Ionic regulation of egg activation

Effect of Kisspeptin on the Developmental Competence and Early Transcript Expression in Porcine Oocytes Parthenogenetically Activated with Different Methods

Recent studies showed the modulatory effect of kisspeptin (KP) on calcium waves through the cell membrane and inside the cell. Spermatozoon can induce similar ooplasmic calcium oscillations at fertilization to trigger meiosis II. Here, we evaluated the effect of KP supplementation with 6-dimethylaminopurine (6-DMAP) for 4 h on embryonic development after oocyte activation with single electric pulse, 5 µM ionomycin, or 8% ethanol. Compared to control nonsupplemented groups, KP significantly improved embryo developmental competence electric- and ethanol-activated oocytes in terms of cleavage (75.3% and 58.6% versus 64% and 48%, respectively, p < 0.05) and blastocyst development (31.3% and 10% versus 19.3% and 4%, respectively, p < 0.05). MOS expression was increased in electrically activated oocytes in presence of KP while it significantly reduced CCNB1 expression. In ionomycin treated group, both MOS and CCNB1 showed significant increase with no difference between KP and control groups. In ethanol-treated group, KP significantly reduced CCNB1 but no effect was observed on MOS expression. The early alterations in MOS and CCNB1 mRNA transcripts caused by KP may explain the significant differences in the developmental competence between the experimental groups. Kisspeptin supplementation may be adopted in protocols for porcine oocyte activation through electric current and ethanol to improve embryonic developmental competence.

Signal transduction in mammalian oocytes during fertilization

Mammalian embryo development begins when the fertilizing sperm triggers a series of elevations in the oocyte's intracellular free Ca(2+) concentration. The elevations are the result of repeated release and re-uptake of Ca(2+) stored in the smooth endoplasmic reticulum. Ca(2+) release is primarily mediated by the phosphoinositide signaling system of the oocyte. The system is stimulated when the sperm causes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG); IP3 then binds its receptor on the surface of the endoplasmic reticulum that induces Ca(2+) release. The manner in which the sperm generates IP3, the Ca(2+) mobilizing second messenger, has been the subject of extensive research for a long time. The sperm factor hypothesis has eventually gained general acceptance, according to which it is a molecule from the sperm that diffuses into the ooplasm and stimulates the phosphoinositide cascade. Much evidence now indicates that the sperm-derived factor is phospholipase C-zeta (PLCζ) that cleaves PIP2 and generates IP3, eventually leading to oocyte activation. A recent addition to the candidate sperm factor list is the post-acrosomal sheath WW domain-binding protein (PAWP), whose role at fertilization is currently under debate. Ca(2+) influx across the plasma membrane is also important as, in the absence of extracellular Ca(2+), the oscillations run down prematurely. In pig oocytes, the influx that sustains the oscillations seems to be regulated by the filling status of the stores, whereas in the mouse other mechanisms might be involved. This work summarizes the current understanding of Ca(2+) signaling in mammalian oocytes.

Egg Activation at Fertilization by a Soluble Sperm Protein

The most fundamental unresolved issue of fertilization is to define how the sperm activates the egg to begin embryo development. Egg activation at fertilization in all species thus far examined is caused by some form of transient increase in the cytoplasmic free Ca2+ concentration. What has not been clear, however, is precisely how the sperm triggers the large changes in Ca2+ observed within the egg cytoplasm. Here, we review the studies indicating that the fertilizing sperm stimulates a cytosolic Ca2+ increase in the egg specifically by delivering a soluble factor that diffuses into the cytosolic space of the egg upon gamete membrane fusion. Evidence is primarily considered in species of eggs where the sperm has been shown to elicit a cytosolic Ca2+ increase by initiating Ca2+ release from intracellular Ca2+ stores. We suggest that our best understanding of these signaling events is in mammals, where the sperm triggers a prolonged series of intracellular Ca2+ oscillations. The strongest empirical studies to date suggest that mammalian sperm-triggered Ca2+ oscillations are caused by the introduction of a sperm-specific protein, called phospholipase C-zeta (PLCζ) that generates inositol trisphosphate within the egg. We will discuss the role and mechanism of action of PLCζ in detail at a molecular and cellular level. We will also consider some of the evidence that a soluble sperm protein might be involved in egg activation in nonmammalian species.

When isolated at full receptivity, in vitro fertilized wheat (Triticum aestivum, L.) egg cells reveal [Ca2+]cyt oscillation of intracellular origin

During in vitro fertilization of wheat (Triticum aestivum, L.) in egg cells isolated at various developmental stages, changes in cytosolic free calcium ([Ca2+]cyt) were observed. The dynamics of [Ca2+]cyt elevation varied, reflecting the difference in the developmental stage of the eggs used. [Ca2+]cyt oscillation was exclusively observed in fertile, mature egg cells fused with the sperm cell. To determine how [Ca2+]cyt oscillation in mature egg cells is generated, egg cells were incubated in thapsigargin, which proved to be a specific inhibitor of the endoplasmic reticulum (ER) Ca2+-ATPase in wheat egg cells. In unfertilized egg cells, the addition of thapsigargin caused an abrupt transient increase in [Ca2+]cyt in the absence of extracellular Ca2+, suggesting that an influx pathway for Ca2+ is activated by thapsigargin. The [Ca2+]cyt oscillation seemed to require the filling of an intracellular calcium store for the onset of which, calcium influx through the plasma membrane appeared essential. This was demonstrated by omitting extracellular calcium from (or adding GdCl3 to) the fusion medium, which prevented [Ca2+]cyt oscillation in mature egg cells fused with the sperm. Combined, these data permit the hypothesis that the first sperm-induced transient increase in [Ca2+]cyt depletes an intracellular Ca2+ store, triggering an increase in plasma membrane Ca2+ permeability, and this enhanced Ca2+ influx results in [Ca2+]cyt oscillation.

