cAMP synthesis in the rat oocyte

cAMP synthesis by the rat oocyte and cumulus-oocyte complex was studied using direct labeling techniques. Cumulus-oocyte complexes synthesized cAMP in response to luteinizing hormone, follicle-stimulating hormone, cholera toxin, and forskolin. However, naked oocytes prepared from cumulus-oocyte complexes by mechanically removing the cumulus cells synthesized cAMP only in response to forskolin and follicle-stimulating hormone; cholera toxin and luteinizing hormone did not stimulate cAMP synthesis. Cholera toxin could augment the response of the oocytes to FSH, indicating an intact, though atypical, adenylate cyclase system. Forskolin was found to inhibit the onset of oocyte maturation in both cumulus-oocyte complexes and naked oocytes. The implications of these findings for the relationship between cAMP synthesis and oocyte maturation in the rat are discussed.     

Spontaneous maturation in vitro of cumulus-enclosed rat oocytes is inhibited by forskolin

We have recently reported that the adenylate cyclase activator, forskolin, induces in the rat ovarian follicle both cAMP accumulation and oocyte maturation. We demonstrate here, on the other hand, that the spontaneous maturation in vitro of isolated rat cumulus-enclosed oocytes is inhibited by forskolin. The inhibitory effect of forskolin is dose dependent with an ED50 at 15 microM. Forskolin inhibition decreases gradually with time, being completely relieved by 20 h of culture. Methylisobutylxanthine significantly prolongs the duration of the inhibitory action of forskolin. In addition to its inhibitory effect on oocyte maturation, forskolin triggers the cumulus-oocyte complex to generate cAMP. Cyclic AMP accumulation is maximally stimulated by 100 microM of forskolin with an ED50 at 60 microM. The potency of the cumulus-oocyte complex to respond to forskolin in terms of cAMP accumulation decreases with time. The pattern of the decrease in the potency of the cumulus-oocyte complexes to generate cAMP corresponds with the relief of its inhibitory influence on the oocyte. These results indicate that inhibition of maturation of the cumulus-enclosed oocyte may be coupled to elevation of cAMP levels in the cumulus-oocyte complex. As isolated cumulus-free oocytes are not inhibited by forskolin, we suggest that in the cumulus-enclosed oocyte system, cAMP generated by the cumulus cells is apparently transferred to the oocyte and maintains it in a meiotically arrested state. Maturation in this system occurs upon relief of inhibition which results from cessation of cAMP generation by the cumulus cells.     

Development of the rat oocyte in vitro: inhibition and induction of maturation in the presence or absence of the cumulus oophorus

The effect in vitro of oocyte maturation inhibitors and the ability of preparations of luteinizing hormone to relieve the arrest induced by these substances was studied in preparations of cumulus-free (naked) rat oocytes, and compared to previously obtained results from oocytes enclosed by their cumulus cells. The development of both the cumulus-oocyte complex and naked oocyte is arrested in vitro by cyclic AMP derivatives or cyclic nucleotide phosphodiesterase inhibitors. While gonadotropins can overcome the effect of these substances in the cumulus oocyte complex, they have no effect on naked oocytes. Cholera enterotoxin, an irreversible activator of adenylate cyclase, maintains developmental arrest in cultured cumulus oocyte complexes but not in naked oocytes. Preparations of luteinizing hormone can partially overcome the effect of cholera enterotoxin in the complexes. Furthermore, the acceleration of oocyte maturation in vitro observed in the presence of gonadotropins, which is seen in cumulus oocyte complexes, can be mimicked by stripping the oocyte of its associated cumulus cells. The results of these and other studies suggest that: (1) the cytoplasmic levels of cyclic AMP in the isolated oocyte are high enough to maintain meiotic arrest if a phosphodiesterase inhibitor is present; (2) the oocyte contains an active phosphodiesterase; (3) the oolemma may lack the adenylate cyclase system and; (4) gonadotropins seem to affect the oocyte indirectly, via the cumulus cells, possibly by interrupting communication between the two cell types.     

