Potential role of mitogen-activated protein kinase in pronuclear envelope assembly and disassembly following fertilization of mouse eggs

Developmental trajectory of monopronucleated zygotes after in vitro fertilization when they include both male and female genomes

OBJECTIVE

To examine the cause of monopronucleated zygote (1PN) formation that includes both maternal and paternal genomes.

DESIGN

Retrospective cohort study.

SETTING

Private fertility clinic.

PATIENT(S)

A total of 44 1PN and 726 2-pronuclear zygotes from 702 patients were observed using 2 different time-lapse observation systems.

INTERVENTION(S)

Previously recorded time lapse data were reviewed to examine the mechanism of 1PN formation.

MAIN OUTCOME MEASURE(S)

The distance between the position of the second polar body extrusion and the fertilization cone or epicenter/starting position of the cytoplasmic wave was measured, and the consequent data were analyzed. Cytoplasmic waves were confirmed using vector analysis software.

RESULT(S)

The cut-off value for the difference in the distance between the position of the second polar body extrusion and the fertilization cone or the epicenter/starting position of the cytoplasmic wave was 17 μm (AUC: 0.987, 95% CI: 0.976-0.999) for the Embryo Scope and 18 μm (AUC: 0.972, 95% CI: 0.955-0.988) for the iBIS time-lapse observation systems.

CONCLUSION(S)

In this study, it was found with a high degree of accuracy that a monopronucleus is formed when the fusion of the sperm takes place within 18 μm from the point of the second polar body extrusion. The theoretical chance of 1PN occurrence after in vitro fertilization is 2.7% when the sperm is considered to be fused anywhere in the plasma membrane of an oocyte.

Initiation of cell volume regulation and unique cell volume regulatory mechanisms in mammalian oocytes and embryos

The period beginning with the signal for ovulation, when a fully-grown oocyte progresses through meiosis to become a mature egg that is fertilized and develops as a preimplantation embryo, is crucial for healthy development. The early preimplantation embryo is unusually sensitive to cell volume perturbations, with even moderate decreases in volume or dysregulation of volume-regulatory mechanisms resulting in developmental arrest. To prevent this, early embryos possess mechanisms of cell volume control that are apparently unique to them. These rely on the accumulation of glycine and betaine (N, N, N-trimethylglycine) as organic osmolytes-compounds that can provide intracellular osmotic support without the deleterious effects of inorganic ions. Preimplantation embryos also have the same mechanisms as somatic cells that mediate rapid responses to deviations in cell volume, which rely on inorganic ion transport. Both the unique, embryo-specific mechanisms that use glycine and betaine and the inorganic ion-dependent mechanisms undergo major changes during meiotic maturation and preimplantation development. The most profound changes occur immediately after ovulation is triggered. Before this, oocytes cannot regulate their volume, since they are strongly attached to their rigid extracellular matrix shell, the zona pellucida. After ovulation is triggered, the oocyte detaches from the zona pellucida and first becomes capable of independent volume regulation. A complex set of developmental changes in each cell volume-regulatory mechanism continues through egg maturation and preimplantation development. The unique cell volume-regulatory mechanisms in eggs and preimplantation embryos and the developmental changes they undergo appear critical for normal healthy embryo development.

Calcineurin role in porcine oocyte activation

Calcineurin is required for oocyte exit from meiotic block in metaphase II (MII) stage in invertebrates and also in lower vertebrates. However, the role of calcineurin in mammalian oocyte activation is still unclear. The aim of this study was to determine whether calcineurin is involved in the processes regulating porcine oocyte activation. Indirect immunofluorescence demonstrated localization of both calcineurin subunits, CnA and CnB, especially in the cortex area of MII oocytes, in vitro fertilized and also parthenogenetically activated oocytes. After activation, the fluorescence intensity of the protein in the cortex area of oocytes remains unchanged; the protein calcineurin in the cytoplasm was recorded mainly around the pronuclei. Treatment of matured oocytes with calcineurin inhibitors, cyclosporin A (CsA) and hymenistatin I (HS-I), followed by activation with calcium ionophore A23187, significantly decreased the rate of activated oocytes compared to oocytes that were treated only with calcium ionophore (Ca-Io), (CsA+Ca-Io 25.0% v. Ca-Io 83.3%; HS-I+Ca-Io 32.5% v. Ca-Io 85.0%). Compared to the control, CsA treatment of matured oocytes followed by activation with Ca-Io did not affect the activity level of metaphase-promoting factor (MPF) and mitogen-activated protein kinase (MAPK) in activated oocytes evaluated by kinase activity assay. Simultaneous staining of calcineurin and cortical granule content in matured oocytes showed that calcineurin distributed in the cortical area of the oocyte has not been colocalized with cortical granules content. On the other hand, the calcineurin inhibition before parthenogenetic activation leads to a reduction of the cortical reaction level compared to oocytes that were not treated with CsA (complete exocytosis: CsA+Ca-Io 2.6% v. Ca-Io 83.9%; sum of cortical granule brightness: CsA + Ca-Io 0.69 v. Ca-Io 0.15). Our results showed that calcineurin is involved in the process of pig oocyte activation and cortical granule exocytosis; however this regulation seems to be MPF and MAPK independent.

Initiation and organization of events during the first cell cycle in mammals: applications in cloning

The technology of cloning involves transplanting a diploid nucleus into a mature oocyte cytoplast. The cytoplast is then activated to initiate the first cell cycle of development as a nuclear transplant embryo. Initiation and regulation of events during the first cell cycle are, therefore, critical for proper reprogramming of the donor nucleus and development as a cloned embryo. Activation is normally induced by the sperm and is mediated by a series of intracellular free calcium ([Ca(2+)](i)) oscillations that last for several hours. Although it is not known precisely how the sperm induces activation, current evidence favors the delivery, by the sperm, of a soluble protein factor that causes the production of IP3. IP3 acts to open a Ca(2+) channel in the endoplasmic reticulum and release Ca(2+) into the cytosol. A variety of methods have been used to duplicate or replace the sperm-induced [Ca(2+)](i) increase to cause activation in nuclear transplant embryos. It has been found that treatments that cause a single transient [Ca(2+)](i) activate some oocytes with the level of activation increasing as the oocyte ages. Attempts have been made to extend the period of time over which [Ca(2+)](i) oscillations occur. This has been successful in increasing activation rates of less mature oocytes but the techniques are still cumbersome. An alternative method, that has been very successful, is the combination of a treatment that elevates [Ca(2+)](i) and a treatment that maintains low levels of maturation promoting factor for several hours after the initial [Ca(2+)](i) elevation. The sperm also contributes the centrosome that organizes microtubules during the first cell cycle. One current hypothesis for regulation of sperm centrosomal activity consists of a dephosphorylation of sperm connecting piece proteins following sperm entry into the oocyte and activation of the oocyte. Dephosphorylation of these proteins results in the disassembly of the connecting piece and assembly of a functional centrosome. In nuclear transfer, centrosomal components are contributed by the donor cell. If the cell is fused to the cytoplast before centriole replication then a single aster forms. If the cell is fused after centriole replication then two asters form. In either case and even in parthenogenetic oocytes, which do not have centrioles, the first cell cycle progresses to metaphase. However, progress is slow and some defects are observed in the assembly of chromosomes into a metaphase plate.

