Telomeric Trans-Silencing: An Epigenetic Repression Combining RNA Silencing and Heterochromatin Formation

The study of P-element repression in Drosophila melanogaster led to the discovery of the telomeric Trans-Silencing Effect (TSE), a repression mechanism by which a transposon or a transgene inserted in subtelomeric heterochromatin (Telomeric Associated Sequence or TAS) has the capacity to repress in trans in the female germline, a homologous transposon, or transgene located in euchromatin. TSE shows variegation among egg chambers in ovaries when silencing is incomplete. Here, we report that TSE displays an epigenetic transmission through meiosis, which involves an extrachromosomal maternally transmitted factor. We show that this silencing is highly sensitive to mutations affecting both heterochromatin formation (Su(var)205 encoding Heterochromatin Protein 1 and Su(var)3–7) and the repeat-associated small interfering RNA (or rasiRNA) silencing pathway (aubergine, homeless, armitage, and piwi). In contrast, TSE is not sensitive to mutations affecting r2d2, which is involved in the small interfering RNA (or siRNA) silencing pathway, nor is it sensitive to a mutation in loquacious, which is involved in the micro RNA (or miRNA) silencing pathway. These results, taken together with the recent discovery of TAS homologous small RNAs associated to PIWI proteins, support the proposition that TSE involves a repeat-associated small interfering RNA pathway linked to heterochromatin formation, which was co-opted by the P element to establish repression of its own transposition after its recent invasion of the D. melanogaster genome. Therefore, the study of TSE provides insight into the genetic properties of a germline-specific small RNA silencing pathway

The Epigenetic Trans-Silencing Effect in Drosophila Involves Maternally-Transmitted Small RNAs Whose Production Depends on the piRNA Pathway and HP1

BACKGROUND: The study of P transposable element repression in Drosophila melanogaster led to the discovery of the Trans-Silencing Effect (TSE), a homology-dependent repression mechanism by which a P-transgene inserted in subtelomeric heterochromatin (Telomeric Associated Sequences, "TAS") has the capacity to repress in trans, in the female germline, a homologous P-lacZ transgene located in euchromatin. Phenotypic and genetic analysis have shown that TSE exhibits variegation in ovaries, displays a maternal effect as well as epigenetic transmission through meiosis and involves heterochromatin (including HP1) and RNA silencing. PRINCIPAL FINDINGS: Here, we show that mutations in squash and zucchini, which are involved in the piwi-interacting RNA (piRNA) silencing pathway, strongly affect TSE. In addition, we carried out a molecular analysis of TSE and show that silencing is correlated to the accumulation of lacZ small RNAs in ovaries. Finally, we show that the production of these small RNAs is sensitive to mutations affecting squash and zucchini, as well as to the dose of HP1. CONCLUSIONS AND SIGNIFICANCE: Thus, our results indicate that the TSE represents a bona fide piRNA-based repression. In addition, the sensitivity of TSE to HP1 dose suggests that in Drosophila, as previously shown in Schizosaccharomyces pombe, a RNA silencing pathway can depend on heterochromatin components

Telomeric Trans-Silencing in Drosophila melanogaster: Tissue Specificity, Development and Functional Interactions between Non-Homologous Telomeres

BACKGROUND: The study of P element repression in Drosophila melanogaster led to the discovery of the telomeric Trans-Silencing Effect (TSE), a homology-dependent repression mechanism by which a P-transgene inserted in subtelomeric heterochromatin (Telomeric Associated Sequences, "TAS") has the capacity to repress in trans, in the female germline, a homologous P-lacZ transgene located in euchromatin. TSE can show variegation in ovaries, displays a maternal effect as well as an epigenetic transmission through meiosis and involves heterochromatin and RNA silencing pathways. PRINCIPAL FINDINGS: Here, we analyze phenotypic and genetic properties of TSE. We report that TSE does not occur in the soma at the adult stage, but appears restricted to the female germline. It is detectable during development at the third instar larvae where it presents the same tissue specificity and maternal effect as in adults. Transgenes located in TAS at the telomeres of the main chromosomes can be silencers which in each case show the maternal effect. Silencers located at non-homologous telomeres functionally interact since they stimulate each other via the maternally-transmitted component. All germinally-expressed euchromatic transgenes tested, located on all major chromosomes, were found to be repressed by a telomeric silencer: thus we detected no TSE escaper. The presence of the euchromatic target transgene is not necessary to establish the maternal inheritance of TSE, responsible for its epigenetic behavior. A single telomeric silencer locus can simultaneously repress two P-lacZ targets located on different chromosomal arms. CONCLUSIONS AND SIGNIFICANCE: Therefore TSE appears to be a widespread phenomenon which can involve different telomeres and work across the genome. It can explain the P cytotype establishment by telomeric P elements in natural Drosophila populations

