The effects of alpha-amanitin and cycloheximide on nuclear progression, protein synthesis, and phosphorylation during bovine oocyte maturation in vitro

Approaches to oocyte meiotic arrest in vitro and impact on oocyte developmental competence

Oocytes are maintained in a state of meiotic arrest following the first meiotic division until ovulation is triggered. Within the antral follicle, meiotic arrest is actively suppressed in a process facilitated by the cyclic nucleotides cGMP and cAMP. If removed from this inhibitory follicular environment and cultured in vitro, mammalian oocytes undergo spontaneous meiotic resumption in the absence of the usual stimulatory follicular stimuli, leading to asynchronicity with oocyte cytoplasmic maturation and lower developmental competence. For more than 50 years, pharmacological agents have been used to attenuate oocyte germinal vesicle (GV) breakdown in vitro. Agents which increase intra-oocyte cAMP or prevent its degradation have been predominantly used, however agents such as kinase and protein synthesis inhibitors have also been trialled. Twenty years of research demonstrates that maintaining GV arrest for a period before in vitro maturation (IVM) improves oocyte developmental competence, and is likely attributed to maintenance of bidirectional communication with cumulus cells leading to improved oocyte metabolic function. However, outcomes are influenced by various factors including the mode of action of the modulators, dose, treatment duration, species, and the degree of hormonal priming of the oocyte donor. Cyclic GMP and/or cAMP modulation in a prematuration step (called pre-IVM) prior to IVM has shown the greatest consistency in improving oocyte developmental competence, whereas kinase and protein synthesis inhibitors have proven less effective at improving IVM outcomes. Such pre-IVM approaches have shown potential to alter current use of artificial reproductive technologies in medical and veterinary practice.

Cruciform DNA in mouse growing oocytes: Its dynamics and its relationship with DNA transcription

Cruciform DNA is a causing factor of genome instability and chromosomal translocation, however, most studies about cruciform DNA in mammalian cells were based on palindromic sequences containing plasmids and reports about endogenous cruciform DNA are rare. In this study we observed the dynamics of endogenous cruciform DNA in mouse growing oocytes using immunofluorescence labeling method. We found cruciform DNA foci exist in transcription active growing oocytes but not in transcription inactive fully grown oocytes and colocalized with Parp1 but not with DNA damage marker γH2A.X. By analyzing the Genotype-Tissue Expression data, we found cruciform DNA-mediated chromosomal translocation in human spermatocytes is associated with the specific DNA transcription in testis. When inhibiting the transcription with α-amanitin in mouse oocytes, we found oocyte cruciform DNA foci decreased significantly. In summary, we observed the endogenous cruciform DNA in growing oocytes and our results showed that the cruciform DNA formation is transcription-dependent.

Short term temperature elevation during IVM affects embryo yield and alters gene expression pattern in oocytes, cumulus cells and blastocysts in cattle

Heat stress causes subfertility in cattle by inducing alterations in steroidogenic capacity, follicular function and ovulation defects, which eventually negatively affect oocyte quality and embryo survival. Here, the effects of short, moderate temperature elevation during IVM, on embryo yield, and on the expression of various genes was evaluated. In 8 replicates, cumulus oocyte complexes (COCs) were matured for 24 h at 39 °C (controls n = 605) or at 41 °C from hour 2 to hour 8 of IVM (treated, n = 912), fertilized, and presumptive zygotes were cultured for 9 days at 39 °C. Cleavage and embryo formation rates were evaluated 48 h post insemination and on days 7, 8, 9 respectively. Cumulus cells, oocytes and blastocysts from 5 replicates were snap frozen for the relative expression analysis of genes related to metabolism, thermal and oxidative stress response, apoptosis, and placentation. In treated group, cleavage and embryo formation rates were statistically significantly lower compared with the control (cleavage 86.7% vs 74.2%; blastocysts: day 7, 29.9% vs 19.7%, day 8, 34.2% vs 22.9% and day 9 35.9% vs 24.5%). Relative mRNA abundance of three genes in cumulus cells (HSP90AA1, CPT1B, G6PD) and three genes in blastocysts (DNMT3A, PLAC8, GPX1) indicated significantly different expression between groups (p < 0.05)., The expression of G6PD, SOD2, GXP1 in oocytes and PTGS2 in blastocysts tended to differ among groups (0.05<p < 0.08). Heat stress altered (p < 0.05) the correlation of expression between HSPs and other genes in oocytes (G6PD, GPX1, CCNB1), cumulus cells (LDH, CCNB1) and blastocysts (AKR1B1, PLAC8). These results imply that exposure of oocytes to elevated temperature, even for only 6 h, disrupts the developmental competence of the oocytes, suppresses blastocyst yield and significantly alters the coordinated pattern of gene expressions.

Methods to Discover and Evaluate Proteasome Small Molecule Stimulators

Protein accumulation has been identified as a characteristic of many degenerative conditions, such as neurodegenerative diseases and aging. In most cases, these conditions also present with diminished protein degradation. The ubiquitin-proteasome system (UPS) is responsible for the degradation of the majority of proteins in cells; however, the activity of the proteasome is reduced in these disease states, contributing to the accumulation of toxic protein. It has been hypothesized that proteasome activity, both ubiquitin-dependent and -independent, can be chemically stimulated to reduce the load of protein in diseased cells. Several methods exist to identify and characterize stimulators of proteasome activity. In this review, we detail the ways in which protease activity can be enhanced and analyze the biochemical and cellular methods of identifying stimulators of both the ubiquitin-dependent and -independent proteasome activities.

Mechanisms by which in vitro meiotic arrest and sexual maturity improve developmental potential of mouse oocytes

To study the relationship between chromatin condensation, gene transcription and developmental competence during oocyte maturation and to explore the mechanisms by which meiotic arrest maintenance (MAM) and sexual maturity improve oocyte competence, we examined effects of MAM with roscovitine or db-cAMP on chromatin condensation, gene transcription and developmental potential of NSN or SN oocytes from prepubertal or adult mice. MAM with roscovitine improved the developmental competence and global gene transcription of prepubertal NSN (prep-NSN) and adult-SN oocytes while having no effect on those of prep-SN oocytes. MAM with db-cAMP facilitated neither development nor transcription in any type of oocytes. MAM with either roscovitine or db-cAMP promoted chromatin condensation of prep-NSN oocytes. MAM with roscovitine promoted gene transcription and chromatin condensation simultaneously through inhibiting cyclin-dependent kinase (CDK) 5 and 2, respectively. The results suggested that MAM with roscovitine improved oocyte competence by promoting gene transcription via inhibiting CDK5. Oocyte cytoplasmic maturation is correlated with gene transcription but not with chromatin condensation. The difference in developmental competence between prepubertal NSN and SN oocytes and between prepubertal and adult SN oocytes was because while the former had not, the latter had completed or acquired the ability for transcription of important genes.

Meiotic arrest with roscovitine and follicular fluid improves cytoplasmic maturation of porcine oocytes by promoting chromatin de-condensation and gene transcription

The developmental capacity of in vitro matured oocytes is inferior to that of the in vivo matured ones due to insufficient cytoplasmic maturation. Although great efforts were made to accomplish better cytoplasmic maturation by meiotic arrest maintenance (MAM) before in vitro maturation (IVM), limited progress has been achieved in various species. This study showed that MAM of porcine oocytes was better achieved with roscovitine than with dibutyryl cyclic adenosine monophosphate (db-cAMP) or 3-isobutyl-1-methylxanthine. Oocyte developmental competence after IVM was significantly improved following MAM in 199 + FF medium (TCM-199 containing 10% porcine follicular fluid and 25 µM roscovitine) to a level even higher than that in control oocytes matured without pre-MAM. Observations on other markers further confirmed the positive effects of MAM in 199 + FF on oocyte cytoplasmic maturation. During MAM culture in 199 + FF, re-decondensation (RDC) of condensed chromatin occurred, and transcription of genes beneficial to cytoplasmic maturation was evident in some of the oocytes with surrounded nucleoli (SN). However, MAM with db-cAMP neither induced RDC nor improved oocyte developmental potential. Together, the results suggest that MAM in the presence of FF and roscovitine improved the developmental competence of porcine oocytes by promoting a pre-GVBD chromatin de-condensation and expression of beneficial genes.

