Coordinated regulation of M phase exit and S phase entry by the Cdc2 activity level in the early embryonic cell cycle

Lamin B receptor-mediated chromatin tethering to the nuclear envelope is detrimental to the Xenopus blastula

In the nucleus of eukaryotic cells, chromatin is tethered to the nuclear envelope (NE), wherein inner nuclear membrane proteins (INMPs) play major roles. However, in Xenopus blastula, chromatin tethering to the NE depends on nuclear filamentous actin that develops in a blastula-specific manner. To investigate whether chromatin tethering operates in the blastula through INMPs, we experimentally introduced INMPs into Xenopus egg extracts that recapitulate nuclear formation in fertilized eggs. When expressed in extracts in which polymerization of actin is inhibited, only lamin B receptor (LBR), among the five INMPs tested, tethered chromatin to the NE, depending on its N2 and N3 domains responsible for chromatin-protein binding. N2-3-deleted LBR did not tether chromatin, although it was localized in the nuclei. We subsequently found that the LBR level was very low in the Xenopus blastula but was elevated after the blastula stage. When the LBR level was precociously elevated in the blastula by injecting LBR mRNA, it induced alterations in nuclear lamina architecture and nuclear morphology and caused DNA damage and abnormal mitotic spindles, depending on the N2-3 domains. These results suggest that LBR-mediated chromatin tethering is circumvented in the Xenopus blastula, as it is detrimental to embryonic development.

Phosphorylation states change Otx2 activity for cell proliferation and patterning in the Xenopus embryo

The homeodomain transcription factor Otx2 has essential roles in head and eye formation via the negative and positive regulation of its target genes, but it remains elusive how this dual activity of Otx2 affects cellular functions. In the current study, we first demonstrated that both exogenous and endogenous Otx2 are phosphorylated at multiple sites. Using Xenopus embryos, we identified three possible cyclin-dependent kinase (Cdk) sites and one Akt site, and analyzed the biological activities of phosphomimetic (4E) and nonphosphorylatable (4A) mutants for those sites. In the neuroectoderm, the 4E but not the 4A mutant downregulated the Cdk inhibitor gene p27xic1 (cdknx) and posterior genes, and promoted cell proliferation, possibly forming a positive-feedback loop consisting of Cdk, Otx2 and p27xic1 for cell proliferation, together with anteriorization. Conversely, the 4A mutant functioned as an activator on its own and upregulated the expression of eye marker genes, resulting in enlarged eyes. Consistent with these results, the interaction of Otx2 with the corepressor Tle1 is suggested to be phosphorylation dependent. These data suggest that Otx2 orchestrates cell proliferation, anteroposterior patterning and eye formation via its phosphorylation state.

Chromatin tethering to the nuclear envelope by nuclear actin filaments: a novel role of the actin cytoskeleton in the Xenopus blastula

The Xenopus oocyte is known to accumulate filamentous or F-actin in the nucleus, but it is currently unknown whether F-actin also accumulates in embryo nuclei. Using fluorescence-labeled actin reporters, we examined the actin distribution in Xenopus embryonic cells and found that F-actin accumulates in nuclei during the blastula stage but not during the gastrula stage. To further investigate nuclear F-actin, we devised a Xenopus egg extract that reproduces the formation of nuclei in which F-actin accumulates. Using this extract, we found that F-actin accumulates primarily at the subnuclear membranous region and is essential to maintain chromatin binding to the nuclear envelope in well-developed nuclei. We also provide evidence that nuclear F-actin increases the structural stability of nuclei and contributes to chromosome alignment on the mitotic spindle at the following M phase. These results suggest the physiological importance of nuclear F-actin accumulation in rapidly dividing large Xenopus blastula cells.

Fission yeast APC/C activators Slp1 and Fzr1 sequentially trigger two consecutive nuclear divisions during meiosis

In meiosis, two rounds of nuclear division occur consecutively without DNA replication between the divisions. We isolated a fission yeast mutant in which the nucleus divides only once to generate two spores, as opposed to four, in meiosis. In this mutant, we found that the initiation codon of the slp1⁺ gene is converted to ATA, producing a reduced amount of Slp1. As a member of the Fizzy family of anaphase-promoting complex/cyclosome (APC/C) activators, Slp1 is essential for vegetative growth; however, the mutant allele shows a phenotype only in meiosis. Slp1 insufficiency delays degradation of maturation-promoting factor at the first meiotic division, and another APC/C activator, Fzr1, which acts late in meiosis, terminates meiosis immediately after the delayed first division to produce two viable spores.

