TOP-2 is differentially required for the proper maintenance of the cohesin subunit REC-8 on meiotic chromosomes in Caenorhabditis elegans spermatogenesis and oogenesis

During meiotic prophase I, accurate segregation of homologous chromosomes requires the establishment of chromosomes with a meiosis-specific architecture. The sister chromatid cohesin complex and the enzyme Topoisomerase II (TOP-2) are important components of meiotic chromosome architecture, but the relationship of these proteins in the context of meiotic chromosome segregation is poorly defined. Here, we analyzed the role of TOP-2 in the timely release of the sister chromatid cohesin subunit REC-8 during spermatogenesis and oogenesis of Caenorhabditis elegans. We show that there is a different requirement for TOP-2 in meiosis of spermatogenesis and oogenesis. The loss-of-function mutation top-2(it7) results in premature REC-8 removal in spermatogenesis, but not oogenesis. This correlates with a failure to maintain the HORMA-domain proteins HTP-1 and HTP-2 (HTP-1/2) on chromosome axes at diakinesis and mislocalization of the downstream components that control REC-8 release including Aurora B kinase. In oogenesis, top-2(it7) causes a delay in the localization of Aurora B to oocyte chromosomes but can be rescued through premature activation of the maturation promoting factor via knockdown of the inhibitor kinase WEE-1.3. The delay in Aurora B localization is associated with an increase in the length of diakinesis bivalents and wee-1.3 RNAi mediated rescue of Aurora B localization in top-2(it7) is associated with a decrease in diakinesis bivalent length. Our results imply that the sex-specific effects of TOP-2 on REC-8 release are due to differences in the temporal regulation of meiosis and chromosome structure in late prophase I in spermatogenesis and oogenesis.

SGK1 is essential for meiotic resumption in mammalian oocytes

In mammalian females, oocytes are stored in the ovary and meiosis is arrested at the diplotene stage of prophase I. When females reach puberty oocytes are selectively recruited in cycles to grow, overcome the meiotic arrest, complete the first meiotic division and become mature (ready for fertilization). At a molecular level, the master regulator of prophase I arrest and meiotic resumption is the maturation-promoting factor (MPF) complex, formed by the active form of cyclin dependent kinase 1 (CDK1) and Cyclin B1. However, we still do not have complete information regarding the factors implicated in MPF activation. In this study we document that out of three mammalian serum-glucocorticoid kinase proteins (SGK1, SGK2, SGK3), mouse oocytes express only SGK1 with a phosphorylated (active) form dominantly localized in the nucleoplasm. Further, suppression of SGK1 activity in oocytes results in decreased CDK1 activation via the phosphatase cell division cycle 25B (CDC25B), consequently delaying or inhibiting nuclear envelope breakdown. Expression of exogenous constitutively active CDK1 can rescue the phenotype induced by SGK1 inhibition. These findings bring new insights into the molecular pathways acting upstream of MPF and a better understanding of meiotic resumption control by presenting a new key player SGK1 in mammalian oocytes.

Microcystin-LR influences the in vitro oocyte maturation of zebrafish by activating the MAPK pathway

Harmful cyanobacteria and their production of microcystins (MCs) exert significant toxicity on reproduction of fish, especially the process of oogenesis. Our previous studies demonstrated that MCs have negative impacts on the quantity and quality of mature oocytes in female zebrafish. However, the underlying mechanisms of MCs disrupting oocyte maturation (OM) have been rarely reported. In the present study, in vitro oocytes (immature) were separated from zebrafish and treated with 1, 10, 100 μg/L MC-LR. The serine/threonine protein phosphatase 2A (PP2A) activity was downregulated significantly in oocytes exposed to 10 and 100 μg/L MC-LR for both 2 and 4 h. The phosphorylation levels of mitogen-activated protein kinase (MAPK) were detected without noticeable change in all oocytes treated with MC-LR for 2 h, whereas the activated levels of MAPK subtypes (ERK, p38 and JNK) increased remarkably in the 100 μg/L MC-LR treatment of 4 h. In the oocytes exposed to 100 μg/L MC-LR for 4 h, germinal vesicle breakdown (GVBD) rates changed abnormally and maturation-promoting factor (MPF) activity increased significantly, in accordance with the upregulation of Cyclin B protein levels. Moreover, the MAPK inhibitors (10 μM) were applied to explore the role of MAPK subtypes during MC-LR influencing OM and results showed that ERK inhibitor U0126 and p38 inhibitor SB203580 mitigated the effects of 100 μg/L MC-LR-induced MAPK hyper-phosphorylation and elevated GVBD in the oocytes. In conclusion, the present study indicates that microcystins disrupt the meiotic maturation by the pathway of MC-PP2A-MAPK-OM due to the phosphorylation disorder in oocytes.

CKS1 Germ Line Exclusion Is Essential for the Transition from Meiosis to Early Embryonic Development

Cell division cycle (Cdc) kinase subunit (CKS) proteins bind cyclin-dependent kinases (CDKs) and play important roles in cell division control and development, though their precise molecular functions are not fully understood. Mammals express two closely related paralogs called CKS1 and CKS2, but only CKS2 is expressed in the germ line, indicating that it is solely responsible for regulating CDK functions in meiosis. Using cks2-/- knockout mice, we show that CKS2 is a crucial regulator of maturation-promoting factor (MPF; CDK1-cyclin A/B) activity in meiosis. cks2-/- oocytes display reduced and delayed MPF activity during meiotic progression, leading to defects in germinal vesicle breakdown (GVBD), anaphase-promoting complex/cyclosome (APC/C) activation, and meiotic spindle assembly. cks2-/- germ cells express significantly reduced levels of the MPF components CDK1 and cyclins A1/B1. Additionally, injection of MPF plus CKS2, but not MPF alone, restored normal GVBD in cks2-/- oocytes, demonstrating that GVBD is driven by a CKS2-dependent function of MPF. Moreover, we generated cks2cks1/cks1 knock-in mice and found that CKS1 can compensate for CKS2 in meiosis in vivo, but homozygous embryos arrested development at the 2- to 5-cell stage. Collectively, our results show that CKS2 is a crucial regulator of MPF functions in meiosis and that its paralog, CKS1, must be excluded from the germ line for proper embryonic development.

Journey of oocyte from metaphase-I to metaphase-II stage in mammals

In mammals, journey from metaphase-I (M-I) to metaphase-II (M-II) is important since oocyte extrude first polar body (PB-I) and gets converted into haploid gamete. The molecular and cellular changes associated with meiotic cell cycle progression from M-I to M-II stage and extrusion of PB-I remain ill understood. Several factors drive oocyte meiosis from M-I to M-II stage. The mitogen-activated protein kinase3/1 (MAPK3/1), signal molecules and Rho family GTPases act through various pathways to drive cell cycle progression from M-I to M-II stage. The down regulation of MOS/MEK/MAPK3/1 pathway results in the activation of anaphase-promoting complex/cyclosome (APC/C). The active APC/C destabilizes maturation promoting factor (MPF) and induces meiotic resumption. Several signal molecules such as, c-Jun N-terminal kinase (JNK2), SENP3, mitotic kinesin-like protein 2 (MKlp2), regulator of G-protein signaling (RGS2), Epsin2, polo-like kinase 1 (Plk1) are directly or indirectly involved in chromosomal segregation. Rho family GTPase is another enzyme that along with cell division cycle (Cdc42) to form actomyosin contractile ring required for chromosomal segregation. In the presence of origin recognition complex (ORC4), eccentrically localized haploid set of chromosomes trigger cortex differentiation and determine the division site for polar body formation. The actomyosin contractile activity at the site of division plane helps to form cytokinetic furrow that results in the formation and extrusion of PB-I. Indeed, oocyte journey from M-I to M-II stage is coordinated by several factors and pathways that enable oocyte to extrude PB-I. Quality of oocyte directly impact fertilization rate, early embryonic development, and reproductive outcome in mammals.

MPF-based meiotic cell cycle control: Half a century of lessons from starfish oocytes

In metazoans that undergo sexual reproduction, genomic inheritance is ensured by two distinct types of cell cycle, mitosis and meiosis. Mitosis maintains the genomic ploidy in somatic cells reproducing within a generation, whereas meiosis reduces by half the ploidy in germ cells to prepare for successive generations. The meiotic cell cycle is believed to be a derived form of the mitotic cell cycle; however, the molecular mechanisms underlying both of these processes remain elusive. My laboratory has long studied the meiotic cell cycle in starfish oocytes, particularly the control of meiotic M-phase by maturation- or M phase-promoting factor (MPF) and the kinase cyclin B-associated Cdk1 (cyclin B-Cdk1). Using this system, we have unraveled the molecular principles conserved in metazoans that modify M-phase progression from the mitotic type to the meiotic type needed to produce a haploid genome. Furthermore, we have solved a long-standing enigma concerning the molecular identity of MPF, a universal inducer of M-phase both in mitosis and meiosis of eukaryotic cells.

