Influence of epidermal growth factor on mammalian oocyte maturation via tyrosine-kinase pathway

Growth factors and female reproduction in vertebrates

Female reproductive efficiency is influenced by the outcomes of various processes, including folliculogenesis, apoptosis, response to gonadotropin signaling, oocyte maturation, and ovulation. The role of hormones in regulating these processes and other reproductive activities has been well established. It is becoming increasingly evident that in addition to well-characterized hormones, growth factors play vital roles in regulating some of these reproductive activities. Growth factors and their receptors are widely distributed in vertebrate ovaries at different stages of ovarian development, indicating their involvement in intraovarian reproductive functions. In the ovary, cell surface receptors allow growth factors to regulate intraovarian reproductive activities. Understanding these actions in the reproductive axis would provide a tool to target growth factors and/or their receptors to yield desirable reproductive outcomes. These include enrichment of in vitro maturation and fertilization culture media, and management of infertility. This review discusses some widely characterized growth factors belonging to the TGF, EGF, IGF, FGF, and BDNF family of peptides and their role in female reproduction in vertebrates, with a focus on mammals.

Interaction between epidermal growth factor receptor and C-C motif chemokine receptor 2 in the ovulatory cascade

The epidermal growth factor receptor (EGFR) signaling pathway is one of the main pathways responsible for propagating the luteinizing hormone (LH) signal throughout the cumulus cells and the oocyte. Recently, we have proposed the C-C motif chemokine receptor 2 (CCR2) and its main ligand (monocyte chemoattractant protein-1, MCP1) as novel mediators of the ovulatory cascade. Our previous results demonstrate that the gonadotropins (GNT), amphiregulin (AREG), and prostaglandin E2 (PGE2) stimulation of periovulatory gene mRNA levels occurs, at least in part, through the CCR2/MCP1 pathway, proposing the CCR2 receptor as a novel mediator of the ovulatory cascade in a feline model. For that purpose, feline cumulus-oocyte complexes (COCs) were cultured in the presence or absence of an EGFR inhibitor, recombinant chemokine MCP1, and gonadotropins [as an inducer of cumulus-oocyte expansion (C-OE), and oocyte maturation] to further assess the mRNA expression of periovulatory key genes, C-OE, oocyte nuclear maturation, and steroid hormone production. We observed that MCP1 was able to revert the inhibition of AREG mRNA expression by an EGFR inhibitor within the feline COC. In accordance, the confocal analysis showed that the GNT-stimulated hyaluronic acid (HA) synthesis, blocked by the EGFR inhibitor, was recovered by the addition of recombinant MCP1 in the C-OE culture media. Also, MCP1 was able to revert the inhibition of progesterone (P4) production by EGFR inhibitor in the C-OE culture media. Regarding oocyte nuclear maturation, recombinant MCP1 could also revert the inhibition triggered by the EGFR inhibitor, leading to a recovery in the percentage of metaphase II (MII)-stage oocytes. In conclusion, our results confirm the chemokine receptor CCR2 as a novel intermediate in the ovulatory cascade and demonstrate that the EGFR/AREG and the CCR2/MCP1 signaling pathways play critical roles in regulating feline C-OE and oocyte nuclear maturation, with CCR2/MCP1 signaling pathway being downstream EGFR/AREG pathway within the ovulatory cascade.

Human antral follicles <6 mm: a comparison between in vivo maturation and in vitro maturation in non-hCG primed cycles using cumulus cell gene expression

Within the context of an oocyte in vitro maturation (IVM) program for reproductive treatment, oocyte cumulus complexes (COCs) derived from follicles <6 mm in patients with PCOS were matured in vitro. Key transcripts related to meiotic maturation (FSHR, LHCGR, EGFR, PGR) and oocyte competence (AREG, ADAMTS, HAS2, PTGS2) were quantified in cumulus cells (CCs) before and after maturation. Control CC samples were collected from PCOS and normo-ovulatory patients who had undergone conventional gonadotrophin stimulation for IVF/ICSI. Additional control samples from a non-stimulated condition were obtained ex vivo from patients undergoing ovariectomy for fertility preservation. Expression data from CCs from follicles with a diameter of <6 mm before (IVM-CCs) and after in vitro maturation (IVM-CCs) were obtained after pooling CCs into four groups in relation to the percentage of matured (MII) oocytes obtained after 40 h of IVM (0; 40-60; 61-80; 100% MII) and values were compared with in vivo matured controls (IVO-CCs). Genes encoding key receptors mediating meiotic resumption are expressed in human antral follicles of <6 mm before and after IVM. The expression levels of FSHR, EGFR and PGR in CCs were significantly down-regulated in the IVO-CCs groups and in the 100% MII IVM group compared with the BM groups; all the receptors studied in the 100% MII IVM group reached an expression profile similar to that of IVO-CCs. However, after maturation in a conventional IVF/ICSI cycle, IVO-CCs from large follicles contained significantly increased levels of ADAMTS1, AREG, HAS2 and PTGS2 compared with IVM-CCs and IVM-CCs; the expression patterns for these genes in all IVM-CCs were unchanged compared with IVM-CCs. In conclusion, genes encoding receptors involved in oocyte meiotic resumption appeared to be expressed in CCs of small human antral follicles. Expression levels of genes-encoding factors reflecting oocyte competence were significantly altered in IVM-CCs compared with in vivo matured oocytes from large follicles. Observed differences might be explained by the different stimulation protocols, doses of gonadotrophin or by the intrinsic differences between in vivo and in vitro maturation.

