Proteolysis of DNA replication licensing factor Cdt1 in S-phase is performed independently of geminin through its N-terminal region

Prostaglandin E2 prevents radiotherapy-induced alopecia by attenuating transit amplifying cell apoptosis through promoting G1 arrest

BACKGROUND

Growing hair follicles (HFs) harbor actively dividing transit amplifying cells (TACs), rendering them highly sensitive to radiotherapy (RT). Clinically, there is still a lack of treatment options for radiotherapy-induced alopecia (RIA).

OBJECTIVE

Our present study aimed to investigated the effect and mechanism of local prostaglandin E2 (PGE2) treatment in RIA prevention.

METHODS

We compared the response of growing HFs to radiation with and without local PGE2 pretreatment in a mouse model in vivo. The effect of PGE2 on the cell cycle was determined in cultured HF cells from fluorescent ubiquitination-based cell cycle indicator mice. We also compared the protective effects of PGE2 and a cyclin-dependent kinases 4/6 (CDK4/6) inhibitor against RIA.

RESULTS

The local cutaneous PGE2 injection reduced RIA by enhancing HF self-repair. Mechanistically, PGE2 did not activate HF stem cells, but it preserved more TACs for regenerative attempts. Pretreatment of PGE2 lessened radiosensitivity of TACs by transiently arresting them in the G1 phase, thereby reducing TAC apoptosis and mitigating HF dystrophy. The preservation of more TACs accelerated HF self-repair and bypassed RT-induced premature termination of anagen. Promoting G1 arrest by systemic administration of palbociclib isethionate (PD0332991), a CDK4/6 inhibitor, offered a similar protective effect against RT.

CONCLUSIONS

Locally administered PGE2 protects HF TACs from RT by transiently inducing G1 arrest, and the regeneration of HF structures lost from RT is accelerated to resume anagen growth, thus bypassing the long downtime of hair loss. PGE2 has the potential to be repurposed as a local preventive treatment for RIA.

CDT1, a licensing factor that limits rereplication

Cells must avoid licensing of neosynthesized DNA to prevent rereplication. In this issue of Molecular Cell, Ratnayeke et al. (2022)1 reveal how the licensing factor CDT1, prior to its degradation, inhibits DNA elongation by suppressing CMG helicase progression at replication forks.

FUCCI Reporter Gene-Based Cell Cycle Analysis

One of the most commonly assessed parameters in cellular analyses is the proliferative activity of a cell population. The fluorescence ubiquitin cell cycle indicator (FUCCI)-based system allows live and in vivo observation of cell cycle progression. Based on the mutually exclusive activity of two fluorescently labeled proteins cdt1 and geminin during the G0/1 and S/G2/M phases of the cell cycle, individual cells can be assigned to their respective cell cycle phase by fluorescence imaging of the nucleus. Here, we describe the generation of NIH/3T3 cells containing the FUCCI reporter system by lentiviral transduction and their use in 3D culture assays. The protocol can be adapted to other cell lines.

Initial characterization of gap phase introduction in every cell cycle of C. elegans embryogenesis

Early embryonic cell cycles usually alternate between S and M phases without any gap phase. When the gap phases are developmentally introduced in various cell types remains poorly defined especially during embryogenesis. To establish the cell-specific introduction of gap phases in embryo, we generate multiple fluorescence ubiquitin cell cycle indicators (FUCCI) in C. elegans. Time-lapse 3D imaging followed by lineal expression profiling reveals sharp and differential accumulation of the FUCCI reporters, allowing the systematic demarcation of cell cycle phases throughout embryogenesis. Accumulation of the reporters reliably identifies both G1 and G2 phases only in two embryonic cells with an extended cell cycle length, suggesting that the remaining cells divide either without a G1 phase, or with a brief G1 phase that is too short to be picked up by our reporters. In summary, we provide an initial picture of gap phase introduction in a metazoan embryo. The newly developed FUCCI reporters pave the way for further characterization of developmental control of cell cycle progression.

A constitutively expressed fluorescent ubiquitination-based cell-cycle indicator (FUCCI) in axolotls for studying tissue regeneration

Regulation of cell cycle progression is essential for cell proliferation during regeneration following injury. After appendage amputation, the axolotl (Ambystoma mexicanum) regenerates missing structures through an accumulation of proliferating cells known as the blastema. To study cell division during blastema growth, we generated a transgenic line of axolotls that ubiquitously expresses a bicistronic version of the fluorescent ubiquitination-based cell-cycle indicator (FUCCI). We demonstrate near-ubiquitous FUCCI expression in developing and adult tissues, and validate these expression patterns with DNA synthesis and mitosis phase markers. We demonstrate the utility of FUCCI for live and whole-mount imaging, showing the predominantly local contribution of cells during limb and tail regeneration. We also show that spinal cord amputation results in increased proliferation at least 5 mm from the site of injury. Finally, we use multimodal staining to provide cell type information for cycling cells by combining fluorescence in situ hybridization, EdU click-chemistry and immunohistochemistry on a single FUCCI tissue section. This new line of animals will be useful for studying cell cycle dynamics using in situ endpoint assays and in vivo imaging in developing and regenerating animals.

In Vivo Methods to Monitor Cardiomyocyte Proliferation

Adult mammalian cardiomyocytes demonstrate scarce cycling and even lower proliferation rates in response to injury. Signals that enhance cardiomyocyte proliferation after injury will be groundbreaking, address unmet clinical needs, and represent new strategies to treat cardiovascular diseases. In vivo methods to monitor cardiomyocyte proliferation are critical to addressing this challenge. Fortunately, advances in transgenic approaches provide sophisticated techniques to quantify cardiomyocyte cycling and proliferation.

Geminin is essential for DNA re-replication in the silk gland cells of silkworms

Endoreplication, known as endocycles or endoreduplication, is a cell cycle variant in which the genomic DNA is re-replicated without mitosis leading to polyploidy. Endoreplication is essential for the development and functioning of the different organs in animals and plants. Deletion of Geminin, a DNA replication licensing inhibitor, causes DNA re-replication or damage. However, the role of Geminin in endoreplication is still unclear. Here, we studied the role of Geminin in the endoreplication of the silk gland cells of silkworms by constructing two transgenic silkworm strains, including BmGeminin1-overexpression and BmGeminin1-RNA interference. Interference of BmGeminin1 led to body weight gain, increased silk gland volume, increased DNA content, and enhanced DNA re-replication activity relative to wild-type Dazao. Meanwhile, overexpression of BmGeminin1 showed an opposite phenotype compared to the BmGem1-RNAi strain. Furthermore, RNA-sequencing of the transgenic strains was carried out to explore how BmGeminin1 regulates DNA re-replication. Our data demonstrated a vital role of Geminin in the regulation of endoreplication in the silk gland of silkworms.

