Reversible disassembly of somatic nucleoli by the germ cell proteins FRGY2a and FRGY2b

Effects of Cellular Extract on Epigenetic Reprogramming

Functional reprogramming of a differentiated cell toward pluripotent cell may have long-term applications in numerous aspects, especially in regenerative medicine. Evidences accumulating from recent studies suggest that cellular extracts from stem cells or pluripotent cells can induce epigenetic reprogramming and facilitate pluripotency in otherwise highly differentiated cell types. Epigenetic reprogramming using cellular extracts has gained increasing attention and applied to recognize the functional factors, acquire the target cell types, and explain the mechanism of reprogramming. Now, more and more researches have proved that cellular extract treatment is an important strategy of cellular reprogramming. Thus, this review mainly focused on the progresses and potential mechanisms in epigenetic reprogramming using cellular extracts.

Combined positive effect of oocyte extracts and brilliant cresyl blue stained recipient cytoplasts on epigenetic reprogramming and gene expression in buffalo nuclear transfer embryos

This study examined the effects of buffalo oocyte extracts (BOE) on donor cells reprogramming and molecular characterisation of oocytes screened via brilliant cresyl blue (BCB) staining and comparison of gene expression profiles of developmentally important genes in blastocysts from IVF and cloned derived from BOE treated donor cells with BCB selected recipient cytoplasts. Relative abundance (RA) of OCT4 and NANOG was increased (P < 0.05) and HDAC-1, DNMT-1, and DNMT-3A decreased (P < 0.05) in extract treated cells (ETCs). This ETCs dedifferentiated into neuron-like lineage under appropriate induction condition. The RA of NASP, EEF1A1, DNMT1, ODC1 and RPS27A was increased (P < 0.05) in BCB+ oocytes, whereas ATP5A1 and S100A10 increased (P < 0.05) in BCB- oocytes. Total cell number and RA of OCT4, NANOG, SOX2, DNMT1, IGF2, IGF2R, MNSOD, GLUT1, BAX and BCL2 in cloned blastocysts derived from BCB+ oocytes with ETC more closely followed that of IVF counterparts compared to BCB+ oocytes with extract untreated cell and BCB- oocytes with ETC derived blastocysts. In conclusion, BOE influenced epigenetic reprogramming of buffalo fibroblasts making them suitable donors for nuclear transfer (NT). BCB staining can be effectively used for selection of developmentally competent oocytes for NT. The combined effects of epigenetic reprogramming of donor nuclei by BOE and higher nuclear reprogramming capacity of BCB+ oocytes improve developmentally important gene expression in cloned blastocysts. Whether these improvements have long-term effects on buffalo calves born following embryo transfer remains unknown.

RE1-silencing Transcription Factor (REST) Is Required for Nuclear Reprogramming by Inhibiting Transforming Growth Factor β Signaling Pathway

Differentiated cells can be reprogrammed by transcription factors, and these factors that are responsible for successful reprogramming need to be further identified. Here, we show that the neuronal repressor RE1-silencing transcription factor (REST) is rich in porcine oocytes and requires for nuclear transfer (NT)-mediated reprogramming through inhibiting TGFβ signaling pathway. REST was dramatically degraded after oocyte activation, but the residual REST was incorporated into the transferred donor nuclei during reprogramming in NT embryos. Inhibition of REST function in oocytes compromised the development of NT embryos but not that of IVF and PA embryos. Bioinformation analysis of putative targets of REST indicated that REST might function on reprogramming in NT embryos by inhibiting TGFβ pathway. Further results showed that the developmental failure of REST-inhibited NT embryos could be rescued by treatment of SB431542, an inhibitor of TGFβ pathway. Thus, REST is a newly discovered transcription factor that is required for NT-mediated nuclear reprogramming.

Long-term effect on in vitro cloning efficiency after treatment of somatic cells with Xenopus egg extract in the pig

In somatic cell nuclear transfer (SCNT), donor cell reprogramming is considered as a biologically important and vulnerable event. Various donor cell pre-treatments with Xenopus egg extracts can promote reprogramming. Here we investigated if the reprogramming effect of one treatment with Xenopus egg extract on donor cells was maintained for several cell passages. The extract treatment resulted in increased cell-colony formation from early passages in treated porcine fibroblasts (ExTES), and increased development of cloned embryos. Partial dedifferentiation was observed in ExTES cells, shown as a tendency towards upregulation of NANOG, c-MYC and KLF-4 and downregulation of DESMIM compared with ExTES at Passage 2. Compared with our routine SCNT, continuously increased development of cloned embryos was observed in the ExTES group, and ExTES cloned blastocysts displayed hypermethylated DNA patterns and hypermethylation of H3K4me3 and H3K27me3 in ICM compared with TE. All seven recipients became pregnant after transferral of ExTES cloned embryos and gave birth to 7-22 piglets per litter (average 12). In conclusion, our results demonstrate that one treatment of porcine fibroblasts with Xenopus egg extract can result in long-term increased ability of the cells to promote their in vitro function in subsequent SCNT. Finally these cells can also result in successful development of cloned embryos to term.

Stress induces cell dedifferentiation in plants

Accumulating evidence lends support to the proposal that a major theme in plant responses to stresses is dedifferentiation, whereby mature cells acquire stem cell features (e.g. open chromatin conformation) prior to acquisition of a new cell fate. In this review, we discuss data addressing plant cell plasticity and provide evidence linking stress, dedifferentiation and a switch in cell fate. We emphasize the epigenetic modifications associated with stress-induced global changes in chromatin structure and conclude with the implications for genetic variation and for induced pluripotent stem cells in animals. It appears that stress is perceived as a signal that directs plant cells to undergo reprogramming (dedifferentiation) as a means for adaptation and in preparation for a stimulus-based acquisition of a new cell fate. This article is part of a Special Issue entitled: Stress as a fundamental theme in cell plasticity.

Reprogrammed peripheral blood mononuclear cells are able to survive longer in irradiated female mice

Induced multipotent stem (iMS) cells are originated from somatic cells and become multipotent by genetic and/or epigenetic modifications. Previous studies have shown that the fish oocytes extracts (FOE) can induce skin fibroblast cells into iMS cells. In this study, we aim to determine whether FOE can similarly induce mouse peripheral blood mononuclear cells (PBMCs) into the iMS state and if so, whether they can survive longer when they are transplanted into the irradiation female mice. PBMCs of GFP-transgenic male mice were cultured and transiently reprogrammed by FOE. They were deemed reaching the iMS state after detection of expression of stem cell markers. The iMS-like PBMCs were transplanted into female C57BL mice by tail vein injection. The spleen wet weights as well as numbers of colonies of the recipient mice were examined. The results showed the spleen wet weights and numbers of spleen colonies of FOE-induced group were all significantly higher than those of the non-induced group and negative control group. On day 90 after transplantation, FISH analysis detected the presence of Y chromosome in the induced group, but not of the other groups. The current findings demonstrate that FOE-induced PBMCs are able to survive longer in irradiated female mice.

