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How eggs arrest at metaphase II: MPF stabilisation plus APC/C inhibition equals Cytostatic Factor


Oocytes from higher chordates, including man and nearly all mammals, arrest at metaphase of the second meiotic division before fertilization. This arrest is due to an activity that has been termed 'Cytostatic Factor'. Cytostatic Factor maintains arrest through preventing loss in Maturation-Promoting Factor (MPF; CDK1/cyclin B). Physiologically, Cytostatic Factor – induced metaphase arrest is only broken by a Ca2+ rise initiated by the fertilizing sperm and results in degradation of cyclin B, the regulatory subunit of MPF through the Anaphase-Promoting Complex/Cyclosome (APC/C). Arrest at metaphase II may therefore be viewed as being maintained by inhibition of the APC/C, and Cytostatic Factor as being one or more pathways, one of which inhibits the APC/C, consorting in the preservation of MPF activity.

Many studies over several years have implicated the c-Mos/MEK/MAPK pathway in the metaphase arrest of the two most widely studied vertebrates, frog and mouse. Murine downstream components of this cascade are not known but in frog involve members of the spindle assembly checkpoint, which act to inhibit the APC/C. Interesting these downstream components appear not to be involved in the arrest of mouse eggs, suggesting a lack of conservation with respect to c-Mos targets. However, the recent discovery of Emi2 as an egg specific APC/C inhibitor whose degradation is Ca2+ dependent has greatly increased our understanding of MetII arrest. Emi2 is involved in both the establishment and maintenance of metaphase II arrest in frog and mouse suggesting a conservation of metaphase II arrest. Its identity as the physiologically relevant APC/C inhibitor involved in Cytostatic Factor arrest prompted us to re-evaluate the role of the c-Mos pathway in metaphase II arrest.

This review presents a model of Cytostatic Factor arrest, which is primarily induced by Emi2 mediated APC/C inhibition but which also requires the c-Mos pathway to set MPF levels within physiological limits, not too high to induce an arrest that cannot be broken, or too low to induce parthenogenesis.


Meiosis is a process in which two consecutive cell divisions (MI and MII) occur in the absence of an intervening S-phase. MI is a reductional division in which homologous chromosomes are segregated, sister chromatids are only resolved following the equational MII division (Fig 1a). On completion of MI, oocytes prevent parthenogenetic activation by arresting their cell cycle at metaphase of MII (MetII) due to an activity termed Cytostatic Factor (CSF) [1, 2]. CSF blocks MetII exit until sperm break arrest via a cytoplasmic Ca2+ signal [35] which induces completion of MII.

Figure 1

The events of female meiosis. (A) Only one pair of homologous chromosomes is shown. After S-phase two cell divisions are required to produce a haploid gamete. During MI, homologous chromosomes segregate between the egg and the first polar body. On MI completion, eggs arrest their cell cycle at MetII. MetII exit is blocked through CSF activity, until sperm break the arrest. Eggs complete MII and in so doing segregate sister chromatids and extrude a second polar body. (B) MPF activity oscillates in time with entry to, and exit from metaphase. (C) At MetII eggs arrest their cell cycle with high levels of CSF activity.

Maturation (or M-Phase) Promoting Factor (MPF; CDK1/cyclin B) [6, 7] activity drives somatic cells into mitosis and eggs into meiosis (for reviews see [810]). MPF is regulated during meiosis, oscillating in time with entry to, and exit from MI and MII (Fig 1b). The activity of MPF can be regulated by both CDK1 phosphorylation and cyclin B degradation (for reviews see [2, 11]). Hereafter cyclin B is used to denote any B-type cyclin degraded at metaphase, in frog this constitutes B1, B2, B4 and B5, while in mammals B1 and B2 [12, 13]. Most is known about cyclin B1 and B2 in frog and cyclin B1 in mammals. Mammalian cyclin B1 appears to be particularly important for eggs in Met II arrest, whereas B2 is non-essential and in all of this review, the use of cyclin B in the context of mammalian eggs actually refers to work carried out using cyclin B1 [8, 14, 15]. At M-phase exit, as would occur at fertilization, loss of MPF is normally associated with the rapid destruction of cyclin B by anaphase [16] since CDK1 has no catalytic function without its regulatory partner [17]. Cyclin B is degraded by a Destruction-box motif (D-box) in its primary sequence, which is recognized by the E3 ligase Anaphase Promoting Complex/Cyclosome (APC/C). The APC/C polyubiquitinates key cell cycle proteins such as cyclin B, targeting them for immediate proteolysis by the 26S proteasome [1820].

Eggs arrest at MetII with high MPF due to CSF activity (Fig 1c). The long-term stability of MPF is unique to eggs since in a mitotic metaphase, the APC/C would be active and cyclin B degraded. Although it is possible to exit CSF mediated MetII arrest by inhibiting the CDK1 component of MPF [21, 22], physiologically a sperm Ca2+ signal induces loss of cyclin B rather than CDK1 inactivation. Interestingly in mouse eggs, the APC/C is not completely inhibited during MetII arrest, such that eggs rely on continual cyclin B synthesis to maintain arrest [3, 23, 24]. Similarly in frog eggs, the APC/C remains active enough to degrade cyclin B [25]. At fertilization, Ca2+ stimulates APC/C activity, in mouse about 6-fold [23], such that cyclin B degradation results in MPF loss. Blocking cyclin B degradation by D-box mutation prevents exit from MetII despite a Ca2+ signal [26, 27].

Unlike MPF, the identity of CSF has never been fully resolved, despite simultaneous identification of both activities in a seminal paper [28]. Observations regarding the relationship between MPF and APC/C activity have led to the conclusion that CSF activity is likely to constitute an APC/C inhibitor [2]. In mitosis, most is known about how the APC/C is inhibited by the spindle assembly checkpoint (SAC) proteins therefore we first discuss evidence that CSF activity is due to activation of the SAC pathway.

