Extracellular ion fluxes during germ cell differentiation and oogenesis in Rhodnius prolixus.

摘要

The roles of extracellular ionic fluxes that accompany oogenesis are poorly understood. The telotrophic meroistic ovarioles of Rhodnius prolixus provide an excellent model to address this problem with the exaggerated morphological and physiological polarity between their syncytial nurse cells and oocytes. In view of the cellular importance and pleiotropic effects of intracellular pH and Ca2+ dynamics, the corresponding extracellular H+ and Ca2+ fluxes were measured non-invasively, at numerous positions along the ovariole throughout an entire oogenesis cycle, using a self-referencing ion-selective probe. Prominent H+ efflux occurred at interfollicular stalks separating adjacent follicles, and changed over the course of oogenesis. This H+ efflux appeared prior to vitellogenesis and stalk formation in the adjacent posterior follicle; efflux peaked during midvitellogenesis and declined during late vitellogenesis. H+ efflux was also observed over follicle cells forming specialized regions of the chorion. Proton fluxes around 5 th instar ovarioles also undergo dynamic changes, which appear to correspond to germ cell differentiation events, as nurse cell and oocyte fates are defined. Extracellular Ca2+ fluxes were also measured around individual ovarioles. Although steady, stage-specific Ca2+ fluxes significantly greater than background noise were not detected, increased variability in flux near the posterior surface of the T-1 follicle during later stages may indicate signalling activity related to interfollicular feedback regulation during this period. In order to understand the molecular basis of extracellular ion fluxes, immunofluorescent labelling of selected H + and Ca2+ pumps and channels was performed. Expression was predominantly restricted to the somatic follicle cells. The proton pump, H+/K+-ATPase, was increasingly expressed by somatic cells as the follicle grew, although no qualitative differences were evident among different populations of cells. Ca2+ pumps and an L-type calcium channel were expressed differently by follicle cells during late previtellogenesis, in a pattern that became more complex as the follicle matured. Regional differences in calcium transport protein expression may mark areas of restricted intercellular communication. This is the first study describing extracellular H+ and Ca2+ fluxes during an oogenesis cycle and during formation of the adult ovariole, providing greater understanding of the ionic and molecular basis of bioelectric currents during oogenesis.