The proteomic investigation of chromatin functional domains reveals novel synergisms among distinct heterochromatin components

The Proteomic Composition and Organization of Constitutive Heterochromatin in Mouse Tissues

Pericentric heterochromatin (PCH) forms spatio-temporarily distinct compartments and affects chromosome organization and stability. Albeit some of its components are known, an elucidation of its proteome and how it differs between tissues in vivo is lacking. Here, we find that PCH compartments are dynamically organized in a tissue-specific manner, possibly reflecting compositional differences. As the mouse brain and liver exhibit very different PCH architecture, we isolated native PCH fractions from these tissues, analyzed their protein compositions using quantitative mass spectrometry, and compared them to identify common and tissue-specific PCH proteins. In addition to heterochromatin-enriched proteins, the PCH proteome includes RNA/transcription and membrane-related proteins, which showed lower abundance than PCH-enriched proteins. Thus, we applied a cut-off of PCH-unspecific candidates based on their abundance and validated PCH-enriched proteins. Amongst the hits, MeCP2 was classified into brain PCH-enriched proteins, while linker histone H1 was not. We found that H1 and MeCP2 compete to bind to PCH and regulate PCH organization in opposite ways. Altogether, our workflow of unbiased PCH isolation, quantitative mass spectrometry, and validation-based analysis allowed the identification of proteins that are common and tissue-specifically enriched at PCH. Further investigation of selected hits revealed their opposing role in heterochromatin higher-order architecture in vivo.

Chemical Biology Approaches to Identify and Profile Interactors of Chromatin Modifications

In eukaryotes, DNA is packaged with histone proteins in a complex known as chromatin. Both the DNA and histone components of chromatin can be chemically modified in a wide variety of ways, resulting in a complex landscape often referred to as the "epigenetic code". These modifications are recognized by effector proteins that remodel chromatin and modulate transcription, translation, and repair of the underlying DNA. In this Review, we examine the development of methods for characterizing proteins that interact with these histone and DNA modifications. "Mark first" approaches utilize chemical, peptide, nucleosome, or oligonucleotide probes to discover interactors of a specific modification. "Reader first" approaches employ arrays of peptides, nucleosomes, or oligonucleotides to profile the binding preferences of interactors. These complementary strategies have greatly enhanced our understanding of how chromatin modifications effect changes in genomic regulation, bringing us ever closer to deciphering this complex language.

Multi-layered chromatin proteomics identifies cell vulnerabilities in DNA repair

The DNA damage response (DDR) is essential to maintain genome stability, and its deregulation predisposes to carcinogenesis while encompassing attractive targets for cancer therapy. Chromatin governs the DDR via the concerted interplay among different layers, including DNA, histone post-translational modifications (hPTMs) and chromatin-associated proteins. Here, we employ multi-layered proteomics to characterize chromatin-mediated functional interactions of repair proteins, signatures of hPTMs and the DNA-bound proteome during DNA double-strand break (DSB) repair at high temporal resolution. Our data illuminate the dynamics of known and novel DDR-associated factors both at chromatin and at DSBs. We functionally attribute novel chromatin-associated proteins to repair by non-homologous end-joining (NHEJ), homologous recombination (HR) and DSB repair pathway choice. We reveal histone reader ATAD2, microtubule organizer TPX2 and histone methyltransferase G9A as regulators of HR and involved in poly-ADP-ribose polymerase-inhibitor sensitivity. Furthermore, we distinguish hPTMs that are globally induced by DNA damage from those specifically acquired at sites flanking DSBs (γH2AX foci-specific) and profiled their dynamics during the DDR. Integration of complementary chromatin layers implicates G9A-mediated monomethylation of H3K56 in DSBs repair via HR. Our data provide a dynamic chromatin-centered view of the DDR that can be further mined to identify novel mechanistic links and cell vulnerabilities in DSB repair.

Cracking chromatin with proteomics: From chromatome to histone modifications

Chromatin is the assembly of genomic DNA and proteins packaged in the nucleus of eukaryotic cells, which together are crucial in regulating a plethora of cellular processes. Histones may be the best known class of protein constituents in chromatin, which are decorated by a range of post-translational modifications to recruit accessory proteins and protein complexes to execute specific functions, ranging from DNA compaction, repair, transcription and duplication, all in a dynamic fashion and depending on the cellular state. The key role of chromatin in cellular fitness is emphasized by the deregulation of chromatin determinants predisposing to different diseases, including cancer. For this reason, deep investigation of chromatin composition is fundamental to better understand cellular physiology. Proteomic approaches have played a crucial role to understand critical aspects of this complex interplay, benefiting from the ability to identify and quantify proteins and their modifications in an unbiased manner. This review gives an overview of the proteomic approaches that have been developed by combining mass spectrometry-based with tailored biochemical and genetic methods to examine overall protein make-up of chromatin, to characterize chromatin domains, to determine protein interactions, and to decipher the broad spectrum of histone modifications that represent the quintessence of chromatin function. This article is protected by copyright. All rights reserved.

Interplay between chromatin marks in development and disease

DNA methylation (DNAme) and histone post-translational modifications (PTMs) have important roles in transcriptional regulation. Although many reports have characterized the functions of such chromatin marks in isolation, recent genome-wide studies reveal surprisingly complex interactions between them. Here, we focus on the interplay between DNAme and methylation of specific lysine residues on the histone H3 tail. We describe the impact of genetic perturbation of the relevant methyltransferases in the mouse on the landscape of chromatin marks as well as the transcriptome. In addition, we discuss the specific neurodevelopmental growth syndromes and cancers resulting from pathogenic mutations in the human orthologues of these genes. Integrating these observations underscores the fundamental importance of crosstalk between DNA and histone H3 methylation in development and disease.

