Molecular organization of tricellular tight junctions

Within an epithelial cellular sheet, the paracellular pathway can be divided into two routes: one between two adjacent cells and one at tricellular contacts, where the vertices of three cells meet. For epithelial barrier function, tight junctions restrict solute permeability through the paracellular pathway between two cells, while tricellular contacts contain specialized structures of tight junctions, named tricellular tight junctions (tTJs). Two types of membrane proteins, tricellulin and angulin family proteins (angulin-1/LSR, angulin-2/ILDR1 and angulin-3/ILDR2) have been identified as molecular components of tTJs. Angulins recruit triellulin to tTJs and these tTJ-associated proteins are required for normal tTJ formation as well as strong epithelial barrier function. Furthermore, mutations in tricellulin and angulin-2/ILDR1 genes cause autosomal recessive familial deafness, DFNB49 and DFNB42, respectively. Further analyses of the angulin-tricellulin system should lead to better understanding of the molecular mechanism and regulation of tTJs.