Characterization of somatic cell nuclear reprogramming by oocytes in which a linker histone is required for pluripotency gene reactivation

Sox2 Localization During Spermatogenesis and Its Association with other Spermatogenesis Markers Using Protein-Protein Network Analysis

Background

Sox2 (SRY box2) is an essential transcription factor that plays a vital role in spermatogenesis and regulates the genes in this process. Sox2 is important for pluripotency, self-renewal, and even spermatogonial stem cell differentiation. This gene is found in pluripotent and specialized cells, and it is involved in their biological activities.

Methods

Protein-protein interaction (PPI) network analysis was performed during spermatogenesis using NCBI, STRING, and Cytoscape databases. Then, after isolating spermatogonial stem cells from 6 C57BL/6 mice, mouse embryonic stem cells and ES-like cells were prepared. In the following, Sox2 expression was examined in differentiated and undifferentiated spermatogonia by immunohistochemistry (IMH), immunocytochemistry (ICC), and Fluidigm PCR (polymerase chain reaction). Finally, the results were compared using the Kruskal-Wallis and Dunn tests at the significance level of p<0.05.

Results

The results of this experiment showed that contrary to expectations, Sox2 has cytoplasmic expression in undifferentiated cells and nuclear expression in differentiated cells in in vitro conditions. In addition, the expression of Sox2 increased during differentiation. Fluidigm PCR showed a significantly higher expression of Sox2 (p<0.05) in differentiated compared to undifferentiated spermatogonia. Sox2 has an interaction with other genes during spermatogenesis such as Oct4, Nanog, Klf4, Stra8, Smad1, Tcf3, and Osm.

Conclusion

Sox2, which is known as a pluripotency marker, has a vital role in spermatogenesis and could be a differential marker. Sox2 has strong connections with other genes such as Oct4, Nanog, Klf4, Tcf3, Osm, Stra8, Lim2, Smad1, Gdnf, and Kit.

Oocyte-specific linker histone H1foo interacts with Esrrb to induce chromatin decondensation at specific gene loci

Linker histone H1 is mainly localized in the linker DNA region, between two nucleosome cores, and regulates chromatin structures linking gene expression. Mammalian oocytes contain the histone H1foo, a distinct member with low sequence similarity to other members in the H1 histone family. Although, from various previous studies, evidence related to H1foo function in chromatin structures is being accumulated, the distribution of H1foo at the target gene loci in a genome-wide manner and the molecular mechanism of H1foo-dependent chromatin architecture remain unclear. In this study, we aimed to identify the target loci and the physiological factor bound to H1foo at the loci. Chromatin immunoprecipitation sequencing analysis of H1foo-overexpressing mouse embryonic stem cells showed that H1foo is enriched around the transcriptional start sites of genes such as oocyte-specific genes and that the chromatin structures at these regions were relaxed. We demonstrated that H1foo was physiologically bound to the nuclear receptor estrogen-related receptor beta (Esrrb), and Esrrb was necessary for H1foo activity of chromatin decondensation at the target loci. The specific localization and interaction with Esrrb were validated in endogenous H1foo of oocytes. Overall, H1foo induces chromatin decondensation in a locus-specific manner and this function is achieved by interacting with Esrrb.

Chromatin accessibility dynamics during cell fate reprogramming

Genome architecture and chromatin dynamics govern the fate and identify of a cell. Recent advances in mapping chromatin landscapes offer valuable tools for the acquisition of accurate information regarding chromatin dynamics. Here we discuss recent findings linking chromatin dynamics to cell fate control. Specifically, chromatin undergoes a binary off/on switch during iPSC reprogramming, closing and opening loci occupied by somatic and pluripotency transcription factors, respectively. This logic of a binary off/on switch may also be operational in cell fate control during normal development and implies that further approaches could potentially be developed to direct cell fate changes both in vitro and in vivo.

The embryonic linker histone dBigH1 alters the functional state of active chromatin

Linker histones H1 are principal chromatin components, whose contribution to the epigenetic regulation of chromatin structure and function is not fully understood. In metazoa, specific linker histones are expressed in the germline, with female-specific H1s being normally retained in the early-embryo. Embryonic H1s are present while the zygotic genome is transcriptionally silent and they are replaced by somatic variants upon activation, suggesting a contribution to transcriptional silencing. Here we directly address this question by ectopically expressing dBigH1 in Drosophila S2 cells, which lack dBigH1. We show that dBigH1 binds across chromatin, replaces somatic dH1 and reduces nucleosome repeat length (NRL). Concomitantly, dBigH1 expression down-regulates gene expression by impairing RNApol II binding and histone acetylation. These effects depend on the acidic N-terminal ED-domain of dBigH1 since a truncated form lacking this domain binds across chromatin and replaces dH1 like full-length dBigH1, but it does not affect NRL either transcription. In vitro reconstitution experiments using Drosophila preblastodermic embryo extracts corroborate these results. Altogether these results suggest that the negatively charged N-terminal tail of dBigH1 alters the functional state of active chromatin compromising transcription.

Dynamic replacement of H3.3 affects nuclear reprogramming in early bovine SCNT embryos

The histone variant H3.3 is an important maternal factor in fertilization of oocytes and reprogramming of somatic cell nuclear transfer (SCNT) embryos. As a crucial replacement histone, maternal H3.3 is involved in chromatin remodeling and zygote genome activation. Litte is, however, known about the replacement of H3.3 in the bovine SCNT embryos. In this study, the maternal H3.3 in mature ooplasm was labeled with HA tag and the donor cells H3.3 was labeled with Flag tag, in order to observe the replacement of H3.3 in the bovine SCNT embryos. Meanwhile, maternal H3.3 knockdown was performed by microinjecting two different interfering fragments before nucleus transfer. It was showed that the dynamic replacement between maternal- and donor nucleus-derived H3.3 was detected after SCNT. And it could be observed that the blastocyst development rate of the cloned embryos decreased from 22.3% to 8.2-10.3% (P < 0.05), the expression of Pou5f1 and Sox2 was down-regulated and the level of H3K9me3 was increased in the interfered embryos. In summary, H3.3 replacement impacted on the process of reprogramming, including embryonic development potential, activation of pluripotency genes and epigenetic modification in bovine SCNT embryos.