SAC proteins as CSF

SAC proteins were identified in budding yeast mutants that lost ability to metaphase arrest after addition of spindle poisons [29, 30]. SAC proteins function in arresting cells in metaphase by inhibiting the APC/C until all chromosomes are biorientated and so under tension from spindle microtubules (reviews see [3133]). Vertebrate homologues of the SAC proteins Bub1 (Budding uninhibited by benzimidazole 1) Mad1 and Mad2 (Metaphase arrest deficient 1 and 2) have been suggested to affect MetII arrest in Xenopus eggs. Immunodepletion of these SAC proteins from egg extracts have all been demonstrated to block CSF arrest [34, 35].

SAC components have been implicated as the downstream effectors the c-Mos/MEK/MAPK/p90rsk pathway, long thought to be essential for frog CSF arrest. c-Mos (pp39mos), a proto-oncogene from a family of kinases functioning in signal transduction regulating cell growth and differentiation [36], is highly expressed during germ cell maturation, and has proposed roles throughout frog oocyte maturation [3743]. Functioning as a MAPK kinase kinase (MEKK), c-Mos is important for activation of the MAPK kinase, MEK1 [4446]. MEK1 serves as the upstream activator of MAPK [4749], which switches on the 90-kD ribosomal protein S6 kinase (p90rsk [50]). At fertilization c-Mos is degraded [43], whilst MEK1, MAPK and p90rsk are inactivated shortly afterwards [51, 52].

The c-Mos ...p90rsk signaling cascade has been shown to aid directly MPF activation and stabilization [5355] making it an ideal CSF candidate. Microinjection of c-Mos RNA into two-cell embryos results in metaphase arrest, and immunodepletion of c-Mos causes a loss of cleavage-arresting activity [43]. Similarly, an injection of an active form of MAPK [56] or constitutively-active rsk [57], into blastomeres of two-cell embryos arrests the injected blastomere in metaphase. Indeed in frog, p90rsk has been suggested to be the only MAPK substrate needed for cyclin B re-accumulation on entry to MetII, MetII spindle formation, and CSF arrest [57, 58]. This is supported by the fact that c-Mos protein is unable to establish CSF arrest in frog egg extracts immunodepleted of p90rsk [59].

The SAC component Bub1 is phosphorlyated and activated by p90rsk [60]. Bub1 [34], Mad1 and Mad2 [35] all appear to be required downstream of c-Mos given that the immunodepletion of these proteins blocked the establishment of CSF arrest by c-Mos in frog egg extracts. Such studies suggest a model in which CSF arrest by c-Mos is mediated by Bub1, Mad1 and Mad2 proteins.

From the above it appears that a well-defined CSF pathway has been identified in frog. However, studying SAC components maybe somewhat misleading with respect to identifying CSF. Although CSF activity and the SAC are similar in being able to induce metaphase arrest through APC/C inhibition, they may use different signalling pathways. Any arrest must be reversed by Ca2+ to prove physiological relevance with respect to CSF. A further issue is how CSF arrest can be achieved at MetII but not at MI metaphase (MetI) since many components of the c-Mos pathway are present and active at MetI. For example, c-Mos, MAPK, p90rsk and Bub1 are essential for suppression of S-phase between meiotic divisions [43, 58, 61, 62] yet do not block eggs at MetI. A possible explanation is the involvement of cyclin E/cdk2, both of which are synthesized during MII [63] and inhibit the APC/C.

In frog eggs cyclin E/Cdk2 activity has been reported to play an essential role in CSF arrest. Cyclin E/Cdk2, like c-Mos, can establish metaphase arrest in egg extracts [34, 63]. Cdk2 antisense prevents CSF arrest [64] and recombinant cyclin E/Cdk2 causes metaphase arrest in egg extracts even in the absence of c-Mos [34]. The two pathways (c-Mos and cyclin E/cdk2) are therefore suggested to be independent of each other but both appear to inhibit the APC/C. CSF activity therefore may result from the coexpression of cyclin E/Cdk2 with the c-Mos/MEK/MAPK/p90rsk pathway. However, the role of cyclin E/cdk2 in CSF arrest remains to be fully elucidated since inhibiting cdk2 [65], and ablation of cyclin E [66] have both been reported not disrupt CSF arrest.

Once CSF arrest has been established then many of the above proteins seem no longer required for maintenance (p90rsk, Mad2, Bub1 and cyclin E/cdk2 are all dispensable for maintenance [34, 35, 59]). This suggests that these proteins act upstream or independently of other effectors of CSF activity. SAC proteins may be essential to improve the efficiency of APC/C inhibition on entry into MetII arrest, yet appear redundant in the maintenance of arrest.

Whilst the c-Mos/MEK/MAPK/p90rsk/(SAC proteins) pathway is well established in the frog, its role in mammalian eggs is less clear. Although eggs from c-Mos knockout mice eventually undergo parthenogenetic activation [67, 68], they do MetII arrest, remaining there for 2–4 h, before going on to exit MII [69]. This suggests that whilst c-Mos is critical for protracted MetII arrest, it is not required for its establishment. Loss of MEK or MAPK activity also results in parthenogenesis [21] suggesting as in frog they are downstream components of the c-Mos pathway. However, p90rsk plays no essential role in mouse because eggs from Rsk knockouts arrest at MetII [70]. Furthermore SAC proteins do not mediate CSF activity since mouse eggs expressing dominant negative mutants of Bub1 and Mad2 arrest at MetII [71]. Therefore the c-Mos/MEK/MAPK pathway acting independently of p90rsk is likely only to be involved in helping maintaining MetII arrest in mammals, rather than having a direct role in its establishment.