Luteolin attenuates neuronal apoptosis in the hippocampi of diabetic encephalopathy rats

Luteolin (3’,4’,5,7-tetrahydroxyflavone) has powerful anti-apoptotic and antioxidant properties. This study aimed to investigate the effects of luteolin on hyperglycemia-mediated apoptosis in the hippocampi of rats with streptozotocin-induced diabetic encephalopathy after injection into the tail veins, and the molecular mechanisms involved. Biochemistry and terminal deoxynucleotidyl transferase mediated dUTP nick end labelling detection results showed that luteolin treatment (given twice daily for 15 days) significantly inhibited hyperglycemia-mediated apoptosis, decreased malondialdehyde levels and increased glutathione levels in the hippocampi of streptozotocin-induced diabetic rats. Western blot analysis revealed that luteolin also inhibited the expression of apoptosis-related factors and cytochrome c release from mitochondria. Luteolin also improved the learning and memory abilities of rats with diabetic encephalopathy in a water maze test. Further western blot analysis revealed that luteolin treatment facilitated neuronal cell survival through activation of the phosphatidylinositol 3-kinase/Akt signaling pathway, an extracellular signal pathway involved in the suppression of cell apoptosis and promotion of cell survival. These experimental findings indicate that luteolin can inhibit apoptosis of hippocampal nerve cells in rats with diabetic encephalopathy, and that this effect is mediated by an indirect antioxidative effect, the inhibition of activation of apoptosis-related factors and the activation of phosphatidylinositol 3-kinase/Akt signal pathway.

The nuclear accumulation of alpha-synuclein is mediated by importin alpha and promotes neurotoxicity by accelerating the cell cycle

α-Synuclein (α-syn), a 14 kDa pre-synaptic protein, is widely involved in the Parkinson's disease (PD) pathogenesis. Recent studies have shown that the nuclear accumulation of α-syn might have a toxic effect. The main purpose of the present study was to explore which amino acid residues in α-syn are associated with its nuclear accumulation, the molecule(s) mediated the nuclear import of α-syn, and the role of α-syn accumulated in the nucleus. It has been noted that the nuclear import of α-syn may be mediated by importin α and that both the amino acid residues 1-60 and 103-140 of α-syn were indispensable for its nuclear import. After imported into the nucleus, the accumulated α-syn played a toxic role in both the PC12 cells and the C57 mice. Furthermore, α-syn-nuclear localization signal-injected mice showed behavioral symptoms associated with PD. Further studies performed in vitro showed that the toxicity of α-syn in the nucleus might be due to an interference of the cell cycle. Thus, it can be concluded that α-syn can accumulate in nucleus, which is mediated by importin α, and promote neurotoxicity by accelerating the cell cycle.

Intranuclear DNA density affects chromosome condensation in metazoans

Quantification of mitotic chromosomes in Caenorhabditis elegans embryos and a Xenopus laevis egg extract system indicates that the chromosome amount per nuclear space, or “intranuclear DNA density,” regulates chromosome condensation. This suggests an adaptive mode of chromosome condensation regulation in metazoans.

Chromosome condensation is critical for accurate inheritance of genetic information. The degree of condensation, which is reflected in the size of the condensed chromosomes during mitosis, is not constant. It is differentially regulated in embryonic and somatic cells. In addition to the developmentally programmed regulation of chromosome condensation, there may be adaptive regulation based on spatial parameters such as genomic length or cell size. We propose that chromosome condensation is affected by a spatial parameter called the chromosome amount per nuclear space, or “intranuclear DNA density.” Using Caenorhabditis elegans embryos, we show that condensed chromosome sizes vary during early embryogenesis. Of importance, changing DNA content to haploid or polyploid changes the condensed chromosome size, even at the same developmental stage. Condensed chromosome size correlates with interphase nuclear size. Finally, a reduction in nuclear size in a cell-free system from Xenopus laevis eggs resulted in reduced condensed chromosome sizes. These data support the hypothesis that intranuclear DNA density regulates chromosome condensation. This suggests an adaptive mode of chromosome condensation regulation in metazoans.

