Meiotic arrest of oocytes depends on cell-to-cell communication in the ovarian follicle

Population Structure and Selection Signatures Underlying Domestication Inferred from Genome-Wide Copy Number Variations in Chinese Indigenous Pigs

Single nucleotide polymorphism was widely used to perform genetic and evolution research in pigs. However, little is known about the effect of copy number variation (CNV) on characteristics in pigs. This study performed a genome-wide comparison of CNVs between Wannan black pigs (WBP) and Asian wild boars (AWB), using whole genome resequencing data. By using Manta, we detected in total 28,720 CNVs that covered approximately 1.98% of the pig genome length. We identified 288 selected CNVs (top 1%) by performing Fst statistics. Functional enrichment analyses for genes located in selected CNVs were found to be muscle related (NDN, TMOD4, SFRP1, and SMYD3), reproduction related (GJA1, CYP26B1, WNT5A, SRD5A2, PTPN11, SPEF2, and CCNB1), residual feed intake (RFI) related (MAP3K5), and ear size related (WIF1). This study provides essential information on selected CNVs in Wannan black pigs for further research on the genetic basis of the complex phenotypic and provides essential information for direction in the protection and utilization of Wannan black pig.

Transcriptional profile of cumulus associated GJA1, PTX3, PRSS35, and SERPINE2 genes with oocytes and embryonic development in water buffalo

BACKGROUND

In the present study, the potential of different groups of cumulus-oocyte complexes (COC's) for in vitro maturation (IVM) and embryonic development was assessed in two groups of COC's of water buffalo. Further, the expression pattern of cumulus-associated GJA1, PTX3, PRSS35, and SERPINE2 genes and their effects on embryonic development was analyzed. Slaughterhouse-derived buffalo COC's were graded into groups A and B. An equal number of 410 COC's were taken in both groups. IVM was carried out using Slaughterhouse-derived buffalo epididymis. A remarkable degree of cumulus expansion was noticed in group A (92.68%) as compared to group B (81.25%) oocytes. On in vitro fertilization (IVF) and embryo culture, group A produced a significantly higher rate of cleavage and blastocyst (92.682 ± 0.7179% and 42.682 ± 0.9683%) as compared to group B (85.365 ± 0.7608% and 31.707 ± 0.9688%). Also, the transcriptional analysis of cumulus-associated genes revealed significantly higher expression in group A as compared to group B.

RESULTS

It was revealed that oocytes having good cumulus mass had a higher developmental potential. Based on differential gene expression of cumulus-associated genes, different quality of COC's, and the resultant embryos after IVF, it was concluded that these genes could be used as a marker for predicting the developmental competence of the oocytes.

CONCLUSION

We concluded that morphologically good quality of COC's had a higher developmental competence, and also the differential expressions of cumulus-associated genes in cumulus cells and embryos. So, we can conclude that these genes could be used as marker genes for predicting the developmental competence of buffalo's oocytes.

Signaling mechanisms and their regulation during in vivo or in vitro maturation of mammalian oocytes

In vitro fertilization (IVF) is currently one of the most effective methods of infertility treatment. An alternative to commonly used ovarian hyperstimulation can become extracorporeal maturation of oocytes (in vitro maturation; IVM). Fertilization and normal development of the embryo depends on the cytoplasmic, nuclear and genomic maturity of the oocyte. The microenvironment of the ovarian follicle and maternal signals, which mediate bidirectional communication between granulosa, cumulus and oocyte cells, influence the growth, maturation and acquisition of oocyte development capability. During oogenesis in mammals, the meiosis is inhibited in the oocyte at the prophase I of the meiotic division due to the high cAMP level. This level is maintained by the activity of C-type natriuretic peptide (CNP, NPPC) produced by granulosa cells. The CNP binds to the NPR2 receptor in cumulus cells and is responsible for the production of cyclic guanosine monophosphate (cGMP). The cGMP penetrating into the oocyte through gap junctions inhibits phosphodiesterase 3A (PDE3A), preventing cAMP hydrolysis responsible for low MPF activity. The LH surge during the reproductive cycle reduces the activity of the CNP/NPR2 complex, which results in a decrease in cGMP levels in cumulus cells and consequently in the oocyte. Reduced cGMP concentration unblocks the hydrolytic activity of PDE3A, which decreases cAMP level inside the oocyte. This leads to the activation of MPF and resumption of meiosis. The latest IVM methods called SPOM, NFSOM or CAPA IVM consist of two steps: prematuration and maturation itself. Taking into account the role of cAMP in inhibiting and then unblocking the maturation of oocytes, they have led to a significant progress in terms of the percentage of mature oocytes in vitro and the proportion of properly developed embryos in both animals and humans.

PSPC1 regulates CHK1 phosphorylation through phase separation and participates in mouse oocyte maturation

Liquid-liquid phase separation (LLPS) underlies the formation of membraneless compartments in mammal cells. However, there are few reports that focus on the correlation of mouse oocyte maturation with LLPS. Previous studies have reported that paraspeckle component 1 (PSPC1) is related to the occurrence and development of tumors, but whether PSPC1 functions in mouse oocyte maturation is still unclear. Sequence analysis of PSPC1 protein showed that it contains a prion-like domain (PrLD) that is required for phase separation of proteins. In this study, we found that PSPC1 could undergo phase separation. Moreover, the loss of PrLD domain of PSPC1 could greatly weaken its phase separation ability. The immunofluorescence assays showed that PSPC1 is present in mouse oocytes in the germinal vesicle (GV) stage. Knockdown of PSPC1 significantly impeded the maturation of mouse oocytes in vitro. CHK1 has been reported to play important roles in the GV stage of mouse oocytes. Co-IP experiment revealed that PSPC1 could interact with phosphatase serine/threonine-protein phosphatase 5 (PPP5C), which regulates CHK1 phosphorylation. Western blot analysis revealed that PSPC1 could regulate the phosphorylation of CHK1 through PPP5C; however, PSPC1 without PrLD domain was inactive, suggesting that the lack of phase separation ability led to the abnormal function of PSPC1 in regulating CHK1 phosphorylation. Thus, we conclude that PSPC1 may undergo phase separation to regulate the phosphorylation level of CHK1 via PPP5C and participate in mouse oocyte maturation. Our study provides new insights into the mechanism of mouse oocyte maturation.

Oocyte Morphometric Assessment and Gene Expression Profiling of Oocytes and Cumulus Cells as Biomarkers of Oocyte Competence in Sheep

Oocyte quality is crucial for subsequent embryo development and so it is a major challenge in assisted reproductive technologies. The aim of the present work was to evaluate the morphometric parameters of oocytes (experiment 1) and the relative gene expression of oocytes and cumulus cells (CCs) (experiment 2) as biomarkers of oocyte quality after individually culturing them (one oocyte or embryo/drop). In experiment 1, individually matured oocytes were measured and classified into small, intermediate, and large oocytes after a cluster analysis, based on total diameter (with zona pellucida, ZP), oocyte diameter (without ZP), and ZP thickness. These oocytes were individually fertilized in vitro and cultured. The embryo development was evaluated up to the blastocyst stage. According to the total diameter, oocyte diameter, and ZP thickness, the blastocyst rate decreased in the small oocytes group (3.1 ± 3.1, 14.1 ± 9.4, and 26.7 ± 3.9, respectively) compared to the intermediate (29.4 ± 5.2, 30.5 ± 10.1, and 28.6 ± 9.6, respectively) and large oocytes groups (54.2 ± 13.5, 44.4 ± 3.9, and 67.6 ± 12.4, respectively). In addition, the probability of reaching the blastocyst stage was positively related to the total diameter (p < 0.001), oocyte diameter (p < 0.05), and ZP thickness (p < 0.001). Furthermore, the relative gene expression of BAX, BCL2, GDF9, and GJA1 was lower in oocytes classified as large. In experiment 2, the mRNA transcript relative abundance pattern of genes in CCs was evaluated according to oocyte total diameter and developmental stage reached. CCs from oocytes classified as large and oocytes capable of developing to the blastocyst stage had a lower relative expression of BAX, STAR, and PTGS2, while a higher expression of HAS2 and SDC2 transcript was observed for those oocytes. In conclusion, oocyte morphometric parameters and gene expression analysis in oocytes and CCs provide methods for the identification of the most competent oocytes for assisted reproductive technologies in sheep.

