Nuclear RNAi contributes to the silencing of off-target genes and repetitive sequences in Caenorhabditis elegans

Distinct regulatory mechanisms by the nuclear Argonautes HRDE-1 and NRDE-3 in the soma of Caenorhabditis elegans

RNA interference (RNAi) is a conserved silencing mechanism that depends on the generation of small RNA molecules that leads to the degradation of the targeted messenger RNAs (mRNAs). Nuclear RNAi is a unique process that triggers regulation through epigenetic alterations to the genome. This pathway has been extensively characterized in Caenorhabditis elegans and involves the nuclear recruitment of H3K9 histone methyltransferases by the Argonautes HRDE-1 and NRDE-3. The coordinate regulation of genetic targets by H3K9 methylation and the nuclear Argonautes is highly complex and has been mainly described based on the small RNA populations that are involved. Recent studies have also linked the nuclear RNAi pathway to the compaction of the hermaphrodite X chromosomes during dosage compensation (DC), a mechanism that balances genetic differences between the biological sexes by repressing X chromosomes in hermaphrodites. This chromosome-wide process provides an excellent opportunity to further investigate the relationship between H3K9 methylation and the nuclear Argonautes. Our work suggests that the nuclear RNAi and the H3K9 methylation pathways each contribute to the condensation of the X chromosomes during DC but the consequences on the transcriptional output of X-linked genes are minimal. Instead, nuclear RNAi mutants exhibit global transcriptional differences, in which HRDE-1 and NRDE-3 affect expression of their mRNA targets through different relationships to H3K9 methylation.

Peri-centrosomal localization of small interfering RNAs in C. elegans

The centrosome is the microtubule-organizing center and a crucial part of cell division. Centrosomal RNAs (cnRNAs) have been reported to enable precise spatiotemporal control of gene expression during cell division in many species. Whether and how cnRNAs exist in C. elegans are unclear. Here, using the nuclear RNAi Argonaute protein NRDE-3 as a reporter, we observed potential peri-centrosome localized small interfering (si)RNAs in C. elegans. NRDE-3 was previously shown to associate with pre-mRNAs and pre-rRNAs via a process involving the presence of complementary siRNAs. We generated a GFP-NRDE-3 knock-in transgene through CRISPR/Cas9 technology and observed that NRDE-3 formed peri-centrosomal foci neighboring the tubulin protein TBB-2, other centriole proteins and pericentriolar material (PCM) components in C. elegans embryos. The peri-centrosomal accumulation of NRDE-3 depends on RNA-dependent RNA polymerase (RdRP)-synthesized 22G siRNAs and the PAZ domain of NRDE-3, which is essential for siRNA binding. Mutation of eri-1, ergo-1, or drh-3 significantly increased the percentage of pericentrosome-enriched NRDE-3. At the metaphase of the cell cycle, NRDE-3 was enriched in both the peri-centrosomal region and the spindle. Moreover, the integrity of centriole proteins and pericentriolar material (PCM) components is also required for the peri-centrosomal accumulation of NRDE-3. Therefore, we concluded that siRNAs could accumulate in the pericentrosomal region in C. elegans and suggested that the peri-centrosomal region may also be a platform for RNAi-mediated gene regulation.

Evaluating the pathogenic and immunological effects of ds-GFP as a control in in vivo RNA interference studies of Schistosoma japonicum

Schistosomiasis affects over 250 million people in 78 countries. Despite praziquantel as the primary treatment, concerns about resistance in schistosomes underscore the need for alternative therapies. The success of RNA interference (RNAi) in schistosomes shows promise for identifying potential drug targets to facilitate drug discovery. Meanwhile, double-stranded RNA (dsRNA) is commonly used in functional gene analysis via RNAi, with double-stranded green fluorescent protein (ds-GFP) widely employed as a control in schistosome-related studies. However, the potential for off-target effects of dsRNAs in various biological systems raises concerns about the reliability of conventional controls in schistosome RNAi experiments. Therefore, this study aims to evaluate the safety and suitability of ds-GFP as an RNAi negative control in Schistosoma japonicum. Our data indicate that ds-GFP is innocuous and exerts no discernible impact on the host's physiology and immune responses. Comprehensive evaluations conducted in mice showed no significant alterations in body and organ weights. While a splenic immune response was observed, histopathological examinations of multiple organs confirmed the absence of significant lesions following ds-GFP treatment. Additionally, S. japonicum morphology, reproductive capacity, and host responses to parasite eggs showed no significant variations. Taken together, these findings bolster the endorsement of ds-GFP as an appropriate negative control in S. japonicum RNAi experiments, offering reliable outcomes crucial for advancing research on schistosomiasis and related parasitic diseases.

