Transduction mechanisms for gonadotrophin-induced oocyte maturation in mammals

Electro-Metabolic Coupling of Cumulus-Oocyte Complex

Oocyte-cumulus cell interaction is essential for oocyte maturation and competence. The bidirectional crosstalk network mediated by gap junctions is fundamental for the metabolic cooperation between these cells. As cumulus cells exhibit a more glycolytic phenotype, they can provide metabolic substrates that the oocyte can use to produce ATP via oxidative phosphorylation. The impairment of mitochondrial activity plays a crucial role in ovarian aging and, thus, in fertility, determining the success or failure of assisted reproductive techniques. This review aims to deepen the knowledge about the electro-metabolic coupling of the cumulus-oocyte complex and to hypothesize a putative role of potassium channel modulators in order to improve fertility, promote intracellular Ca2+ influx, and increase the mitochondrial biogenesis and resulting ATP levels in cumulus cells.

Participation of the adenosine salvage pathway and cyclic AMP modulation in oocyte energy metabolism

A follicular spike in cyclic AMP (cAMP) and its subsequent degradation to AMP promotes oocyte maturation and ovulation. In vitro matured (IVM) oocytes do not receive the cAMP increase that occurs in vivo, and artificial elevation of cAMP in IVM cumulus-oocyte complexes improves oocyte developmental potential. This study examined whether mouse oocytes can use the cAMP degradation product AMP to generate ATP via the adenosine salvage pathway, and examined whether pharmacological elevation of cAMP in IVM cumulus-oocyte complexes alters ATP levels. Oocytes cultured with isotopic ¹³C₅-AMP dose-dependently produced ¹³C₅-ATP, however total cellular ATP remained constant. Pharmacological elevation of cAMP using forskolin and IBMX prior to IVM decreased oocyte ATP and ATP:ADP ratio, and promoted activity of the energy regulator AMPK. Conversely, cumulus cells exhibited higher ATP and no change in AMPK. Culture of oocytes without their cumulus cells or inhibition of their gap-junctional communication yielded lower oocyte ¹³C₅-ATP, indicating that cumulus cells facilitate ATP production via the adenosine salvage pathway. In conclusion, this study demonstrates that mouse oocytes can generate ATP from AMP via the adenosine salvage pathway, and cAMP elevation alters adenine nucleotide metabolism and may provide AMP for energy production via the adenosine salvage pathway during the energetically demanding process of meiotic maturation.

Effect of Acrylamide on Oocyte Nuclear Maturation and Cumulus Cells Apoptosis in Mouse In Vitro

Acrylamide (ACR) is a chemical compound with severe neurotoxicity, genotoxicity, carcinogenicity and reproductive toxicity. Recent studies showed that ACR impairs the function of reproductive organs, e.g., epididymis and testes. In vitro maturation of mouse oocyte is a sensitive assay to identify potential chemical hazard to female fertility. The aim of this study was to evaluate the adverse effects of ACR on the nuclear maturation and cumulus cells apoptosis of mouse oocytes in vitro. Cumulus–oocyte complexes were incubated in a maturation medium containing 0, 5, 10 and 20 μM of ACR. Chromosome alignment and spindle morphology of oocytes was determined by immunofluorescence and confocal microscopy. Our results showed that oocytes exposed to different doses of ACR in vitro were associated with a significant decrease of oocyte maturation, significant increase of chromosome misalignment rate, occurrence of abnormal spindle configurations, and the inhibition of oocyte parthenogenetic activation. Furthermore, apoptosis of cumulus cells was determined by TUNEL and CASPASE-3 assay. Results showed that apoptosis in cumulus cells was enhanced and the expression of CASPASE-3 was increased after cumulus–oocyte complexes were exposed to ACR. Therefore, ACR may affect the nuclear maturation of oocytes via the apoptosis of cumulus cells in vitro.

