Completion of DNA replication is monitored by a feedback system that controls the initiation of mitosis in vitro: studies in Xenopus

Nuclei and microtubule asters stimulate maturation/M phase promoting factor (MPF) activation in Xenopus eggs and egg cytoplasmic extracts

Although maturation/M phase promoting factor (MPF) can activate autonomously in Xenopus egg cytoplasm, indirect evidence suggests that nuclei and centrosomes may focus activation within the cell. We have dissected the contribution of these structures to MPF activation in fertilized eggs and in egg fragments containing different combinations of nuclei, centrosomes, and microtubules by following the behavior of Cdc2 (the kinase component of MPF), the regulatory subunit cyclin B, and the activating phosphatase Cdc25. The absence of the entire nucleus-centrosome complex resulted in a marked delay in MPF activation, whereas the absence of the centrosome alone caused a lesser delay. Nocodazole treatment to depolymerize microtubules through first interphase had an effect equivalent to removing the centrosome. Furthermore, microinjection of isolated centrosomes into anucleate eggs promoted MPF activation and advanced the onset of surface contraction waves, which are close indicators of MPF activation and could be triggered by ectopic MPF injection. Finally, we were able to demonstrate stimulation of MPF activation by the nucleus-centriole complex in vitro, as low concentrations of isolated sperm nuclei advanced MPF activation in cycling cytoplasmic extracts. Together these results indicate that nuclei and microtubule asters can independently stimulate MPF activation and that they cooperate to enhance activation locally.

Dissection of the XChk1 signaling pathway in Xenopus laevis embryos

Checkpoint pathways inhibit cyclin-dependent kinases (Cdks) to arrest cell cycles when DNA is damaged or unreplicated. Early embryonic cell cycles of Xenopus laevis lack these checkpoints. Completion of 12 divisions marks the midblastula transition (MBT), when the cell cycle lengthens, acquiring gap phases and checkpoints of a somatic cell cycle. Although Xenopus embryos lack checkpoints prior to the MBT, checkpoints are observed in cell-free egg extracts supplemented with sperm nuclei. These checkpoints depend upon the Xenopus Chk1 (XChk1)-signaling pathway. To understand why Xenopus embryos lack checkpoints, xchk1 was cloned, and its expression was examined and manipulated in Xenopus embryos. Although XChk1 mRNA is degraded at the MBT, XChk1 protein persists throughout development, including pre-MBT cell cycles that lack checkpoints. However, when DNA replication is blocked, XChk1 is activated only after stage 7, two cell cycles prior to the MBT. Likewise, DNA damage activates XChk1 only after the MBT. Furthermore, overexpression of XChk1 in Xenopus embryos creates a checkpoint in which cell division arrests, and both Cdc2 and Cdk2 are phosphorylated on tyrosine 15 and inhibited in catalytic activity. These data indicate that XChk1 signaling is intact but blocked upstream of XChk1 until the MBT.

Reconstitution of an ATM-dependent checkpoint that inhibits chromosomal DNA replication following DNA damage

Cell cycle checkpoints lead to the inhibition of cell cycle progression following DNA damage. A cell-free system derived from Xenopus eggs has been established that reconstitutes the checkpoint pathway inhibiting DNA replication initiation. DNA containing double-strand breaks inhibits replication initiation in a dose-dependent manner. Upon checkpoint activation, a prereplicative complex is assembled that contains ORC, Cdc6, Cdc7, and MCM proteins but lacks Cdc45. The checkpoint is ATM dependent. Cdk2/CyclinE acts downstream of ATM and is downregulated by Cdk2 phosphorylation on tyrosine 15. Cdk2AF/CyclinE is refractory to checkpoint signaling, and Cdc25A overrides the checkpoint and restores DNA replication. This report provides the description of a DNA damage checkpoint pathway that prevents the onset of S phase independently of the transcriptional function of p53 in a vertebrate organism.

Transition of the blastomere cell cycle from cell size-independent to size-dependent control at the midblastula stage in Xenopus laevis

Dissociated animal cap blastomeres of Xenopus laevis blastulae were cultured at a low Ca level (1 microM) from 9th to 18th cell cycle at 22 +/- 1 degrees C and observed by a time-lapse video recorder. Blastomeres cleaved unequally to increase variability in cell size as cell cycles progressed, but synchronously at a constant cell cycle time of about 30 min up to the 12th cleavage in diploid cells, and up to the 13th cleavage in haploid cells, regardless of their cell sizes. Thereafter, blastomeres cleaved asynchronously at varying cell cycle times in proportion to the inverse square of their radii. The transition from the cell size-independent to -dependent cell cycles occurred at the critical cell radius, 37.5 microm for the diploid and 27.9 microm for the haploid. While the protein synthesis inhibitor, cycloheximide (CHX) lengthened cell cycle times two- to six-fold, epidermal growth factor (EGF) had no significant effect on the cell cycle. CHX-treated blastomeres synchronously cleaved at a constant cell cycle time of 60 min up to the 12th cleavage. Thereafter, cell cycle times became variable in proportion to the inverse square of radii in the presence of CHX at 0.10-0.14 microg/ml, but to the inverse cube of radii at 0.18 microg/ml. The critical cell size of CHX-treated blastomeres for the transition from cell size-independent to -dependent cell cycles remained the same as that of untreated blastomeres. Frequency distributions of cell cycle times of synchronous cell cycles were monomodal with the peak at 30 min, except for CHX-treated blastomeres with the peak at 60 min. In contrast, frequency distributions of asynchronous cell cycles were polymodal with peaks at multiples of a unit time of 30-35 min. To explain these results, we propose that blastomere cytoplasm has 30-min cycles that repeatedly produce mitosis promoting factor (MPF) in a quantity proportional to the cell surface area. MPF is neutralized when it titrates a nuclear inhibitor present in a quantity proportional to the genome size, and sequestered in the nucleus. When the total amount of MPF produced exceeds the threshold required to titrate all of the inhibitor, mitosis is initiated.

Disruption of nuclear lamin organization blocks the elongation phase of DNA replication

The role of nuclear lamins in DNA replication is unclear. To address this, nuclei were assembled in Xenopus extracts containing AraC, a reversible inhibitor that blocks near the onset of the elongation phase of replication. Dominant-negative lamin mutants lacking their NH(2)-terminal domains were added to assembled nuclei to disrupt lamin organization. This prevented the resumption of DNA replication after the release of the AraC block. This inhibition of replication was not due to gross disruption of nuclear envelope structure and function. The organization of initiation factors was not altered by lamin disruption, and nuclei resumed replication when transferred to extracts treated with CIP, an inhibitor of the cyclin-dependent kinase (cdk) 2-dependent step of initiation. This suggests that alteration of lamin organization does not affect the initiation phase of DNA replication. Instead, we find that disruption of lamin organization inhibited chain elongation in a dose-dependent fashion. Furthermore, the established organization of two elongation factors, proliferating cell nuclear antigen, and replication factor complex, was disrupted by DeltaNLA. These findings demonstrate that lamin organization must be maintained in nuclei for the elongation phase of DNA replication to proceed.

