Reinforcement versus fluidization in cytoskeletal mechanoresponsiveness

Sustained Strain Applied at High Rates Drives Dynamic Tensioning in Epithelial Cells

Epithelial cells experience long lasting loads of different magnitudes and rates. How they adapt to these loads strongly impacts tissue health. Yet, much remains unknown about their stress evolution under sustained strain. Here, by subjecting cell pairs to sustained strain, we report a bimodal stress response, where in addition to the typically observed stress relaxation, a subset of cells exhibits a dynamic tensioning process with significant elevation in stress within 100s, resembling active pulling-back in muscle fibers. Strikingly, the fraction of cells exhibiting tensioning increases with increasing strain rate. The tensioning response is accompanied by actin remodeling, and perturbation to actin abrogates it, supporting cell contractility's role in the response. Collectively, our data show that epithelial cells adjust their tensional states over short timescales in a strain-rate dependent manner to adapt to sustained strains, demonstrating that the active pulling-back behavior could be a common protective mechanism against environmental stress.

Cross-scale mechanobiological regulation of cylindrical compressible liquid inclusion via coating

The double-bag theory in modern anatomy suggests that structures with coatings are commonly found in human body at various length scales, such as osteocyte processes covered by pericellular matrix and bones covered by muscle tissue. To understand the mechanical behaviors and physiological responses of such biological structures, we develop an analytical model to quantify surface effects on the deformation of a coated cylindrical compressible liquid inclusion in an elastic matrix subjected to remote loading. Our analytical solution reveals that coating can either amplify or attenuate the volumetric strain of the inclusion, depending on the relative elastic moduli of inclusion, coating, and matrix. For illustration, we utilize this solution to explore amplification/attenuation of volumetric strain in musculoskeletal systems, nerve cells, and vascular tissues. We demonstrate that coating often plays a crucial role in mechanical regulation of the development and repair of human tissues and cells. Our model provides qualitative analysis of cross-scale mechanical response of coated liquid inclusions, helpful for constructing mechanical microenvironment of cells.

Discrete network models of endothelial cells and their interactions with the substrate

Endothelial cell monolayers line the inner surfaces of blood and lymphatic vessels. They are continuously exposed to different mechanical loads, which may trigger mechanobiological signals and hence play a role in both physiological and pathological processes. Computer-based mechanical models of cells contribute to a better understanding of the relation between cell-scale loads and cues and the mechanical state of the hosting tissue. However, the confluency of the endothelial monolayer complicates these approaches since the intercellular cross-talk needs to be accounted for in addition to the cytoskeletal mechanics of the individual cells themselves. As a consequence, the computational approach must be able to efficiently model a large number of cells and their interaction. Here, we simulate cytoskeletal mechanics by means of molecular dynamics software, generally suitable to deal with large, locally interacting systems. Methods were developed to generate models of single cells and large monolayers with hundreds of cells. The single-cell model was considered for a comparison with experimental data. To this end, we simulated cell interactions with a continuous, deformable substrate, and computationally replicated multistep traction force microscopy experiments on endothelial cells. The results indicate that cell discrete network models are able to capture relevant features of the mechanical behaviour and are thus well-suited to investigate the mechanics of the large cytoskeletal network of individual cells and cell monolayers.

Active viscoelastic models for cell and tissue mechanics

Living cells are out of equilibrium active materials. Cell-generated forces are transmitted across the cytoskeleton network and to the extracellular environment. These active force interactions shape cellular mechanical behaviour, trigger mechano-sensing, regulate cell adaptation to the microenvironment and can affect disease outcomes. In recent years, the mechanobiology community has witnessed the emergence of many experimental and theoretical approaches to study cells as mechanically active materials. In this review, we highlight recent advancements in incorporating active characteristics of cellular behaviour at different length scales into classic viscoelastic models by either adding an active tension-generating element or adjusting the resting length of an elastic element in the model. Summarizing the two groups of approaches, we will review the formulation and application of these models to understand cellular adaptation mechanisms in response to various types of mechanical stimuli, such as the effect of extracellular matrix properties and external loadings or deformations.

Mechanosensitive TRPV4 channel guides maturation and organization of the bilayered mammary epithelium

Biophysical cues from the cell microenvironment are detected by mechanosensitive components at the cell surface. Such machineries convert physical information into biochemical signaling cascades within cells, subsequently leading to various cellular responses in a stimulus-dependent manner. At the surface of extracellular environment and cell cytoplasm exist several ion channel families that are activated by mechanical signals to direct intracellular events. One of such channel is formed by transient receptor potential cation channel subfamily V member, TRPV4 that is known to act as a mechanosensor in wide variaty of tissues and control ion-influx in a spatio-temporal way. Here we report that TRPV4 is prominently expressed in the stem/progenitor cell populations of the mammary epithelium and seems important for the lineage-specific differentiation, consequently affecting mechanical features of the mature mammary epithelium. This was evident by the lack of several markers for mature myoepithelial and luminal epithelial cells in TRPV4-depleted cell lines. Interestingly, TRPV4 expression is controlled in a tension-dependent manner and it also impacts differentation process dependently on the stiffness of the microenvironment. Furthermore, such cells in a 3D compartment were disabled to maintain normal mammosphere structures and displayed abnormal lumen formation, size of the structures and disrupted cellular junctions. Mechanosensitive TRPV4 channel therefore act as critical player in the homeostasis of normal mammary epithelium through sensing the physical environment and guiding accordingly differentiation and structural organization of the bilayered mammary epithelium.

Myosin-independent stiffness sensing by fibroblasts is regulated by the viscoelasticity of flowing actin

The stiffness of the extracellular matrix induces differential tension within integrin-based adhesions, triggering differential mechanoresponses. However, it has been unclear if the stiffness-dependent differential tension is induced solely by myosin activity. Here, we report that in the absence of myosin contractility, 3T3 fibroblasts still transmit stiffness-dependent differential levels of traction. This myosin-independent differential traction is regulated by polymerizing actin assisted by actin nucleators Arp2/3 and formin where formin has a stronger contribution than Arp2/3 to both traction and actin flow. Intriguingly, despite only slight changes in F-actin flow speed observed in cells with the combined inhibition of Arp2/3 and myosin compared to cells with sole myosin inhibition, they show a 4-times reduction in traction than cells with myosin-only inhibition. Our analyses indicate that traditional models based on rigid F-actin are inadequate for capturing such dramatic force reduction with similar actin flow. Instead, incorporating the F-actin network's viscoelastic properties is crucial. Our new model including the F-actin viscoelasticity reveals that Arp2/3 and formin enhance stiffness sensitivity by mechanically reinforcing the F-actin network, thereby facilitating more effective transmission of flow-induced forces. This model is validated by cell stiffness measurement with atomic force microscopy and experimental observation of model-predicted stiffness-dependent actin flow fluctuation.

Intracellular tension sensor reveals mechanical anisotropy of the actin cytoskeleton

The filamentous actin (F-actin) cytoskeleton is a composite material consisting of cortical actin and bundled F-actin stress fibers, which together mediate the mechanical behaviors of the cell, from cell division to cell migration. However, as mechanical forces are typically measured upon transmission to the extracellular matrix, the internal distribution of forces within the cytoskeleton is unknown. Likewise, how distinct F-actin architectures contribute to the generation and transmission of mechanical forces is unclear. Therefore, we have developed a molecular tension sensor that embeds into the F-actin cytoskeleton. Using this sensor, we measure tension within stress fibers and cortical actin, as the cell is subject to uniaxial stretch. We find that the mechanical response, as measured by FRET, depends on the direction of applied stretch relative to the cell's axis of alignment. When the cell is aligned parallel to the direction of the stretch, stress fibers and cortical actin both accumulate tension. By contrast, when aligned perpendicular to the direction of stretch, stress fibers relax tension while the cortex accumulates tension, indicating mechanical anisotropy within the cytoskeleton. We further show that myosin inhibition regulates this anisotropy. Thus, the mechanical anisotropy of the cell and the coordination between distinct F-actin architectures vary and depend upon applied load.

