Crystal structure of a PIWI protein suggests mechanisms for siRNA recognition and slicer activity

Mechanistic insights into RNA cleavage by human Argonaute2-siRNA complex

In animals, AGO-clade Argonaute proteins utilize small interfering RNAs (siRNAs) as guides to recognize target with complete complementarity, resulting in target RNA cleavage that is a critical step for target silencing. These proteins feature a constricted nucleic acid-binding channel that limits base pairing between the guide and target beyond the seed region. How the AGO-siRNA complexes overcome this structural limitation and achieve efficient target cleavage remains unclear. We performed cryo-electron microscopy of human AGO-siRNA complexes bound to target RNAs of increasing lengths to examine the conformational changes associated with target recognition and cleavage. Initially, conformational transition propagates from the opening of the PAZ domain and extends through a repositioning of the PIWI-L1-N domain toward the binding channel, facilitating the capture of siRNA-target duplex. Subsequent extension of base pairing drives the downward movement of the PIWI-L1-N domain to enable catalytic activation. Finally, further base pairing toward the 3' end of siRNA destabilizes the PAZ-N domain, resulting in a "uni-lobed" architecture, which might facilitate the multi-turnover action of the AGO-siRNA enzyme complex. In contrast to PIWI-clade Argonautes, the "uni-lobed" structure of the AGO complex makes multiple contacts with the target in the central region of the siRNA-target duplex, positioning it within the catalytic site. Our findings shed light on the stepwise mechanisms by which the AGO-siRNA complex executes target RNA cleavage and offer insights into the distinct operational modalities of AGO and PIWI proteins in achieving such cleavage.

Identifying the interactions conferring functional mechanical rigidity on RNase-resistant RNA from Zika virus

Some viruses counter host-cell efforts to digest invading viral RNA by using special structures resistant to host RNases, known as exoribonuclease-resistant RNAs (xrRNAs). xrRNAs typically form an unusual fold with the 5'-end threaded through a ring consisting of a multihelix junction closed by a pseudoknot. By using single-molecule force spectroscopy (SMFS), we previously showed that a Zika virus xrRNA is extremely rigid mechanically, withstanding very high forces, and that this mechanical resistance-not simply the knot-like fold topology-is essential for RNase resistance. Here, we have determined which interactions are most important for generating mechanical rigidity in the Zika virus xrRNA, by systematically mutating tertiary contacts. We found that removing any of the tertiary contacts involving the threaded 5' end was sufficient to abrogate mechanical resistance. In contrast, breaking a single pseudoknot base pair was not sufficient to do so: Two broken pairs were needed. This hierarchy of interaction importance for mechanical rigidity was supported by simulations mapping how mechanical tension was distributed within the xrRNA. For all mutants, RNase resistance varied in lock-step with mechanical resistance, confirming the primary role of mechanical rigidity in xrRNA function. This work reveals which interactions are most important for Zika xrRNA function, with implications for targeting the xrRNA therapeutically.

Structural insights into RNA cleavage by PIWI Argonaute

Argonaute proteins are categorized into AGO and PIWI clades. Across most animal species, AGO-clade proteins are widely expressed in various cell types, and regulate normal gene expression1. By contrast, PIWI-clade proteins predominantly function during gametogenesis to suppress transposons and ensure fertility1,2. Both clades use nucleic acid guides for target recognition by means of base pairing, crucial for initiating target silencing, often through direct cleavage. AGO-clade proteins use a narrow channel to secure a tight guide-target interaction3. By contrast, PIWI proteins feature a wider channel that potentially allows mismatches during pairing, broadening target silencing capability4,5. However, the mechanism of PIWI-mediated target cleavage remains unclear. Here we demonstrate that after target binding, PIWI proteins undergo a conformational change from an 'open' state to a 'locked' state, facilitating base pairing and enhancing target cleavage efficiency. This transition involves narrowing of the binding channel and repositioning of the PIWI-interacting RNA-target duplex towards the MID-PIWI lobe, establishing extensive contacts for duplex stabilization. During this transition, we also identify an intermediate 'comma-shaped' conformation, which might recruit GTSF1, a known auxiliary protein that enhances PIWI cleavage activity6. GTSF1 facilitates the transition to the locked state by linking the PIWI domain to the RNA duplex, thereby expediting the conformational change critical for efficient target cleavage. These findings explain the molecular mechanisms underlying PIWI-PIWI-interacting RNA complex function in target RNA cleavage, providing insights into how dynamic conformational changes from PIWI proteins coordinate cofactors to safeguard gametogenesis.

Conformational switches in human RNA binding proteins involved in neurodegeneration

Conformational switching in RNA binding proteins (RBPs) is crucial for regulation of RNA processing and transport. Dysregulation or mutations in RBPs and broad RNA processing abnormalities are related to many human diseases including neurodegenerative disorders. Here, we review the role of protein-RNA conformational switches in RBP-RNA complexes. RBP-RNA complexes exhibit wide range of conformational switching depending on the RNA molecule and its ability to induce conformational changes in its partner RBP. We categorize the conformational switches into three groups: rigid body, semi-flexible and full flexible. We also investigate conformational switches in large cellular assemblies including ribosome, spliceosome and RISC complexes. In addition, the role of intrinsic disorder in RBP-RNA conformational switches is discussed. We have also discussed the effect of different disease-causing mutations on conformational switching of proteins associated with neurodegenerative diseases. We believe that this study will enhance our understanding on the role of protein-RNA conformational switches. Furthermore, the availability of a large number of atomic structures of RBP-RNA complexes in near future would facilitate to create a complete repertoire of human RBP-RNA conformational switches.

Dynamic co-evolution of transposable elements and the piRNA pathway in African cichlid fishes

BACKGROUND

East African cichlid fishes have diversified in an explosive fashion, but the (epi)genetic basis of the phenotypic diversity of these fishes remains largely unknown. Although transposable elements (TEs) have been associated with phenotypic variation in cichlids, little is known about their transcriptional activity and epigenetic silencing. We set out to bridge this gap and to understand the interactions between TEs and their cichlid hosts.

RESULTS

Here, we describe dynamic patterns of TE expression in African cichlid gonads and during early development. Orthology inference revealed strong conservation of TE silencing factors in cichlids, and an expansion of piwil1 genes in Lake Malawi cichlids, likely driven by PiggyBac TEs. The expanded piwil1 copies have signatures of positive selection and retain amino acid residues essential for catalytic activity. Furthermore, the gonads of African cichlids express a Piwi-interacting RNA (piRNA) pathway that targets TEs. We define the genomic sites of piRNA production in African cichlids and find divergence in closely related species, in line with fast evolution of piRNA-producing loci.

CONCLUSIONS

Our findings suggest dynamic co-evolution of TEs and host silencing pathways in the African cichlid radiations. We propose that this co-evolution has contributed to cichlid genomic diversity.

The potential emerging role of piRNA/PIWI complex in virus infection

P-element-induced wimpy testis-interacting RNAs (piRNAs), a class of small noncoding RNAs with about 24-32 nucleotides, often interact with PIWI proteins to form a piRNA/PIWI complex that could influence spermiogenesis, transposon silencing, epigenetic regulation, etc. PIWI proteins have a highly conserved function in a variety of species and are usually expressed in germ cells. However, increasing evidence has revealed the important role of the piRNA/PIWI complex in the occurrence and prognosis of various human diseases and suggests its potential application in the diagnosis and treatment of related diseases, becoming a prominent marker for these human diseases. Recent studies have confirmed that piRNA/PIWI complexes or piRNAs are abnormally expressed in some viral infections, effecting disease progression and viral replication. In this study, we reviewed the association between the piRNA/PIWI complex and several human disease-associated viruses, including human papillomavirus, human immunodeficiency virus, human rhinovirus, severe acute respiratory syndrome coronavirus 2, respiratory syncytial virus, and herpes simplex virus type 1.

