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Key Points Small RNAs are defined by their size (~20–30 nucleotides in length) and their association with members of the Argonaute family. They impact nearly every biological process in eukaryotic cells, directly or indirectly. MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) are born from dsRNA precursors, whereas Piwi-interacting RNAs (piRNAs) originate from single-stranded transcripts. To perform their myriad roles, different classes of small RNAs must not only be generated in a precise manner, but must also be sorted into specific Argonaute complexes. An Argonaute protein primed with a single-stranded small RNA is called an RNA-induced silencing complex (RISC). Eukaryotic organisms often encode several Argonaute proteins that function in distinct pathways. They typically show various preferences for the small RNAs they accept, comprising loading determinants that include the identity of terminal nucleotides, small RNA duplex structure and thermodynamic properties. Small RNA duplexes are usually not incorporated into Argonaute proteins without assistance from additional protein factors, known as the RISC-loading machinery. miRNAs and siRNAs arise from small RNA duplexes and are loaded into Argonaute as dsRNA molecules. Thus, during RISC maturation, one strand must be selected specifically, whereas the other strand must be lost or degraded. Mature RISC regulates targets through sequence complementarity. The ultimate impact of accurate strand selection and sorting is that an active RISC is formed, imbued with the ability to regulate target transcripts. Biogenesis and sorting of small RNAs in animals and plants share some key mechanistic features, but have also evolved myriad variations and adaptations. To achieve their diverse regulatory functions, specific small RNA strands need to pair with the correct Argonaute protein partners. This Review discusses the recent substantial progress in understanding how small RNA sorting occurs in animals and plants. Small RNAs directly or indirectly impact nearly every biological process in eukaryotic cells. To perform their myriad roles, not only must precise small RNA species be generated, but they must also be loaded into specific effector complexes called RNA-induced silencing complexes (RISCs). Argonaute proteins form the core of RISCs and different members of this large family have specific expression patterns, protein binding partners and biochemical capabilities. In this Review, we explore the mechanisms that pair specific small RNA strands with their partner proteins, with an eye towards the substantial progress that has been recently made in understanding the sorting of the major small RNA classes — microRNAs (miRNAs) and small interfering RNAs (siRNAs) — in plants and animals.

MicroRNAs are thought to repress mRNA targets through perfect pairing with their seed region, but a sizeable number of miRNA interaction sites are orphans, without a perfect canonical miRNA partner. Now a large number of miRNAs are found to use an alternative binding mode that involves the bulging out of an unpaired mRNA nucleotide, leading to a functional mRNA-miRNA interaction. MicroRNAs (miRNAs) regulate mRNA targets through perfect pairing with their seed region (positions 2–7). Recently, a precise genome-wide map of miRNA interaction sites in mouse brain was generated by high-throughput sequencing and analysis of clusters of ~50-nucleotide mRNA tags cross-linked to Argonaute (Ago HITS-CLIP). By analyzing Ago HITS-CLIP 'orphan clusters'—Ago binding regions from HITS-CLIP that cannot be explained by canonical seed matches—we have now identified an alternative binding mode used by miRNAs. Specifically, G-bulge sites (positions 5–6) are often bound and regulated by miR-124 in brain. More generally, bulged sites comprise ≥15% of all Ago-miRNA interactions in mouse brain and are evolutionarily conserved. We call position 6 the 'pivot' nucleotide and suggest a model in which a transitional 'nucleation bulge' leads to functional bulge mRNA-miRNA interactions, expanding the number of potential miRNA regulatory sites.

Derepression of transposable elements (TEs) in the course of epigenetic reprogramming of the mouse embryonic germline necessitates the existence of a robust defense that is comprised of PIWI/piRNA pathway and de novo DNA methylation machinery. To gain further insight into biogenesis and function of piRNAs, we studied the intracellular localization of piRNA pathway components and used the combination of genetic, molecular, and cell biological approaches to examine the performance of the piRNA pathway in germ cells of mice lacking Maelstrom (MAEL), an evolutionarily conserved protein implicated in transposon silencing in fruit flies and mice. Here we show that principal components of the fetal piRNA pathway, MILI and MIWI2 proteins, localize to two distinct types of germinal cytoplasmic granules and exhibit differential association with components of the mRNA degradation/translational repression machinery. The first type of granules, pi-bodies, contains the MILI-TDRD1 module of the piRNA pathway and is likely equivalent to the enigmatic \"cementing material\" first described in electron micrographs of rat gonocytes over 35 years ago. The second type of granules, piP-bodies, harbors the MIWI2-TDRD9-MAEL module of the piRNA pathway and signature components of P-bodies, GW182, DCP1a, DDX6/p54, and XRN1 proteins. piP-bodies are found predominantly in the proximity of pi-bodies and the two frequently share mouse VASA homolog (MVH) protein, an RNA helicase. In Mael-mutant gonocytes, MIWI2, TDRD9, and MVH are lost from piP-bodies, whereas no effects on pi-body composition are observed. Further analysis revealed that MAEL appears to specifically facilitate MIWI2-dependent aspects of the piRNA pathway including biogenesis of secondary piRNAs, de novo DNA methylation, and efficient downregulation of TEs. Cumulatively, our data reveal elaborate cytoplasmic compartmentalization of the fetal piRNA pathway that relies on MAEL function.

