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Post translational modifications (PTMs) are covalent modifications of proteins that can range from small chemical modifications to addition of entire proteins. PTMs contribute to regulation of protein function and thereby greatly increase the functional diversity of the proteome. In the heart, a few well-studied PTMs, such as phosphorylation and glycosylation, are known to play essential roles for cardiac function. Yet, only a fraction of the ~ 300 known PTMs have been studied in a cardiac context. Here we investigated the proteome-wide map of PTMs present in human hearts by utilizing high-resolution mass spectrometry measurements and a suite of PTM identification algorithms. Our approach led to identification of more than 150 different PTMs across three of the chambers in human hearts. This finding underscores that decoration of cardiac proteins by PTMs is much more diverse than hitherto appreciated and provides insights in cardiac protein PTMs not yet studied. The results presented serve as a catalogue of which PTMs are present in human hearts and outlines the particular protein and the specific amino acid modified, and thereby provides a detail-rich resource for exploring protein modifications in human hearts beyond the most studied PTMs.

Key Points Although long non-coding RNAs (lncRNAs) and mRNAs share many common features, several types of lncRNAs are distinguished from mRNAs by unique features of biogenesis, form and function. lncRNAs exhibit more highly specific expression patterns than mRNAs. Many lncRNAs undergo special processing events, such as backspliced circularization, 5′- and 3′-bookending by processed small nucleolar RNAs (snoRNAs), and cleavage by RNase P. lncRNAs are more enriched in the nucleus than the cytoplasm relative to mRNAs, and although cytoplasmic lncRNAs associate with the ribosome, few are productively translated. Certain classes of lncRNAs are preferentially subject to degradation by nonsense-mediated decay and the nuclear exosome, and the elongation of divergent ncRNA transcripts is co-transcriptionally terminated by premature polyadenylation. lncRNAs are uniquely capable of cis action on the genome and chromatin. This feature of lncRNAs enables such biological phenomena as gene imprinting, dosage compensation of sex chromosomes, transcriptional enhancement, chromosome looping and antisense regulation. Long non-coding RNAs (lncRNAs) are a class of RNAs with great molecular and regulatory diversity. This Review discusses how, beyond their lack of protein-coding potential, some types of lncRNAs are known to exhibit features that are distinct from mRNAs, including their transcriptional regulation, localization, processing, biological capabilities and degradation. Such properties underlie many of the key cellular functions of lncRNAs. Long non-coding RNAs (lncRNAs) are a diverse class of RNAs that engage in numerous biological processes across every branch of life. Although initially discovered as mRNA-like transcripts that do not encode proteins, recent studies have revealed features of lncRNAs that further distinguish them from mRNAs. In this Review, we describe special events in the lifetimes of lncRNAs — before, during and after transcription — and discuss how these events ultimately shape the unique characteristics and functional roles of lncRNAs.

RNA methylation to form N6-methyladenosine (m6A) in mRNA accounts for the most abundant mRNA internal modification and has emerged as a widespread regulatory mechanism that controls gene expression in diverse physiological processes. Transcriptome-wide m6A mapping has revealed the distribution and pattern of m6A in cellular RNAs, referred to as the epitranscriptome. These maps have revealed the specific mRNAs that are regulated by m6A, providing mechanistic links connecting m6A to cellular differentiation, cancer progression and other processes. The effects of m6A on mRNA are mediated by an expanding list of m6A readers and m6A writer-complex components, as well as potential erasers that currently have unclear relevance to m6A prevalence in the transcriptome. Here we review new and emerging methods to characterize and quantify the epitranscriptome, and we discuss new concepts — in some cases, controversies — regarding our understanding of the mechanisms and functions of m6A readers, writers and erasers.

