Spatiotemporal Tension Distribution of Individual Stress Fibers at the Cell-Matrix Interface

Abstract

Actomyosin stress fibers (SFs) enable cells to exert traction on planar extracellular matrices (ECMs) by tensing focal adhesions (FAs) at the cell-ECM interface. While it is widely appreciated that the spatiotemporal distribution of these tensile forces play key roles in polarity, motility, fate choice, and other defining cell behaviors, virtually nothing is known about how an individual SF quantitatively contributes to tensile loads borne by specific molecules within associated FAs. We address this key open question by using femtosecond laser ablation to sever single SFs in cells while tracking tension across vinculin using a fluorescence resonance energy transfer (FRET) -based molecular optical sensor. We show that disruption of a single SF reduces tension across vinculin in FAs located throughout the cell, with enriched vinculin tension reduction in FAs oriented parallel to the targeted SF. Remarkably, however, some subpopulations of FAs exhibit enhanced vinculin tension upon SF irradiation and undergo dramatic, unexpected transitions between tension-enhanced and tension-reduced states. These changes depend strongly on the location of the severed SF, consistent with our earlier finding that different SF pools are regulated by distinct myosin activators. To unify these findings, we present a structural model in which central SFs are more interconnected and mutually reinforced than peripheral SFs due in part to the presence of transverse actomyosin structures that link central SFs into a cohesive network. Tension released upon compromise of a central SF is thus broadly redistributed to other stress fibers and focal adhesions, resulting in cell shape stabilization. These studies represent the most direct and high-resolution intracellular measurements of SF contributions to tension on specific FA proteins to date and offer a new paradigm for investigating regulation of adhesive complexes by cytoskeletal force. (Chang and Kumar, J Cell Sci 2013)