PIWI-Interacting RNA: Its Biogenesis and Functions.

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-piRNA complexes induce stepwise changes in nuclear architecture at target loci.

PIWI-interacting RNAs (piRNAs) are germline-specific small RNAs that form effector complexes with PIWI proteins (Piwi-piRNA complexes) and play critical roles for preserving genomic integrity by repressing transposable elements (TEs). Drosophila Piwi transcriptionally silences specific targets through heterochromatin formation and increases histone H3K9 methylation (H3K9me3) and histone H1 deposition at these loci, with nuclear RNA export factor variant Nxf2 serving as a co-factor. Using ChEP and DamID-seq, we now uncover a Piwi/Nxf2-dependent target association with nuclear lamins. Hi-C analysis of Piwi or Nxf2-depleted cells reveals decreased intra-TAD and increased inter-TAD interactions in regions harboring Piwi-piRNA target TEs. Using a forced tethering system, we analyze the functional effects of Piwi-piRNA/Nxf2-mediated recruitment of piRNA target regions to the nuclear periphery. Removal of active histone marks is followed by transcriptional silencing, chromatin conformational changes, and H3K9me3 and H1 association. Our data show that the Piwi-piRNA pathway can induce stepwise changes in nuclear architecture and chromatin state at target loci for transcriptional silencing.

Hierarchical roles of mitochondrial Papi and Zucchini in Bombyx germline piRNA biogenesis.

PIWI-interacting RNAs (piRNAs) are small regulatory RNAs that bind to PIWI proteins to control transposons and maintain genome integrity in animal germ lines. piRNA 3' end formation in the silkworm Bombyx mori has been shown to be mediated by the 3'-to-5' exonuclease Trimmer (Trim; known as PNLDC1 in mammals), and piRNA intermediates are bound with PIWI anchored onto mitochondrial Tudor domain protein Papi. However, it remains unclear whether the Zucchini (Zuc) endonuclease and Nibbler (Nbr) 3'-to-5' exonuclease, both of which have pivotal roles in piRNA biogenesis in Drosophila, are required for piRNA processing in other species. Here we show that the loss of Zuc in Bombyx had no effect on the levels of Trim and Nbr, but resulted in the aberrant accumulation of piRNA intermediates within the Papi complex, and that these were processed to form mature piRNAs by recombinant Zuc. Papi exerted its RNA-binding activity only when bound with PIWI and phosphorylated, suggesting that complex assembly involves a hierarchical process. Both the 5' and 3' ends of piRNA intermediates within the Papi complex showed hallmarks of PIWI 'slicer' activity, yet no phasing pattern was observed in mature piRNAs. The loss of Zuc did not affect the 5'- and 3'-end formation of the intermediates, strongly supporting the idea that the 5' end of Bombyx piRNA is formed by PIWI slicer activity, but independently of Zuc, whereas the 3' end is formed by the Zuc endonuclease. The Bombyx piRNA biogenesis machinery is simpler than that of Drosophila, because Bombyx has no transcriptional silencing machinery that relies on phased piRNAs.

Piwi Modulates Chromatin Accessibility by Regulating Multiple Factors Including Histone H1 to Repress Transposons.

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.

Mod(mdg4) variants repress telomeric retrotransposon HeT-A by blocking subtelomeric enhancers.

Telomeres in Drosophila are composed of sequential non-LTR retrotransposons HeT-A, TART and TAHRE. Although they are repressed by the PIWI-piRNA pathway or heterochromatin in the germline, the regulation of these retrotransposons in somatic cells is poorly understood. In this study, we demonstrated that specific splice variants of Mod(mdg4) repress HeT-A by blocking subtelomeric enhancers in ovarian somatic cells. Among the variants, we found that the Mod(mdg4)-N variant represses HeT-A expression the most efficiently. Subtelomeric sequences bound by Mod(mdg4)-N block enhancer activity within subtelomeric TAS-R repeats. This enhancer-blocking activity is increased by the tandem association of Mod(mdg4)-N to repetitive subtelomeric sequences. In addition, the association of Mod(mdg4)-N couples with the recruitment of RNA polymerase II to the subtelomeres, which reinforces its enhancer-blocking function. Our findings provide novel insights into how telomeric retrotransposons are regulated by the specific variants of insulator proteins associated with subtelomeric sequences.

Loss of l(3)mbt leads to acquisition of the ping-pong cycle in Drosophila ovarian somatic cells.

