Effect of hexoses and gonadotrophin supplementation on bovine oocyte nuclear maturation during in vitro maturation in a synthetic follicle fluid medium

The relationship between bovine blastocyst formation in vitro and follicular fluid amino acids

Follicular fluid has been found as a possible source of metabolic predictors for oocyte competence, and it is conveniently accessible during ovum pick-up (OPU). We used the OPU procedure to recover oocytes from 41 Holstein heifers for in vitro embryo production in this study. Follicular fluid was collected during OPU in order to establish a link between follicular amino acids and blastocyst formation. Each heifer's oocytes were collected, matured in vitro for 24 h and fertilized separately. The heifers were then divided into two groups based on blastocyst formation: those that produced at least one blastocyst (the blastocyst group, n = 29) and those that did not (the failed group, n = 12). The blastocyst group had higher follicular glutamine concentrations and lower aspartate levels than the failed group. Furthermore, network and Spearman correlation analyses revealed a link between blastocyst formation and aspartate (r = -0.37, p = 0.02) or glutamine (r = 0.38, p = 0.02). The receiver operator characteristic curve revealed that glutamine (AUC = 0.75) was the greatest predictor of blastocyst formation. These findings revealed that follicular amino acid levels in bovines can be used to predict blastocyst development.

Impact of epidermal growth factor and/or β-mercaptoethanol supplementations on the in vitro produced buffaloes' embryos

The present study investigated the effects of epidermal growth factors (EGF) and/or β-Mercaptoethanol (βME) supplementations to oocyte maturation, fertilization, and culture media on the buffalo in vitro embryo production. The ovaries were collected and transferred within 2 h to the laboratory. The cumulus oocytes complexes were aspirated from 3 to 8 mm diameter follicles. Firstly, EGF; 0, 10, 20, or 50 ng/mL or βME; 0, 25, 50, 100, or 200 μM were supplemented to the in vitro maturation (TCM-199), fertilization (IVF-TALP), or culture (IVC: SOF) media. Our results revealed that supplementing EGF (20 ng/mL) to the TCM-199, IVF-TALP, or SOF media could efficiently improve the growth rates and development of buffalos' embryos, while EGF (50 ng/mL) could stimulate the embryo production only after treatment of the IVF-TALP /or SOF media, but not the IVM medium. However, βME was less efficient than EGF; it stimulated the growth rates of buffalo embryos when supplemented with the maturation and fertilization (IVF-TALP) media in a 50 μM concentration. Secondly, combined EGF (20 ng/mL) and βME (50 μM) were supplemented to the maturation media as effective concentration. The combined treatment of EGF (20 ng/mL) and βME (50 μM) showed no significant enhancing effect on the buffalo embryos compared to each alone. For future perspectives, further study is required to examine the effects of combined EGF and βME on the maturation and fertilization of buffalo oocytes at different categories of age and seasonal localities.

Characterization of carbohydrate metabolism in in vivo and in vitro grown and matured mouse antral follicles

Establishing an ideal human follicle culture system for oncofertility patients relies mainly on animal models since donor tissue is scarce and often of suboptimal quality. The in vitro system developed in our laboratory supports the growth of prepubertal mouse secondary follicles up to mature oocytes. Given the importance of glucose in preparing the oocyte for proper maturation, a baseline characterization of follicle metabolism both in the culture system and in vivo was carried out. Markers of glucose-related pathways (glycolysis, tricarboxylic acid (TCA) cycle, pentose phosphate pathway (PPP), polyol pathway, hexosamine biosynthesis pathway (HBP)) as well as for the antioxidant capacity were measured in the different follicle cell types by both enzymatic activities (spectrophotometric detection) and gene expression (qPCR). This study confirmed that in vivo the somatic cells, mainly granulosa, exhibit intense glycolytic activity, while oocytes perform PPP. Throughout the final maturation step, oocytes in vivo and in vitro showed steady levels for all the key enzymes and metabolites. On the other hand, ovulation triggers a boost of pyruvate and lactate uptake in cumulus cells in vivo, consumes reduced nicotinamide adenine dinucleotide phosphate (NADPH) and increases TCA cycle and small molecules antioxidant capacity (SMAC) activities, while in vitro, the metabolic upregulation in all the studied pathways is limited. This altered metabolic pattern might be a consequence of cell exhaustion because of culture conditions, impeding cumulus cells to fulfil their role in providing proper support for acquiring oocyte competence. SUMMARY SENTENCE: In vitro cultured mouse follicles exhibit altered glycolytic activity and redox metabolism in the somatic compartment during meiotic maturation.

Metabolic control of oocyte development

Well balanced and timed metabolism is essential for oocyte development. The effects of extrinsic nutrients on oocyte maturation have been widely reported. In contrast, intrinsic control of oogenesis by intracellular metabolites and metabolic enzymes has received little attention. The comprehensive characterization of metabolic patterns could lead to more complete understanding of regulatory mechanisms underlying oocyte development. A cell's metabolic state is integrated with epigenetic regulation. Epigenetic modifications in germ cells are therefore sensitive to parental environmental exposures. Nevertheless, direct genetic evidence for metabolites involvement in epigenetic establishment during oocyte development is still lacking. Moreover, metabolic disorder-induced epigenetic perturbations during oogenesis might mediate the inter/transgenerational effects of environmental insults. The molecular mechanisms responsible for this deserve further investigation. Here, we summarize the findings on metabolic regulation in oocyte maturation, and how it contributes to oocyte epigenetic modification. Finally, we propose a mouse model that metabolic disorder in oocyte serves as a potential factor mediating the maternal environment effects on offspring health.

