The clam embryo protein cyclin A induces entry into M phase and the resumption of meiosis in Xenopus oocytes

CREB1 promotes proliferation and differentiation by mediating the transcription of CCNA2 and MYOG in bovine myoblasts

The cAMP response element binding protein 1 (CREB1) is an important nuclear transcription factor in eukaryotes. To explore the potential role of CREB1 on Qinchuan bovine skeletal myoblasts, we investigated the function of CREB1 on proliferation and differentiation. In this study, we found that CREB1 promoted cell proliferation by promoting DNA synthesis in S phase and cell division in G2 phase and promoted myogenic differentiation process in bovine myoblasts. Through dual luciferase experiments, we found that CREB1 can bind to the proximal promoter regions of CCNA2 and MyoG, indicating that CREB1 can play a positive regulatory role in the proliferation and differentiation of myoblasts by mediating the transcription of CCNA2 and MyoG. In addition, through downstream target gene analysis and transcriptome sequencing, we found that CREB1 plays a role in cell proliferation, myogenic differentiation, skeletal muscle repair and other related pathways.

Cell Cycle Regulation by Heat Shock Transcription Factors

Cell division and cell cycle mechanism has been studied for 70 years. This research has revealed that the cell cycle is regulated by many factors, including cyclins and cyclin-dependent kinases (CDKs). Heat shock transcription factors (HSFs) have been noted as critical proteins for cell survival against various stresses; however, recent studies suggest that HSFs also have important roles in cell cycle regulation-independent cell-protective functions. During cell cycle progression, HSF1, and HSF2 bind to condensed chromatin to provide immediate precise gene expression after cell division. This review focuses on the function of these HSFs in cell cycle progression, cell cycle arrest, gene bookmarking, mitosis and meiosis.

CDCA7 Facilitates Tumor Progression by Directly Regulating CCNA2 Expression in Esophageal Squamous Cell Carcinoma

Background

CDCA7 is a copy number amplified gene identified not only in esophageal squamous cell carcinoma (ESCC) but also in various cancer types. Its clinical relevance and underlying mechanisms in ESCC have remained unknown.

Methods

Tissue microarray data was used to analyze its expression in 179 ESCC samples. The effects of CDCA7 on proliferation, colony formation, and cell cycle were tested in ESCC cells. Real-time PCR and Western blot were used to detect the expression of its target genes. Correlation of CDCA7 with its target genes in ESCC and various SCC types was analyzed using GSE53625 and TCGA data. The mechanism of CDCA7 was studied by chromatin immunoprecipitation (ChIP), luciferase reporter assays, and rescue assay.

Results

The overexpression of CDCA7 promoted proliferation, colony formation, and cell cycle in ESCC cells. CDCA7 affected the expression of cyclins in different cell phases. GSE53625 and TCGA data showed CCNA2 expression was positively correlated with CDCA7. The knockdown of CCNA2 reversed the malignant phenotype induced by CDCA7 overexpression. Furthermore, CDCA7 was found to directly bind to CCNA2, thus promoting its expression.

Conclusions

Our results reveal a novel mechanism of CDCA7 that it may act as an oncogene by directly upregulating CCNA2 to facilitate tumor progression in ESCC.

Function analysis and molecular characterization of cyclin A in ovary development of oriental river prawn, Macrobrachium nipponense

Macrobrachium nipponense has the characteristics of fast ovarian development cycle, which leads to the coexistence of multiple generations, the reduction of commodity specifications and the low economic benefit. Therefore, the study on the mechanism of ovarian development is of great significance to the development of industry. Cyclin A (CycA)is a key gene regulating ovarian development in vertebrates, but little information was available for its function in crustaceans. In this study, the full-length cDNA of Mn-CycA was obtained from the ovary. The full-length cDNA (2033 bp) with an open reading frame of 1368 bp, encoded a 456-amino acid protein. qRT-PCR revealed tissue-specific expression pattern of Mn-CycA, with abundant expression in the ovary. Results in different developmental stages of ovary indicated that Mn-CycA expression is positively correlated with ovarian maturation. qRT-PCR In different developmental stages, the expression of Mn-CycA mRNA gradually increased during the embryonic stage and decreased significantly on the first day of the hatching stage. At the 25th day of the metamorphosis stage, the expression level of Mn-CycAmRNA in female shrimp was 3.5 times higher than that in male shrimp, which may be related to the proliferation of oogonia and the formation of oocytes. In situ hybridization (ISH) of ovary showed Mn-CycA was examined in all stages and was mainly located in oogonia and oocytes. Compared with the control group, the obvious change of gonad somatic index (GSI) proved that injection of Mn-CycA dsRNA could delay the ovarian development cycle, which provided strong evidence for the involvement of Mn-CycA in ovarian maturation and oogenesis, and expanded a new perspective for studying the fast ovarian development cycle in M. nipponense.

Chatterboxes: the structural and functional diversity of cyclins

Proteins of the cyclin family have divergent sequences and execute diverse roles within the cell while sharing a common fold: the cyclin box domain. Structural studies of cyclins have played a key role in our characterization and understanding of cellular processes that they control, though to date only ten of the 29 CDK-activating cyclins have been structurally characterized by X-ray crystallography or cryo-electron microscopy with or without their cognate kinases. In this review, we survey the available structures of human cyclins, highlighting their molecular features in the context of their cellular roles. We pay particular attention to how cyclin activity is regulated through fine control of degradation motif recognition and ubiquitination. Finally, we discuss the emergent roles of cyclins independent of their roles as cyclin-dependent protein kinase activators, demonstrating the cyclin box domain to be a versatile and generalized scaffolding domain for protein-protein interactions across the cellular machinery.

Functions of cyclins and CDKs in mammalian gametogenesis†

Cyclins and cyclin-dependent kinases (CDKs) are key regulators of the cell cycle. Most of our understanding of their functions has been obtained from studies in single-cell organisms and mitotically proliferating cultured cells. In mammals, there are more than 20 cyclins and 20 CDKs. Although genetic ablation studies in mice have shown that most of these factors are dispensable for viability and fertility, uncovering their functional redundancy, CCNA2, CCNB1, and CDK1 are essential for embryonic development. Cyclin/CDK complexes are known to regulate both mitotic and meiotic cell cycles. While some mechanisms are common to both types of cell divisions, meiosis has unique characteristics and requirements. During meiosis, DNA replication is followed by two successive rounds of cell division. In addition, mammalian germ cells experience a prolonged prophase I in males or a long period of arrest in prophase I in females. Therefore, cyclins and CDKs may have functions in meiosis distinct from their mitotic functions and indeed, meiosis-specific cyclins, CCNA1 and CCNB3, have been identified. Here, we describe recent advances in the field of cyclins and CDKs with a focus on meiosis and early embryogenesis.

