Nuclei from fertilized mouse embryos have calcium-releasing activity

The signal that stimulates mammalian embryo development

Embryo development is stimulated by calcium (Ca2+) signals that are generated in the egg cytoplasm by the fertilizing sperm. Eggs are formed via oogenesis. They go through a cell division known as meiosis, during which their diploid chromosome number is halved and new genetic combinations are created by crossing over. During formation the eggs also acquire cellular components that are necessary to produce the Ca2+ signal and also, to support development of the newly formed embryo. Ionized calcium is a universal second messenger used by cells in a plethora of biological processes and the eggs develop a "toolkit", a set of molecules needed for signaling. Meiosis stops twice and these arrests are controlled by a complex interaction of regulatory proteins. The first meiotic arrest lasts until after puberty, when a luteinizing hormone surge stimulates meiotic resumption. The cell cycle proceeds to stop again in the middle of the second meiotic division, right before ovulation. The union of the female and male gametes takes place in the oviduct. Following gamete fusion, the sperm triggers the release of Ca2+ from the egg's intracellular stores which in mammals is followed by repetitive Ca2+ spikes known as Ca2+ oscillations in the cytosol that last for several hours. Downstream sensor proteins help decoding the signal and stimulate other molecules whose actions are required for proper development including those that help to prevent the fusion of additional sperm cells to the egg and those that assist in the release from the second meiotic arrest, completion of meiosis and entering the first mitotic cell division. Here I review the major steps of egg formation, discuss the signaling toolkit that is essential to generate the Ca2+ signal and describe the steps of the signal transduction mechanism that activates the egg's developmental program and turns it into an embryo.

Longitudinal profiling of human androgenotes through single-cell analysis unveils paternal gene expression dynamics in early embryo development

STUDY QUESTION

How do transcriptomics vary in haploid human androgenote embryos at single cell level in the first four cell cycles of embryo development?

SUMMARY ANSWER

Gene expression peaks at the fourth cell cycle, however some androcytes exhibit unique transcriptional behaviors.

WHAT IS KNOWN ALREADY

The developmental potential of an embryo is determined by the competence of the oocyte and the sperm. However, studies of the contribution of the paternal genome using pure haploid androgenotes are very scarce.

STUDY DESIGN, SIZE, DURATION

This study was performed analyzing the single-cell transcriptomic sequencing of 38 androcytes obtained from 10 androgenote bioconstructs previously produced in vitro (de Castro et al., 2023). These results were analyzed through different bioinformatics software such as g: Profiler, GSEA, Cytoscape, and Reactome.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Single cell sequencing was used to obtain the transcriptomic profiles of the different androcytes. The results obtained were compared between the different cycles studied using the DESeq2 program and functional enrichment pathways using g: Profiler, Cytoscape, and Reactome.

MAIN RESULTS AND THE ROLE OF CHANCE

A wave of paternally driven transcriptomic activation was found during the third-cell cycle, with 1128 upregulated and 225 downregulated genes and the fourth-cell cycle, with 1373 upregulated and 286 downregulated genes, compared to first-cell cycle androcytes. Differentially expressed routes related to cell differentiation, DNA-binding transcription, RNA biosynthesis and RNA polymerase II transcription regulatory complex, and cell death were found in the third and fourth with respect to the first-cell cycle. Conversely, in the fourth cell cycle, 153 downregulated and 332 upregulated genes were found compared with third cell cycle, associated with differentially expressed processes related to E-box binding and zinc finger protein 652 (ZNF652) transcription factor. Further, significant overexpression of LEUTX, PRAMEF1, DUXA, RFPL4A, TRIM43, and ZNF675 found in androgenotes, compared to biparental embryos, highlights the paternal contributions to zygote genome activation.

LARGE SCALE DATA

All raw sequencing data are available through the Gene Expression Omnibus (GEO) under accessions number: GSE216501.

LIMITATIONS, REASONS FOR CAUTION

Extrapolation of biological events from uniparental constructs to biparental embryos should be done with caution. Maternal and paternal genomes do not act independently of each other in a natural condition. The absence of one genome may affect gene transcription of the other. In this sense, the haploid condition of the bioconstructs could mask the transcriptomic patterns of the single cells.

WIDER IMPLICATIONS OF THE FINDINGS

The results obtained demonstrated the level of involvement of the human paternal haploid genome in the early stages of embryo development as well as its evolution at the transcriptomic level, laying the groundwork for the use of these bioconstructs as reliable models to dispel doubts about the genetic role played by the paternal genome in the early cycles of embryo development.

STUDY FUNDING/COMPETING INTEREST(S)

This study was funded by Instituto de Salud Carlos III (ISCIII) through the project 'PI22/00924', co-funded by European Regional Development Fund (ERDF); 'A way to make Europe'. F.D. was supported by the Spanish Ministry of Economy and Competitiveness through the Miguel Servet program (CPII018/00002). M.J.E. was supported by Instituto de Salud Carlos III (PI19/00577 [M.J.E.]) and FI20/00086. P.dC. was supported by a predoctoral grant for training in research into health (PFIS PI19/00577) from the Instituto de Salud Carlos III. All authors declare having no conflict of interest with regard to this trial.

Comparative single-cell transcriptomic profiles of human androgenotes and parthenogenotes during early development

OBJECTIVE

To unravel the differential transcriptomic behavior of human androgenotes (AGs) and parthenogenotes (PGs) throughout the first cell cycles, analyze the differential expression of genes related to key biologic processes, and determine the time frame for embryonic genome activation (EGA) in AGs and PGs.

DESIGN

Laboratory study.