Calcium influx and male fertility in the context of the sperm proteome: an update

Freshly ejaculated spermatozoa are incapable or poorly capable of fertilizing an oocyte. The fertilization aptness of spermatozoa depends on the appropriate and time-dependent acquisition of hyperactivation, chemotaxis, capacitation, and the acrosome reaction, where calcium (Ca²⁺) is extensively involved in almost every step. A literature review showed that several ion channel proteins are likely responsible for regulation of the Ca²⁺ uptake in spermatozoa. Therefore, manipulation of the functions of channel proteins is closely related to Ca²⁺ influx, ultimately affecting male fertility. Recently, it has been shown that, together with different physiological stimuli, protein-protein interaction also modifies the Ca²⁺ influx mechanism in spermatozoa. Modern proteomic analyses have identified several sperm proteins, and, therefore, these findings might provide further insight into understanding the Ca²⁺ influx, protein functions, and regulation of fertility. The objective of this review was to synthesize the published findings on the Ca²⁺ influx mechanism in mammalian spermatozoa and its implications for the regulation of male fertility in the context of sperm proteins. Finally, Pathway Studio (9.0) was used to catalog the sperm proteins that regulate the Ca²⁺ influx signaling by using the information available from the PubMed database following a MedScan Reader (5.0) search.

The use of sea urchin eggs as a model to investigate the effects of crassolide, a diterpene isolated from a soft coral

Crassolide, a monocyclic diterpene isolated and purified from the soft coral Lobophytum crassum, inhibited the cell cleavage of sea urchin eggs without affecting fertilization. The effect was observed with concentrations above 2 x 10(-5)m in egg suspensions. Addition of crassolide between 5 and 40 min post-fertilization totally blocked the first cleavage, which in the control occurs 1 hr after fertilization. When added between 50 and 60 min post-fertilization, crassolide produced polynucleated cells in embryos. Crassolide did not affect the egg permeability to Na(+) and Ca(2+), but caused an increase of 0.2 units in the intracellular pH of fertilized eggs coupled with a proton efflux. Crassolide, which does not affect Ca(2+) influx or permeability at the level of storage in reticular vesicles, could be used as a negative control when analysing calcium changes in short-term toxicological studies. The relationship between the pH increase and the cell cleavage needs further investigation.

Effects of seawater acidification on cell cycle control mechanisms in Strongylocentrotus purpuratus embryos

Previous studies have shown fertilization and development of marine species can be significantly inhibited when the pH of sea water is artificially lowered. Little mechanistic understanding of these effects exists to date, but previous work has linked developmental inhibition to reduced cleavage rates in embryos. To explore this further, we tested whether common cell cycle checkpoints were involved using three cellular biomarkers of cell cycle progression: (1) the onset of DNA synthesis, (2) production of a mitotic regulator, cyclin B, and (3) formation of the mitotic spindle. We grew embryos of the purple sea urchin, Strongylocentrotus purpuratus, in seawater artifically buffered to a pH of ∼7.0, 7.5, and 8.0 by CO(2) infusion. Our results suggest the reduced rates of mitotic cleavage are likely unrelated to common cell cycle checkpoints. We found no significant differences in the three biomarkers assessed between pH treatments, indicating the embryos progress through the G(1)/S, G(2)/M and metaphase/anaphase transitions at relatively similar rates. These data suggest low pH environments may not impact developmental programs directly, but may act through secondary mechanisms such as cellular energetics.

Does the potential for chaos constrain the embryonic cell-cycle oscillator?

Although many of the core components of the embryonic cell-cycle network have been elucidated, the question of how embryos achieve robust, synchronous cellular divisions post-fertilization remains unexplored. What are the different schemes that could be implemented by the embryo to achieve synchronization? By extending a cell-cycle model previously developed for embryos of the frog Xenopus laevis to include the spatial dimensions of the embryo, we establish a novel role for the rapid, fertilization-initiated calcium wave that triggers cell-cycle oscillations. Specifically, in our simulations a fast calcium wave results in synchronized cell cycles, while a slow wave results in full-blown spatio-temporal chaos. We show that such chaos would ultimately lead to an unpredictable patchwork of cell divisions across the embryo. Given this potential for chaos, our results indicate a novel design principle whereby the fast calcium-wave trigger following embryo fertilization synchronizes cell divisions.

Cell divisions across an embryo occur in rapid synchrony - like clockwork - starting within minutes of fertilization. How does an embryo achieve this remarkable uniformity? Simple diffusion is too slow: typical proteins diffuse with a rate of 10 µm²/s, requiring nearly 14 hours to traverse a 1 mm embryo. An exciting idea is that the embryo is an active medium, much like the heart where pulses of electrical activity result in organized contractions. However, just as the heart can have arrhythmias, our model predicts that oscillations in the embryo can become chaotic. What would be the biological consequences of this behavior? How do embryos avoid chaos? Our work provides potential answers to these questions: Chaos would lead to an unpredictable patchwork of cell divisions across the embryo - clearly a fatal defect in development. To avoid chaos then, we predict that cell-cycle oscillations need to be triggered throughout the embryo at almost precisely the same time. The threat that chaos will mar development therefore explains the mystery of why embryos universally employ a fast calcium wave to trigger cell-cycle oscillations. In this way, developing organisms get the synchronizing benefits of an active medium without suffering the destructive consequences of chaotic arrhythmias.