BAZ1B the Protean Protein

The bromodomain adjacent to the zinc finger domain 1B (BAZ1B) or Williams syndrome transcription factor (WSTF) are just two of the names referring the same protein that is encoded by the WBSCR9 gene and is among the 26-28 genes that are lost from one copy of 7q11.23 in Williams syndrome (WS: OMIM 194050). Patients afflicted by this contiguous gene deletion disorder present with a range of symptoms including cardiovascular complications, developmental defects as well as a characteristic cognitive and behavioral profile. Studies in patients with atypical deletions and mouse models support BAZ1B hemizygosity as a contributing factor to some of the phenotypes. Focused analysis on BAZ1B has revealed this to be a versatile nuclear protein with a central role in chromatin remodeling through two distinct complexes as well as being involved in the replication and repair of DNA, transcriptional processes involving RNA Polymerases I, II, and III as well as possessing kinase activity. Here, we provide a comprehensive review to summarize the many aspects of BAZ1B function including its recent link to cancer.

Binding patterns of RNA-binding proteins to repeat-derived RNA sequences reveal putative functional RNA elements

Recent reports have revealed that repeat-derived sequences embedded in introns or long noncoding RNAs (lncRNAs) are targets of RNA-binding proteins (RBPs) and contribute to biological processes such as RNA splicing or transcriptional regulation. These findings suggest that repeat-derived RNAs are important as scaffolds of RBPs and functional elements. However, the overall functional sequences of the repeat-derived RNAs are not fully understood. Here, we show the putative functional repeat-derived RNAs by analyzing the binding patterns of RBPs based on ENCODE eCLIP data. We mapped all eCLIP reads to repeat sequences and observed that 10.75 % and 7.04 % of reads on average were enriched (at least 2-fold over control) in the repeats in K562 and HepG2 cells, respectively. Using these data, we predicted functional RNA elements on the sense and antisense strands of long interspersed element 1 (LINE1) sequences. Furthermore, we found several new sets of RBPs on fragments derived from other transposable element (TE) families. Some of these fragments show specific and stable secondary structures and are found to be inserted into the introns of genes or lncRNAs. These results suggest that the repeat-derived RNA sequences are strong candidates for the functional RNA elements of endogenous noncoding RNAs.

Protein-DNA/RNA Interactions: An Overview of Investigation Methods in the -Omics Era

The fields of application of functional proteomics are not limited to the study of protein–protein interactions; they also extend to those involving protein complexes that bind DNA or RNA. These interactions affect fundamental processes such as replication, transcription, and repair in the case of DNA, as well as transport, translation, splicing, and silencing in the case of RNA. Analytical or preparative experimental approaches, both in vivo and in vitro, have been developed to isolate and identify DNA/RNA binding proteins by exploiting the advantage of the affinity shown by these proteins toward a specific oligonucleotide sequence. The present review proposes an overview of the approaches most commonly employed in proteomics applications for the identification of nucleic acid-binding proteins, such as affinity purification (AP) protocols, EMSA, chromatin purification methods, and CRISPR-based chromatin affinity purification, which are generally associated with mass spectrometry methodologies for the unbiased protein identification.

RNA Epigenetics: Fine-Tuning Chromatin Plasticity and Transcriptional Regulation, and the Implications in Human Diseases

Chromatin structure plays an essential role in eukaryotic gene expression and cell identity. Traditionally, DNA and histone modifications have been the focus of chromatin regulation; however, recent molecular and imaging studies have revealed an intimate connection between RNA epigenetics and chromatin structure. Accumulating evidence suggests that RNA serves as the interplay between chromatin and the transcription and splicing machineries within the cell. Additionally, epigenetic modifications of nascent RNAs fine-tune these interactions to regulate gene expression at the co- and post-transcriptional levels in normal cell development and human diseases. This review will provide an overview of recent advances in the emerging field of RNA epigenetics, specifically the role of RNA modifications and RNA modifying proteins in chromatin remodeling, transcription activation and RNA processing, as well as translational implications in human diseases.

Native Chromatin Proteomics (N-ChroP) to Characterize Histone Post-translational Modification (PTM) Combinatorics at Distinct Genomic Regions

In this chapter, we describe the proteomic approach named "Native Chromatin Proteomics" (N-ChroP) that couples a modified Chromatin ImmunoPrecipitation (ChIP) protocol with the mass spectrometry (MS) analysis of immunoprecipitated proteins to study the combinatorial enrichment or exclusion of histone post-translational modifications (PTMs) at specific genomic regions, such as promoters or enhancers. We describe the protocol steps from the digestion of chromatin and nucleosome immunoprecipitation to histone digestion and peptide enrichment prior to MS analysis, up to the MS raw data analysis. We also discuss current challenges and offer suggestions based on the direct hands-on experience acquired during the method setup.