Ordered dephosphorylation initiated by the selective proteolysis of cyclin B drives mitotic exit

APC/C-mediated proteolysis of cyclin B and securin promotes anaphase entry, inactivating CDK1 and permitting chromosome segregation, respectively. Reduction of CDK1 activity relieves inhibition of the CDK1-counteracting phosphatases PP1 and PP2A-B55, allowing wide-spread dephosphorylation of substrates. Meanwhile, continued APC/C activity promotes proteolysis of other mitotic regulators. Together, these activities orchestrate a complex series of events during mitotic exit. However, the relative importance of regulated proteolysis and dephosphorylation in dictating the order and timing of these events remains unclear. Using high temporal-resolution proteomics, we compare the relative extent of proteolysis and protein dephosphorylation. This reveals highly-selective rapid proteolysis of cyclin B, securin and geminin at the metaphase-anaphase transition, followed by slow proteolysis of other substrates. Dephosphorylation requires APC/C-dependent destruction of cyclin B and was resolved into PP1-dependent categories with unique sequence motifs. We conclude that dephosphorylation initiated by selective proteolysis of cyclin B drives the bulk of changes observed during mitotic exit.

The G2-to-M transition from a phosphatase perspective: a new vision of the meiotic division

Cell division is orchestrated by the phosphorylation and dephosphorylation of thousands of proteins. These post-translational modifications underlie the molecular cascades converging to the activation of the universal mitotic kinase, Cdk1, and entry into cell division. They also govern the structural events that sustain the mechanics of cell division. While the role of protein kinases in mitosis has been well documented by decades of investigations, little was known regarding the control of protein phosphatases until the recent years. However, the regulation of phosphatase activities is as essential as kinases in controlling the activation of Cdk1 to enter M-phase. The regulation and the function of phosphatases result from post-translational modifications but also from the combinatorial association between conserved catalytic subunits and regulatory subunits that drive their substrate specificity, their cellular localization and their activity. It now appears that sequential dephosphorylations orchestrated by a network of phosphatase activities trigger Cdk1 activation and then order the structural events necessary for the timely execution of cell division. This review discusses a series of recent works describing the important roles played by protein phosphatases for the proper regulation of meiotic division. Many breakthroughs in the field of cell cycle research came from studies on oocyte meiotic divisions. Indeed, the meiotic division shares most of the molecular regulators with mitosis. The natural arrests of oocytes in G2 and in M-phase, the giant size of these cells, the variety of model species allowing either biochemical or imaging as well as genetics approaches explain why the process of meiosis has served as an historical model to decipher signalling pathways involved in the G2-to-M transition. The review especially highlights how the phosphatase PP2A-B55δ critically orchestrates the timing of meiosis resumption in amphibian oocytes. By opposing the kinase PKA, PP2A-B55δ controls the release of the G2 arrest through the dephosphorylation of their substrate, Arpp19. Few hours later, the inhibition of PP2A-B55δ by Arpp19 releases its opposing kinase, Cdk1, and triggers M-phase. In coordination with a variety of phosphatases and kinases, the PP2A-B55δ/Arpp19 duo therefore emerges as the key effector of the G2-to-M transition.

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.

Mitotic kinase anchoring proteins: the navigators of cell division

The coordinated activities of many protein kinases, acting on multiple protein substrates, ensures the error-free progression through mitosis of eukaryotic cells. Enormous research effort has thus been devoted to studying the roles and regulation of these mitotic kinases, and to the identification of their physiological substrates. Central for the timely deployment of specific protein kinases to their appropriate substrates during the cell division cycle are the many anchoring proteins, which serve critical regulatory roles. Through direct association, anchoring proteins are capable of modulating the catalytic activity and/or sub-cellular distribution of the mitotic kinases they associate with. The key roles of some anchoring proteins in cell division are well-established, whilst others are still being unearthed. Here, we review the current knowledge on anchoring proteins for some mitotic kinases, and highlight how targeting anchoring proteins for inhibition, instead of the mitotic kinases themselves, could be advantageous for disrupting the cell division cycle.

MiR-200-3p Is Potentially Involved in Cell Cycle Arrest by Regulating Cyclin A during Aestivation in Apostichopus japonicus

The sea cucumber (Apostichopus japonicus) has become a good model organism for studying environmentally induced aestivation in marine invertebrates. We hypothesized that mechanisms that arrest energy-expensive cell cycle activity would contribute significantly to establishing the hypometabolic state during aestivation. Cyclin A is a core and particularly interesting cell cycle regulator that functions in both the S phase and in mitosis. In the present study, negative relationships between miR-200-3p and AjCA expressions were detected at both the transcriptional and the translational levels during aestivation in A. japonicus. Dual-luciferase reporter assays confirmed the targeted location of the miR-200-3p binding site within the AjCA gene transcript. Furthermore, gain- and loss-of-function experiments were conducted in vivo with sea cucumbers to verify the interaction between miR-200-3p and AjCA in intestine tissue by qRT-PCR and Western blotting. The results show that the overexpression of miR-200-3p mimics suppressed AjCA transcript levels and translated protein production, whereas transfection with a miR-200-3p inhibitor enhanced both AjCA mRNA and AjCA protein in A. japonicus intestine. Our findings suggested a potential mechanism that reversibly arrests cell cycle progression during aestivation, which may center on miR-200-3p inhibitory control over the translation of cyclin A mRNA transcripts.

Comprehensive and quantitative analysis of G1 cyclins. A tool for studying the cell cycle

In eukaryotes, the cell cycle is driven by the actions of several cyclin dependent kinases (CDKs) and an array of regulatory proteins called cyclins, due to the cyclical expression patterns of the latter. In yeast, the accepted pattern of cyclin waves is based on qualitative studies performed by different laboratories using different strain backgrounds, different growing conditions and media, and different kinds of genetic manipulation. Additionally, only the subset of cyclins regulating Cdc28 was included, while the Pho85 cyclins were excluded. We describe a comprehensive, quantitative and accurate blueprint of G₁ cyclins in the yeast Saccharomyces cerevisiae that, in addition to validating previous conclusions, yields new findings and establishes an accurate G₁ cyclin blueprint. For the purposes of this research, we produced a collection of strains with all G₁ cyclins identically tagged using the same and most respectful procedure possible. We report the contribution of each G₁ cyclin for a broad array of growing and stress conditions, describe an unknown role for Pcl2 in heat-stress conditions and demonstrate the importance of maintaining the 3’UTR sequence of cyclins untouched during the tagging process.