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.     

Influence of hyaluronan accumulation during cumulus expansion on in vitro porcine oocyte maturation

During oocyte maturation, the cumulus-oocyte complexes (COCs) expand dramatically. This phenomenon, which is known as cumulus expansion, is the result of the synthesis and accumulation of hyaluronan in the extracellular space between cumulus cells. The purpose of this study was to investigate the effect of 6-diazo-5-oxo-l-norleucine (DON), an inhibitor of hyaluronan synthesis, on cumulus expansion during in vitro porcine oocyte maturation and hyaluronan accumulation within COCs. Further, this study aimed to examine the influence of hyaluronan accumulation within COCs on the rate of oocyte maturation. Cumulus expansion was observed during in vitro maturation. However, the addition of DON to the maturation medium significantly inhibited cumulus expansion. The total inhibition of hyaluronan accumulation within COCs was observed with the use of confocal microscopy. Moreover, a positive correlation between the area of cumulus expansion and the rate of oocyte maturation was observed. These results demonstrate that the hyaluronan accumulation within the COCs during oocyte maturation affects oocyte maturation. On the basis of these results, we propose that hyaluronan accumulation within the COCs during cumulus expansion is a necessary step in the porcine oocyte maturation process.     

Effect of forskolin on the spontaneous maturation and cyclic AMP content of rat oocyte-cumulus complexes

Forskolin (0-100 microM), a reversible stimulator of the catalytic subunit of adenylate cyclase, induced dose-response increases in the % germinal vesicle (GV) (ID50 = 2.68 microM, where ID50 is the dose of forskolin which maintained the meiotic arrest at the germinal vesicle stage, determined cytogenetically, of 50% cultured oocytes), and the cAMP content (determined by RIA) of cumulus-enclosed oocytes and cumulus masses. A significant positive correlation was established between the amount of cAMP within the cumulus mass and that in the corresponding enclosed oocyte (r = 0.78). In contrast, neither the % GV nor the cAMP content of cumulus-free oocytes was affected by the drug. The arresting action of forskolin upon cumulus-enclosed oocytes was dependent upon the presence of adherent cumulus cells, and was transient and fully reversible. The gradual decrease in the % GV of cumulus-enclosed oocytes cultured from 4 to 12 h in 25 microM-forskolin (84, 54 and 22% GV at 4, 8 and 12 h, respectively) was accompanied by a drastic fall in intra-oocyte cAMP at 8 h (41.2 +/- 2.5 and 1.1 +/- 0.6 fmol/oocyte at 4 and 8 h, respectively), while the cumulus cell cAMP content remained constant (135.3 +/- 14.7 and 145.9 +/- 28.7 fmol/cumulus at 4 and 8 h, respectively). Moreover, heterologous metabolic coupling, assessed by determination of the fraction of radiolabelled uridine marker that was transferred from the cumulus mass to the oocyte, significantly decreased. These results show that cumulus cell cAMP is transferred to the rat oocyte where it appears to play a pivotal role in the regulation of meiosis and that the rat oolemma does not appear to possess active catalytic subunits of adenylate cyclase in an amount adequate to stimulate measurably cAMP synthesis.     

