Meiosis reinitiation from the first prophase is dependent on the levels of intracellular Ca2+ and pH in oocytes of the bivalves Mactra chinensis and Limaria hakodatensis

Electrophysiology and Fluorescence Spectroscopy Approaches for Evaluating Gamete and Embryo Functionality in Animals and Humans

This review has examined two of the techniques most used by our research group for evaluating gamete and embryo functionality in animal species, ranging from marine invertebrates to humans. Electrophysiology has given access to fundamental information on some mechanisms underpinning the biology of reproduction. This technique demonstrates the involvement of ion channels in multiple physiological mechanisms, the achievement of homeostasis conditions, and the triggering of profound metabolic modifications, often functioning as amplification signals of cellular communication. Fluorescence spectrometry using fluorescent probes to mark specific cell structures allows detailed information to be obtained on the functional characteristics of the cell populations examined. The simple and rapid execution of this methodology allowed us to establish a panel helpful in elucidating functional features in living cells in a simultaneous and multi-parameter way in order to acquire overall drafting of gamete and embryo functionality.

Mechanisms that prevent catastrophic interactions between paternal chromosomes and the oocyte meiotic spindle

Meiosis produces haploid gametes by accurately reducing chromosome ploidy through one round of DNA replication and two subsequent rounds of chromosome segregation and cell division. The cell divisions of female meiosis are highly asymmetric and give rise to a large egg and two very small polar bodies that do not contribute to development. These asymmetric divisions are driven by meiotic spindles that are small relative to the size of the egg and have one pole juxtaposed against the cell cortex to promote polar body extrusion. An additional unique feature of female meiosis is that fertilization occurs before extrusion of the second polar body in nearly all animal species. Thus sperm-derived chromosomes are present in the egg during female meiosis. Here, we explore the idea that the asymmetry of female meiosis spatially separates the sperm from the meiotic spindle to prevent detrimental interactions between the spindle and the paternal chromosomes.

Ocean acidification hampers sperm-egg collisions, gamete fusion, and generation of Ca2+ oscillations of a broadcast spawning bivalve, Tegillarca granosa

Although the effect of ocean acidification on fertilization success of marine organisms is increasingly well documented, the underlying mechanisms are not completely understood. The fertilization success of broadcast spawning invertebrates depends on successful sperm-egg collisions, gamete fusion, and standard generation of Ca²⁺ oscillations. Therefore, the realistic effects of future ocean pCO₂ levels on these specific aspects of fertilization of Tegillarca granosa were investigated in the present study through sperm velocity trials, fertilization kinetics model analysis, and intracellular Ca²⁺ assays, respectively. Results obtained indicated that ocean acidification significantly reduced the fertilization success of T. granosa, which could be accountable by (i) decreased sperm velocity hence reducing the probability for sperm-egg collisions; (ii) lowered probability of gamete fusion for each gamete collision event; and (iii) disrupted intracellular Ca²⁺ oscillations.

Insights into molecular features of Venerupis decussata oocytes: a microarray-based study

The production of Venerupis decussata relies on wild seed collection, which has been recently compromised due to recruitment failure and severe mortalities. To address this issue and provide an alternative source of seed, artificial spawning and larval rearing programs were developed. However, hatchery-based seed production is a relatively new industry and it is still underdeveloped. A major hurdle in the European clam seed production is the control of spawning and reproduction, which is further hindered by the impossibility of obtaining fertile gametes by gonadal "stripping", as meiosis re-initiation is constrained to a maturation process along the genital ducts. In the present study, oocytes were collected from 15 females and microarray analyses was performed to investigate gene expression profiles characterizing released and stripped ovarian oocytes. A total of 198 differentially expressed transcripts between stripped and spawned oocytes were detected. Functional analysis carried out on these transcripts highlighted the importance of a few biological processes, which are most probably implicated in the control of oocyte competence. Significant differences were observed for transcripts encoding proteins involved in meiosis progression (e.g. dual specificity phosphatase CDC25), WNT signalling (e.g. frizzled class receptor 8, wingless-type MMTV integration site family member 4), steroid synthesis (e.g. progestin and adipoQ receptor family member 3, cytochrome P450-C17), mRNA processing (e.g. zinc finger protein XlCOF28), calcium regulation (e.g. regucalcin, calmodulin) and ceramide metabolism (ceramidase B, sphingomyelinase). This study provides new information on transcriptional profiles putatively associated with ovarian egg infertility, and suggests potential mechanisms regulating early oocyte development in clams. Genes which were differentially expressed between stripped and spawned oocytes might have a pivotal role during maturation process in the gonadal duct and could be interesting targets for further functional studies aiming to make ovarian oocytes fertilizable.

