CDC2A (CDK1)-mediated phosphorylation of MSY2 triggers maternal mRNA degradation during mouse oocyte maturation

Multi-omics revealed that DCP1A and SPDL1 determine embryogenesis defects in postovulatory ageing oocytes

Growing evidence indicates that the deterioration of egg quality caused by postovulatory ageing significantly hampers embryonic development. However, the molecular mechanisms by which postovulatory ageing leads to a decline in oocyte quality have not been fully characterized. In this study, we observed an accelerated decay of maternal mRNAs through RNA-seq analyses in postovulatory-aged (PostOA) oocytes. We noted that these downregulated mRNAs should be degraded during the 2-cell stage. Proteomic analyses revealed that the degradation of maternal mRNAs is associated with the accumulation of DCP1A. The injection of exogenous Dcp1a mRNA or siRNA into MII stage oocytes proved that DCP1A could accelerate the degradation of maternal mRNAs. Additionally, we also found that SPDL1 is crucial for maintaining spindle/chromosome structure and chromosome euploidy in PostOA oocytes. Spdl1-mRNA injection remarkably recovered the meiotic defects in PostOA oocytes. Collectively, our findings provide valuable insights into the molecular mechanisms underlying postovulatory ageing.

Identification of YBX2 and TSKS As STK33 Interacting Proteins in Testicular Germ Cells

Spermiogenesis is a unique process, in which round spermatids undergo morphological changes to form spermatozoa. Serine/Threonine Kinase 33 (STK33), a member of the serine/threonine protein kinase family, plays a pivotal role in spermiogenesis, manifested by the infertile phenotype of Stk33 knockout mice and patients carrying STK33 mutations. To date, the mechanism by which STK33 promotes spermiogenesis is not fully understood. Here we aimed to identify germ cell-specific proteins that interact with STK33. Using immunoprecipitation and mass spectrometry, 13 proteins were identified that potentially interact with STK33 in testicular germ cells. By comparing the expression patterns of the candidate genes in testicular germ cells, we selected Y-Box Binding Protein 2 (YBX2) and Testis Specific Serine Kinase Substrate (TSKS) for validation. When co-expressed in cultured cells, TSKS was immunoprecipitated by STK33, and vice versa. Furthermore, STK33 was recruited to the TSKS foci, likely through interaction with TSKS. Although proximity ligation assay demonstrated that STK33 and YBX2 form the complex in germ cells, their interaction was not recapitulated in cultured cells. Phosphorylation assays showed that STK33 was unable to phosphorylate both YBX2 and TSKS in vitro. Overall, these results suggest that STK33 regulates spermiogenesis through TSKS and YBX2, which warrants further investigation in vivo.

ZAR1/2-Regulated Epigenetic Modifications are Essential for Age-Associated Oocyte Quality Maintenance and Zygotic Activation

The developmental competence and epigenetic progression of oocytes gradually become dysregulated with increasing maternal age. However, the mechanisms underlying age-related epigenetic regulation in oocytes remain poorly understood. Zygote arrest proteins 1 and 2 (ZAR1/2) are two maternal factors with partially redundant roles in maintaining oocyte quality, mainly known by regulating mRNA stability. In addition to this known function, it is found that ZAR1/2 is required for oocyte epigenetic maturation and zygotic reprogramming. Zar1/2-deleted oocytes exhibited reduced levels of multiple histone modifications and of the expression of corresponding histone modifiers, along with over-condensed chromatin, leading to compromised minor zygotic genome activation and deficient embryo development following fertilization. Cytoplasmic ZAR1/2 participated in intranuclear epigenetic maturation by binding the transcripts encoding histone modifiers and regulating their stability and translational activity. Moreover, oocytes from aged mice exhibited similar histone-modification deficiencies as the Zar1/2-deleted oocytes. ZAR1/2 mRNA and protein levels are downregulated in oocytes from mice and women with advanced ages, suggesting ZAR1/2 as regulators of epigenetic changes with reproductive aging. This study presents a new nucleo-cytoplasmic interaction mechanism that is involved in preventing oocyte epigenetic aging. Further, ZAR1/2 represents potential gene targets for diagnosis and clinical interventions in age-associated deficiencies in oocyte and embryo development.

Y box-binding protein 1 regulates zebrafish folliculogenesis partly through p21-mediated control of follicle cell proliferation

Y box-binding protein 1 (Ybx1/ybx1) regulates gene expression through DNA/RNA binding. In zebrafish, Ybx1 is highly abundant in primary growth (PG) follicles in the ovary, but decreases precipitously as the follicles enter the secondary growth (SG). To understand Ybx1 function in folliculogenesis, we created a ybx1 mutant using TALEN and observed disrupted folliculogenesis during the previtellogenic (PV) to early vitellogenic (EV) transition of SG, resulting in underdeveloped ovaries and infertility. Expression and western blot analyses revealed differential gene expression between ybx1-/- and control ovaries, with significantly increased expression of cdkn1a (p21), a cell cycle inhibitor, in ybx1-/- follicles. While cdkn1a knockout via CRISPR/Cas9 was embryonically lethal, the heterozygote (cdkn1a+/-) displayed advanced follicle activation and maturation, contrasting with the ybx1-/- phenotype. Partial loss of p21 alleviated the ybx1-/- phenotype, restoring folliculogenesis with normal PG-PV and PV-EV transitions in ybx1-/-;cdkn1a+/- mutants. While ybx1-/- mutant follicle cells displayed poor proliferation in vivo and in vitro, the cells from the ybx1-/-;cdkn1a+/- follicles resumed normal proliferation. In conclusion, Ybx1 is crucial for early folliculogenesis in zebrafish, potentially by repressing cdkn1a expression, either directly or indirectly.

The global phosphorylation landscape of mouse oocytes during meiotic maturation

Phosphorylation is a key post-translational modification regulating protein function and biological outcomes. However, the phosphorylation dynamics orchestrating mammalian oocyte development remains poorly understood. In the present study, we apply high-resolution mass spectrometry-based phosphoproteomics to obtain the first global in vivo quantification of mouse oocyte phosphorylation. Of more than 8000 phosphosites, 75% significantly oscillate and 64% exhibit marked upregulation during meiotic maturation, indicative of the dominant regulatory role. Moreover, we identify numerous novel phosphosites on oocyte proteins and a few highly conserved phosphosites in oocytes from different species. Through functional perturbations, we demonstrate that phosphorylation status of specific sites participates in modulating critical events including metabolism, translation, and RNA processing during meiosis. Finally, we combine inhibitor screening and enzyme-substrate network prediction to discover previously unexplored kinases and phosphatases that are essential for oocyte maturation. In sum, our data define landscape of the oocyte phosphoproteome, enabling in-depth mechanistic insights into developmental control of germ cells.

