Drosophila Chk2 is required for DNA damage-mediated cell cycle arrest and apoptosis

Overdrive is essential for targeted sperm elimination by Segregation Distorter

Intra-genomic conflict driven by selfish chromosomes is a powerful force that shapes the evolution of genomes and species. In the male germline, many selfish chromosomes bias transmission in their own favor by eliminating spermatids bearing the competing homologous chromosomes. However, the mechanisms of targeted gamete elimination remain mysterious. Here, we show that Overdrive (Ovd), a gene required for both segregation distortion and male sterility in Drosophila pseudoobscura hybrids, is broadly conserved in Dipteran insects but dispensable for viability and fertility. In D. melanogaster, Ovd is required for targeted Responder spermatid elimination after the histone-to-protamine transition in the classical Segregation Distorter system. We propose that Ovd functions as a general spermatid quality checkpoint that is hijacked by independent selfish chromosomes to eliminate competing gametes.

A review on L-methioninase in cancer therapy: Precision targeting, advancements and diverse applications for a promising future

Cancer remains a global health challenge, demanding novel therapeutic options due to the debilitating side effects of conventional treatments on healthy tissues. The review highlights the potential of L-methioninase, a pyridoxal-5-phosphate (PLP)-dependent enzyme, as a promising avenue in alternative cancer therapy. L-methioninase offers a unique advantage, its ability to selectively target and inhibit the growth of cancer cells without harming healthy cells. This selectivity arises because tumor cells lack an essential enzyme called methionine synthase, which healthy cells use to make the vital amino acid L-methionine. Several sources harbor L-methioninase, including bacteria, fungi, plants, and protozoa. Future research efforts can explore and exploit this diverse range of sources to improve the therapeutic potential of L-methioninase in the fight against cancer. Despite challenges, research actively explores microbial L-methioninase for its anticancer potential. This review examines the enzyme's side effects, advancements in combination therapies, recombinant technologies, polymer conjugation and novel delivery methods like nanoparticles, while highlighting the success of oral administration in preclinical trials. Beyond its promising role in cancer therapy, L-methioninase holds potential applications in food science, antioxidants, and various health concerns like diabetes, cardiovascular issues, and neurodegenerative diseases. This review provides a piece of current knowledge and future prospects of L-methioninase, exploring its diverse therapeutic potential.

Genetic variation in P-element dysgenic sterility is associated with double-strand break repair and alternative splicing of TE transcripts

The germline mobilization of transposable elements (TEs) by small RNA mediated silencing pathways is conserved across eukaryotes and critical for ensuring the integrity of gamete genomes. However, genomes are recurrently invaded by novel TEs through horizontal transfer. These invading TEs are not targeted by host small RNAs, and their unregulated activity can cause DNA damage in germline cells and ultimately lead to sterility. Here we use hybrid dysgenesis-a sterility syndrome of Drosophila caused by transposition of invading P-element DNA transposons-to uncover host genetic variants that modulate dysgenic sterility. Using a panel of highly recombinant inbred lines of Drosophila melanogaster, we identified two linked quantitative trait loci (QTL) that determine the severity of dysgenic sterility in young and old females, respectively. We show that ovaries of fertile genotypes exhibit increased expression of splicing factors that suppress the production of transposase encoding transcripts, which likely reduces the transposition rate and associated DNA damage. We also show that fertile alleles are associated with decreased sensitivity to double-stranded breaks and enhanced DNA repair, explaining their ability to withstand high germline transposition rates. Together, our work reveals a diversity of mechanisms whereby host genotype modulates the cost of an invading TE, and points to genetic variants that were likely beneficial during the P-element invasion.

Regulation and coordination of the different DNA damage responses in Drosophila

Cells have evolved mechanisms that allow them to respond to DNA damage to preserve genomic integrity and maintain tissue homeostasis. These responses include the activation of the cell cycle checkpoints and the repair mechanisms or the induction of apoptosis that eventually will eliminate damaged cells. These “life” vs. “death” decisions differ depending on the cell type, stages of development, and the proliferation status of the cell. The apoptotic response after DNA damage is of special interest as defects in its induction could contribute to tumorigenesis or the resistance of cancer cells to therapeutic agents such as radiotherapy. Multiples studies have elucidated the molecular mechanisms that mediate the activation of the DNA damage response pathway (DDR) and specifically the role of p53. However, much less is known about how the different cellular responses such as cell proliferation control and apoptosis are coordinated to maintain tissue homeostasis. Another interesting question is how the differential apoptotic response to DNA damage is regulated in distinct cell types. The use of Drosophila melanogaster as a model organism has been fundamental to understand the molecular and cellular mechanisms triggered by genotoxic stress. Here, we review the current knowledge regarding the cellular responses to ionizing radiation as the cause of DNA damage with special attention to apoptosis in Drosophila: how these responses are regulated and coordinated in different cellular contexts and in different tissues. The existence of intrinsic mechanisms that might attenuate the apoptotic pathway in response to this sort of DNA damage may well be informative for the differences in the clinical responsiveness of tumor cells after radiation therapy.

Enhanced germline stem cell longevity in Drosophila diapause

In many species including humans, aging reduces female fertility. Intriguingly, some animals preserve fertility longer under specific environmental conditions. For example, at low temperature and short day-length, Drosophila melanogaster enters a state called adult reproductive diapause. As in other stressful conditions, ovarian development arrests at the yolk uptake checkpoint; however, mechanisms underlying fertility preservation and post-diapause recovery are largely unknown. Here, we report that diapause causes more complete arrest than other stresses yet preserves greater recovery potential. During dormancy, germline stem cells (GSCs) incur DNA damage, activate p53 and Chk2, and divide less. Despite reduced niche signaling, germline precursor cells do not differentiate. GSCs adopt an atypical, suspended state connected to their daughters. Post-diapause recovery of niche signaling and resumption of division contribute to restoring GSCs. Mimicking one feature of quiescence, reduced juvenile hormone production, enhanced GSC longevity in non-diapausing flies. Thus, diapause mechanisms provide approaches to GSC longevity enhancement.

Drosophila enter adult reproductive diapause in low temperatures and short day, halting ovarian development yet preserving fertility. Here the authors show that ovarian arrest in diapause is distinct from other stress responses and that despite DNA damage and decreased division, germline stem cells recover.

dRTEL1 is essential for the maintenance of Drosophila male germline stem cells

Stem cells have the potential to maintain undifferentiated state and differentiate into specialized cell types. Despite numerous progress has been achieved in understanding stem cell self-renewal and differentiation, many fundamental questions remain unanswered. In this study, we identify dRTEL1, the Drosophila homolog of Regulator of Telomere Elongation Helicase 1, as a novel regulator of male germline stem cells (GSCs). Our genome-wide transcriptome analysis and ChIP-Seq results suggest that dRTEL1 affects a set of candidate genes required for GSC maintenance, likely independent of its role in DNA repair. Furthermore, dRTEL1 prevents DNA damage-induced checkpoint activation in GSCs. Finally, dRTEL1 functions to sustain Stat92E protein levels, the key player in GSC maintenance. Together, our findings reveal an intrinsic role of the DNA helicase dRTEL1 in maintaining male GSC and provide insight into the function of dRTEL1.

Differential activation of eMI by distinct forms of cellular stress

As one of the major, highly conserved catabolic pathways, autophagy delivers cytosolic components to lysosomes for degradation. It is essential for development, cellular homeostasis, and coping with stress. Reduced autophagy increases susceptibility to protein aggregation diseases and leads to phenotypes associated with aging. Of the three major forms of autophagy, macroautophagy (MA) can degrade organelles or aggregated proteins, and chaperone-mediated autophagy is specific for soluble proteins containing KFERQ-related targeting motifs. During endosomal microautophagy (eMI), cytoplasmic proteins are engulfed into late endosomes in an ESCRT machinery-dependent manner. eMI can be KFERQ-specific or occur in bulk and be induced by prolonged starvation. Its physiological regulation and function, however, are not understood. Here, we show that eMI in the Drosophila fat body, akin to the mammalian liver, is induced upon oxidative or genotoxic stress in an ESCRT and partially Hsc70-4-dependent manner. Interestingly, eMI activation is selective, as ER stress fails to elicit a response. Intriguingly, we find that reducing MA leads to a compensatory enhancement of eMI, suggesting a tight interplay between these degradative processes. Furthermore, we show that mutations in DNA damage response genes are sufficient to trigger eMI and that the response to oxidative stress is under the control of MAPK/JNK signaling. Our data suggest that, controlled by various signaling pathways, eMI allows an organ to react and adapt to specific types of stress and is thus likely critical to prevent disease.Abbreviations:Atg: autophagy-related; CMA: chaperone-mediated autophagy; DDR: DNA damage repair; Df: deficiency (deletion); (E)GFP: (enhanced) green fluorescent protein; eMI: endosomal microautophagy; ER: endoplasmatic reticulum; ESCRT: endosomal sorting complexes required for transport; Eto: etoposide; FLP: flipase; Hsc: heat shock cognate protein; LAMP2A: lysosomal-associated membrane protein 2A; LE: late endosome; MA: macroautophagy; MI: microautophagy; MVB: multivesicular body; PA: photoactivatable; Para: paraquat; ROS: reactive oxygen species; SEM: standard error of means; Tor: target of rapamycin [serine/threonine kinase]; UPR: unfolded protein response; Vps: vacuolar protein sorting.

