DNA methylation is an essential epigenetic mark in mammals that has to be re-established after each round of DNA replication. The protein UHRF1 is essential for this process; it has been proposed ...that the protein targets newly replicated DNA by cooperatively binding hemi-methylated DNA and H3K9me2/3, but this model leaves a number of questions unanswered. Here, we present evidence for a direct recruitment of UHRF1 by the replication machinery via DNA ligase 1 (LIG1). A histone H3K9-like mimic within LIG1 is methylated by G9a and GLP and, compared with H3K9me2/3, more avidly binds UHRF1. Interaction with methylated LIG1 promotes the recruitment of UHRF1 to DNA replication sites and is required for DNA methylation maintenance. These results further elucidate the function of UHRF1, identify a non-histone target of G9a and GLP, and provide an example of a histone mimic that coordinates DNA replication and DNA methylation maintenance.
Display omitted
•Mammalian DNA ligase 1 (LIG1) contains a conserved H3K9 mimic•LIG1 is a non-histone target of G9a and GLP•Methylated LIG1 is a high-affinity target for the tandem tudor domain of UHRF1•Methylated LIG1 recruits UHRF1 to replication foci to maintain DNA methylation
DNA methylation is a key epigenetic mark that must be re-established after each round of replication. Ferry et al. describe a mechanism directly linking DNA methylation maintenance to DNA replication, in which the histone modification machinery acts on a replication protein, allowing recruitment of a key DNA methylation factor.
Single-strand break repair (SSBR) and base excision repair (BER) of modified bases and abasic sites share several players. Among them is XRCC1, an essential scaffold protein with no enzymatic ...activity, required for the coordination of both pathways. XRCC1 is recruited to SSBR by PARP-1, responsible for the initial recognition of the break. The recruitment of XRCC1 to BER is still poorly understood. Here we show by using both local and global induction of oxidative DNA base damage that XRCC1 participation in BER complexes can be distinguished from that in SSBR by several criteria. We show first that XRCC1 recruitment to BER is independent of PARP. Second, unlike SSBR complexes that are assembled within minutes after global damage induction, XRCC1 is detected later in BER patches, with kinetics consistent with the repair of oxidized bases. Third, while XRCC1-containing foci associated with SSBR are formed both in eu- and heterochromatin domains, BER complexes are assembled in patches that are essentially excluded from heterochromatin and where the oxidized bases are detected.
The integrity of chromatin, which provides a dynamic template for all DNA-related processes in eukaryotes, is maintained through replication-dependent and -independent assembly pathways. To address ...the role of histone deposition in the absence of DNA replication, we deleted the H3.3 chaperone Hira in developing mouse oocytes. We show that chromatin of non-replicative developing oocytes is dynamic and that lack of continuous H3.3/H4 deposition alters chromatin structure, resulting in increased DNase I sensitivity, the accumulation of DNA damage, and a severe fertility phenotype. On the molecular level, abnormal chromatin structure leads to a dramatic decrease in the dynamic range of gene expression, the appearance of spurious transcripts, and inefficient de novo DNA methylation. Our study thus unequivocally shows the importance of continuous histone replacement and chromatin homeostasis for transcriptional regulation and normal developmental progression in a non-replicative system in vivo.
Display omitted
•Histone H3/H4 replacement is continuous and mediated by Hira during mouse oogenesis•Loss of Hira results in chromatin abnormalities and extensive oocyte loss•Hira depletion reduces histone load, which prevents normal transcriptional regulation•Hira-mediated histone replacement is required for normal 5mC deposition in oocytes
To address the extent to which basic cellular processes depend on replication-independent chromatin assembly in vivo, Nashun et al. deleted histone H3.3 chaperone Hira during mouse oogenesis. Their results demonstrate a critical relationship between continuing histone replacement, chromatin homeostasis, transcriptional regulation, and de novo DNA methylation.
