Redundant pathways for Cdc2 activation in Xenopus oocyte: either cyclin B or Mos synthesis

Unraveling the interplay between PKA inhibition and Cdk1 activation during oocyte meiotic maturation

Oocytes are arrested in prophase I. In vertebrates, meiotic resumption is triggered by hormonal stimulation that results in cAMP-dependent protein kinase (PKA) downregulation leading to Cdk1 activation. Yet the pathways connecting PKA to Cdk1 remain unclear. Here, we identify molecular events triggered by PKA downregulation occurring upstream of Cdk1 activation. We describe a two-step regulation controlling cyclin B1 and Mos accumulation, which depends on both translation and stabilization. Cyclin B1 accumulation is triggered by PKA inhibition upstream of Cdk1 activation, while its translation requires Cdk1 activity. Conversely, Mos translation initiates in response to the hormone, but the protein accumulates only downstream of Cdk1. Furthermore, two successive translation waves take place, the first controlled by PKA inhibition and the second by Cdk1 activation. Notably, Arpp19, an essential PKA effector, does not regulate the early PKA-dependent events. This study elucidates how PKA downregulation orchestrates multiple pathways that converge toward Cdk1 activation and induce the oocyte G2/M transition.

Identification of GnRH-like peptide and its potential signaling pathway involved in the oocyte meiotic maturation in the Chinese mitten crab, Eriocheir sinensis

Gonadotropin-releasing hormone (GnRH) plays a pivotal role in reproductive regulation in vertebrates. However, GnRH was rarely isolated and its function remains poorly characterized in invertebrates. The existence of GnRH in ecdysozoa has been controversial for a long. Here, we isolated and identified two GnRH-like peptides from brain tissues in Eriocheir sinensis. Immunolocalization showed that the presence of EsGnRH-like peptide in brain, ovary and hepatopancreas. Synthetic EsGnRH-like peptides can induce germinal vesicle breakdown (GVBD) of oocyte. Similar to vertebrates, ovarian transcriptomic analysis revealed a GnRH signaling pathway in the crab, in which most genes exhibited dramatically high expression at GVBD. RNAi knockdown of EsGnRHR suppressed the expression of most genes in the pathway. Co-transfection of the expression plasmid pcDNA3.1-EsGnRHR with reporter plasmid CRE-luc or SRE-luc into 293T cells showed that EsGnRHR transduces its signal via cAMP and Ca²⁺ signaling transduction pathways. In vitro incubation of the crab oocyte with EsGnRH-like peptide confirmed the cAMP-PKA cascade and Ca²⁺ mobilization signaling cascade but lack of a PKC cascade. Our data present the first direct evidence of the existence of GnRH-like peptides in the crab and demonstrated its conserved role in the oocyte meiotic maturation as a primitive neurohormone.

The translation regulator Zar1l controls timing of meiosis in Xenopus oocytes

Oocyte maturation and early embryo development occur in vertebrates in the near absence of transcription. Thus, sexual reproduction of vertebrates critically depends on the timely translation of mRNAs already stockpiled in the oocyte. Yet how translational activation of specific mRNAs is temporally coordinated is still incompletely understood. Here, we elucidate the function of Zar1l, a yet uncharacterized member of the Zar RNA-binding protein family, in Xenopus oocytes. Employing TRIM-Away, we demonstrate that loss of Zar1l accelerates hormone-induced meiotic resumption of Xenopus oocytes due to premature accumulation of the M-phase-promoting kinase cMos. We show that Zar1l is a constituent of a large ribonucleoparticle containing the translation repressor 4E-T and the central polyadenylation regulator CPEB1, and that it binds directly to the cMos mRNA. Partial, hormone-induced degradation of Zar1l liberates 4E-T from CPEB1, which weakens translational repression of mRNAs encoding cMos and likely additional M-phase-promoting factors. Thus, our study provides fundamental insights into the mechanisms that ensure temporally regulated translation of key cell cycle regulators during oocyte maturation, which is essential for sexual reproductivity.

Regulation of oocyte maturation: Role of conserved ERK signaling

During oogenesis, oocytes arrest at meiotic prophase I to acquire competencies for resuming meiosis, fertilization, and early embryonic development. Following this arrested period, oocytes resume meiosis in response to species-specific hormones, a process known as oocyte maturation, that precedes ovulation and fertilization. Involvement of endocrine and autocrine/paracrine factors and signaling events during maintenance of prophase I arrest, and resumption of meiosis is an area of active research. Studies in vertebrate and invertebrate model organisms have delineated the molecular determinants and signaling pathways that regulate oocyte maturation. Cell cycle regulators, such as cyclin-dependent kinase (CDK1), polo-like kinase (PLK1), Wee1/Myt1 kinase, and the phosphatase CDC25 play conserved roles during meiotic resumption. Extracellular signal-regulated kinase (ERK), on the other hand, while activated during oocyte maturation in all species, regulates both species-specific, as well as conserved events among different organisms. In this review, we synthesize the general signaling mechanisms and focus on conserved and distinct functions of ERK signaling pathway during oocyte maturation in mammals, non-mammalian vertebrates, and invertebrates such as Drosophila and Caenorhabditis elegans.