P-TEFb Kinase Activity Is Essential for Global Transcription, Resumption of Meiosis and Embryonic Genome Activation in Pig

Positive transcription elongation factor b (P-TEFb) is a RNA polymerase II carboxyl-terminal domain (Pol II CTD) kinase that phosphorylates Ser2 of the CTD and promotes the elongation phase of transcription. Despite the fact that P-TEFb has role in many cellular processes, the role of this kinase complex remains to be understood in mammalian early developmental events. In this study, using immunocytochemical analyses, we found that the P-TEFb components, CDK9, Cyclin T1 and Cyclin T2 were localized to nuclear speckles, as well as in nucleolar-like bodies in pig germinal vesicle oocytes. Using nascent RNA labeling and small molecule inhibitors, we showed that inhibition of CDK9 activity abolished the transcription of GV oocytes globally. Moreover, using fluorescence in situ hybridization, in absence of CDK9 kinase activity the production of ribosomal RNAs was impaired. We also presented the evidences indicating that P-TEFb kinase activity is essential for resumption of oocyte meiosis and embryo development. Treatment with CDK9 inhibitors resulted in germinal vesicle arrest in maturing oocytes in vitro. Inhibition of CDK9 kinase activity did not interfere with in vitro fertilization and pronuclear formation. However, when in vitro produced zygotes were treated with CDK9 inhibitors, their development beyond the 4-cell stage was impaired. In these embryos, inhibition of CDK9 abrogated global transcriptional activity and rRNA production. Collectively, our data suggested that P-TEFb kinase activity is crucial for oocyte maturation, embryo development and regulation of RNA transcription in pig.

RNA Polymerase II Inhibitor, α-Amanitin, Affects Gene Expression for Gap Junctions and Metabolic Capabilities of Cumulus Cells, but Not Oocyte, during in vitro Mouse Oocyte Maturation

A specific inhibitor of RNA polymerase II, α-amanitin is broadly used to block transcriptional activities in cells. Previous studies showed that α-amanitin affects in vitro maturation of cumulus-oocyte-complex (COC). In this study, we evaluated the target of α-amanitin, and whether it affects oocytes or cumulus cells (CCs), or both. We treated α-amanitin with different time period during in vitro culture of denuded oocytes (DOs) or COCs in comparison, and observed the changes in morphology and maturation status. Although DOs did not show any change in morphology and maturation rates with α-amanitin treatment, oocytes from COCs were arrested at metaphase I (MI) stage and CCs were more scattered than control groups. To discover causes of meiotic arrest and scattering of CCs, we focused on changes of cumulus expansion, gap junctions, and cellular metabolism which to be the important factors for the successful in vitro maturation of COCs. Expression of genes for cumulus expansion markers (Ptx3, Has2, and Tnfaip6) and gap junctional proteins (Gja1, Gja4, and Gjc1) decreased in α-amanitin-treated CCs. However, these changes were not observed in oocytes. In addition, expression of genes related to metabolism (Prps1, Rpe, Rpia, Taldo1, and Tkt) decreased in α -amanitin-treated CCs but not in oocytes. Therefore, we concluded that the transcriptional activities of CCs for supporting suitable transcripts, especially for its metabolic activities and formation of gap junctions among CCs as well as with oocytes, are important for oocytes maturation in COCs.

Expression of INHβA and INHβB proteins in porcine oocytes cultured in vitro is dependent on the follicle size

The current study aimed to investigate differential expression of inhibin βA (INHβA) and inhibin βB (INHβB) in porcine oocytes before or after in vitro maturation (IVM) isolated from follicles of various sizes. Porcine oocytes isolated from large, medium and small follicles (40 from each) were used to study the INHβA and INHβB protein expression pattern using western blot analysis before or after 44 h of oocyte IVM. An increased expression of INHβA was found in oocytes collected from large and medium follicles compared with small follicles before or after IVM (P < 0.001, P < 0.05, respectively). Similarly, higher INHβB levels were observed in oocytes recovered from large follicles compared with small (P < 0.01). As INHβA and INHβB are expressed in both porcine follicular somatic cells and oocytes, it can be assumed that these transforming growth factor beta (TGFβ) superfamily factors are involved in the regulation of molecular bi-directional pathways during follicle and oocyte development, and can be recognized as markers of follicle and oocyte maturation. Moreover, the current study clearly demonstrated that inhibin expression is substantially associated with porcine follicle growth and development.

Oocyte maturation and fertilization in marine nemertean worms: using similar sorts of signaling pathways as in mammals, but often with differing results

In marine worms belonging to the phylum Nemertea, oocyte maturation and fertilization are regulated by the same general kinds of signals that control such processes in mammals. However, unlike mammalian oocytes that develop within follicles, nemertean oocytes characteristically lack a surrounding sheath of follicle cells and often respond differently to maturation-related cues than do mammalian oocytes. For example, elevators of cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP) levels promote the resumption of meiotic maturation (=germinal vesicle breakdown, GVBD) in nemertean oocytes, whereas increasing intraoocytic cAMP and cGMP typically blocks GVBD in mammals. Similarly, AMP-activated kinase (AMPK) signaling keeps nemertean oocytes from maturing, but in mouse oocytes, AMPK activation triggers GVBD. In addition, protein kinase C (PKC) activity is required for seawater-induced GVBD in nemerteans, whereas some PKCs have been shown to inhibit GVBD in mammals. Furthermore, although fertilization causes both types of oocytes to reorganize their endoplasmic reticulum and generate calcium oscillations that can involve soluble sperm factor activity and inositol 1,4,5-trisphosphate signaling, some discrepancies in the spatiotemporal patterns and underlying mechanisms of fertilization are also evident in nemerteans versus mammals. Thus, to characterize differences and similarities in gamete biology more fully, aspects of oocyte maturation and fertilization in marine nemertean worms are reviewed and briefly compared with related findings that have been published for mammalian oocytes. In addition, possible causes of the alternative responses displayed by oocytes in these two animal groups are addressed.

Bovine oocyte meiotic inhibition before in vitro maturation and its value to in vitro embryo production: does it improve developmental competence?

The efficiency of bovine in vitro embryo production has remained low despite extensive effort to understand the effects of culture conditions, media composition and supplementation. As bovine oocytes resume meiosis spontaneously when cultured, it was hypothesized that preventing meiosis in vitro before in vitro maturation (IVM) and in vitro fertilization (IVF) would allow more oocytes to acquire developmental competence. This article reviews some of the factors involved in meiotic arrest as well as the effects of meiotic inhibition before IVM on bovine oocytes developmental competence following IVF. Follicular components and cAMP-elevating agents can delay or inhibit meiosis in various proportions of oocytes; however, few studies have examined their effects on development following IVM and IVF because they are not practical (follicular components) or have a transient effect on meiosis (cAMP-elevating agents). Protein synthesis or phosphorylation inhibition prevented meiosis in high percentages of oocytes; however, these non-specific inhibitions led to lower developmental competence compared with non-arrested oocytes. Maturation promoting factor (MPF) inhibition with specific inhibitors has been examined in several studies. Despite faster maturation following removal from inhibition and some structural damage to the oocytes, MPF inhibition generally led to blastocyst rates similar to control, non-arrested oocytes. Future work will involve evaluating the effects on arrested oocytes of molecules that can improve developmental competence in non-arrested oocytes. It is also anticipated that new IVM systems that take into consideration new knowledge of the mechanisms involved in the control of meiosis will be developed. Moreover, global gene expression analysis studies will also provide clues to the culture conditions required for optimal expression of developmental competence.

Sequential analysis of global gene expression profiles in immature and in vitro matured bovine oocytes: potential molecular markers of oocyte maturation

Background

Without intensive selection, the majority of bovine oocytes submitted to in vitro embryo production (IVP) fail to develop to the blastocyst stage. This is attributed partly to their maturation status and competences. Using the Affymetrix GeneChip Bovine Genome Array, global mRNA expression analysis of immature (GV) and in vitro matured (IVM) bovine oocytes was carried out to characterize the transcriptome of bovine oocytes and then use a variety of approaches to determine whether the observed transcriptional changes during IVM was real or an artifact of the techniques used during analysis.

