Role of meiosis-activating sterols in rat oocyte maturation: effects of specific inhibitors and changes in the expression of lanosterol 14alpha-demethylase during the preovulatory period

Local effect of allopregnanolone in rat ovarian steroidogenesis, follicular and corpora lutea development

Allopregnanolone (ALLO) is a known neurosteroid and a progesterone metabolite synthesized in the ovary, CNS, PNS, adrenals and placenta. Its role in the neuroendocrine control of ovarian physiology has been studied, but its in situ ovarian effects are still largely unknown. The aims of this work were to characterize the effects of intrabursal ALLO administration on different ovarian parameters, and the probable mechanism of action. ALLO administration increased serum progesterone concentration and ovarian 3β-HSD2 while decreasing 20α-HSD mRNA expression. ALLO increased the number of atretic follicles and the number of positive TUNEL granulosa and theca cells, while decreasing positive PCNA immunostaining. On the other hand, there was an increase in corpora lutea diameter and PCNA immunostaining, whereas the count of TUNEL-positive luteal cells decreased. Ovarian angiogenesis and the immunohistochemical expression of GABAA receptor increased after ALLO treatment. To evaluate if the ovarian GABAA receptor was involved in these effects, we conducted a functional experiment with a specific antagonist, bicuculline. The administration of bicuculline restored the number of atretic follicles and the diameter of corpora lutea to normal values. These results show the actions of ALLO on the ovarian physiology of the female rat during the follicular phase, some of them through the GABAA receptor. Intrabursal ALLO administration alters several processes of the ovarian morpho-physiology of the female rat, related to fertility and oocyte quality.

Follicular fluid meiosis-activating sterol (FF-MAS) promotes meiotic resumption via the MAPK pathway in porcine oocytes

Follicular fluid meiosis-activating sterol (FF-MAS) exerts beneficial effects on the meiotic resumption of mammalian oocytes and their subsequent early embryonic development, but the signaling pathway underlying these effects has not been elucidated. Therefore, the objective of the present study was to investigate whether the mitogen-activated protein kinase (MAPK) pathway is involved in the FF-MAS-induced in vitro resumption of meiosis in porcine oocytes. Porcine cumulus oocyte complexes (COCs) were allocated in several groups cultured in TCM-199 medium with different concentration of AY 9944-A-7 (20, 30, 40 μmol/L) or ketoconazole (20 μmol/L) to increase or decrease endogenous accumulation of FF-MAS. Each experimental condition was repeated at least six times. After maturation for 44 h, the resumption of meiosis was assessed by the frequency of germinal vesicle breakdown (GVBD) and the first polar body (PBI) extrusion, The relative expressions of related genes in MAPK pathway [c-mos, mitogen-activated protein kinase kinase (MEK) and extracellular signal-regulated kinase 1/2 (ERK1/2)] at both transcriptional and translational levels were detected to investigate the kinetic trend of expression throughout oocyte maturation in vitro in response to the addition of AY 9944-A-7 or ketoconazole to the maturation medium. Results indicated that AY 9944-A-7 promoted, while ketoconazole inhibited, the in vitro maturation (IVM) of porcine oocytes. Relative expression of meiosis related genes was upregulated by AY 9944-A-7 and downregulated by ketoconazole. With extended culturing time, c-mos mRNA expression levels reached their peak at 12 h of maturation and decreased gradually thereafter, while MEK, ERK1 and ERK2 expression increased after an initial decrease peaking at 44 h of culture in the AY 9944-A-7-group. And the trend of the protein expression of c-mos, MEK, ERK1/2 was basically consistent with the mRNA expression of these genes. These results imply that the endogenous accumulation of FF-MAS is beneficial to resumption of meiosis in porcine oocytes and that MAPK signaling is involved in FF-MAS-induced meiotic resumption.

Transcriptome analysis of granulosa cells after conventional vs long FSH-induced superstimulation in cattle

BACKGROUND

Prolongation of superstimulatory treatment appears to be associated with a greater superovulatory response and with greater oocyte maturation in cattle. A genome-wide bovine oligo-microarray was used to compare the gene expression of granulosa cells collected from ovarian follicles after differing durations of the growing phase induced by exogenous FSH treatment. Cows were given a conventional (4-day) or long (7-day) superstimulatory treatment (25 mg FSH im at 12-h intervals; n = 6 per group), followed by prostaglandin treatment with last FSH and LH treatment 24 h later. Granulosa cells were harvested 24 h after LH treatment.

RESULTS

The expression of 416 genes was down-regulated and 615 genes was up-regulated in the long FSH group compared to the conventional FSH group. Quantification by RT-PCR of 7 genes (NTS, PTGS2, PTX3, RGS2, INHBA, CCND2 and LRP8) supported the microarrays data. Multigene bioinformatic analysis indicates that markers of fertility and follicle maturity were up-regulated in the long FSH group.

CONCLUSION

Using the large gene expression dataset generated by the genomic analysis and our previous associated with the growth phase and gene expression changes post LH, we can conclude that a prolonged FSH-induced growing phase is associated with transcriptomic characteristics of greater follicular maturity and may therefore be more appropriate for optimizing the superovulatory response and developmental competence of oocytes in cattle.

CYP51A1 induced by growth differentiation factor 9 and follicle-stimulating hormone in granulosa cells is a possible predictor for unfertilization

Growth differentiation factor 9 (GDF9), an oocyte-secreted factor, whose receptors exist in granulosa cells, is involved in follicle progression. Therefore, GDF9 is considered to potentially mediate signals necessary for follicular growth. However, the effect of GDF9 on human granulosa cells is not fully understood. Human immortalized nonluteinized granulosa cell line (HGrC1) which we have previously reported was stimulated with GDF9 and/or follicle-stimulating hormone (FSH). Granulosa cells obtained from in vitro fertilization (IVF) patients were also evaluated with quantitative reverse transcription polymerase chain reaction (RT-PCR). Real-time RT-PCR showed that GDF9 increased messenger RNA (mRNA) levels of enzymes required for cholesterol biosynthesis, such as 3-hydroxy-3-methylglutanyl-CoA synthase 1 (HMGCS1), farnesyl-diphosphate farnesyltransferase 1, squalene epoxidase, lanosterol synthase, and cytochrome P450, family 51, subfamily A, polypeptide 1 (CYP51A1). A greater increase in mRNA levels of HMGCS1 and CYP51A1 was observed by combined treatment with GDF9 and FSH. Clinical samples showed a significant increase in CYP51A1 mRNA in the group of granulosa cells connected with unfertilized oocytes. Our results suggest that GDF9, possibly with FSH, may play significant roles in the regulation of cholesterol biosynthesis and the expression of CYP51A1 which might be a predictor for unfertilization.

The Interplay between Circadian System, Cholesterol Synthesis, and Steroidogenesis Affects Various Aspects of Female Reproduction

Circadian aspect of reproduction has gained much attention in recent years. In mammals, it is very important that the timing of greatest sexual motivation is in line with the highest fertility. Peripheral clocks have been found to reside also in reproductive organs, such as the uterus and ovary. The timing signal from the suprachiasmatic nucleus is suggested to be transmitted via hormonal and neural mechanisms, and could thus mediate circadian expression of target genes in these organs. In turn, estrogens from the ovary have been found to signal back to the hypothalamus, completing the feedback loop. In this review we will focus on the interplay between clock and estrogens. Estradiol has been directly linked with expression of Per1 and Per2 in the uterus. CLOCK, on the other hand, has been shown to alter estradiol signaling. We also present the idea that cholesterol could play a vital role in the regulation of reproduction. Cholesterol synthesis itself is circadially regulated and has been found to interfere with steroidogenesis in the ovary on the molecular level. This review presents a systems view on how the interplay between circadian clock, steroidogenesis, and cholesterol synthesis affect various aspects of mammalian reproduction.

