The cyclin-dependent kinase inhibitor Dacapo promotes replication licensing during Drosophila endocycles

Studying Cellular Senescence Using the Model Organism Drosophila melanogaster

Cellular senescence, a complex biological process characterized by irreversible cell cycle arrest, contributes significantly to the development and progression of aging and of age-related diseases. Studying cellular senescence in vivo can be challenging due to the high heterogeneity and dynamic nature of senescent cells. Recently, Drosophila melanogaster has emerged as a powerful model organism for studying aging and cellular senescence due to its tractability and short lifespan, as well as due to the conservation of age-related genes and of key age-related pathways with mammals. Consequently, several research studies have utilized Drosophila to investigate the cellular mechanisms and pathways implicated in cellular senescence. Herein, we provide an overview of the assays that can be applied to study the different features of senescent cells in D. melanogaster tissues, highlighting the benefits of this model in aging research. We also emphasize the importance of selecting appropriate biomarkers for the identification of senescent cells, and the need for further understanding of the aging process including a more accurate identification and detection of senescent cells at the organismal level; a far more complex process as compared to single cells.

An interplay between cellular growth and atypical fusion defines morphogenesis of a modular glial niche in Drosophila

Neural stem cells (NSCs) live in an intricate cellular microenvironment supporting their activity, the niche. Whilst shape and function are inseparable, the morphogenetic aspects of niche development are poorly understood. Here, we use the formation of a glial niche to investigate acquisition of architectural complexity. Cortex glia (CG) in Drosophila regulate neurogenesis and build a reticular structure around NSCs. We first show that individual CG cells grow tremendously to ensheath several NSC lineages, employing elaborate proliferative mechanisms which convert these cells into syncytia rich in cytoplasmic bridges. CG syncytia further undergo homotypic cell-cell fusion, using defined cell surface receptors and actin regulators. Cellular exchange is however dynamic in space and time. This atypical cell fusion remodels cellular borders, restructuring the CG syncytia. Ultimately, combined growth and fusion builds the multi-level architecture of the niche, and creates a modular, spatial partition of the NSC population. Our findings provide insights into how a niche forms and organises while developing intimate contacts with a stem cell population.

Noncanonical function of Capicua as a growth termination signal in Drosophila oogenesis

Capicua (Cic) proteins are conserved HMG-box transcriptional repressors that control receptor tyrosine kinase (RTK) signaling responses and are implicated in human neurological syndromes and cancer. While Cic is known to exist as short (Cic-S) and long (Cic-L) isoforms with identical HMG-box and associated core regions but distinct N termini, most previous studies have focused on Cic-S, leaving the function of Cic-L unexplored. Here we show that Cic-L acts in two capacities during Drosophila oogenesis: 1) as a canonical sensor of RTK signaling in somatic follicle cells, and 2) as a regulator of postmitotic growth in germline nurse cells. In these latter cells, Cic-L behaves as a temporal signal that terminates endoreplicative growth before they dump their contents into the oocyte. We show that Cic-L is necessary and sufficient for nurse cell endoreplication arrest and induces both stabilization of CycE and down-regulation of Myc. Surprisingly, this function depends mainly on the Cic-L-specific N-terminal module, which is capable of acting independently of the Cic HMG-box-containing core. Mirroring these observations, basal metazoans possess truncated Cic-like proteins composed only of Cic-L N-terminal sequences, suggesting that this module plays unique, ancient roles unrelated to the canonical function of Cic.

Polyploidy in development and tumor models in Drosophila

Polyploidy, a cell status defined as more than two sets of genomic DNA, is a conserved strategy across species that can increase cell size and biosynthetic production, but the functional aspects of polyploidy are nuanced and vary across cell types. Throughout Drosophila developmental stages (embryo, larva, pupa and adult), polyploid cells are present in numerous organs and help orchestrate development while contributing to normal growth, well-being and homeostasis of the organism. Conversely, increasing evidence has shown that polyploid cells are prevalent in Drosophila tumors and play important roles in tumor growth and invasiveness. Here, we summarize the genes and pathways involved in polyploidy during normal and tumorigenic development, the mechanisms underlying polyploidization, and the functional aspects of polyploidy in development, homeostasis and tumorigenesis in the Drosophila model.

Proper CycE-Cdk2 activity in endocycling tissues requires regulation of the cyclin-dependent kinase inhibitor Dacapo by dE2F1b in Drosophila

Polyploidy is an integral part of development and is associated with cellular stress, aging, and pathological conditions. The endocycle, comprised of successive rounds of G and S phases without mitosis, is widely employed to produce polyploid cells in plants and animals. In Drosophila, maintenance of the endocycle is dependent on E2F-governed oscillations of Cyclin E (CycE)-Cdk2 activity, which is known to be largely regulated at the level of transcription. In this study, we report an additional level of E2F-dependent control of CycE-Cdk2 activity during the endocycle. Genetic experiments revealed that an alternative isoform of Drosophila de2f1, dE2F1b, regulates the expression of the p27CIP/KIP-like Cdk inhibitor Dacapo (Dap). We provide evidence showing that dE2F1b-dependent Dap expression in endocycling tissues is necessary for setting proper CycE-Cdk2 activity. Furthermore, we demonstrate that dE2F1b is required for proliferating cell nuclear antigen expression that establishes a negative feedback loop in S phase. Overall, our study reveals previously unappreciated E2F-dependent regulatory networks that are critical for the periodic transition between G and S phases during the endocycle.

Arp2/3 and Unc45 maintain heterochromatin stability in Drosophila polytene chromosomes

Repairing DNA double-strand breaks is particularly challenging in pericentromeric heterochromatin, where the abundance of repeated sequences exacerbates the risk of ectopic recombination. In Drosophila Kc cells, accurate homologous recombination repair of heterochromatic double-strand breaks relies on the relocalization of repair sites to the nuclear periphery before Rad51 recruitment and strand invasion. This movement is driven by Arp2/3-dependent nuclear actin filaments and myosins’ ability to walk along them. Conserved mechanisms enable the relocalization of heterochromatic repair sites in mouse cells, and defects in these pathways lead to massive ectopic recombination in heterochromatin and chromosome rearrangements. In Drosophila polytene chromosomes, extensive DNA movement is blocked by a stiff structure of chromosome bundles. Repair pathways in this context are poorly characterized, and whether heterochromatic double-strand breaks relocalize in these cells is unknown. Here, we show that damage in heterochromatin results in relaxation of the heterochromatic chromocenter, consistent with a dynamic response. Arp2/3, the Arp2/3 activator Scar, and the myosin activator Unc45, are required for heterochromatin stability in polytene cells, suggesting that relocalization enables heterochromatin repair also in this tissue. Together, these studies reveal critical roles for actin polymerization and myosin motors in heterochromatin repair and genome stability across different organisms and tissue types.