Specific activation of the hb4 gene in the Xenopus oocyte through a Nobox-binding element located at the proximal promoter sequence

We isolated and characterized Xenopus tropicalis hb4 flanking DNA and showed that the -3076/+29 sequence was able to drive stage-specific transcription in the developmental process. Transgenic reporter analysis indicated that green fluorescent protein was expressed in the ovaries of female frogs at 3 months of age and in both the ovaries and testis of frogs at 6 months of age. A series of experiments with deletion of the flanking sequence and a subsequent luciferase reporter assay revealed that there were two positive regulatory regions and that the most proximal sequence of the promoter region had a certain level of transcriptional activity in oocytes. Subsequently, we showed that a conserved sequence containing Nobox-binding element (NBE) was essential for transcriptional activation and that Nobox expressed in the ovary had a crucial role in hb4 transcription through the NBE sequence.

The effect of Xenopus laevis egg extracts with/without BRG1 on the development of preimplantation cloned mouse embryos

SummaryMuch effort has been devoted to improving the efficiency of animal cloning. The aim of this study was to investigate the effect of BRG1 contained in Xenopus egg extracts on the development of cloned mouse embryos. The results showed that mouse NIH/3T3 cells were able to express pluripotent genes after treatment with egg extracts, indicating that the egg extracts contained reprogramming factors. After co-injection of Xenopus egg extracts and single mouse cumulus cells into enucleated mouse oocytes, statistically higher pronucleus formation and development rates were observed in the egg Extract- co-injected group compared with those in the no egg extract-injected (NT) group (38-66% vs 18-34%, P&lt;0.001). Removal of BRG1 protein from Xenopus egg extracts was conducted, and the BRG1-depleted extracts were co-injected with single donor cells into recipient oocytes. The results showed that the percentages of pronucleus formation were significantly higher in both BRG1-depleted and BRG1-intact groups than that in the nuclear transfer (NT) group (94, 64% vs 50%, P&lt;0.05). Furthermore, percentages in the BRG1-depleted group were even higher than in the BRG1-intact group (94% vs 64%). More confined expression of Oct4 in the inner cell mass (ICM) was observed in the blastocyst derived from the egg extract-injected groups. However, Nanog expression was more contracted in the ICM of cloned blastocysts in the BRG1-depleted group than in the BGR1-intact group. Based on the present study, BRG1 might not play an essential role in reprogramming, but the factors enhancing pronucleus formation and development of cloned mouse embryos are contained in Xenopus egg extracts.

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.

Various nuclear reprogramming systems using egg and oocyte materials

Maternal factors stored in eggs and oocytes are necessary for reprogramming sperm for embryonic development. This reprogramming activity of maternal factors also works towards somatic cells, including terminally differentiated cells. Several different experimental systems utilizing egg and oocyte materials have been applied to study nuclear reprogramming by maternal factors. Among these systems, the most widely used is the transfer of a somatic cell nucleus to an oocyte arrested at the metaphase II stage, leading to the production of a cloned animal. Nuclear transfer to an unfertilized oocyte thus provides a unique opportunity to examine reprogramming processes involved in acquiring totipotency. Other experimental systems are also available to study maternal reprogramming, such as nuclear transfer to Xenopus laevis oocytes at the germinal vesicle stage, treatment with extracts obtained from eggs or oocytes, and induced pluripotency with overexpressed maternal factors. Each system can be used for answering different types of scientific questions. This review describes currently available reprogramming systems using egg and oocyte materials and discusses how we can deepen our understanding of reprogramming mechanisms by taking advantage of these various experimental systems.

Binding of HMGN proteins to cell specific enhancers stabilizes cell identity

The dynamic nature of the chromatin epigenetic landscape plays a key role in the establishment and maintenance of cell identity, yet the factors that affect the dynamics of the epigenome are not fully known. Here we find that the ubiquitous nucleosome binding proteins HMGN1 and HMGN2 preferentially colocalize with epigenetic marks of active chromatin, and with cell-type specific enhancers. Loss of HMGNs enhances the rate of OSKM induced reprogramming of mouse embryonic fibroblasts (MEFs) into induced pluripotent stem cells (iPSCs), and the ASCL1 induced conversion of fibroblast into neurons. During transcription factor induced reprogramming to pluripotency, loss of HMGNs accelerates the erasure of the MEF-specific epigenetic landscape and the establishment of an iPSCs-specific chromatin landscape, without affecting the pluripotency potential and the differentiation potential of the reprogrammed cells. Thus, HMGN proteins modulate the plasticity of the chromatin epigenetic landscape thereby stabilizing, rather than determining cell identity.

HMGN1 and HMGN2 are ubiquitous nucleosome binding proteins. Here the authors provide evidence that HMGN proteins preferentially localize to chromatin regulatory sites to modulate the plasticity of the epigenetic landscape, proposing that HGMNs stabilize, rather than determine, cell identity.

Dedifferentiation of human uterine polyp stem cells into embryo-like cells during inducing pluripotency

By introduction of Oct4, Sox2, Klf4 and cMyc, human adult somatic cells can be reprogrammed into embryonic stem cell capable of pluripotent differentiation. In several lines of human endometrial polyp- and cervical polyp-mesenchymal stem cells (EPMSCs and CPMSC), we showed introduction of the four transcription factors led to a dedifferentiation of these cells into early embryo-like cells in three days, ranging from one-cell, two-cell, four-cell embryos, and morula to blastocyst. These early embryo-like cells resembled human early embryo derived from in vitro fertilization (IVF) in morphology, and hatching activity. These cells also expressed hypoblast (GATA4) and trophoblast (Cdx2) markers. After culturing the embryo-like cells for one month, the induced pluripotency stem cells (iPSC) could be formed (proved by pluripotency gene expression, by in vitro and in vivo differentiation). C/EBPα expression was also increased in uterine polyps. In contrast, MSCs derived from normal endometrium could not be induced to dedifferentiation to such early embryo-like cells. We conclude that EPMSCs and CPMSCs could be dedifferentiated to early embryo-like cells by the iPSC cocktail. This suggests that polyps of the organ derived from Mullerian duct may harbor epigenetic markers making them vulnerable to reprogramming to the earliest developmental stage. This study provides a simple model to derive early human embryo-like cells by in vitro.