Special vulnerability of somatic niche cells to transposable element activation in Drosophila larval ovaries

In the Drosophila ovary, somatic escort cells (ECs) form a niche that promotes differentiation of germline stem cell (GSC) progeny. The piRNA (Piwi-interacting RNA) pathway, which represses transposable elements (TEs), is required in ECs to prevent the accumulation of undifferentiated germ cells (germline tumor phenotype). The soma-specific piRNA cluster flamenco (flam) produces a substantial part of somatic piRNAs. Here, we characterized the biological effects of somatic TE activation on germ cell differentiation in flam mutants. We revealed that the choice between normal and tumorous phenotypes of flam mutant ovaries depends on the number of persisting ECs, which is determined at the larval stage. Accordingly, we found much more frequent DNA breaks in somatic cells of flam larval ovaries than in adult ECs. The absence of Chk2 or ATM checkpoint kinases dramatically enhanced oogenesis defects of flam mutants, in contrast to the germline TE-induced defects that are known to be mostly suppressed by сhk2 mutation. These results demonstrate a crucial role of checkpoint kinases in protecting niche cells against deleterious TE activation and suggest substantial differences between DNA damage responses in ovarian somatic and germ cells.

Role of p53 isoforms in the DNA damage response during Drosophila oogenesis

The tumor suppressor p53 is involved in the DNA damage response and induces cell cycle arrest or apoptosis upon DNA damage. Drosophila p53 encodes two isoforms, p53A and p53B, that induce apoptosis in somatic cells. To investigate the roles of Drosophila p53 isoforms in female germline cells, the DNA damage response was analyzed in the adult ovary. Early oogenesis was sensitive to irradiation and lok-, p53-, and hid-dependent cell death occurred rapidly after both low- and high-dose irradiation. Both p53 isoforms were responsible for this cell death. On the other hand, delayed cell death in mid-oogenesis was induced at a low level only after high-dose irradiation in a p53-independent manner. The daily egg production, which did not change after low-dose irradiation, was severely reduced after high-dose irradiation in p53 mutant females due to the loss of germline stem cells. When the p53A or p53B isoform was expressed in the germline cells in the p53 mutant females at levels that do not affect normal oogenesis, p53A, but not p53B, restored the fertility of the irradiated female. In summary, moderate expression of p53A is critical to maintain the function of germline stem cells during normal oogenesis as well as after high-dose irradiation.

Overexpression of the Drosophila ATR homologous checkpoint kinase Mei-41 induces a G2/M checkpoint in Drosophila imaginal tissue

Background

DNA damage generally results in the activation of ATM/ATR kinases and the downstream checkpoint kinases Chk1/Chk2. In Drosophila melanogaster, the ATR homologue meiotic 41 (mei-41) is pivotal to DNA damage repair and cell cycle checkpoint signalling. Although various mei-41 mutant alleles have been analyzed in the past, no gain-of-function allele is yet available. To fill this gap, we have generated transgenic flies allowing temporal and tissue-specific induction of mei-41.

Results

Overexpression of mei-41 in wing and eye anlagen affects proliferation and a G2/M checkpoint even in the absence of genomic stress. Similar consequences were observed following the overexpression of the downstream kinase Grapes (Grp) but not of Loki (Lok), encoding the respective Drosophila Chk1 and Chk2 homologues, in agreement with their previously reported activities. Moreover, we show that irradiation induced cell cycle arrest was prolonged in the presence of ectopic mei-41 expression. Similar to irradiation stress, mei-41 triggered the occurrence of a slower migrating form of Grp, implying specific phosphorylation of Grp in response to either signal. Using a p53R-GFP biosensor, we further show that overexpression of mei-41 was sufficient to elicit a robust p53 activation in vivo.

Conclusion

We conclude that overexpression of the Drosophila ATR homologue mei-41 elicits an effectual DNA damage response irrespective of irradiation.

Electronic supplementary material

The online version of this article (10.1186/s41065-018-0066-4) contains supplementary material, which is available to authorized users.

tRNA processing defects induce replication stress and Chk2-dependent disruption of piRNA transcription

RNase P is a conserved endonuclease that processes the 5' trailer of tRNA precursors. We have isolated mutations in Rpp30, a subunit of RNase P, and find that these induce complete sterility in Drosophila females. Here, we show that sterility is not due to a shortage of mature tRNAs, but that atrophied ovaries result from the activation of several DNA damage checkpoint proteins, including p53, Claspin, and Chk2. Indeed, we find that tRNA processing defects lead to increased replication stress and de-repression of transposable elements in mutant ovaries. We also report that transcription of major piRNA sources collapse in mutant germ cells and that this correlates with a decrease in heterochromatic H3K9me3 marks on the corresponding piRNA-producing loci. Our data thus link tRNA processing, DNA replication, and genome defense by small RNAs. This unexpected connection reveals constraints that could shape genome organization during evolution.

The FHA domain determines Drosophila Chk2/Mnk localization to key mitotic structures and is essential for early embryonic DNA damage responses

DNA damage responses, including mitotic centrosome inactivation, cell-cycle delay in mitosis, and nuclear dropping from embryo cortex, maintain genome integrity in syncytial Drosophila embryos. A conserved signaling kinase, Chk2, known as Mnk/Loki, is essential for the responses. Here we demonstrate that functional EGFP-Mnk expressed from a transgene localizes to the nucleus, centrosomes, interkinetochore/centromere region, midbody, and pseudocleavage furrows without DNA damage and in addition forms numerous foci/aggregates on mitotic chromosomes upon DNA damage. We expressed EGFP-tagged Mnk deletion or point mutation variants and investigated domain functions of Mnk in vivo. A triple mutation in the phosphopeptide-binding site of the forkhead-associated (FHA) domain disrupted normal Mnk localization except to the nucleus. The mutation also disrupted Mnk foci formation on chromosomes upon DNA damage. FHA mutations and deletion of the SQ/TQ-cluster domain (SCD) abolished Mnk transphosphorylations and autophosphorylations, indicative of kinase activation after DNA damage. A potent NLS was found at the C-terminus, which is required for normal Mnk function. We propose that the FHA domain in Mnk plays essential dual functions in mediating embryonic DNA damage responses by means of its phosphopeptide-binding ability: activating Mnk in the nucleus upon DNA damage and recruiting Mnk to multiple subcellular structures independently of DNA damage.

Low levels of p53 protein and chromatin silencing of p53 target genes repress apoptosis in Drosophila endocycling cells

Apoptotic cell death is an important response to genotoxic stress that prevents oncogenesis. It is known that tissues can differ in their apoptotic response, but molecular mechanisms are little understood. Here, we show that Drosophila polyploid endocycling cells (G/S cycle) repress the apoptotic response to DNA damage through at least two mechanisms. First, the expression of all the Drosophila p53 protein isoforms is strongly repressed at a post-transcriptional step. Second, p53-regulated pro-apoptotic genes are epigenetically silenced in endocycling cells, preventing activation of a paused RNA Pol II by p53-dependent or p53-independent pathways. Over-expression of the p53A isoform did not activate this paused RNA Pol II complex in endocycling cells, but over-expression of the p53B isoform with a longer transactivation domain did, suggesting that dampened p53B protein levels are crucial for apoptotic repression. We also find that the p53A protein isoform is ubiquitinated and degraded by the proteasome in endocycling cells. In mitotic cycling cells, p53A was the only isoform expressed to detectable levels, and its mRNA and protein levels increased after irradiation, but there was no evidence for an increase in protein stability. However, our data suggest that p53A protein stability is regulated in unirradiated cells, which likely ensures that apoptosis does not occur in the absence of stress. Without irradiation, both p53A protein and a paused RNA pol II were pre-bound to the promoters of pro-apoptotic genes, preparing mitotic cycling cells for a rapid apoptotic response to genotoxic stress. Together, our results define molecular mechanisms by which different cells in development modulate their apoptotic response, with broader significance for the survival of normal and cancer polyploid cells in mammals.

