Analysis of the angulin family consisting of LSR, ILDR1 and ILDR2: tricellulin recruitment, epithelial barrier function and implication in deafness pathogenesis

Abstract

Tricellular tight junctions (tTJs) seal the extracellular space at tricellular contacts (TCs), where the corners of three epithelial cells meet. To date, the transmembrane proteins tricellulin and lipolysis-stimulated lipoprotein receptor (LSR) are known to be molecular components of tTJs. LSR recruits tricellulin to tTJs, and both proteins are required for the full barrier function of epithelial cellular sheets. Here, we show that two LSR-related proteins, immunoglobulin-like domain-containing receptor (ILDR)1 and ILDR2, are also localized at TCs and recruit tricellulin. The expressions of LSR, ILDR1 and ILDR2 were complementary in various epithelial cell types, although LSR and ILDR1 were coexpressed in some epithelia. ILDR1 was required for the establishment of a strong barrier of the epithelium, similar to LSR, when introduced into cultured epithelial cells, while ILDR2 provided a much weaker barrier. We further analyzed human ILDR1, whose mutations cause a familial deafness, DFNB42, and found that most DFNB42-associated ILDR1 mutant proteins were defective in recruitment of tricellulin. We also found that tricellulin mutant proteins associated with another familial deafness, DFNB49, were not recruited to TCs by ILDR1. These findings show the heterogeneity of the molecular organization of tTJs in terms of the content of LSR, ILDR1 or ILDR2, and suggest that ILDR1-mediated recruitment of tricellulin to TCs is required for hearing. Given their common localization at epithelial cell corners and recruitment of tricellulin, we propose to designate LSR, ILDR1 and ILDR2 as angulin family proteins.