'Why have women not returned to use their frozen oocytes?': a 5-year follow-up of women after planned oocyte cryopreservation.

RESEARCH QUESTION: What are the reproductive choices and retrospective reflections of women at least 4 years after planned oocyte cryopreservation (POC)? DESIGN: This was an internet survey, using the REDCap application, of women who underwent POC, at a single-centre university-affiliated IVF unit, 4-8 years before the survey. The questionnaire addressed reproductive choices and outcomes following POC. RESULTS: Seventy-nine women who underwent POC during 2011-2014 were invited to participate, and 70 (89%) responded. Mean age at cryopreservation was 37.1 ± 2.4 (range 30-41) years, mean age at study participation 42.6 ± 2.6 (range 35-48) years, and mean time from first cryopreservation cycle to study participation 5.5 ± 1.3 (range 4-8) years. The main retrospectively reported reason for POC was not wanting to become pregnant without a partner (59, 84%). During the follow-up period, 44 women (63%) attempted to conceive either naturally or by assisted reproductive technology using fresh or cryopreserved oocytes. Of those, 28 women achieved a live birth (64% of those who tried to conceive). Fourteen respondents (20% of all respondents) reported using their cryopreserved oocytes, and three (21%) achieved a birth using those oocytes. Fifteen women (34%) of those who tried to conceive used donor spermatozoa. CONCLUSIONS: The most common reasons for not using frozen oocytes were achieving pregnancy without frozen oocytes or preferring not to have a child without a partner. A considerable proportion of women who had POC and were not interested in being a single parent by choice eventually try to conceive using donor spermatozoa several years later.

Biopsy-induced inflammatory conditions improve endometrial receptivity: the mechanism of action.

A decade ago, we first reported that endometrial biopsy significantly improves the success of pregnancy in IVF patients with recurrent implantation failure, an observation that was later confirmed by others. Recently, we have demonstrated that this treatment elevated the levels of endometrial pro-inflammatory cytokines and increased the abundance of macrophages (Mac) and dendritic cells (DCs). We therefore hypothesised that the biopsy-related successful pregnancy is secondary to an inflammatory response, and aimed at deciphering its mechanism of action. Supporting our hypothesis, we found that the pro-inflammatory TNFα stimulated primary endometrial stromal cells to express cytokines that attracted monocytes and induced their differentiation into DCs. These monocyte-derived DCs stimulated endometrial epithelial cells to express the adhesive molecule SPP1 (osteopontin (OPN)) and its receptors ITGB3 and CD44, whereas MUC16, which interferes with adhesion, was downregulated. Other implantation-associated genes, such as CHST2, CCL4 (MIP1B) and GROA, were upregulated by monocyte-derived Mac. These findings suggest that uterine receptivity is mediated by the expression of molecules associated with inflammation. Such an inflammatory milieu is not generated in some IVF patients with recurrent implantation failure in the absence of local injury provoked by the biopsy treatment.

Endometrial inflammation and effect on implantation improvement and pregnancy outcome.

Implantation failure, which is presently the major barrier in human fertility, is attributed, in many cases, to the failure of the uterus to acquire receptivity. The transition into a receptive uterus includes cellular changes in the endometrium and the modulated expression of different cytokines, growth factors, transcription factors, and prostaglandins. These molecules partake in the generation of an inflammatory response followed by the recruitment of immune cells. These cells have shown to be involved in the maternal immune tolerance toward the implanted embryo as well as in the maternal-fetus interaction during pregnancy. Most of the accumulated evidence indicates that embryo implantation is associated with an active Th1 inflammatory response while a Th2-humoral inflammation is required for pregnancy maintenance. Yet, recent findings suggest that a Th1 inflammatory response is also necessary for the acquisition of uterine receptivity. This notion was originally suggested by reports from our and other clinical centers worldwide that IVF patients with repeated implantation failure subjected to endometrial biopsy exhibit a substantial improvement in their chances to conceive. These findings, followed by the demonstration of an elevated pro-inflammatory cytokine/chemokine expression, as well as an increased abundance of immune cells, in the endometrium of these patients, raised the idea that acquisition of uterine receptivity is closely associated with an inflammatory response. This review summarizes the molecular and biochemical evidence that confirm this notion and proposes a mechanism by which injury-induced inflammation improves uterine receptivity and the subsequent pregnancy outcome.