In order to maintain genome integrity, eukaryotic cells have evolved multiple ways to respond to DNA damage stress. One of the major cellular responses is apoptosis, during which the cell undergoes programmed cell death in order to prevent the propagation of the damaged genome to daughter cells. Although clinical observations and other studies have shown that tissues can differ in their apoptotic response, the molecular mechanisms underlying these differences are largely unknown. We have shown in our model system, Drosophila, that endocycling cells do not initiate cell death in response to DNA damage. The endocycle is a cell cycle variation that is widely found in nature and conserved from plant to animals. During the endocycle, cells duplicate their genomic DNA but do not enter mitosis to segregate chromosomes, resulting in a polyploid genome content. In this study, we investigate how the apoptotic response to DNA damage is repressed in endocycling cells. We find that the Drosophila ortholog of the human p53 tumor suppressor protein is expressed at very low levels in endocycling cells. Moreover, the downstream pro-apoptotic genes that are regulated by p53 are epigenetically silenced in endocycling cells. Our results provide important insights into tissue-specific apoptotic responses in development, with possible broader impact on understanding radiation therapy response and cancer of different tissues.

Histone supply regulates S phase timing and cell cycle progression

Eukaryotes package DNA into nucleosomes that contain a core of histone proteins. During DNA replication, nucleosomes are disrupted and re-assembled with newly synthesized histones and DNA. Despite much progress, it is still unclear why higher eukaryotes contain multiple core histone genes, how chromatin assembly is controlled, and how these processes are coordinated with cell cycle progression. We used a histone null mutation of Drosophila melanogaster to show that histone supply levels, provided by a defined number of transgenic histone genes, regulate the length of S phase during the cell cycle. Lack of de novo histone supply not only extends S phase, but also causes a cell cycle arrest during G2 phase, and thus prevents cells from entering mitosis. Our results suggest a novel cell cycle surveillance mechanism that monitors nucleosome assembly without involving the DNA repair pathways and exerts its effect via suppression of CDC25 phosphatase String expression.

DOI:http://dx.doi.org/10.7554/eLife.02443.001

As a cell prepares to divide, it goes through four distinct stages. First, it grows in size (G1 phase); next it copies its entire DNA content (S phase); then it grows some more (G2 phase); and, last, it splits into two new cells (M phase).

During S phase, groups of histone proteins that normally stick together to tightly package the DNA are pulled apart in order to make the DNA accessible for copying. After the DNA has been duplicated, both copies of the DNA strand need to be repackaged. Therefore, after copying the DNA the cell rapidly reassembles the DNA–histone complexes (called nucleosomes), using a combination of old and newly synthesized histones to do so. A cell can adjust how quickly it copies DNA according to the availability of these histone proteins, which is important because copying DNA without the resources to package it could expose the DNA to damage.

Here, Günesdogan et al. investigate how a cell controls these processes using a mutant of the fruit fly Drosophila melanogaster that completely lacks the genes required to make histones. Cells that lack histones copy their DNA very slowly but adding copies of histone genes back into these flies speeds up the rate at which DNA is copied.

Günesdogan et al. ask whether the slower speed of DNA replication in cells without new histones is connected to preventing DNA damage. However, these cells can still copy all their DNA, despite being unable to package it, so the higher risk of making mistakes is not enough to stop S phase. In fact, indications suggest that DNA damage detection methods continue to work as normal in cells without histones: these cells can get all the way to the end of G2 phase without any problems.

To go one step further and start splitting in two, a cell needs to switch on another gene, called string in the fruit fly and CDC25 in vertebrates, which makes an enzyme required for the cell division process. Normal cells switch on string during G2 phase, but cells that lack histones do not—and therefore do not enter M phase. Günesdogan et al. show that turning on string by a genetic trick is sufficient to overcome this cell cycle arrest and drive the cells into M phase. String could therefore form part of a surveillance mechanism that blocks cell division if DNA–histone complexes are not assembled correctly.

DOI:http://dx.doi.org/10.7554/eLife.02443.002

Biological Effects of Space Radiation and Development of Effective Countermeasures

As part of a program to assess the adverse biological effects expected from astronaut exposure to space radiation, numerous different biological effects relating to astronaut health have been evaluated. There has been major focus recently on the assessment of risks related to exposure to solar particle event (SPE) radiation. The effects related to various types of space radiation exposure that have been evaluated are: gene expression changes (primarily associated with programmed cell death and extracellular matrix (ECM) remodeling), oxidative stress, gastrointestinal tract bacterial translocation and immune system activation, peripheral hematopoietic cell counts, emesis, blood coagulation, skin, behavior/fatigue (including social exploration, submaximal exercise treadmill and spontaneous locomotor activity), heart functions, alterations in biological endpoints related to astronaut vision problems (lumbar puncture/intracranial pressure, ocular ultrasound and histopathology studies), and survival, as well as long-term effects such as cancer and cataract development. A number of different countermeasures have been identified that can potentially mitigate or prevent the adverse biological effects resulting from exposure to space radiation.

Chk2 and p53 regulate the transmission of healed chromosomes in the Drosophila male germline

When a dicentric chromosome breaks in mitosis, the broken ends cannot be repaired by normal mechanisms that join two broken ends since each end is in a separate daughter cell. However, in the male germline of Drosophila melanogaster, a broken end may be healed by de novo telomere addition. We find that Chk2 (encoded by lok) and P53, major mediators of the DNA damage response, have strong and opposite influences on the transmission of broken-and-healed chromosomes: lok mutants exhibit a large increase in the recovery of healed chromosomes relative to wildtype control males, but p53 mutants show a strong reduction. This contrasts with the soma, where mutations in lok and p53 have the nearly identical effect of allowing survival and proliferation of cells with irreparable DNA damage. Examination of testes revealed a transient depletion of germline cells after dicentric chromosome induction in the wildtype controls, and further showed that P53 is required for the germline to recover. Although lok mutant males transmit healed chromosomes at a high rate, broken chromosome ends can also persist through spermatogonial divisions without healing in lok mutants, giving rise to frequent dicentric bridges in Meiosis II. Cytological and genetic analyses show that spermatid nuclei derived from such meiotic divisions are eliminated during spermiogenesis, resulting in strong meiotic drive. We conclude that the primary responsibility for maintaining genome integrity in the male germline lies with Chk2, and that P53 is required to reconstitute the germline when cells are eliminated owing to unrepaired DNA damage.

TRIP/NOPO E3 ubiquitin ligase promotes ubiquitylation of DNA polymerase η

We previously identified a Drosophila maternal effect-lethal mutant named 'no poles' (nopo). Embryos from nopo females undergo mitotic arrest with barrel-shaped, acentrosomal spindles during the rapid cycles of syncytial embryogenesis because of activation of a Chk2-mediated DNA checkpoint. NOPO is the Drosophila homolog of human TNF receptor associated factor (TRAF)-interacting protein (TRIP), which has been implicated in TNF signaling. NOPO and TRIP contain RING domains closely resembling those of known E3 ubiquitin ligases. We herein sought to elucidate the mechanism by which TRIP/NOPO promotes genomic stability by performing a yeast two-hybrid screen to identify potential substrates/interactors. We identified members of the Y-family of DNA polymerases that facilitate replicative bypass of damaged DNA (translesion synthesis) as TRIP interactors. We show that TRIP and NOPO co-immunoprecipitate with human and Drosophila Polη, respectively, from cultured cells. We generated a null mutation in Drosophila Polη (dPolη) and found that dPolη-derived embryos have increased sensitivity to ultraviolet irradiation and exhibit nopo-like mitotic spindle defects. dPolη and nopo interact genetically in that overexpression of dPolη in hypomorphic nopo-derived embryos suppresses nopo phenotypes. We observed enhanced ubiquitylation of Polη by TRIP and NOPO E3 ligases in human cells and Drosophila embryos, respectively, and show that TRIP promotes hPolη localization to nuclear foci in human cells. We present a model in which TRIP/NOPO ubiquitylates Polη to positively regulate its activity in translesion synthesis.

Drosophila MOF controls Checkpoint protein2 and regulates genomic stability during early embryogenesis

Background

In Drosophila embryos, checkpoints maintain genome stability by delaying cell cycle progression that allows time for damage repair or to complete DNA synthesis. Drosophila MOF, a member of MYST histone acetyl transferase is an essential component of male X hyperactivation process. Until recently its involvement in G2/M cell cycle arrest and defects in ionizing radiation induced DNA damage pathways was not well established.

Results

Drosophila MOF is highly expressed during early embryogenesis. In the present study we show that haplo-insufficiency of maternal MOF leads to spontaneous mitotic defects like mitotic asynchrony, mitotic catastrophe and chromatid bridges in the syncytial embryos. Such abnormal nuclei are eliminated and digested in the yolk tissues by nuclear fall out mechanism. MOF negatively regulates Drosophila checkpoint kinase 2 tumor suppressor homologue. In response to DNA damage the checkpoint gene Chk2 (Drosophila mnk) is activated in the mof mutants, there by causing centrosomal inactivation suggesting its role in response to genotoxic stress. A drastic decrease in the fall out nuclei in the syncytial embryos derived from mof¹/+; mnk^p6/+ females further confirms the role of DNA damage response gene Chk2 to ensure the removal of abnormal nuclei from the embryonic precursor pool and maintain genome stability. The fact that mof mutants undergo DNA damage has been further elucidated by the increased number of single and double stranded DNA breaks.

