Dissection of the XChk1 signaling pathway in Xenopus laevis embryos

In vitro cell cycle oscillations exhibit a robust and hysteretic response to changes in cytoplasmic density

The cytoplasm, where most cellular reactions occur, has a variable density. We currently lack an understanding of how density variations affect cellular functions because of the challenge of controlling it experimentally. Here, we systematically modulate the density of an in vitro cytoplasm using microfluidics and analyze how the cell cycle behaves in turn. We found that mitotic cycles maintain their function across 0.2× to 1.2× of the natural density. Higher densities arrest cell cycles, and dilution recovers oscillations. However, the density at which cycles reappear is lower than the natural density. This behavior suggests a history-dependent mechanism called hysteresis, common in physics, chemistry, and engineering. Our approach paves the way for studying the responses of other processes to density changes.

Cells control the properties of the cytoplasm to ensure proper functioning of biochemical processes. Recent studies showed that cytoplasmic density varies in both physiological and pathological states of cells undergoing growth, division, differentiation, apoptosis, senescence, and metabolic starvation. Little is known about how cellular processes cope with these cytoplasmic variations. Here, we study how a cell cycle oscillator comprising cyclin-dependent kinase (Cdk1) responds to changes in cytoplasmic density by systematically diluting or concentrating cycling Xenopus egg extracts in cell-like microfluidic droplets. We found that the cell cycle maintains robust oscillations over a wide range of deviations from the endogenous density: as low as 0.2× to more than 1.22× relative cytoplasmic density (RCD). A further dilution or concentration from these values arrested the system in a low or high steady state of Cdk1 activity, respectively. Interestingly, diluting an arrested cytoplasm of 1.22× RCD recovers oscillations at lower than 1× RCD. Thus, the cell cycle switches reversibly between oscillatory and stable steady states at distinct thresholds depending on the direction of tuning, forming a hysteresis loop. We propose a mathematical model which recapitulates these observations and predicts that the Cdk1/Wee1/Cdc25 positive feedback loops do not contribute to the observed robustness, supported by experiments. Our system can be applied to study how cytoplasmic density affects other cellular processes.

Pluripotent stem cells with low differentiation potential contain incompletely reprogrammed DNA replication

Reprogrammed pluripotent stem cells (PSCs) are valuable for research and potentially for cell replacement therapy. However, only a fraction of reprogrammed PSCs are developmentally competent. Genomic stability and accurate DNA synthesis are fundamental for cell development and critical for safety. We analyzed whether defects in DNA replication contribute to genomic instability and the diverse differentiation potentials of reprogrammed PSCs. Using a unique single-molecule approach, we visualized DNA replication in isogenic PSCs generated by different reprogramming approaches, either somatic cell nuclear transfer (NT-hESCs) or with defined factors (iPSCs). In PSCs with lower differentiation potential, DNA replication was incompletely reprogrammed, and genomic instability increased during replicative stress. Reprogramming of DNA replication did not correlate with DNA methylation. Instead, fewer replication origins and a higher frequency of DNA breaks in PSCs with incompletely reprogrammed DNA replication were found. Given the impact of error-free DNA synthesis on the genomic integrity and differentiation proficiency of PSCs, analyzing DNA replication may be a useful quality control tool.

Sperm-mediated DNA lesions alter metabolite levels in spent embryo culture medium

Paternal genetic alterations may affect embryo viability and reproductive outcomes. Currently it is unknown whether embryo metabolism is affected by sperm-mediated abnormalities. Hence, using a mouse model, this study investigated the response to paternally transmitted DNA lesions on genetic integrity and metabolism in preimplantation embryos. Spent embryo culture media were analysed for metabolites by nuclear magnetic resonance spectroscopy and embryonic genetic integrity was determined by terminal deoxynucleotidyl transferase dUTP nick-end labelling (TUNEL) assay on embryonic Day 4.5 (E4.5). Metabolic signatures were compared between normally derived embryos (control) and embryos derived from spermatozoa carrying induced DNA lesions (SDL). SDL embryos showed a significant reduction in blastocyst formation on E3.5 and E4.5 (P<0.0001) and had an approximately 2-fold increase in TUNEL-positive cells (P<0.01). A cohort of SDL embryos showing delayed development on E4.5 had increased uptake of pyruvate (P<0.05) and released significantly less alanine (P<0.05) to the medium compared with the corresponding control embryos. On the other hand, normally developed SDL embryos had a reduced (P<0.001) pyruvate-to-alanine ratio compared with normally developed embryos from the control group. Hence, the difference in the metabolic behaviour of SDL embryos may be attributed to paternally transmitted DNA lesions in SDL embryos.