The molecular mechanisms underpinning maternal mRNA dormancy

A large number of mRNAs of maternal origin are produced during oogenesis and deposited in the oocyte. Since transcription stops at the onset of meiosis during oogenesis and does not resume until later in embryogenesis, maternal mRNAs are the only templates for protein synthesis during this period. To ensure that a protein is made in the right place at the right time, the translation of maternal mRNAs must be activated at a specific stage of development. Here we summarize our current understanding of the sophisticated mechanisms that contribute to the temporal repression of maternal mRNAs, termed maternal mRNA dormancy. We discuss mechanisms at the level of the RNA itself, such as the regulation of polyadenine tail length and RNA modifications, as well as at the level of RNA-binding proteins, which often block the assembly of translation initiation complexes at the 5' end of an mRNA or recruit mRNAs to specific subcellular compartments. We also review microRNAs and other mechanisms that contribute to repressing translation, such as ribosome dormancy. Importantly, the mechanisms responsible for mRNA dormancy during the oocyte-to-embryo transition are also relevant to cellular quiescence in other biological contexts.

A genome-wide perspective of the maternal mRNA translation program during oocyte development

Transcriptional and post-transcriptional regulations control gene expression in most cells. However, critical transitions during the development of the female gamete relies exclusively on regulation of mRNA translation in the absence of de novo mRNA synthesis. Specific temporal patterns of maternal mRNA translation are essential for the oocyte progression through meiosis, for generation of a haploid gamete ready for fertilization and for embryo development. In this review, we will discuss how mRNAs are translated during oocyte growth and maturation using mostly a genome-wide perspective. This broad view on how translation is regulated reveals multiple divergent translational control mechanisms required to coordinate protein synthesis with progression through the meiotic cell cycle and with development of a totipotent zygote.

eIF4E1b is a non-canonical eIF4E protecting maternal dormant mRNAs

Maternal mRNAs are essential for protein synthesis during oogenesis and early embryogenesis. To adapt translation to specific needs during development, maternal mRNAs are translationally repressed by shortening the polyA tails. While mRNA deadenylation is associated with decapping and degradation in somatic cells, maternal mRNAs with short polyA tails are stable. Here we report that the germline-specific eIF4E paralog, eIF4E1b, is essential for zebrafish oogenesis. eIF4E1b localizes to P-bodies in zebrafish embryos and binds to mRNAs with reported short or no polyA tails, including histone mRNAs. Loss of eIF4E1b results in reduced histone mRNA levels in early gonads, consistent with a role in mRNA storage. Using mouse and human eIF4E1Bs (in vitro) and zebrafish eIF4E1b (in vivo), we show that unlike canonical eIF4Es, eIF4E1b does not interact with eIF4G to initiate translation. Instead, eIF4E1b interacts with the translational repressor eIF4ENIF1, which is required for eIF4E1b localization to P-bodies. Our study is consistent with an important role of eIF4E1b in regulating mRNA dormancy and provides new insights into fundamental post-transcriptional regulatory principles governing early vertebrate development.

Regulation of Oocyte mRNA Metabolism: A Key Determinant of Oocyte Developmental Competence

The regulation of mRNA transcription and translation is uncoupled during oogenesis. The reason for this uncoupling is two-fold. Chromatin is only accessible to the transcriptional machinery during the growth phase as it condenses prior to resumption of meiosis to ensure faithful segregation of chromosomes during meiotic maturation. Thus, transcription rates are high during this time period in order to produce all of the transcripts needed for meiosis, fertilization, and embryo cleavage until the newly formed embryonic genome becomes transcriptionally active. To ensure appropriate timing of key developmental milestones including chromatin condensation, resumption of meiosis, segregation of chromosomes, and polar body extrusion, the translation of protein from transcripts synthesized during oocyte growth must be temporally regulated. This is achieved by the regulation of mRNA interaction with RNA binding proteins and shortening and lengthening of the poly(A) tail. This chapter details the essential factors that regulate the dynamic changes in mRNA synthesis, storage, translation, and degradation during oocyte growth and maturation.

Cordycepin delays postovulatory aging of oocytes through inhibition of maternal mRNAs degradation via DCP1A polyadenylation suppression

Postovulatory aging leads to the decline in oocyte quality and subsequent impairment of embryonic development, thereby reducing the success rate of assisted reproductive technology (ART). Potential preventative strategies preventing oocytes from aging and the associated underlying mechanisms warrant investigation. In this study, we identified that cordycepin, a natural nucleoside analogue, promoted the quality of oocytes aging in vitro, as indicated by reduced oocyte fragmentation, improved spindle/chromosomes morphology and mitochondrial function, as well as increased embryonic developmental competence. Proteomic and RNA sequencing analyses revealed that cordycepin inhibited the degradation of several crucial maternal proteins and mRNAs caused by aging. Strikingly, cordycepin was found to suppress the elevation of DCP1A protein by inhibiting polyadenylation during postovulatory aging, consequently impeding the decapping of maternal mRNAs. In humans, the increased degradation of DCP1A and total mRNA during postovulatory aging was also inhibited by cordycepin. Collectively, our findings demonstrate that cordycepin prevents postovulatory aging of mammalian oocytes by inhibition of maternal mRNAs degradation via suppressing polyadenylation of DCP1A mRNA, thereby promoting oocyte developmental competence.

Spatiotemporal regulation of maternal mRNAs during vertebrate oocyte meiotic maturation

Vertebrate oocytes face a particular challenge concerning the regulation of gene expression during meiotic maturation. Global transcription becomes quiescent in fully grown oocytes, remains halted throughout maturation and fertilization, and only resumes upon embryonic genome activation. Hence, the oocyte meiotic maturation process is largely regulated by protein synthesis from pre-existing maternal messenger RNAs (mRNAs) that are transcribed and stored during oocyte growth. Rapidly developing genome-wide techniques have greatly expanded our insights into the global translation changes and possible regulatory mechanisms during oocyte maturation. The storage, translation, and processing of maternal mRNAs are thought to be regulated by factors interacting with elements in the mRNA molecules. Additionally, posttranscriptional modifications of mRNAs, such as methylation and uridylation, have recently been demonstrated to play crucial roles in maternal mRNA destabilization. However, a comprehensive understanding of the machineries that regulate maternal mRNA fate during oocyte maturation is still lacking. In particular, how the transcripts of important cell cycle components are stabilized, recruited at the appropriate time for translation, and eliminated to modulate oocyte meiotic progression remains unclear. A better understanding of these mechanisms will provide invaluable insights for the preconditions of developmental competence acquisition, with important implications for the treatment of infertility. This review discusses how the storage, localization, translation, and processing of oocyte mRNAs are regulated, and how these contribute to oocyte maturation progression.

