Multiple roles for Piwi in silencing Drosophila transposons

Abstract

Silencing of transposons in the Drosophila ovary relies on three Piwi family proteins-Piwi, Aubergine (Aub), and Ago3-acting in concert with their small RNA guides, the Piwi-interacting RNAs (piRNAs). Aub and Ago3 are found in the germ cell cytoplasm, where they function in the ping-pong cycle to consume transposon mRNAs. The nuclear Piwi protein is required for transposon silencing in both germ and somatic follicle cells, yet the precise mechanisms by which Piwi acts remain largely unclear. We investigated the role of Piwi by combining cell typespecific knockdowns with measurements of steady-state transposon mRNA levels, nascent RNA synthesis, chromatin state, and small RNA abundance. In somatic cells, Piwi loss led to concerted effects on nascent transcripts and transposon mRNAs, indicating that Piwi acts through transcriptional gene silencing (TGS). In germ cells, Piwi loss showed disproportionate impacts on steady-state RNA levels, indicating that it also exerts an effect on post-transcriptional gene silencing (PTGS). Piwi knockdown affected levels of germ cell piRNAs presumably bound to Aub and Ago3, perhaps explaining its post-transcriptional impacts. Overall, our results indicate that Piwi plays multiple roles in the piRNA pathway, in part enforcing transposon repression through effects on local chromatin states and transcription but also participating in germ cell piRNA biogenesis.

Small but sturdy: small RNAs in cellular memory and epigenetics

“…Although it is not yet clear whether piRNAs are indeed required for determining the chromatin state on piRNA clusters, piRNAs associated with PIWI do guide deposition of the H3K9me3 mark on their genomic targets (Klenov et al 2007;Wang and Elgin 2011;Sienski et al 2012;Gu and Elgin 2013;Huang et al 2013;Le Thomas et al 2013;Rozhkov et al 2013;Yakushev et al 2013). According to the model that emerged from studies of nuclear PIWI function, piRNAs guide the nuclear PIWI protein to complementary nascent transcripts followed by recruitment of a histone methyltransferase (e.g., dSETDB1) and heterochromatinization of the target locus (Fig.…”

Antisense Transcription of Retrotransposons in Drosophila: An Origin of Endogenous Small Interfering RNA Precursors

“…In fission yeast and the Drosophila germline, movement of repetitive sequences and retroTns, respectively, are repressed by a transcriptional gene silencing mechanism wherein siRNAs induce heterochromatin formation (Huisinga and Elgin 2009;Wang and Elgin 2011;Sienski et al 2012;Huang et al 2013;Le Thomas et al 2013;Rozhkov et al 2013). In Schizosaccharomyces pombe, siRNAs are produced from RNA-dependent RNA polymerase generated dsRNA precursors by Dicer 1 (Volpe et al 2002;Yu et al 2014;Holoch and Moazed 2015).…”

The RNA helicase MOV10L1 binds piRNA precursors to initiate piRNA processing

“…Piwi-interacting RNAs (piRNAs) are a class of small RNAs (23-30 nucleotides [nt]) that bind to Piwi proteins to form piRNPs (Piwi-piRNA ribonucleoproteins), which silence selfish genetic elements in the germline of metazoans (Aravin et al 2004(Aravin et al , 2006Girard et al 2006;Grivna et al 2006;Lau et al 2006;Vagin et al 2006;KuramochiMiyagawa et al 2008;Ghildiyal and Zamore 2009;Siomi et al 2011;Sienski et al 2012;Huang et al 2013;Rozhkov et al 2013). Loss of Piwi genes results in retrotransposon up-regulation, arrest of gametogenesis, and infertility (Cox et al 1998;Harris and Macdonald 2001;Deng and Lin 2002;Kuramochi-Miyagawa et al 2004;Carmell et al 2007;Li et al 2009).…”