New insights into cell cycle control from the Drosophila endocycle

Endopolyploidy changes with age-related polyethism in the honey bee, Apis mellifera

Honey bees (Apis mellifera) exhibit age polyethism, whereby female workers assume increasingly complex colony tasks as they age. While changes in DNA methylation accompany age polyethism, other DNA modifications accompanying age polyethism are less known. Changes in endopolyploidy (DNA amplification in the absence of cell division) with increased larval age are typical in many insect cells and are essential in adults for creating larger cells, more copies of essential loci, or greater storage capacity in secretory cells. However, changes in endopolyploidy with increased adult worker age and polyethism are unstudied. In this study, we examined endopolyploidy in honey bee workers ranging in age from newly emerged up to 55 days old. We found a nonsignificant increase in ploidy levels with age (P < 0.1) in the most highly endopolyploid secretory cells, the Malpighian tubules. All other cell types decreased ploidy levels with age. Endopolyploidy decreased the least amount (nonsignificant) in neural (brain) cells and the stinger (P < 0.1). There was a significant reduction of endopolyploidy with age in leg (P < 0.05) and thoracic (P < 0.001) muscles. Ploidy in thoracic muscle dropped from an average of 0.5 rounds of replication in newly emerged workers to essentially no rounds of replication (0.125) in the oldest workers. Ploidy reduction in flight muscle cells is likely due to the production of G1 (2C) nuclei by amitotic division in the multinucleate striated flight muscles that are essential to foragers, the oldest workers. We suggest that ploidy is constrained by the shape, size and makeup of the multinucleate striated muscle cells. Furthermore, the presence of multiple 2C nuclei might be optimal for cell function, while higher ploidy levels might be a dead-end strategy of some aging adult tissues, likely used to increase cell size and storage capacity in secretory cells.

DNA copy-number control through inhibition of replication fork progression

Proper control of DNA replication is essential to ensure faithful transmission of genetic material and prevent chromosomal aberrations that can drive cancer progression and developmental disorders. DNA replication is regulated primarily at the level of initiation and is under strict cell-cycle regulation. Importantly, DNA replication is highly influenced by developmental cues. In Drosophila, specific regions of the genome are repressed for DNA replication during differentiation by the SNF2 domain-containing protein SUUR through an unknown mechanism. We demonstrate that SUUR is recruited to active replication forks and mediates the repression of DNA replication by directly inhibiting replication fork progression instead of functioning as a replication fork barrier. Mass spectrometry identification of SUUR-associated proteins identified the replicative helicase member CDC45 as a SUUR-associated protein, supporting a role for SUUR directly at replication forks. Our results reveal that control of eukaryotic DNA copy number can occur through the inhibition of replication fork progression.

Endoreduplication and fruit growth in tomato: evidence in favour of the karyoplasmic ratio theory

The growth of a plant organ depends upon the developmental processes of cell division and cell expansion. The activity of cell divisions sets the number of cells that will make up the organ; the cell expansion activity then determines its final size. Among the various mechanisms that may influence the determination of cell size, endopolyploidy by means of endoreduplication appears to be of great importance in plants. Endoreduplication is widespread in plants and supports the process of differentiation of cells and organs. Its functional role in plant cells is not fully understood, although it is commonly associated with ploidy-dependent cell expansion. During the development of tomato fruit, cells from the (fleshy) pericarp tissue become highly polyploid, reaching a DNA content barely encountered in other plant species (between 2C and 512C). Recent investigations using tomato fruit development as a model provided new data in favour of the long-standing karyoplasmic ratio theory, stating that cells tend to adjust their cytoplasmic volume to the nuclear DNA content. By establishing a highly structured cellular system where multiple physiological functions are integrated, endoreduplication does act as a morphogenetic factor supporting cell growth during tomato fruit development.

Impact of nuclear Piwi elimination on chromatin state in Drosophila melanogaster ovaries

The Piwi-interacting RNA (piRNA)-interacting Piwi protein is involved in transcriptional silencing of transposable elements in ovaries of Drosophila melanogaster. Here we characterized the genome-wide effect of nuclear Piwi elimination on the presence of the heterochromatic H3K9me3 mark and HP1a, as well as on the transcription-associated mark H3K4me2. Our results demonstrate that a significant increase in the H3K4me2 level upon nuclear Piwi loss is not accompanied by the alterations in H3K9me3 and HP1a levels for several germline-expressed transposons, suggesting that in this case Piwi prevents transcription by a mechanism distinct from H3K9 methylation. We found that the targets of Piwi-dependent chromatin repression are mainly related to the elements that display a higher level of H3K4me2 modification in the absence of silencing, i.e. most actively transcribed elements. We also show that Piwi-guided silencing does not significantly influence the chromatin state of dual-strand piRNA-producing clusters. In addition, host protein-coding gene expression is essentially not affected due to the nuclear Piwi elimination, but we noted an increase in small nuclear spliceosomal RNAs abundance and propose Piwi involvement in their post-transcriptional regulation. Our work reveals new aspects of transposon silencing in Drosophila, indicating that transcription of transposons can underpin their Piwi dependent silencing, while canonical heterochromatin marks are not obligatory for their repression.

