Oscillatory CaMKII activity in mouse egg activation

No transcription, no problem: Protein phosphorylation changes and the transition from oocyte to embryo

Although mature oocytes are arrested in a differentiated state, they are provisioned with maternally-derived macromolecules that will start embryogenesis. The transition to embryogenesis, called 'egg activation', occurs without new transcription, even though it includes major cell changes like completing stalled meiosis, translating stored mRNAs, cytoskeletal remodeling, and changes to nuclear architecture. In most animals, egg activation is triggered by a rise in free calcium in the egg's cytoplasm, but we are only now beginning to understand how this induces the egg to transition to totipotency and proliferation. Here, we discuss the model that calcium-dependent protein kinases and phosphatases modify the phosphorylation landscape of the maternal proteome to activate the egg. We review recent phosphoproteomic mass spectrometry analyses that revealed broad phospho-regulation during egg activation, both in number of phospho-events and classes of regulated proteins. Our interspecies comparisons of these proteins pinpoints orthologs and protein families that are phospho-regulated in activating eggs, many of which function in hallmark events of egg activation, and others whose regulation and activity warrant further study. Finally, we discuss key phospho-regulating enzymes that may act apically or as intermediates in the phosphorylation cascades during egg activation. Knowing the regulators, targets, and effects of phospho-regulation that cause an egg to initiate embryogenesis is crucial at both fundamental and applied levels for understanding female fertility, embryo development, and cell-state transitions.

The characteristics of the calcium signals that activate mammalian eggs at fertilization

Gamete membrane fusion in mammals brings the paternal genome into the cytoplasm of the egg. It also enables signals to pass from the sperm into the egg to trigger the completion of meiosis and the start of embryo development. The essential signal to activate development in all mammals studied, consists of a series of transient increases in the cytosolic Ca2+ concentration driven by cycles of InsP3 production. This review focusses on the characteristics of these sperm-induced Ca2+ signals. I consider how some specific features of sperm-derived phospholipase C-zeta (PLCζ), along with the known properties of the type 1 InsP3 receptor, provide a basis for understanding the mechanisms of the dynamic changes in Ca2+ observed in fertilizing eggs. I describe how the PLCζ targeting of cytoplasmic vesicles in the egg cytoplasm, that contain PI(4,5)P2, is necessary to explain the rapid waves associated with the rising phase of each Ca2+ transient. I also discuss the importance of the repetitive Ca2+ rises for egg activation and the way mitochondrial ATP production may modulate Ca2+ release in eggs. Finally, I consider the role that a sperm-induced ATP increase may play in the egg activation process.

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.

Ionophore application for artificial oocyte activation and its potential effect on morphokinetics: a sibling oocyte study

PURPOSE

To evaluate whether ionophore application at the oocyte stage changes the morphokinetics of the associated embryos in cases of artificial oocyte activation.

METHODS

In a prospective sibling oocyte approach, 78 ICSI patients with suspected fertilization problems had half of their MII-oocytes treated with a ready-to-use ionophore (calcimycin) immediately following ICSI (study group). Untreated ICSI eggs served as the control group. Primary analyses focused on morphokinetic behavior and the presence of irregular cleavages. The rates of fertilization, utilization, pregnancy, and live birth rate were also evaluated.

RESULTS

Ionophore-treated oocytes showed a significantly earlier formation of pronuclei (t2PNa) and a better synchronized third cell cycle (s3) (P < .05). The rate of irregular cleavage was unaffected (P > .05). Ionophore treatment significantly improved the overall rates of fertilization (P < .01) and blastocyst utilization (P < .05).

CONCLUSION

Ionophore application does not negatively affect cleavage timing nor is it associated with irregular cleavage.

Knockin' on Egg's Door: Maternal Control of Egg Activation That Influences Cortical Granule Exocytosis in Animal Species

Fertilization by multiple sperm leads to lethal chromosomal number abnormalities, failed embryo development, and miscarriage. In some vertebrate and invertebrate eggs, the so-called cortical reaction contributes to their activation and prevents polyspermy during fertilization. This process involves biogenesis, redistribution, and subsequent accumulation of cortical granules (CGs) at the female gamete cortex during oogenesis. CGs are oocyte- and egg-specific secretory vesicles whose content is discharged during fertilization to block polyspermy. Here, we summarize the molecular mechanisms controlling critical aspects of CG biology prior to and after the gametes interaction. This allows to block polyspermy and provide protection to the developing embryo. We also examine how CGs form and are spatially redistributed during oogenesis. During egg activation, CG exocytosis (CGE) and content release are triggered by increases in intracellular calcium and relies on the function of maternally-loaded proteins. We also discuss how mutations in these factors impact CG dynamics, providing unprecedented models to investigate the genetic program executing fertilization. We further explore the phylogenetic distribution of maternal proteins and signaling pathways contributing to CGE and egg activation. We conclude that many important biological questions and genotype–phenotype relationships during fertilization remain unresolved, and therefore, novel molecular players of CG biology need to be discovered. Future functional and image-based studies are expected to elucidate the identity of genetic candidates and components of the molecular machinery involved in the egg activation. This, will open new therapeutic avenues for treating infertility in humans.

Comparative study of preimplantation development following distinct assisted oocyte activation protocols in a PLC-zeta knockout mouse model

Mammalian fertilization encompasses a series of Ca2+ oscillations initiated by the sperm factor phospholipase C zeta (PLCζ). Some studies have shown that altering the Ca2+ oscillatory regime at fertilization affects preimplantation blastocyst development. However, assisted oocyte activation (AOA) protocols can induce oocyte activation in a manner that diverges profoundly from the physiological Ca2+ profiling. In our study, we used the newly developed PLCζ-null sperm to investigate the independent effect of AOA on mouse preimplantation embryogenesis. Based on previous findings, we hypothesized that AOA protocols with Ca2+ oscillatory responses might improve blastocyst formation rates and differing Ca2+ profiles might alter blastocyst transcriptomes. A total of 326 MII B6D2F1-oocytes were used to describe Ca2+ profiles and to compare embryonic development and individual blastocyst transcriptomes between four control conditions: C1 (in-vivo fertilization), C2 (ICSI control sperm), C3 (parthenogenesis) and C4 (ICSI-PLCζ-KO sperm) and four AOA groups: AOA1 (human recombinant PLCζ), AOA2 (Sr2+), AOA3 (ionomycin) and AOA4 (TPEN). All groups revealed remarkable variations in their Ca2+ profiles; however, oocyte activation rates were comparable between the controls (91.1% ± 13.8%) and AOA (86.9% ± 11.1%) groups. AOA methods which enable Ca2+ oscillatory responses (AOA1: 41% and AOA2: 75%) or single Ca2+ transients (AOA3: 50%) showed no significantly different blastocyst rates compared to ICSI control group (C2: 70%). In contrast, we observed a significant decrease in compaction (53% vs. 83%) and blastocyst rates (41% vs. 70%) in the absence of an initial Ca2+ trigger (AOA4) compared with the C2 group. Transcription profiles did not identify significant differences in gene expression levels between the ICSI control group (C2) and the four AOA groups.