Maturation of Xenopus laevis oocytes under cadmium and lead exposures: Cell biology investigations

Since amphibians are recognised as good models to assess the quality of environments, only few studies have dealt with the impacts of chemical contaminants on their gametes, while toxic effects at this stage will alter all the next steps of their life cycle. Therefore, we propose to investigate the oocyte maturation of Xenopus laevis in cadmium- and lead-contaminated conditions. The impacts of cadmium and lead ions were explored on events involved in the hormone-dependent process of maturation. In time-course experiments, cadmium, at the highest concentration, delayed and prevented the germinal vesicle breakdown. Even in the absence of progesterone this ion could also induce it. No such spontaneous maturation was observed after lead exposures. An acceleration of the process at the highest tested concentration of lead (90μM), in presence of progesterone, was recorded. Cytological observations highlighted that cadmium exposures drove severe disturbances of meiotic spindle morphogenesis. At last, cadmium exposures altered the MAPK pathway, regarding the activation of ERK2 and RSK, but also the activation and the activity of the MPF, by disturbing the state of phosphorylation of Cdc2 and histone H3. Xenopus laevis oocytes were affected by these metal ion exposures, notably by Cd²⁺. Signatures of these metal exposures on the oocyte maturation were detected. This germ cell appeared to be a relevant model to assess the effects of environmental contaminants such as metals.

Maintained MPF Level after Oocyte Vitrification Improves Embryonic Development after IVF, but not after Somatic Cell Nuclear Transfer

Levels of maturation-promoting factor (MPF) in oocytes decline after vitrification, and this decline has been suggested as one of the main causes of low developmental competence resulting from cryoinjury. Here, we evaluated MPF activity in vitrified mouse eggs following treatment with caffeine, a known stimulator of MPF activity, and/or the proteasome inhibitor MG132. Collected MII oocytes were vitrified and divided into four groups: untreated, 10 mM caffeine (CA), 10 μM MG132 (MG), and 10 mM caffeine +10 μM MG132 (CA+MG). After warming, the MPF activity of oocytes and their blastocyst formation and implantation rates in the CA, MG, and CA+MG groups were much higher than those in the untreated group. However, the cell numbers in blastocysts did not differ among groups. Analysis of the effectiveness of caffeine and MG132 for improving somatic cell nuclear transfer (SCNT) technology using cryopreserved eggs showed that supplementation did not improve the blastocyst formation rate of cloned mouse eggs. These results suggest that maintaining MPF activity after cryopreservation may have a positive effect on further embryonic development, but is unable to fully overcome cryoinjury. Thus, intrinsic factors governing the developmental potential that diminish during oocyte cryopreservation should be explored.

Effect of caffeine treatment before vitrification on MPF and MAPK activity and spontaneous parthenogenetic activation of in vitro matured ovine oocytes

BACKGROUND

Molecules that stabilize protein kinases may be useful in overcoming the deleterious effects of cryopreservation.

OBJECTIVE

To evaluate the effect of caffeine treatment before vitrification of in vitro matured ovine oocytes on the activity of MPF and MAPK as well as the spontaneous parthenogenetic activation after 24 h culture.

MATERIALS AND METHODS

Oocytes obtained from slaughterhouse sheep ovaries were in vitro matured for 21 h, incubated for 3 h with or without caffeine and then vitrified. After warming, oocytes were processed for the analysis of chromatin configuration and the evaluation of spontaneous parthenogenetic activation (24 h in vitro culture). Fresh in vitro matured oocytes were used as control.

RESULTS

Caffeine treatment before vitrification maintained the MPF activity at a level similar to that of fresh oocytes, and reduced the spontaneous parthenogenetic activation in comparison with oocytes that were not-treated with caffeine.

CONCLUSION

Caffeine treatment prolongs the meiotic arrest of vitrified MII oocytes, likely via its action of stabilizing the MPF level.

Absence of Reciprocal Feedback Between MPF and ERK2 MAP Kinase in Mitotic Xenopus laevis Embryo Cell-Free Extract

MPF and MAP kinase ERK2 are two major M-phase kinases. They interact with each other in a complex way during meiotic maturation of Xenopus laevis oocytes. Here we study their interrelationship during first mitosis in X. laevis embryo cell-free extract perturbing the polyubiquitination pathway as a tool. Recombinant ubiquitin K48R (Ub-K48R) mutant protein arrests mitotic cyclin B degradation in the extract. This results in both increased accumulation of phosphorylated form of cyclin B2 and MPF activity as well as mitotic phosphorylation of its substrates. Ub-K48R also increased the mitotic phosphorylation of ERK2. Simultaneous addition of Ub-K48R and the proteasome inhibitor MG 132 strengthened and further prolonged MPF activity, MCM4 phosphorylation and accumulation of phosphorylated forms of cyclin B2. ERK2 phosphorylation levels increased and persisted longer than upon action of Ub-K48R alone. This shows a synergistic effect of inhibition of two different steps of ubiquitin-proteasome pathway on MPF activity and mitotic phosphorylation and ubiquitination of specific M-phase proteins. On the other hand, complete inhibition of ERK2 activation using U0126 had no effect either on MPF activity or on MCM4 phosphorylation either in control or in Ub-K48R-supplemented extracts. Experimental reduction of MPF activity by addition of recombinant p21(Cip) protein resulted in significant reduction of ERK2 phosphorylation. Thus, the reciprocal feedback observed between MPF and ERK2 in meiosis is not observed during mitotic M-phase in cell-free Xenopus embryo extracts. ERK2 phosphorylation is regulated by the levels of MPF activity, however no influence of ERK2 on MPF activity could be detected. These results show a fundamental difference in the relationship between the two major M-phase kinases in meiotic and mitotic cell cycle.

Different Thresholds of MPF Inactivation are Responsible for Controlling Different Mitotic Events in Mammalian Cell Division

We present evidence for a paradigm that, during cell division, the decreasing activity of MPF acts as a master signal, which utilizes different thresholds to control the initiation of different mitotic events. The key temporal control here is the degradation of cyclin B1. Using single cell analysis, we measured the kinetics of cyclin B1 degradation and determined quantitatively the thresholds of cyclin B1 level for different mitotic events within a HeLa cell. These observed thresholds were: 1.36 +/- 0.49 microM (for chromosome separation), 0.75 +/- 0.08 microM (for cytokinesis) and 0.54 +/- 0.16 microM (for nuclear reassembly). By comparison, the average concentration of endogenous cyclin B1 within a prometaphase cell was found to be 2.92 +/- 1.7 microM. We suggest that the decreasing order of these thresholds plays an important role in triggering the initiation of successive mitotic events in cell division.

Emi2 at the Crossroads: Where CSF Meets MPF

Vertebrate eggs arrest at metaphase of meiosis II due to an activity known as cytostatic factor (CSF). CSF antagonizes the ubiquitin ligase activity of the anaphase-promoting complex/cyclosome (APC/C), preventing cyclin B destruction and meiotic exit until fertilization occurs. A puzzling feature of CSF arrest is that APC/C inhibition is leaky. Ongoing cyclin B synthesis is counterbalanced by a limited amount of APC/C-mediated cyclin B destruction; thus, cyclin B/Cdc2 activity remains at steady state. How the APC/C can be slightly active toward cyclin B, and yet restrained from ubiquitinating cyclin B altogether, is unknown. Emi2/XErp1 is the critical CSF component directly responsible for APC/C inhibition during CSF arrest. Fertilization triggers the Ca2+-dependent destruction of Emi2, releasing the APC/C to ubiquitinate the full pool of cyclin B and initiate completion of meiosis. Previously, we showed that a phosphatase maintains Emi2's APC/C-inhibitory activity in CSF-arrested Xenopus egg extracts. Here, we demonstrate that phosphatase inhibition permits Emi2 phosphorylation at thr-545 and -551, which inactivates Emi2. Furthermore, we provide evidence that adding excess cyclin B to CSF extracts stimulates Cdc2 phosphorylation of these same residues, antagonizing Emi2-APC/C association. Our findings suggest a model wherein the pool of Emi2 acts analogously to a rheostat by integrating Cdc2 and phosphatase activities to prevent cyclin B overaccumulation and Cdc2 hyperactivity during the indefinite period of time between arrival at metaphase II and eventual fertilization. Finally, we propose that inactivation of Emi2 by Cdc2 permits mitotic progression during early embryonic cleavage cycles.

