Impact of nuclear Piwi elimination on chromatin state in Drosophila melanogaster ovaries

Abstract

The Piwi-interacting RNA (piRNA)-interacting Piwi protein is involved in transcriptional silencing of transposable elements in ovaries of Drosophila melanogaster. Here we characterized the genome-wide effect of nuclear Piwi elimination on the presence of the heterochromatic H3K9me3 mark and HP1a, as well as on the transcription-associated mark H3K4me2. Our results demonstrate that a significant increase in the H3K4me2 level upon nuclear Piwi loss is not accompanied by the alterations in H3K9me3 and HP1a levels for several germline-expressed transposons, suggesting that in this case Piwi prevents transcription by a mechanism distinct from H3K9 methylation. We found that the targets of Piwi-dependent chromatin repression are mainly related to the elements that display a higher level of H3K4me2 modification in the absence of silencing, i.e. most actively transcribed elements. We also show that Piwi-guided silencing does not significantly influence the chromatin state of dual-strand piRNA-producing clusters. In addition, host protein-coding gene expression is essentially not affected due to the nuclear Piwi elimination, but we noted an increase in small nuclear spliceosomal RNAs abundance and propose Piwi involvement in their post-transcriptional regulation. Our work reveals new aspects of transposon silencing in Drosophila, indicating that transcription of transposons can underpin their Piwi dependent silencing, while canonical heterochromatin marks are not obligatory for their repression.

The Ecdysone Receptor Coactivator Taiman Links Yorkie to Transcriptional Control of Germline Stem Cell Factors in Somatic Tissue

“…Intriguingly, the miR bantam is required to maintain female germline stem cells (Yang et al, 2009) and directly represses the hid mRNA (Brennecke et al., 2003), suggesting that Nanos and bantam may coregulate the hid mRNA downstream of hyperactive Yki. Piwi interacts with piRNAs in the germ cell cytoplasm to repress target mRNAs but also has a critical nuclear role in formation of repressive chromatin on specific genomic loci (Klenov et al, 2014; Le Thomas et al, 2013). Both Nanos and Piwi play key roles in blocking differentiation and supporting self-renewing divisions in the germline (Cox et al, 1998; Cox et al, 2000; Forbes and Lehmann, 1998; Kobayashi et al, 1996; Lin and Spradling, 1997), suggesting that disc cells with hyperactive Yki autonomously adopt elements of a germline transcriptional program.…”

piRNA Biogenesis in Drosophila melanogaster

“…Many factors required for piRNA biogenesis in follicular cells such as the putative RNA helicase Armitage (Armi), the Tudor-domain protein Vreteno (Vret) and Yb co-localize in perinuclear foci that were termed Yb bodies [85–90]. Though at steady-state Piwi is present exclusively in the nucleus of follicular cells, deletion of its nuclear localization signal leads to Piwi localization to Yb bodies [40]. Therefore, it is plausible that Yb bodies represent a site of piRNA processing and loading into Piwi before the Piwi/piRNA complex relocates to the nucleus.…”

Transposons, Genome Size, and Evolutionary Insights in Animals

“…Initially, methylation was thought to be the only responsible mechanism for transposon control in plants, and only later was it noted that transposon silencing depends on the interference of siRNAs [Ito, 2013]. The prevailing mechanism in animals is piRNAs [Wheeler, 2013;Klenov et al, 2014], which have a very ancient origin given their presence in the sponge Amphimedon and in the cnidarian Nematostella [Grimson et al, 2008].…”