François J. Richard

Role of cyclic nucleotide signaling in oocyte maturation

The development of the ovarian follicle, oocyte maturation, and ovulation require a complex set of endocrine, paracrine, and autocrine inputs that are translated into the regulation of cyclic nucleotide levels. Changes in intracellular cAMP mediate the gonadotropin regulation of granulosa and theca cell functions. Likewise, a decrease in cAMP concentration in the oocyte has been associated with the resumption of meiosis. Using pharmacological and molecular approaches, we determined that the expression of cyclic nucleotide phosphodiesterases (PDEs), the enzymes that degrade and inactivate cAMP, is compartmentalized in the ovarian follicle of all species studied, with PDE3 present in the oocytes and PDE4s in granulosa cells. The PDE3 expressed in the mouse oocyte was cloned, and the protein expressed in a heterologous system had properties similar to those of a PDE3A derived from somatic cells. Inhibition of the oocyte PDE3 completely blocked oocyte maturation in vitro and in vivo, demonstrating that the activity of this enzyme is essential for oocyte maturation. Heterologous expression of PDE3A in Xenopus oocyte causes morphological changes distinctive of resumption of meiosis (GVBD), as well as activation of mos translation and MAPK phosphorylation. Using mRNA and antibody microinjection in the Xenopus eggs, we have shown that PDE3 is downstream from the kinase PKB/Akt in the pathway that mediates IGF-1 but not progesterone-induced meiotic resumption. The presence of a similar regulatory module in mammalian oocytes is inferred by pharmacological studies with PDE3 inhibitors and measurement of PDE activity. Thus, PDE3 plays an essential role in the signaling pathway that controls resumption of meiosis in amphibians and mammals. Understanding the regulation of this enzyme may shed some light on the signals that trigger oocyte maturation.

Controlling meiotic resumption in bovine oocytes: a review.

Abstract Meiotic arrest refers to the nuclear stage of the oocytes within the follicles. Meiotic resumption occurs when oocytes are isolated from their follicular environment and placed in a simple maturation medium. The sine qua none condition for meiotic resumption is that the oocytes must be competent to resume meiosis. It has been shown that competent oocytes must reach a minimum diameter before been able to resume meiosis. In the bovine, competent oocytes measure at least 110 μm in diameter (24, 39). It appears that the inhibitory factors of meiotic resumption might only be necessary after the oocyte has acquired its competence. However, once the oocytes become competent, they need these factors to maintain meiotic arrest. It is generally recognized that follicular cells produce inhibitors necessary to maintain the oocyte in meiotic arrest. The removal of the oocyte from its follicular environment deprives the oocyte of inhibitory factors. Oocytes then resume meiosis (63). This article will first review the different chemical modulators to emphasize the importance of protein synthesis and the role of different kinases and phosphatases. Then it will review the follicular aspects involved in the control of meiotic arrest of oocytes competent to resume meiosis.

Role of cyclic nucleotide phosphodiesterases in resumption of meiosis

In the follicles of the mammalian and amphibian ovary, oocyte maturation is arrested at the prophase of the first meiotic division. Prior to ovulation, oocytes reenter the cell cycle, complete the meiotic division, and extrude the first polar body. Work from several laboratories including ours has provided evidence that the cAMP-mediated signal transduction pathway plays an important role in regulation of meiosis, the cyclic nucleotide acting as a negative regulator of maturation. Since cAMP can be regulated both at the level of synthesis and degradation, our laboratory is investigating the role of phosphodiesterases (PDE) in the control of cAMP levels of oocytes. Using pharmacological and molecular tools, we have determined that a PDE3 is the enzyme involved in the control of cAMP levels in the oocytes. In vitro and in vivo studies have established that inhibition of the oocyte PDE3 blocks resumption of a PDE is per se sufficient to cause resumption of meiosis in an amphibian oocyte model. The pathways regulating this PDE isoform expressed in the oocyte is under investigation, as they may uncover the physiological signals controlling meiosis.

Characterization of Novel Phosphodiesterases in the Bovine Ovarian Follicle

Abstract 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.

Cloning and Characterization of the Cyclic Guanosine Monophosphate-Inhibited Phosphodiesterase PDE3A Expressed in Mouse Oocyte