Xenopus egg extract treatment reduced global DNA methylation of donor cells and enhanced somatic cell nuclear transfer embryo development in pigs

The efficiency to produce offspring by somatic cell nuclear transfer (SCNT) is low. It has been showed that treatment of donor cells with Xenopus oocyte extract increased live births in ovine and handmade cloned embryo development in pigs. Scriptaid treatment after oocyte activation is another approach to improve SCNT efficiency. The present study was carried out to investigate (a) the effects of treatment of donor cells with Xenopus egg extract on donor cell DNA methylation at days 0 and 4 with two digitonin permeabilization concentrations (10 and 15 μg/mL), (b) the effects of treatment of donor cells with Xenopus egg extract on early development of cloned embryos, and (c) the effects of combined treatments, treating donor cells with extract before nuclear transfer and treatment of cloned embryos with scriptaid after oocyte activation, on embryo development. Compared to the control, a decrease of DNA methylation in donor cells was observed at 2.5 h after extract treatment. However, this effect was not observed after the cells were cultured for four more days. More embryos developed into blastocysts in the Xenopus egg extract-treated group than in the control (13.4±1.9% vs. 9.1±1.9%, p=0.01). Furthermore, scriptaid treatment of cloned embryos further increased the frequency of development to blastocyst, compared to the control reconstructed with the same extract-treated cells (22.5±0.9% vs. 15.3±0.9%, p<0.01). In addition, egg extract treatments increased the cell number in the blastocysts. This study demonstrated that Xenopus egg extract treatment reduced donor cell DNA methylation and enhanced the SCNT embryo development. Moreover, the combined treatments of donor cells with egg extract before nuclear transfer and of cloned embryos with scriptaid could improve cloned embryo development additively.

Y-box-binding protein 1 (YB-1) and its functions

This review describes the structure and functions of Y-box binding protein 1 (YB-1) and its homologs. Interactions of YB-1 with DNA, mRNAs, and proteins are considered. Data on the participation of YB-1 in DNA reparation and transcription, mRNA splicing and translation are systematized. Results on interactions of YB-1 with cytoskeleton components and its possible role in mRNA localization are discussed. Data on intracellular distribution of YB-1, its redistribution between the nucleus and the cytoplasm, and its secretion and extracellular functions are summarized. The effect of YB-1 on cell differentiation, its involvement in extra- and intracellular signaling pathways, and its role in early embryogenesis are described. The mechanisms of regulation of YB-1 expression in the cell are presented. Special attention is paid to the involvement of YB-1 in oncogenic cell transformation, multiple drug resistance, and dissemination of tumors. Both the oncogenic and antioncogenic activities of YB-1 are reviewed. The potential use of YB-1 in diagnostics and therapy as an early cancer marker and a molecular target is discussed.

Nuclear transfer to eggs and oocytes

We review experiments in which somatic cell nuclei are transplanted singly to enucleated eggs (metaphase II) in amphibia and mammals and as multiple nuclei to the germinal vesicle of amphibian oocytes (prophase I). These experiments have shown the totipotency of some somatic cell nuclei, as well as switches in cell type and changes in gene expression. Abnormalities of nuclear transplant embryo development increase greatly as nuclei are taken from progressively more differentiated donor cells. The molecular changes that accompany the reprogramming of transplanted nuclei help to indicate the mechanisms used by eggs and oocytes to reprogram gene expression. We discuss the importance of chromosomal protein exchange, of transcription factor supply, and of chromatin access in reprogramming.

Identification and characterization of an oocyte factor required for development of porcine nuclear transfer embryos

Nuclear reprogramming of differentiated cells can be induced by oocyte factors. Despite numerous attempts, these factors and mechanisms responsible for successful reprogramming remain elusive. Here, we identify one such factor, necessary for the development of nuclear transfer embryos, using porcine oocyte extracts in which some reprogramming events are recapitulated. After incubating somatic nuclei in oocyte extracts from the metaphase II stage, the oocyte proteins that were specifically and abundantly incorporated into the nuclei were identified by mass spectrometry. Among 25 identified proteins, we especially focused on a multifunctional protein, DJ-1. DJ-1 is present at a high concentration in oocytes from the germinal vesicle stage until embryos at the four-cell stage. Inhibition of DJ-1 function compromises the development of nuclear transfer embryos but not that of fertilized embryos. Microarray analysis of nuclear transfer embryos in which DJ-1 function is inhibited shows perturbed expression of P53 pathway components. In addition, embryonic arrest of nuclear transfer embryos injected with anti-DJ-1 antibody is rescued by P53 inhibition. We conclude that DJ-1 is an oocyte factor that is required for development of nuclear transfer embryos. This study presents a means for identifying natural reprogramming factors in mammalian oocytes and a unique insight into the mechanisms underlying reprogramming by nuclear transfer.

Examination of DNA methyltransferase expression in cloned embryos reveals an essential role for Dnmt1 in bovine development

In studies of somatic cell nuclear transfer (SCNT), the ability of factors within the oocyte to epigenetically reprogram transferred nuclei is essential for embryonic development of the clone to proceed. However, irregular patterns of X-chromosome inactivation, abnormal expression of imprinted genes, and genomic DNA hypermethylation are frequently observed in reconstructed embryos, suggesting abnormalities in this process. To better understand the epigenetic events underlying SCNT reprogramming, we sought to determine if the abnormal DNA methylation levels observed in cloned embryos result from a failure of the oocyte to properly reprogram transcription versus differential biochemical regulation of the DNA methyltransferase family of enzymes (DNMTs) between embryonic and somatic nuclei. To address this question, we conducted real-time quantitation of Dnmt transcripts in bovine preimplantation embryos generated though in vitro fertilization (IVF), parthenogenic activation, and SCNT. By the 8-cell stage, transcripts encoding Dnmt1 become significantly down-regulated in cloned embryos, likely in response to the state of genomic hypermethylation, while the de novo methyltransferases maintain an expression pattern indistinguishable from their IVF and parthenote counterparts. Depletion of embryonic/maternal Dnmt1 transcripts within IVF embryos using short-interfering RNAs, while able to lower genomic DNA methylation levels, resulted in developmental arrest at the 8/16-cell stage. In contrast, SCNT embryos derived from a stable, Dnmt1-depleted donor cell line develop to blastocyst stage, but failed to carry to term. Our results indicate an essential role for Dnmt1 during bovine preimplantation development, and suggest proper transcriptional reprogramming of this gene family in SCNT embryos.