The GATOR complex regulates an essential response to meiotic double-stranded breaks in Drosophila

The TORC1 regulator GATOR1/SEACIT controls meiotic entry and early meiotic events in yeast. However, how metabolic pathways influence meiotic progression in metazoans remains poorly understood. Here we examine the role of the TORC1 regulators GATOR1 and GATOR2 in the response to meiotic double-stranded breaks (DSB) during Drosophila oogenesis. We find that in mutants of the GATOR2 component mio, meiotic DSBs trigger the constitutive downregulation of TORC1 activity and a permanent arrest in oocyte growth. Conversely, in GATOR1 mutants, high TORC1 activity results in the delayed repair of meiotic DSBs and the hyperactivation of p53. Unexpectedly, we found that GATOR1 inhibits retrotransposon expression in the presence of meiotic DSBs in a pathway that functions in parallel to p53. Thus, our studies have revealed a link between oocyte metabolism, the repair of meiotic DSBs and retrotransposon expression.

A Cyclin A-Myb-MuvB-Aurora B network regulates the choice between mitotic cycles and polyploid endoreplication cycles

Endoreplication is a cell cycle variant that entails cell growth and periodic genome duplication without cell division, and results in large, polyploid cells. Cells switch from mitotic cycles to endoreplication cycles during development, and also in response to conditional stimuli during wound healing, regeneration, aging, and cancer. In this study, we use integrated approaches in Drosophila to determine how mitotic cycles are remodeled into endoreplication cycles, and how similar this remodeling is between induced and developmental endoreplicating cells (iECs and devECs). Our evidence suggests that Cyclin A / CDK directly activates the Myb-MuvB (MMB) complex to induce transcription of a battery of genes required for mitosis, and that repression of CDK activity dampens this MMB mitotic transcriptome to promote endoreplication in both iECs and devECs. iECs and devECs differed, however, in that devECs had reduced expression of E2F1-dependent genes that function in S phase, whereas repression of the MMB transcriptome in iECs was sufficient to induce endoreplication without a reduction in S phase gene expression. Among the MMB regulated genes, knockdown of AurB protein and other subunits of the chromosomal passenger complex (CPC) induced endoreplication, as did knockdown of CPC-regulated cytokinetic, but not kinetochore, proteins. Together, our results indicate that the status of a CycA-Myb-MuvB-AurB network determines the decision to commit to mitosis or switch to endoreplication in both iECs and devECs, and suggest that regulation of different steps of this network may explain the known diversity of polyploid cycle types in development and disease.

Replication timing in Drosophila and its peculiarities in polytene chromosomes

Drosophila melanogaster is one of the popular model organisms in DNA replication studies. Since the 1960s, DNA replication of polytene chromosomes has been extensively studied by cytological methods. In the recent two decades, the progress in our understanding of DNA replication was associated with new techniques. Use of fluorescent dyes increased the resolution of cytological methods significantly. High-throughput methods allowed analysis of DNA replication on a genome scale, as well as its correlation with chromatin structure and gene activi ty. Precise mapping of the cytological structures of polytene chromosomes to the genome assembly allowed comparison of replication between polytene chromosomes and chromosomes of diploid cells. New features of replication characteristic for D. melanogaster were described for both diploid and polytene chromosomes. Comparison of genomic replication profiles revealed a significant similarity between Drosophila and other well-studi ed eukaryotic species, such as human. Early replication is often confined to intensely transcribed gene-dense regions characterized by multiple replication initiation sites. Features of DNA replication in Drosophila might be explained by a compact genome. The organization of replication in polytene chromosomes has much in common with the organization of replication in chromosomes in diploid cells. The most important feature of replication in polytene chromosomes is its low rate and the dependence of S-phase duration on many factors: external and internal, local and global. The speed of replication forks in D. melanogaster polytene chromosomes is affected by SUUR and Rif1 proteins. It is not known yet how universal the mechanisms associated with these factors are, but their study is very promising.

Bombyx mori cyclin-dependent kinase inhibitor is involved in regulation of the silkworm cell cycle

Cyclin-dependent kinase inhibitors (CKIs) are negative regulators of the cell cycle. They can bind to cyclin-dependent kinase (CDK)-cyclin complexes and inhibit CDK activities. We identified a single homologous gene of the CDK interacting protein/kinase inhibitory protein (Cip/Kip) family, BmCKI, in the silkworm, Bombyx mori. The gene transcribes two splice variants: a 654-bp-long BmCKI-L (the longer splice variant) encoding a protein with 217 amino acids and a 579-bp-long BmCKI-S (the shorter splice variant) encoding a protein with 192 amino acids. BmCKI-L and BmCKI-S contain the Cip/Kip family conserved cyclin-binding domain and the CDK-binding domain. They are localized in the nucleus and have an unconventional bipartite nuclear localization signal at amino acid residues 181-210. Overexpression of BmCKI-L or BmCKI-S affected cell cycle progression; the cell cycle was arrested in the first gap phase of cell cycle (G1). RNA interference of BmCKI-L or BmCKI-S led to cells accumulating in the second gap phase and the mitotic phase of cell cycle (G2/M). Both BmCKI-L and BmCKI-S are involved in cell cycle regulation and probably have similar effects. The transgenic silkworm with BmCKI-L overexpression (BmCKI-L-OE), exhibited embryonic lethal, larva developmental retardation and lethal phenotypes. These results suggest that BmCKI-L might regulate the growth and development of silkworm. These findings clarify the function of CKIs and increase our understanding of cell cycle regulation in the silkworm.

Endoreplication: The Good, the Bad, and the Ugly

To battle adverse internal and external conditions and maintain homeostasis, diploid organisms employ various cellular processes, such as proliferation and apoptosis. In some tissues, an alternative mechanism, endoreplication, is employed toward similar goals. Endoreplication is an evolutionarily conserved cell cycle program during which cells replicate their genomes without division, resulting in polyploid cells. Importantly, endoreplication is reported to be indispensable for normal development and organ formation across various organisms, from fungi to humans. In recent years, more attention has been drawn to delineating its connections to wound healing and tumorigenesis. In this Review, we discuss mechanisms of endoreplication and polyploidization, their essential and positive roles in normal development and tissue homeostasis, and the relationship between polyploidy and cancer.

