Dissemination of ovarian tumors involves the implantation of cancer spheroids into the mesothelial monolayer on the walls of peritoneal and pleural cavity organs. Biopsies of tumors attached to peritoneal organs show that mesothelial cells are not present under tumor masses. We have developed a live, image-based in vitro model in which interactions between tumor spheroids and mesothelial cells can be monitored in real time to provide spatial and temporal understanding of mesothelial clearance. Here we provide evidence that ovarian cancer spheroids utilize integrin – and talin - dependent activation of myosin and traction force to promote mesothelial cells displacement from underneath a tumor cell spheroid. These results suggest that ovarian tumor cell clusters gain access to the sub-mesothelial environment by exerting force on the mesothelial cells lining target organs, driving migration and clearance of the mesothelial cells.

Coupling of the mechanical microenvironment with contractile forces between neighboring cells regulates collective migration.

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.

Force transduction at cell-cell adhesions regulates tissue development, maintenance and adaptation. We developed computational and experimental approaches to quantify, with both sub-cellular and multi-cellular resolution, the dynamics of force transmission in cell clusters. Applying this technology to spontaneously-forming adherent epithelial cell clusters, we found that basal force fluctuations were coupled to E-cadherin localization at the level of individual cell-cell junctions. At the multi-cellular scale, cell-cell force exchange depended on the cell position within a cluster, and was adaptive to reconfigurations due to cell divisions or positional rearrangements. Importantly, force transmission through a cell required coordinated modulation of cell-matrix adhesion and actomyosin contractility in the cell and its neighbors. These data provide insights into mechanisms that could control mechanical stress homeostasis in dynamic epithelial tissues, and highlight our methods as a resource for the study of mechanotransduction in cell-cell adhesions.DOI: http://dx.doi.org/10.7554/eLife.03282.001

Focal adhesions are micrometer-sized protein aggregates that connect actin stress fibers to the extracellular matrix, a network of macromolecules surrounding tissue cells. The actin fibers are under tension due to actin-myosin contractility. Recent measurements have shown that as the actin force is increased, these adhesions grow in size and in the direction of the force. This is in contrast to the growth of condensed domains of surface-adsorbed molecules in which the dynamics are isotropic. We predict these force-sensitive, anisotropic dynamics of focal adhesions from a model for the adsorption of proteins from the cytoplasm to the adhesion site. Our theory couples the mechanical forces and elasticity to the adsorption dynamics via force-induced conformational changes of molecular-sized mechanosensors located in the focal adhesion. We predict the velocity of both the front and back of the adhesion as a function of the applied force. In addition, our results show that the relative motion of the front and back of the adhesion is asymmetric and in different ranges of forces, the adhesion can either shrink or grow in the direction of the force.

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Biochemistry and mechanics are closely coupled in cell adhesion. At sites of cellmatrix adhesion, mechanical force triggers signaling through the Rho-pathway, which leads to structural reinforcement and increased contractility in the actin cytoskeleton.The resulting force acts back to the sites of adhesion, resulting in a positive feedback loop for mature adhesion. Here we model this biochemical-mechanical feedback loop for the special case when the actin cytoskeleton is organized in stress fibers, which are contractile bundles of actin filaments. Activation of myosin II molecular motors through the Rho-pathway is described by a system of reaction-diffusion equations, which are coupled into a viscoelastic model for a contractile actin bundle. We find strong spatial gradients in the activation of contractility and in the corresponding deformation pattern of the stress fiber, in good agreement with experimental findings. I. INTRODUCTIONAdhesion of biological cells to each other and the extracellular matrix is one of the hallmarks of multicellular organisms and a very active area of research in cell biology. Investigating cell adhesion is crucial to understand physiological processes like tissue development and maintenance, but also disease-related processes like growth and migration of cancer cells [1,2]. In general, biological cells have a limited repertoire of possible behaviours, including spreading, adhesion, migration, division or death, but very sophisticated ways of controling the switching between these different processes. Traditionally, investigation of this cellular decision making has focused strongly on the biochemical aspects, including detailed models for signal transduction [3,4]. During recent years, it has become increasingly clear that in

Substrate rigidity profoundly impacts cellular behaviors such as migration, gene expression, and cell fate. Total Internal Reflection Fluorescence (TIRF) microscopy enables selective visualization of the dynamics of substrate adhesions, vesicle trafficking, and biochemical signaling at the cell-substrate interface. Here we apply high-refractive-index silicone gels to perform TIRF microscopy on substrates with a wide range of physiological elastic moduli and simultaneously measure traction forces exerted by cells on the substrate.