Is the mechanical activity of epithelial cells controlled by deformations or forces?

Abstract

The traction forces developed by cells depend strongly on the substrate rigidity. In this letter, we characterize quantitatively this effect on MDCK epithelial cells by using a microfabricated force sensor consisting in a high-density array of soft pillars whose stiffness can be tailored by changing their height and radius to obtain a rigidity range from 2 nN/microm up to 130 nN/microm. We find that the forces exerted by the cells are proportional to the spring constant of the pillars meaning that, on average, the cells deform the pillars by the same amount whatever their rigidity. The relevant parameter may thus be a deformation rather than a force. These dynamic observations are correlated with the reinforcement of focal adhesions that increases with the substrate rigidity.

FIGURE 1

(A) Distribution of fluorescently labeled fibronectin (red). (B) Scanning electron micrograph of edge detail of a MDCK monolayer showing cell-to-substrate interactions. Scale bars correspond to 5 μm.

FIGURE 2

Log-log plot of the force as a function of substrate rigidity. 〈F〉 (blue) and F_max (red) within an island of cells are represented for different surface densities (ratio of the post surface over the total surface) 10% (○), 22% (□), and 40% (▵). Open and solid symbols, respectively, correspond to pillars of 1 and 2 μm in diameter. The slope of the dashed line is 1. (Inset) Typical histogram of force distribution (spring constant 64 nN/μm).

FIGURE 3

(A and C) Confocal images of immunofluorescence staining of the focal adhesion protein vinculin for different substrate rigidities (pillar spring constants, respectively, 2 and 71 nN/μm). (B and D) Details corresponding to the indicated regions in panels A and C, respectively. Scale bars correspond to 10 μm.