DNA Replication Control During Drosophila Development: Insights into the Onset of S Phase, Replication Initiation, and Fork Progression

Proper control of DNA replication is critical to ensure genomic integrity during cell proliferation. In addition, differential regulation of the DNA replication program during development can change gene copy number to influence cell size and gene expression. Drosophila melanogaster serves as a powerful organism to study the developmental control of DNA replication in various cell cycle contexts in a variety of differentiated cell and tissue types. Additionally, Drosophila has provided several developmentally regulated replication models to dissect the molecular mechanisms that underlie replication-based copy number changes in the genome, which include differential underreplication and gene amplification. Here, we review key findings and our current understanding of the developmental control of DNA replication in the contexts of the archetypal replication program as well as of underreplication and differential gene amplification. We focus on the use of these latter two replication systems to delineate many of the molecular mechanisms that underlie the developmental control of replication initiation and fork elongation.

The Enok acetyltransferase complex interacts with Elg1 and negatively regulates PCNA unloading to promote the G1/S transition

KAT6 histone acetyltransferases (HATs) are highly conserved in eukaryotes and are involved in cell cycle regulation. However, information regarding their roles in regulating cell cycle progression is limited. Here, we report the identification of subunits of the Drosophila Enok complex and demonstrate that all subunits are important for its HAT activity. We further report a novel interaction between the Enok complex and the Elg1 proliferating cell nuclear antigen (PCNA)-unloader complex. Depletion of Enok in S2 cells resulted in a G1/S cell cycle block, and this block can be partially relieved by depleting Elg1. Furthermore, depletion of Enok reduced the chromatin-bound levels of PCNA in both S2 cells and early embryos, suggesting that the Enok complex may interact with the Elg1 complex and down-regulate its PCNA-unloading function to promote the G1/S transition. Supporting this hypothesis, depletion of Enok also partially rescued the endoreplication defects in Elg1-depleted nurse cells. Taken together, our study provides novel insights into the roles of KAT6 HATs in cell cycle regulation through modulating PCNA levels on chromatin.

Expression of an S phase-stabilized version of the CDK inhibitor Dacapo can alter endoreplication

In developing organisms, divergence from the canonical cell division cycle is often necessary to ensure the proper growth, differentiation, and physiological function of a variety of tissues. An important example is endoreplication, in which endocycling cells alternate between G and S phase without intervening mitosis or cytokinesis, resulting in polyploidy. Although significantly different from the canonical cell cycle, endocycles use regulatory pathways that also function in diploid cells, particularly those involved in S phase entry and progression. A key S phase regulator is the Cyclin E-Cdk2 kinase, which must alternate between periods of high (S phase) and low (G phase) activity in order for endocycling cells to achieve repeated rounds of S phase and polyploidy. The mechanisms that drive these oscillations of Cyclin E-Cdk2 activity are not fully understood. Here, we show that the Drosophila Cyclin E-Cdk2 inhibitor Dacapo (Dap) is targeted for destruction during S phase via a PIP degron, contributing to oscillations of Dap protein accumulation during both mitotic cycles and endocycles. Expression of a PIP degron mutant Dap attenuates endocycle progression but does not obviously affect proliferating diploid cells. A mathematical model of the endocycle predicts that the rate of destruction of Dap during S phase modulates the endocycle by regulating the length of G phase. We propose from this model and our in vivo data that endo S phase-coupled destruction of Dap reduces the threshold of Cyclin E-Cdk2 activity necessary to trigger the subsequent G-S transition, thereby influencing endocycle oscillation frequency and the extent of polyploidy.

Replication fork progression during re-replication requires the DNA damage checkpoint and double-strand break repair

Replication origins are under tight regulation to ensure activation occurs only once per cell cycle [1, 2]. Origin re-firing in a single S phase leads to the generation of DNA double-strand breaks (DSBs) and activation of the DNA damage checkpoint [2-7]. If the checkpoint is blocked, cells enter mitosis with partially re-replicated DNA that generates chromosome breaks and fusions [5]. These types of chromosomal aberrations are common in numerous human cancers, suggesting that re-replication events contribute to cancer progression. It was proposed that fork instability and DSBs formed during re-replication are the result of head-to-tail collisions and collapse of adjacent replication forks [3]. However, previously studied systems lack the resolution to determine whether the observed DSBs are generated at sites of fork collisions. Here, we utilize the Drosophila ovarian follicle cells, which exhibit re-replication under precise developmental control [8-10], to model the consequences of re-replication at actively elongating forks. Re-replication occurs from specific replication origins at six genomic loci, termed Drosophila amplicons in follicle cells (DAFCs) [10-12]. Precise developmental timing of DAFC origin firing permits identification of forks at defined points after origin initiation [13, 14]. Here, we show that DAFC re-replication causes fork instability and generates DSBs at sites of potential fork collisions. Immunofluorescence and ChIP-seq demonstrate the DSB marker γH2Av is enriched at elongating forks. Fork progression is reduced in the absence of DNA damage checkpoint components and nonhomologous end-joining (NHEJ), but not homologous recombination. NHEJ appears to continually repair forks during re-replication to maintain elongation.

Characterization of a Drosophila ortholog of the Cdc7 kinase: a role for Cdc7 in endoreplication independent of Chiffon

Cdc7 is a serine-threonine kinase that phosphorylates components of the pre-replication complex during DNA replication initiation. Cdc7 is highly conserved, and Cdc7 orthologs have been characterized in organisms ranging from yeast to humans. Cdc7 is activated specifically during late G1/S phase by binding to its regulatory subunit, Dbf4. Drosophila melanogaster contains a Dbf4 ortholog, Chiffon, which is essential for chorion amplification in Drosophila egg chambers. However, no Drosophila ortholog of Cdc7 has yet been characterized. Here, we report the functional and biochemical characterization of a Drosophila ortholog of Cdc7. Co-expression of Drosophila Cdc7 and Chiffon is able to complement a growth defect in yeast containing a temperature-sensitive Cdc7 mutant. Cdc7 and Chiffon physically interact and can be co-purified from insect cells. Cdc7 phosphorylates the known Cdc7 substrates Mcm2 and histone H3 in vitro, and Cdc7 kinase activity is stimulated by Chiffon and inhibited by the Cdc7-specific inhibitor XL413. Drosophila egg chamber follicle cells deficient for Cdc7 have a defect in two types of DNA replication, endoreplication and chorion gene amplification. However, follicle cells deficient for Chiffon have a defect in chorion gene amplification but still undergo endocycling. Our results show that Cdc7 interacts with Chiffon to form a functional Dbf4-dependent kinase complex and that Cdc7 is necessary for DNA replication in Drosophila egg chamber follicle cells. Additionally, we show that Chiffon is a member of an expanding subset of DNA replication initiation factors that are not strictly required for endoreplication in Drosophila.

