Regulation of meiotic metaphase by a cytoplasmic maturation-promoting factor during mouse oocyte maturation

Synthesis and function of Mos: the control switch of vertebrate oocyte meiosis

One distinguishing feature of vertebrate oocyte meiosis is its discontinuity; oocytes are released from their prophase I arrest, usually by hormonal stimulation, only to again halt at metaphase II, where they await fertilization. The product of the c-mos proto-oncogene, Mos, is a key regulator of this maturation process. Mos is a serine-threonine kinase that activates and/or stabilizes maturation-promoting factor (MPF), the master cell cycle switch, through a pathway that involves the mitogen-activated protein kinase (MAPK) cascade. Oocytes arrested at prophase I lack detectable levels of Mos, which must be synthesized from a pool of maternal mRNAs for proper maturation. While Mos is necessary throughout maturation in Xenopus, it seems to be required only for meiosis II in the mouse. The translational activation of c-mos mRNA at specific times during meiosis requires cytoplasmic polyadenylation. Cis- and trans-acting factors for polyadenylation are, therefore, essential elements of maturation.

Regulation of the acquisition of meiotic competence in the mouse: changes in the subcellular localization of cdc2, cyclin B1, cdc25C and wee1, and in the concentration of these proteins and their transcripts

During their development, mammalian oocytes acquire the ability to resume meiosis. We demonstrate that the concentration of p34cdc2 increases during the acquisition of meiotic competence, as determined by immunoblotting, whereas the concentration of cyclin B1 decreases. Laser-scanning confocal microscopy corroborated these changes and furthermore indicate that an increase occurs in the nuclear concentration of each protein. Results of immunoblotting experiments demonstrate that associated with the acquisition of meiotic competence is an increase in the concentration of cdc25C, an activator of p34cdc2/cyclin B kinase, and a decrease in wee1, an inhibitor of cdc2/cyclin B kinase. These changes were again corroborated by laser-scanning confocal microscopy, which also indicates that an increase in the nuclear concentration of wee1 occurs. The concentration of the transcripts encoding these proteins, however, is essentially similar in meiotically incompetent and competent oocytes. Thus, these changes in protein concentration that occur during oocyte development likely reflect changes in the translational efficiency of their mRNAs. Consistent with this is that the relative rate of synthesis of p34cdc2 in meiotically competent oocytes is approximately 3 times greater than that in meiotically incompetent oocytes, whereas the stability of newly synthesized p34cdc2 is essentially the same in each cell type.

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.

Mos/mitogen-activated protein kinase can induce early meiotic phenotypes in the absence of maturation-promoting factor: a novel system for analyzing spindle formation during meiosis I

Mitogen-activated protein kinase (MAPK) is selectively activated by injecting either mos or MAPK kinase (mek) RNA into immature mouse oocytes maintained in the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). IBMX arrests oocyte maturation, but Mos (or MEK) overexpression overrides this block. Under these conditions, meiosis I is significantly prolonged, and MAPK becomes fully activated in the absence of p34cdc2 kinase or maturation-promoting factor. In these oocytes, large openings form in the germinal vesicle adjacent to condensing chromatin, and microtubule arrays, which stain for both MAPK and centrosomal proteins, nucleate from these regions. Maturation-promoting factor activation occurs later, concomitant with germinal vesicle breakdown, the contraction of the microtubule arrays into a precursor of the spindle, and the redistribution of the centrosomal proteins into the newly forming spindle poles. These studies define important new functions for the Mos/MAPK cascade in mouse oocyte maturation and, under these conditions, reveal novel detail of the early stages of oocyte meiosis I.

Morphological differences in nuclear materials released from hamster sperm heads at an early stage of incorporation into immature oocytes, mature oocytes, or fertilized eggs

To elucidate the effects of ooplasmic factors on the early morphological changes in hamster sperm heads within the ooplasm, immature ovarian oocytes at the germinal vesicle stage (GV oocytes), ovulated fully mature oocytes, and fertilized eggs at anaphase II or the pronuclear stage (PN eggs) were examined in detail 15-30 min after insemination or reinsemination. Thin-sectioning studies demonstrated distinct materials released from the sperm nucleus over the entire postacrosomal nuclear surface immediately after disappearance of the sperm nuclear envelope. The release occurred in all of the oocytes and eggs prior to or even in the absence of subsequent chromatin decondensation. Depending upon the stage of the penetrated oocyte or egg, however, the materials varied in morphology: several hemispherical projections of amorphous material within mature oocytes; a number of electron-dense globules within GV oocytes and PN eggs; and both forms within eggs at anaphase II-telophase II. These observations and the fact that only the release of the amorphous material was accompanied by sperm chromatin decondensation indicate that this release was the initial process of chromatin decondensation, whereas the release of the globules resulted from a deficiency or lack of ooplasmic factors affecting the sperm nucleus. Restriction of the release in both forms of material to the late meiotic phase suggests changes in the factors associated with progression of meiosis. To approach an understanding of the mechanism of successful decondensation of sperm chromatin, the ooplasmic factors considered responsible for the stage-dependent release of nuclear materials are discussed.

Inhibition of v-Mos kinase activity by protein kinase A

We investigated the effect of cyclic AMP-dependent protein kinase (PKA ) on v-Mos kinase activity. Increase in PKA activity in vivo brought about either by forskolin treatment or by overexpression of PKA catalytic subunit resulted in a significant inhibition of v-Mos kinase activity. The purified PKA catalytic subunit was able to phosphorylate recombinant p37v-mos in vitro, suggesting that the mechanism of in vivo inhibition of v-Mos kinase involves direct phosphorylation by PKA. Combined tryptic phosphopeptide two-dimensional mapping analysis and in vitro mutagenesis studies indicated that Ser-56 is the major in vivo phosphorylation site on v-Mos. In vivo phosphorylation at Ser-56 correlated with slower migration of the v-Mos protein during sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, even though Ser-56 was phosphorylated by PKA, this phosphorylation was not involved in the inhibition of v-Mos kinase. The alanine-for-serine substitution at residue 56 did not affect the ability of v-Mos to autophosphorylate in vitro or, more importantly, to activate MEK1 in transformed NIH 3T3 cells. We identified Ser-263 phosphorylation, the Ala-263 mutant of v-Mos was not inhibited by forskolin treatment. From our results, we propose that the known inhibitory role of PKA in the initiation of oocyte maturation in mice could be explained at least in part by its inhibition of Mos kinase.