SETTING

Private fertility clinic.

PATIENT(S)

Mature oocytes were retrieved from healthy donors and subjected to artificial oocyte activation using calcium ionophore and puromycin to generate PGs (n = 6) or enucleated and subjected to intracytoplasmic sperm injection to generate AGs (n = 10).

INTERVENTION(S)

Uniparental constructs at different early stages of development were disaggregated into constituent single cells (we suggest the terms parthenocytes and androcytes) to characterize the single-cell transcriptional landscape using next-generation sequencing.

MAIN OUTCOMES MEASURE(S)

Transcriptomic profiles comparison between different stages of early development in AGs and PGs.

RESULT(S)

The uniparental transcriptomic profiles at the first cell cycle showed 68 down-regulated and 26 up-regulated differentially expressed genes (DEGs) in PGs compared with AGs. During the third cell cycle, we found 60 up-regulated and 504 down-regulated DEGs in PGs compared with AGs. In the fourth cell cycle, 1,771 up-regulated and 1,171 down-regulated DEGs were found in PGs compared with AGs. The AGs and PGs had reduced EGA profiles during the first 3 cell cycles, and a spike of EGA at the fourth cell cycle was observed in PGs.

CONCLUSION(S)

Transcriptomic analysis of AGs and PGs revealed their complementary behavior until the fourth cell cycle. Androgenotes undergo a low wave of transcription during the first cell cycle, which reflects the paternal contribution to cell cycle coordination, mechanics of cell division, and novel transcription regulation. Maternal transcripts are most prominent in the third and fourth cell cycles, with amplification of transcription related to morphogenic progression and embryonic developmental competence acquisition. Regarding EGA, in PGs, a primitive EGA begins at the 1-cell stage and gradually progresses until the 4-cell stage, when crucial epigenetic reprogramming (through methylation) is up-regulated. In addition, our longitudinal single-cell transcriptomic analysis challenges that the zygote and early cleavage stages are the only totipotent entities, by revealing potential totipotency in cleavage-stage AGs and implications of paternal transcripts.

Mysteries and unsolved problems of mammalian fertilization and related topics

Mammalian fertilization is a fascinating process that leads to the formation of a new individual. Eggs and sperm are complex cells that must meet at the appropriate time and position within the female reproductive tract for successful fertilization. I have been studying various aspects of mammalian fertilization over 60 years. In this review, I discuss many different aspects of mammalian fertilization, some of my laboratory's contribution to the field, and discuss enigmas and mysteries that remain to be solved.

Phospholipase Cζ (PLCζ) versus postacrosomal sheath WW domain-binding protein (PAWP): Which molecule will survive as a sperm factor?

During mammalian fertilization, sperm is fused with the oocyte's membrane, triggering the resumption of meiosis from the metaphase II arrest, the extrusion of the second polar body, and the exocytosis of cortical granules; these events are collectively called 'oocyte activation.' In all species studied to date, the transient rise in the cytosolic level of calcium (in particular, the repeated calcium increases called 'calcium oscillations' in mammals) is required for these events. Researchers have focused on identifying the factor(s) that can induce calcium oscillations during fertilization. Sperm‐specific phospholipase C, i.e., PLC zeta (PLCζ), is a strong candidate of the factor(s), and several research groups using different species obtained evidence that PLCζ is a sperm factor that can induce calcium oscillations during fertilization. However, postacrosomal sheath Tryptophan‐Tryptophan (WW)—domain‐binding protein (PAWP) was recently shown to have a pivotal role in inducing calcium oscillations in some species. In this review, we focus on PLCζ and PAWP as sperm factors, and we discuss this controversy: Which of these two molecules survives as a sperm factor?

Contrasting effects of heat shock during in vitro maturation on development of in vitro-fertilized and parthenogenetic bovine embryos

This study investigated the influence of heat shock during in vitro maturation on embryo development following in vitro fertilization (IVF) or parthenogenesis (Part). Immature bovine cumulus-oocyte complexes were exposed to heat shock (41.0°C) during the first 12 hr of in vitro maturation (IVM), followed by 12 hr at 38.5°C. Control group consisted of in vitro maturation for 24 hr at 38.5°C. Oocytes were in vitro-fertilized or activated with ionomycin and cultured in vitro for 192 hr post-in vitro insemination or parthenogenetic activation (hpia). There was an interaction (p < .01) between temperature of IVM and method of oocyte activation (IVF or Part) for cleavage at 48 hpia. Heat shock had a negative impact (p < .01) on cleavage of IVF embryos, whereas no (p > .05) effect was found in the Part embryos. Embryo development towards blastocyst stage at 168 and 192 hpia decreased in both IVF and Part embryos derived from heat-shocked oocytes. Heat shock increased (p < .05) the apoptotic index in Part blastocysts, but no effect (p > .05) was found in IVF counterparts. Heat shock also down-regulated the expression of AQP3 (p < .01) and up-regulated the expression of HSP70.1 (p < .01) in Part blastocysts, whereas it down-regulated the expression of ATP1A1 (p < .05) in IVF blastocysts. In conclusion, the effects of heat shock during IVM on early embryo cleavage and blastocyst apoptosis are influenced by the method of oocyte activation and expression of some genes can be disturbed in embryos derived from heat-shocked oocytes.