Mass Spectrometry to Study Chromatin Compaction

Chromatin accessibility is a major regulator of gene expression. Histone writers/erasers have a critical role in chromatin compaction, as they “flag” chromatin regions by catalyzing/removing covalent post-translational modifications on histone proteins. Anomalous chromatin decondensation is a common phenomenon in cells experiencing aging and viral infection. Moreover, about 50% of cancers have mutations in enzymes regulating chromatin state. Numerous genomics methods have evolved to characterize chromatin state, but the analysis of (in)accessible chromatin from the protein perspective is not yet in the spotlight. We present an overview of the most used approaches to generate data on chromatin accessibility and then focus on emerging methods that utilize mass spectrometry to quantify the accessibility of histones and the rest of the chromatin bound proteome. Mass spectrometry is currently the method of choice to quantify entire proteomes in an unbiased large-scale manner; accessibility on chromatin of proteins and protein modifications adds an extra quantitative layer to proteomics dataset that assist more informed data-driven hypotheses in chromatin biology. We speculate that this emerging new set of methods will enhance predictive strength on which proteins and histone modifications are critical in gene regulation, and which proteins occupy different chromatin states in health and disease.

Targeting the scaffolding role of LSD1 (KDM1A) poises acute myeloid leukemia cells for retinoic acid-induced differentiation

The histone demethylase LSD1 is deregulated in several tumors, including leukemias, providing the rationale for the clinical use of LSD1 inhibitors. In acute promyelocytic leukemia (APL), pharmacological doses of retinoic acid (RA) induce differentiation of APL cells, triggering degradation of the PML-RAR oncogene. APL cells are resistant to LSD1 inhibition or knockout, but targeting LSD1 sensitizes them to physiological doses of RA without altering of PML-RAR levels, and extends survival of leukemic mice upon RA treatment. The combination of RA with LSD1 inhibition (or knockout) is also effective in other non-APL, acute myeloid leukemia (AML) cells. Nonenzymatic activities of LSD1 are essential to block differentiation, while RA with targeting of LSD1 releases a differentiation gene expression program, not strictly dependent on changes in histone H3K4 methylation. Integration of proteomic/epigenomic/mutational studies showed that LSD1 inhibitors alter the recruitment of LSD1-containing complexes to chromatin, inhibiting the interaction between LSD1 and the transcription factor GFI1.

Purification and enrichment of specific chromatin loci

Understanding how chromatin is regulated is essential to fully grasp genome biology, and establishing the locus-specific protein composition is a major step toward this goal. Here we explain why the isolation and analysis of a specific chromatin segment are technically challenging, independently of the method. We then describe the published strategies and discuss their advantages and limitations. We conclude by discussing why significant technology developments are required to unambiguously describe the composition of small single loci.

Expression and phase separation potential of heterochromatin proteins during early mouse development

In most eukaryotes, constitutive heterochromatin is associated with H3K9me3 and HP1α. The latter has been shown to play a role in heterochromatin formation through liquid-liquid phase separation. However, many other proteins are known to regulate and/or interact with constitutive heterochromatic regions in several species. We postulate that some of these heterochromatic proteins may play a role in the regulation of heterochromatin formation by liquid-liquid phase separation. Indeed, an analysis of the constitutive heterochromatin proteome shows that proteins associated with constitutive heterochromatin are significantly more disordered than a random set or a full nucleome set of proteins. Interestingly, their expression begins low and increases during preimplantation development. These observations suggest that the preimplantation embryo is a useful model to address the potential role for phase separation in heterochromatin formation, anticipating exciting research in the years to come.

Adaptation of Hybridization Capture of Chromatin-associated Proteins for Proteomics to Mammalian Cells

The hybridization capture of chromatin-associated proteins for proteomics (HyCCAPP) technology was initially developed to uncover novel DNA-protein interactions in yeast. It allows analysis of a target region of interest without the need for prior knowledge about likely proteins bound to the target region. This, in theory, allows HyCCAPP to be used to analyze any genomic region of interest, and it provides sufficient flexibility to work in different cell systems. This method is not meant to study binding sites of known transcription factors, a task better suited for Chromatin Immunoprecipitation (ChIP) and ChIP-like methods. The strength of HyCCAPP lies in its ability to explore DNA regions for which there is limited or no knowledge about the proteins bound to it. It can also be a convenient method to avoid biases (present in ChIP-like methods) introduced by protein-based chromatin enrichment using antibodies. Potentially, HyCCAPP can be a powerful tool to uncover truly novel DNA-protein interactions. To date, the technology has been predominantly applied to yeast cells or to high copy repeat sequences in mammalian cells. In order to become the powerful tool we envision, HyCCAPP approaches need to be optimized to efficiently capture single-copy loci in mammalian cells. Here, we present our adaptation of the initial yeast HyCCAPP capture protocol to human cell lines, and show that single-copy chromatin regions can be efficiently isolated with this modified protocol.

All I's on the RADAR: role of ADAR in gene regulation

Adenosine to inosine (A-to-I) editing is the most abundant form of RNA modification in mammalian cells, which is catalyzed by adenosine deaminase acting on the double-stranded RNA (ADAR) protein family. A-to-I editing is currently known to be involved in the regulation of the immune system, RNA splicing, protein recoding, microRNA biogenesis, and formation of heterochromatin. Editing occurs within regions of double-stranded RNA, particularly within inverted Alu repeats, and is associated with many diseases including cancer, neurological disorders, and metabolic syndromes. However, the significance of RNA editing in a large portion of the transcriptome remains unknown. Here, we review the current knowledge about the prevalence and function of A-to-I editing by the ADAR protein family, focusing on its role in the regulation of gene expression. Furthermore, RNA editing-independent regulation of cellular processes by ADAR and the putative role(s) of this process in gene regulation will be discussed.