Mechanism of caffeine-induced checkpoint override in fission yeast

Mitotic checkpoints restrain the onset of mitosis (M) when DNA is incompletely replicated or damaged. These checkpoints are conserved between the fission yeast Schizosaccharomyces pombe and mammals. In both types of organisms, the methylxanthine caffeine overrides the synthesis (S)-M checkpoint that couples mitosis to completion of DNA S phase. The molecular target of caffeine was sought in fission yeast. Caffeine prevented activation of Cds1 and phosphorylation of Chk1, two protein kinases that enforce the S-M checkpoint triggered by hydroxyurea. Caffeine did not inhibit these kinases in vitro but did inhibit Rad3, a kinase that regulates Cds1 and Chk1. In accordance with this finding, caffeine also overrode the G(2)-M DNA damage checkpoint that requires Rad3 function. Rad3 coprecipitated with Cds1 expressed at endogenous amounts, a finding that supports the hypothesis that Rad3 is involved in direct activation of Cds1.

Requirement of the prolyl isomerase Pin1 for the replication checkpoint

The peptidyl-prolyl isomerase Pin1 has been implicated in regulating cell cycle progression. Pin1 was found to be required for the DNA replication checkpoint in Xenopus laevis. Egg extracts depleted of Pin1 inappropriately transited from the G2 to the M phase of the cell cycle in the presence of the DNA replication inhibitor aphidicolin. This defect in replication checkpoint function was reversed after the addition of recombinant wild-type Pin1, but not an isomerase-inactive mutant, to the depleted extract. Premature mitotic entry in the absence of Pin1 was accompanied by hyperphosphorylation of Cdc25, activation of Cdc2/cyclin B, and generation of epitopes recognized by the mitotic phosphoprotein antibody, MPM-2. Therefore, Pin1 appears to be required for the checkpoint delaying the onset of mitosis in response to incomplete replication.

Mechanisms ensuring rapid and complete DNA replication despite random initiation in Xenopus early embryos

Chromosome replication initiates without sequence specificity at average intervals of approximately 10 kb during the rapid cell cycles of early Xenopus embryos. If the distribution of origins were random, some inter-origin intervals would be too long to be fully replicated before the end of S phase. To investigate what ensures rapid completion of DNA replication, we have examined the replication intermediates of plasmids of various sizes (5.3-42.2 kbp) in Xenopus egg extracts by two-dimensional gel electrophoresis and electron microscopy. We confirm that replication initiates without sequence specificity on all plasmids. We demonstrate for the first time that multiple initiation events occur on large plasmids, but not on small (<10 kb) plasmids, at average intervals of approximately 10 kb. Origin interference may prevent multiple initiation events on small plasmids. Multiple initiation events are neither synchronous nor regularly spaced. Bubble density is higher on later than on earlier replication intermediates, showing that initiation frequency increases throughout S phase, speeding up replication of late intermediates. We suggest that potential origins are abundant and randomly distributed, but that the increase of initiation frequency during S phase, and possibly origin interference, regulate origin activation to ensure rapid completion of replication.

Absence of Wee1 ensures the meiotic cell cycle in Xenopus oocytes

Meiotic cells undergo two successive divisions without an intervening S phase. However, the mechanism of S-phase omission between the two meiotic divisions is largely unknown. Here we show that Wee1, a universal mitotic inhibitor, is absent in immature (but not mature)Xenopus oocytes, being down-regulated specifically during oogenesis; this down-regulation is most likely due to a translational repression. Even the modest ectopic expression of Wee1 in immature (meiosis I) oocytes can induce interphase nucleus reformation and DNA replication just after meiosis I. Thus, the presence of Wee1 during meiosis I converts the meiotic cell cycle into a mitotic-like cell cycle having S phase. In contrast, Myt1, a Wee1-related kinase, is present and directly involved in G2 arrest of immature oocytes, but its ectopic expression has little effect on the meiotic cell cycle. These results strongly indicate that the absence of Wee1 in meiosis I ensures the meiotic cell cycle in Xenopus oocytes. Based on these results and the data published previously in other organisms, we suggest that absence of Wee1 may be a well-conserved mechanism for omitting interphase or S phase between the two meiotic divisions.

Kinetic analysis of a molecular model of the budding yeast cell cycle

The molecular machinery of cell cycle control is known in more detail for budding yeast, Saccharomyces cerevisiae, than for any other eukaryotic organism. In recent years, many elegant experiments on budding yeast have dissected the roles of cyclin molecules (Cln1-3 and Clb1-6) in coordinating the events of DNA synthesis, bud emergence, spindle formation, nuclear division, and cell separation. These experimental clues suggest a mechanism for the principal molecular interactions controlling cyclin synthesis and degradation. Using standard techniques of biochemical kinetics, we convert the mechanism into a set of differential equations, which describe the time courses of three major classes of cyclin-dependent kinase activities. Model in hand, we examine the molecular events controlling "Start" (the commitment step to a new round of chromosome replication, bud formation, and mitosis) and "Finish" (the transition from metaphase to anaphase, when sister chromatids are pulled apart and the bud separates from the mother cell) in wild-type cells and 50 mutants. The model accounts for many details of the physiology, biochemistry, and genetics of cell cycle control in budding yeast.

Components of the spindle-assembly checkpoint are essential in Caenorhabditis elegans

The spindle-assembly checkpoint ensures that, during mitosis and meiosis, chromosomes do not segregate until they are properly attached to the microtubules of the spindle. Here we show that mdf-1 and mdf-2 are components of the spindle-assembly checkpoint in Caenorhabditis elegans, and are essential for the long-term survival and fertility of this organism. Loss of function of either of these genes leads to the accumulation of a variety of defects, including chromosome abnormalities, X-chromosome non-disjunction or loss, problems in gonad development, and embryonic lethality. Antibodies that recognize the MDF-2 protein localize to nuclei of the cleaving embryo in a cell-cycle-dependent manner. mdf-1, a gene encoding a product that interacts with MDF-2, is required for cell-cycle arrest and proper chromosome segregation in premeiotic germ cells treated with nocodazole, a microtubule-depolymerizing agent. In the absence of mdf gene products, errors in chromosome segregation arise and accumulate, ultimately leading to genetic lethality.

