The metaphase II arrest in mouse oocytes is controlled through microtubule-dependent destruction of cyclin B in the presence of CSF.

In unfertilized eggs from vertebrates, the cell cycle is arrested in metaphase of the second meiotic division (metaphase II) until fertilization or activation. Maintenance of the long-term meiotic metaphase arrest requires mechanisms preventing the destruction of the maturation promoting factor (MPF) and the migration of the chromosomes. In frog oocytes, arrest in metaphase II (M II) is achieved by cytostatic factor (CSF) that stabilizes MPF, a heterodimer formed of cdc2 kinase and cyclin. At the metaphase/anaphase transition, a rapid proteolysis of cyclin is associated with MPF inactivation. In Drosophila, oocytes are arrested in metaphase I (M I); however, only mechanical forces generated by the chiasmata seem to prevent chromosome separation. Thus, entirely different mechanisms may be involved in the meiotic arrests in various species. We report here that in mouse oocytes a CSF-like activity is involved in the M II arrest (as observed in hybrids composed of fragments of metaphase II-arrested oocytes and activated mitotic mouse oocytes) and that the high activity of MPF is maintained through a continuous equilibrium between cyclin B synthesis and degradation. In addition, the presence of an intact metaphase spindle is required for cyclin B degradation. Finally, MPF activity is preferentially associated with the spindle after bisection of the oocyte. Taken together, these observations suggest that the mechanism maintaining the metaphase arrest in mouse oocytes involves an equilibrium between cyclin synthesis and degradation, probably controlled by CSF, and which is also dependent upon the three-dimensional organization of the spindle.     

Cell cycle dynamics of an M-phase-specific cytoplasmic factor in Xenopus laevis oocytes and eggs

We have examined the regulation of maturation-promoting factor (MPF) activity in the mitotic and meiotic cell cycles of Xenopus laevis eggs and oocytes. To this end, we developed a method for the small scale extraction of eggs and oocytes and measured MPF activity in extracts by a dilution end point assay. We find that in oocytes, MPF activity appears before germinal vesicle breakdown and then disappears rapidly at the end of the first meiotic cycle. In the second meiotic cycle, MPF reappears before second metaphase, when maturation arrests. Thus, MPF cycling coincides with the abbreviated cycles of meiosis. When oocytes are induced to mature by low levels of injected MPF, cycloheximide does not prevent the appearance of MPF at high levels in the first cycle. This amplification indicates that an MPF precursor is present in the oocyte and activated by posttranslational means, triggered by the low level of injected MPF. Furthermore, MPF disappears approximately on time in such oocytes, indicating that the agent for MPF inactivation is also activated by posttranslational means. However, in the absence of protein synthesis, MPF never reappears in the second meiotic cycle. Upon fertilization or artificial activation of normal eggs, MPF disappears from the cytoplasm within 8 min. For a period thereafter, the inactivating agent remains able to destroy large amounts of MPF injected into the egg. It loses activity just as endogenous MPF appears at prophase of the first mitotic cycle. The repeated reciprocal cycling of MPF and the inactivating agent during cleavage stages is unaffected by colchicine and nocodazole and therefore does not require the effective completion of spindle formation, mitosis, or cytokinesis. However, MPF appearance is blocked by cycloheximide applied before mitosis; and MPF disappearance is blocked by cytostatic factor. In all these respects, MPF and the inactivating agent seem to be tightly linked to, and perhaps participate in, the cell cycle oscillator previously described for cleaving eggs of Xenopus laevis (Hara, K., P. Tydeman, and M. Kirschner, 1980, Proc. Natl. Acad. Sci. USA, 77:462- 466).     

