The mitogen-activated protein kinase signaling pathway stimulates mos mRNA cytoplasmic polyadenylation during Xenopus oocyte maturation

Dissection of the Ovulatory Process Using ex vivo Approaches

Ovulation is a unique physiological phenomenon that is essential for sexual reproduction. It refers to the entire process of ovarian follicle responses to hormonal stimulation resulting in the release of mature fertilization-competent oocytes from the follicles and ovaries. Remarkably, ovulation in different species can be reproduced out-of-body with high fidelity. Moreover, most of the molecular mechanisms and signaling pathways engaged in this process have been delineated using in vitro ovulation models. Here, we provide an overview of the major molecular and cytological events of ovulation observed in frogs, primarily in the African clawed frog Xenopus laevis, using mainly ex vivo approaches, with the focus on meiotic oocyte maturation and follicle rupture. For the purpose of comparison and generalization, we also refer extensively to ovulation in other biological species, most notoriously, in mammals.

Evasion of regulatory phosphorylation by an alternatively spliced isoform of Musashi2

The Musashi family of RNA binding proteins act to promote stem cell self-renewal and oppose cell differentiation predominantly through translational repression of mRNAs encoding pro-differentiation factors and inhibitors of cell cycle progression. During tissue development and repair however, Musashi repressor function must be dynamically regulated to allow cell cycle exit and differentiation. The mechanism by which Musashi repressor function is attenuated has not been fully established. Our prior work indicated that the Musashi1 isoform undergoes site-specific regulatory phosphorylation. Here, we demonstrate that the canonical Musashi2 isoform is subject to similar regulated site-specific phosphorylation, converting Musashi2 from a repressor to an activator of target mRNA translation. We have also characterized a novel alternatively spliced, truncated isoform of human Musashi2 (variant 2) that lacks the sites of regulatory phosphorylation and fails to promote translation of target mRNAs. Consistent with a role in opposing cell cycle exit and differentiation, upregulation of Musashi2 variant 2 was observed in a number of cancers and overexpression of the Musashi2 variant 2 isoform promoted cell transformation. These findings indicate that alternately spliced isoforms of the Musashi protein family possess distinct functional and regulatory properties and suggest that differential expression of Musashi isoforms may influence cell fate decisions.

Paxillin and embryonic PolyAdenylation Binding Protein (ePABP) engage to regulate androgen-dependent Xenopus laevis oocyte maturation - A model of kinase-dependent regulation of protein expression

Steroid-triggered Xenopus laevis oocyte maturation is an elegant physiologic model of nongenomic steroid signaling, as it proceeds completely independent of transcription. We previously demonstrated that androgens are the main physiologic stimulator of oocyte maturation in Xenopus oocytes, and that the adaptor protein paxillin plays a crucial role in mediating this process through a positive feedback loop in which paxillin first enhances Mos protein translation, ensued by Erk2 activation and Erk-dependent phosphorylation of paxillin on serine residues. Phosphoserine-paxillin then further augments Mos protein translation and downstream Erk2 activation, resulting in meiotic progression. We hypothesized that paxillin enhances Mos translation by interacting with embryonic PolyAdenylation Binding Protein (ePABP) on polyadenylated Mos mRNA. Knockdown of ePABP phenocopied paxillin knockdown, with reduced Mos protein expression, Erk2 and Cdk1 activation, as well as oocyte maturation. In both Xenopus oocytes and mammalian cells (HEK-293), paxillin and ePABP constitutively interacted. Testosterone (Xenopus) or EGF (HEK-293) augmented ePABP-paxillin binding, as well as ePABP binding to Mos mRNA (Xenopus), in an Erk-dependent fashion. Thus, ePABP and paxillin work together in an Erk-dependent fashion to enhance Mos protein translation and promote oocyte maturation.

Insertion of inter-domain linkers improves expression and bioactivity of Zygote arrest (Zar) fusion proteins

Developmentally important proteins that are crucial for fertilization and embryogenesis are synthesized through highly regulated translation of maternal mRNA. The Zygote arrest proteins, Zar1 and Zar2, are crucial for embryogenesis and have been implicated in binding mRNA and repressing mRNA translation. To investigate Zar1 and Zar2, the full-length proteins had been fused to glutathione-S-transferase (GST) or MS2 protein tags with minimal inter-domain linkers derived from multiple cloning sites; however, these fusion proteins expressed poorly and/or lacked robust function. Here, we tested the effect of inserting additional linkers between the fusion domains. Three linkers were tested, each 17 amino acids long with different physical and chemical properties: flexible hydrophilic, rigid extended or rigid helical. In the presence of any of the three linkers, GST-Zar1 and GST-Zar2 had fewer breakdown products. Moreover, in the presence of any of the linkers, MS2-Zar1 was expressed to higher levels, and in dual luciferase tethered assays, both MS2-Zar1 and MS2-Zar2 repressed luciferase translation to a greater extent. These data suggest that for Zar fusion proteins, increasing the length of linkers, regardless of their physical or chemical properties, improves stability, expression and bioactivity.

Calcium signaling and meiotic exit at fertilization in Xenopus egg

Calcium is a universal messenger that mediates egg activation at fertilization in all sexually reproducing species studied. However, signaling pathways leading to calcium generation and the mechanisms of calcium-induced exit from meiotic arrest vary substantially among species. Here, we review the pathways of calcium signaling and the mechanisms of meiotic exit at fertilization in the eggs of the established developmental model, African clawed frog, Xenopus laevis. We also discuss calcium involvement in the early fertilization-induced events in Xenopus egg, such as membrane depolarization, the increase in intracellular pH, cortical granule exocytosis, cortical contraction, contraction wave, cortical rotation, reformation of the nuclear envelope, sperm chromatin decondensation and sister chromatid segregation.

Heck products of parthenolide and melampomagnolide-B as anticancer modulators that modify cell cycle progression

(E)-13-(Aryl/heteroaryl)parthenolides (5a-i and 6a-i) were synthesized and evaluated for their ability to modify cell cycle progression during progesterone-stimulated Xenopus oocyte maturation and screened for their anticancer activity against a panel of 60 human cancer cell lines. (E)-13-(4-aminophenyl) parthenolide (5b) caused a significant inhibition of progesterone-stimulated oocyte maturation, and was determined to function downstream of MAP kinase signaling, but upstream of the activation of the universal G2/M regulator, M-phase promoting factor (MPF), cyclin B/Cyclin-dependent kinase (CDK). The compound (E)-13-(2-bromo-phenyl)parthenolide (5c) activates oocyte maturation independently of progesterone stimulation. Compounds 5b and 5c displayed modest growth inhibition on select cancer cell lines at 10 μM dose when tested on the panel of 60 cancer cell lines. By contrast, compounds (5f and 7) did not modulate oocyte maturation but did exhibit micromolar level growth inhibition against most of the human cancer cell lines over a range of doses. Together, our findings indicate that screening of compounds in the oocyte maturation assay may identify additional effective cell cycle regulatory compounds that do not necessarily exert overt cytotoxicity as assessed in traditional drug screening assays.

