Loss of occludin affects tricellular localization of tricellulin

The tricellular tight junction (tTJ) forms at the convergence of bicellular tight junctions (bTJs) where three epithelial cells meet in polarized epithelia, and it is required for the maintenance of the transepithelial barrier. Tricellulin is a four transmembrane domain protein recently identified as the first marker of tTJ, but little is known about how tricellulin is localized at tTJs. As for the molecular mechanism of association of tricellulin with tight junctions (TJs), we found that tricellulin was incorporated into claudin-based TJs independently of binding to zona occludens-1. Unexpectedly, exogenous expression of tricellulin increased cross-links of TJ strands in the plasma membrane. As for the molecular mechanisms for localization of tricellulin at tricellular junctions, we found that knockdown of occludin caused mislocalization of tricellulin to bTJs, implying that occludin supports tricellular localization of tricellulin by excluding tricellulin from bTJs.     

Tricellulin Forms a Barrier to Macromolecules in Tricellular Tight Junctions without Affecting Ion Permeability

Tricellulin is a tight junction protein localized in tricellular tight junctions (tTJs), the meeting points of three cells, but also in bicellular tight junctions (bTJs). To investigate its specific barrier functions in bTJs and tTJs, TRIC-a was expressed in low-level tricellulin–expressing cells, and MDCK II, either in all TJs or only in tTJs. When expressed in all TJs, tricellulin increased paracellular electrical resistance and decreased permeability to ions and larger solutes, which are associated with enhanced ultrastructural integrity of bTJs toward enhanced strand linearity. In tTJs in contrast, ultrastructure was unchanged and tricellulin minimized permeability to macromolecules but not to ions. This paradox is explained by properties of the tTJ central tube which is wide enough for passage of macromolecules, but too rare to contribute significantly to ion permeability. In conclusion, at low tricellulin expression the tTJ central tube forms a pathway for macromolecules. At higher expression, tricellulin forms a barrier in tTJs effective only for macromolecules and in bTJs for solutes of all sizes.     

Deficiency of Angulin-2/ILDR1, a Tricellular Tight Junction-Associated Membrane Protein,Causes Deafness with Cochlear Hair Cell Degeneration in Mice

Tricellular tight junctions seal the extracellular spaces of tricellular contacts, where the vertices of three epithelial cells meet, and are required for the establishment of a strong barrier function of the epithelial cellular sheet. Angulins and tricellulin are known as specific protein components of tricellular tight junctions, where angulins recruit tricellulin. Mutations in the genes encoding angulin-2/ILDR1 and tricellulin have been reported to cause human hereditary deafness DFNB42 and DFNB49, respectively. To investigate the pathogenesis of DFNB42, we analyzed mice with a targeted disruption of Ildr1, which encodes angulin-2/ILDR1. Ildr1 null mice exhibited profound deafness. Hair cells in the cochlea of Ildr1 null mice develop normally, but begin to degenerate by two weeks after birth. Tricellulin localization at tricellular contacts of the organ of Corti in the cochlea was retained in Ildr1 null mice, but its distribution along the depth of tricellular contacts was affected. Interestingly, compensatory tricellular contact localization of angulin-1/LSR was observed in the organ of Corti in Ildr1 null mice although it was hardly detected in the organ of Corti in wild-type mice. The onset of hair cell degeneration in Ildr1 null mice was earlier than that in the reported Tric mutant mice, which mimic one of the tricellulin mutations in DFNB49 deafness. These results indicate that the angulin-2/ILDR1 deficiency causes the postnatal degenerative loss of hair cells in the cochlea, leading to human deafness DFNB42. Our data also suggest that angulin family proteins have distinct functions in addition to their common roles of tricellulin recruitment and that the function of angulin-2/ILDR1 for hearing cannot be substituted by angulin-1/LSR.     

Angulin-1 seals tricellular contacts independently of tricellulin and claudins

Tricellular tight junctions (tTJs) are specialized tight junctions (TJs) that seal the intercellular space at tricellular contacts (TCs), where the vertices of three epithelial cells meet. Tricellulin and angulin family membrane proteins are known constituents of tTJs, but the molecular mechanism of tTJ formation remains elusive. Here, we investigated the roles of angulin-1 and tricellulin in tTJ formation in MDCK II cells by genome editing. Angulin-1–deficient cells lost the plasma membrane contact at TCs with impaired epithelial barrier function. The C terminus of angulin-1 bound to the TJ scaffold protein ZO-1, and disruption of their interaction influenced the localization of claudins at TCs, but not the tricellular sealing. Strikingly, the plasma membrane contact at TCs was formed in tricellulin- or claudin-deficient cells. These findings demonstrate that angulin-1 is responsible for the plasma membrane seal at TCs independently of tricellulin and claudins.     