Caspase-2 regulates S-phase cell cycle events to protect from DNA damage accumulation independent of apoptosis

In addition to its classical role in apoptosis, accumulating evidence suggests that caspase-2 has non-apoptotic functions, including regulation of cell division. Loss of caspase-2 is known to increase proliferation rates but how caspase-2 is regulating this process is currently unclear. We show that caspase-2 is activated in dividing cells in G1-phase of the cell cycle. In the absence of caspase-2, cells exhibit numerous S-phase defects including delayed exit from S-phase, defects in repair of chromosomal aberrations during S-phase, and increased DNA damage following S-phase arrest. In addition, caspase-2-deficient cells have a higher frequency of stalled replication forks, decreased DNA fiber length, and impeded progression of DNA replication tracts. This indicates that caspase-2 protects from replication stress and promotes replication fork protection to maintain genomic stability. These functions are independent of the pro-apoptotic function of caspase-2 because blocking caspase-2-induced cell death had no effect on cell division, DNA damage-induced cell cycle arrest, or DNA damage. Thus, our data supports a model where caspase-2 regulates cell cycle and DNA repair events to protect from the accumulation of DNA damage independently of its pro-apoptotic function.

Replication initiation: Implications in genome integrity

In every cell cycle, billions of nucleotides need to be duplicated within hours, with extraordinary precision and accuracy. The molecular mechanism by which cells regulate the replication event is very complicated, and the entire process begins way before the onset of S phase. During the G1 phase of the cell cycle, cells prepare by assembling essential replication factors to establish the pre-replicative complex at origins, sites that dictate where replication would initiate during S phase. During S phase, the replication process is tightly coupled with the DNA repair system to ensure the fidelity of replication. Defects in replication and any error must be recognized by DNA damage response and checkpoint signaling pathways in order to halt the cell cycle before cells are allowed to divide. The coordination of these processes throughout the cell cycle is therefore critical to achieve genomic integrity and prevent diseases. In this review, we focus on the current understanding of how the replication initiation events are regulated to achieve genome stability.

Cross-regulation of viral kinases with cyclin A secures shutoff of host DNA synthesis

Herpesviruses encode conserved protein kinases (CHPKs) to stimulate phosphorylation-sensitive processes during infection. How CHPKs bind to cellular factors and how this impacts their regulatory functions is poorly understood. Here, we use quantitative proteomics to determine cellular interaction partners of human herpesvirus (HHV) CHPKs. We find that CHPKs can target key regulators of transcription and replication. The interaction with Cyclin A and associated factors is identified as a signature of β-herpesvirus kinases. Cyclin A is recruited via RXL motifs that overlap with nuclear localization signals (NLS) in the non-catalytic N termini. This architecture is conserved in HHV6, HHV7 and rodent cytomegaloviruses. Cyclin A binding competes with NLS function, enabling dynamic changes in CHPK localization and substrate phosphorylation. The cytomegalovirus kinase M97 sequesters Cyclin A in the cytosol, which is essential for viral inhibition of cellular replication. Our data highlight a fine-tuned and physiologically important interplay between a cellular cyclin and viral kinases.

Distinct and sequential re-replication barriers ensure precise genome duplication

Achieving complete and precise genome duplication requires that each genomic segment be replicated only once per cell division cycle. Protecting large eukaryotic genomes from re-replication requires an overlapping set of molecular mechanisms that prevent the first DNA replication step, the DNA loading of MCM helicase complexes to license replication origins, after S phase begins. Previous reports have defined many such origin licensing inhibition mechanisms, but the temporal relationships among them are not clear, particularly with respect to preventing re-replication in G2 and M phases. Using a combination of mutagenesis, biochemistry, and single cell analyses in human cells, we define a new mechanism that prevents re-replication through hyperphosphorylation of the essential MCM loading protein, Cdt1. We demonstrate that Cyclin A/CDK1 can hyperphosphorylate Cdt1 to inhibit MCM re-loading in G2 phase. The mechanism of inhibition is to block Cdt1 binding to MCM independently of other known Cdt1 inactivation mechanisms such as Cdt1 degradation during S phase or Geminin binding. Moreover, our findings suggest that Cdt1 dephosphorylation at the mitosis-to-G1 phase transition re-activates Cdt1. We propose that multiple distinct, non-redundant licensing inhibition mechanisms act in a series of sequential relays through each cell cycle phase to ensure precise genome duplication.

The initial step of DNA replication is loading the DNA helicase, MCM, onto DNA during the first phase of the cell division cycle. If MCM loading occurs inappropriately onto DNA that has already been replicated, then cells risk DNA re-replication, a source of endogenous DNA damage and genome instability. How mammalian cells prevent any sections of their very large genomes from re-replicating is still not fully understood. We found that the Cdt1 protein, one of the critical MCM loading factors, is inhibited specifically in late cell cycle stages through a mechanism involving protein phosphorylation. This phosphorylation prevents Cdt1 from binding MCM; when Cdt1 cannot be phosphorylated MCM is inappropriately re-loaded onto DNA and cells are prone to re-replication. When cells divide and transition into G1 phase, Cdt1 is then dephosphorylated to re-activate it for MCM loading. Based on these findings we assert that the different mechanisms that cooperate to avoid re-replication are not redundant. Instead, different cell cycle phases are dominated by different re-replication control mechanisms. These findings have implications for understanding how genomes are duplicated precisely once per cell cycle and shed light on how that process is perturbed by changes in Cdt1 levels or phosphorylation activity.

Geminin is required for Hox gene regulation to pattern the developing limb

Development of the complex structure of the vertebrate limb requires carefully orchestrated interactions between multiple regulatory pathways and proteins. Among these, precise regulation of 5' Hox transcription factor expression is essential for proper limb bud patterning and elaboration of distinct limb skeletal elements. Here, we identified Geminin (Gmnn) as a novel regulator of this process. A conditional model of Gmnn deficiency resulted in loss or severe reduction of forelimb skeletal elements, while both the forelimb autopod and hindlimb were unaffected. 5' Hox gene expression expanded into more proximal and anterior regions of the embryonic forelimb buds in this Gmnn-deficient model. A second conditional model of Gmnn deficiency instead caused a similar but less severe reduction of hindlimb skeletal elements and hindlimb polydactyly, while not affecting the forelimb. An ectopic posterior SHH signaling center was evident in the anterior hindlimb bud of Gmnn-deficient embryos in this model. This center ectopically expressed Hoxd13, the HOXD13 target Shh, and the SHH target Ptch1, while these mutant hindlimb buds also had reduced levels of the cleaved, repressor form of GLI3, a SHH pathway antagonist. Together, this work delineates a new role for Gmnn in modulating Hox expression to pattern the vertebrate limb.

Metachromins X and Y from a marine sponge Spongia sp. and their effects on cell cycle progression

New sesquiterpene quinones, metachromins X (1) and Y (2), together with the known metachromins C (3), J (4), and T (5), were isolated as inhibitors of cell cycle progression in the HeLa/Fucci2 cells. The structure of 1 was assigned by spectroscopic data and confirmed by a total synthesis. The planar structure of 2 was determined by interpretation of spectroscopic data, whereas its absolute configuration was analyzed by a combination of chiral HPLC and CD spectroscopy. Metachromins X (1) and C (3) arrested the cell cycle progression of HeLa/Fucci2 cells at S/G2/M phase.