Steroidal triterpenes of cholesterol synthesis

Cholesterol synthesis is a ubiquitous and housekeeping metabolic pathway that leads to cholesterol, an essential structural component of mammalian cell membranes, required for proper membrane permeability and fluidity. The last part of the pathway involves steroidal triterpenes with cholestane ring structures. It starts by conversion of acyclic squalene into lanosterol, the first sterol intermediate of the pathway, followed by production of 20 structurally very similar steroidal triterpene molecules in over 11 complex enzyme reactions. Due to the structural similarities of sterol intermediates and the broad substrate specificity of the enzymes involved (especially sterol-Δ²⁴-reductase; DHCR24) the exact sequence of the reactions between lanosterol and cholesterol remains undefined. This article reviews all hitherto known structures of post-squalene steroidal triterpenes of cholesterol synthesis, their biological roles and the enzymes responsible for their synthesis. Furthermore, it summarises kinetic parameters of enzymes (V_max and K_m) and sterol intermediate concentrations from various tissues. Due to the complexity of the post-squalene cholesterol synthesis pathway, future studies will require a comprehensive meta-analysis of the pathway to elucidate the exact reaction sequence in different tissues, physiological or disease conditions. A major reason for the standstill of detailed late cholesterol synthesis research was the lack of several steroidal triterpene standards. We aid to this efforts by summarizing commercial and laboratory standards, referring also to chemical syntheses of meiosis-activating sterols.

Male germ cell-specific knockout of cholesterogenic cytochrome P450 lanosterol 14α-demethylase (Cyp51)

Cytochrome P450 lanosterol 14α-demethylase (CYP51) and its products, meiosis-activating sterols (MASs), were hypothesized by previous in vitro studies to have an important role in regulating meiosis and reproduction. To test this in vivo, we generated a conditional male germ cell-specific knockout of the gene Cyp51 in the mouse. High excision efficiency of Cyp51 allele in germ cells resulted in 85-89% downregulation of Cyp51 mRNA and protein levels in germ cells. Quantitative metabolic profiling revealed significantly higher levels of CYP51 substrates lanosterol and 24,25-dihydrolanosterol and substantially diminished levels of MAS, the immediate products of CYP51. However, germ cell-specific ablation of Cyp51, leading to lack of MAS, did not affect testicular morphology, daily sperm production, or reproductive performance in males. It is plausible that due to the similar structures of cholesterol intermediates, previously proposed biological function of MAS in meiosis progression can be replaced by some other yet-unidentified functionally redundant lipid molecule(s). Our results using the germ cell-specific knockout model provide first in vivo evidence that the de novo synthesis of MAS and cholesterol in male germ cells is most likely not essential for spermatogenesis and reproduction and that MASs, originating from germ cells, do not cell-autonomously regulate spermatogenesis and fertility.

Sterols in spermatogenesis and sperm maturation

Mammalian spermatogenesis is a complex developmental program in which a diploid progenitor germ cell transforms into highly specialized spermatozoa. One intriguing aspect of sperm production is the dynamic change in membrane lipid composition that occurs throughout spermatogenesis. Cholesterol content, as well as its intermediates, differs vastly between the male reproductive system and nongonadal tissues. Accumulation of cholesterol precursors such as testis meiosis-activating sterol and desmosterol is observed in testes and spermatozoa from several mammalian species. Moreover, cholesterogenic genes, especially meiosis-activating sterol-producing enzyme cytochrome P450 lanosterol 14α-demethylase, display stage-specific expression patterns during spermatogenesis. Discrepancies in gene expression patterns suggest a complex temporal and cell-type specific regulation of sterol compounds during spermatogenesis, which also involves dynamic interactions between germ and Sertoli cells. The functional importance of sterol compounds in sperm production is further supported by the modulation of sterol composition in spermatozoal membranes during epididymal transit and in the female reproductive tract, which is a prerequisite for successful fertilization. However, the exact role of sterols in male reproduction is unknown. This review discusses sterol dynamics in sperm maturation and describes recent methodological advances that will help to illuminate the complexity of sperm formation and function.

Reactive oxygen species are indispensable in ovulation

Ovulation is stimulated by the preovulatory surge of the pituitary luteinizing hormone (LH). Because the ovulatory response is commonly identified with inflammation, we explored the involvement of reactive oxygen species (ROS) in this process. Our experiments show that administration of broad-range scavengers of oxidative species into the ovarian bursa of mice, hormonally induced to ovulate, significantly reduced the rate of ovulation. LH-induced cumulus mucification/expansion, a necessary requirement for ovulation, was prevented by antioxidants both in vivo and in an ex vivo system of isolated intact ovarian follicles. Along this line, H(2)O(2) fully mimicked the effect of LH, bringing about an extensive mucification/expansion of the follicle-enclosed cumulus-oocyte complexes. Impaired progesterone production was observed in isolated follicles incubated with LH in the presence of the antioxidant agents. Furthermore, LH-stimulated up-regulation of genes, the expression of which is crucial for ovulation, was substantially attenuated upon ROS ablation. This system was also used for demonstrating the role of ROS in phosphorylation and activation of the EGF receptor as well as its downstream effector, p42/44 MAPK. Together, our results provide evidence that ovarian production of ROS is an essential preovulatory signaling event, most probably transiently triggered by LH.

Gonadotropin-regulated expressions of lanosterol 14alpha-demethylase, sterol Delta14-reductase and C-4 sterol methyl oxidase contribute to the accumulation of meiosis-activating sterol in rabbit gonads

Meiosis-activating sterol (MAS), the intermediate of cholesterol biosynthesis, is an important lipophilic molecule mediating gonadotropins' action in inducing oocyte meiotic resumptions in various mammalian species. With respect to MAS's physiological relevance during oocyte maturation in the rabbit, early study has demonstrated that luteinizing hormone (LH), but not follicle stimulating hormone (FSH) can induce FF-MAS accumulation facilitating oocyte maturation in rabbits. However, the potential underlying mechanism for the MAS accumulation in the rabbit gonad remained unclear. We hypothesized that differential expression of MAS synthetic and metabolic enzymes would contribute to the timely MAS accumulation in the rabbit gonad. To address this issue, in the present investigation, we first cloned the cDNAs encoding there pre- and post-MAS enzymes, lanosterol 14alpha-demethylase (CYP51), sterol Delta14-reductase (14-SR) and C-4 sterol methyl oxidase (C4MO), respectively, using rapid amplification of cDNA ends (RACE) cloning, and then performed northern hybridization experiments to explore their expression profiles in the rabbit ovary, testis, and various other tissues. We observed that CYP51 expression was significantly upregulated only by LH/hCG in the antral follicle exhibiting its peak levels in preovulatory follicles; whereas both FSH and LH significantly downregulated 14-SR expression with the progression of antral follicular development. These findings provided here novel evidence that an inverse upregulation of CYP51 and downregulation of 14-SR expression under FSH/LH stimulation functions as the machinery for FF-MAS accumulation in preovulatory follicles prior to ovulation in the rabbit.

Reducing CYP51 inhibits follicle-stimulating hormone induced resumption of mouse oocyte meiosis in vitro

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.

