Local force and geometry sensing regulate cell functions

Myosin II dynamics are regulated by tension in intercalating cells

Axis elongation in Drosophila occurs through polarized cell rearrangements driven by actomyosin contractility. Myosin II promotes neighbor exchange through the contraction of single cell boundaries, while the contraction of myosin II structures spanning multiple pairs of cells leads to rosette formation. Here we show that multicellular actomyosin cables form at a higher frequency than expected by chance, indicating that cable assembly is an active process. Multicellular cables are sites of increased mechanical tension as measured by laser ablation. Fluorescence recovery after photobleaching experiments show that myosin II is stabilized at the cortex in regions of increased tension. Myosin II is recruited in response to an ectopic force and relieving tension leads to a rapid loss of myosin, indicating that tension is necessary and sufficient for cortical myosin localization. These results demonstrate that myosin II dynamics are regulated by tension in a positive feedback loop that leads to multicellular actomyosin cable formation and efficient tissue elongation.

Crosslinking of cell-derived 3D scaffolds up-regulates the stretching and unfolding of new extracellular matrix assembled by reseeded cells

Elevated levels of tissue crosslinking are associated with numerous diseases (cancer stroma, organ fibrosis), and also eliminate the otherwise remarkable clinical successes of tissue-derived scaffolds, instead eliciting a foreign body reaction. Nevertheless, it is not well understood how the initial physical and biochemical properties of cellular microenvironments, stem cell niches, or of 3D tissue scaffolds guide the assembly and remodeling of new extracellular matrix (ECM) that is ultimately sensed by cells. Here, we incorporated FRET-based mechanical strain sensors, either into cell-derived ECM scaffolds or into the fibronectin (Fn) matrix assembled by reseeded fibroblasts, and demonstrated the following. Cell-generated tensile forces change the conformation of Fn in both 3D scaffolds and new matrix over time. The time course by which new matrix fibers are stretched by reseeded cells is accelerated by scaffold crosslinking. Importantly, stretching Fn fibers increases their elastic modulus (rigidity) and alters their biochemical display. Regulated by Fn fiber unfolding, more soluble Fn binds to the native than to the crosslinked scaffolds. Additionally, matrix assembly of fibroblasts is decreased by scaffold crosslinking. Taken together, scaffold crosslinking has a multifactorial impact on the microenvironment that reseeded cells assemble and respond to, with far-reaching implications for tissue engineering and disease physiology.

Dynamics of gonococcal type IV pili during infection

Type IV pili are important bacterial virulence factors that mediate attachment to mammalian host cells and elicit downstream signals. When adhered to abiotic surfaces, the human pathogen Neisseria gonorrhoeae generates force by retracting these polymeric cell appendages. We recently found that single pili generate stalling forces that exceed 100 pN, but it is unclear whether bacteria generate force once they adhere to their human host cells. Here, we report that pili retract very actively during infection of human epithelial cells. The retraction velocity is bimodal and the high velocity mode persisted at higher forces in contrast to an abiotic environment. Bacteria generate considerable force during infection, but the maximum force is reduced from 120+/-40 pN on abiotic surfaces to 70+/-20 pN on epithelial cells, most likely due to elastic effects. Velocity and maximum force of pilus retraction are largely independent of the infection period within 1 h and 24 h post-infection. Thus, the force generated by type IV pili during infection is high enough to induce cytoskeletal rearrangements in the host cell.

Osteoblast mechanoresponses on Ti with different surface topographies

During implant healing, mechanical force is transmitted to osteogenic cells via implant surfaces with various topographies. This study tested a hypothesis that osteoblasts respond to mechanical stimulation differently on titanium with different surface topographies. Rat bone-marrow-derived osteoblastic cells were cultured on titanium disks with machined or acid-etched surfaces. A loading session consisted of a 3-minute application of a 10- or 20-mum-amplitude vibration. Alkaline phosphatase activity and gene expression increased only when the cells were loaded in 3 sessions/day on machined surfaces, regardless of the vibration amplitude, whereas they were increased with 1 loading session/day on the acid-etched surface. The loading did not affect the osteoblast proliferation on either surface, but selectively enhanced the cell spreading on the machined surface. Analysis of the data suggests that osteoblastic differentiation is promoted by mechanical stimulation on titanium, and that the promotion is disproportionate, depending on the titanium surface topography. The frequency of mechanical stimulation, rather than its amplitude, seemed to have a key role.

Cell mechanics: dissecting the physical responses of cells to force

It is now widely appreciated that normal tissue morphology and function rely upon cells' ability to sense and generate forces appropriate to their correct tissue context. Although the effects of forces on cells have been studied for decades, our understanding of how those forces propagate through and act on different cell substructures remains at an early stage. The past decade has seen a resurgence of interest, with a variety of different micromechanical methods in current use that probe cells' dynamic deformation in response to a time-varying force. The ability of researchers to carefully measure the mechanical properties of cells subjected to a variety of pharmacological and genetic interventions, however, currently outstrips our ability to quantitatively interpret the data in many cases. Despite these challenges, the stage is now set for the development of detailed models for cell deformability, motility, and mechanosensing that are rooted at the molecular level.