A life off the beaten track in biomechanics: Imperfect elasticity, cytoskeletal glassiness, and epithelial unjamming

Textbook descriptions of elasticity, viscosity, and viscoelasticity fail to account for certain mechanical behaviors that typify soft living matter. Here, we consider three examples. First, strong empirical evidence suggests that within lung parenchymal tissues, the frictional stresses expressed at the microscale are fundamentally not of viscous origin. Second, the cytoskeleton (CSK) of the airway smooth muscle cell, as well as that of all eukaryotic cells, is more solid-like than fluid-like, yet its elastic modulus is softer than the softest of soft rubbers by a factor of 104-105. Moreover, the eukaryotic CSK expresses power law rheology, innate malleability, and fluidization when sheared. For these reasons, taken together, the CSK of the living eukaryotic cell is reminiscent of the class of materials called soft glasses, thus likening it to inert materials such as clays, pastes slurries, emulsions, and foams. Third, the cellular collective comprising a confluent epithelial layer can become solid-like and jammed, fluid-like and unjammed, or something in between. Esoteric though each may seem, these discoveries are consequential insofar as they impact our understanding of bronchospasm and wound healing as well as cancer cell invasion and embryonic development. Moreover, there are reasons to suspect that certain of these phenomena first arose in the early protist as a result of evolutionary pressures exerted by the primordial microenvironment. We have hypothesized, further, that each then became passed down virtually unchanged to the present day as a conserved core process. These topics are addressed here not only because they are interesting but also because they track the journey of one laboratory along a path less traveled by.

High-Intensity Focused Ultrasound Increases Collagen and Elastin Fiber Synthesis by Modulating Caveolin-1 in Aging Skin

Caveolin-1 (Cav-1) induces cellular senescence by reducing extracellular signal-regulated kinase (ERK)1/2 phosphorylation and activating p53 via inhibition of mouse double minute 2 homolog (MDM2) and sirtuin 1 (Sirt1), promoting cell cycle arrest and decreasing fibroblast proliferation and collagen synthesis. High-intensity focused ultrasound (HIFU) treatment increases collagen synthesis, rejuvenating skin. Using H2O2-induced senescent fibroblasts and the skin of 12-month-old mice, we tested the hypothesis that HIFU increases collagen production through Cav-1 modulation. HIFU was administered at 0.3, 0.5, or 0.7 J in the LINEAR and DOT modes. In both models, HIFU administration decreased Cav-1 levels, increased ERK1/2 phosphorylation, and decreased the binding of Cav-1 with both MDM2 and Sirt1. HIFU administration decreased p53 activation (acetylated p53) and p21 levels and increased cyclin D1, cyclin-dependent kinase 2, and proliferating cell nuclear antigen levels in both models. HIFU treatment increased collagen and elastin expression, collagen fiber accumulation, and elastin fiber density in aging skin, with 0.5 J in LINEAR mode resulting in the most prominent effects. HIFU treatment increased collagen synthesis to levels similar to those in Cav-1-silenced senescent fibroblasts. Our results suggest that HIFU administration increases dermal collagen and elastin fibers in aging skin via Cav-1 modulation and reduced p53 activity.

Mechanical microscopy of cancer cells: TGF-β induced epithelial to mesenchymal transition corresponds to low intracellular viscosity in cancer cells

Viscosity is an essential parameter that regulates bio-molecular reaction rates of diffusion-driven cellular processes. Hence, abnormal viscosity levels are often associated with various diseases and malfunctions like cancer. For this reason, monitoring intracellular viscosity becomes vital. While several approaches have been developed for in vitro and in vivo measurement of viscosity, analysis of intracellular viscosity in live cells has not yet been well realized. Our research introduces a novel, natural frequency-based, non-invasive method to determine the intracellular viscosity in cells. This method can not only efficiently analyze the differences in intracellular viscosity post modulation with molecules like PEG or glucose but is sensitive enough to distinguish the difference in intra-cellular viscosity among various cancer cell lines such as Huh-7, MCF-7, and MDAMB-231. Interestingly, TGF-β a cytokine reported to induce epithelial to mesenchymal transition (EMT), a feature associated with cancer invasiveness resulted in reduced viscosity of cancer cells, as captured through our method. To corroborate our findings with existing methods of analysis, we analyzed intra-cellular viscosity with a previously described viscosity-sensitive molecular rotor-based fluorophore-TPSII. In parity with our position sensing device (PSD)-based approach, an increase in fluorescence intensity was observed with viscosity enhancers, while, TGF-β exposure resulted in its reduction in the cells studied. This is the first study of its kind that attempts to characterize differences in intracellular viscosity using a novel, non-invasive PSD-based method.

Force propagation between epithelial cells depends on active coupling and mechano-structural polarization

Cell-generated forces play a major role in coordinating the large-scale behavior of cell assemblies, in particular during development, wound healing, and cancer. Mechanical signals propagate faster than biochemical signals, but can have similar effects, especially in epithelial tissues with strong cell-cell adhesion. However, a quantitative description of the transmission chain from force generation in a sender cell, force propagation across cell-cell boundaries, and the concomitant response of receiver cells is missing. For a quantitative analysis of this important situation, here we propose a minimal model system of two epithelial cells on an H-pattern ('cell doublet'). After optogenetically activating RhoA, a major regulator of cell contractility, in the sender cell, we measure the mechanical response of the receiver cell by traction force and monolayer stress microscopies. In general, we find that the receiver cells show an active response so that the cell doublet forms a coherent unit. However, force propagation and response of the receiver cell also strongly depend on the mechano-structural polarization in the cell assembly, which is controlled by cell-matrix adhesion to the adhesive micropattern. We find that the response of the receiver cell is stronger when the mechano-structural polarization axis is oriented perpendicular to the direction of force propagation, reminiscent of the Poisson effect in passive materials. We finally show that the same effects are at work in small tissues. Our work demonstrates that cellular organization and active mechanical response of a tissue are key to maintain signal strength and lead to the emergence of elasticity, which means that signals are not dissipated like in a viscous system, but can propagate over large distances.

Antagonizing cholecystokinin A receptor in the lung attenuates obesity-induced airway hyperresponsiveness

Obesity increases asthma prevalence and severity. However, the underlying mechanisms are poorly understood, and consequently, therapeutic options for asthma patients with obesity remain limited. Here we report that cholecystokinin-a metabolic hormone best known for its role in signaling satiation and fat metabolism-is increased in the lungs of obese mice and that pharmacological blockade of cholecystokinin A receptor signaling reduces obesity-associated airway hyperresponsiveness. Activation of cholecystokinin A receptor by the hormone induces contraction of airway smooth muscle cells. In vivo, cholecystokinin level is elevated in the lungs of both genetically and diet-induced obese mice. Importantly, intranasal administration of cholecystokinin A receptor antagonists (proglumide and devazepide) suppresses the airway hyperresponsiveness in the obese mice. Together, our results reveal an unexpected role for cholecystokinin in the lung and support the repurposing of cholecystokinin A receptor antagonists as a potential therapy for asthma patients with obesity.