Calcium-binding protein CALU-1 is essential for proper collagen formation in Caenorhabditis elegans

Collagen, a major component of the extracellular matrix, is crucial for the structural integrity of the Caenorhabditis elegans cuticle. While several proteins involved in collagen biosynthesis have been identified, the complete regulatory network remains unclear. This study investigates the role of CALU-1, an ER-resident calcium-binding protein, in cuticle collagen formation and maintenance. We employed genetic analyses, including the generation of single and double mutants, scanning electron microscopy, and transcriptome profiling to characterize CALU-1 function. Our results demonstrate that CALU-1 is essential for proper cuticle structure, including annuli, furrows, and alae formation. Synthetic lethality was observed between calu-1 and dpy-18 (encoding a prolyl 4-hydroxylase subunit) mutations, while double mutants of calu-1 with peptidyl-prolyl cis-trans isomerase (PPIase) genes exhibited exacerbated phenotypes. CALU-1 deficiency led to altered collagen stability, increased cuticle permeability, and differential expression of stress response genes similar to collagen mutants. We conclude that CALU-1 plays a critical role in regulating collagen biosynthesis, possibly by modulating the ER environment to optimize the function of collagen-modifying enzymes. These findings provide new insights into the complex regulation of extracellular matrix formation in C. elegans, with potential implications for understanding related processes in other organisms.

Distinct regulatory mechanisms by the nuclear Argonautes HRDE-1 and NRDE-3 in the soma of Caenorhabditis elegans

RNA interference is a conserved silencing mechanism that depends on the generation of small RNA molecules that disrupt synthesis of their corresponding transcripts. Nuclear RNA interference is a unique process that triggers regulation through epigenetic alterations to the genome. This pathway has been extensively characterized in Caenorhabditis elegans and involves the nuclear recruitment of H3K9 histone methyltransferases by the Argonautes HRDE-1 and NRDE-3. The coordinate regulation of genetic targets by H3K9 methylation and the nuclear Argonautes is highly complex and has been mainly described based on the small RNA populations that are involved. Recent studies have also linked the nuclear RNAi pathway to the compaction of the hermaphrodite X chromosomes during dosage compensation, a mechanism that balances genetic differences between the biological sexes by repressing X chromosomes in hermaphrodites. This chromosome-wide process provides an excellent opportunity to further investigate the relationship between H3K9 methylation and the nuclear Argonautes from the perspective of the transcriptome. Our work suggests that the nuclear RNAi and the H3K9 methylation pathways each contribute to the condensation of the X chromosomes during dosage compensation but the consequences on their transcriptional output are minimal. Instead, nuclear RNAi mutants exhibit global transcriptional differences, in which HRDE-1 and NRDE-3 affect expression of their native targets through different modes of regulation and different relationships to H3K9 methylation.

ARTICLE SUMMARY

This study examines the transcriptional consequences during the disruption of the nuclear RNAi silencing mechanism in C. elegans . Through microscopy and bioinformatic work, we demonstrate that although nuclear RNAi mutants exhibit significantly decondensed X chromosomes, chromosome-wide transcriptional de-repression is not detectable. Downstream analyses further explore the global influence of the nuclear RNAi pathway, indicating that the nuclear Argonautes HRDE-1 and NRDE-3 function through two distinct mechanisms.

