DNA damage leads to a Cyclin A-dependent delay in metaphase-anaphase transition in the Drosophila gastrula

Wnt signaling modulates the response to DNA damage in the Drosophila wing imaginal disc by regulating the EGFR pathway

Despite the deep conservation of the DNA damage response (DDR) pathway, cells in different contexts vary widely in their susceptibility to DNA damage and their propensity to undergo apoptosis as a result of genomic lesions. One of the cell signaling pathways implicated in modulating the DDR is the highly conserved Wnt pathway, which is known to promote resistance to DNA damage caused by ionizing radiation in a variety of human cancers. However, the mechanisms linking Wnt signal transduction to the DDR remain unclear. Here, we use a genetically encoded system in Drosophila to reliably induce consistent levels of DNA damage in vivo, and demonstrate that canonical Wnt signaling in the wing imaginal disc buffers cells against apoptosis in the face of DNA double-strand breaks. We show that Wg, the primary Wnt ligand in Drosophila, activates epidermal growth factor receptor (EGFR) signaling via the ligand-processing protease Rhomboid, which, in turn, modulates the DDR in a Chk2-, p53-, and E2F1-dependent manner. These studies provide mechanistic insight into the modulation of the DDR by the Wnt and EGFR pathways in vivo in a highly proliferative tissue. Furthermore, they reveal how the growth and patterning functions of Wnt signaling are coupled with prosurvival, antiapoptotic activities, thereby facilitating developmental robustness in the face of genomic damage.

Bradysia (Sciara) coprophila larvae up-regulate DNA repair pathways and down-regulate developmental regulators in response to ionizing radiation

The level of resistance to radiation and the developmental and molecular responses can vary between species, and even between developmental stages of one species. For flies (order: Diptera), prior studies concluded that the fungus gnat Bradysia (Sciara) coprophila (sub-order: Nematocera) is more resistant to irradiation-induced mutations that cause visible phenotypes than the fruit fly Drosophila melanogaster (sub-order: Brachycera). Therefore, we characterized the effects of and level of resistance to ionizing radiation on B. coprophila throughout its life cycle. Our data show that B. coprophila embryos are highly sensitive to even low doses of gamma-irradiation, whereas late-stage larvae can tolerate up to 80 Gy (compared to 40 Gy for D. melanogaster) and still retain their ability to develop to adulthood, though with a developmental delay. To survey the genes involved in the early transcriptional response to irradiation of B. coprophila larvae, we compared larval RNA-seq profiles with and without radiation treatment. The up-regulated genes were enriched for DNA damage response genes, including those involved in DNA repair, cell cycle arrest, and apoptosis, whereas the down-regulated genes were enriched for developmental regulators, consistent with the developmental delay of irradiated larvae. Interestingly, members of the PARP and AGO families were highly up-regulated in the B. coprophila radiation response. We compared the transcriptome responses in B. coprophila to the transcriptome responses in D. melanogaster from 3 previous studies: whereas pathway responses are highly conserved, specific gene responses are less so. Our study lays the groundwork for future work on the radiation responses in Diptera.

Cyclin A2 Expression as Predictive Biomarker in Muscle-Invasive Upper Tract Urothelial Carcinoma

INTRODUCTION

The aim was to evaluate the prognostic value of altered Cyclin A2 (CCNA2) gene expression in upper tract urothelial carcinoma (UTUC) and to assess its predictive potential as a prognostic factor for overall survival (OS) and disease-free survival.

METHODS

62 patients who underwent surgical treatment for UTUC were included. Gene expression of CCNA2, MKI67, and p53 was analyzed by quantitative reverse transcriptase polymerase chain reaction. Survival analyses were performed using the Kaplan-Meier method and the log-rank test. For Cox regression analyses, uni- and multivariable hazard ratios were calculated. Spearman correlation was used to analyze correlation of CCNA2 expression with MKI67 and p53.

RESULTS

The median age of the cohort was 73 years, and it consisted of 48 males (77.4%) and 14 females (22.6%). Patients with high CCNA2 expression levels showed longer OS (HR 0.33; 95% CI: 0.15-0.74; p = 0.0073). Multivariable Cox regression analyses identified CCNA2 overexpression (HR 0.37; 95% CI: 0.16-0.85; p = 0.0189) and grading G2 (vs. G3) (HR 0.39; 95% CI: 0.17-0.87; p = 0.0168) to be independent predictors for longer OS. CCNA2 expression correlated positively with MKI67 expression (Rho = 0.4376, p = 0.0005).