Boning up on Wolff's Law: mechanical regulation of the cells that make and maintain bone

Bone tissue forms and is remodeled in response to the mechanical forces that it experiences, a phenomenon described by Wolff's Law. Mechanically induced formation and adaptation of bone tissue is mediated by bone cells that sense and respond to local mechanical cues. In this review, the forces experienced by bone cells, the mechanotransduction pathways involved, and the responses elicited are considered. Particular attention is given to two cell types that have emerged as key players in bone mechanobiology: osteocytes, the putative primary mechanosensors in intact bone; and osteoprogenitors, the cells responsible for bone formation and recently implicated in ectopic calcification of cardiovascular tissues. Mechanoregulation of bone involves a complex interplay between these cells, their microenvironments, and other cell types. Thus, dissection of the role of mechanics in regulating bone cell fate and function, and translation of that knowledge to improved therapies, requires identification of relevant cues, multifactorial experimental approaches, and advanced model systems that mimic the mechanobiological environment.

The biomechanical integrin

The integrin lies at the center of our efforts to understand mechanotransduction in the human body. Over the past two decades, a wealth of information has yielded important insights into integrin structure and functioning in biochemical pathways; however, relatively little emphasis has been placed on mechanics. In this article, we review the current knowledge base of integrin mechanobiology by examining the role of integrins in stabilizing tissue structure, the mechanisms of integrin force transfer, the process of cell migration, and the pathology of cancer. In order to successfully address the gaps in cancer and other disease research going forward, future efforts of integrin mechanobiology must focus on examining cells in 3D environments and integrating our current understanding into computational models that predict the behavior of integrins in non-equilibrium interactions.

Single-cell response to stiffness exhibits muscle-like behavior

Living cells sense the rigidity of their environment and adapt their activity to it. In particular, cells cultured on elastic substrates align their shape and their traction forces along the direction of highest stiffness and preferably migrate towards stiffer regions. Although numerous studies investigated the role of adhesion complexes in rigidity sensing, less is known about the specific contribution of acto-myosin based contractility. Here we used a custom-made single-cell technique to measure the traction force as well as the speed of shortening of isolated myoblasts deflecting microplates of variable stiffness. The rate of force generation increased with increasing stiffness and followed a Hill force-velocity relationship. Hence, cell response to stiffness was similar to muscle adaptation to load, reflecting the force-dependent kinetics of myosin binding to actin. These results reveal an unexpected mechanism of rigidity sensing, whereby the contractile acto-myosin units themselves can act as sensors. This mechanism may translate anisotropy in substrate rigidity into anisotropy in cytoskeletal tension, and could thus coordinate local activity of adhesion complexes and guide cell migration along rigidity gradients.

Intrinsic extracellular matrix properties regulate stem cell differentiation

One of the recent paradigm shifts in stem cell biology has been the discovery that stem cells can begin to differentiate into mature tissue cells when exposed to intrinsic properties of the extracellular matrix (ECM), such as matrix structure, elasticity, and composition. These parameters are known to modulate the forces a cell can exert upon its matrix. Mechano-sensitive pathways subsequently convert these biophysical cues into biochemical signals that commit the cell to a specific lineage. Just as with well-studied growth factors, ECM parameters are extremely dynamic and are spatially- and temporally-controlled during development, suggesting that they play a morphogenetic role in guiding differentiation and arrangement of cells. Our ability to dynamically regulate the stem cell niche as the body does is likely a critical requirement for developing differentiated cells from stem cells for therapeutic applications. Here, we present the emergence of stem cell mechanobiology and its future challenges with new biomimetic, three-dimensional scaffolds that are being used therapeutically to treat disease.

Actomyosin contractility controls cell surface area of oligodendrocytes

BACKGROUND

To form myelin oligodendrocytes expand and wrap their plasma membrane multiple times around an axon. How is this expansion controlled?

RESULTS

Here we show that cell surface area depends on actomyosin contractility and is regulated by physical properties of the supporting matrix. Moreover, we find that chondroitin sulfate proteoglycans (CSPG), molecules associated with non-permissive growth properties within the central nervous system (CNS), block cell surface spreading. Most importantly, the inhibitory effects of CSPG on plasma membrane extension were completely prevented by treatment with inhibitors of actomyosin contractility and by RNAi mediated knockdown of myosin II. In addition, we found that reductions of plasma membrane area were accompanied by changes in the rate of fluid-phase endocytosis.

CONCLUSION

In summary, our results establish a novel connection between endocytosis, cell surface extension and actomyosin contractility. These findings open up new possibilities of how to promote the morphological differentiation of oligodendrocytes in a non-permissive growth environment. See related minireview by Bauer and ffrench-Constant: http://www.jbiol.com/content/8/8/78.

