Actin Disassembly is Mechanically Regulated in Spontaneously Fracturing Stress Fibers

Abstract

Stress fibers are contractile bundles of actin, myosin and other components which mechanically interact with the extracellular matrix and maintain the cell's structural integrity. As part of the cytoskeleton they must be able to rapidly remodel in response to dynamic stimuli, but detailed mechanisms are not established. Here we use mathematical modeling and quantitative image analysis of cells expressing tagged stress fiber components to study stress fiber remodeling. We observed occasional acute elongation of stress fibers followed either by repair (82% of the time) or spontaneous fracturing followed by rapid recoil over ∼25 s (18%). Fractured stress fibers shortened by ∼80% total regardless of their initial length, suggesting significant attachments to other cytoskeletal elements were absent. We discovered that fiber shortening is accompanied by substantial actin disassembly with a ∼30 s delay. The disassembly processes shed ∼40% of the actin initially present. Thus, the fiber actin density increases during recoil, peaks, and then decays to twice its initial value. To quantitatively explain this behavior we extended an earlier mathematical model of stress fibers to spontaneous fracture and recoil [Stachowiak and O'Shaughnessy, New J. Phys., v10, p025002 (2008)]. The model predicts that, following breakage, fiber shortening due to myosin contractile force increases actin density which in turn augments actin-actin compressive elastic stresses. These stresses promote actin depolymerization, thus allowing the fiber to remodel its actin. Model predictions agree quantitatively with experimental data. The measured 30 s delay is the time for actin density during contraction to reach the threshold to trigger depolymerization-promoting stresses. These results demonstrate that remodeling of cytoskeletal structures can be mechanically regulated by coupling between shape, elastic stress and component turnover rates.