Heterocyclic aminoparthenolide derivatives modulate G(2)-M cell cycle progression during Xenopus oocyte maturation

Aminoparthenolide derivatives have been prepared by reaction of parthenolide with various heterocyclic amines to afford corresponding Michael addition products. These novel compounds were evaluated for their modulatory effects on Xenopus oocyte maturation. Two compounds, 6e and 6f, were identified that promote G2-M cell cycle progression.

Musashi protein-directed translational activation of target mRNAs is mediated by the poly(A) polymerase, germ line development defective-2

The mRNA-binding protein, Musashi, has been shown to regulate translation of select mRNAs and to control cellular identity in both stem cells and cancer cells. Within the mammalian cells, Musashi has traditionally been characterized as a repressor of translation. However, we have demonstrated that Musashi is an activator of translation in progesterone-stimulated oocytes of the frog Xenopus laevis, and recent evidence has revealed Musashi's capability to function as an activator of translation in mammalian systems. The molecular mechanism by which Musashi directs activation of target mRNAs has not been elucidated. Here, we report a specific association of Musashi with the noncanonical poly(A) polymerase germ line development defective-2 (GLD2) and map the association domain to 31 amino acids within the C-terminal domain of Musashi. We show that loss of GLD2 interaction through deletion of the binding domain or treatment with antisense oligonucleotides compromises Musashi function. Additionally, we demonstrate that overexpression of both Musashi and GLD2 significantly enhances Musashi function. Finally, we report a similar co-association also occurs between murine Musashi and GLD2 orthologs, suggesting that coupling of Musashi to the polyadenylation apparatus is a conserved mechanism to promote target mRNA translation.

Paxillin and steroid signaling: from frog to human

Paxillin is a well-characterized cytoplasmic adaptor protein that is known to play important roles in cytoskeletal rearrangement, cell adhesion, and cell motility. In addition to its structural functions, paxillin has more recently been shown to function as a regulator of cell division-mediating steroid-triggered meiosis in oocytes as well as steroid- and growth factor-induced proliferation in prostate and breast cancer. Paxillin mediates these processes through a conserved pathway that involves both extranuclear (nongenomic) and nuclear (genomic) steroid signaling, as well as both cytoplasmic and nuclear kinase signaling. In fact, paxillin appears to serve as a critical liaison between extranuclear and nuclear signaling in response to multiple stimuli, making it a fascinating molecule to study when trying to determine how growth signals from the membrane lead to important proliferative changes in the nucleus. This chapter outlines recent advances in understanding how paxillin regulates both steroid and growth factor signaling, focusing on the conserved nature of its actions from a frog germ cell to a human cancer cell.

Molecular cloning and mRNA expression of M-phase phosphoprotein 6 gene in black tiger shrimp (Penaeus monodon)

It is widely accepted that protein phosphorylation is a major control event in regulating cell cycle. In the present study, a novel M-phase phosphoprotein 6 (MPP6) was identified from black tiger shrimp Penaeus monodon (designated as PmMPP6) by cDNA library and RACE approaches. The full-length cDNA of PmMPP6 was of 690 bp, including a 5'-terminal un-translated region (5'UTR) of 68 bp, a 3'UTR of 172 bp with a poly (A) tail, and an open reading frame (ORF) of 450 bp encoding a polypeptide of 149 amino acids with a predicted molecular weight of 17.01 kDa. Blastx and phylogenetic analysis together supported that PmMPP6 was a novel member of shrimp MPP6. The mRNA expression of PmMPP6 in thirteen tissues was examined by real-time PCR, and mRNA transcript of PmMPP6 was predominantly detectable in tissues of lymphoid and muscle, to a lesser degree in the tissues of gill, ovary and hepatopancreas, and mainly detected in haemocytes, heart and gonad. The temporal expression of PmMPP6 in different developmental stages of ovary was investigated by real-time PCR. During the six stages of ovary development, two peaks expression of PmMPP6 was detected in stage II with 3.78-fold increase and stage V with 3.48-fold increase compared to that in stage I. All these results indicated that PmMPP6 might be involved in regulating shrimp cell cycle and ovary development.

Autoregulation of Musashi1 mRNA translation during Xenopus oocyte maturation

The mRNA translational control protein, Musashi, plays a critical role in cell fate determination through sequence-specific interactions with select target mRNAs. In proliferating stem cells, Musashi exerts repression of target mRNAs to promote cell cycle progression. During stem cell differentiation, Musashi target mRNAs are de-repressed and translated. Recently, we have reported an obligatory requirement for Musashi to direct translational activation of target mRNAs during Xenopus oocyte meiotic cell cycle progression. Despite the importance of Musashi in cell cycle regulation, only a few target mRNAs have been fully characterized. In this study, we report the identification and characterization of a new Musashi target mRNA in Xenopus oocytes. We demonstrate that progesterone-stimulated translational activation of the Xenopus Musashi1 mRNA is regulated through a functional Musashi binding element (MBE) in the Musashi1 mRNA 3' untranslated region (3' UTR). Mutational disruption of the MBE prevented translational activation of Musashi1 mRNA and its interaction with Musashi protein. Further, elimination of Musashi function through microinjection of inhibitory antisense oligonucleotides prevented progesterone-induced polyadenylation and translation of the endogenous Musashi1 mRNA. Thus, Xenopus Musashi proteins regulate translation of the Musashi1 mRNA during oocyte maturation. Our results indicate that the hierarchy of sequential and dependent mRNA translational control programs involved in directing progression through meiosis are reinforced by an intricate series of nested, positive feedback loops, including Musashi mRNA translational autoregulation. These autoregulatory positive feedback loops serve to amplify a weak initiating signal into a robust commitment for the oocyte to progress through the cell cycle and become competent for fertilization.

Ringo/cyclin-dependent kinase and mitogen-activated protein kinase signaling pathways regulate the activity of the cell fate determinant Musashi to promote cell cycle re-entry in Xenopus oocytes

Cell cycle re-entry during vertebrate oocyte maturation is mediated through translational activation of select target mRNAs, culminating in the activation of mitogen-activated protein kinase and cyclin B/cyclin-dependent kinase (CDK) signaling. The temporal order of targeted mRNA translation is crucial for cell cycle progression and is determined by the timing of activation of distinct mRNA-binding proteins. We have previously shown in oocytes from Xenopus laevis that the mRNA-binding protein Musashi targets translational activation of early class mRNAs including the mRNA encoding the Mos proto-oncogene. However, the molecular mechanism by which Musashi function is activated is unknown. We report here that activation of Musashi1 is mediated by Ringo/CDK signaling, revealing a novel role for early Ringo/CDK function. Interestingly, Musashi1 activation is subsequently sustained through mitogen-activated protein kinase signaling, the downstream effector of Mos mRNA translation, thus establishing a positive feedback loop to amplify Musashi function. The identified regulatory sites are present in mammalian Musashi proteins, and our data suggest that phosphorylation may represent an evolutionarily conserved mechanism to control Musashi-dependent target mRNA translation.

