Retardation Can Quantify Tension in Single Stress Fibers?

Abstract

Tension in stress fibers plays an important role in cell functions such as cell migration, division, and contraction. We devised a method to evaluate a cell contraction force from retardation using a photo-elasticity technique (S. Sugita et al., Scientific Reports 9: 3690, 2019) and confirmed that the method can differentiate the phenotypes of vascular smooth muscle cells: contractile and synthetic types. It, however, remains unclear whether this method quantifies the “force” itself and why the retardation can sense the changes in force. Because the retardation is determined not only by the force but also by the density and alignment degree of birefringent material of stress fibers, cell activities might have changed such density or alignment. In this study, we isolated a single stress fiber from a cell to avoid cell responses and investigated the effect of force on the retardation. The vascular smooth muscle cells were isolated from porcine thoracic aortas, incubated on a dish, and treated with an osmotic shock solution to leave only stress fibers on the dish. The stress fibers were observed under a birefringent imaging system, and their retardation was measured. Application of ATP solutions to increase tension in the stress fibers led to an increase in the retardation, and retardation correlated significantly (P< 0.05) with the concentration of the ATP solutions. To elucidate the mechanism of retardation changes, the width of the stress fibers was evaluated from shadow images of the stress fibers under a phase-contrast microscope. When ATP solutions were applied, the width of the stress fiber decreased while its length was kept constant. These results demonstrated that the elevation of the retardation was attributed to the condensation of stress fibers induced by the tension.