Roles for Tbx3 in regulation of two-cell state and telomere elongation in mouse ES cells

Gene body DNA hydroxymethylation restricts the magnitude of transcriptional changes during aging

DNA hydroxymethylation (5hmC), the most abundant oxidative derivative of DNA methylation, is typically enriched at enhancers and gene bodies of transcriptionally active and tissue-specific genes. Although aberrant genomic 5hmC has been implicated in age-related diseases, its functional role in aging remains unknown. Here, using mouse liver and cerebellum as model organs, we show that 5hmC accumulates in gene bodies associated with tissue-specific function and restricts the magnitude of gene expression changes with age. Mechanistically, 5hmC decreases the binding of splicing associated factors and correlates with age-related alternative splicing events. We found that various age-related contexts, such as prolonged quiescence and senescence, drive the accumulation of 5hmC with age. We provide evidence that this age-related transcriptionally restrictive function is conserved in mouse and human tissues. Our findings reveal that 5hmC regulates tissue-specific function and may play a role in longevity.

Gene body DNA hydroxymethylation restricts the magnitude of transcriptional changes during aging

DNA hydroxymethylation (5hmC), the most abundant oxidative derivative of DNA methylation, is typically enriched at enhancers and gene bodies of transcriptionally active and tissue-specific genes. Although aberrant genomic 5hmC has been implicated in age-related diseases, its functional role in aging remains unknown. Here, using mouse liver and cerebellum as model organs, we show that 5hmC accumulates in gene bodies associated with tissue-specific function and restricts the magnitude of gene expression changes with age. Mechanistically, 5hmC decreases the binding of splicing associated factors and correlates with age-related alternative splicing events. We found that various age-related contexts, such as prolonged quiescence and senescence, drive the accumulation of 5hmC with age. We provide evidence that this age-related transcriptionally restrictive function is conserved in mouse and human tissues. Our findings reveal that 5hmC regulates tissue-specific function and may play a role in longevity.

ADNP modulates SINE B2-derived CTCF-binding sites during blastocyst formation in mice

CTCF is crucial for chromatin structure and transcription regulation in early embryonic development. However, the kinetics of CTCF chromatin occupation in preimplantation embryos have remained unclear. In this study, we used CUT&RUN technology to investigate CTCF occupancy in mouse preimplantation development. Our findings revealed that CTCF begins binding to the genome prior to zygotic genome activation (ZGA), with a preference for CTCF-anchored chromatin loops. Although the majority of CTCF occupancy is consistently maintained, we identified a specific set of binding sites enriched in the mouse-specific short interspersed element (SINE) family B2 that are restricted to the cleavage stages. Notably, we discovered that the neuroprotective protein ADNP counteracts the stable association of CTCF at SINE B2-derived CTCF-binding sites. Knockout of Adnp in the zygote led to impaired CTCF binding signal recovery, failed deposition of H3K9me3, and transcriptional derepression of SINE B2 during the morula-to-blastocyst transition, which further led to unfaithful cell differentiation in embryos around implantation. Our analysis highlights an ADNP-dependent restriction of CTCF binding during cell differentiation in preimplantation embryos. Furthermore, our findings shed light on the functional importance of transposable elements (TEs) in promoting genetic innovation and actively shaping the early embryo developmental process specific to mammals.

ZSCAN4 Regulates Zygotic Genome Activation and Telomere Elongation in Porcine Parthenogenetic Embryos

Zinc finger and SCAN domain-containing 4 (ZSCAN4), a DNA-binding protein, maintains telomere length and plays a key role in critical aspects of mouse embryonic stem cells, including maintaining genomic stability and defying cellular senescence. However, the effect of ZSCAN4 in porcine parthenogenetic embryos remains unclear. To investigate the function of ZSCAN4 and the underlying mechanism in porcine embryo development, ZSCAN4 was knocked down via dsRNA injection in the one-cell stage. ZSCAN4 was highly expressed in the four- and five- to eight-cell stages in porcine embryos. The percentage of four-cell stage embryos, five- to eight-cell stage embryos, and blastocysts was lower in the ZSCAN4 knockdown group than in the control group. Notably, depletion of ZSCAN4 induced the protein expression of DNMT1 and 5-Methylcytosine (5mC, a methylated form of the DNA base cytosine) in the four-cell stage. The H3K27ac level and ZGA genes expression decreased following ZSCAN4 knockdown. Furthermore, ZSCAN4 knockdown led to DNA damage and shortened telomere compared with the control. Additionally, DNMT1-dsRNA was injected to reduce DNA hypermethylation in ZSCAN4 knockdown embryos. DNMT1 knockdown rescued telomere shortening and developmental defects caused by ZSCAN4 knockdown. In conclusion, ZSCAN4 is involved in the regulation of transcriptional activity and is essential for maintaining telomere length by regulating DNMT1 expression in porcine ZGA.