Who guards the guardian? Mechanisms that restrain APC/C during the cell cycle

The cell cycle is principally controlled by Cyclin Dependent Kinases (CDKs), whose oscillating activities are determined by binding to Cyclin coactivators. Cyclins exhibit dynamic changes in abundance as cells pass through the cell cycle. The sequential, timed accumulation and degradation of Cyclins, as well as many other proteins, imposes order on the cell cycle and contributes to genome maintenance. The destruction of many cell cycle regulated proteins, including Cyclins A and B, is controlled by a large, multi-subunit E3 ubiquitin ligase termed the Anaphase Promoting Complex/Cyclosome (APC/C). APC/C activity is tightly regulated during the cell cycle. Its activation state increases dramatically in mid-mitosis and it remains active until the end of G1 phase. Following its mandatory inactivation at the G1/S boundary, APC/C activity remains low until the subsequent mitosis. Due to its role in guarding against the inappropriate or untimely accumulation of Cyclins, the APC/C is a core component of the cell cycle oscillator. In addition to the regulation of Cyclins, APC/C controls the degradation of many other substrates. Therefore, it is vital that the activity of APC/C itself be tightly guarded. The APC/C is most well studied for its role and regulation during mitosis. However, the APC/C also plays a similarly important and conserved role in the maintenance of G1 phase. Here we review the diverse mechanisms counteracting APC/C activity throughout the cell cycle and the importance of their coordinated actions on cell growth, proliferation, and disease.

Cyclin B3 Deficiency Impairs Germline Stem Cell Maintenance and Its Overexpression Delays Cystoblast Differentiation in Drosophila Ovary

It is well known that cyclinB3 (cycB3) plays a key role in the control of cell cycle progression. However, whether cycB3 is involved in stem cell fate determination remains unknown. The Drosophila ovary provides an exclusive model for studying the intrinsic and extrinsic factors that modulate the fate of germline stem cells (GSCs). Here, using this model, we show that DrosophilacycB3 plays a new role in controlling the fate of germline stem cells (GSC). Results from cycB3 genetic analyses demonstrate that cycB3 is intrinsically required for GSC maintenance. Results from green fluorescent protein (GFP)-transgene reporter assays show that cycB3 is not involved in Dad-mediated regulation of Bmp signaling, or required for dpp-induced bam transcriptional silencing. Double mutants of bam and cycB3 phenocopied bam single mutants, suggesting that cycB3 functions in a bam-dependent manner in GSCs. Deficiency of cycB3 fails to cause apoptosis in GSCs or influence cystoblast (CB) differentiation into oocytes. Furthermore, overexpression of cycB3 dramatically increases the CB number in Drosophila ovaries, suggesting that an excess of cycB3 function delays CB differentiation. Given that the cycB3 gene is evolutionarily conserved, from insects to humans, cycB3 may also be involved in controlling the fate of GSCs in humans.

EDAG promotes the expansion and survival of human CD34+ cells

EDAG is multifunctional transcriptional regulator primarily expressed in the lin^loc^-kit⁺Sca-1⁺ hematopoietic stem cells (HSC) and CD34⁺ progenitor cells. Previous studies indicate that EDAG is required for maintaining hematopoietic lineage commitment balance. Here using ex vivo culture and HSC transplantation models, we report that EDAG enhances the proliferative potential of human cord blood CD34⁺ cells, increases survival, prevents cell apoptosis and promotes their repopulating capacity. Moreover, EDAG overexpression induces rapid entry of CD34⁺ cells into the cell cycle. Gene expression profile analysis indicate that EDAG knockdown leads to down-regulation of various positive cell cycle regulators including cyclin A, B, D, and E. Together these data provides novel insights into EDAG in regulation of expansion and survival of human hematopoietic stem/progenitor cells.

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

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

Preterm labor in the absence of acute histologic chorioamnionitis is characterized by cellular senescence of the chorioamniotic membranes

BACKGROUND

Decidual senescence has been considered a mechanism of disease for spontaneous preterm labor in the absence of severe acute inflammation. Yet, signs of cellular senescence have also been observed in the chorioamniotic membranes from women who underwent the physiological process of labor at term.

OBJECTIVE

We aimed to investigate whether, in the absence of acute histologic chorioamnionitis, the chorioamniotic membranes from women who underwent spontaneous preterm labor or labor at term exhibit signs of cellular senescence.

STUDY DESIGN

Chorioamniotic membrane samples were collected from women who underwent spontaneous preterm labor or labor at term. Gestational age-matched nonlabor controls were also included. Senescence-associated genes/proteins were determined using reverse transcription quantitative polymerase chain reaction analysis (n = 7-9 each for array; n = 26-28 each for validation), enzyme-linked immunosorbent assays (n = 7-9 each), immunoblotting (n = 6-7 each), and immunohistochemistry (n = 7-8 each). Senescence-associated β-galactosidase activity (n = 7-11 each) and telomere length (n = 15-22 each) were also evaluated.

RESULTS

In the chorioamniotic membranes without acute histologic chorioamnionitis: (1) the expression profile of senescence-associated genes was different between the labor groups (term in labor and preterm in labor) and the nonlabor groups (term no labor and preterm no labor), yet there were differences between the term in labor and preterm in labor groups; (2) most of the differentially expressed genes among the groups were closely related to the tumor suppressor protein (TP53) pathway; (3) the expression of TP53 was down-regulated in the term in labor and preterm in labor groups compared to their nonlabor counterparts; (4) the expression of CDKN1A (gene coding for p21) was up-regulated in the term in labor and preterm in labor groups compared to their nonlabor counterparts; (5) the expression of the cyclin kinase CDK2 and cyclins CCNA2, CCNB1, and CCNE1 was down-regulated in the preterm in labor group compared to the preterm no labor group; (6) the concentration of TP53 was lower in the preterm in labor group than in the preterm no labor and term in labor groups; (7) the senescence-associated β-galactosidase activity was greater in the preterm in labor group than in the preterm no labor and term in labor groups; (8) the concentration of phospho-S6 ribosomal protein was reduced in the term in labor group compared to its nonlabor counterpart, but no differences were observed between the preterm in labor and preterm no labor groups; and (9) no significant differences were observed in relative telomere length among the study groups (term no labor, term in labor, preterm no labor, and preterm in labor).

