Involvement of purine nucleotide synthetic pathways in gonadotropin-induced meiotic maturation in mouse cumulus cell-enclosed oocytes

Transcriptome analysis of ovarian tissues highlights genes controlling energy homeostasis and oxidative stress as potential drivers of heterosis for egg number and clutch size in crossbred laying hens

Heterosis is the major benefit of crossbreeding and has been exploited in laying hens breeding for a long time. This genetic phenomenon has been linked to various modes of nonadditive gene action. However, the molecular mechanism of heterosis for egg production in laying hens has not been fully elucidated. To fill this research gap, we sequenced mRNAs and lncRNAs of the ovary stroma containing prehierarchical follicles in White Leghorn, Rhode Island Red chickens as well as their reciprocal crossbreds that demonstrated heterosis for egg number and clutch size. We further delineated the modes of mRNAs and lncRNAs expression to identify their potential functions in the observed heterosis. Results showed that dominance was the principal mode of nonadditive expression exhibited by mRNAs and lncRNAs in the prehierarchical follicles of crossbred hens. Specifically, low-parent dominance was the main mode of mRNA expression, while high-parent dominance was the predominant mode of lncRNA expression. Important pathways enriched by genes that showed higher expression in crossbreds compared to either one or both parental lines were cell adhesion molecules, tyrosine and purine metabolism. In contrast, ECM-receptor interaction, focal adhesion, PPAR signaling, and ferroptosis were enriched in genes with lower expression in the crossbred. Protein network interaction identified nonadditively expressed genes including apolipoprotein B (APOB), transferrin, acyl-CoA synthetase medium-chain family member (APOBEC) 3, APOBEC1 complementation factor, and cathepsin S as hub genes. Among these potential hub genes, APOB was the only gene with underdominance expression common to the 2 reciprocal crossbred lines, and has been linked to oxidative stress. LncRNAs with nonadditive expression in the crossbred hens targeted natriuretic peptide receptor 1, epidermal differentiation protein beta, spermatogenesis-associated gene 22, sperm-associated antigen 16, melanocortin 2 receptor, dolichol kinase, glycine amiinotransferase, and prolactin releasing hormone receptor. In conclusion, genes with nonadditive expression in the crossbred may play crucial roles in follicle growth and atresia by improving follicle competence and increasing oxidative stress, respectively. These 2 phenomena could underpin heterosis for egg production in crossbred laying hens.

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.

Effects of glucose metabolism pathways on nuclear and cytoplasmic maturation of pig oocytes

The developmental competence of IVM porcine oocytes is still low compared with that in their in vivo counterparts. Although many studies reported effects of glucose metabolism (GM) on oocyte nuclear maturation, few reported on cytoplasmic maturation. Previous studies could not differentiate whether GM of cumulus cells (CCs) or that of cumulus-denuded oocytes (DOs) supported oocyte maturation. Furthermore, species differences in oocyte GM are largely unknown. Our aim was to address these issues by using enzyme activity inhibitors, RNAi gene silencing and special media that could support nuclear but not cytoplasmic maturation when GM was inhibited. The results showed that GM in CCs promoted pig oocyte maturation by releasing metabolites from both pentose phosphate pathway and glycolysis. Both pyruvate and lactate were transferred into pig DOs by monocarboxylate transporter and pyruvate was further delivered into mitochondria by mitochondrial pyruvate carrier in both pig DOs and CCs. In both pig DOs and CCs, pyruvate and lactate were utilized through mitochondrial electron transport and LDH-catalyzed oxidation to pyruvate, respectively. Pig and mouse DOs differed in their CC dependency for glucose, pyruvate and lactate utilization. While mouse DOs could not, pig DOs could use the lactate-derived pyruvate.

Follicular environment as a predictive tool for embryo development and kinetics in cattle

Follicular fluid composition and the transcription pattern of granulosa cells were analysed to better comprehend associations between embryo development and morphokinetics. Bovine follicles were punctured and their respective follicular fluid and granulosa cells were collected. Cumulus-oocyte complexes derived from these follicles were matured and fertilised invitro. Embryo morphology and kinetics were evaluated at 40h after insemination, when embryos were classified as fast (FCL, four or more cells), slow (SCL, 2-3 cells) or non-cleaved (NCL). Their development was followed until the blastocyst stage. Glucose, pyruvate, cholesterol and oestradiol were quantified in the follicular fluid and the transcription pattern of 96 target genes was evaluated in granulosa cells by large-scale quantitative reverse transcription polymerase chain reaction. Follicular fluid from the blastocyst group had increased levels of glucose, total cholesterol and pyruvate compared to the non-blastocyst group, whereas higher levels of oestradiol were observed in the follicular fluid of embryos and blastocysts with fast cleavage. The transcriptional pattern revealed altered metabolic pathways between groups, such as lipid metabolism, cellular stress and cell signalling. In conclusion, both follicular fluid and granulosa cells are associated with the possibility of identifying follicles that may generate embryos with high potential to properly develop to the blastocyst stage.