The nucleolus

When cells are observed by phase contrast microscopy, nucleoli are among the most conspicuous structures. The nucleolus was formally described between 1835 and 1839, but it was another century before it was discovered to be associated with a specific chromosomal locus, thus defining it as a cytogenetic entity. Nucleoli were first isolated in the 1950s, from starfish oocytes. Then, in the early 1960s, a boomlet of studies led to one of the epochal discoveries in the modern era of genetics and cell biology: that the nucleolus is the site of ribosomal RNA synthesis and nascent ribosome assembly. This epistemologically repositioned the nucleolus as not merely an aspect of nuclear anatomy but rather as a cytological manifestation of gene action-a major heuristic advance. Indeed, the finding that the nucleolus is the seat of ribosome production constitutes one of the most vivid confluences of form and function in the history of cell biology. This account presents the nucleolus in both historical and contemporary perspectives. The modern era has brought the unanticipated discovery that the nucleolus is plurifunctional, constituting a paradigm shift.

RNA content in the nucleolus alters p53 acetylation via MYBBP1A

A number of external and internal insults disrupt nucleolar structure, and the resulting nucleolar stress stabilizes and activates p53. We show here that nucleolar disruption induces acetylation and accumulation of p53 without phosphorylation. We identified three nucleolar proteins, MYBBP1A, RPL5, and RPL11, involved in p53 acetylation and accumulation. MYBBP1A was tethered to the nucleolus through nucleolar RNA. When rRNA transcription was suppressed by nucleolar stress, MYBBP1A translocated to the nucleoplasm and facilitated p53-p300 interaction to enhance p53 acetylation. We also found that RPL5 and RPL11 were required for rRNA export from the nucleolus. Depletion of RPL5 or RPL11 blocked rRNA export and counteracted reduction of nucleolar RNA levels caused by inhibition of rRNA transcription. As a result, RPL5 or RPL11 depletion inhibited MYBBP1A translocation and p53 activation. Our observations indicated that a dynamic equilibrium between RNA generation and export regulated nucleolar RNA content. Perturbation of this balance by nucleolar stress altered the nucleolar RNA content and modulated p53 activity.

Lamin B1 maintains the functional plasticity of nucleoli

The dynamic ability of genomes to interact with discrete nuclear compartments appears to be essential for chromatin function. However, the extent to which structural nuclear proteins contribute to this level of organization is largely unresolved. To test the links between structure and function, we evaluated how nuclear lamins contribute to the organization of a major functional compartment, the nucleolus. HeLa cells with compromised expression of the genes encoding lamins were analyzed using high-resolution imaging and pull-down assays. When lamin B1 expression was depleted, inhibition of RNA synthesis correlated with complex structural changes within the nucleolar active centers until, eventually, the nucleoli were dispersed completely. With normal lamin expression, the nucleoli were highly plastic, with dramatic and freely reversible structural changes correlating with the demand for ribosome biogenesis. Preservation of the nucleolar compartment throughout these structural transitions is shown to be linked to lamin B1 expression, with the lamin B1 protein interacting with the major nucleolar protein nucleophosmin/B23.

Chromatin position in human HepG2 cells: although being non-random, significantly changed in daughter cells

Mammalian chromosomes occupy chromosome territories within nuclear space the positions of which are generally accepted as non-random. However, it is still controversial whether position of chromosome territories/chromatin is maintained in daughter cells. We addressed this issue and investigated maintenance of various chromatin regions of unknown composition as well as nucleolus-associated chromatin, a significant part of which is composed of nucleolus organizer region-bearing chromosomes. The photoconvertible histone H4-Dendra2 was used to label such regions in transfected HepG2 cells, and its position was followed up to next interphase. The distribution of labeled chromatin in daughter cells exhibited a non-random character. However, its distribution in a vast majority of daughter cells extensively differed from the original ones and the labeled nucleolus-associated chromatin differently located into the vicinity of different nucleoli. Therefore, our results were not consistent with a concept of preservation chromatin position. This conclusion was supported by the finding that the numbers of nucleoli significantly differed between the two daughter cells. Our results support a view that while the transfected daughter HepG2 cells maintain some features of the parental cell chromosome organization, there is also a significant stochastic component associated with reassortment of chromosome territories/chromatin that results in their positional rearrangements.

Cell-free extracts from mammalian oocytes partially induce nuclear reprogramming in somatic cells

Nuclear transfer has been regarded as the only reliable tool for studying nuclear reprogramming of mammalian somatic cells by oocytes. However, nuclear transfer is not well suited for biochemical analyses of the molecular mechanisms of reprogramming. A cell-free system from oocytes is an attractive alternative way to mimic reprogramming in vitro, since a large number of cells can be treated and analyzed. Nevertheless, a cell-free system using oocytes has not been developed in mammals. Here, cell extracts from porcine oocytes were prepared and their ability to induce nuclear reprogramming was evaluated. Extracts from metaphase II (MII) oocytes erased the machinery for regulating gene expression in reversibly permeabilized somatic cells. For example, the extracts caused histone deacetylation and the disappearance of TATA box-binding protein from the nuclei. However, MII-extract-treated cells did not show any obvious changes after cell culture. In contrast, extracts from germinal vesicle (GV) oocytes activated pluripotent marker genes, especially NANOG, and induced partial dedifferentiation after cell culture. The activation of pluripotent marker genes by GV extracts was associated with histone acetylation that was induced during extract treatment. These results indicate that GV- and MII-oocyte extracts have different roles on nuclear reprogramming. Furthermore, both oocyte extracts induced site-specific demethylation in the upstream region of NANOG. These results indicate that cell-free extracts derived from GV- and MII-oocytes could be useful for studying the mechanisms involved in nuclear reprogramming.

Arabidopsis cold shock domain proteins: relationships to floral and silique development

Cold shock domain proteins (CSPs) are highly conserved from bacteria to higher plants and animals. Bacterial cold shock proteins function as RNA chaperones by destabilizing RNA secondary structures and promoting translation as an adaptative mechanism to low temperature stress. In animals, cold shock domain proteins exhibit broad functions related to growth and development. In order to understand better the function of CSPs in planta, detailed analyses were performed for Arabidopsis thaliana CSPs (AtCSPs) on the transcript and protein levels using an extensive series of tissue harvested throughout developmental stages within the entire life cycle of Arabidopsis. On both the transcript and protein levels, AtCSPs were enriched in shoot apical meristems and siliques. Although all AtCSPs exhibited similar expression patterns, AtCSP2 was the most abundantly expressed gene. In situ hybridization analyses were also used to confirm that AtCSP2 and AtCSP4 transcripts accumulate in developing embryos and shoot apices. AtCSPs transcripts were also induced during a controlled floral induction study. In vivo ChIP analysis confirmed that an embryo expressed MADS box transcription factor, AGL15, interacts within two AtCSP promoter regions and alters the respective patterns of AtCSP transcription. Comparative analysis of AtCSP gene expression between Landsberg and Columbia ecotypes confirmed a 1000-fold reduction of AtCSP4 gene expression in the Landsberg background. Analysis of the AtCSP4 genomic locus identified multiple polymorphisms in putative regulatory cis-elements between the two ecotypes. Collectively, these data support the hypothesis that AtCSPs are involved in the transition to flowering and silique development in Arabidopsis.

