E(y)2/Sus1 is required for blocking PRE silencing by the Wari insulator in Drosophila melanogaster

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.

Context-dependent role of Pho binding sites in Polycomb complex recruitment in Drosophila

Polycomb group (PcG) proteins maintain the silenced state of key developmental genes, but how these proteins are recruited to specific regions of the genome is still not completely understood. In Drosophila, PcG proteins are recruited to Polycomb response elements (PREs) comprised of a flexible array of sites for sequence-specific DNA binding proteins, "PcG recruiters," including Pho, Spps, Cg, and GAF. Pho is thought to play a central role in PcG recruitment. Early data showed that mutation of Pho binding sites in PREs in transgenes abrogated the ability of those PREs to repress gene expression. In contrast, genome-wide experiments in pho mutants or by Pho knockdown showed that PcG proteins can bind to PREs in the absence of Pho. Here, we directly addressed the importance of Pho binding sites in 2 engrailed (en) PREs at the endogenous locus and in transgenes. Our results show that Pho binding sites are required for PRE activity in transgenes with a single PRE. In a transgene, 2 PREs together lead to stronger, more stable repression and confer some resistance to the loss of Pho binding sites. Making the same mutation in Pho binding sites has little effect on PcG-protein binding at the endogenous en gene. Overall, our data support the model that Pho is important for PcG binding but emphasize how multiple PREs and chromatin environment increase the ability of PREs to function in the absence of Pho. This supports the view that multiple mechanisms contribute to PcG recruitment in Drosophila.

No apparent role for the Wari insulator in transcriptional regulation of the endogenous white gene of Drosophila melanogaster

Chromatin insulators have been proposed to play an important role in chromosome organization and local regulatory interactions. In Drosophila , one of these insulators is known as Wari. It is located immediately downstream of the 3' end of the white transcription unit. Wari has been proposed to interact with the white promoter region, thereby facilitating recycling of the RNA polymerase machinery. We have tested this model by deleting the Wari insulator at the endogenous white locus and could not detect a significant effect on eye pigmentation.

Boundaries potentiate polycomb response element-mediated silencing

Background

Epigenetic memory plays a critical role in the establishment and maintenance of cell identities in multicellular organisms. Polycomb and trithorax group (PcG and TrxG) proteins are responsible for epigenetic memory, and in flies, they are recruited to specialized DNA regulatory elements termed polycomb response elements (PREs). Previous transgene studies have shown that PREs can silence reporter genes outside of their normal context, often by pairing sensitive (PSS) mechanism; however, their silencing activity is non-autonomous and depends upon the surrounding chromatin context. It is not known why PRE activity depends on the local environment or what outside factors can induce silencing.

Results

Using an attP system in Drosophila, we find that the so-called neutral chromatin environments vary substantially in their ability to support the silencing activity of the well-characterized bxdPRE. In refractory chromosomal contexts, factors required for PcG-silencing are unable to gain access to the PRE. Silencing activity can be rescued by linking the bxdPRE to a boundary element (insulator). When placed next to the PRE, the boundaries induce an alteration in chromatin structure enabling factors critical for PcG silencing to gain access to the bxdPRE. When placed at a distance from the bxdPRE, boundaries induce PSS by bringing the bxdPREs on each homolog in close proximity.

Conclusion

This proof-of-concept study demonstrates that the repressing activity of PREs can be induced or enhanced by nearby boundary elements.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01047-8.

The Ubx Polycomb response element bypasses an unpaired Fab-8 insulator via cis transvection in Drosophila

Chromatin insulators or boundary elements protect genes from regulatory activities from neighboring genes or chromatin domains. In the Drosophila Abdominal-B (Abd-B) locus, the deletion of such elements, such as Frontabdominal-7 (Fab-7) or Fab-8 led to dominant gain of function phenotypes, presumably due to the loss of chromatin barriers. Homologous chromosomes are paired in Drosophila, creating a number of pairing dependent phenomena including transvection, and whether transvection may affect the function of Polycomb response elements (PREs) and thus contribute to the phenotypes are not known. Here, we studied the chromatin barrier activity of Fab-8 and how it is affected by the zygosity of the transgene, and found that Fab-8 is able to block the silencing effect of the Ubx PRE on the DsRed reporter gene in a CTCF binding sites dependent manner. However, the blocking also depends on the zygosity of the transgene in that the barrier activity is present when the transgene is homozygous, but absent when the transgene is heterozygous. To analyze this effect, we performed chromatin immunoprecipitation and quantitative PCR (ChIP-qPCR) experiments on homozygous transgenic embryos, and found that H3K27me3 and H3K9me3 marks are restricted by Fab-8, but they spread beyond Fab-8 into the DsRed gene when the two CTCF binding sites within Fab-8 were mutated. Consistent with this, the mutation reduced H3K4me3 and RNA Pol II binding to the DsRed gene, and consequently, DsRed expression. Importantly, in heterozygous embryos, Fab-8 is unable to prevent the spread of H3K27me3 and H3K9me3 marks from crossing Fab-8 into DsRed, suggesting an insulator bypass. These results suggest that in the Abd-B locus, deletion of the insulator in one copy of the chromosome could lead to the loss of insulator activity on the homologous chromosome, and in other loci where chromosomal deletion created hemizygous regions of the genome, the chromatin barrier could be compromised. This study highlights a role of homologous chromosome pairing in the regulation of gene expression in the Drosophila genome.