Impact of gonadotropin supplementation on the expression of germ cell marker genes (MATER, ZAR1, GDF9, and BMP15) during in vitro maturation of buffalo (Bubalus bubalis) oocyte

The present study was designed to investigate whether gonadotropins [follicle-stimulating hormone (FSH) and luteinizing hormone (LH)] and buffalo follicular fluid (bFF) supplementation in maturation medium influences the transcript abundance of germ cell marker genes [maternal antigen that embryos require (MATER), Zygote arrest 1 (ZAR1), growth differentiation factor 9 (GDF9), and bone morphogenetic protein 15 (BMP15)] mRNA in buffalo (Bubalus bubalis) oocytes. Buffalo ovaries were collected from local abattoir, oocytes were aspirated from antral follicles (5-8 mm) and matured in vitro using two different maturation regimens, viz, group A: gonadotropin (FSH and LH) and group B: non-gonadotropin-supplemented maturation medium containing 20% buffalo follicular fluid (bFF). mRNA was isolated from immature (330) and in vitro matured oocytes from both the groups (A, 320; B, 340), and reverse transcribed using Moloney murine leukemia virus reverse transcriptase. Expression levels of MATER, ZAR1, GDF9, and BMP15 mRNA transcripts were analyzed in oocytes of both maturation groups as well as immature oocytes using real-time PCR. QPCR results showed that GDF9 and BMP15 transcripts were significantly (p<0.05) influenced with gonadotropins and bFF supplementation during in vitro maturation of buffalo oocyte; however, MATER and ZAR1 transcripts were not influenced with gonadotropins and bFF supplementation in vitro. These results indicated that the expression levels of MATER, ZAR1, GDF9, and BMP15 mRNA were varied differentially during in vitro maturation of buffalo oocyte and were found to be gonadotropins (FSH and LH) or bFF dependent for GDF9 and BMP15.

Effect of lanosterol on the in vitro maturation in semi-defined culture system of prepubertal ewe oocytes

The choice of medium and supplements can affect meiotic regulation and may have an impact on the regulation of mammalian oocyte growth and embryonic cell function. The aim of the present study was to assess the effects of oxygen concentration and endogenous lanosterol on the in vitro maturation (IVM) media without serum and based on recombinant human chorionic gonadotrophin in prepubertal ewe oocytes. Firstly, the effect of varying oxygen concentrations (5% and 20%) during IVM in TCM-199 supplemented (4 mg/ml bovine serum albumin (BSA), 100 μM cysteamine, 0.3 mM sodium pyruvate, 0.1 UI/ml recombinant follicle-stimulating hormone (r-FSH; Gonal-F® 75 UI, Serono, Italy), 0.1 UI/ml recombinant leuteinizing hormone (r-LH; Lhadi® 75 UI, Serono, Italy) and 1 μg/ml estradiol-17β) on subsequent nuclear maturation of oocytes examined under ultraviolet light following staining with bisbenzimide (Hoechst 33342) was investigated. Secondly, two concentrations of lanosterol (0, 10 and 50 μM) were added to the IVM medium. Nuclear maturation of oocytes was examined as previously. Lipid content in oocytes, an important indicator of cytoplasmic maturity, was also measured using Nile red fluorescent stain. The results showed that low oxygen concentration affected the nuclear maturation. Similarly, a significantly higher rate of meiosis resumption was observed with 10 μM (72.3%) of lanosterol compared with the control (51.8%) or 50 μM of lanosterol (59.4%). A significantly higher content of lipids was also observed with 10 and 50 μM of lanosterol (7.3 ± 0.2 × 106 and 7.4 ± 0.2 × 106 arbitrary units of fluorescence) compared with the control (6.7 ± 0.2 × 106 arbitrary units of fluorescence). The results indicate that 10 μM lanosterol during IVM in medium without serum and based on recombinant human chorionic gonadotrophin has a positive effect on maturation of prepubertal ewe oocytes.

DNA fragmentation in canine immature grade 1 cumulus-oocyte complexes

In this work, we studied the incidence of DNA fragmentation, interpreted as apoptotic changes and assessed by the TUNEL assay, in cumulus cells and oocytes of immature Grade 1 cumulus-oocyte complexes (COCs) obtained from healthy bitches (n = 27) of three age groups: young (1-3 years; n = 13), adult (4-6 years; n = 8) and elderly (7-10 years; n = 6). Age affected (p < 0.05) Grade 1 COCs recovery rates, with young animals yielding more (p < 0.01) Grade 1 COCs than the other two age groups. Conversely, no differences were observed in the incidence of DNA fragmentation (TUNEL-positive) in cumulus cells or oocytes between the three age groups. Overall, more than 80% of Grade 1 COCs presented <15% of TUNEL-positive cumulus cells and enclosed TUNEL-negative (intact DNA) oocytes. Despite a higher proportion of TUNEL-negative oocytes being found in the germinal vesicle stage, most of the oocytes with nuclear material compatible with meiosis resumption (MR) or with non-identifiable nuclear material (ND) did not present DNA fragmentation. No correlation was observed between DNA fragmentations in oocytes and in cumulus cells. We concluded that the morphological parameters used to classify canine Grade 1 COCs are reliable to select a homogeneous population of COCs with low incidence of DNA fragmentation. Furthermore, these results indicate that DNA fragmentation can only explain a minor proportion of the incidence of MR and degeneration in canine oocytes at collection.

