The exit of mouse oocytes from meiotic M-phase requires an intact spindle during intracellular calcium release

Discs large homologue 1 (Dlg1) coordinates mouse oocyte polarisation during maturation

Mouse oocyte meiotic division requires the establishment of asymmetries in the oocyte before division, indicating the presence of polarity-establishing molecules. During mouse oocyte maturation proper orientation and positioning of the meiotic spindle at the oocyte cortex, as well as polarity in the oocyte cytoplasm and its oolemma, are necessary for the formation of functional haploid oocytes. Discs large homologue 1 (Dlg1) protein is a conserved protein that regulates cell polarity. In the present study, we found that Dlg1 was expressed at different stages of oocyte development. The localisation of Dlg1 during mouse oocyte maturation and its relationship with the cytoskeleton were analysed. Our data show that at the germinal vesicle stage, Dlg1 was present in the cytoplasm, prominently surrounding the germinal vesicle membrane. During maturation, Dlg1 became highly polarised by associating with the spindle and formed characteristic crescent-shaped accumulations under the cortex. Addition of nocodazole or cytochalasin B into the culture medium at different stages changed the localisation of Dlg1, indicating that the organisation of Dlg1 is a complex multi-step process and is dependent on microtubules and microfilaments. More importantly, we found that silencing of Dlg1 compromised the G2-M transition.

TRPM7-like channels are functionally expressed in oocytes and modulate post-fertilization embryo development in mouse

The Transient Receptor Potential (TRP) channels are a family of cationic ion channels widely distributed in mammalian tissues. In general, the global genetic disruption of individual TRP channels result in phenotypes associated with impairment of a particular tissue and/or organ function. An exception is the genetic ablation of the TRP channel TRPM7, which results in early embryonic lethality. Nevertheless, the function of TRPM7 in oocytes, eggs and pre-implantation embryos remains unknown. Here, we described an outward rectifying non-selective current mediated by a TRP ion channel in immature oocytes (germinal vesicle stage), matured oocytes (metaphase II eggs) and 2-cell stage embryos. The current is activated by specific agonists and inhibited by distinct blockers consistent with the functional expression of TRPM7 channels. We demonstrated that the TRPM7-like channels are homo-tetramers and their activation mediates calcium influx in oocytes and eggs, which is fundamental to support fertilization and egg activation. Lastly, we showed that pharmacological inhibition of the channel function delays pre-implantation embryo development and reduces progression to the blastocyst stage. Our data demonstrate functional expression of TRPM7-like channels in mouse oocytes, eggs and embryos that may play an essential role in the initiation of embryo development.

Ca(2+) dynamics in oocytes from naturally-aged mice

The ability of human metaphase-II arrested eggs to activate following fertilisation declines with advancing maternal age. Egg activation is triggered by repetitive increases in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the ooplasm as a result of sperm-egg fusion. We therefore hypothesised that eggs from older females feature a reduced ability to mount appropriate Ca²⁺ responses at fertilisation. To test this hypothesis we performed the first examination of Ca²⁺ dynamics in eggs from young and naturally-aged mice. Strikingly, we find that Ca²⁺ stores and resting [Ca²⁺]_i are unchanged with age. Although eggs from aged mice feature a reduced ability to replenish intracellular Ca²⁺ stores following depletion, this difference had no effect on the duration, number, or amplitude of Ca²⁺ oscillations following intracytoplasmic sperm injection or expression of phospholipase C zeta. In contrast, we describe a substantial reduction in the frequency and duration of oscillations in aged eggs upon parthenogenetic activation with SrCl₂. We conclude that the ability to mount and respond to an appropriate Ca²⁺ signal at fertilisation is largely unchanged by advancing maternal age, but subtle changes in Ca²⁺ handling occur that may have more substantial impacts upon commonly used means of parthenogenetic activation.

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.

Regulation of endoplasmic reticulum Ca(2+) oscillations in mammalian eggs

Changes in the intracellular concentration of free calcium ([Ca(2+)]i) regulate diverse cellular processes including fertilization. In mammalian eggs, the [Ca(2+)]i changes induced by the sperm unfold in a pattern of periodical rises, also known as [Ca(2+)]i oscillations. The source of Ca(2+) during oscillations is the endoplasmic reticulum ([Ca(2+)]ER), but it is presently unknown how [Ca(2+)]ER is regulated. Here, we show using mouse eggs that [Ca(2+)]i oscillations induced by a variety of agonists, including PLCζ, SrCl2 and thimerosal, provoke simultaneous but opposite changes in [Ca(2+)]ER and cause differential effects on the refilling and overall load of [Ca(2+)]ER. We also found that Ca(2+) influx is required to refill [Ca(2+)]ER, because the loss of [Ca(2+)]ER was accelerated in medium devoid of Ca(2+). Pharmacological inactivation of the function of the mitochondria and of the Ca(2+)-ATPase pumps PMCA and SERCA altered the pattern of oscillations and abruptly reduced [Ca(2+)]ER, especially after inactivation of mitochondria and SERCA functions. We also examined the expression of SERCA2b protein and found that it was expressed throughout oocyte maturation and attained a conspicuous cortical cluster organization in mature eggs. We show that its overexpression reduces the duration of inositol-1,4,5-trisphosphate-induced [Ca(2+)]i rises, promotes initiation of oscillations and enhances refilling of [Ca(2+)]ER. Collectively, our results provide novel insights on the regulation of [Ca(2+)]ER oscillations, which underlie the unique Ca(2+)-signalling system that activates the developmental program in mammalian eggs.

Suppression of chemically induced and spontaneous mouse oocyte activation by AMP-activated protein kinase

Oocyte activation is an important process triggered by fertilization that initiates embryonic development. However, parthenogenetic activation can occur either spontaneously or with chemical treatments. The LT/Sv mouse strain is genetically predisposed to spontaneous activation. LT oocytes have a cell cycle defect and are ovulated at the metaphase I stage instead of metaphase II. A thorough understanding of the female meiosis defects in this strain remains elusive. We have reported that AMP-activated protein kinase (PRKA) has an important role in stimulating meiotic resumption and promoting completion of meiosis I while suppressing premature parthenogenetic activation. Here we show that early activation of PRKA during the oocyte maturation period blocked chemically induced activation in B6SJL oocytes and spontaneous activation in LT/SvEiJ oocytes. This inhibitory effect was associated with high levels of MAPK1/3 activity. Furthermore, stimulation of PRKA partially rescued the meiotic defects of LT/Sv mouse oocytes in concert with correction of abnormal spindle pole localization of PRKA and loss of prolonged spindle assembly checkpoint activity. Altogether, these results confirm a role for PRKA in helping sustain the MII arrest in mature oocytes and suggest that dysfunctional PRKA contributes to meiotic defects in LT/SvEiJ oocytes.

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.