CONCLUSION

In the absence of acute histologic chorioamnionitis, signs of cellular senescence are present in the chorioamniotic membranes from women who underwent spontaneous preterm labor compared to those who delivered preterm in the absence of labor. However, the chorioamniotic membranes from women who underwent spontaneous labor at term did not show consistent signs of cellular senescence in the absence of histologic chorioamnionitis. These results suggest that different pathways are implicated in the pathological and physiological processes of labor.

Intertwined control of the cell cycle and nucleocytoplasmic transport by the cyclin-dependent kinase Pho85 and RanGTPase Gsp1 in Saccharomyces cerevisiae

Deciphering the molecular mechanisms that connect cell cycle progression and nucleocytoplasmic transport is of particular interest: this intertwined relationship, once understood, may provide useful insight on the diseases resulting from the malfunction of these processes. In the present study we report on findings that indicate a biochemical connection between the cell cycle regulator CDK Pho85 and Ran-GTPase Gsp1, an essential nucleocytoplasmic transport component. When Gsp1 cannot be phosphorylated by Pho85, the cell cycle progression is impaired. Accordingly, a nonphosphorylatable version of Gsp1 abnormally localizes to the nucleus, which impairs the nuclear transport of molecules, including key components of cell cycle progression. Furthermore, our results suggest that the physical interaction of Gsp1 and the Kap95 karyopherin, essential to the release of nuclear cargoes, is altered. Altogether, the present findings point to the involvement of a biochemical mechanism in the interlocked regulation of the cell cycle and nuclear transport.

Cyclin A2 modulates kinetochore-microtubule attachment in meiosis II

Cyclin A2 is a crucial mitotic Cdk regulatory partner that coordinates entry into mitosis and is then destroyed in prometaphase within minutes of nuclear envelope breakdown. The role of cyclin A2 in female meiosis and its dynamics during the transition from meiosis I (MI) to meiosis II (MII) remain unclear. We found that cyclin A2 decreases in prometaphase I but recovers after the first meiotic division and persists, uniquely for metaphase, in MII-arrested oocytes. Conditional deletion of cyclin A2 from mouse oocytes has no discernible effect on MI but leads to disrupted MII spindles and increased merotelic attachments. On stimulation of exit from MII, there is a dramatic increase in lagging chromosomes and an inhibition of cytokinesis. These defects are associated with an increase in microtubule stability in MII spindles, suggesting that cyclin A2 mediates the fidelity of MII by maintaining microtubule dynamics during the rapid formation of the MII spindle.

Bistability of mitotic entry and exit switches during open mitosis in mammalian cells

Mitotic entry and exit are switch-like transitions that are driven by the activation and inactivation of Cdk1 and mitotic cyclins. This simple on/off reaction turns out to be a complex interplay of various reversible reactions, feedback loops, and thresholds that involve both the direct regulators of Cdk1 and its counteracting phosphatases. In this review, we summarize the interplay of the major components of the system and discuss how they work together to generate robustness, bistability, and irreversibility. We propose that it may be beneficial to regard the entry and exit reactions as two separate reversible switches that are distinguished by differences in the state of phosphatase activity, mitotic proteolysis, and a dramatic rearrangement of cellular components after nuclear envelope breakdown, and discuss how the major Cdk1 activity thresholds could be determined for these transitions.

Pursuing the impossible: an interview with Tim Hunt

Tim Hunt took an undergraduate degree in Natural Sciences at Cambridge in 1964, and his PhD and subsequent work focussed on the control of protein synthesis until 1982, when his adventitious discovery of the central cell cycle regulator cyclin, while he was teaching at the Marine Biological Laboratory in Woods Hole, redirected him to the study of cell cycle regulation. From 1990 to his retirement Tim worked in the Clare Hall Laboratories of Cancer Research UK. He shared the Nobel Prize in Physiology and Medicine with Lee Hartwell and Paul Nurse in 2001, and talked to us about the series of coincidences that led him to the prizewinning discovery.

Loss of Cdk2 and cyclin A2 impairs cell proliferation and tumorigenesis

Cell-cycle inhibition has yet to offer a generally effective approach to cancer treatment, but a full evaluation of different combinations of cell-cycle inhibitors has not been evaluated. Cyclin A2, a core component of the cell cycle, is often aberrantly expressed in cancer where it may impact cell proliferation. In this study, we investigated the role of cyclin A2 in tumorigenesis using a conditional genetic knockout mouse model. Cyclin A2 deletion in oncogene-transformed mouse embryonic fibroblasts (MEF) suppressed tumor formation in immunocompromised mice. These findings were confirmed in mice with cyclin A2-deficient hepatocytes, where a delay in liver tumor formation was observed. Because cyclin A2 acts in complex with Cdk2 in the cell cycle, we explored a hypothesized role for Cdk2 dysregulation in this effect through conditional deletions of both genes. In oncogene-transformed MEFs lacking both genes, tumor formation was strongly suppressed in a manner associated with decreased proliferation, premature senescence, and error-prone recovery from serum deprivation after immortalization. Whereas loss of cyclin A2 led to a compensatory increase in Cdk1 activity, this did not occur with loss of both Cdk2 and cyclin A2. Our work offers a rationale to explore combinations of Cdk1 and Cdk2 inhibitors as a general approach in cancer therapy.

Evolution of a developmental mechanism: Species-specific regulation of the cell cycle and the timing of events during craniofacial osteogenesis

Neural crest mesenchyme (NCM) controls species-specific pattern in the craniofacial skeleton but how this cell population accomplishes such a complex task remains unclear. To elucidate mechanisms through which NCM directs skeletal development and evolution, we made chimeras from quail and duck embryos, which differ markedly in their craniofacial morphology and maturation rates. We show that quail NCM, when transplanted into duck, maintains its faster timetable for development and autonomously executes molecular and cellular programs for the induction, differentiation, and mineralization of bone, including premature expression of osteogenic genes such as Runx2 and Col1a1. In contrast, the duck host systemic environment appears to be relatively permissive and supports osteogenesis independently by providing circulating minerals and a vascular network. Further experiments reveal that NCM establishes the timing of osteogenesis by regulating cell cycle progression in a stage- and species-specific manner. Altering the time-course of D-type cyclin expression mimics chimeras by accelerating expression of Runx2 and Col1a1. We also discover higher endogenous expression of Runx2 in quail coincident with their smaller craniofacial skeletons, and by prematurely over-expressing Runx2 in chick embryos we reduce the overall size of the craniofacial skeleton. Thus, our work indicates that NCM establishes species-specific size in the craniofacial skeleton by controlling cell cycle, Runx2 expression, and the timing of key events during osteogenesis.