Chromatin proteomics reveals novel combinatorial histone modification signatures that mark distinct subpopulations of macrophage enhancers

The integrated activity of cis-regulatory elements fine-tunes transcriptional programs of mammalian cells by recruiting cell type–specific as well as ubiquitous transcription factors (TFs). Despite their key role in modulating transcription, enhancers are still poorly characterized at the molecular level, and their limited DNA sequence conservation in evolution and variable distance from target genes make their unbiased identification challenging. The coexistence of high mono-methylation and low tri-methylation levels of lysine 4 of histone H3 is considered a signature of enhancers, but a comprehensive view of histone modifications associated to enhancers is still lacking. By combining chromatin immunoprecipitation (ChIP) with mass spectrometry, we investigated cis-regulatory regions in macrophages to comprehensively identify histone marks specifically associated with enhancers, and to profile their dynamics after transcriptional activation elicited by an inflammatory stimulation. The intersection of the proteomics data with ChIP-seq and RNA-seq analyses revealed the existence of novel subpopulations of enhancers, marked by specific histone modification signatures: specifically, H3K4me1/K36me2 marks transcribed enhancers, while H3K4me1/K36me3 and H3K4me1/K79me2 combinations mark distinct classes of intronic enhancers. Thus, our MS analysis of functionally distinct genomic regions revealed the combinatorial code of histone modifications, highlighting the potential of proteomics in addressing fundamental questions in epigenetics.

Genomic and Proteomic Resolution of Heterochromatin and Its Restriction of Alternate Fate Genes

Heterochromatin is integral to cell identity maintenance by impeding the activation of genes for alternate cell fates. Heterochromatic regions are associated with histone 3 lysine 9 trimethylation (H3K9me3) or H3K27me3, but these modifications are also found in euchromatic regions that permit transcription. We discovered that resistance to sonication is a reliable indicator of the heterochromatin state, and we developed a biophysical method (gradient-seq) to discriminate subtypes of H3K9me3 and H3K27me3 domains in sonication-resistant heterochromatin (srHC) versus euchromatin. These classifications are more accurate than the histone marks alone in predicting transcriptional silence and resistance of alternate fate genes to activation during direct cell conversion. Our proteomics of H3K9me3-marked srHC and functional screens revealed diverse proteins, including RBMX and RBMXL1, that impede gene induction during cellular reprogramming. Isolation of srHC with gradient-seq provides a genome-wide map of chromatin structure, elucidating subtypes of repressed domains that are uniquely predictive of diverse other chromatin properties.

Quantitative assessment of chemical artefacts produced by propionylation of histones prior to mass spectrometry analysis

Histone PTMs play a crucial role in regulating chromatin structure and function, with impact on gene expression. MS is nowadays widely applied to study histone PTMs systematically. Because histones are rich in arginine and lysine, classical shot-gun approaches based on trypsin digestion are typically not employed for histone modifications mapping. Instead, different protocols of chemical derivatization of lysines in combination with trypsin have been implemented to obtain "Arg-C like" digestion products that are more suitable for LC-MS/MS analysis. Although widespread, these strategies have been recently described to cause various side reactions that result in chemical modifications prone to be misinterpreted as native histone marks. These artefacts can also interfere with the quantification process, causing errors in histone PTMs profiling. The work of Paternoster V. et al. is a quantitative assessment of methyl-esterification and other side reactions occurring on histones after chemical derivatization of lysines with propionic anhydride [Proteomics 2016, 16, 2059-2063]. The authors estimate the effect of different solvents, incubation times, and pH on the extent of these side reactions. The results collected indicate that the replacement of methanol with isopropanol or ACN not only blocks methyl-esterification, but also significantly reduces other undesired unspecific reactions. Carefully titrating the pH after propionic anhydride addition is another way to keep methyl-esterification under control. Overall, the authors describe a set of experimental conditions that allow reducing the generation of various artefacts during histone propionylation.

Mass Spectrometry-Based Tools to Characterize DNA-Protein Cross-Linking by Bis-Electrophiles

DNA-protein cross-links (DPCs) are unusually bulky DNA adducts that form in cells as a result of exposure to endogenous and exogenous agents including reactive oxygen species, ultraviolet light, ionizing radiation, environmental agents (e.g. transition metals, formaldehyde, 1,2-dibromoethane, 1,3-butadiene) and common chemotherapeutic agents. Covalent DPCs are cytotoxic and mutagenic due to their ability to interfere with faithful DNA replication and to prevent accurate gene expression. Key to our understanding of the biological significance of DPC formation is identifying the proteins most susceptible to forming these unusually bulky and complex lesions and quantifying the extent of DNA-protein cross-linking in cells and tissues. Recent advances in bottom-up mass spectrometry-based proteomics have allowed for an unbiased assessment of the whole protein DPC adductome after in vitro and in vivo exposures to cross-linking agents. This MiniReview summarizes current and emerging methods for DPC isolation and analysis by mass spectrometry-based proteomics. We also highlight several examples of successful applications of these novel methodologies to studies of DPC lesions induced by bis-electrophiles such as formaldehyde, 1,2,3,4-diepoxybutane, nitrogen mustards and cisplatin.