Glucose metabolism is required for oocyte maturation of zebrafish

Glucose is an essential source of energy production for animal cells. The importance of glucose metabolism in oocyte maturation has been studied extensively in mammals. However, such roles in non-mammalian species are still largely unknown. Here, we used zebrafish as a model, which is phylogenetically distant from mammals, and analyzed the role of glucose metabolism in oocyte maturation. Major glucose transporters (GLUT/Slc2A) were analyzed in zebrafish, two Slc2a1 (Slc2a1a and Slc2a1b), one Slc2a2, and two Slc2a3 (Slc2a3a and Slc2a3b) were identified. Among these five Slc2a genes, slc2a1b exhibited the highest expression level in fully grown follicles. The expression of slc2a1b gradually increased during folliculogenesis, and also significantly increases during the oocyte maturation process. Consistently, the glucose concentration increases during natural oocyte maturation. By using a fluorescent glucose derivative (6-NBDG) to trace glucose transport, the uptake of glucose by ovarian follicles in a time-dependent manner could be observed. Intriguingly, by treatment of glucose in vitro, oocyte maturation could be induced in a time-, dose- and stage-dependent manner. Glucose can be metabolized by glycolysis, the pentose phosphate pathway (PPP), the hexosamine biosynthesis pathway (HBP), and the polyol pathway. Using the inhibitors for these pathways, we found only PPP but not glycolysis, HBP or polyol pathway is essential for oocyte maturation. All these results clearly demonstrate for the first time that the glucose metabolism is required for oocyte maturation of zebrafish, suggesting the highly conserved role of glucose metabolism in control of oocyte maturation between fish and mammals.

Glucose metabolism characterization during mouse in vitro maturation identifies alterations in cumulus cells†

In vitro maturation (IVM) is an assisted reproduction technique with reduced hormone-related side-effects. Several attempts to implement IVM in routine practice have failed, primarily due to its relatively low efficiency compared with conventional in vitro fertilization (IVF). Recently, capacitation (CAPA)-IVM-a novel two-step IVM method-has improved the embryology outcomes through synchronizing the oocyte nuclear and cytoplasmic maturation. However, the efficiency gap between CAPA-IVM and conventional IVF is still noticeable especially in the numerical production of good quality embryos. Considering the importance of glucose for oocyte competence, its metabolization is studied within both in vivo and CAPA-IVM matured mouse cumulus-oocyte-complexes (COCs) through direct measurements in both cellular compartments, from transcriptional and translational perspectives, to reveal metabolic shortcomings within the CAPA-IVM COCs. These results confirmed that within in vivo COC, cumulus cells (CCs) are highly glycolytic, whereas oocytes, with low glycolytic activity, are deviating their glucose towards pentose phosphate pathway. No significant differences were observed in the CAPA-IVM oocytes compared with their in vivo counterparts. However, their CCs exhibited a precocious increase of glycolytic activity during the pre-maturation culture step and activity was decreased during the IVM step. Here, specific alterations in mouse COC glucose metabolism due to CAPA-IVM culture were characterized using direct measurements for the first time. Present data show that, while CAPA-IVM CCs are able to utilize glucose, their ability to support oocytes during final maturation is impaired. Future CAPA-IVM optimization strategies could focus on adjusting culture media energy substrate concentrations and/or implementing co-culture strategies.

The Phosphodiesterase Inhibitor, Isobutyl-1-Methylxanthine Prevents the Sudden Drop in Cyclic Adenosine Monophosphate Concentration and Modulates Glucose Metabolism of Equine Cumulus-Oocyte Complexes Matured in Vitro

Spontaneous nuclear maturation of mammalian oocytes can occur when physically removed from the ovarian follicle during in vitro oocyte maturation (IVM), largely because of a decrease in cyclic adenosine monophosphate (cAMP) concentration. Modulation of oocyte cAMP during IVM by using phosphodiesterase inhibitors has been shown to maintain elevated oocyte cAMP concentrations and control meiotic resumption of bovine and ovine oocytes. This study determined the effect of inclusion of isobutyl-1-methylxanthine (IBMX) during collection and the first 12 hours of incubation of equine oocytes on cAMP concentration and glucose metabolism of cumulus-oocyte complexes (COCs). Abattoir-derived COCs were collected in aspiration medium with (Asp-IBMX) or without (Asp) IBMX. Cumulus-oocyte complexes were then incubated for 12 hours in IVM medium with (Mat-IBMX) or without (Mat) IBMX, followed by additional 24 hours in Mat medium. The cAMP concentration, glucose consumption, lactate production, and metaphase II rates of the COCs were assessed. Cumulus-oocyte complexes aspirated into Asp-IBMX (62.2 ± 2.6 fmol per COC) medium had higher cAMP concentration than Asp (31.8 ± 2.8 fmol per COC) control group (P < .05). Likewise, at 12 hours of IVM, Mat-IBMX group (33.2 ± 2.1 fmol per COC) had higher cAMP concentration than the Mat group (7.68 ± 0.5 fmol per COC; P < .05). Glucose consumption and lactate production were lower during the first 12 hours of incubation in COCs cultured in Mat-IBMX (P < .05). Isobutyl-1-methylxanthine prevented the rapid drop in cAMP concentration and altered metabolism of glucose by the COC. Preventing the sudden drop in cAMP prevents the premature nuclear maturation of in vitro-matured oocytes causing poor developmental competence.

Energy metabolism of follicular environment during oocyte growth and maturation

Oocyte quality is affected by many factors, among which the environment of growth and maturation seems to be crucial. Studies show that well balanced oocyte energy metabolism has a significant impact on several elements of cytoplasmic and nuclear maturation as well as further embryo developmental competence. Therefore homeostasis between metabolism of glucose and fatty acids in the oocyte is being widely described nowadays. This review aims to discuss the follicular (in vivo) or maturation media (in vitro) environments with regard to glucose and fatty acid metabolism, as the main sources of the energy for the oocyte. A great emphasis is given on the balance between those two metabolic pathways and its further impact on female fertility.