Isolation and characterization of Xenopus ATM (X-ATM): expression, localization, and complex formation during oogenesis and early development

ATM, the gene product mutated in Ataxia Telangiectasia (A-T) encodes a 350-kDa protein involved in the regulation of several cellular responses to DNA breaks. We used a degenerate PCR-based strategy to isolate a partial clone of X-ATM, the Xenopus homologue of human ATM. Sequence analysis and confirmed that the clone was most closely related to human ATM. Xenopus ATM protein (X-ATM) is 85% identical to human ATM within the kinase domain and 71% identical over the carboxyl-terminal half of the protein. Polyclonal antibodies raised against recombinant X-ATM are highly specific for the ATM protein and recognize a single polypeptide of 370-kDa in oocytes, embryos, egg extracts and a Xenopus cell line. We found that X-ATM was expressed maternally in eggs and as early as stage II pre-vitellogenic oocytes, and the protein and mRNA were present at relatively constant levels throughout development. Subcellular fractionation showed that the protein was nuclear in both the female and male germlines. The level of X-ATM protein did not change throughout the meiotic divisions or the synchronous mitotic cycles of cleavage stage embryos. In addition, we did not observe any change in the level or mobility of X-ATM protein following gamma-irradiation of embryos. Finally, we also demonstrated that X-ATM was present in a high molecular weight complex of approximately 500 kDa containing the X-ATM protein and other, as yet unidentified component(s).

Inactivation of p42 mitogen-activated protein kinase is required for exit from M-phase after cyclin destruction

By using cycling Xenopus egg extracts, we have previously found that if mitogen-activated protein kinase (p42 MAPK) is activated on entry into mitosis (M-phase), the extract is arrested with condensed chromosomes and spindle microtubules. Here we show that these arrested extracts have high levels of M-phase promoting factor (MPF, Cyclin B/Cdc2) activity, stabilized levels of Cyclin B, and sustained M-phase-specific phosphorylations. We also examined the role of p42 MAPK in DNA damage checkpoint-arrested extracts that were induced to enter M-phase by the addition of Cdc25C protein. In these extracts, Cdc25C protein triggers the abrupt, premature activation of MPF and entry into M-phase. MPF activity then drops suddenly due to Cyclin B proteolysis, just as p42 MAPK is activated. Unexpectedly, however, M-phase is sustained, as judged by maintenance of M-phase-specific phosphorylations and condensed chromosomes. To determine if this M-phase arrest depended on p42 MAPK activation, we added PD98059 (PD), an inhibitor of p42 MAPK activation, to egg extracts with exogenous Cdc25. Both untreated and PD-treated extracts entered M-phase simultaneously, with a sharp peak of MPF activity. However, only PD-treated extracts subsequently exited from M-phase and entered interphase. In PD-treated extracts, p42 MAPK was not activated, and the transition to interphase was accompanied by the formation of decondensed nuclei and the disappearance of M-phase-specific phosphorylation of proteins. These results show that although entry into M-phase requires the activation of MPF, exit from M-phase even after cyclin destruction, is dependent on the inactivation of p42 MAPK.

Distinct, constitutively active MAPK phosphatases function in Xenopus oocytes: implications for p42 MAPK regulation In vivo

Xenopus oocyte maturation requires the phosphorylation and activation of p42 mitogen-activated protein kinase (MAPK). Likewise, the dephosphorylation and inactivation of p42 MAPK are critical for the progression of fertilized eggs out of meiosis and through the first mitotic cell cycle. Whereas the kinase responsible for p42 MAPK activation is well characterized, little is known concerning the phosphatases that inactivate p42 MAPK. We designed a microinjection-based assay to examine the mechanism of p42 MAPK dephosphorylation in intact oocytes. We found that p42 MAPK inactivation is mediated by at least two distinct phosphatases, an unidentified tyrosine phosphatase and a protein phosphatase 2A-like threonine phosphatase. The rates of tyrosine and threonine dephosphorylation were high and remained constant throughout meiosis, indicating that the dramatic changes in p42 MAPK activity seen during meiosis are primarily attributable to changes in MAPK kinase activity. The overall control of p42 MAPK dephosphorylation was shared among four partially rate-determining dephosphorylation reactions, with the initial tyrosine dephosphorylation of p42 MAPK being the most critical of the four. Our findings provide biochemical and kinetic insight into the physiological mechanism of p42 MAPK inactivation.

Caffeine inhibits the checkpoint kinase ATM

The basis of many anti-cancer therapies is the use of genotoxic agents that damage DNA and thus kill dividing cells. Agents that cause cells to override the DNA-damage checkpoint are predicted to sensitize cells to killing by genotoxic agents. They have therefore been sought as adjuncts in radiation therapy and chemotherapy. One such compound, caffeine, uncouples cell-cycle progression from the replication and repair of DNA [1] [2]. Caffeine therefore servers as a model compound in establishing the principle that agents that override DNA-damage checkpoints can be used to sensitize cells to the killing effects of genotoxic drugs [3]. But despite more than 20 years of use, the molecular mechanisms by which caffeine affects the cell cycle and checkpoint responses have not been identified. We investigated the effects of caffeine on the G2/M DNA-damage checkpoint in human cells. We report that the radiation-induced activation of the kinase Cds1 [4] (also known as Chk2 [5]) is inhibited by caffeine in vivo and that ATM kinase activity is directly inhibited by caffeine in vitro. Inhibition of ATM provides a molecular explanation of the attenuation of DNA-damage checkpoint responses and for the increased radiosensitivity of caffeine-treated cells [6] [7] [8].

Alternating oscillations and chaos in a model of two coupled biochemical oscillators driving successive phases of the cell cycle

The animal cell cycle is controlled by the periodic variation of two cyclin-dependent protein kinases, cdk1 and cdk2, which govern the entry into the M (mitosis) and S (DNA replication) phases, respectively. The ordered progression between these phases is achieved thanks to the existence of checkpoint mechanisms based on mutual inhibition of these processes. Here we study a simple theoretical model for oscillations in cdk1 and cdk2 activity, involving mutual inhibition of the two oscillators. Each minimal oscillator is described by a three-variable cascade involving a cdk, together with the associated cyclin and cyclin-degrading enzyme. The dynamics of this skeleton model of coupled oscillators is determined as a function of the strength of their mutual inhibition. The most common mode of dynamic behavior, obtained under conditions of strong mutual inhibition, is that of alternating oscillations in cdk1 and cdk2, which correspond to the physiological situation of the ordered recurrence of the M and S phases. In addition, for weaker inhibition we obtain evidence for a variety of dynamic phenomena such as complex periodic oscillations, chaos, and the coexistence between multiple periodic or chaotic attractors. We discuss the conditions of occurrence of these various modes of oscillatory behavior, as well as their possible physiological significance.

