Adherens junctions formation: The role of actin filament dynamics and myosin contractility

Cell-cell adherens junctions are closely associated with actin filaments and this interaction is required for strong cell-cell adhesion. However, the precise functions of the actin cytoskeleton in formation of cell-cell adherens junctions have not previously been identified. The role of the actin cytoskeleton in formation of cell-cell contacts by normal and oncogene-transformed epithelial cells and fibroblasts was analyzed using immunofluorescence microscopy, video-DIC microscopy, and observations of actin filament dynamics in live cells. We found that non-transformed epithelial cells formed stable cell-cell contacts that were oriented tangentially, along the cell-cell boundary. The establishment of cell-cell adherens junctions in non-transformed epithelial cells was associated with inhibition of actin-dependent lamellar motility and a dramatic reorganization of the actin cytoskeleton. The observed changes in actin cytoskeletal organization included disassembly of marginal actin bundles, formation of lateral actin arcs along the edges of the expanding contact, and de novo assembly of actin bundles along the cell-cell boundary. Localization of myosin II to lateral actin arcs suggested that myosin II-driven contraction of actin arcs might be involved in lateral spreading of the contact while assembly of actin bundles aligned parallel to cell-cell contacts may serve to strengthen and stabilize newly formed adherens junctions.;Unlike normal epithelial cells, fibroblasts and oncogene-transformed epithelial cells did not form stable cell-cell contacts and did not exhibit changes in lamellar motility and overall organization of the actin cytoskeleton. Contacts between fibroblasts were radially oriented and associated with straight bundles of actin filaments. We found that the spatial organization of cell-cell contacts in normal epithelial cells could be converted from the typical tangential pattern to the radial pattern observed in fibroblasts by treatment with TPA and nocodazole, two agents that induce formation of radially oriented actin bundles and/or enhanced myosin II contractility. Inhibition of myosin contractility prevented formation of radial cell-cell contacts in fibroblasts and TPA- or nocodazole-treated epithelial cells. These data open the possibility that modulation of the spatial organization of the actin cytoskeleton may play an important role in regulating organization and stability of cell-cell contacts.