Single Ca2+ transients vs oscillatory Ca2+ signaling for assisted oocyte activation: limitations and benefits

Oocyte activation is a calcium (Ca²⁺)-dependent process that has been investigated in depth, in particular, regarding its impact on assisted reproduction technology (ART). Following a standard model of signal transduction, Ca²⁺ drives the meiotic progression upon fertilization in all species studied to date. However, Ca²⁺ changes during oocyte activation are species specific, and they can be classified in two modalities based on the pattern defined by the Ca²⁺ signature: a single Ca²⁺ transient (e.g. amphibians) or repetitive Ca²⁺ transients called Ca²⁺ oscillations (e.g. mammals). Interestingly, assisted oocyte activation (AOA) methods have highlighted the ability of mammalian oocytes to respond to single Ca²⁺ transients with normal embryonic development. In this regard, there is evidence supporting that cellular events during the process of oocyte activation are initiated by different number of Ca²⁺ oscillations. Moreover, it was proposed that oocyte activation and subsequent embryonic development are dependent on the total summation of the Ca²⁺ peaks, rather than to a specific frequency pattern of Ca²⁺ oscillations. The present review aims to demonstrate the complexity of mammalian oocyte activation by describing the series of Ca²⁺-linked physiological events involved in mediating the egg-to-embryo transition. Furthermore, mechanisms of AOA and the limitations and benefits associated with the application of different activation agents are discussed.

Deficiencies in extrusion of the second polar body due to high calcium concentrations during in vitro fertilization in inbred C3H/He mice

Successful in vitro fertilization (IVF) of all inbred strains of laboratory mice has not yet been accomplished. We have previously shown that a high calcium concentration improved IVF in various inbred mice. However, we also found that in cumulus-free ova of C3H/He mice such IVF conditions significantly increased the deficiency of extrusion of the second polar body (PBII) in a dose-dependent manner (2% at 1.71 mM and 29% at 6.84 mM, P < 0.05) and that PBII extrusion was affected by high calcium levels at 2–3 h post-insemination. While developmental competence of ova without PBII extrusion to blastocysts after 96 h culture was not affected, a significant reduction in the nuclear number of the inner cell mass was observed in blastocyst fertilized under high calcium condition. We also examined how high calcium concentration during IVF affects PBII extrusion in C3H/He mice. Cumulus cells cultured under high calcium conditions showed a significantly alleviated deficient PBII extrusion. This phenomenon is likely to be specific to C3H/He ova because deficient PBII extrusion in reciprocal fertilization between C3H and BDF1 gametes was observed only in C3H/He ova. Sperm factor(s) was still involved in deficient PBII extrusion due to high calcium concentrations, as this phenomenon was not observed in ova activated by ethanol. The cytoskeletal organization of ova without PBII extrusion showed disturbed spindle rotation, incomplete formation of contractile ring and disturbed localization of actin, suggesting that high calcium levels affect the anchoring machinery of the meiotic spindle. These results indicate that in C3H/He mice high calcium levels induce abnormal fertilization, i.e. deficient PBII extrusion by affecting the cytoskeletal organization, resulting in disturbed cytokinesis during the second meiotic division. Thus, use of high calcium media for IVF should be avoided for this strain.

CaV3.2 T-type channels mediate Ca²⁺ entry during oocyte maturation and following fertilization

Initiation of mouse embryonic development depends upon a series of fertilization-induced rises in intracellular Ca(2+). Complete egg activation requires influx of extracellular Ca(2+); however, the channels that mediate this influx remain unknown. Here, we tested whether the α1 subunit of the T-type channel CaV3.2, encoded by Cacna1h, mediates Ca(2+) entry into oocytes. We show that mouse eggs express a robust voltage-activated Ca(2+) current that is completely absent in Cacna1h(-/-) eggs. Cacna1h(-/-) females have reduced litter sizes, and careful analysis of Ca(2+) oscillation patterns in Cacna1h(-/-) eggs following in vitro fertilization (IVF) revealed reductions in first transient length and oscillation persistence. Total and endoplasmic reticulum (ER) Ca(2+) stores were also reduced in Cacna1h(-/-) eggs. Pharmacological inhibition of CaV3.2 in wild-type CF-1 strain eggs using mibefradil or pimozide reduced Ca(2+) store accumulation during oocyte maturation and reduced Ca(2+) oscillation persistence, frequency and number following IVF. Overall, these data show that CaV3.2 T-type channels have prev8iously unrecognized roles in supporting the meiotic-maturation-associated increase in ER Ca(2+) stores and mediating Ca(2+) influx required for the activation of development.

Calcium waves occur as Drosophila oocytes activate

Egg activation is the process by which a mature oocyte becomes capable of supporting embryo development. In vertebrates and echinoderms, activation is induced by fertilization. Molecules introduced into the egg by the sperm trigger progressive release of intracellular calcium stores in the oocyte. Calcium wave(s) spread through the oocyte and induce completion of meiosis, new macromolecular synthesis, and modification of the vitelline envelope to prevent polyspermy. However, arthropod eggs activate without fertilization: in the insects examined, eggs activate as they move through the female's reproductive tract. Here, we show that a calcium wave is, nevertheless, characteristic of egg activation in Drosophila. This calcium rise requires influx of calcium from the external environment and is induced as the egg is ovulated. Pressure on the oocyte (or swelling by the oocyte) can induce a calcium rise through the action of mechanosensitive ion channels. Visualization of calcium fluxes in activating eggs in oviducts shows a wave of increased calcium initiating at one or both oocyte poles and spreading across the oocyte. In vitro, waves also spread inward from oocyte pole(s). Wave propagation requires the IP3 system. Thus, although a fertilizing sperm is not necessary for egg activation in Drosophila, the characteristic of increased cytosolic calcium levels spreading through the egg is conserved. Because many downstream signaling effectors are conserved in Drosophila, this system offers the unique perspective of egg activation events due solely to maternal components.