The M-phase regulatory phosphatase PP2A-B55δ opposes protein kinase A on Arpp19 to initiate meiotic division

Oocytes are held in meiotic prophase for prolonged periods until hormonal signals trigger meiotic divisions. Key players of M-phase entry are the opposing Cdk1 kinase and PP2A-B55δ phosphatase. In Xenopus, the protein Arpp19, phosphorylated at serine 67 by Greatwall, plays an essential role in inhibiting PP2A-B55δ, promoting Cdk1 activation. Furthermore, Arpp19 has an earlier role in maintaining the prophase arrest through a second serine (S109) phosphorylated by PKA. Prophase release, induced by progesterone, relies on Arpp19 dephosphorylation at S109, owing to an unknown phosphatase. Here, we identified this phosphatase as PP2A-B55δ. In prophase, PKA and PP2A-B55δ are simultaneously active, suggesting the presence of other important targets for both enzymes. The drop in PKA activity induced by progesterone enables PP2A-B55δ to dephosphorylate S109, unlocking the prophase block. Hence, PP2A-B55δ acts critically on Arpp19 on two distinct sites, opposing PKA and Greatwall to orchestrate the prophase release and M-phase entry.

Translational Control of Xenopus Oocyte Meiosis: Toward the Genomic Era

The study of oocytes has made enormous contributions to the understanding of the G2/M transition. The complementarity of investigations carried out on various model organisms has led to the identification of the M-phase promoting factor (MPF) and to unravel the basis of cell cycle regulation. Thanks to the power of biochemical approaches offered by frog oocytes, this model has allowed to identify the core signaling components involved in the regulation of M-phase. A central emerging layer of regulation of cell division regards protein translation. Oocytes are a unique model to tackle this question as they accumulate large quantities of dormant mRNAs to be used during meiosis resumption and progression, as well as the cell divisions during early embryogenesis. Since these events occur in the absence of transcription, they require cascades of successive unmasking, translation, and discarding of these mRNAs, implying a fine regulation of the timing of specific translation. In the last years, the Xenopus genome has been sequenced and annotated, enabling the development of omics techniques in this model and starting its transition into the genomic era. This review has critically described how the different phases of meiosis are orchestrated by changes in gene expression. The physiological states of the oocyte have been described together with the molecular mechanisms that control the critical transitions during meiosis progression, highlighting the connection between translation control and meiosis dynamics.

The G2-to-M transition from a phosphatase perspective: a new vision of the meiotic division

Cell division is orchestrated by the phosphorylation and dephosphorylation of thousands of proteins. These post-translational modifications underlie the molecular cascades converging to the activation of the universal mitotic kinase, Cdk1, and entry into cell division. They also govern the structural events that sustain the mechanics of cell division. While the role of protein kinases in mitosis has been well documented by decades of investigations, little was known regarding the control of protein phosphatases until the recent years. However, the regulation of phosphatase activities is as essential as kinases in controlling the activation of Cdk1 to enter M-phase. The regulation and the function of phosphatases result from post-translational modifications but also from the combinatorial association between conserved catalytic subunits and regulatory subunits that drive their substrate specificity, their cellular localization and their activity. It now appears that sequential dephosphorylations orchestrated by a network of phosphatase activities trigger Cdk1 activation and then order the structural events necessary for the timely execution of cell division. This review discusses a series of recent works describing the important roles played by protein phosphatases for the proper regulation of meiotic division. Many breakthroughs in the field of cell cycle research came from studies on oocyte meiotic divisions. Indeed, the meiotic division shares most of the molecular regulators with mitosis. The natural arrests of oocytes in G2 and in M-phase, the giant size of these cells, the variety of model species allowing either biochemical or imaging as well as genetics approaches explain why the process of meiosis has served as an historical model to decipher signalling pathways involved in the G2-to-M transition. The review especially highlights how the phosphatase PP2A-B55δ critically orchestrates the timing of meiosis resumption in amphibian oocytes. By opposing the kinase PKA, PP2A-B55δ controls the release of the G2 arrest through the dephosphorylation of their substrate, Arpp19. Few hours later, the inhibition of PP2A-B55δ by Arpp19 releases its opposing kinase, Cdk1, and triggers M-phase. In coordination with a variety of phosphatases and kinases, the PP2A-B55δ/Arpp19 duo therefore emerges as the key effector of the G2-to-M transition.

Managing the Oocyte Meiotic Arrest-Lessons from Frogs and Jellyfish

During oocyte development, meiosis arrests in prophase of the first division for a remarkably prolonged period firstly during oocyte growth, and then when awaiting the appropriate hormonal signals for egg release. This prophase arrest is finally unlocked when locally produced maturation initiation hormones (MIHs) trigger entry into M-phase. Here, we assess the current knowledge of the successive cellular and molecular mechanisms responsible for keeping meiotic progression on hold. We focus on two model organisms, the amphibian Xenopus laevis, and the hydrozoan jellyfish Clytia hemisphaerica. Conserved mechanisms govern the initial meiotic programme of the oocyte prior to oocyte growth and also, much later, the onset of mitotic divisions, via activation of two key kinase systems: Cdk1-Cyclin B/Gwl (MPF) for M-phase activation and Mos-MAPkinase to orchestrate polar body formation and cytostatic (CSF) arrest. In contrast, maintenance of the prophase state of the fully-grown oocyte is assured by highly specific mechanisms, reflecting enormous variation between species in MIHs, MIH receptors and their immediate downstream signalling response. Convergence of multiple signalling pathway components to promote MPF activation in some oocytes, including Xenopus, is likely a heritage of the complex evolutionary history of spawning regulation, but also helps ensure a robust and reliable mechanism for gamete production.