Meiotic maturation of incompetent prepubertal sheep oocytes is induced by paracrine factor(s) released by gonadotropin-stimulated oocyte-cumulus cell complexes and involves mitogen-activated protein kinase activation

In this study, sheep oocyte-cumulus cell complexes (OCC) derived from medium (M) antral follicles (M-OCC) were in vitro matured alone or in coculture with OCC derived from small (S) antral follicles (S-OCC) to investigate the contribution of cumulus cells (CC) and oocytes to the process of oocyte meiotic maturation and cumulus expansion (CE). Experiments were conducted with or without gonadotropins (FSH/LH). Regardless of culture conditions, about 12% of S-oocytes reached the metaphase II stage, and S-CC showed a low degree of CE. In contrast, both maturational processes were significantly stimulated by gonadotropins in M-OCC. However, about 48% of S-oocytes progressed to metaphase II, and S-CC expanded after coculture with gonadotropin-stimulated M-OCC and M-CC but not with mural granulosa cells. Both maturational processes were inhibited when S-OCC were cocultured with M-denuded oocytes, or when S-denuded oocytes were cocultured with M-CC. The capacity of these paracrine factor(s) to activate the MAPK pathway in somatic and germ cells of S-complexes was investigated. It was found that MAPK kinase/MAPK phosphorylation levels in M-OCC but not in S-OCC were significantly increased by gonadotropins, first in CC and later in the oocytes. Kinase phosphorylations were activated only in S-oocytes cocultured with M-OCC or M-CC. These results demonstrate that soluble factors specifically produced by M-CC are capable to induce meiotic maturation and CE in S-complexes by acting via CC. These factors can induce MAPK activation only in S-oocytes, whose meiotic arrest could be due to the inability of surrounding CC to respond to gonadotropin stimulation.

Spontaneous and LH-induced maturation inBufo arenarumoocytes: importance of gap junctions

It has been demonstrated inBufo arenarumthat fully grown oocytes are capable of meiotic resumption in the absence of a hormonal stimulus if they are deprived of their follicular envelopes. This event, called spontaneous maturation, only takes place in oocytes collected during the reproductive period, which have a metabolically mature cytoplasm.

InBufo arenarum, progesterone acts on the oocyte surface and causes modifications in the activities of important enzymes, such as a decrease in the activity of adenylate cyclase (AC) and the activation of phospholipase C (PLC). PLC activation leads to the formation of diacylglycerol (DAG) and inositol triphosphate (IP3), second messengers that activate protein kinase C (PKC) and cause an increase in intracellular Ca2+. Recent data obtained fromBufo arenarumshow that progesterone-induced maturation causes significant modifications in the level and composition of neutral lipids and phospholipids of whole fully grown ovarian oocytes and of enriched fractions in the plasma membrane. In amphibians, the luteinizing hormone (LH) is responsible for meiosis resumption through the induction of progesterone production by follicular cells.

The aim of this work was to study the importance of gap junctions in the spontaneous and LH-induced maturation inBufo arenarumoocytes. During the reproductive period,Bufo arenarumoocytes are capable of undergoing spontaneous maturation in a similar way to mammalian oocytes while, during the non-reproductive period, they exhibit the behaviour that is characteristic of amphibian oocytes, requiring progesterone stimulation for meiotic resumption (incapable oocytes).

This different ability to mature spontaneously is coincident with differences in the amount and composition of the phospholipids in the oocyte membranes. Capable oocytes exhibit in their membranes higher quantities of phospholipids than incapable oocytes, especially of PC and PI, which are precursors of second messengers such as DAG and IP3.

