Biochemical studies of mammalian oogenesis: synthesis and stability of various classes of RNA during growth of the mouse oocyte in vitro

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.

NLRP14 Safeguards Calcium Homeostasis via Regulating the K27 Ubiquitination of Nclx in Oocyte-to-Embryo Transition

Sperm-induced Ca2+ rise is critical for driving oocyte activation and subsequent embryonic development, but little is known about how lasting Ca2+ oscillations are regulated. Here it is shown that NLRP14, a maternal effect factor, is essential for keeping Ca2+ oscillations and early embryonic development. Few embryos lacking maternal NLRP14 can develop beyond the 2-cell stage. The impaired developmental potential of Nlrp14-deficient oocytes is mainly caused by disrupted cytoplasmic function and calcium homeostasis due to altered mitochondrial distribution, morphology, and activity since the calcium oscillations and development of Nlrp14-deficient oocytes can be rescued by substitution of whole cytoplasm by spindle transfer. Proteomics analysis reveal that cytoplasmic UHRF1 (ubiquitin-like, containing PHD and RING finger domains 1) is significantly decreased in Nlrp14-deficient oocytes, and Uhrf1-deficient oocytes also show disrupted calcium homeostasis and developmental arrest. Strikingly, it is found that the mitochondrial Na+ /Ca2+ exchanger (NCLX) encoded by Slc8b1 is significantly decreased in the Nlrp14mNull oocyte. Mechanistically, NLRP14 interacts with the NCLX intrinsically disordered regions (IDRs) domain and maintain its stability by regulating the K27-linked ubiquitination. Thus, the study reveals NLRP14 as a crucial player in calcium homeostasis that is important for early embryonic development.

Ovarian inflammation mediated by Toll-like receptor 4 increased transcripts of maternal effect genes and decreased embryo development†

Obese women are subfertile and have reduced assisted reproduction success, which may be due to reduced oocyte competence. We hypothesize that consumption of a high-fat/high-sugar diet induces ovarian inflammation, which is a primary contributor to decreased oocyte quality and pre-implantation embryo development. To test this hypothesis, C57BL/6 (B6) mice with a normal inflammatory response and C3H/HeJ (C3H) mice with a dampened inflammatory response due to dysfunctional Toll-like receptor 4 were fed either normal chow or high-fat/high-sugar diet. In both B6 and C3H females, high-fat/high-sugar diet induced excessive adiposity and hyperglycemia compared to normal chow-fed counterparts. Conversely, ovarian CD68 levels and oocyte expression of oxidative stress markers were increased when collected from B6 high-fat/high-sugar but not C3H high-fat/high-sugar mice. Following in vitro fertilization of in vivo matured oocytes, blastocyst development was decreased in B6-high-fat/high-sugar but not C3H high-fat/high-sugar mice. Expression of cumulus cell markers of oocyte quality were altered in both B6 high-fat/high-sugar and C3H high-fat/high-sugar. However, there were no diet-dependent differences in spindle abnormalities in either B6 or C3H mice, suggesting potential defects in cytoplasmic maturation. Indeed, there were significant increases in the abundance of maternal effect gene mRNAs in oocytes from only B6 high-fat/high-sugar mice. These differentially expressed genes encode proteins of the subcortical maternal complex and associated with mRNA metabolism and epigenetic modifications. These genes regulate maternal mRNA degradation at oocyte maturation, mRNA clearance at the zygotic genome activation, and methylation of imprinted genes suggesting a mechanism by which inflammation induced oxidative stress impairs embryo development.

Developmental mRNA m5C landscape and regulatory innovations of massive m5C modification of maternal mRNAs in animals

m⁵C is one of the longest-known RNA modifications, however, its developmental dynamics, functions, and evolution in mRNAs remain largely unknown. Here, we generate quantitative mRNA m⁵C maps at different stages of development in 6 vertebrate and invertebrate species and find convergent and unexpected massive methylation of maternal mRNAs mediated by NSUN2 and NSUN6. Using Drosophila as a model, we reveal that embryos lacking maternal mRNA m⁵C undergo cell cycle delays and fail to timely initiate maternal-to-zygotic transition, implying the functional importance of maternal mRNA m⁵C. From invertebrates to the lineage leading to humans, two waves of m⁵C regulatory innovations are observed: higher animals gain cis-directed NSUN2-mediated m⁵C sites at the 5' end of the mRNAs, accompanied by the emergence of more structured 5'UTR regions; humans gain thousands of trans-directed NSUN6-mediated m⁵C sites enriched in genes regulating the mitotic cell cycle. Collectively, our studies highlight the existence and regulatory innovations of a mechanism of early embryonic development and provide key resources for elucidating the role of mRNA m⁵C in biology and disease.

mRNAs are known to be decorated with m5C at a low-to-medium level. Here, the authors generate atlases of mRNA m5C during animal development in 6 species and identify convergent and unexpected massive methylation of maternal mRNAs by NSUN2 and NSUN6.

