A novel membrane protein Hoka regulates septate junction organization and stem cell homeostasis in the Drosophila gut

Smooth septate junctions (sSJs) regulate the paracellular transport in the intestinal and renal system in arthropods. In Drosophila, the organization and physiological function of sSJs are regulated by at least three sSJ-specific membrane proteins: Ssk, Mesh, and Tsp2A. Here, we report a novel sSJ membrane protein Hoka, which has a single membrane-spanning segment with a short extracellular region having 13-amino acids, and a cytoplasmic region with three repeats of the Tyr-Thr-Pro-Ala motif. The larval midgut in hoka-mutants shows a defect in sSJ structure. Hoka forms a complex with Ssk, Mesh, and Tsp2A and is required for the correct localization of these proteins to sSJs. Knockdown of hoka in the adult midgut leads to intestinal barrier dysfunction, stem cell overproliferation, and epithelial tumors. In hoka-knockdown midguts, aPKC is up-regulated in the cytoplasm and the apical membrane of epithelial cells. The depletion of aPKC and yki in hoka-knockdown midguts results in reduced stem cell overproliferation. These findings indicate that Hoka cooperates with the sSJ-proteins Ssk, Mesh, and Tsp2A to organize sSJs, and is required for maintaining intestinal stem cell homeostasis through the regulation of aPKC and Yki activities in the Drosophila midgut. Summary statementDepletion of hoka, a gene encoding a novel septate junction protein, from the Drosophila midgut results in the disruption of septate junctions, intestinal barrier dysfunction, stem cell overproliferation, and epithelial tumors.