Regulation of inositol 1,4,5-trisphosphate receptor function during mouse oocyte maturation

At the time of fertilization, an increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) underlies egg activation and initiation of development in all species studied to date. The inositol 1,4,5-trisphosphate receptor (IP(3)R1), which is mostly located in the endoplasmic reticulum (ER) mediates the majority of this Ca(2+) release. The sensitivity of IP(3)R1, that is, its Ca(2+) releasing capability, is increased during oocyte maturation so that the optimum [Ca(2+)](i) response concurs with fertilization, which in mammals occurs at metaphase of second meiosis. Multiple IP(3)R1 modifications affect its sensitivity, including phosphorylation, sub-cellular localization, and ER Ca(2+) concentration ([Ca(2+)](ER)). Here, we evaluated using mouse oocytes how each of these factors affected IP(3)R1 sensitivity. The capacity for IP(3)-induced Ca(2+) release markedly increased at the germinal vesicle breakdown stage, although oocytes only acquire the ability to initiate fertilization-like oscillations at later stages of maturation. The increase in IP(3)R1 sensitivity was underpinned by an increase in [Ca(2+)](ER) and receptor phosphorylation(s) but not by changes in IP(3)R1 cellular distribution, as inhibition of the former factors reduced Ca(2+) release, whereas inhibition of the latter had no impact. Therefore, the results suggest that the regulation of [Ca(2+)](ER) and IP(3)R1 phosphorylation during maturation enhance IP(3)R1 sensitivity rendering oocytes competent to initiate oscillations at the expected time of fertilization. The temporal discrepancy between the initiation of changes in IP(3)R1 sensitivity and acquisition of mature oscillatory capacity suggest that other mechanisms that regulate Ca(2+) homeostasis also shape the pattern of oscillations in mammalian eggs.

Rat eggs cannot wait: Spontaneous exit from meiotic metaphase-II arrest

Mammalian eggs await fertilisation while arrested at the second metaphase stage of meiotic division. A network of signalling pathways enables the establishment and maintenance of this metaphase-II arrest. In the absence of fertilisation, mammalian eggs can spontaneously exit metaphase II when parthenogenetically stimulated, or sometimes without any obvious stimulation. Ovulated rat eggs abortively release from metaphase-II arrest once removed from egg donors. Spontaneously activated rat eggs extrude the second polar body and proceed to the so-called metaphase III-'like' stage, with clumps of condensed chromatin scattered in the egg cytoplasm. It is still unclear what makes rat eggs susceptible to spontaneous activation; however, a vague picture of the signalling pathways involved in the process of spontaneous activation is beginning to emerge. Such cell cycle instability is one of the major reasons why it is more difficult to establish nuclear transfer in the rat. This review examines the known predisposing factors and biochemical mechanisms involved in spontaneous activation. The strategies used to prevent spontaneous metaphase-II release in rat eggs will also be discussed.

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

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

A PolScope evaluation of meiotic spindle dynamics in frozen-thawed oocytes

In mature human oocytes, the metaphase II (MII) spindle presence and birefringence signal detected through the PolScope may vary before and after freezing. In particular, spindle dynamics during the first few hours after thawing is still under study. In this study, oocytes from stimulated ovaries were cryopreserved in 1.5 mol/l 1,2-propanediol with 0.3 mol/l sucrose using a slow freezing-rapid thawing method. Oocytes were examined with the PolScope for the presence, intensity of signal birefringence and size of the meiotic spindle before freezing and at 0, 1 and 2 h post-thaw (where 0 h = the time of the end of the thawing procedure). Of the 173 surviving oocytes exhibiting a spindle before freezing, 82.7% (143/173) showed spindle birefringence within 1 h of thawing. However, at the end of the thawing procedure the intensity of spindle birefringence (retardance) and the spindle length were smaller in comparison to the pre-freezing condition. These parameters increased after 1 h, although were not restored to the value observed before freezing. No significant changes were observed by extending the culture to 2 h.

Actin filaments: key players in the control of asymmetric divisions in mouse oocytes

Meiotic maturation is characterized by the succession of two asymmetric divisions each giving rise to a small polar body and a large oocyte. These highly asymmetric divisions are characteristic of meiosis in higher organisms. They allow most of the maternal stores to be retained in the oocyte, a vital property for further embryo development. In mouse oocytes, the asymmetry is ensured by the migration and the anchoring of the division spindle to the cortex in meiosis I and by its anchoring to the cortex in meiosis II. In addition, and subsequent to this off-centre positioning of the spindle, a differentiation of the cortex overhanging the chromosomes takes place and is necessary for the extrusion of small polar bodies. In the present review, we will emphasize the role of the actin cytoskeleton in the control of spindle positioning, spindle anchoring to the cortex and cortical differentiation.

Clinical efficiency of oocyte and embryo cryopreservation

The assessment of the standard for success of in vitro fertilization (IVF) treatment is an arduous task. Clinical efficiency and safety of a given procedure should represent fundamental tools for objective comparison. However, differences in patient populations, laboratory protocols, and expression of clinical data make the analysis of different studies and strategies very difficult. Formulation of the standard for success through the cumulative delivery rate per cycle of stimulation is a very attractive option because it includes the essential contribution of frozen embryos, which can represent 30-40% of all deliveries, while taking into account the need to minimize the proportion of pharmacological and surgical treatments. Embryo cryopreservation may be applied at different postinsemination stages. Larger and more detailed sets of data are available for day 2 embryo freezing, which allows cumulative delivery rates of 50-60% in good prognosis patients. In the last few years, novel freezing methods have improved the overall efficiency of oocyte cryopreservation. Especially in contexts afflicted by legal restrictions to embryo cryo- preservation, this form of preservation has started to have an impact on the IVF standard of success, generating cumulative pregnancy rates approaching 50%. Despite having been applied systematically by some IVF programs for only a few years, oocyte freezing already competes in efficiency with pronuclear-stage cryopreservation, and it does not appear unrealistic to predict that in the future it will challenge the dominance of embryo cryopreservation as the preferred form of conservation.

Second meiotic arrest and exit in frogs and mice

Mature vertebrate oocytes typically undergo programmed arrest at the second meiotic cell cycle until they are signalled to initiate embryonic development at fertilization. Here, we describe the underlying molecular mechanisms of this second meiotic arrest and release in Xenopus, and compare and contrast them with their counterparts in mice.

Overcoming MIII arrest from spontaneous activation in cultured rat oocytes

The rat oocyte spontaneously activates under a wide variety of conditions. This process progresses to MIII arrest that is not responsive to parthenogenetic activation and development. Insofar as activation involves extrusion of the second polar body (PBII), we set out to determine if preventing this step by inhibiting microfilaments would change the course of spontaneous activation (SA). In particular, how long does the effect of SA persist while retaining reversibility of PBII extrusion once inhibitors are removed? We wanted to determine if the eggs would be responsive to parthenogenetic activation and capable of resuming development once a permanent inhibition is achieved. We set out to determine whether SA would depend on the ovular age of oocytes. Inhibiting of PBII extrusion was achieved by affecting microtubules with demecolcine or nocodazole or actin filaments with cytochalasin B (CB) and cytochalasin D (CD). We found that all oocytes undergo SA and progression to MIII; however, the rapidity of spontaneous activation is a function of the ovular age of the oocyte. The resumption of the meiosis period changes dramatically from 20 to 180 min with decreasing ovular age. We established that suppression of PB formation can be effectively achieved in oocytes of younger ovular age, and that inhibition of PB extrusion became irreversible after 3.5 h of treatment. We established that drug-treated oocytes could undergo subsequent reactivation and in vitro development to blastocysts. The rate of in vitro development of cytochalasin-treated group was comparable to parthenogenetic controls, while nocodazole and demecolcine produced oocytes that developed at lower frequencies. Thus, the application of the microfilament inhibiting drugs helps to overcome the negative effect of SA that results in MIII arrest. Here we also show optimized parthenogenetic stimulation that resulted in development to the blastocyst stage at frequency comparable to development of fertilized embryos.