Epigenomic mechanisms regulate distinct aspects of the inflammatory response in immune cells. Despite the central role for microglia in neuroinflammation and neurodegeneration, little is known about ...their epigenomic regulation of the inflammatory response. Here, we show that Ten-eleven translocation 2 (TET2) methylcytosine dioxygenase expression is increased in microglia upon stimulation with various inflammogens through a NF-κB-dependent pathway. We found that TET2 regulates early gene transcriptional changes, leading to early metabolic alterations, as well as a later inflammatory response independently of its enzymatic activity. We further show that TET2 regulates the proinflammatory response in microglia of mice intraperitoneally injected with LPS. We observed that microglia associated with amyloid β plaques expressed TET2 in brain tissue from individuals with Alzheimer’s disease (AD) and in 5xFAD mice. Collectively, our findings show that TET2 plays an important role in the microglial inflammatory response and suggest TET2 as a potential target to combat neurodegenerative brain disorders.
Display omitted
•TET2 is upregulated in microglia cells upon various inflammatory stimuli•TET2 regulates TLR-4-induced type I IFN response and LPS-induced aerobic glycolysis•TET2 regulates the proinflammatory response induced by LPS in vitro and in vivo•TET2 is expressed by amyloid β plaque-associated microglia in AD brains
Microglia play a key role in neuroinflammation and neurodegeneration in different neurodegenerative diseases, but little is known about the epigenetic modifying enzymes regulating their inflammatory response. Carrillo-Jimenez et al. identify TET2 as a major regulator of the microglia proinflammatory response and suggest that, by targeting this protein, microglial activation could be impaired.
Cultured pluripotent stem cells hold great promise for regenerative medicine. Considerable efforts have been invested into the refinement and definition of improved culture systems that sustain ...self-renewal and avoid differentiation of pluripotent cells in vitro. Recent studies have, however, found that the choice of culture condition has a significant impact on epigenetic profiles of cultured pluripotent cells. Mouse and human ESCs (embryonic stem cells) show substantial epigenetic differences that are dependent on the culture condition, including global changes to DNA methylation and histone modifications and, in female human ESCs, to the epigenetic process of X chromosome inactivation. Epigenetic perturbations have also been detected during culture of pre-implantation embryos; limited research undertaken in mouse suggests a direct effect of the in vitro environment on epigenetic processes in this system. Widespread epigenetic changes induced by the culture condition in stem cells thus emphasize the necessity for extensive research into both immediate and long-term epigenetic effects of embryo culture during assisted reproductive technologies.
The DNA glycosylase hOGG1 initiates base excision repair (BER) of oxidised purines in cellular DNA. Using confocal microscopy and biochemical cell fractionation experiments we show that, upon UVA ...irradiation of human cells, hOGG1 is recruited from a soluble nucleoplasmic localisation to the nuclear matrix. More specifically, after irradiation, hOGG1 forms foci colocalising with the nuclear speckles, organelles that are interspersed between chromatin domains and that have been associated with transcription and RNA-splicing processes. The use of mutant forms of hOGG1 unable to bind the substrate showed that relocalisation of hOGG1 does not depend on the recognition of the DNA lesion by the enzyme. The recruitment of hOGG1 to the nuclear speckles is prevented by the presence of antioxidant compounds during UVA irradiation, implicating reactive oxygen species as signals for the relocalisation of hOGG1. Furthermore, APE1, the second enzyme in the BER pathway, is also present in nuclear speckles in UVA-irradiated cells. The recruitment of DNA repair proteins to nuclear speckles after oxidative stress implicates these organelles in the cellular stress response.
