Using the histone H2a transcript as an endogenous standard to study relative transcript abundance during bovine early development

Heat-Stress Impacts on Developing Bovine Oocytes: Unraveling Epigenetic Changes, Oxidative Stress, and Developmental Resilience

Extreme temperature during summer may lead to heat stress in cattle and compromise their productivity. It also poses detrimental impacts on the developmental capacity of bovine budding oocytes, which halt their fertility. To mitigate the adverse effects of heat stress, it is necessary to investigate the mechanisms through which it affects the developmental capacity of oocytes. The primary goal of this study was to investigate the impact of heat stress on the epigenetic modifications in bovine oocytes and embryos, as well as on oocyte developmental capacity, reactive oxygen species, mitochondrial membrane potential, apoptosis, transzonal projections, and gene expression levels. Our results showed that heat stress significantly reduced the expression levels of the epigenetic modifications from histone H1, histone H2A, histone H2B, histone H4, DNA methylation, and DNA hydroxymethylation at all stages of the oocyte and embryo. Similarly, heat stress significantly reduced cleavage rate, blastocyst rate, oocyte mitochondrial-membrane potential level, adenosine-triphosphate (ATP) level, mitochondrial DNA copy number, and transzonal projection level. It was also found that heat stress affected mitochondrial distribution in oocytes and significantly increased reactive oxygen species, apoptosis levels and mitochondrial autophagy levels. Our findings suggest that heat stress significantly impacts the expression levels of genes related to oocyte developmental ability, the cytoskeleton, mitochondrial function, and epigenetic modification, lowering their competence during the summer season.

Mitochondrial-related consequences of heat stress exposure during bovine oocyte maturation persist in early embryo development

Hyperthermia during estrus has direct consequences on the maturing oocyte that carries over to the resultant embryo to compromise its ability to continue in development. Because early embryonic development is reliant upon maternal transcripts and other ooplasmic components, we examined impact of heat stress on bovine oocyte transcripts using microarray. Oocytes were matured at 38.5ºC for 24 h or 41.0ºC for the first 12 h of in vitro maturation; 38.5ºC thereafter. Transcriptome profile was performed on total (adenylated + deadenylated) RNA and polyadenylated mRNA populations. Heat stress exposure altered the abundance of several transcripts important for mitochondrial function. The extent to which transcript differences are coincident with functional changes was evaluated by examining reactive oxygen species, ATP content, and glutathione levels. Mitochondrial reactive oxygen species levels were increased by 6 h exposure to 41.0ºC while cytoplasmic levels were reduced compared to controls (P < 0.0001). Exposure to 41.0ºC for 12 h increased total and reduced glutathione levels in oocytes at 12 h but reduced them by 24 h (time × temperature P < 0.001). ATP content was higher in heat-stressed oocytes at 24 h (P < 0.0001). Heat-induced increases in ATP content of matured oocytes persisted in early cleavage-stage embryos (8- to 16-cell embryos; P < 0.05) but were no longer apparent in blastocysts (P > 0.05). Collectively, results indicate that direct exposure of maturing oocytes to heat stress may alter oocyte mitochondrial processes/function, which is inherited by the early embryo after fertilization.

Accumulation of Chromatin Remodelling Enzyme and Histone Transcripts in Bovine Oocytes

During growth, the oocyte accumulates mRNAs that will be required in the later stages of oogenesis and early embryogenesis until the activation of the embryonic genome. Each of these developmental stages is controlled by multiple regulatory mechanisms that ensure proper protein production. Thus mRNAs are stabilized, stored, recruited, polyadenylated, translated and/or degraded over a period of several days. As a consequence, understanding the biological significance of changes in the abundance of transcripts during oocyte growth and differentiation is rather complex. Nevertheless the availability of transcriptomic platforms applicable to scarce samples such as oocytes has generated large amounts of data that depict the transcriptome of oocytes under different conditions. Despite several technical constrains related to protein determination in oocytes that still limit the possibility to verify certain hypothesis, it is now possible to use mRNA levels to start building plausible scenarios. To start deciphering the changes in the level of specific mRNAs involved in chromatin remodelling, we have performed a meta-analysis of existing microarray datasets from germinal vesicle (GV) stage bovine oocytes during the final stages of oocyte differentiation. We then analysed the expression profiles of histone and histone-remodelling enzyme mRNAs and correlated these with the major histone modifications known to occur at the same period, based on data available in the literature. We believe that this approach could reveal the function of specific enzymes in the oocyte. In turn, this information will be useful in future studies, which final ambitious goal is to decipher the 'oocyte-specific histone code'.