Molecular Features of Polycystic Ovary Syndrome Revealed by Transcriptome Analysis of Oocytes and Cumulus Cells

Polycystic ovary syndrome (PCOS) is typically characterized by a polycystic ovarian morphology, hyperandrogenism, ovulatory dysfunction, and infertility. Furthermore, PCOS patients undergoing ovarian stimulation have more oocytes; however, the poor quality of oocytes leads to lower fertilization and implantation rates, decreased pregnancy rates, and increased miscarriage rates. The complex molecular mechanisms underlying PCOS and the poor quality of oocytes remain to be elucidated. We obtained matched oocytes and cumulus cells (CCs) from PCOS patients, compared them with age-matched controls, and performed RNA sequencing analysis to explore the transcriptional characteristics of their oocytes and CCs. Moreover, we validated our newly confirmed candidate genes for PCOS by immunofluorescence. Unsupervised clustering analysis showed that the overall global gene expression patterns and transposable element (TE) expression profiles of PCOS patients tightly clustered together, clearly distinct from those of controls. Abnormalities in functionally important pathways are found in PCOS oocytes. Notably, genes involved in microtubule processes, TUBB8 and TUBA1C, are overexpressed in PCOS oocytes. The metabolic and oxidative phosphorylation pathways are also dysregulated in both oocytes and CCs from PCOS patients. Moreover, in oocytes, differentially expressed TEs are not uniformly dispersed in human chromosomes. Endogenous retrovirus 1 (ERV1) elements located on chromosomes 2, 3, 4, and 5 are rather highly upregulated. Interestingly, these correlate with the most highly expressed protein-coding genes, including tubulin-associated genes TUBA1C, TUBB8P8, and TUBB8, linking the ERV1 elements to the occurrence of PCOS. Our comprehensive analysis of gene expression in oocytes and CCs, including TE expression, revealed the specific molecular features of PCOS. The aberrantly elevated expression of TUBB8 and TUBA1C and ERV1 provides additional markers for PCOS and may contribute to the compromised oocyte developmental competence in PCOS patients. Our findings may also have implications for treatment strategies to improve oocyte maturation and the pregnancy outcomes for women with PCOS.

Oocyte Competence Biomarkers Associated With Oocyte Maturation: A Review

Oocyte developmental competence is one of the determining factors that influence the outcomes of an IVF cycle regarding the ability of a female gamete to reach maturation, be fertilized, and uphold an embryonic development up until the blastocyst stage. The current approach of assessing the competency of an oocyte is confined to an ambiguous and subjective oocyte morphological evaluation. Over the years, a myriad of biomarkers in the cumulus-oocyte-complex has been identified that could potentially function as molecular predictors for IVF program prognosis. This review aims to describe the predictive significance of several cumulus-oocyte complex (COC) biomarkers in evaluating oocyte developmental competence. A total of eight acclaimed cumulus biomarkers are examined in the study. RT-PCR and microarray analysis were extensively used to assess the significance of these biomarkers in foreseeing oocyte developmental competence. Notably, these biomarkers regulate vital processes associated with oocyte maturation and were found to be differentially expressed in COC encapsulating oocytes of different maturity. The biomarkers were reviewed according to the respective oocyte maturation events namely: nuclear maturation, apoptosis, and extracellular matrix remodeling, and steroid metabolism. Although substantial in vitro evidence was presented to justify the potential use of cumulus biomarkers in predicting oocyte competency and IVF outcomes, the feasibility of assessing these biomarkers as an add-on prognostic procedure in IVF is still restricted due to study challenges.

Mechanisms of Oocyte Maturation and Related Epigenetic Regulation

Meiosis is the basis of sexual reproduction. In female mammals, meiosis of oocytes starts before birth and sustains at the dictyate stage of meiotic prophase I before gonadotropins-induced ovulation happens. Once meiosis gets started, the oocytes undergo the leptotene, zygotene, and pachytene stages, and then arrest at the dictyate stage. During each estrus cycle in mammals, or menstrual cycle in humans, a small portion of oocytes within preovulatory follicles may resume meiosis. It is crucial for females to supply high quality mature oocytes for sustaining fertility, which is generally achieved by fine-tuning oocyte meiotic arrest and resumption progression. Anything that disturbs the process may result in failure of oogenesis and seriously affect both the fertility and the health of females. Therefore, uncovering the regulatory network of oocyte meiosis progression illuminates not only how the foundations of mammalian reproduction are laid, but how mis-regulation of these steps result in infertility. In order to provide an overview of the recently uncovered cellular and molecular mechanism during oocyte maturation, especially epigenetic modification, the progress of the regulatory network of oocyte meiosis progression including meiosis arrest and meiosis resumption induced by gonadotropins is summarized. Then, advances in the epigenetic aspects, such as histone acetylation, phosphorylation, methylation, glycosylation, ubiquitination, and SUMOylation related to the quality of oocyte maturation are reviewed.

Dissection of the Ovulatory Process Using ex vivo Approaches

Ovulation is a unique physiological phenomenon that is essential for sexual reproduction. It refers to the entire process of ovarian follicle responses to hormonal stimulation resulting in the release of mature fertilization-competent oocytes from the follicles and ovaries. Remarkably, ovulation in different species can be reproduced out-of-body with high fidelity. Moreover, most of the molecular mechanisms and signaling pathways engaged in this process have been delineated using in vitro ovulation models. Here, we provide an overview of the major molecular and cytological events of ovulation observed in frogs, primarily in the African clawed frog Xenopus laevis, using mainly ex vivo approaches, with the focus on meiotic oocyte maturation and follicle rupture. For the purpose of comparison and generalization, we also refer extensively to ovulation in other biological species, most notoriously, in mammals.

New markers for regulation of transcription and macromolecule metabolic process in porcine oocytes during in vitro maturation

Oocyte maturation is essential for proper fertilization, embryo implantation and early development. While the physiological conditions of these processes are relatively well-known, its exact molecular mechanisms remain widely undiscovered. Oocyte growth, differentiation and maturation are therefore the subject of scientific debate. Precious literature has indicated that the oocyte itself serves a regulatory role in the mechanisms underlying these processes. Hence, the present study performed expression microarrays to analyze the complete transcriptome of porcine oocytes during their in vitro maturation (IVM). Pig material was used for experimentation, as it possesses similarities to the reproductive processes and general genetic proximities of Sus scrofa to human. Oocytes, isolated from the ovaries of slaughtered animals were assessed via the Brilliant Cresyl Blue test and directed to IVM. A number of oocytes were left to be analyzed as the ‘before IVM’ group. Oocyte mRNA was isolated and used for microarray analysis, which was subsequently validated via RT-qPCR. The current study particularly focused on genes belonging to ‘positive regulation of transcription, DNA-dependent’, ‘positive regulation of gene expression’, ‘positive regulation of macromolecule metabolic process’ and ‘positive regulation of transcription from RNA polymerase II promoter’ ontologies. FOS, VEGFA, ESR1, AR, CCND2, EGR2, ENDRA, GJA1, INHBA, IHH, INSR, APP, WWTR1, SMARCA1, NFAT5, SMAD4, MAP3K1, EGR1, RORA, ECE1, NR5A1, KIT, IKZF2, MEF2C, SH3D19, MITF and PSMB4 were all determined to be significantly altered (fold change, >|2|; P<0.05) among these groups, with their downregulation being observed after IVM. Genes with the most altered expressions were analyzed and considered to be potential markers of maturation associated with transcription regulation and macromolecule metabolism process.

Endothelin-1 promotes human germinal vesicle-stage oocyte maturation by downregulating connexin-26 expression in cumulus cells

STUDY QUESTION

Does endothelin-1 (ET-1) promote human oocyte maturation and by what mechanism?

SUMMARY ANSWER

Addition of ET-1 to the medium in which human germinal vesicle (GV)-stage immature oocytes are cultured enhances the GV breakdown (GVBD) rate; the resumption of meiosis may be initiated by ET-1 downregulating the expression of connexin-26 (Cx26) in cumulus cells via endothelin receptor type B (ETRB), leading to decreased cAMP levels in the oocyte.

WHAT IS KNOWN ALREADY

The paracrine factor ET-1 is secreted by ovarian somatic cells in pre-ovulatory follicles and regulates oocyte maturation in mice. Connexins, or gap junction proteins, form intercellular membrane channels that play important roles in the resumption of meiosis.