Role of the X and Y Chromosomes in the Female Germ Cell Line Development in the Mouse (Mus musculus)

BACKGROUND

In eutherian mammals, the sex chromosome complement, XX and XY, determines sexual differentiation of gonadal primordia into testes and ovaries, which in turn direct differentiation of germ cells into haploid sperm and oocytes, respectively. When gonadal sex is reversed, however, the germ cell sex becomes discordant with the chromosomal sex. XY females in humans are infertile, while XY females in the mouse (Mus musculus) are subfertile or infertile dependent on the cause of sex reversal and the genetic background. This article reviews publications to understand how the sex chromosome complement affects the fertility of XY oocytes by comparing with XX and monosomy X (XO) oocytes.

SUMMARY

The results highlight 2 folds disadvantage of XY oocytes over XX oocytes: (1) the X and Y chromosomes fail to pair during the meiotic prophase I, resulting in sex chromosome aneuploidy at the first meiotic division and (2) expression of the Y-linked genes during oocyte growth affects the transcriptome landscape and renders the ooplasmic component incompetent for embryonic development. Key Message: The XX chromosome complement gives the oocyte the highest competence for embryonic development.

Physiologically relevant miRNAs in mammalian oocytes are rare and highly abundant

miRNAs, ~22nt small RNAs associated with Argonaute (AGO) proteins, are important negative regulators of gene expression in mammalian cells. However, mammalian maternal miRNAs show negligible repressive activity and the miRNA pathway is dispensable for oocytes and maternal-to-zygotic transition. The stoichiometric hypothesis proposed that this is caused by dilution of maternal miRNAs during oocyte growth. As the dilution affects miRNAs but not mRNAs, it creates unfavorable miRNA:mRNA stoichiometry for efficient repression of cognate mRNAs. Here, we report that porcine ssc-miR-205 and bovine bta-miR-10b are exceptional miRNAs, which resist the diluting effect of oocyte growth and can efficiently suppress gene expression. Additional analysis of ssc-miR-205 shows that it has higher stability, reduces expression of endogenous targets, and contributes to the porcine oocyte-to-embryo transition. Consistent with the stoichiometric hypothesis, our results show that the endogenous miRNA pathway in mammalian oocytes is intact and that maternal miRNAs can efficiently suppress gene expression when a favorable miRNA:mRNA stoichiometry is established.

PABPN1L assemble into "ring-like" aggregates in the cytoplasm of MII oocytes and is associated with female infertility†

Infertility affects 10% - 15% of families worldwide. However, the pathogenesis of female infertility caused by abnormal early embryonic development is not clear. A resent study showed that PABPN1L recruited BTG4 to mRNA 3'-poly(A) tails and was essential for maternal mRNA degradation. Here, we generated an PABPN1L-antibody and found "ring-like" PABPN1L aggregates in the cytoplasm of MII oocytes. PABPN1L-EGFP proteins spontaneously formed"ring-like" aggregates in vitro. This phenomenon is similar with CCR4-NOT catalytic subunit, CNOT7, when it starts deadenylation process in vitro. We constructed two mouse model (Pabpn1l  -/- and Pabpn1l  tm1a/tm1a) simulating the intron1-exon2 abnormality of human PABPN1L and found that the female was sterile and the male was fertile. Using RNA-Seq, we observed a large-scale up-regulation of RNA in zygotes derived from Pabpn1l-/- MII oocytes. We found that 9222 genes were up-regulated instead of being degraded in the Pabpn1l-♀/+♂zygote. Both the Btg4 and Cnot61 genes are necessary for the deadenylation process and Pabpn1l  -/- resembled both the Btg4 and Cnot6l knockouts, where 71.2% genes stabilized in the Btg4-♀/+♂ zygote and 84.2% genes stabilized in the Cnot6l-♀/+♂zygote were also stabilized in Pabpn1l-♀/+♂ zygote. BTG4/CNOT7/CNOT6L was partially co-located with PABPN1L in MII oocytes. The above results suggest that PABPN1L is widely associated with CCR4-NOT-mediated maternal mRNA degradation and PABPN1L variants on intron1-exon2 could be a genetic marker of female infertility. Summary sentence. "Ring-like" PABPN1L aggregates was found in the cytoplasm of MII oocytes and in vitro; intron1-exon2 abnormality of Pabpn1l leads female sterile in mice.

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.

MicroRNA dilution during oocyte growth disables the microRNA pathway in mammalian oocytes

MicroRNAs (miRNAs) are ubiquitous small RNAs guiding post-transcriptional gene repression in countless biological processes. However, the miRNA pathway in mouse oocytes appears inactive and dispensable for development. We propose that marginalization of the miRNA pathway activity stems from the constraints and adaptations of RNA metabolism elicited by the diluting effects of oocyte growth. We report that miRNAs do not accumulate like mRNAs during the oocyte growth because miRNA turnover has not adapted to it. The most abundant miRNAs total tens of thousands of molecules in growing (∅ 40 μm) and fully grown (∅ 80 μm) oocytes, a number similar to that observed in much smaller fibroblasts. The lack of miRNA accumulation results in a 100-fold lower miRNA concentration in fully grown oocytes than in somatic cells. This brings a knock-down-like effect, where diluted miRNAs engage targets but are not abundant enough for significant repression. Low-miRNA concentrations were observed in rat, hamster, porcine and bovine oocytes, arguing that miRNA inactivity is not mouse-specific but a common mammalian oocyte feature. Injection of 250,000 miRNA molecules was sufficient to restore reporter repression in mouse and porcine oocytes, suggesting that miRNA inactivity comes from low-miRNA abundance and not from some suppressor of the pathway.

