Dam it's good! DamID profiling of protein-DNA interactions

Beyond assembly: the increasing flexibility of single-molecule sequencing technology

The maturation of high-throughput short-read sequencing technology over the past two decades has shaped the way genomes are studied. Recently, single-molecule, long-read sequencing has emerged as an essential tool in deciphering genome structure and function, including filling gaps in the human reference genome, measuring the epigenome and characterizing splicing variants in the transcriptome. With recent technological developments, these single-molecule technologies have moved beyond genome assembly and are being used in a variety of ways, including to selectively sequence specific loci with long reads, measure chromatin state and protein-DNA binding in order to investigate the dynamics of gene regulation, and rapidly determine copy number variation. These increasingly flexible uses of single-molecule technologies highlight a young and fast-moving part of the field that is leading to a more accessible era of nucleic acid sequencing.

Drosophila DAxud1 Has a Repressive Transcription Activity on Hsp70 and Other Heat Shock Genes

Drosophila melanogaster DAxud1 is a transcription factor that belongs to the Cysteine Serine Rich Nuclear Protein (CSRNP) family, conserved in metazoans, with a transcriptional transactivation activity. According to previous studies, this protein promotes apoptosis and Wnt signaling-mediated neural crest differentiation in vertebrates. However, no analysis has been conducted to determine what other genes it might control, especially in connection with cell survival and apoptosis. To partly answer this question, this work analyzes the role of Drosophila DAxud1 using Targeted-DamID-seq (TaDa-seq), which allows whole genome screening to determine in which regions it is most frequently found. This analysis confirmed the presence of DAxud1 in groups of pro-apoptotic and Wnt pathway genes, as previously described; furthermore, stress resistance genes that coding heat shock protein (HSP) family genes were found as hsp70, hsp67, and hsp26. The enrichment of DAxud1 also identified a DNA-binding motif (AYATACATAYATA) that is frequently found in the promoters of these genes. Surprisingly, the following analyses demonstrated that DAxud1 exerts a repressive role on these genes, which are necessary for cell survival. This is coupled with the pro-apoptotic and cell cycle arrest roles of DAxud1, in which repression of hsp70 complements the maintenance of tissue homeostasis through cell survival modulation.

Epigenetics and the role of nutraceuticals in health and disease

In the post-genomic era, the data provided by complete genome sequencing could not answer several fundamental questions about the causes of many noninfectious diseases, diagnostic biomarkers, and novel therapeutic approaches. The rapidly expanding understanding of epigenetic mechanisms, as well as widespread acceptance of their hypothesized role in disease induction, facilitated the development of a number of novel diagnostic markers and therapeutic concepts. Epigenetic aberrations are reversible in nature, which enables the treatment of serious incurable diseases. Therefore, the interest in epigenetic modulatory effects has increased over the last decade, so about 60,000 publications discussing the expression of epigenetics could be detected in the PubMed database. Out of these, 58,442 were published alone in the last 10 years, including 17,672 reviews (69 historical articles), 314 clinical trials, 202 case reports, 197 meta-analyses, 156 letters to the editor, 108 randomized controlled trials, 87 observation studies, 40 book chapters, 22 published lectures, and 2 clinical trial protocols. The remaining publications are either miscellaneous or a mixture of the previously mentioned items. According to the species and gender, the publications included 44,589 human studies (17,106 females, 14,509 males, and the gender is not mentioned in the remaining papers) and 30,253 animal studies. In the present work, the role of epigenetic modulations in health and disease and the influencing factors in epigenetics are discussed.

Long range inter-chromosomal interaction of Oct4 distal enhancer loci regulates ESCs pluripotency

Nuclear architecture underlies the transcriptional programs within the cell to establish cell identity. As previously demonstrated, long-range chromatin interactions of the Oct4 distal enhancer (DE) are correlated with active transcription in naïve state embryonic stem cells. Here, we identify and characterize extreme long-range interactions of the Oct4 DE through a novel CRISPR labeling technique we developed and chromosome conformation capture to identify lethal giant larvae 2 (Llgl2) and growth factor receptor-bound protein 7 (Grb7) as putative functional interacting target genes in different chromosomes. We show that the Oct4 DE directly regulates expression of Llgl2 and Grb7 in addition to Oct4. Expression of Llgl2 and Grb7 closely correlates with the pluripotent state, where knock down of either result in loss of pluripotency, and overexpression enhances somatic cell reprogramming. We demonstrated that biologically important interactions of the Oct4 DE can occur at extreme distances that are necessary for the maintenance of the pluripotent state.