Intracellular calcium signaling in the fertilized eggs of Annelida

Fertilization is such a universal and indispensable step in sexual reproduction, but a high degree of variability exists in the way it takes place in the animal kingdom. As discussed in other reviews in this issue, recent works on this subject clarified many points. However, important results on the mechanisms of fertilization are obtained mainly from a few restricted model organisms. In this sense, it is utterly important to collect more information from various phyla. In this review, we have re-introduced Annelida as one of the most suitable models for the analysis of fertilization process. We have briefly reviewed the historical works on the fertilization of Annelida. Then, we have described recent findings on the two independent Ca(2+) increases in the fertilized eggs of Annelida, which arise from two different mechanisms and may have distinct physiological roles toward sperm entry and egg activation. We propose that the Ca(2+) increase in the fertilized eggs reflect the specific needs of the zygote in a given species.

FINITE ELEMENT MODEL TO STUDY THE MECHANICS OF CALCIUM REGULATION IN OOCYTE

At various stages of fertilization specific spatial and temporal patterns of Ca 2+ are required for oocyte maturation. It is crucial to understand the mechanics of Ca 2+ regulation in cytosol of oocytes, in order to have better understanding of fertilization process. In this paper, a finite element model of cytosolic calcium regulation in oocyte has been developed for a two-dimensional unsteady state case. The model incorporates the important biophysical processes like diffusion, reaction, leak from endoplasmic recticulum (ER), efflux from cytosol to ER via sarco-ER calcium adenosine triphosphate (SERCA) pumps, buffers and sodium calcium exchanger. Appropriate boundary conditions have been framed. The effect of source, buffer, sodium calcium exchanger, etc. on spatial and temporal patterns of calcium in oocyte have been studied with the help of numerical results.

Comparative biology of sperm factors and fertilization-induced calcium signals across the animal kingdom

Fertilization causes mature oocytes or eggs to increase their concentrations of intracellular calcium ions (Ca²⁺) in all animals that have been examined, and such Ca²⁺ elevations, in turn, provide key activating signals that are required for non-parthenogenetic development. Several lines of evidence indicate that the Ca²⁺ transients produced during fertilization in mammals and other taxa are triggered by soluble factors that sperm deliver into oocytes after gamete fusion. Thus, for a broad-based analysis of Ca²⁺ dynamics during fertilization in animals, this article begins by summarizing data on soluble sperm factors in non-mammalian species, and subsequently reviews various topics related to a sperm-specific phospholipase C, called PLCζ, which is believed to be the predominant activator of mammalian oocytes. After characterizing initiation processes that involve sperm factors or alternative triggering mechanisms, the spatiotemporal patterns of Ca²⁺ signals in fertilized oocytes or eggs are compared in a taxon-by-taxon manner, and broadly classified as either a single major transient or a series of repetitive oscillations. Both solitary and oscillatory types of fertilization-induced Ca²⁺ signals are typically propagated as global waves that depend on Ca²⁺ release from the endoplasmic reticulum in response to increased concentrations of inositol 1,4,5-trisphosphate (IP₃). Thus, for taxa where relevant data are available, upstream pathways that elevate intraoocytic IP3 levels during fertilization are described, while other less-common modes of producing Ca²⁺ transients are also examined. In addition, the importance of fertilization-induced Ca²⁺ signals for activating development is underscored by noting some major downstream effects of these signals in various animals.

