Tcstv1 and Tcstv3 elongate telomeres of mouse ES cells

Rescuing DNMT1 fails to fully reverse the molecular and functional repercussions of its loss in mouse embryonic stem cells

Epigenetic mechanisms are crucial for developmental programming and can be disrupted by environmental stressors, increasing susceptibility to disease. This has sparked interest in therapies for restoring epigenetic balance, but it remains uncertain whether disordered epigenetic mechanisms can be fully corrected. Disruption of DNA methyltransferase 1 (DNMT1), responsible for DNA methylation maintenance, has particularly devastating biological consequences. Therefore, here we explored if rescuing DNMT1 activity is sufficient to reverse the effects of its loss utilizing mouse embryonic stem cells. However, only partial reversal could be achieved. Extensive changes in DNA methylation, histone modifications, and gene expression were detected, along with transposable element derepression and genomic instability. Reduction of cellular size, complexity, and proliferation rate were observed, as well as lasting effects in germ layer lineages and embryoid bodies. Interestingly, by analyzing the impact on imprinted regions, we uncovered 20 regions exhibiting imprinted-like signatures. Notably, while many permanent effects persisted throughout Dnmt1 inactivation and rescue, others arose from the rescue intervention. Lastly, rescuing DNMT1 after differentiation initiation worsened outcomes, reinforcing the need for early intervention. Our findings highlight the far-reaching functions of DNMT1 and provide valuable perspectives on the repercussions of epigenetic perturbations during early development and the challenges of rescue interventions.

Nr5a2 is dispensable for zygotic genome activation but essential for morula development

Early embryogenesis is driven by transcription factors (TFs) that first activate the zygotic genome and then specify the lineages constituting the blastocyst. Although the TFs specifying the blastocyst's lineages are well characterized, those playing earlier roles remain poorly defined. Using mouse models of the TF Nr5a2, we show that Nr5a2-/- embryos arrest at the early morula stage and exhibit altered lineage specification, frequent mitotic failure, and substantial chromosome segregation defects. Although NR5A2 plays a minor but measurable role during zygotic genome activation, it predominantly acts as a master regulator at the eight-cell stage, controlling expression of lineage-specifying TFs and genes involved in mitosis, telomere maintenance, and DNA repair. We conclude that NR5A2 coordinates proliferation, genome stability, and lineage specification to ensure correct morula development.

The role of Zfp352 in the regulation of transient expression of 2-cell specific genes in mouse embryonic stem cells

Mouse ES cell populations contain a minor sub-population that expresses genes specifically expressed in 2-cell stage embryos. This sub-population consists of 2-cell-gene labeled cells (2CLCs) generated by the transient activation of the 2-cell specific genes initiated by the master regulator, Dux. However, the mechanism regulating the transient expression remains largely unclear. Here we reported a novel function of Zfp352, one of the 2-cell specific genes, in regulating the 2CLC sub-population. Zfp352 encodes zinc-finger transcription factor belonging to the Klf family. Dux transiently activates Zfp352 after the activation of Zscan4c in a subset of the 2CLC subpopulation. Interestingly, in the reporter assay, the transcriptional activation of Zscan4c by Dux is strongly repressed by the co-expression of Zfp352. However, the knockout of Zfp352 resulted in the repression of a subset of the 2-cell-specific genes. These data suggest the dual roles of Zfp352 in regulating the transient activation of the 2-cell-specific genes.

Metabolic and cell cycle shift induced by the deletion of Dnm1l attenuates the dissolution of pluripotency in mouse embryonic stem cells

Mitochondria are versatile organelles that continuously change their morphology via fission and fusion. However, the detailed functions of mitochondrial dynamics-related genes in pluripotent stem cells remain largely unclear. Here, we aimed to determine the effects on energy metabolism and differentiation ability of mouse embryonic stem cells (ESCs) following deletion of the mitochondrial fission-related gene Dnml1. Resultant Dnm1l-/- ESCs maintained major pluripotency characteristics. However, Dnm1l-/- ESCs showed several phenotypic changes, including the inhibition of differentiation ability (dissolution of pluripotency). Notably, Dnm1l-/- ESCs maintained the expression of the pluripotency marker Oct4 and undifferentiated colony types upon differentiation induction. RNA sequencing analysis revealed that the most frequently differentially expressed genes were enriched in the glutathione metabolic pathway. Our data suggested that differentiation inhibition of Dnm1l-/- ESCs was primarily due to metabolic shift from glycolysis to OXPHOS, G2/M phase retardation, and high level of Nanog and 2-cell-specific gene expression.

Human endogenous retroviruses in development and disease

Human endogenous retroviruses (HERVs) represent ∼8% of human genome, deriving from exogenous retroviral infections of germ line cells occurred millions of years ago and being inherited by the offspring in a Mendelian fashion. Most of HERVs are nonprotein-coding because of the accumulation of mutations, insertions, deletions, and/or truncations. It has been long thought that HERVs were "junk DNA". However, it is now known that HERVs are involved in various biological processes through encoding proteins, acting as promoters/enhancers, or lncRNAs to affect human health and disease. In this review, we summarized recent findings about HERVs, with implications in embryonic development, pluripotency, cancer, aging, and neurodegenerative diseases.

