Piwi Is Required during Drosophila Embryogenesis to License Dual-Strand piRNA Clusters for Transposon Repression in Adult Ovaries.

Most piRNAs in the Drosophila female germline are transcribed from heterochromatic regions called dual-strand piRNA clusters. Histone 3 lysine 9 trimethylation (H3K9me3) is required for licensing piRNA production by these clusters. However, it is unclear when and how they acquire this permissive heterochromatic state. Here, we show that transient Piwi depletion in Drosophila embryos results in H3K9me3 decrease at piRNA clusters in ovaries. This is accompanied by impaired biogenesis of ovarian piRNAs, accumulation of transposable element transcripts, and female sterility. Conversely, Piwi depletion at later developmental stages does not disturb piRNA cluster licensing. These results indicate that the identity of piRNA clusters is epigenetically acquired in a Piwi-dependent manner during embryonic development, which is reminiscent of the widespread genome reprogramming occurring during early mammalian zygotic development.

In vivo antagonistic role of the Human T-Cell Leukemia Virus Type 1 regulatory proteins Tax and HBZ.

Adult T cell leukemia (ATL) is an aggressive malignancy secondary to chronic infection by the human T-cell leukemia virus type 1 (HTLV-1) infection. Two viral proteins, Tax and HBZ, play central roles in ATL leukemogenesis. Tax expression transforms T cells in vitro and induces ATL-like disease in mice. Tax also induces a rough eye phenotype and increases hemocyte count in Drosophila melanogaster, indicative of transformation. Among multiple functions, Tax modulates the expression of the enhancer of zeste homolog 2 (EZH2), a methyltransferase of the Polycomb Repressive Complex 2 (PRC2), leading to H3K27me3-dependent reprogramming of around half of cellular genes. HBZ is a negative regulator of Tax-mediated viral transcription. HBZ effects on epigenetic signatures are underexplored. Here, we established an hbz transgenic fly model, and demonstrated that, unlike Tax, which induces NF-κB activation and enhanced PRC2 activity creating an activation loop, HBZ neither induces transformation nor NF-κB activation in vivo. However, overexpression of Tax or HBZ increases the PRC2 activity and both proteins directly interact with PRC2 complex core components. Importantly, overexpression of HBZ in tax transgenic flies prevents Tax-induced NF-κB or PRC2 activation and totally rescues Tax-induced transformation and senescence. Our results establish the in vivo antagonistic effect of HBZ on Tax-induced transformation and cellular effects. This study helps understanding long-term HTLV-1 persistence and cellular transformation and opens perspectives for new therapeutic strategies targeting the epigenetic machinery in ATL.

The somatic piRNA pathway controls germline transposition over generations.

Transposable elements (TEs) are parasitic DNA sequences that threaten genome integrity by replicative transposition in host gonads. The Piwi-interacting RNAs (piRNAs) pathway is assumed to maintain Drosophila genome homeostasis by downregulating transcriptional and post-transcriptional TE expression in the ovary. However, the bursts of transposition that are expected to follow transposome derepression after piRNA pathway impairment have not yet been reported. Here, we show, at a genome-wide level, that piRNA loss in the ovarian somatic cells boosts several families of the endogenous retroviral subclass of TEs, at various steps of their replication cycle, from somatic transcription to germinal genome invasion. For some of these TEs, the derepression caused by the loss of piRNAs is backed up by another small RNA pathway (siRNAs) operating in somatic tissues at the post transcriptional level. Derepressed transposition during 70 successive generations of piRNA loss exponentially increases the genomic copy number by up to 10-fold.

MicroRNA-Dependent Transcriptional Silencing of Transposable Elements in Drosophila Follicle Cells.