Transient reactivation of CSF in parthenogenetic one-cell mouse embryos

During meiosis, the cytostatic factor (CSF) activity stabilizes the activity of the M-phase promoting factor (MPF) in metaphase II arrested vertebrate oocytes. Upon oocyte activation, the inactivation of both MPF and CSF enables the entry into the first embryonic mitotic cell cycle. Using a biological assay based on cell-fusion (hybrid between a parthenogenetically activated egg entering the first mitotic division and an activated oocyte), we observed that in activated mouse oocytes a first drop in CSF activity is detectable as early as 20 min post-activation. This suggests that CSF is inactivated upon MPF inactivation. However, CSF activity increases again to reach a maximum 60 min post-activation and gradually disappears during the following 40 min. Thus, in activated mouse oocytes (undergoing the transition to interphase) CSF activity fluctuates before definitive inactivation. We found that hybrids arrested in M-phase, thus containing CSF activity after oocyte activation, have activated forms of MAP kinases while hybrids in interphase have inactive forms of these enzymes. We postulate that CSF inactivation in mouse oocytes proceeds in two steps. The initial inactivation of CSF, required for MPF inactivation, is transient and does not require MAP kinase inactivation. The final inactivation of CSF, required for normal embryonic cell cycle progression, is dependent upon the inactivation of MAP kinases.     

Regulation of mitogen-activated protein kinase phosphorylation, microtubule organization, chromatin behavior, and cell cycle progression by protein phosphatases during pig oocyte maturation and fertilization in vitro

We used okadaic acid (OA), a potent inhibitor of protein phosphatases 1 and 2A, to study the regulatory effects of protein phosphatases on mitogen-activated protein (MAP) kinase phosphorylation, morphological changes in the nucleus, and microtubule assembly during pig oocyte maturation and fertilization in vitro. When germinal vesicle (GV) stage oocytes were exposed to OA, MAP kinase phosphorylation was greatly accelerated, being fully activated at 10 min. However, MAP kinase was dephosphorylated by long-term (>20 h) exposure to OA. Correspondingly, premature chromosome condensation and GV breakdown were accelerated, whereas meiotic spindle assembly and meiotic progression beyond metaphase I stage were inhibited. OA also quickly reversed the inhibitory effects of butyrolactone I, a specific inhibitor of maturation-promoting factor (MPF), on MAP kinase phosphorylation and meiosis resumption. Treatment of metaphase II oocytes triggered metaphase II spindle elongation and disassembly as well as chromosome alignment disruption. OA treatment of fertilized eggs resulted in prompt phosphorylation of MAP kinase, disassembly of microtubules around the pronuclear area, chromatin condensation, and pronuclear membrane breakdown, but inhibited further cleavage. Our results suggest that inhibition of protein phosphatases promptly phosphorylates MAP kinase, induces premature chromosome condensation and meiosis resumption as well as pronucleus breakdown, but inhibits spindle organization and suppresses microtubule assembly by sperm centrosomes in pig oocytes and fertilized eggs.     

UV-C irradiation delays mitotic progression by recruiting Mps1 to kinetochores

The effect of UV irradiation on replicating cells during interphase has been studied extensively. However, how the mitotic cell responds to UV irradiation is less well defined. Herein, we found that UV-C irradiation (254 nm) increases recruitment of the spindle checkpoint proteins Mps1 and Mad2 to the kinetochore during metaphase, suggesting that the spindle assembly checkpoint (SAC) is reactivated. In accordance with this, cells exposed to UV-C showed delayed mitotic progression, characterized by a prolonged chromosomal alignment during metaphase. UV-C irradiation also induced the DNA damage response and caused a significant accumulation of γ-H2AX on mitotic chromosomes. Unexpectedly, the mitotic delay upon UV-C irradiation is not due to the DNA damage response but to the relocation of Mps1 to the kinetochore. Further, we found that UV-C irradiation activates Aurora B kinase. Importantly, the kinase activity of Aurora B is indispensable for full recruitment of Mps1 to the kinetochore during both prometaphase and metaphase. Taking these findings together, we propose that UV irradiation delays mitotic progression by evoking the Aurora B-Mps1 signaling cascade, which exerts its role through promoting the association of Mps1 with the kinetochore in metaphase.     

