Sequential control of meiosis reinitiation by pH and Ca2+ in oocytes of the prosobranch mollusk Patella vulgata

Standardization of a Patella spp. (Mollusca, Gastropoda) embryo-larval bioassay and advantages of its use in marine ecotoxicology

The use of three limpet species, Patella vulgata Linnaeus, 1758, Patella depressa Pennant, 1777 and Patella ulyssiponensis Gmelin, 1791 as model organisms in marine ecotoxicology has been evaluated. Initial laboratory experiments were aimed to standardize a biological test with embryos and larvae of Patella spp, establishing the percentage of normal trochophore larvae as endpoint. Before conducting in vitro fertilization, oocytes must be maturated artificially by incubation in an alkaline solution; therefore, alkalinizing agent, pH and time of eggs alkalinization were evaluated. Moreover, time of sperm activation, optimum sperm and oocytes concentration during fertilization, gamete contact time, use of stirring during the fertilization, egg concentration and incubation temperature were examined. Minimum sample size per treatment was also estimated. Exposure of oocytes for 10min to FSW alkalinized with NH4OH at pH 9.0, the use of undiluted sperm pre-activated during 45min and a concentration of 200 oocytesmL(-1), a gamete-contact time of 180min and egg incubation at 18°C during 24h at a concentration of 80 eggsmL(-1) were the conditions allowing maximal embryo-larval development success. With an error of 0.05, a sampling size ≥320 allows a 95% confidence in the estimate. This Patella spp. acute bioassay fulfills a number of important a priori requirements to be used in ecotoxicological studies. Nevertheless, in vitro fertilization requires considerable handling, which may lead to failure in fecundation. Such difficulties are also addressed, in order to facilitate the routine use of this protocol by other laboratories.

Signaling pathways in ascidian oocyte maturation: the roles of cAMP/Epac, intracellular calcium levels, and calmodulin kinase in regulating GVBD

Most mature ascidian oocytes undergo germinal vesicle breakdown (GVBD) when released by the ovary into sea water (SW). Acidic SW blocks this but they can be stimulated by raising the pH, increasing intracellular cAMP levels by cell permeant forms, inhibiting its breakdown or causing synthesis. Boltenia villosa oocytes undergo GVBD in response to these drugs. However, the cAMP receptor protein kinase A (PKA) does not appear to be involved, as oocytes are not affected by the kinase inhibitor H-89. Also, the PKA independent Epac agonist 8CPT-2Me-cAMP stimulates GVBD in acidic SW. GVBD is inhibited in calcium free sea water (CaFSW). The intracellular calcium chelator BAPTA-AM blocks GVBD at 10 µM. GVBD is also inhibited when the ryanodine receptors (RYR) are blocked by tetracaine or ruthenium red but not by the IP(3) inhibitor D-609. However, dimethylbenzanthracene (DMBA), a protein kinase activator, stimulates GVBD in BAPTA, tetracaine or ruthenium red blocked oocytes. The calmodulin kinase inhibitor KN-93 blocks GVBD at 10 µM. This and preceding papers support the hypothesis that the maturation inducing substance (MIS) produced by the follicle cells in response to increased pH causes activation of a G protein which triggers cAMP synthesis. The cAMP then activates an Epac molecule, which causes an increase in intracellular calcium from the endoplasmic reticulum ryanodine receptor. The increased intracellular calcium subsequently activates calmodulin kinase, which causes an increase in cdc25 phosphatase activity, activating MPF and the progression of the oocyte into meiosis.

