Possible Role of p38 MAPK-MNK1-EMI2 Cascade in Metaphase-II Arrest of Mouse Oocytes¹

Abstract

The Mos-MAPK signaling pathway involving the Mos-MEK1/2-ERK1/2-RSK1/2/3 or MSK1-EMI2 cascade is directly linked to metaphase-II arrest of vertebrate oocytes. In this study, we examined whether p38, a member of the MAPK subfamily, is regulated under the control of Mos and contributes to metaphase-II arrest in the mouse oocyte. Morpholino oligonucleotide-mediated depletion of Mos revealed a remarkable decrease in phosphorylation of p38. Simultaneous treatment of oocytes with two chemical inhibitors of p38 and MEK1/2 induced both release from metaphase II and degradation of cyclin B1, whereas the treatment with each of these two inhibitors had little effect. Moreover, phosphorylation of EMI2 was dramatically abolished by addition of the two inhibitors. Indeed, MNK1, a kinase downstream of p38, exhibited the ability to phosphorylate EMI2. These results suggest that in addition to the Mos-MEK1/2 pathway, the Mos-mediated p38 pathway may be implicated in metaphase-II arrest.

Introduction

Vertebrate oocytes at the germinal-vesicle (GV) stage are arrested at prophase of meiosis I and undergo maturation after hormonal stimulation [1, 2]. Mature oocytes are rearrested at the metaphase of meiosis II until fertilization. The metaphase-II arrest is caused by the activity of cytostatic factor CSF [3] that inhibits the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase required for degradation of cyclin B [4]. As a consequence of cyclin B stabilization, the activity of MPF, the Cdk1-cyclin B complex [5], is maintained at an elevated level that prevents the exit from metaphase II and parthenogenetic activation of oocytes [6]. In Xenopus, CSF-mediated arrest involves the Mos-MAPK pathway (Mos-Mek1/2-Erk1/2-Rsk1/2/3) and Emi2, a direct inhibitor of the APC/C [7–11]. Rsks phosphorylate four Ser/Thr residues of Emi2 at positions S335, T336, S342, and S344 [12, 13]. Multiple phosphorylation of Emi2 upregulates its stability and inhibitory binding to the APC/C, thereby promoting metaphase-II arrest [14].

In mice, the oocytes lacking Mos are activated without stimulation on fertilization [15–19]. The EMI2-depleted oocytes sustain a low level of cyclin B after meiosis I, which leads to the failure to enter metaphase II [20, 21]. Thus, both Mos and EMI2 are essential for metaphase-II arrest in the oocytes, but the role of the components downstream of Mos is still controversial. Inhibition of MEK1/2 synthesis by small interfering RNA (siRNA) has little effect on metaphase-II arrest [22]. Injection of a constitutively active form of MEK1/2 into Mos-deficient oocytes fails to activate ERK1/2 [23]. These findings raise the possibility that another Mos-dependent kinase also plays a key role in metaphase-II arrest.

One of the major members of the MAPK family that negatively regulates cell-cycle progression at both G1/S and G2/M in somatic cells is p38 MAPK [24]. At least five Ser/Thr kinases, MSK1, MSK2, MNK1, MK2, and MK3, have been identified as p38 substrates [25]. Functional analysis of p38 in the mouse oocytes suggests that this kinase regulates spindle assembly and accurate chromosome segregation through phosphorylation of MK2 [26, 27]. In porcine oocytes, p38 has been reported to contribute to the transition of metaphase I to metaphase II [28], although the mechanistic details remain unclear.

In this study, we examined whether p38 is regulated under the control of Mos and contributes to metaphase-II arrest in mouse oocytes. Depletion of Mos revealed a remarkable decrease in phosphorylation of p38. When the Mos-mediated MEK1/2 and p38 pathways were both blocked by specific inhibitors, the metaphase-II arrest was released. The possible role of p38 in metaphase-II arrest is discussed.

Materials and Methods

All animal experiments were performed ethically, and experimentation was in accord with the Guide for the Care and Use of Laboratory Animals at University of Tsukuba.