Defining an optimal control for RNAi experiments with adult Schistosoma mansoni

In parasites such as Schistosoma mansoni, gene knockdown by RNA interference (RNAi) has become an indispensable tool for functional gene characterization. To distinguish target-specific RNAi effects versus off-target effects, controls are essential. To date, however, there is still no general agreement about suitable RNAi controls, which limits the comparability between studies. To address this point, we investigated three selected dsRNAs for their suitability as RNAi controls in experiments with adult S. mansoni in vitro. Two dsRNAs were of bacterial origin, the neomycin resistance gene (neoR) and the ampicillin resistance gene (ampR). The third one, the green fluorescent protein gene (gfp), originated from jellyfish. Following dsRNA application, we analyzed physiological parameters like pairing stability, motility, and egg production as well as morphological integrity. Furthermore, using RT-qPCR we evaluated the potential of the used dsRNAs to influence transcript patterns of off-target genes, which had been predicted by si-Fi (siRNA-Finder). At the physiological and morphological levels, we observed no obvious changes in the dsRNA treatment groups compared to an untreated control. However, we detected remarkable differences at the transcript level of gene expression. Amongst the three tested candidates, we suggest dsRNA of the E. coli ampR gene as the most suitable RNAi control.

The Caenorhabditis elegans TDRD5/7-like protein, LOTR-1, interacts with the helicase ZNFX-1 to balance epigenetic signals in the germline

LOTUS and Tudor domain containing proteins have critical roles in the germline. Proteins that contain these domains, such as Tejas/Tapas in Drosophila, help localize the Vasa helicase to the germ granules and facilitate piRNA-mediated transposon silencing. The homologous proteins in mammals, TDRD5 and TDRD7, are required during spermiogenesis. Until now, proteins containing both LOTUS and Tudor domains in Caenorhabditis elegans have remained elusive. Here we describe LOTR-1 (D1081.7), which derives its name from its LOTUS and Tudor domains. Interestingly, LOTR-1 docks next to P granules to colocalize with the broadly conserved Z-granule helicase, ZNFX-1. The Tudor domain of LOTR-1 is required for its Z-granule retention. Like znfx-1 mutants, lotr-1 mutants lose small RNAs from the 3' ends of WAGO and mutator targets, reminiscent of the loss of piRNAs from the 3' ends of piRNA precursor transcripts in mouse Tdrd5 mutants. Our work shows that LOTR-1 acts with ZNFX-1 to bring small RNA amplifying mechanisms towards the 3' ends of its RNA templates.

Small RNA pathways in the nematode Ascaris in the absence of piRNAs

Small RNA pathways play key and diverse regulatory roles in C. elegans, but our understanding of their conservation and contributions in other nematodes is limited. We analyzed small RNA pathways in the divergent parasitic nematode Ascaris. Ascaris has ten Argonautes with five worm-specific Argonautes (WAGOs) that associate with secondary 5’-triphosphate 22-24G-RNAs. These small RNAs target repetitive sequences or mature mRNAs and are similar to the C. elegans mutator, nuclear, and CSR-1 small RNA pathways. Even in the absence of a piRNA pathway, Ascaris CSR-1 may still function to “license” as well as fine-tune or repress gene expression. Ascaris ALG-4 and its associated 26G-RNAs target and likely repress specific mRNAs during testis meiosis. Ascaris WAGO small RNAs demonstrate target plasticity changing their targets between repeats and mRNAs during development. We provide a unique and comprehensive view of mRNA and small RNA expression throughout spermatogenesis. Overall, our study illustrates the conservation, divergence, dynamics, and flexibility of small RNA pathways in nematodes.

The parasitic nematode Ascaris lacks piRNAs. Here the authors compare Argonaute proteins and small RNAs from C. elegans and Ascaris, expanding our understanding of the conservation, divergence, and flexibility of Argonautes and small RNA pathways in nematodes.

Mechanisms of epigenetic regulation by C. elegans nuclear RNA interference pathways

RNA interference (RNAi) is a highly conserved gene regulatory phenomenon whereby Argonaute/small RNA (AGO/sRNA) complexes target transcripts by antisense complementarity to modulate gene expression. While initially appreciated as a cytoplasmic process, RNAi can also occur in the nucleus where AGO/sRNA complexes are recruited to nascent transcripts. Nuclear AGO/sRNA complexes recruit co-factors that regulate transcription by inhibiting RNA Polymerase II, modifying histones, compacting chromatin and, in some organisms, methylating DNA. C. elegans has a longstanding history in unveiling the mechanisms of RNAi and has become an outstanding model to delineate the mechanisms underlying nuclear RNAi. In this review we highlight recent discoveries in the field of nuclear RNAi in C. elegans and the roles of nuclear RNAi in the regulation of gene expression, chromatin organization, genome stability, and transgenerational epigenetic inheritance.

