Piwi induces piRNA-guided transcriptional silencing and establishment of a repressive chromatin state

California Institute of Technology, Pasadena, California 91125, USA
Genes & development (Impact Factor: 10.8). 02/2013; 27(4). DOI: 10.1101/gad.209841.112
Source: PubMed


In the metazoan germline, piwi proteins and associated piwi-interacting RNAs (piRNAs) provide a defense system against the expression of transposable elements. In the cytoplasm, piRNA sequences guide piwi complexes to destroy complementary transposon transcripts by endonucleolytic cleavage. However, some piwi family members are nuclear, raising the possibility of alternative pathways for piRNA-mediated regulation of gene expression. We found that Drosophila Piwi is recruited to chromatin, colocalizing with RNA polymerase II (Pol II) on polytene chromosomes. Knockdown of Piwi in the germline increases expression of transposable elements that are targeted by piRNAs, whereas protein-coding genes remain largely unaffected. Derepression of transposons upon Piwi depletion correlates with increased occupancy of Pol II on their promoters. Expression of piRNAs that target a reporter construct results in a decrease in Pol II occupancy and an increase in repressive H3K9me3 marks and heterochromatin protein 1 (HP1) on the reporter locus. Our results indicate that Piwi identifies targets complementary to the associated piRNA and induces transcriptional repression by establishing a repressive chromatin state when correct targets are found.

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Available from: Adrien Le Thomas, Mar 06, 2014
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    • "This enhanced L1 insertion has also been demonstrated in vitro in a variety of cancer cell lines, which additionally display increased RNA polymerase II binding to TEs compared to non-malignant cell lines[49]. The PIWI protein localizes to the nucleus and works at the transcriptional level, establishing a repressive chromatin state as the result of an increase in H3K9me3 and HP1 chromatin marks (Fig. 2b)[41]. Overexpression of piRNA has been associated with the recruitment of PIWI, HP1a, and Su(var)3–9, the reduction of RNAPII,) includes: microRNAs (miRNAs), tRNA-derived fragment (tRF), PIWI-interacting RNAs (piRNAs), C/D snoRNA-derived RNA (sdRNAs), and small interfering RNAs (siRNAs). "
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    ABSTRACT: PIWI-interacting RNAs (piRNAs) are emerging players in cancer genomics. Originally described in the germline, there are over 20,000 piRNA genes in the human genome. In contrast to microRNAs, piRNAs interact with PIWI proteins, another member of the Argonaute family, and function primarily in the nucleus. There, they are involved in the epigenetic silencing of transposable elements in addition to the transcriptional regulation of genes. It has recently been demonstrated that piRNAs are also expressed across a variety of human somatic tissue types in a tissue-specific manner. An increasing number of studies have shown that aberrant piRNA expression is a signature feature across multiple tumour types; however, their specific tumorigenic functions remain unclear. In this article, we discuss the emerging functional roles of piRNAs in a variety of cancers, and highlight their potential clinical utilities.
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    • "During PTGS, piRNAs guide Aubergine (Aub) and Argonaute 3 (Ago3) to TE transcripts, hence inducing their cleavage through Aub/Ago3 slicing activity (Brennecke et al, 2007; Li et al, 2009; Malone et al, 2009). During TGS, piRNAs guide Piwi to genomic TE copies, likely through nascent transcript recognition, where it promotes heterochromatinization through the deposition of repressive H3K9 methylation marks (Sienski et al, 2012; Darricarrere et al, 2013; Huang et al, 2013; Le Thomas et al, 2013; Rozhkov et al, 2013). In the absence of piRNAs, TEs become expressed and induce various oogenesis defects (early arrest of egg chamber development, dorsoventral patterning defects, etc.) leading to sterility. "
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    ABSTRACT: RNase P is a conserved endonuclease that processes the 5' trailer of tRNA precursors. We have isolated mutations in Rpp30, a subunit of RNase P, and find that these induce complete sterility in Drosophila females. Here, we show that sterility is not due to a shortage of mature tRNAs, but that atrophied ovaries result from the activation of several DNA damage checkpoint proteins, including p53, Claspin, and Chk2. Indeed, we find that tRNA processing defects lead to increased replication stress and de-repression of transposable elements in mutant ovaries. We also report that transcription of major piRNA sources collapse in mutant germ cells and that this correlates with a decrease in heterochromatic H3K9me3 marks on the corresponding piRNA-producing loci. Our data thus link tRNA processing, DNA replication, and genome defense by small RNAs. This unexpected connection reveals constraints that could shape genome organization during evolution.
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    • "Piwi itself localizes to the nucleus and is involved in transcriptional repression through installation of the repressive H3K9me3 chromatin mark on target loci (Le Thomas et al., 2013; Rozhkov et al., 2013; Shpiz et al., 2011; Sienski et al., 2012). In contrast to Piwi, both Aub and Ago3 are cytoplasmic and endonucleolytically cleave transcripts complementary to their piRNA guide (Brennecke et al., 2007; Gunawardane et al., 2007; Huang et al., 2014). "
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    ABSTRACT: In Drosophila, two Piwi proteins, Aubergine (Aub) and Argonaute-3 (Ago3), localize to perinuclear "nuage" granules and use guide piRNAs to target and destroy transposable element transcripts. We find that Aub and Ago3 are recruited to nuage by two different mechanisms. Aub requires a piRNA guide for nuage recruitment, indicating that its localization depends on recognition of RNA targets. Ago3 is recruited to nuage independently of a piRNA cargo and relies on interaction with Krimper, a stable component of nuage that is able to aggregate in the absence of other nuage proteins. We show that Krimper interacts directly with Aub and Ago3 to coordinate the assembly of the ping-pong piRNA processing (4P) complex. Symmetrical dimethylated arginines are required for Aub to interact with Krimper, but they are dispensable for Ago3 to bind Krimper. Our study reveals a multi-step process responsible for the assembly and function of nuage complexes in piRNA-guided transposon repression. Copyright © 2015 Elsevier Inc. All rights reserved.
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