The role of cyclin B in meiosis I

Molecular cloning and characterization of a cyclin B gene on the ovarian maturation stage of black tiger shrimp (Penaeus monodon)

The techniques of homology cloning and anchored PCR were used to clone the cyclin B gene from black tiger shrimp. The full length cDNA of black tiger shrimp cyclin B (btscyclin B) contained a 5' untranslated region (UTR) of 102 bp, an ORF of 1,206 bp encoding a polypeptide of 401 amino acids with an estimated molecular mass of 45 kDa and a 3' UTR of 396 bp. The searches for protein sequence similarities with BLAST analysis indicated that the deduced amino acid sequence of btscyclin B was homological to the cyclin B of other species and even the mammalians. Two conserved signature sequences of cyclin B gene family were found in the btscyclin B deduced amino acid sequence. The temporal expressions of cyclin B gene in the different tissues, including liver, ovary, muscle, brain stomach, heart and intestine, were measured by RT-PCR. mRNA expression of cyclin B could be detected in liver, ovary, muscle, brain, stomach, heart and strongest in the ovary, but almost not be detected in the intestine. In ovarian maturation stages, the expression of btscyclin B was different. The result indicated that btscyclin B was constitutive expressed and played an important role in the cell division stage.

Cyclin B protein undergoes increased expression and nuclear relocation during oocyte meiotic maturation of the freshwater prawn Macrobrachium rosenbergii and the Chinese mitten crab Eriocheir sinensis

Cyclin B functions as a regulatory protein through association with its catalytic partner Cdc2 kinase forming M-phase promoting factor (MPF), which plays a central role in the meiotic maturation of oocyte. To gain insight into the molecular events, we here cloned a cyclin B cDNA from the ovary of the prawn Macrobrachium rosenbergii and compared its spatial-temporal expression patterns during oocyte maturation with those of crab Eriocheir sinensis. The prawn cyclin B cDNA encodes a 398 amino acid protein with predicted molecular weight of 45.16 kDa. Immunodetection of cyclin B protein by Western blot showed that a target band of approximately 53 kDa protein in the prawn ovaries at both late vitellogenesis (lVt) and germinal vesicle breakdown (GVBD) stages, whereas a 41 kDa band was present in the crab ovaries. Cyclin B protein expression changes indicating that the newly synthesis of cyclin B proteins could be required for GVBD in both prawn and crab. Immunohistochemical analysis revealed that both the prawn and crab cyclin B proteins, were localized in the ooplasm of previtellogenic oocytes, then relocated into germinal vesicle at vitellogenesis stage and localized on meiotic spindle at M phase. These similar behaviors suggested that the prawn and the crab cyclin B proteins associated with Cdc2 kinase have conserved roles in inducing GVBD and regulating the formation of meiotic spindle. The similar expression patterns of the cyclin B proteins during oocyte maturation implicated that the molecular mechanisms for MPF activation could be identical between the prawn and the crab.

Mammalian cell cycle cyclins

Proper progression throughout the cell division cycle depends on the expression level of a family of proteins known as cyclins, and the subsequent activation of cyclin-dependent kinases (Cdks). Among the numerous members of the mammalian cyclin family, only a few of them, cyclins A, B, C, D and E, are known to display critical roles in the cell cycle. These functions will be reviewed here with a special focus on their relevance in different cell types in vivo and their implications in human disease.

Identifying potential maternal genes of Bombyx mori using digital gene expression profiling

Maternal genes present in mature oocytes play a crucial role in the early development of silkworm. Although maternal genes have been widely studied in many other species, there has been limited research in Bombyx mori. High-throughput next generation sequencing provides a practical method for gene discovery on a genome-wide level. Herein, a transcriptome study was used to identify maternal-related genes from silkworm eggs. Unfertilized eggs from five different stages of early development were used to detect the changing situation of gene expression. The expressed genes showed different patterns over time. Seventy-six maternal genes were annotated according to homology analysis with Drosophila melanogaster. More than half of the differentially expressed maternal genes fell into four expression patterns, while the expression patterns showed a downward trend over time. The functional annotation of these material genes was mainly related to transcription factor activity, growth factor activity, nucleic acid binding, RNA binding, ATP binding, and ion binding. Additionally, twenty-two gene clusters including maternal genes were identified from 18 scaffolds. Altogether, we plotted a profile for the maternal genes of Bombyx mori using a digital gene expression profiling method. This will provide the basis for maternal-specific signature research and improve the understanding of the early development of silkworm.

The potential role of As-sumo-1 in the embryonic diapause process and early embryo development of Artemia sinica

During embryonic development of Artemia sinica, environmental stresses induce the embryo diapause phenomenon, required to resist apoptosis and regulate cell cycle activity. The small ubiquitin-related modifier-1 (SUMO), a reversible post-translational protein modifier, plays an important role in embryo development. SUMO regulates multiple cellular processes, including development and other biological processes. The molecular mechanism of diapause, diapause termination and the role of As-sumo-1 in this processes and in early embryo development of Artemia sinica still remains unknown. In this study, the complete cDNA sequences of the sumo-1 homolog, sumo ligase homolog, caspase-1 homolog and cyclin B homolog from Artemia sinica were cloned. The mRNA expression patterns of As-sumo-1, sumo ligase, caspase-1, cyclin B and the location of As-sumo-1 were investigated. SUMO-1, p53, Mdm2, Caspase-1, Cyclin B and Cyclin E proteins were analyzed during different developmental stages of the embryo of A. sinica. Small interfering RNA (siRNA) was used to verify the function of sumo-1 in A. sinica. The full-length cDNA of As-sumo-1 was 476 bp, encoding a 92 amino acid protein. The As-caspases-1 cDNA was 966 bp, encoding a 245 amino-acid protein. The As-sumo ligase cDNA was 1556 bp encoding, a 343 amino acid protein, and the cyclin B cDNA was 739 bp, encoding a 133 amino acid protein. The expressions of As-sumo-1, As-caspase-1 and As-cyclin B were highest at the 10 h stage of embryonic development, and As-sumo ligase showed its highest expression at 0 h. The expression of As-SUMO-1 showed no tissue or organ specificity. Western blotting showed high expression of As-SUMO-1, p53, Mdm2, Caspase-1, Cyclin B and Cyclin E at the 10 h stage. The siRNA caused abnormal development of the embryo, with increased malformation and mortality. As-SUMO-1 is a crucial regulation and modification protein resumption of embryonic diapause and early embryo development of A. sinica.

BmCyclin B and BmCyclin B3 are required for cell cycle progression in the silkworm, Bombyx mori

Cyclin B is an important regulator of the cell cycle G2 to M phase transition. The silkworm genomic database shows that there are two Cyclin B genes in the silkworm (Bombyx mori), BmCyclin B and BmCyclin B3. Using silkworm EST data, the cyclin B3 (EU074796) gene was cloned. Its complete cDNA was 1665 bp with an ORF of 1536 bp derived from seven exons and six introns. The BmCyclin B3 gene encodes 511 amino acids, and the predicted molecular weight is 57.8 kD with an isoelectric point of 9.18. The protein contains one protein damage box and two cyclin boxes. RNA interference-mediated reduction of BmCyclin B and BmCyclin B3 expression induced cell cycle arrest in G2 or M phase in BmN-SWU1 cells, thus inhibiting cell proliferation. These results suggest that BmCyclin B and BmCyclin B3 are necessary for completing the cell cycle in silkworm cells.