STUDY DESIGN, SIZE, DURATION

This laboratory study was conducted over a 1-year period. The effects of ET-1 on meiotic resumption were evaluated in human GV-stage cumulus-oocyte complexes (COCs; 70 oocytes/group). The transcriptome profiles of ET-1-treated or untreated cumulus cells were compared to explore the possible mechanisms by which ET-1 may regulate oocyte maturation.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The ET-1, ETRA and ETRB expression levels in human cumulus cells from oocytes at different stages of maturation were evaluated using real-time quantitative PCR. Human GV-stage COCs collected from patients undergoing IVF at a university-affiliated infertility centre were cultured with or without ET-1, and cumulus cells were subsequently denuded using hyaluronidase and cultured in α-MEM. A GeneChip® Human Transcriptome Array was applied to explore differences in the whole-genome transcriptome profiles of cumulus cells treated with or without ET-1. Real-time quantitative PCR and Western blotting were used respectively to examine Cx26 mRNA and protein levels in cumulus cells. Changes in cAMP levels in both oocytes and cumulus cells after ET-1 treatment were measured using an enzyme-linked immunosorbent assay.

MAIN RESULTS AND THE ROLE OF CHANCE

Cumulus cells from MII-stage oocytes exhibited upregulated ET-1 expression, compared to those from GV-stage oocytes. The addition of ET-1 to the culture medium enhanced the GVBD rate of cumulus cell-enclosed human oocytes. Whole-genome transcriptome microarray analyses revealed significantly downregulated Cx26 expression in cumulus cells after ET-1 treatment, and this action was blocked by an ETRB antagonist. The involvement of Cx26 was further supported by the finding that ET-1 treatment led to decreased cAMP levels in oocytes but increased cAMP levels in cumulus cells.

LARGE SCALE DATA

Microarray data are published in the GEO database (GSE97684).

LIMITATIONS, REASONS FOR CAUTION

The heterogeneity of human COCs collected from patients undergoing IVF might affect the maturation results in vitro. Although we focused on the effects of ET-1 on human oocyte maturation in the present study, mammalian oocyte maturation is a complicated process involving many endocrine and paracrine factors.

WIDER IMPLICATIONS OF THE FINDINGS

Our present study suggests that in vitro, human GV-stage oocyte maturation could be enhanced by adding ET-1 to the culture medium. In the present study, we explored the molecular mechanisms by which ET-1 initiates the resumption of meiosis and demonstrated that ET-1 promotes oocyte maturation by downregulating the expression of the gap junction protein Cx26 in cumulus cells. These results expand our understanding of the molecular mechanisms underlying mammalian oocyte maturation and provide a basis for better in-vitro maturation strategies.

STUDY FUNDING AND COMPETING INTERESTS

This work was supported by grants from the China Natural Science Foundation (Grant Nos. 81170567 and 81370761). The authors declare that they have no conflicts of interest associated with this manuscript.

Connexin 43 coupling in bovine cumulus cells, during the follicular growth phase, and its relationship to in vitro embryo outcomes

Gap junctional coupling between cumulus cells is required for oocytes to reach developmental competence. Multiple connexins, which form these gap junctions, have been found within the ovarian follicles of several species including bovine. The aim of this study was to determine the role of connexin 43 (CX43) and its relationship to embryo development, after in vitro fertilization (IVF). Cumulus-oocyte complexes (COCs) were obtained from abattoir sourced, mixed breed, bovine ovaries. COCs were isolated from follicles ranging from 2 to 5 mm in size, representing the preselected follicle pool. Immediately after isolation, two cumulus cell biopsies were collected and stored for analysis pending determination of developmental outcomes. Using in vitro procedures, COCs were individually matured, fertilized, and cultured to the blastocyst stage. Biopsies were grouped as originating from COCs that arrested at the two-cell stage (low developmental competence [LDC]) or having developed to the late morula/blastocyst stage (high developmental competence [HDC]), after IVF and embryo culture. The expression level of CX43 was found to be significantly higher in cumulus cells from COCs that had an HDC when compared with those that had an LDC. Moreover, the gap junctional intercellular coupling rate was significantly higher in cumulus from COCs deemed to have an HDC. Significantly higher expression of the cumulus health markers luteinizing hormone receptor and cytochrome p450 19A1 was found in the cumulus originating from oocytes with HDC, suggesting that this system may provide a mechanism for noninvasively testing for oocyte health in preselected bovine follicles.

α-Tocopherol modifies the expression of genes related to oxidative stress and apoptosis during in vitro maturation and enhances the developmental competence of rabbit oocytes

The developmental competence of in vitro maturation (IVM) oocytes can be enhanced by antioxidant agents. The present study investigated, for the first time in the rabbit model, the effect of adding α-tocopherol (0, 100, 200 and 400 µM) during IVM on putative transcripts involved in antioxidant defence (superoxide dismutase 2, mitochondrial (SOD2), glutathione peroxidase 1 (GPX1), catalase (CAT)), cell cycle regulation and apoptosis cascade (apoptosis tumour protein 53 (TP53), caspase 3, apoptosis-related cysteine protease (CASP3)), cell cycle progression (cellular cycle V-Akt murine thymoma viral oncogene homologue 1 (AKT1)), cumulus expansion (gap junction protein, alpha 1, 43 kDa (GJA1) and prostaglandin-endoperoxide synthase 2 (prostaglandin G/H synthase and cyclo-oxygenase) (PTGS2)) and metabolism (glucose-6-phosphate dehydrogenase (G6PD)). Meiotic progression, mitochondrial reallocation, cumulus cell apoptosis and the developmental competence of oocytes after IVF were also assessed. Expression of SOD2, CAT, TP53, CASP3 and GJA1 was downregulated in cumulus–oocyte complexes (COCs) after IVM with 100 μM α-tocopherol compared with the group without the antioxidant. The apoptotic rate and the percentage of a non-migrated mitochondrial pattern were lower in COCs cultured with 100 μM α-tocopherol, consistent with better-quality oocytes. In fact, early embryo development was improved when 100 μM α-tocopherol was included in the IVM medium, but remained low compared with in vivo-matured oocytes. In conclusion, the addition of 100 μM α-tocopherol to the maturation medium is a suitable approach to manage oxidative stress and apoptosis, as well as for increasing the in vitro developmental competence of rabbit oocytes.

The switch from cAMP-independent to cAMP-dependent arrest of meiotic prophase is associated with coordinated GPR3 and CDK1 expression in mouse oocytes

Mammalian oocytes are arrested in meiotic prophase from around the time of birth until just before ovulation. Following an extended period of growth, they are stimulated to mature to the metaphase II stage by a preovulatory luteinizing hormone (LH) surge that occurs with each reproductive cycle. Small, growing oocytes are not competent to mature into fertilizable eggs because they do not possess adequate amounts of cell cycle regulatory proteins, particularly cyclin-dependent kinase 1 (CDK1). As oocytes grow, they synthesize CDK1 and acquire the ability to mature. After oocytes achieve meiotic competence, meiotic arrest at the prophase stage is dependent on high levels of cAMP that are generated in the oocyte under the control of the constitutively active G_s-coupled receptor, GPR3. In this study, we examined the switch between GPR3-independent and GPR3-dependent meiotic arrest. We found that the ability of oocytes to mature, as well as oocyte CDK1 levels, were dependent on follicle size, but CDK1 expression in oocytes from preantral follicles was not acutely altered by the activity of follicle stimulating hormone (FSH). Gpr3 was expressed and active in incompetent oocytes within early stage follicles, well before cAMP is required to maintain meiotic arrest. Oocytes from Gpr3^-/- mice were less competent to mature than oocytes from Gpr3^+/+ mice, as assessed by the time course of germinal vesicle breakdown. Correspondingly, Gpr3^-/- oocytes contained significantly lower CDK1 levels than their Gpr3^+/+ counterparts that were at the same stage of follicle development. These results demonstrate that GPR3 potentiates meiotic competence, most likely by raising cAMP.