Excess cholesterol induces mouse egg activation and may cause female infertility

The HDL receptor scavenger receptor, class B type I (SR-BI) controls the structure and fate of plasma HDL. Female SR-BI KO mice are infertile, apparently because of their abnormal cholesterol-enriched HDL particles. We examined the growth and meiotic progression of SR-BI KO oocytes and found that they underwent normal germinal vesicle breakdown; however, SR-BI KO eggs, which had accumulated excess cholesterol in vivo, spontaneously activated, and they escaped metaphase II (MII) arrest and progressed to pronuclear, MIII, and anaphase/telophase III stages. Eggs from fertile WT mice were activated when loaded in vitro with excess cholesterol by a cholesterol/methyl-β-cyclodextrin complex, phenocopying SR-BI KO oocytes. In vitro cholesterol loading of eggs induced reduction in maturation promoting factor and MAPK activities, elevation of intracellular calcium, extrusion of a second polar body, and progression to meiotic stages beyond MII. These results suggest that the infertility of SR-BI KO females is caused, at least in part, by excess cholesterol in eggs inducing premature activation and that cholesterol can activate WT mouse eggs to escape from MII arrest. Analysis of SR-BI KO female infertility raises the possibility that abnormalities in cholesterol metabolism might underlie some cases of human female infertility of unknown etiology.

Calcium in health and disease

Evolution has exploited the chemical properties of Ca(2+), which facilitate its reversible binding to the sites of irregular geometry offered by biological macromolecules, to select it as a carrier of cellular signals. A number of proteins bind Ca(2+) to specific sites: those intrinsic to membranes play the most important role in the spatial and temporal regulation of the concentration and movements of Ca(2+) inside cells. Those which are soluble, or organized in non-membranous structures, also decode the Ca(2+) message to be then transmitted to the targets of its regulation. Since Ca(2+) controls the most important processes in the life of cells, it must be very carefully controlled within the cytoplasm, where most of the targets of its signaling function reside. Membrane channels (in the plasma membrane and in the organelles) mediate the entrance of Ca(2+) into the cytoplasm, ATPases, exchangers, and the mitochondrial Ca(2+) uptake system remove Ca(2+) from it. The concentration of Ca(2+) in the external spaces, which is controlled essentially by its dynamic exchanges in the bone system, is much higher than inside cells, and can, under conditions of pathology, generate a situation of dangerous internal Ca(2+) overload. When massive and persistent, the Ca(2+) overload culminates in the death of the cell. Subtle conditions of cellular Ca(2+) dyshomeostasis that affect individual systems that control Ca(2+), generate cell disease phenotypes that are particularly severe in tissues in which the signaling function of Ca(2+) has special importance, e.g., the nervous system.

Specificity of calcium/calmodulin-dependent protein kinases in mouse egg activation

CaMKIIγ, the predominant CaMKII isoform in mouse eggs, controls egg activation by regulating cell cycle resumption. In this study we further characterize the involvement and specificity of CaMKIIγ in mouse egg activation. Using exogenous expression of different cRNAs in Camk2g(-/-) eggs, we show that the other multifunctional CaM kinases, CaMKI, and CaMKIV, are not capable of substituting CaMKIIγ to initiate cell cycle resumption in response to a rise in intracellular Ca (2+). Exogenous expression of Camk2g or Camk2d results in activation of nearly 80% of Camk2g(-/-) MII eggs after stimulation with SrCl 2, which does not differ from the incidence of activation of wild-type eggs expressing exogenous Egfp. In contrast, none of the Camk2g(-/-) MII eggs expressing Camk1 or Camk4 activate in response to SrCl 2 treatment. Expression of a constitutively active form of Camk4 (ca-Camk4), but not Camk1, triggers egg activation. EMI2, an APC/C repressor, is a key component in regulating egg activation downstream of CaMKII in both Xenopus laevis and mouse. We show that exogenous expression of either Camk2g, Camk2d, or ca-Camk4, but not Camk1, Camk4, or a catalytically inactive mutant form of CaMKIIγ (kinase-dead) in Camk2g(-/-) mouse eggs leads to almost complete degradation (~90%) of exogenously expressed EMI2 followed by cell cycle resumption. Thus, degradation of EMI2 following its phosphorylation specifically by CaMKII is mechanistically linked to and promotes cell cycle resumption in MII eggs.

Release from meiotic arrest in ascidian eggs requires the activity of two phosphatases but not CaMKII

The fertilising sperm triggers a transient Ca(2+) increase that releases eggs from cell cycle arrest in the vast majority of animal eggs. In vertebrate eggs, Erp1, an APC/C(cdc20) inhibitor, links release from metaphase II arrest with the Ca(2+) transient and its degradation is triggered by the Ca(2+)-induced activation of CaMKII. By contrast, many invertebrate groups have mature eggs that arrest at metaphase I, and these species do not possess the CaMKII target Erp1 in their genomes. As a consequence, it is unknown exactly how cell cycle arrest at metaphase I is achieved and how the fertilisation Ca(2+) transient overcomes the arrest in the vast majority of animal species. Using live-cell imaging with a novel cyclin reporter to study cell cycle arrest and its release in urochordate ascidians, the closest living invertebrate group to the vertebrates, we have identified a new signalling pathway for cell cycle resumption in which CaMKII plays no part. Instead, we find that the Ca(2+)-activated phosphatase calcineurin (CN) is required for egg activation. Moreover, we demonstrate that parthenogenetic activation of metaphase I-arrested eggs by MEK inhibition, independent of a Ca(2+) increase, requires the activity of a second egg phosphatase: PP2A. Furthermore, PP2A activity, together with CN, is required for normal egg activation during fertilisation. As ascidians are a sister group of the vertebrates, we discuss these findings in relation to cell cycle arrest and egg activation in chordates.

