Nuclear envelope breakdown may deliver an inhibitor of protein phosphatase 1 which triggers cyclin B translation in starfish oocytes

MAPK signaling couples SCF-mediated degradation of translational regulators to oocyte meiotic progression

RNA-binding proteins (RBPs) are important regulators of gene expression programs, especially during gametogenesis. How the abundance of particular RBPs is restricted to defined stages of meiosis remains largely elusive. Here, we report a molecular pathway that subjects two nonrelated but broadly evolutionarily conserved translational regulators (CPB-3/CPEB and GLD-1/STAR) to proteosomal degradation in Caenorhabditis elegans germ cells at the transition from pachytene to diplotene of meiotic prophase. Both RBPs are recognized by the same ubiquitin ligase complex, containing the molecular scaffold Cullin-1 and the tumor suppressor SEL-10/FBXW7 as its substrate recognition subunit. Destabilization of either RBP through this Skp, Cullin, F-box-containing complex (SCF) ubiquitin ligase appears to loosen its negative control over established target mRNAs, and presumably depends on a prior phosphorylation of CPB-3 and GLD-1 by MAPK (MPK-1), whose activity increases in mid- to late pachytene to promote meiotic progression and oocyte differentiation. Thus, we propose that the orchestrated degradation of RBPs via MAPK-signaling cascades during germ cell development may act to synchronize meiotic with sexual differentiation gene expression changes.

Block of CDK1-dependent polyadenosine elongation of Cyclin B mRNA in metaphase-i-arrested starfish oocytes is released by intracellular pH elevation upon spawning

Meiotic progression requires the translation of maternal mRNAs in a strict temporal order. In isolated animal oocytes, translation of maternal mRNAs containing a cytoplasmic polyadenylation element (CPE), such as cyclin B, is activated by in vitro stimulation of meiotic resumption which induces phosphorylation of CPEB (CPE-binding protein) and elongation of their polyadenosine (poly(A)) tails; whether or not this model can be applied in vivo to oocytes arrested at metaphase of meiosis I in ovaries is unknown. In this study, we found that active CDK1 (cyclin-dependent kinase 1) phosphorylated CPEB in ovarian oocytes arrested at metphase I in the starfish body cavity, but phosphorylation of CPEB was not sufficient for elongation of cyclin B poly(A) tails. Immediately after spawning, however, mRNA was polyadenylated, suggesting that an increase in intracellular pH (pHi ) upon spawning triggers the elongation of poly(A) tails. Using a cell-free system made from maturing oocytes at metaphase I, we demonstrated that polyadenylation was indeed suppressed at pH below 7.0. These results suggest that a pH-sensitive process, functioning after CPEB phosphorylation, is blocked under physiologically low pHi (<7.0) in metaphase-I-arrested oocytes. The increase in pHi (>7.0) that occurs after spawning triggers polyadenylation of cyclin B mRNA and progression into meiosis II.

Structural Analysis of the Pin1-CPEB1 interaction and its potential role in CPEB1 degradation

The Cytoplasmic Polyadenylation Element Binding proteins are RNA binding proteins involved in the translational regulation of mRNA. During cell cycle progression, CPEB1 is labeled for degradation by phosphorylation-dependent ubiquitination by the SCF(β-TrCP) ligase. The peptidyl-prolyl isomerase Pin1 plays a key role in CPEB1 degradation. Conditioned by the cell cycle stage, CPEB1 and Pin1 interactions occur in a phosphorylation-independent or -dependent manner. CPEB1 contains six potential phosphorylatable Pin1 binding sites. Using a set of biophysical techniques, we discovered that the pS210 site is unique, since it displays binding activity not only to the WW domain but also to the prolyl-isomerase domain of Pin1. The NMR structure of the Pin1 WW-CPEB1 pS210 (PDB ID: 2n1o) reveals that the pSerPro motif is bound in trans configuration through contacts with amino acids located in the first turn of the WW domain and the conserved tryptophan in the β3-strand. NMR relaxation analyses of Pin1 suggest that inter-domain flexibility is conferred by the modulation of the interaction with peptides containing the pS210 site, which is essential for degradation.

c-erbB2 and c-myb induce mouse oocyte maturation involving activation of maturation promoting factor