Adenosine Modulates the Oocyte Developmental Competence by Exposing Stages and Synthetic Blocking during In Vitro Maturation

Purine metabolism is known factor for nuclear maturation of oocytes through both follicle cells and oocyte itself. However, it is largely unknown the roles of purine metabolism in the oocyte competence for fertilization and early development. In this study, the effects of adenosine in oocyte competence for development were examined using adenosine and its synthetic inhibitors. Adenosine treatment from GV intact stage for 18 hr (fGV) caused of decrease the fertilization rate but of increase the cleavage rate compared from the other stage treatment groups. Hadacidin did not effect on fertilization rate but increased cleavage rate without stage specificity. Adenosine did not block the effects of hadacidin with the exception of fGV group. By the inhibition of purine synthetic pathways the fertilization rate was decreased in the fGV and fGVB groups but increased in the fMII group. Exogenous adenosine caused of decrease fertilization rate in the fGVB group but increase in the fMII group. Cleavage rate was dramatically increased in the adenosine treatment with synthetic inhibitors. It means that the metabolism of purine has stage specific effects on fertilization and cleavage. Exogenous adenosine had only can improve oocyte developmental competence when it treated at GV intact stage. On the other hand, endogenous synthesis in all maturation stage caused of increase the cleavage rate without effects on fertilization. These data suggest that adenosine at GV stage as a paracrine fashion and inhibitions of endogenous adenosine in all stage improve oocyte developmental competence..

Effects of glucose metabolism during in vitro maturation on cytoplasmic maturation of mouse oocytes

Although there are many reports on the effect of glucose metabolism on oocyte nuclear maturation, there are few studies on its effect on ooplasmic maturation. By manipulating glucose metabolism pathways using a maturation medium that could support oocyte nuclear maturation but only a limited blastocyst formation without glucose, this study determined effects of glucose metabolism pathways on ooplasmic maturation. During maturation of cumulus-oocyte-complexes (COCs) with glucose, the presence of PPP inhibitor, DHEA or glycolysis inhibitor, iodoacetate significantly decreased blastocyst rates, intraoocyte glutathione and ATP. While blastocyst rates, GSH/GSSG ratio and NADPH were higher, ROS was lower significantly in COCs matured with iodoacetate than with DHEA. Fructose-6-phosphate overcame the inhibitory effect of DHEA on PPP. During maturation of COCs with pyruvate, electron transport inhibitor, rotenone or monocarboxylate transfer inhibitor, 4-CIN significantly decreased blastocyst rates. Cumulus-denuded oocytes had a limited capacity to use glucose or lactate, but they could use pyruvate to support maturation. In conclusion, whereas glycolysis promoted ooplasmic maturation mainly by supplying energy, PPP facilitated ooplasmic maturation to a greater extent by both reducing oxidative stress and supplying energy through providing fructose-6-phosphate for glycolysis. Pyruvate was transferred by monocarboxylate transporters and utilized through mitochondrial electron transport to sustain ooplasmic maturation.

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.

Transcriptomic landscape of prophase I sunflower male meiocytes

Meiosis is a form of specialized cell division that generates gametes, allowing recombination of alleles and halving the chromosome number. Arabidopsis and maize are the plant models that have been most extensively studied to determine the genes involved in meiosis. Here we present an RNA-seq study in which gene expression in male meiocytes isolated during prophase I was compared to that in somatic tissues of the sunflower HA89 line. We sampled more than 490 million gene tags from these libraries, assembled them de novo into a sunflower transcriptome. We obtained expression data for 36,304 sunflower genes, of which 19,574 (54%) were differentially expressed (DE) between meiocytes and somatic tissue. We also determined the functional categories and metabolic pathways that are DE in these libraries. As expected, we found large differences between the meiotic and somatic transcriptomes, which is in accordance with previous studies in Arabidopsis and maize. Furthermore, most of the previously implicated meiotic genes were abundantly and DE in meiocytes and a large repertoire of transcription factors (TF) and genes related to silencing are expressed in the sunflower meiocytes. We detected TFs which appear to be exclusively expressed in meiocytes. Our results allow for a better understanding of the conservation and differences in the meiotic transcriptome of plants.

Folic acid facilitates in vitro maturation of mouse and Xenopus laevis oocytes

The water-soluble B vitamins, folate and folic acid, play an important role in reproductive health, but little is known about the effects of folic acid on infertility. The present study tested the hypothesis that folic acid affects oocyte maturation, a possible cause of female infertility. We have studied the in vitro maturation of mouse and Xenopus oocytes. Hypoxanthine (Hx) was used as an inhibitor of mouse oocyte maturation to mimic in vivo conditions by maintaining high levels of cyclic-AMP. The frequency of first polar body (PB1) formation and germinal vesicle breakdown (GVBD) in mouse oocytes was decreased by Hx. This effect was counteracted by folic acid added to the medium. PB1 extrusion and GVBD percentages rose to 27·7 and 40·0% from 12·8 and 19·9%, respectively, by exposure to 500 μM-folic acid. Folic acid also restored the spindle configuration, which had been elongated by Hx, as well as normalising the distribution of cortical granules (CG). In folic acid-treated Xenopus eggs, extracellular signal-regulated kinase 1 was phosphorylated, cyclin B2 and Mos were up-regulated and the frequency of GVBD was accelerated. Taken together, the findings suggest that folic acid facilitates oocyte maturation by altering the expression and phosphorylation of proteins involved in M-phase-promoting factor and mitogen-activated protein kinase pathways, as well as causing changes in spindle configuration and CG migration.

