Essential role of germinal vesicle material in the meiotic cell cycle of Xenopus oocytes

A rare case of intra-ovarian oocyte maturation

The intra-ovarian presence of ootids, i.e. female gametes that have completed meiosis, is considered exceptional in the animal kingdom. The present study explores the first such case to be reported in a sea cucumber (Echinodermata: Holothuroidea). In the overwhelming majority of animals, including holothuroids, oocytes (i.e. immature female gametes) that are developing in the ovary undergo a primary arrest at the prophase stage of meiosis, which may last from days to decades. In free-spawning taxa, this arrest is normally lifted only during or shortly before transit in the gonoduct, when gamete release (spawning) is imminent. However, oocytes of the holothuroid Chiridota laevis were discovered to have resumed the second meiotic division including the completion of germinal vesicle breakdown and polar-body expulsion inside the ovary, effectively reaching the ootid stage concomitantly with ovulation (i.e. escape from follicle cells) prior to spawning. The potential drivers and significance of this exceptionally rare case of full intra-ovarian oogenic maturation are discussed.

Entry into mitosis: a solution to the decades-long enigma of MPF

Maturation or M phase-promoting factor (MPF) is the universal inducer of M phase common to eukaryotic cells. MPF was originally defined as a transferable activity that can induce the G2/M phase transition in recipient cells. Today, however, MPF is assumed to describe an activity that exhibits its effect in donor cells, and furthermore, MPF is consistently equated with the kinase cyclin B-Cdk1. In some conditions, however, MPF, as originally defined, is undetectable even though cyclin B-Cdk1 is fully active. For over three decades, this inconsistency has remained a long-standing puzzle. The enigma is now resolved through the elucidation that MPF, defined as an activity that exhibits its effect in recipient cells, consists of at least two separate kinases, cyclin B-Cdk1 and Greatwall (Gwl). Involvement of Gwl in MPF can be explained by its contribution to the autoregulatory activation of cyclin B-Cdk1 and by its stabilization of phosphorylations on cyclin B-Cdk1 substrates, both of which are essential when MPF induces the G2/M phase transition in recipient cells. To accomplish these tasks, Gwl helps cyclin B-Cdk1 by suppressing protein phosphatase 2A (PP2A)-B55 that counteracts cyclin B-Cdk1. MPF, as originally defined, is thus not synonymous with cyclin B-Cdk1, but is instead a system consisting of both cyclin B-Cdk1 that directs mitotic entry and Gwl that suppresses the anti-cyclin B-Cdk1 phosphatase. The current view that MPF is a synonym for cyclin B-Cdk1 in donor cells is thus imprecise; instead, MPF is best regarded as the entire pathway involved in the autoregulatory activation of cyclin B-Cdk1, with specifics depending on the experimental system.

Interaction of parvoviruses with the nuclear envelope

Parvoviruses are serious pathogens but also serve as platforms for gene therapy or for using their lytic activity in experimental cancer treatment. Despite of their growing importance during the last decade little is known on how the viral genome is transported into the nucleus of the infected cell, which is crucial for replication. As nucleic acids are not karyophilic per se nuclear import must be driven by proteins attached to the viral genome. In turn, presence and conformation of these proteins depend upon the entry pathway of the virus into the cell. This review focuses on the trafficking of the parvoviral genome from the cellular periphery to nucleus. Despite of the uncertainties in knowledge about the entry pathway we show that parvoviruses developed a unique strategy to pass the nuclear envelope by hijacking enzymes involved in mitosis.

