Tensile stress stimulates microtubule outgrowth in living cells

Touch, grasp, deliver and control: functional cross-talk between microtubules and cell adhesions

Cross-talk between microtubule networks and sites of cell-matrix and cell-cell adhesion has profound impact on these structures and is essential for proper cell organization, polarization and motility. Components of adhesion sites can interact directly with microtubules or with proteins that specifically associate with microtubule plus ends and minus ends and in this way capture, stabilize or destabilize microtubules. In their turn, microtubules can serve as routes for delivery of structural and regulatory factors that control adhesion site turnover. In addition, the microtubule lattice or growing microtubule plus ends can serve as diffusional sinks that accumulate and scaffold regulatory molecules, thereby affecting their activity in the vicinity of adhesions. Combination of these mechanisms underlies the functional co-operation between microtubules and adhesion sites and defines their dynamic behavior.

A kinematic model of stretch-induced stress fiber turnover and reorientation

A kinetic model based on constrained mixture theory was developed to describe the reorganization of actin stress fibers in adherent cells in response to diverse patterns of mechanical stretch. The model was based on reports that stress fibers are pre-extended at a "homeostatic" level under normal, non-perturbed conditions, and that perturbations in stress fiber length destabilize stress fibers. In response to a step change in matrix stretch, the model predicts that stress fibers are initially stretched in registry with the matrix, but that these overly stretched fibers are gradually replaced by new fibers assembled with the homeostatic level of stretch in the new configuration of the matrix. In contrast, average fiber stretch is chronically perturbed from the homeostatic level when the cells are subjected to cyclic equibiaxial stretch. The model was able to describe experimentally measured time courses of stress fiber reorientation perpendicular to the direction of cyclic uniaxial stretch, as well as the lack of alignment in response to equibiaxial stretch. The model also accurately described the relationship between stretch magnitude and the extent of stress fiber alignment in endothelial cells subjected to cyclic uniaxial stretch. Further, in the case of cyclic simple elongation with transverse matrix contraction, stress fibers orient in the direction of least perturbation in stretch. In summary, the model predicts that the rate of stretch-induced stress fiber disassembly determines the rate of alignment, and that stress fibers tend to orient toward the direction of minimum matrix stretch where the rate of stress fiber turnover is a minimum.

Probing the origin of tubulin rigidity with molecular simulations

Tubulin heterodimers are the building blocks of microtubules, a major component of the cytoskeleton, whose mechanical properties are fundamental for the life of the cell. We uncover the microscopic origins of the mechanical response in microtubules by probing features of the energy landscape of the tubulin monomers and tubulin heterodimer. To elucidate the structures of the unfolding pathways and reveal the multiple unfolding routes, we performed simulations of a self-organized polymer (SOP) model of tubulin. The SOP representation, which is a coarse-grained description of chains, allows us to perform force-induced simulations at loading rates and time scales that closely match those used in single-molecule experiments. We show that the forced unfolding of each monomer involves a bifurcation in the pathways to the stretched state. After the unfolding of the C-term domain, the unraveling continues either from the N-term domain or from the middle domain, depending on the monomer and the pathway. In contrast to the unfolding complexity of the monomers, the dimer unfolds according to only one route corresponding to the unraveling of the C-term domain and part of the middle domain of beta-tubulin. We find that this surprising behavior is due to the viscoelastic properties of the interface between the monomers. We map precise features of the complex energy landscape of tubulin by surveying the structures of the various metastable intermediates, which, in the dimer case, are characterized only by changes in the beta-tubulin monomer.

Mechanical forces facilitate actin polymerization at focal adhesions in a zyxin-dependent manner

We examined the effects of mechanical forces on actin polymerization at focal adhesions (FAs). Actin polymerization at FAs was assessed by introducing fluorescence-labeled actin molecules into permeabilized fibroblasts cultured on fibronectin. When cell contractility was inhibited by the myosin-II inhibitor blebbistatin, actin polymerization at FAs was diminished, whereas α5β1 integrin remained accumulated at FAs. This suggests that actin polymerization at FAs depends on mechanical forces. To examine the action of mechanical forces more directly, the blebbistatin-treated cells were subjected to a sustained uniaxial stretch, which induced actin polymerization at FAs. These results demonstrate the novel role of mechanical forces in inducing actin polymerization at FAs. To reveal the molecular mechanism underlying the force-induced actin polymerization at FAs, we examined the distribution of zyxin, a postulated actin-regulatory protein. Actin-polymerizing activity was strong at zyxin-rich FAs. Accumulation of zyxin at FAs was diminished by blebbistatin, whereas uniaxial stretching of the cells induced zyxin accumulation. Displacing endogenous zyxin from FAs by expressing the FA-targeting region of zyxin decreased the force-induced actin polymerization at FAs. These results suggest that zyxin is involved in mechanical-force-dependent facilitation of actin polymerization at FAs.