Molecular analysis of a ras-like nuclear (Ran) gene from Penaeus monodon and its expression at the different ovarian stages of development

In the present study, a ras-like nuclear (Ran) gene was obtained from the ovary and neurosecretory organ in eyestalk cDNA library of black tiger prawn (Penaeus monodon). The full-length black tiger prawn Ran (PmRan) cDNA consisted of 1140 nucleotides including an open reading frame (ORF) 648 bp, a 5' untranslated region (5'UTR) of 117 bp and a 3'UTR of 375 bp with a polyadenylation signal sequence "aataaa" and a poly (A) tail. The ORF encoded a peptide of 215 amino acids with molecular mass 24.6 kDa and a theoretical isoelectric point of 7.39. ScanProsite analysis indicated that PmRan protein sequence contained a small GTPase Ran family motif. Homology analysis of the deduced amino acid sequence of the PmRan with other known Ran sequences by MatGAT software revealed that the PmRan show very high homology with the sequences of other animals (92.1-98.6% similarity, 85.6-98.1% identity). Analysis of the tissue expression pattern of the PmRan gene showed that the PmRan mRNA was expressed in all tested tissues, including hepatopancreas, ovary, muscle, intestine, neurosecretory organ in eyestalk, neurosecretory organ in brain, stomach, and heart, with the highest levels in ovary. Furthermore, the PmRan expression was found to be high level in the six ovarian stages of development. The results indicated PmRan might play an important role in ovarian development.

Transcriptome dynamics and molecular cross-talk between bovine oocyte and its companion cumulus cells

BACKGROUND

The bi-directional communication between the oocyte and its companion cumulus cells (CCs) is crucial for development and functions of both cell types. Transcripts that are exclusively expressed either in oocytes or CCs and molecular mechanisms affected due to removal of the communication axis between the two cell types is not investigated at a larger scale. The main objectives of this study were: 1. To identify transcripts exclusively expressed either in oocyte or CCs and 2. To identify those which are differentially expressed when the oocyte is cultured with or without its companion CCs and vice versa.

RESULTS

We analyzed transcriptome profile of different oocyte and CC samples using Affymetrix GeneChip Bovine Genome array containing 23000 transcripts. Out of 13162 genes detected in germinal vesicle (GV) oocytes and their companion CCs, 1516 and 2727 are exclusively expressed in oocytes and CCs, respectively, while 8919 are expressed in both. Similarly, of 13602 genes detected in metaphase II (MII) oocytes and CCs, 1423 and 3100 are exclusively expressed in oocytes and CCs, respectively, while 9079 are expressed in both. A total of 265 transcripts are differentially expressed between oocytes cultured with (OO+CCs) and without (OO-CCs) CCs, of which 217 and 48 are over expressed in the former and the later groups, respectively. Similarly, 566 transcripts are differentially expressed when CCs mature with (CCs+OO) or without (CCs-OO) their enclosed oocytes. Of these, 320 and 246 are over expressed in CCs+OO and CCs-OO, respectively.While oocyte specific transcripts include those involved in transcription (IRF6, POU5F1, MYF5, MED18), translation (EIF2AK1, EIF4ENIF1) and CCs specific ones include those involved in carbohydrate metabolism (HYAL1, PFKL, PYGL, MPI), protein metabolic processes (IHH, APOA1, PLOD1), steroid biosynthetic process (APOA1, CYP11A1, HSD3B1, HSD3B7). Similarly, while transcripts over expressed in OO+CCs are involved in carbohydrate metabolism (ACO1, 2), molecular transport (GAPDH, GFPT1) and nucleic acid metabolism (CBS, NOS2), those over expressed in CCs+ OO are involved in cellular growth and proliferation (FOS, GADD45A), cell cycle (HAS2, VEGFA), cellular development (AMD1, AURKA, DPP4) and gene expression (FOSB, TGFB2).

CONCLUSION

In conclusion, this study has generated large scale gene expression data from different oocyte and CCs samples that would provide insights into gene functions and interactions within and across different pathways that are involved in the maturation of bovine oocytes. Moreover, the presence or absence of oocyte and CC factors during bovine oocyte maturation can have a profound effect on transcript abundance of each cell types, thereby showing the prevailing molecular cross-talk between oocytes and their corresponding CCs.

Mos in the oocyte: how to use MAPK independently of growth factors and transcription to control meiotic divisions

In many cell types, the mitogen-activated protein kinase (MAPK) also named extracellular signal-regulated kinase (ERK) is activated in response to a variety of extracellular growth factor-receptor interactions and leads to the transcriptional activation of immediate early genes, hereby influencing a number of tissue-specific biological activities, as cell proliferation, survival and differentiation. In one specific cell type however, the female germ cell, MAPK does not follow this canonical scheme. In oocytes, MAPK is activated independently of growth factors and tyrosine kinase receptors, acts independently of transcriptional regulation, plays a crucial role in controlling meiotic divisions, and is under the control of a peculiar upstream regulator, the kinase Mos. Mos was originally identified as the transforming gene of Moloney murine sarcoma virus and its cellular homologue was the first proto-oncogene to be molecularly cloned. What could be the specific roles of Mos that render it necessary for meiosis? Which unique functions could explain the evolutionary cost to have selected one gene to only serve for few hours in one very specific cell type? This review discusses the original features of MAPK activation by Mos and the roles of this module in oocytes.

Developmental timing of mRNA translation--integration of distinct regulatory elements

Targeted mRNA translation is emerging as a critical mechanism to control gene expression during developmental processes. Exciting new findings have revealed a critical role for regulatory elements within the mRNA untranslated regions to direct the timing of mRNA translation. Regulatory elements can be targeted by sequence-specific binding proteins to direct either repression or activation of mRNA translation in response to developmental signals. As new regulatory elements continue to be identified it has become clear that targeted mRNAs can contain multiple regulatory elements, directing apparently contradictory translational patterns. How is this complex regulatory input integrated? In this review, we focus on a new challenge area-how sequence-specific RNA binding proteins respond to developmental signals and functionally integrate to regulate the extent and timing of target mRNA translation. We discuss current understanding with a particular emphasis on the control of cell cycle progression that is mediated through a complex interplay of distinct mRNA regulatory elements during Xenopus oocyte maturation.

