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.mechanosensing | adaptation to load | cell migration | cell spreading | cell mechanics A s part of their normal physiological functions, most cells in the organism need to respond to mechanical stimuli such as deformations, forces, and the geometry and stiffness of the extracellular matrix (1, 2). Aberrant mechanical responsiveness is often associated with severe diseases, including cardiovascular disorders, asthma, fibrotic diseases, or cancer metastasis.Since the early 1980s, several techniques have been developed to characterize the forces generated by living cells (3-5) and to investigate the effect of the mechanical properties of twodimensional (2D) substrates (6-8). It was shown that cells are able to sense and respond to the rigidity of their surroundings. For instance, cells cultured on elastic substrates with a rigidity gradient preferably locomote towards stiffer regions and align their shape, their cytoskeletal structures, and their traction forces along the direction of highest stiffness (9-11). Moreover, it has been demonstrated that matrix rigidity could direct stem cells' lineage specification (12).It is generally assumed that rigidity sensing is based on mechanochemical signal-transduction pathways. The search for the mechanosensing element has generated numerous plausible candidates (reviewed in ref.2). The most prominent of them is the focal adhesion complex (13,14). These molecular assemblies consist of numerous proteins that are associated with integrin adhesion receptors (15) and provide the pathway of force transmission from the cytoskeleton to the extracellular matrix (16). Adhesion of integrins to extracellular matrix proteins triggers the formation of focal adhesions, their connection to actin, and the contraction of the cytoskeleton by myosin II (17-19). On a soft substrat...

We developed a new versatile micron-scale rheometer allowing us to measure the creep or the relaxation function (time analysis), as well as to determine the dynamical complex modulus (frequency analysis) of a single living cell. In this setup, a microscopic sample can be stretched or compressed uniaxially between two parallel microplates: one rigid, the other flexible. The flexible microplate is used as a nanonewton force sensor of calibrated stiffness, the force being simply proportional to the plate deflection. An original design of the microplates allows us to achieve an efficient feedback control of either strain or stress applied to the cell. Controlling the flexible plate deflection with a typical precision of less than 200nm, we are able to apply stresses ranging from a few pascals to thousands of pascals with a precision better than 2%. The control of the flexible plate deflexion is achieved by direct imaging of the plate tip on a photosensitive detector mounted on the phototube of an inverted microscope. Thus, the detection principle is suitable to all usual microscopes and very easy to set up. Beyond the creep function, already analyzed in detail in a previous work, we report here the first measurement of the relaxation function, as well as of the storage and the loss dynamic moduli [G′(f) and G″(f), f ranging from 0.02to10Hz] for an isolated living cell. Eventually, the rheometer we built is not limited to cell stretching. It should also be a powerful tool to study the rheology of micron sized samples such as microgels or vesicles, as well as to perform shear experiments.

Plant and animals have evolved different strategies for their development. Whether this is linked to major differences in their cell mechanics remains unclear, mainly because measurements on plant and animal cells relied on independent experiments and setups, thus hindering any direct comparison. In this study we used the same micro-rheometer to compare animal and plant single cell rheology. We found that wall-less plant cells exhibit the same weak power law rheology as animal cells, with comparable values of elastic and loss moduli. Remarkably, microtubules primarily contributed to the rheological behavior of wall-less plant cells whereas rheology of animal cells was mainly dependent on the actin network. Thus, plant and animal cells evolved different molecular strategies to reach a comparable cytoplasmic mechanical core, suggesting that evolutionary convergence could include the internal biophysical properties of cells.

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