Plasmids

DNA fragments encoding EMI2 (GenBank ID: NP_001074772.1) and MSK1 (GenBank ID: AAQ24165.1) were amplified by PCR using a mouse ovary or testis cDNA library as a template. The following sets of oligonucleotides were used as primers: 5′-GGGCGGCCGCGGATGGACTCCTCTGCTGTC-3′ and 5′-GGCTCGAGTCAGAGGCGTTTTAAGTTCCGC-3′ for EMI2; 5′-GCCTCGAGAGGGTGAAGATGGAGGG-3′ and 5′-GCTCTAGAACATACCTCAGGCACATG-3′ for MSK1. The amplified fragments were introduced into a pcDNA3/FLAG-HA vector. To prepare a pGEX4T-1/EMI2 plasmid containing the EMI2 sequence with residues 288–383, DNA fragments were PCR amplified using pcDNA3/FLAG-HA/EMI2 as the template and introduced into pGEX4T-1. A pcDNA3.1-poly(A)/FLAG-HA/EMI2 plasmid was prepared by introducing the Kpn I/Bam HI fragment of pcDNA3/FLAG-HA/EMI2 into pcDNA3.1-poly(A) as previously described [29]. In vitro site-directed mutagenesis was carried out according to the manufacturer's protocol (Agilent Technology, Santa Clara, CA).

Antibodies

A synthetic phosphorylated peptide, Cys-Arg-Leu-Arg-Arg-Leu-phospho Ser-phospho Thr-Leu-Gln-Glu-Gln-Gly, corresponding to the 12-residue sequence of EMI2 at positions 321–332, was conjugated to maleimide-activated keyhole limpet hemocyanin (KLH) or bovine serum albumin (BSA) according to the manufacturer's protocol (Thermo Fisher Scientific, Waltham, MA). The protein KLH-conjugated peptide was emulsified with Freund complete (Difco Laboratories, Detroit, MI) or incomplete adjuvant (Wako, Osaka, Japan), and injected into female Japanese white rabbits (Japan SLC, Shizuoka, Japan). Antibody against the unphosphorylated form of EMI2 was removed from serum using a Sepharose 4B column previously coupled with glutathione S-transferase (GST)-tagged EMI2 at residues 288–383. Anti-phosphorylated EMI2 antibody was purified using a Sepharose 4B column previously coupled with the BSA-conjugated 12-residue peptide. Anti-phosphorylated MNK1, anti-phosphorylated p38, and anti-phosphorylated ERK1/2 antibodies were purchased from Cell Signaling Technology (Danvers, MA), and anti-β-actin, anti-glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and fluorescein isothiocyanate (FITC)-conjugated anti-α-tubulin antibodies were from Sigma-Aldrich (St. Louis, MO). Anti-cyclin B1, anti-HA, and anti-Mos antibodies were purchased from Abcam (Cambridge, MA), Roche (Mannheim, Germany), and Santa Cruz Biotechnology (Santa Cruz, CA), respectively. Horseradish peroxidase-conjugated antibodies against mouse, rat, and rabbit IgG were purchased from Jackson ImmunoResearch Laboratories (West Grove, PA).

Oocyte Collection and Microinjection

GV-stage oocytes were collected from ovaries of 6- to 9-wk-old ICR mice (Japan SLC). The oocytes were placed in a drop of flushing-holding medium (FHM) containing 0.32 mM N⁶, 2′-O-dibutyryladenosine 3′:5′-cyclic monophosphate, dbcAMP (Sigma-Aldrich). The oocytes were microinjected with RNA samples (0.2 μg/μl), 4 mM morpholino oligonucleotide (MO; Gene Tools, Philomath, OR; see Supplemental Table S1; Supplemental Data are available online at www.biolreprod.org), or 50 mM small interfering RNA (siRNA; Sigma-Aldrich; see Supplemental Table S1) using a FemtoJet constant flow system (Eppendorf, Hamburg, Germany). The oocytes were cultured in KSOM medium (SOM with a higher K⁺concentration) containing 5% BSA at 37°C under 5% CO₂ in air.

Immunoblot Analysis

Proteins were separated by SDS-PAGE and transferred onto Immobilon-P polyvinylidene difluoride membranes (Millipore, Bedford, MA). The blots were blocked with 20 mM Tris-HCl, pH 7.5, containing 5% skim milk, 0.1% Tween-20, and 0.15 M NaCl; incubated with primary antibodies; and then treated with secondary antibodies conjugated with horseradish peroxidase. The immunoreactive proteins were visualized using an ECL or ECL Prime Western Blotting Detection kit (GE Healthcare, Piscataway, NJ).

