Involvement of p34cdc2 in establishing the dependency of S phase on mitosis

Origins of DNA replication in eukaryotes

Errors occurring during DNA replication can result in inaccurate replication, incomplete replication, or re-replication, resulting in genome instability that can lead to diseases such as cancer or disorders such as autism. A great deal of progress has been made toward understanding the entire process of DNA replication in eukaryotes, including the mechanism of initiation and its control. This review focuses on the current understanding of how the origin recognition complex (ORC) contributes to determining the location of replication initiation in the multiple chromosomes within eukaryotic cells, as well as methods for mapping the location and temporal patterning of DNA replication. Origin specification and configuration vary substantially between eukaryotic species and in some cases co-evolved with gene-silencing mechanisms. We discuss the possibility that centromeres and origins of DNA replication were originally derived from a common element and later separated during evolution.

Fission yeast cell cycle mutants and the logic of eukaryotic cell cycle control

Cell cycle mutants in the budding and fission yeasts have played critical roles in working out how the eukaryotic cell cycle operates and is controlled. The starting point was Lee Hartwell’s 1970s landmark papers describing the first cell division cycle (CDC) mutants in budding yeast. These mutants were blocked at different cell cycle stages and so were unable to complete the cell cycle, thus defining genes necessary for successful cell division. Inspired by Hartwell’s work, I isolated CDC mutants in the very distantly related fission yeast. This started a program of searches for mutants in fission yeast that revealed a range of phenotypes informative about eukaryotic cell cycle control. These included mutants defining genes that were rate-limiting for the onset of mitosis and of the S-phase, that were responsible for there being only one S-phase in each cell cycle, and that ensured that mitosis only took place when S-phase was properly completed. This is a brief account of the discovery of these mutants and how they led to the identification of cyclin-dependent kinases as core to these cell cycle controls.

CHRONOCRISIS: When Cell Cycle Asynchrony Generates DNA Damage in Polyploid Cells

Polyploid cells contain multiple copies of all chromosomes. Polyploidization can be developmentally programmed to sustain tissue barrier function or to increase metabolic potential and cell size. Programmed polyploidy is normally associated with terminal differentiation and poor proliferation capacity. Conversely, non-programmed polyploidy can give rise to cells that retain the ability to proliferate. This can fuel rapid genome rearrangements and lead to diseases like cancer. Here, the mechanisms that generate polyploidy are reviewed and the possible challenges upon polyploid cell division are discussed. The discussion is framed around a recent study showing that asynchronous cell cycle progression (an event that is named "chronocrisis") of different nuclei from a polyploid cell can generate DNA damage at mitotic entry. The potential mechanisms explaining how mitosis in non-programmed polyploid cells can generate abnormal karyotypes and genetic instability are highlighted.

Transcriptional repression of CDC6 and SLD2 during meiosis is associated with production of short heterogeneous RNA isoforms

Execution of the meiotic and mitotic cell division programs requires distinct gene expression patterns. Unlike mitotic cells, meiotic cells reduce ploidy by following one round of DNA replication with two rounds of chromosome segregation (meiosis I and meiosis II). However, the mechanisms by which cells prevent DNA replication between meiosis I and meiosis II are not fully understood. Here, we show that transcriptional repression of two essential DNA replication genes, CDC6 and SLD2, is associated with production of shorter meiosis-specific RNAs containing the 3' end of both genes. Despite the short CDC6 RNA coding for a short protein (Cdc6^short), this protein is not essential for meiosis and it does not have either a positive or negative impact on DNA replication. Production of CDC6^short mRNA does not require the upstream CDC6 promoter (P_CDC6) and is not a processed form of the full-length RNA. Instead, CDC6^short depends on transcription initiation from within the ORF upon repression of P_CDC6. Finally, using CDC6 genes from related yeast, we show that repression of full-length CDC6 mRNA is evolutionarily conserved and that this repression is consistently associated with production of unique short CDC6 RNAs. Together, these data demonstrate that meiotic cells transcriptionally repress full-length CDC6 and SLD2, and that inactivation of P_CDC6 results in heterogeneous transcription initiation from within the CDC6 ORF.

Polyploidy in tissue homeostasis and regeneration

Polyploid cells, which contain multiple copies of the typically diploid genome, are widespread in plants and animals. Polyploidization can be developmentally programmed or stress induced, and arises from either cell-cell fusion or a process known as endoreplication, in which cells replicate their DNA but either fail to complete cytokinesis or to progress through M phase entirely. Polyploidization offers cells several potential fitness benefits, including the ability to increase cell size and biomass production without disrupting cell and tissue structure, and allowing improved cell longevity through higher tolerance to genomic stress and apoptotic signals. Accordingly, recent studies have uncovered crucial roles for polyploidization in compensatory cell growth during tissue regeneration in the heart, liver, epidermis and intestine. Here, we review current knowledge of the molecular pathways that generate polyploidy and discuss how polyploidization is used in tissue repair and regeneration.

Fission yeast ceramide ts mutants cwh43 exhibit defects in G0 quiescence, nutrient metabolism, and lipid homeostasis

Cellular nutrient states control whether cells proliferate, or whether they enter or exit quiescence. Here, we report characterizations of fission yeast temperature-sensitive (ts) mutants of the evolutionarily conserved transmembrane protein, Cwh43, and explore its relevance to utilization of glucose, nitrogen-source, and lipids. GFP-tagged Cwh43 localizes at ER associated with the nuclear envelope and the plasma membrane, as in budding yeast. We found that cwh43 mutants failed to divide in low glucose and lost viability during quiescence under nitrogen starvation. In cwh43 mutant, comprehensive metabolome analysis demonstrated dramatic changes in marker metabolites that altered under low glucose and/or nitrogen starvation, although cwh43 apparently consumed glucose in the culture media. Furthermore, we found that cwh43 mutant had elevated levels of triacylglycerols (TGs) and coenzyme A, and that it accumulated lipid droplets. Notably, TG biosynthesis was required to maintain cell division in cwh43 mutant. Thus, Cwh43 affects utilization of glucose and nitrogen-sources, as well as storage lipid metabolism. These results may fit to a notion developed in budding yeast that Cwh43 conjugates ceramide to GPI (glycosylphosphatidylinositol)-anchored proteins and maintains integrity of membrane organization.