Pregnancy after the calcium ionophore correction of pronuclei position in oocytes after intracytoplasmic sperm injection

OBJECTIVE

To report a successful pregnancy after transfer of embryos derived from oocytes after calcium ionophore correction of the pronuclei localization after intracytoplasmic sperm injection (ICSI).

DESIGN

Case report.

SETTING

University hospital.

PATIENT(S)

A 30-year-old patient and her 30-year-old husband, diagnosed with asthenoteratozoopermia, underwent ICSI because of three unsuccessful IUI attempts.

INTERVENTION(S)

Thirteen metaphase II oocytes were injected with morphologically normal spermatozoa and immediately divided into two groups. Group 1 (n = 7) was the untreated control and in group 2 (n = 6), the oocytes were treated with 10 μM calcium ionophore solution for 20 minutes at 37°C in 6% CO(2). The fertilization was checked 18 hours later and location of pronuclei in cytoplasm was assessed. Transfer of two embryos was performed on the third day after oocyte retrieval.

MAIN OUTCOME MEASURE(S)

Pregnancy after transfer of embryos after calcium ionophore correction of pronuclei localization.

RESULT(S)

Fertilized zygotes from calcium ionophore-treated oocytes with physiologically normal central localization of pronuclei were chosen for ET. Clinical pregnancy was confirmed at 7 weeks of gestation.

CONCLUSION(S)

Post-ICSI calcium ionophore activation of oocytes can be used for correction of the pronuclei localization, which can increase developmental rate of embryos.

Calcium signalling in early embryos

The onset of development in most species studied is triggered by one of the largest and longest calcium transients known to us. It is the most studied and best understood aspect of the calcium signals that accompany and control development. Its properties and mechanisms demonstrate what embryos are capable of and thus how the less-understood calcium signals later in development may be generated. The downstream targets of the fertilization calcium signal have also been identified, providing some pointers to the probable targets of calcium signals further on in the process of development. In one species or another, the fertilization calcium signal involves all the known calcium-releasing second messengers and many of the known calcium-signalling mechanisms. These calcium signals also usually take the form of a propagating calcium wave or waves. Fertilization causes the cell cycle to resume, and therefore fertilization signals are cell-cycle signals. In some early embryonic cell cycles, calcium signals also control the progress through each cell cycle, controlling mitosis. Studies of these early embryonic calcium-signalling mechanisms provide a background to the calcium-signalling events discussed in the articles in this issue.

Flipping the switch: How a sperm activates the egg at fertilization

Sperm interaction with an egg in animals was first documented 160 years ago in sea urchins by Alphonse Derbès (1847) when he noted the formation of an "envelope" following the sperm's "approach" to the egg. The "envelope" in sea urchins is an obvious phenotype of fertilization in this animal and over the past 35 years has served to indicate a presence of calcium released from cytoplasmic stores essential to activate the egg. The mechanism of calcium release has been intensely studied because it is a universal regulator of cellular activity, and recently several intersecting pathways of calcium release have been defined. Here we examine these various mechanisms with special emphasis on recent work in eggs of both sea urchins and mice.

A functional genomic and proteomic perspective of sea urchin calcium signaling and egg activation

The sea urchin egg has a rich history of contributions to our understanding of fundamental questions of egg activation at fertilization. Within seconds of sperm-egg interaction, calcium is released from the egg endoplasmic reticulum, launching the zygote into the mitotic cell cycle and the developmental program. The sequence of the Strongylocentrotus purpuratus genome offers unique opportunities to apply functional genomic and proteomic approaches to investigate the repertoire and regulation of Ca(2+) signaling and homeostasis modules present in the egg and zygote. The sea urchin "calcium toolkit" as predicted by the genome is described. Emphasis is on the Ca(2+) signaling modules operating during egg activation, but the Ca(2+) signaling repertoire has ramifications for later developmental events and adult physiology as well. Presented here are the mechanisms that control the initial release of Ca(2+) at fertilization and additional signaling components predicted by the genome and found to be expressed and operating in eggs at fertilization. The initial release of Ca(2+) serves to coordinate egg activation, which is largely a phenomenon of post-translational modifications, especially dynamic protein phosphorylation. Functional proteomics can now be used to identify the phosphoproteome in general and specific kinase targets in particular. This approach is described along with findings to date. Key outstanding questions regarding the activation of the developmental program are framed in the context of what has been learned from the genome and how this knowledge can be applied to functional studies.

ITP

I describe how we came to microinject inositol trisphosphate into sea urchin eggs and found that is was a very potent activator of calcium release.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

ERK1 activation is required for S-phase onset and cell cycle progression after fertilization in sea urchin embryos

Fertilization of sea urchin eggs results in a large, transient increase in intracellular free Ca2+ concentration that is responsible for re-initiation of the cell division cycle. We show that activation of ERK1, a Ca2+-dependent MAP kinase response, is required for both DNA synthesis and cell cycle progression after fertilization. We combine experiments on populations of cells with analysis at the single cell level, and develop a proxy assay for DNA synthesis in single embryos, using GFP-PCNA. We compare the effects of low molecular weight inhibitors with a recombinant approach targeting the same signalling pathway. We find that inhibition of the ERK pathway at fertilization using either recombinant ERK phosphatase or U0126, a MEK inhibitor, prevents accumulation of GFP-PCNA in the zygote nucleus and that U0126 prevents incorporation of [3H]-thymidine into DNA. Abrogation of the ERK1 signalling pathway also prevents chromatin decondensation of the sperm chromatin after pronuclear fusion, nuclear envelope breakdown and formation of a bipolar spindle.