Immunostaining Analysis

Oocytes were fixed in PBS containing 4% paraformaldehyde at 4°C for 2 h, treated with 1% Triton X-100 in PBS for 20 min, and blocked with PBS containing 1.5% goat serum and 0.05% Tween-20 at room temperature for 60 min. The oocytes were incubated with FITC-conjugated anti-α-tubulin antibody at room temperature for 60 min, counterstained with Hoechst 33342, and then viewed under an IX71 fluorescence microscope (Olympus, Tokyo, Japan), as previously described [30].

RNA Synthesis In Vitro

RNAs were synthesized by T7 polymerase using a RiboMAX Large Scale RNA Production System-T7 kit (Promega, Madison, WI). The synthesized RNAs were treated with RQ1 RNase-free DNase (Promega) at 37°C for 20 min, extracted with phenol-chloroform, precipitated with ethanol, dissolved in 10 mM Tris-HCl, pH 7.4, containing 0.1 mM EDTA, applied to MicroSpin G-25 columns (GE Healthcare), and stored at −80°C.

Immunoprecipitation

HEK293T cells were cultured in Dulbecco modified Eagle medium supplemented with 10% fetal bovine serum, 100 U/ml of penicillin, and 0.1 mg/ml of streptomycin at 37°C under 5% CO₂ in air. Cells were transfected with expression plasmids encoding FLAG-HA-tagged MSK1, using a PerFectin transfection reagent (Genlantis, San Diego, CA). After 48 h, cells were cultured in serum-free medium for 24 h and then treated with 1 μM 12-O-tetradecanoylphorbol-13-acetate (LC Laboratories, Woburn, MA) for 60 min. Cells were lysed in a lysis buffer containing 25 mM Tris-HCl, pH 7.5, 0.15 M NaCl, 0.1% SDS, 1% NP-40, 1 mM EDTA, 1 μg/ml aprotonin, 1 μg/ml pepstatin, 1 μg/ml leupeptin, 50 μg/ml PMSF, 1 mM dithiothreitol (DTT), 20 mM NaF, and 0.5 mM Na₃ VO₄. The cell lysates were subjected to immunoprecipitation using anti-FLAG M2-agarose beads (Sigma-Aldrich).

RNA Interference

NIH3T3 cells were transfected with siRNA (0.1 μM) using Lipofectamine 2000 (Invitrogen, Carlsbad, CA). After 24 h, cells were treated with 10 μM or 0.5 mM H₂ O₂ for 60 min for oxidative stress and then lysed with the above lysis buffer. Proteins in the cell lysates were subjected to immunoblot analysis.

In Vitro Kinase Assay

In vitro kinase assays were carried out in 10 mM Tris-HCl, pH 7.5, containing 10 mM MgCl₂, 0.1 mM DTT, 2 mM Na₃ VO₄, 50 μM NaF, and 0.2 mM ATP. GST-tagged EMI2 protein (1 μg) was incubated with MSK1 purified from HEK293T cells, MNK1 (Life Technologies, Carlsbad, CA), or MK2 (Enzo Life Sciences, Farmingdale, NY) in the above buffer at 30°C for 30 min, and analyzed by immunoblotting using anti-phosphorylated EMI2 antibody.

Statistical Analysis

Data are presented as mean values ± SEM. The Student t-test was used for statistical analysis; significance was assumed for P< 0.05.