Identification of an active RNAi pathway in Candida albicans

RNA interference (RNAi) is a fundamental regulatory pathway with a wide range of functions, including regulation of gene expression and maintenance of genome stability. Although RNAi is widespread in the fungal kingdom, well-known species, such as the model yeast Saccharomyces cerevisiae, have lost the RNAi pathway. Until now evidence has been lacking for a fully functional RNAi pathway in Candida albicans, a human fungal pathogen considered critically important by the World Health Organization. Here, we demonstrated that the widely used C. albicans reference strain (SC5314) contains an inactivating missense mutation in the gene encoding for the central RNAi component Argonaute. In contrast, most other C. albicans isolates contain a canonical Argonaute protein predicted to be functional and RNAi-active. Indeed, using high-throughput small and long RNA sequencing combined with seamless CRISPR/Cas9-based gene editing, we demonstrate that an active C. albicans RNAi machinery represses expression of subtelomeric gene families. Thus, an intact and functional RNAi pathway exists in C. albicans, highlighting the importance of using multiple reference strains when studying this dangerous pathogen.

Dynamic co-evolution of transposable elements and the piRNA pathway in African cichlid fishes

East African cichlid fishes have diversified in an explosive fashion, but the (epi)genetic basis of the phenotypic diversity of these fishes remains largely unknown. Although transposable elements (TEs) have been associated with phenotypic variation in cichlids, little is known about their transcriptional activity and epigenetic silencing. Here, we describe dynamic patterns of TE expression in African cichlid gonads and during early development. Orthology inference revealed an expansion of piwil1 genes in Lake Malawi cichlids, likely driven by PiggyBac TEs. The expanded piwil1 copies have signatures of positive selection and retain amino acid residues essential for catalytic activity. Furthermore, the gonads of African cichlids express a Piwi-interacting RNA (piRNA) pathway that target TEs. We define the genomic sites of piRNA production in African cichlids and find divergence in closely related species, in line with fast evolution of piRNA-producing loci. Our findings suggest dynamic co-evolution of TEs and host silencing pathways in the African cichlid radiations. We propose that this co-evolution has contributed to cichlid genomic diversity.

The missing part: the Archaeoglobus fulgidus Argonaute forms a functional heterodimer with an N-L1-L2 domain protein

Argonaute (Ago) proteins are present in all three domains of life (bacteria, archaea and eukaryotes). They use small (15-30 nucleotides) oligonucleotide guides to bind complementary nucleic acid targets and are responsible for gene expression regulation, mobile genome element silencing, and defence against viruses or plasmids. According to their domain organization, Agos are divided into long and short Agos. Long Agos found in prokaryotes (long-A and long-B pAgos) and eukaryotes (eAgos) comprise four major functional domains (N, PAZ, MID and PIWI) and two structural linker domains L1 and L2. The majority (∼60%) of pAgos are short pAgos, containing only the MID and inactive PIWI domains. Here we focus on the prokaryotic Argonaute AfAgo from Archaeoglobus fulgidus DSM4304. Although phylogenetically classified as a long-B pAgo, AfAgo contains only MID and catalytically inactive PIWI domains, akin to short pAgos. We show that AfAgo forms a heterodimeric complex with a protein encoded upstream in the same operon, which is a structural equivalent of the N-L1-L2 domains of long pAgos. This complex, structurally equivalent to a long PAZ-less pAgo, outperforms standalone AfAgo in guide RNA-mediated target DNA binding. Our findings provide a missing piece to one of the first and the most studied pAgos.

When Argonaute takes out the ribonuclease sword

Argonaute (AGO) proteins in all three domains of life form ribonucleoprotein or deoxyribonucleoprotein complexes by loading a guide RNA or DNA, respectively. Since all AGOs retain a PIWI domain that takes an RNase H fold, the ancestor was likely an endoribonuclease (i.e., a slicer). In animals, most miRNA-mediated gene silencing occurs slicer independently. However, the slicer activity of AGO is indispensable in specific events, such as development and differentiation, which are critical for vertebrates and thus cannot be replaced by the slicer-independent regulation. This review highlights the distinctions in catalytic activation mechanisms among slicing-competent AGOs, shedding light on the roles of two metal ions in target recognition and cleavage. The precision of the target specificity by the RNA-induced silencing complexes is reevaluated and redefined. The possible coevolutionary relationship between slicer-independent gene regulation and AGO-binding protein, GW182, is also explored. These discussions reveal that numerous captivating questions remain unanswered regarding the timing and manner in which AGOs employ their slicing activity.

The Potential Role of the Piwi Gene in the Development and Reproduction of Plutella xylostella

Piwi proteins play a significant role in germ cell development and the silencing of transposons in animals by associating with small non-coding RNAs known as Piwi-interacting RNAs (piRNAs). While the Piwi gene has been well characterized in various insect species, the role of the Piwi (PxPiwi) gene in the diamondback moth (Plutella xylostella), a globally distributed pest of cruciferous crops, remains unclear. Expression analysis demonstrated the upregulation of PxPiwi in pupae and testes. Furthermore, we generated a PxPiwi-knockout mutant using CRISPR/Cas9 technology, which resulted in a significantly prolonged pupal stage and the failure of pupae to develop into adults. Additionally, the knockdown of PxPiwi, through RNA interference (RNAi), led to a substantial decrease in the oviposition and hatchability of P. xylostella. These findings indicate that PxPiwi is specifically expressed and essential for the development and reproduction of P. xylostella. This is the first report indicating the involvement of the Piwi gene in the development of lepidopteran insects, except for reproduction and germ cell development, which provides a foundation for future investigations into the functions of PxPiwi.

Basic Principles of RNA Interference: Nucleic Acid Types and In Vitro Intracellular Delivery Methods

Since its discovery in 1989, RNA interference (RNAi) has become a widely used tool for the in vitro downregulation of specific gene expression in molecular biological research. This basically involves a complementary RNA that binds a target sequence to affect its transcription or translation process. Currently, various small RNAs, such as small interfering RNA (siRNA), micro RNA (miRNA), small hairpin RNA (shRNA), and PIWI interacting RNA (piRNA), are available for application on in vitro cell culture, to regulate the cells' gene expression by mimicking the endogenous RNAi-machinery. In addition, several biochemical, physical, and viral methods have been established to deliver these RNAs into the cell or nucleus. Since each RNA and each delivery method entail different off-target effects, limitations, and compatibilities, it is crucial to understand their basic mode of action. This review is intended to provide an overview of different nucleic acids and delivery methods for planning, interpreting, and troubleshooting of RNAi experiments.