Emi2 as CSF

When considering all of the above, one may conclude that the c-Mos pathway is unlikely to constitute fully CSF. Recently an egg-specific protein Emi2 (or Early mitotic inhibitor 1-related protein 1; Erp1) has been identified. Emi2 degradation is Ca2+ dependent and likely functions to both establish and maintain CSF arrest by APC/C inhibition [7276]. Interest in Emi2 was generated from work on a related protein Emi1, which prevents premature APC/C activation in G2 of the mitotic cell cycle by binding to the APC/C activator protein Cdc20 [77]. Although Emi1 itself was initially suggested to be involved in MetII arrest [78], this is now known not to be so [79], and was probably due to antibody cross-reactivity between the two Emi proteins [76].

Emi2 is a substrate of a polo-like kinase (Plk), which plays a crucial role in regulating progression through M phase [80], allowing timely activation of the APC/C at the onset of anaphase [8183]. In frog eggs a role for Plx1 in Ca2+-mediated APC/C activation was demonstrated several years ago [81] with the authors proposing the existence of a Plx1-regulated inhibitor of the APC/C active at MetII which was inactivated by Plx1 at fertilization. A later yeast-two hybrid screen for Plx1-interacting proteins identified Emi2. Like CSF activity, Emi2 accumulates during egg maturation, is present and stable in CSF arrested egg extracts, but is rapidly degraded on Ca2+ addition [75]. Plx1 is fully active at metaphase [84], yet at MetII it does not remove inhibition of the APC/C until at fertilization. At fertilization the target of Ca2+ is calmodulin-dependent protein kinase II (CamKII) [85, 86], which acts a priming kinase, directly phosphorylating Emi2 [72]. Plx1 further phosphorylates Emi2 [7274], to generate a degron which is recognised by the SCF ubiquitin ligase, resulting in Emi2 polyubiquitination and destruction [74]. Therefore APC/C inhibition is only removed once both CamKII and Plx1 are active.

In mammals, like frog, a Ca2+ signal breaks MetII arrest through a signalling pathway involving CamKII and activation the APC/C [8789]. Although the full mechanism of Emi2 degradation has not yet been demonstrated in mouse, like frog, mouse Emi2 contains specific motifs for phosphorylation by both Plk and CamKII. Given that ablation of Emi2 in MetII arrested mouse eggs results in parthenogenetic activation [90], it would appear that the target of CamKII in mouse eggs is also Emi2. Supporting a role in maintaining CSF activity, Emi2 is extremely stable in MetII eggs, yet rapidly degraded by Ca2+ [91]. As predicted for an APC/C inhibitor, it follows that on release from MetII, Emi2 destruction precedes that of cyclin B [91]. These findings taken together demonstrate an essential role for Emi2 in the maintenance of MetII arrest.

On entry into MetII Emi2 is also important for APC/C inhibition, allowing cyclin B, and so MPF, accumulation. Emi2 levels are low in oocyte maturation, presumably to allow the APC/C to be active and permit passage through MI [91]. Emi2 morpholinos added to maturing mouse oocytes prevent cyclin B re-accumulation on entry into MII and eggs consequentially fail to form MetII spindles, eventually decondensing their chromatin[91]. In this work, MetII arrest was rescued by re-addition of Emi2, expression of a D-box mutant of cyclin B or by addition of nocodazaole to induce a SAC mediated arrest. Emi2 also appears to be essential for the establishment of arrest in frog eggs [92, 93] suggesting a conserved mechanism in vertebrates.

Conclusion: Our model, MPF stabilisation plus APC/C inhibition equals CSF

A unifying hypothesis would be useful which invokes most of the CSF candidates described in both mouse and frog (the c-Mos/MEK/MAPK/p90rsk/Bub1; Mad1 and Mad2; cyclin E1/cdk2 and Emi2). Having a completely unified mechanism however looks unlikely given that the Mos/MAPK pathway in mouse does not involve p90rsk. To establish whether Emi2 and the c-Mos pathway function independently remains important but here we go on to suggest a working model of how these two pathways interact.

We propose that MetII arrest is established through Emi2-mediated APC/C inhibition, and maintained both by Emi2 and the c-Mos/MAPK pathway, which acts to stabilise MPF (Fig 2). The exact nature of the effect of the c-Mos pathway on MPF stability is still to be fully resolved however Yamamoto et al. [94] in frog eggs showed that when MPF activity reaches a critical lower level, the c-Mos/MAPK pathway suppresses cyclin B degradation in order to elevate MPF levels; whilst elevation of MPF beyond a critical upper level activates APC/C dependent cyclin B degradation [94]. This suggests that Mos may help set the level of MPF activity. A CSF-arrested frog egg extract will exit MetII without cyclin B loss or a Ca2+ stimulus when Greatwall kinase, known to positively affect MPF activity, is immunodepleted [95]. This illustrates the point that when considering the protacted nature of MetII arrest, one must consider mechanisms in the egg which are designed to respond to the sperm (Emi2 mediated APC/C inactivity) as well as those designed to keep MPF active until the time of fertilization. Our suggestion is that the c-Mos pathway may contribute to this second mechanism.

Figure 2

Model of the regulation of MetII arrest in mammalian eggs. High MPF activity is essential for MetII arrest and may be maintained via separate pathways; direct inhibition of the APC/C, and direct stabilization of MPF. The pathway which involves Emi2-mediated CSF arrest is shown in solid lines. In mouse eggs, the c-Mos pathway is not mediated by p90rsk, so its downstream targets remain obscure (dashed lines), but potential target points are shown as either inhibition of the APC/C or inhibition of Emi2 degradation. MPF activity may negatively regulate the c-Mos pathway, as based on studies from frog [94]. See text for further details.



naphase-Promoting Complex/Cyclosome


Budding uninhibited by benzimidazole


Calmodulin-dependent protein kinase II


Cytostatic Factor




Early mitotic inhibitor 2


mitotic-arrest deficient


first meiotic division


second meiotic division


metaphase I


metaphase II


Maturation (M-Phase)-Promoting Factor; CDK1/cyclin B


90-kD ribosomal protein S6 kinase


  1. 1.