Results

8489 transcripts were detected across the two oocyte groups, of which ~25.0% (2117 transcripts) were differentially expressed (p < 0.001); corresponding to 589 over-expressed and 1528 under-expressed transcripts in the IVM oocytes compared to their immature counterparts. Over expression of transcripts by IVM oocytes is particularly interesting, therefore, a variety of approaches were employed to determine whether the observed transcriptional changes during IVM were real or an artifact of the techniques used during analysis, including the analysis of transcript abundance in oocytes in vitro matured in the presence of α-amanitin. Subsets of the differentially expressed genes were also validated by quantitative real-time PCR (qPCR) and the gene expression data was classified according to gene ontology and pathway enrichment. Numerous cell cycle linked (CDC2, CDK5, CDK8, HSPA2, MAPK14, TXNL4B), molecular transport (STX5, STX17, SEC22A, SEC22B), and differentiation (NACA) related genes were found to be among the several over-expressed transcripts in GV oocytes compared to the matured counterparts, while ANXA1, PLAU, STC1and LUM were among the over-expressed genes after oocyte maturation.

Conclusion

Using sequential experiments, we have shown and confirmed transcriptional changes during oocyte maturation. This dataset provides a unique reference resource for studies concerned with the molecular mechanisms controlling oocyte meiotic maturation in cattle, addresses the existing conflicting issue of transcription during meiotic maturation and contributes to the global goal of improving assisted reproductive technology.

Cumulus cell gene expression following the LH surge in bovine preovulatory follicles: potential early markers of oocyte competence

Cumulus cells (CCs) are essential for oocytes to reach full development competency and become fertilized. Many major functional properties of CCs are triggered by gonadotropins and governed by the oocyte. Consequently, cumulus may reflect oocyte quality and is often used for oocyte selection. The most visible function of CCs is their ability for rapid extracellular matrix expansion after the LH surge. Although unexplained, LH induces the final maturation and improves oocyte quality. To study the LH signaling and gene expression cascade patterns close to the germinal vesicle breakdown, bovine CCs collected at 2 h before and 6 h after the LH surge were hybridized to a custom-made microarray to better understand the LH genomic action and find differentially expressed genes associated with the LH-induced oocyte final maturation. Functional genomic analysis of the 141 overexpressed and 161 underexpressed clones was performed according to their molecular functions, gene networks, and cell compartments. Following real-time PCR validation of our gene lists, some interesting pathways associated with the LH genomic action on CCs and their possible roles in oocyte final maturation, ovulation, and fertilization are discussed. A list of early potential markers of oocyte competency in vivo and in vitro is thereafter suggested. These early biomarkers are a preamble to understand the LH molecular pathways that trigger the final oocyte competence acquisition process in bovine.

Analysis of actinomycin D treated cattle oocytes and their use for somatic cell nuclear transfer

The present work aimed to evaluate the transcription and replication inhibitor, actinomycin D, for oocyte chemical enucleation. Cattle oocytes matured in vitro were treated with actinomycin D according to the following treatments: T1, control; T2=1.0 microg/ml for 16 h; T3=1.0 microg/ml for 14 h; T4=2.5 microg/ml for 14 h; T5=5.0 microg/ml for 14 h. The oocytes were denuded and activated during 24-26 h of maturation. Oocytes were fixed to determine the maturation status and for chromosome morphology evaluation. Furthermore, oocytes treated with actinomycin D were used for somatic cell nuclear transfer (SCNT). Parthenogenetic and SCNT embryos were fixed to evaluate the percentage of apoptotic nuclei by the TUNEL assay. The maturation (T1=90.4%; T2=82.3%; T3=79.1%; T4=83.4%; T5=74.7%), cleavage (T1=68.9%; T2=46.0%; T3=49.7%; T4=33.4%; T5=29.3%) and blastocyst rate at D8 (T1=41.1%; T2=1.8%; T3=1.3%; T4=0.9%; T5=0.0%) after actinomycin D treatment were significantly different. There was a significant chromosome uncoiling when treated with greater concentrations (2.5 and 5.0 microg/ml). After SCNT, the cleavage rate (61.3%) was similar to the actinomycin D-treated control group (61.3%) and less than the non-treated control (70.2%), although the blastocyst rate was greater in the SCNT group (11.8%) comparing with the treated control (3.6%) and less than the untreated control (38.0%). Treated parthenogenetic embryos had more apoptotic cells than the parthenogenetic controls (24.2% compared with 4.8%). However, the SCNT group using treated cytoplasts was similar from the SCNT control (9.3 compared with 13.0%). Actinomycin D treatment was efficient in blocking embryonic development. Moreover, it was possible to obtain reconstructed embryos that possess an apoptotic cell index indistinguishable from controls.

The role of transcription in EGF- and FSH-mediated oocyte maturation in vitro

Understanding mechanisms responsible for meiotic resumption in mammalian oocytes is critical for the identification of strategies to enhance developmental competence of in vitro-matured oocytes. Improvement of in vitro oocyte maturation systems is dependent on a better understanding of mechanisms that regulate oocyte maturation both in vivo and in vitro as well as on the identification of methods to manipulate the meiotic progression of oocytes matured in vitro in a physiological manner. The purpose of this review is two-fold: first, to examine the mechanisms that underlie the acquisition of oocyte developmental competence and regulation of oocyte maturation in vivo and in vitro; second, to present data examining the role of transcription in mediating the ability of EGF and FSH to induce oocyte maturation in vitro. Results presented support the conclusions that (1) EGF-induced oocyte maturation does not require nascent gene transcription in both mice and domestic cats; (2) FSH requires gene transcription to induce oocyte maturation in both species; (3) EGF must be present in the maturation medium to optimize the effectiveness of FSH to promote oocyte maturation; (4) the mechanism used by FSH to induce oocyte maturation in vitro appears to predominate over that used by EGF when both EGF and FSH are present in maturation medium used for either murine or feline cumulus oocyte complexes.

Identification of candidate mRNAs associated with gonadotropin-induced maturation of murine cumulus oocyte complexes using serial analysis of gene expression

In cultured cumulus oocyte complexes (COC), FSH induces gene transcription required for germinal vesicle breakdown (GVBD). Experiments were performed to determine the critical period when gene transcription is required for GVBD and to identify candidate mRNAs involved. Experiment I: murine COC were cultured 4 h in the presence of FSH with 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole (DRB) added at different intervals after the start of culture. COC cultured with FSH underwent GVBD (82 ± 7%). When DRB was added at 0, 5, or 10 min after culture initiation, oocyte maturation was blocked (17 ± 7, 14 ± 6, and 21 ± 6% GVBD, respectively). When DRB was added after 15, 20, or 30 min, progressively more COC underwent GVBD (37 ± 6, 39 ± 6, and 66 ± 6%, respectively). The critical period of transcription required for GVBD occurred between 15 and 30 min after culture initiation. Experiment II: COC were cultured for 25 min in the presence (plusDRB) or absence (minusDRB) of DRB. SAGE libraries were generated from COC RNA of each treatment group. A total of 48,431 and 45,367 tags were sequenced for the plusDRB and minusDRB libraries, respectively. Criteria used to identify transcripts of interest included a total tag count of at least 10 across both libraries and a threefold or greater difference in expression between libraries. Using these criteria, 39 and 27 transcripts were identified as differentially expressed at the P ≤ 0.01 and P ≤ 0.001 levels, respectively. Differentially expressed transcripts were classed into major categories that included cell growth, development, and regulation of gene expression. Differentially expressed transcripts represent candidates potentially involved in regulating maturation of murine COC.

