MeCP2 binds to methylated DNA independently of phase separation and heterochromatin organisation.

Correlative evidence has suggested that the methyl-CpG-binding protein MeCP2 contributes to the formation of heterochromatin condensates via liquid-liquid phase separation. This interpretation has been reinforced by the observation that heterochromatin, DNA methylation and MeCP2 co-localise within prominent foci in mouse cells. The findings presented here revise this view. MeCP2 localisation is independent of heterochromatin as MeCP2 foci persist even when heterochromatin organisation is disrupted. Additionally, MeCP2 foci fail to show hallmarks of phase separation in live cells. Importantly, we find that mouse cellular models are highly atypical as MeCP2 distribution is diffuse in most mammalian species, including humans. Notably, MeCP2 foci are absent in Mus spretus which is a mouse subspecies lacking methylated satellite DNA repeats. We conclude that MeCP2 has no intrinsic tendency to form condensates and its localisation is independent of heterochromatin. Instead, the distribution of MeCP2 in the nucleus is primarily determined by global DNA methylation patterns.

Loss of H3K9 trimethylation alters chromosome compaction and transcription factor retention during mitosis.

Recent studies have shown that repressive chromatin machinery, including DNA methyltransferases and polycomb repressor complexes, binds to chromosomes throughout mitosis and their depletion results in increased chromosome size. In the present study, we show that enzymes that catalyze H3K9 methylation, such as Suv39h1, Suv39h2, G9a and Glp, are also retained on mitotic chromosomes. Surprisingly, however, mutants lacking histone 3 lysine 9 trimethylation (H3K9me3) have unusually small and compact mitotic chromosomes associated with increased histone H3 phospho Ser10 (H3S10ph) and H3K27me3 levels. Chromosome size and centromere compaction in these mutants were rescued by providing exogenous first protein lysine methyltransferase Suv39h1 or inhibiting Ezh2 activity. Quantitative proteomic comparisons of native mitotic chromosomes isolated from wild-type versus Suv39h1/Suv39h2 double-null mouse embryonic stem cells revealed that H3K9me3 was essential for the efficient retention of bookmarking factors such as Esrrb. These results highlight an unexpected role for repressive heterochromatin domains in preserving transcription factor binding through mitosis and underscore the importance of H3K9me3 for sustaining chromosome architecture and epigenetic memory during cell division.

Foxd3 controls heterochromatin-mediated repression of repeat elements and 2-cell state transcription.

Repeat element transcription plays a vital role in early embryonic development. The expression of repeats such as MERVL characterises mouse embryos at the 2-cell stage and defines a 2-cell-like cell (2CLC) population in a mouse embryonic stem cell culture. Repeat element sequences contain binding sites for numerous transcription factors. We identify the forkhead domain transcription factor FOXD3 as a regulator of major satellite repeats and MERVL transcription in mouse embryonic stem cells. FOXD3 binds to and recruits the histone methyltransferase SUV39H1 to MERVL and major satellite repeats, consequentially repressing the transcription of these repeats by the establishment of the H3K9me3 heterochromatin modification. Notably, depletion of FOXD3 leads to the de-repression of MERVL and major satellite repeats as well as a subset of genes expressed in the 2-cell state, shifting the balance between the stem cell and 2-cell-like population in culture. Thus, FOXD3 acts as a negative regulator of repeat transcription, ascribing a novel function to this transcription factor.

Complete loss of H3K9 methylation dissolves mouse heterochromatin organization.