Maturation of the rat cumulus-oocyte complex: structure and function

The cumulus cells that surround the mammalian oocyte become dispersed following the preovulatory surge of the pituitary gonadotropin, luteinizing hormone (LH). We have examined cumulus-oocyte complexes of PMSG-primed immature rats before and at 1, 2, 3, 4, 6, and 8 hr after injection of human chorionic gonadotropin (hCG), which acts on the rat ovary like the pituitary gonadotropin. Associations between projections of the cumulus cells and the oocyte were analyzed in thin sections. We observed that some cumulus projections were greatly enlarged where they associate with the oocyte. These enlarged regions were filled with numerous small vesicles. Gap junctions between cumulus cell projections and the oocytes were small. We quantitated the number and size of gap junctions between cumulus cells. The number of small gap junctions (less than 1 microM) between cumulus cells did not change significantly over the 8-hr period after hCG administration. Larger gap junctions, however, showed a general downward trend beginning after the third hour post hCG. Light microscopic observations of plastic sections revealed that dispersion of the cumulus oophorus is not observed until after 4 hr post-hCG, but between 4 and 8 hr after gonadotropin administration the cumulus becomes markedly dispersed. In the majority of the oocytes in these complexes the germinal vesicle (GV) displayed some irregularity in shape at 2 hr post-hCG, although absence of the GV was not observed until later. Our observations suggest a new means of communication in the cumulus-oocyte complex by the vesicle-filled enlargements of the cumulus cell projections at the oocyte surface. They further indicate that the decrease in metabolic coupling observed in rat cumulus-oocyte complexes soon after exposure to LH is not associated with a change in number and size of the gap junctions between the cumulus cells. We suggest that it is either the disruption of the gap junctions at the region of contact of the cumulus cell projections with the oocyte surface or the operation of a gating mechanism that blocks the junctional channels without affecting their morphological appearance that is responsible for uncoupling of the oocyte from the cumulus cells.     

Cell-to-cell communication and ovulation. A study of the cumulus-oocyte complex

Cell-to-cell communication was characterized in cumulus-oocyte complexes from rat ovarian follicles before and after ovulation. Numerous, small gap junctional contacts were present between cumulus cells and oocytes before ovulation. The gap junction are formed on the oocyte surface by cumulus cell processes that transverse the zona pellucida and contact the oolemma. The entire cumulus mass was also connected by gap junctions via cumulus-cumulus interactions. In the hours preceding ovulation, the frequency of gap junctional contacts between cumulus cells and the oocyte was reduced, and the cumulus was disorganized. Electrophysiological measurements indicated that bidirectional ionic coupling was present between the cumulus and oocyte before ovulation. In addition, iontophoretically injected fluorescein dye was tranferred between the oocyte and cumulus cells. Examination of the extent of ionic coupling in cumulus-oocyte specimens before and after ovulation revealed that ionic coupling between the cumulus and oocyte progressively decreased as the time of ovulation approached. In postovulatory specimens, no coupling was detected. Although some proteolytic mechanism may be involved in the disintegration of the cumulus-oocyte complex, neither the cumulus cells nor the oocyte produced detectable levels of plasminogen activator, a protease which is synthesized by membrana granulosa cells. In summary, cell communication is a characterisitc feature of the cumulus-oocyte complex, and this communication is terminated near the time of ovulation. This temporal pattern of the termination of communication between the cumulus and the oocyte may indicate that communication provides a mechanism for regulating the maturation of the oocyte during follicular development before ovulation.     

Effect of forskolin on the spontaneous maturation and cyclic AMP content of hamster oocyte-cumulus complexes