Expression and regulation of lanosterol 14alpha-demethylase in mouse embryo and uterus during the peri-implantation period

Lanosterol 14alpha-demethylase (LDM) is expressed ubiquitously in all mammals and is important in cholesterol biosynthesis. However, whether LDM expression is involved in the interaction between uterus and embryo during implantation remains unknown. In the present study, the expression of LDM was investigated in mouse embryo and uterus during the peri-implantation period using confocal microscopy, immunohistochemistry and western blot methods. Further, regulation of LDM expression was investigated in pseudopregnancy, delayed implantation, artificial decidualisation and ovariectomisation using 17beta-oestradiol and progesterone treatment mouse models. The results showed that LDM was selectively expressed in preimplantation embryos and the uterine subluminal stroma surrounding the implanting blastocyst on Day 5 of pregnancy. No corresponding signal was detected in the uterus on Day 5 of pseudopregnancy. Most notably, once delayed implantation was terminated by oestrogen treatment and the embryo implanted, a high level of LDM expression was induced in the subluminal stroma surrounding the implanting blastocyst, whereas no corresponding signal was detected in the delayed implantation uterus. A high level of LDM expression was observed in the uterus decidua on Days 6-8 of pregnancy. Furthermore, LDM expression was induced in the uterine stroma under artificial decidualisation. Oestrogen, but not progesterone, treatment induced a high level of LDM expression in the uterus of ovariectomised mice. These results indicate that LDM is closely related to mouse embryo implantation and can be upregulated by oestrogen.

3',5'-cyclic adenosine monophosphate response element binding protein up-regulated cytochrome P450 lanosterol 14alpha-demethylase expression involved in follicle-stimulating hormone-induced mouse oocyte maturation

Cytochrome P450 lanosterol 14alpha-demethylase (CYP51) is a key enzyme in sterols and steroids biosynthesis that can induce meiotic resumption in mouse oocytes. The present study investigated the expression mechanism and function of CYP51 during FSH-induced mouse cumulus oocyte complexes (COCs) meiotic resumption. FSH increased cAMP-dependent protein kinase (PKA) RIIbeta level and induced cAMP response element-binding protein (CREB) phosphorylation and CYP51 expression in cumulus cells before oocyte meiotic resumption. Moreover, CYP51 and epidermal growth factor (EGF)-like factor [amphiregulin (AR)] expression were blocked by (2)-naphthol-AS-Ephosphate (KG-501) (a drug interrupting the formation of CREB functional complex). KG-501 and RS21607 (a specific inhibitor of CYP51 activity) inhibited oocyte meiotic resumption, which can be partially rescued by progesterone. These two inhibitors also inhibited FSH-induced MAPK phosphorylation. EGF could rescue the suppression by KG-501 but not RS21607. Furthermore, type II PKA analog pairs, N(6)-monobutyryl-cAMP plus 8-bromo-cAMP, increased PKA RIIbeta level and mimicked the action of FSH, including CREB phosphorylation, AR and CYP51 expression, MAPK activation, and oocyte maturation. All these data suggest that CYP51 plays a critical role in FSH-induced meiotic resumption of mouse oocytes. CYP51 and AR gene expression in cumulus cells are triggered by FSH via a type II PKA/CREB-dependent signal pathway. Our study also implicates that CYP51 activity in cumulus cells participates in EGF receptor signaling-regulated oocyte meiotic resumption.

Are steroids dispensable for meiotic resumption in mammals?

Meiosis of vertebrate oocytes is a protracted process initiated within differentiated oocytes before the first meiotic arrest of the first meiotic division. Meiosis normally resumes in response to the stimulation of ovulation, proceeding to metaphase of the second meiotic division. In fish and amphibian oocytes, this resumption is triggered by follicular steroids. By contrast, the role of steroids in the resumption of mammalian oocyte maturation is less clear. Specifically, mammalian meiotic maturation proceeds undisturbed even when steroid production is severely suppressed. This puzzling mammalian divergence has been reexamined recently. Here, we review the published data and conclude that steroids are not necessary for the resumption of mammalian meiosis. Nevertheless, steroids are probably involved in follicular growth, somatic-cell differentiation and the acquisition of developmental competence of mature ova.

Mechanisms Regulating Oocyte Meiotic Resumption: Roles of Mitogen-Activated Protein Kinase

Oocyte meiotic maturation is one of the important physiological requirements for species survival. However, little is known about the detailed events occurring during this process. A number of studies have demonstrated that MAPK plays a pivotal role in the regulation of meiotic cell cycle progression in oocytes, but controversial findings have been reported in both lower vertebrates and mammals. In this review, we summarized the roles of MAPK cascade and related signal pathways in oocyte meiotic reinitiation in both lower vertebrates and mammals. We also tried to reconcile the paradoxical results and highlight the new findings concerning the function of MAPK in both oocytes and the surrounding follicular somatic cells. The unresolved questions and future research directions regarding the role of MAPK in meiotic resumption are addressed.

Lanosterol 14α-demethylase expression in the mouse ovary and its participation in cumulus-enclosed oocyte spontaneous meiotic maturation in vitro

The expression of lanosterol 14alpha-demethylase (LDM) in the mouse ovary after gonadotrophin administration was examined and the action of follicle fluid meiosis activating sterol (FF-MAS), derived from lanosterol by the action of LDM, on oocyte spontaneous maturation was also evaluated in cumulus cell enclosed oocytes (CEOs). Expression of LDM was primarily in oocytes in primordial and secondary follicles prior to administration of gonadotrophins, but obvious LDM expression was apparent in ovarian somatic cells 48 h after administration of equine chorionic gonadotrophin (eCG), especially in luteal and cumulus cells 54 h after eCG or 48 h after eCG plus 6 h after human chorionic gonadotrophin (hCG). The LDM expression in oocytes was only slightly elevated in larger growing follicles after eCG treatment. On the contrary, 48 h after hCG treatment, the elevated expression of LDM was only detected in interstitial cells. Therefore, eCG may be the primary gonadotrophin for LDM expression, and furthermore for production of FF-MAS in mouse cumulus cells (which are indispensable for oocyte maturation in vivo). Conversely, inhibitors of LDM, either 40 microM azalanstat or 50 microM RS-21745, significantly inhibited oocyte germinal vesicle breakdown (GVB) after 4h of in vitro culture; GVB rates decreased to 14 or 20%, compared to 90% in spontaneous maturation, respectively. There was no significant increase in GVB in CEOs following specific inhibitor of sterol Delta14-reductase and Delta7-reductase, AY9944-A-7 (5-100 microM), until marked oocytes degeneration appeared (50 microM). The phenomena may be ascribed to slow, passive accumulation of FF-MAS by AY9944-A-7, which cannot be associated with fast spontaneous progression. Furthermore, in spontaneous-matured CEOs, LDM was expressed preferentially in cumulus cells instead of oocytes. Therefore, FF-MAS may have a positive role in the spontaneous maturation of CEOs. In conclusion, there was an eCG-dependent dual LDM expression pattern on both oocytes and somatic cells in growing follicles in vivo, which may increase LDM expression and FF-MAS production in cumulus cells for oocyte maturation. For the first time, the inhibitory effect of LDM inhibitors on spontaneous maturation, together with the strong LDM expression in spontaneous matured CEOs, indicated that FF-MAS produced by cumulus cells might participate in spontaneous maturation of mouse CEOs.