PGRMC1 participates in late events of bovine granulosa cells mitosis and oocyte meiosis

Progesterone Receptor Membrane Component 1 (PGRMC1) is expressed in both oocyte and ovarian somatic cells, where it is found in multiple cellular sub-compartments including the mitotic spindle apparatus. PGRMC1 localization in the maturing bovine oocytes mirrors its localization in mitotic cells, suggesting a possible common action in mitosis and meiosis. To test the hypothesis that altering PGRMC1 activity leads to similar defects in mitosis and meiosis, PGRMC1 function was perturbed in cultured bovine granulosa cells (bGC) and maturing oocytes and the effect on mitotic and meiotic progression assessed. RNA interference-mediated PGRMC1 silencing in bGC significantly reduced cell proliferation, with a concomitant increase in the percentage of cells arrested at G2/M phase, which is consistent with an arrested or prolonged M-phase. This observation was confirmed by time-lapse imaging that revealed defects in late karyokinesis. In agreement with a role during late mitotic events, a direct interaction between PGRMC1 and Aurora Kinase B (AURKB) was observed in the central spindle at of dividing cells. Similarly, treatment with the PGRMC1 inhibitor AG205 or PGRMC1 silencing in the oocyte impaired completion of meiosis I. Specifically the ability of the oocyte to extrude the first polar body was significantly impaired while meiotic figures aberration and chromatin scattering within the ooplasm increased. Finally, analysis of PGRMC1 and AURKB localization in AG205-treated oocytes confirmed an altered localization of both proteins when meiotic errors occur. The present findings demonstrate that PGRMC1 participates in late events of both mammalian mitosis and oocyte meiosis, consistent with PGRMC1's localization at the mid-zone and mid-body of the mitotic and meiotic spindle.

Supplementation of bovine embryo culture medium with L-arginine improves embryo quality via nitric oxide production

Nitric oxide (NO) is a cell-signaling molecule that regulates a variety of molecular pathways. We investigated the role of NO during preimplantation embryonic development by blocking its production with an inhibitor or supplementing in vitro bovine embryo cultures with its natural precursor, L-arginine, over different periods. Endpoints evaluated included blastocyst rates, development kinetics, and embryo quality. Supplementation with the NO synthase inhibitor N-Nitro-L-arginine-methyl ester (L-NAME) from Days 1 to 8 of culture decreased blastocyst (P < 0.05) and hatching (P < 0.05) rates. When added from Days 1 to 8, 50 mM L-arginine decreased blastocyst rates (P < 0.001); in contrast, when added from Days 5 to 8, 1 mM L-arginine improved embryo hatching rates (P < 0.05) and quality (P < 0.05) as well as increased POU5F1 gene expression (P < 0.05) as compared to the untreated control. Moreover, NO levels in the medium during this culture period positively correlated with the increased embryo hatching rates and quality (P < 0.05). These data suggest exerts its positive effects during the transition from morula to blastocyst stage, and that supplementing the embryo culture medium with L-arginine favors preimplantation development of bovine embryos.

Paternal breed effects on expression of IGF-II, BAK1 and BCL2-L1 in bovine preimplantation embryos

The effects of the paternal breed on early embryo and later pre- and postnatal development are well documented. Several recent studies have suggested that such paternal effects may be mediated by the paternally induced epigenetic modifications during early embryogenesis. The objective of this study was to investigate the effects of the paternal breed on the early embryonic development and relative expression of the maternally imprinted gene, IGF-II, and the apoptosis-related genes BAK1 and BCL2-L1 in in vitro produced (IVP) bovine embryos derived from two unrelated paternal breeds (Holstein and Brown Swiss). The degree of correlation of IGF-II expression pattern with embryo developmental competence and apoptosis-related genes was also investigated. The relative abundance of IGF-II, BCL2-L1 and BAK1 transcripts in day 8 embryos was measured by quantitative reverse-transcription polymerase chain reaction using the comparative Cp method. Our data revealed that the paternal breed did not influence cleavage rate, blastocyst rate and relative abundance of IGF-II, BAK1 and BCL2-L1 in day 8 blastocysts (P > 0.05). Nevertheless, IGF-II expression levels were highly correlated with embryonic developmental competence (r = 0.66, P < 0.1), relative expression of BCL2-L1 (r = 0.72, P < 0.05) and ratio of BCL2-L1/BAK1 (r = 0.78, P < 0.05). In conclusion, our data show that IGF-II, BCL2-L1 and BAK1 expression is not related to the chosen combination of paternal breed, but that IGF-II expression is correlated with embryonic viability and apoptosis-related gene expression.

