Differential contributions of connexin37 and connexin43 to oogenesis revealed in chimeric reaggregated mouse ovaries

SMAD4 promotes somatic-germline contact during murine oocyte growth

Development of the mammalian oocyte requires physical contact with the surrounding granulosa cells of the follicle, which provide it with essential nutrients and regulatory signals. This contact is achieved through specialized filopodia, termed transzonal projections (TZPs), that extend from the granulosa cells to the oocyte surface. Transforming growth factor (TGFβ) family ligands produced by the oocyte increase the number of TZPs, but how they do so is unknown. Using an inducible Cre recombinase strategy together with expression of green fluorescent protein to verify Cre activity in individual cells, we examined the effect of depleting the canonical TGFβ mediator, SMAD4, in mouse granulosa cells. We observed a 20-50% decrease in the total number of TZPs in SMAD4-depleted granulosa cell-oocyte complexes, and a 50% decrease in the number of newly generated TZPs when the granulosa cells were reaggregated with wild-type oocytes. Three-dimensional image analysis revealed that TZPs of SMAD4-depleted cells were longer than controls and more frequently oriented towards the oocyte. Strikingly, the transmembrane proteins, N-cadherin and Notch2, were reduced by 50% in SMAD4-depleted cells. SMAD4 may thus modulate a network of cell adhesion proteins that stabilize the attachment of TZPs to the oocyte, thereby amplifying signalling between the two cell types.

FOXL2 and NR5A1 induce human fibroblasts into steroidogenic ovarian granulosa-like cells

Human granulosa cells in different stages are essential for maintaining normal ovarian function, and granulosa cell defect is the main cause of ovarian dysfunction. To address this problem, it is necessary to induce functional granulosa cells at different stages in vitro. In this study, we established a reprogramming method to induce early- and late-stage granulosa cells with different steroidogenic abilities. We used an AMH-fluorescence-reporter system to screen candidate factors for cellular reprogramming and generated human induced granulosa-like cells (hiGC) by overexpressing FOXL2 and NR5A1. AMH-EGFP+ hiGC resembled human cumulus cells in transcriptome profiling and secreted high levels of oestrogen and progesterone, similar to late-stage granulosa cells at antral or preovulatory stage. Moreover, we identified CD55 as a cell surface marker that can be used to isolate early-stage granulosa cells. CD55+ AMH-EGFP- hiGC secreted high levels of oestrogen but low levels of progesterone, and their transcriptome profiles were more similar to early-stage granulosa cells. More importantly, CD55+ hiGC transplantation alleviated polycystic ovary syndrome (PCOS) in a mouse model. Therefore, hiGC provides a cellular model to study the developmental program of human granulosa cells and has potential to treat PCOS.

Effect of Cyclic Adenosine Monophosphate on Connexin 37 Expression in Sheep Cumulus-Oocyte Complexes

Gap junctional connection (GJC) in the cumulus-oocyte complex (COC) provides necessary support for message communication and nutrient transmission required for mammalian oocyte maturation. Cyclic adenosine monophosphate (cAMP) is not only a prerequisite for regulating oocyte meiosis, but also the key intercellular factor for affecting GJC function in COCs. However, there are no reports on whether cAMP regulates connexin 37 (Cx37) expression, one of the main connexin proteins, in sheep COCs. In this study, the expression of Cx37 protein and gene in immature sheep COC was detected using immunohistochemistry and PCR. Subsequently, the effect of cAMP on Cx37 expression in sheep COCs cultured in a gonadotropin-free culture system for 10 min or 60 min was evaluated using competitive ELISA, real-time fluorescent quantitative PCR (RT-qPCR), and Western blot. The results showed that the Cx37 protein was present in sheep oocytes and cumulus cells; the same results were found with respect to GJA4 gene expression. In the gonadotropin-free culture system, compared to the control, significantly higher levels of cAMP as well as Cx37 gene and protein expression were found in sheep COCs following treatment in vitro with Forskolin and IBMX (100 μM and 500 μM)) for 10 min (p < 0.05). Compared to the controls (at 10 or 60 min), cAMP levels in sheep COCs were significantly elevated as a result of Forskolin and IBMX treatment (p < 0.05). Following culturing in vitro for 10 min or 60 min, Forskolin and IBMX treatment can significantly promote Cx37 expression in sheep COCs (p < 0.05), a phenomenon which can be counteracted when the culture media is supplemented with RP-cAMP, a cAMP-specific competitive inhibitor operating through suppression of the protein kinase A (PKA). In summary, this study reports the preliminary regulatory mechanism of cAMP involved in Cx37 expression for the first time, and provides a novel explanation for the interaction between cAMP and GJC communication during sheep COC culturing in vitro.

Molecular Mechanisms Determining Mammalian Oocyte Quality with the Treatment of Cancer Therapy

Cancer is a global public health issue and remains one of the leading causes of death in the United States (Siegel et al. CA Cancer J Clin. 72:7-33, 2022). It is estimated in the US in 2022, about 935,000 new cases of cancer will be diagnosed in women, and the probability of developing invasive cancer is 5.8% for females younger than 50 years old (Siegel et al. CA Cancer J Clin. 72:7-33, 2022). However, advances in screening programs, diagnostic methods, and therapeutic options have greatly increased the five-year survival rate in reproductive-age women with a variety of cancers. Given the clinical consequences of gonadotoxic cancer therapies, young, female cancer survivors may face compromised fertility, premature ovarian insufficiency, early-onset menopause, and endocrine dysregulation (Bedoschi et al. Future Oncol. 12:2333-44, 2016). Gonadotoxic side effects may include decreased oocyte quality within surviving follicles, loss of ovarian follicles, and impaired ovarian function. In reproductive-age women, oocyte quality is an important element for successful clinical pregnancies and healthy offspring as poor-quality oocytes may be a cause of infertility (McClam et al. Biol Reprod. 106:328-37, 2022; Marteil et al. Reprod Biol. 9:203-24, 2009; Krisher. J Anim Sci. 82: E14-E23, 2004). Thus, it is critical to determine the quantity and quality of surviving follicles in the ovary after cancer treatment and to assess oocyte quality within those surviving follicles as these are markers for determining the capacity for ovarian function restoration and future fertility, especially for young cancer survivors (Xu et al. Nat Med. 17:1562-3, 2011). The long-term effects of cancer therapeutics on oocyte quality are influenced by factors including, but not limited to, individual patient characteristics (e.g. age, health history, comorbidities, etc.), disease type, or treatment regimen (Marci et al. Reprod Biol Endocrinol. 16:1-112, 2018). These effects may translate clinically into an impaired production of viable oocytes and compromised fertility (Garutti et al. ESMO Open. 6:100276, 2021).

