Cellular Rheology and Hydraulics

The cytoplasm represents the largest part of the cell by volume and hence its rheology sets the rate at which cellular shape change can occur. To date, the cytoplasm has generally been modelled as a single-phase viscoelastic material; however, recent experimental evidence suggests that its rheology is better described using a poroelastic formulation in which the cytoplasm is considered to be a biphasic system constituted of a porous elastic solid meshwork (cytoskeleton, organelles, macromolecules) bathed in an interstitial fluid (cytosol). In this framework, a single parameter, the poroelastic diffusion constant Dp, sets cellular rheology scaling as Dp∼Eξ∧2/μ with E the elastic modulus, ξ the hydraulic pore size, and μ the cytosolic viscosity. Here we measure Dp in cells by fitting experimental stress relaxation curves in response to rapid application of a localised force by atomic force microscopy microindentation. Next, using indentation tests in conjunction with osmotic perturbations, we qualitatively verified the validity of the predicted scaling of Dp with pore size. Using chemical and genetic perturbations, we show that cytoplasmic rheology depends strongly on the integrity of the actin cytoskeleton but not on microtubules or intermediate filaments. Finally, we show that the short-time scale effective viscoelasticity of a cell is due to water redistribution within the cytoplasm and relate the cytoplasmic viscosity to cellular microstructure.