Inactivation of M-phase promoting factor at exit from first embryonic mitosis in the rat is independent of cyclin B1 degradation

Treatment with MG132 prevents spontaneous activation of rat oocyte in culture and promotes embryonic development after intracytoplasmic sperm injection

Intracytoplasmic sperm injection (ICSI) is an effective reproductive technique for obtaining rat offspring using preserved sperm with low or no motility. However, rat oocytes undergo spontaneous activation immediately after retrieval from the oviduct and poorly develop after ICSI unless it is performed quickly. Here, we evaluated whether treatment with MG132, the proteasome inhibitor, suppresses the spontaneous activation of oocytes before and during ICSI. After retrieval from the oviducts, the rate of development into morula and blastocyst from the oocytes cultured in vitro for 1 h prior to ICSI significantly decreased compared with that from the control oocytes subject to ICSI without culture (7% versus 36%). However, a higher proportion of oocytes treated with MG132 for 0, 1, and 3 h before and during ICSI developed into morulae and blastocysts (70%, 60%, and 52%, respectively). Offspring were obtained from oocytes treated with MG132 for 0 and 1 h before and during ICSI (percentage: 31%). Altogether, MG132 could suppress the spontaneous activation of rat oocytes and increase embryonic development after ICSI.

Particulate Matter Exposure During Oocyte Maturation: Cell Cycle Arrest, ROS Generation, and Early Apoptosis in Mice

Particulate matter (PM) is a general atmospheric pollutant released into the air by an anthropogenic and naturally derived mixture of substances. Current studies indicate that fine dust can result in different health defects, including endothelial dysfunction, asthma, lung cancer, cardiovascular diseases, uterine leiomyoma, deterioration in sperm quality, and overall birth impairment. However, the most prominent effects of PM₁₀ (diameter < 10 μM) exposure on the female reproductive system, especially with respect to oocyte maturation, remain unclear. In the present study, maturing mouse oocytes were treated with PM₁₀ and the phenotypes of the resulting toxic effects were investigated. Exposure to PM₁₀ led to impairment of maturation capacity by inducing cell cycle arrest and blocking normal polar body extrusion during in vitro maturation and activation of fertilization of mouse oocytes. Additionally, defects in tubulin formation and DNA alignment were observed in PM₁₀-treated oocytes during metaphase I to anaphase/telophase I transition. Moreover, PM₁₀ induced reactive oxygen species generation, mitochondrial dysfunction, DNA damage, and early apoptosis. Taken together, these results indicate that PM₁₀ exposure leads to a decline in oocyte quality and affects the subsequent embryonic development potential of mammalian oocytes.

Sulforaphane induces reactive oxygen species-mediated mitotic arrest and subsequent apoptosis in human bladder cancer 5637 cells

The present study was undertaken to determine whether sulforaphane-derived reactive oxygen species (ROS) might cause growth arrest and apoptosis in human bladder cancer 5637 cells. Our results show that the reduced viability of 5637 cells by sulforaphane is due to mitotic arrest, but not the G2 phase. The sulforaphane-induced mitotic arrest correlated with an induction of cyclin B1 and phosphorylation of Cdk1, as well as a concomitant increased complex between cyclin B1 and Cdk1. Sulforaphane-induced apoptosis was associated with the activation of caspase-8 and -9, the initiators caspases of the extrinsic and intrinsic apoptotic pathways, respectively, and activation of effector caspase-3 and cleavage of poly (ADP-ribose) polymerase. However, blockage of caspase activation inhibited apoptosis and abrogated growth inhibition in sulforaphane-treated 5637 cells. This study further investigated the roles of ROS with respect to mitotic arrest and the apoptotic effect of sulforaphane, and the maximum level of ROS accumulation was observed 3h after sulforaphane treatment. However, a ROS scavenger, N-acetyl-L-cysteine, notably attenuated sulforaphane-mediated apoptosis as well as mitotic arrest. Overall, these results suggest that sulforaphane induces mitotic arrest and apoptosis of 5637 cells via a ROS-dependent pathway.

