Epidermal growth factor activates cytosolic [Ca2+] elevations and subsequent membrane permeabilization in mouse cumulus-oocyte complexes

Calcium signaling in oocyte quality and functionality and its application

Calcium (Ca2+) is a second messenger for many signal pathways, and changes in intracellular Ca2+ concentration ([Ca2+]i) are an important signaling mechanism in the oocyte maturation, activation, fertilization, function regulation of granulosa and cumulus cells and offspring development. Ca2+ oscillations occur during oocyte maturation and fertilization, which are maintained by Ca2+ stores and extracellular Ca2+ ([Ca2+]e). Abnormalities in Ca2+ signaling can affect the release of the first polar body, the first meiotic division, and chromosome and spindle morphology. Well-studied aspects of Ca2+ signaling in the oocyte are oocyte activation and fertilization. Oocyte activation, driven by sperm-specific phospholipase PLCζ, is initiated by concerted intracellular patterns of Ca2+ release, termed Ca2+ oscillations. Ca2+ oscillations persist for a long time during fertilization and are coordinately engaged by a variety of Ca2+ channels, pumps, regulatory proteins and their partners. Calcium signaling also regulates granulosa and cumulus cells' function, which further affects oocyte maturation and fertilization outcome. Clinically, there are several physical and chemical options for treating fertilization failure through oocyte activation. Additionally, various exogenous compounds or drugs can cause ovarian dysfunction and female infertility by inducing abnormal Ca2+ signaling or Ca2+ dyshomeostasis in oocytes and granulosa cells. Therefore, the reproductive health risks caused by adverse stresses should arouse our attention. This review will systematically summarize the latest research progress on the aforementioned aspects and propose further research directions on calcium signaling in female reproduction.

Luteinizing Hormone Causes Phosphorylation and Activation of the cGMP Phosphodiesterase PDE5 in Rat Ovarian Follicles, Contributing, Together with PDE1 Activity, to the Resumption of Meiosis

The meiotic cell cycle of mammalian oocytes in preovulatory follicles is held in prophase arrest by diffusion of cGMP from the surrounding granulosa cells into the oocyte. Luteinizing hormone (LH) then releases meiotic arrest by lowering cGMP in the granulosa cells. The LH-induced reduction of cGMP is caused in part by a decrease in guanylyl cyclase activity, but the observation that the cGMP phosphodiesterase PDE5 is phosphorylated during LH signaling suggests that an increase in PDE5 activity could also contribute. To investigate this idea, we measured cGMP-hydrolytic activity in rat ovarian follicles. Basal activity was due primarily to PDE1A and PDE5, and LH increased PDE5 activity. The increase in PDE5 activity was accompanied by phosphorylation of PDE5 at serine 92, a protein kinase A/G consensus site. Both the phosphorylation and the increase in activity were promoted by elevating cAMP and opposed by inhibiting protein kinase A, supporting the hypothesis that LH activates PDE5 by stimulating its phosphorylation by protein kinase A. Inhibition of PDE5 activity partially suppressed LH-induced meiotic resumption as indicated by nuclear envelope breakdown, but inhibition of both PDE5 and PDE1 activities was needed to completely inhibit this response. These results show that activities of both PDE5 and PDE1 contribute to the LH-induced resumption of meiosis in rat oocytes, and that phosphorylation and activation of PDE5 is a regulatory mechanism.

Roles of epidermal growth factor (EGF)-like factor in the ovulation process

Luteinizing hormone (LH) surge stimulates preovulatory follicles to induce the ovulation process, including oocyte maturation, cumulus expansion, and granulosa cell luteinization. The matured oocytes surrounded by an expanded cumulus cell layer are released from follicles to the oviduct. However, LH receptors are dominantly expressed in granulosa cells, but less in cumulus cells and are not expressed in oocytes, indicating that the secondary factors expressed and secreted from LH-stimulated granulosa cells are required for the induction of the ovulation process. Prostaglandin and progesterone are well-known factors that are produced in granulosa cells and then stimulate in both granulosa and cumulus cells. The mutant mice of prostaglandin synthase (Ptgs2KO mice) or progesterone receptor (PRKO mice) revealed that the functions were essential to accomplish the ovulation process, but not to induce the ovulation process. To identify the factors initiating the transfer of the stimuli of LH surge from granulosa cells to cumulus cells, M. Conti's lab and our group performed microarray analysis of granulosa cells and identified the epidermal growth factor (EGF)-like factor, amphiregulin (AREG), epiregulin (EREG), and β-cellulin (BTC) that act on EGF receptor (EGFR) and then induce the ERK1/2 and Ca²⁺-PLC pathways in cumulus cells. When each of the pathways was down-regulated using a pharmacological approach or gene targeting study, the induction of cumulus expansion and oocyte maturation were dramatically suppressed, indicating that both pathways are inducers of the ovulation process. However, an in vitro culture study also revealed that the EGFR-induced unphysiological activation of PKC in cumulus cells accelerated oocyte maturation with low cytostatic activity. Thus, the matured oocytes are not arrested at the metaphase II (MII) stage and then spontaneously form pronuclei. The expression of another type of EGF-like factor, neuregulin 1 (NRG1), that does not act on EGFR, but selectively binds to ErbB3 is observed in granulosa cells after the LH surge. NRG1 supports EGFR-induced ERK1/2 phosphorylation, but reduces PKC activity to physiological level in the cumulus cells, which delays the timing of meiotic maturation of oocytes to adjust the timing of ovulation. Thus, both types of EGF-like factor are rapidly induced by LH surge and then stimulate cumulus cells to control ERK1/2 and PKC pathways, which results in the release of matured oocytes with a fertilization competence.

