Inheritance of H3K9 methylation regulates genome architecture in Drosophila early embryos.

Constitutive heterochromatin is essential for transcriptional silencing and genome integrity. The establishment of constitutive heterochromatin in early embryos and its role in early fruitfly development are unknown. Lysine 9 trimethylation of histone H3 (H3K9me3) and recruitment of its epigenetic reader, heterochromatin protein 1a (HP1a), are hallmarks of constitutive heterochromatin. Here, we show that H3K9me3 is transmitted from the maternal germline to the next generation. Maternally inherited H3K9me3, and the histone methyltransferases (HMT) depositing it, are required for the organization of constitutive heterochromatin: early embryos lacking H3K9 methylation display de-condensation of pericentromeric regions, centromere-centromere de-clustering, mitotic defects, and nuclear shape irregularities, resulting in embryo lethality. Unexpectedly, quantitative CUT&Tag and 4D microscopy measurements of HP1a coupled with biophysical modeling revealed that H3K9me2/3 is largely dispensable for HP1a recruitment. Instead, the main function of H3K9me2/3 at this developmental stage is to drive HP1a clustering and subsequent heterochromatin compaction. Our results show that HP1a binding to constitutive heterochromatin in the absence of H3K9me2/3 is not sufficient to promote proper embryo development and heterochromatin formation. The loss of H3K9 HMTs and H3K9 methylation alters genome organization and hinders embryonic development.

The C. elegans SET1 histone methyltransferase SET-2 is not required for transgenerational memory of silencing.

The SET-2 /SET1 histone H3K4 methyltransferase and RNAi pathway components are required to maintain fertility across generations in C. elegans . SET-2 preserves the germline transcriptional program transgenerationally, and RNAi pathways rely on small RNAs to establish and maintain transgenerational gene silencing. We investigated whether the functionality of RNAi-induced transgenerational silencing and the composition of pools of endogenous small RNA are affected by the absence of SET-2 . Our results suggest that defects in RNAi pathways are not responsible for the transcriptional misregulation observed in the absence of SET-2 .

Epigenetic and gene regulatory functions of small RNAs.

In this review article, I summarize the intervention I made during the "Hommage à François Gros" held at the Institut Pasteur in Paris on the 25th of April, 2023. I discuss how the discovery of the existence of an RNA intermediate between genetic information and protein translation has changed our perspective on the role of RNA in gene regulation in these past years. I also discuss new emerging paradigms, highlighting the role of RNA in heritable information similar to the well-known DNA function.

Dans cet article de synthèse, je résume l'intervention que j'ai faite lors du colloque «  Hommage à François Gros  ¼ qui s'est tenu le 25 avril 2023 à l'Institut Pasteur à Paris. J'explique comment la découverte de l'existence d'un ARN intermédiaire entre l'information génétique et la traduction des protéines a changé notre perspective sur le rôle de l'ARN dans la régulation des gènes au cours de ces dernières années. Je discute également de nouveaux paradigmes émergents, en soulignant le rôle de l'ARN dans la transmission d'informations héréditaires, analogue à la fonction bien connue de l'ADN.

Inheritance and maintenance of small RNA-mediated epigenetic effects.

Heritable traits are predominantly encoded within genomic DNA, but it is now appreciated that epigenetic information is also inherited through DNA methylation, histone modifications, and small RNAs. Several examples of transgenerational epigenetic inheritance of traits have been documented in plants and animals. These include even the inheritance of traits acquired through the soma during the life of an organism, implicating the transfer of epigenetic information via the germline to the next generation. Small RNAs appear to play a significant role in carrying epigenetic information across generations. This review focuses on how epigenetic information in the form of small RNAs is transmitted from the germline to the embryos through the gametes. We also consider how inherited epigenetic information is maintained across generations in a small RNA-dependent and independent manner. Finally, we discuss how epigenetic traits acquired from the soma can be inherited through small RNAs.

piRNAs initiate transcriptional silencing of spermatogenic genes during C. elegans germline development.

