Atomic force microscopy study revealed velocity-dependence and nonlinearity of nanoscale poroelasticity of eukaryotic cells

Intracellular network deformation of the cell plays an important role in cellular shape formation. Recent studies suggest that cell reshaping and deformation due to external forces involve cellular volume, pore size, elasticity, and intracellular filaments polymerization degree change. This cell behavior can be described by poroelastic models due to the porous structure of the cytoplasm. In this study, the nanoscale poroelasticity of human mammary basal/claudin low carcinoma cell (MDA-MB-231) was investigated using indentation-based atomic force microscopy. The effects of cell deformation (i.e., indentation) velocity and depth on the poroelasticity of MDA-MB-231 cells were studied. Specifically, the cell poroelastic behavior (i.e., the diffusion coefficient) was quantified at different indenting velocities (0.2, 2, 10, 20, 100, 200 μm/s) and indentation depths (635, 965, and 1313nm) by fitting the force-relaxation curves using a poroelastic model. Cell treated with cytoskeleton inhibitors (latrunculin B, blebbistatin, and nocodazole) were measured to investigate the effect of the cytoskeletal components on the cell poroelasticity. It was found that in general the MDA-MB-231 cells behaved less poroelastic (i.e., with lower diffusion coefficient) at higher indenting velocities due to the local stiffening up and dramatic pore size reduction caused by faster force load, and the cytoplasm is nonlinear in terms of poroelasticity. The poroelastic relaxation was more pronounced when the local cytoplasm porous structure was stretched by higher indentation. Furthermore, inhibition of cytoskeletal components resulted in pronounced poroelastic relaxation when compared with the control, and affected the nonlinearity of cell poroelasticity at different depth range inside of the cell. The comparison between the diffusion coefficient variation and the Young's modulus change under each indentation/treatment condition suggested that the cytoplasm porous geometry is more dominant than the cell Young's modulus in terms of affecting cell poroelasticity.