Stability of adhesion clusters and cell reorientation under lateral cyclic tension

This work is motivated by experimental observations that cells on stretched substrate exhibit different responses to static and dynamic loads. A model of focal adhesion that can consider the mechanics of stress fiber, adhesion bonds, and substrate was developed at the molecular level by treating the focal adhesion as an adhesion cluster. The stability of the cluster under dynamic load was studied by applying cyclic external strain on the substrate. We show that a threshold value of external strain amplitude exists beyond which the adhesion cluster disrupts quickly. In addition, our results show that the adhesion cluster is prone to losing stability under high-frequency loading, because the receptors and ligands cannot get enough contact time to form bonds due to the high-speed deformation of the substrate. At the same time, the viscoelastic stress fiber becomes rigid at high frequency, which leads to significant deformation of the bonds. Furthermore, we find that the stiffness and relaxation time of stress fibers play important roles in the stability of the adhesion cluster. The essence of this work is to connect the dynamics of the adhesion bonds (molecular level) with the cell's behavior during reorientation (cell level) through the mechanics of stress fiber. The predictions of the cluster model are consistent with experimental observations.

Paxillin-dependent stimulation of microtubule catastrophes at focal adhesion sites

An organized microtubule array is essential for the polarized motility of fibroblasts. Dynamic microtubules closely interact with focal adhesion sites in migrating cells. Here, we examined the effect of focal adhesions on microtubule dynamics. We observed that the probability of microtubule catastrophes (transitions from growth to shrinkage) was seven times higher at focal adhesions than elsewhere. Analysis of the dependence between the microtubule growth rate and catastrophe probability throughout the cytoplasm revealed that a nonspecific (mechanical or spatial) factor provided a minor contribution to the catastrophe induction by decreasing microtubule growth rate at adhesions. Strikingly, at the same growth rate, the probability of catastrophes was significantly higher at adhesions than elsewhere, indicative of a site-specific biochemical trigger. The observed catastrophe induction occurred at adhesion domains containing the scaffolding protein paxillin that has been shown previously to interact with tubulin. Furthermore, replacement of full-length paxillin at adhesion sites by microinjected paxillin LIM2-LIM3 domains suppressed microtubule catastrophes exclusively at adhesions. We suggest that paxillin influences microtubule dynamics at focal adhesions by serving as a scaffold for a putative catastrophe factor and/or regulating its exposure to microtubules.

Localized elasticity measured in epithelial cells migrating at a wound edge using atomic force microscopy

Restoration of lung homeostasis following injury requires efficient wound healing by the epithelium. The mechanisms of lung epithelial wound healing include cell spreading and migration into the wounded area and later cell proliferation. We hypothesized that mechanical properties of cells vary near the wound edge, and this may provide cues to direct cell migration. To investigate this hypothesis, we measured variations in the stiffness of migrating human bronchial epithelial cells (16HBE cells) approximately 2 h after applying a scratch wound. We used atomic force microscopy (AFM) in contact mode to measure the cell stiffness in 1.5-microm square regions at different locations relative to the wound edge. In regions far from the wound edge (>2.75 mm), there was substantial variation in the elastic modulus in specific cellular regions, but the median values measured from multiple fields were consistently lower than 5 kPa. At the wound edge, cell stiffness was significantly lower within the first 5 microm but increased significantly between 10 and 15 microm before decreasing again below the median values away from the wound edge. When cells were infected with an adenovirus expressing a dominant negative form of RhoA, cell stiffness was significantly decreased compared with cells infected with a control adenovirus. In addition, expression of dominant negative RhoA abrogated the peak increase in stiffness near the wound edge. These results suggest that cells near the wound edge undergo localized changes in cellular stiffness that may provide signals for cell spreading and migration.

Modeling mechanosensing and its effect on the migration and proliferation of adherent cells

The behavior of normal adherent cells is influenced by the stiffness of the substrate they are anchored to. Cells are able to detect substrate mechanical properties by actively generating contractile forces and use this information to migrate and proliferate. In particular, the speed and direction of cell crawling, as well as the rate of cell proliferation, vary with the substrate compliance and prestrain. In this work, we present an active mechanosensing model based on an extension of the classical Hill's model for skeletal muscle behavior. We also propose a thermodynamical approach to model cell migration regulated by mechanical stimuli and a proliferation theory also depending on the mechanical environment. These contributions give rise to a conceptually simple mathematical formulation with a straightforward and inexpensive computational implementation, yielding results consistent with numerous experiments. The model can be a useful tool for practical applications in biology and medicine in situations where cell-substrate interaction as well as substrate mechanical behavior play an important role, such as the design of tissue engineering applications.

Kinetic model for lamellipodal actin-integrin 'clutch' dynamics

In migrating cells, with especial prominence in lamellipodial protrusions at the cell front, highly dynamic connections are formed between the actin cytoskeleton and the extracellular matrix through linkages of integrin adhesion receptors to actin filaments via complexes of cytosolic "connector" proteins. Myosin-mediated contractile forces strongly influence the dynamic behavior of these adhesion complexes, apparently in two counter-acting ways: negatively as the cell-generated forces enhance complex dissociation, and at the same time positively as force-induced signaling can lead to strengthening of the linkage complexes. The net balance arising from this dynamic interplay is challenging to ascertain a priori, rendering experimental studies difficult to interpret and molecular manipulations of cell and/or environment difficult to predict. We have constructed a kinetics-based model governing the dynamic behavior of this system. We obtained ranges of parameter value sets yielding behavior consistent with that observed experimentally for 3T3 cells and for CHO cells, respectively. Model simulations are able to produce results for the effects of paxillin mutations on the turnover rate of actin/integrin linkages in CHO cells, which are consistent with recent literature reports. Overall, although this current model is quite simple it provides a useful foundation for more detailed models extending upon it.