    Jones KT: Mammalian egg activation: from Ca2+ spiking to cell cycle progression. Reproduction 2005, 130: 813–823. 10.1530/rep.1.00710

    CAS  PubMed  Article  Google Scholar 

  2. 2.

    Tunquist BJ, Maller JL: Under arrest: cytostatic factor (CSF)-mediated metaphase arrest in vertebrate eggs. Genes Dev 2003, 17: 683–710. 10.1101/gad.1071303

    CAS  PubMed  Article  Google Scholar 

  3. 3.

    Jones KT: Ca2+ oscillations in the activation of the egg and development of the embryo in mammals. Int J Dev Biol 1998, 42: 1–10.

    CAS  PubMed  Google Scholar 

  4. 4.

    Runft LL, Jaffe LA, Mehlmann LM: Egg activation at fertilization: where It all begins. Dev Biol 2002, 245: 237–254. 10.1006/dbio.2002.0600

    CAS  PubMed  Article  Google Scholar 

  5. 5.

    Stricker SA: Comparative biology of calcium signaling during fertilization and egg activation in animals. Dev Biol 1999, 211: 157–176. 10.1006/dbio.1999.9340

    CAS  PubMed  Article  Google Scholar 

  6. 6.

    Gautier J, Minshull J, Lohka M, Glotzer M, Hunt T, Maller JL: Cyclin is a component of maturation-promoting factor from Xenopus. Cell 1990, 60: 487–494. 10.1016/0092-8674(90)90599-A

    CAS  PubMed  Article  Google Scholar 

  7. 7.

    Lohka MJ, Hayes MK, Maller JL: Purification of Maturation-Promoting Factor, an intracellular regulator of early mitotic events. Proc Natl Acad Sci USA 1988, 85: 3009–3013. 10.1073/pnas.85.9.3009

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  8. 8.

    Doree M, Hunt T: From Cdc2 to Cdk1: when did the cell cycle kinase join its cyclin partner? J Cell Sci 2002, 115: 2461–2464.

    CAS  PubMed  Google Scholar 

  9. 9.

    Masui Y: From oocyte maturation to the in vitro cell cycle: the history of discoveries of Maturation-Promoting Factor (MPF) and Cytostatic Factor (CSF). Differentiation 2001, 69: 1–17. 10.1046/j.1432-0436.2001.690101.x

    CAS  PubMed  Article  Google Scholar 

  10. 10.

    Pines J: Four-dimensional control of the cell cycle. Nat Cell Biol 1999, 1: E73–79. 10.1038/11041

    CAS  PubMed  Article  Google Scholar 

  11. 11.

    Morgan DO: Principles of CDK regulation. Nature 1995, 374: 131–134. 10.1038/374131a0

    CAS  PubMed  Article  Google Scholar 

  12. 12.

    Chapman DL, Wolgemuth DJ: Isolation of the murine cyclin B2 cDNA and characterization of the lineage and temporal specificity of expression of the B1 and B2 cyclins during oogenesis, spermatogenesis and early embryogenesis. Development 1993, 118: 229–240.

    CAS  PubMed  Google Scholar 

  13. 13.

    Hochegger H, Klotzbucher A, Kirk J, Howell M, le Guellec K, Fletcher K, Duncan T, Sohail M, Hunt T: New B-type cyclin synthesis is required between meiosis I and II during Xenopus oocyte maturation. Development 2001, 128: 3795–3807.

    CAS  PubMed  Google Scholar 

  14. 14.

    Brandeis M, Rosewell I, Carrington M, Crompton T, Jacobs MA, Kirk J, Gannon J, Hunt T: Cyclin B2-null mice develop normally and are fertile whereas cyclin B1-null mice die in utero. Proc Natl Acad Sci USA 1998, 95: 4344–4349. 10.1073/pnas.95.8.4344

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  15. 15.

    Kuroda T, Naito K, Sugiura K, Yamashita M, Takakura I, Tojo H: Analysis of the roles of cyclin B1 and cyclin B2 in porcine oocyte maturation by inhibiting synthesis with antisense RNA injection. Biol Reprod 2004, 70: 154–159. 10.1095/biolreprod.103.021519

    CAS  PubMed  Article  Google Scholar 

  16. 16.

    Clute P, Pines J: Temporal and spatial control of cyclin B1 destruction in metaphase. Nat Cell Biol 1999, 1: 82–87. 10.1038/10049

    CAS  PubMed  Article  Google Scholar 

  17. 17.

    Pines J, Hunter T: Isolation of a human cyclin cDNA: evidence for cyclin mRNA and protein regulation in the cell cycle and for interaction with p34cdc2. Cell 1989, 58: 833–846. 10.1016/0092-8674(89)90936-7

    CAS  PubMed  Article  Google Scholar 

  18. 18.

    Morgan DO: Regulation of the APC and the exit from mitosis. Nat Cell Biol 1999, 1: E47–53. 10.1038/10039

    CAS  PubMed  Article  Google Scholar 

  19. 19.

    Peters JM: The anaphase-promoting complex: proteolysis in mitosis and beyond. Mol Cell 2002, 9: 931–943. 10.1016/S1097-2765(02)00540-3

    CAS  PubMed  Article  Google Scholar 

  20. 20.

    Zachariae W, Nasmyth K: Whose end is destruction: cell division and the anaphase-promoting complex. Genes Dev 1999, 13: 2039–2058.

    CAS  PubMed  Article  Google Scholar 

  21. 21.

    Phillips KP, Petrunewich MAF, Collins JL, Booth RA, Liu XJ, Baltz JM: Inhibition of MEK or cdc2 kinase parthenogenetically activates mouse eggs and yields the same phenotypes as Mos-/- parthenogenotes. Dev Biol 2002, 247: 210–223. 10.1006/dbio.2002.0680

    CAS  PubMed  Article  Google Scholar 

  22. 22.