Germinal vesicle chromatin configurations of bovine oocytes

A common feature in the configuration of germinal vesicle (GV) chromatin in most species is that diffuse chromatin condenses into a perinucleolar ring during follicular growth; however, no such ring was observed in goat oocytes. Reports on whether bovine GV chromatin condenses into a perinucleolar ring are controversial. Besides, it is not known whether the perinucleolar ring in an oocyte represents a step toward final maturation or atresia. Changes in GV chromatin configurations during growth and maturation of bovine oocytes were studied using a new method that allows a clearer visualization of both the nucleolus and the chromatin after Hoechst and chromomycin A(3) staining. On the basis of the degree of condensation and distribution, the GV chromatin of bovine oocytes were classified into five configurations: NSN with diffuse chromatin in the whole nuclear area, N with condensed netlike chromatin, C with clumped chromatin, SN with clumped chromatin surrounding the nucleoli, and F with floccular chromatin near the nucleoli and near the nuclear envelope. Most of the oocytes were at the NSN stage in the <1.4-mm follicles, but the NSN pattern disappeared completely in follicles larger than 1.5mm. The SN pattern began to emerge in 1.5-mm follicles, and the number of SN oocytes increased while the number of oocytes with N and C configurations decreased with follicular growth. During maturation in vivo, while the number of N, C, and SN oocytes decreased, that of the F oocytes increased and reached maximum at 51h post prostaglandin injection. After that, the number of F oocytes decreased significantly because of germinal vesicle breakdown (GVBD). During maturation in vitro, GV chromatin configurations changed in a similar manner as during maturation in vivo. Fewer oocytes were at N, C, and SN stages, but more were at F and GVBD stages in the atretic than in the healthy follicles. Serum starvation slowed the F-GVBD transition of the in vitro maturing oocytes. More oocytes were of the SN or C configuration when ovaries were transported at 45-40 degrees C than at 35-30 degrees C. Most of the heated oocytes were blocked at the SN stage during in vitro maturation. It is concluded that (i) bovine GV chromatin condenses into a perinucleolar ring during follicular growth; (ii) bovine oocytes were synchronized at the F stage before GVBD; (iii) oocyte GV chromatin configurations were affected by serum starvation, high temperature, and follicular atresia.

MATER protein expression and intracellular localization throughout folliculogenesis and preimplantation embryo development in the bovine

Background

Mater (Maternal Antigen that Embryos Require), also known as Nalp5 (NACHT, leucine rich repeat and PYD containing 5), is an oocyte-specific maternal effect gene required for early embryonic development beyond the two-cell stage in mouse. We previously characterized the bovine orthologue MATER as an oocyte marker gene in cattle, and this gene was recently assigned to a QTL region for reproductive traits.

Results

Here we have analyzed gene expression during folliculogenesis and preimplantation embryo development. In situ hybridization and immunohistochemistry on bovine ovarian section revealed that both the transcript and protein are restricted to the oocyte from primary follicles onwards, and accumulate in the oocyte cytoplasm during follicle growth. In immature oocytes, cytoplasmic, and more precisely cytosolic localization of MATER was confirmed by immunohistochemistry coupled with confocal microscopy and immunogold electron microscopy. By real-time PCR, MATER messenger RNA was observed to decrease strongly during maturation, and progressively during the embryo cleavage stages; it was hardly detected in morulae and blastocysts. The protein persisted after fertilization up until the blastocyst stage, and was mostly degraded after hatching. A similar predominantly cytoplasmic localization was observed in blastomeres from embryos up to 8-cells, with an apparent concentration near the nuclear membrane.

Conclusion

Altogether, these expression patterns are consistent with bovine MATER protein being an oocyte specific maternal effect factor as in mouse.

Developmental competence of bovine oocytes after specific inhibition of MPF kinase activity: Effect of estradiol supplementation and follicle size

In the bovine, the concentration of 17beta-estradiol (E2) in the follicular fluid of the dominant follicle is high, indicating a possible role of E2 on the cytoplasmic maturation that occurs before the LH surge. The aim of this study was to investigate the role of E2 on the developmental competence of bovine oocytes originating from different sized follicles and temporarily maintained at the germinal vesicle stage with roscovitine (ROS). First, the efficiency of ROS to inhibit germinal vesicle breakdown (GVBD) in oocytes harvested from small (3-4 mm diameter) and medium (5-8 mm diameter) sized follicles was demonstrated. Next, the effect of E2 during temporary inhibition of GVBD by ROS on the subsequent nuclear maturation was evaluated. Oocytes from small and medium sized follicles were cultured in the presence of ROS, FSH and with or without E2 for 24 h. After this period, oocytes were cultured for another 24 h with FSH but without ROS and E2, after which the nuclear stages and the developmental competence of oocytes were assessed. In conclusion, it is demonstrated that exposure to E2, during temporary inhibition of the GVBD with ROS, affected neither nuclear nor cytoplasmic maturation of oocytes originating from small and medium sized follicles. It might be that in vivo, the increase of E2 during follicle growth is more related to selection of the dominant follicle than to the cytoplamsic maturation of the oocyte as such.

Genes Preferentially Expressed in Bovine Oocytes Revealed by Subtractive and Suppressive Hybridization1

To isolate bovine oocyte marker genes, we performed suppressive and subtractive hybridization between oocytes and somatic tissues (i.e., intestine, lung, muscle, and cumulus cells). The subtracted library was characterized by sequencing 185 random clone inserts, representing 146 nonredundant genes. After Blast analysis within GenBank, 64% could be identified, 21% were homologous to unannotated expressed sequence tag (EST) or genomic sequences, and 15% were novel. Of 768 clone inserts submitted for differential screening by macroarray hybridization, 83% displayed a fourfold overexpression in the oocyte. The 40 most preferential nonredundant ESTs were submitted to GenBank analysis. Several well-known oocyte-specific genes were represented, including growth differentiation factor 9, bone morphogenetic protein 15, or the zona pellucida glycoprotein genes. Other ESTs were not identified. We investigated the expression profile of several candidates in the oocyte and a panel of gonadal and somatic tissues by reverse transcription-polymerase chain reaction. B-cell translocation gene 4, cullin 1, MCF.2 transforming sequence, a locus similar to snail soma ferritin, and three unidentified genes were, indeed, preferentially expressed in the oocyte, even though most were also highly expressed in testis. The transcripts were degraded throughout preimplantation development and were not compensated for by embryonic transcription after the morula stage. These profiles suggest a role in gametogenesis, fertilization, or early embryonic development.

Influence of antral follicle size on oocyte characteristics and embryo development in the bovine

The developmental competence of bovine oocytes isolated from antral follicles of different sizes was assessed in three European laboratories (Belgium, UCL; Denmark, DIAS; France, INRA). Using the same protocol for in vitro production of embryos, the oocytes isolated from follicles with a diameter > or = 6 mm always gave a higher blastocyst rate than oocytes from follicles < 4 mm (UCL: 42% versus 14%, DIAS: 50% versus 35%, INRA: 39% versus 22%; P < 0.05). Blastocyst cell number was not affected by follicle size. Several parameters were investigated for these oocytes. The energy metabolism of cumulus-oocyte-complexes and of denuded oocytes was assessed by the oxygen and pyruvate uptake and by lactate release both at the beginning and the end of the maturation. No effect of follicle size could be detected but lactate release increased after maturation. The global profile of transcripts, the pattern of protein neosynthesis and the kinetics of meiosis resumption were not affected by follicle size. The developmental kinetics of derived embryos was also analysed. Whatever the follicle size, viable embryos had a shorter first and third embryonic cell cycle. Among the viable embryos, the size of the follicle interfered with the fourth cell cycle duration. A higher percentage of blastocysts issued from large follicle presented a short fourth cell cycle (9h) (35% versus 6%; P < 0.05). Beside, blastocysts derived from small follicles had a delayed cavitation and expansion. Thereby, a higher developmental competence for oocytes from follicle > or = 6 mm versus < 4 mm was demonstrated in three laboratories although no differences could be displayed directly at the oocyte level.