Chromatin: linking structure and function in the nucleolus

The nucleolus is an informative model structure for studying how chromatin-regulated transcription relates to nuclear organisation. In this review, we describe how chromatin controls nucleolar structure through both the modulation of rDNA activity by convergently-evolved remodelling complexes and by direct effects upon rDNA packaging. This packaging not only regulates transcription but may also be important for suppressing internal recombination between tandem rDNA repeats. The identification of nucleolar histone chaperones and novel chromatin proteins by mass spectrometry suggests that structure-specific chromatin components remain to be characterised and may regulate the nucleolus in novel ways. However, it also suggests that there is considerable overlap between nucleolar and non-nucleolar-chromatin components. We conclude that a fuller understanding of nucleolar chromatin will be essential for understanding how gene organisation is linked with nuclear architecture.

Strategies and new developments in the generation of patient-specific pluripotent stem cells

Generating pluripotent stem cells directly from cells obtained from patients is one of the ultimate goals in regenerative medicine. Two "reprogramming" strategies for the generation of pluripotent stem cells from somatic cells have been studied extensively: nuclear transfer to oocytes and fusion with ES cells. The recent demonstration that, in mouse, nuclear transfer into zygotes can also be effective if the recipient cells are arrested in mitosis provides an exciting new avenue for this type of approach. Patient-specific pluripotent cells could potentially also be generated by the spontaneous reprogramming of bone marrow cells, spermatogonial cells, and parthenogenetic embryos. A third overall type of strategy arose from the demonstration that pluripotent stem (iPS) cells can be generated from mouse fibroblasts by the introduction of four transcription factors (Oct-3/4, Sox2, c-Myc, and KLF4). Recent work has underlined the potential of this strategy by improving the efficiency of the process and demonstrating that iPS cells can contribute to many different tissues in vivo, including the germline. Taken together, these studies underscore the crucial roles of transcription factors and chromatin remodeling in nuclear reprogramming.

FBXO25-associated nuclear domains: a novel subnuclear structure

Skp1, Cul1, Rbx1, and the FBXO25 protein form a functional ubiquitin ligase complex. Here, we investigate the cellular distribution of FBXO25 and its colocalization with some nuclear proteins by using immunochemical and biochemical approaches. FBXO25 was monitored with affinity-purified antibodies raised against the recombinant fragment spanning residues 2-62 of the FBXO25 sequence. FBXO25 protein was expressed in all mouse tissues tested except striated muscle, as indicated by immunoblot analysis. Confocal analysis revealed that the endogenous FBXO25 was partially concentrated in a novel dot-like nuclear domain that is distinct from clastosomes and other well-characterized structures. These nuclear compartments contain a high concentration of ubiquitin conjugates and at least two other components of the ubiquitin-proteasome system: 20S proteasome and Skp1. We propose to name these compartments FBXO25-associated nuclear domains. Interestingly, inhibition of transcription by actinomycin D or heat-shock treatment drastically affected the nuclear organization of FBXO25-containing structures, indicating that they are dynamic compartments influenced by the transcriptional activity of the cell. Also, we present evidences that an FBXO25-dependent ubiquitin ligase activity prevents aggregation of recombinant polyglutamine-containing huntingtin protein in the nucleus of human embryonic kidney 293 cells, suggesting that this protein can be a target for the nuclear FBXO25 mediated ubiquitination.

Histone methylation controls telomerase-independent telomere lengthening in cells undergoing dedifferentiation

Cellular dedifferentiation underlies topical issues in biology such as regeneration and nuclear cloning and has common features in plants and animals. In plants, this process characterizes the transition of differentiated leaf cells to protoplasts (plant cells devoid of cell walls) and is accompanied by global chromatin reorganization associated with reprogramming of gene expression. A screen for mutants defective in proliferation and callus formation identified kyp-2-a mutant in the KRYPTONITE (KYP)/SUVH4 gene encoding a histone H3 lysine 9 (H3K9) methyltransferase. Analysis of telomere length revealed stochastic telomerase-independent lengthening of telomeres in wild type but not in kyp-2 protoplasts. In kyp-2 mutant, telomeric repeats were no longer associated with dimethylated H3K9. The Arabidopsis telomerase reverse transcriptase (tert) mutant displayed accelerated proliferation despite its short telomeres, though it also showed accelerated cell death. Microarray analysis uncovered several components of the ubiquitin proteolytic system, which are downregulated in kyp-2 compared to wild-type protoplasts. Thus, our results suggest that histone methylation activity is required for the establishment/maintenance of the dedifferentiated state and/or reentry into the cell cycle, at least partly, through activation of genes whose products are involved in the ubiquitin proteolytic pathway. In addition, our results illuminate the complexity of cellular dedifferentiation, particularly the occurrence of DNA recombination that can lead to genome instability.

Epigenomic differentiation in mouse preimplantation nuclei of biparental, parthenote and cloned embryos

Chromosomes, sub-chromosomal regions and genes are repositioned during cell differentiation to acquire a cell-type-specific spatial organization. The constraints that are responsible for this cell-type-specific spatial genome positioning are unknown. In this study we addressed the question of whether epigenetic genome modifications may represent constraints to the acquisition of a specific nuclear organization. The organization of kinetochores, pericentric heterochromatin and the nucleolus was analysed in pre-implantation mouse embryos obtained by in-vitro fertilization (IVF), parthenogenetic activation (P) and nuclear transfer (NT) of differentiated somatic nuclei, which possess different epigenomes. Each stage of pre-implantation embryonic development is characterized by a stage-specific spatial organization of nucleoli, kinetochores and pericentric heterochromatin. Despite differences in the frequencies and the time-course of nuclear architecture reprogramming events, by the eight-cell stage P and NT embryos achieved the same distinct nuclear organization in the majority of embryos as observed for IVF embryos. At this stage the gametic or somatic nuclear architecture of IVF or P and NT embryos, respectively, is replaced by a common embryonic nuclear architecture. This finding suggests that the epigenome of the three types of embryos partially acts as a constraint of the nuclear organization of the three nuclear subcompartments analysed.