Oocyte quality is enhanced by hypoglycosylated FSH through increased cell-to-cell interaction during mouse follicle development

Macroheterogeneity in follicle-stimulating hormone (FSH) β-subunit N-glycosylation results in distinct FSH glycoforms. Hypoglycosylated FSH21 is the abundant and more bioactive form in pituitaries of females under 35 years of age, whereas fully glycosylated FSH24 is less bioactive and increases with age. To investigate whether the shift in FSH glycoform abundance contributes to the age-dependent decline in oocyte quality, the direct effects of FSH glycoforms on folliculogenesis and oocyte quality were determined using an encapsulated in vitro mouse follicle growth system. Long-term culture (10-12 days) with FSH21 (10 ng/ml) enhanced follicle growth, estradiol secretion and oocyte quality compared with FSH24 (10 ng/ml) treatment. FSH21 enhanced establishment of transzonal projections, gap junctions and cell-to-cell communication within 24 h in culture. Transient inhibition of FSH21-mediated bidirectional communication abrogated the positive effects of FSH21 on follicle growth, estradiol secretion and oocyte quality. Our data indicate that FSH21 promotes folliculogenesis and oocyte quality in vitro by increasing cell-to-cell communication early in folliculogenesis, and that the shift in in vivo abundance from FSH21 to FSH24 with reproductive aging may contribute to the age-dependent decline in oocyte quality.

Making a good egg: human oocyte health, aging, and in vitro development

Mammalian eggs (oocytes) are formed during fetal life and establish associations with somatic cells to form primordial follicles that create a store of germ cells (the primordial pool). The size of this pool is influenced by key events during the formation of germ cells and by factors that influence the subsequent activation of follicle growth. These regulatory pathways must ensure that the reserve of oocytes within primordial follicles in humans lasts for up to 50 years, yet only approximately 0.1% will ever be ovulated with the rest undergoing degeneration. This review outlines the mechanisms and regulatory pathways that govern the processes of oocyte and follicle formation and later growth, within the ovarian stroma, through to ovulation with particular reference to human oocytes/follicles. In addition, the effects of aging on female reproductive capacity through changes in oocyte number and quality are emphasized, with both the cellular mechanisms and clinical implications discussed. Finally, the details of current developments in culture systems that support all stages of follicle growth to generate mature oocytes in vitro and emerging prospects for making new oocytes from stem cells are outlined.

Effect of progesterone on in vitro meiotic maturation of canine oocytes associated with Cx37 and Cx43 gene expression

Progesterone (P4) concentrations in canines are exceptionally high in the periovulatory period. However, the mechanisms by which P4 modulates final oocyte development in dogs remain to be characterized. The aim of this study was to evaluate the effect of P4 on meiotic development related to the gene expression of connexin 37 (Cx37) and connexin 43 (Cx43) in the canine cumlus oocyte complexes (COCs). COCs were isolated from 120 canine ovaries after a routine ovariohysterectomy. In each experiment, groups of COCs retrieved from the antral follicles were subjected to in vitro maturation (IVM) for 72 h without (control) or with P4 (50 μg/mL and 100 μg/mL) or the P4 receptor antagonist, aglepristone (RU534 at 1 μM and 10 μM). Some of the COCs recovered (from each group) after 72 h of IVM were subjected to meiotic evaluation; the remaining COCs, and those not subjected to IVM, were used to analyze the gene expression of Cx37 and Cx43 by qPCR. The results were evaluated using ANOVA. The addition of P4 increased (P < 0.05) the meiotic development compared to that in the control or aglepristone groups. The highest (P < 0.05) percentage of oocytes in the MII stage was observed upon P4 supplementation. In contrast, the highest percentage (P < 0.05) of oocytes arrested in the GV stage and the lowest (P < 0.05) percentages in the MII stage were observed for COCs cultured with aglepristone. Although a significant decrease in the mRNA levels of both connexins was observed after culturing, no effect on Cx37 and Cx43 gene expression was observed when exogenous P4 was added compared to those of the control group. However, COCs cultured with aglepristone exhibited higher (P < 0.05) expression of Cx37 and Cx43 than COCs in the control IVM-group, regardless of the concentration. In conclusion, our results suggest that a high dosage of P4 during IVM enhances the nuclear maturation of canine oocytes without altering the gene expression levels of Cx37 and Cx43. However, the increase in their expression upon treatment with a P4 antagonist indicates an in vivo role for this hormone in the endogenous modulation of both Cx37 and Cx43.

Supplementation of EGF, IGF-1, and Connexin 37 in IVM Medium Significantly Improved the Maturation of Bovine Oocytes and Vitrification of Their IVF Blastocysts

The quality and developmental capacity of oocytes derived from in vitro maturation (IVM) remain unsatisfactory, which greatly impairs the efficiency and application of embryo technologies. The present experiment was designed to investigate the effect of the supplementation of EGF, IGF-1, and Cx37 in an IVM medium on the maturation quality and development ability of bovine oocytes. The cytoplasmic maturation events of oocytes and the quality of in vitro fertilization (IVF) blastocysts were examined to investigate the relative mechanisms. Our results showed that the nuclear maturation and blastocyst development after the IVF of oocytes treated with 25 μg/mL Cx37 or the combination of 50 ng/mL EGF and 100 ng/mL IGF-1 were significantly increased compared to those of the control group (p < 0.05). Furthermore, the blastocyst rate, and blastocyst total cell number and survival rate after vitrification of the EGF+IGF-1+Cx37 group, were significantly higher than those of the control group (p < 0.05), but lower than those of the FSH+LH+EGF+IGF-1+Cx37 group (p < 0.05). The transzonal projection (TZP) intensity, glutathione (GSH) level, and mitochondrial function of the EGF+IGF-1+Cx37 group were significantly higher than that of the control group, and lower than those of the FSH+LH+EGF+IGF-1+Cx37 group, in contrast to the results of the reactive oxygen species (ROS) levels. In conclusion, our results showed that the supplementation of 50 ng/mL EGF, 100 ng/mL IGF-1, and 25 μg/mL Cx37 in the IVM of bovine oocytes significantly improved their quality and developmental ability by increasing the TZP, mitochondrial function, and GSH level.

