Gene expression during the oocyte-to-embryo transition in mammals

Padi6 expression patterns in buffalo oocytes and preimplantation embryos

The subcortical maternal complex, which consists of maternal-effect genes, plays a crucial role in the development of oocytes and preimplantation embryo until the activation of the zygote genome. One such gene, known as peptidyl-arginine deiminase VI (Padi6), is involved in the oocyte maturation, fertilization and embryonic development. However, the precise function of Padi6 gene in buffalo is still unclear and requires further investigation. In this study, the sequence, mRNA and protein expression patterns of Padi6 gene were analyzed in oocytes, preimplantation embryos and somatic tissues of buffalo. The coding sequence of gene was successfully cloned and characterized. Real-time quantitative PCR results indicated an absence of Padi6 transcripts in somatic tissues. Notably, the expression levels of Padi6 in oocytes showed an increased from the germinal vesicle stage to metaphase II stage, followed by a rapid decrease during the morula and blastocyst stages. Immunofluorescence analysis confirmed these findings, revealing a noticeable decline in protein expression levels. Our research provides the initial comprehensive expression profile of Padi6 in buffalo oocytes and preimplantation embryos, serving as a solid foundation for further investigations into the functionality of maternal-effect genes in buffalo.

In vivo and in vitro matured bovine oocytes present a distinct pattern of single-cell gene expression

Oocyte gene expression is a well controlled event that promotes gamete competence to undergo maturation, fertilization, and to support early embryo development, directly affecting reproductive outcomes. Considering that in vivo controlled ovarian stimulation or in vitro maturation (IVM) for the acquisition of mature oocytes has distinct implications for gene expression, we sought to evaluate the effects of these procedures on the expression of competence-related genes in single-cell oocytes. Healthy Nelore cows of reproductive age were synchronized to harvest in vivo matured oocytes; ovaries from slaughtered animals were used to obtain cumulus-oocyte complexes that were in vitro matured. Single-cell gene expression was performed using TaqMan Low-Density Arrays and 42 genes were evaluated. In silico analysis of protein interactions and Gene Ontology (GO) analysis was performed. Reduced gene expression was observed for 24 targets in IVM oocytes when compared with those of in vivo matured oocytes (P < 0.05). Differences ranged from 1.5-fold to 4.8-fold higher in in vivo oocytes and the BMP15 (5.28), GDF9 (6.23), NOBOX (7.25), HSPA8 (7.85) and MSX1 (11.00) showed the greatest fold increases. The strongest score of functional interactions was observed between the CDC20 and CKS2, with the differentially expressed gene CDC20 being the main marker behind GO enrichment. IVM negatively affected the expression of important genes related to oocyte competency, and showed higher expression levels in in vivo matured oocytes. In vivo controlled ovarian stimulation may be a better strategy to achieve proper oocyte competence and increase the success of assisted reproductive technologies.

Single‐cell profiling of transcriptomic changes during in vitro maturation of human oocytes

Purpose

In vitro maturation (IVM) of human oocytes offers an invaluable opportunity for infertility treatment. However, in vitro matured oocytes often show lower developmental abilities than their in vivo counterparts, and molecular mechanisms underlying successful maturation remain unclear. In this study, we investigated gene expression profiles of in vitro matured oocytes at the single‐cell level to gain mechanistic insight into IVM of human oocytes.

Methods

Human oocytes were retrieved by follicular puncture and in vitro matured. In total, 19 oocytes from 11 patients were collected and subjected to single‐cell RNA‐seq analyses.

Results

Global gene expression profiles were similar among oocytes at the same maturation stage, while a small number of oocytes showed distinct transcriptomes from those at the corresponding maturation stage. Differential gene expression analysis identified hundreds of transcripts that dynamically altered their expression during IVM, and we revealed molecular pathways and upstream regulators that may govern oocyte maturation. Furthermore, oocytes that were delayed in their maturation showed distinct transcriptomes. Finally, we identified genes whose transcripts were enriched in each stage of oocyte maturation.

Conclusions

Our work uncovers transcriptomic changes during human oocyte IVM and the differential gene expression profile of each oocyte.

Nlrp5 and Tle6 expression patterns in buffalo oocytes and preimplantation embryos

Maternal-effect genes (MEGs) accumulate in oocytes during oogenesis and mediate the preimplantation embryo developmental program until activation of the zygote genome. Nlrp5 and Tle6 are required for normal preimplantation and embryonic development. However, the precise function of these MEGs in buffalo (Bubalus bubalis) remains to be elucidated. The aim of this study was to characterize Nlrp5 and Tle6 sequences, and analyze their mRNA and protein expression patterns in somatic tissues, oocytes, and preimplantation embryos of buffalo. The coding sequences of each gene were successfully cloned and characterized. Real-time quantitative reverse transcription PCR results revealed an absence of Nlrp5 or Tle6 transcripts in somatic tissues, with the exception of ovary. Expression levels of Nlrp5 and Tle6 in oocytes increased from the germinal vesicle stage to metaphase II stage, and then gradually decreased during morula and blastocyst stages. Protein expression patterns were confirmed by immunofluorescence analysis. This study lays a foundation for further validation of the function of MEGs in buffalo.

3D Liquid Marble Microbioreactors Support In Vitro Maturation of Prepubertal Ovine Oocytes and Affect Expression of Oocyte-Specific Factors

Simple Summary

Oocyte in vitro maturation has broad potential for generating embryos for research and for application of assisted reproductive technologies, such as in vitro embryo production. In human, the possibility to efficiently mature oocytes in vitro would solve the reproductive problems of patients with special diseases. Nevertheless, the developmental ability of in vitro matured oocytes is currently lower than those matured in vivo. Here, we used young sheep oocytes as model of low-quality gametes to show that a novel liquid marble 3D culture system is suitable to mature in vitro oocytes with reduced potential, improving the rates of in vitro embryo production. The present findings are useful for the optimization of in vitro maturation systems, and to improve the developmental potential of in vitro matured oocytes. Further applications should be considered also in other species, including human, to mature oocytes with intrinsic low quality.