Control of Eukaryotic DNA Replication Initiation-Mechanisms to Ensure Smooth Transitions

DNA replication differs from most other processes in biology in that any error will irreversibly change the nature of the cellular progeny. DNA replication initiation, therefore, is exquisitely controlled. Deregulation of this control can result in over-replication characterized by repeated initiation events at the same replication origin. Over-replication induces DNA damage and causes genomic instability. The principal mechanism counteracting over-replication in eukaryotes is a division of replication initiation into two steps—licensing and firing—which are temporally separated and occur at distinct cell cycle phases. Here, we review this temporal replication control with a specific focus on mechanisms ensuring the faultless transition between licensing and firing phases.

Improvement of the knock-in effciency in the genome of human induced pluripotent stem cells using the CRISPR/Cas9 system

Human induced pluripotent stem (hiPS) cells are a powerful tool for biomedical research. The ability to create patient-specifc pluripotent cells and their subsequent differentiation into any somatic cell type makes hiPS cells a valuable object for creating in vitro models of human diseases, screening drugs and a future source of cells for regenerative medicine. To realize entirely a potential of hiPScells, effective and precise methods for their genome editing are needed. The CRISPR/Cas9 system is the most widely used method for introducing site-specifc double-stranded breaks into DNA. It allows genes of interest to be knocked out with high efciency. However, knock-in into the target site of the genome is a much more difcult task. Moreover, many researchers have noted a low efciency of introducing target constructs into the hiPS cells’ genome. In this review, I attempt to describe the currently known information regarding the matter of increasing efciency of targeted insertions into hiPS cells’ genome. Here I will describe the most effective strategies for designing the donor template for homology-directed repair, methods to manipulate the double-strand break repair pathways introduced by a nuclease, including control of CRISPR/Cas9 delivery time. A low survival rate of hiPS cells following genome editing experiments is another difculty on the way towards successful knock-in, and here several highly effective approaches addressing it are proposed. Finally, I describe the most promising strategies, one-step reprogramming and genome editing, which allows gene-modifed integration-free hiPS cells to be efciently generated directly from somatic cells.

DNA replication licensing proteins: Saints and sinners in cancer

DNA replication is all-or-none process in the cell, meaning, once the DNA replication begins it proceeds to completion. Hence, to achieve maximum control of DNA replication, eukaryotic cells employ a multi-subunit initiator protein complex known as "pre-replication complex or DNA replication licensing complex (DNA replication LC). This complex involves multiple proteins which are origin-recognition complex family proteins, cell division cycle-6, chromatin licensing and DNA replication factor 1, and minichromosome maintenance family proteins. Higher-expression of DNA replication LC proteins appears to be an early event during development of cancer since it has been a common hallmark observed in a wide variety of cancers such as oesophageal, laryngeal, pulmonary, mammary, colorectal, renal, urothelial etc. However, the exact mechanisms leading to the abnormally high expression of DNA replication LC have not been clearly deciphered. Increased expression of DNA replication LC leads to licensing and/or firing of multiple origins thereby inducing replication stress and genomic instability. Therapeutic approaches where the reduction in the activity of DNA replication LC was achieved either by siRNA or shRNA techniques, have shown increased sensitivity of cancer cell lines towards the anti-cancer drugs such as cisplatin, 5-Fluorouracil, hydroxyurea etc. Thus, the expression level of DNA replication LC within the cell determines a cell's fate thereby creating a paradox where DNA replication LC acts as both "Saint" and "Sinner". With a potential to increase sensitivity to chemotherapy drugs, DNA replication LC proteins have prospective clinical importance in fighting cancer. Hence, in this review, we will shed light on importance of DNA replication LC with an aim to use DNA replication LC in diagnosis and prognosis of cancer in patients as well as possible therapeutic targets for cancer therapy.

Regulation of the initiation of DNA replication in human cells

The origin of species would not have been possible without high fidelity DNA replication and complex genomes evolved with mechanisms that control the initiation of DNA replication at multiple origins on multiple chromosomes such that the genome is duplicated once and only once. The mechanisms that control the assembly and activation of the replicative helicase and the initiation of DNA replication in yeast and Xenopus egg extract systems have been identified and reviewed [1,2]. The goal of this review is to organize currently available data on the mechanisms that control the initiation of DNA replication in human cells.

The replication initiation determinant protein (RepID) modulates replication by recruiting CUL4 to chromatin

Cell cycle progression in mammals is modulated by two ubiquitin ligase complexes, CRL4 and SCF, which facilitate degradation of chromatin substrates involved in the regulation of DNA replication. One member of the CRL4 complex, the WD-40 containing protein RepID (DCAF14/PHIP), selectively binds and activates a group of replication origins. Here we show that RepID recruits the CRL4 complex to chromatin prior to DNA synthesis, thus playing a crucial architectural role in the proper licensing of chromosomes for replication. In the absence of RepID, cells rely on the alternative ubiquitin ligase, SKP2-containing SCF, to progress through the cell cycle. RepID depletion markedly increases cellular sensitivity to SKP2 inhibitors, which triggered massive genome re-replication. Both RepID and SKP2 interact with distinct, non-overlapping groups of replication origins, suggesting that selective interactions of replication origins with specific CRL components execute the DNA replication program and maintain genomic stability by preventing re-initiation of DNA replication.

RepID has previously been shown to promote origin firing. Here the authors reveal that RepID regulates replication origins via the recruitment of the CRL4 complex, and prevents re-initiation and unscheduled DNA replication.

Spatiotemporal Labeling of Melanocytes in Mice

Melanocytes are pigment producing cells in the skin that give rise to cutaneous malignant melanoma, which is a highly aggressive and the deadliest form of skin cancer. Studying melanocytes in vivo is often difficult due to their small proportion in the skin and the lack of specific cell surface markers. Several genetically-engineered mouse models (GEMMs) have been created to specifically label the melanocyte compartment. These models give both spatial and temporal control over the expression of a cellular ‘beacon’ that has an added benefit of inducible expression that can be activated on demand. Two powerful models that are discussed in this review include the melanocyte-specific, tetracycline-inducible green fluorescent protein expression system (iDct-GFP), and the fluorescent ubiquitination-based cell cycle indicator (FUCCI) model that allows for the monitoring of the cell-cycle. These two systems are powerful tools in studying melanocyte and melanoma biology. We discuss their current uses and how they could be employed to help answer unresolved questions in the fields of melanocyte and melanoma biology.

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.

Regulation and Function of Cdt1; A Key Factor in Cell Proliferation and Genome Stability

Successful cell proliferation requires efficient and precise genome duplication followed by accurate chromosome segregation. The Cdc10-dependent transcript 1 protein (Cdt1) is required for the first step in DNA replication, and in human cells Cdt1 is also required during mitosis. Tight cell cycle controls over Cdt1 abundance and activity are critical to normal development and genome stability. We review here recent advances in elucidating Cdt1 molecular functions in both origin licensing and kinetochore–microtubule attachment, and we describe the current understanding of human Cdt1 regulation.