Enforcing temporal control of maternal mRNA translation during oocyte cell-cycle progression

Meiotic cell-cycle progression in progesterone-stimulated Xenopus oocytes requires that the translation of pre-existing maternal mRNAs occur in a strict temporal order. Timing of translation is regulated through elements within the mRNA 3' untranslated region (3' UTR), which respond to cell cycle-dependant signalling. One element that has been previously implicated in the temporal control of mRNA translation is the cytoplasmic polyadenylation element (CPE). In this study, we show that the CPE does not direct early mRNA translation. Rather, early translation is directed through specific early factors, including the Musashi-binding element (MBE) and the MBE-binding protein, Musashi. Our findings indicate that although the cyclin B5 3' UTR contains both CPEs and an MBE, the MBE is the critical regulator of early translation. The cyclin B2 3' UTR contains CPEs, but lacks an MBE and is translationally activated late in maturation. Finally, utilizing antisense oligonucleotides to attenuate endogenous Musashi synthesis, we show that Musashi is critical for the initiation of early class mRNA translation and for the subsequent activation of CPE-dependant mRNA translation.

Mos 3' UTR regulatory differences underlie species-specific temporal patterns of Mos mRNA cytoplasmic polyadenylation and translational recruitment during oocyte maturation

The Mos proto-oncogene is a critical regulator of vertebrate oocyte maturation. The maturation-dependent translation of Mos protein correlates with the cytoplasmic polyadenylation of the maternal Mos mRNA. However, the precise temporal requirements for Mos protein function differ between oocytes of model mammalian species and oocytes of the frog Xenopus laevis. Despite the advances in model organisms, it is not known if the translation of the human Mos mRNA is also regulated by cytoplasmic polyadenylation or what regulatory elements may be involved. We report that the human Mos 3' untranslated region (3' UTR) contains a functional cytoplasmic polyadenylation element (CPE) and demonstrate that the endogenous Mos mRNA undergoes maturation-dependent cytoplasmic polyadenylation in human oocytes. The human Mos 3' UTR interacts with the human CPE-binding protein and exerts translational control on a reporter mRNA in the heterologous Xenopus oocyte system. Unlike the Xenopus Mos mRNA, which is translationally activated by an early acting Musashi/polyadenylation response element (PRE)-directed control mechanism, the translational activation of the human Mos 3' UTR is dependent on a late acting CPE-dependent process. Taken together, our findings suggest a fundamental difference in the 3' UTR regulatory mechanisms controlling the temporal induction of maternal Mos mRNA polyadenylation and translational activation during Xenopus and mammalian oocyte maturation.

Sequential waves of polyadenylation and deadenylation define a translation circuit that drives meiotic progression

The maternal mRNAs that drive meiotic progression in oocytes contain short poly(A) tails and it is only when these tails are elongated that translation takes place. Cytoplasmic polyadenylation requires two elements in the 3′-UTR (3′-untranslated region), the hexanucleotide AAUAAA and the CPE (cytoplasmic polyadenylation element), which also participates in the transport and localization, in a quiescent state, of its targets. However, not all CPE-containing mRNAs are activated at the same time during the cell cycle, and polyadenylation is temporally and spatially regulated during meiosis. We have recently deciphered a combinatorial code that can be used to qualitatively and quantitatively predict the translational behaviour of CPE-containing mRNAs. This code defines positive and negative feedback loops that generate waves of polyadenylation and deadenylation, creating a circuit of mRNA-specific translational regulation that drives meiotic progression.

A novel mRNA 3' untranslated region translational control sequence regulates Xenopus Wee1 mRNA translation

Cell cycle progression during oocyte maturation requires the strict temporal regulation of maternal mRNA translation. The intrinsic basis of this temporal control has not been fully elucidated but appears to involve distinct mRNA 3' UTR regulatory elements. In this study, we identify a novel translational control sequence (TCS) that exerts repression of target mRNAs in immature oocytes of the frog, Xenopus laevis, and can direct early cytoplasmic polyadenylation and translational activation during oocyte maturation. The TCS is functionally distinct from the previously characterized Musashi/polyadenylation response element (PRE) and the cytoplasmic polyadenylation element (CPE). We report that TCS elements exert translational repression in both the Wee1 mRNA 3' UTR and the pericentriolar material-1 (Pcm-1) mRNA 3' UTR in immature oocytes. During oocyte maturation, TCS function directs the early translational activation of the Pcm-1 mRNA. By contrast, we demonstrate that CPE sequences flanking the TCS elements in the Wee1 3' UTR suppress the ability of the TCS to direct early translational activation. Our results indicate that a functional hierarchy exists between these distinct 3' UTR regulatory elements to control the timing of maternal mRNA translational activation during oocyte maturation.

Translational control by cytoplasmic polyadenylation in Xenopus oocytes

Elongation of the poly(A) tails of specific mRNAs in the cytoplasm is a crucial regulatory step in oogenesis and early development of many animal species. The best studied example is the regulation of translation by cytoplasmic polyadenylation elements (CPEs) in the 3′ untranslated region of mRNAs involved in Xenopus oocyte maturation. In this review we discuss the mechanism of translational control by the CPE binding protein (CPEB) in Xenopus oocytes as follows:1.

The cytoplasmic polyadenylation machinery such as CPEB, the subunits of cleavage and polyadenylation specificity factor (CPSF), symplekin, Gld-2 and poly(A) polymerase (PAP).

2.

The signal transduction that leads to the activation of CPE-mediated polyadenylation during oocyte maturation, including the potential roles of kinases such as MAPK, Aurora A, CamKII, cdk1/Ringo and cdk1/cyclin B.

3.

The role of deadenylation and translational repression, including the potential involvement of PARN, CCR4/NOT, maskin, pumilio, Xp54 (Ddx6, Rck), other P-body components and isoforms of the cap binding initiation factor eIF4E.

Finally we discuss some of the remaining questions regarding the mechanisms of translational regulation by cytoplasmic polyadenylation and give our view on where our knowledge is likely to be expanded in the near future.

The Xenopus cell cycle: an overview

Oocytes, eggs and embryos from the frog Xenopus laevis have been an important model system for studying cell-cycle regulation for several decades. First, progression through meiosis in the oocyte has been extensively investigated. Oocyte maturation has been shown to involve complex networks of signal transduction pathways, culminating in the cyclic activation and inactivation of Maturation Promoting Factor (MPF), composed of cyclin B and cdc2. After fertilisation, the early embryo undergoes rapid simplified cell cycles which have been recapitulated in cell-free extracts of Xenopus eggs. Experimental manipulation of these extracts has given a wealth of biochemical information about the cell cycle, particularly concerning DNA replication and mitosis. Finally, cells of older embryos adopt a more somatic-type cell cycle and have been used to study the balance between cell cycle and differentiation during development.

Oocyte quality and maternal control of development

The oocyte is a unique and highly specialized cell responsible for creating, activating, and controlling the embryonic genome, as well as supporting basic processes such as cellular homeostasis, metabolism, and cell cycle progression in the early embryo. During oogenesis, the oocyte accumulates a myriad of factors to execute these processes. Oogenesis is critically dependent upon correct oocyte-follicle cell interactions. Disruptions in oogenesis through environmental factors and changes in maternal health and physiology can compromise oocyte quality, leading to arrested development, reduced fertility, and epigenetic defects that affect long-term health of the offspring. Our expanding understanding of the molecular determinants of oocyte quality and how these determinants can be disrupted has revealed exciting new insights into the role of oocyte functions in development and evolution.