Chromatin remodeling in Drosophila preblastodermic embryo extract

Chromatin is known to undergo extensive remodeling during nuclear reprogramming. However, the factors and mechanisms involved in this remodeling are still poorly understood and current experimental approaches to study it are not best suited for molecular and genetic analyses. Here we report on the use of Drosophila preblastodermic embryo extracts (DREX) in chromatin remodeling experiments. Our results show that incubation of somatic nuclei in DREX induces changes in chromatin organization similar to those associated with nuclear reprogramming, such as rapid binding of the germline specific linker histone dBigH1 variant to somatic chromatin, heterochromatin reorganization, changes in the epigenetic state of chromatin, and nuclear lamin disassembly. These results raise the possibility of using the powerful tools of Drosophila genetics for the analysis of chromatin changes associated with this essential process.

Chaperone-mediated chromatin assembly and transcriptional regulation in Xenopus laevis

Chromatin is the complex of DNA and histone proteins that is the physiological form of the eukaryotic genome. Chromatin is generally repressive for transcription, especially so during early metazoan development when maternal factors are explicitly in control of new zygotic gene expression. In the important model organism Xenopus laevis, maturing oocytes are transcriptionally active with reduced rates of chromatin assembly, while laid eggs and fertilized embryos have robust rates of chromatin assembly and are transcriptionally repressed. As the DNA-to-cytoplasmic ratio decreases approaching the mid-blastula transition (MBT) and the onset of zygotic genome activation (ZGA), the chromatin assembly process changes with the concomitant reduction in maternal chromatin components. Chromatin assembly is mediated in part by histone chaperones that store maternal histones and release them into new zygotic chromatin. Here, we review literature on chromatin and transcription in frog embryos and cell-free extracts and highlight key insights demonstrating the roles of maternal and zygotic histone deposition and their relationship with transcriptional regulation. We explore the central historical and recent literature on the use of Xenopus embryos and the key contributions provided by experiments in cell-free oocyte and egg extracts for the interplay between histone chaperones, chromatin assembly, and transcriptional regulation. Ongoing and future studies in Xenopus cell free extracts will likely contribute essential new insights into the interplay between chromatin assembly and transcriptional regulation.

LC3-Dependent Autophagy in Pig 2-Cell Cloned Embryos Could Influence the Degradation of Maternal mRNA and the Regulation of Epigenetic Modification

In this study, the distribution as well as the effect of autophagy on reprogramming in pig cloned embryos were observed immediately after somatic cell nuclear transfer. Results showed that the LC3 was at the highest level in cloned embryos at 2-cell stage, and it decreased with the development from 2-cell stage to blastocyst. Different to cloned embryos, the intensity of LC3 in parthenogenetic activation (PA) embryos was at the highest level at 4-cell stage. A markedly higher level of Bmp15, H1foo, and Dppa3 was shown in cloned embryos at 2-cell stage (p < 0.05 or p < 0.01), but a significantly lower level of LC3, Sox2, and eIF1A was observed at 4-cell stage (p < 0.05), compared with PA embryos. When the efficient interfering by the LC3 siRNA was performed on the cloned embryos (p < 0.01), not only the mRNA level of maternal Cyclin B, Bmp15, Gdf9, c-mos, H1foo, and Dppa3 was increased significantly (p < 0.05), but also the expression of Dnmt1 and Dnmt3b was obviously upregulated (p < 0.05). Although the expression of Sox2 and Oct4 is not changed, the expression of Stat3 decreased significantly (p < 0.05). Furthermore with the treatment of 200 nM rapamycin, the expression of eIF1A and Stat3 was significantly increased at 4-cell stage. In conclusion, the LC3-dependent autophagy mainly occurred in cloned embryos at 2-cell stage, but at 4-cell stage in PA embryos. In addition, the modulation of autophagy could affect genome activation by influencing the degradation of maternal mRNA and regulating the expression of DNA methyltransferase.

Cycling through developmental decisions: how cell cycle dynamics control pluripotency, differentiation and reprogramming

A strong connection exists between the cell cycle and mechanisms required for executing cell fate decisions in a wide-range of developmental contexts. Terminal differentiation is often associated with cell cycle exit, whereas cell fate switches are frequently linked to cell cycle transitions in dividing cells. These phenomena have been investigated in the context of reprogramming, differentiation and trans-differentiation but the underpinning molecular mechanisms remain unclear. Most progress to address the connection between cell fate and the cell cycle has been made in pluripotent stem cells, in which the transition through mitosis and G1 phase is crucial for establishing a window of opportunity for pluripotency exit and the initiation of differentiation. This Review will summarize recent developments in this area and place them in a broader context that has implications for a wide range of developmental scenarios.

Nuclear Actin in Development and Transcriptional Reprogramming

Actin is a highly abundant protein in eukaryotic cells and dynamically changes its polymerized states with the help of actin-binding proteins. Its critical function as a constituent of cytoskeleton has been well-documented. Growing evidence demonstrates that actin is also present in nuclei, referred to as nuclear actin, and is involved in a number of nuclear processes, including transcriptional regulation and chromatin remodeling. The contribution of nuclear actin to transcriptional regulation can be explained by its direct interaction with transcription machineries and chromatin remodeling factors and by controlling the activities of transcription factors. In both cases, polymerized states of nuclear actin affect the transcriptional outcome. Nuclear actin also plays an important role in activating strongly silenced genes in somatic cells for transcriptional reprogramming. When these nuclear functions of actin are considered, it is plausible to speculate that nuclear actin is also implicated in embryonic development, in which numerous genes need to be activated in a well-coordinated manner. In this review, we especially focus on nuclear actin's roles in transcriptional activation, reprogramming and development, including stem cell differentiation and we discuss how nuclear actin can be an important player in development and cell differentiation.

