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

Agarose-based 3D culture improved the developmental competence of oocyte-granulosa complex isolated from porcine preantral follicle

Various models have been established to culture whole follicles of the Preantral stage; however, the process remains inefficient and is an ongoing challenge formation. It is reported that oocyte-cumulus-granulosa complexes (OCGCs) isolated from Early Antral follicles (EAFs) undergo in vitro growth (IVG) and acquire meiotic competence in some animals. However, IVG for the oocyte-granulosa complexes (OGCs) from Preantral Follicles (PAFs) has not been firmly established. The present study indicated that the use of a modified medium with Ascorbic Acid (50 μM) facilitated granulosa cell proliferation, promoted cumulus cell differentiations, and increased antrum formation for the OGCs isolated from PAFs (0.3-0.4 mm). However, the two-dimensional 96-well plate system (2D) experienced smaller size follicles and could not prolong more than 10 days of IVG. Another method is to use an Agarose matrix 3D system to provide a soft, non-adhesive base that supports the IVG of OGCs isolated from PAFs and promotes cell proliferation, antrum formation, and maintenance for 14 days. OGCs that were grown using this method retained their spherical morphology, which in turn helped to attain healthy granulosa cells and maintain their connection with oocytes, in addition, these oocytes significantly increased diameter and lipid content, indicating developmental competence. Our result indicated that the OGCs from PAFs after IVG undergo a change in chromatin morphology and expression of acetylation of histone H3 at lysine 9 (Ac-H3-K9) and methylation of histone H3 at lysine 4 (Me-H3-K4), similar to the in vivo oocytes isolated from the ovary. Likewise, IVG oocytes cultured for maturation showed full cumulus expansion and reached mature oocytes. Furthermore, after in vitro maturation, IVG oocytes underwent the first cleavage following parthenogenetic activation. In conclusion, while most studies used whole follicles from the Preantral stage for IVG, our research finding was the first to reveal that oocytes isolated from the final stage of PAFs can migrate out of the follicle and undergo IVG under suitable conditions.

Cumulus Cell and Oocyte Gene Expression in Prepubertal Gilts and Sows Identifies Cumulus Cells as a Prime Informative Parameter of Oocyte Quality

Cumulus cells (CCs) are pivotal during oocyte development. This study aimed to identify novel marker genes for porcine oocyte quality by examining the expression of selected genes in CCs and oocytes, employing the model of oocytes from prepubertal animals being of reduced quality compared to those from adult animals. Total RNA was extracted either directly after follicle aspiration or after in vitro maturation, followed by RT-qPCR. Immature gilt CCs accumulated BBOX1 transcripts, involved in L-carnitine biosynthesis, to a 14.8-fold higher level (p < 0.05) relative to sows, while for CPT2, participating in fatty acid oxidation, the level was 0.48 (p < 0.05). While showing no differences between gilt and sow CCs after maturation, CPT2 and BBOX1 levels in oocytes were higher in gilts at both time points. The apparent delayed lipid metabolism and reduced accumulation of ALDOA and G6PD transcripts in gilt CCs after maturation, implying downregulation of glycolysis and the pentose phosphate pathway, suggest gilt cumulus-oocyte complexes have inadequate ATP stores and oxidative stress balance compared to sows at the end of maturation. Reduced expression of BBOX1 and higher expression of CPT2 in CCs before maturation and higher expression of G6PD and ALDOA after maturation are new potential markers of oocyte quality.

Chromatin Configuration in Diplotene Mouse and Human Oocytes during the Period of Transcriptional Activity Extinction

In the oocyte nucleus, called the germinal vesicle (GV) at the prolonged diplotene stage of the meiotic prophase, chromatin undergoes a global rearrangement, which is often accompanied by the cessation of its transcriptional activity. In many mammals, including mice and humans, chromatin condenses around a special nuclear organelle called the atypical nucleolus or formerly nucleolus-like body. Chromatin configuration is an important indicator of the quality of GV oocytes and largely predicts their ability to resume meiosis and successful embryonic development. In mice, GV oocytes are traditionally divided into the NSN (non-surrounded nucleolus) and SN (surrounded nucleolus) based on the specific chromatin configuration. The NSN-SN transition is a key event in mouse oogenesis and the main prerequisite for the normal development of the embryo. As for humans, there is no single nomenclature for the chromatin configuration at the GV stage. This often leads to discrepancies and misunderstandings, the overcoming of which should expand the scope of the application of mouse oocytes as a model for developing new methods for assessing and improving the quality of human oocytes. As a first approximation and with a certain proviso, the mouse NSN/SN classification can be used for the primary characterization of human GV oocytes. The task of this review is to analyze and discuss the existing classifications of chromatin configuration in mouse and human GV oocytes with an emphasis on transcriptional activity extinction at the end of oocyte growth.

Epitranscriptome marks detection and localization of RNA modifying proteins in mammalian ovarian follicles

BACKGROUND

Most of the resources that support the early development of the embryo are stored in the oocyte. Clearing of maternal resources and activation of the embryonic genome to produce its own mRNA transcripts marks the maternal-to-embryo transition. Dependence on stored mRNA can last from a few hours to several days, depending on animal species. The mechanisms regulating stabilization and recruitment of stored maternal transcripts have not yet been described in full detail but are known to involve reversible polyadenylation and modulation of 3'UTR-mediated elements. RNA epigenetic modifications, new players in this field, have an important role in RNA regulation and stabilization.