Reference gene selection for gene expression analysis of oocytes collected from dairy cattle and buffaloes during winter and summer

Oocytes from dairy cattle and buffaloes have severely compromised developmental competence during summer. While analysis of gene expression is a powerful technique for understanding the factors affecting developmental hindrance in oocytes, analysis by real-time reverse transcription PCR (RT-PCR) relies on the correct normalization by reference genes showing stable expression. Furthermore, several studies have found that genes commonly used as reference standards do not behave as expected depending on cell type and experimental design. Hence, it is recommended to evaluate expression stability of candidate reference genes for a specific experimental condition before employing them as internal controls. In acknowledgment of the importance of seasonal effects on oocyte gene expression, the aim of this study was to evaluate the stability of expression levels of ten well-known reference genes (ACTB, GAPDH, GUSB, HIST1H2AG, HPRT1, PPIA, RPL15, SDHA, TBP and YWHAZ) using oocytes collected from different categories of dairy cattle and buffaloes during winter and summer. A normalization factor was provided for cattle (RPL15, PPIA and GUSB) and buffaloes (YWHAZ, GUSB and GAPDH) based on the expression of the three most stable reference genes in each species. Normalization of non-reference target genes by these reference genes was shown to be considerably different from normalization by less stable reference genes, further highlighting the need for careful selection of internal controls. Therefore, due to the high variability of reference genes among experimental groups, we conclude that data normalized by internal controls can be misleading and should be compared to not normalized data or to data normalized by an external control in order to better interpret the biological relevance of gene expression analysis.

Discovery, identification and sequence analysis of RNAs selected for very short or long poly A tail in immature bovine oocytes

A major challenge in applying genomics to oocyte physiology is that many RNAs are present but will not be translated into proteins, making it difficult to draw conclusions from RNAseq and array data. Oocyte maturation and early embryo development rely on maternal storage of specific RNAs with a short poly(A) tail, which must be elongated for translation. To resolve the role of key genes during that period, we aimed to characterize both extremes of mRNA: deadenylated RNA and long polyA tails mRNA population in immature bovine oocytes. Using magnetic beads coupled to oligodT, we isolated deadenylated (A-, 20-50 adenosines) from polyadenylated (A+, up to 200 adenosines) RNAs. After transcriptomic analysis, we observed that A+ candidates are associated with short-term processes required for immediate cell survival (translation or protein transport) or meiotic resumption, while several A- candidates are involved in processes (chromatin modification, gene transcription and post-transcriptional modifications) that will be extremely important in the development of the early embryo. In addition to a list of candidates probably translated early or late, sequence analysis revealed that cytoplasmic polyadenylation element (CPE) and U(3)GU(3) were enriched in A- sequences. Moreover, a motif associated with polyadenylation signals (MAPS, U(5)CU(2)) appeared to be enriched in 3'untranslated regions (UTR) with CPE or U(3)GU(3) sequences in bovine but also in zebrafish and Xenopus tropicalis. To further validate our methodology, we measured specific tail length of known candidates (AURKA, PTTG1, H2A1) but also determined the poly(A) tail length of other candidate RNAs (H3F3A, H1FOO, DAZAP2, ATF1, ATF2, KAT5, DAZL, ELAVL2). In conclusion, we have reported a methodology to isolate deadenylated from polyadenylated RNAs in samples with small total RNA quantities such as mammals. Moreover, we identified deadenylated RNAs in bovine oocytes that may be stored for the long-term process of early embryo development and described a conserved motif enriched in the 3'UTR of deadenylated RNAs.