Amino acid supplementation of a simple inorganic salt solution supports efficient in vitro maturation (IVM) of bovine oocytes

Defining oocyte in vitro maturation (IVM) conditions allows for improved reproducibility and efficiency of bovine embryo production. IVM conditions for bovine oocytes have been extensively studied, but beneficial effects of individual supplements remain controversial. This study compared methods of cumulus oocyte complex (COC) isolation, and culture medium requirements, for IVM in order to define optimal conditions. Antral follicles in ovaries were sliced or aspirated to isolate COCs. Brilliant cresyl blue staining of COCs was used to determine the most effective collection technique and the effect of hormones and groups of amino acids in the culture medium was investigated. Our results showed COCs isolated through aspiration had greater meiotic competency to reach MII. Oocyte maturation was achieved with the addition of 1 µg/mL FSH, while estrogen and human chorionic gonadotrophin did not increase the number of MII oocytes. We also provide novel data, that supplementation of a simple inorganic salt solution with L-proline, L-glutamine and essential amino acids in combination, but not individually, resulted in nuclear maturation comparable to TCM199, a more complex medium containing all 20 common amino acids, vitamins, inorganic salts and FBS. Replacement of FBS with BSA in this simplified medium creates a defined medium which provides conditions for IVM that enable reproducible maturation rates.

High-glucose concentrations change DNA methylation levels in human IVM oocytes

STUDY QUESTION

What are the effects of high-glucose concentrations on DNA methylation of human oocytes?

SUMMARY ANSWER

High-glucose concentrations altered DNA methylation levels of Peg3 and Adiponectin in human in vitro maturation oocytes.

WHAT IS KNOWN ALREADY

Maternal diabetes has a detrimental influence on oocyte quality including epigenetic modifications, as shown in non-human mammalian species.

STUDY DESIGN, SIZE, DURATION

Immature metaphase I (MI) stage oocytes of good quality were retrieved from patients who had normal ovarian potential and who underwent ICSI in the Reproductive Medicine Center of People's Hospital of Zhengzhou University. MI oocytes were cultured in medium with different glucose concentrations (control, 10 mM and 15 mM) in vitro and 48 h later, oocytes with first polar body extrusion were collected to check the DNA methylation levels.

PARTICIPANTS/MATERIALS, SETTING, METHODS

MI oocytes underwent in vitro maturation (IVM) at 37°C with 5% mixed gas for 48 h. Then the mature oocytes were treated with bisulfite buffer. Target sequences were amplified using nested or half-nested PCR and the DNA methylation status was tested using combined bisulfite restriction analysis (COBRA) and bisulfite sequencing (BS).

MAIN RESULTS AND THE ROLE OF CHANCE

High-glucose concentrations significantly decreased the first polar body extrusion rate. Compared to controls, the DNA methylation levels of Peg3 in human IVM oocytes were significantly higher in 10 mM (P < 0.001) and 15 mM (P < 0.001) concentrations of glucose. But the DNA methylation level of H19 was not affected by high-glucose concentrations in human IVM oocytes. We also found that there was a decrease in DNA methylation levels in the promoter of adiponectin in human IVM oocytes between controls and oocytes exposed to 10 mM glucose (P = 0.028).

LARGE SCALE DATA

N/A.

LIMITATIONS REASONS FOR CAUTION

It is not clear whether the alterations are beneficial or not for the embryo development and offspring health. The effects of high-glucose concentrations on the whole process of oocyte maturation are still not elucidated. Another issue is that the number of oocytes used in this study was limited.

WIDER IMPLICATIONS OF THE FINDINGS

This is the first time that the effects of high-glucose concentration on DNA methylation of human oocytes have been elucidated. Our result indicates that in humans, the high risk of chronic diseases in offspring from diabetic mothers may originate from abnormal DNA modifications in oocytes.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the fund of National Natural Science Foundation of China (81401198) and Doctor Foundation of Qingdao Agricultural University (1116008).The authors declare that there are no potential conflicts of interest relevant to this article.

Treatment of Epidermal Growth Factor (EGF) enhances Nuclear Maturation of Porcine Oocytes and Stimulates Expression of ER/Golgi Transport Proteins

This study was conducted to investigate stimulatory effect of epidermal growth factor (EGF) on nuclear maturation and the expression level of EGF-receptor (EGFR), GM-130 (a marker of Golgi apparatus), transport protein Sec61 subunit beta (Sec61β), and coatomer protein complex subunit gamma 2 (COPG2) in porcine oocytes. The cumulus-oocyte complexes were collected from follicle with 3-6 mm in diameter. They were incubated in medium with/without EGF for 22 h (IVMⅠ) and subsequently incubated hormone-free medium with/without EGF for 22 h (IVMⅡ). Nuclear maturation state was checked by aceto-orcein stain. Protein expression of EGFR, GM-130, Sec61β, and COPG2 were measured by immunofluorescence. In results, nuclear maturation of oocytes in EGF non-treated oocytes were significantly lower than EGF-treated groups at IVMⅠ or IVMⅡ stage (P<0.05), whereas maturational rate in EGF treatment groups at both of IVM stage was higher in among the all treatment groups (P<0.05). EGFR, GM-130, Sec61β and COPG2 were expressed in the cytoplasm of oocytes. Especially, GM-130 and EGFR were strongly expressed, but Sec61β and COPG2 were weakly expressed in cortical area of cytoplasm. The protein level of GM-130, Sec61β, and COPG2 were significantly higher in the EGF-treated groups (P<0.05). However EGFR was no difference between non EGF-treated groups and control. In conclusion, EGF plays an important role in the systems for oocyte maturation with endoplasmic reticulum and Golgi apparatus. In addition, the protein levels of Sec61β and COPG2 could be changed by EGF in the porcine oocytes during maturation.

