Tensile stress stimulates microtubule outgrowth in living cells

Tensile Stress on Microtubules Facilitates Dynein-Driven Cargo Transport

Mechanical stress significantly affects the physiological functions of cells, including tissue homeostasis, cytoskeletal alterations, and intracellular transport. As a major cytoskeletal component, microtubules respond to mechanical stimulation by altering their alignment and polymerization dynamics. Previously, we reported that microtubules may modulate cargo transport by one of the microtubule-associated motor proteins, dynein, under compressive mechanical stress. Despite the critical role of tensile stress in many biological functions, how tensile stress on microtubules regulates cargo transport is yet to be unveiled. The present study demonstrates that the low-level tensile stress-induced microtubule deformation facilitates dynein-driven transport. We validate our experimental findings using all-atom molecular dynamics simulation. Our study may provide important implications for developing new therapies for diseases that involve impaired intracellular transport.

Microtubules as a signal hub for axon growth in response to mechanical force

Microtubules are highly polar structures and are characterized by high anisotropy and stiffness. In neurons, they play a key role in the directional transport of vesicles and organelles. In the neuronal projections called axons, they form parallel bundles, mostly oriented with the plus-end towards the axonal termination. Their physico-chemical properties have recently attracted attention as a potential candidate in sensing, processing and transducing physical signals generated by mechanical forces. Here, we discuss the main evidence supporting the role of microtubules as a signal hub for axon growth in response to a traction force. Applying a tension to the axon appears to stabilize the microtubules, which, in turn, coordinate a modulation of axonal transport, local translation and their cross-talk. We speculate on the possible mechanisms modulating microtubule dynamics under tension, based on evidence collected in neuronal and non-neuronal cell types. However, the fundamental question of the causal relationship between these mechanisms is still elusive because the mechano-sensitive element in this chain has not yet been identified.

Anti-tumor and cardiotoxic effects of microtubule polymerization inhibitors: The mechanisms and management strategies

Microtubule polymerization inhibitors (MPIs) have long been used as anticancer agents because they inhibit mitosis. Microtubules are thought to play an important role in the migration of tumor cells and the formation of tumor blood vessels, and new MPIs are being developed. Many clinical trials of novel MPIs have been conducted in humans, while some clinical studies in dogs have also been reported. More attempts to apply MPIs not only in humans but also in the veterinary field are expected to be made in the future. Meanwhile, MPIs have a risk of cardiotoxicity. In this paper, we review findings on the pharmacological effects and cardiotoxicity of MPIs, as well as the mechanisms of their cardiotoxicity. Cardiotoxicity of MPIs involves not only the direct effects of MPIs on cardiomyocytes but also their effects on vascular function. For example, hypertension induced by impaired vascular function also contributes to the exacerbation of myocardial damage, and blood pressure control may be useful in reducing cardiotoxicity. By combined administration of MPIs and other anticancer agents, MPI efficacy may be enhanced, thereby potentially allowing to keep MPI dosage low. Measurement of myocardial injury markers in blood and echocardiography may be useful for monitoring cardiotoxicity. In particular, two-dimensional speckle tracking may have high sensitivity for the early detection of MPI-induced cardiac dysfunction. The exploration of the potential of new MPIs while understanding their toxicity and how to deal with them will lead to the further development of cancer chemotherapy.

Hax1 regulate focal adhesion dynamics through IQGAP1

Cell migration is a highly orchestrated process requiring the coordination between the cytoskeleton, cell membrane and extracellular matrix adhesions. Our previous study demonstrated that Hax1 interacts with EB2, a microtubule end-binding protein, and this interaction regulate cell migration in keratinocytes. However, little is known about the underlying regulatory mechanism. Here, we show that Hax1 links dynamic focal adhesions to regulate cell migration via interacting with IQGAP1, a multidomain scaffolding protein, which was identified by affinity purification coupled with LC-MS/MS. Biochemical characterizations revealed that C-terminal region of Hax1 and RGCT domain of IQGAP1 are the most critical binding determinants for its interaction. IQGAP1/Hax1 interaction is essential for cell migration in MCF7 cells. Knockdown of HAX1 not only stabilizes focal adhesions, but also impairs the accumulation of IQGAP in focal adhesions. Further study indicates that this interaction is critical for maintaining efficient focal adhesion turnover. Perturbation of the IQGAP1/Hax1 interaction in vivo using a membrane-permeable TAT-RGCT peptide results in impaired focal adhesion turnover, thus leading to inhibition of directional cell migration. Together, our findings unravel a novel interaction between IQGAP1 and Hax1, suggesting that IQGAP1 association with Hax1 plays a significant role in focal adhesion turnover and directional cell migration. Video Abstract.

Compressive forces stabilize microtubules in living cells

Microtubules are cytoskeleton components with unique mechanical and dynamic properties. They are rigid polymers that alternate phases of growth and shrinkage. Nonetheless, the cells can display a subset of stable microtubules, but it is unclear whether microtubule dynamics and mechanical properties are related. Recent in vitro studies suggest that microtubules have mechano-responsive properties, being able to stabilize their lattice by self-repair on physical damage. Here we study how microtubules respond to cycles of compressive forces in living cells and find that microtubules become distorted, less dynamic and more stable. This mechano-stabilization depends on CLASP2, which relocates from the end to the deformed shaft of microtubules. This process seems to be instrumental for cell migration in confined spaces. Overall, these results demonstrate that microtubules in living cells have mechano-responsive properties that allow them to resist and even counteract the forces to which they are subjected, being a central mediator of cellular mechano-responses.

Pre-strains and buckling in mechanosensitivity of contractile cells and focal adhesions: A tensegrity model

We demonstrate that several key aspects of the contractile activity of a cell interacting with the substrate can be captured by means of a non linear elastic tensegrity mechanical system made of a tensile element in parallel with a buckling-prone component, and exchanging forces with the surroundings through an extracellular matrix-focal adhesion complex. Mechanosensitivity of the focal adhesion plaque is triggered by pre-strain-driven buckling of the system induced either by pre-contraction or pre-polymerization of the constituents. The impact of pre-polymerization on the mechanical force and the implications of using linear and nonlinear elasticity for the focal adhesion plaque are assessed.