Comparative biology of cAMP-induced germinal vesicle breakdown in marine invertebrate oocytes

During maturation, oocytes must undergo a process of nuclear disassembly, or "germinal vesicle breakdown" (GVBD), that is regulated by signaling pathways involving cyclic AMP (cAMP). In vertebrate and starfish oocytes, cAMP elevation typically prevents GVBD. Alternatively, increased concentrations of intra-oocytic cAMP trigger, rather than inhibit, GVBD in several groups of marine invertebrates. To integrate what is known about the stimulation of GVBD by intra-oocytic cAMP, this article reviews published data for ascidian, bivalve, brittle star, jellyfish, and nemertean oocytes. The bulk of the review concentrates on the three most intensively analyzed groups known to display cAMP-induced GVBD-nemerteans, ascidians, and jellyfish. In addition, this synopsis also presents some previously unpublished findings regarding the stimulatory effects of intra-oocytic cAMP on GVBD in jellyfish and the annelid worm Pseudopotamilla occelata. Finally, factors that may account for the currently known distribution of cAMP-induced GVBD across animal groups are discussed.

Calcium waves and oscillations in eggs

Eggs from several protostomes (molluscs, annelids, nemerteans, etc.) and two deuterostomes (mammals and ascidians) display repetitive calcium signals. Oscillations in the level of intracellular calcium concentration are occasionally triggered by maturing hormones (as in some molluscs) and mostly observed after fertilization which occurs at different stages of the meiotic cell cycle (oocytes are arrested in prophase, metaphase I or metaphase II). In most eggs examined so far, calcium oscillations last until the end of meiosis just before male and female pronuclei form. This ability depends on the sensitivity of InsP3 channels and on the permeability of the plasma membrane to extracellular calcium. In eggs that undergo cytoplasmic reorganization at fertilization (annelids, nemerteans, ascidians, etc.) the repetitive calcium signals are waves that originate from localized cortical sites that become calcium waves pacemakers. In ascidians we have identified the site of initiation of repetitive calcium waves as an accumulation of endoplasmic reticulum sandwiched between the plasma membrane and an accumulation of mitochondria. We compare and discuss the generation of calcium signals in the different eggs, their relationship with the cell cycle and the possible roles they play during development.

A novel mechanism controls the Ca2+ oscillations triggered by activation of ascidian eggs and has an absolute requirement for Cdk1 activity

Fertilisation in ascidians triggers a series of periodic rises in cytosolic Ca2+ that are essential for release from metaphase I arrest and progression through meiosis II. These sperm-triggered Ca2+ oscillations are switched off at exit from meiosis II. Ascidian zygotes provided the first demonstration of the positive feedback loop whereby elevated Cdk1 activity maintained these Ca2+ oscillations. Since then it has been reported that Cdk1 sensitises the type I inositol trisphosphate [Ins(1,4,5)P3] receptor in somatic cells, and that sperm-triggered Ca2+ oscillations in mouse zygotes stop because the forming pronuclei sequester phospholipase C zeta that was delivered to the egg by the fertilising sperm.

Here, using enucleation, we demonstrate in ascidian eggs that Ca2+ spiking stops at the correct time in the absence of pronuclei. Sequestration of sperm factor is therefore not involved in terminating Ca2+ spiking for these eggs. Instead we found that microinjection of the Cdk1 inhibitor p21 blocked Ca2+ spiking induced by ascidian sperm extract (ASE). However, such eggs were still capable of releasing Ca2+ in response to Ins(1,4,5)P3 receptor agonists, indicating that ASE-triggered Ca2+ oscillations can stop even though the response to Ins(1,4,5)P3 remained elevated. These data suggest that Cdk1 activity promotes Ins(1,4,5)P3 production in the presence of the sperm factor, rather than sensitising the Ca2+ releasing machinery to Ins(1,4,5)P3. These findings suggest a new link between this cell cycle kinase and the Ins(1,4,5)P3 pathway.

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.

