A redistribution of plasma membrane-associated innexins takes place during the developmental transition of salivary glands from their embryo to larval stage. In embryos, innexins (..at least Inx2, Ogre and Inx3 (Bauer et al. 2004, Bauer et al. 2006)) are primarily localised toward the apical end of cells (Image: Inx2 in the embryonic salivary gland). In larvae and pupae, innexins are distributed mainly at the basal end (Image: Diagrammatic comparison of Innexin distribution in embryonic and pupal salivary glands). The gap junction plaques observed in embryonic cells are only 'putative' plaques given that it's hard to resolve whether they are indeed plaques or large internal vesicles (annular junctions?) in such small cells. The whole polarity of the cells does not appear to switch during the developmental transition - apical reporter molecules such as Na+/K+ ATPase α remain elevated at the apical pole in both embryo and larval glands. A related observation of dynamic innexin re-distribution has been found in studies of the ovary. Inx2 is detected as a cytoplasmic cloud in nurse cells but is transported to the oocyte around stage 11 (Bohrmann and Zimmermann, 2008). Is there a reason for the age-associated re-distribution of innexins in the salivary gland?
- - Molecules associated with ligand/receptor intercellular signalling pathways cluster at the apical end of embryonic salivary gland cells. Is gap junction-mediated cell-cell communication also required in close proximity to these clusters during this time?...or do innexins form a part of the subcellular structures that anchor ligands/receptors and their associated proteins?
- - Glue is exocytosed into the lumen at the apical end of larval salivary gland cells. Is the innexin redistribution merely required to accomodate this process?
- - A lifecycle pathway has been proposed for innexins (Diagram of a hypothetical Innexin lifecycle). Presumably the whole gap junction maintenance machinery - molecules involved in transport, targeting, endocytosis and degradation - might have to be re-distributed in addition to the gap junctions themselves.
- - On the issue of 'maintenance machinery' - no innexin transport vesicles or identifiable annular junctions have been detected in embryonic salivary gland cells...yet, anyway. Does a different innexin lifecycle pathway exist at this developmental stage?...are the cells just too small to see any of these objects?...is gap junction turnover so slow/or fast in embryonic cells that annular junctions do not get a chance to accumulate?