Expression of Cyclin B1 Messenger RNA Isoforms and Initiation of Cytoplasmic Polyadenylation in the Bovine Oocyte1

Oocytes can synthesize and store maternal mRNA in an inactive translational state until the start of in vitro maturation. Cytoplasmic polyadenylation, driven by 3'-untranslated region (UTR) cis-acting cytoplasmic polyadenylation element (CPE), is associated with translational activation of cyclin B1 mRNA during maturation. The main aim of the present study was to investigate if bovine oocyte cyclin B1 mRNA undergoes cytoplasmic polyadenylation/translation during in vitro maturation, as in other species. We have found that cyclin B1 mRNA is present in two isoforms, consisting of the same open reading frame but with different 3'-UTR lengths. Only the longest isoform (cyclin B1L) has a putative CPE sequence and other regulatory sequences, and its mRNA level decreases during early embryo development. The polyadenylation state of cyclin B1L during in vitro maturation was studied. Results demonstrated that cyclin B1L bears a relatively long poly(A) tail in germinal vesicle-stage oocytes, which is further lengthened at 10 h of maturation, before metaphase I. Interestingly, cyclin B1L bears a short poly(A) tail when the ovaries and the oocytes are transported and manipulated on ice to stop the polyadenylation process. Cytoplasmic polyadenylation most probably occurs during ovary transport in warm saline, when oocytes are still in their follicular environment. Our results also show a link between cytoplasmic polyadenylation of cyclin B1 and translation/appearance of cyclin B1 protein before in vitro maturation.

Poly(A) RNA Is Reduced by Half During Bovine Oocyte Maturation but Increases when Meiotic Arrest Is Maintained with CDK Inhibitors1

Variations in the amount of different RNA species were investigated during in vitro maturation of bovine oocytes. Total RNA content was estimated to be 2 ng before meiosis, and after meiosis resumption, no decrease was observed. Ribosomal RNA did not appear to be degraded either, whereas poly(A) RNA was reduced by half after meiosis resumption, from 53 pg to 25 pg per oocyte. Real-time polymerase chain reaction was performed on growth and differentiation factor-9 (GDF-9), on cyclin B1, and on two genes implicated in the resistance to oxidative stress, glucose-6-phosphate-dehydrogenase (G6PD) and peroxiredoxin-6 (PRDX6). When these transcripts were reverse-transcribed with hexamers, the amplification results were not different before or after in vitro maturation. But when reverse transcription was performed with oligo(dT), amplification was dramatically reduced after maturation, except for cyclin B1 mRNA, implying deadenylation without degradation of three transcripts. Although calf oocytes have a lower developmental competence, their poly(A) RNA contents were not different from that of cow oocytes, nor were they differently affected during maturation. When bovine oocytes were maintained in vitro under meiotic arrest with CDK inhibitors, their poly(A) RNA amount increased, but this rise did not change the poly(A) RNA level once maturation was achieved. The increase could not be observed under transcription inhibition and, when impeding transcription and adenylation, the poly(A) RNA decreased to a level normally observed after maturation, in spite of the maintenance of meiotic arrest. These results demonstrate the importance of adenylation and deadenylation processes during in vitro maturation of bovine oocytes.

Developmental capacity of bovine cumulus oocyte complexes after transcriptional inhibition of germinal vesicle breakdown

Oocytes cultured in the presence of FSH and the transcriptional inhibitor, 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole (DRB), remain in meiotic arrest at the germinal vesicle (GV) stage. The objectives of this study were to assess the kinetics of maturation and the developmental capacity of bovine cumulus oocyte complexes (COC) following release from prolonged meiotic arrest by DRB. In Experiment I, COC were cultured for 20 h in Tissue culture medium (TCM)-199 supplemented with 10% estrus cow serum (ECS), 5 microg/ml FSH and 1 microg/ml estradiol in the presence of 120 microM DRB. COC were then released from meiotic arrest and cultured for 20 h in DRB-free medium. CONTROL COC were cultured for 20 h in DRB-free medium, with culture initiated concomitant to the release of DRB-treated COC from meiotic arrest. Nuclear maturation was assessed after 0, 5, 10, 15, and 20 h of culture in DRB-free medium. The proportion of DRB-arrested oocytes reaching metaphase II (MII) following 20 h culture in DRB-free medium was not significantly different from controls ( 96+/-4% versus 99+/-4%). In Experiment II, COC were cultured for 20 h in TCM-199 supplemented with 10% ECS, 10 microg/ml LH, 5 microg/ml FSH, and 1 microg/ml estradiol in the presence or absence of 120 microM DRB. COC in the DRB-treated group were then washed and matured coincident with a second group of control COC for 20 h in DRB-free medium. COC in both groups were fertilized and then randomly assigned to one of two culture systems: TCM-199 + 10%ECS or mSOF + 0.6% fatty acid-free BSA. Development was assessed at 72 h post insemination (hpi), 168 hpi (Day 7) and 216 hpi (Day 9). In this experiment, culture with DRB-arrested oocyte maturation at the GV stage (DRB, 85+/-3% GV; CONTROL, 2+/-3% GV; P<0.001 ). Following release from arrest, maturation and fertilization, the proportion of COC that cleaved by 72 hpi was decreased by treatment with DRB (DRB: 78+/-3% versus

CONTROL

90+/-3%; P<0.05). However, no effect of DRB was found on the proportion of cleaved zygotes that reached the blastocyst stage on either Day 7 or Day 9 of culture (Day 7: DRB 16+/-2% versus CONTROL, 21+/-2%; Day 9: DRB 23+/-3% versus CONTROL, 31+/-3%). More embryos reached the blastocyst stage in the TCM-199/serum culture system compared to the mSOF/BSA system on both Days 7 and 9 (Day 7: TCM-199, 23+/-2% versus mSOF, 13+/-2%, P<0.05; Day 9: TCM-199, 32+/-3% versus mSOF, 22+/-3%, P<0.05 ). In summary, bovine COC maintained in meiotic arrest for 20 h by culture in the presence of the transcriptional inhibitor DRB retained their capacity to develop to the blastocyst stage after fertilization in vitro.

Gene transcription and regulation of oocyte maturation

The developmental potential of an embryo is dependent on the developmental potential of the oocyte from which it originates. The process of oocyte maturation is critical for the efficient application of biotechnologies such as in vitro embryo production and mammalian cloning. However, the overall efficiency of in vitro maturation remains low because oocytes matured in vitro have a lower developmental competence than oocytes matured in vivo. Furthermore, oocytes that have been exposed to gonadotropins have greater developmental competence than oocytes matured in the absence of gonadotropins. By understanding the molecular mechanisms underlying gonadotropin-induced maturation, improvement in oocyte maturation technologies may be expected as procedures to manipulate specific factors involved in signalling for resumption of meiosis are identified. The present review will focus on transcriptional mechanisms underlying the maturation of mammalian oocytes in vitro, as well as on the acquisition of oocyte developmental competence. In addition, a working model for the transcriptional control of mammalian oocyte maturation is proposed.

Factors affecting oocyte quality and quantity in commercial application of embryo technologies in the cattle breeding industry

With the introduction of multiple ovulation, embryo recovery and transfer techniques (MOET) plus embryo freeze-thaw methods in the early 1980s, the breeding industry has the tools in hand to increase the number of calves from donors of high genetic merit. In the early 1990s, the introduction of ovum pick-up followed by in vitro embryo production (OPU-IVP) opened up even greater possibilities. Using these technologies, we challenge biological mechanisms in reproduction. Where normally one oocyte per estrous cycle will develop to ovulation, now numerous other oocytes that otherwise would have degenerated are expected to develop into an embryo. Completion of oocyte growth and pre-maturation in vivo before final maturation both appear to be essential phases in order to obtain competence to develop into an embryo and finally a healthy offspring. In order to increase oocyte quality and quantity in embryo production technologies, current procedures focus primarily on improving the homogeneity of the population of oocytes with regard to growth and state of pre-maturation at the start of a treatment. In the case of MOET, dominant follicle removal (DFR) before superovulation treatment improves the number of viable embryos per session from 3.9 to 5.4 in cows but not in heifers and a prolonged period of follicle development obtained by preventing release of the endogenous LH surge increases the number of ova but not the number of viable embryos per session. In the case of OPU-IVP, the frequency of OPU clearly affects quantity and quality of the collected oocytes and FSH stimulation prior to OPU every 2 weeks resulted in 3.3 embryos per session. Analysis of 7,800 OPU sessions demonstrated that the oocyte yield is dependent on the team, in particular, the technician manipulating the ovaries. It is concluded that an increased understanding of the processes of oocyte growth, pre- and final maturation will help to improve the efficiency of embryo technologies. However, somewhere we will meet the limits dictated by nature.