Changes in endoplasmic reticulum structure during mouse oocyte maturation are controlled by the cytoskeleton and cytoplasmic dynein

Oocyte maturation in mouse is associated with a dramatic reorganisation of the endoplasmic reticulum (ER) from a network of cytoplasmic accumulations in the germinal vesicle-stage oocyte (GV) to a network of distinctive cortical clusters in the metaphase II egg (MII). Multiple lines of evidence suggest that this redistribution of the ER is important to prepare the oocyte for the generation of repetitive Ca2+ transients which trigger egg activation at fertilisation. The aim of the current study was therefore to investigate the timecourse and mechanism of ER reorganisation during oocyte maturation. The ER is first restructured at the time of GV-breakdown (GVBD) into a dense network of membranes which envelop and invade the developing meiotic spindle. GVBD is essential for the initiation of ER reorganisation, since ER structure does not change in GV-arrested oocytes. ER reorganisation is also prevented by the microtubule inhibitor nocodazole and by the inhibition of cytoplasmic dynein, a microtubule-associated motor protein. ER redistribution at GVBD is therefore dynein-driven and cell cycle-dependent. Following GVBD the dense network of ER surrounds the spindle during its migration to the oocyte cortex. Cortical clusters of ER are formed close to the time of, but independently of the metaphase I-metaphase II transition. Formation of the characteristic ER clusters is prevented by the depolymerisation of microfilaments, but not of microtubules. These experiments reveal that ER reorganisation during oocyte maturation is a complex multi-step process involving distinct microtubule- and microfilament-dependent phases and indicate a role for dynein in the cytoplasmic changes which prepare the oocyte for fertilisation.

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

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

Analysis of oocyte physiology to improve cryopreservation procedures

In contrast to the preimplantation mammalian embryo, it has been notoriously difficult to cryopreserve the metaphase II oocyte. The ability to store oocytes successfully at -196 degrees C has numerous practical and financial advantages, together with ethical considerations, and will positively impact animal breeding programs and assisted conception in the human. Differences in membrane permeability and in physiology are two main reasons why successful oocyte cryopreservation has remained elusive. It is proposed, therefore, that rather than relying on technologies already established for the preimplantation embryo, the development of cryopreservation techniques suitable for the mammalian oocyte needs to take into account the idiosyncratic physiology of this cell. Analysis of intracellular calcium, for example, has revealed that exposure to conventional permeating cryoprotectants, such as propanediol, ethylene glycol and DMSO, all independently result in an increase in calcium, which in turn has the potential to initiate oocyte activation, culminating in zona hardening. Quantification of the metabolome and proteome of the oocyte has revealed that whereas slow freezing has a dramatic effect on cell physiology, vitrification appears to have limited effect. This is plausibly achieved by the limited exposure to cryoprotectants. Analysis of meiotic spindle dynamics and embryo development following IVF, also indicate that vitrification is less traumatic than slow freezing, and therefore has the greatest potential for successful oocyte cryopreservation.

Vitrification of in vitro matured ovine oocytes affects in vitro pre-implantation development and mRNA abundance

The impact of vitrification procedures on in vitro matured (IVM) ovine oocytes mRNA content and ability to undergo successful fertilization, cleavage and embronic development was assessed. Vitrified-warmed (n = 113) and control (n = 140) IVM oocytes were in vitro fertilized and cultured up to blastocyst stage under standard conditions. Vitrified oocytes showed lower cleavage rate (47% vs. 75%, P < 0.001) and development to blastocyst stage (17% vs. 57%, P < 0.001) than controls. In addition, the timings of the first cleavage and blastocysts production were significantly delayed in the vitrified-warmed group (P < 0.001 in both cases). In parallel, we analyzed by reverse transcriptase real-time PCR the relative abundance of beta-actin, H2A.Z histone, Poli A Polimerase (PAP), Heat Shock Protein 90 beta (HSP90 beta), P34(cdc2), Cyclin b, Na/K-ATPase and Type I cadherin (E-Cad) transcripts in single IVM controls (n = 24) and vitrified-warmed oocytes (n = 40). Results were normalized against the exogenous rabbit alpha-globin mRNA standard and the beta-actin housekeeping gene and similarly described a lower abundance of most mRNAs in oocytes subjected to vitrification procedures. When normalized against the exogenous standard mRNA, all transcripts except for beta-actin and H2A.Z showed a significantly different abundance in the two classes of oocytes. The same results were obtained after normalization against the internal standard, except for HSP90 beta and E-Cad transcripts, whose lower abundance in vitrified-warmed oocytes resulted prominent, but not significant (P = 0.083 and P = 0.068, respectively). The oocyte lower transcripts abundance following vitrification might be an early indicator of poor quality in good correlation with the developmental data to blastocyst stage.

Demecolcine-induced enucleation of sheep meiotically maturing oocytes

The objective of this study was to investigate the possible effect of demecolcine, a microtubule-disrupting reagent, on induced enucleation (IE) of sheep meiotically maturing oocytes. Immunofluorescent staining with anti-tubulin antibodies was used to examine the spindle status of the oocytes. When the oocytes with intact germinal vesicles (GV) were cultured in the medium containing various concentrations of demecolcine (0.01 to 0.4 microg.mL-1) for 20 to 22 h, the spindle microtubule organization and first polar body (PB1) extrusion were inhibited by demecolcine in a dose-dependent manner. The highest IE rate (58.1%) was from the treatment with 0.04 microg.mL-1 demecolcine. Demecolcine treatment applied after germinal vesicle breakdown (GVBD) or at metaphase (M) yielded a PB1 extrusion rate and IE efficiency similar to the treatment applied at the onset of maturation. Analysis by immunofluorescence showed that both nonspindle microtubules and spindle microtubules were significantly disorganized by demecolcine. Combination treatment with demecolcine and cycloheximide (CHX) or 6-dimethylaminopurine (6-DMAP) led to single pronuclear formation rather than PB1 extrusion. When demecolcine-treated oocytes were transferred into demecolcine-free medium, the ability to extrude PB1 was quickly restored and a 72.1% IE rate was obtained following such treatment. These results demonstrate that demecolcine can be used as a potential reagent for induced enucleation of sheep meiotically maturing oocytes and may greatly facilitate research in nuclear transfer.

Sucrose concentration influences the rate of human oocytes with normal spindle and chromosome configurations after slow-cooling cryopreservation

BACKGROUND

Recently described slow-cooling cryopreservation protocols involving elevated sucrose concentration have improved survival frequencies of human oocytes, potentially overcoming a major hurdle that has limited the adoption of oocyte storage. Because implantation rates of embryos from frozen oocytes remain generally low, it is still debated whether, irrespective of survival rates, this form of cryopreservation leads inevitably to the disruption or complete loss of the metaphase II (MII) spindle.

METHODS

Human oocytes with an extruded polar body I (PBI) were cryopreserved using a slow-cooling method including 1.5 mol/l propane-1,2-diol (PrOH) and alternative sucrose concentrations (either 0.1 or 0.3 mol/l) in the freezing solution. Fresh control and frozen-thawed survived oocytes were analysed by confocal microscopy to evaluate MII spindle and chromosome organizations.