The forces that shape embryos: physical aspects of convergent extension by cell intercalation

We discuss the physical aspects of the morphogenic process of convergence (narrowing) and extension (lengthening) of tissues by cell intercalation. These movements, often referred to as 'convergent extension', occur in both epithelial and mesenchymal tissues during embryogenesis and organogenesis of invertebrates and vertebrates, and they play large roles in shaping the body plan during development. Our focus is on the presumptive mesodermal and neural tissues of the Xenopus (frog) embryo, tissues for which some physical measurements have been made. We discuss the physical aspects of how polarized cell motility, oriented along future tissue axes, generate the forces that drive oriented cell intercalation and how this intercalation results in convergence and extension or convergence and thickening of the tissue. Our goal is to identify aspects of these morphogenic movements for further biophysical, molecular and cell biological, and modeling studies.

Magnetic microposts for mechanical stimulation of biological cells: fabrication, characterization, and analysis

Cells use force as a mechanical signal to sense and respond to their microenvironment. Understanding how mechanical forces affect living cells requires the development of tool sets that can apply nanoscale forces and also measure cellular traction forces. However, there has been a lack of techniques that integrate actuation and sensing components to study force as a mechanical signal. Here, we describe a system that uses an array of elastomeric microposts to apply external forces to cells through cobalt nanowires embedded inside the microposts. We first biochemically treat the posts' surfaces to restrict cell adhesion to the posts' tips. Then by applying a uniform magnetic field (B<0.3 T), we induce magnetic torque on the nanowires that is transmitted to a cell's adhesion site as an external force. We have achieved external forces of up to 45 nN, which is in the upper range of current nanoscale force-probing techniques. Nonmagnetic microposts, similarly prepared but without nanowires, surround the magnetic microposts and are used to measure the traction forces and changes in cell mechanics. We record the magnitude and direction of the external force and the traction forces by optically measuring the deflection of the microposts, which linearly deflect as cantilever springs. With this approach, we can measure traction forces before and after force stimulation in order to monitor cellular response to forces. We present the fabrication methods, magnetic force characterization, and image analysis techniques used to achieve the measurements.

Myosin IIA regulates cell motility and actomyosin–microtubule crosstalk

Non-muscle myosin II has diverse functions in cell contractility, cytokinesis and locomotion, but the specific contributions of its different isoforms have yet to be clarified. Here, we report that ablation of the myosin IIA isoform results in pronounced defects in cellular contractility, focal adhesions, actin stress fibre organization and tail retraction. Nevertheless, myosin IIA-deficient cells display substantially increased cell migration and exaggerated membrane ruffling, which was dependent on the small G-protein Rac1, its activator Tiam1 and the microtubule moter kinesin Eg5. Myosin IIA deficiency stabilized microtubules, shifting the balance between actomyosin and microtubules with increased microtubules in active membrane ruffles. When microtubule polymerization was suppressed, myosin IIB could partially compensate for the absence of the IIA isoform in cellular contractility, but not in cell migration. We conclude that myosin IIA negatively regulates cell migration and suggest that it maintains a balance between the actomyosin and microtubule systems by regulating microtubule dynamics.

Analysis of microtubule curvature

The microtubule cytoskeleton in living cells generate and resist mechanical forces to mediate fundamental cell processes, including cell division and migration. Recent advances in digital fluorescence microscopy have enabled the direct observation of bending of individual microtubules in living cells, which has enabled quantitative estimation of the mechanical state of the microtubule array. Although a variety of mechanisms have been proposed, the precise origins of microtubule deformation in living cells remain largely obscure. To investigate these mechanisms and their relative importance in cellular processes, a method is needed to accurately quantify microtubule bending within living cells. Here we describe a method for quantification of bending, using digital fluorescence microscope images to estimate the distribution of curvature in the microtubule. Digital images of individual microtubules can be used to obtain a set of discrete x-y coordinates along the microtubule contour, which is then used to estimate the curvature distribution. Due to system noise and digitization error, the estimate will be inaccurate to some degree. To quantify the inaccuracy, a computational model is used to simulate both the bending of thermally driven microtubules and their observation by digital fluorescence microscopy. This allows for direct comparison between experimental and simulated images, a method which we call model convolution microscopy. We assess the accuracy of various methods and present a suitable method for estimating the curvature distribution for thermally driven semiflexible polymers. Finally, we discuss extensions of the method to quantify microtubule curvature in living cells.