Developmental activation of the capability to undergo checkpoint-induced apoptosis in the early zebrafish embryo

In this study, we demonstrate the developmental activation, in the zebrafish embryo, of a surveillance mechanism which triggers apoptosis to remove damaged cells. We determine the time course of activation of this mechanism by exposing embryos to camptothecin, an agent which specifically inhibits topoisomerase I within the DNA replication complex and which, as a consequence of this inhibition, also produces strand breaks in the genomic DNA. In response to an early (pre-gastrula) treatment with camptothecin, apoptosis is induced at a time corresponding approximately to mid-gastrula stage in controls. This apoptotic response to a block of DNA replication can also be induced by early (pre-MBT) treatment with the DNA synthesis inhibitors hydroxyurea and aphidicolin. After camptothecin treatment, a high proportion of cells in two of the embryo's three mitotic domains (the enveloping and deep cell layers), but not in the remaining domain (the yolk syncytial layer), undergoes apoptosis in a cell-autonomous fashion. The first step in this response is an arrest of the proliferation of all deep- and enveloping-layer cells. These cells continue to increase in nuclear volume and to synthesize DNA. Eventually they become apoptotic, by a stereotypic pathway which involves cell membrane blebbing, "margination" and fragmentation of nuclei, and cleavage of the genomic DNA to produce a nucleosomal ladder. Fragmentation of nuclei can be blocked by the caspase-1,4,5 inhibitor Ac-YVAD-CHO, but not by the caspase-2,3,7[, 1] inhibitor Ac-DEVD-CHO. This suggests a functional requirement for caspase-4 or caspase-5 in the apoptotic response to camptothecin. Recently, Xenopus has been shown to display a developmental activation of the capability for stress- or damaged-induced apoptosis at early gastrula stage. En masse, our experiments suggest that the apoptotic responses in zebrafish and Xenopus are fundamentally similar. Thus, as for mammals, embryos of the lower vertebrates exhibit the activation of surveillance mechanisms, early in development, to produce the selective apoptosis of damaged cells.

Nucleo-cytoplasmic interactions that control nuclear envelope breakdown and entry into mitosis in the sea urchin zygote

In sea urchin zygotes and mammalian cells nuclear envelope breakdown (NEB) is not driven simply by a rise in cytoplasmic cyclin dependent kinase 1-cyclin B (Cdk1-B) activity; the checkpoint monitoring DNA synthesis can prevent NEB in the face of mitotic levels of Cdk1-B. Using sea urchin zygotes we investigated whether this checkpoint prevents NEB by restricting import of regulatory proteins into the nucleus. We find that cyclin B1-GFP accumulates in nuclei that cannot complete DNA synthesis and do not break down. Thus, this checkpoint limits NEB downstream of both the cytoplasmic activation and nuclear accumulation of Cdk1-B1. In separate experiments we fertilize sea urchin eggs with sperm whose DNA has been covalently cross-linked to inhibit replication. When the pronuclei fuse, the resulting zygote nucleus does not break down for &gt;180 minutes (equivalent to three cell cycles), even though Cdk1-B activity rises to greater than mitotic levels. If pronuclear fusion is prevented, then the female pronucleus breaks down at the normal time (average 68 minutes) and the male pronucleus with cross-linked DNA breaks down 16 minutes later. This male pronucleus has a functional checkpoint because it does not break down for &gt;120 minutes if the female pronucleus is removed just prior to NEB. These results reveal the existence of an activity released by the female pronucleus upon its breakdown, that overrides the checkpoint in the male pronucleus and induces NEB. Microinjecting wheat germ agglutinin into binucleate zygotes reveals that this activity involves molecules that must be actively translocated into the male pronucleus.

The Drosophila ATM homologue Mei-41 has an essential checkpoint function at the midblastula transition

BACKGROUND

Drosophila embryogenesis is initiated by 13 rapid syncytial mitotic divisions that do not require zygotic gene activity. This maternally directed cleavage phase of development terminates at the midblastula transition (MBT), at which point the cell cycle slows dramatically, membranes surround the cortical nuclei to form a cellular blastoderm, and zygotic gene expression is first required.

RESULTS

We show that embryos lacking Mei-41, a Drosophila homologue of the ATM tumor suppressor, proceed through unusually short syncytial mitoses, fail to terminate syncytial division following mitosis 13, and degenerate without forming cells. A similar cleavage-stage arrest is produced by mutations in grapes, which encodes a homologue of the Checkpoint-1 kinase. We present biochemical, cytological and genetic data indicating that Mei-41 and Grapes are components of a conserved DNA-replication/damage checkpoint pathway that triggers inhibitory phosphorylation of the Cdc2 kinase and mediates resistance to replication inhibitors and DNA-damaging agents. This pathway is nonessential during postembryonic development, but it is required to terminate the cleavage stage at the MBT. Cyclins are required for Cdc2 kinase activity, and mutations in cyclin A and cyclin B bypass the requirement for mei-41 at the MBT. These mutations do not restore wild-type syncytial cell-cycle timing or the embryonic replication checkpoint, however, suggesting that Mei-41-mediated inhibition of Cdc2 has an additional essential function at the MBT.

CONCLUSIONS

The Drosophila DNA-replication/damage checkpoint pathway can be activated by externally triggered DNA damage or replication defects throughout the life cycle, and under laboratory conditions this inducible function is nonessential. During early embryogenesis, however, this pathway is activated by developmental cues and is required for the transition from maternal to zygotic control of development at the MBT.

Characterization of the interactions between human cdc25C, cdks, cyclins and cdk-cyclin complexes

We have overexpressed and purified human dual-specificity phosphatase cdc25C from a prokaryotic expression system at high levels and in a soluble, active form, and have studied and quantified its potential to interact with cdks, cyclins and preformed cdk-cyclin complexes by fluorescence spectroscopy and size-exclusion chromatography. Our data indicate that human cdc25C forms stable complexes, through hydrophobic contacts, with cdk and cyclin monomers, as well as with preformed cdk-cyclin complexes. In vitro, cdc25C interacts with cyclin monomers with high affinity, with tenfold less affinity with cdks, and with intermediate affinity with cdk-cyclin complexes. Moreover, changes observed in the intrinsic fluorescence of cdks, cyclins and cdk-cyclin complexes upon interaction with cdc25C are indicative of concomitant conformational changes within cdks and cyclins. From our results, we propose that in vitro, in the presence of monomeric cdks and cyclins, cdc25C forms stable ternary complexes, first through a high affinity interaction with a cyclin, which may then help target cdc25C towards a cdk. We discuss the biological relevance of our results and propose that a similar, two-step mechanism of interaction between cdc25C and cdk-cyclin complexes may occur in vivo.

Requirement of Cdk2-cyclin E activity for repeated centrosome reproduction in Xenopus egg extracts

The abnormally high number of centrosomes found in many human tumor cells can lead directly to aneuploidy and genomic instability through the formation of multipolar mitotic spindles. To facilitate investigation of the mechanisms that control centrosome reproduction, a frog egg extract arrested in S phase of the cell cycle that supported repeated assembly of daughter centrosomes was developed. Multiple rounds of centrosome reproduction were blocked by selective inactivation of cyclin-dependent kinase 2-cyclin E (Cdk2-E) and were restored by addition of purified Cdk2-E. Confocal immunomicroscopy revealed that cyclin E was localized at the centrosome. These results demonstrate that Cdk2-E activity is required for centrosome duplication during S phase and suggest a mechanism that could coordinate centrosome reproduction with cycles of DNA synthesis and mitosis.

