Activation of mitogen-activated protein kinase during meiotic maturation in porcine oocytes

Dynamic events are differently mediated by microfilaments, microtubules, and mitogen-activated protein kinase during porcine oocyte maturation and fertilization in vitro

The role of microfilaments, microtubules, and mitogen-activated protein (MAP) kinase in regulation of several important dynamic events of porcine oocyte maturation and fertilization is described. Fluorescently labeled microfilaments, microtubules, and cortical granules were visualized using either epifluorescence microscopy or laser scanning confocal microscopy. Mitogen-activated protein kinase phosphorylation was revealed by Western immunoblotting. We showed that 1) microfilament disruption did not affect meiosis resumption and metaphase I meiotic apparatus formation but inhibited further cell cycle progression (chromosome separation) even though MAP kinase was phosphorylated; 2) cortical granule (CG) migration was driven by microfilaments (but not microtubules), and once the chromosomes and CGs were localized beneath the oolemma their anchorage to the cortex was independent of either microfilaments or microtubules; 3) neither microfilaments nor microtubules were involved in CG exocytosis during oocyte activation; 4) sperm incorporation was mediated by microfilaments, while pronuclear (PN) syngamy was controlled by microtubules rather than microfilaments; 5) spindle microtubule organization was temporally correlated with MAP kinase phosphorylation, while the extensive microtubule organization in the sperm aster that is required for PN apposition and syngamy occurred in the absence of MAP kinase activation; and 6) MAP kinase phosphorylation did not change either when microtubules were disrupted by nocodazole or when cytoplasmic microtubule asters were induced by taxol. The present study suggests that the role of the cytoskeleton during porcine oocyte maturation is similar to that of rodents, while the mechanisms of fertilization in pig resemble those of lower vertebrates.

Protein kinase C and meiotic regulation in isolated mouse oocytes

In this study, the possible role of protein kinase C (PKC) in mediating both positive and negative actions on meiotic maturation in isolated mouse oocytes has been examined. When cumulus cell-enclosed oocytes (CEO) were cultured for 17-18 hr in a medium containing 4 mM hypoxanthine (HX) to maintain meiotic arrest, each of the five different activators and five different antagonists of PKC stimulated germinal vesicle breakdown (GVB) in a dose-dependent fashion. One of the activators, phorbol-12-myristate 13-acetate (PMA), also triggered GVB in CEO arrested with isobutylmethylxanthine or guanosine, but not in those arrested with dibutyryl cyclic AMP. When denuded oocytes (DO) were cultured for 3hr in inhibitor-free medium, all PKC activators suppressed maturation (<10% GVB compared to 94% in controls), while the effect of PKC antagonists was negligible. Four of the five antagonists reversed the meiosis-arresting action of HX in DO. PMA transiently arrested the spontaneous maturation of both CEO and DO, with greater potency in DO. The stimulatory action of PMA in HX-arrested oocytes was dependent on cumulus cells, because meiotic induction occurred in CEO but not DO. PKC activators also preferentially stimulated cumulus expansion when compared to antagonists. A cell-cell coupling assay determined that the action of PMA on oocyte maturation was not due to a loss of metabolic coupling between the oocyte and cumulus oophorus. Finally, Western analysis demonstrated the presence of PKCs alpha, beta1, delta, and eta in both cumulus cells and oocytes, but only PKC epsilon was detected in the cumulus cells. It is concluded that direct activation of PKC in the oocyte suppresses maturation, while stimulation within cumulus cells generates a positive trigger that leads to meiotic resumption.

