Compression force induces transient actomyosin activation and compression-time dependent stiffening in cells

Cells are subject to a broad spectrum of mechanical stimuli, such as shear, tension and compression, which arise from their extracellular matrix and neighboring cells. As for today, we are still lacking a comprehensive understanding of the molecular mechanisms by which mechanical stimuli are sensed by cells and translated to adjust specific changes in their mechanical properties. Thus, the effect of transient compression on the mechanical properties of cells is systematically investigated in this work. Two fundamental questions are addressed: whether cells exhibit different mechanical responses depending on the time or force of compression and which cellular compartments are involved in the response to static compression. Consequently, we quantify the Young's modulus of cells before and after compression using beaded microcantilevers mounted on an atomic force microscope. Our experiments suggest that cells respond differently according to the time of compression. Compressing cells for short periods of time (10s) results in a significant but reversible stiffening of cells, whereas for long compression time (300s), cells do not show any significant response or in some cases soften slightly. Neither the force nor the depth of mechanical compression influences the compression-stiffening effect of the cell. Instead, the mechanical response of the cell seems to mainly depend on their initial mechanical state, as stiffer cells resulted to undergo less compression-stiffening compared to softer cells. By performing inhibitory assays, we find the actomyosin cortex to be mainly responsible for the compression-stiffening effect of the cell, whereas the osmotic pressure was not involved in the response. In conclusion, cells seem to have an internal mechanical clock which regulates different reorganizations of the actomyosin cortex in order to trigger different kind of mechanical responses to compression.