Effect of different levels of intracellular cAMP on the in vitro maturation of cattle oocytes and their subsequent development following in vitro fertilization

The formation of aggregated chromatin/chromosomes in mouse oocytes treated with high concentration of IBMX as a model for a chromosome transfer in human

The presence of cyclic adenosine monophosphate (cAMP) has been considered to be a fundamental factor in ensuring meiotic arrest prior to ovulation. cAMP is regarded as a key molecule in the regulation of oocyte maturation. However, it has been reported that increased levels of intracellular cAMP can result in abnormal cytokinesis, with some MI oocytes leading to symmetrically cleaved 2-cell MII oocytes. Consequently, we aimed to investigate the effects of elevated intracellular cAMP levels on abnormal cytokinesis and oocyte maturation during the meiosis of mouse oocytes. This study found that a high concentration of isobutylmethylxanthine (IBMX) also caused chromatin/chromosomes aggregation (AC) after the first meiosis. The rates of AC increased the greater the concentration of IBMX. In addition, AC formation was found to be reversible, showing that the re-formation of the spindle chromosome complex was possible after the IBMX was removed. In human oocytes, the chromosomes aggregate after the germinal vesicle breakdown and following the first and second polar body extrusions (the AC phase), while mouse oocytes do not have this AC phase. The results of our current study may indicate that the AC phase in human oocytes could be related to elevated levels of intracytoplasmic cAMP.

Use of assisted reproductive technologies (ARTs) to shorten the generational interval in ruminants: current status and perspectives

The challenges posed by climate change and increasing world population are stimulating renewed efforts for improving the sustainability of animal production. To meet such challenges, the contribution of genomic selection approaches, in combination with assisted reproductive technologies (ARTs), to spreading and preserving animal genetics is essential. The largest increase in genetic gain can be achieved by shortening the generation interval. This review provides an overview of the current status and progress of advanced ARTs that could be applied to reduce the generation time in both female and male of domestic ruminants. In females, the use of juvenile in vitro embryo transfer (JIVET) enables to generate offspring after the transfer of in vitro produced embryos derived from oocytes of prepubertal genetically superior donors reducing the generational interval and acceleration genetic gain. The current challenge is increasing in vitro embryo production (IVEP) from prepubertal derived oocytes which is still low and variable. The two main factors limiting IVEP success are the intrinsic quality of prepubertal oocytes and the culture systems for in vitro maturation (IVM). In males, advancements in ARTs are providing new strategies to in vitro propagate spermatogonia and differentiate them into mature sperm or even to recapitulate the whole process of spermatogenesis from embryonic stem cells. Moreover, the successful use of immature cells, such as round spermatids, for intracytoplasmic injection (ROSI) and IVEP could allow to complete the entire process in few months. However, these approaches have been successfully applied to human and mouse whereas only a few studies have been published in ruminants and results are still controversial. This is also dependent on the efficiency of ROSI that is limited by the current isolation and selection protocols of round spermatids. In conclusion, the current efforts for improving these reproductive methodologies could lead toward a significant reduction of the generational interval in livestock animals that could have a considerable impact on agriculture sustainability.

A fresh start for IVM: capacitating the oocyte for development using pre-IVM

BACKGROUND

While oocyte IVM is practiced sporadically it has not achieved widespread clinical practice globally. However, recently there have been some seminal advances in our understanding of basic aspects of oocyte biology and ovulation from animal studies that have led to novel approaches to IVM. A significant recent advance in IVM technology is the use of biphasic IVM approaches. These involve the collection of immature oocytes from small antral follicles from minimally stimulated patients/animals (without hCG-priming) and an ∼24 h pre-culture of oocytes in an advanced culture system ('pre-IVM') prior to IVM, followed by routine IVF procedures. If safe and efficacious, this novel procedure may stand to make a significant impact on human ART practices.

OBJECTIVE AND RATIONALE

The objectives of this review are to examine the major scientific advances in ovarian biology with a unique focus on the development of pre-IVM methodologies, to provide an insight into biphasic IVM procedures, and to report on outcomes from animal and clinical human data, including safety data. The potential future impact of biphasic IVM on ART practice is discussed.

SEARCH METHODS

Peer review original and review articles were selected from PubMed and Web of Science searches for this narrative review. Searches were performed using the following keywords: oocyte IVM, pre-IVM, biphasic IVM, CAPA-IVM, hCG-triggered/primed IVM, natural cycle IVF/M, ex-vivo IVM, OTO-IVM, oocyte maturation, meiotic competence, oocyte developmental competence, oocyte capacitation, follicle size, cumulus cell (CC), granulosa cell, COC, gap-junction communication, trans-zonal process, cAMP and IVM, cGMP and IVM, CNP and IVM, EGF-like peptide and IVM, minimal stimulation ART, PCOS.

OUTCOMES

Minimizing gonadotrophin use means IVM oocytes will be collected from small antral (pre-dominant) follicles containing oocytes that are still developing. Standard IVM yields suboptimal clinical outcomes using such oocytes, whereas pre-IVM aims to continue the oocyte's development ex vivo, prior to IVM. Pre-IVM achieves this by eliciting profound cellular changes in the oocyte's CCs, which continue to meet the oocyte's developmental needs during the pre-IVM phase. The literature contains 25 years of animal research on various pre-IVM and biphasic IVM procedures, which serves as a large knowledge base for new approaches to human IVM. A pre-IVM procedure based on c-type natriuretic peptide (named 'capacitation-IVM' (CAPA-IVM)) has undergone pre-clinical human safety and efficacy trials and its adoption into clinical practice resulted in healthy live birth rates not different from conventional IVF.

WIDER IMPLICATIONS

Over many decades, improvements in clinical IVM have been gradual and incremental but there has likely been a turning of the tide in the past few years, with landmark discoveries in animal oocyte biology finally making their way into clinical practice leading to improved outcomes for patients. Demonstration of favorable clinical results with CAPA-IVM, as the first clinically tested biphasic IVM system, has led to renewed interest in IVM as an alternative, low-intervention, low-cost, safe, patient-friendly ART approach, and especially for patients with PCOS. The same new approach is being used as part of fertility preservation in patients with cancer and holds promise for social oocyte freezing.

The combination of rolipram and cilostamide improved the developmental competence of cloned porcine embryos

In vitro maturation of porcine oocytes is characterized by asynchronous cytoplasmic and nuclear maturation, leading to less competent oocytes supporting embryo development. The purpose of this study was to evaluate the combined effect of rolipram and cilostamide as cyclic Adenine monophosphate (cAMP) modulators to find the maximum cAMP levels that temporarily arrest meiosis. We determined the optimal time to maintain functional gap junction communication during pre-in vitro maturation to be four hours. Oocyte competence was evaluated by the level of glutathione, reactive oxygen species, meiotic progression, and gene expression. We evaluated embryonic developmental competence after parthenogenetic activation and somatic cell nuclear transfer. The combined treatment group showed significantly higher glutathione and lower reactive oxygen species levels and a higher maturation rate than the control and single treatment groups. Cleavage and blastocyst formation rates in parthenogenetic activation and somatic cell nuclear transfer embryos were higher in two-phase in vitro maturation than in the other groups. The relative levels of BMP15and GDF9 expression were increased in two-phase in vitro maturation. Somatic cell nuclear transfer blastocysts from two-phase in vitro maturation oocytes showed a lower level of expression of apoptotic genes than the control, indicating better pre-implantation developmental competence. The combination of rolipram and cilostamide resulted in optimal synchrony of cytoplasmic and nuclear maturation in porcine in vitro matured oocytes and there by enhanced the developmental competence of pre-implantation embryos.

Progress toward species-tailored prematuration approaches in carnivores

In the past four decades, the bovine model has been highly informative and inspiring to assisted reproductive technologies (ART) in other species. Most of the recent advances in ART have come from studies in cattle, particularly those unveiling the importance of several processes that must be recapitulated in vitro to ensure the proper development of the oocyte. The maintenance of structural and functional communications between the cumulus cells and the oocyte and a well-orchestrated chromatin remodeling with the gradual silencing of transcriptional activity represent essential processes for the progressive acquisition of oocyte developmental competence. These markers are now considered the milestones of physiological approaches to increase the efficiency of reproductive technologies. Different in vitro approaches have been proposed. In particular, the so-called "pre-IVM" or "prematuration" is a culture step performed before in vitro maturation (IVM) to support the completion of the oocyte differentiation process. Although these attempts only partially improved the embryo quality and yield, they currently represent a proof of principle that oocytes retrieved from an ovary or an ovarian batch shouldn't be treated as a whole and that tailored approaches can be developed for culturing competent oocytes in several species, including humans. An advancement in ART's efficiency would be desirable in carnivores, where the success is still limited. Since the progress in reproductive medicine has often come from comparative studies, this review highlights aspects that have been critical in other species and how they may be extended to carnivores.