The m6A reader PRRC2A is essential for meiosis I completion during spermatogenesis

N6-methyladenosine (m6A) and its reader proteins YTHDC1, YTHDC2, and YTHDF2 have been shown to exert essential functions during spermatogenesis. However, much remains unknown about m6A regulation mechanisms and the functions of specific readers during the meiotic cell cycle. Here, we show that the m6A reader Proline rich coiled-coil 2A (PRRC2A) is essential for male fertility. Germ cell-specific knockout of Prrc2a causes XY asynapsis and impaired meiotic sex chromosome inactivation in late-prophase spermatocytes. Moreover, PRRC2A-null spermatocytes exhibit delayed metaphase entry, chromosome misalignment, and spindle disorganization at metaphase I and are finally arrested at this stage. Sequencing data reveal that PRRC2A decreases the RNA abundance or improves the translation efficiency of targeting transcripts. Specifically, PRRC2A recognizes spermatogonia-specific transcripts and downregulates their RNA abundance to maintain the spermatocyte expression pattern during the meiosis prophase. For genes involved in meiotic cell division, PRRC2A improves the translation efficiency of their transcripts. Further, co-immunoprecipitation data show that PRRC2A interacts with several proteins regulating mRNA metabolism or translation (YBX1, YBX2, PABPC1, FXR1, and EIF4G3). Our study reveals post-transcriptional functions of PRRC2A and demonstrates its critical role in the completion of meiosis I in spermatogenesis.

Mammalian oocytes store mRNAs in a mitochondria-associated membraneless compartment

Full-grown oocytes are transcriptionally silent and must stably maintain the messenger RNAs (mRNAs) needed for oocyte meiotic maturation and early embryonic development. However, where and how mammalian oocytes store maternal mRNAs is unclear. Here, we report that mammalian oocytes accumulate mRNAs in a mitochondria-associated ribonucleoprotein domain (MARDO). MARDO assembly around mitochondria was promoted by the RNA-binding protein ZAR1 and directed by an increase in mitochondrial membrane potential during oocyte growth. MARDO foci coalesced into hydrogel-like matrices that clustered mitochondria. Maternal mRNAs stored in the MARDO were translationally repressed. Loss of ZAR1 disrupted the MARDO, dispersed mitochondria, and caused a premature loss of MARDO-localized mRNAs. Thus, a mitochondria-associated membraneless compartment controls mitochondrial distribution and regulates maternal mRNA storage, translation, and decay to ensure fertility in mammals.

Y-box Binding Protein 1: Looking Back to the Future

Y-box binding protein 1 is a member of the cold shock domain (CSD) protein family and one of the most studied proteins associated with a large number of human diseases. This review aims to critically reassess the growing number of pathological functions ascribed to YB-1 in the past decades. The focus is given on the important role of YB-1 and related CSD proteins in the physiology of normal cells. The functional significance of these proteins is highlighted by their high evolutionary conservation from bacteria to men, where they are ubiquitously expressed and involved in coordinating all steps of mRNA biogenesis, including transcription, translation, storage, and degradation. Their activities are especially important under conditions requiring rapid change in the gene expression programs, such as early embryonic development, differentiation, stress, and adaptation to new environments. Therefore, to define a precise role of YB-1 in tumorigenic transformation and in other pathological conditions, it is important to understand its basic properties and functions in normal cells, and how they are interrupted in complex diseases including cancer.

Role of Y-Box Binding Proteins in Ontogenesis

Y-box binding proteins (YB proteins) are multifunctional DNA/RNA-binding proteins capable of regulating gene expression at multiple levels. At present, the most studied function of these proteins is the regulation of protein synthesis. Special attention in this review has been paid to the role of YB proteins in the control of mRNA translation and stability at the earliest stages of organism formation, from fertilization to gastrulation. Furthermore, the functions of YB proteins in the formation of germ cells, in which they accumulate in large amounts, are summarized. The review then discusses the contribution of YB proteins to the regulation of gene expression during the differentiation of various types of somatic cells. Finally, future directions in the study of YB proteins and their role in ontogenesis are considered.

What defines the maternal transcriptome?

In somatic cells, RNA polymerase II (Pol II) transcription initiation starts by the binding of the general transcription factor TFIID, containing the TATA-binding protein (TBP) and 13 TBP-associated factors (TAFs), to core promoters. However, in growing oocytes active Pol II transcription is TFIID/TBP-independent, as during oocyte growth TBP is replaced by its vertebrate-specific paralog TBPL2. TBPL2 does not interact with TAFs, but stably associates with TFIIA. The maternal transcriptome is the population of mRNAs produced and stored in the cytoplasm of growing oocytes. After fertilization, maternal mRNAs are inherited by the zygote from the oocyte. As transcription becomes silent after oocyte growth, these mRNAs are the sole source for active protein translation. They will participate to complete the protein pool required for oocyte terminal differentiation, fertilization and initiation of early development, until reactivation of transcription in the embryo, called zygotic genome activation (ZGA). All these events are controlled by an important reshaping of the maternal transcriptome. This procedure combines cytoplasmic readenylation of stored transcripts, allowing their translation, and different waves of mRNA degradation by deadenylation coupled to decapping, to eliminate transcripts coding for proteins that are no longer required. The reshaping ends after ZGA with an almost total clearance of the maternal transcripts. In the past, the murine maternal transcriptome has received little attention but recent progresses have brought new insights into the regulation of maternal mRNA dynamics in the mouse. This review will address past and recent data on the mechanisms associated with maternal transcriptome dynamic in the mouse.

An Emerging Role for Post-translational Modifications in Regulating RNP Condensates in the Germ Line

RNA-binding proteins undergo regulated phase transitions in an array of cell types. The phase separation of RNA-binding proteins, and subsequent formation of RNP condensates or granules, occurs during physiological conditions and can also be induced by stress. Some RNP granules have roles in post-transcriptionally regulating mRNAs, and mutations that prevent the condensation of RNA-binding proteins can reduce an organism's fitness. The reversible and multivalent interactions among RNP granule components can result in RNP complexes that transition among diffuse and condensed states, the latter of which can be pathological; for example, in neurons solid RNP aggregates contribute to disease states such as amyotrophic lateral sclerosis (ALS), and the dysregulation of RNP granules in human germ cells may be involved in Fragile X-associated primary ovarian insufficiency. Thus, regulating the assembly of mRNAs and RNA-binding proteins into discrete granules appears to provide important functions at both cellular and physiological levels. Here we review our current understanding of the role of post-translational modifications (PTMs) in regulating the condensation of RNA-binding proteins in the germ line. We compare and contrast the in vitro evidence that methylation inhibits phase separation of RNA binding proteins, with the extent to which these results apply to the in vivo germ line environment of several model systems. We also focus on the role of phosphorylation in modulating the dynamics of RNP granules in the germ line. Finally, we consider the gaps that exist in our understanding of the role of PTMs in regulating germ line RNP granules.

YTHDF2 Regulates Maternal Transcriptome Degradation and Embryo Development in Goat

Maternal mRNA clearance is critical for the early embryo development, which is under the tight control of RNA N6-methyladenosine (m⁶A). However, little information is known regarding the maternal mRNA clearance and mechanisms behind it in farm animals. In the present study, 3362 differentially expressed genes (DEGs) were found during the maternal-to-zygotic transition (MZT) and determined as maternal mRNAs in goat. Of which, 1961 was decreased at the 4-cell stage embryos, while 1401 was trigged down-regulation at the 8-cell stage embryos, which were termed as maternally encoded mRNA decay genes and zygotic genome activation (ZGA)-dependent maternal mRNAs, respectively. The expression of m⁶A reader YTHDF2 was increased during goat ZGA, and knockdown of YTHDF2 resulted in decreased blastocyst rate. In the 8-cell stage YTHDF2 knockdown embryos, the M-decay and Z-decay maternal mRNA clearance were impaired. Specifically, the expression of deadenylase (CNOT1 and CNOT11) and decapping enzymes (DCP1A and DCP2) was decreased. In conclusion, we ascertained maternal mRNAs and inferred that maternal mRNA clearance is also ZGA-dependent in goat. We reported that YTHDF2 is vital for goat early embryogenesis as it advances maternal mRNA clearance, which might through the recruitment of deadenylases and mRNA decapping enzymes. This work will be of great value for understanding the stochastic reprogramming events during MZT and achieving better development of goat embryos in vitro.