Fertilization causes a single Ca2+ increase that fully depends on Ca2+ influx in oocytes of limpets (Phylum Mollusca, Class Gastropoda)

Mature limpet oocytes arrested at the first metaphase (MI) of meiosis are activated by the stimulation of fertilizing sperm. The aim of the present study was to clarify the spatiotemporal property and mechanism of intracellular Ca2+ increase in limpet oocytes, which is a prerequisite signal for initiation of development at fertilization. In all of the five limpet species tested, the initial Ca2+ rising phase just after fertilization took the form of a centripetal Ca2+ wave spreading from the whole cortex to the center (cortical flash), yielding a homogeneous Ca2+ elevation throughout the oocyte. The Ca2+ level remained high during the subsequent plateau phase lasting for several minutes and then returned nearly to the original value. No additional Ca2+ increase followed the plateau phase at least by the time of first cleavage. Both rising and plateau phases of Ca2+ increase at fertilization were inhibited by removal of external Ca2+, suggesting that continuous Ca2+ entry occurs throughout the Ca2+ increase. Injection of inositol 1,4,5-trisphosphate (IP3) was effective in generating a Ca2+ increase in mature limpet oocytes arrested at MI; however, their ability to show an IP3-induced Ca2+ increase was extremely low, as compared with other animals. Responsiveness to IP3 injection in immature oocytes arrested at the first prophase (PI) was similar to that in the mature oocytes, suggesting that the IP3-induced Ca2+ release system does not develop during the process of meiotic maturation in limpet oocytes. Caffeine, cyclic adenosine diphosphate ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP), the agents known to stimulate internal Ca2+ release mechanisms distinct from an IP3-dependent pathway, had no effect on intracellular Ca2+ changes in mature limpet oocytes. Labeling of the endoplasmic reticulum (ER) with DiI revealed that cortical ER clusters are only present in the localized region around meiotic chromosomes in mature oocytes. These data strongly suggest that Ca2+ release and its propagating mechanisms are undeveloped in limpet oocytes and that Ca2+ influx is the only Ca2+-mobilizing system available and functioning at fertilization.

Oocyte and egg organization in the patellogastropod Lottia and its bearing on axial specification during early embryogenesis

In the basal gastropod Lottia, the apical region of the oocyte is normally the site where the meiotic apparatus attaches and polar body formation occurs following fertilization. This site marks the animal-vegetal axis of the egg. A stereotypical cleavage pattern is organized, and the segregation of developmental potential occurs along this axis during early development. The segregation of developmental potential is a relatively late event and probably does not start until after cleavage begins. By compressing oocytes during the process of germinal vesicle breakdown, the position where the meiotic apparatus attaches to the cell membrane can be altered so that it no longer corresponds to the apical end of the oocyte. This new site of polar body formation sets up a new animal-vegetal axis that organizes cleavage and the segregation of developmental potential. The timing of animal-vegetal axis specification in Lottia is much later than it is in derived gastropods with a precocious specification of the D quadrant.

Maturation and fertilization in Lottia gigantea oocytes: intracellular pH, Ca(2+), and electrophysiology

Intracellular pH and Ca(2+) were measured with BCECF- and Calcium Green-dextran during maturation and fertilization of oocytes of the limpet Lottia gigantea. Maturation of oocytes from prophase to metaphase I of meiosis was induced in seawater adjusted to pH 9 with NH(4)OH. Intracellular pH rose during maturation induction, and maturation was also induced by microinjecting pH 8, but not pH 7, HEPES buffer. Intracellular Ca(2+) rose during NH(4)OH-induced maturation, but maturation was not inhibited when the increase was blocked by microinjection of BAPTA. When the metaphase I oocytes were fertilized(), there was an abrupt increase in intracellular Ca(2+), and activation (polar body formation) failed to occur in BAPTA-injected oocytes. Intracellular pH did not rise during fertilization. These observations show that maturation from prophase to metaphase I of meiosis is pH-dependent and activation of the metaphase I oocytes is Ca(2+)-dependent. A Ca(2+) action potential was present in both immature and mature oocytes but was more prominent in mature oocytes whose input resistance was higher. Fertilization produced a long-lasting (17-20 min) Na(+)-dependent fertilization potential with superimposed oscillations resembling Ca(2+) action potentials.