First meiosis of early dictyate nuclei from primordial oocytes in mature and activated mouse oocytes

Nuclei of diplotene (dictyate) primordial oocytes (PO) were transferred to metaphase II oocytes and to activated mouse oocytes using cell fusion techniques. In a metaphase II oocyte, the PO nucleus condenses within 2-3 h to bivalents which become arranged on the first meiotic spindle. After oocyte activation, homologous chromosomes segregate between the oocyte and the first polar body, and a diploid pronucleus-like nucleus reforms from the one set of dyads. This nucleus condenses in the first embryonic mitosis into 40 'somatic' chromosomes which coexist in the common metaphase plate with 20 somatic chromosomes originating from the female pronucleus. Shortening of the time between fusion and activation of about 1 h prevents bivalent differentiation. The PO nucleus condenses only partially and reforms, after oocyte activation, a pronucleus-like nucleus. This nucleus gives rise at the first embryonic mitosis to 20 bivalents which coexist with 20 somatic chromosomes originating from the female pronucleus. A PO nucleus introduced into an activated egg completes the first cell cycle as an intact interphase nucleus. It never condenses in the first embryonic mitosis into bivalents, and undergoes only initial condensation (preceding bivalent differentiation). These results indicate that: (1) condensation into bivalents, meiotic spindle formation and first meiotic division can be greatly accelerated by the introduction of an early diplotene (dictyate) oocyte nucleus into a metaphase II oocyte, and (2) depending on whether the diplotene nucleus enters the first embryonic (mitotic) cell cycle after just initiating or after completing the first meiosis, it gives rise at the first cleavage division to meiotic (bivalents) or 'somatic' chromosomes respectively.

Activation of p34cdc2 kinase around the meiotic resumption in bovine oocytes cultured in vitro

The p34(cdc2) kinase has been identified as a protein factor that is a regulator of meiotic maturation in mammalian oocytes. To investigate the regulatory function of the meiotic resumption in bovine oocytes cultured in vitro, the changes in the phosphorylation states of p34(cdc2) kinase and the histone H1 kinase activity were examined around germinal vesicle breakdown (GVBD). All bovine oocytes just after isolation from their follicles were arrested at the germinal vesicle (GV) stage, and these extracts exhibited two (upper and lower) bands of p34(cdc2) kinase on SDS-PAGE followed by immunoblotting with an antibody against C-terminal peptide of p34(cdc2). When these oocytes were cultured for 24 h in a medium supplemented with 100 microg/ml genistein, tyrosine phosphorylation inhibitor, GVBD was induced in 85% of oocytes, indicating that the upper band of p34(cdc2) kinase in bovine oocytes at the GV stage was already fully phosphorylated tyrosine residue prior to culture. Another (middle) band of p34(cdc2) kinase between the upper and lower bands appeared in the extracts of the oocytes cultured for 4 h, and significant activation of the histone H1 kinase was found in these oocytes (67 +/- 18 fmol/h/oocyte) as compared to that in oocytes cultured for 0 h (46 +/- 11 fmol/h/oocyte). The staining intensity of the middle band and the activity of the histone H1 kinase were further increased after the initiation of GVBD at 6 h of culture, but the quantitative changes of upper and lower bands were not detected throughout the 12 h of culture. Thus, it is concluded that the dephosphorylation of p34(cdc2) kinase followed by activation of the histone H1 kinase after the onset of culture plays a key role in the resumption of meiosis in bovine oocytes.

Association between p34cdc2 levels and meiotic arrest in pig oocytes during early growth

The molecules involved in determining meiotic competence were determined in porcine oocytes isolated from preantral and antral follicles of different sizes. Oocytes isolated from preantral follicles had a mean diameter of 78 microns, contained diffuse filamentous chromatin in the germinal vesicle and were incapable of progressing from the G2 to the M phase of the cycle even after 72 h in culture. Oocytes from early antral follicles had a mean diameter of 105 microns, showed a filamentous chromatin configuration and about half resumed meiosis but arrested at metaphase I (MI) when cultured. Oocytes from mid-antral (3-4 mm) and large antral follicles (5-6 mm) had mean oocyte diameters of 115 and 119 microns respectively, contained condensed chromatin around the nucleolus and progressed to metaphase II (MII) in 48% and 93% of instances respectively. Analysis of p34cdc2, the catalytic subunit of maturation promoting factor (MPF), by immunoblotting indicates that the inability of small (78 microns) oocytes to resume meiosis is due, at least in part, to inadequate levels of the catalytic subunit of MPF. On the other hand, the inability of intermediate-sized (105 microns) oocytes from antral follicles to complete the first meiotic division by progressing beyond MI appears not to be limited by levels of p34cdc2, which are maximal by this stage. We postulate that an inadequacy of molecules other than p34cdc2 limits progression of MI to MII; the acquisition of these molecules during the final stages of growth may be correlated with the formation of the perinucleolar chromatin rim in the germinal vesicle.

UV irradiation of chromosomal DNA and its effect upon MPF and meiosis in mammalian oocytes

Bovine oocytes were irradiated at germinal vesicle (GV) or metaphase II (MII) stage, after Hoechst staining, with chromosomally focused UV-C (254 nm) or UV-A ( > 330 nm). UV-C irradiation at GV stage did not inhibit germinal vesicle breakdown (GVBD) or chromosomal condensation; spindle formation was abolished and maturation promoting factor (MPF) levels failed to increase. UV-A irradiation at GV stage caused meiotic arrest at anaphase I; MPF levels were lower than control. UV-C irradiation at MII stage led to subsequent abnormal parthenogenetic activation when MPF levels failed to decrease. A normal male but no female pronucleus was formed at fertilization. UV-A irradiation at MII stage also caused abnormal activation; MPF levels declined normally. A normal male and abnormal female pronucleus formed at fertilization. UV-A irradiation results have implications for oocyte evaluation during development using Hoechst staining. UV-C irradiation is a potential means for oocyte enucleation in nuclear transfer.

Molecular control of meiosis

The animal cell cycle consists of a round of chromosomal DNA replication in S-phase, followed by segregation of the replicated chromosomes into the daughter nuclei during M-phase. In most animal cells, gap phases termed G(1) and G(2) are introduced between the M- and S-phases, respectively. Meiosis is a particular example of cell division occurring in germ cells. This specialized cell cycle consists of two successive rounds of chromosome segregation that follow a round of DNA replication. Meiosis produces progeny cells with half as many chromosomes as their parents, thus making sexual reproduction possible. This review is concerned with the factors that have been implicated in the control of meiosis, although research in progress may reveal additional regulatory processes.

On the c-mos proto-oncogene product during meiotic maturation in bovine oocytes cultured in vitro

The present study was carried out using dot-blot Western analysis with pp39mos-specific polyclonal antibodies to examine the quantitative and qualitative changes of c-mos proto-oncogene product, Mos, during bovine oocyte maturation in vitro. Mos is present throughout meiotic maturation, is produced from around the onset of meiotic resumption, and is phosphorylated on germinal vesicle breakdown. These results indicate for the first time that the synthesis and phosphorylation of Mos during maturation culture play a key role in the accomplishment of meiosis in bovine oocytes.