Comparative proteomic analysis of proteins involved in oocyte meiotic maturation in mice

After birth, oocytes stay at the diplotene stage in prophase of meiosis I. Meiosis resumes about 1 day before ovulation, and arrests in metaphase II (MII) after ovulation. The mature, MII oocytes are then ready for fertilization and to provide materials for early embryonic development. Proteomic characterization of oocytes can help identify proteins that are important for female meiotic maturation and early embryonic development. In this study, we compared the proteomic profiles between the germinal vesicle and MII mouse oocytes by two-dimensional electrophoresis; 95 differentially expressed protein spots corresponding to 63 proteins were identified. Many of these proteins are known to be essential for oocyte meiosis and early embryonic development, such as adenylosuccinate synthetase, nucleoplasmin-2, and protein-arginine deiminase type-6. Of the 12 proteins that were identified and are highly expressed in oocytes, a novel protein, E330034G19Rik, was found to be oocyte-specific. According to analysis by bioinformatics, it may regulate chromosome segregation during meiosis or cleavage. An in-depth study of these proteins will help us better understand the mechanisms of oocyte meiotic maturation, fertilization, and early embryogenesis. It will also help us understand the mechanisms of diseases that stem from abnormal oocyte maturation, such as polycystic ovary syndrome and premature ovary failure.

Canonical and Alternative Pathways in Cyclin-Dependent Kinase 1/Cyclin B Inactivation upon M-Phase Exit in Xenopus laevis Cell-Free Extracts

Cyclin-Dependent Kinase 1 (CDK1) is the major M-phase kinase known also as the M-phase Promoting Factor or MPF. Studies performed during the last decade have shown many details of how CDK1 is regulated and also how it regulates the cell cycle progression. Xenopus laevis cell-free extracts were widely used to elucidate the details and to obtain a global view of the role of CDK1 in M-phase control. CDK1 inactivation upon M-phase exit is a primordial process leading to the M-phase/interphase transition during the cell cycle. Here we discuss two closely related aspects of CDK1 regulation in Xenopus laevis cell-free extracts: firstly, how CDK1 becomes inactivated and secondly, how other actors, like kinases and phosphatases network and/or specific inhibitors, cooperate with CDK1 inactivation to assure timely exit from the M-phase.

Inhibition of the ubiquitin-proteasome system leads to delay of the onset of ZGA gene expression

In mammalian oocytes, the ubiquitin-proteasome system (UPS) is suggested to play important roles in oocyte meiosis resumption, spindle assembly, polar body emission and pronuclear formation by regulating cyclin B1 degradation. However, little is known about the direct relationship between zygotic gene activation (ZGA) and degradation of maternal proteins. Here, we investigated the role of the UPS in the onset of ZGA in early mouse embryos. First, we found degradation of cyclin B1 protein in fertilized oocytes at 1 hpi by western blot analysis and used these oocytes throughout this study. Subsequently, we determined optimal experimental conditions for transient inhibition of proteasomal activity by specific and reversible proteasomal inhibitor MG132 in the G1 phase of the first cell cycle. Under the selected optimal conditions, we subjected transient MG132-treated embryos to reverse transcription (RT)-PCR analysis of expression of four ZGA genes, i.e., the hsp70.1, MuERV-L, eif-1a and zscan4d genes. As a result, we found that onset of expression of the four examined ZGA genes was delayed in both normally developed 2-cell embryos and arrested 1-cell embryos. Our results indicate that proteasomal degradation of proteins by the UPS plays a pivotal role in the molecular mechanisms of ZGA in early mouse embryos.

Gene expression during the oocyte-to-embryo transition in mammals

The seminal question in modern developmental biology is the origins of new life arising from the unification of sperm and egg. The roots of this question begin from 19th to 20th century embryologists studying fertilization and embryogenesis. Although the revolution of molecular biology has yielded significant insight into the complexity of this process, the overall orchestration of genes, molecules, and cells is still not fully formed. Early mammalian development, specifically the oocyte-to-embryo transition, is essentially under "maternal command" from factors deposited in the cytoplasm during oocyte growth, independent of de novo transcription from the nascent embryo. Many of the advances in understanding this developmental period occurred in tandem with application of new methods and techniques from molecular biology, from protein electrophoresis to sequencing and assemblies of whole genomes. From this bed of knowledge, it appears that precise control of mRNA translation is a key regulator coordinating the molecular and cellular events occurring during oocyte-to-embryo transition. Notably, oocyte transcriptomes share, yet retain some uniqueness, common genetic motifs among all chordates. The common genetic motifs typically define fundamental processes critical for cellular maintenance, whereas the unique genetic features may be a source of variation and a substrate for sexual selection, genetic drift, or gene flow. One purpose for this complex interplay among genes, proteins, and cells may allow for evolution to transform and act upon the underlying processes, at molecular, structural and organismal levels, to increase diversity, which is the ultimate goal of sexual reproduction.