A novel oocyte maturation arresting factor in the central nervous system of scallops inhibits serotonin-induced oocyte maturation and spawning of bivalve mollusks

Serotonin (5-hydroxytriptamine; 5-HT) is a major neurotransmitter that triggers oocyte maturation and sequential spawning in bivalve mollusks. A proteinous and heat-labile substance that proved to be a novel inhibitor of 5-HT-induced egg release from ovarian tissue was found in the cerebral and pedal ganglia (CPG) of the scallop Patinopecten yessoensis. The same inhibitory activity was also observed in the proteinous fraction from the supernatant of hemolymph. Histological observation demonstrated that the novel inhibitor prevented 5-HT from inducing oocyte maturation in the scallop ovary and that no prostaglandin F2alpha (PGF2alpha) inhibited 5-HT-induced oocyte maturation, although PGF2alpha strongly prohibited 5-HT-induced egg release through the gonoduct from ovarian tissue. The novel inhibitor from the scallop CPG also prohibited 5-HT-induced oocyte maturation of other bivalve species as well as scallops. The novel inhibitor, mediated through a receptor mechanism on oocyte membranes, blocked extracellular Ca2+ uptake into oocytes, which was observed in 5-HT-induced oocyte maturation. It is suggested that the novel inhibitor with a molecular mass of 60 kDa, named oocyte maturation arresting factor, which appears to be a universal substance for bivalve species, may be transported from the CPG to the ovary via hemolymph and may prohibit 5-HT-induced oocyte maturation due to the interference of extracellular Ca2+ influx into oocytes, eventually resulting in the inhibition of spawning. On the other hand, it seems that PGF2alpha inhibits 5-HT-induced transport of mature eggs through the gonoduct.

Calcium ion currents mediating oocyte maturation events

During maturation, the last phase of oogenesis, the oocyte undergoes several changes which prepare it to be ovulated and fertilized. Immature oocytes are arrested in the first meiotic process prophase, that is morphologically identified by a germinal vesicle. The removal of the first meiotic block marks the initiation of maturation. Although a large number of molecules are involved in complex sequences of events, there is evidence that a calcium increase plays a pivotal role in meiosis re-initiation. It is well established that, during this process, calcium is released from the intracellular stores, whereas less is known on the role of external calcium entering the cell through the plasma membrane ion channels. This review is focused on the functional role of calcium currents during oocyte maturation in all the species, from invertebrates to mammals. The emerging role of specific L-type calcium channels will be discussed.

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.

Signaling pathways in ascidian oocyte maturation: Effects of various inhibitors and activators on germinal vesicle breakdown

The Ascidiacea, the invertebrate chordates, includes three orders; the Stolidobranchia is the most complex. Until the present study, the onset of oocyte maturation (germinal vesicle breakdown) had been investigated in only a single pyurid (Halocynthia roretzi), in which germinal vesicle breakdown (GVBD) begins when the oocyte contacts seawater (SW); nothing was known about internal events. This study strongly suggests the importance of protein phosphorylation in this process. Herdmania pallida (Pyuridae) functions like H. roretzi; GVBD occurs in SW. Oocytes of Cnemidocarpa irene (Styelidae) do not spontaneously undergo GVBD in SW but must be activated. Herdmania oocytes are inhibited from GVBD by pH 4 SW and subsequently activated by mastoparan (G-protein activator), A23187 (Ca2+ ionophore) or dimethylbenzanthracene (tyrosine kinase activator). This requires maturation promoting factor (MPF) activity; cyclin-dependent kinase inhibitors roscovitine and olomoucine are inhibitory. It also entails dephosphorylation as demonstrated by the ability of the phosphatase inhibitor vitamin K3 to inhibit GVBD. GVBD is also inhibited by the tyrosine kinase inhibitors tyrphostin A23 and genistein, and LY-294002, a phosphatidylinositol-3-kinase inhibitor previously shown to inhibit starfish GVBD. LY-294002 inhibits strongly when activation is by mastoparan or ionophore but not when activated by dimethylbenzanthracene (DMBA). The DMBA is hypothesized to phosphorylate a phosphatase directly or indirectly causing secondary activation, bypassing inhibition.