Imaging of native transcription and transcriptional dynamics in vivo using a tagged Argonaute protein

A flexible method to image unmodified transcripts and transcription in vivo would be a valuable tool to understand the regulation and dynamics of transcription. Here, we present a novel approach to follow native transcription, with fluorescence microscopy, in live C. elegans. By using the fluorescently tagged Argonaute protein NRDE-3, programmed by exposure to defined dsRNA to bind to nascent transcripts of the gene of interest, we demonstrate transcript labelling of multiple genes, at the transcription site and in the cytoplasm. This flexible approach does not require genetic manipulation, and can be easily scaled up by relying on whole-genome dsRNA libraries. We apply this method to image the transcriptional dynamics of the heat-shock inducible gene hsp-4 (a member of the hsp70 family), as well as two transcription factors: ttx-3 (a LHX2/9 orthologue) in embryos, and hlh-1 (a MyoD orthologue) in larvae, respectively involved in neuronal and muscle development.

Intrinsically disordered protein PID-2 modulates Z granules and is required for heritable piRNA-induced silencing in the Caenorhabditis elegans embryo

In Caenorhabditis elegans, the piRNA (21U RNA) pathway is required to establish proper gene regulation and an immortal germline. To achieve this, PRG-1-bound 21U RNAs trigger silencing mechanisms mediated by RNA-dependent RNA polymerase (RdRP)-synthetized 22G RNAs. This silencing can become PRG-1-independent and heritable over many generations, a state termed RNA-induced epigenetic gene silencing (RNAe). How and when RNAe is established, and how it is maintained, is not known. We show that maternally provided 21U RNAs can be sufficient for triggering RNAe in embryos. Additionally, we identify PID-2, a protein containing intrinsically disordered regions (IDRs), as a factor required for establishing and maintaining RNAe. PID-2 interacts with two newly identified and partially redundant eTudor domain-containing proteins, PID-4 and PID-5. PID-5 has an additional domain related to the X-prolyl aminopeptidase APP-1, and binds APP-1, implicating potential N-terminal proteolysis in RNAe. All three proteins are required for germline immortality, localize to perinuclear foci, affect size and appearance of RNA inheritance-linked Z granules, and are required for balancing of 22G RNA populations. Overall, our study identifies three new proteins with crucial functions in C. elegans small RNA silencing.

Epimutations driven by small RNAs arise frequently but most have limited duration in Caenorhabditis elegans

Epigenetic regulation involves changes in gene expression independent of DNA sequence variation that are inherited through cell division. In addition to a fundamental role in cell differentiation, some epigenetic changes can also be transmitted transgenerationally through meiosis. Epigenetic alterations (epimutations) could thus contribute to heritable variation within populations and be subject to evolutionary processes such as natural selection and drift. However, the rate at which epimutations arise and their typical persistence are unknown, making it difficult to evaluate their potential for evolutionary adaptation. Here, we perform a genome-wide study of epimutations in a metazoan organism. We use experimental evolution to characterize the rate, spectrum and stability of epimutations driven by small silencing RNAs in the model nematode Caenorhabditis elegans. We show that epimutations arise spontaneously at a rate approximately 25 times greater than DNA sequence changes and typically have short half-lives of two to three generations. Nevertheless, some epimutations last at least ten generations. Epimutations mediated by small RNAs may thus contribute to evolutionary processes over a short timescale but are unlikely to bring about long-term divergence in the absence of selection.

Predicting gene essentiality in Caenorhabditis elegans by feature engineering and machine-learning

