N-octanoylated ghrelin peptide inhibits bovine oocyte meiotic resumption

Meiosis activating sterol, produced directly by lanosterol 14-alpha-demethylase (CYP51) during cholesterol biosynthesis, has been shown to promote the initiation of oocyte meiosis. However, the physiological significance of CYP51 action on oocyte meiosis in response to gonadotrophins' induction remained to be further explored. Herein, we analyzed the role of CYP51 in gonadotrophin-induced in vitro oocyte maturation via RNA interference (RNAi). We showed that although both luteinizing hormone (LH) and follicle-stimulating hormone (FSH) significantly induced meiotic resumption in follicle-enclosed oocytes (FEOs), the effect of LH on oocyte meiosis resumption in FEOs was weaker than FSH. Moreover, both FSH and LH were able to upregulate CYP51 expression in cultured follicular granulosa cells when examined at 8 h or 12 h posttreatments, respectively. Interestingly, whereas knockdown of CYP51 expression via small interference RNA (siRNA) moderately blocked (23% reduction at 24 h) FSH-induced oocyte maturation [43% germinal vesicle breakdown (GVBD) rate in RNAi vs. 66% in control, P < 0.05] in FEOs, similar treatments showed no apparent effects on LH-induced FEO meiotic maturation (58% GVBD rate in RNAi vs. 63% in control, P > 0.05). Moreover, the results in a cumulus-enclosed oocytes (CEOs) model showed that approximately 30% of FSH-induced CEOs' meiotic resumption was blocked upon CYP51 knockdown by siRNAs. These findings suggest that FSH, partially at least, employs CYP51, and therefore the MAS pathway, to initiate oocyte meiosis.

The Influence of Glucose, Cumulus Cells, and Metabolic Coupling on ATP Levels and Meiotic Control in the Isolated Mouse Oocyte

Effect of lactate dehydrogenase activity and isoenzyme localization in bovine oocytes and utilization of oxidative substrates on in vitro maturation

Mammalian oocytes are surrounded by numerous layers of cumulus cells, and the loss of gap junctional communication in the outer layers of cumulus cells induces meiotic resumption in oocytes. In this study, we investigated the dynamic changes in the gap junctional protein connexin-43 in cumulus cells during the meiotic resumption of porcine oocytes. The amount of connexin-43 in all layers of cumulus cells recovered from cumulus-oocyte complexes was increased after 4-h cultivation. However, at 12-h cultivation, the positive signal for connexin-43 immunoreactivity was markedly reduced in the outer layers of cumulus cells. When these reductions of connexin-43 were blocked by protein kinase C (PKC) or phosphatidylinositol (PI) 3-kinase inhibitor, networks of filamentous bivalents (i.e., advanced chromosomal status) were undetectable in the germinal vesicle of the oocyte. After 28-h cultivation, when the majority of oocytes were reaching the metaphase I (MI) stage, the connexin-43 in the inner layers of cumulus cells was phosphorylated, regardless of mitogen-activated protein (MAP) kinase activation. These results suggest that the initiation of meiotic resumption, namely, the formation of networks of filamentous bivalents in germinal vesicle, is associated with the reduction of gap junctional protein connexin-43 in the outer layers of cumulus cells via the PKC and/or PI 3-kinase pathway. Moreover, the connexin-43 in the inner layers of cumulus cells is phosphorylated during meiotic progression beyond the MI stage, regardless of MAP kinase activation in cumulus cells surrounding the oocyte.