Requirement for protein synthesis during the onset of meiosis in bovine oocytes and its involvement in the autocatalytic amplification of maturation-promoting factor

The present study was carried out using the method of electrofusion, or treatment with okadaic acid (OA), to determine whether protein synthesis at the onset of culture was required for the meiotic resumption of bovine follicular oocytes. Germinal vesicle breakdown (GVBD) occurred in bovine oocytes at 6 hr after separation from their follicles in vitro. Following this, immature germinal vesicle (GV) oocytes, preincubated for 0, 2, 4, and 6 hr, were fused to mature oocytes. When immature oocytes, preincubated for 0 hr, were fused to mature oocytes and then cultured for 3 hr in basic medium, GVBD was observed in all fused cells, whereas in the case of cultivation in medium supplemented with the protein synthesis inhibitor (25 micrograms/ml cycloheximide; CX), 39% of the fused cells possessed an intact GV within their cytoplasm. In immature oocytes preincubated for 4 or 6 hr, however, this proportion was significantly reduced to 7% and 4%, respectively, without protein synthesis after fusion. In addition, the CX-dependent block of GVBD could be overcome in only 13% of bovine follicular oocytes by the addition of 2 microM OA, although 51% of oocytes which synthesized the protein during the first 6 hr of culture induced GVBD in subsequent culture with CX plus OA. Thus, we conclude that the initiation of GVBD in bovine oocytes requires protein synthesized at the onset of meiosis, which is related to the autocatalytic amplification of the maturation-promoting factor.

Inhibition of protein synthesis affects histone H1 kinase, but not chromosome condensation activity, during the first meiotic division of pig oocytes

The influence of protein synthesis on the regulation of the first meiotic division was studied in pig oocytes. We show that histone H1 kinase activity gradually increases during in vitro culture of pig oocytes, reaching maximum in metaphase I stage after 24 hr of culture. However, in the presence of the protein synthesis inhibitor cycloheximide, histone H1 kinase is not activated during the whole culture period, and after 24 hr it is approximately at the same level as in prophase-stage oocytes. The gradual increase in phosphorylation of six proteins of molecular weights 39, 48, 53, 66, 96, and 120 kDa, observed during the first 24 hr of culture, was not detected when cycloheximide was added to the culture medium. Similarly, the decrease in phosphorylation of a 90-kDa protein was not seen in cycloheximide-treated oocytes. On the other hand, the levels of both MPF components, p34cdc2 and cyclin B, which were found to be nearly constant during the first meiotic division, were not influenced by cycloheximide treatment as revealed by Western blotting. The process of germinal vesicle breakdown (GVBD) was totally blocked by cycloheximide. The condensation of chromatin, however, was not influenced, suggesting that GVBD and chromosome condensation could be regulated independently. The different degrees of MPF activation involved in these processes, as well as the nature of the protein(s) which must be synthesized for triggering GVBD, are discussed.

Analysis of the mechanism(s) of metaphase I arrest in maturing mouse oocytes

Fully grown mouse oocytes are normally competent to progress from prophase I to metaphase II without interruption. However, growing mouse oocytes initially become only partially competent to undergo meiotic maturation. Meiotic maturation in these oocytes does not progress beyond metaphase I. In contrast to the oocytes of most strains of mice, most oocytes of strain LT/Sv mice become arrested at metaphase I even when they are fully grown. The initiation of oocyte maturation is correlated with an increase in p34cdc2 kinase activity that continues to rise until metaphase I. The transition into anaphase I is normally correlated with a decrease in p34cdc2 kinase activity. This study demonstrated that metaphase I arrest in both partially competent growing oocytes and fully grown LT/Sv oocytes is correlated with a sustained elevation of p34cdc2 kinase activity. In fact, p34cdc2 activity continued to increase during the time when activity normally decreased. In normally maturing oocytes, some, but not all, of the cyclin B, the regulatory protein associated with p34cdc2, became degraded in oocytes that entered anaphase I. In contrast, the amount of cyclin B present in the metaphase I-arrested oocytes continued to increase at the time when it was being degraded in normal oocytes progressing to metaphase II. These results suggest that the progression of meiosis is arrested at metaphase I in both groups of oocytes because of continued p34cdc2 kinase activity sustained, at least in part, by restricted degradation of cyclin B.(ABSTRACT TRUNCATED AT 250 WORDS)

Nuclei from fertilized mouse embryos have calcium-releasing activity

During mammalian fertilization, the sperm triggers a series of intracellular Ca2+ oscillations which initiate oocyte activation and the formation of pronuclei. Oocyte activation can be induced artificially by a variety of chemical and physical stimuli which elevate intracellular calcium. We show that the transfer of nuclei from 1- and 2-cell-stage fertilized mouse embryos to unfertilized oocytes stimulates the completion of meiosis and the formation of pronuclei. Nuclei from embryos that had developed to the 4-cell stage did not stimulate meiotic resumption. The ability to cause oocyte activation was specific to nuclei transferred from fertilized embryos as nuclei from parthenogenetic embryos or cytoplasts from fertilized or parthenogenetic embryos did not induce activation. Nucleus-induced oocyte activation was associated with the generation of intracellular Ca2+ transients, which were seen after nuclear envelope breakdown of the transferred nuclei. Treatment of the oocyte with the intracellular Ca2+ chelator, BAPTA, prior to nuclear transfer inhibited intracellular Ca2+ transients and oocyte activation. The specific Ca(2+)-releasing activity of the nucleus was not caused by sperm-induced protein synthesis since similar activity was present in nuclei originating from embryos exposed to cycloheximide throughout fertilization. The specific ability of nuclei from fertilized embryos to stimulate Ca2+ transients and oocyte activation was also found in nuclei from embryos parthenogenetically activated by the injection of a partially purified cytosolic sperm factor. The results suggest that the fertilizing sperm introduces Ca(2+)-releasing activity which becomes associated with the nucleus of early mammalian embryos.

Time-dependent effects of cycloheximide and α-amanitin on meiotic resumption and progression in bovine follicular oocytes

To identify the stage during maturation at which new protein and RNA are synthesized for meiotic resumption, follicular oocytes were cultured in TCM-199 with the protein synthesis inhibitor cycloheximide or the hnRNA synthesis inhibitor alpha-amanitin. Although the meiotic resumption of cumulus-enclosed oocytes was completely blocked by the addition of 25 microg/ml cycloheximide at 4 h after the onset of culture, 23% of oocytes cultured from 5 h post cultivation in the medium with cycloheximide underwent germinal vesicle breakdown (GVBD). By further delaying the addition of cycloheximide, the proportion of oocytes which underwent GVBD increased. Addition of the inhibitor at 8 h or more post cultivation resulted in GVBD occurring in more than 87% of oocytes, though none of them were able to proceed beyond the metaphase I stage. In contrast, the addition of 50 microg/ml alpha-amanitin from the onset of culture significantly reduced the proportion of GVBD to 75% in cumulus-enclosed oocytes, while no significant reduction in the proportions of GVBD was noted in the case of its addition from 1 h of culture onward. However, denuded oocytes were almost insensitive to any treatments with alpha-amanitin. These results indicate that protein synthesis in the oocytes and RNA synthesis in the cumulus cells soon after the onset of culture are necessary for GVBD and that continuous protein synthesis following GVBD is indispensable for progression of the meiotic division in bovine oocytes.

