Caffeine promotes premature chromosome condensation formation and in vitro development in porcine reconstructed embryos via a high level of maturation promoting factor activity during nuclear transfer

When the nucleus in G0/G1 phase is transferred to an enucleated oocyte by nuclear transfer (NT), its nuclear envelope is broken, followed by condensation of chromosome structure by maturation promoting factor (MPF). This morphological remodeling of the transferred interphase nucleus seems to be essential for subsequent development of NT embryos. In this study, we treated porcine NT embryos with caffeine, which has been reported to increase MPF activity, to keep their MPF level high during NT. When 2.5 mM caffeine was added to the handling medium, the proportion of NT embryos showing condensed chromosome increased significantly (P < 0.05). In NT embryos treated with caffeine, the activity of p34cdc2 kinase was significantly (P < 0.05) higher than in those without caffeine at 3 h post-injection. In addition, the rate of development to the blastocyst stage after activation was significantly (P < 0.05) higher in NT embryos treated with caffeine. These results indicate that caffeine treatment can increase not only the rate of chromosome condensation but also the developmental rate to the blastocyst stage of porcine NT embryos. This action is most likely due to the support/increase of MPF activity throughout the process of NT.

Protein synthesis is required for the transition to Ca(2+)-dependent regulated secretion in progesterone-matured Xenopus oocytes

Calcium (Ca) ionophores trigger cortical granule exocytosis in progesterone-matured Xenopus oocytes (eggs), but not in immature oocytes. Prior work suggested that this secretory transition involved a Ca-dependent isoform of protein kinase C (PKC). To address this possibility, we treated eggs with several different inhibitors of Ca-dependent PKCs. Although these agents (eg., staurosporine, Ro31-8220) completely blocked cortical granule exocytosis that is triggered in eggs by phorbol esters, they had no impact on ionomycin-evoked secretion of cortical granule lectin. These data suggest that Ca-dependent PKCs do not mediate secretory triggering in eggs. Instead, further investigation revealed that protein synthesis (but not RNA synthesis) was required for eggs to secrete in response to ionomycin. Moreover, we observed that when oocytes were matured by injection of maturation promoting factor (MPF), they failed to secrete in response to ionomycin. Collectively, these results suggest that the progesterone-dependent maturation pathway induces these cells either to synthesize de novo, a protein that mediates Ca-dependent secretory triggering, or that intrinsic Ca-sensing machinery is modified in a protein-synthesis-dependent fashion. Initial efforts to distinguish between these possibilities (using Ca overlay, pharmacological and immunoblot strategies) revealed that such Ca-binding proteins as calmodulin, synaptotagmin1, CAPS, rabphilin-3A and calcineurin were unlikely to transduce the secretory effects of ionomycin in eggs. Thus, the cortical reaction in these cells may rely on a novel mechanism for initiating Ca-dependent exocytosis.

Involvement of calcium/calmodulin-dependent protein kinase II (CaMKII) in meiotic maturation and activation of pig oocytes

Calcium signal is important for the regulation of meiotic cell cycle in oocytes, but its downstream mechanism is not well known. The functional roles of calcium/calmodulin-dependent protein kinase II (CaMKII) in meiotic maturation and activation of pig oocytes were studied by drug treatment, Western blot analysis, kinase activity assay, indirect immunostaining, and confocal microscopy. The results indicated that meiotic resumption of both cumulus-enclosed and denuded oocytes was prevented by CaMKII inhibitor KN-93, Ant-AIP-II, or CaM antagonist W7 in a dose-dependent manner, but only germinal vesicle breakdown (GVBD) of denuded oocytes was inhibited by membrane permeable Ca2+ chelator BAPTA-AM. When the oocytes were treated with KN-93, W7, or BAPTA-AM after GVBD, the first polar body emission was inhibited. A quick elevation of CaMKII activity was detected after electrical activation of mature pig oocytes, which could be prevented by the pretreatment of CaMKII inhibitors. Treatment of oocytes with KN-93 or W7 resulted in the inhibition of pronuclear formation. The possible regulation of CaMKII on maturation promoting factor (MPF), mitogen-activated protein kinase (MAPK), and ribosome S6 protein kinase (p90rsk) during meiotic cell cycles of pig oocytes was also studied. KN-93 and W7 prevented the accumulation of cyclin B and the full phosphorylation of MAPK and p90rsk during meiotic maturation. When CaMKII activity was inhibited during parthenogenetic activation, cyclin B, the regulatory subunit of MPF, failed to be degraded, but MAPK and p90rsk were quickly dephosphorylated and degraded. Confocal microscopy revealed that CaM and CaMKII were localized to the nucleus and the periphery of the GV stage oocytes. Both proteins were concentrated to the condensed chromosomes after GVBD. In oocytes at the meiotic metaphase MI or MII stage, CaM distributed on the whole spindle, but CaMKII was localized only on the spindle poles. After transition into anaphase, both proteins were translocated to the area between separating chromosomes. All these results suggest that CaMKII is a multifunctional regulator of meiotic cell cycle and spindle assembly and that it may exert its effect via regulation of MPF and MAPK/p90rsk activity during the meiotic maturation and activation of pig oocytes.