Structural characterization of the ADAM 16 disintegrin loop active site

ADAM's have various roles in intercellular adhesion and are thought to function by binding integrins through a 13 amino acid motif called the disintegrin loop. Xenopus laevis sperm express the protein ADAM 16, and peptides with the sequence of its disintegrin loop cause downstream events in eggs that require a rise in intracellular calcium similar to that occurring at fertilization. We characterized the portion of the ADAM 16 disintegrin loop responsible for causing egg activation. A peptide based on the C-terminal half of the motif, which includes a known integrin-binding sequence, is a partial agonist of calcium release. A peptide with the N-terminal sequence of the motif activates eggs in a manner virtually identical to the full-length peptide but lacks a recognized integrin-binding sequence. None of these peptides alter the permeability or fluidity of liposomes made from membrane lipids of X. laevis eggs. This result reflects the fact that the peptides do not cause calcium to leak across the egg membrane and indirectly provides evidence that they act through a receptor on the egg surface. The infrared spectrum of the full-length peptide has a strong absorption peak corresponding to a beta-turn. We predict this structure occurs at the N-terminal sequence MPKT. A rearranged peptide lacking any turns fails to activate eggs. These results provide the first structural information about the active site of an ADAM disintegrin loop. We interpret these results in terms of active site sequences from other ADAM's and the role of integrins during fertilization.

The NO pathway acts late during the fertilization response in sea urchin eggs

Both the inositol 1,4,5-trisphosphate (InsP(3)) and ryanodine receptor pathways contribute to the Ca(2+) transient at fertilization in sea urchin eggs. To date, the precise contribution of each pathway has been difficult to ascertain. Evidence has accumulated to suggest that the InsP(3) receptor pathway has a primary role in causing Ca(2+) release and egg activation. However, this was recently called into question by a report implicating NO as the primary egg activator. In the present study we pursue the hypothesis that NO is a primary egg activator in sea urchin eggs and build on previous findings that an NO/cGMP/cyclic ADP-ribose (cADPR) pathway is active at fertilization in sea urchin eggs to define its role. Using a fluorescence indicator of NO levels, we have measured both NO and Ca(2+) at fertilization and establish that NO levels rise after, not before, the Ca(2+) wave is initiated and that this rise is Ca(2+)-dependent. By inhibiting the increase in NO at fertilization, we find not that the Ca(2+) transient is abolished but that the duration of the transient is significantly reduced. The latency and rise time of the transient are unaffected. This effect is mirrored by the inhibition of cGMP and cADPR signaling in sea urchin eggs at fertilization. We establish that cADPR is generated at fertilization, at a time comparable to the time of the rise in NO levels. We conclude that NO is unlikely to be a primary egg activator but, rather, acts after the initiation of the Ca(2+) wave to regulate the duration of the fertilization Ca(2+) transient.

Relationship between intracellular pH and chloride in Xenopus oocytes expressing the chloride channel ClC-0

During maturation of oocytes, Cl(-) conductance (G(Cl)) oscillates and intracellular pH (pH(i)) increases. Elevating pH(i) permits the protein synthesis essential to maturation. To examine whether changes in G(Cl) and pH(i) are coupled, the Cl(-) channel ClC-0 was heterologously expressed. Overexpressing ClC-0 elevates pH(i), decreases intracellular Cl(-) concentration ([Cl(-)](i)), and reduces volume. Acute acidification with butyrate does not activate acid extrusion in ClC-0-expressing or control oocytes. The ClC-0-induced pH(i) change increases after overnight incubation at extracellular pH 8.5 but is unaltered after incubation at extracellular pH 6.5. Membrane depolarization did not change pH(i). In contrast, hyperpolarization elevates pH(i). Thus neither membrane depolarization nor acute activation of acid extrusion accounts for the ClC-0-dependent alkalinization. Overnight incubation in low extracellular Cl(-) concentration increases pH(i) and decreases [Cl(-)](i) in control and ClC-0 expressing oocytes, with the effect greater in the latter. Incubation in hypotonic, low extracellular Cl(-) solutions prevented pH(i) elevation, although the decrease in [Cl(-)](i) persisted. Taken together, our observations suggest that KCl loss leads to oocyte shrinkage, which transiently activates acid extrusion. In conclusion, expressing ClC-0 in oocytes increases pH(i) and decreases [Cl(-)](i). These parameters are coupled via shrinkage activation of proton extrusion. Normal, cyclical changes of oocyte G(Cl) may exert an effect on pH(i) via shrinkage, thus inducing meiotic maturation.

Molecular cloning and characterization of ryanodine receptor from unfertilized sea urchin eggs

Unfertilized eggs of sea urchins (Hemicentrotus pulcherrimus) demonstrated cyclic ADP-ribose (cADPR)-induced Ca(2+) release and caffeine-induced Ca(2+) release, both of which were considered to be mediated through the ryanodine receptor (RyR). We cloned cDNAs for sea urchin egg RyR (suRyR), which encode a 597-kDa protein of 5,317 amino acids. suRyR shares common structural features with known RyRs: the well-conserved COOH-terminal domain, which forms a functional Ca(2+) channel, and a large hydrophilic NH2-terminal domain. suRyR shows amino acid sequence identity (43-45%) similar to the three mammalian RyR isoforms. Phylogenetic analysis indicates that suRyR branched from three isoforms of vertebrates before they diverged, suggesting that suRyR may be the only RyR isoform in the sea urchin. Four in-frame insertions were found in suRyR cDNAs, one of which was novel and unique, in that it had a cluster of serine residues. The transcripts with and without these insertions were found in the egg RNA. These results suggest that suRyR may be expressed as a functional Ca(2+)-induced Ca(2+) release channel, which might also be involved in cADPR-induced Ca(2+) release.