TRPV3 channels mediate strontium-induced mouse-egg activation

In mammals, calcium influx is required for oocyte maturation and egg activation. The molecular identities of the calcium-permeant channels that underlie the initiation of embryonic development are not established. Here, we describe a transient receptor potential (TRP) ion channel current activated by TRP agonists that is absent in TrpV3(-/-) eggs. TRPV3 current is differentially expressed during oocyte maturation, reaching a peak of maximum density and activity at metaphase of meiosis II (MII), the stage of fertilization. Selective activation of TRPV3 channels provokes egg activation by mediating massive calcium entry. Widely used to activate eggs, strontium application is known to yield normal offspring in combination with somatic cell nuclear transfer. We show that TRPV3 is required for strontium influx, because TrpV3(-/-) eggs failed to conduct Sr(2+) or undergo strontium-induced activation. We propose that TRPV3 is a major mediator of calcium influx in mouse eggs and is a putative target for artificial egg activation.

Modulation of cell cycle control during oocyte-to-embryo transitions

Ex ovo omnia--all animals come from eggs--this statement made in 1651 by the English physician William Harvey marks a seminal break with the doctrine that all essential characteristics of offspring are contributed by their fathers, while mothers contribute only a material substrate. More than 360 years later, we now have a comprehensive understanding of how haploid gametes are generated during meiosis to allow the formation of diploid offspring when sperm and egg cells fuse. In most species, immature oocytes are arrested in prophase I and this arrest is maintained for few days (fruit flies) or for decades (humans). After completion of the first meiotic division, most vertebrate eggs arrest again at metaphase of meiosis II. Upon fertilization, this second meiotic arrest point is released and embryos enter highly specialized early embryonic divisions. In this review, we discuss how the standard somatic cell cycle is modulated to meet the specific requirements of different developmental stages. Specifically, we focus on cell cycle regulation in mature vertebrate eggs arrested at metaphase II (MII-arrest), the first mitotic cell cycle, and early embryonic divisions.

Phospho-regulation pathways during egg activation in Drosophila melanogaster

Egg activation is the series of events that transition a mature oocyte to an egg capable of supporting embryogenesis. Increasing evidence points toward phosphorylation as a critical regulator of these events. We used Drosophila melanogaster to investigate the relationship between known egg activation genes and phosphorylation changes that occur upon egg activation. Using the phosphorylation states of four proteins-Giant Nuclei, Young Arrest, Spindly, and Vap-33-1-as molecular markers, we showed that the egg activation genes sarah, CanB2, and cortex are required for the phospho-regulation of multiple proteins. We show that an additional egg activation gene, prage, regulates the phosphorylation state of a subset of these proteins. Finally, we show that Sarah and calcineurin are required for the Anaphase Promoting Complex/Cyclosome (APC/C)-dependent degradation of Cortex following egg activation. From these data, we present a model in which Sarah, through the activation of calcineurin, positively regulates the APC/C at the time of egg activation, which leads to a change in phosphorylation state of numerous downstream proteins.

Molecular changes during egg activation

Egg activation is the final transition that an oocyte goes through to become a developmentally competent egg. This transition is usually triggered by a calcium-based signal that is often, but not always, initiated by fertilization. Activation encompasses a number of changes within the egg. These include changes to the egg's membranes and outer coverings to prevent polyspermy and to support the developing embryo, as well as resumption and completion of the meiotic cell cycle, mRNA polyadenylation, translation of new proteins, and the degradation of specific maternal mRNAs and proteins. The transition from an arrested, highly differentiated cell, the oocyte, to a developmentally active, totipotent cell, the activated egg or embryo, represents a complete change in cellular state. This is accomplished by altering ion concentrations and by widespread changes in both the proteome and the suite of mRNAs present in the cell. Here, we review the role of calcium and zinc in the events of egg activation, and the importance of macromolecular changes during this transition. The latter include the degradation and translation of proteins, protein posttranslational regulation through phosphorylation, and the degradation, of maternal mRNAs.

Ca(2+) homeostasis and regulation of ER Ca(2+) in mammalian oocytes/eggs

The activation of the developmental program in mammalian eggs relies on the initiation at the time of fertilization of repeated rises in the intracellular concentration of free calcium ([Ca(2+)](i)), also known as [Ca(2+)](i) oscillations. The ability to mount the full complement of oscillations is only achieved at the end of oocyte maturation, at the metaphase stage of meiosis II (MII). Over the last decades research has focused on addressing the mechanisms by which the sperm initiates the oscillations and identification of the channels that mediate intracellular Ca(2+) release. This review will describe the up-to-date knowledge of other aspects of Ca(2+) homeostasis in mouse oocytes, such as the mechanisms that transport Ca(2+) out of the cytosol into the endoplasmic reticulum (ER), the Ca(2+) store of the oocyte/egg, into other organelles and also those that extrude Ca(2+). Evidence pointing to channels in the plasma membrane that mediate Ca(2+) entry from the extracellular milieu, which is required for the persistence of the oscillations, is also discussed, along with the modifications that these mechanisms undergo during maturation. Lastly, we highlight areas where additional research is needed to obtain a better understating of the molecules and mechanisms that regulate Ca(2+) homeostasis in this unique Ca(2+) signaling system.

Calcium and egg activation in Drosophila

In many animals, a rise in intracellular calcium levels is the trigger for egg activation, the process by which an arrested mature oocyte transitions to prepare for embryogenesis. In nearly all animals studied to date, this calcium rise, and thus egg activation, is triggered by the fertilizing sperm. However in the insects that have been examined, fertilization is not necessary to activate their oocytes. Rather, these insects' eggs activate as they transit through the female's reproductive tract, regardless of male contribution. Recent studies in Drosophila have shown that egg activation nevertheless requires calcium and that the downstream events and molecules of egg activation are also conserved, despite the difference in initial trigger. Genetic studies have uncovered essential roles for the calcium-dependent enzyme calcineurin and its regulator calcipressin, and have hinted at roles for calmodulin, in Drosophila egg activation. Physiological and in vitro studies have led to a model in which mechanical forces that impact the Drosophila oocyte as it moves through the reproductive tract triggers the influx of calcium from the external environment, thereby initiating egg activation. Future research will aim to test this model, as well as to determine the spatiotemporal dynamics of cytoplasmic calcium flux and mode of signal propagation in this unique system.

