HP1 proteins regulate nucleolar structure and function by secluding pericentromeric constitutive heterochromatin.

Nucleoli are nuclear compartments regulating ribosome biogenesis and cell growth. In embryonic stem cells (ESCs), nucleoli containing transcriptionally active ribosomal genes are spatially separated from pericentromeric satellite repeat sequences packaged in largely repressed constitutive heterochromatin (PCH). To date, mechanisms underlying such nuclear partitioning and the physiological relevance thereof are unknown. Here we show that repressive chromatin at PCH ensures structural integrity and function of nucleoli during cell cycle progression. Loss of heterochromatin proteins HP1α and HP1ß causes deformation of PCH, with reduced H3K9 trimethylation (H3K9me3) and HP1γ levels, absence of H4K20me3 and upregulated major satellites expression. Spatially, derepressed PCH aberrantly associates with nucleoli accumulating severe morphological defects during S/G2 cell cycle progression. Hp1α/ß deficiency reduces cell proliferation, ribosomal RNA biosynthesis and mobility of Nucleophosmin, a major nucleolar component. Nucleolar integrity and function require HP1α/ß proteins to be recruited to H3K9me3-marked PCH and their ability to dimerize. Correspondingly, ESCs deficient for both Suv39h1/2 H3K9 HMTs display similar nucleolar defects. In contrast, Suv4-20h1/2 mutant ESCs lacking H4K20me3 at PCH do not. Suv39h1/2 and Hp1α/ß deficiency-induced nucleolar defects are reminiscent of those defining human ribosomopathy disorders. Our results reveal a novel role for SUV39H/HP1-marked repressive constitutive heterochromatin in regulating integrity, function and physiology of nucleoli.

SUMOylated PRC1 controls histone H3.3 deposition and genome integrity of embryonic heterochromatin.

Chromatin integrity is essential for cellular homeostasis. Polycomb group proteins modulate chromatin states and transcriptionally repress developmental genes to maintain cell identity. They also repress repetitive sequences such as major satellites and constitute an alternative state of pericentromeric constitutive heterochromatin at paternal chromosomes (pat-PCH) in mouse pre-implantation embryos. Remarkably, pat-PCH contains the histone H3.3 variant, which is absent from canonical PCH at maternal chromosomes, which is marked by histone H3 lysine 9 trimethylation (H3K9me3), HP1, and ATRX proteins. Here, we show that SUMO2-modified CBX2-containing Polycomb Repressive Complex 1 (PRC1) recruits the H3.3-specific chaperone DAXX to pat-PCH, enabling H3.3 incorporation at these loci. Deficiency of Daxx or PRC1 components Ring1 and Rnf2 abrogates H3.3 incorporation, induces chromatin decompaction and breakage at PCH of exclusively paternal chromosomes, and causes their mis-segregation. Complementation assays show that DAXX-mediated H3.3 deposition is required for chromosome stability in early embryos. DAXX also regulates repression of PRC1 target genes during oogenesis and early embryogenesis. The study identifies a novel critical role for Polycomb in ensuring heterochromatin integrity and chromosome stability in mouse early development.

Cbx2 targets PRC1 to constitutive heterochromatin in mouse zygotes in a parent-of-origin-dependent manner.

Polycomb repressive complexes PRC1 and PRC2 regulate expression of genes involved in proliferation and development. In mouse early embryos, however, canonical PRC1 localizes to paternal pericentric heterochromatin (pat-PCH), where it represses transcription of major satellite repeats. In contrast, maternal PCH (mat-PCH) is enriched for H3 lysine 9 tri-methylation (H3K9me3) and Hp1ß. How PRC1 is targeted to pat-PCH, yet excluded from mat-PCH, has remained elusive. Here, we identify a PRC1 targeting mechanism that relies on Cbx2 and Hp1ß. Cbx2 directs catalytically active PRC1 to PCH via its chromodomain (CD(Cbx2)) and neighboring AT-hook (AT(Cbx2)) binding to H3K27me3 and AT-rich major satellites, respectively. CD(Cbx2) prevents AT(Cbx2) from interacting with DNA at PCH marked by H3K9me3 and Hp1ß. Loss-of-function studies show that Hp1ß and not H3K9me3 prevents PRC1 targeting to mat-PCH. Our findings indicate that CD(Cbx2) and AT(Cbx2) separated by a short linker function together to integrate H3K9me3/HP1 and H3K27me3 states.

