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(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. RESULTS: Using 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 Ca(2+)-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 Ca(2+)-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 Ca(2+)-channels, whereas only both pH(i)-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(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. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s12861-019-0192-x) contains supplementary material, which is available to authorized users.