Mobility and invasiveness of metastatic esophageal cancer are potentiated by shear stress in a ROCK- and Ras-dependent manner

To metastasize, tumor cells must adopt different morphological responses to resist shear forces encountered in circulating blood and invade through basement membranes. The Rho and Ras GTPases play a critical role in regulating this dynamic behavior. Recently, we demonstrated shear-induced activation of adherent esophageal metastatic cells, characterized by formation of dynamic membrane blebs. Although membrane blebbing has only recently been characterized as a rounded mode of cellular invasion promoted through Rho kinase (ROCK), the role of shear forces in modulating membrane blebbing activity is unknown. To further characterize membrane blebbing in esophageal metastatic cells (OC-1 cell line), we investigated the role of shear in cytoskeletal remodeling and signaling through ROCK and Ras. Our results show that actin and tubulin colocalize to the cortical ring of the OC-1 cell under static conditions. However, under shear, actin acquires a punctuate distribution and tubulin localizes to the leading edge of the OC-1 cell. We show for the first time that dynamic bleb formation is induced by shear alone independent of integrin-mediated adhesion ( P < 0.001, compared with OC-1 cells). Y-27632, a specific inhibitor of ROCK, causes a significant reduction in shear-induced bleb formation and inhibits integrin αvβ3-Ras colocalization at the leading edge of the cell. Direct measurement of Ras activation shows that the level of GTP-bound Ras is elevated in sheared OC-1 cells and that the shear-induced increase in Ras activity is inhibited by Y-27632. Finally, we show that shear stress significantly increases OC-1 cell invasion ( P < 0.007), an effect negated by the presence of Y-27632. Together our findings suggest a novel physiological role for ROCK and Ras in metastatic cell behavior.

Information about a form (on the dynamic laws of morphogenesis)

How a developing embryo becomes "informed" about its form?" This problem remains obscure and controversial. We argue that the "information about a form" is distributed throughout three main components: the dynamic laws, the parameters and the initial/boundary conditions. In the absence of a dynamic law two other components are "blind", that is, do not contain any unambiguous information. We present a version of a dynamic law of morphogenesis, based upon the presumption of a feedback between passive and active mechanical stresses. We explore several models of shape formation based upon this law and show that, as depending upon the parameters values, they generate a large set of realistic shapes. Genetic and epigenetic basis of the models parameters is discussed.

Subcutaneous tissue fibroblast cytoskeletal remodeling induced by acupuncture: Evidence for a mechanotransduction-based mechanism

Acupuncture needle rotation has been previously shown to cause specific mechanical stimulation of subcutaneous connective tissue. This study uses acupuncture to investigate the role of mechanotransduction-based mechanisms in mechanically-induced cytoskeletal remodeling. The effect of acupuncture needle rotation was quantified by morphometric analysis of mouse tissue explants imaged with confocal microscopy. Needle rotation induced extensive fibroblast spreading and lamellipodia formation within 30 min, measurable as an increased in cell body cross sectional area. The effect of rotation peaked with two needle revolutions and decreased with further increases in rotation. Significant effects of rotation were present throughout the tissue, indicating the presence of a response extending laterally over several centimeters. The effect of rotation with two needle revolutions was prevented by pharmacological inhibitors of actomyosin contractility (blebbistatin), Rho kinase (Y-27632 and H-1152), and Rac signaling. The active cytoskeletal response of fibroblasts demonstrated in this study constitutes an important step in understanding cellular mechanotransduction responses to externally applied mechanical stimuli in whole tissue, and supports a previously proposed model for the mechanism of acupuncture involving connective tissue mechanotransduction.

Assembly and mechanosensory function of focal adhesions: experiments and models

Initial integrin-mediated cell-matrix adhesions (focal complexes) appear underneath the lamellipodia, in the regions of the "fast" centripetal flow driven by actin polymerization. Once formed, these adhesions convert the flow behind them into a "slow", myosin II-driven mode. Some focal complexes then turn into elongated focal adhesions (FAs) associated with contractile actomyosin bundles (stress fibers). Myosin II inhibition does not suppress formation of focal complexes but blocks their conversion into mature FAs and further FA growth. Application of external pulling force promotes FA growth even under conditions when myosin II activity is blocked. Thus, individual FAs behave as mechanosensors responding to the application of force by directional assembly. We proposed a thermodynamic model for the mechanosensitivity of FAs, taking into account that an elastic molecular aggregate subject to pulling forces tends to grow in the direction of force application by incorporating additional subunits. This simple model can explain a variety of processes typical of FA behavior. Assembly of FAs is triggered by the small G-protein Rho via activation of two major targets, Rho-associated kinase (ROCK) and the formin homology protein, Dia1. ROCK controls creation of myosin II-driven forces, while Dia1 is involved in the response of FAs to these forces. Expression of the active form of Dia1, allows the external force-induced assembly of mature FAs, even in conditions when Rho is inhibited. Conversely, downregulation of Dia1 by siRNA prevents FA maturation even if Rho is activated. Dia1 and other formins cap barbed (fast growing) ends of actin filaments, allowing insertion of the new actin monomers. We suggested a novel mechanism of such "leaky" capping based on an assumption of elasticity of the formin/barbed end complex. Our model predicts that formin-mediated actin polymerization should be greatly enhanced by application of external pulling force. Thus, the formin-actin complex might represent an elementary mechanosensing device responding to force by enhancement of actin assembly. In addition to its role in actin polymerization, Dia1 seems to be involved in formation of links between actin filaments and microtubules affecting microtubule dynamics. Alpha-tubulin deacetylase HDAC6 cooperates with Dia1 in formation of such links. Since microtubules are known to promote FA disassembly, the Dia1-mediated effect on microtubule dynamics may possibly play a role in the negative feedback loop controlling size and turnover of FAs.