In Drosophila germ cells, PIWI-interacting RNAs (piRNAs) are amplified through a PIWI slicer-dependent feed-forward loop termed the ping-pong cycle, yielding secondary piRNAs. However, the detailed mechanism remains poorly understood, largely because an ex vivo model system amenable to biochemical analyses has not been available. Here, we show that CRISPR-mediated loss of function of lethal (3) malignant brain tumor [l(3)mbt] leads to ectopic activation of the germ-specific ping-pong cycle in ovarian somatic cells. Perinuclear foci resembling nuage, the ping-pong center, appeared following l(3)mbt mutation. This activation of the ping-pong machinery in cultured cells will greatly facilitate elucidation of the mechanism underlying secondary piRNA biogenesis in Drosophila.

Nuclear RNA export factor variant initiates piRNA-guided co-transcriptional silencing.

The PIWI-interacting RNA (piRNA) pathway preserves genomic integrity by repressing transposable elements (TEs) in animal germ cells. Among PIWI-clade proteins in Drosophila, Piwi transcriptionally silences its targets through interactions with cofactors, including Panoramix (Panx) and forms heterochromatin characterized by H3K9me3 and H1. Here, we identified Nxf2, a nuclear RNA export factor (NXF) variant, as a protein that forms complexes with Piwi, Panx, and p15. Panx-Nxf2-P15 complex formation is necessary in the silencing by stabilizing protein levels of Nxf2 and Panx. Notably, ectopic targeting of Nxf2 initiates co-transcriptional repression of the target reporter in a manner independent of H3K9me3 marks or H1. However, continuous silencing requires HP1a and H1. In addition, Nxf2 directly interacts with target TE transcripts in a Piwi-dependent manner. These findings suggest a model in which the Panx-Nxf2-P15 complex enforces the association of Piwi with target transcripts to trigger co-transcriptional repression, prior to heterochromatin formation in the nuclear piRNA pathway. Our results provide an unexpected connection between an NXF variant and small RNA-mediated co-transcriptional silencing.

Krimper Enforces an Antisense Bias on piRNA Pools by Binding AGO3 in the Drosophila Germline.

Piwi-interacting RNAs (piRNAs) suppress transposon activity in animal germ cells. In the Drosophila ovary, primary Aubergine (Aub)-bound antisense piRNAs initiate the ping-pong cycle to produce secondary AGO3-bound sense piRNAs. This increases the number of secondary Aub-bound antisense piRNAs that can act to destroy transposon mRNAs. Here we show that Krimper (Krimp), a Tudor-domain protein, directly interacts with piRNA-free AGO3 to promote symmetrical dimethylarginine (sDMA) modification, ensuring sense piRNA-loading onto sDMA-modified AGO3. In aub mutant ovaries, AGO3 associates with ping-pong signature piRNAs, suggesting AGO3's compatibility with primary piRNA loading. Krimp sequesters ectopically expressed AGO3 within Krimp bodies in cultured ovarian somatic cells (OSCs), in which only the primary piRNA pathway operates. Upon krimp-RNAi in OSCs, AGO3 loads with piRNAs, further showing the capacity of AGO3 for primary piRNA loading. We propose that Krimp enforces an antisense bias on piRNA pools by binding AGO3 and blocking its access to primary piRNAs.

Hamster PIWI proteins bind to piRNAs with stage-specific size variations during oocyte maturation.

In animal gonads, transposable elements are actively repressed to preserve genome integrity through the PIWI-interacting RNA (piRNA) pathway. In mice, piRNAs are abundantly expressed in male germ cells, and form effector complexes with three distinct PIWIs. The depletion of individual Piwi genes causes male-specific sterility with no discernible phenotype in female mice. Unlike mice, most other mammals have four PIWI genes, some of which are expressed in the ovary. Here, purification of PIWI complexes from oocytes of the golden hamster revealed that the size of the PIWIL1-associated piRNAs changed during oocyte maturation. In contrast, PIWIL3, an ovary-specific PIWI in most mammals, associates with short piRNAs only in metaphase II oocytes, which coincides with intense phosphorylation of the protein. An improved high-quality genome assembly and annotation revealed that PIWIL1- and PIWIL3-associated piRNAs appear to share the 5'-ends of common piRNA precursors and are mostly derived from unannotated sequences with a diminished contribution from TE-derived sequences, most of which correspond to endogenous retroviruses. Our findings show the complex and dynamic nature of biogenesis of piRNAs in hamster oocytes, and together with the new genome sequence generated, serve as the foundation for developing useful models to study the piRNA pathway in mammalian oocytes.

miRNA regulatory ecosystem in early development.

MicroRNAs (miRNAs) reshape spatiotemporal gene expression by both modulating the levels of actively transcribed genes and accelerating the clearance of previously transcribed messages, thereby promoting the transition from a preceding stage to subsequent processes during development. Lee et al. (2014) now demonstrate that maternal miRNAs are adenylated by Wispy, which leads to clearing of maternal miRNAs during early embryogenesis.