RESULTS

The objectives of this study were first to determine if some of post-transcriptional methylation of stored mRNA is greater in oocytes than in somatic cells. We found that m⁶A, known to be the most prevalent and involved in various aspects of RNA metabolism and physiological functions, is particularly abundant in porcine oocyte mRNA compared to liver used as a somatic tissue reference. The second objective was to compare the epitranscriptome machinery, such as methyltransferases ("writers"), binding proteins ("readers") and demethylases ("erasers") catalyzing the different process, in follicles and oocytes of different mammalian species by immunofluorescence and confocal microscopy. The expression and localization patterns of these proteins differ between mice, pigs and cows ovaries and oocytes. m⁵C-associated proteins were generally less abundant. In contrast, m⁶A-associated proteins were expressed strongly during the early and late stages of folliculogenesis. Transzonal projections were found to contain more granules bearing the m⁵C mark in mice but both m⁵C and m⁶A methylation marks in association with mature oocytes of pigs and cows. Eraser proteins showed the greatest interspecies diversity in terms of distribution in the germinal tissues.

CONCLUSIONS

So far, few studies have looked at the oocyte and ovarian epitranscriptomic profile. Our findings indicate that a hitherto unrecognized species-specific layer of transcript regulation occurs at the RNA level and might be consequential during the oocyte transcriptional silencing period.

A refined culture system of oocytes from early antral follicles promotes oocyte maturation and embryo development in cattle

In brief

The proposed culture system improves the current state of in vitro culture of growing oocytes in the bovine species and allows access to the untapped gamete reserve, thus improving reproductive efficiency.

Abstract

The present study aimed to improve the in vitro culture of bovine oocytes collected from early antral follicles (EAFs) to support the progressive acquisition of meiotic and developmental competence. The rationale that drove the development of such a culture system was to maintain as much as possible the physiological conditions that support the oocyte growth and differentiation in vivo. To this extent, oocytes were cultured for 5 days, which parallels the transition from early to medium antral follicles (MAFs) in the bovine, and supports promoting a 3D-like structure were provided. Additionally, the main hormones (follicle-stimulating hormone, estradiol, progesterone, and testosterone) were added in concentrations similar to the ones previously observed in bovine EAFs. The meiotic arrest was imposed using cilostamide. The cultured cumulus-oocyte complexes (COCs) reached a mean diameter of 113.4 ± 0.75 µm and showed a progressive condensation of the chromatin enclosed in the germinal vesicle (GV), together with a gradual decrease in the global transcriptional activity, measured by 5-ethynyl uridine incorporation. The described morpho-functional changes were accompanied by an increased ability to mature and develop to the blastocyst stage in vitro, although not matching the rates obtained by MAF-retrieved oocytes. The described system improves the current state of in vitro culture of growing oocytes in the bovine species, and it can be used to increase the number of gametes usable for in vitro embryo production in animals of high genetic merit or with specific desirable traits.

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.

Physiological parameters related to oocyte nuclear differentiation for the improvement of IVM/IVF outcomes in women and cattle

In vitro maturation (IVM) has been applied in numerous different contexts and strategies in humans and animals, but in both cases it represents a challenge still far from being overcome. Despite the large dataset produced over the last two decades on the mechanisms that govern antral follicular development and oocyte metabolism and differentiation, IVM outcomes are still unsatisfactory. This review specifically focuses on data concerning the potential consequences of using supraphysiological levels of FSH during IVM, as well as on the regulation of oocyte chromatin dynamics and its utility as a potential marker of oocyte developmental competence. Taken together, the data revisited herein indicate that a significant improvement in IVM efficacy may be provided by the integration of pre-OPU patient-specific protocols preparing the oocyte population for IVM and more physiological culture systems mimicking more precisely the follicular environment that would be experienced by the recovered oocytes until completion of metaphase II.

The H3.3 chaperone Hira complex orchestrates oocyte developmental competence

Successful reproduction requires an oocyte competent to sustain early embryo development. By the end of oogenesis, the oocyte has entered a transcriptionally silenced state, the mechanisms and significance of which remain poorly understood. Histone H3.3, a histone H3 variant, has unique cell cycle-independent functions in chromatin structure and gene expression. Here, we have characterised the H3.3 chaperone Hira/Cabin1/Ubn1 complex, showing that loss of function of any of these subunits causes early embryogenesis failure in mouse. Transcriptome and nascent RNA analyses revealed that transcription is aberrantly silenced in mutant oocytes. Histone marks, including H3K4me3 and H3K9me3, are reduced and chromatin accessibility is impaired in Hira/Cabin1 mutants. Misregulated genes in mutant oocytes include Zscan4d, a two-cell specific gene involved in zygote genome activation. Overexpression of Zscan4 in the oocyte partially recapitulates the phenotypes of Hira mutants and Zscan4 knockdown in Cabin1 mutant oocytes partially restored their developmental potential, illustrating that temporal and spatial expression of Zscan4 is fine-tuned at the oocyte-to-embryo transition. Thus, the H3.3 chaperone Hira complex has a maternal effect function in oocyte developmental competence and embryogenesis, through modulating chromatin condensation and transcriptional quiescence.

Summary: The H3.3 chaperone Hira complex has a maternal effect function in oocyte developmental competence and embryogenesis by modulating chromatin condensation and transcriptional quiescence.

Effect of season on germinal vesicle stage, quality, and subsequent in vitro developmental competence in bovine cumulus-oocyte complexes

Although the reproductive performance of grazing cattle is lower in summer compared to winter, the effect of season on oocyte developmental competence has not been thoroughly examined. We measured the effect of season on oocyte chromatin compaction, cumulus cell quality, and embryonic development after in vitro fertilization. Cumulus oocytes-complexes (COCs) were collected from abattoir cows' ovaries during the winter and summer months. First, we evaluated the degree of chromatin compaction in germinal vesicle (GV) oocytes (GV1 through GV3), which is associated with different degrees of developmental competence. Then, we determined the apoptotic index in cumulus cells from immature and in vitro matured COCs. Finally, in vitro matured oocytes were fertilized to determine blastocyst rate and embryo quality. During the summer months, we observed a significantly lower proportion of oocytes reaching the GV3 stage and higher levels of DNA fragmentation in cumulus cell. As a result, blastocyst yield and quality were reduced during the summer months. In conclusion, summer negatively affected oocyte GV stage progression, cumulus cell quality, and embryo development. Increased cumulus cell DNA fragmentation during summer, may partially explain the reduced oocyte maturation capacity, considering the relevance of cumulus-oocyte communication during this stage.