Multi-OMICS: a critical technical perspective on integrative lipidomics approaches

During the past decades, high-throughput approaches for analyzing different molecular classes such as nucleic acids, proteins, metabolites, and lipids have grown rapidly. These approaches became powerful tools for getting a fundamental understanding of biological systems. Considering each approach and its results separately, relations and causal connections between these classes have no chance to be revealed, since only separate molecular snapshots are provided. Only a combined approach, not fully established yet, with the integration of the corresponding data, might yield a comprehensive and complete understanding of biological processes, such as crosstalk and interactions in signaling pathways. Taking two or more omics-methods into consideration for analysis is referred to as a multi-omics approach, which is gradually evolving. In this critical note, we briefly discuss the relevance, challenges, current state, and potential of data integration from multi-omics approaches, with a special focus on lipidomics analysis, listing the advantages and gaps in this field. This article is part of a Special Issue entitled: BBALIP_Lipidomics Opinion Articles edited by Sepp Kohlwein.

Chromatin Immunoprecipitation and Quantitative Real-Time PCR to Assess Binding of a Protein of Interest to Identified Predicted Binding Sites Within a Promoter

Chromatin immunoprecipitation (ChIP) has become a widely used methodology for assessment of protein/DNA interactions. The technique allows the analysis of direct binding of transcription factors to gene promoters, identification of histone modifications, and localization of DNA modifying enzymes. Antibodies conjugated to agarose beads can be utilized to immunoprecipitate specific proteins, cross-linked to sheared chromatin regions to which they are bound endogenously. With downstream applications including quantitative real-time polymerase chain reaction (qRT-PCR), genome-wide sequencing (ChIP-seq), microarray analysis (ChIP-chip), and mass spectrometry (ChIP-MS), the technique enables comprehensive assessment of protein/DNA interactions. Here I describe ChIP, followed by qRT-PCR, to assess direct binding of a single protein to multiple predicted binding sites within a gene promoter.

Proteomics to study DNA-bound and chromatin-associated gene regulatory complexes

High-resolution mass spectrometry (MS)-based proteomics is a powerful method for the identification of soluble protein complexes and large-scale affinity purification screens can decode entire protein interaction networks. In contrast, protein complexes residing on chromatin have been much more challenging, because they are difficult to purify and often of very low abundance. However, this is changing due to recent methodological and technological advances in proteomics. Proteins interacting with chromatin marks can directly be identified by pulldowns with synthesized histone tails containing posttranslational modifications (PTMs). Similarly, pulldowns with DNA baits harbouring single nucleotide polymorphisms or DNA modifications reveal the impact of those DNA alterations on the recruitment of transcription factors. Accurate quantitation – either isotope-based or label free – unambiguously pinpoints proteins that are significantly enriched over control pulldowns. In addition, protocols that combine classical chromatin immunoprecipitation (ChIP) methods with mass spectrometry (ChIP-MS) target gene regulatory complexes in their in-vivo context. Similar to classical ChIP, cells are crosslinked with formaldehyde and chromatin sheared by sonication or nuclease digested. ChIP-MS baits can be proteins in tagged or endogenous form, histone PTMs, or lncRNAs. Locus-specific ChIP-MS methods would allow direct purification of a single genomic locus and the proteins associated with it. There, loci can be targeted either by artificial DNA-binding sites and corresponding binding proteins or via proteins with sequence specificity such as TAL or nuclease deficient Cas9 in combination with a specific guide RNA. We predict that advances in MS technology will soon make such approaches generally applicable tools in epigenetics.

The Metabolic Impact on Histone Acetylation and Transcription in Ageing

Loss of cellular homeostasis during aging results in altered tissue functions and leads to a general decline in fitness and, ultimately, death. As animals age, the control of gene expression, which is orchestrated by multiple epigenetic factors, degenerates. In parallel, metabolic activity and mitochondrial protein acetylation levels also change. These two hallmarks of aging are effectively linked through the accumulating evidence that histone acetylation patterns are susceptible to alterations in key metabolites such as acetyl-CoA and NAD(+), allowing chromatin to function as a sensor of cellular metabolism. In this review we discuss experimental data supporting these connections and provide a context for the possible medical and physiological relevance.

HyCCAPP as a tool to characterize promoter DNA-protein interactions in Saccharomyces cerevisiae

Currently available methods for interrogating DNA-protein interactions at individual genomic loci have significant limitations, and make it difficult to work with unmodified cells or examine single-copy regions without specific antibodies. In this study, we describe a physiological application of the Hybridization Capture of Chromatin-Associated Proteins for Proteomics (HyCCAPP) methodology we have developed. Both novel and known locus-specific DNA-protein interactions were identified at the ENO2 and GAL1 promoter regions of Saccharomyces cerevisiae, and revealed subgroups of proteins present in significantly different levels at the loci in cells grown on glucose versus galactose as the carbon source. Results were validated using chromatin immunoprecipitation. Overall, our analysis demonstrates that HyCCAPP is an effective and flexible technology that does not require specific antibodies nor prior knowledge of locally occurring DNA-protein interactions and can now be used to identify changes in protein interactions at target regions in the genome in response to physiological challenges.

Active and Repressive Chromatin-Associated Proteome after MPA Treatment and the Role of Midkine in Epithelial Monolayer Permeability

Mycophenolic acid (MPA) is prescribed to maintain allografts in organ-transplanted patients. However, gastrointestinal (GI) complications, particularly diarrhea, are frequently observed as a side effect following MPA therapy. We recently reported that MPA altered the tight junction (TJ)-mediated barrier function in a Caco-2 cell monolayer model system. This study investigates whether MPA induces epigenetic changes which lead to GI complications, especially diarrhea.

