Appreciating force and shape—the rise of mechanotransduction in cell biology

A Multisensory Network Drives Nuclear Mechanoadaptation

Cells have adapted to mechanical forces early in evolution and have developed multiple mechanisms ensuring sensing of, and adaptation to, the diversity of forces operating outside and within organisms. The nucleus must necessarily adapt to all types of mechanical signals, as its functions are essential for virtually all cell processes, many of which are tuned by mechanical cues. To sense forces, the nucleus is physically connected with the cytoskeleton, which senses and transmits forces generated outside and inside the cell. The nuclear LINC complex bridges the cytoskeleton and the nuclear lamina to transmit mechanical information up to the chromatin. This system creates a force-sensing macromolecular complex that, however, is not sufficient to regulate all nuclear mechanoadaptation processes. Within the nucleus, additional mechanosensitive structures, including the nuclear envelope and the nuclear pore complex, function to regulate nuclear mechanoadaptation. Similarly, extra nuclear mechanosensitive systems based on plasma membrane dynamics, mechanotransduce information to the nucleus. Thus, the nucleus has the intrinsic structural components needed to receive and interpret mechanical inputs, but also rely on extra nuclear mechano-sensors that activate nuclear regulators in response to force. Thus, a network of mechanosensitive cell structures ensures that the nucleus has a tunable response to mechanical cues.

Sticking around: Cell adhesion patterning for energy minimization and substrate mechanosensing

Tissue stiffness (Young's modulus) is a key control parameter in cell behavior and bioengineered gels where defined mechanical properties have become an essential part of the toolkit for interrogating mechanotransduction. Here, we show using a mechanical cell model that the effective substrate stiffness experienced by a cell depends, not just on the engineered mechanical properties of the substrate but critically also on the particular arrangement of adhesions between cell and substrate. In particular, we find that cells with different adhesion patterns can experience two different gel stiffnesses as equivalent and will generate the same mean cell deformations. In considering small patches of adhesion, which mimic focal adhesion complexes, we show how the experimentally observed focal adhesion growth and elongation on stiff substrates can be explained by energy considerations. Relatedly, energy arguments also provide a reason why nascent adhesions do not establish into focal adhesions on soft substrates, as has been commonly observed. Fewer and larger adhesions are predicted to be preferred over more and smaller, an effect enhanced by random spot placing with the simulations predicting qualitatively realistic cell shapes in this case.

Engineering Bright and Mechanosensitive Alkaline-Earth Rare-Earth Upconverting Nanoparticles

Upconverting nanoparticles (UCNPs) are an emerging platform for mechanical force sensing at the nanometer scale. An outstanding challenge in realizing nanometer-scale mechano-sensitive UCNPs is maintaining a high mechanical force responsivity in conjunction with bright optical emission. This Letter reports mechano-sensing UCNPs based on the lanthanide dopants Yb³⁺ and Er³⁺, which exhibit a strong ratiometric change in emission spectra and bright emission under applied pressure. We synthesize and analyze the pressure response of five different types of nanoparticles, including cubic NaYF₄ host nanoparticles and alkaline-earth host materials CaLuF, SrLuF, SrYbF, and BaLuF, all with lengths of 15 nm or less. By combining optical spectroscopy in a diamond anvil cell with single-particle brightness, we determine the noise equivalent sensitivity (GPa/√Hz) of these particles. The SrYb_0.72Er_0.28F@SrLuF particles exhibit an optimum noise equivalent sensitivity of 0.26 ± 0.04 GPa/√Hz. These particles present the possibility of robust nanometer-scale mechano-sensing.

Recent Advances and Emerging Challenges in the Molecular Modeling of Mechanobiological Processes

Many biological processes result from the effect of mechanical forces on macromolecular structures and on their interactions. In particular, the cell shape, motion, and differentiation directly depend on mechanical stimuli from the extracellular matrix or from neighboring cells. The development of experimental techniques that can measure and characterize the tiny forces acting at the cellular scale and down to the single-molecule, biomolecular level has enabled access to unprecedented details about the involved mechanisms. However, because the experimental observables often do not provide a direct atomistic picture of the corresponding phenomena, particle-based simulations performed at various scales are instrumental in complementing these experiments and in providing a molecular interpretation. Here, we will review the recent key achievements in the field, and we will highlight and discuss the many technical challenges these simulations are facing, as well as suggest future directions for improvement.

Factoring in the force: A novel role for eIF6

eIF6 is known for its role as a stimulatory translation initiation factor. In this issue, Keen et al. (2022. J. Cell Biol. https://doi.org/10.1083/jcb.202005213) identify a novel, noncanonical role, whereby eIF6 regulates focal adhesion formation, mechanosensing, and cell mechanics, independent of its translational role.

The revolutionary developmental biology of Wilhelm His, Sr

Swiss‐born embryologist Wilhelm His, Sr. (1831–1904) was the first scientist to study embryos using paraffin histology, serial sectioning and three‐dimensional modelling. With these techniques, His made many important discoveries in vertebrate embryology and developmental neurobiology, earning him two Nobel Prize nominations. He also developed several theories of mechanical and evolutionary developmental biology. His argued that adult form is determined by the differential growth of developmental primordia. Furthermore, he suggested that changes in the growth parameters of those primordia are responsible for generating new phenotypes during evolution. His developed these theories in his book ‘Our Bodily Form’ (Unsere Körperform). Here, we review His's work with special emphasis on its potential importance to the disciplines of evolutionary developmental biology (evo‐devo) and mechanobiology.

Biophysical Approaches for Applying and Measuring Biological Forces

Over the past decades, increasing evidence has indicated that mechanical loads can regulate the morphogenesis, proliferation, migration, and apoptosis of living cells. Investigations of how cells sense mechanical stimuli or the mechanotransduction mechanism is an active field of biomaterials and biophysics. Gaining a further understanding of mechanical regulation and depicting the mechanotransduction network inside cells require advanced experimental techniques and new theories. In this review, the fundamental principles of various experimental approaches that have been developed to characterize various types and magnitudes of forces experienced at the cellular and subcellular levels are summarized. The broad applications of these techniques are introduced with an emphasis on the difficulties in implementing these techniques in special biological systems. The advantages and disadvantages of each technique are discussed, which can guide readers to choose the most suitable technique for their questions. A perspective on future directions in this field is also provided. It is anticipated that technical advancement can be a driving force for the development of mechanobiology.