Functional analysis of chromatin assembled in synthetic nuclei

Numerous regulatory mechanisms contribute to the control of eukaryotic transcription. These controls are manifested through higher-order protein-DNA structure within the nucleus. In vitro assays have proven extremely useful in deciphering the potential regulatory roles of chromatin and nuclear structure in transcription. Embryonic egg extracts of Xenopus with their vast maternal stores and rapid cell-cycle oscillations can be exploited to recapitulate multiple layers of nuclear regulation. Incubation of cloned DNA templates in Xenopus egg extracts promotes a self-ordered assembly of physiologically spaced nucleosomes and synthetic nuclei structure formation. Interaction of membrane vesicles with chromatin leads to formation of a bilayer nuclear envelope encapsulating the DNA. These synthetic nuclei are functional organelles capable of active protein transport and a single round of semiconservative DNA synthesis. This system can be used to directly test the mechanisms by which trans-acting factors promote transcription on nucleosomal DNA, either during chromatin assembly or postassembly or in conjunction with remodeling machinery and/or DNA replication. The functional consequences of trans-acting factor interaction within synthetic nuclei are determined by a coupled in vitro transcription analysis. Immobilizing biotin end-labeled DNA templates on paramagnetic streptavidin beads greatly improves the flexibility of the system. The ease of chromatin-assembled template recovery allows the introduction of wash steps, buffer changes, and specific reaction optimization. These methods for reconstituting gene regulatory mechanisms in vitro are an attempt to strike a balance between biochemical accessibility and physiological relevance.

Purification and cloning of a protein kinase that phosphorylates and activates the polo-like kinase Plx1

The Xenopus polo-like kinase 1 (Plx1) is essential during mitosis for the activation of Cdc25C, for spindle assembly, and for cyclin B degradation. Polo-like kinases from various organisms are activated by phosphorylation by an unidentified protein kinase. A protein kinase, polo-like kinase kinase 1 or xPlkk1, that phosphorylates and activates Plx1 in vitro was purified to near homogeneity and cloned. Phosphopeptide mapping of Plx1 phosphorylated in vitro by recombinant xPlkk1 or in progesterone-treated oocytes indicates that xPlkk1 may activate Plx1 in vivo. The xPlkk1 protein itself was also activated by phosphorylation on serine and threonine residues, and the kinetics of activation of xPlkk1 in vivo closely paralleled the activation of Plx1. Moreover, microinjection of xPlkk1 into Xenopus oocytes accelerated the timing of activation of Plx1 and the transition from G2 to M phase of the cell cycle. These results define a protein kinase cascade that regulates several events of mitosis.

Xenopus Cdc45-dependent loading of DNA polymerase alpha onto chromatin under the control of S-phase Cdk

At the onset of S phase, chromosomal replication is initiated by the loading of DNA polymerase alpha onto replication origins. However, the molecular mechanisms for controlling the initiation are poorly understood. Using Xenopus egg extract, we report here the identification of a Xenopus homolog of Cdc45, a yeast protein essential for the initiation of replication, which is shown to be an essential molecule for the initiation of replication via the loading of DNA polymerase alpha onto chromatin. XCdc45, by physically interacting with the polymerase in the extract, became associated with chromatin only after nuclear formation. During S phase, XCdc45 co-localized with the polymerase in the nuclei, and the loading of the polymerase, which depended on endogenous XCdc45, was facilitated by exogenously added recombinant XCdc45. These findings, together with the apparent requirement of S-phase-cdk activity for the loading of XCdc45, suggest that XCdc45, under the control of S-phase cdk, plays a pivotal role in the loading of DNA polymerase alpha onto chromatin.

The Xenopus Chk1 protein kinase mediates a caffeine-sensitive pathway of checkpoint control in cell-free extracts

We have analyzed the role of the protein kinase Chk1 in checkpoint control by using cell-free extracts from Xenopus eggs. Recombinant Xenopus Chk1 (Xchk1) phosphorylates the mitotic inducer Cdc25 in vitro on multiple sites including Ser-287. The Xchk1-catalyzed phosphorylation of Cdc25 on Ser-287 is sufficient to confer the binding of 14-3-3 proteins. Egg extracts from which Xchk1 has been removed by immunodepletion are strongly but not totally compromised in their ability to undergo a cell cycle delay in response to the presence of unreplicated DNA. Cdc25 in Xchk1-depleted extracts remains bound to 14-3-3 due to the action of a distinct Ser-287-specific kinase in addition to Xchk1. Xchk1 is highly phosphorylated in the presence of unreplicated or damaged DNA, and this phosphorylation is abolished by caffeine, an agent which attenuates checkpoint control. The checkpoint response to unreplicated DNA in this system involves both caffeine-sensitive and caffeine-insensitive steps. Our results indicate that caffeine disrupts the checkpoint pathway containing Xchk1.

Caulerpenyne interferes with microtubule-dependent events during the first mitotic cycle of sea urchin eggs

Caulerpenyne (Cyn), the major secondary metabolite synthesized by the green alga Caulerpa taxifolia proliferating in the Mediterranean Sea, is a cytotoxic sesquiterpene. As this compound has an antiproliferative potency by inhibiting division of many types of cells, we examined the precise effects of Cyn during the early development of the sea urchin Paracentrotus lividus. Whereas Cyn (60 microM) had no effect on fertilization, it blocked the first cell division in the same manner whether added before or after fertilization, provided the drug was added before or during metaphase. Immunofluorescence localization revealed that Cyn had no effect on the microtubular sperm aster formation, pronuclei migration and fusion, chromosome condensation, nuclear envelope breakdown, and bipolar mitotic spindle assembly. However, mitosis was blocked in a metaphase-like stage at which most chromosomes were aligned at the equatorial plate, while a few of them had not even migrated towards the metaphase plate. When added after the metaphase-anaphase transition, the first division occurred normally but the second division was inhibited with the same phenotype as described above. We previously showed that Cyn did not affect protein synthesis or H1 kinase activation or deactivation (Pesando et al., 1996, Aquat. Toxicol. 35, 139), but that it partially inhibited DNA synthesis. Our results establish that Cyn does not affect the microfilament-dependent processes of fertilization and cytokinesis and allows the beginning of mitosis, but prevents normal DNA replication and results in metaphase-like arrest of sea urchin embryos.

