The 18S rRNA Methyltransferase DIMT-1 Regulates Lifespan in the Germline Later in Life.

Ribosome heterogeneity has emerged as an important regulatory control feature for determining which proteins are synthesized, however, the influence of age on ribosome heterogeneity is not fully understood. Whether mRNA transcripts are selectively translated in young versus old cells and whether dysregulation of this process drives organismal aging is unknown. Here we examined the role of ribosomal RNA (rRNA) methylation in maintaining appropriate translation as organisms age. In a directed RNAi screen, we identified the 18S rRNA N6'-dimethyl adenosine (m6,2A) methyltransferase, dimt-1, as a regulator of C. elegans lifespan and stress resistance. Lifespan extension induced by dimt-1 deficiency required a functional germline and was dependent on the known regulator of protein translation, the Rag GTPase, raga-1, which links amino acid sensing to the mechanistic target of rapamycin complex (mTORC)1. Using an auxin-inducible degron tagged version of dimt-1, we demonstrate that DIMT-1 functions in the germline after mid-life to regulate lifespan. We further found that knock-down of dimt-1 leads to selective translation of transcripts important for stress resistance and lifespan regulation in the C. elegans germline in mid-life including the cytochrome P450 daf-9, which synthesizes a steroid that signals from the germline to the soma to regulate lifespan. We found that dimt-1 induced lifespan extension was dependent on the daf-9 signaling pathway. This finding reveals a new layer of proteome dysfunction, beyond protein synthesis and degradation, as an important regulator of aging. Our findings highlight a new role for ribosome heterogeneity, and specific rRNA modifications, in maintaining appropriate translation later in life to promote healthy aging.

18S rRNA methyltransferases DIMT1 and BUD23 drive intergenerational hormesis.

Heritable non-genetic information can regulate a variety of complex phenotypes. However, what specific non-genetic cues are transmitted from parents to their descendants are poorly understood. Here, we perform metabolic methyl-labeling experiments to track the heritable transmission of methylation from ancestors to their descendants in the nematode Caenorhabditis elegans (C. elegans). We find heritable methylation in DNA, RNA, proteins, and lipids. We find that parental starvation elicits reduced fertility, increased heat stress resistance, and extended longevity in fed, naïve progeny. This intergenerational hormesis is accompanied by a heritable increase in N6'-dimethyl adenosine (m6,2A) on the 18S ribosomal RNA at adenosines 1735 and 1736. We identified DIMT-1/DIMT1 as the m6,2A and BUD-23/BUD23 as the m7G methyltransferases in C. elegans that are both required for intergenerational hormesis, while other rRNA methyltransferases are dispensable. This study labels and tracks heritable non-genetic material across generations and demonstrates the importance of rRNA methylation for regulating epigenetic inheritance.

NOP16 is a histone mimetic that regulates Histone H3K27 methylation and gene repression.

Post-translational modifications of histone tails alter chromatin accessibility to regulate gene expression. Some viruses exploit the importance of histone modifications by expressing histone mimetic proteins that contain histone-like sequences to sequester complexes that recognize modified histones. Here we identify an evolutionarily conserved and ubiquitously expressed, endogenous mammalian protein Nucleolar protein 16 (NOP16) that functions as a H3K27 mimic. NOP16 binds to EED in the H3K27 trimethylation PRC2 complex and to the H3K27 demethylase JMJD3. NOP16 knockout selectively globally increases H3K27me3, a heterochromatin mark, without altering methylation of H3K4, H3K9, or H3K36 or acetylation of H3K27. NOP16 is overexpressed and linked to poor prognosis in breast cancer. Depletion of NOP16 in breast cancer cell lines causes cell cycle arrest, decreases cell proliferation and selectively decreases expression of E2F target genes and of genes involved in cell cycle, growth and apoptosis. Conversely, ectopic NOP16 expression in triple negative breast cancer cell lines increases cell proliferation, cell migration and invasivity in vitro and tumor growth in vivo , while NOP16 knockout or knockdown has the opposite effect. Thus, NOP16 is a histone mimic that competes with Histone H3 for H3K27 methylation and demethylation. When it is overexpressed in cancer, it derepresses genes that promote cell cycle progression to augment breast cancer growth.

Prometheus unshackled: Liver regeneration makes you young.