Endocrine control of diurnal oocyte maturation in the kyusen wrasse, Halichoeres poecilopterus

The present study examined diurnal cycles of oocyte development and maturation in the kyusen wrasse, Halichoeres poecilopterus, and investigated the sensitivity of oocytes to maturation-inducing hormone (MIH) and gonadotropic hormone (GTH). Female fish were sampled at fixed intervals throughout the day, revealing that final oocyte maturation and ovulation were completed by 6:00 hr, and that spawning occurred daily between 6:00 and 9:00 hr. In vitro experiments showed that the steroids 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) and 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S) were equally potent and highly effective inducers of germinal vesicle breakdown (GVBD) in kyusen wrasse oocytes. Additionally, circulating levels of 17,20beta-P and 20beta-S increased around the time of GVBD and ovulation, suggesting that 17,20beta-P and 20beta-S act as MIHs in the kyusen wrasse. Moreover, in vitro experiments clearly showed that kyusen wrasse oocytes had a daily developmental cycle of GTH and MIH sensitivity, and that oocytes that completed vitellogenesis acquired GTH-induced maturational competence. An endogenous GTH surge likely occurs between 12:00 and 15:00 hr, and this daily pre-maturational GTH surge probably controls the diurnal maturation cycles of kyusen wrasse oocytes.

Dependence of DNA synthesis and in vitro development of bovine nuclear transfer embryos on the stage of the cell cycle of donor cells and recipient cytoplasts

The effect of the stage of the cell cycle of donor cells and recipient cytoplasts on the timing of DNA replication and the developmental ability in vitro of bovine nuclear transfer embryos was examined. Embryos were reconstructed by fusing somatic cells with unactivated recipient cytoplasts or with recipient cytoplasts that were activated 2 h before fusion. Regardless of whether recipient cytoplasts were unactivated or activated, the embryos that were reconstructed from donor cells at the G0 phase initiated DNA synthesis at 6-9 h postfusion (hpf). The timing of DNA synthesis was similar to that of parthenogenetic embryos, and was earlier than that of the G0 cells in cell culture condition. Most embryos that were reconstructed from donor cells at the G1/S phase initiated DNA synthesis within 6 hpf. The developmental rate of embryos reconstructed by a combination of G1/S cells and activated cytoplasts was higher than the rates of embryos in the other combination of donor cells and recipient cytoplasts. The results suggest that the initial DNA synthesis of nuclear transfer embryos is affected by the state of the recipient oocytes, and that the timing of initiation of the DNA synthesis depends on the donor cell cycle. Our results also suggest that the cell cycles of somatic cells synchronized in the G1/S phase and activated cytoplasts of recipient oocytes are well coordinated after nuclear transfer, resulting in high developmental rates of nuclear transfer embryos to the blastocyst stage in vitro.

Parthenogenetic activation of rhesus monkey oocytes and reconstructed embryos

This study determines the efficiency of sequential calcium treatments (electroporation or ionomycin) combined with protein synthesis (cycloheximide) or phosphorylation inhibitors (6-dimethylaminopurine) or the specific maturation promoting factor (MPF) inhibitor, roscovitine, in inducing artificial activation and development of rhesus macaque parthenotes or nuclear transfer embryos. Exposure of oocytes arrested at metaphase II (MII) to ionomycin followed by 6-dimethylaminopurine or to electroporation followed by cycloheximide and cytochalasin B induced pronuclear formation and development to the blastocyst stage at a rate similar to control embryos produced by intracytoplasmic sperm injection. Parthenotes did not complete meiosis or extrude a second polar body, consistent with their presumed diploid status. In contrast, oocytes treated sequentially with ionomycin and roscovitine extruded the second polar body and formed a pronucleus at a rate higher than that observed in controls. Following reconstruction by nuclear transfer, activation with ionomycin/6-dimethylaminopurine resulted in embryos that contained a single pronucleus and no polar bodies. All nuclear transfer embryos activated with ionomycin/roscovitine contained one large pronucleus. However, a third of these embryos emitted one or two polar bodies, clearly containing chromatin material. In summary, we have identified simple yet effective methods of oocyte or cytoplast activation in the monkey, ionomycin/6-dimethylaminopurine, electroporation/cycloheximide/cytochalasin B, and ionomycin/roscovitine, which are applicable to parthenote or nuclear transfer embryo production.

Rescue of mouse embryos from 2-cell blocks by microinjection of maturation promoting factor

OBJECTIVE

To examine the rescue of mouse embryos from 2-cell blocks by the microinjection of maturation promoting factor (MPF) extracted from matured Xenopus eggs into one of the blastomeres of 2-cell stage mouse embryos.

DESIGN

Controlled laboratory study.

SETTING

First Department of Obstetrics and Gynecology, Toho University School of Medicine, Tokyo, Japan.

ANIMAL(S)

Eight- to 10-week-old female Crj:CD-1(ICR) mice.

INTERVENTION(S)

One of the blastomeres of the mouse 2-cell embryos was injected with MPF (MI group) or mHTF medium (MED group) at 28--32 hours after insemination.

MAIN OUTCOME MEASURE(S)

The developmental rate to blastocyst.

RESULT(S)

The developmental rate to blastocyst in the MI group (48.0%) was significantly higher than that in the MED group (0%).