MMSET is dynamically regulated during cell-cycle progression and promotes normal DNA replication

The timely and precise duplication of cellular DNA is essential for maintaining genome integrity and is thus tightly-regulated. During mitosis and G1, the Origin Recognition Complex (ORC) binds to future replication origins, coordinating with multiple factors to load the minichromosome maintenance (MCM) complex onto future replication origins as part of the pre-replication complex (pre-RC). The pre-RC machinery, in turn, remains inactive until the subsequent S phase when it is required for replication fork formation, thereby initiating DNA replication. Multiple myeloma SET domain-containing protein (MMSET, a.k.a. WHSC1, NSD2) is a histone methyltransferase that is frequently overexpressed in aggressive cancers and is essential for normal human development. Several studies have suggested a role for MMSET in cell-cycle regulation; however, whether MMSET is itself regulated during cell-cycle progression has not been examined. In this study, we report that MMSET is degraded during S phase in a cullin-ring ligase 4-Cdt2 (CRL4(Cdt2)) and proteasome-dependent manner. Notably, we also report defects in DNA replication and a decreased association of pre-RC factors with chromatin in MMSET-depleted cells. Taken together, our results suggest a dynamic regulation of MMSET levels throughout the cell cycle, and further characterize the role of MMSET in DNA replication and cell-cycle progression.

A Cas9 Variant for Efficient Generation of Indel-Free Knockin or Gene-Corrected Human Pluripotent Stem Cells

While Cas9 nucleases permit rapid and efficient generation of gene-edited cell lines, the CRISPR-Cas9 system can introduce undesirable “on-target” mutations within the second allele of successfully modified cells via non-homologous end joining (NHEJ). To address this, we fused the Streptococcus pyogenes Cas9 (SpCas9) nuclease to a peptide derived from the human Geminin protein (SpCas9-Gem) to facilitate its degradation during the G₁ phase of the cell cycle, when DNA repair by NHEJ predominates. We also use mRNA transfection to facilitate low and transient expression of modified and unmodified versions of Cas9. Although the frequency of homologous recombination was similar for SpCas9-Gem and SpCas9, we observed a marked reduction in the capacity for SpCas9-Gem to induce NHEJ-mediated indels at the target locus. Moreover, in contrast to native SpCas9, we demonstrate that transient SpCas9-Gem expression enables reliable generation of both knockin reporter cell lines and genetically repaired patient-specific induced pluripotent stem cell lines free of unwanted mutations at the targeted locus.

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“On-target” deleterious mutations are often encountered with CRISPR-Cas9 gene editing

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SpCas9-Gem retains gene-targeting efficiency with reduced NHEJ activity

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SpCas9-Gem facilitates derivation of “indel-free” gene-edited iPSCs

Regulation of Unperturbed DNA Replication by Ubiquitylation

Posttranslational modification of proteins by means of attachment of a small globular protein ubiquitin (i.e., ubiquitylation) represents one of the most abundant and versatile mechanisms of protein regulation employed by eukaryotic cells. Ubiquitylation influences almost every cellular process and its key role in coordination of the DNA damage response is well established. In this review we focus, however, on the ways ubiquitylation controls the process of unperturbed DNA replication. We summarise the accumulated knowledge showing the leading role of ubiquitin driven protein degradation in setting up conditions favourable for replication origin licensing and S-phase entry. Importantly, we also present the emerging major role of ubiquitylation in coordination of the active DNA replication process: preventing re-replication, regulating the progression of DNA replication forks, chromatin re-establishment and disassembly of the replisome at the termination of replication forks.

Effect of X-Irradiation at Different Stages in the Cell Cycle on Individual Cell-Based Kinetics in an Asynchronous Cell Population

Using an asynchronously growing cell population, we investigated how X-irradiation at different stages of the cell cycle influences individual cell–based kinetics. To visualize the cell-cycle phase, we employed the fluorescent ubiquitination-based cell cycle indicator (Fucci). After 5 Gy irradiation, HeLa cells no longer entered M phase in an order determined by their previous stage of the cell cycle, primarily because green phase (S and G2) was less prolonged in cells irradiated during the red phase (G1) than in those irradiated during the green phase. Furthermore, prolongation of the green phase in cells irradiated during the red phase gradually increased as the irradiation timing approached late G1 phase. The results revealed that endoreduplication rarely occurs in this cell line under the conditions we studied. We next established a method for classifying the green phase into early S, mid S, late S, and G2 phases at the time of irradiation, and then attempted to estimate the duration of G2 arrest based on certain assumptions. The value was the largest when cells were irradiated in mid or late S phase and the smallest when they were irradiated in G1 phase. In this study, by closely following individual cells irradiated at different cell-cycle phases, we revealed for the first time the unique cell-cycle kinetics in HeLa cells that follow irradiation.

FUCCI sensors: powerful new tools for analysis of cell proliferation

Visualizing the cell cycle behavior of individual cells within living organisms can facilitate the understanding of developmental processes such as pattern formation, morphogenesis, cell differentiation, growth, cell migration, and cell death. Fluorescence Ubiquitin Cell Cycle Indicator (FUCCI) technology offers an accurate, versatile, and universally applicable means of achieving this end. In recent years, the FUCCI system has been adapted to several model systems including flies, fish, mice, and plants, making this technology available to a wide range of researchers for studies of diverse biological problems. Moreover, a broad range of FUCCI‐expressing cell lines originating from diverse cell types have been generated, hence enabling the design of advanced studies that combine in vivo experiments and cell‐based methods such as high‐content screening. Although only a short time has passed since its introduction, the FUCCI technology has already provided fundamental insight into how cells establish quiescence and how G1 phase length impacts the balance between pluripotency and stem cell differentiation. Further discoveries using the FUCCI technology are sure to come. WIREs Dev Biol 2015, 4:469–487. doi: 10.1002/wdev.189

This article is categorized under: 1

Adult Stem Cells, Tissue Renewal, and Regeneration > Methods and Principles

2

Technologies > Generating Chimeras and Lineage Analysis

3

Technologies > Analysis of Cell, Tissue, and Animal Phenotypes

DNA replication and oncogene-induced replicative stress

DNA replication must be tightly regulated to ensure that the genome is accurately duplicated during each cell cycle. When these regulatory mechanisms fail, replicative stress and DNA damage ensue. Activated oncogenes promote replicative stress, inducing a DNA damage response (DDR) early in tumorigenesis. Senescence or apoptosis result, forming a barrier against tumour progression. This may provide a selective pressure for acquisition of mutations in the DDR pathway during tumorigenesis. Despite its potential importance in early cancer development, the precise nature of oncogene-induced replicative stress remains poorly understood. Here, we review our current understanding of replication initiation and its regulation, describe mechanisms by which activated oncogenes might interfere with these processes and discuss how replicative stress might contribute to the genomic instability seen in cancers.

SCF-FBXO31 E3 ligase targets DNA replication factor Cdt1 for proteolysis in the G2 phase of cell cycle to prevent re-replication

FBXO31 was originally identified as a putative tumor suppressor gene in breast, ovarian, hepatocellular, and prostate cancers. By screening a set of cell cycle-regulated proteins as potential FBXO31 interaction partners, we have now identified Cdt1 as a novel substrate. Cdt1 DNA replication licensing factor is part of the pre-replication complex and essential for the maintenance of genomic integrity. We show that FBXO31 specifically interacts with Cdt1 and regulates its abundance by ubiquitylation leading to subsequent degradation. We also show that Cdt1 regulation by FBXO31 is limited to the G2 phase of the cell cycle and is independent of the pathways previously described for Cdt1 proteolysis in S and G2 phase. FBXO31 targeting of Cdt1 is mediated through the N terminus of Cdt1, a region previously shown to be responsible for its cell cycle regulation. Finally, we show that Cdt1 stabilization due to FBXO31 depletion results in re-replication. Our data present an additional pathway that contributes to the FBXO31 function as a tumor suppressor.