Egg Activation at Fertilization by a Soluble Sperm Protein

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

Sperm-induced Ca2+ release during egg activation in mammals

This review discusses the role that the sperm-specific phospholipase C zeta (PLCζ) is proposed to play during the fertilization of mammalian eggs. At fertilization, the sperm initiates development by causing a series of oscillations in cytosolic concentrations of calcium [Ca(2)] within the egg. PLCζ mimics the sperm at fertilization, causing the same pattern of Ca(2+) release as seen at fertilization. Introducing PLCζ into mouse eggs also mimics a number of other features of the way in which the fertilizing sperm triggers Ca(2+) oscillations. We discuss the localization of PLCζ within the egg and present a hypothesis about the localization of PLCζ within the sperm before the initiation of fertilization.

Biological transcomplementary activation as a novel and effective strategy applied to the generation of porcine somatic cell cloned embryos

A novel method termed the biological transcomplementary activation (B-TCA) has been recently utilized for the stimulation of porcine oocytes reconstituted by somatic cell nuclear transfer (SCNT). The use of cytosolic components originating from fertilized (FE) rabbit zygotes as the stimuli for the B-TCA of SCNT-derived pig oocytes appeared to be a highly efficient strategy applied to promote the in vitro development of cloned embryos, leading to a significant improvement in the blastocyst yield (43.6%) compared to the yields achieved using the standard protocol of simultaneous fusion and electrical activation (SF-EA; [31.3%]) or the protocol of delayed electrical activation (D-EA) independent of extracellular Ca(2+) ions (0%). The FE rabbit zygote cytoplast-mediated B-TCA resulted in the increased blastocyst formation rate of porcine cloned embryos as compared to the B-TCA triggered by either cytoplasts isolated from pig parthenogenotes (PAs; [27.8%]) or rabbit PA-descended cytoplasts (0%). A considerably lower percentage of blastocysts containing apoptotic and/or necrotic (annexin V-eGFP-positive) cells were obtained from the SCNT-derived oocytes stimulated by the FE rabbit zygote cytoplast-based B-TCA (22.2%) compared to those stimulated using the SF-EA protocol (35.1%). In contrast to the B-TCA induced by FE rabbit zygote cytoplasts, apoptosis/necrosis incidence decreased totally among the cloned pig blastocysts that developed from reconstituted oocytes undergoing the porcine PA cytoplast-evoked B-TCA. In conclusion, the FE rabbit zygote cytoplast-mediated B-TCA turned out to be a relatively effective strategy for the in vitro production of porcine blastocyst clones of higher quality compared to those created using the standard SF-EA approach.

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.

The oocyte activation and Ca2+ oscillation-inducing abilities of mouse and human dead (sonicated) spermatozoa

In ICSI procedures, it is well known that the selection of viable (live) spermatozoa and certain types of immobilization prior to injection is very important for obtaining successful results, but unfortunately there are rare situations when only immotile spermatozoa are available (such as in severe asthenozoospermia or necrozoospermia). In such cases, failure of oocyte activation after ICSI often occurs and may be due to the lack of SOAF (sperm-borne oocyte activating factor) activity. In order to investigate the SOAF activities of dead spermatozoa, mouse and human spermatozoa were immobilized (killed by sonication), maintained in THF medium for varying time intervals (up to 72 h) and then injected into mature unfertilized mouse oocytes. Injected mouse oocytes were examined for their activation, development into blastocysts and Ca2+ responses by imaging and confocal laser scanning microscope. The rates of oocyte activation, blastocyst development and normal patterns of Ca2+ oscillation from the killed-sperm-injected oocytes decreased gradually in accordance with the maintenance interval between sonication and injection. For injection with mouse sonicated spermatozoa, the rate of normal Ca2+ oscillations declined first (after a 3 h maintenance interval) and then blastocyst development was gradually obstructed (after approx. 10 h). The oocyte activation-inducing ability of dead spermatozoa was maintained for a relatively long period, but began to decline after 20 h. The activation rates and Ca2+ response of the oocytes that were injected with human sonicated spermatozoa decreased earlier than those injected with mouse spermatozoa. Although the oocyte activation-inducing ability was maintained for a relatively long time after the death of the spermatozoa, embryo development into blastocysts and the rate of normal Ca2+ oscillations declined after a short maintenance interval between sonication and injection. The Ca2+ response seemed to be the most sensitive indicator for the evaluating the SOAF activity of dead (killed) spermatozoa.

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.

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.

INTRACELLULAR CALCIUM IN THE FERTILIZATION AND DEVELOPMENT OF MAMMALIAN EGGS

1. Mammalian eggs are arrested at metaphase of their second meiotic division when ovulated and remain arrested until fertilized. The sperm delivers into the egg phospholipase C (PLC) zeta, which triggers a series of Ca(2+) spikes lasting several hours. The Ca(2+) spikes provide the necessary and sufficient trigger for all the events of fertilization, including exit from metaphase II arrest and extrusion of cortical granules that block the entry of other sperm. 2. The oscillatory Ca(2+) signal switches on calmodulin-dependent protein kinase II (CaMKII), which phosphorylates the egg-specific protein Emi2, earmarking it for degradation. To remain metaphase II arrested, eggs must maintain high levels of maturation-promoting factor (MPF) activity, a heterodimer of CDK1 and cyclin B1. Emi2 prevents loss of MPF by blocking cyclin B1 degradation, a process that is achieved by inhibiting the activity of the anaphase-promoting complex/cyclosome. However, CaMKII is not the primary initiator in the extrusion of cortical granules. 3. Ca(2+) spiking is also observed in mitosis of one-cell embryos, probably because PLCzeta contains a nuclear localization signal and so is released into the cytoplasm following nuclear envelope breakdown. The function of these mitotic Ca(2+) spikes remains obscure, although they are not absolutely required for passage through mitosis. 4. Intriguingly, the pattern of Ca(2+) spikes observed at fertilization has an effect on both pre- and postimplantation development in a manner that is independent of their ability to activate eggs. This suggests that the Ca(2+) spikes set in train at fertilization are having effects on processes initiated in the newly fertilized egg but whose influences are only observed several cell divisions later. The nature of the signals remains little explored, but their importance is clear and so warrants further investigation.