Differential response of cumulus cell-enclosed and denuded mouse oocytes in a meiotic induction model system

In this study we have examined the effects of denuded oocyte coculture with dissociated cumulus cells (CC) or intact oocyte-CC complexes on meiotic resumption. When denuded oocytes (DO) or cumulus cell-enclosed oocytes (CEO) were cultured in 40-microl drops of medium under oil, and held in meiotic arrest with 4 mM hypoxanthine plus 25 microM dbcAMP, they underwent germinal vesicle breakdown (GVB) at similar frequencies (34%-35%). Coculture of DO with complexes or dissociated CCs stimulated maturation (50% and 61% GVB, respectively), with no effect of DO on maturation of cocultured CEO (32% GVB). This coculture effect was increased with the number of CCs added to the culture drop. When either glucose or glutamine was eliminated from the medium, no meiotic induction resulted from cocultured CCs. When CEO were cultured alone in microdrops, increasing their number from 10 to 50 significantly lowered the percentage resuming maturation, an effect also reduced by removing glucose and/or glutamine from the medium. This effect was not observed with DO. When inhibitory medium was conditioned overnight with complexes, subsequent culture with DO led to higher maturation percentages than culture in unconditioned medium; however, when CEO were cultured in conditioned medium, there was either no effect or increased inhibition of maturation. Assay of glucose and pyruvate in spent medium showed that DO cultured alone consumed glucose and pyruvate, but under CC coculture conditions more glucose was consumed and significant amounts of pyruvate accumulated in the medium, changes that led to an increase in the maturation of DO. Further experiments showed that DO were more sensitive than CEO to the meiosis-inducing effect of pyruvate. These results demonstrate different responsiveness of DO and CEO to coculture conditions and question the physiological relevance of denuded oocyte/CC coculture to study meiotic induction.

Disruption of gap junctional communication within the ovarian follicle induces oocyte maturation

Meiotically arrested mammalian oocytes are stimulated to resume meiosis by LH. This response, which can be reversed by elevation of intraoocyte cAMP levels, is associated with interruption of gap junctional communication (GJC) within the ovarian follicle. In the present study, we examined the hypothesis that disruption of GJC within the ovarian follicle is sufficient for induction of oocyte maturation. For this purpose, we incubated rat follicle-enclosed oocytes with carbenoxolone (CBX), a known blocker of gap junctions. We found that this selective disruptor of GJC promoted maturation of almost all the follicle-enclosed oocytes after 5 h of incubation; this response was also obtained by a transient (2 h) exposure to this agent. CBX-induced oocyte maturation was accompanied by a substantial decrease in intraoocyte concentrations of cAMP that was not associated with elevated activity of type 3A phosphodiesterase (PDE3A). The effect of CBX on reinitiation of meiosis was blocked by isobutylmethylxanthine, a phosphodiesterase inhibitor. Unlike LH, CBX did not activate MAPK in the follicular cells, and inhibition of the MAPK signaling pathway by means of UO126 did not prevent the resumption of meiosis. Injection of CBX into the ovarian bursa of intact animals stimulated maturation in 30% of the oocytes, whereas no maturation was observed in the contralateral ovary injected with PBS. We conclude that, because experimentally induced breakdown of communication within the ovarian follicle is associated with a drop in intraoocyte cAMP concentrations and results in resumption of meiosis, this could be the physiological mechanism employed by LH to stimulate oocyte maturation.

Meiosis activating sterol (MAS) regulate FSH-induced meiotic resumption of cumulus cell-enclosed porcine oocytes via PKC pathway

Meiosis activating sterol (MAS) have been found to be able to promote oocytes meiotic maturation of small animals in vitro, such as mouse, rat and rabbit. But in large animals, whether MAS play the same function, especially the physiological mechanisms of MAS on oocytes maturation are not clear. To our knowledge, this is the first time to investigate the role and signal pathway of MAS on FSH-induced porcine oocytes meiotic resumption. Porcine cumulus-enclosed oocytes (CEOs) isolated from 3 to 5mm follicles were cultured in the FSH-medium for 24h supplemented with 0-50 microM RS21745 or 0-100 microM RS21607 (two specific inhibitors of lanosterol 14alpha-demethylase that converts lanosterol to FF-MAS), or cultured in FSH-medium with 25 microM RS21745 for 0-24h firstly, then transferred into a new FSH-medium (the total culture time is 24h). The results revealed that RS21745 or RS21607 could inhibit FSH-induced porcine CEOs meiotic resumption in a dose and time-dependent manner. Meanwhile, FSH-induced cumulus expansion could also be inhibited dose-dependently by RS21745 or RS21607. Otherwise, AY9944-A-7, an inhibitor of Delta14-reductase which promotes cholesterol accumulation from FF-MAS, had no effect on both denuded oocytes (DOs) cultured for 24 or 44 h and CEOs cultured for 24h meiotic resumption, but it could promote CEOs meiotic resumption after 44 h culture. In addition, we got that 10(-8) to 10(-6)M PMA, an activator of PKC pathway, could reverse the inhibiting effect of RS21745 on FSH-induced CEOs meiotic resumption and enhance the rate of germinal vesicle breakdown (GVBD) of CEOs cultured in medium with hypoxanthine (HX). Moreover, 5-10 microM chelerythrine chloride, an inhibitor of PKC, could enhance the inhibitory effect of RS21745 on FSH-induced porcine oocytes resumption of meiosis. All the data of this study support that endogenous FF-MAS takes part in the FSH-induced porcine oocytes meiotic resumption and might play an active role via PKC signal pathway.

cDNA Cloning, Genomic Structure and Expression Analysis of the Bovine Lanosterol 14.ALPHA.-Demethylase (CYP51) in Gonads

Meiosis activating sterol (MAS), the intermediate of cholesterol biosynthesis, is an important substance to stimulate oocytes maturation in FSH-induced signal transduction pathway. Lanosterol 14alpha-demethylase (CYP51) converts lanosterol to MAS. Although MAS is firstly isolated from bovine testis, the information about bovine CYP51 gene and its expression is little. In present studies, the cDNA cloning, genomic structure, chromosomal mapping, and expression patterns of bovine CYP51 were demonstrated. The cDNA coding bovine CYP51 contains a 1509 bp open reading frame and a 1119 bp 3' untranslated region. And the bovine CYP51 gene includes 10 exons and spans about 17 kb. Screening the cattle RH5000 panel bovine CYP51 is mapped to chromosome 4 (0cR). The sequenced promoter region is TATA-less and contains several highly conserved regulatory elements, such as GC-box, cAMP-responsive elements (CRE), sterol regulatory element (SRE) which is important fragment for its transcription. No evidence of processed pseudogenes is found using long PCR and Southern blot. Northern blot analysis reveals that an approximately 2.7 kb mRNA is expressed in all the examined bovine tissues, while a 1.8 kb mRNA is found only in the mature bovine testis where the MAS is accumulated. Immunochemistry analysis shows that leydig cells express the highest level of the CYP51 protein in testis. Among different stages follicles it is localized primarily to the oocytes with the level varying slightly. Granulosa cells of primordial, primary and secondary follicles show background staining. While granulosa cells facing the antrum and cumulus granulosa cells of antral follicles show considerably heavier staining. The highest level is expressed in corpus lutea. These data indicate a stage- and cell type-specific expression of CYP51 protein in bovine oogenesis.

Oocyte maturation: the coming of age of a germ cell

Normal female fertility relies on proper development of the oocyte. This growth culminates just prior to ovulation, when oocyte maturation occurs. Oocyte maturation refers to a release of meiotic arrest that allows oocytes to advance from prophase I to metaphase II of meiosis. This precisely regulated meiotic progression is essential for normal ovulation and subsequent fertilization, and involves changes in the delicate balance between factors promoting meiotic arrest and others that are stimulating maturation. Most of the inhibitory mechanisms appear to involve the upregulation of intracellular cyclic adenosine monophosphate levels. These processes may include direct transport of the nucleotide into oocytes via gap junctions, G protein-mediated stimulation of adenylyl cyclase, and inhibition of intracellular phosphodiesterases. In contrast, potential factors that play roles in triggering oocyte maturation include gonadotropins (e.g., follicle-stimulating factor and luteinizing hormone), growth factors (e.g., amphiregulin and epiregulin), sterols (e.g., follicular fluid-derived meiosis-activating sterol), and steroids (e.g., testosterone progesterone, and estradiol). Delineating the complex interactions between these positive and negative components is critical for determining the role that oocyte maturation plays in regulating follicle development and ovulation, and may lead to novel methods that can be used to modulate these processes in women with both normal and aberrant fertility.