The effects of epidermal growth factor (EGF) on the in vitro development of isolated goat secondary follicles and the relative mRNA expression of EGF, EGF-R, FSH-R and P450 aromatase in cultured follicles

The effects of varying concentrations of EGF were evaluated in terms of in vitro follicular development and the mRNA expression levels of EGF, EGF-R, FSH-R and P450 aromatase. After 6 days, the addition of 50 ng/mL of EGF to the culture medium increased the antrum formation rates in comparison to cultured control and after 18 days of culture produced oocytes with higher rates of meiosis resumption when compared to the other treatments (P<0.05). The daily follicular growth rates in presence of EGF (50 or 100) were increased in comparison to the cultured control (P<0.05). Treatment with EGF 50 stimulated the expression of EGF mRNA but reduced EGF-R mRNA expression and estradiol secretion as compared to the cultured control (P<0.05). After 18 days of culture, the mRNA levels for FSH-R and P450 aromatase were greater than those of the non-cultured controls (P<0.05). In conclusion, the effects of EGF treatment on the mRNA levels for EGF, EGF-R, FSH-R, and P450 aromatase varied according to the stage of follicle development.

EGF-like factors induce expansion of the cumulus cell-oocyte complexes by activating calpain-mediated cell movement

Cumulus cell-oocyte complex (COC) expansion is obligatory for LH-induced ovulation and is initiated by LH induction of the epidermal growth factor (EGF)-like factors that mediate the synthesis of the hyaluronan-rich matrix and hyaluronan-stabilizing factors. COC expansion also involves the movement of cumulus cells within the matrix by mechanisms that have not been characterized. We document herein that two proteases, calpain 2 and to a lesser extent calpain 1, are expressed in cumulus cells and that the proteolytic activity of these enzymes is rapidly and significantly increased in COC isolated from human chorionic gonadotropin-induced ovulatory follicles in vivo. Stimulation of calpain activity was associated with proteolytic degradation of paxillin and talin (two components of focal adhesion complexes), cell detachment, and the formation of cell surface bleb-like protrusions. Injection of a calpain inhibitor in vivo reduced 1) human chorionic gonadotropin-stimulated calpain enzyme activity, 2) cell detachment, 3) membrane protrusion formation, and 4) COC expansion by mechanisms that did not alter Has2 expression. During EGF-like factor induction of COC expansion in culture, calpain activity was increased by ERK1/2 and intracellular Ca(2+) signaling pathways. Inhibition of calpain activity in cultured COC blocked cumulus cell detachment, protrusion formation, and the vigorous movement of cumulus cells. As a consequence, COC expansion was impaired. Collectively, these results show that two highly coordinated processes control COC expansion. One process involves the synthesis of the hyaluronan matrix, and the other mediates cumulus cell detachment and movement. The latter are controlled by calpain activation downstream of the EGF receptor activation of the Ca(2+) pathway and ERK1/2 pathways.