GLABROUS INFLORESCENCE STEMS regulates trichome branching by genetically interacting with SIM in Arabidopsis

Arabidopsis trichomes are large branched single cells that protrude from the epidermis. The first morphological indication of trichome development is an increase in nuclear content resulting from an initial cycle of endoreduplication. Our previous study has shown that the C2H2 zinc finger protein GLABROUS INFLORESCENCE STEMS (GIS) is required for trichome initiation in the inflorescence organ and for trichome branching in response to gibberellic acid signaling, although GIS gene does not play a direct role in regulating trichome cell division. Here, we describe a novel role of GIS, controlling trichome cell division indirectly by interacting genetically with a key endoreduplication regulator SIAMESE (SIM). Our molecular and genetic studies have shown that GIS might indireclty control cell division and trichome branching by acting downstream of SIM. A loss of function mutation of SIM signficantly reduced the expression of GIS. Futhermore, the overexpression of GIS rescued the trichome cluster cell phenotypes of sim mutant. The gain or loss of function of GIS had no significant effect on the expression of SIM. These results suggest that GIS may play an indirect role in regulating trichome cell division by genetically interacting with SIM.

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.

Cyclin E drives human keratinocyte growth into differentiation

Human epidermis is continuously exposed to environmental mutagenic hazard and is the most frequent target of human cancer. How the epidermis coordinates proliferation with differentiation to maintain homeostasis, even in hyperproliferative conditions, is unclear. For instance, overactivation of the proto-oncogene MYC in keratinocytes stimulates differentiation. Here we explore the cell cycle regulation as proliferating human keratinocytes commit to terminal differentiation upon loss of anchorage or overactivation of MYC. The S-phase of the cell cycle is deregulated as mitotic regulators are inhibited in the onset of differentiation. Experimental inhibition of mitotic kinase cdk1 or kinases of the mitosis spindle checkpoint Aurora B or Polo-like Kinase, triggered keratinocyte terminal differentiation. Furthermore, hyperactivation of the cell cycle by overexpressing the DNA replication regulator Cyclin E induced mitosis failure and differentiation. Inhibition of Cyclin E by shRNAs attenuated the induction of differentiation by MYC. In addition, we present evidence that Cyclin E induces DNA damage and the p53 pathway. The results provide novel clues for the mechanisms committing proliferative keratinocytes to differentiate, with implications for tissue homeostasis maintenance, HPV amplification and tumorigenesis.

The mysterious human epidermal cell cycle, or an oncogene-induced differentiation checkpoint

Fifteen years ago, we reported that proto-oncogene MYC promoted differentiation of human epidermal stem cells, a finding that was surprising to the MYC and the skin research communities. MYC was one of the first human oncogenes identified, and it had been strongly associated with proliferation. However, it was later shown that MYC could induce apoptosis under low survival conditions. Currently, the notion that MYC promotes epidermal differentiation is widely accepted, but the cell cycle mechanisms that elicit this function remain unresolved. We have recently reported that keratinocytes respond to cell cycle deregulation and DNA damage by triggering terminal differentiation. This mechanism might constitute a homeostatic protection face to cell cycle insults. Here, I discuss recent and not-so-recent evidence suggesting the existence of a largely unexplored oncogene-induced differentiation response (OID) analogous to oncogene-induced apoptosis (OIA) or senescence (OIS). In addition, I propose a model for the role of the cell cycle in skin homeostasis maintenance and for the dual role of MYC in differentiation.

Canonical and atypical E2Fs regulate the mammalian endocycle

The endocycle is a variant cell cycle consisting of successive DNA synthesis and Gap phases that yield highly polyploid cells. Although essential for metazoan development, relatively little is known about its control or physiologic role in mammals. Using novel lineage-specific cre mice we identified two opposing arms of the E2F program, one driven by canonical transcription activation (E2F1, E2F2 and E2F3) and the other by atypical repression (E2F7 and E2F8), that converge on the regulation of endocycles in vivo. Ablation of canonical activators in the two endocycling tissues of mammals, trophoblast giant cells in the placenta and hepatocytes in the liver, augmented genome ploidy, whereas ablation of atypical repressors diminished ploidy. These two antagonistic arms coordinate the expression of a unique G₂/M transcriptional program that is critical for mitosis, karyokinesis and cytokinesis. These results provide in vivo evidence for a direct role of E2F family members in regulating non-traditional cell cycles in mammals.

"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.

Dynamics of endoreplication during Drosophila posterior scutellar macrochaete development

Endoreplication is a variant type of DNA replication, consisting only of alternating G1 and S phases. Many types of Drosophila tissues undergo endoreplication. However, the timing and the extent to which a single endocycling macrochaete undergoes temporally programmed endoreplication during development are unclear. Here, we focused on the dynamics of endoreplication during posterior scutellar (pSC) macrochaete development. Quantitative analyses of C values in shaft cells and socket cells revealed a gradual rise from 8C and 4C at 8 hours after pupal formation (APF) to 72C and 24C at 29 hours APF, respectively. The validity of the values was further confirmed by the measurement of DNA content with a confocal laser microscope. BrdU incorporation assays demonstrated that shaft cells undergo four rounds of endoreplication from 18 to 29.5 hours APF. In contrast, socket cells undergo two rounds of endoreplication during the same period. Statistical analyses showed that the theoretical C values, based on BrdU assays, nearly coincide with the actually measured C values in socket cells, but not in shaft cells after 22 hours APF. These analyses suggest that socket cells undergo two rounds of endoreplication. However, the mechanism of endoreplication in the shaft cells may change from 22 hours APF, suggesting the possibility that shaft cells undergo two or four rounds of endoreplication during the periods. We also found that the timing of endoreplication differs, depending on the type of macrochaete. Moreover, endocycling in shaft cells of both the left and right sides of pSC bristle lineages occurs in the same pattern, indicating that the process is synchronized for specific types of macrochaete. Our findings suggest that endocycling in macrochaete cell lineages can be a model for understanding mechanisms of endoreplication at the single-cell level.