Function of the Mos/MAPK pathway during oocyte maturation in the Japanese brown frog Rana japonica

Fully grown immature oocytes acquire the ability to be fertilized with sperm after meiotic maturation, which is finally accomplished by the formation and activation of the maturation-promoting factor (MPF). MPF is the complex of Cdc2 and cyclin B, and its function in promoting metaphase is common among species. The Mos/mitogen-activated protein kinase (MAPK) pathway is also commonly activated during vertebrate oocyte maturation, but its function seems to be different among species. We investigated the function of the Mos/MAPK pathway during oocyte maturation of the frog Rana japonica. Although MAPK was activated in accordance with MPF activation during oocyte maturation, MPF activation and germinal vesicle breakdown (GVBD) was not initiated when the Mos/MAPK pathway was activated in immature oocytes by the injection of c-mos mRNA. Inhibition of Mos synthesis by c-mos antisense RNA and inactivation of MAPK by CL100 phosphatase did not prevent progesterone-induced MPF activation and GVBD. However, continuous MAPK activation and MAPK inhibition through oocyte maturation accelerated and delayed MPF activation, respectively. Furthermore, Mos induced a low level of cyclin B protein synthesis in immature oocytes without the aid of MAPK. These results suggest that the general function of the Mos/MAPK pathway, which is not essential for MPF activation and GVBD in Rana oocytes, is to enhance cyclin B translation by Mos itself and to stabilize cyclin B protein by MAPK during oocyte maturation.

Molecular mechanisms of the initiation of oocyte maturation: general and species-specific aspects

Stimulated by maturation-inducing hormone secreted from follicle cells surrounding the oocytes, fully-grown oocytes mature and become fertilisable. During maturation, immature oocytes resume meiosis arrested at the first prophase and proceed to the first or second metaphase at which they are naturally inseminated. Paying special attention to general and species-specific aspects, we summarise the mechanisms regulating the initial phase of oocyte maturation, from the reception of hormonal signals on the oocyte surface to activation of the maturation-promoting factor in the cytoplasm, in amphibians, fishes, mammals and marine invertebrates.

Butyrolactone I reversibly inhibits meiotic maturation of bovine oocytes,Without influencing chromosome condensation activity

In this study, butyrolactone I (BL I), a potent and specific inhibitor of cyclin-dependent kinases, was shown to block germinal vesicle (GV) breakdown (GVBD) in bovine oocytes in a concentration-dependent manner; GVBD was almost totally inhibited over the course of 24-48 h of culture when 100 microM BL I was included in tissue culture medium 199 containing either polyvinyl alcohol or BSA. Correlated with this inhibition was the failure of either p34(cdc2) kinase or mitogen-activated protein (MAP) kinase to become activated, and it was unlikely that BL I directly inhibited MAP kinase, since 100 microM BL I did not inhibit MAP kinase activity present in extracts obtained from metaphase II-arrested bovine eggs that possess high levels of MAP kinase activity. Nevertheless, the formation of highly condensed bivalents was observed in 78% of the BL I-treated GV-intact oocytes. This result suggests that chromosome condensation during first meiosis in bovine oocytes does not require the activity of either p34(cdc2) kinase or MAP kinase. Treatment of BL I-arrested oocytes with okadaic acid (OA) did not result in either the activation of p34(cdc2) kinase or MAP kinase, or inducement of GVBD. The BL I-induced block of GVBD for 24 h was reversible, and a subsequent 24-h culture resulted in 90% of oocytes reaching metaphase II with emission of the first polar body. Correlated with the progression to and arrest at metaphase II was the full activation of both p34(cdc2) and MAP kinases. The reversibility after 48 h of culture in BL I was partially decreased when compared to that achieved after an initial 24-h culture. Fertilization in vitro of these eggs resulted in a high incidence of both sperm penetration and pronucleus formation (88% and 70%, respectively).

Male pronuclear formation in denuded porcine oocytes after in vitro maturation in the presence of cysteamine

The present study was conducted to examine effects of cysteamine in culture medium on progression of meiosis, glutathione (GSH) content, kinase activities (histone H1 kinase and mitogen-activated protein kinase), and male pronuclear formation after in vitro insemination of cumulus-denuded oocytes (DOs) in the pig. DOs, obtained by mechanically removing cells from cumulus-oocyte complexes (COCs) with a small-bore pipette, were cultured for 45 h in TCM199 supplemented with sodium pyruvate, gonadotropins, estradiol, and 10% porcine follicular fluid, with or without cysteamine (150 microM). Maturation rates of DOs cultured with and without cysteamine were not different (60-70%) but were significantly lower than those of COCs (90-100%) (p < 0.05). GSH content of matured DOs cultured with cysteamine was significantly higher than that of DOs cultured without cysteamine (p < 0.05). Values for both types of kinase activity in matured DOs cultured with and without cysteamine were not different (p > 0.05). After in vitro insemination, DOs cultured with cysteamine showed significantly higher rates of male pronuclear formation (80.3 +/- 3.0%) than DOs cultured without cysteamine (16.4 +/- 0.5%) (p < 0.05). These results indicate that the addition of cysteamine to culture medium increased oocyte GSH content and promoted male pronuclear formation after sperm penetration of porcine DOs but had no effects on their maturation rates or kinase activities.