Mechanotransductive surfaces for reversible biocatalysis activation

Fibronectin, like other proteins involved in mechanotransduction, has the ability to exhibit recognition sites under mechanical stretch. Such cryptic sites are buried inside the protein structure in the native fold and become exposed under an applied force, thereby activating specific signalling pathways. Here, we report the design of new active polymeric nanoassembled surfaces that show some similarities to these cryptic sites. These nanoassemblies consist of a first polyelectrolyte multilayer stratum loaded with enzymes and capped with a second polyelectrolyte multilayer acting as a mechanically sensitive nanobarrier. The biocatalytic activity of the film is switched on/off reversibly by mechanical stretching, which exposes enzymes through the capping barrier, similarly to mechanisms involved in proteins during mechanotransduction. This first example of a new class of biologically inspired surfaces should have great potential in the design of various devices aimed to trigger and modulate chemical reactions by mechanical action with applications in the field of microfluidic devices or mechanically controlled biopatches for example.

Live Cells Exert 3-Dimensional Traction Forces on Their Substrata

The traction forces exerted by an adherent cell on a substrate have been studied only in the two-dimensions (2D) tangential to substrate surface (Txy). We developed a novel technique to measure the three-dimensional (3D) traction forces exerted by live bovine aortic endothelial cells (BAECs) on polyacrylamide deformable substrate. On 3D images acquired by confocal microscopy, displacements were determined with image-processing programs, and traction forces in tangential (XY) and normal (Z) directions were computed by finite element method (FEM). BAECs generated traction force in normal direction (Tz) with an order of magnitude comparable to Txy. Tz is upward at the cell edge and downward under the nucleus, changing continuously with a sign reversal between cell edge and nucleus edge. The method was evaluated regarding accuracy and precision of displacement measurements, effects of FE mesh size, displacement noises, and simple bootstrapping. These results provide new insights into cell-matrix interactions in terms of spatial and temporal variations in traction forces in 3D. This technique can be applied to study live cells to assess their biomechanical dynamics in conjunction with biochemical and functional activities, for investigating cellular functions in health and disease.

Do membrane undulations help cells probe the world?

Cells sense physical properties of their environment including substratum rigidity, roughness, and topography of recognition sites. The cell surface displays continuous deformations of nanometer-scale amplitude and Hz frequency. Recent results support the hypothesis that these surface undulations constitute a powerful strategy for the rapid acquisition of environmental cues: transient contact with surroundings generates forces of piconewton intensity as a result of rapid formation and dissociation of intermolecular bonds. The combination of binding and steric forces is expected to drive conformational changes and lateral reorganization of membrane biomolecules, thus generating signaling cascades. We propose that spontaneous membrane mobility shapes the initial information generated by cell-to-surface contacts, and thereby biases later consequences of these interactions.

The regulation mechanism for the auto-inhibition of binding of human filamin A to integrin

The ability of adhesion receptors to transmit biochemical signals and mechanical force across cell membranes depends on interactions with the actin cytoskeleton. Human filamins are large actin cross-linking proteins that connect integrins to the cytoskeleton. Filamin binding to the cytoplasmic tail of beta integrins has been shown to prevent integrin activation in cells, which is important for controlling cell adhesion and migration. The molecular-level mechanism for filamin binding to integrin has been unclear, however, as it was recently demonstrated that filamin undergoes intramolecular auto-inhibition of integrin binding. In this study, using steered molecular dynamics simulations, we found that mechanical force applied to filamin can expose cryptic integrin binding sites. The forces required for this are considerably lower than those for filamin immunoglobulin domain unfolding. The mechanical-force-induced unfolding of filamin and exposure of integrin binding sites occur through stable intermediates where integrin binding is possible. Accordingly, our results support filamin's role as a mechanotransducer, since force-induced conformational changes allow binding of integrin and other transmembrane and intracellular proteins. This observed force-induced conformational change can also be one of possible mechanisms involved in the regulation of integrin activation.

Mechanical signaling on the single protein level studied using steered molecular dynamics

Efficient communication between the cell and its external environment is of the utmost importance to the function of multicellular organisms. While signaling events can be generally characterized as information exchange by means of controlled energy conversion, research efforts have hitherto mainly been concerned with mechanisms involving chemical and electrical energy transfer. Here, we review recent computational efforts addressing the function of mechanical force in signal transduction. Specifically, we focus on the role of steered molecular dynamics (SMD) simulations in providing details at the atomic level on a group of protein domains, which play a fundamental role in signal exchange by responding properly to mechanical strain. We start by giving a brief introduction to the SMD technique and general properties of mechanically stable protein folds, followed by specific examples illustrating three general regimes of signal transfer utilizing mechanical energy: purely mechanical, mechanical to chemical, and chemical to mechanical. Whenever possible the physiological importance of the example at hand is stressed to highlight the diversity of the processes in which mechanical signaling plays a key role. We also provide an overview of future challenges and perspectives for this rapidly developing field.