CONCLUSION

Low CCNA2 expression is significantly associated with worse OS. Thus, CCNA2 might serve as a potential biomarker in muscle-invasive UTUC and may be used to characterize a subset of patients having an unfavorable outcome and for future risk assessment scores.

Mitotic DNA Damage Response: At the Crossroads of Structural and Numerical Cancer Chromosome Instabilities

DNA double-strand breaks (DSBs) prevent cells from entering mitosis allowing cells to repair their genomic damage. Little is known about the response to DSBs once cells have already committed to mitosis. Here, we review the genome-protective role of the mitotic DNA damage response (DDR) and evidence suggesting that its untimely activation induces chromosome segregation errors and paradoxically undermines genomic integrity. In contrast to normal cells, cancer cells coopt this pathway to propagate structural and numerical chromosomal instabilities. Cells derived from genomically unstable tumors exhibit evidence for a partially activated DDR during mitosis, which leads to ongoing chromosome segregation errors. Thus, a thorough understanding of the consequences of mitotic DNA damage is key to our ability to devise novel anticancer therapeutic strategies.

Maintaining Genome Stability in Defiance of Mitotic DNA Damage

The implementation of decisions affecting cell viability and proliferation is based on prompt detection of the issue to be addressed, formulation and transmission of a correct set of instructions and fidelity in the execution of orders. While the first and the last are purely mechanical processes relying on the faithful functioning of single proteins or macromolecular complexes (sensors and effectors), information is the real cue, with signal amplitude, duration, and frequency ultimately determining the type of response. The cellular response to DNA damage is no exception to the rule. In this review article we focus on DNA damage responses in G2 and Mitosis. First, we set the stage describing mitosis and the machineries in charge of assembling the apparatus responsible for chromosome alignment and segregation as well as the inputs that control its function (checkpoints). Next, we examine the type of issues that a cell approaching mitosis might face, presenting the impact of post-translational modifications (PTMs) on the correct and timely functioning of pathways correcting errors or damage before chromosome segregation. We conclude this essay with a perspective on the current status of mitotic signaling pathway inhibitors and their potential use in cancer therapy.

Crumbs interacts with Xpd for nuclear division control in Drosophila

Crumbs (Crb) family proteins are crucial for cell polarity. Recent studies indicate that they are also involved in growth regulation and cancer. However, it is not well-understood how Crb participates in mitotic processes. Here, we report that Drosophila Crb is critically involved in nuclear division by interacting with Xeroderma pigmentosum D (XPD). A novel gene named galla-1 was identified from a genetic screen for crb modifiers. Galla-1 protein shows homology to MIP18, a subunit of the mitotic spindle-associated MMS19-XPD complex. Loss-of-function galla-1 mutants show abnormal chromosome segregation, defective centrosome positions and branched spindles during nuclear division in early embryos. Embryos with loss-of-function or overexpression of crb show similar mitotic defects and genetic interaction with galla-1. Both Galla-1 and Crb proteins show overlapping localization with spindle microtubules during nuclear division. Galla-1 physically interacts with the intracellular domain of Crb. Interestingly, Galla-1 shows little binding to the Drosophila homolog of XPD, but a related protein Galla-2 binds both Crb and Xpd. Loss-of-function galla-2 mutants show similar mitotic defects as galla-1 and strong genetic interaction with crb. Xpd can form a physical complex with Crb. In imaginal disc, Crb overexpression causes tissue overgrowth as well as DNA damages marked by H2Av phosphorylation. These phenotypes are suppressed by reduction of Xpd. Taken together, this study identifies a novel Crb-Galla-Xpd complex and its function for proper chromosome segregation during nuclear division, implicating a potential link between Crb and Xpd-related genome instability.