Ca(2+)(cyt) negatively regulates the initiation of oocyte maturation

Ca²⁺ is a ubiquitous intracellular messenger that is important for cell cycle progression. Genetic and biochemical evidence support a role for Ca²⁺ in mitosis. In contrast, there has been a long-standing debate as to whether Ca²⁺ signals are required for oocyte meiosis. Here, we show that cytoplasmic Ca²⁺ (Ca²⁺_cyt) plays a dual role during Xenopus oocyte maturation. Ca²⁺ signals are dispensable for meiosis entry (germinal vesicle breakdown and chromosome condensation), but are required for the completion of meiosis I. Interestingly, in the absence of Ca²⁺_cyt signals oocytes enter meiosis more rapidly due to faster activation of the MAPK-maturation promoting factor (MPF) kinase cascade. This Ca²⁺-dependent negative regulation of the cell cycle machinery (MAPK-MPF cascade) is due to Ca²⁺_cyt acting downstream of protein kinase A but upstream of Mos (a MAPK kinase kinase). Therefore, high Ca²⁺_cyt delays meiosis entry by negatively regulating the initiation of the MAPK-MPF cascade. These results show that Ca²⁺ modulates both the cell cycle machinery and nuclear maturation during meiosis.

External Ca2+ is predominantly used for cytoplasmic and nuclear Ca2+ increases in fertilized oocytes of the marine bivalve Mactra chinensis

Oocytes of the marine bivalve Mactra chinensis are spawned and arrested at the germinal vesicle stage (first meiotic prophase) until fertilization, without undergoing a process called oocyte maturation. As is the case of other animals, a fertilized oocyte of the bivalve displays increases in intracellular free Ca(2+). We have clarified here the spatiotemporal patterns and sources of the intracellular Ca(2+) changes at fertilization. Shortly after insemination, increased Ca(2+) simultaneously appeared at the whole cortical region of the oocyte and spread inwardly to the center, attaining the maximal Ca(2+) levels throughout the oocyte, including the cytoplasm and nucleus. The initial maximal Ca(2+) peak was followed by a submaximal plateau phase of cytoplasmic and nuclear Ca(2+) elevations, which persisted for several minutes. The nuclear envelope began to break down shortly before the termination of the plateau phase. These sperm-induced Ca(2+) changes were inhibited by suppression of the influx of external Ca(2+) from seawater but not by disturbance of the release of internal Ca(2+) from inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)]-sensitive stores, suggesting that the increased Ca(2+) is from an external source. In contrast to the situation observed at fertilization, an oocyte artificially stimulated with serotonin (5-hydroxytryptamine, 5-HT) displayed repetitive Ca(2+) transients, each of which started from one cortical region and propagated across the oocyte as a Ca(2+) wave. The 5-HT-induced Ca(2+) transients persisted even in the absence of external Ca(2+). Experiments with caged Ins(1,4,5)P(3) revealed that Ca(2+) release from Ins(1,4,5)P(3)-sensitive stores is another pathway that is sufficient to trigger meiosis reinitiation from the first prophase. These results demonstrate that Mactra oocytes can potentially use two different Ca(2+)-mobilizing pathways: Ca(2+) influx producing a centripetal Ca(2+) wave from the whole cortex and Ca(2+) release from Ins(1,4,5)P(3)-sensitive stores producing a point-source propagating Ca(2+) wave. However, it seems likely that the Ca(2+) influx pathway is predominantly activated at fertilization.

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.