The two-step process of ovarian follicular growth and maturation in mammals can be compared to a fruit ripening where quality depends on the second step

In human IVF, the main uncertainty factor impacting on success is oocyte quality, which largely depends on the follicular status at the time of collection. Decades of debate ensued to find the perfect stimulation protocol demonstrated the complexity of the ovarian response to exogenous gonadotropins and the dynamic nature of late folliculogenesis. Although several follicular markers, proteins, RNA from granulosa cells or microRNA and follicular fluid metabolites have been associated with outcome, the possibility to influence them during stimulation remains elusive. The heterogeneity of the follicle's maturity following control ovarian stimulation is also an important factor to explain average poor oocyte quality still observed today. In this review, the analogy between the apple ripening on the tree and follicular development is presented to focus the attention on a biphasic process: growth and differentiation. The molecular analysis of the progressive follicular differentiation indicates 2 competing phenomena: growth and differentiation where a delicate balance must operate from one to the other to ensure proper maturity at ovulation. As long as FSH stimulates growth, follicles remain green, and it is only when FSH is replaced by LH that the ripening process begins, and "apples" become red. Both fruits, follicles and apples, depend on a perfect timing of events to generate offspring.

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

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

A Nuclear and Cytoplasmic Characterization of Bovine Oocytes Reveals That Cysteamine Partially Rescues the Embryo Development in a Model of Low Ovarian Reserve

Simple Summary

Women’s reproductive performance starts declining in the mid-30s, and by age 40–45, the possibility of becoming pregnant becomes very small. Reproductive aging is a physiological process of fertility decline characterized by a decrease in quality and stockpile of eggs (also called ovarian reserve) in most mammals. However, young individuals too can show an accelerated reproductive aging that similarly results in a low ovarian reserve and hypofertility. This syndrome, called premature ovarian failure (POF), is becoming a relevant problem due to the general tendency to postpone the first pregnancy. In this study, we used bovine ovaries that were classified in two categories, according to the number of follicles visible on the ovarian surface, and analyzed some parameters of egg maturation. We observed that eggs from the ‘aging-like’ ovaries carry several defects that impair maturation. However, one of the parameters was improved upon supplementation with a scavenger of free radicals, providing a proof of concept that in-depth knowledge of the cellular mechanisms is essential to find solutions to everyday-life problems.

Abstract

Decreased oocyte quality is a major determinant of age-associated fertility decline. Similarly, individuals affected by early ovarian aging carry low-quality oocytes. Using an established bovine model of early ovarian aging, we investigated key features of ‘quality’ oocyte maturation, associated with the onset of egg aneuploidy and reproductive aging, such as histone modifications, mitochondria distribution and activity, reduced glutathione (GSH) content, and gap junction functionality. Bovine ovaries were classified according to the antral follicle count (AFC), and the retrieved oocytes were processed immediately or matured in vitro. We observed alterations in several cellular processes, suggesting a multifactorial etiology of the reduced oocyte quality. Furthermore, we performed a rescue experiment for one of the parameters considered. By adding cysteamine to the maturation medium, we experimentally increased the free radical scavenger ability of the ‘low competence’ oocytes and obtained a higher embryo development. Our findings show that adopting culture conditions that counteract the free radicals has a positive impact on the quality of ‘compromised’ oocytes. Specifically, cysteamine treatment seems to be a promising option for treating aging-related deficiencies in embryo development.

Meiosis and beyond - understanding the mechanistic and evolutionary processes shaping the germline genome

The separation of germ cell populations from the soma is part of the evolutionary transition to multicellularity. Only genetic information present in the germ cells will be inherited by future generations, and any molecular processes affecting the germline genome are therefore likely to be passed on. Despite its prevalence across taxonomic kingdoms, we are only starting to understand details of the underlying micro-evolutionary processes occurring at the germline genome level. These include segregation, recombination, mutation and selection and can occur at any stage during germline differentiation and mitotic germline proliferation to meiosis and post-meiotic gamete maturation. Selection acting on germ cells at any stage from the diploid germ cell to the haploid gametes may cause significant deviations from Mendelian inheritance and may be more widespread than previously assumed. The mechanisms that affect and potentially alter the genomic sequence and allele frequencies in the germline are pivotal to our understanding of heritability. With the rise of new sequencing technologies, we are now able to address some of these unanswered questions. In this review, we comment on the most recent developments in this field and identify current gaps in our knowledge.

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.

A prematuration approach to equine IVM: considering cumulus morphology, seasonality, follicle of origin, gap junction coupling and large-scale chromatin configuration in the germinal vesicle

Several studies report that a two-step culture where mammalian oocytes are first kept under meiosis-arresting conditions (prematuration) followed by IVM is beneficial to embryo development. The most promising results were obtained by stratifying the oocyte population using morphological criteria and allocating them to different culture conditions to best meet their metabolic needs. In this study, horse oocytes were characterised to identify subpopulations that may benefit from prematuration. We investigated gap-junction (GJ) coupling, large-scale chromatin configuration and meiotic competence in compact and expanded cumulus-oocyte complexes (COCs) according to follicle size (<1, 1-2, >2cm) and season. Then we tested the effect of cilostamide-based prematuration in compact COCs collected from follicles <1 and 1-2cm in diameter on embryo development. Meiotic competence was not affected by prematuration, whereas COCs from follicles 1-2cm in diameter yielded embryos with a higher number of cells per blastocyst than oocytes that underwent direct IVM (P<0.01, unpaired Mann-Whitney test), suggesting improved developmental competence. Oocytes collected from follicles <1cm in diameter were not affected by prematuration. This study represents an extensive characterisation of the functional properties of immature horse oocytes and is the first report of the effects of cilostamide-based prematuration in horse oocyte IVM on embryo development.