Involvement of G-quadruplex regions in mammalian replication origin activity

Genome-wide studies of DNA replication origins revealed that origins preferentially associate with an Origin G-rich Repeated Element (OGRE), potentially forming G-quadruplexes (G4). Here, we functionally address their requirements for DNA replication initiation in a series of independent approaches. Deletion of the OGRE/G4 sequence strongly decreased the corresponding origin activity. Conversely, the insertion of an OGRE/G4 element created a new replication origin. This element also promoted replication of episomal EBV vectors lacking the viral origin, but not if the OGRE/G4 sequence was deleted. A potent G4 ligand, PhenDC3, stabilized G4s but did not alter the global origin activity. However, a set of new, G4-associated origins was created, whereas suppressed origins were largely G4-free. In vitro Xenopus laevis replication systems showed that OGRE/G4 sequences are involved in the activation of DNA replication, but not in the pre-replication complex formation. Altogether, these results converge to the functional importance of OGRE/G4 elements in DNA replication initiation.

Preserving Genome Integrity During the Early Embryonic DNA Replication Cycles

During the very early stages of embryonic development chromosome replication occurs under rather challenging conditions, including a very short cell cycle, absence of transcription, a relaxed DNA damage response and, in certain animal species, a highly contracted S-phase. This raises the puzzling question of how the genome can be faithfully replicated in such a peculiar metabolic context. Recent studies have provided new insights into this issue, and unveiled that embryos are prone to accumulate genetic and genomic alterations, most likely due to restricted cellular functions, in particular reduced DNA synthesis quality control. These findings may explain the low rate of successful development in mammals and the occurrence of diseases, such as abnormal developmental features and cancer. In this review, we will discuss recent findings in this field and put forward perspectives to further study this fascinating question.

Genomic structure, expression, and functional characterization of checkpoint kinase 1 from Penaeus monodon

Chk1 is a cell-cycle regulator. Chk1 has been identified in organisms ranging from yeast to humans, but few researchers have studied Chk1 in shrimps. We cloned Chk1 from the black tiger shrimp (Penaeus monodon). The full-length cDNA sequence of PmChk1 had 3,334 base pairs (bp), with an open reading frame of 1,455 bp. The complete genomic sequence of PmChk1 (11,081 bp) contained 10 exons separated by nine introns. qRT-PCR showed that PmChk1 was highly expressed in the ovaries and gills of P. monodon. The lowest PmChk1 expression was noted in stage III of ovarian development in P. monodon. PmChk1 expression decreased significantly after injection of 5-hydroxytryptamine and eyestalk ablation in P. monodon ovaries. RNA interference experiments were undertaken to examine the expression of PmChk1, PmCDC2, and PmCyclin B. PmChk1 knockdown in the ovaries and hepatopancreas by dsRNA-Chk1 was successful. The localization and level of PmChk1 expression in the hepatopancreas was studied using in situ hybridization, which showed that data were in accordance with those of qRT-PCR. The Gonadosomatic Index of P. monodon after dsRNA-Chk1 injection was significantly higher than that after injection of dsRNA-GFP or phosphate-buffered saline. These data suggest that PmChk1 may have important roles in the ovarian maturation of P. monodon.