Mechanisms and regulation of DNA replication initiation in eukaryotes

Cellular DNA replication is initiated through the action of multiprotein complexes that recognize replication start sites in the chromosome (termed origins) and facilitate duplex DNA melting within these regions. In a typical cell cycle, initiation occurs only once per origin and each round of replication is tightly coupled to cell division. To avoid aberrant origin firing and re-replication, eukaryotes tightly regulate two events in the initiation process: loading of the replicative helicase, MCM2-7, onto chromatin by the origin recognition complex (ORC), and subsequent activation of the helicase by its incorporation into a complex known as the CMG. Recent work has begun to reveal the details of an orchestrated and sequential exchange of initiation factors on DNA that give rise to a replication-competent complex, the replisome. Here, we review the molecular mechanisms that underpin eukaryotic DNA replication initiation - from selecting replication start sites to replicative helicase loading and activation - and describe how these events are often distinctly regulated across different eukaryotic model organisms.

Historical Perspective of Eukaryotic DNA Replication

The replication of the genome of a eukaryotic cell is a complex process requiring the ordered assembly of multiprotein replisomes at many chromosomal sites. The process is strictly controlled during the cell cycle to ensure the complete and faithful transmission of genetic information to progeny cells. Our current understanding of the mechanisms of eukaryotic DNA replication has evolved over a period of more than 30 years through the efforts of many investigators. The aim of this perspective is to provide a brief history of the major advances during this period.

Analysis of the Schizosaccharomyces pombe Cell Cycle

Schizosaccharomyces pombe cells are rod shaped, and they grow by tip elongation. Growth ceases during mitosis and cell division; therefore, the length of a septated cell is a direct measure of the timing of mitotic commitment, and the length of a wild-type cell is an indicator of its position in the cell cycle. A large number of documented stage-specific changes can be used as landmarks to characterize cell cycle progression under specific experimental conditions. Conditional mutations can permanently or transiently block the cell cycle at almost any stage. Large, synchronously dividing cell populations, essential for the biochemical analysis of cell cycle events, can be generated by induction synchrony (arrest-release of a cell cycle mutant) or selection synchrony (centrifugal elutriation or lactose-gradient centrifugation). Schizosaccharomyces pombe cell cycle studies routinely combine particular markers, mutants, and synchronization procedures to manipulate the cycle. We describe these techniques and list key landmarks in the fission yeast mitotic cell division cycle.

Functional reprogramming of polyploidization in megakaryocytes

Polyploidization is a natural process that frequently accompanies differentiation; its deregulation is linked to genomic instability and cancer. Despite its relevance, why cells select different polyploidization mechanisms is unknown. Here we report a systematic genetic analysis of endomitosis, a process in which megakaryocytes become polyploid by entering mitosis but aborting anaphase. Whereas ablation of the APC/C cofactor Cdc20 results in mitotic arrest and severe thrombocytopenia, lack of the kinases Aurora-B, Cdk1, or Cdk2 does not affect megakaryocyte polyploidization or platelet levels. Ablation of Cdk1 forces a switch to endocycles without mitosis, whereas polyploidization in the absence of Cdk1 and Cdk2 occurs in the presence of aberrant re-replication events. Importantly, ablation of these kinases rescues the defects in Cdc20 null megakaryocytes. These findings suggest that endomitosis can be functionally replaced by alternative polyploidization mechanisms in vivo and provide the cellular basis for therapeutic approaches aimed to discriminate mitotic and polyploid cells.

Unraveling the biology of a fungal meningitis pathogen using chemical genetics

The fungal meningitis pathogen Cryptococcus neoformans is a central driver of mortality in HIV/AIDS. We report a genome-scale chemical genetic data map for this pathogen that quantifies the impact of 439 small-molecule challenges on 1,448 gene knockouts. We identified chemical phenotypes for 83% of mutants screened and at least one genetic response for each compound. C. neoformans chemical-genetic responses are largely distinct from orthologous published profiles of Saccharomyces cerevisiae, demonstrating the importance of pathogen-centered studies. We used the chemical-genetic matrix to predict novel pathogenicity genes, infer compound mode of action, and to develop an algorithm, O2M, that predicts antifungal synergies. These predictions were experimentally validated, thereby identifying virulence genes, a molecule that triggers G2/M arrest and inhibits the Cdc25 phosphatase, and many compounds that synergize with the antifungal drug fluconazole. Our work establishes a chemical-genetic foundation for approaching an infection responsible for greater than one-third of AIDS-related deaths.

Effects of telomerase activity and apoptosis on ex vivo expansion of cord blood CD34(+) cells

OBJECTIVE

Ex vivo expansion of CD34(+) cells has become critically important in order to obtain sufficient haematopoietic stem cells for clinical application. Among major regulators involved in ex vivo expansion, telomerase activity and apoptosis have been revealed to be closely linked to cell cycle progression. However, all exact roles remain to be elucidated. Here, change in telomerase activity and level of apoptosis in cord blood (CB) CD34(+) cells were evaluated together with specific cell population growth rate during ex vivo culture.

MATERIALS AND METHODS

CD34(+) cells isolated from human CB were expanded ex vivo over a 28-day period. Besides monitoring cell proliferation kinetics of the CD34(+) cells, changes in telomerase activity and apoptotic levels were investigated. Several relevant genes were quantified by qRT-PCR during the culture period.

RESULTS

Significant elevation of telomerase activity had close relationship to activation of CB CD34(+) cell expansion. Peak apoptotic level was accompanied by a remarkable decline in cell-specific growth rate, and apoptotic level of differentiated CD34(-) population was significantly higher than that of the CD34(+) population.

CONCLUSION

Although telomerase activity was activated during the culture, expansion of CB CD34(+) cells seemed to be more susceptible to apoptotic suppression when cultured ex vivo, which implied that apoptosis may serve as a rate-limiting factor involved in controlling expansion efficiency.

Cyclin-dependent kinase 1 (Cdk1) is essential for cell division and suppression of DNA re-replication but not for liver regeneration

Cyclin-dependent kinase 1 (Cdk1) is an archetypical kinase and a central regulator that drives cells through G2 phase and mitosis. Knockouts of Cdk2, Cdk3, Cdk4, or Cdk6 have resulted in viable mice, but the in vivo functions of Cdk1 have not been fully explored in mammals. Here we have generated a conditional-knockout mouse model to study the functions of Cdk1 in vivo. Ablation of Cdk1 leads to arrest of embryonic development around the blastocyst stage. Interestingly, liver-specific deletion of Cdk1 is well tolerated, and liver regeneration after partial hepatectomy is not impaired, indicating that regeneration can be driven by cell growth without cell division. The loss of Cdk1 does not affect S phase progression but results in DNA re-replication because of an increase in Cdk2/cyclin A2 activity. Unlike other Cdks, loss of Cdk1 in the liver confers complete resistance against tumorigenesis induced by activated Ras and silencing of p53.

