Regulation of fertilization-induced Ca(2+)spiking in the mouse zygote

Dynamic shapes of the zygote and two-cell mouse and human

Mouse zygote morphokinetics were measured during interphase, the mitotic period, cytokinesis, and two-cell stage. Sequences of rounder-distorted-rounder shapes were revealed, as were changing patterns of cross section area. A calcium chelator and an actin-disrupting agent inhibited the area changes that occurred between pronuclear envelope breakdown and cytokinesis. During cell division, two vortices developed in each nascent cell and they rotated in opposite directions at each end of the cell, a pattern that sometimes persisted for up to 10 h. Exchange with the environment may have been promoted by these shape and area cycles and persisting circulation in the cytoplasm may have a similar function between a cell's interior and periphery. Some of these movements were sporadically also seen in human zygotes with abnormal numbers of pronuclei and the two-cell stages that developed from these compromised human zygotes.

Postovulatory ageing modifies sperm-induced Ca2+ oscillations in mouse oocytes through a conditions-dependent, multi-pathway mechanism

Postovulatory ageing of mammalian oocytes occurs between their ovulation and fertilization and has been shown to decrease their developmental capabilities. Aged oocytes display numerous abnormalities, including altered Ca²⁺ signalling. Fertilization-induced Ca²⁺ oscillations are essential for activation of the embryonic development, therefore maintaining proper Ca²⁺ homeostasis is crucial for the oocyte quality. In the present paper, we show that the mechanism underlying age-dependent alterations in the pattern of sperm-triggered Ca²⁺ oscillations is more complex and multifaceted than previously believed. Using time-lapse imaging accompanied by immunostaining and molecular analyses, we found that postovulatory ageing affects the amount of Ca²⁺ stored in the cell, expression of Ca²⁺ pump SERCA2, amount of available ATP and distribution of endoplasmic reticulum and mitochondria in a manner often strongly depending on ageing conditions (in vitro vs. in vivo). Importantly, those changes do not have to be caused by oxidative stress, usually linked with the ageing process, as they occur even if the amount of reactive oxygen species remains low. Instead, our results suggest that aberrations in Ca²⁺ signalling may be a synergistic result of ageing-related alterations of the cell cycle, cytoskeleton, and mitochondrial functionality.

Assisted oocyte activation following ICSI fertilization failure

The capacity of intracytoplasmic sperm injection (ICSI) to permit almost any type of spermatozoa to fertilize oocytes has made it the most successful treatment for male factor infertility. Despite its high success rates, fertilization failure following ICSI still occurs in 1-3% of couples. Assisted oocyte activation (AOA) is being increasingly applied in human assisted reproduction to restore fertilization and pregnancy rates in couples with a history of ICSI fertilization failure. However, controversy still exists mainly because the artificial activating agents do not mimic precisely the initial physiological processes of mammalian oocyte activation, which has led to safety concerns. This review addresses the mechanism of human oocyte activation and the relatively rare phenomenon of fertilization failure after ICSI. Next, it describes the current diagnostic approaches and focuses on the application, efficiency and safety of AOA in human assisted reproduction.

Changing expression of chloride channels during preimplantation mouse development

Plasma membrane chloride channels (ClCs) play important roles in a broad range of cellular processes including cell volume regulation, proliferation, and transepithelial transport, all of which are critical during preimplantation embryonic development. In this study, the molecular and functional expression of voltage-gated ClCs was analyzed throughout preimplantation development of the mouse conceptus. mRNA transcripts for allClcngenes were detected. OnlyClcn1mRNA showed differential expression in the blastocyst, being detected in the trophectoderm but not in the inner cell mass. CLCN3 protein was detected at low levels in the cytoplasm and plasma membrane in 4-cell embryos and was localized to the apical plasma membrane of the trophoblasts in the blastocyst. Whole-cell patch-clamp recordings demonstrated the presence of a DIDS-sensitive, outwardly rectifying Cl−current throughout development, with this conductance being large at the 1-cell, morula and blastocyst stages. A second DIDS-insensitive Cl−current, which was inactivated by membrane depolarization, was present in cells differentiating into the trophoblast lineage and during blastocyst expansion. Inhibition of the DIDS-sensitive current and the DIDS-insensitive current, with 9-AC, prevented blastocyst expansion.

Calcium signaling in mammalian egg activation and embryo development: the influence of subcellular localization

Calcium (Ca(2+) ) signals drive the fundamental events surrounding fertilization and the activation of development in all species examined to date. Initial studies of Ca(2+) signaling at fertilization in marine animals were tightly linked to new discoveries of bioluminescent proteins and their use as fluorescent Ca(2+) sensors. Since that time, there has been rapid progress in our understanding of the key functions for Ca(2+) in many cell types and of the impact of cellular localization on Ca(2+) signaling pathways. In this review, which focuses on mammalian egg activation, we consider how Ca(2+) is regulated and stored at different stages of oocyte development and examine the functions of molecules that serve as both regulators of Ca(2+) release and effectors of Ca(2+) signals. We then summarize studies exploring how Ca(2+) directs downstream effectors mediating both egg activation and later signaling events required for successful preimplantation embryo development. Throughout this review, we focus attention on how localization of Ca(2+) signals influences downstream signaling events, and attempt to highlight gaps in our knowledge that are ripe for future research.

