How is mpf degraded

Keith T. First, its activation during meiotic progression from prophase I arrest at germinal vesicle breakdown. Second, its role during the transition from meiosis I to meiosis II, a defining feature of meiosis involving segregation of homologous chromosomes. Third, maintenance of its activity at metaphase II arrest and the necessity for its destruction during oocyte activation. An understanding of how oocytes switch it on and turn it off underpins much of the basic cell biology of oocyte maturation.

What makes meiosis interesting is that much of the basic cell cycle machinery employed is analogous to that used in mitosis. The devil therefore is in understanding the detail and defining what actually may be unique to meiosis.

This review concentrates on what is known about the events controlling the two meiotic divisions in mammalian oocytes during their maturation, taking the mouse as the model mammal.

It therefore overlooks some interesting aspects of oocyte biology, such as the transition from mitosis to meiosis in primordial germ cells and the atretic loss of the large majority of oocytes during pachytene I, when the process of recombination is monitored.

More is known about the female than the male gametes due to ease of study and the ability to examine the process in real time. However, recent developments of spermatogonial cell lines that undergo meiosis in culture may help to redress this balance Feng et al.

This review concentrates on the activity of maturation promoting factor MPF during the two meiotic divisions following arrest from the dictyate stage of prophase I in fully grown oocytes. Although binding of CDK1 to cyclin B1 is necessary, it is not sufficient for kinase activity.

In addition, before entry into mitosis, cyclin B1 and so MPF is spatially restricted to the cytoplasm, through a cytoplasmic retention sequence, containing a nuclear export signal. When cells commit to mitosis, cyclin B1 has to become phosphorylated within its cytoplasmic retention sequence, leading to rapid accumulation of cyclin B1 and MPF within the nucleus, and ensuing NEB Hagting et al.

Mammalian oocytes arrest at the dictyate stage of prophase I for most of their lives. The follicular environment is thought to inhibit oocyte maturation because spontaneous cell cycle resumption is observed when fully grown oocytes are removed from their follicles and cultured in vitro.

However, expression of exogenous CDK1 in incompetent oocytes fails to make them competent de Vantery et al. Much more likely is that alterations in how MPF is regulated spatially nucleus versus cytoplasm and temporally CDC25 and Wee kinases will underlie the phenomenon of oocyte competency. The spontaneous resumption of meiosis occurring when competent oocytes are released from their inhibitory follicular environment is believed to be triggered by a fall in oocyte levels of cyclic AMP cAMP: Eppig, ; Conti et al.

In vivo meiotic arrest is achieved by a stimulatory G protein Gs acting on adenylyl cyclase Mehlmann et al. The most likely mechanism by which cAMP maintains arrest has been studied in frog oocytes, and is through activation of protein kinase A, which in turn phosphorylates CDC25 Duckworth et al.

Nebreda and Ferby, There is still contention as to the need for MAPK activation in order for MPF to be activated following progesterone application, the physiological stimulator of oocyte maturation in the frog. Although the changes in the balance of the phosphatases and kinases involved in release from prophase I arrest could also occur in mitosis, there may be differences.

The reason for this is currently unclear. Furthermore, it may be that novel activators of MPF are observed in oocytes. Following GVBD and activation of MPF, recombined homologous chromosomes converge at the spindle equator forming a metaphase configuration. The goal of first meiosis is a reductional division in which recombined homologous chromosomes are segregated, whilst sister chromatids remain attached.

Contrast this with mitosis and the second meiotic division in which sisters are separated. Segregation at meiosis I is therefore treated as a unique division, errors in which are believed to result in many of the aneuploidies observed in early human conceptuses Nicolaidis and Petersen, ; Pellestor et al. It is not currently defined if a similar mechanism operates to monitor homologous chromosomes during meiosis I.

During mitosis, checkpoint components, comprised of the Mad and Bub family of proteins, monitor attachment and tension at kinetochores, which attach sister chromatids to the spindle microtubules. The Mad and Bub proteins orchestrate the spindle checkpoint by preventing premature anaphase prior to full chromosome alignment Yu, ; Musacchio and Hardwick, When metaphase is reached, with both sisters under tension and fully aligned, the checkpoint proteins become inactive.

Therefore during exit from mitosis, MPF activity declines due to loss of cyclin B1. This has led to the hypothesis that in vertebrates, at least, meiosis I is characterized by a poor spindle integrity surveillance mechanism that may underlie the high disjunction rates observed in humans. Furthermore increasing cyclin B1 levels in oocytes during their maturation delays polar body formation, again suggesting that cyclin B1 needs to be efficiently degraded in oocytes for homologous chromosome segregation Polanski et al.