Abstract In the preovulatory follicle, oocyte meiotic resumption occurs soon after the LH surge and is associated with a decrease in cAMP. Inhibition of cAMP degradation blocks germinal vesicle breakdown as well as activation of meiotic promoting factor, both hallmarks of reentry into the cell cycle. In situ and pharmacological analysis of rodent ovaries suggested the presence of a phosphodiesterase 3 (PDE3) in the germ cell but not the somatic cell compartment. Here we have investigated the structure and properties of the PDE form expressed in mouse oocytes. Polymerase chain reactions using a mouse oocyte cDNA library as a template, and primers based on the conserved sequence of rat and human PDE3As, yielded partial fragments corresponding to mouse PDE3A. Further screening of the mouse oocyte cDNA library and subsequent ligation of individual cDNA clones yielded PDE3A cDNA containing the entire coding region of mouse PDE3A. To determine the kinetic properties of this PDE, the cDNAs encoding the full-length PDE3A and NH2-truncation forms Delta 1 (Δ346aa) and Delta 2 (Δ608aa) were expressed in mouse Leydig tumor cells. Whereas the full-length recombinant protein was always found in the particulate fraction, the Delta 1 and Delta 2 truncated PDE3As were recovered mostly in the soluble fraction. The Michaelis constant values for hydrolysis of cAMP of PDE3A Delta 1 and PDE3A Delta 2 were similar to those of intact full-length PDE3A or oocyte PDE (0.2–0.5 μM). More importantly, there was good correlation between the rank of potency of selective and nonselective compounds in inhibiting recombinant PDE3A or PDE activity derived from cumulus-oocyte complexes and in blocking resumption of meiosis. These data provide evidence that the PDE expressed in the oocyte is a soluble form of PDE3A and that activity of this enzyme is involved in the control of resumption of meiosis.

Effect of cycloheximide, 6-DMAP, roscovitine and butyrolactone I on resumption of meiosis in porcine oocytes.

Improvement of the ability to maintain germinal vesicle stage oocytes in vitro is important for the acquisition of developmental competence. Maintaining oocytes at this stage without damaging their quality would allow synchronization of maturation and homogenization of the oocytes population. More investigations are needed to better understand how the oocyte cell cycle is blocked without consequences to future developmental competence. This study tested the efficacy of pharmacological inhibitors of the G2/M cell cycle transition in keeping porcine oocytes at the germinal vesicle (GV) stage and the reversibility of this inhibition. Porcine cumulus-oocyte complexes (COCs) were thus incubated without any hormones for 24 h in the presence or absence of tested inhibitors: 6-DMAP (protein kinase inhibitor, 2 mM), cycloheximide (protein synthesis inhibitor, 2 microg/ml), roscovitine (cyclin-dependent kinase inhibitor, 50 microM) and butyrolactone I (cyclin-dependent kinase inhibitor, 50 microM). Cumulus-oocyte complexes cultured with any of the inhibitors were significantly blocked at the GV stage. The inhibitory effect varied according to the products, with cycloheximide being the most efficient. Reversibility of the pharmacological inhibitors was assessed by culturing COCs an additional 24 h in inhibitor-free culture medium. Examination of oocytes revealed that the inhibitory effect was fully reversible. This study suggests that 6-DMAP, cycloheximide, roscovitine and butyrolactone I can be use to block meiotic resumption in porcine oocytes in NCSU culture medium.

New insight into the role of phosphodiesterase 3A in porcine oocyte maturation

BackgroundThe ovulatory surge of gonadotropins triggers oocyte maturation and rupture of the ovarian follicle. The resumption of nuclear maturation in the oocyte from the prophase stage is characterized by germinal vesicle breakdown (GVBD). It has previously been shown that specific inhibition of cAMP degradation by PDE3 prevents the resumption of oocyte meiosis. However, no report has characterized the activity of PDE3 in the porcine oocyte, or the implication of the cAMP-PDE3 pathway in the entire nuclear maturation process. In this study, PDE3 activity in the oocyte was assessed during in vitro maturation (IVM) and the possible roles of the cAMP-PDE3 pathway in the resumption and progression of meiosis were investigated in terms of different models of oocyte maturation.ResultsCyclic AMP-degrading PDE activity was detected in the cumulus-oocyte complex (COC) and was partially inhibited by a specific PDE3 inhibitor, cilostamide. When measured only in the denuded oocyte, PDE activity was almost completely inhibited by cilostamide, suggesting that cAMP-PDE3 activity is the major cAMP-PDE in porcine oocytes. PDE3A mRNA was detected by RT-PCR. PDE3 activity did not vary significantly during the early hours of IVM, but a maximum was observed at 13 hours. In cumulus-oocyte complexes, meiosis resumed after 20.81 hours of culture. PDE3 inhibition no longer maintained meiotic arrest if sustained beyond 17.65 hours of IVM, 3 hours prior to resumption of meiosis. Thereafter, PDE3 inhibition progressively lost its efficacy in GVBD. When the protein phosphatase 1 and 2A inhibitor okadaic acid was continuously or transiently (3 hours) present during IVM, meiosis resumed prematurely; PDE3 inhibition was unable to prevent GVBD. However, PDE3 inhibition in COC treated with OA for 3 hours significantly delayed meiosis at the intermediate stage.ConclusionThe present investigation has demonstrated that PDE3A is the major cAMP-degrading PDE in the oocyte. It regulates the resumption of meiosis until 3 hours prior to GVBD and transiently affects meiotic progression.