Bacterial Argonaute Proteins Aid Cell Division in the Presence of Topoisomerase Inhibitors in Escherichia coli

Prokaryotic Argonaute (pAgo) proteins are guide-dependent nucleases that function in host defense against invaders. Recently, it was shown that TtAgo from Thermus thermophilus also participates in the completion of DNA replication by decatenating chromosomal DNA. Here, we show that two pAgos from cyanobacteria Synechococcus elongatus (SeAgo) and Limnothrix rosea (LrAgo) are active in heterologous Escherichia coli and aid cell division in the presence of the gyrase inhibitor ciprofloxacin, depending on the host double-strand break repair machinery. Both pAgos are preferentially loaded with small guide DNAs (smDNAs) derived from the sites of replication termination. Ciprofloxacin increases the amounts of smDNAs from the termination region and from the sites of genomic DNA cleavage by gyrase, suggesting that smDNA biogenesis depends on DNA replication and is stimulated by gyrase inhibition. Ciprofloxacin enhances asymmetry in the distribution of smDNAs around Chi sites, indicating that it induces double-strand breaks that serve as a source of smDNA during their processing by RecBCD. While active in E. coli, SeAgo does not protect its native host S. elongatus from ciprofloxacin. These results suggest that pAgo nucleases may help to complete replication of chromosomal DNA by promoting chromosome decatenation or participating in the processing of gyrase cleavage sites, and may switch their functional activities depending on the host species. IMPORTANCE Prokaryotic Argonautes (pAgos) are programmable nucleases with incompletely understood functions in vivo. In contrast to eukaryotic Argonautes, most studied pAgos recognize DNA targets. Recent studies suggested that pAgos can protect bacteria from invader DNA and counteract phage infection and may also have other functions including possible roles in DNA replication, repair, and gene regulation. Here, we have demonstrated that two cyanobacterial pAgos, SeAgo and LrAgo, can assist DNA replication and facilitate cell division in the presence of topoisomerase inhibitors in Escherichia coli. They are specifically loaded with small guide DNAs from the region of replication termination and protect the cells from the action of the gyrase inhibitor ciprofloxacin, suggesting that they help to complete DNA replication and/or repair gyrase-induced breaks. The results show that pAgo proteins may serve as a backup to topoisomerases under conditions unfavorable for DNA replication and may modulate the resistance of host bacterial strains to antibiotics.

cDNA Cloning of Feline PIWIL1 and Evaluation of Expression in the Testis of the Domestic Cat

The PIWI clade of Argonaute proteins is essential for spermatogenesis in all species examined to date. This protein family binds specific classes of small non-coding RNAs known as PIWI-interacting RNAs (piRNAs) which together form piRNA-induced silencing complexes (piRISCs) that are recruited to specific RNA targets through sequence complementarity. These complexes facilitate gene silencing through endonuclease activity and guided recruitment of epigenetic silencing factors. PIWI proteins and piRNAs have been found to play multiple roles in the testis including the maintenance of genomic integrity through transposon silencing and facilitating the turnover of coding RNAs during spermatogenesis. In the present study, we report the first characterization of PIWIL1 in the male domestic cat, a mammalian system predicted to express four PIWI family members. Multiple transcript variants of PIWIL1 were cloned from feline testes cDNA. One isoform shows high homology to PIWIL1 from other mammals, however, the other has characteristics of a "slicer null" isoform, lacking the domain required for endonuclease activity. Expression of PIWIL1 in the male cat appears limited to the testis and correlates with sexual maturity. RNA-immunoprecipitation revealed that feline PIWIL1 binds small RNAs with an average size of 29 nt. Together, these data suggest that the domestic cat has two PIWIL1 isoforms expressed in the mature testis, at least one of which interacts with piRNAs.

N-terminal region of Drosophila melanogaster Argonaute2 forms amyloid-like aggregates

BACKGROUND

Argonaute proteins play a central role in RNA silencing by forming protein-small RNA complexes responsible for the silencing process. While most Argonaute proteins have a short N-terminal region, Argonaute2 in Drosophila melanogaster (DmAgo2) harbors a long and unique N-terminal region. Previous in vitro biochemical studies have shown that the loss of this region does not impair the RNA silencing activity of the complex. However, an N-terminal mutant of Drosophila melanogaster has demonstrated abnormal RNA silencing activity. To explore the causes of this discrepancy between in vitro and in vivo studies, we investigated the biophysical properties of the region. The N-terminal region is highly rich in glutamine and glycine residues, which is a well-known property for prion-like domains, a subclass of amyloid-forming peptides. Therefore, the possibility of the N-terminal region functioning as an amyloid was tested.

RESULTS

Our in silico and biochemical assays demonstrated that the N-terminal region exhibits amyloid-specific properties. The region indeed formed aggregates that were not dissociated even in the presence of sodium dodecyl sulfate. Also, the aggregates enhanced the fluorescence intensity of thioflavin-T, an amyloid detection reagent. The kinetics of the aggregation followed that of typical amyloid formation exhibiting self-propagating activity. Furthermore, we directly visualized the aggregation process of the N-terminal region under fluorescence microscopy and found that the aggregations took fractal or fibril shapes. Together, the results indicate that the N-terminal region can form amyloid-like aggregates.

CONCLUSIONS

Many other amyloid-forming peptides have been reported to modulate the function of proteins through their aggregation. Therefore, our findings raise the possibility that aggregation of the N-terminal region regulates the RNA silencing activity of DmAgo2.

Structural basis for sequence-specific recognition of guide and target strands by the Archaeoglobus fulgidus Argonaute protein

Argonaute (Ago) proteins are found in all three domains of life. The best-characterized group is eukaryotic Argonautes (eAgos). Being the structural core of RNA interference machinery, they use guide RNA molecules for RNA targeting. Prokaryotic Argonautes (pAgos) are more diverse, both in terms of structure (there are eAgo-like 'long' and truncated 'short' pAgos) and mechanism, as many pAgos are specific for DNA, not RNA guide and/or target strands. Some long pAgos act as antiviral defence systems. Their defensive role was recently demonstrated for short pAgo-encoding systems SPARTA and GsSir2/Ago, but the function and action mechanisms of all other short pAgos remain unknown. In this work, we focus on the guide and target strand preferences of AfAgo, a truncated long-B Argonaute protein encoded by an archaeon Archaeoglobus fulgidus. We demonstrate that AfAgo associates with small RNA molecules carrying 5'-terminal AUU nucleotides in vivo, and characterize its affinity to various RNA and DNA guide/target strands in vitro. We also present X-ray structures of AfAgo bound to oligoduplex DNAs that provide atomic details for base-specific AfAgo interactions with both guide and target strands. Our findings broaden the range of currently known Argonaute-nucleic acid recognition mechanisms.

A bacterial Argonaute with efficient DNA and RNA cleavage activity guided by small DNA and RNA

Argonaute proteins are widespread in prokaryotes and eukaryotes with diversified catalytic activities. Here, we describe an Argonaute from Marinitoga hydrogenitolerans (MhAgo) with all eight cleavage activities. Utilization of all four types of guides and efficient cleavage of single-stranded DNA (ssDNA) and RNA targets are revealed. The preference for the 5'-terminus nucleotides of 5'P guides, but no obvious preferences for that in 5'OH guides, is further uncovered. Moreover, the cleavage efficiency is heavily impaired by mismatches in the central and 3'-supplementary regions of guides, and the affinity between guides or guides/target duplex and MhAgo is proved as one of the factors affecting cleavage efficiency. Structural and mutational analyses imply some unknown distinctive structural features behind the cleavage activity of MhAgo. Meanwhile, 5'OH-guide RNA (gRNA)-mediated plasmid cleavage activity is unveiled. Conclusively, MhAgo is versatile, and its biochemical characteristics improve our understanding of pAgos and the pAgo-based techniques.