MAP kinase cascade, but not ERKs, activated during early cleavage of mouse embryos

Mitosis in early embryos is independent of exogenous mitogens, although mitogen stimulations and subsequent activation of a mitogen-activated protein (MAP) kinase cascade are essential for the proliferation of somatic cells. The activation state of the MAP kinase cascade during early cleavage has never been reported. In the present study, factors involved in the MAP kinase cascade--Ras, Raf-1, 14-3-3, MEK, and ERKs--and their activation states were detected by immunoblotting during early cleavage of mouse embryos. We found the constant presence of these molecules in mouse early embryos and the activation of Raf-1 exclusively at the M-phase. An immunoprecipitation study revealed that active Raf-1 in the M-phase was dissociated from 14-3-3, as in somatic cells, whereas inactive Raf-1 was associated with 14-3-3. Surprisingly, the ERKs (MAP kinases) were not activated throughout early cleavage, although M-phase-specific activation of the MAP kinase kinase, MEK was observed. Myelin basic protein kinase activity was, however, significantly higher in the M-phase than in the interphase. These results indicate that the MAP kinase cascade is activated at the M-phase and that some MAP kinases other than ERKs are activated during early cleavage of mouse embryos.

Morphology of porcine cumulus-oocyte-complexes depends on the stage of preovulatory maturation

The aim of this investigation was to determine the relationship between the morphology of the cumulus-oocyte-complexes (COCs) and the meiotic configuration of oocytes as an LH peak mimicked by hCG. Estrus was synchronized in a total of 29 crossbred Landrace gilts by feeding Regumate for 15 d and administering 1000 IU PMSG. The LH peak was simulated by treatment with 500 IU hCG at 80 h after PMSG. Endoscopic oocyte recovery was carried out 2 h before and 10, 22 and 34 h after hCG. Only macroscopically healthy follicles with a diameter of more than 5 mm were punctured. Altogether, 410 follicles from 57 ovaries were punctured and 251 COCs were aspirated. Oocyte recovery rate increased from 48.5% (P < 0.01) of the early, not yet preovulatory follicles (2 h before hCG) to 80.8% of late preovulatory follicles (34 h after hCG). Cumulus morphology in COCs recovered 2 h before and 10 h after hCG was heterogeneous, with most (72.9 to 57.4%; P < 0.01) showing a compact or slightly expanded cumulus. Starting at about 22 h after hCG, COC morphology changed dramatically (86.7% of COCs with expanded cumulus; P < 0.01), and 34 h after hCG, 98.3% of the COCs had only an expanded cumulus. The percentage of oocytes with a mature meiotic configuration increased (11.2; 7.1; 41.4 and 70.2%, respectively, n = 238 oocytes; P < 0.01) as the interval post hCG increased (-2, 10, 22, 34 h, respectively). Meiotic configuration was related to COC morphology: compact COCs--88.9% diplotene, expanded COCs--53.8% metaphase II (M-II), and denuded oocytes--69.2% degenerated chromatin. These results indicate that there is a relationship between oocyte recovery rate, COC morphology, and meiotic configuration and preovulatory follicle maturation after the application of hCG.