Increasingly complex networks of small RNAs act through RNA-interference (RNAi) pathways to regulate gene expression, to mediate antiviral responses, to organize chromosomal domains, and to restrain the spread of selfish genetic elements. Historically, RNAi has been defined as a response to double-stranded RNA. However, some small RNA species may not arise from double-stranded RNA precursors. Yet, like microRNAs and small interfering RNAs, such species guide Argonaute proteins to silencing targets through complementary base-pairing. Silencing can be achieved by corecruitment of accessory factors or through the activity of Argonaute itself, which often has endonucleolytic activity. As a specific and adaptive regulatory system, RNAi is used throughout eukarya, which indicates a long evolutionary history. A likely function of RNAi throughout that history is to protect the genome from both pathogenic and parasitic invaders.

A new type of RNAi In RNA interference, small RNAs (siRNAs or miRNAs) act to regulate gene expression. They serve as specificity factors that direct the RISC (RNA-induced silencing) complex to the complementary mRNA targets. A major component of RISC is a protein of the Argonaute family. Two groups have now identified a new class of small RNAs that interact with one Argonaute subfamily, the Piwi class. These testis-specific small RNAs, called 'piRNAs', are slightly longer than the previously described small RNAs. The function of the piRNAs is not yet known, but they might be involved in sperm production. One of two studies that identifies a new class of small RNAs that interact with one Argonaute subfamily, the Piwi class. These testis-specific small RNAs, called 'piRNAs', are slightly longer than the previously described siRNAs and miRNAs. Small RNAs associate with Argonaute proteins and serve as sequence-specific guides to regulate messenger RNA stability, protein synthesis, chromatin organization and genome structure 1 , 2 , 3 . In animals, Argonaute proteins segregate into two subfamilies 4 . The Argonaute subfamily acts in RNA interference and in microRNA-mediated gene regulation using 21–22-nucleotide RNAs as guides. The Piwi subfamily is involved in germline-specific events such as germline stem cell maintenance and meiosis. However, neither the biochemical function of Piwi proteins nor the nature of their small RNA guides is known. Here we show that MIWI, a murine Piwi protein, binds a previously uncharacterized class of ∼29–30-nucleotide RNAs that are highly abundant in testes. We have therefore named these Piwi-interacting RNAs (piRNAs). piRNAs show distinctive localization patterns in the genome, being predominantly grouped into 20–90-kilobase clusters, wherein long stretches of small RNAs are derived from only one strand. Similar piRNAs are also found in human and rat, with major clusters occurring in syntenic locations. Although their function must still be resolved, the abundance of piRNAs in germline cells and the male sterility of Miwi mutants suggest a role in gametogenesis.

The Piwi-interacting RNA (piRNA) pathway is a small RNA–based innate immune system that defends germ cell genomes against transposons. In Drosophila ovaries, the nuclear Piwi protein is required for transcriptional silencing of transposons, though the precise mechanisms by which this occurs are unknown. Here we show that the CG9754 protein is a component of Piwi complexes that functions downstream of Piwi and its binding partner, Asterix, in transcriptional silencing. Enforced tethering of CG9754 to nascent messenger RNA transcripts causes cotranscriptional silencing of the source locus and the deposition of repressive chromatin marks. We have named CG9754 \"Panoramix,\" and we propose that this protein could act as an adaptor, scaffolding interactions between the piRNA pathway and the general silencing machinery that it recruits to enforce transcriptional repression.