The regulation of maturation promoting factor during prophase I arrest and meiotic entry in mammalian oocytes

Mammalian oocytes arrest at prophase of meiosis I at around birth and they remain arrested at this stage until puberty when the preovulatory surge of luteinizing hormone (LH) causes ovulation. Prophase I arrest in the immature oocyte results from the maintenance of low activity of maturation promoting factor (MPF), which consists of a catalytic subunit (CDK1) and regulatory subunit (cyclin B1). Phosphorylation-mediated inactivation of CDK1 and constant degradation of cyclin B1 keep MPF activity low during prophase I arrest. LH-mediated signaling manipulates a vast array of molecules to activate CDK1. Active CDK1 not only phosphorylates different meiotic phosphoproteins during the resumption of meiosis but also inhibits their rapid dephosphorylation by inhibiting the activities of CDK1 antagonizing protein phosphatases (PPs). In this way, CDK1 both phosphorylates its substrates and protects them from being dephosphorylated. Accumulating evidence suggests that the net MPF activity that drives the resumption of meiosis in oocytes depends on the activation status of CDK1 antagonizing PPs. This review aims to provide a summary of the current understanding of the signaling pathways involved in regulating MPF activity during prophase I arrest and reentry into meiosis of mammalian oocytes.

Winter hibernation and UCHL1-p34cdc2 association in toad oocyte maturation competence

Currently, it is believed that toad oocyte maturation is dependent on the physiological conditions of winter hibernation. Previous antibody-blocking experiments have demonstrated that toad ubiquitin carboxyl-terminal hydrolase L1 (tUCHL1) is necessary for germinal vesicle breakdown during toad oocyte maturation. In this paper, we first supply evidence that tUCHL1 is highly evolutionarily conserved. Then, we exclude protein availability and ubiquitin carboxyl-terminal hydrolase enzyme activity as factors in the response of oocytes to winter hibernation. In the context of MPF (maturation promoting factor) controlling oocyte maturation and to further understand the role of UCHL1 in oocyte maturation, we performed adsorption and co-immunoprecipitation experiments using toad oocyte protein extracts and determined that tUCHL1 is associated with MPF in toad oocytes. Recombinant tUCHL1 absorbed p34(cdc2), a component of MPF, in obviously larger quantities from mature oocytes than from immature oocytes, and p13(suc1) was isolated from tUCHL1 with a dependence on the ATP regeneration system, suggesting that still other functions may be involved in their association that require phosphorylation. In oocytes from hibernation-interrupted toads, the p34(cdc2) protein level was significantly lower than in oocytes from toads in artificial hibernation, providing an explanation for the different quantities isolated by recombinant tUCHL1 pull-down and, more importantly, identifying a mechanism involved in the toad oocyte's dependence on a low environmental temperature during winter hibernation. Therefore, in toads, tUCHL1 binds p34(cdc2) and plays a role in oocyte maturation. However, neither tUCHL1 nor cyclin B1 respond to low temperatures to facilitate oocyte maturation competence during winter hibernation.

Removal of centrosomal PP1 by NIMA kinase unlocks the MPF feedback loop to promote mitotic commitment in S. pombe

BACKGROUND

Activation of the Cdk1/cyclin B complex, also known as mitosis-promoting factor (MPF), drives commitment to mitosis. Interphase MPF is inhibited through phosphorylation of Cdk1 by Wee1-related kinases. Because Cdc25 phosphatases remove this phosphate, Cdc25 activity is an essential part of the switch that drives cells into mitosis. The generation of a critical "trigger" of active MPF promotes a positive feedback loop that employs Polo kinase to boost Cdc25 activity and inhibit Wee1, thereby ensuring that mitotic commitment is a bistable switch. Mutations in the spindle pole body (SPB) component Cut12 suppress otherwise lethal deficiencies in Cdc25.

RESULTS

Cut12 harbors a bipartite protein phosphatase 1 (PP1) docking domain. Mutation of either element alone suppressed the temperature-dependent lethality of cdc25.22, whereas simultaneous ablation of both allowed cells to divide in the complete absence of Cdc25. Late G2 phase phosphorylation between the two elements by MPF and the NIMA kinase Fin1 blocked PP1(Dis2) recruitment, thereby promoting recruitment of Polo to Cut12 and the SPB and elevating global Polo kinase activity throughout the cell.

CONCLUSIONS

PP1 recruitment to Cut12 sets a threshold for Polo's feedback-loop activity that locks the cell in interphase until Cdc25 pushes MPF activity through this barrier to initiate mitosis. We propose that events on the SPB (and, by inference, the centrosome) integrate inputs from diverse signaling networks to generate a coherent decision to divide that is appropriate for the particular environmental context of each cell. PP1 recruitment sets one or more critical thresholds for single or multiple local events within this switch.

Protein kinase assays for measuring MPF and MAPK activities in mouse and rat oocytes and early embryos

Protein phosphorylation plays a pivotal role in cell cycle regulation. MPF (M-phase Promoting Factor) and MAPK (Mitogen-activated protein kinase) are two major kinases driving oocyte maturation and early embryonic divisions. Their activities can be measured experimentally with kinase assays that use specific exogenous substrates. The activities of MPF and MAPK are measured using histone H1 kinase and MBP (Myelin Basic Protein) kinase assays, respectively. Here, we describe detailed procedures for measuring these two activities in mouse and rat oocytes and in early mouse embryos. The assays we describe can be performed using very small amounts of biological material and produce clearly discernible measurements of histone H1 and MBP kinase activities.

Centrosomal MPF triggers the mitotic and morphogenetic switches of fission yeast

Activation of mitosis-promoting factor (MPF) drives mitotic commitment. In human cells active MPF appears first on centrosomes. We show that local activation of MPF on the equivalent organelle of fission yeast, the spindle pole body (SPB), promotes Polo kinase activity at the SPBs long before global MPF activation drives mitotic commitment. Artificially promoting MPF or Polo activity at various locations revealed that this local control of Plo1 activity on G2 phase SPBs dictates the timing of mitotic commitment. Cytokinesis of the rod-shaped fission yeast cell generates a naive, new, cell end. Growth is restricted to the experienced old end until a point in G2 phase called new end take off (NETO) when bipolar growth is triggered. NETO coincided with MPF activation of Plo1 on G2 phase SPBs (ref. 4). Both MPF and Polo activities were required for NETO and both induced NETO when ectopically activated at interphase SPBs. NETO promotion by MPF required polo. Thus, local MPF activation on G2 SPBs directs polo kinase to control at least two distinct and temporally separated, cell-cycle transitions at remote locations.

Nitric oxide-donor SNAP induces Xenopus eggs activation

Nitric oxide (NO) is identified as a signaling molecule involved in many cellular or physiological functions including meiotic maturation and parthenogenetic activation of mammalian oocytes. We observed that nitric oxide donor SNAP was potent to induce parthenogenetic activation in Xenopus eggs. NO-scavenger CPTIO impaired the effects of SNAP, providing evidence for the effects of the latter to be specific upon NO release. In Xenopus eggs, SNAP treatment induced pigment rearrangement, pronucleus formation and exocytosis of cortical granules. At a biochemical level, SNAP exposure lead to MAPK and Rsk inactivation within 30 minutes whereas MPF remained active, in contrast to calcium ionophore control where MPF activity dropped rapidly. MAPK inactivation could be correlated to pronuclear envelope reformation observed. In SNAP-treated eggs, a strong increase in intracellular calcium level was observed. NO effects were impaired in calcium-free or calcium limited medium, suggesting that that parthenogenetic activation of Xenopus oocytes with a NO donor was mainly calcium-dependent.

Roles of MAPK and spindle assembly checkpoint in spontaneous activation and MIII arrest of rat oocytes

Rat oocytes are well known to undergo spontaneous activation (SA) after leaving the oviduct, but the SA is abortive with oocytes being arrested in metaphase III (MIII) instead of forming pronuclei. This study was designed to investigate the mechanism causing SA and MIII arrest. Whereas few oocytes collected from SD rats at 13 h after hCG injection that showed 100% of mitogen-activated protein kinase (MAPK) activities activated spontaneously, all oocytes recovered 19 h post hCG with MAPK decreased to below 75% underwent SA during in vitro culture. During SA, MAPK first declined to below 45% and then increased again to 80%; the maturation-promoting factor (MPF) activity fluctuated similarly but always began to change ahead of the MAPK activity. In SA oocytes with 75% of MAPK activities, microtubules were disturbed with irregularly pulled chromosomes dispersed over the spindle and the spindle assembly checkpoint (SAC) was activated. When MAPK decreased to 45%, the spindle disintegrated and chromosomes surrounded by microtubules were scattered in the ooplasm. SA oocytes entered MIII and formed several spindle-like structures by 6 h of culture when the MAPK activity re-increased to above 80%. While SA oocytes showed one Ca²⁺ rise, Sr²⁺-activated oocytes showed several. Together, the results suggested that SA stimuli triggered SA in rat oocytes by inducing a premature MAPK inactivation, which led to disturbance of spindle microtubules. The microtubule disturbance impaired pulling of chromosomes to the spindle poles, caused spindle disintegration and activated SAC. The increased SAC activity reactivated MPF and thus MAPK, leading to MIII arrest.