Rewiring of chromatin state and gene expression by transposable elements.

Transposable elements form a major fraction of the genome in various eukaryotic species. Although deleterious effects of transpositions within the genome have been reported, recent findings suggest that transposable elements can function as novel regulatory elements to fine-tune gene expression. Transposable elements can impact the chromatin state through processes such as heterochromatin formation, enhancer-promoter interactions, and chromatin boundary formation, mainly because of the functions of chromatin-based pathways that regulate the expression of transposable elements via DNA methylation and repressive histone modification. Therefore, transposable elements can rewire the chromatin state and gene expression depending on their insertions. Here, we review the findings that reveal the role of transposable elements as modifiers of the chromatin state and gene expression as well as the molecular mechanisms capable of inducing these changes.

DmGTSF1 is necessary for Piwi-piRISC-mediated transcriptional transposon silencing in the Drosophila ovary.

The Piwi-piRNA (PIWI-interacting RNA) complex (Piwi-piRISC) 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 Piwi-piRISC. 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-piRISC-mediated transcriptional silencing.

Deep sequencing and high-throughput analysis of PIWI-associated small RNAs.

Small RNAs are now known to be major regulatory factors of gene expression. Emerging methods based on deep-sequencing have enabled the analysis of small RNA expression in a high-throughput manner, leading to the identification of large numbers of small RNAs in various species. Moreover, profiling small RNA data together with transcriptome data enables transcriptional and post-transcriptional regulation mediated by small RNAs to be hypothesized. Here, we isolated PIWIL1 (MIWI)-associated small RNAs from mouse testes, and performed small RNA-seq analysis. In addition, directional RNA-seq was performed using Piwil1 mutant mouse testes. Using these data, we describe protocols for analyzing small RNA-seq reads to obtain profiles of small RNAs associated with PIWI proteins. We also present bioinformatic protocols for analyzing RNA-seq reads that aim to annotate expression of piRNA clusters and identify genes regulated by piRNAs.

piRNAs derived from ancient viral processed pseudogenes as transgenerational sequence-specific immune memory in mammals.

Endogenous bornavirus-like nucleoprotein elements (EBLNs) are sequences within vertebrate genomes derived from reverse transcription and integration of ancient bornaviral nucleoprotein mRNA via the host retrotransposon machinery. While species with EBLNs appear relatively resistant to bornaviral disease, the nature of this association is unclear. We hypothesized that EBLNs could give rise to antiviral interfering RNA in the form of PIWI-interacting RNAs (piRNAs), a class of small RNA known to silence transposons but not exogenous viruses. We found that in both rodents and primates, which acquired their EBLNs independently some 25-40 million years ago, EBLNs are present within piRNA-generating regions of the genome far more often than expected by chance alone (ℙ = 8 × 10(-3)-6 × 10(-8)). Three of the seven human EBLNs fall within annotated piRNA clusters and two marmoset EBLNs give rise to bona fide piRNAs. In both rats and mice, at least two of the five EBLNs give rise to abundant piRNAs in the male gonad. While no EBLNs are syntenic between rodent and primate, some of the piRNA clusters containing EBLNs are; thus we deduce that EBLNs were integrated into existing piRNA clusters. All true piRNAs derived from EBLNs are antisense relative to the proposed ancient bornaviral nucleoprotein mRNA. These observations are consistent with a role for EBLN-derived piRNA-like RNAs in interfering with ancient bornaviral infection. They raise the hypothesis that retrotransposon-dependent virus-to-host gene flow could engender RNA-mediated, sequence-specific antiviral immune memory in metazoans analogous to the CRISPR/Cas system in prokaryotes.

Small RNA profiling and characterization of piRNA clusters in the adult testes of the common marmoset, a model primate.

Small RNAs mediate gene silencing by binding Argonaute/Piwi proteins to regulate target RNAs. Here, we describe small RNA profiling of the adult testes of Callithrix jacchus, the common marmoset. The most abundant class of small RNAs in the adult testis was piRNAs, although 353 novel miRNAs but few endo-siRNAs were also identified. MARWI, a marmoset homolog of mouse MIWI and a very abundant PIWI in adult testes, associates with piRNAs that show characteristics of mouse pachytene piRNAs. As in other mammals, most marmoset piRNAs are derived from conserved clustered regions in the genome, which are annotated as intergenic regions. However, unlike in mice, marmoset piRNA clusters are also found on the X chromosome, suggesting escape from meiotic sex chromosome inactivation by the X-linked clusters. Some of the piRNA clusters identified contain antisense-orientated pseudogenes, suggesting the possibility that pseudogene-derived piRNAs may regulate parental functional protein-coding genes. More piRNAs map to transposable element (TE) subfamilies when they have copies in piRNA clusters. In addition, the strand bias observed for piRNAs mapped to each TE subfamily correlates with the polarity of copies inserted in clusters. These findings suggest that pachytene piRNA clusters determine the abundance and strand-bias of TE-derived piRNAs, may regulate protein-coding genes via pseudogene-derived piRNAs, and may even play roles in meiosis in the adult marmoset testis.