Histone H1 chaperone activity of TAF-I is regulated by its subtype-dependent intramolecular interaction

Linker histone H1 is involved in the regulation of gene activity through the maintenance of higher-order chromatin structure. Previously, we have shown that template activating factor-I (TAF-I or protein SET) is involved in linker histone H1 dynamics as a histone H1 chaperone. In human and murine cells, two TAF-I subtypes exist, namely TAF-Iα and TAF-Iβ. TAF-I has a highly acidic amino acid cluster in its C-terminal region and forms homo- or heterodimers through its dimerization domain. Both dimer formation and the C-terminal region of TAF-I are essential for the histone chaperone activity. TAF-Iα exhibits less histone chaperone activity compared with TAF-Iβ even though TAF-Iα and β differ only in their N-terminal regions. However, it is unclear how subtype-specific TAF-I activities are regulated. Here, we have shown that the N-terminal region of TAF-Iα autoinhibits its histone chaperone activity via intramolecular interaction with its C-terminal region. When the interaction between the N- and C-terminal regions of TAF-Iα is disrupted, TAF-Iα shows a histone chaperone activity similar to that of TAF-Iβ. Taken together, these results provide mechanistic insights into the concept that fine tuning of TAF-I histone H1 chaperone activity relies on the subtype compositions of the TAF-I dimer.

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.

Exposure of Somatic Cells to Cytoplasm Extracts of Porcine Oocytes Induces Stem Cell-Like Colony Formation and Alters Expression of Pluripotency and Chromatin-Modifying Genes

Cell permeabilization followed by exposure to cytoplasmic extracts of oocytes has been proposed as an alternative to transduction of transcription factors for inducing pluripotency in cultured somatic cells. The main goal in this study was to investigate the effect of treating porcine fibroblast cells with cytoplasmic extracts of GV-stage oocyte (OEx) followed by inhibition of histone deacetylases with Scriptaid (Scrip) on the formation of stem cell-like colonies and expression of genes encoding pluripotency and chromatin-modifying enzymes. Stem cell-like colonies start developing ∼2 weeks after treatment in cells exposed to OEx or OEx + Scrip. The number of cell colonies at the first day of appearance and 48 hours later was also similar between OEx and OEx + Scrip treatments. Transcripts for Nanog, Rex1, and c-Myc genes were detected in most cell samples that were analyzed on different days after OEx treatment. However, Sox2 transcripts were not detected and only a small proportion of samples had detectable levels of Oct4 mRNA after OEx treatment. A similar pattern of transcripts for pluripotency genes was observed in cells treated with OEx alone or OEx + Scrip. Transcript levels for Dnmt1 and Ezh2 were reduced at Day 3 after treatment in cells exposed to OEx. These findings revealed that: (a) exposure to OEx can induce a partial reprogramming of fibroblast cells toward pluripotency, characterized by colony formation and activation of pluripotency genes; and (b) inhibition of histone deacetylases does not improve the reprogramming effect of OEx treatment.

H1foo Has a Pivotal Role in Qualifying Induced Pluripotent Stem Cells

Embryonic stem cells (ESCs) are a hallmark of ideal pluripotent stem cells. Epigenetic reprogramming of induced pluripotent stem cells (iPSCs) has not been fully accomplished. iPSC generation is similar to somatic cell nuclear transfer (SCNT) in oocytes, and this procedure can be used to generate ESCs (SCNT-ESCs), which suggests the contribution of oocyte-specific constituents. Here, we show that the mammalian oocyte-specific linker histone H1foo has beneficial effects on iPSC generation. Induction of H1foo with Oct4, Sox2, and Klf4 significantly enhanced the efficiency of iPSC generation. H1foo promoted in vitro differentiation characteristics with low heterogeneity in iPSCs. H1foo enhanced the generation of germline-competent chimeric mice from iPSCs in a manner similar to that for ESCs. These findings indicate that H1foo contributes to the generation of higher-quality iPSCs.

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H1foo enhanced the efficiency of iPSC generation

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H1foo promoted in vitro differentiation characteristics with low heterogeneity

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H1foo enhanced the generation of germline-competent chimeric mice

Role of H1 linker histones in mammalian development and stem cell differentiation

H1 linker histones are key chromatin architectural proteins facilitating the formation of higher order chromatin structures. The H1 family constitutes the most heterogeneous group of histone proteins, with eleven non-allelic H1 variants in mammals. H1 variants differ in their biochemical properties and exhibit significant sequence divergence from one another, yet most of them are highly conserved during evolution from mouse to human. H1 variants are differentially regulated during development and their cellular compositions undergo dramatic changes in embryogenesis, gametogenesis, tissue maturation and cellular differentiation. As a group, H1 histones are essential for mouse development and proper stem cell differentiation. Here we summarize our current knowledge on the expression and functions of H1 variants in mammalian development and stem cell differentiation. Their diversity, sequence conservation, complex expression and distinct functions suggest that H1s mediate chromatin reprogramming and contribute to the large variations and complexity of chromatin structure and gene expression in the mammalian genome.

Regulation of Reprogramming and Cellular Plasticity through Histone Exchange and Histone Variant Incorporation

Early embryonic cells are totipotent and can generate a complete organism including embryonic and extraembryonic tissues. After division, cells lose their potency as they move toward a pluripotent state characterized by decreased cellular plasticity. During this transition, drastic changes in transcriptional programs occur in parallel with global chromatin reorganization. The epigenetic mechanisms governing the changes in chromatin signatures during the transitions of cellular plasticity states are starting to be understood. Among these mechanisms, recent studies highlight the importance of histone variant incorporation and/or eviction from chromatin in the regulation of the chromatin state that is linked to cellular potential. In this review, we discuss the role of histone variants during in vivo and in vitro reprogramming events. These results sustain the hypothesis that histone variants and histone exchange are key actors in the establishment of cellular plasticity programs.