Species-specific differences in the activity and nuclear localization of murine and bovine phospholipase C zeta 1

Injection of mammalian sperm extracts or cRNA of the sperm-specific phospholipase C zeta 1 (PLCZ1) has been shown to trigger repetitive oscillations in the concentration of free calcium ([Ca(2+)](i)), leading to oocyte activation and embryo development in all mammals studied to date. While PLCZ1 has cross-species activity, it has also been observed that species-specific differences may exist in the frequency and pattern of the resulting [Ca(2+)](i) oscillations following PLCZ1 cRNA injection into oocytes of different species. Accordingly, we used a crossover design strategy to directly investigate the activity of murine and bovine PLCZ1 in both murine and bovine oocytes. In murine oocytes, injection of murine Plcz1 cRNA induced [Ca(2+)](i) oscillations at 10-fold lower concentrations than bovine PLCZ1, although in bovine oocytes bovine PLCZ1 was more effective than murine Plcz1 at inducing [Ca(2+)](i) oscillations. Investigation of ITPR1 (IP(3)R1) down-regulation in bovine oocytes by PLCZ1 cRNA also showed that bovine PLCZ1 was more active in homologous oocytes. To determine whether these PLCZs exhibited similar cellular distribution, Venus-tagged PLCZ1 cRNA was injected into oocytes, and PLCZ1 was overexpressed. Bovine PLCZ1 failed to accumulate in the pronucleus (PN) of bovine or murine zygotes, despite possessing a putative nuclear localization signal. Conversely, murine PLCZ1 accumulated in the PN of both murine and bovine zygotes. These results demonstrate that murine PLCZ1 and bovine PLCZ1 possess species-specific differences in activity and suggest potential differences in the mode of action of the protein between the two species. Variation in sperm PLCZ1 protein content among species, along with oocyte-specific differences in the localization and availability of PLCZ1 substrates, may further contribute to optimize the activation stimulus to enhance embryo development.

Inositol 1,4,5-trisphosphate receptor 1 degradation in mouse eggs and impact on [Ca2+]i oscillations

The initiation of normal embryo development depends on the completion of all events of egg activation. In all species to date, egg activation requires an increase(s) in the intracellular concentration of calcium ([Ca(2+)](i)), which is almost entirely mediated by inositol 1,4,5-trisphosphate receptor 1 (IP(3)R1). In mammalian eggs, fertilization-induced [Ca(2+)](i) responses exhibit a periodic pattern that are called [Ca(2+)](i) oscillations. These [Ca(2+)](i) oscillations are robust at the beginning of fertilization, which occurs at the second metaphase of meiosis, but wane as zygotes approach the pronuclear stage, time after which in the mouse oscillations cease altogether. Underlying this change in frequency are cellular and biochemical changes associated with egg activation, including degradation of IP(3)R1, progression through the cell cycle, and reorganization of intracellular organelles. In this study, we investigated the system requirements for IP(3)R1 degradation and examined the impact of the IP(3)R1 levels on the pattern of [Ca(2+)](i) oscillations. Using microinjection of IP(3) and of its analogs and conditions that prevent the development of [Ca(2+)](i) oscillations, we show that IP(3)R1 degradation requires uniform and persistently elevated levels of IP(3). We also established that progressive degradation of the IP(3)R1 results in [Ca(2+)](i) oscillations with diminished periodicity while a near complete depletion of IP(3)R1s precludes the initiation of [Ca(2+)](i) oscillations. These results provide insights into the mechanism involved in the generation of [Ca(2+)](i) oscillations in mouse eggs.

Acetylcholine rescues two-cell block through activation of IP3 receptors and Ca2+/calmodulin-dependent kinase II in an ICR mouse strain

Acetylcholine (ACh) causes early activation events in mouse oocytes, but little is known about its precise role in the early embryonic development of mice. We aimed to determine whether and how ACh is capable of rescuing two-cell block in an in vitro culture system. ACh evoked different transient Ca(2+) patterns showing a higher Ca(2+) peak in the two-cell stage embryos (two-cells) than observed in mature oocytes. In early two-cells subjected to an in vitro two-cell block, xestospongin C (Xes-C), an IP3 receptor antagonist, significantly decreased the level of the ACh-induced Ca(2+) increase. The reduction in the ACh-induced Ca(2+) increase by Xes-C in late two-cells was lower than that in early two-cells. Furthermore, KN62 and KN93, both CaMKII inhibitors, were found to reduce the magnitude of the ACh-induced Ca(2+) increase in early two-cells. The addition of ACh to the culture medium showed an ability to rescue in vitro two-cell block. However, the addition of ACh together with both Xes-C and CaMKII inhibitors or with either inhibitor separately had no effect on the rescue of two-cell block. Long-term exposure of late two-cells to ACh decreased morula and early blastocyst development and ACh had a differential effect on early and late two-cells. These results indicate that ACh likely rescues the in vitro two-cell block through activation of IP3R- and/or CaMKII-dependent signal transduction pathways.