Expression of PLIN2 and PLIN3 during oocyte maturation and early embryo development in cattle

In vitro-produced embryos store high lipid content in cytoplasmic lipid droplets (LD), and reduction or removal of LD has been demonstrated to improve freeze-thaw viability. The Perilipin Adipophilin Tail-interacting Protein of 47 kD (PAT) family of proteins is involved in the formation and regulation of LD in many cell types, but their presence has not been addressed either in cattle oocytes or preimplantation embryos. Therefore, this study aimed to detect the expression of PAT family transcripts (Perilipin-2 [PLIN2] and Perilipin-3 [PLIN3]) in immature and in vitro-matured (IVM) oocytes, and in in vitro-produced embryos at the stages of two to four cells, eight to 16 cells, morulae (MO), and blastocyst (BL). The expression of PLIN3 was downregulated in response to IVM, and PLIN2 was comparatively more expressed than PLIN3 in IVM oocytes (P < 0.001). During the early stages of embryo development, PLIN2 expression reached its peak at the MO stage (P < 0.001) and decreased again at the BL stage. In contrast, PLIN3 was expressed in low levels during the earliest stages of development, slightly upregulated at the MO stage (P < 0.05), and greatly increased its expression at the BL stage (15-fold; P < 0.001). PLIN3 was comparatively more expressed than PLIN2 during embryo culture in most stages analyzed (P < 0.05), except in eight- to 16-cell embryos. These results indicate that PLIN2 might be involved in the maintenance of lipid stocks necessary to support embryo development after fertilization of IVM oocytes. Also, we hypothesize that PLIN3 is the main PAT protein responsible for stabilization of LD formed in consequence of the acute lipid load seen during embryo development. We confirmed the presence of both PLIN2 and PLIN3 proteins in BL at Day 7 using immunocytochemistry: these PAT proteins colocalized with LD stained with BODIPY. PLIN3 seemed to be more ubiquitously spread out in the cytoplasm than PLIN2, consistent with the pattern seen in adipocytes. These findings suggest that both elderly (bigger) and newly formed (smaller) LD, positive for PLIN2 and PLIN3 respectively, coexist in blastocysts. To our knowledge this is the first report showing that transcripts of the PAT family are present in cattle oocytes and embryos.

Analysis of microRNAs and their precursors in bovine early embryonic development

In animals, the maternal-to-embryonic transition (MET) occurs in the first days of early development and involves the degradation of maternal transcripts that have been stored during oogenesis. Moreover, precise and specific control mechanisms govern the adequate synchronization of the MET events to promote the activation of the embryonic genome. These mechanisms are not well understood, but it is believed that microRNAs (miRNAs) could be one of the mechanisms involved. After a microarray screening study, we analysed the expression of specific miRNA during oocyte maturation and early embryo development until preimplantation stages. Two differentially expressed candidates were selected for further analysis. Mature and precursor forms of miR-21 and miR-130a were quantified by qRT-PCR in pools of 20 oocytes at GV (germinal vesicle), GV breakdown and metaphase II stages as well as in pools of embryos at the 2-cell, 4-cell, 8-cell and blastocyst stages. The results showed a linear increase during the 1-8 cell stage for the mature forms of miR-130a and miR-21 (P < 0.05 and P < 0.003, respectively) and for the precursor form of miR-130a (P < 0.002). To see if this increase was due to minor transcriptional activity, 2-cell embryos were exposed to α-amanitin for 30-34 h. Results showed a significant decrease in miR-21, pre-miR-21, miR-130a and SRFS3 in α-amanitin-treated embryos (P < 0.05). Considering the potential regulatory role of these miRNA, the bovine genome was screened to identify putative targets with a 3'UTR exact seed match. This study suggests that miRNAs could be important players in the MET, as expression profiles suggest a potential regulation role during early development steps.

Impact of heat stress exposure during meiotic maturation on oocyte, surrounding cumulus cell, and embryo RNA populations

To determine if reductions in developmental competence related to heat stress exposure were correlated with perturbations in certain RNA populations, poly(A) RNA, total RNA, RNA size distribution, and the abundance of transcripts (cyclin B1, GDF9, BMP15, poly(A) polymerase, HSP70, 18S & 28S rRNA) were examined in oocytes matured at 38.5 or 41 C. Performing in vitro fertilization resulted in embryos for examining RNA. Relative to germinal vesicle-stage oocytes, total amount of poly(A) RNA decreased similarly in oocytes matured at 38.5 or 41 C. Total RNA did not change during meiotic maturation or up through the 4 to 8-cell stage of embryonic development. Blastocyst-stage embryos had more total RNA; those originating from heat-stressed oocytes had more than those from nonheat-stressed oocytes. Oocytes and 4 to 8-cell embryos had similar RIN values and ratios for rRNA, 18S/fast region, and 18S/inter region. Values obtained for blastocyst-stage embryos were similar to those obtained for cumulus cell RNA, which did not change during maturation. Culture at 41 C for the first 12 h of meiotic maturation had no impact on RNA size distribution or transcripts examined from oocytes, surrounding cumulus or resultant 4 to 8-cell embryos. Interestingly, however, RNA from blastocysts originating from heat-stressed oocytes had lower 18S/fast region and 18S/inter region ratios compared to other developmental stages and cumulus cells. Although biological significance of these RNA changes is unclear, differences at the molecular level in embryos from heat-stressed oocytes emphasize the importance of minimizing stress exposure during meiotic maturation, if the intent is to obtain developmentally-competent embryos.