Porcine oocyte activation: differing roles of calcium and pH

Intracellular pH has recently been shown to increase during parthenogenetic activation of the porcine oocyte. In the following set of experiments, intracellular pH was monitored during activation and pronuclear development was assessed following activation treatments with calcium, in the absence of calcium, and in oocytes loaded with the calcium chelator BAPTA-AM in calcium-free medium. Intracellular pH increase was not different among groups when treating with 7% ethanol or 50 microM calcium ionophore, or during treatment with thimerosal for 12 or 25 min. Activation with thimerosal (200 microM, 12 min) followed by 8 mM dithiothreitol (DTT, 30 min) resulted in a decreased pronuclear development in calcium-free medium with or without BAPTA-AM loaded oocytes as compared to controls. Activation with 50 microM calcium ionophore resulted in pronuclear development that was different between the calcium-free and BAPTA-AM loaded oocytes in calcium-free medium. Similar incidences of pronuclear formation were observed in all ethanol treatment groups. It was concluded that external calcium as well as large changes in intracellular free calcium are not necessary for the increase in intracellular pH, but normal intracellular calcium signaling is critical for normal levels of pronuclear development. Finally, oocytes were measured for intracellular pH changes for 30 min following subzonal sperm injection. Intracellular pH did not increase, although pronuclear formation was observed 6 hr post SUZI. This suggested that major differences were still present between sperm-induced and parthenogenetic activation of the porcine oocyte.

Mechanism of intracellular pH increase during parthenogenetic activation of In vitro matured porcine oocytes

Parthenogenetic activation of porcine oocytes by using 7% ethanol, 50 or 100 microM A23187 results in an increase in intracellular pH as does prolonged exposure to thimerosal. We attempt to specify which transporters or mechanisms are involved in the observed increase in intracellular pH during oocyte activation. Experiments were performed in the absence of sodium; the presence of 2.5 mM amiloride, a potent inhibitor of the Na(+)/H(+) antiport; in the absence of bicarbonate; and in the presence of 4, 4'-diisothiocyanatodihydrostilbene-2,2'-di-sulfonic acid, disodium salt (H(2)DIDS) for all three activation methods. These treatments had no effect on the increase in intracellular pH induced by the calcium ionophore or thimerosal, but all reduced the increase in pH (P < 0.001) in the 7% ethanol group. This suggests that the Na(+)/H(+) antiport and the HCO(3)(-)/Cl(-) exchangers are not playing a role during treatment with calcium ionophore or thimerosal, and the pH increase observed during treatment with 7% ethanol may be dependent upon a sodium or bicarbonate flux (or both) into the oocyte. Bafilomycin A1 (500 nm), an inhibitor of vacuolar-type H(+) ATPases, had no effect on 7% ethanol or thimerosal treatments, but significantly reduced the increase in intracellular pH observed during calcium ionophore treatment. This may be the result of an initial local increase in intracellular free calcium levels.

Cytoplasmic Ca2+ oscillation coordinates the formation of actin filaments in the sea urchin eggs activated with phorbol ester

Changes in the intracellular Ca2+ concentration ([Ca2+]i) and the formation of actin filaments were investigated in unfertilized eggs of the sea urchin Hemicentrotus pulcherrimus after activation with a phorbol ester, 12-O-tetradecanoyl phorbol13-acetate (TPA). Intracellular Ca2+ oscillation was observed using a fluorescent Ca2+ indicator dye, calcium green dextran. From about 20 to 80 min after the addition of TPA to 100 microM, there was a rise in [Ca2+]i, which was followed by Ca2+ oscillation. A change in [Ca2+]i in response to TPA was not observed in eggs that had been injected with heparin, an inositol 1,4,5-triphosphate (IP3) receptor antagonist. Therefore, long-term exposure to a high concentration of TPA seems to induce Ca2+ release via the IP3 pathway, as well as causing the release of diacylglycerol from membrane lipids. Moreover, the elongation of actin filaments occurred in the cytoplasm during the rise in [Ca2+]i. Actin filaments also formed when TPA-induced cytoplasmic alkalization was inhibited by exposure to Na(+)-free sea water. These results suggest that the observed cytoplasmic formation of actin filaments may be related to change in the cytoplasmic [Ca2(+)]i, and not intracellular pH, induced by TPA. These phenomena may be similar to the changes in actin construction that occur during cell cycle events.

Evidence that Src-type tyrosine kinase activity is necessary for initiation of calcium release at fertilization in sea urchin eggs

The initiation of Ca(2+) release from internal stores in the egg is a hallmark of egg activation. In sea urchins, PLCgamma activity is necessary for the production of IP(3), which leads to the initial rise in Ca(2+). To examine the possible function of a tyrosine kinase in activating PLCgamma at fertilization, sea urchin eggs were treated with the specific Src kinase inhibitor PP1 or microinjected with recombinant Src-family SH2-domain proteins, which act as dominant interfering inhibitors of Src-family kinase function. Both modes of inhibiting Src-family kinases resulted in a specific and dose-dependent delay in the onset of Ca(2+) release from the endoplasmic reticulum at fertilization. The rise in cytoplasmic pH at fertilization also was inhibited by microinjection of Src-family SH2-domain proteins. Further, an antibody directed against Src-type kinases recognized a protein of ca. M(r) 57K that was enriched in the membrane fraction of eggs. The kinase activity of this protein was stimulated rapidly and transiently at fertilization, as measured by autophosphorylation and by phosphorylation of an exogenous substrate. Together, these data indicate that a Src-type tyrosine kinase is necessary for the initiation of Ca(2+) release from the egg ER at fertilization and identify a Src-type p57 protein as a candidate in the signaling pathway leading to this Ca(2+) release.