Ovulation-selective genes: the generation and characterization of an ovulatory-selective cDNA library.

Ovulation-selective/specific genes, that is, genes preferentially or exclusively expressed during the ovulatory process, have been the subject of growing interest. We report herein studies on the use of suppression subtractive hybridization (SSH) to construct a 'forward' ovulation-selective/specific cDNA library. In toto, 485 clones were sequenced and analyzed for homology to known genes with the basic local alignment tool (BLAST). Of those, 252 were determined to be nonredundant. Of these 252 nonredundant clones, 98 were analyzed by probing mouse preovulatory and postovulatory ovarian cDNA. Twenty-five clones (26%) failed to show any signal, and 43 cDNAs tested thus far display a true ovulation-selective/specific expression pattern. In this communication, we focus on one such ovulation-selective gene, the fatty acid elongase 1 (FAE-1) homolog, found to be localized to the inner periantral granulosa and to the cumulus granulosa cells of antral follicles. The FAE-1 gene is a beta-ketoacyl-CoA synthase belonging to the fatty acid elongase (ELO) family, which catalyzes the initial step of very long-chain fatty acid synthesis. All in all, the present study accomplished systematic identification of those hormonally regulated genes that are expressed in the ovary in an ovulation-selective/specific manner. These ovulation-selective/specific genes may have significant implications for the understanding of ovarian function in molecular terms and for the development of innovative strategies for both the promotion of fertility and its control.

Expression and modification of PKA and AKAPs during meiosis in rat oocytes.

Meiosis in oocytes is initiated during fetal life, arrested around birth and resumed after puberty. Meiotic arrest is controlled by a cAMP-dependent protein kinase (PKA)-mediated cAMP action. We examined oocytes for the presence and modulation of the regulatory (R) subunits of PKA and the A-kinase anchoring proteins (AKAPs) that target PKA to specific subcellular locations. We found that rat oocytes express the two regulatory subunit isoforms, RI and RII of PKA. Immunocytochemistry revealed that the regulatory subunits underwent cellular translocation upon resumption of meiosis. We also demonstrated the presence of a novel 140 kDa AKAP, AKAP140 that exhibited a retarded electrophoretic motility at reinitiation of meiosis. The mobility shift of AKAP140 was susceptible to alkaline phosphatase and prevented by inhibition of p34cdc2 kinase. We conclude that rat oocytes express AKAP140 that is phosphorylated during meiosis. AKAP140 phosphorylation is sensitive to p34cdc2 kinase inhibitors. We hypothesize that AKAP140 and its phosphorylation state may influence the translocation of the R subunits of PKA throughout resumption of meiosis.

Inactivation of M-phase promoting factor at exit from first embryonic mitosis in the rat is independent of cyclin B1 degradation.

Exit from M-phase and completion of cell division requires inactivation of M-phase promoting factor (MPF), a heterodimer composed of the regulatory cyclin B1 and the catalytic p34cdc2 kinase. Inactivation of MPF is associated with cyclin B1 degradation that is brought about by the ubiquitin-proteasome pathway. Our study examined the role of the proteasome in the first mitosis of rat embryos and its participation in the regulation of cyclin B1 degradation and MPF inactivation. We show that in the early zygote the proteasome is evenly distributed in the ooplasm and the nucleus, whereas during mitosis it accumulates on the spindle apparatus. We further demonstrate that inhibition of proteasomal catalytic activity prevents 1-cell embryos from undergoing mitosis. This mitotic arrest is associated with the presence of relatively high amounts of cyclin B1, which unexpectedly does not result in elevated MPF activity. Our findings strongly imply that completion of the first embryonic division depends on proteasomal degradation and that cyclin B1 is included among its target proteins. They also provide the first evidence that MPF inactivation at this stage of development is not solely dependent upon cyclin B1 degradation and is insufficient to allow the formation of the 2-cell embryo.