EAST affects the activity of Su(Hw) insulators by two different mechanisms in Drosophila melanogaster

Recent data suggest that insulators organize chromatin architecture in the nucleus. The best characterized Drosophila insulator, found in the gypsy retrotransposon, contains 12 binding sites for the Su(Hw) protein. Enhancer blocking, along with Su(Hw), requires BTB/POZ domain proteins, Mod(mdg4)-67.2 and CP190. Inactivation of Mod(mdg4)-67.2 leads to a direct repression of the yellow gene promoter by the gypsy insulator. Here, we have shown that such repression is regulated by the level of the EAST protein, which is an essential component of the interchromatin compartment. Deletion of the EAST C-terminal domain suppresses Su(Hw)-mediated repression. Partial inactivation of EAST by mutations in the east gene suppresses the enhancer-blocking activity of the gypsy insulator. The binding of insulator proteins to chromatin is highly sensitive to the level of EAST expression. These results suggest that EAST, one of the main components of the interchromatin compartment, can regulate the activity of chromatin insulators.

EAST Organizes Drosophila Insulator Proteins in the Interchromosomal Nuclear Compartment and Modulates CP190 Binding to Chromatin

Recent data suggest that insulators organize chromatin architecture in the nucleus. The best studied Drosophila insulator proteins, dCTCF (a homolog of the vertebrate insulator protein CTCF) and Su(Hw), are DNA-binding zinc finger proteins. Different isoforms of the BTB-containing protein Mod(mdg4) interact with Su(Hw) and dCTCF. The CP190 protein is a cofactor for the dCTCF and Su(Hw) insulators. CP190 is required for the functional activity of insulator proteins and is involved in the aggregation of the insulator proteins into specific structures named nuclear speckles. Here, we have shown that the nuclear distribution of CP190 is dependent on the level of EAST protein, an essential component of the interchromatin compartment. EAST interacts with CP190 and Mod(mdg4)-67.2 proteins in vitro and in vivo. Over-expression of EAST in S2 cells leads to an extrusion of the CP190 from the insulator bodies containing Su(Hw), Mod(mdg4)-67.2, and dCTCF. In consistent with the role of the insulator bodies in assembly of protein complexes, EAST over-expression led to a striking decrease of the CP190 binding with the dCTCF and Su(Hw) dependent insulators and promoters. These results suggest that EAST is involved in the regulation of CP190 nuclear localization.

Functional Requirements for Fab-7 Boundary Activity in the Bithorax Complex

Chromatin boundaries are architectural elements that determine the three-dimensional folding of the chromatin fiber and organize the chromosome into independent units of genetic activity. The Fab-7 boundary from the Drosophila bithorax complex (BX-C) is required for the parasegment-specific expression of the Abd-B gene. We have used a replacement strategy to identify sequences that are necessary and sufficient for Fab-7 boundary function in the BX-C. Fab-7 boundary activity is known to depend on factors that are stage specific, and we describe a novel ∼700-kDa complex, the late boundary complex (LBC), that binds to Fab-7 sequences that have insulator functions in late embryos and adults. We show that the LBC is enriched in nuclear extracts from late, but not early, embryos and that it contains three insulator proteins, GAF, Mod(mdg4), and E(y)2. Its DNA binding properties are unusual in that it requires a minimal sequence of >65 bp; however, other than a GAGA motif, the three Fab-7 LBC recognition elements display few sequence similarities. Finally, we show that mutations which abrogate LBC binding in vitro inactivate the Fab-7 boundary in the BX-C.

The Drosophila eve insulator Homie promotes eve expression and protects the adjacent gene from repression by polycomb spreading