Mitochondrial fission links ECM mechanotransduction to metabolic redox homeostasis and metastatic chemotherapy resistance

Metastatic breast cancer cells disseminate to organs with a soft microenvironment. Whether and how the mechanical properties of the local tissue influence their response to treatment remains unclear. Here we found that a soft extracellular matrix empowers redox homeostasis. Cells cultured on a soft extracellular matrix display increased peri-mitochondrial F-actin, promoted by Spire1C and Arp2/3 nucleation factors, and increased DRP1- and MIEF1/2-dependent mitochondrial fission. Changes in mitochondrial dynamics lead to increased production of mitochondrial reactive oxygen species and activate the NRF2 antioxidant transcriptional response, including increased cystine uptake and glutathione metabolism. This retrograde response endows cells with resistance to oxidative stress and reactive oxygen species-dependent chemotherapy drugs. This is relevant in a mouse model of metastatic breast cancer cells dormant in the lung soft tissue, where inhibition of DRP1 and NRF2 restored cisplatin sensitivity and prevented disseminated cancer-cell awakening. We propose that targeting this mitochondrial dynamics- and redox-based mechanotransduction pathway could open avenues to prevent metastatic relapse.

Nano-Precision Tweezers for Mechanosensitive Proteins and Beyond

Mechanical forces play pivotal roles in regulating cell shape, function, and fate. Key players that govern the mechanobiological interplay are the mechanosensitive proteins found on cell membranes and in cytoskeleton. Their unique nanomechanics can be interrogated using single-molecule tweezers, which can apply controlled forces to the proteins and simultaneously measure the ensuing structural changes. Breakthroughs in high-resolution tweezers have enabled the routine monitoring of nanometer-scale, millisecond dynamics as a function of force. Undoubtedly, the advancement of structural biology will be further fueled by integrating static atomic-resolution structures and their dynamic changes and interactions observed with the force application techniques. In this minireview, we will introduce the general principles of single-molecule tweezers and their recent applications to the studies of force-bearing proteins, including the synaptic proteins that need to be categorized as mechanosensitive in a broad sense. We anticipate that the impact of nano-precision approaches in mechanobiology research will continue to grow in the future.

Local contractions regulate E-cadherin rigidity sensing

E-cadherin is a major cell-cell adhesion molecule involved in mechanotransduction at cell-cell contacts in tissues. Because epithelial cells respond to rigidity and tension in tissue through E-cadherin, there must be active processes that test and respond to the mechanical properties of these adhesive contacts. Using submicrometer, E-cadherin-coated polydimethylsiloxane pillars, we find that cells generate local contractions between E-cadherin adhesions and pull to a constant distance for a constant duration, irrespective of pillar rigidity. These cadherin contractions require nonmuscle myosin IIB, tropomyosin 2.1, α-catenin, and binding of vinculin to α-catenin. Cells spread to different areas on soft and rigid surfaces with contractions, but spread equally on soft and rigid without. We further observe that cadherin contractions enable cells to test myosin IIA-mediated tension of neighboring cells and sort out myosin IIA-depleted cells. Thus, we suggest that epithelial cells test and respond to the mechanical characteristics of neighboring cells through cadherin contractions.

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

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

Comparison of direct and inverse methods for 2.5D traction force microscopy

Essential cellular processes such as cell adhesion, migration and division strongly depend on mechanical forces. The standard method to measure cell forces is traction force microscopy (TFM) on soft elastic substrates with embedded marker beads. While in 2D TFM one only reconstructs tangential forces, in 2.5D TFM one also considers normal forces. Here we present a systematic comparison between two fundamentally different approaches to 2.5D TFM, which in particular require different methods to deal with noise in the displacement data. In the direct method, one calculates strain and stress tensors directly from the displacement data, which in principle requires a divergence correction. In the inverse method, one minimizes the difference between estimated and measured displacements, which requires some kind of regularization. By calculating the required Green's functions in Fourier space from Boussinesq-Cerruti potential functions, we first derive a new variant of 2.5D Fourier Transform Traction Cytometry (FTTC). To simulate realistic traction patterns, we make use of an analytical solution for Hertz-like adhesion patches. We find that FTTC works best if only tangential forces are reconstructed, that 2.5D FTTC is more precise for small noise, but that the performance of the direct method approaches the one of 2.5D FTTC for larger noise, before both fail for very large noise. Moreover we find that a divergence correction is not really needed for the direct method and that it profits more from increased resolution than the inverse method.

Bridging pico-to-nanonewtons with a ratiometric force probe for monitoring nanoscale polymer physics before damage

Understanding the transmission of nanoscale forces in the pico-to-nanonewton range is important in polymer physics. While physical approaches have limitations in analyzing the local force distribution in condensed environments, chemical analysis using force probes is promising. However, there are stringent requirements for probing the local forces generated before structural damage. The magnitude of those forces corresponds to the range below covalent bond scission (from 200 pN to several nN) and above thermal fluctuation (several pN). Here, we report a conformationally flexible dual-fluorescence force probe with a theoretically estimated threshold of approximately 100 pN. This probe enables ratiometric analysis of the distribution of local forces in a stretched polymer chain network. Without changing the intrinsic properties of the polymer, the force distribution was reversibly monitored in real time. Chemical control of the probe location demonstrated that the local stress concentration is twice as biased at crosslinkers than at main chains, particularly in a strain-hardening region. Due to the high sensitivity, the percentage of the stressed force probes was estimated to be more than 1000 times higher than the activation rate of a conventional mechanophore.