Together with core histones, which make up the nucleosome, the linker histone (H1) is one of the five main histone protein families present in chromatin in eukaryotic cells. H1 binds to the nucleosome to form the next structural unit of metazoan chromatin, the chromatosome, which may help chromatin to fold into higher-order structures. Despite their important roles in regulating the structure and function of chromatin, linker histones have not been studied as extensively as core histones. Nevertheless, substantial progress has been made recently. The first near-atomic resolution crystal structure of a chromatosome core particle and an 11 Å resolution cryo-electron microscopy-derived structure of the 30 nm nucleosome array have been determined, revealing unprecedented details about how linker histones interact with the nucleosome and organize higher-order chromatin structures. Moreover, several new functions of linker histones have been discovered, including their roles in epigenetic regulation and the regulation of DNA replication, DNA repair and genome stability. Studies of the molecular mechanisms of H1 action in these processes suggest a new paradigm for linker histone function beyond its architectural roles in chromatin.

Over the past few decades, epigenetics has evolved from a collection of curious biological phenomena to a functionally dissected research field. In this article, the authors provide a personal perspective on the advances of research into epigenetics — from its historical origins to its modern era — with a focus on molecular breakthroughs. Over the past 20 years, breakthrough discoveries of chromatin-modifying enzymes and associated mechanisms that alter chromatin in response to physiological or pathological signals have transformed our knowledge of epigenetics from a collection of curious biological phenomena to a functionally dissected research field. Here, we provide a personal perspective on the development of epigenetics, from its historical origins to what we define as 'the modern era of epigenetic research'. We primarily highlight key molecular mechanisms of and conceptual advances in epigenetic control that have changed our understanding of normal and perturbed development.

Sophisticated gene-regulatory mechanisms probably evolved in prokaryotes billions of years before the emergence of modern eukaryotes, which inherited the same basic enzymatic machineries. However, the epigenomic landscapes of eukaryotes are dominated by nucleosomes, which have acquired roles in genome packaging, mitotic condensation and silencing parasitic genomic elements. Although the molecular mechanisms by which nucleosomes are displaced and modified have been described, just how transcription factors, histone variants and modifications and chromatin regulators act on nucleosomes to regulate transcription is the subject of considerable ongoing study. We explore the extent to which these transcriptional regulatory components function in the context of the evolutionarily ancient role of chromatin as a barrier to processes acting on DNA and how chromatin proteins have diversified to carry out evolutionarily recent functions that accompanied the emergence of differentiation and development in multicellular eukaryotes.Eukaryotes differ substantially from bacteria and archaea owing to their nucleosome-based packaging of DNA. In this Review, Talbert, Meers and Henikoff place gene regulation in an evolutionary context by discussing how the emergence and diversification of eukaryotic chromatin provided both challenges and opportunities for intricate mechanisms of gene regulation in eukaryotes.

Precise control of gene expression is fundamental to cell function and development. Although ultimately gene expression relies on DNA-binding transcription factors to guide the activity of the transcription machinery to genes, it has also become clear that chromatin and histone post-translational modification have fundamental roles in gene regulation. Polycomb repressive complexes represent a paradigm of chromatin-based gene regulation in animals. The Polycomb repressive system comprises two central protein complexes, Polycomb repressive complex 1 (PRC1) and PRC2, which are essential for normal gene regulation and development. Our early understanding of Polycomb function relied on studies in simple model organisms, but more recently it has become apparent that this system has expanded and diverged in mammals. Detailed studies are now uncovering the molecular mechanisms that enable mammalian PRC1 and PRC2 to identify their target sites in the genome, communicate through feedback mechanisms to create Polycomb chromatin domains and control transcription to regulate gene expression. In this Review, we discuss and contextualize the emerging principles that define how this fascinating chromatin-based system regulates gene expression in mammals.Polycomb repressive complex 1 (PRC1) and PRC2 are important gene regulators in various physiological contexts, especially in development. Recent studies have uncovered the molecular mechanisms that enable mammalian PRC1 and PRC2 to identify their genomic target sites, modify chromatin properties and control transcription.

