Power laws in microrheology experiments on living cells: comparative analysis and modelling

We compare and synthesize the results of two microrheological experiments on the cytoskeleton of single cells. In the first one, the creep function J(t) of a cell stretched between two glass plates is measured after applying a constant force step. In the second one, a micrometric bead specifically bound to transmembrane receptors is driven by an oscillating optical trap, and the viscoelastic coefficient $G_e(\omega)$ is retrieved. Both $J(t)$ and $G_e(\omega)$ exhibit power law behavior: $J(t)= A(t/t_0)^\alpha$ and $\bar G_e(\omega)\bar = G_0 (\omega/\omega_0)^\alpha$, with the same exponent $\alpha\approx 0.2$. This power law behavior is very robust ; $\alpha$ is distributed over a narrow range, and shows almost no dependance on the cell type, on the nature of the protein complex which transmits the mechanical stress, nor on the typical length scale of the experiment. On the contrary, the prefactors $A_0$ and $G_0$appear very sensitive to these parameters. Whereas the exponents $\alpha$ are normally distributed over the cell population, the prefactors $A_0$ and $G_0$ follow a log-normal repartition. These results are compared with other data published in the litterature. We propose a global interpretation, based on a semi-phenomenological model, which involves a broad distribution of relaxation times in the system. The model predicts the power law behavior and the statistical repartition of the mechanical parameters, as experimentally observed for the cells. Moreover, it leads to an estimate of the largest response time in the cytoskeletal network: $\tau_m \approx 1000$ s.Comment: 47 pages, 14 figures // v2: PDF file is now Acrobat Reader 4 (and up) compatible // v3: Minor typos corrected - The presentation of the model have been substantially rewritten (p. 17-18), in order to give more details - Enhanced description of protocols // v4: Minor corrections in the text : the immersion angles are estimated and not measured // v5: Minor typos corrected. Two references were clarifie

Microscopie à l'angle de Brewster : transitions de phases et défauts d'orientation dans des films monomoléculairess

Microscopy at the Brewster angle is a new powerful technique for the study of monolayers at the free surface of water. Taking advantage of the reflectivity properties of an interface, it is sensitive to the thickness, density and anisotopy of the films. This technique was used to study the phase transitions that sometimes occur during the formation of films adsorbed at the surface of aqueous solutionsof amphiphiles. The number, nature and morphology of these phases depend on experimental conditions, including pH.We thus were able to follow the formation and evolution of structures in which the tilt of the molecules creates an observable optical anisotropy. The corresponding phases are "locked tilted mesophases", i.e.the molecular direction is fixed with respect to intermolecular directions. We observed several orientational defects morphologies, includind star structures, which were modeled by continuum elasticity theory developped for thin films of smectic liquid crystals. We also observed structures consisting of kinked defects lines, spirals or stripes of constant width. Layers of a polymer (PDMS) at the free surface of water were also studied. We observed a lateral separation into domains of different surface density, both in the monolayer and in multilayer regimesLa microscopie à l'angle de Brewster est une nouvelle et tres puissante technique d'étude des films monomoléculaires à la surface de l'eau. Son principe est basé sur les propriétés de réflectivité des interfaces. Elle est sensible à l'épaisseur, la densité et l'anisotropie optique des films. Cette technique a été appliquée à l'étude de couches adsorbées à la surface de solutions aqueuses d'acides gras (acides palmitiques et myristiques). Ces couches traversent pendant leur formation des transitions de phases. Le nombre, la nature et la morphologie de ces phases dépendent de nombreux paramètres dont le pH. Nous avons entre autres observé des phases optiquement anisotropes, contituées de molécules inclinées par rapport à la normale à la solution.Ces phases sont sans doute des mésophases "verrouillées", c'est à dire que la direction des molécules est fixée par rapport aux directions intermoléculaires. Elles présentent différents types de défauts d'orientation, dont des structures en étoile. L'existence de telles structures est expliquée par application d'un modèle d'élasticité continue développé pour l'étude des films minces de cristaux liquides smectiques. Des structures en zig-zag, en spirales et en bandes de largeur déterminée ont également été observées. Nous avons également étudié les couches d'un polymère (le PDMS) à la surface de l'eau. Nous y avons observé la séparation latérale en domaines de densités de surface différentes, à la fois dans le régime monocouche et le régime multicouches

The nature and intensity of mechanical stimulation drive different dynamics of MRTF-A nuclear redistribution after actin remodeling in myoblasts.

