Electrochemical patterns during Drosophila oogenesis: ion-transport mechanisms generate stage-specific gradients of pH and membrane potential in the follicle-cell epithelium

BackgroundAlterations of bioelectrical properties of cells and tissues are known to function as wide-ranging signals during development, regeneration and wound-healing in several species. The Drosophila follicle-cell epithelium provides an appropriate model system for studying the potential role of electrochemical signals, like intracellular pH (pH(i)) and membrane potential (V-mem), during development. Therefore, we analysed stage-specific gradients of pH(i) and V-mem as well as their dependence on specific ion-transport mechanisms.ResultsUsing fluorescent indicators, we found distinct alterations of pH(i)- and V-mem-patterns during stages 8 to 12 of oogenesis. To determine the roles of relevant ion-transport mechanisms in regulating pH(i) and V-mem and in establishing stage-specific antero-posterior and dorso-ventral gradients, we used inhibitors of Na+/H+-exchangers and Na+-channels (amiloride), V-ATPases (bafilomycin), ATP-sensitive K+-channels (glibenclamide), voltage-dependent L-type Ca2+-channels (verapamil), Cl--channels (9-anthroic acid) and Na+/K+/2Cl(-)-cotransporters (furosemide). Either pH(i) or V-mem or both parameters were affected by each tested inhibitor. While the inhibition of Na+/H+-exchangers (NHE) and amiloride-sensitive Na+-channels or of V-ATPases resulted in relative acidification, inhibiting the other ion-transport mechanisms led to relative alkalisation. The most prominent effects on pH(i) were obtained by inhibiting Na+/K+/2Cl(-)-cotransporters or ATP-sensitive K+-channels. V-mem was most efficiently hyperpolarised by inhibiting voltage-dependent L-type Ca2+-channels or ATP-sensitive K+-channels, whereas the impact of the other ion-transport mechanisms was smaller. In case of very prominent effects of inhibitors on pH(i) and/or V-mem, we also found strong influences on the antero-posterior and dorso-ventral pH(i)- and/or V-mem-gradients. For example, inhibiting ATP-sensitive K+-channels strongly enhanced both pH(i)-gradients (increasing alkalisation) and reduced both V-mem-gradients (increasing hyperpolarisation). Similarly, inhibiting Na+/K+/2Cl(-)-cotransporters strongly enhanced both pH(i)-gradients and reduced the antero-posterior V-mem-gradient. To minor extents, both pH(i)-gradients were enhanced and both V-mem-gradients were reduced by inhibiting voltage-dependent L-type Ca2+-channels, whereas only both pH(i)-gradients were reduced (increasing acidification) by inhibiting V-ATPases or NHE and Na+-channels.ConclusionsOur data show that in the Drosophila follicle-cell epithelium stage-specific pH(i)- and V-mem-gradients develop which result from the activity of several ion-transport mechanisms. These gradients are supposed to represent important bioelectrical cues during oogenesis, e.g., by serving as electrochemical prepatterns in modifying cell polarity and cytoskeletal organisation.