A fast signal-induced activation of Poly(ADP-ribose) polymerase: a novel downstream target of phospholipase c.

We present the first evidence for a fast activation of the nuclear protein poly(ADP-ribose) polymerase (PARP) by signals evoked in the cell membrane, constituting a novel mode of signaling to the cell nucleus. PARP, an abundant, highly conserved, chromatin-bound protein found only in eukaryotes, exclusively catalyzes polyADP-ribosylation of DNA-binding proteins, thereby modulating their activity. Activation of PARP, reportedly induced by formation of DNA breaks, is involved in DNA transcription, replication, and repair. Our findings demonstrate an alternative mechanism: a fast activation of PARP, evoked by inositol 1,4,5,-trisphosphate-Ca(2+) mobilization, that does not involve DNA breaks. These findings identify PARP as a novel downstream target of phospholipase C, and unveil a novel fast signal-induced modification of DNA-binding proteins by polyADP-ribosylation.

The proteasome is involved in the first metaphase-to-anaphase transition of meiosis in rat oocytes.

The proteasome engages in protein degradation as a regulatory process in biological transactions. Among other cellular processes, the proteasome participates in degradation of ubiquinated cyclins in mitosis. However, its role in meiosis has not been established. Resumption of meiosis in the oocyte involves the activation of maturation promoting factor (MPF), a complex of p34cdc2 and cyclin B. Inactivation of this factor, occurring between the two meiotic divisions, is associated with degradation of cyclin B. In this study, we examined the possible involvement of the proteasome in regulation of the exit from metaphase I in spontaneously maturing rat oocytes. We found that upon resumption of meiosis, proteasomes translocate to the spindle apparatus. We further demonstrated that specific inhibitors of proteasome catalytic activity, MG132 and lactacystin, blocked polar body extrusion. Chromosome and microtubule fluorescent staining verified that MG132-treated oocytes were arrested at metaphase I. Intervention of proteasomal action with this inhibitor also resulted in accumulation of cyclin B and elevated activity of MPF. These data demonstrate that proteasomal catalytic activity is absolutely essential for the decrease in MPF activity and completion of the first meiotic division. Its translocation to the spindle apparatus may facilitate the timely degradation of cyclin B.

Temporal analysis of connexin43 protein and gene expression throughout the menstrual cycle in human endometrium.

OBJECTIVE: To analyze the pattern of connexin43 gene and protein expression in human endometrium throughout the menstrual cycle. DESIGN: Controlled clinical study. SETTING: An academic research center. PATIENT(S): Women with 28-day menstrual cycles who had mechanical infertility and failed to conceive after IVF treatment. INTERVENTION(S): Endometrial and blood samples were collected on days 8, 12, 14, 21, and 25 of spontaneous menstrual cycles. MAIN OUTCOME MEASURE(S): Endometrial expression of connexin43 protein and messenger RNA, endometrial thickness, and serum concentrations of gonadotropins and steroids. RESULT(S): The expression of connexin43 gene and protein decreased on day 12 and day 14 of the menstrual cycle and then increased on day 21 and day 25, respectively. A serum LH surge accompanied by a peak in the FSH concentration was observed on days 12-14. The progesterone concentration increased on days 21-25, but there was no significant change in the E2 concentration. The thickness of the endometrium increased between days 8 and 12 and did not change further between days 21 and 25. CONCLUSION(S): The expression of connexin43 gene and protein in human endometrium changes during the menstrual cycle in a pattern that is associated with the secretion of LH, FSH, and progesterone. This pattern may serve as a marker for implantation competence.

Is hydrosalpinx fluid cytotoxic?