cGMP-inhibited cAMP phosphodiesterase 3A (PDE3A) is expressed in mouse oocytes, and its function is indispensable for meiotic maturation as demonstrated by genetic ablation. Moreover, PDE3 activity is required for insulin/insulin-like growth factor-1 stimulation of Xenopus oocyte meiotic resumption. Here, we investigated the cAMP-dependent protein kinase B (PKB)/Akt regulation of PDE3A and its impact on oocyte maturation. Cell-free incubation of recombinant mouse PDE3A with PKB/Akt or cAMP-dependent protein kinase A catalytic subunits leads to phosphorylation of the PDE3A protein. Coexpression of PDE3A with constitutively activated PKB/Akt (Myr-Akt) increases PDE activity as well as its phosphorylation state. Injection of pde3a mRNA potentiates insulin-dependent maturation of Xenopus oocytes and rescues the phenotype of pde3(-/-) mouse oocytes. This effect is greatly decreased by mutation of any of the PDE3A serines 290-292 to alanine in both Xenopus and mouse. Microinjection of myr-Akt in mouse oocytes causes in vitro meiotic maturation and this effect requires PDE3A. Collectively, these data indicate that activation of PDE3A by PKB/Akt-mediated phosphorylation plays a role in the control of PDE3A activity in mammalian oocytes.

Study question Does the oocyte control the expression of genes regulating the ovulatory cascade and the formation of the extracellular matrix in cumulus cells? Summary answer Oocyte secreted factors (OSFs) suppress the expression of prostaglandin synthase-2 (PTGS2) in cumulus cells, a key mediator of the maturation cascade. What is known already Oocyte quality depends on cytoplasmic and nuclear maturation synchrony. OSFs modulate maturation dynamics by inhibiting the expression of AREG (amphiregulin) and EREG (epiregulin), key mediators of the ovulatory cascade. AREG and EREG stimulate PTGS2 expression in cumulus cells, which catalyses the synthesis of prostaglandins, that amplify and translate AREG/EREG signals leading to meiotic completion and expression of genes required for cumulus expansion, among which HAS2 (hyaluronan synthase-2), essential for the synthesis of hyaluronan, the backbone of cumulus extracellular matrix, as well as PTX3 (pentraxin 3) and TNFAIP6 (tumor necrosis factor alpha-induced protein 6), genes encoding for matrix structural proteins. Study design, size, duration Relative mRNA levels of PTGS2, HAS2, PTX3 and TNFAIP6 were compared in cumulus cells from 3 treatment-groups: 1-COC: Intact cumulus-oocyte complexes subjected to in vitro maturation (IVM); 2-OOX: oocytectomized COC subjected to IVM; 3-OOX+DO: OOX subjected to IVM with the addition of denuded oocytes (1 DO/µL). Four experimental replicates including all three treatment-groups were performed. The inclusion of OOX+DO aimed to assess whether the effects of the oocyte on gene expression are mediated by OSFs. Participants/materials, setting, methods COC were aspirated from 3-8mm follicles of bovine ovaries. Oocytectomy was achieved by ooplasm aspiration with a micromanipulator. Pools of 20 COCs/OOX/OOX+DO were cultured for 22 hours in TCM199 with 100ng/mL AREG and 0.4% BSA. Subsequently, cumulus cells were separated, total RNA extracted, and mRNA abundance assessed by real-time RT-PCR with oligo-dT primers and two internal controls for relative quantification. Treatment effects were tested by ANOVA and groups were compared with the Tukey test. Main results and the role of chance Oocyctectomy drastically increased PTGS2 mRNA abundance, which was reversed by the addition of denuded oocytes (DO) to culture. Relative PTGS2 mRNA levels (mean ± EPM) were 1.02 ± 0.12, 10.40 ± 1.65 and 0.88 ± 0.08 for COC, OOX and OOX+DO, respectively (P = 0.02). Consistently, although in a lower magnitude, oocytectomy increased mRNA levels of PTX3(1.00 ± 0.04, 2.50 ± 0.18 and 0.75 ± 0.15 for COC, OOX and OOX+DO, respectively. P &amp;lt; 0.001) and TNFAIP6 (1.00 ± 0.07, 2.04 ± 0.17 and 0.67 ± 0.04 for COC, OOX and OOX+DO, respectively. P &amp;lt; 0.001), an effect again reversed by the presence of DO in culture. In contrast, oocytectomy decreased the expression of HAS2, which was neutralised in the presence of DO (1.01 ± 0.09, 0.69 ± 0.02 and 1.11 ± 0.08 for COC, OOX and OOX+DO, respectively. P = 0.005). Our data indicate that the oocyte modulates its own maturation pace by inhibiting the expression of PTGS2, which is accompanied by similar negative impacts on genes that strongly respond to prostaglandin signalling such as PTX3 and TNFAIP6. In parallel, our data suggest that OSFs exert a potent stimulatory effect on HAS2 transcription, capable to overcome their concomitant inhibitory influence via supressed AREG/EREG and PTGS2 expression. Limitations, reasons for caution Our study utilised animal and culture models that may not exactly represent human COC physiological maturation. In addition, our data are restricted to mRNA levels. Nevertheless, somatic cells predominantly transcribe poly A-mRNA that is promptly exported and translated. Accordingly, our RT-PCR strategy utilised poly-A reverse transcription with oligo-dT primers. Wider implications of the findings Our data indicate that OSFs inhibit the expression of PTGS2, a key gene for activation and amplification of the maturation cascade in cumulus cells. We speculate that this may represent an effort of the oocyte to prevent precocious meiotic resumption, thus favouring nuclear and cytoplasmic maturation synchrony and developmental competence. Trial registration number Not applicable