Cardiomyocyte Microtubules: Control of Mechanics, Transport, and Remodeling

Microtubules are essential cytoskeletal elements found in all eukaryotic cells. The structure and composition of microtubules regulate their function, and the dynamic remodeling of the network by posttranslational modifications and microtubule-associated proteins generates diverse populations of microtubules adapted for various contexts. In the cardiomyocyte, the microtubules must accommodate the unique challenges faced by a highly contractile, rigidly structured, and long-lasting cell. Through their canonical trafficking role and positioning of mRNA, proteins, and organelles, microtubules regulate essential cardiomyocyte functions such as electrical activity, calcium handling, protein translation, and growth. In a more specialized role, posttranslationally modified microtubules form load-bearing structures that regulate myocyte mechanics and mechanotransduction. Modified microtubules proliferate in cardiovascular diseases, creating stabilized resistive elements that impede cardiomyocyte contractility and contribute to contractile dysfunction. In this review, we highlight the most exciting new concepts emerging from recent studies into canonical and noncanonical roles of cardiomyocyte microtubules.

Viscoelasticity, Like Forces, Plays a Role in Mechanotransduction

Viscoelasticity and its alteration in time and space has turned out to act as a key element in fundamental biological processes in living systems, such as morphogenesis and motility. Based on experimental and theoretical findings it can be proposed that viscoelasticity of cells, spheroids and tissues seems to be a collective characteristic that demands macromolecular, intracellular component and intercellular interactions. A major challenge is to couple the alterations in the macroscopic structural or material characteristics of cells, spheroids and tissues, such as cell and tissue phase transitions, to the microscopic interferences of their elements. Therefore, the biophysical technologies need to be improved, advanced and connected to classical biological assays. In this review, the viscoelastic nature of cytoskeletal, extracellular and cellular networks is presented and discussed. Viscoelasticity is conceptualized as a major contributor to cell migration and invasion and it is discussed whether it can serve as a biomarker for the cells’ migratory capacity in several biological contexts. It can be hypothesized that the statistical mechanics of intra- and extracellular networks may be applied in the future as a powerful tool to explore quantitatively the biomechanical foundation of viscoelasticity over a broad range of time and length scales. Finally, the importance of the cellular viscoelasticity is illustrated in identifying and characterizing multiple disorders, such as cancer, tissue injuries, acute or chronic inflammations or fibrotic diseases.

Microtubule assembly and disassembly dynamics model: Exploring dynamic instability and identifying features of Microtubules' Growth, Catastrophe, Shortening, and Rescue

Microtubules (MTs), a cellular structure element, exhibit dynamic instability and can switch stochastically from growth to shortening; but the factors that trigger these processes at the molecular level are not understood. We developed a 3D Microtubule Assembly and Disassembly DYnamics (MADDY) model, based upon a bead-per-monomer representation of the αβ-tubulin dimers forming an MT lattice, stabilized by the lateral and longitudinal interactions between tubulin subunits. The model was parameterized against the experimental rates of MT growth and shortening, and pushing forces on the Dam1 protein complex due to protofilaments splaying out. Using the MADDY model, we carried out GPU-accelerated Langevin simulations to access dynamic instability behavior. By applying Machine Learning techniques, we identified the MT characteristics that distinguish simultaneously all four kinetic states: growth, catastrophe, shortening, and rescue. At the cellular 25 μM tubulin concentration, the most important quantities are the MT length L , average longitudinal curvatureκ long , MT tip width w , total energy of longitudinal interactions in MT latticeU long , and the energies of longitudinal and lateral interactions required to complete MT to full cylinderU long add andU lat add . At high 250 μM tubulin concentration, the most important characteristics are L ,κ long , number of hydrolyzed αβ-tubulin dimersn hyd and number of lateral interactions per helical pitchn lat in MT lattice, energy of lateral interactions in MT latticeU lat , and energy of longitudinal interactions in MT tipu long . These results allow greater insights into what brings about kinetic state stability and the transitions between states involved in MT dynamic instability behavior.

A Layered View on Focal Adhesions

Simple Summary

The cytoskeleton is a network of protein fibres within cells that provide structure and support intracellular transport. Focal adhesions are protein complexes associated with the outer cell membrane that are found at the ends of specialised actin fibres of this cytoskeleton. They mediate cell adhesion by connecting the cytoskeleton to the extracellular matrix, a protein and sugar network that surrounds cells in tissues. Focal adhesions also translate forces on actin fibres into forces contributing to cell migration. Cell adhesion and migration are crucial to diverse biological processes such as embryonic development, proper functioning of the immune system or the metastasis of cancer cells. Advances in fluorescence microscopy and data analysis methods provided a more detailed understanding of the dynamic ways in which proteins bind and dissociate from focal adhesions and how they are organised within these protein complexes. In this review, we provide an overview of the advances in the current scientific understanding of focal adhesions and summarize relevant imaging techniques. One of the key insights is that focal adhesion proteins are organised into three layers parallel to the cell membrane. We discuss the relevance of this layered nature for the functioning of focal adhesion.

Abstract

The cytoskeleton provides structure to cells and supports intracellular transport. Actin fibres are crucial to both functions. Focal Adhesions (FAs) are large macromolecular multiprotein assemblies at the ends of specialised actin fibres linking these to the extracellular matrix. FAs translate forces on actin fibres into forces contributing to cell migration. This review will discuss recent insights into FA protein dynamics and their organisation within FAs, made possible by advances in fluorescence imaging techniques and data analysis methods. Over the last decade, evidence has accumulated that FAs are composed of three layers parallel to the plasma membrane. We focus on some of the most frequently investigated proteins, two from each layer, paxillin and FAK (bottom, integrin signalling layer), vinculin and talin (middle, force transduction layer) and zyxin and VASP (top, actin regulatory layer). Finally, we discuss the potential impact of this layered nature on different aspects of FA behaviour.

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

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

Mechanoregulation of PDZ Proteins, An Emerging Function

Mechanical forces have emerged as essential regulators of cell organization, proliferation, migration, and polarity to regulate cellular and tissue homeostasis. Changes in forces or loss of the cellular response to them can result in abnormal embryonic development and diseases. Over the past two decades, many efforts have been put in deciphering the molecular mechanisms that convert forces into biochemical signals, allowing for the identification of many mechanotransducer proteins. Here we discuss how PDZ proteins are emerging as new mechanotransducer proteins by altering their conformations or localizations upon force loads, leading to the formation of macromolecular modules tethering the cell membrane to the actin cytoskeleton.