MAPK3/1 (ERK1/2) and Myosin Light Chain Kinase in Mammalian Eggs Affect Myosin-II Function and Regulate the Metaphase II State in a Calcium- and Zinc-Dependent Manner

Vertebrate eggs are arrested at metaphase of meiosis II, a state classically known as cytostatic factor arrest. Maintenance of this arrest until the time of fertilization and then fertilization-induced exit from metaphase II are crucial for reproductive success. Another key aspect of this meiotic arrest and exit is regulation of the metaphase II spindle, which must be appropriately localized adjacent to the egg cortex during metaphase II and then progress into successful asymmetric cytokinesis to produce the second polar body. This study examined the mitogen-activated protein kinases MAPK3 and MAPK1 (also known as ERK1/2) as regulators of these two related aspects of mammalian egg biology, specifically testing whether this MAPK pathway affected myosin-II function and whether myosin-II perturbation would produce some of the same effects as MAPK pathway perturbation. Inhibition of the MEK1/2-MAPK pathway with U0126 leads to reduced levels of phosphorylated myosin-regulatory light chain (pMRLC) and causes a reduction in cortical tension, effects that are mimicked by treatment with the myosin light chain kinase (MLCK) inhibitor ML-7. These data indicate that one mechanism by which the MAPK pathway acts in eggs is by affecting myosin-II function. We further show that MAPK or MLCK inhibition induces loss of normal cortical spindle localization or parthenogenetic egg activation. This parthenogenesis is dependent on cytosolic and extracellular calcium and can be rescued by hyperloading eggs with zinc, suggesting that these effects of inhibition of MLCK or the MAPK pathway are linked with dysregulation of ion homeostasis.

Post-ovulatory aging of oocytes disrupts kinase signaling pathways and lysosome biogenesis

Post-ovulatory aging of oocytes results in the progressive loss of fertilization and developmental competence. This degradation of oocyte quality has been the object of numerous investigations, primarily focused on individual signaling pathways which provide limited insight into the status of global signaling events. The purpose of the present investigation was to comprehensively assess broad patterns of signaling pathway activity during in vitro aging as an initial step in defining control points that can be targeted to prevent the reduction in oocyte quality during prolonged culture. An antibody microarray-based phospho-proteome analysis performed on oocytes before and after eight hours of culture revealed significant changes in the abundance or activation state of 43 proteins that function in a wide variety of protein kinase-mediated signaling pathways. Several of the most significantly affected kinases were studied by Western blot and confocal immunofluorescence to corroborate the array results. Prolonged culture resulted in global changes in the abundance and activity of protein kinases that regulate the response to calcium, stress, and cell-cycle control. Examination of intracellular structures revealed a previously unrecognized increase in the abundance of large autophogagic lysosomes, which correlates with changes in protein kinase pathways. These results provide insight into the stresses experienced by oocytes during culture and the diversity of responses that results from them. The observed increase in autophagy-related activity, together with the disruptions in calcium signaling, cell-cycle, and stress-response pathways, have the potential to negatively impact oocyte quality by interfering with the normal sequence of biochemical changes that constitute egg activation following fertilization.

The dynamics of MAPK inactivation at fertilization in mouse eggs

Egg activation at fertilization in mammals is initiated by prolonged Ca(2+) oscillations that trigger the completion of meiosis and formation of pronuclei. A fall in mitogen-activated protein kinase (MAPK) activity is essential for pronuclear formation, but the precise timing and mechanism of decline are unknown. Here, we have measured the dynamics of MAPK pathway inactivation during fertilization of mouse eggs using novel chemiluminescent MAPK activity reporters. This reveals that the MAPK activity decrease begins during the Ca(2+) oscillations, but MAPK does not completely inactivate until after pronuclear formation. The MAPKs present in eggs are Mos, MAP2K1 and MAP2K2 (MEK1 and MEK2, respectively) and MAPK3 and MAPK1 (ERK1 and ERK2, respectively). Notably, the MAPK activity decline at fertilization is not explained by upstream destruction of Mos, because a decrease in the signal from a Mos-luciferase reporter is not associated with egg activation. Furthermore, Mos overexpression does not affect the timing of MAPK inactivation or pronuclear formation. However, the late decrease in MAPK could be rapidly reversed by the protein phosphatase inhibitor, okadaic acid. These data suggest that the completion of meiosis in mouse zygotes is driven by an increased phosphatase activity and not by a decline in Mos levels or MEK activity.

SRC-family tyrosine kinases in oogenesis, oocyte maturation and fertilization: an evolutionary perspective

The oocyte is a highly specialized cell poised to respond to fertilization with a unique set of actions needed to recognize and incorporate a single sperm, complete meiosis, reprogram maternal and paternal genomes and assemble them into a unique zygotic genome, and finally initiate the mitotic cell cycle. Oocytes accomplish this diverse series of events through an array of signal transduction pathway components that include a characteristic collection of protein tyrosine kinases. The src-family protein kinases (SFKs) figure importantly in this signaling array and oocytes characteristically express certain SFKs at high levels to provide for the unique actions that the oocyte must perform. The SFKs typically exhibit a distinct pattern of subcellular localization in oocytes and perform critical functions in different subcellular compartments at different steps during oocyte maturation and fertilization. While many aspects of SFK signaling are conserved among oocytes from different species, significant differences exist in the extent to which src-family-mediated pathways are used by oocytes from species that fertilize externally vs those which are fertilized internally. The observation that several oocyte functions which require SFK signaling appear to represent common points of failure during assisted reproductive techniques in humans, highlights the importance of these signaling pathways for human reproductive health.

Inactivation of mitogen-activated protein kinase is neither necessary nor sufficient for the onset of pronuclear formation in mouse oocytes

Mammalian oocytes are arrested at metaphase II due to high MAP kinase activity. After fertilization, oocytes resume meiosis, leading to female chromosome segregation, polar body emission and pronuclear (PN) formation. Previous biochemical studies showed that MAP kinase activity remained high for several hours after fertilization and began to decrease in parallel with PN formation. It has been thought that MAP kinase activity is incompatible with PN formation, and its inactivation is required for the initiation of PN formation in mammalian oocytes. In this study, we revisited this hypothesis by examining MAP kinase activity and PN formation in individual mouse oocytes using cytological analysis. We showed that MAP kinase activity in oocytes could be evaluated using phospho-ERK1/2 immunofluorescent staining. Co-immunofluorescent staining of phospho-ERK1/2 and nuclear pore components showed that PN formation preceded MAP kinase inactivation and could be initiated while MAP kinase activity was still high. Moreover, artificial inactivation of MAP kinase or its downstream target, ribosomal S6 kinase, accelerated but did not rapidly induce PN formation. Our results show that although the MAP kinase pathway negatively regulates PN formation, its inactivation is neither necessary nor sufficient for PN formation. These results suggest the involvement of other essential factor(s) in this process.