A Specialized Histone H1 Variant Is Required for Adaptive Responses to Complex Abiotic Stress and Related DNA Methylation in Arabidopsis

Linker (H1) histones play critical roles in chromatin compaction in higher eukaryotes. They are also the most variable of the histones, with numerous nonallelic variants cooccurring in the same cell. Plants contain a distinct subclass of minor H1 variants that are induced by drought and abscisic acid and have been implicated in mediating adaptive responses to stress. However, how these variants facilitate adaptation remains poorly understood. Here, we show that the single Arabidopsis (Arabidopsis thaliana) stress-inducible variant H1.3 occurs in plants in two separate and most likely autonomous pools: a constitutive guard cell-specific pool and a facultative environmentally controlled pool localized in other tissues. Physiological and transcriptomic analyses of h1.3 null mutants demonstrate that H1.3 is required for both proper stomatal functioning under normal growth conditions and adaptive developmental responses to combined light and water deficiency. Using fluorescence recovery after photobleaching analysis, we show that H1.3 has superfast chromatin dynamics, and in contrast to the main Arabidopsis H1 variants H1.1 and H1.2, it has no stable bound fraction. The results of global occupancy studies demonstrate that, while H1.3 has the same overall binding properties as the main H1 variants, including predominant heterochromatin localization, it differs from them in its preferences for chromatin regions with epigenetic signatures of active and repressed transcription. We also show that H1.3 is required for a substantial part of DNA methylation associated with environmental stress, suggesting that the likely mechanism underlying H1.3 function may be the facilitation of chromatin accessibility by direct competition with the main H1 variants.

The H1 linker histones: multifunctional proteins beyond the nucleosomal core particle

The linker histone H1 family members are a key component of chromatin and bind to the nucleosomal core particle around the DNA entry and exit sites. H1 can stabilize both nucleosome structure and higher-order chromatin architecture. In general, H1 molecules consist of a central globular domain with more flexible tail regions at both their N- and C-terminal ends. The existence of multiple H1 subtypes and a large variety of posttranslational modifications brings about a considerable degree of complexity and makes studying this protein family challenging. Here, we review recent progress in understanding the function of linker histones and their subtypes beyond their role as merely structural chromatin components. We summarize current findings on the role of H1 in heterochromatin formation, transcriptional regulation and embryogenesis with a focus on H1 subtypes and their specific modifications.

Molecular dynamics of histone H1

The H1 or linker histones bind dynamically to chromatin in living cells via a process that involves transient association with the nucleosome near the DNA entry/exit site followed by dissociation, translocation to a new location, and rebinding. The mean residency time of H1 on any given nucleosome is about a minute, which is much shorter than that of most core histones but considerably longer than that of most other chromatin-binding proteins, including transcription factors. Here we review recent advances in understanding the kinetic pathway of H1 binding and how it relates to linker histone structure and function. We also describe potential mechanisms by which the dynamic binding of H1 might contribute directly to the regulation of gene expression and discuss several situations for which there is experimental evidence to support these mechanisms. Finally, we review the evidence for the participation of linker histone chaperones in mediating H1 exchange.

H3.3 replacement facilitates epigenetic reprogramming of donor nuclei in somatic cell nuclear transfer embryos

Transfer of a somatic nucleus into an enucleated oocyte is the most efficient approach for somatic cell reprogramming. While this process is known to involve extensive chromatin remodeling of the donor nucleus, the maternal factors responsible and the underlying chromatin-based mechanisms remain largely unknown. Here we discuss our recent findings demonstrating that the histone variant H3.3 plays an essential role in reprogramming and is required for reactivation of key pluripotency genes in somatic cell nuclear transfer (SCNT) embryos. Maternal-derived H3.3 replaces H3 in the donor nucleus shortly after oocyte activation, with the amount of replacement directly related to the differentiation status of the donor nucleus in SCNT embryos. We provide additional evidence to suggest that de novo synthesized H3.3 replaces histone H3 carrying repressive modifications in the donor nuclei of SCNT embryos, and hypothesize that replacement may occur at specific loci that must be reprogrammed for gene reactivation.

The Egg and the Nucleus: A Battle for Supremacy

This brief introduction is followed by a published version of my Nobel Laureate lecture, re-published herein with the kind permission of the Nobel Foundation. Much has happened since my original research, for which that prize was awarded. Hence, I am pleased to offer a few thoughts about the future of my research and its possible impact on humankind.Although the original work on nuclear transfer and reprogramming was done over half a century ago, advances continue to be made. In particular the Takahashi and Yamanaka induced pluripotent stem cells (iPS) procedure has opened up the field of cell replacement to a great extent. Now, more recently, further advances make this whole field come closer to actual usefulness for humans. Recently, in the UK, the government approved the use of mitochondrial replacement therapy to avoid the problems associated with genetically defective mitochondria in certain women. Although the House of Commons (members of Parliament) and the House of Lords had to debate and discuss whether to allow this kind of human therapy, I was very pleased to find that both bodies approved this procedure. This means that a patient can choose to make use of the procedure; it does not in any way force an individual to have a procedure that they are not comfortable with. In my view, this is a great advance in respect to giving patients a choice about the treatment they receive. I am told that the UK is the first country in the world to approve mitochondrial replacement therapy.Now that the Clustered Regularly Interspaced Short Palindromic Repeat (CRISPr) technology is being widely used and works well, one can foresee that there will be those who wish to use this technology to make genetic changes to humans. For example, if a human has a gene that makes it susceptible to infection or any other disorder, the removal of that gene might give such a person immunity from that disease. If this gene deletion is done within the germ line, the genetic change will be inherited. However, one can imagine that various people will strongly object and say that this technology should not be allowed. I would very much hope that various regulatory bodies, governments, etc. will allow the choice to remain with the individual. I can see no argument for such bodies to make a law that removes any choice whatsoever by an individual.

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.

Germline-specific H1 variants: the "sexy" linker histones

The eukaryotic genome is packed into chromatin, a nucleoprotein complex mainly formed by the interaction of DNA with the abundant basic histone proteins. The fundamental structural and functional subunit of chromatin is the nucleosome core particle, which is composed by 146 bp of DNA wrapped around an octameric protein complex formed by two copies of each core histone H2A, H2B, H3, and H4. In addition, although not an intrinsic component of the nucleosome core particle, linker histone H1 directly interacts with it in a monomeric form. Histone H1 binds nucleosomes near the exit/entry sites of linker DNA, determines nucleosome repeat length and stabilizes higher-order organization of nucleosomes into the ∼30 nm chromatin fiber. In comparison to core histones, histone H1 is less well conserved through evolution. Furthermore, histone H1 composition in metazoans is generally complex with most species containing multiple variants that play redundant as well as specific functions. In this regard, a characteristic feature is the presence of specific H1 variants that replace somatic H1s in the germline and during early embryogenesis. In this review, we summarize our current knowledge about their structural and functional properties.