A Comprehensive Review on the Role of ZSCAN4 in Embryonic Development, Stem Cells, and Cancer

ZSCAN4 is a transcription factor that plays a pivotal role during early embryonic development. It is a unique gene expressed specifically during the first tide of de novo transcription during the zygotic genome activation. Moreover, it is reported to regulate telomere length in embryonic stem cells and induced pluripotent stem cells. Interestingly, ZSCAN4 is expressed in approximately 5% of the embryonic stem cells in culture at any given time, which points to the fact that it has a tight regulatory system. Furthermore, ZSCAN4, if included in the reprogramming cocktail along with core reprogramming factors, increases the reprogramming efficiency and results in better quality, genetically stable induced pluripotent stem cells. Also, it is reported to have a role in promoting cancer stem cell phenotype and can prospectively be used as a marker for the same. In this review, the multifaceted role of ZSCAN4 in embryonic development, embryonic stem cells, induced pluripotent stem cells, cancer, and germ cells are discussed comprehensively.

Zscan4 Contributes to Telomere Maintenance in Telomerase-Deficient Late Generation Mouse ESCs and Human ALT Cancer Cells

Proper telomere length is essential for indefinite self-renewal of embryonic stem (ES) cells and cancer cells. Telomerase-deficient late generation mouse ES cells and human ALT cancer cells are able to propagate for numerous passages, suggesting telomerase-independent mechanisms responding for telomere maintenance. However, the underlying mechanisms ensuring the telomere length maintenance are unclear. Here, using late generation telomerase KO (G4 Terc^-/-) ESCs as a model, we show that Zscan4, highly upregulated in G4 Terc^-/- ESCs, is responsible for the prolonged culture of these cells with stably short telomeres. Mechanistically, G4 Terc^-/- ESCs showed reduced levels of DNA methylation and H3K9me3 at Zscan4 promoter and subtelomeres, which relieved the expression of Zscan4. Similarly, human ZSCAN4 was also derepressed by reduced H3K9me3 at its promoter in ALT U2 OS cells, and depletion of ZSCAN4 significantly shortened telomeres. Our results define a similar conserved pathway contributing to the telomere maintenance in telomerase-deficient late generation mESCs and human ALT U2OS cancer cells.

Klf5 establishes bi-potential cell fate by dual regulation of ICM and TE specification genes

Early blastomeres of mouse preimplantation embryos exhibit bi-potential cell fate, capable of generating both embryonic and extra-embryonic lineages in blastocysts. Here we identify three major two-cell-stage (2C)-specific endogenous retroviruses (ERVs) as the molecular hallmark of this bi-potential plasticity. Using the long terminal repeats (LTRs) of all three 2C-specific ERVs, we identify Krüppel-like factor 5 (Klf5) as their major upstream regulator. Klf5 is essential for bi-potential cell fate; a single Klf5-overexpressing embryonic stem cell (ESC) generates terminally differentiated embryonic and extra-embryonic lineages in chimeric embryos, and Klf5 directly induces inner cell mass (ICM) and trophectoderm (TE) specification genes. Intriguingly, Klf5 and Klf4 act redundantly during ICM specification, whereas Klf5 deficiency alone impairs TE specification. Klf5 is regulated by multiple 2C-specific transcription factors, particularly Dux, and the Dux/Klf5 axis is evolutionarily conserved. The 2C-specific transcription program converges on Klf5 to establish bi-potential cell fate, enabling a cell state with dual activation of ICM and TE genes.

Using multiple 2C-specific ERV cell fate markers, Kinisu et al. identify Klf5 as a key transcription factor that confers a 2C-like developmental potential and activates ICM and TE specification genes. Klf5 and Klf4 act redundantly for ICM and TE specification in mouse preimplantation embryos.

Transposable Element Dynamics and Regulation during Zygotic Genome Activation in Mammalian Embryos and Embryonic Stem Cell Model Systems

Transposable elements (TEs) are mobile genetic sequences capable of duplicating and reintegrating at new regions within the genome. A growing body of evidence has demonstrated that these elements play important roles in host genome evolution, despite being traditionally viewed as parasitic elements. To prevent ectopic activation of TE transposition and transcription, they are epigenetically silenced in most somatic tissues. Intriguingly, a specific class of TEs-retrotransposons-is transiently expressed at discrete phases during mammalian development and has been linked to the establishment of totipotency during zygotic genome activation (ZGA). While mechanisms controlling TE regulation in somatic tissues have been extensively studied, the significance underlying the unique transcriptional reactivation of retrotransposons during ZGA is only beginning to be uncovered. In this review, we summarize the expression dynamics of key retrotransposons during ZGA, focusing on findings from in vivo totipotent embryos and in vitro totipotent-like embryonic stem cells (ESCs). We then dissect the functions of retrotransposons and discuss how their transcriptional activities are finetuned during early stages of mammalian development.

A Tale of Two States: Pluripotency Regulation of Telomeres

Inside the nucleus, chromatin is functionally organized and maintained as a complex three-dimensional network of structures with different accessibility such as compartments, lamina associated domains, and membraneless bodies. Chromatin is epigenetically and transcriptionally regulated by an intricate and dynamic interplay of molecular processes to ensure genome stability. Phase separation, a process that involves the spontaneous organization of a solution into separate phases, has been proposed as a mechanism for the timely coordination of several cellular processes, including replication, transcription and DNA repair. Telomeres, the repetitive structures at the end of chromosomes, are epigenetically maintained in a repressed heterochromatic state that prevents their recognition as double-strand breaks (DSB), avoiding DNA damage repair and ensuring cell proliferation. In pluripotent embryonic stem cells, telomeres adopt a non-canonical, relaxed epigenetic state, which is characterized by a low density of histone methylation and expression of telomere non-coding transcripts (TERRA). Intriguingly, this telomere non-canonical conformation is usually associated with chromosome instability and aneuploidy in somatic cells, raising the question of how genome stability is maintained in a pluripotent background. In this review, we will explore how emerging technological and conceptual developments in 3D genome architecture can provide novel mechanistic perspectives for the pluripotent epigenetic paradox at telomeres. In particular, as RNA drives the formation of LLPS, we will consider how pluripotency-associated high levels of TERRA could drive and coordinate phase separation of several nuclear processes to ensure genome stability. These conceptual advances will provide a better understanding of telomere regulation and genome stability within the highly dynamic pluripotent background.