Dynamics of nuclear and cytoplasm maturation of bovine oocytes cultivated in vitro in medium supplemented with fulerol

ABSTRACT The aim of this work was to evaluate, in vitro, the dynamics of nuclear and cytoplasmic maturation of bovine oocytes in traditional IVM medium (CT) and supplemented with fullerol (MF50), for 36 hours. The nuclear maturation of CT (n=300) and MF50 (n=270) every 6 hours, stained with Hoechst33342 and cytoplasmic, the mitochondrial distribution of CT (n=197) and MF50 (n=159) at every 12 hours, stained with Mitotracker Orange. At 6 hours, CT oocytes (19%) were in MI (metaphase I), while in MF50 they were in GV (germ vesicle) or GVB (GV breakeage), repeating at 12 hours. At 18 hours, 46.3% were matured in CT, and 20% in MF50. At 24 hours, 43.9% of maturation was observed in the MF50 group, and 63.8% in the CT. At 30 and 36 hours, the maturation pattern was stable, but with the onset of oocyte degeneration. There was a delay in cytoplasmic maturation with 36 hours (P<0.05) in MF50 (53.9% of mature gametes), compared to CT (69.8%). With immature cytoplasm, they were 10.4% and 31.7% for CT and MF50 (P<0.05), respectively. It was concluded that fullerol possibly interfered in the expansion of cumulus oophorus cells, as well as delayed the meiotic progression and cytoplasmic maturation.

RNA-sequencing reveals genes linked with oocyte developmental potential in bovine cumulus cells

Cumulus cells provide an interesting biological material to perform analyses to understand the molecular clues determining oocyte competence. The objective of this study was to analyze the transcriptional differences between cumulus cells from oocytes exhibiting different developmental potentials following individual in vitro embryo production by RNA-seq. Cumulus cells were allocated into three groups according to the developmental potential of the oocyte following fertilization: (1) oocytes developing to blastocysts (Bl+), (2) oocytes cleaving but arresting development before the blastocyst stage (Bl-), and (3) oocytes not cleaving (Cl-). RNAseq was performed on 4 (Cl-) or 5 samples (Bl+ and Bl-) of cumulus cells pooled from 10 cumulus-oocyte complexes per group. A total of 49, 50, and 18 differentially expressed genes (DEGs) were detected in the comparisons Bl+ versus Bl-, Bl+ versus Cl- and Bl- versus Cl-, respectively, showing a fold change greater than 1.5 at an adjusted p value <0.05. Focussing on DEGs in cumulus cells from Bl+ group, 10 DEGs were common to both comparisons (10/49 from Bl+ vs. Bl-, 10/50 from Bl+ vs. Cl-). These DEGs correspond to 6 upregulated genes (HBE1, ITGA1, PAPPA, AKAP12, ITGA5, and SLC1A4), and 4 downregulated genes (GSTA1, PSMB8, FMOD, and SFRP4) in Bl+ compared to the other groups, from which 7 were validated by quantitative PCR (HBE1, ITGA1, PAPPA, AKAP12, ITGA5, PSMB8 and SFRP4). These genes are involved in critical biological functions such as integrin-mediated cell adhesion, oxygen availability, IGF and Wnt signaling or PKA pathway, highlighting specific biological processes altered in incompetent in vitro maturation 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.

Effect of cumulin and super-GDF9 in standard and biphasic mouse IVM

PURPOSE

In vitro maturation (IVM) is a technology that generates mature oocytes following culture of immature cumulus-oocyte complexes (COC) in vitro. IVM is characterized by minimal patient stimulation, making it attractive for certain patient groups. Recently, a biphasic IVM system, capacitation (CAPA)-IVM, has shown improved clinical outcomes relative to standard IVM; however, it remains less efficient than IVF. This study assessed whether supplementation of CAPA-IVM culture media with the novel TGFβ superfamily proteins cumulin and super-GDF9 improves subsequent mouse embryo development.

METHODS

Immature mouse COCs were cultured by standard IVM or biphasic IVM ± cumulin or super-GDF9.

RESULTS

Both cumulin and super-GDF9 in standard IVM significantly improved day-6 blastocyst rate (53.9% control, 73.6% cumulin, 70.4% super-GDF9; p = 0.006; n = 382-406 oocytes). Cumulin or super-GDF9 in CAPA-IVM did not alter embryo yield or blastocyst cell allocation in an unstimulated model. Moreover, cumulin did not alter these outcomes in a mild PMSG stimulation model. Cumulin in CAPA-IVM significantly increased cumulus cell expression of cumulus expansion genes (Ptgs2, Ptx3, Adamts1, Gfat2) and decreased Lhr expression relative to control. However, cumulin-induced mRNA expression of cumulus cell (Ptgs2, Ptx3) and oocyte genes (Gdf9, Bmp15, Oct4, Stella) in CAPA-IVM remained significantly lower than that of in vivo matured cells.

CONCLUSION

Cumulin did not provide an additional beneficial effect in biphasic IVM in terms of blastocyst yield and cell allocation; however in standard IVM, cumulin and super-GDF9 significantly improve oocyte developmental competence.

Factors Influencing the In Vitro Maturation (IVM) of Human Oocyte

In vitro maturation (IVM) of oocytes is a promising assisted reproductive technology (ART) deemed as a simple and safe procedure. It is mainly used in patients with impaired oocyte maturation and in fertility preservation for women facing the risk of losing fertility. However, to date, it is still not widely used in clinical practice because of its underperformance. The influencing factors, such as biphasic IVM system, culture medium, and the supplementation, have a marked effect on the outcomes of oocyte IVM. However, the role of different culture media, supplements, and follicular priming regimens in oocyte IVM have yet to be fully clarified and deserve further investigation.

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.

Recreating the Follicular Environment: A Customized Approach for In Vitro Culture of Bovine Oocytes Based on the Origin and Differentiation State

The mammalian ovary is a large source of oocytes organized into follicles at various stages of folliculogenesis. However, only a limited number of them can be used for in vitro embryo production (IVEP), while most have yet to complete growth and development to attain full meiotic and embryonic developmental competence. While the in vitro growth of primordial follicles in the ovarian cortex has the potential to produce mature oocytes, it is still at an experimental stage. The population of early antral follicles (EAFs), instead, may represent a reserve of oocytes close to completing the growth phase, which might be more easily exploited in vitro and could increase the number of female gametes dedicated to IVEP.Here we present in vitro culture strategies that have been developed utilizing physiological parameters to support the specific needs of oocytes at distinct stages of differentiation, in order to expand the source of female gametes for IVEP by maximizing the attainment of fertilizable oocytes. Furthermore, these culture systems provide powerful tools to dissect the molecular processes that direct the final differentiation of the mammalian oocyte.

Modulation of phosphatidylinositol 3-kinase activity during in vitro oocyte maturation increases the production of bovine blastocysts

This study aimed to evaluate the effect of regulating phosphatidylinositol 3-kinase (PI3K) activity on the kinetics of oocyte nuclear maturation and the blastocyst rate. To evaluate oocyte viability, nuclear maturation rate and in vitro embryo production, cumulus-oocyte complexes (COCs) were maintained for 0, 10 min, 6 h or 22 h in TCM 199 medium supplemented with 20 nM wortmannin, an inhibitor of PI3K. After each period, COCs were transferred to the same medium without wortmannin and kept under the same conditions until completion of 22 h of in vitro maturation (IVM). To evaluate the effect of time on progression of nuclear maturation, COCs cultivated with 20 nM wortmannin was maintained for 22, 28 or 34 h of IVM. To determine the effect of wortmannin on the activity of maturation-promoting factor (MPF), COCs were kept under IVM conditions in the presence of the inhibitor for 0, 1, 3, 6, or 8 h. Exposure of COCs to wortmannin decreased (P < 0.05) the percentage of oocytes that reached metaphase II (MII) up to 22 h, MPF activity and reduced PI3K activity by 30%. However, after 28 and 34 h, 70% of oocytes reached the MII stage in the presence of inhibitor Moreover, COCs matured in the presence of wortmannin showed an increase (P < 0.05) in the blastocyst rate. These findings suggested that the regulation of the PI3K activity during IVM of bovine COCs interfered with the meiotic progression due to control of MPF activity, positively affecting the blastocyst rate.

