Claudin-based paracellular proton barrier in the stomach.

The claudins comprise a multigene family that consists of at least 27 members. Claudins are responsible for establishing the paracellular barrier--which has permselectivity--at the tight junctions in epithelial cells, and the specific patterns of claudin expression in the epithelial cell sheets that cover the internal and external surfaces of organs contribute to the formation of microenvironments and organs' biological functions. Data on the detailed characterization of individual claudins and their roles in different microenvironments are accumulating. A study on the stomach-specific claudin-18-knockout mouse, which has gastritis, recently revealed that the stomach-type claudin-18 specifically forms the proton barrier in the stomach, consistent with previously reported circumstantial evidence. Combined with previous studies on the specific ionic homeostasis by different types of claudins, our findings support the idea that claudins may regulate ion-specific homeostasis in vivo.