Apical junctional complexes and cell polarity

Recent studies have greatly expanded our knowledge of initial events that lead to epithelial cell polarity. Epithelial polarity is defined, in part, by apical cell–cell tight junctions that separate the plasma membrane into the apical domain and the basolateral domain, as well as the zonula adherens that mediate intercellular adhesion. The process of epithelial polarization is closely coupled to the biogenesis of these junctions. Studies in mammalian epithelial cells and lower organisms have identified two evolutionarily conserved junctional complexes as important epithelia polarity regulators: the Crumbs complex and the partitioning defective complex. Disruption of the components of the two complexes leads to a disorder of epithelial cell polarity and defects in junction formation or maintenance. Recent discoveries have revealed more details of how the two junctional polarity complexes function to establish epithelial polarity. They also raised the question about the relationship between polarity and adhesion. Although it is widely accepted that cell–cell adhesion provides a landmark from which polarity can proceed, there are results pointing to the possibility that polarity complexes can regulate cell–cell adhesion. It seems likely that proteins that control cell adhesion and cell polarity work intimately together to establish final epithelial polarity.