Conclusion

mof mutants exhibited genomic instability as evidenced by the occurance of frequent mitotic bridges in anaphase, asynchronous nuclear divisions, disruption of cytoskeleton, inactivation of centrosomes finally leading to DNA damage. Our findings are consistent to what has been reported earlier in mammals that; reduced levels of MOF resulted in increased genomic instability while total loss resulted in lethality. The study can be further extended using Drosophila as model system and carry out the interaction of MOF with the known components of the DNA damage pathway.

Chk2 and p53 are haploinsufficient with dependent and independent functions to eliminate cells after telomere loss

The mechanisms that cells use to monitor telomere integrity, and the array of responses that may be induced, are not fully defined. To date there have been no studies in animals describing the ability of cells to survive and contribute to adult organs following telomere loss. We developed assays to monitor the ability of somatic cells to proliferate and differentiate after telomere loss. Here we show that p53 and Chk2 limit the growth and differentiation of cells that lose a telomere. Furthermore, our results show that two copies of the genes encoding p53 and Chk2 are required for the cell to mount a rapid wildtype response to a missing telomere. Finally, our results show that, while Chk2 functions by activating the p53-dependent apoptotic cascade, Chk2 also functions independently of p53 to limit survival. In spite of these mechanisms to eliminate cells that have lost a telomere, we find that such cells can make a substantial contribution to differentiated adult tissues.

Identification of dAven, a Drosophila melanogaster ortholog of the cell cycle regulator Aven

Aven is a regulator of the DNA-damage response and G2/M cell cycle progression. Overexpression of Aven is associated with poor prognosis in patients with childhood acute lymphoblastic leukemia and acute myeloid leukemia, and altered intracellular Aven distribution is associated with infiltrating ductal carcinoma and papillary carcinoma breast cancer subtypes. Although Aven orthologs have been identified in most vertebrate species, no Aven gene has been reported in invertebrates. Here, we describe a Drosophila melanogaster open reading frame (ORF) that shares sequence and functional similarities with vertebrate Aven genes. The protein encoded by this ORF, which we named dAven, contains several domains that are highly conserved among Aven proteins of fish, amphibian, bird and mammalian origins. In flies, knockdown of dAven by RNA interference (RNAi) resulted in lethality when its expression was reduced either ubiquitously or in fat cells using Gal4 drivers. Animals undergoing moderate dAven knockdown in the fat body had smaller fat cells displaying condensed chromosomes and increased levels of the mitotic marker phosphorylated histone H3 (PHH3), suggesting that dAven was required for normal cell cycle progression in this tissue. Remarkably, expression of dAven in Xenopus egg extracts resulted in G2/M arrest that was comparable to that caused by human Aven. Taken together, these results suggest that, like its vertebrate counterparts, dAven plays a role in cell cycle regulation. Drosophila could be an excellent model for studying the function of Aven and identifying cellular factors that influence its activity, revealing information that may be relevant to human disease.

Drosophila Chk2 and p53 proteins induce stage-specific cell death independently during oogenesis

In Drosophila, the checkpoint protein-2 kinase (DmChk2) and its downstream effector protein, Dmp53, are required for DNA damage-mediated cell cycle arrest, DNA repair and apoptosis. In this study we focus on understanding the function of these two apoptosis inducing factors during ovarian development. We found that expression of Dmp53, but not DmChk2, led to loss of ovarian stem cells. We demonstrate that expression of DmChk2, but not Dmp53, induced mid-oogenesis cell death. DmChk2 induced cell death was not suppressed by Dmp53 mutant, revealing for the first time that in Drosophila, over-expression of DmChk2 can induce cell death which is independent of Dmp53. We found that over-expression of caspase inhibitors such as DIAP1, p35 and p49 did not suppress DmChk2- and Dmp53-induced cell death. Thus, our study reveals stage-specific effects of Dmp53 and DmChk2 in oogenesis. Moreover, our results demonstrate that although DmChk2 and Dmp53 affect different stages of ovarian development, loss of ovarian stem cells by p53 expression and mid-oogenesis cell death induced by DmChk2 do not require caspase activity.

Genetic architecture of ovary size and asymmetry in European honeybee workers

The molecular basis of complex traits is increasingly understood but a remaining challenge is to identify their co-regulation and inter-dependence. Pollen hoarding (pln) in honeybees is a complex trait associated with a well-characterized suite of linked behavioral and physiological traits. In European honeybee stocks bidirectionally selected for pln, worker (sterile helper) ovary size is pleiotropically affected by quantitative trait loci that were initially identified for their effect on foraging behavior. To gain a better understanding of the genetic architecture of worker ovary size in this model system, we analyzed a series of crosses between the selected strains. The crossing results were heterogeneous and suggested non-additive effects. Three significant and three suggestive quantitative trait loci of relatively large effect sizes were found in two reciprocal backcrosses. These loci are not located in genome regions of known effects on foraging behavior but contain several interesting candidate genes that may specifically affect worker-ovary size. Thus, the genetic architecture of this life history syndrome may be comprised of pleiotropic, central regulators that influence several linked traits and other genetic factors that may be downstream and trait specific.

Tau Ser262 phosphorylation is critical for Abeta42-induced tau toxicity in a transgenic Drosophila model of Alzheimer's disease

The amyloid-beta 42 (Abeta42) peptide has been suggested to promote tau phosphorylation and toxicity in Alzheimer's disease (AD) pathogenesis; however, the underlying mechanisms are not fully understood. Using transgenic Drosophila expressing both human Abeta42 and tau, we show here that tau phosphorylation at Ser262 plays a critical role in Abeta42-induced tau toxicity. Co-expression of Abeta42 increased tau phosphorylation at AD-related sites including Ser262, and enhanced tau-induced neurodegeneration. In contrast, formation of either sarkosyl-insoluble tau or paired helical filaments was not induced by Abeta42. Co-expression of Abeta42 and tau carrying the non-phosphorylatable Ser262Ala mutation did not cause neurodegeneration, suggesting that the Ser262 phosphorylation site is required for the pathogenic interaction between Abeta42 and tau. We have recently reported that the DNA damage-activated Checkpoint kinase 2 (Chk2) phosphorylates tau at Ser262 and enhances tau toxicity in a transgenic Drosophila model. We detected that expression of Chk2, as well as a number of genes involved in DNA repair pathways, was increased in the Abeta42 fly brains. The induction of a DNA repair response is protective against Abeta42 toxicity, since blocking the function of the tumor suppressor p53, a key transcription factor for the induction of DNA repair genes, in neurons exacerbated Abeta42-induced neuronal dysfunction. Our results demonstrate that tau phosphorylation at Ser262 is crucial for Abeta42-induced tau toxicity in vivo, and suggest a new model of AD progression in which activation of DNA repair pathways is protective against Abeta42 toxicity but may trigger tau phosphorylation and toxicity in AD pathogenesis.

A DNA damage-activated checkpoint kinase phosphorylates tau and enhances tau-induced neurodegeneration

Hyperphosphorylation of the microtubule associated protein tau is detected in the brains of individuals with a range of neurodegenerative diseases including Alzheimer's disease (AD). An imbalance in phosphorylation and/or dephosphorylation of tau at disease-related sites has been suggested to initiate the abnormal metabolism and toxicity of tau in disease pathogenesis. However, the mechanisms underlying abnormal phosphorylation of tau in AD are not fully understood. Here, we show that the DNA damage-activated Checkpoint kinase 2 (Chk2) is a novel tau kinase and enhances tau toxicity in a transgenic Drosophila model. Overexpression of Drosophila Chk2 increases tau phosphorylation at Ser262 and enhances tau-induced neurodegeneration in transgenic flies expressing human tau. The non-phosphorylatable Ser262Ala mutation abolishes Chk2-induced enhancement of tau toxicity, suggesting that the Ser262 phosphorylation site is involved in the enhancement of tau toxicity by Chk2. In vitro kinase assays revealed that human Chk2 and a closely related checkpoint kinase 1 (Chk1) directly phosphorylate human tau at Ser262. We also demonstrate that Drosophila Chk2 does not modulate the activity of the fly homolog of microtubule affinity regulating kinase, which has been shown to be a physiological tau Ser262 kinase. Since accumulation of DNA damage has been detected in the brains of AD patients, our results suggest that the DNA damage-activated kinases Chk1 and Chk2 may be involved in tau phosphorylation and toxicity in the pathogenesis of AD.