Duplication of the sperm genome by human androgenetic embryo production: towards testing the paternal genome prior to fertilization

There is currently no technique for evaluating the sperm genome before fertilization. However, sperm genome duplication could offer a way forward, whereby one of the sister blastomeres of a 2-cell haploid androgenetic embryo could be analysed. A method was developed for production of human androgenotes by enucleation of oocytes at telophase II (TII) after intracellular sperm injection (ICSI). The results were compared with those obtained via the more usual procedure of oocyte enucleation at metaphase II (MII) prior to ICSI. TII enucleation led to an improvement in the rate of embryo survival, increased the production rate of 1PN-embryos, and also the production of 2- to 8-cell-stage embryos (85.0, 74.9 and 65.8% in TII enucleation, versus 73.8, 48.9 and 33.3% in MII enucleation). Fluorescence in-situ hybridization (FISH) analysis of 30 2- to 5-cell androgenic embryos for two to seven chromosomes revealed the correct chromosome distribution in 76.7% of haploid human androgenotes.

Activation of fertilized and nuclear transfer eggs

In all animal species, initiation of embryonic development occurs shortly after the joining together of the gametes from each of the sexes. The first of these steps, referred to as "egg activation", is a series of molecular events that results in the syngamy of the two haploid genomes and the beginning of cellular divisions for the new diploid embryo. For many years it has been known that the incoming sperm drives this process, as an unfertilized egg will remain dormant until it can no longer sustain normal metabolic processes. Until recently, it was also believed that the sperm was the only cell capable of creating a viable embryo and offspring. Recent advances in cell biology have allowed researchers to not only understand the molecular mechanisms of egg activation, but to exploit the use of pharmacological agents to bypass sperm-induced egg activation for the creation of animals by somatic cell nuclear transfer. This chapter will focus on the molecular events of egg activation in mammals as they take place during fertilization, and will discuss how these mechanisms are successfully bypassed in processes such as somatic cell nuclear transfer.

The Role of X/Y Linker Region and N-terminal EF-hand Domain in Nuclear Translocation and Ca2+ Oscillation-inducing Activities of Phospholipase Cζ, a Mammalian Egg-activating Factor

Sperm-specific phospholipase C-zeta (PLCzeta) causes intracellular Ca(2+) oscillations and thereby egg activation and is accumulated into the formed pronucleus (PN) when expressed in mouse eggs by injection of cRNA encoding PLCzeta, which consists of four EF-hand domains (EF1-EF4) in the N terminus, X and Y catalytic domains, and C-terminal C2 domain. Those activities were analyzed by expressing PLCzeta mutants tagged with fluorescent protein Venus by injection of cRNA into unfertilized eggs or 1-cell embryos after fertilization. Nuclear localization signal (NLS) existed at 374-381 in the X/Y linker region. Nuclear translocation was lost by replacement of Arg(376), Lys(377), Arg(378), Lys(379), or Lys(381) with glutamate, whereas Ca(2+) oscillations were conserved. Nuclear targeting was also absent for point mutation of Lys(299) and/or Lys(301) in the C terminus of X domain, or Trp(13), Phe(14), or Val(18) in the N terminus of EF1. Ca(2+) oscillation-inducing activity was lost by the former mutation and was remarkably inhibited by the latter. A short sequence 374-383 fused with Venus showed active translocation into the nucleus of COS-7 cells, but 296-309 or 1-19 did not. Despite the presence of these special regions, both activities were deprived by deletion of not only EF1 but also EF2-4 or C2 domain. Thus, PLCzeta is driven into the nucleus primarily by the aid of NLS and putative regulatory sites, but coordinated three-dimensional structure, possibly formed by a folding in the X/Y linker and close EF/C2 contact as in PLCdelta1, seems to be required not only for enzymatic activity but also for nuclear translocation ability.

Calcium signals for egg activation in mammals

A dramatic increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) occurs in eggs at fertilization common to all animal species examined to date, and this serves as a pivotal signal for egg activation characterized by resumption of meiotic cell division and formation of the pronuclei. In mammalian eggs, repetitive [Ca(2+)](i) rises (Ca(2+) oscillations) each of which accompanies a propagating wave across the egg occur due to release of Ca(2+) from the endoplasmic reticulum mainly through type 1 inositol 1,4,5-trisphosphate (IP(3)) receptor. Ca(2+) oscillations are induced by a cytosolic sperm factor driven into the egg cytoplasm upon sperm-egg fusion. A current strong candidate of the sperm factor is a novel sperm-specific isozyme of phospholipase C (IP(3)-producing enzyme), PLCzeta. Recent extensive research has reveled characteristics of PLCzeta such as the Ca(2+) oscillation-inducing activity after injection of PLCzeta-encoding RNA or recombinant PLCzeta into mouse eggs, extremely high Ca(2+)-sensitivity of the enzymatic activity in vitro, and nuclear translocation ability possibly related to cell-cycle-dependent regulation of Ca(2+) oscillations. [Ca(2+)](i) rises cause successive activation of calmodulin-dependent kinase II and E3 ubiquitin ligase, lead to proteolysis of ubiquitinated cyclin B1 and inactivation of metaphase-promoting factor (Cdk1/cyclin B1 complex), and result in the release of eggs from meiotic arrest.

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.