Tpt1 activates transcription of oct4 and nanog in transplanted somatic nuclei

Nuclear transfer to eggs or oocytes provides a potential route for cell-replacement therapies because oocytes directly reprogram transplanted mammalian somatic-cell nuclei such that they have an embryo-like pattern of gene expression. This includes a large increase in the mRNA level of the stem-cell marker gene oct4. We have developed a novel procedure to identify new proteins that greatly increase the level of oct4 mRNA upon nuclear transfer. We have isolated Xenopus oocyte proteins that bind to the regulatory region of the mouse oct4 gene and identified these by mass spectrometry. The proteins include the retinoic-acid-receptor gamma, a known repressor of oct4 transcription, and Tpt1, a cancer-associated factor. The depletion of transcripts of retinoic-acid receptor gamma from oocytes increases oct4 and nanog transcription as expected, and depletion of tpt1 transcripts in oocytes reduces oct4 and nanog transcription in injected HeLa nuclei. An elevation of tpt1 transcripts in oocytes results in an earlier activation of oct4 transcription. Therefore, we identify a novel role for tpt1 in activating pluripotency genes upon nuclear transfer. Our results help to elucidate the mechanism by which somatic-cell nuclei are reprogrammed to have an embryo-like pattern of gene expression.

Spindle formation and microtubule organization during first division in reconstructed rat embryos produced by somatic cell nuclear transfer

The present study was conducted to demonstrate the spindle formation and behavior of chromosomes and microtubules during first division in reconstructed rat embryos produced by somatic cell nuclear transfer (SCNT) with cumulus cell nuclei. To demonstrate the effect of oocyte aging after ovulation on the cleavage of SCNT embryos, micromanipulation was carried out 11, 15 and 18 h after injection of hCG. SCNT oocytes were activated by incubation in culture medium supplemented with 5 microM ionomycin for 5 min followed by treatment with 2 mM 6-dimethylaminopurine (6-DMAP) in mR1ECM for 2-3 h. For immunocytochemical observation, the SCNT embryos were incubated with monoclonal anti-alpha-tubulin antibody and then fluorescein isothiocyanate (FITC)-conjugated goat anti-mouse IgG. Cleavage rates were significantly higher for oocytes collected after 15 and 18 h rather than for those collected 11 h after injection of hCG (56 and 53%, respectively vs. 28%; P<0.05). Premature chromosome condensation occurred before activation of the SCNT oocytes, but adequate spindle formation was only rarely observed. The distribution of microtubules in SCNT embryos after activation was different from those of fertilized and parthenogenic oocytes, i.e., a dense microtubule organization shaped like a ring was observed. Eighteen to 20 h post-activation, most SCNT embryos were in the 2-cell stage, but no nucleoli were clearly visible, which was quite different from the fertilized oocytes. In addition, first division with and without small cellular bodies containing DNA was observed in the rat SCNT embryos in some cases. The present study suggests that reorganization of transferred nuclei in rat SCNT embryos may be inadequate in terms of formation of the mitotic assembly and nucleolar reorganization.

Reprogramming events of mammalian somatic cells induced byXenopus laevis egg extracts

It is known that differentiated cells can be reprogrammed to an undifferentiated state in oocyte cytoplasm after nuclear transfer. Recently, some reports suggested that Xenopus egg extracts have the ability to reprogram mammalian somatic cells. Reprogramming events of mammalian cells after Xenopus egg extract treatment and after cell culture of extract-treated cells have not been elucidated. In this experiment, we examined reprogramming events in reversibly permeabilized or nonpermeabilized porcine fibroblast cells after Xenopus egg extract treatment. The Xenopus egg-specific histone B4 was assembled on porcine chromatin and nuclear lamin LIII was incorporated into nuclei. Deacetylation of histone H3 at lysine 9 in extract-treated cells was detected in nonpermeabilized cells, suggesting that a part of reprogramming may be induced even in nonpermeabilized cells. Following culture of extract-treated cells, the cells began to express the pluripotent marker genes such as POU5F1 (OCT4) and SOX2 and to form colonies. Reactivation of the OCT4 gene in extract-treated cells was also confirmed in bovine fibroblasts transformed with an OCT4-EGFP construct. These results suggest that nuclei of mammalian cells can be partially reprogrammed to an embryonic state by Xenopus egg extracts and the remodeled cells partly dedifferentiate after cell culture. A system using egg extracts may be useful for understanding the mechanisms and processes of dedifferentiation and reprogramming of mammalian somatic cells after nuclear transfer.

Epigenetic discrimination by mouse metaphase II oocytes mediates asymmetric chromatin remodeling independently of meiotic exit

In mammalian fertilization, paternal chromatin is exhaustively remodeled, yet the maternal contribution to this process is unknown. To address this, we prevented the induction of meiotic exit by spermatozoa and examined sperm chromatin remodeling in metaphase II (mII) oocytes. Methylation of paternal H3-K4 and H3-K9 remained low, unlike maternal H3, although paternal H3-K4 methylation increased in zygotes. Thus, mII cytoplasm can sustain epigenetic asymmetry in a cell-cycle dependent manner. Paternal genomic DNA underwent oocyte-mediated cytosine demethylation and acquired maternally-derived K12-acetylated H4 (AcH4-K12) independently of microtubule assembly and maternal chromatin. AcH4-K12 persisted without typical maturation-associated deacetylation, irrespective of paternal pan-genomic cytosine methylation. Contrastingly, somatic cell nuclei underwent rapid H4 deacetylation; sperm and somatic chromatin exhibited asymmetric AcH4-K12 dynamics simultaneously within the same mII oocyte. Inhibition of somatic histone deacetylation revealed endogenous histone acetyl transferase activity. Oocytes thus specify the histone acetylation status of given nuclei by differentially targeting histone deacetylase and acetyl transferase activities. Asymmetric H4 acetylation during and immediately after fertilization was dispensable for development when both parental chromatin sets were hyperacetylated. These studies delineate non-zygotic chromatin remodeling and suggest a powerful model with which to study de novo genomic reprogramming.

'Open minded' cells: how cells can change fate

It has long intrigued researchers why some but not all organisms can regenerate missing body parts. Plants are remarkable in that they can regenerate the entire organism from a small piece of tissue, or even a single cell. Epigenetic mechanisms that control chromatin organization are now known to regulate the cellular plasticity and reprogramming necessary for regeneration. Interestingly, although animals and plants have evolved different strategies and mechanisms to control developmental processes, they have maintained many similarities in the way they regulate chromatin organization. Given that plants can rapidly switch fate, we propose that an understanding of the mechanisms regulating this process in plant cells could provide a new perspective on cellular dedifferentiation in animals.

From Nuclear Transfer to Nuclear Reprogramming: The Reversal of Cell Differentiation

This is a personal historical account of events leading from the earliest success in vertebrate nuclear transfer to the current hope that nuclear reprogramming may facilitate cell replacement therapy. Early morphological evidence in Amphibia for the toti- or multipotentiality of some nuclei from differentiated cells first established the principle of the conservation of the genome during cell differentiation. Molecular markers show that many somatic cell nuclei are reprogrammed to an embryonic pattern of gene expression soon after nuclear transplantation to eggs. The germinal vesicles of oocytes in first meiotic prophase have a direct reprogramming activity on mammalian as well as amphibian nuclei and offer a route to identify nuclear reprogramming molecules. Amphibian eggs and oocytes have a truly remarkable ability to transcribe genes as DNA or nuclei, to translate mRNA, and to modify or localize proteins injected into them. The development of nuclear transplant embryos depends on the ability of cells to interpret small concentration changes of signal factors in the community effect and in morphogen gradients. Many difficulties in a career can be overcome by analyzing in increasing depth the same fundamentally interesting and important problem.