Juvenile hormone-receptor complex acts on mcm4 and mcm7 to promote polyploidy and vitellogenesis in the migratory locust

Juvenile hormone (JH), a sesquiterpenoid produced by the corpora allata, coordinates insect growth, metamorphosis, and reproduction. While JH action for the repression of larval metamorphosis has been well studied, the molecular basis of JH in promoting adult reproduction has not been fully elucidated. Methoprene-tolerant (Met), the JH receptor, has been recently shown to mediate JH action during metamorphosis as well as in vitellogenesis, but again, the precise mechanism underlying the latter has been lacking. We have now demonstrated using Met RNAi to phenocopy a JH-deprived condition in migratory locusts, that JH stimulates DNA replication and increases ploidy in preparation for vitellogenesis. Mcm4 and Mcm7, two genes in the DNA replication pathway were expressed in the presence of JH and Met. Depletion of Mcm4 or Mcm7 inhibited de novo DNA synthesis and polyploidization, and resulted in the substantial reduction of vitellogenin mRNA levels as well as severely impaired oocyte maturation and ovarian growth. By using luciferase reporter and electrophoretic mobility shift assays, we have shown that Met directly regulates the transcription of Mcm4 and Mcm7 by binding to upstream consensus sequences with E-box or E-box-like motifs. Our work suggests that the JH-receptor complex acts on Mcm4 and Mcm7 to regulate DNA replication and polyploidy for vitellogenesis and oocyte maturation.

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.

The CDK regulators Cdh1 and Sic1 promote efficient usage of DNA replication origins to prevent chromosomal instability at a chromosome arm

Robustness and completion of DNA replication rely on redundant DNA replication origins. Reduced efficiency of origin licensing is proposed to contribute to chromosome instability in CDK-deregulated cell cycles, a frequent alteration in oncogenesis. However, the mechanism by which this instability occurs is largely unknown. Current models suggest that limited origin numbers would reduce fork density favouring chromosome rearrangements, but experimental support in CDK-deregulated cells is lacking. We have investigated the pattern of origin firing efficiency in budding yeast cells lacking the CDK regulators Cdh1 and Sic1. We show that each regulator is required for efficient origin activity, and that both cooperate non-redundantly. Notably, origins are differentially sensitive to CDK deregulation. Origin sensitivity is independent on normal origin efficiency, firing timing or chromosomal location. Interestingly, at a chromosome arm, there is a shortage of origin firing involving active and dormant origins, and the extent of shortage correlates with the severity of CDK deregulation and chromosome instability. We therefore propose that CDK deregulation in G1 phase compromises origin redundancy by decreasing the number of active and dormant origins, leading to origin shortage and increased chromosome instability.

Endocycles: a recurrent evolutionary innovation for post-mitotic cell growth

In endoreplication cell cycles, known as endocycles, cells successively replicate their genomes without segregating chromosomes during mitosis and thereby become polyploid. Such cycles, for which there are many variants, are widespread in protozoa, plants and animals. Endocycling cells can achieve ploidies of >200,000 C (chromatin-value); this increase in genomic DNA content allows a higher genomic output, which can facilitate the construction of very large cells or enhance macromolecular secretion. These cells execute normal S phases, using a G1-S regulatory apparatus similar to the one used by mitotic cells, but their capability to segregate chromosomes has been suppressed, typically by downregulation of mitotic cyclin-dependent kinase activity. Endocycles probably evolved many times, and the various endocycle mechanisms found in nature highlight the versatility of the cell cycle control machinery.

High-dose irradiation induces cell cycle arrest, apoptosis, and developmental defects during Drosophila oogenesis

Ionizing radiation (IR) treatment induces a DNA damage response, including cell cycle arrest, DNA repair, and apoptosis in metazoan somatic cells. Because little has been reported in germline cells, we performed a temporal analysis of the DNA damage response utilizing Drosophila oogenesis as a model system. Oogenesis in the adult Drosophila female begins with the generation of 16-cell cyst by four mitotic divisions of a cystoblast derived from the germline stem cells. We found that high-dose irradiation induced S and G2 arrests in these mitotically dividing germline cells in a grp/Chk1- and mnk/Chk2-dependent manner. However, the upstream kinase mei-41, Drosophila ATR ortholog, was required for the S-phase checkpoint but not for the G2 arrest. As in somatic cells, mnk/Chk2 and dp53 were required for the major cell death observed in early oogenesis when oocyte selection and meiotic recombination occurs. Similar to the unscheduled DNA double-strand breaks (DSBs) generated from defective repair during meiotic recombination, IR-induced DSBs produced developmental defects affecting the spherical morphology of meiotic chromosomes and dorsal-ventral patterning. Moreover, various morphological abnormalities in the ovary were detected after irradiation. Most of the IR-induced defects observed in oogenesis were reversible and were restored between 24 and 96 h after irradiation. These defects in oogenesis severely reduced daily egg production and the hatch rate of the embryos of irradiated female. In summary, irradiated germline cells induced DSBs, cell cycle arrest, apoptosis, and developmental defects resulting in reduction of egg production and defective embryogenesis.