Mechanical Forces in Nuclear Organization

Cells generate and sense mechanical forces that trigger biochemical signals to elicit cellular responses that control cell fate changes. Mechanical forces also physically distort neighboring cells and the surrounding connective tissue, which propagate mechanochemical signals over long distances to guide tissue patterning, organogenesis, and adult tissue homeostasis. As the largest and stiffest organelle, the nucleus is particularly sensitive to mechanical force and deformation. Nuclear responses to mechanical force include adaptations in chromatin architecture and transcriptional activity that trigger changes in cell state. These force-driven changes also influence the mechanical properties of chromatin and nuclei themselves to prevent aberrant alterations in nuclear shape and help maintain genome integrity. This review will discuss principles of nuclear mechanotransduction and chromatin mechanics and their role in DNA damage and cell fate regulation.

Non-muscle myosin II isoforms orchestrate substrate stiffness sensing to promote cancer cell contractility and migration

The stiffening of the extracellular matrix (ECM) during tumor progression results in an increase in cancer cell motility. In cell migration, two major isoforms of non-muscle myosin II (NMII), NMIIA and NMIIB, are expressed and assembled into the cytoskeleton. However, the isoform-specific regulatory roles of NMIIA and NMIIB as well as the underlying mechanisms in response to mechanical cues of the ECM are still elusive. Here, based on polyacrylamide (PAA) gels with tunable elastic modulus, we mimicked the mechanical properties of tumor tissue at different stages of breast cancer in vitro and investigated the distinct roles of NMII isoforms in the regulation of substrate stiffness. We demonstrate that NMIIA is engaged in establishing cell polarity by facilitating lamellipodia formation, focal adhesion turnover, and actin polymerization at the cell leading edge, while NMIIB is recruited to the cell perinuclear region and contributes to traction force generation and polarized distribution, both in a substrate stiffness-dependent manner. We further validated that substrate stiffness modulates the distribution and activation of NMII isoforms via the Rac1/p-PAK1/pS1916-NMIIA and PKCζ/pS1935-NMIIB signaling pathways in a site- and kinase-specific phosphoregulation manner. Our study is helpful for understanding the mechanotransduction of cancer cells and provides inspiration for molecular targets in antimetastatic therapy.

Correlative cryo-ET identifies actin/tropomyosin filaments that mediate cell-substrate adhesion in cancer cells and mechanosensitivity of cell proliferation

The actin cytoskeleton is the primary driver of cellular adhesion and mechanosensing due to its ability to generate force and sense the stiffness of the environment. At the cell's leading edge, severing of the protruding Arp2/3 actin network generates a specific actin/tropomyosin (Tpm) filament population that controls lamellipodial persistence. The interaction between these filaments and adhesion to the environment is unknown. Using cellular cryo-electron tomography we resolve the ultrastructure of the Tpm/actin copolymers and show that they specifically anchor to nascent adhesions and are essential for focal adhesion assembly. Re-expression of Tpm1.8/1.9 in transformed and cancer cells is sufficient to restore cell-substrate adhesions. We demonstrate that knock-out of Tpm1.8/1.9 disrupts the formation of dorsal actin bundles, hindering the recruitment of α-actinin and non-muscle myosin IIa, critical mechanosensors. This loss causes a force-generation and proliferation defect that is notably reversed when cells are grown on soft surfaces. We conclude that Tpm1.8/1.9 suppress the metastatic phenotype, which may explain why transformed cells naturally downregulate this Tpm subset during malignant transformation.

3D Bioprinting of Cell-Laden Hydrogels for Improved Biological Functionality

The encapsulation of cells within gel-phase materials to form bioinks offers distinct advantages for next-generation 3D bioprinting. 3D bioprinting has emerged as a promising tool for patterning cells, but the technology remains limited in its ability to produce biofunctional, tissue-like constructs due to a dearth of materials suitable for bioinks. While early demonstrations commonly used viscous polymers optimized for printability, these materials often lacked cell compatibility and biological functionality. In response, advanced materials that exist in the gel phase during the entire printing process are being developed, since hydrogels are uniquely positioned to both protect cells during extrusion and provide biological signals to embedded cells as the construct matures during culture. Here, an overview of the design considerations for gel-phase materials as bioinks is presented, with a focus on their mechanical, biochemical, and dynamic gel properties. Current challenges and opportunities that arise due to the fact that bioprinted constructs are active, living hydrogels composed of both acellular and cellular components are also evaluated. Engineering hydrogels with consideration of cells as an intrinsic component of the printed bioink will enable control over the evolution of the living construct after printing to achieve greater biofunctionality.

Au nanoflower film-based stretchable biosensors for in situ monitoring of superoxide anion release in cell mechanotransduction

Cell mechanotransduction plays an important role in vascular regulation and disease development.

An Easy-to-Fabricate Cell Stretcher Reveals Density-Dependent Mechanical Regulation of Collective Cell Movements in Epithelia

Introduction

Mechanical forces regulate many facets of cell and tissue biology. Studying the effects of forces on cells requires real-time observations of single- and multi-cell dynamics in tissue models during controlled external mechanical input. Many of the existing devices used to conduct these studies are costly and complicated to fabricate, which reduces the availability of these devices to many laboratories.

Methods

We show how to fabricate a simple, low-cost, uniaxial stretching device, with readily available materials and instruments that is compatible with high-resolution time-lapse microscopy of adherent cell monolayers. In addition, we show how to construct a pressure controller that induces a repeatable degree of stretch in monolayers, as well as a custom MATLAB code to quantify individual cell strains.

Results

As an application note using this device, we show that uniaxial stretch slows down cellular movements in a mammalian epithelial monolayer in a cell density-dependent manner. We demonstrate that the effect on cell movement involves the relocalization of myosin downstream of Rho-associated protein kinase (ROCK).

Conclusions

This mechanical device provides a platform for broader involvement of engineers and biologists in this important area of cell and tissue biology. We used this device to demonstrate the mechanical regulation of collective cell movements in epithelia.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-021-00689-6.