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.

The roles of Ca2+, downstream protein kinases, and oscillatory signaling in regulating fertilization and the activation of development

Reviews in Developmental Biology have covered the pathways that generate the all-important intracellular calcium (Ca(2+)) signal at fertilization [Miyazaki, S., Shirakawa, H., Nakada, K., Honda, Y., 1993a. Essential role of the inositol 1,4,5-trisphosphate receptor/Ca(2+) release channel in Ca(2+) waves and Ca(2+) oscillations at fertilization of mammalian eggs. Dev. Biol. 158, 62-78; Runft, L., Jaffe, L., Mehlmann, L., 2002. Egg activation at fertilization: where it all begins. Dev. Biol. 245, 237-254] and the different temporal responses of Ca(2+) in many organisms [Stricker, S., 1999. Comparative biology of calcium signaling during fertilization and egg activation in animals. Dev. Biol. 211, 157-176]. Those reviews raise the importance of identifying how Ca(2+) causes the events of egg activation (EEA) and to what extent these temporal Ca(2+) responses encode developmental information. This review covers recent studies that have analyzed how these Ca(2+) signals are interpreted by specific proteins, and how these proteins regulate various EEA responsible for the onset of development. Many of these proteins are protein kinases (CaMKII, PKC, MPF, MAPK, MLCK) whose activity is directly or indirectly regulated by Ca(2+), and whose amount increases during late oocyte maturation. We cover biochemical progress in defining the signaling pathways between Ca(2+) and the EEA, as well as discuss how oscillatory or multiple Ca(2+) signals are likely to have specific advantages biochemically and/or developmentally. These emerging concepts are put into historical context, emphasizing that key contributions have come from many organisms. The intricate interdependence of Ca(2+), Ca(2+)-dependent proteins, and the EEA raise many new questions for future investigations that will provide insight into the extent to which fertilization-associated signaling has long-range implications for development. In addition, answers to these questions should be beneficial to establishing parameters of egg quality for human and animal IVF, as well as improving egg activation protocols for somatic cell nuclear transfer to generate stem cells and save endangered species.

Difference in Ca2+ oscillation-inducing activity and nuclear translocation ability of PLCZ1, an egg-activating sperm factor candidate, between mouse, rat, human, and medaka fish

Mouse phospholipase C, zeta 1 (PLCZ1), a strong candidate of egg-activating sperm factor, induces Ca(2+) oscillations and accumulates into formed pronucleus (PN) when expressed by cRNA injection. These activities were compared among mouse and human PLCZ1, newly cloned rat Plcz1, and medaka fish plcz1. The PLCZ1 proteins of the four species have an approximately homologous sequence of nuclear localization signal. However, the nuclear translocation ability was defective in rat, human, and medaka PLCZ1 expressed in mouse eggs. Rat PLCZ1 could not enter rat PN, whereas mouse PLCZ1 could. Mouse and human PLCZ1 translocated into the nucleus of COS-7 cells transfected with cDNA. There was little medaka PLCZ1 accumulated in the nucleus, and rat PLCZ1 was never located in the nucleus. All PLCZ1 proteins including fish could induce Ca(2+) oscillations in mouse eggs, but the activity was variable in the order of human >> mouse > medaka >> rat, estimated from minimal RNA concentration to induce Ca(2+) spikes. Ca(2+) oscillations by human PLCZ1 continued far beyond the time of PN formation (T(PN)), whereas those by mouse PLCZ1 ceased slightly before T(PN). High-frequency Ca(2+) spikes by overexpressed rat PLCZ1 stopped far before T(PN), possibly by feedback inhibition. Ca(2+) oscillations by fertilization of rat eggs stopped at T(PN), despite defective nuclear translocation of rat PLCZ1. Thus, PLCZ1 sequestration into PN participates in termination of Ca(2+) oscillations at the interphase of mouse embryos but does not always operate in other mammals, notably in rat embryos.

Release of phospholipase C ζand [Ca2+]i oscillation-inducing activity during mammalian fertilization

During fertilization of mammalian eggs a factor from the sperm, the sperm factor (SF), is released into the ooplasm and induces persistent [Ca2+]ioscillations that are required for egg activation and embryo development. A sperm-specific phospholipase C (PLC), PLCz, is thought to be the SF. Here, we investigated whether the SF activity and PLCζare simultaneously and completely released into the ooplasm soon after sperm entry. To accomplish this, we enucleated sperm heads within 90 min of intracytoplasmic sperm injection (ICSI) and monitored the persistence of the [Ca2+]ioscillations in eggs in which the sperm had been withdrawn. We also stained the enucleatedsperm heads to ascertain the presence/absence of PLCζ. Our results show that by 90 min all the SF activity had been released from the sperm, as fertilized enucleated eggs oscillated as fertilized controls, even in cases in which oscillations were prolonged by arresting eggs at metaphase. In addition, we found that the released SF activity became associated with the pronucleus (PN), as induction of PN envelope breakdown evoked comparable [Ca2+]iresponses in enucleated and non-manipulated zygotes. Lastly, we found that PLCzlocalized to the equatorial area of bull sperm and to the post-acrosomal region of mouse sperm and that by 90 min after ICSI all the sperm’s PLCζimmunoreactivity was lost in both species. Altogether, our findings show that during fertilization the SF activity and PLCζimmunoreactivity are simultaneously released from the sperm, suggesting that PLCζmay be the only [Ca2+]ioscillation-inducing factor of mammalian sperm.

