Stress fibers and Cortical F-actin Coordinate the Mechanical Anisotropy in Cells

The F-actin cytoskeleton is a composite of cortical (disordered) actin and (bundled) F-actin stress fibers which together mediate the mechanical behaviors of the cell, from cell division to cell migration. However, as mechanical stresses are typically measured upon transmission to the extracellular matrix (ECM), the internal distribution of stress within the cytoskeleton is unknown. Likewise, how distinct F-actin architectures contribute to the generation and transmission of mechanical stress is unclear. Therefore, to measure mechanical stresses internally, we have developed a novel tension sensor that embeds into the F-actin cytoskeleton. Using this sensor, we isolate the roles of stress fibers and cortical actin in determining the mechanical response of the cell to bi-axial stretch. At low strain, the mechanical response is anisotropic, as stress-fibers principally bear the applied mechanical load. By contrast, at high strain, the response is isotropic, as stress-fibers and cortical actin bear load uniformly. Thus, these results indicate that despite their mutual integration into a composite material the mechanical roles of distinct F-actin architectures and the coordination between them vary and depend upon the state of the applied load.