SGK phosphorylates Cdc25 and Myt1 to trigger cyclin B-Cdk1 activation at the meiotic G2/M transition

The kinase cyclin B-Cdk1 complex is a master regulator of M-phase in both mitosis and meiosis. At the G2/M transition, cyclin B-Cdk1 activation is initiated by a trigger that reverses the balance of activities between Cdc25 and Wee1/Myt1 and is further accelerated by autoregulatory loops. In somatic cell mitosis, this trigger was recently proposed to be the cyclin A-Cdk1/Plk1 axis. However, in the oocyte meiotic G2/M transition, in which hormonal stimuli induce cyclin B-Cdk1 activation, cyclin A-Cdk1 is nonessential and hence the trigger remains elusive. Here, we show that SGK directly phosphorylates Cdc25 and Myt1 to trigger cyclin B-Cdk1 activation in starfish oocytes. Upon hormonal stimulation of the meiotic G2/M transition, SGK is activated by cooperation between the Gβγ-PI3K pathway and an unidentified pathway downstream of Gβγ, called the atypical Gβγ pathway. These findings identify the trigger in oocyte meiosis and provide insights into the role and activation of SGK.

Multiple Mechanisms Inactivate the LIN-41 RNA-Binding Protein To Ensure a Robust Oocyte-to-Embryo Transition in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, the conserved LIN-41 RNA-binding protein is a translational repressor that coordinately controls oocyte growth and meiotic maturation. LIN-41 exerts these effects, at least in part, by preventing the premature activation of the cyclin-dependent kinase CDK-1. Here we investigate the mechanism by which LIN-41 is rapidly eliminated upon the onset of meiotic maturation. Elimination of LIN-41 requires the activities of CDK-1 and multiple SCF (Skp1, Cul1, and F-box protein)-type E3 ubiquitin ligase subunits, including the conserved substrate adaptor protein SEL-10/Fbw7/Cdc4, suggesting that LIN-41 is a target of ubiquitin-mediated protein degradation. Within the LIN-41 protein, two nonoverlapping regions, Deg-A and Deg-B, are individually necessary for LIN-41 degradation; both contain several potential phosphodegron sequences, and at least one of these sequences is required for LIN-41 degradation. Finally, Deg-A and Deg-B are sufficient, in combination, to mediate SEL-10-dependent degradation when transplanted into a different oocyte protein. Although LIN-41 is a potent inhibitor of protein translation and M phase entry, the failure to eliminate LIN-41 from early embryos does not result in the continued translational repression of LIN-41 oocyte messenger RNA targets. Based on these observations, we propose a model for the elimination of LIN-41 by the SEL-10 E3 ubiquitin ligase and suggest that LIN-41 is inactivated before it is degraded. Furthermore, we provide evidence that another RNA-binding protein, the GLD-1 tumor suppressor, is regulated similarly. Redundant mechanisms to extinguish translational repression by RNA-binding proteins may both control and provide robustness to irreversible developmental transitions, including meiotic maturation and the oocyte-to-embryo transition.

Effect of Kisspeptin on the Developmental Competence and Early Transcript Expression in Porcine Oocytes Parthenogenetically Activated with Different Methods

Recent studies showed the modulatory effect of kisspeptin (KP) on calcium waves through the cell membrane and inside the cell. Spermatozoon can induce similar ooplasmic calcium oscillations at fertilization to trigger meiosis II. Here, we evaluated the effect of KP supplementation with 6-dimethylaminopurine (6-DMAP) for 4 h on embryonic development after oocyte activation with single electric pulse, 5 µM ionomycin, or 8% ethanol. Compared to control nonsupplemented groups, KP significantly improved embryo developmental competence electric- and ethanol-activated oocytes in terms of cleavage (75.3% and 58.6% versus 64% and 48%, respectively, p < 0.05) and blastocyst development (31.3% and 10% versus 19.3% and 4%, respectively, p < 0.05). MOS expression was increased in electrically activated oocytes in presence of KP while it significantly reduced CCNB1 expression. In ionomycin treated group, both MOS and CCNB1 showed significant increase with no difference between KP and control groups. In ethanol-treated group, KP significantly reduced CCNB1 but no effect was observed on MOS expression. The early alterations in MOS and CCNB1 mRNA transcripts caused by KP may explain the significant differences in the developmental competence between the experimental groups. Kisspeptin supplementation may be adopted in protocols for porcine oocyte activation through electric current and ethanol to improve embryonic developmental competence.

MPF-based meiotic cell cycle control: Half a century of lessons from starfish oocytes

In metazoans that undergo sexual reproduction, genomic inheritance is ensured by two distinct types of cell cycle, mitosis and meiosis. Mitosis maintains the genomic ploidy in somatic cells reproducing within a generation, whereas meiosis reduces by half the ploidy in germ cells to prepare for successive generations. The meiotic cell cycle is believed to be a derived form of the mitotic cell cycle; however, the molecular mechanisms underlying both of these processes remain elusive. My laboratory has long studied the meiotic cell cycle in starfish oocytes, particularly the control of meiotic M-phase by maturation- or M phase-promoting factor (MPF) and the kinase cyclin B-associated Cdk1 (cyclin B-Cdk1). Using this system, we have unraveled the molecular principles conserved in metazoans that modify M-phase progression from the mitotic type to the meiotic type needed to produce a haploid genome. Furthermore, we have solved a long-standing enigma concerning the molecular identity of MPF, a universal inducer of M-phase both in mitosis and meiosis of eukaryotic cells.