Cocktails of NSAIDs and 17α Ethinylestradiol at Environmentally Relevant Doses in Drinking Water Alter Puberty Onset in Mice Intergenerationally.

Non-steroidal anti-inflammatory drugs (NSAIDs) and 17α-ethinyl-estradiol (EE2) are among the most relevant endocrine-disrupting pharmaceuticals found in the environment, particularly in surface and drinking water due to their incomplete removal via wastewater treatment plants. Exposure of pregnant mice to NSAID therapeutic doses during the sex determination period has a negative impact on gonadal development and fertility in adults; however, the effects of their chronic exposure at lower doses are unknown. In this study, we investigated the impact of chronic exposure to a mixture containing ibuprofen, 2hydroxy-ibuprofen, diclofenac, and EE2 at two environmentally relevant doses (added to the drinking water from fetal life until puberty) on the reproductive tract in F1 exposed mice and their F2 offspring. In F1 animals, exposure delayed male puberty and accelerated female puberty. In post-pubertal F1 testes and ovaries, differentiation/maturation of the different gonad cell types was altered, and some of these modifications were observed also in the non-exposed F2 generation. Transcriptomic analysis of post-pubertal testes and ovaries of F1 (exposed) and F2 animals revealed significant changes in gene expression profiles and enriched pathways, particularly the inflammasome, metabolism and extracellular matrix pathways, compared with controls (non-exposed). This suggested that exposure to these drug cocktails has an intergenerational impact. The identified Adverse Outcome Pathway (AOP) networks for NSAIDs and EE2, at doses that are relevant to everyday human exposure, will improve the AOP network of the human reproductive system development concerning endocrine disruptor chemicals. It may serve to identify other putative endocrine disruptors for mammalian species based on the expression of biomarkers.

Polycomb function during oogenesis is required for mouse embryonic development.

In mammals, totipotent embryos are formed by fusion of highly differentiated gametes. Acquisition of totipotency concurs with chromatin remodeling of parental genomes, changes in the maternal transcriptome and proteome, and zygotic genome activation (ZGA). The inefficiency of reprogramming somatic nuclei in reproductive cloning suggests that intergenerational inheritance of germline chromatin contributes to developmental proficiency after natural conception. Here we show that Ring1 and Rnf2, components of Polycomb-repressive complex 1 (PRC1), serve redundant transcriptional functions during oogenesis that are essential for proper ZGA, replication and cell cycle progression in early embryos, and development beyond the two-cell stage. Exchange of chromosomes between control and Ring1/Rnf2-deficient metaphase II oocytes reveal cytoplasmic and chromosome-based contributions by PRC1 to embryonic development. Our results strongly support a model in which Polycomb acts in the female germline to establish developmental competence for the following generation by silencing differentiation-inducing genes and defining appropriate chromatin states.

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.

PR-SET7 and SUV4-20H regulate H4 lysine-20 methylation at imprinting control regions in the mouse.

Imprinted genes are important in development and their allelic expression is mediated by imprinting control regions (ICRs). On their DNA-methylated allele, ICRs are marked by trimethylation at H3 Lys 9 (H3K9me3) and H4 Lys 20 (H4K20me3), similar to pericentric heterochromatin. Here, we investigate which histone methyltransferases control this methylation of histone at ICRs. We found that inactivation of SUV4-20H leads to the loss of H4K20me3 and increased levels of its substrate, H4K20me1. H4K20me1 is controlled by PR-SET7 and is detected on both parental alleles. The disruption of SUV4-20H or PR-SET7 does not affect methylation of DNA at ICRs but influences precipitation of H3K9me3, which is suggestive of a trans-histone change. Unlike at pericentric heterochromatin, however, H3K9me3 at ICRs does not depend on SUV39H. Our data show not only new similarities but also differences between ICRs and heterochromatin, both of which show constitutive maintenance of methylation of DNA in somatic cells.