Greatwall kinase and cyclin B-Cdk1 are both critical constituents of M-phase-promoting factor

Maturation/M-phase-promoting factor is the universal inducer of M-phase in eukaryotic cells. It is currently accepted that M-phase-promoting factor is identical to the kinase cyclin B–Cdk1. Here we show that cyclin B–Cdk1 and M-phase-promoting factor are not in fact synonymous. Instead, M-phase-promoting factor contains at least two essential components: cyclin B–Cdk1 and another kinase, Greatwall kinase. In the absence of Greatwall kinase, the M-phase-promoting factor is undetectable in oocyte cytoplasm even though cyclin B–Cdk1 is fully active, whereas M-phase-promoting factor activity is restored when Greatwall kinase is added back. Although the excess amount of cyclin B–Cdk1 alone, but not Greatwall kinase alone, can induce nuclear envelope breakdown, spindle assembly is abortive. Addition of Greatwall kinase greatly reduces the amount of cyclin B–Cdk1 required for nuclear envelope breakdown, resulting in formation of the spindle with aligned chromosomes. M-phase-promoting factor is thus a system consisting of one kinase (cyclin B–Cdk1) that directs mitotic entry and a second kinase (Greatwall kinase) that suppresses the protein phosphatase 2A-B55 which opposes cyclin B–Cdk1.

Cyclin B–Cdk1 is thought to be synonymous with the promoting factor that drives entry into M-phase of the cell cycle. Here, Greatwall kinase is shown to be required for the breakdown of the nuclear envelope and the assembly of the spindle on entry into M-phase, suggesting that it too is a part of the M-phase-promoting factor.

Histone deacetylase induces accelerated maturation in Xenopus laevis oocytes

In oocyte maturation in Xenopus laevis, nuclear material induces rapid maturation and is required for entry into meiosis II. Nuclear material contains a large number of RNAs and proteins, including histone deacetylase (HDAC); however, it is not known which materials induce accelerated maturation. The HDAC activity modifies transcription rate and is required for normal meiosis; however, its function in oocyte maturation is still unclear. We investigated the function of HDAC activity, which is localized in the nuclear material, in the regulation of the speed of oocyte maturation. Inhibition of HDAC activity with trichostatin A (TSA) induced hyperacetylation of histone H3 and prolonged oocyte maturation. In contrast, increase in HDAC activity with an injection of FLAG-tagged maternal histone deacetylase (HDACm-FLAG) mRNA induced deacetylation of histone H3 and reduced the duration of oocyte maturation. Cdc2 kinase, Cdc25C or mitogen-activated protein kinase (MAPK), which are key regulators of the meiosis, were activated coincidently with maturation progression. In oocytes, the mRNA level of Cdc25C, an activator of Cdc2, was increased by HDACm-FLAG mRNA-injection; in contrast, the mRNA level of Cdc2 inhibitor Wee1 was increased by TSA treatment. These results suggest that HDAC activity is involved in the control of maturation speed through the regulation of mRNA levels of cell cycle regulators. Thus, HDACm is a candidate for the nuclear material component that induces rapid maturation in Xenopus oocytes.

Effect of Enucleation on Inactivation of Cytostatic Factor Activity in Matured Rat Oocytes

In mammals, matured oocytes are arrested at the MII stage until fertilization, which is regulated by cytostaticfactor (CSF) activity. Maturation-promoting factor (MPF) and the mitogen-activated protein kinase (MAPK) pathway are known as candidates for CSF. Despite of the results that nuclear and perinuclear materials were dispensable for activation of MPF and MAPK in other species, our previous study in rats demonstrated that MPF activity was rapidly decreased after enucleation. We showed here for the first time that nuclear and perinuclear materials were indispensable for CSF activity in matured rat oocytes. In both cytoplasm-removed and enucleated oocytes, high activity of p34(cdc2) kinase was observed immediately after manipulation, but the activity of enucleated oocytes was dramatically reduced within 1 h. Cyclin B level was also decreased, corresponding with inactivation of p34(cdc2) kinase. In enucleated oocytes, the Mos level was dramatically decreased, and both MEK and MAPK dephosphorylation were also induced. A combined treatment with a proteasome inhibitor, MG132, and a protein phosphatase inhibitor, okadaic acid, dramatically improved both levels of p-MAPK and cyclin B in these enucleated oocytes. These data suggest that nuclear and perinuclear materials of matured rat oocytes suppress proteasome and protein phosphatase activation, which is indispensable for stability of CSF.

