The Claudins: From Tight Junctions to Biological Systems.

Claudins are cell-cell adhesion molecules located at the tight junctions (TJs) between cells in epithelial cell sheets. The claudin family in mammals consists of 27 four-transmembrane domain proteins. Claudins are responsible for the paracellular barrier function of TJs, and in some cases confer paracellular channel functions to the paracellular barriers of TJs. Based on recent breakthroughs in the molecular structure of claudins, the hypothetical 'antiparallel double row model' was proposed, which suggests how claudins polymerize in a linear fashion and form TJ strands with paracellular barrier and channel functions. Meanwhile, ongoing studies at the cell and tissue levels are clarifying how the paracellular barrier and/or channel functions of claudin-based TJs, which are both robust and flexible, organize various biological systems.