    Yu H: Regulation of APC-Cdc20 by the spindle checkpoint. Curr Opin Cell Biol 2002, 14: 706–714. 10.1016/S0955-0674(02)00382-4

    CAS  PubMed  Article  Google Scholar 

  23. 23.

    Nixon VL, Levasseur M, McDougall A, Jones KT: Ca2+ oscillations promote APC/C-dependent cyclin B1 degradation during metaphase arrest and completion of meiosis in fertilizing mouse eggs. Curr Biol 2002, 12: 746–750. 10.1016/S0960-9822(02)00811-4

    CAS  PubMed  Article  Google Scholar 

  24. 24.

    Winston NJ: Stability of cyclin B protein during meiotic maturation and the first mitotic cell division in mouse oocytes. Biol Cell 1997, 89: 211–219. 10.1016/S0248-4900(97)80037-8

    CAS  PubMed  Article  Google Scholar 

  25. 25.

    Thibier C, De Smedt V, Poulhe R, Huchon D, Jessus C, Ozon R: In vivo regulation of cytostatic activity in Xenopus metaphase II-arrested oocytes. Dev Biol 1997, 185: 55–66. 10.1006/dbio.1997.8543

    CAS  PubMed  Article  Google Scholar 

  26. 26.

    Madgwick S, Nixon VL, Chang H-Y, Herbert M, Levasseur M, Jones KT: Maintenance of sister chromatid attachment in mouse eggs through maturation-promoting factor activity. Dev Biol 2004, 275: 68–81. 10.1016/j.ydbio.2004.07.024

    CAS  PubMed  Article  Google Scholar 

  27. 27.

    Murray AW, Solomon MJ, Kirschner MW: The role of cyclin synthesis and degradation in the control of maturation promoting factor activity. Nature 1989, 339: 280–286. 10.1038/339280a0

    CAS  PubMed  Article  Google Scholar 

  28. 28.

    Masui Y, Market CL: Cytoplasmic control of nuclear behaviour during meiotic matutation of frog oocytes. J Exp Zool 1971, 177: 129–145. 10.1002/jez.1401770202

    CAS  PubMed  Article  Google Scholar 

  29. 29.

    Hoyt MA, Totis L, Roberts BT: S. cerevisiae genes required for cell cycle arrest in response to loss of microtubule function. Cell 1991, 66: 507–517. 10.1016/0092-8674(81)90014-3

    CAS  PubMed  Article  Google Scholar 

  30. 30.

    Li R, Murray AW: Feedback control of mitosis in budding yeast. Cell 1991, 66: 519–531. 10.1016/0092-8674(81)90015-5

    CAS  PubMed  Article  Google Scholar 

  31. 31.

    Musacchio A, Hardwick KG: The spindle checkpoint: structural insights into dynamic signalling. Nat Rev Mol Cell Biol 2002, 3: 731–741. 10.1038/nrm929

    CAS  PubMed  Article  Google Scholar 

  32. 32.

    Shah JV, Cleveland DW: Waiting for anaphase: Mad2 and the spindle assembly checkpoint. Cell 2000, 103: 997–1000. 10.1016/S0092-8674(00)00202-6

    CAS  PubMed  Article  Google Scholar 

  33. 33.

    Wassmann K, Benezra R: Mitotic checkpoints: from yeast to cancer. Curr Opin Genet Dev 2001, 11: 83–90. 10.1016/S0959-437X(00)00161-1

    CAS  PubMed  Article  Google Scholar 

  34. 34.

    Tunquist BJ, Schwab MS, Chen LG, Maller JL: The spindle checkpoint kinase Bub1 and cyclin E/Cdk2 both contribute to the establishment of meiotic metaphase arrest by Cytostatic Factor. Curr Biol 2002, 12: 1027–1033. 10.1016/S0960-9822(02)00894-1

    CAS  PubMed  Article  Google Scholar 

  35. 35.

    Tunquist BJ, Eyers PA, Chen LG, Lewellyn AL, Maller JL: Spindle checkpoint proteins Mad1 and Mad2 are required for cytostatic factor-mediated metaphase arrest. J Cell Biol 2003, 163: 1231–1242. 10.1083/jcb.200306153

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  36. 36.

    Hunter T: A thousand and one protein kinases. Cell 1987, 50: 823–829. 10.1016/0092-8674(87)90509-5

    CAS  PubMed  Article  Google Scholar 

  37. 37.

    Goldman DS, Kiessling AA, Millette CF, Cooper GM: Expression of c-mos RNA in germ cells of male and female mice. Proc Natl Acad Sci USA 1987, 84: 4509–4513. 10.1073/pnas.84.13.4509

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  38. 38.

    Keshet E, Rosenberg MP, Mercer JA, Propst F, Vande Woude GF, Jenkins NA, Copeland NG: Developmental regulation of ovarian-specific Mos expression. Oncogene 1988, 2: 235–240.

    CAS  PubMed  Google Scholar 

  39. 39.

    Mutter GL, Wolgemuth DJ: Distinct developmental patterns of c-mos protooncogene expression in female and male mouse germ cells. Proc Natl Acad Sci USA 1987, 84: 5301–5305. 10.1073/pnas.84.15.5301

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  40. 40.

    Paules RS, Buccione R, Moschel RC, Vande Woude GF, Eppig JJ: Mouse Mos protooncogene product is present and functions during oogenesis. Proc Natl Acad Sci USA 1989, 86: 5395–5399. 10.1073/pnas.86.14.5395

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  41. 41.

    Propst F, Rosenberg MP, Iyer A, Kaul K, Vande Woude GF: c-mos proto-oncogene RNA transcripts in mouse tissues: structural features, developmental regulation, and localization in specific cell types. Mol Cell Biol 1987, 7: 1629–1637.

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  42. 42.

    Sagata N, Oskarsson M, Copeland T, Brumbaugh J, Vande Woude GF: Function of c-mos proto-oncogene product in meiotic maturation in Xenopus oocytes. Nature 1988, 335: 519–525. 10.1038/335519a0

    CAS  PubMed  Article  Google Scholar 

  43. 43.