Pentose phosphate pathway activity: effect on in vitro maturation and oxidative status of bovine oocytes

The relationship between pentose phosphate pathway (PPP) activity in cumulus-oocyte complexes (COCs) and oxidative and mitochondrial activity in bovine oocytes was evaluated with the aim of analysing the impact of two inhibitors (NADPH and 6-aminonicotinamide (6-AN)) and a stimulator (NADP) of the key enzymes of the PPP on the maturation rate, oxidative and mitochondrial activity and the mitochondrial distribution in oocytes. The proportion of COCs with measurable PPP activity (assessed using brilliant cresyl blue staining), glucose uptake, lactate production and meiotic maturation rate diminished when 6-AN (0.1, 1, 5 and 10mM for 22h) was added to the maturation medium (P<0.05). The addition of NADPH did not modify glucose uptake or lactate production, but reduced PPP activity in COCs and meiotic maturation rates (P<0.05). The presence of NADP (0.0125, 0.125, 1.25 and 12.5mM for 22h of culture) in the maturation medium had no effect on PPP activity in COCs, glucose uptake, lactate production and meiotic maturation rate. However, in the absence of gonadotropin supplementation, NADP stimulated both glucose uptake and lactate production at 12.5mM (the highest concentration tested; P<0.05). NADP did not modify cleavage rate, but decreased blastocyst production (P<0.05). During IVM, oocyte oxidative and mitochondrial activity was observed to increase at 15 and 22h maturation, which was also related to progressive mitochondrial migration. Inhibiting the PPP with 6-AN or NADPH led to reduced oxidative and mitochondrial activity compared with the respective control groups and inhibition of mitochondrial migration (P<0.05). Stimulation of the PPP with NADP increased oxidative and mitochondrial activity at 9h maturation (P<0.05) and delayed mitochondrial migration. The present study shows the significance of altering PPP activity during bovine oocyte IVM, revealing that there is a link between the activity of the PPP and the oxidative status of the oocyte.

Nutrient pathways regulating the nuclear maturation of mammalian oocytes

Oocyte maturation is defined as that phase of development whereby a fully grown oocyte reinitiates meiotic maturation, completes one meiotic division with extrusion of a polar body, then arrests at MII until fertilisation. Completion of maturation depends on many different factors, not the least of which is the proper provision of energy substrates to fuel the process. Interaction of the oocyte and somatic compartment of the follicle is critical and involves numerous signals exchanged between the two cell types in both directions. One of the prominent functions of the cumulus cells is the channelling of metabolites and nutrients to the oocyte to help stimulate germinal vesicle breakdown and direct development to MII. This entails the careful integration and coordination of numerous metabolic pathways, as well as oocyte paracrine signals that direct certain aspects of cumulus cell metabolism. These forces collaborate to produce a mature oocyte that, along with accompanying physiological changes called cytoplasmic maturation, which impart subsequent developmental competence to the oocyte, can be fertilised and develop to term. This review focuses on nuclear maturation and the metabolic interplay that regulates it, with special emphasis on data generated in the mouse.

Glycolytic pathway activity: effect on IVM and oxidative metabolism of bovine oocytes

The aim of the present study was to determine the effect of altering glycolytic pathway activity during bovine IVM on the meiotic maturation rate, oxidative activity, mitochondrial activity and the mitochondrial distribution within oocytes. Glycolytic activity was manipulated using two inhibitors (ATP, NaF) and a stimulator (AMP) of key enzymes of the pathway. Inhibition of glucose uptake, lactate production and meiotic maturation rates was observed when media were supplemented with ATP or NaF. The addition of AMP to the maturation medium had no effect on glucose uptake, lactate production or meiotic maturation. In the absence of gonadotrophin supplementation, AMP stimulated both glucose uptake and lactate production. However, AMP also decreased cytoplasmic maturation, as determined by early cleavage. During IVM, oocyte oxidative and mitochondrial activity was observed to increase at 15 and 22h maturation. Inhibiting glycolysis with ATP or NaF led to a reduced oxidative and mitochondrial pattern compared with the respective control groups. Stimulation of the pathway with AMP increased oxidative and mitochondrial activity. A progressive mitochondrial migration to the central area was observed during maturation; oocytes treated with ATP, NaF or AMP showed limited migration. The present study reveals the effects of altering glycolytic pathway activity in cumulus-oocyte complexes, revealing the link between glycolysis of the cumulus-oocyte complex and the oxidative and mitochondrial activity of the oocyte.

DHEA-mediated inhibition of the pentose phosphate pathway alters oocyte lipid metabolism in mice

Women with polycystic ovary syndrome (PCOS) and hyperandrogenism have altered hormone levels and suffer from ovarian dysfunction leading to subfertility. We have attempted to generate a model of hyperandrogenism by feeding mice chow supplemented with dehydroepiandrosterone (DHEA), an androgen precursor that is often elevated in women with PCOS. Treated mice had polycystic ovaries, low ovulation rates, disrupted estrous cycles, and altered hormone levels. Because DHEA is an inhibitor of glucose-6-phosphate dehydrogenase, the rate-limiting enzyme in the pentose phosphate pathway, we tested the hypothesis that oocytes from DHEA-exposed mice would have metabolic disruptions. Citrate levels, glucose-6-phosphate dehydrogenase activity, and lipid content in denuded oocytes from these mice were significantly lower than controls, suggesting abnormal tricarboxylic acid and pentose phosphate pathway metabolism. The lipid and citrate effects were reversible by supplementation with nicotinic acid, a precursor for reduced nicotinamide adenine dinucleotide phosphate. These findings suggest that elevations in systemic DHEA can have a negative impact on oocyte metabolism and may contribute to poor pregnancy outcomes in women with hyperandrogenism and PCOS.

Global gene expression profiling of individual human oocytes and embryos demonstrates heterogeneity in early development

Early development in humans is characterised by low and variable embryonic viability, reflected in low fecundity and high rates of miscarriage, relative to other mammals. Data from assisted reproduction programmes provides additional evidence that this is largely mediated at the level of embryonic competence and is highly heterogeneous among embryos. Understanding the basis of this heterogeneity has important implications in a number of areas including: the regulation of early human development, disorders of pregnancy, assisted reproduction programmes, the long term health of children which may be programmed in early development, and the molecular basis of pluripotency in human stem cell populations. We have therefore investigated global gene expression profiles using polyAPCR amplification and microarray technology applied to individual human oocytes and 4-cell and blastocyst stage embryos. In order to explore the basis of any variability in detail, each developmental stage is replicated in triplicate. Our data show that although transcript profiles are highly stage-specific, within each stage they are relatively variable. We describe expression of a number of gene families and pathways including apoptosis, cell cycle and amino acid metabolism, which are variably expressed and may be reflective of embryonic developmental competence. Overall, our data suggest that heterogeneity in human embryo developmental competence is reflected in global transcript profiles, and that the vast majority of existing human embryo gene expression data based on pooled oocytes and embryos need to be reinterpreted.