Function of the pentose phosphate pathway and its key enzyme, transketolase, in the regulation of the meiotic cell cycle in oocytes

Objective

Previously, we identified that transketolase (Tkt), an important enzyme in the pentose phosphate pathway, is highly expressed at 2 hours of spontaneous maturation in oocytes. Therefore, this study was performed to determine the function of Tkt in meiotic cell cycle regulation, especially at the point of germinal vesicle breakdown (GVBD).

Methods

We evaluated the loss-of-function of Tkt by microinjecting Tkt double-stranded RNAs (dsRNAs) into germinal vesicle-stage oocytes, and the oocytes were cultured in vitro to evaluate phenotypic changes during oocyte maturation. In addition to maturation rates, meiotic spindle and chromosome rearrangements, and changes in expression of other enzymes in the pentose phosphate pathway were determined after Tkt RNA interference (RNAi).

Results

Despite the complete and specific knockdown of Tkt expression, GVBD occurred and meiosis was arrested at the metaphase I (MI) stage. The arrested oocytes exhibited spindle loss, chromosomal aggregation, and declined maturation promoting factor and mitogen-activated protein kinase activities. The modified expression of two enzymes in the pentose phosphate pathway, Prps1 and Rbks, after Tkt RNAi and decreased maturation rates were amended when ribose-5-phosphate was supplemented in the culture medium, suggesting that the Tkt and pentose phosphate pathway are important for the maturation process.

Conclusion

We concluded that Tkt and its associated pentose phosphate pathway play an important role in the MI-MII transition of the oocytes' meiotic cell cycle, but not in the process of GVBD.

Proteomic screen for potential regulators of M-phase entry and quality of meiotic resumption in Xenopus laevis oocytes

The quality of oocytes depends largely on the capacity to resume meiotic maturation. In Xenopus laevis, only fully grown oocytes react to progesterone stimulation by resumption of meiotic maturation associated with the entry into the meiotic M-phase. Proteins involved in this process are poorly known. To identify novel proteins regulating M-phase entry, we performed a differential proteomic screen. We compared proteomes of fully grown stage VI oocytes characterized as poorly or highly responsive to progesterone treatment. The comparison of 2-D gels allowed us to identify several spots including two specifically present in highly responsive oocytes and two specifically present in poorly responsive ones. By mass spectrometry we identified the two proteins specifically present in highly responsive oocytes as inosine 5'monophosphate cyclohydrolase and YjgF homologues, and the two specifically present in poorly responsive oocytes as elongation factor 2 (EF2) and S-adenosyl-L-homocysteine hydrolase (SAHH). The proteins specifically expressed in highly responsive oocytes may participate in the stimulation of meiotic maturation and M-phase entry, while the proteins specifically present in poorly maturing oocytes may participate in the inhibition of meiotic resumption.

Effects of adenosine monophosphate-activated kinase activators on bovine oocyte nuclear maturation in vitro

The purpose of this study was to examine the effects of an activator of AMPK (5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR)) on bovine oocyte nuclear maturation in vitro. After 7 hr of culture, AICAR (1 mM) significantly increased the percentages of cumulus-enclosed oocytes (CEO) and denuded oocytes (DO) remaining at the germinal vesicle stage. After 22 hr of culture, AICAR significantly reduced the percentage of CEO reaching metaphase II (MII). AICAR at 1.0 mM also increased the inhibitory effect of the adenylate cyclase activator forskolin in CEO; however, at 0.05 mM, AICAR increased the percentage of oocytes at MII after 22 hr of culture compared to forskolin alone. The adenosine kinase inhibitor 5'-aminodeoxyadenosine reversed the effect of AICAR in CEO and DO showing that phosphorylation of AICAR by adenosine kinase is required for its inhibitory activity. GMP, but not AMP, inhibited meiosis in CEO and DO; however, inhibition of guanyl and adenyl nucleotides synthesis did not reverse the effect of AICAR suggesting that the inhibitory effect of AICAR is not due to increased synthesis of these nucleotides. Metformin, another activator of AMPK, also inhibited GVBD in CEO and DO. The alpha-1 isoform of the catalytic subunit of AMPK was detected in oocytes and cumulus cells, and reverse transcription-polymerase chain reaction experiments showed the presence of transcripts for alpha-1, alpha-2, beta-1, and gamma-3 isoforms of the regulatory subunits in cumulus cells and oocytes. These data show that the AMPK activator AICAR is inhibitory to nuclear maturation in bovine oocytes due to activation of AMPK.

A comparative analysis of metabolism and viability in porcine oocytes during in vitro maturation

The importance of oocyte quality cannot be overstated, because it impacts all subsequent events during development of the embryo, the fetus and even the resulting offspring. Oocyte metabolism plays a critical role in supporting developmental competence via multiple mechanisms. It is beginning to be understood that metabolic pathways not only affect cytoplasmic maturation but may control nuclear maturation as well. A complete understanding of the precise roles that metabolism plays in determining oocyte quality is crucial for developing efficient in vitro maturation systems to support acquisition of oocyte competence. To date, this pursuit has not been entirely successful. Work in our laboratory on porcine oocyte metabolism has elucidated some of the intricate control mechanisms at work within the oocyte, not only for energy production, but also encompassing progression of nuclear maturation, mitochondrial activity and distribution, and oxidative and ionic stresses. We hypothesize that by utilizing oocyte metabolic data, we can develop more appropriate in vitro maturation systems that result in increased oocyte and embryo developmental competence.