Synthesis and accumulation of p34cdc2 and cyclin B in mouse oocytes during acquisition of competence to resume meiosis

This study tests the hypothesis 033 that growing murine oocytes, which are incompetent to resume meiosis, are deficient in their content of p34cdc2 and/or cyclin B, the two subunits of maturation promoting factor (MPF). Accumulation of the two MPF components occurred in an asynchronous manner in growing oocytes. Cyclin B content reached maximal levels in oocytes that were not yet competent to undergo germinal vesicle breakdown (GVB), the first obvious morphological manifestation of the resumption of meiosis. Thus, the amount of cyclin B is not the limiting factor rendering these growing oocytes incompetent to undergo GVB. In contrast, synthesis and accumulation of p34cdc2 increased during the period of oocyte growth in vivo when they became competent to undergo GVB. A similar increase in the amount of p34cdc2 also occurred in cultured granulosa cell-free oocytes despite the lack of oocyte growth, but these cultured oocytes did not become GVB competent. Thus, the accumulation of p34cdc2 is probably necessary, but not sufficient, for mouse oocytes to become competent to undergo GVB. This accumulation occurs autonomously in oocytes independently of growth or of the participation of follicular somatic cells.

Role of tyrosine kinase in the regulation of myogenin expression

Using an affinity-purified anti-myogenin antibody, three stages of mouse myoblast C2C12 cells during myogenesis could be identified: proliferating myoblasts as myogenin-negative mononucleated cells, differentiating myoblasts as myogenin-positive mononucleated cells, and myotubes as myogenin-positive multinucleated cells. We found differential effects of genistein, an inhibitor of protein-tyrosine kinase, on myogenic cells during these three stages. Genistein severely inhibited myotube formation and myogenin production in differentiating myoblasts by inhibiting the transcription of the myogenin gene in a dose-dependent manner. We also found that genistein inactivated mitogen-activated protein kinase (MAP kinase) accompanied by suppression of myogenin expression. In contrast, genistein failed to inactivate MAP kinase and eliminate myogenin from myotubes. The results suggest that protein-tyrosine kinase plays a role in the transcriptional regulation of myogenin through the MAP kinase cascade during myogenesis. Furthermore, genistein inhibited the transactivation of the myosin heavy chain gene by constitutively expressed myogenin. Therefore, it is suggested that protein-tyrosine kinase is involved in the post-translational regulation of myogenin as well as in transcriptional regulation during myogenesis.

Translational regulation of the gradual increase in histone H1 kinase activity in maturing mouse oocytes

In maturing mouse oocytes, p34cdc2-associated histone H1 kinase activity gradually increases until it reaches its maximum at metaphase I (Choi et al., 1991: Development 113:789-795). In this study, treatment of oocytes with cycloheximide resulted in a failure to increase the level of histone H1 activity above that detected at approximately the time of germinal vesicle breakdown (GVB), which is approximately 20-30% of the level normally achieved at metaphase I. Cyclin B was detected in GV-stage oocytes, but there was a 2-2.5-fold increase in the amount of cyclin B in maturing oocytes from GV-stage to metaphase I and a burst of cyclin B synthesis during the first 3 hr of maturation. Okadaic acid-treatment of mouse oocytes did not accelerate activation of histone H1 kinase but rather arrested its activity at the same level observed in cycloheximide-treated oocytes. Thus the components of the p34cdc2 kinase activating system in mouse oocytes are apparently not present in GV-stage oocytes in an amount or configuration that would allow maximum kinase activation when meiosis is reinitiated by okadaic acid. Importantly, okadaic acid-treatment dramatically inhibited protein synthesis. Therefore, the inhibition of protein synthesis by okadaic acid probably abrogates the possibility of de novo synthesis of the regulators of p34cdc2 kinase required to drive its activity to the maximum level normally achieved by metaphase I. It is concluded that there is a critical point in driving the continued activation of histone H1 kinase that occurs at approximately the time of GVB. Progression beyond this point requires de novo protein synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Histone H1 kenase activity during in vitro fertilization of pig follicular oocytes matured in vitro

The present study was conducted to clarify the relationship between histone H1 kinase (H1K) activity and events associated with in vitro fertilization of pig follicular oocytes matured in vitro. Histone H1 kinase has been shown to be homologous with a maturation promoting factor (MPF). Cumulus-oocyte complexes obtained from prepubertal gilts were cultured for 46 h in a modified Waymouth's MB752/1 medium and were then inseminated in vitro with frozen-thawed and preincubated epididymal boar spermatozoa. At 4, 6, 8 and 10 h post insemination, the oocytes were stained with 10 microg/ml Hoechst-33342 and examined under a fluorescent microscope for the stage of fertilization, according to morphological changes of oocyte nuclear chromatin and the extent of sperm penetration. Sperm penetration was observed to occur within 4 h post insemination (20.5%), and the percentage of fertilized oocytes increased (P < 0.01) to 72.9% at 8 h post insemination. Pronuclear formation was observed from 6 h post insemination (3.3%) and the percentage increased (P < 0.01) to 46.8% at 10 h post insemination. In each examination period, H1K activities in unfertilized oocytes at metaphase-II remained unchanged (112.0 fmol/h/oocyte) and were higher (P < 0.01) than those in fertilized oocytes (30.1 fmol/h/oocyte). The H1K activity in fertilized oocytes such as oocytes emitting a second polar body, oocytes with an enlarging sperm head(s) and oocytes with multiple pronuclei did not differ significantly. These results suggest that MPF in pig oocytes is inactivated shortly after sperm penetration and is maintained at the basal level throughout pronuclear formation.

