Novel signaling mechanisms in the ovary during oocyte maturation and ovulation

Evidence for the requirement of 14-3-3eta (YWHAH) in meiotic spindle assembly during mouse oocyte maturation

BACKGROUND

The 14-3-3 (YWHA) proteins are central mediators in various cellular signaling pathways regulating development and growth, including cell cycle regulation. We previously reported that all seven mammalian 14-3-3 isoforms are expressed in mouse oocytes and eggs and that, 14-3-3η (YWHAH) accumulates and co-localizes in the region of meiotic spindle in mouse eggs matured in vivo. Therefore, we investigated the role of 14-3-3η in spindle formation during mouse oocyte maturation.

RESULTS

Examination of oocytes matured in vitro demonstrated that 14-3-3η accumulates in both meiosis I and II spindles. To explore if 14-3-3η interacts directly with α-tubulin in meiotic spindles, we performed an in situ proximity ligation assay that can detect intracellular protein-protein interactions at the single molecule level and which allows visualization of the actual interaction sites. This assay revealed a marked interaction between 14-3-3η and α-tubulin at the metaphase II spindle. To demonstrate a functional role for 14-3-3η in oocyte maturation, mouse oocytes were microinjected with a translation-blocking morpholino oligonucleotide against 14-3-3η mRNA to reduce 14-3-3η protein synthesis during oocyte maturation. Meiotic spindles in those cells were examined by immunofluorescence staining of 14-3-3η and α-tubulin along with observation of DNA. In 76% of cells injected with the morpholino, meiotic spindles were found to be deformed or absent and there was reduced or no accumulation of 14-3-3η in the spindle region. Those cells contained clumped chromosomes, with no polar body formation. Immunofluorescence staining of 14-3-3η and α-tubulin in control eggs matured in vitro from uninjected oocytes and oocytes microinjected with the ineffective, inverted form of a morpholino against 14-3-3η, a morpholino against 14-3-3γ, or deionized water showed normal, bipolar spindles.

CONCLUSIONS

The results indicate that 14-3-3η is essential for normal meiotic spindle formation during in vitro maturation of mouse oocytes, in part by interacting with α-tubulin, to regulate the assembly of microtubules. These data add to our understanding of the roles of 14-3-3 proteins in mouse oocyte maturation and mammalian reproduction.

B-cell translocation gene 2: expression in the rat ovary and potential association with adenine nucleotide translocase 2 in mitochondria

The B-cell translocation gene 2 (Btg2) is an anti-proliferative tumor suppressor gene that behaves as a transcriptional regulator. The present study investigated gonadotropin induction of Btg2 in the rat ovary and the mechanism of Btg2 action as a partner of mitochondrial protein adenine nucleotide translocase 2 (Ant2). Transient induction of Btg2 as well as Btg1 mRNA levels by LH/hCG was observed in ovarian granulosa cells. Btg2 protein levels were also stimulated by LH/hCG. LH-induced gene expression of Btg2 required ERK signal pathway. Studies of deletion mutants in HeLa cells showed that deletion of Btg2 C-terminus (Btg2/ΔC) abolished the interaction with Ant2. In fact, the expression levels of Btg2/ΔC construct were decreased in mitochondrial fraction. Btg2 was also expressed in mitochondria and interacted with Ant2 in preovulatory granulosa cells. Interestingly, a Btg2/ΔC construct inhibited an action of Btg2 wild-type on ATP and H(2)O(2) production. These findings demonstrate the gonadotropin stimulation of Btg2 in the ovary and, the physical interaction of Btg2 with Ant2 in mitochondria.

Ontogeny of the ovary in polycystic ovary syndrome

Activation of primordial follicles into the growing pool, selection of the dominant follicle, and its eventual ovulation require complex endocrine and metabolic interactions as well as intraovarian paracrine signals to coordinate granulosa cell proliferation, theca cell differentiation, and oocyte maturation. Early preantral follicle development relies mostly upon mesenchymal-epithelial cell interactions, intraovarian paracrine signals, and oocyte-secreted factors, whereas development of the antral follicle depends on circulating gonadotropins as well as locally derived regulators. In women with polycystic ovary syndrome (PCOS), ovarian hyperandrogenism, hyperinsulinemia from insulin resistance, and altered intrafollicular paracrine signaling perturb the activation, survival, growth, and selection of follicles, causing accumulation of small antral follicles within the periphery of the ovary, giving it a polycystic morphology. Altered adipocyte-ovarian interactions further compound these adverse events on follicle development and also can harm the oocyte, particularly in the presence of increased adiposity. Finally, endocrine antecedents of PCOS occur in female infants born to mothers with PCOS, which suggests that interactions between genes and the maternal-fetal hormonal environment may program ovarian function after birth.