Role of cyclic AMP in the maturation of Ciona intestinalis oocytes

Immature oocytes are arrested at prophase I of the meiotic process and maturation onset is indicated by oocyte nuclear disassembly (germinal vesicle breakdown or GVBD). Signaling pathways that elevate intracellular cyclic AMP (cAMP) may either prevent or induce oocyte maturation depending on the species. In some marine invertebrates and, in particular, in ascidian oocytes, cAMP triggers GVBD rather than blocking it. In this paper, we tested different cAMP elevators in fully grown oocytes at the germinal vesicle stage (GV) of the ascidian Ciona intestinalis. We demonstrated that through the activation of adenylate cyclase or the inhibition and phosphodiesterases the oocyte remained at the GV stage. This effect was reversible as the GV-arrested oocytes, rinsed and incubated in sea water, are able to undergo spontaneous maturation and extrusion of follicle cells. In addition, oocytes acquire the ability to be fertilized and start early development. However, morphology of follicle cells, embryos and larvae from in vitro matured oocytes showed different morphology from those derived from in vivo mature oocytes. The role and the transduction mechanism of cAMP in the regulation of oocyte maturation were discussed. Finally, we indicated a variation of biological mechanisms present in the ascidian species; moreover, we sustain evidence proving that tunicates share some biological mechanisms with vertebrates. This information provided new hints on the importance of ascidians in the evolution of chordates.

Acrosome reaction in the cumulus oophorus revisited: involvement of a novel sperm-released factor NYD-SP8

Fertilization is a process involving multiple steps that lead to the final fusion of one sperm and the oocyte to form the zygote. One of the steps, acrosome reaction (AR), is an exocytosis process, during which the outer acrosome membrane fuses with the inner sperm membrane, leading to the release of acrosome enzymes that facilitate sperm penetration of the egg investments. Though AR has been investigated for decades, the initial steps of AR in vivo, however, remain largely unknown. A well elucidated model holds the view that AR occurs on the surface of the zona pellucida (ZP), which is triggered by binding of sperm with one of the ZP glycosylated protein, ZP3. However, this model fails to explain the large number of 'falsely' acrosome-reacted sperms found within the cumulus layer in many species examined. With the emerging evidence of cross-talk between sperm and cumulus cells, the potential significance of AR in the cumulus oophorus, the outer layer of the egg, has been gradually revealed. Here we review the acrosome status within the cumulus layer, the cross-talk between sperm and cumulus cells with the involvement of a novel sperm-released factor, NYD-SP8, and re-evaluate the importance and physiological significance of the AR in the cumulus in fertilization.

Pharmacological properties of a pore induced by raising intracellular Ca2+

Recent studies on the P2X(7) receptor in 2BH4 cells and peritoneal macrophages have demonstrated that the raise in intracellular Ca(2+) concentration induces a pore opening similar to P2X(7) receptor pore. Herein, we have investigated whether the pore activated by the elevation of intracellular Ca(2+) concentration is associated to P2X(7) receptor. Using patch clamp in cell attached, whole cell configuration, and dye uptake, we measured the pore opening in cell types that express the P2X(7) receptor (2BH4 cells and peritoneal macrophages) and in cells that do not express this receptor (HEK-293 and IT45-RI cells). In 2BH4 cells, the stimulation with ionomycin (5-10 microM) increased intracellular free Ca(2+) concentration and induced pore formation with conductance of 421 +/- 14 pS, half-time (t(1/2)) for ethidium bromide uptake of 118 +/- 17 s, and t(1/2) for Lucifer yellow of 122 +/- 11 s. P2X(7) receptor antagonists did not block these effects. Stimulation of HEK-293 and IT45-RI cells resulted in pore formation with properties similar to those found for 2BH4 cells. Connexin hemichannel inhibitors (carbenoxolone and heptanol) also did not inhibit the pore-induced effect following the increase in intracellular Ca(2+) concentration. However, 5-(N,N-hexamethylene)-amiloride, a P2X(7) receptor pore blocker, inhibited the induced pore. Moreover, intracellular signaling modulators, such as calmodulin, phospholipase C, mitogen-activated protein kinase, and cytoskeleton components were important for the pore formation. Additionally, we confirmed the results obtained for electrophysiology by using the flow cytometry, and we discarded the possibility of cellular death induced by raising intracellular Ca(2+) at the doses used by using lactate dehydrogenase release assay. In conclusion, increased concentration in intracellular Ca(+2) induces a novel membrane pore pharmacologically different from the P2X(7) associated pore and hemigap-junction pore.

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

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

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

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

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

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

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

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.

Meiotic resumption in response to luteinizing hormone is independent of a Gi family G protein or calcium in the mouse oocyte

The signaling pathway by which luteinizing hormone (LH) acts on the somatic cells of vertebrate ovarian follicles to stimulate meiotic resumption in the oocyte requires a decrease in cAMP in the oocyte, but how cAMP is decreased is unknown. Activation of Gi family G proteins can lower cAMP by inhibiting adenylate cyclase or stimulating a cyclic nucleotide phosphodiesterase, but we show here that inhibition of this class of G proteins by injection of pertussis toxin into follicle-enclosed mouse oocytes does not prevent meiotic resumption in response to LH. Likewise, elevation of Ca2+ can lower cAMP through its action on Ca2+-sensitive adenylate cyclases or phosphodiesterases, but inhibition of a Ca2+ rise by injection of EGTA into follicle-enclosed mouse oocytes does not inhibit the LH response. Thus, neither of these well-known mechanisms of cAMP regulation can account for LH signaling to the oocyte in the mouse ovary.