Assisted reproductive technologies in the reproductive management of small ruminants

In modern agriculture, assisted reproductive technologies are being used for out of season oestrus induction, enhancement of reproductive performance and genetic improvement. In addition, they can have substantial contribution in preservation of endangered species or breeds, as well as in eradication programs of various diseases. While their applications are widespread in cattle, in small ruminants it is almost restricted to artificial insemination. The main limitations of a wider application in small ruminants are the naturally occurring anoestrus period, the variability of response to superovulatory treatments, the fertilisation failure and the need of surgery for collection and transfer of gametes and embryos. Nonetheless, during the last 30 years, considerable progress has been made in sheep and goat embryo technologies, especially in the fields of oestrus synchronisation, superovulation and in vitro embryo production. This paper reviews the status of assisted reproductive technologies in sheep, analysing the prospects offered by recent advances in in vivo and in vitro embryo production from mature and juvenile lambs.

In vitro maturation of dromedary (Camelus dromedarius) oocytes: effect of different protein supplementations and epidermal growth factor*

The present experiment was aimed to compare the effect of different protein supplementation sources, foetal calf serum (FCS), oestrous dromedary serum (EDS) and BSA, in experiment 1, and the effect of different concentrations of epidermal growth factor (EGF), in experiment 2, on in vitro nuclear maturation of the dromedary oocytes. Cumulus oocyte complexes (COCs) were harvested from the ovaries collected from a local slaughterhouse by aspirating the visible follicles in PBS supplemented with 5% FCS. Pooled COCs were randomly distributed to 4-well culture plates containing 500 μl of the maturation medium and cultured at 38.5 °C in an atmosphere of 5% CO(2) in air for 32-36 h. The basic maturation medium consisted of TCM-199 supplemented with 0.1 mg/ml L-glutamine, 0.8 mg/ml sodium bicarbonate, 0.25 mg/ml pyruvate, 50 μg/ml gentamicin, 10 μg/ml bFSH, 10 μg/ml bLH and 1 μg/ml estradiol. In experiment 1, this medium was supplemented with 10% FCS, 10% EDS or 0.4% BSA, whereas in experiment 2, it was supplemented with 0.4% BSA and 0, 10, 20 or 50 ng/ml of EGF. The oocytes were fixed, stained with 1% aceto-orcein stain and their nuclear status was evaluated. Oocytes were classified as germinal vesicle, diakinesis, metaphase-I, anaphase-I (A-I), metaphase-II (M-II) and those with degenerated, fragmented, scattered, activated or without visible chromatin as others. There was no difference (p > 0.05) observed in the proportion of oocytes reaching M-II stage between the media supplemented with FCS (71.5 ± 4.8), EDS (72.8 ± 2.9) and BSA (72.7 ± 6.2). In experiment 2, a higher proportion (p < 0.05) of oocytes reached M-II stage when the medium was supplemented with 20 ng/ml of EGF (81.4 ± 3.2) when compared with the media supplemented with 10 ng/ml (66.9 ± 4.1) and control (67.2 ± 7.1) groups. It may be concluded that the maturation media for dromedary camel oocytes can be supplemented with any of the three protein sources, i.e. FCS, EDS and BSA without any significant differences on the maturation rates. Also, a supplementation of 20 ng/ml of EGF in the maturation medium seems to be optimal and improves the nuclear maturation of dromedary camel oocytes.