CONCLUSION(S)

The 2-cell block was specifically rescued by the microinjection of MPF and not by the insertion of pipettes.

Study of the in vitro maturation of mouse oocytes induced by microinjection of maturation promoting factor (MPF)

PURPOSE

Maturation promoting factor (MPF) acts at the resumption of meiosis and nonspecifically throughout the animal species. There exists a considerable body of literature on MPF, but little work has been done to study the induction of maturation of mammalian oocytes by microinjection of extracted MPF.

METHODS

Immature (GV-stage) mouse oocytes were microinjected MPF extracted from matured Xenopus eggs in the presence of dbcAMP.

RESULTS

The rate of germinal vesicle, breakdown (GVBD) induced at 24 hr after MPF injection was significantly higher (90.5%) than that of the control (2.2%), which was injected with HTF medium containing dbcAMP (P < 0.0001). The rate of extrusion of the first polar body at 24 hr after MPF injection was significantly higher (84.1%) than that of the same control (1.1%) (P < 0.0001).

CONCLUSIONS

From these results, it is concluded that the maturation of mammalian oocytes can be induced by the microinjection of MPF extracted from other species.

A p90(rsk) mutant constitutively interacting with MAP kinase uncouples MAP kinase from p34(cdc2)/cyclin B activation in Xenopus oocytes

The efficient activation of p90(rsk) by MAP kinase requires their interaction through a docking site located at the C-terminal end of p90(rsk). The MAP kinase p42(mpk1) can associate with p90(rsk) in G(2)-arrested but not in mature Xenopus oocytes. In contrast, an N-terminally truncated p90(rsk) mutant named D2 constitutively interacts with p42(mpk1). In this report we show that expression of D2 inhibits Xenopus oocyte maturation. The inhibition requires the p42(mpk1) docking site. D2 expression uncouples the activation of p42(mpk1) and p34(cdc2)/cyclin B in response to progesterone but does not prevent signaling through p90(rsk). Instead, D2 interferes with a p42(mpk1)-triggered pathway, which regulates the phosphorylation and activation of Plx1, a potential activator of the Cdc25 phosphatase. This new pathway that links the activation of p42(mpk1) and Plx1 during oocyte maturation is independent of p34(cdc2)/cyclin B activity but requires protein synthesis. Using D2, we also provide evidence that the sustained activation of p42(mpk1) can trigger nuclear migration in oocytes. Our results indicate that D2 is a useful tool to study MAP kinase function(s) during oocyte maturation. Truncated substrates such as D2, which constitutively interact with MAP kinases, may also be helpful to study signal transduction by MAP kinases in other cellular processes.

Translational recruitment of Xenopus maternal mRNAs in response to poly(A) elongation requires initiation factor eIF4G-1

Xenopus oocytes accumulate maternal mRNAs which are then recruited to ribosomes during meiotic cell cycle progression in response to progesterone and coincident with poly(A) elongation. Prior to stimulation, most protein synthesis ( approximately 70%) does not require intact translation factor eIF4G (B. D. Keiper and R. E. Rhoads, 1997, Nucleic Acids Res. 25, 395-402). In the present study we have addressed the requirement of eIF4G in the recruitment of mRNAs during meiosis. Cleavage of eIF4G by coxsackievirus protease 2A inhibited progesterone-induced meiotic progression in 88% of the oocytes; prevented the recruitment of maternal mRNAs encoding cyclin B1, c-Mos, D7, and B9; and disrupted the association of eIF4G with poly(A)-binding protein. Poly(A) elongation, however, was not inhibited by eIF4G cleavage. Injection of MPF restored meiotic cell cycle progression to >60% of the oocytes but not the recruitment of cyclin B1 or B9 mRNA. Previously recruited maternal mRNAs were removed from polyribosomes following subsequent cleavage of eIF4G, indicating that eIF4G is required both to recruit and also to maintain maternal mRNAs on polyribosomes. The expression of a cleavage-resistant variant of human eIF4G-1 (G486E) significantly restored the ability to synthesize c-Mos in response to progesterone and to translate exogenous beta-globin mRNA, indicating that the inhibition by protease 2A is due to cleavage of eIF4G alone. These results indicate that intact eIF4G is required for the poly(A)-dependent recruitment of several maternal mRNAs (cyclin B1, c-Mos, D7, and B9) during meiotic cell cycle progression but not for the synthesis of most proteins.

Induction of germinal vesicle breakdown in a cell-free preparation from starfish oocytes

Incubation of isolated germinal vesicles in the homogenate from maturing starfish oocytes resulted in synchronous germinal vesicle breakdown (GVBD), and chromosome condensation and gathering within 30 min. GVBD in this cell-free system required aerobic conditions. The endogenous ATP-generation system was preserved in the homogenate and effective under aerobic conditions, and thus exogenous ATP was not added to the homogenate. Injection of the homogenate into immature starfish oocytes induced meiotic maturation without 1-methyladenine, indicating high activity of maturation-promoting factor (MPF) in the homogenate. MPF activity in the homogenate was stable for 2 h at room temperature, while it disappeared within 1 h in the supernatant prepared by centrifugation of the homogenate. This disappearance of MPF activity is regulated by cyclin B destruction, similar to that seen in vivo.