GIGAS CELL1, a novel negative regulator of the anaphase-promoting complex/cyclosome, is required for proper mitotic progression and cell fate determination in Arabidopsis

Increased cellular ploidy is widespread during developmental processes of multicellular organisms, especially in plants. Elevated ploidy levels are typically achieved either by endoreplication or endomitosis, which are often regarded as modified cell cycles that lack an M phase either entirely or partially. We identified GIGAS CELL1 (GIG1)/OMISSION OF SECOND DIVISION1 (OSD1) and established that mutation of this gene triggered ectopic endomitosis. On the other hand, it has been reported that a paralog of GIG1/OSD1, UV-INSENSITIVE4 (UVI4), negatively regulates endoreplication onset in Arabidopsis thaliana. We showed that GIG1/OSD1 and UVI4 encode novel plant-specific inhibitors of the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase. These proteins physically interact with APC/C activators, CDC20/FZY and CDH1/FZR, in yeast two-hybrid assays. Overexpression of CDC20.1 and CCS52B/FZR3 differentially promoted ectopic endomitosis in gig1/osd1 and premature occurrence of endoreplication in uvi4. Our data suggest that GIG1/OSD1 and UVI4 may prevent an unscheduled increase in cellular ploidy by preferentially inhibiting APC/C(CDC20) and APC/C(FZR), respectively. Generation of cells with a mixed identity in gig1/osd1 further suggested that the APC/C may have an unexpected role for cell fate determination in addition to its role for proper mitotic progression.

Separation of stem cell maintenance and transposon silencing functions of Piwi protein

Piwi-interacting RNAs (piRNAs) and Piwi proteins have the evolutionarily conserved function of silencing of repetitive genetic elements in germ lines. The founder of the Piwi subfamily, Drosophila nuclear Piwi protein, was also shown to be required for the maintenance of germ-line stem cells (GSCs). Hence, null mutant piwi females exhibit two types of abnormalities, overexpression of transposons and severely underdeveloped ovaries. It remained unknown whether the failure of GSC maintenance is related to transposon derepression or if GSC self-renewal and piRNA silencing are two distinct functions of the Piwi protein. We have revealed a mutation, piwi(Nt), removing the nuclear localization signal of the Piwi protein. piwi(Nt) females retain the ability of GSC self-renewal and a near-normal number of egg chambers in the ovarioles but display a drastic transposable element derepression and nuclear accumulation of their transcripts in the germ line. piwi(Nt) mutants are sterile most likely because of the disturbance of piRNA-mediated transposon silencing. Analysis of chromatin modifications in the piwi(Nt) ovaries indicated that Piwi causes chromatin silencing only of certain types of transposons, whereas others are repressed in the nuclei without their chromatin modification. Thus, Piwi nuclear localization that is required for its silencing function is not essential for the maintenance of GSCs. We suggest that the Piwi function in GSC self-renewal is independent of transposon repression and is normally realized in the cytoplasm of GSC niche cells.

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.

Invertebrate models of spinal muscular atrophy: insights into mechanisms and potential therapeutics

Invertebrate genetic models with their tractable neuromuscular systems are effective vehicles for the study of human nerve and muscle disorders. This is exemplified by insights made into spinal muscular atrophy (SMA) using the fruit fly Drosophila melanogaster and the nematode worm Caenorhabditis elegans. For speed and economy, these invertebrates offer convenient, whole-organism platforms for genetic screening as well as RNA interference (RNAi) and chemical library screens, permitting the rapid testing of hypotheses related to disease mechanisms and the exploration of new therapeutic routes and drug candidates. Here, we discuss recent developments encompassing synaptic physiology, RNA processing, and screening of compound and genome-scale RNAi libraries, showcasing the importance of invertebrate SMA models.

Safeguarding genetic information in Drosophila

Eukaryotic cells employ a plethora of conserved proteins and mechanisms to ensure genome integrity. In metazoa, these mechanisms must operate in the context of organism development. This mini-review highlights two emerging features of DNA damage responses in Drosophila: a crosstalk between DNA damage responses and components of the spindle assembly checkpoint, and increasing evidence for the effect of DNA damage on the developmental program at multiple points during the Drosophila life cycle.

Mitosis in vertebrates: the G2/M and M/A transitions and their associated checkpoints

In this review, I stress the importance of direct data and accurate terminology when formulating and communicating conclusions on how the G2/M and metaphase/anaphase transitions are regulated. I argue that entry into mitosis (i.e., the G2/M transition) is guarded by several checkpoint control pathways that lose their ability to delay or stop further cell cycle progression once the cell becomes committed to divide, which in vertebrates occurs in the late stages of chromosome condensation. After this commitment, progress through mitosis is then mediated by a single Mad/Bub-based checkpoint that delays chromatid separation, and exit from mitosis (i.e., completion of the cell cycle) in the presence of unattached kinetochores. When cells cannot satisfy the mitotic checkpoint, e.g., when in concentrations of spindle poisons that prohibit the stable attachment of all kinetochores, they are delayed in mitosis for many hours. In normal cells, the duration of this delay depends on the organism and ranges from ∼4 h in rodents to ∼22 h in humans. Recent live cell studies reveal that under this condition, many cancer cells (including HeLa and U2OS) die in mitosis by apoptosis within ∼24 h, which implies that biochemical studies on cancer cell populations harvested in mitosis after a prolonged mitotic arrest are contaminated with dead or dying cells.