Conserved insulin signaling in the regulation of oocyte growth, development, and maturation

Insulin signaling regulates various aspects of physiology, such as glucose homeostasis and aging, and is a key determinant of female reproduction in metazoans. That insulin signaling is crucial for female reproductive health is clear from clinical data linking hyperinsulinemic and hypoinsulinemic condition with certain types of ovarian dysfunction, such as altered steroidogenesis, polycystic ovary syndrome, and infertility. Thus, understanding the signaling mechanisms that underlie the control of insulin-mediated ovarian development is important for the accurate diagnosis of and intervention for female infertility. Studies of invertebrate and vertebrate model systems have revealed the molecular determinants that transduce insulin signaling as well as which biological processes are regulated by the insulin-signaling pathway. The molecular determinants of the insulin-signaling pathway, from the insulin receptor to its downstream signaling components, are structurally and functionally conserved across evolution, from worms to mammals-yet, physiological differences in signaling still exist. Insulin signaling acts cooperatively with gonadotropins in mammals and lower vertebrates to mediate various aspects of ovarian development, mainly owing to evolution of the endocrine system in vertebrates. In contrast, insulin signaling in Drosophila and Caenorhabditis elegans directly regulates oocyte growth and maturation. In this review, we compare and contrast insulin-mediated regulation of ovarian functions in mammals, lower vertebrates, C. elegans, and Drosophila, and highlight conserved signaling pathways and regulatory mechanisms in general while illustrating insulin's unique role in specific reproductive processes.

Translational Control of Germ Cell Decisions

Germline poses unique challenges to gene expression control at the transcriptional level. While the embryonic germline maintains a global hold on new mRNA transcription, the female adult germline produces transcripts that are not translated into proteins until embryogenesis of subsequent generation. As a consequence, translational control plays a central role in governing various germ cell decisions including the formation of primordial germ cells, self-renewal/differentiation decisions in the adult germline, onset of gametogenesis and oocyte maturation. Mechanistically, several common themes such as asymmetric localization of mRNAs, conserved RNA-binding proteins that control translation by 3' UTR binding, translational activation by the cytoplasmic elongation of the polyA tail and the assembly of mRNA-protein complexes called mRNPs have emerged from the studies on Caenorhabditis elegans, Xenopus and Drosophila. How mRNPs assemble, what influences their dynamics, and how a particular 3' UTR-binding protein turns on the translation of certain mRNAs while turning off other mRNAs at the same time and space are key challenges for future work.

Entry into mitosis: a solution to the decades-long enigma of MPF

Maturation or M phase-promoting factor (MPF) is the universal inducer of M phase common to eukaryotic cells. MPF was originally defined as a transferable activity that can induce the G2/M phase transition in recipient cells. Today, however, MPF is assumed to describe an activity that exhibits its effect in donor cells, and furthermore, MPF is consistently equated with the kinase cyclin B-Cdk1. In some conditions, however, MPF, as originally defined, is undetectable even though cyclin B-Cdk1 is fully active. For over three decades, this inconsistency has remained a long-standing puzzle. The enigma is now resolved through the elucidation that MPF, defined as an activity that exhibits its effect in recipient cells, consists of at least two separate kinases, cyclin B-Cdk1 and Greatwall (Gwl). Involvement of Gwl in MPF can be explained by its contribution to the autoregulatory activation of cyclin B-Cdk1 and by its stabilization of phosphorylations on cyclin B-Cdk1 substrates, both of which are essential when MPF induces the G2/M phase transition in recipient cells. To accomplish these tasks, Gwl helps cyclin B-Cdk1 by suppressing protein phosphatase 2A (PP2A)-B55 that counteracts cyclin B-Cdk1. MPF, as originally defined, is thus not synonymous with cyclin B-Cdk1, but is instead a system consisting of both cyclin B-Cdk1 that directs mitotic entry and Gwl that suppresses the anti-cyclin B-Cdk1 phosphatase. The current view that MPF is a synonym for cyclin B-Cdk1 in donor cells is thus imprecise; instead, MPF is best regarded as the entire pathway involved in the autoregulatory activation of cyclin B-Cdk1, with specifics depending on the experimental system.

Phosphorylation of ARPP19 by protein kinase A prevents meiosis resumption in Xenopus oocytes

During oogenesis, oocytes are arrested in prophase and resume meiosis by activating the kinase Cdk1 upon hormonal stimulation. In all vertebrates, release from prophase arrest relies on protein kinase A (PKA) downregulation and on the dephosphorylation of a long-sought but still unidentified substrate. Here we show that ARPP19 is the PKA substrate whose phosphorylation at serine 109 is necessary and sufficient for maintaining Xenopus oocytes arrested in prophase. By downregulating PKA, progesterone, the meiotic inducer in Xenopus, promotes partial dephosphorylation of ARPP19 that is required for the formation of a threshold level of active Cdk1. Active Cdk1 then initiates MPF autoamplification loop that occurs independently of both PKA and ARPP19 phosphorylation at serine 109 but requires the Greatwall-dependent phosphorylation of ARPP19 at serine 67. Therefore, ARPP19 stands at a crossroads in the meiotic M-phase control network by integrating differential effects of PKA and Greatwall, two essential kinases for meiosis resumption.

