LSR defines cell corners for tricellular tight junction formation in epithelial cells

Abstract

SummaryEpithelial cell contacts consist of not only bicellular contacts but also tricellular contacts, where the corners of three cells meet. At tricellular contacts, tight junctions (TJs) generate specialized structures termed tricellular TJs (tTJs) to seal the intercellular space. Tricellulin is the only known molecular component of tTJs and is involved in the formation of tTJs, as well as in the normal epithelial barrier function. However, the detailed molecular mechanism of how tTJs are formed and maintained remains elusive. Using a localization-based expression cloning method, we identified a novel tTJ-associated protein known as lipolysis-stimulated lipoprotein receptor (LSR). Upon LSR knockdown in epithelial cells, tTJ formation was affected and the epithelial barrier function was diminished. Tricellulin accumulation at the tricellular contacts was also diminished in these cells. By contrast, LSR still accumulated at the tricellular contacts upon tricellulin knockdown. Analyses of deletion mutants revealed that the cytoplasmic domain of LSR was responsible for the recruitment of tricellulin. On the basis of these observations, we propose that LSR defines tricellular contacts in epithelial cellular sheets by acting as a landmark to recruit tricellulin for tTJ formation.

The Blood–Brain Barrier

“…Recently, it has been shown that unique molecular components are required to seal the paracellular barrier at the contact points of three cells. These tricellular junctional complexes are made up of lipolysis-stimulated receptor (LSR), which is required to localize marveld2 to tricellular adhesions (Masuda et al 2011). It remains unclear what the nature of the size/charge-selective pore is formed by the specific composition tight junction proteins expressed by CNS ECs, and whether the permeability of this pore is static or whether it is dynamically altered in response to neuronal activity.…”

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.…”

Recent Advances in Claudin-Targeting Technology

“…Whereas bicellular TJs contain biochemical complexes including occludin, claudins (cLs), and junctional adhesion molecules, 5) tricellular TJs include at least tricellulin and members of the angulin family. [6][7][8] Pharmacologic and pathologic research has revealed that cLs offer considerable potential as targets for the delivery of various therapeutic compounds. For instance, cL-1-knockout mice showed increased epidermal permeability to solutes.…”