Genome organization in cardiomyocytes expressing mutated A-type lamins

Cardiomyopathy is a myocardial disorder, in which the heart muscle is structurally and functionally abnormal, often leading to heart failure. Dilated cardiomyopathy is characterized by a compromised left ventricular function and contributes significantly to the heart failure epidemic, which represents a staggering clinical and public health problem worldwide. Gene mutations have been identified in 35% of patients with dilated cardiomyopathy. Pathogenic variants in LMNA, encoding nuclear A-type lamins, are one of the major causative causes of dilated cardiomyopathy (i.e. CardioLaminopathy). A-type lamins are type V intermediate filament proteins, which are the main components of the nuclear lamina. The nuclear lamina is connected to the cytoskeleton on one side, and to the chromatin on the other side. Among the models proposed to explain how CardioLaminopathy arises, the “chromatin model” posits an effect of mutated A-type lamins on the 3D genome organization and thus on the transcription activity of tissue-specific genes. Chromatin contacts with the nuclear lamina via specific genomic regions called lamina-associated domains lamina-associated domains. These LADs play a role in the chromatin organization and gene expression regulation. This review focuses on the identification of LADs and chromatin remodeling in cardiac muscle cells expressing mutated A-type lamins and discusses the methods and relevance of these findings in disease.

Antibody-free profiling of transcription factor occupancy during early embryogenesis by FitCUT&RUN

Key transcription factors (TFs) play critical roles in zygotic genome activation (ZGA) during early embryogenesis, while genome-wide occupancies of only a few factors have been profiled during ZGA due to the limitation of cell numbers or the lack of high-quality antibodies. Here, we present FitCUT&RUN, a modified CUT&RUN method, in which an Fc fragment of immunoglobulin G is used for tagging, to profile TF occupancy in an antibody-free manner and demonstrate its reliability and robustness using as few as five thousand K562 cells. We applied FitCUT&RUN to zebrafish undergoing embryogenesis to generate reliable occupancy profiles of three known activators of zebrafish ZGA: Nanog, Pou5f3 and Sox19b. By profiling the time-series occupancy of Nanog during zebrafish ZGA, we observed a clear trend toward a gradual increase in Nanog occupancy and found that Nanog occupancy prior to the major phase of ZGA is critical for the activation of a significant proportion of early transcribed genes. Our results further suggested that the sequential binding of Nanog may be controlled by replication timing and the presence of Nanog motifs.

DamID transcriptional profiling identifies the Snail/Scratch transcription factor Kahuli as an Alk target in the Drosophila visceral mesoderm

Development of the Drosophila visceral muscle depends on Anaplastic Lymphoma Kinase (Alk) receptor tyrosine kinase (RTK) signaling, which specifies founder cells (FCs) in the circular visceral mesoderm (VM). Although Alk activation by its ligand Jelly Belly (Jeb) is well characterized, few target molecules have been identified. Here, we used targeted DamID (TaDa) to identify Alk targets in embryos overexpressing Jeb versus embryos with abrogated Alk activity, revealing differentially expressed genes, including the Snail/Scratch family transcription factor Kahuli (Kah). We confirmed Kah mRNA and protein expression in the VM, and identified midgut constriction defects in Kah mutants similar to those of pointed (pnt). ChIP and RNA-Seq data analysis defined a Kah target-binding site similar to that of Snail, and identified a set of common target genes putatively regulated by Kah and Pnt during midgut constriction. Taken together, we report a rich dataset of Alk-responsive loci in the embryonic VM and functionally characterize the role of Kah in the regulation of embryonic midgut morphogenesis.

Gene expression profiling of epidermal cell types in C. elegans using Targeted DamID

The epidermis of Caenorhabditis elegans is an essential tissue for survival because it contributes to the formation of the cuticle barrier as well as facilitating developmental progression and animal growth. Most of the epidermis consists of the hyp7 hypodermal syncytium, the nuclei of which are largely generated by the seam cells, which exhibit stem cell-like behaviour during development. How seam cell progenitors differ transcriptionally from the differentiated hypodermis is poorly understood. Here, we introduce Targeted DamID (TaDa) in C. elegans as a method for identifying genes expressed within a tissue of interest without cell isolation. We show that TaDa signal enrichment profiles can be used to identify genes transcribed in the epidermis and use this method to resolve differences in gene expression between the seam cells and the hypodermis. Finally, we predict and functionally validate new transcription and chromatin factors acting in seam cell development. These findings provide insights into cell type-specific gene expression profiles likely associated with epidermal cell fate patterning.

Chromatin Profiling Techniques: Exploring the Chromatin Environment and Its Contributions to Complex Traits

The genetic architecture of complex traits is multifactorial. Genome-wide association studies (GWASs) have identified risk loci for complex traits and diseases that are disproportionately located at the non-coding regions of the genome. On the other hand, we have just begun to understand the regulatory roles of the non-coding genome, making it challenging to precisely interpret the functions of non-coding variants associated with complex diseases. Additionally, the epigenome plays an active role in mediating cellular responses to fluctuations of sensory or environmental stimuli. However, it remains unclear how exactly non-coding elements associate with epigenetic modifications to regulate gene expression changes and mediate phenotypic outcomes. Therefore, finer interrogations of the human epigenomic landscape in associating with non-coding variants are warranted. Recently, chromatin-profiling techniques have vastly improved our understanding of the numerous functions mediated by the epigenome and DNA structure. Here, we review various chromatin-profiling techniques, such as assays of chromatin accessibility, nucleosome distribution, histone modifications, and chromatin topology, and discuss their applications in unraveling the brain epigenome and etiology of complex traits at tissue homogenate and single-cell resolution. These techniques have elucidated compositional and structural organizing principles of the chromatin environment. Taken together, we believe that high-resolution epigenomic and DNA structure profiling will be one of the best ways to elucidate how non-coding genetic variations impact complex diseases, ultimately allowing us to pinpoint cell-type targets with therapeutic potential.