Bacterial origins of human cell-autonomous innate immune mechanisms

The cell-autonomous innate immune system enables animal cells to resist viral infection. This system comprises an array of sensors that, after detecting viral molecules, activate the expression of antiviral proteins and the interferon response. The repertoire of immune sensors and antiviral proteins has long been considered to be derived from extensive evolutionary innovation in vertebrates, but new data challenge this dogma. Recent studies show that central components of the cell-autonomous innate immune system have ancient evolutionary roots in prokaryotic genes that protect bacteria from phages. These include the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway, Toll/IL-1 receptor (TIR) domain-containing pathogen receptors, the viperin family of antiviral proteins, SAMHD1-like nucleotide-depletion enzymes, gasdermin proteins and key components of the RNA interference pathway. This Perspective details current knowledge of the elements of antiviral immunity that are conserved from bacteria to humans, and presents possible evolutionary scenarios to explain the observed conservation.

GTSF1 accelerates target RNA cleavage by PIWI-clade Argonaute proteins

Argonaute proteins use nucleic acid guides to find and bind specific DNA or RNA target sequences. Argonaute proteins have diverse biological functions and many retain their ancestral endoribonuclease activity, cleaving the phosphodiester bond between target nucleotides t10 and t11. In animals, the PIWI proteins-a specialized class of Argonaute proteins-use 21-35 nucleotide PIWI-interacting RNAs (piRNAs) to direct transposon silencing, protect the germline genome, and regulate gene expression during gametogenesis1. The piRNA pathway is required for fertility in one or both sexes of nearly all animals. Both piRNA production and function require RNA cleavage catalysed by PIWI proteins. Spermatogenesis in mice and other placental mammals requires three distinct, developmentally regulated PIWI proteins: MIWI (PIWIL1), MILI (PIWIL2) and MIWI22-4 (PIWIL4). The piRNA-guided endoribonuclease activities of MIWI and MILI are essential for the production of functional sperm5,6. piRNA-directed silencing in mice and insects also requires GTSF1, a PIWI-associated protein of unknown function7-12. Here we report that GTSF1 potentiates the weak, intrinsic, piRNA-directed RNA cleavage activities of PIWI proteins, transforming them into efficient endoribonucleases. GTSF1 is thus an example of an auxiliary protein that potentiates the catalytic activity of an Argonaute protein.

Mechanisms of helicase activated DNA end resection in bacteria

DNA end resection mediated by the coordinated action of nuclease and helicase is a crucial step in initiating homologous recombination. The end-resection apparatus NurA nuclease and HerA helicase are present in both archaea and bacteria. Here, we report the cryo-electron microscopy structure of a bacterial HerA-NurA complex from Deinococcus radiodurans. The structure reveals a barrel-like hexameric HerA and a distinctive NurA dimer subcomplex, which has a unique extended N-terminal region (ENR) involved in bacterial NurA dimerization and activation. In addition to the long protruding linking loop and the C-terminal α helix of NurA, the flexible ENR is close to the HerA-NurA interface and divides the central channel of the DrNurA dimer into two halves, suggesting a possible mechanism of DNA end processing. In summary, this work provides new insights into the structure, assembly, and activation mechanisms of bacterial DNA end resection mediated by a minimal end-resection apparatus.

Anatomy of four human Argonaute proteins

MicroRNAs (miRNAs) bind to complementary target RNAs and regulate their gene expression post-transcriptionally. These non-coding regulatory RNAs become functional after loading into Argonaute (AGO) proteins to form the effector complexes. Humans have four AGO proteins, AGO1, AGO2, AGO3 and AGO4, which share a high sequence identity. Since most miRNAs are found across the four AGOs, it has been thought that they work redundantly, and AGO2 has been heavily studied as the exemplified human paralog. Nevertheless, an increasing number of studies have found that the other paralogs play unique roles in various biological processes and diseases. In the last decade, the structural study of the four AGOs has provided the field with solid structural bases. This review exploits the completed structural catalog to describe common features and differences in target specificity across the four AGOs.

A short prokaryotic Argonaute activates membrane effector to confer antiviral defense

Argonaute (Ago) proteins are widespread nucleic-acid-guided enzymes that recognize targets through complementary base pairing. Although, in eukaryotes, Agos are involved in RNA silencing, the functions of prokaryotic Agos (pAgos) remain largely unknown. In particular, a clade of truncated and catalytically inactive pAgos (short pAgos) lacks characterization. Here, we reveal that a short pAgo protein in the archaeon Sulfolobus islandicus, together with its two genetically associated proteins, Aga1 and Aga2, provide robust antiviral protection via abortive infection. Aga2 is a toxic transmembrane effector that binds anionic phospholipids via a basic pocket, resulting in membrane depolarization and cell killing. Ago and Aga1 form a stable complex that exhibits nucleic-acid-directed nucleic-acid-recognition ability and directly interacts with Aga2, pointing to an immune sensing mechanism. Together, our results highlight the cooperation between pAgos and their widespread associated proteins, suggesting an uncharted diversity of pAgo-derived immune systems.

The Tudor Domain-Containing Protein, Kotsubu (CG9925), Localizes to the Nuage and Functions in piRNA Biogenesis in D. melanogaster

Silencing of transposable elements (TEs) by Piwi-interacting RNAs (piRNAs) is crucial for maintaining germline genome integrity and fertility in animals. To repress TEs, PIWI clade Argonaute proteins cooperate with several Tudor domain-containing (Tdrd) proteins at membraneless perinuclear organelles, called nuage, to produce piRNAs to repress transposons. Here, we identify and characterize Kotsubu (Kots), one of the Drosophila Tudor domain-containing protein-1 (Tdrd1) orthologs, encoded by the CG9925 gene, that localizes to the nuage in gonads. We further show the dynamic localization of Kots in the male germline, where it shows perinuclear signals in spermatogonia but forms large cytoplasmic condensates in the spermatocytes that overlap with components of piNG-body, a nuage-associated organelle. The loss of kots results in a notable upregulation of stellate and a corresponding reduction in the suppressor of stellate piRNAs in the mutants. Furthermore, a moderate yet significant reduction of other piRNAs was observed in kots mutant testes. Taken together, we propose that Kots functions in the piRNA pathway, predominantly in the male germline by forming discrete cytoplasmic granules.

A Structural View of miRNA Biogenesis and Function

Micro-RNAs (miRNAs) are a class of non-coding RNAs (ncRNAs) that act as post-transcriptional regulators of gene expression. Since their discovery in 1993, they have been the subject of deep study due to their involvement in many important biological processes. Compared with other ncRNAs, miRNAs are generated from devoted transcriptional units which are processed by a specific set of endonucleases. The contribution of structural biology methods for understanding miRNA biogenesis and function has been essential for the dissection of their roles in cell biology and human disease. In this review, we summarize the application of structural biology for the characterization of the molecular players involved in miRNA biogenesis (processors and effectors), starting from the X-ray crystallography methods to the more recent cryo-electron microscopy protocols.

Prokaryotic Argonaute Proteins as a Tool for Biotechnology

Programmable nucleases are the most important tool for manipulating the genes and genomes of both prokaryotes and eukaryotes. Since the end of the 20th century, many approaches were developed for specific modification of the genome. The review briefly considers the advantages and disadvantages of the main genetic editors known to date. The main attention is paid to programmable nucleases from the family of prokaryotic Argonaute proteins. Argonaute proteins can recognize and cleave DNA sequences using small complementary guide molecules and play an important role in protecting prokaryotic cells from invading DNA. Argonaute proteins have already found applications in biotechnology for targeted cleavage and detection of nucleic acids and can potentially be used for genome editing.