Spatial regulation of APCCdh1-induced cyclin B1 degradation maintains G2 arrest in mouse oocytes

Within the mammalian ovary, oocytes remain arrested at G2 for several years. Then a peri-ovulatory hormonal cue triggers meiotic resumption by releasing an inhibitory phosphorylation on the kinase Cdk1. G2 arrest, however, also requires control in the concentrations of the Cdk1-binding partner cyclin B1, a process achieved by anaphase-promoting complex (APC(Cdh1)) activity, which ubiquitylates and so targets cyclin B1 for degradation. Thus, APC(Cdh1) activity prevents precocious meiotic entry by promoting cyclin B1 degradation. However, it remains unresolved how cyclin B1 levels are suppressed sufficiently to maintain arrest but not so low that they make oocytes hormonally insensitive. Here, we examined spatial control of this process by determining the intracellular location of the proteins involved and using nuclear-targeted cyclin B1. We found that raising nuclear cyclin B1 concentrations, an event normally observed in the minutes before nuclear envelope breakdown, was a very effective method of inducing the G2/M transition. Oocytes expressed only the alpha-isoform of Cdh1, which was predominantly nuclear, as were Cdc27 and Psmd11, core components of the APC and the 26S proteasome, respectively. Furthermore, APC(Cdh1) activity appeared higher in the nucleus, as nuclear-targeted cyclin B1 was degraded at twice the rate of wild-type cyclin B1. We propose a simple spatial model of G2 arrest in which nuclear APC(Cdh1)-proteasomal activity guards against any cyclin B1 accumulation mediated by nuclear import.

Molecular characterization and expression profiles of cyclin A and cyclin B during ovarian development of the giant tiger shrimp Penaeus monodon

The meiotic maturation of oocytes is regulated by maturation promoting factor (MPF), a complex of cdc2 (Cdk1) and cyclin B and other Cdk/cyclin complexes. To better understand molecular aspects governing reproductive maturation of the giant tiger shrimp (Penaeus monodon), the full length cDNAs and genomic organization of cyclins A and B (PMCyA and PMCyB) were characterized. A single form of PMCyA contained an open reading frame (ORF) of 1326 bp corresponding to a deduced protein of 441 amino acids. Its genomic sequence contained 5 exons, 4 introns and untranslated regions (UTRs) spanning 2586 bp in length. In contrast, PMCyB possessed three isoforms with an identical ORF of 1206 bp (401 amino acids) but three different 3' UTR lengths of 416, 543 and 1117 bp, respectively. Their respective genomic sequences were composed of 8 exons, 7 introns and UTRs covering 4181, 4307 and 4940 bp. Expression levels of both PMCyA and PMCyB in ovaries of broodstock were much greater than those of juveniles (P<0.05). During ovarian development and after spawning of normal shrimp broodstock, PMCyA was not differentially expressed (P>0.05) whereas the level of PMCyB in stage IV was greater than that of stage I ovaries (P<0.05). Unilateral eyestalk ablation, a technique commonly used to induce spawning in P. monodon female brooders, had no effects on transcription of PMCyB (P>0.05) but resulted in a lower expression of PMCyA at stage IV of ovarian development of this economically important species (P<0.05).

Molecular characterization and expression profiles of cyclin B1, B2 and Cdc2 kinase during oogenesis and spermatogenesis in rainbow trout (Oncorhynchus mykiss)

The meiotic maturation of oocyte and spermatocyte in animals is controlled by the maturation promotion factor (MPF), a complex of Cdc2 and cyclin B proteins. To better understand the mechanism of oocyte and spermatocyte maturation in fish, the expression of cyclin B1 (CB1), B2 (CB2) and Cdc2 kinase during oogenesis and spermatogenesis in rainbow trout were examined at both the mRNA and protein levels. Quantitative real-time PCR analysis showed that the amount of CB1 and CB2 mRNA was greater at previtellogenesis and late vitellogenesis stages, but less at early vitellogenesis stage and during early embryogenesis. Cdc2 mRNA was continuously present throughout the processes of oogenesis and early embryogenesis except for a decline at early vitellogenesis. In situ hybridization analysis indicated that CB1, CB2 and Cdc2 transcripts were present in oocytes of different developmental stages as well as in all spermatogenic cells except for spermatogonia. Immunohistochemical analysis revealed that CB1 protein was absent in vitellogenic oocytes, but present in young previtellogenic and mature oocytes. In contrast, CB2 and Cdc2 proteins were present at all stages oocyte development. Similarly, CB2 and Cdc2 proteins were present throughout spermatogenesis, whereas CB1 protein was only detected in spermatogonia and spermatocytes, but not in spermatids. Thus, it appears that CB1, CB2 and Cdc2 transcripts have similar expression patterns during oogenesis and spermatogenesis, but CB1 protein varies in amount during these processes. These data suggest that CB1 may have a leading role in the regulation of meiotic maturation of oocytes and spermotocytes.

Regulation of p34cdc2 protein kinase activity by phosphorylation and cyclin binding

Activation of the protein kinase p34cdc2 is required for entry into meiotic or mitotic M phase in all eukaryotic cells. One important mechanism regulating the activity of p34cdc2 during the cell cycle is based on phosphorylation/dephosphorylation. Avian p34cdc2 is phosphorylated on threonine 14 (Thr14), tyrosine 15 (Tyr15), threonine 161 (Thr161) and serine 277 (Ser277). Dephosphorylation of both Thr14 and Tyr15 is required for activation of p34cdc2 at the G2/M transition, indicating that phosphorylation of these residues negatively regulates p34cdc2 activity. Conversely, phosphorylation of Thr161 is required for kinase activity. Whether modification of this residue is due to intramolecular autophosphorylation or to the action of an as yet unidentified kinase remains unresolved. Likewise, the role of phosphorylation of p34cdc2 on Ser277 during G1 phase of the cell cycle remains to be determined. The function of p34cdc2 is regulated also by cell cycle-dependent complex formation with cyclin proteins. We found that chicken cyclin B2 undergoes a striking redistribution from the cytoplasm to the nucleus just prior to the onset of mitosis. Expression of a non-destructible cyclin B2 mutant causes HeLa cells to arrest in mitosis. Frequently, arrested cells displayed multiple mitotic spindles.

Activation of maturation promoting factor in Bufo arenarum oocytes: injection of mature cytoplasm and germinal vesicle contents

Although progesterone is the established maturation inducer in amphibians, Bufo arenarum oocytes obtained during the reproductive period (spring-summer) resume meiosis with no need of an exogenous hormonal stimulus if deprived of their enveloping follicle cells, a phenomenon called spontaneous maturation. In this species it is possible to obtain oocytes competent and incompetent to undergo spontaneous maturation according to the seasonal period in which animals are captured. Reinitiation of meiosis is regulated by maturation promoting factor (MPF), a complex of the cyclin-dependent kinase p34cdc2 and cyclin B. Although the function and molecule of MPF are common among species, the formation and activation mechanisms of MPF differ according to species. This study was undertaken to evaluate the presence of pre-MPF in Bufo arenarum oocytes incompetent to mature spontaneously and the effect of the injection of mature cytoplasm or germinal vesicle contents on the resumption of meiosis. The results of our treatment of Bufo arenarum immature oocytes incompetent to mature spontaneously with sodium metavanadate (NaVO3) and dexamethasone (DEX) indicates that these oocytes have a pre-MPF, which activates and induces germinal vesicle breakdown (GVBD) by dephosphorylation on Thr-14/Tyr-15 by cdc25 phosphatase and without cyclin B synthesis. The injection of cytoplasm containing active MPF is sufficient to activate an amplification loop that requires the activation of cdc25 and protein kinase C, the decrease in cAMP levels, and is independent of protein synthesis. However, the injection of germinal vesicle content also induces GVBD in the immature receptor oocyte, a process dependent on protein synthesis but not on cdc25 phosphatase or PKC activity.

