Interplay of pericentromeric genome organization and chromatin landscape regulates the expression of Drosophila melanogaster heterochromatic genes

Genomic context-dependent histone H3K36 methylation by three Drosophila methyltransferases and implications for dedicated chromatin readers

Methylation of histone H3 at lysine 36 (H3K36me3) marks active chromatin. The mark is interpreted by epigenetic readers that assist transcription and safeguard the integrity of the chromatin fiber. The chromodomain protein MSL3 binds H3K36me3 to target X-chromosomal genes in male Drosophila for dosage compensation. The PWWP-domain protein JASPer recruits the JIL1 kinase to active chromatin on all chromosomes. Unexpectedly, depletion of K36me3 had variable, locus-specific effects on the interactions of those readers. This observation motivated a systematic and comprehensive study of K36 methylation in a defined cellular model. Contrasting prevailing models, we found that K36me1, K36me2 and K36me3 each contribute to distinct chromatin states. A gene-centric view of the changing K36 methylation landscape upon depletion of the three methyltransferases Set2, NSD and Ash1 revealed local, context-specific methylation signatures. Set2 catalyzes K36me3 predominantly at transcriptionally active euchromatin. NSD places K36me2/3 at defined loci within pericentric heterochromatin and on weakly transcribed euchromatic genes. Ash1 deposits K36me1 at regions with enhancer signatures. The genome-wide mapping of MSL3 and JASPer suggested that they bind K36me2 in addition to K36me3, which was confirmed by direct affinity measurement. This dual specificity attracts the readers to a broader range of chromosomal locations and increases the robustness of their actions.

Transposable elements as scaffold/matrix attachment regions: shaping organization and functions in genomes

The hierarchical structure of eukaryotic genomes has regulatory layers, one of them being epigenetic "indexing" of the genome that leads to cell-type-specific patterns of gene expression. By establishing loops and defining chromatin domains, cells can achieve coordinated control over multi-locus segments of the genome. This is thought to be achieved using scaffold/matrix attachment regions (S/MARs) that establish structural and functional loops and topologically associating domains (TADs) that define a self-interacting region of the genome. Large-scale genome-wide mapping of S/MARs has begun to uncover these aspects of genome organization. A recent genome-wide study showed the association of transposable elements (TEs) with a significant fraction of S/MARs, suggesting that the multitude of TE-derived repeats constitute a class of anchorage sites of chromatin loops to nuclear architecture. In this study, we provide an insight that TE-driven dispersal of S/MARs has the potential to restructure the chromosomes by creating novel loops and domains. The combination of TEs and S/MARs, as elements that can hop through the genome along with regulatory capabilities, may provide an active mechanism of genome evolution leading to the emergence of novel features in biological systems. The significance is that a genome-wide study mapping developmental S/MARs reveals an intriguing link between these elements and TEs. This article highlights the potential of the TE-S/MAR combination to drive evolution by restructuring and shaping the genome.

Study of the Association of Ouib and Nom with Heterochromatin in Drosophila melanogaster

In Drosophila, a large group of actively transcribed genes is located in pericentromeric heterochromatin. It is assumed that heterochromatic proteins recruit transcription factors to gene promoters. Two proteins, Ouib and Nom, were previously shown to bind to the promoters of the heterochromatic genes nvd and spok. Interestingly, Ouib and Nom are paralogs of the M1BP protein, which binds to the promoters of euchromatic genes. We have shown that, like M1BP, the Quib and Nom proteins bind to CP190, which is involved in the recruitment of transcription complexes to promoters. Unlike heterochromatic proteins, Ouib and Nom do not interact with the major heterochromatic protein HP1a and bind to euchromatic promoters on polytene chromosomes from the larval salivary glands. The results suggest a new mechanism for the recruitment of transcription factors into the heterochromatic compartment of the nucleus.

Multiple Roles of dXNP and dADD1-Drosophila Orthologs of ATRX Chromatin Remodeler

The Drosophila melanogaster dADD1 and dXNP proteins are orthologues of the ADD and SNF2 domains of the vertebrate ATRX (Alpha-Thalassemia with mental Retardation X-related) protein. ATRX plays a role in general molecular processes, such as regulating chromatin status and gene expression, while dADD1 and dXNP have similar functions in the Drosophila genome. Both ATRX and dADD1/dXNP interact with various protein partners and participate in various regulatory complexes. Disruption of ATRX expression in humans leads to the development of α-thalassemia and cancer, especially glioma. However, the mechanisms that allow ATRX to regulate various cellular processes are poorly understood. Studying the functioning of dADD1/dXNP in the Drosophila model may contribute to understanding the mechanisms underlying the multifunctional action of ATRX and its connection with various cellular processes. This review provides a brief overview of the currently available information in mammals and Drosophila regarding the roles of ATRX, dXNP, and dADD1. It discusses possible mechanisms of action of complexes involving these proteins.