Mitochondrial signaling and fertilization

The magnitude of the potential difference (polarity) across the inner mitochondrial membrane (DeltaPsim) determines levels of several mitochondrial activities, including ATP generation, focal regulate calcium homeostasis and organelle volume homeostasis. We investigated whether a domain of mitochondria in the mouse oocyte, characterized by high DeltaPsim and a unique location in the subplasmalemmal cytoplasm, is involved in the earliest events of fertilization: sperm attachment, penetration and cortical granule exocytosis. Experimental manipulations of the magnitude of DeltaPsim and the distribution of mitochondria in zona-free MII oocytes, followed by insemination and culture, indicate that high-polarized mitochondria (HPM) are required for penetration and cortical granule exocytosis, but not for persistent attachment to the oolemma. The capacity of subplasmalemmal mitochondria to undergo transient reductions (dissipations) of DeltaPsim appears necessary for penetration and cortical granule exocytosis. We suggest that the HPM normally establish a continuous circumferential circuit of 'reactive' organelles capable of responding to and propagating, triggering or activating signals across the subplasmalemmal cytoplasm, such as those initiated by the fertilizing sperm at the site of penetration. The HPM in the oocyte and early embryo may have functions similar to those of their somatic cell counterparts and promote the focal regulation of developmental activities that are themselves spatially localized. The establishment of high DeltaPsim in the subplasmalemmal cytoplasm may be among the first steps in the preovulatory maturation of the oocyte and defects in this domain may result in fertilization failure or abnormality.

Ultra-structural changes and developmental potential of porcine oocytes following vitrification

This study evaluated the effects of exposure and/or vitrification of porcine metaphase II (MII) oocytes on their in vitro viability and ultra-structural changes with two experiments. Experiment 1 examined the effect of vitrified oocytes on microtubule localization, mitochondrial morphology, chromosome organization and the developmental rate in IVF control and vitrified oocytes. Oocytes matured for 44 h were subjected to IVF (IVF control). Oocytes matured for 42 h were exposed to cryoprotectants (CPA control), followed by 2h culture, and subjected to IVF. Oocytes vitrified at 42 h post-maturation were warmed, cultured for 2h, and subjected to IVF (vitrified). Experiment 2 evaluated the effect of oocytes freezing on development of ICSI with and without activation and parthenotes. Fresh and vitrified oocytes were subjected to ICSI with and without electrical activation. Cleavage and blastocyst rates were significantly (P<0.05) lower in vitrified IVF, parthenote and ICSI embryos than those in fresh counterparts. Between ICSI embryos from fresh oocytes and vitrified oocytes, the rates of blastocyst were significantly higher (P<0.05) in activated group than the group without activation. Significant differences (P<0.05) were observed in normal spindle configuration of vitrified (43.5%) compared to control (81.0%) oocytes, but no significant difference was observed between CPA exposed and control groups. In conclusion, porcine oocytes at MII stage are very sensitive to vitrification with altered microtubule localization and mitochondrial organization thus resulting in impaired fertilization and embryo 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.

Phosphorylation of IP3R1 and the regulation of [Ca2+]i responses at fertilization: a role for the MAP kinase pathway

A sperm-induced intracellular Ca2+ signal ([Ca2+]i) underlies the initiation of embryo development in most species studied to date. The inositol 1,4,5 trisphosphate receptor type 1 (IP3R1) in mammals, or its homologue in other species, is thought to mediate the majority of this Ca2+ release. IP3R1-mediated Ca2+ release is regulated during oocyte maturation such that it reaches maximal effectiveness at the time of fertilization, which, in mammalian eggs, occurs at the metaphase stage of the second meiosis (MII). Consistent with this, the [Ca2+]i oscillations associated with fertilization in these species occur most prominently during the MII stage. In this study, we have examined the molecular underpinnings of IP3R1 function in eggs. Using mouse and Xenopus eggs, we show that IP3R1 is phosphorylated during both maturation and the first cell cycle at a MPM2-detectable epitope(s), which is known to be a target of kinases controlling the cell cycle. In vitro phosphorylation studies reveal that MAPK/ERK2, one of the M-phase kinases, phosphorylates IP3R1 at at least one highly conserved site, and that its mutation abrogates IP3R1 phosphorylation in this domain. Our studies also found that activation of the MAPK/ERK pathway is required for the IP3R1 MPM2 reactivity observed in mouse eggs, and that eggs deprived of the MAPK/ERK pathway during maturation fail to mount normal [Ca2+]i oscillations in response to agonists and show compromised IP3R1 function. These findings identify IP3R1 phosphorylation by M-phase kinases as a regulatory mechanism of IP3R1 function in eggs that serves to optimize [Ca2+]i release at fertilization.