Effect of urokinase type plasminogen activator on in vitro bovine oocyte maturation

This study examines the impacts of the urokinase-type plasminogen activator (uPA) on the in vitro maturation (IVM) of bovine oocytes. Cumulus-oocyte complexes in IVM medium were treated with uPA, amiloride (an uPA inhibitor), dimethyl sulfoxide (DMSO) or left untreated (control group). After 24 h of IVM, oocytes were recovered for testing or were in vitro fertilized and cultured to the blastocyst stage. The factors examined in all groups were: (i) oocyte nuclear maturation (Hoëscht staining); (ii) oocyte cytoplasmic maturation (cortical granules, CGs, distribution assessed by LCA-FITC); (iii) oocyte and cumulus cell (CC) gene expression (RT-qPCR); and (iv) embryo development (cleavage rate and blastocyst yield). Oocytes subjected to uPA treatment showed rates of nuclear maturation and CG distribution patterns similar to controls (P > 0.05), whereas lower rates of oocyte maturation were recorded in the amiloride group (P < 0.05). Both in oocytes and CC, treatment with uPA did not affect the transcription of genes related to apoptosis, cell junctions, cell cycle or serpin protease inhibitors. In contrast, amiloride altered the expression of genes associated with cell junctions, cell cycle, oxidative stress and CC serpins. No differences were observed between the control and uPA group in cleavage rate or in blastocyst yield recorded on Days 7, 8 or 9 post-insemination. However, amiloride led to drastically reduced cleavage rate (28.5% vs 83.2%) and Day 9 embryo production (6.0% vs 21.0%) over the rates recorded for DMSO. These results indicate that the proteolytic activity of uPA is needed for successful oocyte maturation in bovine.

Signaling-Mediated Regulation of Meiotic Prophase I and Transition During Oogenesis

Generation of healthy oocytes requires coordinated regulation of multiple cellular events and signaling pathways. Oocytes undergo a unique developmental growth and differentiation pattern interspersed with long periods of arrest. Oocytes from almost all species arrest in prophase I of oogenesis that allows for long period of growth and differentiation essential for normal oocyte development. Depending on species, oocytes that transit from prophase I to meiosis I also arrest at meiosis I for fairly long periods of time and then undergo a second arrest at meiosis II that is completed upon fertilization. While there are species-specific differences in C. elegans, D. melanogaster, and mammalian oocytes in stages of prophase I, meiosis I, or meiosis II arrest, in all cases cell signaling pathways coordinate the developmental events controlling oocyte growth and differentiation to regulate these crucial phases of transition. In particular, the ERK MAP kinase signaling pathway, cyclic AMP second messengers, and the cell cycle regulators CDK1/cyclin B are key signaling pathways that seem evolutionarily conserved in their control of oocyte growth and meiotic maturation across species. Here, I identify the common themes and differences in the regulation of key meiotic events during oocyte growth and maturation.

Differential gene-expression profiles from canine cumulus cells of ovulated versus in vitro-matured oocytes

We compared the nuclear maturation status and gene-expression profiles of canine cumulus cells (CCs) derived from cumulus-oocyte complexes (COCs) that were spontaneously ovulated versus those that were matured in vitro. Cumulus-oocyte complexes were retrieved from uteri by surgical flushing (after spontaneous ovulation) or by ovariectomy follicle aspiration and in vitro maturation. The objective of Experiment 1 was to investigate the nuclear maturation status of in vivo- versus in vitro-matured oocytes. The objective of Experiment 2 was to compare gene-expression profiles of CCs derived from in vivo- versus in vitro-matured COCs. Genes analysed are related to cell maturation, development and apoptosis, including GDF9, MAPK1, PTX3, CX43, Bcl2 and BAX; mRNA expression for all of these genes, except for GDF9, differed (P<0.05) between in vivo- and in vitro-matured CCs. In conclusion, we found that gene-expression profiles are related to the quality of CCs and therefore posit that monitoring gene expression could be a useful strategy to guide attempts to improve in vitro culture systems.

In vivo and in vitro maturation of rabbit oocytes differently affects the gene expression profile, mitochondrial distribution, apoptosis and early embryo development

In vivo-matured cumulus–oocyte complexes are valuable models in which to assess potential biomarkers of rabbit oocyte quality that contribute to enhanced IVM systems. In the present study we compared some gene markers of oocytes and cumulus cells (CCs) from immature, in vivo-matured and IVM oocytes. Moreover, apoptosis in CCs, nuclear maturation, mitochondrial reallocation and the developmental potential of oocytes after IVF were assessed. In relation to cumulus expansion, gene expression of gap junction protein, alpha 1, 43 kDa (Gja1) and prostaglandin-endoperoxide synthase 2 (Ptgs2) was significantly lower in CCs after in vivo maturation than IVM. In addition, there were differences in gene expression after in vivo maturation versus IVM in both oocytes and CCs for genes related to cell cycle regulation and apoptosis (V-Akt murine thymoma viral oncogene homologue 1 (Akt1), tumour protein 53 (Tp53), caspase 3, apoptosis-related cysteine protease (Casp3)), oxidative response (superoxide dismutase 2, mitochondrial (Sod2)) and metabolism (glucose-6-phosphate dehydrogenase (G6pd), glyceraldehyde-3-phosphate dehydrogenase (Gapdh)). In vivo-matured CCs had a lower apoptosis rate than IVM and immature CCs. Meiotic progression, mitochondrial migration to the periphery and developmental competence were higher for in vivo-matured than IVM oocytes. In conclusion, differences in oocyte developmental capacity after IVM or in vivo maturation are accompanied by significant changes in transcript abundance in oocytes and their surrounding CCs, meiotic rate, mitochondrial distribution and apoptotic index. Some of the genes investigated, such as Gja1, could be potential biomarkers for oocyte developmental competence in the rabbit model, helping improve in vitro culture systems in these species.

Effect of removing cumulus cells from porcine cumulus-oocyte complexes derived from small and medium follicles during IVM on the apoptotic status and meiotic progression of the oocytes

The present study was undertaken to examine the apoptotic status and meiotic progression of oocytes from small follicle (SF; 0.5-2 mm in diameter) and medium follicle (MF; 3-6 mm in diameter) when the oocytes were denuded before, during, and after IVM. Cumulus-oocyte complexes (COCs) were collected from SF or MF of prepubertal gilt ovaries. Before or 20 hours after the start of IVM culture, some oocytes were denuded and cultured for IVM. At the end of IVM, apoptotic status and meiotic progression of the oocytes were compared with oocytes matured in the presence of cumulus cells (CCs) by Annexin-V/PI assay and 4',6-Diamidino-2-phenylindole staining. Apoptotic status of the oocytes was only affected by time when the oocytes were denuded. In both oocytes from SF and MF, although the incidence of early and late apoptotic oocytes was significantly higher when the CCs were removed before IVM, the rate was significantly lower when CCs were removed 20 and 44 hours after the start of IVM. The incidence of mature oocytes was significantly affected by both the origin of COCs and time when oocytes were denuded from the COCs. Although the percentage of mature oocytes was higher in MF than SF, maturation rates were significantly higher when oocytes were denuded 20 hours, as compared with 0 and 44 hours after the start of IVM. However, the percentage of mature oocytes with a morphologically normal spindle was significantly higher when oocytes were denuded 44 hours, rather than 22 hours of IVM. In conclusion, removing CCs 20 hours after the start of IVM seems to promote meiotic progression of the oocytes to the metaphase-II stage even when the COCs were collected from SF, although factor(s) from or communication with CCs during IVM may need to obtain a morphologically normal spindle in mature oocytes.

Studies on lysophosphatidic acid action during in vitro preimplantation embryo development

Assisted reproductive technologies, including in vitro embryo production (IVP), have been successfully used in animal reproduction to optimize breeding strategies for improved production and health in animal husbandry. Despite the progress in IVP techniques over the years, further improvements in in vitro embryo culture systems are required for the enhancement of oocyte and embryo developmental competence. One of the most important issues associated with IVP procedures is the optimization of the in vitro culture of oocytes and embryos. Studies in different species of animals and in humans have identified important roles for receptor-mediated lysophosphatidic acid (LPA) signaling in multiple aspects of human and animal reproductive tract function. The data on LPA signaling in the ovary and uterus suggest that LPA can directly contribute to embryo-maternal interactions via its influence on early embryo development beginning from the influence of the ovarian environment on the oocyte to the influence of the uterine environment on the preimplantation embryo. This review discusses the current status of LPA as a potential supplement in oocyte maturation, fertilization, and embryo culture media and current views on the potential involvement of the LPA signaling pathway in early embryo development.