Tanshinone IIA enhances the ovarian reserve and attenuates ovarian oxidative stress in aged mice

Background

Tanshinone IIA (TSA), a major lipophilic component extracted from the roots of Salvia miltiorrhiza Bunge, has been widely used in China for its various biological activities. However, its effect on ovarian reserve in aged mice was not studied elsewhere.

Objectives

This study aimed to explore the effect of TSA on the ovarian reserve of aged mice as well as young mice. Forty weeks old mice (N = 40) were considered as aged group compared to 4 weeks old mice (N = 40), and these groups were subdivided into four subgroups (N = 10) to receive different doses of TSA (0, 10, 20, and 40 μg/g/day).

Methods

The effect of TSA was evaluated by counting follicular number by histological examination. Basal serum levels of FSH, LH, E2, and anti‐Mullerian hormone (AMH) were measured by ELISA. Moreover, the expression levels of antioxidant genes (CAT, Nrf2, GPX1), gap junction (Cx37), ERK1/2, and Smad5 family gene were examined at both mRNA (qPCR) and protein levels (western blot).

Results

Follicular number, level of AMH and E2, and the expression of CAT, Nrf2, and GPX1 genes increased significantly (p < 0.05) in aged mice administrated with medium (20 μg/g/day) and high (40 μg/g/day) doses of TSA, whereas FSH and LH levels were significantly low compared to low dose (10 μg/g/day) and control (0 μg/g/day) aged subgroups. However, we did not observe any effect of all doses of TSA on young mice.

Conclusions

Administration of TSA with medium and high doses up‐regulates the expression of antioxidative genes, reduces the oxidative injury, increases levels of AMH, and E2 levels that are relatively comparable to those in young mice, and consequently results in a healthy oocyte development.

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

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

Morphology and Biochemistry of Ovulation

The process of ovulation involves multiple and iterrelated genetic, biochemical, and morphological events: cessation of the proliferation of granulosa cells, resumption of oocyte meiosis, expansion of cumulus cell-oocyte complexes, digestion of the follicle wall, and extrusion of the metaphase-II oocyte. The present narrative review examines these interrelated steps in detail. The combined or isolated roles of the follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are highlighted. Genes indiced by the FSH genes are relevant in the cumulus expansion, and LH-induced genes are critical for the resumption of meiosis and digestion of the follicle wall. A non-human model for follicle-wall digestion and oocyte release was provided.

Pannexins and Connexins: Their Relevance for Oocyte Developmental Competence

The oocyte is the major determinant of embryo developmental competence in all mammalian species. Although fundamental advances have been generated in the field of reproductive medicine and assisted reproductive technologies in the past three decades, researchers and clinicians are still trying to elucidate molecular factors and pathways, which could be pivotal for the oocyte’s developmental competence. The cell-to-cell and cell-to-matrix communications are crucial not only for oocytes but also for multicellular organisms in general. This latter mentioned communication is among others possibly due to the Connexin and Pannexin families of large-pore forming channels. Pannexins belong to a protein group of ATP-release channels, therefore of high importance for the oocyte due to its requirements of high energy supply. An increasing body of studies on Pannexins provided evidence that these channels not only play a role during physiological processes of an oocyte but also during pathological circumstances which could lead to the development of diseases or infertility. Connexins are proteins that form membrane channels and gap-junctions, and more precisely, these proteins enable the exchange of some ions and molecules, and therefore they do play a fundamental role in the communication between the oocyte and accompanying cells. Herein, the role of Pannexins and Connexins for the processes of oogenesis, folliculogenesis, oocyte maturation and fertilization will be discussed and, at the end of this review, Pannexin and Connexin related pathologies and their impact on the developmental competence of oocytes will be provided.

Reversing complete mechanical transzonal projections disruption during mouse in vitro follicle culture with unaltered oocyte competence†

In vitro oocyte growth is widely studied as an alternative fertility preservation approach. Several animal models are used to generate extensive information on this complex process regulated by the constant and dynamic interaction between the oocyte and its somatic compartment throughout follicle growth and maturation. A two-dimensional attachment mouse secondary follicle culture system was used to assess the oocyte's capacity to overcome disconnection from its somatic companions at different developmental stages for final competence acquisition. To test this, complete mechanical denudation of oocytes from preantral (PA) and early antral (EA) follicles was performed. Established endpoints were the oocyte's potential to reconnect with somatic cells and the impact of connectivity disruption on mature oocyte quality. This study proves that oocytes from PA and EA cultured mouse follicles can overcome complete denudation, restoring likely functional transzonal projections with no significant differences in meiotic and developmental competence compared with those from intact cultured follicles. These novel findings constitute good premises for developing successful strategies to rescue human oocyte competence in the context of in vitro culture approaches such as nonhuman chorionic gonadotropin triggered in vitro maturation.

John EJ, Favetta LA. BPA and BPS Affect Connexin 37 in Bovine Cumulus Cells

Bisphenol S (BPS) is used as an alternative plasticizer to Bisphenol A (BPA), despite limited knowledge of potential adverse effects. BPA exhibits endocrine disrupting effects during development. This article focuses on the impact of bisphenols during oocyte maturation. Connexins (Cx) are gap junctional proteins that may be affected by bisphenols, providing insight into their mechanism during development. Cxs 37 and 43 are crucial in facilitating cell communication between cumulus cells and oocytes. Cumulus-oocyte complexes (COCs), denuded oocytes, and cumulus cells were exposed to 0.05 mg/mL BPA or BPS for 24 h. Both compounds had no effect on Cx43. Cumulus cells exhibited a significant increase in Cx37 expression following BPA (p = 0.001) and BPS (p = 0.017) exposure. COCs treated with BPA had increased Cx37 protein expression, whilst BPS showed no effects, suggesting BPA and BPS act through different mechanisms. Experiments conducted in in vitro cultured cumulus cells, obtained by stripping germinal vesicle oocytes, showed significantly increased expression of Cx37 in BPA, but not the BPS, treated group. BPA significantly increased Cx37 protein expression, while BPS did not. Disrupted Cx37 following BPA exposure provides an indication of possible effects of bisphenols on connexins during the early stages of development.