Eukaryotic genomes harbor invading transposable elements that are silenced by PIWI-interacting RNAs (piRNAs) to maintain genome integrity in animal germ cells. However, whether piRNAs also regulate endogenous gene expression programs remains unclear. Here, we show that C. elegans piRNAs trigger the transcriptional silencing of hundreds of spermatogenic genes during spermatogenesis, promoting sperm differentiation and function. This silencing signal requires piRNA-dependent small RNA biogenesis and loading into downstream nuclear effectors, which correlates with the dynamic reorganization of two distinct perinuclear biomolecular condensates present in germ cells. In addition, the silencing capacity of piRNAs is temporally counteracted by the Argonaute CSR-1, which targets and licenses spermatogenic gene transcription. The spatial and temporal overlap between these opposing small RNA pathways contributes to setting up the timing of the spermatogenic differentiation program. Thus, our work identifies a prominent role for piRNAs as direct regulators of endogenous transcriptional programs during germline development and gamete differentiation.

A virus-derived microRNA targets immune response genes during SARS-CoV-2 infection.

SARS-CoV-2 infection results in impaired interferon response in patients with severe COVID-19. However, how SARS-CoV-2 interferes with host immune responses is incompletely understood. Here, we sequence small RNAs from SARS-CoV-2-infected human cells and identify a microRNA (miRNA) derived from a recently evolved region of the viral genome. We show that the virus-derived miRNA produces two miRNA isoforms in infected cells by the enzyme Dicer, which are loaded into Argonaute proteins. Moreover, the predominant miRNA isoform targets the 3'UTR of interferon-stimulated genes and represses their expression in a miRNA-like fashion. Finally, the two viral miRNA isoforms were detected in nasopharyngeal swabs from COVID-19 patients. We propose that SARS-CoV-2 can potentially employ a virus-derived miRNA to hijack the host miRNA machinery, which could help to evade the interferon-mediated immune response.

Global Run-On sequencing to measure nascent transcription in C. elegans.

Global Run-On sequencing (GRO-seq) is one of the most sensitive techniques to detect nascent transcription from RNA polymerase (Pol) at a genome-wide level. The protocol incorporates labeled ribonucleotides into nascent RNAs from Pol I, II, and III. We have adapted the GRO-seq protocol to the nematode Caenorhabditis elegans to measure transcription from embryos and adult worms. Here, we provide a detailed overview of the protocol highlighting the critical steps for generating successful libraries. For complete details on the use and execution of this protocol, please refer to Quarato et al. (2021).

Histone variant H2A.Z regulates zygotic genome activation.

During embryogenesis, the genome shifts from transcriptionally quiescent to extensively active in a process known as Zygotic Genome Activation (ZGA). In Drosophila, the pioneer factor Zelda is known to be essential for the progression of development; still, it regulates the activation of only a small subset of genes at ZGA. However, thousands of genes do not require Zelda, suggesting that other mechanisms exist. By conducting GRO-seq, HiC and ChIP-seq in Drosophila embryos, we demonstrate that up to 65% of zygotically activated genes are enriched for the histone variant H2A.Z. H2A.Z enrichment precedes ZGA and RNA Polymerase II loading onto chromatin. In vivo knockdown of maternally contributed Domino, a histone chaperone and ATPase, reduces H2A.Z deposition at transcription start sites, causes global downregulation of housekeeping genes at ZGA, and compromises the establishment of the 3D chromatin structure. We infer that H2A.Z is essential for the de novo establishment of transcriptional programs during ZGA via chromatin reorganization.

Small RNAs in epigenetic inheritance: from mechanisms to trait transmission.

Inherited information is transmitted to progeny primarily by the genome through the gametes. However, in recent years, epigenetic inheritance has been demonstrated in several organisms, including animals. Although it is clear that certain post-translational histone modifications, DNA methylation, and noncoding RNAs regulate epigenetic inheritance, the molecular mechanisms responsible for epigenetic inheritance are incompletely understood. This review focuses on the role of small RNAs in transmitting epigenetic information across generations in animals. Examples of documented cases of transgenerational epigenetic inheritance are discussed, from the silencing of transgenes to the inheritance of complex traits, such as fertility, stress responses, infections, and behavior. Experimental evidence supporting the idea that small RNAs are epigenetic molecules capable of transmitting traits across generations is highlighted, focusing on the mechanisms by which small RNAs achieve such a function. Just as the role of small RNAs in epigenetic processes is redefining the concept of inheritance, so too our understanding of the molecular pathways and mechanisms that govern epigenetic inheritance in animals is radically changing.

