Light induced, localized, and abrupt force relaxations in fibroblast cells on soft substrates

Exposure to fluorescent excitation light is known to adversely affect living cells. Here, we provide experimental evidence showing fibroblast cells relax abruptly during illumination with fluorescent excitation light. Cells were plated on polyacrylamide (PA) gels embedded with fluorescent microbeads. The beads served as fiducial markers to track substrate displacements induced by cell force. When the cells adhere, they apply traction forces to their underlying surface, causing the substrate to contract inward with respect to the cell boundary. We find that contractile cell forces are sporadically interrupted by abrupt force relaxation events, characterized by outward “jumps” in displacement (⩾ 30 nm in 0.5 s) of the underlying substrate during exposure to fluorescent light. Jumps occur more frequently on softer substrates. Additionally, the speed and direction of displacements of particles before and after jumps are conserved, suggesting that the jumps do not involve any change in cytoskeletal or adhesion structure of the cell, and are possibly mediated by disengagement and re-engagement of myosin motors with actin filaments. Interestingly, we find that the frequency of jumps is also modulated by the energy introduced by the excitation light source. We propose a mechanistic hypothesis to explain the observations.