Aquaporins and (in)fertility: More than just water transport

Aquaporins (AQPs) are a family of channel proteins that facilitate the transport of water and small solutes across biological membranes. They are widely distributed throughout the organism, having a number of key functions, some of them unexpected, both in health and disease. Among the various diseases in which AQPs are involved, infertility has been overlooked. According to the World Health Organization (WHO) infertility is a global public health problem with one third of the couples suffering from subfertility or even infertility due to male or female factors alone or combined. Thus, there is an urgent need to unveil the molecular mechanisms that control gametes production, maturation and fertilization-related events, to more specifically determine infertility causes. In addition, as more couples seek for fertility treatment through assisted reproductive technologies (ART), it is pivotal to understand how these techniques can be improved. AQPs are heterogeneously expressed throughout the male and female reproductive tracts, highlighting a possible regulatory role for these proteins in conception. In fact, their function, far beyond water transport, highlights potential intervention points to enhance ART. In this review we discuss AQPs distribution and structural organization, functions, and modulation throughout the male and female reproductive tracts and their relevance to the reproductive success. We also highlight the most recent advances and research trends regarding how the different AQPs are involved and regulated in specific mechanisms underlying (in)fertility. Finally, we discuss the involvement of AQPs in ART-related processes and how their handling can lead to improvement of infertility treatment.

The Relevance of Aquaporins for the Physiology, Pathology, and Aging of the Female Reproductive System in Mammals

Aquaporins constitute a group of water channel proteins located in numerous cell types. These are pore-forming transmembrane proteins, which mediate the specific passage of water molecules through membranes. It is well-known that water homeostasis plays a crucial role in different reproductive processes, e.g., oocyte transport, hormonal secretion, completion of successful fertilization, blastocyst formation, pregnancy, and birth. Further, aquaporins are involved in the process of spermatogenesis, and they have been reported to be involved during the storage of spermatozoa. It is noteworthy that aquaporins are relevant for the physiological function of specific parts in the female reproductive system, which will be presented in detail in the first section of this review. Moreover, they are relevant in different pathologies in the female reproductive system. The contribution of aquaporins in selected reproductive disorders and aging will be summarized in the second section of this review, followed by a section dedicated to aquaporin-related proteins. Since the relevance of aquaporins for the male reproductive system has been reviewed several times in the recent past, this review aims to provide an update on the distribution and impact of aquaporins only in the female reproductive system. Therefore, this paper seeks to determine the physiological and patho-physiological relevance of aquaporins on female reproduction, and female reproductive aging.

Androgen effect on connexin expression in the mammalian female reproductive system: A systematic review

The functions of androgen and connexin in the mammalian female reproductive system are suggested to be related. Previous research has shown that androgen affects connexin expression in the female reproductive system, altering its function. However, no definitive conclusion on their cause-effect relationship has been drawn yet. In addition, a high prevalence of women with polycystic ovary syndrome (PCOS), who are characterized by elevated androgen levels and failure of ovulation, has prompted the studies on the relationship between androgen and connexin in the ovaries. This systematic review aims to investigate the effect of androgen on connexin expression in the mammalian female reproductive system. The literature search was conducted using the MEDLINE via EBSCOhost and the Scopus database and the following keywords: “androgen” or “testosterone” or “androgen blocker” or “anti-androgen” or “androstenedione” or “dehydroepiandrosterone” or “flutamide AND connexin” or “gap junction” or “cell junction”. We only considered in vitro and in vivo studies that involved treatment by androgen or androgen receptor blockers and measured connexin expression as one of the parameters. Our review showed that the exposure to androgen or androgen blocker affects connexin expression but not its localization in the mammalian ovary. However, it is not clear whether androgen downregulates or upregulates connexin expression.

DPAGT1-Mediated Protein N-Glycosylation Is Indispensable for Oocyte and Follicle Development in Mice

Post-translational modification of proteins by N-linked glycosylation is crucial for many life processes. However, the exact contribution of N-glycosylation to mammalian female reproduction remains largely undefined. Here, DPAGT1, the enzyme that catalyzes the first step of protein N-glycosylation, is identified to be indispensable for oocyte development in mice. Dpagt1 missense mutation (c. 497A>G; p. Asp166Gly) causes female subfertility without grossly affecting other functions. Mutant females ovulate fewer eggs owing to defective development of growing follicles. Mutant oocytes have a thin and fragile zona pellucida (ZP) due to the reduction in glycosylation of ZP proteins, and display poor developmental competence after fertilization in vitro. Moreover, completion of the first meiosis is accelerated in mutant oocytes, which is coincident with the elevation of aneuploidy. Mechanistically, transcriptomic analysis reveals the downregulation of a number of transcripts essential for oocyte meiotic progression and preimplantation development (e.g., Pttgt1, Esco2, Orc6, and Npm2) in mutant oocytes, which could account for the defects observed. Furthermore, conditional knockout of Dpagt1 in oocytes recapitulates the phenotypes observed in Dpagt1 mutant females, and causes complete infertility. Taken together, these data indicate that protein N-glycosylation in oocytes is essential for female fertility in mammals by specific control of oocyte development.

Translational activation of maternally derived mRNAs in oocytes and early embryos and the role of embryonic poly(A) binding protein (EPAB)

Transcription ceases upon stimulation of oocyte maturation and gene expression during oocyte maturation, fertilization, and early cleavage relies on translational activation of maternally derived mRNAs. Two key mechanisms that mediate translation of mRNAs in oocytes have been described in detail: cytoplasmic polyadenylation-dependent and -independent. Both of these mechanisms utilize specific protein complexes that interact with cis-acting sequences located on 3'-untranslated region (3'-UTR), and both involve embryonic poly(A) binding protein (EPAB), the predominant poly(A) binding protein during early development. While mechanistic details of these pathways have primarily been elucidated using the Xenopus model, their roles are conserved in mammals and targeted disruption of key regulators in mouse results in female infertility. Here, we provide a detailed account of the molecular mechanisms involved in translational activation during oocyte and early embryo development, and the role of EPAB in this process.