Results

Possible Involvement of Mos in Phosphorylation of p38

We initially examined whether phosphorylation of p38 was affected by depletion of Mos. Because MO is known to effectively induce Mos reduction [31], GV-stage oocytes were injected with 4 mM MO against Mos mRNA (Mos/MO) or 4 mM negative control MO (Control/MO) in FHM containing dbcAMP. After 2 h incubation, the oocytes were cultured in KSOM medium in the absence of dbcAMP for 16 h. Depletion of Mos was verified by immunoblot analysis (Fig. 1A). Importantly, phosphorylation of both ERK1/2 and p38 was remarkably decreased in the Mos-depleted oocytes. Immunostaining analysis indicated that approximately 70% of the Control/MO-injected oocytes were arrested at metaphase II, whereas a significantly low proportion (15%) of the metaphase-II oocytes was found in the Mos/MO-injected oocytes (Fig. 1B). Because almost 60% of the Mos/MO-injected oocytes contained large polar bodies or decondensed chromatin formation, as described previously [31], these oocytes were classified into “oocyte activation.” These results suggest that Mos may be involved in the phosphorylation state of p38, and imply that p38 may partly participate in metaphase-II arrest.

Fig. 1

Possible involvement of Mos in phosphorylation of p38. A) Depletion of Mos by injection with MO. The GV-stage oocytes were injected with 4 mM MO against Mos mRNA (Mos/MO) or negative control MO (Control/MO), and incubated in the presence of dbcAMP. After 2 h incubation, the oocytes were cultured in the absence of dbcAMP for 16 h. Whole protein extracts of the MO-injected oocytes (30 cells/lane) were analyzed by immunoblotting using antibodies against Mos, phosphorylated ERK1/2 (pERK1/2), phosphorylated p38 (pp38), and GAPDH (three independent experiments). B) Immunostaining analysis. The injected oocytes were stained with anti-α-tubulin antibody (green) and Hoechst 33342 (blue). Original magnification ×40. The oocytes at the GV, metaphase I (MI), and metaphase II (MII) stages and activated oocytes (oocyte activation) were counted (three independent experiments). Numbers in parentheses indicate the total number of oocytes examined.

p38 Contributes to Metaphase-II Arrest

To test the possibility that a p38 pathway functions in metaphase-II arrest, we examined the effects of U0126 and SB203580 [32, 33], which specifically interfere with the activity of MEK1/2 and p38, respectively (Fig. 2). As shown in Supplemental Figure S1, ERK1/2 in the metaphase-II oocytes was not phosphorylated by incubation in the presence of 20 or 50 μM U0126, and p38 remained phosphorylated at the same concentrations. On the other hand, 10 μM SB203580 has been demonstrated to sufficiently inhibit p38 in porcine cumulus-oocyte complex [28]. Our preliminary experiments, however, indicated that the treatment of mouse oocytes at GV breakdown (GVBD) stage with 0–50 μM SB203580 has little effect, whereas approximately 40% of the total GVBD oocytes treated with 60 μM SB203580 are arrested at the metaphase-I stage (Supplemental Fig. S2). In this study, we thus examined the effects of 20 μM U0126 and 60 μM SB203580 on oocyte maturation. The metaphase-II oocytes were incubated with U0126 and/or SB203580 for 7 h and subjected to immunostaining analysis (Fig. 2A). SB203580 had little effect on metaphase-II arrest (Fig. 2B). Approximately 20% of the U0126-treated oocytes exhibited cell-cycle resumption, and the remainder (∼80%) maintained the arrest with shrinking bipolar spindle. The simultaneous treatment with U0126 and SB203580 led to chromosome segregation, a hallmark of anaphase progression (∼90% of the total oocytes). Although the level of cyclin B1 ∼45% decreased in the presence of U0126 or SB203580, simultaneous treatment dramatically induced degradation of cyclin B1 (Fig. 2C). Moreover, the phosphorylation state of ERK1/2 was not affected by SB203580, as expected. Thus, the p38 pathway, in addition to the Mos-MAPK pathway, may contribute to the maintenance of metaphase-II arrest.

Fig. 2

Contribution of p38 to metaphase-II arrest. A) Immunostaining analysis (two independent experiments). The metaphase-II oocytes were incubated in the absence (−) or presence (+) of 60 μM SB203580 and 20 μM U0126 for 7 h, and then stained with anti-α-tubulin antibody (green) and Hoechst 33342 (blue). Original magnification ×40. B) Effect of SB203580 and U0126 on metaphase-II release. The metaphase-II oocytes were incubated in the absence (−) or presence (+) of 60 μM SB203580 and 20 μM U0126 for 7 h. The oocytes released from metaphase-II arrest were counted. Numbers in parentheses indicate the total number of oocytes examined. C) Immunoblot analysis. The metaphase-II oocytes were incubated with (+) or without (−) 60 μM SB203580 and 20 μM U0126 for 7 h. Whole protein extracts (30 cells/lane) were analyzed using anti-cyclin B1 (CycB1), anti-phosphorylated ERK1/2 (pERK1/2), and anti-GAPDH antibodies. Chemiluminescence intensity of CycB1 was measured and corrected by GAPDH intensity. Data in Band Care expressed as mean ± SEM of three independent experiments (*P< 0.05).

