A novel signalling mechanism for generating Ca2+ oscillations at fertilization in mammals

Fertilization and Cleavage Axes Differ In Primates Conceived By Conventional (IVF) Versus Intracytoplasmic Sperm Injection (ICSI)

With nearly ten million babies conceived globally, using assisted reproductive technologies, fundamental questions remain; e.g., How do the sperm and egg DNA unite? Does ICSI have consequences that IVF does not? Here, pronuclear and mitotic events in nonhuman primate zygotes leading to the establishment of polarity are investigated by multidimensional time-lapse video microscopy and immunocytochemistry. Multiplane videos after ICSI show atypical sperm head displacement beneath the oocyte cortex and eccentric para-tangential pronuclear alignment compared to IVF zygotes. Neither fertilization procedure generates incorporation cones. At first interphase, apposed pronuclei align obliquely to the animal-vegetal axis after ICSI, with asymmetric furrows assembling from the male pronucleus. Furrows form within 30° of the animal pole, but typically, not through the ICSI injection site. Membrane flow drives polar bodies and the ICSI site into the furrow. Mitotic spindle imaging suggests para-tangential pronuclear orientation, which initiates random spindle axes and minimal spindle:cortex interactions. Parthenogenetic pronuclei drift centripetally and assemble astral spindles lacking cortical interactions, leading to random furrows through the animal pole. Conversely, androgenotes display cortex-only pronuclear interactions mimicking ICSI. First cleavage axis determination in primates involves dynamic cortex-microtubule interactions among male pronuclei, centrosomal microtubules, and the animal pole, but not the ICSI site.

Phosphoinositide signaling in sperm development

Phosphatidylinositol phosphates (PIPs)¹ are membrane lipids with crucial roles during cell morphogenesis, including the establishment of cytoskeletal organization, membrane trafficking, cell polarity, cell-cycle control and signaling. Recent studies in mice (Mus musculus), fruit flies (Drosophila melanogaster) and other organisms have defined germ cell intrinsic requirements for these lipids and their regulatory enzymes in multiple aspects of sperm development. In particular, PIP levels are crucial in germline stem cell maintenance, spermatogonial proliferation and survival, spermatocyte cytokinesis, spermatid polarization, sperm tail formation, nuclear shaping, and production of mature, motile sperm. Here, we briefly review the stages of spermatogenesis and discuss the roles of PIPs and their regulatory enzymes in male germ cell development.

Effect of medium additives during liquid storage on developmental competence of in vitro matured bovine oocytes

Our aim was to improve the developmental competence of bovine oocytes during their liquid storage by using additives. In vitro matured oocytes were stored for 20 h at 25°C in HEPES buffered TCM 199 medium (base medium). After storage, in vitro embryo development after in vitro fertilization was compared to those of non-stored (control) ones. Addition of 10% (v/v) newborn calf serum or 10.27 mmol/L pyruvate alone to the base medium did not improve blastocyst formation rates in stored oocytes; however, their simultaneous addition significantly improved the rate compared with those stored in base medium (P < 0.05). Supplementation of the holding medium with dithiothreitol (DTT) at any concentrations did not improve embryo development from stored oocytes. Although supplementation with cyclosporine A (CsA) significantly reduced apoptosis and membrane damage rates during storage, it did not improve the developmental competence of oocytes. 1,2-bis(2-aminophenoxy) ethane N,N,N',N'-tetraacetic acid tetrakis-acetoxymethyl ester and ruthenium red had no effect on oocyte apoptotic rates. Blastocyst formation rates in all stored groups remained significantly lower than that of the control. In conclusion, pyruvate and serum had a synergic effect to moderate the reduction of oocyte quality during storage, whereas mitochondrial membrane pore inhibitor CsA and the antioxidant DTT did not affect their developmental competence.

Vertebrate Reproduction

Vertebrate reproduction requires a myriad of precisely orchestrated events-in particular, the maternal production of oocytes, the paternal production of sperm, successful fertilization, and initiation of early embryonic cell divisions. These processes are governed by a host of signaling pathways. Protein kinase and phosphatase signaling pathways involving Mos, CDK1, RSK, and PP2A regulate meiosis during maturation of the oocyte. Steroid signals-specifically testosterone-regulate spermatogenesis, as does signaling by G-protein-coupled hormone receptors. Finally, calcium signaling is essential for both sperm motility and fertilization. Altogether, this signaling symphony ensures the production of viable offspring, offering a chance of genetic immortality.

Effects of repetitive ionomycin treatment on in vitro development of bovine somatic cell nuclear transfer embryos

To artificially activate embryos in somatic cell nuclear transfer (SCNT), chemical treatment with ionomycin has been used to induce transient levels of Ca(2+) and initiate reprogramming of embryos. Ca(2+) oscillation occurs naturally several times after fertilization (several times with 15- to 30-min intervals). This indicates how essential additional Ca(2+) influx is for successful reprogramming of embryos. Hence, in this report, the experimental design was aimed at improving the developmental efficiency of cloned embryos by repetitive Ca(2+) transients rather than the commonly used ionomycin treatment (4 min). To determine optimal Ca(2+) inflow conditions, we performed three different repetitive ionomycin (10 µM) treatments in reconstructed embryos: Group 1 (4-min ionomycin treatment, once), Group 2 (30-sec treatment, 4 times, 15-min intervals) and Group 3 (1-min treatment, 4 times, 15-min intervals). Pronuclear formation rates were checked to assess the effects of repetitive ionomycin treatment on reprogramming of cloned embryos. Cleavage rates were investigated on day 2, and the formation rates of blastocysts (BLs) were examined on day 7 to demonstrate the positive effect of repeated ionomycin treatment. In Group 3, a significant increase in BL formation was observed [47/200 (23.50%), 44/197 (22.33%) and 69/195 (35.38%) in Groups 1, 2 and 3, respectively]. Culturing embryos with different ionomycin treatments caused no significant difference among the groups in terms of the total cell number of BLs (164.3, 158.5 and 145.1, respectively). Additionally, expression of the anti-apoptotic Bcl-2 gene and MnSOD increased significantly in Group 3, whereas the expression of the pro-apoptotic Bax decreased statistically. In conclusion, the present study demonstrated that repeated ionomycin treatment is an improved activation method that can increase the developmental competence of SCNT embryos by decreasing the incidence of apoptosis.

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.

Activation of amphibian oocytes by sperm extracts

In the fertilization of most animals, egg activation is accompanied by an increase in cytoplasmatic Ca2+; however, the mechanism through which the fertilizing sperm induce this phenomenon is still controversial. An increase in intracellular free Ca2+is required to trigger egg activation events, a process that includes cortical granule exocytosis, resumption and completion of meiosis and DNA replication, and culminates in the first mitotic cleavage. In this work, we investigated the effect of microinjection and incubation of different fractions of homologous sperm extract on the activation ofBufo arenarumoocytes maturedin vitro. Two heat treatment-sensitive fractions obtained by chromatography were able to induce oocyte activation. The sperm fraction, which contained a 24 kDa protein, induced 90% activation when it was microinjected into the oocytes. Whilst the sperm fraction, which contained a 36 kDa protein, was able to induce about 70% activation only when it was applied on the oocyte surface.

Failed fertilization after ICSI and spermiogenic defects

OBJECTIVE

To evaluate the relationship between late spermiogenic events, including histone-protamine replacement, acrosome integrity, and sperm morphology, with fertilization rate after intracytoplasmic sperm injection (ICSI).

DESIGN

Prospective study.

SETTING

Isfahan Fertility and Infertility Center, Royan Institute, Tehran, Iran.