Unusual cytoskeletal and chromatin configurations in mouse oocytes that are atypical in meiotic progression

Meiotic maturation progresses atypically in oocytes of strain LT/Sv and I/LnJ mice. LT/Sv occytes show a high frequency of metaphase I-arrest and parthenogenetic activation. I/LnJ oocytes display retarded kinetics of meiotic maturation and a high frequency of metaphase I-arrest. Some I/LnJ oocytes fail to resume meiosis. Changes in the configuration of chromatin, microtubules, and centrosomes are associated with specific stages of meiotic progression. In this study, the configuration of these subcellular components was examined in LT/Sv, I/LnJ, and C57BL/6J (control) oocytes either freshly isolated from large antral follicles or after culture for 15 hr to allow progression of spontaneous meiotic maturation. Differences were found in the organization of chromatin, microtubules, and centrosomes in LT/Sv and I/LnJ oocytes compared to control oocytes. For example, rather than exhibiting multiple cytoplasmic and nuclear centrosomes as in the normal germinal vesicle-stage oocytes, LT/Sv oocytes typically contain a single large centrosome. In contrast, I/LnJ oocytes displayed many small centrosomes. The microtubules of normal germinal vesicle-stage oocytes were organized as arrays or asters, but microtubules were shorter in LT/Sv oocytes and absent from I/LnJ oocytes. After a 15-hr culture, centrosomal material of normal metaphase II oocytes was organized at both spindle poles. In contrast, metaphase I-arrested LT/Sv oocytes exhibited an elongated spindle with centrosomal material appearing more organized at one pole of the spindle. Both control and LT/Sv oocytes displayed cytoplasmic centrosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium and meiotic maturation of the mammalian oocyte

The role of calcium in the regulation of both the meiotic and mitotic cell cycles has been the subject of considerable investigation in the nonmammalian field. In contrast, the mechanisms for signalling meiotic maturation in the mammalian oocyte are not as well documented nor as clearly defined. In the mammalian oocyte, calcium is associated with both spontaneous and hormone-induced meiotic maturation. A transient release of endogenously stored calcium precedes germinal vesicle breakdown and can override cyclic AMP maintained meiotic arrest; it thus may signal the resumption of meiosis. Additionally, extracellular calcium is apparently required for meiotic progression past metaphase I. The time sequence for meiotic resumption and progression is very varied between species. The timing of cell cycle protein synthesis during meiosis suggests that cyclins may be expressed in oocytes of some species much earlier in their development than in others. A generic model is proposed for the mechanism for triggering meiotic resumption in the mammalian oocyte. In this model, the critical components of meiotic resumption involve the temporal relationship of cyclin synthesis and the subsequent activation of the MPF complex by the calcium signal generated, which accounts for differences among species.

Meiosis-specific cell cycle regulation in maturing Xenopus oocytes

Meiotic cell cycles differ from mitotic cell cycles in that the former lack S-phase in the interphase between meiosis I and meiosis II. To obtain clues for mechanisms involved in the cell cycle regulation unique to meiosis, we have examined changes in chromosomal morphology and H1 kinase activity during a meiotic period from metaphase I (MI) to metaphase II (MII) in Xenopus oocytes. Using populations of oocytes that underwent germinal vesicle breakdown (GVBD) within a 10 minute interval, we found that the kinase activity declined gradually during the 60 minute period after GVBD and then increased steadily during the following 80 minute interval, showing remarkable differences from the rapid drop and biphasic increase of the kinase activity in intermitotic periods (Solomon et al. (1990) Cell 63, 1013–1024; Dasso and Newport (1990) Cell 61, 811–823). We also found that the exit from MI lagged, by more than 30 minutes, behind the time of lowest H1 kinase activity, whereas the two events took place concomitantly at the end of meiosis II and mitosis. Consequently, the H1 kinase activity was already increasing during the first meiotic division. When H1 kinase activation at MII was delayed by a transient inhibition of protein synthesis after GVBD, oocytes were able to support formation of interphase nuclei and DNA replication between the first meiotic division and the MII arrest, indicating that the cell cycle entered S-phase between meiosis I and meiosis II. These results strongly suggest that the machinery required for entering S-phase has been established in maturing oocytes by the end of meiosis I.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of a proline-directed kinase motif is responsible for structural changes in myogenin

Myogenin, a member of the MyoD family which governs skeletal muscle differentiation, was identified as a pair of phosphorylated bands on SDS-PAGE during myogenesis. The slow migrating form was found to be hyperphosphorylated myogenin. In vitro phosphorylation by CDC2 kinase caused a prominent reduction in electrophoretic mobility of myogenin. Furthermore, we demonstrated that phosphorylation of the serine residue at position 43 contributes to the modification of myogenin in vivo and in vitro resulting in the reduction in electrophoretic mobility. We propose here that a CDC2-like proline-directed kinase regulates myogenin activity through its phosphorylation.

Parthenogenetic activation of oocytes in c-mos-deficient mice

In Xenopus the c-mos proto-oncogene product (Mos) is essential for the initiation of oocyte maturation, for the progression from meiosis I to meiosis II and for the second meiotic metaphase arrest, acting as an essential component of the cytostatic factor CSF. Its function in mouse oocytes is unclear, however, as is the biological significance of c-mos mRNA expression in testes and several somatic tissues. We have generated c-mos-deficient mice by gene targeting in embryonic stem cells. These mice grew at the same rate as their wild-type counterparts and reproduction was normal in the males, but the fertility of the females was very low. The c-mos-deficient female mice developed ovarian teratomas at a high frequency. Oocytes from these females matured to the second meiotic metaphase both in vivo and in vitro, but were activated without fertilization. The results indicate that in mice Mos plays a role in the second meiotic metaphase arrest, but does not seem to be essential for the initiation of oocyte maturation, spermatogenesis or somatic cell cycle.

Morphological and functional changes accompanying the acquisition of meiotic competence in ovarian goat oocyte

In order to determine at which follicular size goat oocytes were capable of resuming and completing meiosis, we evaluated their ability to mature in vitro and measured their maturation promoting factor (MPF) activity by histone H1 kinase assay. The results indicated that goat oocyte meiotic competence developed progressively in follicles ranging from 0.5 to 3 mm; the oocytes acquired the ability to resume meiosis in follicles of 0.5-0.8 mm, to reach metaphase I (MI) in follicles of 1-1.8 mm, and to reach metaphase II (MII) in follicles larger than 3 mm. The presence of MPF activity was first observed in oocytes arrested at early prometaphase I and reached a maximum level in oocytes blocked in metaphase I (MI). In the second part of this study, RNA synthesis and nucleolar changes were analyzed during the growth period. The acquisition of meiotic competence was accompanied by nucleolar compaction and a dramatic decrease in RNA synthesis. Changes in protein patterns were also analyzed, but only slight differences were observed among oocytes from the different classes.

Visualization and cytogenetic analysis of second polar body chromosomes following its fusion with a one-cell mouse embryo

PURPOSE

This study was designed to visualize the second polar body (2PB) chromosomes using its electrofusion with a one-cell-stage mouse embryo to approach preconception diagnosis of chromosomal disorders.

RESULTS

Eighty to 90% hybridization efficiency has been achieved by electrofusion of 2PB with mouse zygotes. 2PB chromosomes were visualized in 40-50% of hybrids. Sixty-five percent of 2PB chromosomes were visualized when fused with the cytoplast obtained microsurgically by removing pronuclei from a one-cell embryo. As much as 33-43% of these resulting metaphases appeared to contain chromosomal aberrations. The follow-up of the development of the reconstructed one cell-stage hybrids in vitro revealed a significant decrease in their viability. The hybrid embryos resulting from 2PB electrofusion with enucleated zygotes did not develop beyond the two-cell stage.