Three forms of cyclin B transcripts in the ovary of the kuruma prawn Marsupenaeus japonicus: Their molecular characterizations and expression profiles during oogenesis

Cyclin B is a well known regulatory factor that plays a crucial role in mitosis and meiosis. Although the existence of cyclin B has been reported to be universal in a wide variety of eukaryotic organisms, no molecular data are available on crustacean species. In this study, three forms of cyclin B transcripts were first identified and characterized in the ovary of the commercially important kuruma prawn Marsupenaeus japonicus. The three transcripts (2.4, 1.9 and 1.7 kb) shared the identical sequence, with variations only in the length of 3' untranslated regions (UTRs), and coexisted in the ovary as demonstrated by Northern blot analysis. The sequences of 3' UTRs indicated that the distinct length UTRs of the transcripts is attributed to an alternative usage of various polyadenylation signals in the 3' UTR. The open reading frame of 1203 bp encoded a putative 401 amino acid peptide. The deduced amino acid sequence shared 45-50% identities with the known B-type cyclin in other animals. Quantitative real-time RT-PCR revealed that the short transcript (1.7 kb) was the most abundant among the three transcripts, followed by the long (2.4 kb) and medium (1.9 kb), and the three forms of the transcripts displayed various expression profiles during oogenesis. In situ hybridization showed that the short transcript commenced expressing in the ova as early as the oogonia stage and accumulated largely at the perinucleolus (PN) stage, whereas almost no expression was found for the medium and long transcripts at the oogonia stage and moderate signals were detected at the PN stage. The differential expression of the three forms of transcripts suggested that various transcripts might perform different roles during oogenesis of the kuruma prawn.

The dynamics of cyclin B1 distribution during meiosis I in mouse oocytes

Cdk1-cyclin B1 kinase activity drives oocytes through meiotic maturation. It is regulated by the phosphorylation status of cdk1 and by its spatial organisation. Here we used a cyclin B1-green fluorescent protein (GFP) fusion protein to examine the dynamics of cdk1-cyclin B1 distribution during meiosis I (MI) in living mouse oocytes. Microinjection of cyclin B1-GFP accelerated germinal vesicle breakdown (GVBD) and, as previously described, overrides cAMP-mediated meiotic arrest. GVBD was pre-empted by a translocation of cyclin B1-GFP from the cytoplasm to the germinal vesicle (GV). After nuclear accumulation, cyclin B1-GFP localised to the chromatin. The localisation of cyclin B1-GFP is governed by nuclear import and export. In GV intact oocytes, cyclin export was demonstrated by showing that cyclin B1-GFP injected into the GV is exported to the cytoplasm while a similar size dextran is retained. Import was revealed by the finding that cyclin B1-GFP accumulated in the GV when export was inhibited using leptomycin B. These studies show that GVBD in mouse oocytes is sensitive to cyclin B1 abundance and that the changes in distribution of cyclin B1 contribute to progression through MI.

Identification of interaction partners and substrates of the cyclin A1-CDK2 complex

The CDK2-associated cyclin A1 is essential for spermatogenesis and contributes to leukemogenesis. The detailed molecular functions of cyclin A1 remain unclear, since the molecular networks involving cyclin A1-CDK2 have not been elucidated. Here, we identified novel cyclin A1/CDK2 interaction partners in a yeast triple-hybrid approach. Several novel proteins (INCA1, KARCA1, and PROCA1) as well as the known proteins GPS2 (G-protein pathway suppressor 2), Ku70, receptor for activated protein kinase C1/guanine nucleotide-binding protein beta-2-like-1, and mRNA-binding motif protein 4 were identified as interaction partners. These proteins link the cyclin A1-CDK2 complex to diverse cellular processes such as DNA repair, signaling, and splicing. Interactions were confirmed by GST pull-down assays and co-immunoprecipitation. We cloned and characterized the most frequently isolated unknown gene, which we named INCA1 (inhibitor of CDK interacting with cyclin A1). The nuclear INCA1 protein is evolutionarily conserved and lacks homology to any known gene. This novel protein and two other interacting partners served as substrates for the cyclin A1-CDK2 kinase complex. Cyclin A1 and all interaction partners were highly expressed in testis with varying degrees of tissue specificity. The highest expression levels were observed at different time points during testis maturation, whereas expression levels in germ cell cancers and infertile testes decreased. Taken together, we identified testicular interaction partners of the cyclin A1-CDK2 complex and studied their expression pattern in normal organs, testis development, and testicular malignancies. Thereby, we establish a new basis for future functional analyses of cyclin A1. We provide evidence that the cyclin A1-CDK2 complex plays a role in several signaling pathways important for cell cycle control and meiosis.

Temperature may influence and regulate NF-YB expression in toad oocyte

Fully grown oocytes derived from Bufo gargarizans maintained at relatively low temperatures (4 degrees C, LTE-oocytes) acquire the competence to resume normal meiosis. In contrast, fully grown oocytes derived from toads maintained at relatively high temperatures (28 degrees C, HTE-oocytes) never acquire maturation competence. By suppression subtractive hybridization, we obtained 18 ESTs preliminarily thought to be preferentially expressed in LTE-oocytes; of these, TS1-4 shared homology with the human and mouse NF-YB genes. We cloned the full-length toad NF-YB gene by RACE and identified three alternatively spliced transcripts: tNF-YB1, tNF-YB2, and tNF-YB3 (GenBank Accession Nos. AY442015, AY442016, and AY442017, respectively). Toad NF-YB was differentially transcribed and translated in LTE-oocytes versus HTE-oocytes, likely resulting in differential CCAAT-binding and/or transcriptional activity of NF-Y. Furthermore, toad cyclin B2 was differentially transcribed at high and low temperatures. Taken together, this report of the differential expression of toad NF-YB at different temperatures is the first evidence that temperature may influence and regulate NF-YB expression.

Analysis of the roles of cyclin B1 and cyclin B2 in porcine oocyte maturation by inhibiting synthesis with antisense RNA injection

The function of cyclin B1 (CB1) and cyclin B2 (CB2) during porcine oocyte maturation was investigated by injecting oocytes with their antisense RNAs (asRNAs). At first, protein levels of both cyclin Bs were examined by immunoblotting, revealing that immature oocytes had only CB2, at a level comparable to 1/20 to 1/40 of that detected in first metaphase oocytes. Both cyclin B syntheses were started around germinal vesicle breakdown (GVBD); CB1 and CB2 peaked at the second metaphase and first metaphase, respectively. We obtained a porcine CB2 cDNA fragment, which was 88% homologous with human CB2, by reverse-transcriptase polymerase chain reaction (RT-PCR) using total RNAs of immature porcine oocytes and a primer set of human CB2. Specific asRNAs of CB1 and CB2 were prepared in vitro. Then one, the other, or both were injected into the cytoplasm of immature oocytes. CB1 asRNA inhibited CB1 synthesis specifically; the injected oocytes underwent first meiosis normally but could not arrest at the second meiotic metaphase. CB2 asRNA inhibited CB2 synthesis specifically, but had almost no effect on the maturation of injected oocytes. When both CB1 and CB2 asRNAs were injected, synthesis of both cyclin Bs was inhibited, and GVBD was significantly suppressed but occurred slowly. These results suggest that CB1 is the principal molecule for regulation in mammalian oocyte maturation, whereas CB2 has only an accessory role. They also show that in porcine oocytes, cyclin B synthesis is not necessary for GVBD induction itself, but synthesis of at least one cyclin B, CB1 or CB2, is necessary for GVBD induction in a normal time course.