The cryopreservation of female gametes is still an open problem because of their structural sensitivity to the cooling-and-freezing process and to the exposure to cryoprotectants. The present work was aimed to study the effect of vitrification on immature bovine oocytes freed of cumulus cell investment before freezing. To verify the feasibility and efficiency of denuded oocyte (DO) cryopreservation, the cytoplasmic alterations eventually induced either by cell removal or by the vitrification process were analyzed. In particular, the migration of cortical granules and Ca++ localization were studied. In addition, the localization and distribution of microtubules and microfilaments in immature fresh and vitrified DOs were evaluated. Finally, to establish whether the removal of cumulus cells influenced developmental competence, DOs were thawed after vitrification, matured in vitro and fertilized; then presumptive zygotes were cultured to reach the blastocyst stage. The results indicate that mechanical removal of cumulus cells from immature bovine oocytes does not affect their maturation competence but reduces the blastocyst rate when compared with intact cumulus oocyte complexes (COCs). The findings indicate further that the vitrification process induces changes of cytoplasmic components. However, the composition of the manipulation medium used to remove cumulus cells plays a crucial role in reducing the injuries caused by cryopreservation in both cytoplasmic and nuclear compartments. In fact, the presence of serum exerts a sort of protection, significantly improving both oocyte maturation and blastocyst rates. In conclusion, we demonstrate that denuded immature oocytes can be vitrified after cumulus cells removal and successfully develop up, after thawing, to the blastocyst stage, following in vitro maturation and fertilization.

Reactive oxygen species (ROS) production is a normal process of cell metabolism. In vitro environments usually increase cell production of ROS, which has been implicated as a main cause of cell damage. Nevertheless, the role of ROS in oocyte in vitro maturation (IVM) is controversial. In most cells, enzymatic antioxidant systems can attenuate the effect of oxidative stress by scavenging ROS. The aim of this work was to determine whether: (1) standard conditions of bovine oocyte IVM are responsible for oxidative stress; (2) cumulus cells participate in protection against oxidative stress of the oocyte; and (3) enzymatic antioxidant activity is present in oocytes and cumulus cells. Cumulus-oocyte complexes (COCs) were matured in TCM-199 + 10% steer serum for 24 h at 39 degrees C in 5% CO2:95% humidified air. Oxidative stress was determined by the 2',7'-dichlorofluorescein diacetate assay. Superoxide dismutase (SOD), glutathione peroxidase, and catalase activities were measured spectrophotometrically. Under standard conditions of in vitro maturation, there was no increase in ROS production per COC (P > 0.05), but ROS level per cumulus cell diminished. There was no modification in ROS levels in oocytes matured in the presence versus the absence of their surrounding cumulus cells ( P > 0.05). To the best of our knowledge, the presence of SOD, glutathione peroxidase and catalase activities were detected in oocytes and cumulus cells for the first time. Enzymatic units were lower in denuded oocytes with respect to cumulus (P < 0.05), accounting for 37% for SOD, 25% for glutathione peroxidase, and 11% for catalase of the total COC units. Specific enzyme activity diminished in cumulus cells (P > 0.05) and increased in oocytes due to maturation (P > 0.05). The presence of activity of an enzymatic antioxidant system in the bovine oocyte would regulate in part ROS levels during IVM. Oocytes could be capable of controlling the increase in ROS because of the presence of their own enzymatic antioxidant system, SOD having the highest specific activity with respect to cumulus cells.