Selective Regulation of Oocyte Meiotic Events Enhances Progress in Fertility Preservation Methods

Following early embryonic germ cell migration, oocytes are surrounded by somatic cells and remain arrested at diplotene stage until luteinizing hormone (LH) surge. Strict regulation of both meiotic arrest and meiotic resumption during dormant stage are critical for future fertility. Inter-cellular signaling system between the somatic compartment and oocyte regulates these meiotic events and determines the follicle quality. As well as the collected number of eggs, their qualities are also important for in vitro fertilization (IVF) outcome. In spontaneous and IVF cycles, germinal vesicle (GV)–stage oocytes, premature GV breakdown, and persistence of first meiotic arrest limit the reproductive performance. Likewise, both women with premature ovarian aging and young cancer women are undergoing chemoradiotherapy under the risk of follicle loss because of unregulated meiotic events. Understanding of oocyte meiotic events is therefore critical for the prevention of functional ovarian reserve. High levels of cyclic guanosine monophophate (cGMP), cyclic adenosine monophophate (cAMP) and low phosphodiesterase (PDE) 3A enzyme activity inside the oocyte are responsible for maintaining of meiotic arrest before the LH surge. cGMP is produced in the somatic compartment, and natriuretic peptide precursor C (Nppc) and natriuretic peptide receptor 2 (Npr2) regulate its production. cGMP diffuses into the oocyte and reduces the PDE3A activity, which inhibits the conversion of cAMP to the 5′AMP, and cAMP levels are enhanced. In addition, oocyte itself has the ability to produce cAMP. Taken together, accumulation of cAMP inside the oocyte induces protein kinase activity, which leads to the inhibition of maturation-promoting factor and meiotic arrest also continues. By stimulating the expression of epidermal growth factor, LH inhibits the Nppc/Npr2 system, blocks cGMP synthesis, and initiates meiotic resumption. Oocytes lacking the functional of this pathway may lead to persistence of the GV oocyte, which reduces the number of good quality eggs. Selective regulation of somatic cell signals and oocyte meiotic events enhance progress in fertility preservation methods, which may give us the opportunity to prevent follicle loss in prematurely aging women and young women with cancer are undergoing chemoradiotherapy.

Correlation of cumulus gene expression of GJA1, PRSS35, PTX3, and SERPINE2 with oocyte maturation, fertilization, and embryo development

BACKGROUND

GJA1 and PTX3 were proposed as gene markers for oocyte and embryo developmental competence, while SERPINE2 was reported to be associated with pregnancy outcome. PRSS35, which is exclusively expressed in the ovary, may be correlated with oocyte competence. This study was conducted to evaluate the correlation of cumulus GJA1, PRSS35, PTX3, and SERPINE2 gene expression levels with oocyte maturation, fertilization, and early embryo development.

METHODS

In total, 308 cumulus cell samples separated from individual cumulus-oocyte complex were obtained from 40 patients undergoing the intracytoplasmic sperm injection treatment procedure. Gene expression levels (mRNA levels) in cumulus cells were assessed using quantitative real-time polymerase chain reaction.

RESULTS

Gene expression levels of GJA1 and SERPINE2 in cumulus cells surrounding mature oocytes were significantly lower than those in cumulus cells enclosing immature oocytes. PRSS35 mRNA levels in cumulus cells of fertilized oocytes were significantly higher than those in cumulus cells of unfertilized oocytes. GJA1 and SERPINE2 seemed to express higher mRNA levels, while PRSS35 showed lower expression in cumulus cells of oocytes that developed into embryos with good morphology; however, the expression levels of all three genes and PTX3 showed no significant differences between embryos with good or poor morphology.

CONCLUSIONS

GJA1 and SERPINE2 represent potential gene markers associated with oocyte maturation. PRSS35 may be correlated with oocyte fertilization potential. However, GJA1, PRSS35, PTX3, and SERPINE2 may not be considered as marker genes for predicting embryo morphology.

Specific deletion of AMP-activated protein kinase (α1AMPK) in murine oocytes alters junctional protein expression and mitochondrial physiology

Oogenesis and folliculogenesis are dynamic processes that are regulated by endocrine, paracrine and autocrine signals. These signals are exchanged between the oocyte and the somatic cells of the follicle. Here we analyzed the role of AMP-activated protein kinase (AMPK), an important regulator of cellular energy homeostasis, by using transgenic mice deficient in α1AMPK specifically in the oocyte. We found a decrease of 27% in litter size was observed in ZP3-α1AMPK-/- (ZP3-KO) female mice. Following in vitro fertilization, where conditions are stressful for the oocyte and embryo, ZP3-KO oocytes were 68% less likely to pass the 2-cell stage. In vivo and in cumulus-oocyte complexes, several proteins involved in junctional communication, such as connexin37 and N-cadherin were down-regulated in the absence of α1AMPK. While the two signalling pathways (PKA and MAPK) involved in the junctional communication between the cumulus/granulosa cells and the oocyte were stimulated in control oocytes, ZP3-KO oocytes exhibited only low phosphorylation of MAPK or CREB proteins. In addition, MII oocytes deficient in α1AMPK had a 3-fold lower ATP concentration, an increase in abnormal mitochondria, and a decrease in cytochrome C and PGC1α levels, suggesting perturbed energy production by mitochondria. The absence of α1AMPK also induced a reduction in histone deacetylase activity, which was associated with an increase in histone H3 acetylation (K9/K14 residues). Together, the results of the present study suggest that absence of AMPK, modifies oocyte quality through energy processes and oocyte/somatic cell communication. The limited effect observed in vivo could be partly due to a favourable follicle microenvironment where nutrients, growth factors, and adequate cell interaction were present. Whereas in a challenging environment such as that of in vitro culture following IVF, the phenotype is revealed.

The fundamental role of bone morphogenetic protein 15 in ovarian function and its involvement in female fertility disorders

BACKGROUND

A large number of studies have contributed to understanding the general mechanisms driving ovarian folliculogenesis in humans and show a complex endocrine dialog between the central nervous system, the pituitary and the ovary, integrated by various intraovarian paracrine messages. The role of intraovarian paracrine regulation has acquired more relevance in the recent years owing to the discovery of previously unknown factors, such as the oocyte-derived bone morphogenetic protein (BMP)15.

METHODS

A thorough literature search was carried out in order to summarize what has been reported so far on the role of BMP15, and the BMP15 paralog, growth and differentiation factor 9 (GDF9), in ovarian function and female fertility. Research articles published in English until March 2014 were included.

RESULTS

The biological actions of BMP15 include: (i) the promotion of follicle growth and maturation starting from the primary gonadotrophin-independent phases of folliculogenesis; (ii) the regulation of follicular granulosa cell (GC) sensitivity to FSH action and the determination of ovulation quota; (iii) the prevention of GC apoptosis and (iv) the promotion of oocyte developmental competence. The existence of biologically active heterodimers with GDF9, and/or the synergistic co-operation of BMP15 and GDF9 homodimers are indeed relevant in this context. Experimental disruption of the bmp15 gene in mice resulted in a mild fertility defect limited to females, whereas natural missense mutations in ewes cause variable phenotypes (ranging from hyperprolificacy to complete sterility) depending on a fine gene dosage mechanism also involving GDF9. Strong evidence supports the concept that such a mechanism plays an important role in the regulation of ovulation rate across mammalian and non-mammalian species. Following the discovery of sheep fecundity genes, several research groups have focused on alterations in human BMP15 associated with primary ovarian insufficiency (POI) or polycystic ovary syndrome. Several variants of BMP15 are significantly associated with POI supporting their pathogenic role, but the underlying biological mechanism is still under investigation and of great interest in medicine. BMP15 maps to the Xp locus involved in the determination of the ovarian defect in Turner syndrome and significantly contributes to the determination of ovarian reserve. Pioneering studies in women undergoing controlled ovarian stimulation indicate that BMP15 may represent a marker of ovarian response or oocyte quality.

CONCLUSIONS

BMP15, an oocyte-derived growth and differentiation factor, is a critical regulator of folliculogenesis and GC activities. Variations in BMP15 gene dosage have a relevant influence on ovarian function and can account for several defects of female fertility. The modulation of BMP15 action may have interesting pharmacological perspectives and the analysis of BMP15 may become a useful marker in IVF procedures. Recent outcomes indicate that the close interactions of BMP15/GDF9 have a critical biological impact that should be taken into account in future studies.