Cell death by bortezomib-induced mitotic catastrophe in natural killer lymphoma cells

The proteasome inhibitor bortezomib (PS-341/Velcade) is used for the treatment of relapsed and refractory multiple myeloma and mantle-cell lymphoma. We recently reported its therapeutic potential against natural killer (NK)-cell neoplasms. Here, we investigated the molecular mechanisms of bortezomib-induced cell death in NK lymphoma cells. NK lymphoma cell lines (SNK-6 and NK-YS) and primary cultures of NK lymphomas treated with bortezomib were examined for alterations in cell viability, apoptosis, cellular senescence, and cell cycle status. Bortezomib primarily induced mitochondrial apoptosis in NK-YS cells and in primary lymphoma cells at the same concentration as reported in myeloma cells. Unexpectedly, SNK-6 cells required a significantly higher median inhibitory concentration of bortezomib (23 nmol/L) than NK-YS and primary lymphoma cells (6-13 nmol/L). Apoptosis was limited in SNK-6 cells due to the extensively delayed turnover of Bcl-2 family members. These cells were killed by bortezomib, albeit at higher pharmacologic concentrations, via mitotic catastrophe-a mitotic cell death associated with M-phase arrest, cyclin B1 accumulation, and increased CDC2/CDK1 activity. Our results suggest that, in addition to cell death by apoptosis at lower bortezomib concentrations, NK lymphoma cells resistant to bortezomib-induced apoptosis can be killed via mitotic catastrophe, an alternative cell death mechanism, at higher pharmacologic concentrations of bortezomib. Hence, activating mitotic catastrophe by bortezomib may provide a novel therapeutic approach for treating apoptosis-resistant NK-cell malignancies and other cancers.

Fertilization differently affects the levels of cyclin B1 and M-phase promoting factor activity in maturing and metaphase II mouse oocytes

Fertilization affects levels of cyclin B1 and M-phase promoting factor (MPF) activity in maturing and metaphase II mouse oocytes in two distinct ways. In metaphase II oocytes, it leads to a Ca2+-dependent, continuous degradation of cyclin B1 and inactivation of cyclin dependent kinase (CDC2A)–cyclin B1 complex (MPF). In this paper, we show that neither mono- nor polyspermic fertilization of prometaphase I and metaphase I oocytes triggered degradation of cyclin B1. However, polyspermic fertilization of prometaphase I oocytes led to a transient decrease in MPF activity that lasted for 2 h. The inactivation of MPF in polyspermic prometaphase I oocytes did not depend on the fertilization-induced increase in the cytoplasmic concentration of free Ca2+ions, but was caused, at least in part, by dephosphorylation of CDC2A at threonine 161 (Thr161). We found that polyspermic fertilization did not affect glutathione levels in prometaphase I oocytes, and concluded that the decrease in MPF activity and dephosphorylation of CDC2A at Thr161 in polyspermic prometaphase I oocytes were not caused by a change in the redox status of the cell induced by an introduction of excessive amount of sperm protamines. Instead, we propose that inactivation of MPF activity in polyspermic maturing oocytes is caused by a change in nucleo-cytoplasmic ratio that leads to a ‘titration’ of kinases and phosphatases responsible for keeping MPF in an active state. This idea is supported by the finding that oocytes fused with thymocytes rather than spermatozoa also showed a transient decrease in MPF activity.

Role of proteasomes in cellular regulation

The 26S proteasome is the key enzyme of the ubiquitin-dependent pathway of protein degradation. This energy-dependent nanomachine is composed of a 20S catalytic core and associated regulatory complexes. The eukaryotic 20S proteasomes demonstrate besides several kinds of peptidase activities, the endoribonuclease, protein-chaperone and DNA-helicase activities. Ubiquitin-proteasome pathway controls the levels of the key regulatory proteins in the cell and thus is essential for life and is involved in regulation of crucial cellular processes. Proteasome population in the cell is structurally and functionally heterogeneous. These complexes are subjected to tightly organized regulation, particularly, to a variety of posttranslational modifications. In this review we will summarize the current state of knowledge regarding proteasome participation in the control of cell cycle, apoptosis, differentiation, modulation of immune responses, reprogramming of these particles during these processes, their heterogeneity and involvement in the main levels of gene expression.