Expression profiling of prospero in the Drosophila larval chemosensory organ: Between growth and outgrowth

Background

The antenno-maxilary complex (AMC) forms the chemosensory system of the Drosophila larva and is involved in gustatory and olfactory perception. We have previously shown that a mutant allele of the homeodomain transcription factor Prospero (prosVoila1, V1), presents several developmental defects including abnormal growth and altered taste responses. In addition, many neural tracts connecting the AMC to the central nervous system (CNS) were affected. Our earlier reports on larval AMC did not argue in favour of a role of pros in cell fate decision, but strongly suggested that pros could be involved in the control of other aspect of neuronal development. In order to identify these functions, we used microarray analysis of larval AMC and CNS tissue isolated from the wild type, and three other previously characterised prospero alleles, including the V1 mutant, considered as a null allele for the AMC.

Results

A total of 17 samples were first analysed with hierarchical clustering. To determine those genes affected by loss of pros function, we calculated a discriminating score reflecting the differential expression between V1 mutant and other pros alleles. We identified a total of 64 genes in the AMC. Additional manual annotation using all the computed information on the attributed role of these genes in the Drosophila larvae nervous system, enabled us to identify one functional category of potential Prospero target genes known to be involved in neurite outgrowth, synaptic transmission and more specifically in neuronal connectivity remodelling. The second category of genes found to be differentially expressed between the null mutant AMC and the other alleles concerned the development of the sensory organs and more particularly the larval olfactory system. Surprisingly, a third category emerged from our analyses and suggests an association of pros with the genes that regulate autophagy, growth and insulin pathways. Interestingly, EGFR and Notch pathways were represented in all of these three functional categories. We now propose that Pros could perform all of these different functions through the modulation of these two antagonistic and synergic pathways.

Conclusions

The current data contribute to the clarification of the prospero function in the larval AMC and show that pros regulates different function in larvae as compared to those controlled by this gene in embryos. In the future, the possible mechanism by which Pros could achieve its function in the AMC will be explored in detail.

The role of p53 gene family in reproduction

The p53 family of genes (p53, p63, and p73) is conserved over evolutionary time scales. Although the functions of p53 gene and its protein as a tumor suppressor have been firmly established, the earliest functions for the p53 ancestral genes in worms and flies are to ensure germ-line genomic integrity and the fidelity of the developmental process. In vertebrates, the p53 family of genes retains those functions in germ-line genomic integrity but have added important functions in regulation of reproduction. Loss of the p53, p63, or p73 genes in female mice leads to a significant decrease of fertility. The p53 gene product regulates maternal reproduction at the implantation stage of the embryo. p63 and p73 play important roles in monitoring the genomic quality of oocytes. The p53 pathway appears to play a similar role in human fertility. In humans, certain alleles containing a functional single-nucleotide polymorphism (SNP) in the p53 pathway are under positive evolutionary selection. Selected alleles of these SNPs in the p53 pathway are associated with decreased fertility. This important function of the p53 pathway in reproduction provides a plausible explanation for the evolution of p53 as a tumor suppressor gene and the positive selection of some alleles in the p53 gene and its pathway. These observations provide a good possible example of antagonistic pleiotrophy for fertility, tumor suppression, and longevity.

no poles encodes a predicted E3 ubiquitin ligase required for early embryonic development of Drosophila

In a screen for cell-cycle regulators, we identified a Drosophila maternal effect-lethal mutant that we named ;no poles' (nopo). Embryos from nopo females undergo mitotic arrest with barrel-shaped, acentrosomal spindles during the rapid S-M cycles of syncytial embryogenesis. We identified CG5140, which encodes a candidate RING domain-containing E3 ubiquitin ligase, as the nopo gene. A conserved residue in the RING domain is altered in our EMS-mutagenized allele of nopo, suggesting that E3 ligase activity is crucial for NOPO function. We show that mutation of a DNA checkpoint kinase, CHK2, suppresses the spindle and developmental defects of nopo-derived embryos, revealing that activation of a DNA checkpoint operational in early embryos contributes significantly to the nopo phenotype. CHK2-mediated mitotic arrest has been previously shown to occur in response to mitotic entry with DNA damage or incompletely replicated DNA. Syncytial embryos lacking NOPO exhibit a shorter interphase during cycle 11, suggesting that they may enter mitosis prior to the completion of DNA replication. We show that Bendless (BEN), an E2 ubiquitin-conjugating enzyme, interacts with NOPO in a yeast two-hybrid assay; furthermore, ben-derived embryos arrest with a nopo-like phenotype during syncytial divisions. These data support our model that an E2-E3 ubiquitination complex consisting of BEN-UEV1A (E2 heterodimer) and NOPO (E3 ligase) is required for the preservation of genomic integrity during early embryogenesis.

p53-independent apoptosis limits DNA damage-induced aneuploidy

DNA damage or unprotected telomeres can trigger apoptosis via signaling pathways that directly sense abnormal DNA structures and activate the p53 transcription factor. We describe a p53-independent mechanism that acts in parallel to the canonical DNA damage response pathway in Drosophila to induce apoptosis after exposure to ionizing radiation. Following recovery from damage-induced cell cycle arrest, p53 mutant cells activate the JNK pathway and expression of the pro-apoptotic gene hid. Mutations in grp, a cell cycle checkpoint gene, and puc, a negative regulator of the JNK pathway, sensitize p53 mutant cells to ionizing radiation (IR)-induced apoptosis. Induction of chromosome aberrations by DNA damage generates cells with segmental aneuploidy and heterozygous for mutations in ribosomal protein genes. p53-independent apoptosis limits the formation of these aneuploid cells following DNA damage. We propose that reduced copy number of haploinsufficient genes following chromosome damage activates apoptosis and helps maintain genomic integrity.

Heterochromatic genome stability requires regulators of histone H3 K9 methylation

Heterochromatin contains many repetitive DNA elements and few protein-encoding genes, yet it is essential for chromosome organization and inheritance. Here, we show that Drosophila that lack the Su(var)3-9 H3K9 methyltransferase display significantly elevated frequencies of spontaneous DNA damage in heterochromatin, in both somatic and germ-line cells. Accumulated DNA damage in these mutants correlates with chromosomal defects, such as translocations and loss of heterozygosity. DNA repair and mitotic checkpoints are also activated in mutant animals and are required for their viability. Similar effects of lower magnitude were observed in animals that lack the RNA interference pathway component Dcr2. These results suggest that the H3K9 methylation and RNAi pathways ensure heterochromatin stability.

HorkaD, a chromosome instability-causing mutation in Drosophila, is a dominant-negative allele of Lodestar

Correct segregation of chromosomes is particularly challenging during the rapid nuclear divisions of early embryogenesis. This process is disrupted by Horka(D), a dominant-negative mutation in Drosophila melanogaster that causes female sterility due to chromosome tangling and nondisjunction during oogenesis and early embryogenesis. Horka(D) also renders chromosomes unstable during spermatogenesis, which leads to the formation of diplo//haplo mosaics, including the gynandromorphs. Complete loss of gene function brings about maternal-effect lethality: embryos of the females without the Horka(D)-identified gene perish due to disrupted centrosome function, defective spindle assembly, formation of chromatin bridges, and abnormal chromosome segregation during the cleavage divisions. These defects are indicators of mitotic catastrophe and suggest that the gene product acts during the meiotic and the cleavage divisions, an idea that is supported by the observation that germ-line chimeras exhibit excessive germ-line and cleavage function. The gene affected by the Horka(D) mutation is lodestar, a member of the helicase-related genes. The Horka(D) mutation results in replacement of Ala777 with Thr, which we suggest causes chromosome instability by increasing the affinity of Lodestar for chromatin.

Telomere loss provokes multiple pathways to apoptosis and produces genomic instability in Drosophila melanogaster

Telomere loss was produced during development of Drosophila melanogaster by breakage of an induced dicentric chromosome. The most prominent outcome of this event is cell death through Chk2 and Chk1 controlled p53-dependent apoptotic pathways. A third p53-independent apoptotic pathway is additionally utilized when telomere loss is accompanied by the generation of significant aneuploidy. In spite of these three lines of defense against the proliferation of cells with damaged genomes a small fraction of cells that have lost a telomere escape apoptosis and divide repeatedly. Evasion of apoptosis is accompanied by the accumulation of karyotypic abnormalites that often typify cancer cells, including end-to-end chromosome fusions, anaphase bridges, aneuploidy, and polyploidy. There was clear evidence of bridge-breakage-fusion cycles, and surprisingly, chromosome segments without centromeres could persist and accumulate to high-copy number. Cells manifesting these signs of genomic instability were much more frequent when the apoptotic mechanisms were crippled. We conclude that loss of a single telomere is sufficient to generate at least two phenotypes of early cancer cells: genomic instability that involves multiple chromosomes and aneuploidy. This aneuploidy may facilitate the continued escape of such cells from the normal checkpoint mechanisms.