The RNA-binding protein, ZFP36L2, influences ovulation and oocyte maturation

ZFP36L2 protein destabilizes AU-rich element-containing transcripts and has been implicated in female fertility. In the C57BL/6NTac mouse, a mutation in Zfp36l2 that results in the decreased expression of a form of ZFP36L2 in which the 29 N-terminal amino acid residues have been deleted, ΔN-ZFP36L2, leads to fertilized eggs that arrest at the two-cell stage. Interestingly, homozygous ΔN-Zfp36l2 females in the C57BL/6NTac strain release 40% fewer eggs than the WT littermates (Ramos et al., 2004), suggesting an additional defect in ovulation and/or oocyte maturation. Curiously, the same ΔN-Zfp36l2 mutation into the SV129 strain resulted in anovulation, prompting us to investigate a potential problem in ovulation and oocyte maturation. Remarkably, only 20% of ΔN-Zfp36l2 oocytes in the 129S6/SvEvTac strain matured ex vivo, suggesting a defect on the oocyte meiotic maturation process. Treatment of ΔN-Zfp36l2 oocytes with a PKA inhibitor partially rescued the meiotic arrested oocytes. Furthermore, cAMP levels were increased in ΔN-Zfp36l2 oocytes, linking the cAMP/PKA pathway and ΔN-Zfp36l2 with meiotic arrest. Since ovulation and oocyte maturation are both triggered by LHR signaling, the downstream pathway was investigated. Adenylyl cyclase activity was increased in ΔN-Zfp36l2 ovaries only upon LH stimulation. Moreover, we discovered that ZFP36L2 interacts with the 3′UTR of LHR mRNA and that decreased expression levels of Zfp36l2 correlates with higher levels of LHR mRNA in synchronized ovaries. Furthermore, overexpression of ZFP36L2 decreases the endogenous expression of LHR mRNA in a cell line. Therefore, we propose that lack of the physiological down regulation of LHR mRNA levels by ZFP36L2 in the ovaries is associated with anovulation and oocyte meiotic arrest.

The regulation of maturation promoting factor during prophase I arrest and meiotic entry in mammalian oocytes

Mammalian oocytes arrest at prophase of meiosis I at around birth and they remain arrested at this stage until puberty when the preovulatory surge of luteinizing hormone (LH) causes ovulation. Prophase I arrest in the immature oocyte results from the maintenance of low activity of maturation promoting factor (MPF), which consists of a catalytic subunit (CDK1) and regulatory subunit (cyclin B1). Phosphorylation-mediated inactivation of CDK1 and constant degradation of cyclin B1 keep MPF activity low during prophase I arrest. LH-mediated signaling manipulates a vast array of molecules to activate CDK1. Active CDK1 not only phosphorylates different meiotic phosphoproteins during the resumption of meiosis but also inhibits their rapid dephosphorylation by inhibiting the activities of CDK1 antagonizing protein phosphatases (PPs). In this way, CDK1 both phosphorylates its substrates and protects them from being dephosphorylated. Accumulating evidence suggests that the net MPF activity that drives the resumption of meiosis in oocytes depends on the activation status of CDK1 antagonizing PPs. This review aims to provide a summary of the current understanding of the signaling pathways involved in regulating MPF activity during prophase I arrest and reentry into meiosis of mammalian oocytes.

Lysophosphatidic acid stimulates hyaluronan production by mouse cumulus-oocyte complexes

Purpose

In mammals, cumulus expansion due to increased synthesis of hyaluronan was suggested to correlate with modification of the gap junction between cumulus cells and the oocyte, leading to cumulus expansion. We examined whether lysophosphatidic acid, a lipid mediator detected in mammalian body fluids, stimulates significant production of hyaluronan and thus affects mouse cumulus expansion in vitro.

Methods

Cumulus-oocyte complexes isolated from the gonadotropin-treated ovaries of B6C3F1 mice were exposed to lysophosphatidic acid in the presence and absence of 0.3 % fetal bovine serum for measurement of cumulus expansion and released hyaluronan, respectively.

Results

Exogenously added lysophosphatidic acid concentration-dependently stimulated production of hyaluronan in the cumulus cell-oocyte complex, and the stimulatory effect of lysophosphatidic acid on hyaluronan production was mediated through the signal pathways, including LPA receptor-Gi coupling, EGF receptor transactivation, and activations of phosphatidylinositol-specific phospholipase C, protein kinase C and mitogen-activated protein kinases. LPA increased mRNA expression of tumor necrosis α-induced protein 6, a hyaluronan-binding protein, and expansion of cumulus cell-oocyte complex.

Conclusions

Lysophosphatidic acid in follicular fluids may participate in physiological cumulus expansion before ovulation by stimulating production of hyaluronan and proteins that enable the association of hyaluronan with cumulus cells and oocytes.

Biology and biotechnology of follicle development

Growth and development of ovarian follicles require a series of coordinated events that induce morphological and functional changes within the follicle, leading to cell differentiation and oocyte development. The preantral early antral follicle transition is the stage of follicular development during which gonadotropin dependence is obtained and the progression into growing or atresia of the follicle is made. Follicular growth during this period is tightly regulated by oocyte-granulosatheca cell interactions. A cluster of early expressed genes is required for normal folliculogenesis. Granulosa cell factors stimulate the recruitment of theca cells from cortical stromal cells. Thecal factors promote granulosa cell proliferation and suppress granulosa cell apoptosis. Cell-cell and cell-extracellular matrix interactions influence the production of growth factors in the different follicular compartments (oocyte, granulosa, and theca cells). Several autocrine and paracrine factors are involved in follicular growth and differentiation; their activity is present even at the time of ovulation, decreasing the gap junction communication, and stimulating the theca cell proliferation. In addition, the identification of the factors that promote follicular growth from the preantral stage to the small antral stage may provide important information for the identification for assisted reproduction techniques.

Developmental control of oocyte maturation and egg activation in metazoan models

Production of functional eggs requires meiosis to be coordinated with developmental signals. Oocytes arrest in prophase I to permit oocyte differentiation, and in most animals, a second meiotic arrest links completion of meiosis to fertilization. Comparison of oocyte maturation and egg activation between mammals, Caenorhabditis elegans, and Drosophila reveal conserved signaling pathways and regulatory mechanisms as well as unique adaptations for reproductive strategies. Recent studies in mammals and C. elegans show the role of signaling between surrounding somatic cells and the oocyte in maintaining the prophase I arrest and controlling maturation. Proteins that regulate levels of active Cdk1/cyclin B during prophase I arrest have been identified in Drosophila. Protein kinases play crucial roles in the transition from meiosis in the oocyte to mitotic embryonic divisions in C. elegans and Drosophila. Here we will contrast the regulation of key meiotic events in oocytes.

The APC/C activator FZR1 coordinates the timing of meiotic resumption during prophase I arrest in mammalian oocytes

FZR1, an activator of the anaphase-promoting complex/cyclosome (APC/C), is recognized for its roles in the mitotic cell cycle. To examine its meiotic function in females we generated an oocyte-specific knockout of the Fzr1 gene (Fzr1Δ/Δ). The total number of fully grown oocytes enclosed in cumulus complexes was 35-40% lower in oocytes from Fzr1Δ/Δ mice and there was a commensurate rise in denuded, meiotically advanced and/or fragmented oocytes. The ability of Fzr1Δ/Δ oocytes to remain prophase I/germinal vesicle (GV) arrested in vitro was also compromised, despite the addition of the phosphodiesterase milrinone. Meiotic competency of smaller diameter oocytes was also accelerated by Fzr1 loss. Cyclin B1 levels were elevated ~5-fold in Fzr1Δ/Δ oocytes, whereas securin and CDC25B, two other APC/CFZR1 substrates, were unchanged. Cyclin B1 overexpression can mimic the effects of Fzr1 loss on GV arrest and here we show that cyclin B1 knockdown in Fzr1Δ/Δ oocytes affects the timing of meiotic resumption. Therefore, the effects of Fzr1 loss are mediated, at least in part, by raised cyclin B1. Thus, APC/CFZR1 activity is required to repress cyclin B1 levels in oocytes during prophase I arrest in the ovary, thereby maintaining meiotic quiescence until hormonal cues trigger resumption.

A combination of FSH and dibutyryl cyclic AMP promote growth and acquisition of meiotic competence of oocytes from early porcine antral follicles

Growing porcine oocytes from early antral follicles (1.2-1.5 mm in diameter) do not mature to metaphase II (MII, 4%) under culture conditions which supported maturation (MII, 95%) of fully grown oocytes from large (4-6 mm) antral follicles. We hypothesized that FSH and dbcAMP supported growth and acquisition of meiotic competence. Growing oocytes (113.0 ± 0.4 μm, mean ± SEM) were cultured for 5 d in medium supplemented with 1 mM dbcAMP, 0.01 IU/mL FSH or both; in these media, oocytes reached, 120.5 ± 0.4, 123.5 ± 0.4 and 125.7 ± 0.2 μm, respectively, after 5 d, and then were matured in vitro for 48 h. Oocytes remained enclosed by cumulus cells when cultured with FSH (82%) or both FSH and dbcAMP (80%), but not with dbcAMP alone (0%). Furthermore, oocytes cultured with FSH maintained trans-zonal projections of cumulus cells. Oocytes remained at the GV stage at higher rates when cultured with dbcAMP and FSH (99%), or dbcAMP (97%), than with FSH (64%), or without either (75%). Following in vitro maturation, oocytes reached MII after in vitro growth with dbcAMP (19%), FSH (11%), or both (68%). When oocytes were cultured with both FSH and dbcAMP, activation of Cdc2 and MAP kinases in growing oocytes was similar to fully grown oocytes. In conclusion, growing porcine oocytes grew and acquired meiotic competence in medium supplemented with dbcAMP and FSH; the former maintained oocytes in meiotic arrest, whereas the latter maintained trans-zonal projections of cumulus cells to oocytes during in vitro growth culture.