Activated polo-like kinase Plx1 is required at multiple points during mitosis in Xenopus laevis

Entry into mitosis depends upon activation of the dual-specificity phosphatase Cdc25C, which dephosphorylates and activates the cyclin B-Cdc2 complex. Previous work has shown that the Xenopus polo-like kinase Plx1 can phosphorylate and activate Cdc25C in vitro. In the work presented here, we demonstrate that Plx1 is activated in vivo during oocyte maturation with the same kinetics as Cdc25C. Microinjection of wild-type Plx1 into Xenopus oocytes accelerated the rate of activation of Cdc25C and cyclin B-Cdc2. Conversely, microinjection of either an antibody against Plx1 or kinase-dead Plx1 significantly inhibited the activation of Cdc25C and cyclin B-Cdc2. This effect could be reversed by injection of active Cdc25C, indicating that Plx1 is upstream of Cdc25C. However, injection of Cdc25C, which directly activates cyclin B-Cdc2, also caused activation of Plx1, suggesting that a positive feedback loop exists in the Plx1 activation pathway. Other experiments show that injection of Plx1 antibody into early embryos, which do not require Cdc25C for the activation of cyclin B-Cdc2, resulted in an arrest of cleavage that was associated with monopolar spindles. These results demonstrate that in Xenopus laevis, Plx1 plays important roles both in the activation of Cdc25C at the initiation of mitosis and in spindle assembly at late stages of mitosis.

Identification of primary initiation sites for DNA replication in the hamster dihydrofolate reductase gene initiation zone

Mammalian replication origins appear paradoxical. While some studies conclude that initiation occurs bidirectionally from specific loci, others conclude that initiation occurs at many sites distributed throughout large DNA regions. To clarify this issue, the relative number of early replication bubbles was determined at 26 sites in a 110-kb locus containing the dihydrofolate reductase (DHFR)-encoding gene in CHO cells; 19 sites were located within an 11-kb sequence containing ori-beta. The ratio of approximately 0.8-kb nascent DNA strands to nonreplicated DNA at each site was quantified by competitive PCR. Nascent DNA was defined either as DNA that was labeled by incorporation of bromodeoxyuridine in vivo or as RNA-primed DNA that was resistant to lambda-exonuclease. Two primary initiation sites were identified within the 12-kb region, where two-dimensional gel electrophoresis previously detected a high frequency of replication bubbles. A sharp peak of nascent DNA occurred at the ori-beta origin of bidirectional replication where initiation events were 12 times more frequent than at distal sequences. A second peak occurred 5 kb downstream at a previously unrecognized origin (ori-beta'). Thus, the DHFR gene initiation zone contains at least three primary initiation sites (ori-beta, ori-beta', and ori-gamma), suggesting that initiation zones in mammals, like those in fission yeast, consist of multiple replication origins.

Modeling M-phase control in Xenopus oocyte extracts: the surveillance mechanism for unreplicated DNA

Alternating phases of DNA synthesis and mitosis, during the first 12 cell divisions of frog embryos, are driven by autonomous cytoplasmic oscillations of M-phase promoting factor (MPF). Cell-free extracts of frog eggs provide a convenient preparation for studying the molecular machinery that generates MPF oscillations and the surveillance mechanism that normally prevents entry into mitosis until chromosomal DNA is fully replicated. Early experiments suggested that unreplicated DNA blocks MPF activity by inducing phosphorylation of a crucial tyrosine residue, but recent evidence implicates a stoichiometric inhibitor (an MPF binding protein) as the 'braking' agent. Using a realistic mathematical model of the mitotic control system in frog egg extracts, we suggest that both tyrosine phosphorylation and a stoichiometric inhibitors are involved in the block of MPF by unreplicated DNA. Both pathways operate by raising the cyclin threshold for MPF activation. As a bonus, in the process of analyzing these experiments, we obtain more direct and reliable estimates of the rate constants in the model.

Histone H1 reduces the frequency of initiation in Xenopus egg extract by limiting the assembly of prereplication complexes on sperm chromatin

Somatic histone H1 reduces both the rate and extent of DNA replication in Xenopus egg extract. We show here that H1 inhibits replication directly by reducing the number of replication forks, but not the rate of fork progression, in Xenopus sperm nuclei. Density substitution experiments demonstrate that those forks that are active in H1 nuclei elongate to form large tracts of fully replicated DNA, indicating that inhibition is due to a reduction in the frequency of initiation and not the rate or extent of elongation. The observation that H1 dramatically reduces the number of replication foci in sperm nuclei supports this view. The establishment of replication competent DNA in egg extract requires the assembly of prereplication complexes (pre-RCs) on sperm chromatin. H1 reduces binding of the pre-RC proteins, XOrc2, XCdc6, and XMcm3, to chromatin. Replication competence can be restored in these nuclei, however, only under conditions that promote the loss of H1 from chromatin and licensing of the DNA. Thus, H1 inhibits replication in egg extract by preventing the assembly of pre-RCs on sperm chromatin, thereby reducing the frequency of initiation. These data raise the interesting possibility that H1 plays a role in regulating replication origin use during Xenopus development.

The role of the Ran GTPase pathway in cell cycle control and interphase nuclear functions

Ran is a small, highly abundant, nuclear GTPase. Mutants in Ran and in proteins that interact with it disrupt the normal checkpoint control of mitosis with respect to the completion of DNA synthesis. Ran and other components of this pathway are also required for numerous nuclear functions such as RNA export, protein import, RNA processing and DNA replication. It will be important to understand how these facets of Ran's activities are linked and how they promote correct control of the cell cycle. This review examines recent progress in discovering other components of the Ran GTPase pathway and considers how this pathway may be required for the control of the cell cycle.

Regulation of cell cycle progression following DNA damage

DNA damage causes an arrest in cell cycle progression. Checkpoints, which monitor the state of the DNA, exist throughout the cycle and negatively regulate cell cycle transitions when damage is detected. The molecular basis of how these checkpoints are activated, and interact with the cell cycle machinery, is just beginning to be understood. Studies in yeast have identified a number of genes involved in a G2 DNA damage checkpoint, while in mammalian cells a G1 checkpoint has been extensively studied.

Function and regulation of cdc25 protein phosphate through mitosis and meiosis

Activation of the cyclin B-cdc2 kinase mitotic inducer involves dephosphorylation of two inhibitory residues, tyrosine 15 and threonine 14, cdc25 is the specific phosphatase that directly dephosphorylates and activates the cdc2 kinase, cdc25 activity is regulated by phosphorylation. Both phosphatases 1 and 2A could act as cdc25-specific inhibitory phosphatases. Although the cyclin B-cdc2 complex plays a role in activating cdc25, it is highly probable that a distinct protein kinase is involved as a trigger in cdc25 activation. The implication of raf kinase as a cdc25-specific activating kinase in human cells and Xenopus oocytes is discussed.