Oocyte nucleus controls progression through meiotic maturation

We analyzed progression through the meiotic maturation in oocytes manipulated to replace the prophase oocyte nucleus with the nucleus from a cumulus cell, a pachytene spermatocyte or the pronucleus from a fertilized egg. Removal of the oocyte nucleus led to a significant reduction in histone H1 kinase activity. Replacement of the oocyte nucleus by a pronucleus followed by culture resulted in premature pseudomeiotic division and occasional abnormal cytokinesis; however, histone H1 kinase activity was rescued, microtubules formed a bipolar spindle, and chromosomes were condensed. In addition to the anomalies observed after pronuclear transfer, those after transfer of the nucleus from a cumulus cell or spermatocyte included a dramatically impaired ability to form the bipolar spindle or to condense chromosomes, and histone H1 kinase activity was not rescued. Expression of a cyclin B-YFP in enucleated oocytes receiving the cumulus cell nucleus rescued histone H1 kinase activity, but spindle formation and chromosome condensation remained impaired, indicating a pleiotropic effect of oocyte nucleus removal. However, when the cumulus cell nucleus was first transformed into pronuclei (transfer into a metaphase II oocyte followed by activation), such pronuclei supported maturation after transfer into the oocyte in a manner similar to that of normal pronuclei. These results show that the oocyte nucleus contains specific components required for the control of progression through the meiotic maturation and that some of these components are also present in pronuclei.

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

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

Study of germinal vesicle requirement for the normal kinetics of maturation/M-phase-promoting factor activity during porcine oocyte maturation

Mammalian immature oocytes contain large nuclei referred to as germinal vesicles (GVs). The translocation of maturation/M-phase promoting factor (MPF) into GVs just before the activation of MPF has been reported in several species. To examine whether the GV is required for MPF activation in mammalian oocytes, porcine immature oocytes were enucleated and their MPF activity and CCNB (also known as cyclin B) levels were investigated. The activation of MPF at the start of maturation was detected at normal levels in enucleated oocytes, whereas reactivation to induce the second meiosis was not observed. Although protein synthesis was found to be normal both qualitatively and quantitatively, even in the absence of the nucleus, CCNB1 did not sufficiently accumulate in the enucleated oocytes. The defects in the enucleated oocytes were reversed by the injection of GV material into the enucleated oocytes. Furthermore, the inhibition of CCNB1 degradation revealed drastic accumulation of CCNB1, indicating active synthesis of CCNB1 in enucleated oocytes. The mitogen-activated protein kinase cascade remained unaffected by enucleation. These results indicate that GV is not required for the activation of MPF during the first meiosis, but that it is required for the second meiosis because of its promotion of CCNB1 accumulation.

Transcription-dependent and transcription-independent functions of the classical progesterone receptor in Xenopus ovaries

Two forms of the classical progesterone receptors (PR), XPR-1 and XPR-2, have been cloned in Xenopus laevis. Their relative roles in mediating progesterone action in the ovaries are not clear. Using antibodies generated against the cloned XPR-2, we demonstrated here that the somatic follicle cells expressed an 80-kDa PR protein, termed XPR-1. Treatment of follicle cells with progesterone resulted in disappearance of this protein, consistent with proteosome-mediated XPR-1 protein degradation. A smaller (approximately 70 kDa) PR protein, termed XPR-2, was expressed in the oocytes, but not in follicle cells. XPR-2 underwent progesterone-induced phosphorylation but not protein degradation. Treating isolated ovarian fragments with progesterone caused oocyte maturation and the release of the mature oocytes from the ovarian tissues ("ovulation"). Inhibition of transcription, with actinomycin D, did not interfere with progesterone-induced oocyte maturation but blocked "ovulation" so that mature oocytes were trapped in the follicles. These results suggest that the dual functions of progesterone, transcription-dependent follicle rupture and transcription-independent oocyte maturation, are mediated by the two forms of PR proteins differentially expressed in the follicle cells and the oocytes, respectively.