DNA damage to a single chromosome end delays anaphase onset

Chromosome ends contain nucleoprotein structures known as telomeres. Damage to chromosome ends during interphase elicits a DNA damage response (DDR) resulting in cell cycle arrest. However, little is known regarding the signaling from damaged chromosome ends (designated here as "TIPs") during mitosis. In the present study, we investigated the consequences of DNA damage induced at a single TIP in mitosis. We used laser microirradiation to damage mitotic TIPs or chromosome arms (non-TIPs) in PtK2 kidney epithelial cells. We found that damage to a single TIP, but not a non-TIP, delays anaphase onset. This TIP-specific checkpoint response is accompanied by differential recruitment of DDR proteins. Although phosphorylation of H2AX and the recruitment of several repair factors, such as Ku70-Ku80, occur in a comparable manner at both TIP and non-TIP damage sites, DDR factors such as ataxia telangiectasia mutated (ATM), MDC1, WRN, and FANCD2 are specifically recruited to TIPs but not to non-TIPs. In addition, Nbs1, BRCA1, and ubiquitin accumulate at damaged TIPs more rapidly than at damaged non-TIPs. ATR and 53BP1 are not detected at either TIPs or non-TIPs in mitosis. The observed delay in anaphase onset is dependent on the activity of DDR kinases ATM and Chk1, and the spindle assembly checkpoint kinase Mps1. Cells damaged at a single TIP or non-TIP eventually exit mitosis with unrepaired lesions. Damaged TIPs are segregated into micronuclei at a significantly higher frequency than damaged non-TIPs. Together, these findings reveal a mitosis-specific DDR uniquely associated with chromosome ends.

UV-C irradiation delays mitotic progression by recruiting Mps1 to kinetochores

The effect of UV irradiation on replicating cells during interphase has been studied extensively. However, how the mitotic cell responds to UV irradiation is less well defined. Herein, we found that UV-C irradiation (254 nm) increases recruitment of the spindle checkpoint proteins Mps1 and Mad2 to the kinetochore during metaphase, suggesting that the spindle assembly checkpoint (SAC) is reactivated. In accordance with this, cells exposed to UV-C showed delayed mitotic progression, characterized by a prolonged chromosomal alignment during metaphase. UV-C irradiation also induced the DNA damage response and caused a significant accumulation of γ-H2AX on mitotic chromosomes. Unexpectedly, the mitotic delay upon UV-C irradiation is not due to the DNA damage response but to the relocation of Mps1 to the kinetochore. Further, we found that UV-C irradiation activates Aurora B kinase. Importantly, the kinase activity of Aurora B is indispensable for full recruitment of Mps1 to the kinetochore during both prometaphase and metaphase. Taking these findings together, we propose that UV irradiation delays mitotic progression by evoking the Aurora B-Mps1 signaling cascade, which exerts its role through promoting the association of Mps1 with the kinetochore in metaphase.

The effect of a DNA damaging agent on embryonic cell cycles of the cnidarian Hydractinia echinata

The onset of gastrulation at the Mid-Blastula Transition can accompany profound changes in embryonic cell cycles including the introduction of gap phases and the transition from maternal to zygotic control. Studies in Xenopus and Drosophila embryos have also found that cell cycles respond to DNA damage differently before and after MBT (or its equivalent, MZT, in Drosophila). DNA checkpoints are absent in Xenopus cleavage cycles but are acquired during MBT. Drosophila cleavage nuclei enter an abortive mitosis in the presence of DNA damage whereas post-MZT cells delay the entry into mitosis. Despite attributes that render them workhorses of embryonic cell cycle studies, Xenopus and Drosophila are hardly representative of diverse animal forms that exist. To investigate developmental changes in DNA damage responses in a distant phylum, I studied the effect of an alkylating agent, Methyl Methanesulfonate (MMS), on embryos of Hydractinia echinata. Hydractinia embryos are found to differ from Xenopus embryos in the ability to respond to a DNA damaging agent in early cleavage but are similar to Xenopus and Drosophila embryos in acquiring stronger DNA damage responses and greater resistance to killing by MMS after the onset of gastrulation. This represents the first study of DNA damage responses in the phylum Cnidaria.

Cellular responses to DNA damage: one signal, multiple choices

DNA double-strand breaks (DSBs) produce a number of cellular responses, some mutually exclusive. Depending on where on the chromosome it occurs, a DSB may become preserved inside a telomere or eliminated by repair. A cell may arrest division via checkpoint activation to fix DSBs or commit suicide by apoptosis. What determines the outcome: to bury, fix, or succumb to DNA DSBs? With this question in mind, we review recent data on cellular responses to DSBs.