The comings and goings of actin: coupling protrusion and retraction in cell motility

Cells utilize actin filaments to produce protrusive and contractile arrays that cooperate to drive cell motility. The generation of the two arrays and the coupling between them result from the unique properties of the lamellipodium, a protrusive leaflet of cytoplasm at the cell edge. From the lamellipodium into the lamella behind, there is a transition from a fast retrograde flow of actin polymer driven by polymerization to a slow flow driven by the interaction of anti-parallel arrays of actin with myosin. In addition to driving protrusion, the lamellipodium appears to play a role in supplying filaments to the lamella for the assembly of the contractile network required for traction.

An introduction to cell motility for the physical scientist

Directed, purposeful movement is one of the qualities that we most closely associate with living organisms, and essentially all known forms of life on this planet exhibit some type of self-generated movement or motility. Even organisms that remain sessile most of the time, like flowering plants and trees, are quite busy at the cellular level, with large organelles, including chloroplasts, constantly racing around within cellular boundaries. Directed biological movement requires that the cell be able to convert its abundant stores of chemical energy into mechanical energy. Understanding how this mechanochemical energy transduction takes place and understanding how small biological forces generated at the molecular level are marshaled and organized for large-scale cellular or organismal movements are the focus of the field of cell motility. This tutorial, aimed at readers with a background in physical sciences, surveys the state of current knowledge and recent advances in modeling cell motility.

Topographically induced direct cell mechanotransduction

This review is designed to introduce the cytoskeleton and then discuss how mechanical forces may be transduced to the cell nucleus. In addition to this, it also tries to explain current thinking as to how the nucleus turns these mechanical cues into gene changes and is especially interested in mechanotransduction arising from topographically induced morphological changes, specifically nanotopography. Thus, this review also describes cell responses to topography.

Quantification of microtubule nucleation, growth and dynamics in wound-edge cells

Mammalian cells develop a polarized morphology and migrate directionally into a wound in a monolayer culture. To understand how microtubules contribute to these processes, we used GFP-tubulin to measure dynamic instability and GFP-EB1, a protein that marks microtubule plus-ends, to measure microtubule growth events at the centrosome and cell periphery. Growth events at the centrosome, or nucleation, do not show directional bias, but are equivalent toward and away from the wound. Cells with two centrosomes nucleated approximately twice as many microtubules/minute as cells with one centrosome. The average number of growing microtubules per microm2 at the cell periphery is similar for leading and trailing edges and for cells containing one or two centrosomes. In contrast to microtubule growth, measurement of the parameters of microtubule dynamic instability demonstrate that microtubules in the trailing edge are more dynamic than those in the leading edge. Inhibition of Rho with C3 transferase had no detectable effect on microtubule dynamics in the leading edge, but stimulated microtubule turnover in the trailing edge. Our data demonstrate that in wound-edge cells, microtubule nucleation is non-polarized, in contrast to microtubule dynamic instability, which is highly polarized, and that factors in addition to Rho contribute to microtubule stabilization.

Use of nanotopography to study mechanotransduction in fibroblasts--methods and perspectives

The environment around a cell during in vitro culture is unlikely to mimic those in vivo. Preliminary experiments with nanotopography have shown that nanoscale features can strongly influence cell morphology, adhesion, proliferation and gene regulation, but the mechanisms mediating this cell response remain unclear. In this perspective article, we attempt to illustrate that a possible mechanism is direct transmittal of forces encountered by cells during spreading to the nucleus via the cytoskeleton. We further try to illustrate that this 'self-induced' mechanotransduction may alter gene expression by changing interphase chromosome positioning. Whilst the observations described here to show how we think nanotopography can be developed as a tool to look at mechanotransduction are preliminary, we feel they indicate that topography may give cell biologists a non-invasive tool with which to investigate in vitro cellular mechanisms.

Activation of a signaling cascade by cytoskeleton stretch

Cells sense and respond to mechanical force. However, the mechanisms of transduction of extracellular matrix (ECM) forces to biochemical signals are not known. After removing the cell membrane and soluble proteins by Triton X-100 extraction, we found that the remaining complex (Triton cytoskeletons) activated Rap1 upon stretch. Rap1 guanine nucleotide exchange factor, C3G, was required for this activation; C3G as well as the adaptor protein, CrkII, in cell extract bound to Triton cytoskeletons in a stretch-dependent manner. CrkII binding, which was Cas dependent, correlated with stretch-dependent tyrosine phosphorylation of proteins in Triton cytoskeletons including Cas at the contacts with ECM. These in vitro findings were compatible with in vivo observations of stretch-enhanced phosphotyrosine signals, accumulation of CrkII at cell-ECM contacts, and CrkII-Cas colocalization. We suggest that mechanical force on Triton cytoskeletons activates local tyrosine phosphorylation, which provides docking sites for cytosolic proteins, and initiates signaling to activate Rap1.

