The observation that an endogenous, neuronal-specific, cell adhesion protein such as FasII ( Fasciclin II, homologue of N-CAM Kurusu et al. 2002) exhibits aberrant expression and subcellular distribution in neurons in an ogre mutant (image at top of this page) suggests that loss of ensheathing glia from the mushroom body axonal tracts has dire consequences for the general cell biology of de-sheathed neurons. Many questions remain about the nature of glial and neuronal defects observed in ogre mutant animals:
- What is the role of innexins such as Ogre in glia? Is it required for cell adhesion? Or formation of gap junctions that mediate spatial buffering of ions (Wallraff et al. 2006) via a pan-glial syncytium network?...Or something else?
- If Ogre is required only in glial cells, why do some rescued brains (expressing wild type Ogre in the glia of ogre mutants) occasionally show structural defects? Here are three, out of many, possibilities: 1, There may be undetectably low levels of innexins in neurons that are essential for normal development/function. This theory could be addressed by expressing wild type Ogre protein in neurons (or both neurons and glia together?) of ogre mutants and looking for phenotypic rescue. 2, Alternatively, the level of Ogre expressed using the GAL4/UAS system may be variable and relative innexin subunit levels could be a very important determinant in mediating rescue. Attempts at rescuing the electroretinogram (ERG) of an ogre mutant by GAL4/UAS -mediated expression of wild type Ogre also notably achieved only partial rescue (Curtin et al, 2002). 3, Expressing transgenic innexins can generate phenotypes that could mask rescue. For example, overexpressing UAS-inx2 can disrupt cell polarity in embryonic epithelia (Bauer et al, 2004) and also causes defects in larval disc cells (Image: Hazards of innexin overexpression).
- Inx2 and Inx3 are also expressed in glia. Will they prove to be as essential as Ogre for normal glial and neuronal structure?
- Dominant gap junction mutants exist, both innexin (see Inx2 summary) and connexin. Presumably they would cause defects by a different mechanism than loss-of-function mutants (...the ogre mutant is essentially loss-of-function in neuropile glial cells due to its absence). What effects do dominant innexin mutants have on nervous system development?...and are these comparable phenotypically and mechanistically to those in vertebrates?
- ogre mutant brains are much smaller than those of wild type animals. Are innexins essential for the proliferation/survival of neurons and/or glia? Obviously neuropile glia remain in the ogre mutant (Image: ogre mutant genotype) used in these studies...but cell numbers were not quantified and there could have been fewer than normal. What happens in other innexin (Inx2/Inx3) null mutants?
- Neuropile glial processes accumulate at the brain midline in the ogre mutant. If Ogre is required for cell adhesion or cell guidance/recognition does it have to exist in the plasma membrane as part of a channel?
- Gap junctions are required in vertebrate glia for the normal expression of other proteins such as glutamate transporters (Figiel et al. 2007). Is this also the case for innexins? Failure to remove excess glutamate from active glutamatergic neurons can result in neurotoxic effects - which might explain some of the structural phenotype observed in the ogre mutant (Images: glial defects, neuronal defects).