Regulation of the aryl hydrocarbon receptor activity in bovine cumulus-oocyte complexes during in vitro maturation: The role of EGFR and post-EGFR ERK1/2 signaling cascade

The aryl hydrocarbon receptor (AhR) has been extensively characterized as an environmental sensor with major roles in xenobiotic-induced toxicity. Evidence is accumulating that these functions serve as adaptive mechanisms overlapping its physiological roles. We previously described a critical role of constitutive AhR activation for the correct progress of mammalian oocyte maturation but the signaling pathway through which AhR controls maturation remains unclear. The aim of this study was to investigate whether the AhR interacts with the epidermal growth factor receptor (EGFR) and p42/44 extracellular regulated kinases (ERK1/2), both key factors in the signaling network that finely regulates the oocyte maturation. As experimental model we used bovine cumulus-oocyte complexes (COCs) during in vitro maturation (IVM). Blocking ERK1/2 signaling in COCs during IVM with the specific EGFR inhibitor AG1478 or the mitogen-activated protein kinase kinase (MEK) inhibitor PD98059 downregulated the expression of the AhR-target gene Cyp1a1. Inhibition of AhR activity was associated with a reduction in the oocytes' ability to progress in meiosis resumption. In contrast, exposure to the AhR antagonist resveratrol reduced both CYP1A1 expression and the oocytes' maturation competence, without affecting ERK1/2 signaling. These findings strongly indicate the EGFR/ERKs signaling network as an upstream regulator of the AhR activation in COCs, offering a new understanding of the finely tuned physiological mechanism leading to oocyte maturation. This information may provide fresh opportunities for improving oocyte in vitro maturation, and therefore boosting the efficiency of assisted reproduction techniques in mammals.

Inhibition of germinal vesicle breakdown using IBMX increases microRNA-21 in the porcine oocyte

BACKGROUND

Germinal vesicle breakdown (GVBD) occurs during oocyte meiotic maturation, a period when transcriptional processes are virtually inactive. Thus, the maturing oocyte is reliant on processes such as post-transcriptional gene regulation (PTGR) to regulate the mRNA and protein repertoire. MicroRNA (miRNA) are a class of functional small RNA that target mRNA to affect their abundance and translational efficiency. Of particular importance is miRNA-21 (MIR21) due to its role in regulating programmed cell death 4 (PDCD4). The objective of this study was to characterize the abundance and regulation of MIR21 in relation to GVBD.

METHODS

Oocytes were collected from aspirated porcine tertiary follicles. Relative abundance of mature MIR21 was quantified at 0, 8, 16, 24, 32, and 42 h of in vitro (IVM) with or without treatment with 3-isobutyl-1-methylxanthine (IBMX).

RESULTS

IBMX increased abundance of MIR21 at 24 h approximately 30-fold compared to control oocytes (P < 0.05), and the induced increase in MIR21 abundance at 24 h was concomitant with premature depletion of PDCD4 protein abundance. To characterize the effect of artificially increasing MIR21 on oocyte competence without inhibiting GVBD, a MIR21 mimic, scrambled microRNA negative control, or nuclease free water was micro-injected into denuded oocytes at 21 h of IVM. The maturation rate of oocytes injected with synthetic MIR21 (63.0 ± 7.5%) was higher than oocytes injected with negative controls (P < 0.05).

CONCLUSIONS

Inhibition of nuclear meiotic maturation via IBMX significantly increased MIR21 and decreased its target, PDCD4. Injection of a MIR21 mimic increased oocyte maturation rate. Our results indicate MIR21 is active and important during meiotic maturation of the oocyte.

Effects of cyclic adenosine monophosphate modulators on maturation and quality of vitrified-warmed germinal vesicle stage mouse oocytes

BACKGROUND

It is still one of the unresolved issues if germinal vesicle stage (GV) oocytes can be successfully cryopreserved for fertility preservation and matured in vitro without damage after warming. Several studies have reported that the addition of cyclic adenosine monophosphate (cAMP) modulators to in vitro maturation (IVM) media improved the developmental potency of mature oocytes though vitrification itself provokes cAMP depletion. We evaluated whether the addition of cAMP modulators after GV oocytes retrieval before vitrification enhances maturation and developmental capability after warming of GV oocytes.

METHODS

Retrieved GV oocytes of mice were divided into cumulus-oocyte complexes (COCs) and denuded oocytes (DOs). Then, GV oocytes were cultured with or without dibutyryl-cAMP (dbcAMP, cAMP analog) and 3-isobutyl-l-methylxanthine (phosphodiesterase inhibitor) during the pre-vitrification period for 30 min.

RESULTS

One hour after warming, the ratio of oocytes that stayed in the intact GV stage was significantly higher in groups treated with cAMP modulators. After 18 h of IVM, the percentage of maturation was significantly higher in the COC group treated with dbcAMP. The expression of F-actin, which is involved in meiotic spindle migration and chromosomal translocation, is likewise increased in this group. However, there was no difference in chromosome and spindle organization integrity or developmental competence between the MII oocytes of all groups.

CONCLUSIONS

Increasing the intracellular cAMP level before vitrification of the GV oocytes maintained the cell cycle arrest, and this process may facilitate oocyte maturation after IVM by preventing cryodamage and synchronizing maturation between nuclear and cytoplasmic components. The role of cumulus cells seems to be essential for this mechanism.

Follicle priming by FSH and pre-maturation culture to improve oocyte quality in vivo and in vitro

Nowadays there is strong demand to produce embryos from premium quality cattle, and we can produce embryos using oocytes collected from living premium animals by ovum-pick up (OPU) followed by in vitro fertilization (IVF). However, the developmental competence of IVF oocytes to form blastocysts is variable. The developmental competence of oocytes depends on the size and stages of follicles, and follicle-stimulating hormone priming (FSH-priming) prior to OPU can promote follicular growth and improve the developmental competence of oocytes. Furthermore, following the induction of ovulation using an injection of luteinizing hormone or gonadotropin-releasing hormone after FSH-priming, we can collect in vivo matured oocytes from ovulatory follicles, which show higher developmental competence than oocytes matured in vitro. However, the conventional protocols for FSH-priming consist of multiple FSH injection for 3-4 days, which is stressful for the animal and labor-intensive for the veterinarian. In addition, these techniques cannot be applied to IVF of oocytes collected from bovine ovaries derived from slaughterhouses, which are important sources of oocytes. Here, we review previous research focused on FSH-priming, especially for collecting in vivo matured oocytes and a simplified method for superstimulation using a single injection of FSH. We also introduce the previous achievements using in vitro pre-maturation culture, which can improve the developmental competence of oocytes derived from non-stimulated animals.

Participation of the adenosine salvage pathway and cyclic AMP modulation in oocyte energy metabolism

A follicular spike in cyclic AMP (cAMP) and its subsequent degradation to AMP promotes oocyte maturation and ovulation. In vitro matured (IVM) oocytes do not receive the cAMP increase that occurs in vivo, and artificial elevation of cAMP in IVM cumulus-oocyte complexes improves oocyte developmental potential. This study examined whether mouse oocytes can use the cAMP degradation product AMP to generate ATP via the adenosine salvage pathway, and examined whether pharmacological elevation of cAMP in IVM cumulus-oocyte complexes alters ATP levels. Oocytes cultured with isotopic ¹³C₅-AMP dose-dependently produced ¹³C₅-ATP, however total cellular ATP remained constant. Pharmacological elevation of cAMP using forskolin and IBMX prior to IVM decreased oocyte ATP and ATP:ADP ratio, and promoted activity of the energy regulator AMPK. Conversely, cumulus cells exhibited higher ATP and no change in AMPK. Culture of oocytes without their cumulus cells or inhibition of their gap-junctional communication yielded lower oocyte ¹³C₅-ATP, indicating that cumulus cells facilitate ATP production via the adenosine salvage pathway. In conclusion, this study demonstrates that mouse oocytes can generate ATP from AMP via the adenosine salvage pathway, and cAMP elevation alters adenine nucleotide metabolism and may provide AMP for energy production via the adenosine salvage pathway during the energetically demanding process of meiotic maturation.

Capacitation IVM improves cumulus function and oocyte quality in minimally stimulated mice

PURPOSE

Oocyte in vitro maturation (IVM) is a patient-friendly reproductive technology but lower success rates than IVF have limited its uptake. Capacitation-IVM (CAPA-IVM) is an innovative new IVM system currently undergoing clinical evaluation. This study aimed to determine temporal effects of the pre-IVM phase of CAPA-IVM on cumulus function and oocyte developmental competence in mildly-stimulated mice.

METHODS

Immature cumulus oocyte complexes (COCs) derived from mildly stimulated (23 h PMSG) 28-day-old mice underwent pre-IVM for 0-24 h in medium containing c-type natriuretic peptide (CNP), E₂, FSH and insulin, prior to IVM (CAPA-IVM). The effect of pre-IVM duration on cumulus cell function and embryo development post-CAPA-IVM/IVF was assessed.

RESULTS

Day 6 blastocyst rate increased incrementally with increasing pre-IVM duration: 40.6 ± 2.0%, 45.8 ± 1.2%, 52.2 ± 3.5%, 53.3 ± 5.9%, and 59.9 ± 2.5% for 0, 2, 6, 12, and 24 h pre-IVM, respectively (P < 0.01). DNA content/COC, a measure of cumulus cell proliferation, was significantly higher with 24 h pre-IVM group compared to 0, 2, or 6 h pre-IVM (P < 0.001). Pre-IVM for 24 h significantly increased cumulus expansion and mRNA expression of matrix genes Has2 and Tnfaip6 and Areg relative to no pre-IVM control (P < 0.01). Cumulus-oocyte gap-junctional communication (GJC) was maintained throughout 24 h pre-IVM (P < 0.0001), and GJC loss was slowed during the subsequent IVM phase, whilst meiotic resumption was accelerated (P < 0.05). Pre-IVM increased COC ATP and ADP content (P < 0.05), but not AMP, ATP/ADP, and energy charge.