Resumption of oocyte meiosis in mammals: on models, meiosis activating sterols, steroids and EGF-like factors

De novo synthesis of meiosis activating sterols (MAS) was stimulated by LH- and AY-9944 in rat cultured follicles and cumulus oocyte complexes (COCs), but could not be measured in denuded oocytes. Thus, MAS synthesized by the somatic compartment of the follicle could serve as a signal for the resumption of meiosis. Nevertheless, the delay in germinal vesicle breakdown (GVB) after MAS or AY-9944 stimulation as compared with gonadotropins, obtained by several groups, remains the strongest evidence against the suggested role of MAS as an essential mediator of LH in meiosis resumption. Recently several studies using mammalian COCs in culture have implied that steroids, like in fish and amphibians, serve as signals in mediating the LH/hCG stimulation of meiosis. However, in these studies there was no clear distinction between the requirement for steroids for the acquisition of meiotic competence, oocyte and follicle wellbeing or as a signal for meiotic resumption. Further, some of the authors overlooked earlier studies showing that blocking ovarian or follicular steroidogenesis does not affect GVB, the first step of meiosis resumption. Finally, in vivo and in vitro studies in the rat confirm and extend recent studies showing that locally produced and released EGF-like factors, such as epiregulin, seem to mediate at least part of the LH/hCG actions on oocyte maturation and release of ova at ovulation.

Dual-action hypoglycemic and hypocholesterolemic agents that inhibit glycogen phosphorylase and lanosterol demethylase

Diabetic dyslipidemia requires simultaneous treatment with hypoglycemic agents and lipid-modulating drugs. We recently described glycogen phosphorylase inhibitors that reduce glycogenolysis in cells and lower plasma glucose in ob/ob mice (J. Med. Chem., 41: 2934, 1998). In evaluating the series prototype, CP-320626, in dogs, up to 90% reduction in plasma cholesterol was noted after 2 week treatment. Cholesterol reductions were also noted in ob/ob mice and in rats. In HepG2 cells, CP-320626 acutely and dose-dependently inhibited cholesterolgenesis without affecting fatty acid synthesis. Inhibition occurred together with a dose-dependent increase in the cholesterol precursor, lanosterol, suggesting that cholesterolgenesis inhibition was due to lanosterol 14alpha-demethylase (CYP51) inhibition. In ob/ob mice, acute treatment with CP-320626 resulted in a decrease in hepatic cholesterolgenesis with concomitant lanosterol accumulation, further implicating CYP51 inhibition as the mechanism of cholesterol lowering in these animals. CP-320626 and analogs directly inhibited rhCYP51, and this inhibition was highly correlated with HepG2 cell cholesterolgenesis inhibition (R2 = 0.77). These observations indicate that CP-320626 inhibits cholesterolgenesis via direct inhibition of CYP51, and that this is the mechanism whereby CP-320626 lowers plasma cholesterol in experimental animals. Dual-action glycogenolysis and cholesterolgenesis inhibitors therefore have the potential to favorably affect both the hyperglycemia and the dyslipidemia of type 2 diabetes.

Meiosis-Activating Sterol Synthesis in Rat Preovulatory Follicle: Is It Involved in Resumption of Meiosis?1

Meiosis-activating sterol (MAS) was shown to overcome the inhibitory effect of hypoxanthine on spontaneous maturation of mouse oocytes and was suggested to mediate the stimulation of meiosis by gonadotropins. Follicular fluid (FF)-MAS is synthesized by cytochrome P450 lanosterol 14alpha-demethylase (LDM). Follicular LDM was preferentially localized in oocytes by immunohistochemistry. Using [3H]acetate or R-[5-3H]mevalonate as precursors as well as high-performance liquid chromatographic and thin-layer chromatographic separation, we have measured the concentrations of de novo-synthesized lanosterol, FF-MAS, and cholesterol in rat graafian follicles, cumulus-oocyte complexes (COCs), and denuded oocytes (DOs) treated with LH, AY-9944 (an inhibitor of Delta14-reductase, which was anticipated to increase FF-MAS levels by inhibiting its metabolism), or both after 8 h of culture. In follicles, both LH and AY-9944 increased the accumulation of FF-MAS as compared to controls. In COCs, AY-9944 caused a marked increase in FF-MAS, but we were unable to detect accumulation of FF-MAS in DOs. Neither the endogenous increases in FF-MAS accumulation nor the addition of FF-MAS to the culture medium could overcome the inhibition on resumption of meiosis by phosphodiesterase inhibitors. Compared to LH-induced resumption of meiosis in follicles, that induced by AY-9944 was much delayed. These results call into question any role of FF-MAS as an obligatory mediator of LH activity on germinal vesicle breakdown. The discrepancy between the positive staining for LDM in oocytes and our inability to detect de novo synthesized FF-MAS in DOs may relate to the sensitivity of the methodology employed and either the number of oocytes used or a deficiency in LDM synthetic activity in such oocytes. Further studies are required to confirm any of these alternatives.

Mouse oocyte meiotic resumption and polar body extrusion in vitro are differentially influenced by FSH, epidermal growth factor and meiosis-activating sterol

BACKGROUND

In this study, we compared the relative ability of FSH (100 mIU/ml), epidermal growth factor (EGF) (10 ng/ml), and follicular-fluid meiosis-activating sterol (FF-MAS, 10 micromol/l) to induce meiotic resumption and polar body I (PBI) extrusion in mouse oocytes.

METHODS

Cumulus-enclosed oocytes (CEO) were co-incubated with meiosis-arresting agents, including 4 mmol/l hypoxanthine (Hx), 0.3 mmol/l dibutyryl cAMP (dbcAMP), and 8.5 micromol/l cilostamide, a selective inhibitor of the oocyte-specific phosphodiesterase 3 (PDE 3).

RESULTS

In Hx-treated oocytes, FSH, EGF and FF-MAS induced meiosis resumption at very high rates, but only FSH and EGF also promoted PBI extrusion with high frequency. In experiments conducted in the presence of dbcAMP, FF-MAS was unable to promote an increase in germinal vesicle breakdown (GVBD) rate, whereas FSH and EGF generated a response similar to the Hx groups. Neither FSH, EGF nor FF-MAS caused any change in the meiotic status of CEO when meiotic arrest at the germinal vesicle (GV) stage was maintained by cilostamide. In the presence of Hx, naked oocytes (NkO) co-cultured with their cumulus cells were able to respond to the GVBD-inducing effect of FSH and EGF by resuming meiosis at high rate.

CONCLUSIONS

Collectively, these results indicate that: (i) a signal triggered in cumulus cells by either FSH or EGF, but not necessarily coincident with FF-MAS, may contribute to meiotic maturation, supporting GVBD and extrusion of PBI; (ii) the transmission of this signal can occur in a paracrine fashion, at least with reference to the breakdown of the GV. It also appears that concomitant regulation of intra-oocyte cAMP degradation is a prerequisite for meiosis resumption.

A synthetic analogue of meiosis-activating sterol (FF-MAS) is a potent agonist promoting meiotic maturation and preimplantation development of mouse oocytes maturing in vitro

BACKGROUND

Follicular fluid-meiosis-activating sterol (FF-MAS) is a factor present in the pre-ovulatory follicle during the time of oocyte maturation. In mouse oocytes maturing in vitro, FF-MAS promotes the completion of meiotic maturation to metaphase II (MII) and improves competence to complete the 2-cell stage to blastocyst transition. We produced analogues of FF-MAS and selected three on the basis of potency to promote the resumption of meiosis by mouse oocytes maintained in meiotic arrest by hypoxanthine. The objective of this study was to determine whether these FF-MAS analogues also affect the quality of oocytes maturing in vitro with respect to the completion of meiotic maturation and augmenting the frequency of development to the blastocyst stage after fertilization in vitro.