An M-phase-specific protein kinase of Xenopus oocytes: partial purification and possible mechanism of its periodic activation

The activity of a Ca2+- and cyclic nucleotide-independent protein kinase(s) which catalyzes hyperphosphorylation of a set of endogenous proteins, including a 95-kDa soluble phosphoprotein, is found to fluctuate in both the meiotic and mitotic cell cycles of Xenopus oocytes and activated eggs. The activity is high in M-phase and hardly detectable in interphase. The activity copurifies with a major histone kinase(s) throughout four purification steps: ammonium sulfate precipitation, DEAE-cellulose chromatography, high-performance liquid chromatography on TSK G3000, and CM-Sepharose chromatography. This suggests that a single enzyme shares activity against endogenous proteins and added histones. Changes in the activity of the M-phase-specific protein kinase(s) as assayed in vitro correlate with changes in the extent of protein phosphorylation in oocytes pulse-labeled with 32P-phosphate by microinjection during meiotic maturation and the early embryonic cell cycle. This suggests that the kinase(s) has a broad specificity and plays a key role in the increased protein phosphorylation which occurs at the transition to M-phase. Microinjection of the maturation-promoting factor (MPF) into immature oocytes triggers, after a 10-min lag period, the activation of the M-phase specific kinase(s), even in the absence of protein synthesis. In contrast MPF microinjection does not induce kinase activation in cycloheximide-treated oocytes arrested after completion of the first meiotic cell cycle or in activated eggs arrested in S-phase by incubation in cycloheximide. This suggests that immature oocytes contain an inactive kinase precursor (prokinase) which is synthesized at each of the following cell cycles. In the absence of MPF addition, the prokinase to kinase transition occurs "spontaneously" after a 2-hr lag period in high-speed supernatants prepared from prophase-arrested oocytes if low-molecular-weight metabolites are eliminated by gel filtration. Addition of ATP, but not of AMP-PNP (adenylyl-imidodiphosphate), prevents spontaneous kinase activation in gel-filtered extracts. We propose that MPF activates the M-phase-specific protein kinase in the intact cell by inactivating a factor which requires phosphorylation conditions to inhibit the prokinase to kinase transition.     

Mitogen-activated protein kinase (MAP kinase) activation in Xenopus oocytes: Roles of MPF and protein synthesis

Mitogen-activated protein kinase (MAP kinase) is a serine/threonine kinase whose enzymatic activity is thought to play a crucial role in mitogenic signal transduction and also in the progesterone-induced meiotic maturation of Xenopus oocytes. We have purified MAP kinase from Xenopus oocytes and have shown that the protein is present in metaphase ll oocytes under two different forms: an inactive 41-kD protein able to autoactivate and to autophosphorylate in vitro, and an active 42-kD kinase resolved into two tyrosine phosphorylated isoforms on 2D gels. During meiotic maturation, MAP kinase becomes tyrosine phosphorylated and activated following the activation of the M-phase promoting factor (MPF), a complex between the p34^cdc2 kinase and cyclin B. In vivo, MAP kinase activity displays a different stability in metaphase l and in metaphase II: protein synthesis is required to maintain MAP kinase activity in metaphase I but not in metaphase II oocytes. Injection of either MPF or cyclin B into prophase oocytes promotes tyrosine phosphorylation of MAP kinase, indicating that its activation is a downstream event of MPF activation. In contrast, injection of okadaic acid, which induces in vivo MPF activation, promotes only a very weak tyrosine phosphorylation of MAP kinase, suggesting that effectors other than MPF are required for the MAP kinase activation. Moreover, in the absence of protein synthesis, cyclin B and MPF are unable to promote in vivo activation of MAP kinase, indicating that this activation requires the synthesis of new protein(s). © 1993 Wiley-Liss, Inc.     