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.

Dynamic Changes in pStat3 are Involved in Meiotic Spindle Assembly in Mouse Oocytes

The signal transducer and activator of transcription 3 (Stat3) is activated upon phosphorylation at Y705 (pStat3) and serves the dual function of signal transduction and transcription activation. Our previous study suggested that pStat3 is functional during oocyte maturation when transcription is silenced. Therefore, we speculated that pStat3 serves other functions. Immunocytochemical analysis revealed that pStat3 emerges at microtubule asters and spindle and is subsequently localized at the spindle poles along with pericentrin during mouse oocyte maturation. Both Stat3 and pStat3 proteins were detected in conditionally knocked out Stat3^-/- mouse oocytes. pStat3 localization was the same in Stat3^+/+ and Stat3^-/-oocytes, and oocyte maturation proceeded normally, suggesting that pStat3 was still functional. Furthermore, the treatment of oocytes with the Stat3-specific inhibitors stattic and BP-1-102 or anti-pStat3 antibody led to significantly abnormal spindle assembly and chromosome mislocation in a dose-dependent manner, and pStat3 was either absent or improperly localized in these oocytes. Moreover, the development of pre-implantation stage embryos derived from inhibitor-treated oocytes was significantly hampered following in vitro fertilization. These findings indicate a novel function of pStat3 in spindle assembly.

Spatio-temporal expression of ANK2 promotes cytokinesis in oocytes

In the absence of transcription, the regulation of gene expression in oocytes is controlled almost exclusively at the level of transcriptome and proteome stabilization, and translation. A subset of maternal transcripts is stored in a translationally dormant state in the oocyte, and temporally driven translation of specific mRNAs propel meiotic progression, oocyte-to-embryo transition and early embryo development. We identified Ank2.3 as the only transcript variant present in the mouse oocyte and discovered that it is translated after nuclear envelope breakdown. Here we show that Ank2.3 mRNA is localized in higher concentration in the oocyte nucleoplasm and, after nuclear envelope breakdown, in the newly forming spindle where its translation occurs. Furthermore, we reveal that Ank2.3 mRNA contains an oligo-pyrimidine motif at 5'UTR that predetermines its translation through a cap-dependent pathway. Lastly, we show that prevention of ANK2 translation leads to abnormalities in oocyte cytokinesis.

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.

Using Single Molecule mRNA Fluorescent in Situ Hybridization (RNA-FISH) to Quantify mRNAs in Individual Murine Oocytes and Embryos

Changes in abundance of mRNAs during oocyte growth and maturation and during pre-implantation embryo development have been documented using quantitative real-time RT-PCR (qPCR), microarray analyses, and whole genome sequencing. However, these techniques require amplification of mRNAs, normalization using housekeeping genes, can be biased for abundant transcripts, and/or require large numbers of oocytes and embryos which can be difficult to acquire from mammalian species. We optimized a single molecule RNA fluorescence in situ hybridization (RNA-FISH) protocol, which amplifies fluorescence signal to detect candidate transcripts, for use with individual oocytes and embryos. Quantification using the software Localize showed patterns of Gdf9 and Pou5f1 mRNA expression in oocytes and embryos that were consistent with previously published data. Interestingly, low levels of Nanog mRNA were also accurately and reproducibly measured in oocytes and one- and two-cell embryos suggesting that RNA-FISH could be used to detect and quantify low abundance transcripts. Unlike other techniques, RNA-FISH is also able to detect changes in the localization patterns of mRNAs which may be used to monitor post-transcriptional regulation of a transcript. Thus, RNA-FISH represents an important technique to investigate potential mechanisms associated with the synthesis and stability of candidate mRNAs in mammalian oocytes and embryos.

Chromatin Modification and Global Transcriptional Silencing in the Oocyte Mediated by the mRNA Decay Activator ZFP36L2

Global transcriptional silencing is a highly conserved mechanism central to the oocyte-to-embryo transition. We report the unexpected discovery that global transcriptional silencing in oocytes depends on an mRNA decay activator. Oocyte-specific loss of ZFP36L2 an RNA-binding protein that promotes AU-rich element-dependent mRNA decay prevents global transcriptional silencing and causes oocyte maturation and fertilization defects, as well as complete female infertility in the mouse. Single-cell RNA sequencing revealed that ZFP36L2 downregulates mRNAs encoding transcription and chromatin modification regulators, including a large group of mRNAs for histone demethylases targeting H3K4 and H3K9, which we show are bound and degraded by ZFP36L2. Oocytes lacking Zfp36l2 fail to accumulate histone methylation at H3K4 and H3K9, marks associated with the transcriptionally silent, developmentally competent oocyte state. Our results uncover a ZFP36L2-dependent mRNA decay mechanism that acts as a developmental switch during oocyte growth, triggering wide-spread shifts in chromatin modification and global transcription.

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.