Localization and dynamics of Cdc2-cyclin B during meiotic reinitiation in starfish oocytes

The Cdc2-cyclin B kinase has a central role in regulating the onset of M phase. In starfish oocytes, Cdc2-cyclin B begins to be activated approximately 10 min after application of maturation hormone, followed by accumulation in the nucleus then nuclear envelope breakdown. By immunofluorescence and by expressing a green fluorescent (GFP) chimera of cyclin B, we find that cyclin B is present in aggregates in the cytoplasm of immature oocytes. The aggregates disperse at approximately 10 min, suggesting that the dispersal is closely related to the activation of the kinase. Using cyclin B-GFP, the dispersion begins from the region containing the centrosomes. Extractability of Cdc2-cyclin B changes with similar kinetics during maturation. Active Cdc25 phosphatase released Cdc2-cyclin B from the detergent-insoluble fraction independently of its phosphatase activity. Live cell imaging also showed that Cdc2-cyclin B begins to accumulate in the nucleus before changes in nuclear pore permeability, consistent with Cdc2-cyclin B-induced disassembly of the pores.

Characterization and expression of mammalian cyclin b3, a prepachytene meiotic cyclin

We report the identification and expression pattern of a full-length human cDNA and a partial mouse cDNA encoding cyclin B3. Cyclin B3 (CCNB3) is conserved from Caenorhabditis elegans to Homo sapiens and has an undefined meiotic function in female, but not male Drosophila melanogaster. We show that H. sapiens cyclin B3 interacts with cdk2, is localized to the nucleus, and is degraded during anaphase entry after the degradation of cyclin B1. Degradation is dependent on sequences conserved in a destruction box motif. Overexpression of nondegradable cyclin B3 blocks the mitotic cell cycle in late anaphase, and at higher doses it can interfere with progression through G(1) and entry into S phase. H. sapiens cyclin B3 mRNA and protein are detected readily in developing germ cells in the human testis and not in any other tissue. The mouse cDNA has allowed us to further localize cyclin B3 mRNA to leptotene and zygotene spermatocytes. The expression pattern of mammalian cyclin B3 suggests that it may be important for events occurring in early meiotic prophase I.

A new transgenic mouse model for the study of cell cycle control in megakaryocytes

During the development of the megakaryocytic lineage, the megakaryoblasts give rise to megakaryocytes which undergo repeated S phases in the absence of cytokinesis (endomitosis). The cellular oncogene myc plays a central role in the proliferation and differentiation of several cell types. In a previous study, we generated transgenic mice carrying c-myc fused to the estrogen receptor under the control of the platelet factor four (PF4) megakaryocyte-specific promoter. The bone marrow of female transgenic mice, but not of male mice, displayed increased megakaryopoiesis. Here we report that beta-estradiol-induced activation of c-myc in cultured bone marrow cells derived from male or female transgenic mice resulted in prolonged survival of the cells in vitro. Addition of a cocktail of hemopoietic growth factors to beta-estradiol-treated cells, including interleukin 6 (IL-6), IL-3 and stem cell factor further improved the survival time in culture and increased the percentage of large mature cells, but did not result in immortalization. The majority of these PF4-expressing cells, however, did not reach the differentiation stage at which acetylcholinesterase is expressed and did not appear as large megakaryocytes. We conclude that cultured megakaryocytes overexpressing myc are induced to proliferate, but have a limited potential to fully differentiate. Under these conditions, cyclin D3 was downregulated while the level of cyclin A was slightly upregulated.

Acquisition of meiotic competence in mouse oocytes: absolute amounts of p34(cdc2), cyclin B1, cdc25C, and wee1 in meiotically incompetent and competent oocytes

M-Phase promoting factor (MPF) is a complex of p34(cdc2) and cyclin B. Results of previous studies in which relative mass amounts of these cell cycle regulators were determined suggested that the accumulation of p34(cdc2), rather than cyclin B, could be a limiting factor in the acquisition of meiotic competence in mouse oocytes. Nevertheless, in the absence of measurements of the absolute amount of these components of MPF, it is possible that the molar amount of p34(cdc2) is in excess to that of cyclin B, i.e., the accumulation of p34(cdc2) is not a limiting factor. We report measurements of the absolute mass of p34(cdc2) and cyclin B1, as well as the two proximal regulators of MPF, namely cdc25C and wee1, in meiotically incompetent and competent mouse oocytes. We find that the numbers of molecules of p34(cdc2), cyclin B1, cdc25C, and wee1 in meiotically incompetent oocytes are 1.4 x 10(6), 11.3 x 10(6), 24.6 x 10(6), 15. 6 x 10(6), respectively, and in meiotically competent oocytes the numbers are 14.3 x 10(6), 95.5 x 10(6), 80.0 x 10(6), 40.1 x 10(6), respectively. Thus, the concentration of cyclin B1 is always in excess to that of p34(cdc2), and this is consistent with the hypothesis that the accumulation of p34(cdc2) plays a role in the acquisition of meiotic competence. Last, the concentration of cdc25C is greater than that of wee1 and the concentration of each is greater than that of p34(cdc2) in both meiotically incompetent and competent oocytes.

Residual Cdc2 activity remaining at meiosis I exit is essential for meiotic M-M transition in Xenopus oocyte extracts

To investigate the regulatory mechanisms of the cell cycle transition from M phase to M phase in meiotic cycles, a Xenopus oocyte extract that performs the M-M transition has been developed. Using the meiotic extract, we found that a low level of Cdc2 activity remained at the exit of meiosis I (MI), due to incomplete degradation of cyclin B. The inactivation of the residual Cdc2 activity induced both entry into S phase and tyrosine phosphorylation on Cdc2 after MI. Quantitative analysis demonstrated that a considerable amount of Wee1 was present at the MI exit and Cdc2 inhibitory phosphorylation during this period was suppressed by the dominance of Cdc2 over Wee1. Consistently, the addition of more than a critical amount of Wee1 to the extract induced Cdc2 inhibitory phosphorylation, changing the M-M transition into an M-S-M transition. Thus, the Cdc2 activity remaining at MI exit is required for suppressing entry into S phase during the meiotic M-M transition period.