Roles of microtubules, cell polarity and adhesion in electric-field-mediated motility of 3T3 fibroblasts

Direct-current electric fields mediate motility (galvanotaxis) of many cell types. In 3T3 fibroblasts, electric fields increased the proportion, speed and cathodal directionality of motile cells. Analogous to fibroblasts' spontaneous migration, we initially hypothesized that reorientation of microtubule components modulates galvanotaxis. However, cells with intact microtubules did not reorient them in the field and cells without microtubules still migrated, albeit slowly, thus disproving the hypothesis. We next proposed that, in monolayers wounded and placed in an electric field, reorientation of microtubule organizing centers and stable, detyrosinated microtubules towards the wound edge is necessary and/or sufficient for migration. This hypothesis was negated because field exposure mediated migration of unoriented, cathode-facing cells and curtailed migration of oriented, anode-facing cells. This led us to propose that ablating microtubule detyrosination would not affect galvanotaxis. Surprisingly, preventing microtubule detyrosination increased motility speed, suggesting that detyrosination inhibits galvanotaxis. Microtubules might enhance adhesion/de-adhesion remodeling during galvanotaxis; thus, electric fields might more effectively mediate motility of cells poorly or dynamically attached to substrata. Consistent with this hypothesis, incompletely spread cells migrated more rapidly than fully spread cells. Also, overexpression of PAK4, a Cdc42-activated kinase that decreases adhesion, enhanced galvanotaxis speed, whereas its lack decreased speed. Thus, electric fields mediate fibroblast migration via participation of microtubules and adhesive components, but their participation differs from that during spontaneous motility.

Dynamic fibroblast cytoskeletal response to subcutaneous tissue stretch ex vivo and in vivo

Cytoskeleton-dependent changes in cell shape are well-established factors regulating a wide range of cellular functions including signal transduction, gene expression, and matrix adhesion. Although the importance of mechanical forces on cell shape and function is well established in cultured cells, very little is known about these effects in whole tissues or in vivo. In this study we used ex vivo and in vivo models to investigate the effect of tissue stretch on mouse subcutaneous tissue fibroblast morphology. Tissue stretch ex vivo (average 25% tissue elongation from 10 min to 2 h) caused a significant time-dependent increase in fibroblast cell body perimeter and cross-sectional area (ANOVA, P < 0.01). At 2 h, mean fibroblast cell body cross-sectional area was 201% greater in stretched than in unstretched tissue. Fibroblasts in stretched tissue had larger, "sheetlike" cell bodies with shorter processes. In contrast, fibroblasts in unstretched tissue had a "dendritic" morphology with smaller, more globular cell bodies and longer processes. Tissue stretch in vivo for 30 min had effects that paralleled those ex vivo. Stretch-induced cell body expansion ex vivo was inhibited by colchicine and cytochalasin D. The dynamic, cytoskeleton-dependent responses of fibroblasts to changes in tissue length demonstrated in this study have important implications for our understanding of normal movement and posture, as well as therapies using mechanical stimulation of connective tissue including physical therapy, massage, and acupuncture.

ACF7: an essential integrator of microtubule dynamics

ACF7 is a member of the spectraplakin family of cytoskeletal crosslinking proteins possessing actin and microtubule binding domains. Here, we show that ACF7 is an essential integrator of MT-actin dynamics. In endodermal cells, ACF7 binds along microtubules but concentrates at their distal ends and at cell borders when polarized. In ACF7's absence, microtubules still bind EB1 and CLIP170, but they no longer grow along polarized actin bundles, nor do they pause and tether to actin-rich cortical sites. The consequences are less stable, long microtubules with skewed cytoplasmic trajectories and altered dynamic instability. In response to wounding, ACF7 null cultures activate polarizing signals, but fail to maintain them and coordinate migration. Rescue of these defects requires ACF7's actin and microtubule binding domains. Thus, spectraplakins are important for controlling microtubule dynamics and reinforcing links between microtubules and polarized F-actin, so that cellular polarization and coordinated cell movements can be sustained.

Podosome formation in cultured A7r5 vascular smooth muscle cells requires Arp2/3-dependent de-novo actin polymerization at discrete microdomains

Phorbol ester triggers the conversion of focal adhesions into podosomes in A7r5 smooth muscle cells. Here we followed the dynamics of podosome formation using dual fluorescence live video and confocal microscopy, as well as interference reflection and evanescent wave microscopy. We show that podosomes form at the outer region of stress fiber bundles, at specialized sites where they are embedded in adhesion plaques at the basal surface of the plasma membrane, and that cortactin resides constitutively at these microdomains. We further demonstrate that the formation of podosomes requires Arp2/3-dependent actin polymerization at the stress fiber-focal adhesion interface. Concentration of Arp2/3 coincides with podosome formation and precedes the engagement of SM22 and alpha-actinin, while the focal adhesion components zyxin and vinculin redistribute only at later stages of podosome development. We thus suggest that the genesis of podosomes includes two steps, one requiring the early de novo polymerization of actin filaments, and a second, late phase characterized by the recruitment of focal adhesion components. Moreover, we provide evidence for the existence of an as yet unidentified region in close proximity to the focal adhesion-stress fiber interface, which marks the site of actin cytoskeleton remodeling and is a novel site of Arp2/3-dependent F-actin polymerization.