Effect of follicular cells, IGF-I and tyrosine kinase blockers on oocyte maturation

The aim of this study was to test the following hypotheses: (i) that oocyte maturation is controlled by surrounding follicular cells; (ii) that a meiosis-regulating factor of follicular origin is not species-specific; (iii) that one of the follicular regulators of oocyte maturation is IGF-I; and, (iv) that Cumulus oophorus and tyrosine kinase-dependent intracellular mechanisms do not mediate IGF-I action on oocytes. It was found that co-culture of cumulus-enclosed bovine oocytes with isolated bovine ovarian follicles or with isolated porcine ovarian follicles significantly increased the proportion of matured oocytes (at metaphase II of meiosis) after culture. Porcine oocytes without cumulus investments had lower maturation rates than cumulus-enclosed oocytes. Co-culture with isolated porcine ovarian follicles resulted in stimulation of maturation of both cumulus-free and cumulus-enclosed porcine oocytes. These observations suggest that follicular cells (whole follicles or Cumulus oophorus) support bovine and porcine oocyte maturation, and that follicular maturation-promoting factor is not species-specific. The release of significant amounts of IGF-I by cultured bovine and porcine isolated follicles and granulosa cells was demonstrated. Addition of IGF-I to culture medium at 10 or 100 (but not 1000) ng/ml stimulated meiotic maturation of both cumulus-enclosed and cumulus-free porcine oocytes. Neither of the tyrosine kinase blockers, genistein or lavendustin (100 ng/ml medium), changed the stimulating effect of IGF-I on porcine oocytes. The present data suggest that at least one of the follicular stimulators of oocyte nuclear maturation is IGF-I, and that its effect is probably not mediated by cumulus investment or by tyrosine kinase-dependent intracellular mechanisms.

Interplay between CDC2 kinase and MAP kinase pathway during maturation of mammalian oocytes

Two principal kinases, p34cdc2 kinase and MAP kinase play a pivotal role in maturation of mammalian oocytes. In the porcine and bovine oocytes both kinases are activated around the time of germinal vesicle breakdown (GVBD). Butyrolactone I (BL I), a specific inhibitor of cdk kinases, prevents effectively and reversibly resumption of meiosis in the porcine and bovine oocytes. Neither p34cdc2 kinase nor MAP kinase are activated in oocytes inhibited in the GV stage. The bovine oocytes maintained for 48 h in the medium supplemented with BL I, progress subsequently to metaphase II in 91%, their cumuli expand optimally and after in vitro fertilization they possess two pronuclei. When the cdc2 kinase is blocked in the porcine oocytes by BL I, MAP kinase, activated by okadaic acid treatment, is able to substitute cdc2 kinase and induce GVBD. The histone H1 kinase activity sharply decreases in the metaphase II oocytes treated by BL I and one or two female pronuclei are formed. These data indicate that BL I is a useful tool either for the two step in vitro culture of mammalian oocytes or for their activation in nuclear transfer experiments.

Mitogen-activated protein kinase activity during goat oocyte maturation and the acquisition of meiotic competence

Changes in MPF and MAPK activities during meiotic maturation of goat oocytes were investigated. Detection of MPF activity occurred concomitantly with GVBD, increased at MI, decreased during anaphase-telophase I transition, and increased thereafter in MII oocytes. The appearance of MAPK activity was delayed compared to MPF activity. MAPK activity increased after GVBD and persisted during the MI-MII transition. Whether MAPK was implicated in goat oocyte meiotic competence was also investigated by using oocytes from different follicle size categories that arrest at specific stages of the maturation process (GV, GVBD, MI, and MII). Results indicate that the ability of goat oocytes to resume meiosis is not directly related to the presence of Erk2. The ability to phosphorylate MAPK is acquired by the oocyte during follicular growth after the ability to resume meiosis. GVBD-arrested oocytes exhibited a high level of MPF activity after 27 hr of culture. However, 28% of oocytes from this group contained inactive MAPK, and 72% exhibited high MAPK activity. In addition, 29% of GVBD-arrested oocytes contained a residual interphasic network without recruitment of microtubules around the condensed chromosomes; 71% of GVBD-arrested oocytes displayed recruitment of microtubules near the condensed chromosomes and contained asters of microtubules distributed throughout the cytoplasm. These results indicate that oocytes arrested at GVBD were not exactly at the same point in the meiotic cell cycle progression, and suggest that MAPK could be implicated in the regulation of microtubule organization. The data presented here suggest that in goat oocytes, MAPK is not implicated in the early events of meiosis resumption, but rather in post-GVBD events such as spindle formation and MII arrest.