Ultraviolet-irradiated spermatozoa activate oocytes but arrest preimplantation development after fertilization and nuclear transplantation in cattle

Artificial means of parthenogenetically activating mammalian oocytes are believed to lack an essential sperm epigenetic component required for normal development. The main goal of this study was to examine the potential of ultraviolet (UV)-irradiated sperm as a means of functionally eliminating the chromatin component of spermatozoa without affecting the ability to induce activation and support parthenogenetic development in cattle. Spermatozoa were stained with a DNA dye, exposed to various UV irradiation doses, and used to fertilize secondary oocytes. Although the percentage of pronuclei at 18 h postinsemination was similar using treated and control sperm, most oocytes fertilized by UV-irradiated sperm failed to develop beyond the 2-cell stage, suggesting that UV irradiation can functionally destroy the genomic component of spermatozoa with limited effects on the ability to induce oocyte activation. However, when oocytes activated with UV-irradiated sperm were used as hosts for nuclear transfer, developmental rates to cleavage and to blastocyst improved only marginally and remained lower than in the controls, indicating that UV-treated spermatozoa blocked development even in the presence of a diploid donor nucleus. Although DNA replication was not inhibited by UV irradiation treatment, abnormal chromatin morphology after cleavage suggests improper segregation of chromatin to daughter blastomeres during the first mitotic division. Together, these results indicate that although sperm exposed to UV can activate oocytes, a developmental block occurs at or soon after the first mitosis in parthenotes and oocytes reconstructed by nuclear transfer.     

The fall of biological maturation promoting factor (MPF) and histone H1 kinase activity during anaphase and telophase in mouse oocytes.

Cell fusions have been used to determine the biological activity of the MPF complex in murine oocytes during their progression through anaphase and telophase to metaphase II. Oocytes (1) at metaphase I, (2) during the anaphase-telophase transition, or (3) at metaphase II were fused to germinal vesicle-staged (immature) oocytes. The hybrids were cultured for 1 h in the presence of db cAMP before fixation and nuclear evaluation. Metaphase I oocytes invariably induced germinal vesicle breakdown (GVBD) in the immature partner. By contrast, anaphase/telophase oocytes never induced GVBD in immature oocytes. The capacity to induce GVBD reappears after the formation of the second metaphase plate. In a second study, histone H1 kinase activity was measured during mouse oocyte maturation in single oocytes. H1 kinase activity was low in GV oocytes, increased sharply at MI, declined during anaphase and telophase and increased again at MII. After egg activation, H1 kinase activity was reduced to basal levels. These results provide direct evidence that a drop in activity of MPF in murine oocytes occurs concomitantly with the exit from metaphase I; MPF activity remains low until the cell re-enters metaphase.     

Interaction of cell cycle kinases, microtubules, and chromatin in ascidian oocytes during meiosis

We used kinase assays and confocal microscopy to study the interaction of cell cycle proteins with microtubule organising centres (MTOC) and chromatin in ascidian oocytes during meiosis. The activity of maturation promoting factor (MPF) and mitogen activated protein kinase (MAPK) appear not to be correlated in control oocytes. MPF activity peaks during metaphase I and II of the meiotic cell cycle whereas the activity of MAPK peaks at telophase I and is subsequently degraded to remain at low levels for the remainder of meiosis. The protein synthesis inhibitor emetine induces the degradation in MPF activity in unfertilized metaphase-I (M-I) oocytes, while MAPK is unaffected. Emetine does not alter the activities of these cell cycle kinases in fertilized oocytes during meiosis I but MPF activity remains low while MAPK activity is high for an elongated time period and oocytes do not complete meiosis I. Emetine induces maternal MTOC duplication in unfertilized M-I oocytes and prevents sperm aster growth in fertilized oocytes, but it does not alter the M-I meiotic apparatus in unfertilized oocytes. These experiments suggest that neither MPF alone nor emetine-sensitive proteins are responsible for M-I arrest in ascidian oocytes, MAPK may ensure this stability. In addition, we showed that the maternal MTOC is present at M-I but suppressed from duplicating in an emetine-sensitive manner.     

Does the M-Phase Promoting Factor (MPF) Activate a Major Ca2+-and Cyclic Nucleotide-Independent Protein Kinase in Starfish Ocytes?

