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

Abstract

SummaryTricellular 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. In the present study, we show that two LSR-related proteins, immunoglobulin-like domain-containing receptor (ILDR) 1 and ILDR2, are also localized at TCs and recruit tricellulin. At least one of LSR, ILDR1 and ILDR2 was expressed in most of the epithelial tissues in mice. The expressions of LSR, ILDR1 and ILDR2 were generally complementary to each other, although LSR and ILDR1 were co-expressed 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, whereas ILDR2 provided a much weaker barrier. We further analyzed human ILDR1, mutations in which 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.

Localization of Angulin-1/LSR and Tricellulin at Tricellular Contacts of Brain and Retinal Endothelial Cells <i>in vivo</i>

“…Two types of integral membrane proteins, tricellulin (Ikenouchi et al, 2005) and angulin family proteins, which include angulin-1/LSR, angulin-2/ILDR1, and angulin-3/ILDR2 (Higashi et al, 2013;Masuda et al, 2011), have been identified as specific molecular components of tTJs. Tricellulin is a member of the TAMP family containing occludin with four transmembrane domains (Ikenouchi et al, 2005;Raleigh et al, 2010;Steed et al, 2009), while angulins are singlepass membrane proteins with an Ig-like domain (Masuda et al, 2011;Higashi et al, 2013). Notably, angulins recruit tricellulin to TCs and at least one of the angulin family members is expressed in each epithelium with tricellulin (Higashi et al, 2013;Masuda et al, 2011).…”

“…Tricellulin is a member of the TAMP family containing occludin with four transmembrane domains (Ikenouchi et al, 2005;Raleigh et al, 2010;Steed et al, 2009), while angulins are singlepass membrane proteins with an Ig-like domain (Masuda et al, 2011;Higashi et al, 2013). Notably, angulins recruit tricellulin to TCs and at least one of the angulin family members is expressed in each epithelium with tricellulin (Higashi et al, 2013;Masuda et al, 2011). shRNA-mediated depletion of tricellulin or angulin-1/LSR in cultured epithelial cells revealed that these molecules are required for normal tTJ formation as well as the full barrier function of epithelial cellular sheets (Higashi et al, 2013;Ikenouchi et al, 2005;Masuda et al, 2011).…”

“…Notably, angulins recruit tricellulin to TCs and at least one of the angulin family members is expressed in each epithelium with tricellulin (Higashi et al, 2013;Masuda et al, 2011). shRNA-mediated depletion of tricellulin or angulin-1/LSR in cultured epithelial cells revealed that these molecules are required for normal tTJ formation as well as the full barrier function of epithelial cellular sheets (Higashi et al, 2013;Ikenouchi et al, 2005;Masuda et al, 2011). Therefore, it is reasonable to consider that the concentrated localization of a combination of tricellulin and angulins at TCs is a good indicator of tTJs, despite their original definition by freeze-fracture replica electron microscopy.…”

29 Immunoglobulin-Like Domain Containing Receptor Mediates Fat-Stimulated Cholecystokinin Secretion

“…Tricellulin is expressed in the inner ear, and mutations in this gene have been linked to DFNB49 (48), suggesting that the pathogenesis of this impairment may involve aberrations in LSR-promoted tricellular tight junction formation, which require proper targeting of tricellulin. Similar to LSR, it now appears that ILDR is also involved in targeting of tricellulin to the tight junctions in the inner ear and elsewhere in the body (49). There is a growing body of evidence to suggest that the lipoprotein receptors possess multiligand, multifunctional capabilities (50).…”

ILDR1 is important for paracellular water transport and urine concentration mechanism

“…The tricellular tight junction (tTJ) is a specialized cell junction different in ultrastructure from that of the bTJ (5). Unlike the bTJ, which is made of claudin and occludin (6), the proteins making the tTJ include tricellulin and angulins (LSR/angulin-1, ILDR1/angulin-2, and ILDR2/angulin-3) (7,8). Transgenic knockout (KO) of either tricellulin or Ildr1 in mice causes hearing loss due to degeneration of mechanosensory cochlear hair cells in the organ of Corti (9, 10).…”