Zinc depletion causes multiple defects in ovarian function during the periovulatory period in mice

Shortly before ovulation, the oocyte acquires developmental competence and granulosa cells undergo tremendous changes including cumulus expansion and luteinization. Zinc is emerging as a key regulator of meiosis in vitro, but a complete understanding of zinc-mediated effects during the periovulatory period is lacking. The present study uncovers the previously unknown role of zinc in maintaining meiotic arrest before ovulation. A zinc chelator [N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine (TPEN)] caused premature germinal vesicle breakdown and associated spindle defects in denuded oocytes even in the presence of a phosphodiesterase 3A inhibitor (milrinone). TPEN also potently blocked cumulus expansion by blocking induction of expansion-related transcripts Has2, Ptx3, Ptgs2, and Tnfaip6 mRNA. Both meiotic arrest and cumulus expansion were rescued by exogenous zinc. Lack of cumulus expansion is due to an almost complete suppression of phospho-Sma- and Mad-related protein 2/3 signaling. Consistent with a decrease in phospho-Sma- and Mad-related protein 2/3 signaling, TPEN also decreased cumulus transcripts (Ar and Slc38a3) and caused a surprising increase in mural transcripts (Lhcgr and Cyp11a1) in cumulus cells. In vivo, feeding a zinc-deficient diet for 10 d completely blocked ovulation and compromised cumulus expansion. However, 42.5% of oocytes had prematurely resumed meiosis before human chorionic gonadotropin injection, underscoring the importance of zinc before ovulation. A more acute 3-d treatment with a zinc-deficient diet did not block ovulation but did increase the number of oocytes trapped in luteinizing follicles. Moreover, 23% of ovulated oocytes did not reach metaphase II due to severe spindle defects. Thus, acute zinc deficiency causes profound defects during the periovulatory period with consequences for oocyte maturation, cumulus expansion, and ovulation.

Function of COP9 signalosome in regulation of mouse oocytes meiosis by regulating MPF activity and securing degradation

The COP9 (constitutive photomorphogenic) signalosome (CSN), composed of eight subunits, is a highly conserved protein complex that regulates processes such as cell cycle progression and kinase signalling. Previously, we found the expression of the COP9 constitutive photomorphogenic homolog subunit 3 (CSN3) and subunit 5 (CSN5) changes as oocytes mature for the first time, and there is no report regarding roles of COP9 in the mammalian oocytes. Therefore, in the present study, we examined the effects of RNA interference (RNAi)-mediated transient knockdown of each subunit on the meiotic cell cycle in mice oocytes. Following knockdown of either CSN3 or CSN5, oocytes failed to complete meiosis I. These arrested oocytes exhibited a disrupted meiotic spindle and misarranged chromosomes. Moreover, down-regulation of each subunit disrupted the activity of maturation-promoting factor (MPF) and concurrently reduced degradation of the anaphase-promoting complex/cyclosome (APC/C) substrates Cyclin B1 and Securin. Our data suggest that the CSN3 and CSN5 are involved in oocyte meiosis by regulating degradation of Cyclin B1 and Securin via APC/C.

Gas6 downregulation impaired cytoplasmic maturation and pronuclear formation independent to the MPF activity

Previously, we found that the growth arrest-specific gene 6 (Gas6) is more highly expressed in germinal vesicle (GV) oocytes than in metaphase II (MII) oocytes using annealing control primer (ACP)-PCR technology. The current study was undertaken to investigate the role of Gas6 in oocyte maturation and fertilization using RNA interference (RNAi). Interestingly, despite the specific and marked decrease in Gas6 mRNA and protein expression in GVs after Gas6 RNAi, nuclear maturation including spindle structures and chromosome segregation was not affected. The only discernible effect induced by Gas6 RNAi was a change in maturation promoting factor (MPF) activity. After parthenogenetic activation, Gas6 RNAi-treated oocytes at the MII stage had not developed further and arrested at MII (90.0%). After stimulation with Sr(2+), Gas6-silenced MII oocytes had markedly reduced Ca(2+) oscillation and exhibited no exocytosis of cortical granules. In these oocytes, sperm penetration occurred during fertilization but not pronucleus (PN) formation. By roscovitine and colcemid treatment, we found that the Gas6 knockdown affected cytoplasmic maturation directly, independent to the changed MPF activity. These results strongly suggest that 1) the Gas6 signaling itself is important to the cytoplasmic maturation, but not nuclear maturation, and 2) the decreased Gas6 expression and decreased MPF activity separately or mutually influence sperm head decondensation and PN formation.

[The crucial role of the proto-oncogene c-mos in regulation of oocyte maturation]

Meiosis arrest before fertilization is a common and unique feature of oogenesis in many animal species. On account of the unclear biological significance of meiosis arrest at various stages and for different durations in different animal species, this process and its regulation are the subject of many scientific studies. Studies on the development of ovarian teratomas proved to be helpful in defining the role of particular genes and biochemical cycles in control of the cell cycle in animals. These benign tumors are a valuable source of information on oocyte maturation. The c-mos proto-oncogene, which is specifically expressed in female and male germ cells, plays a crucial role in control of meiotic cell division in mammals. Its product--Mos protein kinase--acting through mitogen-activated protein kinases (MAPKs) regulates critical cellular functions required for homeostasis and decides about cell survival or apoptosis. The MAPK kinase kinase--MAPK kinase--MAPK (MKKK-MKK-MAPK) phosphorelay system, in view of its role in cells, seems to be the ideal target for therapeutic intervention in cancer and other diseases. The recent research on human oocytes suggests that the basic mechanisms regulating various stages of oocyte maturation are similar to those described in animals.

Different effects of redundant feedback loops on a bistable switch

Bistable switches have important roles in cellular decision-making processes. Bistability can be the consequence of positive or double-negative feedback loops. Although necessary, such feedback is not sufficient for bistability, which also requires nonlinearity. Nonlinearity can be provided by synergy of multiple feedback loops or by an ultrasensitive response within a single feedback loop. However, these two possibilities are not mutually exclusive; a combination of them is also possible. Here we analyze a biochemical regulatory network that controls a crucial cell cycle transition in all eukaryotic cells and contains multiple redundant feedback loops and nonlinearity. We show in this realistic biological example that two redundant feedback loops have different effects on the position of one of the saddle-node bifurcations of the system, which determines where the system switches. This illustrates that even though the roles of positive and double-negative feedbacks have been regarded as equivalent, the difference in their architectures can lead to differences in their effects on the system. We speculate that this conclusion could be general for other bistable systems with redundant feedback loops.

PKCalpha/beta I inhibitor Go6976 induces dephosphorylation of constitutively hyperphosphorylated Rb and G1 arrest in T24 cells

BACKGROUND

Rb functions as a key controller of the G(1)-S transition of the cell cycle, and its inactivation leads to a defective G(1) checkpoint. Bladder cancer frequently displays alterations in Rb such as constitutive hyperphosphorylation which results in inactive Rb and progression of cells to the S-phase. Several protein kinase C (PKC) inhibitors are currently undergoing clinical trials as anticancer drugs.

MATERIALS AND METHODS

T24 urinary bladder carcinoma cells, known to express hyperphosphorylated Rb, were treated with PKCα/βI inhibitor Go6976. The treated cells were subjected to cell cycle analysis, cell growth assay and Western blots for Rb and cdc2 phosphorylation.

RESULTS

The treatment resulted in Rb dephosphorylation at Ser 795 and Ser 807/811, and cdc2 dephosphorylation at Tyr15. Subsequent G(0/1) arrest and reduced proliferation rates were observed.

CONCLUSION

The results show that Go6976 can be used to restore constantly hyperphosphorylated and therefore constantly inactive Rb function in cancer cells.

MPF governs the assembly and contraction of actomyosin rings by activating RhoA and MAPK during chemical-induced cytokinesis of goat oocytes

The interplay between maturation-promoting factor (MPF), mitogen-activated protein kinase (MAPK) and Rho GTPase during actin-myosin interactions has yet to be determined. The mechanism by which microtubule disrupters induce the formation of ooplasmic protrusion during chemical-assisted enucleation of mammalian oocytes is unknown. Moreover, a suitable model is urgently needed for the study of cytokinesis. We have established a model of chemical-induced cytokinesis and have studied the signaling events leading to cytokinesis using this model. The results suggested that microtubule inhibitors activated MPF, which induced actomyosin assembly (formation of ooplasmic protrusion) by activating RhoA and thus MAPK. While MAPK controlled actin recruitment on its own, MPF promoted myosin enrichment by activating RhoA and MAPK. A further chemical treatment of oocytes with protrusions induced constriction of the actomyosin ring by inactivating MPF while activating RhoA. In conclusion, the present data suggested that the assembly and contraction of the actomyosin ring were two separable steps: while an increase in MPF activity promoted the assembly through RhoA-mediated activation of MAPK, a decrease in MPF activity triggered contraction of the ring by activating RhoA.