RNA 2′-O-Methylation (Nm) Modification in Human Diseases

Nm (2′-O-methylation) is one of the most common modifications in the RNA world. It has the potential to influence the RNA molecules in multiple ways, such as structure, stability, and interactions, and to play a role in various cellular processes from epigenetic gene regulation, through translation to self versus non-self recognition. Yet, building scientific knowledge on the Nm matter has been hampered for a long time by the challenges in detecting and mapping this modification. Today, with the latest advancements in the area, more and more Nm sites are discovered on RNAs (tRNA, rRNA, mRNA, and small non-coding RNA) and linked to normal or pathological conditions. This review aims to synthesize the Nm-associated human diseases known to date and to tackle potential indirect links to some other biological defects

Environmentally-Induced Transgenerational Epigenetic Inheritance: Implication of PIWI Interacting RNAs

International audienceEnvironmentally-induced transgenerational epigenetic inheritance is an emerging field. The understanding of associated epigenetic mechanisms is currently in progress with open questions still remaining. In this review, we present an overview of the knowledge of environmentally-induced transgenerational inheritance and associated epigenetic mechanisms, mainly in animals. The second part focuses on the role of PIWI-interacting RNAs (piRNAs), a class of small RNAs involved in the maintenance of the germline genome, in epigenetic memory to put into perspective cases of environmentally-induced transgenerational inheritance involving piRNA production. Finally, the last part addresses how genomes are facing production of new piRNAs, and from a broader perspective, how this process might have consequences on evolution and on sporadic disease development

A Long Terminal Repeat-Containing Retrotransposon of Schizosaccharomyces pombe Expresses a Gag-Like Protein That Assembles into Virus-Like Particles Which Mediate Reverse Transcription

The Tf1 element of Schizosaccharomyces pombe is a long terminal repeat-containing retrotransposon that encodes functional protease, reverse transcriptase, and integrase proteins. Although these proteins are known to be necessary for protein processing, reverse transcription, and integration, respectively, the function of the protein thought to be Gag has not been determined. We present here the first electron microscopy of Tf1 particles. We tested whether the putative Gag of Tf1 was required for particle formation, packaging of RNA, and reverse transcription. We generated deletions of 10 amino acids in each of the four hydrophilic domains of the protein and found that all four mutations reduced transposition activity. The N-terminal deletion removed a nuclear localization signal and inhibited nuclear import of the transposon. The two mutations in the center of Gag destabilized the protein and resulted in no virus-like particles. The C-terminal deletion caused a defect in RNA packaging and, as a result, low levels of cDNA. The electron microscopy of cells expressing a truncated Tf1 showed that Gag alone was sufficient for the formation of virus-like particles. Taken together, these results indicate that Tf1 encodes a Gag protein that is a functional equivalent of the Gag proteins of retroviruses

TSE is sensitive to mutations affecting <i>squash</i> and <i>zucchini</i>.

<p>(<b>A</b>) Expression control in ovaries of the <i>P-lacZ</i> transgene used as a TSE target (<i>BQ16</i>, located on chromosome <i>3</i>). (<b>B</b>) G₁ females produced from the cross between <i>P-1152</i> females and <i>BQ16</i> males. (<b>C–D</b>) Heteroallelic females for mutant alleles of <i>zuc</i> or <i>squ</i>: these females have inherited the <i>BQ16</i> target paternally and the <i>P-1152</i> telomeric silencer from a homozygous <i>P-1152</i> female. The maternally-introduced <i>zuc</i> or <i>squ</i> mutant allele is written first. In each case, the percentage of TSE is given with the total number of egg chambers assayed in parenthesis.</p

TSE is correlated with the presence of small RNAs whose production depends on the piRNA pathway and HP1.