Cumulus Cell Transcripts Transit to the Bovine Oocyte in Preparation for Maturation

ABSTRACT So far, the characteristics of a good quality egg have been elusive, similar to the nature of the physiological, cellular, and molecular cues leading to its production both in vivo and in vitro. Current understanding highlights a strong and complex interdependence between the follicular cells and the gamete. Secreted factors induce cellular responses in the follicular cells, and direct exchange of small molecules from the cumulus cells to the oocyte through gap junctions controls meiotic arrest. Studying the interconnection between the cumulus cells and the oocyte, we previously demonstrated that the somatic cells also contribute transcripts to the gamete. Here, we show that these transcripts can be visualized moving down the transzonal projections (TZPs) to the oocyte, and that a time course analysis revealed progressive RNA accumulation in the TZPs, indicating that RNA transfer occurs before the initiation of meiosis resumption under a timetable fitting with the acquisition of developmental competence. A comparison of the identity of the nascent transcripts trafficking in the TZPs, with those in the oocyte increasing in abundance during maturation, and that are present on the oocytes polyribosomes, revealed transcripts common to all three fractions, suggesting the use of transferred transcripts for translation. Furthermore, the removal of potential RNA trafficking by stripping the cumulus cells caused a significant reduction in maturation rates, indicating the need for the cumulus cell RNA transfer to the oocyte. These results offer a new perspective to the determinants of oocyte quality and female fertility, as well as provide insight that may eventually be used to improve in vitro maturation conditions.

Regulation of Gap Junctional Communication Between Cumulus Cells During In Vitro Maturation in Swine, a Gap-FRAP Study

ABSTRACT Intercellular gap-junctional communication (GJC) plays an important role in ovarian cell physiology. Closure of GJC has been proposed to be involved in oocyte maturation, particularly in the resumption of meiosis, both in vivo and in vitro, by controlling the flow of meiosis inhibitors, such as cAMP and cGMP. Understanding how GJC dynamics are regulated during in vitro maturation (IVM) could provide a powerful tool for controlling meiotic resumption and oocyte maturation in vitro. Since little is known about the GJC dynamic regulation between cumulus cells, we have developed an assay based on recovery of calcein fluorescence in photobleached cumulus cells, a gap-FRAP assay. The GJC profile has been characterized during the first hours of porcine IVM. We showed that equine chorionic gonadotropin (eCG) and epidermal growth factor (EGF) down-regulated GJC effectiveness between cumulus cells. However, human chorionic gonadotropin was not down-regulating GJC effectiveness. We also showed that the GJC network expanded during this period and that this effect was not regulated by gonadotropins. Porcine follicular fluid present in the maturation medium also had an impact on GJC regulation, increasing GJC network establishment and the effectiveness of calcein transfer rate between cumulus cells. These results show that both eCG and EGF are regulating the decrease in GJC effectiveness after 4.5 h of IVM, while the network extension is gonadotropin independent. Regulation of GJC between cumulus cells would then be specifically regulated during in vitro IVM.

New Elements in the C-Type Natriuretic Peptide Signaling Pathway Inhibiting Swine In Vitro Oocyte Meiotic Resumption

ABSTRACT C-type natriuretic peptide (CNP) and its cognate receptor, natriuretic peptide receptor (NPR) B, have been shown to promote cGMP production in granulosa/cumulus cells. Once transferred to the oocyte through the gap junctions, the cGMP inhibits oocyte meiotic resumption. CNP has been shown to bind another natriuretic receptor, NPR-C. NPR-C is known to interact with and degrade bound CNP, and has been reported to possess signaling functions. Therefore, NPR-C could participate in the control of oocyte maturation during swine in vitro maturation (IVM). Here, we examine the effect of CNP signaling on meiotic resumption, the amount of cGMP and gap junctional communication (GJC) regulation during swine IVM. The results show an inhibitory effect of CNP in inhibiting oocyte meiotic resumption in follicle-stimulating hormone (FSH)-stimulated IVM. We also found that an NPR-C-specific agonist (cANP[4–23]) is likely to play a role in maintaining meiotic arrest during porcine IVM when in the presence of a suboptimal dose of CNP. Moreover, we show that, even if CNP can increase intracellular concentration of cGMP in cumulus-oocyte complexes, cANP(4–23) had no impact on cGMP concentration, suggesting a potential cGMP-independent signaling pathway related to NPR-C activation. These data support a potential involvement of cANP(4–23) through NPR-C in inhibiting oocyte meiotic resumption while in the presence of a suboptimal dose of CNP. The regulation of GJC was not altered by CNP, cANP(4–23), or the combination of CNP and cANP(4–23), supporting their potential contribution in sending signals to the oocytes. These findings offer promising insights in to new elements of the signaling pathways that may be involved in inhibiting resumption of meiosis during FSH-stimulated swine IVM.