Histone H3 lysine 9 (H3K9) methylation is a central epigenetic modification that defines heterochromatin from unicellular to multicellular organisms. In mammalian cells, H3K9 methylation can be catalyzed by at least six distinct SET domain enzymes: Suv39h1/Suv39h2, Eset1/Eset2 and G9a/Glp. We used mouse embryonic fibroblasts (MEFs) with a conditional mutation for Eset1 and introduced progressive deletions for the other SET domain genes by CRISPR/Cas9 technology. Compound mutant MEFs for all six SET domain lysine methyltransferase (KMT) genes lack all H3K9 methylation states, derepress nearly all families of repeat elements and display genomic instabilities. Strikingly, the 6KO H3K9 KMT MEF cells no longer maintain heterochromatin organization and have lost electron-dense heterochromatin. This is a compelling analysis of H3K9 methylation-deficient mammalian chromatin and reveals a definitive function for H3K9 methylation in protecting heterochromatin organization and genome integrity.

Human DICER helicase domain recruits PKR and modulates its antiviral activity.

The antiviral innate immune response mainly involves type I interferon (IFN) in mammalian cells. The contribution of the RNA silencing machinery remains to be established, but several recent studies indicate that the ribonuclease DICER can generate viral siRNAs in specific conditions. It has also been proposed that type I IFN and RNA silencing could be mutually exclusive antiviral responses. In order to decipher the implication of DICER during infection of human cells with alphaviruses such as the Sindbis virus and Semliki forest virus, we determined its interactome by proteomics analysis. We show that DICER specifically interacts with several double-stranded RNA binding proteins and RNA helicases during viral infection. In particular, proteins such as DHX9, ADAR-1 and the protein kinase RNA-activated (PKR) are enriched with DICER in virus-infected cells. We demonstrate that the helicase domain of DICER is essential for this interaction and that its deletion confers antiviral properties to this protein in an RNAi-independent, PKR-dependent, manner.

Prevention of dsRNA-induced interferon signaling by AGO1x is linked to breast cancer cell proliferation.

Translational readthrough, i.e., elongation of polypeptide chains beyond the stop codon, was initially reported for viral RNA, but later found also on eukaryotic transcripts, resulting in proteome diversification and protein-level modulation. Here, we report that AGO1x, an evolutionarily conserved translational readthrough isoform of Argonaute 1, is generated in highly proliferative breast cancer cells, where it curbs accumulation of double-stranded RNAs (dsRNAs) and consequent induction of interferon responses and apoptosis. In contrast to other mammalian Argonaute protein family members with primarily cytoplasmic functions, AGO1x exhibits nuclear localization in the vicinity of nucleoli. We identify AGO1x interaction with the polyribonucleotide nucleotidyltransferase 1 (PNPT1) and show that the depletion of this protein further augments dsRNA accumulation. Our study thus uncovers a novel function of an Argonaute protein in buffering the endogenous dsRNA-induced interferon responses, different than the canonical function of AGO proteins in the miRNA effector pathway. As AGO1x expression is tightly linked to breast cancer cell proliferation, our study thus suggests a new direction for limiting tumor growth.

H3K9me3-heterochromatin loss at protein-coding genes enables developmental lineage specification.

Gene silencing by chromatin compaction is integral to establishing and maintaining cell fates. Trimethylated histone 3 lysine 9 (H3K9me3)-marked heterochromatin is reduced in embryonic stem cells compared to differentiated cells. However, the establishment and dynamics of closed regions of chromatin at protein-coding genes, in embryologic development, remain elusive. We developed an antibody-independent method to isolate and map compacted heterochromatin from low-cell number samples. We discovered high levels of compacted heterochromatin, H3K9me3-decorated, at protein-coding genes in early, uncommitted cells at the germ-layer stage, undergoing profound rearrangements and reduction upon differentiation, concomitant with cell type-specific gene expression. Perturbation of the three H3K9me3-related methyltransferases revealed a pivotal role for H3K9me3 heterochromatin during lineage commitment at the onset of organogenesis and for lineage fidelity maintenance.

Characterization of DCL4 missense alleles provides insights into its ability to process distinct classes of dsRNA substrates.