Premature ovarian insufficiency in the XO female mouse on the C57BL/6J genetic background

In humans, all but 1% of monosomy 45.X embryos die in utero and those who reach term suffer from congenital abnormalities and infertility termed Turner's syndrome (TS). By contrast, XO female mice on various genetic backgrounds show much milder physical defects and normal fertility, diminishing their value as an animal model for studying the infertility of TS patients. In this article, we report that XO mice on the C57BL/6J (B6) genetic background showed early oocyte loss, infertility or subfertility and high embryonic lethality, suggesting that the effect of monosomy X in the female germline may be shared between mice and humans. First, we generated XO mice on either a mixed N2(C3H.B6) or B6 genetic background and compared the number of oocytes in neonatal ovaries; N2.XO females retained 45% of the number of oocytes in N2.XX females, whereas B6.XO females retained only 15% of that in B6.XX females. Second, while N2.XO females were as fertile as N2.XX females, both the frequency of delivery and the total number of pups delivered by B6.XO females were significantly lower than those by B6.XX females. Third, after mating with B6 males, both N2.XO and B6.XO females rarely produced XO pups carrying paternal X chromosomes, although a larger percentage of embryos was found to be XO before implantation. Furthermore, B6.XO females delivered 20% XO pups among female progeny after mating with C3H males. We conclude that the impact of monosomy X on female mouse fertility depends on the genetic background.

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.

Y-Box Binding Proteins in mRNP Assembly, Translation, and Stability Control

Y-box binding proteins (YB proteins) are DNA/RNA-binding proteins belonging to a large family of proteins with the cold shock domain. Functionally, these proteins are known to be the most diverse, although the literature hardly offers any molecular mechanisms governing their activities in the cell, tissue, or the whole organism. This review describes the involvement of YB proteins in RNA-dependent processes, such as mRNA packaging into mRNPs, mRNA translation, and mRNA stabilization. In addition, recent data on the structural peculiarities of YB proteins underlying their interactions with nucleic acids are discussed.

Diversity of RNA-Binding Proteins Modulating Post-Transcriptional Regulation of Protein Expression in the Maturing Mammalian Oocyte

The oocyte faces a particular challenge in terms of gene regulation. When oocytes resume meiosis at the end of the growth phase and prior to ovulation, the condensed chromatin state prevents the transcription of genes as they are required. Transcription is effectively silenced from the late germinal vesicle (GV) stage until embryonic genome activation (EGA) following fertilisation. Therefore, during its growth, the oocyte must produce the mRNA transcripts needed to fulfil its protein requirements during the active period of meiotic completion, fertilisation, and the maternal-to zygote-transition (MZT). After meiotic resumption, gene expression control can be said to be transferred from the nucleus to the cytoplasm, from transcriptional regulation to translational regulation. Maternal RNA-binding proteins (RBPs) are the mediators of translational regulation and their role in oocyte maturation and early embryo development is vital. Understanding these mechanisms will provide invaluable insight into the oocyte's requirements for developmental competence, with important implications for the diagnosis and treatment of certain types of infertility. Here, we give an overview of post-transcriptional regulation in the oocyte, emphasising the current knowledge of mammalian RBP mechanisms, and develop the roles of these mechanisms in the timely activation and elimination of maternal transcripts.

The oocyte-to-embryo transition in mouse: past, present, and future

The oocyte-to-embryo transition (OET) arguably initiates with formation of a primordial follicle and culminates with reprogramming of gene expression during the course of zygotic genome activation. This transition results in converting a highly differentiated cell, i.e. oocyte, to undifferentiated cells, i.e. initial blastomeres of a preimplantation embryo. A plethora of changes occur during the OET and include, but are not limited to, changes in transcription, chromatin structure, and protein synthesis; accumulation of macromolecules and organelles that will comprise the oocyte's maternal contribution to the early embryo; sequential acquisition of meiotic and developmental competence to name but a few. This review will focus on transcriptional and post-transcriptional changes that occur during OET in mouse because such changes are likely the major driving force for OET. We often take a historical and personal perspective, and highlight how advances in experimental methods often catalyzed conceptual advances in understanding the molecular bases for OET. We also point out questions that remain open and therefore represent topics of interest for future investigation.

The maternal-to-zygotic transition revisited

The development of animal embryos is initially directed by maternal gene products. Then, during the maternal-to-zygotic transition (MZT), developmental control is handed to the zygotic genome. Extensive research in both vertebrate and invertebrate model organisms has revealed that the MZT can be subdivided into two phases, during which very different modes of gene regulation are implemented: initially, regulation is exclusively post-transcriptional and post-translational, following which gradual activation of the zygotic genome leads to predominance of transcriptional regulation. These changes in the gene expression program of embryos are precisely controlled and highly interconnected. Here, we review current understanding of the mechanisms that underlie handover of developmental control during the MZT.

Post-translational regulation of the maternal-to-zygotic transition

The maternal-to-zygotic transition (MZT) is essential for the developmental control handed from maternal products to newly synthesized zygotic genome in the earliest stages of embryogenesis, including maternal component (mRNAs and proteins) degradation and zygotic genome activation (ZGA). Various protein post-translational modifications have been identified during the MZT, such as phosphorylation, methylation and ubiquitination. Precise post-translational regulation mechanisms are essential for the timely transition of early embryonic development. In this review, we summarize recent progress regarding the molecular mechanisms underlying post-translational regulation of maternal component degradation and ZGA during the MZT and discuss some important issues in the field.