Simultaneous generation of chemical concentration and mechanical shear stress gradients using microfluidic osmotic flow comparable to interstitial flow

Cells are very sensitive to various microenvironmental cues, including mechanical stress and chemical gradients. Therefore, physiologically relevant models of cells should consider how cells sense and respond to microenvironmental cues. This can be accomplished by using microfluidic systems, in which fluid physics can be realized at a nanoliter scale. Here we describe a simple and versatile method to study the generation of chemical concentration and mechanical shear stress gradients in a single microfluidic chip. Our system uses an osmotic pump that produces very slow (<a few microm/s) and controlled flow, allowing a wide and stable diffusion of specific chemical concentration. We also established a shear stress gradient passively via a circular channel in the interstitial level. For evaluation of the system, we used L929 mouse fibroblast cells and simultaneously exposed them to a mechanical stress gradient and a chemical nutrient gradient. The interstitial shear stress level clearly affected cell alignment, mobility velocity, and attachment. At the same time, cell proliferation reflected nutrient concentration level. Our system, which enables continuous and long-term culture of cells in a combined chemical and mechanical gradient, provides physiologically realistic conditions and will be applicable to studies of cancer metastasis and stem cell differentiation.

Determination of cellular tractions on elastic substrate based on an integral Boussinesq solution

Cell-substrate interaction is implicated in many physiological processes. Dynamical monitoring of cellular tractions on substrate is critical in investigating a variety of cell functions such as contraction, migration, and invasion. On account of the inherent ill-posed property as an inverse problem, cellular traction recovery is essentially sensitive to substrate displacement noise and thus likely produces unstable results. Therefore, some additional constraints must be applied to obtain a reliable traction estimate. By integrating the classical Boussinesq solution over a small rectangular area element, we obtain a new analytical solution to express the relation between tangential tractions and induced substrate displacements, and then form an alternative discrete Green's function matrix to set up a new framework of cellular force reconstruction. Deformation images of flexible substrate actuated by a single cardiac myocyte are processed by digital image correlation technique and the displacement data are sampled with a regular mesh to obtain cellular tractions by the proposed solution. Numerical simulations indicate that the 2-norm condition number of the improved coefficient matrix typically does not exceed the order of 100 for actual computation of traction recovery, and that the traction reconstruction is less sensitive to the shift or subdivision of the data sampling grid. The noise amplification arising from ill-posed inverse problem can be restrained and the stability of inverse solution is improved so that regularization operations become less relevant to the present force reconstruction with economical sampling density. The traction recovery for a single cardiac myocyte, which is in good agreement with that obtained by the Fourier transform traction cytometry, demonstrates the feasibility of the proposed method. We have developed a simple and efficient method to recover cellular traction field from substrate deformation. Unlike previous force reconstructions that numerically employ some regularization schemes, the present approach stabilizes the traction recovery by analytically improving the Green's function such that the intricate regularizations can be avoided under proper conditions. The method has potential application to a real-time traction force microscopy in combination with a high-efficiency displacement acquisition technique.

Use of arginine-glycine-aspartic acid adhesion peptides coupled with a new collagen scaffold to engineer a myocardium-like tissue graft

BACKGROUND

Cardiac tissue engineering might be useful in treatment of diseased myocardium or cardiac malformations. The creation of functional, biocompatible contractile tissues, however, remains challenging. We hypothesized that coupling of arginine-glycine-aspartic acid-serine (RGD+) adhesion peptides would improve cardiomyocyte viability and differentiation and contractile performance of collagen-cell scaffolds.

METHODS

Clinically approved collagen scaffolds were functionalized with RGD+ cells and seeded with cardiomyocytes. Contractile performance, cardiomyocyte viability and differentiation were analyzed at days 1 and 8 and/or after culture for 1 month.

RESULTS

The method used for the RGD+ cell-collagen scaffold coupling enabled the following features: high coupling yields and complete washout of excess reagent and by-products with no need for chromatography; spectroscopic quantification of RGD+ coupling; a spacer arm of 36 A, a length reported as optimal for RGD+-peptide presentation and favorable for integrin-receptor clustering and subsequent activation. Isotonic and isometric mechanical parameters, either spontaneous or electrostimulated, exhibited good performance in RGD+ constructs. Cell number and viability was increased in RGD+ scaffolds, and we saw good organization of cell contractile apparatus with occurrence of cross-striation.

CONCLUSIONS

We report a novel method of engineering a highly effective collagen-cell scaffold based on RGD+ peptides cross-linked to a clinically approved collagen matrix. The main advantages were cell contractile performance, cardiomyocyte viability and differentiation.

Differential matrix rigidity response in breast cancer cell lines correlates with the tissue tropism

Metastasis to a variety of distant organs, such as lung, brain, bone, and liver, is a leading cause of mortality in the breast cancer patients. The tissue tropism of breast cancer metastasis has been recognized and studied extensively, but the cellular processes underlying this phenomenon, remain elusive. Modern technologies have enabled the discovery of a number of the genetic factors determining tissue tropism of malignant cells. However, the effect of these genetic differences on the cell motility and invasiveness is poorly understood. Here, we report that cellular responses to the mechanical rigidity of the extracellular matrix correlate with the rigidity of the target tissue. We tested a series of single cell populations isolated from MDA-MB-231 breast cancer cell line in a variety of assays where the extracellular matrix rigidity was varied to mimic the environment that these cells might encounter in vivo. There was increased proliferation and migration through the matrices of rigidities corresponding to the native rigidities of the organs where metastasis was observed. We were able to abolish the differential matrix rigidity response by knocking down Fyn kinase, which was previously identified as a critical component of the FN rigidity response pathway in healthy cells. This result suggests possible molecular mechanisms of the rigidity response in the malignant cells, indicating potential candidates for therapeutic interventions.