Binding of guide piRNA triggers methylation of the unstructured N-terminal region of Aub leading to assembly of the piRNA amplification complex

PIWI proteins use guide piRNAs to repress selfish genomic elements, protecting the genomic integrity of gametes and ensuring the fertility of animal species. Efficient transposon repression depends on amplification of piRNA guides in the ping-pong cycle, which in Drosophila entails tight cooperation between two PIWI proteins, Aub and Ago3. Here we show that post-translational modification, symmetric dimethylarginine (sDMA), of Aub is essential for piRNA biogenesis, transposon silencing and fertility. Methylation is triggered by loading of a piRNA guide into Aub, which exposes its unstructured N-terminal region to the PRMT5 methylosome complex. Thus, sDMA modification is a signal that Aub is loaded with piRNA guide. Amplification of piRNA in the ping-pong cycle requires assembly of a tertiary complex scaffolded by Krimper, which simultaneously binds the N-terminal regions of Aub and Ago3. To promote generation of new piRNA, Krimper uses its two Tudor domains to bind Aub and Ago3 in opposite modification and piRNA-loading states. Our results reveal that post-translational modifications in unstructured regions of PIWI proteins and their binding by Tudor domains that are capable of discriminating between modification states is essential for piRNA biogenesis and silencing.

piR-hsa-211106 Inhibits the Progression of Lung Adenocarcinoma Through Pyruvate Carboxylase and Enhances Chemotherapy Sensitivity

Although the importance of PIWI-interacting RNAs (piRNAs) in cancer has recently been recognized, studies on the role and functional mechanism of piRNAs in lung adenocarcinoma (LUAD) development and progression are limited. In this study, we identified 10 differently expressed piRNAs in LUAD tissues compared to normal tissues, among which, piR-hsa-211106 expression levels were downregulated in LUAD tissues and cell lines. Furthermore, the effects of piR-hsa-211106 on the malignant phenotypes and chemosensitivity of LUAD cells were detected by gain- and loss-of-function analyses in vitro and in vivo, which showed that piR-hsa-211106 inhibited LUAD cell proliferation, tumor growth, and migration, but promoted apoptosis. Moreover, our finding indicated that piR-hsa-211106 is a potential therapeutic target that synergistically imparts anticancer effects with a chemotherapeutic agent for LUAD-cisplatin. Further mechanistic investigation indicated that piR-hsa-211106 could bind to pyruvate carboxylase (PC) by RNA pull down and RNA immunoprecipitation assays and inhibited PC mRNA and protein expression. Our study demonstrates that piR-hsa-211106 inhibits LUAD progression by hindering the expression and function of PC and enhances chemotherapy sensitivity, suggesting that piR-hsa-211106 is a novel diagnostic and therapeutic target for LUAD.

Argonaute proteins: structures and their endonuclease activity

Argonaute proteins are highly conserved and widely expressed in almost all organisms. They not only play a critical role in the biogenesis of small RNAs but also defend against invading nucleic acids via small RNA or DNA-mediated gene silencing pathways. One functional mechanism of Argonaute proteins is acting as a nucleic-acid-guided endonuclease, which can cleave targets complementary to DNA or RNA guides. The cleavage then leads to translational silencing directly or indirectly by recruiting additional silencing proteins. Here, we summarized the latest research progress in structural and biological studies of Argonaute proteins and pointed out their potential applications in the field of gene editing.

Genome-wide identification, evolutionary relationship and expression analysis of AGO, DCL and RDR family genes in tea

Three gene families in plants viz. Argonaute (AGOs), Dicer-like (DCLs) and RNA dependent RNA polymerase (RDRs) constitute the core components of small RNA mediated gene silencing machinery. The present study endeavours to identify members of these gene families in tea and to investigate their expression patterns in different tissues and various stress regimes. Using genome-wide analysis, we have identified 18 AGOs, 5 DCLs and 9 RDRs in tea, and analyzed their phylogenetic relationship with orthologs of Arabidopsis thaliana. Gene expression analysis revealed constitutive expression of CsAGO1 in all the studied tissues and stress conditions, whereas CsAGO10c showed most variable expression among all the genes. CsAGO10c gene was found to be upregulated in tissues undergoing high meristematic activity such as buds and roots, as well as in Exobasidium vexans infected samples. CsRDR2 and two paralogs of CsAGO4, which are known to participate in biogenesis of hc-siRNAs, showed similarities in their expression levels in most of the tea plant tissues. This report provides first ever insight into the important gene families involved in biogenesis of small RNAs in tea. The comprehensive knowledge of these small RNA biogenesis purveyors can be utilized for tea crop improvement aimed at stress tolerance and quality enhancement.

piRNAs as Modulators of Disease Pathogenesis

Advances in understanding disease pathogenesis correlates to modifications in gene expression within different tissues and organ systems. In depth knowledge about the dysregulation of gene expression profiles is fundamental to fully uncover mechanisms in disease development and changes in host homeostasis. The body of knowledge surrounding mammalian regulatory elements, specifically regulators of chromatin structure, transcriptional and translational activation, has considerably surged within the past decade. A set of key regulators whose function still needs to be fully elucidated are small non-coding RNAs (sncRNAs). Due to their broad range of unfolding functions in the regulation of gene expression during transcription and translation, sncRNAs are becoming vital to many cellular processes. Within the past decade, a novel class of sncRNAs called PIWI-interacting RNAs (piRNAs) have been implicated in various diseases, and understanding their complete function is of vital importance. Historically, piRNAs have been shown to be indispensable in germline integrity and stem cell development. Accumulating research evidence continue to reveal the many arms of piRNA function. Although piRNA function and biogenesis has been extensively studied in Drosophila, it is thought that they play similar roles in vertebrate species, including humans. Compounding evidence suggests that piRNAs encompass a wider functional range than small interfering RNAs (siRNAs) and microRNAs (miRNAs), which have been studied more in terms of cellular homeostasis and disease. This review aims to summarize contemporary knowledge regarding biogenesis, and homeostatic function of piRNAs and their emerging roles in the development of pathologies related to cardiomyopathies, cancer, and infectious diseases.

Prokaryotic Argonaute from Archaeoglobus fulgidus interacts with DNA as a homodimer

Argonaute (Ago) proteins are found in all three domains of life. The best-characterized group is eukaryotic Argonautes (eAgos), which are the core of RNA interference. The best understood prokaryotic Ago (pAgo) proteins are full-length pAgos. They are composed of four major structural/functional domains (N, PAZ, MID, and PIWI) and thereby closely resemble eAgos. It was demonstrated that full-length pAgos function as prokaryotic antiviral systems, with the PIWI domain performing cleavage of invading nucleic acids. However, the majority of identified pAgos are shorter and catalytically inactive (encode just MID and inactive PIWI domains), thus their action mechanism and function remain unknown. In this work we focus on AfAgo, a short pAgo protein encoded by an archaeon Archaeoglobus fulgidus. We find that in all previously solved AfAgo structures, its two monomers form substantial dimerization interfaces involving the C-terminal β-sheets. Led by this finding, we have employed various biochemical and biophysical assays, including SEC-MALS, SAXS, single-molecule FRET, and AFM, to show that AfAgo is indeed a homodimer in solution, which is capable of simultaneous interaction with two DNA molecules. This finding underscores the diversity of prokaryotic Agos and broadens the range of currently known Argonaute-nucleic acid interaction mechanisms.

siRNA Specificity: RNAi Mechanisms and Strategies to Reduce Off-Target Effects

Short interfering RNAs (siRNAs) are processed from long double-stranded RNA (dsRNA), and a guide strand is selected and incorporated into the RNA-induced silencing complex (RISC). Within RISC, a member of the Argonaute protein family directly binds the guide strand and the siRNA guides RISC to fully complementary sites on-target RNAs, which are then sequence-specifically cleaved by the Argonaute protein-a process commonly referred to as RNA interference (RNAi). In animals, endogenous microRNAs (miRNAs) function similarly but do not lead to direct cleavage of the target RNA but to translational inhibition followed by exonucleolytic decay. This is due to only partial complementarity between the miRNA and the target RNA. SiRNAs, however, can function as miRNAs, and partial complementarity can lead to miRNA-like off-target effects in RNAi applications. Since siRNAs are widely used not only for screening but also for therapeutics as well as crop protection purposes, such miRNA-like off-target effects need to be minimized. Strategies such as RNA modifications or pooling of siRNAs have been developed and are used to reduce off-target effects.