The landscape of accessible chromatin in bovine oocytes and early embryos

Chromatin reorganization governs the regulation of gene expression during preimplantation development. However, the landscape of chromatin dynamics in this period has not been explored in bovine. In this study, we constructed a genome-wide map of accessible chromatin in bovine oocytes and early embryos using an improved assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) which revealed unique features of the accessible chromatin during bovine early embryo development. We found that chromatin accessibility is low in oocytes and 2-/4-cell embryos, followed by a significant increase in embryos during major embryonic genome activation (EGA), and peaked in elongating day 14 embryos. Genome-wide characteristics of open chromatin showed that ATAC-seq signals in both transcription start sites (TSS) and transcription end sites (TES) were strong. Additionally, the distal ATAC-seq peaks were enriched in repeat elements in a type-specific and stage-specific manner. We further unveiled a series of transcription factor (TF) motifs with distinct variation of enrichment from distal ATAC-seq peaks. By integrated analysis of chromatin accessibility with transcriptomes and DNA methylomes in bovine early embryos, we showed that promoter accessibility was positively correlated with gene expression, especially during major EGA, and was strongly correlated to DNA methylation and CpG density. Finally, we identified the critical chromatin signatures and TFs that differ between in vivo and in vitro derived blastocysts, which provides insights to the potential mechanisms leading to low quality of embryos produced in vitro. Together, this comprehensive analysis revealed critical features of chromatin landscape and epigenetic reprogramming during bovine preimplantation embryo development.

Paeonia lactiflora improves ovarian function and oocyte quality in aged female mice

Although ovarian aging is a key cause of decreased ovarian function and oocyte quality, it remains a problem in infertility treatment. Therefore, this study is aimed to investigate whether Paeonia lactiflora (PL), a herb improves ovarian function and oocyte quality using aged female mice. C57BL/6 female mice aged 8 months were treated orally every day with PL of 26.5 mg/kg (n=7) and 53 mg/kg (n=7) of body weight for 4 weeks using an oral zoned needle. The control group (n=7) was treated with normal saline. Ovaries and serum were collected for the H&E stain and the evaluation of reactive oxygen species (ROS) levels, respectively. In the second experiment, female mice were orally administered with PL (26.5 mg/kg: n=12, 53 mg/kg: n=12, control: n=12) and then superovulated with PMSG and hCG, and mated with male mice. Zygotes were retrieved and cultured for 4 days. Ovaries were provided for examination of expressions of genes associated with angiogenesis (VEGF and visfatin), anti-aging (Sirt1 and Sirt2), and follicular development (c-Kit, BMP-15, and GDF-9). PL significantly increased numbers of surviving follicles (primordial, primary, secondary, and antral), numbers of zygotes retrieved, embryo development rate, and ovarian expression of VEGF, visfatin, c-Kit, BMP-15, and GDF-9 at both doses. However, ovarian expression of Sirt1 and Sirt2 was increased at 53.0 mg/kg of PL. ROS levels were not affected by PL. These results suggest that PL may possess beneficial effects regarding ovarian function and oocyte quality, possibly by activation of ovarian angiogenesis and follicular development.

Heterochromatin Morphodynamics in Late Oogenesis and Early Embryogenesis of Mammals

During the period of oocyte growth, chromatin undergoes global rearrangements at both morphological and molecular levels. An intriguing feature of oogenesis in some mammalian species is the formation of a heterochromatin ring-shaped structure, called the karyosphere or surrounded "nucleolus", which is associated with the periphery of the nucleolus-like bodies (NLBs). Morphologically similar heterochromatin structures also form around the nucleolus-precursor bodies (NPBs) in zygotes and persist for several first cleavage divisions in blastomeres. Despite recent progress in our understanding the regulation of gene silencing/expression during early mammalian development, as well as the molecular mechanisms that underlie chromatin condensation and heterochromatin structure, the biological significance of the karyosphere and its counterparts in early embryos is still elusive. We pay attention to both the changes of heterochromatin morphology and to the molecular mechanisms that can affect the configuration and functional activity of chromatin. We briefly discuss how DNA methylation, post-translational histone modifications, alternative histone variants, and some chromatin-associated non-histone proteins may be involved in the formation of peculiar heterochromatin structures intimately associated with NLBs and NPBs, the unique nuclear bodies of oocytes and early embryos.

A Comparative Analysis of Oocyte Development in Mammals

Sexual reproduction requires the fertilization of a female gamete after it has undergone optimal development. Various aspects of oocyte development and many molecular actors in this process are shared among mammals, but phylogeny and experimental data reveal species specificities. In this chapter, we will present these common and distinctive features with a focus on three points: the shaping of the oocyte transcriptome from evolutionarily conserved and rapidly evolving genes, the control of folliculogenesis and ovulation rate by oocyte-secreted Growth and Differentiation Factor 9 and Bone Morphogenetic Protein 15, and the importance of lipid metabolism.

The potential impact of tumor suppressor genes on human gametogenesis: a case-control study

PURPOSE

To study the incidence of tumor suppressor gene (TSG) mutations in men and women with impaired gametogenesis.

METHODS

Gene association analyses were performed on blood samples in two distinct patient populations: males with idiopathic male infertility and females with unexplained diminished ovarian reserve (DOR). The male study group consisted of men with idiopathic azoospermia, oligozoospermia, asthenozoospermia, or teratozoospermia. Age-matched controls were men with normal semen analyses. The female study group consisted of women with unexplained DOR with anti-Müllerian hormone levels ≤ 1.1 ng/mL. Controls were age-matched women with normal ovarian reserve (> 1.1 ng/mL).