CONCLUSION

Electrofusion is an efficient approach for hybridization of 2PB with a one-cell mouse embryo and may be useful for visualization and cytogenetic analysis of 2PB chromosomes. The visualization rate of 2PB chromosomes is higher if 2PB is fused with enucleated zygotes. However, the method induces over 30% of chromosomal aberrations and may lead to a significant decrease in the viability of the resulting one-cell embryos.

A nuclear factor required for specific translation of cyclin B may control the timing of first meiotic cleavage in starfish oocytes

In most animals, the rate of cyclin B synthesis increases after nuclear envelope breakdown during the first meiotic cell cycle. We have found that cyclin B-cdc2 kinase activity drops earlier in emetine-treated than in control starfish oocytes, although the protein synthesis inhibitor does not activate the cyclin degradation pathway prematurely. Moreover, protein synthesis is required to prevent meiotic cleavage to occur prematurely, sometimes before chromosomes have segregated on the metaphase plate. In normal conditions, increased synthesis of cyclin B after germinal vesicle breakdown (GVBD) balances cyclin degradation and increases the time required for cyclin B-cdc2 kinase to drop below the level that inhibits cleavage. Taken together, these results point to cyclin B as a possible candidate that could explain the need for increased protein synthesis during meiosis I. Although direct experimental evidence was not provided in the present work, cyclin B synthesis after GVBD may be important for correct segregation of homologous chromosomes at the end of first meiotic metaphase, as shown by a variety of cytological disorders that accompany premature cleavage. Although the overall stimulation of protein synthesis because of cdc2 kinase activation is still observed in oocytes from which the germinal vesicle has been removed before hormonal stimulation, the main increase of cyclin B synthesis normally observed after germinal vesicle breakdown is suppressed. The nuclear factor required for specific translation of cyclin B after GVBD is not cyclin B mRNA, as shown by using a highly sensitive reverse transcription followed by polymerase chain reaction procedure that failed to detect any cyclin B mRNA in isolated germinal vesicles.

Acquisition of maturational competence in in vitro grown mouse oocytes

The maturational competence of the in vitro grown mouse oocytes of various sizes was examined, and was compared to that of oocytes of equivalent chronological age which grew in vivo. Oocyte-granulosa cell complexes isolated from 10-day-old female mice were cultured for 10 days in vitro in the presence of 2 mM hypoxanthine to maintain the meiotic arrest. When the in vitro grown oocytes were denuded and cultured for further 24 h in the hypoxanthine-free medium, 9, 37, and 100% of oocytes of 60-64.5, 65-69.5, and over 70 microns in diameter underwent germinal vesicle breakdown (GVBD), respectively. Of those oocytes which underwent GVBD, 19, 37 and 68% of oocytes larger than 60, 65, and 70 microns progressed to metaphase II, respectively. No oocyte less than 60 microns resumed meiosis. The parallelism found in the oocyte size and maturational competence was similar to that of their in vivo counterparts collected from 20-day-old mice. A significant proportion of the in vitro grown oocytes, however, formed a pronucleus-like structure(s) after maturation culture, indicating that they could not maintain a normal arrest at metaphase II.

Competent mouse oocytes isolated from antral follicles exhibit different chromatin organization and follow different maturation dynamics

After labelling DNA with the specific vital fluorophore Hoechst 33342, oocytes, isolated by puncture from antral follicles in adult mice, have two essentially different configurations of their nuclear fluorescence images. These have been called SN (where the nucleolus is surrounded by chromatin) and NSN (where the nucleolus is not surrounded by chromatin). Intermediate configurations are also found, although with a lower frequency. The proportion of each class is on the average equal and depends neither on the presence of cumulus cells nor on the age of the mouse. Electron microscopy confirms several ultrastructural differences between these two nuclear configurations, namely, the structure of the nucleolus, which is vacuolated in NSN-type and compact in SN-type oocytes. Using video-enhanced fluorescence microscopy at low level of excitation light, we could follow directly in vitro the meiotic maturation of both classes, without impairing their viability. We show that in germinal vessicle (GV) state, the chromatin does not change from one configuration into the other and that both classes are able to mature to metaphase II, although the maturation has slightly different characteristics.

Isolation of the murine cyclin B2 cDNA and characterization of the lineage and temporal specificity of expression of the B1 and B2 cyclins during oogenesis, spermatogenesis and early embryogenesis

A cDNA encoding the murine cyclin B2 (cycB2) was isolated from an adult mouse testis cDNA library as part of studies designed to identify cyclins involved in murine germ cell development. This cycB2 cDNA was then used to examine the pattern of cycB2 expression during male and female germ cell development and in early embryogenesis, and to compare this expression with the previously characterized expression of cycB1. A single 1.7 kb cycB2 transcript was detected by northern blot hybridization analysis of total RNA isolated from midgestation embryos and various adult tissues. Northern blot and in situ hybridization analyses revealed that cycB2 expression in the testis was most abundant in the germ cells, specifically in pachytene spermatocytes. This is in contrast to the highest levels of expression of cycB1 being present in early spermatids. In situ analysis of the ovary revealed cycB2 transcripts in both germ cells and somatic cells, specifically in the oocytes and granulosa cells of growing and mature follicles. The pattern of cycB1 and cycB2 expression in ovulated and fertilized eggs was also examined. While the steady state level of cycB1 and cycB2 signal remained constant in oocytes and ovulated eggs, signal of both appeared to decrease following fertilization. In addition, both cycB1 and cycB2 transcripts were detected in the cells of the inner cell mass and the trophectoderm of the blastocyst. These results demonstrate that there are lineage- and developmental-specific differences in the pattern of the B cyclins in mammalian germ cells, in contrast to the co-expression of B cyclins in the early conceptus.

Okadaic acid induces premature chromosome condensation reflecting the cell cycle progression in one-cell stage mouse embryos

Haploid parthenogenetic embryos as well as fertilized mouse eggs were treated in vitro with 1-10 microM okadaic acid (OA) at the one-cell stage. Cytogenetic analysis detected that OA induces nuclear envelope breakdown (NEBD) and premature condensation of interphase chromosomes in pronuclei as well as in 2nd polar body (PB) nuclei. G1-, S-, and G2-type prematurely condensed chromosomes (PCC) were found in pronuclei of embryos of different age, which reflects their progression through the first cell cycle. In nuclei from 2nd PBs only G1- and S-type PCC were observed. Using the types of PCC as a criterion of different phases of the cell cycle, it was possible to estimate that in haploid parthenogenetic embryos G1-phase lasts until 5.5 hr post activation (hpa), S-phase takes from 4.5 to 9.5 hpa, and from 8.5 hpa G2-phase had started. Second PBs were found to be in G1-phase until 6.5 hpa and S-phase started in some as early as 5.5 hpa, but in most not before 7.5 hpa. Treatment with OA visualizes G1-chromosomes in pronuclei as well as in 2nd PBs, and it is easy to count the number of these chromosomes and recognize a T6 marker chromosome. The possibility to apply cytogenetic analysis of G1-chromosomes from 2nd PBs for a more accurate detection of maternal meiotic nondisjunction is discussed.