Molecular Mechanisms of Prophase I Meiotic Arrest Maintenance and Meiotic Resumption in Mammalian Oocytes

Mechanisms of meiotic prophase I arrest maintenance (germinal vesicle [GV] stage) and meiotic resumption (germinal vesicle breakdown [GVBD] stage) in mammalian oocytes seem to be very complicated. These processes are regulated via multiple molecular cascades at transcriptional, translational, and posttranslational levels, and many of them are interrelated. There are many molecular cascades of meiosis maintaining and meiotic resumption in oocyte which are orchestrated by multiple molecules produced by pituitary gland and follicular cells. Furthermore, many of these molecular cascades are duplicated, thus ensuring the stability of the entire system. Understanding mechanisms of oocyte maturation is essential to assess the oocyte status, develop effective protocols of oocyte in vitro maturation, and design novel contraceptive drugs. Mechanisms of meiotic arrest maintenance at prophase I and meiotic resumption in mammalian oocytes are covered in the present article.

mRNA 3' uridylation and poly(A) tail length sculpt the mammalian maternal transcriptome

A fundamental principle in biology is that the program for early development is established during oogenesis in the form of the maternal transcriptome. How the maternal transcriptome acquires the appropriate content and dosage of transcripts is not fully understood. Here we show that 3' terminal uridylation of mRNA mediated by TUT4 and TUT7 sculpts the mouse maternal transcriptome by eliminating transcripts during oocyte growth. Uridylation mediated by TUT4 and TUT7 is essential for both oocyte maturation and fertility. In comparison to somatic cells, the oocyte transcriptome has a shorter poly(A) tail and a higher relative proportion of terminal oligo-uridylation. Deletion of TUT4 and TUT7 leads to the accumulation of a cohort of transcripts with a high frequency of very short poly(A) tails, and a loss of 3' oligo-uridylation. By contrast, deficiency of TUT4 and TUT7 does not alter gene expression in a variety of somatic cells. In summary, we show that poly(A) tail length and 3' terminal uridylation have essential and specific functions in shaping a functional maternal transcriptome.

Preovulatory Aging In Vivo and In Vitro Affects Maturation Rates, Abundance of Selected Proteins, Histone Methylation Pattern and Spindle Integrity in Murine Oocytes

Delayed ovulation and delayed fertilization can lead to reduced developmental competence of the oocyte. In contrast to the consequences of postovulatory aging of the oocyte, hardly anything is known about the molecular processes occurring during oocyte maturation if ovulation is delayed (preovulatory aging). We investigated several aspects of oocyte maturation in two models of preovulatory aging: an in vitro follicle culture and an in vivo mouse model in which ovulation was postponed using the GnRH antagonist cetrorelix. Both models showed significantly reduced oocyte maturation rates after aging. Furthermore, in vitro preovulatory aging deregulated the protein abundance of the maternal effect genes Smarca4 and Nlrp5, decreased the levels of histone H3K9 trimethylation and caused major deterioration of chromosome alignment and spindle conformation. Protein abundance of YBX2, an important regulator of mRNA stability, storage and recruitment in the oocyte, was not affected by in vitro aging. In contrast, in vivo preovulatory aging led to reduction in Ybx2 transcript and YBX2 protein abundance. Taken together, preovulatory aging seems to affect various processes in the oocyte, which could explain the low maturation rates and the previously described failures in fertilization and embryonic development.

Postovulatory aging affects dynamics of mRNA, expression and localization of maternal effect proteins, spindle integrity and pericentromeric proteins in mouse oocytes

STUDY QUESTION

Is the postovulatory aging-dependent differential decrease of mRNAs and polyadenylation of mRNAs coded by maternal effect genes associated with altered abundance and distribution of maternal effect and RNA-binding proteins (MSY2)?

SUMMARY ANSWER

Postovulatory aging results in differential reduction in abundance of maternal effect proteins, loss of RNA-binding proteins from specific cytoplasmic domains and critical alterations of pericentromeric proteins without globally affecting protein abundance.

WHAT IS KNOWN ALREADY

Oocyte postovulatory aging is associated with differential alteration in polyadenylation and reduction in abundance of mRNAs coded by selected maternal effect genes. RNA-binding and -processing proteins are involved in storage, polyadenylation and degradation of mRNAs thus regulating stage-specific recruitment of maternal mRNAs, while chromosomal proteins that are stage-specifically expressed at pericentromeres, contribute to control of chromosome segregation and regulation of gene expression in the zygote.

STUDY DESIGN, SIZE, DURATION

Germinal vesicle (GV) and metaphase II (MII) oocytes from sexually mature C57B1/6J female mice were investigated. Denuded in vivo or in vitro matured MII oocytes were postovulatory aged and analyzed by semiquantitative confocal microscopy for abundance and localization of polyadenylated RNAs, proteins of maternal effect genes (transcription activator BRG1 also known as ATP-dependent helicase SWI/SNF related, matrix associated, actin dependent regulator of chromatin, subfamily a, member 4 (SMARCA4) and NOD-like receptor family pyrin domain containing 5 (NLRP5) also known as MATER), RNA-binding proteins (MSY2 also known as germ cell-specific Y-box-binding protein, YBX2), and post-transcriptionally modified histones (trimethylated histone H3K9 and acetylated histone H4K12), as well as pericentromeric ATRX (alpha thalassemia/mental retardation syndrome X-linked, also termed ATP-dependent helicase ATRX or X-linked nuclear protein (XNP)). For proteome analysis five replicates of 30 mouse oocytes were analyzed by selected reaction monitoring (SRM).

MATERIAL AND METHODS

GV and MII oocytes were obtained from large antral follicles or ampullae of sexually mature mice, respectively. Denuded MII oocytes were aged for 24 h post ovulation. For analysis of distribution and abundance of polyadenylated RNAs fixed oocytes were in situ hybridized to Cy5 labeled oligo(dT)20 nucleotides. Absolute quantification of protein concentration per oocyte of selected proteins was done by SRM proteome analysis. Relative abundance of ATRX was assessed by confocal laser scanning microscopy (CLSM) of whole mount formaldehyde fixed oocytes or after removal of zona and spreading. MSY2 protein distribution and abundance was studied in MII oocytes prior to, during and after exposure to nocodazole, or after aging for 2 h in presence of H2O2 or for 24 h in presence of a glutathione donor, glutathione ethylester (GEE).

MAIN RESULTS AND ROLE OF CHANCE

The significant reduction in abundance of proteins (P < 0.001) translated from maternal mRNAs was independent of polyadenylation status, while their protein localization was not significantly changed by aging. Most of other proteins quantified by SRM analysis did not significantly change in abundance upon aging except MSY2 and GTSF1. MSY2 was enriched in the subcortical RNP domain (SCRD) and in the spindle chromosome complex (SCC) in a distinct pattern, right and left to the chromosomes. There was a significant loss of MSY2 from the SCRD (P < 0.001) and the spindle after postovulatory aging. Microtubule de- and repolymerization caused reversible loss of MSY2 spindle-association whereas H2O2 stress did not significantly decrease MSY2 abundance. Aging in presence of GEE decreased significantly (P < 0.05) the aging-related overall and cytoplasmic loss of MSY2. Postovulatory aging increased significantly spindle abnormalities, unaligned chromosomes, and abundance of acetylated histone H4K12, and decreased pericentromeric trimethylated histone H3K9 (all P < 0.001). Spreading revealed a highly significant increase in pericentromeric ATRX (P < 0.001) upon ageing. Thus, the significantly reduced abundance of MSY2 protein, especially at the SCRD and the spindle may disturb the spatial control and timely recruitment, deadenylation and degradation of developmentally important RNAs. An autonomous program of degradation appears to exist which transiently and specifically induces the loss and displacement of transcripts and specific maternal proteins independent of fertilization in aging oocytes and thereby can critically affect chromosome segregation and gene expression in the embryo after fertilization.