Biomaterials Regulate Mechanosensors YAP/TAZ in Stem Cell Growth and Differentiation

Tissue-resident stem cells are surrounded by a microenvironment known as 'stem cell niche' which is specific for each stem cell type. This niche comprises of cell-intrinsic and -extrinsic factors like biochemical and biophysical signals, which regulate stem cell characteristics and differentiation. Biochemical signals have been thoroughly studied however, the effect of biophysical signals on stem cell regulation is yet to be completely understood. Biomaterials have aided in addressing this issue since they can provide a defined and tuneable microenvironment resembling in vivo conditions. We review various biomaterials used in many studies which have shown a connection between biomaterial-generated mechanical signals and alteration in stem cell behaviour. Researchers probed to understand the mechanism of mechanotransduction and reported that the signals from the extracellular matrix regulate a transcription factor yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ), which is a downstream-regulator of the Hippo pathway and it transduces the mechanical signals inside the nucleus. We highlight the role of the YAP/TAZ as mechanotransducers in stem cell self-renewal and differentiation in response to substrate stiffness, also the possibility of mechanobiology as the emerging field of regenerative medicines and three-dimensional tissue printing.

The Golgi microtubules regulate single cell durotaxis

Current understandings on cell motility and directionality rely heavily on accumulated investigations of the adhesion-actin cytoskeleton-actomyosin contractility cycles, while microtubules have been understudied in this context. Durotaxis, the ability of cells to migrate up gradients of substrate stiffness, plays a critical part in development and disease. Here, we identify the pivotal role of Golgi microtubules in durotactic migration of single cells. Using high-throughput analysis of microtubule plus ends/focal adhesion interactions, we uncover that these non-centrosomal microtubules actively impart leading edge focal adhesion (FA) dynamics. Furthermore, we designed a new system where islands of higher stiffness were patterned within RGD peptide coated polyacrylamide gels. We revealed that the positioning of the Golgi apparatus is responsive to external mechanical cues and that the Golgi-nucleus axis aligns with the stiffness gradient in durotaxis. Together, our work unveils the cytoskeletal underpinning for single cell durotaxis. We propose a model in which the Golgi-nucleus axis serves both as a compass and as a steering wheel for durotactic migration, dictating cell directionality through the interaction between non-centrosomal microtubules and the FA dynamics.

Are microtubules tension sensors?

Mechanical signals play many roles in cell and developmental biology. Several mechanotransduction pathways have been uncovered, but the mechanisms identified so far only address the perception of stress intensity. Mechanical stresses are tensorial in nature, and thus provide dual mechanical information: stress magnitude and direction. Here we propose a parsimonious mechanism for the perception of the principal stress direction. In vitro experiments show that microtubules are stabilized under tension. Based on these results, we explore the possibility that such microtubule stabilization operates in vivo, most notably in plant cells where turgor-driven tensile stresses exceed greatly those observed in animal cells.

Cellular mechanical stress is a key determinant of cell shape and function, but how the cell senses stress direction is unclear. In this Perspective the authors propose that microtubules autonomously sense stress directions in plant cells, where tensile stresses are higher than in animal cells.

Viscoelasticity of single cells-from subcellular to cellular level

This review deals with insights into complex cellular structures and processes obtained by measuring viscoelastic impedances of the cell envelope and the cytoplasm by colloidal bead microrheometry. I first introduce a mechanical cell model that allows us to understand their unique ability of mechanical self-stabilization by actin microtubule crosstalk. In the second part, I show how cell movements can be driven by pulsatile or propagating solitary actin gelatin waves (SAGW) that are generated on nascent adhesion domains by logistically controlled membrane recruitment of functional proteins by electrostatic-hydrophobic forces. The global polarization of cell migration is guided by actin-microtubule crosstalk that is mediated by the Ca⁺⁺ and strain-sensitive supramolecular scaffolding protein IQGAP. In the third part, I introduce the traction force microscopy as a tool to measure the forces between somatic cells and the tissue ´Here I show, how absolute values of viscoelastic impedances of the composite cell envelope can be obtained by deformation field mapping techniques. In the fourth part, it is shown how the dynamic mechanical properties of the active viscoplastic cytoplasmic space can be evaluated using colloidal beads as phantom endosomes. Separate measurements of velocity distributions of directed and random motions of phantom endosomes, yield local values of transport forces, viscosities and life times of directed motion along microtubules. The last part deals with biomimetic experiments allowing us to quantitatively evaluate the mechanical properties of passive and active actin networks on the basis of the percolation theory of gelation.

Talin as a mechanosensitive signaling hub

Cell adhesion to the extracellular matrix (ECM), mediated by transmembrane receptors of the integrin family, is exquisitely sensitive to biochemical, structural, and mechanical features of the ECM. Talin is a cytoplasmic protein consisting of a globular head domain and a series of α-helical bundles that form its long rod domain. Talin binds to the cytoplasmic domain of integrin β-subunits, activates integrins, couples them to the actin cytoskeleton, and regulates integrin signaling. Recent evidence suggests switch-like behavior of the helix bundles that make up the talin rod domains, where individual domains open at different tension levels, exerting positive or negative effects on different protein interactions. These results lead us to propose that talin functions as a mechanosensitive signaling hub that integrates multiple extracellular and intracellular inputs to define a major axis of adhesion signaling.

Single and collective cell migration: the mechanics of adhesions

Chemical and physical properties of the environment control cell proliferation, differentiation, or apoptosis in the long term. However, to be able to move and migrate through a complex three-dimensional environment, cells must quickly adapt in the short term to the physical properties of their surroundings. Interactions with the extracellular matrix (ECM) occur through focal adhesions or hemidesmosomes via the engagement of integrins with fibrillar ECM proteins. Cells also interact with their neighbors, and this involves various types of intercellular adhesive structures such as tight junctions, cadherin-based adherens junctions, and desmosomes. Mechanobiology studies have shown that cell-ECM and cell-cell adhesions participate in mechanosensing to transduce mechanical cues into biochemical signals and conversely are responsible for the transmission of intracellular forces to the extracellular environment. As they migrate, cells use these adhesive structures to probe their surroundings, adapt their mechanical properties, and exert the appropriate forces required for their movements. The focus of this review is to give an overview of recent developments showing the bidirectional relationship between the physical properties of the environment and the cell mechanical responses during single and collective cell migration.

