Drosophila Piwi functions downstream of piRNA production mediating a chromatin-based transposon silencing mechanism in female germ line

Abstract

Transposon control is a critical process during reproduction. The PIWI family proteins can play a key role, using a piRNA-mediated slicing mechanism to suppress transposon activity posttranscriptionally. In Drosophila melanogaster , Piwi is predominantly localized in the nucleus and has been implicated in heterochromatin formation. Here, we use female germ-line–specific depletion to study Piwi function. This depletion of Piwi leads to infertility and to axis specification defects in the developing egg chambers; correspondingly, widespread loss of transposon silencing is observed. Germ-line Piwi does not appear to be required for piRNA production. Instead, Piwi requires Aubergine (and presumably secondary piRNA) for proper localization. A subset of transposons that show significant overexpression in germ-line Piwi-depleted ovaries exhibit a corresponding loss of HP1a and H3K9me2. Germ-line HP1a depletion also leads to a loss of transposon silencing, demonstrating the functional requirement for HP1a enrichment at these loci. Considering our results and those of others, we infer that germ-line Piwi functions downstream of piRNA production to promote silencing of some transposons via recruitment of HP1a. Thus, in addition to its better-known function in posttranscriptional silencing, piRNA also appears to function in a targeting mechanism for heterochromatin formation mediated by Piwi.

Ectopic assembly of heterochromatin in Drosophila melanogaster triggered by transposable elements

“…Other TEs with piRNA signals include HeT-A, I element, and copia (20). All three transposon families are silenced by HP1a and H3K9me2/3 assembly over their promoter elements in the female germ line by a mechanism dependent on RNAi components (22,37). piRNA biogenesis is most active at this stage, and it has been argued that its effects may be limited to germ cells.…”

“…A third component, Piwi, is a nuclear protein that binds mostly antisense piRNAs, a subset of which are generated by ping-pong cycle-dependent piRNA biogenesis (17,20). Piwi interacts with HP1a and has been implicated in a transcriptional silencing role in the germ line (21)(22)(23). These piRNA components are loaded into the oocyte (17,24); their mode of TE repression in the early embryo has been relatively unexplored but could include a role in establishing heterochromatin domains.…”

From guide to target: molecular insights into eukaryotic RNA-interference machinery

“…Subsequently, the RNA products of this reaction may be loaded into the complementary Piwi-clade protein Ago3 for ping-pong amplification [107] (Box 1). Piwi also appears to initiate transcriptional gene silencing (TGS) by shuttling to the nucleus, recognizing transposon transcripts, and recruiting DNA [109,110] and/or chromatin modifiers, [111,112] which repress transcription of transposon loci (Box 2). This link between RNAi and heterochromatin formation is highly reminiscent of RNA induced transcriptional silencing (RITS) in budding yeast.…”

Heterochromatin-Associated Proteins HP1a and Piwi Collaborate to Maintain the Association of Achiasmate Homologs in Drosophila Oocytes

“…Most piRNA sequences in the Drosophila genome map to heterochromatic regions (Saito et al 2006), and several lines of investigation have demonstrated that Piwi and HP1a can physically interact (Brower-Toland et al 2007;Mendez et al 2011). Moreover, transcriptional silencing of retrotransposons by the Piwi-piRNA system involves recruitment of HP1a to transposable elements (Wang and Elgin 2011;Sentmanat and Elgin 2012;Huang et al 2013). Piwi also participates in activities that are not directly related to transposon silencing.…”