[The alp1-1315 mutation of the tubulin-folding cofactor D gene delays the mitosis initiation in cdc25-22 mutant cells of Schizosaccharomyces pombe]

Tubulin-folding cofactor D is necessary for the assembly of tubulin heterodimers and, possibly, plays additional roles in the cell. The effects of cofactor D, microtubules, and/or tubulin dimers on the mitosis initiation were studied in Schizosaccharomyces pombe. It was found for the first time that S. pombe cells with the alp1-1315 and cdc25-22 mutations remained highly viable at 36 degrees C for 8 h, in contrast to cells with the alp1-1315 mutation alone. The progression of cdc25-22 alp1-1315 cells through mitosis after a cell division arrest at 36 degrees C was described. When transferred to 25 degrees C, cdc25-22 alp1-1315 cells displayed a lag of approximately 30 min in Plo1-GFP appearance in the spindle pole body (SPB), 1 h in chromosome condensation, and 75 min in spindle formation. Thus, the initiation of mitosis in cdc25-22 alp1-1315 cells was delayed as compared with cdc25-22 cells. Since treatment of cdc25-22 cells with a microtubule-destabilizing drug during an arrest is known to cause a premitotic arrest with low activity of the mitosis-promoting factor (MPF), it was assumed that an impaired integrity of microtubules and/or lack of tubulin dimers in the nucleus were responsible for the delayed mitosis initiation in cdc25-22 alp1-1315 cells and in cdc25-22 cells treated with a microtubule-destabilizing drug. The progression through mitosis after a cdc25-22 arrest was extremely slow in cdc25-22 alp1-1315 cells, which was attributed to the de novo formation of tubulin dimers.

[The effects of protooncogene on oocyte maturation mediated by cytokines]

AIM

The mechanisms of cytokines in regulating oocyte maturation is still little known. The present study attempt to investigate whether the protooncogene of c-erbB2, c-myb are involved in introducing of cytokines to regulate oocyte maturation.

METHODS

This research used mouse GV stage oocyte culture model in vitro and RT-PCR, Western blotting method to explore the effect of EGF, TNFalpha, ET-1 and NO on oocyte maturation; to analyze the c-erbB2 mRNA and c-myb mRNA expression and the phosphorylation of MAPK and cyclinB1 expression in oocytes affected by above cytokines.

RESULTS

EGF(10 microg/L) stimulated meiosis of oocytes significantly, the level of c-erbB2 mRNA, c-myb mRNA were increased, and promoted the phosphorylation of MAPK and cyclinB1 expression; TNFalpha (1 microg/L) and ET-1 ((10(-1) mol/L) had the results to EGF. Low dose of SNP (10(-5)mol/L) had no effect on oocyte maturation, but could significantly reverse the suppression of dbcAMP on oocyte maturation.

CONCLUSION

c-erbB2 and c-myb were involved in introducing of cytokines to regulate oocyte maturation, might be the middle link in connection of the cytokines with MAPK and MPF in regulation oocyte maturation.

Analysis of the topoisomerase II-dependent decatenation G2 checkpoint and checkpoint kinases in human cells

Inhibition of Topo II function using poisons and catalytic inhibitors triggers checkpoints that act to delay progression of G2 cells into mitosis. Topo II poisons induce Topo II-associated DNA double-strand breaks that activate ATM and the DNA damage G2 checkpoint. Topo II catalytic inhibitors do not induce DNA double-strand breaks but block decatenation of intertwined daughter chromatids. Complete decatenation before anaphase of mitosis is required for chromatid segregation. G2 cells appear to sense the degree of chromatid arm catenations and actively delay the onset of mitosis by sustaining the inhibition of mitosis-promoting factor (MPF) and polo-like kinase 1 (Plk-1) kinase activities that normally propel G2 cells into mitosis. This chapter details the methods for assay of decatenation G2 checkpoint function and checkpoint kinase activities.

Enhanced developmental potential of heat-shocked porcine parthenogenetic embryos is related to accelerated mitogen-activated protein kinase dephosphorylation

Recently, we demonstrated that a 9-h heat shock of 42°C can have marked stimulatory effects on porcine parthenogenetic embryo development if applied immediately after oocyte activation. Developmental discrepancies between heat-shocked (HS) and non-HS embryos were manifest as early as 3 h after activation, suggesting involvement of maturation promoting factor (MPF) and/or mitogen-activated protein kinase (MAPK). Analysis of cdc2 kinase activity showed that MPF inactivation occurred at similar rates in HS and control embryos upon oocyte activation. However, MAPK dephosphorylation was accelerated in HS embryos compared with controls. Okadaic acid, a protein phosphatase inhibitor, maintained MAPK activity at high levels in both non-HS and HS embryos and sensitised HS embryos to the effects of elevated temperatures. No increase in heat shock proteins was observed in pronuclear-stage HS embryos. These data suggest that the acceleration of development observed in HS porcine parthenogenetic embryos is associated with a precocious inactivation of the MAPK signalling cascade. The faster cleavage divisions observed in HS embryos may be linked physiologically to their enhanced developmental potential in vitro.

[c-erbB(2) and c-myb induce oocyte maturation via activation of mitogen-activated protein kinase and maturation promoting factor]

It is important to study the mechanism of oocyte maturation because oocyte maturation is essential for the female procreation. The present study was designed to observe the effects of protooncogenes c-erbB(2) and c-myb on oocyte maturation and the upstream and downstream relationship with mitogen-activated protein kinase (MAPK) and maturation promoting factor (MPF). The investigation was designed as follows: (1) In order to explore the effects of protooncogenes on oocyte maturation, the dose- and time-dependent effects of c-erbB(2) antisense oligodeoxynucleotide (ASODN) and c-myb ASODN on oocyte maturation were examined, and the effects of oocyte microinjection with recombinant c-erbB(2) and c-myb proteins on oocyte maturation were investigated; (2) In order to study the upstream and downstream relationship among protooncogenes of c-erbB(2), c-myb and protein kinases of MAPK and MPF in regulating oocyte maturation, mouse oocytes were cultured in the medium treated with c-erbB(2) ASODN, c-myb ASODN, PD98059 (the MAPK inhibitor) or roscovitine (the MPF inhibitor) for 8 h, respectively, and the expressions of c-erbB(2) mRNA, c-myb mRNA, MAPK and MPF were examined. The results showed that both c-erbB(2) ASODN and c-myb ASODN inhibited the rate of germinal vesicle breakdown (GVBD) and the first polar (PB1) extrusion of denuded oocytes (DOs) in a dose- and time-dependent way, and delayed their maturation time significantly. When recombinant c-erbB(2) and c-myb proteins were microinjected into cytoplasm of germinal vesicle stage oocyte, we found that the GVBD rate increased by 23.1% (P<0.05) and 32.2% (P<0.05), respectively, for 6-hour culture, and the PB1 extrusion rate increased by 17.3% (P<0.05) and 23.5% (P<0.05), respectively, for 12-hour culture. RT-PCR showed that the mRNA expressions of c-erbB(2) and c-myb were detected in oocytes; c c-erbB(2) ASODN inhibited c-erbB(2) mRNA and c-myb mRNA expressions; c-myb ASODN inhibited c-myb mRNA expression but had no effect on c-erbB(2) mRNA expression. Nonsense tat ODN had no effects on the expressions of c-erbB(2) mRNA and c-myb mRNA. Neither PD98059 nor roscovitine changed the expressions of c-erbB(2) mRNA and c-myb mRNA though both of them inhibited recombinant c-erbB(2) and c-myb proteins-induced oocyte maturation. Furthermore, MAPK phosphorylation and cyclin B1 synthesis in oocytes were inhibited remarkably when oocytes were treated with c-erbB(2) ASODN, c-myb ASODN, PD98059 and roscovitine. Nonsense tat ODN had no effects on MAPK phosphorylation and cyclinB1 content. The results suggest that protooncogenes c-erbB(2) and c-myb play an important role in oocyte maturation; the effects of c-erbB(2) and c-myb depend upon the action of MAPK and MPF, and their activation is the event that occurs downstream of c-erbB(2) and c-myb in the maturation signal pathway.

[Cdc25B overexpression reverses two-cell block in mouse embryos]

AIM

To explore the effect of Cdc25B overexpression on the development of mouse two-cell embryos.

METHODS

The pBSK-Cdc25B was in vitro transcribed into 5'-capped mRNA for microinjection by using mMESSAGE mMACHINE kit. The Cdc25B mRNA was microinjected into mouse embryos at two-cell stage in order to observe the embryonic development and cleavage rate. Using protein kinase activity assay and Western blot to detect the MPF activity as well as the phosphorylation status of Cdc2-Tyr15 in Cdc25B overexpression group respectively.