Brr6 drives the Schizosaccharomyces pombe spindle pole body nuclear envelope insertion/extrusion cycle

Insertion into and release of the cytoplasmic domain of the Schizosaccharomyces pombe spindle pole body from a nuclear envelope fenestra during mitosis requires Brr6.

The fission yeast interphase spindle pole body (SPB) is a bipartite structure in which a bulky cytoplasmic domain is separated from a nuclear component by the nuclear envelope. During mitosis, the SPB is incorporated into a fenestra that forms within the envelope during mitotic commitment. Closure of this fenestra during anaphase B/mitotic exit returns the cytoplasmic component to the cytoplasmic face of an intact interphase nuclear envelope. Here we show that Brr6 is transiently recruited to SPBs at both SPB insertion and extrusion. Brr6 is required for both SPB insertion and nuclear envelope integrity during anaphase B/mitotic exit. Genetic interactions with apq12 and defective sterol assimilation suggest that Brr6 may alter envelope composition at SPBs to promote SPB insertion and extrusion. The restriction of the Brr6 domain to eukaryotes that use a polar fenestra in an otherwise closed mitosis suggests a conserved role in fenestration to enable a single microtubule organizing center to nucleate both cytoplasmic and nuclear microtubules on opposing sides of the nuclear envelope.

Endoreduplication in maize endosperm: involvement of m phase--promoting factor inhibition and induction of s phase--related kinases

Endoreduplication is an endonuclear chromosome duplication that occurs in the absence of mitosis and in Zea mays (L.) is required for endosperm development. Induction of DNA synthesis during early stages of endosperm development is maintained by increasing the amount and activity of S phase-related protein kinases, which was demonstrated here by their ability to interact with human E2F or with the adenovirus E1A proteins. In addition it was shown that endoreduplicated endosperm cells contain an inhibitor that suppresses the activity of the M phase-promoting factor (MPF). These results demonstrate that in maize endosperm, endoreduplication proceeds as a result of two events, inhibition of MPF and induction of S phase-related protein kinases.

System-level feedbacks control cell cycle progression

Repetitive cell cycles, which are essential to the perpetuation of life, are orchestrated by an underlying biochemical reaction network centered around cyclin-dependent protein kinases (Cdks) and their regulatory subunits (cyclins). Oscillations of Cdk1/CycB activity between low and high levels during the cycle trigger DNA replication and mitosis in the correct order. Based on computational modeling, we proposed that the low and the high kinase activity states are alternative stable steady states of a bistable Cdk-control system. Bistability is a consequence of system-level feedback (positive and double-negative feedback signals) in the underlying control system. We have also argued that bistability underlies irreversible transitions between low and high Cdk activity states and thereby ensures directionality of cell cycle progression.

Plant geminivirus rep protein induces rereplication in fission yeast

The replication-associated protein (Rep) of geminiviruses, single-stranded DNA viruses of higher plants, is essential for virus replication. Since these viruses do not encode their own polymerases, Rep induces differentiated plant cells to reenter the cell cycle by interacting with the plant homologues of retinoblastoma proteins in order to activate the host DNA synthesis machinery. We have used fission yeast (Schizosaccharomyces pombe) as a model organism to analyze the impact of ectopically expressed African cassava mosaic virus Rep protein on the cell division cycle in closer detail. Upon expression, Rep showed its characteristic DNA cleavage activity, and about 10% of the cells exhibited morphological changes. They were elongated threefold, on average, and possessed a single but enlarged and less compact nucleus in comparison to noninduced or vector-only control cells. Flow cytometry of Rep-expressing cultures revealed a distinct subpopulation of Rep protein-containing cells with aberrant morphology. The other 90% of the cells were indistinguishable from control cells, and no Rep was detectable. Rep-expressing cells exhibited DNA contents beyond 2C, indicating ongoing replication without intervening mitosis. Because a second open reading frame (ORF), AC4, is present within the Rep gene, the role of AC4 was examined by destroying its start codon within the AC1 ORF. The results confirmed that Rep is necessary and sufficient to induce rereplication in fission yeast. The unique potential of this well-investigated model for dissecting the cell cycle control by geminiviral proteins is discussed.

S6K1 is involved in polyploidization through its phosphorylation at Thr421/Ser424

Studies on polyploidization of megakaryocytes have been hampered by the lack of synchronized polyploid megakaryocytes. In this study, a relatively synchronized polyploid cell model was successfully established by employing Dami cells treated with nocodazole. In nocodazole-induced cells, cyclin B expression oscillated normally as in diploid cells and polyploid megakaryocytes. By using the nocodazole-induced Dami cell model, we found that 4E-BP1 and Thr421/Ser424 of ribosomal S6 kinase 1(S6K1) were phosphorylated mostly at M-phase in cytoplasm and oscillated in nocodazole-induced polyploid Dami cells, concomitant with increased expression of p27 and cyclin D3. However, phosphorylation of 4E-BP1 and S6K1 on Thr421/Ser424 was significantly decreased in differentiated Dami cells induced by phorbol 12-myristate 13-acetate (PMA), concomitant with increased expression of cyclin D1 and p21 and cyclin D3. Overexpression of the kinase dead form of S6K1 containing the mutation Lys 100 --> Gln in PMA-induced Dami cells increased ploidy whereas overexpression of rapamycin-resistant form of S6K1 containing the mutations Thr421 --> Glu and Ser424 --> Asp significantly dephosphorylated 4E-BP1 and reduced expression of cyclin D1, cyclin D3, p21 and p27, and slightly decreased the ploidy of PMA-induced Dami cells, compared with treatment with PMA alone. Moreover, overexpression of rapamycin-resistant form of S6K1 significantly reversed polyploidization of nocodazole-induced Dami cells. Furthermore, MAP (a novel compound synthesized recently) partly blocked the phosphorylation of S6K1 on Thr421/Ser424 and decreased the expression of p27 and polyploidization in nocodazole-induced Dami cells. Taken together, these data suggested that S6K1/4E-BP1 pathway may play an important role in polyploidization of megakaryocytes.