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

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

Non-dependence of cyclin E/Cdk2 kinase activity on the initiation of oocyte maturation in goldfish

Cdk2 kinase activity increases during oocyte maturation but neither cyclin A nor B is associated with Cdk2 in mature oocytes in goldfish. As a potential Cdk2 partner in meiosis, a cyclin E homolog was isolated from a goldfish oocyte cDNA library. A monoclonal antibody was raised against bacterially produced full-length goldfish cyclin E. Both cyclin E and Cdk2 were already present in immature oocytes and their protein levels did not change remarkably during oocyte maturation. Cyclin E formed a complex mainly with Cdk2 just at the time of germinal vesicle breakdown (GVBD) in association with the increase in Cdk2 kinase activity, although a fraction of cyclin E bound to Cdk(s) other than Cdk2 and Cdc2. Ectopic activation of cyclin E/Cdk2 by the injection of cyclin E messenger RNA (mRNA) into immature oocytes did not induce maturation-promoting factor (MPF) activation and GVBD. Furthermore, inhibition of cyclin E/Cdk2 kinase activity by the injection of p21SDI1 into the oocytes treated with 17alpha,20beta-dihydroxy-4-pregnen-3-one had no effect on MPF activation and GVBD. These results indicate that cyclin E/Cdk2 kinase activity is insufficient and unnecessary for initiating goldfish oocyte maturation.

Effect of dexamethasone on the expression of p34(cdc2) and cyclin B1 in pig oocytes in vitro

The meiotic division in oocytes is arrested in the G2 phase of the cell cycle. Resumption of meiosis, also known as oocyte maturation, entails a G2 to M transition. At the G2-M boundary, maturation promoting factor (MPF) activation is usually induced via several ways, including tyrosine dephosphorylation of p34(cdc2) and synthesis of cyclin B according to cell type and species. Previous studies in our laboratory demonstrated that glucocorticoids directly inhibit the meiotic maturation of pig oocytes in vitro. The aim of this study was therefore to investigate the influence of glucocorticoids on the expression of p34(cdc2) and cyclin B1 in resumption of meiosis of pig oocytes. We detected the relative levels and association of p34(cdc2) and cyclin B1. Isolated cumulus-enclosed oocytes were cultured in Waymouth MB752/1 medium supplemented with sodium pyruvate (50 microgram/ml), LH (0.5 microgram/ml), FSH (0.5 microgram/ml), and estradiol-17beta (1 microgram/ml) in the presence or absence of dexamethasone (DEX) for 24 hr; they then were cultured without hormonal supplements in the presence or absence of DEX for an additional 24 hr. We found that cyclin B1, as well as p34(cdc2), was already present in fully grown G2-arrested pig oocytes when removed from the follicle. In these oocytes, cyclin B1 and p34(cdc2) were already associated in complex. Treatment with DEX at concentrations of 1 microgram/ml or above decreased the level of cyclin B1, but had no effect on the level of p34(cdc2). The exposure of oocytes to DEX also decreased the amount of complexed p34(cdc2)-cyclin B1. These findings suggest that the inhibitory action of DEX on meiotic maturation could be due, at least in part, to the reduced amount of p34(cdc2)-cyclin B1 complex.

Translational control of cyclin B1 mRNA during meiotic maturation: coordinated repression and cytoplasmic polyadenylation

Translational control is prominent during meiotic maturation and early development. In this report, we investigate a mode of translational repression in Xenopus laevis oocytes, focusing on the mRNA encoding cyclin B1. Translation of cyclin B1 mRNA is relatively inactive in the oocyte and increases dramatically during meiotic maturation. We show, by injection of synthetic mRNAs, that the cis-acting sequences responsible for repression of cyclin B1 mRNA reside within its 3'UTR. Repression can be saturated by increasing the concentration of reporter mRNA injected, suggesting that the cyclin B1 3'UTR sequences provide a binding site for a trans-acting repressor. The sequences that direct repression overlap and include cytoplasmic polyadenylation elements (CPEs), sequences known to promote cytoplasmic polyadenylation. However, the presence of a CPE per se appears insufficient to cause repression, as other mRNAs that contain CPEs are not translationally repressed. We demonstrate that relief of repression and cytoplasmic polyadenylation are intimately linked. Repressing elements do not override the stimulatory effect of a long poly(A) tail, and polyadenylation of cyclin B1 mRNA is required for its translational recruitment. Our results suggest that translational recruitment of endogenous cyclin B1 mRNA is a collaborative effect of derepression and poly(A) addition. We discuss several molecular mechanisms that might underlie this collaboration.

Cyclin F regulates the nuclear localization of cyclin B1 through a cyclin-cyclin interaction

The key regulator of G(2)-M transition of the cell cycle is M-phase promoting factor (MPF), a complex composed of cdc2 and a B-type cyclin. Cyclin B1 nuclear localization involves phosphorylation within a region called the cytoplasmic retention signal, which also contains a nuclear export signal. The mechanism of MPF nuclear localization remains unclear since it contains no functional nuclear localization signal (NLS). We exploited the yeast two-hybrid screen to find protein(s) potentially mediating localization of cyclin B1 and identified a novel interaction between cyclin B1 and cyclin F. We found that cdc2, cyclin B1 and cyclin F form a complex that exhibits histone H1 kinase activity. Cyclin B1 and cyclin F also colocalize through immunofluorescence studies. Additionally, deletion analysis revealed that each putative NLS of cyclin F is functional. Taken together, the data suggest that the NLS regions of cyclin F regulate cyclin B1 localization to the nucleus. The interaction between cyclin B1 and cyclin F represents the first example of direct cyclin-cyclin binding, and elucidates a novel mechanism that regulates MPF localization and function.

Selective requirement for Cdc25C protein synthesis during meiotic progression in porcine oocytes

Fundamental differences between meiosis and mitosis suggest that the shared central cell cycle machinery may be regulated differently during the two division cycles. This paper focuses on unique features of Cdc25C protein function during meiotic progression. We report on the existence of oocyte-specific CDC25C transcripts that differ from their somatic counterparts in the 3' untranslated region. While CDC25C mRNA levels remain constant in fully-grown oocytes, corresponding protein levels increase progressively during maturation to a maximum at metaphase II. Elevation of Cdc25C protein levels in G2-oocytes by mRNA injection failed to increase MPF-kinase levels or to induce premature entry into M-phase. Likewise, antisense-induced arrest of translation (translational arrest) had no effect on chromosome condensation, nucleolar disassembly, or nuclear membrane contraction. By contrast, translational arrest inhibited subsequent events including membrane disassembly and spindle formation. Neither up- nor down-regulation of Cdc25C synthesis after metaphase I plate formation influenced progression to metaphase II. However, translational arrest during metaphase resulted in incomplete chromosome decondensation and abnormal pronuclear membrane assembly after activation. We conclude that Cdc25 protein, translated from unique transcripts, is preferentially located in the oocyte nucleus and is essential for progress through late diakinesis. Subsequently, new synthesis of Cdc25C protein is required for the orderly transition from meiotic to mitotic cell division.