RESULTS

The mouse embryos with Cdc25B overexpression developed to the four-cell stage 48 h after the hCG injection with the percentage of cleavage over 40% compared with the embryos in control groups which still remained at the two-cell stage. Moreover, MPF activity increased significantly after Cdc25B mRNA injection. The phosphorylation status of Cdc2-Tyr15 was coincident with MPF activity.

CONCLUSION

The results indicate that Cdc25B overexpression in early mouse two-cell embryos reverses two-cell block and promotes their development into four-cell stage by activating MPF.

Zinc regulates the ability of Cdc25C to activate MPF/cdk1

Zn(2+) is an essential micronutrient for the growth and development of multicellular organisms, as Zn(2+) deficiencies lead to growth retardation and congenital malformations (Vallee, BL, Falchuk, KH. 1993. Physiol Rev., 73:79-118). At the cellular level Zn(2+) depravation results in proliferation defects in many cell types (Vallee, BL, Falchuk, KH. 1993. Physiol Rev., 73:79-118), however the molecular pathways involved remain poorly defined. Here we show that the transition metal chelator TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl) ethylene diamine) blocks the G2/M transition of the meiotic cell cycle by inhibiting Cdc25C-cdk1 activation. ICP-MS analyses reveal that Cdc25C is a Zn(2+)-binding metalloprotein, and that TPEN effectively strips Zn(2+) away from the enzyme. Interestingly, although apo-Cdc25C (Zn(2+)-deficient) remains fully catalytically active, it is compromised in its ability to dephosphorylate and activate MPF/cdk1. Thus, Zn(2+) is an important regulator of Cdc25C function in vivo. Because of the conserved essential role of the Cdc25C-cdk1 module in the eukaryotic cell cycle, these studies provide fundamental insights into cell cycle regulation.

Cyclin B2/cyclin-dependent kinase1 dissociation precedes CDK1 Thr-161 dephosphorylation upon M-phase promoting factor inactivation in Xenopus laevis cell-free extract

Cyclin-dependent kinase 1 (CDK1) is the enzymatic subunit of M-phase Promoting Factor (MPF). It is positively regulated by phosphorylation on Thr-161 and association with a cyclin B molecule. The role of Thr-161 dephosphorylation upon MPF inactivation remains unclear; nevertheless, degradation of cyclin B is thought to be a direct cause of MPF inactivation. However, MPF inactivation actually precedes cyclin B degradation in Xenopus cell-free extracts. Here we study in details the temporal relationship between histone H1 kinase (reflecting MPF activity) inactivation, Thr-161 dephosphorylation, CDK1-cyclin B2 dissociation and cyclin B2 proteolysis in such extracts. We show an asynchrony between inactivation of histone H1 kinase and degradation of cyclin B2. CDK1 dephosphorylation on Thr 161 is an even later event than cyclin B2 degradation, reinforcing the hypothesis that cyclin B dissociation from CDK1 is the key event inactivating MPF. Cyclins synthesized along with MPF inactivation could deliver shortly living active MPF molecules, potentially increasing the asynchrony between histone H1 kinase inactivation and cyclin B2 degradation. We confirm this by showing that in the absence of protein synthesis, such a tendency is lower, but nevertheless, still detectable. Finally, to characterise better CDK1/cyclin B dissociation, we show that CDK1 begins to dissociate from cyclin B2 before the very beginning of cyclin B2 degradation and that the diminution in CDK1-associated cyclin B2 is faster than the decline of its total pool. Thus, neither cyclin B2 degradation nor Thr-161 dephosphorylation participates directly in CDK1 inactivation as measured by histone H1 kinase decline upon the exit from mitotic M-phase in Xenopus embryo extract.

Physiological temperature variants and culture media modify meiotic progression and developmental potential of pig oocytes in vitro

Ovarian follicles in vivo are cooler than surrounding abdominal and ovarian tissues. This study investigated whether typical follicular temperatures influence the maturation and developmental potential of pig oocytes in vitro. Oocytes were synchronised at the germinal vesicle (GV) stage and incubated at 39, 37 or 35.5 degrees C. When compared with 39 degrees C, which is often used for in vitro studies, lower temperatures delayed spontaneous progression to the metaphase I and II (MI and MII) stages of meiosis. The MII was delayed by about 12 h per degrees C. All oocytes had normal morphology. Oocytes reaching GV breakdown (GVBD) at 39 degrees C were subsequently unaffected by cooling, demonstrating thermal sensitivity during the pre-GVBD stage only. Simultaneous assay of maturation-controlling kinases (maturation promoting factor (MPF) and MAPK) showed that cooling delayed kinase activation, provided it was applied prior to GVBD. Activity profiles remained coupled to the stage of meiosis. Neither enzyme was directly thermally sensitive over this temperature range. Following in vitro fertilisation, fewer blastocysts developed from embryos derived from 35.5 or 37 degrees C oocytes as compared with those from 39 degrees C oocytes. Manipulation of fertilisation timings to allow for delayed maturation showed that over-maturing or aging at lower temperatures compromises subsequent embryo development, despite normal nuclear maturation; the GV stage was again the thermally sensitive period. Cleavage rates were improved by the culture of oocytes with follicle-stimulating hormone (FSH) at 37 but not at 35.5 degrees C. Inclusion of 20% follicular fluid in the oocyte medium restored the blastocyst rate to that seen at higher temperatures. Thus, FSH and follicular fluid may allow oocytes to achieve normal developmental potential at in vivo temperatures.

Involvement of Protein Kinase B/AKT in Early Development of Mouse Fertilized Eggs1

The activation of AKT (also called protein kinase B) is thought to be a critical step in the phosphoinositide 3-kinase pathway that regulates cell growth and differentiation. In this report, we investigated the role of AKT in the regulation of mouse early embryo development. Injection of mRNA coding for a constitutively active myristoylated AKT (myr-Akt1) into one-cell stage fertilized eggs induced cell division more effectively than injection of wild-type AKT (Akt1-WT) mRNA, whereas microinjection of mRNA of kinase-deficient AKT (Akt1-KD) delayed the first mitotic division. Meanwhile, microinjection of different kinds of mRNA of AKT affected the phosphorylation status of CDC2A-Tyr15 and the activation of M-phase promoting factor (MPF). To investigate the intermediate factor between AKT and MPF, we then injected one-cell stage eggs first with Akt1-WT mRNA or myr-Akt1 mRNA and then with mRNA encoding either wild-type CDC25B (Cdc25b-WT) or a AKT-nonphosphorylatable Ser351 to Ala CDC25B mutant (Cdc25b-S351A). Cdc25b-S351A strongly inhibited the effect of AKT. Therefore, AKT causes the activation of MPF and strongly promotes the development of one-cell stage mouse fertilized eggs by inducing AKT-dependent phosphorylation of CDC25B, a member of the CDC25 phosphatase family. Our finding that CDC25B acts as a potential target of AKT provides new insight into the effect of AKT in the regulation of early development of mouse embryos.

Survivin phosphorylation and M-phase promoting factor in oral carcinogenesis

Survivin is a recently described inhibitor of apoptosis and mitotic regulator which is selectively over-expressed in human tumors. Its expression rate is predictive of disease progression, early recurrences and resistance to therapy. Up-regulation of survivin in oral pre-malignant lesions (OPL) and in oral squamous cell carcinoma (OSCC) has already been demonstrated in previous studies. A critical step for activation of survivin has been identified in the phosphorylation on Thr34 by the main mitotic kinase p34cdc2-cyclin B1. The aim of this work was to investigate the relationship between survivin, its phosphorylated active form (p-survivin) and M-phase promoting factor (MPF), p34cdc2-cyclin B1 in oral carcinogenesis. 32 OSCCs and 17 OPLs from surgical specimens were studied for cyclin B1, p-survivin, survivin, and p34cdc2 expression by immunohistochemistry. All cases of OSCC expressed survivin and its expression rate was correlated to p-survivin levels (P<0.05). Cyclin B1 was positive in 80% of cases, while p-34cdc2 was over-expressed in all OSCCs. All OPLs associated with OSCC expressed survivin and its levels were correlated to p-survivin levels (P<0.05). Cyclin B1 was positive in 70% of cases, while p-34cdc2 was positive in all OPLs. In conclusion, this study demonstrated that MPF, survivin and p-survivin are expressed during early and late phase of oral carcinogenesis. MPF proteins, which are co-expressed on mitotic apparatus, could represent a potential target for therapies based on manipulation of survivin phosphorylation, which would induce apoptosis in cancer cells.