Abstract

In vitro oocyte maturation (IVM) is a well-established technique. Despite the high IVM rates obtained in most mammalian species, the developmental competence of IVM oocytes is suboptimal. The aim of this work was to evaluate the potential beneficial effects of a liquid marble microbioreactor (LM) as a 3D culture system to mature in vitro prepubertal ovine oocytes, as models of oocytes with intrinsic low competence. Cumulus–oocyte complexes of prepubertal sheep ovaries were in vitro matured in a LM system with hydrophobic fumed-silica-nanoparticles (LM group) or in standard conditions (4W control group). We evaluated: (a) maturation and (b) developmental rates following in vitro fertilization (IVF) and embryo culture; (c) expression of a panel of genes. LM and 4W groups showed similar IVM and IVF rates, while in vitro development to blastocyst stage approached significance (4W: 14.1% vs. LM: 28.3%; p = 0.066). The expression of GDF9, of enzymes involved in DNA methylation reprogramming and of the subcortical maternal complex was affected by the IVM system, while no difference was observed in terms of cell-stress-response. LM microbioreactors provide a suitable microenvironment to induce prepubertal sheep oocyte IVM and should be considered to enhance the developmental competence of oocytes with reduced potential also in other species, including humans.

Human oocyte meiotic maturation is associated with a specific profile of alternatively spliced transcript isoforms

The transition from a transcriptionally active state (GV) to a transcriptionally inactive state (mature MII oocytes) is required for the acquisition of oocyte developmental competence. We hypothesize that the expression of specific genes at the in vivo matured (MII) stage could be modulated by posttranscriptional mechanisms, particularly regulation of alternative splicing (AS). In this study, we examined the transcriptional activity of GV oocytes after ovarian stimulation followed by oocyte pick-up and the landscape of alternatively spliced isoforms in human MII oocytes. Individual oocytes were processed and analyzed for transcriptional activity (GV), gene expression (GV and MII), and AS signatures (GV and MII) on HTA 2.0 microarrays. Samples were grouped according to maturation stage, and then subgrouped according to women's age and antral follicular count (AFC); array results were validated by quantitative polymerase chain reaction. Differentially expressed genes between GV and MII oocytes clustered mainly in biological processes related to mitochondrial metabolism. Interestingly, 16 genes that were related to the regulation of transcription and mitochondrial translation showed differences in alternatively spliced isoform profiles despite not being differentially expressed between groups. Altogether, our results contribute to our understanding of the role of AS in oocyte developmental competence acquisition.

The Regulation and Functions of Endogenous Retrovirus in Embryo Development and Stem Cell Differentiation

Endogenous retroviruses (ERVs) are repetitive sequences in the genome, belonging to the retrotransposon family. During the course of life, ERVs are associated with multiple aspects of chromatin and transcriptional regulation in development and pathological conditions. In mammalian embryos, ERVs are extensively activated in early embryo development, but with a highly restricted spatial-temporal pattern; and they are drastically silenced during differentiation with exceptions in extraembryonic tissue and germlines. The dynamic activation pattern of ERVs raises questions about how ERVs are regulated in the life cycle and whether they are functionally important to cell fate decision during early embryo and somatic cell development. Therefore, in this review, we focus on the pieces of evidence demonstrating regulations and functions of ERVs during stem cell differentiation, which suggests that ERV activation is not a passive result of cell fate transition but the active epigenetic and transcriptional regulation during mammalian development and stem cell differentiation.

Human eggs, zygotes, and embryos express the receptor angiotensin 1-converting enzyme 2 and transmembrane serine protease 2 protein necessary for severe acute respiratory syndrome coronavirus 2 infection

OBJECTIVE

To study messenger ribonucleic acid (mRNA) and protein expressions of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) entry receptors (angiotensin 1-converting enzyme 2 [ACE2] and CD147) and proteases (transmembrane serine protease 2 [TMPRSS2] and cathepsin L [CTSL]) in human oocytes, embryos, and cumulus (CCs) and granulosa cells (GCs).

DESIGN

Research study.

SETTING

Clinical in vitro fertilization (IVF) treatment center.

PATIENTS

Patients undergoing IVF were treated at the Colorado Center for Reproductive Medicine.

INTERVENTIONS

Oocytes (germinal vesicle and metaphase II [MII]) and embryos (1-cell [1C] and blastocyst [BL]) were donated for research at the disposition by the patients undergoing IVF. Follicular cells (CC and GC) were collected from women undergoing egg retrieval after ovarian stimulation without an ovulatory trigger for in vitro maturation/IVF treatment cycles.

MAIN OUTCOME MEASURES

Presence or absence of ACE2, CD147, TMPRSS2, and CTSL mRNAs detected using quantitative reverse transcription polymerase chain reaction and proteins detected using capillary Western blotting in human oocytes, embryos, and ovarian follicular cells.

RESULTS

The quantitative reverse transcription polymerase chain reaction analysis revealed high abundance of ACE2 gene transcripts in germinal vesicle and MII oocytes than in CC, GC, and BL. ACE2 protein was present only in the MII oocytes, and 1C and BL embryos, but other ACE2 protein variants were observed in all the samples. TMPRSS2 protein was present in all the samples, whereas mRNA was observed only in the BL stage. All the samples were positive for CD147 and CTSL mRNA expressions. However, CCs and GCs were the only samples that showed coexpression of both CD147 and CTSL proteins in low abundance.

CONCLUSIONS

CCs and GCs are the least susceptible to SARS-CoV-2 infection because of lack of the required combination of receptors and proteases (ACE2/TMPRSS2 or CD147/CTSL) in high abundance. The coexpression of ACE2 and TMPRSS2 proteins in the MII oocytes, zygotes, and BLs demonstrated that these gametes and embryos have the cellular machinery required and, thus, are potentially susceptible to SARS-CoV-2 infection if exposed to the virus. However, we do not know whether the infection occurs in vivo or in vitro in an assisted reproductive technology setting yet.

Single-cell analysis of transcriptome and DNA methylome in human oocyte maturation

Oocyte maturation is a coordinated process that is tightly linked to reproductive potential. A better understanding of gene regulation during human oocyte maturation will not only answer an important question in biology, but also facilitate the development of in vitro maturation technology as a fertility treatment. We generated single-cell transcriptome and used our previously published single-cell methylome data from human oocytes at different maturation stages to investigate how genes are regulated during oocyte maturation, focusing on the potential regulatory role of non-CpG methylation. DNMT3B, a gene encoding a key non-CpG methylation enzyme, is one of the 1,077 genes upregulated in mature oocytes, which may be at least partially responsible for the increased non-CpG methylation as oocytes mature. Non-CpG differentially methylated regions (DMRs) between mature and immature oocytes have multiple binding motifs for transcription factors, some of which bind with DNMT3B and may be important regulators of oocyte maturation through non-CpG methylation. Over 98% of non-CpG DMRs locate in transposable elements, and these DMRs are correlated with expression changes of the nearby genes. Taken together, this data indicates that global non-CpG hypermethylation during oocyte maturation may play an active role in gene expression regulation, potentially through the interaction with transcription factors.