Translational Regulation in the Mammalian Oocyte

Fully grown oocytes arrest meiosis at prophase I and deposit maternal RNAs. A subset of maternal transcripts is stored in a dormant state in the oocyte, and the timely driven translation of specific mRNAs guides meiotic progression, the oocyte-embryo transition, and early embryo development. In the absence of transcription, the regulation of gene expression in oocytes is controlled almost exclusively at the level of transcriptome and proteome stabilization and at the level of protein synthesis.This chapter focuses on the recent findings on RNA distribution related to the temporal and spatial translational control of the meiotic cycle progression in mammalian oocytes. We discuss the most relevant mechanisms involved in the organization of the oocyte's maternal transcriptome storage and localization, and the regulation of translation, in correlation with the regulation of oocyte meiotic progression.

Comparative Proteomic Analysis of Buffalo Oocytes Matured in vitro Using iTRAQ Technique

To investigate the protein profiling of buffalo oocytes at the germinal vesicle (GV) stage and metaphase II (MII) stage, an iTRAQ-based strategy was applied. A total of 3,763 proteins were identified, which representing the largest buffalo oocytes proteome dataset to date. Among these proteins identified, 173 proteins were differentially expressed in GV oocytes and competent MII oocytes, and 146 proteins were differentially abundant in competent and incompetent matured oocytes. Functional and KEGG pathway analysis revealed that the up-regulated proteins in competent MII oocytes were related to chromosome segregation, microtubule-based process, protein transport, oxidation reduction, ribosome, and oxidative phosphorylation, etc., in comparison with GV and incompetent MII oocytes. This is the first proteomic report on buffalo oocytes from different maturation stages and developmental competent status. These data will provide valuable information for understanding the molecular mechanism underlying buffalo oocyte maturation, and these proteins may potentially act as markers to predict developmental competence of buffalo oocyte during in vitro maturation.

Obesity-Dependent Increases in Oocyte mRNAs Are Associated With Increases in Proinflammatory Signaling and Gut Microbial Abundance of Lachnospiraceae in Female Mice

RNAs stored in the metaphase II-arrested oocyte play important roles in successful embryonic development. Their abundance is defined by transcriptional activity during oocyte growth and selective degradation of transcripts during LH-induced oocyte maturation. Our previous studies demonstrated that mRNA abundance is increased in mature ovulated oocytes collected from obese humans and mice and therefore may contribute to reduced oocyte developmental competence associated with metabolic dysfunction. In the current study mouse models of diet-induced obesity were used to determine whether obesity-dependent increases in proinflammatory signaling regulate ovarian abundance of oocyte-specific mRNAs. The abundance of oocyte-specific Bnc1, Dppa3, and Pou5f1 mRNAs as well as markers of proinflammatory signaling were significantly increased in ovaries of obese compared with lean mice which were depleted of fully grown preovulatory follicles. Chromatin-immunoprecipitation analyses also demonstrated increased association of phosphorylated signal transducer and activator of transcription 3 with the Pou5f1 promoter in ovaries of obese mice suggesting that proinflammatory signaling regulates transcription of this gene in the oocyte. The cecum microbial content of lean and obese female mice was subsequently examined to identify potential relationships between microbial composition and proinflammatory signaling in the ovary. Multivariate Association with Linear Models identified significant positive correlations between cecum abundance of the bacterial family Lachnospiraceae and ovarian abundance of Tnfa as well as Dppa3, Bnc1, and Pou5f1 mRNAs. Together, these data suggest that diet-induced changes in gut microbial composition may be contributing to ovarian inflammation which in turn alters ovarian gene expression and ultimately contributes to obesity-dependent reduction in oocyte quality and development of infertility in obese patients.

DAZL and CPEB1 regulate mRNA translation synergistically during oocyte maturation

Meiotic progression requires exquisitely coordinated translation of maternal messenger (m)RNA that has accumulated during oocyte growth. A major regulator of this program is the cytoplasmic polyadenylation element binding protein 1 (CPEB1). However, the temporal pattern of translation at different meiotic stages indicates the function of additional RNA binding proteins (RBPs). Here, we report that deleted in azoospermia-like (DAZL) cooperates with CPEB1 to regulate maternal mRNA translation. Using a strategy that monitors ribosome loading onto endogenous mRNAs and a prototypic translation target, we show that ribosome loading is induced in a DAZL- and CPEB1-dependent manner, as the oocyte reenters meiosis. Depletion of the two RBPs from oocytes and mutagenesis of the 3' untranslated regions (UTRs) demonstrate that both RBPs interact with the Tex19.1 3' UTR and cooperate in translation activation of this mRNA. We observed a synergism between DAZL and cytoplasmic polyadenylation elements (CPEs) in the translation pattern of maternal mRNAs when using a genome-wide analysis. Mechanistically, the number of DAZL proteins loaded onto the mRNA and the characteristics of the CPE might define the degree of cooperation between the two RBPs in activating translation and meiotic progression.

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.