Phosphorylation of EMI2 in Mouse Oocytes

Xenopus Emi2 is activated and stabilized by phosphorylation at S335, T336, S342, and S344 during meiosis II [12, 13]. These four Ser/Thr residues are evolutionarily well conserved in the mouse, human, and zebrafish counterparts (Fig. 3A). To examine phosphorylation of EMI2 by the p38 pathway, we prepared anti-phosphorylated EMI2 antibody that recognizes phosphorylated S326 and T327 of mouse EMI2. The antibody specificity was validated by in vitro kinase assays using MSK1 purified from HEK 293T cells and GST-fused protein containing the amino acid sequence of EMI2 at positions 288–383. Consistent with the fact that MSK1 phosphorylates S326, T327, S333, and S335 in the EMI2 peptide [34], MSK1-phosphorylated EMI2 was recognized by anti-phosphorylated EMI2 antibody (Fig. 3B). No protein was immunoreactive with anti-phosphorylated EMI2 antibody, when phosphorylated EMI2 was treated with lambda protein phosphatase (Fig. 3B), and when a GST-fused EMI2 mutant protein, EMI2/2A, in which S326 and T327 are substituted with Ala [34], was analyzed by in vitro kinase assays (Fig. 3C). Thus, this antibody recognizes S326- and T327-phosphorylated EMI2.

Fig. 3

Phosphorylation of EMI2 in mouse oocytes. A) Alignment of the amino acid sequences of EMI2 among mouse, human, Xenopus, and zebrafish. Black boxes indicate the conserved residues. S326, T327, S333, and S335 of mouse EMI2 are phosphorylated by MSK1 and RSKs. The sequence of an EMI2/2A mutant is also aligned. B) In vitro kinase assay. GST-tagged EMI2 was incubated with (+) or without (−) 100 ng purified MSK1, treated with (+) or without (−) lambda protein phosphatase (λpp), and analyzed by immunoblotting using anti-phosphorylated EMI2 (pEMI2) and anti-GST antibodies. C) Recognition of EMI2/2A by anti-pEMI2 antibody. GST-tagged wild-type EMI2 (WT) or EMI2/2A (2A) was incubated with (+) or without (−) MSK1 (two independent experiments). D) Phosphorylation of EMI2 in mouse oocytes. Whole protein extracts from the metaphase-II oocytes (150 cells/lane) were treated with (+) or without (−) λpp and analyzed by immunoblotting using anti-pEMI2 and anti-β-actin antibodies (two independent experiments). Asterisks indicate background proteins. E) Exogenous expression of EMI2 in metaphase-II oocytes. The metaphase II-stage oocytes were injected with (+) or without (−) polyadenylated RNA encoding HA-tagged EMI2/DSG, and cultured for 4 h. The oocytes (30 cells/lane) were subjected to immunoblot analysis using antibodies against pEMI2, HA, and GAPDH (two independent experiments).

To ascertain phosphorylation of EMI2 in mouse oocytes, we carried out immunoblot analysis of 150 metaphase-II oocytes using anti-phosphorylated EMI2 antibody (Fig. 3D). Phosphorylated EMI2 was found as a 90-kDa protein, the immunoreactivity of which was abolished by the treatment with lambda protein phosphatase. When RNA encoding an HA-tagged EMI2 mutant, EMI2/DSG [34], was injected into metaphase II-stage oocytes, the exogenously expressed 90-kDa protein was indeed immunoreactive with anti-phosphorylated EMI2 and anti-HA antibodies (Fig. 3E).