In this issue of Molecular Cell, Yang and colleagues1 discover age-dependent increases in broad regions of the repressive histone modification H3K27me3. They also demonstrate partial reversion to younger H3K27me3 patterns and gene expression upon resection of older livers.

Biological roles of adenine methylation in RNA.

N6-Methyladenosine (m6A) is one of the most abundant modifications of the epitranscriptome and is found in cellular RNAs across all kingdoms of life. Advances in detection and mapping methods have improved our understanding of the effects of m6A on mRNA fate and ribosomal RNA function, and have uncovered novel functional roles in virtually every species of RNA. In this Review, we explore the latest studies revealing roles for m6A-modified RNAs in chromatin architecture, transcriptional regulation and genome stability. We also summarize m6A functions in biological processes such as stem-cell renewal and differentiation, brain function, immunity and cancer progression.

From correlation to causation: The new frontier of transgenerational epigenetic inheritance.

While heredity is predominantly controlled by what deoxyribonucleic acid (DNA) sequences are passed from parents to their offspring, a small but growing number of traits have been shown to be regulated in part by the non-genetic inheritance of information. Transgenerational epigenetic inheritance is defined as heritable information passed from parents to their offspring without changing the DNA sequence. Work of the past seven decades has transitioned what was previously viewed as rare phenomenology, into well-established paradigms by which numerous traits can be modulated. For the most part, studies in model organisms have correlated transgenerational epigenetic inheritance phenotypes with changes in epigenetic modifications. The next steps for this field will entail transitioning from correlative studies to causal ones. Here, we delineate the major molecules that have been implicated in transgenerational epigenetic inheritance in both mammalian and non-mammalian models, speculate on additional molecules that could be involved, and highlight some of the tools which might help transition this field from correlation to causation.

Hypoxia induces transgenerational epigenetic inheritance of small RNAs.

Animals sense and adapt to decreased oxygen availability, but whether and how hypoxia exposure in ancestors can elicit phenotypic consequences in normoxia-reared descendants are unclear. We show that hypoxia educes an intergenerational reduction in lipids and a transgenerational reduction in fertility in the nematode Caenorhabditis elegans. The transmission of these epigenetic phenotypes is dependent on repressive histone-modifying enzymes and the argonaute HRDE-1. Feeding naive C. elegans small RNAs extracted from hypoxia-treated worms is sufficient to induce a fertility defect. Furthermore, the endogenous small interfering RNA F44E5.4/5 is upregulated intergenerationally in response to hypoxia, and soaking naive normoxia-reared C. elegans with F44E5.4/5 double-stranded RNA (dsRNA) is sufficient to induce an intergenerational fertility defect. Finally, we demonstrate that labeled F44E5.4/5 dsRNA is itself transmitted from parents to children. Our results suggest that small RNAs respond to the environment and are sufficient to transmit non-genetic information from parents to their naive children.

N6-methyladenine: A Rare and Dynamic DNA Mark.

Chromatin, consisting of deoxyribonucleic acid (DNA) wrapped around histone proteins, facilitates DNA compaction and allows identical DNA code to confer many different cellular phenotypes. This biological versatility is accomplished in large part by post-translational modifications to histones and chemical modifications to DNA. These modifications direct the cellular machinery to expand or compact specific chromatin regions and mark certain regions of the DNA as important for cellular functions. While each of the four bases that make up DNA can be modified (Iyer et al., Prog Mol Biol Transl Sci. 101:25-104, 2011), this chapter will focus on methylation of the 6th position on adenines (6mA). 6mA is a prevalent modification in unicellular organisms and until recently was thought to be restricted to them. A flurry of conflicting studies have proposed that 6mA either does not exist, is present at low levels, or is present at relatively high levels and regulates complex processes in different multicellular eukaryotes. Here, we will briefly describe the history of 6mA, examine its evolutionary conservation, and evaluate the current methods for detecting 6mA. We will discuss the proteins that have been reported to bind and regulate 6mA and examine the known and potential functions of this modification in eukaryotes. Finally, we will close with a discussion of the ongoing debate about whether 6mA exists as a directed DNA modification in multicellular eukaryotes.

The promises and pitfalls of specialized ribosomes.

The concept of specialized ribosomes has garnered equal amounts of interest and skepticism since it was first introduced. We ask researchers in the field to provide their perspective on the topic and weigh in on the evidence (or lack thereof) and what the future may bring.