Behavior of tricellulin during destruction and formation of tight junctions under various extracellular calcium conditions

Tricellulin is an important component of tricellular tight junctions (TJs) and is involved in the formation of tricellular contacts. However, little is known about its regulation during the assembly and disassembly of tricellular TJs. By using the well-differentiated pancreatic cancer cell line HPAC, which highly expresses tricellulin at tricellular contacts, we have investigated changes in the localization, expression and phosphorylation of tricellulin and in its TJ functions as a barrier and fence during the destruction and formation of TJs induced by changes in the extracellular calcium concentration. During both extracellular Ca²⁺ depletion caused by EGTA treatment and Ca²⁺ repletion after Ca²⁺ starvation, the expression of tricellulin increased in whole lysates and in Triton-X-100-insoluble fractions without any change in its mRNA. The increases in immunoreactivity revealed by Western blotting were prevented by alkaline phosphatase treatment. Immunoprecipitation assays showed that tricellulin was phosphorylated on threonine residues when it increased after Ca²⁺ depletion and repletion. In the early stage after Ca²⁺ repletion, tricellulin was expressed not only at tricellular contacts but also in the cytoplasm and at bicellular borders. In confocal laser microscopy, tricellulin was observed at the apical-most regions and basolateral membranes of tricellular contacts after Ca²⁺ repletion. Knockdown of tricellulin delayed the recovery of the barrier and fence functions after Ca²⁺ repletion. Thus, the dynamic behavior of tricellulin during the destruction and formation of TJs under various extracellular calcium conditions seems to be closely associated with the barrier and fence functions of TJs.     

Tricellulin is a tight-junction protein necessary for hearing

The inner ear has fluid-filled compartments of different ionic compositions, including the endolymphatic and perilymphatic spaces of the organ of Corti; the separation from one another by epithelial barriers is required for normal hearing. TRIC encodes tricellulin, a recently discovered tight-junction (TJ) protein that contributes to the structure and function of tricellular contacts of neighboring cells in many epithelial tissues. We show that, in humans, four different recessive mutations of TRIC cause nonsyndromic deafness (DFNB49), a surprisingly limited phenotype, given the widespread tissue distribution of tricellulin in epithelial cells. In the inner ear, tricellulin is concentrated at the tricellular TJs in cochlear and vestibular epithelia, including the structurally complex and extensive junctions between supporting and hair cells. We also demonstrate that there are multiple alternatively spliced isoforms of TRIC in various tissues and that mutations of TRIC associated with hearing loss remove all or most of a conserved region in the cytosolic domain that binds to the cytosolic scaffolding protein ZO-1. A wild-type isoform of tricellulin, which lacks this conserved region, is unaffected by the mutant alleles and is hypothesized to be sufficient for structural and functional integrity of epithelial barriers outside the inner ear.     

Tricellulin expression in brain endothelial and neural cells

Tricellulin is a tight junction (TJ) protein, which is not only concentrated at tricellular contacts but also present at bicellular contacts between epithelial tissues. We scrutinized the brain for tricellulin expression in endothelial and neural cells by using real-time polymerase chain reaction, Western blot and immunohistochemical and immunocytochemical analysis of cultured brain cells and paraffin sections of brain. Tricellulin mRNA was detected in primary cultures and in a cell line of human brain microvascular endothelial cells. Protein expression was confirmed by Western blot and immunofluorescence analysis, which further highlighted the localization of tricellulin in the cell membrane at tricellular and along bicellular contacts, and in the nucleus and perinuclear region. Compared with the well-studied TJ protein, zonula occludens-1, tricellulin expression was less marked at the cell membrane but more evident in the nuclear and perinuclear regions. The presence of tricellulin in cultured endothelial cells was corroborated by immunohistochemical and immunofluorescence staining in brain blood vessels, where it was colocalized with another TJ protein, claudin-5. Tricellulin mRNA was detected in neurons and astrocytes, whereas protein expression was observed in astrocytes but not in neurons, as shown by immunofluorescence analysis. This study reveals the presence and subcellular distribution of tricellulin in brain endothelial cells, both in vitro and in situ and its colocalization with other relevant TJ proteins. Moreover, it demonstrates the expression of the protein in astrocytes opening new avenues for future research to establish the biological significance of tricellulin expression in glial cells.     