PATIENT(S)

Semen samples from 68 infertile couples undergoing ICSI at Isfahan Fertility and Infertility center were assessed during this study.

INTERVENTION(S)

Semen analysis was carried out according to World Health Organization criteria. Protamine deficiency, acrosin activity, sperm morphology, and acrosome size were assessed by chromomycin A3 (CMA3) staining, gelatinolysis test, and Papanicolaou staining (strict criteria), respectively.

MAIN OUTCOME MEASURE(S)

The correlation between protamine deficiency, sperm morphology, acrosin activity, and acrosome size with each other and fertilization rate were assessed.

RESULT(S)

Percentage CMA3 positivity and mean halo diameter show a significant correlation with fertilization rate. However, no correlation was found between sperm normal morphology and fertilization rate. The mean values of acrosome size and fertilization rate were significantly different when patients were grouped for CMA3 positivity of 40%. Multiple linear regression analysis revealed that only protamine deficiency has direct effect on fertilization rate.

CONCLUSION(S)

Protamine deficiency appears to have a more significant effect on fertilization after ICSI than acrosin activity and semen parameters.

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.

Development and calcium level changes in pre-implantation porcine nuclear transfer embryos activated with 6-DMAP after fusion

This study investigated the effect of treatment with 6-dimethylaminopurine (6-DMAP) following fusion on in vitro development of porcine nuclear transfer (NT) embryos. Frozen thawed ear skin cells were transferred into the perivitelline space of enucleated oocytes. Reconstructed oocytes were fused and activated with electric pulse in 0.3 M mannitol supplemented with either 0.1 or 1.0 mM CaCl(2). In each calcium concentration, activated oocytes were divided into three groups. Two groups of them were exposed to either ionomycin (I + 6-DMAP or 6-DMAP alone. In experiment 2, fused NT embryos in 0.3 M mannitol containing 1.0 mM CaCl(2) were exposed to 6-DMAP either immediately or 20 min after fusion/activation. For 0.1 mM CaCl(2), oocytes activated with either I + 6-DMAP or 6-DMAP alone showed a higher (P < 0.05) developmental rate to the blastocyst stage than those activated with an electric pulse alone (26.7 and 22.5 vs. 12.5%). For 1.0 mM CaCl(2), oocytes activated with either I + 6-DMAP or 6-DMAP alone showed significantly higher (P < 0.05) developmental rate to the blastocyst stage (35.6 and 28.3 vs. 19.8%). Developmental rate to the blastocyst stage was (P < 0.05) increased in NT embryos activated with 6-DMAP 20 min after fusion. 6-DMAP made a higher and wider Ca(2+) transient compared to that induced by electric pulses (Fig. 3). The fluctuation lasted during the time that oocytes were cultured in 6-DMAP. Regardless of Ca(2+) concentration in fusion medium, activation with 6-DMAP following electric pulses supported more development of porcine NT embryos. Activation of NT embryos with 6-DMAP after fusion in the presence of 1.0 mM CaCl(2) could support better developmental rate to the blastocyst stage.

Improved fertilization and embryo development resulting in birth of live piglets after intracytoplasmic sperm injection and in vitro culture in a cysteine-supplemented medium

The effects of cysteine treatment on fertilization rate, intracellular concentration of glutathione, and embryo development in vitro and after embryo transfer were examined following intracytoplasmic sperm injection (ICSI) of in vitro-matured porcine oocytes using a piezo drive unit. Culture of presumed zygotes after ICSI with 1.71-3.71 mM cysteine for 3-12h improved (P<0.05) fertilization rates as compared to treatment with 0.57 mM cysteine or to controls (0mM) (56 to 68%, 48%, 35%, respectively). Extension of treatment time with cysteine beyond 3h did not further increase fertilization rates, suggesting that cysteine promoted early developmental events after ICSI (e.g. decondensation of sperm chromatin). There was no effect of cysteine supplementation on oocyte glutathione levels after ICSI. Pretreatment of spermatozoa for 3h with 1.71 mM cysteine did not improve fertilization rates. The incidence of blastocysts formation when cultured in 1.71 mM cysteine for 3h after ICSI was 31%, which was higher (P<0.05) than controls (18%). Transfer of 20-38 embryos cultured with 1.71 mM cysteine for 3h after ICSI to each of seven recipients yielded three deliveries with an average litter size of 4.0. We concluded that cysteine supplementation for the first 3h after ICSI improved fertilization and embryo development rates, with no influence on glutathione levels in oocytes, and that the cysteine-treated ICSI embryos developed to full term. The study also showed that porcine oocytes matured in a chemically defined medium had the ability for full-term development after piezo-ICSI without additional treatments for oocyte activation.

Secretory mechanisms and Ca2+ signaling in gametes: similarities to regulated neuroendocrine secretion in somatic cells and involvement in emerging pathologies

Recent studies demonstrate that regulated secretion in probably all mammalian cells, from gonadotropes to gametes, utilizes similar signaling systems, intracellular Ca(2+) regulation, Ca(2+)-dependent proteins, cytoskeletal participation, and SNARE-mediated fusion. Thus, highly specialized cells, like sperm and eggs, should no longer be considered to have evolved a cell-type specific secretory mechanism. In gametes, Ca(2+)-dependent proteins and enzymes transduce elevations of intracellular Ca(2+) into secretory events, i.e., exocytosis of the acrosome in sperm and cortical granules in the egg. Just as secretory deficiencies have clinical consequences in endocrine and exocrine cells, failure of secretion of cortical granules or the acrosome can result in failure of normal fertilization or fertilization followed by abnormal development. With the advent of human in vitro fertilization, such gamete pathologies have been recently identified and have led to new clinical procedures to achieve normal fertilization and pregnancies. A better understanding of the common Ca(2+)-dependent secretory pathways in both gametes and somatic cells should be beneficial to investigating mis-regulation in either cell type.

Calmodulin-dependent protein kinase II triggers mouse egg activation and embryo development in the absence of Ca2+ oscillations

Fertilization in mammalian eggs is accompanied by oscillatory changes in intracellular Ca(2+) concentration, which are critical for initiating and completing egg activation events and the developmental program. Ca(2+)/Camodulin-dependent protein kinase II (CaMKII) is a multifunctional enzyme that is postulated to be the downstream transducer of the Ca(2+) signal in many cell types. We tested the hypothesis that CaMKII is the major integrator of Ca(2+)-induced egg activation events and embryo development by microinjecting a cRNA that encodes a constitutively active (Ca(2+)-independent) mutant form of CaMKII (CA-CaMKII) into mouse eggs. Expression of this cRNA, which does not increase intracellular Ca(2+), induced a sustained rise in CaMKII activity and triggered egg activation events, including cell cycle resumption, and degradation and recruitment of maternal mRNAs; cortical granule exocytosis, however, did not occur normally. Furthermore, when mouse eggs were injected with sperm devoid of Ca(2+)-releasing activity and activated with either CA-CaMKII cRNA or by SrCl(2), similar rates and incidence of development to the blastocyst stage were observed. These results strongly suggest that CaMKII is a major integrator of the Ca(2+) changes that occur following fertilization.