METHODS

We employed a Chromatin Immunoprecipitation-O-Proteomics (ChIP-O-Proteomics) approach to identify proteins associated with active (H3K4me3) as well as repressive (H3K27me3) chromatin histone modifications in MPA-treated cells, and further characterized the role of midkine, a H3K4me3-associated protein, in the context of epithelial monolayer permeability.

RESULTS

We identified a total of 333 and 306 proteins associated with active and repressive histone modification marks, respectively. Among them, 241 proteins were common both in active and repressive chromatin, 92 proteins were associated exclusively with the active histone modification mark, while 65 proteins remained specific to repressive chromatin. Our results show that 45 proteins which bind to the active and seven proteins which bind to the repressive chromatin region exhibited significantly altered abundance in MPA-treated cells as compared to DMSO control cells. A number of novel proteins whose function is not known in bowel barrier regulation were among the identified proteins, including midkine. Our functional integrity assays on the Caco-2 cell monolayer showed that the inhibition of midkine expression prior to MPA treatment could completely block the MPA-mediated increase in barrier permeability.

CONCLUSIONS

The ChIP-O-Proteomics approach delivered a number of novel proteins with potential implications in MPA toxicity. Consequently, it can be proposed that midkine inhibition could be a potent therapeutic approach to prevent the MPA-mediated increase in TJ permeability and leak flux diarrhea in organ transplant patients.

Reading the Combinatorial Histone Language

Histones are subject to frequent combinatorial post-translational modifications (PTMs), forming a complex chemical "language" that is interpreted by PTM-specific histone-interacting protein modules (reader domains). These specific interactions are thought to instruct gene expression and downstream biological functions. While the majority of studies have focused on individual modifications, our current understanding of the combinatorial PTM patterns on histones is starting to emerge, benefiting from the convergence of multiple technologies. Here, we review the key technical advances and progress on discovery and characterization of combinatorial histone PTM patterns. We focus on the interactions between reader domains and combinatorial PTMs, which is essential for understanding the mechanism and biological meaning of establishing and interpreting information embedded in histone PTM patterns.

Improving understanding of chromatin regulatory proteins and potential implications for drug discovery

Many epigenetic-based therapeutics, including drugs such as histone deacetylase inhibitors, are now used in the clinic or are undergoing advanced clinical trials. The study of chromatin-modifying proteins has benefited from the rapid advances in high-throughput sequencing methods, the organized efforts of major consortiums and by individual groups to profile human epigenomes in diverse tissues and cell types. However, while such initiatives have carefully characterized healthy human tissue, disease epigenomes and drug-epigenome interactions remain very poorly understood. Reviewed here is how high-throughput sequencing improves our understanding of chromatin regulator proteins and the potential implications for the study of human disease and drug development and discovery.

The contribution of mass spectrometry-based proteomics to understanding epigenetics

Chromatin is a macromolecular complex composed of DNA and histones that regulate gene expression and nuclear architecture. The concerted action of DNA methylation, histone post-translational modifications and chromatin-associated proteins control the epigenetic regulation of the genome, ultimately determining cell fate and the transcriptional outputs of differentiated cells. Deregulation of this complex machinery leads to disease states, and exploiting epigenetic drugs is becoming increasingly attractive for therapeutic intervention. Mass spectrometry (MS)-based proteomics emerged as a powerful tool complementary to genomic approaches for epigenetic research, allowing the unbiased and comprehensive analysis of histone post-translational modifications and the characterization of chromatin constituents and chromatin-associated proteins. Furthermore, MS holds great promise for epigenetic biomarker discovery and represents a useful tool for deconvolution of epigenetic drug targets. Here, we will provide an overview of the applications of MS-based proteomics in various areas of chromatin biology.

Affinity reagents for studying histone modifications & guidelines for their quality control

Histone post-translational modifications (PTMs) have pivotal functions in many chromatin processes, which makes their detection and characterization an imperative in chromatin biology. The established approaches for histone PTM characterization are generally based on affinity reagents specific for modified histone tails such as antibodies and, most recently, recombinant reading domains. Hence, the proper performance of these reagents is a critical precondition for the validity of the generated experimental data. In this review, we evaluate and update the quality criteria for assessment of the binding specificity of histone PTM affinity reagents. In addition, we discuss in detail the advantages and pitfalls of using antibodies and recombinant reading domains in chromatin biology research. Reading domains provide key advantages, such as consistent quality and recombinant production, but the future will tell if this emerging technology keeps its promises.

Proteins that bind regulatory regions identified by histone modification chromatin immunoprecipitations and mass spectrometry

The locations of transcriptional enhancers and promoters were recently mapped in many mammalian cell types. Proteins that bind those regulatory regions can determine cell identity but have not been systematically identified. Here we purify native enhancers, promoters or heterochromatin from embryonic stem cells by chromatin immunoprecipitations (ChIP) for characteristic histone modifications and identify associated proteins using mass spectrometry (MS). 239 factors are identified and predicted to bind enhancers or promoters with different levels of activity, or heterochromatin. Published genome-wide data indicate a high accuracy of location prediction by ChIP-MS. A quarter of the identified factors are important for pluripotency and includes Oct4, Esrrb, Klf5, Mycn and Dppa2, factors that drive reprogramming to pluripotent stem cells. We determined the genome-wide binding sites of Dppa2 and find that Dppa2 operates outside the classical pluripotency network. Our ChIP-MS method provides a detailed read-out of the transcriptional landscape representative of the investigated cell type.