Insulators can block the action of enhancers on promoters and the spreading of repressive chromatin, as well as facilitating specific enhancer-promoter interactions. However, recent studies have called into question whether the activities ascribed to insulators in model transgene assays actually reflect their functions in the genome. The Drosophila even skipped (eve) gene is a Polycomb (Pc) domain with a Pc-group response element (PRE) at one end, flanked by an insulator, an arrangement also seen in other genes. Here, we show that this insulator has three major functions. It blocks the spreading of the eve Pc domain, preventing repression of the adjacent gene, TER94. It prevents activation of TER94 by eve regulatory DNA. It also facilitates normal eve expression. When Homie is deleted in the context of a large transgene that mimics both eve and TER94 regulation, TER94 is repressed. This repression depends on the eve PRE. Ubiquitous TER94 expression is “replaced” by expression in an eve pattern when Homie is deleted, and this effect is reversed when the PRE is also removed. Repression of TER94 is attributable to spreading of the eve Pc domain into the TER94 locus, accompanied by an increase in histone H3 trimethylation at lysine 27. Other PREs can functionally replace the eve PRE, and other insulators can block PRE-dependent repression in this context. The full activity of the eve promoter is also dependent on Homie, and other insulators can promote normal eve enhancer-promoter communication. Our data suggest that this is not due to preventing promoter competition, but is likely the result of the insulator organizing a chromosomal conformation favorable to normal enhancer-promoter interactions. Thus, insulator activities in a native context include enhancer blocking and enhancer-promoter facilitation, as well as preventing the spread of repressive chromatin.

Insulators are specialized DNA elements that can separate the genome into functional units. Most of the current thinking about these elements comes from studies done with model transgenes. Studies of insulators within the specialized Hox gene complexes have suggested that model transgenes can reflect the normal functions of these elements in their native context. However, recent genome-wide studies have called this into question. This work analyzes the native function of an insulator that resides between the Drosophila genes eve and TER94, which are expressed in very different patterns. Also, the eve gene is a Polycomb (Pc) domain, a specialized type of chromatin that is found in many places throughout the genome. We show that this insulator has three major functions. It blocks the spreading of the eve Pc domain, preventing repression of TER94. It prevents activation of TER94 by eve regulatory DNA. It also facilitates normal eve expression. Each of these activities are consistent with those seen with model transgenes, and other known insulators can provide these functions in this context. This work provides a novel and convincing example of the normal role of insulators in regulating the eukaryotic genome, as well as providing insights into their mechanisms of action.

Functional sub-division of the Drosophila genome via chromatin looping: the emerging importance of CP190

Insulators help in organizing the eukaryotic genomes into physically and functionally autonomous regions through the formation of chromatin loops. Recent findings in Drosophila and vertebrates suggest that insulators anchor multiple loci through long-distance interactions which may be mechanistically linked to insulator function. Important to such processes in Drosophila is CP190, a common co-factor of insulator complexes. CP190 is also known to associate with the nuclear matrix, components of the RNAi machinery, active promoters and borders of the repressive chromatin domains. Although CP190 plays a pivotal role in insulator function in Drosophila, vertebrates lack a probable functional equivalent of CP190 and employ CTCF as the major factor to carry out insulator function/chromatin looping. In this review, we discuss the emerging role of CP190 in tethering genome, specifically in the perspective of insulator function in Drosophila. Future studies aiming genome-wide role of CP190 in chromatin looping is likely to give important insights into the mechanism of genome organization.

mRNA export and gene expression: the SAGA-TREX-2 connection

In the gene expression field, different steps have been traditionally viewed as discrete and unconnected events. Nowadays, genetic and functional studies support the model of a coupled network of physical and functional connections to carry out mRNA biogenesis. Gene expression is a coordinated process that comprises different linked steps like transcription, RNA processing, export to the cytoplasm, translation and degradation of mRNAs. Its regulation is essential for cellular survival and can occur at many different levels. Transcription is the central function that occurs in the nucleus, and RNAPII plays an essential role in mRNA biogenesis. During transcription, nascent mRNA is associated with the mRNA-binding proteins involved in processing and export of the mRNA particle. Cells have developed a network of multi-protein complexes whose functions regulate the different factors involved both temporally and spatially. This coupling mechanism acts as a quality control to solve some of the organization problems of gene expression in vivo, where all the factors implicated ensure that mRNAs are ready to be exported and translated. In this review, we focus on the functional coupling of gene transcription and mRNA export, and place particular emphasis on the relationship between the NPC-associated complex, TREX2, and the transcription co-activator, SAGA. We have pinpointed the experimental evidence for Sus1's roles in transcription initiation, transcription elongation and mRNA export. In addition, we have reviewed other NPC-related processes such as gene gating to the nuclear envelope, the chromatin structure and the cellular context in which these processes take place. This article is part of a Special Issue entitled: Nuclear Transport and RNA Processing.

Insulators form gene loops by interacting with promoters in Drosophila

Chromatin insulators are regulatory elements involved in the modulation of enhancer-promoter communication. The 1A2 and Wari insulators are located immediately downstream of the Drosophila yellow and white genes, respectively. Using an assay based on the yeast GAL4 activator, we have found that both insulators are able to interact with their target promoters in transgenic lines, forming gene loops. The existence of an insulator-promoter loop is confirmed by the fact that insulator proteins could be detected on the promoter only in the presence of an insulator in the transgene. The upstream promoter regions, which are required for long-distance stimulation by enhancers, are not essential for promoter-insulator interactions. Both insulators support basal activity of the yellow and white promoters in eyes. Thus, the ability of insulators to interact with promoters might play an important role in the regulation of basal gene transcription.