Matrix Stiffening Enhances DNCB-Induced IL-6 Secretion in Keratinocytes Through Activation of ERK and PI3K/Akt Pathway

Matrix stiffness, a critical physical property of the cellular environment, is implicated in epidermal homeostasis. In particular, matrix stiffening during the pathological progression of skin diseases appears to contribute to cellular responses of keratinocytes. However, it has not yet elucidated the molecular mechanism underlying matrix-stiffness-mediated signaling in coordination with chemical stimuli during inflammation and its effect on proinflammatory cytokine production. In this study, we demonstrated that keratinocytes adapt to matrix stiffening by increasing cell–matrix adhesion via actin cytoskeleton remodeling. Specifically, mechanosensing and signal transduction are coupled with chemical stimuli to regulate cytokine production, and interleukin-6 (IL-6) production is elevated in keratinocytes on stiffer substrates in response to 2,4-dinitrochlorobenzene. We demonstrated that β1 integrin and focal adhesion kinase (FAK) expression were enhanced with increasing stiffness and activation of ERK and the PI3K/Akt pathway was involved in stiffening-mediated IL-6 production. Collectively, our results reveal the critical role of matrix stiffening in modulating the proinflammatory response of keratinocytes, with important clinical implications for skin diseases accompanied by pathological matrix stiffening.

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.

The Emerging Role of Decellularized Plant-Based Scaffolds as a New Biomaterial

The decellularization of plant-based biomaterials to generate tissue-engineered substitutes or in vitro cellular models has significantly increased in recent years. These vegetal tissues can be sourced from plant leaves and stems or fruits and vegetables, making them a low-cost, accessible, and sustainable resource from which to generate three-dimensional scaffolds. Each construct is distinct, representing a wide range of architectural and mechanical properties as well as innate vasculature networks. Based on the rapid rise in interest, this review aims to detail the current state of the art and presents the future challenges and perspectives of these unique biomaterials. First, we consider the different existing decellularization techniques, including chemical, detergent-free, enzymatic, and supercritical fluid approaches that are used to generate such scaffolds and examine how these protocols can be selected based on plant cellularity. We next examine strategies for cell seeding onto the plant-derived constructs and the importance of the different functionalization methods used to assist in cell adhesion and promote cell viability. Finally, we discuss how their structural features, such as inherent vasculature, porosity, morphology, and mechanical properties (i.e., stiffness, elasticity, etc.) position plant-based scaffolds as a unique biomaterial and drive their use for specific downstream applications. The main challenges in the field are presented throughout the discussion, and future directions are proposed to help improve the development and use of vegetal constructs in biomedical research.

Conventional rigid 2D substrates cause complex contractile signals in monolayers of human induced pluripotent stem cell-derived cardiomyocytes

Abstract

Human induced pluripotent stem cell‐derived cardiomyocytes (hiPSC‐CM) in monolayers interact mechanically via cell–cell and cell–substrate adhesion. Spatiotemporal features of contraction were analysed in hiPSC‐CM monolayers (1) attached to glass or plastic (Young's modulus (E) >1 GPa), (2) detached (substrate‐free) and (3) attached to a flexible collagen hydrogel (E = 22 kPa). The effects of isoprenaline on contraction were compared between rigid and flexible substrates. To clarify the underlying mechanisms, further gene expression and computational studies were performed. HiPSC‐CM monolayers exhibited multiphasic contractile profiles on rigid surfaces in contrast to hydrogels, substrate‐free cultures or single cells where only simple twitch‐like time‐courses were observed. Isoprenaline did not change the contraction profile on either surface, but its lusitropic and chronotropic effects were greater in hydrogel compared with glass. There was no significant difference between stiff and flexible substrates in regard to expression of the stress‐activated genes NPPA and NPPB. A computational model of cell clusters demonstrated similar complex contractile interactions on stiff substrates as a consequence of cell‐to‐cell functional heterogeneity. Rigid biomaterial surfaces give rise to unphysiological, multiphasic contractions in hiPSC‐CM monolayers. Flexible substrates are necessary for normal twitch‐like contractility kinetics and interpretation of inotropic interventions.

Key points

Spatiotemporal contractility analysis of human induced pluripotent stem cell‐derived cardiomyocyte (hiPSC‐CM) monolayers seeded on conventional, rigid surfaces (glass or plastic) revealed the presence of multiphasic contraction patterns across the monolayer with a high variability, despite action potentials recorded in the same areas being identical.

These multiphasic patterns are not present in single cells, in detached monolayers or in monolayers seeded on soft substrates such as a hydrogel, where only ‘twitch’‐like transients are observed.

HiPSC‐CM monolayers that display a high percentage of regions with multiphasic contraction have significantly increased contractile duration and a decreased lusotropic drug response.

There is no indication that the multiphasic contraction patterns are associated with significant activation of the stress‐activated NPPA or NPPB signalling pathways.

A computational model of cell clusters supports the biological findings that the rigid surface and the differential cell–substrate adhesion underly multiphasic contractile behaviour of hiPSC‐CMs.

Forces generated by lamellipodial actin filament elongation regulate the WAVE complex during cell migration

Actin filaments generate mechanical forces that drive membrane movements during trafficking, endocytosis and cell migration. Reciprocally, adaptations of actin networks to forces regulate their assembly and architecture. Yet, a demonstration of forces acting on actin regulators at actin assembly sites in cells is missing. Here we show that local forces arising from actin filament elongation mechanically control WAVE regulatory complex (WRC) dynamics and function, that is, Arp2/3 complex activation in the lamellipodium. Single-protein tracking revealed WRC lateral movements along the lamellipodium tip, driven by elongation of actin filaments and correlating with WRC turnover. The use of optical tweezers to mechanically manipulate functional WRC showed that piconewton forces, as generated by single-filament elongation, dissociated WRC from the lamellipodium tip. WRC activation correlated with its trapping, dwell time and the binding strength at the lamellipodium tip. WRC crosslinking, hindering its mechanical dissociation, increased WRC dwell time and Arp2/3-dependent membrane protrusion. Thus, forces generated by individual actin filaments on their regulators can mechanically tune their turnover and hence activity during cell migration.