Visualizing spatiotemporal dynamics of multicellular cell-cycle progressions with fucci technology

The visualization of cell-cycle behavior of individual cells within complex tissues presents an irresistible challenge to biologists studying multicellular structures. However, the transition from G1 to S in the cell cycle is difficult to monitor despite the fact that the process involves the critical decision to initiate a new round of DNA replication. Here, we use ubiquitination oscillators that control cell-cycle transitions to develop genetically encoded fluorescent probes for cell-cycle progression. Fucci (fluorescent ubiquitination-based cell-cycle indicator) probes exploit the regulation of cell-cycle-dependent ubiquitination to effectively label individual nuclei in G1 phase red, and those in S/G2/M phases green. Cultured cells and transgenic mice constitutively expressing the probes have been generated, such that every cell nucleus shows either red or green fluorescence. This protocol details two experiments that use biological samples expressing Fucci probes. One experiment involves time-lapse imaging of cells stably expressing a Fucci derivative (Fucci2), which allows for the exploration of the spatiotemporal patterns of cell-cycle dynamics during structural and behavioral changes of cultured cells. The other experiment involves large-field, high-resolution imaging of fixed sections of Fucci transgenic mouse embryos, which provides maps that illustrate cell proliferation versus differentiation in various developing organs.

Identification of herpesvirus proteins that contribute to G1/S arrest

Lytic infection by herpesviruses induces cell cycle arrest at the G1/S transition. This appears to be a function of multiple herpesvirus proteins, but only a minority of herpesvirus proteins have been examined for cell cycle effects. To gain a more comprehensive understanding of the viral proteins that contribute to G1/S arrest, we screened a library of over 200 proteins from herpes simplex virus type 1, human cytomegalovirus, and Epstein-Barr virus (EBV) for effects on the G1/S interface, using HeLa fluorescent, ubiquitination-based cell cycle indicator (Fucci) cells in which G1/S can be detected colorimetrically. Proteins from each virus were identified that induce accumulation of G1/S cells, predominantly tegument, early, and capsid proteins. The identification of several capsid proteins in this screen suggests that incoming viral capsids may function to modulate cellular processes. The cell cycle effects of selected EBV proteins were further verified and examined for effects on p53 and p21 as regulators of the G1/S transition. Two EBV replication proteins (BORF2 and BMRF1) were found to induce p53 but not p21, while a previously uncharacterized tegument protein (BGLF2) was found to induce p21 protein levels in a p53-independent manner. Proteomic analyses of BGLF2-interacting proteins identified interactions with the NIMA-related protein kinase (NEK9) and GEM-interacting protein (GMIP). Silencing of either NEK9 or GMIP induced p21 without affecting p53 and abrogated the ability of BGLF2 to further induce p21. Collectively, these results suggest multiple viral proteins contribute to G1/S arrest, including BGLF2, which induces p21 levels likely by interfering with the functions of NEK9 and GMIP.

IMPORTANCE

Most people are infected with multiple herpesviruses, whose proteins alter the infected cells in several ways. During lytic infection, the viral proteins block cell proliferation just before the cellular DNA replicates. We used a novel screening method to identify proteins from three different herpesviruses that contribute to this block. Several of the proteins we identified had previously unknown functions or were structural components of the virion. Subsets of these proteins from Epstein-Barr virus were studied for their effects on the cell cycle regulatory proteins p53 and p21, thereby identifying two proteins that induce p53 and one that induces p21 (BGLF2). We identified interactions of BGLF2 with two human proteins, both of which regulate p21, suggesting that BGLF2 induces p21 by interfering with the functions of these two host proteins. Our study indicates that multiple herpesvirus proteins contribute to the cell proliferation block, including components of the incoming virions.

Fucci2a: a bicistronic cell cycle reporter that allows Cre mediated tissue specific expression in mice

Markers of cell cycle stage allow estimation of cell cycle dynamics in cell culture and during embryonic development. The Fucci system incorporates genetically encoded probes that highlight G1 and S/G2/M phases of the cell cycle allowing live imaging. However the available mouse models that incorporate Fucci are beset by problems with transgene inactivation, varying expression level, lack of conditional potential and/or the need to maintain separate transgenes-there is no transgenic mouse model that solves all these problems. To address these shortfalls we re-engineered the Fucci system to create 2 bicistronic Fucci variants incorporating both probes fused using the Thosea asigna virus 2A (T2A) self cleaving peptide. We characterize these variants in stable 3T3 cell lines. One of the variants (termed Fucci2a) faithfully recapitulated the nuclear localization and cell cycle stage specific florescence of the original Fucci system. We go on to develop a conditional mouse allele (R26Fucci2aR) carefully designed for high, inducible, ubiquitous expression allowing investigation of cell cycle status in single cell lineages within the developing embryo. We demonstrate the utility of R26Fucci2aR for live imaging by using high resolution confocal microscopy of ex vivo lung, kidney and neural crest development. Using our 3T3 system we describe and validate a method to estimate cell cycle times from relatively short time-lapse sequences that we then apply to our neural crest data. The Fucci2a system and the R26Fucci2aR mouse model are compelling new tools for the investigation of cell cycle dynamics in cell culture and during mouse embryonic development.

Multi-step loading of human minichromosome maintenance proteins in live human cells

Once-per-cell cycle replication is regulated through the assembly onto chromatin of multisubunit protein complexes that license DNA for a further round of replication. Licensing consists of the loading of the hexameric MCM2-7 complex onto chromatin during G1 phase and is dependent on the licensing factor Cdt1. In vitro experiments have suggested a two-step binding mode for minichromosome maintenance (MCM) proteins, with transient initial interactions converted to stable chromatin loading. Here, we assess MCM loading in live human cells using an in vivo licensing assay on the basis of fluorescence recovery after photobleaching of GFP-tagged MCM protein subunits through the cell cycle. We show that, in telophase, MCM2 and MCM4 maintain transient interactions with chromatin, exhibiting kinetics similar to Cdt1. These are converted to stable interactions from early G1 phase. The immobile fraction of MCM2 and MCM4 increases during G1 phase, suggestive of reiterative licensing. In late G1 phase, a large fraction of MCM proteins are loaded onto chromatin, with maximal licensing observed just prior to S phase onset. Fluorescence loss in photobleaching experiments show subnuclear concentrations of MCM-chromatin interactions that differ as G1 phase progresses and do not colocalize with sites of DNA synthesis in S phase.