Epigenetic Silencing of P-Element Reporter Genes Induced by Transcriptionally Active Domains of Constitutive Heterochromatin in Drosophila melanogaster

Reporter genes inserted via P-element integration into different locations of the Drosophila melanogaster genome have been routinely used to monitor the functional state of chromatin domains. It is commonly thought that P-element-derived reporter genes are subjected to position effect variegation (PEV) when transposed into constitutive heterochromatin because they acquire heterochromatin-like epigenetic modifications that promote silencing. However, sequencing and annotation of the D. melanogaster genome have shown that constitutive heterochromatin is a genetically and molecularly heterogeneous compartment. In fact, in addition to repetitive DNAs, it harbors hundreds of functional genes, together accounting for a significant fraction of its entire genomic territory. Notably, most of these genes are actively transcribed in different developmental stages and tissues, irrespective of their location in heterochromatin. An open question in the genetic and molecular studies on PEV in D. melanogaster is whether functional heterochromatin domains, i.e., heterochromatin harboring active genes, are able to silence reporter genes therein transposed or, on the contrary, can drive their expression. In this work, we provide experimental evidence showing that strong silencing of the Pw⁺ reporters is induced even when they are integrated within or near actively transcribed loci in the pericentric regions of chromosome 2. Interestingly, some Pw⁺ reporters were found insensitive to the action of a known PEV suppressor. Two of them are inserted within Yeti, a gene expressed in the deep heterochromatin of chromosome 2 which carries active chromatin marks. The difference sensitivity to suppressors-exhibited Pw⁺ reporters supports the view that different epigenetic regulators or mechanisms control different regions of heterochromatin. Together, our results suggest that there may be more complexity regarding the molecular mechanisms underlying PEV.

Structural and developmental dynamics of Matrix associated regions in Drosophila melanogaster genome

Background

Eukaryotic genome is compartmentalized into structural and functional domains. One of the concepts of higher order organization of chromatin posits that the DNA is organized in constrained loops that behave as independent functional domains. Nuclear Matrix (NuMat), a ribo-proteinaceous nucleoskeleton, provides the structural basis for this organization. DNA sequences located at base of the loops are known as the Matrix Attachment Regions (MARs). NuMat relates to multiple nuclear processes and is partly cell type specific in composition. It is a biochemically defined structure and several protocols have been used to isolate the NuMat where some of the steps have been critically evaluated. These sequences play an important role in genomic organization it is imperative to know their dynamics during development and differentiation.

Results

Here we look into the dynamics of MARs when the preparation process is varied and during embryonic development of D. melanogaster. A subset of MARs termed as “Core-MARs” present abundantly in pericentromeric heterochromatin, are constant unalterable anchor points as they associate with NuMat through embryonic development and are independent of the isolation procedure. Euchromatic MARs are dynamic and reflect the transcriptomic profile of the cell. New MARs are generated by nuclear stabilization, and during development, mostly at paused RNA polymerase II promoters. Paused Pol II MARs depend on RNA transcripts for NuMat association.

Conclusions

Our data reveals the role of MARs in functionally dynamic nucleus and contributes to the current understanding of nuclear architecture in genomic context.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08944-4.

The Green Valley of Drosophila melanogaster Constitutive Heterochromatin: Protein-Coding Genes Involved in Cell Division Control

Constitutive heterochromatin represents a significant fraction of eukaryotic genomes (10% in Arabidopsis, 20% in humans, 30% in D. melanogaster, and up to 85% in certain nematodes) and shares similar genetic and molecular properties in animal and plant species. Studies conducted over the last few years on D. melanogaster and other organisms led to the discovery of several functions associated with constitutive heterochromatin. This made it possible to revise the concept that this ubiquitous genomic territory is incompatible with gene expression. The aim of this review is to focus the attention on a group of protein-coding genes resident in D. melanogaster constitutive of heterochromatin, which are implicated in different steps of cell division.

The regulation and potential functions of intronic satellite DNA

Satellite DNAs are arrays of tandem repeats found in the eukaryotic genome. They are mainly found in pericentromeric heterochromatin and have been believed to be mostly inert, leading satellite DNAs to be erroneously regarded as junk. Recent studies have started to elucidate the function of satellite DNA, yet little is known about the peculiar case where satellite DNA is found within the introns of protein coding genes, resulting in incredibly large introns, a phenomenon termed intron gigantism. Studies in Drosophila demonstrated that satellite DNA-containing introns are transcribed with the gene and require specialized mechanisms to overcome the burdens imposed by the extremely long stretches of repetitive DNA. Whether intron gigantism confers any benefit or serves any functional purpose for cells and/or organisms remains elusive. Here we review our current understanding of intron gigantism: where it is found, the challenges it imposes, how it is regulated and what purpose it may serve.

The role of insulators and transcription in 3D chromatin organization of flies

The DNA in many organisms, including humans, is shown to be organized in topologically associating domains (TADs). In Drosophila, several architectural proteins are enriched at TAD borders, but it is still unclear whether these proteins play a functional role in the formation and maintenance of TADs. Here, we show that depletion of BEAF-32, Cp190, Chro, and Dref leads to changes in TAD organization and chromatin loops. Their depletion predominantly affects TAD borders located in regions moderately enriched in repressive modifications and depleted in active ones, whereas TAD borders located in euchromatin are resilient to these knockdowns. Furthermore, transcriptomic data has revealed hundreds of genes displaying differential expression in these knockdowns and showed that the majority of differentially expressed genes are located within reorganized TADs. Our work identifies a novel and functional role for architectural proteins at TAD borders in Drosophila and a link between TAD reorganization and subsequent changes in gene expression.