Roles of the cytoskeleton, cell adhesion and rho signalling in mechanosensing and mechanotransduction

All cells sense and respond to various mechanical forces in and mechanical properties of their environment. To respond appropriately, cells must be able to sense the location, direction, strength and duration of these forces. Recent progress in mechanobiology has provided a better understanding of the mechanisms of mechanoresponses underlying many cellular and developmental processes. Various roles of mechanoresponses in development and tissue homeostasis have been elucidated, and many molecules involved in mechanotransduction have been identified. However, the whole picture of the functions and molecular mechanisms of mechanotransduction remains to be understood. Recently, novel mechanisms for sensing and transducing mechanical stresses via the cytoskeleton, cell-substrate and cell-cell adhesions and related proteins have been identified. In this review, we outline the roles of the cytoskeleton, cell-substrate and cell-cell adhesions, and related proteins in mechanosensing and mechanotransduction. We also describe the roles and regulation of Rho-family GTPases in mechanoresponses.

Modeling Active Mechanosensing in Cell-Matrix Interactions

Cells actively sense the mechanical properties of the extracellular matrix, such as its rigidity, morphology, and deformation. The cell-matrix interaction influences a range of cellular processes, including cell adhesion, migration, and differentiation, among others. This article aims to review some of the recent progress that has been made in modeling mechanosensing in cell-matrix interactions at different length scales. The issues discussed include specific interactions between proteins, the structure and mechanosensitivity of focal adhesions, the cluster effects of the specific binding, the structure and behavior of stress fibers, cells' sensing of substrate stiffness, and cell reorientation on cyclically stretched substrates. The review concludes by looking toward future opportunities in the field and at the challenges to understanding active cell-matrix interactions.

Regulation of Focal Adhesion Dynamics and Cell Motility by the EB2 and Hax1 Protein Complex

Cell migration is a fundamental cellular process requiring integrated activities of the cytoskeleton, membrane, and cell/extracellular matrix adhesions. Many cytoskeletal activities rely on microtubule filaments. It has been speculated that microtubules can serve as tracks to deliver proteins essential for focal adhesion turnover. Three microtubule end-binding proteins (EB1, EB2, and EB3) in mammalian cells can track the plus ends of growing microtubules. EB1 and EB3 together can regulate microtubule dynamics by promoting microtubule growth and suppressing catastrophe, whereas, in contrast, EB2 does not play a direct role in microtubule dynamic instability, and little is known about the cellular function of EB2. By quantitative proteomics, we identified mammalian HCLS1-associated protein X-1 (HAX1) as an EB2-specific interacting protein. Knockdown of HAX1 and EB2 in skin epidermal cells stabilizes focal adhesions and impairs epidermal migration in vitro and in vivo. Our results further demonstrate that cell motility and focal adhesion turnover require interaction between Hax1 and EB2. Together, our findings provide new insights for this critical cellular process, suggesting that EB2 association with Hax1 plays a significant role in focal adhesion turnover and epidermal migration.

The many ways adherent cells respond to applied stretch

Cells in various tissues are subjected to mechanical stress and strain that have profound effects on cell architecture and function. The specific response of the cell to applied strain depends on multiple factors, including cell contractility, spatial and temporal strain pattern, and substrate dimensionality and rigidity. Recent work has demonstrated that the cell response to applied strain depends on a complex combination of these factors, but the way these factors interact to elicit a specific response is not intuitive. We submit that an understanding of the integrated response of a cell to these factors will provide new insight into mechanobiology and contribute to the effective design of deformable engineered scaffolds meant to provide appropriate mechanical cues to the resident cells.

Review of cellular mechanotransduction on micropost substrates

As physical entities, living cells can sense and respond to various stimulations within and outside the body through cellular mechanotransduction. Any deviation in cellular mechanotransduction will not only undermine the orchestrated regulation of mechanical responses, but also lead to the breakdown of their physiological function. Therefore, a quantitative study of cellular mechanotransduction needs to be conducted both in experiments and in computational simulations to investigate the underlying mechanisms of cellular mechanotransduction. In this review, we present an overview of the current knowledge and significant progress in cellular mechanotransduction via micropost substrates. In the aspect of experimental studies, we summarize significant experimental progress and place an emphasis on the coupled relationship among cellular spreading, focal adhesion and contractility as well as the influence of substrate properties on force-involved cellular behaviors. In the other aspect of computational investigations, we outline a coupled framework including the biochemically motivated stress fiber model and thermodynamically motivated adhesion model and present their predicted biomechanical responses and then compare predicted simulation results with experimental observations to further explore the mechanisms of cellular mechanotransduction. At last, we discuss the future perspectives both in experimental technologies and in computational models, as well as facing challenges in the area of cellular mechanotransduction.

Molecular control of stress transmission in the microtubule cytoskeleton

In this article, we will summarize recent progress in understanding the mechanical origins of rigidity, strength, resiliency and stress transmission in the MT cytoskeleton using reconstituted networks formed from purified components. We focus on the role of network architecture, crosslinker compliance and dynamics, and molecular determinants of single filament elasticity, while highlighting open questions and future directions for this work.

Mechanotransduction: use the force(s)

Mechanotransduction - how cells sense physical forces and translate them into biochemical and biological responses - is a vibrant and rapidly-progressing field, and is important for a broad range of biological phenomena. This forum explores the role of mechanotransduction in a variety of cellular activities and highlights intriguing questions that deserve further attention.