Chk1 Inhibition of the Replication Factor Drf1 Guarantees Cell-Cycle Elongation at the Xenopus laevis Mid-blastula Transition

The early cell divisions of many metazoan embryos are rapid and occur in the near absence of transcription. At the mid-blastula transition (MBT), the cell cycle elongates and several processes become established including the onset of bulk transcription and cell-cycle checkpoints. How these events are timed and coordinated is poorly understood. Here we show in Xenopus laevis that developmental activation of the checkpoint kinase Chk1 at the MBT results in the SCF^β-TRCP-dependent degradation of a limiting replication initiation factor Drf1. Inhibition of Drf1 is the primary mechanism by which Chk1 blocks cell-cycle progression in the early embryo and is an essential function of Chk1 at the blastula-to-gastrula stage of development. This study defines the downregulation of Drf1 as an important mechanism to coordinate the lengthening of the cell cycle and subsequent developmental processes.

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Activation of Chk1 at the Xenopus MBT results in the degradation of Drf1

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Drf1 degradation is SCF^β-TRCP dependent

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Chk1 blocks the cell cycle in the early embryo through inhibition of Drf1

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Inhibition of Drf1 is an essential function of Chk1 during gastrulation

Regulation of DNA Replication in Early Embryonic Cleavages

Early embryonic cleavages are characterized by short and highly synchronous cell cycles made of alternating S- and M-phases with virtually absent gap phases. In this contracted cell cycle, the duration of DNA synthesis can be extraordinarily short. Depending on the organism, the whole genome of an embryo is replicated at a speed that is between 20 to 60 times faster than that of a somatic cell. Because transcription in the early embryo is repressed, DNA synthesis relies on a large stockpile of maternally supplied proteins stored in the egg representing most, if not all, cellular genes. In addition, in early embryonic cell cycles, both replication and DNA damage checkpoints are inefficient. In this article, we will review current knowledge on how DNA synthesis is regulated in early embryos and discuss possible consequences of replicating chromosomes with little or no quality control.

Regulation of zygotic genome activation and DNA damage checkpoint acquisition at the mid-blastula transition

Following fertilization, oviparous embryos undergo rapid, mostly transcriptionally silent cleavage divisions until the mid-blastula transition (MBT), when large-scale developmental changes occur, including zygotic genome activation (ZGA) and cell cycle remodeling, via lengthening and checkpoint acquisition. Despite their concomitant appearance, whether these changes are co-regulated is unclear. Three models have been proposed to account for the timing of (ZGA). One model implicates a threshold nuclear to cytoplasmic (N:C) ratio, another stresses the importance cell cycle elongation, while the third model invokes a timer mechanism. We show that precocious Chk1 activity in pre-MBT zebrafish embryos elongates cleavage cycles, thereby slowing the increase in the N:C ratio. We find that cell cycle elongation does not lead to transcriptional activation. Rather, ZGA slows in parallel with the N:C ratio. We show further that the DNA damage checkpoint program is maternally supplied and independent of ZGA. Although pre-MBT embryos detect damage and activate Chk2 after induction of DNA double-strand breaks, the Chk1 arm of the DNA damage response is not activated, and the checkpoint is nonfunctional. Our results are consistent with the N:C ratio model for ZGA. Moreover, the ability of precocious Chk1 activity to delay pre-MBT cell cycles indicate that lack of Chk1 activity limits checkpoint function during cleavage cycles. We propose that Chk1 gain-of-function at the MBT underlies cell cycle remodeling, whereas ZGA is regulated independently by the N:C ratio.

Tolerance to Gamma Radiation in the Tardigrade Hypsibius dujardini from Embryo to Adult Correlate Inversely with Cellular Proliferation