Endogenous gradients of resting potential instructively pattern embryonic neural tissue via Notch signaling and regulation of proliferation

Biophysical forces play important roles throughout embryogenesis, but the roles of spatial differences in cellular resting potentials during large-scale brain morphogenesis remain unknown. Here, we implicate endogenous bioelectricity as an instructive factor during brain patterning in Xenopus laevis. Early frog embryos exhibit a characteristic hyperpolarization of cells lining the neural tube; disruption of this spatial gradient of the transmembrane potential (Vmem) diminishes or eliminates the expression of early brain markers, and causes anatomical mispatterning of the brain, including absent or malformed regions. This effect is mediated by voltage-gated calcium signaling and gap-junctional communication. In addition to cell-autonomous effects, we show that hyperpolarization of transmembrane potential (Vmem) in ventral cells outside the brain induces upregulation of neural cell proliferation at long range. Misexpression of the constitutively active form of Notch, a suppressor of neural induction, impairs the normal hyperpolarization pattern and neural patterning; forced hyperpolarization by misexpression of specific ion channels rescues brain defects induced by activated Notch signaling. Strikingly, hyperpolarizing posterior or ventral cells induces the production of ectopic neural tissue considerably outside the neural field. The hyperpolarization signal also synergizes with canonical reprogramming factors (POU and HB4), directing undifferentiated cells toward neural fate in vivo. These data identify a new functional role for bioelectric signaling in brain patterning, reveal interactions between Vmem and key biochemical pathways (Notch and Ca(2+) signaling) as the molecular mechanism by which spatial differences of Vmem regulate organogenesis of the vertebrate brain, and suggest voltage modulation as a tractable strategy for intervention in certain classes of birth defects.

Newt lens transdifferentiation: from lentectomy to immuno-FISH

Newt lens regeneration is achieved by a unique cellular regulation of transdifferentiation where the dorsal iris pigmented epithelial cells (PECs) dedifferentiate and redifferentiate into lens cells. Recent studies have shown that nuclear architecture of PECs is dynamically changed and unique epigenetic regulation in somatic nucleus is crucial in the lens transdifferentiation. Immuno-FISH, detection of protein and gene loci in nucleus, is one of the effective tools to analyze nuclear architecture of PECs. In this chapter a whole process from lentectomy to immuno-FISH is described.

Histone variants enriched in oocytes enhance reprogramming to induced pluripotent stem cells

Expression of Oct3/4, Sox2, Klf4, and c-Myc (OSKM) can reprogram somatic cells into induced pluripotent stem cells (iPSCs). Somatic cell nuclear transfer (SCNT) can also be used for reprogramming, suggesting that factors present in oocytes could potentially augment OSKM-mediated induction of pluripotency. Here, we report that two histone variants, TH2A and TH2B, which are highly expressed in oocytes and contribute to activation of the paternal genome after fertilization, enhance OSKM-dependent generation of iPSCs and can induce reprogramming with Klf4 and Oct3/4 alone. TH2A and TH2B are enriched on the X chromosome during the reprogramming process, and their expression in somatic cells increases the DNase I sensitivity of chromatin. In addition, Xist deficiency, which was reported to enhance SCNT reprogramming efficiency, stimulates iPSC generation using TH2A/TH2B in conjunction with OSKM, but not OSKM alone. Thus, TH2A/TH2B may enhance reprogramming by introducing processes that normally operate in zygotes and during SCNT.

Analysis of nuclear reprogramming following nuclear transfer to Xenopus oocyte

Germinal vesicle of stage V-VI Xenopus Laevis oocytes (at the prophase I stage of meiosis) can be used to transplant mammalian nuclei. In this type of interspecies nuclear transfer no cell division occurs and no new cell types are generated. However, the transplanted nuclei undergo extensive transcriptional reprogramming. Here, it is first explained how to carry out transplantation of multiple mammalian cell nuclei to Xenopus oocytes. It is then described how to perform RT-qPCR, Western Blot, Chromatin Immunoprecipitation, and live imaging analysis to monitor transcriptional reprogramming of the nuclei transplanted to oocytes.

Maternal histone variants and their chaperones promote paternal genome activation and boost somatic cell reprogramming

The mammalian egg employs a wide spectrum of epigenome modification machinery to reprogram the sperm nucleus shortly after fertilization. This event is required for transcriptional activation of the paternal/zygotic genome and progression through cleavage divisions. Reprogramming of paternal nuclei requires replacement of sperm protamines with canonical and non-canonical histones, covalent modification of histone tails, and chemical modification of DNA (notably oxidative demethylation of methylated cytosines). In this essay we highlight the role maternal histone variants play during developmental reprogramming following fertilization. We discuss how reduced maternal histone variant incorporation in somatic nuclear transfer experiments may explain the reduced viability of resulting embryos and how knowledge of repressive and activating maternal factors may be used to improve somatic cell reprogramming.

Chromatin changes in reprogramming of mammalian somatic cells

Somatic cell nuclear transfer (SCNT), cell fusion, and induced pluripotent stem cells (iPSCs) technologies are three strategies that allow reprogramming somatic cells into the pluripotent state; however, the efficiency is low and the mechanisms are not fully clear. In addition, there are reports that changes in chromatin play a critical role in these reprogramming strategies by modulating binding of transcription factors to their targets. In this review, we mainly discuss inactivation of the X chromosome, chromatin decondensation and remodeling, histone modifications, and histone variants in the three strategies. This review will provide an insight for future nuclear reprogramming research.