Recombination-independent recognition of DNA homology for meiotic silencing in Neurospora crassa.

The pairing of homologous chromosomes represents a critical step of meiosis in nearly all sexually reproducing species. In many organisms, pairing involves chromosomes that remain apparently intact. The mechanistic nature of homology recognition at the basis of such pairing is unknown. Using "meiotic silencing by unpaired DNA" (MSUD) as a model process, we demonstrate the existence of a cardinally different approach to DNA homology recognition in meiosis. The main advantage of MSUD over other experimental systems lies in its ability to identify any relatively short DNA fragment lacking a homologous allelic partner. Here, we show that MSUD does not rely on the canonical mechanism of meiotic recombination, yet it is promoted by REC8, a conserved component of the meiotic cohesion complex. We also show that certain patterns of interspersed homology are recognized as pairable during MSUD. Such patterns need to be colinear and must contain short tracts of sequence identity spaced apart at 21 or 22 base pairs. By using these periodicity values as a guiding parameter in all-atom molecular modeling, we discover that homologous DNA molecules can pair by forming quadruplex-based contacts with an interval of 2.5 helical turns. This process requires right-handed plectonemic coiling and additional conformational changes in the intervening double-helical segments. Our results 1) reconcile genetic and biophysical evidence for the existence of direct homologous double-stranded DNA (dsDNA)-dsDNA pairing, 2) identify a role for this process in initiating RNA interference, and 3) suggest that chromosomes can be cross-matched by a precise mechanism that operates on intact dsDNA molecules.

Translation and codon usage regulate Argonaute slicer activity to trigger small RNA biogenesis.

In the Caenorhabditis elegans germline, thousands of mRNAs are concomitantly expressed with antisense 22G-RNAs, which are loaded into the Argonaute CSR-1. Despite their essential functions for animal fertility and embryonic development, how CSR-1 22G-RNAs are produced remains unknown. Here, we show that CSR-1 slicer activity is primarily involved in triggering the synthesis of small RNAs on the coding sequences of germline mRNAs and post-transcriptionally regulates a fraction of targets. CSR-1-cleaved mRNAs prime the RNA-dependent RNA polymerase, EGO-1, to synthesize 22G-RNAs in phase with translating ribosomes, in contrast to other 22G-RNAs mostly synthesized in germ granules. Moreover, codon optimality and efficient translation antagonize CSR-1 slicing and 22G-RNAs biogenesis. We propose that codon usage differences encoded into mRNA sequences might be a conserved strategy in eukaryotes to regulate small RNA biogenesis and Argonaute targeting.

Functional Characterization of the m6A-Dependent Translational Modulator PfYTH.2 in the Human Malaria Parasite.

Posttranscriptional regulation of gene expression is central to the development and replication of the malaria parasite, Plasmodium falciparum, within its human host. The timely coordination of RNA maturation, homeostasis, and protein synthesis relies on the recruitment of specific RNA-binding proteins to their cognate target mRNAs. One possible mediator of such mRNA-protein interactions is the N6-methylation of adenosines (m6A), a prevalent mRNA modification of parasite mRNA transcripts. Here, we used RNA protein pulldowns, RNA modification mass spectrometry, and quantitative proteomics to identify two P. falciparum YTH domain proteins (PfYTH.1 and PfYTH.2) as m6A-binding proteins during parasite blood-stage development. Interaction proteomics revealed that PfYTH.2 associates with the translation machinery, including multiple subunits of the eukaryotic initiation factor 3 (eIF3) and poly(A)-binding proteins. Furthermore, knock sideways of PfYTH.2 coupled with ribosome profiling showed that this m6A reader is essential for parasite survival and is a repressor of mRNA translation. Together, these data reveal an important missing link in the m6A-mediated mechanism controlling mRNA translation in a unicellular eukaryotic pathogen.IMPORTANCE Infection with the unicellular eukaryotic pathogen Plasmodium falciparum causes malaria, a mosquito-borne disease affecting more than 200 million and killing 400,000 people each year. Underlying the asexual replication within human red blood cells is a tight regulatory network of gene expression and protein synthesis. A widespread mechanism of posttranscriptional gene regulation is the chemical modification of adenosines (m6A), through which the fate of individual mRNA transcripts can be changed. Here, we report on the protein machinery that "reads" this modification and "translates" it into a functional outcome. We provide mechanistic insight into one m6A reader protein and show that it interacts with the translational machinery and acts as a repressor of mRNA translation. This m6A-mediated phenotype has not been described in other eukaryotes as yet, and the functional characterization of the m6A interactome will ultimately open new avenues to combat the disease.