Cell mechanical responses to subcellular perturbations generated by ultrasound and targeted microbubbles

The inherently dynamic and anisotropic microenvironment of cells imposes not only global and slow physical stimulations on cells but also acute and local perturbations. However, cell mechanical responses to transient subcellular physical signals remain unclear. In this study, acoustically activated targeted microbubbles were used to exert mechanical perturbations to single cells. The cellular contractile force was sensed by elastic micropillar arrays, while the pillar deformations were imaged using brightfield high-speed video microscopy at a frame rate of 1k frames per second for the first 10s and then confocal fluorescence microscopy. Cell mechanical responses are accompanied by cell membrane integrity changes. Both processes are determined by the perturbation strength generated by microbubble volumetric oscillations. The instantaneous cellular traction force relaxation exhibits two distinct patterns, correlated with two cell fates (survival or permanent damage). The mathematical modeling unveils that force-induced actomyosin disassembly leads to gradual traction force relaxation in the first few seconds. The perturbation may also influence the far end subcellular regions from the microbubbles and may propagate into connected cells with attenuations and delays. This study carefully characterizes the cell mechanical responses to local perturbations induced by ultrasound and microbubbles, advancing our understanding of the fundamentals of cell mechano-sensing, -responsiveness, and -transduction. STATEMENT OF SIGNIFICANCE: Subcellular physical perturbations commonly exist but haven't been fully explored yet. The subcellular perturbation generated by ultrasound and targeted microbubbles covers a wide range of strength, from mild, intermediate to intense, providing a broad biomedical relevance. With µm² spatial sensing ability and up to 1ms temporal resolution, we present spatiotemporal details of the instantaneous cellular contractile force changes followed by attenuated and delayed global responses. The correlation between the cell mechanical responses and cell fates highlights the important role of the instantaneous mechanical responses in the entire cellular reactive processes. Supported by mathematical modeling, our work provides new insights into the dynamics and mechanisms of cell mechanics.

Caldesmon controls stress fiber force-balance through dynamic cross-linking of myosin II and actin-tropomyosin filaments

Contractile actomyosin bundles are key force-producing and mechanosensing elements in muscle and non-muscle tissues. Whereas the organization of muscle myofibrils and mechanism regulating their contractility are relatively well-established, the principles by which myosin-II activity and force-balance are regulated in non-muscle cells have remained elusive. We show that Caldesmon, an important component of smooth muscle and non-muscle cell actomyosin bundles, is an elongated protein that functions as a dynamic cross-linker between myosin-II and tropomyosin-actin filaments. Depletion of Caldesmon results in aberrant lateral movement of myosin-II filaments along actin bundles, leading to irregular myosin distribution within stress fibers. This manifests as defects in stress fiber network organization and contractility, and accompanied problems in cell morphogenesis, migration, invasion, and mechanosensing. These results identify Caldesmon as critical factor that ensures regular myosin-II spacing within non-muscle cell actomyosin bundles, and reveal how stress fiber networks are controlled through dynamic cross-linking of tropomyosin-actin and myosin filaments.

Intrinsic cell rheology drives junction maturation

A fundamental property of higher eukaryotes that underpins their evolutionary success is stable cell-cell cohesion. Yet, how intrinsic cell rheology and stiffness contributes to junction stabilization and maturation is poorly understood. We demonstrate that localized modulation of cell rheology governs the transition of a slack, undulated cell-cell contact (weak adhesion) to a mature, straight junction (optimal adhesion). Cell pairs confined on different geometries have heterogeneous elasticity maps and control their own intrinsic rheology co-ordinately. More compliant cell pairs grown on circles have slack contacts, while stiffer triangular cell pairs favour straight junctions with flanking contractile thin bundles. Counter-intuitively, straighter cell-cell contacts have reduced receptor density and less dynamic junctional actin, suggesting an unusual adaptive mechano-response to stabilize cell-cell adhesion. Our modelling informs that slack junctions arise from failure of circular cell pairs to increase their own intrinsic stiffness and resist the pressures from the neighbouring cell. The inability to form a straight junction can be reversed by increasing mechanical stress artificially on stiffer substrates. Our data inform on the minimal intrinsic rheology to generate a mature junction and provide a springboard towards understanding elements governing tissue-level mechanics.

Gold Nano-Bio-Interaction to Modulate Mechanobiological Responses for Cancer Therapy Applications

In the present study, we investigate the mechanobiological responses of human lung cancer that may occur through their interactions with two different types of gold nanoparticles: nanostars and nanospheres. Hyperspectral images of nanoparticle-treated cells revealed different spatial distributions of nanoparticles in cells depending on their morphology, with nanospheres being more uniformly distributed in cells than nanostars. Gold nanospheres were also found to be more effective in mechanobiological modulations. They significantly suppressed the migratory ability of cells under different incubation times while lowering the bulk stiffness and adhesion of cells. This in vitro study suggests the potential applications of gold nanoparticles to manage cell migration. Nano-bio-interactions appeared to impact the cytoskeletal organization of cells and consequently alter the mechanical properties of cells, which could influence the cellular functions of cells. According to the results and migratory index model, it is thought that nanoparticle-treated cells experience mechanical changes in their body, which largely reduces their migratory potentials. These findings provide a better understanding of nano-bio-interaction in terms of cell mechanics and highlight the importance of mechanobiological responses in designing gold nanoparticles for cancer therapy.

Vimentin intermediate filaments and filamentous actin form unexpected interpenetrating networks that redefine the cell cortex

Filamentous actin (F-actin) and vimentin intermediate filaments (VIFs) are two major cytoskeletal components; they are generally thought to be spatially compartmentalized and to have distinctly different and independent functions. Here we combine two imaging methods, high-resolution structured illumination microscopy and cryo-electron tomography, as well as functional characterizations, to show that unexpectedly, VIFs and F-actin have extensive structural interactions within the cell cortex and form interpenetrating networks. These interactions have very important functional consequences for cells, which are broadly significant given the wide range of processes attributed to F-actin. These results profoundly alter our understanding of the contributions of cytoskeletal components and counter the common belief that VIFs and F-actin are independent in both structure and function.

The cytoskeleton of eukaryotic cells is primarily composed of networks of filamentous proteins, F-actin, microtubules, and intermediate filaments. Interactions among the cytoskeletal components are important in determining cell structure and in regulating cell functions. For example, F-actin and microtubules work together to control cell shape and polarity, while the subcellular organization and transport of vimentin intermediate filament (VIF) networks depend on their interactions with microtubules. However, it is generally thought that F-actin and VIFs form two coexisting but separate networks that are independent due to observed differences in their spatial distribution and functions. In this paper, we present a closer investigation of both the structural and functional interplay between the F-actin and VIF cytoskeletal networks. We characterize the structure of VIFs and F-actin networks within the cell cortex using structured illumination microscopy and cryo-electron tomography. We find that VIFs and F-actin form an interpenetrating network (IPN) with interactions at multiple length scales, and VIFs are integral components of F-actin stress fibers. From measurements of recovery of cell contractility after transient stretching, we find that the IPN structure results in enhanced contractile forces and contributes to cell resilience. Studies of reconstituted networks and dynamic measurements in cells suggest direct and specific associations between VIFs and F-actin. From these results, we conclude that VIFs and F-actin work synergistically, both in their structure and in their function. These results profoundly alter our understanding of the contributions of the components of the cytoskeleton, particularly the interactions between intermediate filaments and F-actin.

Cell monolayer deformation microscopy reveals mechanical fragility of cell monolayers following EMT

Tissue and cell mechanics are crucial factors in maintaining homeostasis and in development, with aberrant mechanics contributing to many diseases. During the epithelial-to-mesenchymal transition (EMT), a highly conserved cellular program in organismal development and cancer metastasis, cells gain the ability to detach from their original location and autonomously migrate. While a great deal of biochemical and biophysical changes at the single-cell level have been revealed, how the physical properties of multicellular assemblies change during EMT, and how this may affect disease progression, is unknown. Here we introduce cell monolayer deformation microscopy (CMDM), a new methodology to measure the planar mechanical properties of cell monolayers by locally applying strain and measuring their resistance to deformation. We employ this new method to characterize epithelial multicellular mechanics at early and late stages of EMT, finding the epithelial monolayers to be relatively compliant, ductile, and mechanically homogeneous. By comparison, the transformed mesenchymal monolayers, while much stiffer, were also more brittle, mechanically heterogeneous, displayed more viscoelastic creep, and showed sharp yield points at significantly lower strains. Here, CMDM measurements identify specific biophysical functional states of EMT and offer insight into how cell aggregates fragment under mechanical stress. This mechanical fingerprinting of multicellular assemblies using new quantitative metrics may also offer new diagnostic applications in healthcare to characterize multicellular mechanical changes in disease.

