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

Abstract

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.

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.

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.