Changes in Aquaporin 1, 5 and 9 Gene Expression in the Porcine Oviduct According to Estrous Cycle and Early Pregnancy

Aquaporins (AQPs) are a group of small, integral membrane proteins which play an important role in fluid homeostasis in the reproductive system. In our previous study, we demonstrated AQP1, 5 and 9 protein expression and localization in the porcine oviduct. The presence of these isoforms could suggest their role in the transport of the ovum to the uterus by influencing the epithelial cells’ production of oviductal fluid. The aim of this study was to evaluate the expression of AQP1, AQP5 and AQP9 in the infundibulum, ampulla and isthmus in the porcine oviduct during the estrous cycle (early luteal phase, days 2–4, medium luteal phase, days 10–12, late luteal phase days 14–16, follicular phase days 18–20) and pregnancy (period before implantation, days 14–16 and after the implantation, days 30–32) using the Real-Time PCR technique. As clearly demonstrated for the first time, AQP1, 5, and 9 gene expression is influenced by the estrus cycle and pregnancy. Furthermore, expression of AQPs in the porcine oviduct may provide the physiological medium that sustains and enhances fertilization and early cleavage-stage embryonic development. Overall, our study provides a characterization of oviduct AQPs, increasing our understanding of fluid homeostasis in the porcine oviduct to successfully establish and maintain pregnancy.

Involvement of GJA1 and Gap Junctional Intercellular Communication between Cumulus Cells and Oocytes from Women with PCOS

Polycystic ovary syndrome (PCOS) is a common female endocrine system disease that affects 17.8% of women of reproductive age and leads to infertility, obesity, glucose metabolic disorders, cardiovascular disease, and body-mind problems. However, the etiology of PCOS remains unclear. Follicular growth is disrupted as a result of ovarian hyperandrogenism and distorted intraovarian paracrine signaling in women with PCOS. Microcommunication between oocytes and cumulus cells plays a critical role in folliculogenesis. Gap junction alpha 1 (GJA1) plays a crucial role in the developing follicles by forming communication channels between cumulus cells and oocytes, but this has not yet been reported in women with PCOS. Therefore, we aimed to study the role of GJA1 in the microcommunication between oocytes and cumulus cells in women with PCOS. In our study, cumulus cell-oocyte complexes (COCs) from women were isolated via ultrasound-guided vaginal puncture, and oocytes were selected from COCs and categorized based on 3 oocyte maturation stages. Then, RT-qPCR and immunofluorescence analysis were performed to detect both the gene expression and protein of GJA1 in oocytes from women with and without PCOS. There was no statistically significant difference in age and BMI (body mass index), but patients with PCOS had a higher ratio of basic LH/FSH (luteinizing hormone/follicle-stimulating hormone), androstenedione, and total ovarian volume. The qRT-PCR results showed higher gene expression of GJA1 in oocytes without PCOS at the germinal vesicle (GV) stage compared with that of oocytes from women with PCOS. Immunofluorescence analysis showed that the expression level of GJA1 in oocytes from women with PCOS was very weak compared with that of oocytes from women without PCOS. In conclusion, GJA1 may play a critical role in the development of oogenesis arrest in women with PCOS throughout the oogenesis processes, including oogenesis and oocyte maturation.

Dynamin 2-dependent endocytosis is essential for mouse oocyte development and fertility

During folliculogenesis, oocytes are dependent on metabolic and molecular support from surrounding somatic cells. Here, we examined the role of the dynamin (DNM) family of mechanoenzymes in mediating endocytotic uptake into growing follicular oocytes. We found DNM1 and DNM2 to be highly expressed in growing follicular oocytes as well as in mature germinal vesicle (GV) and metaphase II (MII) stage oocytes. Moreover, oocyte-specific conditional knockout (cKO) of DNM2 (DNM2Δ) led to complete sterility, with follicles arresting at the preantral stage of development. In addition, DNM2Δ ovaries were characterized by disrupted follicular growth as well as oocyte and follicle apoptosis. Further, the loss of DNM activity, either through DNM2 cKO or through pharmacological inhibition (Dyngo 6a) led to the impairment of endocytotic pathways in preantral oocytes as well as in mature GV and MII oocytes, respectively. Loss of DNM activity resulted in the redistribution of endosomes and the misslocalization of clathrin and actin, suggesting dysfunctional endocytosis. Notably, there was no observable effect on the fertility of DNM1Δ females. Our study has provided new insight into the complex and dynamic nature of oocyte growth during folliculogenesis, suggesting a role for DNM2 in mediating the endocytotic events that are essential for oocyte development.

Role of connexins in female reproductive system and endometriosis

Gap junction form channels between the cells and facilitate the function of cellular cross talk. Connexins, the gap junction proteins play an essential role in female reproductive health and its expression anomalies are correlated with female reproductive disorders like polycystic ovarian syndrome, recurrent miscarriage, pre-term birth and endometriosis. Endometriosis is a chronic gynecologic disorder caused by ectopic endometrial lesions growing outside the uterine cavity. Embryonic implantation is adversely affected in case of endometriosis leading to infertility. Endometriosis also interferes with ovulatory functions, reduces fertilization and impaires blastocyst implantation. There lies a lacunae in understanding of the role of gap junctions protein connexins in endometriosis. Therefore, this study discusses the role of connexins in improving female fertility by taming the processes of oogenesis, germ line development, uterine receptivity, placental growth, implantation, decidualization and concludes by focusing the role of connexins in endometriosis.

Activation of Notch Signaling by Oocytes and Jag1 in Mouse Ovarian Granulosa Cells

The Notch pathway plays diverse and complex roles in cell signaling during development. In the mammalian ovary, Notch is important for the initial formation and growth of follicles, and for regulating the proliferation and differentiation of follicular granulosa cells during the periovulatory period. This study seeks to determine the contribution of female germ cells toward the initial activation and subsequent maintenance of Notch signaling within somatic granulosa cells of the ovary. To address this issue, transgenic Notch reporter (TNR) mice were crossed with Sohlh1-mCherry (S1CF) transgenic mice to visualize Notch-active cells (EGFP) and germ cells (mCherry) simultaneously in the neonatal ovary. To test the involvement of oocytes in activation of Notch signaling in ovarian somatic cells, we ablated germ cells using busulfan, a chemotherapeutic alkylating agent, or investigated KitWv/Wv (viable dominant white-spotting) mice that lack most germ cells. The data reveal that Notch pathway activation in granulosa cells is significantly suppressed when germ cells are reduced. We further demonstrate that disruption of the gene for the Notch ligand Jag1 in oocytes similarly impacts Notch activation and that recombinant JAG1 enhances Notch target gene expression in granulosa cells. These data are consistent with the hypothesis that germ cells provide a ligand, such as Jag1, that is necessary for activation of Notch signaling in the developing ovary.