Role of linker histone H1c during the reprogramming of Chinese swamp buffalo (Bubalus Bubalis) embryos produced by somatic cell nuclear transfer

During reprogramming, there is exchange of histone H1c and the oocyte-specific linker histone, and H1c may play a critically important role in the reprogramming process of somatic cell nuclear transfer (SCNT). The aim of the present study was to investigate the role of the H1c gene in SCNT reprogramming in Chinese swamp buffalo (Bubalus bubalis) using RNA interference (RNAi). Chinese swamp buffalo H1c gene sequences were obtained and H1c-RNAi vectors were designed, synthesised and then transfected into a buffalo fetal skin fibroblast cell line. Expression of H1c was determined by real-time polymerase chain reaction to examine the efficiency of vector interference. These cells were then used as a nuclear donor for SCNT so as to observe the further development of SCNT embryos. Inhibition of H1c gene expression in donor cells significantly improved the developmental speed of embryos from the 1-cell to 8-cell stage. Furthermore, compared with the control group, inhibition of H1c gene expression significantly reduced the blastocyst formation rate. It is concluded that linker histone H1c is very important in SCNT reprogramming in Chinese swamp buffalo. Correct expression of the H1c gene plays a significant role in preimplantation embryonic development in B. bubalis.

Interactions with John Gurdon--muscle as a mesodermal read-out and the community effect

John Gurdon has made major contributions to developmental biology in addition to his Nobel prize winning work on nuclear reprogramming. With the frog, Xenopus, as a vertebrate model, his work on mesoderm induction led him to identify a community effect required for tissue differentiation after progenitor cells have entered a specific mesodermal programme. It is in the context of this biologically important concept, with myogenesis as an example, that we have had most scientific exchanges. Here I trace my contacts with him, from an interest in histone regulation of gene expression and reprogramming, to myogenic determination factors as markers of early mesodermal induction, to the role of the community effect in the spatiotemporal control of skeletal muscle formation. I also recount some personal anecdotes from encounters in Oxford, Paris and Cambridge, to illustrate my appreciation of him as a scientist and a colleague.

Mitosis gives a brief window of opportunity for a change in gene transcription

In mitotic nuclei transplant experiments, many genes undergo major changes in gene expression. This supports the idea that mitosis facilitates new cell fate decisions during normal development.

Cell differentiation is remarkably stable but can be reversed by somatic cell nuclear transfer, cell fusion, and iPS. Nuclear transfer to amphibian oocytes provides a special opportunity to test transcriptional reprogramming without cell division. We show here that, after nuclear transfer to amphibian oocytes, mitotic chromatin is reprogrammed up to 100 times faster than interphase nuclei. We find that, as cells traverse mitosis, their genes pass through a temporary phase of unusually high responsiveness to oocyte reprogramming factors (mitotic advantage). Mitotic advantage is not explained by nuclear penetration, DNA modifications, histone acetylation, phosphorylation, methylation, nor by salt soluble chromosomal proteins. Our results suggest that histone H2A deubiquitination may account, at least in part, for the acquisition of mitotic advantage. They support the general principle that a temporary access of cytoplasmic factors to genes during mitosis may facilitate somatic cell nuclear reprogramming and the acquisition of new cell fates in normal development.

Cells are dividing very actively at a time in development when new gene expression and new cell lineages arise. At mitosis, most transcription factors are temporarily displaced from chromosomes. We show that, after transplantation to oocytes, somatic cell nuclei that have been synchronized in mitosis can be reprogrammed to pluripotency gene expression up to 100 times faster than interphase nuclei. We find that, as cells traverse mitosis, their genes pass through a temporary phase of unusually high responsiveness to oocyte reprogramming factors (mitotic advantage). Many other genes in the genome have also shown a mitotic advantage, which affects the rate rather than the final level of transcriptional enhancement. This is attributable to a chromatin state rather than to more rapid passage of reprogramming factors through the nuclear membrane. Histone H2A deubiquitination at mitosis is required for the acquisition of mitotic advantage. Our results support the general principle that a temporary access of cytoplasmic factors to genes during mitosis facilitates somatic cell nuclear reprogramming and the acquisition of new cell fates in normal development.

Hierarchical molecular events driven by oocyte-specific factors lead to rapid and extensive reprogramming

Nuclear transfer to oocytes is an efficient way to transcriptionally reprogram somatic nuclei, but its mechanisms remain unclear. Here, we identify a sequence of molecular events that leads to rapid transcriptional reprogramming of somatic nuclei after transplantation to Xenopus oocytes. RNA-seq analyses reveal that reprogramming by oocytes results in a selective switch in transcription toward an oocyte rather than pluripotent type, without requiring new protein synthesis. Time-course analyses at the single-nucleus level show that transcriptional reprogramming is induced in most transplanted nuclei in a highly hierarchical manner. We demonstrate that an extensive exchange of somatic- for oocyte-specific factors mediates reprogramming and leads to robust oocyte RNA polymerase II binding and phosphorylation on transplanted chromatin. Moreover, genome-wide binding of oocyte-specific linker histone B4 supports its role in transcriptional reprogramming. Thus, our study reveals the rapid, abundant, and stepwise loading of oocyte-specific factors onto somatic chromatin as important determinants for successful reprogramming.

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Xenopus oocytes induce an oocyte transcription pattern in mouse nuclei in 2 days

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Reprogramming requires a switch from somatic to oocyte transcriptional components

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Unusually high amounts of oocyte-derived RNA polymerase II drive reprogramming

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The pattern of oocyte linker histone binding to somatic chromatin is revealed

Global absolute quantification reveals tight regulation of protein expression in single Xenopus eggs

While recent developments in genomic sequencing technology have enabled comprehensive transcriptome analyses of single cells, single cell proteomics has thus far been restricted to targeted studies. Here, we perform global absolute protein quantification of fertilized Xenopus laevis eggs using mass spectrometry-based proteomics, quantifying over 5800 proteins in the largest single cell proteome characterized to date. Absolute protein amounts in single eggs are highly consistent, thus indicating a tight regulation of global protein abundance. Protein copy numbers in single eggs range from tens of thousands to ten trillion copies per cell. Comparison between the single-cell proteome and transcriptome reveal poor expression correlation. Finally, we identify 439 proteins that significantly change in abundance during early embryogenesis. Downregulated proteins include ribosomal proteins and upregulated proteins include basal transcription factors, among others. Many of these proteins do not show regulation at the transcript level. Altogether, our data reveal that the transcriptome is a poor indicator of the proteome and that protein levels are tightly controlled in X. laevis eggs.

Genome reactivation after the silence in mitosis: recapitulating mechanisms of development?