Calcium microdomains and cell cycle control

The cell division cycle comprises successive rounds of genome replication and segregation that are never error-free. A complex signalling network chaperones cell cycle events to ensure that cell cycle progression does not occur until any errors detected are put right. The signalling network consists of cell cycle control proteins that are phosphorylated and dephosphorylated, synthesized and degraded interactively to generate a set of sensors and molecular switches that are thrown at appropriate times to permit or trigger cell cycle progression. In early embryos, discrete calcium signals have been shown to be a key component of the molecular switch mechanism. In somatic cells in contrast, the participation of calcium signals in cell cycle control is far from clear. Recent experiments in syncytial Drosophila embryos have shown that localised calcium signals in the nucleus and mitotic spindle can be detected. It appears that the nucleus comprises a calcium signalling microdomain bounded by endoplasmic reticulum that isolates the nucleus and spindle. These findings offer a possible explanation for the apparent absence of calcium signals in somatic cells during mitosis.

Calcium oscillations and mammalian egg activation

Fertilization in all species studied to date induces an increase in the intracellular concentration of free calcium ions ([Ca2+]i) within the egg. In mammals, this [Ca2+]i signal is delivered in the form of long-lasting [Ca2+]i oscillations that begin shortly after fusion of the gametes and persist beyond the time of completion of meiosis. While not fully elucidated, recent evidence supports the notion that the sperm delivers into the ooplasm a trigger of oscillations, the so-called sperm factor (SF). The recent discovery that mammalian sperm harbor a specific phospholipase C (PLC), PLCzeta has consolidated this view. The fertilizing sperm, and presumably PLCzeta promote Ca2+ release in eggs via the production of inositol 1,4,5-trisphosphate (IP3), which binds and gates its receptor, the type-1 IP3 receptor, located on the endoplasmic reticulum, the Ca2+ store of the cell. Repetitive Ca2+ release in this manner results in a positive cumulative effect on downstream signaling molecules that are responsible for the completion of all the events comprising egg activation. This review will discuss recent advances in our understanding of how [Ca2+]i oscillations are initiated and regulated in mammals, highlight areas of discrepancies, and emphasize the need to better characterize the downstream molecular cascades that are dependent on [Ca2+]i oscillations and that may impact embryo development.

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.

An increase in [Ca2+]i is sufficient but not necessary for driving mitosis in early mouse embryos

An increase in intracellular Ca2+ concentration ([Ca2+]i) has been shown to drive sea-urchin embryos and some fibroblasts through nuclear-envelope breakdown (NEBD) and the metaphase-to-anaphase transition. Mitotic Ca2+ transients can be pan-cellular global events or localized to the perinuclear region. It is not known whether Ca2+ is a universal regulator of mitosis or whether its role is confined to specific cell types. To test the hypothesis that Ca2+ is a universal regulator of mitosis, we have investigated the role of Ca2+ in mitosis in one-cell mouse embryos. Fertilized embryos generate Ca2+ transients during the first mitotic division. Imposing a Ca2+ transient by photorelease of inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] resulted in acceleration of mitosis entry, suggesting that a [Ca2+]i increase is capable of triggering mitosis. Mitotic Ca2+ transients were inhibited using three independent approaches: injection of intracellular Ca2+ buffers; downregulation of Ins(1,4,5)P3 receptors; and removal of extracellular Ca2+. None of the interventions had any effects on the timing of NEBD or cytokinesis. The possibility that NEBD is driven by localized perinuclear Ca2+ transients was examined using two-photon microscopy but no Ca2+-dependent increases in fluorescence were found to precede NEBD. Finally, the second mitotic division took place in the absence of any detectable [Ca2+]i increase. Thus, although an induced [Ca2+]i increase can accelerate mitosis entry, neither cytosolic nor perinuclear [Ca2+] increases appear to be necessary for progression through mitosis in mouse embryos.

Cell cycle-coupled [Ca(2+)](i) oscillations in mouse zygotes and function of the inositol 1,4,5-trisphosphate receptor-1