Menin is a regulator of the stress response in Drosophila melanogaster

Menin, the product of the multiple endocrine neoplasia type I gene, has been implicated in several biological processes, including the control of gene expression and apoptosis, the modulation of mitogen-activated protein kinase pathways, and DNA damage sensing or repair. In this study, we have investigated the function of menin in the model organism Drosophila melanogaster. We show that Drosophila lines overexpressing menin or an RNA interference for this gene develop normally but are impaired in their response to several stresses, including heat shock, hypoxia, hyperosmolarity and oxidative stress. In the embryo subjected to heat shock, this impairment was characterized by a high degree of developmental arrest and lethality. The overexpression of menin enhanced the expression of HSP70 in embryos and interfered with its down-regulation during recovery at the normal temperature. In contrast, the inhibition of menin with RNA interference reduced the induction of HSP70 and blocked the activation of HSP23 upon heat shock, Menin was recruited to the Hsp70 promoter upon heat shock and menin overexpression stimulated the activity of this promoter in embryos. A 70-kDa inducible form of menin was expressed in response to heat shock, indicating that menin is also regulated in conditions of stress. The induction of HSP70 and HSP23 was markedly reduced or absent in mutant embryos harboring a deletion of the menin gene. These embryos, which did not express the heat shock-inducible form of menin, were also hypersensitive to various conditions of stress. These results suggest a novel role for menin in the control of the stress response and in processes associated with the maintenance of protein integrity.

Regulation of mitosis in response to damaged or incompletely replicated DNA require different levels of Grapes (Drosophila Chk1)

Checkpoints monitor the state of DNA and can delay or arrest the cell cycle at multiple points including G1-S transition, progress through S phase and G2-M transition. Regulation of progress through mitosis, specifically at the metaphase-anaphase transition, occurs after exposure to ionizing radiation (IR) in Drosophila and budding yeast, but has not been conclusively demonstrated in mammals. Here we report that regulation of metaphase-anaphase transition in Drosophila depends on the magnitude of radiation dose and time in the cell cycle at which radiation is applied, which may explain the apparent differences among experimental systems and offer an explanation as to why this regulation has not been seen in mammalian cells. We further document that mutants in Drosophila Chk1 (Grapes) that are capable of delaying the progress through mitosis in response to IR are incapable of delaying progress through mitosis when DNA synthesis is blocked by mutations in an essential replication factor encoded by double park (Drosophila Cdt1). We conclude that DNA damage and replication checkpoints operating in the same cell cycle at the same developmental stage in Drosophila can exhibit differential requirements for the Chk1 homolog. The converse situation exists in fission yeast where loss of Chk1 is more detrimental to the DNA damage checkpoint than to the DNA replication checkpoint. It remains to be seen which of these two different uses of Chk1 homologs are conserved in mammals. Finally, our results demonstrate that Drosophila provides a unique opportunity to study the regulation of the entry into, and progress through, mitosis by DNA structure checkpoints in metazoa.

Mitosis: a matter of getting rid of the right protein at the right time

There are two major problems for the cell to solve in mitosis: how to ensure that each daughter cell receives an equal and identical complement of the genome, and how to prevent cell separation before chromosome segregation. Both these problems are solved by controlling when two specific proteins are destroyed: securin, an inhibitor of chromosome segregation, and cyclin B, which inhibits cell separation (cytokinesis). It has recently become clear that several other proteins are degraded at specific points in mitosis. This review (which is part of the Chromosome Segregation and Aneuploidy series) focuses on how specific proteins are selected for proteolysis at defined points in mitosis and how this contributes to the proper coordination of chromosome segregation and cytokinesis.