Uncovering Regulators of Heterochromatin Mediated Silencing Using a Zebrafish Transgenic Reporter

Heterochromatin formation and maintenance is critical for the repression of transcription from repetitive sequences. However, in vivo tools for monitoring heterochromatin mediated repression of repeats in the context of vertebrate development have been lacking. Here we demonstrate that a large concatemeric transgene integration containing the dsRed fluorescent reporter under the control of a ubiquitous promoter recapitulates molecular hallmarks of heterochromatic silencing, and that expression from the transgene array can be reactivated by depletion of known regulators of heterochromatin. We then use this reporter to identify a previously unappreciated role for the zebrafish NSD1 orthologs, Nsd1a and Nsd1b, in promoting heterochromatin mediated repression. Our results provide proof-principle that this transgenic reporter line can be used to rapidly identify genes with potential roles in heterochromatic silencing in the context of a live, vertebrate organism.

H3K9me2 genome-wide distribution in the holocentric insect Spodoptera frugiperda (Lepidoptera: Noctuidae)

BACKGROUND

Eukaryotic genomes are packaged by Histone proteins in a structure called chromatin. There are different chromatin types. Euchromatin is typically associated with decondensed, transcriptionally active regions and heterochromatin to more condensed regions of the chromosomes. Methylation of Lysine 9 of Histone H3 (H3K9me) is a conserved biochemical marker of heterochromatin. In many organisms, heterochromatin is usually localized at telomeric as well as pericentromeric regions but can also be found at interstitial chromosomal loci. This distribution may vary in different species depending on their general chromosomal organization. Holocentric species such as Spodoptera frugiperda (Lepidoptera: Noctuidae) possess dispersed centromeres instead of a monocentric one and thus no observable pericentromeric compartment. To identify the localization of heterochromatin in such species we performed ChIP-Seq experiments and analyzed the distribution of the heterochromatin marker H3K9me2 in the Sf9 cell line and whole 4th instar larvae (L4) in relation to RNA-Seq data.

RESULTS

In both samples we measured an enrichment of H3K9me2 at the (sub) telomeres, rDNA loci, and satellite DNA sequences, which could represent dispersed centromeric regions. We also observed that density of H3K9me2 is positively correlated with transposable elements and protein-coding genes. But contrary to most model organisms, H3K9me2 density is not correlated with transcriptional repression.

CONCLUSION

This is the first genome-wide ChIP-Seq analysis conducted in S. frugiperda for H3K9me2. Compared to model organisms, this mark is found in expected chromosomal compartments such as rDNA and telomeres. However, it is also localized at numerous dispersed regions, instead of the well described large pericentromeric domains, indicating that H3K9me2 might not represent a classical heterochromatin marker in Lepidoptera. (242 words).

Multi-scale dynamics of heterochromatin repair

Studies across different organisms show that nuclear architecture and dynamics play central roles in different aspects of homologous recombination (HR) repair. Here we review the most recent discoveries in this field, ranging from directed motions mediating relocalization pathways, to global chromatin mobilization, local DNA looping, and changes in repair focus properties associated with clustering and phase separation. We discuss how these dynamics work in different contexts, including molecular mechanisms and regulatory pathways involved. We specifically highlight how they function in pericentromeric heterochromatin, which presents a unique environment for HR repair given the abundance of repeated DNA sequences prone to aberrant recombination, the 'silent' chromatin state, and the phase separation characterizing this domain.

The Organization of Pericentromeric Heterochromatin in Polytene Chromosome 3 of the Drosophilamelanogaster Line with the Rif11; SuURES Su(var)3-906 Mutations Suppressing Underreplication

Although heterochromatin makes up 40% of the Drosophila melanogaster genome, its organization remains little explored, especially in polytene chromosomes, as it is virtually not represented in them due to underreplication. Two all-new approaches were used in this work: (i) with the use of a newly synthesized Drosophila line that carries three mutations, Rif11, SuURES and Su(var)3-906, suppressing the underreplication of heterochromatic regions, we obtained their fullest representation in polytene chromosomes and described their structure; (ii) 20 DNA fragments with known positions on the physical map as well as molecular genetic features of the genome (gene density, histone marks, heterochromatin proteins, origin recognition complex proteins, replication timing sites and satellite DNAs) were mapped in the newly polytenized heterochromatin using FISH and bioinformatics data. The borders of the heterochromatic regions and variations in their positions on arm 3L have been determined for the first time. The newly polytenized heterochromatic material exhibits two main types of morphology: a banding pattern (locations of genes and short satellites) and reticular chromatin (locations of large blocks of satellite DNA). The locations of the banding and reticular polytene heterochromatin was determined on the physical map.