Cyclin I is involved in the regulation of cell cycle progression

Cyclins control cell cycle progression by regulating the activity of cyclin-dependent kinases (Cdks). Cyclin I is a member of the cyclin family because of the presence of a cyclin box motif. It has been suggested that Cyclin I is involved in various biological processes, such as cell survival, angiogenesis, and cell differentiation. However, whether or not Cyclin I has a role in regulating the cell cycle similarly to other cyclins has yet to be clarified. Therefore, we investigated the role for Cyclin I in cell cycle progression. We showed that the protein level of Cyclin I oscillated during the cell cycle, and that Cyclin I was subjected to ubiquitination and degradation in cells. The interaction between Cyclin I and Cdk5 was detected in cells overexpressed with both proteins. Furthermore, depletion of Cyclin I by siRNAs prevented cell proliferation, suggesting the positive role of Cyclin I for the cell cycle progression. In addition, flow cytometric analysis revealed that cells depleted of Cyclin I were accumulated at G₂/M phases. By using HeLa.S-Fucci (fluorescent ubiquitination-based cell cycle indicator) cells, we further confirmed that knockdown of Cyclin I induced cell cycle arrest at S/G₂/M phases. These results strongly suggest that Cyclin I has the role in the regulation of cell cycle progression.

Unusual expression of red fluorescence at M phase induced by anti-microtubule agents in HeLa cells expressing the fluorescent ubiquitination-based cell cycle indicator (Fucci)

Plinabulin (NPI-2358) is a novel microtubule-depolymerizing agent. In HeLa cells, plinabulin arrests the cell-cycle at M phase and subsequently induces mitotic catastrophe. To better understand the effects on this compound on the cell-cycle, we used the fluorescent ubiquitination-based cell cycle indicator (Fucci), which normally enables G1 and S/G2/M cells to emit red and green fluorescence, respectively. When HeLa-Fucci cells were treated with 50 nM plinabulin, cells began to fluoresce both green and red in an unusual pattern; most cells exhibited the new pattern after 24 h of treatment. X-irradiation efficiently induced G2 arrest in plinabulin-treated cells and significantly retarded the emergence of the unusual pattern, suggesting that entering M phase is essential for induction of the pattern. By simultaneously visualizing chromosomes with GFP-histone H2B, we established that the pattern emerges after nuclear envelope breakdown but before metaphase. Pedigree assay revealed a significant relationship between the unusual expression and mitotic catastrophe. Nocodazole, KPU-133 (a more potent derivative of plinabulin), and paclitaxel also exerted similar effects. From these data, we conclude that the unusual pattern may be associated with dysregulation of late M phase-specific E3 ligase activity and mitotic catastrophe following treatment with anti-microtubule agents.

Re-replication induced by geminin depletion occurs from G2 and is enhanced by checkpoint activation

To prevent re-replication of DNA in a single cell cycle, the licensing of replication origins by Mcm2-7 is prevented during S and G2 phases. Animal cells achieve this by cell-cycle-regulated proteolysis of the essential licensing factor Cdt1 and inhibition of Cdt1 by geminin. Here we investigate the consequences of ablating geminin in synchronised human U2OS cells. Following geminin loss, cells complete an apparently normal S phase, but a proportion arrest at the G2-M boundary. When Cdt1 accumulates in these cells, DNA re-replicates, suggesting that the key role of geminin is to prevent re-licensing in G2. If cell cycle checkpoints are inhibited in cells lacking geminin, cells progress through mitosis and less re-replication occurs. Checkpoint kinases thereby amplify re-replication into an all-or-nothing response by delaying geminin-depleted cells in G2. Deep DNA sequencing revealed no preferential re-replication of specific genomic regions after geminin depletion. This is consistent with the observation that cells in G2 have lost their replication timing information. By contrast, when Cdt1 is overexpressed or is stabilised by the neddylation inhibitor MLN4924, re-replication can occur throughout S phase.

Cdt1p, through its interaction with Mcm6p, is required for the formation, nuclear accumulation and chromatin loading of the MCM complex

Regulation of DNA replication initiation is essential for the faithful inheritance of genetic information. Replication initiation is a multi-step process involving many factors including ORC, Cdt1p, Mcm2-7p and other proteins that bind to replication origins to form a pre-replicative complex (pre-RC). As a prerequisite for pre-RC assembly, Cdt1p and the Mcm2-7p heterohexameric complex accumulate in the nucleus in G1 phase in an interdependent manner in budding yeast. However, the nature of this interdependence is not clear, nor is it known whether Cdt1p is required for the assembly of the MCM complex. In this study, we provide the first evidence that Cdt1p, through its interaction with Mcm6p with the C-terminal regions of the two proteins, is crucial for the formation of the MCM complex in both the cytoplasm and nucleoplasm. We demonstrate that disruption of the interaction between Cdt1p and Mcm6p prevents the formation of the MCM complex, excludes Mcm2-7p from the nucleus, and inhibits pre-RC assembly and DNA replication. Our findings suggest a function for Cdt1p in promoting the assembly of the MCM complex and maintaining its integrity by interacting with Mcm6p.

Cdt1 and geminin in DNA replication initiation

One of the mechanisms controlling the initiation of DNA replication is the dynamic interaction between Cdt1, which promotes assembly of the pre-replication license complex, and Geminin, which inhibits it. Specifically, Cdt1 cooperates with the cell cycle protein Cdc6 to promote loading of the minichromosome maintenance helicases (MCM) onto the chromatin-bound origin recognition complex (ORC), by directly interacting with the MCM complex, and by modulating histone acetylation and inducing chromatin unfolding. Geminin, on the other hand, prevents the loading of the MCM onto the ORC both by directly binding to Cdt1, and by modulating Cdt1 stability and activity. Protein levels of Geminin and Cdt1 are tightly regulated through the cell cycle, and the Cdt1-Geminin complex likely acts as a molecular switch that can enable or disable the firing of each origin of replication. In this review we summarize structural studies of Cdt1 and Geminin and subsequent insights into how this molecular switch may function to ensure DNA is faithfully replicated only once during S phase of each cell cycle.

Multiple Cdt1 molecules act at each origin to load replication-competent Mcm2-7 helicases

Eukaryotic origins of replication are selected by loading a head-to-head double hexamer of the Mcm2-7 replicative helicase around origin DNA. Cdt1 plays an essential but transient role during this event; however, its mechanism of action is unknown. Through analysis of Cdt1 mutations, we demonstrate that Cdt1 performs multiple functions during helicase loading. The C-terminus of Cdt1 binds Mcm2-7, and this interaction is required for efficient origin recruitment of both proteins. We show that origin recognition complex (ORC) and Cdc6 recruit multiple Cdt1 molecules to the origin during helicase loading, and disruption of this multi-Cdt1 intermediate prevents helicase loading. Although dispensable for loading Mcm2-7 double hexamers that are topologically linked to DNA, the essential N-terminal domain of Cdt1 is required to load Mcm2-7 complexes that are competent for association with the Cdc45 and GINS helicase-activating proteins and replication initiation. Our data support a model in which origin-bound ORC and Cdc6 recruit two Cdt1 molecules to initiate double-hexamer formation prior to helicase loading and demonstrate that Cdt1 influences the replication competence of loaded Mcm2-7 helicases.