At the crossroads of differentiation and proliferation: precise control of cell-cycle changes by multiple signaling pathways in Drosophila follicle cells

Here, we discuss the findings to date about genes and pathways required for regulation of somatic follicle-cell proliferation and differentiation during Drosophila oogenesis and demonstrate how loss of these genes contributes to the tumorigenic potential of mutant cells. Follicle cells undergo cell-fate determination through stepwise activation of multiple signaling pathways, including the Notch, Hedgehog, Wingless, janus kinase/STAT, and JNK pathways. In addition, changes in DNA replication and cellular growth depend on the spatial and temporal activation of the mitotic cycle-endocycle and endocycle-gene amplification cell-cycle switches and insulin-dependent monitoring of cellular health; systemic loss of these pathways contributes to loss of controlled cellular proliferation, loss of differentiation/growth, and aberrant cell polarity in follicle cells. We also highlight the effects of the neoplastic and Hippo pathways on the cell cycle and cellular proliferation in promoting normal development and conclude that lack of coordination of multiple signaling pathways promotes conditions favorable for tumorigenesis.

C. elegans MCM-4 is a general DNA replication and checkpoint component with an epidermis-specific requirement for growth and viability

DNA replication and its connection to M phase restraint are studied extensively at the level of single cells but rarely in the context of a developing animal. C. elegans lin-6 mutants lack DNA synthesis in postembryonic somatic cell lineages, while entry into mitosis continues. These mutants grow slowly and either die during larval development or develop into sterile adults. We found that lin-6 corresponds to mcm-4 and encodes an evolutionarily conserved component of the MCM2-7 pre-RC and replicative helicase complex. The MCM-4 protein is expressed in all dividing cells during embryonic and postembryonic development and associates with chromatin in late anaphase. Induction of cell cycle entry and differentiation continues in developing mcm-4 larvae, even in cells that went through abortive division. In contrast to somatic cells in mcm-4 mutants, the gonad continues DNA replication and cell division until late larval development. Expression of MCM-4 in the epidermis (also known as hypodermis) is sufficient to rescue the growth retardation and lethality of mcm-4 mutants. While the somatic gonad and germline show substantial ability to cope with lack of zygotic mcm-4 function, mcm-4 is specifically required in the epidermis for growth and survival of the whole organism. Thus, C. elegans mcm-4 has conserved functions in DNA replication and replication checkpoint control but also shows unexpected tissue-specific requirements.

Developmental control of the DNA replication and transcription programs

Polyploid or polytene cells, which have more than 2C DNA content, are widespread throughout nature and present in most differentiated Drosophila tissues. These cells also can display differential replication, that is, genomic regions of increased or decreased DNA copy number relative to overall genomic ploidy. How frequently differential replication is used as a developmental strategy remains unclear. Here, we use genome-wide array-based comparative genomic hybridization (aCGH) to profile differential DNA replication in isolated and purified larval fat body and midgut tissues of Drosophila, and we compare them with recent aCGH profiles of the larval salivary gland. We identify sites of euchromatic underreplication that are common to all three tissues and others that are tissue specific. We demonstrate that both common and tissue-specific underreplicated sites are dependent on the Suppressor of Underreplication protein, SUUR. mRNA-seq profiling shows that whereas underreplicated regions are generally transcriptionally silent in the larval midgut and salivary gland, transcriptional silencing and underreplication have been uncoupled in the larval fat body. In addition to revealing the prevalence of differential replication, our results show that transcriptional silencing and underreplication can be mechanistically uncoupled.

Drug-induced cell cycle modulation leading to cell-cycle arrest, nuclear mis-segregation, or endoreplication

BACKGROUND

Cancer cell responses to chemotherapeutic agents vary, and this may reflect different defects in DNA repair, cell-cycle checkpoints, and apoptosis control. Cytometry analysis only quantifies dye-incorporation to examine DNA content and does not reflect the biological complexity of the cell cycle in drug discovery screens.

RESULTS

Using population and time-lapse imaging analyses of cultured immortalized cells expressing a new version of the fluorescent cell-cycle indicator, Fucci (Fluorescent Ubiquitination-based Cell Cycle Indicator), we found great diversity in the cell-cycle alterations induced by two anticancer drugs. When treated with etoposide, an inhibitor of DNA topoisomerase II, HeLa and NMuMG cells halted at the G2/M checkpoint. HeLa cells remained there, but NMuMG cells then overrode the checkpoint and underwent nuclear mis-segregation or avoided the checkpoint and entered the endoreplication cycle in a drug concentration dependent manner. In contrast, an inhibitor of Cdk4 led to G1 arrest or endoreplication in NMuMG cells depending upon the initial cell-cycle phase of drug exposure.

