The septate junction protein Mesh is required for epithelial morphogenesis, ion transport, and paracellular permeability in the Drosophila Malpighian tubule

摘要

In multicellular organisms, epithelia form sheets that cover external and internal surfaces, serving as barriers and regulating the passage of solutes and water between compartments. Transepithelial solute and water transport can occur through the transcellular pathway (i.e., across cells) or the paracellular pathway (i.e., between cells). The permeability of the paracellular pathway in the epithelia of vertebrates is controlled by tight junctions (TJs; 1, 22, 24, 27). Invertebrate epithelia generally lack TJs and instead possess septate junctions (SJs) as their occluding junctions (32). SJs form circumferential belts around the lateral regions of epithelial cells and, in cross-section electron microscopy, display a ladderlike structure between adjacent cells with septa spanning a 15-20-nm intercellular space (26). SJs are subdivided into several morphological variants that exist across invertebrate phyla, and some animals possess multiple types of SJs depending on the developmental origin of the epithelial cells (26, 33). In arthropods, two types of SJs have been described: pleated (pSJs) and smooth SJs (sSJs). In tangentially cut sections, pSJs reveal undulating rows of septa, whereas septa in the sSJs show regularly spaced parallel lines (26, 38, 47). pSJs are observed in ectodermally derived epithelia such as the epidermis, fo-regut, hindgut, trachea, and salivary glands, whereas sSJs are found in endodermally derived midgut and gastric ceca (37, 69). sSJs are also present in Malpighian tubules, although their epithelial cells are ectodermal and mesodermal derivatives (16, 18, 37).