Study of cellular behaviors on concave and convex microstructures fabricated from elastic PDMS membranes

Cells respond to geometrical cues, as well as to biochemical and mechanical stimuli. Recent progress in micro- and nano-technology has allowed researchers to create microbeads, micro-circular islands, and microposts, that can be used to examine the effect of geometrical cues on cellular behavior. Knowledge of changes in cell mechanics and morphology in response to geometric cues is important for understanding the basic behavior of cells during development and pathological processes. Most previous research in this area has focused on cell responses to two-dimensional planar or rectilinear structures. Very few studies have examined cell responses to three-dimensional curved structures because of the difficulty of fabricating such microstructures. Here we describe a novel method for the fabrication of convex and concave microstructures by use of a thin poly(dimethylsiloxane) (PDMS) membrane, SU-8 shadow mask, and negative air pressure without using any complicated silicon processes. We successfully fabricated concave and convex microstructures, with base diameters of 200-300 microm and depth (or height) of 50-150 microm (aspect ratios up to 1 : 0.5), and used these microstructures to study the responses of cultured L929 mouse fibroblast cells and human mesenchymal stem cells. These cells clearly sensed the three-dimensional microscale curvature and actively "escaped" from concave patterns, but not from those which were convex. Thus, it appears that microscale concave structures suppress cell adhesion and proliferation. We hypothesized that this might relate to deformation of the plasma membrane and subsequent opening of membrane channels. We anticipate that our system will be useful for various bio-MEMS (micro electro mechanical system) applications, including formation of uniformly-sized embryoid bodies, embryonic stem cell differentiation, and the fabrication of cell docking devices, microbioreactors, and microlenses as well as cell mechanics study.

Mechanical tension contributes to clustering of neurotransmitter vesicles at presynaptic terminals

Memory and learning in animals are mediated by neurotransmitters that are released from vesicles clustered at the synapse. As a synapse is used more frequently, its neurotransmission efficiency increases, partly because of increased vesicle clustering in the presynaptic neuron. Vesicle clustering has been believed to result primarily from biochemical signaling processes that require the connectivity of the presynaptic terminal with the cell body, the central nervous system, and the postsynaptic cell. Our in vivo experiments on the embryonic Drosophila nervous system show that vesicle clustering at the neuromuscular presynaptic terminal depends on mechanical tension within the axons. Vesicle clustering vanishes upon severing the axon from the cell body, but is restored when mechanical tension is applied to the severed end of the axon. Clustering increases when intact axons are stretched mechanically by pulling the postsynaptic muscle. Using micro mechanical force sensors, we find that embryonic axons that have formed neuromuscular junctions maintain a rest tension of approximately 1 nanonewton. If the rest tension is perturbed mechanically, axons restore the rest tension either by relaxing or by contracting over a period of approximately 15 min. Our results suggest that neuromuscular synapses employ mechanical tension as a signal to modulate vesicle accumulation and synaptic plasticity.

Substrate topography induces a crossover from 2D to 3D behavior in fibroblast migration

In a three-dimensional environment, cells migrate through complex topographical features. Using microstructured substrates, we investigate the role of substrate topography in cell adhesion and migration. To do so, fibroblasts are plated on chemically identical substrates composed of microfabricated pillars. When the dimensions of the pillars (i.e., the diameter, length, and spacing) are varied, migrating cells encounter alternating flat and rough surfaces that depend on the spacing between the pillars. Consequently, we show that substrate topography affects cell shape and migration by modifying cell-to-substrate interactions. Cells on micropillar substrates exhibit more elongated and branched shapes with fewer actin stress fibers compared with cells on flat surfaces. By analyzing the migration paths in various environments, we observe different mechanisms of cell migration, including a persistent type of migration, that depend on the organization of the topographical features. These responses can be attributed to a spatial reorganization of the actin cytoskeleton due to physical constraints and a preferential formation of focal adhesions on the micropillars, with an increased lifetime compared to that observed on flat surfaces. By changing myosin II activity, we show that actomyosin contractility is essential in the cellular response to micron-scale topographic signals. Finally, the analysis of cell movements at the frontier between flat and micropillar substrates shows that cell transmigration through the micropillar substrates depends on the spacing between the pillars.