Biomimetic DNA Nanotechnology to Understand and Control Cellular Responses

At the cellular level, numerous nanocues guide the cells to adhere, interact, proliferate, differentiate, etc. Understanding and manipulating the cellular functions in vitro, necessitates the elucidation of these nanocues provided to the cells by the extracellular matrix (ECM), neighbouring cells or in the form of ligands. DNA nanotechnology is a biocompatible, flexible and a promising molecular level toolkit for mimicking cell-cell and cell-matrix interactions. In this review, we summarize various advances in cell-matrix, cell-cell and cell receptor-ligand interactions using DNA nanotechnology as a tool. We also provide a brief outlook on the current challenges and the future potentials of these DNA-based nanostructures so as to inspire novel innovations in the field.

The viscoelasticity of adherent cells follows a single power-law with distinct local variations within a single cell and across cell lines

AFM-based force-distance curves are commonly used to characterize the nanomechanical properties of live cells. The transformation of these curves into nanomechanical properties requires the development of contact mechanics models. Spatially-resolved force-distance curves involving 1 to 2 μm deformations were obtained on HeLa and NIH 3T3 (fibroblast) cells. An elastic and two viscoelastic models were used to describe the experimental force-distance curves. The best agreement was obtained by applying a contact mechanics model that accounts for the geometry of the contact and the finite-thickness of the cell and assumes a single power-law dependence with time. Our findings show the shortcomings of elastic and semi-infinite viscoelastic models to characterize the mechanical response of a mammalian cell under micrometer-scale deformations. The parameters of the 3D power-law viscoelastic model, compressive modulus and fluidity exponent showed local variations within a single cell and across the two cell lines. The corresponding nanomechanical maps revealed structures that were not visible in the AFM topographic maps.

Matrix stiffness drives stromal autophagy and promotes formation of a protumorigenic niche

Increased stiffness of solid tissues has long been recognized as a diagnostic feature of several pathologies, most notably malignant diseases. In fact, it is now well established that elevated tissue rigidity enhances disease progression and aggressiveness and is associated with a poor prognosis in patients as documented, for instance, for lung fibrosis or the highly desmoplastic cancer of the pancreas. The underlying mechanisms of the interplay between physical properties and cellular behavior are, however, not very well understood. Here, we have found that switching culture conditions from soft to stiff substrates is sufficient to evoke (macro) autophagy in various fibroblast types. Mechanistically, this is brought about by stiffness-sensing through an Integrin αV-focal adhesion kinase module resulting in sequestration and posttranslational stabilization of the metabolic master regulator AMPKα at focal adhesions, leading to the subsequent induction of autophagy. Importantly, stiffness-induced autophagy in stromal cells such as fibroblasts and stellate cells critically supports growth of adjacent cancer cells in vitro and in vivo. This process is Integrin αV dependent, opening possibilities for targeting tumor-stroma crosstalk. Our data thus reveal that the mere change in mechanical tissue properties is sufficient to metabolically reprogram stromal cell populations, generating a tumor-supportive metabolic niche.

Response of Bacteria to Mechanical Stimuli

Abstract—

Bacteria adapt rapidly to changes in ambient conditions, constantly inspecting their surroundings by means of their sensor systems. These systems are often thought to respond only to signals of a chemical nature. Yet, bacteria are often affected by mechanical forces, e.g., during transition from planktonic to sessile state. Mechanical stimuli, however, have seldom been considered as the signals bacteria can sense and respond to. Nonetheless, bacteria perceive mechanical stimuli, generate signals, and develop responses. This review analyzes the information on the way bacteria respond to mechanical stimuli and outlines how bacteria convert incoming signals into appropriate responses.

Cardiovascular mechanobiology-a Special Issue to look at the state of the art and the newest insights into the role of mechanical forces in cardiovascular development, physiology and disease

There has been much progress recently in the area of cardiovascular mechanobiology and this Special Issue aims at taking stock. This editorial gives context of the main motivation for this special issue as well as a brief summary of its content.

Brick Strex: a robust device built of LEGO bricks for mechanical manipulation of cells

Cellular forces, mechanics and other physical factors are important co-regulators of normal cell and tissue physiology. These cues are often misregulated in diseases such as cancer, where altered tissue mechanics contribute to the disease progression. Furthermore, intercellular tensile and compressive force-related signaling is highlighted in collective cell behavior during development. However, the mechanistic understanding on the role of physical forces in regulation of cellular physiology, including gene expression and signaling, is still lacking. This is partly because studies on the molecular mechanisms of force transmission require easily controllable experimental designs. These approaches should enable both easy mechanical manipulation of cells and, importantly, readouts ranging from microscopy imaging to biochemical assays. To achieve a robust solution for mechanical manipulation of cells, we developed devices built of LEGO bricks allowing manual, motorized and/or cyclic cell stretching and compression studies. By using these devices, we show that [Formula: see text]-catenin responds differentially to epithelial monolayer stretching and lateral compression, either localizing more to the cell nuclei or cell-cell junctions, respectively. In addition, we show that epithelial compression drives cytoplasmic retention and phosphorylation of transcription coregulator YAP1. We provide a complete part listing and video assembly instructions, allowing other researchers to build and use the devices in cellular mechanics-related studies.

Mechanobiological conceptual framework for assessing stem cell bioprocess effectiveness

Fully realizing the enormous potential of stem cells requires developing efficient bioprocesses and optimizations founded in mechanobiological considerations. Here, we emphasize the importance of mechanotransduction as one of the governing principles of stem cell bioprocesses, underscoring the need to further explore the behavioral mechanisms involved in sensing mechanical cues and coordinating transcriptional responses. We identify the sources of intrinsic, extrinsic, and external noise in bioprocesses requiring further study, and discuss the criteria and indicators that may be used to assess and predict cell-to-cell variability resulting from environmental fluctuations. Specifically, we propose a conceptual framework to explain the impact of mechanical forces within the cellular environment, identify key cell state determinants in bioprocesses, and discuss downstream implementation challenges.