The canine oocyte: uncommon features of in vivo and in vitro maturation

The biology of the canine oocyte is unusual compared with that of other mammalian females. The present paper reviews both in vivo and in vitro specificities of canine oocytes. Final follicular growth in the bitch is characterised by an early appearance of LH binding sites in the granulosa, a high proportion of polyovular follicles and a preovulatory luteinisation, starting at the time of the LH surge. Through follicular fluid, preovulatory oocytes are thus exposed to high levels of progesterone, as high as 1000-fold plasma concentrations. The composition of the follicular fluid is affected by the size of the female. The more specific aspect of oocyte biology in the bitch is ovulation: oocytes are expelled immature, at the Prophase I stage. Ovulatory follicles are 6–8 mm in diameter, releasing oocytes from 110 µm, with dark cytoplasm. Resumption of meiosis occurs from 48 h postovulation, MII stages appearing 48–54 h after ovulation. The mechanisms controlling such a late meiotic resumption are still unknown. Granulosa cells seem to play a central role as in other mammalian species, but not with cAMP as the principal mediator. The importance of a transient reactivation of oocyte transcription a few hours before meiotic resumption is to be explored. These specific features may contribute to the low efficiency of IVM. Only 10–20% oocytes reach the metaphase stage and suffer from a poor cytoplasmic maturation. Moreover, in vitro culture of canine oocytes is associated with a high proportion of degeneration. To date, IVM of the oocytes is the main limiting factor for the development of assisted reproductive techniques in the canine. A better knowledge of the basic physiology of folliculogenesis and the molecular mechanisms controlling oocyte meiosis resumption in this species may allow us to overcome this obstacle.

Bidirectional communication between oocytes and follicle cells: ensuring oocyte developmental competence

Female fertility is determined to a large extent by the quality (developmental competence) of the oocyte as reflected in its ability to undergo meiosis, be fertilized, and give rise to a healthy embryo. Growth of the mammalian oocyte is coordinated with that of the follicle that encloses it by the actions of signals that pass in both directions between the germline and somatic components. This review summarizes what is known about the roles played by 2 different modes of intrafollicular signalling in oogenesis: paracrine factors activating receptors on the opposite cell type, and direct sharing of small molecules throughout the follicle via gap junction channels. Recent evidence indicates that these 2 modes of signalling interact to regulate oocyte growth and granulosa cell proliferation and that defects in either can contribute to female infertility.

Connexins: sensors and regulators of cell cycling

It is nowadays well established that gap junctions are critical gatekeepers of cell proliferation, by controlling the intercellular exchange of essential growth regulators. In recent years, however, it has become clear that the picture is not as simple as originally anticipated, as structural precursors of gap junctions can affect cell cycling by performing actions not related to gap junctional intercellular communication. Indeed, connexin hemichannels also foresee a pathway for cell growth communication, albeit between the intracellular compartment and the extracellular environment, while connexin proteins as such can directly or indirectly influence the production of cell cycle regulators independently of their channel activities. Furthermore, a novel set of connexin-like proteins, the pannexins, have lately joined in as regulators of the cell proliferation process, which they can affect as either single units or as channel entities. In the current paper, these multifaceted aspects of connexin-related signalling in cell cycling are reviewed.

RFD Award Lecture 2009. In vitro maturation of farm animal oocytes: a useful tool for investigating the mechanisms leading to full-term development

Due to logistical and economic reasons, assisted reproduction of domestic animals has been based mostly on the use of oocytes isolated from ovaries collected at the slaughterhouse. In order to propagate valuable or rare genetic material, perform somatic cell nuclear transfer or generate genetically modified animals, it is essential to obtain fully competent oocytes that will allow full-term development of the in vitro-produced embryos. Such a need makes clear the crucial role played by oocyte quality. In fact, it is easy to compromise the oocyte's developmental potential but it is impossible to restore once it has been lost. Almost three decades after the first cow, sheep, goat, horse and pig in vitro-generated offspring were born, a large body of information has accumulated on the mechanisms regulating oocyte competence and on how the latter may be preserved during all the required manipulations. The amount of knowledge is far from complete and many laboratories are actively working to further expand it. In this review we will highlight the aspects of the ongoing research in which we have been actively involved.

The Role of Gap Junction Proteins in Infertility

Testis and ovary serve an important role of producing male and female gametes. Their normal functioning is very important for the proper formation of sperm and ovum and thus has a critical role in the successful fertility outcome. Synchronized activity of various cells in the gonads is needed to provide favorable niche for the growth and development of the germ cells. Among various ways of cellular communication, intercellular communication is mediated by gap junctions, which provides open but selective exchange of ion and molecules of restricted size between two adjoining cells. The basic unit of gap junction is connexins. Their important role has been speculated in the maintenance of homeostasis, morphogenesis, cell differentiation, and growth control in higher organisms. The expression of gap junction proteins in reproductive tissues has drawn the attention and interest of researcher to investigate their role in the reproductive outcome. The reports about the correlation of gap junction protein expression pattern in infertility patients and in animal models have suggested their implication in fertility. Some of these gap junction proteins seem to have redundant functions, whereas some could be very critical in the normal fertility and could not be dispensable for the successful outcome of the reproduction.

cAMP pulsing of denuded mouse oocytes increases meiotic resumption via activation of AMP-activated protein kinase

cAMP plays a critical role in the control of oocyte maturation, as a high level of cAMP maintains oocyte arrest at the first meiotic prophase. Yet this study shows that pulsing meiotically arrested denuded oocytes (DO) with cAMP induces oocyte maturation through the activation of AMP-activated protein kinase (PRKA). Short-term (3 h) pulsing of meiotically arrested oocytes with forskolin, an adenyl cyclase (AC) activator, increased oocyte cAMP, led to elevated AMP, and induced oocyte meiotic resumption compared to oocytes continuously cultured in the control medium with or without forskolin. Western analysis showed that germinal vesicle (GV)-stage oocytes after forskolin pulsing contained increased levels of phospho-acetyl CoA carboxylase (pACACA), a primary substrate of PRKA. Pulsing oocytes with the phosphodiesterase (PDE)-sensitive cAMP analog, 8-bromo-cAMP (8-Br-cAMP), also increased pACACA and pPRKA levels in GV-stage oocytes and induced oocyte meiotic resumption. Moreover, the PRKA inhibitors, compound C and araA, prevented 8-Br-cAMP pulsing-induced maturation. The lack of effect on meiotic induction and PRKA activation when oocytes were pulsed with the PDE-resistant activators of cAMP-dependent protein kinase, Sp-cAMP-AM and Sp-5,6-DCI-cBIMPS, suggests that cAMP degradation is required for pulsing-induced maturation. Pulsing oocytes with the exchange protein directly activated by cAMP (Epac)-specific activator, 8-CPT-2'-O-Me-cAMP, had no stimulatory effect on oocyte maturation, suggesting Epac is not involved in the pulsing-induced maturation. Taken together, these data support the idea that a transient increase in oocyte cAMP can induce meiotic resumption via activation of PRKA.

Connexin expression and gap junctional coupling in human cumulus cells: contribution to embryo quality

Gap junctional coupling among cumulus cells is important for oogenesis since its deficiency in mice leads to impaired folliculogenesis. Multiple connexins (Cx), the subunits of gap junction channels, have been found within ovarian follicles in several species but little is known about the connexins in human follicles. The aim of this study was to determine which connexins contribute to gap junctions in human cumulus cells and to explore the possible relationship between connexin expression and pregnancy outcome from in vitro fertilization (IVF). Cumulus cells were obtained from IVF patients undergoing intra-cytoplasmic sperm injection (ICSI). Connexin expression was examined by RT-PCR and confocal microscopy. Cx43 was quantified by immunoblotting and gap junctional coupling was measured by patch-clamp electrophysiology. All but 5 of 20 connexin mRNAs were detected. Of the connexin proteins detected, Cx43 forms numerous gap junction-like plaques but Cx26, Cx30, Cx30.3, Cx32 and Cx40 appeared to be restricted to the cytoplasm. The strength of gap junctional conductance varied between patients and was significantly and positively correlated with Cx43 level, but neither was correlated with patient age. Interestingly, Cx43 level and intercellular conductance were positively correlated with embryo quality as judged by cleavage rate and morphology, and were significantly higher in patients who became pregnant than in those who did not. Thus, despite the presence of multiple connexins, Cx43 is a major contributor to gap junctions in human cumulus cells and its expression level may influence pregnancy outcome after ICSI.