RESULTS

Fifty-seven male cases (mean age = 38.4; mean sperm count = 15.7 ± 12.1; mean motility = 38.2 ± 24.7) and 37 age-matched controls (mean age = 38.0; mean sperm count = 89.6 ± 37.5; mean motility = 56.2 ± 14.3) were compared. Variants observed in CHD5 were found to be enriched in the study group (p = 0.000107). The incidence of CHD5 mutation c.*3198_*3199insT in the 3'UTR (rs538186680) was significantly higher in cases compared to controls (p = 0.0255). 72 DOR cases (mean age = 38.7; mean AMH = 0.5 ± 0.3; mean FSH = 11.7 ± 12.5) and 48 age-matched controls (mean age = 37.6; mean AMH = 4.1 ± 3.0; mean FSH = 7.1 ± 2.2) were compared. Mutations in CHD5 (c.-140A>C), RB1 (c.1422-18delT, rs70651121), and TP53 (c.376-161A>G, rs75821853) were found at significantly higher frequencies in DOR cases compared to controls (p ≤ 0.05). In addition, 363 variants detected in the DOR patients were not present in the control group.

CONCLUSION

Unexplained impaired gametogenesis in both males and females may be associated with genetic variation in TSGs. TSGs, which play cardinal roles in cell-cycle control, might also be critical for normal spermatogenesis and oogenesis. If validated in larger prospective studies, it is possible that TSGs provide an etiological basis for some patients with impaired gametogenesis.

Gaps and barriers: Gap junctions as a channel of communication between the soma and the germline

Gap junctions, expressed in most tissues of the body, allow for the cytoplasmic coupling of adjacent cells and promote tissue cooperation. Gap junctions connect also the soma and the germline in many animals, and transmit somatic signals that are crucial for germline maturation and integrity. In this review, we examine the involvement of gap junctions in the relay of information between the soma and the germline, and ask whether such communication could have consequences for the progeny. While the influence of parental experiences on descendants is of great interest, the possibility that gap junctions participate in the transmission of information across generations is largely unexplored.

Treatment with cyclic adenosine monophosphate modulators prior to in vitro maturation alters the lipid composition and transcript profile of bovine cumulus-oocyte complexes and blastocysts

Mammalian oocytes resume meiosis spontaneously after removal from the ovarian follicle. We tested the effects of a 2-h prematuration treatment (Pre-IVM) with forskolin (FSK) and 3-isobutyl-1-methylxanthine (IBMX) in bovine cumulus-oocyte complexes (COCs) on the lipid content of oocytes and blastocysts, on the membrane lipid composition of blastocysts and on the transcriptional profiling of cumulus cells and blastocysts in a high-throughput platform. Embryonic development rates to the morula (mean 56.1%) or blastocyst (mean 26.3%) stages were unaffected by treatment. Lipid content was not affected after Pre-IVM, but was increased after IVM in treated oocytes. Conversely, the lipid content was reduced in Pre-IVM blastocysts. Pre-IVM COCs generated blastocysts containing blastomeres with more unsaturated lipids in their membranes. Pre-IVM also altered the relative abundance of 31 gene transcripts after 2h and 16 transcripts after 24h in cumulus cells, while seven transcripts were altered in blastocysts. Our results suggest that the Pre-IVM treatment affected the lipid composition and transcriptional profiles of COCs and blastocysts. Therefore, Pre-IVM with FSK and IBMX could be used either to prevent spontaneous meiotic resumption during IVM or to modulate lipid composition in the membrane and cytoplasm of blastocysts, potentially improving bovine embryos.

Ovine oocytes display a similar germinal vesicle configuration and global DNA methylation at prepubertal and adult ages

Epigenetic mechanisms are thought to be involved in the reduced developmental capacity of early prepubertal ewe oocytes compared to their adult counterparts. In this study, we have analyzed the global DNA methylation pattern and in vitro meiotic and developmental competence of oocytes at the germinal vesicle (GV) stage obtained from adult and 3-month-old donors. All oocytes were aspirated from antral follicles with a diameter ≥3 mm, and DNA methylation on 5-methylcytosine was detected by immunofluorescence using an anti-methyl cytosine antibody. The main global chromatin configuration pattern shown by both prepubertal and adult ovine oocytes corresponded to condensed chromatin localized close to the nuclear envelope (the SNE pattern). Immunofluorescence showed that a global bright nuclear staining of 5-methylcytosine (5-mC) occurred in all germinal vesicle stage oocytes and matched the propidium iodide staining pattern. The total fluorescence intensity values of lamb GVs were not lower than those observed in adult GVs. The meiotic competence and cleavage rates were similar in adult and prepubertal oocytes, however, the developmental competence of embryos to reach blastocysts was higher for adult oocytes than lamb oocytes (p<0.0001). In conclusion, our results indicate that adult-size oocytes derived from 3 to 4 month old prepubertal ewes show similar GV morphology and DNA methylation staining patterns to those obtained from adult animals, despite exhibiting a lower developmental competence.

microRNA expression profile in porcine oocytes with different developmental competence derived from large or small follicles

Oocyte developmental competence is acquired during folliculogenesis and regulated by complex molecular mechanisms. Several molecules are involved in these mechanisms, including microRNAs (miRNAs) that are essential for oocyte-specific processes throughout the development. The objective of this study was to identify the expression profile of miRNAs in porcine oocytes derived from follicles of different sizes using RNA deep sequencing. Oocytes were aspirated from large (LO; 3-6 mm) or small (SO; 1.5-1.9 mm) follicles and tested for developmental competence and chromatin configurations. Small RNA libraries were constructed from both groups and then sequenced in an Illumina NextSeq. 500. Oocytes from the LO group exhibited higher developmental competence and different chromatin configuration compared with oocytes from the SO group. In total, 167 and 162 known miRNAs were detected in the LO and SO groups, respectively. MiR-205, miR-16, miR-148a-3p, and miR-125b were among the top 10 highly expressed miRNAs in both groups. Eight miRNAs were differentially expressed (DE) between both groups. Target gene prediction and pathway analysis revealed 46 pathways that were enriched with miRNA-target genes. The oocyte meiosis pathway and signaling pathways including FoxO, PI3K-Akt, and cAMP were predictably targeted by DE miRNAs. These results give more insights into the potential role of miRNAs in regulating the oocyte development.