Functional, biochemical, and chromatographic characterization of the complete [Ca2+]i oscillation-inducing activity of porcine sperm

A cytosolic sperm protein(s), referred to as sperm factor (SF), is delivered into eggs by the sperm during mammalian fertilization to induce repetitive increases in the intracellular concentration of free Ca2+ ([Ca2+]i) that are referred to as [Ca2+]i oscillations. [Ca2+]i oscillations are essential for egg activation and early embryonic development. Recent evidence shows that the novel sperm-specific phospholipase C (PLC), PLCzeta, may be the long sought after [Ca2+]i oscillation-inducing SF. Here, we demonstrate the complete extraction of SF from porcine sperm and show that regardless of the method of extraction a single molecule/complex appears to be responsible for the [Ca2+]i oscillation-inducing activity of these extracts. Consistent with this notion, all sperm fractions that induced [Ca2+]i oscillations, including FPLC-purified fractions, exhibited high in vitro PLC activity at basal Ca2+ levels (0.1-5 microM), a hallmark of PLCzeta. Notably, we detected immunoreactive 72-kDa PLCzeta in an inactive fraction, and several fractions capable of inducing oscillations were devoid of 72-kDa PLCzeta. Nonetheless, in the latter fractions, proteolytic fragments, presumably corresponding to cleaved forms of PLCzeta, were detected by immunoblotting. Therefore, our findings corroborate the hypothesis that a sperm-specific PLC is the main component of the [Ca2+]i oscillation-inducing activity of sperm but provide evidence that the presence of 72-kDa PLCzeta does not precisely correspond with the Ca2+ releasing activity of porcine sperm fractions.

Germinal vesicle material drives meiotic cell cycle of mouse oocyte through the 3′UTR-dependent control of cyclin B1 synthesis

We compared the profile of histone H1 kinase activity, reflecting Maturation Promoting Factor (MPF) activity in oocytes bisected at the germinal vesicle (GV) stage and allowed to mature as separate oocyte halves in vitro. Whereas the oocyte halves containing the nucleus exhibited the same profile of increased kinase activity as that typical for intact oocytes, the anuclear halves revealed strong inhibition of the increase in this activity soon after germinal vesicle breakdown (GVBD). In contrast, the profile of MAP kinase activity did not differ significantly between anuclear and nucleus-containing oocyte halves throughout maturation. Of the two MPF components, CDK1 and cyclin B1, the amount of the latter was significantly reduced in anuclear halves, a reduction due to low-level synthesis and not to enhanced degradation. Expression of three reporter luciferase RNAs constructed, respectively, to contain cyclin B1-specific 3'UTR, the globin-specific 3'UTR, or no 3'UTR sequence was enhanced in nuclear halves, with significantly greater enhancement for the construct containing cyclin B1-specific 3'UTR as compared to the two other RNAs. We conclude that the profile of activity of MPF during mouse oocyte maturation is controlled by an unknown GV-associated factor(s) acting via 3'UTR-dependent control of cyclin B1 synthesis. These results require the revision of the hitherto prevailing view that the control of MPF activity during mouse oocyte maturation is independent of GV-derived material.

Thirty years of calcium signals at fertilization

It was discovered about 30 years ago that a dramatic increase in intracellular calcium ion concentration ([Ca(2+)](i)) occurs at fertilization and that this increase acts as the pivotal signal for egg activation. Later, the Ca(2+) signal at fertilization turned out to be ubiquitous among animal species. Extensive advance has been brought during these 30 years in research on spatiotemporal aspects and signaling mechanisms of the [Ca(2+)](i) increase, sperm factors that induce the Ca(2+) response, and cell cycle resumption caused by the [Ca(2+)](i) rise. I provide a historical account of these advances in mammals, sea urchins, and a few other models.

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.

Delay on the in vitro kinetic development of prepubertal ovine embryos

In the present study we characterize the developmental potential of prepubertal and adult ovine oocytes, analyzing the developmental speed to two-cell and blastocyst stages and its relationship with hatching from the zona pellucida, development after vitrification and the number and allocation of inner mass and trophoblastic cells. Prepubertal and adult ovine oocytes were matured and fertilized in vitro and first cleavage rates at 22, 26 and 32 h were recorded. Cleaved oocytes were cultured and blastocyst production was assessed at 6-9 days post-fertilization (dpf). Blastocysts from the two sources obtained on different days were divided into two groups: the first was vitrified, warmed and cultured in vitro to evaluate re-expansion of the blastocoelic cavity; blastocysts of the second were cultured separately to allow for hatching and count of trophoblastic and inner mass cells of hatched blastocysts by differential staining. We observed a significantly lower rate (P < 0.01) of cleaved prepubertal oocytes at 22 and 26 h after fertilization while it was higher (P<0.01) at 32 h than in the adult ones. Adult blastocyst production was significantly lower (P < 0.01) in prepubertal than in adult groups and began on the seventh dpf, later (P < 0.01) than in the adult group, where they appeared on the sixth dpf. Prepubertal blastocysts hatched at a lower rate than the adult ones (P < 0.01) and in both experimental groups faster blastocysts showed a higher (P < 0.01) hatching rate. Similarly, prepubertal derived blastocysts showed lower viability after vitrification (P < 0.01) compared to the adult counterparts, and in particular slower embryos had reduced viability after vitrification compared to the fastest (P < 0.01). Cell number was not different between blastocysts of both groups obtained at 6 and 7 dpf, which were higher (P < 0.01) than those obtained at 8 and 9 dpf. The ICM/trophoblast cell ratio was similar in 6- and 7-day obtained blastocyst and increased (P < 0.01) in those obtained 1 or 2 days later. These findings show that differences in kinetic development between prepubertal and adult derived embryos reflect differences in developmental capacity of the oocytes from which they derive and could be indicative of embryo quality.