Mechanisms Regulating Oocyte Meiotic Resumption: Roles of Mitogen-Activated Protein Kinase

Oocyte meiotic maturation is one of the important physiological requirements for species survival. However, little is known about the detailed events occurring during this process. A number of studies have demonstrated that MAPK plays a pivotal role in the regulation of meiotic cell cycle progression in oocytes, but controversial findings have been reported in both lower vertebrates and mammals. In this review, we summarized the roles of MAPK cascade and related signal pathways in oocyte meiotic reinitiation in both lower vertebrates and mammals. We also tried to reconcile the paradoxical results and highlight the new findings concerning the function of MAPK in both oocytes and the surrounding follicular somatic cells. The unresolved questions and future research directions regarding the role of MAPK in meiotic resumption are addressed.

MAPK interacts with XGef and is required for CPEB activation during meiosis in Xenopus oocytes

Meiotic progression in Xenopus oocytes, and all other oocytes investigated, is dependent on polyadenylation-induced translation of stockpiled maternal mRNAs. Early during meiotic resumption, phosphorylation of CPE-binding protein (CPEB) is required for polyadenylation-induced translation of mRNAs encoding cell cycle regulators. Xenopus Gef (XGef), a Rho-family guanine-exchange factor, influences the activating phosphorylation of CPEB. An exchange-deficient version of XGef does not, therefore implicating Rho-family GTPase function in early meiosis. We show here that Clostridium difficile Toxin B, a Rho-family GTPase inhibitor, does not impair early CPEB phosphorylation or progression to germinal vesicle breakdown, indicating that XGef does not influence these events through activation of a Toxin-B-sensitive GTPase. Using the inhibitors U0126 for mitogen-activated protein kinase (MAPK), and ZM447439 for Aurora kinase A and Aurora kinase B, we found that MAPK is required for phosphorylation of CPEB, whereas Aurora kinases are not. Furthermore, we do not detect active Aurora kinase A in early meiosis. By contrast, we observe an early, transient activation of MAPK, independent of Mos protein expression. MAPK directly phosphorylates CPEB on four residues (T22, T164, S184, S248), but not on S174, a key residue for activating CPEB function. Notably, XGef immunoprecipitates contain MAPK, and this complex can phosphorylate CPEB. MAPK may prime CPEB for phosphorylation on S174 by an as-yet-unidentified kinase or may activate this kinase.

Paxillin Regulates Steroid-triggered Meiotic Resumption in Oocytes by Enhancing an All-or-None Positive Feedback Kinase Loop

Oocyte maturation is triggered by steroids in a transcription-independent fashion that involves an unusual positive feedback loop whereby MOS (a germ cell-specific Raf) activates MEK1, which in turn activates ERK2. ERK2 then acts back on MOS to enhance its expression and amplify the kinase signaling cascade. To date, little is known regarding other factors that regulate this powerful positive feedback kinase cascade. Here we present the scaffold molecule Paxillin as a newly recognized essential regulator of meiosis in Xenopus laevis oocytes. Reduction of Paxillin expression using RNA interference or antisense oligonucleotides completely abrogates steroid-triggered meiotic resumption. Detailed signaling studies reveal that Paxillin is acting early in the kinase cascade, because it is required for accumulation of MOS protein and complete activation of downstream kinase signaling in response to steroids. Surprisingly, full Paxillin activity also requires serine phosphorylation by a kinase downstream of MOS and MEK1, possibly ERK2. Together, these data suggest that Paxillin is an important regulator of the positive feedback effects of MEK/ERK signaling on MOS protein expression. These experiments reveal a novel and critical function for Paxillin in meiosis and support the notion that Paxillin may be a general modulator of mitogen-activated protein kinase signaling.

Intracellular acidification delays hormonal G2/M transition and inhibits G2/M transition triggered by thiophosphorylated MAPK in Xenopus oocytes

Xenopus oocyte maturation is analogous to G2/M transition and characterized by germinal vesicle breakdown (GVBD), spindle formation, activation of MPF and Mos-Xp42(Mpk1) pathways. It is accompanied prior to GVBD by a transient increase in intracellular pH. We determined that a well known acidifying compound, NH(4)Cl, delayed progesterone-induced GVBD in a dose-dependent manner. GVBD(50) was delayed up to 2.3-fold by 10 mM NH(4)Cl. Cyclin B2 phosphorylation, Cdk1 Tyr15 dephosphorylation as well as p39(Mos) accumulation, Xp42(Mpk1) and p90(Rsk) phosphorylation induced by progesterone were also delayed by incubation of oocyte in NH(4)Cl. The delay induced by NH(4)Cl was prevented by injection of MOPS buffer pH 7.7. In contrast to acidifying medium, alkalyzing treatment such as Tris buffer pH 9 injections, accelerated GVBD, MPF and Xp42(Mpk1) activation, indicating that pHi changes control early steps of G2/M dynamics. When injected in an immature recipient oocyte, egg cytoplasm triggers GVBD through MPF auto-amplification, independently of protein synthesis. In these conditions, GVBD and Xp42(Mpk1) activation were delayed by high concentration of NH(4)Cl, which never prevented or delayed MPF activation. Strickingly, NH(4)Cl strongly inhibited thiophosphorylated active MAPK-induced GVBD and MPF activation. Nevertheless, Tris pH 9 did not have any effects on egg cytoplasm- or active MAPK-induced GVBD. Taken together, our results suggest that dynamic of early events driving Xp42(Mpk1) and MPF activation induced by progesterone may be negatively or positively regulated by pH(i) changes. However Xp42(Mpk1) pathway was inhibited by acidification alone. Finally, MPF auto-amplification loop was not sensitive to pH(i) changes.

Musashi regulates the temporal order of mRNA translation during Xenopus oocyte maturation

A strict temporal order of maternal mRNA translation is essential for meiotic cell cycle progression in oocytes of the frog Xenopus laevis. The molecular mechanisms controlling the ordered pattern of mRNA translational activation have not been elucidated. We report a novel role for the neural stem cell regulatory protein, Musashi, in controlling the translational activation of the mRNA encoding the Mos proto-oncogene during meiotic cell cycle progression. We demonstrate that Musashi interacts specifically with the polyadenylation response element in the 3' untranslated region of the Mos mRNA and that this interaction is necessary for early Mos mRNA translational activation. A dominant inhibitory form of Musashi blocks maternal mRNA cytoplasmic polyadenylation and meiotic cell cycle progression. Our data suggest that Musashi is a target of the initiating progesterone signaling pathway and reveal that late cytoplasmic polyadenylation element-directed mRNA translation requires early, Musashi-dependent mRNA translation. These findings indicate that Musashi function is necessary to establish the temporal order of maternal mRNA translation during Xenopus meiotic cell cycle progression.