Ultrastructure and intercellular contact-mediated communication in cultured human early stage follicles exposed to mTORC1 inhibitor

The reproductive lifespan of a woman is determined by the gradual recruitment of quiescent follicles into the growing pool. In humans, ovarian tissue removal from its in vivo environment induces spontaneous activation of resting follicles. Similarly, pharmacological activation of the PI3K/Akt pathway leads to accelerated follicle recruitment, but has been associated with follicular damage. Recent findings demonstrate that everolimus (EVE), an mTORC1 inhibitor, limits primordial follicle activation. However, its potential benefit regarding growing follicle integrity remains unexplored. Ovarian cortical fragments were exposed to ± EVE for 24 h and cultured for an additional 5 days. After 0, 1 and 6 days of culture, fragments were either processed for ultrastructural analysis or subjected to follicular isolation for gene expression and immunofluorescence assessments. Data from transmission electron microscopy showed that growing follicles displayed similar ultrastructural features irrespective of the conditions and maintained close contacts between germinal and stromal compartments. Establishment of intra-follicular communication was confirmed by detection of a gap junction component, Cx43, in both groups throughout culture, whereas transzonal projections, which physically link granulosa cells to oocyte, formed later in EVE-treated follicles. Importantly, levels of GJA1 mRNA, encoding for the Cx43 protein, significantly increased from Day 0 to Day 1 in the EVE group, but not in the control group. Given that EVE-treated follicles were smaller than controls, these findings suggest that EVE might facilitate the establishment of appropriate intercellular communications without impairing follicle ultrastructure. Therefore, mTORC1 inhibitors might represent an attractive tool to delay the culture-induced primordial follicle activation while maintaining follicles in a functionally integrated state.

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.

Control of Mammalian Oocyte Development by Interactions with the Maternal Follicular Environment

Development of animal germ cells depends critically on continuous contact and communication with the somatic compartment of the gonad. In females, each oocyte is enclosed within a follicle, whose somatic cells supply nutrients that sustain basal metabolic activity of the oocyte and send signals that regulate its differentiation. This maternal microenvironment thus plays an indispensable role in ensuring the production of fully differentiated oocytes that can give rise to healthy embryos. The granulosa cells send signals, likely membrane-associated Kit ligand, which trigger oocytes within resting-stage primordial follicles to initiate growth. During growth, the granulosa cells feed amino acids, nucleotides, and glycolytic substrates to the oocyte. These factors are necessary for the oocyte to complete its growth and are delivered via gap junctions that couple the granulosa cells to the oocyte. In a complementary manner, growing oocytes also release growth factors, notably growth-differentiation factor 9 and bone morphogenetic protein 15, which are necessary for proper differentiation of the granulosa cells and for these cells to support oocyte growth. During the late stages of oocyte growth, cyclic GMP that is synthesized by the granulosa cells and diffuses into the oocyte is required to prevent its precocious entry into meiotic maturation. Finally, at the early stages of maturation, granulosa cell signals promote the synthesis of a subset of proteins within the oocyte that enhance their ability to develop as embryos. Thus, the maternal legacy of the follicular microenvironment is witnessed by the fertilization of the ovulated oocyte and subsequent birth of healthy offspring.

In vitro and in vivo mouse follicle development in ovaries and reaggregated ovaries

Follicle development requires complex and coordinated interactions between both the oocyte and its associated somatic cells. In ovarian dysfunction, follicle development may be abnormal due to defective somatic cell function; for example, premature ovarian insufficiency or malignancies. Replacing defective somatic cells, using the reaggregated ovary (RO) technique, may 'rescue' follicle development. ROs containing mature follicles have been generated when transplanted to a host mouse to develop. We have developed a RO culture technique and the aims were to determine how follicle development differed between transplanted and cultured ROs, and the influence of ovarian age (P2 vs P6). Mouse ROs were cultured for 14 days; P2 and P6 ovaries cultured as Controls. Follicle development was compared to ROs transplanted for 14 days and ovaries from P16 and P20 mice. ROs generated from either P2 or P6 exhibited similar follicle development in culture whereas in vivo follicle development was more advanced in P6 ROs. Follicles were more developed in cultured ROs than transplanted ROs. However, follicles in cultured ROs and ovaries had smaller oocytes with fewer theca and granulosa cells than in vivo counterparts. Our results demonstrate the fluidity of follicle development despite ovary dissociation and that environment is more important to basal lamina formation and theca cell development. Furthermore, follicle development within cultured ROs appears to be independent of oocyte nest breakdown and primordial follicle formation in source ovaries. Our results highlight the need for understanding follicle development in vitro, particularly in the development of the RO technique as a potential fertility treatment.

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

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

A common African variant of human connexin 37 is associated with Caucasian primary ovarian insufficiency and has a deleterious effect in vitro

Folliculogenesis requires communication between granulosa cells and oocytes, mediated by connexin-based gap junctions. Connexin 37 (Cx37)-deficient female mice are infertile. The present study assessed Cx37 deficiency in patients with primary ovarian insufficiency (POI). A candidate gene study was performed in patients and controls from the National Genotyping Center (Evry, France) including 58 Caucasian patients with idiopathic isolated POI and 142 Caucasian controls. Direct genomic sequencing of the coding regions of the GJA4 gene (encoding Cx37) was performed with the aim to identify a deleterious variant associated with POI and absent in ethnically matched controls. A single Cx37 variant absent in the control population was identified, namely a c.946G>A heterozygous substitution leading to a p.Gly316Ser variant that was present in two POI patients. This variant was absent in all Caucasian controls from various databases, and has been observed exclusively in African populations. This variant was identified to have a dominant negative effect in HeLa cells in vitro to alter connexon function (by 67.2±7.17%), as determined by Gap-fluorescence recovery after photobleaching. The alteration principally resulted from a decrease of cell surface connexons due to altered trafficking (by 47.73±8.59%). In marked contrast to this observation, a p.Pro258Ser variant frequent in all ethnic populations in databases had no functional effect in vitro. In conclusion, the present study reported on a Cx37 variant in two Caucasian POI patients, which was absent in control Caucasian populations, and which had a deleterious effect in vitro. It is therefore suggested that in the genetic context of the Caucasian population, this variant may contribute to POI.