The Drosophila Grp/Chk1 DNA damage checkpoint controls entry into anaphase

It is well established that DNA damage induces checkpoint-mediated interphase arrest in higher eukaryotes, but recent studies demonstrate that DNA damage delays entry into anaphase as well. Damaged DNA in syncytial and gastrulating Drosophila embryos delays the metaphase/anaphase transition . In human cultured cells, DNA damage also induces a delay in mitosis . However, the mechanism by which DNA damage delays the anaphase onset is controversial. Some studies implicate a DNA damage checkpoint , whereas other studies invoke a spindle checkpoint . To resolve this issue, we compared the effects of random DNA breaks induced by X-irradiation to site-specific I-CreI endonuclease-induced chromosome breaks on cell-cycle progression in wild-type and checkpoint-defective Drosophila neuroblasts. We found that both the BubR1 spindle checkpoint pathway and the Grp/Chk1 DNA damage checkpoint pathway are involved in delaying the metaphase/anaphase transition after extensive X-irradiation-induced DNA damage, whereas Grp/Chk1, but not BubR1, is required to delay anaphase onset in the presence of I-CreI-induced double-strand breaks. On the basis of these results, we propose that DNA damage in nonkinetochore regions produces a Grp/Chk1 DNA-damage-checkpoint-mediated delay in the metaphase/anaphase transition.

Chromosome damage and progression into and through mitosis in vertebrates

How cells behave as they divide in the presence of chromosome (DNA) damage is only just beginning to be explored. It appears to depend on the cell type and organism, the stage of development, how extensive the damage is and when it occurs. The existing data support the conclusion that vertebrate somatic cells lack a conventional DNA damage checkpoint during mitosis, and that when damaged DNA does prolong mitosis it is mediated by the spindle assembly checkpoint. As a rule, in the presence of DNA damage cells ultimately undergo an aberrant mitosis and enter the ensuing G1. They then either die, via apoptosis or mitotic catastrophe, or survive with an altered genome. To avoid these outcomes, cells with DNA damage are normally prevented from entering mitosis by a number of G2 checkpoint control pathways.

Hypoxia induces major effects on cell cycle kinetics and protein expression inDrosophila melanogasterembryos

Hypoxia induces a stereotypic response in Drosophila melanogaster embryos: depending on the time of hypoxia, embryos arrest cell cycle activity either at metaphase or just before S phase. To understand the mechanisms underlying hypoxia-induced arrest, two kinds of experiments were conducted. First, embryos carrying a kinesin-green fluorescent protein construct, which permits in vivo confocal microscopic visualization of the cell cycle, showed a dose-response relation between O2level and cell cycle length. For example, mild hypoxia (Po2∼55 Torr) had no apparent effect on cell cycle length, whereas severe hypoxia (Po2∼25–35 Torr) or anoxia (Po2= 0 Torr) arrested the cell cycle. Second, we utilized Drosophila embryos carrying a heat shock promoter driving the string ( cdc25) gene (HS-STG3), which permits synchronization of embryos before the start of mitosis. Under conditions of anoxia, we induced a stabilization or an increase in the expression of several G1/S (e.g., dE2F1, RBF2) and G2/M (e.g., cyclin A, cyclin B, dWee1) proteins. This study suggests that, in fruit fly embryos, 1) there is a dose-dependent relationship between cell cycle length and O2levels in fruit fly embryos, and 2) stabilized cyclin A and E2F1 are likely to be the mediators of hypoxia-induced arrest at metaphase and pre-S phase.

Enzyme fragment complementation: a flexible high throughput screening assay technology

High-affinity complementation of a small fragment of beta-galactosidase to an inactive deletion mutant of the enzyme forms a stable heteromeric enzyme complex capable of hydrolyzing substrates to produce either chemiluminescent or fluorescent signals. This review describes a series of screening assays in which the small beta-galactosidase fragment, Enzyme Donor or ProLabel, is either chemically conjugated or recombinantly fused to small molecules or proteins, respectively. Chemical conjugation forms the basis of several HitHunter HTS assays in which competitive displacement of the ProLabel conjugate from either a binding protein (receptor or antibody) is induced by the analyte in question. In this manner, a calibration curve is generated, to measure cellular analytes including 3',5'-cyclic AMP. Changes in this second messenger, occurring due to G protein-coupled receptor (GPCR) activation, can thus be easily measured in a homogeneous assay. Similar assays have been developed for tyrosine kinases, serine threonine kinases, nuclear hormone receptors, and proteases. A second form of assay technology involves measurement of cellular protein expression, in which the protein is fused to ProLabel. Analysis can be undertaken in crude cell lysates, or with intact cells, using beta-galactosidase complementation in a microtiter plate. This homogeneous technology is highly sensitive and has been developed to measure protein expression changes occurring in response to pathway activation by targets such as GPCRs, tyrosine kinase receptors, and proteases. In summary, the DiscoveRx technology using beta-galactosidase complementation provides a robust and flexible assay technology for use in cell-free and cell-based HTS.

