Viscoelastic response of fibroblasts to tension transmitted through adherens junctions

Cytoplasmic deformation was monitored by observing the displacements of 200-nm green fluorescent beads microinjected into the cytoplasm of Swiss 3T3 fibroblasts. We noted a novel protrusion of nonruffling cell margins that was accompanied by axial flow of beads and cytoplasmic vesicles as far as 50 μm behind the protruding plasma membrane. Fluorescent analog cytochemistry and immunofluorescence localization of F-actin, α-actinin, N-cadherin, and β- catenin showed that the protruding margins of deforming cells were mechanically coupled to neighboring cells by adherens junctions. Observations suggested that protrusion resulted from passive linear deformation in response to tensile stress exerted by centripetal contraction of the neighboring cell. The time dependence of cytoplasmic strain calculated from the displacements of beads and vesicles was fit quantitatively by a Kelvin- Voight model for a viscoelastic solid with a mean limiting strain of 0.58 and a mean strain rate of 4.3 x 10^-3 s^-1. In rare instances, the deforming cell and its neighbor spontaneously became uncoupled, and recoil of the protruding margin was observed. The time dependence of strain during recoil also fit a Kelvin-Voight model with similar parameters, suggesting that the kinetics of deformation primarily reflect the mechanical properties of the deformed cell rather than the contractile properties of its neighbor. The existence of mechanical coupling between adjacent fibroblasts through adherens junctions and the viscoelastic responses of cells to tension transmitted directly from cell to cell are factors that must be taken into account to fully understand the role of fibroblasts in such biological processes as wound closure and extracellular matrix remodeling during tissue development.