Regulation of fusion of the nucleolar precursor bodies following activation of mouse oocytes: roles of the maturation-promoting factors and mitogen-activated protein kinases

Fusion of nucleoli or nucleolus precursor bodies (NPBs) has been observed during somatic cell interphase and pronuclear development of human zygotes; however, the underlying mechanism is unknown. NPB fusion and its regulation by mitogen-activated protein kinase (MAPK) and maturation-promoting factor (MPF) were studied in activated mouse oocytes. Small NPBs appeared about 4 h after ethanol activation, and took about 1.5 h to fuse into a large NPB, which persisted for about 10 h before disappearance. Analysis of the temporal windows for kinase action indicated that a high MAPK activity during the first 2 h and a low MPF activity during the first 3-4 h after activation were essential for subsequent NPB fusion. A preactivation decline in MAPK activity was associated with decreased NPB fusion following activation of aged oocytes. While MAPK inactivation by regulator U0126 prevented NPB fusion in oocytes activated by ethanol or 5 min Sr2+ treatments, it had no effect on oocytes fertilized or activated by 6 h Sr2+ treatment. In most cases, while rates of pronuclear formation did not differ, rates of NPB fusion differed significantly between different treatments. Our results suggest that: (i) the MAPK and MPF activities at the initial stage of activation regulate NPB fusion after pronuclear formation; (ii) pronuclear assembly and NPB fusion are two separable events that might be controlled by different mechanisms; and (iii) high MAPK activity and low MPF activity at the initial stage of activation is essential for NPB fusion when only one calcium rise is induced by ethanol, while inhibition of MAPK activity does not affect NPB fusion when the repetitive intracellular Ca2+ rises are induced after fertilization.

Essential role of protein phosphatase 2A in metaphase II arrest and activation of mouse eggs shown by okadaic acid, dominant negative protein phosphatase 2A, and FTY720

Vertebrate eggs arrest at second meiotic metaphase. The fertilizing sperm causes meiotic exit through Ca(2+)-mediated activation of the anaphase-promoting complex/cyclosome (APC/C). Although the loss in activity of the M-phase kinase CDK1 is known to be an essential downstream event of this process, the contribution of phosphatases to arrest and meiotic resumption is less apparent, especially in mammals. Therefore, we explored the role of protein phosphatase 2A (PP2A) in mouse eggs using pharmacological inhibition and activation as well as a functionally dominant-negative catalytic PP2A subunit (dn-PP2Ac-L199P) coupled with live cell imaging. We observed that PP2A inhibition using okadaic acid induced events normally observed at fertilization: degradation of the APC/C substrates cyclin B1 and securin resulting from loss of the APC/C inhibitor Emi2. Although sister chromatids separated, chromatin remained condensed, and polar body extrusion was blocked as a result of a rapid spindle disruption, which could be ameliorated by non-degradable cyclin B1, suggesting that spindle integrity was affected by CDK1 loss. Similar cell cycle effects to okadaic acid were also observed using dominant-negative PP2Ac. Preincubation of eggs with the PP2A activator FTY720 could block many of the actions of okadaic acid, including Emi2, cyclin B1, and securin degradation and sister chromatid separation. Therefore, in conclusion, we used okadaic acid, dn-PP2Ac-L199P, and FTY720 on mouse eggs to demonstrate that PP2A is needed to for both continued metaphase arrest and successful exit from meiosis.

Mos limits the number of meiotic divisions in urochordate eggs

Mos kinase is a universal mediator of oocyte meiotic maturation and is produced during oogenesis and destroyed after fertilization. The hallmark of maternal meiosis is that two successive M phases (meiosis I and II) drive two rounds of asymmetric cell division (ACD). However, how the egg limits the number of meioses to just two, thereby preventing gross aneuploidy, is poorly characterized. Here, in urochordate eggs, we show that loss of Mos/MAPK activity is necessary to prevent entry into meiosis III. Remarkably, maintaining the Mos/MAPK pathway active after fertilization at near physiological levels induces additional rounds of meiotic M phase (meiosis III, IV and V). During these additional rounds of meiosis, the spindle is positioned asymmetrically resulting in further rounds of ACD. In addition, inhibiting meiotic exit with Mos prevents pronuclear formation, cyclin A accumulation and maintains sperm-triggered Ca2+ oscillations, all of which are hallmarks of the meiotic cell cycle in ascidians. It will be interesting to determine whether Mos availability in mammals can also control the number of meioses as it does in the urochordates. Our results demonstrate the power of urochordate eggs as a model to dissect the egg-to-embryo transition.

Okadaic acid-sensitive phosphatase is related to MII/G1 transition in mouse oocytes

It is reported that okadaic acid (OA)-sensitive phosphatase is related to mitogen-activated protein kinase (MAPK)/p90rsk activation in mammalian oocytes. OA is also involved in the positive feedback loop between M phase-promoting factor (MPF) and cdc25c in Xenopus oocytes during meiotic maturation. However, the effect of phosphatase inhibition by OA on MPF and MAPK activities at the MII/G1 in oocytes remains unknown. The aim of this study is to clarify the relationship between OA-sensitive phosphatase and mitosis MII/G1 transition in mouse oocytes. MII-arrested oocytes were, isolated from mice, inseminated and cultured in TYH medium (control group) or TYH medium supplemented with 2.5 μM of OA (OA group). Histone H1 kinase and myelin basic protein (MBP) kinase activities were measured as indicators of MPF and p42 MAPK activities after insemination. Phosphorylation of cdc25c after insemination was analized in OA and control group by western blotting. Seven hours after insemination a pronucleus (PN) was formed in 84.1% (69/85) of oocytes in the control group. However, no PN was formed in oocytes of the OA group (p < 0.001). Although MPF and MAPK activities in the control group significantly decreased at 3, 4, 5, and 7 h after insemination, these decreases were significantly inhibited by OA addition (p < 0.05). Furthermore, OA addition prevented cdc25c dephosphorylation 7 h after insemination. In conclusion, OA-sensitive phosphatase correlates with inactivation of MPF and MAPK, and with the dephosphorylation of cdc25c at the MII/G1 transition in mouse oocytes.