Method to isolate polyribosomal mRNA from scarce samples such as mammalian oocytes and early embryos

Background

Although the transcriptome of minute quantities of cells can be profiled using nucleic acid amplification techniques, it remains difficult to distinguish between active and stored messenger RNA. Transcript storage occurs at specific stages of gametogenesis and is particularly important in oogenesis as stored maternal mRNA is used to sustain de novo protein synthesis during the early developmental stages until the embryonic genome gets activated. In many cases, stored mRNA can be several times more abundant than mRNA ready for translation. In order to identify active mRNA in bovine oocytes, we sought to develop a method of isolating very small amounts of polyribosome mRNA.

Results

The proposed method is based on mixing the extracted oocyte cytoplasm with a preparation of polyribosomes obtained from a non-homologous source (Drosophila) and using sucrose density gradient ultracentrifugation to separate the polyribosomes. It involves cross-linking the non-homologous polyribosomes and neutralizing the cross-linking agent. Using this method, we show that certain stages of oocyte maturation coincide with changes in the abundance of polyribosomal mRNA but not total RNA or poly(A). We also show that the abundance of selected sequences matched changes in the corresponding protein levels.

Conclusions

We report here the successful use of a method to profile mRNA present in the polyribosomal fraction obtained from as little as 75 mammalian oocytes. Polyribosomal mRNA fractionation thus provides a new tool for studying gametogenesis and early development with better representation of the underlying physiological status.

Housekeeping gene transcript abundance in bovine fertilized and cloned embryos

The objective of this study was to compare housekeeping gene expression levels, relative to total mRNA, across different stages of bovine preimplantation development in embryos generated by IVF and somatic cell nuclear transfer (SCNT). We first analyzed the levels of total RNA recovered from different stages of preimplantation development. A similar RNA level was observed from oocytes to 16-cell stage embryos with a significant increase at morula and blastocyst stages. Then we used an absolute mRNA determination method that accounts for the RNA level in the embryo by quantifying copies of transcripts normalized to loaded cDNA amount. The number of housekeeping genes mRNA copies per nanogram of cDNA was compared among samples obtained from different stages of preimplantation IVF-derived embryos. None of the genes analyzed (GAPDH, PPIA, ACTB, RPL15, GUSB, and Histone H2A.2) maintained constant levels throughout preimplantation development, indicating that they are not suitable for normalizing gene expression across developmental stages. We then compared expression of housekeeping genes between IVF and SCNT embryos at different embryonic stages. We found different levels of transcript abundance between IVF and SCNT embryos for GAPDH, RPL15, GUSB, and ACTB. On the other hand, Histone H2A.2 and PPIA were similar between IVF and SCNT embryos at each stage analyzed, although they varied across stages as previously mentioned.

Microarray analysis of gene expression during early development: a cautionary overview

The rise of the 'omics' technologies started nearly a decade ago and, among them, transcriptomics has been used successfully to contrast gene expression in mammalian oocytes and early embryos. The scarcity of biological material that early developmental stages provide is the prime reason why the field of transcriptomics is becoming more and more popular with reproductive biologists. The potential to amplify scarce mRNA samples and generate the necessary amounts of starting material enables the relative measurement of RNA abundance of thousands of candidates simultaneously. So far, microarrays have been the most commonly used high-throughput method in this field. Microarray platforms can be found in a wide variety of formats, from cDNA collections to long or short oligo probe sets. These platforms generate large amounts of data that require the integration of comparative RNA abundance values in the physiological context of early development for their full benefit to be appreciated. Unfortunately, significant discrepancies between datasets suggest that direct comparison between studies is difficult and often not possible. We have investigated the sample-handling steps leading to the generation of microarray data produced from prehatching embryo samples and have identified key steps that significantly impact the downstream results. This review provides a discussion on the best methods for the preparation of samples from early embryos for microarray analysis and focuses on the challenges that impede dataset comparisons from different platforms and the reasons why methodological benchmarking performed using somatic cells may not apply to the atypical nature of prehatching development.