In the model plant Arabidopsis thaliana, four Dicer-like proteins (DCL1-4) mediate the production of various classes of small RNAs (sRNAs). Among these four proteins, DCL4 is by far the most versatile RNaseIII-like enzyme, and previously identified dcl4 missense alleles were shown to uncouple the production of the various classes of DCL4-dependent sRNAs. Yet little is known about the molecular mechanism behind this uncoupling. Here, by studying the subcellular localization, interactome and binding to the sRNA precursors of three distinct dcl4 missense alleles, we simultaneously highlight the absolute requirement of a specific residue in the helicase domain for the efficient production of all DCL4-dependent sRNAs, and identify, within the PAZ domain, an important determinant of DCL4 versatility that is mandatory for the efficient processing of intramolecular fold-back double-stranded RNA (dsRNA) precursors, but that is dispensable for the production of small interfering RNAs (siRNAs) from RDR-dependent dsRNA susbtrates. This study not only provides insights into the DCL4 mode of action, but also delineates interesting tools to further study the complexity of RNA silencing pathways in plants, and possibly other organisms.

[To have and have not, RNA interference as an antiviral defense system in mammals]. / En avoir ou pas, l'interférence par l'ARN comme défense antivirale chez les mammifères.

RNA silencing is a small RNA based mechanism regulating gene expression and involved in many biological processes in most eukaryotes. In plants, nematodes and arthropods, this mechanism participates to antiviral defense. In mammals, although the RNA silencing machinery is present and needed for the microRNA pathway, its importance as an antiviral defense is still debated. In recent years, several studies have attempted to answer to the question of whether RNA silencing as an antiviral pathway is retained in mammals. However, these studies did not provide a clear answer yet. In this review, we will present the arguments for and against a relevant antiviral role of RNA interference (RNAi) in mammals, by discussing examples of active and functional mammalian antiviral RNAi in specific cell types and/or in specific conditions.

New DRB complexes for new DRB functions in plants.

Double-stranded RNA binding (DRB) proteins are generally considered as promoting cofactors of Dicer or Dicer-like (DCL) proteins that ensure efficient and precise production of small RNAs, the sequence-specificity guide of RNA silencing processes in both plants and animals. However, the characterization of a new clade of DRB proteins in Arabidopsis has recently challenged this view by showing that DRBs can also act as potent inhibitors of DCL processing. This is achieved through sequestration of a specific class of small RNA precursors, the endogenous inverted-repeat (endoIR) dsRNAs, thereby selectively preventing production of their associated small RNAs, the endoIR-siRNAs. Here, we concisely summarize the main findings obtained from the characterization of these new DRB proteins and discuss how the existence of such complexes can support a potential, yet still elusive, biological function of plant endoIR-siRNAs.

A specific dsRNA-binding protein complex selectively sequesters endogenous inverted-repeat siRNA precursors and inhibits their processing.

In plants, several dsRNA-binding proteins (DRBs) have been shown to play important roles in various RNA silencing pathways, mostly by promoting the efficiency and/or accuracy of Dicer-like proteins (DCL)-mediated small RNA production. Among the DRBs encoded by the Arabidopsis genome, we recently identified DRB7.2 whose function in RNA silencing was unknown. Here, we show that DRB7.2 is specifically involved in siRNA production from endogenous inverted-repeat (endoIR) loci. This function requires its interacting partner DRB4, the main cofactor of DCL4 and is achieved through specific sequestration of endoIR dsRNA precursors, thereby repressing their access and processing by the siRNA-generating DCLs. The present study also provides multiple lines of evidence showing that DRB4 is partitioned into, at least, two distinct cellular pools fulfilling different functions, through mutually exclusive binding with either DCL4 or DRB7.2. Collectively, these findings revealed that plants have evolved a specific DRB complex that modulates selectively the production of endoIR-siRNAs. The existence of such a complex and its implication regarding the still elusive biological function of plant endoIR-siRNA will be discussed.

Evolutionary history of double-stranded RNA binding proteins in plants: identification of new cofactors involved in easiRNA biogenesis.

