Optical tweezers combined with FRET tension sensor reveal force-dependent vinculin dynamics

Visualizing and quantifying molecular responses to local forces exerted at cell adhesions is crucial to elucidate how physical forces control cellular behavior. Here, we combine optical tweezers with Förster resonance energy transfer (FRET) microscopy of the vinculin tension sensor, VinTS, to measure the response of vinculin, a key mechanical load-bearing protein, to an applied force. Fibroblasts expressing VinTS formed adhesions on fibronectin-coated, 3μm-diameter, polystyrene beads. As the beads were displaced by the cell, we applied an optical trap to counteract this movement and increase the traction force required by the cell to maintain the bead’s displacement. The median bead displacement after 5 min was ∼200nm in all trapping conditions tested, from zero (no laser) up to 0.26 pN/nm, inducing counteracting forces in the 10-100pN range. To maintain this displacement, vinculin recruitment increased at high stiffness (up to 35% in relative intensity) while vinculin tension increased only moderately in all trapping conditions (1-2% decrease in absolute FRET efficiency). Vinculin recruitment was governed by stiffness rather than the magnitude of the traction force and was correlated with vinculin tension at 0.26 pN/nm but not at lower stiffness. In rare instances, vinculin puncta migrated a few micrometers away from the bead, exceeding the bead’s movement speed while experiencing an increase in both vinculin intensity and tension. Taken together, the results suggest that combining an optical trap with vinculin tension measurements in living cells uncovers novel vinculin dynamics in the presence of a force. Why it matters Combining optical tweezers with FRET microscopy we measure vinculin tension in conjunction with its recruitment in response to force at adhesions of living cells. We demonstrate that vinculin recruitment is governed by trap stiffness rather than force and that the cell responds to higher stiffness with an increase in vinculin recruitment but less increase in vinculin tension. At high trap stiffness, a correlation between recruitment and tension is observed but not at low stiffness or with no optical trap. Such dynamic measurements, enabled by the techniques presented here, can help elucidate the mechanisms by which cells sense physical forces and the properties of proteins, such as vinculin, which play a fundamental role in cellular behaviors involving tissue growth and repair.