CONCLUSION

The pre-IVM phase of CAPA-IVM improves the quality of IVM oocytes in a temporally dependent manner and significantly influences cumulus cell function including increased cell proliferation, cumulus expansion, and prolonged cumulus-oocyte GJC.

Mitochondrial function in immature bovine oocytes is improved by an increase of cellular cyclic AMP

Although in vitro maturation (IVM) of oocytes is important for assisted reproduction, the rate of development of embryos from IVM oocytes is lower than from their in vivo counterparts. It has been shown that an artificial increase of intracellular cAMP before culture significantly improves oocyte developmental competence in cattle and mice. Here, we revealed that forskolin and 3-isobutyl-1-methylxanthine treatment of prophase-stage oocytes induced the expression of genes required for glycolysis, fatty acid degradation, and the mitochondrial electron transport system and improved mitochondrial functions and ATP levels in oocytes without involving nuclear maturation. We propose the existence of a comprehensive energy-supply system in oocytes under follicle-stimulating hormone stimulation as a potential explanation of how oocytes acquire developmental competence.

Role of cAMP modulator supplementations during oocyte in vitro maturation in domestic animals

Cyclic adenosine monophosphate (cAMP) is an important molecule in signal transduction within the cell, functioning as a second cell messenger of gonadotrophin stimulation. The concentration of cAMP in cumulus-oocyte complexes (COCs) is known to be controlled through modulation of its synthesis by adenylyl cyclase (AC) and by degradation through the cyclic nucleotide phosphodiesterase (PDE) enzymes. One of the main obstacles for in vitro embryo production is the optimization of reproduction processes that occur in oocyte maturation. The function of cAMP is important in maintaining meiotic arrest in mammalian oocytes. When the oocyte is physically removed from the antral follicle for in vitro maturation (IVM), intra-oocyte cAMP concentrations decrease and spontaneous meiotic resumption begins, due to the depletion of inhibitory factors from the follicle. In many studies, relatively greater cAMP concentrations before IVM has been reported to improve oocyte competence, leading to subsequent benefits in embryonic development in different species. There, therefore, has been an increase in oocyte cAMP concentrations with several treatments and different approaches, such as invasive AC, stimulators of AC activity, PDE inhibitors, and cAMP analogs. The aim of this review is to comprehensively evaluate and provide data related to (i) the use of cAMP modulators during IVM and the effects on completion of meiosis and cytoplasmic reorganization, which are required for development of oocytes with the capacity to contribute to fertilization and subsequent embryonic development; and (ii) the main cAMP modulators and the effects when used in oocyte IVM.

In vitro follicle culture in the context of IVF

The currently available assisted reproduction techniques for fertility preservation (i.e. in vitro maturation (IVM) and in vitro fertilization) are insufficient as stand-alone procedures as only few reproductive cells can be conserved with these techniques. Oocytes in primordial follicles are well suited to survive the cryopreservation procedure and of use as valuable starting material for fertilization, on the condition that these could be grown up to fully matured oocytes. Our understanding of the biological mechanisms directing primordial follicle activation has increased over the last years and this knowledge has paved the way toward clinical applications. New multistep in vitro systems are making use of purified precursor cells and extracellular matrix components and by applying bio-printing technologies, an adequate follicular niche can be built. IVM of human oocytes is clinically applied in patients with polycystic ovary/polycystic ovary syndrome; related knowhow could become useful for fertility preservation and for patients with maturation failure and follicle-stimulating hormone resistance. The expectations from the research on human ovarian tissue and immature oocytes cultures, in combination with the improved vitrification methods, are high as these technologies can offer realistic potential for fertility preservation.

A pre-in vitro maturation medium containing cumulus oocyte complex ligand-receptor signaling molecules maintains meiotic arrest, supports the cumulus oocyte complex and improves oocyte developmental competence

STUDY QUESTION

Can a pre-in vitro maturation (pre-IVM) medium containing signaling molecules rather than chemical/pharmaceutical agents, sustain meiotic arrest and improve developmental competence of in vitro matured oocytes in CF1 outbred mice?

SUMMARY ANSWER

A short 2 h period of pre-IVM prevents spontaneous meiotic resumption, improves mitochondria activity in subsequently matured oocytes, and increases developmental competence, pregnancy rate and implantation of resulting embryos.

WHAT IS KNOWN ALREADY

Spontaneous resumption of meiosis in vitro is detrimental for oocyte developmental competence. Pre-IVM systems that prevent spontaneous meiotic resumption with chemical/pharmaceutical agents are a promising approach to improving IVM oocyte competence; however, the success of these methods has proven to be inconsistent.

STUDY DESIGN, SIZE, DURATION

This study consisted of a series of experiments using cumulus oocyte complexes (COC) derived from outbred mice following ovarian stimulation. The study was designed to examine if a novel, ligand/receptor-based pre-IVM treatment could sustain meiotic arrest in vitro and improve oocyte developmental competence, compared to control IVM. Two pre-IVM durations (2 h and 24 h) were evaluated, and the effect of the mitochondrial stimulator PQQ during 24 h pre-IVM was studied.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Murine (outbred CF1) immature COC were cultured in vitro in the presence of C-type natriuretic peptide (CNP) (30 nM), estradiol (100 nM), FSH (1 × 10-4 IU/ml) and bone morphogenic protein 15 (BMP15) (100 ng/ml) for 2 h or 24 h prior to IVM. Meiotic status during pre-IVM and IVM was analyzed using orcein staining, and functionality of gap junction communication was confirmed using the functional gap junction inhibitor carbenoxolone (CBX). Oocytes exposed to pre-IVM treatment were compared to control oocytes collected on the same day from the same females and undergoing standard IVM. Developmental competence and embryo viability was assessed by oocyte mitochondrial activity and ATP concentration, in vitro embryo development following IVF and in vitro culture, blastocyst cell number and allocation, embryo morphokinetics, and embryo transfer. Differences were determined to be significant when P < 0.05.

MAIN RESULTS AND THE ROLE OF CHANCE

Both a short (2 h) and long (24 h) pre-IVM period successfully prevented spontaneous resumption of meiosis. Moreover, gap junctions remained open during the pre-IVM period, as shown by the resumption of meiosis (95.9 ± 2.1%) in the presence of CBX during pre-IVM. A 2 h pre-IVM treatment improved blastocyst development after 96 h of culture per cleaved embryo compared to control (71.9 ± 7.4% versus 53.3 ± 6.2%, respectively), whereas a longer 24 h pre-IVM had no effect on development. A short 2 h period of pre-IVM increased mitochondrial activity in mature oocytes. On the contrary, mitochondrial activity was reduced in mature oocytes following 24 h of arrest and IVM. Treatment of arrested COC with pyrroloquinoline quinone (PQQ) during the 24 h pre-IVM period successfully maintained mitochondrial activity equal to control. However, PQQ was not able to improve blastocyst development compared to pre-IVM 24 h without PQQ. Moreover, ATP concentration in mature oocytes following pre-IVM and/or IVM, did not differ between treatments. A 2 h pre-IVM period prior to IVM improved pregnancy rate following transfer to recipient females. Implantation was also improved after transfer of embryos derived from oocytes arrested for either 2 h or 24 h prior to IVM, compared to control IVM derived embryos (41.9 ± 9%, 37.2 ± 9.5% and 17.2 ± 8.3%, respectively), although fetal development did not differ.

LIMITATIONS, REASONS FOR CAUTION

Slower meiotic resumption and enhanced mitochondrial activity likely contribute to improved developmental competence of oocytes exposed to pre-IVM for 2 h, but further experiments are required to identify specific mechanisms. Maintaining oocytes in meiotic arrest for 24 h with this approach could be a potential window to improve oocyte quality. However, an initial attempt to utilize this period of arrest to manipulate quality with PQQ, a mitochondrial stimulator, did not improve oocyte competence.

WIDER IMPLICATIONS OF THE FINDINGS

IVM could be an attractive clinical alternative to conventional IVF, with reduced time, cost and reliance on high doses of exogenous hormones to stimulate follicle growth, thus eliminating ovarian hyperstimulation syndrome (OHSS). Currently IVM is not widely used as it results in reduced embryo development and lower pregnancy outcomes compared to embryos produced from in vivo matured oocytes. Our approach to IVM, incorporating a ligand/receptor pre-IVM period, could improve human oocyte quality following IVM leading to routine adoption of this patient friendly technology. In addition, our methodology of pre-IVM containing signaling molecules rather than chemical/pharmaceutical agents may prove to be more consistent at improving oocyte quality than those focusing only on cAMP modulation with pharmacological agents. Finally, a reliable method of maintaining oocytes in meiotic arrest in vitro provides a novel window of opportunity in which the oocyte may be manipulated to address specific physiological deficiencies prior to meiotic resumption.

