Article

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: 12.64). 02/2013; 27(4). DOI: 10.1101/gad.209841.112
Source: PubMed

ABSTRACT 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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    • "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.
    Molecular cell 08/2015; 59(4):564-575. DOI:10.1016/j.molcel.2015.07.017 · 14.46 Impact Factor
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    • "In Schizosaccharomyces pombe, centromeric repeats region–derived siRNAs associate with Argonaute 1 and form the RNA-induced transcriptional silencing (RITS) complex to induce the heterochromatin assembly (Volpe et al. 2002; Motamedi et al. 2004; Verdel et al. 2004). In Caenorhabditis elegans, siRNAs and piRNAs can enter the nucleus and trigger co-transcriptional silencing of genes (Guang et al. 2008, 2010; Le Thomas et al. 2013). SiRNA-directed recruitment of the nuclear RNAi components , the NRDE factors, could inhibit RNA Pol II during the elongation phase of transcription (Guang et al. 2010). "
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    • "In Schizosaccharomyces pombe, centromeric repeats region–derived siRNAs associate with Argonaute 1 and form the RNA-induced transcriptional silencing (RITS) complex to induce the heterochromatin assembly (Volpe et al. 2002; Motamedi et al. 2004; Verdel et al. 2004). In Caenorhabditis elegans, siRNAs and piRNAs can enter the nucleus and trigger co-transcriptional silencing of genes (Guang et al. 2008, 2010; Le Thomas et al. 2013). SiRNA-directed recruitment of the nuclear RNAi components , the NRDE factors, could inhibit RNA Pol II during the elongation phase of transcription (Guang et al. 2010). "
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    ABSTRACT: The TATA box represents one of the most prevalent core promoters where the pre-initiation complexes (PICs) for gene transcription are assembled. This assembly is crucial for transcription initiation and well regulated. Here we show that some cellular microRNAs (miRNAs) are associated with RNA polymerase II (Pol II) and TATA box-binding protein (TBP) in human peripheral blood mononuclear cells (PBMCs). Among them, let-7i sequence specifically binds to the TATA-box motif of interleukin-2 (IL-2) gene and elevates IL-2 mRNA and protein production in CD4+ T-lymphocytes in vitro and in vivo. Through direct interaction with the TATA-box motif, let-7i facilitates the PIC assembly and transcription initiation of IL-2 promoter. Several other cellular miRNAs, such as mir-138, mir-92a or mir-181d, also enhance the promoter activities via binding to the TATA-box motifs of insulin, calcitonin or c-myc, respectively. In agreement with the finding that an HIV-1–encoded miRNA could enhance viral replication through targeting the viral promoter TATA-box motif, our data demonstrate that the interaction with core transcription machinery is a novel mechanism for miRNAs to regulate gene expression.
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