Pre- and postovulatory aging of murine oocytes affect the transcript level and poly(A) tail length of maternal effect genes

Maternal effect genes code for oocyte proteins that are important for early embryogenesis. Transcription in oocytes does not take place from the onset of meiotic progression until zygotic genome activation. During this period, protein levels are regulated posttranscriptionally, for example by poly(A) tail length. Posttranscriptional regulation may be impaired in preovulatory and postovulatory aged oocytes, caused by delayed ovulation or delayed fertilization, respectively, and may lead to developmental defects. We investigated transcript levels and poly(A) tail length of ten maternal effect genes in in vivo- and in vitro- (follicle culture) grown oocytes after pre- and postovulatory aging. Quantitative RT-PCR was performed using random hexamer-primed cDNA to determine total transcript levels and oligo(dT)16-primed cDNA to analyze poly(A) tail length. Transcript levels of in vivo preovulatory-aged oocytes remained stable except for decreases in Brg1 and Tet3. Most genes investigated showed a tendency towards increased poly(A) content. Polyadenylation of in vitro preovulatory-aged oocytes was also increased, along with transcript level declines of Trim28, Nlrp2, Nlrp14 and Zar1. In contrast to preovulatory aging, postovulatory aging of in vivo- and in vitro-grown oocytes led to a shortening of poly(A) tails. Postovulatory aging of in vivo-grown oocytes resulted in deadenylation of Nlrp5 after 12 h, and deadenylation of 4 further genes (Tet3, Trim28, Dnmt1, Oct4) after 24 h. Similarly, transcripts of in vitro-grown oocytes were deadenylated after 12 h of postovulatory aging (Tet3, Trim28, Zfp57, Dnmt1, Nlrp5, Zar1). This impact of aging on poly(A) tail length may affect the timed translation of maternal effect gene transcripts and thereby contribute to developmental defects.

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.

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.

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.

Interrogating the transcriptome of oocytes and preimplantation embryos

During its growth phase, a mouse oocyte accumulates RNA that is the sole template for new protein synthesis in the transcriptionally silent interval between growth completion and transcriptional activation of the embryonic genome. Over this transcriptionally silent interval, almost half the quantity of RNA accumulated in the full-grown oocyte is degraded, while stable messages undergo major transcript-specific polyadenylation fluctuations associated with timely translation of new proteins. These processes, in the background of substantial RNA degradation, create unique pitfalls for transcriptome analysis. Three particular challenges are discussed herein. (1) Systematic errors of relative quantification occur if standard approaches are used, wherein samples are normalized to a constant quantity of RNA, or when computational analyses are normalized to an apparent "constant" endogenous to the sample. We show that use of a fixed quantity of exogenous RNA per oocyte or embryo alleviates this problem. (2) Comparison of large-scale expression analyses from widely disparate platforms highlights how the differing protocols produce correspondingly different lists of genes with significant changes in transcript abundance. Only with careful attention to the differences among experiments can such discrepancies be understood. (3) The complete assessment of changes in expression requires correspondingly comprehensive assessment of the role of isoform-specific changes.

Formation of an antral follicle-like structure of bovine cumulus-oocyte complexes embedded individually or in groups in collagen gels

Culture techniques of antral follicle-like structure (AFLS) derived from cumulus-oocyte complexes (COCs) might provide important insights into follicular development and oocyte maturation. This study was undertaken to investigate the effects of embedding bovine COCs individually (one COC) or in groups (4-5 COCs) in collagen gels on the formation of AFLS and the meiotic status of oocytes. The observations of AFLS formation were performed every second day for 14 days. The AFLS was formed at Day 2 or 4 after the start of culture (Day=0), irrespective of the culture methods. The mean diameters of AFLS during Days 4-14 using the individual culture method were significantly higher (p<0.05) than those using the group culture method. However, the AFLS formation rate in the individual culture method was significantly lower compared to that in the group culture method (26.1% vs 62.7%, p<0.01). Almost all oocytes had undergone the germinal vesicle breakdown stage, irrespective of the culture method or AFLS formation. In conclusion, comparison with the individual culture method revealed that the mean diameters of AFLS in the group culture method were smaller, but more COCs formed AFLS. The group culture method might be useful for evaluating the various hypotheses of follicular formation and interfollicular communication. However, improvement of the group culture system is necessary to prevent the meiotic resumption of oocytes, because the AFLS formation is dependent on the cumulus/granulosa cells surrounding oocytes.

General features of certain RNA populations from gametes and cumulus cells

Results described herein provide insight regarding certain features of gamete RNA and how they compare to cumulus cell RNA. In particular, 28S/18S rRNA ratio and size distribution of RNA molecules differed in total RNA from oocytes versus surrounding cumulus cells. Specifically, oocyte total RNA had a lower rRNA ratio and an increased abundance of smaller RNA sizes compared to RNA from surrounding cumulus. Extensive efforts demonstrated that observed differences were repeatable whether oocyte maturation occurred in vitro or in vivo, and were similar between the nuclear stages examined. Features of oocyte RNA were conserved across six mammalian species, yet differed from surrounding cumulus. Profiles of sperm RNA were also examined but had no discernible ribosomal RNA peaks and were conserved across four mammalian species. Because the oocyte and spermatozoon are highly specialized cells representing unique molecular entities required for proper embryo development, dissimilarities described herein likely represent real gamete versus cumulus RNA differences.