Accumulation of oviductal fluid in the ampullar lumen as a result of occlusion of the infundibulum is referred to as hydrosalpinx. A low pregnancy rate (10%) after in-vitro fertilization (IVF) in hydrosalpinx patients and a relatively high incidence (50%) of abortions during the first trimester suggested that leakage of this fluid into the uterine cavity may exert a cytotoxic effect on the developing embryo. To examine this possibility, we analysed the composition of the hydrosalpinx fluid and tested its effect on human granulosa cells and embryos. Hydrosalpinx fluids and granulosa cells were collected from IVF patients at ovum pick-up. IVF eggs containing three pronuclei (3PN) were employed for this study. Analysis of hydrosalpinx fluids revealed electrolyte concentrations similar to those in serum with lower amounts of total protein and albumin. No blood cells were detected and bacterial cultures were negative. Granulosa cells incubated in hydrosalpinx fluid-containing medium (diluted 1:1) were not morphologically different and showed a steroidogenic capacity that was higher than that of cells incubated in its absence. Fertilized 3PN eggs incubated in IVF culture medium successfully developed into 6- to 8- and 8- to 16-cell embryos within 48 and 72 h, respectively. This rate of embryonal development was not impaired by hydrosalpinx fluid (at either 50 or 100% concentration). In the absence of a demonstrable detrimental effect we suggest that the low implantation rate in hydrosalpinx IVF patients may not be due to an embryotoxic effect. We further suggest that constant passage of fluid into the uterine cavity in these patients could possibly introduce some mechanical interference that may result in implantation failure.

Cell-to-cell communication in the ovarian follicle: developmental and hormonal regulation of the expression of connexin43.

The extensively developed network of cell-to-cell communication in the ovarian follicle is generated by gap junctions. In addition to the transmission of nutrients from the follicular cells to the oocyte, junctional communication in the ovarian follicle mediates the transfer of cAMP, the regulatory signal that maintains the oocyte in meiotic arrest. Luteinizing hormone (LH) interrupts cell-to-cell communication within the ovarian follicle, leading to a decrease in intra-oocyte concentrations of cAMP followed by resumption of meiosis. The developmental and hormonal regulation of the ovarian gap junction protein connexin43 (Cx43) and gene expression throughout folliculogenesis is reviewed in this article. An age-dependent increase in the amount of the Cx43 protein that was accompanied by its phosphorylation in preovulatory follicles has been observed. This protein disappeared after ovulation. The changes in both the amount and phosphorylation state of Cx43 were mimicked by exogenous administration of hormones as follows. Pregnant mare serum gonadotrophin increased Cx43 protein expression with a concurrent induction of its phosphorylation while a further human chorionic gonadotrophin injection resulted in a significant decrease of the protein. Cx43 mRNA showed a similar pattern of expression. In-vitro analysis of isolated ovarian follicles revealed that short time exposure (10 min) to LH stimulates phosphorylation of Cx43 followed by its immediate dephosphorylation, while longer incubations (8 and 24 h) with this hormone result in elimination of the protein. A significant decrease in Cx43 mRNA concentration at 24 h of incubation with LH was observed in these follicles. These results suggest that: (i) the presence of the gap junction protein in the ovary is developmentally regulated; (ii) after sexual maturation, both the amount of the Cx43 ovarian gap junction protein and its phosphorylation state are subjected to regulation by gonadotrophins; (iii) the LH-induced gating mechanism of the gap junctions in rat ovarian follicles is comprised of two steps: the immediate response is represented by a change in the phosphorylation state of the Cx43 protein, and the later response is manifested by a reduction of Cx43 protein concentration, due to attenuation of its gene expression.

Developmental expression and regulation of the gap junction protein and transcript in rat ovaries.