Sperm entry in mammalian eggs initiates oscillations in the concentration of free calcium ([Ca(2+)](i)). In mouse eggs, oscillations start at metaphase II (MII) and conclude as the zygotes progress into interphase and commence pronuclear (PN) formation. The inositol 1,4,5-trisphosphate receptor (IP(3)R-1), which underlies the oscillations, undergoes degradation during this transition, suggesting that one or more of the eggs' Ca(2+)-releasing machinery components may be regulated in a cell cycle-dependent manner, thereby coordinating [Ca(2+)](i) responses with the cell cycle. To ascertain the site(s) of interaction, we initiated oscillations at different stages of the cell cycle in zygotes with different IP(3)R-1 mass. In addition to sperm, we used two other agonists: porcine sperm factor (pSF), which stimulates production of IP(3), and adenophostin A, a non-hydrolyzable analogue of IP(3). None of the agonists tested induced oscillations at interphase, suggesting that neither decreased IP(3)R-1 mass nor lack of production or excessive IP(3) degradation can account for the insensitivity to IP(3) at this stage. Moreover, the releasable Ca(2+) content of the stores did not change by interphase, but it did decrease by first mitosis. More importantly, experiments revealed that IP(3)R-1 sensitivity and possibly IP(3) binding were altered at interphase, and our data demonstrate stage-specific IP(3)R-1 phosphorylation by M-phase kinases. Accordingly, increasing the activity of M-phase kinases restored the oscillatory-permissive state in zygotes. We therefore propose that the restriction of oscillations in mouse zygotes to the metaphase stage may be coordinated at the level of IP(3)R-1 and that this involves cell cycle stage-specific receptor phosphorylation.

Ca2+ signalling and cortical re-organisation during the transition from meiosis to mitosis in mammalian oocytes

In mammals, the mature ovulated egg is arrested in metaphase II of the first meiotic division. The signal that triggers the transition from meiosis to mitosis is provided by the fertilising sperm and takes the form of a series of Ca(2+) oscillations. The pattern of Ca(2+) oscillations is imposed by maternal control mechanisms that ensure Ca(2+) transients occur during M-phase of meiosis II and during the first mitotic division. The transition from meiosis to mitosis involves a major re-organisation. The unfertilised egg is polarised with the meiotic spindle located in the cortex of the animal pole and clusters of endoplasmic reticulum in the vegetal hemisphere. By the time of the first mitotic division some 20h later the spindle has formed in the centre of the embryo and is surrounded by endoplasmic reticulum. These changes in organisation have implications for the inheritance of ER in meiotic and mitotic cell divisions and may reflect different roles and requirements for Ca(2+) in meiosis and mitosis.

Cell cycle-dependent Ca2+ oscillations in mouse embryos are regulated by nuclear targeting of PLCζ

During the first cell cycle Ca2+ oscillations are regulated in a cell cycle-dependent manner, such that the oscillations are unique to M phase. How the Ca2+ oscillations are regulated with such cell cycle stage-dependency is unknown, despite their importance for egg activation and embryo development. We recently identified a novel, sperm-specific phospholipase C (PLCzeta; PLCζ) that triggers Ca2+ oscillations similar to those caused by sperm. We show that PLCζ-induced Ca2+ oscillations also occur exclusively during M phase. The cell cycle-dependency can be explained by PLCζ's localisation to the pronuclei, which depends specifically upon a nuclear localisation signal sequence. Preventing pronuclear localisation of PLCζ by mutation of the nuclear localisation signal, or by inhibiting pronuclear formation/import, can prolong Ca2+ oscillations or allow them to occur during interphase. These data suggest a novel mechanism for regulating a PLC through nuclear sequestration and may explain the cell cycle-dependent regulation of Ca2+ oscillations following fertilisation.

Ca2+ oscillation-inducing phospholipase C zeta expressed in mouse eggs is accumulated to the pronucleus during egg activation

Sperm-specific phospholipase C zeta (PLC zeta) is known to induce intracellular Ca(2+) oscillations and egg activation when expressed in mouse eggs by injection of RNA encoding PLC zeta. We investigated the expression level and spatial distribution of PLC zeta in the egg in real time and in relation to the initiation and termination of Ca(2+) oscillations by monitoring fluorescence of a yellow fluorescent protein 'Venus' fused with PLC zeta. Ca(2+) oscillations similar to those at fertilization were induced at 40-50 min after RNA injection, when expressed PLC zeta reached 10-40 x 10(-15) g in the egg. PLC zeta-Venus increased up to 3 h and attained a steady level at 4-5 h. Interestingly, PLC zeta-Venus is accumulated to the pronucleus (PN) formed at 5-6 h and continuously increased there. Ca(2+) oscillations stopped in most eggs before initiation of the accumulation. A variant of PLC zeta that lacks three EF hand domains was much less effective in induction of Ca(2+) oscillations and little accumulated in the pronucleus, indicating a critical role of those domains. The ability of the accumulation to the pronucleus qualifies PLC zeta for a strong candidate of the Ca(2+) oscillation-inducing sperm factor, which is introduced into the ooplasm upon sperm-egg fusion and concentrated to the pronucleus after inducing egg activation.