Use of heterologous complementary DNA array screening to analyze bovine oocyte transcriptome and its evolution during in vitro maturation

We have analyzed gene expression in bovine oocytes before and after in vitro maturation (IVM) using heterologous hybridization onto cDNA array. Total RNA was purified from pools of over 200 oocytes either immediately after aspiration from follicles at the surface of slaughterhouse cow ovaries or following in vitro maturation. Radiolabeled cDNA probes were generated by reverse-transcription followed by linear PCR amplification and were hybridized to Atlas human cDNA arrays. To our knowledge, this is the first report of gene expression profiling by this technology in the mammalian female germ cell. Our results demonstrate that cDNA array screening is a suitable method for analyzing the transcription pattern in oocytes. About 300 identified genes were reproducibly shown to be expressed in the bovine oocyte, the largest profile available so far in this model. The relative abundance of most messenger RNAs appeared stable during IVM. However, 70 transcripts underwent a significant differential regulation (by a factor of at least two). Their potential role in the context of oocyte maturation is discussed. Together they constitute a molecular signature of the degree of oocyte cytoplasmic maturation achieved in vitro.

In vitro maturation of bovine oocytes requires polyadenylation of mRNAs coding proteins for chromatin condensation, spindle assembly, MPF and MAP kinase activation

In the oocyte mRNA molecules are stored in order to be used during the maturation process when transcription is silenced. Translational activation of stored mRNA templates commonly is correlated with their cytoplasmic polyadenylation. In the present study, the effects of cordycepin, a potent polyadenylation inhibitor, on the in vitro maturation MPF and MAP kinase activation of bovine cumulus oocyte complexes (COCs) were examined. The presence of cordycepin (5 microg/ml) during the whole culture period (24 h) prevents chromatin condensation and germinal vesicle breakdown (GVBD) as well as MPF and MAP kinase activation. When COCs were first cultivated in inhibitor-free medium for 6 or 9 h and subsequently transferred to cordycepin supplemented medium for further 18 or 15 h neither MPF nor MAP kinase became activated and 86 and 85%, respectively, of the oocytes underwent GVBD but failed to form a spindle and hypercondensed their chromatin. Extending the first culture period in inhibitor-free medium to 12 or 15 h before transferring the COCs to cordycepin supplemented medium for a further 12 or 9 h allowed 48 and 79%, respectively of oocytes to reach the metaphase 2 (M 2) stage. From these data, it is concluded that mRNA molecules coding for proteins required for chromatin condensation and GVBD become polyadenylated during the first 6 h following the onset of culture whereas mRNA molecules coding for proteins required for spindle assembly of metaphase 1 (M 1) and MPF and MAP kinase activation become polyadenylated between 9 and 12 h following initiation of culture.

Effects of in vivo prematuration and in vivo final maturation on developmental capacity and quality of pre-implantation embryos

In current in vitro production (IVP) systems, oocytes lack in vivo dominant and preovulatory follicular development, which may compromise pregnancy and viability of calves born. When an oocyte sets off in vivo on the road toward fertilization, it contains numerous transcripts and proteins necessary to survive the first few cell cycles of embryonic development. It is not yet known during which period of development the oocyte builds up the store, possibly primarily during the major growth phase of the oocyte, which is completed at the time a follicle reaches the size of 3 mm. Here, we investigated to what extent the later phases of follicular development, such as prematuration in the dominant follicle before the LH surge and ensuing final maturation in the preovulatory follicle, contribute to oocyte competence and development into viable biastocysts. Recent studies on in vivo vs in vitro oocyte maturation employed oocytes from an identical preovulatory development by applying ovum pick-up (OPU) twice (before and 24 h after the LH surge) in each cow treated for superovulation with a controlled LH surge. The embryo recovery rates at Day 7 of IVC after IVF were similar: 44% (97/219) for in vivo- vs 41% (87/213) for in vitro-matured oocytes, which shows that the natural environment during final maturation is not essential for the mere in vitro development of the prematured oocyte beyond the 8- to 16-cell stage. However, in vivo maturation appeared to contribute to the oocyte's quality in a more subtle way, as indicated by a significant increase in the proportion of expanded blastocysts and a more physiological degree of chromosome aberrations of the embryos. In blastocysts derived from in vivo-matured oocytes, 21% of the embryos were mixoploid vs 50% from in vitro-matured oocytes, concomitant with a higher number of cells (96 vs 54 per normal blastocyst). The expression pattern of a set of six developmentally important genes was, however, not significantly altered in blastocysts derived from in vivo-matured oocytes. Certain deviations were observed compared with the levels of entirely in vivo-developed control blastocysts, which suggests that the beneficial effects of in vivo maturation are possibly exerted at initial stages of embryonic development. Prematuration in vivo, occurring in a dominant follicle developing from about 8 mm into the preovulatory follicle, is accompanied by changes in protein synthesis of the cumulus oocyte complex (COC). Presumably, the differentially expressed proteins are involved in equipping the oocyte with further developmental competence. Although we have unraveled some important biochemical and cellular biological features of the oocyte, further research on in vivo processes is essential to improve in vitro embryo production in practice.

Nuclear ultrastructure in bovine oocytes after inhibition of meiosis by chemical and biological inhibitors

Various components of the ovarian follicle as well as different chemicals can suppress the resumption of meiosis in cumulus-oocyte complexes (COCs). In this study the nuclear ultrastructure of bovine COCs was assessed after 8 h of meiotic inhibition with 50 microM roscovitine (ROSC), 50 microM butyrolactone (BL-I), 2 mM 6-DMAP, 2 microM cycloheximide (CX), or a theca cell monolayer (TC). COCs were recovered according to standard in vitro methods, cultured in a simple and defined medium, and processed for transmission electron microscopy. Control COCs were processed before onset of culture and multiple oocytes were evaluated for each treatment. In all groups, the oocyte nucleus presented a dense fibrillar nucleolus consisting of a fibrillar sphere with a fibrillar center. In TC and 6-DMAP inhibited COCs condensed chromatin adhered to the nucleolus while in all other groups the perinuclear chromatin was separated from the nucleolus. In ROSC inhibited COCs, the nuclear envelope presented only slight small amplitude undulation. The BL-I-inhibited COCs presented an intermediate level of low amplitude undulation of the NE. In CX, 6-DMAP, and TC inhibited COCs the nuclear envelope presented extensively low amplitude undulations. In ROSC inhibited COCs, electron-dense granules formed ring-shaped structures. In some of the BL-I inhibited COCs multiple stellate crystal-like structures were found, and in these COCs the nuclear envelope and the perinuclear cisternae appeared less distinct than in the other BL-I inhibited COCs. In 6-DMAP inhibited COCs interchromatin-like granule clusters were present. In conclusion, the oocyte nuclei in all COCs presented a dense fibrillar nucleolus resembling that in control COCs. However, variations were observed in 1) the nuclear envelope morphology; 2) the chromatin location in relation to the nucleolus; and 3) the presence of different populations of intranuclear granules. Although all treatments inhibited oocyte nucleus breakdown, the mechanisms underlying these effects are different and require further characterization. Mol. Reprod. Dev. 59: 459-467, 2001.

Resumption of meiosis: mechanism involved in meiotic progression and its relation with developmental competence

It has been more than 10 years since the first calf was born following in vitro maturation of a bovine oocyte. During that period, a number of modifications have been made to the culture conditions during the maturation period but still most oocytes failed to produce viable embryos. Recently, the pretreatment of donors leading to clear improvements in oocyte developmental competence has been achieved. These treatments can be either 1) ovarian stimulation with FSH followed by the slaughter of the animal and the incubation of ovaries before oocyte aspiration or 2) the transvaginal aspiration of oocytes from FSH-stimulated animals following a "coasting period" of 48 hours. These 2 treatments indicate the necessity of preparing the immature oocyte before using it. An alternative to the animal pretreatment would be to recreate in vitro the conditions present in more mature follicles. It is believed that the ability of the oocyte to become an embryo depends on the accumulation of specific information in the form of mRNA or proteins. Because the consequences of that change become visible 4 days later at the early 8-cell stage, the favorite hypothesis is that the oocyte accumulates stable mRNA and that accumulation requires an active transcription machinery. Therefore, to improve oocyte's potential, follicular instructions must be given before the germinal vesicle breakdown induced by oocyte aspiration and culture. The culture of bovine oocytes in a state of meiotic arrest could potentially allow the correct follicular signal(s) to change the development potential of an oocyte. The unraveling of the mechanism for cell control of meiotic arrest opens the way to a functional and reversible culture system for immature oocytes. The remaining challenge is to discover follicular signals or factors to apply during the in vitro culture period to enhance the competence of oocytes.