Genetic analysis of CHK1 and CHK2 homologues revealed a unique cross talk between ATM and ATR pathways in Neurospora crassa

DNA damage checkpoint is an important mechanism for organisms to maintain genome integrity. In Neurospora crassa, mus-9 and mus-21 are homologues of ATR and ATM, respectively, which are pivotal factors of DNA damage checkpoint in mammals. A N. crassa clock gene prd-4 has been identified as a CHK2 homologue, but its role in DNA damage response had not been elucidated. In this study, we identified another CHK2 homologue and one CHK1 homologue from the N. crassa genome database. As disruption of these genes affected mutagen tolerance, we named them mus-59 and mus-58, respectively. The mus-58 mutant was sensitive to hydroxyurea (HU), but the mus-59 and prd-4 mutants showed the same HU sensitivity as that of the wild-type strain. This indicates the possibility that MUS-58 is involved in replication checkpoint and stabilization of stalled forks like mammalian CHK1. Phosphorylation of MUS-58 and MUS-59 was observed in the wild-type strain in response to mutagen treatments. Genetic relationships between those three genes and mus-9 or mus-21 indicated that the mus-9 mutation was epistatic to mus-58, and mus-21 was epistatic to prd-4. These relationships correspond to two signal pathways, ATR-CHK1 and ATM-CHK2 that have been established in mammalian cells. However, both the mus-9 mus-59 and mus-21 mus-58 double mutants showed an intermediate level between the two parental strains for CPT sensitivity. Furthermore, these double mutants showed severe growth defects. Our findings suggest that the DNA damage checkpoint of N. crassa is controlled by unique mechanisms.

A long-term flow cytometry assay to analyze the role of specific genes of Drosophila melanogaster S2 cells in surviving genotoxic stress

Drosophila S2 cells are easy to manipulate and culture and are a versatile model system for high-throughput screens such as genome-wide siRNA screens to find genes involved in stress or therapy resistance or for screening through large compound libraries to identify cytotoxins. Clonogenic assays are considered the gold-standard to investigate the cytotoxicity of specific treatments or to compare the sensitivity of various cell types for a specific treatment. However, this assay cannot be used for Drosophila S2 cells as they are virtually unable to grow in distinct colonies. We designed a novel fluorescence-based flow cytometry assay to study long-term proliferation of S2 cells under various conditions and in the presence of specific gene products or after downregulation of specific gene products. Here we validate this assay and we used this novel method to investigate the role of checkpoint genes grapes/Dchk1 and DmChk2 in cell survival responses. Our data demonstrate that Grapes/Dchk1 but not DmChk2 is required to survive hydroxyurea. Our assay will be of use to investigate the long-term effects of various treatments in S2 cells and to evaluate the role of specific proteins therein.

TheDrosophilahomolog ofMCPH1,a human microcephaly gene, is required for genomic stability in the early embryo

Mutation of human microcephalin (MCPH1) causes autosomal recessive primary microcephaly, a developmental disorder characterized by reduced brain size. We identified mcph1, the Drosophila homolog of MCPH1, in a genetic screen for regulators of S-M cycles in the early embryo. Embryos of null mcph1 female flies undergo mitotic arrest with barrel-shaped spindles lacking centrosomes. Mutation of Chk2 suppresses these defects, indicating that they occur secondary to a previously described Chk2-mediated response to mitotic entry with unreplicated or damaged DNA. mcph1 embryos exhibit genomic instability as evidenced by frequent chromatin bridging in anaphase. In contrast to studies of human MCPH1, the ATR/Chk1-mediated DNA checkpoint is intact in Drosophila mcph1 mutants. Components of this checkpoint, however, appear to cooperate with MCPH1 to regulate embryonic cell cycles in a manner independent of Cdk1 phosphorylation. We propose a model in which MCPH1 coordinates the S-M transition in fly embryos: in the absence of mcph1, premature chromosome condensation results in mitotic entry with unreplicated DNA, genomic instability, and Chk2-mediated mitotic arrest. Finally, brains of mcph1 adult male flies have defects in mushroom body structure, suggesting an evolutionarily conserved role for MCPH1 in brain development.

Transgenic rescue of methotrexate-induced teratogenicity in Drosophila melanogaster

The folic acid analog methotrexate (MTX), a competitive inhibitor of dihydrofolate reductase (DHFR), is used to treat a variety of cancers and autoimmune disorders. However, MTX also causes a wide range of toxic effects in healthy cells and is an established teratogen. Efforts to "rescue" the defects caused by MTX by administering a folate analog or by transgenic expression of a DHFR with an altered affinity for MTX have been attempted in a variety of mammals but limited protection was conferred. As a result, our understanding of the effect of MTX at the molecular genetic level remains incomplete and, in addition, continued mammalian sacrifice is not ideal. Due to the similarity of teratogenic effects produced by MTX in Drosophila melanogaster these insects were transformed with DHFR alleles to determine if rescue could be achieved. The resulting "MTX-resistant" flies were subsequently used to investigate changes in gene expression in response to MTX using semiquantitative reverse transcription PCR. The majority (12/14) of key transcripts that were affected in MTX-exposed females including transcripts involved in cell cycle, defense response, and transport were "rescued" in the "MTX-resistant" transgenic flies. These studies illustrate the utility of this invertebrate model for the investigation of molecular effects of MTX-induced teratogenicity, MTX-resistant DHFRs for gene therapy techniques, and teratogenic protection.

Reducing DNA polymerase alpha in the absence of Drosophila ATR leads to P53-dependent apoptosis and developmental defects

The ability to respond to DNA damage and incomplete replication ensures proper duplication and stability of the genome. Two checkpoint kinases, ATM and ATR, are required for DNA damage and replication checkpoint responses. In Drosophila, the ATR ortholog (MEI-41) is essential for preventing entry into mitosis in the presence of DNA damage. In the absence of MEI-41, heterozygosity for the E(mus304) mutation causes rough eyes. We found that E(mus304) is a mutation in DNApol-alpha180, which encodes the catalytic subunit of DNA polymerase alpha. We did not find any defects resulting from reducing Polalpha by itself. However, reducing Polalpha in the absence of MEI-41 resulted in elevated P53-dependent apoptosis, rough eyes, and increased genomic instability. Reducing Polalpha in mutants that lack downstream components of the DNA damage checkpoint (DmChk1 and DmChk2) results in the same defects. Furthermore, reducing levels of mitotic cyclins rescues both phenotypes. We suggest that reducing Polalpha slows replication, imposing an essential requirement for the MEI-41-dependent checkpoint for maintenance of genome stability, cell survival, and proper development. This work demonstrates a critical contribution of the checkpoint function of MEI-41 in responding to endogenous damage.

An essential role for Drosophila hus1 in somatic and meiotic DNA damage responses

The checkpoint proteins Rad9, Rad1 and Hus1 form a clamp-like complex which plays a central role in the DNA-damage-induced checkpoint response. Here we address the function of the 9-1-1 complex in Drosophila. We decided to focus our analysis on the meiotic and somatic requirements of hus1. For that purpose, we created a null allele of hus1 by imprecise excision of a P element found 2 kb from the 3' of the hus1 gene. We found that hus1 mutant flies are viable, but the females are sterile. We determined that hus1 mutant flies are sensitive to hydroxyurea and methyl methanesulfonate but not to X-rays, suggesting that hus1 is required for the activation of an S-phase checkpoint. We also found that hus1 is not required for the G2-M checkpoint and for post-irradiation induction of apoptosis. We subsequently studied the role of hus1 in activation of the meiotic checkpoint and found that the hus1 mutation suppresses the dorsal-ventral pattering defects caused by mutants in DNA repair enzymes. Interestingly, we found that the hus1 mutant exhibits similar oocyte nuclear defects as those produced by mutations in DNA repair enzymes. These results demonstrate that hus1 is essential for the activation of the meiotic checkpoint and that hus1 is also required for the organization of the oocyte DNA, a function that might be independent of the meiotic checkpoint.

Drosophila ATR in double-strand break repair

The ability of a cell to sense and respond to DNA damage is essential for genome stability. An important aspect of the response is arrest of the cell cycle, presumably to allow time for repair. Ataxia telangiectasia mutated (ATM) and ATR are essential for such cell-cycle control, but some observations suggest that they also play a direct role in DNA repair. The Drosophila ortholog of ATR, MEI-41, mediates the DNA damage-dependent G2-M checkpoint. We examined the role of MEI-41 in repair of double-strand breaks (DSBs) induced by P-element excision. We found that mei-41 mutants are defective in completing the later steps of homologous recombination repair, but have no defects in end-joining repair. We hypothesized that these repair defects are the result of loss of checkpoint control. To test this, we genetically reduced mitotic cyclin levels and also examined repair in grp (DmChk1) and lok (DmChk2) mutants. Our results suggest that a significant component of the repair defects is due to loss of MEI-41-dependent cell cycle regulation. However, this does not account for all of the defects we observed. We propose a novel role for MEI-41 in DSB repair, independent of the Chk1/Chk2-mediated checkpoint response.

