Innexin Gap Junctions

Innexin cellular distribution changes with cell age/development

Image showing innexin gap junction plaque position relative to cellular position markers in Drosophila embryonic salivary gland cells
Image of gap junction position in Drosophila embryo salivary gland cells

click image-box for full image in PDF format

Figure legend: Image showing the position of Inx2 (presumably in the form gap junction plaques) in embryonic salivary glands. Focusing on the apical region of cells (near the no innexin 2 protein is consistently detected elsewhere in these cells). Inx2-containing plaques (blue) are located in the lateral plasma membrane at positions slightly more basal than the apical markers Na+/K+ ATPase α subunit (ATPα-GFP, green) and fasciclin III (FasIII, red). In larval and pupal salivary glands Inx2 is observed in the lateral plasma membrane near the basal (non-lumenal) pole of cells and is absent from the whole apical (lumen) end of cells. This is best illustrated in the 3D reconstructions revealing gap junction plaque distribution in pupal salivary glands.

Image showing the subcellular position of Na/K ATPase-alpha-GFP in pupal salivary gland cells. It does not differ much from embryonic salivary gland cells.

Figure legend: Not all apical cell markers become redistributed during the course of development from embryo to pupa in salivary gland cells. The image sequence above shows a population of pupal salivary gland cells from different angles. The last image in the sequence is a side-view of these cells. Green colour indicates the distribution of endogenous Na+/K+ ATPase α subunits (AtpαG00109). The most intense ATPα-GFP staining in pupal salivary gland cells is at the apical pole (the lumen end of the cell), just as it is in embryonic salivary gland cells.

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 ( 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? gap junction turnover so slow/or fast in embryonic cells that annular junctions do not get a chance to accumulate?

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