In contrast, complete inhibition of protein synthesis in the few hours before the first meiotic division leads to nuclear reformation for the vast majority of oocytes following first polar body extrusion Clarke and Masui, These data suggest that continued expression of an unstable protein during meiosis I is needed for the maintenance of chromatin condensation. Oocytes arrest with a fully formed spindle and aligned chromosomes. However, the oocyte has the capacity to match cyclin B1 synthesis with its degradation, in order that MPF levels do not drop.

Thus in mammalian oocytes cyclin B1 needs to be continually synthesized to maintain arrest. What is limiting cyclin B1 destruction, and so maintaining metaphase II arrest? It is an attractive hypothesis that oocytes are held at metaphase II by a spindle checkpoint.

In the mouse this would be further supported by the finding that Mad2, another spindle checkpoint component, is localized to the sister chromatid kinetochores at metaphase II arrest, but is lost following oocyte activation Kallio et al.

Such localization of spindle checkpoint components is normally observed when the spindle checkpoint pathway is active. This is because the former is patently overcome by the sperm at fertilization, but the latter prevents the oocyte from activating.

There are two further confusions to address in understanding how oocytes maintain a metaphase arrest. However, this arrest is in interphase, not mitosis Verlhac et al. What is still unclear is its true contribution to maintaining a metaphase arrest during meiosis II.

Two other candidates that may contribute to metaphase arrest are noteworthy. However, other studies that utilized p21 cip , a potent cdk2 inhibitor, failed to find any effect of cdk2 inhibition on metaphase II arrest Furuno et al.

Therefore further studies are needed to resolve the role of cdk2 in metaphase II arrest. The second candidate worthy of discussion is Emi1, the most recent addition to the group of proteins believed to play a role in metaphase II arrest in frog oocytes.

Depletion of Emi1 from frog oocyte extracts prevents them from arresting at metaphase and excess Emi1 arrests oocytes at metaphase. It will be interesting to determine if Emi1 plays an analogous role in mammalian oocytes. However, increasing evidence suggests that a soluble sperm factor is introduced into the oocyte following sperm fusion e.

Jones et al. Our study examined the role of the proteasome in the first mitosis of rat embryos and its participation in the regulation of cyclin B1 degradation and MPF inactivation. We show that in the early zygote the proteasome is evenly distributed in the ooplasm and the nucleus, whereas during mitosis it accumulates on the spindle apparatus. We further demonstrate that inhibition of proteasomal catalytic activity prevents 1-cell embryos from undergoing mitosis.

A later yeast-two hybrid screen for Plx1-interacting proteins identified Emi2. Plx1 further phosphorylates Emi2 [ 72 — 74 ], to generate a degron which is recognised by the SCF ubiquitin ligase, resulting in Emi2 polyubiquitination and destruction [ 74 ]. 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.

These findings taken together demonstrate an essential role for Emi2 in the maintenance of MetII arrest. 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 ]. Emi2 also appears to be essential for the establishment of arrest in frog eggs [ 92 , 93 ] suggesting a conserved mechanism in vertebrates. 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.

The exact nature of the effect of the c-Mos pathway on MPF stability is still to be fully resolved however Yamamoto et al. This suggests that Mos may help set the level of MPF activity. Our suggestion is that the c-Mos pathway may contribute to this second mechanism. Model of the regulation of MetII arrest in mammalian eggs. The pathway which involves Emi2-mediated CSF arrest is shown in solid lines.

MPF activity may negatively regulate the c-Mos pathway, as based on studies from frog [94]. See text for further details. Reproduction , — Genes Dev , — Int J Dev Biol , 1— Dev Biol , — Stricker SA: Comparative biology of calcium signaling during fertilization and egg activation in animals. Cell , — J Cell Sci , — Differentiation , 1— Pines J: Four-dimensional control of the cell cycle.

Nat Cell Biol , 1: E73— Nature , — 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 , — Biol Reprod , — Nat Cell Biol , 1: 82— Nat Cell Biol , 1: E47— Peters JM: The anaphase-promoting complex: proteolysis in mitosis and beyond. Mol Cell , 9: — Zachariae W, Nasmyth K: Whose end is destruction: cell division and the anaphase-promoting complex. Curr Opin Cell Biol , — Curr Biol , — Winston NJ: Stability of cyclin B protein during meiotic maturation and the first mitotic cell division in mouse oocytes.

Biol Cell , — Dev Biol , 55— Dev Biol , 68— Masui Y, Market CL: Cytoplasmic control of nuclear behaviour during meiotic matutation of frog oocytes.

J Exp Zool , —