Taiwanin A inhibits MCF-7 cancer cell activity through induction of oxidative stress, upregulation of DNA damage checkpoint kinases, and activation of p53 and FasL/Fas signaling pathways

This study investigates the anti-MCF-7 breast cancer cell effects and the underlying pharmacological activity and mechanism of taiwanin A, a major lignan isolated from Taiwania cryptomerioides. Our results show that taiwanin A time-dependently induced reactive oxygen species level and DNA damage in MCF-7 cells, which were likely activated kinases ataxia telangiectasia mutated (ATM) and checkpoint kinase (Chk). Taiwanin A could also up-regulate p53, phosphorylated p53, p21(Cip1), and p27(Kip1) and down-regulate the G(2)/M checkpoint cyclin-dependent kinase1 (Cdk1)-cyclin A/B, leading to induction of G(2)/M cell-cycle arrest in MCF-7 cells. Blockade of p53 gene expression by siRNA further demonstrated that the cell-cycle arrest induced by taiwanin A was p53-dependent. The FasL/Fas-mediated apoptotic signaling cascade was involved in taiwanin A-induced apoptosis via activation of caspases-10 and -7 (but not caspase-8), and proteolytic cleavage of poly(ADP-ribose) polymerase (PARP). In contrast, mitochondria-initiated apoptotic pathway was not involved. This is the first report to delineate novel mechanism of the action of taiwanin A against MCF-7 cells, suggesting this lignan may have value for development as an anti-breast cancer agent.

Replisome progression complex links DNA replication to sister chromatid cohesion in Xenopus egg extracts

Cohesin-mediated sister chromatid cohesion is established during the S-phase, and recent studies demonstrate that a cohesin protein ring concatenates sister DNA molecules. However, little is known about how DNA replication is linked to the establishment of sister chromatid cohesion. Here, we used Xenopus egg extracts to show that AND-1 and Tim1-Tipin, homologues of Saccharomyces cerevisiae Ctf4 and Tof1-Csm3, respectively, are associated with the replisome and are required for proper establishment of the cohesion observed in the M-phase extracts. Immunodepletion of both AND-1 and Tim1-Tipin from the extracts leads to aberrant sister chromatid cohesion, which is similarly induced by the depletion of cohesin. These results demonstrate that AND-1 and Tim1-Tipin are key factors linking DNA replication and establishment of sister chromatid cohesion. On the basis of the physical interactions between AND-1 and DNA polymerases, we discuss a model to describe how replisome progression complex establishes sister chromatid cohesion.

Quantitative characterization of a mitotic cyclin threshold regulating exit from mitosis

Regulation of cyclin abundance is central to eukaryotic cell cycle control. Strong overexpression of mitotic cyclins is known to lock the system in mitosis, but the quantitative behavior of the control system as this threshold is approached has only been characterized in the in vitro Xenopus extract system. Here, we quantitate the threshold for mitotic block in budding yeast caused by constitutive overexpression of the mitotic cyclin Clb2. Near this threshold, the system displays marked loss of robustness, in that loss or even heterozygosity for some regulators becomes deleterious or lethal, even though complete loss of these regulators is tolerated at normal cyclin expression levels. Recently, we presented a quantitative kinetic model of the budding yeast cell cycle. Here, we use this model to generate biochemical predictions for Clb2 levels, asynchronous as well as through the cell cycle, as the Clb2 overexpression threshold is approached. The model predictions compare well with biochemical data, even though no data of this type were available during model generation. The loss of robustness of the Clb2 overexpressing system is also predicted by the model. These results provide strong confirmation of the model's predictive ability.

DNA replication licensing in somatic and germ cells

The DNA replication (or origin) licensing system ensures precise duplication of the genome in each cell cycle and is a powerful regulator of cell proliferation in metazoa. Studies in yeast, Drosophila melanogaster and Xenopus laevis have characterised the molecular machinery that constitutes the licensing system, but it remains to be determined how this important evolutionary conserved pathway is regulated in Homo sapiens. We have investigated regulation of the origin licensing factors Cdc6, Cdt1, Mcm2 and Geminin in human somatic and germ cells. Cdc6 and Cdt1 play an essential role in DNA replication initiation by loading the Mcm2-7 complex, which is required for unwinding the DNA helix, onto chromosomal origins. Geminin is a repressor of origin licensing that blocks Mcm2-7 loading onto origins. Our studies demonstrate that Cdc6, Cdt1 and Mcm2 play a central role in coordinating growth during the proliferation-differentiation switch in somatic self-renewing systems and that Cdc6 expression is rate-limiting for acquisition of replication competence in primary oocytes. In striking contrast, we show that proliferation control during male gametogenesis is not linked to Cdc6 or Mcm2, but appears to be coordinated by the negative regulator Geminin with Cdt1 becoming rate-limiting in late prophase. Our data demonstrate a striking sexual dimorphism in the mechanisms repressing origin licensing and preventing untimely DNA synthesis during meiosis I, implicating a pivotal role for Geminin in maintaining integrity of the male germline genome.