Means, mechanisms and consequences of adenine methylation in DNA.

N6-methyl-2'-deoxyadenosine (6mA or m6dA) has been reported in the DNA of prokaryotes and eukaryotes ranging from unicellular protozoa and algae to multicellular plants and mammals. It has been proposed to modulate DNA structure and transcription, transmit information across generations and have a role in disease, among other functions. However, its existence in more recently evolved eukaryotes remains a topic of debate. Recent technological advancements have facilitated the identification and quantification of 6mA even when the modification is exceptionally rare, but each approach has limitations. Critical assessment of existing data, rigorous design of future studies and further development of methods will be required to confirm the presence and biological functions of 6mA in multicellular eukaryotes.

The adenine methylation debate.

[Figure: see text].

Methylation of viral mRNA cap structures by PCIF1 attenuates the antiviral activity of interferon-ß.

Interferons induce cell-intrinsic responses associated with resistance to viral infection. To overcome the suppressive action of interferons and their effectors, viruses have evolved diverse mechanisms. Using vesicular stomatitis virus (VSV), we report that the host cell N6-adenosine messenger RNA (mRNA) cap methylase, phosphorylated C-terminal domain interacting factor 1 (PCIF1), attenuates the antiviral response. We employed cell-based and in vitro biochemical assays to demonstrate that PCIF1 efficiently modifies VSV mRNA cap structures to m7Gpppm6Am and define the substrate requirements for this modification. Functional assays revealed that the PCIF1-dependent modification of VSV mRNA cap structures is inert with regard to mRNA stability, translation, and viral infectivity but attenuates the antiviral effects of the treatment of cells with interferon-ß. Cells lacking PCIF1 or expressing a catalytically inactive PCIF1 exhibit an augmented inhibition of viral replication and gene expression following interferon-ß treatment. We further demonstrate that the mRNA cap structures of rabies and measles viruses are also modified by PCIF1 to m7Gpppm6Am This work identifies a function of PCIF1 and cap-proximal m6Am in attenuation of the host response to VSV infection that likely extends to other viruses.

Role of epigenetics in unicellular to multicellular transition in Dictyostelium.

BACKGROUND: The evolution of multicellularity is a critical event that remains incompletely understood. We use the social amoeba, Dictyostelium discoideum, one of the rare organisms that readily transits back and forth between both unicellular and multicellular stages, to examine the role of epigenetics in regulating multicellularity. RESULTS: While transitioning to multicellular states, patterns of H3K4 methylation and H3K27 acetylation significantly change. By combining transcriptomics, epigenomics, chromatin accessibility, and orthologous gene analyses with other unicellular and multicellular organisms, we identify 52 conserved genes, which are specifically accessible and expressed during multicellular states. We validated that four of these genes, including the H3K27 deacetylase hdaD, are necessary and that an SMC-like gene, smcl1, is sufficient for multicellularity in Dictyostelium. CONCLUSIONS: These results highlight the importance of epigenetics in reorganizing chromatin architecture to facilitate multicellularity in Dictyostelium discoideum and raise exciting possibilities about the role of epigenetics in the evolution of multicellularity more broadly.

Detection of DNA Methylation in Genomic DNA by UHPLC-MS/MS.

DNA methylation serves to mark DNA as either a directed epigenetic signaling modification or in response to DNA lesions. Methods for detecting DNA methylation have become increasingly more specific and sensitive over time. Conventional methods for detecting DNA methylation, ranging from paper chromatography to differential restriction enzyme digestion preference to dot blots, have more recently been supplemented by ultrahigh performance liquid chromatography coupled with mass spectrometry (UHPLC-MS/MS) to accurately quantify specific DNA methylation. Methylated DNA can also be sequenced by either methylated DNA immunoprecipitation followed by sequencing (MeDIP-seq) or single-molecule real-time sequencing (SMRTseq) for identifying genomic locations of DNA methylation. Here we describe a protocol for the detection and quantification of epigenetic signaling DNA methylation modifications including, N6-methyladenine (6mA), N4-methylcytosine (4mC) and C5-methylcytosine (5mC) in genomic DNA by triple quadrupole liquid chromatography coupled with tandem mass spectrometry (QQQ-LC-MS/MS). The high sensitivity of the UHPLC-MS/MS methodology and the use of calibration standards of pure nucleosides allow for the accurate quantification of DNA methylation.