Subcortical maternal complex (SCMC) expression during folliculogenesis is affected by oocyte donor age in sheep

PURPOSE

The age-associated decline in female fertility is largely ascribable to the decrease in oocyte quality. The subcortical maternal complex (SCMC) is a multiprotein complex essential for early embryogenesis and female fertility and functionally conserved across mammals. The present work evaluated expression dynamics of its components during folliculogenesis in relation to maternal age in sheep.

METHODS

The expression of the SCMC components (KHDC3/FILIA, NLRP2, NLRP5/MATER, OOEP/FLOPED, PADI6, TLE6 and ZBED3) was analyzed by real-time PCR in pools of growing oocytes (GO) of different diameters (70-90 μm (S), 90-110 μm (M), or 110-130 μm (L)) derived from non-hormonally treated adult (Ad; age < 4 years), prepubertal (Pr; age 40 days), or aged ewes (age > 6 years).

RESULTS

Specific expression patterns associated with donor age were observed during folliculogenesis for all genes, except ZBED3. In oocytes of adult donors, the synthesis of NLRP2, NLRP5, PADI6, and ZBED3 mRNAs was complete in S GO, while FILIA, TLE6, and OOEP were actively transcribed at this stage. Conversely, Pr GO showed active transcription of all mRNAs, except for ZBED3, during the entire window of oocyte growth. Notably, aged GO showed a completely inverse pattern, with a decrease of NLRP2, TLE6, FILIA, and PADI6 mRNA abundance during the latest stage of oocyte growth (L GO). Interestingly, MATER showed high expression variability, suggesting large inter-oocyte differences.

CONCLUSION

Our study describes the SCMC expression dynamics during sheep oogenesis and reports age-specific patterns that are likely involved in the age-related decline of oocyte quality.

Seed-Stored mRNAs that Are Specifically Associated to Monosomes Are Translationally Regulated during Germination

The life cycle of many organisms includes a quiescent stage, such as bacterial or fungal spores, insect larvae, or plant seeds. Common to these stages is their low water content and high survivability during harsh conditions. Upon rehydration, organisms need to reactivate metabolism and protein synthesis. Plant seeds contain many mRNAs that are transcribed during seed development. Translation of these mRNAs occurs during early seed germination, even before the requirement of transcription. Therefore, stored mRNAs are postulated to be important for germination. How these mRNAs are stored and protected during long-term storage is unknown. The aim of this study was to investigate how mRNAs are stored in dry seeds and whether they are indeed translated during seed germination. We investigated seed polysome profiles and the mRNAs and protein complexes that are associated with these ribosomes in seeds of the model organism Arabidopsis (Arabidopsis thaliana). We showed that most stored mRNAs are associated with monosomes in dry seeds; therefore, we focus on monosomes in this study. Seed ribosome complexes are associated with mRNA-binding proteins, stress granule, and P-body proteins, which suggests regulated packing of seed mRNAs. Interestingly, ∼17% of the mRNAs that are specifically associated with monosomes are translationally up-regulated during seed germination. These mRNAs are transcribed during seed maturation, suggesting a role for this developmental stage in determining the translational fate of mRNAs during early germination.

Differential gene expression between in vivo and in vitro maturation: a comparative study with bovine oocytes derived from the same donor pool

Objective:

In vitro maturation has been shown to influence gene expression in oocytes, but a common shortcoming in reports on the matter has been the use of different donors in each experimental group thus disregarding donor effects. This study aimed to investigate the abundance of mRNA in oocytes matured in vivo and in vitro obtained from the same group of donors.

Methods:

A bovine model was used to assess the relative abundance of specific transcripts in in vitro-matured (IN VITRO-OPU) and in vivo-matured (IN VIVO-OPU) oocytes collected from the same donors by transvaginal ovum pick-up (OPU). Transcript abundance in oocytes from the IN VIVO-OPU group and oocytes matured in vitro but retrieved from different cows slaughtered at a commercial abattoir (IN VITRO-Abattoir group) was also compared. Total RNA was extracted from denuded oocytes and cDNA was produced via reverse transcription using an oligo(dT) primer for relative quantification of eight target transcripts by real-time PCR.

Results:

Oocytes in the IN VITRO-OPU group had lower (p<0.05) abundance of peroxiredoxin 1 (Prdx1), heat shock protein 70.1 (Hsp70.1), growth and differentiation factor 9 (Gdf9), and maternal antigen that embryo requires (Mater) transcripts than the oocytes in the IN VIVO-OPU group, all obtained from the same pool of donor cows. Similar results were seen in the comparisons involving the IN VIVO-OPU and IN VITRO-Abattoir groups (p<0.05).

Conclusion:

In vitro maturation affected the abundance of polyadenylated transcripts in the oocyte cytoplasm when compared to in vivo maturation induced by exogenous hormones in oocytes collected from the same donor pool.

Delay in maternal transcript degradation in ovine embryos derived from low competence oocytes

Oocytes from prepubertal animals have a reduced ability to undergo embryo development and produce viable offspring. The present work used an ovine model consisting of oocytes derived from adult and prepubertal donors to assess the molecular status of oocytes and preimplantation embryos with different developmental competence. The lower potential of oocytes of young donors was confirmed in terms of in vitro developmental capabilities and kinetics. A panel of genes including maternal effect (DPPA3, GDF9, NMP2, ZAR1) and housekeeping genes (ACTB, RPL19, SDHA, YWHAZ, ATP1A1), genes involved in DNA methylation (DNMT1, DNMT3A, DNMT3B), genomic imprinting (IGF2R), pluripotency (NANOG, POU5F1) and cell cycle regulation (CCNB1, CDK1, MELK) was relatively quantified. Temporal analysis during oocyte maturation and preimplantation embryo development evidenced patterns associated with donor age. With a few gene-specific exceptions, the differential model showed a reduced transcript abundance in immature prepubertal oocytes that completely reversed trend after fertilization, when higher mRNA levels were consistently observed in early embryos, indicating a delay in maternal transcript degradation. We propose that the molecular shortage in the prepubertal oocyte may affect its developmental potential and impair the early pathways of maternal mRNA clearance in the embryo. While confirming the different potential of oocytes derived from adult and prepubertal donors, our work showed for the first time a consistent delay in maternal transcript degradation in embryos derived from low competence oocytes that interestingly recalls the delayed developmental kinetics. Such abnormal transcript persistence may hinder further development and represents a novel perspective on the complexity of developmental competence.