Chromatin proteomic profiling reveals novel proteins associated with histone-marked genomic regions

More than a thousand proteins are thought to contribute to mammalian chromatin and its regulation, but our understanding of the genomic occupancy and function of most of these proteins is limited. Here we describe an approach, which we call "chromatin proteomic profiling," to identify proteins associated with genomic regions marked by specifically modified histones. We used ChIP-MS to identify proteins associated with genomic regions marked by histones modified at specific lysine residues, including H3K27ac, H3K4me3, H3K79me2, H3K36me3, H3K9me3, and H4K20me3, in ES cells. We identified 332 known and 114 novel proteins associated with these histone-marked genomic segments. Many of the novel candidates have been implicated in various diseases, and their chromatin association may provide clues to disease mechanisms. More than 100 histone modifications have been described, so similar chromatin proteomic profiling studies should prove to be valuable for identifying many additional chromatin-associated proteins in a broad spectrum of cell types.

Quantitative phosphoproteomics analysis reveals a key role of insulin growth factor 1 receptor (IGF1R) tyrosine kinase in human sperm capacitation

One of the most important changes during sperm capacitation is the enhancement of tyrosine phosphorylation. However, the mechanisms of protein tyrosine phosphorylation during sperm capacitation are not well studied. We used label-free quantitative phosphoproteomics to investigate the overall phosphorylation events during sperm capacitation in humans and identified 231 sites with increased phosphorylation levels. Motif analysis using the NetworKIN algorithm revealed that the activity of tyrosine phosphorylation kinases insulin growth factor 1 receptor (IGF1R)/insulin receptor is significantly enriched among the up-regulated phosphorylation substrates during capacitation. Western blotting further confirmed inhibition of IGF1R with inhibitors GSK1904529A and NVP-AEW541, which inhibited the increase in tyrosine phosphorylation levels during sperm capacitation. Additionally, sperm hyperactivated motility was also inhibited by GSK1904529A and NVP-AEW541 but could be up-regulated by insulin growth factor 1, the ligand of IGF1R. Thus, the IGF1R-mediated tyrosine phosphorylation pathway may play important roles in the regulation of sperm capacitation in humans and could be a target for improvement in sperm functions in infertile men.

Chromatin enrichment for proteomics

During interphase, chromatin hosts fundamental cellular processes, such as gene expression, DNA replication and DNA damage repair. To analyze chromatin on a proteomic scale, we have developed chromatin enrichment for proteomics (ChEP), which is a simple biochemical procedure that enriches interphase chromatin in all its complexity. It enables researchers to take a 'snapshot' of chromatin and to isolate and identify even transiently bound factors. In ChEP, cells are fixed with formaldehyde; subsequently, DNA together with all cross-linked proteins is isolated by centrifugation under denaturing conditions. This approach enables the analysis of global chromatin composition and its changes, which is in contrast with existing chromatin enrichment procedures, which either focus on specific chromatin loci (e.g., affinity purification) or are limited in specificity, such as the analysis of the chromatin pellet (i.e., analysis of all insoluble nuclear material). ChEP takes half a day to complete and requires no specialized laboratory skills or equipment. ChEP enables the characterization of chromatin response to drug treatment or physiological processes. Beyond proteomics, ChEP may preclear chromatin for chromatin immunoprecipitation (ChIP) analyses.

Advanced methods for the analysis of chromatin-associated proteins

DNA-protein interactions are central to gene expression and chromatin regulation and have become one of the main focus areas of the ENCODE consortium. Advances in mass spectrometry and associated technologies have facilitated studies of these interactions, revealing many novel DNA-interacting proteins and histone posttranslational modifications. Proteins interacting at a single locus or at multiple loci have been targeted in these recent studies, each requiring a separate analytical strategy for isolation and analysis of DNA-protein interactions. The enrichment of target chromatin fractions occurs via a number of methods including immunoprecipitation, affinity purification, and hybridization, with the shared goal of using proteomics approaches as the final readout. The result of this is a number of exciting new tools, with distinct strengths and limitations that can enable highly robust and novel chromatin studies when applied appropriately. The present review compares and contrasts these methods to help the reader distinguish the advantages of each approach.

ChIP-less analysis of chromatin states

Background

Histone post-translational modifications (PTMs) are key epigenetic regulators in chromatin-based processes. Increasing evidence suggests that vast combinations of PTMs exist within chromatin histones. These complex patterns, rather than individual PTMs, are thought to define functional chromatin states. However, the ability to interrogate combinatorial histone PTM patterns at the nucleosome level has been limited by the lack of direct molecular tools.

Results

Here we demonstrate an efficient, quantitative, antibody-free, chromatin immunoprecipitation-less (ChIP-less) method for interrogating diverse epigenetic states. At the heart of the workflow are recombinant chromatin reader domains, which target distinct chromatin states with combinatorial PTM patterns. Utilizing a newly designed combinatorial histone peptide microarray, we showed that three reader domains (ATRX-ADD, ING2-PHD and AIRE-PHD) displayed greater specificity towards combinatorial PTM patterns than corresponding commercial histone antibodies. Such specific recognitions were employed to develop a chromatin reader-based affinity enrichment platform (matrix-assisted reader chromatin capture, or MARCC). We successfully applied the reader-based platform to capture unique chromatin states, which were quantitatively profiled by mass spectrometry to reveal interconnections between nucleosomal histone PTMs. Specifically, a highly enriched signature that harbored H3K4me0, H3K9me2/3, H3K79me0 and H4K20me2/3 within the same nucleosome was identified from chromatin enriched by ATRX-ADD. This newly reported PTM combination was enriched in heterochromatin, as revealed by the associated DNA.