Developmental potential of aged oocyte rescued by nuclear transfer following parthenogenetic activation and in vitro fertilization

Mouse oocyte aged in vitro cannot develop normally following activation. To investigate the roles of nucleus or cytoplasm elements in oocyte aged in vitro process and their subsequent development capability following activation, we reconstructed oocytes with MII chromosome spindle and cytoplasm from aged and fresh oocytes by nuclear transfer. The subsequent developmental potential after parthenogenetic activation (PA) or in vitro fertilization (IVF) was evaluated. After nuclear transfer, more than 75.6% of karyoplast and cytoplast pairs can be fused and reconstructed oocytes have a normal haploid karyotype. Following PA, aged oocytes cannot develop beyond four-cell stage, reconstructed oocytes from fresh nucleus and aged cytoplasm developed to blastocyst with a low percentage (9.1%). Instead, blastocyst formation rate of reconstructed oocyte from aged nucleus and fresh cytoplasm was higher (60.0%). Following IVF, zygote with diploid karyotype can be formed from zona pellucida (ZP)-free oocyte. After cultured in vitro, aged oocytes cannot develop beyond two-cell; reconstructed oocytes from fresh nucleus and aged cytoplasm developed to blastocyst with low percentage (15.0%). However, high blastocyst formation rate (86.2%) can be obtained from reconstructed oocytes from aged nucleus and fresh cytoplasm. Furthermore, after embryo transfer, three viable pups have been obtained, although the efficiency is very low. These observation demonstrated that cytoplasm is more crucial than nucleus to aging process. Fresh cytoplasm could partly rescue nucleus susceptibility to apoptosis from aging in vitro.

Strontium promotes calcium oscillations in mouse meiotic oocytes and early embryos through InsP3 receptors, and requires activation of phospholipase and the synergistic action of InsP3

BACKGROUND

Sr2+ is the most efficient agent for mouse oocyte activation and functions by inducing Ca2+ oscillations. However, its specific mechanism of action remains unknown. Here we investigated the specificity and possible mechanism of Sr2+-induced Ca2+ oscillations in mouse oocytes and early embryos.

METHODS

Ca2+ oscillations in oocytes and embryos were measured by ratiometric fluorescence imaging using fura-2AM. The role of phospholipase C (PLC) and inositol trisphosphate (InsP3) receptors in Sr2+-induced Ca2+ oscillations was examined by selective inhibitors.

RESULTS

Sr2+ can induce Ca2+ oscillations in both immature and mature oocytes, and in early embryos. A cell cycle stage-dependent phenomenon to Sr2+ stimulation was observed in 1-cell embryos. By using a low molecular weight heparin to antagonize the function of InsP3 receptors, we were able to show that InsP3 receptors are essential for Sr2+-induced Ca2+ oscillations. Treating metaphase II (MII) oocytes with the PLC inhibitor, U73122, abolished Sr2+-induced increases in Ca2+. This inhibitory effect of U73122 could be rescued by microinjection of InsP3, indicating that Sr2+-induced Ca2+ oscillations require the synergistic action of InsP3.

CONCLUSIONS

Sr2+-induced calcium oscillations in mouse oocytes and early embryos are mediated through InsP3 receptors, and require PLC activation and the synergistic action of InsP3.

Noninvasive imaging of spindle dynamics during mammalian oocyte activation

OBJECTIVE

To develop a method to evaluate spindle dynamics in living oocytes and in karyoplasts during the initial stages of activation and after pharmacological disruption of cytoskeleton.

DESIGN

Morphological study using a novel microscope.

SETTING

Translational research laboratory at marine biological laboratory.

ANIMAL(S)

Six-week-old CD-1 or B6C3F1 mice superovulated with pregnant mare's serum gonadotropin and human chorionic gonadotropin (hCG).

INTERVENTION(S)

Spindles of living oocytes and karyoplasts were imaged at 5-10 minute intervals using the Pol-Scope during the initial stages of oocyte activation and after pharmacological disruption of cytoskeleton.

MAIN OUTCOME MEASURE(S)

Assessment of spindle dynamics using Pol-Scope imaging.

RESULT(S)

During oocyte activation, spindle mid-region birefringence increased, followed by spindle rotation and second polar body extrusion in both intact oocytes and karyoplasts. Activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate failed to induce spindle activation in 60% of living oocytes and caused spindle disruption in some oocytes. Inhibition of PKC by a myristoylated PKC pseudosubstrate inhibited metaphase II release in most oocytes evaluated (86.7%). Cytochalasin D inhibited only spindle rotation and separation. Nocodazole disrupted spindles in less than 5 minutes after administration.

CONCLUSION(S)

Pol-Scope imaging allows investigation at near real time of spindle dynamics during activation of living oocytes. Spindles also showed evidence of activation even in karyoplasts. The procedure may be useful for detecting functional spindle aberrations in living oocytes. Further studies are needed to determine whether spindle dynamics predict clinical outcome.

Mammalian freeze-dried sperm can maintain their calcium oscillation-inducing ability when microinjected into mouse eggs

Mammalian freeze-dried sperm can maintain their genetic integrity and event support full development to term when microinjected into mature oocytes. However, it is unknown whether freeze-dried sperm can still maintain their calcium oscillation-inducing capability. Here, we microinjected mouse and bovine freeze-dried sperm into mouse MII oocytes and examined their calcium oscillation-inducing ability following intracytoplasmic sperm injection (ICSI). Two pieces of information are revealed. First, nearly all oocytes injected with a freeze-dried mouse sperm head or a bovine sperm showed fertilization-like calcium oscillations, indicating that freeze-drying treatment does not affect the activity of the sperm factor responsible for calcium oscillations. Second, freeze-dried sperm exhibited high resistance to external temperature increase. This is shown by the finding that the freeze-dried sperm can maintain their calcium oscillation-inducing capacity even following exposure to 100 degrees C for 3 h. We therefore conclude that mammalian sperm can maintain their calcium oscillation-inducing capability following freeze-drying, rehydration, and ICSI treatments.

Transgenic RNA interference reveals role for mouse sperm phospholipase Czeta in triggering Ca2+ oscillations during fertilization

A sperm-specific phospholipase (PL) C, termed PLCzeta, is proposed to be the soluble sperm factor that induces Ca(2+) oscillations in mammalian eggs and, thus, initiates egg activation in vivo. We report that sperm from transgenic mice expressing short hairpin RNAs targeting PLCzeta mRNA have reduced amounts of PLCzeta protein. Sperm derived from these transgenic mice trigger patterns of Ca(2+) oscillations following fertilization in vitro that terminate prematurely. Consistent with the perturbation in patterns of Ca(2+) oscillations is the finding that mating of transgenic founder males to females results in lower rates of egg activation and no transgenic offspring. These data strongly suggest that PLCzeta is the physiological trigger of Ca(2+) oscillations required for activation of development.

Molecular dissection of egg fertilization signaling with the aid of tyrosine kinase-specific inhibitor and activator strategies

Fertilization is triggered by sperm-egg interaction and fusion that initiate a transient rise(s) in the free intracellular calcium ([Ca(2+)](i)) that is responsible for a series of biochemical and cell biological events, so-called "egg activation". Calcium-dependent egg activation leads to the initiation of developmental program that culminates in the birth of individuals. A growing body of knowledge has uncovered the molecular mechanisms underlying sperm-induced transient [Ca(2+)](i) increase(s) to some extent; namely, in most animals so far studied, a second messenger inositol 1,4,5-trisphosphate (IP(3)) seems to play a pivotal role in inducing [Ca(2+)](i) transient(s) at fertilization. However, signaling mechanisms used by sperm to initiate IP(3)-[Ca(2+)](i) transient pathway have not been elucidated. To approach this problem, we have employed African clawed frog, Xenopus laevis, as a model animal and conducted experiments designed specifically to determine the role of the Src family protein-tyrosine kinases (SFKs or Src family PTKs) in the sperm-induced egg activation. This review compiles information about the use of PTK-specific inhibitors and activators for analyzing signal transduction events in egg fertilization. Specifically, we focus on molecular identification of Xenopus Src and the signaling mechanism of the Src-dependent egg activation that has been established recently. We also summarize recent advances in understanding the role of the Src family kinases in egg fertilization of other model organisms, and discuss future directions of the field.