Defining genes that are essential for life has major implications for understanding critical biological processes and mechanisms. Although essential genes have been identified and characterised experimentally using functional genomic tools, it is challenging to predict with confidence such genes from molecular and phenomic data sets using computational methods. Using extensive data sets available for the model organism Caenorhabditis elegans, we constructed here a machine-learning (ML)-based workflow for the prediction of essential genes on a genome-wide scale. We identified strong predictors for such genes and showed that trained ML models consistently achieve highly-accurate classifications. Complementary analyses revealed an association between essential genes and chromosomal location. Our findings reveal that essential genes in C. elegans tend to be located in or near the centre of autosomal chromosomes; are positively correlated with low single nucleotide polymorphim (SNP) densities and epigenetic markers in promoter regions; are involved in protein and nucleotide processing; are transcribed in most cells; are enriched in reproductive tissues or are targets for small RNAs bound to the argonaut CSR-1. Based on these results, we hypothesise an interplay between epigenetic markers and small RNA pathways in the germline, with transcription-based memory; this hypothesis warrants testing. From a technical perspective, further work is needed to evaluate whether the present ML-based approach will be applicable to other metazoans (including Drosophila melanogaster) for which comprehensive data sets (i.e. genomic, transcriptomic, proteomic, variomic, epigenetic and phenomic) are available.

A Dual Reporter EndoC-βH1 Human β-Cell Line for Efficient Quantification of Calcium Flux and Insulin Secretion

Human in vitro model systems of diabetes are critical to both study disease pathophysiology and offer a platform for drug testing. We have generated a set of tools in the human β-cell line EndoC-βH1 that allows the efficient and inexpensive characterization of β-cell physiology and phenotypes driven by disruption of candidate genes. First, we generated a dual reporter line that expresses a preproinsulin-luciferase fusion protein along with GCaMP6s. This reporter line allows the quantification of insulin secretion by measuring luciferase activity and calcium flux, a critical signaling step required for insulin secretion, via fluorescence microscopy. Using these tools, we demonstrate that the generation of the reporter human β-cell line was highly efficient and validated that luciferase activity could accurately reflect insulin secretion. Second, we used a lentiviral vector carrying the CRISPR-Cas9 system to generate candidate gene disruptions in the reporter line. We also show that we can achieve gene disruption in ~90% of cells using a CRISPR-Cas9 lentiviral system. As a proof of principle, we disrupt the β-cell master regulator, PDX1, and show that mutant EndoC-βH1 cells display impaired calcium responses and fail to secrete insulin when stimulated with high glucose. Furthermore, we show that PDX1 mutant EndoC-βH1 cells exhibit decreased expression of the β-cell-specific genes MAFA and NKX6.1 and increased GCG expression. The system presented here provides a platform to quickly and easily test β-cell functionality in wildtype and cells lacking a gene of interest.

A Cytoplasmic Argonaute Protein Promotes the Inheritance of RNAi

RNAi-elicited gene silencing is heritable and can persist for multiple generations after its initial induction in C. elegans. However, the mechanism by which parental-acquired trait-specific information from RNAi is inherited by the progenies is not fully understood. Here, we identified a cytoplasmic Argonaute protein, WAGO-4, necessary for the inheritance of RNAi. WAGO-4 exhibits asymmetrical translocation to the germline during early embryogenesis, accumulates at the perinuclear foci in the germline, and is required for the inheritance of exogenous RNAi targeting both germline- and soma-expressed genes. WAGO-4 binds to 22G-RNAs and their mRNA targets. Interestingly, WAGO-4-associated endogenous 22G-RNAs target the same cohort of germline genes as CSR-1 and contain untemplated addition of uracil at the 3' ends. The poly(U) polymerase CDE-1 is required for the untemplated uridylation of 22G-RNAs and inheritance of RNAi. Therefore, we conclude that, in addition to the nuclear RNAi pathway, the cytoplasmic RNAi machinery also promotes RNAi inheritance.

Should dsRNA treatments applied in outdoor environments be regulated?

The New Zealand Environmental Protection Authority (EPA) issued a Decision that makes the use of externally applied double-stranded (ds)RNA molecules on eukaryotic cells or organisms technically out of scope of legislation on new organisms, making risk assessments of such treatments in the open environment unnecessary. The Decision was based on its view that the treatment does not create new or genetically modified organisms and rests on the EPA's conclusions that dsRNA is not heritable and is not a mutagen. For these reasons EPA decided that treatments using dsRNA do not modify genes or other genetic material. I found from an independent review of the literature on the topic indicated, however, that each of the major scientific justifications relied upon by the EPA was based on either an inaccurate interpretation of evidence or failure to consult the research literature pertaining to additional types of eukaryotes. The Decision also did not take into account the unknown and unique eukaryotic biodiversity of New Zealand. The safe use of RNA-based technology holds promise for addressing complex and persistent challenges in public health, agriculture and conservation. However, by failing to restrict the source or means of modifying the dsRNA, the EPA removed regulatory oversight that could prevent unintended consequences of this new technology such as suppression of genes other than those selected for suppression or the release of viral genes or genomes by failing to restrict the source or means of modifying the dsRNA.

