Thin film microrheology of the living actomyosin cortex in the C. elegans embryo

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The cell cortex is a thin layer of cross-linked actin filaments and myosin motor proteins beneath the cell membrane. In addition to providing mechanical stability to the cell, this dynamic network also drives large-scale cortical flows. Cortical flows are crucial to key cellular processes such as cell polarization and cell division. Despite our detailed understanding of individual cortical components, their interplay within the cortex remains obscure because of the difficulty to infer molecular contributions to cortical properties from the observable large-scale behavior of the cortex alone. An intermediate level of description is required, detailing the emergent physical properties of the cortex.In this project, we propose to study the mechanical properties of the cortex directly, using a novel Active THin-film microRheology (ATHUR) approach. We have recently established a method to apply calibrated forces on micrometer-sized magnetic particles in the one-cell C. elegans embryo. The magnetic particles are introduced into embryos by microinjection into the gonad of adult hermaphrodite worms, and external forces are exerted on the incorporated particles by placing the embryo in a magnetic field gradient. To investigate cortical mechanics, we will pull the incorporated magnetic beads to the cortex and study, first, their motion as passive tracers associated with the cortex, and then, by applying a calibrated force on the beads in the cortical plane, the response of the cortex to these active probes.While passive and active microrheology experiments have been used to describe the bulk viscoelasticity of reconstituted actomyosin gels in vitro, the proposed experiments will allow us to access, for the first time, the mechanical properties of the thin, quasi-2D cortical network of a living cell. The mechanical characterization will then be combined with genetic perturbations of actin-binding proteins by RNAi to identify the mechanical parameters that these proteins tune in the cortex. Using a hydrodynamic description of the cortex as a thin film of active complex fluid, we will then relate the changes in the material properties of the cortex to the cortical flow phenotypes.In summary, this proposal aims to provide a link from molecules to the mechanics of cortical behavior, which will be crucial to the understanding of cortical flows in living systems.