DUX-miR-344-ZMYM2-Mediated Activation of MERVL LTRs Induces a Totipotent 2C-like State

Mouse embryonic stem cells (ESCs) sporadically express preimplantation two-cell-stage (2C) transcripts, including MERVL endogenous retrovirus and Zscan4 cluster genes. Such 2C-like cells (2CLCs) can contribute to both embryonic and extraembryonic tissues when reintroduced into early embryos, although the molecular mechanism underlying such an expanded 2CLC potency remains elusive. We examine global nucleosome occupancy and gene expression in 2CLCs and identified miR-344 as the noncoding molecule that positively controls 2CLC potency. We find that activation of endogenous MERVL or miR-344-2 alone is sufficient to induce 2CLCs with activation of 2C genes and an expanded potency. Mechanistically, miR-344 is activated by DUX and post-transcriptionally represses ZMYM2 and its partner LSD1, and ZMYM2 recruits LSD1/HDAC corepressor complex to MERVL LTR for transcriptional repression. Consistently, zygotic depletion of Zmym2 compromises the totipotency-to-pluripotency transition during early development. Our studies establish the previously unappreciated DUX-miR-344-Zmym2/Lsd1 axis that controls MERVL for expanded stem cell potency.

Metabolic control of DNA methylation in naive pluripotent cells

Naive epiblast and embryonic stem cells (ESCs) give rise to all cells of adults. Such developmental plasticity is associated with genome hypomethylation. Here, we show that LIF-Stat3 signaling induces genomic hypomethylation via metabolic reconfiguration. Stat3-/- ESCs show decreased α-ketoglutarate production from glutamine, leading to increased Dnmt3a and Dnmt3b expression and DNA methylation. Notably, genome methylation is dynamically controlled through modulation of α-ketoglutarate availability or Stat3 activation in mitochondria. Alpha-ketoglutarate links metabolism to the epigenome by reducing the expression of Otx2 and its targets Dnmt3a and Dnmt3b. Genetic inactivation of Otx2 or Dnmt3a and Dnmt3b results in genomic hypomethylation even in the absence of active LIF-Stat3. Stat3-/- ESCs show increased methylation at imprinting control regions and altered expression of cognate transcripts. Single-cell analyses of Stat3-/- embryos confirmed the dysregulated expression of Otx2, Dnmt3a and Dnmt3b as well as imprinted genes. Several cancers display Stat3 overactivation and abnormal DNA methylation; therefore, the molecular module that we describe might be exploited under pathological conditions.

Lessons from expanded potential of embryonic stem cells: Moving toward totipotency

Embryonic stem cells possess fascinating capacity of self-renewal and developmental potential, leading to significant progress in understanding the molecular basis of pluripotency, disease modeling, and reprogramming technology. Recently, 2-cell-like embryonic stem cells (ESCs) and expanded potential stem cells or extended pluripotent stem cells (EPSCs) generated from early-cleavage embryos display some features of totipotent embryos. These cell lines provide valuable in vitro models to study underlying principles of totipotency, cell plasticity, and lineage segregation. In this review, we summarize the current progress in this filed and highlight the application potentials of these cells in the future.

The role of telomere-binding modulators in pluripotent stem cells

Pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs), ESCs derived by somatic cell nuclear transfer (ntESCs), and induced pluripotent stem cells (iPSCs) have unlimited capacity for self-renewal and pluripotency and can give rise to all types of somatic cells. In order to maintain their self-renewal and pluripotency, PSCs need to preserve their telomere length and homeostasis. In recent years, increasing studies have shown that telomere reprogramming is essential for stem cell pluripotency maintenance and its induced pluripotency process. Telomere-associated proteins are not only required for telomere maintenance in both stem cells, their extra-telomeric functions have also been found to be critical as well. Here, we will discuss how telomeres and telomere-associated factors participate and regulate the maintenance of stem cell pluripotency.

COL1A2 is a TBX3 target that mediates its impact on fibrosarcoma and chondrosarcoma cell migration

The developmentally important T-box transcription factor TBX3, is overexpressed in several cancers and contributes to tumorigenesis as either a tumour promoter or tumour suppressor. For example, TBX3 promotes cell proliferation, migration and invasion of chondrosarcoma cells but inhibits these processes in fibrosarcoma cells. This suggests that the cellular context influences TBX3 oncogenic functions, but the mechanism(s) involved has not been elucidated. COL1A2 encodes type I collagen and, like TBX3, plays important roles during embryogenesis and can act as either oncogene or tumour suppressor. Here we explore the possibility that COL1A2 may be a TBX3 target gene responsible for mediating its opposing oncogenic roles in chondrosarcoma and fibrosarcoma cells. Results from qRT-PCR, western blotting, luciferase reporter and chromatin immunoprecipitation assays show that TBX3 binds and activates the COL1A2 promoter. Furthermore, we show that TBX3 levels are regulated by AKT1 and that pseudo-phosphorylation of TBX3 at an AKT consensus serine site, enhances its ability to activate COL1A2. Importantly, we demonstrate that COL1A2 mediates the pro- and anti-migratory effects of TBX3 in chondrosarcoma and fibrosarcoma cells respectively. Our data reveal that the AKT1/TBX3/COL1A2 axis plays an important role in sarcomagenesis.