Glucogenic supply increases oocyte developmental competence in sheep

The present study aimed to determine the influence of a glucogenic supply on oocyte developmental competence. Oestrous cycles were synchronised in 22 Sarda ewes by the insertion (Day 0) of one intravaginal progestagen-impregnated sponge that was removed after 6 days. After removal, the ewes were randomly allocated into two experimental groups (treated and control ewes) and, from Day 7 to Day 11, treated ewes received oral administration of a glucogenic mixture, whereas control animals received water. Follicular development was stimulated by FSH administration from Days 8 to 10. Glucose metabolism was assessed from Days 7 to 11, whilst follicle and corpus luteum growth dynamics and functionality were evaluated between Days 6 and 11. At Day 11 ovaries were collected and processed for in vitro embryo production. Glucogenic treatment increased both the plasma levels of glucose, progesterone, oestradiol and the number of 2-3-mm follicles (P < 0.05). Higher fertilisation and blastocyst rates (P < 0.05) were obtained after IVM of oocytes recovered from treated ewes compared with control ones. In conclusion, glucogenic treatment modifies follicle and corpus luteum functionality and improves oocyte quality, as evaluated by in vitro developmental kinetics and blastocyst output.

Effect of varying glucose and glucosamine concentration in vitro on mouse oocyte maturation and developmental competence

The effects of hyper- and hypo-glycaemic conditions during the in vitro maturation of mouse cumulus–oocyte complexes on developmental competence were examined, with an emphasis on the role of the hexosamine biosynthesis pathway. A low (1 mM) glucose concentration achieved optimal oocyte competence (3-fold higher blastocyst development rate compared with high (30 mM) glucose, P < 0.05). In addition, glucose supplementation during only the first hour after release from the follicle was necessary and sufficient to support oocyte maturation and embryo development to the blastocyst stage. Glucosamine (a known hyperglycaemic mimetic and specific activator of the hexosamine pathway) was able to substitute for glucose during this first hour, indicating that flux through the hexosamine pathway is essential for oocyte competence. In the absence of glucose throughout the maturation period, glucosamine was not able to increase developmental competence, and at higher concentrations (2.5 and 5 mM) had a detrimental effect on MII and blastocyst development rates, compared with controls (P < 0.05). These experiments underscore the importance of glucose metabolic pathways during in vitro maturation and support the concept that excess flux through the hexosamine pathway has detrimental consequences.

Influence of selected (pre-)maturational parameters on in vitro development and sex distribution of bovine embryos

The objectives of this research were to study the influence of a reduced oxygen concentration during in vitro maturation (IVM) and examine the effect of follicular glucose concentration on bovine in vitro development and sex distribution. In the first experiment, abattoir-derived cumulus-oocyte complexes (COC) were matured under 5% O2 or 20% O2. Secondly, COC were isolated and the glucose (G) concentration of each follicle was determined. COC were pooled in groups (G (< 1.1 mMol) or G (≥ 1.1 mMol)) according to the glucose content before being subjected to in vitro production (IVP). Cleavage and development rates were assessed on days 3, 7 and 8 post insemination. Blastocysts of each group were sexed by polymerase chain reaction (PCR). Expanded blastocysts were stained to assess total cell numbers and live-dead cell ratio. Cleavage and development rates stayed similar after reducing the O2 concentration during IVM. The sex ratio of embryos generated from oocytes matured under 5% O2 was shifted in favour of the female (♀: 61.9%), whereas the sex ratio of embryos belonging to the IVM 20% O2 group did not differ significantly from the expected 50:50 ratio. Neither a 'higher' nor a 'lower' intrafollicular glucose concentration influenced cleavage and development rates, cell numbers or live-dead cell ratio. Eighty five per cent (G (<1.1)) and 63.6% (G (≥ 1.1)) of the analysed embryos were female. In summary, neither a reduced O2 concentration during IVM nor selection based on follicular glucose concentrations affected the morphological quality of embryos. Although the sex distribution was shifted in favour of female embryos in all three experimental groups, more male embryos could be seen in the G (≥ 1.1) group compared with the G(<1.1) group.

Temporal effects of exogenous oocyte-secreted factors on bovine oocyte developmental competence during IVM

We investigated whether paracrine signalling between the bovine oocyte and cumulus cells is altered during the course of in vitro maturation (IVM). Bovine COCs were cocultured with denuded oocytes or treated with specific oocyte-secreted factors, namely recombinant bone morphogenetic protein (BMP)-15 or growth differentiation factor (GDF)-9, beginning from 0 or 9 h IVM. To generate a 9-h denuded oocyte (DO) group, COCs were cultured intact for the first 9 h of IVM and then denuded. Coculturing intact COCs with DOs denuded immediately after collection or following 9 h of maturation did not affect cleavage rate, but improved blastocyst yield (P < 0.05) on Day 8 (51 and 61%, respectively; P < 0.05) and cell number compared with COCs cultured alone (41%). Significantly, we observed higher levels of endogenous GDF-9 and BMP-15 protein in oocytes of COCs matured for 9 h compared with no incubation. The addition of 175 ng mL–1 GDF-9 or 10% v/v BMP-15 from partially purified transfected 293H cell supernatant for 24 h IVM significantly enhanced development to the blastocyst stage from 40% (control) to 51 and 47%, respectively (P < 0.05). However, treatment of COCs with GDF-9 or BMP-15 between 9 and 24 h of IVM did not increase blastocyst yield. These results provide evidence of quantitative and possibly qualitative temporal changes in oocyte paracrine factor production during IVM.

Oxygen consumption and ROS production are increased at the time of fertilization and cell cleavage in bovine zygotes

BACKGROUND

Oxygen consumption is a key indicator of metabolic activity within embryos. Increased oxidative activity and REDOX changes at the time of fertilization have been suggested to signal Ca(2+) oscillations after sperm penetration. The objective of the present study was to determine the oxygen consumption and the REDOX status of zygotes and early embryos at the time of sperm penetration and cell cleavage and to investigate how metabolism relates to key temporal events and developmental competence.