Exposure of mouse oocytes to bisphenol A causes meiotic arrest but not aneuploidy

Mouse oocytes isolated from large antral follicles were exposed to a wide range of concentrations of bisphenol A (BPA) during maturation in vitro (50 ng/ml to 10 microg/ml BPA in medium). Exposure to high concentrations of BPA (10 microg/ml) affected spindle formation, distribution of pericentriolar material and chromosome alignment on the spindle (termed congression failure), and caused a significant meiotic arrest. However, BPA did not increase hyperploidy at meiosis II at any tested concentration. Some but not all meiosis I arrested oocytes had MAD2-positive foci at centromeres of chromosomes in bivalents, suggesting that they had failed to pass the spindle checkpoint control. In a second set of experiments prepubertal mice were exposed sub-chronically for 7 days to low BPA by daily oral administration, followed by in vitro maturation of the denuded oocytes to metaphase II in the absence of BPA, as this treatment protocol was previously reported to induce chromosome congression failure and therefore suspected to cause aneuploidy in oocytes. The sub-chronic exposure subtly affected spindle morphology and oocyte maturation. However, as with the exposure in vitro, there was no evidence that low BPA doses increased hyperploidy at meiosis II. In conclusion, the data suggest that mouse oocytes from mice respond to BPA-induced disturbances in spindle formation by induction of meiotic arrest. This response might result from an effective checkpoint mechanism preventing the occurrence of chromosome malsegregation and aneuploidy. Low chronic BPA exposure in vivo as such does not appear to pose a risk for induction of errors in chromosome segregation at first meiosis in mouse oocytes. Additional factors besides BPA may have caused the high rate of congression failure and the temporary increase in hyperploidy in mouse metaphase II oocytes reported previously.

Regulation of keratinocyte proliferation in rats with deep, partial-thickness scald: modulation of cyclin D1-cyclin-dependent kinase 4 and histone H1 kinase activity of M-phase promoting factor

BACKGROUND

Keratinocyte proliferation, which is undergone by its cell cycle transition, is considered a major event during re-epithelialization over the wound size. Cyclins, cyclin-dependent kinases, and cyclin-dependent kinase inhibitors interact to regulate the cell cycle. We investigated proliferative events associated with cell-cycle control in keratinocytes during wound healing in rats with deep, partial scald injuries.

MATERIALS AND METHODS

Male Sprague Dawley rats with starting weights of 200 to 220 g were inflicted with standardized deep partial-thickness burns by scalding 10% of the skin surface. The full thickness skin biopsies were harvested for histological evaluation at following time points: 0 d, post-burn day 3, post-burn day 7, and post-burn day 14. Keratinocytes from wound edge were isolated for cell cycle examination. The cell cycle regulators and their activity were detected.

RESULTS

Keratinocytes tended to proliferate and had enlarged nuclei and nucleoli from day 3 after injury. Morphological features became evident on day 14, with an increase in keratinocytes. The percentage of S-phase keratinocytes tended to increase on day 14. The percentage G2/M-phase keratinocytes increased from day 3 and significantly increased on days 7 and 14. Cyclin D1 expression markedly increased from day 3, with down-regulation of cyclin-dependent kinase 4, which re-elevated on day 14. Cyclin B1 expression did not dramatically vary. Histone H1 kinase activity of mitosis phase promoting factor markedly increased on day 14.

CONCLUSIONS

These findings suggested early, active DNA synthesis and mitosis in keratinocytes, with marked proliferation on day 14, that depended on the modulation of cyclin D1-cyclin-dependent kinase 4 and histone H1 kinase activity of mitosis phase promoting factor. During wound healing, patterns of cell-cycle control expression differed from those previously known.

Comparative proteome analysis of changes in the 26S proteasome during oocyte maturation in goldfish

Proteasomes are large, multi-subunit particles that act as the proteolytic machinery for most of the regulated intracellular protein degradation in eukaryotic cells. An alteration of proteasome function may be important for the regulation of the meiotic cell cycle. To study the change at the subunit level of the 26S proteasome during meiotic maturation, we purified 26S proteasomes from immature and mature oocytes of goldfish. Two-dimensional polyacrylamide gel electrophoresis was used to separate proteins. For differential analysis, whole spots of the 26S proteasome from goldfish oocytes were identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry and database analysis. Four spots that were different (only detected in mature oocyte 265 proteasomes and not in immature ones) and four protein spots that were up- or down-regulated were identified unambiguously. The mature-specific spots were not 26S proteasome components but rather their interacting proteins, and were identified as chaperonin-containing TCP-1 subunits and myosin light chain. Minor spots of three subunits of the 20S core particle and one of the 19S regulatory particle showed meiotic cell cycle-dependent changes. These results demonstrate that modifications of proteasomal subunits and cell cycle phase-dependent interactions of proteins with proteasomes occur during oocyte maturation in goldfish.