H3K9/HP1 and Polycomb: two key epigenetic silencing pathways for gene regulation and embryo development.

Proper development of an embryo requires tightly controlled expression of specific sets of genes. In order to generate all the lineages of the adult, populations of pluripotent embryonic stem cells differentiate and activate specific transcriptional programs whereas others are shutdown. The role of transcription factors is obvious in promoting expression of such developmental genes; however maintenance of specific states throughout cell division needs additional mechanisms. Indeed, the nucleoprotein complex of DNA and histones, the chromatin, can act as a facilitator or barrier to transcription depending on its configuration. Chromatin-modifying enzymes regulate accessibility of DNA by establishing specific sets of chromatin, which will be either permissive or repressive to transcription. In this review, we will describe the H3K9/HP1 and Polycomb pathways, which mediate transcriptional repression by modifying chromatin. We discuss how these two major epigenetic silencing modes are dynamically regulated and how they contribute to the early steps of embryo development.

Coupling mitosis to DNA replication: the emerging role of the histone H4-lysine 20 methyltransferase PR-Set7.

To ensure accurate inheritance of genetic information through cell proliferation, chromosomes must be precisely copied only during S phase, and then correctly condensed and segregated during mitosis. Several new findings suggest that this tight coupling between DNA replication and mitosis is in part controlled by cell cycle regulated chromatin modifications, in particular due to the changing activity of lysine methyltransferase PR-Set7/SET8 that is responsible for the monomethylation of histone H4 at lysine 20. Cell cycle oscillation of PR-Set7 is orchestrated by ubiquitin-mediated proteolysis, and interference with this regulatory process leads to unscheduled licensing of replication origins and altered timing of mitotic chromosome compaction. This review provides an overview of how PR-Set7 regulates these two cell cycle events and highlights questions that remain to be addressed.

The histone H4 Lys 20 methyltransferase PR-Set7 regulates replication origins in mammalian cells.

The initiation of DNA synthesis is governed by the licensing of replication origins, which consists of assembling a pre-replication complex (pre-RC) on origins during late M- and G1-phases. In metazoans, functional replication origins do not show defined DNA consensus sequences, thus evoking the involvement of chromatin determinants in the selection of these origins. Here, we show that the onset of licensing in mammalian cells coincides with an increase in histone H4 Lys 20 monomethylation (H4K20me1) at replication origins by the methyltransferase PR-Set7 (also known as Set8 or KMT5A). Indeed, tethering PR-Set7 methylase activity to a specific genomic locus promotes the loading of pre-RC proteins on chromatin. In addition, we demonstrate that PR-Set7 undergoes a PCNA- and Cul4-Ddb1-driven degradation during S phase that contributes to the disappearance of H4K20me1 at origins and the inhibition of replication licensing. Strikingly, expression of a PR-Set7 mutant insensitive to this degradation causes the maintenance of H4K20me1 and repeated DNA replication at origins. These results elucidate a critical role for PR-Set7 and H4K20me1 in the chromatin events that regulate replication origins.

PR-Set7-dependent lysine methylation ensures genome replication and stability through S phase.

PR-Set7/SET8 is a histone H4-lysine 20 methyltransferase required for normal cell proliferation. However, the exact functions of this enzyme remain to be determined. In this study, we show that human PR-Set7 functions during S phase to regulate cellular proliferation. PR-Set7 associates with replication foci and maintains the bulk of H4-K20 mono- and trimethylation. Consistent with a function in chromosome dynamics during S phase, inhibition of PR-Set7 methyltransferase activity by small hairpin RNA causes a replicative stress characterized by alterations in replication fork velocity and origin firing. This stress is accompanied by massive induction of DNA strand breaks followed by a robust DNA damage response. The DNA damage response includes the activation of ataxia telangiectasia mutated and ataxia telangiectasia related kinase-mediated pathways, which, in turn, leads to p53-mediated growth arrest to avoid aberrant chromosome behavior after improper DNA replication. Collectively, these data indicate that PR-Set7-dependent lysine methylation during S phase is an essential posttranslational mechanism that ensures genome replication and stability.