Fine-tuning of Cdc6 accumulation by Cdk1 and MAP kinase is essential for completion of oocyte meiotic divisions

Vertebrate oocytes proceed through the 1st and the 2nd meiotic division without intervening S-phase to become haploid. Although DNA replication does not take place, unfertilized oocytes acquire the competence to replicate DNA one hour after the 1st meiotic division, by accumulating an essential factor of the replicative machinery, Cdc6. Here, we discovered that the turnover of Cdc6 is precisely regulated in oocytes to avoid inhibition of Cdk1. At meiosis resumption, Cdc6 starts to be expressed but cannot accumulate due to a degradation mechanism activated through Cdk1. During transition from 1st to 2nd meiotic division, Cdc6 is under antagonistic regulation of Cyclin B, whose interaction with Cdc6 stabilizes the protein, and Mos/MAPK that negatively controls its accumulation. Since overexpressing Cdc6 inhibits Cdk1 reactivation and drives oocytes into a replicative interphasic state, the fine-tuning of Cdc6 accumulation is essential to ensure two meiotic waves of Cdk1 activation and to avoid unscheduled DNA replication during meiotic maturation.

Translational control of the oogenic program by components of OMA ribonucleoprotein particles in Caenorhabditis elegans

The oocytes of most sexually reproducing animals arrest in meiotic prophase I. Oocyte growth, which occurs during this period of arrest, enables oocytes to acquire the cytoplasmic components needed to produce healthy progeny and to gain competence to complete meiosis. In the nematode Caenorhabditis elegans, the major sperm protein hormone promotes meiotic resumption (also called meiotic maturation) and the cytoplasmic flows that drive oocyte growth. Prior work established that two related TIS11 zinc-finger RNA-binding proteins, OMA-1 and OMA-2, are redundantly required for normal oocyte growth and meiotic maturation. We affinity purified OMA-1 and identified associated mRNAs and proteins using genome-wide expression data and mass spectrometry, respectively. As a class, mRNAs enriched in OMA-1 ribonucleoprotein particles (OMA RNPs) have reproductive functions. Several of these mRNAs were tested and found to be targets of OMA-1/2-mediated translational repression, dependent on sequences in their 3'-untranslated regions (3'-UTRs). Consistent with a major role for OMA-1 and OMA-2 in regulating translation, OMA-1-associated proteins include translational repressors and activators, and some of these proteins bind directly to OMA-1 in yeast two-hybrid assays, including OMA-2. We show that the highly conserved TRIM-NHL protein LIN-41 is an OMA-1-associated protein, which also represses the translation of several OMA-1/2 target mRNAs. In the accompanying article in this issue, we show that LIN-41 prevents meiotic maturation and promotes oocyte growth in opposition to OMA-1/2. Taken together, these data support a model in which the conserved regulators of mRNA translation LIN-41 and OMA-1/2 coordinately control oocyte growth and the proper spatial and temporal execution of the meiotic maturation decision.

The phosphorylation of ARPP19 by Greatwall renders the auto-amplification of MPF independently of PKA in Xenopus oocytes

Entry into mitosis or meiosis relies on the coordinated action of kinases and phosphatases that ultimately leads to the activation of Cyclin-B-Cdk1, also known as MPF for M-phase promoting factor. Vertebrate oocytes are blocked in prophase of the first meiotic division, an arrest that is tightly controlled by high PKA activity. Re-entry into meiosis depends on activation of Cdk1, which obeys a two-step mechanism: a catalytic amount of Cdk1 is generated in a PKA and protein-synthesis-dependent manner; then a regulatory network known as the MPF auto-amplification loop is initiated. This second step is independent of PKA and protein synthesis. However, none of the molecular components of the auto-amplification loop identified so far act independently of PKA. Therefore, the protein rendering this process independent of PKA in oocytes remains unknown. Using a physiologically intact cell system, the Xenopus oocyte, we show that the phosphorylation of ARPP19 at S67 by the Greatwall kinase promotes its binding to the PP2A-B55δ phosphatase, thus inhibiting its activity. This process is controlled by Cdk1 and has an essential role within the Cdk1 auto-amplification loop for entry into the first meiotic division. Moreover, once phosphorylated by Greatwall, ARPP19 escapes the negative regulation exerted by PKA. It also promotes activation of MPF independently of protein synthesis, provided that a small amount of Mos is present. Taken together, these findings reveal that PP2A-B55δ, Greatwall and ARPP19 are not only required for entry into meiotic divisions, but are also pivotal effectors within the Cdk1 auto-regulatory loop responsible for its independence with respect to the PKA-negative control.

Control of oocyte growth and meiotic maturation in Caenorhabditis elegans

In sexually reproducing animals, oocytes arrest at diplotene or diakinesis and resume meiosis (meiotic maturation) in response to hormones. Chromosome segregation errors in female meiosis I are the leading cause of human birth defects, and age-related changes in the hormonal environment of the ovary are a suggested cause. Caenorhabditis elegans is emerging as a genetic paradigm for studying hormonal control of meiotic maturation. The meiotic maturation processes in C. elegans and mammals share a number of biological and molecular similarities. Major sperm protein (MSP) and luteinizing hormone (LH), though unrelated in sequence, both trigger meiotic resumption using somatic Gα(s)-adenylate cyclase pathways and soma-germline gap-junctional communication. At a molecular level, the oocyte responses apparently involve the control of conserved protein kinase pathways and post-transcriptional gene regulation in the oocyte. At a cellular level, the responses include cortical cytoskeletal rearrangement, nuclear envelope breakdown, assembly of the acentriolar meiotic spindle, chromosome segregation, and likely changes important for fertilization and the oocyte-to-embryo transition. This chapter focuses on signaling mechanisms required for oocyte growth and meiotic maturation in C. elegans and discusses how these mechanisms coordinate the completion of meiosis and the oocyte-to-embryo transition.