Regulation of germ cell development by intercellular signaling in the mammalian ovarian follicle

Prior to ovulation, the mammalian oocyte undergoes a process of differentiation within the ovarian follicle that confers on it the ability to give rise to an embryo. Differentiation comprises two phases-growth, during which the oocyte increases more than 100-fold in volume as it accumulates macromolecules and organelles that will sustain early embryogenesis; and meiotic maturation, during which the oocyte executes the first meiotic division and prepares for the second division. Entry of an oocyte into the growth phase appears to be triggered when the adjacent granulosa cells produce specific growth factors. As the oocyte grows, it elaborates a thick extracellular coat termed the zona pellucida. Nonetheless, cytoplasmic extensions of the adjacent granulosa cells, termed transzonal projections (TZPs), enable them to maintain contact-dependent communication with the oocyte. Through gap junctions located where the TZP tips meet the oocyte membrane, they provide the oocyte with products that sustain its metabolic activity and signals that regulate its differentiation. Conversely, the oocyte secretes diffusible growth factors that regulate proliferation and differentiation of the granulosa cells. Gap junction-permeable products of the granulosa cells prevent precocious initiation of meiotic maturation, and the gap junctions also enable oocyte maturation to begin in response to hormonal signals received by the granulosa cells. Development of the oocyte or the somatic compartment may also be regulated by extracellular vesicles newly identified in follicular fluid and at TZP tips, which could mediate intercellular transfer of macromolecules. Oocyte differentiation thus depends on continuous signaling interactions with the somatic cells of the follicle. WIREs Dev Biol 2018, 7:e294. doi: 10.1002/wdev.294 This article is categorized under: Gene Expression and Transcriptional Hierarchies > Cellular Differentiation Signaling Pathways > Cell Fate Signaling Early Embryonic Development > Gametogenesis.

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

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

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

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

Gap junction proteins are key drivers of endocrine function

It has long been known that the main secretory cells of exocrine and endocrine glands are connected by gap junctions, made by a variety of connexin species that ensure their electrical and metabolic coupling. Experiments in culture systems and animal models have since provided increasing evidence that connexin signaling contributes to control the biosynthesis and release of secretory products, as well as to the life and death of secretory cells. More recently, genetic studies have further provided the first lines of evidence that connexins also control the function of human glands, which are central to the pathogenesis of major endocrine diseases. Here, we summarize the recent information gathered on connexin signaling in these systems, since the last reviews on the topic, with particular regard to the pancreatic beta cells which produce insulin, and the renal cells which produce renin. These cells are keys to the development of various forms of diabetes and hypertension, respectively, and combine to account for the exploding, worldwide prevalence of the metabolic syndrome. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Regulation of Mammalian Oocyte Meiosis by Intercellular Communication Within the Ovarian Follicle

Meiotic progression in mammalian preovulatory follicles is controlled by the granulosa cells around the oocyte. Cyclic GMP (cGMP) generated in the granulosa cells diffuses through gap junctions into the oocyte, maintaining meiotic prophase arrest. Luteinizing hormone then acts on receptors in outer granulosa cells to rapidly decrease cGMP. This occurs by two complementary pathways: cGMP production is decreased by dephosphorylation and inactivation of the NPR2 guanylyl cyclase, and cGMP hydrolysis is increased by activation of the PDE5 phosphodiesterase. The cGMP decrease in the granulosa cells results in rapid cGMP diffusion out of the oocyte, initiating meiotic resumption. Additional, more slowly developing mechanisms involving paracrine signaling by extracellular peptides (C-type natriuretic peptide and EGF receptor ligands) maintain the low level of cGMP in the oocyte. These coordinated signaling pathways ensure a fail-safe system to prepare the oocyte for fertilization and reproductive success.

Embryonic poly(A)-binding protein is required at the preantral stage of mouse folliculogenesis for oocyte-somatic communication

Embryonic poly(A)-binding protein (EPAB)-deficient mice are infertile due to defects in both the oocyte and the somatic cells of the ovary. Since EPAB is oocyte specific, the abnormalities in the somatic compartment of Epab−/− mice are likely due to factors inherent to the oocyte. Herein, we investigated whether oocyte–somatic communication is disrupted as a result of EPAB deficiency. We found that gap junctions are disrupted at the late preantral stage of folliculogenesis in Epab−/– mice and remain disrupted in cumulus-enclosed oocytes (COCs) from antral follicles. Consistent with the timing of gap junction dysfunction, F-actin staining of transzonal processes (TZPs) is lower in Epab−/− follicles at the late preantral stage and completely absent in Epab−/− COCs. Epab−/− oocytes express significantly lower levels of the junction protein E-cadherin, which is likely to be a contributing factor leading to premature TZP retraction. Overall, these results demonstrate that EPAB is important for oocyte–somatic communication by maintaining TZPs and gap junctions at the preantral stage of folliculogenesis.

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

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

Driving folliculogenesis by the oocyte-somatic cell dialog: Lessons from genetic models

This review focuses on the role of the dialog between the oocyte and its companion somatic cells in driving folliculogenesis from the primordial to the preovulatory follicle stage. Mouse and sheep genetic models have brought complementary evidence of these cell interactions and their consequences for ovarian function. In mouse, the deletion of genes encoding connexins has shown that functional gap junction channels between oocytes and granulosa cells and between granulosa cells themselves maintain the follicle in a functionally integrated state. Targeted deletions in oocytes or granulosa cells have revealed the cell- and stage-specific role of ubiquist factors belonging to the phosphatidylinositol 3 kinase signaling pathway in primordial follicle activation, oocyte growth and follicle survival. Various models of transgenic mice and sheep carrying natural loss-of-function mutations associated with sterility have established that the oocyte-derived factors, bone morphogenetic protein (BMP) 15 and growth differentiation factor 9 orchestrate follicle development, support cumulus metabolism and maturation and participate in oocyte meiosis arrest. Unexpectedly in sheep, mutations resulting in the attenuation of BMP signaling lead to enhanced ovulation rate, likely resulting from a lowered follicular atresia rate and the enhancement of FSH-regulated follicular maturation. Both the activation level of BMP signaling and an adequate equilibrium between BMP15 and growth differentiation factor 9 determine follicle survival, maturation, and development toward ovulation. The physiological approaches which were implemented on genetic animal models during the last 20 years have opened up new perspectives for female fertility by identifying the main signaling pathways of the oocyte-somatic cell dialog.