Fertilization-induced activation of phospholipase C in the sea urchin egg

Fertilization results in the biphasic activation of polyphosphoinositide-specific phospholipase C (PLC) activity with an initial increase in activity coincident with the sperm-induced calcium transient, followed by a more sustained increase prior to mitosis. Immunoprecipitation studies demonstrated that the gamma isoform of PLC is present in both the unfertilized and the fertilized egg and contributes to the initial phase of PLC activation. Fertilization also resulted in translocation of a significant fraction of PLC-gamma from the cytosol to the membrane compartment of the egg.

Mechanism of Ca2+ release at fertilization in mammals

At fertilization in mammals the sperm triggers a series of oscillations in intracellular Ca2+ within the egg. These Ca2+ oscillations activate the development of the egg into an embryo. It is not known how the sperm triggers these Ca2+ oscillations. There are currently three different theories for Ca2+ signaling in eggs at fertilization. One idea is that the sperm acts as a conduit for Ca2+ entry into the egg after membrane fusion. Another idea is that the sperm acts upon plasma membrane receptors to stimulate a phospholipase C (PLC) within the egg which generates inositol 1,4, 5-trisphosphate (InsP(3)). We present a third idea that the sperm causes Ca2+ release by introducing a soluble protein factor into the egg after gamete membrane fusion. In mammals this sperm factor is also referred to as an oscillogen because, after microinjection, the factor causes sustained Ca2+ oscillations in eggs. Our recent data in sea urchin egg homogenates and intact eggs suggests that this sperm factor has phospholipase C activity that leads to the generation of InsP(3). We then present a new version of the soluble sperm factor theory of signaling at fertilization. J. Exp. Zool. (Mol. Dev. Evol.) 285:267-275, 1999.

Combined imaging and chemical sensing of fertilization-induced acid release from single sea urchin eggs

We demonstrate a microarray sensor capable of obtaining both chemical and visual information on multiple cells simultaneously with single-cell resolution. The array was fabricated by covalently immobilizing a thin, pH-sensitive polymer layer on the distal end of an optical imaging fiber. The sensor's ability to measure localized chemical dynamics in real-time was evaluated using sea urchin fertilization biochemistry as a model system. Following sea urchin fertilization, the Na(+)/H(+) transporter is activated to exchange extracellular sodium ions for intracellular hydrogen ions, causing a release of hydrogen ions at the egg's surface. By placing the pH sensor proximal to the egg and switching between a fluorescence image and a white light image, we were able to observe both localized pH changes following fertilization as well as morphological transformations during cell division.

Activation of the proteasomes of sand dollar eggs at fertilization depends on the intracellular pH rise

The mechanism of the activation of intracellular proteasomes at fertilization was measured in living sand dollar eggs using the membrane-impermeant fluorogenic substrate, succinyl-Phe-Leu-Arg-coumarylamido-4-methanesulfonic acid. When the substrate was microinjected into unfertilized eggs, the initial velocity of hydrolysis of the substrate (V0) was low. V0 measured 5 to 10 min after fertilization was five to nine times the prefertilization level and remained high throughout the first cell cycle. Hydrolysis of the substrate was inhibited by clasto-lactacystin beta-lactone, a specific inhibitor of the proteasome. There has been in vitro evidence that calcium may be involved in regulation of proteasome activity to either inhibit the increase in peptidase activity associated with PA 28 binding to the 20S proteasome or stimulate activity of the PA 700-proteasome complex. Since both intracellular free Ca2+ concentration ([Ca2+]i) and intracellular pH (pHi) increase after fertilization, hydrolysis of the proteasome substrate was measured under conditions in which [Ca2+]i and pHi were varied independently during activation. When the pHi of unfertilized eggs was elevated by exposure to 15 mM ammonium chloride in pH 9 seawater, V0 increased to a level comparable to that measured after fertilization. In contrast, [Ca2+]i elevation without pHi change, induced by calcium ionophore in sodium-free seawater, had no effect on V0 in the unfertilized egg. Moreover, when unfertilized eggs were microinjected with buffers modulating pHi, V0 increased in a pH-dependent manner. These results indicate that the pHi rise at fertilization is the necessary prerequisite for activation of the proteasome, an essential component in the regulation of the cell cycle.

Reconstitution of regulated exocytosis in cell-free systems: a critical appraisal

Regulated exocytosis involves the tightly controlled fusion of a transport vesicle with the plasma membrane. It includes processes as diverse as the release of neurotransmitters from presynaptic nerve endings and the sperm-triggered deposition of a barrier preventing polyspermy in oocytes. Cell-free model systems have been developed for studying the biochemical events underlying exocytosis. They range from semi-intact permeabilized cells to the reconstitution of membrane fusion from isolated secretory vesicles and their target plasma membranes. Interest in such cell-free systems has recently been reinvigorated by new evidence suggesting that membrane fusion is mediated by a basic mechanism common to all intracellular fusion events. In this chapter, we review some of the literature in the light of these new developments and attempt to provide a critical discussion of the strengths and limitations of the various cell-free systems.