Regulation of Tension-induced Mechanotranscriptional Signals by the Microtubule Network in Fibroblasts

Mechanical loading of connective tissues induces the expression of extracellular matrix and cytoskeletal genes that are involved in matrix remodeling. These processes depend in part on force transmission through beta1 integrins and actin filaments, but the role of microtubules in regulating mechanotranscriptional responses is not well defined. We assessed the involvement of microtubules in the mechanotranscriptional regulation of filamin A, an actin-cross-linking protein that protects cells against force-induced apoptosis by stabilizing cell membranes. Collagen-coated magnetite beads and magnetic fields were used to apply tensile forces to cultured fibroblasts at focal adhesions. Force enhanced recruitment of alpha-tubulin and the plus end microtubule-binding protein cytoplasmic linker protein-170 (CLIP-170) at focal adhesions. Immunoprecipitation studies demonstrated no direct binding of tubulin to actin or filamin A, but CLIP-170 interacted with tubulin, filamin A, and beta-actin. The association of CLIP-170 with beta-actin was enhanced by force. Force activated the p38 mitogen-activated protein kinase, increased filamin A expression, and induced the relocation of p38 and filamin A to focal adhesions. Disruption of microtubules with nocodazole, independent of force application, enhanced filamin A expression and Sp1-mediated filamin A promoter activity, while stabilization of microtubules with Taxol inhibited force induction of both filamin A mRNA and protein. We conclude that in response to tensile forces applied through beta1 integrins and actin the microtubule network modulates mechanotranscriptional coupling of filamin A.

Tensile force-dependent neurite elicitation via anti-beta1 integrin antibody-coated magnetic beads

Previous work using glass microneedles to apply calibrated, localized force to neurons showed that tensile force is a sufficient signal for neurite initiation and elongation. However, previous studies did not examine the kinetics or probability of neurite initiation as a function of force or the rate of force application. Here we report the use of a new technique-magnetic bead force application-to systematically investigate the role of force in these phenomena with better ease of use and control over force than glass microneedles. Force-induced neurite initiation from embryonic chick forebrain neurons appeared to be a first-order random process whose rate increased with increasing force, and required the presence of peripheral microtubules. In addition, the probability of initiation was more than twofold lower for neurons exposed to rapid initial force ramps (450 pN/s) than for neurons exposed to slower ramps (1.5 and 11 pN/s). We observed a low force threshold for elongation (15-100 pN), which was not previously detected in chick forebrain neurites elongated by glass microneedles. Finally, neurites subjected to constant force elongated at variable instantaneous rates, and switched abruptly between elongation and retraction, similar to spontaneous, growth-cone-mediated outgrowth and microtubule dynamic instability.

Nanometer targeting of microtubules to focal adhesions

Although cell movement is driven by actin, polarization and directional locomotion require an intact microtubule cytoskeleton that influences polarization by modulating substrate adhesion via specific targeting interactions with adhesion complexes. The fidelity of adhesion site targeting is precise; using total internal reflection fluorescence microscopy (TIRFM), we now show microtubule ends (visualized by incorporation of GFP tubulin) are within 50 nm of the substrate when polymerizing toward the cell periphery, but not when shrinking from it. Multiple microtubules sometimes followed similar tracks, suggesting guidance along a common cytoskeletal element. Use of TIRFM with GFP- or DsRed-zyxin in combination with either GFP-tubulin or GFP-CLIP-170 further revealed that the polymerizing microtubule plus ends that tracked close to the dorsal surface consistently targeted substrate adhesion complexes. This supports a central role for the microtubule tip complex in the guidance of microtubules into adhesion foci, and provides evidence for an intimate cross-talk between microtubule tips and substrate adhesions in the range of molecular dimensions.

Tensegrity II. How structural networks influence cellular information processing networks

The major challenge in biology today is biocomplexity: the need to explain how cell and tissue behaviors emerge from collective interactions within complex molecular networks. Part I of this two-part article, described a mechanical model of cell structure based on tensegrity architecture that explains how the mechanical behavior of the cell emerges from physical interactions among the different molecular filament systems that form the cytoskeleton. Recent work shows that the cytoskeleton also orients much of the cell's metabolic and signal transduction machinery and that mechanical distortion of cells and the cytoskeleton through cell surface integrin receptors can profoundly affect cell behavior. In particular, gradual variations in this single physical control parameter (cell shape distortion) can switch cells between distinct gene programs (e.g. growth, differentiation and apoptosis), and this process can be viewed as a biological phase transition. Part II of this article covers how combined use of tensegrity and solid-state mechanochemistry by cells may mediate mechanotransduction and facilitate integration of chemical and physical signals that are responsible for control of cell behavior. In addition, it examines how cell structural networks affect gene and protein signaling networks to produce characteristic phenotypes and cell fate transitions during tissue development.

Tensegrity I. Cell structure and hierarchical systems biology

In 1993, a Commentary in this journal described how a simple mechanical model of cell structure based on tensegrity architecture can help to explain how cell shape, movement and cytoskeletal mechanics are controlled, as well as how cells sense and respond to mechanical forces (J. Cell Sci. 104, 613-627). The cellular tensegrity model can now be revisited and placed in context of new advances in our understanding of cell structure, biological networks and mechanoregulation that have been made over the past decade. Recent work provides strong evidence to support the use of tensegrity by cells, and mathematical formulations of the model predict many aspects of cell behavior. In addition, development of the tensegrity theory and its translation into mathematical terms are beginning to allow us to define the relationship between mechanics and biochemistry at the molecular level and to attack the larger problem of biological complexity. Part I of this two-part article covers the evidence for cellular tensegrity at the molecular level and describes how this building system may provide a structural basis for the hierarchical organization of living systems--from molecule to organism. Part II, which focuses on how these structural networks influence information processing networks, appears in the next issue.