Germline inherited small RNAs facilitate the clearance of untranslated maternal mRNAs in C. elegans embryos.

Inheritance and clearance of maternal mRNAs are two of the most critical events required for animal early embryonic development. However, the mechanisms regulating this process are still largely unknown. Here, we show that together with maternal mRNAs, C. elegans embryos inherit a complementary pool of small non-coding RNAs that facilitate the cleavage and removal of hundreds of maternal mRNAs. These antisense small RNAs are loaded into the maternal catalytically-active Argonaute CSR-1 and cleave complementary mRNAs no longer engaged in translation in somatic blastomeres. Induced depletion of CSR-1 specifically during embryonic development leads to embryonic lethality in a slicer-dependent manner and impairs the degradation of CSR-1 embryonic mRNA targets. Given the conservation of Argonaute catalytic activity, we propose that a similar mechanism operates to clear maternal mRNAs during the maternal-to-zygotic transition across species.

Small-RNA-mediated transgenerational silencing of histone genes impairs fertility in piRNA mutants.

PIWI-interacting RNAs (piRNAs) promote fertility in many animals. However, whether this is due to their conserved role in repressing repetitive elements (REs) remains unclear. Here, we show that the progressive loss of fertility in Caenorhabditis elegans lacking piRNAs is not caused by derepression of REs or other piRNA targets but, rather, is mediated by epigenetic silencing of all of the replicative histone genes. In the absence of piRNAs, downstream components of the piRNA pathway relocalize from germ granules and piRNA targets to histone mRNAs to synthesize antisense small RNAs (sRNAs) and induce transgenerational silencing. Removal of the downstream components of the piRNA pathway restores histone mRNA expression and fertility in piRNA mutants, and the inheritance of histone sRNAs in wild-type worms adversely affects their fertility for multiple generations. We conclude that sRNA-mediated silencing of histone genes impairs the fertility of piRNA mutants and may serve to maintain piRNAs across evolution.

CRISPR editing of sftb-1/SF3B1 in Caenorhabditis elegans allows the identification of synthetic interactions with cancer-related mutations and the chemical inhibition of splicing.

SF3B1 is the most frequently mutated splicing factor in cancer. Mutations in SF3B1 likely confer clonal advantages to cancer cells but they may also confer vulnerabilities that can be therapeutically targeted. SF3B1 cancer mutations can be maintained in homozygosis in C. elegans, allowing synthetic lethal screens with a homogeneous population of animals. These mutations cause alternative splicing (AS) defects in C. elegans, as it occurs in SF3B1-mutated human cells. In a screen, we identified RNAi of U2 snRNP components that cause synthetic lethality with sftb-1/SF3B1 mutations. We also detected synthetic interactions between sftb-1 mutants and cancer-related mutations in uaf-2/U2AF1 or rsp-4/SRSF2, demonstrating that this model can identify interactions between mutations that are mutually exclusive in human tumors. Finally, we have edited an SFTB-1 domain to sensitize C. elegans to the splicing modulators pladienolide B and herboxidiene. Thus, we have established a multicellular model for SF3B1 mutations amenable for high-throughput genetic and chemical screens.

Global effects of the CSR-1 RNA interference pathway on the transcriptional landscape.

Argonaute proteins and their small RNA cofactors short interfering RNAs are known to inhibit gene expression at the transcriptional and post-transcriptional levels. In Caenorhabditis elegans, the Argonaute CSR-1 binds thousands of endogenous siRNAs (endo-siRNAs) that are antisense to germline transcripts. However, its role in gene expression regulation remains controversial. Here we used genome-wide profiling of nascent RNA transcripts and found that the CSR-1 RNA interference pathway promoted sense-oriented RNA polymerase II transcription. Moreover, a loss of CSR-1 function resulted in global increase in antisense transcription and ectopic transcription of silent chromatin domains, which led to reduced chromatin incorporation of centromere-specific histone H3. On the basis of these findings, we propose that the CSR-1 pathway helps maintain the directionality of active transcription, thereby propagating the distinction between transcriptionally active and silent genomic regions.

