[Physiological Functions of Claudins].

Tight junctions(TJs)are a type of intercellular junction that present in vertebrate epithelial and endothelial cells, and contribute to the barrier function of the epithelium and endothelium by limiting the leakage of water-soluble molecules through the intercellular spaces. Claudins are membrane proteins that play a central role in the structure and function of TJs. Claudins form the claudin family, which consists of more than 20 subtypes. Most epithelial cells express multiple claudin subtypes, and different cell types express different combinations of claudins. Importantly, claudin subtypes can be classified into barrier-forming types, which form TJs that block electrolyte permeation, and pore-forming types, which form small pores in the TJs that allow electrolyte permeation. Different combinations of claudins create functional diversity of TJs, tuning the permeability of the intercellular space according to the physiological requirement for each epithelium. Analyses of the phenotype of various claudin gene knockout mice and genetic diseases caused by claudin genes mutations have led to a better understanding of the organ-level function of TJs and the pathologies caused by TJ dysfunction.