Tardigrades are highly tolerant to desiccation and ionizing radiation but the mechanisms of this tolerance are not well understood. In this paper, we report studies on dose responses of adults and eggs of the tardigrade Hypsibius dujardini exposed to gamma radiation. In adults the LD50/48h for survival was estimated at ~ 4200 Gy, and doses higher than 100 Gy reduced both fertility and hatchability of laid eggs drastically. We also evaluated the effect of radiation (doses 50 Gy, 200 Gy, 500 Gy) on eggs in the early and late embryonic stage of development, and observed a reduced hatchability in the early stage, while no effect was found in the late stage of development. Survival of juveniles from irradiated eggs was highly affected by a 500 Gy dose, both in the early and the late stage. Juveniles hatched from eggs irradiated at 50 Gy and 200 Gy developed into adults and produced offspring, but their fertility was reduced compared to the controls. Finally we measured the effect of low temperature during irradiation at 4000 Gy and 4500 Gy on survival in adult tardigrades, and observed a slight delay in the expressed mortality when tardigrades were irradiated on ice. Since H. dujardini is a freshwater tardigrade with lower tolerance to desiccation compared to limno-terrestrial tardigrades, the high radiation tolerance in adults, similar to limno-terrestrial tardigrades, is unexpected and seems to challenge the idea that desiccation and radiation tolerance rely on the same molecular mechanisms. We suggest that the higher radiation tolerance in adults and late stage embryos of H. dujardini (and in other studied tardigrades) compared to early stage embryos may partly be due to limited mitotic activity, since tardigrades have a low degree of somatic cell division (eutely), and dividing cells are known to be more sensitive to radiation.

RAD18 Is a Maternal Limiting Factor Silencing the UV-Dependent DNA Damage Checkpoint in Xenopus Embryos

In early embryos, the DNA damage checkpoint is silent until the midblastula transition (MBT) because of maternal limiting factors of unknown identity. Here we identify the RAD18 ubiquitin ligase as one such factor in Xenopus. We show, in vitro and in vivo, that inactivation of RAD18 function leads to DNA damage-dependent checkpoint activation, monitored by CHK1 phosphorylation. Moreover, we show that the abundance of both RAD18 and PCNA monoubiquitylated (mUb) are developmentally regulated. Increased DNA abundance limits the availability of RAD18 close to the MBT, thereby reducing PCNA(mUb) and inducing checkpoint derepression. Furthermore, we show that this embryonic-like regulation can be reactivated in somatic mammalian cells by ectopic RAD18 expression, therefore conferring resistance to DNA damage. Finally, we find high RAD18 expression in cancer stem cells highly resistant to DNA damage. Together, these data propose RAD18 as a critical embryonic checkpoint-inhibiting factor and suggest that RAD18 deregulation may have unexpected oncogenic potential.

Phosphorylation of Claspin is triggered by the nucleocytoplasmic ratio at the Xenopus laevis midblastula transition

At the Xenopus midblastula transition (MBT), cell cycles lengthen, and checkpoints that respond to damaged or unreplicated DNA are established. The MBT is triggered by a critical nucleocytoplasmic (N/C) ratio; however, the molecular basis for its initiation remains unknown. In egg extracts, activation of Chk1 checkpoint kinase requires the adaptor protein Claspin, which recruits Chk1 for phosphorylation by ATR. At the MBT in embryos, Chk1 is transiently activated to lengthen the cell cycle. We show that Xenopus Claspin is phosphorylated at the MBT at both DNA replication checkpoint-dependent and -independent sites. Further, in egg extracts, Claspin phosphorylation depends on a threshold N/C ratio, but occurs even when ATR is inhibited. Not all phosphorylation that occurs at the MBT is reproduced in egg extracts. Our results identify Claspin as the most upstream molecule in the signaling pathway that responds to the N/C ratio and indicate that Claspin may also respond to an independent timer to trigger the MBT and activation of cell cycle checkpoints.