Tensegrity, cellular biophysics, and the mechanics of living systems

The recent convergence between physics and biology has led many physicists to enter the fields of cell and developmental biology. One of the most exciting areas of interest has been the emerging field of mechanobiology that centers on how cells control their mechanical properties, and how physical forces regulate cellular biochemical responses, a process that is known as mechanotransduction. In this article, we review the central role that tensegrity (tensional integrity) architecture, which depends on tensile prestress for its mechanical stability, plays in biology. We describe how tensional prestress is a critical governor of cell mechanics and function, and how use of tensegrity by cells contributes to mechanotransduction. Theoretical tensegrity models are also described that predict both quantitative and qualitative behaviors of living cells, and these theoretical descriptions are placed in context of other physical models of the cell. In addition, we describe how tensegrity is used at multiple size scales in the hierarchy of life—from individual molecules to whole living organisms—to both stabilize three-dimensional form and to channel forces from the macroscale to the nanoscale, thereby facilitating mechanochemical conversion at the molecular level.

Morphogenesis can be driven by properly parametrised mechanical feedback

A fundamental problem of morphogenesis is whether it presents itself as a succession of links that are each driven by its own specific cause-effect relationship, or whether all of the links can be embraced by a common law that is possible to formulate in physical terms. We suggest that a common biophysical background for most, if not all, morphogenetic processes is based upon feedback between mechanical stresses (MS) that are imposed to a given part of a developing embryo by its other parts and MS that are actively generated within that part. The latter are directed toward hyper-restoration (restoration with an overshoot) of the initial MS values. We show that under mechanical constraints imposed by other parts, these tendencies drive forth development. To provide specificity for morphogenetic reactions, this feedback should be modulated by long-term parameters and/or initial conditions that are set up by genetic factors. The experimental and model data related to this concept are reviewed.

Cyclic tensile strain controls cell shape and directs actin stress fiber formation and focal adhesion alignment in spreading cells

The actin cytoskeleton plays a crucial role for the spreading of cells, but is also a key element for the structural integrity and internal tension in cells. In fact, adhesive cells and their actin stress fiber–adhesion system show a remarkable reorganization and adaptation when subjected to external mechanical forces. Less is known about how mechanical forces alter the spreading of cells and the development of the actin–cell-matrix adhesion apparatus. We investigated these processes in fibroblasts, exposed to uniaxial cyclic tensile strain (CTS) and demonstrate that initial cell spreading is stretch-independent while it is directed by the mechanical signals in a later phase. The total temporal spreading characteristic was not changed and cell protrusions are initially formed uniformly around the cells. Analyzing the actin network, we observed that during the first phase the cells developed a circumferential arc-like actin network, not affected by the CTS. In the following orientation phase the cells elongated perpendicular to the stretch direction. This occurred simultaneously with the de novo formation of perpendicular mainly ventral actin stress fibers and concurrent realignment of cell-matrix adhesions during their maturation. The stretch-induced perpendicular cell elongation is microtubule-independent but myosin II-dependent. In summary, a CTS-induced cell orientation of spreading cells correlates temporary with the development of the acto-myosin system as well as contact to the underlying substrate by cell-matrix adhesions.

Determining the structure-mechanics relationships of dense microtubule networks with confocal microscopy and magnetic tweezers-based microrheology

The microtubule (MT) cytoskeleton is essential in maintaining the shape, strength, and organization of cells. Its spatiotemporal organization is fundamental for numerous dynamic biological processes, and mechanical stress within the MT cytoskeleton provides an important signaling mechanism in mitosis and neural development. This raises important questions about the relationships between structure and mechanics in complex MT structures. In vitro, reconstituted cytoskeletal networks provide a minimal model of cell mechanics while also providing a testing ground for the fundamental polymer physics of stiff polymer gels. Here, we describe our development and implementation of a broad tool kit to study structure-mechanics relationships in reconstituted MT networks, including protocols for the assembly of entangled and cross-linked MT networks, fluorescence imaging, microstructure characterization, construction and calibration of magnetic tweezers devices, and mechanical data collection and analysis. In particular, we present the design and assembly of three neodymium iron boron (NdFeB)-based magnetic tweezers devices optimized for use with MT networks: (1) high-force magnetic tweezers devices that enable the application of nano-Newton forces and possible meso- to macroscale materials characterization; (2) ring-shaped NdFeB-based magnetic tweezers devices that enable oscillatory microrheology measurements; and (3) portable magnetic tweezers devices that enable direct visualization of microscale deformation in soft materials under applied force.

Mechanical cues in cellular signalling and communication

Multicellular organisms comprise an organized array of individual cells surrounded by a meshwork of biomolecules and fluids. Cells have evolved various ways to communicate with each other, so that they can exchange information and thus fulfil their specified and unique functions. At the same time, cells are also physical entities that are subjected to a variety of local and global mechanical cues arising in the microenvironment. Cells are equipped with several different mechanisms to sense the physical properties of the microenvironment and the mechanical forces arising within it. These mechanical cues can elicit a variety of responses that have been shown to play a crucial role in vivo. In this review, we discuss the current views and understanding of cell mechanics and demonstrate the emerging evidence of the interplay between physiological mechanical cues and cell-cell communication pathways.

Targeting and transport: how microtubules control focal adhesion dynamics

Directional cell migration requires force generation that relies on the coordinated remodeling of interactions with the extracellular matrix (ECM), which is mediated by integrin-based focal adhesions (FAs). Normal FA turnover requires dynamic microtubules, and three members of the diverse group of microtubule plus-end-tracking proteins are principally involved in mediating microtubule interactions with FAs. Microtubules also alter the assembly state of FAs by modulating Rho GTPase signaling, and recent evidence suggests that microtubule-mediated clathrin-dependent and -independent endocytosis regulates FA dynamics. In addition, FA-associated microtubules may provide a polarized microtubule track for localized secretion of matrix metalloproteases (MMPs). Thus, different aspects of the molecular mechanisms by which microtubules control FA turnover in migrating cells are beginning to emerge.

Microtubules regulate GEF-H1 in response to extracellular matrix stiffness

Breast epithelial cells sense the stiffness of the extracellular matrix through Rho-mediated contractility. In turn, matrix stiffness regulates RhoA activity. However, the upstream signaling mechanisms are poorly defined. Here we demonstrate that the Rho exchange factor GEF-H1 mediates RhoA activation in response to extracellular matrix stiffness. We demonstrate the novel finding that microtubule stability is diminished by a stiff three-dimensional (3D) extracellular matrix, which leads to the activation of GEF-H1. Surprisingly, activation of the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathway did not contribute to stiffness-induced GEF-H1 activation. Loss of GEF-H1 decreases cell contraction of and invasion through 3D matrices. These data support a model in which matrix stiffness regulates RhoA through microtubule destabilization and the subsequent release and activation of GEF-H1.