Cyclin-dependent kinase-associated proteins Cks1 and Cks2 are essential during early embryogenesis and for cell cycle progression in somatic cells

Cks proteins associate with cyclin-dependent kinases and have therefore been assumed to play a direct role in cell cycle regulation. Mammals have two paralogs, Cks1 and Cks2, and individually deleting the gene encoding either in the mouse has previously been shown not to impact viability. In this study we show that simultaneously disrupting CKS1 and CKS2 leads to embryonic lethality, with embryos dying at or before the morula stage after only two to four cell division cycles. RNA interference (RNAi)-mediated silencing of CKS genes in mouse embryonic fibroblasts (MEFs) or HeLa cells causes cessation of proliferation. In MEFs CKS silencing leads to cell cycle arrest in G(2), followed by rereplication and polyploidy. This phenotype can be attributed to impaired transcription of the CCNB1, CCNA2, and CDK1 genes, encoding cyclin B1, cyclin A, and Cdk1, respectively. Restoration of cyclin B1 expression rescues the cell cycle arrest phenotype conferred by RNAi-mediated Cks protein depletion. Consistent with a direct role in transcription, Cks2 is recruited to chromatin in general and to the promoter regions and open reading frames of genes requiring Cks function with a cell cycle periodicity that correlates with their transcription.

Genome re-duplication and irregular segregation occur during the cell cycle of Entamoeba histolytica

Heterogeneity of genome content is commonly observed in axenic cultures of Entamoeba histolytica. Cells with multiple nuclei and nuclei with heterogenous genome contents suggest that regulatory mechanisms that ensure alternation of DNA synthesis and mitosis are absent in this organism. Therefore, several endo-reduplicative cycles may occur without mitosis. The data also shows that unlike other endo-reduplicating organisms, E.histolytica does not undergo a precise number of endo-reduplicative cycles. We propose that irregular endo-reduplication and genome partitioning lead to heterogeneity in the genome content of E.histolytica trophozoites in their proliferative phase. The goal of future studies should be aimed at understanding the mechanisms that are involved in (a) accumulation of multiple genome contents in a single nucleus; (b) genome segregation in nuclei that contain multiple genome contents and (c) maintenance of genome fidelity in E. histolytica.

Chemical genetic analysis of the regulatory role of Cdc2p in the S. pombe septation initiation network

The protein kinase Cdc2p is the master regulator of cell cycle progression in the fission yeast Schizosaccharomyces pombe. It is required both for entry into mitosis and for onset of DNA replication. Cdc2p must be inactivated to permit exit from mitosis, licensing of replication origins and cytokinesis. To study the role of Cdc2p in greater detail, we generated a cdc2 allele that is sensitive to an inhibitory ATP analogue. We show that the inhibitor-induced cell cycle arrest is reversible and examine the effect of inhibiting Cdc2p on the regulation of the septation initiation network (SIN), which controls the initiation of cytokinesis in S. pombe. We found that specific inactivation of Cdc2p in a mitotically arrested cell promotes the asymmetrical recruitment of SIN proteins to the spindle poles and the recruitment of the most downstream SIN components and beta-(1,3) glucan synthase to the contractile ring. Thus, we conclude that inactivation of Cdc2p is sufficient to activate the SIN and promote cytokinesis.

Preventing DNA over-replication: a Cdk perspective

The cell cycle is tightly controlled to ensure that replication origins fire only once per cycle and that consecutive S-phases are separated by mitosis. When controls fail, DNA over-replication ensues: individual origins fire more than once per S-phase (re-replication) or consecutive S-phases occur without intervening mitoses (endoreduplication). In yeast the cell cycle is controlled by a single cyclin dependent kinase (Cdk) that prevents origin licensing at times when it promotes origin firing, and that is inactivated, via proteolysis of its partner cyclin, as cells undergo mitosis. A quantitative model describes three levels of Cdk activity: low activity allows licensing, intermediate activity allows firing but prevents licensing, and high activity promotes mitosis. In higher eukaryotes the situation is complicated by the existence of additional proteins (geminin, Cul4-Ddb1Cdt2, and Emi1) that control licensing. A current challenge is to understand how these various control mechanisms are co-ordinated and why the degree of redundancy between them is so variable. Here the experimental induction of DNA over-replication is reviewed in the context of the quantitative model of Cdk action. Endoreduplication is viewed as a consequence of procedures that cause Cdk activity to fall below the threshold required to prevent licensing, and re-replication as the result of procedures that increase that threshold value. This may help to explain why over-replication does not necessarily require reduced Cdk activity and how different mechanisms conspire to prevent over-replication. Further work is nevertheless required to determine exactly how losing just one licensing control mechanism often causes over-replication, and why this varies between cell systems.

Emi1 is needed to couple DNA replication with mitosis but does not regulate activation of the mitotic APC/C

Ubiquitin-mediated proteolysis is critical for the alternation between DNA replication and mitosis and for the key regulatory events in mitosis. The anaphase-promoting complex/cyclosome (APC/C) is a conserved ubiquitin ligase that has a fundamental role in regulating mitosis and the cell cycle in all eukaryotes. In vertebrate cells, early mitotic inhibitor 1 (Emi1) has been proposed as an important APC/C inhibitor whose destruction may trigger activation of the APC/C at mitosis. However, in this study, we show that the degradation of Emi1 is not required to activate the APC/C in mitosis. Instead, we uncover a key role for Emi1 in inhibiting the APC/C in interphase to stabilize the mitotic cyclins and geminin to promote mitosis and prevent rereplication. Thus, Emi1 plays a crucial role in the cell cycle to couple DNA replication with mitosis, and our results also question the current view that the APC/C has to be inactivated to allow DNA replication.

Strength in numbers: preventing rereplication via multiple mechanisms in eukaryotic cells

In eukaryotic cells, prereplication complexes (pre-RCs) are assembled on chromatin in the G1 phase, rendering origins of DNA replication competent to initiate DNA synthesis. When DNA replication commences in S phase, pre-RCs are disassembled, and multiple initiations from the same origin do not occur because new rounds of pre-RC assembly are inhibited. In most experimental organisms, multiple mechanisms that prevent pre-RC assembly have now been identified, and rereplication within the same cell cycle can be induced through defined perturbations of these mechanisms. This review summarizes the diverse array of inhibitory pathways used by different organisms to prevent pre-RC assembly, and focuses on the challenge of understanding how in any one cell type, various mechanisms cooperate to strictly enforce once per cell cycle regulation of DNA replication.