Effect of phosphate on the second cleavage division of the rat embryo

Development of the rat embryo is arrested at the 2-cell stage in vitro in the presence of inorganic phosphate (Pi). Rat embryos were affected by exposure to 1.19 mM KH2PO4 in modified hamster embryo culture medium-1 at the late 2-cell stage only. When exposure durations were 6 h, embryos whose exposure timings were prior to cleavage had a reduced rate of development to the blastocyst stage (2-8%) when compared with embryos with no exposure to Pi (97%, P < 0.05). When exposure durations were 18 h, all embryos were arrested at the 2- to 4-cell stage. These timings would correspond to the G2 to M phase of the second cell cycle. Maturation-promoting factor (MPF), which is regulated by a phosphorylation cascade, controls cell division, and its kinase activity is necessary in order for the cell to enter the M phase. However, the histone H1 kinase activity levels and the patterns of the state of phosphorylation of cdc2 were the same in blocked and non-blocked embryos. Because MPF was active in blocked embryos, the developmental block in rat 2-cell embryos caused by phosphate was not due to MPF activity or its phosphorylation cascade.

Micro-injection of recombinant lysyl oxidase blocks oncogenic p21-Ha-Ras and progesterone effects on Xenopus laevis oocyte maturation

Previous evidence suggested an anti-oncogenic role for lysyl oxidase, mainly in ras-transformed cells. Here we prove that recombinant lysyl oxidase is actually able to antagonize p21-Ha-Ras-induced Xenopus laevis oocyte maturation. Lysyl oxidase was also effective on progesterone-dependent maturation, indicating a block lying downstream of Ras. Maturation induced by activated 'maturation promoting factor', normally triggered by progesterone, was also inhibited by lysyl oxidase. Finally, lysyl oxidase did not abolish p42Erk2 phosphorylation upon maturation triggering, suggesting a block downstream of Erk2. Further investigation showed that lysyl oxidase action depends on protein synthesis and is therefore probably mediated by a newly synthesized protein.

Nuclear transplantation in mammals: remodelling of transplanted nuclei under the influence of maturation promoting factor

Whilst the role of Maturation or M-phase Promoting Factor (MPF) as a universal M-phase regulator is well documented, much less attention has been paid to its role in nuclear transplantation experiments and especially to its influence upon remodelling of transplanted nuclei. There is currently wide acceptance that successful nuclear transplantation using differentiated nuclei is possible only in a cytoplasmic environment that is capable of inducing rapid nuclear de-differentiation to a pronuclear-like form. In this review our purpose is firstly, to outline the conditions under which such remodelling can be induced, and secondly, to extend the debate to include a consideration of whether complete nuclear remodelling is an absolute necessity for clonal development.

Plant mitosis promoting factor disassembles the microtubule preprophase band and accelerates prophase progression in Tradescantia

The regulation of mitosis in higher plant cells has been investigated by microinjecting protein kinase from the metaphase-arresting (met1) mutant of Chlamydomonas. Biochemical characterization of this enzyme complex confirms the presence of a p34cdc2/cyclin B-like kinase. The enzyme was injected into living stamen hair cells of Tradescantia virginiana in which microtubules (MTs) were visualized using fluorescent analogue cytochemistry and confocal laser scanning microscopy. Microinjection of this p34cdc2/cyclin B-like kinase caused rapid disassembly of the preprophase band of MTs but not of interphase-cortical, spindle or phragmoplast MTs. Effects of the enzyme on the cytomorphology of live prophase cells were also monitored using video microscopy. We found that injection of this enzyme accelerated chromatin condensation and nuclear envelope breakdown. This indicates the presence and function in plants of an enzyme that can initiate nuclear division similar to the maturation or mitosis promoting factor (MPF) of animal cells. These studies provide the first direct evidence that the mitotically-active form of plant MPF can drive disassembly of preprophase band MTs, chromosome condensation and initiation of mitosis in plant cells.

MAP kinase does not inactivate, but rather prevents the cyclin degradation pathway from being turned on in Xenopus egg extracts

Unfertilized frog eggs arrest at the second meiotic metaphase, due to cytostatic activity of the c-mos proto-oncogene (CSF). MAP kinase has been proposed to mediate CSF activity in suppressing cyclin degradation. Using an in vitro assay to generate CSF activity, and recombinant CL 100 phosphatase to inactivate MAP kinase, we confirm that the c-mos proto-oncogene blocks cyclin degradation through MAP kinase activation. We further show that for MAP kinase to suppress cyclin degradation, it must be activated before cyclin B-cdc2 kinase has effectively promoted cyclin degradation. Thus MAP kinase does not inactivate, but rather prevents the cyclin degradation pathway from being turned on. Using a constitutively active mutant of Ca2+/calmodulin dependent protein kinase II, which mediates the effects of Ca2+ at fertilization, we further show that the kinase can activate cyclin degradation in the presence of both MPF and the c-mos proto-oncogene without inactivating MAP kinase.