RESULTS

Of the 104 oocytes included in the unfrozen group, 76 (73.1%) displayed normal bipolar spindles with equatorially aligned chromosomes. Spindle and chromatin organizations were significantly affected (50.8%) after cryopreservation involving lower sucrose concentration (61 oocytes), whereas these parameters were unchanged (69.7%) using the 0.3 mol/l sucrose protocol (152 oocytes).

CONCLUSIONS

Partial disruption of the MII spindle and associated chromosomes accompanies inadequate cryopreservation during slow cooling. However, protocols adopting higher sucrose concentration in the freezing solution promote the retention of an intact chromosome segregation apparatus comparable in incidence to freshly collected oocytes.

Effect of maturation stage at cryopreservation on post-thaw cytoskeleton quality and fertilizability of equine oocytes

Oocyte cryopreservation is a potentially valuable technique for salvaging the germ-line when a valuable mare dies, but facilities for in vitro embryo production or oocyte transfer are not immediately available. This study examined the influence of maturation stage and freezing technique on the cryopreservability of equine oocytes. Cumulus oocyte complexes were frozen at the immature stage (GV) or after maturation in vitro for 30 hr (MII), using either conventional slow freezing (CF) or open pulled straw vitrification (OPS); cryoprotectant-exposed and untreated nonfrozen oocytes served as controls. After thawing, GV oocytes were matured in vitro, and MII oocytes were incubated for 0 or 6 hr, before staining to examine meiotic spindle quality by confocal microscopy. To assess fertilizability, CF MII oocytes were subjected to intracytoplasmic sperm injection (ICSI) and cultured in vitro. At 12, 24, and 48 hr after ICSI, injected oocytes were fixed to examine their progression through fertilization. Both maturation stage and freezing technique affected oocyte survival. The meiosis resumption rate was higher for OPS than CF for GV oocytes (28% vs. 1.2%; P < 0.05), but still much lower than for controls (66%). Cryopreserving oocytes at either stage induced meiotic spindle disruption (37%-67% normal spindles vs. 99% in controls; P < 0.05). Among frozen oocytes, however, spindle quality was best for oocytes frozen by CF at the MII stage and incubated for 6 hr post-thaw (67% normal); since this combination of cryopreservation/IVM yielded the highest proportion of oocytes reaching MII with a normal spindle (35% compared to <20% for other groups), it was used when examining the effects of cryopreservation on fertilizability. In this respect, the rate of normal fertilization for CF MII oocytes after ICSI was much lower than for controls (total oocyte activation rate, 26% vs. 56%; cleavage rate at 48 hr, 8% vs. 42%: P < 0.05). Thus, although IVM followed by CF yields a respectable percentage of normal-looking MII oocytes (35%), their ability to support fertilization is severely compromised.

Cytoskeleton and cell cycle control during meiotic maturation of the mouse oocyte: integrating time and space

During meiotic maturation of mammalian oocytes, two successive divisions occur without an intermediate phase of DNA replication, so that haploid gametes are produced. Moreover, these two divisions are asymmetric, to ensure that most of the maternal stores are retained within the oocyte. This leads to the formation of daughter cells with different sizes: the large oocyte and the small polar bodies. All these events are dependent upon the dynamic changes in the organization of the oocyte cytoskeleton (microtubules and microfilaments) and are highly regulated in time and space. We review here the current knowledge of the interplay between the cytoskeleton and the cell cycle machinery in mouse oocytes, with an emphasis on the two major activities that control meiotic maturation in vertebrates, MPF (Maturation promoting factor) and CSF (Cytostatic factor).

LKB1/PAR4 protein is asymmetrically localized in mouse oocytes and associates with meiotic spindle

The mouse secondary oocyte is polarized at the ultrastructural and molecular level, but very little is known about mechanisms involved in the establishment of this polarity. We showed that the LKB1 kinase, a mouse homologue of Caenorhabditis elegans PAR4 protein is asymmetrically localized to the animal pole of the mouse oocyte and during oocyte maturation associates with the microtubules of metaphase I and metaphase II meiotic spindles. Therefore, we suggest that LKB1/PAR4 protein, may participate in the polarization of the oocyte and in the regulation of the asymmetry of meiotic divisions during mouse oogenesis.

Polar body morphology and spindle imaging as predictors of oocyte quality

It has been suggested that first polar body (PBI) morphology reflects oocyte competence. Oocytes with an intact normal-sized PBI have been described as generating better day 2 embryos, higher blastocyst yield, and increased pregnancy and implantation rates. In other studies, PBI morphology was found to be unrelated to fertilization rate, embryo quality, and blastocyst formation. In a prospective analysis, the predictive value of the PBI was investigated by comparing the development of oocytes retrieved from intracytoplasmic sperm injection patients and displaying different PBI morphology, classified according to the following characteristics: normal size and smooth surface (I), fragmented (II), rough surface (III), or large size (IV). Fertilization rates were 59, 57, 64 and 60% respectively. No significant differences were found between the various groups. The proportions of high quality (grade A) day 2 embryos were also comparable among groups I-III (14, 12 and 17% respectively), while the low number of grade A embryos in group IV (two embryos) did not allow comparison with the other classes. These data do not suggest that PBI selection can contribute to identification of embryos with high developmental ability. In order to establish alternative criteria for oocyte selection, a metaphase II (MII) spindle analysis was also conducted via Polscope. In oocytes of patients of different age, spindle retardance (which reflects the high order and density of microtubules) was compared with parameters of embryo development. In aged patients, a trend was observed between low retardance and poor embryo quality, although in general the association between retardance and oocyte developmental performance did not reach statistical significance.

Calmodulin-dependent protein kinase II, and not protein kinase C, is sufficient for triggering cell-cycle resumption in mammalian eggs

Mouse eggs arrest at metaphase II following ovulation and are only triggered to complete meiosis when fertilized. Sperm break the cell-cycle arrest by a long-lasting series of Ca2+ spikes that lead to an activation of the anaphase-promoting complex/cyclosome. The signal transduction pathway is not fully resolved but both protein kinase C (PKC) and calmodulin-dependent protein kinase II (CamKII) activities increase at fertilization and previous pharmacological studies have implicated both in cell-cycle resumption. We have used a combination of pharmacological inhibitors and constitutively active cRNA constructs of PKCα and CamKIIα microinjected into mouse eggs to show that it is CamKII and not PKC that is the sufficient trigger for cell-cycle resumption from metaphase II arrest.

Constitutively active PKC constructs had no effect on the resumption of meiosis but caused an immediate and persistent elevation in intracellular Ca2+ when store-operated Ca2+ entry was stimulated. With respect to resumption of meiosis, the effects of constitutively active CamKII on eggs were the same as sperm. Eggs underwent second polar body extrusion and pronucleus formation with normal timings; while both securin and cyclin B1 destruction, visualised by coupling to fluorescent protein tags, were complete by the time of polar body extrusion. Induction of a spindle checkpoint by overexpression of Mad2 or by spindle poisons blocked CamKII-induced resumption of meiosis, but the Ca2+ chelator BAPTA did not. Furthermore direct measurement of Ca2+ levels showed that CamKII did not induce exit from metaphase II arrest by raising Ca2+. Therefore, we conclude that PKCs may play an important role in maintaining Ca2+ spiking at fertilization by promoting store-operated Ca2+ entry, while CamKII transduces cell-cycle resumption, and lies downstream of sperm-induced Ca2+ release but upstream of a spindle checkpoint. These data, combined with the knowledge that CamKII activity increase at fertilization, suggest that mouse eggs undergo cell-cycle resumption through stimulation of CamKII.