Calcium influx-mediated signaling is required for complete mouse egg activation

Mammalian fertilization is accompanied by oscillations in egg cytoplasmic calcium (Ca(2+)) concentrations that are critical for completion of egg activation. These oscillations are initiated by Ca(2+) release from inositol 1,4,5-trisphosphate (IP(3))-sensitive intracellular stores. We tested the hypothesis that Ca(2+) influx across the plasma membrane was a requisite component of egg activation signaling, and not simply a Ca(2+) source for store repletion. Using intracytoplasmic sperm injection (ICSI) and standard in vitro fertilization (IVF), we found that Ca(2+) influx was not required to initiate resumption of meiosis II. However, even if multiple oscillations in intracellular Ca(2+) occurred, in the absence of Ca(2+) influx, the fertilized eggs failed to emit the second polar body, resulting in formation of three pronuclei. Additional experiments using the Ca(2+) chelator, BAPTA/AM, demonstrated that Ca(2+) influx is sufficient to support polar body emission and pronucleus formation after only a single sperm-induced Ca(2+) transient, whereas BAPTA/AM-treated ICSI or fertilized eggs cultured in Ca(2+)-free medium remained arrested in metaphase II. Inhibition of store-operated Ca(2+) entry had no effect on ICSI-induced egg activation, so Ca(2+) influx through alternative channels must participate in egg activation signaling. Ca(2+) influx appears to be upstream of CaMKIIγ activity because eggs can be parthenogenetically activated with a constitutively active form of CaMKIIγ in the absence of extracellular Ca(2+). These results suggest that Ca(2+) influx at fertilization not only maintains Ca(2+) oscillations by replenishing Ca(2+) stores, but also activates critical signaling pathways upstream of CaMKIIγ that are required for second polar body emission.

Src protein kinases in mouse and rat oocytes and embryos

Meiosis of the mammalian oocytes is a specialized cell division, initiated during the female's embryonic life. It arrests at the germinal vesicle (GV) stage and resumes with GV breakdown, followed by segregation of the chromosomes and extrusion of the first polar body in an asymmetric cell division that concludes the first meiotic division, before arresting at metaphase of the second meiotic division (MII). Once fertilized, the oocyte exits from MII, extrudes the second polar body, and the developing zygote will continue dividing to create a blastocyst. Although the two processes of meiosis and mitosis have different developmental functions, it is believed that they share similar mechanisms. Src family kinases (SFKs) are nine non-receptor protein tyrosine kinases that regulate many key cellular functions including meiotic and mitotic cell cycles. In this review we discuss the involvement of SFKs in meiotic and mitotic cell cycle key processes as nuclear envelope breakdown, spindle stabilization, karyokinetic exit from metaphase, regulation of cortical actin, and cytokinetic cleavage furrow ingression.

Dynamic analysis of Ca²+ level during bovine oocytes maturation and early embryonic development

Mammalian oocyte maturation and early embryo development processes are Ca²⁺-dependent. In this study, we used confocal microscopy to investigate the distribution pattern of Ca²⁺ and its dynamic changes in the processes of bovine oocytes maturation, in vitro fertilization (IVF), parthenogenetic activation (PA) and somatic cell nuclear transfer (SCNT) embryo development. During the germinal vesicle (GV) and GV breakdown stage, Ca²⁺ was distributed in the cortical ooplasm and throughout the oocytes from the MI to MII stage. In IVF embryos, Ca²⁺ was distributed in the cortical ooplasm before the formation of the pronucleus. In 4-8 cell embryos and morulas, Ca²⁺ was present throughout the blastomere. In PA embryos, Ca²⁺ was distributed throughout the blastomere at 48 h, similar to in the 4-cell and 8-cell phase and the morula. At 6 h after activation, there was almost no distribution of Ca²⁺ in the SCNT embryos. However, Ca²⁺ was distributed in the donor nucleus at 10 h and it was distributed throughout the blastomere in the 2-8 cell embryos. In this study, Ca²⁺ showed significant fluctuations with regularity of IVF and SCNT groups, but PA did not. Systematic investigation of the Ca²⁺ location and distribution changes during oocyte maturation and early embryo development processes should facilitate a better understanding of the mechanisms involved in oocyte maturation, reconstructed embryo activation and development, ultimately improving the reconstructed embryo development rate.

Anaphase-promoting complex control in female mouse meiosis

Entry into, and passage through, the two meiotic divisions of the oocyte has to be highly coordinated to ensure proper segregation of chromosomes. This coordination ensures that the hallmark stops and starts of the meiotic process occur at the right time to prevent aneuploidy. The Anaphase-Promoting Complex is an activity mostly studied in the mitotic cell cycle division, where it has essential functions during mitosis. As detailed here the Anaphase-Promoting Complex also plays vital roles in controlling at least three meiotic events: maintenance of prophase I arrest, timely and faithful segregation of homologous chromosomes in meiosis I, and the meiotic arrest following ovulation.

Rhythmic actomyosin-driven contractions induced by sperm entry predict mammalian embryo viability

Fertilization-induced cytoplasmic flows are a conserved feature of eggs in many species. However, until now the importance of cytoplasmic flows for the development of mammalian embryos has been unknown. Here, by combining a rapid imaging of the freshly fertilized mouse egg with advanced image analysis based on particle image velocimetry, we show that fertilization induces rhythmical cytoplasmic movements that coincide with pulsations of the protrusion forming above the sperm head. We find that these movements are caused by contractions of the actomyosin cytoskeleton triggered by Ca²⁺ oscillations induced by fertilization. Most importantly, the relationship between the movements and the events of egg activation makes it possible to use the movements alone to predict developmental potential of the zygote. In conclusion, this method offers, thus far, the earliest and fastest, non-invasive way to predict the viability of eggs fertilized in vitro and therefore can potentially improve greatly the prospects for IVF treatment.

Cytoplasmic flows—the movement of cytoplasmic material—can be detected following the fertilization of an egg by a sperm in many species. In this study, rhythmic cytoplasmic flows are shown to be induced in mice by calcium-induced cytoskeleton contractions which could be used to predict the successful outcome of fertilization.

Zinc sparks are triggered by fertilization and facilitate cell cycle resumption in mammalian eggs

In last few hours of maturation, the mouse oocyte takes up over twenty billion zinc atoms and arrests after the first meiotic division, until fertilization or pharmacological intervention stimulates cell cycle progression toward a new embryo. Using chemical and physical probes, we show that fertilization of the mature, zinc-enriched egg triggers the ejection of zinc into the extracellular milieu in a series of coordinated events termed zinc sparks. These events immediately follow the well-established series of calcium oscillations within the activated egg and are evolutionarily conserved in several mammalian species, including rodents and nonhuman primates. Functionally, the zinc sparks mediate a decrease in intracellular zinc content that is necessary for continued cell cycle progression, as increasing zinc levels within the activated egg results in the reestablishment of cell cycle arrest at metaphase. The mammalian egg thus uses a zinc-dependent switch mechanism to toggle between metaphase arrest and resumption of the meiotic cell cycle at the initiation of embryonic development.