The extensively developed network of cell-to-cell communication, generated by gap junctions, mediates transmission of small molecules between the cells of the ovarian follicle. Our study aimed at the analysis of the ontogeny and regulation of connexin43 (Cx43) the ovarian gap junction protein and its gene expression throughout folliculogenesis. Developmental analysis was performed using ovaries of immature rats at different ages and selected ovarian follicles of sexually mature female rats at different phases of their estrous cycle. In order to establish the effect of hormones involved in regulation of folliculogenesis on Cx43 modulation, the experimental animal model of sexually immature female rats administered with exogenous gonadotropins was employed. Developmental and hormonal modulations of Cx43 protein and its mRNA expression were studied by Western and Northern blot analysis, respectively. We found that Cx43 was undetectable in ovaries of rats on the first postnatal day, with a low level of this protein observed in 11-day-old rats ovaries. Some increase in the amount of Cx43 was observed at the age of 25 days with a dramatic elevation accompanied by phosphorylation of this protein that was specific to large antral follicles of sexually mature proestrous rats. Elimination of the protein was observed at estrus and could be prevented by cancellation of the preovulatory surge of luteinizing hormone (LH). This pattern of Cx43 modifications was mimicked by exogenous administration of hormones as follows: Pregnant mare's serum gonadotropin (PMSG) increased the Cx43 protein expression with a concurrent induction of its phosphorylation while a further human chorionic gonadotropin (hCG) injection resulted in a decrease of the signal. Analysis of the Cx43 mRNA showed a direct correlation between the Cx43 protein level and its gene expression. We conclude that: 1) At early folliculogenesis the ovarian gap junction protein Cx43 and its gene are developmentally regulated; and 2) After antrum formation, transcription, translation, and posttranslational modifications of Cx43 are regulated by gonadotropins.

Evidence for endogenous ADP-ribosylation of GTP-binding proteins in neuronal cell nucleus. Possible induction by membrane depolarization.

GTP-binding protein(s) recognized by antibodies against the alpha-subunits of Gi- and Go-proteins were detected in crude nuclei isolated from rat brain stem and cortex. Immunohistochemical staining indicated that in the cortex these proteins are perinuclear, or are embedded in the nuclear membrane. Evidence is presented for an endogenous ADP-ribosylation of these proteins, which competes with their PTX-catalyzed ADP-ribosylation. The endogenous reaction has the characteristics of nonenzymatic ADP-ribosylation of cysteine residues, known to involve NAD-glycohydrolase activity. In vitro experiments showed that the alpha-subunit of Go-proteins in the cell membrane also acts as a substrate of this endogenous ADP-ribosylation. The in situ effect of membrane depolarization on the nuclear GTP-binding proteins may be attributable to their depolarization-induced endogenous ADP-ribosylation, suggesting a novel signaling mechanism in neuronal cells in the central nervous system.

Protein phosphorylation/dephosphorylation in the meiotic cell cycle of mammalian oocytes.

The meiotic cell cycle in mammalian oocytes commences in the fetal ovary, proceeds up to the diplotene stage of the first prophase, and is arrested around birth at a G2-like phase. Reinitiation of meiosis, which occurs immediately before ovulation, represents the transition from G2 to M phase of the cell cycle. Resumption of meiosis is associated with a reduction in cAMP concentrations within the oocyte and is negatively regulated by an activated cAMP-dependent protein kinase (PKA). These findings led to the hypothesis that meiotic arrest is mediated by a PKA-phosphoprotein substrate, which undergoes dephosphorylation under conditions of decreasing intra-oocyte concentrations of cAMP. Thus far, a phosphoprotein that serves as a substrate for PKA and maintains meiotic arrest has not been identified. Nevertheless, the idea that regulation of enzyme activities of a series of protein kinases and phosphatases governs the progression through the meiotic cell cycle is now commonly accepted. The present knowledge of the specific phosphorylation/dephosphorylation biochemical events that participate in the control of meiosis in mammalian oocytes is discussed in this review.

Experimental extension of the time interval between oocyte maturation and ovulation: effect on fertilization and first cleavage.

OBJECTIVE: To test the hypothesis that impaired fertility in human patients with high LH concentrations throughout the follicular phase of the menstrual cycle reflects premature maturation of their oocytes. DESIGN: Previous information that resumption of meiosis is induced by lower hCG concentrations than that required for stimulation of follicular rupture was confirmed and used for establishment of a rat animal model in which oocyte maturation and ovulation can be separated experimentally. In further experiments hypophysectomized, pregnant mare serum gonadotropin (PMSG)-primed, immature female rats injected with 1.1 IU of hCG, a dose found to induce maturation in 72.9% +/- 6% of the rats with no effect on ovulation, were administered with a second injection of an ovulatory dose (4 IU) of hCG, 24 hours later. The ovulated eggs were subjected to IVF. RESULTS: Fertilization and first cleavage in oocytes recovered from our experimental animal model were similar to that observed in control PMSG-primed, either hypophysectomized or intact rats, treated by a single injection of 4 IU of hCG. CONCLUSIONS: The extension of the time interval between oocyte maturation and ovulation in the rat does not result in a lower rate of fertilization or a reduced incidence of cleavage. However, an inferior developmental capacity of these embryos cannot be ruled out.