Cell cycle-regulated proteolysis of mitotic target proteins

The ubiquitin-dependent proteolysis of mitotic cyclin B, which is catalyzed by the anaphase-promoting complex/cyclosome (APC/C) and ubiquitin-conjugating enzyme H10 (UbcH10), begins around the time of the metaphase-anaphase transition and continues through G1 phase of the next cell cycle. We have used cell-free systems from mammalian somatic cells collected at different cell cycle stages (G0, G1, S, G2, and M) to investigate the regulated degradation of four targets of the mitotic destruction machinery: cyclins A and B, geminin H (an inhibitor of S phase identified in Xenopus), and Cut2p (an inhibitor of anaphase onset identified in fission yeast). All four are degraded by G1 extracts but not by extracts of S phase cells. Maintenance of destruction during G1 requires the activity of a PP2A-like phosphatase. Destruction of each target is dependent on the presence of an N-terminal destruction box motif, is accelerated by additional wild-type UbcH10 and is blocked by dominant negative UbcH10. Destruction of each is terminated by a dominant activity that appears in nuclei near the start of S phase. Previous work indicates that the APC/C-dependent destruction of anaphase inhibitors is activated after chromosome alignment at the metaphase plate. In support of this, we show that addition of dominant negative UbcH10 to G1 extracts blocks destruction of the yeast anaphase inhibitor Cut2p in vitro, and injection of dominant negative UbcH10 blocks anaphase onset in vivo. Finally, we report that injection of dominant negative Ubc3/Cdc34, whose role in G1-S control is well established and has been implicated in kinetochore function during mitosis in yeast, dramatically interferes with congression of chromosomes to the metaphase plate. These results demonstrate that the regulated ubiquitination and destruction of critical mitotic proteins is highly conserved from yeast to humans.

Two distinct classes of mitotic cyclin homologues, Cyc1 and Cyc2, are involved in cell cycle regulation in the ciliate Paramecium tetraurelia

The eukaryotic cell cycle is regulated by the sequential activation of different CDK/cyclin complexes. Two distinct classes of mitotic cyclin homologues, CYC1 and CYC2, have been identified and cloned for the first time in the ciliate Paramecium. Cyc1 is 324 amino acids long with a predicted molecular mass of 38 kDa, whereas Cyc2 is 336 amino acids long with a predicted molecular mass of 40 kDa. They display 42-51% sequence identity to other eukaryotic mitotic cyclins within the 'cyclin box' region. The conserved 'cyclin box' and 'destruction box' elements can be identified within each of the sequences. Genomic Southern blot analysis indicated that the CYC1 gene has two isoforms, with 92.3% and 85.9% identify at the amino acid level and at the nucleotide level, respectively. Both Cyc1 and Cyc2 proteins showed characteristic patterns of accumulation and destruction during the vegetative cell cycle, with Cyc1 peaking at the point of commitment to division (PCD), and Cyc2 reaching the maximal level late in the cell cycle. Immunoprecipitation experiments with antibodies specific to Cyc1 and Cyc2 indicated that Cyc1 and Cyc2 associate with distinct CDK homologues. Both immunoprecipitates exhibited histone H1 kinase activity that oscillated in the cell cycle in parallel with the respective amount of cyclins present. Histone H1 kinase activity associated with Cyc1 reached a peak at PCD while Cyc2 showed maximal activity when about 75% cells have completed cytokinesis. We propose that Cyc1 may be involved in commitment to division, in association with the CDK that binds to p13suc1, Cdk3, and that the Cyc2/Cdk2 complex may regulate cytokinesis. PCR-amplification revealed similar sequences in Tetrahymena, Sterkiella, Colpoda and Blepharisma, suggesting the conservation of the cyclin genes within ciliates. Although cell cycle regulation in ciliates differs in some respects from that of other eukaryotes, the cyclin motifs have clearly been conserved during evolution.

Effects of hormones and osmolarity in the culture medium on germinal vesicle breakdown of porcine oocytes

The present study was conducted to examine effects of hormones and osmolarity on germinal vesicle breakdown (GVBD) and histone H1 kinase (H1K) activity in porcine oocytes cultured in vitro. The basic medium used for culture of oocytes was modified Tyrode's solution in which the osmolarity was adjusted to 134 to 495 mOsm by changing the concentration of sodium chloride (NaCl). When the hormones were present, osmolarity of medium that allows GVBD of oocytes was less than 400 mOsm. However, the range of osmolarity of medium that allows meiotic maturation of oocytes was 210 to 362 mOsm. On the other hand, without hormonal supplement, the incidence of GVBD in oocytes decreased as the osmolarity of the medium increased in the rage of 210 to 362 mOsm. By increasing the osmolarity of the medium from 210 to 362 mOsm by addition with sorbitol instead of NaCl, the incidence decreased from 89.1% to 13.3%. In oocytes cultured in medium of 210 mOsm without hormones, the percentage of oocytes that underwent GVBD and had increased H1K activity 20 h after culture was significantly higher (P < 0.05) than those of oocytes cultured in the same medium supplemented with hormones or medium of 362 mOsm. These results indicate that in vitro induction of GVBD in porcine oocytes is strongly affected by osmolarity of the medium in the absence of hormones. The results also suggest that, under low osmolarity (210 mOsm), GVBD is accelerated with rapid increase of H1K activity.

Cytoplasmic polyadenylation elements mediate masking and unmasking of cyclin B1 mRNA

During oocyte maturation, cyclin B1 mRNA is translationally activated by cytoplasmic polyadenylation. This process is dependent on cytoplasmic polyadenylation elements (CPEs) in the 3' untranslated region (UTR) of the mRNA. To determine whether a titratable factor might be involved in the initial translational repression (masking) of this mRNA, high levels of cyclin B1 3' UTR were injected into oocytes. While this treatment had no effect on the poly(A) tail length of endogenous cyclin B1 mRNA, it induced cyclin B1 synthesis. A mutational analysis revealed that the most efficient unmasking element in the cyclin 3' UTR was the CPE. However, other U-rich sequences that resemble the CPE in structure, but which do not bind the CPE-binding polyadenylation factor CPEB, failed to induce unmasking. When fused to the chloramphenical acetyl transferase (CAT) coding region, the cyclin B1 3' UTR inhibited CAT translation in injected oocytes. In addition, a synthetic 3' UTR containing multiple copies of the CPE also inhibited translation, and did so in a dose-dependent manner. Furthermore, efficient CPE-mediated masking required cap-dependent translation. During the normal course of progesterone-induced maturation, cytoplasmic polyadenylation was necessary for mRNA unmasking. A model to explain how cyclin B1 mRNA masking and unmasking could be regulated by the CPE is presented.

Cloning and sequence analysis of two cDNAs encoding cyclin A and cyclin B in the zebra mussel Dreissena polymorpha

Cyclins are key components in the progression of both mitotic and meiotic cell cycle control. Full-length cDNA clones encoding cyclin A and cyclin B were isolated from a zebra mussel testis cDNA library. The clones contained open reading frames of 419 and 434 amino acids, had similarity to cyclins A and B from other species, but also some unique features in their sequences. Cyclin A and B mRNA was expressed in testis, ovary, gill, mantle, muscle, and eggs, as shown by specific polymerase chain reaction.

The clam 3' UTR masking element-binding protein p82 is a member of the CPEB family

During early development gene expression is controlled principally at the translational level. Oocytes of the surf clam Spisula solidissima contain large stockpiles of maternal mRNAs that are translationally dormant or masked until meiotic maturation. Activation of the oocyte by fertilization leads to translational activation of the abundant cyclin and ribonucleotide reductase mRNAs at a time when they undergo cytoplasmic polyadenylation. In vitro unmasking assays have defined U-rich regions located approximately centrally in the 3' UTRs of these mRNAs as translational masking elements. A clam oocyte protein of 82 kDa, p82, which selectively binds the masking elements, has been proposed to act as a translational repressor. Importantly, mRNA-specific unmasking in vitro occurs in the absence of poly(A) extension. Here we show that clam p82 is related to Xenopus CPEB, an RNA-binding protein that interacts with the U-rich cytoplasmic polyadenylation elements (CPEs) of maternal mRNAs and promotes their polyadenylation. Cloned clam p82/CPEB shows extensive homology to Xenopus CPEB and related polypeptides from mouse, goldfish, Drosophila and Caenorhabditis elegans, particularly in their RNA-binding C-terminal halves. Two short N-terminal islands of sequence, of unknown function, are common to vertebrate CPEBs and clam p82. p82 undergoes rapid phosphorylation either directly or indirectly by cdc2 kinase after fertilization in meiotically maturing clam oocytes, prior to its degradation during the first cell cleavage. Phosphorylation precedes and, according to inhibitor studies, may be required for translational activation of maternal mRNA. These data suggest that clam p82 may be a functional homolog of Xenopus CPEB.