In this work, we retrace the evolutionary history of plant double-stranded RNA binding proteins (DRBs), a group of non-catalytic factors containing one or more double-stranded RNA binding motif (dsRBM) that play important roles in small RNA biogenesis and functions. Using a phylogenetic approach, we show that multiple dsRBM DRBs are systematically composed of two different types of dsRBMs evolving under different constraints and likely fulfilling complementary functions. In vascular plants, four distinct clades of multiple dsRBM DRBs are always present with the exception of Brassicaceae species, that do not possess member of the newly identified clade we named DRB6. We also identified a second new and highly conserved DRB family (we named DRB7) whose members possess a single dsRBM that shows concerted evolution with the most C-terminal dsRBM domain of the Dicer-like 4 (DCL4) proteins. Using a BiFC approach, we observed that Arabidopsis thaliana DRB7.2 (AtDRB7.2) can directly interact with AtDRB4 but not with AtDCL4 and we provide evidence that both AtDRB7.2 and AtDRB4 participate in the epigenetically activated siRNAs pathway.

Convergent evolution of complex regulatory landscapes and pleiotropy at Hox loci.

Hox genes are required during the morphogenesis of both vertebrate digits and external genitals. We investigated whether transcription in such distinct contexts involves a shared enhancer-containing landscape. We show that the same regulatory topology is used, yet with some tissue-specific enhancer-promoter interactions, suggesting the hijacking of a regulatory backbone from one context to the other. In addition, comparable organizations are observed at both HoxA and HoxD clusters, which separated through genome duplication in an ancestral invertebrate animal. We propose that this convergent regulatory evolution was triggered by the preexistence of some chromatin architecture, thus facilitating the subsequent recruitment of the appropriate transcription factors. Such regulatory topologies may have both favored and constrained the evolution of pleiotropic developmental loci in vertebrates.

Hox gene regulation and timing in embryogenesis.

Hox genes are critical regulators of embryonic development in bilaterian animals. They exhibit a unique mode of transcriptional regulation where the position of the genes along the chromosome corresponds to the time and place of their expression during development. The sequential temporal activation of these genes in the primitive streak helps determining their subsequent pattern of expression along the anterior-posterior axis of the embryo, yet the precise correspondence between these two collinear processes is not fully understood. In addition, vertebrate Hox genes evolved similar modes of regulation along secondary body axes, such as the developing limbs. We review the current understanding of the mechanisms operating during activation, maintenance and silencing of Hox gene expression in these various contexts, and discuss the evolutionary significance of their genomic organization.

RNA silencing is resistant to low-temperature in grapevine.

RNA silencing is a natural defence mechanism against viruses in plants, and transgenes expressing viral RNA-derived sequences were previously shown to confer silencing-based enhanced resistance against the cognate virus in several species. However, RNA silencing was shown to dysfunction at low temperatures in several species, questioning the relevance of this strategy in perennial plants such as grapevines, which are often exposed to low temperatures during the winter season. Here, we show that inverted-repeat (IR) constructs trigger a highly efficient silencing reaction in all somatic tissues in grapevines. Similarly to other plant species, IR-derived siRNAs trigger production of secondary transitive siRNAs. However, and in sharp contrast to other species tested to date where RNA silencing is hindered at low temperature, this process remained active in grapevine cultivated at 4°C. Consistently, siRNA levels remained steady in grapevines cultivated between 26°C and 4°C, whereas they are severely decreased in Arabidopsis grown at 15°C and almost undetectable at 4°C. Altogether, these results demonstrate that RNA silencing operates in grapevine in a conserved manner but is resistant to far lower temperatures than ever described in other species.

Multiple enhancers regulate Hoxd genes and the Hotdog LncRNA during cecum budding.