Ca2+ signaling during mammalian fertilization: requirements, players, and adaptations

Changes in the intracellular concentration of calcium ([Ca(2+)](i)) represent a vital signaling mechanism enabling communication among cells and between cells and the environment. The initiation of embryo development depends on a [Ca(2+)](i) increase(s) in the egg, which is generally induced during fertilization. The [Ca(2+)](i) increase signals egg activation, which is the first stage in embryo development, and that consist of biochemical and structural changes that transform eggs into zygotes. The spatiotemporal patterns of [Ca(2+)](i) at fertilization show variability, most likely reflecting adaptations to fertilizing conditions and to the duration of embryonic cell cycles. In mammals, the focus of this review, the fertilization [Ca(2+)](i) signal displays unique properties in that it is initiated after gamete fusion by release of a sperm-derived factor and by periodic and extended [Ca(2+)](i) responses. Here, we will discuss the events of egg activation regulated by increases in [Ca(2+)](i), the possible downstream targets that effect these egg activation events, and the property and identity of molecules both in sperm and eggs that underpin the initiation and persistence of the [Ca(2+)](i) responses in these species.

The role of Fyn kinase in the release from metaphase in mammalian oocytes

Meiosis in mammalian oocytes starts during embryonic life and arrests for the first time before birth, at prophase of the first meiotic division. The second meiotic arrest occurs after spindle formation at metaphase of the second meiotic division (MII) in selected oocytes designated for ovulation. The fertilizing spermatozoon induces the release from MII arrest only after the oocyte's spindle assembly checkpoint (SAC) was deactivated. Src family kinases (SFKs) are nine non-receptor protein tyrosine kinases that regulate many key cellular functions. Fyn is an SFK expressed in many cell types, including oocytes. Recent studies, including ours, imply a role for Fyn in exit from meiotic and mitotic metaphases. Other studies demonstrate that SFKs, particularly Fyn, are required for regulation of microtubules polymerization and spindle stabilization. Altogether, Fyn is suggested to play an essential role in signaling events that implicate SAC pathway and hence in regulating the exit from metaphase in oocytes and zygote.

Calmodulin-dependent protein kinase gamma 3 (CamKIIgamma3) mediates the cell cycle resumption of metaphase II eggs in mouse

Mature mammalian eggs are ovulated arrested at meiotic metaphase II. Sperm break this arrest by an oscillatory Ca(2+) signal that is necessary and sufficient for the two immediate events of egg activation: cell cycle resumption and cortical granule release. Previous work has suggested that cell cycle resumption, but not cortical granule release, is mediated by calmodulin-dependent protein kinase II (CamKII). Here we find that mouse eggs contain detectable levels of only one CamKII isoform, gamma 3. Antisense morpholino knockdown of CamKIIgamma3 during oocyte maturation produces metaphase II eggs that are insensitive to parthenogenetic activation by Ca(2+) stimulation and insemination. The effect is specific to this morpholino, as a 5-base-mismatch morpholino is without effect, and is rescued by CamKIIgamma3 or constitutively active CamKII cRNAs. Although CamKII-morpholino-treated eggs fail to exit metaphase II arrest, cortical granule exocytosis is not blocked. Therefore, CamKIIgamma3 plays a necessary and sufficient role in transducing the oscillatory Ca(2+) signal into cell cycle resumption, but not into cortical granule release.

Members of the WNT signaling pathways are widely expressed in mouse ovaries, oocytes, and cleavage stage embryos

The mammalian oocyte-to-embryo transition, characterized by a period of transcriptional silence, is dependent on maternal RNAs and proteins produced during the growth phase of the oocyte. Signaling pathways control timely transcription and translation of RNA, as well as post-translational modification of proteins. The WNT/beta-catenin pathway is clearly not active during preimplantation embryo development. However, alternative Wnt signaling pathways may play a role during early embryo development. This study describes the extensive expression, at the transcript and protein level, of receptors, ligands, and intracellular molecules known to play a role in WNT signaling, as well as those known to negatively regulate the canonical WNT/beta-catenin pathway in developing oocytes and preimplantation embryos. This expression of a wide array of molecules involved in WNT signaling suggests that the alternative WNT pathways may be active during oogenesis and the oocyte-to-embryo transition.

Regulation of peripheral spindle movement and spindle rotation during mouse oocyte meiosis: new perspectives

Spindle movement, including spindle migration during first meiosis and spindle rotation during second meiosis, is essential for asymmetric divisions in mouse oocytes. Previous studies by others and us have shown that microfilaments are required for both spindle migration and rotation. In the present study, we aimed to further investigate the mechanism controlling spindle movement during mouse oocyte meiosis. By employing drug treatment and immunofluorescence microscopy, we showed that dynamic microtubule assembly was involved in both spindle migration and rotation. Furthermore, we found that the calcium/CaM/CaMKII pathway was important for regulating spindle rotation.

Myristoylated alanine-rich C kinase substrate, but not Ca2+/calmodulin-dependent protein kinase II, is the mediator in cortical granules exocytosis

Sperm–egg fusion induces cortical granules exocytosis (CGE), a process that ensures the block to polyspermy. CGE can be induced independently by either a rise in intracellular calcium concentration or protein kinase C (PKC) activation. We have previously shown that myristoylated alanine-rich C kinase substrate (MARCKS) cross-links filamentous actin (F-actin) and regulates its reorganization. This activity is reduced either by PKC-induced MARCKS phosphorylation (PKC pathway) or by its direct binding to calmodulin (CaM; CaM pathway), both inducing MARCKS translocation, F-actin reorganization, and CGE. Currently, we examine the involvement of Ca2+/CaM-dependent protein kinase II (CaMKII) and MARCKS in promoting CGE and show that PKC pathway can compensate for lack of Ca2+/CaM pathway. Microinjecting eggs with either overexpressed protein or complementary RNA of constitutively active αCaMKII triggered resumption of second meiotic division, but induced CGE of an insignificant magnitude compared with CGE induced by wt αCaMKII. Microinjecting eggs with mutant-unphosphorylatable MARCKS reduced the intensity of 12-O-tetradecanoylphorbol 13-acetate or ionomycin-induced CGE by 50%, indicating that phosphorylation of MARCKS by novel and/or conventional PKCs (n/cPKCs) is a pivotal event associated with CGE. Moreover, we were able to demonstrate cPKCs involvement in ionomycin-induced MARCKS translocation and CGE. These results led us to propose that MARCKS, rather than CaMKII, as a key mediator of CGE.