GADD45 promotes locus-specific DNA demethylation and 2C cycling in embryonic stem cells

In this study, Schüle et al. report an unexpected role of GADD45 proteins in regulation of the cycling of ESCs in the 2C state. Using methylome analysis of Gadd45 triple-mutant ESCs, they found a role for GADD45 in demethylation of specific TET targets and partial deregulation of ZGA genes at the two-cell stage.

Mouse embryonic stem cell (ESC) cultures contain a rare cell population of “2C-like” cells resembling two-cell embryos, the key stage of zygotic genome activation (ZGA). Little is known about positive regulators of the 2C-like state and two-cell stage embryos. Here we show that GADD45 (growth arrest and DNA damage 45) proteins, regulators of TET (TET methylcytosine dioxygenase)-mediated DNA demethylation, promote both states. Methylome analysis of Gadd45a,b,g triple-knockout (TKO) ESCs reveal locus-specific DNA hypermethylation of ∼7000 sites, which are enriched for enhancers and loci undergoing TET–TDG (thymine DNA glycosylase)-mediated demethylation. Gene expression is misregulated in TKOs, notably upon differentiation, and displays signatures of DNMT (DNA methyltransferase) and TET targets. TKOs manifest impaired transition into the 2C-like state and exhibit DNA hypermethylation and down-regulation of 2C-like state-specific genes. Gadd45a,b double-mutant mouse embryos display embryonic sublethality, deregulated ZGA gene expression, and developmental arrest. Our study reveals an unexpected role of GADD45 proteins in embryonic two-cell stage regulation.

Induced Pluripotent Stem Cells Reprogrammed with Three Inhibitors Show Accelerated Differentiation Potentials with High Levels of 2-Cell Stage Marker Expression

Although pluripotent stem cells can generate various types of differentiated cells, it is unclear why lineage-committed stem/progenitor cells derived from pluripotent stem cells are decelerated and why the differentiation-resistant propensity of embryonic stem cell (ESC)/induced pluripotent stem cell (iPSC)-derived cells is predominant compared with the in vivo equivalents derived from embryonic/adult tissues. In this study, we demonstrated that iPSCs reprogrammed and maintained with three chemical inhibitors of the fibroblast growth factor 4-mitogen-activated protein kinase cascade and GSK3β (3i) could be differentiated into all three germ layers more efficiently than the iPSCs reprogrammed without the 3i chemicals, even though they were maintained with 3i chemicals once they were reprogrammed. Although the iPSCs reprogrammed with 3i had increased numbers of Zscan4-positive cells, the Zscan4-positive cells among iPSCs that were reprogrammed without 3i did not have an accelerated differentiation ability. These observations suggest that 3i exposure during the reprogramming period determines the accelerated differentiation/maturation potentials of iPSCs that are stably maintained at the distinct state.

Inferring Cell-State Transition Dynamics from Lineage Trees and Endpoint Single-Cell Measurements

As they proliferate, living cells undergo transitions between specific molecularly and developmentally distinct states. Despite the functional centrality of these transitions in multicellular organisms, it has remained challenging to determine which transitions occur and at what rates without perturbations and cell engineering. Here, we introduce kin correlation analysis (KCA) and show that quantitative cell-state transition dynamics can be inferred, without direct observation, from the clustering of cell states on pedigrees (lineage trees). Combining KCA with pedigrees obtained from time-lapse imaging and endpoint single-molecule RNA-fluorescence in situ hybridization (RNA-FISH) measurements of gene expression, we determined the cell-state transition network of mouse embryonic stem (ES) cells. This analysis revealed that mouse ES cells exhibit stochastic and reversible transitions along a linear chain of states ranging from 2C-like to epiblast-like. Our approach is broadly applicable and may be applied to systems with irreversible transitions and non-stationary dynamics, such as in cancer and development.

Novel HDAC5-interacting motifs of Tbx3 are essential for the suppression of E-cadherin expression and for the promotion of metastasis in hepatocellular carcinoma

Tbx3, a transcriptional repressor, is essential in the organogenesis of vertebrates, stem cell self-renewal and differentiation, and the carcinogenesis of multiple tumor types. However, the mechanism by which Tbx3 participates in the metastasis of hepatocellular carcinoma (HCC) remains largely unknown. In this study, we show that Tbx3 was dramatically upregulated in clinical HCC samples and that elevated expression of Tbx3 promoted cancer progression. To determine the underlying mechanism, systematic glycine scan mutagenesis and deletion assays were performed. We identified two critical motifs, ⁵⁸⁵LFSYPYT⁵⁹¹ and ⁶⁰⁴HRH⁶⁰⁶, that contribute to the repression of transcriptional activity. These motifs are also essential for Tbx3 to promote cell migration and metastasis both in vitro and in vivo via the suppression of E-cadherin expression. More importantly, Tbx3 directly interacts with HDAC5 via these motifs, and an HDAC inhibitor blocks Tbx3-mediated cell migration and the downregulation of E-cadherin in HCC. As Tbx3 is involved in the carcinogenesis of multiple types of human cancers, our findings suggest an important target for anti-cancer drug development.