Protein synthesis requirements during resumption of meiosis in the hamster oocyte: early nuclear and microtubule configurations

The organization of chromatin and cytoplasmic microtubules changes abruptly at M-phase entry in both mitotic and meiotic cell cycles. To determine whether the early nuclear and cytoplasmic events associated with meiotic resumption are dependent on protein synthesis, cumulus-enclosed hamster oocytes were cultured in the presence of 100 micrograms/ml puromycin or cycloheximide for 5 hr. Both control (untreated) and treated oocytes were analyzed by fluorescence microscopy after staining with Hoechst 33258 and tubulin antibodies. Freshly isolated oocytes exhibit prominent nucleoli and diffuse chromatin within the germinal vesicle as well as an interphase network of cytoplasmic microtubules. After 4-4.5 hr in culture, most oocytes were in prometaphase I of meiosis as characterized by a prominent spindle with fully condensed chromosomes and numerous cytoplasmic asters. After 5-5.5 hr in culture, microtubule asters are no longer detected in most cells, and the spindle is the only tubulin-positive structure. Incubation for 5 hr in the presence of inhibitors does not impair germinal vesicle breakdown, chromatin condensation, kinetochore microtubule assembly, or cytoplasmic aster formation in the majority of oocytes examined; however, under these conditions, a population of oocytes retains a germinal vesicle, exhibiting variable degrees of chromatin condensation and cytoplasmic aster formation. Meiotic spindle formation is inhibited in all oocytes. These effects are fully reversible upon culture of treated oocytes in drug-free medium for 5 hr. The data indicate that meiotic spindle assembly is dependent on ongoing protein synthesis in the cumulus-enclosed hamster oocyte; in contrast, chromatin condensation and aster formation are not as sensitive to protein synthesis inhibitors during meiotic resumption.

Okadaic acid and p13suc1 modulate the reinitiation of meiosis in mouse oocytes

Short-term exposure to okadaic acid (OA), a specific inhibitor of protein phosphatases 1 and 2A, induced resumption of meiosis, including metaphase spindle formation, in mouse oocytes treated with a phosphodiesterase inhibitor, while long incubations with OA arrested oocyte maturation at a step prior to spindle formation. To explore the basis for this difference, the overall patterns of protein synthesis and phosphorylation and the production of tissue-type plasminogen activator (tPA), the synthesis of which is induced after germinal vesicle breakdown (GVBD), were analyzed under various OA treatments. Short-term exposure to OA led to tPA production and did not greatly affect the maturation-associated changes in protein phosphorylation. By contrast, a long application of OA did not result in tPA production and induced more marked changes in protein phosphorylation. Microinjection into prophase oocytes of the product of the fission yeast gene p13suc1, known to inhibit p34cdc2 kinase activation and/or activity, prevented meiotic reinitiation. This effect was overcome by microinjection of OA, at concentrations higher than those required for induction of maturation in the absence of p13suc1. These observations suggest that inhibition of phosphatase 1 or 2A or both triggers meiotic resumption by acting at the same site or at a site proximal to the p13suc1-sensitive step of cdc2 kinase activation.

Requirement of mosXe protein kinase for meiotic maturation of Xenopus oocytes induced by a cdc2 mutant lacking regulatory phosphorylation sites

The p34cdc2 protein kinase is a component of maturation-promoting factor, the master regulator of the cell cycle in all eukaryotes. The activity of p34cdc2 is itself tightly regulated by phosphorylation and dephosphorylation. Predicted regulatory phosphorylation sites of Xenopus p34cdc2 were mutated in vitro, and in vitro-transcribed RNAs were injected into Xenopus oocytes. The cdc2 single mutants Thr-14----Ala and Tyr-15----Phe did not induce germinal vesicle breakdown (BVBD) upon microinjection into oocytes. In contrast, the cdc2 double mutant Ala-14/Phe-15 did induce GVBD. Both the Ala-14 and Ala-14/Phe-15p34cdc2 mutants were shown to coimmunoprecipitate cyclin B1 and to phosphorylate histone H1 in immune complex kinase assays. Microinjection of antisense oligonucleotides to c-mosXe was used to demonstrate the role of mos protein synthesis in the induction of GVBD by the Ala-14/Phe-15 cdc2 mutant. Thr-161 was also mutated. p34cdc2 single mutants Ala-161 and Glu-161 and triple mutants Ala-14/Phe-15/Ala-161 and Ala-14/Phe-15/Glu-161 failed to induce GVBD in oocytes and showed a decreased binding to cyclin B1 in coimmunoprecipitations. Each of the cdc2 mutants was also assayed by coinjection with cyclin B1 or c-mosXe RNA into oocytes. Several of the cdc2 mutants were found to affect the kinetics of cyclin B1 and/or mos-induced GVBD upon coinjection, although none affected the rate of progesterone-induced maturation. We demonstrate here the significance of Thr-14, Tyr-15, and Thr-161 of p34cdc2 in Xenopus oocyte maturation. In addition, these results suggest a regulatory role for mosXe in induction of oocyte maturation by the cdc2 mutant Ala-14/Phe-15.

The fall of biological maturation promoting factor (MPF) and histone H1 kinase activity during anaphase and telophase in mouse oocytes

Cell fusions have been used to determine the biological activity of the MPF complex in murine oocytes during their progression through anaphase and telophase to metaphase II. Oocytes (1) at metaphase I, (2) during the anaphase-telophase transition, or (3) at metaphase II were fused to germinal vesicle-staged (immature) oocytes. The hybrids were cultured for 1 h in the presence of db cAMP before fixation and nuclear evaluation. Metaphase I oocytes invariably induced germinal vesicle breakdown (GVBD) in the immature partner. By contrast, anaphase/telophase oocytes never induced GVBD in immature oocytes. The capacity to induce GVBD reappears after the formation of the second metaphase plate. In a second study, histone H1 kinase activity was measured during mouse oocyte maturation in single oocytes. H1 kinase activity was low in GV oocytes, increased sharply at MI, declined during anaphase and telophase and increased again at MII. After egg activation, H1 kinase activity was reduced to basal levels. These results provide direct evidence that a drop in activity of MPF in murine oocytes occurs concomitantly with the exit from metaphase I; MPF activity remains low until the cell re-enters metaphase.