Forskolin, a reversible stimulator of the catalytic subunit of adenylate cyclase, has been used to determine: whether an increase in hamster cumulus cell cyclic adenosine monophosphate (cAMP) results in an elevation of intraoocyte cAMP and an accompanying increase in the maintenance of meiotic arrest (%GV where GV is germinal vesicle) when heterologous coupling is maintained, whether the hamster oolemma possesses the catalytic subunit of adenylate cyclase in an amount adequate to stimulate sufficient cAMP synthesis to maintain arrest, and whether release from meiotic arrest is accompanied by a decrease in the content of intraoocyte cAMP. Intracellular cAMP was determined by RIA, functional metabolic coupling was assessed by determination of the fraction of radiolabeled uridine marker transferred from the cumulus mass to the oocyte, and meiotic stage was determined cytogenetically. While the %GV of both cumulus-enclosed (intact) and cumulus-free (denuded) oocytes was dose-dependent upon forskolin, that of intact oocytes was much more sensitive to the drug (intact: ID50 3.4 microM; denuded: ID50 65.0 microM, where ID50 is the dose of forskolin that inhibits the maturation of 50% of cultured oocytes). Forskolin stimulated a significant, dose-dependent increase in the amount of cAMP within the cumulus mass [(r) = 0.789, P less than 0.001)], the intact oocyte [(r) = 0.715, P less than 0.001], and the denuded oocyte [(r) = 0.673, P less than 0.01)]. The cAMP content of intact oocytes was significantly greater than that of denuded oocytes above 6.25 microM forskolin (25 microM forskolin: 9.28 +/- 1.01 vs. 3.98 +/- 0.15 fmol cAMP, intact and denuded oocytes, respectively; P less than 0.001, paired t test). A highly significant positive correlation was established between the amount of cAMP in groups of cumulus masses and that in the corresponding enclosed oocytes [(r) = 0.635, P less than 0.001]. The enhanced sensitivity of meiotic arrest in intact, as compared to denuded, oocytes was due to the presence of adherent cumulus cells but was not attributable to a significant increase in the cAMP content of intact oocytes (at 6.25 microM forskolin; %GV intact = 73.0 +/- 10.7, denuded = 20.3 +/- 7.4; fmol cAMP intact = 5.02 +/- 1.50; denuded = 4.63 +/- 0.81). The arresting action of forskolin on intact oocytes was transient and fully reversible, but release from arrest was not accompanied by a decrease in either intraoocyte cAMP or heterologous metabolic coupling.(ABSTRACT TRUNCATED AT 400 WORDS)     

The effects of plasma-derived extracellular vesicles on cumulus expansion and oocyte maturation in mice

Cumulus cell expansion is required for the ovulation of a fertilizable oocyte. Extracellular vesicles (EVs) are bilayer-lipid membrane vesicles that may be found in a variety of bodily fluids and play an important role in biological processes. This study aimed to examine the effects of plasma-derived EVs on cumulus expansion and in vitro maturation (IVM) of the oocyte. EVswere isolated using ultracentrifugation from the plasma of female mice. The morphology and size of EVs were analyzed by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Western blotting allowed us to identify CD63, CD81, CD9, and HSP70 protein markers of EVs; the expression of the genes related to cumulus cell expansion, including hyaluronan synthase 2 (Has2) and prostaglandinendoperoxide synthase 2 (Ptgs2), were assessed using real-time polymerase chain reaction. Plasma-derived EVs labeled with Dil dye were successfully incorporated with cumulus cells during IVM. Plasma-derived EVs significantly induced cumulus expansion and maturation of oocytes. The percentage of oocytes that reached the MII stage was significantly greater in the EVs treatment group compared with other groups. Although treatment with epidermal growth factor (EGF) significantly increased cumulus expansion in cumulus-oocyte complexes (COCs), the impact was less than that seen with plasma-derived EVs. Furthermore, EVs generated from plasma substantially enhanced Has2 and Ptgs2 mRNA expression in the cumulus-oocyte complex. This research indicates that EVs derived from plasma are capable of promoting cumulus expansion and oocyte maturation.     

Role of cyclic adenosine monophosphate (cAMP) in vitro on bovine oocyte maturation

This experiment attempted to determine the effect of cAMP on maturation of bovine oocytes in chemically-defined, serum-free medium. Cumulus-oocyte complexes were incubated in modified DME/Ham F-12 medium containing dbcAMP at 0 (control), 10(-6), 10(-4) and 10(-2) M. After 18 and 24 hours of culture, the percentage of oocyte maturation between 0 (control) and 10(-2) M dbcAMP-treated groups were significant. Some oocytes were cultured with dbcAMP (10(-2) M) for 6, 12 and 24 hours followed by incubation in control medium to test the reversibility of inhibition or of any harmful effect of dbcAMP. The inhibitory effect of 10(-2) M dbcAMP on bovine oocyte maturation was reversed by transferring cumulus-oocyte complexes to the control medium. In addition, forskolin (0.12 and 0.24 mM) was effective (P < 0.01) in preventing the resumption of meiosis. The cAMP content of oocytes cultured with forskolin was not increased, although cumulus cells responded to forskolin with an increase in cAMP content. These results indicate that elevated levels of cAMP in the culture medium are important in regulating resumption of meiosis of bovine oocytes in vitro.     