Cold shock domain family members YB-1 and MSY4 share essential functions during murine embryogenesis

Three cold shock domain (CSD) family members (YB-1, MSY2, and MSY4) exist in vertebrate species ranging from frogs to humans. YB-1 is expressed throughout embryogenesis and is ubiquitously expressed in adult animals; it protects cells from senescence during periods of proliferative stress. YB-1-deficient embryos die unexpectedly late in embryogenesis (embryonic day 18.5 [E18.5] to postnatal day 1) with a runting phenotype. We have now determined that MSY4, but not MSY2, is also expressed during embryogenesis; its abundance declines substantially from E9.5 to E17.5 and is undetectable on postnatal day 1(adult mice express MSY4 in testes only). Whole-mount analysis revealed similar patterns of YB-1 and MSY4 RNA expression in E11.5 embryos. To determine whether MSY4 delays the death of YB-1-deficient embryos, we created and analyzed MSY4-deficient mice and then generated YB-1 and MSY4 double-knockout embryos. MSY4 is dispensable for normal development and survival, but the testes of adult mice have excessive spermatocyte apoptosis and seminiferous tubule degeneration. Embryos doubly deficient for YB-1 and MSY4 are severely runted and die much earlier (E8.5 to E11.5) than YB-1-deficient embryos, suggesting that MSY4 indeed shares critical cellular functions with YB-1 in the embryonic tissues where they are coexpressed.

Structure and function of the nucleolus in the spotlight

The nucleolus is the most obvious and clearly differentiated nuclear sub-compartment. It is where ribosome biogenesis takes place, but it is becoming clear that the nucleolus also has non-ribosomal functions. In this review we discuss recent progress in our understanding of how both ribosome biosynthesis and some non-ribosomal functions relate to observable nucleolar structure. We still do not have detailed enough information about the in situ organization of the various processes taking place in the nucleolus. However, the present power of light and electron microscopy techniques means that a description of the organization of nucleolar processes at the molecular level is now achievable, and the time is ripe for such an effort.

Chromatin decondensation and nuclear reprogramming by nucleoplasmin

Somatic cell nuclear cloning has repeatedly demonstrated striking reversibility of epigenetic regulation of cell differentiation. Upon injection into eggs, the donor nuclei exhibit global chromatin decondensation, which might contribute to reprogramming the nuclei by derepressing dormant genes. Decondensation of sperm chromatin in eggs is explained by the replacement of sperm-specific histone variants with egg-type histones by the egg protein nucleoplasmin (Npm). However, little is known about the mechanisms of chromatin decondensation in somatic nuclei that do not contain condensation-specific histone variants. Here we found that Npm could widely decondense chromatin in undifferentiated mouse cells without overt histone exchanges but with specific epigenetic modifications that are relevant to open chromatin structure. These modifications included nucleus-wide multiple histone H3 phosphorylation, acetylation of Lys 14 in histone H3, and release of heterochromatin proteins HP1beta and TIF1beta from the nuclei. The protein kinase inhibitor staurosporine inhibited chromatin decondensation and these epigenetic modifications with the exception of H3 acetylation, potentially linking these chromatin events. At the functional level, Npm pretreatment of mouse nuclei facilitated activation of four oocyte-specific genes from the nuclei injected into Xenopus laevis oocytes. Future molecular elucidation of chromatin decondensation by Npm will significantly contribute to our understanding of the plasticity of cell differentiation.

Compartmentation of the nucleolar processing proteins in the granular component is a CK2-driven process

To analyze the compartmentation of nucleolar protein complexes, the mechanisms controlling targeting of nucleolar processing proteins onto rRNA transcription sites has been investigated. We studied the reversible disconnection of transcripts and processing proteins using digitonin-permeabilized cells in assays capable of promoting nucleolar reorganization. The assays show that the dynamics of nucleolar reformation is ATP/GTP-dependent, sensitive to temperature, and CK2-driven. We further demonstrate the role of CK2 on the rRNA-processing protein B23. Mutation of the major CK2 site on B23 induces reorganization of nucleolar components that separate from each other. This was confirmed in assays using extracts containing B23 mutated in the CK2-binding sites. We propose that phosphorylation controls the compartmentation of the rRNA-processing proteins and that CK2 is involved in this process.

Requirement of the protein B23 for nucleolar disassembly induced by the FRGY2a family proteins

In Xenopus somatic cell nuclear cloning, the nucleoli of donor nuclei rapidly and almost completely disappear in egg cytoplasm. We previously showed that the germ cell-specific proteins FRGY2a and FRGY2b were responsible for this unusually drastic nucleolar disassembly. The nucleolar disassembly occurs without inhibition of pre-rRNA transcription, a well known trigger for nucleolar segregation, and the mechanism for the nucleolar disassembly by FRGY2a and FRGY2b remains largely unknown. In this study, we searched for FRGY2a-interacting proteins and investigated the functional consequences of their interactions through a series of experiments. We showed that during the nucleolar disassembly, FRGY2a localized to the nucleoli of isolated nuclei and was capable of disassembling purified nucleoli, suggesting a direct interaction between FRGY2a and nucleolar components. Using a His tag pulldown approach, we identified the abundant and multifunctional nucleolar protein B23 as a potential target of FRGY2a and its related human protein YB1. A specific interaction between FRGY2a/YB1 and B23 was confirmed by co-immunoprecipitation. Finally, B23 knockdown using short interfering RNA and a subsequent add-back experiment confirmed that B23 was necessary for nucleolar disassembly by YB1. We propose that FRGY2a and YB1 disassemble nucleoli by sequestering B23, which is associated with pre-ribosomes and other structurally important nucleolar components.

Human Embryonic Stem Cells Reprogram Myeloid Precursors Following Cell-Cell Fusion

Here, we examine the ability of undifferentiated human embryonic stem cells (hESCs) to reprogram the nuclei of hESC-derived myeloid precursors following cell-cell fusion. Using an OP9 coculture system, we produced CD45+ CD33+ myeloperoxidase+ myeloid precursors from an Oct4-enhanced green fluorescent protein (EGFP) knock-in hESC line and demonstrated that Oct4-EGFP expression was extinguished in these precursors. Upon fusion with undifferentiated hESCs, EGFP expression from the endogenous Oct4 promoter/regulatory region was re-established, ESC-specific surface antigens and marker genes were expressed, and myeloid precursor-specific antigens were no longer detectable. When the hybrid cells were formed into embryoid bodies, upregulation of genes characteristic of the three germ layers and extraembryonic tissues occurred, indicating that the hybrid cells had the potential to differentiate into multiple lineages. Interestingly, the hybrid cells were capable of redifferentiating into myeloid precursors with efficiency comparable with that of diploid hESCs despite their neartetraploid chromosome complement. These results indicate that hESCs are capable of reprogramming nuclei from differentiated cells and that hESC hybrid cells provide a new model system for studying the mechanisms of nuclear reprogramming.