Mad2 is required for inhibiting securin and cyclin B degradation following spindle depolymerisation in meiosis I mouse oocytes

Mad2 is a pivotal component of the spindle assembly checkpoint (SAC) which inhibits anaphase promoting complex/cyclo-some (APC/C) activity by sequestering Cdc20 thereby regulating the destruction of securin and cyclin B. During mitosis, spindle depolymerisation induces a robust Mad2-dependent arrest due to inhibition of securin and cyclin B destruction. In contrast to mitosis, the molecular details underpinning the meiosis I arrest experienced by mouse oocytes exposed to spindle depolymerisation remain incompletely characterised. Notably, the role of Mad2 and the fate of the anaphase-marker, securin, are unexplored. As shown previously, we find that spindle depolymerisation by nocodazole inhibits first polar body extrusion (PBE) and stabilises cyclin B and cyclin-dependent kinase 1 activity in mouse oocytes. Here we show that stabilisation of cyclin B in nocodazole can be sustained for several hours and is associated with stabilisation of securin. These effects are SAC-mediated as, in oocytes depleted of the majority of Mad2 by morpholino antisense, securin and cyclin B are destabilised and 15% of oocytes undergo PBE. This reflects premature APC/C activation as a mutant form of cyclin B lacking its APC/C degradation signal is stable in Mad2-depleted oocytes. Moreover, homologues do not disjoin during the prolonged meiosis I arrest (> 18 h) induced by nocodaozole indicating that a non-cleavage mechanism is insufficient on its own for resolution of arm cohesion in mammalian oocytes. In conclusion, when all kinetochores lack attachment and tension, mouse oocytes mount a robust Mad2-dependent meiosis I arrest which inhibits the destruction of securin and cyclin B.

The effects of proteasome inhibitor lactacystin on mouse oocyte meiosis and first cleavage

In order to study the effects of ubiquitin-proteasome pathway (UPP) on mouse oocyte meiosis and cleavage, oocytes undergoing maturation and parthenogenetic activation and 1-cell embryos were treated with lactacystin, a specific inhibitor of proteasome. The results indicated that the rate of GVBD was not influenced by the treatment, but polar body extrusion, parthenogenesis and first cleavage were inhibited. Immunofluorescent staining using anti beta-tubulin antibody indicated that the continuous treatment of lactacystin from GV stage disorganized microtubules and spindle assembly. When metaphase stage oocytes were treated with the drug, the already formed spindle structure was not affected, but the oocytes were arrested at metaphases. The 1-cell embryos were arrested at interphase or metaphase of first mitosis when they were incubated in the drug. Proteasome regulatory subunit PA700 was located in the spindle region, as indicated by immunofluorescence. These results suggest that UPP has effects on the process of oocyte meiosis and early cleavage in many aspects, including normal organization of spindle at prophase and segregation of chromosomes at anaphase for normal meiosis.

Inducible nitric oxide synthase-derived nitric oxide regulates germinal vesicle breakdown and first polar body emission in the mouse oocyte