Irofulven induces replication-dependent CHK2 activation related to p53 status

CHK2 and p53 are frequently mutated in human cancers. CHK2 is known to phosphorylate and stabilize p53. CHK2 has also been implicated in DNA repair and apoptosis induction. However, whether p53 affects CHK2 activation and whether CHK2 activation modulates chemosensitivity are unclear. In this study, we found that in response to the DNA damage agent, irofulven, CHK2 activation, rather than its expression, is inversely correlated to p53 status. Irofulven inhibits DNA replication and induces chromosome aberrations (breaks and radials) and p53-dependent cell cycle arrest. Pretreatment of cells with the DNA polymerase inhibitor, aphidicolin, resulted in reduction of irofulven-induced CHK2 activation and foci formation, indicating that CHK2 activation by irofulven is replication-dependent. Furthermore, by using ovarian cancer cell lines expressing dominant-negative CHK2 and CHK2-knockout HCT116 cells, we found that CHK2 activation contributes to the control of S and G2/M cell cycle arrests, but not chemosensitivity to irofulven. Overall, this study demonstrates that in response to irofulven-induced DNA damage, the activation of CHK2 is dependent on DNA replication and related to p53 status. By controlling cell cycle arrest and DNA replication, p53 affects CHK2 activation. CHK2 activation contributes to cell cycle arrest, but not chemosensitivity.

Intrinsic Kinase Activity and SQ/TQ Domain of Chk2 Kinase as Well as N-terminal Domain of Wip1 Phosphatase Are Required for Regulation of Chk2 by Wip1

The anti-oncogenic Chk2 kinase plays a crucial role in DNA damage-induced cell cycle checkpoint regulation. Recently, we have shown that Chk2 associates with the oncogenic Wip1 (PPM1D) phosphatase and that Wip1 acts as a negative regulator of Chk2 during DNA damage response by dephosphorylating phosphorylated Thr-68 in activated Chk2 (Fujimoto, H., Onishi, N., Kato, N., Takekawa, M., Xu, X. Z., Kosugi, A., Kondo, T., Imamura, M., Oishi, I., Yoda, A., and Minami, Y. (2006) Cell Death Differ. 13, 1170-1180). Here, we performed structure-function analyses of Chk2 and Wip1 by using a series of deletion or amino acid-substituted mutant proteins of Chk2 and Wip1. We show that nuclear localization of both Chk2 and Wip1 is required for their association in cultured cells and that the serine-glutamine (SQ)/threonine-glutamine (TQ) domain of Chk2, containing Thr-68, and the N-terminal domain of Wip1, comprising about 100 amino acids, are necessary and sufficient for the association of both molecules. However, it was found that an intrinsic kinase activity of Chk2, but not phosphatase activity of Wip1, is required for the association of fulllength Chk2 and Wip1. Interestingly, we also show that the mutant Wip1 proteins, bearing the N-terminal domain of Wip1 alone or lacking an intrinsic phosphatase activity, exhibit dominant negative effects on the functions of the wild-type Wip1, i.e. ectopic expression of either of these Wip1 mutants inhibits dephosphorylation of Thr-68 in Chk2 by Wip1 and anti-apoptotic function of Wip1. These results provide a molecular basis for developing novel anti-cancer drugs, targeting oncogenic Wip1 phosphatase.

Ionizing radiation induces caspase-dependent but Chk2- and p53-independent cell death in Drosophila melanogaster

Ionizing radiation (IR) can induce apoptosis via p53, which is the most commonly mutated gene in human cancers. Loss of p53, however, can render cancer cells refractory to therapeutic effects of IR. Alternate p53-independent pathways exist but are not as well understood as p53-dependent apoptosis. Studies of how IR induces p53-independent cell death could benefit from the existence of a genetically tractable model. In Drosophila melanogaster, IR induces apoptosis in the imaginal discs of larvae, typically assayed at 4-6 hr after exposure to a LD(50) dose. In mutants of Drosophila Chk2 or p53 homologs, apoptosis is severely diminished in these assays, leading to the widely held belief that IR-induced apoptosis depends on these genes in Drosophila. In this article, we show that IR-induced apoptosis still occurs in the imaginal discs of chk2 and p53 mutant larvae, albeit with a delay. We demonstrate that this phenomenon is a true apoptotic response because it requires caspase activity and the chromosomal locus that encodes the pro-apoptotic genes reaper, hid, and grim. We also show that Chk2- and p53-independent apoptosis is IR dose-dependent and is therefore probably triggered by a DNA damage signal. We conclude that Drosophila has Chk2- and p53-independent pathways to activate caspases and induce apoptosis in response to IR. This work establishes Drosophila as a model for p53-independent apoptosis, which is of potential therapeutic importance for inducing cell death in p53-deficient cancer cells.

DNA damage-induced programmed cell death: potential roles in germ cell development

The detection of DNA damage is necessary to protect against proliferation of potentially harmful cells and often results in cell cycle arrest and programmed cell death. Key components of DNA damage signaling networks include ATM, CHK2, p53, and Bax. Mutations in these damage signaling systems are linked to tumorigenesis and developmental abnormalities. Expression of some of these genes in primordial germ cells (PGCs) argues that PGCs may utilize DNA damage-induced signaling mechanisms to select against germ cells that are genetically defective, thus maintaining the integrity of the germline. This paper summarizes the roles of these DNA damage signaling molecules and addresses their potential involvement in germ cell development.

Epigenetic telomere protection by Drosophila DNA damage response pathways

Analysis of terminal deletion chromosomes indicates that a sequence-independent mechanism regulates protection of Drosophila telomeres. Mutations in Drosophila DNA damage response genes such as atm/tefu, mre11, or rad50 disrupt telomere protection and localization of the telomere-associated proteins HP1 and HOAP, suggesting that recognition of chromosome ends contributes to telomere protection. However, the partial telomere protection phenotype of these mutations limits the ability to test if they act in the epigenetic telomere protection mechanism. We examined the roles of the Drosophila atm and atr-atrip DNA damage response pathways and the nbs homolog in DNA damage responses and telomere protection. As in other organisms, the atm and atr-atrip pathways act in parallel to promote telomere protection. Cells lacking both pathways exhibit severe defects in telomere protection and fail to localize the protection protein HOAP to telomeres. Drosophila nbs is required for both atm- and atr-dependent DNA damage responses and acts in these pathways during DNA repair. The telomere fusion phenotype of nbs is consistent with defects in each of these activities. Cells defective in both the atm and atr pathways were used to examine if DNA damage response pathways regulate telomere protection without affecting telomere specific sequences. In these cells, chromosome fusion sites retain telomere-specific sequences, demonstrating that loss of these sequences is not responsible for loss of protection. Furthermore, terminally deleted chromosomes also fuse in these cells, directly implicating DNA damage response pathways in the epigenetic protection of telomeres. We propose that recognition of chromosome ends and recruitment of HP1 and HOAP by DNA damage response proteins is essential for the epigenetic protection of Drosophila telomeres. Given the conserved roles of DNA damage response proteins in telomere function, related mechanisms may act at the telomeres of other organisms.

Grp/DChk1 is required for G2-M checkpoint activation in Drosophila S2 cells, whereas Dmnk/DChk2 is dispensable

Cell-cycle checkpoints are signal-transduction pathways required to maintain genomic stability in dividing cells. Previously, it was reported that two kinases essential for checkpoint signalling, Chk1 and Chk2 are structurally conserved. In contrast to yeast, Xenopus and mammals, the Chk1- and Chk2-dependent pathways in Drosophila are not understood in detail. Here, we report the function of these checkpoint kinases, referred to as Grp/DChk1 and Dmnk/DChk2 in Drosophila Schneider's cells, and identify an upstream regulator as well as downstream targets of Grp/DChk1. First, we demonstrate that S2 cells are a suitable model for G(2)/M checkpoint studies. S2 cells display Grp/DChk1-dependent and Dmnk/DChk2-independent cell-cycle-checkpoint activation in response to hydroxyurea and ionizing radiation. S2 cells depleted for Grp/DChk1 using RNA interference enter mitosis in the presence of impaired DNA integrity, resulting in prolonged mitosis and mitotic catastrophe. Grp/DChk1 is phosphorylated in a Mei-41/DATR-dependent manner in response to hydroxyurea and ionizing radiation, indicating that Mei-41/ATR is an upstream component in the Grp/DChk1 DNA replication and DNA-damage-response pathways. The level of Cdc25(Stg) and phosphorylation status of Cdc2 are modulated in a Grp/DChk1-dependent manner in response to hydroxyurea and irradiation, indicating that these cell-cycle regulators are downstream targets of the Grp/DChk1-dependent DNA replication and DNA-damage responses. By contrast, depletion of Dmnk/DChk2 by RNA interference had little effect on checkpoint responses to hydroxyurea and irradiation. We conclude that Grp/DChk1, and not Dmnk/DChk2, is the main effector kinase involved in G(2)/M checkpoint control in Drosophila cells.