Nucleocytoplasmic ratio of fully grown germinal vesicle oocytes is essential for mouse meiotic chromosome segregation and alignment, spindle shape and early embryonic development

BACKGROUND

This study examined the effect of nucleocytoplasmic ratio of fully grown germinal vesicle (GV) oocytes on meiotic chromosome segregation and alignment, spindle shape, Ca(2+) oscillations and capacity of early embryonic development in mouse.

METHODS

GV oocytes with reduced volume (equal to 1/5 to 4/5 of an intact oocyte) were made by micromanipulation to remove different amounts of cytoplasm, and then matured and fertilized in vitro.

RESULTS

When >1/2 of GV oocyte cytoplasm was removed, the time-course of GV breakdown (GVBD) was delayed and oocyte maturation rate decreased significantly. Abnormal chromosome segregation rate increased if >1/2 of the cytoplasm was removed from the oocyte. Length and structure of meiotic spindle and chromosome alignment were also impaired by the reduction of cytoplasmic volume. Once matured in vitro, the oocytes could undergo Sr(2+)-induced Ca(2+) oscillations and form pronuclei in a manner independent of nucleocytoplasmic ratio, but their ability to develop to 2-cell embryos was affected if >1/2 of their cytoplasm was removed from the GV oocytes.

CONCLUSIONS

These results suggest that nucleocytoplasmic ratio is essential for normal meiotic chromosome segregation, spindle formation and chromosome alignment over the metaphase spindle, and development to 2-cell stage, for which 1/2 of the volume of the GV oocyte appears to be a threshold.

Pre-M phase-promoting factor associates with annulate lamellae in Xenopus oocytes and egg extracts

We have used complementary biochemical and in vivo approaches to study the compartmentalization of M phase-promoting factor (MPF) in prophase Xenopus eggs and oocytes. We first examined the distribution of MPF (Cdc2/CyclinB2) and membranous organelles in high-speed extracts of Xenopus eggs made during mitotic prophase. These extracts were found to lack mitochondria, Golgi membranes, and most endoplasmic reticulum (ER) but to contain the bulk of the pre-MPF pool. This pre-MPF could be pelleted by further centrifugation along with components necessary to activate it. On activation, Cdc2/CyclinB2 moved into the soluble fraction. Electron microscopy and Western blot analysis showed that the pre-MPF pellet contained a specific ER subdomain comprising "annulate lamellae" (AL): stacked ER membranes highly enriched in nuclear pores. Colocalization of pre-MPF with AL was demonstrated by anti-CyclinB2 immunofluorescence in prophase oocytes, in which AL are positioned close to the vegetal surface. Green fluorescent protein-CyclinB2 expressed in oocytes also localized at AL. These data suggest that inactive MPF associates with nuclear envelope components just before activation. This association may explain why nuclei and centrosomes stimulate MPF activation and provide a mechanism for targeting of MPF to some of its key substrates.

Nuclear envelope dynamics in oocytes: from germinal vesicle breakdown to mitosis

We have recently gained new insight into the mechanisms involved in nuclear envelope breakdown, the irreversible step that commits a cell to the M phase. Results from mammalian cell and starfish oocyte studies suggest that mechanical forces of the cytoskeleton, as well as biochemical disassembly of nuclear envelope protein complexes, play important roles in this process.

A receptor linked to a Gi-family G-protein functions in initiating oocyte maturation in starfish but not frogs