The present study investigated the subcellular localization of inducible nitric oxide synthase (iNOS) during mouse oocyte meiotic maturation and fertilization using confocal microscopy, and further studied the roles of iNOS-derived NO in oocyte maturation by using an iNOS-specific inhibitor aminoguanidine (AG) and iNOS antibody microinjection. In germinal vesicle-stage oocytes, iNOS immunoreactivity was mainly localized in the germinal vesicle. Shortly after germinal vesicle breakdown, the iNOS immunoreactivity accumulated around the condensed chromosomes. At metaphase I and metaphase II, with the organization of chromosomes to the equatorial plate, iNOS immunoreactivity was concentrated around the aligned chromosomes, putatively the position of the metaphase spindle. The accumulation of iNOS immunoreactivity could not be detected at anaphase I and anaphase II. However, at telophase I and telophase II, the staining of iNOS was concentrated in the region between the separating chromosomes/chromatids. Furthermore, the staining of iNOS also accumulated in the male and female pronuclei in fertilized eggs. Germinal vesicle breakdown and the first polar body emission of the oocytes were significantly blocked by the iNOS-specific inhibitor AG in a dose-dependent manner. The germinal vesicle breakdown in oocytes injected with iNOS antibody was also inhibited. We found that the phosphorylation of mitogen-activated protein kinase in oocytes after germinal vesicle breakdown was inhibited by AG treatment. The control oocytes extruded a normal first polar body, while the AG-treated oocytes exhibited an elongated protrusion or no elongated protrusion. The results of confocal microscopy showed that the AG-treated oocytes were arrested at anaphase I–telophase I. Our results suggest that the iNOS-derived NO pathway plays important roles in mouse oocyte meiotic maturation, especially in germinal vesicle breakdown and the anaphase–telophase transition.

Meiotic maternal chromosomes introduced to the late mouse zygote are recruited to later embryonic divisions

The aim of this study was to investigate the fate of an additional female genome introduced to a dividing zygote. Maternal chromatin in the form of karyoplasts containing a metaphase II spindle were fused to zygotes blocked in anaphase or telophase of the first cleavage. Permanent preparations made 20-40 min after fusion at anaphase revealed that the donor maternal chromosomes had entered anaphase or telophase in 16 out of 18 cases. A further two groups of embryos that were fused at either anaphase or anaphase/telophase were cultured to the first division. Division occurred 50 min after fusion in both groups of embryos (86 and 85.1%, respectively), of which most divided to two cells (80 and 71.6% of total) and the remainder divided to three cells. About two thirds of two-cell embryos contained an extra nucleus in one blastomere. Nuclei containing donor maternal chromosomes reached a similar size to recipient nuclei in 68% of embryos derived from anaphase-blocked zygotes, in contrast to 31.1% of embryos derived from anaphase/telophase-blocked embryos. Replication of DNA in donor nuclei closely followed the timing and intensity of that in control embryos. When fixed 24 hr after fusion, one third of embryos were still at the two-cell stage, with one or both blastomeres showing a single metaphase plate of the second cleavage. In the remaining embryos, three or four cells were present, some containing two nuclei. Blastocysts developed in 50% of fused embryos and three young were born after transfer of cleaving hybrid embryos to recipients. Chromosome preparations from bone marrow of the young contained 3-4 tetraploid metaphase plates per several hundred plates counted compared with none in control embryos. In conclusion, additional maternal chromosomes can be introduced at the late-dividing zygote and join the embryonic cell cycles during subsequent divisions. This method may provide a useful approach for studying changes specific to the maternal genome during early cell cycles of the mammalian embryo.

Ubiquitin-proteasome pathway modulates mouse oocyte meiotic maturation and fertilization via regulation of MAPK cascade and cyclin B1 degradation

Degradation of proteins mediated by ubiquitin-proteasome pathway (UPP) plays important roles in the regulation of eukaryotic cell cycle. In this study, the functional roles and regulatory mechanisms of UPP in mouse oocyte meiotic maturation, fertilization, and early embryonic cleavage were studied by drug-treatment, Western blot, antibody microinjection, and confocal microscopy. The meiotic resumption of both cumulus-enclosed oocytes and denuded oocytes was stimulated by two potent, reversible, and cell-permeable proteasome inhibitors, ALLN and MG-132. The metaphase I spindle assembly was prevented, and the distribution of ubiquitin, cyclin B1, and polo-like kinase 1 (Plk1) was also distorted. When UPP was inhibited, mitogen-activated protein kinase (MAPK)/p90rsk phosphorylation was not affected, but the cyclin B1 degradation that occurs during normal metaphase-anaphase transition was not observed. During oocyte activation, the emission of second polar body (PB2) and the pronuclear formation were inhibited by ALLN or MG-132. In oocytes microinjected with ubiquitin antibodies, PB2 emission and pronuclear formation were also inhibited after in vitro fertilization. The expression of cyclin B1 and the phosphorylation of MAPK/p90rsk could still be detected in ALLN or MG-132-treated oocytes even at 8 h after parthenogenetic activation or insemination, which may account for the inhibition of PB2 emission and pronuclear formation. We also for the first time investigated the subcellular localization of ubiquitin protein at different stages of oocyte and early embryo development. Ubiquitin protein was accumulated in the germinal vesicle (GV), the region between the separating homologous chromosomes, the midbody, the pronuclei, and the region between the separating sister chromatids. In conclusion, our results suggest that the UPP plays important roles in oocyte meiosis resumption, spindle assembly, polar body emission, and pronuclear formation, probably by regulating cyclin B1 degradation and MAPK/p90rsk phosphorylation.

