Paracellular barrier and channel functions of TJ claudins in organizing biological systems: Advances in the field of barriology revealed in knockout mice

Claudin was first identified as a four-transmembrane protein in the tight junctions (TJs) between epithelial cells. The claudin family has 27 members, which are specifically expressed depending on the epithelial cell type. Accumulating evidence has revealed that claudins are responsible for the paracellular barrier that prevents molecules from passing through epithelial cell sheets. In addition, the extracellular domains of some claudins enable them to act as a permselective paracellular channel for specific molecules, including ions and/or non-ionic solutes. Recent studies using claudin knockout mice revealed that the loss of claudins' specific paracellular barrier and/or channel functions affects specific biological functions and leads to pathological states. In this review, considering recent findings in vivo, we describe how, sometimes in concert with canonical transporters and channels, the paracellular barrier and channel functions of claudins sophisticatedly organize biological systems.