Germ Granules Govern Small RNA Inheritance

In C. elegans nematodes, components of liquid-like germ granules were shown to be required for transgenerational small RNA inheritance. Surprisingly, we show here that mutants with defective germ granules can nevertheless inherit potent small RNA-based silencing responses, but some of the mutants lose this ability after many generations of homozygosity. Animals mutated in pptr-1, which is required for stabilization of P granules in the early embryo, display extraordinarily strong heritable RNAi responses, lasting for tens of generations. Intriguingly, the RNAi capacity of descendants derived from mutants defective in the core germ granule proteins MEG-3 and MEG-4 is determined by the genotype of the ancestors and changes transgenerationally. Further, whether the meg-3/4 mutant alleles were present in the paternal or maternal lineages leads to different transgenerational consequences. Small RNA inheritance, rather than maternal contribution of the germ granules themselves, mediates the transgenerational defects in RNAi of meg-3/4 mutants and their progeny. Accordingly, germ granule defects lead to heritable genome-wide mis-expression of endogenous small RNAs. Upon disruption of germ granules, hrde-1 mutants can inherit RNAi, although HRDE-1 was previously thought to be absolutely required for RNAi inheritance. We propose that germ granules sort and shape the RNA pool, and that small RNA inheritance maintains this activity for multiple generations.

Maternal and zygotic gene regulatory effects of endogenous RNAi pathways

Endogenous small RNAs (sRNAs) and Argonaute proteins are ubiquitous regulators of gene expression in germline and somatic tissues. sRNA-Argonaute complexes are often expressed in gametes and are consequently inherited by the next generation upon fertilization. In Caenorhabditis elegans, 26G-RNAs are primary endogenous sRNAs that trigger the expression of downstream secondary sRNAs. Two subpopulations of 26G-RNAs exist, each of which displaying strongly compartmentalized expression: one is expressed in the spermatogenic gonad and associates with the Argonautes ALG-3/4; plus another expressed in oocytes and in embryos, which associates with the Argonaute ERGO-1. The determinants and dynamics of gene silencing elicited by 26G-RNAs are largely unknown. Here, we provide diverse new insights into these endogenous sRNA pathways of C. elegans. Using genetics and deep sequencing, we dissect a maternal effect of the ERGO-1 branch of the 26G-RNA pathway. We find that maternal primary sRNAs can trigger the production of zygotic secondary sRNAs that are able to silence targets, even in the absence of zygotic primary triggers. Thus, the interaction of maternal and zygotic sRNA populations, assures target gene silencing throughout animal development. Furthermore, we explore other facets of 26G-RNA biology related to the ALG-3/4 branch. We find that sRNA abundance, sRNA pattern of origin and the 3’ UTR length of target transcripts are predictors of the regulatory outcome by the Argonautes ALG-3/4. Lastly, we provide evidence suggesting that ALG-3 and ALG-4 regulate their own mRNAs in a negative feedback loop. Altogether, we provide several new regulatory insights on the dynamics, target regulation and self-regulation of the endogenous RNAi pathways of C. elegans.

Small RNAs (sRNAs) and their partner Argonaute proteins regulate the expression of target RNAs. When sperm and egg meet upon fertilization, a diverse set of proteins and RNA, including sRNA-Argonaute complexes, is passed on to the developing progeny. Thus, these two players are important to initiate specific gene expression programs in the next generation. The nematode Caenorhabditis elegans expresses several classes of sRNAs. 26G-RNAs are a particular class of sRNAs that are divided into two subpopulations: one expressed in the spermatogenic gonad and another expressed in oocytes and in embryos. In this work, we describe the dynamics whereby oogenic 26G-RNAs setup gene silencing in the next generation. In addition, we show several ways that spermatogenic 26G-RNAs and their partner Argonautes, ALG-3 and ALG-4, use to regulate their targets. Finally, we show that ALG-3 and ALG-4 are fine-tuning their own expression, a rare role of Argonaute proteins. Overall, we provide new insights into how sRNAs and Argonautes are regulating gene expression.