LIMITATION, REASONS FOR CAUTION

We used the mouse oocyte to study processes associated with postovulatory aging, which may not entirely reflect processes in aging human oocytes. However, increases in spindle abnormalities, unaligned chromosomes and H4K12 acetylated histones, as well as in mRNA abundance and polyadenylation have been observed also in aged human oocytes suggesting conserved processes in aging.

WIDER IMPLICATIONS OF THE FINDINGS

Postovulatory aging precociously induces alterations in expression and epigenetic modifications of chromatin by ATRX and in histone pattern in MII oocytes that normally occur after fertilization, possibly contributing to disturbances in the oocyte-to-embryo transition (OET) and the zygotic gene activation (ZGA). These observations in mouse oocytes are also relevant to explain disturbances and reduced developmental potential of aged human oocytes and caution to prevent oocyte aging in vivo and in vitro.

STUDY FUNDING/COMPETING INTERESTS

The study has been supported by the German Research Foundation (DFG) (EI 199/7-1 | GR 1138/12-1 | HO 949/21-1 and FOR 1041). There is no competing interest.

Sculpting the Transcriptome During the Oocyte-to-Embryo Transition in Mouse

In mouse, the oocyte-to-embryo transition entails converting a highly differentiated oocyte to totipotent blastomeres. This transition is driven by degradation of maternal mRNAs, which results in loss of oocyte identity, and reprogramming of gene expression during the course of zygotic gene activation, which occurs primarily during the two-cell stage and confers blastomere totipotency. Full-grown oocytes are transcriptionally quiescent and mRNAs are remarkably stable in oocytes due to the RNA-binding protein MSY2, which stabilizes mRNAs, and low activity of the 5' and 3' RNA degradation machinery. Oocyte maturation initiates a transition from mRNA stability to instability due to phosphorylation of MSY2, which makes mRNAs more susceptible to the RNA degradation machinery, and recruitment of dormant maternal mRNAs that encode for critical components of the 5' and 3' RNA degradation machinery. Small RNAs (miRNA, siRNA, and piRNA) play little, if any, role in mRNA degradation that occurs during maturation. Many mRNAs are totally degraded but a substantial fraction is only partially degraded, their degradation completed by the end of the two-cell stage. Genome activation initiates during the one-cell stage, is promiscuous, low level, and genome wide (and includes both inter- and intragenic regions) and produces transcripts that are inefficiently spliced and polyadenylated. The major wave of genome activation in two-cell embryos involves expression of thousands of new genes. This unique pattern of gene expression is the product of maternal mRNAs recruited during maturation that encode for transcription factors and chromatin remodelers, as well as dramatic changes in chromatin structure due to incorporation of histone variants and modified histones.

Mobilization of Dormant Cnot7 mRNA Promotes Deadenylation of Maternal Transcripts During Mouse Oocyte Maturation

Maternal mRNAs in oocytes are remarkably stable. In mouse, oocyte maturation triggers a transition from mRNA stability to instability. This transition is a critical event in the oocyte-to-embryo transition in which a differentiated oocyte loses its identity as it is transformed into totipotent blastomeres. We previously demonstrated that phosphorylation of MSY2, an RNA-binding protein, and mobilization of mRNAs encoding the DCP1A-DCP2 decapping complex contribute to maternal mRNA destruction during meiotic maturation. We report here that Cnot7, Cnot6l, and Pan2, key components of deadenylation machinery, are also dormant maternal mRNAs that are recruited during oocyte maturation. Inhibiting the maturation-associated increase in CNOT7 (or CNOT6L) using a small interference RNA approach inhibits mRNA deadenylation, whereas inhibiting the increase in PAN2 has little effect. Reciprocally, expressing CNOT7 (or CNOT6L) in oocytes prevented from resuming meiosis initiates deadenylation of mRNAs. These effects on deadenylation are also observed when the total amount of poly (A) is quantified. Last, inhibiting the increase in CNOT7 protein results in an ~70% decrease in transcription in 2-cell embryos.

Embryonic Poly(A)-Binding Protein Is Required During Early Stages of Mouse Oocyte Development for Chromatin Organization, Transcriptional Silencing, and Meiotic Competence

During oocyte maturation, fertilization, and early embryo development until zygotic genome activation (ZGA), transcription is suppressed, and gene expression is dependent upon the timely activation of stored mRNAs. Embryonic poly(A)-binding protein (EPAB) is the predominant poly(A)-binding protein in Xenopus, mouse, and human oocytes and early embryos and is important for regulating translational activation of maternally stored mRNAs. EPAB is critical for early development because Epab(-/-) female mice do not produce mature eggs and are infertile. In this study, we further characterize morphological and molecular aspects of Epab(-/-) oocytes. We demonstrated that Epab(-/-) oocytes are smaller in size, contain peripheral germinal vesicles, and are loosely associated with cumulus cells. The chromatin reorganization of the surrounded nucleolus (SN) configuration and transcriptional silencing that normally occurs during oocyte growth does not occur in Epab(-/-) oocytes. Interestingly, microinjection of Epab mRNA into Epab(-/-) preantral follicle-enclosed oocytes rescues reorganization of chromatin and oocyte maturation to metaphase II. Overall, these results demonstrate an important role for EPAB during oocyte growth and the acquisition of meiotic competence.

Expression profiles of select genes in cumulus–oocyte complexes from young and aged mares

There is compelling evidence that oocytes from mares >18 years of age have a high incidence of inherent defects that result in early embryonic loss. In women, an age-related decrease in oocyte quality is associated with an increased incidence of aneuploidy and it has recently been determined that the gene expression profile of human oocytes is altered with advancing age. We hypothesised that similar age-related aberrations in gene expression occur in equine oocytes. Therefore, the aim of the present study was to compare gene expression profiles of individual oocytes and cumulus cells from young and aged mares, specifically evaluating genes that have been identified as being differentially expressed with advancing maternal age and/or aneuploidy in human oocytes. Expression of 48 genes was compared between 14 cumulus–oocyte complexes (COCs) from mares aged 3–12 years and 10 COCs from mares ≥18 years of age. Three genes (mitochondrial translational initiation factor 3 (IF3), heat shock transcription factor 5 (HSF5) and Y box binding protein 2 (YBX2)) were differentially expressed in oocytes, with all being more abundant in oocytes from young mares. Three genes (ADP-ribosylation factor-like 6 interacting protein 6 (ARL6IP6), BCL2-associated X protein (BAX) and hypoxia upregulated 1 (HYOU1)) were differentially expressed in cumulus cells, with all being more abundant in aged mares. The results of the present study confirm there are age-related differences in gene expression in equine COCs, which may be associated with the lower quality and decreased developmental competence of oocytes from aged mares.

Restarting life: fertilization and the transition from meiosis to mitosis

Fertilization triggers a complex cellular programme that transforms two highly specialized meiotic germ cells, the oocyte and the sperm, into a totipotent mitotic embryo. Linkages between sister chromatids are remodelled to support the switch from reductional meiotic to equational mitotic divisions; the centrosome, which is absent from the egg, is reintroduced; cell division shifts from being extremely asymmetric to symmetric; genomic imprinting is selectively erased and re-established; and protein expression shifts from translational control to transcriptional control. Recent work has started to reveal how this remarkable transition from meiosis to mitosis is achieved.

