robust and tunable movement for cell mechanics study. The rotational movement frequency of the micropillars could be precisely controlled by a home-built magnetic actuation apparatus. Cells cultured in the mMPA could be suspended in between two micropillars in a single level and exhibited a 3D structure. With the rotational movement of the micropillar, a homogeneous stretchable force could be applied to a single cell along it long axis with various frequencies. We exclusively studied the influence of dynamic properties of the micropillar movement on cell behaviors. We found that, by fixing the amplitude of the stretchable force, the frequency-based properties of the cell microenvironment could significantly change cell functions. The cell behaviors are dependent on the micropillar movement frequency and a transition from proliferation to apoptosis/death exhibited with the increment of the force application frequency.

Flow-induced stress on adherent cells in microfluidic devices

Transduction of mechanical forces and chemical signals affect every cell in the human body. Fluid flow in systems such as the lymphatic or circulatory systems modulates not only cell morphology, but also gene expression patterns, extracellular matrix protein secretion and cell-cell and cell-matrix adhesions. Similar to the role of mechanical forces in adaptation of tissues, shear fluid flow orchestrates collective behaviours of adherent cells found at the interface between tissues and their fluidic environments. These behaviours range from alignment of endothelial cells in the direction of flow to stem cell lineage commitment. Therefore, it is important to characterize quantitatively fluid interface-dependent cell activity. Common macro-scale techniques, such as the parallel plate flow chamber and vertical-step flow methods that apply fluid-induced stress on adherent cells, offer standardization, repeatability and ease of operation. However, in order to achieve improved control over a cell's microenvironment, additional microscale-based techniques are needed. The use of microfluidics for this has been recognized, but its true potential has emerged only recently with the advent of hybrid systems, offering increased throughput, multicellular interactions, substrate functionalization on 3D geometries, and simultaneous control over chemical and mechanical stimulation. In this review, we discuss recent advances in microfluidic flow systems for adherent cells and elaborate on their suitability to mimic physiologic micromechanical environments subjected to fluid flow. We describe device design considerations in light of ongoing discoveries in mechanobiology and point to future trends of this promising technology.

Cell spreading area regulates clathrin-coated pit dynamics on micropatterned substrate

Clathrin-mediated endocytosis (CME) is the most characterized pathway for the endocytic entry of proteins and lipids at the plasma membrane of eukaryotic cells. Numerous studies have probed the roles of different endocytic accessory proteins in regulating the dynamics of clathrin-coated pit (CCP) assembly. However, it is not completely clear how physical cues regulate CCP dynamics. Here we employ microcontact printing to control cell shape and examine CCP dynamics as a function of cell spreading area for three differently sized cells. Cells with a large spreading area had more short-lived CCPs but a higher CCP initiation rate. Interestingly, we found that fluorescence intensity of CCPs decreased with increasing cell spreading area in a manner that was dependent on the cortical actin network. Our results point to another facet of the regulation of CCP dynamics, suggesting that CME may be modulated while cells change their mechanical state and remodel their actin cytoskeleton during various processes.

Competition between cap and basal actin fiber orientation in cells subjected to contact guidance and cyclic strain

In vivo, adhesive cells continuously respond to a complex range of physical cues coming from the surrounding microenvironment by remodeling their cytoskeleton. Topographical and mechanical cues applied separately have been shown to affect the orientation of the actin stress fibers. Here we investigated the combined effects of contact guidance by topographical cues and uniaxial cyclic strain on actin cytoskeleton orientation of vascular derived cells. We devised a modular setup of stretchable circular and elliptic elastomeric microposts, capable to expose the cells to both contact guidance and uniaxial cyclic strain. A competition occurs between these cues when both contact guidance and strain are oriented along the same direction. For the first time we show that this competition originates from the distinct response of perinuclear basal and actin cap fibers: While basal fibers follow the contact guidance cue, actin cap fibers respond to the cyclic strain by strain avoidance. We also show that nuclear orientation follows actin cap fiber orientation, suggesting that actin cap fibers are responsible for cellular reorientation. Taken together, these findings may have broad implications in understanding the response of cells to combined topographical and mechanical cues.

Cyclic stretching of soft substrates induces spreading and growth

In the body, soft tissues often undergo cycles of stretching and relaxation that may affect cell behaviour without changing matrix rigidity. To determine whether transient forces can substitute for a rigid matrix, we stretched soft pillar arrays. Surprisingly, 1–5% cyclic stretching over a frequency range of 0.01–10 Hz caused spreading and stress fibre formation (optimum 0.1 Hz) that persisted after 4 h of stretching. Similarly, stretching increased cell growth rates on soft pillars comparative to rigid substrates. Of possible factors linked to fibroblast growth, MRTF-A (myocardin-related transcription factor-A) moved to the nucleus in 2 h of cyclic stretching and reversed on cessation; but YAP (Yes-associated protein) moved much later. Knockdown of either MRTF-A or YAP blocked stretch-dependent growth. Thus, we suggest that the repeated pulling from a soft matrix can substitute for a stiff matrix in stimulating spreading, stress fibre formation and growth.

Cells grown on a stiff substrate are stimulated through physical cues to spread, create actin stress fibres and proliferate. Here Cui et al. show that cyclic stretching cells on a soft pillar substrate has the same effect as growth on a stiff substrate, and results in nuclear translocation of YAP and MRTF-A.

Standing surface acoustic wave based cell coculture

Precise reconstruction of heterotypic cell-cell interactions in vitro requires the coculture of different cell types in a highly controlled manner. In this article, we report a standing surface acoustic wave (SSAW)-based cell coculture platform. In our approach, different types of cells are patterned sequentially in the SSAW field to form an organized cell coculture. To validate our platform, we demonstrate a coculture of epithelial cancer cells and endothelial cells. Real-time monitoring of cell migration dynamics reveals increased cancer cell mobility when cancer cells are cocultured with endothelial cells. Our SSAW-based cell coculture platform has the advantages of contactless cell manipulation, high biocompatibility, high controllability, simplicity, and minimal interference of the cellular microenvironment. The SSAW technique demonstrated here can be a valuable analytical tool for various biological studies involving heterotypic cell-cell interactions.