Expression of messenger RNAs for insulin-like growth factors and their receptors in bovine fetuses at early gestation from embryos produced in vivo or in vitro

The objective of this study was to determine the effects of in vitro embryo production on physical development and levels of expression of mRNAs for insulin-like growth factor (IGF) ligands (IGF1, IGF2), their receptors (IGF1R, IGF2R), and IGF binding protein-2 (IGFBP2) in bovine fetuses during early gestation. In vivo embryos were recovered from superovulated Holstein cows. For production of embryos in vitro, Holstein oocytes were matured, fertilized, and subsequently cultured in M199 with 10% serum to 168 hpi. On Day 70 of gestation, fetuses (in vivo, n = 14; in vitro, n = 13) were recovered, serum samples collected, and physical measurements recorded. Semi-quantitative RT-PCR assays were used to determine the levels of expression of mRNAs for IGF1, IGF2, IGF1R, and IGF2R in fetal liver and skeletal muscle. Western blots were used to assess levels of IGFBP2 in fetal serum. Fetal body weight did not differ with treatment; however, production of embryos in vitro was associated with decreased crown-nose length and a tendency for increased paired kidney weight, which became significant when expressed on a per bodyweight basis. There was no effect of treatment on levels of IGFBP2 in fetal serum. Levels of IGF1 mRNA in fetal liver were decreased (P < 0.001) in the in vitro group. Levels of IGF2R mRNA in both liver and skeletal muscle were also decreased (P < 0.01) in fetuses from the in vitro group. In summary, fetuses at Day 70 of gestation from embryos produced in vitro had shortened crown-nose length and increased kidney weight on a per bodyweight basis, as well as decreased expression of mRNAs for IGF1 in liver and IGF2R in both liver and skeletal muscle, compared with fetuses from embryos produced in vivo. In conclusion, in vitro embryo culture was associated with subtle changes in fetal development as well as altered expression of both imprinted and non-imprinted genes.

Providing a stable methodological basis for comparing transcript abundance of developing embryos using microarrays

High throughput methods deliver large amount of data serving to describe the physiological treatment that is being studied. In the case of microarrays, there would be a clear benefit to integrate the published data sets. However, the numerous methodological discrepancies between microarray platforms make this comparison impossible. This incompatibility is magnified when considering the peculiar context of transcript management in early embryogenesis. The total RNA content is known to profoundly fluctuate during development. In addition, the mRNA population is subjected to poly(A) tail shortening and elongating events, a characteristic of stored and recruited messengers. These intrinsic factors need to be considered when interpreting any transcript abundance profiles during early development. As a consequence, many methodological details affect microarray platform performances and prevent compatibility. In an effort to maximize our microarray platform performance, we determined the various sources of variation for every one of the main steps leading to the production of microarray data. The five main steps involved in sample preparation were evaluated, as well as conditions for post-hybridization validation by qRT-PCR. These determinations were essential for the implementation of standardized procedures for our Research Network but they can also provide insight into the compatibility issues that the microarray community is now facing.

The dynamics of gene products fluctuation during bovine pre-hatching development

Early embryonic development, spanning fertilization to blastocyst hatching, is a very dynamic developmental window that is characterized, especially in large mammals, by a period of transcriptional incompetence that ends during the maternal to embryonic transition (MET). Prior to the MET, the first cell cycles are supported by stored RNA and proteins pools accumulated during oogenesis. Therefore, RNA and protein content are different between developmental stages. It is also known that the stability of the stored mRNA and the mechanisms for translation recruitment are partly controlled by the length of the poly(A) tail. To date, little is known about RNA and protein content fluctuations during the pre-hatching period. In this report we present measurements of total RNA, mRNA, poly(A) bearing mRNA and protein contents, as well as estimations of the proportions of both mRNA fractions to total RNA contents within these developmental stages. We found that while the ontogenic profiles of the different transcript contents were expected, their amounts were considerably lower than the reported values. Additionally, low 28S rRNA abundance and a tendency for diminishing protein content prior to the MET, suggest a limited potential for ribosomal turnover and translation. We consider the overall fluctuations in RNA and protein contents to be reference points that are essential for downstream interpretation of gene expression data across stages whether it be through candidates or high throughput approaches.