A look at tricellulin and its role in tight junction formation and maintenance

Tight junctions are elaborate networks of transmembrane and cytosolic proteins that regulate epithelial permeability. Tricellulin was the first tight junction protein found at tricellular tight junctions, the specialized structures occurring where three cells meet together. Here, we summarize the current knowledge about tricellulin (marvelD2), a MARVEL domain protein. We address tricellulin location at tricellular junctions, and establish the comparison with the other members of the MARVEL family, occludin (marvelD1) and marvelD3. The structure of tricellulin and its membrane folding, as well as the proposed molecular interactions of tricellulin with other tight junction proteins, together with the interplay between those proteins are also discussed. In addition, we address the role of tricellulin in barrier properties, discriminating the involvement of the protein in paracellular permeability at bicellular and at tricellular tight junctions. Moreover, the key importance of the protein for hearing is highlighted based on the fact that mutations in TRIC, the human tricellulin gene, lead to deafness. Furthermore, this review points to some of the aspects that still deserve clarification for a better understanding of the biology of tight junctions in general and of tricellulin in particular.     

Evidence of tricellulin expression by immune cells, particularly microglia

Tight junctions (TJs) are elaborate structures located on the apical region of epithelial cells that limit paracellular permeability. Tricellulin is a recently discovered TJ protein, which is concentrated at the structurally specialized tricellular TJs but also present at bicellular contacts between epithelial cells, namely in the stomach. Interestingly, several TJ proteins have been found in other than epithelial cells, as astrocytes, and tricellulin mRNA expression was reported in mature dendritic cells. These findings prompted us to look for tricellulin expression in both epithelial and immune cells in the stomach, as well as in microglia, the brain resident immunocompetent cells. Immunohistochemical analysis of human stomach tissue sections revealed peroxidase staining at three-corner contact sites, as well as at the contact between two adjacent epithelial cells, thus evidencing the expression of tricellulin not only at tricellullar but at bicellular junctions as well. Such analysis, further revealed tricellulin immunostaining in cells of the monocyte/macrophage lineage, scattered throughout the lamina propria. Cultured rat microglia exhibited a notorious tricellulin staining, consistent with an extensive expression of the protein along the cell, which was not absolutely coincident with the lysosomal marker CD68. Detection of mRNA expression by real-time PCR provided supportive evidence for the expression of the TJ protein in microglia. These data demonstrate for the first time that microglia express a TJ protein. Moreover, the expression of tricellulin both in microglia and in the stomach immune cells point to a possible role of this new TJ protein in the immune system.     

c‐Jun N‐terminal kinase is largely involved in the regulation of tricellular tight junctions via tricellulin in human pancreatic duct epithelial cells

Tricellulin (TRIC) is a tight junction protein at tricellular contacts where three epithelial cells meet, and it is required for the maintenance of the epithelial barrier. To investigate whether TRIC is regulated via a c‐Jun N‐terminal kinase (JNK) pathway, human pancreatic HPAC cells, highly expressed at tricellular contacts, were exposed to various stimuli such as the JNK activators anisomycin and 12‐O‐tetradecanoylphorbol 13‐acetate (TPA), and the proinflammatory cytokines IL‐1β, TNFα, and IL‐1α. TRIC expression and the barrier function were moderated by treatment with the JNK activator anisomycin, and suppressed not only by inhibitors of JNK and PKC but also by siRNAs of TRIC. TRIC expression was induced by treatment with the PKC activator TPA and proinflammatory cytokines IL‐1β, TNFα, and IL‐1α, whereas the changes were inhibited by a JNK inhibitor. Furthermore, in normal human pancreatic duct epithelial cells using hTERT‐transfected primary cultured cells, the responses of TRIC expression to the various stimuli were similar to those in HPAC cells. TRIC expression in tricellular tight junctions is strongly regulated together with the barrier function via the JNK transduction pathway. These findings suggest that JNK may be involved in the regulation of tricellular tight junctions including TRIC expression and the barrier function during normal remodeling of epithelial cells, and prevent disruption of the epithelial barrier in inflammation and other disorders in pancreatic duct epithelial cells. J. Cell. Physiol. 225: 720–733, 2010. © 2010 Wiley‐Liss, Inc.     