In starfish, the activity of a major Ca²⁺-and cyclic nuleotide-independent protein kinase has been shown to fluctuate in phase with that of MPF along meiotic and mitotic cell cycle (23, 25). Microinjection of α-naphthylphosphate (α-NP), a potent phosphatase inhibitor, increased considerably (from 15 to 546 picomoles/min/mg protein) the activity of this major cycling kinase in homogenates. Although this result supported the view that kinase phosphorylation might induce its own activation, this hypothesis was eliminated because injection of cytoplasm from hormone-stimulated enucleated oocytes, which contained the fully activated kinase but no MPF, failed to trigger kinase activation in recipient oocytes. In contrast, kinase activation was induced in recipient oocytes injected with either cytoplasm taken from nucleated maturing oocytes, which contained high MPF and kinase activities, or cytoplasm taken later from hormone-stimulated and ATP-γ-S-injected oocytes which contained high MPF but low kinase activites. These results indicate that inhibiting dephosphorylation of some regulatory protein activates the M-phase-specific protein kinase. The possibility that the M-phase or maturation-promoting factor (MPF) might be this regulatory protein is discussed.     

Involvement of calcium/calmodulin-dependent protein kinase II (CaMKII) in meiotic maturation and activation of pig oocytes

Calcium signal is important for the regulation of meiotic cell cycle in oocytes, but its downstream mechanism is not well known. The functional roles of calcium/calmodulin-dependent protein kinase II (CaMKII) in meiotic maturation and activation of pig oocytes were studied by drug treatment, Western blot analysis, kinase activity assay, indirect immunostaining, and confocal microscopy. The results indicated that meiotic resumption of both cumulus-enclosed and denuded oocytes was prevented by CaMKII inhibitor KN-93, Ant-AIP-II, or CaM antagonist W7 in a dose-dependent manner, but only germinal vesicle breakdown (GVBD) of denuded oocytes was inhibited by membrane permeable Ca2+ chelator BAPTA-AM. When the oocytes were treated with KN-93, W7, or BAPTA-AM after GVBD, the first polar body emission was inhibited. A quick elevation of CaMKII activity was detected after electrical activation of mature pig oocytes, which could be prevented by the pretreatment of CaMKII inhibitors. Treatment of oocytes with KN-93 or W7 resulted in the inhibition of pronuclear formation. The possible regulation of CaMKII on maturation promoting factor (MPF), mitogen-activated protein kinase (MAPK), and ribosome S6 protein kinase (p90rsk) during meiotic cell cycles of pig oocytes was also studied. KN-93 and W7 prevented the accumulation of cyclin B and the full phosphorylation of MAPK and p90rsk during meiotic maturation. When CaMKII activity was inhibited during parthenogenetic activation, cyclin B, the regulatory subunit of MPF, failed to be degraded, but MAPK and p90rsk were quickly dephosphorylated and degraded. Confocal microscopy revealed that CaM and CaMKII were localized to the nucleus and the periphery of the GV stage oocytes. Both proteins were concentrated to the condensed chromosomes after GVBD. In oocytes at the meiotic metaphase MI or MII stage, CaM distributed on the whole spindle, but CaMKII was localized only on the spindle poles. After transition into anaphase, both proteins were translocated to the area between separating chromosomes. All these results suggest that CaMKII is a multifunctional regulator of meiotic cell cycle and spindle assembly and that it may exert its effect via regulation of MPF and MAPK/p90rsk activity during the meiotic maturation and activation of pig oocytes.     