Co-culture with pig membrana granulosa cells modulates the activity of cdc2 and MAP kinase in maturing cattle oocytes

Bovine cumulus-enclosed oocytes, initially cultured up to diakinesis (8 h of initial culture) or metaphase I (12 h of initial culture), were subsequently co-cultured for 6 h in contact with pig membrana granulosa (PMG) cells and then assayed for histone H1 and MAP kinase activities. In addition, the phosphorylation state of ERK 1,2 proteins was determined by Western blotting. The alterations in nuclear envelope breakdown, meiotic spindle formation and the patterns of chromosome condensation were analysed by immunofluorescence and transmission electron microscopy. The diakinesis-stage oocytes (initially cultured for 8 h) already possessed high histone H1 kinase and MAP kinase activities that were correlated with condensed and partially individualised chromosomes. The ERK 1 and most ERK 2 proteins were partly phosphorylated. Following the 6 h co-culture of these oocytes with PMG a rapid decrease in MAP kinase activity and a slower decrease in histone H1 kinase occurred, as well as ERK 1 and ERK 2 dephosphorylation. Both kinase activities and ERK 1,2 phosphorylation were fully restored following the release of the oocytes from co-culture and a subsequent culture in the absence of PMG. Moreover, the clumped bivalents were reindividualised and 56% of these oocytes reached metaphase II after 20 h of culture without PMG. The metaphase I oocytes, initially cultured for 12 h, displayed a fusiform meiotic spindle and a metaphase array of chromosomal bivalents, accompanied by high levels of both histone H1 and MAP kinase activity. Co-culture of MI oocytes with PMG abolished the activity of both kinases and caused the dephosphorylation of ERK 1 and ERK 2. Furthermore, the spindle microtubules were depolymerised and the chromosomal bivalents clumped into a single mass. Neither of the protein kinase activities nor the meiotic spindle were restored following subsequent culture in the absence of PMG for up to 20 h. These observations indicate that under in vitro conditions membrana granulosa cells can cause a prompt decrease in histone H1 and MAP kinase activities, and metaphase I oocytes. While these events are fully reversible in late diakinesis oocytes, metaphase I oocytes did not complete maturation after release from co-culture.

Mitogen-activated protein kinase activity and microtubule organisation are altered by protein synthesis inhibition in maturing porcine oocytes

Previously we have shown that mitogen-activated protein (MAP) kinase activity abruptly increases at the first metaphase (M1) and remains significantly higher than that at the germinal vesicle (GV) stage until the second metaphase (M2) in porcine oocytes cultured in vitro. The present paper describes how the mechanism of the blockage of meiotic maturation by protein synthesis inhibition involves MAP kinase regulation. Cycloheximide arrested both germinal vesicle breakdown (GVBD) and the normal transition from M1 to M2. MAP kinase activation was also reduced in these maturation-inhibited oocytes. By using immunofluorescence microscopy with the monoclonal antibody raised against rat alpha-tubulin, we showed that cycloheximide caused morphological abnormality in a spindle at M1, but not at M2. All these results indicate that in porcine oocytes: (1) GV blockage by protein synthesis inhibition involves the suppression of both histone H1 kinase and MAP kinase activation, (2) during the transition from M1 to M2, maintenance of a normal metaphasic spindle and high MAP kinase activity require protein synthesis, and (3) once the M2 cytoskeletal structures have been completed, and/or after the 'critical period', cytostatic factor activity is independent of protein synthesis.

Localization and function of tyrosine-phosphorylated protein in pig oocytes

Several proteins with phosphorylated tyrosine residues have been shown to be closely involved in the control meiotic nuclear division. We identified a 42-kD protein in pig oocytes, using a polyclonal antibody to a synthetic phosphotyrosine construct that increases significantly in amount after 12 hr of maturation culture, and is discretely localized to condensing and condensed chromosomes. However, since microinjection of the antibody into oocytes blocks spindle formation, the role of this protein appears to be at that stage rather than directly in chromosome condensation. Specificity of action of the 42-kD protein indicates that it may be a phosphorylation-dependent component necessary for successful spindle assembly.