Nascent transcription of MCM2-7 is important for nuclear localization of the minichromosome maintenance complex in G1

The minichromosome maintenance genes (MCM2-7) are transcribed at M/G1 just as the Mcm complex is imported into the nucleus to be assembled into prereplication complexes, during a period of low cyclin-dependent kinase (CDK) activity. The CDKs trigger DNA replication and prevent rereplication in part by exporting Mcm2-7 from the nucleus during S phase. We have found that repression of MCM2-7 transcription in a single cell cycle interferes with the nuclear import of Mcms in the subsequent M/G1 phase. This suggests that nascent Mcm proteins are preferentially imported into the nucleus. Consistent with this, we find that loss of CDK activity in G2/M is not sufficient for nuclear import, there is also a requirement for new protein synthesis. This requirement is not met by constitutive production of Cdc6 and does not involve synthesis of new transport machinery. The Mcm proteins generated in the previous cell cycle, which are unable to reaccumulate in the nucleus, are predominantly turned over by ubiquitin-mediated proteolysis in late mitosis/early G1. Therefore, the nuclear localization of Mcm2-7 is dependent on nascent transcription and translation of Mcm2-7 and the elimination of CDK activity which occurs simultaneously as cells enter G1.

Induced and repressed genes after irradiation sensitizing by pentoxyphylline

Aim in cancer therapy is to increase the therapeutic ratio eliminating the disease while minimizing toxicity to normal tissues. Radiation therapy is a main component in targeting cancer. Radiosensitizing agents like pentoxyphylline (PTX) have been evaluated to improve radiotherapy. Commonly, cells respond to radiation by the activation of specific early and late response genes as well as by inhibition of genes, which are expressed under normal conditions. A display of the genetic distinctions at the level of transcription is given here to characterize the molecular events underlying the radiosensitizing mechanisms. The method of suppression subtractive hybridization allows the visualization of both induced and repressed genes in irradiated cells compared with cells sensitized immediately after irradiation. The genes were isolated by cDNA-cloning, differential analysis and sequence similarity search. Genes involved in protein synthesis, metabolism, proteolysis and transcriptional regulation were detected. It is important that genes like KIAA280, which were only known as unidentified EST sequences before without function, but inaccessible by array technology were recovered as functional genes. Database searches for PTX-induced genes detected a human mRNA completely unknown. In case of suppressed genes, we detected several mRNAs; one thereof shows homology to a hypothetical protein possibly involved in signal transduction. A further mRNA encodes the protein BM036 supposed to associate with the E2F transcription factor. A hypothetical protein H41 was detected, which may repress the Her-2/neu receptor influencing breast cancer, gliomas and prostate tumors. Radiation combined with PTX may lead to a better prognosis by down regulation of the Her-2/neu, which will be proven by clinical studies in the near future.

Cyclin and cyclin-dependent kinase substrate requirements for preventing rereplication reveal the need for concomitant activation and inhibition

DNA replication initiation in S. cerevisiae is promoted by B-type cyclin-dependent kinase (Cdk) activity. In addition, once-per-cell-cycle replication is enforced by cyclin-Cdk-dependent phosphorylation of the prereplicative complex (pre-RC) components Mcm2-7, Cdc6, and Orc1-6. Several of these controls must be simultaneously blocked by mutation to obtain rereplication. We looked for but did not obtain strong evidence for cyclin specificity in the use of different mechanisms to control rereplication: both the S-phase cyclin Clb5 and the mitotic cyclins Clb1-4 were inferred to be capable of imposing ORC-based and MCM-based controls. We found evidence that the S-phase cyclin Clb6 could promote initiation of replication without blocking reinitiation, and this activity was highly toxic when the ability of other cyclins to block reinitiation was prevented by mutation. The failure of Clb6 to regulate reinitiation was due to rapid Clb6 proteolysis, since this toxic activity of Clb6 was lost when Clb6 was stabilized by mutation. Clb6-dependent toxicity is also relieved when early accumulation of mitotic cyclins is allowed to impose rereplication controls. Cell-cycle timing of rereplication control is crucial: sufficient rereplication block activity must be available as soon as firing begins. DNA rereplication induces DNA damage, and when rereplication controls are compromised, the DNA damage checkpoint factors Mre11 and Rad17 provide additional mechanisms that maintain viability and also prevent further rereplication, and this probably contributes to genome stability.

The CENP-B homolog, Abp1, interacts with the initiation protein Cdc23 (MCM10) and is required for efficient DNA replication in fission yeast

Abp1, and the closely related Cbh1 and Cbh2 are homologous to the human centromere-binding protein CENP-B that has been implicated in the assembly of centromeric heterochromatin. Fission yeast cells lacking Abp1 show an increase in mini-chromosome instability suggesting that Abp1 is important for chromosome segregation and/or DNA synthesis. Here we show that Abp1 interacts with the DNA replication protein Cdc23 (MCM10) in a two-hybrid assay, and that the Deltaabp1 mutant displays a synthetic phenotype with a cdc23 temperature-sensitive mutant. Moreover, genetic interactions were also observed between abp1+ and four additional DNA replication initiation genes cdc18+, cdc21+, orc1+, and orc2+. Interestingly, we find that S phase is delayed in cells deleted for abp1+ when released from a G1 block. However, no delay is observed when cells are released from an early S phase arrest induced by hydroxyurea suggesting that Abp1 functions prior to, or coincident with, the initiation of DNA replication.

Interactions between the RNA interference effector protein Ago1 and 14-3-3 proteins: consequences for cell cycle progression

The Argonaute family member Ago1 is required for formation of pericentric heterochromatin and small interfering RNA (siRNA)-mediated post-transcriptional gene silencing in the fission yeast Schizosaccharomyces pombe. In addition, we have recently demonstrated that Ago1 function is required for enactment of cell cycle checkpoints (Carmichael, J. B., Provost, P., Ekwall, K., and Hobman, T. C. (2004) Mol. Biol. Cell 15, 1425-1435). Here, we provide evidence that the amino terminus of Ago1 binds to proteins that function in cell cycle regulation including 14-3-3 proteins. Interestingly, the amino terminus of human Ago2, the endonuclease that cleaves siRNA-targeted mRNAs, was also demonstrated to bind 14-3-3 proteins. Overexpression of the Ago1 amino terminus in yeast resulted in cell cycle delay at the G(2)/M boundary. Further investigation revealed that nuclear import of the mitosis-inducing phosphatase Cdc25 is inhibited by overexpression of the Ago1 amino terminus. Under these conditions, we found that the cyclin-dependent kinase Cdc2 is constitutively phosphorylated on tyrosine 15, thereby reducing the activity of this kinase, a situation that delays entry into mitosis. We hypothesize that 14-3-3 proteins are required for Argonaute protein functions in cell cycle and/or gene-silencing pathways.