Spectraplakins: master orchestrators of cytoskeletal dynamics

The dynamics of different cytoskeletal networks are coordinated to bring about many fundamental cellular processes, from neuronal pathfinding to cell division. Increasing evidence points to the importance of spectraplakins in integrating cytoskeletal networks. Spectraplakins are evolutionarily conserved giant cytoskeletal cross-linkers, which belong to the spectrin superfamily. Their genes consist of multiple promoters and many exons, yielding a vast array of differential splice forms with distinct functions. Spectraplakins are also unique in their ability to associate with all three elements of the cytoskeleton: F-actin, microtubules, and intermediate filaments. Recent studies have begun to unveil their role in a wide range of processes, from cell migration to tissue integrity.

An in vitro correlation of metastatic capacity, substrate rigidity, and ECM composition

The process of metastasis requires a metastatic cancer cell to invade a variety of micro-environments of variable stiffnesses. Unlike metastatic cells, normal cell function and viability is dependent on the stiffness of the environment and used as a cue to maintain cell health and proper tissue organization. In this study we have asked if metastatic cells can ignore the parameter of stiffness and if this ability is gradually acquired and if so, through what mechanism. Using a panel of mouse mammary tumor cells derived from the same parental tumor, but possessing different metastatic abilities, we cultured the cells on hard and soft substrates conjugated with collagen or fibronectin. Normal and non-metastatic tumor cells responded to changes in stiffness on fibronectin, but not collagen. However, the more metastatic cells ignored the change in stiffness on fibronectin-coated substrates. This lack of response on fibronectin correlated with a change in the expression level of the α3 integrin subunit, activation of the β1 subunit, and phosphorylation of FAKpY397. We conclude that through fibronectin, changes in the activation and tethering of the beta-1 integrin provides a mechanism for metastatic cells to disregard changes in compliance to survive and navigate in environments of different stiffness.

Mechanobiology in lung epithelial cells: measurements, perturbations, and responses

Epithelial cells of the lung are located at the interface between the environment and the organism and serve many important functions including barrier protection, fluid balance, clearance of particulate, initiation of immune responses, mucus and surfactant production, and repair following injury. Because of the complex structure of the lung and its cyclic deformation during the respiratory cycle, epithelial cells are exposed to continuously varying levels of mechanical stresses. While normal lung function is maintained under these conditions, changes in mechanical stresses can have profound effects on the function of epithelial cells and therefore the function of the organ. In this review, we will describe the types of stresses and strains in the lungs, how these are transmitted, and how these may vary in human disease or animal models. Many approaches have been developed to better understand how cells sense and respond to mechanical stresses, and we will discuss these approaches and how they have been used to study lung epithelial cells in culture. Understanding how cells sense and respond to changes in mechanical stresses will contribute to our understanding of the role of lung epithelial cells during normal function and development and how their function may change in diseases such as acute lung injury, asthma, emphysema, and fibrosis.

Microtubule dynamics regulate cyclic stretch-induced cell alignment in human airway smooth muscle cells

Microtubules are structural components of the cytoskeleton that determine cell shape, polarity, and motility in cooperation with the actin filaments. In order to determine the role of microtubules in cell alignment, human airway smooth muscle cells were exposed to cyclic uniaxial stretch. Human airway smooth muscle cells, cultured on type I collagen-coated elastic silicone membranes, were stretched uniaxially (20% in strain, 30 cycles/min) for 2 h. The population of airway smooth muscle cells which were originally oriented randomly aligned near perpendicular to the stretch axis in a time-dependent manner. However, when the cells treated with microtubule disruptors, nocodazole and colchicine, were subjected to the same cyclic uniaxial stretch, the cells failed to align. Lack of alignment was also observed for airway smooth muscle cells treated with a microtubule stabilizer, paclitaxel. To understand the intracellular mechanisms involved, we developed a computational model in which microtubule polymerization and attachment to focal adhesions were regulated by the preexisting tensile stress, pre-stress, on actin stress fibers. We demonstrate that microtubules play a central role in cell re-orientation when cells experience cyclic uniaxial stretching. Our findings further suggest that cell alignment and cytoskeletal reorganization in response to cyclic stretch results from the ability of the microtubule-stress fiber assembly to maintain a homeostatic strain on the stress fiber at focal adhesions. The mechanism of stretch-induced alignment we uncovered is likely involved in various airway functions as well as in the pathophysiology of airway remodeling in asthma.

Management of cytoskeleton architecture by molecular chaperones and immunophilins

Cytoskeletal structure is continually remodeled to accommodate normal cell growth and to respond to pathophysiological cues. As a consequence, several cytoskeleton-interacting proteins become involved in a variety of cellular processes such as cell growth and division, cell movement, vesicle transportation, cellular organelle location and function, localization and distribution of membrane receptors, and cell-cell communication. Molecular chaperones and immunophilins are counted among the most important proteins that interact closely with the cytoskeleton network, in particular with microtubules and microtubule-associated factors. In several situations, heat-shock proteins and immunophilins work together as a functionally active heterocomplex, although both types of proteins also show independent actions. In circumstances where homeostasis is affected by environmental stresses or due to genetic alterations, chaperone proteins help to stabilize the system. Molecular chaperones facilitate the assembly, disassembly and/or folding/refolding of cytoskeletal proteins, so they prevent aberrant protein aggregation. Nonetheless, the roles of heat-shock proteins and immunophilins are not only limited to solve abnormal situations, but they also have an active participation during the normal differentiation process of the cell and are key factors for many structural and functional rearrangements during this course of action. Cytoskeleton modifications leading to altered localization of nuclear factors may result in loss- or gain-of-function of such factors, which affects the cell cycle and cell development. Therefore, cytoskeletal components are attractive therapeutic targets, particularly microtubules, to prevent pathological situations such as rapidly dividing tumor cells or to favor the process of cell differentiation in other cases. In this review we will address some classical and novel aspects of key regulatory functions of heat-shock proteins and immunophilins as housekeeping factors of the cytoskeletal network.