Lrrc34 Interacts with Oct4 and Has an Impact on Telomere Length in Mouse Embryonic Stem Cells

Telomere length maintenance in pluripotent stem cells (PSCs) is a main characteristic and a major premise for their undifferentiated long-term survival. However, little is known about the factors that control telomere length and elongation in these cells. Here, I describe Lrrc34 (leucine-rich repeat 34) as a novel telomere length regulating gene in murine embryonic stem cells. Downregulation of Lrrc34 results in significant reduction of telomerase activity and telomere length over time while also influencing the expression of known telomere length-associated genes. Generating induced PSCs (iPSCs) with Lrrc34 as a fifth factor in classical Yamanaka reprogramming increases the efficiency but did not have an impact on telomere length in the resulting iPSCs. Moreover, Lrrc34 was found to interact with Oct4, connecting the pluripotency network to telomere length regulation.

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.

From pluripotency to totipotency: an experimentalist's guide to cellular potency

Embryonic stem cells (ESCs) are derived from the pre-implantation mammalian blastocyst. At this point in time, the newly formed embryo is concerned with the generation and expansion of both the embryonic lineages required to build the embryo and the extra-embryonic lineages that support development. When used in grafting experiments, embryonic cells from early developmental stages can contribute to both embryonic and extra-embryonic lineages, but it is generally accepted that ESCs can give rise to only embryonic lineages. As a result, they are referred to as pluripotent, rather than totipotent. Here, we consider the experimental potential of various ESC populations and a number of recently identified in vitro culture systems producing states beyond pluripotency and reminiscent of those observed during pre-implantation development. We also consider the nature of totipotency and the extent to which cell populations in these culture systems exhibit this property.

ATR expands embryonic stem cell fate potential in response to replication stress

Unrepaired DNA damage during embryonic development can be potentially inherited by a large population of cells. However, the quality control mechanisms that minimize the contribution of damaged cells to developing embryos remain poorly understood. Here, we uncovered an ATR- and CHK1-mediated transcriptional response to replication stress (RS) in mouse embryonic stem cells (ESCs) that induces genes expressed in totipotent two-cell (2C) stage embryos and 2C-like cells. This response is mediated by Dux, a multicopy retrogene defining the cleavage-specific transcriptional program in placental mammals. In response to RS, DUX triggers the transcription of 2C-like markers such as murine endogenous retrovirus-like elements (MERVL) and Zscan4. This response can also be elicited by ETAA1-mediated ATR activation in the absence of RS. ATR-mediated activation of DUX requires GRSF1-dependent post-transcriptional regulation of Dux mRNA. Strikingly, activation of ATR expands ESCs fate potential by extending their contribution to both embryonic and extra-embryonic tissues. These findings define a novel ATR dependent pathway involved in maintaining genome stability in developing embryos by controlling ESCs fate in response to RS.

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.

Sphingosine kinases protect murine embryonic stem cells from sphingosine-induced cell cycle arrest

Sphingosine‐1‐phosphate (S1P) is a bioactive lipid molecule regulating organogenesis, angiogenesis, cell proliferation, and apoptosis. S1P is generated by sphingosine kinases (SPHK1 and SPHK2) through the phosphorylation of ceramide‐derived sphingosine. Phenotypes caused by manipulating S1P metabolic enzymes and receptors suggested several possible functions for S1P in embryonic stem cells (ESCs), yet the mechanisms by which S1P and related sphingolipids act in ESCs are controversial. We designed a rigorous test to evaluate the requirement of S1P in murine ESCs by knocking out both Sphk1 and Sphk2 to create cells incapable of generating S1P. To accomplish this, we created lines mutant for Sphk2 and conditionally mutant (floxed) for Sphk1, allowing evaluation of ESCs that transition to double‐null state. The Sphk1/2‐null ESCs lack S1P and accumulate the precursor sphingosine. The double‐mutant cells fail to grow due to a marked cell cycle arrest at G2/M. Mutant cells activate expression of telomere elongation factor genes Zscan4, Tcstv1, and Tcstv3 and display longer telomeric repeats. Adding exogenous S1P to the medium had no impact, but the cell cycle arrest is partially alleviated by the expression of a ceramide synthase 2, which converts excess sphingosine into ceramide. The results indicate that sphingosine kinase activity is essential in mouse ESCs for limiting the accumulation of sphingosine that otherwise drives cell cycle arrest.