Nuclear remodeling assay in Xenopus egg extract

Xenopus egg extract is an ideal material to identify nuclear remodeling activities important for nuclear cloning. Since the protocol for egg extract preparation was established more than 20 yr ago, egg extract has been widely used as a source for the purification of factors associated with a number of physiological activities. The eggs are large, easily obtained in large quantities, and are abundant in a variety of bioactive proteins. Many aspects of somatic nuclear remodeling observed in nuclear cloning are recapitulated in somatic nuclei incubated in egg extract. Our in vitro nuclear remodeling assay has proven effective for the purification of two novel nuclear remodeling activities, ISWI, a key player in the dissociation of TATA binding protein from chromatin, and FRGY2a and FRGY2b, two proteins capable of nucleolar disassembly. Here we outline our protocol of egg extract preparation and in vitro nuclear remodeling assay, as well as the purification method for FRGY2a and FRGY2b. Our in vitro nuclear remodeling assay in combination with egg extract will serve as a powerful tool with which to biochemically uncover molecular events involved in nuclear cloning.

New Insights into Nucleolar Architecture and Activity

The nucleolus is the most obvious and clearly differentiated nuclear subcompartment. It is where ribosome biogenesis takes place and has been the subject of research over many decades. In recent years progress in our understanding of ribosome biogenesis has been rapid and is accelerating. This review discusses current understanding of how the biochemical processes of ribosome biosynthesis relate to an observable nucleolar structure. Emerging evidence is also described that points to other, unconventional roles for the nucleolus, particularly in the biogenesis of other RNA-containing cellular machinery, and in stress sensing and the control of cellular activity. Striking recent observations show that the nucleolus and its components are highly dynamic, and that the steady state structure observed by microscopical methods must be interpreted as the product of these dynamic processes. We still do not have detailed enough information to understand fully the organization and regulation of the various processes taking place in the nucleolus. However, the present power of light and electron microscopy (EM) techniques means that a description of nucleolar processes at the molecular level is now achievable, and the time is ripe for such an effort.

An adaptive multirate algorithm for acquisition of fluorescence microscopy data sets

We propose an algorithm for adaptive efficient acquisition of fluorescence microscopy data sets using a multirate (MR) approach. We simulate acquisition as part of a larger system for protein classification based on their subcellular location patterns and, thus, strive to maintain the achieved level of classification accuracy as much as possible. This problem is similar to image compression but unique due to additional restrictions, namely causality; we have access only to the information scanned up to that point. While we do want to acquire fewer samples with as low distortion as possible to achieve compression, our goal is to do so while affecting the overall classification accuracy as little as possible. We achieve this by using an adaptive MR scanning scheme which samples the regions of the image area that hold the most pertinent information. Our results show that we can achieve significant compression which we can then use to aquire faster or to increase space resolution of our data set, all while minimally affecting the classification accuracy of the entire system.

YB-1 is important for late-stage embryonic development, optimal cellular stress responses, and the prevention of premature senescence

Proteins containing "cold shock" domains belong to the most evolutionarily conserved family of nucleic acid-binding proteins known among bacteria, plants, and animals. One of these proteins, YB-1, is widely expressed throughout development and has been implicated as a cell survival factor that regulates the transcription and/or translation of many cellular growth and death-related genes. For these reasons, YB-1 deficiency has been predicted to be incompatible with cell survival. However, the majority of YB-1(-/-) embryos develop normally up to embryonic day 13.5 (E13.5). After E13.5, YB-1(-/-) embryos exhibit severe growth retardation and progressive mortality, revealing a nonredundant role of YB-1 in late embryonic development. Fibroblasts derived from YB-1(-/-) embryos displayed a normal rate of protein synthesis and minimal alterations in the transcriptome and proteome but demonstrated reduced abilities to respond to oxidative, genotoxic, and oncogene-induced stresses. YB-1(-/-) cells under oxidative stress expressed high levels of the G(1)-specific CDK inhibitors p16Ink4a and p21Cip1 and senesced prematurely; this defect was corrected by knocking down CDK inhibitor levels with specific small interfering RNAs. These data suggest that YB-1 normally represses the transcription of CDK inhibitors, making it an important component of the cellular stress response signaling pathway.

The slender lobes gene, identified by retarded mushroom body development, is required for proper nucleolar organization in Drosophila

The nucleolus dynamically alters its shape through the assembly and disassembly of a variety of nucleolar components in proliferating cells. While the nucleolus is known to function in vital cellular events, little is known about how its components are correctly assembled. Through the analysis of a Drosophila mutant that exhibits a reduced number of mushroom body (MB) neurons in the brain, we reveal that the slender lobes (sle) gene encodes a novel nuclear protein that affects nucleolar organization during development. In sle mutant neuroblasts, the nucleolus was packed more tightly, forming a dense sphere, and the nucleolar proteins fibrillarin and Nop60B were abnormally distributed in the interphase nucleolus. Moreover, another nucleolar marker, Aj1 antigen, was localized to the center of the nucleolus in a manner complementary to the Nop60B distribution, and also formed a large aggregate in the cytoplasm. While developmental defects were limited to a few tissues in sle mutants, including MBs and nurse cells, the altered organization of the nucleolar components were evident in most developing tissues. Therefore, we conclude that Sle is a general factor of nuclear architecture in Drosophila that is required for the correct organization of the nucleolus during development.

Reprogrammationet épigenèse

The fact that the nucleus of a differentiated somatic cell can be reprogrammed in order to sustain embryonic development is now well established. Experiments of somatic cell nuclear transfer (cloning) have proved that a foreign nucleus introduced into an enucleated oocyte can give rise to physiologically normal offsprings, with a normal lifespan. Such evidence of genome expression plasticity is also observed experimentally with heterokaryons, created by the fusion or the nuclear transfer between two somatic cells, where differentiated nuclei are able to express genes characteristic of the host cell. However, the epigenetic mechanisms that permit nuclear plasticity remain poorly understood. In this paper we present the main evidences showing important modifications of the large scale organisation of chromosomal domains and of the DNA methylation pattern upon nuclear transfer and during the first cleavages. These modifications of epigenetic marks, brought by an intimate contact between the chromatin and the recipient oocyte cytoplasmic factors, appear essential for further development. They are established over the first cell cycles of development. The onset of embryonic genome activation and the first cellular differentiation events that occur over the implantation period are two additional check-points of reprogramming that appear to be also highly dependent on epigenetic alterations. Beyond those stages, defective placental functions might be directly responsible for the fetal and postnatal physiopathologies frequently observed in cloned animals. No direct link between preimplantation reprogramming defaults, placental dysfunctions and low development to term has been established yet. The epigenetics studies which are now used to characterise loci specific and probably genotype dependent alterations in cloned animals of different species will provide invaluable help to define the role of epigenesis in the achievement of a developmental program.