Function and Evolution of Nematode RNAi Pathways

Selfish genetic elements, like transposable elements or viruses, are a threat to genomic stability. A variety of processes, including small RNA-based RNA interference (RNAi)-like pathways, has evolved to counteract these elements. Amongst these, endogenous small interfering RNA and Piwi-interacting RNA (piRNA) pathways were implicated in silencing selfish genetic elements in a variety of organisms. Nematodes have several incredibly specialized, rapidly evolving endogenous RNAi-like pathways serving such purposes. Here, we review recent research regarding the RNAi-like pathways of Caenorhabditis elegans as well as those of other nematodes, to provide an evolutionary perspective. We argue that multiple nematode RNAi-like pathways share piRNA-like properties and together form a broad nematode toolkit that allows for silencing of foreign genetic elements.

GTSF-1 is required for formation of a functional RNA-dependent RNA Polymerase complex in Caenorhabditis elegans

Argonaute proteins and their associated small RNAs (sRNAs) are evolutionarily conserved regulators of gene expression. Gametocyte-specific factor 1 (Gtsf1) proteins, characterized by two tandem CHHC zinc fingers and an unstructured C-terminal tail, are conserved in animals and have been shown to interact with Piwi clade Argonautes, thereby assisting their activity. We identified the Caenorhabditis elegans Gtsf1 homolog, named it gtsf-1 and characterized it in the context of the sRNA pathways of C. elegans We report that GTSF-1 is not required for Piwi-mediated gene silencing. Instead, gtsf-1 mutants show a striking depletion of 26G-RNAs, a class of endogenous sRNAs, fully phenocopying rrf-3 mutants. We show, both in vivo and in vitro, that GTSF-1 interacts with RRF-3 via its CHHC zinc fingers. Furthermore, we demonstrate that GTSF-1 is required for the assembly of a larger RRF-3 and DCR-1-containing complex (ERIC), thereby allowing for 26G-RNA generation. We propose that GTSF-1 homologs may act to drive the assembly of larger complexes that act in sRNA production and/or in imposing sRNA-mediated silencing activities.

A new layer of rRNA regulation by small interference RNAs and the nuclear RNAi pathway

Ribosome biogenesis drives cell growth and proliferation, but mechanisms that modulate this process remain poorly understood. For a long time, small rRNA sequences have been widely treated as non-specific degradation products and neglected as garbage sequences. Recently, we identified a new class of antisense ribosomal siRNAs (risiRNAs) that downregulate pre-rRNA through the nuclear RNAi pathway in C. elegans. risiRNAs exhibit sequence characteristics similar to 22G RNA while complement to 18S and 26S rRNA. risiRNAs elicit the translocation of the nuclear Argonaute protein NRDE-3 from the cytoplasm to nucleus and nucleolus, in which the risiRNA/NRDE complex binds to pre-rRNA and silences rRNA expression. Interestingly, when C. elegans is exposed to environmental stimuli, such as cold shock and ultraviolet illumination, risiRNAs accumulate and further turn on the nuclear RNAi-mediated gene silencing pathway. risiRNA may act in a quality control mechanism of rRNA homeostasis. When the exoribonuclease SUSI-1(ceDis3L2) is mutated, risiRNAs are dramatically increased. In this Point of View article, we will summarize our understanding of the small antisense ribosomal siRNAs in a variety of organisms, especially C. elegans, and their possible roles in the quality control mechanism of rRNA homeostasis.

Targeted Chromosomal Translocations and Essential Gene Knockout Using CRISPR/Cas9 Technology in Caenorhabditis elegans

Many genes play essential roles in development and fertility; their disruption leads to growth arrest or sterility. Genetic balancers have been widely used to study essential genes in many organisms. However, it is technically challenging and laborious to generate and maintain the loss-of-function mutations of essential genes. The CRISPR/Cas9 technology has been successfully applied for gene editing and chromosome engineering. Here, we have developed a method to induce chromosomal translocations and produce genetic balancers using the CRISPR/Cas9 technology and have applied this approach to edit essential genes in Caenorhabditis elegans. The co-injection of dual small guide RNA targeting genes on different chromosomes resulted in reciprocal translocation between nonhomologous chromosomes. These animals with chromosomal translocations were subsequently crossed with animals that contain normal sets of chromosomes. The F1 progeny were subjected to a second round of Cas9-mediated gene editing. Through this method, we successfully produced nematode strains with specified chromosomal translocations and generated a number of loss-of-function alleles of two essential genes (csr-1 and mes-6). Therefore, our method provides an easy and efficient approach to generate and maintain loss-of-function alleles of essential genes with detailed genetic background information.