A novel role for glucocorticoid-induced leucine zipper protein in epithelial sodium channel-mediated sodium transport

The steroid hormone aldosterone stimulates sodium (Na+) transport in tight epithelia by altering the expression of target genes that regulate the activity and trafficking of the epithelial sodium channel (ENaC). We performed microarray analysis to identify aldosterone-regulated transcripts in mammalian kidney epithelial cells (mpkC-CD(c14)). One target, glucocorticoid-induced leucine zipper protein (GILZ), was previously identified by serial analysis of gene expression (SAGE); however, its function in epithelial ion transport was unknown. Here we show that GILZ expression is rapidly stimulated by aldosterone in mpkCCD(c14) and that GILZ, in turn, strongly stimulates ENaC-mediated Na+ transport by inhibiting extracellular signal-regulated kinase (ERK) signaling. In Xenopus oocytes with activated ERK, heterologous GILZ expression consistently inhibited phospho-ERK expression and markedly stimulated ENaC-mediated Na+ current, in a manner similar to that of U0126 (a pharmacologic inhibitor of ERK signaling). In mpkCCD(c14) cells, GILZ transfection similarly consistently inhibited phospho-ERK expression and stimulated transepithelial Na+ transport. Furthermore, aldosterone treatment of mpkCCD(c14) cells suppressed phospho-ERK levels with a time course that paralleled their increase of Na+ transport. Finally, GILZ expression markedly increased cell surface ENaC expression in epidermal growth factor-treated mammalian kidney epithelial cells, HEK 293. These observations suggest a novel link between GILZ and regulation of epithelial sodium transport through modulation of ERK signaling and could represent an important pathway for mediating aldosterone actions in health and disease.

Differential roles of p39Mos-Xp42Mpk1 cascade proteins on Raf1 phosphorylation and spindle morphogenesis in Xenopus oocytes

Fully-grown G2-arrested Xenopus oocytes resume meiosis upon hormonal stimulation. Resumption of meiosis is characterized by germinal vesicle breakdown, chromosome condensation, and organization of a bipolar spindle. These cytological events are accompanied by activation of MPF and the p39(Mos)-MEK1-Xp42(Mpk1)-p90(Rsk) pathways. The latter cascade is activated upon p39(Mos) accumulation. Using U0126, a MEK1 inhibitor, and p39(Mos) antisense morpholino and phosphorothioate oligonucleotides, we have investigated the role of the members of the p39(Mos)-MEK1-Xp42(Mpk1)-p90(Rsk) in spindle morphogenesis. First, we have observed at a molecular level that prevention of p39(Mos) accumulation always led to MEK1 phosphorylation defects, even when meiosis was stimulated through the insulin Ras-dependent pathway. Moreover, we have observed that Raf1 phosphorylation that occurs during meiosis resumption was dependent upon the activity of MEK1 or Xp42(Mpk1) but not p90(Rsk). Second, inhibition of either p39(Mos) accumulation or MEK1 inhibition led to the formation of a cytoplasmic aster-like structure that was associated with condensed chromosomes. Spindle morphogenesis rescue experiments using constitutively active Rsk and purified murine Mos protein suggested that p39(Mos) or p90(Rsk) alone failed to promote meiotic spindle organization. Our results indicate that activation of the p39(Mos)-MEK1-Xp42(Mpk1)-p90(Rsk) pathway is required for bipolar organization of the meiotic spindle at the cortex.

Translational regulation of MOS messenger RNA in pig oocytes

The temporal and spatial translation control of stored mRNA in oocytes is regulated by elements in their 3'-untranslated region (3'-UTR). The MOS 3'-UTR in pig oocytes is both heterogeneous (180, 480, or 530 nucleotides), and it contains multiple U-rich elements and extensive A-rich sequences (CA13CA5CA5CA6). We have examined the role of these potential regulatory elements by fusing wild-type or mutant MOS 3'-UTRs to luciferase mRNA and then injecting these chimeric transcripts into oocytes. We draw six main conclusions. First, the length of the MOS 3'-UTR tightly controls the level of translation of luciferase during oocyte maturation. Second, two U-rich (U5A) elements and the hexanucleotide signal (AAUAAA) are required for translation. Third, mutations, duplications, or relocations of the A-rich sequence reduce or block translation. Fourth, the relative importance of the A-rich and U-rich elements in controlling the level of translation differs. Fifth, none of our MOS 3'-UTR manipulations relieved translational repression before germinal vesicle breakdown. Sixth, all the MOS mRNA variants underwent polyadenylation during maturation. Whereas mutations to the hexanucleotide signal block both polyadenylation and translation, mutations to either the A-rich sequence or the U-rich elements block translation without fully blocking polyadenylation. We conclude that MOS mRNA translation in pig oocytes is subject to a more extensive series of controls than that in lower vertebrates.

The distinct stage-specific effects of 2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid on the activation of MAP kinase and Cdc2 kinase in Xenopus oocyte maturation

2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid (PACA), pharmacological inhibitor of phospholipase A(2) (PLA(2)), inhibits epinephrine-stimulated thromboxane production in human platelets. In this study, we investigated the effect of PACA on meiotic maturation individually in stages V and VI oocytes. PACA prevented the maturation in stage V but merely delayed the process in stage VI oocytes. This was associated with the strong inhibition of Mos synthesis at both stages. Besides, PACA-induced inhibition of MAPK activation was evident in stage V but not in stage VI oocytes. PACA also inhibited the activation of Cdc2 kinase (Cdc2) in stage V but merely delayed the process in stage VI oocytes. Furthermore, 5 microM and higher concentrations of PACA completely inhibited the activation of MAPK and Cdc2 only in stage V, not in stage VI, oocytes. Moreover, we propose PACA as a new tool for the study of Xenopus oocyte maturation, which can also play a unique role for the studies of the stage-specific activation of MAPK and Cdc2.

XGef mediates early CPEB phosphorylation during Xenopus oocyte meiotic maturation

Polyadenylation-induced translation is an important regulatory mechanism during metazoan development. During Xenopus oocyte meiotic progression, polyadenylation-induced translation is regulated by CPEB, which is activated by phosphorylation. XGef, a guanine exchange factor, is a CPEB-interacting protein involved in the early steps of progesterone-stimulated oocyte maturation. We find that XGef influences early oocyte maturation by directly influencing CPEB function. XGef and CPEB interact during oogenesis and oocyte maturation and are present in a c-mos messenger ribonucleoprotein (mRNP). Both proteins also interact directly in vitro. XGef overexpression increases the level of CPEB phosphorylated early during oocyte maturation, and this directly correlates with increased Mos protein accumulation and acceleration of meiotic resumption. To exert this effect, XGef must retain guanine exchange activity and the interaction with CPEB. Overexpression of a guanine exchange deficient version of XGef, which interacts with CPEB, does not enhance early CPEB phosphorylation. Overexpression of a version of XGef that has significantly reduced interaction with CPEB, but retains guanine exchange activity, decreases early CPEB phosphorylation and delays oocyte maturation. Injection of XGef antibodies into oocytes blocks progesterone-induced oocyte maturation and early CPEB phosphorylation. These findings indicate that XGef is involved in early CPEB activation and implicate GTPase signaling in this process.