P-body loss is concomitant with formation of a messenger RNA storage domain in mouse oocytes

In mammalian somatic cells, several pathways that converge on deadenylation, decapping, and 5'-3' degradation are found in cytoplasmic foci known as P-bodies. Because controlled mRNA stability is essential for oocyte-to-zygote transition, we examined the dynamics of P-body components in mouse oocytes. We report that oocyte growth is accompanied by loss of P-bodies and a subcortical accumulation of several RNA-binding proteins, including DDX6, CPEB, YBX2 (MSY2), and the exon junction complex. These proteins form transient RNA-containing aggregates in fully grown oocytes with a surrounded nucleolus chromatin configuration. These aggregates disperse during oocyte maturation, consistent with recruitment of maternal mRNAs that occurs during this time. In contrast, levels of DCP1A are low during oocyte growth, and DCP1A does not colocalize with DDX6 in the subcortical aggregates. The amount of DCP1A markedly increases during meiosis, which correlates with the first wave of destabilization of maternal mRNAs. We propose that the cortex of growing oocytes serves as an mRNA storage compartment, which contains a novel type of RNA granule related to P-bodies.

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

Degradation of maternal mRNA is thought to be essential to undergo the maternal-to-embryonic transition. Messenger RNA is extremely stable during oocyte growth in mouse and MSY2, an abundant germ cell-specific RNA-binding protein, likely serves as a mediator of global mRNA stability. Oocyte maturation, however, triggers an abrupt transition in which most mRNAs are significantly degraded. We report that CDC2A (CDK1)-mediated phosphorylation of MSY2 triggers this transition. Injecting Cdc2a mRNA, which activates CDC2A, overcomes milrinone-mediated inhibition of oocyte maturation, induces MSY2 phosphorylation and the maturation-associated degradation of mRNAs. Inhibiting CDC2A following its activation with roscovitine inhibits MSY2 phosphorylation and prevents mRNA degradation. Expressing non-phosphorylatable dominant-negative forms of MSY2 inhibits the maturation-associated decrease in mRNAs, whereas expressing constitutively active forms induces mRNA degradation in the absence of maturation and phosphorylation of endogenous MSY2. A positive-feedback loop of CDK1-mediated phosphorylation of MSY2 that leads to degradation of Msy2 mRNA that in turn leads to a decrease in MSY2 protein may ensure that the transition is irreversible.

Molecular Analysis of Maturation Processes by Protein and Phosphoprotein Profiling during In Vitro Maturation of Bovine Oocytes: A Proteomic Approach

Cellular maturation and differentiation processes are accompanied by the expression of specific proteins. Especially in oocytes, there is no reliable strict linear correlation between mRNA levels and the abundance of proteins. Furthermore, the activity of proteins is modulated by specific kinases and phosphatases which control cellular processes like cellular growth, differentiation, cell cycle and meiosis. During the meiotic maturation of oocytes, the activation of protein kinases, namely of the MPF and MAPK play a predominant role. Therefore, the present study was performed to analyze meiotic maturation at a molecular level, concerning alterations of the proteom and phosphoproteom during IVM. Using a proteomic approach by combining two-dimensional gel electrophoresis followed by selective protein and phosphoprotein staining and mass spectrometry, we identified proteins which were differentially expressed and/or phosphorylated during IVM. Furthermore, we used the MPF inhibitor butyrolactone I, to reveal new molecular effects which are potentially essential for successful maturation. The results show that approximately 550 protein spots could be visualized by the fluorescent dye Sypro ruby at any maturation stage (GV, M I, M II) investigated. From GV stage to M II, ProQ diamond staining indicate in GV 30%, in M I 50%, and in M II 45% of the spots were phosphorylated. The Identity of 40 spots could be established. These proteins belong to different families, for example, cytoskeleton, molecular chaperons, redox, energy and metabolism related proteins, nucleic acid binding proteins, cell cycle regulators, and protein kinases. Four of them were differentially expressed (alteration higher than factor 2) during IVM, namely tubulin beta-chain, cyclin E(2), protein disulfide isomerase and one of two different forms of peroxiredoxin 2. Seven proteins were differentially stained by ProQ diamond, indicating a differential phosphorylation. These are tubulin beta-chain, beta-actin, cyclin E(2), aldose reductase and UMP-synthase, protein disulfide isomerase 2, and peroxiredoxin 2. Furthermore, the results indicate that the phosphorylation of at least peroxiredoxin 2 respond to BL I treatment. This indicates that its phosphorylation is under the control of MPF or MAPK. In summary these results indicates that the reduction of cyclin Eexpression and the (partially) inactivation of peroxiredoxin 2 by phosphorylation, hence alterations in the peroxide levels which can mediate signal transduction are essential components for successful maturation.

Abundant transcripts from retrotransposons are unstable in fully grown mouse oocytes

One physiological function proposed for RNA interference (RNAi) is to constrain expression of repetitive elements and thereby reduce the incidence of retrotransposition. Consistent with this model is that inhibiting the RNAi pathway results in an increase in expression of repetitive elements in preimplantation mouse embryos. Mouse oocytes are essentially transcriptionally quiescent providing a unique opportunity to assess the stability of repetitive element-derived transcripts in these cells. We compared the transcriptome of freshly isolated fully grown germinal vesicle (GV)-intact oocytes to that of oocytes in which meiotic maturation in vitro was inhibited for 48 h by milrinone. Consistent with the aforementioned function for RNAi is that the abundance of only a relatively small number of transcripts decreased in the cultured oocytes, when compared to changes that occur during maturation or following fertilization, and of those, several belonged to mobile elements.