Long and small noncoding RNAs during oocyte-to-embryo transition in mammals

Oocyte-to-embryo transition is a process during which an oocyte ovulates, is fertilized, and becomes a developing embryo. It involves the first major genome reprogramming event in life of an organism where gene expression, which gave rise to a differentiated oocyte, is remodeled in order to establish totipotency in blastomeres of an early embryo. This remodeling involves replacement of maternal RNAs with zygotic RNAs through maternal RNA degradation and zygotic genome activation. This review is focused on expression and function of long noncoding RNAs (lncRNAs) and small RNAs during oocyte-to-embryo transition in mammals. LncRNAs are an assorted rapidly evolving collection of RNAs, which have no apparent protein-coding capacity. Their biogenesis is similar to mRNAs including transcriptional control and post-transcriptional processing. Diverse molecular and biological roles were assigned to lncRNAs although most of them probably did not acquire a detectable biological role. Since some lncRNAs serve as precursors for small noncoding regulatory RNAs in RNA silencing pathways, both types of noncoding RNA are reviewed together.

Larval Red Drum (Sciaenops ocellatus) Sublethal Exposure to Weathered Deepwater Horizon Crude Oil: Developmental and Transcriptomic Consequences

The Deepwater Horizon (DWH) incident resulted in extensive oiling of the pelagic zone and shoreline habitats of many commercially important fish species. Exposure to the water-accommodated fraction (WAF) of oil from the spill causes developmental toxicity through cardiac defects in pelagic fish species. However, few studies have evaluated the effects of the oil on near-shore estuarine fish species such as red drum (Sciaenops ocellatus). Following exposure to a certified weathered slick oil (4.74 μg/L ∑PAH₅₀) from the DWH event, significant sublethal impacts were observed ranging from impaired nervous system development [average 17 and 22% reductions in brain and eye area at 48 h postfertilization (hpf), respectively] to abnormal cardiac morphology (100% incidence at 24, 48, and 72 hpf) in red drum larvae. Consistent with the phenotypic responses, significantly differentially expressed transcripts, enriched gene ontology, and altered functions and canonical pathways predicted adverse outcomes in nervous and cardiovascular systems, with more pronounced changes at later larval stages. Our study demonstrated that the WAF of weathered slick oil of DWH caused morphological abnormalities predicted by a suite of advanced bioinformatic tools in early developing red drum and also provided the basis for a better understanding of molecular mechanisms of crude oil toxicity in fish.

Developmental transcriptomic analyses for mechanistic insights into critical pathways involved in embryogenesis of pelagic mahi-mahi (Coryphaena hippurus)

Mahi-mahi (Coryphaena hippurus) is a commercially and ecologically important species of fish occurring in tropical and temperate waters worldwide. Understanding early life events is crucial for predicting effects of environmental stress, which is largely restricted by a lack of genetic resources regarding expression of early developmental genes and regulation of pathways. The need for anchoring developmental stages to transcriptional activities is highlighted by increasing evidence on the impacts of recurrent worldwide oil spills in this sensitive species during early development. By means of high throughput sequencing, we characterized the developmental transcriptome of mahi-mahi at three critical developmental stages, from pharyngula embryonic stage (24 hpf) to 48 hpf yolk-sac larva (transition 1), and to 96 hpf free-swimming larva (transition 2). With comparative analysis by multiple bioinformatic tools, a larger number of significantly altered genes and more diverse gene ontology terms were observed during transition 2 than transition 1. Cellular and tissue development terms were more significantly enriched in transition 1, while metabolism related terms were more enriched in transition 2, indicating a switch progressing from general embryonic development to metabolism during the two transitions. Special focus was given on the most significant common canonical pathways (e.g. calcium signaling, glutamate receptor signaling, cAMP response element-binding protein signaling, cardiac β-adrenergic signaling, etc.) and expression of developmental genes (e.g. collagens, myosin, notch, glutamate metabotropic receptor etc.), which were associated with morphological changes of nervous, muscular, and cardiovascular system. These data will provide an important basis for understanding embryonic development and identifying molecular mechanisms of abnormal development in fish species.

Visualizing the structure of RNA-seq expression data using grade of membership models

Grade of membership models, also known as “admixture models”, “topic models” or “Latent Dirichlet Allocation”, are a generalization of cluster models that allow each sample to have membership in multiple clusters. These models are widely used in population genetics to model admixed individuals who have ancestry from multiple “populations”, and in natural language processing to model documents having words from multiple “topics”. Here we illustrate the potential for these models to cluster samples of RNA-seq gene expression data, measured on either bulk samples or single cells. We also provide methods to help interpret the clusters, by identifying genes that are distinctively expressed in each cluster. By applying these methods to several example RNA-seq applications we demonstrate their utility in identifying and summarizing structure and heterogeneity. Applied to data from the GTEx project on 53 human tissues, the approach highlights similarities among biologically-related tissues and identifies distinctively-expressed genes that recapitulate known biology. Applied to single-cell expression data from mouse preimplantation embryos, the approach highlights both discrete and continuous variation through early embryonic development stages, and highlights genes involved in a variety of relevant processes—from germ cell development, through compaction and morula formation, to the formation of inner cell mass and trophoblast at the blastocyst stage. The methods are implemented in the Bioconductor package CountClust.

Gene expression profile of a biological sample (either from single cells or pooled cells) results from a complex interplay of multiple related biological processes. Consequently, for example, distal tissue samples may share a similar gene expression profile through some common underlying biological processes. Our goal here is to illustrate that grade of membership (GoM) models—an approach widely used in population genetics to cluster admixed individuals who have ancestry from multiple populations—provide an attractive approach for clustering biological samples of RNA sequencing data. The GoM model allows each biological sample to have partial memberships in multiple biologically-distinct clusters, in contrast to traditional clustering methods that partition samples into distinct subgroups. We also provide methods for identifying genes that are distinctively expressed in each cluster to help biologically interpret the results. Applied to a dataset of 53 human tissues, the GoM approach highlights similarities among biologically-related tissues and identifies distinctively-expressed genes that recapitulate known biology. Applied to gene expression data of single cells from mouse preimplantation embryos, the approach highlights both discrete and continuous variation through early embryonic development stages, and genes involved in a variety of relevant processes. Our study highlights the potential of GoM models for elucidating biological structure in RNA-seq gene expression data.