CONCLUSIONS

Drug-induced cell cycle modulation varied not only between different cell types or following treatment with different drugs, but also between cells treated with different concentrations of the same drug or following drug addition during different phases of the cell cycle. By combining cytometry analysis with the Fucci probe, we have developed a novel assay that fully integrates the complexity of cell cycle regulation into drug discovery screens. This assay system will represent a powerful drug-discovery tool for the development of the next generation of anti-cancer therapies.

Control of neuronal ploidy during vertebrate development

Somatic tetraploid neurons are present in different structures of the vertebrate nervous system, including cortex and retina. In this chapter, we provide evidence that these neurons can be widely detected in the chick nervous system. We also discuss mechanisms creating neuronal tetraploidy in vertebrates, concluding that the neurotrophin receptor p75 could be responsible for the generation of these neurons in most neural tissues, as previously observed in the retina. Somatic tetraploidy in the chick retina correlates with increased neurons' soma size and dendritic arborization, giving rise to neurons known to innervate a specific layer of the optic tectum. Tetraploidy could therefore account for neuronal diversity in the normal nervous system. De novo generation of tetraploid neurons has been shown to occur in Alzheimer's disease. This suggests that the morphological changes expected to occur in the affected neurons could lead to altered neuronal function, thus providing a basis for neurodegeneration.

A mitosis block links active cell cycle with human epidermal differentiation and results in endoreplication

How human self-renewal tissues co-ordinate proliferation with differentiation is unclear. Human epidermis undergoes continuous cell growth and differentiation and is permanently exposed to mutagenic hazard. Keratinocytes are thought to arrest cell growth and cell cycle prior to terminal differentiation. However, a growing body of evidence does not satisfy this model. For instance, it does not explain how skin maintains tissue structure in hyperproliferative benign lesions. We have developed and applied novel cell cycle techniques to human skin in situ and determined the dynamics of key cell cycle regulators of DNA replication or mitosis, such as cyclins E, A and B, or members of the anaphase promoting complex pathway: cdc14A, Ndc80/Hec1 and Aurora kinase B. The results show that actively cycling keratinocytes initiate terminal differentiation, arrest in mitosis, continue DNA replication in a special G2/M state, and become polyploid by mitotic slippage. They unambiguously demonstrate that cell cycle progression coexists with terminal differentiation, thus explaining how differentiating cells increase in size. Epidermal differentiating cells arrest in mitosis and a genotoxic-induced mitosis block rapidly pushes epidermal basal cells into differentiation and polyploidy. These observations unravel a novel mitosis-differentiation link that provides new insight into skin homeostasis and cancer. It might constitute a self-defence mechanism against oncogenic alterations such as Myc deregulation.

Cell type-dependent requirement for PIP box-regulated Cdt1 destruction during S phase

Previous studies have shown that Cdt1 overexpression in cultured cells can trigger re-replication, but not whether CRL4^Cdt2-triggered destruction of Cdt1 is required for normal mitotic cell cycle progression in vivo. We demonstrate that PIP box–mediated destruction of Cdt1^Dup during S phase is necessary for the cell division cycle in Drosophila.

DNA synthesis–coupled proteolysis of the prereplicative complex component Cdt1 by the CRL4^Cdt2 E3 ubiquitin ligase is thought to help prevent rereplication of the genome during S phase. To directly test whether CRL4^Cdt2-triggered destruction of Cdt1 is required for normal cell cycle progression in vivo, we expressed a mutant version of Drosophila Cdt1 (Dup), which lacks the PCNA-binding PIP box (Dup^ΔPIP) and which cannot be regulated by CRL4^Cdt2. Dup^ΔPIP is inappropriately stabilized during S phase and causes developmental defects when ectopically expressed. Dup^ΔPIP restores DNA synthesis to dup null mutant embryonic epidermal cells, but S phase is abnormal, and these cells do not progress into mitosis. In contrast, Dup^ΔPIP accumulation during S phase did not adversely affect progression through follicle cell endocycles in the ovary. In this tissue the combination of Dup^ΔPIP expression and a 50% reduction in Geminin gene dose resulted in egg chamber degeneration. We could not detect Dup hyperaccumulation using mutations in the CRL4^Cdt2 components Cul4 and Ddb1, likely because these cause pleiotropic effects that block cell proliferation. These data indicate that PIP box–mediated destruction of Dup is necessary for the cell division cycle and suggest that Geminin inhibition can restrain Dup^ΔPIP activity in some endocycling cell types.

Endoreplication: a molecular trick during animal neuron evolution

The occurrence of endoreplication has been repeatedly reported in many organisms, including protists, plants, worms, arthropods, molluscs, fishes, and mammals. As a general rule, cells possessing endoreplicated genomes are large-sized and highly metabolically active. Endoreplication has not been frequently reported in neuronal cells that are typically considered to be fully differentiated and non-dividing, and which normally contain a diploid genome. Despite this general statement, various papers indicate that giant neurons in molluscs, as well as supramedullary and hypothalamic magnocellular neurons in fishes, contain DNA amounts larger than 2C. In order to study this issue in greater detail here, we review the available data about endoreplication in invertebrate and vertebrate neurons, and discuss its possible functional significance. As a whole, endoreplication seems to be a sort of molecular trick used by neurons in response to the high functional demands that they experience during evolution.