The Serine Protease Homolog, Scarface, Is Sensitive to Nutrient Availability and Modulates the Development of the Drosophila Blood-Brain Barrier

The adaptable transcriptional response to changes in food availability not only ensures animal survival but also lets embryonic development progress. Interestingly, the CNS is preferentially protected from periods of malnutrition, a phenomenon known as “brain sparing.” However, the mechanisms that mediate this response remain poorly understood. To get a better understanding of this, we used Drosophila melanogaster as a model, analyzing the transcriptional response of neural stem cells (neuroblasts) and glia of the blood–brain barrier (BBB) from larvae of both sexes during nutrient restriction using targeted DamID. We found differentially expressed genes in both neuroblasts and glia of the BBB, although the effect of nutrient deficiency was primarily observed in the BBB. We characterized the function of a nutritional sensitive gene expressed in the BBB, the serine protease homolog, scarface (scaf). Scaf is expressed in subperineurial glia in the BBB in response to nutrition. Tissue-specific knockdown of scaf increases subperineurial glia endoreplication and proliferation of perineurial glia in the blood–brain barrier. Furthermore, neuroblast proliferation is diminished on scaf knockdown in subperineurial glia. Interestingly, reexpression of Scaf in subperineurial glia is able to enhance neuroblast proliferation and brain growth of animals in starvation. Finally, we show that loss of scaf in the blood–brain barrier increases sensitivity to drugs in adulthood, suggesting a physiological impairment. We propose that Scaf integrates the nutrient status to modulate the balance between neurogenesis and growth of the BBB, preserving the proper equilibrium between the size of the barrier and the brain.

SIGNIFICANCE STATEMENT The Drosophila BBB separates the CNS from the open circulatory system. The BBB glia are not only acting as a physical segregation of tissues but participate in the regulation of the metabolism and neurogenesis during development. Here we analyze the transcriptional response of the BBB glia to nutrient deprivation during larval development, a condition in which protective mechanisms are switched on in the brain. Our findings show that the gene scarface reduces growth in the BBB while promoting the proliferation of neural stem, assuring the balanced growth of the larval brain. Thus, Scarface would link animal nutrition with brain development, coordinating neurogenesis with the growth of the BBB.

Time-Based Systems Biology Approaches to Capture and Model Dynamic Gene Regulatory Networks

All aspects of transcription and its regulation involve dynamic events. However, capturing these dynamic events in gene regulatory networks (GRNs) offers both a promise and a challenge. The promise is that capturing and modeling the dynamic changes in GRNs will allow us to understand how organisms adapt to a changing environment. The ability to mount a rapid transcriptional response to environmental changes is especially important in nonmotile organisms such as plants. The challenge is to capture these dynamic, genome-wide events and model them in GRNs. In this review, we cover recent progress in capturing dynamic interactions of transcription factors with their targets-at both the local and genome-wide levels-and how they are used to learn how GRNs operate as a function of time. We also discuss recent advances that employ time-based machine learning approaches to forecast gene expression at future time points, a key goal of systems biology.

μDamID: A Microfluidic Approach for Joint Imaging and Sequencing of Protein-DNA Interactions in Single Cells

DNA adenine methyltransferase identification (DamID) measures a protein's DNA-binding history by methylating adenine bases near each protein-DNA interaction site and then selectively amplifying and sequencing these methylated regions. Additionally, these interactions can be visualized using m6A-Tracer, a fluorescent protein that binds to methyladenines. Here, we combine these imaging and sequencing technologies in an integrated microfluidic platform (μDamID) that enables single-cell isolation, imaging, and sorting, followed by DamID. We use μDamID and an improved m6A-Tracer protein to generate paired imaging and sequencing data from individual human cells. We validate interactions between Lamin-B1 protein and lamina-associated domains (LADs), observe variable 3D chromatin organization and broad gene regulation patterns, and jointly measure single-cell heterogeneity in Dam expression and background methylation. μDamID provides the unique ability to compare paired imaging and sequencing data for each cell and between cells, enabling the joint analysis of the nuclear localization, sequence identity, and variability of protein-DNA interactions. A record of this paper's transparent peer review process is included in the Supplemental Information.