Shigella targets epithelial tricellular junctions and uses a noncanonical clathrin-dependent endocytic pathway to spread between cells

Bacteria move between cells in the epithelium using a sequential pseudopodium-mediated process but the underlying mechanisms remain unclear. We show that during cell-to-cell movement, Shigella-containing pseudopodia target epithelial tricellular junctions, the contact point where three epithelial cells meet. The bacteria-containing pseudopodia were engulfed by neighboring cells only in the presence of tricellulin, a protein essential for tricellular junction integrity. Shigella cell-to-cell spread, but not pseudopodium protrusion, also depended on phosphoinositide 3-kinase, clathrin, Epsin-1, and Dynamin-2, which localized beneath the plasma membrane of the engulfing cell. Depleting tricellulin, Epsin-1, clathrin, or Dynamin-2 expression reduced Shigella cell-to-cell spread, whereas AP-2, Dab2, and Eps15 were not critical for this process. Our findings highlight a mechanism for Shigella dissemination into neighboring cells via targeting of tricellular junctions and a noncanonical clathrin-dependent endocytic pathway.     

Occludin and tricellulin facilitate formation of anastomosing tight-junction strand network to improve barrier function

Tight junctions (TJs) are composed of a claudin-based anastomosing network of TJ strands at which plasma membranes of adjacent epithelial cells are closely attached to regulate the paracellular permeability. Although the TJ proteins occludin and tricellulin have been known to be incorporated in the TJ strand network, their molecular functions remain unknown. Here, we established tricellulin/occludin-double knockout (dKO) MDCK II cells using a genome editing technique and evaluated the structure and barrier function of these cells. In freeze-fracture replica electron microscopy, the TJ strands of tricellulin/occludin-dKO cells had fewer branches and were less anastomosed compared with the controls. The paracellular permeability of ions and small tracers was increased in the dKO cells. A single KO of tricellulin or occludin had limited effects on the morphology and permeability of TJs. Mathematical simulation using a simplified TJ strand network model predicted that reduced cross-links in TJ strands lead to increased permeability of ions and small macromolecules. Furthermore, overexpression of occludin increased the complexity of TJ strand network and strengthened barrier function. Taken together, our data suggest that tricellulin and occludin mediate the formation and/or stabilization of TJ-strand branching points and contribute to the maintenance of epithelial barrier integrity.     

Downsloping High-Frequency Hearing Loss Due to Inner Ear Tricellular Tight Junction Disruption by a Novel ILDR1 Mutation in the Ig-Like Domain

The immunoglobulin (Ig)-like domain containing receptor 1 (ILDR1) gene encodes angulin-2/ILDR1, a recently discovered tight junction protein, which forms tricellular tight junction (tTJ) structures with tricellulin and lipolysis-stimulated lipoprotein receptor (LSR) at tricellular contacts (TCs) in the inner ear. Previously reported recessive mutations within ILDR1 have been shown to cause severe to profound nonsyndromic sensorineural hearing loss (SNHL), DFNB42. Whole-exome sequencing of a Korean multiplex family segregating partial deafness identified a novel homozygous ILDR1 variant (p.P69H) within the Ig-like domain. To address the pathogenicity of p.P69H, the angulin-2/ILDR1 p.P69H variant protein, along with the previously reported pathogenic ILDR1 mutations, was expressed in angulin-1/LSR knockdown epithelial cells. Interestingly, partial mislocalization of the p.P69H variant protein and tricellulin at TCs was observed, in contrast to a severe mislocalization and complete failure of tricellulin recruitment of the other reported ILDR1 mutations. Additionally, three-dimensional protein modeling revealed that angulin-2/ILDR1 contributed to tTJ by forming a homo-trimer structure through its Ig-like domain, and the p.P69H variant was predicted to disturb homo-trimer formation. In this study, we propose a possible role of angulin-2/ILDR1 in tTJ formation in the inner ear and a wider audiologic phenotypic spectrum of DFNB42 caused by mutations within ILDR1.     