Gap junctions in the ovary: expression, localization and function

Gap junctions that allow the direct communication between cytoplasmic compartments of neighboring cells are present in a variety of tissues and organs and play pivotal roles in a wide range of physiological processes. In the ovary, gap junctions consist mainly of connexin (Cx) 43 and Cx37, and their indispensable role in regulating folliculogenesis and oogenesis is well established. The ovarian Cx43 is regulated by gonadotropins at the transcriptional, translational and post-translational levels whereas the regulation of the ovarian Cx37 is yet unknown. In addition to their involvement in normal ovarian functions, gap junction proteins, particularly Cx43, seem to act as cancer suppressors. A summary of our present knowledge regarding gap junctional communication (GJC) and the ovarian gap junction proteins in normally developing ovaries and under pathological conditions is presented in this review.

Spindle formation, chromosome segregation and the spindle checkpoint in mammalian oocytes and susceptibility to meiotic error

The spindle assembly checkpoint (SAC) monitors attachment to microtubules and tension on chromosomes in mitosis and meiosis. It represents a surveillance mechanism that halts cells in M-phase in the presence of unattached chromosomes, associated with accumulation of checkpoint components, in particular, Mad2, at the kinetochores. A complex between the anaphase promoting factor/cylosome (APC/C), its accessory protein Cdc20 and proteins of the SAC renders APC/C inactive, usually until all chromosomes are properly assembled at the spindle equator (chromosome congression) and under tension from spindle fibres. Upon release from the SAC the APC/C can target proteins like cyclin B and securin for degradation by the proteasome. Securin degradation causes activation of separase proteolytic enzyme, and in mitosis cleavage of cohesin proteins at the centromeres and arms of sister chromatids. In meiosis I only the cohesin proteins at the sister chromatid arms are cleaved. This requires meiosis specific components and tight regulation by kinase and phosphatase activities. There is no S-phase between meiotic divisions. Second meiosis resembles mitosis. Mammalian oocytes arrest constitutively at metaphase II in presence of aligned chromosomes, which is due to the activity of the cytostatic factor (CSF). The SAC has been identified in spermatogenesis and oogenesis, but gender-differences may contribute to sex-specific differential responses to aneugens. The age-related reduction in expression of components of the SAC in mammalian oocytes may act synergistically with spindle and other cell organelles' dysfunction, and a partial loss of cohesion between sister chromatids to predispose oocytes to errors in chromosome segregation. This might affect dose-response to aneugens. In view of the tendency to have children at advanced maternal ages it appears relevant to pursue studies on consequences of ageing on the susceptibility of human oocytes to the induction of meiotic error by aneugens and establish models to assess risks to human health by environmental exposures.

Mechanisms Regulating Oocyte Meiotic Resumption: Roles of Mitogen-Activated Protein Kinase

Oocyte meiotic maturation is one of the important physiological requirements for species survival. However, little is known about the detailed events occurring during this process. A number of studies have demonstrated that MAPK plays a pivotal role in the regulation of meiotic cell cycle progression in oocytes, but controversial findings have been reported in both lower vertebrates and mammals. In this review, we summarized the roles of MAPK cascade and related signal pathways in oocyte meiotic reinitiation in both lower vertebrates and mammals. We also tried to reconcile the paradoxical results and highlight the new findings concerning the function of MAPK in both oocytes and the surrounding follicular somatic cells. The unresolved questions and future research directions regarding the role of MAPK in meiotic resumption are addressed.

Meiotic resumption of porcine immature oocytes is prevented by ooplasmic Gsalpha functions

A high cyclic adenosine monophosphate (cAMP) level in fully-grown immature oocytes prevents meiotic resumption. In Xenopus, inhibitory cAMP is synthesized within oocytes depending on a stimulatory alpha-subunit of G-protein (Gsalpha). In the present study, we examined whether ooplasmic Gsalpha is involved in meiotic arrest of porcine oocytes. First, we studied the presence of Gsalpha molecules in porcine oocytes by immunoblotting, and this suggested the presence of reported isoforms (45 and 48 kDa) not only in cumulus cells but also in porcine oocytes. Then we injected an anti-Gsalpha antibody into porcine immature oocytes and found that inhibition of ooplasmic Gsalpha functions significantly promoted germinal vesicle breakdown of the oocytes, whose spontaneous meiotic resumption was prevented by 3-isobutyl-l-methylxanthine (IBMX) treatment. Although cyclin B synthesis and M-phase promoting factor (MPF) activation were largely prevented until 30 h of culture in IBMX-treated oocytes, injection of anti-Gsalpha antibody into these oocytes partially recovered cyclin B synthesis and activated MPF activity at 30 h. These results suggest that meiotic resumption of porcine oocytes is prevented by ooplasmic Gsalpha, which may stimulate cAMP synthesis within porcine oocytes, and that synthesized cAMP prevents meiotic resumption of oocytes through the signaling pathways involved in MPF activation.

Engineering the follicle microenvironment

In vitro ovarian follicle culture provides a tool to investigate folliculogenesis, and may one day provide women with fertility-preservation options. The application of tissue engineering principles to ovarian follicle maturation may enable the creation of controllable microenvironments that will coordinate the growth of the multiple cellular compartments within the follicle. Three-dimensional culture systems can preserve follicle architecture, thereby maintaining critical cell-cell and cell-matrix signaling lost in traditional two-dimensional attached follicle culture systems. Maintaining the follicular structure while manipulating the biochemical and mechanical environment will enable the development of controllable systems to investigate the fundamental biological principles underlying follicle maturation. This review describes recent advances in ovarian follicle culture, and highlights the tissue engineering principles that may be applied to follicle culture, with the ultimate objective of germline preservation for females facing premature infertility.

New insight into the role of phosphodiesterase 3A in porcine oocyte maturation

Background

The ovulatory surge of gonadotropins triggers oocyte maturation and rupture of the ovarian follicle. The resumption of nuclear maturation in the oocyte from the prophase stage is characterized by germinal vesicle breakdown (GVBD). It has previously been shown that specific inhibition of cAMP degradation by PDE3 prevents the resumption of oocyte meiosis. However, no report has characterized the activity of PDE3 in the porcine oocyte, or the implication of the cAMP-PDE3 pathway in the entire nuclear maturation process. In this study, PDE3 activity in the oocyte was assessed during in vitro maturation (IVM) and the possible roles of the cAMP-PDE3 pathway in the resumption and progression of meiosis were investigated in terms of different models of oocyte maturation.

Results

Cyclic AMP-degrading PDE activity was detected in the cumulus-oocyte complex (COC) and was partially inhibited by a specific PDE3 inhibitor, cilostamide. When measured only in the denuded oocyte, PDE activity was almost completely inhibited by cilostamide, suggesting that cAMP-PDE3 activity is the major cAMP-PDE in porcine oocytes. PDE3A mRNA was detected by RT-PCR. PDE3 activity did not vary significantly during the early hours of IVM, but a maximum was observed at 13 hours. In cumulus-oocyte complexes, meiosis resumed after 20.81 hours of culture. PDE3 inhibition no longer maintained meiotic arrest if sustained beyond 17.65 hours of IVM, 3 hours prior to resumption of meiosis. Thereafter, PDE3 inhibition progressively lost its efficacy in GVBD. When the protein phosphatase 1 and 2A inhibitor okadaic acid was continuously or transiently (3 hours) present during IVM, meiosis resumed prematurely; PDE3 inhibition was unable to prevent GVBD. However, PDE3 inhibition in COC treated with OA for 3 hours significantly delayed meiosis at the intermediate stage.

Conclusion

The present investigation has demonstrated that PDE3A is the major cAMP-degrading PDE in the oocyte. It regulates the resumption of meiosis until 3 hours prior to GVBD and transiently affects meiotic progression.