Regulation of oocyte meiotic maturation by somatic cells

In preovulatory follicles, each oocyte is surrounded by numerous layers of cumulus cells, forming the cumulus cell-oocyte complex. An LH surge induces meiotic resumption of the oocyte to progress to metaphase II. Because the expression of LH receptors is not detected in the oocyte and is minimal (negligible) in cumulus cells as compared with granulosa cells, secondary factors from granulosa cells are required to induce the ovulation process. One of the key factors secreted from granulosa cells is an EGF-like factor that activates the EGFR-ERK1/2 pathway in cumulus cells. The activated ERK1/2 pathway is not only involved in gene expression but also essential for the close of gap-junctional communication among cumulus cells and between cumulus cells and the oocyte. Closing gap-junctional communication decreases the amount of cGMP and/or cAMP to transfer into the oocyte, which requires activation of phosphodiesterase type III (PDE3) in the oocyte. PDE3 brakes down cAMP to decrease PKA activity in the oocyte. This decrease in PKA activity induces activation of CDK1 to resume meiosis from the germinal vesicle stage. Thus, the functions of cumulus cells that are regulated by granulosa cell-secreted factors are essential for oocyte meiotic resumption and maturation with developmental competence.

Ca2+ signaling during mammalian fertilization: requirements, players, and adaptations

Changes in the intracellular concentration of calcium ([Ca(2+)](i)) represent a vital signaling mechanism enabling communication among cells and between cells and the environment. The initiation of embryo development depends on a [Ca(2+)](i) increase(s) in the egg, which is generally induced during fertilization. The [Ca(2+)](i) increase signals egg activation, which is the first stage in embryo development, and that consist of biochemical and structural changes that transform eggs into zygotes. The spatiotemporal patterns of [Ca(2+)](i) at fertilization show variability, most likely reflecting adaptations to fertilizing conditions and to the duration of embryonic cell cycles. In mammals, the focus of this review, the fertilization [Ca(2+)](i) signal displays unique properties in that it is initiated after gamete fusion by release of a sperm-derived factor and by periodic and extended [Ca(2+)](i) responses. Here, we will discuss the events of egg activation regulated by increases in [Ca(2+)](i), the possible downstream targets that effect these egg activation events, and the property and identity of molecules both in sperm and eggs that underpin the initiation and persistence of the [Ca(2+)](i) responses in these species.

Function of the A-type cyclins during gametogenesis and early embryogenesis

The cyclins and their cyclin-dependent kinase partners, the Cdks, are the basic components of the machinery that regulates the passage of cells through the cell cycle. Among the cyclins, those known as the A-type cyclins are unique in that in somatic cells, they appear to function at two stages of the cell cycle, at the G1-S transition and again as the cells prepare to enter M-phase. Higher vertebrate organisms have two A-type cyclins, cyclin A1 and cyclin A2, both of which are expressed in the germ line and/or early embryo, following highly specialized patterns that suggest functions in both mitosis and meiosis. Insight into their in vivo functions has been obtained from gene targeting experiments in the mouse model. Loss of cyclin A1 results in disruption of spermatogenesis and male sterility due to cell arrest in the late diplotene stage of the meiotic cell cycle. In contrast, cyclin A2-deficiency is marked by early embryonic lethality; thus, understanding the function of cyclin A2 in the adult germ line awaits conditional mutagenesis or other approaches to knock down its expression.

The roles of cyclin A2, B1, and B2 in early and late mitotic events

This paper presents evidence that chromatin condensation, like nuclear envelope breakdown, is brought about through the combined effects of cyclins A2 and B1, and that cyclins B1 and B2 are largely responsible for maintenance of a spindle assembly checkpoint arrest.

Here we have used siRNAs and time-lapse epifluorescence microscopy to examine the roles of various candidate mitotic cyclins in chromatin condensation in HeLa cells. Knocking down cyclin A2 resulted in a substantial (∼7 h) delay in chromatin condensation and histone H3 phosphorylation, and expressing an siRNA-resistant form of cyclin A2 partially rescued chromatin condensation. There was no detectable delay in DNA replication in the cyclin A2 knockdowns, arguing that the delay in chromatin condensation is not secondary to a delay in S-phase completion. Cyclin A2 is required for the activation and nuclear accumulation of cyclin B1-Cdk1, raising the possibility that cyclin B1-Cdk1 mediates the effects of cyclin A2. Consistent with this possibility, we found that chromatin condensation was tightly associated temporally with the redistribution of cyclin B1 to the nucleus. Moreover, a constitutively nuclear cyclin B1 rescued chromatin condensation in cyclin A2 knockdown cells. On the other hand, knocking down cyclin B1 delayed chromatin condensation by only about one hour. Our working hypothesis is that active, nuclear cyclin B1-Cdk1 normally cooperates with cyclin A2 to bring about early mitotic events. Because cyclin A2 is present only during the early stages of mitosis, we asked whether cyclin B knockdown might have more dramatic defects on late mitotic events. Consistent with this possibility, we found that cyclin B1- and cyclin B1/B2-knockdown cells had difficulty in maintaining a mitotic arrest in the presence of nocodazole. Taken together, these data suggest that cyclin A2 helps initiate mitosis, in part through its effects on cyclin B1, and that cyclins B1 and B2 are particularly critical for the maintenance of the mitotic state.

Discovery of a distinct domain in cyclin A sufficient for centrosomal localization independently of Cdk binding

Centrosomes have recently emerged as key regulators of the cell cycle. The G1/S transition requires a functional centrosome, and centrosomal localization of numerous proteins, including cyclin/Cdk complexes, is important for the G2/M transition. Here we identify a modular centrosomal localization signal (CLS) localizing cyclin A to centrosomes independently of Cdk binding. The cyclin A CLS is located in a distinct part of the molecule compared with the cyclin E CLS and includes the MRAIL hydrophobic patch involved in substrate recognition. The cyclin A CLS interacts with p27(KIP1), and expression of p27(KIP1) removes cyclin A but not cyclin E from centrosomes. Expression of the cyclin A CLS displaces both endogenous cyclin A and E from centrosomes and inhibits DNA replication, supporting an emerging concept that DNA replication is linked to centrosomal events. Structural analysis indicates that differences in surface charge and length of the C-terminal helix explain why the MRAIL region in cyclin E is not a functional CLS. These results indicate that the cyclin A CLS may contribute to targeting and recognition of centrosomal Cdk substrates and is required for specific effects of p27(KIP1) on cyclin A-Cdk2.