Emerging Principles in the Transcriptional Control by YAP and TAZ

Simple Summary

YAP and TAZ are transcriptional cofactors that integrate several upstream signals to generate context-dependent transcriptional responses. This requires extensive integration with epigenetic regulators and other transcription factors. The molecular and genomic characterization of YAP and TAZ nuclear function has broad implications both in physiological and pathological settings.

Abstract

Yes-associated protein (YAP) and TAZ are transcriptional cofactors that sit at the crossroad of several signaling pathways involved in cell growth and differentiation. As such, they play essential functions during embryonic development, regeneration, and, once deregulated, in cancer progression. In this review, we will revise the current literature and provide an overview of how YAP/TAZ control transcription. We will focus on data concerning the modulation of the basal transcriptional machinery, their ability to epigenetically remodel the enhancer–promoter landscape, and the mechanisms used to integrate transcriptional cues from multiple pathways. This reveals how YAP/TAZ activation in cancer cells leads to extensive transcriptional control that spans several hallmarks of cancer. The definition of the molecular mechanism of transcriptional control and the identification of the pathways regulated by YAP/TAZ may provide therapeutic opportunities for the effective treatment of YAP/TAZ-driven tumors.

Electroactive Biomaterials and Systems for Cell Fate Determination and Tissue Regeneration: Design and Applications

During natural tissue regeneration, tissue microenvironment and stem cell niche including cell-cell interaction, soluble factors, and extracellular matrix (ECM) provide a train of biochemical and biophysical cues for modulation of cell behaviors and tissue functions. Design of functional biomaterials to mimic the tissue/cell microenvironment have great potentials for tissue regeneration applications. Recently, electroactive biomaterials have drawn increasing attentions not only as scaffolds for cell adhesion and structural support, but also as modulators to regulate cell/tissue behaviors and function, especially for electrically excitable cells and tissues. More importantly, electrostimulation can further modulate a myriad of biological processes, from cell cycle, migration, proliferation and differentiation to neural conduction, muscle contraction, embryogenesis, and tissue regeneration. In this review, endogenous bioelectricity and piezoelectricity are introduced. Then, design rationale of electroactive biomaterials is discussed for imitating dynamic cell microenvironment, as well as their mediated electrostimulation and the applying pathways. Recent advances in electroactive biomaterials are systematically overviewed for modulation of stem cell fate and tissue regeneration, mainly including nerve regeneration, bone tissue engineering, and cardiac tissue engineering. Finally, the significance for simulating the native tissue microenvironment is emphasized and the open challenges and future perspectives of electroactive biomaterials are concluded.

A review of regulated self-organizing approaches for tissue regeneration

Tissue and organ regeneration is the dynamic process by which a population of cells rearranges into a specific form with specific functions. Traditional tissue regeneration utilizes tissue grafting, cell implantation, and structured scaffolds to achieve clinical efficacy. However, tissue grafting methods face a shortage of donor tissue, while cell implantation may involve leakage of the implanted cells without a supportive 3D matrix. Cell migration, proliferation, and differentiation in structured scaffolds may disorganize and frustrate the artificially pre-designed structures, and sometimes involve immunogenic reactions. To overcome this limitation, the self-organizing properties and innate regenerative capability of tissue/organism formation in the absence of guidance by structured scaffolds has been investigated. This review emphasizes the growing subfield of the regulated self-organizing approach for neotissue formation and describes advances in the subfield using diverse, cutting-edge, inter-disciplinarity technologies. We cohesively summarize the directed self-organization of cells in the micro-engineered cell-ECM system and 3D/4D cell printing. Mathematical modeling of cellular self-organization is also discussed for providing rational guidance to intractable problems in tissue regeneration. It is envisioned that future self-organization approaches integrating biomathematics, micro-nano engineering, and gene circuits developed from synthetic biology will continue to work in concert with self-organizing morphogenesis to enhance rational control during self-organizing in tissue and organ regeneration.

Role of integrin‑linked kinase in static compressive stress‑induced autophagy via phosphatidylinositol 3 kinase in human periodontal ligament cells

Orthodontic tooth movement (OTM) is achieved by using mechanical stimuli, which lead to the remodeling of periodontal tissues. Previous findings have demonstrated that autophagy may be one of the cell responses to mechanical stress. As a key structure in the integrin pathway, integrin linked‑kinase (ILK) may play a role in the transmission of these mechanical signals. In addition, ILK is an important upstream molecule that regulates autophagy, under the influence of phosphatidylinositol 3 kinase (PI3K). Therefore, exploring the effect of mechanical stress on autophagy and the associated role of ILK/PI3K is of utmost significance to understanding the mechanism behind OTM. In the present study, human periodontal ligament cells (hPDLCs) were embedded into a collagen‑alginate complex hydrogel for three‑dimensional (3D) culturing. Static compressive stress (2.5 g/cm²) was loaded using the uniform weight method for 5, 15, 30, and 60 min. The autophagy of hPDLCs was detected by the expression of Beclin‑1 (BECN1) and ATG‑5 using RT‑qPCR and LC3, respectively, using immunofluorescence. The results showed that the level of autophagy and gene expression of ILK increased significantly under static compressive stress. In ILK‑silenced cells, static compressive stress could also upregulate ILK expression and increase the levels of autophagy. After PI3K inhibition, the increase in the autophagy level and the upregulation of ILK expression disappeared. These findings suggest that static compressive stress can induce autophagy in hPDLCs in a rapid, transient process, regulated by ILK and PI3K. Moreover, this static stress can upregulate ILK expression in a PI3K‑dependent manner.