Maternally recruited DCP1A and DCP2 contribute to messenger RNA degradation during oocyte maturation and genome activation in mouse

The oocyte-to-zygote transition entails transforming a highly differentiated oocyte into totipotent blastomeres and represents one of the earliest obstacles that must be successfully hurdled for continued development. Degradation of maternal mRNAs, which likely lies at the heart of this transition, is characterized by a transition from mRNA stability to instability during oocyte maturation. Although phosphorylation of the oocyte-specific RNA-binding protein MSY2 during maturation is implicated in making maternal mRNAs more susceptible to degradation, mechanisms underlying mRNA degradation during oocyte maturation remain poorly understood. We report that DCP1A and DCP2, proteins responsible for decapping mRNA, are encoded by maternal mRNAs recruited for translation during maturation via cytoplasmic polyadenylation elements located in their 3' untranslated regions. Both DCP1A and DCP2 are phosphorylated during maturation, with CDC2A being the kinase likely responsible for both, although MAPK may be involved in DCP1A phosphorylation. Inhibiting accumulation of DCP1A and DCP2 by RNA interference or morpholinos decreases not only degradation of mRNAs during meiotic maturation but also transcription of the zygotic genome. The results indicate that maternally recruited DCP1A and DCP2 are critical players in the transition from mRNA stability to instability during meiotic maturation and that proper maternal mRNA degradation must be successful to execute the oocyte-to-zygote transition.

Association of the transcription profile of bovine oocytes and embryos with developmental potential

Although improvements in culture system have enhanced in vitro embryo production, success rates are still not adequate. The reasons for developmental arrest of a part of in vitro produced embryos are unknown, but are connected in part with low cytoplasmic competence of oocytes. The immaturity of cytoplasm can negatively influence fertilization efficiency and subsequent progression through embryonic genome activation (EGA), which are necessary steps in further pre-implantation development. A large number of studies have compared mRNA abundance among oocytes with different developmental competence with the aim to find markers of the normal embryo development. The amount of mitochondrial DNA (mtDNA) and mRNA for mitochondrial transcriptional factors directing oxidative phosphorylation belongs to such promising markers. Nevertheless, recently published studies revealed that the mammalian embryo is able to compensate for a reduced level of mtDNA in oocyte during subsequent pre-implantation development. The search for other molecular markers is in progress. Characterization of oocyte and embryonic mRNA expression patterns during the pre-implantation period, and their relationship to the successful in vitro and in vivo development will be essential for defining the optimized culture conditions or the nuclear transfer protocols. Microarrays technology enables us to reveal the differentially expressed genes during EGA, and to compare the expression profile of in vivo and in vitro produced embryos. Recent evidence indicates that the depletion of the pool of stored maternal mRNAs is critical for subsequent embryo development. All these experiments gradually offer a list of possible candidates for quality and developmental competence markers for mammalian oocytes and pre-implantation embryos.

Vitrification at the pre-antral stage transiently alters inner mitochondrial membrane potential but proteome of in vitro grown and matured mouse oocytes appears unaffected

BACKGROUND

Vitrification is a fast and effective method to cryopreserve ovarian tissue, but it might influence mitochondrial activity and affect gene expression to cause persistent alterations in the proteome of oocytes that grow and mature following cryopreservation.

METHODS

In part one of the study, the inner mitochondrial membrane potential (Ψ(mit)) of JC-1 stained oocytes from control and CryoTop vitrified pre-antral follicles was analyzed by confocal microscopy at Day 0, or after culture of follicles for 1 or 12 days. In part two, proteins of in vivo grown germinal vesicle (GV) oocytes were subjected to proteome analysis by SDS polyacrylamide gel electrophoresis, tryptic in-gel digestion of gel slices, and one-dimensional-nano-liquid chromatography of peptides on a multi-dimensional-nano-liquid chromatography system followed by mass spectrometry (LC-MS/MS) and Uniprot Gene Ontology (GO) analysis. In part three, samples containing the protein amount of 40 GV and metaphase II (MII) oocytes, respectively, from control and vitrified pre-antral follicles cultured for 12 or 13 days were subjected to 2D DIGE saturation labeling and separated by isoelectric focusing and SDS gel electrophoresis (2D DIGE), followed by DeCyder(Tm) analysis of spot patterns in three independent biological replicates. Statistical and hierarchical cluster analysis was employed to compare control and vitrified groups.

RESULTS

(i) Mitochondrial inner membrane potential differs significantly between control and vitrified GV oocytes at Day 0 and Day 1, but is similar at Day 12 of culture. (ii) LC-MS/MS analysis of SDS gel fractionated protein lysates of 988 mouse GV oocytes revealed identification of 1123 different proteins with a false discovery rate of <1%. GO analysis assigned 811 proteins to the 'biological process' subset. Thirty-five percent of the proteins corresponded to metabolic processes, about 15% to mitochondrion and transport, each, and close to 8% to oxidation-reduction processes. (iii) From the 2D-saturation DIGE analysis 1891 matched spots for GV-stage and 1718 for MII oocyte proteins were detected and the related protein abundances in vitrified and control oocytes were quantified. None of the spots was significantly altered in intensity, and hierarchical cluster analysis as well as histograms of p and q values suggest that vitrification at the pre-antral stage does not significantly alter the proteome of GV or MII oocytes compared with controls.

CONCLUSIONS

Vitrification appears to be associated with a significant transient increase in Ψ(mit) in oocyte mitochondria, which disappears when oocyte/cumulus cell apposition is restored upon development to the antral stage. The nano-LC-MS/MS analysis of low numbers of oocytes is useful to obtain information on relevant biological signaling pathways based on protein identifications. For quantitative comparisons, saturation 2D DIGE analysis is superior to LC-MS/MS due to its high sensitivity in cases where the biological material is very limited. Genetic background, age of the female, and/or stimulation protocol appear to influence the proteome pattern. However, the quantitative 2D DIGE approach provides evidence that vitrification does not affect the oocyte proteome after recovery from transient loss of cell-cell interactions, in vitro growth and in vitro maturation under tested conditions. Therefore, transient changes in mitochondrial activity by vitrification do not appear causal to persistent alterations in the mitochondrial or overall oocyte proteome.

Post-transcriptional control of gene expression during mouse oogenesis

Post-transcriptional mechanisms play a central role in regulating gene expression during oogenesis and early embryogenesis. Growing oocytes accumulate an enormous quantity of messenger RNAs (mRNAs), but transcription decreases dramatically near the end of growth and is undetectable during meiotic maturation. Following fertilization, the embryo is initially transcriptionally inactive and then becomes active at a species-specific stage of early cleavage. Meanwhile, beginning during maturation and continuing after fertilization, the oocyte mRNAs are eliminated, allowing the embryonic genome to assume control of development. How the mammalian oocyte manages the storage, translation, and degradation of the huge quantity and diversity of mRNAs that it harbours has been the focus of enormous research effort and is the subject of this review. We discuss the roles of sequences within the 3'-untranslated region of certain mRNAs and the proteins that bind to them, sequence-non-specific RNA-binding proteins, and recent studies implicating ribonucleoprotein processing (P-) bodies and cytoplasmic lattices. We also discuss mechanisms that may control the temporally regulated translational activation of different mRNAs during meiotic maturation, as well as the signals that trigger silencing and degradation of the oocyte mRNAs. We close by highlighting areas for future research including the potential key role of small RNAs in regulating gene expression in oocytes.

