Innexin Gap Junctions

Innexin protein levels change over time in some tissues

Image highlighting the difference between third instar and pupal salivary gland cells

Figure legend: Image illustrating the ease with which co-stained larval and pupal salivary gland cells can be distinguished. It is therefore possible to co-antibody-stain larval and pupal salivary glands and identify the two developmental stages post-staining. This would allow a direct comparison of innexin protein levels in each developmental stage. Larval salivary gland cells are filled with glue vesicles ('granules') (image - left panel above). The glue vesicle membranes incorporate synaptobrevin-GFP and appear green. Glue vesicles are secreted from cells at the end of the larval stage allowing pupae to attach to surfaces. Pupal cells are devoid of glue vesicles (image- right panel). Plasma membranes are stained with DiI (red).

Image comparing the detection levels of Innexin 2 protein between larval and pupal stages of development
Image showing Inx2 protein detection in third instar larval versus pupal salivary glands

click image-box for full image in PDF format

Figure legend: Comparison of Innexin 2 protein levels in salivary gland cells at two different developmental stages. Tissues were antibody-stained and mounted together and were scanned using identical confocal microscopy settings. Larval and pupal salivary glands were subsequently distinguished using the criteria described in the figure at the top of the page (presence vs. absence of glue vesicles). The intensity of detected Inx2 is much greater in pupal than in larval cells.

Innexins are detected at all developmental stages in salivary glands throughout the life of Drosophila. When glands of similar size, but different developmental stage (eg. larval vs. pupal), are stained side-by-side to detect an innexin protein it becomes apparent that there is a massive difference in the protein level in each developmental stage. Hypothetically, gap junctions may play a role in synchronizing the release of glue vesicles in a coordinated fashion from salivary glands near the end of the larval stage. So why would innexin protein levels increase so significantly after this stage when there is nothing left for these cells to do except enter the apoptotic pathway and disintegrate?

  • - Gap junctions might be important for coordinating entry into the apoptototic pathway. Ensuring that all cells of the gland are switched to auto-destruct at the same time. But if a lower amount of innexins successfully coordinate glue release (...if they have any role in that process)...why would so much more protein be required to coordinate apoptosis?
  • - Do innexins play an important role in the actual process of apoptosis (at least in salivary gland cells)?
    • - Why do putative annular junctions accumulate at the basal end of pupal cells instead of being broken down by the lysosomal and proteasomal pathways? Do these large vesicles have role in apoptosis?
    • - Is any of the innexin in the pupal salivary gland present as regulated hemichannels? - synchronised opening of such channels and the rapid influx of ions after initiation of the auto-destruct signal would have a significant impact in mediating cell death.
    • - Do larger molecules such as proteins pass through gap junctions during the pupal stage? After all, there's no published evidence suggesting that all innexin-based channels have similar pore sizes (we don't even know if they're generally constructed from six subunits, like connexins, Diagram of gap junction channels). There are reports of large molecules passing through insect gap junctions (Brooks and Woodruff, 2004).

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