TRPV Protein Family-From Mechanosensing to Cancer Invasion

Biophysical cues from the cellular microenvironment are detected by mechanosensitive machineries that translate physical signals into biochemical signaling cascades. At the crossroads of extracellular space and cell interior are located several ion channel families, including TRP family proteins, that are triggered by mechanical stimuli and drive intracellular signaling pathways through spatio-temporally controlled Ca2+-influx. Mechanosensitive Ca2+-channels, therefore, act as critical components in the rapid transmission of physical signals into biologically compatible information to impact crucial processes during development, morphogenesis and regeneration. Given the mechanosensitive nature of many of the TRP family channels, they must also respond to the biophysical changes along the development of several pathophysiological conditions and have also been linked to cancer progression. In this review, we will focus on the TRPV, vanilloid family of TRP proteins, and their connection to cancer progression through their mechanosensitive nature.

Mechano-induced cell metabolism promotes microtubule glutamylation to force metastasis

Mechanical signals from the tumor microenvironment modulate cell mechanics and influence cell metabolism to promote cancer aggressiveness. Cells withstand external forces by adjusting the stiffness of their cytoskeleton. Microtubules (MTs) act as compression-bearing elements. Yet how cancer cells regulate MT dynamic in response to the locally constrained environment has remained unclear. Using breast cancer as a model of a disease in which mechanical signaling promotes disease progression, we show that matrix stiffening rewires glutamine metabolism to promote MT glutamylation and force MT stabilization, thereby promoting cell invasion. Pharmacologic inhibition of glutamine metabolism decreased MT glutamylation and affected their mechanical stabilization. Similarly, decreased MT glutamylation by overexpressing tubulin mutants lacking glutamylation site(s) decreased MT stability, thereby hampering cancer aggressiveness in vitro and in vivo. Together, our results decipher part of the enigmatic tubulin code that coordinates the fine-tunable properties of MT and link cell metabolism to MT dynamics and cancer aggressiveness.

Feeling the force: Multiscale force sensing and transduction at the cell-cell interface

A universal principle of all living cells is the ability to sense and respond to mechanical stimuli which is essential for many biological processes. Recent efforts have identified critical mechanosensitive molecules and response pathways involved in mechanotransduction during development and tissue homeostasis. Tissue-wide force transmission and local force sensing need to be spatiotemporally coordinated to precisely regulate essential processes during development such as tissue morphogenesis, patterning, cell migration and organogenesis. Understanding how cells identify and interpret extrinsic forces and integrate a specific response on cell and tissue level remains a major challenge. In this review we consider important cellular and physical factors in control of cell-cell mechanotransduction and discuss their significance for cell and developmental processes. We further highlight mechanosensitive macromolecules that are known to respond to external forces and present examples of how force responses can be integrated into cell and developmental programs.

The impact of biomechanics on corneal endothelium tissue engineering

The integrity of innermost layer of the cornea, the corneal endothelium, is key to sustaining corneal transparency. Therefore, disease or injury causing loss or damage to the corneal endothelial cell population may threaten vision. Transplantation of corneal tissue is the standard treatment used to replace malfunctioning corneal endothelial cells. However, this surgery is dependent upon donor tissue, which is limited in supply. Hence, tissue engineers have attempted to construct alternative transplantable tissues or cell therapies to alleviate this problem. Nevertheless, the intrinsic non-dividing nature of corneal endothelial cells continues to foil scientists in their attempts to yield large numbers of cells in the laboratory for use in such novel therapies. Interestingly, the contribution of the biomechanical properties of the underlying extracellular matrix (ECM) on cell division, tissue development and maintenance has been extensively investigated in other many cell types. However, the impact of biomechanics on corneal endothelial cell behaviour is relatively unexplored. Here, we describe contemporary tissue engineering solutions aimed at circumventing donor tissue scarcity. We review the ECM structure and biomechanical features of corneal endothelial cells. We discuss the alterations of ECM in endothelial disease development and progression and point out the role of ECM in developing a tissue-engineered corneal endothelium. We highlight the main biomechanical cues, including topographical and mechanical features, that impact cellular behaviors. Finally, we discuss the influence of biomechanical cues on cell and tissue development, and how corneal endothelial cells response to individual biomechanical stimuli in tissue engineering, which have implications for designing an engineered endothelium and maintaining cell function.

Dose-independent threshold illumination for non-invasive time-lapse fluorescence imaging of live cells

Fluorescent microscopy employs monochromatic light for excitation, which can adversely affect the cells being observed. We reported earlier that fibroblasts relax their contractile force in response to green light of typical intensity. Here we show that such effects are independent of extracellular matrix and cell lines. In addition, we establish a threshold intensity that elicits minimal or no adverse effect on cell contractility even for long-time exposure. This threshold intensity is wavelength dependent. We cultured fibroblasts on soft 2D elastic hydrogels embedded with fluorescent beads to trace substrate deformation and cell forces. The beads move towards cell center when cells contract, but they move away when cells relax. We use relaxation/contraction ratio (λ _r), in addition to traction force, as measures of cell response to red (wavelength, λ=635-650 nm), green (λ=545-580 nm) and blue (λ=455-490 nm) lights with varying intensities. Our results suggest that intensities below 57, 31 and 3.5 W/m² for red, green and blue lights, respectively, do not perturb force homeostasis. To our knowledge, these intensities are the lowest reported safe thresholds, implying that cell traction is a highly sensitive readout of the effect of light on cells. Most importantly, we find these threshold intensities to be dose-independent; i.e., safe regardless of the energy dosage or time of exposure. Conversely, higher intensities result in widespread force-relaxation in cells with λ _r > 1. Furthermore, we present a photo-reaction based model that simulates photo-toxicity and predicts threshold intensity for different wavelengths within the visible spectra. In conclusion, we recommend employing illumination intensities below aforementioned wavelength-specific thresholds for time-lapse imaging of cells and tissues in order to avoid light-induced artifacts in experimental observations.