Attributes and dynamics of the endoplasmic reticulum in mammalian eggs

The endoplasmic reticulum is a multifunctional continuous network of membrane-enclosed sacs and tubules that extends throughout the cell. The endoplasmic reticulum is the site of protein synthesis and assembly, as well as lipid and membrane synthesis. Additionally, the endoplasmic reticulum contains calcium pumps, intraluminal calcium storage proteins, and specific calcium-releasing channels. Thus, this membrane system plays a central role in intracellular signaling through the storage and release of calcium. At fertilization, the sperm triggers a large and dramatic release of calcium from the endoplasmic reticulum, which activates the egg to begin development. The ability of the egg to fully elevate calcium depends on biochemical and structural changes during oocyte maturation. The sensitivity of the calcium-releasing system increases and the endoplasmic reticulum is reorganized during maturation of the oocyte; together, these dynamic changes place a substantial calcium storage compartment just beneath the membrane, near the site of sperm-egg fusion. Localization of the calcium store may also contribute to the long-lasting calcium oscillations that are characteristic of mammalian fertilization. Examination of the endoplasmic reticulum in living eggs is leading to a better understanding of calcium release at fertilization.

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.

Sperm-induced calcium oscillations at fertilisation in ascidians are controlled by cyclin B1-dependent kinase activity

The generation of calcium oscillations at fertilisation and during mitosis appears to be controlled by the cell cycle machinery. For example, the calcium oscillations in oocytes and embryos occur during metaphase and terminate upon entry into interphase. Here we report the manipulation of sperm-triggered calcium oscillations by cyclin-dependent kinase (CDK) activity, the major component of maturation/M phase promoting factor (MPF). To control the CDK activity we microinjected mRNAs encoding full-length GFP-tagged cyclin B1 or a truncated and therefore stabilised form of cyclin B1 ((delta)90) into unfertilised oocytes. In the presence of full-length cyclin B1, the calcium oscillations terminate when cyclin B1 levels fall along with the concomitant fall in the associated CDK activity. In addition, when the CDK activity is elevated indefinitely with (delta)90 cyclin B1, the calcium oscillations also continue indefinitely. Finally, in oocytes that contain low mitogen-activated protein (MAP) kinase activity and elevated CDK activity, the sperm-triggered calcium oscillations are again prolonged. We conclude that the CDK activity of the ascidian oocyte can be regarded as a positive regulator of sperm-triggered calcium oscillations, a finding that may apply to other oocytes that display sperm-triggered calcium oscillations at fertilisation. Furthermore, these findings may have a bearing upon the mitotic calcium signals of early embryos.

Comparative biology of calcium signaling during fertilization and egg activation in animals

During animal fertilizations, each oocyte or egg must produce a proper intracellular calcium signal for development to proceed normally. As a supplement to recent synopses of fertilization-induced calcium responses in mammals, this paper reviews the spatiotemporal properties of calcium signaling during fertilization and egg activation in marine invertebrates and compares these patterns with what has been reported for other animals. Based on the current database, fertilization causes most oocytes or eggs to generate multiple wavelike calcium oscillations that arise at least in part from the release of internal calcium stores sensitive to inositol 1,4,5-trisphosphate (IP3). Such calcium waves are modulated by upstream pathways involving oolemmal receptors and/or soluble sperm factors and in turn regulate calcium-sensitive targets required for subsequent development. Both "protostome" animals (e.g., mollusks, annelids, and arthropods) and "deuterostomes" (e.g., echinoderms and chordates) display fertilization-induced calcium waves, IP3-mediated calcium signaling, and the ability to use a combination of external calcium influx and internal calcium release. Such findings fail to support the dichotomy in calcium signaling modes that had previously been proposed for protostomes vs deuterostomes and instead suggest that various features of fertilization-induced calcium signals are widely shared throughout the animal kingdom.

The intracellular calcium increase at fertilization in Urechis caupo oocytes: activation without waves