The thiol reagent, thimerosal, irreversibly inhibits meiosis reinitiation in mouse oocyte when applied during a very early and narrow temporal window: a pharmacological analysis

The effect of the sulfhydryl reagent, thimerosal (TMS) on meiosis resumption in germinal vesicle (GV)-stage denuded mouse oocytes was studied. It irreversibly inhibits both GV breakdown (GVBD) and the first polar body (pb1) extrusion in concentration- and time-dependent manners, the most striking result being the very early and narrow temporal window during which denuded primary oocytes released from their follicle are susceptible to a pulse of the drug. This inhibition is bypassed by dithiothreitol (DTT) with an efficiency declining with time, while thiosalicylic acid (TA), an analog of TMS devoid of the mercury atom, has no effect on meiosis reinitiation. These results strongly suggest that the inhibitory effect of TMS is a consequence of its sulfhydryl group oxidising activity. The molecular target(s) of this inhibitory oxidation should however be identified. In contrast to DTT, okadaic acid (OA), known to bypass the inhibitory effect of drugs interfering with protein kinase activities, only induces chromatin condensation and GVBD in TMS-pulsed oocytes with a delay of about 8 hr as compared to the control situation. This confirms that a very early thiol oxidation induced by TMS exerts a much more dramatic effect on resumption on meiosis than any pharmacological manipulation of protein kinase activities leading to activation of MPF.

Calcium signalling during embryonic development

Consider a hypothetical design specification for an integrated communication-control system within an embryo. It would require short-range (subcellular) and long-range (pan-embryonic) abilities, it would have to be flexible and, at the same time, robust enough to operate in a dynamically changing environment without information being lost or misinterpreted. Although many signalling elements appear, disappear and sometimes reappear during development, it is becoming clear that embryos also depend on a ubiquitous, persistent and highly versatile signalling system that is based around a single messenger, Ca2+.

Teleost fish spermatozoa contain a cytosolic protein factor that induces calcium release in sea urchin egg homogenates and triggers calcium oscillations when injected into mouse oocytes

Established studies in a variety of organisms including amphibians, fish, ascidians, nemerteans, echinoderms, mammals, and even a species of flowering plant, clearly demonstrate that an increase in intracellular egg calcium is crucial to the process of egg activation at fertilization. In echinoderms, egg activation appears to involve an egg phospholipase C gamma (PLCgamma). However, numerous studies in mammalian species suggest that calcium is released from internal egg stores at fertilization by a sperm-derived cytosolic protein factor. Recent studies in the mouse have identified this sperm-derived factor as being a novel sperm-specific PLC isoform with distinctive properties (PLCzeta). Homologues of PLCzeta have since been isolated from human and cynomolgus monkey sperm. In addition, sperm factor activity has been detected in non-mammalian species such as chicken, Xenopus, and a flowering plant. Here we report evidence for the existence of a similar sperm-derived factor in a commercially important species of teleost fish, the Nile tilapia Oreochromis niloticus (L). Using an established bioassay for calcium release, the sea urchin egg homogenate, we demonstrate that protein extracts obtained from tilapia spermatozoa exhibit PLC activity similar to that seen in mammalian sperm extracts, and also induce calcium release when added directly to the homogenate. Further, tilapia sperm extracts induced calcium oscillations when injected into mouse oocytes.

Fate of postacrosomal perinuclear theca recognized by monoclonal antibody MN13 after sperm head microinjection and its role in oocyte activation in mice

Monoclonal antibody (mAb) MN13 labels mouse sperm head postacrosomal perinuclear theca (PT), which is possibly involved in oocyte activation during fertilization. The antigenic site is expressed after mild sonication followed by treatment with dithiothreitol (DTT) or heat (45 degrees C), and is visible as a thick band in the postacrosomal region. The presence of protease inhibitors in the sonication medium suppresses the exposure of MN13 epitope (MN13p), suggesting the involvement of a proteolytic reaction in this process. Spermatozoa do not express MN13p after the induction of acrosome exocytosis by Ca(2+) ionophore, zona binding, or during zona penetration, a strategy that ensures safe delivery of postacrosomal PT proteins to oocytes after fusion. MN13 labeling was not detectable during fertilization by zona-free in vitro fertilization, suggesting that the antigenic site does not react with proteolytic enzymes during sperm-oocyte fusion and the antibody does not recognize the nascent epitope. Microinjection of sperm heads prepared by sonication and DTT treatment led to the activation of metaphase II oocytes. The oocyte activating function of such sperm heads was significantly diminished after labeling with MN13 prior to intracytoplasmic sperm injection (ICSI), but labeling with antiequatorin antibody MN9 activated oocytes with a frequency similar to that of unlabeled sperm heads. The sperm heads in inactive oocytes formed premature chromosome condensations (PCCs), which were invested by independent metaphase-like spindles. These observations indicate that the postacrosomal PT recognized by mAb MN13 is involved in oocyte activation. MN13p is dissociated from sperm heads during the early stages of decondensation after ICSI. In activated oocytes, MN13-labeled fine granules were redistributed in the midzone spindle region, whereas in inactive oocytes they formed a ring around the polar regions of the metaphase II and PCC spindles.

Fate of sperm components during assisted reproduction: implications for infertility

Studies in non-human primates highlight their suitability as preclinical models for investigating assisted reproduction techniques. The cytoskeletal events of fertilization in non-human primates are similar to those in humans in that they require a paternally derived centrosome. The centrosome, introduced by the sperm at fertilization, organizes a microtubule array that is responsible for bringing the parental genomes together at first mitosis. Incomplete functioning of the sperm centrosome during fertilization has been identified as a novel form of infertility that would not necessarily benefit from intracytoplasmic sperm injection (ICSI). The global use of ICSI to overcome male infertility has been very successful, although concerns remain regarding the long-term effects on children born after ICSI. The cytoskeletal events that occur during ICSI are quite different from the events of in vitro fertilization: a sperm selected for ICSI does not undergo typical oocyte interactions, and abnormal remodelling of the male pronucleus may result. The implications of these findings are discussed in relation to the safety of the ICSI technique.

Intracellular calcium oscillations signal apoptosis rather than activation in in vitro aged mouse eggs

We have previously demonstrated that initiation of intracellular calcium ([Ca2+]i) oscillations in mouse eggs signals activation or apoptotic death depending on the age of the eggs in which the oscillations are induced. To extend these studies, mouse eggs were aged in vitro to 24, 32, and 40 h post-hCG and injected with sperm cytosolic factor (SF), adenophostin A, or sperm (intracytoplasmic sperm injection), and the times at which signs of apoptosis first appeared were examined. These treatments, which induced [Ca2+]i oscillations, caused fragmentation and other signs of programmed cell death in eggs as early as 32 h post-hCG. The susceptibility of aged eggs to apoptosis appeared to be due to cytoplasmic deficiencies, because fusion of recently ovulated eggs with aged, SF-injected eggs prevented fragmentation. Evaluation of mRNA and protein levels of the apoptotic regulatory proteins Bcl-2 and Bax showed a prominent decrease in the amounts of Bcl-2 mRNA and protein in aged eggs, whereas Bax mRNA levels did not appear to be changed. Lastly, the Ca2+ responses induced by the aforementioned Ca2+ agonists ceased in advance in aged eggs. Together, these results suggest that one or several critical cytosolic molecules involved in the regulation of Ca2+ homeostasis, and in maintaining the equilibrium between anti- and proapoptotic proteins, is either lost or inactivated during postovulatory egg aging, rendering the fertilizing Ca2+ signal into an apoptosis-inducing signal.