Response of avian nuclei to mammalian maturation promoting factor (MPF)

Chicken blastodermal cells (stage X) were fused to mouse enucleated oocytes with either no or high maturation promoting factor (MPF) activity. High MPF levels always induced premature chromosome condensation (PCC) irrespective of the number of nuclei fused to a single oocyte. When a single blastodermal cell was fused to a single oocyte without MPF activity the nucleus remained intact for up to 3 h and thereafter underwent PCC. A quite different situation was observed after multiple fusion of several blastodermal cells to a single oocyte without MPF activity. Here, the transplanted nuclei remained intact even after prolonged culture but underwent extensive swelling. DNA synthesis was detected in almost all unfused blastodermal cells. However, after the fusion of several blastodermal cells to a single oocyte no DNA synthesis could be detected. These results provide further evidence that MPF is the universal cell-cycle regulator in the animal kingdom. Its activity is blocked (or neutralised) after fusion to several S-phase cells. Interestingly, our results further suggest that DNA synthesis is suppressed in meiotic cytoplasm even in the presence of an intact nuclear envelope.

[Experimental analysis of mechanism concerning the environmental temperature effect on acquisition of capability in toad oocyte to resume meiotic division]

Full-grown oocytes derived from Bufo bufo gargarizans rearing in high temperature environment (28-30 degrees C), called high temperature oocytes, never underwent germinal vesicle breakdown (GVBD) after progesterone stimulation, no MPF was detected in their ooplasm, but some events which manifested normally at the beginning of progesterone induced maturation process were revealed in these oocytes. It is worth notice an another kind of maturation promoting substance(s) appeared in the ooplasm of high temperature oocyte after the hormone treatment, which was capable of triggering the resumption of meiotic division of the full-grown oocytes derived from hibernating toad (called low temperature oocytes). It is a hibernation factor-dependent maturation promoting substance (HF-MPS), which appeared after decrease of the oocyte endogenous cAMP level. Its appearance depended upon the oocyte protein synthesis, and its activity to inducing GVBD of low temperature oocytes did not inhibited by puromycin. HF-MPS differs from MPF in maturation promoting activity, as low temperature (10 degrees C) delayed obviously HF-MPS' activity but did not influence the rate of GVBD induced by MPF. Further more, probably due to the lack of "hibernation factor(s)", no expression of p34cdc2 gene was detected in high temperature oocytes (unpublished data), neither HF-MPS nor MPF could amplify autocatalytically in the oocytes. So the low temperature (below 15 degrees C) was found to be indispensable for the toad oocyte maturation. If one day we can prove HF-MPS appeared also in the course of oocyte maturation induced by progesterone, the relationship between HF-MPS and MPF may be: [formula: see text] All these discoveries indicated above make a reasonable explanation of the geographical distribution of the toad which was restricted in the region north to the 23 degrees north latitude and east to the 100 degrees east longitude in China.

Mitogen-activated protein kinase (MAP kinase) activation in Xenopus oocytes: roles of MPF and protein synthesis

Mitogen-activated protein kinase (MAP kinase) is a serine/threonine kinase whose enzymatic activity is thought to play a crucial role in mitogenic signal transduction and also in the progesterone-induced meiotic maturation of Xenopus oocytes. We have purified MAP kinase from Xenopus oocytes and have shown that the protein is present in metaphase II oocytes under two different forms: an inactive 41-kD protein able to autoactivate and to autophosphorylate in vitro, and an active 42-kD kinase resolved into two tyrosine phosphorylated isoforms on 2D gels. During meiotic maturation, MAP kinase becomes tyrosine phosphorylated and activated following the activation of the M-phase promoting factor (MPF), a complex between the p34cdc2 kinase and cyclin B. In vivo, MAP kinase activity displays a different stability in metaphase I and in metaphase II: protein synthesis is required to maintain MAP kinase activity in metaphase I but not in metaphase II oocytes. Injection of either MPF or cyclin B into prophase oocytes promotes tyrosine phosphorylation of MAP kinase, indicating that its activation is a downstream event of MPF activation. In contrast, injection of okadaic acid, which induces in vivo MPF activation, promotes only a very weak tyrosine phosphorylation of MAP kinase, suggesting that effectors other than MPF are required for the MAP kinase activation. Moreover, in the absence of protein synthesis, cyclin B and MPF are unable to promote in vivo activation of MAP kinase, indicating that this activation requires the synthesis of new protein(s).

Inhibition of mos-induced oocyte maturation by protein kinase A

The relationship between the mos protooncogene protein and cAMP-dependent protein kinase (PKA) during the maturation of Xenopus oocytes was investigated. Microinjection of the PKA catalytic subunit (PKAc) into Xenopus oocytes inhibited oocyte maturation induced by the mos product but did not markedly affect the autophosphorylation activity of injected mos protein. By contrast, PKAc did not inhibit maturation promoting factor (MPF) activation or germinal vesicle breakdown (GVBD) that was initiated by injecting crude MPF preparations. In addition, inhibiting endogenous PKA activity by microinjecting the PKA regulatory subunit (PKAr) induced oocyte maturation that was dependent upon the presence of the endogenous mos product. Moreover, PKAr potentiated mos protein-induced MPF activation in the absence of progesterone and protein synthesis. These data are consistent with the hypothesis that progesterone-induced release from G2/M is regulated via PKAc and that PKAc negatively regulates a downstream target that is positively regulated by mos.