Induction of meiotic maturation in mouse oocytes by adenosine analogs

In this study we have examined the meiosis-inducing influence of adenosine analogs in mouse oocytes. When a varied group of nucleosides and nucleotides were tested on overnight cultures of hypoxanthine-arrested, cumulus cell-enclosed oocytes (CEO), halogenated adenosine nucleosides, but not native adenosine, exhibited a significant meiosis-inducing capability. When tested under a variety of conditions, meiotic induction by 8-bromo-adenosine (8-Br-Ado) and a second adenosine analog, methylmercaptopurine riboside (MMPR), was especially potent in denuded oocytes (DO) compared to CEO and was not dependent on the type of inhibitor chosen to maintain meiotic arrest. Germinal vesicle breakdown (GVB) was stimulated with rapid kinetics and was preceded by an increase in AMP-activated protein kinase (AMPK) activity. Moreover, compound C, an inhibitor of AMPK, blocked the meiosis-inducing activities of both adenosine analogs. When tested for an effect on meiotic progression to metaphase II (MII) in spontaneously maturing CEO, 8-Br-Ado and the AMPK activator, 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR), increased the percentage of MII-stage oocytes, but MMPR decreased this number. Adenosine and inhibitors of de novo purine synthesis had no effect on the completion of maturation, while compound C suppressed this process. These results support the proposition that oocyte AMPK mediates the positive influence of AICAR and 8-Br-Ado on both the initiation and completion of meiotic maturation. The role of AMPK in MMPR action is less clear.

Regulation of meiotic maturation

Mammalian oocytes are arrested at prophase of the first meiotic division before induction of maturation by the preovulatory LH surge. In vitro, oocyte maturation occurs spontaneously. The first meiotic arrest is characterized by a large nucleus called the germinal vesicle. One important signaling molecule for resumption of meiosis is cyclic AMP (cAMP). High levels of cAMP block spontaneous meiotic resumption. Research investigating the regulation of oocyte cAMP has led to the discovery of new receptors, guanosine 5'-triphosphate-binding (G) proteins, cyclases, and phosphodiesterases. Leydig insulin-like 3, a polypeptide growth factor of the insulin family, is expressed in thecal cells. Leydig insulin-like 3 activates the Leu-rich, repeat-containing, G protein-coupled receptor 8, which is expressed in the oocyte. Coupled to the inhibitory GTP binding protein, this receptor leads to a decrease in cAMP production. Treatment with Leydig insulin-like 3 polypeptide initiates meiotic progression of oocytes in preovulatory follicles, demonstrating the importance of cAMP management for meiotic resumption. Furthermore, microinjection of an antibody against stimulatory G protein (Gs) into mouse oocytes results in meiotic resumption, suggesting that meiotic arrest of the oocyte is dependent on Gs activity. The orphan Gs-linked receptor, GPR3, is expressed in the oocyte. The oocytes of GPR3-null mice resume meiosis when still in their follicles, suggesting that GPR3 is involved in the control of cAMP production and thus meiotic arrest. Cyclic nucleotides are synthesized by cyclases and degraded by phosphodiesterases. Mouse and rat oocytes express isoform 3 of adenylyl cyclase. In the mouse, the null mutation results in approximately 50% of the oocytes resuming meiosis, demonstrating the importance of the synthesis of cAMP in controlling nuclear maturation. The null mutation of the major phosphodiesterase expressed in mouse oocytes results in female sterility due to ovulation of meiotically arrested oocytes that cannot be fertilized. Maintenance of meiotic arrest is explained by constitutive cAMP signaling associated with undetectable cAMP-phosphodiesterase activity. Collectively, these results are beginning to illuminate the key signaling molecules involved in the control of intraoocyte cAMP levels, thus regulating the arrest and resumption of meiosis.

Chemical manipulation of glucose metabolism in porcine oocytes: effects on nuclear and cytoplasmic maturation in vitro