Three-Dimensional Visualization of the Podocyte Actin Network Using Integrated Membrane Extraction, Electron Microscopy, and Machine Learning

BACKGROUND

Actin stress fibers are abundant in cultured cells, but little is known about them in vivo. In podocytes, much evidence suggests that mechanobiologic mechanisms underlie podocyte shape and adhesion in health and in injury, with structural changes to actin stress fibers potentially responsible for pathologic changes to cell morphology. However, this hypothesis is difficult to rigorously test in vivo due to challenges with visualization. A technology to image the actin cytoskeleton at high resolution is needed to better understand the role of structures such as actin stress fibers in podocytes.

METHODS

We developed the first visualization technique capable of resolving the three-dimensional cytoskeletal network in mouse podocytes in detail, while definitively identifying the proteins that comprise this network. This technique integrates membrane extraction, focused ion-beam scanning electron microscopy, and machine learning image segmentation.

RESULTS

Using isolated mouse glomeruli from healthy animals, we observed actin cables and intermediate filaments linking the interdigitated podocyte foot processes to newly described contractile actin structures, located at the periphery of the podocyte cell body. Actin cables within foot processes formed a continuous, mesh-like, electron-dense sheet that incorporated the slit diaphragms.

CONCLUSIONS

Our new technique revealed, for the first time, the detailed three-dimensional organization of actin networks in healthy podocytes. In addition to being consistent with the gel compression hypothesis, which posits that foot processes connected by slit diaphragms act together to counterbalance the hydrodynamic forces across the glomerular filtration barrier, our data provide insight into how podocytes respond to mechanical cues from their surrounding environment.

The force loading rate drives cell mechanosensing through both reinforcement and cytoskeletal softening

Cell response to force regulates essential processes in health and disease. However, the fundamental mechanical variables that cells sense and respond to remain unclear. Here we show that the rate of force application (loading rate) drives mechanosensing, as predicted by a molecular clutch model. By applying dynamic force regimes to cells through substrate stretching, optical tweezers, and atomic force microscopy, we find that increasing loading rates trigger talin-dependent mechanosensing, leading to adhesion growth and reinforcement, and YAP nuclear localization. However, above a given threshold the actin cytoskeleton softens, decreasing loading rates and preventing reinforcement. By stretching rat lungs in vivo, we show that a similar phenomenon may occur. Our results show that cell sensing of external forces and of passive mechanical parameters (like tissue stiffness) can be understood through the same mechanisms, driven by the properties under force of the mechanosensing molecules involved.

Mechanical compression enhances ciliary beating through cytoskeleton remodeling in human nasal epithelial cells

Nasal inflammatory diseases, including nasal polyps and acute/chronic sinusitis, are characterized by impaired mucociliary clearance and eventually inflammation and infection. Contact of nasal polyps with adjacent nasal mucosa or stagnated mucus within the maxillary sinus produces compressive mechanical stresses on the apical surface of epithelium which can induce cytoskeleton remodeling in epithelial cells. In this study, we hypothesized that compressive stress modulates ciliary beating by altering the mechanical properties of the cytoskeleton of ciliated cell basal bodies. For the primary human nasal epithelial cells, we found that the applied compressive stress higher than the critical value of 1.0 kPa increased the stroke speed of cilia leading to the enhancement of ciliary beating frequency and mucociliary transportability. Immunostained images of the cytoskeleton showed reorganization and compactness of the actin filaments in the presence of compressive stress. Analysis of beating trajectory with the computational modeling for ciliary beating revealed that the stroke speed of cilium increased as the relative elasticity to viscosity of the surrounding cytoskeleton increases. These results suggest that the compressive stress on epithelial cells increases the ciliary beating speed through cytoskeleton remodeling to prevent mucus stagnation at the early stage of airway obstruction. Our study provides an insight into the defensive mechanism of airway epithelium against pathological conditions. STATEMENT OF SIGNIFICANCE: Cilia dynamics of the nasal epithelium is critical for not only maintaining normal breathing but preventing inflammatory diseases. It has been shown that mechanical compressive stresses can alter the shape and phenotype of epithelial cells. However, the effect of compressive stress on cilia dynamics is unclear. In this study, we demonstrated that the oscillation speed of cilia in human nasal epithelial cells was increased by the applied compressive stress experimentally. The computational simulation revealed that the change of ciliary beating dynamics was attributed to the viscoelastic properties of the reorganized cytoskeleton in response to compressive stress. Our results will be beneficial in understanding the defensive mechanism of airway epithelium against pathological conditions.

Assembly of Peripheral Actomyosin Bundles in Epithelial Cells Is Dependent on the CaMKK2/AMPK Pathway

Defects in the maintenance of intercellular junctions are associated with loss of epithelial barrier function and consequent pathological conditions, including invasive cancers. Epithelial integrity is dependent on actomyosin bundles at adherens junctions, but the origin of these junctional bundles is incompletely understood. Here we show that peripheral actomyosin bundles can be generated from a specific actin stress fiber subtype, transverse arcs, through their lateral fusion at cell-cell contacts. Importantly, we find that assembly and maintenance of peripheral actomyosin bundles are dependent on the mechanosensitive CaMKK2/AMPK signaling pathway and that inhibition of this route leads to disruption of tension-maintaining actomyosin bundles and re-growth of stress fiber precursors. This results in redistribution of cellular forces, defects in monolayer integrity, and loss of epithelial identity. These data provide evidence that the mechanosensitive CaMKK2/AMPK pathway is critical for the maintenance of peripheral actomyosin bundles and thus dictates cell-cell junctions through cellular force distribution.

Stiffening of the extracellular matrix is a sufficient condition for airway hyperreactivity

The current therapeutic approach to asthma focuses exclusively on targeting inflammation and reducing airway smooth muscle force to prevent the recurrence of symptoms. However, even when inflammation is brought under control, airways in an asthmatic can still hyperconstrict when exposed to a low dose of agonist. This suggests that there are mechanisms at play that are likely triggered by inflammation and eventually become self-sustaining so that even when airway inflammation is brought back under control, these alternative mechanisms continue to drive airway hyperreactivity in asthmatics. In this study, we hypothesized that stiffening of the airway extracellular matrix is a core pathological change sufficient to support excessive bronchoconstriction even in the absence of inflammation. To test this hypothesis, we increased the stiffness of the airway extracellular matrix by photo-crosslinking collagen fibers within the airway wall of freshly dissected bovine rings using riboflavin (vitamin B2) and Ultraviolet-A radiation. In our experiments, collagen crosslinking led to a twofold increase in the stiffness of the airway extracellular matrix. This change was sufficient to cause airways to constrict to a greater degree, and at a faster rate when they were exposed to 10^-5 M acetylcholine for 5 min. Our results show that stiffening of the extracellular matrix is sufficient to drive excessive airway constriction even in the absence of inflammatory signals.NEW & NOTEWORTHY Targeting inflammation is the central dogma on which current asthma therapy is based. Here, we show that a healthy airway can be made to constrict excessively and at a faster rate in response to the same stimulus by increasing the stiffness of the extracellular matrix, without the use of inflammatory agents. Our results provide an independent mechanism by which airway remodeling in asthma can sustain airway hyperreactivity even in the absence of inflammatory signals.

A Fully Integrated Arduino-Based System for the Application of Stretching Stimuli to Living Cells and Their Time-Lapse Observation: A Do-It-Yourself Biology Approach

Mechanobiology has nowadays acquired the status of a topic of fundamental importance in a degree in Biological Sciences. It is inherently a multidisciplinary topic where biology, physics and engineering competences are required. A course in mechanobiology should include lab experiences where students can appreciate how mechanical stimuli from outside affect living cell behaviour. Here we describe all the steps to build a cell stretcher inside an on-stage cell incubator. This device allows exposing living cells to a periodic mechanical stimulus similar to what happens in physiological conditions such as, for example, in the vascular system or in the lungs. The reaction of the cells to the periodic mechanical stretching represents a prototype of a mechanobiological signal integrated by living cells. We also provide the theoretical and experimental aspects related to the calibration of the stretcher apparatus at a level accessible to researchers not used to dealing with topics like continuum mechanics and analysis of deformations. We tested our device by stretching cells of two different lines, U87-MG and Balb-3T3 cells, and we analysed and discussed the effect of the periodic stimulus on both cell reorientation and migration. We also discuss the basic aspects related to the quantitative analysis of the reorientation process and of cell migration. We think that the device we propose can be easily reproduced at low-cost within a project-oriented course in the fields of biology, biotechnology and medical engineering.

