Atomic force microscopy-based microrheology reveals significant differences in the viscoelastic response between malign and benign cell lines

Abstract

Mechanical phenotyping of cells by atomic force microscopy (AFM) was proposed as a novel tool in cancer cell research as cancer cells undergo massive structural changes, comprising remodelling of the cytoskeleton and changes of their adhesive properties. In this work, we focused on the mechanical properties of human breast cell lines with different metastatic potential by AFM-based microrheology experiments. Using this technique, we are not only able to quantify the mechanical properties of living cells in the context of malignancy, but we also obtain a descriptor, namely the loss tangent, which provides model-independent information about the metastatic potential of the cell line. Including also other cell lines from different organs shows that the loss tangent (G″/G') increases generally with the metastatic potential from MCF-10A representing benign cells to highly malignant MDA-MB-231 cells.

Keywords: atomic force microscopy; cancer; microrheology; viscoelasticity.

Figure 1.

(a) Illustration of force mapping on cells. A deflection AFM image (contact mode) of subconfluent NMuMG cells is shown. During force mapping a force–distance curve is taken at every yellow spot. (b) Schematic drawing of the experiment: the cantilever oscillates around the indentation depth δ₀ with an amplitude δ at the frequency ω. (c) Time course of force during the measurement of a force–distance curve. When the cantilever gets into contact with the sample, the force increases rapidly until the present trigger point is reached. During dwell in contact, the cantilever is excited to sinusoidal oscillations with frequencies from 5 to 200 Hz. Afterwards, the cantilever is retracted and the procedure repeated at a different position. (d) Indentation oscillation δ(ω) with frequencies from 5 to 200 Hz around the indentation depth δ₀ and corresponding force signal F(ω) after detrending.

Figure 2.

(a) AFM height image of subconfluent epithelial NMuMG cells (contact mode). (b,c) Height image overlaid with the force map data of (b) G′ and (c) G″ at an oscillation frequency of 20 Hz.

Figure 3.

(a) AFM-deflection images of MCF-10A, MCF-7 and MDA-MB-231. Cells were imaged in constant force mode using pyramidal cantilever-tip geometry. (b) Median values of the storage modulus G′ (filled symbols) and loss modulus G″ (open symbols) as a function of oscillation frequency (two force maps, more than 10 cells). The data of the complex shear modulus were fitted using the power-law structural damping model (solid lines).

Figure 4.

Loss tangent η = G″/G′ of the human breast cell lines MCF-10A, MCF-7 and MDA-MB-231 as a function of frequency. Continuous lines represent results of fitting the parameters of the power-law structural damping model to the experimental data.

Figure 5.

Correlation plots of the parameters G₀, α and μ for all cell lines. A correlation between G₀ and the power-law exponent α is found for human breast cell lines (coloured symbols connected by dotted line).

Figure 6.

Loss tangent η of all cell lines with different metastatic potential computed at an oscillation frequency of 100 Hz (two force maps, corresponding to 5–20 cells, depending on cell size).