Characterization of the sperm-induced calcium wave in Xenopus eggs using confocal microscopy

We have used confocal microscopy to examine the [Ca2+]i increase in the albino eggs of the frog Xenopus laevis after fertilization. Eggs were placed in agar wells with their animal poles downward so that fertilization occurred preferentially in the equatorial plane, and confocal microscopy was used to provide a two-dimensional optical section through the three-dimensional Ca2+ wave. These data indicate that the wave of increased [Ca2+]i traverses the entire egg and converges uniformly on the antipode. We show that ratioing two different fluorescent dyes to correct for variations in cell thickness is not a reliable technique for this very thick cell due to differential absorption with depth. Indo-1-dextran proves to be a more reliable Ca2+ indicator in this respect. Indo-1-dextran measurements indicate that the resting [Ca2+]i is not uniform throughout the egg but exhibits a 15% higher [Ca2+]i in the cortex than deep in the cytoplasm. This difference is accentuated during wave propagation and is not dependent on extracellular Ca2+. The average peak [Ca2+]i in the center of the egg as the wave propagates through it is 0.7 microM, approximately 60% of the peak cortical [Ca2+]i. The wave velocity through the center of the egg (5.7 micron/s) is slower than that in the cortex (8.9 micron/s), and both velocities vary slightly during transit. The cortical wave speed is particularly high at the beginning (15.7 micron/s) and end (17.2 micron/s) of the wave. Eggs injected with 30-80 microM of 3 kD heparin to compete with inositol-1,4,5,-trisphosphate for binding to its receptor exhibited multiple localized spots of elevated [Ca2+]i, and many of these did not initiate a wave. For those that did lead to a wave, it was usually slow moving and exhibited a reduced (60% reduction) amplitude compared with controls.

MAP kinase activity increases during mitosis in early sea urchin embryos

A MBP kinase activity increases at mitosis during the first two embryonic cell cycles of the sea urchin embryo. The activity profile of the MBP kinase is the same both in whole cell extracts and after immunoprecipitation with an anti-MAP kinase antibody (2199). An in-gel assay of MBP activity also shows the same activity profile. The activity is associated with the 44 kDa protein that cross-reacts with anti-MAP kinase antibodies. The 44 kDa protein shows cross-reactivity to anti-phosphotyrosine and MAP kinase-directed anti-phosphotyrosine/phosphothreonine antibodies at the times that MBP kinase activity is high. The 2199 antibody co-precipitates some histone H1 kinase activity, but the MBP kinase activity cannot be accounted for by histone H1 kinase-dependent phosphorylation of MBP. The MAP kinase 2199 antibody was used to purify the MBP kinase activity. Peptide sequencing after partial digestion shows the protein to be homologous to MAP kinases from other species. These data demonstrate that MAP kinase activation during nuclear division is not confined to meiosis, but also occurs during mitotic cell cycles. MAP kinase activity in immunoprecipitates also increases immediately after fertilization, which in the sea urchin egg occurs at interphase of the cell cycle. Treating unfertilized eggs with the calcium ionophore A23187 stimulates the increase in MAP kinase activity, demonstrating that a calcium signal can activate MAP kinase and suggesting that the activation of MAP kinase at fertilization is due to the fertilization-induced increase in cytoplasmic free calcium concentration. This signalling pathway must differ from the pathway responsible for calcium-induced inactivation of MAP kinase activity that is found in eggs that are fertilized in meiotic metaphase.

Calcium and mitosis

Calcium signals often accompany mitosis. The most obvious example of calcium as a mitotic signal is at fertilization in vertebrate eggs, where the calcium transient induces anaphase onset. New imaging methods have demonstrated smaller calcium signals that control mitosis entry and mitosis exit in sea urchin embryos. Other experiments in mouse and frog embryos indicate that similar signals with similar function may play a part in these embryos, too. The links between these calcium control signals and mitotic kinase activation are adumbrated. It appears that calcium oscillations are a property of the mitotic state. A case is made that calcium may be a universal mitotic signal, with the possible exception of early meiotic events in oocytes.

A novel protein for Ca2+ signaling at fertilization

At fertilization in all species studied the sperm activates the egg by causing an increase in the level of cytoplasmic free Ca2+ concentration. It is still not established how the sperm causes the changes in Ca2+ in the egg, which in the majority of eggs is due to release from internal stores. Current hypotheses about the signaling molecules involved in fertilization are confounded by the fact that for many eggs the fertilization-associated Ca2+ increase is readily mimicked by parthenogenetic activating agents. One exception to this is found for mammalian eggs where there are a series of Ca2+ oscillations observed at fertilization that have distinct characteristics. In this context we discuss three different theories of how sperm trigger Ca2+ release in eggs. We present the case that the sperm mediates its Ca2+ mobilization effects after gamete membrane fusion by introducing a specific protein into the egg cytoplasm. Our argument is based upon the fact that only the mammalian sperm protein factor can trigger a pattern of Ca2+ oscillations that is similar to that induced by the sperm in mammalian eggs. The sperm factor activity is correlated with a novel signaling protein that we have called oscillin and which may mediate Ca2+ release via a novel mechanism.

Non-specific currents at fertilisation in sea urchin oocytes

Using the whole-cell voltage-clamp technique to clamp sea urchin oocytes we show that the fertilising spermatozoon triggers an inward current of -521 +/- 56.7 pA (n = 8) at activation. Simultaneously, the plasma membrane depolarises and the conductance increases from 23.4 +/- 1.4 to 40.6 +/- 1.2 nS (n = 8). The I/V curve for the peak activation current is linear and the current reverses between 0 and +20 mV, suggesting a non-specific ion current. Since injection of inositol triphosphate induced an inward current of -1062 +/- 314 pA (n = 4), and the current was inhibited by preloading oocytes with the calcium chelator BAPTA, the non-specific activation current in sea urchin appears to be calcium dependent.

Tyrosine kinase signaling at fertilization

The unfertilized egg is a highly differentiated cell that retains unlimited developmental potential. The execution of that potential requires signal transduction pathways that release the egg from its quiescent metabolic state, direct the union of the maternal and paternal genome, and initiate a developmental program that will guide embryogenesis. The egg is equipped with an array of cytosolic as well as cell surface receptor protein tyrosine kinases as part of a preassembled signal transduction mechanism. These protein tyrosine kinases have been found to act at several points during this egg activation process, beginning as early as the initial sperm-egg interaction. While many of these kinase functions are common to all cells, several functions unique to fertilization demonstrate the versatility of this class of protein kinases.