Hox genes are required for the development of the intestinal cecum, a major organ of plant-eating species. We have analyzed the transcriptional regulation of Hoxd genes in cecal buds and show that they are controlled by a series of enhancers located in a gene desert flanking the HoxD cluster. The start site of two opposite long noncoding RNAs (lncRNAs), Hotdog and Twin of Hotdog, selectively contacts the expressed Hoxd genes in the framework of a topological domain, coinciding with robust transcription of these genes during cecum budding. Both lncRNAs are specifically transcribed in the cecum, albeit bearing no detectable function in trans. Hedgehogs have kept this regulatory potential despite the absence of the cecum, suggesting that these mechanisms are used in other developmental situations. In this context, we discuss the implementation of a common "budding toolkit" between the cecum and the limbs.

Transgene- and locus-dependent imprinting reveals allele-specific chromosome conformations.

When positioned into the integrin α-6 gene, an Hoxd9lacZ reporter transgene displayed parental imprinting in mouse embryos. While the expression from the paternal allele was comparable with patterns seen for the same transgene when present at the neighboring HoxD locus, almost no signal was scored at this integration site when the transgene was inherited from the mother, although the Itga6 locus itself is not imprinted. The transgene exhibited maternal allele-specific DNA hypermethylation acquired during oogenesis, and its expression silencing was reversible on passage through the male germ line. Histone modifications also corresponded to profiles described at known imprinted loci. Chromosome conformation analyses revealed distinct chromatin microarchitectures, with a more compact structure characterizing the maternally inherited repressed allele. Such genetic analyses of well-characterized transgene insertions associated with a de novo-induced parental imprint may help us understand the molecular determinants of imprinting.

A switch between topological domains underlies HoxD genes collinearity in mouse limbs.

Hox genes are major determinants of the animal body plan, where they organize structures along both the trunk and appendicular axes. During mouse limb development, Hoxd genes are transcribed in two waves: early on, when the arm and forearm are specified, and later, when digits form. The transition between early and late regulations involves a functional switch between two opposite topological domains. This switch is reflected by a subset of Hoxd genes mapping centrally into the cluster, which initially interact with the telomeric domain and subsequently swing toward the centromeric domain, where they establish new contacts. This transition between independent regulatory landscapes illustrates both the modularity of the limbs and the distinct evolutionary histories of its various pieces. It also allows the formation of an intermediate area of low HOX proteins content, which develops into the wrist, the transition between our arms and our hands. This regulatory strategy accounts for collinear Hox gene regulation in land vertebrate appendages.

Chromatin organization and global regulation of Hox gene clusters.

During development, a properly coordinated expression of Hox genes, within their different genomic clusters is critical for patterning the body plans of many animals with a bilateral symmetry. The fascinating correspondence between the topological organization of Hox clusters and their transcriptional activation in space and time has served as a paradigm for understanding the relationships between genome structure and function. Here, we review some recent observations, which revealed highly dynamic changes in the structure of chromatin at Hox clusters, in parallel with their activation during embryonic development. We discuss the relevance of these findings for our understanding of large-scale gene regulation.

Impact of copy number variations (CNVs) on long-range gene regulation at the HoxD locus.

Copy number variations are genomic structural variants that are frequently associated with human diseases. Among these copy number variations, duplications of DNA segments are often assumed to lead to dosage effects by increasing the copy number of either genes or their regulatory elements. We produced a series of large targeted duplications within a conserved gene desert upstream of the murine HoxD locus. This DNA region, syntenic to human 2q31-32, contains a range of regulatory elements required for Hoxd gene transcription, and it is often disrupted and/or reorganized in human genetic conditions collectively known as the 2q31 syndrome. Unexpectedly, one such duplication led to a transcriptional down-regulation in developing digits by impairing physical interactions between the target genes and their upstream regulatory elements, thus phenocopying the effect obtained when these enhancer sequences are deleted. These results illustrate the detrimental consequences of interrupting highly conserved regulatory landscapes and reveal a mechanism where genomic duplications lead to partial loss of function of nearby located genes.