Key Points Nucleosomes are highly dynamic nucleoprotein complexes involved in almost every genomic process across all eukaryotic organisms. Both individual histone and full nucleosome turnover is under tight regulation that is mediated by a diverse set of chaperones and remodellers. Subnucleosomal structures are dynamically generated as intermediaries during transcription and replication and are predicted to exist throughout the genome at specific loci. The precise positioning of a nucleosome is a highly regulated process owing to numerous chaperones and remodeller complexes. Perturbations of nucleosome positioning have been linked to changes in gene expression levels. Nucleosome disassembly and inheritance is carefully regulated during DNA replication with the restoration of the pre-replication positioning linked to transcription. The presence of nucleosomes and their substructures affects local chromatin structure and function. Thus, nucleosome occupancy, their exact positioning and composition need to be dynamically regulated. Advances in genomic technologies have improved our understanding of nucleosome dynamics in various cellular processes, most notably DNA replication and transcription. Advances in genomics technology have provided the means to probe myriad chromatin interactions at unprecedented spatial and temporal resolution. This has led to a profound understanding of nucleosome organization within the genome, revealing that nucleosomes are highly dynamic. Nucleosome dynamics are governed by a complex interplay of histone composition, histone post-translational modifications, nucleosome occupancy and positioning within chromatin, which are influenced by numerous regulatory factors, including general regulatory factors, chromatin remodellers, chaperones and polymerases. It is now known that these dynamics regulate diverse cellular processes ranging from gene transcription to DNA replication and repair.

Key Points Chromatin integrity and functionality is governed by the controlled assembly and disassembly of nucleosomes. An elaborate histone chaperone network governs histone provision, chromatin assembly, histone recycling and histone turnover. Histone chaperone networks operate through histone-dependent co-chaperone interactions and direct chaperone–chaperone contacts. The mode of action of histone chaperones is interpreted from structural and biochemical studies of histone–chaperone complexes. Key molecular functions of histone chaperones include the shielding of functional histone interfaces and trapping histones in non-nucleosomal conformations. The integration of histone chaperone function across DNA metabolic processes acts to maintain genome and epigenome integrity. Histone chaperones safeguard the chromatin template and shield histones from promiscuous interactions to ensure their proper storage, transport, post-translational modification, nucleosome assembly and turnover. The association of histones with specific chaperone complexes is important for their folding, oligomerization, post-translational modification, nuclear import, stability, assembly and genomic localization. In this way, the chaperoning of soluble histones is a key determinant of histone availability and fate, which affects all chromosomal processes, including gene expression, chromosome segregation and genome replication and repair. Here, we review the distinct structural and functional properties of the expanding network of histone chaperones. We emphasize how chaperones cooperate in the histone chaperone network and via co-chaperone complexes to match histone supply with demand, thereby promoting proper nucleosome assembly and maintaining epigenetic information by recycling modified histones evicted from chromatin.

The idea that epigenetic determinants such as DNA methylation, histone modifications or RNA can be passed to the next generation through meiotic products (gametes) is long standing. Such meiotic epigenetic inheritance (MEI) is fairly common in yeast, plants and nematodes, but its extent in mammals has been much debated. Advances in genomics techniques are now driving the profiling of germline and zygotic epigenomes, thereby improving our understanding of MEI in diverse species. Whereas the role of DNA methylation in MEI remains unclear, insights from genome-wide studies suggest that a previously underappreciated fraction of mammalian genomes bypass epigenetic reprogramming during development. Notably, intergenerational inheritance of histone modifications, tRNA fragments and microRNAs can affect gene regulation in the offspring. It is important to note that MEI in mammals rarely constitutes transgenerational epigenetic inheritance (TEI), which spans multiple generations. In this Review, we discuss the examples of MEI in mammals, including mammalian epigenome reprogramming, and the molecular mechanisms of MEI in vertebrates in general. We also discuss the implications of the inheritance of histone modifications and small RNA for embryogenesis in metazoans, with a particular focus on insights gained from genome-wide studies.