Glyburide crosses the placenta in vivo in pregnant rats.

Non-insulin-dependent diabetes mellitus (NIDDM) is normally treated by oral hypoglycaemic agents, but their use is excluded during pregnancy because of their potential teratogenic and hypoglycaemic effects on the fetus. This caveat was recently questioned as glyburide was shown to cross an isolated cotyledon in vitro in insignificant amounts. In the present study, placental transport of glyburide in vivo was examined as an indispensable step towards clinical trials. Tritiated glyburide, C14 albumin or C14-labelled diazepam were injected into 13, 9 and 11 pregnant rats, respectively and the radioactivity was measured thereafter in maternal blood and in whole fetal extracts. The ratios between radioactivity in fetal tissue to that in maternal blood for glyburide (0.535 +/- 0.068) were similar to those of diazepam (0.641 +/- 0.057) which readily crosses the placenta. However, they differed significantly from those for albumin (0.048 +/- 0.0004) which does not cross. Moreover, glyburide in fetal tissue consistently reflected its concentration in maternal blood when measured at consecutive intervals after intravenous injection in the mother. In contrast, albumin in fetal tissue was low at all time points regardless of its levels in maternal blood when measured at different times after injection. These data suggest that glyburide crosses the placenta of pregnant rats and should therefore be considered with caution as a hypoglycaemic agent in the treatment of gestational diabetes.

Molecular control of meiosis.

The animal cell cycle consists of a round of chromosomal DNA replication in S-phase, followed by segregation of the replicated chromosomes into the daughter nuclei during M-phase. In most animal cells, gap phases termed G(1) and G(2) are introduced between the M- and S-phases, respectively. Meiosis is a particular example of cell division occurring in germ cells. This specialized cell cycle consists of two successive rounds of chromosome segregation that follow a round of DNA replication. Meiosis produces progeny cells with half as many chromosomes as their parents, thus making sexual reproduction possible. This review is concerned with the factors that have been implicated in the control of meiosis, although research in progress may reveal additional regulatory processes.

The effect of pre-treatment with a gonadotrophin-releasing hormone agonist on reproductive functions in mature cycling rats.

In order to investigate the performance of follicles in a rat model in which gonadotrophin-releasing hormone agonist (GnRHa) was used for hypothalamic-pituitary-ovarian axis suppression, three groups of mature cycling rats were studied. One group was treated with buserelin followed by pregnant mare's serum gonadotrophin (PMSG), and the second group was treated with PMSG alone. Both these hormonally treated groups received human chorionic gonadotropin for induction of ovulation. The third group received no hormonal treatment. The average number of ovulated oocytes recovered from rat oviducts pre-treated with GnRHa was significantly higher than that in rats treated with the gonadotrophin alone, in spite of the larger number of pre-ovulatory follicles present in the gonadotrophin-treated group. The morphology of both the pre-ovulatory and the post-ovulatory cumulus-oocyte complexes in the three groups appeared similar. No difference in the capacity of follicles of the three groups to synthesize progesterone in vitro in response to luteinizing hormone could be observed. We conclude that ovarian morphology and function are not impaired by pre-treatment with buserelin.

Phosphorylation and expression of connexin-43 ovarian gap junction protein are regulated by luteinizing hormone.