Effect of procaine on membrane potential and intracellular pH in Xenopus laevis oocytes

The effect of the local anaesthetic procaine on the intracellular pH, pHi, and electrophysiological properties of full-grown Xenopus oocytes was studied. In spite of its interference with both the pH-sensitive microelectrodes and fluorescent probe BCECF, we have shown that procaine induced an intracellular acidification rather than the alkalization commonly observed in most cells. The resting pHi of Xenopus oocytes loaded with BCECF was 7.36 +/- 0.04 (n = 16). Addition of 10 mM procaine to the bath at pH 7.5 caused pHi to decrease to a new steady state value of 6.97 +/- 0.05 (n = 9). A similar behaviour of pHi was observed with microelectrodes. Procaine also promoted a rise in membrane conductance and a membrane depolarization. These changes in membrane potential and conductance were not caused by the decrease in pHi since the addition of sodium propionate at pH 7.5 produced the same decrease of pHi as procaine, but resulted in only a slight depolarization with superimposed oscillations. Current measurements using two-electrode voltage clamp showed that the depolarization was associated with an inward current. No significant effect on this current was observed when replacing Cl, K or Na in the external medium. The absence of effect of Cl and K channel inhibitors argues against the involvement of Cl and K currents during the procaine response.

The oocyte metaphase arrest

Usually, oocyte meiosis reinitiation appears as a two step process during which release from the prophase block is followed by a second arrest in metaphase I or II. In this review, we will examine the mechanisms required to maintain the metaphase arrest and stabilize MPF activity at this stage. Then, we will analyse the processes required to exit from the metaphase block. These may drive the cells forward to the metaphase-anaphase transition, as a result of fertilization, activation or protein synthesis inhibition. Instead, inhibiting protein phosphorylation drives the oocyte back to interphase. All these treatments result in derepression of DNA synthesis.

Intracellular pH change does not accompany egg activation in the mouse

In the sea urchin, some other marine invertebrates, and the frog, Xenopus, egg activation at fertilization is accompanied by an increase in intracellular pH (pHi). We measured pHi in germinal vesicle (GV)-intact mouse oocytes, ovulated eggs, and in vivo fertilized zygotes using the pH indicator dye, SNARF-1. The mean pH, was 6.96 +/- 0.004 (+/- SEM) in GV-intact oocytes, 7.00 +/- 0.01 in ovulated, unfertilized eggs, and 7.02 +/- 0.01 in fertilized zygotes, indicating no sustained changes in pHi after germinal vesicle breakdown (GVBD) or fertilization. To examine whether transient changes in pHi occur shortly after egg activation, mouse eggs were parthenogenetically activated by 7% ethanol in phosphate buffered saline (PBS); no significant change in pHi followed ethanol activation. Since increased Na+/H+ antiporter activity is responsible for pHi increase in the sea urchin, pHi was measured in the absence of added bicarbonate or CO2 (a condition under which the antiporter would be the only major pHi regulatory mechanism able to operate, since the others were bicarbonate-dependent) in GV-intact oocytes, ovulated eggs, and in vivo fertilized zygotes to determine whether a Na+/H+ antiporter was activated. There was no physiologically significant difference in pHi after GVBD or fertilization, when pHi was measured in bicarbonate-free medium, nor any change upon parthenogenetic activation. Thus, a change in pHi is not a feature of egg activation in the mouse.

Activation of polyphosphoinositide metabolism at artificial maturation of Patella vulgata oocytes

The metabolism of polyphosphoinositides (PPI) has been investigated during the meiosis reinitiation of the oocytes of a prosobranch mollusk, the limpet Patella vulgata. Meiosis reinitiation which leads to germinal vesicle breakdown (GVBD) and metaphase-1 spindle formation was artificially induced by treating the prophase-blocked oocytes with 10 mM NH4Cl, pH 8.2. This treatment, which results in a rise in intracellular pH, triggered a general increase in polyphosphoinositide synthesis. Determinations of phosphorus content showed that maturation induced a 30 to 50% increase in both phosphatidylinositol (PI) and phosphatidylinositol-1 monophosphate (PIP) concentrations. Incorporations of 32PO4 and [3H]inositol have been measured in three classes of polyphosphoinositides: PI, PIP, and phosphatidylinositol 4,5-bisphosphate (PIP2). By comparing incorporation rates of the radiolabeled precursors into PPI before and after meiosis reinitiation, we found that artificial maturation by ammonia induced a 50-fold increase in the turnover of these lipids. No significant burst of inositol 1,4,5-trisphosphate (IP3) was observed after maturation. We suggest that modifications in PPI metabolism occurring at maturation of Patella oocytes might ensure the formation of an important stock of PPI that would be available for the profuse production of IP3, the messenger responsible for the Ca2+ signal at fertilization.