Phosphorylation of EMI2 by the p38 Pathway

At least five Ser/Thr kinases have been identified as substrates of p38 [25]. We speculated that these five kinases are involved in phosphorylation of EMI2. To ask whether EMI2 is targeted by the p38 pathway in the CSF arrest, we utilized an established experimental system to evaluate the activity of phosphorylated EMI2 [34, 35]. The GV-stage oocytes were injected with HA-tagged EMI2/DSG RNA in the presence of dbcAMP, cultured in the absence of dbcAMP for 12 h, and then transferred to a medium containing U0126 and/or SB203580 (Fig. 4A). SB203580 had little effect on phosphorylation of EMI2, whereas EMI2 phosphorylation was approximately 30% inhibited by the presence of U0126 (Fig. 4B). A significantly great reduction (∼70%) of EMI2 phosphorylation was induced by simultaneous treatment with U0126 and SB203580. Thus, EMI2 phosphorylation may be coregulated by the p38 pathway, in addition to the MEK1/2 pathway.

Fig. 4

Phosphorylation of EMI2 by the p38 pathway. A) Experimental protocol for expression of HA-tagged EMI2/DSG mutant. The GV-stage oocytes were injected with polyadenylated RNA encoding HA-tagged EMI2/DSG in the presence of dbcAMP, and cultured in the absence of dbcAMP for 12 h. After 12 h of culture, the oocytes were incubated in the presence of kinase inhibitors for 6 h. B) Immunoblot analysis. The injected oocytes were incubated in the absence (−) or presence (+) of 60 μM SB203580 and 20 μM U0126 for 6 h. Protein extracts (30 oocytes/lane) were subjected to immunoblot analysis using antibodies against phosphorylated EMI2 (pEMI2) and HA. Chemiluminescence intensity of pEMI2 was measured and corrected by HA intensity. Data are expressed as mean ± SEM of four independent experiments (*P< 0.05). C) Immunostaining analysis (two independent experiments). The injected oocytes were incubated with (+) or without (−) 60 μM SB203580 and 20 μM U0126 for 6 h, stained with anti-α-tubulin antibody (green), and counterstained with Hoechst 33342 (blue). Original magnification ×40.

To examine whether metaphase-I arrest is released by kinase inhibitors, we performed immunostaining analysis of inhibitor-treated oocytes (Fig. 4C). The bipolar spindle with chromosome alignment was normally observed after the treatment with each of U0126 and SB203580, indicating the maintenance of metaphase-I arrest. Despite significantly reduced phosphorylation of EMI2, the oocytes treated with both U0126 and SB203580 underwent no chromosome segregation but contained irregularly scattered chromosomes.

Direct Phosphorylation of EMI2 by MNK1

Because MNK1 and MK2 as substrates of p38 are activated during oocyte maturation [26, 36], we asked if these two kinases directly phosphorylate EMI2. In vitro kinase assays indicated that both MNK1 and MK2 are capable of phosphorylating EMI2 (Fig. 5, A and B). We also examined the effects of two kinase inhibitors, CGP57380 and CMPD1, on the phosphorylation state of EMI2 in mouse oocytes (Fig. 5, C and D). CGP57380 is known to inhibit the activity of MNK1 and exhibit no inhibitory effect on p38 and ERK1/2 [37]. On the other hand, p38-catalyzed phosphorylation of MK2 is inhibited by CMPD1 in a non-ATP-competitive manner; this inhibitor specifically blocks the p38 activity toward MK2 [38]. The GV-stage oocytes were injected with HA-tagged EMI2/DSG RNA, matured to the metaphase-I stage, and then incubated in the presence of 20 μM U0126 and 60 μM CGP57380 or 60 μM CMPD1 for 6 h. Although HA-tagged EMI2 remained phosphorylated in the oocytes treated with CGP57380 alone (Fig. 5C), the treatment with both U0126 and CGP57380 inhibited phosphorylation of EMI2 (Fig. 5E). Moreover, CMPD1 had little effect on EMI2 phosphorylation, regardless of the presence or absence of U0126 (Fig. 5, D and E). These data suggest that MNK1, in addition to other kinases downstream of MEK1/2, may phosphorylate EMI2 during the CSF arrest.