Induction of endocycles represses apoptosis independently of differentiation and predisposes cells to genome instability

The endocycle is a common developmental cell cycle variation wherein cells become polyploid through repeated genome duplication without mitosis. We previously showed that Drosophila endocycling cells repress the apoptotic cell death response to genotoxic stress. Here, we investigate whether it is differentiation or endocycle remodeling that promotes apoptotic repression. We find that when nurse and follicle cells switch into endocycles during oogenesis they repress the apoptotic response to DNA damage caused by ionizing radiation, and that this repression has been conserved in the genus Drosophila over 40 million years of evolution. Follicle cells defective for Notch signaling failed to switch into endocycles or differentiate and remained apoptotic competent. However, genetic ablation of mitosis by knockdown of Cyclin A or overexpression of fzr/Cdh1 induced follicle cell endocycles and repressed apoptosis independently of Notch signaling and differentiation. Cells recovering from these induced endocycles regained apoptotic competence, showing that repression is reversible. Recovery from fzr/Cdh1 overexpression also resulted in an error-prone mitosis with amplified centrosomes and high levels of chromosome loss and fragmentation. Our results reveal an unanticipated link between endocycles and the repression of apoptosis, with broader implications for how endocycles may contribute to genome instability and oncogenesis.

PRC2 goes solo in the Drosophila female germline

Polycomb-group proteins silence gene expression through epigenetic modification of chromatin. In this issue of Developmental Cell, Iovino et al. (2013) demonstrate that Polycomb repressive complex 2 (PRC2) is required for maintenance of oocyte fate by repressing expression of two critical targets, Cyclin E and dacapo, during the early meiotic cycle.

The development of a monoclonal antibody recognizing the Drosophila melanogaster phosphorylated histone H2A variant (γ-H2AV)

The recognition of DNA double-strand breaks (DSBs) using a phospho-specific antibody to the histone 2A variant has become the gold standard assay for DNA damage detection. Here we report on the development of the first monoclonal antibody to the phospho-specific form of Drosophila H2AV and characterize the specificity of this antibody to programmed DSBs in oocytes and rereplication sites in endocycling cells by immunofluorescence assays and to DSBs resulting from irradiation in both cell culture and whole tissue by Western blot assays. These studies show that the antibody derived in the study is highly specific for this modification that occurs at DSB sites, and therefore will be a new useful tool within the Drosophila community for the study of DNA damage response, DSB repair, meiotic recombination and chemical agents that cause DNA damage.

Endoreplication and polyploidy: insights into development and disease

Polyploid cells have genomes that contain multiples of the typical diploid chromosome number and are found in many different organisms. Studies in a variety of animal and plant developmental systems have revealed evolutionarily conserved mechanisms that control the generation of polyploidy and have recently begun to provide clues to its physiological function. These studies demonstrate that cellular polyploidy plays important roles during normal development and also contributes to human disease, particularly cancer.

Endoreplication

Developmentally programmed polyploidy occurs by at least four different mechanisms, two of which (endoreduplication and endomitosis) involve switching from mitotic cell cycles to endocycles by the selective loss of mitotic cyclin-dependent kinase (CDK) activity and bypassing many of the processes of mitosis. Here we review the mechanisms of endoreplication, focusing on recent results from Drosophila and mice.

"The Octet": Eight Protein Kinases that Control Mammalian DNA Replication

Development of a fertilized human egg into an average sized adult requires about 29 trillion cell divisions, thereby producing enough DNA to stretch to the Sun and back 200 times (DePamphilis and Bell, 2011)! Even more amazing is the fact that throughout these mitotic cell cycles, the human genome is duplicated once and only once each time a cell divides. If a cell accidentally begins to re-replicate its nuclear DNA prior to cell division, checkpoint pathways trigger apoptosis. And yet, some cells are developmentally programmed to respond to environmental cues by switching from mitotic cell cycles to endocycles, a process in which multiple S phases occur in the absence of either mitosis or cytokinesis. Endocycles allow production of viable, differentiated, polyploid cells that no longer proliferate. What is surprising is that among the 516 (Manning et al., 2002) to 557 (BioMart web site) protein kinases encoded by the human genome, only eight regulate nuclear DNA replication directly. These are Cdk1, Cdk2, Cdk4, Cdk6, Cdk7, Cdc7, Checkpoint kinase-1 (Chk1), and Checkpoint kinase-2. Even more remarkable is the fact that only four of these enzymes (Cdk1, Cdk7, Cdc7, and Chk1) are essential for mammalian development. Here we describe how these protein kinases determine when DNA replication occurs during mitotic cell cycles, how mammalian cells switch from mitotic cell cycles to endocycles, and how cancer cells can be selectively targeted for destruction by inducing them to begin a second S phase before mitosis is complete.

Regulation of DNA replication during development

As development unfolds, DNA replication is not only coordinated with cell proliferation, but is regulated uniquely in specific cell types and organs. This differential regulation of DNA synthesis requires crosstalk between DNA replication and differentiation. This dynamic aspect of DNA replication is highlighted by the finding that the distribution of replication origins varies between differentiated cell types and changes with differentiation. Moreover, differential DNA replication in some cell types can lead to increases or decreases in gene copy number along chromosomes. This review highlights the recent advances and technologies that have provided us with new insights into the developmental regulation of DNA replication.

Control of Drosophila endocycles by E2F and CRL4(CDT2)

Endocycles are variant cell cycles comprised of DNA Synthesis (S)- and Gap (G)- phases but lacking mitosis^1,2. Such cycles facilitate post-mitotic growth in many invertebrate and plant cells, and are so ubiquitous that they may account for up to half the world’s biomass^3,4. DNA replication in endocycling Drosophila cells is triggered by Cyclin E/Cyclin Dependent Kinase 2 (CycE/Cdk2), but this kinase must be inactivated during each G-phase to allow the assembly of pre-Replication Complexes (preRCs) for the next S-phase^5,6. How CycE/Cdk2 is periodically silenced to allow re-replication has not been established. Here, using genetic tests in parallel with computational modeling, we show that Drosophila’s endocycles are driven by a molecular oscillator in which the E2F1 transcription factor promotes CycE expression and S-phase initiation, S-phase then activates the CRL4^Cdt2 ubiquitin ligase, and this in turn mediates the destruction of E2F1⁷. We propose that it is the transient loss of E2F1 during S-phases that creates the window of low Cdk activity required for preRC formation. In support of this model over-expressed E2F1 accelerated endocycling, whereas a stabilized variant of E2F1 blocked endocycling by de-regulating target genes including CycE, as well as Cdk1 and mitotic Cyclins. Moreover, we find that altering cell growth by changing nutrition or TOR signaling impacts E2F1 translation, thereby making endocycle progression growth-dependent. Many of the regulatory interactions essential to this novel cell cycle oscillator are conserved in animals and plants^1,2,8, suggesting that elements of this mechanism act in most growth-dependent cell cycles.