Meiotic resumption of porcine immature oocytes is prevented by ooplasmic Gsalpha functions

A high cyclic adenosine monophosphate (cAMP) level in fully-grown immature oocytes prevents meiotic resumption. In Xenopus, inhibitory cAMP is synthesized within oocytes depending on a stimulatory alpha-subunit of G-protein (Gsalpha). In the present study, we examined whether ooplasmic Gsalpha is involved in meiotic arrest of porcine oocytes. First, we studied the presence of Gsalpha molecules in porcine oocytes by immunoblotting, and this suggested the presence of reported isoforms (45 and 48 kDa) not only in cumulus cells but also in porcine oocytes. Then we injected an anti-Gsalpha antibody into porcine immature oocytes and found that inhibition of ooplasmic Gsalpha functions significantly promoted germinal vesicle breakdown of the oocytes, whose spontaneous meiotic resumption was prevented by 3-isobutyl-l-methylxanthine (IBMX) treatment. Although cyclin B synthesis and M-phase promoting factor (MPF) activation were largely prevented until 30 h of culture in IBMX-treated oocytes, injection of anti-Gsalpha antibody into these oocytes partially recovered cyclin B synthesis and activated MPF activity at 30 h. These results suggest that meiotic resumption of porcine oocytes is prevented by ooplasmic Gsalpha, which may stimulate cAMP synthesis within porcine oocytes, and that synthesized cAMP prevents meiotic resumption of oocytes through the signaling pathways involved in MPF activation.

Effect of vitrification solutions and cooling upon in vitro matured prepubertal ovine oocytes

The vitrification procedure effects on molecular and cytoskeletal components and on developmental ability of in vitro matured prepubertal ovine oocytes were evaluated. MII oocytes were divided into three groups: (1) vitrified in cryoloops (VTR); (2) exposed to vitrification solutions and rehydrated without being plunged into liquid nitrogen (EXP); (3) without further treatment as a control (CTR). Two hours after treatment, membrane integrity, assessed by propidium iodide/Hoechst staining, was lower in VTR and EXP than in CTR (70.6%, 88.5% and 95.2%, respectively). Cleavage rate after fertilization was statistically different among all groups (21.4%, 45.4% and 82.8% for VTR, EXP and CTR groups respectively; P<0.01). Blastocyst rate in VTR (0.0%) and EXP (2.8%) groups was lower (P<0.01) than in CTR (22.8%). Maturation promoting factor activity was lower (P<0.01) in VTR and EXP groups compared with CTR at both 0 h (82.2%, 83.6% and 100%, respectively) and 2 h (60% and 53.9% and 100%, respectively) after warming. Immediately after warming VTR and EXP oocytes showed a lower rate of normal spindle and chromosome configuration compared to CTR (59.1%, 48.0% and 83.3%, respectively; P<0.01). After 2 h of culture in standard conditions the percentage of oocytes with normal spindle and chromosome organization decreased in both VTR and EXP groups compared to CTR (36.4%, 42.8% versus 87.5%, respectively). In conclusion the exposition to the tested cryoprotectant solution and the vitrification in cryoloops modified cytoskeletal components and alter biochemical pathways that compromise the developmental capacity of prepubertal in vitro matured ovine oocytes.

[The expression of M-phase promoting factor in salivary adenoid cystic carcinoma]

PURPOSE

To study the expression and clinical significance of M-phase promoting factor (MPF) in salivary adenoid cystic carcinoma (SACC).

METHODS

The expression of MPF was investigated in 40 salivary adenoid cystic carcinomas and 40 normal salivary tissues by immunohistochemistry. The expression of MPF was detected in SACC-83 and SACC-LM with Western blot. Pearson's Chi-square test, paired t test and linear correlation analysis were used to analyze the data with SPSS 11.5 software package.

RESULTS

The expression of MPF was significantly higher in salivary adenoid cystic carcinoma than in normal salivary tissues(P<0.05). There was significant correlation between the level of MPF expression and pathological type(P<0.05). The expression of MPF was significantly higher in SACC-LM than in SACC-83 (P<0.05).

CONCLUSIONS

MPF highly expressed in salivary adenoid cystic carcinoma, salivary adenoid cystic carcinoma correlated with the expression of MPF and the abnormal activation of MPF was one of the factors for the proliferation of salivary adenoid cystic carcinoma. Metastasis in salivary adenoid cystic carcinoma correlated with the expression of MPF.

Two phases of calcium requirement during starfish meiotic maturation

During meiosis in oocytes of the starfish, Asterina pectinifera, a Ca(2+) transient has been observed. To clarify the role of Ca(2+) during oocyte maturation in starfish, an intracellular Ca(2+) blocker, TMB-8, was applied. The oocyte maturation induced by 1-methyladenine (1-MA) was blocked by 100 microM TMB-8. Reinitiation of meiosis with germinal vesicle breakdown (GVBD) and the following chromosome condensation did not take place. Maturation-promoting factor (MPF) activity did not increase and GVBD and chromosome condensation did not occur. Ca(2+) transient observed immediately after 1-MA application in control oocytes was also blocked by TMB-8. When calyculin A, which activate the MPF directly, was applied to the oocytes instead of 1-MA in seawater containing 100 microM TMB-8, GVBD and chromosome condensation were blocked. Cytoplasmic transplantation studies confirmed that MPF was activated, although TMB-8 blocked GVBD. These results show that TMB-8 blocked the increase of MPF activity induced by 1-MA and the process of active MPF inducing GVBD and subsequent chromosome condensation. Together with the above phenomena, it is conceivable that there are two phases of Ca(2+) requirement during starfish oocyte maturation. These are the activation of MPF, moreover, GVBD, and the subsequent chromosome condensation.

Changes in MPF and MAPK activities in porcine oocytes activated by different methods

The effect of different oocyte activation methods on the dynamics of M-phase promoting factor (MPF) and mitogen-activated protein kinase (MAPK) activity in porcine oocytes were examined. Three activativation methods were tested: (1) electroporation (EP); (2) electroporation combined with butyrolactone I (BL), an inhibitor of cdc2 and cdk2 kinases; (3) electroporation followed by a treatment with cycloheximide (CHX), a protein synthesis blocker. The activity of cdc2 in MII oocytes was 0.067+/-0.011pmol/oocyte/min (mean+/-S.E.M.), which by 1h decreased in every treatment group (P<0.05) and stayed at low levels until 6h post-activation, approximately the time of pronuclear formation. The initial MAPK activity (0.123+/-0.017pmol/oocyte/min) also decreased 1h after each type of activation treatment (P<0.005). However, in the electroporation only group, activity reached its lowest level at 3h; thereafter, it started to recover and at later time points, MAPK activity did not differ from that in non-treated oocytes (P>0.1). In contrast, oocytes where electroporation was followed by protein kinase or protein synthesis inhibition had low MAPK activity by the time pronuclei were to be formed. Pronuclear formation in these groups (86.3+/-3.3% for EP+BL and 87.6+/-3.7% for EP+CHX) was higher compared to that found in the EP-only oocytes (69.4+/-3.3%; P<0.05). These findings demonstrated that electroporation alone efficiently triggered the inactivation of MPF but not that of MAPK. In order to achieve low MAPK activity to allow high frequency of pronuclear formation, electroporation should be followed by a treatment that inhibits protein synthesis or specific protein kinases. The combined activation methods provided stimuli that efficiently induced both MPF and MAPK inactivation and triggered pronuclear formation with high frequencies.

Specialized roles of the two mitotic cyclins in somatic cells: cyclin A as an activator of M phase-promoting factor

The role of cyclin B-CDC2 as M phase-promoting factor (MPF) is well established, but the precise functions of cyclin A remain a crucial outstanding issue. Here we show that down-regulation of cyclin A induces a G2 phase arrest through a checkpoint-independent inactivation of cyclin B-CDC2 by inhibitory phosphorylation. The phenotype is rescued by expressing cyclin A resistant to the RNA interference. In contrast, down-regulation of cyclin B disrupts mitosis without inactivating cyclin A-CDK, indicating that cyclin A-CDK acts upstream of cyclin B-CDC2. Even when ectopically expressed, cyclin A cannot replace cyclin B in driving mitosis, indicating the specific role of cyclin B as a component of MPF. Deregulation of WEE1, but not the PLK1-CDC25 axis, can override the arrest caused by cyclin A knockdown, suggesting that cyclin A-CDK may tip the balance of the cyclin B-CDC2 bistable system by initiating the inactivation of WEE1. These observations show that cyclin A cannot form MPF independent of cyclin B and underscore a critical role of cyclin A as a trigger for MPF activation.