Regulation of sheep oocyte maturation using cAMP modulators

Physical removal of mammalian cumulus-oocyte complexes (COCs) from ovarian follicles results in spontaneous resumption of meiosis, largely because of a decrease in cAMP concentrations, causing asynchrony between cytoplasmic and nuclear maturation and decreased oocyte developmental competence. The aim of this study was to modulate cAMP concentrations within ovine COCs to delay spontaneous nuclear maturation and improve developmental competence. Abattoir-derived sheep COCs were cultured for 2 hours (pre-IVM) in 100 μM forskolin (FSK) plus 500 μM 3-isobutyl-1-methylxanthine (IBMX). Pre-IVM (100 μM FSK and 500 μM IBMX) culture increased COC cAMP concentrations 10-fold compared with controls (P < 0.05). With regard to nuclear maturation, with FSK and IBMX and/or with FSH and cilostamide delayed completion of meiosis (metaphase II) by 3 to 4 hours compared with standard IVM (FSH-stimulated induction of meiosis). In this study, pre-IVM (with FSK and IBMX) followed by IVM (with FSH and cilostamide), increased ovine COC cAMP concentrations and delayed, but did not inhibit, completion of nuclear maturation. This did not affect embryo development rates, but increased total cell number of blastocysts compared with IVM with FSH alone (103 ± 6 vs. 66 ± 4 cells, respectively; mean ± SEM; P < 0.05). We inferred that regulation of ovine oocyte cAMP concentrations during IVM improved embryo quality compared with embryos produced by standard IVM methods.

Oocyte-somatic cells interactions, lessons from evolution

Background

Despite the known importance of somatic cells for oocyte developmental competence acquisition, the overall mechanisms underlying the acquisition of full developmental competence are far from being understood, especially in non-mammalian species. The present work aimed at identifying key molecular signals from somatic origin that would be shared by vertebrates.

Results

Using a parallel transcriptomic analysis in 4 vertebrate species - a teleost fish, an amphibian, and two mammals - at similar key steps of developmental competence acquisition, we identified a large number of species-specific differentially expressed genes and a surprisingly high number of orthologous genes exhibiting similar expression profiles in the 3 tetrapods and in the 4 vertebrates. Among the evolutionary conserved players participating in developmental competence acquisition are genes involved in key processes such as cellular energy metabolism, cell-to-cell communications, and meiosis control. In addition, we report many novel molecular actors from somatic origin that have never been studied in the vertebrate ovary. Interestingly, a significant number of these new players actively participate in Drosophila oogenesis.

Conclusions

Our study provides a comprehensive overview of evolutionary-conserved mechanisms from somatic origin participating in oocyte developmental competence acquisition in 4 vertebrates. Together our results indicate that despite major differences in ovarian follicular structure, some of the key players from somatic origin involved in oocyte developmental competence acquisition would be shared, not only by vertebrates, but also by metazoans. The conservation of these mechanisms during vertebrate evolution further emphasizes the important contribution of the somatic compartment to oocyte quality and paves the way for future investigations aiming at better understanding what makes a good egg.

Aurora kinase A drives MTOC biogenesis but does not trigger resumption of meiosis in mouse oocytes matured in vivo

Aurora kinase A (AURKA) is an important mitotic kinase involved in the G2/M transition, centrosome maturation and separation, and spindle formation in somatic cells. We used transgenic models that specifically overexpress in mouse oocytes either wild-type (WT-AURKA) or a catalytically inactive (kinase-dead) (KD-AURKA) AURKA to gain new insights regarding the role of AURKA during oocyte maturation. AURKA activation occurs shortly after hCG administration that initiates maturation in vivo. Although AURKA activity is increased in WT-AURKA oocytes, resumption of meiosis is not observed in the absence of hCG administration. Control oocytes contain one to three microtubule organizing centers (MTOCs; centrosome equivalent) at prophase I. At the time of germinal vesicle breakdown (GVBD), the first visible marker of resumption of meiosis, the MTOC number increases. In WT-AURKA oocytes, the increase in MTOC number occurs prematurely but transiently without GVBD, whereas the increase in MTOC number does not occur in control and KD-AURKA oocytes. AURKA activation is biphasic with the initial activation not requiring CDC25B-CDK1 activity, whereas full activation, which is essential for the increase in MTOCs number, depends on CDK1 activity. AURKA activity also influences spindle length and regulates, independent of its protein kinase activity, the amount of MTOC associated with gamma-tubulin. Both WT-AURKA and KD-AURKA transgenic mice have normal fertility during first 6 mo of life. These results suggest that although AURKA is not a trigger kinase for G2/M transition in mouse oocytes, it regulates MTOC number and spindle length, and, independent of its protein kinase activity, gamma-tubulin recruitment to MTOCs.

