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

Abstract

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.

A novel method for sensor-based quantification of single/multicellular force dynamics and stiffening in 3D matrices

“…Note that the gauges are placed away from the center of tissue location so that the design allows measurement of force as a function of time without directly illuminating the cell(s). This eliminates the possibility of light-induced response of the cells (44,45).…”

A novel method for sensor-based quantification of single/multi-cellular traction dynamics and remodeling in 3D matrices

“…It should be noticed that the gauges are placed away from the center of tissue location so that the design allows measurement of force as a function of time without directly illuminating the cell(s). This eliminates the possibility of light induced response of the cells 54,55 . Fig.…”

A novel technique for in situ uniaxial tests of self-assembled soft biomaterials

“…The stage allows us to measure force of the biomimetic tissue as a function of time without "seeing" the tissue. This saves the tissue from light exposure which may result in light induced tissue response 20 . However, the tissue can be imaged by programming the microscope stage to step between the tissue and the gauge locations precisely.…”

“…Such in-situ quantitative inspection may offer new insights that cannot be achieved with existing methods. In addition, force-deformation of the tissue can be measured without exposing the tissue to any light which might affect the measured forces 20 . Small thickness of the sensor stage makes it compatible with live cell microscopy and avoid any histological sectioning needed in the case of thick samples.…”