LARGE SCALE DATA

N/A.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by the Colorado Center for Reproductive Medicine (CCRM, Lone Tree, Colorado USA). We declare no conflict of interest.

Advances in in vitro production of sheep embryos

Sheep is an important livestock in the world providing meat, milk and wool for human beings. With increasing human population, the worldwide needs of production of sheep have elevated. To meet the needs, the assistant reproductive technology including ovine in vitro embryo production (ovine IVP) is urgently required to enhance the effective production of sheep in the world. To learn the status of ovine IVP, we collected some publications related to ovine IVP through PubMed and analyzed the progress in ovine IVP made in the last five years (2012-2017). We made comparisons of these data and found that the recent advances in ovine IVP has been made slowly comparable to that of ovine IVP two decades ago. Therefore, we suggested two strategies or approaches to tackle the main problems in ovine IVP and expect that the efficiency of ovine IVP could be improved significantly when the approaches would be implemented.

Follistatin Rescues Blastocyst Development of Poor Quality Porcine Cumulus-Oocyte Complexes by Delaying Meiotic Resumption With Decreased cGMP

Mammalian oocytes resume maturation when removed from follicles and cultured in vitro. During folliculogenesis, oocytes are bathed in follicular fluid (FF), which provides an important and specialized microenvironment for oocyte competence. Follistatin (FST) is one component of FF that may play a role in oocyte maturation and embryo development. This study was conducted to examine possible effects of FST on porcine oocyte competence and embryo development. Exogenous FST in oocyte maturation medium for 22 or 44 hours did not improve nuclear maturation and had no effect on good quality cumulus-oocyte complexes (COCs). However, FST improved blastocyst rates in embryos derived from oocytes with less than 2 layers of cumulus. Follistatin treatment of the poor quality COC group increased transcript levels for genes indicative of oocyte quality. Transcript levels were also altered for cumulus expansion-related genes in response to FST when measured during the germinal vesicle breakdown stage. Interestingly, high-quality oocytes remained at germinal vesicle stage much longer than low-quality oocytes, FST treatment induced temporary blockage of spontaneous meiotic resumption when added during culture of both good and poor quality COCs, and levels of cyclic guanosine monophosphate (cGMP) were higher in FST-treated versus untreated groups for both good and poor quality oocytes. In conclusion, FST treatment of porcine oocytes during in vitro maturation can rescue competency of poor quality oocytes to develop to blastocyst stage following in vitro fertilization. Beneficial effects of addition of FST to culture medium may be mediated by inhibiting degradation of cGMP and temporarily delaying nuclear maturation.

Influence of nitric oxide and phosphodiesterases during in vitro maturation of bovine oocytes on meiotic resumption and embryo production

This study aimed to examine the effects of nitric oxide (NO) and different phosphodiesterase (PDE) families on meiosis resumption, nucleotides levels and embryo production. Experiment I, COCs were matured in vitro with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) associated or not with the soluble guanylate cyclase (sGC) inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), meiotic resumption and nucleotides levels were assessed. SNAP delayed germinal vesicle breakdown (GVBD) (53.4 ± 1.2 versus 78.4 ± 2.4% for controls, P 0.05). Cyclic GMP levels were higher in SNAP (3.94 ± 0.18, P 0.05). Embryo development did not differ from the control for SNAP and cilostamide groups (38.7 ± 5.8, 37.9 ± 6.2 and 40.5 ± 5.8%, P > 0.05), but SNAP + cilostamide decreased embryo production (25.7 ± 6.9%, P < 0.05). In conclusion, SNAP was confirmed to delay meiosis resumption by the NO/sGC/cGMP pathway, by increasing cGMP, but not cAMP. Inhibiting different PDEs to further increase nucleotides in association with SNAP did not show any additive effects on meiosis resumption, indicating that other pathways are involved. Moreover, SNAP + cilostamide affected the meiosis progression and decreased embryo development.

Transporting cumulus complexes using novel meiotic arresting conditions permits maintenance of oocyte developmental competence

PURPOSE

The aim of this study is to evaluate the effect of a novel bovine cumulus oocyte complex (COC) shipping media designed to arrest meiotic resumption during transport on meiotic arrest, as well as meiotic resumption, subsequent embryonic development, and embryo quality.

METHODS

Bovine cumulus oocyte complexes were transported overnight from the collection facility to the laboratory. COCs were placed in control in vitro maturation (IVM) or in shipping arrest medium (SAM) containing multiple meiotic inhibitors, and then shipped to our laboratory. Upon arrival, meiotic status was assessed, control COCs were inseminated, and arrested COCs were matured and inseminated the next day. Embryonic development and quality were analyzed.

RESULTS

When bovine COC arrived at the laboratory after overnight shipment (21 h) in SAM, the majority of oocytes remained at the GV stage (75.6 ± 2.9% GV). Arrested oocytes successfully resumed and completed meiosis during IVM after removal from SAM (96.8 ± 0.5% metaphase II compared to control 88.3 ± 5.0%). Moreover, the development of blastocysts per COC was not different from control (22.3 ± 2.4% for control and 18.7 ± 2.1% for SAM), nor was any difference detected in blastocyst quality as determined by cell number and allocation.

CONCLUSIONS

Our study demonstrates that a physiological system incorporating cyclic adenosine monophosphate and cyclic guanosine monophosphate modulators can be used to maintain meiotic arrest followed by successful nuclear maturation and pre-implantation embryo development equal to control IVM-derived embryos. Our results offer promising insights for the development of pre-IVM media that may improve oocyte developmental competence in vitro.

Beneficial Effects of Melatonin on the In Vitro Maturation of Sheep Oocytes and Its Relation to Melatonin Receptors

(1) Background: The binding sites of melatonin, as a multifunctional molecule, have been identified in human, porcine, and bovine samples. However, the binding sites and mechanisms of melatonin have not been reported in sheep; (2) Methods: Cumulus–oocyte complexes (COCs) were cultured in TCM-199 supplemented with melatonin at concentrations of 0, 10⁻³, 10⁻⁵, 10⁻⁷, 10⁻⁹, and 10⁻¹¹ M. Melatonin receptors (MT1 and MT2) were evaluated via immunofluorescence and Western blot. The effects of melatonin on cumulus cell expansion, nuclear maturation, embryo development, and related gene (GDF9, DNMT1, PTX3, HAS2, and EGFR) expression were investigated. The level of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) were evaluated in oocytes and cumulus, respectively; (3) Results: Both MT1 and MT2 were expressed in oocytes, cumulus cells, and granulosa cells. Melatonin with a concentration of 10⁻⁷ M significantly enhanced the rates of nuclear maturation, cumulus cells expansion, cleavage, and blastocyst. Melatonin enhanced the expression of BMP15 in oocytes and of PTX3, HAS2, and EGFR in cumulus cells. Melatonin decreased the cAMP level of oocytes but enhanced the cGMP level in oocytes and cumulus cells; (4) Conclusion: The higher presence of MT1 in GV cumulus cells and the beneficial effects of melatonin indicated that its roles in regulating sheep oocyte maturation may be mediated mainly by the MT1 receptor.

New approaches regarding the in vitro maturation of oocytes: manipulating cyclic nucleotides and their partners in crime

Several discoveries have been described recently (5-10 years) about the biology of ovarian follicles (oocyte, cumulus cells and granulosa cells), including new aspects of cellular communication, the control of oocyte maturation and the acquisition of oocyte competence for fertilization and further embryo development. These advances are nourishing assisted reproduction techniques (ART) with new possibilities, in which novel culture systems are being developed and tested to improve embryo yield and quality. This mini-review aims to describe how the recent knowledge on the physiological aspects of mammalian oocyte is reflecting as original or revisited approaches into the context of embryo production. These new insights include recent findings on the mechanisms that control oocyte maturation, especially modulating intraoocyte levels of cyclic nucleotides during in vitro maturation using endogenous or exogenous agents. In this mini-review we also discuss the positive and negative effects of these manipulations on the outcoming embryo.

Oocyte maturation and quality: role of cyclic nucleotides

The cyclic nucleotides, cAMP and cGMP, are the key molecules controlling mammalian oocyte meiosis. Their roles in oocyte biology have been at the forefront of oocyte research for decades, and many of the long-standing controversies in relation to the regulation of oocyte meiotic maturation are now resolved. It is now clear that the follicle prevents meiotic resumption through the actions of natriuretic peptides and cGMP - inhibiting the hydrolysis of intra-oocyte cAMP - and that the pre-ovulatory gonadotrophin surge reverses these processes. The gonadotrophin surge also leads to a transient spike in cAMP in the somatic compartment of the follicle. Research over the past two decades has conclusively demonstrated that this surge in cAMP is important for the subsequent developmental capacity of the oocyte. This is important, as oocyte in vitro maturation (IVM) systems practised clinically do not recapitulate this cAMP surge in vitro, possibly accounting for the lower efficiency of IVM compared with clinical IVF. This review particularly focuses on this latter aspect - the role of cAMP/cGMP in the regulation of oocyte quality. We conclude that clinical practice of IVM should reflect this new understanding of the role of cyclic nucleotides, thereby creating a new generation of ART and fertility treatment options.