Maturation of Xenopus laevis oocyte by progesterone requires poly(A) tail elongation of mRNA

Meiotic maturation of Xenopus laevis oocytes by progesterone requires translation of stored maternal mRNAs. We investigated the role of poly(A) tail elongation of mRNAs during this process using cordycepin, which inhibits poly(A) tail elongation of mRNAs. When oocytes were treated with the buffer containing 10 mM cordycepin for 12 h, concentration of 3'-dATP in cytosol of oocytes increased to 0.7 mM, while that of ATP remained constant at around 1.2 mM. Incorporation of [32P]AMP into poly(A) mRNA was inhibited almost completely by this treatment. Progesterone-induced germinal vesicle breakdown (GVBD) was also abolished. Dose dependence of inhibition of progesterone-induced GVBD on cordycepin was similar to that of [32P]AMP incorporation into poly(A) mRNA. However, maturation-promoting factor-induced GVBD was unaffected by treatment of oocytes with cordycepin. Furthermore, the inhibition of GVBD by cordycepin was rescued by removal of cordycepin even in the presence of actinomycin D. Therefore, we concluded that poly(A) tail elongation of mRNA is required for induction of meiotic maturation of X. laevis oocytes. In addition, progesterone induced a 2.7-fold activation of [32P]AMP incorporation into the poly(A) tail of mRNA after a lag period of 3 h whereas GVBD was induced after 6-8 h from the progesterone treatment. Syntheses of most of the proteins were unaffected by treatment of oocytes with progesterone or cordycepin. However, syntheses of several proteins were increased or decreased by progesterone and cordycepin treatment.

The antibiotic azatyrosine suppresses progesterone or [Val12]p21 Ha-ras/insulin-like growth factor I-induced germinal vesicle breakdown and tyrosine phosphorylation of Xenopus mitogen-activated protein kinase in oocytes

The antibiotic azatyrosine [DL-3-(5-hydroxy-2-pyridyl)alanine] suppressed meiotic maturation in oocytes induced by progesterone or the combination of [Val12]p21Ha-ras microinjection and insulin-like growth factor I. The suppression was dose-dependent in the range of 20-250 microM azatyrosine. In addition, azatyrosine blocked the tyrosine phosphorylation of Xp42, a member of the mitogen-activated protein kinase family, after progesterone or [Val12]p21Ha-ras/insulin-like growth factor I stimulation. Activation of maturation-promoting factor, as shown by a decrease in the tyrosine phosphorylation of the Xenopus homolog of p34cdc2, was also suppressed by azatyrosine. Azatyrosine had no effect in vivo or in vitro on the growth factor-induced autophosphorylation of the oocyte insulin-like growth factor I receptor. Azatyrosine has been shown by others [Shindo-Okada, N., Makabe, O., Nagahara, H. & Nishimura, S. (1989) Mol. Carcinog. 2, 159-167] to inhibit the growth of ras-transformed cells without affecting that of nontransformed cells. In oocytes, the antibiotic exerts an inhibitory action on both a ras-dependent and a ras-independent pathway. Lack of an effect of azatyrosine on germinal vesicle breakdown induced by the microinjection of an extract from mature oocytes, however, suggests that azatryosine is acting upstream of maturation-promoting factor activation.

Sister chromatid separation in frog egg extracts requires DNA topoisomerase II activity during anaphase

We have produced metaphase spindles and induced them to enter anaphase in vitro. Sperm nuclei were added to frog egg extracts, allowed to replicate their DNA, and driven into metaphase by the addition of cytoplasm containing active maturation promoting factor (MPF) and cytostatic factor (CSF), an activity that stabilizes MPF. Addition of calcium induces the inactivation of MPF, sister chromatid separation and anaphase chromosome movement. DNA topoisomerase II inhibitors prevent chromosome segregation at anaphase, demonstrating that the chromatids are catenated at metaphase and that decatenation occurs at the start of anaphase. Topoisomerase II activity towards exogenous substrates does not increase at the metaphase to anaphase transition, showing that chromosome separation at anaphase is not triggered by a bulk activation of topoisomerase II.

Control of microtubule nucleating activity in the cytoplasm of maturing mouse oocytes

Taxol, a drug which promotes microtubule assembly, was used to assess the microtubule nucleating activity of pericentriolar material (PCM) in mouse oocytes prevented from undergoing germinal vesicle breakdown (GVBD), compared with oocytes allowed to proceed normally through GVBD and also in nucleate and anucleate oocyte fragments. Both immunofluorescence staining and ultrastructural analysis reveal that taxol induces aster formation in the cortex of oocytes undergoing GVBD, while formation of a continuous sheet of microtubule bundles parallel to the membrane is induced in metabolically GV-arrested oocytes. Since taxol also induces the formation of asters in anucleate as well as in nucleate oocyte fragments, provided they are not treated with activators of protein kinases A or C, it is concluded that microtubule nucleating activity is related to the acquisition of Maturation Promoting Factor (MPF) and does not require mixing between the nucleoplasm and cytoplasm.