Regulation of ubiquitin-proteasome pathway on pig oocyte meiotic maturation and fertilization

Degradation of proteins mediated by the ubiquitin-proteasome pathway (UPP) plays essential roles in the eukaryotic cell cycle. The main aim of the present study was to analyze the functional roles and regulatory mechanisms of the UPP in pig oocyte meiotic maturation, activation, and early embryo mitosis by drug treatment, Western blot analysis, and confocal microscopy. By using the hypoxanthine-maintained meiotic arrest model, we showed that the meiotic resumption of both cumulus-enclosed oocytes and denuded oocytes was stimulated in a dose- and time-dependent manner by two potent and cell-permeable proteasome inhibitors. Both the mitogen-activated protein kinase (MAPK) kinase inhibitor U0126 and the maturation-promoting factor inhibitor roscovitine overcame the stimulation of germinal vesicle breakdown induced by proteasome inhibitors. The phosphorylation of MAPK and p90rsk and the expression of cyclin B1 increased in a dose- and time-dependent manner when treated with proteasome inhibitors during oocyte in vitro-maturation culture. Both U0126 and roscovitine inhibited the phosphorylation of MAPK and p90rsk, and the synthesis of cyclin B1 stimulated by proteasome inhibitors. When matured oocytes were pretreated with proteasome inhibitors and then fertilized or artificially activated, the second polar body emission and the pronuclear formation were inhibited, and the dephosphorylation of MAPK and p90rsk as well as the degradation of cyclin B1 that should occur after oocyte activation were also inhibited. We also investigated, to our knowledge for the first time, the subcellular localization of 20S proteasome alpha subunits at different stages of oocyte and early embryo development. The 20S proteasome alpha subunits were accumulated in the germinal vesicle, around the condensed chromosomes at prometaphase, with spindle at metaphase I and II, the region between the separating chromosomes, and especially the midbody at anaphase I and telophase I, the pronucleus, and the nucleus in early embryonic cells. In conclusion, our results suggest that the UPP is important at multiple steps of pig oocyte meiosis, fertilization, and early embryonic mitosis and that it may play its roles by regulating cyclin B1 degradation and MAPK/p90rsk phosphorylation.

Involvement of mitogen-activated protein kinase cascade during oocyte maturation and fertilization in mammals

Mitogen-activated protein kinase (MAPK) is a family of Ser/Thr protein kinases that are widely distributed in eukaryotic cells. Studies in the last decade revealed that MAPK cascade plays pivotal roles in regulating the meiotic cell cycle progression of oocytes. In mammalian species, activation of MAPK in cumulus cells is necessary for gonadotropin-induced meiotic resumption of oocytes, while MAPK activation is not required for spontaneous meiotic resumption. After germinal vesicle breakdown (GVBD), MAPK is involved in the regulation of microtubule organization and meiotic spindle assembly. The activation of this kinase is essential for the maintenance of metaphase II arrest, while its inactivation is a prerequisite for pronuclear formation after fertilization or parthenogenetic activation. MAPK cascade interacts extensively with other protein kinases such as maturation-promoting factor, protein kinase A, protein kinase C, and calmodulin-dependent protein kinase II, as well as with protein phosphatases in oocyte meiotic cell cycle regulation. The cross talk between MAPK cascade and other protein kinases is discussed. The review also addresses unsolved problems and discusses future directions.

Regulated interaction between polypeptide chain elongation factor-1 complex with the 26S proteasome during Xenopus oocyte maturation

BACKGROUND

During Xenopus oocyte maturation, the amount of a 48 kDa protein detected in the 26S proteasome fraction (p48) decreased markedly during oocyte maturation to the low levels seen in unfertilized eggs. The results indicate that the interaction of at least one protein with the 26S proteasome changes during oocyte maturation and early development. An alteration in proteasome function may be important for the regulation of developmental events, such as the rapid cell cycle, in the early embryo. In this study, we identified p48.