Methylation dynamics during folliculogenesis and early embryo development in sheep

Genome-wide DNA methylation reprogramming occurs during mammalian gametogenesis and early embryogenesis. Post-fertilization demethylation of paternal and maternal genomes is considered to occur by an active and passive mechanism respectively, in most mammals but sheep; in this species no loss of methylation was observed in either pronucleus. Post-fertilization reprogramming relies on methylating and demethylating enzymes and co-factors that are stored during oocyte growth, concurrently with the re-methylation of the oocyte itself. The crucial remodelling of the oocyte epigenetic baggage often overlaps with potential interfering events such as exposure to assisted reproduction technologies or environmental changes. Here, we report a temporal analysis of methylation dynamics during folliculogenesis and early embryo development in sheep. We characterized global DNA methylation and hydroxymethylation by immunofluorescence and relatively quantified the expression of the enzymes and co-factors mainly responsible for their remodelling (DNA methyltransferases (DNMTs), ten-eleven translocation (TET) proteins and methyl-CpG-binding domain (MBD) proteins). Our results illustrate for the first time the patterns of hydroxymethylation during oocyte growth. We observed different patterns of methylation and hydroxymethylation between the two parental pronuclei, suggesting that male pronucleus undergoes active demethylation also in sheep. Finally, we describe gene-specific accumulation dynamics for methylating and demethylating enzymes during oocyte growth and observe patterns of expression associated with developmental competence in a differential model of oocyte potential. Our work contributes to the understanding of the methylation dynamics during folliculogenesis and early embryo development and improves the overall picture of early rearrangements that will originate the embryo epigenome.

Friend or Foe: Epigenetic Regulation of Retrotransposons in Mammalian Oogenesis and Early Development

Epigenetics is the study of phenotypic variation arising from developmental and environmental factors regulating gene transcription at molecular, cellular, and physiological levels. A naturally occurring biological process driven by epigenetics is the egg-to-embryo developmental transition when two fully differentiated adult cells - egg and sperm - revert to an early stem cell type with totipotency but subsequently differentiates into pluripotent embryonic stem cells that give rise to any cell type. Transposable elements (TEs) are active in mammalian oocytes and early embryos, and this activity, albeit counterintuitive because TEs can lead to genomic instability in somatic cells, correlates to successful development. TEs bridge genetic and epigenetic landscapes because TEs are genetic elements whose silencing and de-repression are regulated by epigenetic mechanisms that are sensitive to environmental factors. Ultimately, transposition events can change size, content, and function of mammalian genomes. Thus, TEs act beyond mutagenic agents reshuffling the genomes, and epigenetic regulation of TEs may act as a proximate mechanism by which evolutionary forces increase a species' hidden reserve of epigenetic and phenotypic variability facilitating the adaptation of genomes to their environment.

Cloning of Porcine Pituitary Tumor Transforming Gene 1 and Its Expression in Porcine Oocytes and Embryos

The maternal-to-embryonic transition (MET) is a complex process that occurs during early mammalian embryogenesis and is characterized by activation of the zygotic genome, initiation of embryonic transcription, and replacement of maternal mRNA with embryonic mRNA. The objective of this study was to reveal the temporal expression and localization patterns of PTTG1 during early porcine embryonic development and to establish a relationship between PTTG1 and the MET. To achieve this goal, reverse transcription-polymerase chain reaction (RT-PCR) was performed to clone porcine PTTG1. Subsequently, germinal vesicle (GV)- and metaphase II (MII)-stage oocytes, zygotes, 2-, 4-, and 8-cell-stage embryos, morulas, and blastocysts were produced in vitro and their gene expression was analyzed. The results revealed that the coding sequence of porcine PTTG1 is 609-bp in length and that it encodes a 202-aa polypeptide. Using qRT-PCR, PTTG1 mRNA expression was observed to be maintained at high levels in GV- and MII-stage oocytes. The transcript levels in oocytes were also significantly higher than those in embryos from the zygote to blastocyst stages. Immunohistochemical analyses revealed that porcine PTTG1 was primarily localized to the cytoplasm and partially localized to the nucleus. Furthermore, the PTTG1 protein levels in MII-stage oocytes and zygotes were significantly higher than those in embryos from the 2-cell to blastocyst stage. After fertilization, the level of this protein began to decrease gradually until the blastocyst stage. The results of our study suggest that porcine PTTG1 is a new candidate maternal effect gene (MEG) that may participate in the processes of oocyte maturation and zygotic genome activation during porcine embryogenesis.

Competition between target sites of regulators shapes post-transcriptional gene regulation

Post-transcriptional gene regulation (PTGR) of mRNA turnover, localization and translation is mediated by microRNAs (miRNAs) and RNA-binding proteins (RBPs). These regulators exert their effects by binding to specific sequences within their target mRNAs. Increasing evidence suggests that competition for binding is a fundamental principle of PTGR. Not only can miRNAs be sequestered and neutralized by the targets with which they interact through a process termed 'sponging', but competition between binding sites on different RNAs may also lead to regulatory crosstalk between transcripts. Here, we quantitatively model competition effects under physiological conditions and review the role of endogenous sponges for PTGR in light of the key features that emerge.

Expression of maternally derived KHDC3, NLRP5, OOEP and TLE6 is associated with oocyte developmental competence in the ovine species

BACKGROUND

The sub-cortical maternal complex (SCMC), located in the subcortex of mouse oocytes and preimplantation embryos, is composed of at least four proteins encoded by maternal effect genes: OOEP, NLRP5/MATER, TLE6 and KHDC3/FILIA. The SCMC assembles during oocyte growth and was seen to be essential for murine zygote progression beyond the first embryonic cell divisions; although roles in chromatin reprogramming and embryonic genome activation were hypothesized, the full range of functions of the complex in preimplantation development remains largely unknown.

RESULTS

Here we report the expression of the SCMC genes in ovine oocytes and pre-implantation embryos, describing for the first time its expression in a large mammalian species. We report sheep-specific patterns of expression and a relationship with the oocyte developmental potential in terms of delayed degradation of maternal SCMC transcripts in pre-implantation embryos derived from developmentally incompetent oocytes. In addition, by determining OOEP full length cDNA by Rapid Amplification of cDNA Ends (RACE) we identified two different transcript variants (OOEP1 and OOEP2), both expressed in oocytes and early embryos, but with different somatic tissue distributions. In silico translation showed that 140 aminoacid peptide OOEP1 shares an identity with orthologous proteins ranging from 95% with the bovine to 45% with mouse. Conversely, OOEP2 contains a premature termination codon, thus representing an alternative noncoding transcript and supporting the existence of aberrant splicing during ovine oogenesis.