Major binding sites for the nuclear import receptor are the internal nucleoporin Nup153 and the adjacent nuclear filament protein Tpr

A major question in nuclear import concerns the identity of the nucleoporin(s) that interact with the nuclear localization sequences (NLS) receptor and its cargo as they traverse the nuclear pore. Ligand blotting and solution binding studies of isolated proteins have attempted to gain clues to the identities of these nucleoporins, but the studies have from necessity probed binding events far from an in vivo context. Here we have asked what binding events occur in the more physiological context of a Xenopus egg extract, which contains nuclear pore subcomplexes in an assembly competent state. We have then assessed our conclusions in the context of assembled nuclear pores themselves. We have used immunoprecipitation to identify physiologically relevant complexes of nucleoporins and importin subunits. In parallel, we have demonstrated that it is possible to obtain immunofluorescence localization of nucleoporins to subregions of the nuclear pore and its associated structures. By immunoprecipitation, we find the nucleoporin Nup153 and the pore-associated filament protein Tpr, previously shown to reside at distinct sites on the intranuclear side of assembled pores, are each in stable subcomplexes with importin alpha and beta in Xenopus egg extracts. Importin subunits are not in stable complexes with nucleoporins Nup62, Nup93, Nup98, or Nup214/CAN, either in egg extracts or in extracts of assembled nuclear pores. In characterizing the Nup153 complex, we find that Nup153 can bind to a complete import complex containing importin alpha, beta, and an NLS substrate, consistent with an involvement of this nucleoporin in a terminal step of nuclear import. Importin beta binds directly to Nup153 and in vitro can do so at multiple sites in the Nup153 FXFG repeat region. Tpr, which has no FXFG repeats, binds to importin beta and to importin alpha/beta heterodimers, but only to those that do not carry an NLS substrate. That the complex of Tpr with importin beta is fundamentally different from that of Nup153 is additionally demonstrated by the finding that recombinant beta or beta45-462 fragment freely exchanges with the endogenous importin beta/Nup153 complex, but cannot displace endogenous importin beta from a Tpr complex. However, the GTP analogue GMP-PNP is able to disassemble both Nup153- and Tpr-importin beta complexes. Importantly, analysis of extracts of isolated nuclei indicates that Nup153- and Tpr-importin beta complexes exist in assembled nuclear pores. Thus, Nup153 and Tpr are major physiological binding sites for importin beta. Models for the roles of these interactions are discussed.

The mitotic peptidyl-prolyl isomerase, Pin1, interacts with Cdc25 and Plx1

The cis/trans peptidyl-prolyl isomerase, Pin1, is a regulator of mitosis that is well conserved from yeast to man. Here we demonstrate that depletion of Pin1-binding proteins from Xenopus egg extracts results in hyperphosphorylation and inactivation of the key mitotic regulator, Cdc2/cyclin B. We show biochemically that this phenotype is a consequence of Pin1 interaction with critical upstream regulators of Cdc2/cyclin B, including the Cdc2-directed phosphatase, Cdc25, and its known regulator, Plx1. Although Pin1 could interact with Plx1 during interphase and mitosis, only the phosphorylated, mitotically active form of Cdc25 was able to bind Pin1, an event we have recapitulated using in vitro phosphorylated Cdc25. Taken together, these data suggest that Pin1 may modulate cell cycle control through interaction with Cdc25 and its activator, Plx1.

Regulated chromosomal DNA replication in the absence of a nucleus

Using Xenopus egg extracts, we have developed a completely soluble system for eukaryotic chromosomal DNA replication. In the absence of a nuclear envelope, a single, complete round of ORC-dependent DNA replication is catalyzed by cytosolic and nuclear extracts added sequentially to demembranated sperm chromatin or prokaryotic plasmid DNA. The absence of rereplication is explained by an activity present in the nucleus that prevents the binding of MCM to chromatin. Our results indicate that the role of the nuclear envelope in DNA replication is to concentrate activators and inhibitors of replication inside the nucleus. In addition, they provide direct evidence that metazoans use the same strategy as yeast to activate DNA replication and to restrict it to a single round per cell cycle.

14-3-3 proteins act as negative regulators of the mitotic inducer Cdc25 in Xenopus egg extracts

Cdc25, the dual-specificity phosphatase that dephosphorylates the Cdc2-cyclin B complex at mitosis, is highly regulated during the cell cycle. In Xenopus egg extracts, Cdc25 is associated with two isoforms of the 14-3-3 protein. Cdc25 is complexed primarily with 14-3-3epsilon and to a lesser extent with 14-3-3zeta. The association of these 14-3-3 proteins with Cdc25 varies dramatically during the cell cycle: binding is high during interphase but virtually absent at mitosis. Interaction with 14-3-3 is mediated by phosphorylation of Xenopus Cdc25 at Ser-287, which resides in a consensus 14-3-3 binding site. Recombinant Cdc25 with a point mutation at this residue (Cdc25-S287A) is incapable of binding to 14-3-3. Addition of the Cdc25-S287A mutant to Xenopus egg extracts accelerates mitosis and overrides checkpoint-mediated arrests of mitotic entry due to the presence of unreplicated and damaged DNA. These findings indicate that 14-3-3 proteins act as negative regulators of Cdc25 in controlling the G2-M transition.

Overexpression of a kinase-inactive ATR protein causes sensitivity to DNA-damaging agents and defects in cell cycle checkpoints

ATR, a phosphatidylinositol kinase-related protein homologous to ataxia telangiectasia mutated (ATM), is important for the survival of human cells following many forms of DNA damage. Expression of a kinase-inactive allele of ATR (ATRkd) in human fibroblasts causes increased sensitivity to ionizing radiation (IR), cis-platinum and methyl methanesulfonate, but only slight UV radiation sensitivity. ATRkd overexpression abrogates the G2/M arrest after exposure to IR, and overexpression of wild-type ATR complements the radioresistant DNA synthesis phenotype of cells lacking ATM, suggesting a potential functional overlap between these proteins. ATRkd overexpression also causes increased sensitivity to hydroxyurea that is associated with microtubule-mediated nuclear abnormalities. These observations are consistent with uncoupling of certain mitotic events from the completion of S-phase. Thus, ATR is an important component of multiple DNA damage response pathways and may be involved in the DNA replication (S/M) checkpoint.

Cyclin D2 arrests Xenopus early embryonic cell cycles

Xenopus cyclin D2 mRNA is a member of the class of maternal RNAs. It is rare and stable during early embryonic development. To investigate the potential role of cyclin D2 during early embryonic cell cycles, cyclin D2 was injected into one blastomere of a two-cell embryo. This injection induced a cell cycle arrest in the injected blastomere. To analyze more precisely the mechanism of this arrest, we took advantage of cycling egg extracts that recapitulate major events of the cell cycle when supplemented with demembranated sperm heads. When Xenopus cyclin D2 is added to egg extracts, the first round of DNA replication occurs as in control extracts. However, Xenopus cyclin D2 blocks subsequent rounds of DNA replication and the oscillations of histone H1 kinase activity associated with cdc2 kinase, indicating that the cell cycle is arrested after the first S-phase. The block induced by Xenopus cyclin D2 is not due to a lack of the mitotic cyclin B2 that accumulates normally. Radiolabeled Xenopus cyclin D2 enters nuclei after completion of the first S-phase and remains stable over the entire period of the arrest. These features suggest that Xenopus cyclin D2 could play an original role during early development, controlling the G2-phase and/or the G2/M transition.