Intracellular pH change does not appear as a prerequisite for triggering activation of Barnea candida (Mollusca, Pelecypoda) oocytes

Barnea candida oocytes, exposed to excess KCl, ammonia, or digitonin, exhibit germinal vesicle breakdown (GVBD) and reinitiate meiosis, at least up to first polar body extrusion. While we confirm that KCl- but not ammonia-induced activation required external calcium, our findings that digitonin is effective at any pH from 6 to 8, in the presence of calcium, while the phorbol ester TPA and diacylglycerol fail to reinitiate meiosis, strongly suggests calcium as the main trigger for this process. Preliminary experiments using the fluorescent probes fluorescein diacetate and Quin 2/AM show, moreover, that KCl and ammonia produce both an intracellular calcium surge (30 nM) and a slight alkalinization of the intracellular cytoplasm from 7.84 to 8.05.

The relationships between early ionic events, the pattern of protein synthesis, and oocyte activation in the surf clam, Spisula solidissima

The ionic events linked to activation of surf clam (Spisula solidissima) oocytes include a transient increased Ca2+ influx and an acid release. The aim of the present work was to further elucidate the respective roles of these two ionic events and to clarify the possible role of protein kinase C in the sequence of events leading to oocyte activation. K+-enriched seawater, ammonium chloride, and the phorbol ester 12-O-tetradecanoyl-13-phorbol acetate (TPA), a protein kinase C activator, were tested for their ability to promote germinal vesicle breakdown (GVBD), an acid release, increased 45Ca2+ uptake, and a shift in the pattern of protein synthesis. Oocytes activated by addition of K+ ions release an amount of H+ similar to that induced by fertilization, with the same time course, show an increased, verapamil-sensitive, 45Ca2+ uptake that is proportional to the amount of added K+, and undergo a shift in their pattern of protein synthesis, which requires the presence of external Ca2+. Ammonium chloride, at concentrations causing a higher production of acid than that induced by K+ ions or fertilization, does not trigger GVBD nor any increased 45Ca2+ uptake or any detectable shift in the pattern of protein synthesis. Combined additions of ammonium chloride with subthreshold concentrations of K+ ions allow GVBD to occur, thus revealing a synergistic effect of ammonia and K+ ions. TPA slowly induces GVBD, an Na+-dependent acid release, and a shift in the pattern of protein synthesis, in the absence of increased 45Ca2+ uptake. Our results lead us to propose the following sequence of events for the activation of Spisula oocytes: an increased Ca2+ influx contributes to activate protein kinase C which causes a Na+-dependent acid release leading to a rise of pHi. This rise of pHi, although insufficient by itself, may set the pHi in a permissive range for activation to occur through the action of other protein kinase C-sensitive events leading to the production of meiosis-inducing proteins.

Action of calcium-binding proteins in egg maturation and fertilisation

There is considerable evidence that calcium acts as a primary trigger for egg maturation and fertilisation in diverse phyla. Calcium regulation has been demonstrated or suggested for numerous specific events in fertilisation, including: sperm motility, the acrosome reaction, sperm-egg binding and fusion, metabolic activation of the egg, etc. However, very little is known concerning the mechanisms whereby calcium exerts its effects. Some calcium-regulated events are mediated through calmodulin and others are likely to be as well. Additionally, protein kinase C has recently been implicated in some processes related to egg maturation and activation, although the evidence presented thus far has been indirect. Other pathways dependent upon calcium but not involving either CaM or PKC have also been identified. Much more research will be required before the multiple involvement of calcium-binding proteins in egg maturation and fertilisation are clarified.