CONCLUSIONS

These findings confirm the existence of the SCMC in sheep and its key role for the oocyte developmental potential, deepening our understanding on the molecular differences underlying cytoplasmic vs nuclear maturation of the oocytes. Describing differences and overlaps in transcriptome composition between model organisms advance our comprehension of the diversity/uniformity between mammalian species during early embryonic development and provide information on genes that play important regulatory roles in fertility in nonmurine models, including the human.

Global characterization of the oocyte-to-embryo transition in Caenorhabditis elegans uncovers a novel mRNA clearance mechanism

The oocyte-to-embryo transition (OET) is thought to be mainly driven by post-transcriptional gene regulation. However, expression of both RNAs and proteins during the OET has not been comprehensively assayed. Furthermore, specific molecular mechanisms that regulate gene expression during OET are largely unknown. Here, we quantify and analyze transcriptome-wide, expression of mRNAs and thousands of proteins in Caenorhabditis elegans oocytes, 1-cell, and 2-cell embryos. This represents a first comprehensive gene expression atlas during the OET in animals. We discovered a first wave of degradation in which thousands of mRNAs are cleared shortly after fertilization. Sequence analysis revealed a statistically highly significant presence of a polyC motif in the 3' untranslated regions of most of these degraded mRNAs. Transgenic reporter assays demonstrated that this polyC motif is required and sufficient for mRNA degradation after fertilization. We show that orthologs of human polyC-binding protein specifically bind this motif. Our data suggest a mechanism in which the polyC motif and binding partners direct degradation of maternal mRNAs. Our data also indicate that endogenous siRNAs but not miRNAs promote mRNA clearance during the OET.

Normalization of RNA-sequencing data from samples with varying mRNA levels

Methods for normalization of RNA-sequencing gene expression data commonly assume equal total expression between compared samples. In contrast, scenarios of global gene expression shifts are many and increasing. Here we compare the performance of three normalization methods when polyA⁺ RNA content fluctuates significantly during zebrafish early developmental stages. As a benchmark we have used reverse transcription-quantitative PCR. The results show that reads per kilobase per million (RPKM) and trimmed mean of M-values (TMM) normalization systematically leads to biased gene expression estimates. Biological scaling normalization (BSN), designed to handle differences in total expression, showed improved accuracy compared to the two other methods in estimating transcript level dynamics. The results have implications for past and future studies using RNA-sequencing on samples with different levels of total or polyA⁺ RNA.

Proteomic analysis of eggs from Mytilus edulis females differing in mitochondrial DNA transmission mode

Many bivalves have an unusual mechanism of mitochondrial DNA (mtDNA) inheritance called doubly uniparental inheritance (DUI) in which distinctly different genomes are inherited through the female (F genome) and male (M genome) lineages. In fertilized eggs that will develop into male embryos, the sperm mitochondria remain in an aggregation, which is believed to be delivered to the primordial germ cells and passed to the next generation through the sperm. In fertilized eggs that will develop into female embryos, the sperm mitochondria are dispersed throughout the developing embryo and make little if any contribution to the next generation. The frequency of embryos with the aggregated or dispersed mitochondrial type varies among females. Previous models of DUI have predicted that maternal nuclear factors cause molecular differences among unfertilized eggs from females producing embryos with predominantly dispersed or aggregated mitochondria. We test this hypothesis using females of each of the two types from a natural population. We have found small, yet detectable, differences of the predicted type at the proteome level. We also provide evidence that eggs of females giving the dispersed pattern have consistently lower expression for different proteasome subunits than eggs of females giving the aggregated pattern. These results, combined with those of an earlier study in which we used hatchery lines of Mytilus, and with a transcriptomic study in a clam that has the DUI system of mtDNA transmission, reinforce the hypothesis that the ubiquitin-proteasome system plays a key role in the mechanism of DUI and sex determination in bivalves. We also report that eggs of females giving the dispersed pattern have higher expression for arginine kinase and enolase, enzymes involved in energy production, whereas ferritin, which is involved in iron homeostasis, has lower expression. We discuss these results in the context of genetic models for DUI and suggest experimental methods for further understanding the role of these proteins in DUI.

Mouse zygote-specific proteasome assembly chaperone important for maternal-to-zygotic transition

During the maternal-to-zygotic transition (MZT), maternal proteins in oocytes are degraded by the ubiquitin–proteasome system (UPS), and new proteins are synthesized from the zygotic genome. However, the specific mechanisms underlying the UPS at the MZT are not well understood. We identified a molecule named zygote-specific proteasome assembly chaperone (ZPAC) that is specifically expressed in mouse gonads, and expression of ZPAC was transiently increased at the mouse MZT. ZPAC formed a complex with Ump1 and associated with precursor forms of 20S proteasomes. Transcription of ZPAC genes was also under the control of an autoregulatory feedback mechanism for the compensation of reduced proteasome activity similar to Ump1 and 20S proteasome subunit gene expression. Knockdown of ZPAC in early embryos caused a significant reduction of proteasome activity and decrease in Ump1 and mature proteasomes, leading to accumulation of proteins that need to be degraded at the MZT and early developmental arrest. Therefore, a unique proteasome assembly pathway mediated by ZPAC is important for progression of the mouse MZT.

Dynamic alterations in the paternal epigenetic landscape following fertilization

Embryonic development is a complex and dynamic process with frequent changes in gene expression, ultimately leading to cellular differentiation and commitment of various cell lines. These changes are likely preceded by changes to signaling cascades and/or alterations to the epigenetic program in specific cells. The process of epigenetic remodeling begins early in development. In fact, soon after the union of sperm and egg massive epigenetic changes occur across the paternal and maternal epigenetic landscape. The epigenome of these cells includes modifications to the DNA itself, in the form of DNA methylation, as well as nuclear protein content and modification, such as modifications to histones. Sperm chromatin is predominantly packaged by protamines, but following fertilization the sperm pronucleus undergoes remodeling in which maternally derived histones replace protamines, resulting in the relaxation of chromatin and ultimately decondensation of the paternal pronucleus. In addition, active DNA demethylation occurs across the paternal genome prior to the first cell division, effectively erasing many spermatogenesis derived methylation marks. This complex interplay begins the dynamic process by which two haploid cells unite to form a diploid organism. The biology of these events is central to the understanding of sexual reproduction, yet our knowledge regarding the mechanisms involved is extremely limited. This review will explore what is known regarding the post-fertilization epigenetic alterations of the paternal chromatin and the implications suggested by the available literature.