The stimulation of oocyte maturation by 1-methyladenine in starfish, and by a steroid in frogs, has been proposed to involve G-protein-coupled receptors. To examine whether activation of receptors linked to G(i) or G(z) was sufficient to cause oocyte maturation, we expressed mammalian G(i)- and G(z)-linked receptors in starfish and frog oocytes. Application of the corresponding agonists caused meiosis to resume in the starfish but not the frog oocytes. We confirmed that the receptors were effectively expressed in the frog oocytes by using a chimeric G-protein, G(qi), that converts input from G(i)- and G(z)-linked receptors to a G(q) output and results in a contraction of the oocyte's pigment. These results argue against G(i) or G(z) functioning to cause maturation in frog oocytes. Consistently, maturation-inducing steroids did not cause pigment contraction in frog oocytes expressing G(qi), and G(z) protein was not detectable in frog oocytes. For starfish oocytes, however, our results support the conclusion that G(i) functions in 1-methyladenine signaling and suggest the possibility of using frog oocyte pigment contraction as an assay to identify the 1-methyladenine receptor. To test this concept, we coexpressed G(qi) and a starfish adenosine receptor in frog oocytes and showed that applying adenosine caused pigment contraction.

Germinal vesicle material is essential for nucleus remodeling after nuclear transfer

Successful cloning by nuclear transfer has been reported with somatic or embryonic stem (ES) cell nucleus injection into enucleated mouse metaphase II oocytes. In this study, we enucleated mouse oocytes at the germinal vesicle (GV) or pro-metaphase I (pro-MI) stage and cultured the cytoplasm to the MII stage. Nuclei from cells of the R1 ES cell line were injected into both types of cytoplasm to evaluate developmental potential of resulting embryos compared to MII cytoplasmic injection. Immunocytochemical staining revealed that a spindle started to organize 30 min after nucleus injection into all three types of cytoplasm. A well-organized bipolar spindle resembling an MII spindle was present in both pro-MI and MII cytoplasm 1 h after injection with ES cells. However, in the mature GV cytoplasm, chromosomes were distributed throughout the cytoplasm and a much bigger spindle was formed. Pseudopronucleus formation was observed in pro-MI and MII cytoplasm after activation treatment. Although no pronucleus formation was found in GV cytoplasm, chromosomes segregated into two groups in response to activation. Only 8.1% of reconstructed embryos with pro-MI cytoplasm developed to the morula stage after culture in CZB medium. In contrast, 53.5% of embryos reconstructed with MII cytoplasm developed to the morula/blastocyst stage, and 5.3% of transferred embryos developed to term. These results indicate that GV material is essential for nucleus remodeling after nuclear transfer.

The classical progesterone receptor associates with p42 MAPK and is involved in phosphatidylinositol 3-kinase signaling in Xenopus oocytes

The induction of Xenopus laevis oocyte maturation by progesterone is a striking example of a steroid hormone-mediated event that does not require transcription. Here we have investigated the role of the classical progesterone receptor in this nongenomic signaling. The Xenopus progesterone receptor (XPR) was predominantly cytoplasmic; however, a significant fraction ( approximately 5%) of one form of the receptor (p82 XPR) was associated with the plasma membrane-containing P-10,000 fraction, compatible with the observation that membrane-impermeant derivatives of progesterone can induce maturation. XPR co-precipitated with active phosphatidylinositol 3-kinase. The phosphatidylinositol 3-kinase (PI3-K) inhibitor wortmannin delayed progesterone-induced maturation and completely blocked the insulin-dependent maturation, indicating that the association of XPR with PI3-K could be functionally important. We also examined whether the nongenomic signaling properties of XPR can account for the ability of glucocorticoids and the progesterone antagonist RU486 to induce maturation. We found that none of these steroids cause XPR to become associated with active PI3-K; thus, association of XPR with active PI3-K is progesterone-specific. Finally, we showed that p42 mitogen-activated protein kinase (MAPK) associates with XPR after progesterone-induced germinal vesicle breakdown and that active recombinant MAPK is able to phosphorylate p110 XPR in vitro. These findings demonstrate that the classical progesterone receptor is involved in progesterone-induced nongenomic signaling in Xenopus oocytes and provide evidence that p42 MAPK and PI3-K activity are directly associated with the classical progesterone receptor.

