The specialization of septate junctions in regions of tricellular junctions

SummaryEstablishment and maintenance of permeability barriers is one of the most important functions of epithelial cells. Tricellular junctions (TCJs) maintain the permeability barriers at the contact site of three epithelial cells. Gliotactin, a member of the Neuroligin family, is the only known Drosophila protein exclusively localized to the TCJ and is necessary for maintenance of the permeability barrier. Overexpression triggers the spread of Gliotactin away from the TCJ and causes epithelial cells to delaminate, migrate and die. Furthermore, excess Gliotactin at the cell membrane results in an extensive downregulation of Discs large (Dlg) at the septate junctions. The intracellular domain of Gliotactin contains two highly conserved tyrosine residues and a PDZ binding motif. We previously found that phosphorylation of the tyrosine residues is necessary to control the level of Gliotactin at the TCJ. In this study we demonstrate that the phenotypes associated with excess Gliotactin are due to a functional interaction between Gliotactin and Dlg that is dependent on both tyrosine phosphorylation as well as the PDZ binding motif. We further show that elevated levels of Dlg strongly enhance Gliotactin overexpression phenotypes to the point where tissue over-growth is observed. The exhibition of these phenotypes require phosphorylation of Dlg on serine 797, a known Par1 phosphorylation target. Blocking this phosphorylation completely suppresses the cell invasiveness and apoptotic phenotypes associated with Gliotactin overexpression. Additionally, we show that Drosophila JNK acts downstream of Gliotactin and Dlg to mediate the overgrowth and apoptosis caused by the functional interaction of Gliotactin and Dlg.

Section: Introductionmentioning

confidence: 99%

Gliotactin and Discs large are co-regulated to maintain epithelial integrity

Barmchi

Charish

Que

et al. 2013

“…In epithelial cells of invertebrates, permeability barriers are formed by pleated septate junctions (SJs), which are functionally analogous to tight junctions in vertebrates (Noirot-Timothee et al, 1982;Tsukita et al, 2001). In Drosophila, SJs form ladder-like junctions between cells in a region just basal to the adherens junctions.…”

Section: Introductionmentioning

confidence: 99%

“…Analysis of epithelial cells with electron microscopy has shown that the continuity of SJ strands is interrupted at the convergence point of three cells (Fristrom, 1982;Noirot-Timothee et al, 1982). This junction, known as the tricellular junction (TCJ), is also vital to create an intact permeability barrier to ensure that solutes fail to flow in the channel formed between the three cell corners.…”

Section: Introductionmentioning

confidence: 99%

Control of Gliotactin localization and levels by tyrosine phosphorylation and endocytosis is necessary for survival of polarized epithelia

Barmchi

Browne

Sturgeon

et al. 2010

SummaryThe tricellular junction (TCJ) forms at the convergence of bicellular junctions from three adjacent cells in polarized epithelia and is necessary for maintaining the transepithelial barrier. In the fruitfly Drosophila, the TCJ is generated at the meeting point of bicellular septate junctions. Gliotactin was the first identified component of the TCJ and is necessary for TCJ and septate junction development. Gliotactin is a member of the neuroligin family and associates with the PDZ protein discs large. Beyond this interaction, little is known about the mechanisms underlying Gliotactin localization and function at the TCJ. In this study, we show that Gliotactin is phosphorylated at conserved tyrosine residues, a process necessary for endocytosis and targeting to late endosomes and lysosomes for degradation. Regulation of Gliotactin levels through phosphorylation and endocytosis is necessary as overexpression results in displacement of Gliotactin away from the TCJ throughout the septate junction domain. Excessive Gliotactin in polarized epithelia leads to delamination, paired with subsequent migration, and apoptosis. The apoptosis and the resulting compensatory proliferation resulting from high levels of Gliotactin are mediated by the Drosophila JNK pathway. Therefore, Gliotactin levels within the cell membrane are regulated to ensure correct protein localization and cell survival.

“…EM studies determined that pleated SJs are discontinuous and interrupted at the conjunction of three cells by channels at a region termed the tricellular junction (TCJ) (Fristrom, 1982;Noirot-Timothee et al, 1982;Walker et al, 1985). The TCJ forms where the ribbon-like septate junctions make a hairpin turn and run in an apical-basal direction.…”

Section: Introductionmentioning

confidence: 99%

“…The TCJ forms where the ribbon-like septate junctions make a hairpin turn and run in an apical-basal direction. The permeability barrier at this junction is created by a series of plugs or diaphragms that span the junction and are presumably held in position by interaction with the descending septa from the SJ (Fristrom, 1982;Noirot-Timothee et al, 1982). Gliotactin was the first protein shown to specifically localize to this region in Drosophila (Schulte et al, 2003).…”

Section: Introductionmentioning

confidence: 99%

Gliotactin and Discs large form a protein complex at the tricellular junction of polarized epithelial cells inDrosophila

Schulte

Charish

Que

et al. 2006

The tricellular junction (TCJ) forms at the convergence of pleated septate junctions (SJs) from three adjacent cells in polarized epithelia and is necessary for maintaining the transepithelial barrier. In Drosophila, the transmembrane protein Gliotactin was the first identified marker of the TCJ, but little is known about other molecular constituents. We now show that Gliotactin associates with Discs large at the TCJ in a Ca2+-dependent manner. Discs large is essential for the formation of the TCJ and the localization of Gliotactin. Surprisingly, Gliotactin localization at the TCJ was independent of its PDZ-binding motif and Gliotactin did not bind directly to Discs large. Therefore Gliotactin and Discs large association is through intermediary proteins at the TCJ. Gliotactin can associate with other septate junction proteins but this was detected only when Gliotactin was overexpressed and spread throughout the septate junction domain. Gliotactin overexpression and spread also resulted in a reduction of Discs large staining but not vice versa. These results suggest that Discs large participates in different protein interactions in the SJ and the TCJ. Finally this work supports a model where Gliotactin and Dlg are components of a larger protein complex that links the converging SJs with the TCJ to create the transepithelial barrier.