Nuclear responses to MPF activation and inactivation in Xenopus oocytes and early embryos

The cell cycle of most organisms is highlighted by characteristic changes in the appearance and activity of the nucleus. Structural changes in the nucleus are particularly evident when a cell begins to divide. At this time, the nuclear envelope is disassembled, the chromatin condenses into metaphase chromosomes, and the chromosomes associate with a newly formed spindle. Upon completion of cell division the nuclear envelope reassembles around the chromosomes as they form telophase nuclei, and subsequently interphase nuclei, in the daughter cells. The cytoplasmic control of nuclear behavior has been the theme of Yoshio Masui's research for much of his career. His pioneering demonstration that the cytoplasm of maturing amphibian oocytes causes the resumption of the meiotic cell cycle when it is injected into an immature oocyte provided unequivocal evidence that a cytoplasmic factor could initiate the transition from interphase to metaphase (M-phase) in intact cells. As described in several reviews in this and the previous issue of Biology of the Cell (see Beckhelling and Ford; Duesbery and Vande Woude; Maller), Masui initially called this activity maturation promoting factor (MPF), but when it was realized that it was a ubiquitous regulator of both mitotic and meiotic cell cycles, MPF came to stand for M-phase promoting factor. Biochemical evidence indicates that MPF activity is composed of a mitotic B-type cyclins and cyclin-dependent kinase 1. The increase in the protein kinase activity of cdk1 initiates the changes in the nucleus associated with oocyte maturation and with the entry into mitosis. This article will attempt to provide a brief summary of the responses of the nucleus to the activation of MPF. In addition, the effect of MPF inactivation on nuclear envelope assembly at the end of mitosis will be discussed. This article is written as a tribute to Yoshio Masui on his retirement from the University of Toronto, and as an expression of gratitude for his guidance while I was a student in his laboratory. I have felt very privileged to have known him as a mentor and a friend.     

Meiotic cell cycle in Xenopus oocytes is independent of cdk2 kinase.

In vertebrates, M phase-promoting factor (MPF), a universal G2/M regulator in eukaryotic cells, drives meiotic maturation of oocytes, while cytostatic factor (CSF) arrests mature oocytes at metaphase II until fertilization. Cdk2 kinase, a G1/S regulator in higher eukaryotic cells, is activated during meiotic maturation of Xenopus oocytes and, like Mos (an essential component of CSF), is proposed to be involved in metaphase II arrest in mature oocytes. In addition, cdk2 kinase has been shown recently to be essential for MPF activation in Xenopus embryonic mitosis. Here we report injection of Xenopus oocytes with the cdk2 kinase inhibitor p21Cip in order to (re)evaluate the role of cdk2 kinase in oocyte meiosis. Immature oocytes injected with p21Cip can enter both meiosis I and meiosis II normally, as evidenced by the typical fluctuations in MPF activity. Moreover, mature oocytes injected with p21Cip are retained normally in metaphase II for a prolonged period, whereas those injected with neutralizing anti-Mos antibody are released readily from metaphase II arrest. These results argue strongly against a role for cdk2 kinase in MPF activation and its proposed role in metaphase II arrest, in Xenopus oocyte meiosis. We discuss the possibility that cdk2 kinase stored in oocytes may function, as a maternal protein, solely for early embryonic cell cycles.     

Inhibition of protein synthesis affects histone H1 kinase,but not chromosome condensation activity,during the first meiotic division of pig oocytes

The influence of protein synthesis on the regulation of the first meiotic division was studied in pig oocytes. We show that histone H1 kinase activity gradually increases during in vitro culture of pig oocytes, reaching maximum in metaphase I stage after 24 hr of culture. However, in the presence of the protein synthesis inhibitor cycloheximide, histone H1 kinase is not activated during the whole culture period, and after 24 hr it is approximately at the same level as in prophase-stage oocytes. The gradual increase in phosphorylation of six proteins of molecular weights 39, 48, 53, 66, 96, and 120 kDa, observed during the first 24 hr of culture, was not detected when cycloheximide was added to the culture medium. Similarly, the decrease in phosphorylation of a 90-kDa protein was not seen in cycloheximide-treated oocytes. On the other hand, the levels of both MPF components, p34^cdc2 and cyclin B, which were found to be nearly constant during the first meiotic division, were not influenced by cycloheximide treatment as revealed by Western blotting. The process of germinal vesicle breakdown (GVBD) was totally blocked by cycloheximide. The condensation of chromatin, however, was not influenced, suggesting that GVBD and chromosome condensation could be regulated independently. The different degrees of MPF activation involved in these processes, as well as the nature of the protein(s) which must be synthesized for triggering GVBD, are discussed. © 1995 Wiley-Liss, Inc.     