Dynamic sorting of nuclear components into distinct nucleolar caps during transcriptional inhibition

Nucleolar segregation is observed under some physiological conditions of transcriptional arrest. This process can be mimicked by transcriptional arrest after actinomycin D treatment leading to the segregation of nucleolar components and the formation of unique structures termed nucleolar caps surrounding a central body. These nucleolar caps have been proposed to arise from the segregation of nucleolar components. We show that contrary to prevailing notion, a group of nucleoplasmic proteins, mostly RNA binding proteins, relocalized from the nucleoplasm to a specific nucleolar cap during transcriptional inhibition. For instance, an exclusively nucleoplasmic protein, the splicing factor PSF, localized to nucleolar caps under these conditions. This structure also contained pre-rRNA transcripts, but other caps contained either nucleolar proteins, PML, or Cajal body proteins and in addition nucleolar or Cajal body RNAs. In contrast to the capping of the nucleoplasmic components, nucleolar granular component proteins dispersed into the nucleoplasm, although at least two (p14/ARF and MRP RNA) were retained in the central body. The nucleolar caps are dynamic structures as determined using photobleaching and require energy for their formation. These findings demonstrate that the process of nucleolar segregation and capping involves energy-dependent repositioning of nuclear proteins and RNAs and emphasize the dynamic characteristics of nuclear domain formation in response to cellular stress.

Dynamics and compartmentation of the nucleolar processing machinery

In active nucleoli, machineries involved in the biogenesis of ribosomal RNAs (rRNAs) are compartmentalized. The late rRNA processing proteins are localized in the granular component (GC). Here we investigate the behavior of these proteins when production of 28S is impaired and when this blockage is reversed. The 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) provokes dispersion of rDNA clusters and we demonstrate that DRB induces disconnection of the late rRNA processing proteins from the transcription sites. These processing proteins are still associated in independent masses without detectable 28S rRNA, indicating that compartmentation of the late rRNA processing machinery is not necessarily linked to processing activity. Removing DRB reverses this disconnection and promotes rRNA processing. Nucleolar reformation occurs in two successive steps, dynamic recruitment to transcription sites of the processing proteins, followed by rDNA compaction. We demonstrate that both steps are sensitive to temperature, suggesting an energy-dependent process. Traffic of processing proteins analyzed by fluorescence recovery after photobleaching is similar in masses disconnected from transcription sites and in the granular component of the active nucleolus. This suggests that protein dynamics and interactions, and not only their processing activity, determine compartmentation of the nucleolar machineries.

Nuclear reprogramming of human somatic cells by xenopus egg extract requires BRG1

Animal cloning by nuclear transplantation in amphibia was demonstrated almost half a century ago and raised the question of the mechanisms and genes involved in nuclear reprogramming. Here, we demonstrate nuclear reprogramming of permeabilized human cells using extracts from Xenopus laevis eggs and early embryos. We show upregulation of pluripotency markers Oct-4 and germ cell alkaline phosphatase (GCAP) in 293T cells and human primary leukocytes. Reprogrammed leukocytes had a limited life span and did not express surface antigens characteristic of pluripotent cells, indicating that reprogramming was incomplete. Reprogramming activity was detected in egg and early embryo extracts until early blastula stage. Late blastula-stage extracts were not only inactive but also inhibitory to reprogramming. Screening for factors required for reprogramming identified the chromatin remodeling ATPase BRG1. Antibody depletion of BRG1 protein or expression of dominant-negative BRG1 abolished the reprogramming ability of amphibian extracts. Conversely, overexpression of BRG1 in Xenopus animal caps extended their competence from blastula to gastrula stage to respond to basic fibroblast growth factor (bFGF) treatment with induction of the mesodermal marker Xbra. Dissection of the molecular machinery using a simplified assay system may aid in achieving complete nuclear reprogramming of somatic cells for regenerative medicine.

DNA demethylation is necessary for the epigenetic reprogramming of somatic cell nuclei

Nuclear transplantation experiments in amphibia and mammals have shown that oocyte and egg cytoplasm can extensively reprogram somatic cell nuclei with new patterns of gene expression and new pathways of cell differentiation; however, very little is known about the molecular mechanism of nuclear reprogramming. Here we have used nuclear and DNA transfer from mammalian somatic cells to analyse the mechanism of activation of the stem cell marker gene oct4 by Xenopus oocytes. We find that the removal of nuclear protein accelerates the rate of reprogramming, but even more important is the demethylation of somatic cell DNA. DNA demethylation seems to precede gene reprogramming, and is absolutely necessary for oct4 transcription. Reprogramming by oocytes occurs in the absence of DNA replication and RNA/protein synthesis. It is also selective, operating only on the promoter, but not enhancers, of oct4; both a putative Sp1/Sp3 and a GGGAGGG binding site are required for demethylation and transcription. We conclude that the demethylation of promoter DNA may be a necessary step in the epigenetic reprogramming of somatic cell nuclei.

Quantitative kinetic analysis of nucleolar breakdown and reassembly during mitosis in live human cells

One of the great mysteries of the nucleolus surrounds its disappearance during mitosis and subsequent reassembly at late mitosis. Here, the relative dynamics of nucleolar disassembly and reformation were dissected using quantitative 4D microscopy with fluorescent protein-tagged proteins in human stable cell lines. The data provide a novel insight into the fates of the three distinct nucleolar subcompartments and their associated protein machineries in a single dividing cell. Before the onset of nuclear envelope (NE) breakdown, nucleolar disassembly started with the loss of RNA polymerase I subunits from the fibrillar centers. Dissociation of proteins from the other subcompartments occurred with faster kinetics but commenced later, coincident with the process of NE breakdown. The reformation pathway also follows a reproducible and defined temporal sequence but the order of reassembly is shown not to be dictated by the order in which individual nucleolar components reaccumulate within the nucleus after mitosis.

How cells dedifferentiate: a lesson from plants

The remarkable regenerative capacity displayed by plants and various vertebrates, such as amphibians, is largely based on the capability of somatic cells to undergo dedifferentiation. In this process, mature cells reverse their state of differentiation and acquire pluripotentiality--a process preceding not only reentry into the cell cycle but also a commitment for cell death or trans- or redifferentiation. Recent studies provide a new perspective on cellular dedifferentiation, establishing chromatin reorganization as its fundamental theme.