No increased susceptibility to breast cancer from combined CHEK2 1100delC genotype and the HLA class III region risk factors

CHEK2 is low-penetrance breast cancer susceptibility gene. The 1100delC mutation may interact with variants/mutations in other breast cancer susceptibility loci. We identified a risk haplotype in the HLA class III region in breast cancer patients [de Jong MM, Nolte IM, de Vries EGE, et al. The HLA class III subregion is responsible for an increased breast cancer risk. Hum Mol Genet 2003, 12, 2311-2319] and tested whether it interacted with 1100delC mutation. The CHEK2 1100delC mutation was analysed in the same series of patients and controls as in the HLA breast cancer study. In 962 unselected breast cancer patients, the 1100delC mutation was observed in 2.9% and in 367 controls in 1.4% (NS). The highest 1100delC frequency occurred in high-risk (4.4%), followed by moderate-risk (3.8%), and lowest in low genetic risk patients (2.4%, P(trend) 0.029). In HLA risk haplotype carriers no increased breast cancer risk was observed in the presence of 1100delC mutation. Patients more often had one than both genetic risk factors. The 1100delC mutation and the HLA risk haplotype confer increased breast cancer risks, but an interactive effect on breast cancer between both factors is unlikely. In contrast, the effect of 1100delC mutation on breast cancer risk was limited to individuals without HLA risk haplotype, suggesting a mutual excluding effect between these risk factors.

The Chk2 protein kinase

Checkpoint kinase 2 (Chk2) is a multifunctional enzyme whose functions are central to the induction of cell cycle arrest and apoptosis by DNA damage. Insight into Chk2 has derived from multiple approaches. Biochemical studies have addressed Chk2 structure, domain organization and regulation by phosphorylation. Extensive work has been done to identify factors that recognize and respond to DNA damage in order to activate Chk2. In turn a number of substrates and targets of Chk2 have been identified that play roles in the checkpoint response. The roles and regulation of Chk2 have been elucidated by studies in model genetic systems extending from worms and flies to mice and humans. The relationship of Chk2 to human cancer studies is developing rapidly with increasing evidence that Chk2 plays a role in tumor suppression.

ATM-dependent CHK2 activation induced by anticancer agent, irofulven

Irofulven (6-hydroxymethylacylfulvene, HMAF, MGI 114) is one of a new class of anticancer agents that are semisynthetic derivatives of the mushroom toxin illudin S. Preclinical studies and clinical trials have demonstrated that irofulven is effective against several tumor types. Mechanisms of action studies indicate that irofulven induces DNA damage, MAPK activation, and apoptosis. In this study we found that in ovarian cancer cells, CHK2 kinase is activated by irofulven while CHK1 kinase is not activated even when treated at higher concentrations of the drug. By using GM00847 human fibroblast expressing tetracycline-controlled, FLAG-tagged kinase-dead ATR (ATR.kd), it was demonstrated that ATR kinase does not play a major role in irofulven-induced CHK2 activation. Results from human fibroblasts proficient or deficient in ATM function (GM00637 and GM05849) indicated that CHK2 activation by irofulven is mediated by the upstream ATM kinase. Phosphorylation of ATM on Ser(1981), which is critical for kinase activation, was observed in ovarian cancer cell lines treated with irofulven. RNA interference results confirmed that CHK2 activation was inhibited after introducing siRNA for ATM. Finally, experiments done with human colon cancer cell line HCT116 and its isogenic CHK2 knockout derivative; and experiments done by expressing kinase-dead CHK2 in an ovarian cancer cell line demonstrated that CHK2 activation contributes to irofulven-induced S phase arrest. In addition, it was shown that NBS1, SMC1, and p53 were phosphorylated in an ATM-dependent manner, and p53 phosphorylation on serine 20 is dependent on CHK2 after irofulven treatment. In summary, we found that the anticancer agent, irofulven, activates the ATM-CHK2 DNA damage-signaling pathway, and CHK2 activation contributes to S phase cell cycle arrest induced by irofulven.

Of flies and men; p53, a tumour suppressor

The completion of the Drosophila genome sequencing project [Science 287 (2000) 2185] has reconfirmed the fruit fly as a model organism to study human disease. Comparison studies have shown that two thirds of genes implicated in human cancers have counterparts in the fly [Curr. Opin. Genet. Dev. 11 (2001) 274; J. Cell Biol. 150 (2000) F23], including the tumour suppressor, p53. The suitability of the fruit fly to study the function of the tumour suppressor p53 is further exemplified by the lack of p53 family members within the fly genome, i.e., no homologues to p63 and p73 have been identified. Hence, there is no redundancy between family members greatly facilitating the analysis of p53 function. In addition, studying p53 in Drosophila provides an opportunity to learn about the evolution of tumour suppressors. Here, we will discuss what is known about Drosophila p53 in relation to human p53.

Enhanced 7-Ethyl-10-hydroxycamptothecin (SN-38) Lethality by Methylselenocysteine Is Associated with Chk2 Phosphorylation at Threonine-68 and Down-Regulation of Cdc6 Expression

Methylselenocysteine (MSC) is an organic selenium compound in preventative clinical trials involving prostate, lung, and colon carcinoma. We found that methioninase-activated MSC potentiates 7-ethyl-10-hydroxycamptothecin (SN-38)-induced cell lethality in vitro in the p53-defective human head and neck carcinoma A253 cells. Activated MSC increases chk2 phosphorylation at threonine-68 induced by SN-38, with no significant effect on chk1 phosphorylation. Cell cycle arrest induced by SN-38, however, was not abrogated or potentiated by MSC. These results suggest that the enhanced cellular lethality of SN-38 by MSC was not associated with cell cycle regulation pathways. Because chk2, in addition to its role in cell cycle arrest, can induce apoptosis by phosphorylation/activation, we examined whether increased chk2 phosphorylation could induce preapoptotic DNA fragmentation. DNA damage analysis showed that megabase DNA fragmentation is decreased, accompanied by the increased 30 to 300 kilobase pairs of DNA fragmentation after exposure to SN-38 with MSC, compared with SN-38 alone. No significant changes in the amount of DNA fragments were observed in cells treated with SN-38 or MSC alone. Moreover, proteolytic destruction of DNA replication-associated proteins cdc6, MCM2, and cdc25A may induce a DNA damage checkpoint response. The observed down-regulation of DNA replication proteins cdc6, MCM2, and cdc25A after exposure to SN-38 with MSC further indicates a relationship between drug response and DNA damage. Exposure to SN-38 with MSC resulted in a significant increase of poly(ADP-ribose) polymerasecleavage and caspase 3 activation. All together, the data support the hypothesis that enhanced lethality of this combination is associated with increased chk2 phosphorylation at Thr68 and down-regulation of specific DNA replication-associated proteins, which result in poly(ADP-ribose) polymerase cleavage, caspase 3 activation, and the induction of 30 to 300 kilobase pairs of DNA fragmentation.

Drosophila melanogaster MNK/Chk2 and p53 regulate multiple DNA repair and apoptotic pathways following DNA damage

We have used genetic and microarray analysis to determine how ionizing radiation (IR) induces p53-dependent transcription and apoptosis in Drosophila melanogaster. IR induces MNK/Chk2-dependent phosphorylation of p53 without changing p53 protein levels, indicating that p53 activity can be regulated without an Mdm2-like activity. In a genome-wide analysis of IR-induced transcription in wild-type and mutant embryos, all IR-induced increases in transcript levels required both p53 and the Drosophila Chk2 homolog MNK. Proapoptotic targets of p53 include hid, reaper, sickle, and the tumor necrosis factor family member EIGER: Overexpression of Eiger is sufficient to induce apoptosis, but mutations in Eiger do not block IR-induced apoptosis. Animals heterozygous for deletions that span the reaper, sickle, and hid genes exhibited reduced IR-dependent apoptosis, indicating that this gene complex is haploinsufficient for induction of apoptosis. Among the genes in this region, hid plays a central, dosage-sensitive role in IR-induced apoptosis. p53 and MNK/Chk2 also regulate DNA repair genes, including two components of the nonhomologous end-joining repair pathway, Ku70 and Ku80. Our results indicate that MNK/Chk2-dependent modification of Drosophila p53 activates a global transcriptional response to DNA damage that induces error-prone DNA repair as well as intrinsic and extrinsic apoptosis pathways.