Cell cycle in ascidian eggs and embryos

In ascidians the cell cycle machinery has been studied mainly in oocytes while ascidian embryos have been used to dissect the mechanism that controls asymmetric cell division (ACD). Here we overview the most specific and often exceptional points and events in cell cycle control in ascidian oocytes and early embryos. Mature stage IV eggs are arrested at metaphase I due to cytostatic factor (CSF). In vertebrates, unfertilized eggs are arrested at metaphase II by CSF. Meta II-CSF is mediated by the Mos/MEK/MAPK/Erp1 pathway, which inhibits the ubiquitin ligase APC/C(cdc20) preventing cyclin B destruction thus stabilizing MPF activity. CSF is inactivated by the fertilization Ca(2+) transient that stimulates the destruction of Erp1 thus releasing APC/C(cdc20) from inhibition. Although many of the components of CSF are conserved between the ascidian and the vertebrates, the lack of Erp1 in the ascidians (and indeed other invertebrates) is notable since the Mos/MAPK pathway nonetheless mediates Meta I-CSF. Moreover, since the fertilization Ca(2+) transient targets Erp1, it is not clear how the sperm-triggered Ca(2+) transient in ascidians (and again other invertebrates) stimulates cyclin B destruction in the absence of Erp1. Nonetheless, like mammalian eggs, sperm trigger a series of Ca(2+) oscillations that increases the rate of cyclin B destruction and the subsequent loss of MAPK activity leading to meiotic exit in ascidians. Positive feedback from MPF maintains the Ca(2+) oscillations in fertilized ascidian eggs ensuring the eventual loss of MPF stimulating the egg-to-embryo transition. Embryonic cell cycles in the ascidian are highly stereotyped where both the rate of cell division and the orientation of cell division planes are precisely controlled. Three successive rounds of ACD generate two small posterior germ cell precursors at the 64 cell stage. The centrosome-attracting body (CAB) is a macroscopic cortical structure visible by light microscopy that causes these three rounds of ACD. Entry into mitosis activates the CAB causing the whole mitotic spindle to rotate and migrate toward the cortical CAB leading to a highly ACD whereby one small cell is formed that inherits the CAB and approximately 40 maternal postplasmic/PEM RNAs including the germ cell marker vasa.

A-type lamin dynamics in bovine somatic cell nuclear transfer embryos

The persistence of A-type nuclear lamin in somatic cell nuclear transfer (SCNT) embryos has been proposed as a marker for incomplete nuclear reprogramming. Using monoclonal antibodies to A/C- (A/C-346 and A/C-131C3) and B-type lamin, we compared distribution during early development of bovine IVF, parthenogenetic and SCNT embryos. A/C-346 staining was observed in the pronuclei of IVF embryos and in nuclei at the two-cell stage, but was not detected in subsequent cleavage stages up to and including hatched blastocysts. In contrast, A/C-131C3 and anti-lamin B2 stained all preimplantation stage embryos. Parthenogenetic and SCNT embryos had similar staining patterns to IVF embryos for all three antibodies, demonstrating correct nuclear architecture reprogramming. Inhibiting protein synthesis with cycloheximide (CHX) in parthenogenetic and SCNT embryos did not affect lamin A/C localisation, suggesting that lamin A/C is maternal in origin. However, activation with CHX delayed lamin A/C incorporation compared with 6-dimethylaminopurine activation. In SCNT embryos, staining for both A/C- and B-type lamin was delayed compared with parthenotes, although lamin B2 incorporation preceded lamin A/C in both. In conclusion, the lamin A/C distribution in SCNT bovine embryos paralleled that of IVF and parthenogenetic controls and therefore is not a marker of incomplete reprogramming.

HCO3(-)/Cl(-) exchange inactivation and reactivation during mouse oocyte meiosis correlates with MEK/MAPK-regulated Ae2 plasma membrane localization

Background

Germinal Vesicle (GV) stage mouse oocytes in first meiotic prophase exhibit highly active HCO₃⁻/Cl⁻ exchange—a class of transport nearly ubiquitously involved in regulation of intracellular pH and cell volume. During meiosis, however, oocyte HCO₃⁻/Cl⁻ exchange becomes inactivated during first metaphase (MI), remains inactive in second metaphase (MII), and is reactivated only after egg activation. Previous work using pharmacological manipulations had indicated that activity of the MEK/MAPK signaling pathway was negatively correlated with HCO₃⁻/Cl⁻ exchange activity during meiosis. However, the mechanism by which the exchanger is inactivated during meiotic progression had not been determined, nor had the role of MEK/MAPK been directly established.

Methodology/Principal Findings

Expression of a constitutively active form of MEK (MAP kinase kinase), which prevented the normal downregulation of MAPK after egg activation, also prevented reactivation of HCO₃⁻/Cl⁻ exchange. Conversely, suppression of endogenous MAPK activity with dominant negative MEK activated the normally quiescent HCO₃⁻/Cl⁻ exchange in mature MII eggs. A GFP-tagged form of the HCO₃⁻/Cl⁻ exchanger isoform Ae2 (Slc4a2) was strongly expressed at the GV oocyte plasma membrane, but membrane localization decreased markedly during meiotic progression. A similar pattern for endogenous Ae2 was confirmed by immunocytochemistry. The loss of membrane-localized Ae2 appeared selective, since membrane localization of a GFP-tagged human dopamine D1 receptor did not change during meiotic maturation.

Conclusions

Direct manipulation of MAPK activity indicated that GFP-tagged Ae2 localization depended upon MAPK activity. Inactivation of HCO₃⁻/Cl⁻ exchange during the meiotic cell cycle may therefore reflect the loss of Ae2 from the oocyte plasma membrane, downstream of MEK/MAPK signaling. This identifies a novel role for MEK/MAPK-mediated cytostatic factor (CSF) activity during meiosis in membrane protein trafficking in mouse oocytes, and shows for the first time that selective retrieval of membrane proteins is a feature of meiosis in mammalian oocytes.

Regulation of intracellular pH during oocyte growth and maturation in mammals

Regulation of intracellular pH (pH(i)) is a fundamental homeostatic process essential for the survival and proliferation of virtually all cell types. The mammalian preimplantation embryo, for example, possesses Na(+)/H(+) and HCO(3)(-)/Cl(-) exchangers that robustly regulate against acidosis and alkalosis respectively. Inhibition of these transporters prevents pH corrections and, perhaps unsurprisingly, leads to impaired embryogenesis. However, recent studies have revealed that the role and regulation of pH(i) is somewhat more complex in the case of the developing and maturing oocyte. Small meiotically incompetent growing oocytes are apparently incapable of regulating their own pH(i), and instead rely upon the surrounding granulosa cells to correct ooplasmic pH, until such a time that the oocyte has developed the capacity to regulate its own pH(i). Later, during meiotic maturation, pH(i)-regulating activities that were developed during growth are inactivated, apparently under the control of MAPK signalling, until the oocyte is successfully fertilized. Here, we will discuss pH homeostasis in early mammalian development, focussing on recent developments highlighting the unusual and unexpected scenario of pH regulation during oocyte growth and maturation.