Gene expression during minor genome activation in preimplantation bovine development

The main goal of this study was to identify mRNA transcripts whose content increases during bovine minor embryonic genome activation. We compared the gene expression profile of the bovine 4-cell-stage embryo and MII oocyte using the technique of suppression subtractive hybridization. Differentially expressed amplicons were subcloned, and 60 of them were sequenced. The resulting DNA sequences were compared with GenBank databases using BLAST search. The expression of five differentially expressed genes with an apparent function in cell cycle progression, chromatin remodeling, and splicing or translation initiation was further characterized by a real-time RT-PCR. Centromere protein F, 350/400ka (CENPF), and splicing factor arginine/serine-rich 3 (SRFS3) show an increase in mRNA content during the 2- to 4-cell and late 8-cell stages. For the high mobility group nucleosomal binding domain 2 (HMGN2), the level of mRNA increases in 2- to 4-cell and morula embryos. The transcription of splicing factor SRFS3 is alpha-amanitin sensitive both during 4-cell and late 8-cell stages. The transcription of CENPF and HMGN2 is alpha-amanitin sensitive only at late 8-cell stage and morula, respectively. SRFS3 represents the first described gene with an important function in preimplantation development, which is also expressed during bovine minor genome activation, and it is alpha-amanitin sensitive during this period. All described genes can play an important role in the preimplantation development of bovine embryos.

The effect of different zwitterionic buffers and PBS used for out-of-incubator procedures during standard in vitro embryo production on development, morphology and gene expression of bovine embryos

The effect of the zwitterionic buffers HEPES, TES and MOPS and of PBS used for out-of-incubator procedures during standard in vitro embryo production on bovine oocytes and embryo development, morphology and on the expression patterns of eight selected genes: Fgf-4, Lama1, Ube2a, Gsta4, Il6, Sod1, Prss11 and Hspb1, was evaluated. All buffers were prepared at a concentration of 10 mM in TALP medium, with the exception of PBS. The total time of oocyte/embryo exposure to each buffer was approximately 41 min. The cleavage rates and number of embryos that developed to > or =8 cells at day 4 were no different among the buffers tested, however, more blastocysts developed at day 7, 8 and 9 in HEPES and MOPS treatments than in PBS and TES (P<0.05). No difference between buffers in total and apoptotic cell number was found. Except for Hspb1 and Ube2a genes, the levels of expression of the six remaining transcripts were higher in in vivo than in in vitro embryos irrespective of buffer used (P<0.05). In addition, higher expression of Hspb1 and lower expression of Ube2a and Lama1 were observed in PBS and TES than in MOPS and HEPES treatments (P<0.05). Expression of Fgf-4 and Gsta4 in the in vitro embryos was lower in PBS than in the remaining three buffers (P<0.05) and the level of expression of the Il6 gene was not affected by any buffer tested but was lower in in vitro than in in vivo derived embryos. Expression of both Sod1 and Prss11 genes in MOPS were at the level of the in vivo embryos. These results showed that the choice of buffer and short exposure time of approximately 41 min, affects mRNA expression of in vitro produced bovine embryos.

In vitro and in vivo derived porcine embryos possess similar, but not identical, patterns of Oct4, Nanog, and Sox2 mRNA expression during cleavage development

In vitro culture conditions stress the cleavage stage mammalian embryo and can contribute to reduced developmental potential of cultured embryos. One process that may be altered during embryo culture is the establishment and maintenance of pluripotency. Pluripotency is largely controlled by three genes: Oct4, Nanog, and Sox2. The objective of this study was to determine the expression pattern of Oct4, Nanog, and Sox2 in cleavage stage porcine embryos obtained in vivo or by in vitro fertilization and parthenogenetic activation. We used quantitative, real time PCR to assess the relative amount of each transcript in cleavage stage embryos. We found that Oct4 was transiently activated at the 2-cell stage (P-value <0.05) while Nanog and Sox2 were activated at the 4-cell stage (P-value <0.05) in in vitro embryos. Embryos derived in vivo showed a similar but not identical pattern of expression of Nanog mRNA been in highest abundance both at the 4 cell and the blastocyst stage. The activation observed at the 4-cell stage for Nanog and Sox2 was shown to be RNA polymerase II dependent (P-value <0.05). This study showed that Oct4, Nanog, and Sox2 possess similar, but not identical, patterns of expression between in vitro and in vivo derived porcine embryos. The difference between the amount of transcripts may reflect the reduced developmental potential observed in in vitro cultured embryos.