Traffic safety for the cell: influence of cyclin-dependent kinase activity on genomic stability

Genomic instability has long been considered a key factor in tumorigenesis. Recent evidence suggests that DNA damage may be widespread in early pre-neoplastic states, with deregulation of cyclin-dependent kinase (Cdk) activity a driving force. Increased Cdk activity may critically reduce licensing of origins of DNA replication, drive re-replication, or mediate overexpression of checkpoint proteins, inducing deleterious cell cycle delay. Conversely, inhibition of Cdk activity may compromise replication efficiency, expression of checkpoint proteins, or activation of DNA repair proteins. These vital functions point to the impact of Cdk activity on the stability of the genome. Insight into these pathways may improve our understanding of tumorigenesis and lead to more rational cancer therapies.

CDK phosphorylation inhibits the DNA-binding and ATP-hydrolysis activities of the Drosophila origin recognition complex

Faithful propagation of eukaryotic chromosomes usually requires that no DNA segment be replicated more than once during one cell cycle. Cyclin-dependent kinases (Cdks) are critical for the re-replication controls that inhibit the activities of components of the pre-replication complexes (pre-RCs) following origin activation. The origin recognition complex (ORC) initiates the assembly of pre-RCs at origins of replication and Cdk phosphorylation of ORC is important for the prevention of re-initiation. Here we show that Drosophila melanogaster ORC (DmORC) is phosphorylated in vivo and is a substrate for Cdks in vitro. Cdk phosphorylation of DmORC subunits DmOrc1p and DmOrc2p inhibits the intrinsic ATPase activity of DmORC without affecting ATP binding to DmOrc1p. Moreover, Cdk phosphorylation inhibits the ATP-dependent DNA-binding activity of DmORC in vitro, thus identifying a novel determinant for DmORC-DNA interaction. DmORC is a substrate for both Cdk2 x cyclin E and Cdk1 x cyclin B in vitro. Such phosphorylation of DmORC by Cdk2 x cyclin E, but not by Cdk1 x cyclin B, requires an "RXL" motif in DmOrc1p. We also identify casein kinase 2 (CK2) as a kinase activity in embryonic extracts targeting DmORC for modification. CK2 phosphorylation does not affect ATP hydrolysis by DmORC but modulates the ATP-dependent DNA-binding activity of DmORC. These results suggest molecular mechanisms by which Cdks may inhibit ORC function as part of re-replication control and show that DmORC activity may be modulated in response to phosphorylation by multiple kinases.

Levels of the origin-binding protein Double parked and its inhibitor Geminin increase in response to replication stress

The regulation of a pre-replicative complex (pre-RC) at origins ensures that the genome is replicated only once per cell cycle. Cdt1 is an essential component of the pre-RC that is rapidly degraded at G1-S and also inhibited by Geminin (Gem) protein to prevent re-replication. We have previously shown that destruction of the Drosophila homolog of Cdt1, Double-parked (Dup), at G1-S is dependent upon cyclin-E/CDK2 and important to prevent re-replication and cell death. Dup is phosphorylated by cyclin-E/Cdk2, but this direct phosphorylation was not sufficient to explain the rapid destruction of Dup at G1-S. Here, we present evidence that it is DNA replication itself that triggers rapid Dup destruction. We find that a range of defects in DNA replication stabilize Dup protein and that this stabilization is not dependent on ATM/ATR checkpoint kinases. This response to replication stress was cell-type specific, with neuroblast stem cells of the larval brain having the largest increase in Dup protein. Defects at different steps in replication also increased Dup protein during an S-phase-like amplification cell cycle in the ovary, suggesting that Dup stabilization is sensitive to DNA replication and not an indirect consequence of a cell-cycle arrest. Finally, we find that cells with high levels of Dup also have elevated levels of Gem protein. We propose that, in cycling cells, Dup destruction is coupled to DNA replication and that increased levels of Gem balance elevated Dup levels to prevent pre-RC reformation when Dup degradation fails.

Preventing re-replication of chromosomal DNA

To ensure its duplication, chromosomal DNA must be precisely duplicated in each cell cycle, with no sections left unreplicated, and no sections replicated more than once. Eukaryotic cells achieve this by dividing replication into two non-overlapping phases. During late mitosis and G1, replication origins are 'licensed' for replication by loading the minichromosome maintenance (Mcm) 2-7 proteins to form a pre-replicative complex. Mcm2-7 proteins are then essential for initiating and elongating replication forks during S phase. Recent data have provided biochemical and structural insight into the process of replication licensing and the mechanisms that regulate it during the cell cycle.

DNA replication and progression through S phase

Initiation and completion of DNA replication defines the beginning and ending of S phase of the cell cycle. Successful progression through S phase requires that replication be properly regulated and monitored to ensure that the entire genome is duplicated exactly once, without errors, in a timely fashion. Given the immense size and complexity of eukaryotic genomes, this presents a significant challenge for the cell. As a result, DNA replication has evolved into a tightly regulated process involving the coordinated action of numerous factors that function in all phases of the cell cycle. We will review our current understanding of these processes from the formation of prereplicative complexes in preparation for S phase to the series of events that culminate in the loading of DNA polymerases during S phase. We will incorporate structural data from archaeal and bacterial replication proteins and discuss their implications for understanding the mechanism of action of their corresponding eukaryotic homologues. We will also describe the concept of replication licensing which protects against genomic instability by limiting initiation events to once per cell cycle. Lastly, we will review our knowledge of checkpoint pathways that maintain the integrity of stalled forks and relay defects in replication to the rest of the cell cycle.