The objectives of this study were to manipulate metabolism of glucose through glycolysis and the pentose phosphate pathway (PPP) in porcine oocytes during in vitro maturation, and determine the effects of this manipulation on meiotic progression, intracellular glutathione (GSX) concentrations and embryonic development. Cumulus-oocyte complexes isolated from abattoir ovaries were matured (40–44 h) in Purdue Porcine Medium for maturation alone (control) or supplemented with pyrroline-5 carboxylate (PC, 0.1 μM; PPP stimulator), diphenyleneiodonium (DPI, 0.1 μM; PPP inhibitor), dinitrophenol (DNP, 10 μM; glycolytic stimulator), hexametaphosphate (HMP, 100 μM; glycolytic inhibitor), PC + HMP or DNP + DPI. At the conclusion of in vitro maturation, cumulus cells were removed and oocytes were randomly allocated for analysis of GSX, metabolism and nuclear maturation, or in vitro fertilization and embryo culture. Both DPI and DNP + DPI decreased (P ≤ 0.05) the activity of glycolysis and the PPP, increased (P ≤ 0.05) the percentage of immature oocytes, and decreased (P ≤ 0.05) the proportion of mature oocytes compared with control oocytes and oocytes from the other treatments. Embryonic development (cleavage and blastocyst stage) and the intracellular content of GSX were also decreased (P ≤ 0.05) following exposure to DPI or DNP + DPI compared with control oocytes and oocytes from the other treatments. Oocyte metabolism, nuclear maturation, GSX content and embryonic development were unaffected (P > 0.05) following exposure to PC, DNP, HMP or PC + HMP. Our results suggest that metabolism of glucose through the PPP and/or glycolysis plays a key role in the control of nuclear and cytoplasmic maturation of porcine oocytes in vitro.

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

In vitro oocyte maturation (IVM) culture conditions have been relatively unchanged over the past few decades and remain suboptimal. In contrast, studies of the in vivo environment have led to significant improvements to in vitro embryo culture technologies. The aim of the present study was to determine the effect of maturing bovine cumulus–oocyte complexes (COCs) in medium based on the composition of bovine follicular fluid (Bovine VitroMat; Cook Australia, Eight Mile Plain, Qld, Australia). In particular, the effect of different glucose concentrations and glucosamine supplementation on meiotic maturation was determined. Culturing COCs in the presence of gonadotrophins in Bovine VitroMat, containing either physiological glucose concentrations (2.3 mm) or 5.6 mm (equivalent to levels in Tissue Culture Medium 199 (TCM199)) supplemented with glucosamine resulted in comparable cumulus expansion to COCs cultured in TCM199 plus gonadotrophins. However, nuclear maturation was 1.3-fold lower in Bovine VitroMat cultures containing 2.3 mm glucose compared with 5.6 mm glucose and this effect was independent of glucosamine supplementation. Investigations into the effects of different glucose concentrations and gonadotrophin supplementation during the initial 6 h of maturation demonstrated that COCs cultured in Bovine VitroMat with 5.6 mm glucose without gonadotrophins had a twofold acceleration of the rate of meiotic resumption, yet the rate of polar body formation was decreased by approximately 20% compared with cultures in 2.3 mm glucose and TCM199. However, this effect was not seen when COCs were cultured for the initial 16 h in Bovine VitroMat + 5.6 mm minus gonadotrophins or in Bovine VitroMat + 2.3 mm glucose ± gonadotrophins. These data demonstrate that glucose concentrations and the timing of the introduction of gonadotrophin during IVM have variable effects on nuclear maturation. Manipulation of glucose concentrations may be a mechanism to influence oocyte meiotic progression and may lead to the development of improved IVM systems, allowing for an increased developmental capacity of bovine oocytes.

Potential role for the sorbitol pathway in the meiotic dysfunction exhibited by oocytes from diabetic mice

Complications common to type I diabetes, such as cataracts and cardiovascular disorders, have been associated with activation of the polyol pathway, which converts glucose to fructose via the intermediate, sorbitol. Under normal glycemic conditions, glucose is typically targeted for glycolysis or the pentose phosphate pathway through phosphorylation by hexokinase. When glucose levels are elevated under diabetic conditions, hexokinase becomes saturated, and the excess glucose is then shunted to aldose reductase, which converts glucose to sorbitol. In the present study, we examined the potential effects of this pathway on the maturation process in mouse oocytes. Increasing concentrations of sorbitol suppressed FSH-induced maturation in oocytes from control mice. Culturing oocytes from diabetic mice in the presence of inhibitors of aldose reductase reversed the suppression of FSH-induced meiotic maturation. When oocytes from control mice were cultured with activators of aldose reductase, FSH-induced maturation was compromised. In addition, treatment with sorbitol or activators of the polyol pathway led to reduced cell-cell communication between the oocyte and the cumulus cells, as well as compromised FSH-mediated cAMP production and de novo purine synthesis. These data indicate that the suppression of FSH-induced meiotic maturation observed in oocytes from diabetic mice may result from a shunting of glucose through the polyol pathway.

Protein Kinase C and Mitogen-Activated Protein Kinase Cascade in Mouse Cumulus Cells: Cross Talk and Effect on Meiotic Resumption of Oocyte1

Protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) in cumulus cells are involved in FSH-induced meiotic resumption of cumulus-enclosed oocytes (CEOs), but their regulation and cross talk are unknown. The present experiments were designed to investigate 1) the possible involvement of MAPK cascade in PKC-induced meiotic resumption; 2) the regulation of PKC on MAPK activity in FSH-induced oocyte maturation; and 3) the pattern of PKC and MAPK function in induced meiotic resumption of mouse oocytes. PKC activators, phorbol 12-myristate 13-acetate (PMA) and 1-oleoyl-2-acetyl-sn-glycerol (OAG), induced the meiotic resumption of CEOs and activation of MAPK in cumulus cells, whereas this effect could be abolished by PKC inhibitors, calphostin C and chelerythrine, or MEK inhibitor U0126. These results suggest that PKC might induce the meiotic reinitiation of CEOs by activating MAPK in cumulus cells. Both PKC inhibitors and U0126 inhibited the FSH-induced germinal vesicle breakdown (GVBD) of oocytes and MAPK activation in cumulus cells, suggesting that PKC and MAPK are involved in FSH-induced GVBD of mouse CEOs. Protein synthesis inhibitor cycloheximide (CHX) inhibited FSH- or PMA-induced oocyte meiotic resumption, but not the MAPK activation in cumulus cells. FSH and PKC activators induced the GVBD in denuded oocytes cocultured with cumulus cells in hypoxanthine (HX)-supplemented medium, and this effect could be reversed by U0126. Thus, when activated by FSH and PKC, MAPK may stimulate the synthesis of specific proteins in cumulus cells followed by secretion of an unknown positive factor that is capable of inducing GVBD in oocytes.

Physiological changes in oocyte-cumulus cell complexes from diabetic mice that potentially influence meiotic regulation

We have previously shown that the type I diabetic condition significantly alters meiotic regulation in mouse oocytes. In the present study, possible physiological deficiencies underlying such meiotic dysfunction were examined in oocyte-cumulus cell complexes (OCC) from type I diabetic mice. Whereas the diabetic condition did not affect glycolysis or the tricarboxylic acid cycle, the increased flux of glucose through the pentose phosphate pathway in response to FSH treatment was suppressed. De novo purine synthesis was also compromised, and ATP levels were reduced in freshly isolated OCC. Additionally, diabetes resulted in a reduction in FSH-mediated cAMP synthesis. The responsiveness of the oocyte to cAMP was also affected; fewer oocytes were induced to resume maturation after a stimulatory pulse with cAMP analogs. Meiotic induction triggered by FSH was significantly reduced, but that stimulated by phorbol ester or epidermal growth factor was affected to a much lesser extent. In addition to metabolic deficiencies, the cell-cell communication between the oocyte and the cumulus cells was reduced in diabetic mice as determined by coupling assays. Thus, numerous physiological parameters are affected by type I diabetes, and these changes may collectively contribute to altered meiotic regulation.

Glutamine and the maintenance of meiotic arrest in mouse oocytes: influence of culture medium, glucose, and cumulus cells

The selection of culture media and supplements therein has a tremendous impact on the regulation of oocyte maturation in vitro. In the present study, we have evaluated how altering the levels of glutamine in the presence or absence of glucose affects meiotic arrest in cumulus cell-enclosed oocytes (CEO) and denuded oocytes (DO) when cultured in either the simple medium M16 or the more complex Eagle's minimum essential medium (MEM). We have also tested the effectiveness of follicle-stimulating hormone (FSH) in triggering germinal vesicle breakdown (GVB) and purine de novo synthesis in differing MEM culture conditions. When DO were cultured 17-18 hr in hypoxanthine (HX)- or dbcAMP-supplemented M16 medium, neither glucose nor glutamine had any effect on oocyte maturation, with dbcAMP the more effective inhibitor. In the absence of glutamine, cumulus cells promoted meiotic resumption, since significantly lower levels of meiotic arrest were maintained in CEO than in DO by either HX or dbcAMP, but addition of the amino acid dose-dependently decreased the maturation percentage in CEO below that observed in DO. In MEM, glutamine and glucose again had little effect on the maturation of DO, although the percentage of maturing DO in HX-supplemented medium was about 20% lower than that in M16 medium. In the absence of glucose, high levels of maturation were observed in CEO in glutamine-free medium that were dose-dependently lowered by the amino acid. However, when glucose was present, CEO were as effectively arrested as DO when glutamine was absent, with no further effect of the amino acid. This inhibitory action of glucose was dependent on the essential amino acids present in MEM. The effects of glutamine were not due to changes in metabolic coupling between the oocyte and cumulus cells. Measurement of purine de novo synthesis indicated that the maintenance of meiotic arrest as well as FSH induction of meiotic resumption were associated with increases in purine synthesis. We conclude that glucose and glutamine act cooperatively to promote the synthesis of new purine compounds within the somatic compartment and that the timing and duration of such synthesis determines whether meiotic resumption will be suppressed or promoted.

A potential role for AMP-activated protein kinase in meiotic induction in mouse oocytes