MG132 treatment during oocyte maturation improves embryonic development after somatic cell nuclear transfer and alters oocyte and embryo transcript abundance in pigs

The objective of this study was to examine the effect of treating pig oocytes during in vitro maturation (IVM) with a proteasome inhibitor, MG132, on oocyte maturation and embryonic development. In one series of experiments, oocytes from medium-sized follicles (3-8 mm in diameter) were untreated (MCO) or treated with MG132 during 0-22 hr (M0-22) or 30-42 hr (M30-42) of IVM. There was no significant effect of MG132 on nuclear maturation or cytoplasmic maturation (as assessed by intracellular amounts of glutathione and p34cdc2 kinase activity). Blastocyst formation after parthenogenetic activation (PA) and somatic cell nuclear transfer (SCNT), however, was increased for M30-42 (65.2% and 27.7% for PA and SCNT, respectively) compared to MCO (42.6% and 13.6%, respectively) and M0-22 (45.3% and 19.5%, respectively; P<0.05). Expression of PCNA and ERK2 was increased in M30-42 for IVM oocytes while transcript abundance for POUF51, DNMT1, FGFR2, and PCNA was increased in M30-42 for 4-cell SCNT embryos. When oocytes derived from small follicles (<3 mm in diameter) were untreated (SCO) or treated with MG132 during 0-22 hr (S0-22), 30-42 hr (S30-42) of IVM, or 0-22 and 30-42 hr of IVM (S0-22/30-42), expression of POU5F1, DNMT1, FGFR2, and PCNA and blastocyst formation were increased for SCNT embryos derived from S30 to 42 (16.5%) and S0-22/30-42 oocytes (20.8%) as compared to embryos from SCO (8.7%) or S0-22 oocytes (8.8%; P<0.05). Results demonstrate that treatment of oocytes with MG132 during the later stage of IVM improves embryonic development and alters gene expression in pigs.

Absence of MSY2 in mouse oocytes perturbs oocyte growth and maturation, RNA stability, and the transcriptome

Messenger RNA is remarkably stable during oocyte growth, thus enabling mRNAs to accumulate during the growth phase and thereby provide mRNAs that support early embryonic development. MSY2, a germ cell-specific RNA-binding protein, is implicated in regulating mRNA stability. MSY2 is essential for development because female Msy2(-/-) mice are infertile. We describe here the characterization of Msy2(-/-) oocytes. Mutant oocytes grow more slowly during the first wave of folliculogenesis, and maturation to and arrest at metaphase II is severely compromised because of aberrant spindle formation and chromosome congression. Consistent with MSY2 conferring mRNA stability is that the amount of poly(A)-containing RNA is reduced by ~25% in mutant oocytes. Stability of an exogenous mRNA injected into mutant oocytes is lower than when compared to their wild-type counterparts, and moreover, expression of wild-type MSY2 in mutant oocytes increases mRNA stability, whereas injection of a mutant form of MSY2 not capable of binding RNA does not. Transcription quiescence that normally occurs during the course of oocyte growth is not observed in mutant oocytes, and the transcriptome of mutant oocytes is markedly perturbed. These results, and those of previous studies, strongly implicate a central role of MSY2 in regulating mRNA stability.

Transcription and post-transcriptional regulation of spermatogenesis

Spermatogenesis in mammals is achieved by multiple players that pursue a common goal of generating mature spermatozoa. The developmental processes acting on male germ cells that culminate in the production of the functional spermatozoa are regulated at both the transcription and post-transcriptional levels. This review addresses recent progress towards understanding such regulatory mechanisms and identifies future challenges to be addressed in this field. We focus on transcription factors, chromatin-associated factors and RNA-binding proteins necessary for spermatogenesis and/or sperm maturation. Understanding the molecular mechanisms that govern spermatogenesis has enormous implications for new contraceptive approaches and treatments for infertility.

Recruitment of Orc6l, a dormant maternal mRNA in mouse oocytes, is essential for DNA replication in 1-cell embryos

Mouse oocytes acquire the ability to replicate DNA during meiotic maturation, presumably to ensure that DNA replication does not occur precociously between MI and MII and only after fertilization. Acquisition of DNA replication competence requires protein synthesis, but the identity of the proteins required for DNA replication is poorly described. In Xenopus, the only component missing for DNA replication competence is CDC6, which is synthesized from a dormant maternal mRNA recruited during oocyte maturation, and a similar situation also occurs during mouse oocyte maturation. We report that ORC6L is another component required for acquisition of DNA replication competence that is absent in mouse oocytes. The dormant maternal Orc6l mRNA is recruited during maturation via a CPE present in its 3' UTR. RNAi-mediated ablation of maternal Orc6l mRNA prevents the maturation-associated increase in ORC6L protein and inhibits DNA replication in 1-cell embryos. These results suggest that mammalian oocytes have more complex mechanisms to establish DNA replication competence when compared to their Xenopus counterparts.

Analysis of mRNA associated factors during bovine oocyte maturation and early embryonic development

Regulation of gene expression at the translational level is particularly essential during developmental periods, when transcription is impaired. According to the closed-loop model of translational initiation, we have analyzed components of the 5 -mRNA cap-binding complex eIF4F (eIF4E, eIF4G, eIF4A), the eIF4E repressor 4E-BP1, and 3 -mRNA poly-(A) tail-associated proteins (PABP1 and 3, PAIP1 and 2, CPEB1, Maskin) during in vitro maturation of bovine oocytes and early embryonic development up to the 16-cell stage. Furthermore, we have elucidated the activity of distinct kinases which are potentially involved in their phosphorylation. Major phosphorylation of specific target sequences of PKA, PKB, PKC, CDKs, ATM/ATR, and MAPK were observed in M II stage oocytes. Furthermore, main changes in the abundance and/or phosphorylation of distinct mRNA-binding factors occur at the transition from M II stage oocytes to 2-cell embryos. In conclusion, the results indicate that, at the transition from oocyte to embryonic development, translational initiation is regulated by striking differences in the abundance and/or phosphorylation of 5 -end and 3 -end mRNA associated factors, mainly the poly-(A) bindings proteins PABP1 and 3, their repressor PAIP2 and a Maskin-like protein with distinct eIF4E-binding properties which prevents eIF4E/cap binding and eIF4F formation in vitro. Nevertheless, from the M II stage to 16-cell embryos a substantial amount of eIF4E and, to a lesser extent, of eIF4G was precipitated by (7)m-GTP-Separose indicating eIF4F complex formation. Therefore, it is likely that in general the reduction in PABP1 and 3 abundance represses overall translation during early embryonic development.