Requirement of mosXe protein kinase for meiotic maturation of Xenopus oocytes induced by a cdc2 mutant lacking regulatory phosphorylation sites

The p34cdc2 protein kinase is a component of maturation-promoting factor, the master regulator of the cell cycle in all eukaryotes. The activity of p34cdc2 is itself tightly regulated by phosphorylation and dephosphorylation. Predicted regulatory phosphorylation sites of Xenopus p34cdc2 were mutated in vitro, and in vitro-transcribed RNAs were injected into Xenopus oocytes. The cdc2 single mutants Thr-14----Ala and Tyr-15----Phe did not induce germinal vesicle breakdown (BVBD) upon microinjection into oocytes. In contrast, the cdc2 double mutant Ala-14/Phe-15 did induce GVBD. Both the Ala-14 and Ala-14/Phe-15p34cdc2 mutants were shown to coimmunoprecipitate cyclin B1 and to phosphorylate histone H1 in immune complex kinase assays. Microinjection of antisense oligonucleotides to c-mosXe was used to demonstrate the role of mos protein synthesis in the induction of GVBD by the Ala-14/Phe-15 cdc2 mutant. Thr-161 was also mutated. p34cdc2 single mutants Ala-161 and Glu-161 and triple mutants Ala-14/Phe-15/Ala-161 and Ala-14/Phe-15/Glu-161 failed to induce GVBD in oocytes and showed a decreased binding to cyclin B1 in coimmunoprecipitations. Each of the cdc2 mutants was also assayed by coinjection with cyclin B1 or c-mosXe RNA into oocytes. Several of the cdc2 mutants were found to affect the kinetics of cyclin B1 and/or mos-induced GVBD upon coinjection, although none affected the rate of progesterone-induced maturation. We demonstrate here the significance of Thr-14, Tyr-15, and Thr-161 of p34cdc2 in Xenopus oocyte maturation. In addition, these results suggest a regulatory role for mosXe in induction of oocyte maturation by the cdc2 mutant Ala-14/Phe-15.

Changes in protein synthesis pattern during in vitro maturation of goat oocytes

The regulation of meiotic events of goat oocytes from prophase I to metaphase II was studied by inhibiting protein synthesis at different times of the transition and by analyzing the changes in the protein synthesis pattern during maturation. Protein synthesis was required for germinal vesicle breakdown (GVBD). Nevertheless, the concomitant event to the rupture of germinal vesicle, i.e., chromosome condensation, took place even in a cycloheximide-containing medium. The transition from metaphase I to metaphase II was also protein synthesis dependent as evidenced by experiments using this protein synthesis inhibitor. The inhibition was partly reversible, i.e., after removal of the drug, oocytes were able to progress until metaphase I but could not proceed beyond this stage. Changes in the protein synthesis pattern were studied by radiolabelling of oocytes with [35S]methionine. These changes were correlated with the nuclear status of the oocyte: At GVBD, a polypeptide of 25 kD disappeared, while one of 27 kD appeared. At the same time, a polypeptide of 33 kD appeared, whereas concomitantly one of 34 kD became barely detectable and finally disappeared as the maturation progressed. During maturation, the synthesis of a 67 kD polypeptide increased and became predominant at the end of the maturation process. The synthesis of actin decreased after 18 hr of culture from a very high to a low level of synthesis.

M-phase-specific histone H1 kinase in fish oocytes. Purification, components and biochemical properties

We demonstrate, for the first time in fish, that a Ca(2+)-independent and cyclic-nucleotide-independent histone H1 kinase activity oscillates according to the cell cycle of the oocyte, peaking at the first and the second meiotic metaphase with a transient drop between them. The kinase, M-phase-specific histone H1 kinase (M-H1K), was purified from mature carp oocytes by using two exogenous substrates for assaying its activity: histone H1 and a synthetic peptide (SP peptide, KKAAKSPKKAKK) containing the sequence KSPKK, which includes the consensus sequence of the site phosphorylated by a serine/threonine-specific protein kinase encoded by the fission yeast cdc2+ gene (cdc 2 kinase). The M-H1K and maturation-promoting factor (MPF) activities coincided closely throughout four steps of purification, strongly suggesting the identity of M-H1K and MPF. The final preparation was purified 5000-fold with a recovery of 4%, when histone H1 was used for the kinase assay, and 10,000-fold with a recovery of 7% when SP peptide was used. The purified molecular mass of the kinase was estimated to be 100 kDa by gel filtration and contained four proteins of 33, 34, 46 and 48 kDa. Anti-PSTAIR antibody recognizing cdc2 kinase cross-reacted with the 33-kDa and 34-kDa proteins, while the 46-kDa and 48-kDa bands cross-reacted with monoclonal antibodies raised against cyclin B. The 33-kDa protein was also recognized by an antibody against a goldfish cdk2 (Eg1) kinase, a cdc2-related kinase which has the PSTAIR sequence and binds to p13suc1 but does not form a complex with cyclin B. M-H1K activity corresponded well to the 34-kDa, 46-kDa and 48-kDa proteins but not to the 33-kDa protein. These results strongly suggest that M-H1K consists of cdc2 kinase forming a complex with cyclin B, and that cdk2 kinase is not a component of M-H1K, although it is found in the highly purified M-H1K. The purified M-H1K utilized Mg2+, Mn2+, ATP and GTP, and had a wide pH optimum ranging over 8.0-10.5. The kinase was thermolabile and sensitive to freezing/thawing.

Regulation of meiotic maturation in the mammalian oocyte: interplay between exogenous cues and the microtubule cytoskeleton

Mammalian oocytes exhibit a series of cell cycle transitions that coordinate the penultimate events of meiosis with the onset of embryogenesis at fertilization. The execution of these cell cycle transitions, at G2/M of meiosis-I and metaphase/anaphase of meiosis I and II, involve both biosynthetic and post-translational modifications that directly modulate centrosome and microtubule behavior. Specifically, somatic cells alter the signal transduction pathways in the oocyte and influence the expression of maturation promoting factor (MPF) and cytostatic factor (CSF) activity through a microtubule-dependent mechanism. The regulation of the oocytes' cell cycle machinery by hormone-mediated somatic cell signals, involving both positive and negative stimuli, ensures that meiotic cell cycle progression is synchronized with the earliest pivotal events of mammalian reproduction.

Oscillation of MPF is accompanied by periodic association between cdc25 and cdc2-cyclin B

Activation of maturation-promoting factor at the onset of mitosis requires the tyrosine dephosphorylation of one of its components, the cdc2 protein kinase. cdc25 is the specific tyrosine phosphatase that activates cdc2. We find that Xenopus oocytes contain a relative of cdc25, p72. In Xenopus embryos the abundance of p72 does not oscillate during the cell cycle. However, p72 directly associates with cdc2-cyclin B in a cell cycle-dependent manner, reaching a peak at M phase. The M phase kinase that associates with p72 is catalytically active. These results suggest that the mechanism by which cdc25 triggers cdc2 activation involves a periodic physical association between cdc25 and the cyclin B-cdc2 complex and also that mitotic control can be affected by mechanisms other than transcriptional regulation of the cdc25 gene.