METHODS

Cumulus cell-enclosed oocytes were isolated from the small antral follicles of 18 or 20 day post-natal mice. These oocytes normally have a reduced competence to complete meiotic maturation and preimplantation embryo development. Oocytes were isolated at the germinal vesicle stage and matured in vitro using media supplemented with 0.1% ethanol, 1 micromol/l FF-MAS, or 0.1-10 micromol/l FF-MAS analogues ZK255884 (884), ZK255933 (933) and ZK255991 (991). Oocytes that progressed to MII were fertilized in vitro and the percentage developing to the 2-cell and blastocyst stages was determined.

RESULTS

At 1 micromol/l, 991 and 933 increased the portion of oocytes progressing to MII, whereas the lowest dose of 991 and 884 was ineffective. Treatment of maturing oocytes with either 0.1 or 1 micromol/l 933 dramatically increased oocyte competence to complete preimplantation development.

CONCLUSIONS

The synthetic analogue of FF-MAS, ZK255933, is a potent agonist that improves the quality of mouse oocytes matured in vitro. This compound may therefore have therapeutic value for treatment of oocytes from women undergoing therapy for infertility owing to poor oocyte quality.

Action of hypoxanthine and meiosis-activating sterol on oocyte maturation in the mouse is strain specific

Follicular fluid meiosis-activating sterol (FF-MAS) is regarded as an important compound relevant to meiotic resumption in mammalian oocytes. The objective of this study was to investigate the influence of FF-MAS on germinal vesicle breakdown (GVBD) and first polar body (PBI) extrusion with regard to culture conditions, state of the oocyte and mouse strain. Denuded oocytes (DO) and cumulus-enclosed oocytes (CEO) were retrieved from PMSG-primed Quackenbush or C57BL/6J x DBA/2 (C57) mice and cultured for 20 h in alpha-MEM medium under the following conditions: (i) 250 micromol/l dibutyryl cAMP (dbcAMP) +/- EGF, 1 ng/ml or FF-MAS, 20 micromol/l; (ii) 4 mmol/l hypoxanthine (HX) +/- EGF or FF-MAS; (iii) HX + EGF + FF-MAS; and (iv) HX + FF-MAS 5 h priming and subsequent culture with HX + EGF. Oocyte GVBD and PBI emission were recorded and stained with Hoechst 33342. Very limited meiotic inhibition was observed in Quackenbush mice in comparison with C57 mice. FF-MAS promoted maturation in C57 DO and CEO and Quackenbush DO. In Quackenbush DO and CEO and C57 DO a significant increase in atypical PBI extrusion occurred, but not in C57 CEO as well as in EGF-treated Quackenbush CEO primed or co-cultured with FF-MAS. These results support a meiosis resumption function for FF-MAS and suggest that in its presence, the quality of the MII oocytes retrieved appears to be influenced by the strain of the mice, the state of the oocyte and the presence or absence of growth factors in the culture medium.

Meiosis-Activating Sterol Promotes the Metaphase I to Metaphase II Transition and Preimplantation Developmental Competence of Mouse Oocytes Maturing in Vitro1

The objective of this study was to determine the effects of a sterol found in ovarian follicular fluid, known as meiosis-activating sterol (FF-MAS), on the maturation of mouse oocytes in vitro. Possible effects of FF-MAS in promoting the metaphase I (MI) to metaphase II (MII) transition (nuclear maturation) and the competence of oocytes to complete preimplantation embryo development to the blastocyst stage (cytoplasmic maturation) were assessed. Cumulus cell-enclosed oocytes that were compromised in their ability to undergo nuclear maturation and subsequent development because of the age or genotype of the female were isolated at the germinal vesicle stage and matured in vitro using media supplemented with 0 to 20 microM FF-MAS. Oocytes that progressed to MII were inseminated in vitro, and the percentages developing to the 2-cell and blastocyst stages were determined. The sterol was omitted from the media used for oocyte insemination or preimplantation development. FF-MAS promoted a significantly higher percentage of oocytes in all groups to progress to MII in vitro. Moreover, FF-MAS treatment of oocytes maturing in vitro dramatically increased the competence of all but one of the groups of oocytes to complete preimplantation development. Therefore, FF-MAS improved mouse oocyte quality by promoting both nuclear and cytoplasmic maturation in vitro.

Roles of gonadotropins and meiosis-activating sterols in meiotic resumption of cultured follicle-enclosed mouse oocytes

This study describes a model for short-term culture of intact mouse follicles under serum-free conditions. Follicles were either obtained from immature mice receiving no ovarian stimulation (i.e. no eCG-primed protocol, group I) or from mice undergoing ovarian stimulation (i.e. eCG-primed protocol, group II). Follicles were grouped according to size (100-170, 180-200, 210-250, 260-350 and 360-400 microm, respectively) and cultured for 24h (group I) or for only 6h (group II). Induced meiotic resumption of follicle-enclosed oocytes were evaluated following stimulation with gonadotropins (i.e. FSH and hCG), AY9944-A-7, an inhibitor of Delta14-reductase, and RS-21745, an inhibitor of lanosterol 14alpha-demethylase; both enzymes affect synthesis of the meiosis activating sterols (MAS) that induce oocyte maturation. The frequency of oocyte degeneration was also recorded. In group I, FSH (10-200 IUl-1) and AY9944-A-7 (5, 25 and 50 microM) separately induced resumption of meiosis in oocytes derived from follicles with a diameter of 180-400 microm. hCG (1.0 and 10 IUml-1) exhibited a similar but weaker effect on oocytes present in follicles with a diameter of 260-400 microm. Irrespective of follicular diameter oocytes obtained from follicles in group II responded to hCG and FSH by resuming meiosis. FSH (50 IUl-1) alone or hCG (10 IUml-1) alone both increased the GVBD percentage of oocytes enclosed in follicles with a diameter 260-400 microm, but the response to hCG was not significant compared to control. FSH (50 IUl-1) combination with hCG (10 IUml-1) showed an additive effect raising the rate of GVBD after 6h culture. Addition of 50 or 100 microM RS-21745 was able to attenuate gonadotropins-induced resumption of meiosis to below background levels. In conclusion, the ability of FSH to induce meiotic resumption of follicle-enclosed mouse oocytes is correlated to follicle size, being most pronounced in larger follicles. hCG caused a similar but less pronounced effect. The ability of RS-21745 to inhibit and the ability of AY9944-A-7 to enhance oocyte maturation of follicle-enclosed oocytes support the concept of FSH employing MAS as a downstream signal transduction molecule for initiation of oocyte maturation in mice.

In-vitro maturation of human oocytes

Immature human oocytes can be matured and fertilized in vitro. However, subsequent embryonic development is different when the immature oocytes are retrieved in different situations. Exposure to the LH surge in vivo may be important for the oocytes to acquire the competence for maturation and subsequent embryonic development. The size of the follicles may also be an important feature for subsequent embryonic development. However, the developmental competence of oocytes derived from small antral follicles does not seem to be adversely affected by the presence of a dominant follicle. Oocyte maturation in vitro is profoundly affected by culture conditions. Gonadotrophins are required for oocyte maturation in vivo, but any requirement in vitro is still unclear. Recent clinical results from in-vitro matured (IVM) human oocytes are promising, although further research remains to be done in order to address the mechanisms of oocyte maturation and to improve culture conditions and also the implantation rate of embryos generated from IVM oocytes.

Two syntheses of FF-MAS

[reaction: see text] Follicular fluid-meiosis activating sterol (FF-MAS) has been shown to be an efficient inducer of meiotic maturation. It can potentially be used for improvements of in vitro fertilization techniques. Two short synthesis of FF-MAS are presented in this article. Both syntheses are based on microbiological degradations of sterol side chains. FF-MAS can be synthesized in nine steps from commercially available starting materials by both routes.