Cyclin A is redundant in fibroblasts but essential in hematopoietic and embryonic stem cells

Cyclins are regulatory subunits of cyclin-dependent kinases. Cyclin A, the first cyclin ever cloned, is thought to be an essential component of the cell-cycle engine. Mammalian cells encode two A-type cyclins, testis-specific cyclin A1 and ubiquitously expressed cyclin A2. Here, we tested the requirement for cyclin A function using conditional knockout mice lacking both A-type cyclins. We found that acute ablation of cyclin A in fibroblasts did not affect cell proliferation, but led to prolonged expression of another cyclin, cyclin E, across the cell cycle. However, combined ablation of all A- and E-type cyclins extinguished cell division. In contrast, cyclin A function was essential for cell-cycle progression of hematopoietic and embryonic stem cells. Expression of cyclin A is particularly high in these compartments, which might render stem cells dependent on cyclin A, whereas in fibroblasts cyclins A and E play redundant roles in cell proliferation.

The hunt for cyclin

It is 25 years since Tim Hunt discovered cyclin, the oscillating protein that drives activation of cyclin-dependent kinases and entry into mitosis (Evans et al., 1983).

The roles of Ca2+, downstream protein kinases, and oscillatory signaling in regulating fertilization and the activation of development

Reviews in Developmental Biology have covered the pathways that generate the all-important intracellular calcium (Ca(2+)) signal at fertilization [Miyazaki, S., Shirakawa, H., Nakada, K., Honda, Y., 1993a. Essential role of the inositol 1,4,5-trisphosphate receptor/Ca(2+) release channel in Ca(2+) waves and Ca(2+) oscillations at fertilization of mammalian eggs. Dev. Biol. 158, 62-78; Runft, L., Jaffe, L., Mehlmann, L., 2002. Egg activation at fertilization: where it all begins. Dev. Biol. 245, 237-254] and the different temporal responses of Ca(2+) in many organisms [Stricker, S., 1999. Comparative biology of calcium signaling during fertilization and egg activation in animals. Dev. Biol. 211, 157-176]. Those reviews raise the importance of identifying how Ca(2+) causes the events of egg activation (EEA) and to what extent these temporal Ca(2+) responses encode developmental information. This review covers recent studies that have analyzed how these Ca(2+) signals are interpreted by specific proteins, and how these proteins regulate various EEA responsible for the onset of development. Many of these proteins are protein kinases (CaMKII, PKC, MPF, MAPK, MLCK) whose activity is directly or indirectly regulated by Ca(2+), and whose amount increases during late oocyte maturation. We cover biochemical progress in defining the signaling pathways between Ca(2+) and the EEA, as well as discuss how oscillatory or multiple Ca(2+) signals are likely to have specific advantages biochemically and/or developmentally. These emerging concepts are put into historical context, emphasizing that key contributions have come from many organisms. The intricate interdependence of Ca(2+), Ca(2+)-dependent proteins, and the EEA raise many new questions for future investigations that will provide insight into the extent to which fertilization-associated signaling has long-range implications for development. In addition, answers to these questions should be beneficial to establishing parameters of egg quality for human and animal IVF, as well as improving egg activation protocols for somatic cell nuclear transfer to generate stem cells and save endangered species.

Regulation of CYL/cyclin D genes by colony-stimulating factor 1

The proliferative effects of colony-stimulating factor 1 (CSF-1) on macrophages are exerted only throughout the G1 phase of the cell cycle. Genetic targets of the delayed early response to CSF-1 include novel G1 cyclin (CYL or cyclin D) genes. In macrophages, cyclin D1 is induced early in G1 and is expressed throughout the cell cycle as long as CSF-1 is present. The cyclin D1 protein turns over rapidly in CSF-1-stimulated cells and its level declines precipitously upon CSF-1 withdrawal. Cyclin D2 is induced later in G1 and its expression is periodic, whereas cyclin D3 is not expressed in macrophages but is regulated by growth factors in other cell types. The cyclin D1 protein associates during G1 with a polypeptide antigenically related to p34cdc2 and binds in vitro to a histone H1 kinase present in lysates of CSF-1-starved macrophages. The instability of the cyclin D1 protein and its ability to rescue a cyclin-dependent kinase activity from growth factor-deprived macrophages together suggest that the cyclin D protein is the dynamic partner in the complex. The timing of expression of cyclin D genes suggests that they act to link growth factor signals with cell cycle transitions during G1.

The genomic repertoire for cell cycle control and DNA metabolism in S. purpuratus

A search of the Strongylocentrotus purpuratus genome for genes associated with cell cycle control and DNA metabolism shows that the known repertoire of these genes is conserved in the sea urchin, although with fewer family members represented than in vertebrates, and with some cases of echinoderm-specific gene diversifications. For example, while homologues of the known cyclins are mostly encoded by single genes in S. purpuratus (unlike vertebrates, which have multiple isoforms), there are additional genes encoding novel cyclins of the B and K/L types. Almost all known cyclin-dependent kinases (CDKs) or CDK-like proteins have an orthologue in S. purpuratus; CDK3 is one exception, whereas CDK4 and 6 are represented by a single homologue, referred to as CDK4. While the complexity of the two families of mitotic kinases, Polo and Aurora, is close to that found in the nematode, the diversity of the NIMA-related kinases (NEK proteins) approaches that of vertebrates. Among the nine NEK proteins found in S. purpuratus, eight could be assigned orthologues in vertebrates, whereas the ninth is unique to sea urchins. Most known DNA replication, DNA repair and mitotic checkpoint genes are also present, as are homologues of the pRB (two) and p53 (one) tumor suppressors. Interestingly, the p21/p27 family of CDK inhibitors is represented by one homologue, whereas the INK4 and ARF families of tumor suppressors appear to be absent, suggesting that these evolved only in vertebrates. Our results suggest that, while the cell cycle control mechanisms known from other animals are generally conserved in sea urchin, parts of the machinery have diversified within the echinoderm lineage. The set of genes uncovered in this analysis of the S. purpuratus genome should enhance future research on cell cycle control and developmental regulation in this model.