    Sagata N, Watanabe N, Vande Woude GF, Ikawa Y: The c-mos proto-oncogene product is a cytostatic factor responsible for meiotic arrest in vertebrate eggs. Nature 1989, 342: 512–518. 10.1038/342512a0

    CAS  PubMed  Article  Google Scholar 

  44. 44.

    Nebreda AR, Hunt T: The c-mos proto-oncogene protein kinase turns on and maintains the activity of MAP kinase, but not MPF, in cell-free extracts of Xenopus oocytes and eggs. EMBO J 1993, 12: 1979–1986.

    CAS  PubMed Central  PubMed  Google Scholar 

  45. 45.

    Posada J, Yew N, Ahn NG, Vande Woude GF, Cooper JA: Mos stimulates MAP kinase in Xenopus oocytes and activates a MAP kinase kinase in vitro. Mol Cell Biol 1993, 13: 2546–2553.

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  46. 46.

    Shibuya EK, Ruderman JV: Mos induces the in vitro activation of mitogen-activated protein kinases in lysates of frog oocytes and mammalian somatic cells. Mol Biol Cell 1993, 4: 781–790.

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  47. 47.

    Kosako H, Gotoh Y, Matsuda S, Ishikawa M, Nishida E: Xenopus MAP kinase activator is a serine/threonine/tyrosine kinase activated by threonine phosphorylation. EMBO J 1992, 11: 2903–2908.

    CAS  PubMed Central  PubMed  Google Scholar 

  48. 48.

    Matsuda S, Kosako H, Takenaka K, Moriyama K, Sakai H, Akiyama T, Gotoh Y, Nishida E: Xenopus MAP kinase activator: identification and function as a key intermediate in the phosphorylation cascade. EMBO J 1992, 11: 973–982.

    CAS  PubMed Central  PubMed  Google Scholar 

  49. 49.

    Tobe K, Kadowaki T, Hara K, Gotoh Y, Kosako H, Matsuda S, Tamemoto H, Ueki K, Akanuma Y, Nishida E, et al.: Sequential activation of MAP kinase activator, MAP kinases, and S6 peptide kinase in intact rat liver following insulin injection. J Biol Chem 1992, 267: 21089–21097.

    CAS  PubMed  Google Scholar 

  50. 50.

    Sturgill TW, Ray LB, Erikson E, Maller JL: Insulin-stimulated MAP-2 kinase phosphorylates and activates ribosomal protein S6 kinase II. Nature 1988, 334: 715–718. 10.1038/334715a0

    CAS  PubMed  Article  Google Scholar 

  51. 51.

    Ferrell JE Jr, Wu M, Gerhart JC, Martin GS: Cell cycle tyrosine phosphorylation of p34cdc2 and a microtubule-associated protein kinase homolog in Xenopus oocytes and eggs. Mol Cell Biol 1991, 11: 1965–1971.

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  52. 52.

    Hartley RS, Lewellyn AL, Maller JL: MAP kinase is activated during mesoderm induction in Xenopus laevis. Dev Biol 1994, 163: 521–524. 10.1006/dbio.1994.1168

    CAS  PubMed  Article  Google Scholar 

  53. 53.

    Chau AS, Shibuya EK: Mos-induced p42 mitogen-activated protein kinase activation stabilizes M-phase in Xenopus egg extracts after cyclin destruction. Biol Cell 1998, 90: 565–572. 10.1016/S0248-4900(99)80014-8

    CAS  PubMed  Article  Google Scholar 

  54. 54.

    Palmer A, Gavin AC, Nebreda AR: A link between MAP kinase and p34cdc2/cyclin B during oocyte maturation: p90rsk phosphorylates and inactivates the p34cdc2 inhibitory kinase Myt1. EMBO J 1998, 17: 5037–5047. 10.1093/emboj/17.17.5037

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  55. 55.

    Peter M, Labbe JC, Doree M, Mandart E: A new role for Mos in Xenopus oocyte maturation: targeting Myt1 independently of MAPK. Development 2002, 129: 2129–2139.

    CAS  PubMed  Google Scholar 

  56. 56.

    Haccard O, Sarcevic B, Lewellyn A, Hartley R, Roy L, Izumi T, Erikson E, Maller JL: Induction of metaphase arrest in cleaving Xenopus embryos by MAP kinase. Science 1993, 262: 1262–1265. 10.1126/science.8235656

    CAS  PubMed  Article  Google Scholar 

  57. 57.

    Gross SD, Schwab MS, Lewellyn AL, Maller JL: Induction of metaphase arrest in cleaving Xenopus embryos by the protein kinase p90Rsk. Science 1999, 286: 1365–1367. 10.1126/science.286.5443.1365

    CAS  PubMed  Article  Google Scholar 

  58. 58.

    Gross SD, Schwab MS, Taieb FE, Lewellyn AL, Qian Y-W, Maller JL: The critical role of the MAP kinase pathway in meiosis II in Xenopus oocytes is mediated by p90Rsk. Curr Biol 2000, 10: 430–438. 10.1016/S0960-9822(00)00425-5

    CAS  PubMed  Article  Google Scholar 

  59. 59.

    Bhatt RR, Ferrell JE Jr: The protein kinase p90 rsk as an essential mediator of cytostatic factor activity. Science 1999, 286: 1362–1365. 10.1126/science.286.5443.1362

    CAS  PubMed  Article  Google Scholar 

  60. 60.

    Schwab MS, Roberts BT, Gross SD, Tunquist BJ, Taieb FE, Lewellyn AL, Maller JL: Bub1 is activated by the protein kinase p90Rsk during Xenopus oocyte maturation. Curr Biol 2001, 11: 141–150. 10.1016/S0960-9822(01)00045-8

    CAS  PubMed  Article  Google Scholar 

  61. 61.