A developmentally regulated gene, ASI2, is required for endocycling in the macronuclear anlagen of Tetrahymena

Ciliated protozoa contain two types of nuclei, germ line micronuclei (Mic) and transcriptionally active macronuclei (Mac). During sexual reproduction, the parental Mac degenerates and a new Mac develops from a mitotic product of the zygotic Mic. Macronuclear development involves extensive endoreplication of the genome. The present study shows that endoreplication of macronuclear DNA in Tetrahymena is an example of endocyling, a variant of the mitotic cycle with alternating S and G phases in the absence of cell division. Thus, endocycling is conserved from ciliates to multicellular organisms. The gene ASI2 in Tetrahymena thermophila encodes a putative signal transduction receptor. ASI2 is nonessential for vegetative growth, but it is upregulated during development of the new Mac. Cells that lack ASI2 in the developing Mac anlagen are arrested in endoreplication of the DNA and die. This study shows that ASI2 is also transcribed in the parental Mac early in conjugation and that transcription of ASI2 in the parental Mac supports endoreplication of the DNA during early stages of development of the Mac anlagen. Other molecular events in Mac anlage development, including developmentally regulated DNA rearrangement, occur normally in matings between ASI2 knockouts, suggesting that ASI2 specifically regulates endocycling in Tetrahymena.

Somatic polyploidy promotes cell function under stress and energy depletion: evidence from tissue-specific mammal transcriptome

Polyploid cells show great among-species and among-tissues diversity and relation to developmental mode, suggesting their importance in adaptive evolution and developmental programming. At the same time, excessive polyploidization is a hallmark of functional impairment, aging, growth disorders, and numerous pathologies including cancer and cardiac diseases. To shed light on this paradox and to find out how polyploidy contributes to organ functions, we review here the ploidy-associated shifts in activity of narrowly expressed (tissue specific) genes in human and mouse heart and liver, which have the reciprocal pattern of polyploidization. For this purpose, we use the modular biology approach and genome-scale cross-species comparison. It is evident from this review that heart and liver show similar traits in response to polyploidization. In both organs, polyploidy protects vitality (mainly due to the activation of sirtuin-mediated pathways), triggers the reserve adenosine-5'-triphosphate (ATP) production, and sustains tissue-specific functions by switching them to energy saving mode. In heart, the strongest effects consisted in the concerted up-regulation of contractile proteins and substitution of energy intensive proteins with energy economic ones. As a striking example, the energy intensive alpha myosin heavy chain (providing fast contraction) decreased its expression by a factor of 10, allowing a 270-fold increase of expression of beta myosin heavy chain (providing slow contraction), which has approximately threefold lower ATP-hydrolyzing activity. The liver showed the enhancement of immunity, reactive oxygen species and xenobiotic detoxication, and numerous metabolic adaptations to long-term energy depletion. Thus, somatic polyploidy may be an ingenious evolutionary instrument for fast adaptation to stress and new environments allowing trade-offs between high functional demand, stress, and energy depletion.

Drosophila microRNAs 263a/b confer robustness during development by protecting nascent sense organs from apoptosis

miR-263a/b are members of a conserved family of microRNAs that are expressed in peripheral sense organs across the animal kingdom. Here we present evidence that miR-263a and miR-263b play a role in protecting Drosophila mechanosensory bristles from apoptosis by down-regulating the pro-apoptotic gene head involution defective. Both microRNAs are expressed in the bristle progenitors, and despite a difference in their seed sequence, they share this key common target. In miR-263a and miR-263b deletion mutants, loss of bristles appears to be sporadic, suggesting that the role of the microRNAs may be to ensure robustness of the patterning process by promoting survival of these functionally specified cells. In the context of the retina, this mechanism ensures that the interommatidial bristles are protected during the developmentally programmed wave of cell death that prunes excess cells in order to refine the pattern of the pupal retina.

Persistent p21 expression after Nutlin-3a removal is associated with senescence-like arrest in 4N cells

Nutlin-3a is a preclinical drug that stabilizes p53 by blocking the interaction between p53 and MDM2. In our previous study, Nutlin-3a promoted a tetraploid G(1) arrest in two p53 wild-type cell lines (HCT116 and U2OS), and both cell lines underwent endoreduplication after Nutlin-3a removal. Endoreduplication gave rise to stable tetraploid clones resistant to therapy-induced apoptosis. Prior knowledge of whether cells are susceptible to Nutlin-induced endoreduplication and therapy resistance could help direct Nutlin-3a-based therapies. In the present study, Nutlin-3a promoted a tetraploid G(1) arrest in multiple p53 wild-type cell lines. However, some cell lines underwent endoreduplication to relatively high extents after Nutlin-3a removal whereas other cell lines did not. The resistance to endoreduplication observed in some cell lines was associated with a prolonged 4N arrest after Nutlin-3a removal. Knockdown of either p53 or p21 immediately after Nutlin-3a removal could drive endoreduplication in otherwise resistant 4N cells. Finally, 4N-arrested cells retained persistent p21 expression; expressed senescence-associated beta-galactosidase; displayed an enlarged, flattened phenotype; and underwent a proliferation block that lasted at least 2 weeks after Nutlin-3a removal. These findings demonstrate that transient Nutlin-3a treatment can promote an apparently permanent proliferative block in 4N cells of certain cell lines associated with persistent p21 expression and resistance to endoreduplication.

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.

