Breaking the Dynamic Instability of the Actomyosin Cortex Triggered by a Single Mechanical Cue Drives Cell Polarity in Non-Adherent Cells

Abstract

The ability of cells to establish and maintain polarized states is essential for numerous developmental and physiological processes. A key role in animal cell polarization is played by myosinII which generate contractile forces that result in producing a tensile stress in the actin network. Polarity establishment requires a symmetry-breaking event, resulting in an axis along which the cell will organize itself. While the polarization can occur spontaneously it could be triggered by a mechanical cue implying the presence of a mechanical threshold beyond which the cortex ruptures.We propose to study the role of asymmetric and weak mechanical cue on the first steps of 3T3 fibroblast polarity establishment. We setup a dual-objective system that combines quantitative micromanipulation using optical tweezers and three-dimensional fluorescence imaging. 3T3 cells were maintained in suspension using glass coverslips coated with a hydrophobic co-polymer.In absence of mechanical and chemical perturbations, cells exhibit an oscillatory behavior driven by the dynamical instability of the actomyosin cortex. Application of a single fibronectin-coated trapped bead is sufficient to induce the growth of an irreversible protrusion in the direction of the cue. In addition, the MTOC reorients toward the newly cue-defined polarity axis. Inhibition of myosinII contractile activity prevents any deformations and MTOC reorientation, suggesting thus far that an “unimpaired” actomyosin contractile activity is necessary for driving cell polarity. Finally, we report a directed cortical flow of actomyosin at the opposite side of the polarizing cue which suggest the idea of a latent migration polarity axis. Depletion of actomyosin filament as well as constant flow away of the polarizing cue zone supposes that the actomyosin cortex is a self-renewing contractile engine rather than a pre-tensioned network releasing once stored tension.