Live-cell monitoring of tyrosine phosphorylation in focal adhesions following microtubule disruption

Tyrosine phosphorylation of focal adhesion components is involved in the regulation of focal adhesion formation and turnover, yet the underlying molecular mechanisms are still poorly defined. In the present study, we have used quantitative fluorescence microscopy to investigate the dynamic relationships between the incorporation of new components into growing focal adhesions and tyrosine phosphorylation of these sites. For this purpose, a new approach for monitoring phosphotyrosine levels in live cells was developed, based on a 'phosphotyrosine reporter' consisting of yellow fluorescent protein fused to two consecutive phosphotyrosine-binding Src-homology 2 (SH2)-domains derived from pp60(c-Src). This YFP-dSH2 localized to cell-matrix adhesions and its intensity was linearly correlated with that of an anti-phosphotyrosine antibody labeling. The differential increase in vinculin and phosphotyrosine levels was examined in live cells by two-color time-lapse movies of CFP-vinculin and YFP-dSH2. In this study, focal adhesion growth was triggered by microtubule disruption, which was previously shown to stimulate focal adhesion development by inducing cellular contraction. We show here that, 2 minutes after addition of the microtubule-disrupting drug nocodazole, the local densities of the focal adhesion-associated proteins vinculin, paxillin and focal adhesion kinase (FAK) are significantly elevated and the focal adhesion area is increased, whereas elevation in tyrosine phosphorylation inside the growing adhesions occurs only a few minutes later. Phosphotyrosine and FAK density reach their maximum levels after 10 minutes of treatment, whereas vinculin and paxillin levels as well as focal adhesion size continue to grow, reaching a plateau at about 30 minutes. Our findings suggest that protein recruitment and growth of focal adhesions are an immediate and direct result of increased contractility induced by microtubule disruption, whereas tyrosine phosphorylation is activated later.

External mechanical strain regulates membrane targeting of Rho GTPases by controlling microtubule assembly

Transmission of externally applied mechanical forces to the interior of a cell requires coordination of biochemical signaling pathways with changes in cytoskeletal assembly and organization. In this study, we addressed one potential mechanism for this signal integration by applying uniform single external mechanical strains to aortic smooth muscle cells (SMCs) via their adhesion substrate. A tensile strain applied to the substrate for 15 min significantly increased microtubule (MT) assembly by 32 +/- 7%, with no apparent effect on the cells' focal adhesions as revealed by immunofluorescence and quantitative analysis of Triton X-100-insoluble vinculin levels. A compressive strain decreased MT mass by 24 +/- 9% but did not influence the level of vinculin in focal adhesions. To understand the decoupling of these two cell responses to mechanical strain, we examined a redistribution of the small GTPases RhoA and Rac. Tensile strain was found to decrease the amount of membrane-associated RhoA and Rac by 70 +/- 9% and 45 +/- 11%, respectively, compared with static controls. In contrast, compressive strain increased membrane-associated RhoA and Rac levels by 74 +/- 17% and 36 +/- 13%, respectively. Disruption of the MT network by prolonged treatments with low doses of either nocodazole or paclitaxel before the application of strain abolished the redistribution of RhoA and Rac in response to the applied forces. Combined, these results indicate that the effects of externally applied mechanical strain on the distribution and activation of the Rho family GTPases require changes in the state of MT polymerization.

Microtubules meet substrate adhesions to arrange cell polarity

Cell movement is driven by the regulated and polarised turnover of the actin cytoskeleton and of the adhesion complexes that link it to the extracellular matrix. For most cells, polarisation requires the engagement of microtubules, which exert their effect by mediating changes in the activity of the Rho GTPases. Evidence suggests that these changes are effected in a very localised fashion at sites of substrate adhesion, via specific microtubule-targeting interactions. Targeting serves to bring molecular complexes bound at the tips and along microtubules in close proximity with adhesion complexes, to promote adhesion disassembly and remodelling of the actin cytoskeleton.

Adhesion-dependent cell mechanosensitivity

The conversion of physical signals, such as contractile forces or external mechanical perturbations, into chemical signaling events is a fundamental cellular process that occurs at cell-extracellular matrix contacts, known as focal adhesions. At these sites, transmembrane integrin receptors are associated via their cytoplasmic domains with the actin cytoskeleton. This interaction with actin is mediated by a submembrane plaque, consisting of numerous cytoskeletal and signaling molecules. Application of intrinsic or external forces to these structures dramatically affects their assembly and triggers adhesion-mediated signaling. In this review, we discuss the structure-function relationships of focal adhesions and the possible mode of action of the putative mechanosensor associated with them. We also discuss the general phenomenon of mechanosensitivity, and the approaches used to measure local forces at adhesion sites, the cytoskeleton-mediated regulation of local contractility, and the nature of the signaling networks that both affect contractility and are affected by it.

How do microtubules guide migrating cells?

Microtubules have long been implicated in the polarization of migrating cells, but how they carry out this role is unclear. Here, we propose that microtubules determine cell polarity by modulating the pattern of adhesions that a cell develops with the underlying matrix, through focal inhibitions of contractility.