Characterization of the membrane binding and fusion events during nuclear envelope assembly using purified components

At the end of mitosis membrane vesicles are targeted to the surface of chromatin and fuse to form a continuous nuclear envelope. To investigate the molecular mechanisms underlying these steps in nuclear envelope assembly, we have developed a defined cell-free system in which the binding and fusion steps in nuclear envelope assembly can be examined separately. We have found that extensively boiled Xenopus egg extracts efficiently promote the decondensation of demembranated Xenopus sperm chromatin. When isolated membranes are added to this decondensed chromatin a specific subfraction of membrane vesicles (approximately 70 nM in diameter) bind to the chromatin, but these vesicles do not fuse to each other. Vesicle binding is independent of ATP and insensitive to N-ethylmalamide. Quantitative analysis of these sites by EM suggests that there is at least one vesicle binding site per 100 kb of chromosomal DNA. We show by tryptic digestion that vesicle-chromatin association requires proteins on both the vesicle and on the chromatin. In addition, we show that the vesicles bound under these conditions will fuse into an intact nuclear envelope when incubated with the soluble fraction of a Xenopus egg nuclear assembly extract. With respect to vesicle fusion, we have found that vesicles prebound to chromatin will fuse to each other when ATP and GTP are present in the boiled extract. These results indicate that nuclear envelope assembly is mediated by a subset of approximately 70-nM-diam vesicles which bind to chromatin sites spaced 100 kb apart and that fusion of these vesicles is regulated by membrane-associated GTP-binding proteins.

MPF is activated in growing immature Xenopus oocytes in the absence of detectable tyrosine dephosphorylation of P34cdc2

Tyrosine-phosphorylated p34cdc2 and cyclin B2 are present and physically associated in small growing stage IV oocytes (800 microns in diameter) of Xenopus laevis. Microinjection of M-phase promoting factor (MPF) into stage IV oocytes induces germinal vesicle breakdown and the activation of the kinase activity of the p34cdc2/cyclin B2 complex measured on p13suc1 beads. During the in vivo activation of MPF in stage IV oocytes, p34cdc2 tyrosine dephosphorylation is not detectable, in contrast to stage VI oocytes. Addition of cycloheximide in MPF-injected stage IV oocytes induces neither the inhibition of histone H1 kinase activity nor the cyclin B2 degradation. Therefore, the activation mechanism of histone H1 kinase in stage IV oocytes does not require detectable tyrosine dephosphorylation of p34cdc2. It is suggested rather that the tyrosine phosphorylation of p34cdc2 plays a role in inhibiting cyclin B2 degradation.

Xenopus M phase MAP kinase: isolation of its cDNA and activation by MPF

MAP kinase is activated and phosphorylated during M phase of the Xenopus oocyte cell cycle, and induces the interphase-M phase transition of microtubule dynamics in vitro. We have carried out molecular cloning of Xenopus M phase MAP kinase and report its entire amino acid sequence. There is no marked change in the MAP kinase mRNA level during the cell cycle. Moreover, studies with an anti-MAP kinase antiserum indicate that MAP kinase activity may be regulated posttranslationally, most likely by phosphorylation. We show that MAP kinase can be activated by microinjection of MPF into immature oocytes or by adding MPF to cell-free extracts of interphase eggs. These results suggest that MAP kinase functions as an intermediate between MPF and the interphase-M phase transition of microtubule organization.

Antibodies to phosphotyrosine injected in Xenopus laevis oocytes modulate maturation induced by insulin/IGF-I

Xenopus oocytes carry IGF-I receptors, and undergo meiotic maturation in response to binding of IGF-I or insulin to the IGF-I receptor. Maturation is initiated upon activation of the IGF-I receptor tyrosine kinase and requires tyrosine dephosphorylation of p34cdc2, the kinase component of maturation promoting factor (MPF). To further evaluate the role of tyrosine phosphorylation in the signalling pathway triggered by insulin/IGF-I, we have injected antibodies to phosphotyrosine into oocytes and examined their effects on oocyte maturation. Antibodies at a low concentration (40 ng/oocyte, corresponding to a concentration of 40 micrograms/ml), enhanced specifically insulin-, but not progesterone-induced maturation. In contrast, at 150 ng/oocyte, the same antibodies decreased maturation induced by insulin, progesterone, or microinjected MPF. In cell-free systems, antibodies to phosphotyrosine recognized the oocyte IGF-I receptor and modulated its ligand-induced tyrosine kinase activity in a biphasic manner, with a stimulation at 40 micrograms/ml and an inhibition at higher concentrations. Moreover, antibodies at 150 ng/oocyte neutralized the kinase activity of a crude MPF extract. This neutralization was not accompanied by a rephosphorylation of p34cdc2, but by a decrease in tyrosine phosphorylation of a 60-kDa protein, which was present in M phase extracts and undetectable in G2-arrested oocytes. Taken together, these results point to at least two levels of anti-phosphotyrosine antibody action: (i) the IGF-I receptor signalling system, and (ii) a regulatory step of MPF activation, which might be distinct of the well-documented inactivating phosphorylation of p34cdc2.

