THE SMALL GTPASE-RHO AND GTPASE-RAC ARE REQUIRED FOR THE ESTABLISHMENT OF CADHERIN-DEPENDENT CELL-CELL CONTACTS

Cadherins are calcium-dependent cell-cell adhesion molecules that requ ire the interaction of the cytoplasmic tail with the actin cytoskeleto n for adhesive activity. Because of the functional relationship betwee n cadherin receptors and actin filament organization, we investigated whether members of the Rho family of small GTPases are necessary for c adherin adhesion. In fibroblasts, the Rho family members Rho and Rac r egulate actin polymerization to produce stress fibers and lamellipodia , respectively. In epithelial cells, we demonstrate that Rho and Rac a re required for the establishment of cadherin-mediated cell-cell adhes ion and the actin reorganization necessary to stabilize the receptors at sites of intercellular junctions. Blocking endogenous Rho or Rac se lectively removed cadherin complexes from junctions induced for up to 3 h, while desmosomes were not perturbed. In addition, withdrawal of c adherins from intercellular junctions temporally precedes the removal of CD44 and integrins, other microfilament-associated receptors. Our d ata showed that the concerted action of Rho and Rac modulate the estab lishment of cadherin adhesion: a constitutively active form of Rac was not sufficient to stabilize cadherin-dependent cell-cell contacts whe n endogenous Rho was inhibited. Upon induction of calcium-dependent in tercellular adhesion, there was a rapid accumulation of actin at sites of cell-cell contacts, which was prevented by blocking cadherin funct ion, Rho or Rac activity. However, if cadherin complexes are clustered by specific antibodies attached to beads, actin recruitment to the re ceptors was perturbed by inhibiting Rac but not Rho. Our results provi de new insights into the role of the small GTPases in the cadherin-dep endent cell-cell contact formation and the remodelling of actin filame nts in epithelial cells.