Egg activation at fertilization: where it all begins

A centrally important factor in initiating egg activation at fertilization is a rise in free Ca(2+) in the egg cytosol. In echinoderm, ascidian, and vertebrate eggs, the Ca(2+) rise occurs as a result of inositol trisphosphate-mediated release of Ca(2+) from the endoplasmic reticulum. The release of Ca(2+) at fertilization in echinoderm and ascidian eggs requires SH2 domain-mediated activation of a Src family kinase (SFK) and phospholipase C (PLC)gamma. Though some evidence indicates that a SFK and PLC may also function at fertilization in vertebrate eggs, SH2 domain-mediated activation of PLC gamma appears not to be required. Much work has focused on identifying factors from sperm that initiate egg activation at fertilization, either as a result of sperm-egg contact or sperm-egg fusion. Current evidence from studies of ascidian and mammalian fertilization favors a fusion-mediated mechanism; this is supported by experiments indicating that injection of sperm extracts into eggs causes Ca(2+) release by the same pathway as fertilization.

In vitro spontaneous parthenogenetic activation of golden hamster oocytes

Parthenogenetic activation is a major hurdle to be cleared for the examination of the human sperm chromosome after intracytoplasmic injection (ICSI) into golden hamster oocytes. Various factors that affect spontaneous activation of hamster oocytes were, therefore, investigated in this study. We collected cumulus-oocyte complexes (COC) from the oviducts of superovulated females and washed them thoroughly with Ca2+-containing or Ca2+-free TALP-HEPES medium (handling media). We cultured oocytes with intact cumulus or those without cumulus (removed by previous hyaluronidase treatment) in Ca2+-containing or -free m-TALP-3 for 6 or 12 h before examining for their activation. Among the oocytes recovered 17 h post-hCG, 92-94% were parthenogenetically activated by 6 h of in vitro culture. Activation rate in the oocytes collected at 13.5 h post-hCG (53%) was significantly (P < 0.05) lower than that in the oocytes collected 17 h post-hCG (92%), indicating that the spontaneous activation rate increased as the oocytes became older. Both cumulus-intact and cumulus-free oocytes had similar (P > 0.05) activation rates when cultured in vitro, suggesting that hyaluronidase treatment had no effect on the rate of oocyte activation. Omission of Ca2+ from the handling medium also had no effect on the activation of the oocytes. The rate of spontaneous activation of the oocytes cultured in calcium-free medium for 6 (9%) and 12 h (16%) was significantly (P < 0.01) lower than that (94%) of the control oocytes cultured in Ca2+-containing medium, implying a positive influence of Ca2+ on in vitro activation of hamster oocytes. When we cultured the oocytes first in calcium-free medium for 6 h, and then in calcium-containing medium for 6 h, 94% were activated, which is comparable to the rate for oocytes continuously cultured in Ca2+-containing medium. This indicates that the inhibition of hamster oocyte activation in Ca2+-free medium is reversible and can be used to control spontaneous activation of golden hamster oocytes.

Mitochondrial modulation of calcium signaling at the initiation of development

Fertilization triggers cytosolic Ca(2+) oscillations that activate mammalian eggs and initiate development. Extensive evidence demonstrates that Ca(2+) is released from endoplasmic reticulum stores; however, less is known about how the increased Ca(2+) is restored to its resting level, forming the Ca(2+) oscillations. We investigated whether mitochondria also play a role in activation-associated Ca(2+) signaling. Mitochondrial dysfunction induced by the mitochondrial uncoupler FCCP or antimycin A disrupted cytosolic Ca(2+) oscillations, resulting in sustained increase in cytosolic Ca(2+), followed by apoptotic cell death. This suggests that functional mitochondria may participate in sequestering the released Ca(2+), contributing to cytosolic Ca(2+) oscillations and preventing cell death. By centrifugation, mouse eggs were stratified and separated into fractions containing both endoplasmic reticulum and mitochondria and fractions containing endoplasmic reticulum with no mitochondria. The former showed Ca(2+) oscillations by activation, whereas the latter exhibited sustained elevation in cytosolic Ca(2+) but no Ca(2+) oscillations, suggesting that mitochondria take up released cytosolic Ca(2+). Further, using Rhod-2 for detection of mitochondrial Ca(2+), we found that mitochondria exhibited Ca(2+) oscillations, the frequency of which was not different from that of cytosolic Ca(2+) oscillations, indicating that mitochondria are involved in Ca(2+) signaling during egg activation. Therefore, we propose that mitochondria play a crucial role in Ca(2+) signaling that mediates egg activation and development, and apoptotic cell death.

Flowering plant sperm contains a cytosolic soluble protein factor which can trigger calcium oscillations in mouse eggs

There is evidence showing that the sperm-induced Ca(2+) oscillations in mammalian eggs at fertilization are triggered by a sperm-derived protein factor. It was established recently that the activity of the putative sperm protein in causing Ca(2+) oscillations in mammalian eggs is not species-specific in vertebrates (1, 16). Here we report that cytosolic soluble extracts derived from flowering plant sperms in Brassica campestris can also induce fertilization-like Ca(2+) oscillations when microinjected into mouse eggs. The factor responsible for inducing Ca(2+) oscillations in the plant sperm was sperm-specific and heat- or trypsin-labile. Eight to ten sperm equivalents of the plant sperm extracts had enough activity to trigger Ca(2+) oscillations in mouse eggs. Our study suggests that, although plant and mammal are evolutionary divergent species, the activity of the putative sperm protein factor in triggering Ca(2+) signaling in mammalian eggs is not specific to the animal kingdom.

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].

Ethylenediamine-N,N,N',N'-tetraacetic acid induces parthenogenetic activation of porcine oocytes at the germinal vesicle stage, leading to formation of blastocysts

The present study showed that treatment with a cell membrane-impermeable metal ion chelator, EDTA, of porcine oocytes at the germinal vesicle (GV) stage collected from follicles 2-6 mm in diameter induced artificial activation followed by formation of a pronucleus (PN). When the oocytes were cultured for 48 h in medium containing 0.1 to 2 mM EDTA disodium salt (Na-EDTA), they were activated to form PN, and the maximum PN formation rate (63%, n = 68) was achieved in oocytes cultured with 1 mM Na-EDTA. More than 90% of oocytes activated by 1 mM Na-EDTA treatment formed 1 PN without emission of the first and the second polar bodies (PB). This result suggests that EDTA at 1 mM may force the maturing (meiosis I) oocytes to form a PN without chromosome segregation. When oocytes at the GV stage that had been cultured with 1 mM Na-EDTA for 48 h were further cultured in 0.4% BSA-containing NCSU23 medium for 144 h, blastocysts that appeared to be morphologically normal were formed at the rate of 10%, whereas no blastocysts were formed from oocytes that had not been cultured with Na-EDTA. Next we examined the effects of Ca2+, Zn2+, Fe3+, or Cu2+-saturated EDTA (Ca-EDTA, Zn-EDTA, Fe-EDTA, and Cu-EDTA, respectively), and a Ca2+-specific chelator, EGTA, at a concentration of 1 mM. The Ca-EDTA, Fe-EDTA, and Cu-EDTA, but not Zn-EDTA or EGTA, had the ability to activate the oocytes. From these results, it is suggested that extracellular chelation of Zn2+ with EDTA of maturing (meiosis I) porcine oocytes results in parthenogenetic activation of the oocytes, which induces PN formation followed by development to blastocysts.

