Overexpressed Gliotactin activates BMP signaling through interfering with the Tkv–Dad association

Epithelial junctions ensure cell–cell adhesion and establish permeability barriers between cells. At the corners of epithelia, the tricellular junction (TCJ) is formed by three adjacent epithelial cells and generates a functional barrier. In Drosophila, a key TCJ protein is Gliotactin (Gli) where loss of Gli disrupts barrier formation and function. Conversely, overexpressed Gli spreads away from the TCJ and triggers apoptosis, delamination, and cell migration. Thus, Gli protein levels are tightly regulated and by two mechanisms, at the protein levels by tyrosine phosphorylation and endocytosis and at the mRNA level through microRNA-184. Regulation of Gli mRNA is mediated through a Gli–BMP–miR184 feedback loop. Excessive Gli triggers BMP signaling pathway through the activation of Tkv type-I BMP receptor and Mad. Elevated level of pMad induces micrRNA-184 expression which in turn targets the Gli 3′UTR and mRNA degradation. Gli activation of Tkv is not through its ligand Dpp but rather through the inhibition of Dad, an inhibitory-Smad. Here, we show that ectopic expression of Gli interferes with Tkv–Dad association by sequestering Dad away from Tkv. The reduced inhibitory effect of Dad on Tkv results in the increased Tkv–pMad signaling activity, and this effect is continuous through larval and pupal wing formation.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author