A regulatory protein that represses gene activity interacts with an enzyme involved in chromosome remodeling to promote the migration and metastasis of liver cancer cells. Ming-Liang He from the City University of Hong Kong and colleagues found that levels of the T-box transcription factor Tbx3 were dramatically increased in tissue biopsies of liver tumors. They injected Tbx3-expressing human liver cancer cells into mice and saw a positive correlation between Tbx3 activity and cancer progression. By mutating and deleting parts of Tbx3, the researchers identified two particular stretches of the protein that bind histone deacetylase 5, an enzyme involved in ensuring DNA coils, are wound tight to suppress gene activity. This interaction is needed for Tbx3’s tumor-promoting function and may be targetable with drugs in order to prevent metastasis in patients with aggressive liver cancer.

Feeders facilitate telomere maintenance and chromosomal stability of embryonic stem cells

Feeder cells like mouse embryonic fibroblasts (MEFs) have been widely applied for culture of pluripotent stem cells, but their roles remain elusive. Noticeably, ESCs cultured on the feeders display transcriptional heterogeneity. We investigated roles of feeder cells by examining the telomere maintenance. Here we show that telomere is longer in mESCs cultured with than without the feeders. mESC cultures without MEF feeders exhibit telomere loss, chromosomal fusion, and aneuploidy with increasing passages. Notably, feeders facilitate heterogeneous transcription of 2-cell genes including Zscan4 and telomere elongation. Moreover, feeders produce Fstl1 that together with BMP4 periodically activate Zscan4. Interestingly, Zscan4 is repressed in mESCs cultured in 2i (inhibitors of Mek and Gsk3β signaling) media, associated with shorter telomeres and increased chromosome instability. These data suggest the important role of feeders in maintaining telomeres for long-term stable self-renewal and developmental pluripotency of mESCs.

Feeder cells are widely used for the culture of embryonic stem cells (ESCs), but their specific effects are not well known. Here, the authors demonstrate that mouse ESCs exhibit telomere loss and chromosomal aberrations associated with reduced Zscan4 with increasing passages in the absence of feeders

Leukemia inhibitory factor regulates marmoset induced pluripotent stem cell proliferation via a PI3K/‌Akt‑dependent Tbx‑3 activation pathway

Leukemia inhibitory factor (LIF) is the most pleiotropic cytokine of the interleukin-6 family, and is widely used to establish and maintain pluripotent stem cells, particularly mouse pluripotent stem cells. However, no reports have fully elucidated the application of LIF in marmoset induced pluripotent stem cell (iPSC) culture, particularly the underlying mechanisms. To demonstrate the feasibility of the application of LIF to marmoset iPSCs, the present study assessed these cells in the presence of LIF. Cell proliferation was measured using MTT assay, cell apoptosis was determined by flow cytometric analysis of fluorescein isothiocyanate Annexin V staining and the differentially expressed genes were analysed using Digital Gene Expression (DGE) analysis. The altered expression of pluripotency-associated genes was confirmed by reverse transcription-quantitative polymerase chain reaction and western blot analysis. Furthermore, following treatment with LY294002, cell proliferation was measured by MTT assay and protein levels were confirmed by western blot analysis. The results showed that LIF significantly promoted the number of proliferating cells, but had no effect on apoptosis. Digital Gene Expression analysis was used to examine the differentially expressed genes of marmoset iPSCs in the presence of LIF. The results showed that the pluripotency-associated transcription factor-encoding gene T-box 3 (Tbx-3) was activated by LIF. Notably, LIF increased the levels of phosphorylated (p-)AKT and Tbx-3 in the marmoset iPSCs. Furthermore, pretreatment with LY294002, an inhibitor of phosphoinositide 3-kinase (PI3K), significantly impaired the LIF-induced upregulation of p-AKT and Tbx-3 in the marmoset iPSCs, suggesting that the PI3K/Akt signaling pathway is involved in this regulation. Taken together, the results suggested that LIF is effective in maintaining marmoset iPSCs in cultures, which is associated with the activation of Tbx-3 through regulation of the PI3K/Akt signaling pathway.

Telomere chromatin establishment and its maintenance during mammalian development

Telomeres are specialized structures that evolved to protect the end of linear chromosomes from the action of the cell DNA damage machinery. They are composed of tandem arrays of repeated DNA sequences with a specific heterochromatic organization. The length of telomeric repeats is dynamically regulated and can be affected by changes in the telomere chromatin structure. When telomeres are not properly controlled, the resulting chromosomal alterations can induce genomic instability and ultimately the development of human diseases, such as cancer. Therefore, proper establishment, regulation, and maintenance of the telomere chromatin structure are required for cell homeostasis. Here, we review the current knowledge on telomeric chromatin dynamics during cell division and early development in mammals, and how its proper regulation safeguards genome stability.