Dnmt1 binds and represses genomic retroelements via DNA methylation in mouse early embryos

Genome-wide passive DNA demethylation in cleavage-stage mouse embryos is related to the cytoplasmic localization of the maintenance methyltransferase DNMT1. However, recent studies provided evidences of the nuclear localization of DNMT1 and its contribution to the maintenance of methylation levels of imprinted regions and other genomic loci in early embryos. Using the DNA adenine methylase identification method, we identified Dnmt1-binding regions in four- and eight-cell embryos. The unbiased distribution of Dnmt1 peaks in the genic regions (promoters and CpG islands) as well as the absence of a correlation between the Dnmt1 peaks and the expression levels of the peak-associated genes refutes the active participation of Dnmt1 in the transcriptional regulation of genes in the early developmental period. Instead, Dnmt1 was found to associate with genomic retroelements in a greatly biased fashion, particularly with the LINE1 (long interspersed nuclear elements) and ERVK (endogenous retrovirus type K) sequences. Transcriptomic analysis revealed that the transcripts of the Dnmt1-enriched retroelements were overrepresented in Dnmt1 knockdown embryos. Finally, methyl-CpG-binding domain sequencing proved that the Dnmt1-enriched retroelements, which were densely methylated in wild-type embryos, became demethylated in the Dnmt1-depleted embryos. Our results indicate that Dnmt1 is involved in the repression of retroelements through DNA methylation in early mouse development.

Transient genome-wide interactions of the master transcription factor NLP7 initiate a rapid nitrogen-response cascade

Dynamic reprogramming of gene regulatory networks (GRNs) enables organisms to rapidly respond to environmental perturbation. However, the underlying transient interactions between transcription factors (TFs) and genome-wide targets typically elude biochemical detection. Here, we capture both stable and transient TF-target interactions genome-wide within minutes after controlled TF nuclear import using time-series chromatin immunoprecipitation (ChIP-seq) and/or DNA adenine methyltransferase identification (DamID-seq). The transient TF-target interactions captured uncover the early mode-of-action of NIN-LIKE PROTEIN 7 (NLP7), a master regulator of the nitrogen signaling pathway in plants. These transient NLP7 targets captured in root cells using temporal TF perturbation account for 50% of NLP7-regulated genes not detectably bound by NLP7 in planta. Rapid and transient NLP7 binding activates early nitrogen response TFs, which we validate to amplify the NLP7-initiated transcriptional cascade. Our approaches to capture transient TF-target interactions genome-wide can be applied to validate dynamic GRN models for any pathway or organism of interest.

Conventional methods cannot reveal transient transcription factors (TFs) and targets interactions. Here, Alvarez et al. capture both stable and transient TF-target interactions by time-series ChIP-seq and/or DamID-seq in a cell-based TF perturbation system and show NLP7 as a master TF to initiate a rapid nitrogen-response cascade.

MadID, a Versatile Approach to Map Protein-DNA Interactions, Highlights Telomere-Nuclear Envelope Contact Sites in Human Cells

Mapping the binding sites of DNA- or chromatin-interacting proteins is essential to understanding biological processes. DNA adenine methyltransferase identification (DamID) has emerged as a comprehensive method to map genome-wide occupancy of proteins of interest. A caveat of DamID is the specificity of Dam methyltransferase for GATC motifs that are not homogenously distributed in the genome. Here, we developed an optimized method named MadID, using proximity labeling of DNA by the methyltransferase M.EcoGII. M.EcoGII mediates N6-adenosine methylation in any DNA sequence context, resulting in deeper and unbiased coverage of the genome. We demonstrate, using m6A-specific immunoprecipitation and deep sequencing, that MadID is a robust method to identify protein-DNA interactions at the whole-genome level. Using MadID, we revealed contact sites between human telomeres, repetitive sequences devoid of GATC sites, and the nuclear envelope. Overall, MadID opens the way to identification of binding sites in genomic regions that were largely inaccessible.

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MadID: mapping of protein-DNA interactions in vivo using proximity labeling

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Deeper and unbiased genome-wide coverage using M.EcoGII, a methyltransferase

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Identification of binding sites in previously inaccessible regions of the genome

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Identification of telomere-nuclear envelope contact sites

Going low to reach high: Small-scale ChIP-seq maps new terrain

Chromatin immunoprecipitation (ChIP) enables mapping of specific histone modifications or chromatin-associated factors in the genome and represents a powerful tool in the study of chromatin and genome regulation. Importantly, recent technological advances that couple ChIP with whole-genome high-throughput sequencing (ChIP-seq) now allow the mapping of chromatin factors throughout the genome. However, the requirement for large amounts of ChIP-seq input material has long made it challenging to assess chromatin profiles of cell types only available in limited numbers. For many cell types, it is not feasible to reach high numbers when collecting them as homogeneous cell populations in vivo. Nonetheless, it is an advantage to work with pure cell populations to reach robust biological conclusions. Here, we review (a) how ChIP protocols have been scaled down for use with as little as a few hundred cells; (b) which considerations to be aware of when preparing small-scale ChIP-seq and analyzing data; and (c) the potential of small-scale ChIP-seq datasets for elucidating chromatin dynamics in various biological systems, including some examples such as oocyte maturation and preimplantation embryo development. This article is categorized under: Laboratory Methods and Technologies > Genetic/Genomic Methods Developmental Biology > Developmental Processes in Health and Disease Biological Mechanisms > Cell Fates.