Cyclic adenosine monophosphate (cAMP) has been implicated as an important regulator of meiotic maturation in mammalian oocytes. A decrease in cAMP, brought about by the action of cAMP phosphodiesterase (PDE), is thought to initiate germinal vesicle breakdown (GVB) by the inactivation of cAMP-dependent protein kinase. However, the product of PDE activity, 5'-AMP, is a potent activator of an important regulatory enzyme, AMP-activated protein kinase (AMPK). The aim of this study was to evaluate a possible role for AMPK in meiotic induction, using oocytes obtained from eCG-primed, immature mice. Alpha-1 and -2 isoforms of the catalytic subunit of AMPK were detected in both oocytes and cumulus cells. When 5-aminoimidazole-4-carboxamide 1-beta-d-ribofuranoside (AICA riboside), an activator of AMPK, was tested on denuded oocytes (DO) and cumulus cell-enclosed oocytes (CEO) maintained in meiotic arrest by dbcAMP or hypoxanthine, GVB was dose-dependently induced. Meiotic induction by AICA riboside in dbcAMP-supplemented medium was initiated within 3 h in DO and 4 h in CEO and was accompanied by increased AMPK activity in the oocyte. AICA riboside also triggered GVB when meiotic arrest was maintained with hypoxanthine, 8-AHA-cAMP, guanosine, or milrinone, but was ineffective in olomoucine- or roscovitine-arrested oocytes, indicating that it acts upstream of maturation-promoting factor. Adenosine monophosphate dose-dependently stimulated GVB in DO when meiotic arrest was maintained with dbcAMP or hypoxanthine. This effect was not mimicked by other monophosphate or adenosine nucleotides and was not affected by inhibitors of ectophosphatases. Combined treatment with adenosine and deoxycoformycin, an adenosine deaminase inhibitor, stimulated GVB in dbcAMP-arrested CEO, suggesting AMPK activation due to AMP accumulation. It is concluded that phosphodiesterase-generated AMP may serve as a transducer of the meiotic induction process through activation of AMPK.

Low oxygen tension during in vitro maturation is beneficial for supporting the subsequent development of bovine cumulus-oocyte complexes

The effects of carbohydrates on meiotic maturation and ATP content of bovine oocytes under low oxygen tension (5%) were investigated. Furthermore, the developmental competence or intracellular H(2)O(2) contents of the oocytes matured under 5% or 20% O(2) was assessed. In vitro maturation of bovine cumulus-oocyte complexes was performed in synthetic oviduct fluid (SOF) containing 20 amino acids and hormones (SOFaa). The proportion of the oocytes that matured to the metaphase II stage in SOFaa containing 1.5 mM glucose, 0.33 mM pyruvate, and 3.3 mM lactate under 5% O(2) was dramatically lower than that of oocytes matured under 20% O(2) (P < 0.01). Similarly, the ATP content of the oocytes that matured under 5% O(2) was much lower than that of oocytes matured under 20% O(2) (P < 0.05). Under 5% O(2) the proportion of metaphase II oocytes increased with increasing glucose concentration (0-20 mM) in SOFaa without pyruvate or lactate. In addition, the ATP content of oocytes cultured in 20 mM glucose was higher (P < 0.05) than that of oocytes cultured in 1. 5 mM glucose. Two glucose metabolites (pyruvate and lactate) and a nonmetabolizable glucose analog (2-deoxy-glucose), however, had no noticeable effects on meiotic maturation under 5% O(2). These results suggest that ATP production under 5% O(2) is not dependent on the TCA cycle. Addition of iodoacetate, a glycolytic inhibitor, to SOFaa containing 20 mM glucose significantly reduced (P < 0.01) the proportion of metaphase II and ATP content. Moreover, the proportion of the development to the blastocyst stage of oocytes matured under 5% O(2) was higher (P < 0.05) than that of oocytes matured under 20% O(2). H(2)O(2) contents of oocytes matured under 5% O(2) was lower (P < 0.05) than that of oocytes matured under 20% O(2). The results of the present study demonstrate that glucose plays important roles in supporting the completion of meiotic maturation in bovine cumulus-oocyte complexes under low oxygen tension and that low oxygen tension during in vitro maturation is beneficial for supporting the subsequent development of bovine oocytes.

Multiple triggers of oocyte maturation in nemertean worms: the roles of calcium and serotonin

To analyze the process of oocyte maturation in nemertean worms, oocytes with a large nucleus (=germinal vesicle, or GV) were removed from gravid ovaries of Cerebratulus lacteus and Micrura alaskensis. Following transfer to natural seawater (NSW), fully grown oocytes spontaneously matured as indicated by their completion of germinal vesicle breakdown (GVBD), whereas GVBD was reversibly blocked if the oocytes were initially placed in calcium-free seawater (CaFSW). Similarly, calcium ionophore treatments triggered GVBD in calcium-containing artificial seawater (ASW) but not in CaFSW, suggesting that external calcium influx may facilitate maturation. However, compared to the overall levels of maturation elicited by ASW, significantly higher percentages of GVBD were achieved with NSW or with ASW that had been conditioned with marine sediment. Moreover, calcium channel blockers decreased GVBD rates in ASW but not in NSW, which is consistent with the view that substances other than external calcium ions can trigger maturation. Accordingly, oocytes underwent equally high levels of GVBD when treated with serotonin (=5-hydroxytryptamine, or 5-HT) in ASW or CaFSW. The 5-HT-induced maturation was blocked by inhibitors of 5-HT receptors but continued to occur in the presence of calcium channel blockers or the calcium chelator BAPTA. In addition, oocytes microinjected with fluorescent calcium indicators underwent GVBD in response to 5-HT without displaying marked calcium transients during confocal imaging runs. Collectively, such findings suggest that nemertean oocytes can mature via multiple pathways that may include external calcium influx or a 5-HT-induced signaling cascade that lacks prominent calcium fluctuations. J. Exp. Zool. 287:243-261, 2000.