Increased levels of the cell cycle inhibitor protein, dacapo, accompany 20-hydroxyecdysone-induced G1 arrest in a mosquito cell line

When treated with the steroid hormone 20-hydroxyecdysone (20E), C7-10 cells from the mosquito, Aedes albopictus, arrest in the G1 phase of the cell cycle. To explore whether 20E-mediated cell cycle arrest proceeds through increased levels of cell cycle inhibitor (CKI) proteins, we cloned the Ae. albopictus homolog of dacapo, the single member of the Cip/Kip family of CKI proteins known from Drosophila melanogaster. The Ae. albopictus dacapo cDNA encoded a 261-amino acid homolog of the Aedes aegypti protein XP_001651102.1, which is encoded by an ∼23 kb gene containing three exons. Like dacapo from D. melanogaster, the ∼27 kDa protein from Aedes and Culex mosquitoes contained several S/TXXE/D motifs corresponding to potential protein kinase CK2 phosphorylation sites, and a binding site for proliferating cell nuclear antigen (PCNA). When extracts from cells treated with 20E were analyzed by western blotting, using a primary antibody to synthetic peptides from the mosquito dacapo protein, up-regulation of an ∼27 kDa protein was observed within 24 h, and the abundance of the protein further increased by 48 h after hormone treatment. This is the first investigation of a cell cycle inhibitory protein in mosquitoes. The results reinforce growing evidence that 20E affects expression of proteins that regulate cell cycle progression.

Dampened activity of E2F1-DP and Myb-MuvB transcription factors in Drosophila endocycling cells

The endocycle is a variant cell cycle comprised of alternating gap (G) and DNA synthesis (S) phases (endoreplication) without mitosis (M), which results in DNA polyploidy and large cell size. Endocycles occur widely in nature, but much remains to be learned about the regulation of this modified cell cycle. Here, we compared gene expression profiles of mitotic cycling larval brain and disc cells with the endocycling cells of fat body and salivary gland of the Drosophila larva. The results indicated that many genes that are positively regulated by the heterodimeric E2F1-DP or Myb-MuvB complex transcription factors are expressed at lower levels in endocycling cells. Many of these target genes have functions in M phase, suggesting that dampened E2F1 and Myb activity promote endocycles. Many other E2F1 target genes that are required for DNA replication were also repressed in endocycling cells, an unexpected result given that these cells must duplicate up to thousands of genome copies during each S phase. For some EF2-regulated genes, the lower level of mRNA in endocycling cells resulted in lower protein concentration, whereas for other genes it did not, suggesting a contribution of post-transcriptional regulation. Both knockdown and overexpression of E2F1-DP and Myb-MuvB impaired endocycles, indicating that transcriptional activation and repression must be balanced. Our data suggest that dampened transcriptional activation by E2F1-DP and Myb-MuvB is important to repress mitosis and coordinate the endocycle transcriptional and protein stability oscillators.

One-hit wonders of genomic instability

Recent data show that cells from many cancers exhibit massive chromosome instability. The traditional view is that the gradual accumulation of mutations in genes involved in transcriptional regulation and cell cycle controls results in tumor development. This, however, does not exclude the possibility that some mutations could be more potent than others in destabilizing the genome by targeting both chromosomal integrity and corresponding checkpoint mechanisms simultaneously. Three such examples of "single-hit" lesions potentially leading to heritable genome destabilization are discussed. They include: failure to release sister chromatid cohesion due to the incomplete proteolytic cleavage of cohesin; massive merotelic kinetochore misattachments upon condensin depletion; and chromosome under-replication. In all three cases, cells fail to detect potential chromosomal bridges before anaphase entry, indicating that there is a basic cell cycle requirement to maintain a degree of sister chromatid bridging that is not recognizable as chromosomal damage.

SIAMESE cooperates with the CDH1-like protein CCS52A1 to establish endoreplication in Arabidopsis thaliana trichomes

Endoreplication, also known as endoreduplication, is a phyogenetically widespread modified version of the cell cycle in which DNA replication is not followed by cell division. The SIAMESE (SIM) gene of Arabidopsis thaliana encodes the founding member of a novel class of plant-specific cyclin-dependent kinase (CDK) inhibitors and is a key regulator of endoreplication during the development of trichomes (shoot epidermal hairs). Here, we have identified mutations in the CCS52A1 gene as genetic modifiers of the multicellular trichome phenotype of sim mutants. Loss-of-function ccs52A1 mutations dramatically enhance the multicellularity of sim mutants trichomes in double mutants, whereas overexpression of CCS52A1 completely suppresses the sim mutant phenotype. CCS52A1 encodes a CDH1/FZR-like protein, a class of proteins that function as activators of the anaphase-promoting complex. Unicellular ccs52A1 trichomes become multicellular upon overexpression of B-type cyclin, consistent with repression of the accumulation of mitotic cyclins in the developing trichome by CCS52A1. As these M-phase-specific cyclins are known to accumulate in sim mutant trichomes, our data suggest that CCS52A1 and SIM cooperate in repressing accumulation of mitotic cyclins to establish the trichome endocycle. Comparison with endoreplication pathways in Drosophila and mammals indicates that while these organisms all use similar components to initiate endoreplication, the components are deployed differently in each organism.

Endoreplication: polyploidy with purpose

A great many cell types are necessary for the myriad capabilities of complex, multicellular organisms. One interesting aspect of this diversity of cell type is that many cells in diploid organisms are polyploid. This is called endopolyploidy and arises from cell cycles that are often characterized as "variant," but in fact are widespread throughout nature. Endopolyploidy is essential for normal development and physiology in many different organisms. Here we review how both plants and animals use variations of the cell cycle, termed collectively as endoreplication, resulting in polyploid cells that support specific aspects of development. In addition, we discuss briefly how endoreplication occurs in response to certain physiological stresses, and how it may contribute to the development of cancer. Finally, we describe the molecular mechanisms that support the onset and progression of endoreplication.

The Drosophila hindgut lacks constitutively active adult stem cells but proliferates in response to tissue damage

The adult Drosophila hindgut was recently reported to contain active, tissue-replenishing stem cells, like those of the midgut, but located within an anterior ring so as to comprise a single giant crypt. In contrast to this view, we observed no active stem cells and little cell turnover in adult hindgut tissue based on clonal marking and BrdU incorporation studies. Again contradicting the previous proposal, we showed that the adult hindgut is not generated by anterior stem cells during larval/pupal development. However, severe tissue damage within the hindgut elicits cell proliferation within a ring of putative quiescent stem cells at the anterior of the pylorus. Thus, the hindgut does not provide a model of tissue maintenance by constitutively active stem cells, but has great potential to illuminate mechanisms of stress-induced tissue repair.