Transcription is silenced during mitosis and reactivated at mitotic exit. The dynamics and identities of "bookmarking" transcription factors and chromatin marks that mediate reactivation often recapitulate those observed during cell identity establishment in development. Thus, features of postmitotic gene reactivation can provide insights into mechanisms of developmental cell fate establishment.

Small molecule compound induces chromatin de-condensation and facilitates induced pluripotent stem cell generation

The revolutionary induced pluripotent stem cell (iPSC) technology provides a new means for cell replacement therapies and drug screening. Small molecule compounds have been found extremely useful to improve the generation of iPSCs and understand the reprogramming mechanism. Here we report the identification of a novel chemical, CYT296, which improves OSKM-mediated induction of iPSCs for >10 folds and enables efficient reprogramming with only Oct4 in combination with other small molecules. The derived iPSCs are genuinely pluripotent and support the development of two 'All-iPSC' mice by tetraploid complementation. CYT296 profoundly impacts heterochromatin formation without affecting cell viability. MEFs treated with CYT296 exhibit de-condensed chromatin structure with markedly reduced loci containing heterochromatin protein 1α (HP1α) and H3K9me3, which is very similar to the chromatin configuration in embryonic stem cells (ESCs). Given that an open chromatin structure serves as a hallmark of pluripotency and has to be acquired to fulfill reprogramming, we propose that CYT296 might facilitate this process by disrupting condensed chromatin, thereby creating a more favorable environment for reprogramming. In agreement of this idea, shRNA targeting HP1α also promotes the generation of iPSCs. Thus current findings not only provide a novel chemical for efficient iPSC induction, but also suggest a new approach to regulate somatic cell reprogramming by targeting chromatin de-condensation with small molecules.

Histone variant H3.3 is an essential maternal factor for oocyte reprogramming

Mature oocyte cytoplasm can reprogram somatic cell nuclei to the pluripotent state through a series of sequential events including protein exchange between the donor nucleus and ooplasm, chromatin remodeling, and pluripotency gene reactivation. Maternal factors that are responsible for this reprogramming process remain largely unidentified. Here, we demonstrate that knockdown of histone variant H3.3 in mouse oocytes results in compromised reprogramming and down-regulation of key pluripotency genes; and this compromised reprogramming for developmental potentials and transcription of pluripotency genes can be rescued by injecting exogenous H3.3 mRNA, but not H3.2 mRNA, into oocytes in somatic cell nuclear transfer embryos. We show that maternal H3.3, and not H3.3 in the donor nucleus, is essential for successful reprogramming of somatic cell nucleus into the pluripotent state. Furthermore, H3.3 is involved in this reprogramming process by remodeling the donor nuclear chromatin through replacement of donor nucleus-derived H3 with de novo synthesized maternal H3.3 protein. Our study shows that H3.3 is a crucial maternal factor for oocyte reprogramming and provides a practical model to directly dissect the oocyte for its reprogramming capacity.

Every amino acid matters: essential contributions of histone variants to mammalian development and disease

Despite a conserved role for histones as general DNA packaging agents, it is now clear that another key function of these proteins is to confer variations in chromatin structure to ensure dynamic patterns of transcriptional regulation in eukaryotes. The incorporation of histone variants is particularly important to this process. Recent knockdown and knockout studies in various cellular systems, as well as direct mutational evidence from human cancers, now suggest a crucial role for histone variant regulation in processes as diverse as differentiation and proliferation, meiosis and nuclear reprogramming. In this Review, we provide an overview of histone variants in the context of their unique functions during mammalian germ cell and embryonic development, and examine the consequences of aberrant histone variant regulation in human disease.

H1foo is essential for in vitro meiotic maturation of bovine oocytes

Oocyte-specific linker histone, H1foo, is localized on the oocyte chromosomes during the process of meiotic maturation, and is essential for mouse oocyte maturation. Bovine H1foo has been identified, and its expression profile throughout oocyte maturation and early embryo development has been established. However, it has not been confirmed if H1foo is indispensable during bovine oocyte maturation. Effective siRNAs against H1foo were screened in HeLa cells, and then siRNA was microinjected into bovine oocytes to down-regulate H1foo expression. H1foo overexpression was achieved via mRNA injection. Reverse transcription polymerase chain reaction (RT-PCR) results indicated that H1foo was up-regulated by 200% and down-regulated by 70%. Based on the first polar body extrusion (PB1E) rate, H1foo overexpression apparently promoted meiotic progression. The knockdown of H1foo significantly impaired bovine oocyte maturation compared with H1foo overexpression and control groups (H1foo overexpression = 88.7%, H1foo siRNA = 41.2%, control = 71.2%; P < 0.05). This decrease can be rescued by co-injection of a modified H1foo mRNA that has escaped from the siRNA target. However, the H1e (somatic linker histone) overexpression had no effect on PB1E rate when compared with the control group. Therefore we concluded that H1foo is essential for bovine oocyte maturation and its overexpression stimulates the process.

[Pluripotency and induced nuclear reprogramming in vertebrates: new perspectives]

Pluripotency is a transitory state during vertebrate development. A pluripotent cell can theoretically acquire all cell fates of the organism. During ontogenetic dynamics, loss of pluripotency is associated with a progressive acquisition of a specific genetic program, which is determined both by instructions received and by cell position in the whole organism. Pluripotent embryonic stem cells can be isolated and cultured in vitro indefinitely. Using mammalian embryonic stem cells (ESCs), it has been possible to identify the factors involved in the establishment and maintenance of pluripotency state. In this review, we will describe recent scientific advances in the understanding of pluripotency, the molecular actors involved in such a regulation and their functional conservation during evolution. We shall focus on new concepts, obtained from the study of vertebrate model organisms, to shed light on the cell transition from pluripotency to differentiated state, and shall recapitulate fundamental and clinical applications of pluripotent cells, of "somatic cell nuclear transfer" (SCNT), of induced nuclear reprogramming in vitro and future perspectives of in vivo applications. Our results, in the xenopus, concerning the first in vivo induced nuclear reprogramming might open new perspectives about the understanding of cell plasticity in an integrated context. Our analyses sought to encourage new and alternative clinical approaches to achieve in situ tissue regeneration.