Voltage sensitive phosphoinositide phosphatases of Xenopus: their tissue distribution and voltage dependence

Voltage-sensitive phosphatases (VSPs) are unique proteins in which membrane potential controls enzyme activity. They are comprised of the voltage sensor domain of an ion channel coupled to a lipid phosphatase specific for phosphoinositides, and for ascidian and zebrafish VSPs, the phosphatase activity has been found to be activated by membrane depolarization. The physiological functions of these proteins are unknown, but their expression in testis and embryos suggests a role in fertilization or development. Here we investigate the expression pattern and voltage dependence of VSPs in two frog species, Xenopus laevis and Xenopus tropicalis, that are well suited for experimental studies of these possible functions. X. laevis has two VSP genes (Xl-VSP1 and Xl-VSP2), whereas X. tropicalis has only one gene (Xt-VSP). The highest expression of these genes was observed in testis, ovary, liver, and kidney. Our results show that while Xl-VSP2 activates only at positive membrane potentials outside of the physiological range, Xl-VSP1 and Xt-VSP phosphatase activity is regulated in the voltage range that regulates sperm-egg fusion at fertilization.

The transcriptome of a human polar body accurately reflects its sibling oocyte

Improved methods are needed to reliably and accurately evaluate oocyte quality prior to fertilization and transfer into the woman of human embryos created through in vitro fertilization (IVF). All oocytes that are retrieved and matured in culture are exposed to sperm with little in the way of evaluating the oocyte quality. Furthermore, embryos created through IVF are currently evaluated for developmental potential by morphology, a criterion lacking in quantitation and accuracy. With the recent successes in oocyte vitrification and storage, clear metrics are needed to determine oocyte quality prior to fertilizing. The first polar body (PB) is extruded from the oocyte before fertilization and can be biopsied without damaging the oocyte. Here, we tested the hypothesis that the PB transcriptome is representative of that of the oocyte. Polar body biopsy was performed on metaphase II (MII) oocytes followed by single-cell transcriptome analysis of the oocyte and its sibling PB. Over 12,700 unique mRNAs and miRNAs from the oocyte samples were compared with the 5,431 mRNAs recovered from the sibling PBs (5,256 shared mRNAs or 97%, including miRNAs). The results show that human PBs reflect the oocyte transcript profile and suggests that mRNA detection and quantification through high-throughput quantitative PCR could result in the first molecular diagnostic for gene expression in MII oocytes. This could allow for both oocyte ranking and embryo preferences in IVF applications.

Transcriptome profiling of embryonic development rate in rainbow trout advanced backcross introgression lines

In rainbow trout (Oncorhynchus mykiss) and other fishes, embryonic development rate is an ecologically and evolutionarily important trait that is closely associated with survival and physiological performance later in life. To identify genes differentially regulated in fast and slow-developing embryos of rainbow trout, we examined gene expression across developmental time points in rainbow trout embryos possessing alleles linked to a major quantitative trait loci (QTL) for fast versus slow embryonic development rate. Whole genome expression microarray analyses were conducted using embryos from a fourth generation backcross family, whereby each backcross generation involved the introgression of the fast-developing alleles for a major development rate QTL into a slow-developing clonal line of rainbow trout. Embryos were collected at 15, 19, and 28 days post-fertilization; sex and QTL genotype were determined using molecular markers, and cDNA from 48 embryos were used for microarray analysis. A total of 183 features were identified with significant differences between embryonic development rate genotypes. Genes associated with cell cycle growth, muscle contraction and protein synthesis were expressed significantly higher in embryos with the fast-developing allele (Clearwater) than those with the slow-developing allele (Oregon State University), which may associate with fast growth and early body mass construction in embryo development. Across time points, individuals with the fast-developing QTL allele appeared to have earlier onset of these developmental processes when compared to individuals with the slow development alleles, even as early as 15 days post-fertilization. Differentially expressed candidate genes chosen for linkage mapping were localized primarily to regions outside of the major embryonic development rate QTL, with the exception of a single gene (very low-density lipoprotein receptor precursor).

EGG molecules couple the oocyte-to-embryo transition with cell cycle progression

The oocyte-to-embryo transition is a precisely coordinated process in which an oocyte becomes fertilized and transitions to an embryonic program of events. The molecules involved in this process have not been well studied. Recently, a group of interacting molecules in C. elegans have been described as coordinating the oocyte-to-embryo transition with the advancement of the cell cycle. Genes egg-3, egg-4, and egg-5 represent a small class of regulatory molecules known as protein-tyrosine phosphase-like proteins, which can bind phosphorylated substrates and act as scaffolding molecules or inhibitors. These genes are responsible for coupling the movements and activities of regulatory kinase mbk-2 with advancement of the cell cycle during the oocyte-to-embryo transition.

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.

Aging of Xenopus tropicalis eggs leads to deadenylation of a specific set of maternal mRNAs and loss of developmental potential

As first shown more than 100 years ago, fertilization of an aged (overripe) egg increases the rate of malformations and embryonic loss in several vertebrates, including possibly humans as well. Since the molecular events in aging eggs may be similar in these species, we established in the frog Xenopus tropicalis a defined protocol for delayed fertilization of eggs. A three-hour delayed fertilization led to a dramatic increase in malformation and mortality. Gene expression profiling revealed that 14% of the polyadenylated maternal transcripts were downregulated upon aging. These transcripts were not degraded, but rather deadenylated as shown for specific maternal mRNAs. The affected transcripts are characterized by a relatively short 3′UTR and a paucity of cytoplasmic polyadenylation elements (CPE) and polyadenylation signals (PAS). Furthermore, maternal mRNAs known to be deadenylated during egg maturation as well as after fertilization were preferentially deadenylated in aged eggs. Taken together our analysis of aging eggs reveals that unfertilized eggs are in a dynamic state that was previously not realized. On the one hand deadenylation of transcripts that are typically deadenylated during egg maturation continues and this implies overripeness of the aged egg in the truest sense of the word. On the other hand transcripts that normally are deadenylated after fertilization loose their poly(A) in the aged egg and this implies that the egg awaiting fertilization starts processes that are normally only observed after fertilization. Based on our novel finding we postulate that the imbalance of the polyadenylated maternal transcripts upon egg aging contributes to the loss of developmental potential. Based on this hypothesis the developmental consequences of downregulation of specific transcripts can be analyzed in future.