Fig. 5

Direct phosphorylation of EMI2 by MNK1. Aand B) In vitro kinase assays for phosphorylation of EMI2 by MNK1 and MK2. GST-tagged EMI2 was incubated with (+) or without (−) 350 ng MNK1 or 100 ng MK2, and subjected to immunoblot analysis using anti-phosphorylated EMI2 (pEMI2) and anti-GST antibodies (two independent experiments). C–E) Effect of CMPD1 and CGP57380 on EMI2 phosphorylation in the oocytes. The GV-stage oocytes were injected with RNA encoding HA-tagged EMI2/DSG in the presence of dbcAMP, and cultured in the absence of dbcAMP for 12 h. The oocytes (30 cells/lane) were incubated in the absence (−) or presence (+) of 60 μM CMPD1, 60 μM CGP57380, or 20 μM U0126, and subjected to immunoblot analysis using antibodies against pEMI2 and HA (two independent experiments). F) Effect of SB203580 and U0126 on phosphorylation of MNK1. The metaphase-II oocytes (30 cells/lane) were incubated with (+) or without (−) 60 μM SB203580 and 20 μM U0126 for 7 h, and subjected to immunoblot analysis using anti-phosphorylated MNK1 (pMNK1), anti-cyclin B1 (CycB1), and anti-GAPDH antibodies (two independent experiments). G) Mos regulates phosphorylation of MNK1. The GV-stage oocytes were injected with 4 mM MO against Mos mRNA (Mos/MO) or negative control MO (Control/MO), and incubated in the presence of dbcAMP. After 2 h incubation, the oocytes (30 cells/lane) were cultured in the absence of dbcAMP for 16 h, and subjected to immunoblot analysis using anti-pMNK1 and anti-GAPDH antibodies (two independent experiments).

Phosphorylation of MNK1 at positions T197 and T202 by ERK1/2 and p38, respectively, is required for the activity of MNK1 in somatic cells [39, 40]. We examined whether MNK1 phosphorylation in the oocytes depends on ERK1/2 and p38. When the metaphase-II oocytes were treated with both SB203580 and U0126 for 6 h, the level of phosphorylated MNK1 was remarkably decreased (Fig. 5F). The injection of Mos/MO into the GV-stage oocytes also induced downregulation of phosphorylated MNK1 (Fig. 5G). Thus, phosphorylation of MNK1 may be regulated by the Mos-MAPK and p38 MAPK pathways in metaphase-II oocytes.

Discussion

This study describes that p38 plays a potential role in EMI2-mediated metaphase-II arrest in the mouse oocytes. Although Mos activates MEK1/2 during oocyte maturation [41–43], Mos has been postulated to inhibit the activity of an unknown phosphatase [23]. As shown in Figure 1A, the inhibition of Mos synthesis results in the failure of p38 phosphorylation, suggesting that Mos may regulate phosphorylation of p38. Activation of p38 requires phosphorylation at T180 and Y182 in Thr-Gly-Tyr motif by MKK3/6 [44, 45]. It is unlikely that Mos directly phosphorylates p38 in metaphase-II oocytes, because Mos is a Ser/Thr kinase incapable of phosphorylating Y182 in p38. Thus, we assume that phosphorylation of p38 is regulated by a Mos-mediated phosphatase, or that Mos regulates a kinase(s) upstream of p38.

The metaphase-II arrest was released by inhibition of MEK1/2 and p38 (Fig. 2B). Inhibition of each kinase activity was insufficient to exit from metaphase II, implying the presence of a compensatory mechanism. Because the MEK1/2-depleted oocytes develop to metaphase II and maintain metaphase-II arrest [22], the p38 pathway may compensate for phosphorylation of EMI2 in the MEK1/2-depleted oocytes. Approximately 20% of the oocytes treated with U0126 were released from metaphase-II arrest, whereas SB203580 had little effect (Fig. 2B). The reason why the metaphase-II release of oocytes is affected by U0126 alone more effectively than by SB203580 is still unclear at the present time, although these two inhibitors similarly exhibit the effect on the level of cyclin B1 (Fig. 2C). Further experiments are required to clarify the correlation between oocyte activation and cyclin B1 level.