Nono, a Bivalent Domain Factor, Regulates Erk Signaling and Mouse Embryonic Stem Cell Pluripotency

Nono is a component of the para-speckle, which stores and processes RNA. Mouse embryonic stem cells (mESCs) lack para-speckles, leaving the function of Nono in mESCs unclear. Here, we find that Nono functions as a chromatin regulator cooperating with Erk to regulate mESC pluripotency. We report that Nono loss results in robust self-renewing mESCs with epigenomic and transcriptomic features resembling the 2i (GSK and Erk inhibitors)-induced "ground state." Erk interacts with and is required for Nono localization to a subset of bivalent genes that have high levels of poised RNA polymerase. Nono loss compromises Erk activation and RNA polymerase poising at its target bivalent genes in undifferentiated mESCs, thus disrupting target gene activation and differentiation. These findings argue that Nono collaborates with Erk signaling to regulate the integrity of bivalent domains and mESC pluripotency.

Essential roles of Tbx3 in embryonic skin development during epidermal stratification

Stepwise differentiation of epidermal cells is essential for development of stratified epithelium, but the underlying mechanisms remain unclear. Here, we show that Tbx3, a member of the T-box family of transcription factors, plays a pivotal role in this mechanism. Tbx3 is expressed in both basal and suprabasal cells in the interfollicular epidermis of mouse embryos. Epidermis-specific Tbx3 conditional knockout (cKO) embryos are small in size and display a thinner epidermis with an impaired barrier function. In the Tbx3 cKO epidermis, keratin 5-positive undifferentiated cells, which reside in both basal and suprabasal layers of wild-type embryos, are localized exclusively in the basal layer. In addition, mRNA expression levels of granular cell markers are increased in the Tbx3 cKO epidermis, suggesting that Tbx3 prevents premature differentiation of spinous cells. We further show that Tbx3 maintains the proliferative potential of basal cells and ensures their planar-oriented cell division. Moreover, Tbx3 is shown to be required for the expression of Hes1, a well-known Notch signaling target protein that is essential for epidermal development. We therefore propose that Tbx3 functions upstream of Hes1 to regulate proliferation and differentiation of basal and suprabasal cells during epidermal development.

Linking Telomere Regulation to Stem Cell Pluripotency

Embryonic stem cells (ESCs), somatic cell nuclear transfer ESCs, and induced pluripotent stem cells (iPSCs) represent the most studied group of PSCs. Unlimited self-renewal without incurring chromosomal instability and pluripotency are essential for the potential use of PSCs in regenerative therapy. Telomere length maintenance is critical for the unlimited self-renewal, pluripotency, and chromosomal stability of PSCs. While telomerase has a primary role in telomere maintenance, alternative lengthening of telomere pathways involving recombination and epigenetic modifications are also required for telomere length regulation, notably in mouse PSCs. Telomere rejuvenation is part of epigenetic reprogramming to pluripotency. Insights into telomere reprogramming and maintenance in PSCs may have implications for understanding of aging and tumorigenesis. Here, I discuss the link between telomere elongation and homeostasis to the acquisition and maintenance of stem cell pluripotency, and their regulatory mechanisms by epigenetic modifications.

The T-Box factor TBX3 is important in S-phase and is regulated by c-Myc and cyclin A-CDK2

The transcription factor, TBX3, is critical for the formation of, among other structures, the heart, limbs and mammary glands and haploinsufficiency of the human TBX3 gene result in ulnar-mammary syndrome which is characterized by hypoplasia of these structures. On the other hand, the overexpression of TBX3 is a feature of a wide range of cancers and it has been implicated in several aspects of the oncogenic process. This includes its ability to function as an immortalizing gene and to promote proliferation through actively repressing negative cell cycle regulators. Together this suggests that TBX3 levels may need to be tightly regulated during the cell cycle. Here we demonstrate that this is indeed the case and that TBX3 mRNA and protein levels peak at S-phase and that the TBX3 protein is predominantly localized to the nucleus of S-phase cells. The increased levels of TBX3 in S-phase are shown to occur transcriptionally through activation by c-Myc at E-box motifs located at -1210 and -701 bps and post-translationally by cyclin A-CDK2 phosphorylation. Importantly, when TBX3 is depleted by shRNA the cells accumulate in S-phase. These results suggest that TBX3 is required for cells to transit through S-phase and that this function may be linked to its role as a pro-proliferative factor.

The T-box transcription factor TBX3 drives proliferation by direct repression of the p21(WAF1) cyclin-dependent kinase inhibitor

BACKGROUND

TBX3, a member of the T-box family of transcription factors, is essential in development and has emerged as an important player in the oncogenic process. TBX3 is overexpressed in several cancers and has been shown to contribute directly to tumour formation, migration and invasion. However, little is known about the molecular basis for its role in development and oncogenesis because there is a paucity of information regarding its target genes. The cyclin-dependent kinase inhibitor p21(WAF1) plays a pivotal role in a myriad of processes including cell cycle arrest, senescence and apoptosis and here we provide a detailed mechanism to show that it is a direct and biologically relevant target of TBX3.

RESULTS

Using a combination of luciferase reporter gene assays and in vitro and in vivo binding assays we show that TBX3 directly represses the p21(WAF1) promoter by binding a T-element close to its initiator. Furthermore, we show that the TBX3 DNA binding domain is required for the transcriptional repression of p21(WAF1) and that pseudo-phosphorylation of a serine proline motif (S190) located within this domain may play an important role in regulating this ability. Importantly, we demonstrate using knockdown and overexpression experiments that p21(WAF1) repression by TBX3 is biologically significant and required for TBX3-induced cell proliferation of chondrosarcoma cells.