Signaling pathways regulating FSH- and amphiregulin-induced meiotic resumption and cumulus cell expansion in the pig

To define signaling pathways that drive FSH- and epidermal growth factor (EGF)-like peptide-induced cumulus expansion and oocyte meiotic resumption, in vitro cultured pig cumulus-oocyte complexes were treated with specific protein kinase inhibitors. We found that FSH-induced maturation of oocytes was blocked in germinal vesicle (GV) stage by protein kinase A (PKA), MAPK14, MAPK3/1, and EGF receptor (EGFR) tyrosine kinase inhibitors (H89, SB203580, U0126, and AG1478 respectively) whereas phosphoinositide-3-kinase/v-akt murine thymoma viral oncogene homolog (PI3K/AKT) inhibitor (LY294002) blocked maturation of oocytes in metaphase I (MI). Amphiregulin (AREG)-induced maturation of oocytes was efficiently blocked in GV by U0126, AG1478, and low concentrations of LY294002; H89, SB203580, and high concentrations of LY294002 allowed the oocytes to undergo breakdown of GV and blocked maturation in MI. Both FSH- and AREG-induced cumulus expansion was incompletely inhibited by H89 and completely inhibited by SB203580, U0126, AG1478, and LY294002. The inhibitors partially or completely inhibited expression of expansion-related genes (HAS2, PTGS2, and TNFAIP6) with two exceptions: H89 inhibited only TNFAIP6 expression and LY294002 increased expression of PTGS2. The results of this study are consistent with the idea that PKA and MAPK14 pathways are essential for FSH-induced transactivation of the EGFR, and synthesis of EGF-like peptides in cumulus cells and MAPK3/1 is involved in regulation of transcriptional and posttranscriptional events in cumulus cells required for meiotic resumption and cumulus expansion. PI3K/AKT signaling is important for regulation of cumulus expansion, AREG-induced meiotic resumption, and oocyte MI/MII transition. The present data also indicate the existence of an FSH-activated and PKA-independent pathway involved in regulation of HAS2 and PTGS2 expression in cumulus cells.

Luteinizing hormone reduces the activity of the NPR2 guanylyl cyclase in mouse ovarian follicles, contributing to the cyclic GMP decrease that promotes resumption of meiosis in oocytes

In preovulatory ovarian follicles of mice, meiotic prophase arrest in the oocyte is maintained by cyclic GMP from the surrounding granulosa cells that diffuses into the oocyte through gap junctions. The cGMP is synthesized in the granulosa cells by the transmembrane guanylyl cyclase natriuretic peptide receptor 2 (NPR2) in response to the agonist C-type natriuretic peptide (CNP). In response to luteinizing hormone (LH), cGMP in the granulosa cells decreases, and as a consequence, oocyte cGMP decreases and meiosis resumes. Here we report that within 20 min, LH treatment results in decreased guanylyl cyclase activity of NPR2, as determined in the presence of a maximally activating concentration of CNP. This occurs by a process that does not reduce the amount of NPR2 protein. We also show that by a slower process, first detected at 2h, LH decreases the amount of CNP available to bind to the receptor. Both of these LH actions contribute to decreasing cGMP in the follicle, thus signaling meiotic resumption in the oocyte.

Impact of the LH surge on granulosa cell transcript levels as markers of oocyte developmental competence in cattle

In the case of in vitro embryonic production, it is known that not all oocytes detain the developmental capacity to form an embryo. This capacity appears to be acquired through completion of folliculogenesis, during which the oocyte and follicular cells influence their respective destinies. The differentiation status of granulosa cells (GCs) could therefore offer an indicator of oocyte quality. The aim of this study was to compare mRNA transcript abundance in GCs associated with oocytes that subsequently reach or not the blastocyst stage. GCs were collected from cattle following an ovarian stimulation protocol that did or did not include the administration of LH. GCs were classified according to the developmental stage achieved by the associated oocytes. Transcript abundance was measured by microarray. Follicles (n=189) obtained from cows before and after the LH surge were essentially similar and the rates of oocytes reaching the blastocyst stage were not significantly different (52 vs 41%), but blastocyst quality was significantly better in the post-LH-surge group. In GCs from the pre-LH-surge group and associated with developmentally competent oocytes, 18 overexpressed and 22 underexpressed transcripts were found, including novel uncharacterized transcripts, whereas no differentially expressed transcripts were associated with developmentally different oocytes in the post-LH-surge group. The novel transcriptomic response associated with LH appeared to mask the difference. Based on oocyte developmental competence, the period prior to the LH surge appears best suited for studying competence-associated mRNA transcripts in bovine follicle cells.