PI3-kinase and mitogen-activated protein kinase in cumulus cells mediate EGF-induced meiotic resumption of porcine oocyte

Previous studies have shown that epidermal growth factor (EGF) has the ability to promote in vitro cultured porcine oocyte maturation. However, little is known about the detailed downstream events in EGF-induced meiotic resumption. We designed this study to determine the relationship of EGF, EGFR, phosphatidylinositol 3-kinase (PI3-kinase), MAPK, and germinal vesicle breakdown (GVBD) during oocyte maturation. Our results showed that GVBD in cumulus-enclosed oocytes (CEOs) but not in denuded oocytes (DOs) was induced by EGF in a dose-dependent manner, which indicated that cumulus cells but not oocyte itself were the main target for EGF-induced meiotic resumption. Furthermore, we found that MAPK in cumulus cells rather than in oocyte was activated immediately after EGF administration. To explore whether EGF exerts its functions through MAPK pathway, the activities of EGF receptor (EGFR) and MAPK were inhibited by employing AG1478 and U0126, respectively. Inhibition of MAPK blocked EGF-induced GVBD, whereas inhibition of EGFR prevented MAPK activation. Both AG1478 and U0126 could lead to the failure of EGF-induced GVBD singly. Notably, we found that LY294002, a specific inhibitor of PI3-kinase, effectively inhibited EGF-induced MAPK activation as well as subsequent oocyte meiotic resumption and this inhibition could not be reversed by adding additional EGF. Thus, PI3-kinase-induced MAPK activation in cumulus cells mediated EGF-induced meiotic resumption in porcine CEOs. Together, this study provides evidences demonstrating a linear relationship of EGF/EGFR, PI3-kinase, MAPK and GVBD and presents a relatively definitive mechanism of EGF-induced meiotic resumption of porcine oocyte.

Endocrine and ovarian response after a 2-day controlled suckling and eCG treatment in lactating rabbit does

Synchronization methods are used to obtain higher fertility when artificial insemination (AI) is applied to lactating rabbit does. The most common methods are eCG administration or temporary doe-litter separation. Nevertheless, drawbacks have been reported, such as negative side effects of hormonal treatment in the doe and low litter growth due to absence of suckling, respectively. Recently, improved reproductive performance (without visible consequences on young rabbit growth), has been obtained by applying a 2-day controlled nursing method before AI, by allowing for a 10 min nursing of the litter 24 h of separation. The present study was undertaken to examine the pituitary (PRL, LH, FSH) and the ovarian response (follicle size and number) to those methods. A total of 442 lactating does inseminated on day 11 post-partum were distributed in three experimental groups: 2CN (closing of nest box on day 9, controlled nursing on days 10 and 11), eCG (20 IU administered on day 9 post-partum) and CONTROL (untreated). Blood samples were obtained from 10 does per group at 48, 24 and 0 h before AI, and 1h after AI. Both 2CN and eCG treatments similarly improved sexual receptivity (76.3, 77.5 and 58.2%, respectively; P<0.001) and fertility (63.1, 64.1 and 48.4%, respectively; P<0.05) in lactating does, compared to the CONTROL group. Similar plasma FSH levels in all groups of does and sampling times were observed. Due to the absence of suckling, plasma concentration of PRL on day 10 post-partum in the 2CN group was lower than in the CONTROL group (P<0.05); this endocrine change in PRL levels could explain the better reproductive performances obtained with 2CN treatment. At 1h after exogenous administration of GnRH (at the moment of AI) a high LH response was observed in all groups (P<0.001). Ovaries from 20 rabbits treated in the same way but uninseminated (2CN, n=10; eCG, n=5; CONTROL, n=5 does) were obtained on day 11 post-partum in order to check the morphometric status (weight, width and height) and to make histological and immunohistochemical studies to detect growth hormone receptor (GH-R). As a result, synchronization methods did not show any significant difference in relation to the CONTROL group. However, a small increase in the number of primary follicles was evidenced in the 2CN group with respect to the eCG group, similarly to the CONTROL group (23.0+/-3.7, 9.4+/-4.9 and 14.8+/-4.92 primary follicles, respectively; P=0.1). GH-R immunostaining-presence was more evident in the 2CN and the eCG groups, including primordial follicles and oocytes themselves. Thus, there could have been some direct effects of GH on follicular development, as described in other species. Some ovarian parameters described open new ways to study intra-ovarian mechanism of follicular development in the post-partum period of rabbit does.

Molecular mechanisms of ovulation: co-ordination through the cumulus complex

Successful ovulation requires that developmentally competent oocytes are released with appropriate timing from the ovarian follicle. Somatic cells of the follicle sense the ovulatory stimulus and guide resumption of meiosis and release of the oocyte, as well as structural remodelling and luteinization of the follicle. Complex intercellular communication co-ordinates critical stages of oocyte maturation and links this process with release from the follicle. To achieve these outcomes, ovulation is controlled through multiple inputs, including endocrine hormones, immune and metabolic signals, as well as intrafollicular paracrine factors from the theca, mural and cumulus granulosa cells and the oocyte itself. This review focuses on the recent advances in understanding of molecular mechanisms that commence after the gonadotrophin surge and culminate with release of the oocyte. These mechanisms include intracellular signalling, gene regulation and remodelling of tissue structure in each of the distinct ovarian compartments. Most critical ovulatory mediators exert effects through the cumulus cell complex that surrounds and connects with the oocyte. The convergence of ovulatory signals through the cumulus complex co-ordinates the key mechanistic processes that mediate and control oocyte maturation and ovulation.