The present study was undertaken to establish an effective method for in vitro maturation (IVM) of denuded oocytes (DOs) by simulating the ovarian three-dimensional status in vivo using buffalo ovarian tissues or cumulus cells, so as to provide a model for investigating the mechanisms of oocyte maturation. Buffalo cumulus-oocyte complexes from ovaries taken at slaughter were denuded by pipetting, and then allocated randomly into four groups for IVM by direct culture in maturation medium (M1, control group), co-culture with a monolayer of cumulus cells (M2), embedded in cumulus cell clumps (M3) and ovarian tissue (M4) for 24 h. The nuclear maturation of DOs was assessed by the extrusion of the first polar body and the cytoplasmic maturation was evaluated by subsequently developmental capacity after parthenogenetic activation. More DOs matured to MII (56.89%) and developed to blastocysts (25.75%) when they were matured in vitro with M3 in comparison with DOs matured in vitro with M1 (45.14 and 15.97%) and M4 (40.48 and 13.49%). Further detection of gap junctions by injecting Lucifer yellow directly into cytoplasm of matured DOs with adherent cumulus cells and scanning with confocal microscope showed that Lucifer yellow were found in nine out of 11 the adherent cumulus cells in M3, indicating that the gap junctions between oocytes and cumulus cells was reconstructed in vitro. These results indicate that co-culture of DOs embedded in cumulus cell clumps can improve their nuclear and cytoplasmic maturation of DOs, possibly through the reconstruction of gap junctions in vitro.

This study was carried out to compare the possible role of a secreted paracrine factor versus that of a gap-junction-transmitted signal in mediating meiotic induction in isolated mouse oocytes from PMSG-primed, immature mice. In the first set of experiments, oocyte-cumulus cell complexes (OCC) were pretreated for 3 h with 2 mM dbcAMP or FSH, washed, and the oocytes then cultured for 17-18 h in 40 microl drops containing either 300 microM dbcAMP or 4 mM hypoxanthine (HX). Each set of pretreated oocytes was cultured under three different conditions: (1) intact cumulus-cell-enclosed oocytes (CEO); (2) denuded oocytes (DO), cultured alone after removal of cumulus cells; and (3) co-cultured cumulus cells and oocytes (CC/DO), where the cumulus cells were removed in the same drop with a mouth-operated pipette and cultured alongside the oocytes. When pretreated with high dbcAMP or FSH, maturation was stimulated in CEO when cultured in either inhibitor (by 41.4-53.7%). Pretreatment failed to affect the maturation rate in DO. DO maturation was not altered appreciably by co-cultured cumulus cells when arrest was maintained with dbcAMP. However, an increase in maturation of 21-23% was observed in CC/DO in the HX-containing cultures that was not dependent on prior treatment with a meiosis-inducing stimulus. When DO were co-cultured with intact, FSH-treated OCC, there was no evidence of a positive factor secreted by the stimulated complexes, despite the fact that oocytes within the OCC were induced to resume maturation. In a second series of experiments the gap junction inhibitor, 18alpha-glycyrrhetinic acid (GA), was utilised. An initial experiment determined that GA dose-dependently blocked OCC metabolic coupling (0.2% coupling at 10 microM compared with 13.6% in controls). When HX-arrested CEO and DO were cultured for 17-18 h in medium containing increasing concentrations of GA, meiotic maturation was induced in CEO but not DO, suggesting that the cumulus cells provided a positive stimulus in the absence of functional gap junctional communication. No effect of GA was seen in dbcAMP-arrested oocytes. A kinetics experiment showed that when CEO were cultured in dbcAMP +/- FSH, meiotic induction was initiated after 3 h and germinal vesicle breakdown reached 60% by 6 h. When GA was added to the cultures at different times after the initiation of culture (0, 2, 3, 4 and 5 h), meiotic induction was immediately blocked. In addition, measurement of OCC coupling revealed that no reduction in coupling occurred during this induction period in the absence of GA. It is concluded that cumulus cells can secrete a positive factor, but that this is normally overridden by inhibitory influences transmitted through the gap junction pathway in intact complexes. Furthermore, upon exposure of complexes to a meiosis-inducing stimulus, a positive gap-junction-mediated signal now predominates to trigger germinal vesicle breakdown, and this signal is utilised throughout the induction period.