Proto-oncogenes are involved in cell growth, proliferation, and differentiation. In the present study, we investigated the roles and mediating pathways of proto-oncogenes c-erbB(2) and c-myb in mouse oocyte maturation by RT-PCR, real-time quantitative PCR, western blot, and recombinant proto-oncogene protein microinjection. Results showed that both c-erbB(2) and c-myb antisense oligodeoxynucleotides (c-erbB(2) ASODN and c-myb ASODN) inhibited germinal vesicle breakdown and the first polar body extrusion in a dose-dependent manner. However, microinjection of recombinant c-erbB(2) or c-myb protein into germinal vesicle stage oocytes stimulated oocyte meiotic maturation. In addition, the expression of c-erbB(2) and c-myb mRNA was detected in oocytes; and c-erbB(2) ASODN and c-myb ASODN inhibited c-erbB(2) mRNA and c-myb mRNA expression, respectively. Maturation promoting factor (MPF) inhibitor roscovitine did not affect the expression of c-erbB(2) mRNA and c-myb mRNA, but blocked the effects of recombinant c-erbB(2) and c-myb protein-induced oocyte maturation. Further, cyclin B1 protein expression in oocytes was remarkably inhibited by c-erbB(2) ASODN, c-myb ASODN, and roscovitine. Nonsense tat ODN had no effect on the expression of c-erbB(2), c-myb, and cyclin B1. These results suggest that c-erbB(2) and c-myb may induce oocyte maturation through mediating a pathway involving the activation of MPF.

Start of the embryonic cell cycle is dually locked in unfertilized starfish eggs

A key event in the oocyte-to-embryo transition is the start of the embryonic mitotic cell cycle. Prior to this start, the cell cycle in oocytes is generally arrested at a particular stage during meiosis, and the meiotic arrest is released by fertilization. However, it remains unclear how release from the meiotic arrest is implicated in the start of the embryonic cell cycle. To elucidate this link, we have used starfish eggs, in which G1 phase arrest occurs after completion of meiosis if the mature oocytes are not fertilized, and fertilization simply directs the start of the embryonic cell cycle. The starfish G1 arrest is known to rely on the Mos-MAPK-Rsk (p90 ribosomal S6 kinase) pathway, and inactivation of Rsk induces S phase in the absence of fertilization. However, here we show that this S phase is not followed by M phase when MAPK remains active, owing to poly(A)-independent repression of cyclin A and B synthesis. By contrast, inactivation of MAPK alone induces M phase, even when S phase is inhibited by constitutively active Rsk. Thus, there is a divergence of separate pathways downstream of MAPK that together block the start of the embryonic mitotic cycle. One is the previously known Rsk-dependent pathway that prevents S phase, and the other is a novel pathway that is not mediated by Rsk and that leads to prevention of the first mitotic M phase through suppression of protein synthesis of M phase cyclins. Release from such a 'dual-lock' by fertilization results in the start of the embryonic cell cycle.

Mechanism of degradation of CPEB during Xenopus oocyte maturation

CPEB, a cytoplasmic polyadenylation element-binding protein, plays an important role in translational control of maternal mRNAs in early animal development. During Xenopus oocyte maturation, CPEB undergoes a Cdc2-mediated phosphorylation- and ubiquitin-dependent degradation that is required for proper entry into meiosis II. However, the precise mechanism of CPEB degradation, including the identity of the responsible E3 ubiquitin ligase, is not known. Here, we show that the SCF(beta-TrCP) E3 ubiquitin ligase complex targets CPEB for degradation during Xenopus oocyte maturation. beta-TrCP, the F-box protein of SCF(beta-TrCP), specifically binds to a sequence (190)TSGFSS(195) (termed here the TSG motif) of CPEB, thereby targeting CPEB for degradation. beta-TrCP binding depends on phosphorylation of Thr-190, Ser-191, and Ser-195 in the TSG motif. Among these residues, Ser-191 is phosphorylated by the Polo-like kinase Plx1, which binds CPEB at a specific Thr-125 residue prephosphorylated by Cdc2. Finally, Cdc2-mediated phosphorylation of other multiple Ser residues, previously implicated in CPEB degradation, is required for both Thr-125 phosphorylation and beta-TrCP binding, presumably causing conformational changes of CPEB. We propose that Cdc2 and Plx1 sequentially phosphorylate CPEB and target it for SCF(beta-TrCP)-dependent degradation in Xenopus oocytes. We suggest that many other proteins carrying the TSG-like motif may be targeted by SCF(beta-TrCP).