Porcine oocyte maturation in vitro: role of cAMP and oocyte-secreted factors - A practical approach

Polyspermy or the penetration of more than one sperm cell remains a problem during porcine in vitro fertilization (IVF). After in vitro culture of porcine zygotes, only a low percentage of blastocysts develop and their quality is inferior to that of in vivo derived blastocysts. It is unknown whether the cytoplasmic maturation of the oocyte is sufficiently sustained in current in vitro maturation (IVM) procedures. The complex interplay between oocyte and cumulus cells during IVM is a key factor in this process. By focusing on this bidirectional communication, it is possible to control the coordination of cumulus expansion, and nuclear and cytoplasmic maturation during IVM to some extent. Therefore, this review focuses on the regulatory mechanisms between oocytes and cumulus cells to further the development of new in vitro embryo production (IVP) procedures, resulting in less polyspermy and improved oocyte developmental potential. Specifically, we focused on the involvement of cAMP in maturation regulation and function of oocyte-secreted factors (OSFs) in the bidirectional regulatory loop between oocyte and cumulus cells. Our studies suggest that maintaining high cAMP levels in the oocyte during the first half of IVM sustained improved oocyte maturation, resulting in an enhanced response after IVF and cumulus matrix disassembly. Recent research indicated that the addition of OSFs during IVM enhanced the developmental competence of small follicle-derived oocytes, which was stimulated by epidermal growth factor (EGF) via developing EGF-receptor signaling.

Extending prematuration with cAMP modulators enhances the cumulus contribution to oocyte antioxidant defence and oocyte quality via gap junctions

STUDY QUESTION

Can bovine oocyte antioxidant defence and oocyte quality be improved by extending the duration of pre-in vitro maturation (IVM) with cyclic adenosine mono-phosphate (cAMP) modulators?

SUMMARY ANSWER

Lengthening the duration of cAMP-modulated pre-IVM elevates intra-oocyte reduced glutathione (GSH) content and reduces hydrogen peroxide (H2O2) via increased cumulus cell-oocyte gap-junctional communication (GJC), associated with an improvement in subsequent embryo development and quality.

WHAT IS KNOWN ALREADY

Oocytes are susceptible to oxidative stress and the oocyte's most important antioxidant glutathione is supplied, at least in part, by cumulus cells. A temporary inhibition of spontaneous meiotic resumption in oocytes can be achieved by preventing a fall in cAMP, and cyclic AMP-modulated pre-IVM maintains cumulus-oocyte GJC and improves subsequent embryo development.

STUDY DESIGN, SIZE, DURATION

This study consisted of a series of 10 experiments using bovine oocytes in vitro, each with multiple replicates. A range of pre-IVM durations were examined as the key study treatments which were compared with a control. The study was designed to examine if one of the oocyte's major antioxidant defences can be enhanced by pre-IVM with cAMP modulators, and to examine the contribution of cumulus-oocyte GJC on these processes.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Immature bovine cumulus-oocyte complexes were treated in vitro without (control) or with the cAMP modulators; 100 µM forskolin (FSK) and 500 µM 3-isobutyl-1-methyxanthine (IBMX), for 0, 2, 4 or 6 h (pre-IVM phase) prior to IVM. Oocyte developmental competence was assessed by embryo development and quality post-IVM/IVF. Cumulus-oocyte GJC, intra-oocyte GSH and H2O2 were quantified at various time points during pre-IVM and IVM, in the presence and the absence of functional inhibitors: carbenoxolone (CBX) to block GJC and buthionine sulfoximide (BSO) to inhibit glutathione synthesis.

MAIN RESULTS AND THE ROLE OF CHANCE

Pre-IVM with FSK + IBMX increased subsequent blastocyst formation rate and quality compared with standard IVM (P < 0.05), regardless of pre-IVM duration. The final blastocyst yields (proportion of blastocysts/immature oocyte) were 26.3% for the control, compared with 39.2, 35.2 and 34.2%, for the 2, 4 and 6 h pre-IVM FSK + IBMX treatments, respectively. In contrast to standard IVM (control), pre-IVM with cAMP modulators maintained open gap junctions between cumulus cells and oocytes for the duration (6 h) of pre-IVM examined, and persisted for a further 8 h in the IVM phase. Cyclic AMP-modulated pre-IVM increased intra-oocyte GSH levels at the completion of both pre-IVM and IVM, in a pre-IVM duration-dependent manner (P < 0.05), which was ablated when GJC was blocked using CBX (P < 0.05). By 4 h of pre-IVM treatment with cAMP modulators, oocyte H2O2 levels were reduced compared the control (P < 0.05), although this beneficial effect was lost when oocytes were co-treated with BSO. Inhibiting glutathione synthesis with BSO during pre-IVM ablated any positive benefits of cAMP-mediated pre-IVM on oocyte developmental competence (P < 0.01).

LIMITATIONS, REASONS FOR CAUTION

It is unclear if the improvement in oocyte antioxidant defence and developmental competence reported here is due to direct transfer of total and/or reduced glutathione from cumulus cells to the oocyte via gap junctions, or whether a GSH synthesis signal and/or amino acid substrates are supplied to the oocyte via gap junctions. Embryo transfer experiments are required to determine if the cAMP-mediated improvement in blastocyst rates leads to improved live birth rates.

WIDER IMPLICATIONS OF THE FINDINGS

IVM offers significant benefits to infertile and cancer patients and has the potential to significantly alter ART practice, if IVM efficiency in embryo production could be improved closer to that of conventional IVF (using ovarian hyperstimulation). Pre-IVM with cAMP modulators is a simple and reliable means to improve IVM outcomes.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by grants and fellowships from the National Health and Medical Research Council of Australia (1007551, 627007, 1008137, 1023210) and by scholarships from the Chinese Scholarship Council (CSC) awarded to H.J.L. and the Japanese Society for the Promotion of Science Postdoctoral Fellowship for Research Abroad awarded to S.S. The Fluoview FV10i confocal microscope was purchased as part of the Sensing Technologies for Advanced Reproductive Research (STARR) facility, funded by the South Australian Premier's Science and Research Fund. We acknowledge partial support from the Australian Research Council Centre of Excellence for Nanoscale BioPhotonics (CE140100003). We declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

Transcriptomic analysis of cyclic AMP response in bovine cumulus cells

Acquisition of oocyte developmental competence needs to be understood to improve clinical outcomes of assisted reproduction. The stimulation of cumulus cell concentration of cyclic adenosine 3'5'-monophosphate (cAMP) by pharmacological agents during in vitro maturation (IVM) participates in improvement of oocyte quality. However, precise coordination and downstream targets of cAMP signaling in cumulus cells are largely unknown. We have previously demonstrated better embryo development after cAMP stimulation for first 6 h during IVM. Using this model, we investigated cAMP signaling in cumulus cells through in vitro culture of cumulus-oocyte complexes (COCs) in the presence of cAMP raising agents: forskolin, IBMX, and dipyridamole (here called FID treatment). Transcriptomic analysis of cumulus cells indicated that FID-induced differentially expressed transcripts were implicated in cumulus expansion, steroidogenesis, cell metabolism, and oocyte competence. Functional genomic analysis revealed that protein kinase-A (PKA), extracellular signal regulated kinases (ERK1/2), and calcium (Ca(2+)) pathways as key regulators of FID signaling. Inhibition of PKA (H89) in FID-supplemented COCs or substitution of FID with calcium ionophore (A23187) demonstrated that FID activated primarily the PKA pathway which inhibited ERK1/2 phosphorylation and was upstream of calcium signaling. Furthermore, inhibition of ERK1/2 phosphorylation by FID supported a regulation by dual specific phosphatase (DUSP1) via PKA. Our findings imply that cAMP (FID) regulates cell metabolism, steroidogenesis, intracellular signaling and cumulus expansion through PKA which modulates these functions through optimization of ERK1/2 phosphorylation and coordination of calcium signaling. These findings have implications for development of new strategies for improving oocyte in vitro maturation leading to better developmental competence.