CK2 beta, which inhibits Mos function, binds to a discrete domain in the N-terminus of Mos

Progesterone stimulates G2-arrested Xenopus oocytes to synthesize Mos, a MAPK kinase kinase required for the coordinated activation of cdc2 and the G2/Meiosis I (MI) transition. Mos leads to activation of MAPK, Rsk, and the inhibition of the cdc2 inhibitor Myt1. Previous work identified CK2 beta as a Mos-interacting protein, and suggested that CK2 beta acts as a negative regulator by setting a threshold above which newly made Mos must accumulate to activate MAPK. However, it had not been demonstrated that CK2 beta directly inhibits Mos. We report here that Mos (52-115) is required for CK2 beta binding and can serve as a portable binding domain. To test whether CK2 beta acts at the level of Mos or on a downstream component, we took advantage of previous work that showed injection of Mos arrests rapidly dividing embryonic cells. We find that coinjection of CK2 beta and Mos into embryonic cells inhibits the ability of Mos to arrest cell division. In contrast, CK2 beta does not inhibit the mitotic arrest induced by injection of active Rsk. These results argue that CK2 beta directly binds and inhibits Mos rather than a downstream component, and support that CK2 beta functions as a molecular buffer that prevents premature MAPK activation and oocyte maturation.

Ca(2+)(cyt) negatively regulates the initiation of oocyte maturation

Ca²⁺ is a ubiquitous intracellular messenger that is important for cell cycle progression. Genetic and biochemical evidence support a role for Ca²⁺ in mitosis. In contrast, there has been a long-standing debate as to whether Ca²⁺ signals are required for oocyte meiosis. Here, we show that cytoplasmic Ca²⁺ (Ca²⁺_cyt) plays a dual role during Xenopus oocyte maturation. Ca²⁺ signals are dispensable for meiosis entry (germinal vesicle breakdown and chromosome condensation), but are required for the completion of meiosis I. Interestingly, in the absence of Ca²⁺_cyt signals oocytes enter meiosis more rapidly due to faster activation of the MAPK-maturation promoting factor (MPF) kinase cascade. This Ca²⁺-dependent negative regulation of the cell cycle machinery (MAPK-MPF cascade) is due to Ca²⁺_cyt acting downstream of protein kinase A but upstream of Mos (a MAPK kinase kinase). Therefore, high Ca²⁺_cyt delays meiosis entry by negatively regulating the initiation of the MAPK-MPF cascade. These results show that Ca²⁺ modulates both the cell cycle machinery and nuclear maturation during meiosis.

Cytoplasmic polyadenylation element (CPE)- and CPE-binding protein (CPEB)-independent mechanisms regulate early class maternal mRNA translational activation in Xenopus oocytes

Meiotic cell cycle progression during vertebrate oocyte maturation requires the correct temporal translation of maternal mRNAs encoding key regulatory proteins. The mechanism by which specific mRNAs are temporally activated is unknown, although both cytoplasmic polyadenylation elements (CPE) within the 3'-untranslated region (3'-UTR) of mRNAs and the CPE-binding protein (CPEB) have been implicated. We report that in progesterone-stimulated Xenopus oocytes, the early cytoplasmic polyadenylation and translational activation of multiple maternal mRNAs occur in a CPE- and CPEB-independent manner. We demonstrate that polyadenylation response elements, originally identified in the 3'-UTR of the mRNA encoding the Mos proto-oncogene, direct CPE- and CPEB-independent polyadenylation of an early class of Xenopus maternal mRNAs. Our findings refute the hypothesis that CPE sequences alone account for the range of temporal inductions of maternal mRNAs observed during Xenopus oocyte maturation. Rather, our data indicate that the sequential action of distinct 3'-UTR-directed translational control mechanisms coordinates the complex temporal patterns and extent of protein synthesis during vertebrate meiotic cell cycle progression.

Design of genetic networks with specified functions by evolution in silico

Recent studies have provided insights into the modular structure of genetic regulatory networks and emphasized the interest of quantitative functional descriptions. Here, to provide a priori knowledge of the structure of functional modules, we describe an evolutionary procedure in silico that creates small gene networks performing basic tasks. We used it to create networks functioning as bistable switches or oscillators. The obtained circuits provide a variety of functional designs, demonstrate the crucial role of posttranscriptional interactions, and highlight design principles also found in known biological networks. The procedure should prove helpful as a way to understand and create small functional modules with diverse functions as well as to analyze large networks.

Xp42(Mpk1) activation is not required for germinal vesicle breakdown but for Raf complete phosphorylation in insulin-stimulated Xenopus oocytes

Fully grown G2-arrested Xenopus oocytes resume meiosis in vitro upon exposure to hormonal stimulation. Progesterone triggers oocyte meiosis resumption through a Ras-independent pathway that involves a p39Mos-dependent activation of the mitogen-activated protein (MAP) kinases. Insulin also triggers meiosis resumption through a tyrosine kinase receptor that activates a Ras-dependent pathway leading to the MAP kinases activation. Antisense phosphorothioate oligonucleotides were used to prevent p39Mos accumulation and Erk-like Xp42(Mpk1) activation during insulin-induced Xenopus oocytes maturation. In contrast to previous works, prevention of p39Mos-induced activation of Xp42(Mpk1) in insulin-treated oocytes did not inhibit but delayed meiotic resumption, like in progesterone-stimulated oocytes. Activations of Xp42(Mpk1), the unique Erk of the oocyte, and of its downstream target p90Rsk, were impaired and phosphorylation of the MAPKK kinase Raf was partially inhibited. Similarly, oocytes treated with the MEK inhibitor U0126, stimulated by insulin exhibited delayed germinal vesicle breakdown, absence of Xp42(Mpk1) activation, and partial phosphorylation of Raf. To summarize, whereas p39Mos-induced activation of MEK/MAPK pathway is dispensable for insulin-induced germinal vesicle breakdown, Xp42(Mpk1) activation induced by insulin is dependent upon p39Mos synthesis. Raf complete phosphorylation appears to require the MEK/MAPK pathway activation both in progesterone and insulin-stimulated oocytes.

