Cortical cell stiffness is independent of substrate mechanics

Abstract

Cortical stiffness is an important cellular property that changes during migration, adhesion, and growth. Previous atomic force microscopy (AFM) indentation measurements of cells cultured on deformable substrates suggested that cells adapt their stiffness to that of their surroundings. Here we show that the force applied by AFM onto cells results in a significant deformation of the underlying substrate if it is softer than the cells. This 'soft substrate effect' leads to an underestimation of a cell's elastic modulus when analyzing data using a standard Hertz model, as confirmed by finite element modelling (FEM) and AFM measurements of calibrated polyacrylamide beads, microglial cells, and fibroblasts. To account for this substrate deformation, we developed the 'composite cell-substrate model' (CoCS model). Correcting for the substrate indentation revealed that cortical cell stiffness is largely independent of substrate mechanics, which has significant implications for our interpretation of many physiological and pathological processes. Two-sentence summary AFM indentation measurements of cells cultured on soft substrates may lead to significant substrate deformations, resulting in an underestimation of cell stiffness. The CoCS model developed in this study, which takes this soft substrate effect into account, revealed that cortical cell stiffness is largely independent of substrate mechanics.

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