In our research, we combined optical tweezers to apply force on a cell with Fӧrster resonance energy transfer to measure the internal tension on vinculin, a protein involved in the focal adhesion of the cell on its substrate. In the cover image of the September 9 issue of Biophysical Reports, we used confocal fluorescence microscopy to reconstruct the 3D surface of a fibroblast cell showing semi-spherical depressions corresponding to the location of micron-sized beads attached to the cell via focal adhesions. The fluorophore is the vinculin tension sensor, which is expressed at focal adhesions but is also present throughout the cytoplasm, allowing us to visualize the whole volume of the cell. The polystyrene beads, 6 µm in diameter, are functionalized with fibronectin to activate the integrins of the cells and create focal adhesions on the bead. Through this reconstruction, we see that the beads remain outside of the cell, with a large surface of contact with the cell membrane, on which small puncta of vinculin accumulate, corresponding to the focal adhesions. In our paper, we also labeled actin and confirmed that actin fibers are connected to these focal adhesions. This is the initial state of our experiment, before we apply the external force on the bead by using the optical tweezers.
In the research shown in this article, we demonstrate that when a force is applied to the bead via an optical trap, the cell reacts mostly by recruiting more vinculin proteins at focal adhesions, with only a small increase in the tension on each one. Surprisingly, this vinculin recruitment is governed by trap stiffness rather than force. Measuring both recruitment and tension helps us understand the mechanical behavior of cells under load, which plays a fundamental role in cellular behaviors involving tissue growth and repair.
— Camille Dubois, Rick I. Cohen, Nada N. Boustany, and Nathalie Westbrook