Predictive factors for embryo implantation potential

In spite of recent improvements in IVF, pregnancy rates have not increased significantly and one of the major problems remains the high multiple pregnancy rate. Better criteria are therefore necessary to establish the viability of a transferable embryo. Early prognosis of the developmental fate of the oocyte would help in selecting the best embryos to transfer, but non-invasive selection at the oocyte stage (extracytoplasmic and intracytoplasmic morphology) has proved to be of little prognostic value. Recently, it has been shown that follicular vascularization appears to be predictive of oocyte developmental fate, making it a good first-step approach for selection. Observation of pronuclei patterns at the zygote stage appears to offer an additional prognostic tool, correlating well with IVF outcome. Morphological evaluation of the embryo at days 2-3 remains the most used and valid method of selection, even though it is not sufficient to select embryos with the higher implantation potential. Blastocyst culture is another possible strategy for selecting the best embryos with reduced risk of aneuploidies, though not all major chromosomal aberrations are excluded by prolonged in-vitro culture. In summary, selecting the best embryo for transfer is a decision that should be based on choices made during the different stages of assisted reproductive technologies.

What Criteria for the Definition of Oocyte Quality?

Although the spermatozoon provides an essential contribution to the generation of a new individual, the developmental fate of the embryo is principally dictated by the oocyte. Oocyte competencies are acquired throughout oogenesis, via the interaction with somatic cells. The ability to reinitiate the meiotic process and undergo preimplantation development is progressively determined during the antral phase. It is known that these changes involve the nuclear and cytoplasmic compartments, respectively, but the underlying cellular and molecular mechanisms are still poorly understood. Analysis of various aspects of oocyte morphology (cytoplasm, zona pellucida, and polar body) via conventional phase-contrast microscopy has generated contrasting evidence on the possibility of establishing reliable criteria for the prediction of developmental potential. The introduction of a newly developed microscopy technique based on the detection of polarized light generated by birefringent cell structures has offered the possibility of visualizing noninvasively the meiotic spindle, whose presence is critical for fertilization and later developmental stages. However, further studies are needed to standardize and interpret the information accessible through such a technique. Although unable to preserve cell viability and therefore provide a method by which to select oocytes with superior developmental competence, invasive techniques can make a fundamental contribution to defining objective criteria of oocyte quality. In particular, immunofluorescence analysis, which is able to identify critical anomalies of the meiotic spindle and cytoskeleton organization that can account for oocyte quality, is an important method for assessing the efficiency of in vitro maturation systems.

Two PAR6 proteins become asymmetrically localized during establishment of polarity in mouse oocytes

Meiotic maturation in mammals is characterized by two asymmetric divisions, leading to the formation of two polar bodies and the female gamete. Whereas the mouse oocyte is a polarized cell, molecules implicated in the establishment of this polarity are still unknown. PAR proteins have been demonstrated to play an important role in cell polarity in many cell types, where they control spindle positioning and asymmetric distribution of determinants. Here we show that two PAR6-related proteins have distinct polarized distributions in mouse oocytes. mPARD6a is first localized on the spindle and then accumulates at the pole nearest the cortex during spindle migration. In the absence of microtubules, the chromosomes still migrate to the cortex, and mPARD6a was found associated with the chromosomes and was facing the cortex. mPARD6a is the first identified protein to associate with the spindle during spindle migration and to relocalize to the chromosomes in the absence of microtubule behavior, suggesting a role in spindle migration. The other protein, mPARD6b, was found on spindle microtubules until entry into meiosis II and relocalized to the cortex at the animal pole during metaphase II arrest. mPARD6b is the first identified protein to localize to the animal pole of the mouse oocyte and likely contributes to the polarization of the cortex.

Ca2+-promoted cyclin B1 degradation in mouse oocytes requires the establishment of a metaphase arrest

CDK1-cyclin B1 is a universal cell cycle kinase required for mitotic/meiotic cell cycle entry and its activity needs to decline for mitotic/meiotic exit. During their maturation, mouse oocytes proceed through meiosis I and arrest at second meiotic metaphase with high CDK1-cyclin B1 activity. Meiotic arrest is achieved by the action of a cytostatic factor (CSF), which reduces cyclin B1 degradation. Meiotic arrest is broken by a Ca2+ signal from the sperm that accelerates it. Here we visualised degradation of cyclin B1::GFP in oocytes and found that its degradation rate was the same for both meiotic divisions. Ca2+ was the necessary and sufficient trigger for cyclin B1 destruction during meiosis II; but it played no role during meiosis I and furthermore could not accelerate cyclin B1 destruction during this time. The ability of Ca2+ to trigger cyclin B1 destruction developed in oocytes following a restabilisation of cyclin B1 levels at about 12 h of culture. This was independent of actual first polar body extrusion. Thus, in metaphase I arrested oocytes, Ca2+ would induce cyclin B1 destruction and the first polar body would be extruded. In contrast to some reports in lower species, we found no evidence that oocyte activation was associated with an increase in 26S proteasome activity. We therefore conclude that Ca2+ mediates cyclin B1 degradation by increasing the activity of an E3 ubiquitin ligase. However, this stimulation occurs only in the presence of the ubiquitin ligase inhibitor CSF. We propose a model in which Ca2+ directly stimulates destruction of CSF during mammalian fertilisation.

Involvement of mitogen-activated protein kinase cascade during oocyte maturation and fertilization in mammals

Mitogen-activated protein kinase (MAPK) is a family of Ser/Thr protein kinases that are widely distributed in eukaryotic cells. Studies in the last decade revealed that MAPK cascade plays pivotal roles in regulating the meiotic cell cycle progression of oocytes. In mammalian species, activation of MAPK in cumulus cells is necessary for gonadotropin-induced meiotic resumption of oocytes, while MAPK activation is not required for spontaneous meiotic resumption. After germinal vesicle breakdown (GVBD), MAPK is involved in the regulation of microtubule organization and meiotic spindle assembly. The activation of this kinase is essential for the maintenance of metaphase II arrest, while its inactivation is a prerequisite for pronuclear formation after fertilization or parthenogenetic activation. MAPK cascade interacts extensively with other protein kinases such as maturation-promoting factor, protein kinase A, protein kinase C, and calmodulin-dependent protein kinase II, as well as with protein phosphatases in oocyte meiotic cell cycle regulation. The cross talk between MAPK cascade and other protein kinases is discussed. The review also addresses unsolved problems and discusses future directions.