Expression of mRNA and protein localization of epidermal growth factor and its receptor in goat ovaries

To examine the possibility that epidermal growth factor (EGF) and its receptor (EGF-R) are expressed throughout folliculogenesis, we studied the presence and distribution of EGF and EGF-R in goat ovaries. Ovaries of goats were collected and either fixed in paraformaldehyde for immunohistochemical localization of proteins, or used for the isolation of follicles, luteal cells and ovarian surface epithelium to study mRNA expression for EGF and EGF-R, using the reverse transcriptase polymerase chain reaction. EGF protein and mRNA were found in primordial, primary and secondary follicles as well as in small and large antral follicles and in surface epithelium, but in corpora lutea only the protein could be detected. Antral follicles expressed EGF mRNA in oocyte, cumulus, mural granulosa and theca cells. For EGF-R, both protein and mRNA were present at all stages of follicular development and in all antral follicular compartments. EGF-R protein and mRNA were also found in corpora lutea and surface epithelium. It is concluded that EGF and its receptor are expressed in goat ovarian follicles at all stages of follicle development, in corpora lutea, and in ovarian surface epithelium.

Meiotic spindle configuration is differentially influenced by FSH and epidermal growth factor during in vitro maturation of mouse oocytes

BACKGROUND

To ascertain whether different hormonal treatment protocols could affect metaphase II (MII) spindle morphology, meiotic spindle organization was detected in prepubertal mouse oocytes matured under conditions allowing spontaneous, FSH- or epidermal growth factor (EGF)-dependent meiotic maturation.

METHODS

Oocyte-cumulus complexes (OCCs) were matured either spontaneously (control; n=270) or in the presence of hypoxanthine (Hx) plus FSH (n=400) or EGF (n=370). Spindles were detected by immunofluorescence analysis. In vivo ovulated (IVO) oocytes were processed similarly.

RESULTS

IVO oocytes displayed spindles underlying the oolemma and with focused poles marked by spots of gamma-tubulin, whereas the majority (89%) of control oocytes had barrel-shaped spindles, positioned away from the oolemma, and with gamma-tubulin distributed along microtubules. Similar configuration/localization was found in 85% of the oocytes matured in vitro in the presence of Hx and FSH. In the presence of Hx-EGF, 35% of the oocytes showed spindles with an IVO-like configuration, although gamma-tubulin was homogeneously distributed throughout microtubules. Independently of spindle shape, 52% of EGF-stimulated oocytes had spindles positioned near the oolemma, in comparison to just 24% of FSH-treated and 13% of control oocytes.

CONCLUSIONS

These results indicate that FSH and EGF can differently affect meiotic spindle morphology, and that EGF might be a stronger contributor than FSH to the acquisition of oocyte competence.

Epidermal growth factor receptor downregulation in cultured bovine cumulus cells: reconstitution of calcium signaling and stimulated membrane permeabilization

Cumulus cell-oocyte complexes (COCs), cultured in vitro, are competent for maturation and fertilization. Inclusion of epidermal growth factor (EGF) in the COC culture medium enhances in vitro maturation and subsequent embryonic development. It has been shown that isolated COCs exposed to EGF respond with a prolonged and pulsatile release of Ca2+ into the extra-cellular medium and that cumulus cells (CCs) of complexes exhibit both a slow rise in intracellular [Ca2+] ([Ca2+]i) and plasma membrane permeabilization in response to EGF. These unusual signaling responses were examined in isolated, cultured bovine CCs. Few individual CCs showed [Ca2+]i increases; the lack of response was found to be due to decrease of expression of endogenous EGF receptors after dissociation. CCs transfected with a human EGF receptor-GFP fusion protein showed robust, prolonged, EGF-stimulated [Ca2+]i elevations characteristic of CC responses in intact COCs. Many CCs that responded to EGF stimulation with a [Ca2+]i rise also released entrapped fura-2 dye at the peak of the [Ca2+]i response, suggesting that CC permeabilization and death follows activation of the EGF receptor. The [Ca2+]i elevation due to EGF stimulation and subsequent membrane permeabilization was shown to be mediated by the inositol triphosphate signaling pathway.