One important role of the junctional communication in the ovarian follicle is to mediate transmission of cAMP, the regulatory signal that maintains the oocyte in meiotic arrest. Luteinizing hormone (LH) interrupts cell-to-cell communication within the ovarian follicle, leading to a decrease in intraoocyte concentrations of cAMP followed by resumption of meiosis. Our experiments were directed at exploration of mechanisms involved in the LH-induced communication breakdown in the preovulatory ovarian follicle. Immunofluorescence and Western blot analysis, using highly specific antibodies, showed that connexin-43 (Cx43), the ovarian gap junction protein, is present in the cytoplasmic membranes of the follicular cells in multiple phosphorylated forms. The relative amounts of the different forms of Cx43 vary in response to LH: short time exposure (10 min) stimulated phosphorylation of Cx43 followed by immediate dephosphorylation, while longer incubations (8 and 24 h) with this hormone resulted in elimination of the protein. Forskolin mimicked the LH-induced phosphorylation/dephosphorylation, as well as the decrease of Cx43 protein level. A gonadotropin-releasing hormone analog (GnRHa) also induced an immediate phosphorylation/dephosphorylation of Cx43 and a later reduction of the amount of Cx43. The direct PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), induced phosphorylation of Cx43 that was completely blocked by the protein kinase C inhibitor, staurosporine. This kinase inhibitor partially interfered with LH, but not forskolin-induced phosphorylation of Cx43. Analysis of the effect of LH on Cx43 gene expression revealed a significant decrease (45%) in Cx43 mRNA level at 24 h of incubation. A drop of Cx43 mRNA was also induced by GnRHa. Our results suggest that the LH-induced gating mechanism of the gap junctions in rat ovarian follicles is comprised of two steps: the immediate response is represented by a change in the phosphorylation state of the Cx43 protein, and the later response is manifested by a reduction of Cx43 protein level, due to attenuation of its gene expression. Phosphorylation of Cx43 may occur through PKA-, as well as PKC-dependent pathways.

Meiotic arrest in incompetent rat oocytes is not regulated by cAMP.

Fully grown, but not growing, mammalian oocytes spontaneously resume meiosis in vitro. Resumption of meiosis, also known as oocyte maturation, is associated with a drop in intraoocyte concentrations of cAMP followed by activation of the maturation promoting factor (MPF). Microtubule-associated-protein (MAP) kinase has been suggested as a substrate for the active p34cdc2 kinase, the catalytic subunit of MPF. Our study was designed to explore the mechanism of regulation of meiotic arrest in growing rat oocytes. Confirming previous observations we showed that in our rat colony oocytes do not acquire the competence to spontaneously resume meiosis earlier than 22 days postpartum. We further demonstrated that follicle-enclosed oocytes from 20-day-old female rats fail to resume meiosis in response to luteinizing hormone, follicle-stimulating hormone, a gonadotropin-releasing hormone analog, or forskolin, all of which are known to induce maturation in competent oocytes. Immunoblot analysis using highly specific anti p34cdc2 antibodies revealed that incompetent oocytes express the catalytic subunit of MPF at amounts that are not different from that found in competent oocytes. In addition, highly specific anti MAP kinase antibodies detected the presence of similar quantities of two isoforms (42 and 44 kDa) of MAP kinase in competent and incompetent oocytes. Measurements of cAMP revealed that as compared to competent oocytes, incompetent oocytes contain somewhat lower levels of this nucleotide (1.42 +/- 0.3 and 1.17 +/- 0.07 fmole/oocyte, respectively). However, considering the difference in protein content, the calculated concentrations seem to be similar. Furthermore, similar to competent oocytes, intracellular concentrations of cAMP in incompetent oocytes dropped significantly (from 1.17 +/- 0.07 to 0.77 +/- 0.12 fmole/oocyte) 2 hr after isolation from the follicle. We hereby suggest that (a) in mammals, similar to amphibians, the term meiotic incompetence can be extended to include inability to resume meiosis in response to hormonal stimulation; (b) it is not the lack of p34cdc2 or downstream regulatory elements, such as MAP kinase, that prevents growing oocytes from resuming meiosis; and (c) unlike fully grown oocytes, resumption of meiosis in growing oocytes is not subjected to negative regulation by cAMP.