Ca2+-independent cell-adhesion activity of claudins, a family of integral membrane proteins localized at tight junctions

In multicellular organisms, various compositionally distinct fluid compartments are established by epithelial and endothelial cellular sheets, For these cells to function as barriers, tight junctions (TJs) are considered to create a primary barrier for the diffusion of solutes through the paracellular pathway [1-3]. In ultrathin sections viewed under electron microscopy, TJs appear as a series of apparent fusions, involving the outer leaflets of plasma membranes of adjacent cells, to form the so-called kissing points of ns, where the intercellular space is completely obliterated [4]. Claudins are a family of 16 proteins whose members have been identified as major integral membrane proteins localized exclusively at TJs [5-8]. It remains unclear, however, whether claudins have the cell adhesion activity that would explain the unusual intercellular adhesion at TJs. Using mouse L-fibroblast transfectants expressing various amounts of claudin-1, -2 or -3, we found that these claudins possess Ca2+-independent cell-adhesion activity. Using ultrathin-section electron microscopy, we observed many kissing points of TJs between adjacent transfectants. Furthermore, the cell-adhesion activity of occludin, another integral membrane protein localized at TJs [9-11], was negligible when compared with that of claudins. Thus, claudins are responsible for TJ-specific obliteration of the intercellular space.