Mechanism of CRL4(Cdt2), a PCNA-dependent E3 ubiquitin ligase

Eukaryotic cell cycle transitions are driven by E3 ubiquitin ligases that catalyze the ubiquitylation and destruction of specific protein targets. For example, the anaphase-promoting complex/cyclosome (APC/C) promotes the exit from mitosis via destruction of securin and mitotic cyclins, whereas CRL1(Skp2) allows entry into S phase by targeting the destruction of the cyclin-dependent kinase (CDK) inhibitor p27. Recently, an E3 ubiquitin ligase called CRL4(Cdt2) has been characterized, which couples proteolysis to DNA synthesis via an unusual mechanism that involves display of substrate degrons on the DNA polymerase processivity factor PCNA. Through its destruction of Cdt1, p21, and Set8, CRL4(Cdt2) has emerged as a master regulator that prevents rereplication in S phase. In addition, it also targets other factors such as E2F and DNA polymerase η. In this review, we discuss our current understanding of the molecular mechanism of substrate recognition by CRL4(Cdt2) and how this E3 ligase helps to maintain genome integrity.

Geminin overexpression prevents the completion of topoisomerase IIα chromosome decatenation, leading to aneuploidy in human mammary epithelial cells

INTRODUCTION

The nuclear enzyme topoisomerase IIα (TopoIIα) is able to cleave DNA in a reversible manner, making it a valuable target for agents such as etoposide that trap the enzyme in a covalent bond with the 5' DNA end to which it cleaves. This prevents DNA religation and triggers cell death in cancer cells. However, development of resistance to these agents limits their therapeutic use. In this study, we examined the therapeutic targeting of geminin for improving the therapeutic potential of TopoIIα agents.

METHODS

Human mammary epithelial (HME) cells and several breast cancer cell lines were used in this study. Geminin, TopoIIα and cell division cycle 7 (Cdc7) silencing were done using specific small interfering RNA. Transit or stable inducible overexpression of these proteins and casein kinase Iε (CKIε) were also used, as well as several pharmacological inhibitors that target TopoIIα, Cdc7 or CKIε. We manipulated HME cells that expressed H2B-GFP, or did not, to detect chromosome bridges. Immunoprecipitation and direct Western blot analysis were used to detect interactions between these proteins and their total expression, respectively, whereas interactions on chromosomal arms were detected using a trapped in agarose DNA immunostaining assay. TopoIIα phosphorylation by Cdc7 or CKIε was done using an in vitro kinase assay. The TopoGen decatenation kit was used to measure TopoIIα decatenation activity. Finally, a comet assay and metaphase chromosome spread were used to detect chromosome breakage and changes in chromosome condensation or numbers, respectively.

RESULTS

We found that geminin and TopoIIα interact primarily in G2/M/early G1 cells on chromosomes, that geminin recruits TopoIIα to chromosomal decatenation sites or vice versa and that geminin silencing in HME cells triggers the formation of chromosome bridges by suppressing TopoIIα access to chromosomal arms. CKIε kinase phosphorylates and positively regulates TopoIIα chromosome localization and function. CKIε kinase overexpression or Cdc7 kinase silencing, which we show phosphorylates TopoIIα in vitro, restored DNA decatenation and chromosome segregation in geminin-silenced cells before triggering cell death. In vivo, at normal concentration, geminin recruits the deSUMOylating sentrin-specific proteases SENP1 and SENP2 enzymes to deSUMOylate chromosome-bound TopoIIα and promote its release from chromosomes following completion of DNA decatenation. In cells overexpressing geminin, premature departure of TopoIIα from chromosomes is thought to be due to the fact that geminin recruits more of these deSUMOylating enzymes, or recruits them earlier, to bound TopoIIα. This triggers premature release of TopoIIα from chromosomes, which we propose induces aneuploidy in HME cells, since chromosome breakage generated through this mechanism were not sensed and/or repaired and the cell cycle was not arrested. Expression of mitosis-inducing proteins such as cyclin A and cell division kinase 1 was also increased in these cells because of the overexpression of geminin.

CONCLUSIONS

TopoIIα recruitment and its chromosome decatenation function require a normal level of geminin. Geminin silencing induces a cytokinetic checkpoint in which Cdc7 phosphorylates TopoIIα and inhibits its chromosomal recruitment and decatenation and/or segregation function. Geminin overexpression prematurely deSUMOylates TopoIIα, triggering its premature departure from chromosomes and leading to chromosomal abnormalities and the formation of aneuploid, drug-resistant cancer cells. On the basis of our findings, we propose that therapeutic targeting of geminin is essential for improving the therapeutic potential of TopoIIα agents.

Contribution of caspase(s) to the cell cycle regulation at mitotic phase

Caspases have been suggested to contribute to not only apoptosis regulation but also non-apoptotic cellular phenomena. Recently, we have reported the involvement of caspase-7 to the cell cycle progression at mitotic phase by knockdown of caspase-7 using small interfering RNAs and short hairpin RNA. Here we showed that chemically synthesized broad-spectrum caspase inhibitors, which have been used to suppress apoptosis, prevented the cell proliferation in a dose-dependent manner, and that the subtype-specific peptide-based caspase inhibitor for caspase-3 and -7, but not for caspase-9, inhibited cell proliferation. It was also indicated that the BIR2 domain of X-linked inhibitor of apoptosis protein, functioning as an inhibitor for caspase-3 and -7, but not the BIR3 domain which plays as a caspase-9 inhibitor, induced cell cycle arrest. Furthermore, flow cytometry revealed that the cells treated with caspase inhibitors arrested at G₂/M phase. By using HeLa.S-Fucci (fluorescent ubiquitination-based cell cycle indicator) cells, the prevention of the cell proliferation by caspase inhibitors induced cell cycle arrest at mitotic phase accompanying the accumulation of the substrates for APC/C, suggesting the impairment of the APC/C activity at the transition from M to G₁ phases. These results indicate that caspase(s) contribute to the cell cycle regulation at mitotic phase.

Coordination of mitosis and morphogenesis: role of a prolonged G2 phase during chordate neurulation

Chordates undergo a characteristic morphogenetic process during neurulation to form a dorsal hollow neural tube. Neurulation begins with the formation of the neural plate and ends when the left epidermis and right epidermis overlying the neural tube fuse to close the neural fold. During these processes, mitosis and the various morphogenetic movements need to be coordinated. In this study, we investigated the epidermal cell cycle in Ciona intestinalis embryos in vivo using a fluorescent ubiquitination-based cell cycle indicator (Fucci). Epidermal cells of Ciona undergo 11 divisions as the embryos progress from fertilization to the tadpole larval stage. We detected a long G2 phase between the tenth and eleventh cell divisions, during which fusion of the left and right epidermis occurred. Characteristic cell shape change and actin filament regulation were observed during the G2 phase. CDC25 is probably a key regulator of the cell cycle progression of epidermal cells. Artificially shortening this G2 phase by overexpressing CDC25 caused precocious cell division before or during neural tube closure, thereby disrupting the characteristic morphogenetic movement. Delaying the precocious cell division by prolonging the S phase with aphidicolin ameliorated the effects of CDC25. These results suggest that the long interphase during the eleventh epidermal cell cycle is required for neurulation.