The PIWI-interacting RNA (piRNA) pathway prevents endogenous genomic parasites, i.e. transposable elements, from damaging the genetic material of animal gonadal cells. Specific regions in the genome, called piRNA clusters, are thought to define each species’ piRNA repertoire and therefore its capacity to recognize and silence specific transposon families. The unistrand cluster flamenco ( flam ) is essential in the somatic compartment of the Drosophila ovary to restrict Gypsy -family transposons from infecting the neighbouring germ cells. Disruption of flam results in transposon de-repression and sterility, yet it remains unknown whether this silencing mechanism is present more widely. Here, we systematically characterise 119 Drosophila species and identify five additional flam -like clusters separated by up to 45 million years of evolution. Small RNA-sequencing validated these as bona-fide unistrand piRNA clusters expressed in somatic cells of the ovary, where they selectively target transposons of the Gypsy family. Together, our study provides compelling evidence of a widely conserved transposon silencing mechanism that co-evolved with virus-like Gypsy -family transposons. To control transposable elements, fruit flies rely on distinct genomic regions called piRNA clusters. Here, new piRNA clusters were identified across diverse Drosophila species, displaying a conserved and specialised role in the control of endogenous retroviruses in ovarian somatic cells.

The RNase III enzyme DICER generates both microRNAs (miRNAs) and endogenous short interfering RNAs (endo-siRNAs). Both small RNA species silence gene expression post-transcriptionally in association with the ARGONAUTE (AGO) family of proteins. In mammals, there are four AGO proteins (AGO1-4), of which only AGO2 possesses endonucleolytic activity. siRNAs trigger endonucleolytic cleavage of target mRNAs, mediated by AGO2, whereas miRNAs cause translational repression and mRNA decay through association with any of the four AGO proteins. Dicer deletion in mouse oocytes leads to female infertility due to defects during meiosis I. Because mouse oocytes express both miRNAs and endo-siRNAs, this phenotype could be due to the absence of either class of small RNA, or both. However, we and others demonstrated that miRNA function is suppressed in mouse oocytes, which suggested that endo-siRNAs, not miRNAs, are essential for female meiosis. To determine if this was the case we generated mice that express a catalytically inactive knock-in allele of Ago2 (Ago2ADH) exclusively in oocytes and thereby disrupted the function of siRNAs. Oogenesis and hormonal response are normal in Ago2ADH oocytes, but meiotic maturation is impaired, with severe defects in spindle formation and chromosome alignment that lead to meiotic catastrophe. The transcriptome of these oocytes is widely perturbed and shows a highly significant correlation with the transcriptome of Dicer null and Ago2 null oocytes. Expression of the mouse transcript (MT), the most abundant transposable element in mouse oocytes, is increased. This study reveals that endo-siRNAs are essential during meiosis I in mouse females, demonstrating a role for endo-siRNAs in mammals.

In plants and mammals, small RNAs indirectly mediate epigenetic inheritance by specifying cytosine methylation. We found that small RNAs themselves serve as vectors for epigenetic information. Crosses between Drosophila strains that differ in the presence of a particular transposon can produce sterile progeny, a phenomenon called hybrid dysgenesis. This phenotype manifests itself only if the transposon is paternally inherited, suggesting maternal transmission of a factor that maintains fertility. In both P- and I-element-mediated hybrid dysgenesis models, daughters show a markedly different content of Piwi-interacting RNAs (piRNAs) targeting each element, depending on their parents of origin. Such differences persist from fertilization through adulthood. This indicates that maternally deposited piRNAs are important for mounting an effective silencing response and that a lack of maternal piRNA inheritance underlies hybrid dysgenesis.

RNA silencing pathways play critical roles in gene regulation, virus infection, and transposon control. RNA interference (RNAi) is mediated by small interfering RNAs (siRNAs), which are liberated from double-stranded (ds)RNA precursors by Dicer and guide the RNA-induced silencing complex (RISC) to targets. Although principles governing small RNA sorting into RISC have been uncovered, the spectrum of RNA species that can be targeted by Dicer proteins, particularly the viral RNAs present during an infection, are poorly understood. Dicer-2 potently restricts viral infection in insects by generating virus-derived siRNAs from viral RNA. To better characterize the substrates of Dicer-2, we examined the virus-derived siRNAs produced during the Drosophila antiviral RNAi response to four different viruses using high-throughput sequencing. We found that each virus was uniquely targeted by the RNAi pathway; dicing substrates included dsRNA replication intermediates and intramolecular RNA stem loops. For instance, a putative intergenic RNA hairpin encoded by Rift Valley Fever virus generates abundant small RNAs in both Drosophila and mosquito cells, while repetitive sequences within the genomic termini of Vaccinia virus, which give rise to abundant small RNAs in Drosophila, were found to be transcribed in both insect and mammalian cells. Moreover, we provide evidence that the RNA species targeted by Dicer-2 can be modulated by the presence of a viral suppressor of RNAi. This study uncovered several novel, heavily targeted features within viral genomes, offering insight into viral replication, viral immune evasion strategies, and the mechanism of antiviral RNAi.