Focal adhesion kinase regulation of mechanotransduction and its impact on endothelial cell functions

Vascular endothelial cells lining the blood vessels form the interface between the bloodstream and the vessel wall and as such they are continuously subjected to shear and cyclic stress from the flowing blood in the lumen. Additional mechanical stimuli are also imposed on these cells in the form of substrate stiffness transmitted from the extracellular matrix components in the basement membrane, and additional mechanical loads imposed on the lung endothelium as the result of respiration or mechanical ventilation in clinical settings. Focal adhesions (FAs) are complex structures assembled at the abluminal endothelial plasma membrane which connect the extracellular filamentous meshwork to the intracellular cytoskeleton and hence constitute the ideal checkpoint capable of controlling or mediating transduction of bidirectional mechanical signals. In this review we focus on focal adhesion kinase (FAK), a component of FAs, which has been studied for a number of years with regards to its involvement in mechanotransduction. We analyzed the recent advances in the understanding of the role of FAK in the signaling cascade(s) initiated by various mechanical stimuli with particular emphasis on potential implications on endothelial cell functions.

The emerging role of forces in axonal elongation

An understanding of how axons elongate is needed to develop rational strategies to treat neurological diseases and nerve injury. Growth cone-mediated neuronal elongation is currently viewed as occurring through cytoskeletal dynamics involving the polymerization of actin and tubulin subunits at the tip of the axon. However, recent work suggests that axons and growth cones also generate forces (through cytoskeletal dynamics, kinesin, dynein, and myosin), forces induce axonal elongation, and axons lengthen by stretching. This review highlights results from various model systems (Drosophila, Aplysia, Xenopus, chicken, mouse, rat, and PC12 cells), supporting a role for forces, bulk microtubule movements, and intercalated mass addition in the process of axonal elongation. We think that a satisfying answer to the question, "How do axons grow?" will come by integrating the best aspects of biophysics, genetics, and cell biology.

Distinct ECM mechanosensing pathways regulate microtubule dynamics to control endothelial cell branching morphogenesis

The compliance and dimensionality of the ECM regulate distinct changes in microtubule growth speed and growth persistence.

During angiogenesis, cytoskeletal dynamics that mediate endothelial cell branching morphogenesis during vascular guidance are thought to be regulated by physical attributes of the extracellular matrix (ECM) in a process termed mechanosensing. Here, we tested the involvement of microtubules in linking mechanosensing to endothelial cell branching morphogenesis. We used a recently developed microtubule plus end–tracking program to show that specific parameters of microtubule assembly dynamics, growth speed and growth persistence, are globally and regionally modified by, and contribute to, ECM mechanosensing. We demonstrated that engagement of compliant two-dimensional or three-dimensional ECMs induces local differences in microtubule growth speed that require myosin II contractility. Finally, we found that microtubule growth persistence is modulated by myosin II–mediated compliance mechanosensing when cells are cultured on two-dimensional ECMs, whereas three-dimensional ECM engagement makes microtubule growth persistence insensitive to changes in ECM compliance. Thus, compliance and dimensionality ECM mechanosensing pathways independently regulate specific and distinct microtubule dynamics parameters in endothelial cells to guide branching morphogenesis in physically complex ECMs.

Stress transmission within the cell

An outstanding problem in cell biology is how cells sense mechanical forces and how those forces affect cellular functions. During past decades, it has become evident that the deformable cytoskeleton (CSK), an intracellular network of various filamentous biopolymers, provides a physical basis for transducing mechanical signals into biochemical responses. To understand how mechanical forces regulate cellular functions, it is necessary to first understand how the CSK develops mechanical stresses in response to applied forces, and how those stresses are propagated through the CSK where various signaling molecules are immobilized. New experimental techniques have been developed to quantify cytoskeletal mechanics, which together with new computational approaches have given rise to new theories and models for describing mechanics of living cells. In this article, we discuss current understanding of cell biomechanics by focusing on the biophysical mechanisms that are responsible for the development and transmission of mechanical stresses in the cell and their effect on cellular functions. We compare and contrast various theories and models of cytoskeletal mechanics, emphasizing common mechanisms that those theories are built upon, while not ignoring irreconcilable differences. We highlight most recent advances in the understanding of mechanotransduction in the cytoplasm of living cells and the central role of the cytoskeletal prestress in propagating mechanical forces along the cytoskeletal filaments to activate cytoplasmic enzymes. It is anticipated that advances in cell mechanics will help developing novel therapeutics to treat pulmonary diseases like asthma, pulmonary fibrosis, and chronic obstructive pulmonary disease.

Slowing of axonal regeneration is correlated with increased axonal viscosity during aging

Background

As we age, the speed of axonal regeneration declines. At the biophysical level, why this occurs is not well understood.

Results

To investigate we first measured the rate of axonal elongation of sensory neurons cultured from neonatal and adult rats. We found that neonatal axons grew 40% faster than adult axons (11.5 µm/hour vs. 8.2 µm/hour). To determine how the mechanical properties of axons change during maturation, we used force calibrated towing needles to measure the viscosity (stiffness) and strength of substrate adhesion of neonatal and adult sensory axons. We found no significant difference in the strength of adhesions, but did find that adult axons were 3 times intrinsically stiffer than neonatal axons.

Conclusions

Taken together, our results suggest decreasing axonal stiffness may be part of an effective strategy to accelerate the regeneration of axons in the adult peripheral nervous system.

The kinetics of force-induced cell reorganization depend on microtubules and actin

The cytoskeleton is an important factor in the functional and structural adaption of cells to mechanical forces. In this study we investigated the impact of microtubules and the acto-myosin machinery on the kinetics of force-induced reorientation of NIH3T3 fibroblasts. These cells were subjected to uniaxial stretching forces that are known to induce cellular reorientation perpendicular to the stretch direction. We found that disruption of filamentous actin using cytochalasin D and latrunculin B as well as an induction of a massive unpolarized actin polymerization by jasplakinolide, inhibited the stretch-induced reorientation. Similarly, blocking of myosin II activity abolished the stretch-induced reorientation of cells but, interestingly, increased their motility under stretching conditions in comparison to myosin-inhibited nonstretched cells. Investigating the contribution of microtubules to the cellular reorientation, we found that, although not playing a significant role in reorientation itself, microtubule stability had a significant impact on the kinetics of this event. Overall, we conclude that acto-myosin, together with microtubules, regulate the kinetics of force-induced cell reorientation. © 2010 Wiley-Liss, Inc.