In vitro maturation of bovine oocytes requires polyadenylation of mRNAs coding proteins for chromatin condensation, spindle assembly, MPF and MAP kinase activation

In the oocyte mRNA molecules are stored in order to be used during the maturation process when transcription is silenced. Translational activation of stored mRNA templates commonly is correlated with their cytoplasmic polyadenylation. In the present study, the effects of cordycepin, a potent polyadenylation inhibitor, on the in vitro maturation MPF and MAP kinase activation of bovine cumulus oocyte complexes (COCs) were examined. The presence of cordycepin (5 microg/ml) during the whole culture period (24 h) prevents chromatin condensation and germinal vesicle breakdown (GVBD) as well as MPF and MAP kinase activation. When COCs were first cultivated in inhibitor-free medium for 6 or 9 h and subsequently transferred to cordycepin supplemented medium for further 18 or 15 h neither MPF nor MAP kinase became activated and 86 and 85%, respectively, of the oocytes underwent GVBD but failed to form a spindle and hypercondensed their chromatin. Extending the first culture period in inhibitor-free medium to 12 or 15 h before transferring the COCs to cordycepin supplemented medium for a further 12 or 9 h allowed 48 and 79%, respectively of oocytes to reach the metaphase 2 (M 2) stage. From these data, it is concluded that mRNA molecules coding for proteins required for chromatin condensation and GVBD become polyadenylated during the first 6 h following the onset of culture whereas mRNA molecules coding for proteins required for spindle assembly of metaphase 1 (M 1) and MPF and MAP kinase activation become polyadenylated between 9 and 12 h following initiation of culture.

Dissolution of the maskin-eIF4E complex by cytoplasmic polyadenylation and poly(A)-binding protein controls cyclin B1 mRNA translation and oocyte maturation

Cytoplasmic polyadenylation stimulates the translation of several dormant mRNAs during oocyte maturation in Xenopus. Polyadenylation is regulated by the cytoplasmic polyadenylation element (CPE), a cis-acting element in the 3'-untranslated region of responding mRNAs, and its associated factor CPEB. CPEB also binds maskin, a protein that in turn interacts with eIF4E, the cap-binding factor. Here, we report that based on antibody and mRNA reporter injection assays, maskin prevents oocyte maturation and the translation of the CPE-containing cyclin B1 mRNA by blocking the association of eIF4G with eIF4E. Dissociation of the maskin-eIF4E complex is essential for cyclin B1 mRNA translational activation, and requires not only cytoplasmic polyadenylation, but also the poly(A)-binding protein. These results suggest a molecular mechanism by which CPE- containing mRNA is activated in early development.

A novel regulatory element determines the timing of Mos mRNA translation during Xenopus oocyte maturation

Progression through vertebrate oocyte maturation requires that pre-existing, maternally derived mRNAs be translated in a strict temporal order. The mechanism that controls the timing of oocyte mRNA translation is unknown. In this study we show that the early translational induction of the mRNA encoding the Mos proto-oncogene is mediated through a novel regulatory element within the 3' untranslated region of the Mos mRNA. This novel element is responsive to the MAP kinase signaling pathway and is distinct from the late acting, cdc2-responsive, cytoplasmic polyadenylation element. Our findings suggest that the timing of maternal mRNA translation is controlled through signal transduction pathways targeting distinct 3' UTR mRNA elements.

A new role for Mos in Xenopus oocyte maturation: targeting Myt1 independently of MAPK

The resumption of meiosis in Xenopus arrested oocytes is triggered by progesterone, which leads to polyadenylation and translation of Mos mRNA, then activation of MAPK pathway. While Mos protein kinase has been reported to be essential for re-entry into meiosis in Xenopus, arrested oocytes can undergo germinal vesicle breakdown (GVBD) independently of MAPK activation, leading us to question what the Mos target might be if Mos is still required. We now demonstrate that Mos is indeed necessary, although is independent of the MAPK cascade, for conversion of inactive pre-MPF into active MPF. We have found that Myt1 is likely to be the Mos target in this process, as Mos interacts with Myt1 in oocyte extracts and Mos triggers Myt1 phosphorylation on some sites in vivo, even in the absence of MAPK activation. We propose that Mos is involved, not only in the MAPK cascade pathway, but also in a mechanism that directly activates MPF in Xenopus oocytes.

Inhibition of Xenopus oocyte meiotic maturation by catalytically inactive protein kinase A

Progesterone induces G2-arrested Xenopus oocytes to develop into fertilizable eggs in a process called meiotic maturation. Protein kinase A (PKA), the cAMP-dependent protein kinase, has long been known to be a potent inhibitor of meiotic maturation, but little information is available on how PKA functions. We have cloned two Xenopus PKA catalytic subunit isoforms, XPKAalpha and XPKAbeta. These proteins are 89% identical and both inhibit progesterone-induced meiotic maturation when overexpressed at low levels, suggesting that PKA activity is tightly regulated in the oocyte. Unexpectedly, catalytically inactive XPKA mutants are able to block progesterone-induced maturation as efficiently as the wild-type active XPKA. These mutants also block meiotic maturation induced by Mos, but are less efficient at inhibiting Cdc25C-induced maturation. Our results indicate that PKA can inhibit meiotic maturation by a novel mechanism, which does not require its kinase activity and is also independent of binding to the PKA regulatory subunits.

Self-perpetuating states in signal transduction: positive feedback, double-negative feedback and bistability

Cell signaling systems that contain positive-feedback loops or double-negative feedback loops can, in principle, convert graded inputs into switch-like, irreversible responses. Systems of this sort are termed "bistable". Recently, several groups have engineered artificial bistable systems into Escherichia coli and Saccharomyces cerevisiae, and have shown that the systems exhibit interesting and potentially useful properties. In addition, two naturally occurring signaling systems, the p42 mitogen-activated protein kinase and c-Jun amino-terminal kinase pathways in Xenopus oocytes, have been shown to exhibit bistable responses. Here we review the basic properties of bistable circuits, the requirements for construction of a satisfactory bistable switch, and the recent progress towards constructing and analysing bistable signaling systems.

Translational and post-translational modifications in meiosis of the mammalian oocyte

The fully-grown oocyte is transcriptionally inactive. Therefore, translational and post-translational modifications furnish the control mechanism of key components governing meiosis. Regulation by protein synthesis provides an irreversible unidirectional mechanism for an extended period that can be restricted by a complementary degradation of the same protein. Both processes utilize tight measures to ensure precise expression at the right time in the right place. Rapid modifications such as phosphorylation and dephosphorylation supply reversible means to regulate protein action. Information regarding these extremely exciting issues is being accumulated recently in an exponential rate. However, the vast majority of these data is generated from studies conducted on Xenopus oocytes. We fully agree with Andrew Murray's statement that "The modern trend of promoting research on a small number of 'model' organisms will eventually deprive us of the opportunity to study interesting biology" [Cell 92 (1992) 157]. Thus, despite of the enormous technical difficulties resulting from the limited availability of biological material we extended our interest to mammalian model systems. Our review will attend to certain examples of such modifications in the regulatory pathway of meiosis in mammalian oocytes.