DNA damage during the spindle-assembly checkpoint degrades CDC25A, inhibits cyclin-CDC2 complexes, and reverses cells to interphase

Cell cycle checkpoints that monitor DNA damage and spindle assembly are essential for the maintenance of genetic integrity, and drugs that target these checkpoints are important chemotherapeutic agents. We have examined how cells respond to DNA damage while the spindle-assembly checkpoint is activated. Single cell electrophoresis and phosphorylation of histone H2AX indicated that several chemotherapeutic agents could induce DNA damage during mitotic block. DNA damage during mitotic block triggered CDC2 inactivation, histone H3 dephosphorylation, and chromosome decondensation. Cells did not progress into G1 but seemed to retract to a G2-like state containing 4N DNA content, with stabilized cyclin A and cyclin B1 binding to Thr14/Tyr15-phosphorylated CDC2. The loss of mitotic cells was not due to cell death because there was no discernible effect on caspase-3 activation, DNA fragmentation, or viability. Extensive DNA damage during mitotic block inactivated cyclin B1-CDC2 and prevented G1 entry when the block was removed. The mitotic DNA damage responses were independent of p53 and pRb, but they were dependent on ATM. CDC25A that accumulated during mitosis was rapidly destroyed after DNA damage in an ATM-dependent manner. Ectopic expression of CDC25A or nonphosphorylatable CDC2 effectively inhibited the dephosphorylation of histone H3 after DNA damage. Hence, although spindle disruption and DNA damage provide conflicting signals to regulate CDC2, the negative regulation by the DNA damage checkpoint could overcome the positive regulation by the spindle-assembly checkpoint.

Phenotypic analysis of separation-of-function alleles of MEI-41, Drosophila ATM/ATR

ATM/ATR kinases act as signal transducers in eukaryotic DNA damage and replication checkpoints. Mutations in ATM/ATR homologs have pleiotropic effects that range from sterility to increased killing by genotoxins in humans, mice, and Drosophila. Here we report the generation of a null allele of mei-41, Drosophila ATM/ATR homolog, and the use of it to document a semidominant effect on a larval mitotic checkpoint and methyl methanesulfonate (MMS) sensitivity. We also tested the role of mei-41 in a recently characterized checkpoint that delays metaphase/anaphase transition after DNA damage in cellular embryos. We then compare five existing mei-41 alleles to the null with respect to known phenotypes (female sterility, cell cycle checkpoints, and MMS resistance). We find that not all phenotypes are affected equally by each allele, i.e., the functions of MEI-41 in ensuring fertility, cell cycle regulation, and resistance to genotoxins are genetically separable. We propose that MEI-41 acts not in a single rigid signal transduction pathway, but in multiple molecular contexts to carry out its many functions. Sequence analysis identified mutations, which, for most alleles, fall in the poorly characterized region outside the kinase domain; this allowed us to tentatively identify additional functional domains of MEI-41 that could be subjected to future structure-function studies of this key molecule.

The telomere protein Taz1 is required to prevent and repair genomic DNA breaks

One fundamental function of telomeres is to prevent the ends of chromosomes from being sensed and treated as DNA damage. Here we present evidence for additional roles of telomeres in promoting proper chromosome segregation and DNA repair. We find that the fission yeast telomere protein Taz1p is required for cell cycle progression at 20 degrees C, a temperature at which taz1Delta cells exhibit a G(2)/M DNA damage checkpoint delay, chromosome missegregation, and DNA double-strand breaks (DSBs). Spindle assembly checkpoint components and a checkpoint-independent function of Rad3p are required for taz1Delta cells to survive at 20 degrees C. Disruption of topoisomerase II activity suppresses the cold sensitivity of taz1Delta cells, suggesting a scenario in which telomeric entanglement is the primary defect. Furthermore, hypersensitivity to treatments that induce DSBs suggests that Taz1p is involved in DSB repair. Our observations imply roles for Taz1p-containing telomeres in preventing and repairing DNA breaks throughout the genome.