Enhanced developmental potential of heat-shocked porcine parthenogenetic embryos is related to accelerated mitogen-activated protein kinase dephosphorylation

Recently, we demonstrated that a 9-h heat shock of 42°C can have marked stimulatory effects on porcine parthenogenetic embryo development if applied immediately after oocyte activation. Developmental discrepancies between heat-shocked (HS) and non-HS embryos were manifest as early as 3 h after activation, suggesting involvement of maturation promoting factor (MPF) and/or mitogen-activated protein kinase (MAPK). Analysis of cdc2 kinase activity showed that MPF inactivation occurred at similar rates in HS and control embryos upon oocyte activation. However, MAPK dephosphorylation was accelerated in HS embryos compared with controls. Okadaic acid, a protein phosphatase inhibitor, maintained MAPK activity at high levels in both non-HS and HS embryos and sensitised HS embryos to the effects of elevated temperatures. No increase in heat shock proteins was observed in pronuclear-stage HS embryos. These data suggest that the acceleration of development observed in HS porcine parthenogenetic embryos is associated with a precocious inactivation of the MAPK signalling cascade. The faster cleavage divisions observed in HS embryos may be linked physiologically to their enhanced developmental potential in vitro.

Second meiotic spindle integrity requires MEK/MAP kinase activity in mouse eggs

ERK-type MAP kinase activity is required for normal first meiotic (MI) metaphase spindle dynamics and first polar body formation at the MI/MII transition, and for MII arrest until egg activation. MEK and MAPK, however, remain active until meiosis is completed and pronuclei form, but whether MEK/MAPK activity affects MII spindle function during egg activation has been unknown. Polarized light microscopy revealed that the MII spindle rapidly (within approximately 15 min) lost birefringence upon treatment of the egg with U0126, indicating decreased organization at the molecular level upon MEK inhibition. In contrast, birefringence rapidly increased when MPF was inhibited with roscovitine, and this was similar to the increased birefringence previously shown after fertilization or parthenogenetic activation with Sr(2+). Confocal microscopy indicated that many spindles in U0126-activated eggs had failed to rotate or were dissociated from the egg cortex. Subsequently, abnormally-located midbodies were evident in U0126-induced parthenogenotes. Thus, MEK/MAPK activity is required to maintain the ordered structure of the MII spindle and for normal spindle dynamics during second polar body formation.

CDC2A (CDK1)-mediated phosphorylation of MSY2 triggers maternal mRNA degradation during mouse oocyte maturation

Degradation of maternal mRNA is thought to be essential to undergo the maternal-to-embryonic transition. Messenger RNA is extremely stable during oocyte growth in mouse and MSY2, an abundant germ cell-specific RNA-binding protein, likely serves as a mediator of global mRNA stability. Oocyte maturation, however, triggers an abrupt transition in which most mRNAs are significantly degraded. We report that CDC2A (CDK1)-mediated phosphorylation of MSY2 triggers this transition. Injecting Cdc2a mRNA, which activates CDC2A, overcomes milrinone-mediated inhibition of oocyte maturation, induces MSY2 phosphorylation and the maturation-associated degradation of mRNAs. Inhibiting CDC2A following its activation with roscovitine inhibits MSY2 phosphorylation and prevents mRNA degradation. Expressing non-phosphorylatable dominant-negative forms of MSY2 inhibits the maturation-associated decrease in mRNAs, whereas expressing constitutively active forms induces mRNA degradation in the absence of maturation and phosphorylation of endogenous MSY2. A positive-feedback loop of CDK1-mediated phosphorylation of MSY2 that leads to degradation of Msy2 mRNA that in turn leads to a decrease in MSY2 protein may ensure that the transition is irreversible.

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

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

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

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

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

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

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.

The presence of X- and Y-chromosomes in oocytes leads to impairment in the progression of the second meiotic division

The oocytes of B6.Y(TIR) sex-reversed female mice can be fertilized but the resultant embryos die at early cleavage stages. In the present study, we examined chromosome segregation at meiotic divisions in the oocytes of XY female mice, compared to those of XX littermates. The timing and frequency of oocyte maturation in culture were comparable between the oocytes from both types of females. At the first meiotic division, the X- and Y-chromosomes segregated independently and were retained in oocytes at equal frequencies. However, more oocytes retained the correct number of chromosomes than anticipated from random segregation. The oocytes that had reached MII-stage were activated by fertilization or incubation with SrCl(2). As expected, the majority of oocytes from XX females completed the second meiotic division and reached the 2-cell stage in 24 h. By contrast, more than half of oocytes from XY females initially remained at the MII-stage while the rest precociously entered interphase after SrCl(2) activation; very few oocytes were seen at the second anaphase or telophase and they often showed impairment of sister-chromatid separation. Eventually the majority of oocytes entered interphase and formed pronuclei, but very few reached the 2-cell stage. Similar results were obtained after fertilization. We conclude that the XY chromosomal composition in oocyte leads to impairment in the progression of the second meiotic division.

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.

When a sperm meets an egg: block to polyspermy

Embryonic development is initiated after the fertilizing spermatozoon enters the egg and triggers a series of events 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. CGE is an evolutionary developed mechanism that causes modification of the zona pellucida to prevent penetration of additional spermatozoa, ensuring successful egg activation and embryo development. The egg CGE is a unique and convenient mammalian model for studying the different proteins participating at the membrane fusion cascade, which, unlike other secretory cells, occurs only once in the egg's lifespan. This article highlights a number of proteins, ascribed to participate in CGE and thus the block to polyspermy. CGE can be triggered either by a calcium dependent pathway, or via protein kinase C (PKC) activation that requires a very low calcium concentration. In a recent study, we suggested that the filamentous actin (F-actin) at the egg's cortex is a dynamic network. It can be maneuvered towards allowing CGE by activated actin associated proteins and/or by activated PKC and its down stream proteins, such as myristoylated alanine-rich C kinase substrate (MARCKS). MARCKS, a protein known to cross-link F-actin in other cell types, was found to be expressed and colocalized with actin in non-activated MII eggs. We further demonstrated MARCKS dissociation from actin after activation by ionomycin, a process that can lead to the breakdown of the actin network, thus allowing CGE. The more we know of the intricate process of CGE and of the proteins participating in it, the more the assisted reproductive procedures might benefit from that knowledge.