Piezo1 channel activation in response to mechanobiological acoustic radiation force in osteoblastic cells

Mechanobiological stimuli, such as low-intensity pulsed ultrasound (LIPUS), have been shown to promote bone regeneration and fresh fracture repair, but the fundamental biophysical mechanisms involved remain elusive. Here, we propose that a mechanosensitive ion channel of Piezo1 plays a pivotal role in the noninvasive ultrasound-induced mechanical transduction pathway to trigger downstream cellular signal processes. This study aims to investigate the expression and role of Piezo1 in MC3T3-E1 cells after LIPUS treatment. Immunofluorescence analysis shows that Piezo1 was present on MC3T3-E1 cells and could be ablated by shRNA transfection. MC3T3-E1 cell migration and proliferation were significantly increased by LIPUS stimulation, and knockdown of Piezo1 restricted the increase in cell migration and proliferation. After labeling with Fluo-8, MC3T3-E1 cells exhibited fluorescence intensity traces with several high peaks compared with the baseline during LIPUS stimulation. No obvious change in the fluorescence intensity tendency was observed after LIPUS stimulation in shRNA-Piezo1 cells, which was similar to the results in the GsMTx4-treated group. The phosphorylation ratio of ERK1/2 in MC3T3-E1 cells was significantly increased (P < 0.01) after LIPUS stimulation. In addition, Phalloidin-iFluor-labeled F-actin filaments immediately accumulated in the perinuclear region after LIPUS stimulation, continued for 5 min, and then returned to their initial levels at 30 min. These results suggest that Piezo1 can transduce LIPUS-induced mechanical signals into intracellular calcium. The influx of Ca²⁺ serves as a second messenger to activate ERK1/2 phosphorylation and perinuclear F-actin filament polymerization, which regulate the proliferation of MC3T3-E1 cells.

Single Cell Forces after Electroporation

Exogenous high-voltage pulses increase cell membrane permeability through a phenomenon known as electroporation. This process may also disrupt the cell cytoskeleton causing changes in cell contractility; however, the contractile signature of cell force after electroporation remains unknown. Here, single-cell forces post-electroporation are measured using suspended extracellular matrix-mimicking nanofibers that act as force sensors. Ten, 100 μs pulses are delivered at three voltage magnitudes (500, 1000, and 1500 V) and two directions (parallel and perpendicular to cell orientation), exposing glioblastoma cells to electric fields between 441 V cm-1 and 1366 V cm-1. Cytoskeletal-driven force loss and recovery post-electroporation involves three distinct stages. Low electric field magnitudes do not cause disruption, but higher fields nearly eliminate contractility 2-10 min post-electroporation as cells round following calcium-mediated retraction (stage 1). Following rounding, a majority of analyzed cells enter an unusual and unexpected biphasic stage (stage 2) characterized by increased contractility tens of minutes post-electroporation, followed by force relaxation. The biphasic stage is concurrent with actin disruption-driven blebbing. Finally, cells elongate and regain their pre-electroporation morphology and contractility in 1-3 h (stage 3). With increasing voltages applied perpendicular to cell orientation, we observe a significant drop in cell viability. Experiments with multiple healthy and cancerous cell lines demonstrate that contractile force is a more dynamic and sensitive metric than cell shape to electroporation. A mechanobiological understanding of cell contractility post-electroporation will deepen our understanding of the mechanisms that drive recovery and may have implications for molecular medicine, genetic engineering, and cellular biophysics.

Generation of stress fibers through myosin-driven reorganization of the actin cortex

Contractile actomyosin bundles, stress fibers, govern key cellular processes including migration, adhesion, and mechanosensing. Stress fibers are thus critical for developmental morphogenesis. The most prominent actomyosin bundles, ventral stress fibers, are generated through coalescence of pre-existing stress fiber precursors. However, whether stress fibers can assemble through other mechanisms has remained elusive. We report that stress fibers can also form without requirement of pre-existing actomyosin bundles. These structures, which we named cortical stress fibers, are embedded in the cell cortex and assemble preferentially underneath the nucleus. In this process, non-muscle myosin II pulses orchestrate the reorganization of cortical actin meshwork into regular bundles, which promote reinforcement of nascent focal adhesions, and subsequent stabilization of the cortical stress fibers. These results identify a new mechanism by which stress fibers can be generated de novo from the actin cortex and establish role for stochastic myosin pulses in the assembly of functional actomyosin bundles.

Adipokine Leptin Co-operates With Mechanosensitive Ca2 +-Channels and Triggers Actomyosin-Mediated Motility of Breast Epithelial Cells

In postmenopausal women, a major risk factor for the development of breast cancer is obesity. In particular, the adipose tissue-derived adipokine leptin has been strongly linked to tumor cell proliferation, migration, and metastasis, but the underlying mechanisms remain unclear. Here we show that treatment of normal mammary epithelial cells with leptin induces EMT-like features characterized by higher cellular migration speeds, loss of structural ordering of 3D-mammo spheres, and enhancement of epithelial traction forces. Mechanistically, leptin triggers the phosphorylation of myosin light chain kinase-2 (MLC-2) through the interdependent activity of leptin receptor and Ca2+ channels. These data provide evidence that leptin-activated leptin receptors, in co-operation with mechanosensitive Ca2+ channels, play a role in the development of breast carcinomas through the regulation of actomyosin dynamics.

An Acoustic Platform for Single-Cell, High-Throughput Measurements of the Viscoelastic Properties of Cells

Cellular processes including adhesion, migration, and differentiation are governed by the distinct mechanical properties of each cell. Importantly, the mechanical properties of individual cells can vary depending on local physical and biochemical cues in a time-dependent manner resulting in significant inter-cell heterogeneity. While several different methods have been developed to interrogate the mechanical properties of single cells, throughput to capture this heterogeneity remains an issue. Here, single-cell, high-throughput characterization of adherent cells is demonstrated using acoustic force spectroscopy (AFS). AFS works by simultaneously, acoustically driving tens to hundreds of silica beads attached to cells away from the cell surface, allowing the user to measure the stiffness of adherent cells under multiple experimental conditions. It is shown that cells undergo marked changes in viscoelasticity as a function of temperature, by altering the temperature within the AFS microfluidic circuit between 21 and 37 °C. In addition, quantitative differences in cells exposed to different pharmacological treatments specifically targeting the membrane-cytoskeleton interface are shown. Further, the high-throughput format of the AFS is utilized to rapidly probe, in excess of 1000 cells, three different cell lines expressing different levels of a mechanosensitive protein, Piezo1, demonstrating the ability to differentiate between cells based on protein expression levels.

G Protein-Coupled Estrogen Receptor Regulates Actin Cytoskeleton Dynamics to Impair Cell Polarization

Mechanical forces regulate cell functions through multiple pathways. G protein-coupled estrogen receptor (GPER) is a seven-transmembrane receptor that is ubiquitously expressed across tissues and mediates the acute cellular response to estrogens. Here, we demonstrate an unidentified role of GPER as a cellular mechanoregulator. G protein-coupled estrogen receptor signaling controls the assembly of stress fibers, the dynamics of the associated focal adhesions, and cell polarization via RhoA GTPase (RhoA). G protein-coupled estrogen receptor activation inhibits F-actin polymerization and subsequently triggers a negative feedback that transcriptionally suppresses the expression of monomeric G-actin. Given the broad expression of GPER and the range of cytoskeletal changes modulated by this receptor, our findings position GPER as a key player in mechanotransduction.

Anxiety and depression: A matter of stiffness?