A dynamical model of oocyte maturation unveils precisely orchestrated meiotic decisions

Maturation of vertebrate oocytes into haploid gametes relies on two consecutive meioses without intervening DNA replication. The temporal sequence of cellular transitions driving eggs from G2 arrest to meiosis I (MI) and then to meiosis II (MII) is controlled by the interplay between cyclin-dependent and mitogen-activated protein kinases. In this paper, we propose a dynamical model of the molecular network that orchestrates maturation of Xenopus laevis oocytes. Our model reproduces the core features of maturation progression, including the characteristic non-monotonous time course of cyclin-Cdks, and unveils the network design principles underlying a precise sequence of meiotic decisions, as captured by bifurcation and sensitivity analyses. Firstly, a coherent and sharp meiotic resumption is triggered by the concerted action of positive feedback loops post-translationally activating cyclin-Cdks. Secondly, meiotic transition is driven by the dynamic antagonism between positive and negative feedback loops controlling cyclin turnover. Our findings reveal a highly modular network in which the coordination of distinct regulatory schemes ensures both reliable and flexible cell-cycle decisions.

In the life cycle of sexual organisms, a specialized cell division -meiosis- reduces the number of chromosomes in gametes or spores while fertilization or mating restores the original number. The essential feature that distinguishes meiosis from mitosis (the usual division) is the succession of two rounds of division following a single DNA replication, as well as the arrest at the second division in the case of oocyte maturation. The fact that meiosis and mitosis are similar but different raises several interesting questions: What is the meiosis-specific dynamics of cell-cycle regulators? Are there mechanisms which guarantee the occurence of two and only two rounds of division despite the presence of intrinsic and extrinsic noises ? The study of a model of the molecular network that underlies the meiotic maturation process in Xenopus oocytes provides unexpected answers to these questions. On the one hand, the modular organization of this network ensures separate controls of the first and second divisions. On the other hand, regulatory synergies ensure that these two stages are precisely and reliably sequenced during meiosis. We conclude that cells have evolved a sophisticated regulatory network to achieve a robust, albeit flexible, meiotic dynamics.

A critical balance between Cyclin B synthesis and Myt1 activity controls meiosis entry in Xenopus oocytes

In fully grown oocytes, meiosis is arrested at first prophase until species-specific initiation signals trigger maturation. Meiotic resumption universally involves early activation of M phase-promoting factor (Cdc2 kinase-Cyclin B complex, MPF) by dephosphorylation of the inhibitory Thr14/Tyr15 sites of Cdc2. However, underlying mechanisms vary. In Xenopus oocytes, deciphering the intervening chain of events has been hampered by a sensitive amplification loop involving Cdc2-Cyclin B, the inhibitory kinase Myt1 and the activating phosphatase Cdc25. In this study we provide evidence that the critical event in meiotic resumption is a change in the balance between inhibitory Myt1 activity and Cyclin B neosynthesis. First, we show that in fully grown oocytes Myt1 is essential for maintaining prophase I arrest. Second, we demonstrate that, upon upregulation of Cyclin B synthesis in response to progesterone, rapid inactivating phosphorylation of Myt1 occurs, mediated by Cdc2 and without any significant contribution of Mos/MAPK or Plx1. We propose a model in which the appearance of active MPF complexes following increased Cyclin B synthesis causes Myt1 inhibition, upstream of the MPF/Cdc25 amplification loop.

Nongenomic steroid- and ceramide-induced maturation in amphibian oocytes involves functional caveolae-like microdomains associated with a cytoskeletal environment

Stimulation of full-grown amphibian oocytes with progesterone initiates a nontranscriptional signaling pathway that converges in the activation of Cdc2/cyclin B and reentry into meiosis. We observed that cholesterol depletion mediated by methyl-beta-cyclodextrin (MbetaCD) inhibited meiotic maturation, suggesting involvement of membrane rafts. In the present study, we further characterized caveolae-like membranes from Rhinella arenarum oocytes biochemically and functionally. The identification by mass spectrometry of a nonmuscle myosin heavy-chain associated with caveolar membranes showed evidence of direct involvement of the underlying cytoskeletal environment in the structure of oocyte rafts. Biophysical analysis using the fluorescent probe Laurdan revealed that MbetaCD-mediated cholesterol depletion affected membrane lipid order. In line with this finding, cholesterol removal also affected the localization of the raft marker lipid GM1. Results demonstrated that ceramide is an effective inducer of maturation that alters the distribution of the raft markers caveolin-1, SRC, and GM1, while progesterone seems not to affect membrane microdomain integrity. Cholesterol depletion had a greater effect on ceramide-induced maturation, thus suggesting that ceramide is an inducer more vulnerable to changes in the plasma membrane. MbetaCD treatment delayed tyrosine phosphorylation and MAPK activation in progesterone-induced maturation. Functional studies regarding tyrosine phosphorylation raise the possibility that the hormone receptor is located in the nonraft membrane in the absence of ligand and that it translocates to the caveola when it binds to progesterone. The presence of raft markers and the finding of signaling molecules from MAPK cascade functionally associated to oocyte light membranes suggest that this caveolae-rich fraction efficiently recreates, in part, maturation signaling.