The intracellular Ca2+ (Cai) increase at fertilization of the marine worm Urechis caupo (Echiura) was studied with conventional and confocal epifluorescence microscopy in oocytes microinjected with calcium green dextran or dually labeled with the calcium-insensitive dye tetramethylrhodamine dextran. Calcium green fluorescence was also measured with a photomultiplier system while the oocyte membrane potential was recorded and manipulated. The results show that Cai rises simultaneously around the oocyte cortex and peaks slightly later in the nucleoplasm. The Cai rise coincides with the initiation of the fertilization potential and we conclude that it is due primarily to external Ca2+ entering through the voltage-gated Ca2+ action potential channels that open during the fertilization potential because: (1) current clamping the oocyte membrane potential to positive values in the absence of sperm produces a similar Cai increase, (2) external Ca2+ is required, (3) and the confocal images are consistent with this mechanism. External application of sperm acrosomal peptide (P23) also caused a Cai increase that was inhibited in the presence of CoCl2. Cai and pHi (measured with BCECF dextran) were manipulated in experiments employing microinjection of BAPTA (to chelate Cai), external application of NH4Cl (to increase pHi) and CoCl2 (to block Ca2+ channels), and fertilization of eggs in pH 7 seawater (Cai increase without pHi increase). The results showed that increases in both Cai and pHi are required for GVBD; neither alone is sufficient. However, although nuclear and cytoplasmic Ca2+ levels tended to parallel each other in oocytes fertilized at pH 7, and during the initial Cai response in oocytes fertilized at pH 8, there was a disproportionate fluorescence increase in the nucleoplasm of the latter prior to GVBD which could not be explained by any artifact we tested, suggesting there may be a selective increase in nuclear Ca2+ associated with GVBD. Finally, electrophysiological experiments with BAPTA-injected oocytes showed that the opening of the fertilization potential Na+ channels was Ca2+-independent, (although they did not close at the normal time). These and earlier results suggest that Urechis sperm may activate oocytes by interacting directly with the Na+ channels or associated receptors.

Intracellular pH increase driven by an Na+/H+ exchanger upon activation of surf clam oocytes

Intracellular pH (pHi) measurements were performed in surf clam (Spisula solidissima) oocytes before and after artificial activation or fertilization [evidenced by germinal vesicle breakdown (GVBD)] by the dimethyloxazolidinedione (DMO) and 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) methods. Results using both methods showed increases of pHi of 0.3 pH unit after activation by excess K+. Using BCECF, we found an increase of similar magnitude after fertilization or after the addition of serotonin. By contrast, GVBD did not occur when the pHi was increased to similar or even higher levels by exposing the oocytes to ammonia. In sodium-free seawater, excess K+ induced GVBD but the pHi of K+-activated oocytes decreased significantly below the resting level of unactivated oocytes. The pHi increases in K+-activated oocytes were otherwise proportional to the external Na+ concentration. The amiloride derivatives dimethylamiloride and hexamethylene amiloride (at 10-50 microM) efficiently inhibited the K+-induced increase of pHi but did not block GVBD. These two derivatives were able, however, to retard K+-induced GVBD, hexamethylene amiloride being the more efficient. This retardation of K+-induced GVBD could be abolished by the simultaneous addition of ammonia. Taken altogether, these results show that a pHi increase, driven by a typical Na+/H+ exchanger, follows activation of surf clam oocytes but that this pHi increase is neither sufficient nor required for GVBD, though it does allow its progression at an optimal rate.

Parthenogenesis in Urechis caupo (Echiura). II. Role of intracellular pH in parthenogenesis induction

A peptide (P23) isolated from sperm acrosomal protein initiates development in eggs of the marine worm Urechis caupo. We have shown previously that eggs exposed to P23 for > or = 3 min complete meiosis but fail to cleave. However, a brief (1.5-2 min) exposure to P23 at pH 8, followed by either acidification of the seawater of pH 7 or dilution of P23 at pH 8 causes germinal vesicle breakdown (GVBD), but eggs fail to complete meiosis and many then later advance to mitosis. In the present study we investigated the hypothesis that partial activation leading to parthenogenesis occurs when there is a partial intracellular alkalinization. Measurements with the fluorescent pH indicator bis(carboxyethyl)-carboxyfluorescein (BCECF) showed that P23 induces a pHi increase similar to that occurring during fertilization and the parthenogenesis-inducing treatments interrupt this rise in pHi. In eggs exposed to P23 for > 3 min the pHi increase was 0.31-0.49 units, slightly higher than in fertilized eggs. In partially activated eggs exposed to P23 for 1.5-2 min at pH 8, pHi began to rise but then returned to control values or remained only partially elevated (< 0.2 pH units average increase). Electrophysiological measurements revealed that removal of P23 during the first few minutes of exposure caused the activation potential to terminate and experiments with [14C]-P23 confirmed that dilution results in a rapid unbinding of P23 from eggs. If proton export is driven by membrane potential as well as the pH gradient, these results explain why dilution of P23 at pH 8 also interrupts the pHi increase.