Progression from meiosis I to meiosis II in Xenopus oocytes requires de novo translation of the mosxe protooncogene

The meiotic maturation of Xenopus oocytes exhibits an early requirement for expression of the mosxe protooncogene. The mosxe protein has also been shown to be a component of cytostatic factor (CSF), which is responsible for arrest at metaphase of meiosis II. In this study, we have assayed the appearance of CSF activity in oocytes induced to mature either by progesterone treatment or by overexpression of mosxe. Progesterone-stimulated oocytes did not exhibit CSF activity until 30-60 min after germinal vesicle breakdown (GVBD). Both the appearance of CSF activity and the progression from meiosis I to meiosis II were inhibited by microinjection of mosxe antisense oligonucleotides just prior to GVBD. These results demonstrate a translational requirement for mosxe, which is temporally distinct from the requirement for mosxe expression at the onset of meiotic maturation. In contrast to progesterone-treated oocytes, oocytes that were induced to mature by overexpression of mosxe exhibited CSF activity at least 3 hr prior to GVBD. Despite the early appearance of CSF, these oocytes were not arrested at meiosis I. These results indicate that, although CSF activity is capable of stabilizing maturation-promoting factor (MPF) at meiosis II and in cleaving embryos, it is incapable of stabilizing MPF prior to or at meiosis I. These studies show that the complex regulation of the cell cycle during meiosis differs significantly from the regulation of the cell cycle during mitosis.

Cyclic changes in amphibian egg microvilli occur during the division cycle: implication of MPF

The microvilli (MV) of Pleurodeles (amphibian) eggs were examined following fertilization and compared with those of artificially activated eggs and enucleated eggs using scanning and transmission electron microscopy. The MV pattern in fertilized eggs was found to undergo a cyclic transformation during the course of the first few division cycles. Similar changes also occurred in the MV of artificially activated eggs and enucleated eggs. The reorganization of the MV was sensitive to cycloheximide and cytochalasin B, but was unaffected by colchicine. Thus, this MV alteration requires protein synthesis and microfilaments but microtubules are not implicated in this process. In addition, the effects on the MV pattern of the maturation or mitosis promoting factor (MPF) were tested. Injection of MPF into eggs at different times during the first division cycle nearly always induced an elongation of the MV. This observation suggests that MPF could regulate either directly or indirectly, via a MPF-sensitive factor, the cyclic transformation of amphibian egg MV.

High-mobility-group proteins P1, I and Y as substrates of the M-phase-specific p34cdc2/cyclincdc13 kinase

All dividing cells entering the M phase of the cell cycle undergo the transient activation of an M-phase-specific histone H1 kinase which was recently shown to be constituted of at least two subunits, p34cdc2 and cyclincdc13. The DNA-binding high-mobility-group (HMG) proteins 1, 2, 14, 17, I, Y and an HMG-like protein, P1, were investigated as potential substrates of H1 kinase. Among these HMG proteins, P1 and HMG I and Y are excellent substrates of the M-phase-specific kinase obtained from both meiotic starfish oocytes and mitotic sea urchin eggs. Anticyclin immunoprecipitates, extracts purified on specific p34cdc2-binding p13suc1-Sepharose and affinity-purified H1 kinase display strong HMG I, Y and P1 phosphorylating activities, demonstrating that the p34cdc2/cyclincdc13 complex is the active kinase phosphorylating these HMG proteins. HMG I and P1 phosphorylation is competitively inhibited by a peptide mimicking the consensus phosphorylation sequence of H1 kinase. HMG I, Y and P1 all possess the consensus sequence for phosphorylation by the p34cdc2/cyclincdc13 kinase (Ser/Thr-Pro-Xaa-Lys/Arg). HMG I is phosphorylated in vivo at M phase on the same sites phosphorylated in vitro by H1 kinase. P1 is phosphorylated by H1 kinase on sites different from the sites of phosphorylation by casein kinase II. The three thermolytic phosphopeptides of P1 phosphorylated in vitro by purified H1 kinase are all present in thermolytic peptide maps of P1 phosphorylated in vivo in proliferating HeLa cells. These phosphopeptides are absent in nonproliferating cells. These results demonstrate that the DNA-binding proteins HMG I, Y and P1 are natural substrates for the M-phase-specific protein kinase. The phosphorylation of these proteins by p34cdc2/cyclincdc13 may represent a crucial event in the intense chromatin condensation occurring as cells transit from the G2 to the M phase of the cell cycle.

Control of sperm nuclear behavior in physiologically polyspermic newt eggs: possible involvement of MPF

We have studied the mechanism controlling the behavior of accessory sperm nuclei in physiologically polyspermic eggs of the newt, Cynops pyrrhogaster. Our approach was to identify cytoplasmic components which would prevent the usual degeneration of accessory sperm nuclei. Injection of cytoplasm from unfertilized eggs, but not fertilized ones, induced multipolar cleavage in polyspermically fertilized eggs as well as centrosome separation and formation of extra bipolar spindles in accessory sperm nuclei. Cytosols extracted from unfertilized Cynops or Xenopus eggs also were active in inducing multipolar cleavage, as were germinal vesicle materials from oocytes of the frogs Xenopus or Rana or of Cynops. In all of these cases, the nuclear cycle as well as the onset of first cleavage was delayed relative to those in control eggs. In contrast, injection of an extract with maturation-promoting factor (MPF) activity, prepared from unfertilized Xenopus eggs, induced precocious and multipolar cleavage when injected into fertilized Cynops eggs. Injection of the MPF-containing extract caused acceleration of the nuclear cycle as well as formation of extra bipolar spindles by the accessory sperm nuclei. These results suggest that a local deficiency of MPF may lead to the degeneration of accessory sperm nuclei in physiologically polyspermic eggs.