Cloned mice derived from embryonic stem cell karyoplasts and activated cytoplasts prepared by induced enucleation

Our objective was to induce enucleation (IE) of activated mouse oocytes to yield cytoplasts capable of supporting development following nuclear transfer. Fluorescence microscopy for microtubules, microfilaments, and DNA was used to evaluate meiotic resumption after ethanol activation and the effect of subsequent transient treatments with 0.4 micro g/ml of demecolcine. Using oocytes from B6D2F1 (C57BL/6 x DBA/2) donors, the success of IE of chromatin into polar bodies (PBs) was dependent on the duration of demecolcine treatment and the time that such treatment was initiated after activation. Similarly, variations in demecolcine treatment altered the proportions of oocytes exhibiting a reversible compartmentalization of chromatin into PBs. Treatment for 15 min begun immediately after activation yielded an optimized IE rate of 21% (n = 80) when oocytes were evaluated after overnight recovery in culture. With this protocol, 30-50% of oocytes were routinely scored as compartmentalized when assessed 90 min postactivation. No oocytes could be scored as such following overnight recovery, with 66% of treated oocytes cleaving to the 2-cell stage (n = 80). Activated cytoplasts were prepared by mechanical removal of PBs from oocytes whose chromatin had undergone IE or compartmentalization. These cytoplasts were compared with mechanically enucleated, metaphase (M) II cytoplasts whose activation was delayed in nuclear transfer experiments using HM-1 embryonic stem cells. Using oocytes from either B6D2F1 or B6CBAF1 (C57BL/6 x CBA) donors, the in vitro development of cloned embryos using activated cytoplasts was consistently inferior to that observed using MII cytoplasts. Live offspring were derived from both oocyte strains using the latter, whereas a single living mouse was cloned from activated B6CBAF1 cytoplasts.

The intracellular pH-regulatory HCO3-/Cl- exchanger in the mouse oocyte is inactivated during first meiotic metaphase and reactivated after egg activation via the MAP kinase pathway

The HCO(3)(-)/Cl(-) exchanger is quiescent in the unfertilized mouse egg but is highly active in regulating intracellular pH in the early embryo and required for normal development. We show here that the HCO(3)(-)/Cl(-) exchanger is active in first meiotic prophase (GV) oocyte but inactivated during meiotic metaphase before the MI to MII transition. Reactivation does not occur until the activated egg enters interphase. A quiescent HCO(3)(-)/Cl(-) exchanger is not simply a general feature of metaphase, because activity did not decrease during first mitotic metaphase. Inactivation of the HCO(3)(-)/Cl(-) exchanger during MI coincided with the activation of MAP kinase (MAPK), whereas its reactivation coincided with the loss of MAPK activity after egg activation. Maintaining high MAPK activity after egg activation prevented the normal reactivation of the HCO(3)(-)/Cl(-) exchanger. Inactivating MAPK in unfertilized MII eggs resulted in HCO(3)(-)/Cl(-) exchanger activation. Preventing MAPK activation during first meiotic metaphase prevented the inactivation of HCO(3)(-)/Cl(-) exchange. Conversely, activating MAPK in the GV oocyte resulted in inactivation of HCO(3)(-)/Cl(-) exchange. These results imply that the HCO(3)(-)/Cl(-) exchanger in mouse oocytes is negatively regulated by MAPK. Thus, suppression of pH-regulatory mechanisms during meiosis is a novel function of MAPK and cytostatic factor activity in the oocyte.

Inhibition of MEK or cdc2 kinase parthenogenetically activates mouse eggs and yields the same phenotypes as Mos(-/-) parthenogenotes

Mammalian eggs are arrested in metaphase II of meiosis until fertilization. Arrest is maintained by cytostatic factor (CSF) activity, which is dependent on the MOS-MEK-MAPK pathway. Inhibition of MEK1/2 with a specific inhibitor, U0126, parthenogenetically activated mouse eggs, producing phenotypes similar to Mos(-/-) parthenogenotes (premature, unequal cleavages and large polar bodies). U0126 inactivated MAPK in eggs within 1 h, in contrast to the 5 h required after fertilization, while the time course of MPF inactivation was similar in U0126-activated and fertilized eggs. We also found that inactivation of MPF by the cdc2 kinase inhibitor roscovitine induced parthenogenetic activation. Inactivation of MPF by roscovitine resulted in the subsequent inactivation of MAPK with a time course similar to that following fertilization. Notably, roscovitine also produced some Mos(-/-)-like phenotypes, indistinguishable from U0126 parthenogenotes. Simultaneous inhibition of both MPF and MAPK in eggs treated with roscovitine and U0126 produced a very high proportion of eggs with the more severe phenotype. These findings confirm that MEK is a required component of CSF in mammalian eggs and imply that the sequential inactivation of MPF followed by MAPK inactivation is required for normal spindle function and polar body emission.

Signalling in mammalian egg activation: role of protein kinases

Embryonic development is initiated after the fertilizing spermatozoon enters the egg and triggers a process known as 'egg activation'. Activation results in an increase in intracellular calcium concentration, cortical granule exocytosis (CGE), cell cycle resumption and recruitment of maternal mRNA. Various treatments can induce parthenogenetic activation characterized by the same manifestations. Signal transduction pathways similar to those known for somatic cells mediate the mammalian egg activation. This review focuses on the signal transduction pathways that occur during mammalian fertilization and during parthenogenetic egg activation. We discuss the possibility that members of the protein tyrosine kinase (PTKs) families, the Src family PTKs in particular, operate during egg activation and that protein kinase C can induce CGE.

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

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

A specific inhibitor of p34(cdc2)/cyclin B suppresses fertilization-induced calcium oscillations in mouse eggs

Fertilization-induced Ca(2+) oscillations in mouse eggs cease at the time of pronuclear formation when maturation-promoting factor (MPF) is inactivated, but the Ca(2+) oscillations are ceaseless if eggs are arrested at metaphase by colcemid, which maintains the activity of MPF. To determine the possible role of MPF in regulation of cytoplasmic Ca(2+) excitability, roscovitine, a specific inhibitor of p34(cdc2)/cyclin B kinase, was used to inactivate MPF, and its effect on fertilization-induced Ca(2+) oscillations was investigated. Our results showed that roscovitine at >/= 50 microM suppressed fertilization-induced Ca(2+) oscillations in normal and colcemid-treated metaphase II (MII) eggs after the first 1-2 Ca(2+) spikes. Roscovitine inhibition of fertilization-induced Ca(2+) oscillations could be reversed by extensive washing of the eggs. Histone H1 kinase activity in colcemid-treated MII eggs was similarly inhibited by roscovitine, which suggested that the cessation of fertilization-induced Ca(2+) oscillations is due to the inactivation of MPF. Thimerosal-induced Ca(2+) oscillations in Ca(2+)-, Mg(2+)-free medium was also suppressed by roscovitine, suggesting a general inhibitory effect of roscovitine on Ca(2+) oscillations. The inhibition may be achieved by disruption of Ca(2+) release and refilling of the calcium store. Thapsigargin, an inhibitor of the endoplasmic reticulum Ca-ATPase, induced significantly less Ca(2+) release in roscovitine-treated eggs than in the non-drug-treated eggs. Taken together, our results suggest that MPF plays an important role in regulation of the cytoplasmic Ca(2+) excitability in mouse eggs.