Excitable waves at the margin of the contact area between a cell and a substrate

In this paper, we study a new physical mechanism to generate an activator field which signals the extreme margin of the contact area between an adherent cell and the substrate. This mechanism is based on the coupling between the adhesive bridges connecting the substrate to the cytoskeleton and a cytosolic activator. Once activated by adhesion on the adhesive bridges, this activator is free to diffuse on the membrane. We propose that this activator is part of the mecano-transduction pathway which links adhesion to actin polymerization and, thus, to cellular motility. The consequences of our model are as follows: (a) the activator is localized at the rim of the contact area, (b) the adhesion is reinforced at the margin of the contact area between the cell and the substrate, (c) excitable waves of the activator can propagate along the adhesion rim.

Spatial patterning of cell proliferation and differentiation depends on mechanical stress magnitude

Mechanical stress has been proposed as a major regulator of tissue morphogenesis; however, it remains unclear what is the exact mechanical signal that leads to local tissue pattern formation. We explored this question by using a micropatterned cell aggregate model in which NIH 3T3 fibroblasts were cultured on micropatterned adhesive islands and formed cell aggregates (or "cell islands") of triangular, square, and circular shapes. We found that the cell islands generated high levels of mechanical stresses at their perimeters compared to their inner regions. Regardless of the shape of cell islands, the mechanical stress patterns corresponded to both cell proliferation and differentiation patterns, meaning that high level of cell proliferation and differentiation occurred at the locations where mechanical stresses were also high. When mechanical stretching was applied to cell islands to elevate overall mechanical stress magnitudes, cell proliferation and differentiation generally increased with the relatively higher mechanical stresses, but neither cell proliferation nor differentiation patterns followed the new mechanical stress pattern. Thus, our findings indicate that a certain range of mechanical stress magnitudes, termed window stress threshold, drives formation of cell proliferation and differentiation patterns and hence possibly functions as a morphogenetic cue for local tissue pattern formation in vivo.

Cellular traction force recovery: An optimal filtering approach in two-dimensional Fourier space

Quantitative estimation of cellular traction has significant physiological and clinical implications. As an inverse problem, traction force recovery is essentially susceptible to noise in the measured displacement data. For traditional procedure of Fourier transform traction cytometry (FTTC), noise amplification is accompanied in the force reconstruction and small tractions cannot be recovered from the displacement field with low signal-noise ratio (SNR). To improve the FTTC process, we develop an optimal filtering scheme to suppress the noise in the force reconstruction procedure. In the framework of the Wiener filtering theory, four filtering parameters are introduced in two-dimensional Fourier space and their analytical expressions are derived in terms of the minimum-mean-squared-error (MMSE) optimization criterion. The optimal filtering approach is validated with simulations and experimental data associated with the adhesion of single cardiac myocyte to elastic substrate. The results indicate that the proposed method can highly enhance SNR of the recovered forces to reveal tiny tractions in cell-substrate interaction.

Mechanosensing in actin stress fibers revealed by a close correlation between force and protein localization

The mechanics of the actin cytoskeleton have a central role in the regulation of cells and tissues, but the details of how molecular sensors recognize deformations and forces are elusive. By performing cytoskeleton laser nanosurgery in cultured epithelial cells and fibroblasts, we show that the retraction of stress fibers (SFs) is restricted to the proximity of the cut and that new adhesions form at the retracting end. This suggests that SFs are attached to the substrate. A new computational model for SFs confirms this hypothesis and predicts the distribution and propagation of contractile forces along the SF. We then analyzed the dynamics of zyxin, a focal adhesion protein present in SFs. Fluorescent redistribution after laser nanosurgery and drug treatment shows a high correlation between the experimentally measured localization of zyxin and the computed localization of forces along SFs. Correlative electron microscopy reveals that zyxin is recruited very fast to intermediate substrate anchor points that are highly tensed upon SF release. A similar acute localization response is found if SFs are mechanically perturbed with the cantilever of an atomic force microscope. If actin bundles are cut by nanosurgery in living Drosophila egg chambers, we also find that zyxin redistribution dynamics correlate to force propagation and that zyxin relocates at tensed SF anchor points, demonstrating that these processes also occur in living organisms. In summary, our quantitative analysis shows that force and protein localization are closely correlated in stress fibers, suggesting a very direct force-sensing mechanism along actin bundles.

Periodic forces trigger a complex mechanical response in ubiquitin

Mechanical forces govern physiological processes in all living organisms. Many cellular forces, for example, those generated in cyclic conformational changes of biological machines, have repetitive components. In apparent contrast, little is known about how dynamic protein structures respond to periodic mechanical information. Ubiquitin is a small protein found in all eukaryotes. We developed molecular dynamics simulations to unfold single and multimeric ubiquitins with periodic forces. By using a coarse-grained representation, we were able to model forces with periods about 2 orders of magnitude longer than the protein's relaxation time. We found that even a moderate periodic force weakened the protein and shifted its unfolding pathways in a frequency- and amplitude-dependent manner. A complex dynamic response with secondary structure refolding and an increasing importance of local interactions was revealed. Importantly, repetitive forces with broadly distributed frequencies elicited very similar molecular responses compared to fixed-frequency forces. When testing the influence of pulling geometry on ubiquitin's mechanical stability, it was found that the linkage involved in the mechanical degradation of cellular proteins renders the protein remarkably insensitive to periodic forces. We also devised a complementary kinetic energy landscape model that traces these observations and explains periodic-force, single-molecule measurements. In turn, this analytical model is capable of predicting dynamic protein responses. These results provide new insights into ubiquitin mechanics and a potential mechanical role during protein degradation, as well as first frameworks for dynamic protein stability and the modeling of repetitive mechanical processes.