Intracellular Ca2+ increase induces post-fertilization events via MAP kinase dephosphorylation in eggs of the hydrozoan jellyfish Cladonema pacificum

Naturally spawned eggs of the hydrozoan jellyfish Cladonema pacificum are arrested at G1-like pronuclear stage until fertilization. Fertilized eggs of Cladonema undergo a series of post-fertilization events, including loss of sperm-attracting ability, expression of adhesive materials on the egg surface, and initiation of cell cycle leading to DNA synthesis and cleavage. Here, we investigate whether these events are regulated by changes in intracellular Ca2+ concentration and mitogen-activated protein kinase (MAP kinase) activity in Cladonema eggs. We found that MAP kinase is maintained in the phosphorylated form in unfertilized eggs. Initiation of sperm-induced Ca2+ increase, which is the first sign of fertilization, was immediately followed by MAP kinase dephosphorylation within a few minutes of fertilization. The fertilized eggs typically stopped sperm attraction by an additional 5 min and became sticky around this time. They further underwent cytokinesis yielding 2-cell embryos at approximately 1 h post-fertilization, which was preceded by DNA synthesis evidenced by BrdU incorporation into the nuclei. Injection of inositol 1,4,5-trisphosphate (IP3) into unfertilized eggs, which produced a Ca2+ increase similar to that seen at fertilization, triggered MAP kinase dephosphorylation and the above post-fertilization events without insemination. Conversely, injection of BAPTA/Ca2+ into fertilized eggs at approximately 10 s after the initiation of Ca2+ increase immediately lowered the elevating Ca2+ level and inhibited the subsequent post-fertilization events. Treatment with U0126, an inhibitor of MAP kinase kinase (MEK), triggered the post-fertilization events in unfertilized eggs, where MAP kinase dephosphorylation but not Ca2+ increase was generated. Conversely, preinjection of the glutathione S-transferase (GST) fusion protein of MAP kinase kinase kinase (Mos), which maintained the phosphorylated state of MAP kinase, blocked the post-fertilization events in fertilized eggs without preventing a Ca2+ increase. These results strongly suggest that all of the three post-fertilization events, cessation of sperm attraction, expression of surface adhesion, and progression of cell cycle, lie downstream of MAP kinase dephosphorylation that is triggered by a Ca2+ increase.

Antagonists of myosin light chain kinase and of myosin II inhibit specific events of egg activation in fertilized mouse eggs

Although recent studies have demonstrated the importance of calcium/calmodulin (Ca(2+)/CAM) signaling in mammalian fertilization, many targets of Ca(2+)/CAM have not been investigated and represent potentially important regulatory pathways to transduce the Ca2+ signal that is responsible for most events of egg activation. A well-established Ca(2+)/CAM-dependent enzyme is myosin light chain kinase (MYLK2), the downstream target of which is myosin II, an isoform of myosin known to be important in cytokinesis. In fertilized mouse eggs, established inhibitors of MYLK2 and myosin II were investigated for their effects on events of egg activation. The MYLK2 antagonist, ML-7, did not decrease the activity of Ca(2+)/CAM protein kinase II or the elevation of intracellular Ca2+, and it did not delay the onset of Ca2+ oscillations. In contrast, ML-7 inhibited second polar body (PB) formation in a dose-dependent manner and reduced cortical granule (CG) exocytosis by a mean of approximately 50%. The myosin II isoform-specific inhibitor, blebbistatin, had similar inhibitory effects. Although both antagonists had no effect on anaphase onset, they inhibited second PB formation by preventing spindle rotation before telophase II and normal contractile ring constriction. To our knowledge, this is the first report that MYLK2 and myosin II are involved in regulating the position of the meiotic spindle, formation of the second PB, and CG exocytosis. The present results suggest that MYLK2 is one of a family of CAM-dependent proteins that act as multifunctional regulators and transduce the Ca2+ signal at fertilization.

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.

Egg activation events are regulated by the duration of a sustained [Ca2+]cyt signal in the mouse

Although the dynamics of oscillations of cytosolic Ca2+ concentration ([Ca2+]cyt) play important roles in early mammalian development, the impact of the duration when [Ca2+]cyt is elevated is not known. To determine the sensitivity of fertilization-associated responses [i.e., cortical granule exocytosis, resumption of the cell cycle, Ca2+/calmodulin-dependent protein kinase II (CaMKII) activity, recruitment of maternal mRNAs] and developmental competence of the parthenotes to the duration of a [Ca2+]cyt transient, unfertilized mouse eggs were subjected to a prolonged [Ca2+]cyt change for 15, 25, or 50 min by means of repetitive Ca2+ electropermeabilization at 2-min intervals. The initiation and completion of fertilization-associated responses are correlated with the duration of time in which the [Ca2+]cyt is elevated, with the exception that autonomous CaMKII activity is down-regulated with prolonged elevated [Ca2+]cyt. Activated eggs from 25- or 50-min treatments readily develop to the blastocyst stage with no sign of apoptosis or necrosis and some implant. Ca2+ influx into unfertilized eggs causes neither Ca2+ release from intracellular stores nor rapid removal of cytosolic Ca2+. Thus, the total Ca2+ signal input appears to be an important regulatory parameter that ensures completion of fertilization-associated events and oocytes have a surprising degree of tolerance for a prolonged change in [Ca2+]cyt.

Timing of MAP kinase inactivation effects on emission of polar body in porcine oocytes activated by Ca2+ ionophore

Artificial activation is required for successful intracytoplasmic sperm injection (ICSI) to induce haploidy pronuclear formation with extraction of second polar body. The present study showed that an additional treatment with Phorbol 12-myristate 13-acetate (PMA) followed by Ca(2+) ionophore treatment improved the rate of pronuclear formation, however, these oocytes had more than two pronuclei because of the suppression of polar body emission. The cultivation with MEK inhibitor U0126 followed by Ca(2+) ionophore also increased the rate of pronuclear formation but suppressed the emission of second polar body. These results suggested that the decrease of MAP kinase activity at early stage of artificial activation, concomitantly with decreasing p34(cdc2) kinase activity, prevented the second polar body extraction. We investigated that the timing of MAP kinase inactivation affected the extraction of the polar body and pronuclear formation rate. The addition of PMA 8 hr after Ca(2+) ionophore treatment induced the delay of MAP kinase inactivation, which resulted in haploidy pronuclear formation with emission of polar body. These results demonstrated for the first time that the delay of MAP kinase inactivation induced by PMA improved pronuclear formation with the extraction of second polar body in porcine oocytes activated by Ca(2+) ionophore. This method can be available for successfully ICSI in low response species of oocyte activation to Ca(2+) ionophore including pig.

Role of MAP kinase and myosin light chain kinase in chromosome-induced development of mouse egg polarity

During maturation, the mouse oocyte is transformed into a highly polarized egg, characterized by an actin cap and cortical granule-free domain (CGFD) overlying the meiotic spindle that is in close proximity to the cortex. The presence of spindle/chromosomes or microinjected sperm chromatin in the cortical region initiates this cortical reorganization, but the pathway is unknown. We report that cortical reorganization induced by microinjected sperm chromatin is blocked by inhibitors of microfilament assembly or disassembly. Active mitogen-activated protein kinase (MAPK), which becomes enriched in the region of sperm chromatin, is required for cortical reorganization, because microinjected sperm chromatin fails to induce cortical reorganization in Mos-/- eggs, which lack MAPK activity. Last, myosin light chain kinase (MLCK), which can be directly phosphorylated and activated by MAPK, appears involved, because the MLCK inhibitors ML-7 and Peptide 18 prevent sperm chromatin-induced cortical reorganization. These results provide new insights into how cortical reorganization occurs independently of extracellular signals to generate egg polarity.

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.