Synthetic dysmobility screen unveils an integrated STK40-YAP-MAPK system driving cell migration

Integrating signals is essential for cell survival, leading to the concept of synthetic lethality. However, how signaling is integrated to control cell migration remains unclear. By conducting a "two-hit" screen, we revealed the synergistic reduction of cell migration when serine-threonine kinase 40 (STK40) and mitogen-activated protein kinase (MAPK) were simultaneously suppressed. Single-cell analyses showed that STK40 knockdown reduced cell motility and coordination by strengthening focal adhesion (FA) complexes. Furthermore, STK40 knockdown reduced the stability of yes-associated protein (YAP) and subsequently decreased YAP transported into the nucleus, while MAPK inhibition further weakened YAP activities in the nucleus to disturb FA remodeling. Together, we unveiled an integrated STK40-YAP-MAPK system regulating cell migration and introduced "synthetic dysmobility" as a novel strategy to collaboratively control cell migration.

Fascin1 empowers YAP mechanotransduction and promotes cholangiocarcinoma development

Mechanical forces control cell behavior, including cancer progression. Cells sense forces through actomyosin to activate YAP. However, the regulators of F-actin dynamics playing relevant roles during mechanostransduction in vitro and in vivo remain poorly characterized. Here we identify the Fascin1 F-actin bundling protein as a factor that sustains YAP activation in response to ECM mechanical cues. This is conserved in the mouse liver, where Fascin1 regulates YAP-dependent phenotypes, and in human cholangiocarcinoma cell lines. Moreover, this is relevant for liver tumorigenesis, because Fascin1 is required in the AKT/NICD cholangiocarcinogenesis model and it is sufficient, together with AKT, to induce cholangiocellular lesions in mice, recapitulating genetic YAP requirements. In support of these findings, Fascin1 expression in human intrahepatic cholangiocarcinomas strongly correlates with poor patient prognosis. We propose that Fascin1 represents a pro-oncogenic mechanism that can be exploited during intrahepatic cholangiocarcinoma development to overcome a mechanical tumor-suppressive environment.

Calcium signaling mediates a biphasic mechanoadaptive response of endothelial cells to cyclic mechanical stretch

The vascular system is precisely regulated to adjust blood flow to organismal demand, thereby guaranteeing adequate perfusion under varying physiological conditions. Mechanical forces, such as cyclic circumferential stretch, are among the critical stimuli that dynamically adjust vessel distribution and diameter, but the precise mechanisms of adaptation to changing forces are unclear. We find that endothelial monolayers respond to cyclic stretch by transient remodeling of the vascular endothelial cadherin–based adherens junctions and the associated actomyosin cytoskeleton. Time-resolved proteomic profiling reveals that this remodeling is driven by calcium influx through the mechanosensitive Piezo1 channel, triggering Rho activation to increase actomyosin contraction. As the mechanical stimulus persists, calcium signaling is attenuated through transient down-regulation of Piezo1 protein. At the same time, filamins are phosphorylated to increase monolayer stiffness, allowing mechanoadaptation to restore junctional integrity despite continuing exposure to stretch. Collectively, this study identifies a biphasic response to cyclic stretch, consisting of an initial calcium-driven junctional mechanoresponse, followed by mechanoadaptation facilitated by monolayer stiffening.

A reversible shearing DNA probe for visualizing mechanically strong receptors in living cells

In the last decade, DNA-based tension sensors have made significant contributions to the study of the importance of mechanical forces in many biological systems. Albeit successful, one shortcoming of these techniques is their inability to reversibly measure receptor forces in a higher regime (that is, >20 pN), which limits our understanding of the molecular details of mechanochemical transduction in living cells. Here, we developed a reversible shearing DNA-based tension probe (RSDTP) for probing molecular piconewton-scale forces between 4 and 60 pN transmitted by cells. Using these probes, we can easily distinguish the differences in force-bearing integrins without perturbing adhesion biology and reveal that a strong force-bearing integrin cluster can serve as a 'mechanical pivot' to maintain focal adhesion architecture and facilitate its maturation. The benefits of the RSDTP include a high dynamic range, reversibility and single-molecule sensitivity, all of which will facilitate a better understanding of the molecular mechanisms of mechanobiology.

Go with the flow: modeling unique biological flows in engineered in vitro platforms

Interest in recapitulating in vivo phenomena in vitro using organ-on-a-chip technology has grown rapidly and with it, attention to the types of fluid flow experienced in the body has followed suit. These platforms offer distinct advantages over in vivo models with regards to human relevance, cost, and control of inputs (e.g., controlled manipulation of biomechanical cues from fluid perfusion). Given the critical role biophysical forces play in several tissues and organs, it is therefore imperative that engineered in vitro platforms capture the complex, unique flow profiles experienced in the body that are intimately tied with organ function. In this review, we outline the complex and unique flow regimes experienced by three different organ systems: blood vasculature, lymphatic vasculature, and the intestinal system. We highlight current state-of-the-art platforms that strive to replicate physiological flows within engineered tissues while introducing potential limitations in current approaches.

Estrogen Modulates Epithelial Breast Cancer Cell Mechanics and Cell-to-Cell Contacts

Excessive estrogen exposure is connected with increased risk of breast cancer and has been shown to promote epithelial-mesenchymal-transition. Malignant cancer cells accumulate changes in cell mechanical and biochemical properties, often leading to cell softening. In this work we have employed atomic force microscopy to probe the influence of estrogen on the viscoelastic properties of MCF-7 breast cancer cells cultured either in normal or hormone free-medium. Estrogen led to a significant softening of the cells in all studied cases, while growing cells in hormone free medium led to an increase in the studied elastic and viscoelastic moduli. In addition, fluorescence microscopy shows that E-cadherin distribution is changed in cells when culturing them under estrogenic conditions. Furthermore, cell-cell contacts seemed to be weakened. These results were supported by AFM imaging showing changes in surfaces roughness, cell-cell contacts and cell height as result of estrogen treatment. This study therefore provides further evidence for the role of estrogen signaling in breast cancer.