Background: We have recently identified a novel cell-cell adhesion system, named NAP system, which is localized at cadherin-based cell-cell adherens junctions (AJs). The NAP system is composed of at least nectin, afadin and ponsin. Nectin is an immunoglobulin-like cell adhesion molecule. Afadin is an actin filament-binding protein which associates nectin with the actin cytoskeleton. Ponsin is an afadin-binding protein which furthermore binds to vinculin and provides a possible linkage of nectin-afadin to cadherin-catenin through vinculin. We compared here the behaviour of the NAP and cadherin-catenin systems during the formation and disruption of the polarized junctional alignment in epithelial cells.
Results: At the early stage of the formation of the polarized junctional alignment in MTD-1 A cells, primordial spot-like junctions were formed at the tips of thin cellular protrusions radiating from adjacent cells. Nectin, afadin, ponsin, cadherin and catenin were simultaneously recruited to these junctions. As the cell polarization proceeded, the spot-like junctions were gradually fused to form belt-like AJs where all these proteins were concentrated. The disruption of cell-cell AJs in MDCK cells by culturing at a low Ca2+ concentration caused rapid endocytosis of cadherin, but not that of nectin or afadin. Addition of 12-O-tetradecanoylphorbol-13-acetate to the cells formed a tight junction-like structure where nectin and afadin, but not cadherin, accumulated.
Conclusion: These results indicate that the NAP and cadherin-catenin systems show similar and differential behaviour during the formation and disruption of the polarized junctional alignment in epithelial cells.