N6-adenosine methylation of ribosomal RNA affects lipid oxidation and stress resistance.

During stress, global translation is reduced, but specific transcripts are actively translated. How stress-responsive mRNAs are selectively translated is unknown. We show that METL-5 methylates adenosine 1717 on 18S ribosomal RNA in C. elegans, enhancing selective ribosomal binding and translation of specific mRNAs. One of these mRNAs, CYP-29A3, oxidizes the omega-3 polyunsaturated fatty acid eicosapentaenoic acid to eicosanoids, key stress signaling molecules. While metl-5-deficient animals grow normally under homeostatic conditions, they are resistant to a variety of stresses. metl-5 mutant worms also show reduced bioactive lipid eicosanoids and dietary supplementation of eicosanoid products of CYP-29A3 restores stress sensitivity of metl-5 mutant worms. Thus, methylation of a specific residue of 18S rRNA by METL-5 selectively enhances translation of cyp-29A3 to increase production of eicosanoids, and blocking this pathway increases stress resistance. This study suggests that ribosome methylation can facilitate selective translation, providing another layer of regulation of the stress response.

Adenine DNA methylation, 3D genome organization, and gene expression in the parasite Trichomonas vaginalis.

Trichomonas vaginalis is a common sexually transmitted parasite that colonizes the human urogenital tract causing infections that range from asymptomatic to highly inflammatory. Recent works have highlighted the importance of histone modifications in the regulation of transcription and parasite pathogenesis. However, the nature of DNA methylation in the parasite remains unexplored. Using a combination of immunological techniques and ultrahigh-performance liquid chromatography (UHPLC), we analyzed the abundance of DNA methylation in strains with differential pathogenicity demonstrating that N6-methyladenine (6mA), and not 5-methylcytosine (5mC), is the main DNA methylation mark in T. vaginalis Genome-wide distribution of 6mA reveals that this mark is enriched at intergenic regions, with a preference for certain superfamilies of DNA transposable elements. We show that 6mA in T. vaginalis is associated with silencing when present on genes. Interestingly, bioinformatics analysis revealed the presence of transcriptionally active or repressive intervals flanked by 6mA-enriched regions, and results from chromatin conformation capture (3C) experiments suggest these 6mA flanked regions are in close spatial proximity. These associations were disrupted when parasites were treated with the demethylation activator ascorbic acid. This finding revealed a role for 6mA in modulating three-dimensional (3D) chromatin structure and gene expression in this divergent member of the Excavata.

Transgenerational epigenetic inheritance: from phenomena to molecular mechanisms.

Inherited information not encoded in the DNA sequence can regulate a variety of complex phenotypes. However, how this epigenetic information escapes the typical epigenetic erasure that occurs upon fertilization and how it regulates behavior is still unclear. Here we review recent examples of brain related transgenerational epigenetic inheritance and delineate potential molecular mechanisms that could regulate how non-genetic information could be transmitted.

The demethylase NMAD-1 regulates DNA replication and repair in the Caenorhabditis elegans germline.

The biological roles of nucleic acid methylation, other than at the C5-position of cytosines in CpG dinucleotides, are still not well understood. Here, we report genetic evidence for a critical role for the putative DNA demethylase NMAD-1 in regulating meiosis in C. elegans. nmad-1 mutants have reduced fertility. They show defects in prophase I of meiosis, which leads to reduced embryo production and an increased incidence of males due to defective chromosomal segregation. In nmad-1 mutant worms, nuclear staging beginning at the leptotene and zygotene stages is disorganized, the cohesin complex is mislocalized at the diplotene and diakinesis stages, and chromosomes are improperly condensed, fused, or lost by the end of diakinesis. RNA sequencing of the nmad-1 germline revealed reduced induction of DNA replication and DNA damage response genes during meiosis, which was coupled with delayed DNA replication, impaired DNA repair and increased apoptosis of maturing oocytes. To begin to understand how NMAD-1 regulates DNA replication and repair, we used immunoprecipitation and mass spectrometry to identify NMAD-1 binding proteins. NMAD-1 binds to multiple proteins that regulate DNA repair and replication, including topoisomerase TOP-2 and co-localizes with TOP-2 on chromatin. Moreover, the majority of TOP-2 binding to chromatin depends on NMAD-1. These results suggest that NMAD-1 functions at DNA replication sites to regulate DNA replication and repair during meiosis.