Excessive concentration of glucose during in vitro maturation impairs the developmental competence of bovine oocytes after in vitro fertilization: relevance to intracellular reactive oxygen species and glutathione contents

The effect of glucose (0, 1.5, 5.6 or 20.0 mM) in synthetic oviduct fluid supplemented with 20 amino acids (SOFaa) on the developmental competence of bovine oocytes after in vitro fertilization was investigated. Intracellular reactive oxygen species (ROS) and the glutathione content of bovine oocytes matured in SOFaa containing 0-20.0 mM glucose were also examined. When oocytes were matured in SOFaa without glucose, the nuclear maturation rate was lower than that in oocytes matured in glucose-containing medium. The developmental competence to the blastocyst stage of oocytes matured in 1.5 mM glucose was higher than that of oocytes matured in 20.0 mM glucose. In addition, the intracellular ROS content of oocytes matured in 0, 1.5 or 5.6 mM glucose was lower than that of oocytes matured in 20.0 mM glucose. Furthermore, the intracellular glutathione content of oocytes matured in 0, 1.5 or 5.6 mM glucose was higher than that of oocytes matured in 20.0 mM glucose. These results show that excessive glucose in the medium for oocyte maturation impairs the development of bovine oocytes to the blastocyst stage, possibly due to the increase of ROS and the decrease in the intracellular glutathione content of bovine oocytes.

Adenosine blocks hormone-induced meiotic maturation by suppressing purine de novo synthesis

We have examined adenosine (Ado) suppression of FSH-induced germinal vesicle breakdown (GVB) and its relationship to purine de novo synthesis. Oocyte-cumulus cell complexes (OCC) from PMSG-primed, immature mice were cultured 17-18 hr in medium containing 4 mM hypoxanthine (HX) or 300 microM dibutyryl cAMP (dbcAMP) to maintain meiotic arrest, and FSH was added to stimulate meiotic maturation. In the absence of FSH, Ado (1-250 microM) had no effect in dbcAMP-arrested oocytes but dose-dependently suppressed maturation in HX-treated oocytes. FSH-induced maturation was prevented by Ado, though more effectively in dbcAMP-supplemented cultures. Ado affected the magnitude, but not the kinetics pattern, of the response to FSH. Inosine also blocked meiotic induction, but only in dbcAMP-arrested oocytes. Purine de novo synthesis was nearly doubled in OCC by FSH treatment, and this response was completely prevented by Ado. FSH had no effect on HX salvage, although Ado reduced this activity by 98%. Inosine effects on metabolism were intermediate between the control and Ado groups. Experiments with radiolabeled energy substrates showed that Ado suppressed FSH activation of the pentose phosphate pathway but did not prevent significant activation of glycolysis or oxidation of pyruvate. Finally, in cultured follicles from primed mice, hCG-induced maturation was blocked by Ado as effectively as by the purine de novo synthesis inhibitor, azaserine. It is concluded that Ado has an inhibitory action on hormone-induced maturation that is due, at least in part, to suppression of glucose metabolism, leading to compromised purine de novo synthesis.

Metabolism of radiolabeled glucose by mouse oocytes and oocyte-cumulus cell complexes

This study was carried out to examine the metabolism of [1-14C]-, [6-14C]-, and [5-3H]glucose by oocyte-cumulus cell complexes (OCC) and denuded oocytes (DO) and to test the hypothesis that metabolism of glucose through the pentose phosphate pathway is associated with meiotic induction. OCC or DO were cultured in hanging drops suspended from the cap of a microfuge tube, with NaOH serving as a trap to collect released 3H2O or 14CO2. Preliminary experiments established that this culture system supports both spontaneous and ligand-induced meiotic maturation. An initial time course experiment (1.5-6 h) showed that hypoxanthine-treated OCC from eCG-primed animals metabolized glucose principally via glycolysis, with an increase to 2.7-fold in response to FSH. Though more [1-14C]glucose was oxidized than [6-14C]glucose, its metabolism was about two orders of magnitude less than that of [5-3H]glucose. Also, FSH significantly increased oxidation of [1-14C]glucose but not [6-14C]glucose, indicating a preferential activation of the pentose phosphate pathway. Pyrroline carboxylate, an activator of the pentose phosphate pathway, increased the activity of this pathway to over 2-fold but failed to affect glucose oxidation through the tricarboxylic acid cycle. Glycolytic metabolism was increased by 25%. The addition of pyruvate to pyruvate-free medium resulted in significant reduction in the metabolism of all three glucose analogues. In OCC retrieved from hCG-injected, primed mice and cultured under hormone-free conditions, metabolic responses were similar to those in FSH-treated complexes cultured in hypoxanthine. DO metabolized glucose, but at a much reduced rate when compared to OCC. Pyruvate reduced the consumption of all three glucose analogues by DO. Pyrroline carboxylate reduced [5-3H]glucose metabolism by DO but had little effect on [1-14C]- and [6-14C]glucose oxidation. These data demonstrate metabolism of glucose by both DO and OCC, but reveal that cumulus cells are more active than the oocyte in this regard. In addition, induction of maturation by FSH, hCG, or pyrroline carboxylate was accompanied by a significant increase in the oxidation of [1-14C]glucose but not [6-14C]glucose by OCC, supporting a proposed role for the pentose phosphate pathway in meiotic induction.