ASAS-SSR Triennial Reproduction Symposium: The use of natural cycle's follicular dynamic to improve oocyte quality in dairy cows and heifers

The selection of the best dairy heifers is mainly driven by the genetic value of their parents. The phenotype analysis of cows and of the daughters of bulls has been used to identify the best genetic value for decades before being replaced by genomic selection of individuals that are not yet parents. Because it is possible to predict the future value of an individual by its genetic makeup, it becomes feasible to do it as early as the blastocyst stage and to decide which should be transferred or not. Because we know the genotype of an animal at birth, or even before, it is becoming desirable to reproduce this animal as soon as possible to reduce generation interval and improve selection speed. Nature provides constraints that can be overcome: a single oocyte per cycle and age at puberty. Indeed, it is now possible to super-stimulate the ovary at any age and to start collecting oocytes at 6 mo by trans-vaginal ultrasonography. The challenge becomes the production of good eggs and embryos capable of implanting and developing into healthy calves. Our understanding of ovarian follicular physiology has been instrumental in designing stimulation protocols that may be adjusted to any physiological context including age, and even the individual animal, to obtain a good response. Therefore, the combination of procedures developed in cows to optimize oocyte quality, for example, FSH coasting, in association with in vitro fertilization and optimal culture conditions can now result in the production of several female embryos twice a month from animals 6 to12 mo of age. The transcriptomic and epigenetic analyses of embryos produced from the same females at different ages were compared and few differences were noted in particular in relation to embryo metabolism. These embryos are as good as the ones obtained from adult animals and can be produced with sexed sperm of bulls 12 mo of age. This combination of these technical optimizations with blastocyst genotyping allows the selection of a second generation within a year.

An improved IVM method for cumulus-oocyte complexes from small follicles in polycystic ovary syndrome patients enhances oocyte competence and embryo yield

STUDY QUESTION

Are meiotic and developmental competence of human oocytes from small (2-8 mm) antral follicles improved by applying an optimized IVM method involving a prematuration step in presence of C-Type Natriuretic Peptide (CNP) followed by a maturation step in presence of FSH and Amphiregulin (AREG)?

SUMMARY ANSWER

A strategy involving prematuration culture (PMC) in the presence of CNP followed by IVM using FSH + AREG increases oocyte maturation potential leading to a higher availability of Day 3 embryos and good-quality blastocysts for single embryo transfer.

WHAT IS KNOWN ALREADY

IVM is a minimal-stimulation ART with reduced hormone-related side effects and risks for the patients, but the approach is not widely used because of an efficiency gap compared to conventional ART. In vitro systems that enhance synchronization of nuclear and cytoplasmic maturation before the meiotic trigger are crucial to optimize human IVM systems. However, previous PMC attempts have failed in sustaining cumulus-oocyte connections throughout the culture period, which prohibited a normal cumulus-oocyte communication and precluded an adequate response by the cumulus-oocyte complex (COC) to the meiotic trigger.

STUDY DESIGN, SIZE, DURATION

A first prospective study involved sibling oocytes from a group of 15 patients with polycystic ovary syndrome (PCOS) to evaluate effects of a new IVM culture method on oocyte nuclear maturation and their downstream developmental competence. A second prospective study in an additional series of 15 women with polycystic ovaries characterized and fine-tuned the culture conditions.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Fifteen women with PCOS (according to Rotterdam criteria) underwent IVM treatment after 3-5 days of highly purified human menopausal gonadotropin (HP-hMG) stimulation and no human chorionic gonadotropin (hCG) trigger before oocyte retrieval. A first study was designed with sibling oocytes to prospectively evaluate the impact of an IVM culture method: 24 h PMC with CNP + 30 h IVM with FSH and AREG, on embryo yield, in comparison to the standard (30 h) IVM clinical protocol (Group I, n = 15). A second prospective study was performed in 15 women with polycystic ovaries, to characterize and optimize the PMC conditions (Group II, n = 15). The latter study involved the evaluation of oocyte meiotic arrest, the preservation of cumulus-oocyte transzonal projections (TZPs), the patterns of oocyte chromatin configuration and cumulus cells apoptosis following the 24 and 46 h PMC. Furthermore, oocyte developmental potential following PMC (24 and 46 h) + IVM was also evaluated. The first 20 good-quality blastocysts from PMC followed by IVM were analysed by next generation sequencing to evaluate their aneuploidy rate.

MAIN RESULTS AND THE ROLE OF CHANCE

PMC in presence of CNP followed by IVM using FSH and AREG increased the meiotic maturation rate per COC to 70%, which is significantly higher than routine standard IVM (49%; P ≤ 0.001). Hence, with the new system the proportion of COCs yielding transferable Day 3 embryos and good-quality blastocysts increased compared to routine standard IVM (from 23 to 43%; P ≤ 0.001 and from 8 to 18%; P ≤ 0.01, respectively). CNP was able to prevent meiosis resumption for up to 46 h. After PMC, COCs had preserved cumulus-oocyte TZPs. The blastocysts obtained after PMC + IVM did not show increased aneuploidy rates as compared to blastocysts from conventional ART.