Absence of the DNA-/RNA-binding protein MSY2 results in male and female infertility

MSY2, a germ-cell-specific member of the Y-box family of DNA-/RNA-binding proteins, is proposed to function as a coactivator of transcription in the nucleus and to stabilize and store maternal and paternal mRNAs in the cytoplasm. In mice lacking Msy2, a normal Mendelian ratio is observed after matings between heterozygotes with equal numbers of phenotypically normal but sterile male and female homozygotes (Msy2-/-). Spermatogenesis is disrupted in postmeiotic null germ cells with many misshapen and multinucleated spermatids, and no spermatozoa are detected in the epididymis. Apoptosis is increased in the testes of homozygotes, and real-time RT-PCR assays reveal large reductions in the mRNA levels of postmeiotic male germ cell mRNAs and smaller reductions of meiotic germ cell transcripts. In females, there is no apparent decrease in either the number of follicles or their morphology in ovaries obtained from 2- and 8-day-old Msy2-/- mice. In contrast, follicle number and progression are reduced in 21-day-old Msy2-/- ovaries. In adult Msy2-/- females, oocyte loss increases, anovulation is observed, and multiple oocyte and follicle defects are seen. Thus, Msy2 represents one of a small number of germ-cell-specific genes whose deletion leads to the disruption of both spermatogenesis and oogenesis.

Susceptibility of Bovine Germinal Vesicle-Stage Oocytes from Antral Follicles to Direct Effects of Heat Stress In Vitro1

Delineation of maternal versus direct effects of heat stress in reducing development at the germinal vesicle (GV) stage is challenging, because oocytes spontaneously resume meiosis after removal from antral follicles. The use of S-roscovitine (inhibitor of p34(cdc2)/cyclin B kinase) to hold bovine oocytes at the GV stage without compromising early embryo development was previously validated in our laboratory. The objective of the present study was to assess the direct effects of an elevated temperature commonly seen in heat-stressed dairy cows on cumulus-oocyte complexes (COCs) held at the GV stage using 50 microM S-roscovitine. During roscovitine culture, GV-stage COCs (antral follicle diameter, 3-8 mm) were cultured at 38.5 or 41 degrees C. Thereafter, oocytes were removed from roscovitine medium and allowed to undergo in vitro maturation, fertilization, and culture. Zona pellucida hardening (solubility to 0.5% pronase), nuclear stage (Hoechst 33342), cortical granule type (lens culinaris agglutinin-fluorescein isothiocyanate [FITC]), and early embryo development were evaluated. Culture of GV-stage COCs at 41 degrees C increased the proportion that had type III cortical granules and reduced the proportion that progressed to metaphase II after in vitro maturation. Effects of 41 degrees C on zona pellucida hardening, fertilization (penetration, sperm per oocyte, pronuclear formation, and monospermic and putative embryos), and cleavage of putative zygotes were not noted. However, culture of GV-stage COCs at 41 degrees C for 6 h decreased the proportion of 8- to 16-cell embryos, whereas 41 degrees C for 12 h reduced blastocyst development. In summary, antral follicle COCs are susceptible to direct effects of elevated body temperature, which may account in part for reduced fertility in heat-stressed cows.

Transgenic RNAi-mediated reduction of MSY2 in mouse oocytes results in reduced fertility

MSY2 is implicated in regulating the stability and translation of maternal mRNAs during mouse oogenesis. We report here that by driving the expression of a transgene encoding an Msy2 hairpin dsRNA in growing oocytes using the oocyte-specific Zp3 promoter, the amount of MSY2 protein was reduced by at least 60% in fully grown oocytes. The decrease appeared specific because no decrease was observed in either non-targeted mRNAs or proteins. Fertility of transgenic females was severely reduced. Although transgenic eggs could be inseminated, the eggs did not exhibit the normal series of oscillations in intracellular Ca2+, resume meiosis, undergo cortical granule exocytosis, or ZP2 cleavage to ZP2f. Transgenic oocytes also displayed a higher incidence of both the non-surrounded nucleolus chromatin morphology, and abnormal meiotic spindle formation was observed following oocyte maturation. Transgenic oocytes contained less total mRNA (approximately 75-80% that of non-transgenic oocytes) and displayed a reduced level of protein synthesis. Moreover, several of the maturation-associated changes in protein synthesis failed to occur in the transgenic oocytes. These results support a role for MSY2 in stabilizing maternal mRNAs in growing oocytes, a process essential to generate meiotically and developmentally competent oocytes.

H1oo: a pre-embryonic H1 linker histone in search of a function

The mouse oocyte-specific linker histone H1oo (1) constitutes a novel mammalian homologue of the oocyte-specific linker histone B4 of the frog and of the cs-H1 linker histone of the sea urchin; (2) is expressed as early as the germinal vesicle (PI) stage oocyte, persisting into the MII stage oocyte, the oocytic polar bodies, and the 2-cell embryo, extinction becoming apparent at the 4-8 cell embryonic stage; and (3) may play a key role in the control of gene expression during oogenesis and early embryogenesis, presumably through the perturbation of chromatin structure.