Cytoplasmic occurrence of the Chk1/Cdc25 pathway and regulation of Chk1 in Xenopus oocytes

Chk1, a nuclear DNA damage/replication G2 checkpoint kinase, phosphorylates Cdc25 and causes its nuclear exclusion in yeast and mammalian cells, thereby arresting the cell at the G2 phase until DNA repair/replication is completed. Chk1 is also involved, at least in part, in the natural G2 arrest of immature Xenopus oocytes, but it is unknown how Chk1 inhibits Cdc25 function and undergoes regulation during oocyte maturation. By using enucleated oocytes, we show here that Chk1 inhibits Cdc25 function in the cytoplasm of G2-arrested oocytes and that Cdc25 is activated exclusively in the cytoplasm of maturing oocytes. Moreover, we show that Chk1 activity is not appreciably altered during maturation, being maintained at basal levels, and that C-terminal truncation mutants of Chk1 have very high kinase activities, strong abilities to inhibit maturation, and altered subcellular localization in oocytes. These results, together with other results, suggest that the Chk1/Cdc25 pathway is involved cytoplasmically in G2 arrest of Xenopus oocytes, but moderately and independent of the G2 checkpoint, and that the C-terminal region of Chk1 negatively regulates its kinase activity and also determines its subcellular localization. Based on these results, we discuss the possibility that Chk1 (with the basal activity) may function as an ordinary regulator of Cdc25 in oocytes (and in other cell types) and that Chk1 might be hyperactivated in response to the G2 checkpoint via its dramatic conformational change.

Re-staging mitosis: a contemporary view of mitotic progression

The process of cell division, or mitosis, has fascinated biologists since its discovery in the late 1870s. Progress through mitosis is traditionally divided into stages that were defined over 100 years ago from analyses of fixed material from higher plants and animals. However, this terminology often leads to ambiguity, especially when comparing different systems. We therefore suggest that mitosis can be re-staged to reflect more accurately the molecular pathways that underlie key transitions.

Identification of XPR-1, a progesterone receptor required for Xenopus oocyte activation

Quiescent full-grown Xenopus oocytes remain arrested at the G(2)/M border of meiosis I until exposed to progesterone, their natural mitogen. Progesterone triggers rapid, nontranscriptional responses that lead to the translational activation of stored mRNAs, resumption of the meiotic cell cycles, and maturation of the oocyte into a fertilizable egg. It has long been presumed that progesterone activates the oocyte through a novel nontranscriptional signaling receptor. Here, we provide evidence that a conventional transcriptional progesterone receptor cloned from Xenopus oocytes, XPR-1, is required for oocyte activation. Overexpression of XPR-1 through mRNA injection increases sensitivity to progesterone and accelerates progesterone-activated cell cycle reentry. Injection of XPR-1 antisense oligonucleotides blocks the ability of oocytes to respond to progesterone; these oocytes are rescued by subsequent injection of XPR-1 or the human progesterone receptor PR-B. Antisense-treated oocytes can be activated in response to inhibition of protein kinase A, one of the earliest known changes occurring downstream of progesterone stimulation. These results argue that the conventional progesterone receptor also functions as the signaling receptor that is responsible for the rapid nontranscriptional activation of frog oocytes.

Nuclei and microtubule asters stimulate maturation/M phase promoting factor (MPF) activation in Xenopus eggs and egg cytoplasmic extracts

Although maturation/M phase promoting factor (MPF) can activate autonomously in Xenopus egg cytoplasm, indirect evidence suggests that nuclei and centrosomes may focus activation within the cell. We have dissected the contribution of these structures to MPF activation in fertilized eggs and in egg fragments containing different combinations of nuclei, centrosomes, and microtubules by following the behavior of Cdc2 (the kinase component of MPF), the regulatory subunit cyclin B, and the activating phosphatase Cdc25. The absence of the entire nucleus-centrosome complex resulted in a marked delay in MPF activation, whereas the absence of the centrosome alone caused a lesser delay. Nocodazole treatment to depolymerize microtubules through first interphase had an effect equivalent to removing the centrosome. Furthermore, microinjection of isolated centrosomes into anucleate eggs promoted MPF activation and advanced the onset of surface contraction waves, which are close indicators of MPF activation and could be triggered by ectopic MPF injection. Finally, we were able to demonstrate stimulation of MPF activation by the nucleus-centriole complex in vitro, as low concentrations of isolated sperm nuclei advanced MPF activation in cycling cytoplasmic extracts. Together these results indicate that nuclei and microtubule asters can independently stimulate MPF activation and that they cooperate to enhance activation locally.