Induction of a G2-phase arrest in Xenopus egg extracts by activation of p42 mitogen-activated protein kinase

Previous work has established that activation of Mos, Mek, and p42 mitogen-activated protein (MAP) kinase can trigger release from G2-phase arrest in Xenopus oocytes and oocyte extracts and can cause Xenopus embryos and extracts to arrest in mitosis. Herein we have found that activation of the MAP kinase cascade can also bring about an interphase arrest in cycling extracts. Activation of the cascade early in the cycle was found to bring about the interphase arrest, which was characterized by an intact nuclear envelope, partially condensed chromatin, and interphase levels of H1 kinase activity, whereas activation of the cascade just before mitosis brought about the mitotic arrest, with a dissolved nuclear envelope, condensed chromatin, and high levels of H1 kinase activity. Early MAP kinase activation did not interfere significantly with DNA replication, cyclin synthesis, or association of cyclins with Cdc2, but it did prevent hyperphosphorylation of Cdc25 and Wee1 and activation of Cdc2/cyclin complexes. Thus, the extracts were arrested in a G2-like state, unable to activate Cdc2/cyclin complexes. The MAP kinase-induced G2 arrest appeared not to be related to the DNA replication checkpoint and not to be mediated through inhibition of Cdk2/cyclin E; evidently a novel mechanism underlies this arrest. Finally, we found that by delaying the inactivation of MAP kinase during release of a cytostatic factor-arrested extract from its arrest state, we could delay the subsequent entry into mitosis. This finding suggests that it is the persistence of activated MAP kinase after fertilization that allows the occurrence of a G2-phase during the first mitotic cell cycle.

DNA-replication checkpoint control at the Drosophila midblastula transition

Embryogenesis is typically initiated by a series of rapid mitotic divisions that are under maternal genetic control. The switch to zygotic control of embryogenesis at the midblastula transition is accompanied by significant increases in cell-cycle length and gene transcription, and changes in embryo morphology. Here we show that mutations in the grapes (grp) checkpoint 1 kinase homologue in Drosophila block the morphological and biochemical changes that accompany the midblastula transition, lead to a continuation of the maternal cell-cycle programme, and disrupt DNA-replication checkpoint control of cell-cycle progression. The timing of the midblastula transition is controlled by the ratio of nuclei to cytoplasm (the nucleocytoplasmic ratio), suggesting that this developmental transition is triggered by titration of a maternal factor by the increasing mass of nuclear material that accumulates during the rapid embryonic mitoses. Our observations support a model for cell-cycle control at the midblastula transition in which titration of a maternal component of the DNA-replication machinery slows DNA synthesis and induces a checkpoint-dependent delay in cell-cycle progression. This delay may allow both completion of S phase and transcription of genes that initiate the switch to zygotic control of embryogenesis.

Growth-related signaling in vascular smooth muscle cells is deregulated by TCDD during the G0/G1 transition

Experiments have been conducted to examine the impact of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on growth-related signaling in vascular smooth muscle cells (SMCs). A 40% reduction of peak DNA synthesis was observed in SMCs only when TCDD was added during the G0/G1 transition of the cell cycle. Enhanced phosphorylation of several endogenous proteins during this period was coincident with increased tyrosine kinase activity as early as 15 min following TCDD challenge. No changes in protein phosphorylation status occurred in cells treated with TCDD during the G1/S transition or during S phase. Cotreatment of quiescent SMCs with 10 nM TCDD and serum for 3 h reduced serum-inducible binding activity to a 12-O-tetradecanoyl phorbol 13-acetate responsive element (TRE) by approximately 40%. No alterations of constitutive TRE binding were observed in quiescent SMCs treated with TCDD for up to 5 h. These data show that mitogen-related signaling in vascular SMCs is modulated by TCDD selectively during the G0/G1 transition, and these effects influence the growth behavior of these cells.

DNA topoisomerase II as the cellular target of a novel antitumor agent ICRF-193, a bisdioxopiperazine derivative, in Xenopus egg extract

We have investigated the molecular target of an antitumor agent ICRF-193, a bisdioxopiperazine derivative, in in vitro chromosome condensation system of Xenopus egg extract (XEE), where DNA topoisomerase II was previously demonstrated to play a crucial role. Demembranated Xenopus sperm head chromatin is converted to metaphase chromosome-like structure in XEE in two steps, i.e., swelling of the chromatin followed by condensation of chromosome. When ICRF-193 was added to the reaction, swelling of the chromatin was not affected but chromosome condensation was completely blocked. This blockade was reversed by exogenous supplement of calf thymus topoisomerase II, which was in turn neutralized by anti-topoisomerase II monoclonal antibody. These results demonstrate that topoisomerase II is the molecular target of the drug ICRF-193.

The Drosophila grapes gene is related to checkpoint gene chk1/rad27 and is required for late syncytial division fidelity

BACKGROUND

Cell cycle checkpoints maintain the fidelity of the somatic cell cycle by ensuring that one step in the cell cycle is not initiated until a previous step has been completed. The extent to which cell cycle checkpoints play a role in the initial rapid embryonic divisions of higher eukaryotes is unclear. The initial syncytial divisions of Drosophila embryogenesis provide an excellent opportunity to address this issue as they are amenable to both genetic and cellular analysis. In order to study the relevance of cell cycle checkpoints in early Drosophila embryogenesis, we have characterized the maternal-effect grapes (grp) mutation, which may affect feedback control during early syncytial divisions.

RESULTS

The Drosophila grp gene encodes a predicted serine/threonine kinase and has significant homology to chk1/rad27, a gene required for a DNA damage checkpoint in Schizosaccharomyces pombe. Relative to normal embryos, embryos derived from grp-mutant mothers exhibit elevated levels of DNA damage. During nuclear cycles 12 and 13, alignment of the chromosomes on the metaphase plate was disrupted in grp-derived embryos, and the embryos underwent a progression of cytological events that were indistinguishable from those observed in normal syncytial embryos exposed to X-irradiation. The mutant embryos also failed to progress through a regulatory transition in Cdc2 activity that normally occurs during interphase of nuclear cycle 14.

CONCLUSION

We propose that the primary defect in grp-derived embryos is a failure to replicate or repair DNA completely before mitotic entry during the late syncytial divisions. This suggests that wild-type grp functions in a developmentally regulated DNA replication/damage checkpoint operating during the late syncytial divisions. These results are discussed with respect to the proposed function of the chk1/rad27 gene.