Maintenance of CSF arrest: A role for Cdc2 and PP2A-mediated regulation of Emi2

Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.
Current Biology (Impact Factor: 9.57). 03/2007; 17(3):213-24. DOI: 10.1016/j.cub.2006.12.045
Source: PubMed


Vertebrate oocytes are arrested in metaphase II of meiosis prior to fertilization by cytostatic factor (CSF). CSF enforces a cell-cycle arrest by inhibiting the anaphase-promoting complex (APC), an E3 ubiquitin ligase that targets Cyclin B for degradation. Although Cyclin B synthesis is ongoing during CSF arrest, constant Cyclin B levels are maintained. To achieve this, oocytes allow continuous slow Cyclin B degradation, without eliminating the bulk of Cyclin B, which would induce release from CSF arrest. However, the mechanism that controls this continuous degradation is not understood.
We report here the molecular details of a negative feedback loop wherein Cyclin B promotes its own destruction through Cdc2/Cyclin B-mediated phosphorylation and inhibition of the APC inhibitor Emi2. Emi2 bound to the core APC, and this binding was disrupted by Cdc2/Cyclin B, without affecting Emi2 protein stability. Cdc2-mediated phosphorylation of Emi2 was antagonized by PP2A, which could bind to Emi2 and promote Emi2-APC interactions.
Constant Cyclin B levels are maintained during a CSF arrest through the regulation of Emi2 activity. A balance between Cdc2 and PP2A controls Emi2 phosphorylation, which in turn controls the ability of Emi2 to bind to and inhibit the APC. This balance allows proper maintenance of Cyclin B levels and Cdc2 kinase activity during CSF arrest.

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Available from: Hiro Yamano, Sep 30, 2015
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    • "In Xenopus, depletion of Cdc25C phosphatase (the only Cdc25 isoform present in CSF extract) results in inhibitory Thr-14/Tyr-15 phosphorylation of Cdk1, as well as in dephosphorylation of MPF substrates , such as Cdc27/Apc3 and XErp1/Emi2 (Lorca et al, 2010). As described above, this relieve of XErp1/Emi2 from Cdk1-mediated inhibitory effects efficiently inhibits the APC/ C (Wu et al, 2007b), leading to loss of the meiotic state in the absence of cyclin B destruction under these conditions. These data suggest that during the MII-arrest in mature Xenopus eggs, Cdc25 activity is continuously required to maintain Cdk1 in its active state (Gautier et al, 1991; Izumi et al, 1992; Smythe and Newport, 1992). "
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    ABSTRACT: Ex ovo omnia-all animals come from eggs-this statement made in 1651 by the English physician William Harvey marks a seminal break with the doctrine that all essential characteristics of offspring are contributed by their fathers, while mothers contribute only a material substrate. More than 360 years later, we now have a comprehensive understanding of how haploid gametes are generated during meiosis to allow the formation of diploid offspring when sperm and egg cells fuse. In most species, immature oocytes are arrested in prophase I and this arrest is maintained for few days (fruit flies) or for decades (humans). After completion of the first meiotic division, most vertebrate eggs arrest again at metaphase of meiosis II. Upon fertilization, this second meiotic arrest point is released and embryos enter highly specialized early embryonic divisions. In this review, we discuss how the standard somatic cell cycle is modulated to meet the specific requirements of different developmental stages. Specifically, we focus on cell cycle regulation in mature vertebrate eggs arrested at metaphase II (MII-arrest), the first mitotic cell cycle, and early embryonic divisions.
    The EMBO Journal 07/2013; 32(16). DOI:10.1038/emboj.2013.164 · 10.43 Impact Factor
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    • "The control of meiotic cyclin B translation in the unfertilized egg, however, is different from that of mitotic cycles. Polyadenylation of cyclin B1 mRNA has been reported in the unfertilized egg (Sheets et al., 1994; Ballantyne et al., 1997; Kim and Richter, 2007; Belloc et al., 2008), and we confirmed that meiotic CSF extracts are capable of continuous cyclin translation in the presence of elevated CDK1 activity, in agreement with previously published studies (Hansen et al., 2007; Wu et al., 2007; Isoda et al., 2011). Therefore, although translation of cyclin B in meiotic M phase is continuous despite CDK1 activity, it is attenuated during M phase of mitotic cycles. "
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    ABSTRACT: Cyclin B activates cyclin-dependent kinase 1 (CDK1) at mitosis, but conflicting views have emerged on the dynamics of its synthesis during embryonic cycles, ranging from continuous translation to rapid synthesis during mitosis. Here we show that a CDK1-mediated negative-feedback loop attenuates cyclin production before mitosis. Cyclin B plateaus before peak CDK1 activation, and proteasome inhibition caused minimal accumulation during mitosis. Inhibiting CDK1 permitted continual cyclin B synthesis, whereas adding nondegradable cyclin stalled it. Cycloheximide treatment before mitosis affected neither cyclin levels nor mitotic entry, corroborating this repression. Attenuated cyclin production collaborates with its destruction, since excess cyclin B1 mRNA accelerated cyclin synthesis and caused incomplete proteolysis and mitotic arrest. This repression involved neither adenylation nor the 3' untranslated region, but it corresponded with a shift in cyclin B1 mRNA from polysome to nonpolysome fractions. A pulse-driven CDK1-anaphase-promoting complex (APC) model corroborated these results, revealing reduced cyclin levels during an oscillation and permitting more effective removal. This design also increased the robustness of the oscillator, with lessened sensitivity to changes in cyclin synthesis rate. Taken together, the results of this study underscore that attenuating cyclin synthesis late in interphase improves both the efficiency and robustness of the CDK1-APC oscillator.
    Molecular biology of the cell 11/2011; 23(2):284-96. DOI:10.1091/mbc.E11-09-0768 · 4.47 Impact Factor
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    • "In the CSF-arrested eggs of vertebrates , a steady level of maturation-promoting factor (MPF, a complex of catalytic subunit CDC2 and regulatory subunit cyclin B) is maintained via a continuous equilibrium between degradation and synthesis of cyclin B (Kubiak 798 Mol Reprod Dev 78:795–807 (2011) Molecular Reproduction & Development et al., 1993; Yamamoto et al., 2005). The ability of EMI2 to bind and inhibit APC/C is controlled via MPF and protein phosphatase 2A (PP2A), recruited through a MOS-mediated pathway (Wu et al., 2007; Tang et al., 2008; Chang et al., 2011). "
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    ABSTRACT: Mammalian eggs await fertilisation while arrested at the second metaphase stage of meiotic division. A network of signalling pathways enables the establishment and maintenance of this metaphase-II arrest. In the absence of fertilisation, mammalian eggs can spontaneously exit metaphase II when parthenogenetically stimulated, or sometimes without any obvious stimulation. Ovulated rat eggs abortively release from metaphase-II arrest once removed from egg donors. Spontaneously activated rat eggs extrude the second polar body and proceed to the so-called metaphase III-'like' stage, with clumps of condensed chromatin scattered in the egg cytoplasm. It is still unclear what makes rat eggs susceptible to spontaneous activation; however, a vague picture of the signalling pathways involved in the process of spontaneous activation is beginning to emerge. Such cell cycle instability is one of the major reasons why it is more difficult to establish nuclear transfer in the rat. This review examines the known predisposing factors and biochemical mechanisms involved in spontaneous activation. The strategies used to prevent spontaneous metaphase-II release in rat eggs will also be discussed.
    Molecular Reproduction and Development 10/2011; 78(10-11):795-807. DOI:10.1002/mrd.21385 · 2.53 Impact Factor
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