Effects of MAP kinase pathway and other factors on meiosis ofUrechis unicinctus eggs

The eggs of Urechis unicinctus Von Drasche, an echiuroid, are arrested at P-I stage in meiosis. The meiosis is reinitiated by fertilization. Immunoblotting analysis using anti-ERK2 and anti-phospho-MAPK antibodies revealed a 44 kDa MAP kinase species that was constantly expressed in U. unicinctus eggs, quickly phosphorylated after fertilization, and dephosphorylated slowly before the completion of meiosis I. Phosphorylation of the protein was not depressed by protein synthesis inhibitor Cycloheximide (CHX), but was depressed by the MEK1 inhibitor PD98059. Under PD98059 treatment, polar body extrusion was suppressed and the function of centrosome and spindle was abnormal though GVBD was not affected, indicating that MAP kinase cascade was important for meiotic division of U. unicinctus eggs. Other discovery includes: A23187 and OA could parthenogenetically activate U. unicinctus eggs and phosphorylated 44 kDa MAP kinase species, indicating that the effect of fertilization on reinitiating meiosis and phosphorylation of 44 kDa MAP kinase specie is mediated by raising intracellular free calcium and by phosphorylation of some proteins, and that phosphotase(s) sensitive to OA is responsible for arresting U. unicinctus eggs in prophase I. diC8, an activator of PKC, accelerated the process of U. unicinctus egg meiotic division after fertilization and accelerated the dephosphorylation of 44 kDa MAP kinase specie, which implied that the acceleration effect of PKC on meiotic division was mediated by inactivation of MAP kinase cascade. Elevating cAMP/PKA level in U. unicinctus eggs had no effect on meiotic division of the eggs.

Effect of genetic background and activating stimulus on the timing of meiotic cell cycle progression in parthenogenetically activated mouse oocytes

With the aim of investigating the effects of oocyte genotype and activating stimulus on the timing of nuclear events after activation, oocytes collected from hybrid B6D2F1, inbred C57BL/6 and outbred CF-1 and immunodeficient nude (NU/+) females were activated using ethanol or strontium and fixed at various time-points. Meiotic status, spindle rotation and second polar body (PB2) extrusion were monitored by fluorescence microscopy using DNA-, microtubule- and microfilament-selective probes. Although activation efficiency was similar in all groups of oocytes, a significant percentage of CF-1 and NU/+ oocytes treated with ethanol and of C57BL/6 oocytes treated either with ethanol or strontium failed to complete activation and became arrested at a new metaphase stage (MIII) after PB2 extrusion. C57BL/6 oocytes also showed slower release from MII arrest but faster progression to telophase (TII) after ethanol exposure, and they exhibited the most rapid exit from TII under both activation treatments. Strontium caused delayed meiotic resumption, spindle rotation and PB2 extrusion, but rapid TII exit, in B6D2F1, CF-1 and NU/+ oocytes when compared with ethanol. Compared with all other strains, NU/+ oocytes were significantly slower in completing spindle rotation and PB2 extrusion, irrespective of the activating stimulus, and a significant decrease in activation rates and pace of meiotic progression was observed after strontium exposure. Thus, our findings demonstrated that the kinetics of meiosis resumption and completion, spindle rotation and PB2 extrusion following parthenogenetic activation depends on both genotype-specific factors and on the activation treatment applied.

The dynamics of cyclin B1 distribution during meiosis I in mouse oocytes

Cdk1-cyclin B1 kinase activity drives oocytes through meiotic maturation. It is regulated by the phosphorylation status of cdk1 and by its spatial organisation. Here we used a cyclin B1-green fluorescent protein (GFP) fusion protein to examine the dynamics of cdk1-cyclin B1 distribution during meiosis I (MI) in living mouse oocytes. Microinjection of cyclin B1-GFP accelerated germinal vesicle breakdown (GVBD) and, as previously described, overrides cAMP-mediated meiotic arrest. GVBD was pre-empted by a translocation of cyclin B1-GFP from the cytoplasm to the germinal vesicle (GV). After nuclear accumulation, cyclin B1-GFP localised to the chromatin. The localisation of cyclin B1-GFP is governed by nuclear import and export. In GV intact oocytes, cyclin export was demonstrated by showing that cyclin B1-GFP injected into the GV is exported to the cytoplasm while a similar size dextran is retained. Import was revealed by the finding that cyclin B1-GFP accumulated in the GV when export was inhibited using leptomycin B. These studies show that GVBD in mouse oocytes is sensitive to cyclin B1 abundance and that the changes in distribution of cyclin B1 contribute to progression through MI.

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

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

The role of calcium/calmodulin-dependent protein kinase II on the inactivation of MAP kinase and p34cdc2 kinase during fertilization and activation in pig oocytes

The objective of this study was to investigate the role of calmodulin-dependent protein kinase II (CaMKII) during fertilization in the pig. Since it has been reported that CaMKII is involved in the capacitation and acrosome reaction of spermatozoa, we tested whether supplementation with the CaMKII inhibitor, KN-93, in the fertilization medium affected sperm penetration. The results showed that the addition of KN-93 in the fertilization medium significantly reduced the rate of sperm penetration into oocytes. However, pre-treatment with KN-93 beforein vitrofertilization (IVF) did not significantly affect sperm penetration, but it did affect pronuclear formation in a dose-dependent manner. In the oocytes pre-treated with KN-93 before IVF and then co-cultured with spermatozoa without the drug, the decrease in p34cdc2kinase and the cyclin B1 level were significantly suppressed as compared with those in penetrated oocytes without treatment with KN-93. However, the decrease in MAP kinase activity was not affected by KN-93. Additional treatment with KN-93 after Ca2+ionophore treatment also inhibited the reduction in p34cdc2kinase activity and the cyclin B1 level, but not MAP kinase activity. Treatment with KN-92, an inactive KN-93 analogue, did not significantly affect sperm penetration and pronuclear formation. In conclusion, the activation of CaMKII by artificial stimuli or sperm stimulated the disruption of cyclin B1 and the inactivation of p34cdc2kinase, but did not affect MAP kinase inactivation during oocyte activation in pigs.

Protein synthesis and mRNA storage in cattle oocytes maintained under meiotic block by roscovitine inhibition of MPF activity

Roscovitine, a specific inhibitor of MPF kinase activity, has been shown to block efficiently and reversibly the meiotic resumption of oocytes from different species, including cattle. In view to verify that oocytes maintain germinal vesicle like molecular activities under roscovitine treatment, we compared in the present study the M-phase Promoting Factor (MPF) and Mitogen Activated Protein (MAP) kinase activities; protein synthesis and phosphorylation patterns in oocytes and cumulus cells; and CDK1 and Cyclin B messengers storage under control culture and under roscovitine inhibition. We observed that roscovitine induced a full and reversible inhibition of MPF kinase activity and of the activating phosphorylation of both ERK1/2 MAPK. During in vivo maturation, there was a highly significant increase in the relative mRNA level of both cyclin B1 and CDK1 whereas during in vitro culture, the relative amount of CDK1 messenger was reduced. These messengers may be used as markers for the optimization of in vitro maturation treatment. Roscovitine reversibly prevented this drop in relative quantities of CDK1 messenger. Oocytes cultured in the presence of roscovitine maintained a GV like profile of protein synthesis except that two proteins of 48 and 64 kDa specific of matured oocytes also appeared under roscovitine treatment. However, roscovitine did not prevent most of the modifications of protein phosphorylation pattern observed during maturation. In conclusion, results of this study revealed that the use of roscovitine did not prevent all the events related to maturation of bovine oocytes.

