A critical role for nucleoporin 358 (Nup358) in transposon silencing and piRNA biogenesis in Drosophila.

Piwi-interacting RNAs (piRNAs) are a class of small noncoding RNAs that bind Piwi proteins to silence transposons and to regulate gene expression. In Drosophila germ cells, the Aubergine (Aub)-Argonaute 3 (Ago3)-dependent ping-pong cycle generates most germline piRNAs. Loading of antisense piRNAs amplified by this cycle enables Piwi to enter the nucleus and silence transposons. Nuclear localization is crucial for Piwi function in transposon silencing, but how this process is regulated remains unknown. It is also not known whether any of the components of the nuclear pore complex (NPC) directly function in the piRNA pathway. Here, we show that nucleoporin 358 (Nup358) and Piwi interact with each other and that a germline knockdown (GLKD) of Nup358 with short hairpin RNA prevents Piwi entry into the nucleus. The Nup358 GLKD also activated transposons, increased genomic instability, and derailed piRNA biogenesis because of a combination of decreased piRNA precursor transcription and a collapse of the ping-pong cycle. Our results point to a critical role for Nup358 in the piRNA pathway, laying the foundation for future studies to fully elucidate the mechanisms by which Nup358 contributes to piRNA biogenesis and transposon silencing.

Parikh RY ，Lin H ，Gangaraju VK 《-》

Co-chaperone Hsp70/Hsp90-organizing protein (Hop) is required for transposon silencing and Piwi-interacting RNA (piRNA) biogenesis.

Piwi-interacting RNAs (piRNAs) are 26-30-nucleotide germ line-specific small non-coding RNAs that have evolutionarily conserved function in mobile genetic element (transposons) silencing and maintenance of genome integrity. Drosophila Hsp70/90-organizing protein homolog (Hop), a co-chaperone, interacts with piRNA-binding protein Piwi and mediates silencing of phenotypic variations. However, it is not known whether Hop has a direct role in piRNA biogenesis and transposon silencing. Here, we show that knockdown of Hop in the germ line nurse cells (GLKD) of Drosophila ovaries leads to activation of transposons. Hop GLKD females can lay eggs at the same rate as wild-type counterparts, but the eggs do not hatch into larvae. Hop GLKD leads to the accumulation of γ-H2Av foci in the germ line, indicating increased DNA damage in the ovary. We also show that Hop GLKD-induced transposon up-regulation is due to inefficient piRNA biogenesis. Based on these results, we conclude that Hop is a critical component of the piRNA pathway and that it maintains genome integrity by silencing transposons.

Karam JA ，Parikh RY ，Nayak D ，Rosenkranz D ，Gangaraju VK ... -  《-》

Biogenesis pathways of piRNAs loaded onto AGO3 in the Drosophila testis.

Nagao A ，Mituyama T ，Huang H ，Chen D ，Siomi MC ，Siomi H ... -  《-》

Heterotypic piRNA Ping-Pong requires qin, a protein with both E3 ligase and Tudor domains.

piRNAs guide PIWI proteins to silence transposons in animal germ cells. Reciprocal cycles of piRNA-directed RNA cleavage--catalyzed by the PIWI proteins Aubergine (Aub) and Argonaute3 (Ago3) in Drosophila melanogaster--expand the population of antisense piRNAs in response to transposon expression, a process called the Ping-Pong cycle. Heterotypic Ping-Pong between Aub and Ago3 ensures that antisense piRNAs predominate. We show that qin, a piRNA pathway gene whose protein product contains both E3 ligase and Tudor domains, colocalizes with Aub and Ago3 in nuage, a perinuclear structure implicated in transposon silencing. In qin mutants, less Ago3 binds Aub, futile Aub:Aub homotypic Ping-Pong prevails, antisense piRNAs decrease, many families of mobile genetic elements are reactivated, and DNA damage accumulates in nurse cells and oocytes. We propose that Qin enforces heterotypic Ping-Pong between Aub and Ago3, ensuring that transposons are silenced and maintaining the integrity of the germline genome.

Zhang Z ，Xu J ，Koppetsch BS ，Wang J ，Tipping C ，Ma S ，Weng Z ，Theurkauf WE ，Zamore PD ... -  《-》

Two distinct transcriptional controls triggered by nuclear Piwi-piRISCs in the Drosophila piRNA pathway.

Transposons occupy a large proportion of eukaryotic genomes. Spontaneous movement of transposons within the genome leads to genomic mutations that are often life threatening. To avoid such outcomes, host species have acquired an elaborate molecular pathway during evolution, in which PIWI proteins and PIWI-interacting RNAs (piRNAs) collaboratively control transposons to maintain germline genome integrity. Studies of PIWI-piRNA-based transposon silencing have uncovered the molecular mechanisms underlying piRNA biogenesis and the silencing mode-of-action in the pathway. Interestingly, the bilateral events require transcriptional controls triggered by nuclear PIWI-piRNA complexes in positive and negative manners, respectively. This review summarizes the mechanisms of PIWI-piRNA-mediated transcriptional controls in Drosophila ovaries.

Sato K ，Siomi MC 《-》