DNA damage during mitosis in human cells delays the metaphase/anaphase transition via the spindle-assembly checkpoint

BACKGROUND

DNA damage during mitosis triggers an ATM kinase-mediated cell cycle checkpoint pathway in yeast and fly embryos that delays progression through division. Recent data suggest that this is also true for mammals. Here we used laser microsurgery and inhibitors of topoisomerase IIalpha to break DNA in various mammalian cells after they became committed to mitosis. We then followed the fate of these cells and emphasized the timing of mitotic progression, spindle structure, and chromosome behavior.

RESULTS

We find that DNA breaks generated during late prophase do not impede entry into prometaphase. If the damage is minor, cells complete mitosis on time. However, more significant damage substantially delays exit from mitosis in many cell types. In human (HeLa, CFPAC-1, and hTERT-RPE) cells, this delay occurs during metaphase, after the formation of a bipolar spindle and the destruction of cyclin A, and it is not dependent on a functional p53 pathway. Pretreating cells with ATM kinase inhibitors does not abrogate the metaphase delay due to chromosome damage. Immunofluorescence studies reveal that cells blocked in metaphase by chromosome damage contain one or more Mad2-positive kinetochores, and the block is rapidly overridden when the cells are microinjected with a dominant-negative construct of Mad2 (Mad2deltaC).

CONCLUSIONS

We conclude that the delay in mitosis induced by DNA damage is not due to an ATM-mediated DNA damage checkpoint pathway. Rather, the damage leads to defects in kinetochore attachment and function that, in turn, maintain the intrinsic Mad-2-based spindle assembly checkpoint.

Cyclin F is degraded during G2-M by mechanisms fundamentally different from other cyclins

Cyclin F, a cyclin that can form SCF complexes and bind to cyclin B, oscillates in the cell cycle with a pattern similar to cyclin A and cyclin B. Ectopic expression of cyclin F arrests the cell cycle in G(2)/M. How the level of cyclin F is regulated during the cell cycle is completely obscure. Here we show that, similar to cyclin A, cyclin F is degraded when the spindle assembly checkpoint is activated and accumulates when the DNA damage checkpoint is activated. Cyclin F is a very unstable protein throughout much of the cell cycle. Unlike other cyclins, degradation of cyclin F is independent of ubiquitination and proteasome-mediated pathways. Interestingly, proteolysis of cyclin F is likely to involve metalloproteases. Rapid destruction of cyclin F does not require the N-terminal F-box motif but requires the COOH-terminal PEST sequences. The PEST region alone is sufficient to interfere with the degradation of cyclin F and confer instability when fused to cyclin A. These data show that although cyclin F is degraded at similar time as the mitotic cyclins, the underlying mechanisms are entirely distinct.

mei-41 and bub1 block mitosis at two distinct steps in response to incomplete DNA replication in Drosophila embryos

Drosophila double park encodes a homolog of Cdt1 that functions in initiation of DNA replication in fission yeast and Xenopus. dup mutants complete the first 15 embryonic cell cycles, presumably via maternal dup products, and show defects in the 16(th) S phase (S16). Cells carrying dup(a1) allele forgo S16 altogether but enter mitosis 16 (M16). We find that the timing of entry into M16 is similar in dup(a1) and heterozygous or wild-type (wt) controls. In contrast, we find that mutant cells carrying another allele, dup(a3), undergo a partial S16 and delay the entry into M16. Thus, initiation of S16 appears necessary for delaying M16. This delay is absent in double mutants of dup(a3) and mei-41 (Drosophila ATR), indicating that a mei-41-dependent checkpoint acts to delay the entry into mitosis in response to incomplete DNA replication. dup(a3) and dup(a1) mutant cells that enter M16 become arrested in M16. We find that mitotic cyclins are stabilized and that a spindle checkpoint protein, Bub1, localizes onto chromosomes during mitotic arrest in dup mutants. These features suggest an arrest prior to metaphase-anaphase transition. dup(a3) bub1 double mutant cells exit M16, indicating that a bub1-mediated checkpoint acts to block mitotic exit in dup mutants. To our knowledge, this is the first report of (1) incomplete DNA replication affecting both the entry into and the exit from mitosis in a single cell cycle via different mechanisms and (2) the role of bub1 in regulating mitotic exit in response to incomplete DNA replication.

