Innexin Gap Junctions

Innexin relationships with other cell components

Image showing the relationship between secretory vesicles and innexins in Drosophila salivary gland cells
Innexins and secretory vesicles

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Figure legend: The image above shows secretory vesicles marked with synaptobrevin-GFP (n-syb-GFP, green) at two cellular depths (Z sections 1+2, upper diagram) across the intercellular boundary between two pupal salivary gland cells. The cells are also antibody-stained for Inx2 (red). There is generally very little positional overlap between the synaptobrevin and Inx2 vesicles near the basal cell surface (Z section 1, left panel). However, there appears to be a quite a lot of secretory activity at both cell depths (ie. lots of green spots, left and right image panels). Note that the depth of Z sections 1+2 are above and below, respectively, the positions occupied by large gap junction plaques in salivary gland cells (Image: Inx2 plaques in the lateral cell membrane). Where plaques exist at the plasma membrane there is a marked decrease in the appearance of secretory vesicles compared to areas where plaques are common (Image: exocytotic vesicles and gap junction plaques). Does this mean that overloading cells with gap junction proteins (in transgenic experiments) could disrupt exo-/endo-cytosis, unwittingly perturbing unrelated intercellular signalling pathways?

Innexins co-localise with other proteins within the cell
- such as Grasp65

Innexins and Grasp65
Innexins and Grasp65

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Figure legend: The image above shows that transgenically expressed UAS-Grasp65 (green) is distributed in a globular pattern throughout pupal salivary gland cells but accumulates in some areas that have high levels of Inx2 (red) protein. A number of different cell depths can be seen here. Grasp65 is reported to be a golgi complex-associated protein (Kondylis et al. 2005). Whether the relationship observed here underlies a functional interaction (...a new role for Grasp65?) or is a spurious observation arising from Grasp65-GFP overexpression remains to be seen.

It is likely that gap junction proteins, connexins, innexins and pannexins, interact with many more molecular partners than have so far been described. From the published literature we know that gap junction proteins associate with:

  • transport vesicles and associated molecules (Martin et al. 2001).
  • annular junction vesicles and the molecular machinery that targets old channels and endocytoses double-membrane vesicles (Piehl et al. 2007).
  • plaques and the molecules that make them dynamic cadherin and β-catenin (Bauer et al. 2004).
  • the actin cytoskeleton (Butkevich et al. 2004)
  • other subcellular oganelles, such as mitochondria (Goubaeva et al. 2007) and presumably other molecules in the mitochondrial membrane.

One approach that could be employed to identify some new molecular partners would be to exploit some of the useful features of Drosophila salivary glands:

  • - Endogenous gap junction proteins are expressed in these cells and occupy physically distinct subdomains associated with specific aspects of the innexin lifecycle - transport vesicles, putative annular junctions and plaques.
  • - The cells are easy to dissect in bulk and are very large making it easy to visualise subcellular domains.
  • - Many UAS-protein-GFP tagged lines are available that can be expressed in these cells.
  • - Many new fluorescently-tagged endogenous genes are being generated regularly by a gene-tagging consortium (Flytrap).
  • - Detected proximity of tagged-proteins to innexins would reveal potential gap junction-interacting molecules that could merit further study.

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This is the end of the Innexins at the cellular level section

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