RNA interference-related silencing mechanisms concern very diverse and distinct biological processes, from gene regulation (via the microRNA pathway) to defense against molecular parasites (through the small interfering RNA and the Piwi-interacting RNA pathways). Small non-coding RNAs serve as specificity factors that guide effector proteins to ribonucleic acid targets via base-pairing interactions, to achieve transcriptional or post-transcriptional regulation. Because of the small sequence complementarity required for microRNA-dependent post-transcriptional regulation, thousands of microRNA (miRNA) putative targets have been annotated in Drosophila. In Drosophila somatic ovarian cells, genomic parasites, such as transposable elements (TEs), are transcriptionally repressed by chromatin changes induced by Piwi-interacting RNAs (piRNAs) that prevent them from invading the germinal genome. Here we show, for the first time, that a functional miRNA pathway is required for the piRNA-mediated transcriptional silencing of TEs in this tissue. Global miRNA depletion, caused by tissue- and stage-specific knock down of drosha (involved in miRNA biogenesis), AGO1 or gawky (both responsible for miRNA activity), resulted in loss of TE-derived piRNAs and chromatin-mediated transcriptional de-silencing of TEs. This specific TE de-repression was also observed upon individual titration (by expression of the complementary miRNA sponge) of two miRNAs (miR-14 and miR-34) as well as in a miR-14 loss-of-function mutant background. Interestingly, the miRNA defects differentially affected TE- and 3' UTR-derived piRNAs. To our knowledge, this is the first indication of possible differences in the biogenesis or stability of TE- and 3' UTR-derived piRNAs. This work is one of the examples of detectable phenotypes caused by loss of individual miRNAs in Drosophila and the first genetic evidence that miRNAs have a role in the maintenance of genome stability via piRNA-mediated TE repression.

piRNA-mediated transgenerational inheritance of an acquired trait.

The maintenance of genome integrity is an essential trait to the successful transmission of genetic information. In animal germ cells, piRNAs guide PIWI proteins to silence transposable elements (TEs) in order to maintain genome integrity. In insects, most TE silencing in the germline is achieved by secondary piRNAs that are produced by a feed-forward loop (the ping-pong cycle), which requires the piRNA-directed cleavage of two types of RNAs: mRNAs of functional euchromatic TEs and heterochromatic transcripts that contain defective TE sequences. The first cleavage that initiates such an amplification loop remains poorly understood. Taking advantage of the existence of strains that are devoid of functional copies of the LINE-like I-element, we report here that in such Drosophila ovaries, the initiation of a ping-pong cycle is exclusively achieved by secondary I-element piRNAs that are produced in the ovary and deposited in the embryonic germline. This unusual secondary piRNA biogenesis, detected in the absence of functional I-element copies, results from the processing of sense and antisense transcripts of several different defective I-element. Once acquired, for instance after ancestor aging, this capacity to produce heterochromatic-only secondary piRNAs is partially transmitted through generations via maternal piRNAs. Furthermore, such piRNAs acting as ping-pong initiators in a chromatin-independent manner confer to the progeny a high capacity to repress the I-element mobility. Our study explains, at the molecular level, the basis for epigenetic memory of maternal immunity that protects females from hybrid dysgenesis caused by transposition of paternally inherited functional I-element.

Maternally deposited germline piRNAs silence the tirant retrotransposon in somatic cells.

Transposable elements (TEs), whose propagation can result in severe damage to the host genome, are silenced in the animal gonad by Piwi-interacting RNAs (piRNAs). piRNAs produced in the ovaries are deposited in the embryonic germline and initiate TE repression in the germline progeny. Whether the maternally transmitted piRNAs play a role in the silencing of somatic TEs is however unknown. Here we show that maternally transmitted piRNAs from the tirant retrotransposon in Drosophila are required for the somatic silencing of the TE and correlate with an increase in histone H3K9 trimethylation an active tirant copy.

A user-friendly chromatographic method to purify small regulatory RNAs.

The discovery of the small regulatory RNAs has changed our vision of cellular regulations. Indeed, when loaded on Argonaute proteins they form ribonucleoprotein complexes (RNPs) that target complementary sequences to achieve widespread silencing mechanisms conserved in most eukaryotes. The recent development of deep sequencing approaches highly contributed to their detection. Small RNA isolation from cells and/or tissues remains a crucial stage to generate robust and relevant sequencing data. In 2006, a novel strategy based on anion-exchange chromatography has been proposed as an alternative to the standard size-isolation purification procedure. Using bioinformatic comparative analysis, we here demonstrate that anion-exchange chromatographic RNP purification prior to small RNA extraction unbiasedly enriches datasets in bona fide reads (small regulatory RNA sequences) and depletes endogenous contaminants (ribosomal RNAs and degradation RNA products). The resulting increase in sequencing depth provides a major benefit to study rare populations. We then developed a fast and basic manual procedure to purify such small non-coding RNAs using anion-exchange chromatography at the bench. We validated the efficiency of this new method and used this strategy to purify small RNAs from various tissues and organisms. We moreover determined that our manual purification increases the output of the previously described anion-exchange chromatography procedure.