Development of cloned embryos from adult rabbit fibroblasts: effect of activation treatment and donor cell preparation

This research was to study the in vitro and in vivo development of cloned embryos derived from adult rabbit fibroblasts following various activation protocols. Effects of serum starvation and passage number of donor cells on the efficiency of cloning were also examined. In experiment I, oocytes were activated either by electric pulses or by electric pulses followed by culture with 6-dimethylaminopurin (DMAP). For experiment II, the best activation protocol from experiment I was employed for cloning using adult rabbit fibroblasts that were cultured for 0-15 passages. In experiment III, the effect of serum starvation of the donor cells on cloning was examined. Finally, in experiment IV, embryo transfers were conducted. These experiments showed that combined electrical pulse and DMAP treatment resulted in superior parthenogenetic blastocyst development (up to 29%), and that activation of the cytoplast before versus after fusion was not different in supporting the in vitro development of nuclear transferred embryos (16%-18% blastocysts). Adult fibroblasts derived from nonpassaged cells were less capable of developing into blastocysts than passaged cells (6% vs. 17%). Serum starvation of donor cells improved cleavage (up to 71%) but did not improve blastocyst development (13%), and no progeny was obtained, irrespective of the treatment. Cell-cycle analysis of adult rabbit fibroblast cells showed that passage 6 and 12 cells were more likely to be in G(0)/G(1) than passage 0 cells, which agrees with the improved embryo development in the passaged-cell groups.

Molecular and biochemical regulation of early mammalian development

Fertilization initiates a rapid series of changes that restructures the egg into the zygote and initiates the program of early development. These changes in the cell occur while the genetic complement of the egg and sperm are in a highly condensed state and unable to participate in transcription. The egg cytoplasm, formed by the maternal genome, contains the necessary components that mediate the early restructuring of egg into zygote. These changes are mediated by a series of cytoplasmic signal transduction events initiated by the rise in [Ca2+]i caused when the sperm penetrates the egg. The structural changes that the egg undergoes are rapid and result in the extensive remodeling of this specialized cell. Protein kinase C (PKC) and calcium/calmodulin-dependent protein kinase II (CaM KII) are two pivotal signaling agents that mediate several of these rapid modifications in cell structure. Studies indicate the meiotic spindle serves as an architectural element in the egg that acts to colocalize elements from several of the key signaling pathways and may provide a means for these pathways to interact. In mammals, transcription begins earlier than in zygotes from other classes of organisms, starting several hours after fertilization in the male and female pronuclei and continuing in the embryonic nuclei. Studies indicate that nuclei undergo an initial state that is permissive for transcription, and then in Gap 2 of the two-cell embryo, enter a transcriptionally repressive state. These changes have been linked to the times during the cell cycle when the DNA is replicated, and also have been proposed as a requirement for proper initiation of the program of early development.

Porcine oocyte activation induced by a cytosolic sperm factor

It is not known how the fertilizing sperm elicits the release of Ca(2+) from the oocyte's intracellular stores. We investigated whether a crude extract isolated from boar sperm could induce the Ca(2+) release and trigger subsequent early and late activation events upon injection into matured porcine oocytes. The sperm extract induced an immediate rise in the intracellular free Ca(2+) concentration in all oocytes tested, which was followed by repetitive Ca(2+) transients in 11 out of 14 oocytes. Heat or trypsin treatment of the extract totally abolished the Ca(2+) releasing activity of the sperm factor. The injected oocytes showed cortical granule exocytosis, they resumed meiosis and entered first interphase: pronuclei were formed in 89.2% (132/148) of the cases. Pronuclear formation was accompanied by the appearance of a new 22 kDa protein as normally seen at fertilization. Of the successfully injected oocytes 51.7% (105/203) cleaved and 2.0% (4/203) developed to the blastocyst stage after being cultured for 7 days in NCSU 23 medium. Injection of the carrier medium could not trigger these changes. The results indicate that the sperm might activate porcine oocytes by introducing a soluble factor into the oocyte's cytoplasm after gamete fusion.

In-vitro development of mouse zygotes following reconstruction by sequential transfer of germinal vesicles and haploid pronuclei

We evaluated whether mouse oocytes reconstructed by germinal vesicle (GV) transfer can develop to blastocyst stage. The oocytes were artificially activated with sequential treatment of A23187 and anisomycin; fertilization was then established by transfer or exchange of pronuclei with those of zygotes fertilized in vivo. Type 1 zygotes were constructed by placing the male haploid pronucleus from a zygote into the cytoplasm of an oocyte that underwent GV transfer, in-vitro maturation and activation; for type 2 zygotes, the female pronucleus was removed from a zygote and replaced with the female pronucleus of an oocyte subjected to GV transfer, in-vitro maturation and activation. Karyotypes of activated oocytes and type 2 zygotes were also subjected to analysis. When cultured in human tubal fluid (HTF) medium, reconstructed oocytes matured and, following artificial activation, consistently developed a pronucleus with a haploid karyotype; the activation rate for this medium was two- to three-fold higher than that of oocytes cultured in M199 (87% versus 30% respectively). Following transfer of a male pronucleus, only 47% of the type 1 zygotes developed to morula or blastocyst stage and embryo morphology was poor. In contrast, 73% of the type 2 zygotes developed to morula or blastocyst stage, many even hatching, with few morphological anomalies. Normal karyotypes were observed in 88% of the type 2 zygotes analysed. These observations demonstrate that the nucleus of a mouse oocyte subjected to sequential nuclear transfer at GV and pronucleus stages is, nonetheless, capable of maturing meiotically, activating normally and supporting embryonic development to hatching blastocyst stage. In contrast, the developmental potential of the cytoplasm of such oocytes appears to be compromised by these procedures.

Sperm-induced Ca(2+) oscillations in mouse oocytes and eggs can be mimicked by photolysis of caged inositol 1,4,5-trisphosphate: evidence to support a continuous low level production of inositol 1, 4,5-trisphosphate during mammalian fertilization

During mouse fertilization the spermatozoon induces a series of low-frequency long-lasting Ca(2+) oscillations. It is generally accepted that these oscillations are due to Ca(2+) release through the inositol 1,4,5-trisphosphate (InsP(3)) receptor. However, InsP(3) microinjection does not mimic sperm-induced Ca(2+) oscillations, leading to the suggestion that the spermatozoon causes Ca(2+) release by sensitizing the InsP(3) receptor to basal levels of InsP(3). This contradicts recent evidence that the spermatozoon triggers Ca(2+) oscillations by introducing a phospholipase C or else an activator of phospholipase C. Here we show for the first time that sperm-induced Ca(2+) oscillations may be mimicked by the photolysis of caged InsP(3) in both mouse metaphase II eggs and germinal vesicle stage oocytes. Eggs, and also oocytes that had displayed spontaneous Ca(2+) oscillations, gave long-lasting Ca(2+) oscillations when fertilized or when caged InsP(3) was photolyzed. In contrast, oocytes that had shown no spontaneous Ca(2+) oscillations did not generate many oscillations when fertilized or following photolysis of caged InsP(3). Fertilization in eggs was most closely mimicked when InsP(3) was uncaged at relatively low amounts for extended periods. Here we observed an initial Ca(2+) transient with superimposed spikes, followed by a series of single transients with a low frequency; all characteristics of the Ca(2+) changes at fertilization. We therefore show that InsP(3) can mimic the distinctive pattern of Ca(2+) release in mammalian eggs at fertilization. It is proposed that a sperm Ca(2+)-releasing factor operates by generating a continuous small amount of InsP(3) over an extended period of time, consistent with the evidence for the involvement of a phospholipase C.