Deadenylation of maternal mRNAs mediated by miR-427 in Xenopus laevis embryos

We show that microRNA-427 (miR-427) mediates the rapid deadenylation of maternal mRNAs after the midblastula transition (MBT) of Xenopus laevis embryogenesis. By MBT, the stage when the embryonic cell cycle is remodeled and zygotic transcription of mRNAs is initiated, each embryo has accumulated approximately 10(9) molecules of miR-427 processed from multimeric pri-miR-427 transcripts synthesized after fertilization. We demonstrate that the maternal mRNAs for cyclins A1 and B2 each contain a single miR-427 target sequence, spanning less than 30 nucleotides, that is both necessary and sufficient for deadenylation, and that inactivation of miR-427 leads to stabilization of the mRNAs. Although this deadenylation normally takes place after MBT, exogenous miRNAs produced prematurely in vivo can promote deadenylation prior to MBT, indicating that turnover of the maternal mRNAs is limited by the amount of accumulated miR-427. Injected transcripts comprised solely of the cyclin mRNA 3' untranslated regions or bearing a 5' ApppG cap undergo deadenylation, showing that translation of the targeted RNA is not required. miR-427 is not unique in promoting deadenylation, as an unrelated miRNA, let-7, can substitute for miR-427 if the reporter RNA contains an appropriate let-7 target site. We propose that miR-427, like the orthologous miR-430 of zebrafish, functions to down-regulate expression of maternal mRNAs early in development.

Expression of exogenous mRNA in Xenopus laevis embryos for the study of cell cycle regulation

The microinjection of mRNA that is transcribed and capped in vitro into fertilized eggs and embryos of Xenopus laevis provides a powerful means for discovering the function of proteins during early development. Proteins may be overexpressed for a gain-of-function effect or exogenous protein function may be compromised by the microinjection of mRNA encoding "dominant-negative" proteins. This methodology is particularly suited for the investigation of the regulation of the cell cycle, checkpoints, and apoptosis in early development.

Wee1 kinase alters cyclin E/Cdk2 and promotes apoptosis during the early embryonic development of Xenopus laevis

Background

The cell cycles of the Xenopus laevis embryo undergo extensive remodeling beginning at the midblastula transition (MBT) of early development. Cell divisions 2–12 consist of rapid cleavages without gap phases or cell cycle checkpoints. Some remodeling events depend upon a critical nucleo-cytoplasmic ratio, whereas others rely on a maternal timer controlled by cyclin E/Cdk2 activity. One key event that occurs at the MBT is the degradation of maternal Wee1, a negative regulator of cyclin-dependent kinase (Cdk) activity.

Results

In order to assess the effect of Wee1 on embryonic cell cycle remodeling, Wee1 mRNA was injected into one-cell stage embryos. Overexpression of Wee1 caused cell cycle delay and tyrosine phosphorylation of Cdks prior to the MBT. Furthermore, overexpression of Wee1 disrupted key developmental events that normally occur at the MBT such as the degradation of Cdc25A, cyclin E, and Wee1. Overexpression of Wee1 also resulted in post-MBT apoptosis, tyrosine phosphorylation of Cdks and persistence of cyclin E/Cdk2 activity. To determine whether Cdk2 was required specifically for the survival of the embryo, the cyclin E/Cdk2 inhibitor, Δ34-Xic1, was injected in embryos and also shown to induce apoptosis.

Conclusion

Taken together, these data suggest that Wee1 triggers apoptosis through the disruption of the cyclin E/Cdk2 timer. In contrast to Wee1 and Δ34-Xic1, altering Cdks by expression of Chk1 and Chk2 kinases blocks rather than promotes apoptosis and causes premature degradation of Cdc25A. Collectively, these data implicate Cdc25A as a key player in the developmentally regulated program of apoptosis in X. laevis embryos.

Cyclin E2 is required for embryogenesis in Xenopus laevis

In mammalian cells, E-type cyclins (E1 and E2) are generally believed to be required for entry into S phase. However, in mice, cyclin E is largely dispensable for normal embryogenesis. Moreover, Drosophila cyclin E plays a critical role in cell fate determination in neural lineages independently of proliferation. Thus, the functions of cyclin E, particularly during early development, remain elusive. Here, we investigated the requirement for E-type cyclins during Xenopus embryogenesis. Although cyclin E1 has been reported as a maternal cyclin, inhibition of its translation in the embryo caused no serious defects. We isolated a Xenopus homologue of human cyclin E2, which was zygotically expressed. Sufficient inhibition of its expression led to death at late gastrula, while partial inhibition allowed survival. These observations indicate distinct roles for Xenopus cyclins E1 and E2, and an absolute requirement of cyclin E2 for Xenopus embryogenesis.