Cip/Kip cyclin-dependent protein kinase inhibitors and the road to polyploidy

Cyclin-dependent kinases (CDKs) play a central role in the orderly transition from one phase of the eukaryotic mitotic cell division cycle to the next. In this context, p27Kip1 (one of the CIP/KIP family of CDK specific inhibitors in mammals) or its functional analogue in other eukarya prevents a premature transition from G1 to S-phase. Recent studies have revealed that expression of a second member of this family, p57Kip2, is induced as trophoblast stem (TS) cells differentiate into trophoblast giant (TG) cells. p57 then inhibits CDK1 activity, an enzyme essential for initiating mitosis, thereby triggering genome endoreduplication (multiple S-phases without an intervening mitosis). Expression of p21Cip1, the third member of this family, is also induced in during differentiation of TS cells into TG cells where it appears to play a role in suppressing the DNA damage response pathway. Given the fact that p21 and p57 are unique to mammals, the question arises as to whether one or both of these proteins are responsible for the induction and maintenance of polyploidy during mammalian development.

Developmentally programmed endoreduplication in animals

Development of a fertilized egg into an adult human requires trillions of cell divisions, the vast majority of which duplicate their genome once and only once. Nevertheless, trophoblast giant cells and megakaryocytes in mammals circumvent this rule by duplicating their genome multiple times without undergoing cell division, a process generally referred to as 'endoreduplication'. In contrast, arthropods such as Drosophila endoreduplicate their genome in most larval tissues, as well as in many adult tissues. Endoreduplication requires that cells prevent entrance into or completion of mitosis and cytokinesis under conditions that permit assembly of prereplication complexes. In addition, cells must prevent induction of apoptosis in response to incomplete DNA replication or DNA damage that may occur during the ensuing sequence of 'endocycles'. Thus, developmentally regulated endoreduplication results in terminal cell differentiation. Recent progress has revealed both differences and similarities in the mechanisms employed by flies and mammals to change from mitotic cell cycles to 'endocycles'. The critical step, however, appears to be switching from a CDK-dependent form of the anaphase promoting complex (APC) to one that functions only in the absence of CDK activity.

Endocycling cells do not apoptose in response to DNA rereplication genotoxic stress

Initiation of DNA replication at origins more than once per cell cycle results in rereplication and has been implicated in cancer. Here we use Drosophila to examine the checkpoint responses to rereplication in a developmental context. We find that increased Double-parked (Dup), the Drosophila ortholog of Cdt1, results in rereplication and DNA damage. In most cells, this rereplication triggers caspase activation and apoptotic cell death mediated by both p53-dependent and -independent pathways. Elevated Dup also caused DNA damage in endocycling cells, which switch to a G/S cycle during normal development, indicating that rereplication and the endocycling DNA reduplication program are distinct processes. Unexpectedly, however, endocycling cells do not apoptose regardless of tissue type. Our combined evidence suggests that endocycling apoptosis is repressed in part because proapoptotic gene promoters are silenced. Normal endocycling cells had DNA lesions near heterochromatin, which increased after rereplication, explaining why endocycling cells must constantly repress the genotoxic apoptotic response. Our results reveal a novel regulation of apoptosis in development and new insights into the little-understood endocycle. Similar mechanisms may operate during vertebrate development, with implications for cancer predisposition in certain tissues.

Differentiation of trophoblast stem cells into giant cells is triggered by p57/Kip2 inhibition of CDK1 activity

Genome endoreduplication during mammalian development is a rare event for which the mechanism is unknown. It first appears when fibroblast growth factor 4 (FGF4) deprivation induces differentiation of trophoblast stem (TS) cells into the nonproliferating trophoblast giant (TG) cells required for embryo implantation. Here we show that RO3306 inhibition of cyclin-dependent protein kinase 1 (CDK1), the enzyme required to enter mitosis, induced differentiation of TS cells into TG cells. In contrast, RO3306 induced abortive endoreduplication and apoptosis in embryonic stem cells, revealing that inactivation of CDK1 triggers endoreduplication only in cells programmed to differentiate into polyploid cells. Similarly, FGF4 deprivation resulted in CDK1 inhibition by overexpressing two CDK-specific inhibitors, p57/KIP2 and p21/CIP1. TS cell mutants revealed that p57 was required to trigger endoreduplication by inhibiting CDK1, while p21 suppressed expression of the checkpoint protein kinase CHK1, thereby preventing induction of apoptosis. Furthermore, Cdk2(-/-) TS cells revealed that CDK2 is required for endoreduplication when CDK1 is inhibited. Expression of p57 in TG cells was restricted to G-phase nuclei to allow CDK activation of S phase. Thus, endoreduplication in TS cells is triggered by p57 inhibition of CDK1 with concomitant suppression of the DNA damage response by p21.

The Cyclin-dependent kinase inhibitor Dacapo promotes genomic stability during premeiotic S phase

The proper execution of premeiotic S phase is essential to both the maintenance of genomic integrity and accurate chromosome segregation during the meiotic divisions. However, the regulation of premeiotic S phase remains poorly defined in metazoa. Here, we identify the p21(Cip1)/p27(Kip1)/p57(Kip2)-like cyclin-dependent kinase inhibitor (CKI) Dacapo (Dap) as a key regulator of premeiotic S phase and genomic stability during Drosophila oogenesis. In dap(-/-) females, ovarian cysts enter the meiotic cycle with high levels of Cyclin E/cyclin-dependent kinase (Cdk)2 activity and accumulate DNA damage during the premeiotic S phase. High Cyclin E/Cdk2 activity inhibits the accumulation of the replication-licensing factor Doubleparked/Cdt1 (Dup/Cdt1). Accordingly, we find that dap(-/-) ovarian cysts have low levels of Dup/Cdt1. Moreover, mutations in dup/cdt1 dominantly enhance the dap(-/-) DNA damage phenotype. Importantly, the DNA damage observed in dap(-/-) ovarian cysts is independent of the DNA double-strands breaks that initiate meiotic recombination. Together, our data suggest that the CKI Dap promotes the licensing of DNA replication origins for the premeiotic S phase by restricting Cdk activity in the early meiotic cycle. Finally, we report that dap(-/-) ovarian cysts frequently undergo an extramitotic division before meiotic entry, indicating that Dap influences the timing of the mitotic/meiotic transition.