Regulation of Cdc28 cyclin-dependent protein kinase activity during the cell cycle of the yeast Saccharomyces cerevisiae

The cyclin-dependent protein kinase (CDK) encoded by CDC28 is the master regulator of cell division in the budding yeast Saccharomyces cerevisiae. By mechanisms that, for the most part, remain to be delineated, Cdc28 activity controls the timing of mitotic commitment, bud initiation, DNA replication, spindle formation, and chromosome separation. Environmental stimuli and progress through the cell cycle are monitored through checkpoint mechanisms that influence Cdc28 activity at key cell cycle stages. A vast body of information concerning how Cdc28 activity is timed and coordinated with various mitotic events has accrued. This article reviews that literature. Following an introduction to the properties of CDKs common to many eukaryotic species, the key influences on Cdc28 activity-cyclin-CKI binding and phosphorylation-dephosphorylation events-are examined. The processes controlling the abundance and activity of key Cdc28 regulators, especially transcriptional and proteolytic mechanisms, are then discussed in detail. Finally, the mechanisms by which environmental stimuli influence Cdc28 activity are summarized.

Either cyclin B1 or B2 is necessary and sufficient for inducing germinal vesicle breakdown during frog (Rana japonica) oocyte maturation

Oocyte maturation is finally triggered by the maturation-promoting factor (MPF), which consists of Cdc2 and cyclin B. We have cloned cDNAs encoding frog (Rana japonica) cyclins B1 and B2 and produced antibodies against their products. Using the antibodies, we investigated changes in protein states and levels of Cdc2 and cyclins B1 and B2 during oocyte maturation. In immature oocytes, all Cdc2 was a monomeric unphosphorylated inactive 35 kDa form and neither cyclin B1 nor cyclin B2 was present. Mature oocytes contained the MPF complex consisting of an active 34 kDa Cdc2 phosphorylated on threonine161 and a 49 kDa cyclin B1 or a 51 kDa cyclin B2. After progesterone stimulation, both cyclins B1 and B2 were synthesized from their stored mRNAs and bound to the preexisting 35 kDa Cdc2. The binding of Cdc2 with cyclin B and its activation probably through the phosphorylation on threonine161 occurred at almost the same time, in accordance with an electrophoretic mobility shift of Cdc2 from 35 to 34 kDa. Microinjection into immature oocytes of cyclin B1 or B2 mRNA alone, or a mixture of them, induced germinal vesicle breakdown (GVBD) with similar dose-dependence. When the translation of endogenous mRNAs of both cyclins B1 and B2 was inhibited with antisense RNAs, progesterone failed to induce GVBD in the oocytes, but the inhibition of only one of the two was unable to inhibit the progesterone-induced GVBD. These results indicate that either cyclin B1 or B2 is necessary and sufficient for inducing GVBD during Rana oocyte maturation.

Cap ribose methylation of c-mos mRNA stimulates translation and oocyte maturation in Xenopus laevis

In Xenopus oocytes, progesterone stimulates the cytoplasmic polyadenylation and resulting translational activation of c-mos mRNA, which is necessary for the induction of oocyte maturation. Although details of the biochemistry of polyadenylation are beginning to emerge, the mechanism by which 3' poly(A) addition stimulates translation initiation is enigmatic. A previous report showed that polyadenylation induced cap-specific 2'-O-methylation, and suggested that this 5' end modification was important for translational activation. Here, we demonstrate that injected c-mos RNA undergoes polyadenylation and cap ribose methylation. Inhibition of this methylation by S-isobutylthioadenosine (SIBA), a methyltransferase inhibitor, has little effect on progesterone-induced c-mos mRNA polyadenylation or general protein synthesis, but prevents the synthesis of Mos protein as well as oocyte maturation. Maturation can be rescued, however, by the injection of factors that act downstream of Mos, such as cyclin A and B mRNAs. Most importantly, we show that the translational efficiency of injected mRNAs containing cap-specific 2'-O-methylation (cap I) is significantly enhanced compared to RNAs that do not contain the methylated ribose (cap 0). These results suggest that cap ribose methylation of c-mos mRNA is important for translational recruitment and for the progression of oocytes through meiosis.

The Polo-like kinase Plx1 is a component of the MPF amplification loop at the G2/M-phase transition of the cell cycle in Xenopus eggs

We have investigated whether Plx1, a kinase recently shown to phosphorylate cdc25c in vitro, is required for activation of cdc25c at the G2/M-phase transition of the cell cycle in Xenopus. Using immunodepletion or the mere addition of an antibody against the C terminus of Plx1, which suppressed its activation (not its activity) at G2/M, we show that Plx1 activity is required for activation of cyclin B-cdc2 kinase in both interphase egg extracts receiving recombinant cyclin B, and cycling extracts that spontaneously oscillate between interphase and mitosis. Furthermore, a positive feedback loop allows cyclin B-cdc2 kinase to activate Plx1 at the G2/M-phase transition. In contrast, activation of cyclin A-cdc2 kinase does not require Plx1 activity, and cyclin A-cdc2 kinase fails to activate Plx1 and its consequence, cdc25c activation in cycling extracts.

In vivo regulation of cyclin A/Cdc2 and cyclin B/Cdc2 through meiotic and early cleavage cycles in starfish

In starfish, fertilization occurs naturally at late meiosis I. In the absence of fertilization, however, oocytes complete meiosis I and II, resulting in mature eggs arrested at the pronucleus stage, which are still fertilizable. In this study, we isolated cDNAs of starfish cyclin A and Cdc2, and monitored extensively the cell cycle dynamics of cyclin A and cyclin B levels and their associated Cdc2 kinase activity, Tyr phosphorylation of Cdc2, and Cdc25 phosphorylation states throughout meiotic and early embryonic cleavage cycles in vivo. In meiosis I, cyclin A was undetectable and cyclin B/Cdc2 alone exhibited histone H1 kinase activity, while thereafter both cyclin A/Cdc2 and cyclin B/Cdc2 kinase activity oscillated along with the cell cycle. Cyclin B-, but not cyclin A-, associated Cdc2 was subjected to regulation via Tyr phosphorylation, and phosphorylation states of Cdc25 correlated with cyclin B/Cdc2 kinase activity with some exceptions. Between meiosis I and II and at the pronucleus stage, cyclin A and B levels remained low, Cdc2 Tyr phosphorylation was undetectable, and Cdc25 remained phosphorylated depending on MAP kinase activity, showing a good correlation between these two stages. Upon fertilization of mature eggs, Cdc2 Tyr phosphorylation reappeared and Cdc25 was dephosphorylated. In the first cleavage cycle, under conditions which prevented Cdc25 activity, cyclin A/Cdc2 was activated with a normal time course and then cyclin B/Cdc2 was activated with a significant delay, resulting in the delayed completion of M-phase. Thus, in contrast to meiosis I, both cyclin A and cyclin B appear to be involved in the embryonic cleavage cycles. We propose that regulation of cyclin A/Cdc2 and cyclin B/Cdc2 is characteristic of meiotic and early cleavage cycles.