CONCLUSIONS

Results from this study provide a detailed mechanism of how TBX3 transcriptionally represses p21(WAF1) which adds to our understanding of how it may contribute to oncogenesis.

Retinoic Acid Specifically Enhances Embryonic Stem Cell Metastate Marked by Zscan4

Pluripotency confers Embryonic Stem Cells (ESCs) the ability to differentiate in ectoderm, endoderm, and mesoderm derivatives, producing the majority of cell types. Although the majority of ESCs divide without losing pluripotency, it has become evident that ESCs culture consists of multiple cell populations with different degrees of potency that are spontaneously induced in regular ESC culture conditions. Zscan4, a key pluripotency factor, marks ESC subpopulation that is referred to as high-level of pluripotency metastate. Here, we report that in ESC cultures treated with retinoic acid (RA), Zscan4 ESCs metastate is strongly enhanced. In particular, we found that induction of Zscan4 metastate is mediated via RA receptors (RAR-alpha, RAR-beta, and RAR-gamma), and it is dependent on phosphoinositide-3-kinase (PI3K) signaling. Remarkably, Zscan4 metastate induced by RA lacks canonical pluripotency genes Oct3/4 and Nanog but retained both self-renewal and pluripotency capabilities. Finally we demonstrated that the conditional ablation of Zscan4 subpopulation is dispensable for both endoderm and mesoderm but is required for ectoderm lineage. In conclusion, our research provides new insights about the role of RA signaling during ESCs high pluripotency metastate fluctuation.

Tcstv1 and Tcstv3 elongate telomeres of mouse ES cells

Mouse embryonic stem cell (ESC) cultures exhibit a heterogeneous mixture of metastable cells sporadically entering the 2-cell (2C)-embryo-like state, critical for ESC potency. One of 2-cell genes, Zscan4, has been shown to be responsible for telomere maintenance, genomic stability and pluripotency of mouse ESCs. Functions of other 2C-genes in ESCs remain elusive. Here we show that 2C-genes Tcstv1 and Tcstv3 play a role in regulation of telomere lengths. Overexpression or knockdown Tcstv1 and Tcstv3 does not immediately affect proliferation, pluripotency and differentiation in vitro of ESCs. However, ectopic expression of Tcstv1 or Tcstv3 results in telomere elongation, whereas Tcstv1/3 knockdown shortens telomeres of ESCs. Overexpression of Tcstv1 or Tcstv3 does not alter telomere stability. Furthermore, Tcstv1 can increase Zscan4 protein levels and telomere recombination by telomere sister chromatid exchange (T-SCE). Depletion of Tcstv1/3 reduces Zscan4 protein levels. Together, Tcstv1 and Tcstv3 are involved in telomere maintenance that is required for long-term self-renewal of mouse ESCs. Our data also suggests that Tcstv1/3 may co-operate and stabilize Zscan4 protein but the molecular bases remain to be determined.

ETO family protein Mtgr1 mediates Prdm14 functions in stem cell maintenance and primordial germ cell formation

Prdm14 is a sequence-specific transcriptional regulator of embryonic stem cell (ESC) pluripotency and primordial germ cell (PGC) formation. It exerts its function, at least in part, through repressing genes associated with epigenetic modification and cell differentiation. Here, we show that this repressive function is mediated through an ETO-family co-repressor Mtgr1, which tightly binds to the pre-SET/SET domains of Prdm14 and co-occupies its genomic targets in mouse ESCs. We generated two monobodies, synthetic binding proteins, targeting the Prdm14 SET domain and demonstrate their utility, respectively, in facilitating crystallization and structure determination of the Prdm14-Mtgr1 complex, or as genetically encoded inhibitor of the Prdm14-Mtgr1 interaction. Structure-guided point mutants and the monobody abrogated the Prdm14-Mtgr1 association and disrupted Prdm14's function in mESC gene expression and PGC formation in vitro. Altogether, our work uncovers the molecular mechanism underlying Prdm14-mediated repression and provides renewable reagents for studying and controlling Prdm14 functions.

DOI:http://dx.doi.org/10.7554/eLife.10150.001

In animals, there are many different types of cells that perform different roles. For example, stem cells divide to produce new cells that may then become other types of cells such as muscle or skin cells. Most stem cells can only produce a limited range of other cell types, except for a subset known as ‘pluripotent’ stem cells that can give rise to cells of any type in the body.

A protein called Prdm14 helps to keep stem cells in a pluripotent state. In mouse embryos, Prdm14 binds to and represses particular genes that promote a process by which the stem cells can change into other cell types. If Prdm14 is missing from pluripotent stem cells, these cells become more sensitive to signals that encourage them to become other types of cells, which can lead to the loss of pluripotency. Prdm14 contains a region called the SET domain. In other proteins, this domain can alter how DNA is packaged to help switch particular genes on or off. However, such activity has not been found for the SET domain of Prdm14, raising questions about how it actually works.