LIMITATIONS REASONS FOR CAUTION

The novel IVM approach in PCOS patients was tested in oocytes derived from small antral follicles which have an intrinsically low developmental potential. Validation of the system would be required for COCs from different (larger) follicular sizes, which may involve further adjustment of PMC conditions. Furthermore, considering that this is a novel strategy in human IVM treatment, its global efficiency needs to be confirmed in large prospective randomized controlled trials. The further application in infertile patients without PCOS, e.g. cancer patients, remains to be evaluated.

WIDER IMPLICATIONS OF THE FINDINGS

The findings of this pilot study suggest that the efficiency gap between IVM and conventional IVF can be reduced by fine-tuning of the culture methods. This novel strategy opens new perspectives for safe and patient-friendly ART in patients with PCOS.

STUDY FUNDING/COMPETING INTEREST(S)

IVM research at the Vrije Universiteit Brussel has been supported by grants from: the Institute for the Promotion of Innovation by Science and Technology in Flanders (Agentschap voor Innovatie door Wetenschap en Technologie-IWT, project 110680); the Fund for Research Flanders (Fonds Wetenschappelijk Onderzoek-Vlaanderen-FWO, project G.0343.13), the Belgian Foundation Against Cancer (HOPE project, Dossier C69). The authors have no conflicts of interest.

Inhibition of PDE3A sustains meiotic arrest and gap junction of bovine growing oocytes in in vitro growth culture

Bovine growing oocytes with a diameter of 105-115 μm from early antral follicles (1.2-1.8 mm) are able to resume meiosis, but lack the competence to mature to metaphase II. To confer full maturation competence onto the oocytes, culture systems which can support their growth and prevent their meiotic resumption during culture are needed. In this study, we cultured growing oocytes for 5 days to examine the effects of different phosphodiesterase (PDE) inhibitors on meiotic arrest and acquisition of full maturation competence of growing oocytes, and their gap junctional communication with cumulus cells. Growing oocyte-cumulus complexes (OCCs) were cultured with 3-isobutyl-1-methylxanthine (IBMX; broad-spectrum PDE inhibitor), rolipram (PDE4D inhibitor), cilostamide and milrinone (PDE3A inhibitors). The mean diameters of oocytes increased similarly in all groups. IBMX, cilostamide and milrinone induced antrum formation by OCCs and maintained meiotic arrest of oocytes during culture, whereas rolipram neither promoted antrum formation nor maintained oocyte meiotic arrest. Gap junctional communication between oocytes and cumulus cells was maintained by IBMX and cilostamide, but not by rolipram as judged by the transfer of injected lucifer yellow dye from oocytes to cumulus cells. In subsequent in vitro maturation, oocytes grown with IBMX, cilostamide and milrinone showed full maturation competence. These results suggest that PDE3A inhibition maintains the meiotic arrest of bovine growing oocytes and sustains their gap junctional communication with cumulus cells for 5 days, thereby contributing to their acquisition of full maturation competence.

Karyosphere (Karyosome): A Peculiar Structure of the Oocyte Nucleus

The karyosphere, aka the karyosome, is a meiosis-specific structure that represents a "knot" of condensed chromosomes joined together in a limited volume of the oocyte nucleus. The karyosphere is an evolutionarily conserved but morphologically rather "multifaceted" structure. It forms at the diplotene stage of meiotic prophase in many animals, from hydra and Drosophila to human. Karyosphere formation is generally linked with transcriptional silencing of the genome. It is believed that karyosphere/karyosome is a prerequisite for proper completion of meiotic divisions and further development. Here, a brief review on the karyosphere features in some invertebrates and vertebrates is provided. Special emphasis is made on terminology, since current discrepancies in this field may lead to confusions. In particular, it is proposed to distinguish the karyosphere with a capsule and the karyosome (a karyosphere devoid of a capsule). The "inverted" karyospheres are also considered, in which the chromosomes situate externally to an extrachromosomal structure (e.g., in human oocytes).

Differences in cumulus cell gene expression indicate the benefit of a pre-maturation step to improve in-vitro bovine embryo production

STUDY QUESTION

Does the gene expression profile of cumulus cells (CC) accompanying oocytes with different degrees of chromatin compaction within the germinal vesicle (GV) reflect the oocyte's quality and response in culture during in-vitro embryo production (IVP).

SUMMARY ANSWER

The transcriptomic profile of the CC is related to oocyte competence, setting the stage for the development of customized pre-maturation strategies to improve IVP.

WHAT IS KNOWN ALREADY

Oocytes complete the acquisition of their competence during antral follicle development. During this period, the chromatin configuration within the GV changes dynamically and is indicative of oocyte's developmental potential. The interactions between somatic and germ cells modulate chromatin morphology and function and are critical for acquisition of oocyte competence.

STUDY DESIGN, SIZE, DURATION

Bovine cumulus-oocyte complexes (COC) were isolated from 0.5 to 6 mm antral follicles. Surrounding CC were separated from the oocyte and classified as GV0, GV1, GV2 and GV3 according to the degree of the oocyte's chromatin compaction.