Genomic RNA profiling and the programme controlling preimplantation mammalian development

Preimplantation development shifts from a maternal to embryonic programme rapidly after fertilization. Although the majority of oogenetic products are lost during the maternal to embryonic transition (MET), several do survive this interval to contribute directly to supporting preimplantation development. Embryonic genome activation (EGA) is characterized by the transient expression of several genes that are necessary for MET, and while EGA represents the first major wave of gene expression, a second mid-preimplantation wave of transcription that supports development to the blastocyst stage has been discovered. The application of genomic approaches has greatly assisted in the discovery of stage specific gene expression patterns and the challenge now is to largely define gene function and regulation during preimplantation development. The basic mechanisms controlling compaction, lineage specification and blastocyst formation are defined. The requirement for embryo culture has revealed plasticity in the developmental programme that may exceed the adaptive capacity of the embryo and has fostered important research directions aimed at alleviating culture-induced changes in embryonic programming. New levels of regulation are emerging and greater insight into the roles played by RNA-binding proteins and miRNAs is required. All of this research is relevant due to the necessity to produce healthy preimplantation embryos for embryo transfer, to ensure that assisted reproductive technologies are applied in the most efficient and safest way possible.

Challenges of functional genomics applied to farm animal gametes and pre-hatching embryos

The genomes of many commercially important farm animals have already been or are in the process of being decrypted. The genomic era is generating an important wave of downstream developments and derived disciplines are also progressing at a very fast pace. The post-genomic era is already ongoing as exemplified by the introduction of new concepts such as phenomics and functional genomics. These new fields are complementary but do not necessarily target similar applications even though they are often used to refer to one another. In an attempt to categorize the fields according to their respective potential applications, a brief comparative description of phenomics and functional genomics has been put together. However, the focus of this paper is mainly directed toward the introduction of functional genomics specifically applied to the study of the molecular mechanisms underlying gamete and early mammalian developments. Many aspects of the peculiar nature of these cells are introducing numerous methodological challenges to the applicability of functional genomics to unravel their molecular physiology. This is particularly true for transcriptomic studies and it is currently of high relevance for the field of reproductive biology to take into consideration these technical hurdles before tackling the implementation of this technology on a large scale. Nonetheless, functional genomics should prove to be up to the expectations in providing sound information to better understand the fascinating window spanning gamete development that leads to the first weeks of life.

Differential regulation of abundance and deadenylation of maternal transcripts during bovine oocyte maturation in vitro and in vivo

Background

In bovine maturing oocytes and cleavage stage embryos, gene expression is mostly controlled at the post-transcriptional level, through degradation and deadenylation/polyadenylation. We have investigated how post transcriptional control of maternal transcripts was affected during in vitro and in vivo maturation, as a model of differential developmental competence.

Results

Using real time PCR, we have analyzed variation of maternal transcripts, in terms of abundance and polyadenylation, during in vitro or in vivo oocyte maturation and in vitro embryo development. Four genes are characterized here for the first time in bovine: ring finger protein 18 (RNF18) and breast cancer anti-estrogen resistance 4 (BCAR4), whose oocyte preferential expression was not previously reported in any species, as well as Maternal embryonic leucine zipper kinase (MELK) and STELLA. We included three known oocyte marker genes (Maternal antigen that embryos require (MATER), Zygote arrest 1 (ZAR1), NACHT, leucine rich repeat and PYD containing 9 (NALP9)). In addition, we selected transcripts previously identified as differentially regulated during maturation, peroxiredoxin 1 and 2 (PRDX1, PRDX2), inhibitor of DNA binding 2 and 3 (ID2, ID3), cyclin B1 (CCNB1), cell division cycle 2 (CDC2), as well as Aurora A (AURKA). Most transcripts underwent a moderate degradation during maturation. But they displayed sharply contrasted deadenylation patterns that account for variations observed previously by DNA array and correlated with the presence of a putative cytoplasmic polyadenylation element in their 3' untranslated region. Similar variations in abundance and polyadenylation status were observed during in vitro maturation or in vivo maturation, except for PRDX1, that appears as a marker of in vivo maturation. Throughout in vitro development, oocyte restricted transcripts were progressively degraded until the morula stage, except for MELK ; and the corresponding genes remained silent after major embryonic genome activation.

Conclusion

Altogether, our data emphasize the extent of post-transcriptional regulation during oocyte maturation. They do not evidence a general alteration of this phenomenon after in vitro maturation as compared to in vivo maturation, but indicate that some individual messenger RNA can be affected.