Cells react to stress by the universal responses of "fluidization" or "reinforcement" (stiffening) of the cytoplasm, through dramatic re-arrangements of the cytoskeleton. Here it is suggested that, at a supracellular level, the brain exhibits such a fundamental behavior as part of its complex response to stress: it is hypothesized that the soft gel formed by brain cell cytoskeletons and the surrounding extracellular matrix (the "cytoskeletons-matrix system") undergoes transitions either to sol (fluidization) or stiff gel (reinforcement) as a very fundamental and evolutionarily conserved brain response to stress, alongside more sophisticated neural pathways. Sol state corresponds to increased cell activity (a sort of "fight or flight" response), whereas stiff gel state corresponds to inactivity (an "immobility" strategy). Psychological stress, through simple stress signals such as pH changes, would lead to an initial tissue fluidization in key regions of the brain, followed, if the stress stimuli persist, by reinforcement (slow formation of actomyosin stress fibers and matrix stiffening). It is also hypothesized that the cytoskeletons-matrix system is one of the biological correlates of so-called "background feelings", i.e conscious feelings built on inner chemical-physical states of the body. Optimal dynamics of the cytoskeletons-matrix system would contribute to a core feeling of well-being, while shifts towards fluidization (activation) or stiffening (inactivation) would contribute to background feelings at the basis of anxiety and stress-induced depression, respectively. It is suggested that the cytoskeletons-matrix system behaves as a "self-organized critical system", anxiety and depression arising whenever the system is driven too far from the optimal critical point. Finally, some application hints from the proposed ideas are given.

Nonlinear Elastic and Inelastic Properties of Cells

Mechanical forces play an important role in various physiological processes, such as morphogenesis, cytokinesis, and migration. Thus, in order to illuminate mechanisms underlying these physiological processes, it is crucial to understand how cells deform and respond to external mechanical stimuli. During recent decades, the mechanical properties of cells have been studied extensively using diverse measurement techniques. A number of experimental studies have shown that cells are far from linear elastic materials. Cells exhibit a wide variety of nonlinear elastic and inelastic properties. Such complicated properties of cells are known to emerge from unique mechanical characteristics of cellular components. In this review, we introduce major cellular components that largely govern cell mechanical properties and provide brief explanations of several experimental techniques used for rheological measurements of cell mechanics. Then, we discuss the representative nonlinear elastic and inelastic properties of cells. Finally, continuum and discrete computational models of cell mechanics, which model both nonlinear elastic and inelastic properties of cells, will be described.

Intercellular communication controls agonist-induced calcium oscillations independently of gap junctions in smooth muscle cells

In this study, we report the existence of a communication system among human smooth muscle cells that uses mechanical forces to frequency modulate long-range calcium waves. An important consequence of this mechanical signaling is that changes in stiffness of the underlying extracellular matrix can interfere with the frequency modulation of Ca²⁺ waves, causing smooth muscle cells from healthy human donors to falsely perceive a much higher agonist dose than they actually received. This aberrant sensing of contractile agonist dose on stiffer matrices is completely absent in isolated smooth muscle cells, although the isolated cells can sense matrix rigidity. We show that the intercellular communication that enables this collective Ca²⁺ response in smooth muscle cells does not involve transport across gap junctions or extracellular diffusion of signaling molecules. Instead, our data support a collective model in which mechanical signaling among smooth muscle cells regulates their response to contractile agonists.

Mechanopharmacology of Rho-kinase antagonism in airway smooth muscle and potential new therapy for asthma

The principle of mechanopharmacology of airway smooth muscle (ASM) is based on the premise that physical agitation, such as pressure oscillation applied to an airway, is able to induce bronchodilation by reducing contractility and softening the cytoskeleton of ASM. Although the underlying mechanism is not entirely clear, there is evidence to suggest that large-amplitude stretches are able to disrupt the actomyosin interaction in the crossbridge cycle and weaken the cytoskeleton in ASM cells. Rho-kinase is known to enhance force generation and strengthen structural integrity of the cytoskeleton during smooth muscle activation and plays a key role in the maintenance of force during prolonged muscle contractions. Synergy in relaxation has been observed when the muscle is subject to oscillatory length change while Rho-kinase is pharmacologically inhibited. In this review, inhibition of Rho-kinase coupled to therapeutic pressure oscillation applied to the airways is explored as a combination treatment for asthma.

Inhibiting Airway Smooth Muscle Contraction Using Pitavastatin: A Role for the Mevalonate Pathway in Regulating Cytoskeletal Proteins

Despite maximal use of currently available therapies, a significant number of asthma patients continue to experience severe, and sometimes life-threatening bronchoconstriction. To fill this therapeutic gap, we examined a potential role for the 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) inhibitor, pitavastatin. Using human airway smooth muscle (ASM) cells and murine precision-cut lung slices, we discovered that pitavastatin significantly inhibited basal-, histamine-, and methacholine (MCh)-induced ASM contraction. This occurred via reduction of myosin light chain 2 (MLC2) phosphorylation, and F-actin stress fiber density and distribution, in a mevalonate (MA)- and geranylgeranyl pyrophosphate (GGPP)-dependent manner. Pitavastatin also potentiated the ASM relaxing effect of a simulated deep breath, a beneficial effect that is notably absent with the β2-agonist, isoproterenol. Finally, pitavastatin attenuated ASM pro-inflammatory cytokine production in a GGPP-dependent manner. By targeting all three hallmark features of ASM dysfunction in asthma—contraction, failure to adequately relax in response to a deep breath, and inflammation—pitavastatin may represent a unique asthma therapeutic.

Structural and mechanical remodeling of the cytoskeleton maintains tensional homeostasis in 3D microtissues under acute dynamic stretch

When stretched, cells cultured on 2D substrates share a universal softening and fluidization response that arises from poorly understood remodeling of well-conserved cytoskeletal elements. It is known, however, that the structure and distribution of the cytoskeleton is profoundly influenced by the dimensionality of a cell's environment. Therefore, in this study we aimed to determine whether cells cultured in a 3D matrix share this softening behavior and to link it to cytoskeletal remodeling. To achieve this, we developed a high-throughput approach to measure the dynamic mechanical properties of cells and allow for sub-cellular imaging within physiologically relevant 3D microtissues. We found that fibroblast, smooth muscle and skeletal muscle microtissues strain softened but did not fluidize, and upon loading cessation, they regained their initial mechanical properties. Furthermore, microtissue prestress decreased with the strain amplitude to maintain a constant mean tension. This adaptation under an auxotonic condition resulted in lengthening. A filamentous actin cytoskeleton was required, and responses were mirrored by changes to actin remodeling rates and visual evidence of stretch-induced actin depolymerization. Our new approach for assessing cell mechanics has linked behaviors seen in 2D cultures to a 3D matrix, and connected remodeling of the cytoskeleton to homeostatic mechanical regulation of tissues.

Characterizing poroelasticity of biological tissues by spherical indentation: an improved theory for large relaxation

Flow of fluids within biological tissues often meets with resistance that causes a rate- and size-dependent material behavior known as poroelasticity. Characterizing poroelasticity can provide insight into a broad range of physiological functions, and is done qualitatively in the clinic by palpation. Indentation has been widely used for characterizing poroelasticity of soft materials, where quantitative interpretation of indentation requires a model of the underlying physics, and such existing models are well established for cases of small strain and modest force relaxation. We showed here that existing models are inadequate for large relaxation, where the force on the indenter at a prescribed depth at long-time scale drops to below half of the initially peak force (i.e., F(0)/F(∞) > 2). We developed an indentation theory for such cases of large relaxation, based on Biot theory and a generalized Hertz contact model. We demonstrated that our proposed theory is suitable for biological tissues (e.g., spleen, kidney, skin and human cirrhosis liver) with both small and large relaxations. The proposed method would be a powerful tool to characterize poroelastic properties of biological materials for various applications such as pathological study and disease diagnosis.