The uncoupling of the gap junctions with 1-octanol or halothane fails to induce maturation in follicles from the non-reproductive period, whose oocytes are incapable of maturing spontaneously. However, if the treatment is performed during the reproductive period, with oocytes capable of undergoing spontaneous maturation, meiosis resumption occurs in high percentages, similar to those obtained by manual defolliculation.

Interestingly, results show that LH is capable of inducing GVBD in both incapable oocytes and in oocytes capable of maturing spontaneously as long as follicle cells are present, which would imply the need for a communication pathway between the oocyte and the follicle cells. This possibility was analysed by combining LH treatment with uncoupling agents such as 1-octanol or halothane. Results show that maturation induction with LH requires a cell–cell coupling, as the uncoupling of the gap junctions decreases GVBD percentages. Experiments with LH in the presence of heparin, BAPTA/AM and theophylline suggest that the hormone could induce GVBD by means of the passage of IP3or Ca2+through the gap junctions, which would increase the Ca2+level in the oocyte cytoplasm and activate phosphodiesterase (PDE), thus contributing to the decrease in cAMP levels and allowing meiosis resumption.

Apoptosis in cumulus cells, but not in oocytes, may influence bovine embryonic developmental competence

Aim of our study was to clarify if the occurrence of apoptosis in oocytes and cumulus cells is correlated to bovine oocyte developmental competence. The cumulus-oocyte complexes (COCs) were selected according to cumulus status: G1 with more than five layers of compact cumulus cells, G2 with one to five layers of compact cumulus cells and G3 with expanded cumulus cells. The degree of apoptosis in cumulus cells and oocytes measured by caspase staining and TUNEL assay before and after maturation, and 24 h post-insemination was compared to the cleavage, blastocyst formation and hatching rates of each group. Highest cleavage, blastocyst and hatching rates were found in cumulus-oocyte complexes with more than five layers of compact cumulus cells, but no apoptosis was detected in immature or in vitro matured oocytes, regardless of the cumulus status. Many cumulus cells contained active caspases before maturation, but caspase activity declined dramatically after maturation. TUNEL positive cells were rarely observed in each cumulus-oocyte complex upon oocyte recovery, but a huge increase of them was seen after in vitro maturation. Significantly more TUNEL and caspase positive cells were found in G2 cumulus-oocyte complexes. Our results suggest that: (i) oocyte apoptosis does not account for the inferior oocyte quality of G2 and G3; (ii) apoptosis occurs in cumulus cells regardless of the number and compactness of cumulus cells; and (iii) the degree of apoptosis in the compact cumulus-oocyte complexes (G1 and G2) is negatively correlated to the developmental competence of oocyte.

Expression of hyaluronan synthase-3 in porcine oviducal epithelium during oestrus

Hyaluronan (HA) has been related to fertilization and embryo development in the pig. Furthermore, HA is present in pig oviduct fluid and the lining epithelium, particularly of the pre-ovulatory sperm reservoir. Because the mechanisms that regulate HA synthesis have not yet been clarified, semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) was conducted to assess the expression of mRNAs of two HA-synthesizing enzymes (has2 and has3) in the oviduct epithelium (uterotubal junction, isthmus, ampullary-isthmic junction and ampulla segments) of non-inseminated (control) and inseminated (treatment) sows at pre-, peri- and post-ovulatory oestrus. Only has3 mRNA was detected; it was present in all tubal segments of both control and treatment samples. The level of has3 expression did not vary significantly between non-inseminated and inseminated specimens, but there was a tendency (NS) for increased mean values during the peri- and post-ovulatory stages compared with pre-ovulation. It is concluded that has3 is expressed by the porcine endosalpinx epithelium and the levels of expression do not vary during the critical periods of sperm transport and fertilization, despite fluctuating levels of HA in the tubal fluid at corresponding periods.

LH reduces proliferative activity of cumulus cells and accelerates GVBD of porcine oocytes

It has been reported that LH receptor (LHR) mRNA is not detected in cumulus cells of porcine cumulus-oocyte complexes (COCs) just after collection from small antral follicles. The present study showed that the formation of LHR in cumulus cells was up-regulated by the cultivation with 20 ng/ml FSH. When the newly synthesized receptors were stimulated by 1.0 microg/ml LH, significantly higher levels of cAMP and progesterone production in cumulus cells were observed as compared with those of COCs cultured with FSH. A loss of proliferative activity of cumulus cells was induced by the additional LH to FSH-containing medium; however, the inhibitory effect was overcome by progesterone receptor antagonist RU486. Furthermore, the addition of LH also accelerated ongoing GVBD in cumulus cells-enclosed oocytes. These results revealed that during in vitro meiotic maturation of porcine COCs, progesterone secreted by FSH- and LH-stimulated cumulus cells reduced proliferative activity of cumulus cells; the changes of cumulus cells might be involved in inducing meiotic resumption of porcine oocytes.