Shigella navigates tight corners

In this issue, Fukumatsu and colleagues (2012) find that Shigella preferentially spread from cell-to-cell at unique intercellular junctions. Shigella protrusions invade adjacent cells at junctions where three cells meet, the tricellular junction. The tight junction protein tricellulin marks these sites and is important for Shigella spread.     

γ-Tubulin-like molecules in the mouse duodenal epithelium

A mouse monoclonal antibody (G9, Horio et al. in Cell Motil Cytoskel 44:284–295, 1999) that was raised against the γ-tubulin from a fission yeast, Schizosaccharomyces pombe, showed a unique staining in the mouse small intestine. Similar to another anti-γ-tubulin antibody that is commercially available, G9 showed typical dot-like staining corresponding to the microtubule-organizing center in the free cells of the epithelium and the connective tissue under it. In addition, G9 stained the cell–cell contacts in the epithelium. This stained region was not bicellular but tricellular junctions of the enterocytes. This staining was unique to G9 and was diminished on the sample of the mouse small intestine, which had lost most of its filamentous microtubules through the preparation process. The tricellular junction is thought to be the weakest point of the epithelial barrier, and no other junctional structures have been identified except for the central sealing elements extending from the tight junctions between the two cells. Our results suggest the existence of a new molecule underlying the tricellular junctions, which may relate to γ-tubulin and the microtubules.     

Septate junctions in imaginal disks of Drosophila: a model for the redistribution of septa during cell rearrangement

The organization of septate junctions during morphogenesis of imaginal disks is described from freeze-fracture replicas and thin sections with a view to understanding junction modulation during rearrangements of cells in epithelia. The septate junctions of each epithelial cell of the disk are distributed in a number of discrete domains equal to the number of neighboring cells. Individual septa traverse domains of contact between pairs of adjacent cells, turn downwards at the lateral boundary of the domain and run parallel to the intersection with a third cell. This arrangement leaves small channels at three-cell intersections that are occupied by specialized structures termed "tricellular plugs." Cell rearrangement involves a progressive change in the width of contact domains between adjacent cells, until old contacts are broken and new ones established. It is proposed that the septate junction adjusts to the changing width of domains by the compaction or extension of existing septa. This redistribution of septa theoretically allows a transepithelial barrier to be maintained during cell rearrangements. The applicability of this model to other epithelial tissues is discussed.     

Remodeling the zonula adherens in response to tension and the role of afadin in this response

Morphogenesis requires dynamic coordination between cell–cell adhesion and the cytoskeleton to allow cells to change shape and move without losing tissue integrity. We used genetic tools and superresolution microscopy in a simple model epithelial cell line to define how the molecular architecture of cell–cell zonula adherens (ZA) is modified in response to elevated contractility, and how these cells maintain tissue integrity. We previously found that depleting zonula occludens 1 (ZO-1) family proteins in MDCK cells induces a highly organized contractile actomyosin array at the ZA. We find that ZO knockdown elevates contractility via a Shroom3/Rho-associated, coiled-coil containing protein kinase (ROCK) pathway. Our data suggest that each bicellular border is an independent contractile unit, with actin cables anchored end-on to cadherin complexes at tricellular junctions. Cells respond to elevated contractility by increasing junctional afadin. Although ZO/afadin knockdown did not prevent contractile array assembly, it dramatically altered cell shape and barrier function in response to elevated contractility. We propose that afadin acts as a robust protein scaffold that maintains ZA architecture at tricellular junctions.     

Fat2 acts through the WAVE regulatory complex to drive collective cell migration during tissue rotation

Directional cell movements during morphogenesis require the coordinated interplay between membrane receptors and the actin cytoskeleton. The WAVE regulatory complex (WRC) is a conserved actin regulator. Here, we found that the atypical cadherin Fat2 recruits the WRC to basal membranes of tricellular contacts where a new type of planar-polarized whip-like actin protrusion is formed. Loss of either Fat2 function or its interaction with the WRC disrupts tricellular protrusions and results in the formation of nonpolarized filopodia. We provide further evidence for a molecular network in which the receptor tyrosine phosphatase Dlar interacts with the WRC to couple the extracellular matrix, the membrane, and the actin cytoskeleton during egg elongation. Our data uncover a mechanism by which polarity information can be transduced from a membrane receptor to a key actin regulator to control collective follicle cell migration during egg elongation. 4D-live imaging of rotating MCF10A mammary acini further suggests an evolutionary conserved mechanism driving rotational motions in epithelial morphogenesis.