The Nrde Pathway Mediates Small-RNA-Directed Histone H3 Lysine 27 Trimethylation in Caenorhabditis elegans

Small-RNA-mediated chromatin modifications have been widely studied in plants and S. pombe. However, direct evidence of small-RNA-guided sequence-specific chromatin alterations is scarce in animals. In C. elegans, the nuclear RNAi defective (Nrde) pathway functions to transport siRNA from the cytoplasm to the nucleus, modulate transcription elongation, induce histone H3 lysine 9 (H3K9) trimethylation, and mediate transgenerational inheritance of RNAi. Here, we show that both exogenous RNAi and NRDE-bound endogenous 22G RNAs can direct sequence-specific histone H3 lysine 27 (H3K27) trimethylation at targeted loci through the Nrde pathway. The resulting H3K27me3 status can be inherited by progeny for multiple generations. piRNAs and WAGO-1-associated siRNAs induce H3K27 methylation as well. Interestingly, CSR-1-associated endogenous siRNAs fail to trigger H3K27 methylation, whereas exogenous provision of dsRNAs can induce H3K27 methylation at the CSR-1-targeted loci via the Nrde pathway. We further observed distinct genetic requirements of H3K9 and H3K27 trimethylation. Whereas set-25 and met-2 are required for K9 methylation, mes-2 is required for K27 methylation. The depletion of mes-2 leads to a nuclear RNAi defective phenotype. These results indicate that dsRNA-triggered chromatin modification is a sequence-specific response that engages the Nrde pathway in C. elegans.

Dual sgRNA-directed gene knockout using CRISPR/Cas9 technology in Caenorhabditis elegans

The CRISPR RNA-guided Cas9 nuclease gene-targeting system has been successfully used for genome editing in a variety of organisms. Here, we report the use of dual sgRNA-guided Cas9 nuclease to generate knockout mutants of protein coding genes, noncoding genes, and repetitive sequences in C. elegans. Co-injection of C. elegans with dual sgRNAs results in the removal of the interval between two sgRNAs and the loss-of-function phenotype of targeted genes. We sought to determine how large an interval can be eliminated and found that at least a 24 kb chromosome segment can be deleted using this dual sgRNA/Cas9 strategy. The deletion of large chromosome segments facilitates mutant screening by PCR and agarose electrophoresis. Thus, the use of the CRISPR/Cas9 system in combination with dual sgRNAs provides a powerful platform with which to easily generate gene knockout mutants in C. elegans. Our data also suggest that encoding multiple sgRNA sequences into a single CRISPR array to simultaneously edit several sites within the genome may cause the off-target deletion of chromosome sequences.

Clone mapper: an online suite of tools for RNAi experiments in Caenorhabditis elegans

RNA interference (RNAi), mediated by the introduction of a specific double-stranded RNA, is a powerful method to investigate gene function. It is widely used in the Caenorhabditis elegans research community. An expanding number of laboratories conduct genome-wide RNAi screens, using standard libraries of bacterial clones each designed to produce a specific double-stranded RNA. Proper interpretation of results from RNAi experiments requires a series of analytical steps, from the verification of the identity of bacterial clones, to the identification of the clones’ potential targets. Despite the popularity of the technique, no user-friendly set of tools allowing these steps to be carried out accurately, automatically, and at a large scale, is currently available. We report here the design and production of Clone Mapper, an online suite of tools specifically adapted to the analysis pipeline typical for RNAi experiments with C. elegans. We show that Clone Mapper overcomes the limitations of existing techniques and provide examples illustrating its potential for the identification of biologically relevant genes. The Clone Mapper tools are freely available via http://www.ciml.univ-mrs.fr/EWBANK_jonathan/software.html.