Chk2/Cds1 protein kinase blocks apoptosis during early development ofXenopus laevis

Early Xenopus laevis embryos possess cell cycles that do not arrest at checkpoints in response to damaged DNA. At the midblastula transition (MBT), embryos with damaged DNA undergo apoptosis. After the MBT, DNA damage triggers cell cycle arrest rather than apoptosis. The transition from checkpoint-unregulated to checkpoint-regulated cycles makes Xenopus embryos compelling for studying mechanisms regulating response to genomic damage. The DNA damage checkpoint is mediated by the Chk2/Cds1 kinase. Conflicting evidence implicates Chk2 as an inhibitor or promoter of apoptosis. To better understand the developmental function of Chk2, we expressed wild-type (wt) and dominant-negative (DN) Chk2 in Xenopus embryos. Wt-Chk2 created a pre-MBT checkpoint due to degradation of Cdc25A and phosphorylation of cyclin-dependent kinases. Embryos expressing DN-Chk2 developed normally until gastrulation and then underwent apoptosis. Conversely, low doses of wt-Chk2 blocked radiation-induced apoptosis. Therefore, Chk2 operates at a switch between cell cycle arrest or apoptosis in response to genomic assaults.

The DNA damage checkpoint in embryonic cell cycles is dependent on the DNA-to-cytoplasmic ratio

In Xenopus, cell cycle checkpoints monitoring DNA damage, DNA replication, and spindle assembly do not appear until after the midblastula transition (MBT; 4000 cells). We show that a DNA damage checkpoint can slow the cell cycle even in 2-cell embryos when the DNA content is increased. Slowing follows caffeine-sensitive activation of the checkpoint kinase, Chk1; degradation of the cell cycle phosphatase, Cdc25A; and inhibitory phosphorylation of Cdc25C and cyclin-dependent kinases (Cdks). Alterations in the DNA-to-cytoplasmic ratio elicit a dose-dependent DNA damage checkpoint, and the ratio required to activate Chk1 for the damage response is lower than that associated with "developmental" activation of Chk1 shortly after the MBT. Our results indicate that a maternal damage response, independent of zygotic transcription, is present even in very early embryos, and requires both double-stranded DNA ends and a threshold DNA-to-cytoplasmic ratio to significantly affect the cell cycle.

A kinetic model of the cyclin E/Cdk2 developmental timer in Xenopus laevis embryos

Early cell cycles of Xenopus laevis embryos are characterized by rapid oscillations in the activity of two cyclin-dependent kinases. Cdk1 activity peaks at mitosis, driven by periodic degradation of cyclins A and B. In contrast, Cdk2 activity oscillates twice per cell cycle, despite a constant level of its partner, cyclin E. Cyclin E degrades at a fixed time after fertilization, normally corresponding to the midblastula transition. Based on published data and new experiments, we constructed a mathematical model in which: (1) oscillations in Cdk2 activity depend upon changes in phosphorylation, (2) Cdk2 participates in a negative feedback loop with the inhibitory kinase Wee1; (3) cyclin E is cooperatively removed from the oscillatory system; and (4) removed cyclin E is degraded by a pathway activated by cyclin E/Cdk2 itself. The model's predictions about embryos injected with Xic1, a stoichiometric inhibitor of cyclin E/Cdk2, were experimentally validated.

Loss of XChk1 function triggers apoptosis after the midblastula transition in Xenopus laevis embryos

Prior to the midblastula transition (MBT), Xenopus laevis embryos do not engage cell cycle checkpoints, although overexpression of the kinase XChk1 arrests cell divisions. At the MBT, XChk1 transiently activates and promotes cell cycle lengthening. In this study, endogenous XChk1 was inhibited by the expression of dominant-negative XChk1 (DN-XChk1). Development appeared normal until the early gastrula stage, when cells lost attachments and chromatin condensed. TUNEL and caspase assays indicated these embryos died by apoptosis during gastrulation. Embryos with unreplicated DNA likewise died by apoptosis. Embryos expressing DN-XChk1 proceeded through additional rapid rounds of DNA replication but initiated zygotic transcription on schedule. Therefore, XChk1 is essential in the early Xenopus embryo for cell cycle remodeling and for survival after the MBT.