yap1b, a divergent Yap/Taz family member, cooperates with yap1 in survival and morphogenesis via common transcriptional targets

Yap1/Taz are well-known Hippo effectors triggering complex transcriptional programs controlling growth, survival and cancer progression. Here, we describe yap1b, a new Yap1/Taz family member with a unique transcriptional activation domain that cannot be phosphorylated by Src/Yes kinases. We show that yap1b evolved specifically in euteleosts (i.e. including medaka but not zebrafish) by duplication and adaptation of yap1. Using DamID-seq, we generated maps of chromatin occupancy for Yap1, Taz (Wwtr1) and Yap1b in gastrulating zebrafish and medaka embryos. Our comparative analyses uncover the genetic programs controlled by Yap family proteins during early embryogenesis, and show largely overlapping targets for Yap1 and Yap1b. CRISPR/Cas9-induced mutation of yap1b in medaka does not result in an overt phenotype during embryogenesis or adulthood. However, yap1b mutation strongly enhances the embryonic malformations observed in yap1 mutants. Thus yap1 ^-/-; yap1b ^-/- double mutants display more severe body flattening, eye misshaping and increased apoptosis than yap1 ^-/- single mutants, thus revealing overlapping gene functions. Our results indicate that, despite its divergent transactivation domain, Yap1b cooperates with Yap1 to regulate cell survival and tissue morphogenesis during early development.

DamID as a versatile tool for understanding gene regulation

The interaction of proteins and RNA with chromatin underlies the regulation of gene expression. The ability to profile easily these interactions is fundamental for understanding chromatin biology in vivo. DNA adenine methyltransferase identification (DamID) profiles genome-wide protein-DNA interactions without antibodies, fixation or protein pull-downs. Recently, DamID has been adapted for applications beyond simple assaying of protein-DNA interactions, such as for studying RNA-chromatin interactions, chromatin accessibility and long-range chromosome interactions. Here, we provide an overview of DamID and introduce improvements to the technology, discuss their applications and compare alternative methodologies.

Summary: This Primer provides an overview of DNA adenine methyltransferase identification (DamID), which is used to profile genome-wide chromatin interactions, and introduces recent improvements to the technology.

TaDa! Analysing cell type-specific chromatin in vivo with Targeted DamID

The emergence of neuronal diversity during development of the nervous system relies on dynamic changes in the epigenetic landscape of neural stem cells and their progeny. Targeted DamID (TaDa) is proving invaluable in identifying the genome-wide binding sites of chromatin-associated proteins in vivo, without fixation, cell isolation, or immunoprecipitation. The simplicity and efficiency of the technique have led to an ever-expanding TaDa toolbox. These tools enable profiling of gene expression and chromatin accessibility, as well as the identification of the genome-wide binding sites of chromatin complexes, transcription factors and RNAs. Here, we review these new developments, with particular emphasis on the use of TaDa in studying neuronal specification.

Targeted DamID reveals differential binding of mammalian pluripotency factors

The precise control of gene expression by transcription factor networks is crucial to organismal development. The predominant approach for mapping transcription factor-chromatin interactions has been chromatin immunoprecipitation (ChIP). However, ChIP requires a large number of homogeneous cells and antisera with high specificity. A second approach, DamID, has the drawback that high levels of Dam methylase are toxic. Here, we modify our targeted DamID approach (TaDa) to enable cell type-specific expression in mammalian systems, generating an inducible system (mammalian TaDa or MaTaDa) to identify genome-wide protein/DNA interactions in 100 to 1000 times fewer cells than ChIP-based approaches. We mapped the binding sites of two key pluripotency factors, OCT4 and PRDM14, in mouse embryonic stem cells, epiblast-like cells and primordial germ cell-like cells (PGCLCs). PGCLCs are an important system for elucidating primordial germ cell development in mice. We monitored PRDM14 binding during the specification of PGCLCs, identifying direct targets of PRDM14 that are key to understanding its crucial role in PGCLC development. We show that MaTaDa is a sensitive and accurate method for assessing cell type-specific transcription factor binding in limited numbers of cells.

An Inducible DamID System for Profiling Interactions of Nuclear Lamina Protein Component Lamin B1 with Chromosomes in Mouse Cells

At the level of DNA organization into chromatin, there are mechanisms that define gene expression profiles in specialized cell types. Genes within chromatin regions that are located at the nuclear periphery are generally expressed at lower levels; however, the nature of this phenomenon remains unclear. These parts of chromatin interact with nuclear lamina proteins like Lamin B1 and, therefore, can be identified in a given cell type by chromatin profiling of these proteins. In this study, we created and tested a Dam Identification (DamID) system induced by Cre recombinase using Lamin B1 and mouse embryonic fibroblasts. This inducible system will help to generate genome-wide profiles of chromatin proteins in given cell types and tissues with no need to dissect tissues from organs or separate cells from tissues, which is achieved by using specific regulatory DNA elements and due to the high sensitivity of the method.