METHODS

Individual oxygen-consumption rates of bovine in vitro matured oocytes and presumptive zygotes (n = 101) were measured using the Nanorespirometer at 0, 7, 12, 17 and 24 h after IVF. Using the Embryoscope, oxygen-consumption profiles of individual oocytes and embryos (n = 75) were recorded repeatedly from 6 h until 30 h after IVF and time-lapse images were acquired, at intervals of ∼36 min. Oocytes and embryos were stained with Hoechst 33342 and visualization of nuclear stage was performed by fluorescence microscopy. To determine the REDOX status, cohorts of oocytes and zygotes (n = 55) were individually stained with REDOX-Sensor Red CC-1 and Hoechst 33342 at 0, 7, 12, 17 and 24 h after IVF and subsequently imaged by confocal microscopy.

RESULTS

A peak of oxygen consumption was observed at the time of fertilization and a smaller rise and fall in oxygen consumption could be detected prior to the first cell cleavage. Increased reactive oxygen species production was also observed at 7 h and then at 24 h after IVF, just preceding the first embryonic cleavage.

CONCLUSIONS

There are specific events during embryo development that appear to be associated with a change in oxygen consumption and REDOX state, indicating that both have a role in sperm-mediated oocyte activation and cell cleavage in bovine embryos.

Prognostic value of follicular fluid 25-OH vitamin D and glucose levels in the IVF outcome

OBJECTIVES

The aim of the present study was to measure serum and follicular fluid 25-OH vitamin D and glucose levels in women who underwent IVF-ET treatment and to further investigate whether the circulating 25-OH vitamin D and glucose levels correlate with IVF success.

METHODS

This prospective observational study included 101 consecutive women who underwent 101 IVF-ICSI ovarian stimulation cycles and were allocated to one of the three groups according to their follicular fluid 25-OH vitamin D concentrations. Group A (n = 31) with less than 20 ng/ml, group B (n = 49) with vitamin levels between 20.1 and 30 ng/ml and group C (n = 21) with more than 30 ng/ml vitamin concentration.

RESULTS

Follicular fluid vitamin levels significantly correlated with the quality of embryos in total (r = -0.27, p = 0.027), while the quality of embryos of group C were of lower quality as compared to those of groups A and B (p = 0.009). Follicular fluid glucose levels were lower in women of group C as compared to the respective levels of groups A and B (p = 0.003). Clinical pregnancy rate demonstrated in 14.5% in women of group C and 32.3% and 32.7% in groups A and B, respectively (p = 0.047).

CONCLUSION

The data suggests that excess serum and follicular fluid vitamin levels in combination with decreased follicular fluid glucose levels have a detrimental impact on the IVF outcome.

The pivotal role of glucose metabolism in determining oocyte developmental competence

The environment that the cumulus oocyte complex (COC) is exposed to during either in vivo or in vitro maturation (IVM) can have profound effects on the success of fertilisation and subsequent embryo development. Glucose is a pivotal metabolite for the COC and is metabolised by glycolysis, the pentose phosphate pathway (PPP), the hexosamine biosynthesis pathway (HBP) and the polyol pathway. Over the course of oocyte maturation, a large proportion of total glucose is metabolised via the glycolytic pathway to provide substrates such as pyruvate for energy production. Glucose is also the substrate for many cellular functions during oocyte maturation, including regulation of nuclear maturation and redox state via the PPP and for the synthesis of substrates of extracellular matrices (cumulus expansion) and O-linked glycosylation (cell signalling) via the HBP. However, the oocyte is susceptible to glucose concentration-dependent perturbations in nuclear and cytoplasmic maturation, leading to poor embryonic development post-fertilisation. For example, glucose concentrations either too high or too low result in precocious resumption of nuclear maturation. This review will discuss the relevant pathways of glucose metabolism by COCs during in vivo maturation and IVM, including the relative contribution of the somatic and gamete compartments of the COC to glucose metabolism. The consequences of exposing COCs to abnormal glucose concentrations will also be examined, either during IVM or by altered maternal environments, such as during hyperglycaemia induced by diabetes and obesity.

Characterization of novel phosphodiesterases in the bovine ovarian follicle

The phosphodiesterase (PDE) family is a group of enzymes that catalyzes the transformation of cyclic nucleotides into 5' nucleotides. Based on rodents, the current mammalian model of PDE distribution in the ovarian follicle predicts Pde3a in the oocyte and Pde4d in the somatic cells. Using bovine as an experimental model, the present results showed that PDE3 was the predominant PDE activity in oocytes. However, cumulus cell cAMP-PDE activity was predominantly resistant to inhibition by 3-isobutyl-methylxantine, indicating PDE8 activity (60% of total PDE activity) and a minor role for PDE4 (<5%). A total of 20% of total oocyte PDE activity was also attributed to PDE8. The PDE activity measurements in mural granulosa cells from 2 to 6 mm in diameter suggest the presence of PDE4 and PDE8. In granulosa cells from follicles >10 mm, total PDE and PDE8 activities along with PDE8A protein level were increased compared with smaller follicles. The RT-PCR experiments showed that cumulus cells expressed PDE8A, PDE8B, and PDE10A. Western blot experiments showed PDE8A, PDE8B, and PDE4D proteins in mural granulosa cells and cumulus-oocyte complexes. PDE8 inhibition using dipyridamole in a dose-dependent manner increased cAMP levels in the cumulus-oocyte complexes and delayed oocyte nuclear maturation. These results are the first to demonstrate the functional presence of PDE8 in the mammalian ovarian follicle. This challenges the recently described cell-specific expression of cAMP-PDEs in the ovarian follicle and the notion that PDE4 is the predominant granulosa/cumulus cell PDE. These findings have implications for our understanding of hormonal regulation of folliculogenesis and the potential application of PDE inhibitors as novel contraceptives.

Heparin effect onin vitronuclear maturation of bovine oocytes

Oocytes undergo numerous biochemical and morphological changes during their development from preantral to preovulatory phases.In vitrostudies have suggested several compounds that might induce oocyte maturation. Heparin is a natural component of ooplasm, follicular fluid and uterine fluid and previous studies indicated that it might act as a chromatin maturation factor in bovine oocytes. We tested this hypothesisin vitroby timing germinal vesicle breakdown (GVBD) and first polar body (PB) formation without any other natural or introduced factors that might influence the rate of oocyte maturation. We also determined if these oocytes could be fertilized.