It is proved that epidermal growth factor (EGF)-like factors mediate gonadotropin-induced rodent oocyte maturation via EGF receptor (EGFR). However, the detail kinetics and signal pathway between FSH and EGF/EGFR is not clear in large animals. In the present study, we investigated the roles of EGFR and protein kinase C (PKC) in FSH-induced porcine oocyte meiotic resumption. Porcine cumulus-oocyte complexes were cultured in NCSU37 medium containing 10% porcine follicular fluid and germinal vesicle breakdown (meiotic resumption) was detected after different treatments. The results showed that EGF-like factor amphiregulin (AR) and EGFR mRNA were expressed in porcine cumulus cells, but not oocytes. FSH significantly induced AR mRNA expression with maximum at 4 h and activated EGFR phosphorylation at 8 h. AR (1-100 ng/ml) dose-dependently induced meiosis resumption of porcine oocyte. The specific EGFR inhibitor, AG1478, but not AG43 (the inactive analog of AG1478), completely blocked FSH, EGF, and AR-induced oocyte meiotic resumption; the inhibitory effect of AG1478 on FSH action gradually decreased when the inhibitor was added at 6 h or later and disappeared when it was added at 11 h; EGF reversed the inhibitory effect on FSH when AG1478 was added within 6 h. FSH triggered porcine oocyte meiotic resumption (at 20 h) later than that of EGF and AR (at 18 h). All these results supported that endogenously produced EGFR activator(s), possibly AR (maximum at 4 h) and EGFR activation (began at 6 h and finished within 11 h), in cumulus cells is necessary for FSH-induced porcine oocyte meiotic resumption (began at 18 h). Furthermore, PKC activator PMA mimicked but PKC inhibitor chelerythrine chloride inhibited FSH action, and AG1478 also suppressed PMA-induced porcine oocyte meiotic resumption. These data together suggested that EGFR activation, by PKC signal pathway, participates in FSH-induced porcine oocyte meiotic resumption.

Phospholipid turnover and ultrastructural correlates during spontaneous germinal vesicle breakdown of the bovine oocyte: Effects of a cyclic AMP phosphodiesterase inhibitor