    Furuno N, Nishizawa M, Okazaki K, Tanaka H, Iwashita J, Nakajo N, Ogawa Y, Sagata N: Suppression of DNA replication via Mos function during meiotic divisions in Xenopus oocytes. EMBO J 1994, 13: 2399–2410.

    CAS  PubMed Central  PubMed  Google Scholar 

  62. 62.

    Tachibana K, Tanaka D, Isobe T, Kishimoto T: c-Mos forces the mitotic cell cycle to undergo meiosis II to produce haploid gametes. Proc Natl Acad Sci USA 2000, 97: 14301–14306. 10.1073/pnas.97.26.14301

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  63. 63.

    Rempel RE, Sleight SB, Maller JL: Maternal Xenopus Cdk2-cyclin E complexes function during meiotic and early embryonic cell cycles that lack a G1 phase. J Biol Chem 1995, 270: 6843–6855. 10.1074/jbc.270.28.16918

    CAS  PubMed  Article  Google Scholar 

  64. 64.

    Gabrielli BG, Roy LM, Maller JL: Requirement for Cdk2 in cytostatic factor-mediated metaphase II arrest. Science 1993, 259: 1766–1769. 10.1126/science.8456304

    CAS  PubMed  Article  Google Scholar 

  65. 65.

    Furuno N, Ogawa Y, Iwashita J, Nakajo N, Sagata N: Meiotic cell cycle in Xenopus oocytes is independent of cdk2 kinase. EMBO J 1997, 16: 3860–3865. 10.1093/emboj/16.13.3860

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  66. 66.

    Grimison B, Liu J, Lewellyn AL, Maller JL: Metaphase arrest by cyclin E-Cdk2 requires the spindle-checkpoint kinase Mps1. Curr Biol 2006, 16: 1968–1973. 10.1016/j.cub.2006.08.055

    CAS  PubMed  Article  Google Scholar 

  67. 67.

    Colledge WH, Carlton MBL, Udy GB, Evans MJ: Disruption of c-mos causes parthenogenetic development of unfertilized mouse eggs. Nature 1994, 370: 65–68. 10.1038/370065a0

    CAS  PubMed  Article  Google Scholar 

  68. 68.

    Hashimoto N, Watanabe N, Furuta Y, Tamemoto H, Sagata N, Yokoyama M, Okazaki K, Nagayoshi M, Takedat N, Ikawatll Y, Aizawai S: Parthenogenetic activation of oocytes in c-mos-deficient mice. Nature 1994, 370: 68–71. 10.1038/370068a0

    CAS  PubMed  Article  Google Scholar 

  69. 69.

    Verlhac MH, Kubiak JZ, Weber M, Geraud G, Colledge WH, Evans MJ, Maro B: Mos is required for MAP kinase activation and is involved in microtubule organization during meiotic maturation in the mouse. Development 1996, 122: 815–822.

    CAS  PubMed  Google Scholar 

  70. 70.

    Dumont J, Umbhauer M, Rassinier P, Hanauer A, Verlhac M-H: p90Rsk is not involved in cytostatic factor arrest in mouse oocytes. J Cell Biol 2005, 169: 227–231. 10.1083/jcb.200501027

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  71. 71.

    Tsurumi C, Hoffmann S, Geley S, Graeser R, Polanski Z: The spindle assembly checkpoint is not essential for CSF arrest of mouse oocytes. J Cell Biol 2004, 167: 1037–1050. 10.1083/jcb.200405165

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  72. 72.

    Hansen DV, Tung JJ, Jackson PK: CaMKII and Polo-like kinase 1 sequentially phosphorylate the cytostatic factor Emi2/XErp1 to trigger its destruction and meiotic exit. Proc Natl Acad Sci USA 2006, 103: 608–613. 10.1073/pnas.0509549102

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  73. 73.

    Liu J, Maller JL: Calcium elevation at fertilization coordinates phosphorylation of XErp1/Emi2 by Plx1 and CaMK II to release metaphase arrest by Cytostatic Factor. Curr Biol 2005, 15: 1458–1468. 10.1016/j.cub.2005.07.030

    CAS  PubMed  Article  Google Scholar 

  74. 74.

    Rauh NR, Schmidt A, Bormann J, Nigg EA, Mayer TU: Calcium triggers exit from meiosis II by targeting the APC/C inhibitor XErp1 for degradation. Nature 2005, 437: 1048–1052. 10.1038/nature04093

    CAS  PubMed  Article  Google Scholar 

  75. 75.

    Schmidt A, Duncan PI, Rauh NR, Sauer G, Fry AM, Nigg EA, Mayer TU: Xenopus polo-like kinase Plx1 regulates XErp1, a novel inhibitor of APC/C activity. Genes Dev 2005, 19: 502–513. 10.1101/gad.320705

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  76. 76.

    Tung JJ, Hansen DV, Ban KH, Loktev AV, Summers MK, Adler JR III, Jackson PK: A role for the anaphase-promoting complex inhibitor Emi2/XErp1, a homolog of early mitotic inhibitor 1, in cytostatic factor arrest of Xenopus eggs. Proc Natl Acad Sci USA 2005, 102: 4318–4323. 10.1073/pnas.0501108102

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  77. 77.

    Reimann JDR, Freed E, Hsu JY, Kramer ER, Peters J-M, Jackson PK: Emi1 Is a mitotic regulator that interacts with Cdc20 and inhibits the Anaphase Promoting Complex. Cell 2001, 105: 645–655. 10.1016/S0092-8674(01)00361-0

    CAS  PubMed  Article  Google Scholar 

  78. 78.

    Reimann JD, Jackson PK: Emi1 is required for cytostatic factor arrest in vertabrate eggs. Nature 2002, 416: 850–854. 10.1038/416850a

    CAS  PubMed  Article  Google Scholar 

  79. 79.

    Ohsumi K, Koyanagi A, Yamamoto TM, Gotoh T, Kishimoto T: Emi1-mediated M-phase arrest in Xenopus eggs is distinct from cytostatic factor arrest. Proc Natl Acad Sci USA 2004, 101: 12531–12536. 10.1073/pnas.0405300101

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  80. 80.