Direct roles of the signaling kinase RSK2 in Cdc25C activation during Xenopus oocyte maturation

The induction of M phase in eukaryotic cell cycles requires robust activation of Cdc2/cyclin B by Cdc25, which itself is robustly activated by serine/threonine phosphorylations. Although multiple protein kinases that directly activate Cdc25C have been identified, whether the combination of different primary phosphorylations of Cdc25C is sufficient to fully activate Cdc25C has not been determined. By analyzing the GST-Cdc25C phosphorylating activity in Xenopus egg extracts, we previously defined roles of MAPK and Cdc2/cyclin B in partially activating Cdc25C and predicted the presence of another major Cdc25C-activating kinase. In this study, we demonstrate that this missing kinase is RSK2, which phosphorylates three sites in Cdc25C and also partially activates Cdc25C. However, the phosphorylations catalyzed by MAPK, Cdc2, and RSK2 fail to fully activate Cdc25C, suggesting that additional biochemical events are required to fully activate this key cell cycle regulator.

Role of oocyte quality in meiotic maturation and embryonic development

The quality of oocytes plays a key role in a proper embryo development. In humans, oocytes of poor quality may be the cause of women infertility and an important obstacle in successful in vitro fertilization (IVF). The competence of oocytes depends on numerous processes taking place during the whole oogenesis, but its final steps such as oocyte maturation, seem to be of key importance. In this paper, we overview factors involved in the development of a fully functional female gamete with Xenopus laevis as a major experimental model. Modern approaches, e.g. proteomic analysis, enable the identification of novel proteins involved in oocyte development. EP45, called also Seryp or pNiXa, which belongs to the serpin (serine protease inhibitors) super-family is one of such recently analyzed proteins. This protein seems to be involved in the stimulation of meiotic maturation and embryo development. EP45 is potentially a key factor in correct oocyte development and determining the quality of oocytes.

Kicked by Mos and tuned by MPF-the initiation of the MAPK cascade in Xenopus oocytes

The mitogen-activated protein kinase (MAPK) cascade is a paradigmatic signaling cascade, which plays a crucial role in many aspects of cellular events. The main initiator of the cascade in Xenopus oocytes is the oncoprotein Mos. After activation of the cascade, Mos activity is stabilized by MAPK via a feedback loop. Mos concentration levels are, however, not controlled by MAPK alone. In this paper we show, by imposing either a sustained or a peaked activity of M-phase promoting factor (MPF) (Cdc2-cyclin B), how the latter regulates the dynamics of Mos. Our experiments are supported by a detailed kinetic model for the Mos-MPF-MAPK network, which takes into account the three different phosphorylation states of Mos and, as a consequence, allows us to determine the time evolution of Mos under control of MPF. Our work opens a path toward a more complete and biologically realistic quantitative understanding of the dynamic interdependence of Mos and MPF in Xenopus oocytes.

Role of Bcl2-like 10 (Bcl2l10) in Regulating Mouse Oocyte Maturation

Previously, we have shown that Bcl2l10 is highly expressed in metaphase II (MII)-stage oocytes. The objective of this study was to characterize Bcl2l10 expression in ovaries and to examine the function of Bcl2l10 in oocyte maturation using RNA interference. Bcl2l10 transcript expression was ovary and oocyte specific. Bcl2l10 was highly expressed in oocytes and pronuclear-stage embryos; however, its expression decreased at the two-cell stage and dramatically disappeared thereafter. Microinjection of Bcl2l10 double-stranded RNA into the cytoplasm of germinal vesicle oocytes resulted in a marked decrease in Bcl2l10 mRNA and protein and metaphase I (MI) arrest (78.9%). Most MI-arrested oocytes exhibited abnormalities in their spindles and chromosome configurations. Bcl2l10 RNA interference had an obvious effect on the activity of maturation-promoting factor but not on that of mitogen-activated protein kinase. We concluded that the role of Bcl2l10 is strongly associated with oocyte maturation, especially at the MI-MII transition.

Luteinizing hormone causes MAP kinase-dependent phosphorylation and closure of connexin 43 gap junctions in mouse ovarian follicles: one of two paths to meiotic resumption

Luteinizing hormone (LH) acts on ovarian follicles to reinitiate meiosis in prophase-arrested mammalian oocytes, and this has been proposed to occur by interruption of a meioisis-inhibitory signal that is transmitted through gap junctions into the oocyte from the somatic cells that surround it. To investigate this idea, we microinjected fluorescent tracers into live antral follicle-enclosed mouse oocytes, and we demonstrate for the first time that LH causes a decrease in the gap junction permeability between the somatic cells, prior to nuclear envelope breakdown (NEBD). The decreased permeability results from the MAP kinase-dependent phosphorylation of connexin 43 on serines 255, 262 and 279/282. We then tested whether the inhibition of gap junction communication was sufficient and necessary for the reinitiation of meiosis. Inhibitors that reduced gap junction permeability caused NEBD, but an inhibitor of MAP kinase activation that blocked gap junction closure in response to LH did not prevent NEBD. Thus, both MAP kinase-dependent gap junction closure and another redundant pathway function in parallel to ensure that meiosis resumes in response to LH.