Requirement for RNA-binding activity of MSY2 for cytoplasmic localization and retention in mouse oocytes

MSY2, a mouse germ cell-specific Y-box protein, is implicated in the global regulation of the stability and/or translation of maternal mRNAs in the mouse oocyte. We report here that in the oocyte approximately 75% of MSY2 protein is associated with a Triton-insoluble preparation, whereas in either male germ cells or when exogenously expressed in transfected somatic cells almost all MSY2 is soluble. This retention in the oocyte, which is unlikely mediated either by microfilaments or by microtubules, markedly decreases beyond the two-cell stage of development. By microinjecting mutant MSY2-EGFP chimeric mRNAs into mouse oocytes and then assaying the expressed protein's localization by laser-scanning confocal microscopy, we find that an intact cold-shock domain (CSD), containing two RNA-binding motifs, is required to localize MSY2 to the oocyte cytoplasm. In addition, an additional basic/aromatic amino acid island (B/A), which can also interact with RNA, in the C-terminal tail domain is necessary to retain MSY2 following Triton permeabilization. Intact mRNA appeared required for this retention, since RNase A treatment of Triton-permeabilized oocytes or microinjection of RNase A into the oocyte released essentially all of the endogenous MSY2 protein. Furthermore, there is a positive correlation between the ability of the mutant MSY2-EGFP protein to remain associated with the Triton-insoluble preparations and its increased affinity for RNA, as determined by RNA electrophoretic mobility shift assays. These results suggest that binding of intact maternal mRNA by MSY2 is required for its cytoplasmic retention.

RNAi: mammalian oocytes do it without RNA-dependent RNA polymerase

Studies in mutant organisms deficient in RNA interference (RNAi) and related post-transcriptional gene silencing implicated a role for a single class of RNA-dependent RNA polymerases (RdRp). Nevertheless, sequence homologs to these RdRps have not been found in coelomate organisms such as Drosophila or mammals. This lack of homologous sequences does not exclude that an RdRp functions in RNAi in these organisms because an RdRp could be acquired by horizontal transfer from an RNA virus. In fact, such a sequence is found in mice (Aquarius) and we observe that it is expressed in mouse oocytes and early embryos, which exhibit RNAi. We report here that cordycepin, an inhibitor of RNA synthesis, does not prevent Mos double-strand RNA (dsRNA) to target endogenous Mos mRNA in mouse oocytes and that targeting a chimeric Mos-EGFP mRNA with dsRNA to EGFP does not reduce the endogenous Mos mRNA, but does target the chimeric mRNA. These results indicate that an RdRp is not involved in dsRNA-mediated mRNA degradation in mammalian oocytes, and possibly in mammals in general, and therefore that only homologous sequences to the dsRNA are targeted for degradation.

RNA-Binding Properties and Translation Repression In Vitro by Germ Cell-Specific MSY2 Protein1

The large amount of MSY2 protein, a mouse germ cell-specific Y-box protein, in oocytes and its degradation by the late two-cell stage suggest that MSY2 may stabilize and/or regulate the translation of maternal mRNAs. We report here the ability of bacterially expressed recombinant MSY2 protein to bind to mRNA and repress translation in vitro. Although MSY2 displays some sequence specificity in binding to short RNA sequences derived from the 3' untranslated region (UTR) of the protamine 1 (Prm1) mRNA, as determined by both gel shift and filter binding assays, essentially no sequence specificity is observed when full-length Prm1 mRNA is used. The binding of MSY2 is approximately 10-fold greater to the full-length Prm1 mRNA than to a 37-nucleotide sequence derived from the 3' UTR, and gel shift assays indicate that multiple MSY2 molecules bind to a single Prm1 mRNA. MSY2 binding to luciferase mRNA at ratios of protein to mRNA that are likely to exist in the oocyte also leads to a moderate inhibition of protein synthesis in vitro. Given the abundance of MSY2 in mouse oocytes (2% of total oocyte protein), these data suggest that MSY2 packages mRNAs in vivo with relatively little sequence specificity, which may lead to both stabilization and translation repression of maternal mRNAs.

Expression of MSY2 in mouse oocytes and preimplantation embryos

Translational control plays a central role during oocyte maturation and early embryogenesis, as these processes occur in the absence of transcription. MSY2, a member of a multifunctional Y-box protein family, is implicated in repressing the translation of paternal mRNAs. Here, we characterize MSY2 expression in mouse oocytes and preimplantation embryos. Northern blot analysis indicates that MSY2 expression is highly restricted and essentially confined to the oocyte in the female mouse. MSY2 transcript and protein, as assessed by reverse transcription-polymerase chain reaction and immunoblotting, respectively, are expressed in growing oocytes, metaphase II-arrested eggs, and 1-cell embryos, but then are degraded by the late 2-cell stage; no expression is detectable in the blastocysts. During oocyte maturation, MSY2 is phosphorylated and following fertilization it is dephosphorylated. Quantification of the mass amount of MSY2 reveals that it represents 2% of the total protein in the fully grown oocyte, i.e., it is a very abundant protein. Both endogenous MSY2 and MSY2-enhanced green fluorescent protein (EGFP), which is synthesized following microinjection of an mRNA encoding MSY2-EGFP, are primarily localized in the cytoplasm, and about 75% of the MSY2 remains associated with oocyte cytoskeletal preparations. Results of these studies are consistent with the proposal that MSY2 functions by stabilizing and/or repressing the translation of maternal mRNAs.