Characterization of five polyamine oxidase isoforms in Arabidopsis thaliana

The genome of Arabidopsis thaliana contains five genes (AtPAO1 to AtPAO5) encoding polyamine oxidase (PAO) which is an enzyme responsible for polyamine catabolism. To understand the individual roles of the five AtPAOs, here we characterized their tissue-specific and space-temporal expression. AtPAO1 seems to have a specific function in flower organ. AtPAO2 was expressed in shoot meristem and root tip of seedlings, and to a higher extent in the later growth stage within restricted parts of the organs, such as shoot meristem, leaf petiole and also in anther. The expression of AtPAO3 was constitutive, but highest in flower organ. AtPAO3 promoter activity was detected in cotyledon, distal portion of root, boundary region of mature rosette leaf and in filaments of flower. AtPAO4 was expressed at higher level all over young seedlings including roots, and in the mature stage its expression was ubiquitous with rather lower level in stem. AtPAO5 expression was observed in the whole plant body throughout various growth stages. Its highest expression was in flowers, particularly in sepals, but not in petals. Furthermore, we determined the substrate specificity of AtPAO1 to AtPAO4. None of the AtPAO enzymes recognized putrescine (Put). AtPAO2 and AtPAO3 showed almost similar substrate recognition patterns in which the most preferable substrate is spermidine (Spd) followed by less specificity to other tetraamines tested. AtPAO4 seemed to be spermine (Spm)-specific. More interestingly, AtPAO1 preferred thermospermine (T-Spm) and norspermine (NorSpm) to Spm, but did not recognize Spd. Based on the results, the individual function of AtPAOs is discussed.

Life without geminin

The interplay of proliferation and differentiation is essential for normal development and organogenesis. Geminin is a cell cycle regulator which controls licensing of origins for DNA replication, safeguarding genomic stability. Geminin has also been shown to regulate cellular decisions of self-renewal versus commitment of neuronal progenitor cells. We discuss here our recent analysis of mice with conditional inactivation of the Geminin gene in the immune system. Our data indicate that Geminin is not indispensable for every cell division: in the absence of Geminin, development of progenitor T cells appears largely unaffected. In contrast, rapid cell divisions, taking place in vitro upon TCR receptor activation or in vivo during homeostatic proliferation, are defective.

DNA replication origins, ORC/DNA interaction, and assembly of pre-replication complex in eukaryotes

Chromosomal DNA replication in eukaryotic cells is highly complicated and sophisticatedly regulated. Owing to its large size, a typical eukaryotic genome contains hundreds to tens of thousands of initiation sites called DNA replication origins where DNA synthesis takes place. Multiple initiation sites remove the constraint of a genome size because only a certain amount of DNA can be replicated from a single origin in a limited time. The activation of these multiple origins must be coordinated so that each segment of chromosomal DNA is precisely duplicated only once per cell cycle. Although DNA replication is a vital process for cell growth and its mechanism is highly conserved, recent studies also reveal significant diversity in origin structure, assembly of pre-replication complex (pre-RC) and regulation of replication initiation along evolutionary lines. The DNA replication origins in the fission yeast Schizosaccharomyces pombe are found to contain a second essential element that is bound by Sap1 protein besides the essential origin recognition complex-binding site. Sap1 is recently demonstrated to be a novel replication initiation protein that plays an essential role in loading the initiation protein Cdc18 to origins and thus directly participates in pre-RC formation. In this review, we summarize the recent advance in understanding how DNA replication origins are organized, how pre-RC is assembled and how DNA replication is initiated and regulated in yeast and metazoans.

Differential geminin requirement for proliferation of thymocytes and mature T cells

Stem/progenitor cells coordinate proliferation and differentiation, giving rise to appropriate cell numbers of functionally specialized cells during organogenesis. In different experimental systems, Geminin was shown to maintain progenitor cells and participate in fate determination decisions and organogenesis. Although the exact mechanisms are unclear, Geminin has been postulated to influence proliferation versus differentiation decisions. To gain insight into the in vivo role of Geminin in progenitor cell division and differentiation, we have generated mice that specifically lack Geminin in cells of lymphoid lineage through Cre-mediated recombination. T cells lacking Geminin expression upregulate early activation markers efficiently upon TCR stimulation in vitro and are able to enter the S phase of cell cycle, but show a marked defect in completing the cycle, leading to a large proportion of T cells accumulating in S/G2/M phases. Accordingly, T cells deficient in Geminin show a reduced ability to repopulate lymphopenic hosts in vivo. Contrary to expectations, Geminin deficiency does not alter development and differentiation of T cells in vivo. Our data suggest that Geminin is required for the proliferation events taking place either in vitro upon TCR receptor activation or during homeostatic expansion, but appears to be redundant for the proliferation and differentiation of the majority of progenitor T cell populations.

Quaternary structure of the human Cdt1-Geminin complex regulates DNA replication licensing

All organisms need to ensure that no DNA segments are rereplicated in a single cell cycle. Eukaryotes achieve this through a process called origin licensing, which involves tight spatiotemporal control of the assembly of prereplicative complexes (pre-RCs) onto chromatin. Cdt1 is a key component and crucial regulator of pre-RC assembly. In higher eukaryotes, timely inhibition of Cdt1 by Geminin is essential to prevent DNA rereplication. Here, we address the mechanism of DNA licensing inhibition by Geminin, by combining X-ray crystallography, small-angle X-ray scattering, and functional studies in Xenopus and mammalian cells. Our findings show that the Cdt1:Geminin complex can exist in two distinct forms, a "permissive" heterotrimer and an "inhibitory" heterohexamer. Specific Cdt1 residues, buried in the heterohexamer, are important for licensing. We postulate that the transition between the heterotrimer and the heterohexamer represents a molecular switch between licensing-competent and licensing-defective states.

Illuminating cell-cycle progression in the developing zebrafish embryo

By exploiting the cell-cycle-dependent proteolysis of two ubiquitination oscillators, human Cdt1 and geminin, which are the direct substrates of SCF(Skp2) and APC(Cdh1) complexes, respectively, Fucci technique labels mammalian cell nuclei in G(1) and S/G(2)/M phases with different colors. Transgenic mice expressing these G(1) and S/G(2)/M markers offer a powerful means to investigate the coordination of the cell cycle with morphogenetic processes. We attempted to introduce these markers into zebrafish embryos to take advantage of their favorable optical properties. However, although the fundamental mechanisms for cell-cycle control appear to be well conserved among species, the G(1) marker based on the SCF(Skp2)-mediated degradation of human Cdt1 did not work in fish cells, probably because the marker was not ubiquitinated properly by a fish E3 ligase complex. We describe here the generation of a Fucci derivative using zebrafish homologs of Cdt1 and geminin, which provides sweeping views of cell proliferation in whole fish embryos. Remarkably, we discovered two anterior-to-posterior waves of cell-cycle transitions, G(1)/S and M/G(1), in the differentiating notochord. Our study demonstrates the effectiveness of using the Cul4(Ddb1)-mediated Cdt1 degradation pathway common to all metazoans for the development of a G(1) marker that works in the nonmammalian animal model.