Nuclear translocation of phospholipase C-zeta, an egg-activating factor, during early embryonic development

Phospholipase C-zeta (PLCzeta), a strong candidate of the egg-activating sperm factor, causes intracellular Ca2+ oscillations and egg activation, and is subsequently accumulated into the pronucleus (PN), when expressed in mouse eggs by injection of RNA encoding PLCzeta. Changes in the localization of expressed PLCzeta were investigated by tagging with a fluorescent protein. PLCzeta began to translocate into the PN formed at 5-6 h after RNA injection and increased there. Observation in the same embryo revealed that PLCzeta in the PN dispersed to the cytoplasm upon nuclear envelope breakdown and translocated again into the nucleus after cleavage. The dynamics was found in the second mitosis as well. When RNA was injected into fertilization-originated 1-cell embryos or blastomere(s) of 2-8-cell embryos, the nuclear localization of expressed PLCzeta was recognized in every embryo up to blastocyst. Thus, PLCzeta exhibited alternative cytoplasm/nucleus localization during development. This supports the view that the sperm factor could control cell cycle-dependent generation of Ca2+ oscillations in early embryogenesis.

Sperm-derived activating ability does not persist in mouse oocytes inseminated during in vitro maturation

Activity of the sperm-derived oocyte-activating factor persists in zygotes and can be detected by a fusion with metaphase II (MII) oocytes leading to the activation of the hybrids. We have shown, that in the great majority of oocytes inseminated 1-2 hr after germinal vesicle breakdown (GVBD) the sperm-derived activating ability was eliminated. Only few hybrids produced by fusion of MII oocytes with oocytes inseminated during in vitro maturation (M x IVM-P + sperm hybrids) underwent activation, whereas almost all of MII oocyte x zygote hybrids entered interphase. However, frequency of activation of M x IVM-P + sperm hybrids was higher than that of control hybrids, which were obtained by fusion of MII oocytes with oocytes uninseminated during in vitro maturation. Although the difference was not statistically significant, it suggested that in a certain number of oocytes inseminated after GVBD the sperm-derived oocyte-activating factor remained partially active. This was confirmed by our observation that several oocytes, which were inseminated during in vitro maturation and managed to accomplish MII, underwent activation and formed pronuclei when examined 25-26 hr after the beginning of maturation. We have also demonstrated that parthenogenotes, could acquire the sperm-derived activity, as a consequence of sperm injection. MII oocytes were fused with parthenogenotes inseminated by ICSI and all hybrids underwent activation. This result indicated that the ability to induce activation in hybrid, was sperm-derived.

Maintenance of sister chromatid attachment in mouse eggs through maturation-promoting factor activity

Mammalian eggs naturally arrest at metaphase of the second meiotic division, until sperm triggers a series of Ca(2+) spikes that result in activation of the anaphase-promoting complex/cyclosome (APC/C). APC/C activation at metaphase targets destruction-box containing substrates, such as cyclin B1 and securin, for degradation, and as such eggs complete the second meiotic division. Cyclin B1 degradation reduces maturation (M-phase)-promoting factor (MPF) activity and securin degradation allows sister chromatid separation. Here we examined the second meiotic division in mouse eggs following expression of a cyclin B1 construct with an N-terminal 90 amino acid deletion (Delta 90 cyclin B1) that was visualized by coupling to EGFP. This cyclin construct was not an APC/C substrate, and so following fertilization, sperm were incapable of stimulating Delta 90 cyclin B1 degradation. In these eggs, chromatin remained condensed and no pronuclei formed. As a consequence of the lack of pronucleus formation, sperm-triggered Ca(2+) spiking continued indefinitely, consistent with a current model in which the sperm-activating factor is localized to the nucleus. Because Ca(2+) spiking was not inhibited by Delta 90 cyclin B1, the degradation timing of securin, visualized by coupling it to EGFP, was unaffected. However, despite rapid securin degradation, sister chromatids remained attached. This was a direct consequence of MPF activity because separation was induced following application of the MPF inhibitor roscovitine. Similar observations regarding the ability of MPF to prevent sister chromatid separation have recently been made in Xenopus egg extracts and in HeLa cells. The results presented here show this mechanism can also occur in intact mammalian eggs and further that this mechanism appears conserved among vertebrates. We present a model in which metaphase II arrest is maintained primarily by MPF levels only.

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.

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.

Meiosis resumption, calcium-sensitive period, and PLC-β1 relocation into the nucleus in the mouse oocyte

We aimed to determine whether the breakdown of the germinal vesicle of the mouse oocyte and the nuclear import of phospholipase C-beta1 were calcium-dependent. We chelated Ca2+ ions with BAPTA-dextran at different times after the release of the oocyte from the ovarian follicle, i.e. after meiosis resumption has started, and we studied the effects on the kinetics of germinal vesicle breakdown, and on the migration of phospholipase C-beta1. We discriminate between two key-periods of calcium-sensitivity during the process of meiosis resumption. During the first hour, changes in the cytosolic Ca2+ especially promoted the migration of phospholipase C-beta1 into the nucleus, whereas changes in the nuclear concentration of Ca2+ were not implicated. Moreover, at this time, the cytosolic calcium pathway is PLC-beta1-dependent. By contrast, during the second hour following the onset of meiosis resumption, and thus just previous GVBD, the PLC-beta1-dependent Ca2+ signals in both cellular compartments were equally necessary for the resumption of meiosis. This particular period of the meiotic process corresponds to the moment when the phospholipase C-beta1 has strongly migrated into the nucleus. Our results highlight also the role played by the nucleus during the second key-period in the control of the GVBD via a Ca2+-dependent pathway.