Co-Expression of Cytokeratins and Vimentin in Sheep Cumulus-Oocyte Complexes. Alteration of Intermediate Filament Distribution by Acrylamide

The association between germ cells and somatic granulosa cells persists throughout the growth of the oocyte by means of foot processes of the cumulus corona cells that cross the zona pellucida. During meiotic maturation important nuclear and cytoplasmic events occur in cumulus-oocyte complex suggesting implication of cytoskeletal elements. Immunoblotting analysis of cytoskeletal proteins of the cumulus cells revealed the presence of vimentin polypeptide and of at least two cytokeratin polypeptides. Using immunofluorescence techniques on cryostat sections through frozen tissue, we provided evidence for the presence of cytokeratins of the simple epithelial type in addition to vimentin in sheep cumulus cells. These two types of intermediate filaments were localized throughout the cytoplasm and especially in the foot processes which cross the zona pellucida. The contact area between the two cell types was also labelled with the antibodies. Acrylamide treatment of cumulus-oocyte complexes involved a drastic disorganization of the intermediate filament network and triggered the isolation of the oocyte from its cumulus cells. This isolation resulted in resumption of meiosis. From these results it appears that intermediate filaments could participate in the process of gap junction loss and indirectly in the control of meiosis resumption.     

EGF-induced EGF-receptor and MAP kinase phosphorylation in goat cumulus cells during in vitro maturation

EGF has been shown to influence meiotic maturation and development competence of oocyte in various mammalian species. We previously reported, in goat, that the EGF receptor (EGF-R) was present both on cumulus cells and oocytes. Here, EGF-induced signaling was investigated during the in vitro maturation process in goat cumulus-oocyte complexes (COCs). Cumulus cells and oocytes were subjected to Western immunoblotting analysis using anti-MAP kinase, anti-phosphotyrosine, anti-phospho MAP kinase, and anti-phospho EGF-R antibodies. We demonstrated that treatment with EGF during the in vitro maturation process induced rapid tyrosine phosphorylation of EGF-R in a time and concentration dependent manner in cumulus cells. A similar pattern of activation by phosphorylation was observed for MAP kinase upon EGF stimulation. AG 1478, an inhibitor of the EGF kinase, suppressed EGF-stimulated phosphorylation of EGF-R and also affected the MAP kinase activation. Treatment with the MEK inhibitor PD 98059 abolished EGF-induced MAP kinase activation. We did not observe oocyte EGF-R phosphorylation in our experiments during the in vitro maturation process. Our data indicate, in goat cumulus cells, that activation of EGF-R by EGF triggers signaling through the MAP kinase pathway during in vitro maturation. This supports the hypothesis that the major site of action for EGF, that regulates oocyte maturation, is the cumulus cell.     

Stimulation and inhibition of rat oocyte meiosis by forskolin

The adenylate cyclase activator forskolin was used to study the role of cAMP for oocyte meiosis and follicular steroid secretion. Follicular and cumulus cAMP production was stimulated dose-dependently by forskolin, as was the follicular secretion of progesterone, testosterone and estradiol. Forskolin induced meiosis in follicle-enclosed oocytes with a maximal effect at 1 microM, with lower and higher concentrations being less effective. The spontaneous resumption of meiosis in isolated cumulus-enclosed oocytes was dose-dependently retarded by forskolin. Meiosis of cumulus-free oocytes was also retarded but only slightly. These data support the earlier hypothesis that a limited increase in follicular cAMP levels triggers meiosis, whereas sustained levels of cAMP in the oocyte itself prevent meiosis.     