The activation of MAP kinase and p34cdc2/cyclin B during the meiotic maturation of Xenopus oocytes

G2-arrested Xenopus oocytes are induced to enter M-phase of meiosis by progesterone stimulation. This process, known as meiotic maturation, requires the activation of p34cdc2/cyclin B complexes (pre-MPF) which is brought about by the prior translation of specific maternal mRNAs stored in the oocyte. One of these mRNAs encodes for the protein kinase Mos which has an essential role in oocyte maturation, most likely due to its ability to activate MAP kinase (MAPK). Here we review our current knowledge on the Mos/MAPK signalling pathway and a recently found connection between MAPK-activated p90rsk and the p34cdc2 inhibitory kinase Myt1. We also discuss a pathway that involves the protein kinase Plx1 and leads to the activation of the phosphatase Cdc25, as well as other regulators of p34cdc2/cyclin B activity which may have a role in oocyte maturation.

Xenopus oocyte maturation: new lessons from a good egg

Fully grown Xenopus oocytes can remain in their immature state essentially indefinitely, or, in response to the steroid hormone progesterone, can be induced to develop into fertilizable eggs. This process is termed oocyte maturation. Oocyte maturation is initiated by a novel plasma membrane steroid hormone receptor. Progesterone brings about inhibition of adenylate cyclase and activation of the Mos/MEK1/p42 MAP kinase cascade, which ultimately brings about the activation of the universal M phase trigger Cdc2/cyclin B. Oocyte maturation provides an interesting example of how signaling cascades entrain the cell cycle clock to environmental changes.

Activation of MPF at meiosis reinitiation in starfish oocytes

Although maturation or M-phase-promoting factor (MPF) was originally identified as a cytoplasmic activity responsible for induction of maturation or meiosis reinitiation in oocytes, MPF is now thought to be the universal trigger of G2/M-phase transition in all eukaryotic cells, and its activity is ascribed to cyclin B. Cdc2 kinase. Here, the activation process of cyclin B. Cdc2 at meiosis reinitiation in starfish oocytes is compared with that at G2/M-phase transition in mitotic somatic cells. Based on this comparison, the role of cyclin B. Cdc2 in the original cytoplasmic MPF activity is reexamined.

MPF localization is controlled by nuclear export

In eukaryotes, mitosis is initiated by M phase promoting factor (MPF), composed of B-type cyclins and their partner protein kinase, CDK1. In animal cells, MPF is cytoplasmic in interphase and is translocated into the nucleus after mitosis has begun, after which it associates with the mitotic apparatus until the cyclins are degraded in anaphase. We have used a fusion protein between human cyclin B1 and green fluorescent protein (GFP) to study this dynamic behaviour in real time, in living cells. We found that when we injected cyclin B1-GFP, or cyclin B1-GFP bound to CDK1 (i.e. MPF), into interphase nuclei it is rapidly exported into the cytoplasm. Cyclin B1 nuclear export is blocked by leptomycin B, an inhibitor of the recently identified export factor, exportin 1 (CRM1). The nuclear export of MPF is mediated by a nuclear export sequence in cyclin B1, and an export-defective cyclin B1 accumulates in interphase nuclei. Therefore, during interphase MPF constantly shuttles between the nucleus and the cytoplasm, but the bulk of MPF is retained in the cytoplasm by rapid nuclear export. We found that a cyclin mutant with a defective nuclear export signal does not enhance the premature mitosis caused by interfering with the regulatory phosphorylation of CDK1, but is more sensitive to inhibition by the Wee1 kinase.