Association of female aging with decreased parthenogenetic activation, raised MPF, and MAPKs activities and reduced levels of glutathione S-transferases activity and thiols in mouse oocytes

This study aims to determine in the mouse whether oocytes from reproductively old females exhibit a different susceptibility to be parthenogenetically activated when compared to oocytes from young females. At the age of 10-12 (young-female group) or 60-62 (old-female group) weeks, hybrid female mice were superovulated using pregnant mare's serum gonadotropin (PMSG) followed by human chorionic gonadotropin (hCG) 48 hr later. After removing the cumulus cells, oocytes were exposed to any of two different activating protocols: (a) 6-min exposure to 8% ethanol; and (b) treatment with 200 microM thimerosal for 15 min followed by 8 mM dithiothreitol (DTT) for 30 min. Oocytes from old female mice displayed (1) lower total percentage of parthenogenetic activation and extrusion of the second polar body after treatment with either thimerosal + DTT or ethanol; (2) higher M-phase-promoting factor (MPF) and mitogen-activated protein kinases (MAPKs) activities; and (3) lower intracytoplasmic levels of glutathione S-transferases (GSTs) activity and thiols than oocytes from young females. These data show that female aging is associated with higher resistance of oocytes to be parthenogenetically activated, higher MPF and MAPKs activities and lower intracytoplasmic levels of GSTs activity and thiols.

Transient exposure to the Eg5 kinesin inhibitor monastrol leads to syntelic orientation of chromosomes and aneuploidy in mouse oocytes

Aneuploidy may result from abnormalities in the biochemical pathways and cellular organelles associated with chromosome segregation. Monastrol is a reversible, cell-permeable, non-tubulin interacting inhibitor of the mitotic kinesin Eg5 motor protein which is required for assembling and maintaining the mitotic spindle. Monastrol can also impair centrosome separation and induce monoastral spindles in mammalian somatic cells. The ability of monastrol to alter kinesin Eg5 and centrosome activities and spindle geometry may lead to abnormal chromosome segregation. Mouse oocytes were exposed to 0 (control), 15, 30, and 45 microg/ml monastrol in vitro for 6 h during meiosis I and subsequently cultured for 17 h in monastrol-free media prior to cytogenetic analysis of metaphase II oocytes. A subset of oocytes was cultured for 5 h prior to processing cells for meiotic I spindle analysis. Monastrol retarded oocyte maturation by significantly (P < 0.05) decreasing germinal vesicle breakdown and increasing the frequencies of arrested metaphase I oocytes. Also, significant (P < 0.05) increases in the frequencies of monoastral spindles and chromosome displacement from the metaphase plate were found in oocytes during meiosis I. In metaphase II oocytes, monastrol significantly (P < 0.05) increased the frequencies of premature centromere separation and aneuploidy. These findings suggest that abnormal meiotic spindle geometry predisposes oocytes to aneuploidy.

Okadaic acid, an inhibitor of protein phosphatase 1 and 2A, induces premature separation of sister chromatids during meiosis I and aneuploidy in mouse oocytes in vitro

Recent advances in understanding some of the molecular aspects of chromosome segregation during mitosis and meiosis provide a background for investigating potential mechanisms of aneuploidy in mammalian germ cells. Numerous protein kinases and phosphatases have important functions during mitosis and meiosis. Alterations in these enzyme activities may upset the normal temporal sequence of biochemical reactions and cellular organelle modifications required for orderly chromosome segregation. Protein phosphatases 1 (PP1) and 2A (PP2A) play integral roles in regulating oocyte maturation (OM) and the metaphase-anaphase transitions. Mouse oocytes were transiently exposed in vitro to different dosages (0, 0.01, 0.1, or 1.0 microg/ml) of the PP1 and PP2A phosphatase inhibitor okadaic acid (OA) during meiosis I and oocytes were cytogenetically analyzed. Significant (p < 0.05) OA dose-response increases in the frequencies of metaphase I (MI) arrested oocytes, MI oocytes with 80 chromatids instead of the normal 20 tetrads, and anaphase I telophase I (AI-TI) oocytes with two groups of an unequal number of chromatids were found. Analysis of MII oocytes revealed significant (p < 0.05) increases in the frequencies of premature sister chromatid separation, single-unpaired chromatids, and hyperploidy. Besides showing that OA is aneugenic, these data suggest that OA-induced protein phosphatase inhibition upsets the normal kinase-phosphatase equilibrium during mouse OM, resulting in precocious removal of cohesion proteins from chromosomes.

Effects of post-treatment with 6-Dimethylaminopurine (6-DMAP) on ethanol activation of mouse oocytes at different ages

To study the effect of post-treatment with 6-Dimethylaminopurine (6-DMAP) on oocyte activation and development, mouse oocytes collected at different times post human chorion gonadotropin (hCG) injection were incubated in 6-DMAP-containing Chatot-Ziomek-Bavister (CZB) medium for different periods after ethanol exposure, and activation and development were observed. When oocytes were cultured in 6-DMAP without prior ethanol exposure, the highest activation rate was only 40%. Incubation in 6-DMAP for 6 h following ethanol exposure significantly (P < 0.05) increased the activation rate in oocytes recovered 15 and 18 h post hCG, but this effect was not significant in the 21 h oocytes. When oocytes were incubated in 6-DMAP for 1 h at different time points after ethanol, a 6-DMAP susceptible temporal window was found to be located from the second to the fifth h in the 18 h oocytes and from the fourth to the fifth h in the 15 h oocytes, and within the window, the duration of 6-DMAP incubation can be reduced to 0.5 h with more than 80% activation. With the 13 h oocytes, however, 6-DMAP-incubation can only be shortened to 3 h and no specific temporal window was identified. Oocytes that were incubated in 6-DMAP for 1 or 2 h after ethanol exposure developed to morula/blastocyst stages at significantly (P < 0.05) higher rates than those incubated in 6-DMAP for 6 h. Our results suggested that (i) long duration of 6-DMAP incubation impaired the development of mouse parthenogenotes; (ii) the effect of 6-DMAP alone was limited without prior ethanol exposure; (iii) the egg age affected both the timing of 6-DMAP susceptibility and the duration of exposure required to obtain a maximal activating effect; (iv) the most effective activating protocols varied for oocytes of different ages.