Adherens junctions and desmosomes are two types of adhesive junctions using homophilic (like to like) interactions of cadherins to bind epithelial cells to their neighbors. Cytoplasmic actin filaments reinforce adherens junctions, whereas cytoplasmic intermediate filaments anchor desmosomes.
Homophilic interactions between densely clustered E-cad-herins bind adjacent cells together at adherens junctions. β- Catenin and a related protein called plakoglobin bind the cytoplasmic domains of E-cadherin. ß-Catenin not only regulates gene expression when it enters the nucleus as part of the Wnt signaling pathway but also interacts with several Cytoskeletal and signaling proteins associated with adherens junctions. α-Catenin has been a candidate to connect cadherins to actin filaments, since it can bind both ß-catenin and actin filaments. However, these two interactions appear to be mutually exclusive, so the link between cadherins and actin is still under investigation.
Adherens junctions are the first connections that are established between developing sheets of epithelial cells. Contact begins when cadherins on the tips of filopodia engage partner cadherins of the same type on another cell. The contact spreads laterally as more cadherins are recruited along with associated actin filaments, as is illustrated by dorsal closure of the ectoderm by Drosophila embryos. These pioneering adherens junctions eventually allow like cells to associate in epithelial sheets and to influence the maturation of the epithelium. Adherens junctions are a prerequisite for the tight junctions that allow epithelial cells to establish polarity with different proteins and lipids in the apical and basal plasma membranes. The shape of the cells depends on Rho family GTPases and protein kinases associated with the adherens junction, which regulate the assembly and contraction of the associated actin cytoskeleton. The junctions and polarity of the cells determine the orientation of the mitotic spindle and the plane of division. This allows for asymmetrical division of stem cells, such as those at the base of stratified epithelia. In mature columnar epithelia, a belt-like adherens junction, called the zonula adherens, encircles the cells near their apical surface and maintains the physical integrity of the epithelium.