Timing of calcium and protein synthesis requirements for the first mitotic cell cycle in fertilised Xenopus eggs

Mitosis is governed by the activity of the M-phase promoting factor (MPF). In some systems, particularly early embryos, transient increases in calcium concentration have been shown to be necessary for mitosis and regulate its timing. By microinjection of the calcium buffer, dibromoBAPTA, into fertilised Xenopus eggs, we have assessed whether calcium events are required to initiate MPF activation and inactivation. Since initial experiments showed that this buffer inhibited protein synthesis, we measured when mitosis and cleavage became independent of translation. We found that, after a period of protein synthesis essential for cleavage, there was a phase during which continued translation affected the timing of cleavage, but was not essential for its occurrence. Measurement of MPF activity in single embryos injected with calcium buffer at different times in the first cell cycle, showed that there were two sensitive periods. The first period of sensitivity blocked MPF activation and coincided with the time at which cleavage became completely independent of protein synthesis. The second sensitive period occurred just before histone kinase activity peaked, and was necessary for kinase inactivation. Preventing inactivation in this way arrested egg extracts in mitosis. These results support the view that transient increases in free calcium concentration contribute to mitotic progression by first triggering MPF activation and subsequently, with elevated MPF activity, inducing its inactivation.

Differential inactivation of maturation-promoting factor and mitogen-activated protein kinase following parthenogenetic activation of bovine oocytes

Bovine oocytes matured for 24 h (young) or 40 h (aged) were treated with calcium ionophore (A23187) alone or followed with 6-dimethylaminopurine (6-DMAP), a protein phosphorylation inhibitor, and were then assayed for histone H1 kinase and mitogen-activated protein kinase (MAPK) activities. Additionally, the changes in chromatin, meiotic spindle, and microfilament were assessed by immunofluoresence microscopy. In both young and aged oocytes, treatment with 6-DMAP following A23187 treatment abolished the activities of both H1 and MAPKs; the decline of H1 kinase preceded the decline in MAPK activity. However, A23187 treatment alone caused a slower decrease in H1 kinase activity and no evident MAPK alteration in young oocytes. In contrast, activities of both kinases decreased in aged oocytes after A23187 treatment, similar to the response in the combined treatments. The inactivation of MAPK was caused by dephosphorylation of MAP42/extracellular signal-regulated kinase 2 (ERK2) as detected by gel mobility shift in the Western blot assay. A23187 treatment of young oocytes led to chromosome separation and second polar body extrusion, but not pronuclear development, with the majority of the oocytes arrested at a transitional stage of metaphase to anaphase known as metaphase III (MIII). However, most of the A23187-treated aged oocytes developed to the pronuclear stage. When oocytes, regardless of age, were treated by A23187 plus 6-DMAP, bivalent chromosomes were clumped into a single mass, the spindle was disassembled, microtubule networks were distributed in the cytoplasm, and a pronucleus appeared. It is suggested that the decrease in H1 kinase activity is involved in the initiation of oocyte activation, i.e., the exit from metaphase II, whereas the decrease in MAPK activity correlates with onset of pronuclear formation. In conclusion, inactivation of maturation-promoting factor and MAPKs probably occurs via two independent processes, and the inactivation of both kinases is required for the metaphase II oocytes to progress through interphase. High MAPK activity might contribute to spindle stabilization, and inactivation of MAPK is associated with microtubular network formation in the cytoplasm.

Cytostatic activity develops during meiosis I in oocytes of LT/Sv mice

Oocytes of wild-type mice are ovulated as the secondary oocytes arrested at metaphase of the second meiotic division. Their fertilization or parthenogenetic activation triggers the completion of the second meiotic division followed by the first embryonic interphase. Oocytes of the LT/Sv strain of mice are ovulated either at the first meiotic metaphase (M I) as primary oocytes or in the second meiotic metaphase (M II) as secondary oocytes. We show here that during in vitro maturation a high proportion of LT/Sv oocytes progresses normally only until metaphase I. In these oocytes MAP kinase activates shortly after histone H1 kinase (MPF) activation and germinal vesicle breakdown. However, MAP kinase activation is slightly earlier than in oocytes from wild-type F1 (CBA/H x C57Bl/10) mice. The first meiotic spindle of these oocytes forms similarly to wild-type oocytes. During aging, however, it increases in size and finally degenerates. In those oocytes which do not remain in metaphase I the extrusion of first polar bodies is highly delayed and starts about 15 h after germinal vesicle breakdown. Most of the oocytes enter interphase directly after first polar body extrusion. Fusion between metaphase I LT/Sv oocytes and wild-type mitotic one-cell embryos results in prolonged M-phase arrest of hybrids in a proportion similar to control LT/Sv oocytes and control hybrids made by fusion of two M I LT/Sv oocytes. This indicates that LT/Sv oocytes develop cytostatic factor during metaphase I. Eventually, anaphase occurs spontaneously and the hybrids extrude the polar body and form pronuclei in a proportion similar as in controls. In hybrids between LT/Sv metaphase I oocytes and wild-type metaphase II oocytes (which contain cytostatic factor) anaphase I proceeds at the time observed in control LT/Sv oocytes and hybrids between two M I LT/Sv oocytes, and is followed by the parthenogenetic activation and formation of interphase nuclei. Also the great majority of hybrids between M I and M II wild-type oocytes undergoes the anaphase but further arrests in a subsequent M-phase. These observations suggest that an internally triggered anaphase I occurs despite the presence of the cytostatic activity both in LT/Sv and wild-type M I oocytes. Anaphase I triggering mechanism must therefore either inactivate or override the CSF activity. The comparison between spontaneous and induced activation of metaphase I LT/Sv oocytes shows that mechanisms involved in anaphase I triggering are altered in these oocytes. Thus, the prolongation of metaphase I in LT/Sv oocytes seems to be determined by delayed anaphase I triggering and not provoked directly by the cytostatic activity.

Link between fertilization-induced Ca2+ oscillations and relief from metaphase II arrest in mammalian eggs: a model based on calmodulin-dependent kinase II activation

Mammalian eggs are ovulated in metaphase II of meiosis, in a state characterized by high levels of cyclin B and of active maturation promoting factor (MPF). This arrest is mediated by an activity referred to as cytostatic factor (CSF) which prevents the degradation of cyclin. Fertilization triggers a train of Ca2+ spikes which is responsible for the decrease in activity of both MPF and CSF. The decline in MPF however much precedes that in CSF. Experimental observations on mammalian eggs indicate that the kinetics of cell cycle resumption much depends on the temporal pattern of the repetitive Ca2+ spikes. Here, we propose a theoretical model which accounts for Ca(2+)-induced relief from metaphase II arrest in mammalian eggs. The model is based on the fact that Ca2+/calmodulin kinase II (CaMKII) activation is the primary event leading to inactivation of both CSF and MPF. To account for experimental observations, it has to be assumed that CaMKII activation affects the level of the active form of the anaphase promoting complex (APC), which initiates the degradation of cyclin, through two pathways characterized by different time scales. Thus, we hypothesize that CaMKII activation by Ca2+ leads to the transformation of a mediator protein from a form which stimulates the inactivation of the APC into a form which gradually and indirectly induces the deactivation of CSF. In consequence, a sufficient number of Ca2+ spikes first triggers the decrease of MPF, thus allowing the egg to enter in interphase, and later that of CSF. Finally, when CSF is low and when Ca2+ oscillations have stopped, the level of MPF can increase again, a phenomenon that would correspond to the first mitosis. This model also accounts for the observed dependence of the time of entry in interphase (marked by the appearance of the pronuclei) on the frequency of Ca2+ spikes, as well as for the possible entry in metaphase III arrest, a pathological state of the egg which results from an insufficient activation by Ca2+. This study provides some theoretical prediction as to the time of the first mitosis as a function of the temporal pattern of Ca2+ oscillations.