Effect of Enucleation on Inactivation of Cytostatic Factor Activity in Matured Rat Oocytes

In mammals, matured oocytes are arrested at the MII stage until fertilization, which is regulated by cytostaticfactor (CSF) activity. Maturation-promoting factor (MPF) and the mitogen-activated protein kinase (MAPK) pathway are known as candidates for CSF. Despite of the results that nuclear and perinuclear materials were dispensable for activation of MPF and MAPK in other species, our previous study in rats demonstrated that MPF activity was rapidly decreased after enucleation. We showed here for the first time that nuclear and perinuclear materials were indispensable for CSF activity in matured rat oocytes. In both cytoplasm-removed and enucleated oocytes, high activity of p34(cdc2) kinase was observed immediately after manipulation, but the activity of enucleated oocytes was dramatically reduced within 1 h. Cyclin B level was also decreased, corresponding with inactivation of p34(cdc2) kinase. In enucleated oocytes, the Mos level was dramatically decreased, and both MEK and MAPK dephosphorylation were also induced. A combined treatment with a proteasome inhibitor, MG132, and a protein phosphatase inhibitor, okadaic acid, dramatically improved both levels of p-MAPK and cyclin B in these enucleated oocytes. These data suggest that nuclear and perinuclear materials of matured rat oocytes suppress proteasome and protein phosphatase activation, which is indispensable for stability of CSF.

The genomic repertoire for cell cycle control and DNA metabolism in S. purpuratus

A search of the Strongylocentrotus purpuratus genome for genes associated with cell cycle control and DNA metabolism shows that the known repertoire of these genes is conserved in the sea urchin, although with fewer family members represented than in vertebrates, and with some cases of echinoderm-specific gene diversifications. For example, while homologues of the known cyclins are mostly encoded by single genes in S. purpuratus (unlike vertebrates, which have multiple isoforms), there are additional genes encoding novel cyclins of the B and K/L types. Almost all known cyclin-dependent kinases (CDKs) or CDK-like proteins have an orthologue in S. purpuratus; CDK3 is one exception, whereas CDK4 and 6 are represented by a single homologue, referred to as CDK4. While the complexity of the two families of mitotic kinases, Polo and Aurora, is close to that found in the nematode, the diversity of the NIMA-related kinases (NEK proteins) approaches that of vertebrates. Among the nine NEK proteins found in S. purpuratus, eight could be assigned orthologues in vertebrates, whereas the ninth is unique to sea urchins. Most known DNA replication, DNA repair and mitotic checkpoint genes are also present, as are homologues of the pRB (two) and p53 (one) tumor suppressors. Interestingly, the p21/p27 family of CDK inhibitors is represented by one homologue, whereas the INK4 and ARF families of tumor suppressors appear to be absent, suggesting that these evolved only in vertebrates. Our results suggest that, while the cell cycle control mechanisms known from other animals are generally conserved in sea urchin, parts of the machinery have diversified within the echinoderm lineage. The set of genes uncovered in this analysis of the S. purpuratus genome should enhance future research on cell cycle control and developmental regulation in this model.

Germinal vesicle material drives meiotic cell cycle of mouse oocyte through the 3′UTR-dependent control of cyclin B1 synthesis

We compared the profile of histone H1 kinase activity, reflecting Maturation Promoting Factor (MPF) activity in oocytes bisected at the germinal vesicle (GV) stage and allowed to mature as separate oocyte halves in vitro. Whereas the oocyte halves containing the nucleus exhibited the same profile of increased kinase activity as that typical for intact oocytes, the anuclear halves revealed strong inhibition of the increase in this activity soon after germinal vesicle breakdown (GVBD). In contrast, the profile of MAP kinase activity did not differ significantly between anuclear and nucleus-containing oocyte halves throughout maturation. Of the two MPF components, CDK1 and cyclin B1, the amount of the latter was significantly reduced in anuclear halves, a reduction due to low-level synthesis and not to enhanced degradation. Expression of three reporter luciferase RNAs constructed, respectively, to contain cyclin B1-specific 3'UTR, the globin-specific 3'UTR, or no 3'UTR sequence was enhanced in nuclear halves, with significantly greater enhancement for the construct containing cyclin B1-specific 3'UTR as compared to the two other RNAs. We conclude that the profile of activity of MPF during mouse oocyte maturation is controlled by an unknown GV-associated factor(s) acting via 3'UTR-dependent control of cyclin B1 synthesis. These results require the revision of the hitherto prevailing view that the control of MPF activity during mouse oocyte maturation is independent of GV-derived material.