The schedule of destruction of three mitotic cyclins can dictate the timing of events during exit from mitosis

BACKGROUND

Degradation of the mitotic cyclins is a hallmark of the exit from mitosis. Induction of stable versions of each of the three mitotic cyclins of Drosophila, cyclins A, B, and B3, arrests mitosis with different phenotypes. We tested a recent proposal that the destruction of the different cyclins guides progress through mitosis.

RESULTS

Real-time imaging revealed that arrest phenotypes differ because each stable cyclin affects specific mitotic events differently. Stable cyclin A prolonged or blocked chromosome disjunction, leading to metaphase arrest. Stable cyclin B allowed the transition to anaphase, but anaphase A chromosome movements were slowed, anaphase B spindle elongation did not occur, and the monooriented disjoined chromosomes began to oscillate between the spindle poles. Stable cyclin B3 prevented normal spindle maturation and blocked major mitotic exit events such as chromosome decondensation but nonetheless allowed chromosome disjunction, anaphase B, and formation of a cytokinetic furrow, which split the spindle.

CONCLUSIONS

We conclude that degradation of distinct mitotic cyclins is required to transit specific steps of mitosis: cyclin A degradation facilitates chromosome disjunction, cyclin B destruction is required for anaphase B and cytokinesis and for directional stability of univalent chromosome movements, and cyclin B3 degradation is required for proper spindle reorganization and restoration of the interphase nucleus. We suggest that the schedule of degradation of cyclin A, cyclin B, and then cyclin B3 contributes to the temporal coordination of mitotic events.

Cell cycle: how, when and why cells get rid of cyclin A

Sequences outside the 'destruction box' direct the degradation of cyclin A to completion before the metaphase-anaphase transition; cyclin A that escapes timely degradation can block the metaphase-anaphase transition, impede anaphase and telophase, and impair a cell's ability to arrest in G1 of the next cell cycle.

Mitotic degradation of cyclin A is mediated by multiple and novel destruction signals

Exit from mitosis requires Cdk1 inactivation, with the most prominent mechanism of Cdk1 inactivation being proteolysis of mitotic cyclins [1]. In higher eukaryotes this involves sequential destruction of A- and B-type cyclins. CycA is destroyed first, and CycA/Cdk1 inactivation is required for the metaphase-to-anaphase transition [2]. The degradation of CycA is delayed in response to DNA damage but is not prevented when the spindle checkpoint is activated [3, 4]. Cyclin destruction is thought to be mediated by a conserved motif, the destruction box (D box). Like B-type cyclins, A-type cyclins contain putative destruction box sequences in their N termini [5]. However, no detailed in vivo analysis of the sequence requirements for CycA destruction has been described so far. Here we tested several mutations in the CycA coding region for destruction in Drosophila embryos. We show that D box sequences are not essential for mitotic destruction of CycA. Destruction is mediated by at least three different elements that act in an overlapping fashion to mediate its mitotic degradation.

Cyclin A is destroyed in prometaphase and can delay chromosome alignment and anaphase

Mitosis is controlled by the specific and timely degradation of key regulatory proteins, notably the mitotic cyclins that bind and activate the cyclin-dependent kinases (Cdks). In animal cells, cyclin A is always degraded before cyclin B, but the exact timing and the mechanism underlying this are not known. Here we use live cell imaging to show that cyclin A begins to be degraded just after nuclear envelope breakdown. This degradation requires the 26S proteasome, but is not affected by the spindle checkpoint. Neither deletion of its destruction box nor disrupting Cdk binding prevents cyclin A proteolysis, but Cdk binding is necessary for degradation at the correct time. We also show that increasing the levels of cyclin A delays chromosome alignment and sister chromatid segregation. This delay depends on the proteolysis of cyclin A and is not caused by a lag in the bipolar attachment of chromosomes to the mitotic spindle, nor is it mediated via the spindle checkpoint. Thus, proteolysis that is not under the control of the spindle checkpoint is required for chromosome alignment and anaphase.