A JNK inhibitor SP600125 induces defective cytokinesis and enlargement in P19 embryonal carcinoma cells

While analyzing the role of c-Jun NH(2)-terminal kinase (JNK) in neurogenesis in P19 embryonal carcinoma cells, we noticed that treatment with SP600125, a JNK inhibitor, increased the cell size markedly. SP600125-induced enlargement of P19 cells was time- and dose-dependent. The increased cell size in response to SP600125 was also detected in B6mt-1 embryonic stem cells. SP600125 treatment inhibited cell growth and increased DNA contents, indicating the inhibition of cell proliferation resulting from endoreduplication. Concurrently, the gene expression of p21, a regulator of G2/M arrest as well as G1 arrest, was increased in cells treated with SP600125. The increased cell size in response to SP600125 was detected even in P19 cells treated with colcemide, an inhibitor of cell cycle progression at the metaphase. The present study suggests that treatment with SP600125 progresses the cell cycle, skipping cytokinesis in P19 cells.

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.

Coupling the cell cycle to development

The core machinery that drives the eukaryotic cell cycle has been thoroughly investigated over the course of the past three decades. It is only more recently, however, that light has been shed on the mechanisms by which elements of this core machinery are modulated to alter cell cycle progression during development. It has also become increasingly clear that, conversely, core cell cycle regulators can play a crucial role in developmental processes. Here, focusing on findings from Drosophila melanogaster and Caenorhabditis elegans, we review the importance of modulating the cell cycle during development and discuss how core cell cycle regulators participate in determining cell fates.

Translational repression of cyclin E prevents precocious mitosis and embryonic gene activation during C. elegans meiosis

Germ cells, the cells that give rise to sperm and egg, maintain the potential to recreate all cell types in a new individual. This wide developmental potential, or totipotency, is manifested in unusual tumors called teratomas, in which germ cells undergo somatic differentiation. Although recent studies have implicated RNA regulation, the mechanism that normally prevents the loss of germ cell identity remains unexplained. In C. elegans, a teratoma is induced in the absence of the conserved RNA-binding protein GLD-1. Here, we demonstrate that GLD-1 represses translation of CYE-1/cyclin E during meiotic prophase, which prevents germ cells from re-entering mitosis and inducing embryonic-like transcription. We describe a mechanism that prevents precocious mitosis in germ cells undergoing meiosis, propose that this mechanism maintains germ cell identity by delaying the onset of embryonic gene activation until after fertilization, and provide a paradigm for the possible origin of human teratomas.

Control of division and differentiation of plant stem cells and their derivatives

The core mechanism of the plant cell cycle is conserved with all other eukaryotes but several aspects are unique to plant cells. Key characteristics of plant development include indeterminate growth and repetitive organogenesis derived from stem cell pools and they may explain the existence of the high number of cell cycle regulators in plants. In this review, we give an overview of the plant cell cycle and its regulatory components. Furthermore, we discuss the cell cycle aspects of plant stem cell maintenance and how the cell cycle relates to cellular differentiation during development. We exemplify this transition by focusing on organ initiation in the shoot.

Logical modelling of cell cycle control in eukaryotes: a comparative study

Dynamical modelling is at the core of the systems biology paradigm. However, the development of comprehensive quantitative models is complicated by the daunting complexity of regulatory networks controlling crucial biological processes such as cell division, the paucity of currently available quantitative data, as well as the limited reproducibility of large-scale experiments. In this context, qualitative modelling approaches offer a useful alternative or complementary framework to build and analyse simplified, but still rigorous dynamical models. This point is illustrated here by analysing recent logical models of the molecular network controlling mitosis in different organisms, from yeasts to mammals. After a short introduction covering cell cycle and logical modelling, we compare the assumptions and properties underlying these different models. Next, leaning on their transposition into a common logical framework, we compare their functional structure in terms of regulatory circuits. Finally, we discuss assets and prospects of qualitative approaches for the modelling of the cell cycle.

Drosophila Minus is required for cell proliferation and influences Cyclin E turnover

Turnover of cyclins plays a major role in oscillatory cyclin-dependent kinase (Cdk) activity and control of cell cycle progression. Here we present a novel cell cycle regulator, called minus, which influences Cyclin E turnover in Drosophila. minus mutants produce defects in cell proliferation, some of which are attributable to persistence of Cyclin E. Minus protein can interact physically with Cyclin E and the SCF Archipelago/Fbw7/Cdc4 ubiquitin-ligase complex. Minus does not affect dMyc, another known SCF(Ago) substrate in Drosophila. We propose that Minus contributes to cell cycle regulation in part by selectively controlling turnover of Cyclin E.

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.

Integration of Insulin receptor/Foxo signaling and dMyc activity during muscle growth regulates body size in Drosophila

Drosophila larval skeletal muscles are single, multinucleated cells of different sizes that undergo tremendous growth within a few days. The mechanisms underlying this growth in concert with overall body growth are unknown. We find that the size of individual muscles correlates with the number of nuclei per muscle cell and with increasing nuclear ploidy during development. Inhibition of Insulin receptor (InR; Insulin-like receptor) signaling in muscles autonomously reduces muscle size and systemically affects the size of other tissues, organs and indeed the entire body, most likely by regulating feeding behavior. In muscles, InR/Tor signaling, Foxo and dMyc (Diminutive) are key regulators of endoreplication, which is necessary but not sufficient to induce growth. Mechanistically, InR/Foxo signaling controls cell cycle progression by modulating dmyc expression and dMyc transcriptional activity. Thus, maximal dMyc transcriptional activity depends on InR to control muscle mass, which in turn induces a systemic behavioral response to allocate body size and proportions.

