The Drosophila tricellular junction protein Gliotactin regulates its own mRNA levels through BMP-mediated induction of miR-184

Abstract

Epithelial bicellular and tricellular junctions are essential for establishing and maintaining permeability barriers. Tricellular junctions are formed by the convergence of three bicellular junctions at the corners of neighbouring epithelia. Gliotactin, a member of the Neuroligin family, is located to the Drosophila tricellular junction and is critical for the formation of tricellular and septate junctions and permeability barrier function. Gliotactin protein levels are tightly controlled by tyrosine phosphorylation and endocytosis. Blocking endocytosis or overexpression of Gliotactin triggers spread away from the tricellular junction, resulting in apoptosis, delamination and migration of epithelial cells. We show that Gliotactin levels are also regulated at the mRNA level by microRNA-mediated degradation targeted to a short region in the 3'UTR that includes a conserved miR-184 target site. miR-184 also targets a suite of septate junction proteins including Neurexin-IV, coracle and Mcr. miR-184 expression is triggered when Gliotactin is overexpressed leading to activation of the BMP signaling pathway. Gliotactin specifically interferes with Dad, an inhibitory SMAD, leading to activation of the Tkv type-I receptor, and Mad to elevate the biogenesis and expression of miR-184.

Transcriptional dynamics of microRNAs and their targets during Drosophila neurogenesis

“…The highly conserved mir-184 is the most abundant microRNA in neurons and glia and the third most abundant in neuroblasts. It has previously been found to be highly expressed in the early embryo and the following stages [30] and is part of a regulatory network controlling adult neural stem/progenitor cell differentiation [38], but is also involved in inhibiting the overexpression of Glioactin [39]. Our comparison between the three embryonic neural cell types reveals that mir-184 is up-regulated in glia compared with neuroblasts and neurons at both time points.…”

C-terminal Src kinase (Csk) regulates the tricellular junction protein Gliotactin independent of Src

“…The level and the correct localization of Gliotactin is tightly controlled by phosphorylation followed by endocytosis ( Padash-Barmchi et al ., 2010 ) and microRNA-mediated degradation of Gliotactin mRNA ( Sharifkhodaei et al ., 2016 ). When overexpressed, the spread of Gliotactin away from the TCJ leads to aberrant phenotypes disrupting the entire epithelial architecture.…”

“…Overall, loss-of-function and overexpression experiments show that the level and the localization of Gliotactin to the TCJ must be tightly controlled to ensure proper barrier function and normal epithelial development. One mechanism of regulation is through the Gliotactin mRNA, which is degraded by miR-184, in response to a feedback loop controlled by bone morphogenetic protein (BMP) signaling ( Sharifkhodaei et al ., 2016 ). Another mechanism is employed at the Gliotactin protein level, where the phosphorylation of two highly conserved tyrosine residues leads to endocytosis and lysosome-mediated degradation ( Padash-Barmchi et al ., 2010 ).…”

Identification of the septate junction protein gliotactin in the mosquito, Aedes aegypti: evidence for a role in increased paracellular permeability in larvae

“…Lack of correlation between mRNA transcript and protein abundance has been recently reported for A. aegypti Kune (Jonusaite et al, 2016b), and such a phenomenon may occur as a result of mRNAregulatory mechanisms or differences in protein degradation rate (Fournier et al, 2010). Indeed, in addition to tight control of D. melanogaster Gli protein levels by tyrosine phosphorylation and endocytosis (Padash-Barmchi et al, 2010, Gli levels are also regulated at the mRNA level by microRNA-mediated degradation (Sharifkhodaei et al, 2016). Our observation of increased Gli abundance in A. aegypti larval anterior midgut and Malpighian tubules in response to salinity (see Fig.…”