Conclusions

Our results suggest the broad utility of recombinant reader domains as an enrichment tool specific to combinatorial PTM patterns, which are difficult to probe directly by antibody-based approaches. The reader affinity platform is compatible with several downstream analyses to investigate the physical coexistence of nucleosomal PTM states associated with specific genomic loci. Collectively, the reader-based workflow will greatly facilitate our understanding of how distinct chromatin states and reader domains function in gene regulatory mechanisms.

The ChroP approach combines ChIP and mass spectrometry to dissect locus-specific proteomic landscapes of chromatin

Chromatin is a highly dynamic nucleoprotein complex made of DNA and proteins that controls various DNA-dependent processes. Chromatin structure and function at specific regions is regulated by the local enrichment of histone post-translational modifications (hPTMs) and variants, chromatin-binding proteins, including transcription factors, and DNA methylation. The proteomic characterization of chromatin composition at distinct functional regions has been so far hampered by the lack of efficient protocols to enrich such domains at the appropriate purity and amount for the subsequent in-depth analysis by Mass Spectrometry (MS). We describe here a newly designed chromatin proteomics strategy, named ChroP (Chromatin Proteomics), whereby a preparative chromatin immunoprecipitation is used to isolate distinct chromatin regions whose features, in terms of hPTMs, variants and co-associated non-histonic proteins, are analyzed by MS. We illustrate here the setting up of ChroP for the enrichment and analysis of transcriptionally silent heterochromatic regions, marked by the presence of tri-methylation of lysine 9 on histone H3. The results achieved demonstrate the potential of ChroP in thoroughly characterizing the heterochromatin proteome and prove it as a powerful analytical strategy for understanding how the distinct protein determinants of chromatin interact and synergize to establish locus-specific structural and functional configurations.

Biochemical systems approaches for the analysis of histone modification readout

Chromatin is the macromolecular nucleoprotein complex that governs the organization of genetic material in the nucleus of eukaryotic cells. In chromatin, DNA is packed with histone proteins into nucleosomes. Core histones are prototypes of hyper-modified proteins, being decorated by a large number of site-specific reversible and irreversible post-translational modifications (PTMs), which contribute to the maintenance and modulation of chromatin plasticity, gene activation, and a variety of other biological processes and disease states. The observations of the variety, frequency and co-occurrence of histone modifications in distinct patterns at specific genomic loci have led to the idea that hPTMs can create a molecular barcode, read by effector proteins that translate it into a specific transcriptional state, or process, on the underlying DNA. However, despite the fact that this histone-code hypothesis was proposed more than 10 years ago, the molecular details of its working mechanisms are only partially characterized. In particular, two questions deserve specific investigation: how the different modifications associate and synergize into patterns and how these PTM configurations are read and translated by multi-protein complexes into a specific functional outcome on the genome. Mass spectrometry (MS) has emerged as a versatile tool to investigate chromatin biology, useful for both identifying and validating hPTMs, and to dissect the molecular determinants of histone modification readout systems. We review here the MS techniques and the proteomics methods that have been developed to address these fundamental questions in epigenetics research, emphasizing approaches based on the proteomic dissection of distinct native chromatin regions, with a critical evaluation of their present challenges and future potential. This article is part of a Special Issue entitled: Molecular mechanisms of histone modification function.

Combining genomic and proteomic approaches for epigenetics research

Epigenetics is the study of changes in gene expression or cellular phenotype that do not change the DNA sequence. In this review, current methods, both genomic and proteomic, associated with epigenetics research are discussed. Among them, chromatin immunoprecipitation (ChIP) followed by sequencing and other ChIP-based techniques are powerful techniques for genome-wide profiling of DNA-binding proteins, histone post-translational modifications or nucleosome positions. However, mass spectrometry-based proteomics is increasingly being used in functional biological studies and has proved to be an indispensable tool to characterize histone modifications, as well as DNA-protein and protein-protein interactions. With the development of genomic and proteomic approaches, combination of ChIP and mass spectrometry has the potential to expand our knowledge of epigenetics research to a higher level.

Mass spectrometry-based proteomics for the analysis of chromatin structure and dynamics

Chromatin is a highly structured nucleoprotein complex made of histone proteins and DNA that controls nearly all DNA-dependent processes. Chromatin plasticity is regulated by different associated proteins, post-translational modifications on histones (hPTMs) and DNA methylation, which act in a concerted manner to enforce a specific "chromatin landscape", with a regulatory effect on gene expression. Mass Spectrometry (MS) has emerged as a powerful analytical strategy to detect histone PTMs, revealing interplays between neighbouring PTMs and enabling screens for their readers in a comprehensive and quantitative fashion. Here we provide an overview of the recent achievements of state-of-the-art mass spectrometry-based proteomics for the detailed qualitative and quantitative characterization of histone post-translational modifications, histone variants, and global interactomes at specific chromatin regions. This synopsis emphasizes how the advances in high resolution MS, from "Bottom Up" to "Top Down" analysis, together with the uptake of quantitative proteomics methods by chromatin biologists, have made MS a well-established method in the epigenetics field, enabling the acquisition of original information, highly complementary to that offered by more conventional, antibody-based, assays.