Release of the Ca(2+) oscillation-inducing sperm factor during mouse fertilization

A cytosolic sperm protein(s), referred to as the sperm factor (SF), is thought to induce intracellular calcium ([Ca(2+)](i)) oscillations during fertilization in mammalian eggs. These oscillations, which are responsible for inducing complete egg activation, persist for several hours. Nevertheless, whether a protracted release of SF is responsible for the duration of the oscillations is unknown. Using a combination of intracytoplasmic sperm injection (ICSI), in vitro fertilization (IVF), sperm removal, reinjection of the withdrawn sperm, and [Ca(2+)](i) monitoring, we determined that 30 min was necessary for establishing oscillations. Importantly, a significant portion of the Ca(2+) activity became dissociated from the sperm within 15-60 min after entry, and by 120 min post-ICSI or IVF, sperm were unable to induce oscillations. The initiation of oscillations coincided with exposure and solubilization of the perinuclear theca (PT), as evidenced by transmission electron microscopy, although disassembly of the PT was not required for commencement of the [Ca(2+)](i) responses. Remarkably, despite its complete release into the ooplasm, SF associated with nuclear structures at the time of pronuclear formation. Lastly, release of SF was not affected by the cell cycle. We conclude that mouse sperm serves as a carrier for SF, which is rapidly and completely solubilized to establish [Ca(2+)](i) oscillations.

Endoplasmic reticulum reorganizations and Ca2+ signaling in maturing and fertilized oocytes of marine protostome worms: the roles of MAPKs and MPF

Before a proper Ca(2+) response is produced at fertilization, oocytes typically undergo a maturation process during which their endoplasmic reticulum (ER) is restructured. In marine protostome worms belonging to the phylum Nemertea, the ER of maturing oocytes forms numerous distinct clusters that are about 5 micro m in diameter. After fertilization, mature oocytes with such aggregates generate a normal series of Ca(2+) oscillations and eventually disassemble their ER clusters at around the time that the oscillations cease. Immature oocytes, however, lack prominent ER clusters and fail to exhibit repetitive Ca(2+) oscillations upon insemination, collectively suggesting that cell cycle-related changes in ER structure may play a role in Ca(2+) signaling. To assess the effects of meiotic regulators on the morphology of the ER and the type of Ca(2+) response that is produced at fertilization, nemertean oocytes were treated with pharmacological modulators of mitogen-activated protein kinases (MAPKs) or maturation-promoting factor (MPF) prior to confocal microscopic analyses. Based on such imaging studies and correlative assays of kinase activities, MAPKs of the ERK1/2 type (extracellular signal regulated kinases 1/2) do not seem to be essential for either structural reorganizations of the ER or repetitive Ca(2+) signaling at fertilization. Conversely, MPF levels appear to modulate both ER structure and the capacity to produce normal Ca(2+) oscillations. The significance of these findings is discussed with respect to other reports on ER structure, MPF cycling and Ca(2+) signaling in oocytes of deuterostome animals.

Ca2+ oscillations at fertilization in mammals are regulated by the formation of pronuclei

In mammals, the sperm triggers a series of cytosolic Ca(2+) oscillations that continue for approximately 4 hours, stopping close to the time of pronucleus formation. Ca(2+) transients are also seen in fertilized embryos during the first mitotic division. The mechanism that controls this pattern of sperm-induced Ca(2+) signalling is not known. Previous studies suggest two possible mechanisms: first, regulation of Ca(2+) oscillations by M-phase kinases; and second, regulation by the presence or absence of an intact nucleus. We describe experiments in mouse oocytes that differentiate between these mechanisms. We find that Ca(2+) oscillations continue after Cdk1-cyclin B1 activity falls at the time of polar body extrusion and after MAP kinase has been inhibited with UO126. This suggests that M-phase kinases are not necessary for continued Ca(2+) oscillations. A role for pronucleus formation in regulating Ca(2+) signalling is demonstrated in experiments where pronucleus formation is inhibited by microinjection of a lectin, WGA, without affecting the normal inactivation of the M-phase kinases. In oocytes with no pronuclei but with low M-phase kinase activity, sperm-induced Ca(2+) oscillations persist for nearly 10 hours. Furthermore, a dominant negative importin beta that inhibits nuclear transport, also prevents pronucleus formation and causes Ca(2+) oscillations that continue for nearly 12 hours. During mitosis, fluorescent tracers that mark nuclear envelope breakdown and the subsequent reformation of nuclei in the newly formed two-cell embryo establish that Ca(2+) oscillations are generated only in the absence of a patent nuclear membrane. We conclude by suggesting a model where nuclear sequestration and release of a Ca(2+)-releasing activity contributes to the temporal organization of Ca(2+) transients in meiosis and mitosis in mice.