Hysteresis drives cell-cycle transitions in Xenopus laevis egg extracts

Cells progressing through the cell cycle must commit irreversibly to mitosis without slipping back to interphase before properly segregating their chromosomes. A mathematical model of cell-cycle progression in cell-free egg extracts from frog predicts that irreversible transitions into and out of mitosis are driven by hysteresis in the molecular control system. Hysteresis refers to toggle-like switching behavior in a dynamical system. In the mathematical model, the toggle switch is created by positive feedback in the phosphorylation reactions controlling the activity of Cdc2, a protein kinase bound to its regulatory subunit, cyclin B. To determine whether hysteresis underlies entry into and exit from mitosis in cell-free egg extracts, we tested three predictions of the Novak-Tyson model. (i) The minimal concentration of cyclin B necessary to drive an interphase extract into mitosis is distinctly higher than the minimal concentration necessary to hold a mitotic extract in mitosis, evidence for hysteresis. (ii) Unreplicated DNA elevates the cyclin threshold for Cdc2 activation, indication that checkpoints operate by enlarging the hysteresis loop. (iii) A dramatic "slowing down" in the rate of Cdc2 activation is detected at concentrations of cyclin B marginally above the activation threshold. All three predictions were validated. These observations confirm hysteresis as the driving force for cell-cycle transitions into and out of mitosis.

Establishment of dependence relationships between genome replication and mitosis

Although budding yeast cell biology and genetics provided a powerful system to isolate S-phase checkpoint mutants, initial studies relied on a defect not likely to be relevant in higher eukaryotes. The first mutants were isolated for their inability to restrain mitotic spindle elongation in S-phase. Since most eukaryotes do not assemble spindles until prometaphase the validity of this approach might have been questioned. However, these early studies were designed with a highly valid assumption in mind; that checkpoints have a variety of targets, but comprise conserved kinase cascades that make up these signaling pathways. The task that lies ahead is to determine targets of the S-phase checkpoint relevant to mammals. One step forward might be the realization that the budding yeast S-phase checkpoint prevents loss of sister chromatid cohesion while DNA replication is ongoing. If this mechanism is conserved in mammals, it could prove vital for chromosome segregation fidelity.

Zygotic control of maternal cyclin A1 translation and mRNA stability

Cyclin mRNAs are unstable in the adult cell cycle yet are stable during the first 12 cell divisions in Xenopus laevis. We recently reported that cyclin A1 and B2 maternal mRNAs are deadenylated upon completion of the 12th division (Audic et al. [2001] Mol. Cell Biol. 21:1662-1671). Deadenylation is mediated by the 3' untranslated region (UTR) of the mRNA and precedes the terminal disappearance of the cyclin proteins, with both processes requiring zygotic transcription. The purpose of the current study was (1) to ask whether deadenylation leads to translational repression and/or destabilization of endogenous cyclin A1 and B2 mRNAs, and (2) to further characterize the regulatory sequences required. We show that zygote-driven deadenylation leads to translational repression and mRNA destabilization. A 99-nucleotide region of the 3'UTR of the cyclin A1 mRNA mediates both deadenylation and destabilization. Surprisingly, two AU-rich consensus elements within this region are dispensable for this activity. These results suggest that zygote-dependent deadenylation, translational repression, and mRNA destabilization by means of novel 3'UTR elements contribute to the disappearance of maternal cyclins. They also suggest that translational control of cyclins may play a role in the transition to the adult cell cycle. These data concur with previous studies in Drosophila showing that zygote-mediated degradation of maternal cdc25 mRNA may be a general mechanism whereby transition to the adult cell cycle proceeds.

