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UBC Theses and Dissertations

Tricellular junction regulation, signaling and scaffolding Sharifkhodaei, Zohreh


The focus of this thesis is to understand the regulatory mechanisms of that control permeability barriers in epithelia. A key role of epithelia is to maintain permeability barriers between tissues. Epithelial junctions are formed to establish a functional barrier between the cells and to ensure cell-cell adhesion. In Drosophila, the tricellular junction (TCJ) generates a barrier at the contact site of three adjacent epithelial cells. Gliotactin is localized exclusively at the corner of three epithelial cells and loss of Gliotactin from the TCJ disrupts the barrier function. Conversely, overexpression of Gliotactin triggers the spread of Gliotactin away from the TCJ leading to apoptosis, delamination, overproliferation and cell migration. Therefore, the expression level of Gliotactin needs to be tightly regulated. Gliotactin protein levels are controlled by tyrosine phosphorylation and subsequent protein endocytosis and degradation. Here we found that Gliotactin expression is also tightly regulated at the mRNA level through microRNA-184. miR-184 targets the Gliotactin 3’UTR and other septate junction mRNAs including NrxIV and Mcr. Gliotactin overexpression triggers BMP signaling through inhibition of Dad, an inhibitory SMAD, and activation of the Tkv type-I receptor and Mad. Elevated level of phosphorylated MAD leads to induction of miR-184 expression. Regulation of Gliotactin at the TCJ is mediated through a Gliotactin-BMP-miR184 feedback loop. We identified a new complex at the TCJ, which regulates junction assembly and function. The scaffolding proteins Scribbled (Scrib) and Discs Large (Dlg) are in close proximity with two TCJ components, Gliotactin and Bark beetle (Bark). The presence of the Scrib PDZ1-2 and the Dlg GUK domains are required for proper formation of the TCJ complex. Loss of Bark or Gliotactin from the TCJ leads to basolateral spread of Scrib and Dlg, while Scrib or Dlg knockdown disrupts the integrity of the complex and promotes the loss of Bark or Gliotactin from the TCJ. Our proposed model suggests that Scrib and Dlg recruit Bark to the TCJ, which in turn leads to Gliotactin recruitment to the TCJ. Overall, we propose that tricellular junction is regulated through two distinct mechanisms, signaling and scaffolding.

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