Problems of Oocyte Maturation and the Control of Chromosome Cycles*

During oocyte maturation and zygote development chromosomes undergo cyclic changes, alternaing the condensed and decondensed states. In oocytes, zygotes and perhaps in other cells, the chromosome cycle appears to be controlled in same way by common cytoplasmic factors. Among them, maturation-promoting factor (MPF) plays a particularly important role, although the germinal vesicle substances and cytoplasmic membrane vesicles are indispensable for the chromosomal changes. MPF precursor is stored in fully grown oocytes of most species, but replenishing MPF after its fall during cell cycles requires protein synthesis. During oocyte maturation protein synthesis increases following the activation of MPF, and the synthesized proteins bind with chromosomes that have condensed to a metaphase state. The temporal correlation between the appearance of MPF with chromosome condensation activity and spindle formation observed in various cells suggest a major role played by MPF in the control of chromosome and microtubule assembly cycles. Thus, MPF is a regulator that coordinates the functions of various cell components to advance the chromosome cycle from interphase to metaphase. Therefore, a key to understanding the control of the chromosome cycle lies in knowing factors on which MPF activity is dependent. Although some physiological parameters of the cell are known to affect MPF activity, including Ca ion levels, intracellular pH, protein synthesis activity, cAMP levels, and protein phosphorylation, it seems difficult to assign the control of MPF cycles to any of these parameters. On the contrary, MPF cycles appear to regulate changes in these parameters. Rather, since MPF has the ability to amplify itself by activating its precursor, thus being involved in the MPF-generating system in the cell, the MPF cycle may be an autonomous process. This notion may be supported by the recent observation of the oscillatory activity of MPF in cytosols extracted from frog eggs. We propose theoretical models to explain the MPF oscillator in the cell.     

Oocyte aging in a marine protostome worm: The roles of maturation‐promoting factor and extracellular signal regulated kinase form of mitogen‐activated protein kinase

The roles of maturation‐promoting factor (MPF) and an extracellular signal regulated kinase form of mitogen‐activated protein kinase (ERK MAPK) are analyzed during oocyte aging in the marine protostome worm Cerebratulus. About a day after removal from the ovary, unfertilized metaphase‐I‐arrested oocytes of Cerebratulus begin to flatten and swell before eventually lysing, thereby exhibiting characteristics of a necroptotic mode of regulated cell death. Based on immunoblots probed with phospho‐specific antibodies, MPF and ERK are initially active in freshly mature specimens. However, as oocytes age, both kinase activities decline, with ERK deactivation occurring well before MPF downregulation. Experiments using pharmacological modulators indicate that oocyte degradation is promoted by the maturation‐initiated activation of ERK as well as by the deactivation of MPF that occurs in extensively aged specimens. The potential significance of these findings is discussed relative to previously published results for apoptotic eggs and oocytes of echinoderm and vertebrate deuterostomes.     

c-mos proto-oncogene product is partly degraded after release from meiotic arrest and persists during interphase in mouse zygotes.

Recently, it has been shown that the product of the c-mos proto-oncogene is a component of cytostatic factor, an activity present in unfertilized eggs from vertebrates that arrests the cell cycle in metaphase of the second meiotic division (metaphase II) possibly by stabilizing maturation-promoting factor (MPF). We have studied the behavior of the c-mos product in metaphase II mouse oocytes and soon after activation. The amount of c-mos in the oocyte was still very high after second polar body extrusion, when cyclin B has been degraded and MPF activity had decreased dramatically. Degradation of c-mos takes place later, during the G1 phase of the first cell cycle and a residual amount of c-mos is detectable during the first zygotic interphase. Our data show that the degradation of c-mos is not involved in the release from the metaphase arrest.