Genome-wide analysis of cyclin family in rice (Oryza Sativa L.)

The cyclins together with highly conserved cyclin-dependent kinases regulate cell cycle progression in plants. Although extensive and systematic study on cell cycle mechanisms and cyclin functions in yeasts and animals has been carried out, only a small number of plant cyclins have been characterized and classified functionally and phylogenetically. We identified several types of cyclin genes in the rice genome and characterized them by phylogenetic, tandem and segmental duplications analyses. Our results indicated that there were at least 49 predicted rice cyclin genes in the rice genome, and they were distributed on 12 chromosomes. Of these cyclins, one possessed only cyclin_C domain and no cyclin_N domain, and the remaining 48 cyclins with cyclin_N domains were classified as nine types based on evolutionary relationships. Eight of these nine types were common between rice and Arabidopsis, whereas only one, known as F-type cyclins, was unique to rice. No homologues of the F-type cyclins in plants could be retrieved from the public databases, and reverse transcription-PCR analysis supported an existence of the F-type cyclin genes. Sequence alignment suggested that the cyclin genes in the rice genome experienced a mass of gene tandem and segmental duplications occurred on seven chromosomes related to the origins of new cyclin genes. Our study provided an opportunity to facilitate assessment and classification of new members, serving as a guide for further functional elucidation of rice cyclins.

The ubiquitin-proteasome pathway in cell cycle control

Ubiquitin-mediated proteolysis is one of the key mechanisms underlying cell cycle control. The removal of barriers posed by accumulation of negative regulators, as well as the clearance of proteins when they are no longer needed or deleterious, are carried out via the ubiquitin-proteasome system. Ubiquitin conjugating enzymes and protein-ubiquitin ligases collaborate to mark proteins destined for degradation by the proteasome by covalent attachment of multi-ubiquitin chains. Most regulated proteolysis during the cell cycle can be attributed to two families of protein-ubiquitin ligases. The anaphase promoting complex/cyclosome (APC/C) is activated during mitosis and G1 where it is responsible for eliminating proteins that impede mitotic progression and that would have deleterious consequences if allowed to accumulate during G1. SCF (Skp1/Culin/F-box protein) protein-ubiquitin ligases ubiquitylate proteins that are marked by phosphorylation at specific sequences known as phosphodegrons. Targeting of proteins for destruction by phosphorylation provides a mechanism for linking cell cycle regulation to internal and external signaling pathways via regulated protein kinase activities.

Quantification of fluorescence in thick specimens, with an application to cyclin B-GFP expression in starfish oocytes

BACKGROUND INFORMATION

Fluorescence imaging of living cells is widely used in cell biology. It is now being extended to thick specimens such as large cells or tissues where it is important to establish methods for obtaining quantitative fluorescence data due to the increasing importance of computational and systems biology approaches.

RESULTS

Fluorescent solutions were used as a calibration standard for determining cellular fluorescence concentrations from z series image sequences. The accuracy of the measurements was evaluated using quantitatively injected cells. Different fluorescence attenuation rates of the cytoplasm and nucleoplasm were documented, and autofluorescence levels were determined. This method was used to characterize the effect of cyclin B overexpression on cell-cycle timing in starfish oocytes. The time interval between application of maturation hormone and germinal vesicle breakdown decreased with increasing cyclin B-GFP (green fluorescent protein) concentration to a level of 100-300 nM, beyond which there was no effect.

CONCLUSIONS

Conditions for determining fluorescent probe concentrations in large cells or multicellular tissues were established, which will facilitate the collection of data for quantitative studies. This method was used to characterize the effect of cyclin B-GFP expression levels on cell-cycle timing in starfish oocytes.

Recycling the cell cycle: cyclins revisited

I discuss advances in the cell cycle in the 21 years since cyclin was discovered. The surprising redundancy amongst the classical cyclins (A, B, and E) and cyclin-dependent kinases (Cdk1 and Cdk2) show that the important differences between these proteins are when and where they are expressed rather than the proteins they phosphorylate. Although the broad principles of the cell cycle oscillator are widely accepted, we are surprisingly ignorant of its detailed mechanism. This is especially true of the anaphase promoting complex (APC), the machine that triggers chromosome segregation and the exit of mitosis by targeting securin and mitotic cyclins for destruction. I discuss how a cyclin/Cdk-based engine could have evolved to assume control of the cell cycle from other, older protein kinases.

G2/M transition of pig oocytes: How do oocytes initiate maturation?

In the ovary, mammalian oocytes resume meiosis and mature to the second metaphase when they are stimulated with gonadotrophins. Similarly, oocytes can mature in vitro when they are liberated from ovarian follicles and cultured under appropriate conditions. Early in the process of maturation, oocytes undergo dramatic but well-ordered changes at the G2/M transition in the cell cycle including: (i) chromosome condensation; (ii) nucleolus disassembly; (iii) germinal vesicle breakdown (GVBD); and (iv) spindle formation in the first metaphase (MI-spindle). These events have been thought to be induced by MPF (maturation-promoting factor or M-phase promoting factor), now known as Cdc2 kinase or Cdk1 kinase, which consists of a catalytic subunit, Cdc2, and a cyclin B regulatory subunit. In fact, nuclear lamins are phosphorylated by Cdc2 kinase, and nuclear membrane breakdown occurs concomitantly with the activation of Cdc2 kinase in the M-phase of both somatic cells and oocytes. Based on the classical and recent studies of the pig oocyte, however, the chromosomes start to condense and the nucleolus disassembles before full activation of Cdc2 kinase, and the MI-spindle is formed after activation of both Cdc2 kinase and MAP kinase; another kinase known to become activated during oocyte maturation. These findings suggest that chromosome condensation and nucleolus disassembly in oocytes are induced by either some kinase(s) other than Cdc2 kinase and MAP kinase or some phosphatase(s). The accumulation of new results regarding the molecular nature of oocyte maturation is important for improving the reproductive technologies in domestic animals as well as in humans. (Reprod Med Biol 2003; 2: 91-99).

Cyclin aggregation and robustness of bio-switching

During the cell cycle, Cdc2-cyclin B kinase abruptly becomes active and triggers the entry into mitosis/meiosis. Recently, it was found that inactive Cdc2-cyclin B is present in aggregates in immature starfish oocytes and becomes disaggregated at the time of its activation during maturation. We discuss a possible scenario in which aggregation of Cdc2-cyclin B dramatically enhances robustness of this activation. In this scenario, only inactive Cdc2-cyclin B can form aggregates, and the aggregates are in equilibrium with inactive Cdc2-cyclin B in solution. During maturation, the hormone-triggered inactivation of Myt1 depletes the soluble inactive Cdc2-cyclin B and the turnover leads to dissolution of the aggregates. This phase change, when coupled with the instability of the signaling network, provides a robust bio-switch.