Detection and quantification of mRNA in single human polar bodies: a minimally invasive test of gene expression during oogenesis

Proteins and mRNA produced in oogenesis support embryonic development until the zygotic transition, 3 days after fertilization. Since polar bodies can be biopsied with little if any harm to the oocyte, we tested the hypothesis that mRNA originating from expression in the meiotic oocyte is present and detectable in a single polar body prior to insemination. Human oocytes were obtained from patients undergoing controlled ovarian hyperstimulation and intracytoplasmic sperm injection. Immature oocytes were cultured overnight and inspected the following day for maturation. Metaphase II oocytes underwent polar body biopsy followed by reverse transcription without RNA isolation. Sibling oocytes were similarly prepared. Complementary DNA from all samples were pre-amplified over 15 cycles for candidate genes using selective primers. Real-time PCR was performed to detect and quantify relative single-cell gene expression. Polar body mRNA was detected in 11 of 12 candidate genes. Transcripts that were present in greater abundance in the oocyte were more likely to be detected in qPCR replicates from single polar bodies. Pre-amplification of cDNA synthesized without RNA isolation can facilitate the quantitative detection of mRNA in single human polar bodies.

KHDC1B is a novel CPEB binding partner specifically expressed in mouse oocytes and early embryos

mRNAs required for meiotic maturation and early embryonic development are stored in growing oocytes. These transcripts are translationally repressed until hormonal cues trigger ovulation. Errors in translation underlie some cases of human infertility and are associated with ovarian germ cell tumors. However, it remains unclear how maternal transcripts are kept quiescent in mammals. This study describes a potential translational regulator, KHDC1B. KHDC1B is a member of a small family of KH-domain containing proteins specific to eutherian mammals. Two family members, KHDC1A and 1B, are highly expressed in oocytes. KHDC1A and 1B bind polyU agarose and form oligomers like other KH-domain proteins. The functions of these proteins were tested by expression in Xenopus embryos. KHDC1A caused cell death, whereas KHDC1B caused cleavage arrest. This arrest phenotype was rescued by coexpression of the mouse translational regulator cytoplasmic polyadenylation binding protein 1 (mCPEB1). Coimmunoprecipitation and coimmunostaining experiments confirmed the functional interaction between KHDC1B and mCPEB1. Finally, KHDC1B levels and binding partners were shown to fluctuate with the cell cycle. KHDC1B, via its interaction with mCEPB1, may regulate translation of mRNA targets required for oocyte maturation.

Does heat stress provoke the loss of a continuous layer of cortical granules beneath the plasma membrane during oocyte maturation?

The objective of the present study was to evaluate the influence of heat stress on bovine oocyte maturation. Both nuclear stage and distribution of cortical granules (CG) were simultaneously evaluated in each oocyte. Oocyte overmaturation under standard conditions of culture was also evaluated. For this purpose, logistic regression procedures were used to evaluate possible effects of factors such as heat stress, overmaturation, replicate, CG distribution and metaphase II (MII) morphology on oocyte maturation. Based on the odds ratio, oocytes on heat stressed (HSO) and overmaturated (OMO) oocyte group were, respectively, 14.5 and 5.4 times more likely to show anomalous MII morphology than those matured under control conditions (CO). The likelihood for an oocyte of showing the CG distribution pattern IV (aging oocyte) was 6.3 and 9.3 times higher for HSO and OMO groups, respectively, than for the CO group. The risk of undergoing anomalous oocyte maturation, considering both nuclear stage and distribution of CG was 17.1 and 18 times greater in oocytes cultured in HSO and OMO groups, respectively, than those in the CO group. In conclusion, heat stress proved to be valuable in aging oocytes. Heat stress advanced age for nuclear and cytoplasmic processes in a similar form to that of oocyte overmaturation.

Gene expression profiles of single human mature oocytes in relation to age

BACKGROUND

The development competence of human oocytes declines with increasing age. The objective of this study was to investigate the effect of age on gene expression profile in mature human oocytes.

METHODS

mRNA was isolated for whole genome gene expression microarray analysis from metaphase II (MII) oocytes donated by IVF or ICSI patients [10 women aged <36 years (younger) and five women aged 37-39 years (both inclusive) (older)] undergoing controlled ovarian stimulation. The oocytes were donated and prepared immediately after recovery from the follicle. RT-PCR on additional four younger and two older oocytes confirmed the array analysis.

RESULTS

On the basis of 15 independent replicates of single MII oocytes, 7470 genes (10 428 transcripts) were identified as present in the MII oocytes. Of these, 342 genes showed a significantly different expression level between the two age groups; notably, genes annotated to be involved in cell cycle regulation, chromosome alignment (e.g. MAD2L1 binding protein), sister chromatid separation (e.g. separase), oxidative stress and ubiquitination. The top signaling network affected by age was 'cell cycle and organism development' (e.g. SMAD2 and activin B1 receptor).

CONCLUSION

There is a substantial difference between younger and older oocytes in the transcriptional level of genes involved in central biological functions of the oocytes, thus providing information on processes that may be associated with the ageing phenomenon and possibly contributing to decreased fertility.

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.

Cell surface changes in the egg at fertilization

An egg changes dramatically at fertilization. These changes include its developmental potential, its physiology, its gene expression profile, and its cell surface. This review highlights the changes in the cell surface of the egg that occur in response to sperm. These changes include modifications to the extracellular matrix, to the plasma membrane, and to the secretory vesicles whose contents direct many of these events. In some species, these changes occur within minutes of fertilization, and are sufficiently dramatic so that they can be seen by the light microscope. Many of these morphological changes were documented in remarkable detail early in the 1900 s by Ernest Everett Just. A recent conference in honor of his contributions stimulated this overview. We highlight the major cell surface changes that occur in echinoderms, one of Just's preferred research organisms.

Characterization of maternal antigen that embryos require (MATER/NLRP5) gene and protein in pig somatic tissues and germ cells

Maternal effect genes produce mRNA or proteins that accumulate in the egg during oogenesis and control the developmental program until embryonic genome activation takes place. NLRP5 (NLR family, Pyrin domain containing 5), also called MATER (Maternal Antigen That Embryos Require) is one of the genes required for normal early embryonic development, although its precise function remains to be elucidated. The aim of the present study was to analyze the NLRP5 gene expression pattern and protein distribution in somatic tissues and germ cells in the pig. Reverse transcription was performed on mRNA from germinal vescicle (GV) oocytes and total RNA from spermatozoa and tissues from different organs. The transcript for NLRP5 gene was identified only in ovaries and oocytes. The presence of NLRP5 protein was detected only in ovaries by western blot analysis and immunohistochemistry.