Bovine oocytes were incubated in a salt medium and TCM 199 supplemented with different concentrations of heparin for 24 h at 37.5 °C in a humidified atmosphere of 5% CO2. With 1.0 and 6.5 mg/ml heparin, the time of GVBD was reduced from 4.7 ± 1.1 h to about 1.5 h and the time of first PB formation was reduced from 22.0 ± 1.1 h to 9.0–11.0 h in salt medium. In TCM 199, only 6.5 mg/ml heparin significantly reduced the time of PB formation. In both incubation media, 1.0 and 6.5 mg/ml heparin induced GVBD, extrusion of the first PB and formation of the metaphase II nucleus. Moreover, heparin did not interfere with the fertilization of oocytes matured in TCM 199. Based on the results, we propose that heparin plays an important role in the rearrangement of the oocyte chromatin and acts as an oocyte maturation factor.

Oocyte maturation: Emerging concepts and technologies to improve developmental potential in vitro

Oocyte in vitro maturation (IVM) is an important reproductive technology that generates mature oocytes that are capable of supporting preimplantation embryo development and full development to term. There is great clinical and commercial incentive to improve the efficiency of the technology, however, progress has been slow over the past decade. A critical challenge is to understand what constitutes oocyte developmental competence and the mechanisms governing it. We have taken the approach of studying in detail oocyte-somatic cell interactions; including, oocyte-cumulus cell (CC) gap-junctional communication, and bidirectional paracrine signalling between the two cell types. It is becoming clear that, compared to oocytes matured in vivo, IVM oocytes undergo maturation prematurely as they are still in the process of acquiring developmental competence in vivo, and the molecular cascade reinitiating meiosis differs entirely to that in vivo. Attempts to enhance oocyte developmental competence by attenuating the spontaneous meiotic resumption of oocytes in vitro have been met with mixed success. Kinase inhibitors that prevent maturation-promoting factor activity have, in general, been ineffectual on promoting oocyte developmental potential post-IVM. In contrast, agents that modulate oocyte cAMP during IVM show greater potential, possibly as these compounds extend oocyte-CC gap-junctional communication. An important concept that is now emerging is that the oocyte secretes potent growth factors that regulate fundamental aspects of CC function and thereby determine the distinctive phenotype of the cumulus-oocyte complex. The capacity of an oocyte to regulate its own microenvironment by oocyte-secreted factors (OSFs) may constitute an important component of oocyte developmental competence. In support of this notion, we have recently demonstrated that supplementing IVM media with exogenous OSFs improves oocyte developmental potential, as evidenced by enhanced pre- and post-implantation embryo development.

The effect of glucosamine concentration on the development and sex ratio of bovine embryos

Glucosamine is a component of hyaluronic acid and an alternative substrate to glucose for the extracellular matrix synthesis of COCs. Its addition to an IVM medium reduces the glucose consumption of bovine COCs. Glucosamine is also metabolized to UDP-N-acetyl glucosamine (UDP-GlcNAc) via the hexosamine biosynthesis pathway and is utilized for O-linked glycosylation by the X-linked enzyme, O-linked GlcNAc transferase (OGT). Moreover, the inactivation of the second X chromosome in female embryos is influential in producing the sex ratio bias observed in vitro when embryos are cultured in the presence of glucose above 2.5mM. Accordingly, the aim of this study is to examine whether the presence of glucosamine during maturation or embryo culture causes a sex ratio bias in bovine blastocysts. Glucosamine was added to the medium in three different embryo developmental periods: in vitro maturation, the one-cell to eight-cell stage (before the maternal-zygotic transition, MZT), and the eight-cell to blastocyst stage (after MZT). When glucosamine was added during in vitro maturation, the developmental competence of oocytes was severely compromised. However, the sex ratio of embryos was not influenced. When glucosamine was added to embryo culture medium during development from one-cell to eight-cell stage (before MZT), it affected neither the development nor the sex ratio of bovine embryos. Finally, when glucosamine was added after MZT, the development rate of embryos was severely decreased, and the sex ratio was skewed toward males. Moreover, an inhibitor of OGT, benzyl-2-acetamido-2-deoxy-alpha-D-galactopyranoside (BADGP), negated the effect of glucosamine on the sex ratio when it was added to embryo culture medium from the eight-cell to blastocyst stage (after MZT). These results suggest that, like glucose, the supplementation of glucosamine into the medium skewed the sex ratio to males and that OGT, an X-linked enzyme, was involved in this phenomenon. Moreover, this effect of glucosamine was limited only to when it was present in the embryo culture medium after MZT.

Adenosine 5'-monophosphate kinase-activated protein kinase (PRKA) activators delay meiotic resumption in porcine oocytes

Adenosine monophosphate-activated kinase (PRKA) is a serine/threonine kinase that functions as a metabolic switch in a number of physiological functions. The present study was undertaken to assess the role of this kinase in nuclear maturation of porcine oocytes. RT-PCR and immunoblotting revealed the expression of the PRKAA1 subunit in granulosa cells, cumulus-oocyte complexes (COC), and denuded oocytes (DO). Porcine COC and DO contained transcripts that corresponded to the expected sizes of the designed primers for PRKAB1 and PRKAG1. The PRKAA2 subunit was detected in granulosa cells and COC, whereas the PRKAG3 subunit was not detected in granulosa cells, COC or DO, whereas it was detected in the heart. The PRKAA1 protein was detected in granulosa cells, COC, DO, and zona pellucida (ZP). In the presence of the pharmacological activator of PRKA 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranosyl 5'-monophosphate (ZMP), COC were transiently maintained in meiotic arrest in a fully reversible manner. This inhibitory effect was not observed in DO. Other known PRKA activators, 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) and metformin, also blocked meiotic resumption in COC. In contrast to mouse oocytes, in which PRKA activators reverse the inhibitory effect of PDE3 inhibitors, this combination still blocked meiotic resumption in porcine COC. These results demonstrate that the meiotic resumption of porcine COC is transiently blocked by PRKA activators in a dose-dependent manner, and that this effect is dependent on PRKA activity in cumulus cells. The present study describes a new role for PRKA in regulating meiotic resumption in COC and strongly suggests that cumulus cells play an essential role in the control of porcine oocyte maturation through the PRKA metabolic switch.