TrEMOLO: accurate transposable element allele frequency estimation using long-read sequencing data combining assembly and mapping-based approaches.

Transposable Element MOnitoring with LOng-reads (TrEMOLO) is a new software that combines assembly- and mapping-based approaches to robustly detect genetic elements called transposable elements (TEs). Using high- or low-quality genome assemblies, TrEMOLO can detect most TE insertions and deletions and estimate their allele frequency in populations. Benchmarking with simulated data revealed that TrEMOLO outperforms other state-of-the-art computational tools. TE detection and frequency estimation by TrEMOLO were validated using simulated and experimental datasets. Therefore, TrEMOLO is a comprehensive and suitable tool to accurately study TE dynamics. TrEMOLO is available under GNU GPL3.0 at https://github.com/DrosophilaGenomeEvolution/TrEMOLO .

Does looping and clustering in the nucleus regulate gene expression?

There has been considerable interest in the way that chromatin is spatially organised within the cell nucleus and how that may relate to gene expression and its control. New molecular techniques have identified looped chromatin domains at the mammalian beta-globin and the Drosophila hsp70 loci. Looped domains may insulate chromatin from the influence of neighbouring domains, and the bases of loops may also act to concentrate proteins locally within the nucleus. The spatial clustering of sequences from the Drosophila bithorax complex, located in trans, has also been demonstrated. An emerging theme is that bringing DNA and proteins together within a defined sub-region of the nuclear volume facilitates both the activation and the repression of gene expression. Nuclear compartments may also be involved in the post-translational modification of proteins by sumoylation and ubiquitylation.

piRNA-mediated nuclear accumulation of retrotransposon transcripts in the Drosophila female germline.

Germline silencing of transposable elements is essential for the maintenance of genome integrity. Recent results indicate that this repression is largely achieved through a RNA silencing pathway that involves Piwi-interacting RNAs (piRNAs). However the repressive mechanisms are not well understood. To address this question, we used the possibility to disrupt the repression of the Drosophila I element retrotransposon by hybrid dysgenesis. We show here that the repression of the functional I elements that are located in euchromatin requires proteins of the piRNA pathway, and that the amount of ovarian I element piRNAs correlates with the strength of the repression in the female germline. Antisense RNAs, which are likely used to produce antisense piRNAs, are transcribed by heterochromatic defective I elements, but efficient production of these antisense small RNAs requires the presence in the genome of euchromatic functional I elements. Finally, we demonstrate that the piRNA-induced silencing of the functional I elements is at least partially posttranscriptional. In a repressive background, these elements are still transcribed, but some of their sense transcripts are kept in nurse cell nuclear foci together with those of the Doc retrotransposon. In the absence of I element piRNAs, either in dysgenic females or in mutants of the piRNA silencing pathway, sense I element transcripts are transported toward the oocyte where retrotransposition occurs. Our results indicate that piRNAs are involved in a posttranscriptional gene-silencing mechanism resulting in RNA nuclear accumulation.

A Transposon Story: From TE Content to TE Dynamic Invasion of Drosophila Genomes Using the Single-Molecule Sequencing Technology from Oxford Nanopore.

Transposable elements (TEs) are the main components of genomes. However, due to their repetitive nature, they are very difficult to study using data obtained with short-read sequencing technologies. Here, we describe an efficient pipeline to accurately recover TE insertion (TEI) sites and sequences from long reads obtained by Oxford Nanopore Technology (ONT) sequencing. With this pipeline, we could precisely describe the landscapes of the most recent TEIs in wild-type strains of Drosophila melanogaster and Drosophila simulans. Their comparison suggests that this subset of TE sequences is more similar than previously thought in these two species. The chromosome assemblies obtained using this pipeline also allowed recovering piRNA cluster sequences, which was impossible using short-read sequencing. Finally, we used our pipeline to analyze ONT sequencing data from a D. melanogaster unstable line in which LTR transposition was derepressed for 73 successive generations. We could rely on single reads to identify new insertions with intact target site duplications. Moreover, the detailed analysis of TEIs in the wild-type strains and the unstable line did not support the trap model claiming that piRNA clusters are hotspots of TE insertions.

The Mi-2 nucleosome remodeler and the Rpd3 histone deacetylase are involved in piRNA-guided heterochromatin formation.

