Investigation into local cell mechanics by atomic force microscopy mapping and optical tweezer vertical indentation

Abstract

Investigating the mechanical properties of cells could reveal a potential source of label-free markers of cancer progression, based on measurable viscoelastic parameters. The Young's modulus has proved to be the most thoroughly studied so far, however, even for the same cell type, the elastic modulus reported in different studies spans a wide range of values, mainly due to the application of different experimental conditions. This complicates the reliable use of elasticity for the mechanical phenotyping of cells. Here we combine two complementary techniques, atomic force microscopy (AFM) and optical tweezer microscopy (OTM), providing a comprehensive mechanical comparison of three human breast cell lines: normal myoepithelial (HBL-100), luminal breast cancer (MCF-7) and basal breast cancer (MDA-MB-231) cells. The elastic modulus was measured locally by AFM and OTM on single cells, using similar indentation approaches but different measurement parameters. Peak force tapping AFM was employed at nanonewton forces and high loading rates to draw a viscoelastic map of each cell and the results indicated that the region on top of the nucleus provided the most meaningful results. OTM was employed at those locations at piconewton forces and low loading rates, to measure the elastic modulus in a real elastic regime and rule out the contribution of viscous forces typical of AFM. When measured by either AFM or OTM, the cell lines' elasticity trend was similar for the aggressive MDA-MB-231 cells, which were found to be significantly softer than the other two cell types in both measurements. However, when comparing HBL-100 and MCF-7 cells, we found significant differences only when using OTM.

Quantitative Deformability Cytometry: Rapid, Calibrated Measurements of Cell Mechanical Properties

“…We also measure MDA-MB-231 cells within the same size range, and find that E a ¼ 0.97 5 0.50 kPa, which is 40% lower than MCF-7 cells (p << 0.001). Our findings that MDA-MB-231 cells are more compliant than MCF-7 cells are in agreement with previous reports (62,63) (Table S2). Using AFM and magnetic twisting cytometry, E values for MCF-7 cells typically range from 0.2 to 1 kPa (62,64-67), whereas for MDA-MB-231 cells, the E value varies from 0.2 to 0.69 kPa (11,31,62,64,65,67).…”

Fluid shear stress coupled with narrow constrictions induce cell type-dependent morphological and molecular changes in SK-BR-3 and MDA-MB-231 cells

“…They are consistent with the "transportability coefficients" computed by Liu et al, showing that MDA-MB-231 cells are much more deformable than SK-BR-3 cells 26 . Our results are therefore in line with the accumulation in the literature of evidence showing that the deformability of both cells 27 and nuclei increases with their metastatic potential 28 .…”

The influence of lateral forces on the cell stiffness measurement by optical tweezers vertical indentation

“…All our measurements have been performed on the top of the cell, in a region above the nucleus. In a recently published paper [27] , using also AFM measurements at different positions of the cell, we have indicated this region as the most reliable to make a comparison between different cell lines in terms of cell elasticity. However, we expect that the differences between the cell elasticity results obtained when the total forces are considered instead of the vertical forces, be considerably bigger in the transition regions between the nucleus and the leading edge.…”

“…This highlights the fact that the cells are sensitive to the applied forces and the loading rates (measured in N/s) [24] because of their viscoelastic, inhomogeneous and anisotropic nature. Therefore, for characterization of different cell lines it is important to complement AFM vertical indentation measurements by OT measurements following the same scheme [25][26][27] .…”