Production of progesterone from de novo-synthesized cholesterol in cumulus cells and its physiological role during meiotic resumption of porcine oocytes

To investigate the role of factors secreted by cumulus cells during meiotic resumption of porcine oocytes, 1, 5, 10, or 20 cumulus-oocyte complexes (COCs) were cultured in each well of a culture dish containing 300 microl of maturation medium for 20 h. There was a significant positive correlation between the rate of germinal vesicle breakdown (GVBD) and the number of COCs cultured in each well for 20 h. The level of progesterone in the medium in which COCs had been cultured for 20 h also rose significantly with an increase in the number of COCs cultured in each well. A significantly small proportion of GVBD in oocytes when one COC was cultured in each well for 20 h was improved by the addition of progesterone. This proportion of GVBD was fully comparable to that of COCs cultured in the absence of additional progesterone with 20 COCs. Thus, progesterone secreted by COCs plays a positive role in GVBD induction in porcine oocytes. Furthermore, we also examined the role of sterol biosynthesis on progesterone production by cumulus cells and in oocyte GVBD. The results showed that the addition of ketoconazole, which suppressed the sterol biosynthetic pathway produced by demethylation of lanosterol, decreased the rate of GVBD, as well as progesterone production in COCs cultured for 20 h. However, the suppression of GVBD by ketoconazole was overtaken by the addition of progesterone. These results demonstrate that a high level of progesterone produced by cumulus cells was responsible for an acceleration of GVBD in porcine oocytes.

Many facets of mammalian lanosterol 14alpha-demethylase from the evolutionarily conserved cytochrome P450 family CYP51

Lanosterol 14alpha-demethylase is a cytochrome P450 enzyme of the cholesterol biosynthetic pathway belonging to the CYP51 gene family which is the most evolutionarily conserved member of the CYP superfamily. Mammalian (human, mouse, rat, pig) CYP51 genes are unique in sharing several common characteristics: highly conserved exon/intron borders and proximal promoter structures, ubiquitous expression at the highest level in the testis, and appearance of testis-specific transcripts that arise from differential polyadenylation site usage. CYP51 protein demethylates lanosterol to form follicular fluid meiosis-activating sterol, FF-MAS, which is, besides being an intermediate of cholesterol biosynthesis, also a signaling sterol that accumulates in ovaries. CYP51 protein resides in the endoplasmatic reticulum of most cells and also in acrosomal membranes of spermatids where transport through the Golgi apparatus is suggested. While sterol regulatory element binding protein (SREBP)-dependent transcriptional regulation of CYP51 contributes to synthesis of cholesterol, the germ-cell-specific cAMP/CREMtau-dependent upregulation might contribute to increased production of MAS.

Physiology of meiosis-activating sterol: endogenous formation and mode of action

BACKGROUND

In the context of mammalian oocyte maturation, it has been suggested that intermediates of cholesterol biosynthesis may represent the physiological signal that instructs the oocyte to reinitiate meiosis.

METHODS

Endogenous levels of follicular fluid meiosis-activating sterol (FF-MAS) were monitored in rabbit ovarian tissue, and the influence of exogenous gonadotrophins on sterol formation was assessed. The involvement of cAMP in FF-MAS-induced versus spontaneous oocyte maturation in vitro in mice was also investigated, as was the direct microinjection of FF-MAS into mouse oocytes.

RESULTS

Levels of FF-MAS in rabbit ovaries were significantly elevated 1 h after hCG/LH induction and remained so for 4 and 12 h after induction. In naked oocytes undergoing spontaneous maturation, a significant decrease in cAMP was detected after 30 min of culture. However, FF-MAS-mediated induction of oocyte maturation in hypoxanthine-arrested naked oocytes was not associated with any detectable decrease in intracellular cAMP levels. Microinjected FF-MAS failed to induce any noticeable meiosis.

CONCLUSIONS

A rapid increase in FF-MAS level occurred in vivo in the rabbit ovary in response to LH, and clear differences were seen in the cAMP pattern during spontaneous and induced oocyte maturation in mice.

Progesterone and 17 alpha-OH-progesterone in concentrations similar to that of preovulatory follicular fluid is without effect on resumption of meiosis in mouse cumulus enclosed oocytes cultured in the presence of hypoxanthine

Some intermediates in the cholesterol biosynthesis between lanosterol and cholesterol are capable of inducing resumption of meiosis in cultured mouse oocytes without the presence of gonadotropins. The mechanism by which these so-called Meiosis Activating Sterols (MAS) activate the meiotic process is unknown, and it is uncertain whether they participate in the physiological control of resumption of meiosis. Recently, it has been shown that accumulation of MAS occurs in a liver cell line and in rat testis tissue cultured in the presence of micromolar concentrations of progesterone and 17 alpha-OH-progesterone. Such high concentrations of progesterone and 17 alpha-OH-progesterone only occur in fluid of preovulatory follicles. In connection with the mid-cycle surge of gonadotropins, this may represent one mechanism whereby follicular accumulation of MAS takes place. In the present study, the effect of 10 micro M progesterone and 10 micro M 17 alpha-OH-progesterone on resumption of meiosis was evaluated using mouse cumulus enclosed oocytes (CEO) cultured in the presence of 4mM hypoxanthine. By the end of the 24-h culture period, the frequency by which oocytes had resumed meiosis was assessed by the determination of germinal vesicle breakdown (GVBD). Neither progesterone nor 17 alpha-OH-progesterone or a combination showed any effect on GVBD. In addition, progesterone and 17 alpha-OH-progesterone in combination with a sub-optimal dose of FSH (4 IU/l) did not affect GVBD. In conclusion, accumulation of MAS to an extent that allows resumption of meiosis to occur in CEO is unlikely to be induced by progesterone and 17 alpha-OH-progesterone or a combination.

Role of meiosis activating sterols, MAS, in induced oocyte maturation

Meiosis of follicle enclosed oocytes is maintained in the prophase of the first meiotic division and oocytes do not spontaneously resume meiosis during oocyte growth and follicle development. Arrest of the meiotic process is most likely secured by the presence of follicular purines, e.g. hypoxanthine, which maintain high levels of cAMP in the oocyte and which also in vitro prevent oocytes from resuming meiosis. Only in response to the mid-cycle surge of gonadotropins will oocytes of preovulatory follicles overcome the meiosis arresting effect of hypoxanthine and resume meiosis proceeding to the metaphase of the second meiotic division. Morphologically, resumption of meiosis is observed by the disappearance of the oocyte's nuclear membrane (germinal vesicle), a process called germinal vesicle breakdown (GVB). The molecular mechanism down-stream to receptor activation by which the mid-cycle surge of gonadotropins induces oocytes to resume meiosis is, however, only partly understood. The oocyte itself lacks gonadotropin receptors and its action is mediated through the attached cumulus cells. In vitro it has been shown that FSH induces synthesis of a signal in the cumulus cells, which overcomes the meiosis arresting effect of hypoxanthine. We have shown that a group of sterols, meiosis activating sterols (MAS), induces oocyte maturation in vitro even in oocytes depleted of cumulus cells. MAS were identified as intermediates in the cholesterol biosynthesis between lanosterol and cholesterol. The two best characterized members of the MAS family are FF-MAS purified from human follicular fluid (4,4-dimethyl-5alpha-cholest-8,14,24-triene-3beta-ol) and T-MAS purified from bull testicular tissue (4,4-dimethyl-5alpha-cholest-8,24-diene-3beta-ol). The synthesis, quantification, localization and tissue-accumulation of MAS are reviewed. Several publications have documented the pharmacological effect of MAS in different species, including oocytes from mouse, rat and human. Conflicting results obtained by the use of sterol synthesis inhibitors, which prevent MAS-accumulation, are also discussed. Whether FSH actually uses MAS as a signal transduction molecule for inducing oocyte maturation and the mechanism by which MAS induce resumption of meiosis is currently unknown, but data to support that MAS is part of the FSH induced signal transduction pathway are presented.