Geminin deficiency causes a Chk1-dependent G2 arrest in Xenopus

Geminin is an unstable inhibitor of DNA replication that gets destroyed at the metaphase/anaphase transition. The biological function of geminin has been difficult to determine because it is not homologous to a characterized protein and has pleiotropic effects when overexpressed. Geminin is thought to prevent a second round of initiation during S or G2 phase. In some assays, geminin induces uncommitted embryonic cells to differentiate as neurons. In this study, geminin was eliminated from developing Xenopus embryos by using antisense techniques. Geminin-deficient embryos show a novel and unusual phenotype: they complete the early cleavage divisions normally but arrest in G2 phase immediately after the midblastula transition. The arrest requires Chk1, the effector kinase of the DNA replication/DNA damage checkpoint pathway. The results indicate that geminin has an essential function and that loss of this function prevents entry into mitosis by a Chk1-dependent mechanism. Geminin may be required to maintain the structural integrity of the genome or it may directly down-regulate Chk1 activity. The data also show that during the embryonic cell cycles, rereplication is almost entirely prevented by geminin-independent mechanisms.

Chk1 is activated transiently and targets Cdc25A for degradation at the Xenopus midblastula transition

In Xenopus embryos, cell cycle elongation and degradation of Cdc25A (a Cdk2 Tyr15 phosphatase) occur naturally at the midblastula transition (MBT), at which time a physiological DNA replication checkpoint is thought to be activated by the exponentially increased nucleo-cytoplasmic ratio. Here we show that the checkpoint kinase Chk1, but not Cds1 (Chk2), is activated transiently at the MBT in a maternal/zygotic gene product-regulated manner and is essential for cell cycle elongation and Cdc25A degradation at this transition. A constitutively active form of Chk1 can phosphorylate Cdc25A in vitro and can target it rapidly for degradation in pre-MBT embryos. Intriguingly, for this degradation, however, Cdc25A also requires a prior Chk1-independent phosphorylation at Ser73. Ectopically expressed human Cdc25A can be degraded in the same way as Xenopus Cdc25A. Finally, Cdc25A degradation at the MBT is a prerequisite for cell viability at later stages. Thus, the physiological replication checkpoint is activated transiently at the MBT by developmental cues, and activated Chk1, only together with an unknown kinase, targets Cdc25A for degradation to ensure later development.

Inactivation of the checkpoint kinase Cds1 is dependent on cyclin B-Cdc2 kinase activation at the meiotic G2/M-phase transition in Xenopus oocytes

Checkpoint controls ensure chromosomal integrity through the cell cycle. Chk1 and Cds1/Chk2 are effector kinases in the G2-phase checkpoint activated by damaged or unreplicated DNA, and they prevent entry into M-phase through inhibition of cyclin B-Cdc2 kinase activation. However, little is known about how the effector kinases are regulated when the checkpoint is attenuated. Recent studies indicate that Chk1 is also involved in the physiological G2-phase arrest of immature Xenopus oocytes via direct phosphorylation and inhibition of Cdc25C, the activator of cyclin B-Cdc2 kinase. Bearing in mind the overlapping functions of Chk1 and Cds1, here we have studied the involvement of Xenopus Cds1 (XCds1) in the G2/M-phase transition of immature oocytes and the regulation of its activity during this period. Protein levels of XCds1 remained constant throughout oocyte maturation and early embryonic development. The levels of XCds1 kinase activity were high in immature oocytes and decreased at the meiotic G2/M-phase transition. Consistently, when overexpressed in immature oocytes, wild-type, but not kinase-deficient, XCds1 significantly delayed entry into M-phase after progesterone treatment. The inactivation of XCds1 depended on the activation of cyclin B-Cdc2 kinase, but not MAP kinase. Although XCds1 was not directly inactivated by cyclin B-Cdc2 kinase in vitro, XCds1 was inactivated by overexpression of cyclin B, which induces the activation of cyclin B-Cdc2 kinase without progesterone. Thus, the present study is the first indication of Cds1 activity in cells that are physiologically arrested at G2-phase, and of its downregulation at entry into M-phase.