Global microRNA elevation by inducible Exportin 5 regulates cell cycle entry.

Proper regulation of gene expression during cell cycle entry ensures the successful completion of proliferation, avoiding risks such as carcinogenesis. The microRNA (miRNA) network is an emerging molecular system regulating multiple genetic pathways. We demonstrate here that the global elevation of miRNAs is critical for proper control of gene expression program during cell cycle entry. Strikingly, Exportin 5 (XPO5) is promptly induced during cell cycle entry by a PI3K-dependent post-transcriptional mechanism. Inhibition of XPO5 induction interfered with global miRNA elevation and resulted in a proliferation defect associated with delayed G1/S transition. During cell cycle entry, XPO5 therefore plays a paramount role as a critical molecular hub controlling the gene expression program through global regulation of miRNAs. Our data suggest that XPO5-mediated global miRNA elevation might be involved in a broad range of cellular events associated with cell cycle control.

PIGB maintains nuclear lamina organization in skeletal muscle of Drosophila.

The nuclear lamina (NL) plays various roles and participates in nuclear integrity, chromatin organization, and transcriptional regulation. Lamin proteins, the main components of the NL, form a homogeneous meshwork structure under the nuclear envelope. Lamins are essential, but it is unknown whether their homogeneous distribution is important for nuclear function. Here, we found that PIGB, an enzyme involved in glycosylphosphatidylinositol (GPI) synthesis, is responsible for the homogeneous lamin meshwork in Drosophila. Loss of PIGB resulted in heterogeneous distributions of B-type lamin and lamin-binding proteins in larval muscles. These phenotypes were rescued by expression of PIGB lacking GPI synthesis activity. The PIGB mutant exhibited changes in lamina-associated domains that are large heterochromatic genomic regions in the NL, reduction of nuclear stiffness, and deformation of muscle fibers. These results suggest that PIGB maintains the homogeneous meshwork of the NL, which may be essential for chromatin distribution and nuclear mechanical properties.

Generation of Stable Drosophila Ovarian Somatic Cell Lines Using the piggyBac System.

Transposable elements (TEs) constitute a large proportion of the genome in multiple organisms. Therefore, anti-transposable element machineries are essential to maintain genomic integrity. PIWI-interacting RNAs (piRNAs) are a major force to repress TEs in Drosophila ovaries. Ovarian somatic cells (OSC), in which nuclear piRNA regulation is functional, have been used for research on piRNA pathway as a cell culture system to elucidate the molecular mechanisms underlying the piRNA pathway. Analysis of piRNA pathway using a reporter system to monitor the gene regulation or overexpression of specific genes would be a powerful approach. Here, we present the technical protocol to establish stable cell lines using the piggyBac system, adopted for OSCs. This easy, consistent, and timesaving protocol may accelerate research on the piRNA pathway.

Maelstrom functions in the production of Siwi-piRISC capable of regulating transposons in Bombyx germ cells.

PIWI-interacting RNAs (piRNAs) bind to PIWI proteins to assemble the piRISC, which represses germline transposons. Maelstrom (Mael) is necessary for piRISC biogenesis in germ cells, but its function remains unclear. Here, we show that Mael interconnects Spindle-E (Spn-E), a key piRISC biogenesis factor, with unloaded Siwi, one of two silkworm PIWI members. Mael also assembles a subset of nuage, a non-membranous organelle involved in piRISC biogenesis. Loss of Mael abrogated the Spn-E-Siwi interaction and Ago3-piRISC biogenesis, but Siwi-piRISC was produced. Bioinformatic analysis showed that Siwi-bound piRNAs in Mael-lacking cells were rich in transposon-targeting piRNAs as in normal cells but were biased toward transposons that are marginally controlled by Siwi-piRISC. This explains the impairment in Ago3-piRISC production because transposon mRNAs cleaved by Siwi are the origin of Ago3-loaded piRNAs. We argue that Mael plays a role in the production of primary Siwi-piRISC capable of regulating transposon expression in germ cells.