High-resolution probing of cellular force transmission

Cells actively probe mechanical properties of their environment by exerting internally generated forces. The response they encounter profoundly affects their behavior. Here we measure in a simple geometry the forces a cell exerts suspended by two optical traps. Our assay quantifies both the overall force and the fraction of that force transmitted to the environment. Mimicking environments of varying stiffness by adjusting the strength of the traps, we found that the force transmission is highly dependent on external compliance. This suggests a calibration mechanism for cellular mechanosensing.

Mechanical network in titin immunoglobulin from force distribution analysis

The role of mechanical force in cellular processes is increasingly revealed by single molecule experiments and simulations of force-induced transitions in proteins. How the applied force propagates within proteins determines their mechanical behavior yet remains largely unknown. We present a new method based on molecular dynamics simulations to disclose the distribution of strain in protein structures, here for the newly determined high-resolution crystal structure of I27, a titin immunoglobulin (IG) domain. We obtain a sparse, spatially connected, and highly anisotropic mechanical network. This allows us to detect load-bearing motifs composed of interstrand hydrogen bonds and hydrophobic core interactions, including parts distal to the site to which force was applied. The role of the force distribution pattern for mechanical stability is tested by in silico unfolding of I27 mutants. We then compare the observed force pattern to the sparse network of coevolved residues found in this family. We find a remarkable overlap, suggesting the force distribution to reflect constraints for the evolutionary design of mechanical resistance in the IG family. The force distribution analysis provides a molecular interpretation of coevolution and opens the road to the study of the mechanism of signal propagation in proteins in general.

Bacterial cell curvature through mechanical control of cell growth

The cytoskeleton is a key regulator of cell morphogenesis. Crescentin, a bacterial intermediate filament-like protein, is required for the curved shape of Caulobacter crescentus and localizes to the inner cell curvature. Here, we show that crescentin forms a single filamentous structure that collapses into a helix when detached from the cell membrane, suggesting that it is normally maintained in a stretched configuration. Crescentin causes an elongation rate gradient around the circumference of the sidewall, creating a longitudinal cell length differential and hence curvature. Such curvature can be produced by physical force alone when cells are grown in circular microchambers. Production of crescentin in Escherichia coli is sufficient to generate cell curvature. Our data argue for a model in which physical strain borne by the crescentin structure anisotropically alters the kinetics of cell wall insertion to produce curved growth. Our study suggests that bacteria may use the cytoskeleton for mechanical control of growth to alter morphology.

Noninjurious mechanical ventilation activates a proinflammatory transcriptional program in the lung

Mechanical ventilation is a life-saving intervention in patients with respiratory failure. However, human and animal studies have demonstrated that mechanical ventilation using large tidal volumes (>or=12 ml/kg) induces a potent inflammatory response and can cause acute lung injury. We hypothesized that mechanical ventilation with a "noninjurious" tidal volume of 10 ml/kg would still activate a transcriptional program that places the lung at risk for severe injury. To identify key regulators of this transcriptional response, we integrated gene expression data obtained from whole lungs of spontaneously breathing mice and mechanically ventilated mice with computational network analysis. Topological analysis of the gene product interaction network identified Jun and Fos families of proteins as potential regulatory hubs. Electrophoretic mobility gel shift assay confirmed protein binding to activator protein-1 (AP-1) consensus sequences, and supershift experiments identified JunD and FosB as components of ventilation-induced AP-1 binding. Specific recruitment of JunD to the regulatory region of the F3 gene by mechanical ventilation was confirmed by chromatin immunoprecipitation assay. In conclusion, we demonstrate a novel computational framework to systematically dissect transcriptional programs activated by mechanical ventilation in the lung, and show that noninjurious mechanical ventilation initiates a response that can prime the lung for injury from a subsequent insult.

Neurosensory mechanotransduction

Neurons that sense touch, sound and acceleration respond rapidly to specific mechanical signals. The proteins that transduce these signals and underlie these senses, however, are mostly unknown. Research over the past decade has suggested that members of three families of channel proteins are candidate transduction molecules. Current studies are directed towards characterizing these candidates, determining how they are mechanically gated and discovering new molecules that are involved in mechanical sensing.

Mechanotransduction gone awry

Cells sense their physical surroundings through mechanotransduction - that is, by translating mechanical forces and deformations into biochemical signals such as changes in intracellular calcium concentration or by activating diverse signalling pathways. In turn, these signals can adjust cellular and extracellular structure. This mechanosensitive feedback modulates cellular functions as diverse as migration, proliferation, differentiation and apoptosis, and is crucial for organ development and homeostasis. Consequently, defects in mechanotransduction - often caused by mutations or misregulation of proteins that disturb cellular or extracellular mechanics - are implicated in the development of various diseases, ranging from muscular dystrophies and cardiomyopathies to cancer progression and metastasis.