Adenylyl cyclases in oocyte maturation: a characterization of AC isoforms in bovine cumulus cells

Mammalian oocytes are arrested at the G(2)/M transition in the meiotic cell cycle. It is well known that a decrease in intraoocyte cAMP concentrations accompanies resumption of meiosis, but the precise trigger of this decrease remains a mystery. Follicular somatic cells are intimately coupled to the oocyte and are thought to transmit maturation signals to the oocyte in response to hormonal stimulation. Here, we investigate the nature of the follicular somatic cell response to hormonal stimulation by identifying and characterizing the adenylate cyclase isoforms present in bovine cumulus cells. RT-PCR and Western blot analysis revealed the presence of multiple adenylyl cyclase isoforms in bovine granulosa and cumulus cells. Pharmacological manipulation of the AC isoforms showed that multiple isoforms were indeed active. Our data indicate that the PKC inhibited adenylate cyclases IV and VI and the calcium-stimulated isoform I predominate in bovine cumulus cells.     

Glucosamine supplementation during in vitro maturation inhibits subsequent embryo development: possible role of the hexosamine pathway as a regulator of developmental competence

Glucose concentration during cumulus-oocyte complex (COC) maturation influences several functions, including progression of oocyte meiosis, oocyte developmental competence, and cumulus mucification. Glucosamine (GlcN) is an alternative hexose substrate, specifically metabolized through the hexosamine biosynthesis pathway, which provides the intermediates for extracellular matrix formation during cumulus cell mucification. The aim of this study was to determine the influence of GlcN on meiotic progression and oocyte developmental competence following in vitro maturation (IVM). The presence of GlcN during bovine IVM did not affect the completion of nuclear maturation and early cleavage, but severely perturbed blastocyst development. This effect was subsequently shown to be dose-dependent and was also observed for porcine oocytes matured in vitro. Hexosamine biosynthesis upregulation using GlcN supplementation is well known to increase O-linked glycosylation of many intracellular signaling molecules, the best-characterized being the phosphoinositol-3-kinase (PI3K) signaling pathway. We observed extensive O-linked glycosylation in bovine cumulus cells, but not oocytes, following IVM in either the presence or the absence of GlcN. Inhibition of O-linked glycosylation significantly reversed the effect of GlcN-induced reduction in developmental competence, but inhibition of PI3K signaling had no effect. Our data are the first to link hexosamine biosynthesis, involved in cumulus cell mucification, to oocyte developmental competence during in vitro maturation.     

Effects of fetal calf serum and bovine serum albumin on in vitro maturation and fertilization of bovine and hamster cumulus-oocyte complexes

Bovine serum albumin (BSA) and fetal calf serum (FCS) were evaluated as protein supplements for in vitro maturation and fertilization of oocytes from cows and hamsters. BSA and low doses of FCS (0.1 or 1.0%) did not support viability or maturation of the cumulus-oocyte complex as well as higher doses of FCS (5, 10, or 20%) for either species. BSA failed to support cumulus expansion for bovine or hamster cumulus-oocyte complexes. All doses of FCS examined supported cumulus expansion in bovine cumulus-oocyte complexes, whereas the hamster complexes required at least 1.0% FCS to induce cumulus expansion. The addition of a serum filtrate, Solcoseryl, with BSA improved viability of the cumulus in the bovine but did not support cumulus expansion or completion of Meiosis I in bovine complexes. In vitro fertilization could be accomplished in media containing FCS by increasing the heparin concentration in the bovine system or reducing FCS for the hamster system. Polyspermy was increased when FCS was the protein supplement. It is not known whether this is an interaction of FCS with the sperm or oocyte. In conclusion, FCS was found necessary for follicle-stimulating-hormone (FSH)-induced cumulus expansion. It also improved cumulus cell viability and completion of the first meiotic division in complexes of both species compared with BSA.