Role of growth hormone and growth hormone receptor in oocyte maturation

Near the completion of growth, mammalian oocytes acquire the competence to resume and complete meiosis. In vivo the preovulatory LH surge triggers the resumption of meiosis in the oocyte contained in preovulatory follicles. When immature oocytes and the surrounding cumulus cells are released from their follicular environment, resumption of meiosis is induced spontaneously. Culture of bovine cumulus oocyte complexes (COCs) obtained from antral follicles results in blastocyst formation following in vitro maturation, in vitro fertilisation and in vitro embryo culture. Addition of growth hormone (GH) to the maturation medium accelerates nuclear maturation of cumulus enclosed bovine oocytes, induces cumulus expansion and promotes early embryonic development following in vitro fertilisation. The effect of GH is exerted through the cumulus cells and not mediated by IGF-I. Cumulus cells and the oocyte express mRNA for GH receptor. Using specific inhibitors it has been shown that the effect of GH on oocyte maturation and cumulus expansion is mediated by the cyclic AMP signal transduction pathway. Within COCs both cumulus cells and oocyte show GH immunoreactivity while expression of GH mRNA is only found in the oocyte. These observations point to a paracrine and/or autocrine action of GH in oocyte maturation.

Signal transduction mechanism for LH in the cumulus-oocyte complex

The paper reviews recently described signalling mechanisms by which cumulus cells exposed to peak levels of gonadotropins, activate oocyte maturation. Cumulus cells react to LH with a prompt Ca raise which diffuses through gap junctions in a few minutes also into the oocyte where a local amplification system spreads the signal all over the cell. Few h later, still as a consequence of LH stimulation, cumulus cells undergo aprogressive depolarisation of their plasma membrane potential. Due to the electric coupling with these cells the oocyte depolarises too and this open specific voltage gated Ca channels responsible for a second wider and more sustained intracellular Ca rise. As a result of changes throughout maturation with a consequent modification of the size and charge of the molecules that can diffuse from one cell compartment to the other. This cell to cell interaction is further modified with cumulus expansion that leads to a progressive uncoupling of outer cumulus cells while the inner cell layer, corona radiata, remains in oocyte maturation by addressing to the oocyte nutrients and instructions in a well-orchestrated sequence. The identification of these mechanisms are a fundamental prerequisite for the development of in vitro systems suitable to produce oocytes matured in vitro with normal developmental competence.

Calcium elevation in sheep cumulus-oocyte complexes after luteinising hormone stimulation

We investigated Ca2+ levels in intact cumulus-oocyte complexes (COCs) on exposure to peak levels of luteinising hormone (LH). Specific preparations were used where cumulus corona cells were loaded with a membrane-permeant Ca(2+)-sensitive dye (FLUO-3AM), whereas the oocyte was injected directly with the nonpermeant form of the dye (FLUO-3). After exposure to LH, cumulus and corona radiata cells showed distinct rises in intracellular Ca2+ in 50-200 sec. The pattern of Ca2+ response varied in the different cells both for the duration of the transients and for their persistence. Interestingly, Ca2+ elevations were recorded in all the layers of the cumulus mass, including the innermost layer of corona cells, demonstrating the wide diffusion of LH receptors. Following the Ca2+ raise in somatic cells, an intracellular Ca2+ elevation also was recorded within the oocyte with a delay of 100-300 sec. The elevation started at the cortex of the oocyte and then spread all over the ooplasm. The addition of verapamil or manganese chloride did not prevent LH-induced Ca2+ elevation in the COC, whereas mechanical uncoupling of cumulus cells from the oocyte prevented any Ca2+ response within the oocyte. The results indicate that cumulus corona cells are capable of transducing LH message by rising intracellular Ca2+ and show that this signal is rapidly transferred into the oocyte through gap junctions. This may result from the direct diffusion of Ca2+ or its putative releaser IP3 from cumulus cells to the oocyte.