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.

Porcine sperm factor supports activation and development of bovine nuclear transfer embryos

A study was undertaken to determine whether injection of porcine sperm factors (pSF), which trigger oscillations in intracellular calcium concentration ([Ca(2+)](i)) in mammalian oocytes, could be used to activate bovine oocytes during nuclear transfer. To date, only combined treatments that induce a monotonic rise in [Ca(2+)](i) and inhibit either phosphorylation or protein synthesis have been utilized in nuclear transfer. Several doses of pSF were tested. Injection of 5 mg/ml pSF triggered [Ca(2+)](i) oscillations that resembled those associated with fertilization with respect to amplitude and periodicity, and as a result, a high percentage of oocytes underwent activation. Furthermore, this concentration of pSF supported in vitro and in vivo development up to 60-90 days of gestation, comparable to development in control nuclear transfer embryos. Nevertheless, neither activation procedure supported development as well as did fertilization. The effectiveness of pSF as an activating agent in bovine oocytes may have been compromised because pSF was unable to support oscillations past 3-5 h postinjection and a second injection was necessary to extend the [Ca(2+)](i) oscillations. Likewise, a single injection of pSF failed to trigger downregulation of the inositol 1,4,5-trisphosphate receptor 1 subtype, whereas a second injection downregulated the receptor in a manner similar to that seen in fertilized oocytes. These results demonstrate that soluble factor(s) from porcine sperm can support early development in bovine nuclear transfer embryos; however, the efficacy may be limited because of the premature cessation of the induced oscillations.

Activity of a sperm-borne oocyte-activating factor in spermatozoa and spermatogenic cells from cynomolgus monkeys and its localization after oocyte activation

It is widely accepted that mature mammalian oocytes are induced to resume meiosis by a sperm-borne oocyte-activating factor(s) (sperm factor, SF) immediately after normal fertilization or intracytoplasmic sperm injection. The SF is most likely a soluble factor that is localized within the cytoplasm of mature spermatozoa, but the exact stage at which it appears during spermatogenesis and its localization after oocyte activation is not fully understood, except in the mouse. First, we injected mature spermatozoa and spermatogenic cells from cynomolgus monkeys into mouse oocytes to assess their oocyte-activating capacity. More than 90% of mouse oocytes were activated after injection of monkey spermatozoa. Round spermatids and primary spermatocytes (late pachytene to diplotene) also activated oocytes (93% and 79%, respectively). Injection of monkey spermatozoa and spermatids induces intracellular Ca(2+) oscillations in a pattern similar to that seen following normal fertilization. Most spermatocytes did not produce typical intracellular Ca(2+) oscillations. Second, we transferred pronuclei or cytoplasts from mouse oocytes that had been activated by monkey spermatozoa or spermatids into intact mature mouse oocytes by electrofusion in order to examine the localization of the SF after pronuclear formation. Some of the SF was localized within the pronuclei, but some stayed in the ooplasm. This study demonstrated that spermatogenic cells of cynomolgus monkeys acquire oocyte-activating capacity at much earlier stages than those of mice, and that the monkey SF has a pronucleus-directing nature, although to a lesser extent than the mouse SF.

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.

The initiation and regulation of Ca2+ signalling at fertilization in mammals

The transition from oocyte to embryo in mammals is triggered by a series of calcium transients. There are two distinguishing features of this signal transduction pathway. First, it appears to be triggered by a cell fusion event between egg and sperm that allows the direct introduction of a factor that leads to the release of intracellular Ca2+. Second, it features a slow-frequency calcium oscillator (one transient every 10-20 min) that persists for 3-4 h. In this review I report on recent developments in our understanding of how the Ca2+ oscillations are started and on the regulation of the overall temporal organization. The review focuses on mammalian fertilization and (inevitably) it is fertilization in the mouse that will be predominantly discussed. Relevant and topical contributions from the excellent body of literature available on other species will be utilized where appropriate but extensive reviews can be found elsewhere [Stricker S A (1999) Comparative biology of calcium signaling during fertilization and egg activation in animals Dev Biol 211: 57-76; Jaffe et al., this issue].

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

Fertilization-induced Ca(2+)spiking in mouse zygotes ceases at the end of pre-G1 as pronuclei (PN) form. In the present studies we found that there was no consistent temporal relationship between PN formation and cessation of spiking. We also show that nucleate and anucleate fragments of zygotes, obtained by bisection of fertilized eggs prior to PN formation, both ceased spiking at times that did not depend on the presence of the PN. We, therefore, concluded that formation of the PN does not cause spiking cessation. The possibility that cessation of the fertilization-induced Ca(2+)spiking may be mediated by a redox sensitive mechanism affecting the sensitivity of Ca(2+)release from internal stores is proposed. At first mitosis, a small proportion of zygotes show low amplitude calcium spikes prior to pronuclear envelope breakdown (PNEBD), whereas all zygotes spiked at this time in the presence of high extracellular Ca(2+)and dithiothreitol. Nucleated zygotic fragments also spiked before PNEBD whereas anucleated ones rarely did. Exit from G2 was required for this spiking to be observed in nucleated zygotes or fragments. Arrest in M-phase resulted in the appearance of a prolonged series of small amplitude spikes. It is concluded that the spiking at mitosis is cell cycle regulated and may differ qualitatively in its control from that at fertilization.