Energy balance is regulated by ghrelin which is a neuroendocrine modulator. Ghrelin is expressed in repro-ductive organs. However, the role of ghrelin during in vitro maturation (IVM) and bovine preimplantationaldevelopment is limited. The purpose of this study was to measure the expression of ghrelin (GHRL) andits receptor growth hormone secretagogue receptor 1A (GHS-R1A) mRNA, and determine cumulus oocytecomplex (COC) viability after IVM with 0, 20, 40 and 60 pM of ghrelin. Also, pronuclear formation wasrecorded after in vitro fertilization (IVF). GHRL and GHS-R1A mRNA expression in oocyte and cumu-lus cells (CCs) was assessed using reverse transcription-polymerase chain reaction (PCR). Oocyte andCC viability were analyzed with the fluorescein diacetate fluorochrome-trypan blue technique. Pronuclearformation was determined 18 hours after IVF with Hoechst 33342. The results demonstrated that ghrelinmRNA is present in oocyte and CCs before and after 24 hours IVM with all treatments. Ghrelin receptor,GHS-R1A, was only detected in oocytes and CCs after 24 hours IVM with 20, 40 and 60 pM of ghrelin.Oocyte viability was not significantly different (P=0.77) among treatments. However, CC viability wassignificantly lower (P=0.04) when COCs were matured with ghrelin (77.65, 72.10, 66.32 and 46.86% for0, 20, 40, and 60 pM of ghrelin, respectively). The chance of two pronuclei forming were higher (P=0.03)when ghrelin was not be added to the IVM medium. We found that ghrelin negatively impacts CC viabilityand pronuclear formation.

Interactions between oocytes and cumulus cells during in vitro maturation of porcine cumulus-oocyte complexes in a chemically defined medium: Effect of denuded oocytes on cumulus expansion and oocyte maturation

Drastic changes in intracellular cAMP and cGMP levels play a critical role in the regulation of meiotic resumption. The objective of this study was to compare cAMP and cGMP contents in cumulus-oocyte complexes (COC), denuded oocytes (DO), and cumulus cell masses (CC) derived from small (SF) and middle follicles (MF) during in vitro maturation (IVM). The COC were aspirated from SF (1–3 mm in diameter) or MF (3–6 mm in diameter) of prepubertal gilt ovaries. The COC were cultured in modified porcine oocyte medium (mPOM) with eCG, hCG, and dibutyryl cAMP for 20 h and then in fresh mPOM without those supplements for 4 h in an atmosphere of 5% CO2 in air at 39°C. At 0, 10, 20, and 24 h of IVM, COC, DO, and CC were collected. The DO were prepared by removal of cumulus cells and zona pellucida. The CC were prepared by puncturing ooplasm by using 18-gauge needle. Intracellular contents of cAMP and cGMP were determined by direct enzyme immunoassay kits. Statistical analyses of 3 to 7 replicated data were performed by ANOVA. There were no significant differences in contents of cAMP and cGMP between DO from SF and MF in all observation points (P &gt; 0.05). Cyclic AMP contents in COC and CC derived from MF were higher than those from SF at 20 h of IVM (MF 33.0 ± 0.5 fmol/COC v. SF 28.4 ± 1.0 fmol/COC, MF 20.9 ± 0.9 fmol/CC v. SF 14.6 ± 0.8 fmol/CC; P &lt; 0.05), whereas there were no significant differences between origins of those (SF v. MF, P &gt; 0.05) at 0, 10, and 24 h of IVM. Furthermore, although cAMP content in CC from MF was not significantly different between 10 and 20 h of IVM (25.4 ± 1.7 and 20.9 ± 0.9 fmol/CC, respectively; P &gt; 0.05), the content in CC from SF significantly decreased between 10 and 20 h (23.1 ± 1.2 , and 14.6 ± 0.8 fmol/CC, respectively; P &lt; 0.05). At 0 and 10 h of IVM, cGMP contents in COC and CC from MF were significantly higher than those from SF (0 h: 81.8 ± 4.5 fmol/COC from MF v. 41.7 ± 10.6 fmol/COC from SF and 82.7 ± 7.5 fmol/CC from MF v. 10.7 ± 2.7 fmol/CC from SF; 10 h: 64.8 ± 8.4 fmol/COC from MF v. 24.8 ± 8.2 fmol/COC from SF, 49.3 ± 14.9 fmol/CC from MF v. 13.5 ± 4.8 fmol/CC from SF; P &lt; 0.05). However, there were no significant differences in cGMP contents in COC and CC between the origins (MF v. SF) at 20 and 24 h of IVM (P &gt; 0.05). From these results, we conclude that cAMP and cGMP contents in cumulus cells are significantly differences between the origins (MF v. SF) during IVM.