Understanding the role of miRNAs for improvement of tea quality and stress tolerance

MicroRNAs (miRNAs) are an emerging class of small non-coding RNAs that exhibit important role in regulation of gene expression, mostly through the mechanism of cleavage and/or inhibition of translation of target mRNAs during or after transcription. Although much has been unravelled about the role of miRNAs in diverse biological processes like maintenance of functional integrity of genes and genome, growth and development, metabolism, and adaptive responses towards biotic and abiotic stresses in plants, not much is known on their specific roles in majority of cash crops - an area of investigation with potentially significant and gainful economic implications. Tea (Camellia sinensis) is globally the second most consumed beverage after water and its cultivation has major agro-economic and social ramifications. In recent years, global tea production has been greatly challenged by many biotic and abiotic stress factors and a deeper understanding of molecular processes regulating stress adaptation in this largely under investigated crop stands to significantly facilitate potential crop improvement strategies towards durable stress tolerance. This review endeavours to highlight recent advances in our understanding of the role of miRNAs in regulating stress tolerance traits in tea plant with additional focus on their role in determining tea quality attributes.

Homologues of Piwi control transposable elements and development of male germline in Penaeus monodon

PIWI belongs to the Argonaute protein family, which is a major protein component in RNA silencing pathway. Piwi proteins play roles in the control of transposons and germline development. They have been widely studied in vertebrates and flies, while very little is known in crustacean so far. We have previously identified and characterized a cDNA encoding Piwi protein (PmPiwi1) in the black tiger shrimp Penaeus monodon. In this study, a cDNA encoding another Piwi protein namely PmPiwi2 was identified by rapid amplification of cDNA ends (RACEs). PmPiwi2 was expressed solely in shrimp testis and ovary, indicating its potential role in germ cell development. Similar to PmPiwi1, PmPiwi2 also plays a part in the control of transposons as PmPiwi2-knockdown shrimp showed a significant increase in the expression of gypsy2 retrotransposon and mariner element in the testis. In addition, a reduction of sperm numbers in the spermatophore of PmPiwi2-knockdown shrimp suggests that PmPiwi2 is required for spermatogenesis similar to PmPiwi1. This study further demonstrated that apoptotic cell death was strongly detected in spermatogonia and spermatocyte cells of both PmPiwi-knockdown shrimp and thus, could be the cause of reduced sperm count. Investigation of sperm morphology showed a remarkably high proportion of abnormal sperms in the spermatophore of the PmPiwi1-knockdown shrimp, while PmPiwi2-knockdown shrimp had comparable percentage of abnormal sperms to the control shrimp. Consistently, the expression of KIFC1, a gene that is necessary for spermiogenesis was significantly reduced upon PmPiwi1 silencing, but not in the PmPiwi2-knockdown shrimp. Our results suggested that while both PmPiwis are required for the development of spermatid, only PmPiwi1 is possibly involved in the final stage of sperm maturation.

PIWIL1 promotes gastric cancer via a piRNA-independent mechanism

Targeted cancer therapy aims to achieve specific elimination of cancerous but not normal cells. Recently, PIWI proteins, a subfamily of the PAZ-PIWI domain (PPD) protein family, have emerged as promising candidates for targeted cancer therapy. PPD proteins are essential for small noncoding RNA pathways. The Argonaute subfamily partners with microRNA and small interfering RNA, whereas the PIWI subfamily partners with PIWI-interacting RNA (piRNA). Both PIWI proteins and piRNA are mostly expressed in the germline and best known for their function in transposon silencing, with no detectable function in mammalian somatic tissues. However, PIWI proteins become aberrantly expressed in multiple types of somatic cancers, thus gaining interest in targeted therapy. Despite this, little is known about the regulatory mechanism of PIWI proteins in cancer. Here we report that one of the four PIWI proteins in humans, PIWIL1, is highly expressed in gastric cancer tissues and cell lines. Knocking out the PIWIL1 gene (PIWIL1-KO) drastically reduces gastric cancer cell proliferation, migration, metastasis, and tumorigenesis. RNA deep sequencing of gastric cancer cell line SNU-1 reveals that KO significantly changes the transcriptome, causing the up-regulation of most of its associated transcripts. Surprisingly, few bona fide piRNAs exist in gastric cancer cells. Furthermore, abolishing the piRNA-binding activity of PIWIL1 does not affect its oncogenic function. Thus, PIWIL1 function in gastric cancer cells is independent of piRNA. This piRNA-independent regulation involves interaction with the UPF1-mediated nonsense-mediated mRNA decay (NMD) mechanism. Altogether, our findings reveal a piRNA-independent function of PIWIL1 in promoting gastric cancer.

Molecular crowding and conserved interface interactions of human argonaute protein-miRNA-target mRNA complex

RNA interference (RNAi) has been of interest given its role in genetic interference. More significantly, recent studies provided evidence of it being one of the antiviral response mechanisms in humans. Argonaute (Ago) protein plays a central role in the RNA-induced silencing complex (RISC) that cleaves mRNA. Molecular crowding in cellular systems is known to impact dynamics and interactions of biomolecules. We present here the results from our molecular dynamics simulations based study on the interfaces between Ago, miRNA and Target RNA in presence of molecular crowders. 6 simulations at 3 crowder concentrations, including the aqueous condition, were performed. Our results indicate that crowding changes the dynamics, makes the complex stabler and aids binding free energy. More importantly, features conserved across the three systems and amino acid residues with crowding resilient interactions with RNA are identified.Communicated by Ramaswamy H. Sarma.

PIWIL4 Maintains HIV-1 Latency by Enforcing Epigenetically Suppressive Modifications on the 5' Long Terminal Repeat

Although substantial progress has been made in depicting the molecular pathogenesis of human immunodeficiency virus type 1 (HIV-1) infection, the comprehensive mechanism of HIV-1 latency and the most promising therapeutic strategies to effectively reactivate the HIV-1 latent reservoir to achieve a functional cure for AIDS remain to be systematically illuminated. Here, we demonstrated that piwi (P element-induced Wimpy)-like RNA-mediated gene silencing 4 (PIWIL4) played an important role in suppressing HIV-1 transcription and contributed to the latency state in HIV-1-infected cells through its recruitment of various suppressive factors, including heterochromatin protein 1α/β/γ, SETDB1, and HDAC4. The knockdown of PIWIL4 enhanced HIV-1 transcription and reversed HIV-1 latency in both HIV-1 latently infected Jurkat T cells and primary CD4⁺ T lymphocytes and resting CD4⁺ T lymphocytes from HIV-1-infected individuals on suppressive combined antiretroviral therapy (cART). Furthermore, in the absence of PIWIL4, HIV-1 latently infected Jurkat T cells were more sensitive to reactivation with vorinostat (suberoylanilide hydroxamic acid, or SAHA), JQ1, or prostratin. These findings indicated that PIWIL4 promotes HIV-1 latency by imposing repressive marks at the HIV-1 5' long terminal repeat. Thus, the manipulation of PIWIL4 could be a novel strategy for developing promising latency-reversing agents (LRAs).IMPORTANCE HIV-1 latency is systematically modulated by host factors and viral proteins. During this process, the suppression of HIV-1 transcription plays an essential role in promoting HIV-1 latency. In this study, we found that PIWIL4 repressed HIV-1 promoter activity and maintained HIV-1 latency. In particular, we report that PIWIL4 can regulate gene expression through its association with the suppressive activity of HDAC4. Therefore, we have identified a new function for PIWIL4: it is not only a suppressor of endogenous retrotransposons but also plays an important role in inhibiting transcription and leading to latent infection of HIV-1, a well-known exogenous retrovirus. Our results also indicate a novel therapeutic target to reactivate the HIV-1 latent reservoir.