In eukaryotes, trimethylation of lysine 9 on histone H3 (H3K9) is associated with transcriptional silencing of transposable elements (TEs). In drosophila ovaries, this heterochromatic repressive mark is thought to be deposited by SetDB1 on TE genomic loci after the initial recognition of nascent transcripts by PIWI-interacting RNAs (piRNAs) loaded on the Piwi protein. Here, we show that the nucleosome remodeler Mi-2, in complex with its partner MEP-1, forms a subunit that is transiently associated, in a MEP-1 C-terminus-dependent manner, with known Piwi interactors, including a recently reported SUMO ligase, Su(var)2-10. Together with the histone deacetylase Rpd3, this module is involved in the piRNA-dependent TE silencing, correlated with H3K9 deacetylation and trimethylation. Therefore, drosophila piRNA-mediated transcriptional silencing involves three epigenetic effectors, a remodeler, Mi-2, an eraser, Rpd3 and a writer, SetDB1, in addition to the Su(var)2-10 SUMO ligase.

Transposable Element Misregulation Is Linked to the Divergence between Parental piRNA Pathways in Drosophila Hybrids.

Interspecific hybridization is a genomic stress condition that leads to the activation of transposable elements (TEs) in both animals and plants. In hybrids between Drosophila buzzatii and Drosophila koepferae, mobilization of at least 28 TEs has been described. However, the molecular mechanisms underlying this TE release remain poorly understood. To give insight on the causes of this TE activation, we performed a TE transcriptomic analysis in ovaries (notorious for playing a major role in TE silencing) of parental species and their F1 and backcrossed (BC) hybrids. We find that 15.2% and 10.6% of the expressed TEs are deregulated in F1 and BC1 ovaries, respectively, with a bias toward overexpression in both cases. Although differences between parental piRNA (Piwi-interacting RNA) populations explain only partially these results, we demonstrate that piRNA pathway proteins have divergent sequences and are differentially expressed between parental species. Thus, a functional divergence of the piRNA pathway between parental species, together with some differences between their piRNA pools, might be at the origin of hybrid instabilities and ultimately cause TE misregulation in ovaries. These analyses were complemented with the study of F1 testes, where TEs tend to be less expressed than in D. buzzatii. This can be explained by an increase in piRNA production, which probably acts as a defence mechanism against TE instability in the male germline. Hence, we describe a differential impact of interspecific hybridization in testes and ovaries, which reveals that TE expression and regulation are sex-biased.

Identification of misexpressed genetic elements in hybrids between Drosophila-related species.

Crosses between close species can lead to genomic disorders, often considered to be the cause of hybrid incompatibility, one of the initial steps in the speciation process. How these incompatibilities are established and what are their causes remain unclear. To understand the initiation of hybrid incompatibility, we performed reciprocal crosses between two species of Drosophila (D. mojavensis and D. arizonae) that diverged less than 1 Mya. We performed a genome-wide transcriptomic analysis on ovaries from parental lines and on hybrids from reciprocal crosses. Using an innovative procedure of co-assembling transcriptomes, we show that parental lines differ in the expression of their genes and transposable elements. Reciprocal hybrids presented specific gene categories and few transposable element families misexpressed relative to the parental lines. Because TEs are mainly silenced by piwi-interacting RNAs (piRNAs), we hypothesize that in hybrids the deregulation of specific TE families is due to the absence of such small RNAs. Small RNA sequencing confirmed our hypothesis and we therefore propose that TEs can indeed be major players of genome differentiation and be implicated in the first steps of genomic incompatibilities through small RNA regulation.

Chimeric RNA transposition intermediates of the I factor produce precise retrotransposed copies.

I elements in Drosophila melanogaster are non-long terminal repeat (LTR) retrotransposons of particular interest because high levels of transposition can be induced by appropriate crosses. They use a full-length RNA transposition intermediate as a template for reverse transcription. Detailed molecular characterization of this intermediate is rendered difficult because of the many transcripts produced by defective elements. The use of an active I element marked with a sequence encoding the HA epitope solves this problem. We used an RNA circularization procedure followed by RT-PCR to analyze the transcripts produced by actively transposing tagged I elements. Most start at the 5' end at the second nucleotide of the I element and all are polyadenylated at a site located in genomic sequences downstream of the 3' end. One of the tagged I elements, inserted in locus 88A, produces chimeric transcripts that carry sequences from both 5'- and 3'-flanking genomic DNA. We show that synthesis of these chimeric transcripts is controlled by the I element itself. Analysis of full-length transposed copies of this element shows that the extra sequences at the 5' and 3' ends are not integrated during retrotransposition. This suggests that initiation and arrest of reverse transcription during retrotransposition are precise processes.

