High-Resolution Probing of Cellular Force Transmission

Cells actively probe mechanical properties of their environment by exerting internally generated forces. The response they encounter profoundly affects their behavior. Here we measure in a simple geometry the forces a cell exerts suspended by two optical traps. Our assay quantifies both the overall force and the fraction of that force transmitted to the environment. Mimicking environments of varying stiffness by adjusting the strength of the traps, we found that the force transmission is highly dependent on external compliance. This suggests a calibration mechanism for cellular mechanosensing.

Figure 1

(a) Differential interference contrast image of a MLO-Y4 cell suspended by two optically trapped spherical particles (polystyrene, diameter $2R=4\mu \mathrm{m}$) attached to opposing sides of the cell. Scale bar: $5\mu \mathrm{m}$. (b) Fluctuations of the force exerted on both probe particles ($k=5.0\times {10}^{-5}\mathrm{N}/\mathrm{m}$). Forces generated by the cell result in anticorrelated displacements. (c) Fluorescence image of a lipid vesicle coated with rhodamine-phalloidin stabilized and biotinylated actin. Scale bar: $5\mu \mathrm{m}$. (d) Thermal force fluctuations seen by streptavidin-coated particles (spherical silica, diameter $2R=1.5\mu \mathrm{m}$) attached via the biotinylated actin.

Figure 2

$A_{12}$ measured with AMR (filled symbols) compared to the normalized fluctuation power spectral density measured with PMR (open symbols). Circles and squares show real and imaginary parts, respectively.

Figure 3

(a) Frequency dependence of the (attenuated) force fluctuations detected by the particles $-k^2⟨u_1{u}_2^{*}⟩$. The power spectral density of the total fluctuating force $⟨F{F}^{*}⟩$ (filled circles) follows an ${\omega }^{-2}$ power law. (b) Trap-stiffness dependence of the transmitted force fluctuations $-k^2⟨u_1{u}_2^{*}⟩$ at 0.19 Hz (filled circles) and of the total fluctuating force $⟨F{F}^{*}⟩$ (open circles). The solid line is the fit of Eq. (3) to the data.

Figure 4

Simple model of the mechanical components in our experimental configuration. $k_{12}$ and $k$ denote the stiffnesses of the cell and the optical traps, respectively, $u_1$, $u_2$ are the displacements of each particle. $F$ denotes the total traction force as a sum of local forces as sketched in the lower panel.