AFM-based microrheology of biological cells : correlation of local viscoelasticity and motility
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Local viscoelasticity of a cell is important in understanding the extension of the lamellipodium, which contributes to the cell’s motility. It has been a challenge to accurately measure viscoelastic properties of a thin sample such as the lamellipodium of a cell (<1000 nm) due to the strong substrate effects and high stresses (>1 kPa). We account for the substrate effects by applying the two advanced models – the Chen and Tu models. The tightly regulated elastic moduli shown in the lamellipodium of fibroblasts manifestly display the successful adoption of these two models. In addition, these models provide the local Poisson ratio and adhesive state of a cell: the regions near the lamellipodium are well adhered while the regions further back to the main body are non-adhered. Our AFM technique successfully illuminates the heterogeneous nature of the cytoskeleton over the entire regions of the cell. By extending these models to the frequency-dependent microrheology technique, we decompose the elastic moduli into the loss and the storage moduli. Our AFM microrheology technique distinctly differentiates the malignantly transformed fibroblasts from the normal fibroblasts: the malignantly transformed fibroblasts display a decrease in viscoelastic moduli of the lamellipodium. Considering that motilities as well as viscoelastic properties of cells are induced by cytoskeletal changes, we focus our attention to illuminating on the cell’s protrusive mechanism correlated with the viscoelastic properties. To do this, we quantify the parameters of cell’s motility by analyzing time-lapsed phase contrast images. The resulting data show an increase in the motile activity caused by malignant transformation. In conclusion, these results are combined to suggest the correlation between the enhanced motility and the decrease in viscoelastic moduli. This conclusion is successfully explained by considering the microscopic model of the cell motility, i.e ‘Elastic Brownian Ratchet’ (Mogilner et al., 1996). It is understood that the lack of actin cross-linking proteins observed in malignantly transformed fibroblasts causes a cell to be softer and more motile. An increase in thermal fluctuations of softer cells can expedite the intercalation of G-actin that leads the cell’s protrusive motility.