Serum response factor and its cofactor myocardin-related transcription factor (MRTF) are key elements of muscle-mass adaptation to workload. The transcription of target genes is activated when MRTF is present in the nucleus. The localization of MRTF is controlled by its binding to G-actin. Thus, the pathway can be mechanically activated through the mechanosensitivity of the actin cytoskeleton. The pathway has been widely investigated from a biochemical point of view, but its mechanical activation and the timescales involved are poorly understood. Here, we applied local and global mechanical cues to myoblasts through two custom-built set-ups, magnetic tweezers and stretchable substrates. Both induced nuclear accumulation of MRTF-A. However, the dynamics of the response varied with the nature and level of mechanical stimulation and correlated with the polymerization of different actin sub-structures. Local repeated force induced local actin polymerization and nuclear accumulation of MRTF-A by 30 minutes, whereas a global static strain induced both rapid (minutes) transient nuclear accumulation, associated with the polymerization of an actin cap above the nucleus, and long-term accumulation, with a global increase in polymerized actin. Conversely, high strain induced actin depolymerization at intermediate times, associated with cytoplasmic MRTF accumulation

Thickness of epithelia on wavy substrates: measurements and continuous models

We measured the thickness of MDCK epithelia grown on substrates with a sinusoidal profile. We show that while at long wavelength the profile of the epithelium follows that of the substrate, at short wavelengths cells are thicker in valleys than on ridges. This is reminiscent of the so-called «healing length in the case of a thin liquid film wetting a rough solid substrate. We explore the ability of continuum mechanics models to account for these observations. Modeling the epithelium as a thin liquid film, with surface tension, does not fully account for the measurements. Neither does modeling the epithelium as a thin incompressible elastic film. On the contrary, the addition of an apical active stress gives satisfactory agreement with measurements, with one fitting parameter, the ratio between the active stress and the elastic modulus

Microrheology of living cells at different time and length scales

Chapter 1International audienc

L’outillage de mouture va-et-vient des Feuilly à Saint-Priest (Rhône)

L’étude des moulins va-et-vient des Feuilly, à Saint-Priest (Rhône), permet de présenter l’évolution typologique et pétrographique de l’outillage de mouture d’un même site de culture bourguignonne depuis le Néolithique moyen jusqu’à la charnière du premier et second âge du Fer en passant par le Bronze final 2b-3a. La réduction de la courbure de la surface active semble se confirmer entre le Néolithique et la Protohistoire. Une molette débordante du Néolithique moyen est identifiée. Alors qu’aucune évolution typologique n’apparaît entre le matériel du Bronze final et celui de l’âge du Fer, des différences pétrographiques se font dans le choix des matériaux, avec une plus grande variété de roches durant la Protohistoire, bien que le granite reste le matériau de prédilection depuis le Néolithique moyen.The saddle querns of the site of Feuilly, Saint-Priest (Rhône, France) were classified from the standpoint of typology and petrography. This site was occupied from the Burgundian Middle Neolithic through the Early to Late Iron Age. Of note is an occupation from the Late Bronze Age 2b-3a. A decrease of the angle of the grinding surface takes place between the Neolithic and Protohistory. An “overlapping upper stone”, dated to the Middle Neolithic, stands out among the assemblage. While no typological evolution is apparent from the Late Bronze Age to the Iron Age, there are differences in the choice of rock, in particular in Protohistory. Granite, nonetheless, remains the main rock type since the Middle Neolithic

Beyond mice: Emerging and transdisciplinary models for the study of early-onset myopathies

International audienceThe use of the adapted models to decipher patho-physiological mechanisms of human diseases is always a great challenge. This is of particular importance for early-onset myopathies, in which pathological mutations often impact not only on muscle structure and function but also on developmental processes. Mice are currently the main animal model used to study neuromuscular disorders including the early-onset myopathies. However strategies based on simple animal models and on transdisciplinary approaches exploring mechanical muscle cell properties emerge as attractive, non-exclusive alternatives. These new ways provide valuable opportunities to improve our knowledge on how mechanical, biochemical, and genetic/epigenetic cues modulate the formation, organization and function of muscle tissues. Here we provide an overview of how single cell and micro-tissue engineering in parallel to non-mammalian, Drosophila and zebrafish models could contribute to filling gaps in our understanding of pathogenic mechanisms underlying early-onset myopathies. We also discuss their potential impact on designing new diagnostic and therapeutic strategies