    Barr FA, Sillje HH, Nigg EA: Polo-like kinases and the orchestration of cell division. Nat Rev Mol Cell Biol 2004, 5: 429–440. 10.1038/nrm1401

    CAS  PubMed  Article  Google Scholar 

  81. 81.

    Descombes P, Nigg EA: The polo-like kinase Plx is required for M phase exit and destruction of mitotic regulators in Xenopus egg extracts. EMBO J 1998, 17: 1328–1335. 10.1093/emboj/17.5.1328

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  82. 82.

    Donaldson MM, Tavares AAM, Ohkura H, Deak P, Glover DM: Metaphase arrest with centromere separation in polo mutants of Drosophila . J Cell Biol 2001, 153: 663–676. 10.1083/jcb.153.4.663

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  83. 83.

    Shirayama M, Zachariae W, Ciosk R, Nasmyth K: The Polo-like kinase Cdc5p and the WD-repeat protein Cdc20p/fizzy are regulators and substrates of the anaphase promoting complex in Saccharomyces cerevisiae. EMBO J 1998, 17: 1336–1349. 10.1093/emboj/17.5.1336

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  84. 84.

    Qian Y-W, Erikson E, Li C, Maller JL: Activated Polo-Like Kinase Plx1 Is Required at Multiple Points during Mitosis in Xenopus laevis. Mol Cell Biol 1998, 18: 4262–4271.

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  85. 85.

    Lorca T, Cruzalegui FH, Fesquet D, Cavadore JC, Mery J, Means A, Doree M: Calmodulin-dependent protein kinase II mediates inactivation of MPF and CSF upon fertilization of Xenopus eggs. Nature 1993, 366: 270–273. 10.1038/366270a0

    CAS  PubMed  Article  Google Scholar 

  86. 86.

    Lorca T, Galas S, Fesquet D, Devault A, Cavadore JC, Doree M: Degradation of the proto-oncogene product p39mos is not necessary for cyclin proteolysis and exit from meiotic metaphase: requirement for a Ca2+ -calmodulin dependent event. EMBO J 1991, 10: 2087–2093.

    CAS  PubMed Central  PubMed  Google Scholar 

  87. 87.

    Madgwick S, Levasseur M, Jones KT: Calmodulin-dependent protein kinase II, and not protein kinase C, is sufficient for triggering cell-cycle resumption in mammalian eggs. J Cell Sci 2005, 118: 3849–3859. 10.1242/jcs.02506

    CAS  PubMed  Article  Google Scholar 

  88. 88.

    Markoulaki S, Matson S, Abbott AL, Ducibella T: Oscillatory CaMKII activity in mouse egg activation. Dev Biol 2003, 258: 464–474. 10.1016/S0012-1606(03)00133-7

    CAS  PubMed  Article  Google Scholar 

  89. 89.

    Markoulaki S, Matson S, Ducibella T: Fertilization stimulates long-lasting oscillations of CaMKII activity in mouse eggs. Dev Biol 2004, 272: 15–25. 10.1016/j.ydbio.2004.04.008

    CAS  PubMed  Article  Google Scholar 

  90. 90.

    Shoji S, Yoshida N, Amanai M, Ohgishi M, Fukui T, Fujimoto S, Nakano Y, Kajikawa E, Perry A: Mammalian Emi2 mediates cytostatic arrest and transduces the signal for meiotic exit via Cdc20. EMBO J 2006, 25: 834–845. 10.1038/sj.emboj.7600953

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  91. 91.

    Madgwick S, Hansen DV, Levasseur M, Jackson PK, Jones KT: Mouse Emi2 is required to enter meiosis II by reestablishing cyclin B1 during interkinesis. J Cell Biol 2006, 174: 791–801. 10.1083/jcb.200604140

    CAS  PubMed Central  PubMed  Article  Google Scholar 

  92. 92.

    Ohe M, Inoue D, Kanemori Y, Sagata N: Erp1/Emi2 is essential for the meiosis I to meiosis II transition in Xenopus oocytes. Dev Biol 2007, in press.

    Google Scholar 

  93. 93.

    Liu J, Grimison B, Lewellyn AL, Maller JL: The Anaphase-promoting Complex/Cyclosome inhibitor Emi2 is essential for meiotic but not mitotic cell cycles. J Biol Chem 2006, 281: 34736–34741. 10.1074/jbc.M606607200

    CAS  PubMed  Article  Google Scholar 

  94. 94.

    Yamamoto TM, Iwabuchi M, Ohsumi K, Kishimoto T: APC/C-Cdc20-mediated degradation of cyclin B participates in CSF arrest in unfertilized Xenopus eggs. Dev Biol 2005, 279: 345–355. 10.1016/j.ydbio.2004.12.025

    CAS  PubMed  Article  Google Scholar 

  95. 95.

    Yu J, Fleming SL, Williams B, Williams EV, Li Z, Somma P, Rieder CL, Goldberg ML: Greatwall kinase: a nuclear protein required for proper chromosome condensation and mitotic progression in Drosophila. J Cell Biol 2004, 164: 487–492. 10.1083/jcb.200310059

    CAS  PubMed Central  PubMed  Article  Google Scholar 

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We would like to acknowledge continued support to the KTJ lab from the Wellcome Trust.

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Correspondence to Suzanne Madgwick.

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SM wrote the review. Both authors contributed to the drafting of the text.

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Madgwick, S., Jones, K.T. How eggs arrest at metaphase II: MPF stabilisation plus APC/C inhibition equals Cytostatic Factor. Cell Div 2, 4 (2007).

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  • Spindle Assembly Checkpoint
  • Parthenogenetic Activation
  • Metaphase Arrest
  • Spindle Assembly Checkpoint Protein
  • Spindle Assembly Checkpoint Component