Glucocorticoid- and protein kinase A-dependent transcriptome regulation in airway smooth muscle

Glucocorticoids (GCs) and protein kinase A (PKA)-activating agents (beta-adrenergic receptor agonists) are mainstream asthma therapies based on their ability to prevent or reverse excessive airway smooth muscle (ASM) constriction. Their abilities to regulate another important feature of asthma--excessive ASM growth--are poorly understood. Recent studies have suggested that GCs render agents of inflammation such as IL-1 beta and TNF-alpha mitogenic to ASM, via suppression of (antimitogenic) induced cyclooxygenase-2-dependent PKA activity. To further explore the mechanistic basis of these observations, we assessed the effects of epidermal growth factor and IL-1 beta stimulation, and the modulatory effects of GC treatment and PKA inhibition, on the ASM transcriptome by microarray analysis. Results demonstrate that ASM stimulated with IL-1 beta, in a manner that is often cooperative with stimulation with epidermal growth factor, exhibit a profound capacity to function as immunomodulatory cells. Moreover, results implicate an important role for induced autocrine/paracrine factors (many whose regulation was minimally affected by GCs or PKA inhibition) as regulators of both airway inflammation and ASM growth. Induction of numerous chemokines, in conjunction with regulation of proteases and agents of extracellular matrix remodeling, is suggested as an important mechanism promoting upregulated G protein-coupled receptor signaling capable of stimulating ASM growth. Additional functional assays suggest that intracellular PKA plays a critical role in suppressing the promitogenic effects of induced autocrine factors in ASM. Finally, identification and comparison of GC- and PKA-sensitive genes in ASM provide insight into the complementary effects of beta-agonist/GC combination therapies, and suggest specific genes as important targets for guiding the development of new generations of GCs and adjunct asthma therapies.

Oocyte selection is concurrent with meiosis resumption in the coenocystic oogenesis of Oikopleura

Oogenesis in the tunicate, Oikopleura, is unusual for a chordate, in that the thousands of nuclei comprising the entire germline are contained in a unique giant cell, the coenocyst. We examined progression through meiotic prophase I in concert with cellular mechanisms implicated in selection, growth and maturation of oocytes in this shared cytoplasm. Unlike sister vertebrates, no germinal vesicle was formed and maternal transcripts were instead synthesized by polyploid nurse nuclei present in equal numbers to transcriptionally quiescent meiotic nuclei. Meiosis resumption was concomitant with MAPK cascade activation during which pERK translocated to nurse nuclei. Simultaneously, the coenocyst partitioned into hundreds of synchronously growing oocytes. Significantly, only the subset of meiotic nuclei selected to populate maturing oocytes displayed histone H3 serine 28 phosphorylation. Disruption of the MAPK cascade, or microtubule dynamics, did not inhibit meiotic resumption but generated oocytes with multiple nurse and meiotic nuclei. As these supernumerary nuclei also became H3S28P enriched, growing oocytes defined a selective kinase environment in the common coenocyst cytoplasm. Vitellogenesis preceded the timing of oocyte selection among excess germ line nuclei in contrast to Drosophila and vertebrates. This unique feature enables late adjustment of oocyte number in accordance with the cytoplasmic volume of the germline cyst accumulated during vitellogenesis.

Roles of Greatwall kinase in the regulation of cdc25 phosphatase

We previously reported that immunodepletion of Greatwall kinase prevents Xenopus egg extracts from entering or maintaining M phase due to the accumulation of inhibitory phosphorylations on Thr14 and Tyr15 of Cdc2. M phase-promoting factor (MPF) in turn activates Greatwall, implying that Greatwall participates in an MPF autoregulatory loop. We show here that activated Greatwall both accelerates the mitotic G2/M transition in cycling egg extracts and induces meiotic maturation in G2-arrested Xenopus oocytes in the absence of progesterone. Activated Greatwall can induce phosphorylations of Cdc25 in the absence of the activity of Cdc2, Plx1 (Xenopus Polo-like kinase) or mitogen-activated protein kinase, or in the presence of an activator of protein kinase A that normally blocks mitotic entry. The effects of active Greatwall mimic in many respects those associated with addition of the phosphatase inhibitor okadaic acid (OA); moreover, OA allows cycling extracts to enter M phase in the absence of Greatwall. Taken together, these findings support a model in which Greatwall negatively regulates a crucial phosphatase that inhibits Cdc25 activation and M phase induction.

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.

Deciphering the H-Ras pathway in Xenopus oocyte

Xenopus oocytes are arrested in prophase of the first meiotic division. In response to progesterone, they re-enter meiosis and arrest again in metaphase of the second meiotic division. This process, called meiotic maturation, is under the control of the Cyclin B-Cdc2 complex, M phase promoting factor (MPF). Injection of a constitutively active Xenopus H-Ras protein activates MPF, suggesting that Ras proteins could be implicated in the progesterone transduction pathway. The aim of this study was (1) to elucidate the pathway triggered by H-Ras leading to MPF activation in Xenopus oocytes and (2) to investigate whether endogenous H-Ras is involved in the physiological process of meiotic maturation. We generated three constitutively active double mutants, each of them recruiting a single effector in mammalian cells, mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K) or RalGDS. Our results show that the activation of a PI3K-related enzyme is crucial for H-Ras-induced MPF activation, whereas the recruitment of either MAPK or RalGDS is not. However, although the H-Ras/PI3K pathway is functional in Xenopus oocytes, it is not the physiological transducer of progesterone responsible for meiotic resumption.