Despite the inhibition of ERK1/2 phosphorylation (Fig. 2Cand Supplemental Fig. S1), only 20% of the metaphase-II oocytes progressed to the anaphase stage in the presence of 20 μM U0126 after incubation for 7 h (Fig. 2B). Our data are apparently consistent with the previous finding that the rates of metaphase-II oocyte activation in the presence of 10 and 50 μM U0126 are approximately 20% and 60% of total oocytes examined 8 h after incubation, respectively, although almost all oocytes treated with 50 μM U0126 are activated after 24 h incubation [46]. Because p38 in the oocytes remains phosphorylated even in the presence of 20 or 50 μM U0126 (Supplemental Fig. S1), phosphorylation of p38 appears unaffected by the concentrations of U0126.

A model experimental system using GV-stage oocytes [34, 35] indicated that the simultaneous treatment with U0126 and SB203580 induces ∼70% reduction in phosphorylation of EMI2 in metaphase I-arrested oocytes (Fig. 4B). Unexpectedly, metaphase-I arrest was maintained in the presence of these two inhibitors, despite the remarkable decrease in EMI2 phosphorylation (Fig. 4C). The maintenance of metaphase-I arrest may be due to the possibility that a relatively low level (∼30%) of phosphorylated EMI2 is enough to interfere with the cell-cycle resumption. Moreover, the metaphase-I-arrested oocytes displayed abnormality in spindle formation and chromosome alignment in the presence of both SB203580 and U0126 (Fig. 4C). These results suggest that in addition to phosphorylation of EMI2, MEK1/2 and p38 may have other unique functions. Mos is indispensable for activation of MISS and DOC1R, which are localized at the metaphase spindle and are involved in spindle formation [47, 48]. Our data reveal that phosphorylated p38 is colocalized with α-tubulin at metaphases I and II (Supplemental Fig. S3). Thus, the p38 pathway as well as the Mos-MAPK pathway may be implicated in spindle formation through activation of MISS and DOC1R.

Phosphorylated MK2 is known to be present during oocyte maturation, and CMPD1 (30 μM) is sufficient to inhibit specific phosphorylation of MK2 by p38 in mouse oocytes [26]. We found that although MK2 phosphorylates EMI2 in vitro (Fig. 5B), EMI2 is phosphorylated even in the presence of 60 μM CMPD1 in the oocytes (Fig. 5, D and E). Thus, EMI2 phosphorylation may be unaffected by MK2 in CSF-arrested oocytes. It should be noted that our data concerning the function of EMI2 phosphorylated via the p38 MAPK-MNK1 pathway (Figs. 4and 5) were obtained by using a model experimental system [34, 35] because of a very low abundance of EMI2 in the metaphase-II oocytes (Fig. 3) and some technical reasons, as described above. Further experiments using metaphase-II oocytes are therefore required for conclusive evidence.

To clarify the role of p38 and ERK1/2 in establishing metaphase-II arrest, we designed siRNAs against the three mRNAs (Supplemental Table S1). The levels of phosphorylated p38, ERK1, and ERK2 were 70% ∼ 85% decreased by introduction of the siRNAs into NIH3T3 cells (Supplemental Fig. S4A). In the mouse oocytes, the levels of these three phosphorylated kinases were unchanged by siRNA injection (Supplemental Fig. S4B). The siRNA knockdown of Mos and EMI2 was enough to interfere with normal maturation of oocytes (Fig. 1and Supplemental Fig. S5). ERK1/2 and p38 are already present in the GV-stage oocytes [27, 49], whereas Mos and EMI2 are synthesized after GVBD and metaphase-I stages, respectively [20, 50]. We thus speculate that proteins already present at the GV stage are not efficiently reduced by siRNA injection into oocytes.

Mos has been demonstrated to trigger the MEK1/2-ERK1/2 cascade [41–43]. Our results suggest that Mos may function in phosphorylation of p38 involved in activation of EMI2 through phosphorylation of MNK1. We have previously reported the functional redundancy of MSK1 and RSKs in EMI2 phosphorylation [34]. Because MNK1 is one of the kinases that participate in phosphorylation of EMI2, the ERK1/2-(MSK1 or RSK1/2/3 or MNK1)-EMI2 and p38-MNK1-EMI2 cascades governed by Mos may mediate metaphase-II arrest in mouse oocytes.

Author notes

¹

Supported in part by grants from Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology in Japan (MEXT) to Y.M., Y.K., and T.B.