Production of piglets using intracytoplasmic sperm injection (ICSI) with flowcytometrically sorted boar semen and artificially activated oocytes

The purpose of the present study was to develop a protocol for the successful production of piglets employing intracytoplasmic sperm injection (ICSI) with flowcytometrically sexed spermatozoa and artificially activated porcine oocytes. In vitro matured oocytes were fertilized by ICSI using non-sorted frozen/thawed epididymal semen. Oocytes were either activated by CaCl(2), Ca(2+)-ionophore or electrical pulse. Activation and fertilization rates of sperm injected oocytes stimulated by CaCl(2)-injection were significantly higher than those without activation (70.4% versus 45.9%; 49.9% versus 33.2%, respectively; P<0.001). Activation rate of sham injected oocytes increased in parallel (11.2% versus 26.3%, P<0.05), parthenogenetic development remained low (2.8% versus 8%). Co-incubation in Ca(2+)-ionophore did not improve activation rates as compared to non-activated oocytes (44.8% versus 42.5%). Fertilization rate decreased as compared to non-treated sperm injected oocytes (36.8% versus 24.5%, P<0.05). Activation of oocytes with a single electrical pulse resulted in significantly higher activation rates in all groups of oocytes as compared to non-stimulated ones (sperm injected oocytes: 65.6% versus 43.1%, P<0.001; sham injected oocytes: 48.5% versus 5.6%, P<0.001; control oocytes: 50.7% versus 0.0%, P<0.001). Fertilization rates (32.3% versus 48.2%) and parthenogenetic development (0.7% versus 38.9%, 0.0% versus 30.9%, P<0.001) increased significantly in parallel. In addition, in four replicates of flowcytometrically sorted Y-chromosome bearing spermatozoa were injected into in vivo matured oocytes, activated with 1.2 pl of a 30 mM CaCl(2) solution. On average 85.3 fertilized oocytes were transferred surgically into four recipients. Pregnancies delivered a total of 13 male piglets. These are the first piglets born from ICSI with sorted spermatozoa.

Time and development

The role that biological timers play in gametogenesis and development is reviewed through use of selected examples. Some general features of biological timers are also reviewed, and two types of timing mechanism are discussed in more detail: circadian rhythms and cell-cycle-based timers. In particular, the recent evidence that oscillatory ion channel activity may play an important role in timing mechanisms is summarized. The activity and properties of an oscillatory K(+) channel present during preimplantation mouse development are described, and preliminary results from its neutralization are discussed.

Cell cycle-dependent regulation of structure of endoplasmic reticulum and inositol 1,4,5-trisphosphate-induced Ca2+ release in mouse oocytes and embryos

The organization of endoplasmic reticulum (ER) was examined in mouse eggs undergoing fertilization and in embryos during the first cell cycle. The ER in meiosis II (MII)-arrested mouse eggs is characterized by accumulations (clusters) that are restricted to the cortex of the vegetal hemisphere of the egg. Monitoring ER structure with DiI18 after egg activation has demonstrated that ER clusters disappear at the completion of meiosis II. The ER clusters can be maintained by inhibiting the decrease in cdk1-cyclin B activity by using the proteasome inhibitor MG132, or by microinjecting excess cyclin B. A role for cdk1-cyclin B in ER organization is further suggested by the finding that the cdk inhibitor roscovitine causes the loss of ER clusters in MII eggs. Cortical clusters are specific to meiosis as they do not return in the first mitotic division; rather, the ER aggregates around the mitotic spindle. Inositol 1,4,5-trisphosphate-induced Ca(2+) release is also regulated in a cell cycle-dependent manner where it is increased in MII and in the first mitosis. The cell cycle dependent effects on ER structure and inositol 1,4,5-trisphosphate-induced Ca(2+) release have implications for understanding meiotic and mitotic control of ER structure and inheritance, and of the mechanisms regulating mitotic Ca(2+) signaling.

Activation of rabbit oocytes: the impact of the Ca2+ signal regime on development

Postfertilization manipulation of mammalian embryos results in various developmental alterations. To determine whether the manipulation of the Ca2+ regime causing oocyte activation is a valuable experimental means in helping understand the biological process by which embryos integrate signals from outside and later regulate gene expression, we linked Ca2+ signal parameters i.e. amplitude, number and frequency, with the efficiency and quality of postimplantation development. Freshly ovulated rabbit oocytes were subjected to repetitive and modulated Ca2+ influx. The results provide three major pieces of information. Firstly, the Ca2+ stimulus is the most efficient signal activating mammalian eggs when it is applied in a repetitive manner, the amplitude being the crucial factor. Secondly, the dynamics of early cleavage does not appear to be determined by either the frequency or the amplitude of modulation of the Ca2+ signal that activates the oocyte. Thirdly, amplitude and temporal modulation of the Ca2+ signal in the early minutes influences the developmental performance and the morphology of the rabbit parthenogenetic conceptus at day 11.5 of pregnancy. The results demonstrate the importance of epigenetic events during postfertilization as well as the possible uses of Ca2+ modulation in studying long term developmental effects.

Egg timers: how is developmental time measured in the early vertebrate embryo?

Eggs and early embryos appear to be programmed to undertake particular developmental decisions at characteristic times, although precisely how these decisions are timed is unknown. We discuss the possible roles and interactions during early vertebrate development of two broad categories of timers: 1) those that involve cyclic or sequential mechanisms, referred to as clocks; and 2) those that require an increase or decrease in some factor to a threshold level for progression of time, referred to as hourglass timers. It is concluded that both clock-like timers linked to various features of the cell cycle and hourglass timers are involved in early developmental timing. The possible involvement of elements of circadian clock timers is also considered. BioEssays 22:57-63, 2000.