Activation of mouse oocytes requires multiple sperm factors but not sperm PLCgamma1

A sperm cytosolic factor is responsible for oocyte activation at fertilization in mammals. The molecular identity of this factor is not yet known, although a sperm phospholipase Cgamma (PLCgamma) is a potential candidate. In this study, cation-exchange chromatography with a Heparin column was used for the fractionation of porcine sperm cytosolic extracts. Oocyte activation potential of the resulting fractions was tested and active fractions were subjected to Western blot analysis using antibodies specific to PLCgamma1. PLCgamma1 was detected in fractions other than those supporting oocyte activation (Ca(2+)-release and pronuclear formation). The active Heparin fraction was then purified on a Mono Q anion-exchange column. One of the resulting fractions still contained Ca(2+)-releasing activity, but pronuclear formation did not occur. We conclude that sperm PLCgamma1 is not involved in oocyte activation and that Ca(2+)-release and pronuclear formation requires multiple factors from sperm cytosol.

Down-regulation of the inositol 1,4,5-trisphosphate receptor in mouse eggs following fertilization or parthenogenetic activation

Fertilization in mammalian eggs is characterized by the presence of intracellular calcium ([Ca(2+)]i) oscillations. In mouse eggs, these oscillations cease after a variable period of time and this is accompanied by a decrease in inositol 1,4,5-trisphosphate receptor (IP3R) responsiveness and down-regulation of the IP3R type 1 (IP3R-1). To investigate the signaling pathway responsible for inducing IP3R-1 down-regulation during fertilization, mouse eggs were exposed to or injected with several Ca(2+)-releasing agonists and the amounts of IP3R-1 immunoreactivity evaluated by Western blotting. Exposure to ethanol or ionomycin, which induce a single [Ca(2+)]i rise, failed to signal down-regulation of IP3R-1. However, [Ca(2+)]i oscillations induced by injection of boar sperm fractions (SF), which presumably stimulate production of IP3, or adenophostin A, an IP3R agonist, both induced down-regulation of IP3R-1 of a magnitude similar to or greater than that observed after fertilization. Exposure to thimerosal, an oxidizing agent that modifies the IP3R without stimulating production of IP3, also initiated down-regulation of IP3R-1, although oscillations initiated by SrCl(2) failed to evoke down-regulation of IP3R-1. The degradation of IP3R-1 in mouse eggs appears to be mediated by the proteasome pathway because it was inhibited by preincubation with lactacystin, a very specific proteasome inhibitor. We therefore suggest that persistent stimulation of the phosphoinositide pathway in mouse eggs by the sperm during fertilization or by injection of SF leads to down-regulation of the IP3R-1.

Increased birefringence in the meiotic spindle provides a new marker for the onset of activation in living oocytes

The newly developed Pol-Scope allows imaging of spindle retardance, which is an optical property of organized macromolecular structures that can be observed in living cells without fixation or staining. Experiments were undertaken to examine changes in meiotic spindles during the initial stages of activation of living mouse oocytes using the Pol-Scope. Parthenogenetic activation of oocytes treated with calcium ionophore evoked a dynamic increase in meiotic spindle retardance, particularly of the midregion, before spindle rotation and second polar body extrusion. The pronounced increase in spindle retardance, which could, for the first time to our knowledge, be quantified in living oocytes, was maintained during polar body extrusion. Spindle retardance of newly in vivo fertilized oocytes was significantly higher than that of ovulated, metaphase II oocytes. Pol-Scope imaging of fertilized oocytes did not affect subsequent development. These results establish that increased spindle retardance precedes polar body extrusion and pronuclear formation. The increased birefringence in the spindle provides an early indicator of oocyte activation. Thus, noninvasive, quantitative imaging of the onset of activation in living oocytes might improve the efficiency of assisted fertilization and other embryo technologies.

Cloning, sequencing, and expression analysis of mouse glucosamine-6-phosphate deaminase (GNPDA/oscillin)

It was reported that a hamster protein, called "oscillin," with a sequence related to that of an Escherichia coli GNPDA triggered Ca(2+) oscillations in mammalian oocytes when introduced into their cytoplasm upon fertilization. Recently, it was shown that GNPDA/oscillin is ubiquitously expressed in rat tissues and that a recombinant hamster GNPDA/oscillin protein does not exhibit oscillin activity when injected into oocytes. In the mouse, the nature and role of such a GNPDA/oscillin is not known, but another candidate protein, tr-kit, has been proposed as a sperm factor causing oocyte activation. In order to clarify this issue, we have characterized the mouse homolog of hamster and human GNPDA/oscillin, and examined its expression along with that of tr-kit, in parallel. We report here the molecular cloning and sequencing of mouse GNPDA/oscillin, which shows over 96% identity with the hamster and human homologs. Using specific primers, we performed an RT-PCR analysis to determine the tissue distribution of mouse GNPDA/oscillin mRNA. Unlike tr-kit mRNA which is expressed solely in mouse testis, GNPDA/oscillin mRNA is detected in unfertilized oocytes and in all tissues examined including testis, heart, thymus, liver, ovary, uterus, kidney, spleen, and lung. The protein itself is also detected in all tissues examined by Western blots. Indirect immunofluorescence studies, using an antibody raised against hamster GNPDA, demonstrate that GNPDA is lost with the acrosome reaction of mouse spermatozoa, is localized in the equatorial and neck regions of the human spermatozoa and the post-acrosomal region of the hamster spermatozoa. Our results thus indicate that mouse GNPDA/oscillin, the homolog of hamster oscillin, unlike tr-kit, does not exhibit some of the required characteristics expected from a putative sperm-derived oocyte-activating factor.

Injection of sperm cytosolic factor into mouse metaphase II oocytes induces different developmental fates according to the frequency of [Ca(2+)](i) oscillations and oocyte age

Intracellular calcium ([Ca(2+)](i)) rises are a hallmark of mammalian fertilization and are associated with normal activation of embryonic development. Injection of mammalian sperm cytosolic factor (SCF) into oocytes has been shown to trigger [Ca(2+)](i) rises similar to those observed during fertilization, and to initiate normal embryonic development. However, Ca(2+) release has also been shown to be associated with cell death, but the mechanisms of the detrimental effects of Ca(2+) stimulation on development have not yet been investigated. Thus, studies were undertaken using SCF to test the effects of [Ca(2+)](i) oscillations on oocyte activation in freshly ovulated and aged oocytes. Injections of 1 mg/ml SCF into freshly ovulated mouse metaphase II oocytes, which evoked Ca(2+) responses with low frequency and short duration, induced normal activation and cleavage to the two-cell stage. Conversely, injection of 15 mg/ml SCF, which triggered high-frequency and persistent Ca(2+) responses, induced abnormal activation that was characterized by abnormal chromatin configurations, inhibition of DNA synthesis, and lack of first mitotic spindle assembly. More importantly, fertilization-like Ca(2+) responses induced by injection of 1 mg/ml SCF triggered cell death, rather than activation, in in vitro-aged oocytes. These oocytes exhibited extensive cytoplasmic and DNA fragmentation that was accompanied by activation of protein caspases, all of which are signs of apoptotic cell death. Fewer similarly aged oocytes that were either unstimulated or activated with 7% ethanol underwent fragmentation. Together, these results suggest that [Ca(2+)](i) oscillations are required to activate freshly ovulated oocytes, but if initiated at abnormally high frequency and duration or if induced in aged oocytes, the [Ca(2+)](i) oscillations may trigger premature termination of embryonic development.