Oocyte-secreted factors enhance oocyte developmental competence

The capacity of fully grown oocytes to regulate their own microenvironment by paracrine factors secreted by the oocyte (oocyte-secreted factors, OSFs) may in turn contribute to oocyte developmental competence. Here, we investigated if OSFs have a direct influence on oocyte developmental competence during in vitro maturation (IVM). Bovine cumulus-oocyte complexes (COCs) were aspirated from abattoir-derived ovaries and matured in serum-free medium. COCs were either co-cultured with denuded oocytes (DOs) or treated with specific OSFs: recombinant bone morphogenetic protein 15 (BMP15) and/or growth differentiation factor 9 (GDF9). Following maturation, embryos were fertilized and cultured in vitro and blastocyst development and cell number were assessed on day 8. Co-culturing intact COCs with DOs did not affect cleavage rate, but increased (P<0.001) the proportion of cleaved embryos that reached the blastocyst stage post-insemination from 39% to 51%. OSFs also altered blastocyst cell allocation as co-culture of COCs with DOs significantly increased total and trophectoderm cell numbers, compared to control COCs. BMP15 alone, GDF9 alone or the two combined all (P<0.05) increased the proportion of oocytes that reached the blastocyst stage post-insemination from 41% (controls) to 58%, 50% and 55%, respectively. These results were further verified in neutralization experiments of the exogenous growth factors and of the native OSFs. Follistatin and the kinase inhibitor SB-431542, which antagonize BMP15 and GDF9, respectively, neutralized the stimulatory effects of the exogenous growth factors and impaired the developmental competence of control COCs. These results demonstrate that OSFs, and particularly BMP15 and GDF9, enhance oocyte developmental competence and provide evidence that OSF regulation of the COC microenvironment is an important determinant of oocyte developmental programming.

Mathematical modelling of oxygen concentration in bovine and murine cumulus–oocyte complexes

Immature oocytes benefit from nutrient modification of the follicular environment by the surrounding cumulus mass. However, the oxygen concentration that the oocyte may be exposed to could be lower than the antral follicular concentration due to the metabolism of surrounding cumulus cells. Using metabolic data previously determined, we have developed a mathematical model of O2diffusion across the bovine and murine cumulus–oocyte complex. From this we have determined that across a physiological range of external pO2, less than 0.25% and 0.5% O2is removed by cumulus cells within the bovine and murine cumulus–oocyte complex respectively. Our model differs from others as it: incorporates a term that allows for nonlinear variation of the oxygen consumption rate with oxygen concentration; considers two regions (oocyte and cumulus) sharing a common boundary, both of which consume oxygen at different non linear rates. Cumulus cells therefore remove little O2, thus sparing this essential gas for the oocyte, which is dependent on ATP generation via oxidative phosphorylation.

Glucosamine Supplementation During In Vitro Maturation Inhibits Subsequent Embryo Development: Possible Role of the Hexosamine Pathway as a Regulator of Developmental Competence1

Glucose concentration during cumulus-oocyte complex (COC) maturation influences several functions, including progression of oocyte meiosis, oocyte developmental competence, and cumulus mucification. Glucosamine (GlcN) is an alternative hexose substrate, specifically metabolized through the hexosamine biosynthesis pathway, which provides the intermediates for extracellular matrix formation during cumulus cell mucification. The aim of this study was to determine the influence of GlcN on meiotic progression and oocyte developmental competence following in vitro maturation (IVM). The presence of GlcN during bovine IVM did not affect the completion of nuclear maturation and early cleavage, but severely perturbed blastocyst development. This effect was subsequently shown to be dose-dependent and was also observed for porcine oocytes matured in vitro. Hexosamine biosynthesis upregulation using GlcN supplementation is well known to increase O-linked glycosylation of many intracellular signaling molecules, the best-characterized being the phosphoinositol-3-kinase (PI3K) signaling pathway. We observed extensive O-linked glycosylation in bovine cumulus cells, but not oocytes, following IVM in either the presence or the absence of GlcN. Inhibition of O-linked glycosylation significantly reversed the effect of GlcN-induced reduction in developmental competence, but inhibition of PI3K signaling had no effect. Our data are the first to link hexosamine biosynthesis, involved in cumulus cell mucification, to oocyte developmental competence during in vitro maturation.

The Impact of Nutrition of the Cumulus Oocyte Complex and Embryo on Subsequent Development in Ruminants

Cumulus-oocyte complexes (COCs) and early embryos rely on a histotrophic nutrition source for energy production and the synthesis of macromolecules. There is accumulating evidence suggesting that the balance of supply and demand for energy and other anabolic substrates during oocyte maturation and very early stages of development programmes subsequent developmental potential, and this may include subsequent fetal growth trajectory. One example is the role of glucose (Glc) during cumulus-oocyte complex maturation. Glucose is an essential nutrient for maturation, especially its role during cumulus expansion. Our laboratory has shown that during in vitro culture, too little glucose during cumulus-oocyte complex maturation affects meiotic competence. We have focussed on glucose (Glc) metabolism through the hexosamine biosynthesis pathway (HBP) during COC maturation in vitro. The HBP in somatic cells is regarded as a "fuel-sensing" pathway and its interaction with cell signalling systems and transcriptional regulation is increasingly apparent. Up-regulation of the HBP during oocyte maturation in vitro has negative consequences for subsequent development. Another example is the role of hypoxia (low O2) during peri-compaction development. My laboratory believes that ruminant embryos during compaction, blastulation and subsequent development in the uterine cavity lack a key hypoxia responsive element. Because of this, hypoxia is important for normal development in ruminants but perturbs further development in rodents. The implication of these examples to the fundamental concept of peri-conception nutritional programming of development are discussed.