Degradation of Cdt1 during S phase is Skp2-independent and is required for efficient progression of mammalian cells through S phase

Previous reports have shown that the N terminus of Cdt1 is required for its degradation during S phase (Li, X., Zhao, Q., Liao, R., Sun, P., and Wu, X. (2003) J. Biol. Chem. 278, 30854-30858; Nishitani, H., Lygerou, Z., and Nishimoto, T. (2004) J. Biol. Chem. 279, 30807-30816). The stabilization was attributed to deletion of the cyclin binding motif (Cy motif), which is required for its phosphorylation by cyclin-dependent kinases. Phosphorylated Cdt1 is subsequently recognized by the F-box protein Skp2 and targeted for proteasomal mediated degradation. Using phosphopeptide mapping and mutagenesis studies, we found that threonine 29 within the N terminus of Cdt1 is phosphorylated by Cdk2 and required for interaction with Skp2. However, threonine 29 and the Cy motif are not necessary for proteolysis of Cdt1 during S phase. Mutants of Cdt1 that do not stably associate with Skp2 or cyclins are still degraded in S phase to the same extent as wild type Cdt1, indicating that other determinants within the N terminus of Cdt1 are required for degrading Cdt1. We localized the region necessary for Cdt1 degradation to the first 32 residues. Overexpression of stable forms of Cdt1 significantly delayed entry into and completion of S phase, suggesting that failure to degrade Cdt1 prevents normal progression through S phase. In contrast, Cdt1 mutants that fail to interact with Skp2 and cyclins progress through S phase with similar kinetics as wild type Cdt1 but stimulate the re-replication caused by overexpressing Cdt1. Therefore, a Skp2-independent pathway that requires the N-terminal 32 residues of Cdt1 is critical for the degradation of Cdt1 in S phase, and this degradation is necessary for the optimum progression of cells through S phase.

The roles of fission yeast ase1 in mitotic cell division, meiotic nuclear oscillation, and cytokinesis checkpoint signaling

The Ase1/Prc1 proteins constitute a conserved microtubule-associated protein family that is implicated in central spindle formation and cytokinesis. Here we characterize a role for fission yeast Ase1. Ase1 localizes to microtubule overlapping zones and displays dynamic alterations of localization during the cell cycle. In particular, its spindle localization during metaphase is reduced substantially, followed by robust appearance at the spindle midzone in anaphase. ase1 deletions are viable but defective in nuclear and septum positioning and completion of cytokinesis, which leads to diploidization and chromosome loss. Time-lapse imaging shows that elongating spindles collapse abruptly in the middle of anaphase B. Either absence or overproduction of Ase1 results in profound defects on microtubule bundling in an opposed manner, indicating that Ase1 is a dose-dependent microtubule-bundling factor. In contrast microtubule nucleating activities are not noticeably compromised in ase1 mutants. During meiosis astral microtubules are not bundled and oscillatory nuclear movement is impaired significantly. The Aurora kinase does not correctly localize to central spindles in the absence of Ase1. Finally Ase1 acts as a regulatory component in the cytokinesis checkpoint that operates to inhibit nuclear division when the cytokinesis apparatus is perturbed. Ase1, therefore, couples anaphase completion with cytokinesis upon cell division.

Regulation of early events in chromosome replication

Eukaryotic genomes are replicated from large numbers of replication origins distributed on multiple chromosomes. The activity of these origins must be coordinated so that the entire genome is efficiently and accurately replicated yet no region of the genome is ever replicated more than once. The past decade has seen significant advances in understanding how the initiation of DNA replication is regulated by key cell-cycle regulators, including the cyclin dependent kinases (CDKs) and the anaphase promoting complex/cyclosome (APC/C). The assembly of essential prereplicative complexes (pre-RCs) at origins only occurs when CDK activity is low and APC/C activity is high. Origin firing, however, can only occur when the APC/C is inactivated and CDKs become active. This two step mechanism ensures that no origin can fire more than once in a cell cycle. In all eukaryotes tested, CDKs can contribute to the inhibition of pre-RC assembly. This inhibition is characterised both by high degrees of redundancy and evolutionary plasticity. Geminin plays a crucial role in inhibiting licensing in metazoans and, like cyclins, is inactivated by the APC/C. Strategies involved in preventing re-replication in different organisms will be discussed.

Replication-dependent destruction of Cdt1 limits DNA replication to a single round per cell cycle in Xenopus egg extracts

In eukaryotes, prereplication complexes (pre-RCs) containing ORC, Cdc6, Cdt1, and MCM2-7 are assembled on chromatin in the G1 phase. In S phase, when DNA replication initiates, pre-RCs are disassembled, and new pre-RC assembly is restricted until the following G1 period. As a result, DNA replication is limited to a single round per cell cycle. One inhibitor of pre-RC assembly, geminin, was discovered in Xenopus, and it binds and inactivates Cdt1 in S phase. However, removal of geminin from Xenopus egg extracts is insufficient to cause rereplication, suggesting that other safeguards against rereplication exist. Here, we show that Cdt1 is completely degraded by ubiquitin-mediated proteolysis during the course of the first round of DNA replication in Xenopus egg extracts. Degradation depends on Cdk2/Cyclin E, Cdc45, RPA, and polymerase alpha, demonstrating a requirement for replication initiation. Cdt1 is ubiquitinated on chromatin, and this process also requires replication initiation. Once replication has initiated, Cdk2/Cyclin E is dispensable for Cdt1 degradation. When fresh Cdt1 is supplied after the first round of DNA replication, significant rereplication results, and rereplication is enhanced in the absence of geminin. Our results identify a replication-dependent proteolytic pathway that targets Cdt1 and that acts redundantly with geminin to inactivate Cdt1 in S phase.

Loss of rereplication control in Saccharomyces cerevisiae results in extensive DNA damage

To maintain genome stability, the entire genome of a eukaryotic cell must be replicated once and only once per cell cycle. In many organisms, multiple overlapping mechanisms block rereplication, but the consequences of deregulating these mechanisms are poorly understood. Here, we show that disrupting these controls in the budding yeast Saccharomyces cerevisiae rapidly blocks cell proliferation. Rereplicating cells activate the classical DNA damage-induced checkpoint response, which depends on the BRCA1 C-terminus checkpoint protein Rad9. In contrast, Mrc1, a checkpoint protein required for recognition of replication stress, does not play a role in the response to rereplication. Strikingly, rereplicating cells accumulate subchromosomal DNA breakage products. These rapid and severe consequences suggest that even limited and sporadic rereplication could threaten the genome with significant damage. Hence, even subtle disruptions in the cell cycle regulation of DNA replication may predispose cells to the genomic instability associated with tumorigenesis.

A screen for Schizosaccharomyces pombe mutants defective in rereplication identifies new alleles of rad4+, cut9+ and psf2+

Fission yeast mutants defective in DNA replication have widely varying morphological phenotypes. We designed a screen for temperature-sensitive mutants defective in the process of replication regardless of morphology by isolating strains unable to rereplicate their DNA in the absence of cyclin B (Cdc13). Of the 42 rereplication-defective mutants analyzed, we were able to clone complementing plasmids for 10. This screen identified new alleles of the APC subunit cut9(+), the initiation/checkpoint factor rad4(+)/cut5(+), and the first mutant allele of psf2(+), a subunit of the novel GINS replication complex. Other genes identified are likely to play general roles in gene expression and protein localization.