Ca(2+) oscillations induced by a cytosolic sperm protein factor are mediated by a maternal machinery that functions only once in mammalian eggs

At fertilization in mammals, the sperm activates the egg by inducing a series of oscillations in the intracellular free Ca(2+) concentration. There is evidence showing that this oscillatory event is triggered by a sperm-derived protein factor which diffuses into egg cytoplasm after gamete membrane fusion. At present the identity of this factor and its precise mechanism of action is unknown. Here, we studied the specificity of action of the sperm factor in triggering Ca(2+) oscillations in mammalian eggs. In doing so, we examined the patterns of Ca(2+) signaling in mouse eggs, zygotes, parthenogenetic eggs and maturing oocytes following the stimulation of bovine sperm extracts which contain the sperm factor. It is observed that the sperm factor could induce Ca(2+) oscillations in metaphase eggs, maturing oocytes and parthenogenetically activated eggs but not in the zygotes. We present evidence that Ca(2+) oscillations induced by the sperm factor require a maternal machinery. This machinery functions only once in mammalian oocytes and eggs, and is inactivated by sperm-derived components but not by parthenogenetic activation. In addition, it is found that neither InsP(3) receptor sensitivity to InsP(3) nor Ca(2+) pool size are the determinants that cause the fertilized egg to lose its ability to generate sperm-factor-induced Ca(2+) oscillations at metaphase. In conclusion, our study suggests that the orderly sequence of Ca(2+) oscillations in mammalian eggs at fertilization is critically dependent upon the presence of a functional maternal machinery that determines whether the sperm-factor-induced Ca(2+) oscillations can persist.

The gamete fusion process is defective in eggs of Cd9-deficient mice

The cell-surface molecule Cd9, a member of the transmembrane-4 superfamily, interacts with the integrin family and other membrane proteins. and is postulated to participate in cell migration and adhesion. Expression of Cd9 enhances membrane fusion between muscle cells and promotes viral infection in some cells. Fertilization also involves membrane fusion, between gametes. In mammals, the sperm binds to microvilli on the egg surface, and sperm-egg membrane fusion first occurs around the equatorial region of the sperm head12. The fused membrane is then disrupted, and the sperm nucleus as well as the cytoplasm is incorporated into the egg. Cd9 is expressed on the plasma membrane of the mouse egg, and an anti-Cd9 monoclonal antibody inhibits sperm-egg surface interactions. We generated Cd9 mice and found that homozygous mutant females were infertile. Sperm-egg binding was normal, but sperm-egg fusion was almost entirely inhibited in eggs from Cd9 females. Intracellular Ca2 oscillations, which signal fertilization, were absent in almost all mutant eggs; in rare cases, a response occurred after a long time period. In normal animals, Cd9 molecules were expressed on the egg microvilli and became densely concentrated at the sperm attachment site. Thus, our results show that Cd9 is important in the gamete fusion process at fertilization.

Xenopus and chicken sperm contain a cytosolic soluble protein factor which can trigger calcium oscillations in mouse eggs

There is evidence showing that at fertilization the sperm introduces into egg cytoplasm a protein-based cytosolic factor, which serves as the physiological trigger for inducing Ca(2+) oscillations in mammalian eggs. Here we show that sperm of nonmammalian vertebrates also contain a cytosolic protein factor that can induce Ca(2+) oscillations when introduced into mammalian eggs. We have observed that cytosolic extracts derived from Xenopus or chicken sperm could induce mouse eggs to undergo Ca(2+) oscillations similar to those induced by bovine sperm extracts. The factor responsible for inducing Ca(2+) oscillations was of high molecular weight and heat- or proteinase K-labile. We show that 0.5 chicken sperm-equivalents or 1-2 Xenopus sperm-equivalents of the extracts had enough activity to trigger Ca(2+) oscillations in mouse eggs. Our findings illustrate that although Xenopus, chicken, and mammals are evolutionarily divergent species, the function of the sperm protein factor in triggering Ca(2+) oscillations in mammalian eggs appears not to be species specific in vertebrates.

Spatiotemporal analysis of Ca(2+) waves in relation to the sperm entry site and animal-vegetal axis during Ca(2+) oscillations in fertilized mouse eggs

Fertilized mouse eggs exhibit repetitive rises in intracellular Ca(2+) concentration ([Ca(2+)](i)) necessary for egg activation. Precise spatiotemporal dynamics of each [Ca(2+)](i) rise were investigated by high-speed Ca(2+) imaging during early development of monospermic eggs. Every [Ca(2+)](i) rise involved a Ca(2+) wave. In the first Ca(2+) transient, [Ca(2+)](i) increased in two steps separated by a "shoulder" point, suggesting two distinct Ca(2+) release mechanisms. The first step was a Ca(2+) wave that propagated from the sperm-fusion site to its antipode in 4-5 s (velocity, approximately 20 microm/s in most eggs). The second step from the shoulder to the peak was a nearly uniform [Ca(2+)](i) rise of 12-15 s. A slight cytoplasmic movement followed the Ca(2+) wave in the same direction and recovered in 25-35 s. These characteristics changed as follows, as Ca(2+) oscillations progressed during the second meiosis up to their cessation at the stage of pronuclei formation ( approximately 3 h after fertilization). (1) The duration of Ca(2+) transients became shorter. (2) The shoulder point shifted to higher levels and the first step occupied most of the rising phase. (3) The rate of [Ca(2+)](i) rise became greater and wave speeds increased up to 80-100 microm/s or more. (4) The transient cytoplasmic movement always resulted from the Ca(2+) wave, although its displacement became smaller. (5) The Ca(2+) wave initiation site was freed from the sperm-fusion or -entry site and eventually localized in the cortex of the vegetal hemisphere. Since the shift of the wave initiation site to the vegetal cortex is observed in fertilized eggs of nemertean worms and ascidians, this might be an evolutionarily conserved feature.

Calcium signalling during zebrafish embryonic development

Calcium signals appear throughout the first 24 hours of zebrafish development. These begin at egg activation, then continue to be generated throughout the subsequent zygote, cleavage, blastula, gastrula, and segmentation periods. They are thus associated with the major phases of pattern formation: cell proliferation, cell differentiation, axis determination, the generation of primary germ layers, the emergence of rudimentary organ systems, and therefore the establishment of the basic vertebrate body plan. When signals need to be transmitted across significant distances they take the form of waves, either intracellular waves when the cell size is large, or later in development when the cell size is reduced, intercellular waves. We will consider both types of calcium signals and their integration into signalling networks, and discuss their possible functions and developmental significance with regard to pattern formation. BioEssays 22:113-123, 2000.

Sperm entry induces polarity in fucoid zygotes

Fucoid zygotes establish a rhizoid-thallus growth axis in response to environmental signals; however, these extrinsic cues are not necessary for polarization, suggesting that zygotes may have inherent polarity. The hypothesis that sperm entry provides a default pathway for polarization of zygotes cultured in the absence of environmental signals was tested, and was supported by several lines of evidence. First, an F-actin patch, a cortical marker of the rhizoid pole, formed at the sperm entry site within minutes of fertilization. Second, the sperm entry site predicted the site of polar adhesive secretion (the first morphological manifestation of the rhizoid pole) and the position of rhizoid outgrowth. Third, when fertilization was restricted to one hemisphere of the egg, rhizoid outgrowth always occurred from that hemisphere. Fourth, delivery of sperm to one location within a population of eggs resulted in polarization of both adhesive secretion and rhizoid outgrowth toward the sperm source. Finally, induction of polyspermy using low sodium seawater increased the frequency of formation of two rhizoids. Sperm entry therefore provides an immediate default axis that can later be overridden by environmental cues.

Establishing a growth axis in fucoid algae

Recent studies indicate that fucoid zygotes establish developmental polarity much earlier than previously thought. A growth axis is first set in place at fertilization, with the site of sperm entry defining the rhizoid pole of the axis. This initial axis is a default axis, which is only used as the final growth axis if the zygote fails to detect spatial cues (such as sunlight) in its intertidal environment. However, the zygote usually senses vectorial information; it then abandons the sperm-induced axis and assembles a new axis de novo in accordance with the perceived vector(s).