Argonaute Proteins: From Structure to Function in Development and Pathological Cell Fate Determination

The highly conserved Argonaute protein family members play a central role in the regulation of gene expression networks, orchestrating the establishment and the maintenance of cell identity throughout the entire life cycle, as well as in several human disorders, including cancers. Four functional Argonaute proteins (AGO1-4), with high structure similarity, have been described in humans and mice. Interestingly, only AGO2 is robustly expressed during human and mouse early development, in contrast to the other AGOs. Consequently, AGO2 is indispensable for early development in vivo and in vitro. Here, we review the roles of Argonaute proteins during early development by focusing on the interplay between specific domains of the protein and their function. Moreover, we report recent works highlighting the importance of AGO posttranslational modifications in cancer.

AGONOTES: A Robot Annotator for Argonaute Proteins

The argonaute protein (Ago) exists in almost all organisms. In eukaryotes, it functions as a regulatory system for gene expression. In prokaryotes, it is a type of defense system against foreign invasive genomes. The Ago system has been engineered for gene silencing and genome editing and plays an important role in biological studies. With an increasing number of genomes and proteomes of various microbes becoming available, computational tools for identifying and annotating argonaute proteins are urgently needed. We introduce AGONOTES (Argonaute Notes). It is a web service especially designed for identifying and annotating Ago. AGONOTES uses the BLASTP similarity search algorithm to categorize all submitted proteins into three groups: prokaryotic argonaute protein (pAgo), eukaryotic argonaute protein (eAgo), and non-argonaute protein (non-Ago). Argonaute proteins can then be aligned to the corresponding standard set of Ago sequences using the multiple sequence alignment program MUSCLE. All functional domains of Ago can further be curated from the alignment results and visualized easily through Bio::Graphic modules in the BioPerl bundle. Compared with existing tools such as CD-Search and available databases such as UniProt and AGONOTES showed a much better performance on domain annotations, which is fundamental in studying the new Ago. AGONOTES can be freely accessed at http://i.uestc.edu.cn/agonotes/. AGONOTES is a friendly tool for annotating Ago domains from a proteome or a series of protein sequences.

Promising approaches of small interfering RNAs (siRNAs) mediated cancer gene therapy

RNA interference (RNAi) has extensive potential to revolutionize every aspect of clinical application in biomedical research. One of the promising tools is the Small interfering RNA (siRNA) molecules within a cellular component. Principally, siRNA mediated innovative advances are increasing rapidly in support of cancer diagnosis and therapeutic purposes. Conversely, it has some delivery challenges to the site of action within the cells of a target organ, due to the progress of nucleic acids engineering and advance material science research contributing to the exceptional organ-specific targeted therapy. This siRNA based therapeutic technique definitely favors a unique and effective prospect to cancer patients. Herein, the significant drive also takes to review and summarize the major organ specific targets of diverse siRNAs based gene silencing mechanism. This machinery promisingly served as the inhibitor components for cancer development in the human model. Furthermore, the focus is also given to current applications on siRNA based quantifiable therapy leading to the silencing of cancer related gene expression in a sequence dependent and selective manner for cancer treatment. That might be a potent tool against the traditional chemotherapy techniques. Therefore, the siRNA mediated cancer gene therapy definitely require sharp attention like future weapons in opposition to cancer by the method of non-invasive siRNA delivery and effective gene silencing approaches.

The Prognosis Value of PIWIL1 and PIWIL2 Expression in Pancreatic Cancer

Pancreatic cancer is a highly aggressive manifestation of cancer, and currently presents poor clinical outcome due to its late diagnosis with metastasic disease. Surgery is the only approach with a curative intend; however, the survival rates seen in this type of patient are still low. After surgery, there is a lack of predictive prognosis biomarkers to predict treatment response and survival to establish a personalized medicine. Human P-element-induced wimpy testis 1 (PIWIL1) and P-element-induced wimpy testis 2 (PIWIL2) proteins act as protectors of germline, and their aberrant expression has been described in several types of tumors. In this study, we aimed to assess an association between PIWIL1 and PIWIL2 expression and the prognosis of biliopancreatic cancer patients. For this, we analyzed protein expression in complete resected tumor samples, and found a significant association between PIWIL2 expression and both progression-free and overall survival (p = 0.036 and p = 0.012, respectively). However, PIWIL2 expression was significantly associated with progression-free survival (p = 0.029), and overall survival (p = 0.025) of such tumors originated in the pancreas, but not in the bile duct or ampulla of Vater. Further analysis revealed that PIWIL1 and PIWIL2, at both mRNA and protein expression levels, correlated positively with factors associated to the progenitor molecular subtype of pancreatic cancer. Based on these findings, PIWIL1 and PIWIL2 expression may be considered a potential prognostic biomarker for resectable pancreatic cancer and may serve to guide subsequent adjuvant treatment decisions.

The emerging role of the piRNA/piwi complex in cancer

Piwi interacting RNAs (piRNAs) constitute novel small non-coding RNA molecules of approximately 24-31 nucleotides in length that often bind to members of the piwi protein family to play regulatory roles. Recently, emerging evidence suggests that in addition to the mammalian germline, piRNAs are also expressed in a tissue-specific manner in a variety of human tissues and modulate key signaling pathways at the transcriptional or post-transcriptional level. In addition, a growing number of studies have shown that piRNA and PIWI proteins, which are abnormally expressed in various cancers, may serve as novel biomarkers and therapeutic targets for tumor diagnostics and treatment. However, the functions of piRNAs in cancer and their underlying mechanisms remain incompletely understood. In this review, we discuss current findings regarding piRNA biogenetic processes, functions, and emerging roles in cancer, providing new insights regarding the potential applications of piRNAs and piwi proteins in cancer diagnosis and clinical treatment.

PIWI-interacting RNAs: small RNAs with big functions

In animals, PIWI-interacting RNAs (piRNAs) of 21-35 nucleotides in length silence transposable elements, regulate gene expression and fight viral infection. piRNAs guide PIWI proteins to cleave target RNA, promote heterochromatin assembly and methylate DNA. The architecture of the piRNA pathway allows it both to provide adaptive, sequence-based immunity to rapidly evolving viruses and transposons and to regulate conserved host genes. piRNAs silence transposons in the germ line of most animals, whereas somatic piRNA functions have been lost, gained and lost again across evolution. Moreover, most piRNA pathway proteins are deeply conserved, but different animals employ remarkably divergent strategies to produce piRNA precursor transcripts. Here, we discuss how a common piRNA pathway allows animals to recognize diverse targets, ranging from selfish genetic elements to genes essential for gametogenesis.

