The cytoskeleton (CSK) is a crowded network of structural proteins that stabilizes cell shape and drives cell motions. Recent studies on the dynamics of the CSK have established that a wide variety of cell types exhibit rheology in which responses are not tied to any particular relaxation times and are thus scale-free. Scale-free rheology is often found in a class of materials called soft glasses, but not all materials expressing scale-free rheology are glassy (see plastics, wood, concrete or some metals for example). As such, the extent to which dynamics of the CSK might be regarded as glassy remained an open question. Here we report both forced and spontaneous motions of microbeads tightly bound to the CSK of human muscle cells. Large oscillatory shear fluidized the CSK matrix, which was followed by slow scale-free recovery of rheological properties (aging). Spontaneous bead motions were subdiffusive at short times but superdiffusive at longer times; intermittent motions reflecting nanoscale CSK rearrangements depended on both the approach to kinetic arrest and energy release due to ATP hydrolysis. Aging, intermittency, and approach to kinetic arrest establish a striking analogy between the behaviour of the living CSK and that of inert non-equilibrium systems, including soft glasses, but with important differences that are highly ATP-dependent. These mesoscale dynamics link integrative CSK functions to underlying molecular events, and represent an important intersection of topical issues in condensed matter physics and systems biology.

We have used the nanometer scale alpha-Hemolysin pore to study the unzipping kinetics of individual DNA hairpins under constant force or constant loading rate. Using a dynamic voltage control method, the entry rate of polynucleotides into the pore and the voltage pattern applied to induce hairpin unzipping are independently set. Thus, hundreds of unzipping events can be tested in a short period of time (few minutes), independently of the unzipping voltage amplitude. Because our method does not entail the physical coupling of the molecules under test to a force transducer, very high throughput can be achieved. We used our method to study DNA unzipping kinetics at small forces, which have not been accessed before. We find that in this regime the static unzipping times decrease exponentially with voltage with a characteristic slope that is independent of the duplex region sequence, and that the intercept depends strongly on the duplex region energy. We also present the first nanopore dynamic force measurements (time varying force). Our results are in agreement with the approximately logV dependence at high V (where V is the loading rate) observed by other methods. The extension of these measurements to lower loading rates reveals a much weaker dependence on V.

Force measurements are performed on single DNA molecules with an optical trapping interferometer that combines subpiconewton force resolution and millisecond time resolution. A molecular construction is prepared for mechanically unzipping several thousand-basepair DNA sequences in an in vitro configuration. The force signals corresponding to opening and closing the double helix at low velocity are studied experimentally and are compared to calculations assuming thermal equilibrium. We address the effect of the stiffness on the basepair sensitivity and consider fluctuations in the force signal. With respect to earlier work performed with soft microneedles, we obtain a very significant increase in basepair sensitivity: presently, sequence features appearing at a scale of 10 basepairs are observed. When measured with the optical trap the unzipping force exhibits characteristic flips between different values at specific positions that are determined by the base sequence. This behavior is attributed to bistabilities in the position of the opening fork; the force flips directly reflect transitions between different states involved in the time-averaging of the molecular system.

Mechanical stress is increasingly being shown to be a potent modulator of cell-cell junctional morphologies in developmental and homeostatic processes. Intercellular force sensing is thus expected to be an important regulator of cell signalling and tissue integrity. In particular, the interplay between myosin contractility, actin dynamics and E-cadherin recruitment largely remains to be uncovered. We devised a suspended cell doublet assay to quantitatively assess the correlation between myosin II activity and local E-cadherin recruitment. The single junction of the doublet exhibited a stereotypical morphology, with E-cadherin accumulating into clusters of varied concentrations at the rim of the circular contact. This local recruitment into clusters derived from the sequestration of E-cadherin through a myosin-II-driven modulation of actin turnover. We exemplify how the regulation of actin dynamics provides a mechanism for the mechanosensitive response of cell contacts.

We report the modifications of the microscopic dynamics of a colloidal glass submitted to shear. We use multispeckle diffusing wave spectroscopy to monitor the evolution of the spontaneous slow relaxation processes after the sample have been submitted to various straining. We show that high shear rejuvenates the system and accelerates its dynamics whereas moderate shear overage the system. We analyze this phenomena within the frame of the Bouchaud's trap model. PACS numbers: 64.70PF, 83.80Hj, 83.10Pp The physical properties of glassy systems such as supercooled liquids, spin glasses, amorphous polymers and colloidal glasses are well known to evolve slowly with time. This phenomena is called aging. The out-ofequilibrium nature of theses systems compels their physical properties to depend on two times as shown both by theoretical and experimental studies. The first time is the age of the system i.e. the time spent in the glassy phase. The second time is the time elapsed since the measurement started. Consequently, a well controlled history is a key requirement to obtain reproducible results. The most common way to control the history as the age of the system is to quench it from an equilibrium state at high temperature into an aging state at low temperature. Since the system is at equilibrium at high temperature, all the history preceding the quench is erased and a complete rejuvenation of the physical properties is achieved. For colloidal glasses however, temperature may not be a practical parameter. Liu and Nagel recently suggested [1] that shear may act equivalently to temperature for such materials. Indeed, a high shear proves to be able to erased the memory for theses systems and thus completely rejuvenate them [2] [3]. In that sense the cessation of a shear is similar to a temperature quench. Moreover, different approaches were recently introduced to describe the coupling between mechanical deformations and aging phenomenon [4] [5] . However quantitative experiments are still lacking to determine unambiguously how shear acts on a microscopic level and how it should be introduced in a mean field model.In this Letter we report non trivial shear effects on a dense solution of polybeads. We show that theses effects can be mimicked by temperature changes in the Bouchaud's trap model [6]. Our underlying physical picture is the following: slow relaxations, of characteristic time τ , are determined by the structural rearrangements of the particles. The dynamics slows down with the age t w of the system as the beads find more and more stable configurations. τ is thus an increasing function of t w . Since a shear flow seems to be able to rejuvenate completely the system, one could imagine that it shuffles the beads arrangements. The resulting configurations could be less stable. The dynamics of rearrangements would be then accelerated and the relaxation time τ would decrease. Oppositely, one could imagine that a moderate oscillatory strain is able to help the system to find more stable, though always non-crystallin...