Role of calcium oscillations in mammalian egg activation: experimental approach

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

Protein kinase C is required for the disappearance of MPF upon artificial activation in mouse eggs

The aim of the present study was to investigate the implication of protein kinase C (PKC) in the mouse egg activation process. We used OAG (1-oleoyl-2-acetyl-sn-glycerol) as a PKC activator, calphostin C as a specific PKC inhibitor, and the calcium ionophore A23187 as a standard parthenogenetic agent. The exposure of zona-free eggs to 150 microM or 50 microM OAG for 10 min resulted in meiosis II completion in approximately 80% of instances. By contrast, at a lower concentration (25 microM), the PKC stimulator was ineffective as parthenogenetic agent. Shortly after the application of 150 microM OAG, the cytosolic Ca2+ concentration ([Ca2+]i) increased transiently in all the eggs examined, whereas after the addition of 50 microM OAG, [Ca2+]i remained unchanged for at least 20 min. During this period, the activity of M-phase promoting factor (MPF) dramatically decreased and most of the eggs entered anaphase except when the PKC was inhibited by calphostin C. Similarly, MPF inactivation and meiosis resumption were prevented in calphostin C-loaded eggs following treatment with A23187, even though the ionophore-induced Ca2+ signalling was not affected. Taken together, our results indicate that stimulation of PKC is a sufficient and necessary event to induce meiosis resumption in mouse eggs and strongly suggest that, in this species, the mechanism by which a transient calcium burst triggers MPF inactivation involves a PKC-dependent pathway.

Nuclear transplantation in mammals: remodelling of transplanted nuclei under the influence of maturation promoting factor

Whilst the role of Maturation or M-phase Promoting Factor (MPF) as a universal M-phase regulator is well documented, much less attention has been paid to its role in nuclear transplantation experiments and especially to its influence upon remodelling of transplanted nuclei. There is currently wide acceptance that successful nuclear transplantation using differentiated nuclei is possible only in a cytoplasmic environment that is capable of inducing rapid nuclear de-differentiation to a pronuclear-like form. In this review our purpose is firstly, to outline the conditions under which such remodelling can be induced, and secondly, to extend the debate to include a consideration of whether complete nuclear remodelling is an absolute necessity for clonal development.

Factors affecting the role of the spindle during early response of mouse oocytes to ethanol stimulation

Winston et al. ([1995] J. Cell Sci., 108:143-151) have shown recently that short (6 min) exposure of spindle intact oocytes from Swiss mice to 8% ethanol induced activation of most oocytes, while disruption of the spindles in these oocytes by nocodazole, before and during ethanol exposure, completely inhibited oocyte activation. We compared the activation rates (ARs) of nocodazole-treated and intact oocytes recovered from SJL and B6D2 F1 hybrid mice under the same experimental conditions. The difference between the ARs of nocodazole-treated and intact SLJ oocytes was about the same as reported for Swiss oocytes (2% vs. 82%, respectively). In contrast, this difference was minor for B6D2 oocytes (87% vs. 100%, respectively). Moreover, 41% of these oocytes underwent activation when the spindle was absent, not only before and during, but also 2 h after ethanol exposure. Shortened exposure (2 min) of B6D2 oocytes to ethanol, however, increased the difference in the ARs of nocodazole-treated and intact oocytes (18% vs. 67%, respectively). We conclude that at least two parameters affect the necessity of the presence of the spindle during ethanol exposure for the activation of mouse oocytes. They are the genotype of the oocytes and the duration of exposure to ethanol. Under one set of these parameters the presence of the spindle is absolutely necessary, while under the other the appearance of the spindle a few hours after ethanol exposure is sufficient to allow the activation of some oocytes.

Maturation promoting factor in ascidian oocytes is regulated by different intracellular signals at meiosis I and II

Using the fluorescent dye Calcium Green-dextran, we measured intracellular Ca2+ in oocytes of the ascidian Ciona intestinalis at fertilization and during progression through meiosis. The relative fluorescence intensity increased shortly after insemination in a single transient, the activation peak, and this was followed by several smaller oscillations that lasted for approximately 5 minutes (phase 1). The first polar body was extruded after the completion of the phase 1 transients, about 9 minutes after insemination, and then the intracellular calcium level remained at baseline for a period of 5 minutes (phase 2). At 14 minutes postinsemination a second series of oscillations was initiated that lasted 11 minutes (phase 3) and terminated at the time of second polar body extrusion. Phases 1 and 3 were inhibited by preloading oocytes with 5 mM heparin. Simultaneous measurements of membrane currents, in the whole-cell clamp configuration, showed that the 1–2 nA inward fertilization current correlated temporally with the activation peak, while a series of smaller oscillations of 0.1-0.3 nA amplitude were generated at the time of the phase 3 oscillations. Biochemical characterization of Maturation Promoting Factor (MPF) in ascidian oocytes led to the identification of a Cdc2-like kinase activity. Using p13suc1-sepharose as a reagent to precipitate the MPF complex, a 67 kDa (67 × 10(3) Mr) protein was identified as cyclin B. Histone H1 kinase activity was high at metaphase I and decreased within 5 minutes of insemination reaching a minimum level during phase 2, corresponding to telophase I. During phase 3, H1 kinase activity increased and then decayed again during telophase II. Oocytes preloaded with BAPTA and subsequently inseminated did not generate any calcium transients, nonetheless H1 kinase activity decreased 5 minutes after insemination, as in the controls, and remained low for at least 30 minutes. Injection of BAPTA during phase 2 suppressed the phase 3 calcium transients, and inhibited both the increase in H1 kinase activity normally encountered at metaphase II and second polar body extrusion.

Cycloheximide-induced activation of mouse eggs: effects on cdc2/cyclin B and MAP kinase activities

Fertilization of metaphase II-arrested mouse eggs results in resumption of meiosis and a decrease in both cdc2/cyclin B kinase and MAP kinase activities; the decrease in cdc2/cyclin B kinase activity precedes the decrease in MAP kinase activity. Cycloheximide treatment of metaphase II-arrested mouse eggs also results in resumption of meiosis but bypasses the fertilization-induced Ca2+ transient. However, it is not known if cycloheximide treatment results in the same temporal changes in cdc2/cyclin B kinase and MAP kinase activities that are intimately associated with resumption of meiosis. We report that cycloheximide-treated mouse eggs manifest similar temporal changes in the decrease in both cdc2/cyclin B kinase and MAP kinase activities that occur following fertilization, although cortical granule exocytosis is not stimulated. The decrease in cdc2/cyclin B kinase activity, however, does not seem to be required for the decrease in MAP kinase activity, since the decrease in MAP kinase activity still occurs in cycloheximide-treated eggs that are also incubated in the presence of nocodazole, which inhibits cyclin B degradation and hence the decrease in cdc2/cyclin B kinase. Following removal of these drugs, cdc2/cyclin B kinase activity remains high, MAP kinase activity increases to levels similar to that in the metaphase II-arrested eggs, and a spindle(s) forms with the chromosomes aligned on a metaphase plate. Results of these experiments suggest that some other protein with a relatively short half-life, e.g. cmos, a known upstream activator of MAP kinase, may be responsible for events leading to the decrease in MAP kinase activity.