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

Background: Alterations 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) and membrane potential (V), during development. Therefore, we analysed stage-specific gradients of pH and V as well as their dependence on specific ion-transport mechanisms.

Results: Using fluorescent indicators, we found distinct alterations of pH- and V-patterns during stages 8 to 12 of oogenesis. To determine the roles of relevant ion-transport mechanisms in regulating pH and V 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 Ca-channels (verapamil), Cl-channels (9-anthroic acid) and Na/K/2Cl-cotransporters (furosemide). Either pH or V 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 were obtained by inhibiting Na/K/2Cl-cotransporters or ATP-sensitive K-channels. V was most efficiently hyperpolarised by inhibiting voltage-dependent L-type Ca-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 and/or V, we also found strong influences on the antero-posterior and dorso-ventral pH- and/or V-gradients. For example, inhibiting ATP-sensitive K-channels strongly enhanced both pH-gradients (increasing alkalisation) and reduced both V-gradients (increasing hyperpolarisation). Similarly, inhibiting Na/K/2Cl-cotransporters strongly enhanced both pH-gradients and reduced the antero-posterior V-gradient. To minor extents, both pH-gradients were enhanced and both V-gradients were reduced by inhibiting voltage-dependent L-type Ca-channels, whereas only both pH-gradients were reduced (increasing acidification) by inhibiting V-ATPases or NHE and Na-channels.

Conclusions: Our data show that in the Drosophila follicle-cell epithelium stage-specific pH- and V-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.