PARTICIPANTS/MATERIALS, SETTING, METHOD

RNA extracted from CC of each group was amplified and hybridized on a bovine embryo-specific 44 K Agilent slide. The CC_GV1, CC_GV2 and CC_GV3 classes were each hybridized against the CC_GV0 class, representing an early oocyte differentiation stage with poor development competence. The data were normalized and fold changes of the differentially expressed genes were determined. Microarray data were validated using quantitative RT-PCR on selected targets. Microarray data were further analyzed through: (i) between-group analysis (BGA), which classifies the samples according to their transcriptomic profiles; (ii) cluster analysis according to the expression profile of each gene; and (iii) Ingenuity Pathway Analysis (IPA) to study gene regulation patterns and predicted functions. Furthermore, CC of each GV group were cultured and apoptotic cells were assessed after 3 h by caspase analysis. Finally, based on the analysis of CC transcriptomic profiles and the relationship between morphological features of the COC and the oocyte chromatin configuration, a customized, stage-dependent oocyte pre-maturation (pre-IVM) system was used to improve oocyte developmental potential before IVM. For this, the blastocyst rate and quality were assessed after in-vitro maturation and fertilization of pre-matured oocytes.

MAIN RESULTS AND THE ROLE OF CHANCE

Overall, quantitative RT-PCR results of a subset of five selected genes were consistent with the microarray data. Clustering analysis generated 16 clusters representing the main profiles of transcription modulation. Of the 5571 significantly differentially expressed probes, the majority (25.49%) best fitted with cluster #6 (downregulation between CC_GV0 and CC_GV1 and stable low levels in successive groups). IPA identified the most relevant functions associated with each cluster. Genes included in cluster #1 were mostly related to biological processes such as 'cell cycle' and 'cell death and survival', whereas genes included in cluster #5 were mostly related to 'gene expression'. Interestingly, 'lipid metabolism' was the most significant function identified in clusters #6, #9 and #12. IPA of gene lists obtained from each contrast (i.e., CC_GV0 vs. CC_GV1; CC_GV0 vs. CC_GV2; CC_GV0 vs. CC_GV3) revealed that the main affected function in each contrast was 'cell death and survival'. Importantly, apoptosis was predicted to be inhibited in CC_GV1 and CC_GV2, but activated in CC_GV3. Caspase analysis indicated that a low percentage of CC_GV0 was prone to undergo apoptosis but apoptosis increased significantly in CC from oocytes with condensed chromatin, reaching a peak in CC_GV3 (P < 0.05). Finally, the tailored oocyte pre-maturation strategy, based on morphological features of the COC and the oocyte chromatin configuration, demonstrated that pre-IVM improved the developmental capability of oocytes at early stages of differentiation (GV1-enriched COC) but was detrimental for oocytes at more advanced stages of development (GV2 and GV3-enriched COC).

LARGE SCALE DATA

The data are available through the GEO series accession number GSE79886.

LIMITATIONS, REASONS FOR CAUTION

This study was conducted with bovine samples. Whether or not the results are applicable to human oocytes requests further elucidation. Embryo transfer experiments are required to determine whether the improvement in blastocyst rates in the tailored system leads to increased live birth rates.

WIDER IMPLICATIONS OF THE FINDINGS

The identification of multiple non-invasive biomarkers predictive of oocyte quality can greatly strengthen the pre-IVM approach aimed to improve IVM outcomes. These results have potentially important implications in treating human infertility and in developing breeding schemes for domestic mammals.

STUDY FUNDING/COMPETING INTERESTS

This work was supported in part by NSERC Strategic Network EmbryoGENE, Canada and in part by CIG-Marie Curie Actions-Reintegration Grants within the EU 7FP (n. 303640, 'Pro-Ovum'). The authors declare no potential conflict of interest.

Epigenetics: A key paradigm in reproductive health

It is well established that there is a heritable element of susceptibility to chronic human ailments, yet there is compelling evidence that some components of such heritability are transmitted through non-genetic factors. Due to the complexity of reproductive processes, identifying the inheritance patterns of these factors is not easy. But little doubt exists that besides the genomic backbone, a range of epigenetic cues affect our genetic programme. The inter-generational transmission of epigenetic marks is believed to operate via four principal means that dramatically differ in their information content: DNA methylation, histone modifications, microRNAs and nucleosome positioning. These epigenetic signatures influence the cellular machinery through positive and negative feedback mechanisms either alone or interactively. Understanding how these mechanisms work to activate or deactivate parts of our genetic programme not only on a day-to-day basis but also over generations is an important area of reproductive health research.

High-resolution microscopy of active ribosomal genes and key members of the rRNA processing machinery inside nucleolus-like bodies of fully-grown mouse oocytes

Nucleolus-like bodies (NLBs) of fully-grown (germinal vesicle, GV) mammalian oocytes are traditionally considered as morphologically distinct entities, which, unlike normal nucleoli, contain transcribed ribosomal genes (rDNA) solely at their surface. In the current study, we for the first time showed that active ribosomal genes are present not only on the surface but also inside NLBs of the NSN-type oocytes. The "internal" rRNA synthesis was evidenced by cytoplasmic microinjections of BrUTP as precursor and by fluorescence in situ hybridization with a probe to the short-lived 5'ETS segment of the 47S pre-rRNA. We further showed that in the NLB mass of NSN-oocytes, distribution of active rDNA, RNA polymerase I (UBF) and rRNA processing (fibrillarin) protein factors, U3 snoRNA, pre-rRNAs and 18S/28S rRNAs is remarkably similar to that in somatic nucleoli capable to make pre-ribosomes. Overall, these observations support the occurrence of rDNA transcription, rRNA processing and pre-ribosome assembly in the NSN-type NLBs and so that their functional similarity to normal nucleoli. Unlike the NSN-type NLBs, the NLBs of more mature SN-oocytes do not contain transcribed rRNA genes, U3 snoRNA, pre-rRNAs, 18S and 28S rRNAs. These results favor the idea that in a process of transformation of NSN-oocytes to SN-oocytes, NLBs cease to produce pre-ribosomes and, moreover, lose their rRNAs. We also concluded that a denaturing fixative 70% ethanol used in the study to fix oocytes could be more appropriate for light microscopy analysis of nucleolar RNAs and proteins in mammalian fully-grown oocytes than a commonly used cross-linking aldehyde fixative, formalin.