Cell-cycle responses to DNA damage in G2

Cellular reproduction, at its basic level, is simply the passing of genetic information from a single parent cell into two daughter cells. As the cellular genome encodes all the information that defines a cell, it is crucial that the genome be accurately replicated. Furthermore, the duplicated genome must be properly segregated so that each daughter cell contains the exact same information as the parent cell. The processes by which this occurs is known as the cell cycle. The failure of either duplication or segregation of the genome can have disastrous consequences for an organism, including cancer and death. This article discusses what is known about checkpoints, the surveillance mechanisms that monitor both the fidelity and accuracy of DNA replication and segregation. Specifically, we will focus on the G2 checkpoint that is responsible for ensuring proper segregation of the duplicated genome into the daughter cells and how this checkpoint functions to arrest entry into mitosis in response to DNA damage.

The developmental expression of cell cycle regulators in Xenopus laevis

Several cyclins and cdks have been cloned in Xenopus, but their developmental expression has not been thoroughly examined. We have analyzed the temporal and spatial expression of cdk1, cdk2, cdk4 and cyclins D1, D2, E, A1, A2 and B1 by in situ hybridization. The transcripts of these cyclins and cdks exhibit striking tissue-restricted expression patterns very early in development that cannot be strictly correlated with proliferation. While the cdks and their activating cyclins are expressed in somewhat overlapping patterns, they are not precisely coincident. Additionally, maternal and zygotic cyclin forms demonstrate markedly different expression patterns.

Cyclin B synthesis is required for sea urchin oocyte maturation

Sea urchins are members of a limited group of animals in which meiotic maturation of oocytes is completed prior to fertilization. This is different from oocytes of most animals such as mammals and amphibians in which fertilization reactivates an arrested meiotic cycle. Using a recently developed technique for in vitro maturation of sea urchin oocytes, we analyzed the role of cyclin B, the regulatory component of maturation-promoting factor, in the control of sea urchin oocyte meiotic induction and progression. Oocytes of the sea urchin Lytechinus variegatus accumulate significant amounts of cyclin B mRNA and protein during oogenesis. We analyzed cyclin B synthetic requirements in oocytes and early embryos by inhibiting cyclin B synthesis with DNA and morpholino antisense oligonucleotides. Cyclin B synthesis is not necessary for the entry of G2-arrested oocytes into meiosis; however, it is required for the proper progression through meiotic divisions. Surprisingly, mature sea urchin eggs contain significant cyclin B protein following meiosis that serves as a maternal store for early cleavage divisions. We also find that cyclin A can functionally substitute for cyclin B in early embryos but not in oocytes. These studies provide a foundation for understanding the mechanism of meiotic maturation independent of the zygotic cell cycle.

Cdc6 synthesis regulates replication competence in Xenopus oocytes

The early division cycles of an embryo rely on the oocyte's ability to replicate DNA. During meiosis, oocytes temporarily lose this ability. After a single round of pre-meiotic S-phase, oocytes enter meiosis and rapidly arrest at prophase of meiosis I (G2). Upon hormonal stimulation, arrested oocytes resume meiosis, re-establish DNA replication competence in meiosis I shortly after germinal vesicle breakdown (GVBD), but repress replication until fertilization. How oocytes lose and regain replication competence during meiosis are important questions underlying the production of functional gametes. Here we show that the inability of immature Xenopus oocytes to replicate is linked to the absence of the Cdc6 protein and the cytoplasmic localization of other initiation proteins. Injection of Cdc6 protein into immature oocytes does not induce DNA replication. However, injection of Cdc6 into oocytes undergoing GVBD is sufficient to induce DNA replication in the absence of protein synthesis. Our results show that GVBD and Cdc6 synthesis are the only events that limit the establishment of the oocyte's replication competence during meiosis.

Antiproliferative action of valproate is associated with aberrant expression and nuclear translocation of cyclin D3 during the C6 glioma G1 phase

Cell cycle progression is tightly regulated by cyclins, cyclin-dependent kinases (cdks) and related inhibitory phophatases. Here, we employed mitotic selection to synchronize the C6 glioma cell cycle at the start of the G1 phase and mapped the temporal regulation of selected cyclins, cdks and inhibitory proteins throughout the 12 h of G1 by immunoblot analysis. The D-type cyclins, D3 and D1, were differentially expressed during the C6 glioma G1 phase. Cyclin D1 was up-regulated in the mid-G1 phase (4-6 h) while cyclin D3 expression emerged only in late G1 (9-12 h). The influence of the anticonvulsant agent valproic acid (VPA) on expression of cyclins and related proteins was determined, since its teratogenic potency has been linked to cell cycle arrest in the mid-G1 phase. Exposure of C6 glioma to VPA induced a marked up-regulation of cyclin D3 and decreased expression of the proliferating cell nuclear antigen. In synchronized cell populations, increased expression of cyclin D3 by VPA was detected in the mid-G1 phase (3-5 h). Immunocytochemical localization demonstrated rapid intracellular translocation of cyclin D3 to the nucleus following VPA exposure, suggesting that VPA-induced cell cycle arrest may be mediated by precocious activation of cyclin D3 in the G1 phase.

Homolog interaction during meiotic prophase I in Arabidopsis requires the SOLO DANCERS gene encoding a novel cyclin-like protein

Interactions between homologs in meiotic prophase I, such as recombination and synapsis, are critical for proper homolog segregation and involve the coordination of several parallel events. However, few regulatory genes have been identified; in particular, it is not clear what roles the proteins similar to the mitotic cell cycle regulators might play during meiotic prophase I. We describe here the isolation and characterization of a new Arabidopsis mutant called solo dancers that exhibits a severe defect in homolog synapsis, recombination and bivalent formation in meiotic prophase I, subsequently resulting in seemingly random chromosome distribution and formation of abnormal meiotic products. We further demonstrate that the mutation affects a meiosis-specific gene encoding a novel protein of 578 amino acid residues with up to 31% amino acid sequence identity to known cyclins in the C-terminal portion. These results argue strongly that homolog interactions during meiotic prophase I require a novel meiosis-specific cyclin in Arabidopsis.