Eukaryotic MCM proteins: beyond replication initiation

The minichromosome maintenance (or MCM) protein family is composed of six related proteins that are conserved in all eukaryotes. They were first identified by genetic screens in yeast and subsequently analyzed in other experimental systems using molecular and biochemical methods. Early data led to the identification of MCMs as central players in the initiation of DNA replication. More recent studies have shown that MCM proteins also function in replication elongation, probably as a DNA helicase. This is consistent with structural analysis showing that the proteins interact together in a heterohexameric ring. However, MCMs are strikingly abundant and far exceed the stoichiometry of replication origins; they are widely distributed on unreplicated chromatin. Analysis of mcm mutant phenotypes and interactions with other factors have now implicated the MCM proteins in other chromosome transactions including damage response, transcription, and chromatin structure. These experiments indicate that the MCMs are central players in many aspects of genome stability.

Non-proteolytic inactivation of geminin requires CDK-dependent ubiquitination

In late mitosis and G1, a complex of the essential initiation proteins Mcm2-7 are assembled onto replication origins to 'license' them for initiation. At other times licensing is inhibited by cyclin-dependent kinases (CDKs) and geminin, thus ensuring that origins fire only once per cell cycle. Here we show that, paradoxically, CDKs are also required to inactivate geminin and activate the licensing system. On exit from metaphase in Xenopus laevis egg extracts, CDK-dependent activation of the anaphase-promoting complex (APC/C) results in the transient polyubiquitination of geminin. This ubiquitination triggers geminin inactivation without requiring ubiquitin-dependent proteolysis, and is essential for replication origins to become licensed. This reveals an unexpected role for CDKs and ubiquitination in activating chromosomal DNA replication.

Regulated mRNA Stability of the Cdk Inhibitor Rum1 Links Nutrient Status to Cell Cycle Progression

BACKGROUND

The survival of a cell depends on continuous sensing of the nutritional environment and appropriate coordination of the cell cycle. The fission yeast Schizosaccharomyces pombe is an excellent model system in which to study these processes. In the presence of nutrients, fission yeast cells grow and divide, spending most of their time in G2; when nutrients are limiting, they are promoted into mitosis and arrest the cell cycle in G1. The molecular mechanisms underlying this response are currently unknown.

RESULTS

Here, we show that expression of the fission yeast Cdk inhibitor Rum1, a key regulator of Cdc2/cyclin B in G1, is subject to regulated mRNA stability in response to nutrient deprivation. In complete minimal medium, rum1 mRNAs are very unstable. Following nitrogen starvation, rum1 mRNAs are rapidly stabilized, allowing the accumulation of Rum1 protein to delay the G1 phase of the subsequent cell cycle. Instability of rum1 mRNAs in complete minimal medium depends on the presence of AU-rich elements in the 3'UTR. We also show that lack of this mechanism has consequences in the mitotic cell cycle, in meiosis, and in the control of ploidy.

CONCLUSION

We propose that mRNA stability is an important mechanism to fine tune the expression of the rum1 gene, in order to allow the production of appropriate levels of Rum1 protein in response to changes in the nutritional environment.

Cell cycle-dependent phosphorylation of the large subunit of replication factor C (RF-C) leads to its dissociation from the RF-C complex

The five subunit replication factor C (RF-C) complex plays a critical role in DNA elongation. We find that the large subunit of RF-C (RF-Cp145) is phosphorylated in vivo whereas the smaller RF-C subunits are not phosphorylated. The phosphorylation of endogenous RFCp145 is modulated in a cell cycle-dependent manner. Phosphorylation is maximal in G2/M and is inhibited by an inhibitor of cyclin-dependent kinases. Phosphorylation of purified recombinant RF-C complex in vitro reveals that RF-Cp145 is preferentially phosphorylated by cdc2-cyclin B but not by cdk2-cyclin A or cdk2-cyclin E. In vitro phosphorylation of RF-C complex by cdc2-cyclin B kinases leads to dissociation of phosphorylated RFCp145 from the RF-C complex. Using different approaches we demonstrate that phosphorylated RFCp145 is indeed dissociated from RF-Cp40 and RF-Cp37 in vivo. These results suggest that destabilization of the RF-C complex by CDKs may inactivate the RF-C complex at the end of S phase.

Human wee1 kinase is directly transactivated by and increased in association with c-Fos/AP-1: rheumatoid synovial cells overexpressing these genes go into aberrant mitosis

Wee1 kinase downregulates the M-phase promoting factor, a complex of cdc2 and cyclin B kinase, that controls mitotic cell division. We isolated human wee1 kinase gene promoter and found that it contained one AP-1-binding motif in its promoter region (5'-CGAGTCA-3'; -823/-817), through which wee1 kinase gene was directly transactivated by c-Fos/AP-1. In rheumatoid synovial cells, wee1 kinase was increased in conjunction with the increase of c-Fos/AP-1 and the substrate of wee1, cdc2, was phosphorylated. The amount of wee1 and c-Fos and the phosphorylation of cdc2 were decreased after treatment of the cells with an inhibitor of AP-1, curcumin. A significant proportion of cultured synovial cells of the patients with rheumatoid arthritis, but not those of osteoarthritis, shifted to a tetraploid (4C) state upon long-term culture. Thus, human wee1 kinase gene is directly transactivated by and increased in association with c-Fos/AP-1, and rheumatoid synovial cells overexpressing these genes go into aberrant mitosis.

The development of diabetes in E2f1/E2f2 mutant mice reveals important roles for bone marrow-derived cells in preventing islet cell loss

Our studies of mice deficient for the E2F1 and E2F2 transcription factors have revealed essential roles for these proteins in the cell cycle control of pancreatic exocrine cells and the regulation of pancreatic beta cell maintenance. Pancreatic exocrine cells in E2f1-/-E2f2 mutant mice become increasingly polyploid with age, coinciding with severe exocrine atrophy. Furthermore, mice deficient for both E2F1 and E2F2 develop nonautoimmune, insulin-dependent diabetes with high penetrance. Surprisingly, transplantation of wild-type bone marrow can prevent or rescue diabetes in E2f1-/-E2f2-/-mice. We hypothesize that exocrine degeneration results in a destructive environment for beta cells, which can be alleviated by restoration of the hematopoietic system that is also defective in E2f1-/-E2f2-/-mice The demonstration that beta cell maintenance under conditions of stress is influenced by bone marrow-derived cells may provide important insight into the design of therapies to boost islet mass and function in diabetic patients.