Enhanced focal adhesion assembly reflects increased mechanosensation and mechanotransduction at maternal–conceptus interface and uterine wall during ovine pregnancy

The integrity of the fetal–maternal interface is critical for proper fetal nourishment during pregnancy. Integrins are important adhesion molecules present at the interface during implantation; however,in vivoevidence for integrin activation and focal adhesion formation at the maternal–conceptus interface is limited. We hypothesized that focal adhesion assembly in uterine luminal epithelium (LE) and conceptus trophectoderm (Tr) results from integrin binding of extracellular matrix (ECM) at this interface to provide increased tensile forces and signaling to coordinate utero-placental development. An ovine model of unilateral pregnancy was used to evaluate mechanotransduction events leading to focal adhesion assembly at the maternal–conceptus interface and within the uterine wall. Animals were hysterectomized on days 40, 80, or 120 of pregnancy, and uteri immunostained for integrins (ITGAV, ITGA4, ITGA5, ITGB1, ITGB3, and ITGB5), ECM proteins (SPP1, LGALS15, fibronectin (FN), and vitronectin (VTN)), cytoskeletal molecules (ACTN and TLN1), and a signal generator (PTK2). Focal adhesion assembly in myometrium and stroma was also studied to provide a frame of reference for mechanical stretch of the uterine wall. Large focal adhesions containing aggregates of ITGAV, ITGA4, ITGA5, ITGB1, ITGB5, ACTN, and PTK2 were detected in interplacentomal uterine LE and Tr of gravid but not non-gravid uterine horns and increased during pregnancy. SPP1 and LGALS15, but not FN or VTN, were present along LE and Tr interfaces in both uterine horns. These data support the idea that focal adhesion assembly at the maternal–conceptus interface reflects adaptation to increasing forces caused by the growing fetus. Cooperative binding of multiple integrins to SPP1 deposited at the maternal–conceptus interface forms an adhesive mosaic to maintain a tight connection between uterine and placental surfaces along regions of epitheliochorial placentation in sheep.

Stretching single talin rod molecules activates vinculin binding

The molecular mechanism by which a mechanical stimulus is translated into a chemical response in biological systems is still unclear. We show that mechanical stretching of single cytoplasmic proteins can activate binding of other molecules. We used magnetic tweezers, total internal reflection fluorescence, and atomic force microscopy to investigate the effect of force on the interaction between talin, a protein that links liganded membrane integrins to the cytoskeleton, and vinculin, a focal adhesion protein that is activated by talin binding, leading to reorganization of the cytoskeleton. Application of physiologically relevant forces caused stretching of single talin rods that exposed cryptic binding sites for vinculin. Thus in the talin-vinculin system, molecular mechanotransduction can occur by protein binding after exposure of buried binding sites in the talin-vinculin system. Such protein stretching may be a more general mechanism for force transduction.

Mechanogenetic coupling of Hydra symmetry breaking and driven Turing instability model

The freshwater polyp Hydra can regenerate from tissue fragments or random cell aggregates. We show that the axis-defining step ("symmetry breaking") of regeneration requires mechanical inflation-collapse oscillations of the initial cell ball. We present experimental evidence that axis definition is retarded if these oscillations are slowed down mechanically. When biochemical signaling related to axis formation is perturbed, the oscillation phase is extended and axis formation is retarded as well. We suggest that mechanical oscillations play a triggering role in axis definition. We extend earlier reaction-diffusion models for Hydra regrowth by coupling morphogen transport to mechanical stress caused by the oscillations. The modified reaction-diffusion model reproduces well two important experimental observations: 1), the existence of an optimum size for regeneration, and 2), the dependence of the symmetry breaking time on the properties of the mechanical oscillations.

Cell fate regulation by coupling mechanical cycles to biochemical signaling pathways

Many aspects of cellular motility and mechanics are cyclic in nature such as the extension and retraction of lamellipodia or filopodia. Inherent to the cycles of extension and retraction that test the environment is the production of mechano-chemical signals that can alter long-term cell behavior, transcription patterns, and cell fate. We are just starting to define such cycles in several aspects of cell motility, including periodic contractions, integrin cycles of binding and release as well as the normal oscillations in motile activity. Cycles of local cell contraction and release are directly coupled to cycles of stressing and releasing extracellular contacts (matrix or cells) as well as cytoplasmic mechanotransducers. Stretching can alter external physical properties or sites exposed by matrix molecules as well as internal networks; thus, cell contractions can cause a secondary wave of mechano-regulated outside-in and internal cell signal changes. In some cases, the integration of both external and internal signals in space and time can stimulate a change in cell state from quiescence to growth or differentiation. In this review we will develop the basic concept of the mechano-chemical cycles and the ways in which they can be described and understood.