Mapping transcription factor occupancy using minimal numbers of cells in vitro and in vivo

The identification of transcription factor (TF) binding sites in the genome is critical to understanding gene regulatory networks (GRNs). While ChIP-seq is commonly used to identify TF targets, it requires specific ChIP-grade antibodies and high cell numbers, often limiting its applicability. DNA adenine methyltransferase identification (DamID), developed and widely used in Drosophila, is a distinct technology to investigate protein–DNA interactions. Unlike ChIP-seq, it does not require antibodies, precipitation steps, or chemical protein–DNA crosslinking, but to date it has been seldom used in mammalian cells due to technical limitations. Here we describe an optimized DamID method coupled with next-generation sequencing (DamID-seq) in mouse cells and demonstrate the identification of the binding sites of two TFs, POU5F1 (also known as OCT4) and SOX2, in as few as 1000 embryonic stem cells (ESCs) and neural stem cells (NSCs), respectively. Furthermore, we have applied this technique in vivo for the first time in mammals. POU5F1 DamID-seq in the gastrulating mouse embryo at 7.5 d post coitum (dpc) successfully identified multiple POU5F1 binding sites proximal to genes involved in embryo development, neural tube formation, and mesoderm-cardiac tissue development, consistent with the pivotal role of this TF in post-implantation embryo. This technology paves the way to unprecedented investigation of TF–DNA interactions and GRNs in specific cell types of limited availability in mammals, including in vivo samples.

CATaDa reveals global remodelling of chromatin accessibility during stem cell differentiation in vivo

During development eukaryotic gene expression is coordinated by dynamic changes in chromatin structure. Measurements of accessible chromatin are used extensively to identify genomic regulatory elements. Whilst chromatin landscapes of pluripotent stem cells are well characterised, chromatin accessibility changes in the development of somatic lineages are not well defined. Here we show that cell-specific chromatin accessibility data can be produced via ectopic expression of E. coli Dam methylase in vivo, without the requirement for cell-sorting (CATaDa). We have profiled chromatin accessibility in individual cell-types of Drosophila neural and midgut lineages. Functional cell-type-specific enhancers were identified, as well as novel motifs enriched at different stages of development. Finally, we show global changes in the accessibility of chromatin between stem-cells and their differentiated progeny. Our results demonstrate the dynamic nature of chromatin accessibility in somatic tissues during stem cell differentiation and provide a novel approach to understanding gene regulatory mechanisms underlying development.

For an embryo to successfully develop into an adult animal, specific genes must act in different types of cells. Though all the cells have the same genes encoded within their DNA, looking at the way that the DNA is packaged can indicate which parts of the DNA are important for that particular cell type. If regions of DNA are “open” one can infer that those regions are actively involved in gene regulation, whereas “closed” regions are considered less important.

It is currently difficult to determine which parts of the DNA are open within an individual cell type in a complex organ, such as the brain. Existing methods require the cells to be physically isolated from the tissue, which is technically challenging.

To overcome this issue, Aughey et al. have now developed a method that does not require isolation of the cells. The new technique involves using genetic engineering to introduce an enzyme called Dam into specific cell types in living fruit flies. This enzyme adds a chemical label on regions of open DNA, which can then be detected. Aughey et al. tested this technique on various cells of the developing brain and gut, and were able to see differences in the openness of DNA that corresponded to the action of genes that are important in each cell type. The data also contain trends that help to understand the role of open DNA in development. For example, mature cells were shown to overall have less open DNA than the stem cells that divide to generate them.

Aughey et al. hope their new technique will be of use to other researchers working with either fruit flies or mammalian tissues. The knowledge that scientists will gain from identifying how open DNA contributes to gene regulation, in both healthy and diseased tissues, will further our understanding of human development and the biology of diseases such as cancer.

Single cell epigenome sequencing technologies

Epigenetic regulation plays crucial roles in the development and disease processes. Since different cell types with distinct epigenetic characteristics are always intermingled together in the tissues or organs, the single cell analysis provides a universal resolution for dissecting their intrinsic complexities. In this review, we discuss recent advances of developing single cell epigenome sequencing techniques for profiling DNA modifications (5mC, 5hmC, 5fC and 5caC), chromatin accessibility, histone modifications, protein-DNA interactions and three-dimensional genome architecture (Hi-C) in an individual mammalian cell. Recent progresses of single cell multi-omics sequencing techniques are also reviewed. These techniques are emerging as powerful tools for unravelling the unique epigenomic features of rare cell types and epigenetic heterogeneity within a seemingly homogenous cell population. We also discussed the current limitations and future development trend of single cell epigenome sequencing techniques.