Atypical E2Fs: new players in the E2F transcription factor family

As major regulators of the cell cycle, apoptosis and differentiation, E2F transcription factors have been studied extensively in a broad range of organisms. The recent identification of atypical E2F family members further expands our structural, functional and molecular view of the cellular E2F activity. Unlike other family members, atypical E2Fs have a duplicated DNA-binding domain and control gene expression without heterodimerization with dimerization partner proteins. Recently, knockout strategies in plants and mammals have pinpointed that atypical E2Fs have a crucial role in plant cell size control, endocycle regulation, proliferation and apoptotic response upon DNA stress. Their position at the crossroads of proliferation and DNA stress response marks these novel E2F proteins as interesting study objects in the field of tumor biology.

Cell biology and evolution: molecular modules link it all?

Classical studies comparing developing embryos have suggested the importance of modified cell biological processes in the evolution of new phenotypes. Here, I revisit this connection focusing on embryonic development, in particular nematode embryogenesis. I compare phenotypic differences in nematode embryogenesis in two basic cell biological processes, the cell cycle and the localization of the first division axis. The analysis of these and other processes shows that, at the cell biological level, exhaustive variation is found that does not necessarily translate into morphological differences. Modern molecular analyses have led to a view in which molecular complexes, made up of groups of proteins, or modules, that are working together, are responsible for the proper execution of cell biological programs. I discuss how this modular architecture could facilitate the phenotypic changes observed in cell biological processes. Ultimately, understanding the connection between cellular behavior and phenotypic outcome will further elucidate the mechanisms responsible for phenotypic evolution.

Atypical E2F activity restrains APC/CCCS52A2 function obligatory for endocycle onset

The endocycle represents an alternative cell cycle that is activated in various developmental processes, including placental formation, Drosophila oogenesis, and leaf development. In endocycling cells, mitotic cell cycle exit is followed by successive doublings of the DNA content, resulting in polyploidy. The timing of endocycle onset is crucial for correct development, because polyploidization is linked with cessation of cell division and initiation of terminal differentiation. The anaphase-promoting complex/cyclosome (APC/C) activator genes CDH1, FZR, and CCS52 are known to promote endocycle onset in human, Drosophila, and Medicago species cells, respectively; however, the genetic pathways governing development-dependent APC/C(CDH1/FZR/CCS52) activity remain unknown. We report that the atypical E2F transcription factor E2Fe/DEL1 controls the expression of the CDH1/FZR orthologous CCS52A2 gene from Arabidopsis thaliana. E2Fe/DEL1 misregulation resulted in untimely CCS52A2 transcription, affecting the timing of endocycle onset. Correspondingly, ectopic CCS52A2 expression drove cells into the endocycle prematurely. Dynamic simulation illustrated that E2Fe/DEL1 accounted for the onset of the endocycle by regulating the temporal expression of CCS52A2 during the cell cycle in a development-dependent manner. Analogously, the atypical mammalian E2F7 protein was associated with the promoter of the APC/C-activating CDH1 gene, indicating that the transcriptional control of APC/C activator genes by atypical E2Fs might be evolutionarily conserved.

The anaphase-promoting complex/cyclosome (APC/C) is required for rereplication control in endoreplication cycles

Endoreplicating cells undergo multiple rounds of DNA replication leading to polyploidy or polyteny. Oscillation of Cyclin E (CycE)-dependent kinase activity is the main driving force in Drosophila endocycles. High levels of CycE-Cdk2 activity trigger S phase, while down-regulation of CycE-Cdk2 activity is crucial to allow licensing of replication origins. In mitotic cells relicensing in S phase is prevented by Geminin. Here we show that Geminin protein oscillates in endoreplicating salivary glands of Drosophila. Geminin levels are high in S phase, but drop once DNA replication has been completed. DNA licensing is coupled to mitosis through the action of the anaphase-promoting complex/cyclosome (APC/C). We demonstrate that, even though endoreplicating cells never enter mitosis, APC/C activity is required in endoreplicating cells to mediate Geminin oscillation. Down-regulation of APC/C activity results in stabilization of Geminin protein and blocks endocycle progression. Geminin is only abundant in cells with high CycE-Cdk2 activity, suggesting that APC/C-Fzr activity is periodically inhibited by CycE-Cdk2, to prevent relicensing in S-phase cells.

Hepatic endopolyploidy as a cellular consequence of age-specific selection for rate of development in mice

Endopolyploidy is the generation of polyploid cells by DNA replication without subsequent cell division and is correlated with hypertrophic growth or growth via cell size. Thus, selection that alters growth may also change onset and frequency of endopolyploidy as a correlated response. We search for endopolyploidy in the liver in response to age-specific restricted index selection for the rate of development. Polyploidy changes over ontogeny are described in five mouse lines: two selected for divergence in early growth (0-10 days of age), two selected for divergence in late growth (28-56 days of age), and one randombred control. Polyploid cell frequency within each line increased as ontogeny continued, as expected from previous research. However, selection for altered growth clearly plays a role in the frequency and onset of polyploid cells. Lines selected for divergence in early growth have polyploidy differences after weaning that are not seen in adult mice. However, lines selected for divergence in late growth are divergent in frequency of polyploid cells, starting near sexual maturity and continuing into adulthood.

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.