Pre-stress and cortical stiffness affect fluidity of living cells

Shape, dynamics and viscoelastic properties of eukaryotic cells are largely governed by a thin reversibly cross-linked actomyosin cortex located directly beneath the plasma membrane. We obtain time-dependent rheological responses of weakly adhered mesenchymal cells (fibroblasts) and epithelial cells (MDCK II) from parallel-plate compression and force relaxation experiments. We introduce an analytical expression for the compression and force relaxation based on the elastic-viscoelastic correspondence principle by treating the cell as a closed liquid-filled shell and assuming a power law to describe the change of surface area during deformation. This approach gives access to pre-stress, area compressibility modulus and the power law (fluidity) coefficient, which we modulate by interfering with myosin activity. We find that the fluidity of cells decreases with increasing intrinsic pre-stress as shown for isolated actin networks subject to external stress.