Emi1-mediated M-phase arrest in Xenopus eggs is distinct from cytostatic factor arrest

Oocytes of most vertebrates arrest at metaphase of the second meiosis (meta-II) to await fertilization, thus preventing parthenogenetic activation. This arrest is caused by a cytoplasmic activity called cytostatic factor (CSF), which was first identified in the frog Rana pipiens oocyte >30 years ago. CSF arrest is executed by maintaining the activity of cyclin B-Cdc2 at elevated levels largely through prevention of cyclin B destruction. Although CSF arrest is established by the Mos-mitogen-activated protein kinase pathway and is released by the Ca-calmodulin kinase II pathway, it remains unclear precisely how cyclin B destruction is regulated. Recently, an early mitotic inhibitor, Emi1, was reported to be a critical component of CSF. This report has been expected to provide a final resolution to the CSF problem because Emi1 inhibits the anaphase-promoting complex/cyclosome, a ubiquitin ligase for cyclin B destruction, through sequestration of Cdc20, an activator for the anaphase-promoting complex/cyclosome. In mitotic cycles, however, Emi1 is destroyed in every pro-metaphase, and accordingly, it is unclear why Emi1 should be required for CSF activity, which is seen only in meta-II. Here, we show that Emi1 is absent in unfertilized mature Xenopus eggs and that exogenous Emi1 is destroyed in meta-II and mitotic metaphase. The expression of Emi1 in oocytes hinders meiotic progression. Although both Emi1 and Mos can inhibit progression through M phase, the Emi1-mediated arrest does not require mitogen-activated protein kinase activity and is not released by Ca. Together, our results indicate that Emi1 is unlikely to be a component of CSF.

Cell cycle aberrations by alpha-synuclein over-expression and cyclin B immunoreactivity in Lewy bodies

alpha-Synuclein is a presynaptic protein that accumulates abnormally in Lewy bodies of Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Its physiological function and role in neuronal death remain poorly understood. Recent immunohistochemical studies suggest that cell cycle-related phenomena may play a role in the pathogenesis of Alzheimer's disease and perhaps other neurodegenerative disorders. In this investigation, we examined the effects of alpha-synuclein expression levels on cell cycle indices in PC12 cells engineered to conditionally induce alpha-synuclein expression upon withdrawal of doxycycline. Over-expression of alpha-synuclein resulted in enhanced proliferation rate and enrichment of cells in the S phase of the cell cycle. This was associated with increased accumulation of the mitotic factor cyclin B and down-regulation of the tumor suppressor retinoblastoma 2. Additionally, ERK1/2, key molecules in proliferation signaling, were highly phosphorylated. Immunohistochemical studies on postmortem brains revealed intense cyclin B immunoreactivity in Lewy bodies in cases with DLB and to a lesser extent in PD. We propose that elevated expression of alpha-synuclein causes changes in cell cycle regulators through ERK activation leading to apoptosis of postmitotic neurons. These changes in cell cycle proteins are also associated with ectopic expression of cyclin B in Lewy bodies.

Two redundant oscillatory mechanisms in the yeast cell cycle

The cell division cycle requires oscillations in activity of B-type cyclin (Clb)-Cdk1 kinases. Oscillations are due to periodic cyclin degradation by the anaphase-promoting complex (APC) activated by Cdc20 or Cdh1, and to cyclical accumulation of the Sic1 inhibitor. The results presented here suggest that the regulatory machinery controlling Clb kinase levels embeds two distinct oscillatory mechanisms. One, a "relaxation oscillator," involves alternation between two meta-stable states: Clb high/inhibitors (Sic1/APC-Cdh1) low, and Clb low/inhibitors high. The other, a "negative feedback oscillator," involves Clb kinase activation of APC-Cdc20, leading to Clb degradation. Genetic analysis suggests that these two mechanisms can function independently, and inactivation of both mechanisms is required to prevent mitosis. Computational modeling confirms that two such mechanisms can be linked to yield a robust cell cycle control system.