The epithelial barrier, formed by septate junctions in invertebrates, compartmentalizes the body and provides a barrier to pathogens. To sustain distinct compartment environments, the epithelial barrier must be stably maintained. Therefore, we are interested in how epithelia regulate barrier function during tissue regeneration. To address this, we have been examining the dynamics of the epithelial barrier during regeneration in the Drosophila wing imaginal disc. Following X-irradiation, we observed a temporary depletion and/or mislocalization of septate junction components in wing discs. Normal localization of these septate junction components is recovered prior to pupation. To determine if the epithelial barrier is also functionally impaired following damage, we examined whether the imaginal disc epithelia of irradiated discs is permeable to a fluorescently-labeled dextran. We observed infiltration of labeled dextran into the lumen of irradiated discs, but not control discs, indicating that the epithelial barrier is compromised following damage. To determine if this disruption of barrier function in irradiated discs is a physical response to apoptotic damage or a regulated process during regeneration, we irradiated larvae late in larval development after a point when they can no longer initiate regenerative repair on imaginal discs, and examined the localization of septate junction components. Following late irradiation, we observed that septate junction components of imaginal discs are not disrupted or mislocalized; suggesting that septate junction disruption following damage is a regulated process, and dependent on the regenerative activity of the imaginal disc.
We also observed that septate junction disruption causes a synergistic increase in Drosophila insulin-like peptide 8 (dilp8)-mediated developmental delay. This suggests that the epithelial barrier limits dilp8 signaling. Dilp8 is upregulated following damage and accumulates in the imaginal disc lumen following expression, but acts on the brain and ring-gland. Therefore, we hypothesize that damage-induced Dilp8 is produced and apically secreted into the lumen of wing imaginal discs, then leaks through disrupted septate junctions to enter the hemolymph and travel to the brain and ring-gland where it signals to regulate developmental timing and growth. It is possible that the subsequent recovery of the epithelial barrier following damage serves as a method to covey the time course of regeneration to the brain and other organs by controlling the release of Dilp8.