Cell size and actin architecture determine force generation in optogenetically activated cells

Adherent cells use actomyosin contractility to generate mechanical force and to sense the physical properties of their environment, with dramatic consequences for migration, division, differentiation, and fate. However, the organization of the actomyosin system within cells is highly variable, with its assembly and function being controlled by small GTPases from the Rho family. To understand better how activation of these regulators translates into cell-scale force generation in the context of different physical environments, here we combine recent advances in non-neuronal optogenetics with micropatterning and traction force microscopy on soft elastic substrates. We find that, after whole-cell RhoA activation by the CRY2/CIBN optogenetic system with a short pulse of 100 ms, single cells contract on a minute timescale in proportion to their original traction force, before returning to their original tension setpoint with near perfect precision, on a longer timescale of several minutes. To decouple the biochemical and mechanical elements of this response, we introduce a mathematical model that is parametrized by fits to the dynamics of the substrate deformation energy. We find that the RhoA response builds up quickly on a timescale of 20 s, but decays slowly on a timescale of 50 s. The larger the cells and the more polarized their actin cytoskeleton, the more substrate deformation energy is generated. RhoA activation starts to saturate if optogenetic pulse length exceeds 50 ms, revealing the intrinsic limits of biochemical activation. Together our results suggest that adherent cells establish tensional homeostasis by the RhoA system, but that the setpoint and the dynamics around it are strongly determined by cell size and the architecture of the actin cytoskeleton, which both are controlled by the extracellular environment.

Bio-functionalization of silicon carbide nanostructures for SiC nanowire-based sensors realization

The bio-functionalization process consisting in grafting desoxyribo nucleic acid via aminopropyl-triethoxysilane is performed on several kinds of silicon carbide nanostructures. Prior, the organic layer is characterized on planar surface with fluorescence microscopy and X-ray photoelectron spectroscopy. Then, the functionalization is performed on two kinds of nanopillar arrays. One is composed of top-down SiC nanopillars with a wide pitch of 5 microm while the other one is a dense array (pitch: 200 nm) of core-shell Si-SiC nanowires obtained by carburization of silicon nanowires. Depending on both the pillar morphology and the pitch, different results in term of DNA surface coverages are obtained, as seen from fluorescence microscopy images. Particularly, in the case of the wide pitch array, it has been shown that the DNA molecules are located all along the nanopillars. To achieve a DNA sensor based on a nanowire-field effect transistor, the functionalization must be conducted on a single SiC nanowire or nanopillar that constitutes the channel of the field effect transistor. The localization of the functionalization in a small area around the nanostructures guarantees high performances to the sensor. In this aim, the functionalization process is combined with common microelectronics techniques of lithography and lift-off. The DNA immobilization is investigated by fluorescence microscopy and atomic force microscopy.

A light sterilizable pipette device for the in vivo estimation of human soft tissues constitutive laws

This paper introduces a new light device for the in vivo estimation of human soft tissues constitutive laws. It consists of an aspiration pipette able to meet the very severe sterilization and handling issues imposed during surgery. The simplicity of the device, free of any electronic circuitry, allows using it as an ancillary instrument. The deformation of the aspired tissue is imaged via a mirror using an external camera. The paper describes the experimental setup as well as the protocol that should be used during surgery. First feasibility measurements are shown for human tongue and forearm skin.

Influences of adhesion area and biological sample size on the estimation of Young's modulus and Poisson's ratio assessed by micropipette aspiration technique

The micropipette aspiration experiment remains a widely used micromanipulation technique for quantifying the mechanical properties of biological samples. Our study extends previous results by investigating the influence of sample size and adhesion area on the mechanical response of compressible thin biological samples. We thus defined a nonlinear relationship between aspirated length, Young's modulus, Poisson's ratio and sample thickness which allowed us to develop an original experimental protocol for simultaneous quantification of the Poisson's ratio and Young's modulus of adherent samples. We first validated our method by characterizing mechanical properties of polyacrylamide gels with tunable stiffness. We then considered application of these results to the quantification of cell elasticity, focusing on the influence of cell adhesion area onto the measured apparent cell stiffness.