International Congress on Transposable elements (ICTE 2016) in Saint Malo: mobile elements under the sun of Brittany.

The third international conference on Transposable Elements (ICTE) was held 16-19 April 2016 in Saint Malo, France. Organized by the French Transposition Community (Research group of the CNRS: "Mobile genetic elements: from mechanism to populations, an integrative approach") and the French Society of Genetics, the conference's goal was to bring together researchers who study transposition in diverse organisms, using multiple experimental approaches. The meeting gathered 180 participants from all around the world. Most of them contributed through poster presentations, invited talks and short talks selected from poster abstracts. The talks were organized into six scientific sessions: "Taming mobile DNA: self and non-self recognition"; "Trans-generational inheritance"; "Mobile DNA genome structure and organization, from molecular mechanisms to applications"; "Remembrance of (retro)transposon past: mobile DNA in genome evolution"; and finally "The yin and the yang of mobile DNA in human health".

Gene silencing triggered by non-LTR retrotransposons in the female germline of Drosophila melanogaster.

Several studies have recently shown that the activity of some eukaryotic transposable elements is sensitive to the presence of homologous transgenes, suggesting the involvement of homology-dependent gene-silencing mechanisms in their regulation. Here we provide data indicating that two non-LTR retrotransposons of Drosophila melanogaster are themselves natural triggers of homology-dependent gene silencing. We show that, in the female germline of D. melanogaster, fragments from the R1 or from the I retrotransposons can mediate silencing of chimeric transcription units into which they are inserted. This silencing is probably mediated by sequence identity with endogenous copies of the retrotransposons because it does not occur with a fragment from the divergent R1 elements of Bombyx mori, and, when a fragment of I is used, it occurs only in females containing functional copies of the I element. This silencing is not accompanied by cosuppression of the endogenous gene homologous to the chimeric transcription unit, which contrasts to some other silencing mechanisms in Drosophila. These observations suggest that in the female germline of D. melanogaster the R1 and I retrotransposons may self-regulate their own activity and their copy number by triggering homology-dependent gene silencing.

Aubergine iCLIP Reveals piRNA-Dependent Decay of mRNAs Involved in Germ Cell Development in the Early Embryo.

The Piwi-interacting RNA (piRNA) pathway plays an essential role in the repression of transposons in the germline. Other functions of piRNAs such as post-transcriptional regulation of mRNAs are now emerging. Here, we perform iCLIP with the PIWI protein Aubergine (Aub) and identify hundreds of maternal mRNAs interacting with Aub in the early Drosophila embryo. Gene expression profiling reveals that a proportion of these mRNAs undergo Aub-dependent destabilization during the maternal-to-zygotic transition. Strikingly, Aub-dependent unstable mRNAs encode germ cell determinants. iCLIP with an Aub mutant that is unable to bind piRNAs confirms piRNA-dependent binding of Aub to mRNAs. Base pairing between piRNAs and mRNAs can induce mRNA cleavage and decay that are essential for embryonic development. These results suggest general regulation of maternal mRNAs by Aub and piRNAs, which plays a key developmental role in the embryo through decay and localization of mRNAs encoding germ cell determinants.

Insect small non-coding RNA involved in epigenetic regulations.

Small regulatory RNAs can not only guide post-transcriptional repression of target genes, but some of them can also direct heterochromatin formation of specific genomic loci. Here we review the published literature on small RNA-guided epigenetic regulation in insects. The recent development of novel analytical technologies (deep sequencing and RNAi screens) has led to the identification of some of the factors involved in these processes, as well as their molecular mechanism and subcellular localization. Other findings uncovered an additional mode of epigenetic control, where maternally inherited small RNAs can affect phenotypes in a stable, transgenerational manner. The evolutive history of small RNA effector proteins in insects suggests that these two modes of regulation are variably conserved among species.

Fast and accurate method to purify small noncoding RNAs from Drosophila ovaries.

The recent development of High Throughput Sequencing technology has boosted the study of small regulatory RNA populations. A critical step prior to cloning and sequencing is purification of small RNA populations. Here, we report the optimization of an anion-exchange chromatography procedure in order to purify small regulatory RNAs bound on proteins. We developed this procedure to make it less time-consuming since our improved method no longer requires specific equipment and can easily be performed at the bench. We believe that our procedure will increase the robustness and accuracy of small RNA libraries in the future.