Estimating Cell Mechanical Anisotropy via Spherical Indentation and F-actin Imaging

Mechanical properties reflect the physiological state of cells, yet, commonly reported Young's modulus (Es), does not represent the mechanical anisotropy. In this work we sought to determine if standard spherical probe indentation, combined with common image-based F-actin alignment quantification, could be used to estimate the anisotropic cell elastic moduli (E1, E2). Toroidal indentation was performed on porcine aortic valvular interstitial cells (PAVICs) and dermal fibroblasts and the F-actin alignment was measured by fluorescent microscopy. A multivariable regression model was developed to predict the anisotropic elastic moduli from the F-actin alignment and Es estimated from a finite element simulation of spherical indentation. We found a moderate correlation between F-actin alignment and the degree of cell anisotropy (E1/E2) for PAVICs but not dermal fibroblasts. Grouping the PAVICs by aspect ratio yielded strong correlations and allowed accurate prediction on the group level. Overall, we conclude that the average anisotropic elastic moduli for the grouped cells of similar morphology can be predicted with high accuracy for some cell types, whereas the properties of individual cells exhibit high variation leading to poor predictive accuracy necessitating direct measure of mechanical anisotropy using specialized methods. This study clarifies the relationship between the F-actin structure, the isotropic Young's modulus, and the anisotropic elastic moduli of cells, providing broader insight into cell mechanics and mechanobiology.