Human telomerase reverse transcriptase is a novel target of Hippo-YAP pathway

Telomerase plays a pivotal role in tumorigenesis by maintaining telomere homeostasis, a hallmark of cancer. However, the mechanisms by which telomerase is reactivated or upregulated during tumorigenesis remain incompletely understood. Here, we report that the Hippo pathway effector Yes-associated protein (YAP) regulates the expression of human telomerase reverse transcriptase (hTERT). Ectopic expression or physiological activation of YAP increases hTERT expression, whereas knockdown of YAP decreases the expression of hTERT. YAP binds to the hTERT promoter through interaction with the TEA domain family transcription factors and activates hTERT transcription. Furthermore, sustained YAP hyperactivation promotes telomerase activity and extends telomere length, with increased hTERT expression. In addition, we show that hTERT expression is positively correlated with YAP activation in human liver cancer tissues. Together, our results demonstrate that YAP promotes hTERT expression, which could contribute to tumor progression.

Single-Cell RNA-Seq Reveals Dynamic Early Embryonic-like Programs during Chemical Reprogramming

Chemical reprogramming provides a powerful platform for exploring the molecular dynamics that lead to pluripotency. Although previous studies have uncovered an intermediate extraembryonic endoderm (XEN)-like state during this process, the molecular underpinnings of pluripotency acquisition remain largely undefined. Here, we profile 36,199 single-cell transcriptomes at multiple time points throughout a highly efficient chemical reprogramming system using RNA-sequencing and reconstruct their progression trajectories. Through identifying sequential molecular events, we reveal that the dynamic early embryonic-like programs are key aspects of successful reprogramming from XEN-like state to pluripotency, including the concomitant transcriptomic signatures of two-cell (2C) embryonic-like and early pluripotency programs and the epigenetic signature of notable genome-wide DNA demethylation. Moreover, via enhancing the 2C-like program by fine-tuning chemical treatment, the reprogramming process is remarkably accelerated. Collectively, our findings offer a high-resolution dissection of cell fate dynamics during chemical reprogramming and shed light on mechanistic insights into the nature of induced pluripotency.

Dynamics of Telomere Rejuvenation during Chemical Induction to Pluripotent Stem Cells

Chemically induced pluripotent stem cells (CiPSCs) may provide an alternative and attractive source for stem cell-based therapy. Sufficient telomere lengths are critical for unlimited self-renewal and genomic stability of pluripotent stem cells. Dynamics and mechanisms of telomere reprogramming of CiPSCs remain elusive. We show that CiPSCs acquire telomere lengthening with increasing passages after clonal formation. Both telomerase activity and recombination-based mechanisms are involved in the telomere elongation. Telomere lengths strongly indicate the degree of reprogramming, pluripotency, and differentiation capacity of CiPSCs. Nevertheless, telomere damage and shortening occur at a late stage of lengthy induction, limiting CiPSC formation. We find that histone crotonylation induced by crotonic acid can activate two-cell genes, including Zscan4; maintain telomeres; and promote CiPSC generation. Crotonylation decreases the abundance of heterochromatic H3K9me3 and HP1α at subtelomeres and Zscan4 loci. Taken together, telomere rejuvenation links to reprogramming and pluripotency of CiPSCs. Crotonylation facilitates telomere maintenance and enhances chemically induced reprogramming to pluripotency.

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CiPSCs acquire telomere elongation after clonal formation with increasing passages

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Both telomerase and recombination mechanisms are involved in the telomere elongation

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Telomere damage and shortening can occur during late stage of lengthy induction

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Crotonylation activates Zscan4 and promotes telomere elongation and CiPSC induction

Aurora Kinase B, a novel regulator of TERF1 binding and telomeric integrity

AURKB (Aurora Kinase B) is a serine/threonine kinase better known for its role at the mitotic kinetochore during chromosome segregation. Here, we demonstrate that AURKB localizes to the telomeres in mouse embryonic stem cells, where it interacts with the essential telomere protein TERF1. Loss of AURKB function affects TERF1 telomere binding and results in aberrant telomere structure. In vitro kinase experiments successfully identified Serine 404 on TERF1 as a putative AURKB target site. Importantly, in vivo overexpression of S404-TERF1 mutants results in fragile telomere formation. These findings demonstrate that AURKB is an important regulator of telomere structural integrity.

Long Terminal Repeats: From Parasitic Elements to Building Blocks of the Transcriptional Regulatory Repertoire

The life cycle of endogenous retroviruses (ERVs), also called long terminal repeat (LTR) retrotransposons, begins with transcription by RNA polymerase II followed by reverse transcription and re-integration into the host genome. While most ERVs are relics of ancient integration events, "young" proviruses competent for retrotransposition-found in many mammals, but not humans-represent an ongoing threat to host fitness. As a consequence, several restriction pathways have evolved to suppress their activity at both transcriptional and post-transcriptional stages of the viral life cycle. Nevertheless, accumulating evidence has revealed that LTR sequences derived from distantly related ERVs have been exapted as regulatory sequences for many host genes in a wide range of cell types throughout mammalian evolution. Here, we focus on emerging themes from recent studies cataloging the diversity of ERV LTRs acting as important transcriptional regulatory elements in mammals and explore the molecular features that likely account for LTR exaptation in developmental and tissue-specific gene regulation.