PIWI-interacting RNAs (piRNAs) are a class of small RNAs that are 24-31 nucleotides in length. They associate with PIWI proteins, which constitute a germline-specific subclade of the Argonaute family, to form effector complexes known as piRNA-induced silencing complexes, which repress transposons via transcriptional or posttranscriptional mechanisms and maintain germline genome integrity. In addition to having a role in transposon silencing, piRNAs in diverse organisms function in the regulation of cellular genes. In some cases, piRNAs have shown transgenerational inheritance to pass on the memory of "self" and "nonself," suggesting a contribution to various cellular processes over generations. Many piRNA factors have been identified; however, both the molecular mechanisms leading to the production of mature piRNAs and the effector phases of gene silencing are still enigmatic. Here, we summarize the current state of our knowledge on the biogenesis of piRNA, its biological functions, and the underlying mechanisms.

PIWI-interacting RNA (piRNA) biogenesis consists of two sequential steps: primary piRNA processing and the ping-pong cycle that depends on reciprocal Slicer-mediated RNA cleavage by PIWI proteins. However, the molecular functions of the factors involved remain elusive. Here, we show that RNAs cleaved by a Bombyx mori PIWI, Siwi, remain bound to the protein upon cleavage but are released by a DEAD box protein BmVasa. BmVasa copurifies with Siwi but not another PIWI BmAgo3. A lack of BmVasa does not affect primary piRNA processing but abolishes the ping-pong cycle. Siwi also forms a complex with BmSpn-E and BmQin. This complex is physically separable from the Siwi/BmVasa complex. BmSpn-E, unlike BmVasa, is necessary for primary piRNA production. We propose a model for piRNA biogenesis, where the BmSpn-E/BmQin dimer binds Siwi to function in primary piRNA processing, whereas BmVasa, by associating with Siwi, ensures target RNA release upon cleavage to facilitate the ping-pong cycle.

The Piwi-piRNA (PIWI-interacting RNA) complex (PiwipiRISC) in Drosophila ovarian somatic cells represses transposons transcriptionally to maintain genome integrity; however, the underlying mechanisms remain obscure. Here, we reveal that DmGTSF1, a Drosophila homolog of gametocyte-specific factor 1 (GTSF1) (which is required for transposon silencing in mouse testes), is necessary for Piwi-piRISC to repress target transposons and neighboring genes. DmGTSF1 depletion affected neither piRNA biogenesis nor nuclear import of PiwipiRISC. DmGTSF1 mutations caused derepression of transposons and loss of ovary follicle layers, resulting in female infertility. We suggest that DmGTSF1, a nuclear Piwi interactor, is an integral factor in Piwi-piRISCmediated transcriptional silencing.

PIWI-interacting RNAs (piRNAs) mediate transcriptional and post-transcriptional silencing of transposable element (TE) in animal gonads. In Drosophila ovaries, Piwi-piRNA complexes (Piwi-piRISCs) repress TE transcription by modifying the chromatin state, such as by H3K9 trimethylation. Here, we demonstrate that Piwi physically interacts with linker histone H1. Depletion of Piwi decreases H1 density at a subset of TEs, leading to their derepression. Silencing at these loci separately requires H1 and H3K9me3 and heterochromatin protein 1a (HP1a). Loss of H1 increases target loci chromatin accessibility without affecting H3K9me3 density at these loci, while loss of HP1a does not impact H1 density. Thus, Piwi-piRISCs require both H1 and HP1a to repress TEs, and the silencing is correlated with the chromatin state rather than H3K9me3 marks. These findings suggest that Piwi-piRISCs regulate the interaction of chromatin components with target loci to maintain silencing of TEs through the modulation of chromatin accessibility.

Persistent colonization of the gastric mucosa by Helicobacter pylori (Hp) elicits chronic inflammation and aberrant epithelial cell proliferation, which increases the risk of gastric cancer. Here we examine the ability of microRNAs to modulate gastric cell proliferation in response to persistent Hp infection and find that epigenetic silencing of miR-210 plays a key role in gastric disease progression. Importantly, DNA methylation of the miR-210 gene is increased in Hp-positive human gastric biopsies as compared with Hp-negative controls. Moreover, silencing of miR-210 in gastric epithelial cells promotes proliferation. We identify STMN1 and DIMT1 as miR-210 target genes and demonstrate that inhibition of miR-210 expression augments cell proliferation by activating STMN1 and DIMT1 . Together, our results highlight inflammation-induced epigenetic silencing of miR-210 as a mechanism of induction of chronic gastric diseases, including cancer, during Hp infection.