Bovine cumulus-oocyte-complex-quality is reflected in sensitivity for alpha-amanitin, oocyte-diameter and developmental capacity

The aim of the present study was to find more parameters to define developmental competence of cumulus-oocyte-complexes (COCs). Bovine COCs were divided into five groups based on their morphology. In order of increasing level of atresia: COC-A had a bright, compact cumulus investment; COC-B1 also had a compact cumulus investment, but was darker than COC-A; the color of COC-B2 was comparable with COC-B1 but the corona radiata appeared to dissociate from the rest of the cumulus investment; the cumulus of COC-B3 was almost black and the corona radiata was almost completely dissociated from the rest of the cumulus investment; COC-C had a strongly expanded cumulus investment with dark spots of degenerated cells. An increasing level of atresia was accompanied by: (1) an increasing zona pellucida diameter (147.6, 150.8, 151.0, 154.3 and 155.1 microm, respectively, for COC-A, COC-B1, COC-B2, COC-B3 and COC-C); (2) an increasing oocyte diameter except for COC-C (120.9, 122.8, 122.8, 123.9 and 118.4 microm, respectively); (3) an increasing developmental competence except for COC-C (13.9, 14.7, 17.4, 19.1 and 11.5%, respectively, development to morula and blastocyst after in vitro embryo production (IVP)) and (4) by a increasing percentage of oocytes exhibiting germinal vesicle breakdown (GVBD) after 24h culture with alpha-amanitin and FSH, except for COC-A (34.4, 68.6, 51.0, 22.0 and 4.8%, respectively, oocytes arrested in GV). In general, embryo-quality, expressed in nuclei-count, was significantly affected by COC-quality (P < 0.05) with B3 > B1> B2 > A > C. However, if the developmental stages were compared separately, this effect was less evident. It was found that, in absence of FSH, alpha-amanitin was unable to inhibit GVBD and that the success of GVBD inhibition was positively correlated to the amount of cumulus cells surrounding the oocyte. COCs that had been exposed to alpha-amanitin and FSH during maturation retained the ability to cleave after IVF, but were unable to develop any further.

Effects of ethanol and dimethylsulphoxide on nuclear and cytoplasmic maturation of bovine cumulus-oocyte complexes

The influence of small doses of ethanol or dimethylsulphoxide (DMSO) on in vitro maturation (IVM) of bovine cumulus-oocyte complexes (COC) was examined, either after spontanous maturation or after inhibition of meiosis with 6-dimethylaminopurine (6-DMAP) or 3-isobutyl-1-methylxanthine (IBMX). Subsequent to IVM for 23 hr in semidefined serum-free Earle's TCM199 medium, nuclear maturation was assessed cytogenetically, while the combined cytoplasmic and nuclear maturation was measured indirectly by the oocytes' ability to undergo fertilization and further embryonic development. Embryo development was followed until the blastocyst stages at day 9 after insemination. Neither spontanous nuclear maturation nor cleavage was compromised by IVM in </=1% (v/v) ethanol or </=1% (v/v) DMSO, nor was the frequency of polyspermy altered. However, IVM in 0.3% or 1% (v/v) ethanol or in 0.4 or 1% (v/v) DMSO negatively affected blastocyst formation, compared to 0% in the control groups (22% and 23% vs. 34%, P < 0.0001, and 29% and 22% vs. 34%, P < 0.01, respectively), whereas the speed of blastocyst formation, assessed as the D7/D9 blastocyst proportion, was not compromised. In oocytes meiotically inhibited with 2 mM 6-DMAP, the presence of ethanol (0. 5%, 1%, and 2% [v/v]) induced germinal vesicle breakdown in a dose-dependent manner (32%, 45%, and 68%, vs. 22%, P < 0.0001), however, the oocytes exhibited no further meiotic progression. In oocytes inhibited with 1 and 2 mM IBMX, the presence of ethanol (0. 5%, 1%, and 2% [v/v]) significantly (P < 0.05) enhanced the inhibitory effect in a dose-dependent manner by reducing the proportion of the mature (AI-MII) stages (77%, 68%, and 56% vs. 79%, and 33%, 29%, and 18% vs. 39%, respectively). It is concluded that even small doses of ethanol or DMSO can cause profound negative effects on bovine in vitro maturation and subsequent embryo development.

Meiosis-activating sterol-mediated resumption of meiosis in mouse oocytes in vitro is influenced by protein synthesis inhibition and cholera toxin

To explore the possible signaling pathways of meiosis-activating sterol (MAS)-induced oocyte maturation and to elucidate whether the MAS pathway involves transcription or translation, arrested immature mouse oocytes were cultured with either the protein synthesis inhibitor cycloheximide or the heteronuclear RNA inhibitors alpha-amanitin or actinomycin D, respectively. Moreover, the possible involvement of a G protein-coupled receptor mechanism in MAS-mediated oocyte maturation was explored by influencing oocyte maturation with cholera toxin (CT). MAS-induced oocyte maturation was completely blocked by the addition of 50 microg/ml cycloheximide 4 h before the addition of MAS. Simultaneous addition of MAS and the protein synthesis inhibitor also significantly reduced the meiotic resumption compared to that in MAS-treated controls. In contrast, neither of the treatment regimens to inhibit transcription of DNA to RNA was observed to have any effect on the MAS-induced resumption of meiosis. CT was observed to inhibit MAS-induced, but not spontaneous, oocyte maturation in vitro, suggesting a putative involvement of G protein-coupled receptor mechanism in the MAS mode of action. In conclusion, protein synthesis was found to be an essential requirement for maintaining the oocytes' responsiveness to MAS-induced resumption of meiosis, in contrast to transcription.

Effect of protein synthesis inhibition before or during in vitro maturation on subsequent development of bovine oocytes

The overall objective of this study was to assess the effect of maintaining meiotic arrest in bovine oocytes in vitro on developmental competence. In Experiment 1 the effect of inhibition of meiotic resumption using cycloheximide (CX), on subsequent was examined. Immature cumulus oocyte complexes (COCs, n = 804) were cultured in the absence (24 h) or presence of CX for 6, 12, 18 or 24 h. The control was inseminated 24 h later, while CX-treated oocytes were cultured for a further 24 h before insemination. In Experiment 2 the effect of exposing the oocyte (n = 1239) during meiotic arrest to putative stimulatory substances (pFSH and FCS) was examined. In Experiment 3, to study the importance of protein synthesis during maturation, synthesis was blocked for a 6-h period at various times (6, 12, 18 h) after start of culture (n = 1117). In Experiment 1, there was no difference in cleavage rate between treatments. However, the percentage of 5 to 8 cell embryos at 72 h post insemination was significantly lower after CX treatment (64 vs 42 to 51%; P < 0.05). This was reflected in a lower rate of blastocysts at Day 6 (9 to 15 vs 31%, P < 0.002). While the blastocyst rate at Day 8 was lower in CX-treated oocytes, the effect was only significant when CX was present for longer than 12 h. A marked decrease in development was noted following inhibition for 18 h or more compared with the control (17 to 19 vs 40%; P < 0.0002). In Experiment 2, addition of either FSH or FCS to oocytes in the presence of CX had no effect on any of the parameters studied, even though there was a positive effect in control oocytes. In Experiment 3, treatment with CX after the oocytes had matured for varying periods resulted in decreased blastocyst rates at Days 6 and 8 of culture. The most significant drop in development occurred when oocytes were cultured for 12 h before exposure to CX (15 vs 40%; P < 0.0001). In conclusion, CX-blocked oocytes retained their developmental competence, although final blastocyst yields were reduced.

