The effect of ooplasmic pH regulation on the formation of yolk spheres in the telotrophic ovariole of Dysdercus intermedius

Extracellular H+ dynamics during oogenesis in Rhodnius prolixus ovarioles

The spatiotemporal dynamics of transmembrane hydrogen ion (H+)fluxes during oogenesis were investigated in the telotrophic ovarioles of the insect, Rhodnius prolixus. Although Rhodnius ovarioles possess exaggerated morphological and electrical polarity between nurse cells and oocytes, little is known about H+ changes during oogenesis,despite the regulatory role played by H+ and pH in many relevant cellular processes. A number of dynamic extracellular H+ fluxes were measured along Rhodnius ovarioles, representing an oogenesis cycle, using a non-invasive, self-referencing, H+-selective probe. The interfollicular stalk separating adjacent follicles exhibited prominent H+ efflux that peaked during midvitellogenesis and declined during late vitellogenesis. H+ efflux in this region preceded stalk formation and, importantly, preceded the onset of vitellogenesis in the adjacent posterior follicle. H+ efflux was also observed over the terminal follicle, where specialized regions of the chorion were forming, and was still detected around follicle cells after ovulation, indicating that the somatic follicular epithelium produced this flux. Transmembrane H+fluxes may drive intracellular pH changes or may stabilize pHi in response to pH-altering events. H+ fluxes may play a role in processes that coincide spatially and temporally, including the onset of vitellogenesis, endocytosis, follicle cell cytoskeletal dynamics, and regulation of interfollicular feedback mechanisms.

The proton/sodium antiporter (exchanger) in the oocyte membrane of Dysdercus intermedius is electrogenic (2H(+)/Na(+)) and causes perioocytic proton accumulation

Vitellogenic follicles of Dysdercus intermedius (Heteroptera: Pyrrhocoridae) were treated with sodium azide (NaN(3)) or 2,4 dinitrophenol (DNP) in order to load the ooplasm with protons along their electrochemical gradient. Ooplasmic pH (pH(OOC)) was recorded using proton-specific microelectrodes. Treatment for six min with 0.5 mM of DNP (dissolved in physiological saline solution; PSS) resulted in acidification of the ooplasm from 7.41+/-0.05 in PSS to pH(OOC(DNP))=7.09+/-0.04. Immersing follicles in PSS after DNP treatment resulted in reactivation of a proton/sodium antiporter and recovery of the initial pH(OOC). Additionally, the proton-specific microelectrodes were placed at a distance of approximately 10 &mgr;m from the surface of the vitellogenic follicle. The extracellular pH (pH(EX)) was measured before (pH(EX(PSS))), during (pH(EX(DNP))) and after (pH(EX(PSS))) DNP treatment. Along the lateral surface of the follicle, the recorded pH(EX(PSS)) was initially 6.79+/-0.02, similar to the pH of the medium (pH(MED)=6.80; recorded at a distance of 300 &mgr;m from the surface of the follicle) and higher than the pH(EX(PSS)) of 6.52+/-0.03 measured in the interfollicular constriction between individual vitellogenic follicles (interfollicular region). During DNP treatment, values changed to 6.80+/-0.03 in the constriction and 6.80+/-0.01 along the lateral surface. After removal of DNP the initial control pH values were reestablished. These extrafollicular H(+) distributions fit into a model of extrafollicular currents reported earlier for D. intermedius.Proton distribution between the ooplasm and the medium was also affected in the presence of 5 mM NaN(3), resulting in a drop in ooplasmic pH from 7.40+/-0.05 down to pH(OOC(NaN3))=7.07+/-0.03. Changes in cytosolic proton activities after DNP or NaN(3) treatment were evidenced by monitoring both the increase in ooplasmic pH (DeltapH) and, simultaneously, the change in the resting potential (DeltaEm). Recovery of the ooplasmic pH depended on the transfer of approximately 6x10(9) H(+)/oocyte (after DNP treatment) or approximately 3x10(9) H(+)/oocyte (after NaN(3) treatment), whereas recovery of Em by charging the capacitance of the oocyte membrane could be attributed to a net efflux of approximately 3x10(9) H(+)/oocyte (after DNP treatment) or approximately 1.7x10(9) H(+)/oocyte (after NaN(3) treatment). In the light of previous reports on the monensin-sensitive proton/sodium antiporter (external Na(+) for ooplasmic H(+)), the operating efficiency of this antiporter is 2H(+)/Na(+).VITELLOGENESIS DURING AND AFTER DNP TREATMENT WAS DEMONSTRATED BY THE ACCUMULATION OF FLUORESCENCE LABELLED HEMOLYMPH PROTEINS IN YOLK SPHERES IN THE CORTEX OF THE OOCYTE: vitellogenesis came to a halt in PSS containing DNP when the ooplasm was acidified and no H(+) accumulation around the follicle was detectable. Vitellogenesis stopped under the condition of DNP(MED)=0.5 mM, but resumed again by exchanging the medium for PSS without DNP. Simultaneously with the appearance of the regular pH(OOC(PSS))=7.40+/-0.03 (efflux of H(+) out of the ooplasm), extrafollicular proton accumulation by H(+) influx into the constriction reappeared within 10 minutes. The results obtained with proton-specific microelectrodes and the in vitro assay to detect vitellogenesis indicate that electrogenic H(+) extrusion out of the ooplasm plays an important role in both maintaining the ooplasmic pH 0.6 units above pH(MED)=6.8 and in the generation of the external current pattern. A model is discussed explaining the acidification of endosomes as a prerequisite for endosomal processing leading to yolk spheres.