<p>(<b>A–B</b>) RNAse protection was carried out using a <i>lacZ</i> sense riboprobe (150 nt) hybridized to RNAs extracted from ovaries from 3–6 day-old females. Data concerning the 20–30 nt region are shown together with aspecific bands used as a loading control (shown below). Canton^y was used as an M strain, (devoid of any <i>P</i> element or <i>P</i> transgene). (<b>A</b>) Small RNA detection and effect of mutations in <i>squash</i> and <i>zucchini</i>. WT corresponds to <i>P-1152</i> females which are wild-type for both <i>squ</i> and <i>zuc</i>. Two small RNAs (arrows) are highly abundant in females carrying the <i>P-1152</i> telomeric TSE silencer at the homozygous state (WT), but are not detected in ovaries of females devoid of the <i>P-1152</i> transgene (M). Females carrying the <i>P-1152</i> telomeric silencer at the homozygous state and mutant for <i>squash</i> and <i>zucchini</i> were analyzed. The same two abundant small RNAs found in <i>P-1152</i> (WT) can be detected in females carrying one functional allele of <i>squ</i> and <i>zuc</i>, but are undetectable in <i>squ</i> or <i>zuc</i> heteroallelic mutant females. Thus, accumulation of <i>lacZ</i> small RNAs occurring in <i>P-1152</i> ovaries requires <i>squ</i> and <i>zuc</i> functions. (<b>B</b>) TSE maternal effect and effect of mutations affecting HP1. TSE<b>+</b> indicates that this cross allows a strong TSE in G₁ females due to the maternal transmission of the telomeric <i>P-1152</i> silencer, whereas TSE<b>-</b> means that only a weak TSE is recovered from this cross in which <i>P-1152</i> is inherited paternally. <i>P-1152</i> homozygous females and M females were analyzed as positive and negative controls, respectively. The two most abundant small RNAs are indicated by arrows. A strong signal for these small RNAs is obtained for <i>P-1152</i> homozygous females and for females having inherited a <i>P-1152</i> transgene maternally (TSE+), but is undetectable in negative control M females. The signal for the small RNAs is significantly reduced for females having inherited <i>P-1152</i> paternally (TSE-), as well as for <i>P-1152</i> homozygous females carrying one null allele of <i>Su(var)205</i> which encodes HP1. Therefore, accumulation of <i>lacZ</i> small RNAs is correlated to the maternal effect of TSE and depends on HP1 dose.</p

piRNAs and epigenetic conversion in Drosophila

International audienceTransposable element (TE) activity is repressed in the Drosophila germline by Piwi-Interacting RNAs (piRNAs), a class of small non-coding RNAs. These piRNAs are produced by discrete genomic loci containing TE fragments. In a recent publication, we tested for the existence of a strict epigenetic induction of piRNA production capacity by a locus in the D. melanogaster genome. We used 2 lines carrying a transgenic 7-copy tandem cluster (P-lacZ-white) at the same genomic site. This cluster generates in both lines a local heterochromatic sector. One line (T-1) produces high levels of ovarian piRNAs homologous to the P-lacZ-white transgenes and shows a strong capacity to repress homologous sequences in trans, whereas the other line (BX2) is devoid of both of these capacities. The properties of these 2 lines are perfectly stable over generations. We have shown that the maternal transmission of a cytoplasm carrying piRNAs from the first line can confer to the inert transgenic locus of the second, a totally de novo capacity to produce high levels of piRNAs as well as the ability to induce homology-dependent silencing in trans. These new properties are stably inherited over generations (n > 50). Furthermore, the converted locus has itself become able to convert an inert transgenic locus via cytoplasmic maternal inheritance. This results in a stable epigenetic conversion process, which can be performed recurrently-a phenomenon termed paramutation and discovered in Maize 60 y ago. Paramutation in Drosophila corresponds to the first stable paramutation in animals and provides a model system to investigate the epigenetically induced emergence of a piRNA-producing locus, a crucial step in epigenome shaping. In this Extra View, we discuss some additional functional aspects and the possible molecular mechanism of this piRNA-linked paramutation

Inheritance of paramutation in plants and Drosophila

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