Genome-wide Mapping of Protein-DNA Interactions with ChEC-seq in Saccharomyces cerevisiae

Genome-wide mapping of protein-DNA interactions is critical for understanding gene regulation, chromatin remodeling, and other chromatin-resident processes. Formaldehyde crosslinking followed by chromatin immunoprecipitation and high-throughput sequencing (X-ChIP-seq) has been used to gain many valuable insights into genome biology. However, X-ChIP-seq has notable limitations linked to crosslinking and sonication. Native ChIP avoids these drawbacks by omitting crosslinking, but often results in poor recovery of chromatin-bound proteins. In addition, all ChIP-based methods are subject to antibody quality considerations. Enzymatic methods for mapping protein-DNA interactions, which involve fusion of a protein of interest to a DNA-modifying enzyme, have also been used to map protein-DNA interactions. We recently combined one such method, chromatin endogenous cleavage (ChEC), with high-throughput sequencing as ChEC-seq. ChEC-seq relies on fusion of a chromatin-associated protein of interest to micrococcal nuclease (MNase) to generate targeted DNA cleavage in the presence of calcium in living cells. ChEC-seq is not based on immunoprecipitation and so circumvents potential concerns with crosslinking, sonication, chromatin solubilization, and antibody quality while providing high resolution mapping with minimal background signal. We envision that ChEC-seq will be a powerful counterpart to ChIP, providing an independent means by which to both validate ChIP-seq findings and discover new insights into genomic regulation.

Genomic mapping of chromatin proteins by using Daminv modification of an FLP-dependent DamID approach

To identify interactions of chromatin proteins with the genome of the cell type of interest that is a part of heterologous tissues and organs of Drosophila, an FLP-dependent DamID approach was recently developed [4], which does not require sorting of cells or nuclei. Here, a modification of this approach, Dam^inv, is described. The modified approach was validated by generating the binding pattern of the LAM protein, a component of the inner membrane of the nuclear envelope, with the genome of glial cells of the Drosophila larval central brains.

An efficient targeted nuclease strategy for high-resolution mapping of DNA binding sites

We describe Cleavage Under Targets and Release Using Nuclease (CUT&RUN), a chromatin profiling strategy in which antibody-targeted controlled cleavage by micrococcal nuclease releases specific protein-DNA complexes into the supernatant for paired-end DNA sequencing. Unlike Chromatin Immunoprecipitation (ChIP), which fragments and solubilizes total chromatin, CUT&RUN is performed in situ, allowing for both quantitative high-resolution chromatin mapping and probing of the local chromatin environment. When applied to yeast and human nuclei, CUT&RUN yielded precise transcription factor profiles while avoiding crosslinking and solubilization issues. CUT&RUN is simple to perform and is inherently robust, with extremely low backgrounds requiring only ~1/10th the sequencing depth as ChIP, making CUT&RUN especially cost-effective for transcription factor and chromatin profiling. When used in conjunction with native ChIP-seq and applied to human CTCF, CUT&RUN mapped directional long range contact sites at high resolution. We conclude that in situ mapping of protein-DNA interactions by CUT&RUN is an attractive alternative to ChIP-seq.

DOI:http://dx.doi.org/10.7554/eLife.21856.001

The DNA in a person’s skin cell will contain the same genes as the DNA in their muscle or brain cells. However, these cells have different identities because different genes are active in skin, muscle and brain cells. Proteins called transcription factors dictate the patterns of gene activation in the different kinds of cells by binding to DNA and switching nearby genes on or off. Transcription factors interact with other proteins such as histones that help to package DNA into a structure known as chromatin. Together, transcription factors, histones and other chromatin-associated proteins determine whether or not nearby genes are active.

Sometimes transcription factors and other chromatin-associated proteins bind to the wrong sites on DNA; this situation can lead to diseases in humans, such as cancer. This is one of the many reasons why researchers are interested in working out where specific DNA-binding proteins are located in different situations. A technique called chromatin immunoprecipitation (or ChIP for short) can be used to achieve this goal, yet despite being one of the most widely used techniques in molecular biology, ChIP is hampered by numerous problems. As such, many researchers are keen to find alternative approaches.

Skene and Henikoff have now developed a new method, called CUT&RUN (which is short for “Cleavage Under Targets & Release Using Nuclease”) to map specific interactions between protein and DNA in a way that overcomes some of the problems with ChIP. In CUT&RUN, unlike in ChIP, the DNA in the starting cells does not need to be broken up first; this means that protein-DNA interactions are more likely to be maintained in their natural state. With CUT&RUN, as in ChIP, a specific antibody identifies the protein of interest. But in CUT&RUN, this antibody binds to the target protein in intact cells and cuts out the DNA that the protein is bound to, releasing the DNA fragment from the cell. This new strategy allows the DNA fragments to be sequenced and identified more efficiently than is currently possible with ChIP.

Skene and Henikoff showed that their new method could more accurately identify where transcription factors bind to DNA from yeast and human cells. CUT&RUN also identified a specific histone that is rarely found in yeast chromatin and the technique can be used with a small number of starting cells. Given the advantages that CUT&RUN offers over ChIP, Skene and Henikoff anticipate that the method will be viewed as a cost-effective and versatile alternative to ChIP. In future, the method could be automated so that multiple analyses can be performed at once.