Expression and localization of Na+/H+ exchangers in rat central nervous system

Neurons in the central nervous system regulate their intracellular pH using particular membrane proteins of which two, namely the Na+-dependent Cl-/HCO3- exchanger and the Na+/H+ exchanger, are essential. In this study we examined messenger RNA expression and distribution of Na+/H+ exchanger in the newborn rat central nervous system and with maturation using Northern blot analysis and in situ hybridization. Our study clearly shows that each Na+/H+ exchanger has a different expression pattern in the rat central nervous system. As in non-excitable tissues, Na+/H+ exchanger 1 is by far the most abundant of all Na+/H+ exchangers in the rat central nervous system. Its expression is ubiquitous although its messenger RNA appears at higher levels in the hippocampus, in the 2nd/3rd layers of periamygdaloid cortex and in the cerebellum. The low level of messenger RNAs encoding Na+/H+ exchanger 2 and 4 is mainly expressed in the cerebral cortex and in the brainstem-diencephalon, while Na+/H+ exchanger 3 transcripts are found only in the cerebellar Purkinje cells. From a developmental point of view, Na+/H+ exchanger 1, 2 and 4 showed an increased level in their transcripts in the cerebral cortex while an opposite trend existed in the cerebellum from postnatal day 0 to postnatal day 30. The messenger RNA for Na+/H+ exchanger 3, however, increased its level with age in cerebellum. From our data we conclude that: i) the expression of the Na+/H+ exchanger is age-, region-, and subtype-specific, with Na+/H+ exchanger 1 being the most prevalent in the rat central nervous system; ii) specialization of groups of neurons with respect to the type of Na+/H+ exchanger is clearly illustrated by Na+/H+ exchanger 3 which is almost totally localized in cerebellar Purkinje cells; and iii) the developmental increase in the messenger RNA for Na+/H+ exchanger 1 in the cerebral cortex and hippocampus is consistent with our previous studies on intracellular pH physiology in neonatal and mature neurons. Together this study indicates that expression of each Na+/H+ exchanger messenger RNA is differentially regulated both during development and in the different regions of rat central nervous system.

A soluble extract from human spermatozoa activates ascidian oocytes

A soluble extract from human spermatozoa induced calcium oscillations and extrusion of the first polar body when injected into oocytes of the ascidian Ciona intestinalis. The properties of calcium oscillations and time of polar body extrusion precisely mimic oocyte activation induced by C. intestinalis sperm or sperm extracts. The data suggest that human sperm extracts can activate oocytes of different phyla by the same mechanism as homologous spermatozoa. Injection of inositol 1,4,5-trisphosphate (IP3) into C. intestinalis oocytes mimicked to some extent the initial stages of oocyte activation, but the results demonstrate that ascidian oocyte activation by human sperm extract cannot be explained solely in terms of IP3-induced calcium release. Injection of other calcium releasing second messengers, cyclic adenosine diphosphate ribose, or calcium ions, does not lead to oocyte activation or release intracellular calcium in ascidian oocyte. It was concluded that human spermatozoa contain one or more molecules than can trigger intracellular calcium release in oocytes from different phyla.

Sperm-egg fusion is the prelude to the initial Ca2+ increase at fertilization in the mouse

Fusion of sperm and egg plasma membranes is an early and essential event at fertilization but it is not known if it plays a part in the signal transduction mechanism that leads to the oscillations in the cytoplasmic free Ca2+ concentration ([Ca2+]i) that accompany mammalian egg activation. We have used two independent fluorescence methods and confocal microscopy to show that cytoplasmic continuity of egg and sperm precedes the onset of the first [Ca2+]i increase in mouse eggs. The Ca2+ indicator dye Ca2+-green dextran was microinjected and its transfer from egg to sperm was monitored. We found that it occurred before, and without a requirement for, any detectable [Ca2+]i increase in the egg. In separate experiments [Ca2+]i changes were recorded in populations of eggs, using fura red, and the eggs fixed at various times after some of the eggs had shown a [Ca2+]i transient. Fusion of the sperm and egg was then assessed by Hoechst dye transfer. All eggs that showed a [Ca2+]i increase had a fused sperm but more than half of the eggs contained a sperm but had not undergone a [Ca2+]i increase. These data indicate that sperm-egg fusion precedes [Ca2+]i changes and we estimate that the elapsed time between sperm-egg fusion and the onset of the [Ca2+li oscillations is 1–3 minutes. Finally, sperm-egg fusion was prevented by using low pH medium which reversibly prevented [Ca2+]i oscillations in eggs that had been inseminated. This was not due to disruption of signalling mechanisms, since [Ca2+]i changes still occurred if low pH was applied after the onset of oscillations at fertilization. [Ca2+]i changes also occurred in eggs in low pH in response to the muscarinic agonist carbachol. These data are consistent with the idea that the [Ca2+]i signals that occur in mammalian eggs at fertilization are initiated by events that are closely coupled to the fusion of the sperm and egg membranes.

Calcium and cell cycle control in early embryos

Over the past few years, we have witnessed a burgeoning series of papers addressing the role of calcium signalling in cell cycle control. In this review I will attempt to bring together all the diverse threads and discuss new concepts that have arisen from the most recent data. Because the major part of the data concerns mitosis/meiosis entry and exit, I have focused on these areas. I will jointly refer to meiotic and mitotic phases of the cell cycle as M-phase because these phases are highly comparable. Studies of the cell cycle involve a huge range of species, from plants to humans. I will, however, restrict this review to the work performed in early embryos. I apologise in advance to contributors to this field whose names I do not mention because they do not work on embryos.