Physiological roles of connexins and pannexins in reproductive organs

Reproductive organs are complex and well-structured tissues essential to perpetuate the species. In mammals, the male and female reproductive organs vary on their organization, morphology and function. Connectivity between cells in such tissues plays pivotal roles in organogenesis and tissue functions through the regulation of cellular proliferation, migration, differentiation and apoptosis. Connexins and pannexins can be seen as major regulators of these physiological processes. In the present review, we assembled several lines of evidence demonstrating that these two families of proteins are essential for male and female reproduction.

Follicle-Stimulating Hormone Increases Gap Junctional Communication Between Somatic and Germ-Line Follicular Compartments During Murine Oogenesis

Germ cells develop in intimate contact and communication with somatic cells of the gonad. In female mammals, oocyte development depends crucially on gap junctions that couple it to the surrounding somatic granulosa cells of the follicle, yet the mechanisms that regulate this essential intercellular communication remain incompletely understood. Follicle-stimulating hormone (FSH) drives the terminal stage of follicular development. We found that FSH increases the steady-state levels of mRNAs encoding the principal connexins that constitute gap junctions and cadherins that mediate cell attachment. This increase occurs both in granulosa cells, which express the FSH-receptor, and in oocytes, which do not. FSH also increased the number of transzonal projections that provide the sites of granulosa cell-oocyte contact. Consistent with increased connexin expression, FSH increased gap junctional communication between granulosa cells and between the oocyte and granulosa cells, and it accelerated oocyte development. These results demonstrate that FSH regulates communication between the female germ cell and its somatic microenvironment. We propose that FSH-regulated gap junctional communication ensures that differentiation processes occurring in distinct cellular compartments within the follicle are precisely coordinated to ensure production of a fertilizable egg.

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

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

Gap junction connexins in female reproductive organs: implications for women's reproductive health

BACKGROUND

Connexins comprise a family of ~20 proteins that form intercellular membrane channels (gap junction channels) providing a direct route for metabolites and signalling molecules to pass between cells. This review provides a critical analysis of the evidence for essential roles of individual connexins in female reproductive function, highlighting implications for women's reproductive health.

METHODS

No systematic review has been carried out. Published literature from the past 35 years was surveyed for research related to connexin involvement in development and function of the female reproductive system. Because of the demonstrated utility of genetic manipulation for elucidating connexin functions in various organs, much of the cited information comes from research with genetically modified mice. In some cases, a distinction is drawn between connexin functions clearly related to the formation of gap junction channels and those possibly linked to non-channel roles.

RESULTS AND CONCLUSIONS

Based on work with mice, several connexins are known to be required for female reproductive functions. Loss of connexin43 (CX43) causes an oocyte deficiency, and follicles lacking or expressing less CX43 in granulosa cells exhibit reduced growth, impairing fertility. CX43 is also expressed in human cumulus cells and, in the context of IVF, has been correlated with pregnancy outcome, suggesting that this connexin may be a determinant of oocyte and embryo quality in women. Loss of CX37, which exclusively connects oocytes with granulosa cells in the mouse, caused oocytes to cease growing without acquiring meiotic competence. Blocking of CX26 channels in the uterine epithelium disrupted implantation whereas loss or reduction of CX43 expression in the uterine stroma impaired decidualization and vascularization in mouse and human. Several connexins are important in placentation and, in the human, CX43 is a key regulator of the fusogenic pathway from the cytotrophoblast to the syncytiotrophoblast, ensuring placental growth. CX40, which characterizes the extravillous trophoblast (EVT), supports proliferation of the proximal EVTs while preventing them from differentiating into the invasive pathway. Furthermore, women with recurrent early pregnancy loss as well as those with endometriosis exhibit reduced levels of CX43 in their decidua. The antimalaria drug mefloquine, which blocks gap junction function, is responsible for increased risk of early pregnancy loss and stillbirth, probably due to inhibition of intercellular communication in the decidua or between trophoblast layers followed by an impairment of placental growth. Gap junctions also play a critical role in regulating uterine blood flow, contributing to the adaptive response to pregnancy. Given that reproductive impairment can result from connexin mutations in mice, it is advised that women suffering from somatic disease symptoms associated with connexin gene mutations be additionally tested for impacts on reproductive function. Better knowledge of these essential connexin functions in human female reproductive organs is important for safeguarding women's reproductive health.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Prophase I arrest of mouse oocytes mediated by natriuretic peptide precursor C requires GJA1 (connexin-43) and GJA4 (connexin-37) gap junctions in the antral follicle and cumulus-oocyte complex

Fully grown germinal vesicle stage mouse oocytes remain arrested in meiotic prophase I until ovulation. This arrest is maintained by cGMP produced in cumulus granulosa cells surrounding the oocyte. Recently, it was found that cGMP production in cumulus cells depends on NPR2 guanylate cyclase activated by its ligand natriuretic peptide precursor C (NPPC). It is assumed that cGMP reaches the oocyte through gap junctions that couple cumulus granulosa cells to each other and to the oocyte. Previous work identified two main types of gap junctions in the follicle, connexin-43 gap junctions (GJA1 protein) between granulosa cells and connexin-37 gap junctions (GJA4) between cumulus cells and the oocyte. However, it had not been established that both types are required for meiotic arrest mediated by NPPC/NPR2 signaling. To investigate this, we used connexin mimetic peptides (CMPs) that specifically disrupt gap junction isoforms within cumulus-oocyte complexes (COCs) and isolated antral follicles in culture. We furthermore developed a punctured antral follicle preparation to permit CMP access to the antral cavity in an otherwise intact follicle. CMP directed against connexin-43 (Cx43 CMP) overcame NPPC-mediated meiotic arrest in both isolated COCs and antral follicles. Cx37 CMP, in contrast, had no effect when present in the medium, but released oocyte arrest in the presence of NPPC when microinjected into the perivitelline space near the oocyte surface in COCs. This is consistent with both connexin isoforms being required for meiotic arrest and with the reported localization of connexin-43 throughout the cumulus cells and connexin-37 at the oocyte surface.