DOI:http://dx.doi.org/10.7554/eLife.21856.002

Genome-Wide Cell Type-Specific Mapping of In Vivo Chromatin Protein Binding Using an FLP-Inducible DamID System in Drosophila

A thorough study of the genome-wide binding patterns of chromatin proteins is essential for understanding the regulatory mechanisms of genomic processes in eukaryotic nuclei, including DNA replication, transcription, and repair. The DNA adenine methyltransferase identification (DamID) method is a powerful tool to identify genomic binding sites of chromatin proteins. This method does not require fixation of cells and the use of specific antibodies, and has been used to generate genome-wide binding maps of more than a hundred different proteins in Drosophila tissue culture cells. Recent versions of inducible DamID allow performing cell type-specific profiling of chromatin proteins even in small samples of Drosophila tissues that contain heterogeneous cell types. Importantly, with these methods sorting of cells of interest or their nuclei is not necessary as genomic DNA isolated from the whole tissue can be used as an input. Here, I describe in detail an FLP-inducible DamID method, namely generation of suitable transgenic flies, activation of the Dam transgenes by the FLP recombinase, isolation of DNA from small amounts of dissected tissues, and subsequent identification of the DNA binding sites of the chromatin proteins.

Past Roadblocks and New Opportunities in Transcription Factor Network Mapping

One of the principal mechanisms by which cells differentiate and respond to changes in external signals or conditions is by changing the activity levels of transcription factors (TFs). This changes the transcription rates of target genes via the cell's TF network, which ultimately contributes to reconfiguring cellular state. Since microarrays provided our first window into global cellular state, computational biologists have eagerly attacked the problem of mapping TF networks, a key part of the cell's control circuitry. In retrospect, however, steady-state mRNA abundance levels were a poor substitute for TF activity levels and gene transcription rates. Likewise, mapping TF binding through chromatin immunoprecipitation proved less predictive of functional regulation and less amenable to systematic elucidation of complete networks than originally hoped. This review explains these roadblocks and the current, unprecedented blossoming of new experimental techniques built on second-generation sequencing, which hold out the promise of rapid progress in TF network mapping.

iDamIDseq and iDEAR: an improved method and computational pipeline to profile chromatin-binding proteins

DNA adenine methyltransferase identification (DamID) has emerged as an alternative method to profile protein-DNA interactions; however, critical issues limit its widespread applicability. Here, we present iDamIDseq, a protocol that improves specificity and sensitivity by inverting the steps DpnI-DpnII and adding steps that involve a phosphatase and exonuclease. To determine genome-wide protein-DNA interactions efficiently, we present the analysis tool iDEAR (iDamIDseq Enrichment Analysis with R). The combination of DamID and iDEAR permits the establishment of consistent profiles for transcription factors, even in transient assays, as we exemplify using the small teleost medaka (Oryzias latipes). We report that the bacterial Dam-coding sequence induces aberrant splicing when it is used with different promoters to drive tissue-specific expression. Here, we present an optimization of the sequence to avoid this problem. This and our other improvements will allow researchers to use DamID effectively in any organism, in a general or targeted manner.

Summary: Critical improvements to the DamID protocol improve specificity and sensitivity in determining genome-wide protein-DNA interactions in transient or stable transgenic animal lines.

Cell type-specific epigenome profiling using affinity-purified nuclei

The development of a multicellular organism from a single zygote depends on the differentiation of progenitor cells to specialized cell types. The differentiation of these cell types is associated with changes in gene expression and the underlying chromatin landscape. To understand how these processes are regulated, it is desirable to understand how the chromatin features that constitute the epigenome differ between cell types at any given time during development. INTACT, a method for the cell type-specific purification of nuclei that can be used for the isolation of both RNA and chromatin, has emerged as a powerful tool to simultaneously study gene expression and chromatin profiles specifically in cell types of interest. In this review, we focus on the application of INTACT to different model organisms and discuss its potential for profiling cell types in their developmental context.

Identification of Tox chromatin binding properties and downstream targets by DamID-Seq

In recent years, DNA adenine methyltransferase identification (DamID) has emerged as a powerful tool to profile protein-DNA interaction on a genome-wide scale. While DamID has been primarily combined with microarray analyses, which limits the spatial resolution and full potential of this technique, our group was the first to combine DamID with sequencing (DamID-Seq) for characterizing the binding loci and properties of a transcription factor (Tox) (sequencing data available at NCBI's Gene Expression Omnibus under the accession number GSE64240). Our approach was based on the combination and optimization of several bioinformatics tools that are here described in detail. Analysis of Tox proximity to transcriptional start sites, profiling on enhancers and binding motif has allowed us to identify this transcription factor as an important new regulator of neural stem cells differentiation and newborn neurons maturation during mouse cortical development. Here we provide a valuable resource to study the role of Tox as a novel key determinant of mammalian somatic stem cells during development of the nervous and lymphatic system, in which this factor is known to be active, and describe a useful pipeline to perform DamID-Seq analyses for any other transcription factor.