Involvement of cumulus cells stimulated by FSH in chromatin condensation and the activation of maturation-promoting factor in bovine oocytes

The effects of FSH-stimulated cumulus cells on the regulatory mechanisms of chromatin condensation and maturation-promoting factor (MPF) activation around the time of germinal vesicle breakdown (GVBD) in bovine oocytes were examined. Chromatin condensation occurred in oocytes arrested at the germinal vesicle (GV) stage by protein synthesis inhibitor, cycloheximide, but this condensation was blocked by FSH-stimulated cumulus cells. However, treatment with cyclic AMP (cAMP)-dependent protein kinase inhibitor, H-8, dramatically increased the proportion of oocytes possessing GVs with condensed bivalents. Under the condition of inhibited protein synthesis, the phosphorylation form of p34cdc2 kinase was not changed due to chromatin condensation, although the activity of histone H1 kinase was significantly increased compared with that of oocytes possessing GVs with filamentous bivalents. The cycloheximide-dependent GVBD block was overcome by okadaic acid (OA) in 48 and 13% of the oocytes in the absence and presence of FSH, respectively. An initial 6-h culture period critical for protein synthesis was necessary for OA to counteract the inhibitory effect exerted by cycloheximide on the induction of GVBD and activation of histone H1 kinase in the absence of FSH, whereas this first culture period was prolonged for 2 h in the presence of FSH. Furthermore, even in FSH-stimulated oocytes, H-8 facilitated an OA-counteracted overcome of the cycloheximide-dependent GVBD block after 2 h of initial culture for protein synthesis. From these results, it is concluded that cAMP-dependent protein kinase activity regulated by cumulus cells following FSH-stimulation requests plays a role in the complex mechanism of chromatin condensation and MPF activation leading to meiotic resumption in bovine oocytes.

Responses of oocytes and embryos to the culture environment

Embryo development is strongly influenced by events occurring during oocyte maturation. Although many immature oocytes are capable of completing meiosis in vitro, only a small percentage of the original pool of immature oocytes is competent to continue development to the blastocyst stage and subsequently result in a pregnancy. This indicates that maturation of oocytes in vitro may not be occurring in an entirely normal manner. Cytoplasmic changes occurring during maturation, collectively termed cytoplasmic maturation, are essential for embryonic development. The cytoplasm of the oocyte may play a crucial role in assembling the correct metabolic machinery for production of sufficient energy for cellular functions during maturation, cleavage and blastocyst formation. A better understanding of the structural, functional and metabolic characteristics of the oocyte during maturation, and the consequence of changes in these parameters on developmental competence is needed. Understanding the role of cytoplasmic changes during oocyte maturation will help increase the efficiency of in vitro embryo production. Better embryo production strategies will facilitate basic research into the control of early development, improve implementation in endangered species, provide a source of high quality oocytes for nuclear transfer and transgenic technologies and benefit the commercial embryo transfer industry.

Developmental capacity of bovine oocytes following inhibition of meiotic resumption by cycloheximide or 6-dimethylaminopurine

This study was conducted to assess the fertilizability and developmental capacity of bovine oocytes which had been maintained in meiotic arrest by either a protein synthesis inhibitor, cycloheximide (CHX), or an inhibitor of serine/threonine protein kinases, 6-dimethylaminopurine (6-DMAP). Both CHX and 6-DMAP reversibly prevented nuclear maturation of nearly all oocytes for 24 h. After the reversal of arrest, CHX-treated oocytes could be successfully matured and fertilized. They developed to the blastocyst stage at slightly lower rates than oocytes cultured without inhibition for 22 h prior to sperm addition but at higher rates than those incubated in a medium containing no inhibitors for 46 h prior to fertilization. Oocytes inhibited by CHX for 48 h matured and fertilized normally but failed to develop into blastocysts. Even though 6-DMAP-treated oocytes completed meiosis I after removal from the drug, the rates of fertilization and blastocyst formation were lower than for untreated oocytes or CHX-treated oocytes. Effects of adding FSH and/or estradiol-17 beta (E(2)) during CHX-inhibition for 24 h were also examined. Embryos from oocytes treated with CHX and E(2) or with CHX and FSH + E(2) developed into blastocysts at similar rates as the controls. Further development of inhibited oocytes was examined by transferring blastocysts derived from oocytes inhibited by CHX with FSH and E(2) for 24 h to recipient heifers. Two calves were obtained following transfer. These results indicate that CHX-inhibited oocytes retain developmental competence, while 6-DMAP-inhibited oocytes after the reversal of arrest have reduced capacities for fertilization and further development. The addition of FSH and E(2) during CHX-inhibition improves development to the blastocyst stage of the oocytes that are capable of initiating and maintaining pregnancy after embryo transfer to recipient animals.

Fertilisation competence of bovine normally matured or aged oocytes derived from different antral follicles: morphology, protein synthesis, H1 and MBP kinase activity

We have investigated the fertilisation competence, protein synthesis, histone H1 kinase and myelin basic protein (MBP) kinase activities in three categories of bovine oocytes (derived from three size categories of follicles: M--medium, 2.5-5.0 mm; S--small, 1.5-2.5 mm; T--tiny, 1.0-1.5 mm). In contrast to more or less normal meiotic maturation (85.6%) and fertilisation (70.8%) of M oocytes cultured for 24 h, the fertilisation of M oocytes cultured for 40 h was associated with increased rates of retarded male pronuclear development and retention of the second polar body. The S and T oocytes cultured for 24 h or 40 h were mostly arrested at defective late diakinesis-metaphase I (77.5-100%) stage. After fertilisation of S and T oocytes cultured for 24 h no polar body was extruded and formation of one, three or four female pronuclei, together with mostly normal male pronuclei, was observed. The fertilisation of S and T oocytes after 40 h culture resulted in a higher number of female and a decreased number of male pronuclei. A major change in the pattern of protein synthesis was associated with the resumption of meiosis. There were no significant differences in the profile of protein synthesis between oocyte categories in all groups either matured or fertilised. The H1 kinase activity reached comparable increased levels in oocytes of all categories matured for 24 h and decreased during the 40 h culture, most significantly in M oocytes. The MBP kinase activity was at approximately the same high level in all categories of oocytes after 24 h of culture and remained stable until 40 h. The fertilisation after 24 h of culture resulted, in M oocytes, in low levels of both H1 and MBP kinase activities; in S oocytes, only H1 kinase was completely inactivated while MBP kinase activity decreased to some extent; in T oocytes, both H1 and MBP kinase activity decreased. Fertilisation of all oocyte categories after 40 h culture resulted in complete inactivation of both these kinases to their basal levels.

Nucleus ultrastructure and transcriptional activity of bovine oocytes in preantral and early antral follicles

An understanding of the recruitment and growth of follicles within the bovine ovary is crucial to their successful exploitation in vitro. The aim of the present study was to describe the nuclear ultrastructure and transcriptional activity of primordial to early tertiary follicular oocytes from bovine adult ovaries. Small blocks of ovarian cortex were incubated in medium enriched with 3H-uridine for 30 min. Subsequently, the tissue blocks were fixed in Karnowsky's fixative, dehydrated, epon embedded, sectioned (2 microns), processed for autoradiography, and examined under light microscopy. Sections showing preantral follicles with presumptive oocyte nucleoli were reembedded for transmission electron microscopy. The follicles were divided into five categories: 1) resting primordial, with a single layer of flattened granulosa cells, 2) activated primordial, with a single layer of flattened and some cuboidal granulosa cells, 3) primary, with a single layer of cuboidal granulosa cells, 4) secondary, with a complete or incomplete bilayer of cuboidal cells, and 5) tertiary, with more than two layers of granulosa cells delineating one or more intercellular cavities. The granulosa cells of all follicle classes were transcriptionally active. However, the oocytes did not display transcriptional activity, as measured by the present means, until the secondary and tertiary follicular stages. The oocyte nucleolus was granular in the primordial follicles. Following follicular activation, fibrillar centres invaded the nucleolus and, in the early tertiary follicle, numerous fibrillar centres were distributed throughout the nucleolus. In conclusion, the oocyte nucleolar function is gradually activated at follicle activation, and oocyte transcription is initiated at approximately the time of the secondary follicle stage.