AFM Indentation Reveals Actomyosin-Based Stiffening of Metastatic Cancer Cells during Invasion into Collagen I Matrices

Abstract

Atomic force microscopy (AFM) is a widely used technique to directly probe the mechanical response of mammalian cells to external forces to determine their elasticity. To date, the majority of AFM-based studies are limited to cells that are adhered to flat substrates, however these measurements lose information regarding cell mechanobiology in a physiologically relevant 3D microenvironment. We have performed combined AFM indentation and confocal fluorescence measurements on MDA-MB-231 metastatic breast cancer cells which have either partially or fully invaded into polymerized bovine collagen I matrices. In order to interpret the raw data from the experiments to determine the cells’ elastic modulus, we have developed numerous analytical and simulation techniques. A sphero-conical tip geometry to represent an AFM probe with a spherical cap transitioning to a cone is derived and applied to analyze deep indentations into the cell-collagen layer. For partially invaded cells, a generalized bonded two-layered elastic half-space model is numerically solved to assist with decoupling the mechanical response of the collagen matrix from the cell. For fully embedded cells, finite element analysis is used to simulate an AFM indentation to extract their elastic moduli. Using these techniques, we demonstrate that the elastic modulus of MDA-MB-231 cells significantly increases by ∼80% as they invade into collagen compared to cells on glass and cells on top of collagen. Inhibiting ROCK decreases the rigidity of cells on a surface as well as the magnitude of stiffening during invasion into collagen. These results corroborate recent actomyosin-based rounded cell motility models in 3D and demonstrate the ability of AFM to study cell mechanics in tissue-like environments.