Extracellular Ca2+ entry and Ca2+ release from inositol 1,4,5-trisphosphate-sensitive stores function at fertilization in oocytes of the marine bivalve Mytilus edulis

An oocyte of the marine bivalve Mytilus edulis, which is arrested at metaphase I, reinitiates meiosis at fertilization. The fertilized oocyte shows increases in intracellular Ca2+ ([Ca2+]i) comprising three different phases: an initial large [Ca2+]i transient, a subsequent low but sustained [Ca2+]i elevation, and repetitive small [Ca2+]i transients. In this study, we have investigated the sources and mechanisms of the sperm-induced [Ca2+]i increases. Application of methoxyverapamil (D-600), an inhibitor of voltage-dependent Ca2+ influx, suppressed the initial [Ca2+]i transient but did not affect the following two phases of [Ca2+]i changes. Injection of heparin, an antagonist of the inositol 1,4,5-trisphosphate (IP3) receptor, inhibited the later two phases without much affecting the initial transient. Combined application of D-600 and heparin almost completely abolished the three phases of the sperm-induced [Ca2+]i changes. Furthermore, Ca2+ influx caused by seawater containing excess K+ was blocked by D-600 but not by heparin, and IP3-induced Ca2+ release caused by photolysis of injected ‘caged’ derivatives of IP3 was blocked by heparin but not by D-600. These results strongly suggest that two types of Ca2+ mobilization systems, the extracellular Ca2+ entry responsible for an initial [Ca2+]i transient and the IP3 receptor-mediated Ca2+ release responsible for the following two phases of [Ca2+]i changes, function at fertilization of Mytilus oocytes.

Comparative patterns of protein phosphorylation during activation of surf clam oocytes by different artificial agents

Oocytes from the surf clam Spisula solidissima are arrested at prophase I of meiotic maturation, until fertilization, We analyzed the patterns of phosphorylated proteins under procedures mimicking, to various degrees, the normal sperm-induced activation process. High K(+)-seawater, the phorbol ester TPA, serotonin, or a combination of these were used to analyze their effects on both germinal vesicle breakdown (GVBD) and protein phosphorylation. Oocytes were preloaded with 35S-methionine or 32P-phosphate, and the pattern of labeled proteins was analyzed by polyacrylamide gel electrophoresis followed by autoradiography. When comparing, in high K(+)-activated oocytes, the pattern of phosphorylated proteins with that of synthesized proteins, it appeared that these two processes were largely unrelated to one another. Activation induced by TPA was slower (60 min for GVBD) than that induced by high K+ or serotonin (12-15 min for GVBD), but was similarly sensitive to the protein phosphorylation inhibitor, 6-dimethylaminopurine, and resulted in a qualitatively similar pattern of phosphorylated proteins appearing with slower kinetics, reflecting slower GVBD. When both serotonin and TPA were added to oocytes, the kinetics of GVBD was intermediate (30 min), and so was the appearance of phosphorylated proteins. Finally, the kinetics of development of H1 kinase activities was evaluated in oocytes activated by serotonin, TPA, or both. Similar to the general pattern of phosphorylated proteins, increased histone H1 kinase activities developed to similar degrees but with kinetics reflecting those of GVBD in each case. In conclusion, activations by different artificial agents, utilizing different pathways, resulted in GVBD with different kinetics but similar overall patterns of phosphorylated proteins after a lag typical of the agent used. This suggests that diverse pathways may initially be used to activate oocytes, but that these different pathways eventually merge into a common one, resulting in a highly conserved and regulated sequence of phosphorylation processes.