The oocyte metaphase arrest

Usually, oocyte meiosis reinitiation appears as a two step process during which release from the prophase block is followed by a second arrest in metaphase I or II. In this review, we will examine the mechanisms required to maintain the metaphase arrest and stabilize MPF activity at this stage. Then, we will analyse the processes required to exit from the metaphase block. These may drive the cells forward to the metaphase-anaphase transition, as a result of fertilization, activation or protein synthesis inhibition. Instead, inhibiting protein phosphorylation drives the oocyte back to interphase. All these treatments result in derepression of DNA synthesis.

On cyclins, oocytes, and eggs

Oocyte and egg are suitable model systems for studying cell division since meiotic maturation resembles a G2/M transition and early embryonic divisions are precisely timed and occur without zygotic transcription. The analysis of oocytes and eggs from different species provides the opportunity to understand the roles of proteins that the critical to the progression and maintenance of the cell cycle. Among them, cyclins are certainly worthy of investigation. Mitotic cyclins (cyclins A and B) are clearly implicated in meiosis and early embryonic cell cycles. More recent studies have revealed that G1-type cyclins (cyclins E and D) could also play a role in both processes and cyclin H has been suggesed to participate to CAK activity (cdc2-activating kinase) in oocytes. The study of cyclins in oocytes and eggs clearly offer insights into their roles during the cell cycle.

Intracellular pH increase driven by an Na+/H+ exchanger upon activation of surf clam oocytes

Intracellular pH (pHi) measurements were performed in surf clam (Spisula solidissima) oocytes before and after artificial activation or fertilization [evidenced by germinal vesicle breakdown (GVBD)] by the dimethyloxazolidinedione (DMO) and 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) methods. Results using both methods showed increases of pHi of 0.3 pH unit after activation by excess K+. Using BCECF, we found an increase of similar magnitude after fertilization or after the addition of serotonin. By contrast, GVBD did not occur when the pHi was increased to similar or even higher levels by exposing the oocytes to ammonia. In sodium-free seawater, excess K+ induced GVBD but the pHi of K+-activated oocytes decreased significantly below the resting level of unactivated oocytes. The pHi increases in K+-activated oocytes were otherwise proportional to the external Na+ concentration. The amiloride derivatives dimethylamiloride and hexamethylene amiloride (at 10-50 microM) efficiently inhibited the K+-induced increase of pHi but did not block GVBD. These two derivatives were able, however, to retard K+-induced GVBD, hexamethylene amiloride being the more efficient. This retardation of K+-induced GVBD could be abolished by the simultaneous addition of ammonia. Taken altogether, these results show that a pHi increase, driven by a typical Na+/H+ exchanger, follows activation of surf clam oocytes but that this pHi increase is neither sufficient nor required for GVBD, though it does allow its progression at an optimal rate.

Alternative splicing of cyclin transcripts in maize endosperm

Cyclins are the regulatory subunits of cyclin-dependent protein kinases. In investigations of the expression of a cyclin gene during maize endosperm development, we detected a cyclin transcript with a 63-bp deletion in the region encoding the conserved 'cyclin box' where cyclin interacts with p34cdc2, the catalytic domain of the cyclin-dependent protein kinase. Analysis of cDNA and genomic sequences, and other observations, indicated that the deletion was caused by alternative splicing of a retained intron in the normally spliced transcript. Whereas the normally spliced cyclin RNA was mitotically functional, as indicated by its ability to promote maturation of Xenopus oocytes, the alternatively spliced transcript was unable to promote maturation. In addition to maize endosperm, the alternatively spliced cyclin was detected in apical meristem, mature leaf, root tip and mature root.

A dependent pathway of cytoplasmic polyadenylation reactions linked to cell cycle control by c-mos and CDK1 activation

During oocyte maturation and early development, mRNAs receive poly(A) in the cytoplasm at distinct times relative to one another and to the cell cycle. These cytoplasmic polyadenylation reactions do not occur during oogenesis, but begin during oocyte maturation and continue throughout early development. In this report, we focus on the link between cytoplasmic polyadenylation and control of the cell cycle during meiotic maturation. Activation of maturation promoting factor, a complex of CDK1 and cyclin, is required for maturation and dependent on c-mos protein kinase. We demonstrate here that two classes of polyadenylation exist during oocyte maturation, defined by their dependence of c-mos and CDK1 protein kinases. Polyadenylation of the first class of mRNAs (class I) is independent of c-mos and CDK1 kinase activities, whereas polyadenylation of the second class (class II) requires both of these activities. Class I polyadenylation, through its effects on c-mos mRNA, is required for class II polyadenylation. cis-acting elements responsible for this distinction reside in the 3'-untranslated region, upstream of the polyadenylation signal AAUAAA. Cytoplasmic polyadenylation elements (CPEs) are sufficient to specify class I polyadenylation, and subtle changes in the CPE can substantially, though not entirely, shift an RNA from class I to class II. Activation of class I polyadenylation events is independent of hyperphosphorylation of CPE-binding protein or poly(A) polymerase, and requires cellular protein synthesis. The two classes of polyadenylation and of mRNA define a dependent pathway, in which polyadenylation of certain mRNAs requires the prior polyadenylation of another. We propose that this provides one method of regulating the temporal order of polyadenylation events, and links polyadenylation to the control of the meiotic cell cycle.

Acquisition of meiotic competence in growing mouse oocytes is controlled at both translational and posttranslational levels

Full-grown mouse oocytes spontaneously resume meiosis in vitro when released from their follicular environment. By contrast, growing oocytes are not competent to resume meiosis; the molecular basis of meiotic competence is not known. Entry into M phase of the eukaryotic cell cycle is controlled by MPF, a catalytically active complex comprising p34cdc2 kinase and cyclin B. Incompetent oocytes contain levels of cyclin B comparable to those in competent oocytes, while their level of p34cdc2 is markedly lower; p34cdc2 accumulates abruptly at the end of oocyte growth, at the time of meiotic competence acquisition. We show here that this change in p34cdc2 concentration is not secondary to a corresponding change in the concentration of the cognate mRNA, indicating that translational control may be involved. Microinjection of translatable p34cdc2 mRNA into incompetent oocytes yielded high levels of the protein, but it did not lead to resumption of meiosis. Similarly, microinjection of cyclin B1 mRNA resulted in accumulation of the protein, but not in the acquisition of meiotic competence. By contrast, the microinjection of both p34cdc2 and cyclin B1 mRNAs in incompetent oocytes induced histone H1 and MAP kinase activation, germinal vesicle breakdown, and entry into M-phase including the translational activation of a dormant mRNA. Thus, endogenous cyclin B1 in incompetent oocytes is not available for interaction with p34cdc2, suggesting that a posttranslational event must occur to achieve meiotic competence. Microinjection of either p34cdc2 or cyclin B1 mRNAs accelerated meiotic reinitiation of okadaic acid-treated incompetent oocytes. Taken together, these results suggest that acquisition of meiotic competence by mouse oocytes is regulated at both translational and posttranslational levels.