RESULTS

p48 was purified by conventional column chromatography. The resulting purified fraction contained two other proteins with molecular masses of 30 (p30) and 37 (p37) kDa. cDNAs encode elongation factor-1gamma and delta were obtained by an immuno-screening method using polyclonal antibodies against purified p48 complex, which recognized p48 and p37. N-terminal amino acid sequence analysis of p30 revealed that it was identical to EF-1beta. To identify the p48 complex bound to the 26S proteasome as EF-1betagammadelta, antibodies were raised against the components of purified p48 complex. Recombinant EF-1 beta,gamma and delta were expressed in Escherichia coli, and an antibody was raised against purified recombinant EF-1gamma. Cross-reactivity of the antibodies toward the p48 complex and recombinant proteins showed it to be specific for each component. These results indicate that the p48 complex bound to the 26S proteasome is the EF-1 complex. MPF phosphorylated EF-1gamma was shown to bind to the 26S proteasome. When EF-1gamma is phosphorylated by MPF, the association is stabilized.

CONCLUSION

p48 bound to the 26S proteasome is identified as the EF-1gamma. EF-1 complex is associated with the 26S proteasome in Xenopus oocytes and the interaction is stabilized by MPF-mediated phosphorylation.

Maturation-promoting factor governs mitogen-activated protein kinase activation and interphase suppression during meiosis of rat oocytes

Meiosis is a particular example of a cell cycle, characterized by two successive divisions without an intervening interphase. Resumption of meiosis in oocytes is associated with activation of maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK). The activity of MPF declines during the transition between the two meiotic divisions, whereas the activity of MAPK is sustained. Attempts to disclose the interplay between these key regulators of meiosis in both amphibian and mammalian oocytes generated contradictory results. Furthermore, the enzyme that governs the suppression of interphase in mammals is still unidentified. To our knowledge, we provide herein the first demonstration in a mammalian system that inhibition of MPF at reinitiation of meiosis abrogated Mos expression and MAPK activation. We also show that oocytes, in which reactivation of MPF at completion of the first telophase was prevented, exhibited an interphase nucleus with decondensed chromosomes. Inhibition of MAPK did not interfere with the progression to the second meiotic metaphase but, rather, resulted in parthenogenic activation. We conclude that in rat oocytes, MPF regulates MAPK activation and its timely reactivation prevents the oocytes from entering interphase.

Role and function of the 26S proteasome in proliferation and apoptosis

The 26S proteasome constitutes the central proteolytic machinery of the highly conserved ubiquitin/proteasome system, the cell's major tool for extralysosomal protein degradation. Recently, a plethora of cell proteins implicated in the regulation of basic cellular processes, such as proliferation, differentiation, cell cycling, and apoptosis have been discovered to undergo processing and functional limitation by entering the ubiquitin/proteasome pathway with the final destination to be proteolytically degraded by the 26S proteasome. Because both negative and positive regulators of proliferation and apoptosis undergo proteasomal degradation in a tightly regulated and temporally controlled fashion, the 26S proteasome can play opposite roles in the regulation of proliferation and apoptosis. These roles are apparently defined by the cell's environment and proliferative state. Finally, proteasomal protein degradation is deregulated in a number of human diseases, including cancer and neurodegenerative and myodegenerative diseases, which all exhibit an imbalance of proliferation and apoptosis. An improved understanding of the modes of proteasomal action should lead to the development of beneficial therapeutic and diagnostic strategies in the future.

Translational and post-translational modifications in meiosis of the mammalian oocyte

The fully-grown oocyte is transcriptionally inactive. Therefore, translational and post-translational modifications furnish the control mechanism of key components governing meiosis. Regulation by protein synthesis provides an irreversible unidirectional mechanism for an extended period that can be restricted by a complementary degradation of the same protein. Both processes utilize tight measures to ensure precise expression at the right time in the right place. Rapid modifications such as phosphorylation and dephosphorylation supply reversible means to regulate protein action. Information regarding these extremely exciting issues is being accumulated recently in an exponential rate. However, the vast majority of these data is generated from studies conducted on Xenopus oocytes. We fully agree with Andrew Murray's statement that "The modern trend of promoting research on a small number of 'model' organisms will eventually deprive us of the opportunity to study interesting biology" [Cell 92 (1992) 157]. Thus, despite of the enormous technical difficulties resulting from the limited availability of biological material we extended our interest to mammalian model systems. Our review will attend to certain examples of such modifications in the regulatory pathway of meiosis in mammalian oocytes.