Statin-mediated cholesterol depletion exerts coordinated effects on the alterations in rat vascular smooth muscle cell biomechanics and migration

KEY POINTS

This study demonstrates and evaluates the changes in rat vascular smooth muscle cell biomechanics following statin-mediated cholesterol depletion. Evidence is presented to show correlated changes in migration and adhesion of vascular smooth muscle cells to extracellular matrix proteins fibronectin and collagen. Concurrently, integrin α5 expression was enhanced but not integrin α2. Atomic force microscopy analysis provides compelling evidence of coordinated reduction in vascular smooth muscle cell stiffness and actin cytoskeletal orientation in response to statin-mediated cholesterol depletion. Proof is provided that statin-mediated cholesterol depletion remodels total vascular smooth muscle cell cytoskeletal orientation that may additionally participate in altering ex vivo aortic vessel function. It is concluded that statin-mediated cholesterol depletion may coordinate vascular smooth muscle cell migration and adhesion to different extracellular matrix proteins and regulate cellular stiffness and cytoskeletal orientation, thus impacting the biomechanics of the cell.

ABSTRACT

Not only does cholesterol induce an inflammatory response and deposits in foam cells at the atherosclerotic plaque, it also regulates cellular mechanics, proliferation and migration in atherosclerosis progression. Statins are HMG-CoA reductase inhibitors that are known to inhibit cellular cholesterol biosynthesis and are clinically prescribed to patients with hypercholesterolemia or related cardiovascular conditions. Nonetheless, the effect of statin-mediated cholesterol management on cellular biomechanics is not fully understood. In this study, we aimed to assess the effect of fluvastatin-mediated cholesterol management on primary rat vascular smooth muscle cell (VSMC) biomechanics. Real-time measurement of cell adhesion, stiffness, and imaging were performed using atomic force microscopy (AFM). Cellular migration on extra cellular matrix (ECM) protein surfaces was studied by time-lapse imaging. The effect of changes in VSMC biomechanics on aortic function was assessed using an ex vivo myograph system. Fluvastatin-mediated cholesterol depletion (-27.8%) lowered VSMC migration distance on a fibronectin (FN)-coated surface (-14.8%) but not on a type 1 collagen (COL1)-coated surface. VSMC adhesion force to FN (+33%) and integrin α5 expression were enhanced but COL1 adhesion and integrin α2 expression were unchanged upon cholesterol depletion. In addition, VSMC stiffness (-46.6%) and ex vivo aortic ring contraction force (-40.1%) were lowered and VSMC actin cytoskeletal orientation was reduced (-24.5%) following statin-mediated cholesterol depletion. Altogether, it is concluded that statin-mediated cholesterol depletion may coordinate VSMC migration and adhesion to different ECM proteins and regulate cellular stiffness and cytoskeletal orientation, thus impacting the biomechanics of the cell and aortic function.

Mechanobiology of dynamic enzyme systems

This Perspective paper advances a hypothesis of mechanosensation by endothelial cells in which the cell is a dynamic crowded system, driven by continuous enzyme activity, that can be shifted from one non-equilibrium state to another by external force. The nature of the shift will depend on the direction, rate of change, and magnitude of the force. Whether force induces a pathophysiological or physiological change in cell biology will be determined by whether the dynamics of a cellular system can accommodate the dynamics and magnitude of the force application. The complex interplay of non-static cytoskeletal structures governs internal cellular rheology, dynamic spatial reorganization, and chemical kinetics of proteins such as integrins, and a flaccid membrane that is dynamically supported; each may constitute the necessary dynamic properties able to sense external fluid shear stress and reorganize in two and three dimensions. The resulting reorganization of enzyme systems in the cell membrane and cytoplasm may drive the cell to a new physiological state. This review focuses on endothelial cell mechanotransduction of shear stress, but may lead to new avenues of investigation of mechanobiology in general requiring new tools for interrogation of mechanobiological systems, tools that will enable the synthesis of large amounts of spatial and temporal data at the molecular, cellular, and system levels.

Tropomodulins Control the Balance between Protrusive and Contractile Structures by Stabilizing Actin-Tropomyosin Filaments

Eukaryotic cells have diverse protrusive and contractile actin filament structures, which compete with one another for a limited pool of actin monomers. Numerous actin-binding proteins regulate the dynamics of actin structures, including tropomodulins (Tmods), which cap the pointed end of actin filaments. In striated muscles, Tmods prevent actin filaments from overgrowing, whereas in non-muscle cells, their function has remained elusive. Here, we identify two Tmod isoforms, Tmod1 and Tmod3, as key components of contractile stress fibers in non-muscle cells. Individually, Tmod1 and Tmod3 can compensate for one another, but their simultaneous depletion results in disassembly of actin-tropomyosin filaments, loss of force-generating stress fibers, and severe defects in cell morphology. Knockout-rescue experiments reveal that Tmod's interaction with tropomyosin is essential for its role in the stabilization of actin-tropomyosin filaments in cells. Thus, in contrast to their role in muscle myofibrils, in non-muscle cells, Tmods bind actin-tropomyosin filaments to protect them from depolymerizing, not elongating. Furthermore, loss of Tmods shifts the balance from linear actin-tropomyosin filaments to Arp2/3 complex-nucleated branched networks, and this phenotype can be partially rescued by inhibiting the Arp2/3 complex. Collectively, the data reveal that Tmods are essential for the maintenance of contractile actomyosin bundles and that Tmod-dependent capping of actin-tropomyosin filaments is critical for the regulation of actin homeostasis in non-muscle cells.

Tissue traction microscopy to quantify muscle contraction within precision-cut lung slices

In asthma, acute bronchospasm is driven by contractile forces of airway smooth muscle (ASM). These forces can be imaged in the cultured ASM cell or assessed in the muscle strip and the tracheal/bronchial ring, but in each case, the ASM is studied in isolation from the native airway milieu. Here, we introduce a novel platform called tissue traction microscopy (TTM) to measure ASM contractile force within porcine and human precision-cut lung slices (PCLS). Compared with the conventional measurements of lumen area changes in PCLS, TTM measurements of ASM force changes are 1) more sensitive to bronchoconstrictor stimuli, 2) less variable across airways, and 3) provide spatial information. Notably, within every human airway, TTM measurements revealed local regions of high ASM contraction that we call "stress hotspots". As an acute response to cyclic stretch, these hotspots promptly decreased but eventually recovered in magnitude, spatial location, and orientation, consistent with local ASM fluidization and resolidification. By enabling direct and precise measurements of ASM force, TTM should accelerate preclinical studies of airway reactivity.

Stiffening and inelastic fluidization in vimentin intermediate filament networks

Intermediate filaments are cytoskeletal proteins that are key regulators of cell mechanics, a role which is intrinsically tied to their hierarchical structure and their unique ability to accommodate large axial strains. However, how the single-filament response to applied strains translates to networks remains unclear, particularly with regards to the crosslinking role played by the filaments' disordered "tail" domains. Here we test the role of these noncovalent crosslinks in the nonlinear rheology of reconstituted networks of the intermediate filament protein vimentin, probing their stress- and rate-dependent mechanics. Similarly to previous studies we observe elastic stress-stiffening but unlike previous work we identify a characteristic yield stress σ*, above which the networks exhibit rate-dependent softening of the network, referred to as inelastic fluidization. By investigating networks formed from tail-truncated vimentin, in which noncovalent crosslinking is suppressed, and glutaraldehyde-treated vimentin, in which crosslinking is made permanent, we show that rate-dependent inelastic fluidization is a direct consequence of vimentin's transient crosslinking. Surprisingly, although the tail-tail crosslinks are individually weak, the effective timescale for stress relaxation of the network exceeds 1000 s at σ*. Vimentin networks can therefore maintain their integrity over a large range of strains (up to ∼1000%) and loading rates (10-3 to 10-1 s-1). Our results provide insight into how the hierarchical structure of vimentin networks contributes to the cell's ability to be deformable yet strong.