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.

The involvement of Src family kinases (SFKs) in the events leading to resumption of meiosis

Ovulated mammalian eggs remain arrested at the second meiotic metaphase (MII) until fertilization. The fertilizing spermatozoon initiates a sequence of biochemical events, collectively referred to as 'egg activation', which overcome this arrest. The initial observable change within the activated egg is a transient rise in intracellular Ca2+ concentration ([Ca2+]i) followed by cortical granule exocytosis (CGE) and resumption of the second meiotic division (RMII). To date, the mechanism by which the fertilizing spermatozoon activates the signaling pathways upstream to the Ca2+ release and the manner by which the signals downstream to Ca2+ release evoke RMII are not well documented. Protein tyrosine kinases (PTKs) were suggested as possible inducers of some aspects of egg activation. Src family kinases (SFKs) constitute a large family of evolutionarily conserved PTKs that mediate crucial biological functions. At present, the theory that one or more SFKs are necessary and sufficient for Ca2+ regulation at fertilization is documented in eggs of marine invertebrates. The mechanism leading to Ca2+ release during fertilization is less established in mammalian eggs. A controversy still exists as to whether SFKs within the mammalian egg are sufficient and/or necessary for Ca2+ release, or whether they play a role during egg activation via other signaling pathways. This article summarizes the possible signaling pathways involved upstream to Ca2+ release but focuses mainly on the involvement of SFKs downstream to Ca2+ release toward RMII, in invertebrate and vertebrate eggs.

Alterations of PLCbeta1 in mouse eggs change calcium oscillatory behavior following fertilization

Inositol 1,4,5-trisphosphate generated by the action of a phospholipase C (PLC) mediates release of intracellular Ca2+ that is essential for sperm-induced activation of mammalian eggs. Much attention currently focuses on the role of sperm-derived PLCzeta in generating changes in egg intracellular Ca2+ despite the fact that PLCzeta constitutes a very small fraction of the total amount of PLC in a fertilized egg. Eggs express several isoforms of PLC, but a role for an egg-derived PLC in sperm-induced Ca2+ oscillations has not been examined. Reducing egg PLCbeta1 by a transgenic RNAi approach resulted in a significant decrease in Ca2+ transient amplitude, but not duration or frequency, following insemination. Furthermore, overexpressing PLCbeta1 by microinjecting a Plcb1 cRNA significantly perturbed the duration and frequency of Ca2+ transients and disrupted the characteristic shape of the first transient. These results provide the first evidence for a role of an egg-derived PLC acting in conjunction with a sperm-derived PLCzeta in egg activation.

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.

CaMKII can participate in but is not sufficient for the establishment of the membrane block to polyspermy in mouse eggs

Fertilization triggers initiation of development and establishment of blocks on the egg coat and plasma membrane to prevent fertilization by multiple sperm (polyspermy). The mechanism(s) by which mammalian eggs establish the membrane block to polyspermy is largely unknown. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) appears to be the key regulator of several egg activation events (completion of meiosis, progression to embryonic interphase, recruitment of maternal mRNAs). Since sperm-induced increases in cytosolic Ca(2+) play a role in establishment of the membrane block to polyspermy in mouse eggs, we hypothesized that CaMKII was a Ca(2+)-dependent effector leading to this change in egg membrane function. To test this hypothesis, we modulated CaMKII activity in two ways: activating eggs parthenogenetically by introducing constitutively active CaMKIIalpha (CA-CaMKII) into unfertilized eggs, and inhibiting endogenous CaMKII in fertilized eggs with myristoylated autocamtide 2-related inhibitory peptide (myrAIP). We find that eggs treated with myrAIP establish a less effective membrane block to polyspermy than do control eggs, but that CA-CaMKII is not sufficient for membrane block establishment, despite the fact that CA-CaMKII-activated eggs undergo other egg activation events. This suggests that: (1) CaMKII activity contributes to the membrane block, but this not faithfully mimicked by CA-CaMKII and furthermore, other pathways, in addition to those activated by Ca(2+) and CaMKII, also participate in membrane block establishment; (2) CA-CaMKII has a range of effects as a parthenogenetic trigger of egg activation (high levels of cell cycle resumption, modest levels of cortical granule exocytosis, and no membrane block establishment).

Extracellular calcium induces activation of Ca(2+)/calmodulin-dependent protein kinase II and mediates spontaneous activation in rat oocytes

Ovulated rat oocytes are activated spontaneously soon after recovery from the oviducts. To investigate the kinetics and mechanism of rat oocyte spontaneous activation (OSA), we investigated the effect of aging in oviducts, hyaluronidase treatment, and extracellular and intracellular calcium, and examined the activity of CaMKII and the effect of its inhibitor on OSA. Oocyte aging in oviducts and hyaluronidase did not affect OSA. However, OSA was significantly decreased in calcium-free medium and in calcium-containing medium containing L-type calcium channel blocker and IP(3)R inhibitor. Moreover, significantly lower OSA was shown with an inhibitor of CaMKII. There was a significant increase of CaMKII activity at 30min after oocyte recovery and constitutively active CaMKII was located near the meiotic spindle in freshly recovered oocytes. Therefore, CaMKII is one of the upstream signals to activate rat oocytes spontaneously after recovery and rat oocytes respond very sensitively to extracellular calcium.