EGF-like factor epiregulin and amphiregulin expression is regulated by gonadotropins/cAMP in human ovarian follicular cells

Epiregulin and amphiregulin are growth factors involved in cancer development, but their potential role in signaling in the gonads is still obscure. We report here that basal expression of these growth factors is evident in human granulosa cells obtained from women treated for in vitro fertilization, when examined by RT-PCR using RNA isolated from primary cultures of ovarian granulosa cells. Expression of these factors was elevated concomitantly with elevation of progesterone production in these cells upon stimulation with luteinizing hormone (LH), and to a lesser extent with follicle stimulating hormone (FSH), both essential stimulants for ovulation and luteinization. Epiregulin and amphiregulin gene expression was dose- and time-dependent when measured subsequent to LH stimulation. Moreover, forskolin, which activates adenylate cyclase, was as efficient as LH in stimulating expression of these growth factors. It is suggested that upregulation of the epiregulin and amphiregulin expression is part of the signal transduction pathway which leads to ovulation and luteinization in the human ovary.

Effect of EGF on in vitro maturation of domestic cat oocytes

The objective of this study was to evaluate the influence of different concentrations of epidermal growth factor (EGF) on in vitro maturation of domestic cat oocytes. A total of 444 cat oocytes were matured in MSOF (maturation synthetic oviductal fluid) in the presence of varying EGF concentrations: (1) MSOF (control); (2) MSOF+10 ng/mL EGF (EGF10); (3) MSOF+25 ng/mL EGF (EGF25); and (4) MSOF+50 ng/mL EGF (EGF50). After IVM, oocytes were in vitro fertilized to verify the effect of adding EGF on cytoplasmic maturation. Cleavage rate was recorded and noncleaving oocytes were stained with Hoechst 33258 and examined to determine nuclear maturation rate. Cleaved zygotes were cultured in vitro and embryo stages were evaluated on days 6 and 7. There was no difference among groups in the total number of oocytes reaching the metaphase II (MII) stage (P>0.05). The EGF25 group had the highest (P<0.01) blastocyst yield (37.5%) and developmental competence (60.9%). Cleavage rate and resulting morulae and blastocysts on day 6 for EGF25 group were higher (P<0.01) than control and EGF50 groups. Although EGF did not significantly enhance nuclear maturation rate, it had a dose-related positive effect on cytoplasmic maturation, since the oocyte's ability to cleave and reach the blastocyst stage was improved at 25 ng/mL, with intermediate improvement at 10 ng/mL, but 50 ng/mL had no significant benefit. In conclusion, the addition of EGF to the maturation medium enhanced cytoplasmic maturation of cat oocytes in vitro.

Epidermal growth factor-receptor tyrosine kinase activity regulates expansion of porcine oocyte-cumulus cell complexes in vitro

We have recently shown that epidermal growth factor (EGF) strongly stimulates expansion of porcine oocyte-cumulus complexes (OCCs) isolated from large follicles (>6 mm) and does not promote expansion of OCCs from small (3-4-mm) follicles. In order to elucidate the role of EGF in OCCs expansion, in the present study, we first examined the presence of EGF receptors (EGFRs) in cumulus cells isolated from follicles of different sizes. Surprisingly, immunoblotting showed that cumulus cells obtained from all follicular size categories contained similar amounts of EGFR protein. On the other hand, we found a dramatic difference in the pattern of protein tyrosine phosphorylation in a comparison of cumulus cells isolated from small and large follicles treated by EGF. Furthermore, tyrosine-phosphorylated EGFR was specifically immunoprecipitated with antiphosphotyrosine antibodies from EGF-treated cumulus cells isolated from the large follicles. This result strongly indicates that only OCCs from the large follicles contain mature EGFRs that are capable of becoming activated by EGF. Remarkably, preincubation of cumulus cells from small follicles (3-4 mm) with FSH strongly increased EGF-stimulated tyrosine phosphorylation to levels comparable with OCCs from large follicles. The FSH-dependent activation of EGFRs was beneficial for expansion of OCCs isolated from the small follicles since OCCs treated sequentially by FSH (3 h) and EGF (1 h) underwent expansion significantly better then OCCs cultured in FSH or EGF alone. We conclude that a FSH-dependent pathway has an important role in the maturation of the EGFR in cumulus cells and that activation of EGFR-dependent signaling is sufficient to induce expansion.