Extracellular matrix stiffness regulates human airway smooth muscle contraction by altering the cell-cell coupling

For an airway or a blood vessel to narrow, there must be a connected path that links the smooth muscle (SM) cells with each other, and transmits forces around the organ, causing it to constrict. Currently, we know very little about the mechanisms that regulate force transmission pathways in a multicellular SM ensemble. Here, we used extracellular matrix (ECM) micropatterning to study force transmission in a two-cell ensemble of SM cells. Using the two-SM cell ensemble, we demonstrate (a) that ECM stiffness acts as a switch that regulates whether SM force is transmitted through the ECM or through cell-cell connections. (b) Fluorescent imaging for adherens junctions and focal adhesions show the progressive loss of cell-cell borders and the appearance of focal adhesions with the increase in ECM stiffness (confirming our mechanical measurements). (c) At the same ECM stiffness, we show that the presence of a cell-cell border substantially decreases the overall contractility of the SM cell ensemble. Our results demonstrate that connectivity among SM cells is a critical factor to consider in the development of diseases such as asthma and hypertension.

Forces and constraints controlling podosome assembly and disassembly

Podosomes are a singular category of integrin-mediated adhesions important in the processes of cell migration, matrix degradation and cancer cell invasion. Despite a wealth of biochemical studies, the effects of mechanical forces on podosome integrity and dynamics are poorly understood. Here, we show that podosomes are highly sensitive to two groups of physical factors. First, we describe the process of podosome disassembly induced by activation of myosin-IIA filament assembly. Next, we find that podosome integrity and dynamics depends upon membrane tension and can be experimentally perturbed by osmotic swelling and deoxycholate treatment. We have also found that podosomes can be disrupted in a reversible manner by single or cyclic radial stretching of the substratum. We show that disruption of podosomes induced by osmotic swelling is independent of myosin-II filaments. The inhibition of the membrane sculpting protein, dynamin-II, but not clathrin, resulted in activation of myosin-IIA filament formation and disruption of podosomes. The effect of dynamin-II inhibition on podosomes was, however, independent of myosin-II filaments. Moreover, formation of organized arrays of podosomes in response to microtopographic cues (the ridges with triangular profile) was not accompanied by reorganization of myosin-II filaments. Thus, mechanical elements such as myosin-II filaments and factors affecting membrane tension/sculpting independently modulate podosome formation and dynamics, underlying a versatile response of these adhesion structures to intracellular and extracellular cues. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

A practical review on the measurement tools for cellular adhesion force

Cell-cell and cell-matrix adhesions are fundamental in all multicellular organisms. They play a key role in cellular growth, differentiation, pattern formation and migration. Cell-cell adhesion is substantial in the immune response, pathogen-host interactions, and tumor development. The success of tissue engineering and stem cell implantations strongly depends on the fine control of live cell adhesion on the surface of natural or biomimetic scaffolds. Therefore, the quantitative and precise measurement of the adhesion strength of living cells is critical, not only in basic research but in modern technologies, too. Several techniques have been developed or are under development to quantify cell adhesion. All of them have their pros and cons, which has to be carefully considered before the experiments and interpretation of the recorded data. Current review provides a guide to choose the appropriate technique to answer a specific biological question or to complete a biomedical test by measuring cell adhesion.

Traction Force Screening Enabled by Compliant PDMS Elastomers

Actomyosin contractility is an essential element of many aspects of cellular biology and manifests as traction forces that cells exert on their surroundings. The central role of these forces makes them a novel principal therapeutic target in diverse diseases. This requires accurate and higher-capacity measurements of traction forces; however, existing methods are largely low throughput, limiting their utility in broader applications. To address this need, we employ Fourier-transform traction force microscopy in a parallelized 96-well format, which we refer to as contractile force screening. Critically, rather than the frequently employed hydrogel polyacrylamide, we fabricate these plates using polydimethylsiloxane rubber. Key to this approach is that the polydimethylsiloxane used is very compliant, with a lower-bound Young's modulus of ∼0.4 kPa. We subdivide these monolithic substrates spatially into biochemically independent wells, creating a uniform multiwell platform for traction force screening. We demonstrate the utility and versatility of this platform by quantifying the compound and dose-dependent contractility responses of human airway smooth muscle cells and retinal pigment epithelial cells. By directly quantifying the endpoint of therapeutic intent, airway-smooth-muscle contractile force, this approach fills an important methodological void in current screening approaches for bronchodilator drug discovery, and, more generally, in measuring contractile response for a broad range of cell types and pathologies.

A β2-Integrin/MRTF-A/SRF Pathway Regulates Dendritic Cell Gene Expression, Adhesion, and Traction Force Generation

β2-integrins are essential for immune system function because they mediate immune cell adhesion and signaling. Consequently, a loss of β₂-integrin expression or function causes the immunodeficiency disorders, Leukocyte Adhesion Deficiency (LAD) type I and III. LAD-III is caused by mutations in an important integrin regulator, kindlin-3, but exactly how kindlin-3 regulates leukocyte adhesion has remained incompletely understood. Here we demonstrate that mutation of the kindlin-3 binding site in the β2-integrin (TTT/AAA-β2-integrin knock-in mouse/KI) abolishes activation of the actin-regulated myocardin related transcription factor A/serum response factor (MRTF-A/SRF) signaling pathway in dendritic cells and MRTF-A/SRF-dependent gene expression. We show that Ras homolog gene family, member A (RhoA) activation and filamentous-actin (F-actin) polymerization is abolished in murine TTT/AAA-β2-integrin KI dendritic cells, which leads to a failure of MRTF-A to localize to the cell nucleus to coactivate genes together with SRF. In addition, we show that dendritic cell gene expression, adhesion and integrin-mediated traction forces on ligand coated surfaces is dependent on the MRTF-A/SRF signaling pathway. The participation of β2-integrin and kindlin-3-mediated cell adhesion in the regulation of the ubiquitous MRTF-A/SRF signaling pathway in immune cells may help explain the role of β2-integrin and kindlin-3 in integrin-mediated gene regulation and immune system function.

Mechanical response of an epithelial island subject to uniaxial stretch on a hybrid silicone substrate

Introduction

The mechanical response of large multi-cellular collectives to external stretch has remained largely unexplored, despite its relevance to normal function and to external challenges faced by some tissues. Here, we introduced a simple hybrid silicone substrate to enable external stretch while providing a physiologically relevant physical micro-environment for cells.

Methods

We micropatterned epithelial islands on the substrate using a stencil to allow for a circular island shape without restraining island edges. We then used traction force microscopy to determine the strain energy and the inter-cellular sheet tension within the island as a function of time after stretch.

Results

While the strain energy stored in the substrate for unstretched cell islands stayed constant over time, a uniaxial 10% stretch resulted in an abrupt increase, followed by sustained increase in the strain energy of the islands over tens of minutes, indicating slower dynamics than for single cells reported previously. The sheet tension at the island mid-line perpendicular to the stretch direction also more than doubled compared to unstretched islands. Interestingly, the sheet tension at the island mid-line parallel to the stretch direction also reached similar levels over tens of minutes indicating the tendency of the island to homogenize its internal stress.

Conclusions

We found that the sheet tension within large epithelial islands depends on its direction relative to that of the stretch initially, but not at longer times. We suggest that the hybrid silicone substrate provides for an accessible substrate for studying the mechanobiology of large epithelial cell islands.

The Other Side of the Fascia: The Smooth Muscle Part 1

According to current scientific standards, the fascia is a connective tissue derived from two separate germ layers, the mesoderm (trunk and limbs, part of the neck) and the ectoderm (cervical tract and skull). The fascia has the property of maintaining the shape and function of its anatomical district, but it also can adapt to mechanical-metabolic stimuli. Smooth muscle and non-voluntary striated musculature originated from the mesoderm have never been properly considered as a type of fascia. They are some of the viscera present in the mediastinum, in the abdomen and in the pelvic floor. This text represents the first article in the international scientific field that discusses the inclusion of some viscera in the context of what is considered fascia, thanks to the efforts of our committee for the definition and nomenclature of the fascial tissue of the Foundation of Osteopathic Research and Clinical Endorsement (FORCE).