Molecular perspective on tight-junction assembly and epithelial polarity

Apical-basal polarity and a highly organized actin cytoskeleton are main characteristics of epithelial cells that support exchange of ions and nutrients from one body compartment to another. The junctional complexes, localized to the apical end of the basolateral domain of the plasma membrane, are not simply epithelial barriers in paracellular transport or fences preventing diffusion of integral proteins in the plasma membrane, but also contain proteins involved in the maintenance of the physiologic epithelial cell state and signal transduction. Claudin-based tight junctions and E-cadherin-based adherens junctions have been extensively studied. Nectins, along with a unique scaffolding protein, afadin, form homophilic and heterophilic trans-dimers and play a key role in identifying cell partners in the primordial cell-cell adhesion. Nectin-based cell-cell adhesion participates in the epithelial morphogenesis, both independently and cooperatively with claudin-based tight junctions and cadherin-based adherens junctions. This review discusses how these adhesion systems interact with each other to form apical junctional complexes, and how they reorganize the actin cytoskeleton in a multistage process of cell adhesion, migration, and polarization.