Promotion of EGF receptor signaling improves the quality of low developmental competence oocytes

Oocytes acquire developmental competence with progressive folliculogenesis. Cumulus oocyte complexes (COCs) from small antral follicles have inherent low competence and are poorly responsive to amphiregulin (AREG) which normally mediates oocyte maturation and ovulation. Using low competence porcine COCs, in an in vitro AREG-induced oocyte maturation system, the combined exposure to N(6),2'-O-dibutyryladenosine 3':5' cyclic monophosphate (cAMP) and bone morphogenetic protein 15 (B15) and growth differentiation factor 9 (G9) was necessary to enhance the rate of oocyte meiotic maturation and blastocyst formation. Furthermore, the combination of cAMP+B15+G9 enabled AREG-stimulated cumulus expansion and increased expression of the matrix-related genes HAS2, TNFIPA6 and PTGS2. Additionally, the combination enhanced p-ERK1/2 which is downstream of the EGF receptor. The enhanced nuclear maturation and blastocyst formation rates with the combinational treatment were ablated by an EGF receptor phosphorylation inhibitor. These results indicate that cAMP and oocyte-secreted factors cooperate to promote EGF receptor functionality in developing COCs, representing a key component of the acquisition of oocyte developmental competence.

Developmental Competence of Vitrified-Warmed Bovine Oocytes at the Germinal-Vesicle Stage is Improved by Cyclic Adenosine Monophosphate Modulators during In Vitro Maturation

Cryopreservation of mature oocytes and embryos has provided numerous benefits in reproductive medicine. Although successful cryopreservation of germinal-vesicle stage (GV) oocytes holds promise for further advances in reproductive biology and clinical embryology fields, reports regarding cryopreservation of immature oocytes are limited. Oocyte survival and maturation rates have improved since vitrification is being performed at the GV stage, but the subsequent developmental competence of GV oocytes is still low. The purpose of this study was to evaluate the effects of supplementation of the maturation medium with cyclic adenosine monophosphate (cAMP) modulators on the developmental competence of vitrified-warmed GV bovine oocytes. GV oocytes were vitrified-warmed and cultured to allow for oocyte maturation, and then parthenogenetically activated or fertilized in vitro. Our results indicate that addition of a cAMP modulator forskolin (FSK) or 3-isobutyl-1-methylxanthine (IBMX) to the maturation medium significantly improved the developmental competence of vitrified-warmed GV oocytes. We also demonstrated that vitrification of GV oocytes led to a decline in cAMP levels and maturation-promoting factor (MPF) activity in the oocytes during the initial and final phases of maturation, respectively. Nevertheless, the addition of FSK or IBMX to the maturation medium significantly elevated cAMP levels and MPF activity during IVM. Taken together, our results suggest that the cryopreservation-associated meiotic and developmental abnormalities observed in GV oocytes may be ameliorated by an artificial increase in cAMP levels during maturation culture after warming.

Chromosome constitution of human embryos generated after in vitro maturation including 3-isobutyl-1-methylxanthine in the oocyte collection medium

STUDY QUESTION

Do cleavage-stage embryos obtained from oocytes matured in vitro after pre-incubation with a phosphodiesterase inhibitor (IBMX) carry more chromosomal abnormalities than those generated from oocytes matured in vivo?

SUMMARY ANSWER

The rate and type of chromosomal abnormalities in normally developing cleavage-stage embryos generated with an in vitro maturation (IVM) system including pre-incubation with IBMX are not different from those observed in supernumerary embryos obtained from oocytes matured in vivo.

WHAT IS KNOWN ALREADY

Very limited information is available about the chromosomal constitution of IVM embryos. Previous studies were carried out using FISH on single biopsied blastomeres or arrested whole embryos and only provided fragmentary information on chromosomal abnormalities in IVM embryos. There is no systematic study of chromosomal abnormalities in all blastomeres of human Day 3 embryos with good morphology.

STUDY DESIGN, SIZE, DURATION

Between July 2012 and December 2012, 16 young (age <35 years old) egg donors underwent 18 IVM cycles for the generation of research embryos. Eighteen embryos developed to Day 3 and were analysed using array comparative genomic hybridization (aCGH).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Immature oocytes were retrieved from 2 to 10 mm follicles after mild ovarian stimulation with gonadotrophins but without hCG ovulation trigger. At collection, oocytes were pre-incubated with 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor and matured in vitro. After IVM culture, mature oocytes were microinjected with sperm from a single donor. Embryos were cultured to Day 3 after ICSI and all blastomeres of 18 good-morphology embryos were collected individually for aCGH.

MAIN RESULTS AND THE ROLE OF CHANCE

Oocyte maturation rate in vitro was 50.2% (120/239). The mean fertilization rate was 68.3% (82/120) and 30.5% (25/82) of fertilized oocytes developed into a morphologically good quality embryo on Day 3 after ICSI. Of these, 18 embryos that developed well up to Day 3 were analysed using aCGH. Eighty of the 123 blastomeres analysed showed at least one chromosomal abnormality. Three out of eighteen embryos had completely normal cells. A single embryo carried a meiotic abnormality, 11 embryos were mosaic and three were chaotic. Although the aneuploidy data of this study are too limited to allow statistical analysis, these data are comparable to our own published data on the chromosome constitution of whole day 3 and day 4 embryos after conventional ART.

LIMITATIONS, REASONS FOR CAUTION

Array CGH technology determines relative quantification of chromosomal domains but does not allow for the visualization of chromosomal rearrangements, assessment of ploidy or detection of uniparental isodisomy. Conclusions drawn on segmental abnormalities should be treated with caution. Although the limited number of embryos analysed here precludes firm conclusions, they provide valuable data on possible causes of the reduced potential of IVM embryos.

WIDER IMPLICATIONS OF THE FINDINGS

This is the first study to describe the complete chromosome complement of all single blastomeres of good-morphology day 3 embryos obtained with IVM (including the presence of IBMX in a pre-incubation medium). The results demonstrate that a high proportion of good-morphology embryos are aneuploid and that there is no obvious increase in aneuploidies as a result of IVM which seems to suggest that the reduced efficiency of IVM technology compared with standard IVF may be accounted for by factors other than aneuploidy, such as cytoplasmic defects or reduced endometrial receptivity.

STUDY FUNDING/COMPETING INTERESTS

This study was funded by the TBM (Applied Biomedical Research with Societal Finality) programme of the IWT (Agency for Innovation through Science and Technology - Flanders, 110680) and by a Methusalem grant of the Vrije Universiteit Brussel. C.S. is a post-doctoral fellow of the Fund for Scientific Research Flanders (FWO - Vlaanderen). K.J. is a PhD student funded by the FWO. The University of Adelaide owns a patent family associated with IVM technologies that is licensed to Cook Medical. R.B.G. and J.G.T. are inventors. The remaining authors have no conflict of interest to declare.

Increasing the cAMP concentration during in vitro maturation of pig oocytes improves cumulus maturation and subsequent fertilization in vitro

Porcine IVF faces various problems such as incomplete cytoplasmic maturation of the oocyte and polyspermy. Previous studies proved the importance of cAMP in regulating nuclear and cytoplasmic maturation of oocytes. This study investigated the effect of the cAMP-modulating agents 3-isobutyl-1-methylxanthine (IBMX) and dibutyryl cAMP sodium salt (dbcAMP) on several parameters during in vitro production of porcine embryos. First, we wanted to see if oocyte collection in IBMX could meiotically arrest oocytes and, as such, improve synchronization of nuclear and cytoplasmic maturation. To this end, cumulus-oocyte complexes (COCs) were collected from gilts in HEPES-buffered Tyrode balanced salt solution medium with 0.5-mM IBMX or without IBMX. At the end of oocyte collection, the effect of IBMX on chromatin configuration was evaluated. However, no differences could be observed in nuclear configuration between IBMX- and IBMX+ oocytes (P > 0.05). Second, we added dbcAMP during IVM to improve cytoplasmic maturation and evaluated cumulus expansion (lack of adhesion), a disintegrin and metalloproteinase with thrombospondin-like repeats (ADAMTS-1) levels in cumulus cells, fertilization, and blastocyst rates. Cumulus-oocyte complexes were matured in modified North Carolina State University medium 37 with or without 1-mM dbcAMP. Frozen-thawed, epididymal, boar spermatozoa were used for IVF. After IVF, presumed zygotes were cultured for 7 days in North Carolina State University medium 23. Penetration rate decreased in dbcAMP+ (57.3%) compared with dbcAMP- (67.8%), but the polyspermy rate also decreased (43.3% vs. 53.4%, respectively) leading to an increased normal fertilization rate (56.7% vs. 46.6%, respectively; P < 0.05). Only 7.2% of the COCs showed adhesion in dbcAMP+ which was lower than 15.7% in dbcAMP- (P < 0.05) probably because of an upregulation of the ADAMTS-1 protein by dbcAMP. When the adherent oocytes were removed during maturation, no difference could be detected between the blastocyst rate of dbcAMP- and dbcAMP+ (17.1% and 21.0% on Day 7, respectively; P > 0.05). In conclusion, the use of IBMX during collection did not cause a meiotic arrest. Using dbcAMP during IVM caused a greater normal fertilization rate, a lower rate of adherent COCs during IVM, higher levels of ADAMTS-1 in cumulus cells, and an equal blastocyst rate after screening out adherent COCs. These findings contribute to a better understanding of cAMP involvement in porcine oocyte maturation and provide a basis to develop an improved system with less polyspermy and higher blastocyst rates.