The MEK inhibitor, U0126, alters fertilization-induced [Ca2+]i oscillation parameters and secretion: differential effects associated with in vivo and in vitro meiotic maturation

Although mitogen-activated protein kinase (MAPK) is a well-known cell cycle regulator, emerging studies have also implicated its activity in the regulation of intracellular calcium concentration ([Ca2+](i)) and secretion. Those studies raise the hypothesis that MAPK activity during oocyte maturation and early fertilization is required for normal egg Ca2+ oscillations and cortical granule (CG) secretion. We extend the findings of [Lee, B., Vermassen, E., Yoon, S.-Y., Vanderheyden, V., Ito, J., Alfandari, D., De Smedt, H., Parys, J.B., Fissore, R.A., 2006. Phosphorylation of IP(3)R1 and the regulation of [Ca2+](i) responses at fertilization: a role for the MAP kinase pathway. Development 133, 4355-4365] by demonstrating acute effects on Ca2+ oscillation frequency, amplitude, and duration in fertilized mouse eggs matured in vitro with the MAPK inhibitor, U0126. Frequency was increased, whereas amplitude and duration were greatly decreased. These effects were significantly reduced in eggs matured in vivo and fertilized in the presence of the inhibitor. Ionomycin studies indicated that intracellular Ca2+ stores were differentially affected in eggs matured in vitro with U0126. Consistent with these effects on [Ca2+](i) elevation, fertilization-induced CG exocytosis and metaphase II exit were also reduced in in vitro-matured eggs with U0126, but not in those similarly treated after in vivo maturation. These results indicate that MAPK targets Ca2+ regulatory proteins during both maturation and fertilization, as well as provide a new hypothesis for MAPK function, which is to indirectly regulate events of early development by controlling Ca2+ oscillation parameters.

Comparison of Ca2+ and CaMKII responses in IVF and ICSI in the mouse

Novel methods of egg activation in human assisted reproductive technologies and animal somatic cell nuclear transfer are likely to alter the signalling process that occurs during normal fertilization. Intracytoplasmic sperm injection (ICSI) bypasses the normal processes of the acrosome reaction, sperm-egg fusion, and processing of the sperm plasma membrane, as well as alters some parameters of intracellular calcium ([Ca(2+)](i)) dynamics (reported previously by Kurokawa and Fissore (2003)). Herein, we extend these studies to determine if ICSI alters the activity of the Ca(2+)-dependent protein, Ca(2+)/calmodulin-dependent kinase II (CaMKII), which is responsible for the completion of meiosis in vertebrate eggs. After ICSI or in vitro fertilization (IVF), individual mouse eggs were monitored for their relative changes in both [Ca(2+)](i) and CaMKII activity during the first [Ca(2+)](i) rise and a subsequent rise associated with second polar body extrusion. The duration of the first [Ca(2+)](i) rise was greater in ICSI than in IVF, but the amplitude of the rise was transiently higher for IVF than ICSI. However, a similar mean CaMKII activity was observed in both procedures. During polar body extrusion, the amplitude and duration of the Ca(2+) rises were increased by a small amount in ICSI compared with IVF, whereas the CaMKII activities were similar. Thus, compared with IVF, ICSI is not associated with decreased or delayed CaMKII activity in response to these Ca(2+) signals in the mouse.

How eggs arrest at metaphase II: MPF stabilisation plus APC/C inhibition equals Cytostatic Factor

Oocytes from higher chordates, including man and nearly all mammals, arrest at metaphase of the second meiotic division before fertilization. This arrest is due to an activity that has been termed 'Cytostatic Factor'. Cytostatic Factor maintains arrest through preventing loss in Maturation-Promoting Factor (MPF; CDK1/cyclin B). Physiologically, Cytostatic Factor – induced metaphase arrest is only broken by a Ca²⁺ rise initiated by the fertilizing sperm and results in degradation of cyclin B, the regulatory subunit of MPF through the Anaphase-Promoting Complex/Cyclosome (APC/C). Arrest at metaphase II may therefore be viewed as being maintained by inhibition of the APC/C, and Cytostatic Factor as being one or more pathways, one of which inhibits the APC/C, consorting in the preservation of MPF activity.

Many studies over several years have implicated the c-Mos/MEK/MAPK pathway in the metaphase arrest of the two most widely studied vertebrates, frog and mouse. Murine downstream components of this cascade are not known but in frog involve members of the spindle assembly checkpoint, which act to inhibit the APC/C. Interesting these downstream components appear not to be involved in the arrest of mouse eggs, suggesting a lack of conservation with respect to c-Mos targets. However, the recent discovery of Emi2 as an egg specific APC/C inhibitor whose degradation is Ca²⁺ dependent has greatly increased our understanding of MetII arrest. Emi2 is involved in both the establishment and maintenance of metaphase II arrest in frog and mouse suggesting a conservation of metaphase II arrest. Its identity as the physiologically relevant APC/C inhibitor involved in Cytostatic Factor arrest prompted us to re-evaluate the role of the c-Mos pathway in metaphase II arrest.

This review presents a model of Cytostatic Factor arrest, which is primarily induced by Emi2 mediated APC/C inhibition but which also requires the c-Mos pathway to set MPF levels within physiological limits, not too high to induce an arrest that cannot be broken, or too low to induce parthenogenesis.

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

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

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.

Epigenetic discrimination by mouse metaphase II oocytes mediates asymmetric chromatin remodeling independently of meiotic exit

In mammalian fertilization, paternal chromatin is exhaustively remodeled, yet the maternal contribution to this process is unknown. To address this, we prevented the induction of meiotic exit by spermatozoa and examined sperm chromatin remodeling in metaphase II (mII) oocytes. Methylation of paternal H3-K4 and H3-K9 remained low, unlike maternal H3, although paternal H3-K4 methylation increased in zygotes. Thus, mII cytoplasm can sustain epigenetic asymmetry in a cell-cycle dependent manner. Paternal genomic DNA underwent oocyte-mediated cytosine demethylation and acquired maternally-derived K12-acetylated H4 (AcH4-K12) independently of microtubule assembly and maternal chromatin. AcH4-K12 persisted without typical maturation-associated deacetylation, irrespective of paternal pan-genomic cytosine methylation. Contrastingly, somatic cell nuclei underwent rapid H4 deacetylation; sperm and somatic chromatin exhibited asymmetric AcH4-K12 dynamics simultaneously within the same mII oocyte. Inhibition of somatic histone deacetylation revealed endogenous histone acetyl transferase activity. Oocytes thus specify the histone acetylation status of given nuclei by differentially targeting histone deacetylase and acetyl transferase activities. Asymmetric H4 acetylation during and immediately after fertilization was dispensable for development when both parental chromatin sets were hyperacetylated. These studies delineate non-zygotic chromatin remodeling and suggest a powerful model with which to study de novo genomic reprogramming.