Altered PIWI-LIKE 1 and PIWI-LIKE 2 mRNA expression in ejaculated spermatozoa of men with impaired sperm characteristics

In about half the cases of involuntary childlessness, a male infertility factor is involved. The PIWI-LIKE genes, a subclade of the Argonaute protein family, are involved in RNA silencing and transposon control in the germline. Knockout of murine Piwi-like 1 and 2 homologs results in complete infertility in males. The aim of this study was to analyze whether the mRNA expression of human PIWI-LIKE 1-4 genes is altered in ejaculated spermatozoa of men with impaired sperm characteristics. Ninety male participants were included in the study, among which 47 were with normozoospermia, 36 with impaired semen characteristics according to the World Health Organization (WHO) manual, 5^th edition, and 7 with azoospermia serving as negative control for the PIWI-LIKE 1-4 mRNA expression in somatic cells in the ejaculate. PIWI-LIKE 1-4 mRNA expression in the ejaculated spermatozoa of the participants was measured by quantitative real-time PCR. In nonazoospermic men, PIWI-LIKE 1-4 mRNA was measurable in ejaculated spermatozoa in different proportions. PIWI-LIKE 1 (100.0%) and PIWI-LIKE 2 (49.4%) were more frequently expressed than PIWI-LIKE 3 (9.6%) and PIWI-LIKE 4 (15.7%). Furthermore, a decreased PIWI-LIKE 2 mRNA expression showed a significant correlation with a decreased sperm count (P = 0.022) and an increased PIWI-LIKE 1 mRNA expression with a decreased progressive motility (P = 0.048). PIWI-LIKE 1 and PIWI-LIKE 2 mRNA expression exhibited a significant association with impaired sperm characteristics and may be a useful candidate for the evaluation of the impact of PIWI-LIKE 1-4 mRNA expression on male infertility.

The AGO proteins: an overview

Small RNAs govern almost every biological process in eukaryotes associating with the Argonaute (AGO) proteins to form the RNA-induced silencing complex (mRISC). AGO proteins constitute the core of RISCs with different members having variety of protein-binding partners and biochemical properties. This review focuses on the AGO subfamily of the AGOs that are ubiquitously expressed and are associated with small RNAs. The structure, function and role of the AGO proteins in the cell is discussed in detail.

Two symmetric arginine residues play distinct roles in Thermus thermophilus Argonaute DNA guide strand-mediated DNA target cleavage

Bacterium Thermus thermophilus Argonaute (Ago; TtAgo) is a prokaryotic Ago (pAgo) that acts as the host defense against the uptake and propagation of foreign DNA by catalyzing the DNA cleavage reaction. The TtAgo active site consists of a plugged-in glutamate finger with two arginine residues (R545 and R486) located symmetrically around it. An interesting challenge is to understand how they can collaboratively facilitate enzymatic catalysis. In Kluyveromyces polysporus Ago, a eukaryotic Ago, the evolutionarily symmetrical residues are arginine and histidine, both of which function to stabilize the plugged-in catalytic tetrad conformation. Surprisingly, our simulation results indicated that, in TtAgo, only R545 is involved in the cleavage reaction by serving as a critical structural anchor to stabilize the catalytic tetrad Asp-Glu-Asp-Asp that is completed by the insertion of the glutamate finger, whereas R486 is not involved in target cleavage. The TtAgo-mediated target DNA cleavage occurs in a substrate-assisted mechanism, in which the pro-Rp (Rp, a tetrahedral phosphorus center with "R-type" chirality) oxygen of scissile phosphate acts as a general base to activate the nucleophilic water. Our unexpected theoretical findings on distinct roles played by R545 and R486 in TtAgo catalysis have been validated by single-point site-mutagenesis experiments, wherein the target cleavage is abolished for all mutants of R545. In sharp contrast, the cleavage activity is maintained for all mutants of R486. Our work provides mechanistic insights on the catalytic specificity of Ago proteins and could facilitate the design of new gene-editing tools in the long term.

The Expanded Universe of Prokaryotic Argonaute Proteins

The eukaryotic Ago proteins and the RNA interference pathways they are involved in are widely used as a powerful tool in research and as potential therapeutics. In contrast, the properties and functions of prokaryotic Ago (pAgo) proteins have remained poorly understood. Understanding the diversity and functions of pAgos holds a huge potential for discovery of new cellular pathways and novel tools for genome manipulations. Only few pAgos have been characterized by structural or biochemical approaches, while previous genomic studies discovered about 300 proteins in archaeal and eubacterial genomes. Since that time the number of bacterial strains with sequenced genomes has greatly expanded, and many previously sequenced genomes have been revised. We undertook comprehensive analysis of pAgo proteins in sequenced genomes and almost tripled the number of known genes of this family. Our research thus forms a foundation for further experimental characterization of pAgo functions that will be important for understanding of the basic biology of these proteins and their adoption as a potential tool for genome engineering in the future.

Members of the ancient family of Argonaute (Ago) proteins are present in all domains of life. The common feature of Ago proteins is the ability to bind small nucleic acid guides and use them for sequence-specific recognition—and sometimes cleavage—of complementary targets. While eukaryotic Ago (eAgo) proteins are key players in RNA interference and related pathways, the properties and functions of these proteins in archaeal and bacterial species have just started to emerge. We undertook comprehensive exploration of prokaryotic Ago (pAgo) proteins in sequenced genomes and revealed their striking diversity in comparison with eAgos. Many pAgos contain divergent variants of the conserved domains involved in interactions with nucleic acids, while having extra domains that are absent in eAgos, suggesting that they might have unusual specificities in the nucleic acid recognition and cleavage. Many pAgos are associated with putative nucleases, helicases, and DNA binding proteins in the same gene or operon, suggesting that they are involved in target processing. The great variability of pAgos revealed by our analysis opens new ways for exploration of their functions in host cells and for their use as potential tools in genome editing.

DNA silencing by prokaryotic Argonaute proteins adds a new layer of defense against invading nucleic acids

Argonaute (Ago) proteins are encoded in all three domains of life and are responsible for the regulation of intracellular nucleic acid levels. Whereas some Ago variants are able to cleave target nucleic acids by their endonucleolytic activity, others only bind to their target nucleic acids while target cleavage is mediated by other effector proteins. Although all Ago proteins show a high degree of overall structural homology, the nature of the nucleic acid binding partners differs significantly. Recent structural and functional data have provided intriguing new insights into the mechanisms of archaeal and bacterial Ago variants demonstrating the mechanistic diversity within the prokaryotic Ago family with astonishing differences in nucleic acid selection and nuclease specificity. In this review, we provide an overview of the structural organisation of archaeal Ago variants and discuss the current understanding of their biological functions that differ significantly from their eukaryotic counterparts.

DNA interference and beyond: structure and functions of prokaryotic Argonaute proteins

Recognition and repression of RNA targets by Argonaute proteins guided by small RNAs is the essence of RNA interference in eukaryotes. Argonaute proteins with diverse structures are also found in many bacterial and archaeal genomes. Recent studies revealed that, similarly to their eukaryotic counterparts, prokaryotic Argonautes (pAgos) may function in cell defense against foreign genetic elements but, in contrast, preferably act on DNA targets. Many crucial details of the pAgo action, and the roles of a plethora of pAgos with non-conventional architecture remain unknown. Here, we review available structural and biochemical data on pAgos and discuss their possible functions in host defense and other genetic processes in prokaryotic cells.

In this review, Aravin and colleagues examine bacterial and archaeal Argonaute proteins, discuss their diverse architectures and their possible roles in host defense, proposing additional functions for Argonaute proteins in prokaryotic cells.