Changes in large-scale chromatin structure and function during oogenesis: a journey in company with follicular cells

The mammalian oocyte nucleus or germinal vesicle (GV) exhibits characteristic chromatin configurations, which are subject to dynamic modifications through oogenesis. Aim of this review is to highlight how changes in chromatin configurations are related to both functional and structural modifications occurring in the oocyte nuclear and cytoplasmic compartments. During the long phase of meiotic arrest at the diplotene stage, the chromatin enclosed within the GV is subjected to several levels of regulation. Morphologically, the chromosomes lose their individuality and form a loose chromatin mass. The decondensed configuration of chromatin then undergoes profound rearrangements during the final stages of oocyte growth that are tightly associated with the acquisition of meiotic and developmental competence. Functionally, the discrete stages of chromatin condensation are characterized by different level of transcriptional activity, DNA methylation and covalent histone modifications. Interestingly, the program of chromatin rearrangement is not completely intrinsic to the oocyte, but follicular cells exert their regulatory actions through gap junction mediated communications and intracellular messenger dependent mechanism(s). With this in mind and since oocyte growth mostly relies on the bidirectional interaction with the follicular cells, a connection between cumulus cells gene expression profile and oocyte developmental competence, according to chromatin configuration is proposed. This analysis can help in identifying candidate genes involved in the process of oocyte developmental competence acquisition and in providing non-invasive biomarkers of oocyte health status that can have important implications in treating human infertility as well as managing breeding schemes in domestic mammals.

Prematuration with cyclic adenosine monophosphate modulators alters cumulus cell and oocyte metabolism and enhances developmental competence of in vitro-matured mouse oocytes

Oocyte in vitro maturation (IVM) is an important assisted reproductive technology and research tool. The adoption of IVM into routine clinical practice has been hindered by its significantly lower success rates compared to conventional in vitro fertilization. Cyclic AMP (cAMP) modulation and follicle-stimulating hormone (FSH), independently, have long been known to improve IVM oocyte developmental competence. This study comprehensively examined the effects of FSH and cAMP/cGMP modulation, alone and in combination, on IVM oocyte metabolism and developmental outcomes. Mouse cumulus-oocyte complexes (COCs) were subjected to a 1 h prematuration phase ± the cAMP modulator forskolin and cAMP/cGMP modulator 3-isobutyl-1-methylxanthine followed by IVM ± FSH. Prematuration with these cyclic nucleotide modulators or IVM with FSH significantly improved oocyte developmental competence and reduced spindle abnormalities compared to spontaneous IVM (no treatment); however, these two treatments in combination endowed even greater developmental competence (improved subsequent blastocyst rates and quality; P < 0.05), albeit blastocyst yield and quality remained significantly lower than that of oocytes matured in vivo. A significant additive effect of combined IVM treatments was evident as increased COC lactate production and oxygen consumption and enhanced oocyte oxidative metabolism, ATP production, ATP:ADP ratio, and glutathione levels (P < 0.05). Nevertheless, IVM increased reactive oxygen species production, particularly as a consequence of FSH addition, relative to in vivo matured oocytes. In conclusion, improvements in the embryo yield following IVM is associated with increased COC oxygen consumption and oocyte oxidative metabolism, but these remain metabolically and developmentally less competent relative to in vivo derived oocytes.

Pre-maturation with cAMP modulators in conjunction with EGF-like peptides during in vitro maturation enhances mouse oocyte developmental competence

Recent studies have independently shown that cyclic adenosine 3'5'-monophosphate (cAMP) modulation prior to in vitro maturation (IVM) and epidermal growth factor (EGF)-like peptide supplementation during IVM improve subsequent oocyte developmental outcomes. This study investigated the effects of an IVM system that incorporates these two concepts. Cumulus-oocyte complexes (COCs) were collected from pre-pubertal mice either 46 hr post-equine chorionic gonadotropin (eCG) (IVM) or post-eCG + post-human chorionic gonadotropin (hCG) stimulation (in vivo maturation; IVV). IVM COCs were treated with the cAMP modulators forskolin and IBMX for 1, 2, or 4 hr (pre-IVM phase) prior to IVM. COCs then underwent IVM with the EGF-like peptides amphiregulin or epiregulin, or with the common IVM stimulants follicle-stimulating hormone (FSH) or EGF. A pre-IVM phase increased the size of the subsequent blastocysts' inner-cell-mass compared to standard IVM, regardless of IVM treatment (P < 0.05). Unlike FSH or EGF, amphiregulin or epiregulin significantly increased blastocyst quality (trophectoderm and total cell numbers) and/or yield (P < 0.01) compared to standard IVM, and were the only treatments that produced blastocysts comparable to IVV-derived blastocysts. Forskolin acutely up-regulated EGF-like peptide mRNA expression after a 2-hr pre-IVM phase (P < 0.001), although EGF receptor and ERK1/2 activities were not significantly different than control. IVV-like levels of EGF-like peptide mRNA expression during IVM were maintained only by supplementing with EGF-like peptides and EGF, since expression levels induced by FSH were significantly lower in vitro than during IVV. However, EGF receptor and ERK1/2 phosphorylation levels were not significantly different across treatment groups. In conclusion, a pre-IVM phase in conjunction with IVM in the presence of EGF-like peptides endows high oocyte developmental competence, as evidenced by increased embryo yield and/or quality relative to FSH and EGF.

The effect of cilostamide on gap junction communication dynamics, chromatin remodeling, and competence acquisition in pig oocytes following parthenogenetic activation and nuclear transfer

In the pig, the efficiency of in vitro embryo production and somatic cell nuclear transfer (SCNT) procedures remains limited. It has been suggested that prematuration treatments (pre-IVM) based on the prolongation of a patent, bidirectional crosstalk between the oocyte and the cumulus cells through gap junction mediate communication (GJC), with the maintenance of a proper level of cAMP, could improve the developmental capability of oocytes. The aim of this study was to assess: 1) dose-dependent effects of cilostamide on nuclear maturation kinetics, 2) the relationship between treatments on GJC functionality and large-scale chromatin configuration changes, and 3) the impact of treatments on developmental competence acquisition after parthenogenetic activation (PA) and SCNT. Accordingly, cumulus-oocyte complexes were collected from 3- to 6-mm antral follicles and cultured for 24 h in defined culture medium with or without 1 μM cilostamide. GJC functionality was assessed by Lucifer yellow microinjection, while chromatin configuration was evaluated by fluorescence microscopy after nuclear staining. Cilostamide administration sustained functional coupling for up to 24 h of culture and delayed meiotic resumption, as only 25.6% of cilostamide-treated oocytes reached the pro-metaphase I stage compared to the control (69.7%; P < 0.05). Moreover, progressive chromatin condensation was delayed before meiotic resumption based upon G2/M biomarker phosphoprotein epitope acquisition using immunolocalization. Importantly, cilostamide treatment under these conditions improved oocyte developmental competence, as reflected in higher blastocyst quality after both parthenogenetic activation and SCNT.

Role of gap junction-mediated communications in regulating large-scale chromatin configuration remodeling and embryonic developmental competence acquisition in fully grown bovine oocyte

PURPOSE

This study was aimed to test the hypothesis that gap junction mediated communications (GJC) are required to allow the progressive chromatin configuration remodeling (from GV1 to GV3) process to occur in fully grown oocytes in order to gain the final step of developmental competence acquisition, and that a premature disruption of GJC can alter this process.

METHODS

Bovine cumulus-oocytes complexes collected from medium antral follicles were cultured for 2, 4, 6 and 8 h in the presence of 10(-4) IU/ml of r-hFSH and with 2 mM of the non-selective PDE inhibitor 3-isobutyl-1-methyl-xanthine (IBMX) to prevent meiotic resumption. GJC functionality and chromatin configuration were monitored during the culture period. After meiotic arrest, the developmental capability of oocytes was assessed after IVM and IVF.

RESULTS

IBMX was effective in significantly sustaining GJC up to 6 h and maintaining meiotic arrest, when compared to control group. Moreover, the percentage of oocytes with less condensed chromatin (GV1) decreased within 4 h of culture, while the proportion of GV2 oocytes gradually increased up to 6 h. Interestingly, a decline in the proportion of GV2 oocytes and an increase in the proportion of GV3 oocytes were observed after 6 h of culture, when the major drop of GJC occurred. On the contrary, when GJC were uncoupled by adding 3 mM of 1-heptanol or through cumulus cells removal, chromatin condensation occurred rapidly throughout the culture period, more promptly in denuded oocytes. Moreover, the maintenance of GJC during meiotic arrest was accompanied by a significant increase of developmental competence compared to the control, as indicated by a higher percentage of hatched blastocysts and blastocyst cell number.

CONCLUSIONS

Altogether, our data indicate that both paracrine and junctional mechanisms are involved in modulating large-scale chromatin structure during the final phase of oocyte differentiation.