A nuclear perspective on RNAi pathways in metazoans.

The role of RNA interference (RNAi) in post-transcriptional regulation of complementary targets is well known. However, less is known about transcriptional silencing mechanisms mediated by RNAi. Such mechanisms have been characterized in yeast and plants, which suggests that similar RNA silencing mechanisms might operate in animals. A growing amount of experimental evidence indicates that short RNAs and their co-factor Argonaute proteins can regulate many nuclear processes in metazoans. PIWI-interacting RNAs (piRNAs) initiate transcriptional silencing of transposable elements, which leads to heterochromatin formation and/or DNA methylation. In addition, Argonaute proteins and short RNAs directly regulate Pol II transcription and splicing of euchromatic protein-coding genes and also affect genome architecture. Therefore, RNAi pathways can have a profound global impact on the transcriptional programs in cells during animal development. This article is part of a Special Issue entitled: Chromatin and epigenetic regulation of animal development.

RNA Chromatin Immunoprecipitation (RNA-ChIP) in Caenorhabditis elegans.

The RNA chromatin immunoprecipitation assay (RNA-ChIP) allows detection and quantification of RNA-protein interactions using in vivo cross-linking with formaldehyde followed by immunoprecipitation of the RNA-protein complexes. Here we describe the RNA-ChIP protocol that we have adapted for Caenorhabditis elegans (C. elegans) to detect interaction between the nuclear Argonaute CSR-1 (chromosome segregation and RNAi deficient) protein and its target nascent RNAs. We have used a transgenic strain expressing a recombinant long isoform of CSR-1 protein fused with N-terminal 3x FLAG epitope.

The ZFP-1(AF10)/DOT-1 complex opposes H2B ubiquitination to reduce Pol II transcription.

The inhibition of transcriptional elongation plays an important role in gene regulation in metazoans, including C. elegans. Here, we combine genomic and biochemical approaches to dissect a role of ZFP-1, the C. elegans AF10 homolog, in transcriptional control. We show that ZFP-1 and its interacting partner DOT-1.1 have a global role in negatively modulating the level of polymerase II (Pol II) transcription on essential widely expressed genes. Moreover, the ZFP-1/DOT-1.1 complex contributes to progressive Pol II pausing on essential genes during development and to rapid Pol II pausing during stress response. The slowing down of Pol II transcription by ZFP-1/DOT-1.1 is associated with an increase in H3K79 methylation and a decrease in H2B monoubiquitination, which promotes transcription. We propose a model wherein the recruitment of ZFP-1/DOT-1.1 and deposition of H3K79 methylation at highly expressed genes initiates a negative feedback mechanism for the modulation of their expression.

The conserved PHD1-PHD2 domain of ZFP-1/AF10 is a discrete functional module essential for viability in Caenorhabditis elegans.

Plant homeodomain (PHD)-type zinc fingers play an important role in recognizing chromatin modifications and recruiting regulatory proteins to specific genes. A specific module containing a conventional PHD finger followed by an extended PHD finger exists in the mammalian AF10 protein, among a few others. AF10 has mostly been studied in the context of the leukemic MLL-AF10 fusion protein, which lacks the N-terminal PHD fingers of AF10. Although this domain of AF10 is the most conserved region of the protein, its biological significance has not been elucidated. In this study, we used genetic and biochemical approaches to examine the PHD1-PHD2 region of the Caenorhabditis elegans ortholog of AF10, zinc finger protein 1 (ZFP-1). We demonstrate that the PHD1-PHD2 region is essential for viability and that the first PHD finger contributes to the preferred binding of PHD1-PHD2 to lysine 4-methylated histone H3 tails. Moreover, we show that ZFP-1 localization peaks overlap with H3K4 methylation-enriched promoters of actively expressed genes genomewide and that H3K4 methylation is important for ZFP-1 localization to promoters in the embryo. We predict that the essential biological role of the PHD1-PHD2 module of ZFP-1/AF10 is connected to the regulation of actively expressed genes during early development.