Irradiation of mammalian cells with solar light is associated with the generation of reactive oxygen species (ROS) and oxidative stress, which is mediated in part by endogenous photosensitizers ...absorbing in the visible range of the solar spectrum. Accordingly, oxidative DNA base modifications such as 7,8-dihydro-8-oxoguanine (8-oxoG) are the predominant types of DNA damage in cells irradiated at wavelengths >400
nm. We have analysed the repair of oxidative purine modifications in human skin fibroblasts and melanoma cells using an alkaline elution technique, both under normal conditions and after depletion of glutathione. Similar repair rates were observed in fibroblasts and melanoma cells from three different patients (
t
1/2 approximately 4
h). In both cell types, glutathione depletion (increased oxidative stress) caused a pronounced repair retardation even under non-toxic irradiation conditions. Furthermore, the cleavage activity at 8-oxoG residues measured in protein extracts of the cells dropped transiently after irradiation. An addition of dithiothreitol restored normal repair rates. Interestingly, the repair rates of cyclobutane pyrimidine dimers (
t
1/2 approximately 18
h), AP sites (
t
1/2 approximately 1
h) and single-strand breaks (
t
1/2 <30
min) were not affected by the light-induced oxidative stress. We conclude that the base excision repair of oxidative purine modifications is surprisingly vulnerable to oxidative stress, while the nucleotide excision repair of pyrimidine dimers is not.
Epigenetic reprogramming involves processes that lead to the erasure of epigenetic information, reverting the chromatin template to a less differentiated state. Extensive epigenetic reprogramming ...occurs both naturally during mammalian development in the early embryo and the developing germ line, and artificially in various in vitro reprogramming systems. Global DNA demethylation appears to be a shared attribute of reprogramming events, and understanding DNA methylation dynamics is thus of considerable interest. Recently, the Tet enzymes, which catalyse the iterative oxidation of 5-methylcytosine to 5-hydroxymethylcytosine, 5-formylcytosine and 5-carboxylcytosine, have emerged as potential drivers of epigenetic reprogramming. Although some of the recent studies point towards the direct role of Tet proteins in the removal of DNA methylation, the accumulating evidence suggests that the processes underlying DNA methylation dynamics might be more complex. Here, we review the current evidence, highlighting the agreements and the discrepancies between the suggested models and the experimental evidence.
•Tet enzymes and 5hmC are implicated in epigenetic reprogramming in vivo and in vitro.•The relationship between 5hmC and DNA demethylation isn’t as simple as models suggest.•5hmC-independent functions of Tet(1-3) should be considered in future investigations.
Gametes are highly specialized cells that can give rise to the next generation through their ability to generate a totipotent zygote. In mice, germ cells are first specified in the developing embryo ...around embryonic day (E) 6.25 as primordial germ cells (PGCs). Following subsequent migration into the developing gonad, PGCs undergo a wave of extensive epigenetic reprogramming around E10.5-E11.5, including genome-wide loss of 5-methylcytosine. The underlying molecular mechanisms of this process have remained unclear, leading to our inability to recapitulate this step of germline development in vitro. Here we show, using an integrative approach, that this complex reprogramming process involves coordinated interplay among promoter sequence characteristics, DNA (de)methylation, the polycomb (PRC1) complex and both DNA demethylation-dependent and -independent functions of TET1 to enable the activation of a critical set of germline reprogramming-responsive genes involved in gamete generation and meiosis. Our results also reveal an unexpected role for TET1 in maintaining but not driving DNA demethylation in gonadal PGCs. Collectively, our work uncovers a fundamental biological role for gonadal germline reprogramming and identifies the epigenetic principles of the PGC-to-gonocyte transition that will help to guide attempts to recapitulate complete gametogenesis in vitro.
Zygotic epigenetic reprogramming entails genome-wide DNA demethylation that is accompanied by Tet methylcytosine dioxygenase 3 (Tet3)-driven oxidation of 5-methylcytosine (5mC) to ...5-hydroxymethylcytosine (5hmC; refs 1-4). Here we demonstrate using detailed immunofluorescence analysis and ultrasensitive LC-MS-based quantitative measurements that the initial loss of paternal 5mC does not require 5hmC formation. Small-molecule inhibition of Tet3 activity, as well as genetic ablation, impedes 5hmC accumulation in zygotes without affecting the early loss of paternal 5mC. Instead, 5hmC accumulation is dependent on the activity of zygotic Dnmt3a and Dnmt1, documenting a role for Tet3-driven hydroxylation in targeting de novo methylation activities present in the early embryo. Our data thus provide further insights into the dynamics of zygotic reprogramming, revealing an intricate interplay between DNA demethylation, de novo methylation and Tet3-driven hydroxylation.
PDF