Is meiosis activating sterol (MAS) an obligatory mediator of meiotic resumption in mammals

In-vitro studies of mouse oocytes have provided evidence that two closely related sterols, subsequently named meiosis-activating sterols (MAS), can overcome the inhibitory effect of hypoxanthine on resumption of meiosis. These sterols are synthesized by cytochrome P450 (CYP) lanosterol 14alpha-demethylase (LDM), a key enzyme in cholesterol biosynthesis. Our studies in the rat with specific inhibitors and molecular approaches did not support the hypothesis that MAS is an obligatory step in the stimulation of the resumption of meiosis. (i) Specific inhibitors of MAS synthesizing enzymes did not prevent spontaneous or LH-stimulated meiosis at doses that have previously been shown to effectively suppress LDM activity. At higher doses, they caused degeneration of oocytes. (ii) The timing of LDM expression in the ovary was incompatible with a role for MAS in meiosis. (iii) The preferential localization of LDM protein in the oocytes suggests MAS production in oocytes, rather than its transport from the somatic compartment as expected by the suggested role of MAS in the regulation of meiosis as a putative cumulus-oocyte signal molecule. (iv) AY-9944, which supposedly increases MAS levels by inhibiting its metabolism, induced the maturation of follicle-enclosed oocytes that was much delayed as compared with gonadotropic stimulation. Thus, the resumption of meiosis induced by added MAS [Biol. Reprod. 61 (1999) 1362, Biol. Reprod. 64 (2001) 418] or presumed endogenous MAS accumulation by AY-9944, resulted in oocyte maturation with remarkably slower kinetics than observed with LH stimulation. This delay in meiosis after MAS stimulation, the studies with LDM inhibitors and its spatial and temporal expression, cast serious doubts whether MAS is indeed mediating the meiosis inducing action of the gonadotropins, as suggested.

Lanosterol 14alpha-demethylase and MAS sterols in mammalian gametogenesis

The lanosterol 14alpha-demethylase protein complex is composed of a cytochrome P450 enzyme CYP51 and its redox partner NADPH cytochrome P450 reductase. The complex participates in cholesterol biosynthesis and produces folicular fluid meiosis activating sterol (FF-MAS) from lanosterol. FF-MAS is metabolized further by sterol Delta14-reductase to testis-meiosis activating sterol (T-MAS). Additional enzymatic steps are needed before cholesterol is produced. Using the anti-human CYP51 antibody we have studied CYP51 protein expression by confocal microscopy in male and female mouse gonads. Leydig cells and acrosomes of spermatids express the highest levels of the CYP51 protein. CYP51 protein is also detected in primary mouse oocytes of non-treated mice and in some granulosa cells. While regulatory mechanisms responsible for FF-MAS accumulation in the ovary are not yet established, two mechanisms contributing to production the of T-MAS in the testis have been found. Potential in vivo roles of FF-MAS and T-MAS in fertilization are discussed.

Production of meiosis-activating sterols from metabolically engineered yeast

Meiosis-activating sterols (MAS), a class of potent regulators of reproductive processes, are difficult to obtain by chemical synthesis or isolation from natural sources. We demonstrate the development of metabolically engineered strains of Saccharomyces cerevisiae that accumulate MAS as the predominant sterol product. Homologous recombination was used to construct an erg24Delta erg25Delta hem1Delta mutant RXY4.3, which lacked sterol Delta14 reductase, C-4 oxidase, and delta-aminolevulinate synthase. The HEM1 deletion allowed sterol import and rendered RXY4.3 viable under aerobic conditions. This mutant accumulated 4,4-dimethyl-5alpha-cholesta-8,14,24-trien-3beta-ol (FF-MAS), and a similar erg25Delta hem1Delta mutant produced 4,4-dimethyl-5alpha-cholesta-8,24-dien-3beta-ol (T-MAS). Based on consistent yields of approximately 5 mug of FF-MAS per mL of culture, fermentation of genetically modified yeast compares favorably with other approaches to produce MAS.

Gonadotropin-induced accumulation of 4,4-dimethylsterols in mouse ovaries and its temporal relation to meiosis

The resumption of oocyte meiosis is triggered by a number of 4,4-dimethylsterols termed meiosis-activating sterols (MAS). The levels of meiosis active (follicular fluid [FF]-MAS and bull testes [T]-MAS) and inactive (lanosterol) 4,4-dimethylsterols, free cholesterol, and progesterone were determined in gonadotropin-primed prepubertal mouse ovaries in vivo by high-performance liquid chromatography. Ovaries responded to an ovulatory stimulation by increasing their content of 4,4-dimethylsterols but not of free cholesterol. The ovarian 4,4-dimethylsterol response was followed with regard to time and dose-response to the gonadotropins and the resumption of meiosis was evaluated using histologic sections. All 4,4-dimethylsterols accumulated in a time-dependent manner in gonadotropin-primed mice after a subsequent stimulation with hCG. The peak of 4,4-dimethylsterol accumulation appeared postmeiotically but coincided roughly with ovulation, and the resumption of meiosis was triggered when the intraovarian level of MAS was <20% of its maximum. The ovarian accumulation of progesterone preceded the 4,4-dimethylsterol accumulation. The FF-MAS accumulation displayed a dose-response maximum with respect to hCG, and a variation of the follicular priming regime revealed that, in contrast to progesterone production, 4,4-dimethylsterol accumulation is dependent on previous follicular growth beyond the gonadotropin-dependent stage. The FF-MAS was not liberated from esterified stores during the accumulatory response and appeared to be synthesized de novo from a precursor (or precursors) metabolically upstream to lanosterol. The data remain inconsistent with a model in which MAS is regarded as the physiological trigger of meiosis. The 4,4-dimethylsterol accumulation is suggested to influence maturation processes by affecting membrane sterol composition.

Involvement of MEK-mitogen-activated protein kinase pathway in follicle-stimulating hormone-induced but not spontaneous meiotic resumption of mouse oocytes

Mitogen-activated protein (MAP) kinase has been reported to be activated during oocyte meiotic maturation in a variety of mammalian species. However, the mechanism(s) responsible for MAP kinase activation and the consequence of its premature activation during gonadotropin-induced oocyte meiotic resumption have not been examined. The present experiments were conducted to investigate the possible role of MAP kinase in FSH-induced and spontaneous oocyte meiotic resumption in the mouse. MAP kinase kinase (MAPKK, MEK) inhibitor, PD98059 or U0126, produced a dose-dependent inhibitory effect on both FSH-induced oocyte meiotic resumption and MAP kinase activation in the oocytes. However, the same inhibitor did not block spontaneous meiotic resumption of either denuded or cumulus cell-enclosed mouse oocytes, despite the activity of MAP kinase being totally inhibited. Immunoblotting the oocytes and the cumulus cells with the anti-active MAP kinase antibody showed that MAP kinase activity in the oocytes was detected at 8 h of FSH treatment, prior to germinal vesicle breakdown and increased as maturation progressed in the following culture period. In the cumulus cells, MAP kinase was activated even faster, its activity was detected at 1 h of FSH stimulation and increased gradually until 8 h of FSH treatment, then decreased and diminished after 12 h of FSH action. These data demonstrated that the MEK-MAP kinase pathway is implicated in FSH-induced but not spontaneous oocyte meiotic resumption.