Here, Nady, Gupta et al. show that Prdm14 tightly interacts with a protein called Mtgr1, which belongs to a family of proteins known to be involved in leukemia. The loss of Mtgr1 also leads to the loss of pluripotency in mouse stem cells and disrupts the formation of reproductive stem cells. Furthermore, Mtgr1 binds to the same genes as Prdm14. Next, Nady, Gupta et al. made synthetic proteins, termed monobodies, that bind to the Prdm14 SET domain. One such monobody enabled the authors to determine the three-dimensional structure of Prdm1 and Mtgr1, which revealed that the SET domain of Prdm14 has many points of contact with Mtgr1. Importantly, interaction between the two partners is crucial for these proteins to maintain pluripotency and promote the production of reproductive stem cells.

Altogether, these findings identify Mtgr1 as a key binding partner of Prdm14 in pluripotent stem cells and uncover a role for the SET domain in this interaction. A future challenge will be to understand the roles of these proteins in leukemia and other diseases.

DOI:http://dx.doi.org/10.7554/eLife.10150.002

Roles for Histone Acetylation in Regulation of Telomere Elongation and Two-cell State in Mouse ES Cells

Mammalian telomeres and subtelomeres are marked by heterochromatic epigenetic modifications, including repressive DNA methylation and histone methylation (e.g., H3K9me3 and H4K20me3). Loss of these epigenetic marks results in increased rates of telomere recombination and elongation. Other than these repressive epigenetic marks, telomeric and subtelomeric H3 and H4 are underacetylated. Yet, whether histone acetylation also regulates telomere length has not been directly addressed. We thought to test the effects of histone acetylation levels on telomere length using histone deacetylase (HDAC) inhibitor (sodium butyrate, NaB) that mediates histone hyperacetylation and histone acetyltransferase (HAT) inhibitor (C646) that mediates histone hypoacetylation. We show that histone hyperacetylation dramatically elongates telomeres in wild-type ES cells, and only slightly elongates telomeres in Terc(-/-) ES cells, suggesting that Terc is involved in histone acetylation-induced telomere elongation. In contrast, histone hypoacetylation shortens telomeres in both wild-type and Terc(-/-) ES cells. Additionally, histone hyperacetylation activates 2-cell (2C) specific genes including Zscan4, which is involved in telomere recombination and elongation, whereas histone hypoacetylation represses Zscan4 and 2C genes. These data suggest that histone acetylation levels affect the heterochromatic state at telomeres and subtelomeres, and regulate gene expression at subtelomeres, linking histone acetylation to telomere length maintenance.

The molecular underpinnings of totipotency

Embryonic stem (ES) cells are characterized by their functional potency and capacity to self-renew in culture. Historically, ES cells have been defined as pluripotent, able to make the embryonic but not the extraembryonic lineages (such as the yolk sac and the placenta). The functional capacity of ES cells has been judged based on their ability to contribute to all somatic lineages when they are introduced into an embryo. However, a number of recent reports have suggested that under certain conditions, ES cells, and other reprogrammed cell lines, can also contribute to the extraembryonic lineages and, therefore, can be said to be totipotent. Here, we consider the molecular basis for this totipotent state, its transcriptional signature and the signalling pathways that define it.

Transcription factor heterogeneity in pluripotent stem cells: a stochastic advantage

When pluripotent cells are exposed to a uniform culture environment they routinely display heterogeneous gene expression. Aspects of this heterogeneity, such as Nanog expression, are linked to differences in the propensity of individual cells to either self-renew or commit towards differentiation. Recent findings have provided new insight into the underlying causes of this heterogeneity, which we summarise here using Nanog, a key regulator of pluripotency, as a model gene. We discuss the role of transcription factor heterogeneity in facilitating the intrinsically dynamic and stochastic nature of the pluripotency network, which in turn provides a potential benefit to a population of cells that needs to balance cell fate decisions.

Telomere length reprogramming in embryos and stem cells

Telomeres protect and cap linear chromosome ends, yet these genomic buffers erode over an organism's lifespan. Short telomeres have been associated with many age-related conditions in humans, and genetic mutations resulting in short telomeres in humans manifest as syndromes of precocious aging. In women, telomere length limits a fertilized egg's capacity to develop into a healthy embryo. Thus, telomere length must be reset with each subsequent generation. Although telomerase is purportedly responsible for restoring telomere DNA, recent studies have elucidated the role of alternative telomeres lengthening mechanisms in the reprogramming of early embryos and stem cells, which we review here.

Tbx3 isoforms are involved in pluripotency maintaining through distinct regulation of Nanog transcriptional activity

Tbx3, a member of T-box gene family, has been reported to play critical roles in embryonic development and cell fate determination. In mammalian tissues, Tbx3 is expressed as two isoforms called Tbx3 and Tbx3+2a. However, the differences between the two isoforms in pluripotency maintaining remain obscure. Here we show that both Tbx3 and Tbx3+2a are highly expressed in mouse embryonic stem cells (mESCs) or induced pluripotent stem cells (iPSCs). Overexpression of either Tbx3 or Tbx3+2a could induce the differentiation of mESCs. Mechanistic studies suggest both Tbx3 and Tbx3+2a inhibit the transcriptional activity of pluripotency related transcription factor Nanog. Moreover, Tbx3+2a could directly interact with Nanog while Tbx3 couldn't, indicating the requirement of the 2a domain in Nanog binding. In summary, our results not only reveal the essential roles of Tbx3 and Tbx3+2a in pluripotency maintaining, but also point out the different mechanistic modes between these two isoforms.