Effect of dehydroleucodine on meiosis reinitiation inBufo arenarumdenuded oocytes

In amphibian oocytes meiosis, the transition from G2to M phase is regulated by the maturation promoting factor (MPF), a complex of the cyclin-dependent kinase p34/cdc2 and cyclin B. In immature oocytes there is an inactive complex (pre-MPF), in which cdc2 is phosphorylated on both Thr-161 and Thr-14/Tyr-15 residues. The dephosphorylation of Thr-14/Tyr-15 is necessary for the start of MPF activation and it is induced by the activation of cdc25 phosphatase. Late, to complete the activation, a small amount of active MPF induces an auto-amplification loop of MPF stimulation (MPF amplification). Dehydroleucodine (DhL) is a sesquiterpenic lactone that inhibits mammalian cell proliferation in G2. We asked whether DhL interferes with MPF activation. For this question, the effect of DhL (up to 30 μM) on the resumption of meiosis was evaluated, and visualized by germinal vesicle break down (GVBD), ofBufo arenarumoocytes inducedin vitroby either: (i) removing follicle cells; (ii) progesterone stimulation; (iii) VG-content injection; or (iv) injection of mature cytoplasm. The results show that DhL induced GVBD inhibition, in a dose-dependent manner, in spontaneous and progesterone-induced oocyte maturation. Nevertheless, DhL at the doses assayed had no effect on GVBD induced by mature cytoplasm injection, but exerted an inhibitory effect on GVBD induced by GV content. On the basis of these results, we interpreted that DhL does not inhibit MPF amplification and that the target of DhL is any event in the early stages of the cdc25 activation cascade.

O-Linked N-Acetylglucosaminyltransferase Inhibition Prevents G2/M Transition in Xenopus laevis Oocytes

Full-grown Xenopus oocytes are arrested at the prophase of the first meiotic division in a G(2)-like state. Progesterone triggers meiotic resumption also called the G(2)/M transition. This event is characterized by germinal vesicle breakdown (GVBD) and by a burst in phosphorylation level that reflects activation of M-phase-promoting factor (MPF) and MAPK pathways. Besides phosphorylation and ubiquitin pathways, increasing evidence has suggested that the cytosolic and nucleus-specific O-GlcNAc glycosylation also contributes to cell cycle regulation. To investigate the relationship between O-GlcNAc and cell cycle, Xenopus oocyte, in which most of the M-phase regulators have been discovered, was used. Alloxan, an O-GlcNAc transferase inhibitor, blocked G(2)/M transition in a concentration-dependent manner. Alloxan prevented GVBD and both MPF and MAPK activations, either triggered by progesterone or by egg cytoplasm injection. The addition of detoxifying enzymes (SOD and catalase) did not rescue GVBD, indicating that the alloxan effect did not occur through reactive oxygen species production. These results were strengthened by the use of a benzoxazolinone derivative (XI), a new O-GlcNAc transferase inhibitor. Conversely, injection of O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate, an O-GlcNAcase inhibitor, accelerated the maturation process. Glutamine:fructose-6-phosphate amidotransferase inhibitors, azaserine and 6-diazo-5-oxonorleucine, failed to prevent GVBD. Such a strategy appeared to be inefficient; indeed, UDP-GlcNAc assays in mature and immature oocytes revealed a constant pool of the nucleotide sugar. Finally, we observed that cyclin B2, the MPF regulatory subunit, was associated with an unknown O-GlcNAc partner. The present work underlines a crucial role for O-GlcNAc in G(2)/M transition and strongly suggests that its function is required for cell cycle regulation.

Oocyte spontaneous activation in different rat strains

Oocyte spontaneous activation (OSA) has been reported to occur during in vitro culture of ovulated rat oocytes. The objective of this study was to compare the rate of oocyte spontaneous activation and the level of maturation promoting factor (MPF) activity in oocytes from different strains. Twelve strains were selected from two commercial sources. Females were superovulated and oocytes collected 17 h after hCG injection. Denuded oocytes were cultured in M16 medium under oil at 37 degrees C and 5% CO(2) in air. The proportion of activated oocytes was determined after 6 h of in vitro culture. Data were compared by analysis of variance (ANOVA), considering each animal as an experimental unit. MPF activity was determined in oocytes from the different strains at 0, 1.5, and 3 h after oocyte collection. The log ratio of the MPF activity at 1.5 and 3 h relative to 0 hours for each animal was analyzed by ANOVA. While significant (p < 0.01) differences were observed between strains in the rate of OSA, there were no differences between strains in the level of MPF during the time points measured (p > 0.3).

Effect of oocyte diameter on meiotic competence, embryo development, p34 (cdc2) expression and MPF activity in prepubertal goat oocytes

The aim of this study was to analyze the relationship between oocyte diameter, meiotic and embryo developmental competence and the expression of the catalytic subunit of MPF, the p34(cdc2), at mRNA, RNA and protein level, as well as its kinase activity, in prepubertal (1-2 months old) goat oocytes. MPF is the main meiotic regulator and a possible regulator of cytoplasmic maturation; therefore, it could be a key factor in understanding the differences between competent and incompetent oocytes. Oocytes were classified according to oocyte diameter in four categories: <110, 110-125, 125-135 and >135 microm and matured, fertilized and cultured in vitro. The p34(cdc2) was analyzed in oocytes at the time of collection (0 h) and after 27 h of IVM (27 h) in each of the oocyte diameter categories. The oocyte diameter was positively related to the percentage of oocytes at MII after IVM (0, 20.7, 58 and 78%, respectively) and the percentage of blastocysts obtained at 8 days postinsemination (0, 0, 1.95 and 12.5%, respectively). The expression of RNA and mRNA p34(cdc2) did not vary between oocyte diameters at 0 and 27h. Protein expression of p34(cdc2) increased in each oocyte category after 27 h of maturation. MPF activity among diameter groups did not vary at 0h but after IVM there was a clear and statistically significant increase of MPF activity in the biggest oocytes.

Molecular mechanism of oocyte maturation

Maturation of vertebrate oocytes is regulated by maturation inducing hormone (MIH), which is progesterone in all vertebrates except in fish, where it is 17alpha, 20beta dihydroxy progesterone. Once the full growth of the oocytes is achieved, they arrest at prophase of meiosis I. MIH releases oocytes from this arrest. MIH promotes the formation of a dimeric protein kinase complex known as maturation promoting factor (MPF), the regulatory component of which is cyclin B and the catalytic component is cell division cycle (Cdc2) kinase. This complex is activated by phosphorylation at Thrl61 but remains inactive due to the inhibitory phosphorylation at Thrl4 and Tyrl5. MIH stimulates Cdc25, a dual specific phosphatase, that dephosphorylates both Thrl4 and Tyrl5 and converts pre- or inactive MPF to active MPF. Germinal vesicle break down (GVBD) is the marker of oocyte maturation. In an Indian freshwater perch, Anabas testudineus, MIH induced GVBD between 18-20 h. MIH induced oocytes extract in SDS-PAGE showed over-expression of a 30 kDa protein, which is confirmed to be cyclin B by using both monoclonal and polyclonal anti-cyclin B antibodies from various sources. The size of cyclin B in other vertebrates including mammals lies between 46-55 kDa. We have cloned cyclin B gene from perch oocyte and found it to contain the domains required for its function and immunological recognition. We also cloned Cdkl gene, which is very similar to other vertebrates Cdkl. Perch oocyte Cdc25 is overexpressed prior to GVBD converting inactive MPF to active MPF that affect GVBD. The objective of this overview is to deal with the molecular regulation of MPF activation which causes final maturation of oocytes.

Aberrant spindle assembly checkpoint in bovine somatic cell nuclear transfer oocytes

Nuclear, microtubular dynamics and spindle assembly checkpoint (SAC) in bovine somatic cell nuclear transfer (SCNT) oocytes receiving G1/0 or M phase somatic cell nuclei were studied. SCNT oocytes assembled microtubules, however, the spindles were structurally abnormal, including bi-, tri-polar or elongated spindles with scattered premature chromosome condensation (PCC) in G1/0 phase nuclei, and some miniature spindles with unaligned chromosomes in M phase nuclei. In contrast, demecolcine-treated SCNT oocytes formed chromosome clusters with membrane protrusion and significantly induced maturation-promoting factor (MPF) activity elevation (up to 177%) for 3 hours, indicating that first SAC at second metaphase (MII) is established upon spindle disruption in SCNT oocytes. After parthenogenetic stimuli, unlike MII oocytes which prevent exit from MII arrest with high MPF activity upon spindle disruption by second SAC, demecolcine-treated SCNT oocytes could not prevent exit from MII arrest with inactivation of MPF activities, whereas MG132-treated SCNT oocytes could persist at MII arrest, indicating that SCNT oocytes lack the ability for second SAC establishment, however, two G1/0 phase nuclei in an ooplasm restored second SAC establishment upon spindle disruption. Furthermore, the developmental potential of demecolcine-treated SCNT oocytes receiving G1/0 phase nuclei to blastocyst stage was not significantly different than untreated SCNT oocytes (29% vs 31%). These results indicate that unlike MII oocytes, SCNT oocytes have aberrant spindle morphology and SAC at MII due to insufficient SAC signals from somatic cell nuclei, thus aberrant remodeling has started immediately after somatic cell nuclear transfer and may be responsible for chromosome instability in SCNT embryos as well as the low successful efficiency of cloning.