Methods for expressing and analyzing GFP-tubulin and GFP-microtubule-associated proteins

Important advances in our understanding of the organization and dynamics of the cytoskeleton have been made by direct observations of fluorescently tagged cytoskeletal proteins in living cells. In early experiments, the cytoskeletal protein of interest was purified, covalently modified with a fluorescent dye, and microinjected into living cells. In the mid-1990s, a powerful new technology arose: Researchers developed methods for expressing chimeric proteins consisting of the gene of interest fused to green fluorescent protein (GFP). This approach has become a standard method for characterizing protein localization and dynamics. More recently, a profusion of "XFP" (spectral variants of GFP) has been developed, allowing researchers straightforwardly to perform experiments ranging from simultaneous co-observation of protein dynamics to fluorescence recovery after photobleaching (FRAP), fluorescence resonance energy transfer (FRET), and subresolution techniques such as stimulated emission-depletion microscopy (STED) and photoactivated localization microscopy (PALM). In this article, the methods used to express and analyze GFP- and/or XFP-tagged tubulin and microtubule-associated proteins (MAPs) are discussed. Although some details may be system-specific, the methods and considerations outlined here can be adapted to a wide variety of proteins and organisms.

The heel and toe of the cell's foot: a multifaceted approach for understanding the structure and dynamics of focal adhesions

Focal adhesions (FAs) are large clusters of transmembrane receptors of the integrin family and a multitude of associated cytoplasmic "plaque" proteins, which connect the extracellular matrix-bound receptors with the actin cytoskeleton. The formation of nearly stationary FAs defines a boundary between the dense and highly dynamic actin network in lamellipodium and the sparser and more diverse cytoskeletal organization in the lamella proper, creating a template for the organization of the entire actin network. The major "mechanical" and "sensory" functions of FAs; namely, the nucleation and regulation of the contractile, myosin-II-containing stress fibers and the mechanosensing of external surfaces depend, to a major extent, on the dynamics of molecular components within FAs. A central element in FA regulation concerns the positive feedback loop, based on the most intriguing feature of FAs; that is, their dependence on mechanical tension developing by the growing stress fibers. FAs grow in response to such tension, and rapidly disassemble upon its relaxation. In this article, we address the mechanistic relationships between the process of FA development, maturation and dissociation and the dynamic molecular events, which take place in different regions of the FA, primarily in the distal end of this structure (the "toe") and the proximal "heel," and discuss the central role of local mechanical forces in orchestrating the complex interplay between FAs and the actin system.

Force-induced cell polarisation is linked to RhoA-driven microtubule-independent focal-adhesion sliding

Mechanical forces play a crucial role in controlling the integrity and functionality of cells and tissues. External forces are sensed by cells and translated into signals that induce various responses. To increase the detailed understanding of these processes, we investigated cell migration and dynamic cellular reorganisation of focal adhesions and cytoskeleton upon application of cyclic stretching forces. Of particular interest was the role of microtubules and GTPase activation in the course of mechanotransduction. We showed that focal adhesions and the actin cytoskeleton undergo dramatic reorganisation perpendicular to the direction of stretching forces even without microtubules. Rather, we found that microtubule orientation is controlled by the actin cytoskeleton. Using biochemical assays and fluorescence resonance energy transfer (FRET) measurements, we revealed that Rac1 and Cdc42 activities did not change upon stretching, whereas overall RhoA activity increased dramatically, but independently of intact microtubules. In conclusion, we demonstrated that key players in force-induced cellular reorganisation are focal-adhesion sliding, RhoA activation and the actomyosin machinery. In contrast to the importance of microtubules in migration, the force-induced cellular reorganisation, including focal-adhesion sliding, is independent of a dynamic microtubule network. Consequently, the elementary molecular mechanism of cellular reorganisation during migration is different to the one in force-induced cell reorganisation.

Mechanical Stability Determines Stress Fiber and Focal Adhesion Orientation

It is well documented in a variety of adherent cell types that in response to anisotropic signals from the microenvironment cells alter their cytoskeletal organization. Previous theoretical studies of these phenomena were focused primarily on the elasticity of cytoskeletal actin stress fibers (SFs) and of the substrate while the contribution of focal adhesions (FAs) through which the cytoskeleton is linked to the external environment has not been considered. Here we propose a mathematical model comprised of a single linearly elastic SF and two identical linearly elastic FAs of a finite size at the endpoints of the SF to investigate cytoskeletal realignment in response to uniaxial stretching of the substrate. The model also includes the contribution of the chemical potential energies of the SF and the FAs to the total potential energy of the SF-FA assembly. Using the global (Maxwell's) stability criterion, we predict stable configurations of the SF-FA assembly in response to substrate stretching. Model predictions obtained for physiologically feasible values of model parameters are consistent with experimental data from the literature. The model shows that elasticity of SFs alone can not predict their realignment during substrate stretching and that geometrical and elastic properties of SFs and FAs need to be included.

Collagen I membranes for tendon repair: effect of collagen fiber orientation on cell behavior

Tendons have poor spontaneous regenerative capabilities, and complete regeneration is never achieved despite intensive remodeling. In this in vitro study, we characterized two multilamellar collagen I membranes differing in the arrangement of collagen fiber deposition (oriented vs. nonoriented) and compared their mechanical properties. Human dermal fibroblasts and tenocytes were seeded on the two membranes to evaluate the effect of fiber orientation on cell viability and cytoskeletal organization. Results demonstrate that the multilamellar collagen I membrane with oriented fibers has the better mechanical properties and affords optimum cell proliferation and adhesion. Its fiber arrangement provides an instructive pattern for cell growth and may serve to guide the alignment of cells migrating from the ends of a crushed or frayed tendon to obtain a strong, correctly structured tendon, thus providing a viable clinical option for tendon repair.