CAF-1 is required for efficient replication of euchromatic DNA in Drosophila larval endocycling cells

The endocycle constitutes an effective strategy for cell growth during development. In contrast to the mitotic cycle, it consists of multiple S-phases with no intervening mitosis and lacks a checkpoint ensuring the replication of the entire genome. Here, we report an essential requirement of chromatin assembly factor-1 (CAF-1) for Drosophila larval endocycles. This complex promotes histone H3-H4 deposition onto newly synthesised DNA in vitro. In metazoans, the depletion of its large subunit leads to the rapid accumulation of cells in S-phase. However, whether this slower S-phase progression results from the activation of cell cycle checkpoints or whether it reflects a more direct requirement of CAF-1 for efficient replication in vivo is still debated. Here, we show that, strikingly, Drosophila larval endocycling cells depleted for the CAF-1 large subunit exhibit normal dynamics of progression through endocycles, although accumulating defects, such as perturbation of nucleosomal organisation, reduction of the replication efficiency of euchromatic DNA and accumulation of DNA damage. Given that the endocycle lacks a checkpoint ensuring the replication of the entire genome, the biological context of Drosophila larval development offered a unique opportunity to highlight the requirement of CAF-1 for chromatin organisation and efficient replication processes in vivo, independently of checkpoint activation.

The origin recognition complex is dispensable for endoreplication in Drosophila

The origin recognition complex (ORC) is an essential component of the prereplication complex (pre-RC) in mitotic cell cycles. The role of ORC as a foundation to assemble the pre-RC is conserved from yeast to human. Furthermore, in metazoans ORC plays a key role in determining the timing of replication initiation and origin usage. In this report we have produced and analyzed a Drosophila orc1 allele to investigate the roles of ORC1 in three different modes of DNA replication during development. As expected, ORC1 is essential for mitotic replication and proliferation in brains and imaginal discs, as well as for gene amplification in ovarian follicle cells. Surprisingly, however, ORC1 is not required for endoreplication. Decreased cell number in orc1 mutant salivary glands is consistent with the idea that undetectable levels of maternal ORC1 during embryogenesis fail to support further proliferation. Nevertheless, these cells begin endoreplicating normally and reach a final ploidy of >1000C in the absence of zygotic synthesis of ORC1. The dispensability of ORC is further supported by an examination of other ORC members, whereas Double-parked protein/Cdt1 and minichromosome maintenance proteins are apparently essential for endoreplication, implying that some aspects of initiation are shared among the three modes of DNA replication. This study provides insight into the physiologic roles of ORC during metazoan development and proposes that DNA replication initiation is governed differently in mitotic and endocycles.

Local DNA underreplication correlates with accumulation of phosphorylated H2Av in the Drosophila melanogaster polytene chromosomes

DNA in Drosophila melanogaster polytene chromosomes is known to be locally underreplicated in both pericentric and intercalary heterochromatin. When the SuUR gene is mutant, complete and partial suppression of underreplication are observed in intercalary and pericentric heterochromatin, respectively; in contrast, overexpression of SuUR results in stronger underreplication. Using antibodies against phosphorylated histone H2Av and flies with different levels of SuUR expression, we demonstrated a clear correlation between the extent of underreplication in specific chromosome regions and the accumulation of H2Av phosphorylated at S137 (gamma-H2AX) at the same sites. Phosphorylated H2Av is a well-established marker of DNA double-stranded breaks (DSB). Our data thus argue that DNA underreplication leads to DSBs and that DSBs accumulate as salivary gland cells progress throughout repeated endocycles. We speculate that ligation of free double-stranded DNA termini causes the formation of ectopic contacts between the underreplicated regions in heterochromatin.

APC/CFzr/Cdh1 promotes cell cycle progression during the Drosophila endocycle

The endocycle is a commonly observed variant cell cycle in which cells undergo repeated rounds of DNA replication with no intervening mitosis. How the cell cycle machinery is modified to transform a mitotic cycle into endocycle has long been a matter of interest. In both plants and animals, the transition from the mitotic cycle to the endocycle requires Fzr/Cdh1, a positive regulator of the Anaphase-Promoting Complex/Cyclosome (APC/C). However, because many of its targets are transcriptionally downregulated upon entry into the endocycle, it remains unclear whether the APC/C functions beyond the mitotic/endocycle boundary. Here, we report that APC/C Fzr/Cdh1 activity is required to promote the G/S oscillation of the Drosophila endocycle. We demonstrate that compromising APC/C activity, after cells have entered the endocycle, inhibits DNA replication and results in the accumulation of multiple APC/C targets, including the mitotic cyclins and Geminin. Notably, our data suggest that the activity of APC/C Fzr/Cdh1 during the endocycle is not continuous but is cyclic, as demonstrated by the APC/C-dependent oscillation of the pre-replication complex component Orc1. Taken together, our data suggest a model in which the cyclic activity of APC/C Fzr/Cdh1 during the Drosophila endocycle is driven by the periodic inhibition of Fzr/Cdh1 by Cyclin E/Cdk2. We propose that, as is observed in mitotic cycles, during endocycles, APC/C Fzr/Cdh1 functions to reduce the levels of the mitotic cyclins and Geminin in order to facilitate the relicensing of DNA replication origins and cell cycle progression.

Normal regulation of Rbf1/E2f1 target genes in Drosophila type 1 protein phosphatase mutants

G1 Cyclin/Cdk complexes phosphorylate and inactivate the pRb tumor suppressor by preventing its ability to bind and repress E2F transcription factors. Current molecular and biochemical evidence suggests that type 1 protein phosphatases (PP1) dephosphorylate and thereby activate pRb, but the functional significance of this has not been addressed in the context of animal development. Here, we use genetic analyses to determine the role of PP1 in the regulation of Rbf1 activity during Drosophila development. While Rbf1 is required for E2f1 inhibition and G1 arrest in the embryonic epidermis and for the periodic expression of E2f1 target genes during endocycle S phase in the embryonic midgut and larval salivary gland, PP1 is not. PP1 regulates periodic cyclin E protein accumulation in ovarian nurse cells independently of Rbf1, which is dispensable for endocycle regulation in this tissue. We conclude that PP1 is not a major regulator of the Rbf1/E2F1 pathway in Drosophila.