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.

Ultraviolet-irradiated spermatozoa activate oocytes but arrest preimplantation development after fertilization and nuclear transplantation in cattle

Artificial means of parthenogenetically activating mammalian oocytes are believed to lack an essential sperm epigenetic component required for normal development. The main goal of this study was to examine the potential of ultraviolet (UV)-irradiated sperm as a means of functionally eliminating the chromatin component of spermatozoa without affecting the ability to induce activation and support parthenogenetic development in cattle. Spermatozoa were stained with a DNA dye, exposed to various UV irradiation doses, and used to fertilize secondary oocytes. Although the percentage of pronuclei at 18 h postinsemination was similar using treated and control sperm, most oocytes fertilized by UV-irradiated sperm failed to develop beyond the 2-cell stage, suggesting that UV irradiation can functionally destroy the genomic component of spermatozoa with limited effects on the ability to induce oocyte activation. However, when oocytes activated with UV-irradiated sperm were used as hosts for nuclear transfer, developmental rates to cleavage and to blastocyst improved only marginally and remained lower than in the controls, indicating that UV-treated spermatozoa blocked development even in the presence of a diploid donor nucleus. Although DNA replication was not inhibited by UV irradiation treatment, abnormal chromatin morphology after cleavage suggests improper segregation of chromatin to daughter blastomeres during the first mitotic division. Together, these results indicate that although sperm exposed to UV can activate oocytes, a developmental block occurs at or soon after the first mitosis in parthenotes and oocytes reconstructed by nuclear transfer.

Sperm-triggered calcium oscillations during meiosis in ascidian oocytes first pause, restart, then stop: correlations with cell cycle kinase activity

We have investigated the relationship between the sperm-triggered Ca2+ oscillations and the activities of two cell cycle kinases (MPF activity and MAP kinase activity) at fertilisation of Ascidiella aspersa oocytes. Maturation Promoting Factor (MPF) activity is elevated in the metaphase I (MI)-arrested unfertilised oocyte (as measured by phosphorylation of exogenous histone H1) and falls 5 minutes after fertilisation to remain at low levels for 5 minutes. The first polar body (pb1) is extruded when the MPF activity is low. The MPF activity is elevated again 15 minutes after fertilisation and finally becomes inactivated 25 minutes after fertilisation when the pb2 is extruded. MAP kinase activity increases from an initially elevated level to reach maximal activity 10 minutes after fertilisation and subsequently falls to reach low levels 25 minutes after fertilisation. Sperm trigger a series of Ca2+ oscillations that pause for 5 minutes while only the MPF activity is low and are present when both MPF and MAP kinase activity are elevated. We next attempted to determine whether the second phase of calcium oscillations is required to reactivate the MPF activity that precedes extrusion of the second polar body. To do this, we triggered a monotonic Ca2+ signal. This leads to the inactivation of MPF followed by MPF reactivation. The MPF activity then remains elevated for an extended period of time. During this period, the chromatin remains condensed and a metaphase II (MII) spindle forms. Fertilisation of these MII oocytes triggers extrusion of pb2 in 7 minutes. Interestingly, the second phase of Ca2+ oscillations is completely absent when MII oocytes are fertilised. Thus, in both MI and MII oocytes, the sperm-triggered Ca2+ oscillations follow the MPF activity. Finally we discuss our finding that the Ca2+ release system remains sensitive during the metaphase-like state (including the period when the Ca2+ oscillations pause).

Increase of intracellular Ca2+ and relocation of E-cadherin during experimental decompaction of mouse embryos

To determine the role of intracellular Ca2+ in compaction, the first morphogenetic event in embryogenesis, we analyzed preimplantation mouse embryos under several decompacting conditions, including depletion of extracellular Ca2+, blocking of Ca2+ channels, and inhibition of microfilaments, calmodulin, and intracellular Ca2+ release. Those treatments induced decompaction of mouse morulae and simultaneously induced changes in cytosolic free Ca2+ concentration and deregionalization of E-cadherin and fodrin. When morulae were allowed to recompact, the location of both proteins recovered. In contrast, actin did not change its cortical location with compaction nor with decompaction-recompaction. Calmodulin localized in areas opposite to cell-cell contacts in eight-cell stage embryos before and after compaction. Inhibition of calmodulin with trifluoperazine induced its delocalization while morulae decompacted. A nonspecific rise of intracellular free Ca2+ provoked by ionomycin did not affect the compacted shape. Moreover, the same decompacting treatments when applied to uncompacted embryos did not produce any change in intracellular Ca2+. Our results demonstrate that in preimplantation mouse embryos experimentally induced stage-specific changes of cell shape are accompanied by changes of intracellular free Ca2+ and redistribution of the cytoskeleton-related proteins E-cadherin, fodrin, and calmodulin. We conclude that intracellular Ca2+ specifically is involved in compaction and probably regulates the function and localization of cytoskeleton elements.

Role of calcium oscillations in mammalian egg activation: experimental approach

Biological rhythms are everywhere; the pulsatility of intracellular signals appears to maximise the cellular processes better than constant signaling. The aim of this paper is, firstly, to review the cellular mechanisms that modulate calcium oscillator activity during fertilisation and, secondly, to describe recent results we have obtained by artificially imposing rhythmical calcium stimulation on fertilised rabbit eggs during in vitro culture. The key finding in these experiments is that the egg appears to be sensitive to repetitive signalling during a period that goes far beyond the time of meiosis reinitiation. When delivered at the proper rhythm transient signalling can optimise developmental processes.

Calcium and mitosis

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

A novel protein for Ca2+ signaling at fertilization

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